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.

Process

Difficulties faced

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&nbsp;°C (3762&nbsp;°F) so electrolysing it is impractical. In the Hall–Héroult process, alumina, Al<sub>2</sub>O<sub>3</sub>, is dissolved in molten synthetic cryolite, Na<sub>3</sub>AlF<sub>6</sub>, to lower its melting point for easier electrolysis.

Cell operation

Cells in factories are operated 24 hours per day so that the molten material in them will not solidify. Temperature within the cell is maintained via electrical resistance. Oxidation of the carbon anode increases the electrical efficiency at a cost of consuming the carbon electrodes and producing carbon dioxide. Bars of aluminium were exhibited alongside the French crown jewels at the Exposition Universelle of 1855, and Emperor Napoleon III of France was said to have reserved his few sets of aluminium dinner plates and eating utensils for his most honored guests.

Production costs using older methods did come down, but when aluminium was selected as the material for the cap/lightning rod to sit atop the Washington Monument in Washington, D.C., upon its completion in 1884, it was still more expensive than silver.

Independent discovery

The Hall–Héroult process was discovered independently and almost simultaneously in 1886 by the American chemist Charles Martin Hall and by the Frenchman Paul Héroult—both 22 years old. Some authors claim Hall was assisted by his sister Julia Brainerd Hall; however, the extent to which she was involved has been disputed. In 1888, Hall opened the first large-scale aluminium production plant in Pittsburgh. It later became the Alcoa corporation.

In 1997, the Hall–Héroult process was designated a National Historic Chemical Landmark by the American Chemical Society in recognition of the importance of the process in the commercialization of aluminum.

Economic impact

Aluminium produced via the Hall–Héroult process, in combination with cheaper electric power, helped make aluminium (and incidentally magnesium) an inexpensive commodity rather than a precious metal.

This, in turn, helped make it possible for pioneers like Hugo Junkers to utilize aluminium and aluminium-magnesium alloys to make items like metal airplanes by the thousands, or Howard Lund to make aluminium fishing boats. In 2012 it was estimated that 12.7 tons of CO<sub>2</sub> emissions are generated per ton of aluminium produced.

In the 20th and 21st century the aluminum industry due to its large-scale requirements for cheap electricity has often been sited in locations where such electricity is available. For example Iceland, a country with no notable bauxite reserves and a population of less than half a million, is the world's twelfth largest aluminum producer due to the availability of cheap and plentiful electricity, particularly hydropower. Similarly Aluminerie Alouette in Sept-Îles, Quebec, is dependent for its electricity needs on the 5,428&nbsp;MW Churchill Falls Generating Station operated by Churchill Falls (Labrador) Corporation Limited. The company town of Kitimat in British Columbia was built by Alcan to meet the growing demand for aluminum in the postwar era. It makes use of the Kenney Dam built to power the smelters. The Tiwai Point Aluminium Smelter on the South Island of New Zealand consumes some 570&nbsp;MW of electricity, most of which is supplied by nearby Manapōuri Power Station. This amounts to around a third of the electricity demand of South Island and 13% of that of the entirety of New Zealand. Borssele Nuclear Power Station was built primarily to supply electricity to an aluminum smelter operated by French Pechiney at the time.

See also

  • Bayer process
  • History of aluminium
  • Solid oxide Hall–Héroult process
  • Hoopes process
  • Downs cell

References

Further reading

  • Grjotheim, U and Kvande, H., Introduction to Aluminium Electrolysis. Understanding the Hall–Heroult Process, Aluminium Verlag GmbH, (Germany), 1993, pp.&nbsp;260.

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