Making aluminium

Bauxite

Bauxite is the mineral form of aluminium. It is the most abundant mineral in the earth's crust. Bauxite contains about 50 per cent alumina. It is formed by the weathering of sedimentary rocks that contain a high proportion of aluminium bearing minerals.

Bauxite is normally found in a layer averaging three to five metres deep, located about half a metre beneath the topsoil. Mined bauxite resembles small red pebbles, called pisolites, averaging about five millimetres in diameter.

Mining and processing bauxite

Initially, samples are collected by drilling a series of holes to a depth of 3.5 metres, 75 metres apart in a grid formation. Mining engineers determine where to mine based on the amount of alumina, silica and iron in the bauxite samples.

Once the area to be mined has been determined, vegetation is removed and the topsoil stripped. Both are used to regenerate land in keeping with our commitment to sustainable development. This topsoil contains beneficial bacteria and organisms that assist newly planted trees to grow over previously mined areas.

Bauxite exists in free flowing form, which makes it relatively easy to dig out of the ground. Front end loaders load the bauxite into 150 tonne bottom dump trucks. The trucks take the ore to a dump station for transportation to the processing (beneficiation) plant by rail and belt conveyor.

At the beneficiation plant, the bauxite is screened and washed to remove fine particles. It is then placed in stockpiles, which are loaded onto ships.

Alumina

Alumina is the name given to aluminium oxide (Al2O3), which is extracted from bauxite via a refining process known as the Bayer process. In general, it takes about two tonnes of bauxite to produce one tonne of alumina.

Refining takes place over four main stages.

1. Digestion: The bauxite is ground in mills and mixed with hot caustic soda at high temperatures and pressures to dissolve alumina in the ore. This separates it from non soluble impurities such as silica, iron and titanium compounds in the ore.
2. Clarification: The caustic soda and alumina solution passes into rows of thickener tanks where solid impurities sink to the bottom as a fine, red mud. The impurities are washed several times with water and disposed of in tailings dams on site. The remaining solution of alumina trihydrate is filtered to make it even clearer.
3. Precipitation: The alumina trihydrate solution is then cooled, concentrated and stirred in open top tanks until it forms into crystals. This part of the process can take several days. Pure alumina is added to this mixture to assist with the formation of alumina trihydrate crystals.
4. Calcination: The crystals are then washed, filtered and heated in gas-fired kilns at temperatures greater than 1,100°C to remove water molecules. The final product is a fine, dry, white powder, which is alumina. Alumina is then cooled and stored.

Aluminium

Alumina is composed of both aluminium and oxygen, which is separated during the smelting process to produce aluminium metal. About two tonnes of alumina are required to produce one tonne of aluminium.

The smelting process uses electrical energy to separate the aluminium metal from the oxygen. This occurs in reduction cells, which are large, steel, carbon lined furnaces. Alumina is fed into the cells where it is dissolved in molten cryolite. This liquid can dissolve alumina and conduct electricity at 970°C.

Electricity is introduced into each cell via carbon anodes - carbon blocks manufactured by the smelters. All reduction cells are connected in a series by an aluminium busbar, which carries electrical current to the cells.

A continuous electrical current of 100,000 to 320,000 amps flows from the anode through the alumina/cryolite mixture, to the carbon cell lining, to the anode of the next cell, and so on. This current causes the alumina in the mixture to react with the carbon anode, forming aluminium and carbon dioxide.

The aluminium, in molten form, sinks to the bottom of the reduction cell, while the carbon dioxide and other gaseous by products form at the top of the cell. The gases are cleaned to remove contaminants before being released into the atmosphere.

Molten aluminium is siphoned from the bottom of the cell by a process called tapping. The aluminium is then transported to a holding furnace to be cast into products. Casting takes place in temperatures of just over 700°C.

Aluminium is cast into ingots, large blocks, t-bars or long cylindrical logs called extrusion billets. Special ingot casting machines automatically cast, stack, strap and weigh the ingots into one tonne bundles ready for transport. Extrusion billets and t-bars are cast according to customer requirements using a process known as vertical drill chill casting.

In these forms, the metal is known as primary aluminium. Primary aluminium can be rolled, extruded or cast as pure aluminium or aluminium alloys. Alloys are formed by combining pure aluminium with magnesium, silicon, manganese or other elements to produce properties such as extra strength or additional corrosion resistance.

Aluminium and its uses

Unique and versatile metal

Aluminium is one of the most important and widely used metals in the transport, construction, packaging and electrical sectors.

• In transport, aluminium is used in cars (engine blocks, cylinder heads, transmission housings and body panels), trucks and buses (sheet and plate for bodies), in railway stock and in aircraft.
• In construction, aluminium is used in sheet products for roofing and wall cladding, in extrusions for windows and doors, and in castings for builders' hardware.
• In packaging, aluminium is used in the form of alloy sheet for beverage can bodies and tops, as foil for household and commercial wrap, and in manufactured packaging products such as cartons for fruit juice and packaging for pharmaceuticals.
• In the electrical sector, aluminium is used in the form of wire, normally reinforced with steel to form cables.