Use
Pure Aluminum oxide, needed to produce aluminum by the Hall process, is made by the Bayer process. The starting material is bauxite (Al2O3 nH2O). The ore contains impurities, such as, SiO2, Fe2O3, TiO2, and Na2O. Most impurities are removed following treatment with caustic soda solution. Bauxite is dissolved in NaOH solution. Silica, iron oxides and other impurities are filtered out of the solution. CO2 is then bubbled through this solution. This precipitates areheated to remove water and produce Al2O3. These impurities are removed. Calcinations of bauxite produce Aluminum oxide of abrasive and refractory grades. Activated Aluminum oxide of amorphous type, as well as the transition Aluminum oxides of , , , and forms, are obtained from various aluminum hydroxides, such as, - and -trihydrates, -monohydrate and Aluminum oxide gel. Such chemicals arealsoobtained from bauxite by the Bayer process.