Aluminum is produced in two stages: the Bayer process, which involves refining bauxite ore to produce aluminium oxide, and the Hall-Heroult process, which involves smelting the aluminium oxide to produce pure aluminium.
The Bayer procedure
1 The bauxite ore is mechanically crushed first. The crushed ore is then mixed with caustic soda and ground in a grinding mill to create a slurry (a watery suspension) containing very fine ore particles.
2 The slurry is pumped into a digester, which is a pressure-cooking tank. The slurry is heated to 230-520°F (110-270°C) under 50 lb/in 2 pressure (340 kPa). These conditions are maintained for a period of time ranging from 30 minutes to several hours. To ensure that all aluminum-containing compounds are dissolved, more caustic soda may be added.
3 The hot slurry, which is now a sodium aluminate solution, is routed through a series of flash tanks, which reduce pressure and recover heat for reuse in the refining process.
4 The slurry is transferred to a settling tank. Impurities that will not dissolve in the caustic soda settle to the bottom of the tank as the slurry rests in this tank. This process, according to one manufacturer, is analogous to fine sand settling to the bottom of a glass of sugar water; the sugar does not settle out because it is dissolved in the water, just as the aluminium in the settling tank remains dissolved in the caustic soda. The residue (dubbed "red mud") that accumulates at the tank's bottom is made up of fine sand, iron oxide, and trace element oxides such as titanium.
5 Once the impurities have been removed, the remaining liquid, which resembles coffee, is pumped through a series of cloth filters. The filters capture any fine particles of impurities that remain in the solution. This material is washed to recover reusable alumina and caustic soda.
6 The filtered liquid is pumped through a six-story-tall series of precipitation tanks. Alumina hydrate seed crystals (alumina bonded to water molecules) are added through the top of each tank. As the seed crystals settle through the liquid, dissolved alumina adheres to them.
7 The crystals precipitate (fall to the tank's bottom) and are removed. They are washed and then placed in a kiln to be calcined (heating to release the water molecules that are chemically bonded to the alumina molecules). A screw conveyor transports crystals in a continuous stream into a rotating, cylindrical kiln that is tilted to allow gravity to move the material through it. A temperature of 2,000° F (1,100° C) causes the water molecules to evaporate, resulting in anhydrous (waterless) alumina crystals. The crystals are cooled after they exit the kiln.
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