How can you separate alumina from silica in a bauxite ore associated with silica? Give equations, if any.

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Solution

Bauxite ore containing silica is heated with conc. $N a O H$ solution at $473 - 523$ K and $35 - 36$ bar pressure. Alumina forms sodium aluminate and silica forms sodium silicate. Impurities are left behind.
$A l_{2} O_{3} + 2 N a O H + 3 H_{2} O \to 2 N a [A l (O H)_{4}]$
Carbon dioxide neutralizes the aluminate to precipitate hydrated alumina.
$2 N a [A l (O H)_{4}] + 2 C O_{2} \to A l_{2} O_{3} . x H_{2} O + 2 N a H C O_{3}$
Sodium silicate remains in the solution and hydrated alumina is filtered and dried. On heating, hydrated alumina gives pure alumina.
$A l_{2} O_{3} . x H_{2} O 1473 K - --- \to A l_{2} O_{3} (s) + x H_{2} O$

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