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Hall-Heroult Cell for the Production of Aluminium

Key Concepts

Pure alumina, Al₂O₃, is extracted from the mineral bauxite using hot concentrated sodium hydroxide solution in the Bayer Process.
Amphoteric alumina dissolves in the sodium hydroxide while other substances present such as iron oxides and silicates do not dissolve.
Alumina has a very high melting point, 2045^oC, so it is dissolved in cryolite, Na₃AlF₆, to lower the melting point to around 970^oC.
Aluminium is produced by the electrolysis of molten alumina in a Hall-Heroult cell.
A Hall-Heroult Cell is a carbon lined reaction vessel which acts as the cathode with carbon anodes dipped into the alumina-cryolite electrolyte.
The amount of aluminium produced can be calculated using Faraday's Laws of Electrolysis.

Hall-Heroult Cell

Anode: (positive electrode)
C_(s) + 2O^2-_(l) ---> CO_2(g) + 4e
Cathode: (negative electrode)
Al³⁺_(l) + 3e ---> Al_(l)

Overall Reaction:
2Al₂O_3(l) + 3C_(s) ---> 4Al_(l) + 3CO_2(g)

At the carbon anodes: solid carbon is oxidized to carbon dioxide gas.
Carbon anodes disintegrate over time and need to be replaced periodically.
Carbon dioxide gas is released into the atmosphere.
At the carbon cathode: aluminium ions are reduced to elemental aluminium.
More dense liquid aluminium sinks to the bottom of the vessel where it can be siphoned off periodically in a process called tapping.
Impurities which are stronger oxidants than Al³⁺ such as water, silica, and iron oxides must be removed before the electrolysis of alumina because these would be reduced instead of the Al³⁺.
The overall REDOX reaction requires a very large energy input, between 100,000 and 320,000 amps is required to produce aluminium and carbon dioxide from alumina and carbon.

Uses for Aluminium

cars, buses, trucks, trains, planes: body panels
buildings: roofing, wall-cladding, windows and doors
packaging: cans, bottle tops, aluminium foil, pharmaceutical packaging
electrical wire and cables

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