The finely divided bauxite ore is heated with concentrated solution of sodium hydroxide in an autoclave at about 150°C. The aluminium oxide present in the ore gets dissolved in NaOH solution forming sodium aluminates. The impuritieS being insoluble separate out and removed by filtrations.
Al2O3. 2H0 + 2 NaOH → 2 Na Al02 + 3H2O
The filtrate containing sodium aluminate solution is diluted with water, agitated and cooled. As a result a precipitate of Al(OH)3 is obtained.
Na AlO+2 2H2O → NaOH + Al(OH)3↓
The filtratetontaining sodium hydroxide is concentrated and used again.