Rapid growth for light-weight metal

Press, 12 February 1986, Page 35

Rapid growth for light-weight metal

First produced commercially in 1888, aluminium has enjoyed a phenomenal rate of growth worldwide. It has been estimated that, in the United States, annual aluminium ' comsumption averages 22kg per head of population.

Aluminium occurs naturally throughout the Earth’s crust, the third most abundant element behind oxygen and silicon. It has a strong tendency to combine with other common elements and so is never found in its metalic state.

Its compounds, however, are an important constituent of virtually all common rocks, and can be found in clay, slate, schist and ganite. The most important aluminium ore, bauxite, was discovered in 1821 near Les Baux in southern France. By 1825, a Danish scientist, Hans Christian Orsted, had isolated aluminium from its natural surroundings for the first time, he described his discovery as “a lump of metal which in colour and

lusture somewhat ressembles tin.” Bauxite has since been found in all continents except Antarctica. The richest deposits generally lie in areas that, during formation, enjoyed tropical and sub-tropical climates.

Large deposits of bauxite are to be found in the United States, Australia, the Soviet Union, southeast Asia, northern South America and southern Europe.

The raw material undergoes a refining process in order to separate the white powdered alumina from impurities such as iron oxide, silica and water. The alumina thus recovered then becomes the raw material in the smelting process. Refining four tonnes of bauxite yields about two tonnes of alumina. This end product is quite pure, containing only infinitesimal amounts of iron and silicon.

Alumina from Australia forms the raw material for the Tiwai Point smelter operated by New

Zealand Aluminium Smelters, Ltd. The smelting process reduces (that is removes oxygen from) the alumina by passing a large electric current through each smelting cell. The resultant fierce heat causes the reduction and oxydation that create the end product aluminium. The long rows of smelting cells, or potlines, consume huge amounts of electricity, necessitating the siting of aluminium setters near plentiful supplies of relatively cheap electric power. Tiwai Point is thus strategically placed near both Manapouri hydro-electric scheme and the port at Bluff. Known as the billet, the standard lengths of aluminium are purchased by Comalco Extrusions, Ltd, and heated to about 500 deg. Centigrade. The metal is then forced through a preheated steel extrusion die, emerging in the desired profile. The section is straightened, cut to length and artificially aged to stabilise mechanical properties.