Q.What is the utility of leaching in the extraction of Al metal ?

Leaching concentrates bauxite: hot NaOH dissolves out the amphoteric Al2O3 as sodium aluminate, leaving Fe2O3/SiO2 impurities behind; pure alumina is recovered and then reduced electrolytically. This is the Bayer process.

What leaching is:

• Leaching is a chemical method of ore concentration in which the ore is treated with a suitable reagent that dissolves the desired metal compound but not the impurities (or vice-versa).

Utility of leaching in the extraction of aluminium (purification of bauxite — the Bayer process):

1. The chief ore of aluminium, bauxite (Al2O3·xH2O), usually contains impurities such as iron oxide (Fe2O3), silica (SiO2) and titanium oxide.
2. The powdered ore is heated with hot, concentrated sodium hydroxide solution (around 473–523 K under pressure). Aluminium oxide is amphoteric, so it dissolves as soluble sodium aluminate: Al2O3 (s) + 2NaOH (aq) + 3H2O (l) → 2Na[Al(OH)4] (aq)
3. The impurities (Fe2O3, etc.) do not dissolve and are filtered off, giving a clear sodium aluminate solution.
4. The solution is neutralised (seeded with Al(OH)3 / by passing CO2) to precipitate pure aluminium hydroxide: 2Na[Al(OH)4] + CO2 → Al2O3·xH2O (hydrated) + Na2CO3 ... (Al(OH)3 precipitates) …