The Bayer process is the principal industrial means of producing alumina, itself important in the Hall-Heroult process for producing aluminum.

Bauxite, the most important ore of aluminum, contains only 40-60% alumina, the rest being a mixture of silica, various iron oxides, and titanium oxide. The alumina must be purifed before it can be refined to aluminum metal. In the Bayer process, bauxite is washed with a hot solution of sodium hydroxide, NaOH, at 250°C. This converts the alumina to aluminum hydroxide, Al(OH)₃, which dissolves in the hydroxide solution according to the chemical equation

Al₂O₃ + 2 OH^- + 3 H₂O → 2 [Al(OH)₄]^-

The other components of bauxite do not dissolve and can be filtered out as solid impurities. Next, the hydroxide solution is cooled, and the aluminum hydroxide dissolved in it precipitates out as a white, fluffy solid. When then heated to 1050°C, the aluminum hydroxide decomposes to alumina, giving off water vapor in the process:

2 Al(OH)₃ → Al₂O₃ + 3 H₂O

The Bayer process was developed in 1888 by the German chemist Karl Bayer . Today, it produces nearly all the world's alumina supply as an intermediate in aluminum production.