The process used for purification of bauxite ore containing high silica content as impurity is 

This is process is used for the bauxite ore which contain silica impurity.
The powdered bauxite ore is mixed with coke and heated at high temperature in the presence of nitrogen  gas. Aluminium nitride, carbon monoxide and water are formed. Hydrolysis of aluminum nitride produce Aluminium hydroxide. Strongly heating of Aluminium hydroxide produce Alumina.