Primary aluminum smelting involves a process that begins with the extraction of the metal from the bauxite ore.
Aluminum smelting is an industrial process used to produce metal. How this is done depends on whether the aluminum is being produced through a primary process, involving bauxite, or a secondary process, involving scrap. The primary process is the most complex and consumes a lot of energy.
Aluminum production 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 obtaining a raw material known as alumina from bauxite.
For this, the ore that has been separated and ground is mixed with sodium hydroxide under high temperature and high pressure conditions. This step is known as digestion, and the machine used is often called a digester. The agitation and conditions within the digester result in the separation of the sodium aluminate and bauxite residues. Residues tend to sink into the mix. It is then necessary to separate the residues from the alumina, which is done by means of filtration.
When alumina is first filtered, it can be found in crystallized form. These crystals undergo 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 the impurities and removing the water from the crystals. This part of the process requires the use of high levels of heat and will eventually result in a white powder, which is 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 known as a reduction pot. The metal that is produced in the reduction pot usually settles to the bottom and is siphoned off periodically. Casting is generally a non-stop process.
Secondary aluminum smelting is slightly different because it involves extracting the metal from scrap materials or by-products known as slag, which are created during the smelting of primary aluminum. When the recovery is simply from scrap, the material is usually melted in a gas reverberatory, an oil reverberatory or a furnace. Chemical solutions are then used to remove impurities and produce aluminum of varying purities. When slag is involved, it must first be processed similarly to bauxite ore before the metal can be mined using the heat source.
Smelting primary aluminum is generally an energy-intensive process. Secondary smelting requires only a fraction of the energy needed for a primary process. The aluminum produced is generally not 100% pure. At best, it can be 99.7% pure.