Aluminium is produced via two different routes: primary aluminium production from ore and recycling aluminium from process scrap and used aluminium products.

Aluminium ore, most commonly bauxite, is plentiful and occurs mainly in tropical and sub-tropical areas such as West Africa, West Indies, South America and Australia. It is mined then refined into alumina using the Bayer Process, and then electrolytically reduced into metallic aluminium. Primary aluminium reduction or smelting plants are located all over the world; however, because it is an energy-intensive process, they are often in areas where there are abundant supplies of inexpensive energy, such as hydroelectric power. Once aluminium is formed it is alloyed with other materials to make an array of metals with differing properties which can be processed in any number of ways – rolled, cast, extruded. After use it can be recycled repeatedly without loss of its properties.

Primary Production

Before it can be converted to aluminium by electrolysis, bauxite ore has to be processed into pure aluminium oxide (alumina). This is achieved through the employment of the Bayer chemical process in facilities known as “alumina refineries”. The aluminium oxide is released from the other compounds present in bauxite by dissolution in caustic soda liquor, which is filtered to remove all insoluble particles. Aluminium hydroxide is precipitated from the soda solution, washed and dried, while the soda solution is recycled. After calcination, the end-product, aluminium oxide (Al₂O₃), is left as a fine-grained, white powder.

Primary aluminium is produced in reduction plants (or “smelters”), where pure aluminium is extracted from alumina by the Hall-Héroult process.

Khakas aluminium smelter, Russia © RUSAL

The reduction of alumina into liquid aluminium occurs at around 950 degrees Celsius in a fluorinated bath under high amperage electrical direct current. This process takes place in electrolytic cells (or “pots”), where carbon cathodes form the bottom of the pot and act as the negative electrode. Anodes (positive electrodes) are held at the top of the pot and are consumed during the process when they react with the oxygen molecules in the alumina. There are two types of anodes currently in use. In Prebake technology facilities, the anodes, manufactured from a mixture of petroleum coke and coal tar pitch (acting as a binder), are pre-baked in separate anode production plants. In Søderberg technology facilities, the carbonaceous mixture is fed directly into the top part of the pot, where self-baking anodes are produced using the heat released by the electrolytic process.

At regular intervals, molten aluminium tapped from the pots is transported to the cast house where it is alloyed in holding furnaces by the addition of other metals (according to the customer’s needs), cleaned of oxides and gases, and then cast into ingots. These can take the form of billets, for extruded products, or ingots, for rolled products, depending on the way they are to be further processed.

Recycling

Aluminium has been recycled since the days it was first commercially produced and today recycled aluminium accounts for one-third of global aluminium usage per year. Recycling is an essential part of the aluminium industry and makes sense economically, technically and ecologically.

Scrap melting

As the final link in the recycling chain, the refiners and remelters contribute significantly to the protection of our environment. It is they who ensure the production of a material that can be reabsorbed into the aluminium life cycle. Both refiners and remelters treat and melt scrap. The refiners then supply foundries with casting alloys, and the remelters provide rolling mills and extruders with wrought alloys. Refiners use scrap, almost exclusively, as a raw material, while remelters use both aluminium scrap and primary metal. For this reason, the size of the remelting industry is measured by the extent of its scrap intake rather than by production volumes of extrusion billets or rolling slabs.

New and old scrap

Aluminium scrap destined for recycling can be described as either “new” or “old”.

New scrap is the surplus material that is discarded during the fabrication and manufacturing of aluminium alloys (e.g. the splinters of sheet edge trimmings). Most new scrap reaching the recycling industry comes directly from the manufacturing industry. It is usually of known quality and composition and often uncoated. It can therefore be melted down with little preparation. 100 % of the scrap arising from fabrication and manufacturing processes is remelted by the aluminium industry.

Old scrap is the material that is collected after an aluminium containing product has reached the end of its useful life. Such scrap could be window frames, a façade from a demolished building, a drinks can or car engine block.

Processing of scrap

Scrap must be of appropriate quality before it can be melted down. To obtain this level of quality, all adherent materials must be removed and the scrap sorted according to alloy type and content. For example, turnings are centrifuged and dried to remove the oil and water that may be present and then magnetically separated from any iron present.

aluMATTER

aluMATTER is a freely-accessible, award-winning website that provides innovative and interactive e-learning resources for aluminium science and technology.