What aluminium is and where it comes from

A metal that is never found pure

Aluminium is the most abundant metal in the Earth's crust: it makes up about 8% of its weight. But it is always combined with oxygen and other elements, which is why large-scale production only began in the late nineteenth century.

The raw material is bauxite, a reddish rock rich in aluminium oxide. The metal is obtained from it in two steps:

1. Refining (Bayer process). The bauxite is dissolved in caustic soda and the alumina is separated out: a white powder of aluminium oxide.
2. Electrolysis (Hall-Héroult process). The alumina is dissolved in pots holding a bath of molten salts at about 950 °C, and a very high electric current separates the aluminium from the oxygen. The liquid metal collects at the bottom of the pot.

The approximate ratio is 4 tonnes of bauxite → 2 of alumina → 1 of aluminium. Electrolysis uses a great deal of energy, in the order of 13 to 15 MWh per tonne. That is why aluminium smelters are built where energy is plentiful and cheap.

Aluminium in Argentina

In Argentina, primary aluminium is produced by Aluar, in Puerto Madryn, Chubut province. The plant started up in 1974 and today produces about 400,000 tonnes a year. Argentina has no bauxite: Aluar imports the alumina, and much of the electricity comes from the Futaleufú hydroelectric plant.

The liquid aluminium is cast into ingots and billets (cylindrical bars of alloy). Billets are the raw material for extruders, who turn them into profiles.

Why it is so widely used

• Lightweight: its density is 2.70 g/cm³, a third of that of steel.
• It does not rust like iron: in contact with air it forms a thin oxide layer that protects it. Anodising thickens that layer on purpose.
• It can be extruded: it can be given almost any cross-section, with thin walls and fine details, in a single step.
• A good conductor of heat and electricity.
• It can be recycled again and again: remelting aluminium scrap takes about 5% of the energy needed to produce it from bauxite.