making metal
Bauxite ore contains 30-54% alumina from which aluminium is extracted. A mixture of silica, various iron oxides, and titanium dioxide is removed from the alumina before it is refined to aluminium metal.
In the Bayer Process, 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 which are removed as "red mud". The clear sodium aluminate solution is seeded with fine particles of alumina in a large tank to precipitate pure alumina particles as the liquor cools. Alumina particles are removed and passed through a rotary calciner to drive off the water to produce a white powder, pure alumina.
The alumina is smelted in the Hall-Héroult process to produce aluminium. Alumina is dissolved in a carbon-lined bath of molten sodium aluminium fluoride and the mixture is electrolyzed. This causes the liquid aluminium to be deposited at the cathode as a precipitate, while the carbon anode is oxidized to carbon dioxide. The liquid aluminium is removed by a vacuum syphon and transferred in batches or via a continuous hot flow line to casting facilities and then on to mills to produce the rolled coils ready for the can makers.
Iron ore is combined with coke and heated to produce an iron rich material called ‘sinter’. This is fed with more iron ore, coke and limestone into a blast furnace. Hot air is blasted into the bottom to raise the temperature in the furnace to around 2200 degrees C. This white hot temperature produces a chemical reduction and melting of the sinter and iron ore, forming a pool of molten iron in the lower part of the furnace – the base material from which steel is made.
The iron undergoes further heating with recycled material being added. Other elements are added to adjust the steel’s composition to make it suitable for can production. The cooled metal transformed into coils by a process of hot and cold rolling, annealing, tempering and finishing before finally coating with tin, chromium or polymers ready for conversion into cans.
