Q.Leaching is a process of concentration of ores. Explain the leaching of alumina from bauxite. (Scores : 3)

Leaching (Baeyer's process) dissolves alumina out of bauxite as sodium aluminate using hot NaOH, leaving the impurities behind as an insoluble residue; CO2 is then used to precipitate pure hydrated alumina back out.

Leaching of alumina from bauxite

Bauxite ore (Al2O3.2H2O) is usually contaminated with iron oxide (Fe2O3), silica (SiO2) and titanium oxide (TiO2). It is concentrated by the chemical method of leaching, which exploits the fact that Al2O3 is amphoteric (dissolves in both acid and alkali) while Fe2O3 is not.

Steps:

1. Powdered bauxite is digested with a concentrated NaOH solution at about 473-523 K and 35-36 bar pressure. The amphoteric Al2O3 dissolves as soluble sodium aluminate (sodium meta-aluminate): Al2O3(s) + 2NaOH(aq) + 3H2O(l) -> 2NaAl(OH)4
2. The impurities Fe2O3, TiO2 etc. do not dissolve in NaOH and remain as an insoluble residue called red mud, which is filtered off.
3. The clear filtrate (sodium aluminate solution) is neutralised by passing CO2 gas through it. This hydrolyses the aluminate and precipitates out hydrated alumina, while the by-product sodium carbonate/bicarbonate stays in solution: 2NaAl(OH)4 + CO2(g) -> Al2O3.xH2O(s) + 2NaHCO3(aq) …