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Introduction to Aluminium and its alloys

Aluminium is the world’s most abundant metal, The major advantages of using aluminium are tied directly to its’ remarkable properties. Some of these properties are outlined in the following sections.

CORROSION RESISTANCE

When the surface of aluminium metal is exposed to air,a protective oxide coating forms almost instantaneously. This oxide layer is corrosion resistant and can be further enhanced with surface treatments such as anodising.

ELECTRICAL AND THERMAL CONDUCTIVITY

Aluminium is an excellent conductor of both heat and electricity. The great advantage of aluminium is that by weight, the conductivity of aluminium is around twice that of copper. This means that aluminium is now the most commonly used material in large power

transmission lines.The best alternatives to copper are aluminium alloys in the 1000 or 6000 series. These can be used for all electrical conduction applications including domestic

wiring.Weight considerations mean that a large proportion of overhead, high voltage power lines now use aluminium rather than copper. They do however, have a low

strength and need to be reinforced with a galvanised or aluminium coated high tensile steel wire in each strand.

LIGHT AND HEAT REFLECTIVITY

Aluminium is a good reflector of both visible light and heat making it an ideal material for light fittings,thermal rescue blankets and architectural insulation.

TOXICITY

Aluminium is not only non-toxic but also does not release any odours or taint products with which it is in contact. This makes aluminium suitable for use in packaging for sensitive products such as food or pharmaceuticals where aluminium foil is used.

RECYCLING

The recyclability of aluminium is unparalleled. When recycled there is no degradation in properties when recycled aluminium is compared to virgin aluminium. Furthermore, recycling of aluminium only requires around 5 percent of the input energy required to

produce virgin aluminium metal. The combination of two remarkable properties of

aluminium makes the need to recycle the metal obvious. These first of these factors is that there is no difference between virgin and recycled aluminium. The second factor is that recycled aluminium only uses 5% of the energy required to produce virgin material.

Currently around 60% of aluminium metal is recycled at the end of its lifecycle but this percentage can still be vastly improved.

ALUMINIUM PRODUCTION

Aluminium is extracted from the principal ore, bauxite. Significant bauxite deposits are found throughout Australia, the Caribbean, Africa, China and South America. Open cut techniques are commonly used to mine the bauxite. The bauxite is purified using the Bayer process. This

process involves dissolving aluminium trihydrate to leave alumina plus iron and titanium oxides. The iron and titanium oxides are by-products of the process and

are often referred to as ‘red mud’. Red mud must be disposed of with strong consideration given to environmental concerns. Approximately two tonnes of bauxite are required to

yield one tonne of alumina.

Smelting

The extraction of aluminium from alumina is achieved using an electrolytic process. A cell or pot is used that consists of a carbon lined steel shell. This shell forms a cathode. A consumable carbon anode is suspended in liquid cryolite (sodium aluminium fluoride) held within the pot at 950°C. Alumina is dissolved in the cryolite by passing low voltages at high amperages through the pot. This results in pure aluminium being deposited at

the cathode.

ENVIRONMENTAL CONSIDERATIONS

The aluminium industry is very conscious of the environmental impact of its activities. The mining and smelting of aluminium, plus the disposal of red mud can have a major environmental impact if not done properly. The industry is proud of its efforts and achievements in rehabilitating open cut mine sites and the restoring flora and fauna to these sites. Such efforts have been rewarded with awards from the United Nations Environment Programme and red mud disposal areas are now being successfully revegetated. Environmental requirements are met on pot line emissions through the use of specialist scrubbing

system.

 

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