Aluminium oxide is the most commonly occurring chemical compound among the various aluminium oxides, existing with the chemical formula Al2O3. It is also known as alumina and can be called aloxite or alundum depending on their type of forms or applications. It occurs naturally in its crystalline polymorphic phase α-Al2O3 as the mineral corundum, and may exist in different varieties to form crystals such as ruby and sapphire.
One of the most common naturally occurring crystalline form of aluminium oxide is corundum. Examples of higher quality forms of corundum are rubies and sapphires, which owe their characteristic colours and properties on the type and quantity of trace impurities.
One of the most common ways to manufacture aluminium oxide is the Bayer process. This involves the refining of bauxite to produce aluminium oxide. Bauxite is the most important ore of aluminium which contains only 30-50% of aluminium oxide with the rest of the composition being a mixture of silica, titanium dioxide and various forms of iron oxides.
In the Bayer process, sodium hydroxide solution (caustic soda) is added to bauxite ore and the mixture is subjected to elevated temperature of 150 to 200°C in a pressurised vessel. In this range of temperature, aluminium can be dissolved in the mixture to form sodium aluminate. The aluminium compounds in the bauxite may be present as gibbsite (Al(OH)3), boehmite (AlOOH) or diaspore (AlOOH). The different forms of aluminium component will determine the extraction conditions.
After the residue is removed from the solution through filtration, gibbsite is precipitated upon cooling and then seeded with fine-grained aluminium hydroxide. The extraction process converts the aluminium oxide in the ore to soluble sodium aluminate, producing water as a byproduct. This process also dissolves silica, while the other components remain insoluble.
The resulting alkaline solution is cooled and treated by bubbling carbon dioxide gas through it, precipitating aluminium hydroxide. This will give a supersaturated solution with high-purity aluminium hydroxide crystals.
However, for bauxites with a composition more than 10% silica, the Bayer process becomes uneconomic due to insoluble sodium aluminium silicate being formed, reducing yield.
Aluminium oxide is used in the ceramic industry as an insulating material. Ceramics formed from the use of aluminium oxide have a good resistance against most acids (except for phosphoric acid and hydrofluoric acid). Their inertness made the ceramics suitable for use in many process applications.
Traditional glass is typically made from silica, which is a main component of sand. Recent research has made a breakthrough that aluminium oxide can be used to make glass which is much tougher and harder than the traditional glass, and can be comparable to metals.
Aluminium oxide can be used as a catalyst for a variety of industrial reactions. One of largest scale application that aluminium oxide is involved as a catalyst is the Claus process, which is the conversion of hydrogen sulphide waste gases into elemental sulfur.
Aluminium oxide also serves as a catalyst support for other industrial catalysts, such as those used in hydrodesufurisation and some Ziegler-Natta polymerisations.
Due to its inertness and white appearance, it is a favoured filler for plastics. It is also a common ingredient in sunscreens and sometimes present in cosmetics such as blush, lipstick, and nail polish. Aluminium oxide is also used to coat titanium oxide, which is a pigment for paints and plastic papers. This layer of coat prevents the catalytic reactions between the products and the atmosphere.