The Hall–Héroult process is the major industrial process for smelting aluminium. It involves dissolving aluminium oxide (alumina) (obtained most often from bauxite, aluminium's chief ore, through the Bayer process) in molten cryolite and electrolyzing the molten salt bath, typically in a purpose-built cell. The process, conducted at an industrial scale, happens at 940–980 °C (1700–1800 °F) and produces aluminium with a purity of 99.5–99.8%. Recycling aluminum, which does not require electrolysis, is thus not treated using this method.

The Hall–Héroult process consumes substantial electrical energy, and its electrolysis stage can produce significant amounts of carbon dioxide if the electricity is generated from high-emission sources. Furthermore, the process generates fluorocarbon compounds as byproducts, contributing to both air pollution and climate change.

Elemental aluminium cannot be produced by the electrolysis of an aqueous aluminium salt, because hydronium ions readily oxidize elemental aluminium. Although a molten aluminium salt could be used instead, aluminium oxide has a melting point of 2072 °C (3762 °F) so electrolysing it is impractical. In the Hall–Héroult process, alumina, Al₂O₃, is dissolved in molten synthetic cryolite, Na₃AlF₆, to lower its melting point for easier electrolysis. The carbon source is generally a coke (fossil fuel).

In the Hall–Héroult process the following simplified reactions take place at the carbon electrodes:

Cathode:

Anode:

Overall:

In reality, much more CO₂ is formed at the anode than CO: