The Bayer process is the principal industrial means of refining bauxite to produce alumina (aluminium oxide). Bauxite, the most important ore of aluminium, contains only 30–54% aluminium oxide, (alumina), Al2O3, the rest being a mixture of silica, various iron oxides, and titanium dioxide. The aluminium oxide must be purified before it can be refined to aluminium metal.
In the Bayer process, bauxite ore is heated in a pressure vessel along with a sodium hydroxide solution at a temperature of 150 to 200 °C. At these temperatures, the aluminium is dissolved as sodium aluminate. The aluminium compounds in the bauxite may be present as gibbsite(Al(OH)3), boehmite(AlOOH) or diaspore(AlOOH); the different forms of the aluminium component will dictate the extraction conditions. After separation of the residue by filtering, gibbsite (aluminium hydroxide) is precipitated when the liquid is cooled, and then seeded with fine-grained aluminium hydroxide. This converts the aluminium oxide in the ore to soluble sodium aluminate, 2NaAlO2, according to the chemical equation: