Primary aluminium is produced in reduction plants (or ‘smelters’), where pure aluminium is extracted from alumina by the Hall-Héroult process.
Reduction of Alumina to Form Aluminium
The reduction of alumina into liquid aluminium is operated at around 950 degrees Celsius in a fluorinated bath under high intensity electrical 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 coming from the alumina. There are two types of anodes currently in use. All potlines built since the early 1970s use the prebake anode technology, where the anodes, manufactured from a mixture of petroleum coke and coal tar pitch (acting as a binder), are ‘pre-baked’ in separate anode plants. In the Soederberg technology, 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.
Alloying and Casting
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 user’s needs), cleaned of oxides and gases, and then cast into ingots. These can take the form of extrusion billets, for extruded products, or rolling ingots, for rolled products, depending on the way it is to be further processed.
Aluminium mould castings are produced by foundries which use this technique to manufacture shaped components.
Global Trends in Aluminium Production
World-wide trends in production are shown in the following graph. Aluminium output has increased by a factor of 13 since 1950, making aluminium the most widely used non-ferrous metal. In 1998, world-wide production of primary aluminium was about 22.7 million tonnes per year for and installed capacity of 24.8 million tonnes.
Figure 1. Production of primary aluminium (million tonnes/year).
The Main Aluminium Producing Regions
The main production areas are North America (6 million tonnes), Europe (3.6 million tonnes) Africa, Australia, CIS, Central and South America, and the Middle East. In Europe the main producing countries are France, Germany and Norway. World-wide, production plants are mainly located where suitable electrical energy resources are available.