Explain the extraction of aluminium from purified alumina by Hall-Heroult process.

Explanation

Aluminium is extracted by electrometallurgy. Pure bauxite is a bad conductor of electricity so it dissolved in cryolite (`Na_(3) AlF_6`) which acts as an electrolyte. Little C`aF_(2) ` (fluorspar) is added to lower the melting point of the mixture and to increase conductivity. A mixture of purified bauxite, molten cryolite and `CaF_(2)`, is taken in a steel vessel coated with carbon lining which acts as cathode. A number of graphite rods acts as anode. On passing electricity aluminium is obtained at the cathode and oxygen is liberated at the anode. The oxygen liberated reacts with carbon of the anode to form carbon monoxide and carbon dioxide. So for each kg of aluminium produced about 0.5 kg of anode is burnt off hence extraction of aluminium is costlier,
The electrode reactions are
At anode : (oxidation - loss of electrons )`C(s) + O^(-2) to CO(g) + 2e.` (melt)
`C(s) +2 O^(-2)`(melt)` to CO_(2) (g) + 4e^(-)`
At cathode : (Reduction - gain of electrons `Al^(3+) + 3^(e -)to AI(l)`
The molten aluminium obtained is removed through the outlet.