Aluminium Fact Sheet
|Minerals Downunder | Rock Files | Fact Sheets|
Aluminium (Al) comprises about 8.2% of the Earth's crust. It is the third most abundant element in the crust and the most plentiful metallic element, but is never found on its own in nature. Bauxite is the ore most commonly mined for aluminium in which aluminium occurs as hydroxide minerals. Aluminium is a silvery-white, tough and lightweight metal (specific gravity 2.7). It is a good conductor of heat and electricity, is very resistant to atmospheric corrosion (it never rusts), is nontoxic, inflammable, nonmagnetic and non-sparking, and it is malleable and ductile. Because of these properties it has become an important metal.
Metallic aluminium was first isolated in 1825 by Hans Christian Oersted from aluminium chloride but it was not commercially produced until 1888 with the establishment of the Pittsburgh Reduction Company (known as Alcoa since 1907). Aluminium can be mixed with almost any other metal and these alloys typically combine lightness with strength. As a result, they are used in a great variety of industries. In Australia, aluminium is produced in Tasmania, Queensland, Victoria and New South Wales with the building and construction industries being the most important consumers.
In 1821, French geologist Pierre Berthier discovered an aluminium-rich material near the village of Les Baux in Provence, France. It was named bauxite after the village. Bauxite occurs as a weathered cover or blanket, known as laterite or duricrust, over a variety of alumina-bearing rocks. It forms when large amounts of rainfall leach away the more mobile elements in the host rock leaving the relatively immobile aluminium with some silicon, iron and titanium. Because of the way it forms, bauxite deposits can be very extensive and are found on almost every continent. The largest known economic resources occur in Guinea and Australia, followed by Brazil, Vietnam and Jamaica.
The main aluminium minerals in bauxite are gibbsite [Al(OH)3, also written as Al2O3.H2O in oxide notation], boehmite [AlO(OH), written as Al2O3.H2O in oxide notation] and diaspore, which is a polymorph of boehmite but is denser and harder. Pure alumina (Al2O3) contains 52.9% aluminium and 47.1% oxygen. Bauxite may be as hard as rock or as soft as mud and may occur as compacted earth (both friable and re-cemented), small balls (pisolites), or hollow, twig-like material (tubules). Its colours may be buff, pink, yellow, red, or white, or any combination of these.
Bauxite ore refers to bauxite that contains sufficiently high levels of Al2O3 and suitably low levels of iron oxide (Fe2O3) and silica (SiO2) to be economically mineable. The amount of reactive silica is particularly important as this form of silica consumes the caustic soda needed to make alumina, thus low reactive silica is desirable. Other potential sources of aluminium include a variety of rocks and minerals such as aluminous shale and slate, aluminium phosphate rock and high-alumina clays.
In 2014, Australia was the world’s largest producer of bauxite (30%) and the second largest producer of alumina (19%). Bauxite is mined from established open cut operations at Weipa in Queensland, Gove in the Northern Territory and the Darling Range in Western Australia. Additionally, new mines have begun operation in the Cape York region of Queensland and in central Tasmania. Other bauxite deposits occur in northern Western Australia, New South Wales and eastern Queensland but are currently uneconomic to mine.
Australia's bauxite resources are assessed each year by Geoscience Australia. The assessment is based on published and unpublished data. These estimates are published each year in Australia’s Identified Mineral Resources.
Once mined, the next stage in aluminium production is the chemical Bayer Process where alumina (Al2O3) is extracted from bauxite in a refinery. Australia has six alumina refineries: QAL and Yarwun are located in Queensland near Gladstone, and Kwinana, Pinjarra, Worsley and Wagerup are all located in Western Australia in the Darling Range.
The final stage of aluminium production takes place in a smelter. The electrochemical Hall-Héroult Process uses huge amounts of power to extract aluminium metal from alumina. (Aluminium is sometimes referred to as ‘solid electricity’ owing to the large amount of power used in its production.) Australia has four aluminium smelters: Bell Bay in Tasmania, Boyne Island in Queensland, Portland in Victoria and Tomago in New South Wales.
In 2014, Australia produced the largest amount of bauxite in the world followed by China, Brazil, India and Guinea and the second largest amount of alumina after China. China is the world’s largest consumer of aluminium and, despite strong domestic production, imports large amounts of alumina and raw bauxite, accounting for more than 40% of global consumption. Other large aluminium markets are the United States of America, Japan and Europe but these regions possess few economic bauxite deposits and also rely on imports for their aluminium smelters. In 2014, Australia produced the seventh highest amount of aluminium in the world after China, Russia, Canada, the United Arab Emirates, India and the United States of America. Australian bauxite, alumina and aluminium statistics are published each year by the Office of the Chief Economist.
Extraction of aluminium metal takes place in three main stages - mining of bauxite ore, refining the ore to recover alumina and smelting alumina to produce aluminium.
Bauxite is mined by surface methods (open-cut mining) in which the topsoil and overburden are removed by bulldozers and scrapers. The topsoil is then stored and later used for revegetating and restoring the area after mining is completed. The underlying bauxite is mined by front-end loaders, power shovels or hydraulic excavators. Some bauxite ores are merely crushed, dried and shipped. Other bauxite is beneficiated after crushing by washing to remove some of the clay, reactive silica and sand waste and then dried in rotary kilns. The ore is then loaded into trucks, railway cars or onto conveyor belts and transported to ships or refineries.
In almost all commercial operations, alumina is extracted from the bauxite by the Bayer refining process. The process, discovered by Karl Josef Bayer in 1888, consists of four stages.
All commercial production of aluminium is based on the Hall-Héroult smelting process in which the aluminium and oxygen in the alumina are separated by electrolysis. This consists of passing an electric current through a molten solution of alumina and natural or synthetic cryolite (sodium aluminium fluoride). The molten solution is contained in reduction cells or pots which are lined at the bottom with carbon (the cathode) and are connected in an electrical series called a potline. Inserted into the top of each pot are carbon anodes, the bottoms of which are immersed in the molten solution.
The passage of an electric current causes the oxygen from the alumina to combine with the carbon of the anode forming carbon dioxide gas. The remaining molten metallic aluminium collects at the cathode on the bottom of the pot. Periodically, it is siphoned off and transferred to large holding furnaces. Impurities are removed, alloying elements added and the molten aluminium is cast into ingots.
The smelting process is a continuous one. As the alumina content of the cryolite bath is reduced more is added. Heat generated by the passage of the electric current maintains the cryolite bath in its molten state so that it will dissolve the alumina. A great amount of energy is consumed during the smelting process; from 14,000 - 16,000 kilowatt hours of electrical energy is needed to produce one tonne of aluminium from about two tonnes of alumina. The availability of cheap electricity is therefore essential for economic production.
Aluminium ingots are produced in various shapes and sizes depending on their end use. They may be rolled into plate, sheet, foil, bars or rods. They may be drawn into wire which is stranded into cable for electrical transmission lines. Presses extrude the ingots into hundreds of different useful and decorative forms or fabricating plants may convert them into large structural shapes.
A number of factors in the aluminium production cycle relate to the environment and considerable resources are allocated to minimise the impact of mining, refining and smelting on the surrounding environment. Mine rehabilitation is carried out, making every effort to return the area to at least its original condition. Extreme care is taken also with the handling and disposal of red mud from the refineries. This is usually pumped into dams which are sealed with impervious material to prevent pollution of the surrounding countryside. Strict measures are taken also to minimise fluoride emissions from smelters and dusty or corrosive material from the refineries.
The production of alumina consumes more than 90% of the world's production of bauxite. The remainder is used by the abrasive, refractory, and chemical industries. Bauxite is used also in the production of high-alumina cement, as an absorbent or catalyst by the oil industry, in welding rod coatings and fluxes and as a flux in making steel and ferroalloys.
Uses of aluminium include the following: - electrical equipment; car, ship, aircraft construction; metallurgical and chemical processes; domestic and industrial construction; packaging (aluminium foil, cans); kitchen utensils (cutlery, pans).
The aluminium industry initiated the development of technology for recycling aluminium containing material and for setting up its own drink can collection centres. One of the industry's main incentives has been the reduced amount of energy it takes to produce one tonne of secondary aluminium compared with one tonne of primary aluminium. This involves a saving of 95% of the energy required to produce molten aluminium from bauxite. Each tonne of recycled aluminium also means a saving of seven tonnes of bauxite.