Hall-Héroult process

The Hall-Héroult process is the major industrial process for the production of aluminium. It involves dissolving alumina in molten cryolite, and electrolysing the solution to obtain pure aluminium metal.

Process

Aluminium cannot be produced by the electrolysis of an aluminium salt dissolved in water because of the high reactivity of aluminium. An alternative is the electrolysis of a molten aluminium compound.

In the Hall-Héroult process alumina, Al₂O₃ is dissolved in a carbon-lined bath of molten cryolite, Na₃AlF₆. Aluminium oxide has a very high melting point (over 2000 °C) while cryolite has a much lower melting point; a small percentage of aluminium oxide dissolved in cryolite has a melting point of about 1000 °C. Aluminium fluoride, AlF₃ is also present to reduce the melting point of the cryolite.

The mixture is electrolyzed. This causes the liquid aluminium to be deposited at the cathode as a precipitate, while the carbon anode is oxidized to carbon dioxide. The electrical source used by many smelters has a very low voltage (typically 3-5 volts), but a considerable amount of current (150,000 amperes) is drawn by the circuit.Fact|date=June 2008

While solid cryolite is denser than solid aluminium at room temperature, the liquid aluminium product is denser than the molten cryolite, at about 1000 °C, and sinks to the bottom of the bath, where it is periodically collected. [. This is advantageous because the aluminium will be protected from atmospheric oxidation by the 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.Fact|date=June 2008

History

The Hall-Héroult process was discovered independently and almost simultaneously in 1886 by the American chemist Charles Martin HallUS patent reference|number = 400664|y = 1889 | m=04|d=02|inventor=Charles Martin Hall|title=Process of Reducing Aluminium from its Fluoride Salts by Electrolysis] and the Frenchman Paul Héroult. In 1888, Hall opened the first large-scale aluminium production plant in Pittsburgh, which would eventually evolve into the Alcoa corporation.

In 1997 the Hall-Héroult process was designated an ACS National Historical Chemical Landmark in recognition of the importance of the commercialization of aluminium. [ [http://acswebcontent.acs.org/landmarks/landmarks/cmh/index.html A National Historic Chemical Landmark: Production Of Aluminum Metal By Electrochemistry - Charles Martin Hall Solves The Aluminum Challenge ] ]

Development

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 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.

Legacy

Although aluminium is one of the most commonly occurring elements on Earth, before the invention of the Hall-Héroult process, 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 as expensive as silver [cite journal | author = George J. Binczewski | title = The Point of a Monument: A History of the Aluminum Cap of the Washington Monument | journal = JOM | volume = 47 | issue = 11 | pages = 20–25 | year = 1995 | url = http://www.tms.org/pubs/journals/JOM/9511/Binczewski-9511.html] . 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.

ee also

*Bayer process
*

References

*Grjotheim, U and Kvande, H., [http://www.csa.com/partners/viewrecord.php?requester=gs&collection=TRD&recid=199501420050MD&recid=199501P10015AI Introduction to Aluminium Electrolysis. Understanding the Hall-Heroult Process] , Aluminium Verlag GmbH, (Germany), 1993, pp. 260.