Q.What is the significance of leaching in the extraction of aluminium?

Bauxite is digested with hot concentrated NaOH under pressure; alumina reacts to give soluble sodium aluminate, $Al_2O_3 + 2NaOH + 3H_2O \rightarrow 2Na[Al(OH)_4]$, while $Fe_2O_3$ and other insoluble impurities are filtered off. $CO_2$ is then passed to precipitate pure hydrated alumina, which is calcined to give pure $Al_2O_3$ — the essential feed for the electrolytic Hall–Héroult step.

Step 1 – Why bauxite needs purification first

Bauxite ($Al_2O_3.2H_2O$) as mined is impure, usually contaminated with iron oxide ($Fe_2O_3$) and silica. Feeding this directly into the electrolytic cell would contaminate the aluminium and waste energy, so the alumina must first be concentrated/purified — this is done by leaching (Baeyer's/Hall's process).

Step 2 – The leaching (digestion) step

Crushed bauxite is digested with a concentrated NaOH solution at 473–523 K under 35–36 bar pressure. Alumina, being amphoteric, dissolves as soluble sodium aluminate:

Iron oxide and most other impurities are not amphoteric and remain undissolved as a solid residue ("red mud"), which is filtered off.

Step 3 – Recovering pure alumina

The clear sodium aluminate filtrate is neutralised by passing $CO_2$ gas, seeded with freshly precipitated $Al(OH)_3$ to induce precipitation of pure hydrated alumina:

This hydrated alumina is filtered, dried, and calcined at about 1470 K to give pure anhydrous $Al_2O_3$.

Step 4 – Significance

Leaching is therefore the key concentration/purification step that converts impure bauxite into the pure $Al_2O_3$ required as feedstock for the Hall–Héroult electrolytic reduction — without it, the aluminium extracted would be impure and the process inefficient.

Recognise that bauxite is impure and that leaching (Baeyer's process) is the chemical concentration step that dissolves only the amphoteric $Al_2O_3$ as sodium aluminate, separating it from insoluble impurities like $Fe_2O_3$.

• Confusing leaching with the electrolytic reduction step itself — leaching only purifies/concentrates the ore into pure alumina, it does not produce metallic aluminium.
• Forgetting that the aluminate solution must be treated with $CO_2$ (not just filtered) to precipitate hydrated alumina back out of solution.