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Aluminium oxide or aluminum oxide is a chemical compound of aluminium and oxygen with the chemical formula Al2O3. It is also commonly referred to as alumina in the mining, ceramic, and materials science communities.
Aluminium oxide is the main component of bauxite, the principal ore of aluminium. Industrially, bauxite is purified to aluminium oxide via the Bayer process, and then converted to aluminium metal in the Hall-Heroult process.
The bauxite ore is made up of impure Al2O3 + Fe2O3 + SiO2. This is then purified by the Bayer Process: Al2O3 + 3H2O + 2NaOH --(heated)--> 2NaAl(OH)4. The Fe2O3 does not dissolve in the base. The SiO2 dissolves as silicate Si(OH)6-6. Upon filtering, Fe2O3 is removed. With the addition of an acid, Al(OH)3 precipitates. The silicate remains in solution. Then, Al(OH)3 --(heated)--> Al2O3 + 3H2O. The Al2O3 is of course, alumina.
The gems ruby and sapphire are mostly aluminium oxide, given their characteristic colors by trace impurities.
Aluminium oxide is responsible for metallic aluminium's resistance to weathering. Metallic aluminium is very reactive with atmospheric oxygen, and a thin layer of aluminium oxide quickly forms on any exposed aluminium surface. This layer protects the metal from further oxidation. The thickness and properties of this oxide layer can be enhanced using a process called anodising. A number of alloys, such as aluminum bronzes, exploit this property by including a proportion of aluminum in the alloy to enhance corrosion resistance.
Aluminium oxide is an excellent thermal and electrical insulator. In its crystalline form, called corundum, its hardness makes it suitable for use as an abrasive and as a component in cutting tools.
Powdered aluminium oxide is frequently used as a medium for chromatography.
In August 2004, scientists in the United States working for 3M developed a technique for making an alloy of alumina and rare earth elements to produce a strong glass called transparent alumina.
Aluminium oxide was taken off the EPA's chemicals lists in 1988.
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