Refining Processes

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Part XIII - Manufacturing Industries

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Chapter 82 - Metal Processing and Metal Working Industry
GENERAL PROFILE The metal smelting and refining industry processes metal ores and scrap metal to obtain pure metals. The metal working industries process metals in order to manufacture machine components, machinery, instruments and tools which are needed by other industries as well as by the other different sectors of the economy. Various types of metals and alloys are used as starting materials, including rolled stock (bars, strips, light sections, sheets or tubes) and drawn stock (bars, light sections, tubes or wire). Basic metal processing techniques include: · smelting and refining of metal ores and scrap · casting molten metals into a given shape (foundry) · hammering or pressing metals into the shape of a die (hot or cold forging) · welding and cutting sheet metal · sintering (compressing and heating materials in powder form, including one or more metals) · shaping metals on a lathe. A wide variety of techniques are used to finish metals, including grinding and polishing, abrasive blasting and many surface finishing and coating techniques (electroplating, galvanizing, heat treatment, anodizing, powder coating and so forth).

SMELTING AND REFINING
Pekka Roto* *Adapted from the 3rd edition, Encyclopaedia of Occupational Health and Safety.

In the production and refining of metals, valuable components are separated from worthless material in a series of different physical and chemical reactions. The end-product is metal containing controlled amounts of impurities. Primary smelting and refining produces metals directly from ore concentrates, while secondary smelting and refining produces metals from scrap and process waste. Scrap includes bits and pieces of metal parts, bars, turnings, sheets and wire that are off-specification or worn-out but are capable of being recycled (see the article ―Metal reclamation‖ in this chapter).

Overview of Processes
Two metal recovery technologies are generally used to produce refined metals, pyrometallurgical and hydrometallurgical. Pyrometallurgical processes use heat to separate desired metals from other materials. These processes use differences between oxidation potentials, melting points, vapour pressures, densities and/or miscibility of the ore components when melted. Hydrometallurgical technologies differ from pyrometallurgical processes in that the desired metals are separated from other materials using techniques that capitalize on differences between constituent solubilities and/or electrochemical properties while in aqueous solutions. Pyrometallurgy During pyrometallic processing, an ore, after being beneficiated (concentrated by crushing, grinding, floating and drying), is sintered or roasted (calcined) with other materials such as baghouse dust and flux. The concentrate is then smelted, or melted, in a blast furnace in order to fuse the desired metals into an impure molten bullion. This bullion then undergoes a third pyrometallic process to refine the metal to the desired level of purity. Each time the ore or bullion is heated, waste materials are created. Dust from ventilation and process gases may be captured in a baghouse and are either disposed of or returned to the process, depending upon the residual metal content. Sulphur in the gas is also captured, and when concentrations are above 4% it can be turned into sulphuric acid. Depending upon the origin of the ore and its residual metals content, various metals such as gold and silver may also be produced as by-products. Roasting is an important pyrometallurgical process. Sulphating roasting is used in the production of cobalt and zinc. Its purpose is to separate the metals so that they can be transformed into a water-soluble form for further hydrometallurgical processing. The smelting of sulphidic ores produces a partially oxidized metal concentrate (matte). In smelting, the worthless material, usually iron, forms a slag with fluxing material and is converted into the oxide. The valuable metals acquire the metallic form at the converting stage, which takes place in converting furnaces. This method is used in copper and nickel production. Iron, ferrochromium, lead, magnesium and ferrous compounds are produced by reduction of the ore with charcoal and a flux (limestone), the smelting process usually taking place in an electric furnace. (See also the Iron and steel industry chapter.) Fused salt electrolysis, used in aluminium production, is another example of a pyrometallurgical process. The high temperature required for the pyrometallurgical treatment of metals is obtained by burning fossil fuels or by using the exothermic reaction of the ore itself (e.g., in the flash

smelting process). The flash smelting process is an example of an energy-saving pyrometallurgical process in which iron and sulphur of the ore concentrate are oxidized. The exothermic reaction coupled with a heat recovery system saves a lot of energy for smelting. The high sulphur recovery of the process is also beneficial for environmental protection. Most of the recently built copper and nickel smelters use this process. Hydrometallurgy Examples of hydrometallurgical processes are leaching, precipitation, electrolytic reduction, ion exchange, membrane separation and solvent extraction. The first stage of hydrometallurgical processes is the leaching of valuable metals from less valuable material, for example, with sulphuric acid. Leaching is often preceded by pre-treatment (e.g., sulphating roasting). The leaching process often requires high pressure, the addition of oxygen or high temperatures. Leaching may also be carried out with electricity. From the leaching solution the desired metal or its compound is recovered by precipitation or reduction using different methods. Reduction is carried out, for example, in cobalt and nickel production with gas. Electrolysis of metals in aqueous solutions is also considered to be a hydrometallurgical process. In the process of electrolysis the metallic ion is reduced to the metal. The metal is in a weak acid solution from which it precipitates on cathodes under the influence of an electrical current. Most non-ferrous metals can also be refined by electrolysis. Often metallurgical processes are a combination of pyro- and hydrometallurgical processes, depending on the ore concentrate to be treated and the type of metal to be refined. An example is nickel production.

Hazards and Their Prevention
Prevention of health risks and accidents in the metallurgical industry is primarily an educational and technical question. Medical examinations are secondary and have only a complementary role in the prevention of health risks. A harmonious exchange of information and collaboration between the planning, line, safety and occupational health departments within the company give the most efficient result in the prevention of health risks. The best and least costly preventive measures are those taken at the planning stage of a new plant or process. In planning of new production facilities, the following aspects should be taken into account as a minimum: · The potential sources of air contaminants should be enclosed and isolated. · The design and placement of the process equipment should allow easy access for maintenance purposes. · Areas in which a sudden and unexpected hazard may occur should be monitored continuously. Adequate warning notices should be included. For example, areas in which arsine or hydrogen cyanide exposure might be possible should be under continuous monitoring.

· Addition and handling of poisonous process chemicals should be planned so that manual handling can be avoided. · Personal occupational hygiene sampling devices should be used in order to evaluate the real exposure of the individual worker, whenever possible. Regular fixed monitoring of gases, dusts and noise gives an overview of exposure but has only a complementary role in the evaluation of exposure dose. · In space planning, the requirements of future changes or extensions of the process should be taken into account so that the occupational hygiene standards of the plant will not worsen. · There should be a continuous system of training and education for safety and health personnel, as well as for foremen and workers. New workers in particular should be thoroughly informed about potential health risks and how to prevent them in their own working environments. In addition, training should be done whenever a new process is introduced. · Work practices are important. For example, poor personal hygiene by eating and smoking in the worksite may considerably increase personal exposure. · The management should have a health and safety monitoring system which produces adequate data for technical and economic decision making. The following are some of the specific hazards and precautions that are found in smelting and refining. Injuries The smelting and refining industry has a higher rate of injuries than most other industries. Sources of these injuries include: splattering and spills of molten metal and slag resulting in burns; gas explosions and explosions from contact of molten metal with water; collisions with moving locomotives, wagons, travelling cranes and other mobile equipment; falls of heavy objects; falls from a height (e.g., while accessing a crane cab); and slipping and tripping injuries from obstruction of floors and passageways. Precautions include: adequate training, appropriate personal protective equipment (PPE) (e.g., hard hats, safety shoes, work gloves and protective clothing); good storage, housekeeping and equipment maintenance; traffic rules for moving equipment (including defined routes and an effective signal and warning system); and a fall protection programme. Heat Heat stress illnesses such as heat stroke are a common hazard, primarily due to infrared radiation from furnaces and molten metal. This is especially a problem when strenuous work must be done in hot environments.

Prevention of heat illnesses can involve water screens or air curtains in front of furnaces, spot cooling, enclosed air-conditioned booths, heat-protective clothing and air-cooled suits, allowing sufficient time for acclimatization, work breaks in cool areas and an adequate supply of beverages for frequent drinking. Chemical hazards Exposure to a wide variety of hazardous dusts, fumes, gases and other chemicals can occur during smelting and refining operations. Crushing and grinding ore in particular can result in high exposures to silica and toxic metal dusts (e.g., containing lead, arsenic and cadmium). There can also be dust exposures during furnace maintenance operations. During smelting operations, metal fumes can be a major problem. Dust and fume emissions can be controlled by enclosure, automation of processes, local and dilution exhaust ventilation, wetting down of materials, reduced handling of materials and other process changes. Where these are not adequate, respiratory protection would be needed. Many smelting operations involve the production of large amounts of sulphur dioxide from sulphide ores and carbon monoxide from combustion processes. Dilution and local exhaust ventilation (LEV) are essential. Sulphuric acid is produced as a by-product of smelting operations and is used in electrolytic refining and leaching of metals. Exposure can occur both to the liquid and to sulphuric acid mists. Skin and eye protection and LEV is needed. The smelting and refining of some metals can have special hazards. Examples include nickel carbonyl in nickel refining, fluorides in aluminium smelting, arsenic in copper and lead smelting and refining, and mercury and cyanide exposures during gold refining. These processes require their own special precautions. Other hazards Glare and infrared radiation from furnaces and molten metal can cause eye damage including cataracts. Proper goggles and face shields should be worn. High levels of infrared radiation may also cause skin burns unless protective clothing is worn. High noise levels from crushing and grinding ore, gas discharge blowers and high-power electric furnaces can cause hearing loss. If the source of the noise cannot be enclosed or isolated, then hearing protectors should be worn. A hearing conservation program including audiometric testing and training should be instituted. Electrical hazards can occur during electrolytic processes. Precautions include proper electrical maintenance with lockout/tagout procedures; insulated gloves, clothing and tools; and ground fault circuit interrupters where needed.

Manual lifting and handling of materials can cause back and upper extremity injuries. Mechanical lifting aids and proper training in lifting methods can reduce this problem.

Pollution and Environmental Protection
Emissions of irritant and corrosive gases like sulphur dioxide, hydrogen sulphide and hydrogen chloride may contribute to air pollution and cause corrosion of metals and concrete within the plant and in the surrounding environment. The tolerance of vegetation to sulphur dioxide varies depending on the type of forest and soil. In general, evergreen trees tolerate lower concentrations of sulphur dioxide than deciduous ones. Particulate emissions may contain non-specific particulates, fluorides, lead, arsenic, cadmium and many other toxic metals. Wastewater effluent may contain a variety of toxic metals, sulphuric acid and other impurities. Solid wastes can be contaminated with arsenic, lead, iron sulphides, silica and other pollutants. Smelter management should include evaluation and control of emissions from the plant. This is specialized work which should be carried out only by personnel thoroughly familiar with the chemical properties and toxicities of the materials discharged from the plant processes. The physical state of the material, the temperature at which it leaves the process, other materials in the gas stream and other factors must all be considered when planning measures to control air pollution. It is also desirable to maintain a weather station, to keep meteorological records and to be prepared to reduce output when weather conditions are unfavourable for dispersal of stack effluents. Field trips are necessary to observe the effect of air pollution on residential and farming areas. Sulphur dioxide, one of the major contaminants, is recovered as sulphuric acid when present in sufficient quantity. Otherwise, to meet emission standards, sulphur dioxide and other hazardous gaseous wastes are controlled by scrubbing. Particulate emissions are commonly controlled by fabric filters and electrostatic precipitators. Large amounts of water are used in flotation processes such as copper concentration. Most of this water is recycled back into the process. Tailings from the flotation process are pumped as slurry into sedimentation ponds. Water is recycled in the process. Metal-containing process water and rainwater are cleaned in water-treatment plants before discharging or recycling. Solid-phase wastes include slags from smelting, blowdown slurries from sulphur dioxide conversion to sulphuric acid and sludges from surface impoundments (e.g., sedimentation ponds). Some slags can be reconcentrated and returned to smelters for reprocessing or recovery of other metals present. Many of these solid-phase wastes are hazardous wastes that must be stored according to environmental regulations.

COPPER, LEAD AND ZINC SMELTING AND REFINING*
*Adapted from EPA 1995.

Copper
Copper is mined in both open pits and underground mines, depending upon the ore grade and the nature of the ore deposit. Copper ore typically contains less that 1% copper in the form of sulphide minerals. Once the ore is delivered above the ground, it is crushed and ground to a powdery fineness and then concentrated for further processing. In the concentration process, ground ore is slurried with water, chemical reagents are added and air is blown through the slurry. The air bubbles attach themselves to the copper minerals and are then skimmed off the top of the flotation cells. The concentrate contains between 20 and 30% copper. The tailings, or gangue minerals, from the ore fall to the bottom of the cells and are removed, dewatered by thickeners and transported as a slurry to a tailings pond for disposal. All water used in this operation, from dewatering thickeners and the tailings pond, is recovered and recycled back into the process. Copper can be produced either pyrometallurgically or hydrometallurgically depending upon the ore-type used as a charge. The ore concentrates, which contain copper sulphide and iron sulphide minerals, are treated by pyrometallurgical processes to yield high purity copper products. Oxide ores, which contain copper oxide minerals that may occur in other parts of the mine, together with other oxidized waste materials, are treated by hydrometallurgical processes to yield high purity copper products. Copper conversion from the ore to metal is accomplished by smelting. During smelting the concentrates are dried and fed into one of several different types of furnaces. There the sulphide minerals are partially oxidized and melted to yield a layer of matte, a mixed copper-iron sulphide and slag, an upper layer of waste. The matte is further processed by converting. The slag is tapped from the furnace and stored or discarded in slag piles onsite. A small amount of slag is sold for railroad ballast and for sand blasting grit. A third product of the smelting process is sulphur dioxide, a gas which is collected, purified and made into sulphuric acid for sale or for use in hydrometallurgical leaching operations. Following smelting, the copper matte is fed into a converter. During this process the copper matte is poured into a horizontal cylindrical vessel (approximately 10×4 m) fitted with a row of pipes. The pipes, known as tuyères, project into the cylinder and are used to introduce air into the converter. Lime and silica are added to the copper matte to react with the iron oxide produced in the process to form slag. Scrap copper may also be added to the converter. The furnace is rotated so that the tuyères are submerged, and air is blown into the molten matte causing the remainder of the iron sulphide to react with oxygen to form iron oxide and sulphur dioxide. Then the converter is rotated to pour off the iron silicate slag. Once all of the iron is removed, the converter is rotated back and given a second blow of air during which the remainder of the sulphur is oxidized and removed from the copper sulphide. The converter is then rotated to pour off the molten copper, which at this point is called blister copper (so named because if allowed to solidify at this point, it will have a bumpy surface due to the presence of gaseous oxygen and sulphur). Sulphur dioxide from the converters is collected

and fed into the gas purification system together with that from the smelting furnace and made into sulphuric acid. Due to its residual copper content, slag is recycled back to the smelting furnace. Blister copper, containing a minimum of 98.5% copper, is refined to high purity copper in two steps. The first step is fire refining, in which the molten blister copper is poured into a cylindrical furnace, similar in appearance to a converter, where first air and then natural gas or propane are blown through the melt to remove the last of the sulphur and any residual oxygen from the copper. The molten copper is then poured into a casting wheel to form anodes pure enough for electrorefining. In electrorefining, the copper anodes are loaded into electrolytic cells and interspaced with copper starting sheets, or cathodes, in a bath of copper sulphate solution. When a direct current is passed through the cell the copper is dissolved from the anode, transported through the electrolyte and re-deposited on the cathode starting sheets. When the cathodes have built-up to sufficient thickness they are removed from the electrolytic cell and a new set of starting sheets is put in their place. Solid impurities in the anodes fall to the bottom of the cell as a sludge where they are ultimately collected and processed for the recovery of precious metals such as gold and silver. This material is known as anode slime. The cathodes removed from the electrolytic cell are the primary product of the copper producer and contain 99.99+% copper. These may be sold to wire-rod mills as cathodes or processed further to a product called rod. In manufacturing rod, cathodes are melted in a shaft furnace and the molten copper is poured onto a casting wheel to form a bar suitable for rolling into a 3/8 inch diameter continuous rod. This rod product is shipped to wire mills where it is extruded into various sizes of copper wire. In the hydrometallurgical process, the oxidized ores and waste materials are leached with sulphuric acid from the smelting process. Leaching is performed in situ, or in specially prepared piles by distributing acid across the top and allowing it to percolate down through the material where it is collected. The ground under the leach pads is lined with an acid-proof, impermeable plastic material to prevent leach liquor from contaminating groundwater. Once the copper-rich solutions are collected they can be processed by either of two processes—the cementation process or the solvent extraction/electrowinning process (SXEW). In the cementation process (which is rarely used today), the copper in the acidic solution is deposited on the surface of scrap iron in exchange for the iron. When sufficient copper has been cemented out, the copper-rich iron is put into the smelter together with the ore concentrates for copper recovery via the pyrometallurgical route. In the SXEW process, the pregnant leach solution (PLS) is concentrated by solvent extraction, which extracts copper but not impurity metals (iron and other impurities). The copper-laden organic solution is then separated from the leachate in a settling tank. Sulphuric acid is added to the pregnant organic mixture, which strips the copper into an electrolytic solution. The leachate, containing the iron and other impurities, is returned to the leaching operation where its acid is used for further leaching. The copper-rich strip solution is passed into an electrolytic cell known as an electrowinning cell. An electrowinning cell differs from an electrorefining cell in that it

uses a permanent, insoluble anode. The copper in solution is then plated onto a starting sheet cathode in much the same manner as it is on the cathode in an electrorefining cell. The copperdepleted electrolyte is returned to the solvent extraction process where it is used to strip more copper from the organic solution. The cathodes produced from the electrowinning process are then sold or made into rods in the same manner as those produced from the electrorefining process. Electrowinning cells are used also for the preparation of starting sheets for both the electrorefining and electrowinning processes by plating the copper onto either stainless steel or titanium cathodes and then stripping off the plated copper. Hazards and their prevention The major hazards are exposure to ore dusts during ore processing and smelting, metal fumes (including copper, lead and arsenic) during smelting, sulphur dioxide and carbon monoxide during most smelting operations, noise from crushing and grinding operations and from furnaces, heat stress from the furnaces and sulphuric acid and electrical hazards during electrolytic processes. Precautions include: LEV for dusts during transfer operations; local exhaust and dilution ventilation for sulphur dioxide and carbon monoxide; a noise control and hearing protection programme; protective clothing and shields, rest breaks and fluids for heat stress; and LEV, PPE and electrical precautions for electrolytic processes. Respiratory protection is commonly worn to protect against dusts, fumes and sulphur dioxide. Table 82.1 lists environmental pollutants for various steps in copper smelting and refining. Table 82.1 Process materials inputs and pollution outputs for copper smelting and refining Process Copper concentration Material input Copper ore, water, chemical reagents, thickeners Copper concentrate, sulphuric acid Copper concentrate, siliceous flux Sulphur dioxide, particulate matter containing arsenic, antimony, cadmium, lead, mercury and zinc Sulphur dioxide, particulate matter containing arsenic, antimony, cadmium, lead, mercury and zinc Air emissions Process wastes Flotation wastewaters

Copper leaching Copper smelting

Uncontrolled leachate

Copper conversion

Copper matte, scrap copper, siliceous flux

Electrolytic copper refining Blister copper, sulphuric acid

Lead
The primary lead production process consists of four steps: sintering, smelting, drossing and pyrometallurgical refining. To begin, a feedstock comprising mainly of lead concentrate in the form of lead sulphide is fed into a sintering machine. Other raw materials may be added including iron, silica, limestone flux, coke, soda, ash, pyrite, zinc, caustic and particulates gathered from pollution control devices. In the sintering machine the lead feedstock is subjected to blasts of hot air which burn off the sulphur, creating sulphur dioxide. The lead oxide material existing after this process contains about 9% of its weight in carbon. The sinter is then fed along with coke, various recycled and cleanup materials, limestone and other fluxing agents into a blast furnace for reducing, where the carbon acts as a fuel and smelts or melts the lead material. The molten lead flows to the bottom of the furnace where four layers form: ―speiss‖ (the lightest material, basically arsenic and antimony); ―matte‖ (copper sulphide and other metal sulphides); blast furnace slag (primarily silicates); and lead bullion (98% lead, by weight). All layers are then drained off. The speiss and matte are sold to copper smelters for recovery of copper and precious metals. The blast furnace slag which contains zinc, iron, silica and lime is stored in piles and partially recycled. Sulphur oxide emissions are generated in blast furnaces from small quantities of residual lead sulphide and lead sulphates in the sinter feed. Rough lead bullion from the blast furnace usually requires preliminary treatment in kettles before undergoing refining operations. During drossing, the bullion is agitated in a drossing kettle and cooled to just above its freezing point (370 to 425°C). A dross, which is composed of lead oxide, along with copper, antimony and other elements, floats to the top and solidifies above the molten lead. The dross is removed and fed into a dross furnace for recovery of the non-lead useful metals. To enhance copper recovery, drossed lead bullion is treated by adding sulphur-bearing materials, zinc, and/or aluminium, lowering the copper content to approximately 0.01%. During the fourth step, the lead bullion is refined using pyrometallurgical methods to remove any remaining non-lead saleable materials (e.g., gold, silver, bismuth, zinc, and metal oxides such as antimony, arsenic, tin and copper oxide). The lead is refined in a cast iron kettle by five stages. Antimony, tin and arsenic are removed first. Then zinc is added and gold and silver are removed in the zinc slag. Next, the lead is refined by vacuum removal (distillation) of zinc. Refining continues with the addition of calcium and magnesium. These two materials combine with bismuth to form an insoluble compound that is skimmed from the kettle. In the final step caustic soda and/or nitrates may be added to the lead to remove any remaining traces of metal impurities. The refined lead will have a purity of 99.90 to 99.99% and may be mixed with other metals to form alloys or it may be directly cast into shapes.

Hazards and their prevention The major hazards are exposure to ore dusts during ore processing and smelting, metal fumes (including lead, arsenic and antimony) during smelting, sulphur dioxide and carbon monoxide during most smelting operations, noise from grinding and crushing operations and from furnaces, and heat stress from the furnaces. Precautions include: LEV for dusts during transfer operations; local exhaust and dilution ventilation for sulphur dioxide and carbon monoxide; a noise control and hearing protection programme; and protective clothing and shields, rest breaks and fluids for heat stress. Respiratory protection is commonly worn to protect against dusts, fumes and sulphur dioxide. Biological monitoring for lead is essential. Table 82.2 lists environmental pollutants for various steps in lead smelting and refining. Table 82.2 Process materials inputs and pollution outputs for lead smelting and refining Process Lead sintering Material input Lead ore, iron, silica, limestone flux, coke, soda, ash, pyrite, zinc, caustic, baghouse dust Lead sinter, coke Air emissions Sulphur dioxide, particulate matter contain-ing cadmium and lead Sulphur dioxide, particulate Plant washdown matter contain-ing wastewater, slag cadmium and lead granulation water Process wastes

Lead smelting

Lead drossing

Lead bullion, soda ash, sulphur, baghouse dust, coke Lead drossing bullion

Lead refining

Zinc
Zinc concentrate is produced by separating the ore, which may contain as little as 2% zinc, from waste rock by crushing and flotation, a process normally performed at the mining site. The zinc concentrate is then reduced to zinc metal in one of two ways: either pyrometallurgically by distillation (retorting in a furnace) or hydrometallurgically by electrowinning. The latter accounts for approximately 80% of total zinc refining. Four processing stages are generally used in hydrometallurgic zinc refining: calcining, leaching, purification and electrowinning. Calcining, or roasting, is a high-temperature process (700 to 1000 °C) that converts zinc sulphide concentrate to an impure zinc oxide called calcine. Roaster

types include multiple-hearth, suspension or fluidized-bed. In general, calcining begins with the mixing of zinc-containing materials with coal. This mixture is then heated, or roasted, to vaporize the zinc oxide which is then moved out of the reaction chamber with the resulting gas stream. The gas stream is directed to the baghouse (filter) area where the zinc oxide is captured in baghouse dust. All of the calcining processes generate sulphur dioxide, which is controlled and converted to sulphuric acid as a marketable process by-product. Electrolytic processing of desulphurized calcine consists of three basic steps: leaching, purification and electrolysis. Leaching refers to the dissolving of the captured calcine in a solution of sulphuric acid to form a zinc sulphate solution. The calcine may be leached once or twice. In the double-leach method, the calcine is dissolved in a slightly acidic solution to remove the sulphates. The calcine is then leached a second time in a stronger solution which dissolves the zinc. This second leaching step is actually the beginning of the third step of purification because many of the iron impurities drop out of the solution as well as the zinc. After leaching, the solution is purified in two or more stages by adding zinc dust. The solution is purified as the dust forces deleterious elements to precipitate so that they can be filtered out. Purification is usually conducted in large agitation tanks. The process takes place at temperatures ranging from 40 to 85°C and pressures ranging from atmospheric to 2.4 atmospheres. The elements recovered during purification include copper as a cake and cadmium as a metal. After purification the solution is ready for the final step, electrowinning. Zinc electrowinning takes place in an electrolytic cell and involves running an electric current from a lead-silver alloy anode through the aqueous zinc solution. This process charges the suspended zinc and forces it to deposit onto an aluminium cathode which is immersed in the solution. Every 24 to 48 hours, each cell is shut down, the zinc-coated cathodes removed and rinsed, and the zinc mechanically stripped from the aluminium plates. The zinc concentrate is then melted and cast into ingots and is often as high as 99.995% pure. Electrolytic zinc smelters contain as many as several hundred cells. A portion of the electrical energy is converted into heat, which increases the temperature of the electrolyte. Electrolytic cells operate at temperature ranges from 30 to 35°C at atmospheric pressure. During electrowinning a portion of the electrolyte passes through cooling towers to decrease its temperature and to evaporate the water it collects during the process. Hazards and their prevention The major hazards are exposure to ore dusts during ore processing and smelting, metal fumes (including zinc and lead) during refining and roasting, sulphur dioxide and carbon monoxide during most smelting operations, noise from crushing and grinding operations and from furnaces, heat stress from the furnaces and sulphuric acid and electrical hazards during electrolytic processes.

Precautions include: LEV for dusts during transfer operations; local exhaust and dilution ventilation for sulphur dioxide and carbon monoxide; a noise control and hearing protection programme; protective clothing and shields, rest breaks and fluids for heat stress; and LEV, PPE, and electrical precautions for electrolytic processes. Respiratory protection is commonly worn to protect against dusts, fumes and sulphur dioxide. Table 82.3 lists environmental pollutants for various steps in zinc smelting and refining. Table 82.3 Process materials inputs and pollution outputs for zinc smelting and refining Process Zinc calcining Material input Zinc ore, coke Air emissions Sulphur dioxide, particulate matter containing zinc and lead Wastewaters containing sulphuric acid Wastewaters containing sulphuric acid, iron Dilute sulphuric acid Process wastes

Zinc leaching

Zinc calcine, sulphuric acid, limestone, spent electrolyte Zinc-acid solution, zinc dust Zinc in a sulphuric acid/aqueous solution, lead-silver alloy anodes, aluminium cathodes, barium carbonate or strontium, colloidal additives

Zinc purification Zinc electrowinning

ALUMINIUM SMELTING AND REFINING
Bertram D. Dinman

Process Overview
Bauxite is extracted by open-pit mining. The richer ores are used as mined. The lower grade ores may be beneficiated by crushing and washing to remove clay and silica waste. The production of the metal comprises two basic steps: 1. Refining. Production of alumina from bauxite by the Bayer process in which bauxite is digested at high temperature and pressure in a strong solution of caustic soda. The resulting hydrate is crystallized and calcined to the oxide in a kiln or fluid bed calciner. 2. Reduction. Reduction of alumina to virgin aluminium metal employing the Hall-Heroult electrolytic process using carbon electrodes and cryolite flux.

Experimental development suggests that in the future aluminium may be reduced to the metal by direct reduction from the ore. There are presently two major types of Hall-Heroult electrolytic cells in use. The so-called ―prebake‖ process utilizes electrodes manufactured as noted below. In such smelters exposure to polycyclic hydrocarbons normally occurs in the electrode manufacturing facilities, especially during mixing mills and forming presses. Smelters utilizing the Soderberg-type cell do not require facilities for the manufacture of baked carbon anodes. Rather, the mixture of coke and pitch binder is put into hoppers whose lower ends are immersed in the molten cryolite-alumina bath mixture. As the mixture of pitch and coke is heated by the molten metal-cryolite bath within the cell, this mixture bakes into a hard graphitic mass in situ. Metal rods are inserted into the anodic mass as conductors for a direct current electric flow. These rods must be replaced periodically; in extracting these, considerable amounts of coal tar pitch volatiles are evolved into the cell room environment. To this exposure is added those pitch volatiles generated as the baking of the pitch-coke mass proceeds. Within the last decade the industry has tended to either not replace or to modify existent Soderberg type reduction facilities as a consequence of the demonstrated carcinogenic hazard they present. In addition, with the increasing automation of reduction cell operations— particularly the changing of anodes, tasks are more commonly performed from enclosed mechanical cranes. Consequently worker exposures and the risk of developing those disorders associated with aluminium smelting are gradually decreasing in modern facilities. By contrast, in those economies wherein adequate capital investment is not readily available, the persistence of older, manually operated reduction processes will continue to present the risks of those occupational disorders (see below) previously associated with aluminium reduction plants. Indeed, this tendency will tend to become more aggravated in such older, unimproved operations, especially as they age. Carbon electrode manufacture The electrodes required by pre-bake electrolytic reduction to pure metal are normally made by a facility associated with this type of aluminium smelting plant. The anodes and cathodes are most frequently made from a mixture of ground petroleum-derived coke and pitch. Coke first is ground in ball mills, then conveyed and mixed mechanically with the pitch and finally cast into blocks in a moulding presses. These anode or cathode blocks are next heated in a gas-fired furnace for several days until they form hard graphitic masses with essentially all volatiles having been driven off. Finally they are attached to anode rods or saw-grooved to receive the cathode bars. It should be noted that the pitch used to form such electrodes represents a distillate which is derived from coal or petroleum tar. In the conversion of this tar to pitch by heating, the final pitch product has boiled off essentially all of its low-boiling point inorganics, e.g., SO2, as well as aliphatic compounds and one- and two ring aromatic compounds. Thus, such pitch should not present the same hazards in its use as coal or petroleum tars since these classes of compounds ought not to be present. There are some indications that the carcinogenic potential of such pitch

products may not be as great as the more complex mixture of tars and other volatiles associated with the incomplete combustion of coal.

Hazards and Their Prevention
The hazards and preventive measures for aluminium smelting and refining processes are basically the same as those found in smelting and refining in general; however, the individual processes present certain specific hazards. Mining Although sporadic references to ―bauxite lung‖ occur in the literature, there is little convincing evidence that such an entity exists. However, the possibility of the presence of crystalline silica in bauxite ores should be considered. Bayer process The extensive use of caustic soda in the Bayer process presents frequent risks of chemical burns of the skin and eyes. Descaling of tanks by pneumatic hammers is responsible for severe noise exposure. The potential hazards associated with the inhalation of excessive doses of aluminium oxide produced in this process are discussed below. All workers involved in the Bayer process should be well informed of the hazards associated with handling caustic soda. In all sites at risk, eyewash fountains and basins with running water and deluge showers should be provided, with notices explaining their use. PPE (e.g., goggles, gloves, aprons and boots) should be supplied. Showers and double locker accommodations (one locker for work clothing, the other for personal clothing) should be provided and all employees encouraged to wash thoroughly at the end of the shift. All workers handling molten metal should be supplied with visors, respirators, gauntlets, aprons, armlets and spats to protect them against burns, dust and fumes. Workers employed on the Gadeau low-temperature process should be supplied with special gloves and suits to protect them from hydrochloric acid fumes given off when the cells start up; wool has proved to have a good resistance to these fumes. Respirators with charcoal cartridges or alumina-impregnated masks give adequate protection against pitch and fluorine fumes; efficient dust masks are necessary for protection against carbon dust. Workers with more severe dust and fume exposure, particularly in Soderberg operations, should be provided with air-supplied respiratory protective equipment. As mechanized potroom work is remotely performed from enclosed cabins, these protective measures will become less necessary. Electrolytic reduction Electrolytic reduction exposes workers to the potential for skin burns and accidents due to molten metal splashes, heat stress disorders, noise, electrical hazards, cryolite and hydrofluoric acid fumes. Electrolytic reduction cells may emit large quantities of dusts of fluoride and alumina.

In carbon-electrode manufacturing shops, exhaust ventilation equipment with bag filters should be installed; enclosure of pitch and carbon grinding equipment further effectively minimizes exposures to heated pitches and carbon dusts. Regular checks on atmospheric dust concentrations should be made with a suitable sampling device. Periodic x-ray examinations should be carried out on workers exposed to dust, and these should be followed up by clinical examinations when necessary. In order to reduce the risk of handling pitch, transport of this material should be mechanized as far as possible (e.g., heated road tankers can be used to transport liquid pitch to the works where it is pumped automatically into heated pitch tanks). Regular skin examinations to detect erythema, epitheliomata or dermatitis are also prudent, and extra protection can be provided by alginate-base barrier creams. Workers doing hot work should be instructed prior to the onset of hot weather to increase fluid intake and heavily salt their food. They and their supervisors should also be trained to recognise incipient heat-induced disorders in themselves and their co-workers. All those working here should be trained to take the proper measure necessary to prevent the occurrence or progression of the heat disorders. Workers exposed to high noise levels should be supplied with hearing protection equipment such as earplugs which allow the passage of low-frequency noise (to allow perception of orders) but reduce the transmission of intense, high-frequency noise. Moreover, workers should undergo regular audiometric examination to detect hearing loss. Finally, personnel should also be trained to give cardiopulmonary resuscitation to victims of electric shock accidents. The potential for molten metal splashes and severe burns are widespread at many sites in reduction plants and associated operations. In addition to protective clothing (e.g., gauntlets, aprons, spats and face visors) the wearing of synthetic apparel should be prohibited, since the heat of molten metal causes such heated fibers to melt and adhere to the skin, further intensifying skin burns. Individuals using cardiac pacemakers should be excluded from reduction operations because of the risk of magnetic field induced dysrhythmias.

Other Health Effects
The hazards to workers, the general population and the environment resulting from the emission of fluoride-containing gases, smokes and dusts due to the use of cryolite flux have been widely reported (see table 82.4). In children living in the vicinity of poorly controlled aluminium smelters, variable degrees of mottling of permanent teeth have been reported if exposure occurred during the developmental phase of permanent teeth growth. Among smelter workers prior to 1950, or where inadequate control of fluoride effluents continued, variable degrees of bony fluorosis have been seen. The first stage of this condition consists of a simple increase in bone density, particularly marked in the vertebral bodies and pelvis. As fluoride is further absorbed into bone, calcification of the ligaments of the pelvis is next seen. Finally, in the event of extreme and protracted exposure to fluoride, calcification of the paraspinal and other

ligamentous structures as well as joints are noted. While this last stage has been seen in its severe form in cryolite processing plants, such advanced stages have rarely if ever been seen in aluminium smelter workers. Apparently the less severe x-ray changes in bony and ligamentous structures are not associated with alterations of the architectural or metabolic function of bone. By proper work practices and adequate ventilatory control, workers in such reduction operations can be readily prevented from developing any of the foregoing x-ray changes, despite 25 to 40 years of such work. Finally, mechanization of potroom operations should minimize if not totally eliminate any fluoride associated hazards. Table 82.4 Process materials inputs and pollution outputs for aluminium smelting and refining Process Bauxite refining Material input Air emissions Process wastes

Bauxite, sodium hydroxide Particulates, caustic/water vapour Alumina slurry, starch, water Aluminium hydrate Alumina, carbon anodes, electrolytic cells, cryolite Particulates and water vapour Fluoride—both gaseous and particulates, carbon dioxide, sulphur dioxide, carbon monoxide, C2F6 ,CF4 and perfluorinated carbons (PFC) Wastewater containing starch, sand and caustic

Alumina clarification and precipitation Alumina calcination Primary electrolytic aluminium smelting

Since the early 1980s an asthma-like condition has been definitively demonstrated among workers in aluminium reduction potrooms. This aberration, referred to as occupational asthma associated with aluminium smelting (OAAAS), is characterized by variable airflow resistance, bronchial hyperresponsiveness, or both, and is not precipitated by stimuli outside the workplace. Its clinical symptoms consist of wheezing, chest tightness and breathlessness and non-productive cough which are usually delayed some several hours following work exposures. The latent period between commencement of work exposure and the onset of OAAAS is highly variable, ranging from 1 week to 10 years, depending upon the intensity and character of the exposure. The condition usually is ameliorated with removal from the workplace following vacations and so on, but will become more frequent and severe with continued work exposures. While the occurrence of this condition has been correlated with potroom concentrations of fluoride, it is not clear that the aetiology of the disorder arises specifically from exposure to this chemical agent. Given the complex mixture of dusts and fumes (e.g., particulate and gaseous fluorides, sulphur dioxide, plus low concentrations of the oxides of vanadium, nickel and

chromium) it is more likely that such fluorides measurements represent a surrogate for this complex mixture of fumes, gases and particulates found in potrooms. It presently appears that this condition is one of an increasingly important group of occupational diseases: occupational asthma. The causal process which results in this disorder is determined with difficulty in an individual case. Signs and symptoms of OAAAS may result from: preexisting allergy-based asthma, non-specific bronchial hyperresponsiveness, the reactive airway dysfunction syndrome (RADS), or true occupational asthma. Diagnosis of this condition is presently problematic, requiring a compatible history, the presence of variable airflow limitation, or in its absence, production of pharmacologically induced bronchial hyperresponsivity. But if the latter is not demonstrable, this diagnosis is unlikely. (However, this phenomenon can eventually disappear after the disorder subsides with removal from work exposures.) Since this disorder tends to become progressively more severe with continued exposure, affected individuals most usually need be removed from continued work exposures. While individuals with pre-existent atopic asthma should initially be restricted from aluminium reduction cell rooms, the absence of atopy cannot predict whether this condition will occur subsequent to work exposures. There are presently reports suggesting that aluminium may be associated with neurotoxicity among workers engaged in smelting and welding this metal. It has been clearly shown that aluminium is absorbed via the lungs and excreted in the urine at levels greater than normal, particularly in reduction cell room workers. However, much of the literature regarding neurological effects in such workers derives from the presumption that aluminium absorption results in human neurotoxicity. Accordingly, until such associations are more reproducibly demonstrable, the connection between aluminium and occupational neurotoxicity must be considered speculative at this time. Because of the occasional need to expend in excess of 300 kcal/h in the course of changing anodes or performing other strenuous work in the presence of molten cryolite and aluminium, heat disorders may be seen during periods of hot weather. Such episodes are most likely to occur when the weather initially changes from the moderate to hot, humid conditions of summer. In addition, work practices which result in accelerated anode changing or employment over two successive work shifts during hot weather will also predispose workers to such heat disorders. Workers inadequately heat acclimatized or physically conditioned, whose salt intake is inadequate or who have intercurrent or recent illness are particularly prone to development of heat exhaustion and/or heat cramps while performing such arduous tasks. Heat stroke has occurred but rarely among aluminium smelter workers except among those with known predisposing health alterations (e.g., alcoholism, ageing). Exposure to the polycyclic aromatics associated with breathing of pitch fume and particulates have been demonstrated to place Soderberg-type reduction cell personnel in particular at an excessive risk of developing urinary bladder cancer; the excess cancer risk is less wellestablished. Workers in carbon electrode plants where mixtures of heated coke and tar are heated are assumed to also be at such risk. However, after electrodes have been baked for several days at about 1,200 °C, polycyclic aromatic compounds are practically totally combusted or

volatilized and are no longer associated with such anodes or cathodes. Hence the reduction cells utilizing prebaked electrodes have not been as clearly shown to present an undue risk of development of these malignant disorders. Other neoplasia (e.g., non-granulocytic leukaemia and brain cancers) have been suggested to occur in aluminium reduction operations; at present such evidence is fragmentary and inconsistent. In the vicinity of the electrolytic cells, the use of pneumatic crust breakers in the potrooms produce noise levels of the order of 100 dBA. The electrolytic reduction cells are run in series from a low-voltage high-amperage current supply and, consequently, cases of electric shock are not usually severe. However, in the power house at the point where the high-voltage supply joins the series-connection network of the potroom, severe electrical shock accidents may occur particularly as the electrical supply is an alternating, high voltage current. Because health concerns have been raised regarding exposures associated with electromagnetic power fields, the exposure of workers in this industry has been brought into question. It must be recognized that the power supplied to electrolytic reduction cells is direct current; accordingly, the electromagnetic fields generated in the potrooms are mainly of the static or standing field type. Such fields, in contrast to low frequency electromagnetic fields, are even less readily shown to exert consistent or reproducible biological effects, either experimentally or clinically. In addition, the flux levels of the magnetic fields measured in present day cell rooms are commonly found to be within presently proposed, tentative threshold limit values for static magnetic fields, sub-radio frequency and static electric fields. Exposure to ultra-low frequency electromagnetic fields also occur in reduction plants, especially at the far-ends of these rooms adjacent to rectifier rooms. However, the flux levels found in the nearby potrooms are minimal, well below present standards. Finally, coherent or reproducible epidemiological evidence of adverse health effects due to electromagnetic fields in aluminium reduction plants have not been convincingly demonstrated. Electrode manufacture Workers in contact with pitch fumes may develop erythema; exposure to sunlight induces photosensitization with increased irritation. Cases of localized skin tumours have occurred among carbon electrode workers where inadequate personal hygiene was practised; after excision and change of job no further spread or recurrence is usually noted. During electrode manufacture, considerable quantities of carbon and pitch dust can be generated. Where such dust exposures have been severe and inadequately controlled, there have been occasional reports that carbon electrode makers may develop simple pneumoconiosis with focal emphysema, complicated by the development of massive fibrotic lesions. Both the simple and complicated pneumoconioses are indistinguishable from the corresponding condition of coalworkers’ pneumoconiosis. The grinding of coke in ball mills produces noise levels of up to 100 dBA. Editor’s note: The aluminium production industry has been classified as a Group 1 known cause of human cancers by the International Agency for Research on Cancer (IARC). A variety of exposures have been associated with other diseases (e.g., ―potroom asthma‖) which are described elsewhere in this Encyclopaedia.

GOLD SMELTING AND REFINING
I.D. Gadaskina and L.A. Ryzik* *Adapted from the 3rd edition, Encyclopaedia of Occupational Health and Safety. Gold mining is carried out on a small scale by individual prospectors (e.g., in China and Brazil) and on a large scale in underground mines (e.g., in South Africa) and in open pit mining (e.g., in the United States). The simplest method of gold mining is panning, which involves filling a circular dish with goldbearing sand or gravel, holding it under a stream of water and swirling it. The lighter sand and gravel are gradually washed off, leaving the gold particles near the centre of the pan. More advanced hydraulic gold mining consists of directing a powerful stream of water against the gold-bearing gravel or sand. This crumbles the material and washes it away through special sluices in which the gold settles, while the lighter gravel is floated off. For river mining, elevator dredges are used, consisting of flat-bottomed boats which use a chain of small buckets to scoop up material from the river bottom and empty it into a screening container (trommel). The material is rotated in the trommel as water is directed on it. The gold-bearing sand sinks through perforations in the trommel and drops onto shaking tables for further concentration. There are two main methods for the extraction of gold from ore. These are the processes of amalgamation and cyanidation. The process of amalgamation is based on the ability of gold to alloy with metallic mercury to form amalgams of varying consistencies, from solid to liquid. The gold can be fairly easily removed from the amalgam by distilling off the mercury. In internal amalgamation, the gold is separated inside the crushing apparatus at the same time as the ore is crushed. The amalgam removed from the apparatus is washed free of any admixtures by water in special bowls. Then the remaining mercury is pressed out of the amalgam. In external amalgamation, the gold is separated outside the crushing apparatus, in amalgamators or sluices (an inclined table covered with copper sheets). Before the amalgam is removed, fresh mercury is added. The purified and washed amalgam is then pressed. In both processes the mercury is removed from the amalgam by distillation. The amalgamation process is rare today, except in small scale mining, because of environmental concerns. Extraction of gold by means of cyanidation is based on the ability of gold to form a stable watersoluble double salt KAu(CN)2 when combined with potassium cyanide in association with oxygen. The pulp resulting from the crushing of gold ore consists of larger crystalline particles, known as sands, and smaller amorphous particles, known as silt. The sand, being heavier, is deposited at the bottom of the apparatus and allows solutions (including silt) to pass through. The gold extraction process consists of feeding finely ground ore into a leaching tub and filtering a solution of potassium or sodium cyanide through it. The silt is separated from the gold cyanide solutions by adding thickeners and by vacuum filtration. Heap leaching, in which the cyanide solution is poured over a levelled heap of coarsely crushed ore, is becoming more popular, especially with low grade ores and mine tailings. In both instances, the gold is recovered from the gold cyanide solution by adding aluminium or zinc dust. In a separate operation, concentrated acid is added in a digest reactor to dissolve the zinc or aluminium, leaving behind the solid gold.

Under the influence of carbonic acid, water and air, as well as the acids present in the ore, the cyanide solutions decompose and give off hydrogen cyanide gas. In order to prevent this, alkali is added (lime or caustic soda). Hydrogen cyanide is also produced when the acid is added to dissolve the aluminium or zinc. Another cyanidation technique involves the use of activated charcoal to remove the gold. Thickeners are added to the gold cyanide solution before slurrying with activated charcoal in order to keep the charcoal in suspension. The gold-containing charcoal is removed by screening, and the gold extracted using concentrated alkaline cyanide in alcoholic solution. The gold is then recovered by electrolysis. The charcoal can be reactivated by roasting, and the cyanide can be recovered and reused. Both amalgamation and cyanidation produce metal that contains a considerable quantity of impurities, the pure gold content rarely exceeding 900 per mil fineness, unless it is further electrolytically refined in order to produce a degree of fineness of up to 999.8 per mil and more. Gold is also recovered as a by-product from the smelting of copper, lead and other metals (see the article ―Copper, lead and zinc smelting and refining‖ in this chapter).

Hazards and Their Prevention
Gold ore occurring in great depths is extracted by underground mining. This necessitates measures to prevent the formation and spread of dust in mine workings. The separation of gold from arsenical ores gives rise to arsenic exposure of mine workers and to pollution of air and soil with arsenic-containing dust. In the mercury extraction of gold, workers may be exposed to high airborne mercury concentrations when mercury is placed in or removed from the sluices, when the amalgam is purified or pressed and when the mercury is distilled off; mercury poisoning has been reported amongst amalgamation and distilling workers. The risk of mercury exposure in amalgamation has become a serious problem in several countries in the Far East and South America. In amalgamation processes the mercury must be placed on the sluices and the amalgam removed in such a manner as to ensure that the mercury does not come in contact with the skin of the hands (by using shovels with long handles, protective clothing impervious to mercury and so on). The processing of the amalgam and the removal or pressing of mercury must also be as fully mechanized as possible, with no possibility of the hands being touched by mercury; the processing of amalgam and the distilling off of mercury must be carried out in separate isolated premises in which the walls, ceilings, floors, apparatus and work surfaces are covered with material which will not absorb mercury or its vapours; all surfaces must be regularly cleaned so as to remove all mercury deposits. All premises intended for operations involving the use of mercury must be equipped with general and local exhaust ventilation. These ventilation systems must be particularly efficient in premises where mercury is distilled off. Stocks of mercury must be kept in hermetically sealed metal containers under a special exhaust hood; workers must be provided with the PPE necessary for work with mercury; and the air must be monitored

systematically in premises used for amalgamation and distilling. There should also be medical monitoring. Contamination of the air by hydrogen cyanide in cyanidation plants is dependent on air temperature, ventilation, the volume of material being processed, the concentration of the cyanide solutions in use, the quality of the reagents and the number of open installations. Medical examination of workers in gold-extracting factories has revealed symptoms of chronic hydrogen cyanide poisoning, in addition to a high frequency of allergic dermatitis, eczema and pyoderma (an acute inflammatory skin disease with pus formation). Proper organization of the preparation of cyanide solutions is particularly important. If the opening of drums containing cyanide salts and the feeding of these salts into dissolving tubs is not mechanized, there can be substantial contamination by cyanide dust and hydrogen cyanide gas. Cyanide solutions should be fed in through closed systems by automatic proportioning pumps. In gold cyanidation plants, the correct degree of alkalinity must be maintained in all cyanidation apparatus; in addition, cyanidation apparatus must be hermetically sealed and equipped with LEV backed up by adequate general ventilation and leak monitoring. All cyanidation apparatus and the walls, floors, open areas and stairs of the premises must be covered with non-porous materials and regularly cleaned with weak alkaline solutions. The use of acids to break down zinc in the processing of gold slime may give off hydrogen cyanide and arsine. These operations must therefore be performed in specially equipped and separated premises, with the use of local exhaust hoods. Smoking should be prohibited and workers should be provided with separate facilities for eating and drinking. First-aid equipment should be available and should contain material for immediately removing any cyanide solution that comes in contact with workers’ bodies and antidotes for cyanide poisoning. Workers must be supplied with personal protective clothing impervious to cyanide compounds.

Environmental Effects
There is evidence of exposure to metallic mercury vapour and methylation of mercury in nature, particularly where the gold is processed. In one study of water, settlements and fish from gold mining areas of Brazil, the mercury concentrations in edible parts of locally consumed fish surpassed by almost 6 times the Brazilian advisory level for human consumption (Palheta and Taylor 1995). In a contaminated area of Venezuela, gold prospectors have been using mercury to separate gold from auriferous sand and rock powders for many years. The high level of mercury in the surface soil and rubber sediments of the contaminated area constitutes a serious occupational and public health risk. Cyanide contamination of wastewater is also a great concern. Cyanide solutions should be treated before being released or should be recovered and reused. Emissions of hydrogen cyanide gas, for example, in the digest reactor, are treated with a scrubber before being exhausted out the stack.

FOUNDRIES
Franklin E. Mirer Founding, or metal casting, involves the pouring of molten metal into the hollow inside of a heatresistant mould which is the outside or negative shape of the pattern of the desired metal object. The mould may contain a core to determine the dimensions of any internal cavity in the final casting. Foundry work comprises: · making a pattern of the desired article · making the mould and cores and assembling the mould · melting and refining the metal · pouring the metal into the mould · cooling the metal casting · removing the mould and core from the metal casting · removing extra metal from the finished casting. The basic principles of foundry technology have changed little in thousands of years. However, processes have become more mechanized and automatic. Wooden patterns have been replaced by metal and plastic, new substances have been developed for producing cores and moulds, and a wide range of alloys are used. The most prominent foundry process is sand moulding of iron. Iron, steel, brass and bronze are traditional cast metals. The largest sector of the foundry industry produces grey and ductile iron castings. Gray iron foundries use iron or pig iron (new ingots) to make standard iron castings. Ductile iron foundries add magnesium, cerium or other additives (often called ladle additives) to the ladles of molten metal before pouring to make nodular or malleable iron castings. The different additives have little impact on workplace exposures. Steel and malleable iron make up the balance of the ferrous foundry industrial sector. The major customers of the largest ferrous foundries are the auto, construction and agricultural implement industries. Iron foundry employment has decreased as engine blocks become smaller and can be poured in a single mould, and as aluminium is substituted for cast iron. Non-ferrous foundries, especially aluminium foundry and die-cast operations, have heavy employment. Brass foundries, both free standing and those producing for the plumbing equipment industry, are a shrinking sector which, however, remains important from an occupational health perspective. In recent years, titanium, chromium, nickel and magnesium, and even more toxic metals such as beryllium, cadmium and thorium, are used in foundry products. Although the metal founding industry may be assumed to start by remelting solid material in the form of metal ingots or pigs, the iron and steel industry in the large units may be so integrated that the division is less obvious. For instance, the merchant blast furnace may turn all its output

into pig iron, but in an integrated plant some iron may be used to produce castings, thus taking part in the foundry process, and the blast furnace iron may be taken molten to be turned into steel, where the same thing can occur. There is in fact a separate section of the steel trade known for this reason as ingot moulding. In the normal iron foundry, the remelting of pig iron is also a refining process. In the non-ferrous foundries the process of melting may require the addition of metals and other substances, and thus constitutes an alloying process. Moulds made from silica sand bound with clay predominate in the iron foundry sector. Cores traditionally produced by baking silica sand bound with vegetable oils or natural sugars have been substantially replaced. Modern founding technology has developed new techniques to produce moulds and cores. In general, the health and safety hazards of foundries can be classified by type of metal cast, moulding process, size of casting and degree of mechanization.

Process Overview
On the basis of the designer’s drawings, a pattern conforming to the external shape of the finished metal casting is constructed. In the same way, a corebox is made that will produce suitable cores to dictate the internal configuration of the final article. Sand casting is the most widely used method, but other techniques are available. These include: permanent mould casting, using moulds of iron or steel; die casting, in which the molten metal, often a light alloy, is forced into a metal mould under pressures of 70 to 7,000 kgf/cm2; and investment casting, where a wax pattern is made of each casting to be produced and is covered with refractory which will form the mould into which the metal is poured. The ―lost foam‖ process uses polystyrene foam patterns in sand to make aluminium castings. Metals or alloys are melted and prepared in a furnace which may be of the cupola, rotary, reverberatory, crucible, electric arc, channel or coreless induction type (see table 82.5). Relevant metallurgical or chemical analyses are performed. Molten metal is poured into the assembled mould either via a ladle or directly from the furnace. When the metal has cooled, the mould and core material are removed (shakeout, stripping or knockout) and the casting is cleaned and dressed (despruing, shot-blasting or hydro-blasting and other abrasive techniques). Certain castings may require welding, heat treatment or painting before the finished article will meet the specifications of the buyer. Table 82.5 Types of foundry furnaces Furnace Cupola furnace Description A cupola furnace is a tall, vertical furnace, open at the top with hinged doors at the bottom. It is charged from the top with alternate layers of coke, limestone and metal; the molten metal is removed at the bottom. Special hazards include carbon monoxide and heat.

Electric arc furnace The furnace is charged with ingots, scrap, alloy metals and fluxing

agents. An arc is produced between three electrodes and the metal charge, melting the metal. A slag with fluxes covers the surface of the molten metal to prevent oxidation, to refine the metal and protect the furnace roof from excessive heat. When ready, the electrodes are raised and the furnace tilted to pour the molten metal into the receiving ladle. Special hazards include metal fumes and noise. Induction furnace An induction furnace melts the metal by passing a high electric current through copper coils on the outside of the furnace, inducing an electric current in the outer edge of the metal charge that heats the metal because of the high electrical resistance of the metal charge. Melting progresses from the outside of the charge to the inside. Special hazards include metal fumes. The crucible or container holding the metal charge is heated by a gas or oil burner. When ready, the crucible is lifted out of the furnace and tilted for pouring into moulds. Special hazards include carbon monoxide, metal fumes, noise and heat. A long, inclined rotating cylindrical furnace that is charged from the top and fired from the lower end. A type of induction furnace. This horizontal furnace consists of a fireplace at one end, separated from the metal charge by a low partition wall called the fire-bridge, and a stack or chimney at the other end. The metal is kept from contact with the solid fuel. Both the fireplace and metal charge are covered by an arched roof. The flame in its path from the fireplace to the stack is reflected downwards or reverberated on the metal beneath, melting it.

Crucible furnace

Rotary furnace Channel furnace Reverberatory furnace

Hazards such as the danger arising from the presence of hot metal are common to most foundries, irrespective of the particular casting process employed. Hazards may also be specific to a particular foundry process. For example, the use of magnesium presents flare risks not encountered in other metal founding industries. This article emphasizes iron foundries, which contain most of the typical foundry hazards. The mechanized or production foundry employs the same basic methods as the conventional iron foundry. When moulding is done, for example, by machine and castings are cleaned by shot blasting or hydroblasting, the machine usually has built-in dust control devices, and the dust hazard is reduced. However, sand is frequently moved from place to place on an open-belt conveyor, and transfer points and sand spillage may be sources of considerable quantities of airborne dust; in view of the high production rates, the airborne dust burden may be even higher than in the conventional foundry. A review of air sampling data in the middle 1970s showed higher dust levels in large American production foundries than in small foundries sampled during the same period. Installation of exhaust hoods over transfer points on belt conveyors, combined with scrupulous housekeeping, should be normal practice. Conveying by pneumatic

systems is sometimes economically possible and results in a virtually dust-free conveying system.

Iron Foundries
For simplicity, an iron foundry can be presumed to comprise the following six sections: 1. metal melting and pouring 2. pattern-making 3. moulding 4. coremaking 5. shakeout/knockout 6. casting cleaning. In many foundries, almost any of these processes may be carried out simultaneously or consecutively in the same workshop area. In a typical production foundry, iron moves from melting to pouring, cooling, shakeout, cleaning and shipping as a finished casting. Sand is cycled from sand mix, moulding, shakeout and back to sand mixing. Sand is added to the system from core making, which starts with new sand. Melting and pouring The iron founding industry relies heavily on the cupola furnace for metal melting and refining. The cupola is a tall, vertical furnace, open at the top with hinged doors at the bottom, lined with refractory and charged with coke, scrap iron and limestone. Air is blown through the charge from openings (tuyers) at the bottom; combustion of coke heats, melts and purifies the iron. Charge materials are fed into the top of the cupola by crane during operation and must be stored close at hand, usually in compounds or bins in the yard adjacent to the charging machinery. Tidiness and efficient supervision of the stacks of raw materials are essential to minimize the risk of injury from slippages of heavy objects. Cranes with large electromagnets or heavy weights are often used to reduce the scrap metal to manageable sizes for charging into the cupola and for filling the charging hoppers themselves. The crane cab should be well protected and the operators properly trained. Employees handling raw materials should wear hand leathers and protective boots. Careless charging can overfill the hopper and can cause dangerous spillage. If the charging process is found to be too noisy, the noise of metal-on-metal impact can be reduced by fitting rubber noisedampening liners to storage skips and bins. The charging platform is necessarily above ground level and can present a hazard unless it is level and has a non-slip surface and strong rails around it and any floor openings.

Cupolas generate large quantities of carbon monoxide, which may leak from the charging doors and be blown back by local eddy currents. Carbon monoxide is invisible, odourless and can quickly produce toxic ambient levels. Employees working on the charging platform or surrounding catwalks should be well trained in order to recognize the symptoms of carbon monoxide poisoning. Both continuous and spot monitoring of exposure levels are needed. Selfcontained breathing apparatus and resuscitation equipment should be maintained in readiness, and operators should be instructed in their use. When emergency work is carried out, a confinedspace entry system of contaminant monitoring should be developed and enforced. All work should be supervised. Cupolas are usually sited in pairs or groups, so that while one is being repaired the others operate. The period of use must be based on experience with durability of refractories and on engineering recommendations. Procedures must be worked out in advance for tapping out iron and for shutting down when hot spots develop or if the water cooling system is disabled. Cupola repair necessarily involves the presence of employees inside the cupola shell itself to mend or renew refractory linings. These assignments should be considered confined-space entries and appropriate precautions taken. Precautions should also be taken to prevent the discharge of material through the charging doors at such times. To protect the workers from falling objects, they should wear safety helmets and, if working at a height, safety harnesses. Workers tapping cupolas (transferring molten metal from the cupola well to a holding furnace or ladle) must observe rigorous personal protection measures. Goggles and protective clothing are essential. The eye protectors should resist both high velocity impact and molten metal. Extreme caution should be exercised in order to prevent remaining molten slag (the unwanted debris removed from the melt with the aid of the limestone additives) and metal from coming into contact with water, which will cause a steam explosion. Tappers and supervisors must ensure that any person not involved in the operation of the cupola remains outside the danger area, which is delineated by a radius of about 4 m from the cupola spout. Delineation of a nonauthorized no-entry zone is a statutory requirement under the British Iron and Steel Foundries Regulations of 1953. When the cupola run is at an end, the cupola bottom is dropped to remove the unwanted slag and other material still inside the shell before employees can carry out the routine refractory maintenance. Dropping the cupola bottom is a skilled and dangerous operation requiring trained supervision. A refractory floor or layer of dry sand on which to drop the debris is essential. If a problem occurs, such as jammed cupola bottom doors, great caution must be exercised to avoid risks of burns to workers from the hot metal and slag. Visible white-hot metal is a danger to workers’ eyes due to the emission of infrared and ultraviolet radiation, extensive exposure to which can cause cataracts. The ladle must be dried before filling with molten metal, to prevent steam explosions; a satisfactory period of flame heating must be established. Employees in metal and pouring sections of the foundry should be provided with hard hats, tinted eye protection and face shields, aluminized clothing such as aprons, gaiters or spats

(lower-leg and foot coverings) and boots. Use of protective equipment should be mandatory, and there should be adequate instruction in its use and maintenance. High standards of housekeeping and exclusion of water to the highest degree possible are needed in all areas where molten metal is being manipulated. Where large ladles are slung from cranes or overhead conveyors, positive ladle-control devices should be employed to ensure that spillage of metal cannot occur if the operator releases his or her hold. Hooks holding molten metal ladles must be periodically tested for metal fatigue to prevent failure. In production foundries, the assembled mould moves along a mechanical conveyor to a ventilated pouring station. Pouring may be from a manually controlled ladle with mechanical assist, an indexing ladle controlled from a cab, or it can be automatic. Typically, the pouring station is provided with a compensating hood with a direct air supply. The poured mould proceeds along the conveyor through an exhausted cooling tunnel until shakeout. In smaller, job shop foundries, moulds may be poured on a foundry floor and allowed to burn off there. In this situation, the ladle should be equipped with a mobile exhaust hood. Tapping and transport of molten iron and charging of electric furnaces creates exposure to iron oxide and other metal oxide fumes. Pouring into the mould ignites and pyrolyses organic materials, generating large amounts of carbon monoxide, smoke, carcinogenic polynuclear aromatic hydrocarbons (PAHs) and pyrolysis products from core materials which may be carcinogenic and also respiratory sensitizers. Moulds containing large polyurethane bound cold box cores release a dense, irritating smoke containing isocyanates and amines. The primary hazard control for mould burn off is a locally exhausted pouring station and cooling tunnel. In foundries with roof fans for exhausting pouring operations, high metal fume concentrations may be found in the upper regions where crane cabs are located. If the cabs have an operator, the cabs should be enclosed and provided with filtered, conditioned air. Pattern making Pattern making is a highly skilled trade translating the two-dimensional design plans to a threedimensional object. Traditional wooden patterns are made in standard workshops containing hand tools and electric cutting and planing equipment. Here, all reasonably practicable measures should be taken to reduce the noise to the greatest extent possible, and suitable ear protectors must be provided. It is important that the employees are aware of the advantages of using such protection. Power-driven wood cutting and finishing machines are obvious sources of danger, and often suitable guards cannot be fitted without preventing the machine from functioning at all. Employees must be well versed in normal operating procedure and should also be instructed in the hazards inherent in the work.

Wood sawing can create dust exposure. Efficient ventilation systems should be fitted to eliminate wood dust from the pattern shop atmosphere. In certain industries using hard woods, nasal cancer has been observed. This has not been studied in the founding industry. Casting in permanent metal moulds, as in die-casting, has been an important development in the foundry industry. In this case, pattern making is largely replaced by engineering methods and is really a die manufacture operation. Most of the pattern-making hazards and the risks from sand are eliminated, but are replaced by the risk inherent in the use of some sort of refractory material to coat the die or mould. In modern die-foundry work, increasing use is made of sand cores, in which case the dust hazards of the sand foundry are still present. Moulding The most common moulding process in the iron founding industry uses the traditional ―green sand‖ mould made from silica sand, coal dust, clay and organic binders. Other methods of mould production are adapted from coremaking: thermosetting, cold self-setting and gas-hardened. These methods and their hazards will be discussed under coremaking. Permanent moulds or the lost foam process may also be used, especially in the aluminium foundry industry. In production foundries, sand mix, moulding, mould assembly, pouring and shakeout are integrated and mechanized. Sand from shakeout is recycled back to the sand mix operation, where water and other additives are added and the sand is mixed in mullers to maintain the desired physical properties. For ease of assembly, patterns (and their moulds) are made in two parts. In manual mouldmaking, the moulds are enclosed in metal or wooden frames called flasks. The bottom half of the pattern is placed in the bottom flask (the drag), and first fine sand and then heavy sand are poured around the pattern. The sand is compacted in the mould by a jolt-squeeze, sand slinger or pressure process. The top flask (the cope) is prepared similarly. Wooden spacers are placed in the cope to form the sprue and riser channels, which are the pathway for the molten metal to flow into the mould cavity. The patterns are removed, the core inserted, and then the two halves of the mould assembled and fastened together, ready for pouring. In production foundries, the cope and drag flasks are prepared on a mechanical conveyor, cores are placed in the drag flask, and the mould assembled by mechanical means. Silica dust is a potential problem wherever sand is handled. Moulding sand is usually either damp or mixed with liquid resin, and is therefore less likely to be a significant source of respirable dust. A parting agent such as talc is sometimes added to promote the ready removal of the pattern from the mould. Respirable talc causes talcosis, a type of pneumoconiosis. Parting agents are more widespread where hand moulding is employed; in the larger, more automatic processes they are rarely seen. Chemicals are sometimes sprayed onto the mould surface, suspended or dissolved in isopropyl alcohol, which is then burned off to leave the compound, usually a type of graphite, coating the mould in order to achieve a casting with a finer surface finish. This involves an immediate fire risk, and all employees involved in applying these coatings should be provided with fire-retardant protective clothing and hand protection, as organic solvents can also cause dermatitis. Coatings should be applied in a ventilated booth to

prevent the organic vapours from escaping into the workplace. Strict precautions should also be observed to ensure that the isopropyl alcohol is stored and used with safety. It should be transferred to a small vessel for immediate use, and the larger storage vessels should be kept well away from the burning-off process. Manual mould making can involve the manipulation of large and cumbersome objects. The moulds themselves are heavy, as are the moulding boxes or flasks. They are often lifted, moved and stacked by hand. Back injuries are common, and power assists are needed so employees do not need to lift objects too heavy to be carried safely. Standardized designs are available for enclosures of mixers, conveyors and pouring and shakeout stations with appropriate exhaust volumes and capture and transport velocities. Adherence to such designs and strict preventive maintenance of control systems will attain compliance with international recognized limits for dust exposure. Coremaking Cores inserted into the mould determine the internal configuration of a hollow casting, such as the water jacket of an engine block. The core must withstand the casting process but at the same time must not be so strong as to resist removal from the casting during the knocking-out stage. Prior to the 1960s, core mixtures comprised sand and binders, such as linseed oil, molasses or dextrin (oil sand). The sand was packed in a core box with a cavity in the shape of the core, and then dried in an oven. Core ovens evolve harmful pyrolysis products and require a suitable, well maintained chimney system. Normally, convection currents within the oven will be sufficient to ensure satisfactory removal of fumes from the workplace, although they contribute enormously to air pollution After removal from the oven, the finished oil sand cores can still give rise to a small amount of smoke, but the hazard is minor; in some cases, however, small amounts of acrolein in the fumes may be a considerable nuisance. Cores may be treated with a ―flare-off coating‖ to improve the surface finish of the casting, which calls for the same precautions as in the case of moulds. Hot box or shell moulding and coremaking are thermosetting processes used in iron foundries. New sand may be mixed with resin at the foundry, or resin-coated sand may be shipped in bags for addition to the coremaking machine. Resin sand is injected into a metal pattern (the core box). The pattern is then heated—by direct natural gas fires in the hot box process or by other means for shell cores and moulding. Hot boxes typically use a furfuryl alcohol (furan), urea- or phenol-formaldehyde thermosetting resin. Shell moulding uses a urea- or phenol-formaldehyde resin. After a short curing time, the core hardens considerably and can be pushed clear of the pattern plate by ejector pins. Hot box and shell coremaking generate substantial exposure to formaldehyde, which is a probable carcinogen, and other contaminants, depending on the system. Control measures for formaldehyde include direct air supply at the operator station, local exhaust at the corebox, enclosure and local exhaust at the core storage station and low-formaldehydeemission resins. Satisfactory control is difficult to achieve. Medical surveillance for respiratory conditions should be provided to coremaking workers. Phenol- or urea-formaldehyde resin

contact with the skin or eyes must be prevented because the resins are irritants or sensitizers and can cause dermatitis. Copious washing with water will help to avoid the problem. Cold-setting (no-bake) hardening systems presently in use include: acid-catalyzed urea- and phenol-formaldehyde resins with and without furfuryl alcohol; alkyd and phenolic isocyanates; Fascold; self-set silicates; Inoset; cement sand and fluid or castable sand. Cold-setting hardeners do not require external heating to set. The isocyanates employed in binders are normally based on methylene diphenyl isocyanate (MDI), which, if inhaled, can act as a respiratory irritant or sensitizer, causing asthma. Gloves and protective goggles are advisable when handling or using these compounds. The isocyanates themselves should be carefully stored in sealed containers in dry conditions at a temperature between 10 and 30°C. Empty storage vessels should be filled and soaked for 24 hours with a 5% sodium carbonate solution in order to neutralize any residual chemical left in the drum. Most general housekeeping principles should be strictly applied to resin moulding processes, but the greatest caution of all should be exercised when handling the catalysts used as setting agents. The catalysts for the phenol and oil isocyanate resins are usually aromatic amines based on pyridine compounds, which are liquids with a pungent smell. They can cause severe skin irritation and renal and hepatic damage and can also affect the central nervous system. These compounds are supplied either as separate additives (three-part binder) or are ready mixed with the oil materials, and LEV should be provided at the mixing, moulding, casting and knockout stages. For certain other no-bake processes the catalysts used are phosphoric or various sulphonic acids, which are also toxic; accidents during transport or use should be adequately guarded against. Gas-hardened coremaking comprises the carbon dioxide (CO2)-silicate and the Isocure (or ―Ashland‖) processes. Many variations of the CO2-silicate process have been developed since the 1950s. This process has generally been used for the production of medium to large moulds and cores. The core sand is a mixture of sodium silicate and silica sand, usually modified by adding such substances as molasses as breakdown agents. After the core box is filled, the core is cured by passing carbon dioxide through the core mixture. This forms sodium carbonate and silica gel, which acts as a binder. Sodium silicate is an alkaline substance, and can be harmful if it comes into contact with the skin or eyes or is ingested. It is advisable to provide an emergency shower close to areas where large quantities of sodium silicate are handled and gloves should always be worn. A readily available eye-wash fountain should be located in any foundry area where sodium silicate is used. The CO2 can be supplied as a solid, liquid or gas. Where it is supplied in cylinders or pressure tanks, a great many housekeeping precautions should be taken, such as cylinder storage, valve maintenance, handling and so on. There is also the risk from the gas itself, since it can lower the oxygen concentration in the air in enclosed spaces. The Isocure process is used for cores and moulds. This is a gas-setting system in which a resin, frequently phenol-formaldehyde, is mixed with a di-isocyanate (e.g., MDI) and sand. This is injected into the core box and then gassed with an amine, usually either triethylamine or dimethylethylamine, to cause the crosslinking, setting reaction. The amines, often sold in drums, are highly volatile liquids with a strong smell of ammonia. There is a very real risk of fire or explosion, and extreme care should be taken, especially where the material is stored in bulk. The

characteristic effect of these amines is to cause halo vision and corneal swelling, although they also affect the central nervous system, where they can cause convulsions, paralysis and, occasionally, death. Should some of the amine come into contact with the eyes or skin, first-aid measures should include washing with copious quantities of water for at least 15 minutes and immediate medical attention. In the Isocure process, the amine is applied as a vapour in a nitrogen carrier, with excess amine scrubbed through an acid tower. Leakage from the corebox is the principle cause of high exposure, although offgassing of amine from manufactured cores is also significant. Great care should be taken at all times when handling this material, and suitable exhaust ventilation equipment should be installed to remove vapours from the working areas. Shakeout, casting extraction and core knockout After the molten metal has cooled, the rough casting must be removed from the mould. This is a noisy process, typically exposing operators well above 90 dBA over an 8 hour working day. Hearing protectors should be provided if it is not practicable to reduce the noise output. The main bulk of the mould is separated from the casting usually by jarring impact. Frequently the moulding box, mould and casting are dropped onto a vibrating grid to dislodge the sand (shakeout). The sand then drops through the grid into a hopper or onto a conveyor where it can be subjected to magnetic separators and recycled for milling, treatment and re-use, or merely dumped. Sometimes hydroblasting can be used instead of a grid, creating less dust. The core is removed here, also sometimes using high-pressure water streams. The casting is then removed and transferred to the next stage of the knockout operation. Often small castings can be removed from the flask by a ―punch-out‖ process before shakeout, which produces less dust. The sand gives rise to hazardous silica dust levels because it has been in contact with molten metal and is therefore very dry. The metal and sand remain very hot. Eye protection is needed. Walking and working surfaces must be kept free of scrap, which is a tripping hazard, and of dust, which can be resuspended to pose an inhalation hazard. Relatively few studies have been carried out to determine what effect, if any, the new core binders have on the health of the de-coring operator in particular. The furanes, furfuryl alcohol and phosphoric acid, urea- and phenol-formaldehyde resins, sodium silicate and carbon dioxide, no-bakes, modified linseed oil and MDI, all undergo some type of thermal decomposition when exposed to the temperatures of the molten metals. No studies have yet been conducted on the effect of the resin-coated silica particle on the development of pneumoconiosis. It is not known whether these coatings will have an inhibiting or accelerating effect on lung-tissue lesions. It is feared that the reaction products of phosphoric acid may liberate phosphine. Animal experiments and some selected studies have shown that the effect of the silica dust on lung tissue is greatly accelerated when silica has been treated with a mineral acid. Urea- and phenol-formaldehyde resins can release free phenols, aldehydes and carbon monoxide. The sugars added to increase collapsibility produce significant amounts of carbon monoxide. No-bakes will release isocyanates (e.g., MDI) and carbon monoxide. Fettling (cleaning)

Casting cleaning, or fettling, is carried out following shakeout and core knockout. The various processes involved are variously designated in different places but can be broadly classified as follows: · Dressing covers stripping, roughing or mucking-off, removal of adherent moulding sand, core sand, runners, risers, flash and other readily disposable matter with hand tools or portable pneumatic tools. · Fettling covers removal of burnt-on moulding sand, rough edges, surplus metal, such as blisters, stumps of gates, scabs or other unwanted blemishes, and the hand cleaning of the casting using hand chisels, pneumatic tools and wire brushes. Welding techniques, such as oxyacetyleneflame cutting, electric arc, arc-air, powder washing and the plasma torch, may be employed for burning off headers, for casting repair and for cutting and washing. Sprue removal is the first dressing operation. As much as half of the metal cast in the mould is not part of the final casting. The mould must include reservoirs, cavities, feeders and sprue in order that it be filled with metal to complete the cast object. The sprue usually can be removed during the knockout stage, but sometimes this must be carried out as a separate stage of the fettling or dressing operation. Sprue removal is done by hand, usually by knocking the casting with a hammer. To reduce noise, the metal hammers can be replaced by rubber-covered ones and the conveyors lined with the same noise-damping rubber. Hot metal fragments are thrown off and pose an eye hazard. Eye protection must be used. Detached sprues should normally be returned to the charging region of the melting plant and should not be permitted to accumulate at the despruing section of the foundry. After despruing (but sometimes before) most castings are shot blasted or tumbled to remove mould materials and perhaps to improve the surface finish. Tumbling barrels generate high noise levels. Enclosures may be necessary, which can also require LEV. Dressing methods in steel, iron and non-ferrous foundries are very similar, but special difficulties exist in the dressing and fettling of steel castings owing to greater amounts of burnt-on fused sand compared to iron and non-ferrous castings. Fused sand on large steel castings may contain cristobalite, which is more toxic than the quartz found in virgin sand. Airless shot blasting or tumbling of castings before chipping and grinding is needed to prevent overexposure to silica dust. The casting must be free of visible dust, although a silica hazard may still be generated by grinding if silica is burnt into the apparently clean metal surface of the casting. The shot is centrifugally propelled at the casting, and no operator is required inside the unit. The blast cabinet must be exhausted so no visible dust escapes. Only when there is a breakdown or deterioration of the shot-blast cabinet and/or the fan and collector is there a dust problem. Water or water and sand or pressure shot blasting may be used to remove adherent sand by subjecting the casting to a high-pressure stream of either water or iron or steel shot. Sand blasting has been banned in several countries (e.g., the United Kingdom) because of the silicosis risk as the sand particles become finer and finer and the respirable fraction thus continually increases. The water or shot is discharged through a gun and can clearly present a risk to

personnel if not handled correctly. Blasting should always be carried out in an isolated, enclosed space. All blasting enclosures should be inspected at regular intervals to ensure that the dust extraction system is functioning and that there are no leaks through which shot or water could escape into the foundry. Blasters’ helmets should be approved and carefully maintained. It is advisable to post a notice on the door to the booth, warning employees that blasting is under way and that unauthorized entry is prohibited. In certain circumstances delay bolts linked to the blast drive motor can be fitted to the doors, making it impossible to open the doors until blasting has ceased. A variety of grinding tools are used to smooth the rough casting. Abrasive wheels may be mounted on floor-standing or pedestal machines or in portable or swing-frame grinders. Pedestal grinders are used for smaller castings that can be easily handled; portable grinders, surface disc wheels, cup wheels and cone wheels are used for a number of purposes, including smoothing of internal surfaces of castings; swing-frame grinders are used primarily on large castings that require a great deal of metal removal.

Other Foundries
Steel founding Production in the steel foundry (as distinct from a basic steel mill) is similar to that in the iron foundry; however, the metal temperatures are much higher. This means that eye protection with coloured lenses is essential and that the silica in the mould is converted by heat to tridymite or crystobalite, two forms of crystalline silica which are particularly dangerous to the lungs. Sand often becomes burnt on to the casting and has to be removed by mechanical means, which give rise to dangerous dust; consequently, effective dust exhaust systems and respiratory protection are essential. Light-alloy founding The light-alloy foundry uses mainly aluminium and magnesium alloys. These often contain small amounts of metals which may give off toxic fumes under certain circumstances. The fumes should be analysed to determine their constituents where the alloy might contain such components. In aluminium and magnesium foundries, melting is commonly done in crucible furnaces. Exhaust vents around the top of the pot for removing fumes are advisable. In oil-fired furnaces, incomplete combustion due to faulty burners may result in products such as carbon monoxide being released into the air. Furnace fumes may contain complex hydrocarbons, some of which may be carcinogenic. During furnace and flue cleaning there is the hazard of exposure to vanadium pentoxide concentrated in furnace soot from oil deposits. Fluorspar is commonly used as a flux in aluminium melting, and significant quantities of fluoride dust may be released to the environment. In certain cases barium chloride has been used as a flux for magnesium alloys; this is a significantly toxic substance and, consequently, considerable care is required in its use. Light alloys may occasionally be degassed by passing sulphur dioxide or

chlorine (or proprietary compounds that decompose to produce chlorine) through the molten metal; exhaust ventilation and respiratory protective equipment are required for this operation. In order to reduce the cooling rate of the hot metal in the mould, a mixture of substances (usually aluminium and iron oxide) which react highly exothermically is placed on the mould riser. This ―thermite‖ mixture gives off dense fumes which have been found to be innocuous in practice. When the fumes are brown in colour, alarm may be caused due to suspicion of the presence of nitrogen oxides; however, this suspicion is unfounded. The finely divided aluminium produced during the dressing of aluminium and magnesium castings constitutes a severe fire hazard, and wet methods should be used for dust collection. Magnesium casting entails considerable potential fire and explosion hazard. Molten magnesium will ignite unless a protective barrier is maintained between it and the atmosphere; molten sulphur is widely employed for this purpose. Foundry workers applying the sulphur powder to the melting pot by hand may develop dermatitis and should be provided with gloves made of fireproof fabric. The sulphur in contact with the metal is constantly burning, so considerable quantities of sulphur dioxide are given off. Exhaust ventilation should be installed. Workers should be informed of the danger of a pot or ladle of molten magnesium catching fire, which may give rise to a dense cloud of finely divided magnesium oxide. Protective clothing of fireproof materials should be worn by all magnesium foundry workers. Clothing coated with magnesium dust should not be stored in lockers without humidity control, since spontaneous combustion may occur. The magnesium dust should be removed from the clothing.French chalk is used extensively in mould dressing in magnesium foundries; the dust should be controlled to prevent talcosis. Penetrating oils and dusting powders are employed in the inspection of lightalloy castings for the detection of cracks. Dyes have been introduced to improve the effectiveness of these techniques. Certain red dyes have been found to be absorbed and excreted in sweat, thus causing soiling of personal clothing; although this condition is a nuisance, no effects on health have been observed. Brass and bronze foundries Toxic metal fumes and dust from typical alloys are a special hazard of brass and bronze foundries. Exposures to lead above safe limits in both melting, pouring and finishing operations are common, especially where alloys have a high lead composition. The lead hazard in furnace cleaning and dross disposal is particularly acute. Overexposure to lead is frequent in melting and pouring and can also occur in grinding. Zinc and copper fumes (the constituents of bronze) are the most common causes of metal fume fever, although the condition has also been observed in foundry workers using magnesium, aluminium, antimony and so on. Some high-duty alloys contain cadmium, which can cause chemical pneumonia from acute exposure and kidney damage and lung cancer from chronic exposure. Permanent-mould process Casting in permanent metal moulds, as in die-casting, has been an important development in the foundry. In this case, pattern making is largely replaced by engineering methods and is really a die-sinking operation. Most of the pattern making hazards are thereby removed and the risks

from sand are also eliminated but are replaced by a degree of risk inherent in the use of some sort of refractory material to coat the die or mould. In modern die-foundry work, increasing use is made of sand cores, in which case the dust hazards of the sand foundry are still present. Die casting Aluminium is a common metal in die casting. Automotive hardware such as chrome trim is typically zinc die cast, followed by copper, nickel and chrome plating. The hazard of metal fume fever from zinc fumes should be constantly controlled, as must be chromic acid mist. Pressure die-casting machines present all the hazards common to hydraulic power presses. In addition, the worker may be exposed to the mist of oils used as die lubricants and must be protected against the inhalation of these mists and the danger of oil-saturated clothing. The fireresistant hydraulic fluids used in the presses may contain toxic organophosphorus compounds, and particular care should be taken during maintenance work on hydraulic systems. Precision founding Precision foundries rely on the investment or lost-wax casting process, in which patterns are made by injection moulding wax into a die; these patterns are coated with a fine refractory powder which serves as a mould-facing material, and the wax is then melted out prior to casting or by the introduction of the casting metal itself. Wax removal presents a definite fire hazard, and decomposition of the wax produces acrolein and other hazardous decomposition products. Wax-burnout kilns must be adequately ventilated. Trichloroethylene has been used to remove the last traces of wax; this solvent may collect in pockets in the mould or be absorbed by the refractory material and vaporize or decompose during pouring. The inclusion of asbestos investment casting refractory materials should be eliminated due to the hazards of asbestos.

Health Problems and Disease Patterns
Foundries stand out among industrial processes because of a higher fatality rate arising from molten metal spills and explosions, cupola maintenance including bottom drop and carbon monoxide hazards during relining. Foundries report a higher incidence of foreign body, contusion and burn injuries and a lower proportion of musculoskeletal injuries than other facilities. They also have the highest noise exposure levels. A study of several dozen fatal injuries in foundries revealed the following causes: crushing between mould conveyor cars and building structures during maintenance and trouble-shooting, crushing while cleaning mullers which were remotely activated, molten metal burns after crane failure, mould cracking, overflowing transfer ladle, steam eruption in undried ladle, falls from cranes and work platforms, electrocution from welding equipment, crushing from materialhandling vehicles, burns from cupola bottom drop, high-oxygen atmosphere during cupola repair and carbon monoxide overexposure during cupola repair.

Abrasive wheels The bursting or breaking of abrasive wheels may cause fatal or very serious injuries: gaps between the wheel and the rest at pedestal grinders may catch and crush the hand or forearm. Unprotected eyes are at risk at all stages. Slips and falls, especially when carrying heavy loads, may be caused by badly maintained or obstructed floors. Injuries to the feet may be caused by falling objects or dropped loads. Sprains and strains may result from overexertion in lifting and carrying. Badly maintained hoisting appliances may fail and cause materials to fall on workers. Electric shock may result from badly maintained or unearthed (ungrounded) electrical equipment, especially portable tools. All dangerous parts of machinery, especially abrasive wheels, should have adequate guarding, with automatic lockout if the guard is removed during processing. Dangerous gaps between the wheel and the rest at pedestal grinders should be eliminated, and close attention should be paid to all precautions in the care and maintenance of abrasive wheels and in regulation of their speed (particular care is required with portable wheels). Strict maintenance of all electrical equipment and proper grounding arrangements should be enforced. Workers should be instructed in correct lifting and carrying techniques and should know how to attach loads to crane hooks and other hoisting appliances. Suitable PPE, such as eye and face shields and foot and leg protection, should also be provided. Provision should be made for prompt first aid, even for minor injuries, and for competent medical care when needed. Dust Dust diseases are prominent among foundry workers. Silica exposures are often close to or exceed prescribed exposure limits, even in well-controlled cleaning operations in modern production foundries and where castings are free of visible dust. Exposures many times above the limit occur where castings are dusty or cabinets leak. Overexposures are likely where visible dust escapes venting in shakeout, sand preparation or refractory repair. Silicosis is the predominant health hazard in the steel fettling shop; a mixed pneumoconiosis is more prevalent in iron fettling (Landrigan et al. 1986). In the foundry, the prevalence increases with length of exposure and higher dust levels. There is some evidence that conditions in steel foundries are more likely to cause silicosis than those in iron foundries because of the higher levels of free silica present. Attempts to set an exposure level at which silicosis will not occur have been inconclusive; the threshold is probably less than 100 micrograms/m3 and perhaps as low as half that amount. In most countries, the occurrence of new cases of silicosis is declining, in part because of changes in technology, a move away from silica sand in foundries and a shift away from silica brick and towards basic furnace linings in steel melting. A major reason is the fact that automation has resulted in the employment of fewer workers in steel production and foundries. Exposure to respirable silica dust remains stubbornly high in many foundries, however, and in countries where processes are labour intensive, silicosis remains a major problem.

Silico-tuberculosis has long been reported in foundry workers. Where the prevalence of silicosis has declined, there has been a parallel falling off in reported cases of tuberculosis, although that disease has not been completely eradicated. In countries where dust levels have remained high, dusty processes are labour intensive and the prevalence of tuberculosis in the general population is elevated, tuberculosis remains an important cause of death amongst foundry workers. Many workers suffering from pneumoconiosis also have chronic bronchitis, often associated with emphysema; it has long been thought by many investigators that, in some cases at least, occupational exposures may have played a part. Cancer of the lung, lobar pneumonia, bronchopneumonia and coronary thrombosis have also been reported to be associated with pneumoconiosis in foundry workers. A recent review of mortality studies of foundry workers, including the American auto industry, showed increased deaths from lung cancer in 14 of 15 studies. Because high lung cancer rates are found among cleaning room workers where the primary hazard is silica, it is likely that mixed exposures are also found. Studies of the carcinogens in the foundry environment have concentrated on polycyclic aromatic hydrocarbons formed in the thermal breakdown of sand additives and binders. It has been suggested that metals such as chromium and nickel, and dusts such as silica and asbestos, may also be responsible for some of the excess mortality. Differences in moulding and core-making chemistry, sand type and the composition of iron and steel alloys may be responsible for different levels of risk in different foundries (IARC 1984). Increased mortality from non-malignant respiratory disease was found in 8 of 11 studies. Silicosis deaths were recorded as well. Clinical studies found x-ray changes characteristic of pneumoconiosis, lung function deficits characteristic of obstruction, and increased respiratory symptoms among workers in modern ―clean‖ production foundries. These resulted from exposures after the l960s and strongly suggest that the health risks prevalent in the older foundries have not yet been eliminated. Prevention of lung disorders is essentially a matter of dust and fume control; the generally applicable solution is providing good general ventilation coupled with efficient LEV. Lowvolume, high-velocity systems are most suitable for some operations, particularly portable grinding wheels and pneumatic tools. Hand or pneumatic chisels used to remove burnt-on sand produce much finely divided dust. Brushing off excess materials with revolving wire brushes or hand brushes also produces much dust; LEV is required. Dust control measures are readily adaptable to floor-standing and swing-frame grinders. Portable grinding on small castings can be carried out on exhaust-ventilated benches, or ventilation may be applied to the tools themselves. Brushing can also be carried out on a ventilated bench. Dust control on large castings presents a problem, but considerable progress has been made with lowvolume, high-velocity ventilation systems. Instruction and training in their use is needed to

overcome the objections of workers who find these systems cumbersome and complain that their view of the working area is impaired. Dressing and fettling of very large castings where local ventilation is impracticable should be done in a separate, isolated area and at a time when few other workers are present. Suitable PPE that is regularly cleaned and repaired, should be provided for each worker, along with instruction in its proper use. Since the 1950s, a variety of synthetic resin systems have been introduced into foundries to bind sand in cores and moulds. These generally comprise a base material and a catalyst or hardener which starts the polymerization. Many of these reactive chemicals are sensitizers (e.g., isocyanates, furfuryl alcohol, amines and formaldehyde) and have now been implicated in cases of occupational asthma among foundry workers. In one study, 12 out of 78 foundry workers exposed to Pepset (cold-box) resins had asthmatic symptoms, and of these, six had a marked decline in airflow rates in a challenge test using methyl di-isocyanate (Johnson et al. 1985). Welding Welding in fettling shops exposes workers to metal fumes with the consequent hazard of toxicity and metal fever, depending on the composition of the metals involved. Welding on cast iron requires a nickel rod and creates exposure to nickel fumes. The plasma torch produces a considerable amount of metal fumes, ozone, nitrogen oxide and ultraviolet radiation, and generates high levels of noise. An exhaust-ventilated bench can be provided for welding small castings. Controlling exposures during welding or burning operations on large castings is difficult. A successful approach involves creating a central station for these operations and providing LEV through a flexible duct positioned at the point of welding. This requires training the worker to move the duct from one location to another. Good general ventilation and, when necessary, the use of PPE will aid in reducing the overall dust and fume exposures. Noise and vibration The highest levels of noise in the foundry are usually found in knockout and cleaning operations; they are higher in mechanized than in manual foundries. The ventilation system itself may generate exposures close to 90 dBA. Noise levels in the fettling of steel castings may be in the range of 115 to 120 dBA, while those actually encountered in the fettling of cast iron are in the 105 to 115 dBA range. The British Steel Casting Research Association established that the sources of noise during fettling include: · the fettling tool exhaust · the impact of the hammer or wheel on the casting · resonance of the casting and vibration against its support

· transmission of vibration from the casting support to surrounding structures · reflection of direct noise by the hood controlling air flow through the ventilation system. Noise control strategies vary with the size of the casting, the type of metal, the work area available, the use of portable tools and other related factors. Certain basic measures are available to reduce noise exposure of individuals and co-workers, including isolation in time and space, complete enclosures, partial sound-absorbing partitions, execution of work on sound-absorbing surfaces, baffles, panels and hoods made from sound-absorbing or other acoustical materials. The guidelines for safe daily exposure limits should be observed and, as a last resort, personal protective devices may be used. A fettling bench developed by the British Steel Casting Research Association reduces the noise in chipping by about 4 to 5 dBA. This bench incorporates an exhaust system to remove dust. This improvement is encouraging and leads to hope that, with further development, even greater noise reductions will become possible. Hand-arm vibration syndrome Portable vibrating tools may cause Raynaud’s phenomenon (hand-arm vibration syndrome— HAVS). This is more prevalent in steel fettlers than in iron fettlers and more frequent among those using rotating tools. The critical vibratory rate for the onset of this phenomenon is between 2,000 and 3,000 revolutions per minute and in the range of 40 to 125 Hz. HAVS is now thought to involve effects on a number of other tissues in the forearm apart from peripheral nerves and blood vessels. It is associated with carpal tunnel syndrome and degenerative changes in the joints. A recent study of steelworks chippers and grinders showed they were twice as likely to develop Dupuytren’s contracture than a comparison group (Thomas and Clarke 1992). Vibration transmitted to the hands of the worker can be considerably reduced by: selection of tools designed to reduce the harmful ranges of frequency and amplitude; direction of the exhaust port away from the hand; use of multiple layers of gloves or an insulating glove; and shortening of exposure time by changes in work operations, tools and rest periods. Eye problems Some of the dusts and chemicals encountered in foundries (e.g., isocyanates, formaldehyde and tertiary amines, such as dimethlyethylamine, triethylamine and so on) are irritants and have been responsible for visual symptoms among exposed workers. These include itchy, watery eyes, hazy or blurred vision or so called ―blue-grey vision‖. On the basis of the occurrence of these effects, reducing time-weighted average exposures below 3 ppm has been recommended. Other problems

Formaldehyde exposures at or above the US exposure limit are found in well-controlled hot-box core-making operations. Exposures many times above the limit may be found where hazard control is poor. Asbestos has been used widely in the foundry industry and, until recently, it was often used in protective clothing for heat-exposed workers. Its effects have been found in x-ray surveys of foundry workers, both among production workers and maintenance workers who have been exposed to asbestos; a cross-sectional survey found the characteristic pleural involvement in 20 out of 900 steel workers (Kronenberg et al. 1991). Periodic examinations Preplacement and periodic medical examinations, including a survey of symptoms, chest x rays, pulmonary function tests and audiograms, should be provided for all foundry workers with appropriate follow-up if questionable or abnormal findings are detected. The compounding effects of tobacco smoke on the risk of respiratory problems among foundry workers mandate inclusion of advice on smoking cessation in a programme of health education and promotion.

Conclusion
Foundries have been an essential industrial operation for centuries. Despite continuing advances in technology, they present workers with a panoply of hazards to safety and health. Because hazards continue to exist even in the most modern plants with exemplary prevention and control programmes, protecting the health and well-being of workers remains an ongoing challenge to management and to the workers and their representatives. This remains difficult both in industry downturns (when concerns for worker health and safety tend to give way to economic stringencies) and in boom times (when the demand for increased output may lead to potentially dangerous short cuts in the processes). Education and training in hazard control, therefore, remain a constant necessity.

FORGING AND STAMPING
Robert M. Park

Process Overview
Forming metal parts by application of high compressive and tensile forces is common throughout industrial manufacturing. In stamping operations, metal, most often in the form of sheets, strips or coils, is formed into specific shapes at ambient temperatures by shearing, pressing and stretching between dies, usually in a series of one or more discrete impact steps. Cold-rolled steel is the starting material in many stamping operations creating sheet metal parts in the automotive and appliance and other industries. Approximately 15% of workers in the automotive industry work in stamping operations or plants. In forging, compressive force is applied to pre-formed blocks (blanks) of metal, usually heated to high temperatures, also in one or more discrete pressing steps. The shape of the final piece is

determined by the shape of the cavities in the metal die or dies used. With open impression dies, as in drop hammer forging, the blank is compressed between one die attached to the bottom anvil and the vertical ram. With closed impression dies, as in press forging, the blank is compressed between the bottom die and an upper die attached to the ram. Drop hammer forges use a steam or air cylinder to raise the hammer, which is then dropped by gravity or is driven by steam or air. The number and force of the hammer blows are manually controlled by the operator. The operator often holds the cold end of the stock while operating the drop hammer. Drop hammer forging once comprised about two-thirds of all forging done in the United States, but is less common today. Press forges use a mechanical or hydraulic ram to shape the piece with a single, slow, controlled stroke (see figure 82.1). Press forging is usually controlled automatically. It can be done hot or at normal temperatures (cold-forging, extruding). A variation on normal forging is rolling, where continuous applications of force are used and the operator turns the part. Figure 82.1 Press forging

Die lubricants are sprayed or otherwise applied to die faces and blank surfaces before and between hammer or press strokes. High-strength machine parts such as shafts, ring gears, bolts and vehicle suspension components are common steel forging products. High-strength aircraft components such as wing spars, turbine disks and landing gear are forged from aluminium, titanium or nickel and steel alloys. Approximately 3% of automotive workers are in forging operations or plants.

Working Conditions
Many hazards common in heavy industry are present in stamping and forging operations. These include repetitive strain injuries (RSIs) from repeated handling and processing of parts and operation of machine controls such as palm buttons. Heavy parts place workers at risk for back and shoulder problems as well as upper extremity musculoskeletal disorders. Press operators in automotive stamping plants have rates of RSIs that are comparable to those of assembly plant workers in high-risk jobs. High-impulse vibration and noise are present in most stamping and some forging (e.g., steam or air hammer) operations, causing hearing loss and possible cardiovascular illness; these are among the highest-noise industrial environments (over 100 dBA). As in other forms of automation-driven systems, worker energy loads can be high, depending on the parts handled and machine cycling rates. Catastrophic injuries resulting from unanticipated machine movements are common in stamping and forging. These can be due to: (1) mechanical failure of machine control systems, such as clutch mechanisms in situations where workers are routinely expected to be within the machine operating envelope (an unacceptable process design); (2) deficiencies in machine design or

performance that invite unprogrammed worker interventions such as moving jammed or misaligned parts; or (3) improper, high-risk maintenance procedures performed without adequate lockout of the entire machine network involved, including parts transfer automation and the functions of other connected machines. Most automated machine networks are not configured for quick, efficient and effective lockout or safe trouble-shooting. Mists from machine lubricating oils generated during normal operation are another generic health hazard in stamping and forging press operations powered by compressed air, potentially putting workers at risk for respiratory, dermatological and digestive diseases.

Health and Safety Problems
Stamping Stamping operations have high risk of severe laceration due to the required handling of parts with sharp edges. Possibly worse is the handling of the scrap resulting from cut-off perimeters and punched out sections of parts. Scrap is typically collected by gravity-fed chutes and conveyors. Clearing occasional jams is a high-risk activity. Chemical hazards specific to stamping typically arise from two main sources: drawing compounds (i.e., die lubricants) in actual press operations and welding emissions from assembly of the stamped parts. Drawing compounds (DCs) are required for most stamping. The material is sprayed or rolled onto sheet metal and further mists are generated by the stamping event itself. Like other metalworking fluids, drawing compounds may be straight oils or oil emulsions (soluble oils). Components include petroleum oil fractions, special lubricity agents (e.g., animal and vegetable fatty acid derivatives, chlorinated oils and waxes), alkanolamines, petroleum sulphonates, borates, cellulose-derived thickeners, corrosion inhibitors and biocides. Air concentrations of mist in stamping operations may reach those of typical machining operations, although these levels tend to be lower on average (0.05 to 2.0 mg/m3). However, visible fog and accumulated oil film on building surfaces are often present, and skin contact may be higher due to extensive handling of parts. Exposures most likely to present hazards are chlorinated oils (possible cancer, liver disease, skin disorders), rosin or tall oil fatty acid derivatives (sensitizers), petroleum fractions (digestive cancers) and, possibly, formaldehyde (from biocides) and nitrosamines (from alkanolamines and sodium nitrite, either as DC ingredients or in surface coatings on incoming steel). Elevated digestive cancer has been observed in two automotive stamping plants. Microbiological blooms in systems that apply DCs by rolling it onto sheet metal from an open reservoir can pose risks to workers for respiratory and dermatological problems analogous to those in machining operations. Welding of stamped parts is often performed in stamping plants, usually without intermediate washing. This produces emissions that include metal fumes and pyrolysis and combustion products from drawing compound and other surface residues. Typical (primarily resistance) welding operations in stamping plants generate total particulate air concentrations in the range 0.05 to 4.0 mg/m3. Metal content (as fumes and oxides) usually makes up less than half of that particulate matter, indicating that up to 2.0 mg/m3 is poorly characterized chemical debris. The result is haze visible in many stamping plant welding areas. The presence of chlorinated

derivatives and other organic ingredients raises serious concerns over the composition of welding smoke in these settings and strongly argues for ventilation controls. Application of other materials prior to welding (such as primer, paint and epoxy-like adhesives), some of which are then welded over, adds further concern. Welding production repair activities, usually done manually, often pose higher exposures to these same air contaminants. Excess rates of lung cancer have been observed among welders in an automotive stamping plant. Forging Like stamping, forging operations can pose high laceration risks when workers handle forged parts or trim the flash or unwanted edges off parts. High impact forging can also eject fragments, scale or tools, causing injury. In some forging activities, the worker grasps the working piece with tongs during the pressing or impact steps, increasing the risk for musculoskeletal injuries. In forging, unlike stamping, furnaces for heating parts (for forging and annealing) as well as bins of hot forgings are usually nearby. These create potential for high heat stress conditions. Additional factors in heat stress are the worker’s metabolic load during manual handling of materials and, in some cases, heat from combustion products of oil-based die lubricants. Die lubrication is required in most forging and has the added feature that the lubricant comes in contact with high-temperature parts. This causes immediate pyrolysis and aerosolization not only in the dies but also subsequently from smoking parts in cooling bins. Forging die lubricant ingredients can include graphite slurries, polymeric thickeners, sulphonate emulsifiers, petroleum fractions, sodium nitrate, sodium nitrite, sodium carbonate, sodium silicate, silicone oils and biocides. These are applied as sprays or, in some applications, by swab. Furnaces used for heating metal to be forged are usually fired by oil or gas, or they are induction furnaces. Emissions can result from fuel-fired furnaces with inadequate draft and from non-ventilated induction furnaces when incoming metal stock has surface contaminants, such as oil or corrosion inhibitors, or if, prior to forging, it was lubricated for shearing or sawing (as in the case of bar stock). In the US, total particulate air concentrations in forging operations typically range from 0.1 to 5.0 mg/m3 and vary widely within forging operations due to thermal convection currents. An elevated lung cancer rate was observed among forging and heat treatment workers from two ball-bearing manufacturing plants.

Health and Safety Practices
Few studies have evaluated actual health effects in workers with stamping or forging exposures. Comprehensive characterization of the toxicity potential of most routine operations, including identification and measurement of priority toxic agents, has not been done. Evaluating the longterm health effects of die lubrication technology developed in the 1960s and 1970s has only recently become feasible. As a result, regulation of these exposures defaults to generic dust or total particulate standards such as 5.0 mg/m3 in the US. While probably adequate in some circumstances, this standard is not demonstrably adequate for many stamping and forging applications. Some reduction in die lubricant mist concentrations is possible with careful management of the application procedure in both stamping and forging. Roll application in stamping is preferred

when feasible, and using minimal air pressure in sprays is beneficial. Possible elimination of priority hazardous ingredients should be investigated. Enclosures with negative pressure and mist collectors can be highly effective but may be incompatible with parts handling. Filtering air released from high-pressure air systems in presses would reduce press oil mist (and noise). Skin contact in stamping operations can be reduced with automation and good personal protective wear, providing protection against both laceration and liquid saturation. For stamping plant welding, washing parts prior to welding is highly desirable, and partial enclosures with LEV would reduce smoke levels substantially. Controls to reduce heat stress in stamping and hot forging include minimizing the amount of manual material handling in high-heat areas, shielding of furnaces to reduce radiation of heat, minimizing the height of furnace doors and slots and using cooling fans. The location of cooling fans should be an integral part of the design of air movement to control mist exposures and heat stress; otherwise, cooling may be obtained only at the expense of higher exposures. Mechanization of material handling, switching from hammer to press forging when possible and adjusting the work rate to ergonomically practical levels can reduce the number of musculoskeletal injuries. Noise levels can be reduced through a combination of switching from hammer to press forges when possible, well-designed enclosures and quieting of furnace blowers, air clutches, air leads and parts handling. A hearing conservation programme should be instituted. PPE needed includes head protection, foot protection, goggles, hearing protectors (around are as with excessive noise), heat- and oil-proof aprons and leggings (with heavy use of oil-based die lubricants) and infrared eye and face protection (around furnaces).

Environmental Health Hazards
The environmental hazards arising from stamping plants, relatively minor compared to those from some other types of plants, include disposal of waste drawing compound and washing solutions and the exhausting of welding smoke without adequate cleaning. Some forging plants historically have caused acute degradation of local air quality with forging smoke and scale dust. However, with appropriate air cleaning capacity, this need not occur. Disposition of stamping scrap and forging scale containing die lubricants is another potential issue.

WELDING AND THERMAL CUTTING
Philip A. Platcow and G.S. Lyndon* *This article is a revision of the 3rd edition of the Encyclopaedia of Occupational Health and Safety article ―Welding and thermal cutting‖ by G.S. Lyndon.

Process Overview

Welding is a generic term referring to the union of pieces of metal at joint faces rendered plastic or liquid by heat or pressure, or both. The three common direct sources of heat are: 1. flame produced by the combustion of fuel gas with air or oxygen 2. electrical arc, struck between an electrode and a workpiece or between two electrodes 3. electrical resistance offered to passage of current between two or more workpieces. Other sources of heat for welding are discussed below (see table 82.6). Table 82.6 Description and hazards of welding processes Welding Process Gas welding and cutting Welding The torch melts the metal surface and filler rod, causing a joint to be formed. Description Hazards

Metal fumes, nitrogen dioxide, carbon monoxide, noise, burns infrared radiation, fire, explosions Metal fumes (especially cadmium), fluorides, fire, explosion, burns

Brazing

The two metal surfaces are bonded without melting the metal. The melting temperature of the filler metal is above 450 °C. Heating is done by flame heating, resistance heating and induction heating.

Soldering

Similar to brazing, except the melting temperature Fluxes, lead fumes, burns of the filler metal is below 450 °C. Heating is also done using a soldering iron. In one variation, the metal is heated by a flame, and a jet of pure oxygen is directed onto the point of cutting and moved along the line to be cut. In flame gouging, a strip of surface metal is removed but the metal is not cut through. The parts are heated by gas jets while under pressure, and become forged together.

Metal cutting and flame gouging

Metal fumes, nitrogen dioxide, carbon monoxide, noise, burns infrared radiation, fire, explosions

Gas pressure welding

Metal fumes, nitrogen dioxide, carbon monoxide, noise, burns infrared radiation, fire, explosions

Flux-shielded arc welding Shielded metal arc welding Uses a consumable electrode consisting of a metal (SMAC); "stick" arc core surrounded by a flux coating welding; manual metal arc welding (MMA); open arc Metal fumes, fluorides (especially with low-hydrogen electrodes), infrared and ultraviolet radiation, burns,

welding Submerged arc welding (SAW) A blanket of granulated flux is deposited on the workpiece, followed by a consumable bare metal wire electrode. The arc melts the flux to produce a protective molten shield in the welding zone.

electrical, fire; also noise, ozon nitrogen dioxide

Fluorides, fire, burns, infrared radiation, electrical; also metal fumes, noise, ultraviolet radiation, ozone, and nitrogen dioxide

Gas-shielded arc welding Metal inert gas (MIG); gas metal arc welding (GMAC)

The electrode is normally a bare consumable wire Ultraviolet radiation, metal of similar composition to the weld metal and is fed fumes, ozone, carbon monoxid continuously to the arc. (with CO2 gas), nitrogen dioxid fire, burns, infrared radiation, electrical, fluorides, noise The tungsten electrode is non-consumable, and filler metal is introduced as a consumable into the arc manually.

Tungsten inert gas (TIG); gas tungsten arc welding (GTAW); heliarc

Ultraviolet radiation, metal fumes, ozone, nitrogen dioxide fire, burns, infrared radiation, electrical, noise, fluorides, carbon monoxide

Plasma arc welding (PAW) Similar to TIG welding, except that the arc and and plasma arc spraying; stream of inert gases pass through a small orifice tungsten arc cutting before reaching the workpiece, creating a "plasma" of highly ionized gas which can achieve temperatures of over 33,400°C.This is also used for metallizing. Flux core arc welding (FCAW); metal active gas welding (MAG) Uses a flux-cored consumable electrode; may have carbon dioxide shield (MAG)

Metal fumes, ozone, nitrogen dioxide, ultraviolet and infrared radiation, noise; fire, burns, electrical, fluorides, carbon monoxide, possible x rays

Ultraviolet radiation, metal fumes, ozone, carbon monoxid (with CO2 gas), nitrogen dioxid fire, burns, infrared radiation, electrical, fluorides, noise

Electric resistance welding Resistance welding (spot, seam, projection or butt welding)

A high current at low voltage flows through the Ozone, noise (sometimes), two components from electrodes. The heat machinery hazards, fire, burns, generated at the interface between the components electrical, metal fumes brings them to welding temperatures. During the passage of the current, pressure by the electrodes produces a forge weld. No flux or filler metal is used. Used for vertical butt welding. The workpieces are Burns, fire, infrared radiation, set vertically, with a gap between them, and electrical, metal fumes copper plates or shoes are placed on one or both

Electro-slag welding

sides of the joint to form a bath. An arc is established under a flux layer between one or more continuously fed electrode wires and a metal plate. A pool of molten metal is formed, protected by molten flux or slag, which is kept molten by resistance to the current passing between the electrode and the workpieces. This resistancegenerated heat melts the sides of the joint and the electrode wire, filling the joint and making a weld. As welding progresses, the molten metal and slag are retained in position by shifting the copper plates. Flash welding The two metal parts to be welded are connected to Electrical, burns, fire, metal a low-voltage, high-current source. When the ends fumes of the components are brought into contact, a large current flows, causing "flashing" to occur and bringing the ends of the components to welding temperatures. A forge weld is obtained by pressure.

Other welding processes Electron beam welding A workpiece in an vacuum chamber is bombarded X rays at high voltages, by a beam of electrons from an electron gun at electrical, burns, metal dusts, high voltages. The energy of the electrons is confined spaces transformed into heat upon striking the workpiece, thus melting the metal and fusing the workpiece. An arc is struck between the end of a carbon electrode (in a manual electrode holder with its own supply of compressed air) and the workpiece. The molten metal produced is blown away by jets of compressed air.

Arcair cutting

Metal fumes, carbon monoxide nitrogen dioxide, ozone, fire, burns, infrared radiation, electrical

Friction welding

A purely mechanical welding technique in which Heat, burns, machinery hazard one component remains stationary while the other is rotated against it under pressure. Heat is generated by friction, and at forging temperature the rotation ceases. A forging pressure then effects the weld. Laser beams can be used in industrial applications requiring exceptionally high precision, such as miniature assemblies and micro techniques in the electronics industry or spinnerets for the artificial fibre industry. The laser beam melts and joins the workpieces.

Laser welding and drilling

Electrical, laser radiation, ultraviolet radiation, fire, burns metal fumes, decomposition products of workpiece coatings

Stud welding

An arc is struck between a metal stud (acting as the electrode) held in a stud welding gun and the metal plate to be joined, and raises the temperature of the ends of the components to melting point. The gun forces the stud against the plate and welds it. Shielding is provided by a ceramic ferrule surrounding the stud.

Metal fumes, infrared and ultraviolet radiation, burns, electrical, fire, noise, ozone, nitrogen dioxide

Thermite welding

A mixture of aluminium powder and a metal oxide Fire, explosion, infrared powder (iron, copper, etc.) is ignited in a crucible, radiation, burns producing molten metal with the evolution of intense heat. The crucible is tapped and the molten metal flows into the cavity to be welded (which is surrounded by a sand mould). This is often used to repair castings or forgings.

In gas welding and cutting, oxygen or air and a fuel gas are fed to a blowpipe (torch) in which they are mixed prior to combustion at the nozzle. The blowpipe is usually hand held (see figure 82.2). The heat melts the metal faces of the parts to be joined, causing them to flow together. A filler metal or alloy is frequently added. The alloy often has a lower melting point than the parts to be joined. In this case, the two pieces are generally not brought to fusion temperature (brazing, soldering). Chemical fluxes may be used to prevent oxidation and facilitate the joining. Figure 82.2 Gas welding with a torch and rod of filter metal. The welder is protected by a leather apron, gauntlets and goggles

In arc welding, the arc is struck between an electrode and the workpieces. The electrode can be connected to either an alternating current (AC) or direct current (DC) electric supply. The temperature of this operation is about 4,000°C when the workpieces fuse together. Usually it is necessary to add molten metal to the joint either by melting the electrode itself (consumable electrode processes) or by melting a separate filler rod which is not carrying current (nonconsumable electrode processes). Most conventional arc welding is done manually by means of a covered (coated) consumable electrode in a hand-held electrode holder. Welding is also accomplished by many semi or fully automatic electric welding processes such as resistance welding or continuous electrode feed. During the welding process, the welding area must be shielded from the atmosphere in order to prevent oxidation and contamination. There are two types of protection: flux coatings and inert gas shielding. In flux-shielded arc welding, the consumable electrode consists of a metal core surrounded by a flux coating material, which is usually a complex mixture of mineral and other components. The flux melts as welding progresses, covering the molten metal with slag and

enveloping the welding area with a protective atmosphere of gases (e.g., carbon dioxide) generated by the heated flux. After welding, the slag must be removed, often by chipping. In gas-shielded arc welding, a blanket of inert gas seals off the atmosphere and prevents oxidation and contamination during the welding process. Argon, helium, nitrogen or carbon dioxide are commonly used as the inert gases. The gas selected depends upon the nature of the materials to be welded. The two most popular types of gas-shielded arc welding are metal- and tungsten inert gas (MIG and TIG). Resistance welding involves using the electrical resistance to the passage of a high current at low voltage through components to be welded to generate heat for melting the metal. The heat generated at the interface between the components brings them to welding temperatures.

Hazards and Their Prevention
All welding involves hazards of fire, burns, radiant heat (infrared radiation) and inhalation of metal fumes and other contaminants. Other hazards associated with specific welding processes include electrical hazards, noise, ultraviolet radiation, ozone, nitrogen dioxide, carbon monoxide, fluorides, compressed gas cylinders and explosions. See table 82.6 for additional detail. Much welding is not done in shops where conditions can generally be controlled, but in the field in the construction or repair of large structures and machinery (e.g., frameworks of buildings, bridges and towers, ships, railroad engines and cars, heavy equipment and so on). The welder may have to carry all his or her equipment to the site, set it up and work in confined spaces or on scaffolds. Physical strain, inordinate fatigue and musculoskeletal injuries may follow being required to reach, kneel or work in other uncomfortable and awkward positions. Heat stress may result from working in warm weather and the occlusive effects of the personal protective equipment, even without the heat generated by the welding process. Compressed gas cylinders In high-pressure gas welding installations, oxygen and the fuel gas (acetylene, hydrogen, town gas, propane) are supplied to the torch from cylinders. The gases are stored in these cylinders at high pressure. The special fire and explosion hazards and precautions for the safe use and storage of the fuel gases are also discussed elsewhere in this Encyclopaedia. The following precautions should be observed: · Only pressure regulators designed for the gas in use should be fitted to cylinders. For example, an acetylene regulator should not be used with coal gas or hydrogen (although it may be used with propane). · Blowpipes must be kept in good order and cleaned at regular intervals. A hardwood stick or soft brass wire should be used for cleaning the tips. They should be connected to regulators with special canvas-reinforced hoses placed in such a way that they are unlikely to be damaged.

· Oxygen and acetylene cylinders must be stored separately and only on fire-resistant premises devoid of flammable material and must be so located that they may be readily removed in case of fire. Local building and fire protection codes must be consulted. · The colour coding in force or recommended for identification of cylinders and accessories should be scrupulously observed. In many countries, the internationally accepted colour codes used for the transport of dangerous materials are applied in this field. The case for enforcement of uniform international standards in this respect is strengthened by safety considerations bound up with the increasing international migration of industrial workers. Acetylene generators In the low-pressure gas welding process, acetylene is generally produced in generators by reaction of calcium carbide and water. The gas is then piped to the welding or cutting torch into which oxygen is fed. Stationary generating plants should be installed either in the open air or in a well-ventilated building away from the main workshops. The ventilation of the generator house should be such as to prevent the formation of an explosive or toxic atmosphere. Adequate lighting should be provided; switches, other electrical gear and electrical lamps should either be located outside the building or be explosion-proof. Smoking, flames, torches, welding plant or flammable materials must be excluded from the house or from the vicinity of an open-air generator. Many of these precautions also apply to portable generators. Portable generators should be used, cleaned and recharged only in the open air or in a well-ventilated shop, away from any flammable material. Calcium carbide is supplied in sealed drums. The material should be stored and kept dry, on a platform raised above the floor level. Stores must be situated under cover, and if they adjoin another building the party wall must be fireproof. The storeroom should be suitably ventilated through the roof. Drums should be opened only immediately before the generator is charged. A special opener should be provided and used; a hammer and chisel should never be used to open drums. It is dangerous to leave calcium carbide drums exposed to any source of water. Before a generator is dismantled, all calcium carbide must be removed and the plant filled with water. The water should remain in the plant for at least half an hour to ensure that every part is free from gas. The dismantling and servicing should be carried out only by the manufacturer of the equipment or by a specialist. When a generator is being recharged or cleaned, none of the old charge must be used again. Pieces of calcium carbide wedged in the feed mechanism or adhering to parts of the plant should be carefully removed, using non-sparking tools made of bronze or another suitable non-ferrous alloy. All concerned should be fully conversant with the manufacturer’s instructions, which should be conspicuously displayed. The following precautions should also be observed:

· A properly designed back-pressure valve must be fitted between the generator and each blowpipe to prevent backfire or reverse flow of gas. The valve should be regularly inspected after backfire, and the water level checked daily. · Only blowpipes of the injector type designed for low-pressure operation should be used. For heating and cutting, town gas or hydrogen at low pressure are sometimes employed. In these cases, a non-return valve should be placed between each blowpipe and the supply main or pipeline. · An explosion may be caused by ―flash-back‖, which results from dipping the nozzle-tip into the molten metal pool, mud or paint, or from any other stoppage. Particles of slag or metal that become attached to the tip should be removed. The tip should also be cooled frequently. · Local building and fire codes should be consulted. Fire and explosion prevention In locating welding operations, consideration should be given to surrounding walls, floors, nearby objects and waste material. The following procedures should be followed: · All combustible material must be removed or adequately protected by sheet metal or other suitable materials; tarpaulins should never be used. · Wood structures should be discouraged or similarly protected. Wood floors should be avoided. · Precautionary measures should be taken in the case of openings or cracks in walls and floors; flammable material in adjoining rooms or on the floor below should be removed to a safe position. Local building and fire codes should be consulted. · Suitable fire-extinguishing apparatus should always be at hand. In the case of low-pressure plant using an acetylene generator, buckets of dry sand should also be kept available; fire extinguishers of dry powder or carbon dioxide types are satisfactory. Water must never be used. · Fire brigades may be necessary. A responsible person should be assigned to keep the site under observation for at least half an hour after completion of the work, in order to deal with any outbreak of fire. · Since explosions can occur when acetylene gas is present in air in any proportion between 2 and 80%, adequate ventilation and monitoring are required to ensure freedom from gas leaks. Only soapy water should be used to search for gas leaks. · Oxygen must be carefully controlled. For example, it should never be released into the air in a confined space; many metals, clothing and other materials become actively combustible in the presence of oxygen. In gas cutting, any oxygen which may not be consumed will be released into the atmosphere; gas cutting should never be undertaken in a confined space without proper ventilation arrangements.

· Alloys rich in magnesium or other combustible metals should be kept away from welding flames or arcs. · Welding of containers can be extremely hazardous. If the previous contents are unknown, a vessel should always be treated as if it had contained a flammable substance. Explosions may be prevented either by removing any flammable material or by making it non-explosive and nonflammable. · The mixture of aluminium and iron oxide used in thermite welding is stable under normal conditions. However, in view of the ease with which aluminium powder will ignite, and the quasi-explosive nature of the reaction, appropriate precautions should be taken in handling and storage (avoidance of exposure to high heat and possible ignition sources). · A written hot-work permit programme is required for welding in some jurisdictions. This programme outlines the precautions and procedures to be followed during welding, cutting, burning and so on. This programme should include the specific operations conducted along with the safety precautions to be implemented. It must be plant specific and may include an internal permit system that must be completed with each individual operation. Protection from heat and burn hazards Burns of the eyes and exposed parts of the body may occur due to contact with hot metal and spattering of incandescent metal particles or molten metal. In arc welding, a high-frequency spark used to initiate the arc can cause small, deep burns if concentrated at a point on the skin. Intense infrared and visible radiation from a gas welding or cutting flame and incandescent metal in the weld pool can cause discomfort to the operator and persons in the vicinity of the operation. Each operation should be considered in advance, and necessary precautions designed and implemented. Goggles made specifically for gas welding and cutting should be worn to protect the eyes from heat and light radiated from the work. Protective covers over filter glass should be cleaned as required and replaced when scratched or damaged. Where molten metal or hot particles are emitted, the protective clothing being worn should deflect spatter. The type and thickness of fire-resistant clothing worn should be chosen according to the degree of hazard. In cutting and arc welding operations, leather shoe coverings or other suitable spats should be worn to prevent hot particles from falling into boots or shoes. For protecting the hands and forearms against heat, spatter, slag and so on, the leather gauntlet type of glove with canvas or leather cuffs is sufficient. Other types of protective clothing include leather aprons, jackets, sleeves, leggings and head covering. In overhead welding, a protective cape and cap are necessary. All protective clothing should be free from oil or grease, and seams should be inside, so as not to trap globules of molten metal. Clothing should not have pockets or cuffs that could trap sparks, and it should be worn so sleeves overlap gloves, leggings overlap shoes and so on. Protective clothing should be inspected for burst seams or holes through which molten metal or slag may enter. Heavy articles left hot on completion of welding should always be marked ―hot‖ as a warning to other workers. With resistance welding, the heat produced may not be visible, and burns can result from handling of hot assemblies. Particles of hot or molten metal should not fly out of spot, seam or projection welds if conditions are correct, but non-flammable screens should be used and precautions taken. Screens also protect passers-by from eye burns. Loose parts

should not be left in the throat of the machine because they are liable to be projected with some velocity. Electrical safety Although no-load voltages in manual arc welding are relatively low (about 80 V or less), welding currents are high, and transformer primary circuits present the usual hazards of equipment operated at power supply line voltage. The risk of electric shock should therefore not be ignored, especially in cramped spaces or in insecure positions. Before welding commences, the grounding installation on arc welding equipment should always be checked. Cables and connections should be sound and of adequate capacity. A proper grounding clamp or bolted terminal should always be used. Where two or more welding machines are grounded to the same structure, or where other portable electric tools are also in use, grounding should be supervised by a competent person. The working position should be dry, secure and free from dangerous obstructions. A well-arranged, well-lighted, properly ventilated and tidy workplace is important. For work in confined spaces or dangerous positions, additional electrical protection (no-load, low-voltage devices) can be installed in the welding circuit, ensuring that only extremely low-voltage current is available at the electrode holder when welding is not taking place. (See discussion of confined spaces below.) Electrode holders in which the electrodes are held by a spring grip or screw thread are recommended. Discomfort due to heating can be reduced by effective heat insulation on that part of the electrode holder which is held in the hand. Jaws and connections of electrode holders should be cleaned and tightened periodically to prevent overheating. Provision should be made to accommodate the electrode holder safely when not in use by means of an insulated hook or a fully insulated holder. The cable connection should be designed so that continued flexing of the cable will not cause wear and failure of the insulation. Dragging of cables and plastic gas supply tubes (gas-shielded processes) across hot plates or welds must be avoided. The electrode lead should not come in contact with the job or any other earthed object (ground). Rubber tubes and rubber-covered cables must not be used anywhere near the high-frequency discharge, because the ozone produced will rot the rubber. Plastic tubes and polyvinyl chloride (PVC) covered cables should be used for all supplies from the transformer to the electrode holder. Vulcanized or tough rubbersheathed cables are satisfactory on the primary side. Dirt and metallic or other conducting dust can cause a breakdown in the high-frequency discharge unit. To avoid this condition, the unit should be cleaned regularly by blowing-out with compressed air. Hearing protection should be worn when using compressed air for more than a few seconds. For electron-beam welding, the safety of the equipment used must be checked prior to each operation. To protect against electric shock, a system of interlocks must be fitted to the various cabinets. A reliable system of grounding of all units and control cabinets is necessary. For plasma welding equipment used for cutting heavy thicknesses, the voltages may be as high as 400 V and danger should be anticipated. The technique of firing the arc by a high-frequency pulse exposes the operator to the dangers of an unpleasant shock and a painful, penetrating high-frequency burn. Ultraviolet radiation

The brilliant light emitted by an electric arc contains a high proportion of ultraviolet radiation. Even momentary exposure to bursts of arc flash, including stray flashes from other workers’ arcs, may produce a painful conjunctivitis (photo-ophthalmia) known as ―arc eye‖ or ―eye flash‖. If any person is exposed to arc flash, immediate medical attention must be sought. Excessive exposure to ultraviolet radiation may also cause overheating and burning of the skin (sunburn effect). Precautions include: · A shield or helmet fitted with correct grade of filter should be used (see the article ―Eye and face protection‖ elsewhere in this Encyclopaedia). For the gas-shielded arc welding processes and carbon-arc cutting, flat handshields provide insufficient protection from reflected radiation; helmets should be used. Filtered goggles or eyeglasses with sideshields should be worn under the helmet to avoid exposure when the helmet is lifted up for inspection of the work. Helmets will also provide protection from spatter and hot slag. Helmets and handshields are provided with a filter glass and a protective cover glass on the outside. This should be regularly inspected, cleaned and replaced when scratched or damaged. · The face, nape of the neck and other exposed parts of the body should be properly protected, especially when working close to other welders. · Assistants should wear suitable goggles at a minimum and other PPE as the risk requires. · All arc welding operations should be screened to protect other persons working nearby. Where the work is carried out at fixed benches or in welding shops, permanent screens should be erected where possible; otherwise, temporary screens should be used. All screens should be opaque, of sturdy construction and of a flame-resistant material. · The use of black paints for the inside of welding booths has become an accepted practice, but the paint should produce a matte finish. Adequate ambient lighting should be provided to prevent eye strain leading to headaches and accidents. · Welding booths and portable screens should be checked regularly to ensure that there is no damage which might result in the arc affecting persons working nearby. Chemical hazards Airborne contaminants from welding and flame cutting, including fumes and gases, arise from a variety of sources: · the metal being welded, the metal in the filler rod or constituents of various types of steel such as nickel or chromium) · any metallic coating on the article being welded or on the filler rod (e.g., zinc and cadmium from plating, zinc from galvanizing and copper as a thin coating on continuous mild steel filler rods)

· any paint, grease, debris and the like on the article being welded (e.g., carbon monoxide, carbon dioxide, smoke and other irritant breakdown products) · flux coating on the filler rod (e.g., inorganic fluoride) · the action of heat or ultraviolet light on the surrounding air (e.g., nitrogen dioxide, ozone) or on chlorinated hydrocarbons (e.g., phosgene) · inert gas used as a shield (e.g., carbon dioxide, helium, argon). Fumes and gases should be removed at the source by LEV. This can be provided by partial enclosure of the process or by the installation of hoods which supply sufficiently high air velocity across the weld position so as to ensure capture of the fumes. Special attention should be paid to ventilation in the welding of non-ferrous metals and certain alloy steels, as well as to protection from the hazard of ozone, carbon monoxide and nitrogen dioxide which may be formed. Portable as well as fixed ventilation systems are readily available. In general, the exhausted air should not be recirculated. It should be recirculated only if there are not hazardous levels of ozone or other toxic gases and the exhaust air is filtered through a highefficiency filter. With electron-beam welding and if materials being welded are of a toxic nature (e.g., beryllium, plutonium and so on), care must be taken to protect the operator from any dust cloud when opening the chamber. When there is a risk to health from toxic fumes (e.g., lead) and LEV is not practicable—for example, when lead-painted structures are being demolished by flame cutting—the use of respiratory protective equipment is necessary. In such circumstances, an approved, highefficiency full-facepiece respirator or ahigh-efficiency positive pressure powered air-purified respirator (PAPR) should be worn. A high standard of maintenance of the motor and the battery is necessary, especially with the original high-efficiency positive pressure power respirator. The use of positive pressure compressed air line respirators should be encouraged where a suitable supply of breathing-quality compressed air is available. Whenever respiratory protective equipment is to be worn, the safety of the workplace should be reviewed to determine whether extra precautions are necessary, bearing in mind the restricted vision, entanglement possibilities and so on of persons wearing respiratory protective equipment. Metal fume fever Metal fume fever is commonly seen in workers exposed to the fumes of zinc in the galvanizing or tinning process, in brass founding, in the welding of galvanized metal and in metallizing or metal spraying, as well as from exposure to other metals such as copper, manganese and iron. It occurs in new workers and those returning to work after a weekend or holiday hiatus. It is an acute condition that occurs several hours after the initial inhalation of particles of a metal or its oxides. It starts with a bad taste in the mouth followed by dryness and irritation of the respiratory mucosa resulting in cough and occasionally dyspnoea and ―tightness‖ of the chest. These may be

accompanied by nausea and headache and, some 10 to 12 hours after the exposure, chills and fever which may be quite severe. These last several hours and are followed by sweating, sleep and often by polyuria and diarrhoea. There is no particular treatment, and recovery is usually complete in about 24 hours with no residua. It can be prevented by keeping exposure to the offending metallic fumes well within the recommended levels through the use of efficient LEV. Confined spaces For entry into confined spaces, there may be a risk of the atmosphere being explosive, toxic, oxygen deficient or combinations of the above. Any such confined space must be certified by a responsible person as safe for entry and for work with an arc or flame. A confined-space entry programme, including an entry permit system, may be required and is highly recommended for work that must be carried out in spaces that are typically not constructed for continuous occupancy. Examples include, but are not limited to, manholes, vaults, ship holds and the like. Ventilation of confined spaces is crucial, since gas welding not only produces airborne contaminants but also uses up oxygen. Gas-shielded arc welding processes can decrease the oxygen content of the air. (See figure 82.3 .) Figure 82.3 Welding in an enclosed space

S.F. Gilman Noise Noise is a hazard in several welding processes, including plasma welding, some types of resistance welding machines and gas welding. In plasma welding, the plasma jet is ejected at very high speeds, producing intense noise (up to 90 dBA), particularly in the higher frequency bands. The use of compressed air to blow off dust also creates high noise levels. To prevent hearing damage, ear plugs or muffs must be worn and a hearing conservation programme should be instituted, including audiometric (hearing capacity) examinations and employee training. Ionizing radiation In welding shops where welds are inspected radiographically with x-ray or gamma-ray equipment, the customary warning notices and instructions must be strictly observed. Workers must be kept at a safe distance from such equipment. Radioactive sources must be handled only with the required special tools and subject to special precautions. Local and governmental regulations must be followed. See the chapter Radiation, ionizing elsewhere in this Encyclopaedia. Sufficient shielding must be provided with electron-beam welding to prevent x rays from penetrating the walls and windows of the chamber. Any parts of the machine providing shields against x-ray radiation should be interlocked so that the machine cannot be energized unless they

are in position. Machines should be checked at the time of installation for leaks of x-ray radiation, and regularly thereafter. Other hazards Resistance welding machines have at least one electrode, which moves with considerable force. If a machine is operated while a finger or hand is lying between the electrodes, severe crushing will result. Where possible, a suitable means of guarding must be devised to safeguard the operator. Cuts and lacerations can be minimized by first deburring components and by wearing protective gloves or gauntlets. Lockout/tagout procedures should be used when machinery with electrical, mechanical or other energy sources is being maintained or repaired. When slag is being removed from welds by chipping and so on, the eyes should be protected by goggles or other means.

LATHES
Toni Retsch* *Adapted from the 3rd edition, Encyclopaedia of Occupational Health and Safety. The important part lathes play in metalworking shops is best illustrated by the fact that 90 to 95% of the swarf (metal shavings) produced in the valves and fittings industry originates from lathes. About one-tenth of the accidents reported in this industry are due to lathes; this corresponds to one-third of all machine accidents. According to a study of the relative accident frequency per machine unit carried out in a plant manufacturing small precision parts and electrical equipment, lathes rank fifth after woodworking machines, metal-cutting saws, power presses and drilling machines. The need for protective measures on lathes is therefore beyond doubt. Turning is a machine process in which the diameter of material is reduced by a tool with a special cutting edge. The cutting movement is produced by rotating the workpiece, and the feed and traverse movements are produced by the tool. By varying these three basic movements, and also by choosing the appropriate tool cutting-edge geometry and material, it is possible to influence the rate of stock removal, surface quality, shape of the chip formed and tool wear.

Structure of Lathes
A typical lathe consists of: · a bed or base with machined slideways for the saddle and tailstock · a headstock mounted on the bed, with the spindle and chuck

· a feed gearbox attached to the front of the bed for transmitting the feed movement as a function of the cutting speed through the leadscrew or feed shaft and apron to the saddle · a saddle (or carriage) carrying the cross slide which performs the traverse movement · a toolpost mounted on the cross slide (see figure 82.4). Figure 82.4 Lathes, cutting-off machines, threading machines

This basic model of a lathe can be infinitely varied, from the universal machine to the special automatic lathe designed for one type of work only. The most important types of lathe are as follows: · Centre lathe. This is the most frequently used turning machine. It corresponds to the basic model with horizontal turning axis. The work is held between centres, by a faceplate or in a chuck. · Multiple-tool lathe. This enables several tools to be engaged at the same time. · Turret lathe, capstan lathe. Machines of this type enable a workpiece to be machined by several tools which are engaged one after the other. The tools are held in the turret, which rotates for bringing them into cutting position. The turrets are generally of the disc or crown type, but there are also drum-type turret lathes. · Copy-turning lathes. The desired shape is transmitted by tracer control from a template to the work. · Automatic lathe. The various operations, including the change of the work, are automated. There are bar automatics and chucking automatics. · Vertical lathe (boring and turning mill). The work turns about a vertical axis; it is clamped to a horizontal revolving table. This type of machine is generally used for machining large castings and forgings. · NC and CNC lathes. All the aforementioned machines can be equipped with a numerical control (NC) or computer-assisted numerical control (CNC) system. The result is a semiautomated or fully automated machine which can be used rather universally, thanks to the great versatility and easy programmability of the control system. The future development of the lathe will probably concentrate on control systems. Contact controls will be increasingly replaced by electronic control systems. As regards the latter, there is a trend in evolution from interpolation-programmed to memory-programmed controls. It is

foreseeable in the long run that the use of increasingly efficient process computers will tend to optimize the machining process.

Accidents
Lathe accidents are generally caused by: · disregard for safety regulations when the machines are installed in workshops (e.g., not enough space between machines, no power disconnect switch for each machine) · missing guards or the absence of auxiliary devices (severe injuries have been caused to workers who tried to brake the spindle of their lathes by pressing one of their hands against unguarded belt pulleys and to operators who inadvertently engaged unguarded clutch levers or pedals; injuries due to flying chips because of the absence of hinged or sliding covers have also occurred) · inadequately located control elements (e.g., a turner’s hand can be pierced by the tailstock centre if the pedal controlling the chuck is mistaken for the one controlling the hydraulic circuit of the tailstock centre movement) · adverse conditions of work (i.e., shortcomings from the point of view of occupational physiology) · lack of PPE or wearing unsuitable work clothing (severe and even fatal injuries have been caused to lathe operators who wore loose clothes or had long, free-hanging hair) · insufficient instruction of personnel (an apprentice was fatally injured when he filed a short shaft which was fixed between centres and rotated by a cranked carrier on the spindle nose and a straight one on the shaft; the lathe carrier seized his left-hand sleeve, which was wrapped around the workpiece, dragging the apprentice violently into the lathe) · poor work organization leading to the use of unsuitable equipment (e.g., a long bar was machined on a conventional production lathe; it was too long for this lathe, and it projected more than 1 m beyond the headstock; moreover, the chuck aperture was too large for the bar and was made up by inserting wooden wedges; when the lathe spindle started rotating, the free bar end bent by 45° and struck the operator’s head; the operator died during the following night) · defective machine elements (e.g., a loose carrier pin in a clutch may cause the lathe spindle to start rotating while the operator is adjusting a workpiece in the chuck).

Accident Prevention
The prevention of lathe accidents starts at the design stage. Designers should give special attention to control and transmission elements. Control elements

Each lathe must be equipped with a power disconnect (or isolating) switch so that maintenance and repair work may be carried out safely. This switch must disconnect the current on all poles, reliably cut the pneumatic and hydraulic power and vent the circuits. On large machines, the disconnect switch should be so designed that it can be padlocked in its out position—a safety measure against accidental reconnection. The layout of the machine controls should be such that the operator can easily distinguish and reach them, and that their manipulation presents no hazard. This means that controls must never be arranged at points which can be reached only by passing the hand over the working zone of the machine or where they may be hit by flying chips. Switches which monitor guards and interlock them with the machine drive should be chosen and installed in such a way that they positively open the circuit as soon as the guard is shifted from its protecting position. Emergency stop devices must cause the immediate standstill of the dangerous movement. They must be designed and located in such a way that they can be easily operated by the threatened worker. Emergency stop buttons must be easily reached and should be in red. The actuating elements of control gear which may trip a dangerous machine movement must be guarded so as to exclude any inadvertent operation. For instance, the clutch engaging levers on the headstock and apron should be provided with safety locking devices or screens. A pushbutton can be made safe by lodging it in a recess or by shrouding it with a protective collar. Hand-operated controls should be designed and located in such a way that the hand movement corresponds to the controlled machine movement. Controls should be identified with easily readable and understandable markings. To avoid misunderstandings and linguistic difficulties, it is advisable to use symbols. Transmission elements All moving transmission elements (belts, pulleys, gears) must be covered with guards. An important contribution to the prevention of lathe accidents can be made by the persons responsible for the installation of the machine. Lathes should be so installed that the operators tending them do not hinder or endanger each other. The operators should not turn their backs towards passageways. Protective screens should be installed where neighbouring workplaces or passageways are within the range of flying chips. Passageways must be clearly marked. Enough space should be left for materials-handling equipment, for stacking workpieces and for tool boxes. Bar-stock guides must not protrude into the passageways. The floor on which the operator stands must be insulated against cold. Care should be taken that the insulation forms no stumbling obstacle, and the flooring should not become slippery even when covered with a film of oil.

Conduit and pipework should be installed in such a way that they do not become obstacles. Temporary installations should be avoided. Safety engineering measures on the shop floor should be directed in particular at the following points: · work-holding fixtures (faceplates, chucks, collets) should be dynamically balanced before use · the maximum permissible speed of a chuck should be indicated on the chuck by the manufacturer and respected by the lathe operator · when scroll chucks are used, it should be ensured that the jaws cannot be slung out when the lathe is started · chucks of this type should be designed in such a manner that the key cannot be taken off before the jaws have been secured. The chuck keys in general should be so designed that it is impossible to leave them in the chuck. It is important to provide for auxiliary lifting equipment to facilitate mounting and removing of heavy chucks and faceplates. To prevent chucks from running off the spindle when the lathe is suddenly braked, they must be securely fixed. This can be achieved by putting a retaining nut with left-hand thread on the spindle nose, by using a ―Camlock‖ quick-action coupling, by fitting the chuck with a locking key or by securing it with a two-part locking ring. When powered work-holding fixtures are used, such as hydraulically operated chucks, collets and tailstock centres, measures must be taken which make it impossible for the hands to be introduced into the danger zone of closing fixtures. This can be achieved by limiting the travel of the clamping element to 6 mm, by choosing the location of deadman’s controls so as to exclude the introduction of the hands into the danger zone or by providing a moving guard which has to be closed before the clamping movement can be started. If starting the lathe while the chuck jaws are open presents a danger, the machine should be equipped with a device which prevents the spindle rotation being started before the jaws are closed. The absence of power must not cause the opening or closure of a powered work-holding fixture. If the gripping force of a power chuck diminishes, the spindle rotation must be stopped, and it must be impossible to start the spindle. Reversing the gripping direction from inside to outside (or vice versa) while the spindle rotates must not cause the chuck to be dislodged from the spindle. Removal of holding fixtures from the spindle should be possible only when the spindle has ceased rotating. When machining bar stock, the portion projecting beyond the lathe must be enclosed by barstock guides. Bar feed weights must be guarded by hinged covers extending to the floor. Carriers

To prevent serious accidents—in particular, when filing work in a lathe—unprotected carriers must not be used. A centring safety carrier should be used, or a protective collar should be fitted to a conventional carrier. It is also possible to use self-locking carriers or to provide the carrier disc with a protective cover. Working zone of the lathe Universal-lathe chucks should be guarded by hinged covers. If possible, protective covers should be interlocked with spindle drive circuits. Vertical boring and turning mills should be fenced with bars or plates to prevent injury from revolving parts. To enable the operator to watch the machining process safely, platforms with railings must be provided. In certain cases, TV cameras can be installed so that the operator may monitor the tool edge and tool in-feed. The working zones of automatic lathes, NC and CNC lathes should be completely enclosed. Enclosures of fully automatic machines should only have openings through which the stock to be machined is introduced, the turned part ejected and the swarf removed from the working zone. These openings must not constitute a hazard when work passes through them, and it must be impossible to reach through them into the danger zone. The working zones of semi-automatic, NC and CNC lathes must be enclosed during the machining process. The enclosures are generally sliding covers with limit switches and interlocking circuit. Operations requiring access to the working zone, such as change of work or tools, gauging and so on, must not be carried out before the lathe has been safely stopped. Zeroing a variable-speed drive is not considered a safe standstill. Machines with such drives must have locked protective covers that cannot be unlocked before the machine is safely stopped (e.g., by cutting the spindlemotor power supply). If special tool-setting operations are required, an inching control is to be provided which enables certain machine movements to be tripped while the protective cover is open. In such cases, the operator can be protected by special circuit designs (e.g., by permitting only one movement to be tripped at a time). This can be achieved by using two-hand controls. Turning swarf Long turning chips are dangerous because they may get entangled with arms and legs and cause serious injury. Continuous and ravelled chips can be avoided by choosing appropriate cutting speeds, feeds and chip thicknesses or by using lathe tools with chip breakers of the gullet or step type. Swarf hooks with handle and buckle should be used for removing chips.

Ergonomics
Every machine should be so designed that it enables a maximal output to be obtained with a minimum of stress on the operator. This can be achieved by adapting the machine to the worker.

Ergonomic factors must be taken into account when designing the human-machine interface of a lathe. Rational workplace design also includes providing for auxiliary handling equipment, such as loading and unloading attachments. All controls must be located within the physiological sphere or reach of both hands. The controls must be clearly laid out and should be logical to operate. Pedal-operated controls should be avoided in machines tended by standing operators. Experience has shown that good work is performed when the workplace is designed for both standing and sitting postures. If the operator has to work standing up, he or she should be given the possibility of changing posture. Flexible seats are in many cases a welcome relief for strained feet and legs. Measures should be taken to create optimal thermal comfort, taking into account the air temperature, relative humidity, air movement and radiant heat. The workshop should be adequately ventilated. There should be local exhaust devices to eliminate gaseous emanations. When machining bar stock, sound-absorbent-lined guide tubes should be used. The workplace should preferably be provided with uniform lighting, affording an adequate level of illumination.

Work Clothing and Personal Protection
Overalls should be close fitting and buttoned or zipped to the neck. They should be without breast pockets, and the sleeves must be tightly buttoned at the wrists. Belts should not be worn. No finger rings and bracelets should be worn when working on lathes. Wearing of safety spectacles should be obligatory. When heavy workpieces are machined, safety shoes with steel toe caps must be worn. Protective gloves must be worn whenever swarf is being collected.

Training
The lathe operator’s safety depends to a large extent on working methods. It is therefore important that he or she should receive thorough theoretical and practical training to acquire skills and develop a behaviour affording the best possible safeguards. Correct posture, correct movements, correct choice and handling of tools should become routine to such an extent that the operator works correctly even if his or her concentration is temporarily relaxed. Important points in a training programme are an upright posture, the proper mounting and removal of the chuck and the accurate and secure fixing of workpieces. Correct holding of files and scrapers and safe working with abrasive cloth must be intensively practised. Workers must be well informed about the hazards of injury which may be caused when gauging work, checking adjustments and cleaning lathes.

Maintenance

Lathes must be regularly maintained and lubricated. Faults must be corrected immediately. If safety is at stake in the event of a fault, the machine should be put out of operation until corrective action has been taken. Repair and maintenance work must be carried out only after the machine has been isolated from the power supply.

GRINDING AND POLISHING
K. Welinder* *Adapted from the 3rd edition, Encyclopaedia of Occupational Health and Safety. Grinding generally involves the use of a bonded abrasive to wear away parts of a workpiece. The aim is to give the work a certain shape, correct its dimensions, increase the smoothness of a surface or improve the sharpness of cutting edges. Examples include removal of sprues and rough edges from a foundry casting, removal of surface scale from metals before forging or welding and deburring of parts in sheet metal and machine shops. Polishing is used to remove surface imperfections such as tool marks. Buffing does not remove metal, but uses a soft abrasive blended in a wax or grease base to produce a high-lustre surface. Grinding is the most comprehensive and diversified of all machining methods and is employed on many materials—predominantly iron and steel but also other metals, wood, plastics, stone, glass, pottery and so on. The term covers other methods of producing very smooth and glossy surfaces, such as polishing, honing, whetting and lapping. The tools used are wheels of varying dimensions, grinding segments, grinding points, sharpening stones, files, polishing wheels, belts, discs and so on. In grinding wheels and the like, the abrasive material is held together by bonding agents to form a rigid, generally porous body. In the case of abrasive belts, the bonding agent holds the abrasive secured to a flexible base material. Buffing wheels are made from cotton or other textile disks sewn together. The natural abrasives—natural corundum or emery (aluminium oxides), diamond, sandstone, flint and garnet—have been largely superseded by artificial abrasives including aluminium oxide (fused alumina), silicon carbide (carborundum) and synthetic diamonds. A number of finegrained materials such as chalk, pumice, tripoli, tin putty and iron oxide are also used, especially for polishing and buffing. Aluminium oxide is most widely used in grinding wheels, followed by silicon carbide. Natural and artificial diamonds are used for important special applications. Aluminium oxide, silicon carbide, emery, garnet and flint are used in grinding and polishing belts. Both organic and inorganic bonding agents are used in grinding wheels. The main type of inorganic bonds are vitrified silicate and magnesite. Notable among organic bonding agents are phenol- or urea- formaldehyde resin, rubber and shellac. The vitrified bonding agents and phenolic resin are completely dominating within their respective groups. Diamond grinding

wheels can also be metal bonded. The various bonding agents give the wheels different grinding properties, as well as different properties with regard to safety. Abrasive and polishing belts and discs are composed of a flexible base of paper or fabric to which the abrasive is bonded by means of a natural or synthetic adhesive. Different machines are used for different types of operations, such as surface grinding, cylindrical (including centreless) grinding, internal grinding, rough grinding and cutting. The two main types are: those where either the grinder or the work is moved by hand and machines with mechanical feeds and chucks. Common equipment types include: surface-type grinders; pedestal-type grinders, polishers and buffers; disk grinders and polishers; internal grinders; abrasive cut-off machines; belt polishers; portable grinders, polishers and buffers; and multiple polishers and buffers.

Hazards and Their Prevention
Bursting The major injury risk in the use of grinding wheels is that the wheel may burst during grinding. Normally, grinding wheels operate at high speeds. There is a trend towards ever-increasing speeds. Most industrialized nations have regulations limiting the maximum speeds at which the various types of grinding wheels may be run. The fundamental protective measure is to make the grinding wheel as strong as possible; the nature of the bonding agent is most important. Wheels with organic bonds, in particular phenolic resin, are tougher than those with inorganic bonds and more resistant to impacts. High peripheral speeds may be permissible for wheels with organic bonds. Very high-speed wheels, in particular, often incorporate various types of reinforcement. For example, certain cup wheels are fitted with steel hubs to increase their strength. During rotation the major stress develops around the centre hole. To strengthen the wheel, the section around the centre hole, which takes no part in the grinding, can thus be made of an especially strong material which is not suitable for grinding. Large wheels with a centre section reinforced in this way are used particularly by the steel works for grinding slabs, billets and the like at speeds up to 80 m/s. The most common method of reinforcing grinding wheels, however, is to include glass fibre fabric in their construction. Thin wheels, such as those used for cutting, may incorporate glass fibre fabric at the centre or at each side, while thicker wheels have a number of fabric layers depending on the thickness of the wheel. With the exception of some grinding wheels of small dimensions, either all wheels or a statistical sampling of them must be given speed tests by the manufacturer. In tests the wheels are run over a certain period at a speed exceeding that permitted in grinding. Test regulations vary from country to country, but usually the wheel has to be tested at a speed 50% above the working speed. In some countries, regulations require special testing of wheels that are to operate at

higher speeds than normal at a central testing institute. The institute may also cut specimens from the wheel and investigate their physical properties. Cutting wheels are subjected to certain impact tests, bending tests and so on. The manufacturer is also obliged to ensure that the grinding wheel is well balanced prior to delivery. The bursting of a grinding wheel may cause fatal or very serious injuries to anyone in the vicinity and heavy damage to plant or premises. In spite of all precautions taken by the manufacturers, occasional wheel bursts or breaks may still occur unless proper care is exercised in their use. Precautionary measures include: · Handling and storing. A wheel may become damaged or cracked during transit or handling. Moisture may attack the bonding agent in phenolic resin wheels, ultimately reducing their strength. Vitrified wheels may be sensitive to repeated temperature variations. Irregularly absorbed moisture may throw the wheel out of balance. Consequently, it is most important that wheels are carefully handled at all stages and kept in an orderly manner in a dry and protected place. · Checking for cracks. A new wheel should be checked to ensure that it is undamaged and dry, most simply by tapping with a wooden mallet. A faultless vitrified wheel will give a clear ring, an organic bonded wheel a less ringing tone; but either can be differentiated from the cracked sound of a defective wheel. In case of doubt, the wheel should not be used and the supplier should be consulted. · Testing. Before the new wheel is put into service, it should be tested at full speed with due precautions being observed. After wet grinding, the wheel should be run idle to eject the water; otherwise the water may collect at the bottom of the wheel and cause imbalance, which may result in bursting when the wheel is next used. · Mounting. Accidents and breakages occur when grinding wheels are mounted on unsuitable apparatus—for example, on spindle ends of buffing machines. The spindle should be of adequate diameter but not so large as to expand the centre hole of the wheel; flanges should be not less than one-third the diameter of the wheel and made of mild steel or of similar material. · Speed. In no circumstances should the maximum permissible operating speed specified by the makers be exceeded. A notice indicating the spindle speed should be fitted to all grinding machines, and the wheel should be marked with the maximum permissible peripheral speed and the corresponding number of revolutions for a new wheel. Special precautions are necessary with variable speed grinding machines and to ensure the fitting of wheels of appropriate permissible speeds in portable grinders. · Work rest. Wherever practicable, rigidly mounted work rests of adequate dimensions should be provided. They should be adjustable and kept as close as possible to the wheel to prevent a trap in which the work might be forced against the wheel and break it or, more probable, catch and injure the operator’s hand.

· Guarding. Abrasive wheels should be provided with guards strong enough to contain the parts of a bursting wheel (see figure 82.5). Some countries have detailed regulations regarding the design of the guards and the materials to be used. In general, cast iron and cast aluminium are to be avoided. The grinding opening should be as small as possible, and an adjustable nose piece may be necessary. Exceptionally, where the nature of the work precludes the use of a guard, special protective flanges or safety chucks may be used. The spindles and tapered ends of double-ended polishing machines can cause entanglement accidents unless they are effectively guarded. Figure 82.5 A well guarded, vitrified abrasive wheel mounted in a surface grinder and operating at a peripheral speed of 33 m/s

Eye injuries Dust, abrasives, grains and splinters are a common hazard to the eyes in all dry-grinding operations. Effective eye protection by goggles or spectacles and fixed eye shields at the machine are essential; fixed eye shields are particularly useful when wheels are in intermittent use—for example, for tool grinding. Fire Grinding of magnesium alloys carries a high fire risk unless strict precautions are taken against accidental ignition and in the removal and drenching of dust. High standards of cleanliness and maintenance are required in all exhaust ducting to prevent risk of fire and also to keep ventilation working efficiently. Textile dust released from buffing operations is a fire hazard requiring good housekeeping and LEV. Vibration Portable and pedestal grinders carry a risk of hand-arm vibration syndrome (HAVS), also known as ―white finger‖ from its most noticeable sign. Recommendations include limiting intensity and duration of exposure, redesigning tools, protective equipment and monitoring exposure and health. Health hazards Although modern grinding wheels do not themselves create the serious silicosis hazard associated in the past with sandstone wheels, highly dangerous silica dust may still be given off from the materials being ground—for example, sand castings. Certain resin-bonded wheels may contain fillers which create a dangerous dust. In addition, formaldehyde-based resins can emit formaldehyde during grinding. In any event, the volume of dust produced by grinding makes efficient LEV essential. It is more difficult to provide local exhaust for portable wheels, although some success in this direction has been achieved by use of low-volume, high-velocity capture systems. Prolonged work should be avoided and respiratory protective equipment provided if

necessary. Exhaust ventilation is also required for most belt sanding, finishing, polishing and similar operations. With buffing in particular, combustible textile dust is a serious concern. Protective clothing and good sanitary and washing facilities with showers should be provided, and medical supervision is desirable, especially for metal grinders.

INDUSTRIAL LUBRICANTS, METAL WORKING FLUIDS AND AUTOMOTIVE OILS
Richard S. Kraus The industrial revolution could not have occurred without the development of refined petroleumbased industrial oils, lubricants, cutting oils and greases. Prior to the discovery in the 1860s that a superior lubricant could be produced by distilling crude oil in a vacuum, industry depended on naturally occurring oils and animal fats such as lard and whale sperm oil for lubricating moving parts. These oils and animal products were especially susceptible to melting, oxidation and breakdown from exposure to heat and moisture produced by the steam engines which powered almost all industrial equipment at that time. The evolution of petroleum-based refined products has continued from the first lubricant, which was used to tan leather, to modern synthetic oils and greases with longer service life, superior lubricating qualities and better resistance to change under varying temperatures and climatic conditions.

Industrial Lubricants
All moving parts on machinery and equipment require lubrication. Although lubrication may be provided by dry materials such as Teflon or graphite, which are used in parts such as small electrical motor bearings, oils and greases are the most commonly used lubricants. As the complexity of the machinery increases, the requirements for lubricants and metal process oils become more stringent. Lubricating oils now range from clear, very thin oils used to lubricate delicate instruments, to thick, tar-like oils used on large gears such as those which turn steel mills. Oils with very specific requirements are used both in the hydraulic systems and to lubricate large computer-operated machine tools such as those used in the aerospace industry to produce parts with extremely close tolerances. Synthetic oils, fluids and greases, and blends of synthetic and petroleum-based oils, are used where extended lubricant life is desired, such as sealed-for-life electric motors, where the increased time between oil changes offsets the difference in cost; where extended temperature and pressure ranges exist, such as in aerospace applications; or where it is difficult and expensive to re-apply the lubricant.

Industrial Oils
Industrial oils such as spindle and lubricating oils, gear lubricants, hydraulic and turbine oils and transmission fluids are designed to meet specific physical and chemical requirements and to operate without discernible change for extended periods under varying conditions. Lubricants for aerospace use must meet entirely new conditions, including cleanliness, durability, resistance to

cosmic radiation and the ability to operate in extremely cold and hot temperatures, without gravity and in a vacuum. Transmissions, turbines and hydraulic systems contain fluids which transfer force or power, reservoirs to hold the fluids, pumps to move the fluids from one place to another and auxiliary equipment such as valves, piping, coolers and filters. Hydraulic systems, transmissions and turbines require fluids with specific viscosities and chemical stability to operate smoothly and provide the controlled transfer of power. The characteristics of good hydraulic and turbine oils include a high viscosity index, thermal stability, long life in circulating systems, deposit resistance, high lubricity, anti-foam capabilities, rust protection and good demulsibility. Gear lubricants are designed to form strong, tenacious films which provide lubrication between gears under extreme pressure. The characteristics of gear oils include good chemical stability, demulsibility and resistance to viscosity increase and deposit formation. Spindle oils are thin, extremely clean and clear oils with lubricity additives. The most important characteristics for way oils—used to lubricate two flat sliding surfaces where there is high pressure and slow speed—are lubricity and tackiness to resist squeezing out and resistance to extreme pressure. Cylinder and compressor oils combine the characteristics of both industrial and automotive oils. They should resist accumulation of deposits, act as a heat transfer agent (internal combustion engine cylinders), provide lubrication for cylinders and pistons, provide a seal to resist blowback pressure, have chemical and thermal stability (especially vacuum pump oil), have a high viscosity index and resist water wash (steam-operated cylinders) and detergency.

Automotive Engine Oils
Manufacturers of internal combustion engines and organizations, such as the Society of Automotive Engineers (SAE) in the United States and Canada, have established specific performance criteria for automotive engine oils. Automotive gasoline and diesel engine oils are subjected to a series of performance tests to determine their chemical and thermal stability, corrosion resistance, viscosity, wear protection, lubricity, detergency and high and low temperature performance. They are then classified according to a code system which allows consumers to determine their suitability for heavy-duty use and for different temperatures and viscosity ranges. Oils for automotive engines, transmissions and gear cases are designed with high viscosity indexes to resist changes in viscosity with temperature changes. Automotive engine oils are especially formulated to resist breakdown under heat as they lubricate internal combustion engines. Internal combustion engine oils must not be too thick to lubricate the internal moving parts when an engine starts up in cold weather, and they must not thin out as the engine heats up when operating. They should resist carbon build-up on valves, rings and cylinders and the formation of corrosive acids or deposits from moisture. Automotive engine oils contain detergents designed to hold carbon and metallic wear particles in suspension so that they can be filtered out as the oil circulates and not accumulate on internal engine parts and cause damage.

Cutting Fluids

The three types of cutting fluids used in industry are mineral oils, soluble oils and synthetic fluids. Cutting oils are typically a blend of high-quality, high-stability mineral oils of various viscosities together with additives to provide specific characteristics depending on the type of material being machined and the work performed. Soluble water-in-oil cutting fluids are mineral oils (or synthetic oils) which contain emulsifiers and special additives including defoamants, rust inhibitors, detergents, bactericides and germicides. They are diluted with water in varying ratios before being used. Synthetic cutting fluids are solutions of non-petroleum-based fluids, additives and water, rather than emulsions, some of which are fire resistant for machining specific metals. Semi-synthetic fluids contain 10 to 15% mineral oil. Some special fluids have both lubricating oil and cutting fluid characteristics due to the tendency of fluids to leak and intermix in certain machine tools such as multi-spindle, automatic screw machines. The desired characteristics of cutting fluids depend on the composition of the metal being worked on, the cutting tool being used and the type of cutting, planing or shaping operation performed. Cutting fluids improve and enhance the metal working process by cooling and lubrication (i.e., protecting the edge of the cutting tool). For example, when working on a soft metal which creates a lot of heat, cooling is the most important criterion. Improved cooling is provided by using a light oil (such as kerosene) or water-based cutting fluid. Control of the builtup edge on cutting tools is provided by anti-weld or anti-wear additives such as sulphur, chlorine or phosphorus compounds. Lubricity, which is important when working on steel to overcome the abrasiveness of iron sulphide, is provided by synthetic and animal fats or sulphurized sperm oil additives.

Other Metal Working and Process Oils
Grinding fluids are designed to provide cooling and prevent metal build-up on grinding wheels. Their characteristics include thermal and chemical stability, rust protection (soluble fluids), preventing gummy deposits upon evaporation and a safe flashpoint for the work performed. Quench oils, which require high stability, are used in metal treating to control the change of the molecular structure of steel as it cools. Quenching in lighter oil is used to case harden small, inexpensive steel parts. A slower quench rate is used to produce machine tool steels which are fairly hard on the outside with lower internal stress. A gapped or multi-phase quenching oil is used to treat high carbon and alloy steels. Roll oils are specially formulated mineral or soluble oils which lubricate and provide a smooth finish to metal, particularly aluminium, copper and brass, as it goes through hot and cold rolling mills. Release oils are used to coat dies and moulds to facilitate the release of the formed metal parts. Tanning oils are still used in the felt and leather-making industry. Transformer oils are specially formulated dielectric fluids used in transformers and large electric breakers and switches. Heat transfer oils are used in open or closed systems and may last up to 15 years in service. The primary characteristics are good thermal stability as systems operate at temperatures from 150 to 315°C, oxidation stability and high flashpoint. Heat transfer oils are normally too viscous to be pumped at ambient temperatures and must be heated to provide fluidity.

Petroleum solvents are used to clean parts by spraying, dripping or dipping. The solvents remove oil and emulsify dirt and metal particles. Rust preventive oils may be either solvent or water based. They are applied to stainless steel coils, bearings and other parts by dipping or spraying, and leave polarized or wax films on the metal surfaces for fingerprint and rust protection and water displacement.

Greases
Greases are mixtures of fluids, thickeners and additives used to lubricate parts and equipment which cannot be made oil-tight, which are hard to reach or where leaking or splashed liquid lubricants might contaminate products or create a hazard. They have a wide range of applications and performance requirements, from lubricating jet engine bearings at sub-zero temperatures to hot rolling mill gears, and resisting acid or water washout, as well as the continuous friction created by railroad car wheel roller bearings. Grease is made by the blending of metallic soaps (salts of long-chained fatty acids) into a lubricating oil medium at temperatures of 205 to 315°C. Synthetic greases may use di-esters, silicone or phosphoric esters and polyalkyl glycols as fluids. The characteristics of the grease depend to a great extent upon the particular fluid, metallic element (e.g., calcium, sodium, aluminium, lithium and so on) in the soap and the additives used to improve performance and stability and to reduce friction. These additives include extreme-pressure additives which coat the metal with a thin layer of non-corrosive metallic sulphur compounds, lead naphthenate or zinc dithiophosphate, rust inhibitors, anti-oxidants, fatty acids for added lubricity, tackiness additives, colour dyes for identification and water inhibitors. Some greases may contain graphite or molybdenum fillers which coat the metallic parts and provide lubrication after the grease has run out or decomposed.

Industrial Lubricants, Grease and Automotive Engine Oil Additives
In addition to using high-quality lubricant base stocks with chemical and thermal stability and high viscosity indexes, additives are needed to enhance the fluid and provide specific characteristics required in industrial lubricants, cutting fluids, greases and automotive engine oils. The most commonly used additives include but are not limited to the following: · Anti-oxidants. Oxidation inhibitors, such as 2,6-ditertiary butyl, paracresol and phenyl naphthylamine, reduce the rate of deterioration of oil by breaking up the long-chain molecules which form when exposed to oxygen. Oxidation inhibitors are used to coat metals such as copper, zinc and lead to prevent contact with the oil so they will not act as catalysts, speeding up oxidation and forming acids which attack other metals. · Foam inhibitors. Defoamants, such as silicones and polyorganic silioxanes, are used in hydraulic oils, gear oils, transmission fluids and turbine oils to reduce surface film tension and remove air entrapped in the oil by pumps and compressors, in order to maintain constant hydraulic pressure and prevent cavitation.

· Corrosion inhibitors. Anti-rust additives, such as lead naphthenate and sodium sulphonate, are used to prevent rust from forming on metallic parts and systems where circulating oil has been contaminated with water or by moist air which entered system reservoirs as they cooled down when the equipment or machinery was not in use. · Anti-wear additives. Anti-wear additives, such as tricresylphosphate, form polar compounds which are attracted to metal surfaces and provide a physical layer of additional protection in the event that the oil film is not sufficient. · Viscosity index improvers. Viscosity index improvers help oils resist the effects of temperature changes. Unfortunately, their effectiveness diminishes with extended use. Synthetic oils are designed with very high viscosity indexes, allowing them to maintain their structure over wider temperature ranges and for much longer periods of time than mineral oils with viscosity index improver additives. · Demulsifiers. Water inhibitors and special compounds separate water out of oil and prevent gum formation; they contain waxy oils which provide added lubricity. They are used where equipment is subject to water wash or where a large amount of moisture is present, such as in steam cylinders, air compressors and gear cases contaminated by soluble cutting fluids. · Colour dyes. Dyes are used to assist users to identify different oils used for specific purposes, such as transmission fluids and gear oils, in order to prevent misapplication. · Extreme pressure additives. Extreme pressure additives, such as non-corrosive sulphurized fatty compounds, zinc dithiophosphate and lead naphthenate, are used in automotive, gear and transmission oils to form coatings which protect metal surfaces when the protective oil film thins or is squeezed out and cannot prevent metal to metal contact. · Detergents. Metal sulphonate and metal phenate detergents are used to hold dirt, carbon and metallic wear particles in suspension in hydraulic oils, gear oils, engine oils and transmission fluids. These contaminants are typically removed when the oil passes through a filter to prevent their being recirculated through the system where they could cause damage. · Tackiness additives. Adhesive or tackiness additives are used to enable oils to adhere to and resist leakage from bearing assemblies, gear cases, large open gears on mills and construction equipment, and overhead machinery. Their tackiness diminishes with extended service. · Emulsifiers. Fatty acids and fatty oils are used as emulsifiers in soluble oils to help form solutions with water. · Lubricity additives. Fat, lard, tallow, sperm and vegetable oils are used to provide a higher degree of oiliness in cutting oils and some gear oils. · Bactericides. Bactericides and germicides, such as phenol and pine oil, are added to soluble cutting oils to prolong the life of the fluid, maintain stability, reduce odours and prevent dermatitis.

Manufacturing Industrial Lubricants and Automotive Oils
Industrial lubricants and oils, grease, cutting fluids and automotive engine oils are manufactured in blending and packaging facilities, also called ―lube plants‖ or ―blending plants‖. These facilities may be located either in or adjacent to refineries which produce lubricant base stocks, or they may be some distance away and receive the base stocks by marine tankers or barges, railroad tank cars or tank trucks. Blending and packaging plants blend and compound additives into lubricating oil base stocks to manufacture a wide range of finished products, which are then shipped in bulk or in containers. The blending and compounding processes used to manufacture lubricants, fluids and greases depend on the age and sophistication of the facility, the equipment available, the types and formulation of the additives used and the variety and volume of products produced. Blending may require only physical mixing of base stocks and additive packages in a kettle using mixers, paddles or air agitation, or auxiliary heat from electric or steam coils may be needed to help dissolve and blend in the additives. Other industrial fluids and lubricants are produced automatically by mixing base stocks and pre-blended additive and oil slurries through manifold systems. Grease may be either batch produced or continuously compounded. Lube plants may compound their own additives from chemicals or purchase pre-packaged additives from specialty companies; a single plant may use both methods. When lube plants manufacture their own additives and additive packages, there may be a need for high temperatures and pressures in addition to chemical reactions and physical agitation to compound the chemicals and materials. After production, fluids and lubricants may be held in the blending kettles or placed in holding tanks to ensure that the additives remain in suspension or solution, to allow time for testing to determine whether the product meets quality specifications and certification requirements, and to allow process temperatures to return to ambient levels before products are packaged and shipped. When testing is completed, finished products are released for bulk shipment or packaging into containers. Finished products are shipped in bulk in railroad tank cars or in tank trucks directly to consumers, distributors or outside packaging plants. Finished products also are shipped to consumers and distributors in railroad box cars or package delivery trucks in a variety of containers, as follows: · Metal, plastic and combination metal/plastic or plastic/fibre intermediate bulk containers, which range in size from 227 l to approximately 2,840 l, are shipped as individual units on builtin or separate pallets, stacked 1 or 2 high. · Metal, fibre or plastic drums with a capacity of 208 l, 114 l or 180 kg are typically shipped 4 to a pallet. · Metal or plastic drums with a capacity of 60 l or 54 kg, and 19 l or 16 kg metal or plastic pails, are stacked on pallets and banded or stretch wrapped to maintain stability.

· Metal or plastic containers with a capacity of 8 l or 4 l, 1 l plastic, metal and fibre bottles and cans and 2 kg grease cartridges are packaged in cartons which are stacked on pallets and banded or stretch wrapped for shipment. Some blending and packaging plants may ship pallets of mixed products and mixed sizes of containers and packages directly to small consumers. For example, a single-pallet shipment to a service station could include 1 drum of transmission fluid, 2 kegs of grease, 8 cases of automotive engine oil and 4 pails of gear lubricant.

Product Quality
Lubricant product quality is important to keep machines and equipment operating properly and to produce quality parts and materials. Blending and packaging plants manufacture finished petroleum products to strict specifications and quality requirements. Users should maintain the level of quality by establishing safe practices for the handling, storage, dispensing and transfer of lubricants from their original containers or tanks to the dispensing equipment and to the point of application on the machine or equipment to be lubricated or the system to be filled. Some industrial facilities have installed centralized dispensing, lubrication and hydraulic systems which minimize contamination and exposure. Industrial oils, lubricants, cutting oils and grease will deteriorate from water or moisture contamination, exposure to excessively high or low temperatures, inadvertent mixing with other products and long-term storage which allows additive drop-out or chemical changes to occur.

Health and Safety
Because they are used and handled by consumers, finished industrial and automotive products must be relatively free of hazards. There is a potential for hazardous exposures when blending and compounding products, when handling additives, when using cutting fluids and when operating oil mist lubrication systems. The chapter Oil and natural gas refineries in this Encyclopaedia gives information regarding potential hazards associated with auxiliary facilities at blending and packaging plants such as boiler rooms, laboratories, offices, oil-water separators and waste treatment facilities, marine docks, tank storage, warehouse operations, railroad tank car and tank truck loading racks and railroad box car and package truck loading and unloading facilities. Safety Manufacturing additives and slurries, batch compounding, batch blending and in-line blending operations require strict controls to maintain desired product quality and, along with the use of PPE, to minimize exposure to potentially hazardous chemicals and materials as well as contact with hot surfaces and steam. Additive drums and containers should be stored safely and kept tightly sealed until ready for use. Additives in drums and bags need to be handled properly to avoid muscular strain. Hazardous chemicals should be properly stored, and incompatible chemicals should not be stored where they can mix with one another. Precautions to be taken when operating filling and packaging machinery include using gloves and avoiding catching

fingers in devices which crimp covers on kegs and pails. Machine guards and protective systems should not be removed, disconnected or by-passed to expedite work. Intermediate bulk containers and drums should be inspected before filling to make sure they are clean and suitable. A confined-space permit system should be established for entry into storage tanks and blending kettles for cleaning, inspection, maintenance or repair. A lockout/tagout procedure should be established and implemented before working on packaging machinery, blending kettles with mixers, conveyors, palletizers and other equipment with moving parts. Leaking drums and containers should be removed from the storage area and spills cleaned up to prevent slips and falls. Recycling, burning and disposal of waste, spilled and used lubricants, automotive engine oils and cutting fluids should be in accordance with government regulations and company procedures. Workers should use appropriate PPE when cleaning spills and handling used or waste products. Drained motor oil, cutting fluids or industrial lubricants which may be contaminated with gasoline and flammable solvents should be stored in a safe place away from sources of ignition, until proper disposal. Fire protection While the potential for fire is less in industrial and automotive lubricant blending and compounding than in refining processes, care must be taken when manufacturing metal working oils and greases due to the use of high blending and compounding temperatures and lower flashpoint products. Special precautions should be taken to prevent fires when products are dispensed or containers filled at temperatures above their flashpoints. When transferring flammable liquids from one container to another, proper bonding and grounding techniques should be applied to prevent static build-up and electrostatic discharge. Electrical motors and portable equipment should be properly classified for the hazards present in the area in which they are installed or used. The potential for fire exists if a leaking product or vapour release in the lube blending and grease processing or storage areas reaches a source of ignition. The establishment and implementation of a hot-work permit system should be considered to prevent fires in blending and packaging facilities. Storage tanks installed inside buildings should be constructed, vented and protected in accordance with government requirements and company policy. Products stored on racks and in piles should not block fire protection systems, fire doors or exit routes. Storage of finished products, both in bulk and in containers and packages, should be in accordance with recognized practices and fire prevention regulations. For example, flammable liquids and additives which are in solutions of flammable liquids may be stored in outside buildings or separate, specially designed inside or attached storage rooms. Many additives are stored in warm rooms (38 to 65°C) or in hot rooms (over 65°C) in order to keep the ingredients in suspension, to reduce the viscosity of thicker products or to provide for easier blending or compounding. These storage rooms should comply with electrical classification, drainage, ventilation and explosion venting requirements, especially when flammable liquids or combustible liquids are stored and dispensed at temperatures above their flashpoints.

Health When blending, sampling and compounding, personal and respiratory protective equipment should be considered to prevent exposures to heat, steam, dusts, mists, vapours, fumes, metallic salts, chemicals and additives. Safe work practices, good hygiene and appropriate personal protection may be needed for exposure to oil mists, fumes and vapours, additives, noise and heat when conducting inspection and maintenance activities while sampling and handling hydrocarbons and additives during the production and packaging and when cleaning up spills and releases: · Work shoes with oil- or slip-resistant soles should be worn for general work, and approved protective toe safety shoes with oil- or slip-resistant soles should be worn where hazards of foot injuries from rolling or falling objects or equipment exist. · Safety goggles and respiratory protection may be needed for hazardous exposures to chemicals, dust or steam. · Impervious gloves, aprons, footwear, face shields and chemical goggles should be worn when handling hazardous chemicals, additives and caustic solutions and when cleaning up spills. · Head protection may be needed when working in pits or areas where the potential exists for injury to the head. · Ready access to appropriate cleaning and drying facilities to handle splashes and spills should be provided. Oil is a common cause of dermatitis, which can be controlled through the use of PPE and good personal hygiene practices. Direct skin contact with any formulated greases or lubricants should be avoided. Lighter oils such as kerosene, solvents and spindle oils defat the skin and cause rashes. Thicker products, such as gear oils and greases, block the pores of the skin, leading to folliculitis. Health hazards due to microbial contamination of oil may be summarized as follows: · Pre-existing skin conditions may be aggravated. · Lubricant aerosols of respirable size may cause respiratory illness. · Organisms may change the composition of the product so that it becomes directly injurious. · Harmful bacteria from animals, birds or humans may be introduced. Contact dermatitis may occur when employees are exposed to cutting fluids during production, work or maintenance and when they wipe oil-covered hands with rags embedded with minute metal particles. The metal causes small lacerations in the skin which may become infected. Water-based cutting fluids on skin and clothing may contain bacteria and cause infections, and

the emulsifiers may dissolve fats from the skin. Oil folliculitis is caused by prolonged exposure to oil-based cutting fluids, such as from wearing oil-soaked clothing. Employees should remove and launder clothing that is soaked with oil before wearing it again. Dermatitis may also be caused by using soaps, detergents or solvents to clean the skin. Dermatitis is best controlled by good hygiene practices and minimizing exposure. Medical advice should be sought when dermatitis persists. In the extensive review conducted as a basis for its criteria document, the US National Institute for Occupational Safety and Health (NIOSH) found an association between exposure to metal working fluids and the risk of developing cancer at several organ sites, including the stomach, pancreas, larynx and rectum (NIOSH 1996). The specific formulations responsible for the elevated cancer risks remain to be determined. Occupational exposure to oil mists and aerosols is associated with a variety of non-malignant respiratory effects, including lipoid pneumonia, asthma, acute airways irritation, chronic bronchitis and impaired pulmonary function (NIOSH 1996). Metal working fluids are readily contaminated by bacteria and fungi. They may affect the skin or, when inhaled as contaminated aerosols, they may have systemic effects. Refinery processes such as hydrofinishing and acid treatment are used to remove aromatics from industrial lubricants, and the use of naphthenic base stocks has been restricted in order to minimize carcinogenicity. Additives introduced in blending and compounding may also create a potential risk to health. Exposures to chlorinated compounds and leaded compounds, such as those used in some gear lubricants and greases, cause irritation of the skin and may be potentially hazardous. Tri-orthocresyl phosphate has caused outbreaks of nerve palsies when lubricating oil was accidentally used for cooking. Synthetic oils consist mainly of sodium nitrite and triethanolamine and additives. Commercial triethanolamine contains diethanolamine, which can react with sodium nitrite to form a relatively weak carcinogen, N-nitrosodiethanolamine, which may create a hazard. Semi-synthetic lubricants present the hazards of both products, as well as the additives in their formulations. Product safety information is important to employees of both manufacturers and users of lubricants, oils and greases. Manufacturers should have material safety data sheets (MSDSs) or other product information available for all of the additives and base stocks used in blending and compounding. Many companies have conducted epidemiological and toxicological testing to determine the degree of hazards associated with any acute and chronic health effects of their products. This information should be available to workers and users through warning labels and product safety information.

SURFACE TREATMENT OF METALS
J.G. Jones, J.R. Bevan, J.A. Catton, A. Zober, N. Fish, K.M. Morse, G. Thomas, M.A. El Kadeem and Philip A. Platcow* *Adapted from the 3rd edition, Encyclopaedia of Occupational Health and Safety.

There is a wide variety of techniques for finishing the surfaces of metal products so that they resist corrosion, fit better and look better (see table 82.7). Some products are treated by a sequence of several of these techniques. This article will briefly describe some of those most commonly used. Table 82.7 Summary of the hazards associated with the different metal treatment methods Metal treatment method Electrolytic polishing Electroplating Hazards Burns and irritation from caustic and corrosive chemicals Exposure to potentially cancer causing chromium and nickel; exposure to cyanides; burns and irritation from caustic and corrosive chemicals; electric shock; the process can be wet, causing slip and fall hazards; potential explosive dust generation; ergonomic hazards Physical hazards from grinders, conveyers, mills; burn hazard from high temperature liquids and equipment; exposure to dusts that may cause lung disease Exposure to hydrofluoric acid; burns and irritation from caustic and corrosive chemicals; burn hazard from high temperature liquids and equipment Burn hazard from high temperature liquids, metals, and equipment; burns and irritation from caustic and corrosive chemicals; metal fume fever; potential lead exposure Burn hazard from high temperature liquids, metals and equipment; burns and irritation from caustic and corrosive Precautions Use appropriate personal protective equipment. Install effective exhaust ventilation. Use appropriate personal protective equipment. Install effective exhaust ventilation, often slotted, push-pull system. Clean up spills immediately. Install non-skid flooring. Use effective design of work procedures and stations to avoid ergonomic stress. Install proper machine guards, including interlocks. Use appropriate personal protective equipment. Install effective exhaust ventilation to avoid dust exposure. HEPA-filtered equipment may be necessary. Implement a programme to avoid exposure to hydrofluoric acid. Use appropriate personal protective equipment. Install effective exhaust ventilation. Use appropriate personal protective equipment. Install effective exhaust ventilation. Implement a lead exposure reduction/monitoring programme. Use appropriate personal protective equipment. Install effective exhaust ventilation. Display signs warning of high temperature equipment and

Enamels and glazing

Etching

Galvanizing

Heat treatment

chemicals; possible explosive atmospheres of hydrogen; potential exposure to carbon monoxide; potential exposure to cyanides; fire hazard from oil quenching Metallizing Burn hazard from high temperature metals and equipment; possible explosive atmospheres of dust, acetylene; zinc metal fume fever Burns and irritation from caustic and corrosive chemicals Exposure to chemical sensitizers

surfaces. Install systems to monitor the concentration of carbon monoxide. Install adequate firesuppression systems.

Install adequate fire suppression systems. Properly separate chemicals and gases. Use appropriate personal protective equipment. Install effective exhaust ventilation. Use appropriate personal protective equipment. Install effective exhaust ventilation. Seek alternatives to sensitizers. Use appropriate personal protective equipment. Install effective exhaust ventilation. Seek alternatives to sensitizers. Use appropriate personal protective equipment. Install effective exhaust ventilation. Properly separate chemicals/gases.

Phosphating

Plastics coating

Priming

Exposure to various solvents which are potentially toxic and flammable, exposure to chemical sensitizers, exposure to potentially carcinogenic chromium

Before any of these techniques can be applied, the products must be thoroughly cleaned. A number of methods of cleaning are used, individually or in sequence. They include mechanical grinding, brushing and polishing (which produce metallic or oxidic dust—aluminium dust may be explosive), vapour degreasing, washing with organic grease solvents, ―pickling‖ in concentrated acid or alkaline solutions and electrolytic degreasing. The last involves immersion in baths containing cyanide and concentrated alkali in which electrolytically formed hydrogen or oxygen remove the grease, resulting in ―blank‖ metal surfaces that are free from oxides and grease. The cleaning is followed by adequate rinsing and drying of the product. Proper design of the equipment and effective LEV will reduce some of the risk. Workers exposed to the hazard of splashes must be provided with protective goggles or eye shields and protective gloves, aprons and clothing. Showers and eyewash fountains should be nearby and in good working order, and splashes and spills should be washed away promptly. With electrolytic equipment, the gloves and shoes must be non-conducting, and other standard electrical precautions, such as the installation of ground fault circuit interrupters and lockout/tagout procedures should be followed.

Treatment Processes

Electrolytic polishing Electrolytic polishing is used to produce a surface of improved appearance and reflectivity, to remove excess metal to accurately fit the required dimensions and to prepare the surface for inspection for imperfections. The process involves preferential anodic dissolution of high spots on the surface after vapour degreasing and hot alkaline cleaning. Acids are frequently used as the electrolyte solutions; accordingly, adequate rinsing is required afterwards. Electroplating Electroplating is a chemical or electrochemical process for applying a metallic layer to the product—for example, nickel to protect against corrosion, hard chromium to improve the surface properties or silver and gold to beautify it. Occasionally, non-metallic materials are used. The product, wired as the cathode, and an anode of the metal to be deposited are immersed in an electrolyte solution (which can be acidic, alkaline or alkaline with cyanide salts and complexes) and connected externally to a source of direct current. The positively charged cations of the metallic anode migrate to the cathode, where they are reduced to the metal and deposited as a thin layer (see figure 82.6). The process is continued until the new coating reaches the desired thickness, and the product is then washed, dried and polished. Anode: Cu → Cu+2 + 2e- ; Cathode: Cu2+ + 2e- → Cu In electroforming, a process closely related to electroplating, objects moulded of, for example, plaster or plastic are made conductive by the application of graphite and then are connected as the cathode so that the metal is deposited on them. Figure 82.6 Schematic representation of electroplating

In anodization, a process that has become increasingly important in recent years, products of aluminium (titanium and other metals are also used) are connected as the anode and immersed in dilute sulphuric acid. However, instead of the formation of positive aluminium ions and migrating for deposition on the cathode, they are oxidized by the oxygen atoms arising at the anode and become bound to it as an oxide layer. This oxide layer is partially dissolved by the sulphuric acid solution, making the surface layer porous. Subsequently, coloured or lightsensitive materials can be deposited in these pores, as in the fabrication of nameplates, for example. Enamels and glazes Vitreous enamel or porcelain enamel is used to give a high heat-, stain- and corrosion-resistant covering to metals, usually iron or steel, in a wide range of fabricated products including bath tubs, gas and electric cookers, kitchen ware, storage tanks and containers, and electrical equipment. In addition, enamels are used in the decoration of ceramics, glass, jewellery and decorative ornaments. The specialized use of enamel powders in the production of such

ornamental ware as Cloisonné and Limoges has been known for centuries. Glazes are applied to pottery ware of all kinds. The materials used in the manufacture of vitreous enamels and glazes include: · refractories, such as quartz, feldspar and clay · fluxes, such as borax (sodium borate decahydrate), soda ash (anhydrous sodium carbonate), sodium nitrate, fluorspar, cryolite, barium carbonate, magnesium carbonate, lead monoxide, lead tetroxide and zinc oxide · colours, such as oxides of antimony, cadmium, cobalt, iron, nickel, manganese, selenium, vanadium, uranium and titanium · opacifiers, such as oxides of antimony, titanium, tin and zirconium, and sodium antimoninate · electrolytes, such as borax, soda ash, magnesium carbonate and sulphate, sodium nitrite and sodium aluminate · flocculating agents, such as clay, gums, ammonium alginate, bentonite and colloidal silica. The first step in all types of vitreous enamelling or glazing is the making of the frit, the enamel powder. This involves preparation of the raw materials, smelting and frit handing. After careful cleaning of the metal products (e.g., shot blasting, pickling, degreasing), the enamel may be applied by a number of procedures: · In the wet process, the object is dipped into the aqueous enamel slip, withdrawn and allowed to drain or, in ―slushing‖, the enamel slip is thicker and must be shaken from the object. · In the dry process, the ground-coated object is heated to the enamelling temperature and then dry enamel powder is dusted through sieves onto it. The enamel sinters into place and, when the object is returned to the furnace, it melts down to a smooth surface. · Spray application is being used increasingly, usually in a mechanized operation. It requires a cabinet under exhaust ventilation. · Decorative enamels are usually applied by hand, using brushes or similar tools. · Glazes for porcelain and pottery articles are usually applied by dipping or spraying. Although some dipping operations are being mechanized, pieces are usually dipped by hand in the domestic porcelain industry. The object is held in the hand, dipped into a large tub of glaze, the glaze is removed by a flick of the wrist and the object is placed in a dryer. An enclosed hood or cabinet with efficient exhaust ventilation should be provided when the glaze is sprayed. The prepared objects are then ―fired‖ in a furnace or kiln, which usually is gas fuelled.

Etching Chemical etching produces a satin or matte finish. Most frequently, it is used as a pre-treatment prior to anodizing, lacquering, conversion coating, buffing or chemical brightening. It is most frequently applied to aluminium and stainless steel, but is also used for many other metals. Aluminium is usually etched in alkaline solutions containing various mixtures of sodium hydroxide, potassium hydroxide, trisodium phosphate and sodium carbonate, together with other ingredients to prevent sludge formation. One of the most common processes uses sodium hydroxide at a concentration of 10 to 40 g/l maintained at a temperature of 50 to 85°C with an immersion time as long as 10 minutes. The alkaline etching is usually preceded and followed by treatment in various mixtures of hydrochloric, hydrofluoric, nitric, phosphoric, chromic or sulphuric acid. A typical acid treatment involves immersions of 15 to 60 seconds in a mixture of 3 parts by volume of nitric acid and 1 part by volume of hydrofluoric acid that is maintained at a temperature of 20°C. Galvanizing Galvanizing applies a zinc coating to a variety of steel products to protect against corrosion. The product must be clean and oxide-free for the coating to adhere properly. This usually involves a number of cleaning, rinsing, drying or annealing processes before the product enters the galvanizing bath. In ―hot dip‖ galvanizing, the product is passed through a bath of molten zinc; ―cold‖ galvanizing is essentially electroplating, as described above. Manufactured products are usually galvanized in a batch process, while the continuous strip method is used for steel strip, sheet or wire. Flux may be employed to maintain satisfactory cleaning of both the product and the zinc bath and to facilitate drying. A prefluxing step may be followed by an ammonium chloride flux cover on the surface of the zinc bath, or the latter may be used alone. In galvanizing pipe, the pipe is immersed in a hot solution of zinc ammonium chloride after cleaning and before the pipe enters the molten zinc bath. The fluxes decompose to form irritating hydrogen chloride and ammonia gas, requiring LEV. The various types of continuous hot-dip galvanizing differ essentially in how the product is cleaned and whether the cleaning is done on-line: · cleaning by flame oxidation of the surface oils with subsequent reduction in the furnace and annealing done in-line · electrolytic cleaning done prior to in-line annealing · cleaning by acid pickling and alkali cleaning, using a flux prior to the preheat furnace and annealing in a furnace before galvanizing · cleaning by acid pickling and alkali cleaning, eliminating the flux and preheating in a reducing gas (e.g., hydrogen) prior to galvanizing.

The continuous galvanizing line for light-gauge strip steel omits pickling and the use of flux; it uses alkaline cleaning and maintains the clean surface of the strip by heating it in a chamber or furnace with a reducing atmosphere of hydrogen until it passes below the surface of the molten zinc bath. Continuous galvanizing of wire requires annealing steps, usually with a molten lead pan in front of the cleaning and galvanizing tanks; air or water cooling; pickling in hot, dilute hydrochloric acid; rinsing; application of a flux; drying; and then galvanizing in the molten zinc bath. A dross, an alloy of iron and zinc, settles to the bottom of the molten zinc bath and must be removed periodically. Various types of materials are floated on the surface of the zinc bath to prevent oxidation of the molten zinc. Frequent skimming is needed at the points of entry and exit of the wire or strip being galvanized. Heat treatment Heat treatment, the heating and cooling of a metal which remains in the solid state, is usually an integral part of the processing of metal products. It almost always involves a change in the crystalline structure of the metal which results in a modification of its properties (e.g., annealing to make the metal more malleable, heating and slow cooling to reduce hardness, heating and quenching to increase hardness, low-temperature heating to minimize internal stresses).
Annealing

Annealing is a ―softening‖ heat treatment widely used to allow further cold working of the metal, improve machinability, stress-relieve the product before it is used and so on. It involves heating the metal to a specific temperature, holding it at that temperature for a specific length of time and allowing it to cool at a particular rate. A number of annealing techniques are used: · Blue annealing, in which a layer of blue oxide is produced on the surface of iron-based alloys · Bright annealing, which is carried out in a controlled atmosphere to minimize surface oxidation · Close annealing or box annealing, a method in which both ferrous and non-ferrous metals are heated in a sealed metal container with or without a packing material and then slowly cooled · Full annealing, usually carried out in a protective atmosphere, aimed at obtaining the maximum softness economically feasible · Malleablizing, a special kind of anneal given to iron castings to make them malleable by transforming the combined carbon in the iron to fine carbon (i.e., graphite) · Partial annealing, a low-temperature process to remove internal stresses induced in the metal by cold working

· Sub-critical or spheroidizing annealing, which produces improved machinability by allowing the iron carbide in the crystalline structure to acquire a spheroid shape.
Age-hardening

Age-hardening is a heat treatment often used on aluminium-copper alloys in which the natural hardening that takes place in the alloy is accelerated by heating to about 180°C for about 1 hour.
Homogenizing

Homogenizing, usually applied to ingots or powdered metal compacts, is designed to remove or greatly reduce segregation. It is achieved by heating to a temperature about 20°C below the metal’s melting point for about 2 hours or more and then quenching.
Normalizing

A process similar to full annealing, ensures the uniformity of the mechanical properties to be obtained and also produces greater toughness and resistance to mechanical loading.
Patenting

Patenting is a special type of annealing process that is usually applied to materials of small crosssection which are intended to be drawn (e.g., 0.6% carbon steel wire). The metal is heated in an ordinary furnace to above the transformation range and then passes from the furnace directly into, for example, a lead bath held at a temperature of about 170°C.
Quench-hardening and tempering

An increase in hardness can be produced in an iron-based alloy by heating to above the transformation range and rapidly cooling to room temperature by quenching in oil, water or air. The article is often too highly stressed to be put into service and, in order to increase its toughness, it is tempered by reheating to a temperature below the transformation range and allowing it to cool at the desired rate. Martempering and austempering are similar processes except that the article is quenched, for example, in a salt or lead bath held at a temperature of 400°C.
Surface- and case-hardening

This is another heat-treatment process applied most frequently to iron-based alloys, which allows the surface of the object to remain hard while its core remains relatively ductile. It has a number of variations: · Flame hardening involves hardening the surfaces of the object (e.g., gear teeth, bearings, slideways) by heating with a high-temperature gas torch and then quenching in oil, water or another suitable medium.

· Electrical induction hardening is similar to flame hardening except that the heating is produced by eddy currents induced in the surface layers. · Carburizing increases the carbon content of the surface of an iron-based alloy by heating the object in a solid, liquid or gaseous carbonaceous medium (e.g., solid charcoal and barium carbonate, liquid sodium cyanide and sodium carbonate, gaseous carbon monoxide, methane and so on) at a temperature of about 900°C. · Nitriding increases the nitrogen content of the surface of a special low-alloy cast iron or steel object by heating it in a nitrogenous medium, usually ammonia gas, at about 500 to 600°C. · Cyaniding is a method of case-hardening in which the surface of a low-carbon steel object is enriched in both carbon and nitrogen simultaneously. It usually involves heating the object for 1 hour in a bath of molten 30% sodium cyanide at 870°C, and then quenching in oil or water. · Carbo-nitriding is a gaseous process for the simultaneous absorption of carbon and nitrogen into the surface layer of steel by heating it to 800 to 875°C in an atmosphere of a carburizing gas (see above) and a nitriding gas (e.g., 2 to 5% anhydrous ammonia). Metallizing Metallizing, or metal spraying, is a technique for applying a protective metallic coating to a mechanically roughened surface by spraying it with molten droplets of metal. It is also used to build up worn or corroded surfaces and for salvaging badly-machined component parts. The process is widely known as Schooping, after the Dr. Schoop who invented it. It uses the Schooping gun, a hand-held, pistol-shaped spray gun through which the metal in wire form is fed into a fuel gas/oxygen blowpipe flame which melts it and, using compressed air, sprays it onto the object. The heat source is a mixture of oxygen and either acetylene, propane or compressed natural gas. The coiled wire is usually straightened before being fed into the gun. Any metal that can be made into a wire may be used; the gun can also accept the metal in powder form. Vacuum metallizing is a process in which the object is placed in a vacuum jar into which the coating metal is sprayed. Phosphating Phosphating is used mainly on mild and galvanized steel and aluminium to augment the adhesion and corrosion resistance of paint, wax and oil finishes. It is also used to form a layer which acts as a parting film in the deep drawing of sheet metal and improves its wear resistance. It essentially consists of allowing the metal surface to react with a solution of one or more phosphates of iron, zinc, manganese, sodium or ammonium. Sodium and ammonium phosphate solutions are used for combined cleaning and phosphating. The need to phosphate multi-metal objects and the desire to increase line speeds in automated operations have led to reducing reaction times by the addition of accelerators such as fluorides, chlorates, molybdates and nickel

compounds to the phosphating solutions.To reduce crystal size and, consequently, increase the flexibility of zinc phosphate coatings, crystal refining agents such as tertiary zinc phosphate or titanium phosphate are added to the pre-treatment rinse. The phosphating sequence typically includes the following steps: · hot caustic cleaning · brushing and rinsing · further hot caustic cleaning · conditioning water rinse · spraying or dipping in hot solutions of acid phosphates · cold water rinse · warm chromic acid rinse · another cold water rinse · drying. Priming Organic paint primers are applied to metal surfaces to promote the adhesion of subsequently applied paints and to retard corrosion at the paint-metal interface. The primers usually contain resins, pigments and solvents and may be applied to the prepared metal surfaces by brush, spray, immersion, roller coating or electrophoresis. The solvents may be any combination of aliphatic and aromatic hydrocarbons, ketones, esters, alcohols and ethers. The most commonly used resins are polyvinyl butynol, phenolic resins, drying oil alkyds, epoxidized oils, epoxyesters, ethyl silicates and chlorinated rubbers. In complex primers, cross-linking agents such as tetraethylene pentamine, pentaethylene hexamine, isocyanates and urea formaldehyde are used. Inorganic pigments used in primer formulations include lead, barium, chromium, zinc and calcium compounds. Plastic coating Plastic coatings are applied to metals in liquid form, as powders which are subsequently cured or sintered by heating, or in the form of fabricated sheets which are laminated to the metal surface with an adhesive. The most commonly used plastics include polyethylene, polyamides (nylons) and PVC. The latter may include plasticizers based on monomeric and polymeric esters and stabilizers such as lead carbonate, fatty acid salts of barium and cadmium, dibutyltin dilaurate,

alkyltin mercaptides and zinc phosphate. Although generally of low toxicity and non-irritating, some of the plasticizers are skin sensitizers.

Hazards and Their Prevention
As might be deduced from the complexity of the processes outlined above, there is a large variety of safety and health hazards associated with the surface treatment of metals. Many are regularly encountered in manufacturing operations; others are presented by the uniqueness of the techniques and materials employed. Some are potentially life threatening. By and large, however, they can be prevented or controlled. Workplace design The workplace should be designed to allow the delivery of raw materials and supplies and the removal of the finished products without interfering with the ongoing processing. Since many of the chemicals are flammable or prone to react when mixed, proper separation in storage and in transit is essential. Many of the metal finishing operations involve liquids, and when leaks, spills or splashes of acids or alkalis occur they must be washed away promptly. Accordingly, adequately drained, slip-resistant floors must be provided. Housekeeping must be diligent to keep the work areas and other spaces clean and free from accumulations of materials. Systems for disposal of solid and liquid wastes and effluents from furnaces and exhaust ventilation must be designed with environmental concerns in mind. Work stations and work assignments should use ergonomic principles to minimize strains, sprains, excessive fatigue and RSIs. Machine guards must have automatic lockout so the machine is de-energized if the guard is removed. Splash guards are essential. Because of the danger of splashes of hot acid and alkali solutions, eyewash fountains and whole-body showers must be installed within easy reach. Signs should be posted to warn other production and maintenance personnel of such dangers as chemical baths and hot surfaces. Chemical assessment All chemicals should be evaluated for potential toxicity and physical hazards, and less hazardous materials should be substituted where possible. However, since the less toxic material may be more flammable, the hazard of fire and explosion must also be considered. In addition, the chemical compatibility of materials must be considered. For example, mixing of nitrate and cyanide salts by accident could cause an explosion due to the strong oxidizing properties of nitrates. Ventilation Most of the metal coating processes require LEV that is strategically placed to draw the vapours or other contaminants away from the worker. Some systems push fresh air across the tank to ―push‖ airborne contaminants to the exhaust side of the system. Fresh air intakes must be located away from exhaust vents so that potentially toxic gases are not recirculated.

Personal protective equipment Processes should be engineered to prevent potentially toxic exposures, but since they cannot always be totally avoided, employees will have to be provided with appropriate PPE (e.g., goggles with or without face shields as appropriate, gloves, aprons or coveralls and shoes). Because many of the exposures involve hot corrosive or caustic solutions, the protective items should be insulated and chemical-resistant. If there is possible exposure to electricity, PPE should be non-conductive. PPE must be available in adequate quantity to allow contaminated, wet items to be cleaned and dried before re-using them. Insulated gloves and other protective clothing should be available where there is the risk of thermal burns from hot metal, furnaces and so on. An important adjunct is the availability of wash-up facilities and clean lockers and dressing rooms, so that workers’ clothing remains uncontaminated and workers do not carry toxic materials back into their homes. Employee training and supervision Employee education and training are essential both when new to the job or when there have been changes in the equipment or the process. MSDSs must be provided for each of the chemical products which explain the chemical and physical hazards, in languages and at educational levels that ensure they will be understood by the workers. Competence testing and periodic retraining will assure that workers have retained the needed information. Close supervision is advisable to make sure that the proper procedures are being followed. Selected hazards Certain hazards are unique to the metal coating industry and deserve special consideration.
Alkaline and acid solutions

The heated alkaline and acid solutions used in cleaning and treatment of metals are particularly corrosive and caustic. They are irritating to the skin and mucous membranes and are especially dangerous when splashed into the eye. Eyewash fountains and emergency showers are essential. Proper protective clothing and goggles will guard against the inevitable splashes; when a splash reaches the skin, the area should be immediately and copiously rinsed with cool, clean water for at least 15 minutes; medical attention may be necessary, particularly when the eye is involved. Care should be exercised when utilizing chlorinated hydrocarbons as phosgene may result from a reaction of the chlorinated hydrocarbon, acids and metals. Nitric and hydrofluoric acid are particularly dangerous when their gases are inhaled, because it may take 4 hours or more before the effects on the lungs become apparent. Bronchitis, pneumonitis and even potentially fatal pulmonary oedema may appear belatedly in a worker who apparently had no initial effect from the exposure. Prompt prophylactic medical treatment and, often, hospitalization are advisable for workers who have been exposed. Skin contact with hydrofluoric acid can cause severe burns without pain for several hours. Prompt medical attention is essential.

Dust

Metallic and oxidic dusts are a particular problem in grinding and polishing operations, and are most effectively removed by LEV as they are created. Ductwork should be designed to be smooth and air velocity should be sufficient to keep the particulates from settling out of the air stream. Aluminium and magnesium dust may be explosive and should be collected in a wet trap. Lead has become less of a problem with the decline of its use in ceramics and porcelain glazes, but it remains the ubiquitous occupational hazard and must always be guarded against. Beryllium and its compounds have received interest recently due to the possibility of carcinogenicity and chronic beryllium disease. Certain operations present a risk of silicosis and pneumoconiosis: the calcining, crushing and drying of flint, quartz or stone; the sieving, mixing and weighing out of these substances in the dry state; and the charging of furnaces with such materials. They also represent a danger when they are used in a wet process and are splashed about the workplace and on workers’ clothing, to become dusts again when they dry out. LEV and rigorous cleanliness and personal hygiene are important preventive measures.
Organic solvents

Solvents and other organic chemicals used in degreasing and in certain processes are dangerous when inhaled. In the acute phase, their narcotic effects may lead to respiratory paralysis and death. In chronic exposure, toxicity of the central nervous system and liver and kidney damage are most frequent. Protection is provided by LEV with a safety zone of at least 80 to 100 cm between the source and the breathing area of the worker. Bench ventilation must also be installed to remove residual vapours from the finished workpieces. Defatting of the skin by organic solvents may be a precursor of dermatitis. Many solvents are also flammable.
Cyanide

Baths containing cyanides are frequently used in electrolytic degreasing, electroplating and cyaniding. Reaction with acid will form the volatile, potentially lethal hydrogen cyanide (prussic acid). The lethal concentration in air is 300 to 500 ppm. Fatal exposures may also result from skin absorption or ingestion of cyanides. Optimum cleanliness is essential for workers using cyanide. Food should not be eaten before washing, and should never be in the work area. Hands and clothing must be carefully cleaned following a potential cyanide exposure. First aid measures for cyanide poisoning include transport into the open air, removal of contaminated clothing, copious washing of the exposed areas with water, oxygen therapy and inhalation of amyl nitrite. LEV and skin protection are essential.
Chromium and nickel

Chromic and nickel compounds used in galvanic baths in electroplating may be hazardous. Chromium compounds can cause burns, ulceration and eczema of the skin and mucosa and a characteristic perforation of the nasal septum. Bronchial asthma may occur. Nickel salts can

cause obstinate allergic or toxic-irritative skin injury. There is evidence that both chromium and nickel compounds may be carcinogenic. LEV and skin protection are essential.
Furnaces and ovens

Special precautions are needed when working with the furnaces employed, for example, in the heat treatment of metals where components are handled at high temperatures and the materials used in the process may either be toxic or explosive or both. The gaseous media (atmospheres) in the furnace may react with the metal charge (oxidizing or reducing atmospheres) or they may be neutral and protective. Most of the latter contain up to 50% hydrogen and 20% carbon monoxide, which, in addition to being combustible, form highly explosive mixtures with air at elevated temperatures. The ignition temperature varies from 450 to 750 °C, but a local spark may cause ignition even at lower temperatures. The danger of explosion is greater when the furnace is being started up or shut down. Since a cooling furnace tends to suck in air (a particular danger when the fuel or power supply is interrupted), a supply of inert gas (e.g., nitrogen or carbon dioxide) should be available for purging when the furnace is shut down as well as when a protective atmosphere is introduced into a hot furnace. Carbon monoxide is perhaps the greatest hazard from furnaces and ovens. Since it is colourless and odourless, it frequently reaches toxic levels before the worker becomes aware of it. Headache is one of the earliest symptoms of toxicity, and, therefore, a worker developing a headache on the job should immediately be removed into fresh air. Danger zones include recessed pockets in which the carbon monoxide may collect; it should be remembered that brickwork is porous and may retain the gas during normal purging and emit it when the purging is completed. Lead furnaces may be dangerous since lead tends to vaporize quite rapidly at temperatures above 870°C. Accordingly, an effective fume extraction system is required. A pot breakage or failure may also be hazardous; a sufficiently large well or pit should be provided to capture the molten metal if this occurs.
Fire and explosion

Many of the compounds used in metal coating are flammable and, under certain circumstances, explosive. For the most part, the furnaces and drying ovens are gas fired, and special precautions such as flame-failure devices at burners, low-pressure cut-off valves in the supply lines and explosion relief panels in the structure of the stoves should be installed. In electrolytic operations, hydrogen formed in the process may collect at the surface of the bath and, if not exhausted, may reach explosive concentrations. Furnaces should be properly ventilated and burners protected from being clogged by dripping material. Oil quenching is also a fire hazard, especially if the metal charge is not completely immersed. Quenching oils should have a high flashpoint, and their temperature should not exceed 27°C. Compressed oxygen and fuel gas cylinders used in metallizing are fire and explosion hazards if not stored and operated properly. See the article ―Welding and thermal cutting‖ in this chapter for detailed precautions.

As required by local ordinances, firefighting equipment, including alarms, should be provided and maintained in working order, and the workers drilled in using it properly.
Heat

The use of furnaces, open flames, ovens, heated solutions and molten metals inevitably presents the risk of excessive heat exposure, which is compounded in hot, humid climates and, particularly, by occlusive protective garments and gear. Complete air conditioning of a plant may not be economically feasible, but supplying cooled air in local ventilation systems is helpful. Rest breaks in cool surroundings and adequate fluid intake (fluids taken at the work station should be free of toxic contaminants) will help to avert heat toxicity. Workers and supervisors should be trained in the recognition of heat stress symptoms.

Conclusion
Surface treatment of metals involves a multiplicity of processes entailing a broad range of potentially toxic exposures, most of which can be prevented or controlled by the diligent application of well-recognized preventive measures.

METAL RECLAMATION
Melvin E. Cassady and Richard D. Ringenwald, Jr. Metal reclamation is the process by which metals are produced from scrap. These reclaimed metals are not distinguishable from the metals produced from primary processing of an ore of the metal. However, the process is slightly different and the exposure could be different. The engineering controls are basically the same. Metal reclamation is very important to the world economy because of the depletion of raw materials and the pollution of the environment created by scrap materials. Aluminium, copper, lead and zinc comprise 95% of the production in the secondary non-ferrous metal industry. Magnesium, mercury, nickel, precious metals, cadmium, selenium, cobalt, tin and titanium are also reclaimed. (Iron and steel are discussed in the chapter Iron and steel industry. See also the article ―Copper, lead and zinc smelting and refining‖ in this chapter.)

Control Strategies
Emission/exposure control principles Metal reclamation involves exposures to dust, fumes, solvents, noise, heat, acid mists and other potential hazardous materials and risks. Some process and/or material handling modifications may be feasible to eliminate or reduce the generation of emissions: minimizing handling, lowering pot temperatures, decreasing dross formation and surface generation of dust, and modifying plant layout to reduce material handling or re-entrainment of settled dust.

Exposure can be reduced in some cases if machines are selected to perform high-exposure tasks so that employees may be removed from the area. This can also reduce ergonomic hazards due to materials handling. To prevent cross contamination of clean areas in the plant, it is desirable to isolate processes generating significant emissions. A physical barrier will contain emissions and reduce their spread. Thus, fewer people are exposed, and the number of emission sources contributing to exposure in any one area will be reduced. This simplifies exposure evaluations and makes the identification and control of major sources easier. Reclaim operations are often isolated from other plant operations. Occasionally, it is possible to enclose or isolate a specific emission source. Because enclosures are seldom air tight, a negative draught exhaust system is often applied to the enclosure. One of the most common ways to control emissions is to provide local exhaust ventilation at the point of emission generation. Capturing emissions at their source reduces the potential for emissions to disperse into the air. It also prevents secondary employee exposure created by the re-entrainment of settled contaminants. The capture velocity of an exhaust hood must be great enough to prevent fumes or dust from escaping the air flow into the hood. The air flow should have enough velocity to carry fume and dust particles into the hood and to overcome the disrupting effects of cross drafts and other random air movements. The velocity required to accomplish this will vary from application to application. The use of recirculation heaters or personal cooling fans which can overcome local exhaust ventilation should be restricted. All exhaust or dilution ventilation systems also require replacement air (known also as ―makeup‖ air systems). If the replacement air system is well designed and integrated into natural and comfort ventilation systems, more effective control of exposures can be expected. For example, replacement air outlets should be placed so clean air flows from the outlet across the employees, towards the emission source and to the exhaust. This technique is often used with supplied-air islands and places the employee between clean incoming air and the emission source. Clean areas are intended to be controlled through direct emission controls and housekeeping. These areas exhibit low ambient contaminant levels. Employees in contaminated areas can be protected by supplied-air service cabs, islands, stand-by pulpits and control rooms, supplemented by personal respiratory protection. The average daily exposure of workers can be reduced by providing clean areas such as breakrooms and lunchrooms that are supplied with fresh filtered air. By spending time in a relatively contaminant-free area, the employees’ time-weighted average exposure to contaminants can be reduced. Another popular application of this principle is the supplied-air island, where fresh filtered air is supplied to the breathing zone of the employee at the workstation. Sufficient space for hoods, duct work, control rooms, maintenance activities, cleaning and equipment storage should be provided.

Wheeled-vehicles are significant sources of secondary emissions. Where wheeled-vehicle transport is used, emissions can be reduced by paving all surfaces, keeping surfaces free of accumulated dusty materials, reducing vehicle travel distances and speed, and by re-directing vehicle exhaust and cooling fan discharge. Appropriate paving material such as concrete should be selected after considering factors such as load, use and care of surface. Coatings may be applied to some surfaces to facilitate wash down of roadways. All exhaust, dilution and make-up air ventilation systems must be properly maintained in order to effectively control air contaminants. In addition to maintaining general ventilation systems, process equipment must be maintained to eliminate spillage of material and fugitive emissions. Work practice programme implementation Although standards emphasize engineering controls as a means of achieving compliance, work practice controls are essential to a successful control programme. Engineering controls can be defeated by poor work habits, inadequate maintenance and poor housekeeping or personal hygiene. Employees who operate the same equipment on different shifts can have significantly different airborne exposures because of differences in these factors between shifts. Work practice programmes, although often neglected, represent good managerial practice as well as good common sense; they are cost effective but require a responsible and cooperative attitude on the part of employees and line supervisors. The attitude of senior management toward safety and health is reflected in the attitude of line supervisors. Likewise, if supervisors do not enforce these programmes, employees attitudes may suffer. Fostering good health and safety attitudes can be accomplished through: · a cooperative atmosphere in which employees participate in the programmes · formal training and educational programmes · emphasizing the plant safety and health programme. Motivating employees and obtaining their trust is necessary in order to have an effective programme. Work practice programmes cannot be simply ―installed‖. Just as with a ventilation system, they must be maintained and continually checked to insure that they are functioning properly. These programmes are the responsibility of management and employees. Programmes should be established to teach, encourage and supervise ―good‖ (i.e., low exposure) practices.
Personal protective equipment

Safety glasses with side shields, coveralls, safety shoes and work gloves should be routinely worn for all jobs. Those engaged in casting and melting, or in casting alloys, should wear aprons and hand protection made of leather or other suitable materials to protect against the splatter of molten metal.

In operations where engineering controls are not adequate to control dust or fume emissions, appropriate respiratory protection should be worn. If noise levels are excessive, and cannot be engineered out or noise sources cannot be isolated, hearing protection should be worn. There should also be a hearing conservation programme, including audiometric testing and training.

Processes
Aluminium The secondary aluminium industry utilizes aluminium-bearing scrap to produce metallic aluminium and aluminium alloys. The processes used in this industry include scrap pretreatment, remelting, alloying and casting. The raw material used by the secondary aluminium industry includes new and old scrap, sweated pig and some primary aluminium. New scrap consists of clippings, forging and other solids purchased from the aircraft industry, fabricators and other manufacturing plants. Borings and turnings are by-product of the machining of castings, rods and forging by the aircraft and automobile industry. Drosses, skimmings and slags are obtained from primary reduction plants, secondary smelting plants and foundries. Old scrap includes automobile parts, household items and airplane parts. The steps involved are as follows: · Inspection and sorting. Purchased aluminium scrap undergoes inspection. Clean scrap requiring no pre-treatment is transported to storage or is charged directly into the smelting furnace. The aluminium that needs pre-treatment is manually sorted. Free iron, stainless steel, zinc, brass and oversized materials are removed. · Crushing and screening. Old scrap, especially casting and sheet contaminated with iron, are inputs to this process. Sorted scrap is conveyed to a crusher or hammer mill where the material is shredded and crushed, and the iron is torn away from the aluminium. The crushed material is passed over vibrating screens to remove dirt and fines. · Baling. Specially designed baling equipment is used to compact bulky aluminium scrap such as scrap sheet, castings and clippings. · Shredding/classifying. Pure aluminium cable with steel reinforcement or insulation is cut with alligator-type shears, then granulated or further reduced in hammer mills to separate the iron core and plastic coating from the aluminium. · Burning/drying. Borings and turning are pre-treated in order to remove cutting oils, greases, moisture and free iron. The scrap is crushed in a hammer mill or ring crusher, the moisture and organics are volatilized in a gas- or oil-fired rotary dryer, the dried chips are screened to remove aluminium fines, the remaining material is magnetically treated for iron removal, and the clean, dried borings are sorted in tote boxes. · Hot-dross processing. Aluminium can be removed from the hot dross discharged from the refining furnace by batch fluxing with a salt-cryolite mixture. This process is carried out in a mechanically rotated, refractory-lined barrel. The metal is tapped periodically through a hole in its base.

· Dry milling. In the dry-milling process, cold aluminium-laden dross and other residues are processed by milling, screening and concentrating to obtain a product containing a minimum aluminium content of 60 to 70%. Ball mills, rod mills or hammer mills can be used to reduce the oxides and non-metallics to fine powders. Separation of dirt and other non-recoverables from the metal is achieved by screening, air classification and/or magnetic separation. · Roasting. Aluminium foil backed with paper, gutta-percha or insulation is an input in this process. In the roasting process, carboneous materials associated with aluminium foils are charged and then separated from the metal product. · Aluminium sweating. Sweating is a pyrometallurgical process which is used to recover aluminium from high-iron-content scrap. High-iron aluminium scrap, castings and dross are inputs in this process. Open-flame reverberatory furnaces with sloping hearths are generally employed. Separation is accomplished as aluminium and other low-melting constituents melt and trickle down the hearth, through a grate and into air-cooled moulds, collecting pots or holding wells. The product is termed ―sweated pig‖. The higher-melting materials including iron, brass and oxidation products formed during the sweating process are periodically tapped from the furnace. · Reverberatory (chlorine) smelting-refining. Reverberatory furnaces are used to convert clean sorted scrap, sweated pigs or, in some cases, untreated scrap into specification alloys. The scrap is charged to the furnace by mechanical means. Materials are added for processing by batch or continuous feed. After the scrap is charged a flux is added to prevent contact with and subsequent oxidation of the melt by air (cover flux). Solvent fluxes are added which react with non-metallics, such as residues from burned coatings and dirt, to form insolubles which float to the surface as slag. Alloying agents are then added, depending on the specifications. Demagging is the process which reduces the magnesium content of the molten charge. When demagging with chlorine gas, chlorine is injected through carbon tubes or lances and reacts with magnesium and aluminium as it bubbles. In the skimming step impure semi-solid fluxes are skimmed off the surface of the melt. · Reverberatory (fluorine) smelting-refining. This process is similar to the reverberatory (chlorine) smelting-refining process except that aluminium fluoride rather than chlorine is employed. Table 82.8 lists exposure and controls for aluminium reclamation operations. Table 82.8 Engineering/administrative controls for aluminium, by operation Process equipment Sorting Exposure Torch desoldering—metal fumes such as lead and cadmium Non-specific dusts and aerosol, Engineering/administrative controls Local exhaust ventilation during desoldering; PPE—respiratory protection when desoldering Local exhaust ventilation and

Crushing/screening

oil mists, metal particulates, and general area ventilation, noise isolation of noise source; PPE— hearing protection Baling Burning/drying No known exposure Non-specific particulate matter which may include metals, soot, and condensed heavy organics. Gases and vapours containing fluorides, sulphur dioxide, chlorides, carbon monoxide, hydrocarbons and aldehydes No controls Local exhaust ventilation, general area ventilation, heat stress work/rest regimen, fluids, isolation of noise source; PPE— hearing protection

Hot-dross processing Some fumes Dry milling Roasting Dust Dust

Local exhaust ventilation, general area ventilation Local exhaust ventilation, general area ventilation Local exhaust ventilation, general area ventilation, heat stress work/rest regimen, fluids, isolation of noise source; PPE— hearing protection Local exhaust ventilation, general area ventilation, heat stress work/rest regimen, fluids, isolation of noise source; PPE— hearing protection and respiratory protection Local exhaust ventilation, general area ventilation, heat stress work/rest regimen, fluids, isolation of noise source; PPE— hearing protection and respiratory protection Local exhaust ventilation, general area ventilation, heat stress work/rest regimen, fluids, isolation of noise source; PPE— hearing protection and respiratory protection

Sweating

Metal fumes and particulates, non-specific gases and vapours, heat and noise

Reverberatory (chlorine) smeltingrefining

Products of combustion, chlorine, hydrogen chlorides, metal chlorides, aluminium chlorides, heat and noise

Reverberatory (fluorine) smeltingrefining

Products of combustion, fluorine, hydrogen flluorides, metal fluorides, aluminium fluorides, heat and noise

Copper reclamation

The secondary copper industry utilizes copper-bearing scrap to produce metallic copper and copper based alloys. The raw materials used can be classified as new scrap produced in the fabrication of finished products or old scrap from obsolete worn out or salvaged articles. Old scrap sources include wire, plumbing fixtures, electrical equipment, automobiles and domestic appliances. Other materials with copper value include slags, drosses, foundry ashes and sweepings from smelters. The following steps are involved: · Stripping and sorting. Scrap is sorted on the bases of its copper content and cleanliness. Clean scrap may be manually separated for charging directly to a melting and alloying furnace. Ferrous components can be separated magnetically. Insulation and lead cable coverings are stripped by hand or by specially designed equipment. · Briquetting and crushing. Clean wire, thin plate, wire screen, borings, turnings and chips are compacted for easier handling. The equipment used includes hydraulic baling presses, hammer mills and ball mills. · Shredding. The separation of copper wire from insulation is accomplished by reducing the size of the mixture. The shredded material is then sorted by air or hydraulic classification with magnetic separation of any ferrous materials. · Grinding and gravity separation. This process accomplishes the same function as shredding but uses an aqueous separation medium and different input materials such as slags, drosses, skimmings, foundry ashes, sweepings and baghouse dust. · Drying. Borings, turnings and chips containing volatile organic impurities such as cutting fluids, oils and greases are removed. · Insulation burning. This process separates insulation and other coatings from copper wire by burning these materials in furnaces. The wire scrap is charged in batches to a primary ignition chamber or afterburner. Volatile combustion products are then passed through a secondary combustion chamber or baghouse for collection. Non-specific particulate matter is generated which may include smoke, clay and metal oxides. Gases and vapours may contain oxides of nitrogen, sulphur dioxide, chlorides, carbon monoxide, hydrocarbons and aldehydes. · Sweating. The removal of low vapour-melting components from scrap is accomplished by heating the scrap to a controlled temperature which is just above the melting point of the metals to be sweated out. The primary metal, copper, is generally not the melted component. · Ammonium carbonate leaching. Copper can be recovered from relatively clean scrap by leaching and dissolution in a basic ammonium carbonate solution. Cupric ions in an ammonia solution will react with metallic copper to produce cuprous ions, which can be reoxidized to the cupric state by air oxidation. After the crude solution is separated from the leach residue, the copper oxide is recovered by steam distillation. · Steam distillation. Boiling the leached material from the carbonate leaching process precipitates the copper oxide. The copper oxide is then dried.

· Hydrothermal hydrogen reduction. Ammonium carbonate solution containing copper ions is heated under pressure in hydrogen, precipitating the copper as a powder. The copper is filtered, washed, dried and sintered under a hydrogen atmosphere. The powder is ground and screened. · Sulphuric acid leaching. Scrap copper is dissolved in hot sulphuric acid to form a copper sulphate solution for feed to the electrowinning process. After digestion, the undissolved residue is filtered off. · Converter smelting. Molten black copper is charged to converter, which is a pear-shaped or cylindrical steel shell lined refractory brick. Air is blown into the molten charges through nozzles called tuyères. The air oxidizes copper sulphide and other metals. A flux containing silica is added to react with the iron oxides to form an iron silicate slag. This slag is skimmed from the furnace, usually by tipping the furnace and then there is a secondary blow and skim. The copper from this process is called blister copper. The blister copper is generally further refined in a fire refining furnace. · Fire refining. The blister copper from the converter is fire refined in a cylindrical tilting furnace, a vessel like a reverberatory furnace. The blister copper is charged to the refining vessel in an oxidizing atmosphere. The impurities are skimmed from the surface and a reducing atmosphere is created by the addition of green logs or natural gas. The resulting molten metal is then cast. If the copper is to be electrolytically refined, the refined copper will be cast as an anode. · Electrolytic refining. The anodes from the fire refining process are placed in a tank containing sulphuric acid and a direct current. The copper from the anode is ionized and the copper ions are deposited on a pure copper starter sheet. As the anodes dissolve in the electrolyte the impurities settle to the bottom of the cell as a slime. This slime can be additionally processed to recover other metal values. The cathode copper produced is melted and cast into a variety of shapes. Table 82.9 lists exposures and controls for copper reclamation operations. Table 82.9 Engineering/administrative controls for copper, by operation Process equipment Stripping and sorting Exposures Air contaminants from material handling and desoldering or scrap cutting Engineering/administrative controls Local exhaust ventilation, general area ventilation

Briquetting and crushing

Non-specific dusts and aerosol, Local exhaust ventilation and oil mists, metal particulates general area ventilation, and noise isolation of noise source; PPE—hearing protection and respiratory protection Non-specific dusts, wire Local exhaust ventilation and

Shredding

insulation material, metal particulates and noise

general area ventilation, isolation of noise source; PPE—hearing protection and respiratory protection Local exhaust ventilation and general area ventilation, isolation of noise source; PPE—hearing protection and respiratory protection Local exhaust ventilation, general area ventilation, work/rest regimen, fluids, isolation of noise source; PPE—hearing protection and respiratory protection

Grinding and gravity separation

Non-specific dusts, metal particulates from fluxes, slags and drosses, and noise

Drying

Non-specific particulate matter, which may include metals, soot and condensed heavy organics Gases and vapours containing fluorides, sulphur dioxide, chlorides, carbon monoxide, hydrocarbons and aldehydes Non-specific particulate matter which may include smoke, clay and metal oxides Gases and vapours containing oxides of nitrogen, sulphur dioxide, chlorides, carbon monoxide, hydrocarbons and aldehydes Metal fumes and particulates, non-specific gases, vapours and particulates

Insulation burning

Local exhaust ventilation, general area ventilation, work/rest regimen, fluids, isolation of noise source; PPE—respiratory protection

Sweating

Local exhaust ventilation, general area ventilation, work/rest regimen, fluids, isolation of noise source; PPE—hearing protection and respiratory protection Local exhaust ventilation, general area ventilation; PPE—respiratory protection Local exhaust ventilation, general area ventilation; PPE—glasses with side shields Local exhaust ventilation, general area ventilation; PPE—respiratory protection Local exhaust ventilation, general area ventilation

Ammonium carbonate leaching Steam distillation

Ammonia

Ammonia

Hydrothermal hydrogen Ammonia reduction Sulphuric acid leaching Sulphuric acid mists

Converter smelting

Volatile metals, noise

Local exhaust ventilation, general area ventilation; PPE—respiratory protection and hearing protection Local exhaust ventilation, general area ventilation; PPE—hearing protection

Electric crucible smelting Fire refining

Particulate, sulphur and nitrogen oxides, soot, carbon monoxide, noise

Sulphur oxides, hydrocarbons, Local exhaust ventilation, particulates general area ventilation; PPE—hearing protection Sulphuric acid and metals from Local exhaust ventilation, sludge general area ventilation

Electrolytic refining

Lead reclamation Raw materials purchased by secondary lead smelters may require processing prior to being charged into a smelting furnace. This section discusses the most common raw materials which are purchased by secondary lead smelters and feasible engineering controls and work practices to limit employee exposure to lead from raw materials processing operations. It should be noted that lead dust can generally be found throughout lead reclamation facilities and that any vehicular air is likely to stir up lead dust which can then be inhaled or adhere to shoes, clothing, skin and hair.
Automotive batteries

The most common raw material at a secondary lead smelter is junk automotive batteries. Approximately 50% of the weight of a junk automotive battery will be reclaimed as metallic lead in the smelting and refining process. Approximately 90% of the automotive batteries manufactured today utilize a polypropylene box or case. The polypropylene cases are reclaimed by almost all secondary lead smelters due to the high economic value of this material. Most of these processes can generate metal fumes, in particular lead and antimony. In automotive battery breaking there is a potential for forming arsine or stibine due to the presence of arsenic or antimony used as hardening agents in grid metal and the potential for having nascent hydrogen present. The four most common processes for breaking automotive batteries are: 1. high speed saw 2. slow speed saw 3. shear

4. whole battery crushing (Saturn crusher or shredder or hammer mill). The first three of these processes involve cutting the top off of the battery, then dumping the groups, or lead-bearing material. The fourth process involves crushing the entire battery in a hammer mill and separating the components by gravity separation. Automotive battery separation takes place after automotive batteries have been broken in order that the lead-bearing material can be separated from the case material. Removing the case may generate acid mists. The most widely used techniques for accomplishing this task are: · The manual technique. This is used by the vast majority of secondary lead smelters and remains the most widely used technique in small to mid-sized smelters. After the battery passes through the saw or shear, an employee manually dumps the groups or lead-bearing material into a pile and places the case and top of the battery into another pile or conveyance system. · A tumbler device. Batteries are placed into a tumbler device after the tops have been sawed/sheared off to separate the groups from the cases. Ribs inside the tumbler dump the groups as it slowly rotates. Groups fall through the slots in the tumbler while the cases are conveyed to the far end and are collected as they exit. Plastic and rubber battery cases and tops are further processed after being separated from the lead bearing material. · A sink/float process. The sink/float process typically is combined with the hammer mill or crushing process for battery breaking. Battery pieces, both lead bearing and cases, are placed in a series of tanks filled with water. Lead bearing material sinks to the bottom of the tanks and is removed by screw conveyor or drag chain while the case material floats and is skimmed off the tank surface. Industrial batteries which were used to power mobile electric equipment or for other industrial uses are purchased periodically for raw material by most secondary smelters. Many of these batteries have steel cases which require removal by cutting the case open with a cutting torch or a hand-held gas powered saw.
Other purchased lead-bearing scrap

Secondary lead smelters purchase a variety of other scrap materials as raw materials for the smelting process. These materials include battery manufacturing plant scrap, drosses from lead refining, scrap metallic lead such as linotype and cable covering, and tetraethyl lead residues. These types of materials may be charged directly into smelting furnaces or mixed with other charge materials.
Raw material handling and transport

An essential part of the secondary lead smelting process is the handling, transportation and storage of raw material. Materials are transported by fork-lifts, front-end loaders or mechanical conveyors (screw, bucket elevator or belt). The primary method of material transporting in the secondary lead industry is mobile equipment.

Some common mechanical conveyance methods which are used by secondary lead smelters include: belt conveying systems that can be used to transport furnace feed material from storage areas to the furnace charring area; screw conveyors for transporting flue dust from the baghouse to an agglomeration furnace or a storage area or bucket elevators and drag chains/lines.
Smelting

The smelting operation at a secondary lead smelter involves the reduction of lead-bearing scrap into metallic lead in a blast furnace or reverberatory. Blast furnaces are charged with lead-bearing material, coke (fuel) limestone and iron (flux). These materials are fed into the furnace at the top of the furnace shaft or through a charge door in the side of the shaft neat the top of the furnace. Some environmental hazards associated with blast furnace operations are metal fumes and particulates (especially lead and antimony), heat, noise and carbon monoxide. A variety of charge material conveying mechanisms are used in the secondary lead industry. The skip hoist is probably the most common. Other devices in use include vibratory hoppers, belt conveyors and bucket elevators. Blast furnace tapping operations involve removing the molten lead and slag from the furnace into moulds or ladles. Some smelters tap metal directly into a holding kettle which keeps the metal molten for refining. The remaining smelters cast the furnace metal into blocks and allow the blocks to solidify. Blast air for the combustion process enters the blast furnace through tuyères which occasionally begin to fill with accretions and must be physically punched, usually with a steel rod, to keep them from being obstructed. The conventional method to accomplish this task is to remove the cover of the tuyères and insert the steel rod. After the accretions have been punched, the cover is replaced. Reverberatory furnaces are charged with lead-bearing raw material by a furnace charging mechanism. Reverberatory furnaces in the secondary lead industry typically have a sprung arch or hanging arch constructed of refractory brick. Many of the contaminants and physical hazards associated with reverberatory furnaces are similar to those of blast furnaces. Such mechanisms can be a hydraulic ram, a screw conveyor or other devices similar to those described for blast furnaces. Reverberatory furnace tapping operations are very similar to blast-furnace tapping operations.
Refining

Lead refining in secondary lead smelters is conducted in indirect fired kettles or pots. Metal from the smelting furnaces is typically melted in the kettle, then the content of trace elements is adjusted to produce the desired alloy. Common products are soft (pure) lead and various alloys of hard (antimony) lead.

Virtually all secondary lead refining operations employ manual methods for adding alloying materials to the kettles and employ manual drossing methods. Dross is swept to the rim of the kettle and removed by shovel or large spoon into a container. Table 82.10 lists exposures and controls for lead reclamation operations. Table 82.10 Engineering/administrative controls for lead, by operation Process equipment Vehicles Exposures Lead dust from roads and splashing water containing lead Engineering/administrative controls Water washdown and keeping areas wetted down. Operator training, prudent work practices and good housekeeping are key elements in minimizing lead emissions when operating mobile equipment. Enclose equipment and provide a positive pressure filtered air system. It is also preferable to equip belt conveyor systems with selfcleaning tail pulleys or belt wipes if they are used to transport furnace feed materials or flue dusts. Local exhaust ventilation, general area ventilation Local exhaust ventilation, general area ventilation

Conveyors

Lead dust

Battery decasing Charge preparation Blast furnace

Lead dust, acid mists Lead dust

Metal fumes and particulates Local exhaust ventilation, (lead, antimony), heat and noise, general area ventilation, carbon monoxide work/rest regimen, fluids, isolation of noise source; PPE— respiratory protection and hearing protection Metal fumes and particulates (lead, antimony), heat and noise Local exhaust ventilation, general area ventilation, work/rest regimen, fluids, isolation of noise source; PPE— respiratory protection and hearing protection Local exhaust ventilation,

Reverberatory furnace

Refining

Lead particulates and possibly

alloying metals and fluxing agents, noise Casting Lead particulates and possibly alloying metals

general area ventilation; PPE— hearing protection Local exhaust ventilation, general area ventilation

Zinc reclamation The secondary zinc industry utilizes new clippings, skimmings and ashes, die-cast skimmings, galvanizers’ dross, flue dust and chemical residue as sources of zinc. Most of the new scrap processed is zinc- and copper-based alloys from galvanizing and die-casting pots. Included in the old scrap category are old zinc engravers’ plates, die castings, and rod and die scrap. The processes are as follows: · Reverberatory sweating. Sweating furnaces are used to separate zinc from other metals by controlling the furnace temperature. Scrap die-cast products, such as automobile grilles and licence plate frames, and zinc skins or residues are starting materials for the process. The scrap is charged to the furnace, flux is added and the contents melted. The high-melting residue is removed and the molten zinc flows out of the furnace directly to subsequent processes, such as melting, refining or alloying, or to collecting vessels. Metal contaminants include zinc, aluminium, copper, iron, lead, cadmium, manganese and chromium. Other contaminants are fluxing agents, sulphur oxides, chlorides and fluorides. · Rotary sweating. In this process zinc scrap, die-cast products, residues and skimmings are charged to a direct-fired furnace and melted. The melt is skimmed, and zinc metal is collected in kettles situated outside the furnace. Unmeltable material, the slag, is then removed prior to recharging. The metal from this process is sent to distillation or alloying process. Contaminants are similar to those of reverberatory sweating. · Muffle sweating and kettle (pot) sweating. In these processes zinc scrap, die-vapour-cast products, residues and skimmings are charged to the muffle furnace, the material sweated and the sweated zinc is sent to refining or alloying processes. The residue is removed by a shaker screen which separates the dross from the slag. Contaminants are similar to those of reverberatory sweating. · Crushing/screening. Zinc residues are pulverized or crushed to break down physical bonds between metallic zinc and contaminant fluxes. The reduced material is then separated in a screening or pneumatic classification step. Crushing can produce zinc oxide and minor amounts of heavy metals and chlorides. · Sodium carbonate leaching. Residues are chemically treated to leach out and convert zinc to zinc oxide. The scrap is first crushed and washed. In this step, the zinc is leached out of the material. The aqueous portion is treated with sodium carbonate, causing zinc to precipitate. The precipitate is dried and calcined to yield crude zinc oxide. The zinc oxide is then reduced to zinc metal. Various zinc salt contaminants can be produced.

· Kettle (pot), crucible, reverberatory, electric induction melting. The scrap is charged to the furnace and fluxes are added. The bath is agitated to form a dross that can be skimmed from the surface. After the furnace has been skimmed the zinc metal is poured into ladles or moulds. Zinc oxide fumes, ammonia and ammonium chloride, hydrogen chloride and zinc chloride can be produced. · Alloying. The function of this process is to produce zinc alloys from pre-treated scrap zinc metal by adding to it in a refining kettle fluxes and alloying agents either in the solidified or molten form. The contents are then mixed, the dross skimmed, and the metal is cast into various shapes. Particulates containing zinc, alloying metals, chlorides, non-specific gases and vapours, as well as heat, are potential exposures. · Muffle distillation. The muffle distillation process is used to reclaim zinc from alloys and to manufacture pure zinc ingots. The process is semi-continuous which involves charging molten zinc from a melting pot or sweating furnace to the muffle section and vaporizing the zinc and condensing the vaporized zinc and tapping from the condenser to moulds. The residue is removed periodically from the muffle. · Retort distillation/oxidation and muffle distillation/oxidation. The product of the retort distillation/oxidation and muffle distillation/oxidation processes is zinc oxide. The process is similar to retort distillation through the vaporization step, but, in this process, the condenser is bypassed and combustion air is added. The vapour is discharged through an orifice into an air stream. Spontaneous combustion occurs inside a refractory vapour-lined chamber. The product is carried by the combustion gases and excess air into a baghouse where the product is collected. Excess air is present to insure complete oxidation and to cool the product. Each of these distillation processes can lead to zinc oxide fume exposures, as well as other metal particulate and oxides of sulphur exposure. Table 82.11 lists exposures and controls for zinc reclamation operations. Table 82.11 Engineering/administrative controls for zinc, by operation Process equipment Exposures Engineering/administrative controls Local exhaust ventilation, general area ventilation, heat stress–work/rest regimen, fluids

Reverberatory sweating Particulates containing zinc, aluminium, copper, iron, lead, cadmium, manganese and chromium, contaminants from fluxing agents, sulphur oxides, chlorides and fluorides Rotary sweating

Particulates containing zinc, Local exhaust ventilation, aluminium, copper, iron, lead, general area ventilation, cadmium, manganese and work/rest regimen, fluids chromium, contaminants from fluxing agents, sulphur oxides,

chlorides and fluorides Muffle sweating and kettle (pot) sweating Particulates containing zinc, Local exhaust ventilation, aluminium, copper, iron, lead, general area ventilation, cadmium, manganese and work/rest regimen, fluids chromium, contaminants from fluxing agents, sulphur oxides, chlorides and fluorides Zinc oxide, minor amounts of heavy metals, chlorides Local exhaust ventilation, general area ventilation

Crushing/screening Sodium carbonate leaching

Zinc oxide, sodium carbonate, Local exhaust ventilation, zinc carbonate, zinc hydroxide, general area ventilation hydrogen chloride, zinc chloride Zinc oxide fumes, ammonia, ammonia chloride, hydrogen chloride, zinc chloride Particulates containing zinc, alloying metals, chlorides; non-specific gases and vapours; heat Local exhaust ventilation, general area ventilation, work/rest regimen, fluids Local exhaust ventilation, general area ventilation, work/rest regimen, fluids Local exhaust ventilation, general area ventilation, work/rest regimen, fluids Local exhaust ventilation, general area ventilation, work/rest regimen, fluids

Kettle (pot) melting crucible, reverberatory, electric induction melting Alloying

Retort distillation, retort Zinc oxide fumes, other metal distillation/oxidation particulates, oxides of sulphur and muffle distillation Graphite rod resistor distillation Zinc oxide fumes, other metal particulates, oxides of sulphur

Magnesium reclamation Old scrap is obtained from sources such as scrap automobile and aircraft parts and old and obsolete lithographic plates, as well as some sludges from primary magnesium smelters. New scrap consists of clippings, turnings, borings, skimmings, slags, drosses and defective articles from sheet mills and fabrication plants. The greatest danger in handling magnesium is that of fire. Small fragments of the metal can readily be ignited by a spark or flame. · Hand sorting. This process is used to separate magnesium and magnesium-alloy fractions from other metals present in the scrap. The scrap is spread out manually, sorted on the basis of weight. · Open pot melting. This process is used to separate magnesium from contaminants in the sorted scrap. Scrap is added to a crucible, heated and a flux consisting of a mixture of calcium, sodium and potassium chlorides is added. The molten magnesium is then cast into ingots.

Table 82.12 lists exposures and controls for magnesium reclamation operations. Table 82.12 Engineering/administrative controls for magnesium, by operation Process equipment Scrap sorting Open pot melting Exposures Dust Fumes and dust, a high potential for fires Dust and fumes, heat and a high potential for fires Engineering/administrative controls Water washdown Local exhaust ventilation and general area ventilation and work practices Local exhaust ventilation, general area ventilation, work/rest regimen, fluids

Casting

Mercury reclamation The major sources for mercury are dental amalgams, scrap mercury batteries, sludges from electrolytic processes that use mercury as a catalyst, mercury from dismantled chlor-alkali plants and mercury-containing instruments. Mercury vapour can contaminate each of these processes. · Crushing. The crushing process is used to release residual mercury from metal, plastic and glass containers. After the containers are crushed, the contaminated liquid mercury is sent to the filtering process. · Filtration. Insoluble impurities such as dirt are removed by passing the mercury-vapour bearing scrap through a filter media. The filtered mercury is fed to the oxygenation process and the solids which do not pass through the filters are sent to retort distillation. · Vacuum distillation. Vacuum distillation is employed to refine contaminated mercury when the vapour pressures of the impurities are substantially lower than that of mercury. Mercury charge is vaporized in a heating pot and the vapours are condensed using a water-cooled condenser. Purified mercury is collected and sent to the bottling operation. The residue remaining in the heating pot is sent to the retorting process to recover the trace amounts of mercury that were not recovered in the vacuum distillation process. · Solution purification. This process removes metallic and organic contaminants by washing the raw liquid mercury with a dilute acid. The steps involved are: leaching the raw liquid mercury with dilute nitric acid to separate metallic impurities; agitating the acid-mercury with compressed air to provide good mixing; decanting to separate the mercury from the acid; washing with water to remove the residual acid; and filtering the mercury in a medium such as activated carbon or silica gel to remove the last traces of moisture. In addition to mercury vapour there can be exposure to solvents, organic chemicals and acid mists.

· Oxygenation. This process refines the filtered mercury by removing metallic impurities by oxidation with sparging air. The oxidation process involves two steps, sparging and filtering. In the sparging step, contaminated mercury is agitated with air in a closed vessel to oxidize the metallic contaminants. After sparging, the mercury is filtered in a charcoal bed to remove the solid metal oxides. · Retorting. The retorting process is used to produce pure mercury by volatilizing the mercury found in solid mercury-bearing scrap. The steps involved in retorting are: heating the scrap with an external heat source in a closed still pot or stack of trays to vaporize the mercury; condensing the mercury vapour in water-cooled condensers; collecting the condensed mercury in a collecting vessel. Table 82.13 lists exposures and controls for mercury reclamation operations. Table 82.13 Engineering/administrative controls for mercury, by operation Process equipment Crushing Filtration Exposures Volatile mercury Volatile mercury Engineering/administrative controls Local exhaust; PPE—respiratory protection Local exhaust ventilation; PPE— respiratory protection Local exhaust ventilation; PPE— respiratory protection Local exhaust ventilation, general area ventilation; PPE—respiratory protection Local exhaust ventilation; PPE— respiratory protection Local exhaust ventilation; PPE— respiratory protection

Vacuum distillation Volatile mercury Solution purification Oxidation Retorting Volatile mercury, solvents, organics and acid mists Volatile mercury Volatile mercury

Nickel reclamation The principal raw materials for nickel reclamation are nickel-, copper- and aluminium-vapour based alloys, which can be found as old or new scrap. Old scrap comprises alloys that are salvaged from machinery and airplane parts, while new scrap refers to sheet scrap, turnings and solids which are by-products of the manufacture of alloy products. The following steps are involved in nickel reclamation: · Sorting. The scrap is inspected and manually separated from the non-metallic and non-nickel materials. Sorting produces dust exposures.

· Degreasing. Nickel scrap is degreased by using trichloroethylene. The mixture is filtrated or centrifuged to separate the nickel scrap. The spent solvent solution of trichloroethylene and grease goes through a solvent recovery system. There can be solvent exposure during degreasing. · Smelting (electric arc or rotary reverberatory) furnace. Scrap is charged to an electric arc furnace and a reducing agent added, usually lime. The charge is melted and is either cast into ingots or sent directly to a reactor for additional refining. Fumes, dust, noise and heat exposures are possible. · Reactor refining. The molten metal is introduced into a reactor where cold-base scrap and pig nickel are added, followed by lime and silica. Alloying materials such as manganese, columbium or titanium are then added to produce the desired alloy composition. Fumes, dust, noise and heat exposures are possible. · Ingot casting. This process involves casting the molten metal from the smelting furnace or the refining reactor into ingots. The metal is poured into moulds and allowed to cool. The ingots are removed from the moulds. Heat and metal fume exposures are possible. Exposures and control measures for nickel reclamation operations are listed in table 82.14 . Table 82.14 Engineering/administrative controls for nickel, by operation Process equipment Sorting Degreasing Exposures Dust Solvent Engineering/administrative controls Local exhaust and solvent substitution Local exhaust ventilation and solvent substitution and/or recovery, general area ventilation Local exhaust ventilation, work/rest regimen, fluids; PPE—respiratory protection and hearing protection Local exhaust ventilation, general area ventilation, work/rest regimen, fluids; PPE—respiratory protection and hearing protection Local exhaust ventilation, general area ventilation, work/rest regimen, fluids

Smelting

Fumes, dust, noise, heat

Refining

Fumes, dust, heat, noise

Casting

Heat, metal fumes

Precious metals reclamation The raw materials for the precious metal industry consist of both old and new scrap. Old scrap includes electronic components from obsolete military and civilian equipment and scrap from the dental industry. New scrap is generated during the fabrication and manufacturing of precious

metal products. The products are the elemental metals such as gold, silver, platinum and palladium. Precious metal processing includes the following steps: · Hand sorting and shredding. Precious metal-bearing scrap is hand sorted and crushed and shredded in a hammer mill. Hammer mills are noisy. · Incineration process. Sorted scrap is incinerated to remove paper, plastic and organic liquid contaminants. Organic chemicals, combustion gases and dust exposures are possible. · Blast-furnace smelting. Treated scrap is charged to a blast furnace, along with coke, flux and recycled slag metal oxides. The charge is melted and slagged, producing black copper which contains the precious metals. The hard slag that is formed contains most of the slag impurities. Dust and noise may be present. · Converter smelting. This process is designed to further purify the black copper by blowing air through the melt in a converter. Slag-containing metal contaminants are removed and recycled to the blast furnace. The copper bullion containing the precious metals is cast into moulds. · Electrolytic refining. Copper bullion serves as the anode of an electrolytic cell. Pure copper thus plates out on the cathode while the precious metals fall to the bottom of the cell and are collected as slimes. The electrolyte used is copper sulphate. Acid mist exposures are possible. · Chemical refining. The precious metal slime from the electrolytic refining process is chemically treated to recover the individual metals. Cyanide-based processes are used to recover gold and silver, which can also be recovered by dissolving them in aqua regia solution and/or nitric acid, followed by precipitation with ferrous sulphate or sodium chloride to recover the gold and silver, respectively. The platinum-group metals can be recovered by dissolving them in molten lead, which is then treated with nitric acid and leaves a residue from which the platinumgroup metals can be selectively precipitated. The precious metal precipitates are then either melted or ignited in order to collect the gold and silver as grains and the platinum metals as sponge. There can be acid exposures. Exposures and controls are listed, by operation, in table 82.15 (see also ―Gold smelting and refining‖). Table 82.15 Engineering/administrative controls for precious metals, by operation Process equipment Sorting and shredding Incineration Blast furnace smelting Exposures Engineering/administrative controls

Hammermill is a potential Noise control material; PPE—hearing noise hazard protection Organics, combustion gases and dust Dust, noise Local exhaust ventilation and general area ventilation Local exhaust ventilation; PPE— hearing protection and respiratory

protection Electrolytic refining Chemical refining Acid mists Acid Local exhaust ventilation, general area ventilation Local exhaust ventilation, general area ventilation; PPE—acid-resistant clothing, chemical goggles and face shield

Cadmium reclamation Old cadmium-bearing scrap includes cadmium-plated parts from junked vehicles and boats, household appliances, hardware and fasteners, cadmium batteries, cadmium contacts from switches and relays and other used cadmium alloys. New scrap is normally cadmium vapour bearing rejects and contaminated by-products from industries which handle the metals. The reclamation processes are: · Pre-treatment. The scrap pre-treatment step involves the vapour degreasing of alloy scrap. Solvent vapours generated by heating recycled solvents are circulated through a vessel containing scrap alloys. The solvent and stripped grease are then condensed and separated with the solvent being recycled. There can be exposure to cadmium dust and solvents. · Smelting/refining. In the smelting/refining operation, pre-treated alloy scrap or elemental cadmium scrap is processed to remove any impurities and produce cadmium alloy or elemental cadmium. Products of oil and gas combustion exposures and zinc and cadmium dust may be present. · Retort distillation. Degreased scrap alloy is charged to a retort and heated to produce cadmium vapours which are subsequently collected in a condenser. The molten metal is then ready for casting. Cadmium dust exposures are possible. · Melting/dezincing. Cadmium metal is charged to a melting pot and heated to the molten stage. If zinc is present in the metal, fluxes and chlorinating agents are added to remove the zinc. Among potential exposures are cadmium fumes and dust, zinc fumes and dust, zinc chloride, chlorine, hydrogen chloride and heat. · Casting. The casting operation forms the desired product line from the purified cadmium alloy or cadmium metal produced in the previous step. Casting can produce cadmium dust and fumes and heat. Exposures in cadmium reclamation processes and the necessary controls are summarized in table 82.16 . Table 82.16 Engineering/administrative controls for cadmium, by operation

Process equipment Scrap degreasing Alloy smelting/refining Retort distillation Melting/dezincing

Exposures Solvents and cadmium dust Products of oil and gas combustion, zinc fumes, cadmium dust and fumes Cadmium fumes

Engineering/administrative controls Local exhaust and solvent substitution Local exhaust ventilation and general area ventilation; PPE— respiratory protection Local exhaust ventilation; PPE— respiratory protection

Cadmium fumes and dust, Local exhaust ventilation, general zinc fumes and dust, zinc area ventilation, work/rest regimen, chloride, chlorine, hydrogen fluids; PPE—respiratory protection chloride, heat stress Cadmium dust and fumes, heat Local exhaust ventilation, general area ventilation, work/rest regimen, fluids; PPE—respiratory protection

Casting

Selenium reclamation Raw materials for this segment are used xerographic copying cylinders and scrap generated during the manufacture of selenium rectifiers. Selenium dusts may be present throughout. Distillation and retort smelting can produce combustion gases and dust. Retort smelting is noisy. Sulphur dioxide mist and acid mist are present in refining. Metal dusts can be produced from casting operations (see table 82.17).The reclamation processes are as follows: · Scrap pre-treatment. This process separates selenium by mechanical processes such as the hammer mill or shot blasting. · Retort smelting. This process purifies and concentrates pre-treated scrap in a retort distillation operation by melting the scrap and separating selenium from the impurities by distillation. · Refining. This process achieves a purification of scrap selenium based on leaching with a suitable solvent such as aqueous sodium sulphite. Insoluble impurities are removed by filtration and the filtrate is treated to precipitate selenium. · Distillation. This process produces a high vapour purity selenium. The selenium is melted, distilled and the selenium vapours are condensed and transferred as molten selenium to a product formation operation. Table 82.17 Engineering/administrative controls for selenium, by operation Process equipment Scrap pretreatment Exposures Dust Engineering/administrative controls Local exhaust

Retort smelting

Combustion gases and dust, noise SO2, acid mist Dust and combustion products Metal dust Selenium fumes

Local exhaust ventilation and general area ventilation; PPE—hearing protection; control of burner noise Local exhaust ventilation; PPE— chemical goggles Local exhaust ventilation, general area ventilation Local exhaust ventilation, general area ventilation Local exhaust ventilation, general area ventilation

Refining Distillation Quenching Casting

· Quenching. This process is used to produce purified selenium shot and powder. The selenium melt is used in producing a shot. The shot is then dried. The steps required to produce powder are the same, except that selenium vapour, rather than molten selenium, is the material which is quenched. · Casting. This process is used to produce selenium ingots or other shapes from the molten selenium. These shapes are produced by pouring molten selenium into moulds of the proper size and shape and cooling and solidifying the melt. Cobalt reclamation The sources of cobalt scrap are super alloy grindings and turnings, and obsolete or worn engine parts and turbine blades. The processes of reclamation are: · Hand sorting. Raw scrap is hand sorted to identify and separate the cobalt-base, nickel-base and non-processable components. This is a dusty operation. · Degreasing. Sorted dirty scrap is charged to a degreasing unit where vapours of perchloroethylene are circulated. This solvent removes the grease and oil on the scrap. The solvent-oil-grease vapour mixture is then condensed and the solvent is recovered. Solvent exposures are possible. · Blasting. Degreased scrap is blasted with grit to remove dirt, oxides and rust. Dusts can be present, depending on the grit used. · Pickling and chemical treatment process. Scrap from the blasting operation is treated with acids to remove residual rust and oxide contaminants. Acid mists are a possible exposure. · Vacuum melting. Cleaned scrap is charged to a vacuum furnace and melted by electric arc or induction furnace. There can be exposure to heavy metals.

· casting. Molten alloy is cast into ingots. Heat stress is possible. See table 82.18 for a summary of exposures and controls for cobalt reclamation. Table 82.18 Engineering/administrative controls for cobalt, by operation Process equipment Hand sorting Degreasing Blasting Exposures Dust Solvents Engineering/administrative controls Water washdown Solvent recovery, local exhaust and solvent substitution

Dust—toxicity dependent upon Local exhaust ventilation; PPE the grit used for physical hazard and respiratory protection depending on grit used Acid mists Local exhaust ventilation, general area ventilation; PPE— respiratory protection Local exhaust ventilation, general area ventilation Local exhaust ventilation, general area ventilation, work/rest regimen, fluids

Pickling and chemical treatment process Vacuum melting Casting

Heavy metals Heat

Tin reclamation The major sources of raw materials are tin-plated steel trimmings, rejects from tin-can manufacturing companies, rejected plating coils from the steel industry, tin drosses and sludges, solder drosses and sludges, used bronze and bronze rejects and metal type scrap. Tin dust and acid mists can be found in many of the processes. · Dealuminization. In this process hot sodium hydroxide is used to leach aluminium from tin-can scrap by contacting the scrap with hot sodium hydroxide, separating the sodium aluminate solution from the scrap residue, pumping the sodium aluminate to a refining operation to recover soluble tin and recovering the dealuminized tin scrap for feed. · Batch mixing. This process is a mechanical operation which prepares a feed suitable for charging to the smelting furnace by mixing drosses and sludges with a significant tin content. · Chemical detinning. This process extracts the tin in scrap. A hot solution of sodium hydroxide and sodium nitrite or nitrate is added to dealuminized or raw scrap. Draining and pumping the

solution to a refining/casting process are performed when the detinning reaction is complete. The detinned scrap is then washed. · Dross smelting. This process is used to partially purify drosses and produce crude furnace metal by melting the charge, tapping the crude furnace metal and tapping the mattes and slags. · Dust leaching and filtration. This process removes the zinc and chlorine values from flue dust by leaching with sulphuric acid to remove zinc and chlorine, filtering the resulting mixture to separate the acid and dissolved zinc and chlorine from the leached dust, drying the leached dust in a dryer and conveying the tin and lead rich dust back to the batch mixing process. · Settling and leaf filtration. This process purifies the sodium stannate solution produced in the chemical detinning process. Impurities such as silver, mercury, copper, cadmium, some iron, cobalt and nickel are precipitated as sulphides. · Evapocentrifugation. The sodium stannate is concentrated from the purified solution by evaporation, crystallization of sodium stannate and recovery of sodium stannate is by centrifugation. · Electrolytic refining. This process produces cathodic-pure tin from the purified sodium stannate solution by passing the sodium stannate solution through electrolytic cells, removing the cathodes after the tin has been deposited and stripping the tin from the cathodes. · Acidification and filtration. This process produces a hydrated tin oxide from the purified sodium stannate solution. This hydrated oxide can either be processed to produce the anhydrous oxide or smelted to produce elemental tin. The hydrated oxide is neutralized with sulphuric acid to form the hydrated tin oxide and filtered to separate the hydrate as filter cake. · Fire refining. This process produces purified tin from the cathodic tin by melting the charge, removing the impurities as slag and dross, pouring the molten metal and casting the metallic tin. · Smelting. This process is used to produce tin when electrolytic refining is not feasible. This is accomplished by reducing the hydrated tin oxide with a reducing agent, melting the tin metal formed, skimming the dross, pouring the molten tin and casting the molten tin. · Calcining. This process converts the hydrated tin oxides to anhydrous stannic oxide by calcining the hydrate and removing and packaging the stannic oxides. · Kettle refining. This process is used to purify crude furnace metal by charging a preheated kettle with it, drying the dross to remove the impurities as slag and matte, fluxing with sulphur to remove copper as matte, fluxing with aluminium to remove antimony and casting molten metal into desired shapes. See table 82.19 for a summary of exposures and controls for tin reclamation. Table 82.19 Engineering/administrative controls for tin, by operation

Process equipment Dealuminization Batch mixing

Exposures Sodium hydroxide Dust

Engineering/administrative controls Local exhaust; PPE—chemical goggles and/or face shield Local exhaust ventilation and general area ventilation Local exhaust ventilation; PPE—chemical goggles and/or face shield Local exhaust ventilation, general area ventilation, work/rest regimen, fluids Local exhaust ventilation, general area ventilation None identified None identified Local exhaust ventilation and general area ventilation; PPE—chemical goggles and/or face shield Local exhaust ventilation and general area ventilation; PPE—chemical goggles and/or face shield Work/rest regimen, PPE Local exhaust ventilation and general area ventilation, work/rest regimen, PPE Local exhaust ventilation and general area ventilation work/rest regimen, PPE Local exhaust ventilation and general area ventilation, work/rest regimen, PPE

Chemical detinning Caustic Dross smelting Dust leaching and filtration Settling and leaf filtration Dust and heat Dust None identified

Evapocentrifugation None identified Electrolytic refining Acid mist

Acidification and filtration Fire refining Smelting Calcining Kettle refining

Acid mists

Heat Combustion gases, fumes and dust, heat Dust, fumes, heat Dust, fumes, heat

Titanium reclamation The two primary sources of titanium scrap are the home and titanium consumers. Home scrap which is generated by the milling and manufacturing of titanium products includes trim sheets, plank sheet, cuttings, turnings and borings. Consumer scrap consists of recycled titanium products. The reclamation operations include: · Degreasing. In this process sized scrap is treated with vapourized organic solvent (e.g., trichloroethylene). Contaminant grease and oil are stripped from the scrap by the solvent vapour. The solvent is recirculated until it can no longer has an ability to degrease. Spent solvent can then be regenerated. The scrap can also be degreased by steam and detergent.

· Pickling. The acid-pickling process removes oxide scale from the degreasing operation by leaching with a solution of hydrochloric and hydrofluoric acids. The acid treatment scrap is washed with water and dried. · Electrorefining. Electrorefining is a titanium scrap pre-treatment process which electro-refines scrap in a fused salt. · Smelting. Pre-treated titanium scrap and alloying agents are melted in a electric-arc vacuum furnace to form a titanium alloy. The input materials include pre-treated titanium scrap and alloying materials such as aluminium, vanadium, molybdenum, tin, zirconium, palladium, columbium and chromium. · Casting. Molten titanium is poured into moulds. The titanium solidifies into a bar called an ingot. Controls for exposures in titanium reclamation procedures are listed in table 82.20 . Table 82.20 Engineering/administrative controls for titanium, by operation Process equipment Solvent degreasing Pickling Electrorefining Smelting Exposures Solvent Acids None known Volatile metals, noise Engineering/administrative controls Local exhaust and solvent recovery Face shields, aprons, long sleeves, safety glasses or goggles None known Local exhaust ventilation and control of noise from burners; PPE—hearing protection PPE

Casting

Heat

ENVIRONMENTAL ISSUES IN METAL FINISHING AND INDUSTRIAL COATINGS
Stewart Forbes

Metal Finishing
The surface treatment of metals increases their durability and improves their appearance. A single product may undergo more than one surface treatment—for example, an auto body panel may be phosphated, primed and painted. This article deals with the processes used for surface treatment of metals and the methods used to reduce their environmental impact.

Operating a metal finishing business requires cooperation between company management, employees, government and the community to effectively minimize the environmental effect of the operations. Society is concerned with the amount and the long-term effects of pollution entering the air, water and land environment. Effective environmental management is established through detailed knowledge of all elements, chemicals, metals, processes and outputs. Pollution prevention planning shifts the environmental management philosophy from reacting to problems to anticipating solutions focusing on chemical substitution, process change and internal recycling, using the following planning sequence: 1. Initiate pollution prevention across all aspects of the business. 2. Identify waste streams. 3. Set priorities for action. 4. Establish root cause of the waste. 5. Identify and implement changes that reduce or eliminate the waste. 6. Measure the results. Continuous improvement is achieved by setting new priorities for action and repeating the sequence of actions. Detailed process documentation will identify the waste streams and allow priorities to be set for waste reduction opportunities. Informed decisions about potential changes will encourage: · easy and practical operational improvements · process changes involving customers and suppliers · changes to less harmful activities where possible · reuse and recycling where change is not practical · using landfilling of hazardous wastes only as a last resort. Major processes and standard operating processes Cleaning is required because all metal finishing processes require that parts to be finished be free from organic and inorganic soils, including oils, scale, buffing and polishing compounds. The three basic types of cleaners in use are solvents, vapour degreasers and alkaline detergents. Solvents and vapour degreasing cleaning methods have been almost totally replaced by alkaline materials where the subsequent processes are wet. Solvents and vapour degreasers are still in use

where parts must be clean and dry with no further wet processing. Solvents such as terpenes are in some instances replacing volatile solvents. Less toxic materials such as 1,1,1-trichloroethane have been substituted for more hazardous materials in vapour degreasing (although this solvent is being phased out as an ozone depleter). Alkaline cleaning cycles usually include a soak immersion followed by an anodic electroclean, followed by a weak acid immersion. Non-etching, non-silicated cleaners are typically used to clean aluminium. The acids are typically sulphuric, hydrochloric and nitric. Anodizing, an electrochemical process to thicken the oxide film on the metal surface (frequently applied to aluminium), treats the parts with dilute chromic or sulphuric acid solutions. Conversion coating is used to provide a base for subsequent painting or to passivate for protection against oxidation. With chromating, parts are immersed in a hexavalent chrome solution with active organic and inorganic agents. For phosphating, parts are immersed in dilute phosphoric acid with other agents. Passivating is accomplished through immersion in nitric acid or nitric acid with sodium dichromate. Electroless plating involves a deposition of metal without electricity. Copper or nickel electroless deposition is used in the manufacture of printed circuit boards. Electroplating involves the deposition of a thin coat of metal (zinc, nickel, copper, chromium, cadmium, tin, brass, bronze, lead, tin-lead, gold, silver and other metals such as platinum) on a substrate (ferrous or non-ferrous). Process baths include metals in solution in acid, alkaline neutral and alkaline cyanide formulations (see figure 82.7). Figure 82.7 Inputs and outputs for a typical electroplating line

Chemical milling and etching are controlled dissolution immersion processes using chemical reagents and etchants. Aluminium is typically etched in caustic prior to anodizing or chemically brightened in a solution which could contain nitric, phosphoric and sulphuric acids. Hot-dip coatings involve the application of metal to a workpiece by immersion in molten metal (zinc or tin galvanizing of steel). Good management practices Important safety, health and environmental improvements can be achieved through process improvements, such as: · using counter-current rinsing and conductivity controls · increasing drainage time

· using more or better wetting agents · keeping process temperatures as high as possible to lower viscosity, thus increasing drag-out recovery (i.e., recovery of solution left on metal) · using air agitation in rinsing to increase rinsing efficiency · using plastic balls in plating tanks to reduce misting · using improved filtration on plating tanks to reduce the frequency of purification treatment · placing a curb around all process areas to contain spills · using separate treatments for recoverable metals such as nickel · installing recovery systems such as ion exchange, atmospheric evaporation, vacuum evaporation, electrolytic recovery, reverse osmosis and electrodialysis · complementing drag-out recovery systems with reductions in drag-in of contaminants and improved cleaning systems · using modern inventory controls to reduce waste and workplace hazards · applying standard procedures (i.e., written procedures, regular operating reviews and sound operating logs) to provide the basis for a sound environmental management structure. Environmental planning for specific wastes Specific waste streams, usually spent plating solutions, can be reduced by: · Filtration. Cartridge or diatomaceous earth filters can be used to remove the accumulation of solids, which reduce the efficiency of the process. · Carbon treatment can be used to remove organic contaminants (most commonly applied in nickel plating, copper electroplating and zinc and cadmium plating). · Purified water. The natural contaminants in water make-up and rinses (e.g., calcium, iron, magnesium, manganese, chlorine and carbonates) can be removed by using deionization, distillation or reverse osmosis. Improving rinse water efficiency reduces the volume of bath sludges requiring treatment. · Cyanide bath carbonate freezing. Lowering the bath temperature to –3 °C crystallizes the carbonates formed in cyanide bath by the breakdown of cyanide, excessive anode current densities and the adsorption of carbon dioxide from the air and facilitates their removal.

· Precipitation. Removal of metal contaminants entering the bath as impurities in anodes can be achieved through precipitation with barium cyanide, barium hydroxide, calcium hydroxide, calcium sulphate or calcium cyanide. · Hexavalent chrome alternatives. Hexavalent chromium can be replaced with trivalent chromium plating solutions for decorative plating. Chrome conversion coatings for paint pretreatments can sometimes be replaced by non-chrome conversion coatings or no-rinse chrome chemistries. · Non-chelated process chemistries. Instead of chelators being added to process baths to control the concentration of free ions in the solution, non-chelated process chemistries can be used so that it may not be necessary to keep metals in solution. These metals can be allowed to precipitate and can be removed by continuous filtration. · Non-cyanide process chemicals. Waste streams containing free cyanide are typically treated using hypochlorite or chlorine to accomplish oxidation, and complex cyanides are commonly precipitated using ferrous sulphate. Using non-cyanide process chemistries both eliminates a treatment step and reduces the sludge volume. · Solvent degreasing. Hot alkaline cleaning baths can be used in place of solvent degreasing of workpieces before processing. The effectiveness of alkaline cleaners can be enhanced by applying electrocurrent or ultrasonics. The benefits of avoiding solvent vapours and sludges often outweigh any additional operating costs. · Alkaline cleaners. Having to discard alkaline cleaners when the accumulation of oil, grease and soils from use reaches a level which impairs the cleaning efficiency of the bath can be avoided by using skimming devices to remove free-floating oils, settling devices or cartridge filters to remove particulates and oil-water coalescers and by using microfiltration or ultrafiltration to remove emulsified oils. · Drag-out reduction. Reducing the volume of drag-out from process baths serves to reduce the amount of valuable process chemicals that contaminates the rinse water, which in turn reduces the amount of sludge that is generated by a conventional metal precipitation treatment process. Several methods of reducing drag-out include: · Process bath operating concentration. The chemical concentration should be kept as low as possible to minimize the viscosity (for quicker draining) and the quantity of chemicals (in the film). · Process bath operating temperature. The viscosity of the process solution can be reduced by increasing the bath temperature. · Wetting agents. The surface tension of the solution can be reduced by adding wetting agents to the process bath.

· Workpiece positioning. The workpiece should be positioned on the rack so that the adhering film drains freely and does not get trapped in grooves or cavities. · Withdrawal or drainage time. The faster a workpiece is removed from the process bath, the thicker the film on the workpiece surface. · Air knives. Blowing air at the workpiece as the workpiece rack is raised above the process tank can improve drainage and drying. · Spray rinses. These can be used above heated baths so that the rinse flow rate equals the evaporation rate of the tank. · Plating baths. Carbonates and organic contaminants should be removed to prevent accumulation of contamination that increases the viscosity of the plating bath. · Drainage boards. The spaces between process tanks should be covered with drainage boards to capture process solutions and to return them to the process bath. · Drag-out tanks. The workpieces should be placed in drag-out tanks (―static rinse‖ tanks) before the standard rinsing operation. Drag-out recovery of chemicals uses a variety of technologies. These include: · Evaporation. Atmospheric evaporators are most common, and vacuum evaporators offer energy savings. · Ion exchange is used for chemical recovery of rinse water. · Electrowinning. This is an electrolytic process whereby the dissolved metals in the solution are reduced and deposited on the cathode. The deposited metal is then recovered. · Electrodialysis. This utilizes ion-permeable membranes and applied current in order to separate ionic species from the solution. · Reverse osmosis. This utilizes a semi-permeable membrane to produce purified water and a concentrated ionic solution. High pressure is used to force the water through the membrane, while most dissolved salts are retained by the membrane. Rinse water Most of the hazardous waste produced in a metal finishing facility comes from waste water generated by the rinsing operations that follow cleaning and plating. By increasing rinse efficiency, a facility can significantly reduce waste water flow. Two basic strategies improve rinsing efficiency. First, turbulence can be generated between the workpiece and the rinse water through spray rinses and rinse water agitation. Movement of the

rack or forced water or air are used. Second, the contact time between the workpiece and the rinse water can be increased. Multiple rinse tanks set countercurrent in series will reduce the amount of rinse water used.

Industrial Coatings
The term coatings includes paints, varnishes, lacquers, enamels and shellacs, putties, wood fillers and sealers, paint and varnish removers, paint brush cleaners and allied paint products. Liquid coatings contain pigments and additives dispersed in a liquid binder and solvent mixture. Pigments are inorganic or organic compounds that provide coating colour and opacity and influence coating flow and durability. Pigments often contain heavy metals such as cadmium, lead, zinc, chromium and cobalt. The binder increases coating adhesiveness, cohesiveness and consistency and is the primary component that remains on the surface when coating is completed. Binders include a variety of oils, resins, rubbers and polymers. Additives such as fillers and extenders may be added to coatings to reduce manufacturing costs and increase coating durability. The types of organic solvents used in coatings include aliphatic hydrocarbons, aromatic hydrocarbons, esters, ketones, glycol ethers and alcohols. Solvents disperse or dissolve the binders and decrease the coating viscosity and thickness. Solvents used in coatings formulations are hazardous because many are human carcinogens and are flammable or explosive. Most solvents contained in a coating evaporate when the coating cures, which generates volatile organic compound (VOC) emissions. VOC emissions are becoming increasingly regulated because of the negative effects on human health and the environment. Environmental concerns associated with conventional ingredients, coating application technologies and coating wastes are a driving force for developing pollution prevention alternatives. Most coatings are used on architectural, industrial or special products. Architectural coatings are used in buildings and building products and for decorative and protective services such as varnishes to protect wood. Industrial facilities incorporate coating operations in various production processes. The automotive, metal can, farm machinery, coil coating, wood and metal furniture and fixtures, and household appliance industries are the major industrial coatings consumers. Design of a coating formulation depends on the purpose of the coating application. Coatings provide aesthetics, and corrosion and surface protection. Cost, function, product safety, environmental safety, transfer efficiency and drying and curing speed determine formulations. Coating processes There are five operations comprising most coating processes: raw materials handling and preparation, surface preparation, coating, equipment cleaning and waste management.
Raw material handling and preparation

Raw material handling and preparation involves inventory storage, mixing operations, thinning and adjusting of coatings and raw material transfer through the facility. Monitoring and handling procedures and practices are needed to minimize the generation of wastes from spoilage, off specification and improper preparation that can result from excessive thinning and consequent wastage. Transfer, whether manual or through a piped system, must be scheduled to avoid spoilage.
Surface preparation

The type of surface preparation technique used depends on the surface being coated—previous preparation, amount of soil, grease, the coating to be applied and the surface finish required. Common preparation operations include degreasing, precoating or phosphating and coating removal. For metal finishing purposes, degreasing involves solvent wiping, cold cleaning or vapour degreasing with halogenated solvents, aqueous alkaline cleaning, semi-aqueous cleaning or aliphatic hydrocarbon cleaning to remove organic soil, dirt, oil and grease. Acid pickling, abrasive cleaning or flame cleaning are used to remove mill scale and rust. The most common preparation operation for metal surfaces, other than cleaning, is phosphate coating, used to promote adhesion of organic coatings onto metal surfaces and retard corrosion. Phosphate coatings are applied by immersing or spraying metal surfaces with zinc, iron or manganese phosphate solution. Phosphating is a surface finishing process similar to electroplating, consisting of a series of process chemical and rinse baths in which parts are immersed to achieve the desired surface preparation. See the article ―Surface treatment of metals‖ in this chapter. Coating removal, chemical or mechanical, is conducted on surfaces that require recoating, repair or inspection. The most common chemical coating removal method is solvent stripping. These solutions usually contain phenol, methylene chloride and an organic acid to dissolve the coating from the coated surface. A final water wash to remove the chemicals can generate large quantities of wastewater. Abrasive blasting is the common mechanical process, a dry operation that uses compressed air to propel a blasting medium against the surface to remove the coating. Surface preparation operations affect the quantity of waste from the specific preparation process. If the surface preparation is inadequate, resulting in poor coating, then removal of the coating and recoating adds to waste generation.
Coating

The coating operation involves transferring the coating to the surface and curing the coating on the surface. Most coating technologies fall into 1 of 5 basic categories: dip coating, roll coating, flow coating, spray coating, and the most common technique, air-atomized spray coating using solvent-based coatings. Air-atomized spray coatings are usually conducted in a controlled environment because of solvent emissions and overspray. Overspray control devices are fabric filters or water walls, generating either used filters or wastewater from air scrubbing systems.

Curing is performed to convert the coating binder into a hard, tough, adherent surface. Curing mechanisms include: drying, baking or exposure to an electron beam or infrared or ultraviolet light. Curing generates significant VOCs from solvent-based coatings and poses a potential for explosion if the solvent concentrations rise above the lower explosive limit. Consequently, curing operations are equipped with air pollution control devices to prevent VOC emissions and for safety control to prevent explosions. Environmental and health concerns, increased regulations affecting conventional coating formulations, high solvent costs and expensive hazardous waste disposal have created a demand for alternative coating formulations that contain less hazardous constituents and generate less waste when applied. Alternative coating formulations include: · High-solid coatings, containing twice the amount of pigment and resin in the same volume of solvent as conventional coatings. Application lowers VOC emissions between 62 and 85% compared to conventional low-solid solvent-based coatings because the solvent content is reduced. · Water-based coatings using water and an organic solvent mixture as the carrier with water used as the base. Compared to solvent-based coatings, water-based coatings generate between 80 and 95% less VOC emissions and spent solvents than conventional low-solid solvent-based coatings. · Powder coatings containing no organic solvent, consisting of finely pulverized pigment and resin particles. They are either thermoplastic (high molecular weight resin for thick coatings) or thermosetting (low molecular weight compounds that form a thin layer before chemically crosslinking) powders.
Equipment cleaning

Equipment cleaning is a necessary, routine maintenance operation in coating processes. This creates significant amounts of hazardous waste, particularly if halogenated solvents are used for cleaning. Equipment cleaning for solvent-based coatings has traditionally been conducted manually with organic solvents to remove coatings from process equipment. Piping requires flushing with solvent in batches until clean. Coating equipment must be cleaned between product changes and after process shutdowns. The procedures and practices used will determine the level of waste generated from these activities.
Waste management

Several waste streams are generated by coating processes. Solid waste includes empty coating containers, coating sludge from overspray and equipment cleaning, spent filters and abrasive materials, dry coating and cleaning rags. Liquid wastes include waste water from surface preparation, overspray control or equipment cleaning, off-specification or excess coating or surface preparation materials, overspray, spills and spent cleaning solutions. Onsite closed-loop recycling is becoming more popular for spent solvents as disposal costs rise. Water-based liquids are usually treated onsite prior to discharge to publicly owned treatment systems.

VOC emissions are generated by all conventional coating processes that use solvent-based coatings, requiring control devices such as carbon adsorption units, condensers or thermal catalytic oxidizers.

REFERENCES
Buonicore, AJ and WT Davis (eds.). 1992. Air Pollution Engineering Manual. New York: Van Nostrand Reinhold/Air and Waste Management Association. Environmental Protection Agency (EPA). 1995. Profile of the Nonferrous Metals Industry. EPA/310-R-95-010. Washington, DC: EPA. International Association for Research on Cancer (IARC). 1984. Monographs on the Evaluation of Carcinogenic Risks to Humans. Vol. 34. Lyon: IARC. Johnson A, CY Moira, L MacLean, E Atkins, A Dybunico, F Cheng, and D Enarson. 1985. Respiratory abnormalities amongst workers in iron and steel industry. Brit J Ind Med 42:94–100. Kronenberg RS, JC Levin, RF Dodson, JGN Garcia, and DE Griffith. 1991. Asbestos-related disease in employees of a steel mill and a glass bottle manufacturing plant. Ann NY Acad Sci 643:397–403. Landrigan, PJ, MG Cherniack, FA Lewis, LR Catlett, and RW Hornung. 1986. Silicosis in a grey iron foundry. The persistence of an ancient disease. Scand J Work Environ Health 12:32–39. National Institute for Occupational Safety and Health (NIOSH). 1996. Criteria for a Recommended Standard: Occupational Exposures to Metalworking Fluids. Cincinatti, OH: NIOSH. Palheta, D and A Taylor. 1995. Mercury in environmental and biological samples from a gold mining area in the Amazon Region of Brazil. Science of the Total Environment 168:63-69. Thomas, PR and D Clarke. 1992 Vibration white finger and Dupuytren’s contracture: Are they related? Occup Med 42(3):155–158.

OTHER RELEVANT READINGS
American Conference of Government Industrial Hygienists (ACGIH) Committee on Industrial Ventilation. 1992. Industrial Ventilation: A Manual of Recommended Practice, 22nd ed. Cincinnati, OH: ACGIH. American National Standards Institute (ANSI). 1976. Safety Requirements for the Construction, Care, and Use of Lathes. ANSI B11.6-1976. New York: ANSI.

—. 1988a. The Use, Care, and Protection of Abrasive Wheels. ANSI B7.1-1988. New York: ANSI. —.1988b. Safety in Welding and Cutting. ANSI Z49.1-1988. New York: ANSI. American Petroleum Institute (API). 1971. Chemistry and Petroleum for Classroom Use in Chemistry Courses. Washington, DC: API. —. 1980. Facts about Oil. Manual 4200. Washington, DC: API. —. 1984. Safe Operation of Inland Bulk Plants. Publication 2008. Washington, DC: API. Antoni, H. 1978. Massnahmen zu höherer Sicherheit beim Spannen mit Backenfuttern [Measures to increase safety in gripping with jaw chucks]. Zeitschrift für industrielle Fertigung 10:611–615. Burgess, WA. 1995. Recognition of Health Hazards in Industry, 2nd edition. New York: John Wiley & Sons. Exxon Company. 1987. Encyclopedia for the User of Petroleum Products. Houston, TX: Exxon Company, USA, Marketing Technical Services. Goldsmith, AH, KW Vorpahl, KA French, PT Jordan, and NB Jurinski. 1976. Health hazards from oil, soot and metals at a hot forging operation. Am Ind Hyg Assoc J 37:217–226. Gulf Publishing Company. 1964. Petroleum Marketing and Transportation, 1964. Houston TX: Gulf Publishing Company. Harten, GA. 1976. Een nieuwe, ergonomisch verbeterde draaibank [A new, ergonomically improved lathe]. Tijdschrift voor sociale geneeskunde (Amstelveen) 54(17):575–578. Kusiak, RA, J Springer, AC Ritchie, and J Muller. 1991. Carcinoma of the lung in Ontario gold miners: Possible aetiological factors. Brit J Ind Med 48(12):808-817. Mobil Oil Corporation. 1990. Handling, Storing and Dispensing Industrial Lubricants. Mobil Technical Bulletin. Fairfax, VA: Mobil Oil Corporation. National Institute for Occupational Safety and Health (NIOSH). 1975. Ventilation Requirements for Grinding, Buffing and Polishing Operations. NIOSH Publ. No. 75-105. Cincinnati, OH: NIOSH. National Safety Council. 1995. Petroleum Section Safety and Health Fact Sheets, 1988-95. Itasca, IL: National Safety Council. Occupational Safety and Health Administration (OSHA). 1979. Prudent Practices for Controlling Lead Exposure in the Secondary Lead Smelting Industry: A Guide for Employers and Employees. Washington, DC: OSHA.

—. 1982. Cooperative Assessment Program Manual for the Secondary Lead Smelting Industry. Washington, DC: OSHA. —. 1984. Cooperative Assessment Program Manual for the Battery Manufacturing Industry. Washington, DC: OSHA. Ontario Metal Finishing Industry Pollution Prevention Project. 1995. Metal Finishing Pollution Prevention Guide. Ottawa: Environment Canada, Water Technology International, Sheridan Environmental Technology Institute. Simonato, L, JJ Moulin, B Javeland, G Ferro, P Wild, R Winkelmann, and R Saracci. 1994. A retrospective mortality study of workers exposed to arsenic in a gold mine and refinery in France. Am J Ind Med 25:652-633.

Part XIII - Manufacturing Industries

Français

Ladle refining
Dr. Dmitri Kopeliovich


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Vacuum ladle degassing o Recirculation Degassing (RH) o Recirculation Degassing with oxygen top lance (RH-OB) o Ladle Degassing (VD, Tank Degassing) o Vacuum Oxygen Decarburization (VOD) Ladle Furnace (LF) Ladle desulfurization by injection of active agents o Powder injection o Cored wire injection Ladle-to-mold degassing

Vacuum ladle degassing Methods of vacuum ladle degassing utilize the reaction of deoxidation by carbon dissolved in steel according to the equation: [C] + [O] = {CO} where: [C] and [O] - carbon and oxygen dissolved in liquid steel; {CO} - gaseous carbon monoxide. Vacuum treatment of molten steel decreases the partial pressure of CO, which results in shifting equilibrium of the reaction of carbon oxidation. Bubbles of carbon monoxide form in the liquid steel, float up and then they are removed by the vacuum system. In addition to deoxidation vacuum treatment helps to remove Hydrogen dissolved in liquid steel. Hydrogen diffuses into the CO bubbles and the gas is then evacuated by the vacuum pump. Movement of the molten steel caused by CO bubbles also results in refining the steel from nonmetallic inclusions, which agglomerate, float up and are absorbed by the slag. CO bubbles also favor the process of floating and removal of nitride inclusions and gaseous Nitrogen. Steels refined in vacuum are characterized by homogeneous structure, low content of nonmetallic inclusions and low gas porosity. Vacuum degassing methods are used for manufacturing large steel ingots, rails, ball bearings and other high quality steels. Vacuum ladle degassing methods:



Recirculation Degassing (RH)

Recirculation degassing unit uses a vacuum chamber having two snorkels connected to the chamber bottom. One of the snorkels is equipped with pipes supplying Argon through its refractory lining. The snorkels of the vacuum chamber are immersed into the ladle with molten steel. Liquid metal fills the chamber to a level determined by the atmospheric pressure (4.2ft/1.3m). Argon bubbles floating up in one of the snorkels (up-leg) force the melt to rise in the snorkel. Through the second snorkel (down-leg) the molten steel flows down back to the ladle producing circulation. The circulation rate may reach 150-200 t/min. The recirculation degassing vacuum chambers are usually equipped with addition hoppers, through which alloying elements or/and desulfurization slag may be added. Benefits of Recirculation Degassing (RH): -Hydrogen removal (degassing); -Oxygen removal (deoxidation); -Carbon removal (decarburization); -Sulfur removal (desulfurization); -Precise alloying; -Non-metallic inclusions removal; -Temperature and chemical homogenizing.



Recirculation Degassing with oxygen top lance (RH-OB)

In this method a conventional Recirculation degassing (RH) vessel (chamber) is equipped with a vertical water cooled lance for blowing oxygen on the molten steel surface. Oxygen intensifies the reaction [C] + [O] = {CO} resulting in fast and effective decarburization. Oxygen also oxidizes phosphorus like in Basic Oxygen Process (BOP) or in oxidizing slag stage in Electric-arc furnace. Oxidation reactions have also heating effect therefore the treated metal may be heated to a required temperature without any additional energy source. Benefits of Recirculation degassing with oxygen top lance (RH-OB): -Hydrogen removal (degassing); -Fast carbon removal (decarburization); -Phosphorus removal (dephosporization); -Sulfur removal (desulfurization); -Reheating; -Precise alloying; -Non-metallic inclusions removal; -Temperature and chemical homogenizing.


Ladle Degassing (VD, Tank Degassing)

In the Tank Degassing method the ladle with molten steel is placed into a vacuum chamber. The ladle is equipped with a porous refractory plug mounted in the ladle bottom. Through the plug argon is supplied during vacuum treatment. There is an addition hopper with vacuum lock on the chamber cover. The hopper is used for adding alloying elements and/or slag components. The reaction [C] + [O] = {CO} starting in the steel under vacuum conditions causes stirring, which is additionally intensified by argon blown through the bottom porous plug. Intensive stirring of the melt and the slag results in deep desulfurization of the steel. Desulfurizing slags possessing high sulfur solubility are used in this process. Argon and CO bubbles also favor the process of floating and removal of nitride inclusions and gaseous nitrogen. Benefits of Ladle Degassing (VD, Tank Degassing): -Hydrogen removal (degassing); -Oxygen removal (deoxidation); -Deep sulfur removal (desulfurization); -Carbon removal (decarburization); -Precise alloying; -Non-metallic inclusions (oxides and nitrides) removal; -Temperature and chemical homogenizing.



Vacuum Oxygen Decarburization (VOD)

In this method a conventional Ladle Degassing (VD, Tank Degassing) chamber is equipped with a vertical water cooled lance for blowing oxygen on the molten steel surface. Vacuum Oxygen Decarburization (VOD) method is used for manufacturing Stainless steels. Oxidation of liquid steel components under vacuum differs from that at normal pressure: oxygen is consumed mainly by the reaction [C] + [O] = {CO} rather than by oxidation of chromium, which is the main constituent of stainless steels. VOD process allows to decarburize the steel with minor chromium losses. Oxidation reactions have also heating effect therefore the treated metal may be heated to a required temperature without any additional energy source. After having the decarburization (oxidation) stage completed deoxidizers are added to the steel in order to remove excessive oxygen. Then a Desulfurizing slag is added to the molten steel surface. Stirring of the melt and the slag caused by argon blown through the porous bottom plug results in deep desulfurization of the steel. Benefits of Vacuum Oxygen Decarburization (VOD): -Deep carbon removal (decarburization); -Low losses of chromium in treatment of stainless steels; -Hydrogen removal (degassing); -Sulfur removal (desulfurization);

-Precise alloying; -Reheating; -Non-metallic inclusions (oxides and nitrides) removal; -Temperature and chemical homogenizing. to top Ladle Furnace (LF)

Molten steel in a ladle may be treated (refined) in a device called Ladle Furnace (LF). The ladle is transported to the Ladle Furnace stand where it is placed under a cover equipped with three graphite electrodes connected to a three-phase arc transformer. The ladle bottom has a porous refractory plug, which is connected to the argon supply pipe at the Ladle Furnace stand. The LF stand is also equipped with an addition hopper mounted on the cover and a lance for injection of desulfurizing agents. Fumes formed during the operation are extracted through the cover. Molten steel treated in Ladle Furnace is covered by a layer of desulfurizing slag. The graphite

electrodes are submerged into the slag, which protects the ladle lining from overheating produced by the electric arcs. The arcs are capable to heat the steel at the rate about 5°F/min (3°C/min). During the treatment process argon is blown through the bottom porous plug providing continuous metal stirring. Stirring results in distribution of heat produced by the arcs, chemical homogenization and desulfurization of the steel by the slag. Alloying elements and/or slag components may be added through the addition hopper. If deep desulfurization is required active desulfurizing agents are injected into the melt through the injection lance or in form of cored wire. Besides refining operations Ladle Furnace (LF) may serve as a buffer station before Continuous casting. Benefits of Ladle Furnace (LF):
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Deep sulfur removal (desulfurization); Controllable reheating by electric power; Alloying; Temperature and chemical homogenizing; Non-metallic inclusions removal.

Ladle Furnace process is used for refining a wide variety of steels, in which degassing (hydrogen removal) is not required. to top Ladle desulfurization by injection of active agents Injection of desulfurizing agents (Ca, Mg, CaSi, CaC2, CaF2+CaO) to a molten steel is the most effective method of sulfur removal. Injection methods usually combine supply of a disperse desulfurizing agent (powder) with stirring by argon blowing. A ladle with deoxidized (killed) molten steel is transported to the injection stand where it is placed under a cover, through which the injection lance may lower and immerse into the melt. Steel treated in the stand is covered by a layer of desulfurizing slag having high solubility of sulfur and capable to absorb sulfides formed as a result of active agents injection. Desulfurization agents are injected in argon stream. Argon bubbles produce stirring of the molten steel and the slag promoting desulfurization. Stirring also provides thermal and chemical homogenization of the melt. When the desulfurizing agents are injected into molten steel in form of a cored wire containing

powder of desulfurizing agent stirring by argon bubbling from the porous plug mounted in the ladle bottom is used. Fumes formed during the operation are extracted through the cover. Injection of desulfurizing agents allows to achieve ultra-low concentrations of sulfur in steel (0.0002%). Benefits of Ladle desulfurization by injection of active agents:
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Deep sulfur removal (desulfurization); Temperature and chemical homogenizing; Non-metallic inclusions removal.

to top Ladle-to-mold degassing

Ladle-to-mold degassing is a vacuum degassing method, in which the mold is placed in a vacuum chamber. The molten steel is poured from a tundish attached to the cover of the chamber. The tundish is continuously filled with the melt poured from the ladle. The steel stream ―boils‖ when it is falling to the mold cavity in vacuum due to the deoxidation reaction [C] + [O] = {CO}. Hydrogen dissolved in steel diffuses into the CO bubbles and the gas is then evacuated by the vacuum pump. Intensity of the deoxidation and degassing during Ladle-to-mold pouring is indicated by the angle, at which the melt stream ―opens‖ as a result of CO bubbles formation.

Electric Arc Furnace (EAF)
Dr. Dmitri Kopeliovich Electric Arc Furnace (EAF) is a steel making furnace, in which steel scrap is heated and melted by heat of electric arcs striking between the furnace electrodes and the metal bath. Two kinds of electric current may be used in Electric Arc Furnaces: direct (DC) and alternating (AC). Three-phase AC Electric Arc Furnaces with graphite electrodes are commonly used in steel making. The main advantage of the Electric Arc Furnaces over the Basic Oxygen Furnaces (BOF) is their capability to treat charges containing up to 100% of scrap. About 33% of the crude steel in the world is made in the Electric Arc Furnaces (EAF). Capacity of Electric Arc Furnace may reach 400 t.
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Structure of an Electric Arc Furnace Refractory lining of an Electric Arc Furnace Chemical and physical processes in an Electric Arc Furnace Operation of an Electric Arc Furnace

Structure of an Electric Arc Furnace

The scheme of a Electric Arc Furnace (EAF) is presented in the picture. The furnace consists of a spherical hearth (bottom), cylindrical shell and a swinging watercooled dome-shaped roof. The roof has three holes for consumable graphite electrodes held by a clamping mechanism. The mechanism provides independent lifting and lowering of each electrode. The water-cooled electrode holders serve also as contacts for transmitting electric current supplied by water-cooled cables (tubes). The electrode and the scrap form the star connection of three-phase current, in which the scrap is common junction. The furnace is mounted on a tilting mechanism for tapping the molten steel through a tap hole with a pour spout located on the back side of the shell. The charge door, through which the slag components and alloying additives are charged, is located on the front side of the furnace shell. The charge door is also used for removing the slag (de-slagging). The scrap is charged commonly from the furnace top. The roof with the electrodes is swung aside before the scrap charging. The scrap arranged in the charge basket is transferred to the furnace by a crane and then dropped into the shell. to top Refractory lining of an Electric Arc Furnace Refractory linings of Electric Arc Furnaces are made generally of resin-bonded magnesia-carbon bricks. Fused magnesite grains and flake graphite are used as raw materials. When the bricks are heated the bonding material is coked and turns into a carbon network binding the refractory grains, preventing wetting by the slag and protecting the lining the from erosion and chemical attack of the molten metal and slag. to top Chemical and physical processes in an Electric Arc Furnace Melting Melting process starts at low voltage (short arc) between the electrodes and the scrap. The arc during this period is unstable. In order to improve the arc stability small pieces of the scrap are placed in the upper layer of the charge. The electrodes descend melting the charge and penetrating into the scrap forming bores. The molten metal flows down to the furnace bottom. When the electrodes reach the liquid bath the arc becomes stable and the voltage may be increased (long arc). The electrodes are lifting together with the melt level. Most of scrap (85%) melt during this period. Temperature of the arc reaches 6300ºF (3500ºC).

Oxidizing stage At this stage excessive carbon, phosphorous, silicon and manganese oxidize. The process is similar to that in Basic Oxygen Furnace. Basic oxidizing slag composed of lime (CaO) and ion ore (FeO) is used during the oxidizing period. Gaseous oxygen may be blown into the melt for additional oxidizing. Iron oxide causes increase of Oxygen content in the molten steel according to the reaction: (square brackets [ ] - signify solution in steel, round brackets ( ) - in slag, curly brackets {} - in gas) (FeO) = [Fe] + [O] Oxygen dissolved in the melt oxidizes carbon, phosphorous, silicon and manganese: [C] + [O] = {CO} [Si] + {O2} = (SiO2) [Mn] + 1/2{O2} = (MnO) 2[P] + 5/2{O2} = (P2O5) Carbon monoxide partially burns in the atmosphere: {CO} + {O2} = {CO2} The formed oxides are absorbed by the slag. CO bubbles floating up through the melt result in refining of the steel from non-metallic inclusions and hydrogen removal. Gaseous products CO and CO2 are removed by the exhausting system. Oxidizing potential of the atmosphere is characterized by the post-combustion ratio: {CO2}/({CO2}+{CO}). The oxidizing slag enriched with phosphorous and other oxides formed during this period is removed from the furnace to a slag pot (de-slagging). Reducing stage New slag composed mainly of lime (CaO), CaF2 (as slag fluidizer) is added at this stage for formation of basic reducing conditions. The function of this slag is refining of the steel from sulfur and absorption of oxides, formed as a result of deoxidation (‖killing‖).

The excessive oxygen dissolved in the melt during oxidizing period is removed by metallic deoxidizers Mn, Si, Al: [Mn] + [O] = (MnO) [Si] + 2[O] = (SiO2) 2[Al] + 3[O] = (Al2O3) Basic reducing slag is favorable for desulfurization in accordance to the reaction: [S] + (CaO) = (CaS) + [O] Oxide and sulfide non-metallic inclusions are absorbed by the slag. Alloying elements (Cr, Ni, Mo, V, etc.) are added after deoxidation. In many cases the processes of ―killing‖ (deoxidation), desulfurization, alloying and final heating are performed outside of the furnace - Ladle refining. to top Operation of an Electric Arc Furnace
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Scrap charging; Melting; Sampling and chemical analysis of the melt; Oxidizing slag formation; Oxidation of C, P, Mn, Si, Al. Sampling and temperature measurement; De-slagging; Basic slag formation; Deoxidizing (‖killing‖); Alloying; Tapping the steel; Refractory lining maintenance.

Steel making (introduction)
Dr. Dmitri Kopeliovich Steel is an alloy of iron, containing up to 2% of carbon (usually up to 1%). Steel contains lower (compared to pig iron) quantities of impurities like phosphorous, sulfur and silicon.

Steel is produced from pig iron by processes, involving reducing the amounts of carbon, silicon and phosphorous. The main steel making methods are:
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Basic Oxygen Process (BOP) Electric-arc furnace Ladle refining (ladle metallurgy, secondary refining)

Basic Oxygen Process (BOP) The Basic Oxygen Process is the most powerful and effective method of steel manufacturing.

The scheme of the Basic Oxygen Furnace (BOF) (basic oxygen furnace, basic oxygen converter) is presented in the picture. Typical basic oxygen converter has a vertical steel shell lined with refractory lining. The furnace is capable to rotate about its horizontal axis on trunnions.

This rotation is necessary for charging raw materials and fluxes, sampling the melt and pouring the steel and the slag out of the furnace. The Basic Oxygen is equipped with the water cooled oxygen lance for blowing oxygen into the melt. The basic oxygen converter uses no additional fuel. The pig iron impurities (carbon, silicon, manganese and phosphorous) serve as fuel. The steel making process in the oxygen converter consists of:
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Charging steel scrap. Pouring liquid pig iron into the furnace. Charging fluxes. Oxygen blowing. Sampling and temperature measurement Tapping the steel to a ladle. De-slagging.

The iron impurities oxidize, evolving heat, necessary for the process. The forming oxides and sulfur are absorbed by the slag. The oxygen converter has a capacity up to 400 t and production cycle of about 40 min. to top Electric-arc furnace

The electric-arc furnace employs three vertical graphite electrodes for producing arcs, striking on to the charge and heating it to the required temperature. As the electric-arc furnace utilizes the external origin of energy (electric current), it is capable to melt up to 100% of steel scrap. The steel making process in the electric-arc furnace consists of:
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Charging scrap metal, pig iron, limestone Lowering the electrodes and starting the power (melting) Oxidizing stage

At this stage the heat, produced by the arcs, causes oxidizing phosphorous, silicon and manganese. The oxides are absorbed into the slag. By the end of the stage the slag is removed.
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De-slagging Reducing stage

New fluxes (lime and anthracite) are added at this stage for formation of basic reducing slag. The function of this slag is refining of the steel from sulfur and absorption of oxides, formed as a result of deoxidation.


Tapping



Lining maintenance

The advantages of the electric-arc furnace are as follows:
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Unlimited scrap quantity may be melt; Easy temperature control; Deep desulfurization; Precise alloying.

to top Ladle refining (ladle metallurgy, secondary refining) Ladle refining is post steel making technological operations, performed in the ladle prior to casting with the purposes of desulfurization, degassing, temperature and chemical homogenization, deoxidation and others. Ladle refining may be carried out at atmospheric pressure, at vacuum, may involve heating, gas purging and stirring. Sulfur refining (desulfurization) in the ladle metallurgy is performed by addition of fluxes (CaO, CaF2 and others) into the ladle and stirring the steel together with the slag, absorbing sulfur. In the production of high quality steel the operation of vacuum treatment in ladle is widely used. Vacuum causes proceeding chemical reaction within the molten steel: [C] + [O] = {CO} This reaction results in reduction of the quantity of oxide inclusions. The bubbles of carbon oxide remove Hydrogen, diffusing into the CO phase. An example of ladle refining method is Recirculation Degassing (RH) vacuum degasser, which consists of a vacuum vessel with two tubes (snorkels), immersed in the steel. In one of the tubes argon is injected. Argon bubbles, moving upwards, cause steel circulation through the vacuum vessel. Additions of fluxes in the vacuum vessel permits conducting desulfurization treatment by this method.

Basic Oxygen Furnace (BOF)

Dr. Dmitri Kopeliovich Basic Oxygen Furnace (BOF) is a steel making furnace, in which molten pig iron and steel scrap convert into steel due to oxidizing action of oxygen blown into the melt under a basic slag. The Basic Oxygen Process (Basic Oxygen Furnace, Basic Oxygen Steelmaking, Basic Oxygen Converter) is the most powerful and effective steel making method. About 67% of the crude steel in the world is made in the Basic Oxygen Furnaces (BOF).
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Furnace structure Refractory lining Chemical and physical processes Operation

Structure of a Basic Oxygen Furnace

The scheme of a Basic Oxygen Furnace (BOF) is presented in the picture. Typical basic oxygen furnace has a vertical vessel lined with refractory lining. Only 8-12% of the furnace volume is filled with the treated molten metal. The bath depth is

about 4-6.5 ft (1.2-1.9 m). The ratio between the height and diameter of the furnace is 1.2-1.5. The typical capacity of the Basic Oxygen Furnace is 250-400 t. The vessel consists of three parts: spherical bottom, cylindrical shell and upper cone. The vessel is attached to a supporting ring equipped with trunnions. The supporting ring provides stable position of the vessel during oxygen blowing. The converter is capable to rotate about its horizontal axis on trunnions driven by electric motors. This rotation (tilting) is necessary for charging raw materials, sampling the melt and pouring the steel out of the converter. The top blown basic oxygen furnace is equipped with the water cooled oxygen for blowing oxygen into the melt through 4-6 nozzles. Oxygen flow commonly reaches 200-280 ft3/(min*t) (6-8 m3/(min*t)). The oxygen pressure is 150-220 psi (1-1.5 MPa). Service life of oxygen lance is about 400 heats. The bottom blown basic oxygen furnace is equipped with 15-20 tuyeres for injection of oxygen (or oxygen with lime powder). The tuyeres are cooled by either hydrocarbon gas (propane, methane) or oil supplied to the outer jacket of the tube. to top Refractory lining of a Basic Oxygen Furnace The refractory lining of basic oxygen furnaces work in severe conditions of high temperature and oxidizing atmosphere. The lining wear is fastest in the zone of contact with the oxidizing slag (slag line). Refractory bricks for lining basic oxygen furnaces are made of either resin bonded magnesite or tar bonded mixtures of magnesite (MgO) and burnt lime (CaO). The bonding material (resin, tar) is coked and turns into a carbon network binding the refractory grains, preventing wetting by the slag and protecting the lining the from chemical attack of the molten metal. The following measures allows to prolong the service life of the lining:
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Control of the content of aggressive oxidizing oxide FeO in the slags at low level. Addition of MgO to the slags. Performing ―slag splashes‖ - projecting residual magnesia saturated slag to the lining walls by Nitrogen blown through the lance. Repair the damaged zones of the lining by gunning refractory materials.

Properly maintained lining may serve 20000 heats. to top Chemical and physical processes in a Basic Oxygen Furnace

The basic oxygen furnace uses no additional fuel. The pig iron impurities (carbon, silicon, manganese and phosphorous) serve as fuel. Iron and its impurities oxidize evolving heat necessary for the process. Oxidation of the molten metal and the slag is complicated process proceeding in several stages and occurring simultaneously on the boundaries between different phases (gas-metal, gas-slag, slag-metal). Finally the reactions may be presented as follows: (square brackets [ ] - signify solution in steel, round brackets ( ) - in slag, curly brackets {} - in gas) 1/2{O2} = [O] [Fe] + 1/2{O2} = (FeO) [Si] + {O2} = (SiO2) [Mn] + 1/2{O2} = (MnO) 2[P] + 5/2{O2} = (P2O5) [C] + 1/2{O2} = {CO} {CO} + 1/2{O2} = {CO2} Most oxides are absorbed by the slag. Gaseous products CO and CO2 are transferred to the atmosphere and removed by the exhausting system. Oxidizing potential of the atmosphere is characterized by the post-combustion ratio: {CO2}/({CO2}+{CO}). Basic Oxygen Process has limiting ability for desulfurization. The most popular method of desulfurization is removal of sulfur from molten steel to the basic reducing slag. However the slag formed in the Basic Oxygen Furnace is oxidizing (not reducing) therefore maximum value of distribution coefficient of sulfur in the process is about 10, which may be achieved in the slags containing high concentrations of CaO. to top Operation of a Basic Oxygen Furnace
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Charging steel scrap (25-30% of the total charge weight). Pouring molten pig iron from blast furnace. Charging fluxes. Starting oxygen blowing. Duration of the blowing is about 20 min. Sampling. Temperature measurements (by disposable thermocouple) and taking samples for chemical analysis are made through the upper cone in tilted position of the furnace.

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Tapping - pouring the steel to a ladle. Special devices (plugs, slag detectors) prevent penetration (carry-over) of the slag into the ladle. De-slagging - pouring the residual slag into the slag pot. The furnace is turned upside down in the direction opposite to the tapping hole.

The Basic Oxygen Furnace has a capacity up to 400 t and production cycle (tap-to-tap) of about 40 min.

Ladle refining
Dr. Dmitri Kopeliovich


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Vacuum ladle degassing o Recirculation Degassing (RH) o Recirculation Degassing with oxygen top lance (RH-OB) o Ladle Degassing (VD, Tank Degassing) o Vacuum Oxygen Decarburization (VOD) Ladle Furnace (LF) Ladle desulfurization by injection of active agents o Powder injection o Cored wire injection Ladle-to-mold degassing

Vacuum ladle degassing Methods of vacuum ladle degassing utilize the reaction of deoxidation by carbon dissolved in steel according to the equation: [C] + [O] = {CO} where: [C] and [O] - carbon and oxygen dissolved in liquid steel; {CO} - gaseous carbon monoxide. Vacuum treatment of molten steel decreases the partial pressure of CO, which results in shifting equilibrium of the reaction of carbon oxidation. Bubbles of carbon monoxide form in the liquid steel, float up and then they are removed by the vacuum system. In addition to deoxidation vacuum treatment helps to remove Hydrogen dissolved in liquid steel. Hydrogen diffuses into the CO bubbles and the gas is then evacuated by the vacuum pump. Movement of the molten steel caused by CO bubbles also results in refining the steel from nonmetallic inclusions, which agglomerate, float up and are absorbed by the slag. CO bubbles also favor the process of floating and removal of nitride inclusions and gaseous Nitrogen.

Steels refined in vacuum are characterized by homogeneous structure, low content of nonmetallic inclusions and low gas porosity. Vacuum degassing methods are used for manufacturing large steel ingots, rails, ball bearings and other high quality steels. Vacuum ladle degassing methods:


Recirculation Degassing (RH)

Recirculation degassing unit uses a vacuum chamber having two snorkels connected to the chamber bottom. One of the snorkels is equipped with pipes supplying Argon through its refractory lining. The snorkels of the vacuum chamber are immersed into the ladle with molten steel. Liquid metal fills the chamber to a level determined by the atmospheric pressure (4.2ft/1.3m). Argon bubbles floating up in one of the snorkels (up-leg) force the melt to rise in the snorkel. Through the second snorkel (down-leg) the molten steel flows down back to the ladle producing circulation. The circulation rate may reach 150-200 t/min. The recirculation degassing vacuum chambers are usually equipped with addition hoppers, through which alloying elements or/and desulfurization slag may be added. Benefits of Recirculation Degassing (RH):

-Hydrogen removal (degassing); -Oxygen removal (deoxidation); -Carbon removal (decarburization); -Sulfur removal (desulfurization); -Precise alloying; -Non-metallic inclusions removal; -Temperature and chemical homogenizing.


Recirculation Degassing with oxygen top lance (RH-OB)

In this method a conventional Recirculation degassing (RH) vessel (chamber) is equipped with a vertical water cooled lance for blowing oxygen on the molten steel surface. Oxygen intensifies the reaction [C] + [O] = {CO} resulting in fast and effective decarburization. Oxygen also oxidizes phosphorus like in Basic Oxygen Process (BOP) or in oxidizing slag stage in Electric-arc furnace. Oxidation reactions have also heating effect therefore the treated metal may be heated to a required temperature without any additional energy source. Benefits of Recirculation degassing with oxygen top lance (RH-OB): -Hydrogen removal (degassing); -Fast carbon removal (decarburization);

-Phosphorus removal (dephosporization); -Sulfur removal (desulfurization); -Reheating; -Precise alloying; -Non-metallic inclusions removal; -Temperature and chemical homogenizing.


Ladle Degassing (VD, Tank Degassing)

In the Tank Degassing method the ladle with molten steel is placed into a vacuum chamber. The ladle is equipped with a porous refractory plug mounted in the ladle bottom. Through the plug argon is supplied during vacuum treatment. There is an addition hopper with vacuum lock on the chamber cover. The hopper is used for adding alloying elements and/or slag components. The reaction [C] + [O] = {CO} starting in the steel under vacuum conditions causes stirring, which is additionally intensified by argon blown through the bottom porous plug. Intensive stirring of the melt and the slag results in deep desulfurization of the steel. Desulfurizing slags possessing high sulfur solubility are used in this process. Argon and CO bubbles also favor the process of floating and removal of nitride inclusions and gaseous nitrogen. Benefits of Ladle Degassing (VD, Tank Degassing):

-Hydrogen removal (degassing); -Oxygen removal (deoxidation); -Deep sulfur removal (desulfurization); -Carbon removal (decarburization); -Precise alloying; -Non-metallic inclusions (oxides and nitrides) removal; -Temperature and chemical homogenizing.


Vacuum Oxygen Decarburization (VOD)

In this method a conventional Ladle Degassing (VD, Tank Degassing) chamber is equipped with a vertical water cooled lance for blowing oxygen on the molten steel surface. Vacuum Oxygen Decarburization (VOD) method is used for manufacturing Stainless steels. Oxidation of liquid steel components under vacuum differs from that at normal pressure: oxygen is consumed mainly by the reaction [C] + [O] = {CO} rather than by oxidation of chromium, which is the main constituent of stainless steels. VOD process allows to decarburize the steel with minor chromium losses. Oxidation reactions have also heating effect therefore the treated metal may be heated to a required temperature without any additional energy source. After having the decarburization (oxidation) stage completed deoxidizers are added to the steel in order to remove excessive oxygen. Then a Desulfurizing slag is added to the molten steel

surface. Stirring of the melt and the slag caused by argon blown through the porous bottom plug results in deep desulfurization of the steel. Benefits of Vacuum Oxygen Decarburization (VOD): -Deep carbon removal (decarburization); -Low losses of chromium in treatment of stainless steels; -Hydrogen removal (degassing); -Sulfur removal (desulfurization); -Precise alloying; -Reheating; -Non-metallic inclusions (oxides and nitrides) removal; -Temperature and chemical homogenizing. to top Ladle Furnace (LF)

Molten steel in a ladle may be treated (refined) in a device called Ladle Furnace (LF).

The ladle is transported to the Ladle Furnace stand where it is placed under a cover equipped with three graphite electrodes connected to a three-phase arc transformer. The ladle bottom has a porous refractory plug, which is connected to the argon supply pipe at the Ladle Furnace stand. The LF stand is also equipped with an addition hopper mounted on the cover and a lance for injection of desulfurizing agents. Fumes formed during the operation are extracted through the cover. Molten steel treated in Ladle Furnace is covered by a layer of desulfurizing slag. The graphite electrodes are submerged into the slag, which protects the ladle lining from overheating produced by the electric arcs. The arcs are capable to heat the steel at the rate about 5°F/min (3°C/min). During the treatment process argon is blown through the bottom porous plug providing continuous metal stirring. Stirring results in distribution of heat produced by the arcs, chemical homogenization and desulfurization of the steel by the slag. Alloying elements and/or slag components may be added through the addition hopper. If deep desulfurization is required active desulfurizing agents are injected into the melt through the injection lance or in form of cored wire. Besides refining operations Ladle Furnace (LF) may serve as a buffer station before Continuous casting. Benefits of Ladle Furnace (LF):
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Deep sulfur removal (desulfurization); Controllable reheating by electric power; Alloying; Temperature and chemical homogenizing; Non-metallic inclusions removal.

Ladle Furnace process is used for refining a wide variety of steels, in which degassing (hydrogen removal) is not required. to top Ladle desulfurization by injection of active agents Injection of desulfurizing agents (Ca, Mg, CaSi, CaC2, CaF2+CaO) to a molten steel is the most effective method of sulfur removal. Injection methods usually combine supply of a disperse desulfurizing agent (powder) with stirring by argon blowing. A ladle with deoxidized (killed) molten steel is transported to the injection stand where it is

placed under a cover, through which the injection lance may lower and immerse into the melt. Steel treated in the stand is covered by a layer of desulfurizing slag having high solubility of sulfur and capable to absorb sulfides formed as a result of active agents injection. Desulfurization agents are injected in argon stream. Argon bubbles produce stirring of the molten steel and the slag promoting desulfurization. Stirring also provides thermal and chemical homogenization of the melt. When the desulfurizing agents are injected into molten steel in form of a cored wire containing powder of desulfurizing agent stirring by argon bubbling from the porous plug mounted in the ladle bottom is used. Fumes formed during the operation are extracted through the cover. Injection of desulfurizing agents allows to achieve ultra-low concentrations of sulfur in steel (0.0002%). Benefits of Ladle desulfurization by injection of active agents:
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Deep sulfur removal (desulfurization); Temperature and chemical homogenizing; Non-metallic inclusions removal.

to top Ladle-to-mold degassing

Ladle-to-mold degassing is a vacuum degassing method, in which the mold is placed in a vacuum chamber. The molten steel is poured from a tundish attached to the cover of the chamber. The tundish is continuously filled with the melt poured from the ladle. The steel stream ―boils‖ when it is falling to the mold cavity in vacuum due to the deoxidation reaction [C] + [O] = {CO}. Hydrogen dissolved in steel diffuses into the CO bubbles and the gas is then evacuated by the vacuum pump. Intensity of the deoxidation and degassing during Ladle-to-mold pouring is indicated by the angle, at which the melt stream ―opens‖ as a result of CO bubbles formation. to top

Deoxidation of steel
Dr. Dmitri Kopeliovich The main sources of Oxygen in steel are as follows:
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Oxygen blowing (example: Basic Oxygen Furnace (BOF)); Oxidizing slags used in steel making processes(example: Electric-arc furnace; Atmospheric oxygen dissolving in liquid steel during pouring operation; Oxidizing refractories (lining of furnaces and ladles); Rusted and wet scrap.

Solubility of oxygen in molten steel is 0.23% at 3090°F (1700°C). However it decreases during cooling down and then drops sharply in Solidification reaching 0.003% in solid steel. Oxygen liberated from the solid solution oxidizes the steel components (C, Fe, alloying elements) forming gas pores (blowholes) and non-metallic inclusions entrapped within the ingot structure. Both blowholes and inclusions adversely affect the steel quality. In order to prevent oxidizing of steel components during solidification the oxygen content should be reduced. Deoxidation of steel is a steel making technological operation, in which concentration (activity) of oxygen dissolved in molten steel is reduced to a required level. There are three principal deoxidation methods:
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Deoxidation by metallic deoxidizers Deoxidation by vacuum Diffusion deoxidation.

Deoxidation by metallic deoxidizers This is the most popular deoxidation method. It uses elements forming strong and stable oxides. Manganese (Mn), silicone (Si), aluminum (Al), cerium (Ce), calcium (Ca) are commonly used as deoxidizers. Deoxidation by an element (D) may be presented by the reaction: n[D] + k[O] = (DnOk) The equilibrium constant KD-O of the reaction is: KD-O = aox/(aDn x aOk) or log KD-O = log aox - n*log aD - k*log aO

where: aox - activity of the oxide (DnOk) in the resulted non-metallic inclusion; aD - activity of the deoxidizer in liquid steel; aO - activity of oxygen in liquid steel. Thermodynamic activity of a solute in a solution is a parameter related to the solute concentration. Activity substitutes concentration in thermodynamic equations describing chemical reactions in non-ideal solutions (activities of solutes in a diluted solution are close to their concentrations). The equilibrium constant of a deoxidation reaction is determined by the steel temperature: log KD-O = AD/T - BD where: AD, BD - characteristic parameters determined for the particular deoxidizer D; T - steel temperature, °K The table presents parameters of the deoxidation reactions for some metallic oxidizers: Equilibrium constant at 1873 °K (2912°F, 1600°C) Manganese [Mn] + [O] = (MnO) 12440 5.33 1.318 Silicone [Si] + 2[O] = (SiO2) 30000 11.5 4.518 2[Al] + 3[O] = Aluminum 62780 20.5 13.018 (Al2O3) Deoxidizer Reaction A B

Values of the equilibrium constant parameters are used for calculation of equilibrium concentrations of oxygen and the deoxidizer by the equation: AD/T - BD = log aox - n*log aD - k*log aO In the simplest case aox=1, aD=[D], aO=[O], therefore: AD/T - BD = n*log [D] - k*log [O] According to the degree of deoxidation Carbon steels may be subdivided into three groups:




Killed steels - completely deoxidized steels, solidification of which does not cause formation of carbon monoxide (CO). Ingots and castings of killed steel have homogeneous structure and no gas porosity (blowholes). Semi-killed steels - incompletely deoxidized steels containing some amount of excess oxygen, which forms carbon monoxide during last stages of solidification.



Rimmed steels - partially deoxidized or non-deoxidized low carbon steels evolving sufficient amount of carbon monoxide during solidification. Ingots of rimmed steels are characterized by good surface quality and considerable quantity of blowholes.

Deoxidation in vacuum Method of deoxidation in vacuum utilizes carbon dissolved in steel as the deoxidizer according to the equation: [C] + [O] = {CO} where: [C] and [O] - carbon and oxygen dissolved in liquid steel; {CO} - gaseous carbon monoxide. The equilibrium constant of this chemical reaction is expressed as follows: KCO = pCO/(aC x aO) where: pCO - partial pressure of carbon monoxide in the atmosphere; aC and aO - activities of carbon and oxygen in liquid steel. Temperature dependence of KCO is insufficient. For approximate calculations the following equation may be used: [C]*[O] = 0.0025*pCO at 2948°F (1620°C) According to the above expressions the oxygen activity (concentration) is proportional to the partial pressure of carbon monoxide therefore decrease of the latter will cause reduction of the oxygen activity. Vacuum treatment of molten steel decreases the partial pressure of CO, which results in shifting equilibrium of the reaction of carbon oxidation. Bubbles of carbon monoxide form in the liquid steel, float up and then they are removed by the vacuum system. In addition to deoxidation vacuum treatment helps to remove Hydrogen dissolved in liquid steel. Hydrogen diffuses into the CO bubbles and the gas is then evacuated by the vacuum pump. Vacuum deoxidation is used mainly in Ladle refining. Steels deoxidized in vacuum are characterized by homogeneous structure, low content of nonmetallic inclusions and low gas porosity. Vacuum treatment is used for manufacturing large steel ingots, rails, ball bearings and other high quality steels. Diffusion deoxidation Oxygen dissolves in both steel and slag. Equilibrium between the two systems may be presented by the equation:

[O] = (O) The equilibrium constant of the reaction: KFeO = a[O]/a(O) or a[O] = KFeO*a(O) Thus reduction of the oxygen activity (concentration) in steel may be achieved by decreasing the oxygen activity in the slag. When the oxygen activity in the slag is reduced oxygen ions dissolved in steel begin to diffuse from the steel into the slag, and the equilibrium conditions are restored. In other words, deoxidation of slag results in deoxidation of the steel. Carbon (coke), silicone, aluminum and other elements are used for slag deoxidation. Since deoxidizers in the difusion method are not introduced directly into the steel melt, oxide non-metallic inclusions do not form. Diffusion deoxidation allows to produce steel less contaminated by non-metallic inclusions.

Desulfurization of steel
Dr. Dmitri Kopeliovich
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Sulfur in steel Desulfurization of steel by slags Desulfurization of steel by injection of active agents

Sulfur in steel Sulfur (S) may dissolve in liquid iron (Fe) at any concentration. However solubility of sulfur in solid iron is limited: 0.002% in α-iron at room temperature and 0.013% in γ-iron at 1832°F (1000°C). When a liquid steel cools down and solidifies the solubility of sulfur drops and it is liberated from the solution in form of iron sulfide (FeS) forming an eutectic with the surrounding iron. The eutectic is segregated at the iron grain boundaries. The eutectic temperature is relatively low - about 1810°F (988°C). Fe-FeS eutectic weakens the bonding between the grains and causes sharp drop of the steel properties (brittleness) at the temperatures of hot deformation (Rolling, Forging etc.). Brittleness of steel at hot metal forming operations due to the presence of low-melting iron sulfides segregated at grain boundaries is called hot shortness. In order to prevent formation of low-melting iron sulfide manganese (Mn) is added to steel to a content not less than 0.2%. Manganese actively reacts with iron sulfides during solidification of steel transforming FeS to

MnS according to the reaction: (FeS) + [Mn] = (MnS) + Fe (square brackets [ ] - signify concentration in steel, round brackets ( ) signify concentration in slag) The melting temperature of manganese sulfide is relatively high - about 2930°F (1610°C) therefore the steels containing manganese may be deformed in hot state (no hot shortness). Unfortunately MnS inclusions are:
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Brittle (less ductile than steel); They may have sharp edges; They are located between the steel grains.

All these factors determine negative influence of sulfide inclusions on the mechanical properties. Cracks may be initiated at brittle sharp edge inclusions. Sulfide inclusions especially arranged in a chain form also make easier the cracks propagation along the grain boundaries. The negative effect of sulfur on the steel properties becomes more significant in large ingots and castings, some zones of which are enriched by sulfur (macrosegregation of sulfur). The properties negatively affected by sulfur:
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Ductility; Impact toughness; corrosion resistance; Weldability.

to top Desulfurization of steel by slags The most popular method of desulfurization is removal of sulfur from molten steel to the basic reducing slag. Basic slag is a slag containing mainly basic oxides: CaO, MgO, MnO, FeO. A typical basic slag consists of 35-60% CaO + MgO, 10-25% FeO, 15-30% SiO2, 5-20% MnO. Transition of sulfur from steel to slag may be presented by the chemical equation: [S] + (CaO) = (CaS) + [O] The equilibrium constant KS1 of the reaction is: KS1 = a[O]*a(CaS)/a[S]*a(CaO)

Where: a[O], a[S] - activities of oxygen and sulfur in the liquid steel; a(CaS), a(CaO) - activities of CaS and CaO in the slag. The same reaction in ionic form: [S] + (O2-) = (S2-) + [O] The equilibrium constant KS2 of the reaction is: KS2 = a[O]*a(S2-)/a[S]*a(O2-) Where: a(S2-), a(O2-) - activities of S2- and O2- in the slag. Capability of a slag to remove sulfur from steel is characterized by the distribution coefficient of sulfur: LS = (S)/[S] Where: (S) - concentration of sulfur in slag; [S] - concentration of sulfur in steel; As appears from the above equations desulfurization is effective in deoxidized (low (O)) basic (high (CaO)) slags. Therefore ability of Basic Oxygen Process (BOP) to remove sulfur is low due to its highly oxidized slag. Desulfurization may be effectively conducted in the reducing slag stage of the steel making process in Electric-arc furnace. At this stage the oxidizing slag is removed and then lime flux is added to form basic slag with high CaO content. Deep desulfurization by slags may be achieved in ladle:
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The refining (desulfurizing) slag with high content of CaO and no FeO is prepared and placed in an empty ladle. The molten steel is poured into the ladle filled with the refining slag. Energy of the falling steel stream causes mixing the slag with the steel, during which sulfur is removed from the steel to slag phase.

Effect of desulfurization may be enhanced by additional stirring, for which electromagnetic (induction) stirrers or argon bubbling are used. to top Desulfurization of steel by injection of active agents

Injection of desulfurizing agents to a molten steel is the most effective method of sulfur removal. Injection methods usually combine supply of a disperse desulfurizing agent (powder) with stirring by argon blowing. Deep desulfurization by injection of active agents are achieved due to the following factors:
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High chemical activity of the desulfurization agents (Ca, Mg); High contact area between the steel and slag phases; Stirring providing good kinetic conditions of desulfurization; Presence of basic non-oxidized slag capable to absorb the products of the desulfurization reaction (CaS, MgS).

The following materials are used as desulfurizing agents:
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Slag mixtures CaO (50-90%) + CaF2 (10-20%) + A2lO3 (0-30%); CaSi; CaC2; CaC2 + Mg; Lime (CaO) + Mg; Ca + Al; Ca; Mg.

The desulfurizing agents are injected into molten steel either in form of powder transported by an argon blown to the steel through a lance or in form of a cored wire containing powder of desulfurizing agent. In the latter method stirring by argon bubbling from the porous plug mounted in the ladle bottom is used.

Chemical reactions between desulfurizing agents and sulfur dissolved in steel may be presented by the following equations: Ca + [S] = (CaS) Mg + [S] = (MgS) Injection of desulfurizing agents allows to achieve ultra-low concentrations of sulfur in steel (0.0002%).

Non-metallic inclusions in steel
Dr. Dmitri Kopeliovich Non-metallic inclusions are chemical compounds of metals (Fe, Mn, Al, Si, Ca) with nonmetals (O, S, C, H, N). Non-metallic inclusions form separate phases. The non-metallic phases containing more than one compound (eg. different oxides, oxide+sulfide) are called complex non-metallic inclusions (spinels, silicates, oxysulfides, carbonitrides). Despite of small content of non-metallic inclusions in steel (0.01-0.02%) they exert significant effect on the steel properties such as:
       

Tensile strength Deformability (ductility) Toughness Fatigue strength corrosion resistance Weldability Polishability Machinability

Depending on the source, from which non-metallic inclusion are derived, they are subdivided into two groups: indigenous and exogenous inclusions. Indigenous inclusions are formed in liquid, solidified or solid steel as a result of chemical reactions (deoxidation, desulfurization) between the elments dissolved in steel. Exogenous inclusions are derived from external sources such as furnace refractories, ladle lining, mold materials etc. Amount of exogenous inclusions and their influence on the steel properties are insufficient. Types of non-metallic inclusions


Oxides

FeO, Al2O3, SiO2, MnO, Cr2O3 etc. Al2O3*SiO2, Al2O3*FeO, Cr2O3*FeO, MgO*Al2O3, MnO*SiO2 etc.


Sulfides

FeS, MnS, CaS, MgS, Ce2S3 etc.


Oxysulfides

MnS*MnO, Al2O3*CaS, FeS*FeO etc.



Carbides

Fe3C, WC, Cr3C2, Mn3C, Fe3W3C etc.


Nitrides

TiN, AlN, VN, BN etc.


Carbonitrides

Titanium carbonitrides, vanadium carbonitrides, niobium carbonitrides etc.


Phosphides

Fe3P, Fe2P, Mn5P2 Formation of non-metallic inclusions There are three stages of inclusions formation: 1. Nucleation At this stage nuclei of new phase are formed as a result of supersaturation of the solution (liquid or solid steel) with the solutes (eg. Al and O) due to dissolution of the additives (deoxidation or desulfurization reagents) or cooling down of the metal. The nucleation process is determined by surface tension on the boundary inclusion-liquid steel. The less the surface tension, the lower supersaturation is required for formation of the new phase nuclei. The nucleation process is much easier in the presence of other phase (other inclusions) in the melt. In this case the new phase formation is determined by the wetting angle between a nucleus and the substrate inclusion. Wetting condition (low wetting angle) are favorable for the new phase nucleation. 2. Growth Growth of a separate inclusion continues until the chemical equilibrium is achieved (no supersaturation). Growth of inclusions in solid steel is very slow process therefore a certain level of nonequilibrium supersaturation may be retained (eg. martensite). 3. Coalescence and agglomeration Motion of the molten steel due to thermal convection or forced stirring causes collisions of the inclusions, which may result in their coalescence (merging of liquid inclusions) or agglomeration (merging of solid inclusions). The coalescence/agglomeration process is driven by the energetic benefit obtained from decrease of the boundary surface between the inclusion and the liquid steel. Inclusions with higher surface energy have higher chance to merge when collide. Large inclusions float up faster than the smaller ones. The floating inclusions are absorbed by the slag. The floating process may be intensified by moderate stirring. Vigorous stirring will result in

breaking the large inclusions into small droplets/fragments. Gas bubbles moving up through the molten steel also promote the inclusions floating and absorption by the slag. Blowing inert gas in Ladle Furnace (LF) or Vacuum ladle degassing of steel in result in obtaining cleaner steel. Morphology of non-metallic inclusions


Globular shape of inclusions is preferable since their effect on the mechanical properties of steel is moderate. Spherical shape of globular inclusions is a result of their formation in liquid state at low content of aluminum. Examples of globular inclusions are manganese sulfides and oxysulfides formed during solidification in the spaces between the dendrite arms, iron aluminates and silicates. Platelet shaped inclusions. Steels deoxidized by aluminum contain manganese sulfides and oxysulfides in form of thin films (platelets) located along the steel grain boundaries. Such inclusions are formed as a result of eutectic transformation during solidification. Platelet shaped inclusions are most undesirable. They considerably weaken the grain boundaries and exert adverse effect on the mechanical properties particularly in hot state (hot shortness). Dendrite shaped inclusions. Excessive amount of strong deoxidizer (aluminum) results in formation of dendrite shaped oxide and sulfide inclusions (separate and aggregated). These inclusions have melting point higher than that of steel. Sharp edges and corners of the dendrite shaped inclusions may cause local concentration of internal stress, which considerably decrease of ductility, toughness and fatigue strength of the steel part. Polyhedral inclusions. Morphology of dendrite shaped inclusions may be improved by addition (after deep deoxidation by aluminum) of small amounts of rare earth (Ce,La) or alkaline earth (Ca, Mg) elements. Due to their more globular shape polyhedral inclusions exert less effect on the steel properties than dendrite shape inclusions.







Distribution of non-metallic inclusions Besides of the shape of non-metallic inclusions their distribution throughout the steel grain structure is very important factor determining mechanical properties of the steel.


Homogeneous distribution of small inclusions is the most desirable type of distribution. In some steels microscopic carbides or nitrides homogeneously distributed in the steel are created by purpose in order to increase the steel strength. Location of inclusions along the grain boundaries is undesirable since this type of distribution weakens the metal. Clusters of inclusions are also unfavorable since they may result in local drop of mechanical properties such as toughness and fatigue strength.





Distribution of non-metallic inclusions may change as a result of metal forming (eg. Rolling). Ductile inclusions are deformed and elongated in the rolling direction. The less ductile inclusions are breaking forming chains of fragments. Elongated inclusions and chains result anisotropy of mechanical and other properties. Mechanical properties in transverse direction are lower than those parallel to the rolling direction.

Structure of killed steel ingot
Dr. Dmitri Kopeliovich

Typical ingot structure consists of five zones:


Zone of small equiaxed grains

The thin layer of small crysrtals forms when a melt comes to a contact with a wall of a cold metallic mold. The crystals (grains) have no favorable direction (equiaxed) and their chemical composition is close to that of the liquid steel. Heat liberated as a result of Crystallization depresses the nucleation and crystal growth.



Zone of columnar grains

Columnar grains start to grow when a stable and directed heat flow is formed as a result of heat transfer through the zone of small equiaxed grains. Direction of the columnar grains growth is oppsite to the direction of heat flow. Columnar grains continue to grow untill the heat flow decreases due to the following causes:
  

Large width of the solidified metal; Heating the mold wall; Formation of an air gap between the ingot and the mold wall. The air gap is a result of shrinkage caused by solidification.

When the temperature of the melt, adjacent to the solidification front, increases due to the liberation of the latent heat, constitutional undercooling will end and the columnar grains growth will stop.


Zone of large equiaxed grains

Low temperature gradient (low heat flow) and low cooling rate of the solute-enriched liquid in the cenral zone of the ingot result in formation of equiaxed grains. This process is slow due to slow heat extraction therefore the number of nuclei (seed crystals) is low and the grains size is large. Zone of large equiaxed grains is enriched by the impurities (sulfur, phosphorous, carbon).


Bottom cone

This cone-shaped zone is a mixture of small equiaxed garins grown as a result of the contact with a bottom of a cold metallic mold and crystals and crystals fragments, which sedimintate from other ingot zones. Bottom cone is characterized by negative segregation of the impurities.


Shinkage cavity zone

Shrinkage cavity is located in the top part of the ingot (which is later discarded) where last portion of liquid solidifies. The mold design should provide upwards direction of solidification at its last stage. Below the shrinkage cavity the zone of shrinkage porosity is located. This zone forms when the feeding of solidifying metal by the residual liquid is insufficient. Isolated pockets of liquid metal separated from the liquid pool by ―bridges‖ form their own shrinkage cavities (shrinkage pores). The mold shape, which is wider in upper levels and thermal isolation of the ―hot top‖ favor to diminish the shrinkage porosity.

Macrosegregation in steel ingots

Dr. Dmitri Kopeliovich Solidification of a liquid alloy is characterized by a phenomenon called segregation. Segregation is a separation of impurities and alloying elements in different regions of solidified alloy. Segregation is caused by the rejection of the solutes from a solidified alloy into the liquid phase. This rejection is a result of different solubility of impurities in liquid and solid phases at the equilibrium temperature. Partition of solutes between the dendrite arms and interdendritic liquid is called Microsegregation. Causes of macrosegregation Microsegregation is the basic cause of macrosegregation. Microsegregation results in formation of numerous small volumes of liquid enriched with the solute. The liquid is surrounded by the solid dendrites having low solute concentration (solute-poor). Transfer of the solute-rich liquid phase or solute-poor solid phase to large distances over the solidifying mass results in enrichment (positive macrosegregation) or impoverishment (negative macrosegregation) of particular zones of the ingot/casting with the solute. The macro movements of the liquid and solid phases may be caused by the following effects:


Solidification shrinkage

Shrinkage of solidifying alloy causes suction of the liquid phase through the interdendritic structure. The moving liquid feeds the shrinking metal.


Buoyancy forces

Liquid phase may move as a result of a gradient of the density caused by gradients of temperature and solute concentration.


Zone melting effect

A separate volume of the liquid phase having different temperatures at its opposite sides may ―move‖ in the direction of the higher temperature. The volume solidifies at the colder side and melts solid phase at the hotter side like in zone melting.


CO bubbles

CO bubbles form in solidifying steel as a result of either low content of metallic deoxidizers in rimmed steel or Ladle-to-mold degassing in vacuum. Floating gas bubbles cause movement of the solidifying alloy.


External forces

Metal movement may be caused by the energy of the stream poured into the mold from the ladle. Solidified metal may be also forced by electro-magnetic field, mechanical stirrers, vibration.


Sedimentation of solid phase

Solid crystals homogeneously nucleated in liquid phase and fragments of broken or melted off dendrites settle down (they are commonly heavier than liquid. Macrosegregation zones in steel ingot

Development of macrosegregation zones in a steel ingot and their locations are associated with the ingot grain structure.


Bottom negative segregation

Bottom negative segregation is a result of low solute concentration in the crystals formed in the early stage of solidification and comprising bottom cone. The bottom cone is a mixture of small equiaxed garins grown as a result of the contact with a bottom of a cold metallic mold and crystals and crystals fragments, which sedimintate from other ingot zones.


V-segregation

The central zone of ingot is enriched with solute rejected by the solidification front progressing from the mold wall to its center. The central zone consists of large equiaxed grains, which settle down to the V-shaped solidification front. The residual liquid surrounding the large equiaxed grains is solute-rich and it forms V-segregates when solidifies.


A-segregation

A-segregates (freckles) form in the Zone of columnar grains at the regions with structure characterized by the transition from the columnar grains to large equiaxed grains. A-segrgates present channels enriched by sulfur, carbon, phosphorus and other impurities.


Hot top segregation

Hot top segregation zone is located in the top central ingot region below the shrinkage cavity. Hot top segregation is formed at the final solidification stage from the residual liquid enriched by the solutes as a result of microsegregation (rejection by solidifying dendrites) followed by penetration of the liquid through the dendrite skeleton.

Fabrication of large steel ingots
Dr. Dmitri Kopeliovich Large steel ingots are required for manufacturing electric power plant turbine shafts, generator rotor shafts, nuclear pressure vessels, chemical pressure vessels, ship parts and other heavy machinery parts. Metalforming technology used for final shaping of large ingots is Forging. The largest ingot (570 metric tons) was produced in 1980 by ―Kawasaki Steel‖ (Japan). Solidification of a large mass of steel is characterized by significant development of micro and macro-defects of the ingot structure:
  

Non-metallic inclusions Macrosegregation Hydrogen in steel

The structure defects decrease the reliability of the part manufactured from the ingot. Since such parts work in equipment, failure of which is potentially catastrophic (nuclear equipment, electric power plants, chemical equipment, large scale machinery), Technology of large ingots fabrication should provide minimum degree of steel structure defects. Non-metallic inclusions Non-metallic inclusions in steel are chemical compounds of metals (Fe, Mn, Al, Si, Ca) with non-metals (O, S, C, H, N). Non-metallic inclusions form separate phases. The non-metallic phases containing more than one compound (eg. different oxides, oxide+sulfide) are called complex non-metallic inclusions (spinels, oxysulfides, carbonitrides). Despite small content of non-metallic inclusions in steel (0.01-0.02%) they exert significant effect on the steel properties such as:
       

Tensile strength Deformability (ductility) Toughness Fatigue strength corrosion resistance Weldability Polishability Machinability

The following parameters of non-metallic inclusions influence on the properties of parts made of large steel ingots:




Chemical composition (oxides, sulfides etc.) o Oxides o Sulfides o Oxysulfides o Carbides o Nitrides o Carbonitrides o Phosphides Inclusions size

Size of non-metallic inclusions is determined by the processes of nucleation, growth and coalescence/agglomeration. High surface energy causes the nucleation at higher supersaturation of the solutes (oxygen, sulfur, nitrogen, aluminum, silicon, titanium, vanadium, etc.) and favors coalescence and agglomeration of the inclusions.


Shape (morphology) of non-metallic inclusions



Globular inclusion form in liquid state at low concentration of aluminum. Globular shape inclusions exert moderate influence on the steel properties therefore globular morphology is preferable. o Platelet shaped inclusions form at the grain boundaries as a result of eutectic transformation during solidification. Platelet shaped inclusion exert adverse effect on the steel properties and therefore this morphology is undesirable. o Dendrite shaped inclusions form at high concentration of aluminum. Their shape is characterized by sharp edges, which may cause concentration of stresses during the ingot forging and decrease of ductility, toughness and fatigue strength of the steel part fabricated from the ingot. o Polyhedral inclusions are result of modification of the dendrite shaped inclusions by addition of strong deoxidizers and rare earth (Ce,La) or alkaline earth (Ca, Mg) elements. The effect of polyhedral inclusions on the steel properties is less than that of the dendrite shaped inclusions. Distribution (homogeneous, clusters, chains)

o

Homogeneous distribution of non-metallic inclusions is most desirable. Clusters of inclusions are unfavorable since they may result in local drop of mechanical properties such as toughness and fatigue strength.


Physical and mechanical properties (hardness, ductility, melting point)

Microscopic hard inclusions (carbides, nitrides) strengthen the metal however larger hard inclusions may cause drop of the steel ductility without increase of the strength and hardness. Ductile and brittle inclusions behave different during plastic deformation (steel Forging). Ductile inclusions elongate in the direction of deformation. Brittle inclusions break to fragments and form chains. Macrosegregation The following factors favors macrosegregation in large steel ingots:






Large absolute amount of solutes (sulfur, phosphorous, carbon). Steel is being enriched with the solutes rejected by the moving solidification front therefore at the final solidification stage the residual liquid contain large solute content. Low cooling rate of the metal in the central ingot zone. Low cooling rate results in larger spaces between the dendrite arms filled with the liquid metal enriched with the solutes. Partition of solutes between the dendrite arms and interdendritic liquid is called Microsegregation, which is the main cause of macrosegregation. Additionally solidification conditions at low cooling rate are closer to equilibrium resulting in decrease of Equilibrium Partition Coefficient and higher microsegregation. Large distances, which liquid metal and separate dendrite crystals and their fragments may be transferred. Transfer of liquid and solid phases in a solidifying ingot is the result of solidification shrinkage, buoyancy forces acting on the liquid metal, sedimentation of solid crystals homogeneously nucleated in liquid phase and fragments of broken or melted off dendrites.

Development of macrosegregation zones in a steel ingot and their locations are associated with the ingot grain structure.


Bottom negative segregation

Bottom negative segregation is a result of low solute concentration in the crystals formed in the early stage of solidification and comprising bottom cone. The bottom cone is a mixture of small equiaxed garins grown as a result of the contact with a bottom of a cold metallic mold and crystals and crystals fragments, which sedimintate from other ingot zones.


V-segregation

The central zone of ingot is enriched with solute rejected by the solidification front progressing from the mold wall to its center. The central zone consists of large equiaxed grains, which settle down to the V-shaped solidification front. The residual liquid surrounding the large equiaxed grains is solute-rich and it forms V-segregates when solidifies.


A-segregation

A-segregates (freckles) form in the Zone of columnar grains at the regions with structure characterized by the transition from the columnar grains to large equiaxed grains. A-segrgates present channels enriched by sulfur, carbon, phosphorus and other impurities.



Hot top segregation

Hot top segregation zone is located in the top central ingot region below the shrinkage cavity. Hot top segregation is formed at the final solidification stage from the residual liquid enriched by the solutes as a result of microsegregation (rejection by solidifying dendrites) followed by penetration of the liquid through the dendrite skeleton. Factors allowing to diminish macrosegregation in large steel ingots:


    

Lowering contents of impurities. Steel for large ingots is treated in the melting furnace and in Ladle refining stands in order to remove undesirable impurities such as sulfur (desulfurization]]), phosphorous (dephosphorization), hydrogen (degassing). Steel for large steel ingots commonly contains not more than 0.005% of sulfur, 0.005 of phosphorous and up to 2 ppm of hydrogen. Alloying of steel by alkaline (Ca, Mg) or rare earth (Ce,La) elements in amount of about 2*[S]. Low concentration of silicon in steel (about 0.1%). Using steel grades with lower carbon content. Modification of the ingot dimensions. If low level of A-segregation is required the ratio of the ingot height to its diameter should be as low as possible (about 1.0-1.2). Decrease of the pouring temperature. Lower pouring temperature results in increase of the cooling rate, which is favorable for depressing macrosegregation.

Hydrogen in steel Sources of hydrogen in liquid steel:
    

Damp scrap; Fluxes and alloying additives; Furnace and ladle refractories; Atmospheric humidity; Fuel (if used).

Hydrogen is easily dissolved in liquid steel in dissociated (atomic) state. Solubility of hydrogen in steel drops sharply during solidification resulting in formation of gaseous hydrogen form H2. In solid steel hydrogen is dissolved in form of interstitial solution. Carbon, nickel, chromium (up to 10%), vanadium, titanium, zirconium, columbium, tantalum increase the solubility of hydrogen in solid steel. Silicon, aluminum, tungsten, chromium (10% and higher) decrease the solubility of hydrogen in solid steel. Solubility of hydrogen in austenite is much higher than in ferrite. Both gaseous and dissolved forms of hydrogen exert adverse effect on mechanical properties of steels:



Hydrogen flakes

Solubility of hydrogen decrease during solidification and cooling down of steel ingot. Hydrogen atoms possessing high mobility are collected at internal voids such as non-metallic inclusions (sulfides, oxides) and their clusters, shrinkage pores, cracks caused by internal stresses. Hydrogen atoms collected at internal voids combine and form gaseous hydrogen H2, which may cause formation of cracks (flakes) when the gas pressure exceeds the steel strength. Hydrogen flakes is particularly dangerous for parts fabricated from large ingots. Vacuum ladle degassing methods allow to decrease the content of hydrogen to 2 ppm, which does not cause flaking formation.


Hydrogen embrittlement

Hydrogen in dissolved form also decreases steel properties such as ductility, Fracture Toughness and fatigue strength. Technology of large ingots fabrication The following tasks are accomplished by the technology of large ingots fabrication:
 

     

Dephosphorization (to about 0.005%) in a steel making furnace (melting furnace). Desulfurization of steel (to about 0.005%) in the melting furnace and then by a ladle refining desulfurization method (eg. Ladle desulfurization by injection of active agents, Ladle Furnace (LF)). Deoxidation of steel by metallic deoxidizers to the required concentrations of aluminum, silicon and other deoxidizing elements. Removal of non-metallic inclusions from the liquid steel in both melting furnace and Ladle refining stand. Vacuum treatment of steel in ladle by one of the degassing methods: Recirculation Degassing (RH), Ladle Degassing (VD). *Achievement of the required temperature of steel before pouring to the mold by using Ladle Furnace (LF). Final vacuum treatment of the steel in the Ladle-to-mold degassing operation. Providing non-intermittent supply of liquid steel to the pouring-to-mold operation.

The possible technological schemes of large steel ingots fabrication are presented in the figure:

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Electroslag Remelting
Dr. Dmitri Kopeliovich Electroslag Remelting (ESR) is a process of remelting a consumable electrode utilizing the heat generated by an electric current passing through a molten slag between the electrode and the solidifying ingot. ESR process description

The consumable electrods are as cast or forged cylindrical parts made of an alloy to be remelt. An electroslag remelting process (ESR) starts when the lower tip of a consumable electrode is immersed into a pool of molten slag. The premelted slag possessing electrical conductivity is located on the water-cooled mold base connected to a power supply. The electric current (commonly AC) passing through the the slag keeps it at high temperature, which is about 360ºF (200ºC) higher than the melting point of the remelted metal. The electrode tip is heated by the hot slag and starts to melt forming droplets of liquid metal, which disconnect from the electrode and sink through the slag layer.

The slag composition is based on calcium fluoride (CaF2), lime (CaO) and alumina (Al2O3). The slag composition provides the following properties:
    

Melting point lower than that of the remelted alloy; Required level ov viscosity; Required level of electrical conductivity; High solubility of sulfur; Capability to adsorb non-metallic inclusions.

The molten steel in form of both liquid film on the electrode tip and descending droplets contacts with the slag and get refined due to desulfurization and removal of non-metallic inclusions (sulfides and oxides). The droplets enters the molten steel pool, bottom of which is progressively solidifying. The water-cooled copper mold provides relatively high gradient of temperature resulting in high solidification rate. Solidification front is moving upwards (unidirectional solidification) forming sound homogeneous metal structure. The ingot has a good surface quality due to a thin slag film covering it. Advantages of Electroslag Remelting
       

Deep desulfurization; Refining non-metallic inclusions; Homogeneous Distribution of non-metallic inclusions Fine Grain structure; No Shrinkage defects; Low macrosegregation; Good surface quality; Controllable process.

Applications of Electroslag Remelting
 

    

Superalloys (nickel base alloys possessing high strength, Creep resistance and oxidation resistance at high temperatures. Large Forging ingots. Fabrication of large steel ingots requires technologies providing high purity steel with low level of macrosegregation and shrinkage porosity. The technology of electroslag remelting (ESR) meets these demands. Ingots up to 200 metric tons are produced by electroslag remelting utilizing simultaneous remelting of tens of electrodes. The ingots are used for manufacturing heavy parts for electric power generating units (turbine shafts, generator rotor shafts). Rolling mill rolls. Tool and die steels. Stainless steels. Titanium alloys for aerospace applications. Titanium scrap remelting. Titnanium machining turnings and foundry scrap may be recycled by ESR providing cost effective secondary titanium.

Electroslag Remelting under controllable atmosphere






Electroslag Remelting under vacuum. The main limitation of the conventional ESR process is difficulty of removal of hydrogen exerting adverse effect on mechanical properties of steels. Low content of Hydrogen in conventional ESR is obtained by control of hydrogen in the consumable electrode. Additionally Electroslag Remelting under atmospere may cause oxidation by Oxygen penetrating into the slag contacting the pool of liquid metal. Electroslag Remelting under vacuum provides both vacuum degassing and oxidation protection of the remelted metal. Electroslag Remelting under inert gas atmosphere. Inert gas protects the slag and the metal from oxidation and from pick-up of atmospheric hydrogen. The method is simpler and cheaper than ESR under vacuum but it does not allow hydrogen removal. Electroslag Remelting under increased pressure. The method is used for remelting nitride hardened steels (alloyed by Nitrogen). During ESR process such steels are continuously alloyed by nitrogen containing compounds. Increased pressure prevents removal of dissolved nitrogen from the liquid steel to the atmosphere.

Steel strip processing
Dr. Dmitri Kopeliovich Steel strips are commonly manufactured of continuously cast slabs. Coiled steel strips have the thickness (gauge) up to 0.2‖ (5 mm). The strips are supplied in different conditions:
   

Hot rolled Pickled Cold rolled Coated

Hot rolling Continuously cast steel slabs are processed in a hot Rolling line:
 



Heating in a gas/oil fired reheating furnace to the temperature 2200-2280ºF (12001250ºC). Removing oxide scale in a scale breaker. Iron oxides form on the slab surface as a result of high temperature oxidation in the reheating furnace. Scale breaker utilizes high pressure water jets directed to the slab surface. Rough rolling in either reversing or tandem roughing rolling mill. Rough rolling mills may have either two-high or four-high configuration. The strip temperature decreases in the rough rolling operation to about 1830ºF (1000ºC).

  

Finishing rolling in a tandem four-high rolling mill (5-7 stands). The strip dimensions (thickness, width) are strictly controlled in this operation. Cooling by water jets. The temperature is reduced to max. 1110ºF (600ºC). Coiling the finished strip. Hot rolled strips have a thin scale of iron oxides on their surface. Part of the strips is used in as-rolled condition. Other part is processed in a pickling line where the oxide scale is removed.

to top Pickling of steel strip Hot rolled strip is treated in a pickling line:
  

   

Uncoiling. Pickling in a solution of hydrochloric or sulfuric acid. The acid dissolutes the oxide scale from the strip surface according to the reaction: Fe2O3 + 6HCl = 2FeCl3 + 3H2O. Rinsing and passivation of the strip. At this stage the acid residuals are washed by water from the strip. Passivation film is formed on the strip surface providing corrosion protection. Drying. Edge trimming. Oiling. Recoiling.

to top Cold rolling of steel strip Cold rolled steel strips are used mainly in the Deep drawing process. The purposes of cold rolling processing are:
   

Further reduction of the strip thickness; Improvement of the surface finish; Improvement of the surface flatness; Achievement of a required level of work hardening.

Cold rolling processing includes the following stages:
 



Cold rolling in a tandem rolling mill. The rolling mill consists of 5-7 stands having fourhigh configuration (two large back-up rolls and two small work rolls). Annealing at a temperature 1100-1300ºF (600-700ºC) in a controlled reducing atmosphere (commonly a mixture of Hydrogen and Nitrogen) preventing oxidation of the steel surface. Annealing results in recrystallization of the steel Grain structure and in stress relief. There are two alternatives of annealing process: continuous annealing line and batch annealing furnace. In the continuous annealing line (see the picture below) steel strip passes annealing furnace at a controlled speed. In the batch process steel strip coils are stacked on top of each other in the bell type furnace. Batch annealing allows to achieve lower hardness and higher ductility of the steel than in alternative continuous annealing. Temper rolling (skin pass rolling) - final cold rolling operation with low thickness reduction conducted in order to impart to the steel required levels of hardness, evenness and surface finish. Four-high rolling mill is used for tempering.

Most of annealed low carbon steel strips are tempered since they are too soft (HV<110) in annealed condition. Bending and deep drawing operations of soft annealed steel may cause formation of kinks (cross breaks) and stretcher strains, which are the result of localized stretching of the strip at low cold deformation beyond the yield point. Light tempering of annealed strip (non-kinking temper, pinch pass) produces strip surface conditions, which do not cause formation of cross breaks and stretcher strains. Hardness of pinch passed steel is about 115 HV. Other temper conditions of steel strip are: eighth hard (105<HV<125), quarter hard (115<HV<130), half hard (130<HV<160), three quarter hard (150<HV<185).

Surface finish conditions of cold rolled steel strip:
  



Mirror. Superior luster mirror finish is produced by rolling between fine polished rolls. Mirror finish strips are used mainly for electroplating. Bright. Bright finish is produced by rolling between polished rolls. It is the common surface finish condition of cold rolled strip. Matt. Matt (dull) surface finish is produced by rolling between roughened rolls. Matt surface is suitable for enameling and painting. Matt finish strips are also used in deep drawing due to the ability of the rough surface to hold lubricant providing low friction between the strip and the drawing tools (punch, blank holder, die). Blued. Blued surface finish is produced by controlled heating and cooling of bright finish strip resulting in formation of thin blue oxide film on the steel surface.

Strip edges conditions:
   

Natural mill edges, which are result of cold rolling without any edge cutting operation. Natural mill edges may have micro-cracks due to non-uniform work hardening. Slit edges (shared edges), which are produced by rotary slitter. Slit edges are square with slight sharp burr. Dressed edges - slit edges, from which burr has been removed. Rolled edges, which are formed by edge rolling.

to top Steel strip coating The most popular techniques of continuous steel strip coating are hot dip galvanizing and Electroplating. Hot dip galvanizing process
 



Uncoiling. Cleaning. This operation commonly combines alkaline cleaning (degreasing) and mechanical cleaning by rotating cylindrical brushes. Oil and other contaminants are removed from the steel surface. Pickling. Degreased and rinsed strip enters a pickling bath where oxides and rust are dissolved by hydrochloric acid and removed according to the reactions:

Iron oxide reaction: Fe2O3 + 6HCl = 2FeCl3 + 3H2O Rust reaction: Fe2O3*nH2O + 6HCl = 2FeCl3 + (n+3)H2O
 

Zinc coating (hot galvanizing). Clean strip passes through a bath with molten zinc (Zn). Wiping excessive zinc by air knives.

  

Cooling. The coated strip is cooled by air and water. Levelling. Leveller produces smooth and flat strip surface. Recoiling.

Continuous electroplating process
 



Uncoiling. Cleaning. This operation commonly combines alkaline cleaning (degreasing) and mechanical cleaning by rotating cylindrical brushes. Oil and other contaminants are removed from the steel surface. Pickling. Degreased and rinsed strip enters a pickling bath where oxides and rust are dissolved by hydrochloric acid and removed according to the reactions:

Iron oxide reaction: Fe2O3 + 6HCl = 2FeCl3 + 3H2O Rust reaction: Fe2O3*nH2O + 6HCl = 2FeCl3 + (n+3)H2O




Electroplating. Clean strip passes through a series of vertical electroplating baths. The strip is connected to the DC power supply as a cathode (negative). The positively connected anodes are arranged in parallel to the strip opposite to its surface. The bath is filled with an electrolytic solution. Electrolyte and anodes compositions are determined by the the deposited material and the electroplating method (Tin alloy electroplating, Decorative chromium electroplating|chromium electroplating, zinc electroplating). Recoiling.

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