The process of making aluminium

 

Aluminum is produced in two phases: the Bayer process, which refines bauxite ore to produce aluminium oxide, and the Hall-Heroult process, which smelts the aluminium oxide to produce pure aluminium.


The Bayer method

The bauxite ore is first mechanically crushed. The crushed ore is then combined with caustic soda and ground in a mill to produce a slurry (a watery suspension containing extremely fine ore particles).

2 The slurry is pumped into a digester, which is a pressure cooker-like tank. The slurry is heated to 230-520°F (110-270°C) at 50 lb-in 2 of pressure (340 kPa). These conditions are maintained for anywhere between thirty minutes and several hours. Additional caustic soda may be added to ensure complete dissolution of all aluminum-containing compounds.

3 The hot slurry, which is now a sodium aluminate solution, is passed through a series of flash tanks that lower the pressure and recover heat for reuse in the refining process.

4 Pump the slurry into a settling tank. Impurities that do not dissolve in the caustic soda settle to the bottom of this tank as the slurry rests. One manufacturer compares this process to fine sand settling to the bottom of a glass of sugar water. Since the sugar is dissolved in the water, it does not settle out, just as the aluminium in the settling tank remains dissolved in the caustic soda. Red mud is the accumulation of fine sand, iron oxide, and trace element oxides such as titanium at the bottom of the tank.

5 Following the removal of impurities, 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 series of precipitation tanks that are six stories tall. Through the top of each tank, seed crystals of alumina hydrate (alumina bonded to water molecules) are added. As the seed crystals settle through the liquid, dissolved alumina adheres to them, causing them to grow.

7 The crystals settle to the bottom of the tank and are subsequently removed. Following washing, the grains are transferred to a kiln for calcining (heating to release the water molecules that are chemically bonded to the alumina molecules). A continuous stream of crystals is transported by a screw conveyor into a rotating, cylindrical kiln that is tilted to allow gravity to move the material through it. At a temperature of 2,000° F (1,100° C), the water molecules are driven off, leaving crystals of anhydrous (waterless) alumina. After exiting the kiln, the crystals are cooled in a cooler.

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