? Nature and use
? Production of alumina
? Aluminum electrolysis
The content of aluminum in the earth's crust is second only to oxygen and silicon. There are about 250 kinds of aluminum-containing minerals in nature, the most common being aluminosilicates and their weathering products, clay (Table 2). Bauxite is an ore with alumina hydrate as its main component and has always been the main raw material for aluminum smelting. Natural alumina hydrates include gibbsite, boehmite and diaspore. Their physical and chemical properties vary widely. Therefore, the bauxite is classified into a gibbsite type, a boehmite type, a diaspore type, and a mixed type according to the mineral form of the alumina hydrate therein. The industrial reserves of bauxite found in the world are about 25 billion tons, and the prospective reserves are about 35 billion tons. Countries with abundant reserves and large production volumes include Guinea, Australia, Brazil, Jamaica, and India. The bauxites in these countries are mostly high-iron, low-silicon gibbsite-type, suitable for the production of alumina using the simpler Bayer process.
China's proven bauxite is mainly distributed in Henan, Shanxi, Guizhou, Shandong, Guangxi and other places. Except for some areas, it is a low-iron high-alumina diaspore type, and the impurities are mainly SiO2 in kaolinite. And a small amount of Fe2O3, TiO2. China also has a rich alum stone mine. Zhejiang and Anhui provinces have billion tons of potassium alumite resources, which are raw materials for the production of alumina and potash. In addition, there are abundant nepheline resources found in Yunnan and other places. Compared with various common non-ferrous metal ores, the aluminum content in aluminum ore is much higher, and the reserves of aluminum ore are very rich, which makes the aluminum industry have a large advantage in resources. Properties and Uses The standard electrode potential of aluminum (25 ° C) - 1.662 volts, electrochemical equivalent of 0.3356 g / (A ? hour).
Among various commonly used metals, the density of aluminum is small, and the electrical conductivity, thermal conductivity and reflective properties are very good; the electrical conductivity of aluminum is equivalent to 62-65% of the international standard annealed copper, about half of the silver, if equal weight In terms of aluminum, the electrical conductivity of the aluminum exceeds that of the two metals. Aluminum does not become brittle at low temperatures (-198 ° C). A dense and hard aluminum oxide film is formed on the surface of aluminum in the air to a thickness of 0.005 to 0.02 μm, which becomes a natural protective layer of aluminum, so that aluminum has good corrosion resistance. In addition, anodized or electroplated methods can be used to create a colorful surface on the surface of aluminum and aluminum products. The surface of the aluminum can also be plated with other metals. Aluminum and a variety of aluminum alloys have good ductility and can be processed in various plastics to form aluminum wire, aluminum foil and aluminum. Aluminum has a low melting point and good casting properties, and the amount of cast aluminum alloy used is also large (see aluminum processing). Aluminum has a great affinity for oxygen. The heat of formation of alumina is -400.9±1.5 kcal/mol, so aluminum can be used as a deoxidizer for steelmaking and some metals with high melting point metal oxides (such as MnO2, Cr2O3). Agent. Aluminum reacts with nitrogen, sulfur and halogen elements at high temperatures to form compounds such as AlN, Al2S3, AlCl3. When these compounds (except AlN) and aluminum are heated to above 1000 ° C in a vacuum, a corresponding low-valent aluminum compound is formed. These low-cost compounds undergo disproportionation decomposition at low temperatures to form aluminum metal and its trivalent compounds (for example, AlCl3+2Al匊3AlCl). When AlN is heated to a temperature above 2000 ° C, it begins to decompose into monomeric elements.
Aluminum is an amphoteric element that reacts slowly with most dilute acids and dissolves quickly in concentrated hydrochloric acid. However, concentrated nitric acid deactivates aluminum (see metal corrosion). Aluminum reacts strongly with caustic solution and dissolves rapidly to form aluminate ions: 2Al+2OH-+6H2O-→2Al(OH)嬄+3H2↑Al is widely used in various industrial sectors and daily life. The aviation industry is a traditional aluminum sector. In the construction industry, aluminum alloy is used as the door, window and siding of the house. Various vehicles made of aluminum and aluminum alloys can reduce the energy consumption of transportation because of their light weight, thereby compensating for the energy consumed in aluminum smelting. In terms of power transmission, aluminum is already in the first place, and now 90% of high-voltage wires are made of aluminum. In the food industry, from warehouses, storage tanks to cans, to beverage containers, etc., can be made of aluminum.
The production of aluminum includes the production of alumina and the production of metallic aluminum from alumina electrolysis. The production of alumina has a variety of methods for extracting alumina from ore, such as: Bayer process, soda lime sintering method, Bayer-sintering combined method. The Bayer process has been the main method of producing alumina, and its production accounts for about 95% of the world's total alumina production. Since the 1970s, research on the acid method has made great progress, but it has not been applied in industry.
The Bayer method was invented in 1888 by K.J. Bayer. The principle is to use a caustic soda (NaOH) solution to warm out the alumina in the bauxite to obtain a sodium aluminate solution. After the solution is separated from the residue (red mud), the temperature is lowered, and aluminum hydroxide is added as a seed crystal. After stirring for a long time, the sodium aluminate is separated into aluminum hydroxide, washed, and calcined at a temperature of 950 to 1200 ° C. Alumina finished product. The solution after precipitation of aluminum hydroxide is referred to as a mother liquor, and is concentrated by evaporation and recycled.
The brief chemical reaction of the Bayer method is as follows:
Due to the different crystal structures of gibbsite, boehmite and diaspore, their solubility in caustic soda solution varies greatly, so different dissolution conditions are required, mainly different dissolution temperatures. The gibbsite-type bauxite can be dissolved at 125-140 ° C, and the diaspore-type bauxite is dissolved at 240-260 ° C with the addition of lime (3 to 7%).
The main progress of the modern Bayer process is: 1 large-scale and continuous operation of equipment; 2 automation of production process; 3 energy saving, such as high-pressure enhanced dissolution and fluidized roasting; 4 production of sand-like alumina to meet aluminum electrolysis and flue gas The need for dry purification. The process of Bayer process is shown in Figure 1.
The advantages of Bayer's method are mainly simple process, low investment and low energy consumption. The lowest energy consumption per ton of alumina is only about 3×106 kcal, and the alkali consumption is generally about 100 kg (calculated as Na2CO3). The economic effect of the Bayer process is determined by the quality of the bauxite, mainly the SiO2 content of the ore, usually expressed as the weight ratio of Al2O3 to SiO2 in the ore. Because in the dissolution process of the Bayer process, SiO2 is converted into sodalite-type hydrated sodium aluminosilicate (Na2O?Al2O3?1.7SiO2?nH2O), which is discharged along with the red mud. About 1 kg of Al2O3 and 0.8 kg of NaOH are lost per kg of SiO2 in the ore. The lower the aluminum to silicon ratio of bauxite, the worse the economic effect of the Bayer process. Until the late 1970s, the bauxite treated by the Bayer process had an aluminum to silicon ratio greater than 7-8. Due to the gradual reduction of high-grade gibbsite-type bauxite resources, how to use other types of low-grade aluminum ore resources and new energy-saving technologies is an important direction for research and development. The soda lime sintering method is suitable for treating high-silicon bauxite, mixing bauxite, sodium carbonate and lime in a certain proportion, and sintering into sodium aluminate (Na2O?Al2O3) and sodium ferrite (Na2O) in a rotary kiln. ? Fe2O3, calcium orthosilicate (2CaO?SiO2) and sodium titanate (CaO?TiO2 clinker. Then use a dilute alkali solution to dissolve the sodium aluminate in the clinker. At this time, the NaOH obtained by hydrolysis of sodium ferrite also enters. Solution: If the dissolution conditions are properly controlled, the calcium orthosilicate will not react with the sodium aluminate solution in a large amount, but will be discharged with the composition of calcium titanate, Fe2O3?H2O, etc. The sodium aluminate solution obtained by dissolving the clinker passes through. In a special desiliconization process, SiO2O forms a hydrated aluminosilicate (called sodium silicon slag) or a hydrated garnet 3CaO?Al2O3?xSiO2?(6-2x)H2O precipitate (where x≈0.1), and the solution is purified. The CO2 gas is passed into the refined sodium aluminate solution, and the seed crystal is stirred to obtain the aluminum hydroxide precipitate and the mother liquor whose main component is sodium carbonate. The aluminum hydroxide is calcined to become the finished alumina product. The Al2O3 in the hydrated garnet can be It is extracted and recovered with Na2CO3 mother liquor. The main chemical reaction is as follows:
+3 Ca(OH)2+2NaAl(OH)4+x Na2SiO3─→ 3CaO?Al2O3?x SiO2?(6-2x)H2O↓+2(1+x)NaOH
The main technical achievements in the production of alumina by the Chinese soda lime sintering method are: low alkali ratio formulation in clinker burning, and two-stage abrasive and low molecular ratio solution in the clinker dissolution process to suppress side reaction loss during dissolution. The dissolution rates of Na2O and Al2O3 in the clinker are 94-96% and 92-90.4, respectively. The total recovery of Al2O3 is about 90%, and the consumption of Na2CO3 per ton of alumina is about 95 kg. The soda lime sintering method can treat the low-grade ore which cannot be economically utilized by the Bayer method, and the aluminum-to-silicon ratio can be as low as 3.5, and the comprehensive utilization of the raw materials is better, and has its own characteristics. The common process of soda lime sintering is shown in Figure 2.
The Bayer-Sintering Combined Method can give full play to the advantages of the two methods, take advantage of the length and complement each other, and use the relatively low-alumina bauxite to obtain better economic results. The joint method has many forms, all based on the Bayer method, supplemented by the sintering method. According to the purpose and process connection method of the joint method, it can be divided into three processes: series method, parallel method and hybrid method.
1 Tandem method is to recover Na2O and Al2O3 in Bayer red mud by sintering method, which is used to treat gibbsite-type bauxite which cannot be economically utilized by Bayer method. Expanded raw material resources, reduced alkali consumption, replaced caustic soda with cheaper soda ash, and the recovery rate of Al2O3 was also higher.
2 Parallel method is parallel operation between Bayer method and sintering method to treat bauxite separately, but the sintering method only accounts for 10-15% of the total production capacity. The NaOH produced by the sintering process is used to supplement the consumption of NaOH in the Bayer process.
3 The hybrid method is a synthesis of the first two joint methods. The sintering method in this method treats a part of low-grade ore in addition to the Bayer process red mud.
China has developed a variety of alumina production methods based on the characteristics of its own aluminum resources. In the early 1950s, the soda lime sintering process was used to treat aluminum-silicone-only bauxite-type bauxite with a ratio of only 3.5, creating a characteristic alumina production system. With the Chinese sintering method, the total recovery of Al2O3 can reach 90%; the alkali consumption per ton of alumina (Na2CO3) is about 90 kg; the SiO2 content of alumina drops to 0.02-0.04%; and in the 1950s, the process has been The comprehensive recovery of metal gallium and the use of red mud to produce cement. In the early 1960s, the Bayer Sintering and Mixing Alumina Plant was built to achieve a total recovery of 91% for Al2O3 and a reduction of 60 kg per ton of alumina. This is a highly efficient treatment of higher grade diaspore. Bauxite has created a new road. China has also accumulated a lot of experience in dealing with high-grade diaspore-type bauxite in the Bayer process. Depending on the physical properties, alumina for electrolysis can be divided into three categories: sand, powder and intermediate (Table 3).
At present, the aluminum industry is developing and adopting sand-like alumina, because this alumina has high activity, is easy to dissolve in cryolite solution, and can well absorb hydrogen fluoride in the flue gas of the electrolysis tank, which is beneficial to flue gas purification. . The chemical composition of alumina for aluminum smelting is generally as follows:
Al2O3 >98.35% Fe2O3 0.01～0.04%
SiO2 0.01～0.04% TiO2 <0.005%
ZnO 0.003～0.02% CaO 0.007～0.07%
Na2O 0.3~0.65% V2O5 <0.003%
P2O5 <0.003% Cr2O3 <0.002%
Burning 0.2 to 1.5%
The principle of aluminum electrolysis is to make the direct current into an electrolyte composed of alumina and cryolite as a solvent, and decompose the alumina in the electrolyte melt into aluminum and oxygen at 950 to 970 °C. The aluminum liquid deposited on the cathode is collected in the bottom of the electrolytic cell due to the difference in specific gravity, and carbon dioxide and carbon monoxide gas are precipitated on the anode (see molten salt electrolysis). The aluminum liquid is sucked out from the electrolytic cell, purified to remove hydrogen, non-metal and metal impurities and clarified, and then cast into various aluminum ingots.
Electrochemical Reaction of Aluminum Electrolysis Process The cryolite-alumina melt has an ionic structure in which the cation has Na+ and a small amount of Al3+, the anion has AlF咶, AlF嬄 and Al-OF complex ions, and a small amount of O2- and F- (see melting salt). At a temperature of 1000 ° C, sodium is about 250 mV lower than aluminum at the precipitation potential. Since there is no large overvoltage in the discharge of ions on the cathode, the cathode reaction is: Al3+(complexed)+3e-→Al and the anode reaction is: 6O2-(complexed)+3C-12e-→3CO2 aluminum The total reaction formula of the electrolysis process is: 2Al2O3+3C-→4Al+3CO2 Since the Al dissolved in the electrolyte melt is oxidized by CO2, causing a decrease in current efficiency, the total reaction formula of aluminum electrolysis should actually be: It is the volume percentage of CO2/(CO+CO2) in the anode gas.
In the cryolite-alumina solution, the content of Al2O3 is generally maintained at 3 to 5%, and in order to improve the properties of the electrolyte, fluorides of aluminum, magnesium, calcium and lithium are usually added.
The development of industrial aluminum electrolyzers is divided into three types: the side-inserted anode rod self-baking tank, the upper anode rod self-baking tank and the prebaked anode tank. The prebaked anode electrolysis cell is divided into a side feeding tank and an intermediate feeding tank.