Hall-Héroult process
The Hall-Héroult process is a crucial industrial method for extracting aluminum from alumina (Al₂O₃) through electrolysis. This process begins with bauxite ore, which is refined into alumina using the Bayer method. The alumina is then dissolved in a molten cryolite (Na₃AlF₆) bath, where the addition of aluminum fluoride lowers the melting point. Developed independently in 1886 by Charles Martin Hall and Paul Héroult, this method allows aluminum to be economically separated due to its efficient use of energy — operating at low voltages (3 to 5 volts) and high amperages (up to 350,000 amps).
As electrolysis occurs, liquid aluminum collects at the cathode, while oxygen is produced at the anode, which reacts with the carbon electrodes to generate carbon dioxide. This gas is subsequently treated before being released into the atmosphere. The process involves continuously adding alumina as it gets consumed, while the carbon-lined electrolytic cell facilitates the collection of the liquid aluminum. The end product is typically 99.7% pure aluminum, which can then be cast or alloyed as needed. The production of carbon anodes can be achieved through two methods: the Söderberg and prebake techniques.
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Hall-Héroult process
Aluminum is second only to iron as the most used metal. Although aluminum is the most common metal in the Earth’s surface, 8 percent by weight, it is almost always combined with other elements. Aluminum is difficult to separate from the common ores of aluminum such as oxides and silicates. The Hall-Héroult process separates aluminum from bauxite ore. The process is the only industrial source of aluminum.
Definition
The Hall-Héroult process is the process by which aluminum is separated from alumina, A12O3, through electrolysis. The alumina is made from ore. The alumina is dissolved in a carbon-lined bath of molten cryolite, Na3AlF6. Aluminum fluoride is added to reduce the melting point of cryolite. During electrolysis, liquid aluminum is deposited at the cathode.
Overview
In 1886, Charles Martin Hall, an American, and Paul Héroult, a Frenchman, developed the process separately. The key ideas were that aluminum can be isolated by electrolysis and that a small amount of alumina could be dissolved in molten cryolite at a much lower temperature than the melting point of alumina. The energy saved allowed aluminum to be separated at an economical price. Carbon electrodes are used with a low voltage of 3 to 5 volts and a high amperage of up to 350,000 amps. Oxygen is produced at the anode and reacts with the electrode to produce carbon dioxide. The carbon dioxide is exhausted into the after it is cleaned. Some hydrogen fluoride is also produced and is removed in a water bath before the carbon dioxide is exhausted. As the alumina is used up, new alumina is added by breaking through the solid crust that develops on the surface. The electrolytic cell is lined with carbon, but a layer of cryolite forms on the carbon. The liquid aluminum falls to the bottom of the cell, where it is siphoned off by a vacuum system. Then it is transferred to a casting area where it is either poured into a mold to solidify or alloyed with other elements and cast. The aluminum produced by most smelters is about 99.7 percent pure.
There are two technologies for producing the carbon anodes: Söderberg and prebake. In the Söderberg method, coke and coal tar are added continuously to the anode. The heat from the electrolytic cell bakes the electrode to the form needed for electrolysis. In the prebake method, the electrodes are baked in large ovens before being placed in the electolytic cell.
The alumina used in the electrolytic cell is prepared by the Bayer method. Bauxite is dissolved in concentrated sodium hydroxide. The insoluble compounds in bauxite are filtered off. The alumina in the filtrate is precipitated, washed, dried, and ground into a fine white powder. The sodium hydroxide can be recycled for further use. About 3.6 metric tons of bauxite are required to form 1.8 metric tons of alumina, and 1.8 metric tons of alumina are required to form 0.9 metric ton of aluminum.
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Ratvik, Arne Petter, et al. "Alumin Production Process: From Hall-Héroult to Modern Smelters." ChemTexts, 25 Feb. 2022, doi.org/10.1007/s40828-022-00162-5. Accessed 27 Dec. 2024.