Name the principal ore of aluminium. Explain the significance of leaching in the extraction of aluminium.

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Solution

Bauxite is the principal ore of Al.
The leaching of Bauxite helps in the removal of impurities of silica $(S i O_{2})$ , iron oxide $(F e_{2} O_{3})$ and titanium dioxide $(T i O_{2})$ from Bauxite. The powdered ore is heated with conc. NaOH solutions (45%) at 473 - 523 K. $A l_{2} O_{3}$ dissolves to form sodium meta-aluminate and silica dissolves to form sodium silicate leaving behind $F e_{2} O_{3}, T i O_{2}$ etc. which are filtered off. The filtrate is neutralised by passing $C O_{2}$ when hydrated alumina separates out while sodium silicate remains in the solution.
$A l_{2} O_{3} (s) Alumina + 2 N a O H (a q) + 3 H_{2} O (l) 473 - 523 K - ----- \to 2 N a [A l (O H)_{4}] (a q .) Sodium meta-aluminate$
$S i O_{2} (l) + 2 N a O H (a q) 473 - 523 K - ----- \to N a_{2} S i O_{3} (a q) Sodium silicate + H_{2} O (l)$
$2 N a [A l (O H)_{4}] (a q) + 2 C O_{2} (g) \to A l_{2} O_{3} . x H_{2} O (s) ↓ + 2 N a H C O_{3} (a q)$
Hydrated alumina is filtered, dried and heated to give back pure $A l_{2} O_{3} .$
$A l_{2} O_{3} . x H_{2} O (s) 1473 K - --- \to A l_{2} O_{3} (s) + x H_{2} O (s)$

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

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