Electrolysis is used in industrial aluminum smelting. The main principle is Hall-Elu aluminum electrolysis: pure alumina is used as raw material to produce aluminum by electrolysis. Because pure alumina has a high melting point (about 2045℃), it is difficult to melt. Therefore, molten cryolite (Na3AlF6) is used as a flux in industry, so that alumina can be dissolved in liquid cryolite at about 1000℃ and become a melt of cryolite and alumina.
A comprehensive introduction is as follows:
Production and processing of aluminum
There are four links in the production process of aluminum: aluminum mining-alumina production-electrolytic aluminum smelting-aluminum processing production.
Generally speaking, two tons of aluminum ore produce one ton of alumina; Two tons of alumina produce one ton of electrolytic aluminum.
(A) the production method of alumina
So far, many methods have been proposed to extract alumina from aluminum ore or other aluminum-containing raw materials. Due to technical and economic reasons, some methods have been eliminated, and some are still in the experimental research stage. The proposed methods for producing alumina can be divided into four categories, namely, alkali method, acid-base combination method and thermal method. At present, only alkali method is used in large-scale industrial production.
Bauxite is the most important aluminum resource in the world, followed by alunite, nepheline and clay. At present, in the world alumina industry, except for some alumina produced by Russian nepheline, almost all alumina in the world is produced by bauxite.
Bauxite is an ore mainly composed of gibbsite, diaspore or diaspore. Up to now, all bauxite resources available for alumina production in China are diaspore bauxite.
The content of alumina in bauxite varies greatly, ranging from about 30% to over 70%. The main impurities in bauxite are silicon oxide, iron oxide and titanium oxide besides alumina. In addition, it also contains a small amount or trace of calcium and magnesium carbonate, potassium, sodium, vanadium, chromium, zinc, phosphorus, gallium, scandium, sulfur and other compounds and organic substances, among which gallium.
One of the main indicators to measure the quality of bauxite is the ratio of alumina content to silica content in bauxite, commonly known as aluminum-silicon ratio.
When producing alumina by alkali method, aluminum ore is treated with alkali (NaOH or Na2CO3) to convert alumina in the ore into sodium aluminate solution. Impurities such as iron and titanium and most of silicon in the ore become insoluble compounds. Insoluble residue (red mud) is separated from the solution, and its useful components are recovered after washing, waste or comprehensive treatment. Pure sodium aluminate solution can be decomposed to separate aluminum hydroxide. After separation, washing and calcination, alumina products can be obtained. The decomposed mother liquor can be recycled to treat another batch of ore. The processes of producing alumina by alkaline method include Bayer process, sintering process and Bayer-sintering combined process.
Bayer process was invented by Austrian chemist K·J· Bayer in 1889 ~ 1892 to extract alumina from bauxite. 100 years, there have been many improvements in technology, but the basic principle has not changed. In order to commemorate Bayer's great contribution, the name Bayer method has been used to this day.
Bayer process includes two main processes. First of all, alumina is dissolved from bauxite under certain conditions (the commonly used term in alumina industry is leaching. The same below), and then aluminum hydroxide is dissolved from the supersaturated sodium aluminate. These are two patents proposed by Bayer. The essence of Bayer process is to extract alumina from bauxite by hydrometallurgy. In the process of alumina production by Bayer process, silicon-containing minerals will cause the loss of Al2O3 and Na2O.
In Bayer process, bauxite, after crushing, enters wet grinding together with lime and circulating mother liquor to make qualified pulp. After pre-desilication, the pulp is preheated to the dissolution temperature for dissolution. After self-evaporation, the dissolved pulp is cooled and enters the process of dilution, precipitation and separation of red mud (dissolved solid residue). The secondary steam generated in the process of self-evaporation is used for early preheating of pulp. After settling and separation, the red mud is washed into the red mud yard. The separated crude liquid (sodium aluminate solution containing solid suspension, the same below) is sent to the vane filter. The crude liquid after removing most of the suspended solids by the blade filter is called semen. Semen enters the decomposition process and is decomposed by seeds to obtain aluminum hydroxide. After the decomposed aluminum hydroxide is classified, separated and washed, a part of it is returned to the seed crystal decomposition process as a seed crystal, and the other part is roasted to obtain an alumina product. The decomposed mother liquor separated after seed separation evaporates and returns to the dissolution process to form a closed cycle. Roasting to obtain aluminum hydroxide.
Different types of bauxite require different dissolution conditions. Gibbsite bauxite can be well dissolved at 105℃, diaspore bauxite can be dissolved at a faster speed at 200℃, while diaspore bauxite can only be dissolved at a temperature higher than 240℃, and its typical industrial dissolution temperature is 260℃. Dissolution time is not less than.
Bayer process for treating bauxite with high Al-Si ratio has the advantages of simple process, high product quality and far better economic effect than other methods. When it is used to treat gibbsite bauxite which is easy to dissolve out, its advantages are more prominent. At present, more than 90% of alumina and aluminum hydroxide produced in the world are produced by Bayer process. Due to the particularity of bauxite resources in China, about 50% of alumina in China is produced by Bayer process at present.
The combined process of Bayer process and sintering process is called combined production process. Combination technology can be divided into parallel combination technology, series combination technology and mixed combination technology. The method of producing alumina mainly depends on the grade of bauxite (that is, the aluminum-silicon ratio of ore). From the comprehensive technical and economic point of view, when the Al-Si ratio of ore is about 3, the sintering process is usually selected. Bayer process can be used for ores with Al/Si ratio higher than 10. When the grade of bauxite is between the two, the combined method can give full play to the advantages of Bayer process and sintering process and obtain better technical and economic indicators.
At present, the annual output of alumina in the world is about 55 million tons, and that in China is about 6.8 million tons.
(2) Production methods of primary aluminum, aluminum alloy and aluminum material.
At present, the only method for industrial production of primary aluminum is Hall-Elu aluminum electrolysis, which was invented by American Hall and French Elu in 1886. Hall-Elu aluminum electrolysis is an electrolyte composed of alumina as raw material and cryolite (Na3AlF6) as flux. Alumina in the electrolyte melt is decomposed into aluminum and oxygen by electrolysis at 950-970℃, and aluminum is precipitated at the carbon cathode in liquid form. Oxygen escapes as carbon dioxide gas on the carbon anode. For every ton of primary aluminum produced, 1.5 tons of carbon dioxide can be produced, and the comprehensive power consumption is about 15000kwh.
Industrial aluminum reduction cells can be roughly divided into three types: side-inserted anode self-baking cell, up-inserted anode self-baking cell and prebaked anode cell. Self-baking cell technology is gradually being eliminated because of its high power consumption in electrolysis process, which is not conducive to environmental protection At present, the annual output of primary aluminum in the world is about 28 million tons, and that in China is about 7 million tons.
If necessary, the primary aluminum obtained by electrolysis can be refined to obtain high purity aluminum. At present, the main production method of aluminum alloy is melt blending. Because aluminum and its alloys have excellent machinability, plates, strips, foils, tubes, wires and other profiles are produced by forging, casting, rolling, stamping and pressing.