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Aluminium oxide

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Aluminium 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.

Properties


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.
Aluminium oxide is responsible for metallic aluminium's resistance to weathering. Metallic aluminium is very reactive with atmospheric oxygen, and a thin passivation 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 aluminium bronzes, exploit this property by including a proportion of aluminium in the alloy to enhance corrosion resistance.
Aluminium oxide was taken off the EPA's chemicals lists in 1988.

Natural occurrence


Corundum is the naturally occurring mineral form of aluminium oxide. Rubies and sapphires are gem-quality forms of corundum with their characteristic colors due to trace impurities in the corundum structure.

Industrial Fabrication Process


Aluminium oxide, also known as alumina, is the main component of bauxite, the principal ore of aluminium. The largest manufacturers in the world of alumina are Alcoa, Alcan and Rusal. Companies which specialise in the production of speciality aluminium oxides and aluminium hydroxides include Alcan and Almatis. The bauxite ore is made up of impure Al2O3, Fe2O3, and SiO2. These are then purified by the Bayer Process:
Al2O3 + 3H2O + 2NaOH + heat → 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:
2Al(OH)3 + heat → Al2O3 + 3H2O
The formed Al2O3 is alumina.
In 1961, GE developed "Lucalox", a transparent alumina used in sodium vapor lamps.
In August 2004, scientists in the United States working for 3M developed a technique for making an alloy of aluminium oxide and rare earth elements to produce a strong glass called transparent alumina.

Uses


Annual world production of aluminia is approximately 60 million tonnes, over 90% of which is used in the manufacture of aluminium metal. Major uses for aluminium hydroxide include the manufacture of water treatment chemicals such as aluminium sulphate, Poly Aluminium Chloride and sodium aluminate. Large tonnnages are also used in the manufacture of zeolites, coating titania pigments and as a fire retardants/smoke suppressant. The major uses of speciality aluminium oxides are in refractories, ceramics, polishing and abrasive applications. Minor uses include use in toothpast formulations, and as a medium for chromatography. In this latter application alumina is available in the so-called Brockmann types neutral, basic (pH 9.5) and acidic (pH 4.5 when in water).

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