Bauxite is usually strip mined because it is almost always found near the terrain’s surface, with little or no overburden. Approximately 70% to 80% of the world’s dry bauxite production is processed first into alumina and then into aluminum by electrolysis as of 2015. Bauxite rocks are typically classified according to their intended commercial application: metallurgical, abrasive, cement, chemical, and refractory.
Usually, bauxite ore is heated in a pressure vessel and a sodium hydroxide solution at a temperature of 150 to 200 °C. At these temperatures, the aluminum is dissolved as an aluminate (the Bayer process). After separation of ferruginous residue (red mud) by filtering, pure gibbsite is precipitated when the liquid is cooled and then seeded with fine-grained aluminum hydroxide. The gibbsite is usually converted into aluminum oxide, Al2O3, by heating. This mineral is dissolved at a temperature of about 960 °C in molten cryolite. Next, this molten substance can yield metallic aluminum by passing an electric current through it in electrolysis, which is called the Hall–Héroult process after its American and French discoverers in 1886.
Before the Hall–Héroult process, elemental aluminum was made by heating ore along with essential sodium or potassium in a vacuum. The method was complicated and consumed materials that were themselves expensive at that time. This made early elemental aluminum more costly than gold.
