Conversion of impure bauxite to sodium aluminate:
Al 2 O 3 . 2 H 2 O [Impure bauxite] Amphoteric oxide + 2 NaOH [conc. soln] Base → 150 − 200 ° C 2 NaAlO 2 [sodium aluminate] Salt + 3H 2 O Water \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}} Amphoteric oxide [Impure bauxite] Al 2 O 3 .2 H 2 O + Base [conc. soln] 2 NaOH 150 − 200° C Salt [sodium aluminate] 2 NaAlO 2 + Water 3H 2 O
Conversion of sodium aluminate to aluminium hydroxide:
NaAlO 2 [sodium aluminate] + 2 H 2 O → 50 − 60 ° C NaOH [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} [sodium aluminate] NaAlO 2 + 2 H 2 O 50 − 60° C [sodium hydroxide] NaOH + [aluminium hydroxide] Al(OH) 3
Conversion of Al(OH)3 - to pure alumina - heat on Al(OH)3 :
2Al(OH) 3 [aluminium hydroxide] → 1100 ° C Al 2 O 3 [pure alumina] + 3H 2 O \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} [aluminium hydroxide] 2Al(OH) 3 1100° C [pure alumina] Al 2 O 3 + 3H 2 O
Precipitated Al(OH)3 is ignited to give pure alumina [Al2 O3 ]
