Question

Explain electrolytic reduction of alumina with the help of diagram.

Answer 1

1. In this method electrolysis of molten mixture of alumina (melting point > 2000${}^0$ C) is done in a steel tank. The tank has a graphite lining on the inner side. This lining does the work of a cathode. A set of graphite rods dipped in the molten electrolyte works as anode. Cryolite (${\mathrm{Na}}_3{\mathrm{AlF}}_6$) and fluorspar (${\mathrm{CaF}}_2$ ) are added in the mixture to lower its melting point up to ${1000}^0$ C.
2. Aluminium is deposited on the cathode on passing electric current. Molten aluminium being heavier than the electrolyte, is collected at the bottom of the tank. It is taken out from there from time to time, Oxygen gas is liberated at the anode. The electrode reactions are as shown below. Anode reaction $2O^{2-}\to {O_2}^{+}4e^{-}$(Oxidation) Cathode reaction $A{l}^{3+}+3e^{-}\to Al\left(l\right)$(Reduction) The liberated oxygen reacts with the anodes to form carbon dioxide gas. The anodes have to be changed from time to time as they get oxidised during the electrolysis of alumina.