2 The main production process Bayer process production process mainly consists of three stages of dissolution, decomposition and melting. The principal processes the whole process: ore crushing and wet milling, high temperature and high pressure stripping, washed red mud separation, decomposing seeds, and the mother liquor was evaporated aluminum hydroxide was calcined.
After the aluminum ore enters the plant, it is crushed, homogenized and stored, and the crushed ore is sent to the next step of wet grinding. The purpose of this process is to break the aluminum ore to a particle size of ≤15mm and to uniformly supply the chemical composition to the wet mill. The control index is: the weighted average A/S (aluminum to silicon ratio) of the ore supply per seven days fluctuates at ± Within 0.5 range.
Wet grinding is to further mill the aluminum ore and carry out three components (aluminum ore, lime, circulating alkali liquor), so that the obtained product-original slurry meets the requirements of high-pressure dissolution. The technical conditions of the process control are: the amount of lime added is 7% of dry aluminum ore; the amount of circulating alkali solution is αk (caustic coefficient) of controlling the eluate is 1.55; the fineness of grinding is +170 # sieve<15 %, +100 # sieve <5%.
High-pressure dissolution is the core part of the Bayer process, requiring high heat utilization, low construction investment, easy operation, and low operating costs. For the dissolution of diaspore type ore, there are currently three forms of high pressure dissolution: pipeline preheating and residence dissolution (ie full pipeline); pipeline preheating and mechanical agitation poking tank preheating, new steam heating , stop preheating, molten salt heating and residence tank (without mechanical stirring) dissolution. All three forms are practiced in China. The main technical conditions for the control of this process are: the original ore slurry should be pressed and desiliconized frequently, so as to avoid the tube wall "crust" when the pipeline is preheated; the dissolution temperature is 260-280 ° C; the dissolution time is 15-60 min.
The slurry obtained after the completion of the dissolution is cooled, depressurized and diluted to be treated under normal pressure—the separation and washing of the red mud. The separation and washing are generally carried out using settling tanks. The most advanced settling tank currently used in industry is deep. Cone settling tank. The overflow of the separation settling tank is the product crude liquid, and the refined liquid obtained by controlled filtration is sent to the seed for decomposition; the underflow is a solid residue (called red mud), which is settled by 4-5 times and backwashed to recover the liquid in the liquid. The alkali is sent to the yard for storage. The main technical conditions for the separation and washing process of red mud sedimentation are: the temperature of the material in the process is above 95 °C; the mass percentage of the underflow solids in the separation settling tank is ≥41%, and the suspended matter content in the overflow is ≤200mg/L; the final washing sedimentation The bottom solids mass percentage of the tank is ≥48%, and the Na 2 O ≤5 kg per ton of dry red mud is taken; in order to improve the sedimentation performance, a flocculant is added during the production process.
Seed decomposition is a process in which a sodium aluminate solution is added to a seed (fine aluminum hydroxide) at a reduced temperature and stirred for a long time to separate the solid aluminum hydroxide and the liquid caustic. The main technical conditions controlled by this process are: temperature at which decomposition begins (70 ° C) and final temperature (45 ° C); decomposition time 55-60 h; seed addition amount (seed ratio is 2.5-5.5); decomposition rate (45%-50 %).
After the seed is decomposed, a mixture of solid (aluminum hydroxide) and liquid (caustic alkali) is obtained. After classification and filtration, the seed is obtained (fine aluminum hydroxide) and the product aluminum hydroxide and decomposition mother liquid (caustic solution). . The seed is returned to the seeding trough, and the product aluminum hydroxide is filtered and washed to obtain an alumina product, and the decomposed mother liquid is sent to the evaporation station for treatment. [next]
The purpose of evaporation is threefold: one is to increase the concentration of the solution, and a part of the water is distilled off to meet the requirement of high-pressure dissolution for alkali concentration (Na 2 O 180-230 g/L); the second is to exclude Na 2 CO 3 and Na accumulated during the production process. 2 SO 4 , their solubility is inversely proportional to the alkali concentration. When the alkali concentration reaches a certain level, they are solid phase precipitated from the solution and separated. Third, the organic matter accumulated in the production process is excluded, and the organic matter generally follows Na 2 CO 3 and The precipitation of Na 2 SO 4 precipitated. Evaporation is done in a high efficiency vacuum evaporator.
Regardless of the production method, the aluminum hydroxide obtained is calcined to obtain the product alumina. The purpose of the calcination is twofold: one is to remove the attached water and the crystal water in the aluminum hydroxide, and the other is to convert the crystal form of the alumina into a crystal form required for electrolysis. The calcination operation mainly controls the calcination temperature and the amount of alumina reduction. The equipment used for roasting used to be a rotary kiln. Now it is a fluidized roaster. The main improvement is that the heat consumption is greatly reduced. The heat consumption of the rotary kiln is 5.02 MJ/tA1 2 O 3 , and the fluidized roaster is 3.1 MJ/t Al 2 O 3 . The fuel used in the calciner coal gas, heavy oil or natural gas.
3 Technical and economic indicators Bayer method is the shortest and most economical production method in the world for processing alumina from bauxite, and is also the most important production method. At present, there are 57 Bayer process plants and 7 joint law plants in the world. The Bayer process has an annual production capacity of 49.38 million tons of alumina, accounting for 91.4% of the world's total alumina production.
4 Bauxite ore treated by the technical and economic indicators of Bayer Process, China: diaspore type A1 2 O 3 62.2%, A/S=14.2. Factory capacity: 300,000 tons / year of alumina. Product quality: sand-like alumina, 15% for +125μm and 12% for -45μm. Bauxite single consumption 1.85t / t (dry mine); soda single consumption 50kg Na 2 O / t; lime single consumption 200kgCaO / t; new water consumption 3.6t / t; power consumption 257kW.h / t; roasting heat Consumption 3.2MJ / t; other heat consumption (calculated by steam) 7.2 MJ / t.
(2) Sintering sintering method is the only production method that is applied industrially when dealing with various low-grade aluminum resources. This is because the addition of lime (CaO) or limestone (CaCO 3 ) and alkali powder (Na 2 CO 3 ) by the ingredients, during the firing process, produces new mineral components that are different from the mineral components in the ore and are easy to handle later. In the cooked product - clinker. Alumina can be produced by treating the clinker with a wet process later.
The process flow of the sintering method is shown in Figure 2. [next]
The basic principle of the sintering method is as follows.
1 When the clinker is fired into ingredients, the composition of the clinker meets the following requirements (molar ratio): [next]
Fe 2 O 3 +Na 2 CO 3 →2NaFeO 2 +CO 2
SiO 2 +Na 2 CO 3 →Na 2 SiO 3 +CO2
2Na 2 SiO 3 +2Al 2 O 3 →Na 2 O·Al 2 O 3 ·2SiO 2 +2NaAlO 2
Na 2 O·Al 2 O 3 ·2SiO 2 +4CaO→2NaAlO 2 +2(2CaO·SiO 2 )
The main components in the clinker are NaAlO 2 , NaFeO 2 , 2CaO·SiO 2 .
2 Clinker dissolution The clinker is dissolved in hot water, and the reaction is as follows.
NaAlO 2 is soluble in water and undergoes hydrolysis when the dissolution conditions are unfavorable:
NaAlO 2 +2H 2 O====NaOH+Al(OH) 3 ↓
NaFeO 2 is soluble in water and the degree of hydrolysis is more intense:
NaFeO 2 +2H 2 O====NaOH+Fe(OH) 3 ↓
Hydration of 2CaO·SiO 2 :
2CaO·SiO 2 +2H 2 O====CaO·SiO 2 ·H 2 O↓+Ca(OH) 2 ↓
Acting with Na 2 CO 3 :
2CaO·SiO 2 +2Na 2 CO 3 +H 2 O====2CaCO 3 +Na 2 SiO 3 +2NaOH
Also reacts with NaAlO 2 :
3(2CaO·SiO 2 )+6NaAlO 2 +15H 2 O====
3Na 2 SiO 3 +2Al(OH) 3 +2(3CaO·A1 2 O 3 ·6H 2 O)[next]
Na 2 SiO 3 is dissolved in the NaA1O 2 solution, and the solubility is related to the concentration of Al 2 O 3 :
2Na 2 SiO 3 +2NaAlO 2 +4H 2 O====Na 2 O·Al 2 O 3 ·2SiO 2 ·2H 2 O↓+4NaOH
Hydration of CaO·A1 2 O 3 :
3(CaO·Al 2 O 3 )+12H 2 O====3CaO·Al 2 O 3 ·6H 2 O+4Al(OH) 3 ↓
Acting with Na 2 CO 3 :
CaO·Al 2 O 3 +Na 2 CO 3 ====2NaAlO 2 +CaCO 3 ↓
In summary, the solution contains NaAlO 2 , NaOH, Na 2 SiO 3 ; the solid phase precipitate contains CaCO 3 , 2CaO·SiO 2 , Fe(OH) 3 , Al(OH) 3 , Na 2 O·Al 2 O 3 · 2SiO 2 · 2H 2 O.
3 medium pressure desiliconization The solution obtained after the clinker is dissolved is called the crude liquid because it contains the SiO 2 component in the state of Na 2 SiO 3 . If the SiO 2 is not removed, the quality of the product alumina is not pure, so it is necessary to The crude liquid is subjected to purification treatment, that is, medium pressure desiliconization.
The principle of medium pressure desiliconization is that when the solution is heated, an insoluble solid compound is formed and then separated.
2Na 2 SiO 3 +2NaA1O 2 +4H 2 O Na 2 O·A1 2 O 3 ·2SiO 2 ·2H 2 O↓+4NaOH
When lime (CaO) is added, the following reaction occurs:
2Na 2 SiO 3 +2NaA1O 2 +Ca(OH) 2 +4H 2 O=====CaO·A1 2 O 3 ·2SiO 2 ·2H 2 O↓+6NaOH
The conditions for medium pressure desiliconization control are: temperature 170 ° C, desiliconization time 2 h, solution concentration A1 2 O 3 <120 g / L.
4 The purpose of carbonation decomposition is to introduce CO 2 gas into the sodium aluminate semen, so that the semen is decomposed, and the A1 2 O 3 in the semen is precipitated as much as possible. The principle of action is as follows.
The first step: neutralize the free NaOH in the semen to reduce the ak of the solution.
2NaOH+CO 2 ====Na 2 CO 3 +H 2 O
The second step: the solution is in a metastable state due to the decrease in the ak value, and the solution begins to hydrolyze.
NaAlO 2 +2H 2 O====Al(OH) 3 ↓+NaOH
Continuously introducing CO 2 , continuously hydrolyzing, and continuously depositing Al(OH) 3 .
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