Aluminium oxide, also known as alumina, is the white odorless and water-insoluble crystalline form of aluminium hydroxide minerals. It is a major component of bauxite, the principal aluminium ore.
Its chemical and physical properties are well suited to engineering ceramics, where it provides strength, resistance to abrasion and thermal conductivity. It is used for a variety of applications including high-temperature electrical and voltage insulators, instrumentation parts for thermal test machines, seal rings, gas laser tubes and laboratory equipment.
Alumina is also used as a refractory material due to its high melting point. It can be smelted into sheets, bricks and loose fiber forms to suit different application requirements.
In addition, it is used as a coating that prevents oxidation of aluminium metal. This coating, which can be anodized or grown via plasma electrolytic oxidation, adds strength and makes the material less susceptible to deterioration.
Crystallized alumina is usually in the form of diaspore (Al2O3), boehmite (g-AlO3) and gibbsite (Al(OH)3). The process for crystallizing alumina from bauxite involves the soaking of the alumina in solutions of sodium aluminate, which allows physical separation of the dissolved impurities and the precipitation of pure aluminium oxide.
The alumina smelting process can produce a large quantity of oxide flakes. These flakes are widely used in paint for decorative effects and can be coated onto bicycle rims to provide abrasion and wear resistance.
The use of aluminium oxide as abrasive has been reported to cause some workers to develop a condition called Shaver's disease, a type of non-specific chronic industrial bronchitis. It is believed that this condition may result from exposure to fumes that contain finely divided aluminium oxide as well as silica and other substances.