The main production process is briefly described below.
1 Raw material preparation system Prepare raw materials for clinker burning - raw slurry, to meet the requirements of moisture, ratio and fineness.
The aluminum ore from the mine is first crushed, homogenized and stored to meet the requirements of particle size less than 15mm and stable chemical composition, and then sent to the tube mill for raw slurry grinding. At the same time, there are 5 kinds of materials added to the mill. : industrial base powder (added in the production process the alkali losses), coal desulfurization (desulfurization firing kiln), lime (by reaction with SiO 2), the mother liquor evaporated (circulating lye) and silicon residue (intermediate product production process ). The main technical indicators controlled by this process are: slurry moisture 38%; fineness 120# sieve residue <10%; formula ensures clinker 
2 Clinker burning is the core part of the sintering method. All the burning chemical reactions take place here. The quality of the fired product-clinker directly affects the technical specifications of the subsequent processes. The fuel used for the firing is pulverized coal, but the generated ash directly enters the material to participate in the chemical reaction. The equipment used for firing is a rotary kiln. The raw slurry is sprayed from the kiln tail. The pulverized coal is sprayed from the kiln head and flows in opposite directions. Finally, the flue gas is discharged from the kiln tail and purified by dust collection. The chimney is emptied and burned. The clinker of the product is discharged from the kiln head and cooled by the cooling machine, and then stored in the warehouse for clinker dissolution treatment. The technical conditions for clinker firing control are: firing temperature 1240-1260 ° C; kiln tail temperature 200-300 ° C; exhaust gas containing CO <0.5%; cooling mill clinker temperature <200 ° C; kiln coal powder fine Degree 170 # sieve residue <10%.
3 Clinker Dissolution Wet grinding dissolution is the most common method. The clinker from the clinker silo is fed into the ball mill (dissolution mill) through the disc feeder, and an appropriate amount of the dissolution liquid-adjustment liquid is added to the mill, and the slurry from the dissolution mill is classified by a classifier. Returning sand back to the mill, the overflow is the dissolution of the slurry to the next process of red mud separation and washing. The main technical conditions controlled by this process are: dissolution temperature (80±5) °C; concentration of the eluent A1 2 O 3 (120±5) g/L, αk=1.2-1.3; the composition of the adjustment liquid is Na 2 O (28 ±3)g/L, A1 2 O 3 (30±5) g/L; the dissolution rate of useful components in clinker is A1 2 O 3 >90%, and Na 2 O>93%.
4 Preparation of the adjustment liquid Although the useful components in the clinker can be dissolved in the hot water, in order to avoid the loss of the secondary reaction in order to prevent the stability of the dissolution slurry, it is necessary to maintain a certain Na 2 O concentration and caustic ratio of the dissolution solution, which depends on Adjust the fluid to complete. The preparation of the adjustment solution is to mix the four solutions in proportion to meet the requirements of the adjustment solution. The four solutions are aluminum hydroxide washing solution, seed decomposition mother liquor, red mud washing liquid and carbonic acid decomposition mother liquor. The equipment used for the preparation is a storage tank and a pump.
5 Red mud separation and washing The process of separating the mixture of solid (clinking residue after clinker - red mud) solution (solution after dissolution - sodium aluminate solution) and washing the red mud is called red mud separation and washing . The separated solution is weighed and sent to the medium pressure desiliconization process; the residue after washing is red mud to the yard or cement plant, and red mud can be used as a raw material for cement production. The requirement for this process is “quickâ€, trying to shorten the time of solid-liquid contact to prevent a secondary reaction between the solid and liquid, and returning the alumina in the solution to the solid phase. The equipment commonly used in this process is a settling tank, a vacuum filter. The main technical indicators of this process control are: separation of sedimentation tank bottom flow solid content percentage 30%-40%; process temperature 95 °C; abandoning red mud liquid alkali content Na 2 O ≤ 5kg / t dry mud. [next]
6 Solution desiliconization This is a means of purifying the solution. According to the different quality requirements of the purified solution, a method of desiliconization (medium pressure desiliconization) and two-stage desiliconization can be adopted. Medium-pressure desiliconization is to heat the crude liquid (adding the silicon residue seed and part of the seed liquid) to 170 ° C and keep it for 1.5-2 h, so that the components in the solution react chemically to produce solid phase silicon residue, and then the silicon residue is separated and returned. The ingredients are subjected to controlled filtration and separation of the fine solid suspension to obtain a purified liquid, which is sent to a decomposition process. The main equipment used for medium pressure desiliconization is desiliconizer, separation settling tank and leaf filter.
The second-stage desiliconization system overflows the separation settling tank obtained by medium-pressure desiliconization, and is added to the lime milk and stirred for 2 hours under normal pressure to further precipitate SiO 2 in the solid phase to obtain a purer solution. The solution has a ratio of A1 2 O 3 /SiO 2 (mass ratio) of 1,500, and then separates the solid phase and the liquid phase. In general, a medium-pressure desiliconization is used. When there are special requirements for alumina products, the second-stage desiliconization is used.
7 Seed Decomposition The purpose of seed decomposition in the sintering method is to obtain a caustic solution (seed mother liquor) to return to the use to ensure the stability of the solution, while obtaining solid aluminum hydroxide is a by-product thereof. The decomposition rate of seed decomposition is low (less than 50%), the decomposition time is long (55h or more), and the occupation of equipment is insufficient. The equipment and process flow for seed decomposition are the same as those of the Bayer process.
8 Carbonic acid decomposition Compared with seed decomposition, the decomposition rate of carbonic acid decomposition is high (more than 90%), the decomposition time is short (2-3h), and the equipment used is small. However, the solution (carbon fraction liquid) obtained after the decomposition is an aqueous solution of Na 2 CO 3 , which can only be concentrated by evaporation, and then reacted with the components in the ore in a calcining kiln after being milled by the raw material. The main equipment used for the decomposition of carbonic acid is a carbon partition, which can be decomposed intermittently or continuously. The CO 2 gas used for the decomposition comes from the purified lime furnace flue gas at a concentration of CO 2 >38% (volume percent).
There are currently 3 sintering process plants for processing bauxite mines, 7 joint process plants, and 3 plants for nepheline mines.
The main problem of the sintering method is high energy consumption, and the comprehensive energy consumption of the process is 46.05 MJ/t alumina. The main technical and economic indicators are: total alumina recovery rate of 87%; bauxite single consumption 2t / t; limestone single consumption 1.8t / t; soda single consumption 108kg / t; coke single consumption 95kg / t; burnt coal consumption 770kg/t; raw coal plus 100kg/t; roasting fuel consumption 78kg/t; power consumption 450kW.h/t; steam consumption 4.2t/t; compressed air consumption 980m 3 /t; new water consumption 18t/ t. [next]
(3) The combined method is a method in which the Bayer process is combined with the sintering method to produce alumina. The most important feature of the method is that the sodium aluminate solution obtained by the sintering system can be used to supplement the alkali loss in the Bayer process. The process is suitable for large scale production and for the treatment of A1 2 O 3 /SiO 2 =5-7.
There are three forms of joint methods, namely parallel method, series method and hybrid method. Only the United States, the former Soviet Union, and China adopt the United Law. The United States has used the tandem method, and China has developed the hybrid method.
1 Parallel method Parallel method refers to the Bayer method and the sintering method are carried out in parallel, each processing high-grade and low-grade ore, each discharging its own waste residue - red mud. The Bayer method and the sintering method are mutually utilized: the alkali precipitated by the Bayer method is not treated by causticization, but is sent to the sintering method; the alkali consumption of the Bayer method is supplemented by the sodium aluminate semen of the sintering method; Bayer method and The aluminum hydroxide produced by the sintering method is washed and calcined.
When using the parallel method, the factory must have high-grade ore and low-grade ore supply, high-grade ore for Bayer process, and low-grade ore for sintering.
2 series method series method refers to the series connection of Bayer method and sintering method. The ore is treated by Bayer method, and the residue obtained - red mud is treated by sintering method, and the final residue is discharged by sintering method.
The difference between this production method and the pure Bayer method and the pure sintering method is:
a. The red mud of the Bayer method is not sent out but sent to the sintering method, and then processed by the sintering method, without adding ore when the ingredients are added;
b. After the precipitation of the alkali (Na 2 CO 3 ) accumulated in the cycle of the Bayer process, the sintering method is added without causticizing, which simplifies the Bayer process;
c. The sodium aluminate semen produced by the sintering method is not subjected to carbonation decomposition treatment, but is sent to the Bayer process seed decomposition process, which simplifies the sintering process and supplements the alkali consumption of the Bayer process.
The advantage of the series method is that the ore is treated by two passes, and the recovery rate of alumina in the ore is high; the capacity of the Bayer method is large, and the capacity of the sintering method is small, so that the factory investment is small and the product cost is low.
At present, there is only one tandem production plant in the world - the Pavlodar alumina plant in Kazakhstan. The factory has also been successfully researched and improved over the years. The process of the plant is shown in Figure 3. [next]
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