Aluminium, production and refinement
Content
JU Gimnazija „Meša Selimović“ Tuzla
Chemistry essay on:
Aluminium, production and refinement
Student: Atić Armin
1
Content: 1. Basic characteristics of Aluminium 2. History of Aluminium:
Discovery of Aluminium First production of Aluminium; Woehler's process Electrolysis Bayer's process
Page: 3
3 4 5 5 6 9 10
3. Production of Aluminium
Aluminium ores Verneuil's process Eloxation-pasiviation
Chemistry essay on „Aluminium, production and refinement“-Atić Armin
2
Basic characteristics of Aluminium
Aluminium is a chemical element with the symbol Al, its atomic number is 13, and atomic mass is 26,9815386(13). It's a metal element with a silver-white glow and it's placed in the third group (former IIIB) of the Periodic system of the elements. Aluminium's melting point is 660 °C, and it's boiling point is 2519 °C. As a metal it is very reactive, but it's protected by a thin transparent layer of oxygen, which quickly forms when exposed to air. Aluminium and it's oxides are amfotermic. Characteristics of Aluminium:
1.
2. 3. High electricity conductivity (0,377 Ωμ-1 • cm-1 on 20ºC)
Small density (3,702 g/cm³), which makes it a light meta High heat conductivity (235 W • mK)
4. It's alloys are very hard 5. It hardly wears out and is corosion resistant 6. Easily shaped and formed 7. Highly represented: Earth core: O Si Al Fe
45,5% 25,7% 8,3% 6,2%
History of Aluminium
Discovery of Aluminium
Aluminium (lat. alumen) compared to many other materials, was unknown for for a very long time. In the year 1808. Aluminium was discovered and described by Sir Humphry Davy, and named „aluminum“. He tried to find a way for production of Aluminium, but he failed.
3
First production of Aluminium; Woehler's process
Hans Christian Ørsted was the first person to produce Aluminium. He did it through a reaction with Aluminium-chloride (AlCl3) and Potassiumamalgam, but he didn't get Aluminium in an entirely pure state. . Wohler used the same method in 1827., but he made Al2O3 to AlCl3 and used metallic potassium as a reductive agent. That way he got purer Aluminium. His historical revelation can be proven with an experiment: Put 1,2g of metallic potassium (in small pieces without the oxigen layer) and 1,33g of anhydride Aluminium-chloride in a clay pot. Put the pot on a tripod with a clay triangle and carefully heat with a Bunsen burner. Once the reaction starts, remove the burner immediately. In case of a fast reaction the pot can easily brake. Once the reaction ended, dissolve the solid materials (including the gotten powdered Aluminium) in a glass jar with water and filter it slowly using a suction pump and a Büchner funnel. After the filter gets dry, scrape off the gray powder and spread it with your fingertips over the Bunsen burner. Powdered Aluminium burns to Aluminium-oxide (Al2O3). As a proof that Aluminium is there, it's affinity to oxygen is used. This experiment must in all cases be performed in a laboratory with an air drain. Reaction display: a) Display: AlCl3 + 3K Al
powdered
+
3 KCl
b) Proof of Aluminium: 4Al
1854.
+
3O2
2Al2O3
R.W. Bunsen and Henri Etienne Sainte-Claire-Deville got pure Aluminium by electrolysis of Aluminium-sodium-chloride smelt. Sainte-Claire-Deville technically got Aluminium by reducting Aluminium-chloride with sodium. To actively produce Aluminium at the time was impossible because of insufficient energy sources in the time.
4
Electrolysis
After Bunsen and Sainte-Claire-Deville produced Aluminium by electrolysis, C.M. Hall and P.T.Herault created and described the process of electrolysis for producing Aluminium from a cryolite smelt using carbon cathodes and anodes.
Reaction: Cathode: 2 Al 3+ + 6 e- —> 2 Al Anode: 3 O2- —> 3O2 + 6 e3O2 + C (3- X) CO2 + (2X-3)CO Alltogether: 2Al2O3 4Al + 3O2
Bayer's process
Karl Josef Bayer developed a process that allowed production of large amounts of Aluminium (mass production). In the process bauxite is filtered to a pure bauxite (which contains anhydride Aluminiumhidroxide). First the iron oxides hematite (Fe2O3) and boehmite (Al2O3H2O) are removed from bauxite. The process is based on easy dissolution of Aluminium-hydroxide in ground up bauxite in sodium base (NaOH) and under 180 ºC and 7 bar of pressure creates sodiumaluminate.
5
Non-dissolved remains (red mud; mostly non-disolved iron compound) are removed. Galium is possible as a by-product. From the aluminate base during cooling pure Aluminium-hydroxide is produced. The filtrate is diluted with NaOH, the temperature is dropped to 78ºC, and the pressure is reduced to normal. By „vaccinating“ solid Aluminium-hydroxide as a crystallisation core, pure Aluminium-oxide is produced. Produced solid Aluminiumhydroxide is fried in rotating furnaces at a temperature of 1200ºC up to 1300ºC, forming Aluminium-oxide, called alumina.
From this pure bauxite by electrolysis of the smelt by the Hall-Herault process pure Aluminium is produced. Anode oxidation of Aluminium-De Saint Martin 1923. Technical use of anode oxidation using chromic acid
1911.
Production of Aluminium Aluminium ores
1) Bauxite is a natural material made from one or more Aluminium hydroxide minerals, plus a combination of various silicium-dioxide, iron oxide, titanium, aluminosilicates and other impurities. Aluminium hydroxide is found in bauxite together with gibbsite, boehmite and diaspore.
6
Bauxite is classified by it's intended use as an abrasive or in chemical, metal and other industries. World production of bauxite (85%) is used in producing Aluminium in the Bayer process. Supporting method in making Aluminium from bauxite is electrolysis of Aluminium in a liquid bath in natural or artificial cryolite (Na3AlF6), called the Hall-Heroult process. Bauxite was named by a village Les Baux de Provence in southern France where it was discovered in 1821. by a geologist Pierre Berthier. 2) Feldspar is a name for a group of very important petrogenic minerals, that make around 60% of Earth's core. It's name comes from German words „feld“ which means field and „spar“, a term for brightly coloured minerals with a smooth surface. Feldspar minerals are commonly brightly coloured, with a hardness of 6 on Mohs' scale. It's main chemical formula is x Al(Al,Si)3O8 where x can be natrium (Na) and/or Calcium (Ca) and/or Potassium (K). Feldspars can occur in intrusive and efusive magma rocks, which means they can crystalise while magma is in Earth's core, and when it comes out on the surface. They can also appear in a specific group of metamorf rocks, and in many sediment rocks. The Feldspar group is fivided in two subgroups: Alkali Feldspars and Plagioclase. The following minerals are in the Alkali Feldspars group:
•
sanidine -
(K,Na)Si3O8 • orthoclase KSi3O8 • microcline KSi3O8 Minerals of this subgroup are divided by their Crystal lattice arrangement, which is a result of the way of crystallisation and the temperature it is performed in. Sanidine is stabile in higher temperatures, and microcline in lower. That means that Sanidine is found in intrusive magma rocks, in the deep, where magma is warmer, and where there is more time for crystallisation. That makes it's Crystal lattice ordered. Microcline,
7
unlike Sanidine, is crystallised in lower temperatures, when the magma comes out on the surface, ie. is found in efusive magma rocks. Also, the temperature in which Orthoclase start crystallisation is higher that in microcline, and lower than in sanidine. That makes Orthoclase usually appear in form of large Fenocrystals (have a lot of time for crystallisation) in a porfiroid structure. Subgroup of Plagioclase represents and isomorphic series of Albite, which is a pure Sodium Alumosilicate, to Anorthite, a pure Calcium Alumosilicate. Other members of this isomorphic series have a certain percentage of Albite and Anorthite. Members of the Plagioclase group are:
• • • • • •
Albite - NaAlSi3O8 (0-10% of Anorthite component) Oligoclase (10-30% of Anorthite component) Andesine (30-50% of Anorthite component) Labradorite (50-70% of Anorthite component) Bytownite (70-90% of Anorthite component) Anorthite- CaAl2Si2O8 (90-100% of Anorthite component) Plagioclase are a very important group of petrogenic minerals, and enter the composition of almost all magma rocks (exception are the most basic ones). Albite enters the composition of acid magma rocks, Oligoclase and Andesine in the composition of intermediate, Labradorite, Bytownite and Anorthite in the composition of base magma rocks (in extremely rare cases Anorthite can be found in ultrabase rocks). 3) Mica (KAl2(OH,F)2[AlSi3O10]) is a name for a group of complex Hydro Sodium-Aluminium silicate minerals that slightly differ in their chemical composition. Examples are Biothite-(H2K)(Mg, Fe)3Al(SiO4)3, Lepidolite-K Li Al(OH, F)2Al(SiO4)3, Muscovite (the most common Mica)H2KAl3(SiO4)3, Flogopite-(H2K)(Mg, Fe)3Al(SiO4)3 and Vermiculite. They have a low expanding coefficient, high dielectric power, good electrical resistance, unique dielectric constant and capacitative stability, and they are known as best electrical and thermal isolators. They are used in equipment that must be highly resistant to high temperatures like in rockets, rocket engines etc. 4) We can find Aluminium in Corund, because Corund is a crystallised fom of Aluminium-oxyde (Al2O3). Transparent Corunds are gem stones. Red is Ruby, and all other are Saphire. On Mohs' hardness scale they have the amount 9,0, which makes them the second hardest thing on the
8
World. In 1837. Gaudin made first artificial Rubies by binding Aluminium on high temperatures with a small amount of chrome as a pigment. In 1847. Ebelman made a white saphire by binding Aluminium to Boronic acid ((H3BO3 or B(OH)3). Auguste Verneuil produced artificial ruby by binding BaF2 and Al2O3 with a small amount of chrome at temperatures of 2000ºC. 1903. Verneuil published that he can make artificial rubies at a commercial level by using his flame fusion process. 5) Cryolite (Na3AlF6) is a rare mineral exploited on a large mine in Ivigtute, Greenland, but was fully exploited in 1987. It was used as an Aluminium ore and in producing bauxite with electrolysis. Problems in separating Oxygen from aluminum were overcome by using Cryolite as oxyde materials separator. Pure Cryolite is melted at 1012ºC(1285 K), and can separate Aluminium-oxydes well enough to enable extraction of Aluminium by electrolysis. The process takes a lot of energy, but is much more efficient than by warming oxides themselves.
Verneuil's process
One of the most important factors for successfull crystallisation of artificial gemstones is starting with very pure starting material, with minimal pureness of 99,9995%. The starting material is thoroughly ground up and put in a box in Verneuil's furnace with an opening in the bottom which enables it to come out when the box vibrates. While the powder is released, oxygen enters the furnace and travels alongside the powder down the tube. The tube is within an even bigger tube which contains Hidrogen. On the spot where the smaller tube opens to the bigger tube, thrust with flames emerges with a temperature of min. 2000ºC inside the core. As the powder goes through the fire it melts to small pieces, which slowly form a „cone“ which top is positioned just close enough to the core to keep it liquid. That top forms the crystal. As more of those pieces fall to the top, a single crystal is created, and the rest slowly moves down which enables
9
crystallisation of the base of the crystal, while it's top is liquid. Verneuil has after he perfected the process summed up the most important info: temperature of the fire mustn't be higher that the one needed for fusion; always keep the melted product in the same part of the oxy-hydrogen fire and to lower the point of contact between the melted product and the base as much as possible.
Eloxation-pasivation of Aluminium
Eloxation (from eloxal, short from electrical oxidation of aluminium) is a method of protecting Aluminium by anode oxidation, turning the surface zone of the metal to oxide or hydroxide. It creates a layer of thickness from 5 to 25 micrometers which protects from corosion. Natural thickness of the oxide layers is a few nanometers. Pasivation means increasing resistance of Aluminium to outer influence (disables chemical reactions). The pasivation and depasivation phenomenon can be shown in the following experiment: Pasivation with HNO3 Three test tubes are filled with following solutions: 1) Nitric acid; concentration c=6 Mol/l 2) Hydrochloric acid; concentration c=6 Mol/l 3) Copper sulfate solution; concentration c=0,5 Mol/l Strips of Aluminium are held in Nitric acid for a few minutes and immediately transferred to the Copper sulfate solution. The look of Aluminium strips doesn't change, clearly there was no reaction. The reason is shown in the following reaction equations: 2Al + 2NO3- + 2H30+ Al2O3 + 2NO + H2O 2 NO + O2 2NO2 The Aluminium reacts with Nitric acid creating a small layer of Aluminium-oxide which protects metal from further attack of oxidising acids. No creation of Hydrogen is noticed. Attack of copper (Cu2+) ions stops the layer of oxides, but when dipping in Copper sulfate the removal of the copper cover can't be seen. After that the strips are held in Hydrochloric acid for a short time. After that hydrogen is developed which can be identified with a test for burst gasses. Later the Aluminium strips are put in Copper sulfate solution and the metal is immediately stripped with a Copper cover. Al2O3 + 6H3O+ + 6Cl- + 2aq. 2Al + 6 H3O+ + 6Cl- + 2aq. 2Al3+aq + 6Cl- + 9H2O 2Al3+aq + 3H2 + 6H2O + 6Cl-
10
2Al + 3Cu2+aq
2Al3+aq + 3Cu + aq
First the Aluminium oxide layer is removed on the Aluminium with creation of hexaqua complex. Then the Hydrochloric acid attac the free Aluminium with release of Hydrogen. Developing of the gas can't be seen. Due to the lack of the protective layer of oxide, the Copper (Cu 2+) ion can attack Aluminium. According to the group of elements, the more noble Copper dissolves on the Aluminium in form of a rusty red cover.
Sources:
Protokoll zum Lehramtsvortrag "Chemie des Aluminiums" - Ute Babbel Wikipedia- the free encyclopedia North Carolina Geological Survey (NCGS)-official data www.chemistrydaily.com Mineral Information Institute (MII)-official data
11
Chemistry essay on:
Aluminium, production and refinement
Student: Atić Armin
1
Content: 1. Basic characteristics of Aluminium 2. History of Aluminium:
Discovery of Aluminium First production of Aluminium; Woehler's process Electrolysis Bayer's process
Page: 3
3 4 5 5 6 9 10
3. Production of Aluminium
Aluminium ores Verneuil's process Eloxation-pasiviation
Chemistry essay on „Aluminium, production and refinement“-Atić Armin
2
Basic characteristics of Aluminium
Aluminium is a chemical element with the symbol Al, its atomic number is 13, and atomic mass is 26,9815386(13). It's a metal element with a silver-white glow and it's placed in the third group (former IIIB) of the Periodic system of the elements. Aluminium's melting point is 660 °C, and it's boiling point is 2519 °C. As a metal it is very reactive, but it's protected by a thin transparent layer of oxygen, which quickly forms when exposed to air. Aluminium and it's oxides are amfotermic. Characteristics of Aluminium:
1.
2. 3. High electricity conductivity (0,377 Ωμ-1 • cm-1 on 20ºC)
Small density (3,702 g/cm³), which makes it a light meta High heat conductivity (235 W • mK)
4. It's alloys are very hard 5. It hardly wears out and is corosion resistant 6. Easily shaped and formed 7. Highly represented: Earth core: O Si Al Fe
45,5% 25,7% 8,3% 6,2%
History of Aluminium
Discovery of Aluminium
Aluminium (lat. alumen) compared to many other materials, was unknown for for a very long time. In the year 1808. Aluminium was discovered and described by Sir Humphry Davy, and named „aluminum“. He tried to find a way for production of Aluminium, but he failed.
3
First production of Aluminium; Woehler's process
Hans Christian Ørsted was the first person to produce Aluminium. He did it through a reaction with Aluminium-chloride (AlCl3) and Potassiumamalgam, but he didn't get Aluminium in an entirely pure state. . Wohler used the same method in 1827., but he made Al2O3 to AlCl3 and used metallic potassium as a reductive agent. That way he got purer Aluminium. His historical revelation can be proven with an experiment: Put 1,2g of metallic potassium (in small pieces without the oxigen layer) and 1,33g of anhydride Aluminium-chloride in a clay pot. Put the pot on a tripod with a clay triangle and carefully heat with a Bunsen burner. Once the reaction starts, remove the burner immediately. In case of a fast reaction the pot can easily brake. Once the reaction ended, dissolve the solid materials (including the gotten powdered Aluminium) in a glass jar with water and filter it slowly using a suction pump and a Büchner funnel. After the filter gets dry, scrape off the gray powder and spread it with your fingertips over the Bunsen burner. Powdered Aluminium burns to Aluminium-oxide (Al2O3). As a proof that Aluminium is there, it's affinity to oxygen is used. This experiment must in all cases be performed in a laboratory with an air drain. Reaction display: a) Display: AlCl3 + 3K Al
powdered
+
3 KCl
b) Proof of Aluminium: 4Al
1854.
+
3O2
2Al2O3
R.W. Bunsen and Henri Etienne Sainte-Claire-Deville got pure Aluminium by electrolysis of Aluminium-sodium-chloride smelt. Sainte-Claire-Deville technically got Aluminium by reducting Aluminium-chloride with sodium. To actively produce Aluminium at the time was impossible because of insufficient energy sources in the time.
4
Electrolysis
After Bunsen and Sainte-Claire-Deville produced Aluminium by electrolysis, C.M. Hall and P.T.Herault created and described the process of electrolysis for producing Aluminium from a cryolite smelt using carbon cathodes and anodes.
Reaction: Cathode: 2 Al 3+ + 6 e- —> 2 Al Anode: 3 O2- —> 3O2 + 6 e3O2 + C (3- X) CO2 + (2X-3)CO Alltogether: 2Al2O3 4Al + 3O2
Bayer's process
Karl Josef Bayer developed a process that allowed production of large amounts of Aluminium (mass production). In the process bauxite is filtered to a pure bauxite (which contains anhydride Aluminiumhidroxide). First the iron oxides hematite (Fe2O3) and boehmite (Al2O3H2O) are removed from bauxite. The process is based on easy dissolution of Aluminium-hydroxide in ground up bauxite in sodium base (NaOH) and under 180 ºC and 7 bar of pressure creates sodiumaluminate.
5
Non-dissolved remains (red mud; mostly non-disolved iron compound) are removed. Galium is possible as a by-product. From the aluminate base during cooling pure Aluminium-hydroxide is produced. The filtrate is diluted with NaOH, the temperature is dropped to 78ºC, and the pressure is reduced to normal. By „vaccinating“ solid Aluminium-hydroxide as a crystallisation core, pure Aluminium-oxide is produced. Produced solid Aluminiumhydroxide is fried in rotating furnaces at a temperature of 1200ºC up to 1300ºC, forming Aluminium-oxide, called alumina.
From this pure bauxite by electrolysis of the smelt by the Hall-Herault process pure Aluminium is produced. Anode oxidation of Aluminium-De Saint Martin 1923. Technical use of anode oxidation using chromic acid
1911.
Production of Aluminium Aluminium ores
1) Bauxite is a natural material made from one or more Aluminium hydroxide minerals, plus a combination of various silicium-dioxide, iron oxide, titanium, aluminosilicates and other impurities. Aluminium hydroxide is found in bauxite together with gibbsite, boehmite and diaspore.
6
Bauxite is classified by it's intended use as an abrasive or in chemical, metal and other industries. World production of bauxite (85%) is used in producing Aluminium in the Bayer process. Supporting method in making Aluminium from bauxite is electrolysis of Aluminium in a liquid bath in natural or artificial cryolite (Na3AlF6), called the Hall-Heroult process. Bauxite was named by a village Les Baux de Provence in southern France where it was discovered in 1821. by a geologist Pierre Berthier. 2) Feldspar is a name for a group of very important petrogenic minerals, that make around 60% of Earth's core. It's name comes from German words „feld“ which means field and „spar“, a term for brightly coloured minerals with a smooth surface. Feldspar minerals are commonly brightly coloured, with a hardness of 6 on Mohs' scale. It's main chemical formula is x Al(Al,Si)3O8 where x can be natrium (Na) and/or Calcium (Ca) and/or Potassium (K). Feldspars can occur in intrusive and efusive magma rocks, which means they can crystalise while magma is in Earth's core, and when it comes out on the surface. They can also appear in a specific group of metamorf rocks, and in many sediment rocks. The Feldspar group is fivided in two subgroups: Alkali Feldspars and Plagioclase. The following minerals are in the Alkali Feldspars group:
•
sanidine -
(K,Na)Si3O8 • orthoclase KSi3O8 • microcline KSi3O8 Minerals of this subgroup are divided by their Crystal lattice arrangement, which is a result of the way of crystallisation and the temperature it is performed in. Sanidine is stabile in higher temperatures, and microcline in lower. That means that Sanidine is found in intrusive magma rocks, in the deep, where magma is warmer, and where there is more time for crystallisation. That makes it's Crystal lattice ordered. Microcline,
7
unlike Sanidine, is crystallised in lower temperatures, when the magma comes out on the surface, ie. is found in efusive magma rocks. Also, the temperature in which Orthoclase start crystallisation is higher that in microcline, and lower than in sanidine. That makes Orthoclase usually appear in form of large Fenocrystals (have a lot of time for crystallisation) in a porfiroid structure. Subgroup of Plagioclase represents and isomorphic series of Albite, which is a pure Sodium Alumosilicate, to Anorthite, a pure Calcium Alumosilicate. Other members of this isomorphic series have a certain percentage of Albite and Anorthite. Members of the Plagioclase group are:
• • • • • •
Albite - NaAlSi3O8 (0-10% of Anorthite component) Oligoclase (10-30% of Anorthite component) Andesine (30-50% of Anorthite component) Labradorite (50-70% of Anorthite component) Bytownite (70-90% of Anorthite component) Anorthite- CaAl2Si2O8 (90-100% of Anorthite component) Plagioclase are a very important group of petrogenic minerals, and enter the composition of almost all magma rocks (exception are the most basic ones). Albite enters the composition of acid magma rocks, Oligoclase and Andesine in the composition of intermediate, Labradorite, Bytownite and Anorthite in the composition of base magma rocks (in extremely rare cases Anorthite can be found in ultrabase rocks). 3) Mica (KAl2(OH,F)2[AlSi3O10]) is a name for a group of complex Hydro Sodium-Aluminium silicate minerals that slightly differ in their chemical composition. Examples are Biothite-(H2K)(Mg, Fe)3Al(SiO4)3, Lepidolite-K Li Al(OH, F)2Al(SiO4)3, Muscovite (the most common Mica)H2KAl3(SiO4)3, Flogopite-(H2K)(Mg, Fe)3Al(SiO4)3 and Vermiculite. They have a low expanding coefficient, high dielectric power, good electrical resistance, unique dielectric constant and capacitative stability, and they are known as best electrical and thermal isolators. They are used in equipment that must be highly resistant to high temperatures like in rockets, rocket engines etc. 4) We can find Aluminium in Corund, because Corund is a crystallised fom of Aluminium-oxyde (Al2O3). Transparent Corunds are gem stones. Red is Ruby, and all other are Saphire. On Mohs' hardness scale they have the amount 9,0, which makes them the second hardest thing on the
8
World. In 1837. Gaudin made first artificial Rubies by binding Aluminium on high temperatures with a small amount of chrome as a pigment. In 1847. Ebelman made a white saphire by binding Aluminium to Boronic acid ((H3BO3 or B(OH)3). Auguste Verneuil produced artificial ruby by binding BaF2 and Al2O3 with a small amount of chrome at temperatures of 2000ºC. 1903. Verneuil published that he can make artificial rubies at a commercial level by using his flame fusion process. 5) Cryolite (Na3AlF6) is a rare mineral exploited on a large mine in Ivigtute, Greenland, but was fully exploited in 1987. It was used as an Aluminium ore and in producing bauxite with electrolysis. Problems in separating Oxygen from aluminum were overcome by using Cryolite as oxyde materials separator. Pure Cryolite is melted at 1012ºC(1285 K), and can separate Aluminium-oxydes well enough to enable extraction of Aluminium by electrolysis. The process takes a lot of energy, but is much more efficient than by warming oxides themselves.
Verneuil's process
One of the most important factors for successfull crystallisation of artificial gemstones is starting with very pure starting material, with minimal pureness of 99,9995%. The starting material is thoroughly ground up and put in a box in Verneuil's furnace with an opening in the bottom which enables it to come out when the box vibrates. While the powder is released, oxygen enters the furnace and travels alongside the powder down the tube. The tube is within an even bigger tube which contains Hidrogen. On the spot where the smaller tube opens to the bigger tube, thrust with flames emerges with a temperature of min. 2000ºC inside the core. As the powder goes through the fire it melts to small pieces, which slowly form a „cone“ which top is positioned just close enough to the core to keep it liquid. That top forms the crystal. As more of those pieces fall to the top, a single crystal is created, and the rest slowly moves down which enables
9
crystallisation of the base of the crystal, while it's top is liquid. Verneuil has after he perfected the process summed up the most important info: temperature of the fire mustn't be higher that the one needed for fusion; always keep the melted product in the same part of the oxy-hydrogen fire and to lower the point of contact between the melted product and the base as much as possible.
Eloxation-pasivation of Aluminium
Eloxation (from eloxal, short from electrical oxidation of aluminium) is a method of protecting Aluminium by anode oxidation, turning the surface zone of the metal to oxide or hydroxide. It creates a layer of thickness from 5 to 25 micrometers which protects from corosion. Natural thickness of the oxide layers is a few nanometers. Pasivation means increasing resistance of Aluminium to outer influence (disables chemical reactions). The pasivation and depasivation phenomenon can be shown in the following experiment: Pasivation with HNO3 Three test tubes are filled with following solutions: 1) Nitric acid; concentration c=6 Mol/l 2) Hydrochloric acid; concentration c=6 Mol/l 3) Copper sulfate solution; concentration c=0,5 Mol/l Strips of Aluminium are held in Nitric acid for a few minutes and immediately transferred to the Copper sulfate solution. The look of Aluminium strips doesn't change, clearly there was no reaction. The reason is shown in the following reaction equations: 2Al + 2NO3- + 2H30+ Al2O3 + 2NO + H2O 2 NO + O2 2NO2 The Aluminium reacts with Nitric acid creating a small layer of Aluminium-oxide which protects metal from further attack of oxidising acids. No creation of Hydrogen is noticed. Attack of copper (Cu2+) ions stops the layer of oxides, but when dipping in Copper sulfate the removal of the copper cover can't be seen. After that the strips are held in Hydrochloric acid for a short time. After that hydrogen is developed which can be identified with a test for burst gasses. Later the Aluminium strips are put in Copper sulfate solution and the metal is immediately stripped with a Copper cover. Al2O3 + 6H3O+ + 6Cl- + 2aq. 2Al + 6 H3O+ + 6Cl- + 2aq. 2Al3+aq + 6Cl- + 9H2O 2Al3+aq + 3H2 + 6H2O + 6Cl-
10
2Al + 3Cu2+aq
2Al3+aq + 3Cu + aq
First the Aluminium oxide layer is removed on the Aluminium with creation of hexaqua complex. Then the Hydrochloric acid attac the free Aluminium with release of Hydrogen. Developing of the gas can't be seen. Due to the lack of the protective layer of oxide, the Copper (Cu 2+) ion can attack Aluminium. According to the group of elements, the more noble Copper dissolves on the Aluminium in form of a rusty red cover.
Sources:
Protokoll zum Lehramtsvortrag "Chemie des Aluminiums" - Ute Babbel Wikipedia- the free encyclopedia North Carolina Geological Survey (NCGS)-official data www.chemistrydaily.com Mineral Information Institute (MII)-official data
11
