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Bauxite ore is a mixture of hydrated aluminum oxide derivatives and compounds of other elements such as iron. The aluminum compounds in the bauxite may be present as gibbsite [Al(OH)₃], boehmite [or böhmite, γ-AlO(OH)], or diaspore [α-AlO(OH)]. The different forms of the aluminum component and the impurities dictate the extraction conditions. Aluminum oxides and hydroxides are amphoteric – having both acidic and basic character.

In the Bayer process, a mixture of the bauxite ore and a sodium hydroxide solution is heated in a pressure vessel at a temperature of 150 to 200°C (300 to 390°F), causing the aluminum to be dissolved as sodium aluminate. After separation of the residue by filtering, gibbsite is precipitated when the liquid is cooled and then seeded with fine-grained aluminum hydroxide crystals from previous extractions.

The extraction process converts the aluminum oxide in the ore to soluble sodium aluminate (NaAlO₂), and at the same time, silica is dissolved to form sodium silicate (Na₂SiO₃):

The other components of the bauxite ore do not dissolve.

Lime may be added to precipitate the silica as calcium silicate (CaSiO₃). The undissolved waste (bauxite tailings) after the aluminum compounds are extracted contains iron oxides, silica, lime (CaO), titania (TiO₂), and some unreacted alumina.

See also: Bauxite.