alumina

 

(əlū'mĭnə) or aluminum oxide, Al₂O₃, chemical compound with m.p. about 2,000°C and sp. gr. about 4.0. It is insoluble in water and organic liquids and very slightly soluble in strong acids and alkalies. Alumina occurs in two crystalline forms. Alpha alumina is composed of colorless hexagonal crystals with the properties given above; gamma alumina is composed of minute colorless cubic crystals with sp. gr. about 3.6 that are transformed to the alpha form at high temperatures. Alumina powder is formed by crushing crystalline alumina; it is white when pure. Alumina is widely distributed in nature. Combined with silica and other minerals it occurs in clays, feldspars, and micas. It is the major component of bauxite and occurs in an almost pure form as corundum. Alumina is commercially important. A major use is in the production of aluminum metal. It is also used for abrasives; corundum and emery are widely used, as are artificially prepared alumina abrasives. Trade names for alumina abrasives include Alundum and Aloxite. Alumina is also used in ceramics, in pigments, and in the manufacture of chemicals. Clays containing alumina are used in porcelain, pottery, and bricks. Pure alumina is used in making crucibles and other refractory apparatus. Hydrated alumina is used in mordant dyeing to make lake pigments; it is also used in glassmaking, in cosmetics, and in medicine as an antacid.

Aluminium oxide
Identifiers
CAS number	1344-28-1
Properties
Molar mass	101.96 g/mol
Density	3.97 g cm−3, solid
Melting point	2054 °C
Boiling point	2980 °C [1]
Solubility in water	insoluble
Structure
Coordination geometry	octahedral
Thermochemistry
Std enthalpy of formation ΔfHo298	−1675.7 kJ mol−1
Standard molar entropy So298	50.92 J mol−1 K−1
Hazards
MSDS	External MSDS
EU classification	Not listed.
Flash point	non-flammable
Related Compounds
Other anions	aluminium hydroxide
Other cations	boron trioxide gallium oxide indium oxide thallium oxide
Supplementary data page
Structure and properties	n, εr, etc.
Thermodynamic data	Phase behaviour Solid, liquid, gas
Spectral data	UV, IR, NMR, MS
Except where noted otherwise, data are given for materials in their standard state (at 25 °C, 100 kPa) Infobox disclaimer and references

Aluminium oxide is an amphoteric oxide of aluminium 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 electrical insulator but has a relatively high thermal conductivity. In its most commonly occurring crystalline form, called corundum or α-aluminum oxide, 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 alumina 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. The alumina generated by anodising is typically amorphous, but discharge assisted oxidation processes such as plasma electrolytic oxidation result in a significant proportion of crystalline alumina in the coating, enhancing its hardness.

Aluminium oxide was taken off the United States Environmental Protection Agency's chemicals lists in 1988. Aluminum oxide is on EPA's TRI list if it is a fibrous form.

Natural occurrence

Corundum is the naturally occurring crystalline 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.

Crystal structure

The most common form of crystalline alumina, α-aluminum oxide, is known as corundum and has a hexagonal close packed (HCP) lattice 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)₆^2-. Upon filtering, Fe₂O₃ is removed. When the Bayer liquor is cooled, 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.

As discussed above (in the "Properties" section), alumina can be grown as a coating on aluminium by anodising or by plasma electrolytic oxidation.

Uses

Annual world production of alumina is approximately 65 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 tonnages 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 toothpaste 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). Aluminium oxide is also used in preparation of coating suspensions in compact fluorescent lamps.

Al₂O₃ is also used in fluoride water filters. It is one of the few methods available to filter water soluble fluorides out of water.

Aluminum oxide is also used as a material in hip replacements.

Commercial uses

Because of aluminium oxide's position on the Mohs scale of mineral hardness, (9), it is very widely used as an abrasive as a significantly less expensive replacement for industrial diamonds. Many types of sandpaper use aluminium oxide crystals. In addition, its low heat retention and specific heat makes it widely used in almost all grinding operations, particularly cutoff tools.

Aluminium oxide is used in certain CD/DVD cleaning kits. Alumina will polish the media surface, leaving it clean and relatively scratch-free.

Alumina is also the byproduct of hydrogen generation for the purposes of fuel generation when water is added to pellets comprised of aluminum and gallium. The other byproduct of the reaction is gallium.

Most pre-finished wood flooring now uses aluminum oxide as a hard protective coating.

See also

• Bayer process
• List of Alumina Refineries

References

1. ^ Mallickrodt Backer, MSDS. Aluminum Oxide: Material Safety Data Sheet (A28440).

External links

• International Chemical Safety Card 0351
• PhysicsWeb article on Transparent alumina

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