Metallurgy

Principle of Extraction of Metals
OCCURRENCE OF METALS:

The earth's crust is the biggest source of metals. Some soluble salts of the metals are also found in sea -water. Metals occur in
nature sometimes free but mostly in combined state. The natural mode of occurrence of a metals is largely dependent on its nature.
Those metals which are least reactive and have little or no affinity for oxygen, moisture and other chemical reagents occur in free
or metallic or native state i.e., in uncombined state. Most of the metals are reactive and hence are found in combined state i.e., as
compounds.
The natural substances in which the metals or their compounds occur in the earth are called minerals. The mineral has a definite
composition. It may be a single compound or a complex mixture. The minerals from which the metals can be conveniently and
economically extracted are known as ores. All the ores are minerals but all minerals cannot be ores. For example, both bauxite
(Al2O3. 2H2O) and clay (Al2O3.2SiO2.2H2O) are minerals of aluminium. It is bauxite which is used for extraction of aluminium and
not clay. Thus, bauxite is an ore of aluminium. Ores may be divided into four groups.

1. Native ores : These ores contain metals in free state, e.g., silver, gold, platinum, mercury, copper, etc. These are found
usually associated with rock or alluvial materials like clay, sand, etc. Sometimes lumps of pure metals are also found.
These are termed nuggets.

2. Oxidised ores : In these ores. Metals re present as their oxides or oxysalts such as carbonates, nitrates, sulphates,
phosphates, silicates, etc.
3. Halide ores : Metallic halides are very few in nature. Chlorides are most common.


2. METALLURGY:

The whole process of obtaining a pure metal from one of its ores is known as metallurgy.
In order to extract the metal from ores, several physical and chemical methods are used. The method used depends upon the nat ure
of the ore, the properties of the metal and the local conditions, Thus, it is not possible to have a universal method for the extraction
of all the metals from their ores. However, the metallurgy of a metal involves three main operations:

1. Concentration or dressing of Ore,

2. Reduction of Ore

3. Purification or refining of Ore


1. Concentration or dressing of ores: Ores usually contain soil, sand, stones and other useless silicates. These undesired
impurities present in ores are called Gangue or Matrix. The removal of these impurities from the ores is known as
concentration. Before the ore is subjected to concentration, it is crushed into small pieces in gyratory crushers. The
crushed ore is then grinded with the help of rollers or in the stamp mill to powder form.


Physical Methods

The following physical methods are generally employed for the concentration of the ores depending upon the nature of
the ore.

1. Gravity separation : The separation is based on the difference in the specific gravities of the gangue particles
and the ore particles. The powdered ore is agitated with water or washed with a running stream of water. The
heavy ore particles settle down while the lighter particles of sand, clay, etc., are washed away. For this either
Wilfley table is used. It a wooden table having slanting floor on which long wooden strips called riffles are
fixed.



























The powdered ore is suspended in a stream of water. The heavier ore particles collect behind the riffles and
the gangue particles are carried away with the stream of water. Hydraulic classifier is shown in this. Powdered
ore is dropped from the top of classifier and strong stream of water is introduced from the bottom. The lighter
gangue particles are carried away by the water while the heavier ore particles settle down. Generally, Oxide
and carbonate ores are concentrated by this method. For example, tin ore (cassiterite) and iron ore
(haematite) are concentrated by gravity method.






Electromagnetic Separation


 Electromagnetic Separation : When one component either the ore or impurity is magnetic in nature, this method can
be used for separation. A magnetic separator consisting a belt moving on two rollers is used. One of which is a strong
magnet. The powdered ore is dropped on the belt from one end (non-magnetic) and at the other end (magnetic), the
magnetic particles are attracted and fall nearer the roller while non-magnetic particles fall away from the roller. Ferro-
magnetic ores are concentrated by this method. For example, wolframite (FeWO4) - a magnetic ore, is separated from
the non-magnetic ore, cassiterite SnO2, by this method.





 Froth floatation process : This method is used for the concentration of sulphide ores. The method is based on the
preferential wetting properties with the frothing agent and water. The powdered ore is added to water containing pine oil
(frothing agent) and sodium ethyl xanthate (collecting agent). A vigorous stream of air is now passed through, which
thoroughly agitates the mixture and disposes the oil into colloid sized particles. As a result of this, the sulphide particles
of the ore stick to the oil droplets and rise to the surface in the form of froth supported by air bubbles. Water wets the
gangue particles which sink to the bottom. With this method, it is possible to concentrate the dense ores such as galena
and zinc blende.




Chemical methods

1. Calcination : It involves heating of the ore below its fusion temperature in absence of air. This step expels organic
matter and moisture from the ores. It can remove moisture from hydrated oxides or carbon dioxide from carbonates. For
example :




Al2O3· 2H2O Al2O3 + 2H2O

2Fe2O3· 3H2O 2Fe2O3 + 6H2O

CuCO3· Cu(OH)2 2CuO + CO2 + H2O



Calcination makes the ore porous. The step is generally performed in reverberatory furnace.

2. Roasting : It is also heating of the ore either alone or with some other material usually in presence of air below its
fusion temperature. roasting is generally done in a reverberatory furnace or in a blast furnace. In roasting definite
chemical changes like oxidation, chlorination, etc., take place while in calcination these occurs only expulsion of organic
matter, water, carbon dioxide etc., i.e., it does not involve any major chemical change.



The roasting process may be any one of the following types :





1. Oxidising roasting : This is very common type of roasting in metallurgy and is carried out to remove sulphur
and arsenic in the form of their volatile oxides such as SO2 and As2O3 respectively. The ores are
simultaneously converted into corresponding oxides. This type of roasting is generally applied in ores of lead,
zinc, nickel, copper, etc.
S + O2 SO2



4As + 3O2 2As2O3



2PbS + 3O2 2PbO + 2SO2



2CuS + 3O2 2Cu2O + 2SO2


2. Partial oxidising or sulphating roasting : This type of roasting is carried out at a temperature below the
melting point of the charge and air is admitted. Part of the sulphide ore is oxidised to sulphate and the rest is
converted into oxide. For example, the roasting of galena leads to the formation of a mixture of lead oxide and
lead sulphate.

2PbS + 3O2 2PbO + 2SO2



PbS + 2O2 PbSO4

3. Chlorinating roasting : This type of roasting is done in the case of silver ore. The ore, argentite is mixed
with common salt and the mixture is heated in the presence of air. The final product is the chloride of the
metal.

Ag2S + 2NaCl 2AgCl + Na2S













Leaching :


 Leaching : It involves the treatment of the ore with a suitable reagent as to make it soluble while impurities remain
insoluble. The ore is recovered from the solution by suitable chemical method. For example, bauxite ore contains ferric
oxide, titanium oxide and silica as impurities. When the powdered ore is digested with an aqueous solution of sodium
hydroxide at about 150°C under pressure, the alumina (Al2O3) dissolves forming soluble sodium meaaluminate while
ferric oxide (Fe2O3), TiO2 and silica remain as insoluble part.
Al2O3 + 2NaOH 2NaAlO2 + H2O



Pure alumina is recovered from the filtrate
NaAlO2 + 2H2O Al(OH)3 + NaOH

2Al(OH)3 Al2O3 + 3H2O



Gold and silver are also extracted from their native ores by Leaching (Mac-Arthur Forrest cyanide process). Both silver
and gold particles dissolves in dilute solution of sodium cyanide in presence of oxygen of the air forming complex
cyanides.





4Ag + 8NaCN + 2H
2
O + O
2


4NaAg(CN)
2
+ 4NaOH

Sod. argento cyanide

4Au + 8NaCN + 2H2O + O2 4NaAu(CN)2 + 4NaOH





Ag or Au is recovered from the solution by the addition of electropositive metal like zinc.
2NaAg(CN)2 + Zn Na2Zn(CN)4 + 2Ag

2NaAu(CN)2 + Zn Na2Zn(CN)4+ 2Au



2. Second operation––Reduction to free metal

Some of the methods commonly used to get free metal the concentrated ore are given below :

(a) Smelting : This involves the reduction of the ore to the molten metal at a high temperature. For the extraction of less
electropositive metals such as Pb, Zn, Fe, Sn, etc., powerful reducing agents such as C, H2, CO, water gas, Na, K, Mg,
Al may be used. Some examples are given below :
PbO + C Pb + CO

WO3 + 3H2 W + 3H2O

CuO + CO Cu + CO2

TiCl4 + 2Mg Ti + 2MgCl2

Cr2O3 + 2Al 2Cr + Al2O3



Out of these carbon reduction and aluminium reduction are important processes.
(i) Carbon reduction process : It is generally called as smelting. The oxides of the less electropositive metals are
reduced by strongly heating them with coal or coke.
PbO + C Pb + CO

PbO + CO Pb + CO2

Similarly,
Fe2O3 + 3C 2Fe + 3CO

Fe2O3 + 3CO 2Fe + 3CO2

The ores, even after concentration, contain some gangue. To remove gangue, certain substances are mixed with
concentrated ore which combine with gangue to form fusible material which is not soluble in molten metal. The
substances used are called fluxes and the fusible material formed during reduction process is called slag. Slag is usually
lighter and floats on the surface of the molten metal.
An acidic flux (e.g., silica, borax, etc.) is the chemical substance which removes the basic impurities.








SiO
2
+ CaO

CaSiO3

Acidic flux

Basic

impurity

(Gangue)
Slag
The basic flux (e.g., limestone, magnesite, ferric oxide, etc.) is the chemical substance which removes the acidic
impurities.









MgCO3 + SiO
2



MgSiO3 + CO
2


Basic flux

Acidic

impurity

(Gangue)
Slag
(ii) Reduction by aluminium (Goldschmidt aluminothermic process) : This process is employed in the case of those
metals which have very high melting points and are to be extracted from their oxides. Their reduct ion with carbon is not
satisfactory. A mixture of concentrated oxide ore and aluminium powder, commonly called as thermite, is taken in a
steel crucible placed in a bed of sand. The reaction is started by the use of an ignition mixture containing magnesium
powder and barium peroxide.
Cr2O3 + 2Al 2Cr + Al2O3

3Mn3O4 + 8Al 9Mn + 4Al2O3
Large amount of heat energy is released during reduction, which fuses both the alumina and the metal.

(b) Self reduction process : This process is also called autoreduction process or air reduction process. The sulphide ores
of less electropositive metals like Hg, Pb, Cu, etc., are heated in air as to convert part of the ore into oxide or sulphate
which then reacts with the remaining sulphide ore to give the metal and sulphur dioxide. No external reducing agent is























used in this process.
2HgS + 3O2 2HgO + 2SO2 Extraction of Hg

2HgO + HgS 3Hg + SO2 from cinnabar ore

2PbS + 3O2 2PbO + 2SO2 Extraction of lead

2PbO + PbS 3Pb + SO2 from galena ore
Electrolytic reduction :
Electrolytic reduction : The oxides of the highly electropositive metals like Na, K, Mg, Ca, Al, etc., cannot be reduced easily with
carbon at moderate temperatures. For reduction, a very high temperature is required at which the metal may combine with carbon
to form a carbide. These metals are thus extracted by the electrolysis of their oxides, hydroxides or chlorides in fused state.
Sometimes, a small amount of some other salt is added as to lower the fusion temperature or to increase the conductivity or both.
The metal is liberated at the cathode. Sodium is obtained by the electrolysis of fused mixture of NaCl and CaCl2 (Down's process)
or by electrolysis of fused sodium hydroxide (Castner's process).


(d) Hydrometallurgy :
This process is based on the fact that more electropositive metal can displace less electropositive metal from its salt solution. The
ore is teated with such chemical reagents which convert it into soluble compound. By the addition of more electropositive met al to
the filtrate, the metal present in the ore can be precipitated. The following two examples illustrate this process.
(i) Extraction of copper : Malachite ore is first roasted.
CuCO3· Cu(OH)2 2CuO + H2O + CO2
Copper oxide obtained is dissolved in sulphuric acid.










CuO + H
2
SO
4
CuSO
4
+ H
2
O
(Soluble)
To the solution of copper sulphate, scrap iron is added which precipitates copper.










CuSO
4
+ Fe Cu + FeSO4
ppt. Soluble
(ii) Extraction of silver : The ore is dissolved in sodium cyanide solution.
Ag2S + 4NaCN 2NaAg(CN)2 + Na2S

By the addition of zinc turnings, Ag is precipitated










2NaAg(CN)2 + Zn Na2Zn(CN)4 + 2Ag
Soluble ppt.

(e) Amalgamation process : This method is used for the extraction of noble metals like gold, silver, etc., from the native ores. The
finely powdered ore is brought in contact with mercury which combines with the particles of the metal present in the ore and form
amalgam. The metal is recovered from the amalgam by subjecting it to distillation, where the mercury distills over leaving behind
the metal.


3. Third operation-Refining or purification : The metals obtained by the application of above reduction methods from the
concentrated ores are usually impure. These impure metals may be associated with small amounts of (a) unchanged ore, (b) other
metals produced by the simultaneous reduction of their compounds originally present in the ore, (c) non-metals like silicon, carbon,
phosphorus, etc. (d) residual slag, flux, etc. The impure metal is thus subjected to some purifying processes known as refining in
order to remove the undesired impurities. The following refining processes may be applied depending upon the nature of the metal
under treatment and the nature of the impurities.
(a) Liquation process : This process is based on the difference in fusibility of the metal and impurities. When the impurities are
less fusible than the metal itself, this process is employed. The metal melts and flows down leaving behind the impurities on the
hearth. This method is used to purify the metals like Bi, Sn, Pb, Hg, etc.
(b) Distillation : This process is used for those metals which are easily volatile. The impure metal is heated in a retort and its
vapours are separately condensed in a receiver. The non-volatile impurities are left-behind in the retort. This is used for the
purification of Zn, Cd, Hg, etc.
(c) Pyrometallurgical oxidation process : This process is used when the impurities have a rgeater affinity for oxygen than the
metal itself. This method is usually employed for refining the metals like Fe, Cu, Ag, etc. The oxidation is done by various ways.
(i) Cupellation : The impure metal is heated in a cupel or oval shaped crucible made of bone ash or cement and a blast of air is
passed over the molten mass. The impurities get oxidised and removed with the blast of air. For example the impurity of lead
present in silver is removed by cupellation process.
(ii) Bessemerisation : The impure-metal is heated in a furnace and a blast of compressed air is blown through the molten mass.
The impurities get oxidised. For example, the molten pig iron is taken in a bassemer converter and compressed air is passed which
oxidises the impurities.






2Mn + O2 2MnO, Si + O2 SiO
2


2C + O2 2CO, MnO + SiO
2
MnSiO3
Slag


(iii) Poling : The impure metal containing oxides as impurity can be purified by this method. The molten impure metal is stirred
with green poles of wood. The green poles of wood release the hydrocarbon gases which reduce the oxide impurities. This method
is especially used in the purification of copper. (old method)
(d) Electrolytic refining of metals : Many of the metals such as copper, silver, gold, aluminium, lead, etc., are purified by this
method. This is perhaps the most important method. The impure metal is made anode while a thin sheet of pure metal acts as a
cathode. The electrolytic solution consists of generally an aqueous solution of a salt or a complex of the metal. On passing the
current, the pure metal is deposited on the cathode and equivalent amount of the metal gets dissolved from the anode. Thus, t he
metal is transferred from anode to cathode through solution. The soluble impurities pass into the solution while the insoluble one,
especially less electropositive impurities collect below the anode as anodic mud or anode sludge,. Some examples are given
below :
(i) Purification of copper





Impure metal—Anode; Thin sheets of copper—Cathode
Electrolyte—An aqueous solution of copper sulphate containing H
2
SO
4
.

A current of 1.3 volt is used. Anodic mud contains Ag, Au, Pt, Pd, etc., and impurities like Fe, Zn, Ni, etc., pass into the solution.
99.9% pure copper is obtained.

(ii) Purification of lead





Impure metal—Anode; Thin sheets of pure lead—Cathode
















Electrolyte—A solution of lead silico fluoride PbSiF6 containing 8 – 10% of
H
2<>/subSiF6

(e) Special methods

(i) Mond’s process : Nickel is purified by this method. Impure nickel is treated with carbon monoxide at 60–80°C when volatile
compound, nickel carbonyl, is formed. Nickel carbonyl decomposes at 180°C to form pure nickel and carbon monoxide which can
again be used.

(ii) Van-Arkel process : This methods is generally applied for obtaining ultrapure metals. The impure metal is converted into a
volatile compound while the impurities are not affected. The volatile compound is then decomposed electrically to get the pur e
metal.

Ti, Zr, Hf, Si, etc., have been refined by this method. The method is quite expensive.









Zone refining of Fractional crystallisation :
Zone refining of Fractional crystallisation : Elements such as Si, Ge, Ga, etc., Which are used as semi-conductors are refined by
this method. Highly pure metals are obtained. The method is based on the difference in solubility of impurities in molten and solid
state of the metal. A movable heater is fitted around a rod of the impure metal. The heater is slowly moved across the rod. The
metal melts at the point of heating and as the heater moves on from one end of the rod to the other end, the pure metal cryst allises
while the impurities pass on the adjacent melted zone.
Summary of the Extraction of Metals

Different metallurgical processes can be broadly divided into three main types.
(i) Pyrometallurgy : Extraction is done using heat energy. The metals like Cu, Fe, Zn, Pb, Sn, Ni, Cr, Hg, etc., whi ch are found in
nature in the form of oxides, carbonates, sulphides are extracted by this process.
(ii) Hydrometallurgy : Extraction of metals involving aqueous solution is known as hydrometallurgy. Silver, gold, etc., are
extracted by this process.
(iii) Electrometallurgy : Extraction of highly reactive metals such as Na, K, Ca, Mg, Al, etc., by carrying electrolysis of one of the
suitable compound in fused or molten state.
FURNACES
Furnace is a device in which high temperature is produced either by burning a fuel or by using electricity. Several types of furnaces
are used in the extraction of metals. The important ones are described below :
(i) Reverberatory furnace : This is the kind of a furnace in which fuel does not come in direct contact with the charge. The flames
are deflected from the roof of the furnace to the charge undergoing reaction. Thus, this furnace can be used for reduction as well as
for oxidation purposes.
The furnace consists of three main parts namely fire place, hearth and chimney. The fire place is built at one end of the furnace at
slightly lower level than that of the hearth. The roof is made slanting and connects with the chimney on the other end. The hot
gases from the fire place are reflected by the concave ceiling over the hearth. The furnace is surrounded on all sides by walls of fire
bricks. Air supply can be controlled by vents and direct blast.
The furnace is used for smelting (reduction) and roasting of the ores. The reduction is done by the use of some suitable reducing
agent. The furnace is used (a) for reducing the roasted tin stone (SnO2) to molten tin metal by the use


of

coke (b) for roasting the galena ore (PbS) as to convert
it into PbO and PbSO
4
by the use of air (c) for roasting of copper pyrites
(CuFeS
2
) as to convert it into Cu
2
O and FeO by means of air.


(ii) Blast furnace :
It is a huge chimney like structure which can be
between 25 and 60 meters in height and 5 to 10 metres
in diameter. It is constructed of steel plates and the inner
regions lined with fire-bricks. It has a double cup and
cone arrangement at the top for the introduction of
charge. Preheated air at a temperature of about 600°C is
injected into the furnace through a number of pipes
called tuyeres in the bottom of the furnace. It is
provided with two tap holes plugged with clay; molten metal is tapped from the lower one and molten slag from the other. The
temperature range in the furnace is from 1500°C at the bottom and 200–300°C at the top. Carbon and carbon monoxide reduce the
metallic oxides to the free metals.
Blast furnace is frequently used for the extraction of iron and copper from their ores. Slag formation plays an important rol e in the
blast furnace as it covers the melted metal and thus protects the metal from being reoxidised.

(iii) Muffle furnace :
This furnace is used when high temperature is required and the fuel and its products of combustion are not to be desired to come
into contact with the material to be heated. The muffle is a chamber made of refractory material. The muffle is surrounded by hot
flames and hot gases all around. In an electric muffle furnace, the closed chamber is surrounded by heating electric coils. Such a


















furnace is used for the extraction of zinc, for annealing and gold and silver assaying.
Electric furnaces :
Electric furnaces : In these furnaces, electrical energy is converted into heat energy. Such furnaces are largely used where cheap
power is available and very high temperatures are required and also for carrying electrolytic reduction. The electric furnaces are
generally of three types :

(a) Induction furnace (b) Arc furnace and (c) Resistance furnace.
(a) Induction furnace : In this furnace the charge lying on the furnace bed or in a crucible constitute the secondary coil of an
induction unit and the induced currents produced by making and breaking the primary circuit, heat up the material.
(b) Arc furnaces : Heat is generated by arcs and a temperature over 3000°C may be obtained. Carbon electrodes are used to carry
the current and an arc is struck between them and the charge. Arc furnaces are of two types :
(i) Direct-heat (ii) Indirect-heat.
In the direct heat arc furnaces, arc is used to heat up a gas in which the arc is burning and in indirect heat arc furnace, t he arc burns
above the charge i.e., the arc radiates heat towards the charge.
(c) Resistance furnaces : Heat is generated by the resistance in the electric circuit. In some cases the material forming the charge
may act as the resistance and in other cases, the body of furnace is made up of resistance material and t his material cause heating.
In some cases, rods of poorly conducting material are embedded into the charge which become intensely hot on passage of the
current.
REFRACTORY MATERIALS
The materials which can withstand very high temperatures without melting or becoming soft are known as refractory materials.
These are not affected by slags formed during the extraction of metals. These are used in the form of bricks for the internal linings
of furnaces. Refractory materials used are of three types:



















(i) Acid refractories

(ii) Basic refractories

(iii) Neutral refractories
Silica, quartz, silicious Lime, dolomite, magnesite, Graphite, chromite,
sand stones, etc. etc., are the examples. bone ash, etc., are the
are the examples. examples.















Silica (92% SiO2, 2.7% Al2O3) and quartz can tolerate temperatures upto about 1750°C, bauxite upto 1800°C, alumina upto
2000°C and magnesite, chromite, etc., upto 2200°C. Some carbides such as silicon carbide is used as refractory for special
purposes.
SINTERING
The conversion of small pieces of substance into large one. The substance is partially fused so that smaller pieces join together to
form larger one.
COLLECTORS
These compounds adsorb themselves on polar groups to grains of ores and thus derive them on the surface to pass on into the froth
eg. ethyl xanthate
ACTIVATORS AND DEPRESSANTS
These compounds activate or depress the floating property of one of the components of the ore and thus help in the sepration of
different minerals present in the same ore. CuSO4 is an example of activators, while sod and pot cyanides are the examples of
depressants eg. in galena (PbS) is usually associated with sphalerite (ZnS) and FeS22. Floatation is carried out in presence of
potassium ethyl xanthate (collector) and sod. Cyanide and alkali (depressants) the later compounds depress the floatation property
of ZnS and FeS22 particals and hence only PbS particals go into the froth when air is blown in. After the removal of galena the
procen is repeated by adding CuSO4 (activators) which activates the floating character of ZnS particals and thus time ZnS comes
with the froth. The acidification of remaining slurry leads to the floatation of FeS22.
Purification of lead
(ii) Purification of lead












Impure metal—Anode; Thin sheets of pure lead—Cathode
Electrolyte—A solution of lead silico fluoride PbSiF6 containing 8 – 10% of
H
2<>/subSiF6


(e) Special methods

(i) Mond’s process : Nickel is purified by this method. Impure nickel is treated with carbon monoxide at 60–80°C when volatile
compound, nickel carbonyl, is formed. Nickel carbonyl decomposes at 180°C to form pure nickel and carbon monoxide which can
again be used.

(ii) Van-Arkel process : This methods is generally applied for obtaining ultrapure metals. The impure metal is converted into a
volatile compound while the impurities are not affected. The volatile compound is then decomposed electrically to get the pur e
metal.

Ti, Zr, Hf, Si, etc., have been refined by this method. The method is quite expensive.


(iii) Zone refining of Fractional crystallisation : Elements such as Si, Ge, Ga, etc., Which are used as semi-conductors are
refined by this method. Highly pure metals are obtained. The method is based on the difference in solubility of impurities in molten
and solid state of the metal. A movable heater is fitted around a rod of the impure metal. The heater is slowly moved across the rod.
The metal melts at the point of heating and as the heater moves on from one end of the rod to the other end, the pure metal
crystallises while the impurities pass on the adjacent melted zone.
Summary of the Extraction of Metals














Different metallurgical processes can be broadly divided into three main types.











OCCURRENCE OF METALS
(i) Pyrometallurgy : Extraction is done using heat energy. The metals like Cu, Fe, Zn, Pb, Sn, Ni, Cr, Hg, etc., which are found in
nature in the form of oxides, carbonates, sulphides are extracted by this process.
(ii) Hydrometallurgy : Extraction of metals involving aqueous solution is known as hydrometallurgy. Silver, gold, etc., are
extracted by this process.
(iii) Electrometallurgy : Extraction of highly reactive metals such as Na, K, Ca, Mg, Al, etc., by carrying electrolysis of one of the
suitable compound in fused or molten state.
FURNACES
Furnace is a device in which high temperature is produced either by burning a fuel or by using electricity. Several types of furnaces
are used in the extraction of metals. The important ones are described below :
(i) Reverberatory furnace : This is the kind of a furnace in which fuel does not come in direct contact with the charge. The flames
are deflected from the roof of the furnace to the charge undergoing reaction. Thus, this furnace can be used for reduction as well as
for oxidation purposes.
The furnace consists of three main parts namely fire place, hearth and chimney. The fire place is built at one end of the furnace at
slightly lower level than that of the hearth. The roof is made slanting and connects with the chimney on the other end. The hot
gases from the fire place are reflected by the concave ceiling over the hearth. The furnace is surrounded on all sides by walls of fire
bricks. Air supply can be controlled by vents and direct blast.
The furnace is used for smelting (reduction) and roasting of the ores. The reduction is done by the use of some suitable reducing
agent. The furnace is used (a) for reducing the roasted tin stone (SnO2) to molten tin metal by the use





of
coke (b) for roasting the galena ore (PbS) as to convert
it into PbO and PbSO
4
by the use of air (c) for roasting of copper pyrites
(CuFeS
2
) as to convert it into Cu
2
O and FeO by means of air.


(ii) Blast furnace :
It is a huge chimney like structure which can be
between 25 and 60 meters in height and 5 to 10 metres in diameter. It is constructed of steel plates and the inner regions lined with
fire-bricks. It has a double cup and cone arrangement at the top for the introduction of charge. Preheated air at a temperature of
about 600°C is injected into the furnace through a number of pipes called tuyeres in the bottom of the furnace. It is provided with
two tap holes plugged with clay; molten metal is tapped from the lower one and molten slag from the other. The temperature range
in the furnace is from 1500°C at the bottom and 200–300°C at the top. Carbon and carbon monoxide reduce the metallic oxides to
the free metals.

Blast furnace is frequently used for the extraction of iron and copper from their ores. Slag formation plays an important rol e in the
blast furnace as it covers the melted metal and thus protects the metal from being reoxidised.


















(iii) Muffle furnace : This furnace is used when high temperature is required and the fuel
and its products of combustion are not to be desired to come into contact with the material to
be heated. The muffle is a chamber made of refractory material. The muffle is surrounded by
hot flames and hot gases all around. In an electric muffle furnace, the closed chamber is
surrounded by heating electric coils. Such a furnace is used for the extraction of zinc, for
annealing and gold and silver assaying.

(iv) Electric furnaces : In these furnaces, electrical energy is
converted into heat energy. Such furnaces are largely used where
cheap power is available and very high temperatures are required
and also for carrying electrolytic reduction. The electric furnaces are
generally of three types :
(a) Induction furnace (b) Arc furnace and (c) Resistance furnace.
(a) Induction furnace : In this furnace the charge lying on the
furnace bed or in a crucible constitute the secondary coil of an induction unit and the induced
currents produced by making and breaking the primary circuit, heat up the material.
(b) Arc furnaces : Heat is generated by arcs and a temperature over 3000°C may be
obtained. Carbon electrodes are used to carry the current and an arc is struck between them
and the charge. Arc furnaces are of two types :
(i) Direct-heat (ii) Indirect-heat.
In the direct heat arc furnaces, arc is used to heat up a gas in which the arc is burning and in
indirect heat arc furnace, the arc burns above the charge i.e., the arc radiates heat towards the
charge.
(c) Resistance furnaces : Heat is generated by the resistance in the electric circuit. In some
cases the material forming the charge may act as the resistance and in other cases, the body of
furnace is made up of resistance material and this material cause heating. In some cases, rods
of poorly conducting material are embedded into the charge which become intensely hot on
passage of the current.
REFRACTORY MATERIALS
The materials which can withstand very high temperatures without melting or becoming soft
are known as refractory materials. These are not affected by slags formed during the
extraction of metals. These are used in the form of bricks for the internal linings of furnaces.
Refractory materials used are of three types:
(i) Acid refractories (ii) Basic refractories (iii) Neutral refractories
Silica, quartz, silicious Lime, dolomite, magnesite, Graphite, chromite,
sand stones, etc. etc., are the examples. bone ash, etc., are the
are the examples. examples.
Silica (92% SiO
2
, 2.7% Al
2
O
3
) and quartz can tolerate temperatures upto about 1750°C,
bauxite upto 1800°C, alumina upto 2000°C and magnesite, chromite, etc., upto 2200°C.
Some carbides such as silicon carbide is used as refractory for special purposes.
SINTERING
The conversion of small pieces of substance into large one. The substance is partially fused
so that smaller pieces join together to form larger one.
COLLECTORS
These compounds adsorb themselves on polar groups to grains of ores and thus derive them
on the surface to pass on into the froth eg. ethyl xanthate
ACTIVATORS AND DEPRESSANTS
These compounds activate or depress the floating property of one of the components of the
ore and thus help in the sepration of different minerals present in the same ore. CuSO
4
is an
example of activators, while sod and pot cyanides are the examples of depressants eg. in
galena (PbS) is usually associated with sphalerite (ZnS) and FeS2
2
. Floatation is carried out
in presence of potassium ethyl xanthate (collector) and sod. Cyanide and alkali (depressants)
the later compounds depress the floatation property of ZnS and FeS2
2
particals and hence
only PbS particals go into the froth when air is blown in. After the removal of galena the
procen is repeated by adding CuSO
4
(activators) which activates the floating character of ZnS
particals and thus time ZnS comes with the froth. The acidification of remaining slurry leads
to the floatation of FeS2
2
.