industrial aluminum smelting: the main principle is Hall-Elu aluminum electrolysis: pure aluminum oxide is used as raw material to produce aluminum by electrolysis. Because pure aluminum oxide has a high melting point (about 245℃), it is difficult to melt. Therefore, molten cryolite (Na3AlF6) is used as a flux in industry, so that aluminum oxide is dissolved in liquid cryolite at about 1℃ to become a molten body of cryolite and aluminum oxide, and then it is placed in an electrolytic cell.
The overall introduction is as follows:
Production and Processing of Aluminum
There are four links in the production process of aluminum to form a complete industrial chain: aluminum ore mining-alumina production-electrolytic aluminum smelting-aluminum processing and production.
generally speaking, two tons of aluminum ore produce one ton of alumina; Two tons of alumina produce one ton of electrolytic aluminum.
(I) production method of alumina
Up to now, many methods for extracting alumina from aluminum ore or other aluminum-containing raw materials have been proposed. Due to technical and economic reasons, some methods have been eliminated, and some are still in the experimental research stage. The proposed alumina production methods can be classified into four categories, namely, alkali method, acid method, acid-base combined method and thermal method. At present, only alkali method is used for 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 nepheline in Russia, almost all alumina in the world is produced from 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 3% to over 7%. In addition to alumina, the main impurities in bauxite are silicon oxide, iron oxide and titanium oxide. In addition, it also contains a small or trace amount of calcium and magnesium carbonate, potassium, sodium, vanadium, chromium, zinc, phosphorus, gallium, scandium, sulfur and other compounds and organic substances. Although the content of gallium in bauxite is small, it will gradually accumulate in circulating mother liquor during alumina production, so it can be effectively recovered and become the main source of gallium production.
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 alumina-silica ratio.
when producing alumina by alkali method, aluminum ore is treated with alkali (NaOH or Na2CO3) to transform alumina in the ore into sodium aluminate solution. Impurities such as iron and titanium and most of silicon in the ore become insoluble compounds. The insoluble residue (red mud) is separated from the solution, washed and discarded or comprehensively treated to recover the useful components. Pure sodium aluminate solution can decompose and separate out aluminum hydroxide, and after separation, washing and calcination, alumina products can be obtained. The decomposition mother liquor is recycled to treat another batch of ore. There are many processes to produce alumina by alkali method, such as Bayer process, sintering process and Bayer-sintering combined process. Bayer process is a method for extracting alumina from bauxite, which was invented by Austrian chemist K·J·Bayer in 1889 ~ 1892. There have been many improvements in technology for more than one hundred years, but the basic principle has not changed. In order to commemorate this great contribution of Bayer, this method has always used the name Bayer method.
Bayer process includes two main processes. The first is the dissolution of alumina from bauxite under certain conditions (the term used in alumina industry is leaching). The following is the same) process, and then the process of aluminum hydroxide dissolving from supersaturated sodium aluminate solution, which is the two patents put forward 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 is crushed, and then 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 the dissolved pulp is cooled by self-evaporation, it enters the processes of dilution and settling and separation of red mud (solid residue after dissolution). The secondary steam generated in the self-evaporation process is used for the pre-heating of pulp. After settling and separation, the red mud is washed into the red mud yard, and the separated crude liquid (sodium aluminate solution containing solid suspended matter, the same below) is sent to the leaf filter. The crude liquid is called semen after most of the floating matter is removed by leaf filtration. The semen enters the decomposition process and is decomposed by seed crystals to obtain aluminum hydroxide. After classification, separation and washing, a part of the decomposed aluminum hydroxide returns to the seed decomposition process as a seed, and the other part is roasted to obtain alumina products. The decomposed mother liquor separated after seed crystal decomposition is evaporated and returned to the dissolution process to form a closed cycle. Alumina is obtained by roasting aluminum hydroxide.
different types of bauxite require different dissolution conditions. The gibbsite type bauxite can be well dissolved at 15℃, and the diaspore type bauxite can have a faster dissolution rate at 2℃, while the diaspore type bauxite must be dissolved at a temperature higher than 24℃, and its typical industrial dissolution temperature is 26℃. The dissolution time is not less than 6 minutes.
Bayer process is used to treat bauxite with high Al-Si ratio, with simple process and high product quality, and its economic effect is far better than other methods. When it is used to treat gibbsite bauxite which is easy to dissolve out, its advantages are even more outstanding. At present, more than 9% of alumina and aluminum hydroxide produced in the world are produced by Bayer process. Due to the particularity of bauxite resources in China, at present about 5% of alumina in China is produced by Bayer process.
The process of combining Bayer process and sintering process is called combined production process. The combination method can be divided into parallel combination method, series combination method and mixed combination method. What method is used to produce alumina is mainly determined by the grade of bauxite (that is, the aluminum-silicon ratio of ore). From the general technical and economic point of view, the sintering method is usually used when the aluminum-silicon ratio of ore is about 3; Bayer process can be used for ores with Al-Si ratio higher than 1. When the grade of bauxite is between the two, the combined method can be used to give full play to the respective advantages of Bayer process and sintering process and achieve 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.
(II) Production methods of primary aluminum, aluminum alloys and aluminum materials
At present, the only method for industrial production of primary aluminum is Hall-Elu aluminum electrolysis. It was invented in 1886 by Hall of America and Elu of France. Hall-Elu aluminum electrolysis method is an electrolyte composed of alumina as raw material and cryolite (Na3AlF6) as flux. At 95-97℃, the alumina in the electrolyte melt is decomposed into aluminum and oxygen by electrolysis. Aluminum precipitates in the form of liquid phase at the carbon cathode, and oxygen escapes in the form of carbon dioxide gas at 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 15kwh.
industrial aluminum reduction cells can be generally 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 being gradually eliminated because of its high electricity consumption in electrolysis process and unfavorable 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 fusion blending. Because aluminum and its alloys have excellent machinability, plates, strips, foils, pipes, wires and other profiles are produced by forging, casting, rolling, punching and pressing.