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

(i) Function of Sodium hydroxide — Sodium hydroxide is added to bauxite ore during purification of bauxite. Bauxite is reacted with a conc. solution of Sodium hydroxide under pressure for 2 hours as a first step in obtaining Al₂O₃.

The impurities present in bauxite mainly Fe₂O₃ and SiO₂ remain unaffected with conc. NaOH as impurities are not amphoteric.

Bauxite, being amphoteric reacts with the base forming sodium salt [sodium aluminate] and water. Hence, impurities are separated out.

$\underset{\text{Amphoteric oxide}}{\underset{\text{[Impure bauxite]}}{\text{Al}$2\text{O}3.2\text{H}2\text{O }}} + \underset{\text{Base}}{\underset{\text{[conc. soln]}}{2\text{NaOH }}} \xrightarrow{150 - 200 \degree\text{C}} \underset{\text{Salt}}{\underset{\text{[sodium aluminate]}}{2\text{NaAlO}2}} + \underset{\text{Water}}{ \text{3H}_2\text{O}}

(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.