Aluminium Fact Sheet

• Introduction
• Occurence
• Australian Resources and Deposits
• Mining
• Uses
• Suggestions for Further Reading

Aluminium (Al) is the most plentiful metallic element in the Earth's crust. Combined with oxygen and hydrogen, it forms bauxite, the ore most commonly mined for aluminium. Metallic aluminium was first isolated in 1829 from aluminium chloride but it was not commercially produced until 1886.

Aluminium is a silvery-white, tough, but lightweight metal (specific gravity 2.7). It is a good conductor of electricity and is very resistant to atmospheric corrosion. Because of these properties it has become an important metal. Aluminium alloys combine lightness with strength and as a result are used in a great variety of industries. In Australia, the building and construction industry is the most important consumer.

The main minerals in bauxite are gibbsite (Al₂0₃.3H₂0), boehmite (Al₂0₃.H₂0), and diaspore, which has the same composition as boehmite but is denser and harder. The pure anhydrous oxide of aluminium, alumina (Al₂0₃), contains 52.9% aluminium and 47.1% oxygen. Bauxite may be as hard as rock or as soft as mud and it 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 Al₂0₃ and suitably low levels of Fe₂0₃ and silica to be economically mineable. 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.

Named after the French district of Les Baux, where it was first discovered in 1821, bauxite is produced by tropical or semitropical weathering of alumina-bearing rocks. It occurs as a weathered cover or blanket known as laterite, over a variety of rocks. Because of the way it forms, bauxite deposits are generally very extensive. Bauxite is found on almost every continent with the largest known economic resources in Australia and the Republic of Guinea. In terms of ranking, these countries are followed by Brazil, Jamaica, and India. Although the USA, Japan, and the Federal Republic of Germany are the world's largest consumers of aluminium they possess little or no bauxite deposits of their own.

Australia produces about 40% of world bauxite and more than 30% of world alumina, making it the largest producer of bauxite and alumina. Bauxite is mined from open cut operations at Weipa (QLD), Gove (NT), and the Darling Range (WA). There are very large bauxite deposits in the Mitchell Plateau and Cape Bougainville regions of Western Australia, although none are currently economic 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 shown on the Mineral Statistics Fact Sheet.

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 and then used for revegetating the area and returning it to sometimes better than original condition or converting it to agricultural land. The underlying bauxite is mined by front-end loaders, power shovels or hydraulic excavators. Sometimes the bauxite is crushed and washed to remove some of the clay and sand waste and then dried in rotary kilns. Other bauxites may just be crushed or dried. 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 Bayer in 1888, consists of four stages.

1. Digestion - in which the finely ground bauxite is fed into a steam-heated unit called a digester. Here it is mixed, under pressure, with a hot solution of caustic soda. The aluminium oxide of the bauxite and some of the silica react with the caustic soda forming a solution of sodium aluminate or green liquor and a precipitate of sodium aluminium silicate.
2. Clarification - in which the green liquor or alumina-bearing solution is separated from the waste (the undissolved iron oxides and silica which were part of the original bauxite and now make up the sand and red mud waste). This stage involves three steps: firstly, the coarse sand-sized waste is removed and washed to recover caustic soda; secondly, the red mud is separated out; and, thirdly the remaining green liquor is pumped through filters to remove any residual impurities. The sand and mud are pumped together to residue lakes and the green liquor is pumped to heat exchangers where it is cooled from 1000¡C to around 65O¡-790¡C.
3. Precipitation - in this stage the alumina is precipitated from the liquor as crystals of alumina hydrate. To do this, the green liquor solution is mixed in tall precipitator vessels with small amounts of fine crystalline alumina, which stimulates the precipitation of solid alumina hydrate, as the solution cools. When completed the solid alumina hydrate is passed on to the next stage and the remaining liquor, which contains caustic soda and some alumina, goes back to the digesters.
4. Calcination - in the final stage the alumina hydrate is washed to remove any remaining liquor and then dried. Finally it is heated to about 1000¡C to drive off the water of crystallisation, leaving the alumina, which is a dry, pure white, sandy material. A portion of the alumina may be left in the hydrate form or further processed for the chemical industry.

All commercial production of aluminium is based on the Hall-Heroult 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 absorbant 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. In Australia about 10% of aluminium production comes from recycled material.

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