Aluminium smelting
Encyclopedia
Aluminium
Aluminium or aluminum is a silvery white member of the boron group of chemical elements. It has the symbol Al, and its atomic number is 13. It is not soluble in water under normal circumstances....
from its oxide alumina, generally by the Hall-Héroult process
Hall-Héroult process
The Hall–Héroult process is the major industrial process for the production of aluminium. It involves dissolving alumina in molten cryolite, and electrolysing the molten salt bath to obtain pure aluminium metal.-Process:...
. Alumina is extracted from the ore Bauxite
Bauxite
Bauxite is an aluminium ore and is the main source of aluminium. This form of rock consists mostly of the minerals gibbsite Al3, boehmite γ-AlO, and diaspore α-AlO, in a mixture with the two iron oxides goethite and hematite, the clay mineral kaolinite, and small amounts of anatase TiO2...
by means of the Bayer process
Bayer process
The Bayer process is the principal industrial means of refining bauxite to produce alumina .Bauxite, the most important ore of aluminium, contains only 30–54% alumina, Al2O3, the rest being a mixture of silica, various iron oxides, and titanium dioxide. The alumina must be purified before it can...
at an alumina refinery.
This is an electrolytic
Electrolysis
In chemistry and manufacturing, electrolysis is a method of using a direct electric current to drive an otherwise non-spontaneous chemical reaction...
process, so an aluminium smelter
Smelting
Smelting is a form of extractive metallurgy; its main use is to produce a metal from its ore. This includes iron extraction from iron ore, and copper extraction and other base metals from their ores...
uses prodigious amounts of electricity; they tend to be located very close to large power stations, often hydro-electric ones, and near ports since almost all of them use imported alumina.
Layout of an aluminium smelter
An aluminium smelter consists of a large number of pots, steel containers lined with carbon, in which the electrolysis takes place; smelting is run as a batch process, with the aluminium metal deposited at the bottom of the pots and periodically drained off. Power must be constantly available, since the pots have to be repaired at significant cost if the liquid metal solidifies.The anode
Anode
An anode is an electrode through which electric current flows into a polarized electrical device. Mnemonic: ACID ....
s are made of carbon, generally derived from pitch.
A typical smelter contains about 300 pots, each of which produces about a ton of aluminium a day, though the largest proposed smelters are up to five times that capacity.
Environmental issues of aluminium smelters
The process produces a quantity of fluorideFluoride
Fluoride is the anion F−, the reduced form of fluorine when as an ion and when bonded to another element. Both organofluorine compounds and inorganic fluorine containing compounds are called fluorides. Fluoride, like other halides, is a monovalent ion . Its compounds often have properties that are...
waste: perfluorocarbons and hydrogen fluoride
Hydrogen fluoride
Hydrogen fluoride is a chemical compound with the formula HF. This colorless gas is the principal industrial source of fluorine, often in the aqueous form as hydrofluoric acid, and thus is the precursor to many important compounds including pharmaceuticals and polymers . HF is widely used in the...
as gases, and sodium
Sodium fluoride
Sodium fluoride is an inorganic chemical compound with the formula NaF. A colorless solid, it is a source of the fluoride ion in diverse applications. Sodium fluoride is less expensive and less hygroscopic than the related salt potassium fluoride....
and aluminium fluoride
Aluminium fluoride
Aluminium fluoride is an inorganic compound used primarily in the production of aluminium. This colourless solid can be prepared synthetically but also occurs in nature.-Production and occurrence:...
s and unused cryolite
Cryolite
Cryolite is an uncommon mineral identified with the once large deposit at Ivigtût on the west coast of Greenland, depleted by 1987....
as particulates. This can be as small as 0.5 kg per ton of aluminium in the best plants in 2007, up to 4 kg per ton of aluminium in older designs in 1974. Unless carefully controlled, these fluorides tend to be very toxic to vegetation around the plants.
The Soderburgh process which pre-bakes pitch to form the anode, produces significant emissions of polycyclic aromatic hydrocarbon
Polycyclic aromatic hydrocarbon
Polycyclic aromatic hydrocarbons , also known as poly-aromatic hydrocarbons or polynuclear aromatic hydrocarbons, are potent atmospheric pollutants that consist of fused aromatic rings and do not contain heteroatoms or carry substituents. Naphthalene is the simplest example of a PAH...
s as the pitch is consumed in the smelter.
The linings of the pots end up contaminated with cyanide-forming materials; Alcoa
Alcoa
Alcoa Inc. is the world's third largest producer of aluminum, behind Rio Tinto Alcan and Rusal. From its operational headquarters in Pittsburgh, Pennsylvania, Alcoa conducts operations in 31 countries...
has a process for converting spent linings into aluminium fluoride for reuse and synthetic sand usable for building purposes and inert waste.
Example aluminium smelters
- Alcan Lynemouth Aluminium SmelterAlcan Lynemouth Aluminium SmelterThe Alcan Lynemouth Aluminium Smelter is situated near Ashington, Northumberland, on the coast of North East England, south of the village of Lynemouth. The smelter is owned by Canadian aluminium company Alcan, which is part of Rio Tinto...
, powered by the coal-fired Lynemouth Power StationLynemouth Power StationAlcan Lynemouth Power Station is a coal and biomass fired power station which provides electricity for the Alcan Lynemouth Aluminium Smelter and the National Grid. It is located on the coast of Northumberland, north east of the town of Ashington in North East England...
in North East EnglandNorth East EnglandNorth East England is one of the nine official regions of England. It covers Northumberland, County Durham, Tyne and Wear, and Teesside . The only cities in the region are Durham, Newcastle upon Tyne and Sunderland... - Anglesey AluminiumAnglesey AluminiumAnglesey Aluminium Metal Ltd, shut down on 30 September 2009, was a joint venture between Rio Tinto Group and Kaiser Aluminium.Its aluminium smelter, located on the outskirts of Holyhead, was one of the largest employers in North Wales, with 540 staff members, and began to produce aluminium in 1971...
, powered by WylfaWylfaThe Wylfa Nuclear Power Station is situated just west of Cemaes Bay on the island of Anglesey, North Wales. Its location on the coast provides an excellent cooling source for its operation...
nuclear power station in north-west WalesWalesWales is a country that is part of the United Kingdom and the island of Great Britain, bordered by England to its east and the Atlantic Ocean and Irish Sea to its west. It has a population of three million, and a total area of 20,779 km²...
(shut down as of 30/09/09) - The Valco aluminium smelter in GhanaGhanaGhana , officially the Republic of Ghana, is a country located in West Africa. It is bordered by Côte d'Ivoire to the west, Burkina Faso to the north, Togo to the east, and the Gulf of Guinea to the south...
, powered by the Akosombo DamAkosombo DamThe Akosombo Dam , is a hydroelectric dam on the Volta River in southeastern Ghana in the Akosombo gorge and part of the Volta River Authority. The construction of the dam flooded part of the Volta River Basin, and the subsequent creation of Lake Volta...
hydro-electric dam - Fjarðaál in IcelandIcelandIceland , described as the Republic of Iceland, is a Nordic and European island country in the North Atlantic Ocean, on the Mid-Atlantic Ridge. Iceland also refers to the main island of the country, which contains almost all the population and almost all the land area. The country has a population...
, to be powered by the Kárahnjúkar dam - JharsugudaJharsugudaJharsuguda is the district headquarters of Jharsuguda district of Orissa, India. It is the administrative headquarters of Jharsuguda district. It is an upcoming industrial hub, basically in the metal and cement sectors. Jharsuguda is well connected to all major cities of India through rail network...
in OrissaOrissaOrissa , officially Odisha since Nov 2011, is a state of India, located on the east coast of India, by the Bay of Bengal. It is the modern name of the ancient nation of Kalinga, which was invaded by the Maurya Emperor Ashoka in 261 BC. The modern state of Orissa was established on 1 April...
, IndiaIndiaIndia , officially the Republic of India , is a country in South Asia. It is the seventh-largest country by geographical area, the second-most populous country with over 1.2 billion people, and the most populous democracy in the world...
, to be powered by its own 1215 MW coal-fired power station. - AlcoaAlcoa World Alumina and ChemicalsAlcoa World Alumina and Chemicals is a joint venture between Alumina Limited and Alcoa and is abbreviated to AWAC. AWAC's business is the mining of bauxite, the extraction of alumina and the smelting of aluminium. It has about 25% of the global alumina market...
's Point Henry smelterPoint Henry smelterThe Point Henry aluminium smelter is located near Geelong, Victoria in the suburb of Moolap. The smelter has a production capacity of 185,000 tonnes of aluminium a year. It is operated by Alcoa World Alumina and Chemicals Australia, a joint venture between Alcoa and Alumina Limited...
near GeelongGeelong, VictoriaGeelong is a port city located on Corio Bay and the Barwon River, in the state of Victoria, Australia, south-west of the state capital; Melbourne. It is the second most populated city in Victoria and the fifth most populated non-capital city in Australia...
, AustraliaAustraliaAustralia , officially the Commonwealth of Australia, is a country in the Southern Hemisphere comprising the mainland of the Australian continent, the island of Tasmania, and numerous smaller islands in the Indian and Pacific Oceans. It is the world's sixth-largest country by total area...
, powered by its own brown coal fueled power stationAnglesea Power Station, Victoria (Australia)Anglesea Power Station is located at Anglesea, in Victoria, Australia within the floristically rich Anglesea Heath area. It is brown coal powered with one steam turbine with a capacity of 150MW of electricity, supplying almost 40% of the electricity used by the nearby Point Henry aluminium...
and grid electricity.