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Hall-Heroult process

The Hall-Heroult process is the major industrial process for the production of aluminum.

In the Hall-Heroult process alumina, Al2O3 is dissolved in a carbon-lined bath of molten cryolite, Na3AlF6. Aluminum fluoride[?], AlF3 is also present to reduce the melting point of the cryolite. The mixture is electrolyzed, and liquid aluminum is produced at the cathode. The carbon anode is oxidized and bubbles away as carbon dioxide. The overall chemical reaction is

2Al2O3 + 3C → 4Al + 3CO2

The liquid aluminum product is denser than the molten cryolite and sinks to the bottom of the bath, where it is periodically collected. The top and sides of the bath are covered with a crust of solid cryolite which acts as thermal insulation. Electrical resistance within the bath provides sufficient heat to keep the cryolite molten.

The demand of electrical power and the polution of the surounding were problems with this reaction. The use of hydroelectric power plants and new filtersystems changed this.

The Hall-Heroult process was discovered independently and almost simultaneously in 1886 by the American chemist Charles Hall[?] and the Frenchman Paul Heroult. In 1888, Hall opened the first large-scale aluminum production plant in Pittsburg.

See also Bayer process

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