Hall–Héroult process
Furthermore, the process generates fluorocarbon compounds as byproducts, contributing to both air pollution and climate change.
In the Hall–Héroult process, alumina, Al2O3, is dissolved in molten synthetic cryolite, Na3AlF6, to lower its melting point for easier electrolysis.
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 (1700 to 1800°F).
The liquid aluminium is removed from the cell via a siphon every 1 to 3 days in order to avoid having to use extremely high temperature valves and pumps.
Collected aluminium from different cells in a factory is finally melted together to ensure uniform product and made into metal sheets.
The electrolytic mixture is sprinkled with coke to prevent the anode's oxidation by the oxygen involved.
The exhaust is primarily CO2 produced from the anode consumption and hydrogen fluoride (HF) from the cryolite and flux (AlF3).
Properly designed cells can leverage magnetohydrodynamic forces induced by the electrolysing current to agitate the electrolyte.
The materials most often used in anodes, coke and pitch resin, are mainly residues from the petroleum industry and need to be of high enough purity so no impurities end up into the molten aluminum or the electrolyte.
More material to the top of the anode is continuously added in the form of briquettes made from coke and pitch.
The lost heat from the smelting operation is used to bake the briquettes into the carbon form required for the reaction with alumina.
[1] Prebaked anodes are baked in very large gas-fired ovens at high temperature before being lowered by various heavy industrial lifting systems into the electrolytic solution.
This heats up the gas layer and causes it to expand, thus further reducing the surface area where electrolyte and anode are in contact with each other.
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.
This meant that the cost to produce the small amount of aluminium made in the early 19th century was very high, higher than for gold or platinum.
[6] Bars of aluminium were exhibited alongside the French crown jewels at the Exposition Universelle of 1855, and Emperor Napoleon III of France was said[citation needed] to have reserved his few sets of aluminium dinner plates and eating utensils for his most honored guests.
[7] The Hall–Héroult process was invented independently and almost simultaneously in 1886 by the American chemist Charles Martin Hall[8] and by the Frenchman Paul Héroult[9]—both 22 years old.
