Question

For the substance given below, describe the role played in the extraction of aluminium.

Sodium hydroxide

Answer 1

In the extraction of aluminium, sodium hydroxide plays two roles. First, finely grinded bauxite (ore of aluminium) is heated under pressure with conc. Caustic soda solution (NaOH solution) for 2-8 hours at 140°C to 150°C to produce sodium aluminate. The chemical equation is as follows:

\[\ce{Al2O3.2H2O + 2NaOH → 2NaAlO2 + 3H2O}\]

Second, when the sodium aluminate is diluted with water and cooled to 50°C, the sodium aluminate is hydrolysed to produce aluminium hydroxide as the precipitate. Here, the impurities dissolve in sodium hydroxide.