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.
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 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, 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 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 α-form. The conversion to the α-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.
If aluminium hydroxide is heated to a temperature in excess of 1100ºC, then it passes through the transition phases of alumina referred to above.
The final product, if a high enough temperature is used, is α-alumina. The manufacturing process is commercially undertaken in long rotary kilns. Mineralisers are frequently added to catalyse the reaction and bring down the temperature at which the α-alumina phase forms; fluoride salts are the most commonly used mineralisers.
These calcined alumina products are used in a wide range of ceramic and refractory applications. The main impurity present is sodium oxide. Various grades are produced which differ in crystallite size, morphology and chemical impurities.
The calcined grades are often sub-divided into ordinary soda, medium soda (soda level 0.15-0.25% wt%) and low soda alumina.
Low Soda Alumina
Many applications, particularly in the electrical/electronic areas, require a low level of soda to be present in the alumina. A low soda alumina is generally defined as an alumina with soda content of <0.1% by weight. This can be manufactured by many different routes including acid washing, chlorine addition, boron addition, and utilisation of soda adsorbing compounds.
“Reactive” alumina is the terms normally given to a relatively high purity and small crystal size (<1 μm) alumina which sinters to a fully dense body at lower temperatures than low soda, medium-soda or ordinary-soda aluminas. These powders are normally supplied after intensive ball-milling which breaks up the agglomerates produced after calcination. They are utilised where exceptional strength, wear resistance, temperature resistance, surface finish or chemical inertness are required.
Tabular alumina is recrystallised or sintered α-alumina, so called because its morphology consists of large, 50-500 μm, flat tablet-shaped crystals of corundum. It is produced by pelletising, extruding, or pressing calcined alumina into shapes and then heating these shapes to a temperature just under their fusion point, 1700-1850ºC in shaft kilns.
After calcination, the spheres of shapes of sintered alumina can be used as they are for some applications, e.g. catalyst beds, or they can be crushed, screened and ground to produce a wide range of sizes. As the material has been sintered it has an especially low porosity, high density, low permeability, good chemical inertness, high refractoriness and is especially suitable for refractory applications.
Fused alumina is made in electric arc furnaces by passing a current between vertical carbon electrodes. The heat generated melts the alumina. The furnace consists of a water cooled steel shell and 3-20 tonne batches of material are fused at any one time. The fused alumina has a high density, low porosity, low permeability and high refractoriness. As a result these characteristics, it is used in the manufacture of abrasives and refractories.
High Purity Aluminas
High purity aluminas are normally classified as those with a purity of 99.99% and can be manufactured by routes starting from Bayer hydrate using successive activations and washings, or via a chloride to achieve the necessary degree of purity. Even higher purities are manufactured by calcining ammonium aluminium sulfate or from aluminium metal. In the case of the route via ammonium aluminium sulfate, the necessary degree of purity is obtained by successive recrystallisations. Especially high purities can be made from aluminium by reacting the metal with an alcohol, purifying the aluminium alkoxide by distillation, hydrolysing and the calcination. A minor route involves subjecting super purity aluminium metal pellets under distilled water to a spark discharge.
Applications include the manufacture of synthetic gem stones such as rubies and yttrium aluminium garnets for lasers, and sapphires for instrument windows and lasers.