Aluminium Oxide (Al2O3 ) or alumina is one of the most versatile of refractory ceramic oxides and finds use in a wide range of applications.

It is found in nature as corundum in emery, topaz, amethyst, and emerald and as the precious gemstones ruby and sapphire, but it is from the more abundant ores such as bauxite, cryolite and clays that the material is commercially extracted and purified.

Corundum exists as rhombohedral crystals with hexagonal structure. The unit cell is an acute rhombohedron of side length 5.2Å and plane angle ~55°. It is the close packing of the aluminium and oxygen atoms within this structure that leads to its good mechanical and thermal properties.
Extraction Routes

The most common process for the extraction and purification of alumina is the ‘Bayer’ process. The first step in the process is the mixing of ground bauxite into a solution of sodium hydroxide. By applying steam and pressure in tanks containing the mixture, the bauxite slowly dissolves. The alumina released reacts with the sodium hydroxide to form sodium aluminate. After the contents of the tank have passed through other vessels where the pressure and temperature are reduced and impurities are removed, the solution of sodium aluminate is placed in a special tank where the alumina is precipitated out. The precipitate is removed from the tank, washed, and heated in a kiln to drive off any water present. The residue is a commercially pure alumina.

Other extraction processes are used including pyrogenic treatment of bauxite with soda, and the extraction of aluminium hydroxide from meta kaolin via either the chloride or sulphate.

The yield of alumina from these processes can approach 90%.

For advanced ceramics uses, the alumina manufactured by these processes requires further purification. This is often achieved by recrystallisation from ammonium alum.