Model Paper 2 — Question 5

(a) Caustic soda is used in the Bayer purification of bauxite. Bauxite is heated under pressure with conc. caustic soda (NaOH) for 2 to 8 hours. Hot, concentrated NaOH dissolves the amphoteric alumina present in the ore, forming soluble sodium aluminate:

$\underset{\text{Amphoteric oxide}}{\underset{\text{[Impure bauxite]}}{\text{Al}_2\text{O}_3.2\text{H}_2\text{O }}} + \underset{\text{Base}}{\underset{\text{[conc. soln]}}{2\text{NaOH }}} \xrightarrow{150 - 200 \degree\text{C}} \underset{\text{Salt}}{\underset{\text{[sodium aluminate]}}{2\text{NaAlO}_2}} + \underset{\text{Water}}{ \text{3H}_2\text{O}}$

The insoluble impurities mainly Fe₂O₃ and SiO₂ are left behind and are referred to as Red mud.

(b) Fluorspar is added to the electrolyte mixture as it lowers the fusion point of the mixture i.e., the mixture fuses around 950°C instead of 2050°C.