The aluminium industry relies on the Bayer process to produce alumina from bauxite. It remains the most economic means of obtaining alumina, which in turn is used for the production of aluminium metal - some two tonnes of alumina are required to produce on tonne of aluminium.

The primary aluminium industry is dependent on a regular supply of alumina for four functions:

• basic raw material for aluminium production
• thermal insulator for the top of electrolytic cells
• coating for prebaked anodes
• absorbent filter for cell emissions

Alumina production

Bauxite is washed, ground and dissolved in caustic soda (sodium hydroxide) at high pressure and temperature. The resulting liquor contains a solution of sodium aluminate and undissolved bauxite residues containing iron, silicon, and titanium. These residues sink gradually to the bottom of the tank and are removed. They are known colloquially as red mud.

The clear sodium aluminate solution is pumped into a huge tank called a precipitator. Fine particles of alumina are added to seed the precipitation of pure alumina particles as the liquor cools. The particles sink to the bottom of the tank, are removed, and are then passed through a rotary or fluidised calciner at 1100°C to drive off the chemically combined water. The result is a white powder, pure alumina.

The caustic soda is returned to the start of the process and used again.

Top of page