Aluminum has a ton of properties that make it one of the most used materials in the world. It is widespread 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 found, but to produce is difficult, energy-intensive and expensive.
Raw materials for production
What raw materials do aluminum get from ? The production of aluminum from all minerals containing it is expensive and unprofitable. It is mined from bauxites, which contain up to 50% of aluminum oxides and are deposited directly on the surface of the earth by significant 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 in the range from 2 to 50%, titanium (up to 10%).
Alumina, and this is aluminum oxide, is composed of hydroxides, corundum and kaolinite.
Recently, alumina began to be obtained from nepheline, which also contains oxides of sodium, potassium, silicon, and alunites.
To produce 1 ton of pure aluminum, about two tons of alumina is needed, which in turn is obtained from about 4.5 tons of bauxite.
Bauxite deposits
World bauxite reserves are limited. All over the globe there are only seven districts with its rich deposits. These are 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 may also be developed. Russia produces bauxite in the Urals, in the Altai and Krasnoyarsk Territories, in one of the districts of the Leningrad Region, nepheline - on the Kola Peninsula.
The richest deposits belong to the Russian combined company UC RUSAL. Behind her are the giants Rio Tinto (England-Australia), combined with Canadian Alcan and CVRD. In fourth place is the company Chalco from China, then the US-Australian corporation Alcoa, which are also large producers of aluminum.
The origin of production
Danish physicist Oersted was the first to release aluminum in free form in 1825. The chemical reaction took place with aluminum chloride and potassium amalgam, instead of which, two years later, the German chemist Weler used potassium metal.
Potassium is a rather expensive material, therefore, in the industrial production of aluminum, the Frenchman St. Clair Deville used sodium instead of potassium in 1854, the element is much cheaper, and a stable double chloride of aluminum and sodium.
Russian scientist NN Beketov was able to displace aluminum from molten cryolite with magnesium. In the late eighties of the same century, this chemical reaction was used by the Germans in the first aluminum smelter. In the second half of the 18th century, about 20 tons of pure metal were obtained by chemical methods. It was a very expensive aluminum.
The production of aluminum by electrolysis began in 1886, when almost the same patent applications were filed simultaneously by the founders of this method, the American scientist Hall and the Frenchman Eru. They proposed dissolving alumina in molten cryolite and then producing aluminum by electrolysis.
From this began the aluminum industry, which has become over one 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. At the first of aluminum ores, whether bauxite or nepheline, they obtain alumina - aluminum oxide Al 2 O 3 .
Then, industrial aluminum with a purity of 99.5% is isolated from the oxide, which for some purposes is not enough.
Therefore, in the last stage, aluminum is refined. Aluminum production is completed by its purification up to 99.99%.
Alumina production
There are three methods for producing aluminum oxide from ores:
- acidic;
- electrolytic;
- alkaline.
The latter method is the most common, developed back in the same XVIII century, but since then it has been repeatedly modified and significantly improved, it is used for processing bauxite of high grades. So get about 85% of alumina.
The essence of the alkaline method is 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 is again used to dissolve alumina;
First, bauxite is crushed and ground in mills with caustic alkali and lime, then in autoclaves at temperatures up to 250 ° C it decomposes chemically 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 the sludge. Then it decomposes. Through the filters, the solution is pumped into containers with mixers to constantly mix the composition into which solid aluminum hydroxide is added for seed.
In hydrocyclones and vacuum filters, aluminum hydroxide is released, part of which is returned as seed material, and part is used for calcination. The filtrate remaining after separation of the hydroxide is also returned 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, 8.4 kWh of electricity is consumed.
A strong chemical compound with a melting point of 2050 ° C is not yet aluminum. Aluminum production ahead.
Electrolysis of Alumina
The main equipment for electrolysis is a special bath lined with carbon blocks. An electric current is supplied to it. Carbon anodes are immersed in the bath, which burn out when pure oxygen is separated from the oxide and form carbon monoxide and carbon dioxide. Bathtubs, or electricizers, as specialists call them, are connected in series to the electrical circuit, forming a series. The current strength is 150 thousand amperes.
Anodes can be of two types: fired from large coal blocks, the mass of which can be more than a ton, and self-fired, consisting of coal briquettes in an aluminum shell, which are sintered during electrolysis under the influence of high temperatures.
The operating voltage on the bath is usually about 5 volts. It takes into account the voltage required for the decomposition of oxide, and the inevitable losses in a branched network.
From a solution of cryolite alumina dissolved in a melt, a liquid metal, which is heavier than electrolyte salts, settles on the coal base of the bath. It is periodically pumped out.
The process of aluminum production requires large amounts of electricity. To get one ton of aluminum from alumina, you need to use about 13.5 thousand kWh of direct current electricity. Therefore, another condition for the creation of large production centers is a powerful power plant operating nearby.
Aluminum refining
The most famous method is three-layer electrolysis. It also takes place in electrolytic baths with coal bottom, 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 hearth. Pure aluminum, which dissolves from the electrolyte in the anode layer, is understood upward and serves as a cathode. Current is supplied to it using a graphite electrode.
The electrolyte in the intermediate layer is aluminum fluorides, either pure or with the addition of sodium and barium chloride. It is heated to a temperature of 800 ° C.
Electricity consumption for three-layer refining is 20 kW * h per kg of metal, that is, 20 thousand kW * h are needed per ton. That is why, like no metal production, aluminum requires not just a source of electricity, but a large power plant in close proximity.
Refined aluminum contains very small amounts of iron, silicon, copper, zinc, titanium and magnesium.
After refining, aluminum is processed into commercial products. These are ingots, and wire, and sheet, and ingots.
The segregation products obtained as a result of refining, partly in the form of a solid precipitate, are used for deoxidation, and partly are discharged 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 high electrical conductivity at cryogenic temperatures.
Recycling
A quarter of the total aluminum demand is met by recycling. Shaped casting is poured from recycled products.
Pre-sorted raw materials are smelted in a threshold furnace. It remains metals with a higher melting point than aluminum, for example, nickel and iron. Various non-metallic inclusions are removed from molten aluminum by purging with chlorine or nitrogen.
More fusible metal impurities are removed by additives of magnesium, zinc or mercury and vacuum. Magnesium is removed from the melt by chlorine.
The desired cast alloy is obtained by introducing additives, which are determined by the composition of the molten aluminum.
Aluminum Production Centers
In terms of aluminum consumption, China ranks first, leaving Germany far behind in the second place and the third-place winner.
China is also a country of aluminum production, with a huge margin of leadership in this area.
The top ten, except the PRC, includes Russia, Canada, the UAE, India, the USA, Australia, Norway, Brazil and Bahrain.
In Russia , RUSAL is a 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 its assets are concentrated in their native country.
Norwegian Norsk Hydro's Hydro Aluminum division owns aluminum smelters 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) - perhaps 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 producers of aluminum are international companies that own bauxite reserves. And the extremely energy-intensive process itself consists of producing alumina from aluminum ores, producing fluoride salts, which include cryolite, carbon anode mass and carbon anode, cathode, lining materials, and the actual electrolytic production of pure metal, which is the main component of aluminum metallurgy.