(i) (a) Cryolite lowers the melting point of alumina from 2050° to 950°C. This in turn increases the conductivity of electrolyte and saves on electricity, which is the main source of energy in the reduction of alumina.
(b) Sodium hydroxide dissolves the alumina (Al2O3 ), but not the impurities to form sodium aluminate. The impurities are filtered out and the sodium aluminate is treated with carbon dioxide to form pure aluminium hydroxide. Alumina is recovered from sodium hydroxide by strong heating.
(c) Graphite is used as cathode as it is a good conductor of electricity and is not attacked by the chemicals.
