Notes On Metallurgy of Aluminium - ICSE Class 10 Chemistry

Aluminium is the most abundant metal in the earth’s crust, where it is found in the form of hydrated oxides like bauxite. 

Important ores of Aluminium:
        •  Bauxite: Chemial formula is:  Al2O3.2H20
        •  Bauxite is the principal ore of aluminium. Bauxite is chemically known as hydrated aluminium oxide.
        •  Corundum: Chemial formula is: Al2O3. Corundum is the aluminium oxide.
        •  Cryolite: Chemial formula is: Na3AlF6Cryolite is sodium aluminium fluoride

The extraction of aluminium from bauxite involves three steps:
        •  The purification of bauxite using Bayer’s process.
        •  The electrolytic reduction of anhydrous Al2O3 by Hall and Herault’s process.
        •  The last step is the purification of impure aluminium by Hoope’s process  

In Baeyer’s process, impure bauxite is converted into pure alumina. The conversion process involves mainly three reactions:

  • Concentrated sodium hydroxide solution is heated with amphoteric bauxite at a temperature of 150 to 200° C under pressure to get sodium aluminate.
  • Sodium aluminate is then diluted with water at a temperature of 50 to 60° C. Thus, sodium aluminate gets hydrolysed to form solid aluminium hydroxide and sodium hydroxide.
  • Finally, dried aluminium hydroxide precipitate is ignited at a temperature of 1100° C to remove water molecules to get alumina.

Bauxite is also concentrated through Hall’s process, in which sodium aluminate is prepared by adding sodium carbonate to bauxite. When excess carbon dioxide is passed through sodium aluminate in the presence of water, it forms aluminium hydroxide along with sodium carbonate. When dried aluminium hydroxide is ignited, alumina is formed.

Alumina is a very stable oxide and aluminium shows a strong affinity to combine with oxygen. Therefore, reducing it with reducing agents is practically difficult. Hence, it is reduced by electrolytic reduction through Hall-Heroult’s process.

Hall-Heroult’s Process: 

In this process, alumina is heated with cryolite and fluorspar in an iron tank with a lining of carbon inside. Once the oxide melts, a carbon rod fixed with a copper clamp, which acts as the anode, is introduced into it. The carbon lining of the container behaves as the cathode. When current is passed, metallic aluminium is liberated at the cathode and oxygen at the anode. The aluminium so formed is collected at the bottom of the tank in molten stage, and is taken out through a small outlet.

The molten aluminium obtained after electrolysis is more than 99 per cent pure, and is refined further by Hoope’s process to make it 99.9 per cent pure.


At Cathode:
2Al+3 + 6e- → 2Al

At Anode:
6O-2 - 12e- →3O2



Hoope’s Process: 
In Hoope’s process, electro-positive aluminium is refined in a tank with three immiscible layers. The upper layer is pure molten aluminium with a carbon electrode immersed into it, which functions as the cathode. The middle layer is an electrolyte, which is a mixture of cryolite, barium fluoride, aluminium fluoride and calcium fluoride. The bottom layer is of impure aluminium. The tank has a lining of carbon, which acts as the anode.

When current is passed, aluminium ions from the electrolyte move towards the cathode, where they gain electrons to form pure aluminium. More aluminium ions from the impure aluminium in the bottom layer enter the middle layer to get electrons and get collected at the cathode. The bottom layer is replenished with impure aluminium at frequent intervals.
Impure aluminium is refined by Hoope’s process to produce 99.9 per cent pure aluminium.

Aluminium is used in:
        •  Manufacture of automobile components
        •  Construction process
        •  Manufacture of electric wires
        •  Packing medicines and pharmaceutical products
        •  Manufacture of soft drink cans and espresso coffee makers
        •  Manufacture of utensils

Summary

Aluminium is the most abundant metal in the earth’s crust, where it is found in the form of hydrated oxides like bauxite. 

Important ores of Aluminium:
        •  Bauxite: Chemial formula is:  Al2O3.2H20
        •  Bauxite is the principal ore of aluminium. Bauxite is chemically known as hydrated aluminium oxide.
        •  Corundum: Chemial formula is: Al2O3. Corundum is the aluminium oxide.
        •  Cryolite: Chemial formula is: Na3AlF6Cryolite is sodium aluminium fluoride

The extraction of aluminium from bauxite involves three steps:
        •  The purification of bauxite using Bayer’s process.
        •  The electrolytic reduction of anhydrous Al2O3 by Hall and Herault’s process.
        •  The last step is the purification of impure aluminium by Hoope’s process  

In Baeyer’s process, impure bauxite is converted into pure alumina. The conversion process involves mainly three reactions:

  • Concentrated sodium hydroxide solution is heated with amphoteric bauxite at a temperature of 150 to 200° C under pressure to get sodium aluminate.
  • Sodium aluminate is then diluted with water at a temperature of 50 to 60° C. Thus, sodium aluminate gets hydrolysed to form solid aluminium hydroxide and sodium hydroxide.
  • Finally, dried aluminium hydroxide precipitate is ignited at a temperature of 1100° C to remove water molecules to get alumina.

Bauxite is also concentrated through Hall’s process, in which sodium aluminate is prepared by adding sodium carbonate to bauxite. When excess carbon dioxide is passed through sodium aluminate in the presence of water, it forms aluminium hydroxide along with sodium carbonate. When dried aluminium hydroxide is ignited, alumina is formed.

Alumina is a very stable oxide and aluminium shows a strong affinity to combine with oxygen. Therefore, reducing it with reducing agents is practically difficult. Hence, it is reduced by electrolytic reduction through Hall-Heroult’s process.

Hall-Heroult’s Process: 

In this process, alumina is heated with cryolite and fluorspar in an iron tank with a lining of carbon inside. Once the oxide melts, a carbon rod fixed with a copper clamp, which acts as the anode, is introduced into it. The carbon lining of the container behaves as the cathode. When current is passed, metallic aluminium is liberated at the cathode and oxygen at the anode. The aluminium so formed is collected at the bottom of the tank in molten stage, and is taken out through a small outlet.

The molten aluminium obtained after electrolysis is more than 99 per cent pure, and is refined further by Hoope’s process to make it 99.9 per cent pure.


At Cathode:
2Al+3 + 6e- → 2Al

At Anode:
6O-2 - 12e- →3O2



Hoope’s Process: 
In Hoope’s process, electro-positive aluminium is refined in a tank with three immiscible layers. The upper layer is pure molten aluminium with a carbon electrode immersed into it, which functions as the cathode. The middle layer is an electrolyte, which is a mixture of cryolite, barium fluoride, aluminium fluoride and calcium fluoride. The bottom layer is of impure aluminium. The tank has a lining of carbon, which acts as the anode.

When current is passed, aluminium ions from the electrolyte move towards the cathode, where they gain electrons to form pure aluminium. More aluminium ions from the impure aluminium in the bottom layer enter the middle layer to get electrons and get collected at the cathode. The bottom layer is replenished with impure aluminium at frequent intervals.
Impure aluminium is refined by Hoope’s process to produce 99.9 per cent pure aluminium.

Aluminium is used in:
        •  Manufacture of automobile components
        •  Construction process
        •  Manufacture of electric wires
        •  Packing medicines and pharmaceutical products
        •  Manufacture of soft drink cans and espresso coffee makers
        •  Manufacture of utensils

References

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