The finely divided bauxite ore is treated with concentrated solution of Na2CO3 in an autoclave. Aluminium Oxide present in the ore dissolves in the solution as it is amphoteric. The impurities Fe2O3 and SiO2 are insoluble hence settles down and removed by filtration.
Al2O3. 2H2O+ Na2 CO3 → Na AlO2+2H2O + CO2.
The filtrate is warmed at a temperature of 50-60°C and a current of CO2 gas is passed. As a result, a precipitate of Aluminium hydroxide, Al(OH)3 is obtained.
2Na AlO2 + 3H2O + CO2 → Na2CO3 + 2AI (OH)3
This precipitate of Al(OH)3 is filleted, washed and dried.