Question

For each of the substances listed below, explain its role in the extraction of aluminium:
(a) bauxite
(b) sodium hydroxide
(c) cryolite
(d) graphite

Answer 1

(a) Bauxite is the main ore of aluminium. It is a hydrated aluminium oxide (${\mathrm{Al}}_2{\mathrm{O}}_3.x{\mathrm{H}}_2\mathrm{O}$).

(b) A sodium hydroxide solution is used for the purification of bauxite ore. The powdered impure bauxite is dissolved in the sodium hydroxide solution and is heated under pressure to form sodium aluminate.

(c) Cryolite is a sodium aluminium fluoride and its chemical formula is ${\mathrm{Na}}_3{\mathrm{AlF}}_6$. Cryolite acts as a solvent in the electrolytic mixture. It helps dissolve the fused pure alumina and allows the mixture to melt at ${950}^{\mathrm{o}}\mathrm{C}$ instead of ${2050}^{\mathrm{o}}\mathrm{C}$ (melting point of alumina). At this temperature, a molten alumina mixture is obtained that can conduct electricity more easily than fused alumina.

(d) Graphite (carbon) is taken as the electrode in the electrolytic extraction of aluminium from pure alumina. The inner gas carbon layer acts as the cathode and the thick carbon rods dipped in the fused electrolyte constitute the anode.