••• Write chemical equations for the reactions involved in obtaining pure alumina from the mineral bauxite which has impurities of iron oxide and silica.... ??
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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.
Al2O3.2H2O→2NaAlO2+3H2O
∙ 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.
NaAlO2+2H2O→NaOH+Al(OH)3
∙ Conversion of Al(OH)3 to pure alumina;
The precipitates are washed repeatedly with water, dried and on heating at 1473 K give pure aluminium oxide (alumina).
2Al(OH)3→Al2O3+3H2O