Extracting alumina from a variety of ores methods, for example: Bayer process, soda lime sintering, Bayer - combined sintering method and the like. 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. Bayer method It was invented in 1888 by KJ 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 flow of the 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 Na 2 CO 3 ). The economic effect of the Bayer process is determined by the quality of the bauxite, mainly the SiO 2 content of the ore, usually expressed as the aluminum to silicon ratio of the ore, ie the weight ratio of Al 2 O 3 to SiO 2 in the ore. Because in the dissolution process of the Bayer process, SiO 2 is converted into a sodalite type hydrated sodium aluminosilicate (Na 2 O·Al 2 O 3 ·1.7SiO 2 ·nH 2 O), which is discharged along with the red mud. Approximately 1 kg of Al 2 O 3 and 0.8 kg of NaOH are lost per kg of SiO 2 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. Soda lime sintering Applicable to the treatment of high-silicon bauxite, mixing bauxite, sodium carbonate and lime in a certain proportion, and sintering in a rotary kiln from sodium aluminate (Na 2 O·Al 2 O 3 ), sodium ferrite ( Clinker made of Na 2 O·Fe 2 O 3 , calcium orthosilicate (2CaO·SiO 2 ) and sodium titanate (CaO·TiO 2 ). The sodium aluminate in the clinker is then dissolved in a dilute alkali solution. The NaOH obtained by hydrolysis of sodium ferrite also enters the solution. If the dissolution conditions are properly controlled, the calcium orthosilicate does not react with the sodium aluminate solution in large amounts, but is composed of calcium titanate, Fe 2 O 3 ·H 2 O, etc. The red mud is discharged. The sodium aluminate solution obtained by dissolving the clinker undergoes a special desiliconization process, and SiO 2 O forms hydrated sodium aluminosilicate (referred to as sodium silicon slag) or hydrated garnet 3CaO·Al 2 O 3 ·xSiO 2 · (6-2x) H 2 O precipitation (where x ≈ 0.1) to purify the solution. Pass the CO 2 gas into the refined sodium aluminate solution, and add the seed crystal to stir to obtain the aluminum hydroxide precipitate and the main component is carbonic acid. The mother liquor of sodium. The aluminum hydroxide is calcined to become a finished product of alumina. The Al 2 O 3 in the hydrated garnet can be extracted and recovered by the mother liquor containing Na 2 CO 3 . The main chemical reactions of soda lime sintering are as follows: sintering: Al 2 O 3 +Na 2 CO 3 ─→Na 2 O·Al 2 O 3 +CO 2 Fe 2 O 3 +Na 2 CO 3 ─→Na 2 O·Fe 2 O 3 +CO 2 SiO 2 +2CaCO 3 ─→2CaO·SiO 2 +2CO 2 TiO 2 +CaCO 3 ─→CaO·TiO 2 +CO 2 Clinker dissolution: Na 2 O·Al 2 O 3 +4H 2 O─→2NaAl(OH) 4 (dissolved) Na 2 O·Fe 2 O 3 +2H 2 O─→Fe 2 O 3 ·H 2 O↓+2NaOH (hydrolysis) Desiliconization: 1.7 Na 2 SiO 3 +2NaAl(OH) 4 ─→Na 2 O·Al 2 O 3 ·1.7SiO 2 ·nH 2 O↓+3.4NaOH 3 Ca(OH) 2 +2NaAl(OH) 4 +x Na 2 SiO 3 ─→ 3CaO·Al 2 O 3 ·x SiO 2 ·(6-x)H 2 O↓+2(1+x)NaOH break down: 2NaOH+CO 2 ─→Na 2 CO 3 +H 2 O NaAl(OH) 4 ─→Al(OH) 3 ↓+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 Na 2 O and Al 2 O 3 in the clinker are 94 to 96% and 92 to 94%, respectively. The total recovery of Al 2 O 3 is about 90%, and the consumption of Na 2 CO 3 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-silicon ratio can be as low as 3.5, and the comprehensive utilization of the raw materials is better, and has its own characteristics. Bayer-sintering method The advantages of the two methods can be fully utilized, and the advantages and disadvantages can be complemented. The bauxite with lower aluminum and silicon can be used to obtain better economic effects. 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 The tandem method is to recover Na 2 O and Al 2 O 3 in the Bayer red mud by sintering method, and to treat the gibbsite-type bauxite which cannot be economically utilized by the 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 Al 2 O 3 can reach 90%; the alkali consumption per ton of alumina (Na 2 CO 3 ) is about 90 kg; the SiO 2 content of alumina drops to 0.02-0.04%; and it has comprehensive recovery of metal gallium from the process in the 1950s and utilization of red mud cement production. In the early 1960s, the Bayer Sintering Mixing Alumina Plant was built to achieve a total recovery of 91% for Al 2 O 3 and a reduction of 60 kg per ton of alumina for high-efficiency treatment of higher grades. Aluminite-type bauxite has opened up a new path. 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 1). Table 1 Physical properties of different types of industrial alumina 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 better absorb hydrogen fluoride in the flue gas of the electrolyzer, which is beneficial to flue gas purification. . The chemical composition of alumina for aluminum smelting is generally as follows: Al 2 O 3 >98.35% Fe 2 O 3 0.01～0.04% SiO 2 0.01 to 0.04% TiO 2 <0.005% ZnO 0.003～0.02% CaO 0.007～0.07% Na 2 O 0.3 to 0.65% V 2 O 5 <0.003% P 2 O 5 <0.003% Cr 2 O 3 <0.002% Burning 0.2 to 1.5%
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