Aluminum, aluminum production: technology, process and description

Aluminum has a mass of properties that make it one of the most used materials in the world. It is widely distributed in nature, taking first place among metals. It would seem that there should not be any difficulties with its production. But the high chemical activity of the metal leads to the fact that in its pure form it is not met, and it is difficult to produce - it is energy-intensive and costly.

Raw materials for production

What raw materials are aluminum produced from? The production of aluminum from all minerals containing it is expensive and unprofitable. They extract it from bauxites, which contain up to 50% of aluminum oxides and lie directly on the surface of the earth with considerable masses.

These aluminum ores have a rather complex chemical composition. They contain alumina in an amount of 30-70% of the total mass, silica, which can be up to 20%, iron oxide ranging from 2 to 50%, titanium (up to 10%).

Alumina, and this is aluminum oxide, is composed of hydroxides, corundum and kaolinite.

Recently, aluminum oxides have been obtained from nephelines, which also contain sodium, potassium, silicon, and alunite oxides.

To produce 1 ton of pure aluminum, about two tons of alumina are needed, which in turn is obtained from about 4.5 tons of bauxite.

Deposits of bauxite

The reserves of bauxite in the world are limited. On the whole globe there are only seven regions with its rich deposits. This is Guinea in Africa, Brazil, Venezuela and Suriname in South America, Jamaica in the Caribbean, Australia, India, China, Greece and Turkey in the Mediterranean and Russia.

In countries where there are rich deposits of bauxite, aluminum production can also be developed. Russia extracts bauxites in the Urals, in the Altai and Krasnoyarsk regions, in one of the districts of the Leningrad region, nepheline - on the Kola Peninsula.

The richest deposits belong to the Russian United Company UC RUSAL. It is followed by the giants Rio Tinto (England-Australia), merged with the Canadian Alcan and CVRD. The fourth place is the company Chalco from China, then the American-Australian corporation Alcoa, which are also major aluminum producers.

Origin of production

The Danish physicist Oersted singled out the first aluminum in free form in 1825. The chemical reaction was carried out with aluminum chloride and potassium amalgam, instead of which two years later the German chemist Weller used metallic potassium.

Potassium - the material is quite expensive, so in the industrial production of aluminum, the Frenchman Saint Clair Deville instead of potassium in 1854 used sodium, an element much cheaper, and resistant double chloride of aluminum and sodium.

Russian scientist NN Beketov was able to force out aluminum from molten cryolite by magnesium. In the late eighties of the same century, this chemical reaction was used by the Germans at the first aluminum plant. In the second half of the 18th century, 20 tons of pure metal were obtained by chemical means. It was very expensive aluminum.

Aluminum production with the help of electrolysis originated in 1886, when virtually identical patent applications were filed by the founders of this method by American scholar Hall and the Frenchman Eru. They proposed dissolving alumina in molten cryolite, and then obtaining aluminum by electrolysis.

With this, the aluminum industry began, which became for more than a century of history one of the largest branches of metallurgy.

The main stages of production technology

In general terms, aluminum production technology has not changed since its inception.

The process consists of three stages. On the first of the aluminum ores, be it bauxites or nephelines, alumina-aluminum oxide Al ₂ O _{3 is} obtained.

Then, industrial aluminum is extracted from the oxide with a purification degree of 99.5%, which for some purposes is insufficient.

Therefore, in the last stage, aluminum is refined. The production of aluminum is completed by its purification to 99.99%.

Obtaining alumina

There are three ways to produce aluminum oxide from ores:

- acidic;

- Electrolytic;

- alkaline.

The latter method - the most common, developed in the same XVIII century, but since then repeatedly improved and significantly improved, is used for processing high grade bauxite. So get about 85% of alumina.

The essence of the alkaline method lies in the fact that aluminum solutions decompose at high speed when aluminum hydroxide is introduced into them. The solution remaining after the reaction is evaporated at a high temperature of about 170 ° C. and again used to dissolve the alumina;

First bauxite is crushed and crushed in mills with caustic alkali and lime, then in the autoclaves at temperatures up to 250 ° C, its chemical decomposition takes place and sodium aluminate is formed, which is diluted with an alkaline solution even at a lower temperature - only 100 ° C. The aluminate solution is washed in Special thickeners, separated from sludge. Then its decomposition takes place. Through the filters, the solution is pumped into a stirred tank to continuously mix the formulation into which solid aluminum hydroxide is added for seeding.

Hydrocyclones and vacuum filters produce aluminum hydroxide, some of which are returned as a seed material, and some are used for calcination. The filtrate remaining after separation of the hydroxide also returns to circulation to leach the next batch of bauxites.

The process of calcination (dehydration) of hydroxide in rotary kilns occurs at temperatures up to 1300 ° C.

To produce two tons of aluminum oxide consumes 8.4 kWh of electricity.

A strong chemical compound, whose melting point is 2050 ° C, is not yet aluminum. Production of aluminum in front.

Electrolysis of aluminum oxide

The main equipment for electrolysis is a special bath, lined with carbon blocks. Electric current is supplied to it. In the bath, the carbon anodes are burned when they are separated from the pure oxygen oxide and form oxide and carbon dioxide. Baths, or electrolyzers, as they are called specialists, are included in the electrical circuit in series, forming a series. The current strength is 150 thousand amperes.

Anodes can be of two types: baked from large coal blocks, the mass of which can be more than a ton and self-baking, consisting of coal briquettes in an aluminum shell, which are sintered during electrolysis under high temperatures.

The operating voltage in the bath is usually about 5 volts. It takes into account both the voltage required for the decomposition of the oxide, and the inevitable losses in the branched network.

From the dissolved in the melt on the basis of cryolite of aluminum oxide, a liquid metal that is heavier than electrolyte salts settles on the coal base of the bath. It is periodically pumped out.

The process of aluminum production requires a lot of electricity. To get one ton of aluminum from alumina, you need to spend about 13.5 thousand kWh of DC electricity. Therefore, one more condition for creating large production centers is a nearby powerful power station.

Refining of aluminum

The most famous method is three-layer electrolysis. It also takes place in electrolysis baths with carbon pads, lined with magnesite. The anode in the process is the molten metal itself, which is subjected to purification. It is located in the lower layer on the conductive base. Pure aluminum, which dissolves from the electrolyte in the anode layer, is understood upward and serves as a cathode. The current to it is supplied by means of a graphite electrode.

The electrolyte in the intermediate layer is aluminum fluorides or pure or with the addition of sodium and barium chloride. It is heated to 800 ° C.

Electricity consumption for three-layer refining is 20 kWh per 1 kg of metal, that is 20 thousand kWh for one ton. That is why, like no metal production, aluminum requires the presence not only of a source of electricity, but of a large power plant in close proximity.

Refined aluminum contains iron, silicon, copper, zinc, titanium and magnesium in very small quantities.

After refining, aluminum is processed into commercial products. This is ingots, and wire, and a sheet, and chushki.

Segregation products obtained as a result of refining, in part, in the form of a solid precipitate, are used for deoxidation, and partially depart as an alkaline solution.

Absolutely pure aluminum is obtained by subsequent zone melting of the metal in an inert gas or vacuum. Its remarkable characteristic is the high electrical conductivity at cryogenic temperatures.

Recycling of secondary raw materials

A quarter of the total demand for aluminum is met by secondary processing of raw materials. Of products recyclable molding molded.

Pre-sorted raw materials are melted in a threshold furnace. In it there are metals having a higher melting point than aluminum, for example, nickel and iron. Various nonmetallic inclusions are removed from the molten aluminum by purging with chlorine or nitrogen.

More fusible metal impurities are removed by additives of magnesium, zinc or mercury and evacuation. Magnesium is removed from the melt by chlorine.

A predetermined foundry alloy is produced by introducing additives which are determined by the composition of the molten aluminum.

Aluminum production centers

In terms of aluminum consumption, China occupies the first place, leaving behind the USA in second place and the third place holder Germany.

China is also a country of aluminum production, with a huge margin leading in this area.

The top ten, except the PRC, includes Russia, Canada, the United Arab Emirates, India, the United States, Australia, Norway, Brazil and Bahrain.

In Russia, the unified company RUSAL is the monopolist in the production of alumina and aluminum . It produces up to 4 million tons of aluminum per year and exports products to seventy countries, and is present on five continents in seventeen countries.

The American company Alcoa in Russia owns two metallurgical plants.

The largest aluminum producer in China is Chalco. Unlike foreign competitors, all of its assets are concentrated in its home country.

The Hydro Aluminum division of the Norwegian company Norsk Hydro owns aluminum plants in Norway, Germany, Slovakia, Canada, and Australia.

Australian BHP Billiton owns aluminum production in Australia, South Africa and South America.

In Bahrain is Alba (Aluminum Bahrain BSC) - almost the largest production. Aluminum of this manufacturer occupies more than 2% of the total volume of "winged" metal produced in the world.

So, summing up, we can say that the main aluminum producers are international companies that own bauxite reserves. And the exceptionally energy-intensive process consists of obtaining alumina from aluminum ores, producing fluoride salts, which include cryolite, carbonaceous anodic mass and carbon anodic, cathode, lining materials, and the actual electrolytic production of pure metal, which is the main component of aluminum metallurgy.