The Manufacturing Basics of Aluminium
MANUFACTURING PROCESS
1. Bauxite Mining
Bauxite is used as the main raw material in the manufacture of aluminium. It is a clay mineral found in tropical and sub-tropical areas such as Australia and the West Indies.
The bauxite is often mined from only a few metres below the ground level. About 4-5 tonnes of bauxites are required for 1 tonne of aluminium.
To extract pure alumina, the Bayer process is used.
2. Bauxite grinding
The bauxite mineral is transported to refineries where the clay is washed off and the bauxite passes through a grinder to produce a more consistent material.
3. Bauxite crushing and digesting – producing sodium aluminate
The ground mineral is pumped into large pressure tanks with a caustic soda or sodium hydroxide solution, and a steam heat is applied. The caustic soda reacts with the aluminium compounds in the bauxite material to produce a solution of sodium aluminate (also known as slurry). The unwanted residues (also known as red mud), containing iron, silicon and titanium, gradually sink to the bottom of the tank and are removed.
4. Settling
The sodium aluminate solution is then passed through into lower pressure settling tanks. The solution at the top of the tanks is directed downwards through a series of filers to remove excess red mud. The remaining alumina is then passed through huge “leaves” or cloth filters to remove any solids in the solution.
5. Precipitation
The sodium aluminate solution is then cooled and pumped into large precipitators (sometimes as tall as a 6-story building). Aluminium hydroxide seed crystals added to the solution to start the precipitation process. At this point, large aluminium crystals are formed.
6. Calcination
The crystals are then heated in rotary kilns to temperatures over 960°C. This extracts the last impurities and creates a white powder, known as alumina or aluminium oxide.
The refined alumina is transformed into aluminium through the smelting or Hall–Héroult process.
7. Smelting process
The alumina is poured into a reduction cell with 950°C molten cryolite. 400kA electrical currents are passed through the mixture to break the bond between the aluminium and oxygen. The result is 99.8% pure aluminium.
