The basic element for production of aluminium is bauxite. Bauxite is an element formed by the weathering of limestone and silicate rocks. Bauxite must be processed into pure aluminium oxide (alumina) before it can be converted to aluminium by electrolysis, usually in alumina refineries located near the bauxite mine. The molten aluminium is cast into ingots. These can take the form of extruded billets (cylindrical) for extruded products, or rolled ingots (rectangular) for rolled products. This is preceded by the addition of appropriate alloy components which provide the aluminium with unique properties, depending on how it is to be further processed.
Extrusion means that a heated aluminium billet, usually between 450-500° C, is forced through a shaped die. After extrusion the profile is immediately cooled with air or water depending on the size, shape and alloy of the profile. Following the cooling process the extrusion is stretched to remove any possible tensions and twists. The profile is then cut to length before being despatched for age hardening. By natural or artificial ageing, the material achieves its final strength.
For the production of aluminium sheets and band (strip), the process of flattening the ingot, is carried out by either hot or cold-rolling. For hot rolling the ingot is preheated to between 450-550° C and is then passed back and forth through massive rollers that reduce the thickness of the ingot. The distance between the rollers is reduced until the required thickness is achieved. The production of aluminium foil means that the sheet is even further rolled. The last pass of foil rolling is done with a double web. The finished foil typically consists of shiny and a matt sides.
Continuous casting is another method for the production of coils for rolling into sheets and band (strips). Molten metal is used and this sets when passed through the cleft of two rotating water-cooled rollers.
