Smelting aluminum can be done by thermal reduction, but the cost is too high. Electrolysis method is used in industrial aluminum smelting. The main principle is Hall-Heru aluminum electrolysis method: Pure alumina as raw material using electrolytic aluminum, because pure alumina melting point high (about 2045℃), it is difficult [SCALE STEEL]to melt, so industrial melting cryolite (Na3AlF6) as a flux, so that alumina at about 1000℃ dissolved in liquid cryolite, cryolite and alumina melting body, and then in the electrolytic cell, Use carbon block as Yin and Yang poles for electrolysis.
There are four links in the production process of aluminum forming a complete industrial chain: aluminum ore mining - alumina production - electrolytic aluminum smelting - aluminum processing and production.
In general, two tons of aluminum ore produce one ton of alumina; Two tons of alumina produce one ton of electrolytic aluminum.
To date, many methods have been proposed to extract alumina from aluminium ore or other aluminum-containing raw materials. Some[SCALE STEEL] methods have been phased out for technical and economic reasons, while others are still in the experimental research stage. The proposed alumina production methods can be classified into four types, namely, alkali process, acid process, acid-base combined process and thermal process. At present, only the alkali process is used in large-scale industrial production.
Bauxite is one of the most important aluminum resources in the world, followed by alum, nepheline, clay and so on. At present, the [SCALE STEEL]world alumina industry, with the exception of Russia, which uses nepheline to produce part of the alumina, almost all of the world's alumina is produced from bauxite as raw material.
Bauxite is an ore consisting mainly of diaspore, diaspore, or diaspore. Up to now, all the bauxite resources which can be used for alumina production in China are diaspore type bauxite.
The content of alumina in bauxite varies greatly, from about 30% to more than 70%. Bauxite contains chemical composition in addition to alumina, the main impurities are silicon oxide, iron oxide and titanium oxide. In addition, there are a small amount or trace of calcium and magnesium carbonate, potassium, sodium, vanadium, chromium, zinc, phosphorus, gallium, scandium, sulfur and other elements of the [SCALE STEEL]compound and organic compounds. Gallium content in bauxite is small, but in the process of alumina production will gradually accumulate in the circulating mother liquor, so that it can be effectively recovered and become the main source of gallium production.
One of the main indicators to measure the [SCALE STEEL]quality of bauxite is the ratio of alumina content and silica content in bauxite, commonly known as aluminum-silicon ratio.
In the production of alumina by the alkali process, aluminium ore is treated with a base (NaOH or Na2CO3) to convert the alumina in the ore into a solution of sodium aluminate. Impurities such as iron and titanium and most of the silicon in the ore become insoluble compounds. The insoluble residue (red mud) is separated from [SCALE STEEL]the solution, then washed and discarded or treated comprehensively to recover the useful components. Pure sodium aluminate solution can be decomposed into aluminum hydroxide, after separation, washing and calcination, alumina products are obtained. The decomposing mother liquor is then recycled to treat another batch of ore. Alumina production by alkali process includes Bayer process, sintering process and Bayer - sintering combined process.
Bayer process is a method of extracting alumina from bauxite, which was invented by the Austrian chemist K. J. Bayer in 1889 ~ 1892. There have been many improvements in process technology for over a hundred years, but[SCALE STEEL] the basic principles have not changed. In honor of Bayer's great contribution, the method has been using the name Bayer method.
The Bayer process consists of two main processes. The first is the dissolution of alumina from bauxite under certain conditions (the term used by the alumina industry is leaching). The following is the same) process, and then the process of water resolution in the solution of sodium aluminate supersaturated by aluminum hydroxide, which is the two patents filed by Bayer. The essence of Bayer process is to extract[SCALE STEEL] alumina from bauxite by hydrometallurgy. In the Bayer process of alumina production, siliceous minerals can cause the loss of Al2O3 and Na2O.
In the Bayer process, bauxite is crushed and wet-milled along with lime and circulating mother liquor to make qualified pulp. After predesilication, the pulp is preheated to dissolution temperature for dissolution.
After the dissolution of the pulp after evaporation and cooling into the dilution and red mud (solid residue after dissolution) settlement separation process. The secondary steam produced by the self-evaporation process is used to preheat the pulp. After settlement and separation, the red mud is washed into the red mud yard, and the separated crude liquid (sodium aluminate solution containing solid floating matter, the same below) is sent to the leaf filter. Coarse liquid through the leaf filter to remove most of the floating matter is called semen. Semen enters the decomposition process to get aluminum [SCALE STEEL]hydroxide by seed decomposition. After the decomposed alumina is graded and separated and washed, part of it is returned to the crystal seed decomposition process, and the other part is roasted to get alumina products. After the seed decomposition, the mother liquor is returned to the dissolution process through evaporation to form a closed cycle. Alumina is obtained by roasting aluminum hydroxide.
The dissolution conditions required for different types of bauxite vary greatly. The diaspore type bauxite can be well dissolved at 105℃, diaspore type bauxite can have a faster dissolution rate at 200℃, and diaspore type bauxite must [SCALE STEEL]be dissolved at a temperature higher than 240℃, its typical industrial dissolution temperature is 260℃. Dissolution time is not less than 60 minutes.
The Bayer process is used to treat bauxite with high aluminum-Si ratio. The process is simple, the product quality is high, and the economic [SCALE STEEL]effect is far better than other methods. The advantages are more outstanding when it is used to treat the easily dissolved bauxite. At present, more than 90 percent of the world's production of alumina and aluminum hydroxide is produced by the Bayer process. Due to the special nature of China's bauxite resources, about 50% of China's alumina is produced by Bayer process. The process combining Bayer process and sintering process is called combined process. The combined method can be divided into parallel combined method, series combined method and hybrid combined method. The method used to produce alumina is mainly determined by the grade of the bauxite (i.e. the aluminum-silicon ratio of the ore). From the general technical and [SCALE STEEL]economic point of view, the sintering method is usually used when the ratio of aluminum to silicon of ore is about 3. The Bayer method can be used for ore with Al/Si ratio higher than 10. When the grade of bauxite is between the two, the combined method can be used to give full play to the advantages of Bayer method and sintering method, and achieve better technical and economic indicators. At present, the global annual alumina production is about 55 million tons, and China's alumina production is about 6.8 million tons.
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