Describe electrolytic reduction of alumina.
i) Aluminium is extracted from alumina by electrolysis of mixture of alumina (20%) , Cryolite (60%) & Calcium fluoride (20%). Addition of cryolite& fluorspar not only makes alumina a good conductor of electricity but also lowers melting point. ii)The molten electrolyte is covered with a layer of powder coke to prevent oxidation. Following reactions taking place during electrolysis are:
iii) The oxygen liberated at anode reacts with carbon of anode producing CO and CO2. Hence anode get consumed therefore it should be replaced time to time. iv) Aluminium set free at the cathode is removed periodically in the molten condition from bottom of the cell.