Discuss the Bayer's method for extraction of alumina from bauxite ore with chemical equation.

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Solution

Bayer's method for the extraction of Alumina:

$∙$ Conversion of impure bauxite to sodium aluminate;
Bauxite in the powdered form is added to conc. sodium hydroxide and then heated in a closed vessel at $433 K$ and $5 - 6$ bar pressure for $6 - 8$ hours.
The aluminium oxide in bauxite is converted to sodium aluminate which is soluble in water.
$A l_{2} O_{3} .2 H_{2} O \to 2 N a A l O_{2} + 3 H_{2} O$
$∙$ Conversion of sodium aluminate to aluminium hydroxide;
Iron oxide which is insoluble in sodium hydroxide is removed by filtration.
Silica forms sodium silicate which is soluble in water.
This sodium aluminate and sodium silicate are here in the filtrate.
Excess of water is added to it and continuously stirred so that hydrolysis of sodium aluminate occurs and precipitates of aluminium hydroxide are obtained and the sodium silicate remains as impurity in the solution.
Some precipitates of aluminium hydroxide are added from outside to make the reaction simple and fast.
$N a A l O_{2} + 2 H_{2} O \to N a O H + A l (O H)_{3}$
$∙$ Conversion of $A l (O H)_{3}$ to pure alumina;
The precipitates are washed repeatedly with water, dried and on heating at $1473 K$ give pure aluminium oxide (alumina).
$2 A l (O H)_{3} \to A l_{2} O_{3} + 3 H_{2} O$

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