This process is used for the purification of bauxite ore containing silica (SiO2) as the main impurity. The powdered ore is mixed with coke and the mixture is heated at about 1800°C in the presence of Nitrogen gas, when aluminium nitride is formed.
Al2O3. 2H2O + 3C + N2 → 2AI N + 3CO+ 2H2O
Aluminium nitride thus obtained is hydrolysed with water to get a precipitate of Al(OH)3.
2Al N + 6H2O → 2NH3 + Al(OH)3
The precipitate of Al (OH)3 is filtered, washed and dried. The silica present as impurity in bauxite is reduced to silicon which being volatile at high temperature. is removed easily.