Alumina (Aluminium Oxide) is the most widely used oxide ceramic material. Its applications are widespread, and include spark plugs, tap washers, abrasion resistant tiles, and cutting tools.
Very large tonnages are also used in the manufacture of monolithic and brick refractories. It is also mixed with other materials such as flake graphite for other, more difficult applications are envisaged, such as pouring spouts and sliding gate valves.
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Key Properties
Aluminium oxides key properties are shown below.
High compression strengthHigh hardnessResistant to abrasionResistant to chemical attack by a wide range of chemicals even at elevated temperaturesHigh thermal conductivityResistant to thermal shockHigh degree of refractorinessHigh dielectric strengthHigh electrical resistivity even at elevated temperaturesTransparent to microwave radio frequenciesLow neutron cross section capture areaRaw material readily available and price not subject to violent fluctuation
Annual Production
Annual production of aluminum oxide is 45 million tonnes. 90% of this is used in the manufacture of aluminium metal by electrolysis.
Where Does Alumina Come From?
Most of the aluminium oxide produced commercially is obtained by the calcination of aluminium hydroxide (frequently termed alumina trihydrate or ATH).
The aluminium hydroxide is virtually all made by the Bayer Process.
This involves the digestion of bauxite in caustic soda and the subsequent precipitation of aluminium hydroxide by the addition of fine seed crystals of aluminium hydroxide.
Phases
Aluminium oxide exists in many forms, α, χ, η, δ, κ, θ, γ, ρ; these arise during the heat treatment of aluminium hydroxide or aluminium oxy hydroxide. The most thermodynamically stable form is α-aluminium oxide.
Aluminium Hydroxides
Aluminium forms a range of hydroxides; some of these are well characterised crystalline compounds, whilst others are ill-defined amorphous compounds. The most common trihydroxides are gibbsite, bayerite and nordstrandite, whilst the more common oxide hydroxide forms are boehmite and diaspore.
Commercially the most important form is gibbsite, although bayerite and boehmite are also manufactured on an industrial scale.
Aluminium hydroxide has a wide range of uses, such as flame retardants in plastics and rubber, paper fillers and extenders, toothpaste filler, antacids, titania coating and as a feedstock for the manufacture of aluminium chemicals, e.g. aluminium sulfate, aluminium chlorides, poly aluminium chloride, aluminium nitrate.
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