Aluminium is produced from ore (primary aluminium) and scrap (recycled aluminium). Today total aluminium production (primary and recycled) is over 55 million tonnes. Aluminium ore, most commonly bauxite, is plentiful and occurs mainly in tropical and sub-tropical areas - Africa, West Indies, South America and Australia - with some deposits in Europe.
Bauxite is mined then refined into aluminium oxide trihydrate (alumina) using the Bayer Process, which has changed very little since the first plant opened more than 100 years ago. Alumina is then electrolytically reduced into aluminium. Two to three tonnes of bauxite are required to produce one tonne of alumina and two tonnes of alumina are required to produce one tonne of aluminium metal.
In order to produce alumina, 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.
Alumina is then electrolytically reduced via a smelting process into metallic aluminium. The basis for all primary aluminium-smelting plants is the Hall-Héroult Process, invented in 1886. Alumina is dissolved in an electrolytic bath of molten cryolite (sodium aluminium fluoride) within a large carbon or graphite lined steel container known as a "pot".
An electric current is passed through the electrolyte at low voltage, but very high current, typically 200,000 amperes and up to 350,000 amperes for the latest generations. The electric current flows between a carbon anode (positive), made of petroleum coke and pitch, and a cathode (negative), formed by the thick carbon or graphite lining of the pot.
Molten aluminium is deposited at the bottom of the pot and is siphoned off periodically, taken to a holding furnace, often but not always blended to an alloy specification, cleaned and then generally cast.
An aluminium smelter consists of one or more “potlines”. Each of them typically counts around 300 pots and produces some 150,000 tonnes of aluminium annually. However, some of the latest generation of potlines are in the 200-300,000 tonnes range. A typical smelter will produce 300,000 tonnes per year and the largest ones up to one million tonnes.
The smelting process is continuous. A smelter cannot easily be stopped and restarted. If production is interrupted by a power supply failure of more than four hours, the metal in the pots will solidify, often requiring an expensive rebuilding process.
On average it takes some 15.7 kWh of electricity to produce one kilogram of aluminium from alumina. Design and process improvements have progressively reduced this figure from about 21kWh in the 1950s. Aluminium is formed at about 900°C, but once formed has a melting point of only 660°C. In some smelters this spare heat is used to melt recycled metal.
Recycled aluminium requires only 5% of the energy required to make "new" aluminium. Blending recycled metal with new metal allows considerable energy savings, as well as the efficient use of process heat. There is no difference between primary and recycled aluminium in terms of quality or properties.
Primary aluminium smelting plants are located all over the world, however, because it is an energy-intensive process, they are often in areas where there are abundant supplies of inexpensive energy, such as hydroelectric power. Once aluminium is formed it is alloyed with other materials to make an array of metals with different properties and it can be processed in any number of ways - rolled, cast, extruded.
Text: International Aluminium Institute
Photos: City of Euless, International Rivers, Met Soc