Hall-Héroult process

The Hall-Héroult process is the major industrial process for the production of aluminium.

In the Hall-Héroult process alumina, Al₂O₃ is dissolved in a carbon-lined bath of molten cryolite, Na₃AlF₆. Aluminium fluoride, AlF₃ is also present to reduce the melting point of the cryolite. The mixture is electrolyzed, which reduces the liquid aluminium. This causes the liquid aluminium to be deposited at the cathode as a precipitate. The carbon anode is oxidized and bubbles away as carbon dioxide. The electrical current used by many smelters, has a very low voltage, but massive amperage. This is typically 3-5 volts, but 150,000 amperes. The overall chemical reaction is

2 Al₂O₃ + 3 C → 4 Al + 3 CO₂

The liquid aluminium 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 need of electrical power and pollution of the surroundings were early problems with this reaction. The use of hydroelectric power plants and new filter systems has resolved this to some extent, but the problem still exists.

The Hall-Héroult process was discovered independently and almost simultaneously in 1886 by the American chemist Charles Martin Hall and the Frenchman Paul Héroult. In 1888, Hall opened the first large-scale aluminium production plant in Pittsburgh.

The Hall-Héroult process is used all over the world, and is the only method of aluminium smelting currently used in the industry. Today, there are two primary technologies using the Hall-Héroult process: Soderberg and prebake. Soderberg uses a continuously created anode made by addition of "pitch" to the top of the anode. The lost heat from the smelting operation is used to bake the pitch into the carbon form required for reaction with alumina. Prebake technology is named after its anodes, which are baked in very large gas-fired ovens at high temperature before being lowered by various heavy industrial lifting systems into the electrolytic solution. In both technologies, the anode, attached to a very large electrical bus, is slowly used up by the process. Prebake technology tends to be very slightly more efficient, but is more labor intensive. Prebake technology is becoming preferred in the industry because of the various pollutant emissions related to the creation of the anode from liquid pitch.

See also

*Bayer process
*Aluminum separation

Patents

* US400664 C. M. Hall