Hall–Héroult process

Hall–Héroult process is the major industrial process for smelting aluminium. It involves dissolving aluminium oxide (alumina) in molten cryolite, and electrolysing the molten salt bath, typically in a purpose-built cell.

Process

Theory

A Hall–Héroult industrial cell

Elemental aluminium cannot be produced by the electrolysis of an aqueous aluminium salt because hydronium ions readily oxidize elemental aluminium. Although a molten aluminium salt could be used instead, aluminium oxide has a melting point of over 2,000 °C (3,600 °F) so electrolyzing it is impractical. In the Hall–Héroult process alumina, Al₂O₃, is dissolved in molten cryolite, Na₃AlF₆, to lower its melting point for easier electrolysis. This also increases conductivity of the solution.

Pure cryolite has a melting point of 1,012 °C (1,854 °F). With a small percentage of alumina dissolved in it, its melting point drops to about 1,000 °C (1,800 °F). Aluminium fluoride, AlF₃ is added to the mixture to further reduce the melting point.

A molten mixture of cryolite, alumina and aluminium fluoride is electrolyzed by passing low voltage (3-5V) direct current through it. This causes liquid aluminium metal to be deposited at the cathode while the oxygen from the alumina combines with carbon from the anode to produce carbon dioxide. An industrial-scale smelter typically operates at about 150,000 amperes per cell.^[1][2]

Cell operation

Temperature within the cell is maintained via electrical resistance. Oxidation of the carbon anode increases the electrical efficiency at a cost of consuming the carbon electrodes and producing carbon dioxide.

While solid cryolite is denser than solid aluminium at room temperature, liquid aluminium is denser than molten cryolite at temperatures around 1,000 °C (1,830 °F). The aluminium sinks to the bottom of the electrolytic cell, where it is periodically collected. The liquid aluminium is removed from the cell via a siphon in order to avoid having to use extremely high temperature valves and pumps. Alumina is added to the cells as the aluminum is removed.

The cell produces gases at the anode. The exhaust is primarily CO₂ produced from the anode consumption and hydrogen fluoride (HF) from the cryolite and flux. The gases are either treated or vented into the atmosphere; the former involving neutralization of the HF to its sodium salt, sodium fluoride. Particulates are captured using electrostatic or bag filters. The CO₂ is usually vented into the atmosphere.

Agitation of the molten material in the cell increases its production rate at the expense of an increase in cryolite impurities in the product. Properly designed cells can leverage magnetohydrodynamic forces induced by the electrolyzing current to agitate the electrolyte. In non-agitating static pool cells the impurities either rise to the top of the metallic aluminium, or else sink to the bottom, leaving high-purity aluminium in the middle area.

Variations

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 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.

History

An existing need

Aluminium is the most abundant metallic element in the Earth's crust, but it is rarely found in its elemental state. It occurs in many minerals but its primary commercial source is bauxite, a mixture of hydrated aluminium oxides and compounds of other elements such as iron.

Prior to the Hall–Héroult process, elemental aluminium was made by heating ore along with elemental sodium or potassium in a vacuum. The method was complicated and consumed materials that were in themselves expensive at that time. This meant the cost to produce the small amount of aluminium made in the early 19th century was very high, higher than for 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.

Production costs using older methods did come down, but when aluminium was selected as the material for the cap/lightning rod to sit atop the Washington Monument in Washington, D.C., it was still more expensive than silver.^[3]

Independent discovery

The Hall–Héroult process was invented independently and almost simultaneously in 1886 by the American chemist Charles Martin Hall,^[4] assisted by his sister Julia Brainerd Hall,^[5] and by the Frenchman Paul Héroult. In 1888, Hall opened the first large-scale aluminium production plant in Pittsburgh. It later became the Alcoa corporation.

In 1997 the Hall–Héroult process was designated a National Historic Chemical Landmark by the American Chemical Society in recognition of the importance of the process in the commercialization of aluminium.^[6]

Economic impact

Aluminum produced via the Hall–Héroult process, in combination with cheaper electric power, helped make aluminium (and incidentally magnesium) an inexpensive commodity rather than a precious metal.

This in turn helped make it possible for pioneers like Hugo Junkers to utilize aluminium and aluminium-magnesium alloys to make items like metal airplanes by the thousands, or Howard Lund to make aluminium fishing boats.^[7]

See also

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

References