Aluminium was once worth more than gold, until chemistry cracked bauxite
Bauxite, a dull reddish rock named after a village in Provence, is where nearly all our aluminium starts. Before industrial refining, extracting the metal needed costly sodium or potassium heated in a vacuum, which made early aluminium pricier than gold. Two nineteenth-century processes turned it into the stuff of drink cans.
The French geologist Pierre Berthier identified bauxite in 1821 near Les Baux in southern France. It is a sedimentary rock rich in aluminium minerals such as gibbsite, mixed with iron oxides and clay. Two main kinds form in different settings. Lateritic bauxites, found mostly in the tropics, develop when rocks like granite or basalt weather intensely in well-drained ground, leaching away other minerals. Carbonate bauxites, common in Europe, Jamaica, Guyana and Suriname, collect as limestone dissolves and leaves concentrated clay behind. Jamaica's may even derive from volcanic ash blown from Central America during the Miocene.
Refining is a two-step affair. In the Bayer process, crushed ore is cooked with caustic soda under pressure at 150 to 200 °C, dissolving the aluminium; after the waste is filtered out, cooling and seeding the liquid makes pure gibbsite crystallise. Heated beyond 1,000 °C, that becomes alumina, which is then dissolved in molten cryolite and split by electric current in the Hall–Héroult process, named for its American and French inventors. Roughly 70 to 80 percent of dry bauxite output went this route as of 2010. The same caustic liquor is also the world's main source of the rare metal gallium, although only about 15 percent can be recovered at typical concentrations.
Guinea is the largest producer, ahead of Australia and China. Because deposits usually lie within 20 metres of the surface, they are strip-mined, stripping vegetation and topsoil and permanently altering habitat. The Bayer process leaves red mud, a caustic sludge with a pH around 13; a 2010 spill in Hungary killed all life in the Marcal River. Mining on indigenous and traditional lands has fuelled disputes in Guinea, Ghana, Vietnam and India.
Even shipping it is risky. Damp bauxite can liquefy in a ship's hold and shift suddenly, destabilising the vessel; the MS Bulk Jupiter, lost in 2015, is suspected to have sunk this way. Recycling aluminium uses far less electricity and could stretch reserves considerably.
Source: Bauxite
