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Aluminum smelting is an industrial process used to produce metal. The manner in which this is done depends on whether the aluminum is being produced by way of a primary process, involving bauxite, or a secondary process, involving scrap. The primary process is the most complex and energy intensive.
The production of aluminum is generally divided into two categories, primary and secondary. Primary aluminum smelting involves a process that begins with extracting the metal from bauxite ore, a type of rock most commonly found in tropical and subtropical climates. This process begins by first obtaining a raw material known as alumina from the bauxite.
To achieve this, ore that has been sorted and ground is mixed with sodium hydroxide under high-temperature and high-pressure conditions. This step is known as digesting, and the machine that is used is often called a digester. The agitation and conditions inside the digester result in the separation of sodium aluminate and bauxite residues. The residues tend to sink to the bottom of the mixture. It is then necessary to separate the residues from the alumina, which is achieved by way of filtering.
When alumina is first filtered, it can be found in a crystallized form. These crystals are subjected to a process known as precipitation, which involves using alumina hydrate to create a purer form of alumina crystals. The next step is calcination, which involves washing away impurities and removing the water from the crystals. This part of the process requires the use of high levels of heat and will ultimately result in a white powder, which is the alumina.
At this point, aluminum smelting will involve converting the powder into a metallic form. This requires large amounts of direct current (DC), which is done in a contraption referred to as a reduction pot. The metal that is produced in the reduction pot generally settles to the bottom and is siphoned off periodically. Smelting is usually a non-stop process.
Secondary aluminum smelting is a bit different because it involves extracting the metal from scrap materials or byproducts known as dross, which are created during primary aluminum smelting. When recovery is simply from scrap, the material is usually melted in a gas reverberatory, an oil reverberatory, or a hearth furnace. Chemical solutions are then used to remove the impurities and produce aluminum of varying purities. When dross is involved, it must first must be processed in a similar manner as bauxite ore before the metal can be extracted using the heat source.
Primary aluminum smelting is generally an energy-intensive process. Secondary smelting requires only a fraction of the energy required for a primary process. The aluminum produced is not usually 100-percent pure. At best, it may be 99.7-percent pure.