Aluminium industry. How is high purity aluminium obtained?

Published: 6-06-2018

Aluminium is one of the most widespread elements in the Earth’s crust – constituting 7% of its elemental composition, it is third most abundant element after oxygen and silicon. It is obtained from bauxite, or sedimentary rock, in which it is mainly in the form of an oxide. This metal has been known for over 2000 years and is characterized by a wide technical application. So what can we use it for?

Aluminium is used in industry mainly in the form of alloys, which improves its usable properties. In such a form it becomes a universal construction material with a very versatile application. Among the aluminium alloys, casting alloys and alloys used for plastic forming can be distinguished. In addition to aluminium, elements such as copper, magnesium, silicon and manganese are included in their composition. Aluminium alloys are used, among others, in aviation, chemical industry, automotive industry and even in shipbuilding.

Aluminium is widely used in the industry also in its pure form. In such form, it is used for the production of various objects of everyday use, such as mirrors, cans for drinks and food, kitchen utensils or commonly known aluminium foil. It is also used for the production of chemical equipment, electric wires and even explosives. To isolate this element from bauxite ore, it is necessary to carry out two steps. The first of these is the Bayer process, which allows aluminium oxide from the mineral to be obtained. The compound is then subjected to electrolysis, resulting in the production of high purity aluminium.

What is aluminium made of?

Pure aluminium does not naturally occur in nature due to its ability to passivate. This phenomenon involves the oxidation of metal in the presence of air, resulting in a passive protective layer on its surface. In the case of aluminium, it is first coated with a layer of aluminium oxide (Al2O3) with a thickness of few nm. Then under the influence of moisture, the outer layer undergoes partial hydrolysis, which additionally forms hydroxide, i.e. Al(OH)3.

Aluminium is part of various mineral rocks found in nature in the form of ores. To produce pure aluminium, mainly clayey bauxite ores are used. They most often appear in places of weathering of aluminosilicate rocks in a hot climate and also contain iron compounds. They are rocks with a characteristic red or brown colour, which occur in two varieties: silicate and carbonate.

Production of high purity aluminium

High purity aluminium (over 99%) is obtained industrially as a result of two consecutive processes. In the first one aluminium oxide is obtained (Bayer process), and in the next stage an electrolytic reduction process is carried out (Hall–Héroult process), thanks to which pure aluminium is obtained. Due to the reduction of costs associated with the transport of bauxite ore, most processing plants are built in the vicinity of the mines.

The Bayer process

The first stage after extraction of the ore is washing it with water. In this way, most of the water-soluble impurities are removed. Then CaO, or calcium oxide, is added to such prepared raw material. Next it is crushed using special tube mills until the grains have a very small diameter, i.e. below 300 μm. It is extremely important to finely grind the raw material, as it provides a sufficiently large specific surface area of grains, which in turn translates into a more efficient process of extraction.

The next stage in the production of aluminium oxide is the dissolution of grains with an aqueous solution of caustic soda. In the PCC Group, sodium hydroxide is produced by membrane electrolysis. The product obtained in this way is characterized by unusually high quality and purity, while meeting the requirements of the latest edition of the European Pharmacopoeia. The mixture containing milled grains and sodium hydroxide is stored for several hours in special reactors called autoclaves. During the precipitation process, high pressure and elevated temperature are maintained in the reactors. In this way, sodium aluminate is obtained, which is then purified using various filters.

In the next step, the purified solution of sodium aluminate decomposes. As a result, soda lye is obtained (it is an aqueous solution of caustic soda) and crystals of aluminium hydroxide with a high degree of purity. The precipitate obtained by crystallization is then filtered and washed with water. In turn, the remaining soda lye is heated and recycled for re-use in the process.

The last stage of aluminium oxide production is calcination. It consists in heating aluminium hydroxide at a temperature above 1000oC, which results in its decomposition to Al2O3, which is obtained in the form of a pure white powder. Aluminium oxide prepared in this way is transported to furnaces in order to obtain metallic aluminium in the electrolytic reduction process.

Electrolysis of aluminium oxide

The next step in obtaining pure aluminium is to conduct the electrolysis process using the Hall–Héroult method. Firstly, the Al2O3 obtained in the Bayer process is melted with cryolite and then undergoes electrolysis at a temperature not exceeding 900°C.

The liquid aluminium obtained in this way is separated from the electrolyte and removed from the electrolytic baths by means of the so-called vacuum siphons. The raw material is then sent to a foundry device, where it is then introduced into heated furnaces in which the refining process takes place. It consists of purifying aluminium in order to obtain its maximum purity. Aluminium can be purified industrially using two methods. The first one consists of melting aluminium and passing chlorine through it, thanks to which impurities are bound in the form of chlorides and removed from the process. The second method involves the electrolytic reduction of aluminium – copper alloy. The end product obtained in this way is characterized by very high purity.

Aluminium – the material of the future

The development of a method for the production of pure aluminium from bauxite using the Bayer process and Hall–Héroult electrolysis has expanded the application of this element. In addition, due to the combination of high strength and lightness of aluminium, in some applications it can replace more expensive steel. What is more, because of its resistance to weather conditions, aluminium is used for the production of window and door profiles. Another advantage is the possibility of it being able to be recycled repeatedly, making it a relatively environmentally friendly material.

In summary, aluminium is an extremely versatile material, widely used in the food, energy, chemical, transport, construction, automotive and aerospace industries. Due to its numerous advantages, the scope of its use probably has not been exhausted yet and aluminium will continue to gain in popularity in the near future.

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