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ICSE Class 10 Chemistry Question 1 of 64

Metallurgy — Question 15

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Question 6

In the extraction of aluminium from bauxite, the first step is the dressing of the ore by Baeyer's process. Give balanced equations for the conversion of impure bauxite to pure alumina using a concentrated solution of NaOH.

Answer

Conversion of impure bauxite to sodium aluminate:

Al2O3.2H2O [Impure bauxite]Amphoteric oxide+2NaOH [conc. soln]Base→150−200°C2NaAlO2[sodium aluminate]Salt+3H2OWater\underset{\text{Amphoteric oxide}}{\underset{\text{[Impure bauxite]}}{\text{Al}_2\text{O}_3.2\text{H}_2\text{O }}} + \underset{\text{Base}}{\underset{\text{[conc. soln]}}{2\text{NaOH }}} \xrightarrow{150 - 200 \degree\text{C}} \underset{\text{Salt}}{\underset{\text{[sodium aluminate]}}{2\text{NaAlO}_2}} + \underset{\text{Water}}{ \text{3H}_2\text{O}}

Conversion of sodium aluminate to aluminium hydroxide:

NaAlO2[sodium aluminate]+2H2O→50−60°CNaOH[sodium hydroxide]+Al(OH)3[aluminium hydroxide]\underset{\text{[sodium aluminate]}}{\text{NaAlO}_2} + 2\text{H}_2\text{O} \xrightarrow{50 - 60 \degree\text{C}} \underset{\text{[sodium hydroxide]}}{\text{NaOH}} + \underset{\text{[aluminium hydroxide]}}{\text{Al(OH)}_3}

Conversion of Al(OH)3 - to pure alumina - heat on Al(OH)3:

2Al(OH)3[aluminium hydroxide]→1100°CAl2O3[pure alumina]+3H2O\underset{\text{[aluminium hydroxide]}}{\text{2Al(OH)}_3} \xrightarrow{1100\degree\text{C}} \underset{\text{[pure alumina]}}{\text{Al}_2\text{O}_3} + \text{3H}_2\text{O}