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Aluminum Oxide

Aluminum oxide, with the chemical formula \(Al_2O_3\), is an amphoteric oxide and is commonly referred to as alumina. Corundum (α-aluminum oxide), emery, sapphire, amethyst, topaz, as well as many other names are reflecting its widespread occurrence in nature and industry. Corundum is the most common naturally occurring crystalline form of aluminum oxide. Rubies and sapphires are gem-quality forms of corundum, which owe their characteristic colors to trace impurities. Rubies are given their characteristic deep red color and their laser qualities by traces of chromium. Sapphires come in different colors given by various other impurities, such as iron and titanium.

Its most significant use is in the production of aluminum metal, although it is also used as an abrasive due to its hardness and as a refractory material due to its high melting point.

Properties

Aluminum oxide is an electrical insulator but has a relatively high thermal conductivity (\(30\, W m^{-1} K^{-1}\)) for a ceramic material. It is thus used as insulating material in power electronics. Aluminum oxide is responsible for resistance of metallic aluminum to weathering. Since metallic aluminum is very reactive with atmospheric oxygen, a thin passivation layer of alumina (4 nm thickness) forms on any exposed aluminum surface, protecting the metal from further oxidation. The thickness and properties of this oxide layer can be enhanced using a process called anodizing.

Production

The production of aluminum oxide is mainly from bauxite (the main aluminum ore), which is a mixture of various minerals including gibbsite (\(Al(OH)_3\), boehmite (\(\gamma-AlO(OH)\)), and diaspore (\(\alpha-AlO(OH)\)) along with impurities of iron oxides, quartz, and silicates.

Bauxite is purified by the Bayer process which is the principal industrial refining process. As bauxite contains only about 40 to 50% of alumina, the rest has been removed. This is achieved by washing bauxite with hot sodium hydroxide, which dissolves the alumina by converting it to aluminum hydroxide which forms a solution in a strong base:

\[Al_2O_3 + 2 OH^- + 3 H_2O \rightarrow 2 [Al(OH)_4]^-\]

The other components of the bauxite do not dissolve and are filtered off (the residues usually form a red sludge which presents a disposal problem, since it contains, for example, arsen and cadmium(1)). Next the solution is cooled which causes precipitation of a fluffy solid (aluminum hydroxide). The aluminum hydroxide is then heated to 1050°C which causes it to decompose into aluminum oxide and water:

\[2 Al(OH)_3 \rightarrow Al_2O_3 + 3 H_2O\]

 

References

An accident at an Hungarian alumina plant at Devecser caused several fatalities and many injuries. The wall of a waste-retaining pond broke, releasing a torrent of the toxic red sludge down a local stream. The sludge stream can be seen on a NASA satellite photo (see Earth from Space).

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