Alumina (Aluminium Oxide) - The Different Types of Commercially Available Grades

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.

Key Properties

Aluminium Oxides key properties 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

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.

Commercial Grades

Low Soda

Many applications, particularly in the electrical/electronic areas, require a low level of soda to be present in the aluminum oxide. A low soda alumina is generally defined as with a 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.

Fused

White 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 of these characteristics, it is used in the manufacture of abrasives and refractories.

The only compound of silicon and carbon is Silicon Carbide (SiC), or carborundum. SiC does occur naturally as the mineral moissanite, but this is extremely rare. However, it has been mass produced in powder form for use as an abrasive since 1893. As an abrasive, it has been used for over one hundred years in grinding wheels and many other abrasive applications.

With today’s technology, high-quality technical grade ceramics have been developed with SiC which exhibit very advantageous mechanical properties such as:

• Exceptional hardness

• High strength

• Low density

• High elastic modulus

• High thermal shock resistance

• Superior chemical inertness

• High thermal conductivity

• Low thermal expansion

These high strength and very durable ceramics are widely used in applications such as automotive brakes and clutches along with ceramic plates embedded in bulletproof vests. Silicon carbide is also used in semiconductor electronic devices operating at high temperatures and/or high voltages such as flame igniters, resistance heating, and harsh environment electronic components.