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Processes - Non-Ferrous (Aluminum / Copper / Zinc / Lead / Magnesium): Material Processing - Primary Aluminum

Primary aluminum is produced using two main processes to refine the bauxite ore (the Bayer Process) into alumina and then from the alumina to produce aluminum (the Hall-Heroult Process). Summary descriptions of each main process are as follows:

Bayer
Pre-processed (sometimes dried, crushed, and screened) bauxite ore, from the mine, is further crushed and ground to particle size for extraction of alumina through digestion by hot sodium hydroxide liquor. After removal of "red mud" and fine solids from the process liquor, aluminum trihydrate crystals are precipitated and then calcined in rotary kilns or fluidized bed calciners to produce alumina (Al2O3). Two to three tons of bauxite is required to make one ton of alumina, depending on the grade of the bauxite ore.

Hall Heroult
Primary aluminum is produced in the Hall Heroult process via a series of furnaces called pots or cells. The pots are carbon-lined steel tube that hold a molten electrolyte, called cryolite, which is infused with alumina (from the Bayer process). Electrical current flows into the pot through a carbon anode (positive) inserted in the cryolite, exiting through the cathode (negative), the thick carbon or graphite lining of the pot. The carbon of the anode combines with the oxygen of the alumina, producing molten aluminum, CO2, and CO. Molten aluminum is deposited at the bottom of the pot and is siphoned off periodically to a holding furnace. Alloy additions are usually made and then the aluminum is cast.