In the Hall-Heroult process alumina, Al2O3 is dissolved in a carbon-lined bath of molten cryolite, Na3AlF6. Aluminum fluoride, AlF3 is also present to reduce the melting point of the cryolite. The mixture is electrolyzed, and liquid aluminum is produced at the cathode. The carbon anode is oxidized and bubbles away as carbon dioxide. The overall chemical reaction is
The liquid aluminum product is denser than the molten cryolite and sinks to the bottom of the bath, where it is periodically collected. The top and sides of the bath are covered with a crust of solid cryolite which acts as thermal insulation. Electrical resistance within the bath provides sufficient heat to keep the cryolite molten.
The need of electrical power and pollution of the surroundings were early problems with this reaction. The use of hydroelectric power plants and new filter systems has resolved this to some extent.
The Hall-Heroult process was discovered independently and almost simultaneously in 1886 by the American chemist Charles Hall and the Frenchman Paul Heroult. In 1888, Hall opened the first large-scale aluminum production plant in Pittsburg.
The Hall-Héroult process is used all over the world, and is the only method of aluminum smelting currently used in the industry. Today, there are two primary technologies using the Hall-Héroult process: Soderberg and prebake. Soderberg uses liquid anodes, poured continuously ontop of the hot electrolytic bath. 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 electircal bus, is slowly used up by the process. Prebake technology tends to be very slightly more efficient, but is more labor intensive.