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Hall-H%C3%A9roult 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.

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 an industrial carbon-lined vat of molten cryolite, Na₃AlF₆, called a "cell". Aluminium oxide has a melting point of over 2,000 °C (3,630 °F) while pure cryolite has a melting point of 1,012 °C (1,854 °F). With a small percentage of alumina dissolved in it, cryolite has a melting point of about 1,000 °C (1,830 °F). Some Aluminium fluoride, AlF₃ is also added into the process to reduce the melting point of the cryolite-alumina mixture.

The molten mixture of cryolite, alumina, aluminum floride is next electrolyzed by passing a direct electric current through it. The electrochemical reaction causes liquid aluminium metal to be deposited at the cathode as a precipitate, while the oxygen from the alumina combines with carbon from the anode to produce carbon dioxide, CO₂, -- a waste product to be disposed of. The electric voltage across each electrolytic cell in the factory is just three to five volts, but a very high DC current is needed to pass through the cells in order to support the electrochemical reaction. In current electrochemical cells for refinining aluminum, the current through easch cell can range from 220 kA to 340 kA. The oxidation of the carbon anode reduces the required voltage across each cell, increasing the electrical efficiency, at a cost of continually replacing the carbon electrodes with new ones, and also the cost of releasing carbon dioxide into the atmosphere. Hundreds of Hall-Heroult cells are usually connected electrically in series, and they are supplied with direct current from a single set of rectifiers that the convert the alternating current (AC) supplied to the factory into direct current (DC). The the very high electric current is supplied to the cells through heavy, low electrical resistance metal busbars made of pure aluminum or copper. 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's resistance. If needed any cell can be bypassed by shunt busbars.

Hall-Heroult Industrial Cell/Pot

The liquid aluminium is taken out with the help of a siphon operating with a vacuum, in order to avoid having to use extremely high temperature valves and pumps. The liquid aluminium then may be transferred in batches or via a continuous hot flow line to a location where it is cast into aluminum ingots. The aluminum can either be cast into the form of final cast-aluminum products, or the ingots can be sent elsewhere such as a rolling mill for being pressed into sheets, or the a wire-drawing mill for producing aluminum wires and cables.

While solid cryolite is denser than solid aluminium at room temperature, the liquid aluminium product is denser than the molten cryolite at tempertures around 1000 celsius, and the aluminum sinks to the bottom of the electrolytic cell, where it is periodically collected. The tops and sides of the cells are covered with layers of solid cryolite which also act as thermal insulation. The unavoidable electric resistance within each cell produces sufficient heat to keep the cryolite-alumina mixture molten.

With the percentage of aluminum dissolved in each cell being depleted by the electrolysis in the molton cryolite, additional alumina is continually dropped into the cells to maintain the required level of alumina. Whenever a solid crust forma across the surface of the molten cryolite-alumina, this crust is broken from time to time to allow the added alumina to fall into the molten cryolite and dissolve there.

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 usually vented into the atmosphere.

The very large electric current passing through the electrolytic cells generates a powerful magnetic field, and can this can stir the molten aluminium with magneto-hydrodynamic forces in properly-designed cells. The stirring of the molten aluminium in each cell typically increases its performance, but the purity of the aluminum is reduced, since it gets mixed with small amounts of cryolite and aluminum fluoride. If the cells are designed for no stirring, they can be operated with static pools of molten aluminium so that the impurities either rise to the top of the metallic aluminium, or else sink to the bottom, leaving high-purity aluminium in the middle.

Aluminium smelters are usually sited where inexpensive hydroelectric power is available. For some European smelters, the electric power produced by hydroelectric plants in countries such as Norway, Switzerland, and Austria is transmitted by high-voltage power lines to such places as Denmark, Sweden, Germany , and Italy to be used by aluminum and magnesium factories. Since aluminium factories require nearly-uniform supplies of electric current, they make the most of nearly-constant supplies of electric power, and these are also available close to many hydroelectric power plants. To give an example of such use of hydroelectric power, the three main regions for aluminum production in North America have always been in the Tennessee River Valley of the Southeastern United States, the Columbia River Valley of Washington State and Oregon, and the St. Lawrence River Valley of southeastern Canada and the Northeastern United States.

Many decades ago, before the existence of the Tennessee Valley Authority, aluminum companies such as Alcoa even built their own hydroelectric dams and powerhouses in the Appalachian Mountains of North Carolina and Tennessee.

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.

The Legacy of the Hall-Heroult Process

Although aluminium is one of the most commonly-occurring metallic elements on Earth, before the invention of the Hall-Héroult process, it had been very difficult to extract from its ores. It was extracted by heating the ores in a vacuum with pure sodium or potassium, both of which were expensive to obtain, requiring electrolysis.

This made the little pure aluminium that had been produced in the 19th Century (and none before that) more valuable than gold or platinum. Bars of aluminium were exhibited alongside the French crown jewels at the Exposition Universelle of 1855, and Emperor Napoleon III of France was said to have reserved his few sets of aluminium dinner plates and eating utensils for his most honored guests.

Also, the pyramidal cap at the tip of the Washington Monument in Washington, D.C., was made of pure aluminium that was obtained before the time of Hall and Heroult. At the time of the Washington Monument's completion, metallic aluminium was as expensive as silver.

However, the invention of the Hall-Héroult process and the development of large sources of electric power for aluminum factories lead to production of aluminum (and incidentally of magnesium) as an inexpensive commodity. These came just in time of the great expansion in the use of electric power in homes and factories, and also for the development of metal airplanes, the production of which by the tens of thousands requires large amounts of aluminum and alumninum-magnesium alloys.

See also

• Bayer process
• Aluminium:Production and refinement

Notes

1. ^ DUBAL 2003 installed cell amperage for D20 cells
2. ^ ABB Aluminium Smelter Project Qatalum PL1 and 2 in Qatar
3. ^ This is advantageous because the aluminium is always protected from oxidation by atmospheric oxygen by the layer of molten cryolite.
4. ^ US400,664 (PDF version) (1889-04-02) Charles Martin Hall, Process of Reducing Aluminium from its Fluoride Salts by Electrolysis. 
5. ^ A National Historic Chemical Landmark: Production Of Aluminum Metal By Electrochemistry - Charles Martin Hall Solves The Aluminum Challenge
6. ^ George J. Binczewski (1995). "The Point of a Monument: A History of the Aluminum Cap of the Washington Monument". JOM 47 (11): 20–25. http://www.tms.org/pubs/journals/JOM/9511/Binczewski-9511.html. 

References

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

External links

• Hall-Héroult process description