State the function of (i) Sodium hydroxide (ii) Cryolite (iii) Fluorspar in the metallurgy of aluminium in Hall Herault’s process.

(i) Sodium hydroxide: Used to purify bauxite by dissolving alumina and separating impurities.

(ii) Addition of Cryolite & Fluorspar

1. Lowers the fusion point of the mixture i.e., mixture fuses around 950°C instead of 2050°C.
2. Enhances the mobility of the fused mixture by acting as a solvent for the electrolytic mixture.
3. Addition of Cryolite enhances the conductivity of the mixture since, pure alumina is almost a non-conductor of electricity.