The main principle is the Hall-Hellul aluminum electrolysis method: pure alumina is used as raw material to produce aluminum by electrolysis. Because pure alumina has a high melting point (about 2045°C), it is difficult to melt, so the industry Molten cryolite (Na3AlF6) is used as a flux to dissolve the aluminum oxide in the liquid cryolite at about 1000°C to become a melt of cryolite and alumina. Then in the electrolytic cell, carbon blocks are used as the cathode and anode. Perform electrolysis.
A comprehensive 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.
(1) Alumina production methods
So far, many methods for extracting alumina from aluminum ores 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 summarized into four categories, namely alkali method, acid method, acid-base combined method and thermal method. Currently, only the alkali process is used for large-scale industrial production.
Bauxite is the most important aluminum resource in the world, followed by alunite, nepheline, clay, etc. At present, in the world's alumina industry, except for Russia's use of nepheline to produce some alumina, almost all alumina in the world is produced from bauxite as raw material.
Bauxite is an ore mainly composed of gibbsite, boehmite or diaspore. So far, all the bauxite resources available for alumina production in my country are diaspore-type bauxite.
The content of alumina in bauxite varies greatly, ranging from a low of only about 30% to a high of more than 70%. In addition to alumina, the main impurities contained in bauxite are silicon oxide, iron oxide and titanium oxide. In addition, it also contains small or trace amounts of carbonates of calcium and magnesium, compounds of potassium, sodium, vanadium, chromium, zinc, phosphorus, gallium, scandium, sulfur and other elements, as well as organic matter. Although the content of gallium in bauxite is small, it will gradually accumulate in the circulating mother liquor during the alumina production process, so that 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 and silicon oxide content in bauxite, commonly known as the aluminum-silicon ratio.
When producing alumina by the alkali method, the aluminum ore is treated with alkali (NaOH or Na2CO3) to convert the alumina in the ore into a sodium aluminate solution. Impurities such as iron and titanium and most of the silicon in the ore become insoluble compounds. Separate the insoluble residue (red mud) from the solution and discard it after washing or conduct comprehensive treatment to recover the useful components. Pure sodium aluminate solution can be decomposed to release aluminum hydroxide. After separation, washing and calcination, the alumina product is obtained. The decomposition mother liquor is recycled to process another batch of ore. There are various processes for producing alumina by alkali method, such as Bayer process, sintering process and Bayer-sinter combined process. The Bayer process is a method for extracting alumina from bauxite invented by the Austrian chemist K.J. Bayer from 1889 to 1892. There have been many improvements in process technology over the past 100 years, but the basic principles have not changed. In order to commemorate this great contribution of Bayer, the method has always been named Bayer method.
The Bayer process consists of two main processes. First, it is the process of dissolution of aluminum oxide from bauxite under certain conditions (a term commonly used in the alumina industry, namely leaching. The same below), and then the dissolution of aluminum hydroxide from supersaturated sodium aluminate solution. process, these are the two patents proposed by Bayer. The essence of the Bayer process is to extract alumina from bauxite using hydrometallurgy. In the Bayer process alumina production process, silicon-containing minerals will cause the loss of Al2O3 and Na2O.
In the Bayer process, after the bauxite is crushed, it is wet-milled together with lime and circulating mother liquor to make qualified slurry. The slurry is pre-desiliconized and then preheated to the dissolution temperature for dissolution. After dissolution, the slurry undergoes self-evaporation and cooling and then enters the process of dilution and sedimentation separation of red mud (solid phase residue after dissolution).
The secondary steam generated from the evaporation process is used for preheating of the slurry. After sedimentation and separation, the red mud is washed and entered 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. The crude liquid is called semen after passing through a leaf filter to remove most of the floating matter. The semen enters the decomposition process and is decomposed by seed crystals to obtain aluminum hydroxide. After the decomposed aluminum hydroxide is classified, separated and washed, part of it is returned to the seed decomposition process as seed crystals, and the other part is roasted to obtain alumina products. The decomposition mother liquor separated after the seed crystal is decomposed is evaporated and returned to the dissolution process to form a closed loop. Aluminum oxide is obtained after roasting aluminum hydroxide.
The dissolution conditions required for different types of bauxite vary greatly. Gibbsite type bauxite can dissolve well at 105℃, monohydrated boehmite type bauxite can have a faster dissolution rate at 200℃ dissolution temperature, and monohydrated boehmite type bauxite can have a faster dissolution rate at a dissolution temperature of 200℃. Aluminite-type bauxite must be dissolved at a temperature higher than 240°C, and its typical industrial dissolution temperature is 260°C. The dissolution time should not be less than 60 minutes.
The Bayer process is used to process bauxite with a high aluminum-to-silicon ratio. The process is simple, the product quality is high, and its economic effect is far better than other methods. The advantages are even more prominent when used to process easily soluble gibbsite bauxite. At present, more than 90% of the alumina and aluminum hydroxide produced in the world are produced by the Bayer process. Due to the particularity of China's bauxite resources, about 50% of China's alumina is currently produced by the Bayer process.
The process that combines the Bayer process and the sintering process is called the combined process production process. The combination method can be divided into parallel combination method, series combination method and mixed combination method. The method used to produce alumina is mainly determined by the grade of bauxite (i.e. the aluminum-to-silicon ratio of the ore). From a general technical and economic point of view, the sintering method is usually used for ores with an aluminum-to-silicon ratio of about 3; the Bayer method can be used for ores with an aluminum-to-silicon ratio higher than 10; when the grade of bauxite is between the two, the combined method can be used Method treatment, in order to give full play to the respective advantages of the Bayer process and the sintering method, and achieve better technical and economic indicators.
At present, the annual global alumina production is about 55 million tons, and my country's alumina production is about 6.8 million tons.
(2) Production methods of primary aluminum, aluminum alloys and aluminum materials
At present, the only method for industrial production of primary aluminum is the Hall-Hellul aluminum electrolysis method. It was invented in 1886 by Hall of the United States and Erou of France. The Hall-Hellul aluminum electrolysis method is an electrolyte composed of alumina as raw material and cryolite (Na3AlF6) as flux. The alumina in the electrolyte melt is decomposed into aluminum and Oxygen and aluminum precipitate in the form of liquid phase on the carbon cathode, and oxygen escapes in the form of carbon dioxide gas on the carbon anode. Every ton of raw aluminum produced can produce 1.5 tons of carbon dioxide, and the comprehensive power consumption is about 15,000kwh.
Industrial aluminum electrolytic cells can generally be divided into three categories: side-inserted anode self-baking tanks, top-inserted anode self-baking tanks and pre-baked anode tanks. Since the self-baking tank technology consumes high power during the electrolysis process and is not conducive to environmental protection, the self-baking tank technology is being gradually eliminated. At present, the global annual output of primary aluminum is about 28 million tons, and my country's annual output of primary aluminum is about 7 million tons.
If necessary, the raw aluminum obtained by electrolysis can be refined to obtain high-purity aluminum. The current aluminum alloy production method is mainly based on the melting method. Since aluminum and its alloys have excellent machinability, plates, strips, foils, tubes, wires and other profiles are produced through forging, casting, rolling, stamping, pressing and other methods.