How to obtain aluminum – main methods
History and methods of producing aluminum
Aluminum is a metal that is used in many spheres of human activity. This element is widespread in nature, but cannot be found in pure form. This is because aluminum has a high chemical activity and can enter into chemical reactions with various elements. Owing to this feature, obtaining free aluminum is a laborious and complex process. Let’s examine it in detail.
Raw material for producing the metal
Aluminum is produced from bauxites, which are found in significant layers on the earth’s surface, and contain around 50% of aluminum oxides and oxide hydrates. The chemical composition of bauxites is complex; this aluminum ore contains around 20% silicon earth, from 30% to 70% alum shale, and up to 10% titanium oxide. Iron oxides in bauxites may make up from 2% to 50%. Alum shales, which contain aluminum oxide, consist of kaolinite, corundum and aluminum hydroxide.
Since recent times aluminum has been produced from nephelines and alunites, consisting of silicon, potassium oxide and sodium oxide. To produce one ton of pure aluminum, two tons of alum shale must be processed. To produce two tons of alum shale, 4.5 tons of bauxite must be processed.
Supplies of bauxites on Earth are relatively scarce. Countries with large bauxites fields can produce aluminum in industrial scale – Brazil, Australia, Guinea, China, India, Turkey, Greece, Surinam, Venezuela, Jamaica and Russia.
How the production of aluminum was born
The first scientist to isolate aluminum in free form was the Danish physicist Ørsted – in 1825. Ørsted carried out a chemical reaction of an amalgam of potassium and aluminum chloride. Two years later the German physicist Wöhler replaced the potassium amalgam with metallic potassium. As potassium is an expensive material, which it is not profitable to use in industrial production, scientists looked for a replacement for this element. In 1854, the French scientist Deville began to use sodium, and also a durable double chloride of sodium and aluminum.
Later, production of aluminum was carried out using magnesium from melted cryolite – this technology was invented by the Russian scientist Beketov, but was first used in industrial production by the Germans in the 1880s. In Germany, the first factory for the production of aluminum was built, but production of the metal remained very expensive.
In 1886, aluminum was first produced using the electrolysis reaction. This method was patented practically simultaneously by two scientists – Héroult from France and Hall from the USA.
Both physicists proposed to dissolve aluminum oxide in melted cryolite, after which the aluminum was to be produced by electrolysis. Thanks to Héroult and Hall, in 1886 the era of the industrial production of aluminum began. Click here to find out more interesting facts about aluminum.
Technology of aluminum production - stages
In the 21st century, the technology of aluminum production developed in 1886 is still used. The process includes three stages: first aluminum oxide is obtained from nephelines or bauxites, then industrial aluminum is extracted from it, with a level of purity of 99.5%. This is insufficient for some industrial purposes, so in the last stage the metal is purified to 99.99%.
Obtaining aluminum oxide
Aluminum oxide can be obtained by three methods:
The third method is the most widespread, which was improved and elaborated on from the moment it was invented in the 18th century. Bauxite is crushed with lime and caustic alkali, then melted in an autoclave at a temperature of 250 °С, as a result of which the chemical breakdown of bauxite takes place, and sodium aluminate forms. The obtained substance is diluted with an alkaline solution and separated from sludge, and rinsed in drainers. After sodium aluminate breaks down, it is pumped through filters and mixed, adding solid aluminum hydroxide as a seed into the remaining aluminum hydroxide residue from the solution. Then the aluminum hydroxide is heated to obtain aluminum oxide. This is the first stage for obtaining metallic aluminum.
Electrolysis of aluminum oxide
The reaction of electrolysis is carried out in a special bath with an electric current, lined with hydrocarbon blocks. Coal anodes are immersed in the bath, which burn with the release of pure oxygen from the aluminum oxide, and form carbon monoxide and dioxide.
Electrolysers are gradually included in the electrical chain, forming a series. Anodes can be calcinated from large coal block with a mass of over one ton, and self-calcinating, consisting of coal bricks in an aluminum shell, which are baked at high temperatures in the electrolysis process. From the aluminum oxide dissolved in an alloy with a cryolite base, the liquid metal settles on the coal base of the bath. This is aluminum in the form in which it is used in industry.
Obtaining aluminum of high purity
In order to obtain pure aluminum, industrial aluminum must be additionally purified – shaped castings are molded from the obtained product. First industrial aluminum is sorted, then melted down in a furnace. Metals with a higher melting point than aluminum remain in the furnace – they may be nickel or iron. Nitrogen or chlorine is bubbled through molten aluminum, removing non-metallic inclusions from it. To remove low-melting metallic mixtures, alloys of mercury, zinc or magnesium are used. Magnesium can be removed from the alloy by chlorine. To obtain a cast alloy of aluminum, additives are introduced that determine its composition.