Aluminium (Aluminum), Earth's Commonest Metal: Only Non-metals Oxygen and Silicon are More Abundant in the Crust
Aluminium (aluminum) is present in soils and clays as well as some precious stones such as rubies and sapphires. Its compounds have been known for centuries.
The ancient Greeks and Romans used alums, made from potassium and aluminium sulphates, in tanning leather and to help dyes adhere to fabrics. Alums were also used to stop bleeding. The pure metal, however, was not isolated until the mid-nineteenth century.
Aluminium was Worth More than Silver
During the 1860s, the most honoured guests at a royal banquet in Paris were given aluminium cutlery to eat with, while the less favoured had to be content with silver. This was due to the difficulty, and hence expense of its extraction from its ore. The pure metal is obtained by passing electricity through the molten ore. During the nineteenth century, electricity was a new phenomenon and not well understood. However, the technology advanced rapidly, and during the fifty years following its discovery, the price of aluminium dropped from 545 dollars per pound to 18 cents.
The purified ore of aluminium, bauxite, melts at 2045 degrees Centigrade, though this temperature is reduced by mixing it with another mineral, cryolite. This, together with the large quantity of electricity required means that the extraction is highly energy-intensive, and aluminium smelters need to be supplied by their own power stations. They are often built in mountainous regions, where hydro-electricity can be generated cheaply.
Uses of Aluminium
Aluminium’s uses are largely determined by its rare combination of high strength and low density. It is approximately one-third the density of iron. It is ideal for use in aeroplanes and space rockets, where weight needs to be held to a minimum, as well as in window frames and drinks cans.
It is an excellent heat conductor, and finds extensive use in cooking utensils. When used as kitchen foil, its shiny surface reflects heat back into food, which cooks more rapidly. This reflective nature is also employed to prevent loss of body heat from a person suffering from exposure. Aluminium paint on the walls of buildings in polar regions helps minimise heat loss through radiation.
The electrical conductivity is also high, so that aluminium is used for overhead cables in preference to heavier metals that could break under their own weight.
Aluminium is a Highly Reactive Metal
Despite aluminium’s high reactivity, it does not readily corrode, as it is protected by a coherent layer of aluminium oxide that forms on its surface. If this layer is removed, however, it corrodes very rapidly, even catastrophically. Because of their complex weaponry, the superstructure of modern warships, to prevent their becoming top-heavy, are built of aluminium alloys. During the Falklands conflict of 1982, when a British ship was struck by a missile, the heat generated by the explosion ignited the metal, which soon burned out of control.
Aluminium in the Diet
Aluminium compounds have been present in food and water throughout human history. They provide no beneficial effects and are normally excreted through the kidneys. Neither is there any hard evidence that they are in any way toxic. A possible link between aluminium and Alzheimer’s disease has been suggested, but it is not known whether the higher-than-normal concentration of aluminium in the brain tissue of sufferers is a cause or an effect of the disease.
Recycling Cans and Foil
Like all natural resources, bauxite reserves are being rapidly depleted. Recycling of aluminium costs 5% of the energy needed to extract new metal. Salvaging drinks cans and cooking foil should be encouraged as a contribution toward decreasing the prodigal use of energy. It might also postpone the date when aluminium would again become more expensive than silver and gold.
Reference: John Emsley, Nature’s Building Blocks, Oxford University Press 2001