Aluminium Smelting

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	Anode Effect Survey 1994 - 1997

The basis for all modern 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 150,000 amperes. 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.

A typical aluminium smelter consists of around 300 pots. These will produce some 125,000 tonnes of aluminium annually. However, some of the latest generation of smelters are in the 350-400,000 tonne range.

On average, around the world, 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 1950's.

Smelter Energy Use

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.

La Grande Baie Smelter in Quebec, Canada

Recycled aluminium requires only 5 per cent 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.

Aluminium smelting is energy intensive, which is why the world's smelters are located in areas which have access to abundant power resources (hydro-electric, natural gas, coal or nuclear). Many locations are remote and the electricity is generated specifically for the aluminium plant.

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.

From time to time individual pot linings reach the end of their useful life and the pots are then taken out of service and relined.

Most smelters produce aluminium of 99.7% purity, which is acceptable for most applications. However, super purity aluminium (99.99%) is used for some special applications, typically those where high ductility or conductivity is required. The marginal difference in the purities of smelter grade aluminium and super purity aluminium results in significant changes in the properties of the metal.

Typical Applications