Alumina is the most widely used oxide ceramic material. Its applications are widespread, and include spark plugs, tap washers, pump seals, electronic substrates, grinding media, abrasion resistant tiles, cutting tools, bioceramics, (hip-joints), body armour, laboratory ware and wear parts for the textile and paper industries. Very large tonnages are also used in the manufacture of monolithic and brick refractories. It is also used mixed with other materials such as flake graphite where even more severe applications are envisaged, such as pouring spouts and sliding gate valves.
Key Properties

The characteristics which alumina has and which are important for these applications are shown below.

· High compression strength

· High hardness

· Resistant to abrasion

· Resistant to chemical attack by a wide range of chemicals even at elevated temperatures

· High thermal conductivity

· Resistant to thermal shock

· High degree of refractoriness

· High dielectric strength

· High electrical resistivity even at elevated temperatures

· Transparent to microwave radio frequencies

· Low neutron cross section capture area

· Raw material readily available and price not subject to violent fluctuation
Annual Production

Annual production of alumina is some 45 million tonnes, of which 90% 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 of Alumina

Aluminium oxide exists in many forms, a, c, h, d, k, q, g, r; these arise during the heat treatment of aluminium hydroxide or aluminium oxy hydroxide. The most thermodynamically stable form is a-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.
Commercial Grades of Alumina
Smelter Grade Alumina

Smelter or metallurgical grade alumina is the name given to alumina utilised in the manufacture of aluminium metal. Historically it was manufactured from aluminium hydroxide using rotary kilns but is now generally produced in fluid bed or fluid flash calciners. In the fluid flash processes the aluminium hydroxide is fed into a counter-current stream of hot air obtained by burning fuel oil or gas. The first effect is that of removing the free water and this is followed by removal of the chemically combined water; this occurs over a range of temperatures between 180-600ºC. The dehydrated alumina is principally in the form of activated alumina and the surface area gradually decreases as the temperature rises towards 1000ºC. Further calcination at temperatures > 1000ºC converts this to the more stable a-form. The conversion to the a-form is typically of the order of 25% and the specific surface area is relatively high at >50m²/g due to the presence of transition aluminas.