Hall-Héroult process

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

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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 considerable amperage. This is typically 3-5 volts, but 150,000 amperes, however there are plans to construct a 500,000 Ampere pilot smelter in Saguenay, Canada [1]. 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. (Note: Solid cryolite at room temperature is denser than solid aluminium at room temperature. However, molten cryolite at about 1000°C is less dense than molten aluminium at the same temperature. Thus molten aluminium sinks to the bottom. This is good because the aluminium will be protected from oxidation by the covering molten cryolite.) 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^[1] 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: Söderberg and prebake. Söderberg 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 because the oxygen generated by the electrolytic process can oxidize the carbon anode. 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.

A book was published on this process by some Norwegian scientists: "Understanding the Hall-Heroult process for production of aluminium", edited by Kai Grjotheim and Halvor Kvande, published by Aluminium-Verlag, Dusseldorf in 1986.

In 1997 the Hall-Héroult process was designated an ACS National Historical Chemical Landmark in recognition of the importance of the commercialization of aluminum.[2]

Precious metal status

Although aluminium is one of the most commonly occurring elements on Earth, before Hall-Héroult it was initially found to be exceedingly difficult to extract from its various ores. This made the little available pure aluminium which had been discovered (or refined at great expense) more valuable than gold. Bars of aluminium were exhibited alongside the French crown jewels at the Exposition Universelle of 1855, and Napoleon III was said to have reserved a set of aluminium dinner plates for his most honored guests. Additionally, the pyramidal top to the Washington Monument is made of pure aluminium. At the time of the monument's construction, aluminium was more expensive than silver, gold, or platinum. Over time, however, the price of the metal has dropped; the invention of the Hall-Héroult process caused the high price of aluminium to permanently collapse.

See also

• Bayer process
• Aluminum separation

References

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

1. ^ US400,664 (PDF version) (1889-04-02) Charles Martin Hall Process of Reducing Aluminium from its Fluoride Salts by Electrolysis 

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