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Hall-Héroult process
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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.
inium 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.
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Encyclopedia
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 melting point of over while cryolite has a much lower melting point; a small percentage of aluminium oxide dissolved in cryolite has a melting point of about . 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 is drawn by the circuit - in state of the art cells the cell current can be from 220kA to 340kA. Hundreds of these cells are usually arranged in series and supplied from a single transformer set that generates the current with a voltage of 1-2kV from 110kV or more high voltage supply lines. The heavy current is supplied through heavy busbars usually made of cast aluminum. The cells are electrically heated to reach the operating temperature with this current, and the anode regulator system varies the current passing through the cell by raising or lowering the anodes and changing the cell resistance. If needed any cell can be bypassed by shunt busbars.
The liquid aluminium is taken out with the help of a siphon operating with a vacuum to avoid having to use high temperature valves and pumps. The liquid aluminium then may be transferred in batches or via a continuous hot flow line to the casting facilities. The metal is then either cast into the final forms with any alloying materials needed, or cast into ingots that are remelted.
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. 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.
As the proportion of alumina is depleted in the cryolite additional alumina is added by a hopper system to maintain the alumina composition. The solid crust at the top of the bath prevents this and the crust is periodically broken to allow the added alumina to mix in with the electrolyte.
The electrolysis process produces exhaust which escapes into the fume hood and is evacuated. The exhaust is primarily CO2 produced from the anode consumption and hydrogen fluoride (HF) from the cryolite & flux. HF is a highly corrosive gas and attacks glass surfaces which means that cranes and heavy equipment used in the plant need glass windscreens and windows to be covered with plastic film. The gases are usually treated in adjacent treatment plants which dissolve the HF in water and neutralize it. The particulates are also captured and reused using electrostatic or bag filters. The remaining CO2 is exhausted into the atmosphere.
The large current causes heavy magenetic fields, and can stir the aluminium with magnetohydrodynamic (MHD) forces. The stirring of aluminium in the cell increases its performance, but reduces the purity, since the materials get evenly mixed. Otherwise the cell can be operated with static aluminium pool so that the impurities either rise to the top of the aluminium or sink to the bottom leaving high purity aluminium in the middle.
Aluminium smelters are usually sited where economical hydroelectric, wind or fossil fuel power is available. In some european smelters, the electrical energy produced in countries such as norway is transported via high voltage lines to germany and other areas and used by smelters. Since aluminium smelters require constant supply they allow the best use of constant generation capacity, and can be used to increase the base load to make the demand more constant and less cyclical. This can make the overall electrical generation and transmission system more economical for end users.
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.
History
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, 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.
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. 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
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