Production of aluminium
In 1821, geologist Pierre Berthier discovered a new mineral in the French village of Les Baux, which he subsequently named Bauxite. Bauxite is a sedimentary rock formed by the weathering of silicate material in tropical conditions. It is composed, to a greater or lesser degree, of hydrated oxides of aluminium, iron and titanium. Aluminium does not occur naturally. Instead, it usually appears as an impurity in silicon and oxygen, and is also present in alkaline metals and alkaline earth metals.
Bauxite is usually extracted from large open cast mines. It is not used in its extracted state for aluminium manufacture, but is first converted to aluminium oxide (alumina). This is done using the Bayer method, usually at the bauxite mine. The biggest deposits can be found in the tropical and sub-tropical areas of Central America, South America, Western and Central Africa and Australia. Mineable bauxite contains in excess of 20–30% aluminium and two tonnes of alumina can be obtained from around four tonnes of bauxite.
To meet Sweden’s aluminium requirement, approximately 180,000m³ of bauxite has to be mined every year. Compare this to the Swedish peat industry, which produces a volume of approximately 4,500,000m³ every year, some 25 times greater than its bauxite equivalent.
In 1886, American Charles Martin Hall and Frenchman Paul T. Heroult, working independently of each other, discovered an electrolysis process that enabled alumina to be converted to pure aluminium. The method, known as the Hall-Heroult method, is still used today for the commercial production of aluminium.
Alumina is converted by electrolysis in molten aluminium fluoride and cryolite at a temperature of 960–980°C. The voltage passed between the anode and cathode is set to approximately 5 V. Current strength during the electrolysis process is dependent on the size of the smelter, but is between 30 and 300 kA.
Depending on the type of smelter used, energy consumption during electrolysis is between 13 and 17 kWh/kg. Electrolysis is usually carried out in locations in the world where there is good access to cheap electricity. Two tonnes of alumina produces around one tonne of aluminium.
Primary aluminium has a purity of at least 99.5%. Producers then add a variety of alloying materials to produce a range of aluminium alloys. Aluminium is now the second most widely used metal in the world after iron.