Hall–Héroult process
The Hall–Héroult process is the major industrial process for smelting aluminium. It involves dissolving aluminium oxide (alumina) (obtained most often from bauxite, aluminium's chief ore, through the Bayer process) in molten cryolite, and electrolysing the molten salt bath, typically in a purpose-built cell. The Hall–Héroult process applied at industrial scale happens at 940–980°C and produces 99.5–99.8% pure aluminium. Recycled aluminum requires no electrolysis, thus it does not end up in this process.
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 2072 °C so electrolysing it is impractical. In the Hall–Héroult process, alumina, Al2O3, is dissolved in molten synthetic cryolite, Na3AlF6, to lower its melting point for easier electrolysis.
In the Hall–Héroult process the following simplified reactions take place at the carbon electrodes:
Overall:
In reality much more CO2 is formed at the anode than CO:
Pure cryolite has a melting point of 1009 ± 1°C. With a small percentage of alumina dissolved in it, its melting point drops to about 1000°C. Besides having a relatively low melting point, cryolite is used as an electrolyte because among other things it also solves alumina well, conducts electricity, dissociates electrolytically at higher voltage than alumina and has a lighter density than aluminum at the temperatures required by the electrolysis.
Aluminium fluoride (AlF3) is usually added to the electrolyte. The ratio NaF/AlF3 is called the cryolite ratio and it is 3 in pure cryolite. In industrial production, AlF3 is added so that the cryolite ratio is 2–3 to further reduce the melting point so that the electrolysis can happen at temperatures between 940 and 980°C. The density of liquid aluminum is 2.3 g/ml at temperatures between 950 and 1000°C. The density of the electrolyte should be less than 2.1 g/ml so that the molten aluminum separates from the electrolyte and settles properly to the bottom of the electrolysis cell. In addition to AlF3, other additives like lithium fluoride may be added to alter different properties (melting point, density, conductivity etc.) of the electrolyte.
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