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Fly ash aluminum extraction technology
Powder coal ash is a residue of coal combustion in coal-fired boilers solid waste, a byproduct of coal-fired power plants mainly. By 2007, the annual emissions of fly ash in China have exceeded 200 million tons (and are still increasing year by year), with a cumulative stock of more than 2.5 billion tons and an area of ​​over 50,000 hm 2 . Fly ash takes up a lot of arable land and causes serious pollution to soil, water resources and air. The comprehensive utilization of fly ash is an important issue that has been studied and solved in China for many years. At present, the content of aluminum oxide in pulverized coal-based is generally 17% to 35%, ash content of the aluminum parts can be as high 40% to 60%, non-traditional alumina a very important resource. The extraction of alumina from high-aluminum fly ash is a fine utilization technology of fly ash, which has positive significance for reducing the pollution of fly ash, expanding the utilization of fly ash, and expanding the source of alumina industrial raw materials in China. Priority theme requirements for key areas of the medium and long-term science and technology development plan (2006-2020). With the increasingly strict national environmental protection policy and the intensified crisis of high-grade bauxite resources shortage, the technical method of extracting alumina from high-aluminum fly ash has become a hot spot of concern and research in recent years.
1. Chemical composition and phase morphology of fly ash
The chemical composition and phase morphology of fly ash are the basis for studying the aluminum extraction technology of fly ash. China's fly ash is mainly low calcium ash (CaO<10%), and high calcium ash is only produced in some areas. Table 1 and Table 2 give the general range of chemical composition and phase morphology of low calcium fly ash in China.
Table 1 Chemical composition of low calcium fly ash in China
ingredient
SiO 2
Al 2 O 3
Fe 2 O 3
CaO
MgO
Na 2 O and K 2 O
SO 3
LOI
content
40~60
17~35
2 to 15
1 to 10
0.5 to 2
0.5 to 4
0.1 to 2
1~26
Table 2 Basic mineral composition of low calcium fly ash in China
ingredient
Glass phase
Mullite
quartz
Hematite
magnetite
range
average value
5~79
60.4
2.7~34.1
21.2
0.9 to 18.5
8.1
0 to 4.7
1.1
0.4 to 13.8
2.8
It can be seen from Tables 1 and 2 that fly ash differs not only in chemical composition and elemental composition, but also in phase composition. The chemical composition and phase morphology of fly ash are greatly affected by factors such as coal production area, coal type, combustion mode and combustion degree. Fly ash in East China and North China is generally high alumina fly ash with alumina content exceeding 30%. High alumina fly ash with alumina content exceeding 40% in Shanxi and Inner Mongolia is also found in large quantities. In terms of phase composition, Barbara G·Kutchko analyzed 12 F-class fly ash from different coal-fired power plants and found that the amorphous matter (mainly vitreous) content exceeds 65%, and the crystalline phase (including quartz and mullite) Etc.) are below 50%. Zhang Zhanjun and other researches on high-alumina fly ash in a thermal power plant in Inner Mongolia showed that the Al 2 O 3 content was as high as 48.5%, the mullite- corundum phase in fly ash accounted for 73.7%, and the glass phase only accounted for 24.6%. The uncertainty of aluminum content and phase composition of fly ash is difficult for the in-depth study and promotion of fly ash aluminum extraction technology. At the same time, the main phase of fly ash is mullite (2 Al 2 O 3 ·2SiO 2 ) and aluminosilicate glass phase (the sum of the two is >80%), the mullite properties are relatively stable, and the aluminosilicate glass phase The metastable structure which maintains the arrangement of the high temperature liquid structure also exhibits high chemical stability, so that the soluble SiO 2 and Al 2 O 3 activities in the fly ash are low. Therefore, the ordinary acid or alkali method is directly used, and the effect of extracting alumina from the high aluminum fly ash is very poor. It is necessary to take certain measures to first modify the minerals of fly ash, break the stable structure of Al-O-Si, and improve the activity of aluminum in fly ash.
Second, the research status of fly ash aluminum extraction technology
Since the 1950s, Professor J.Grzymek of Poland has extracted alumina from high alumina gangue or high alumina fly ash (Al 2 O 3 >30%) and used its residue to produce cement. Many scholars at home and abroad have been A lot of research has been done on the technology of aluminum ash from fly ash. There are many processes for extracting alumina (aluminum hydroxide) or aluminum salt from fly ash, but there are mainly two types of alkali sintering and acid leaching, and most of the processes are still in the laboratory research stage, and industrial application is rare.
(1) Alkaline sintering
At present, the research on alkali-sintering fly ash aluminum extraction technology can be divided into two categories: calcium salt additive sintering method and sodium salt additive sintering method.
Calcium salt aid sintering method is to sinter one or more of limestone , lime, gypsum and other calcium salts with fly ash at 1200 ~ 1400 ° C, so that the aluminosilicate with low activity in fly ash is at high temperature. Aluminium-silicon separation is achieved by forming calcium aluminate soluble in the Na 2 CO 3 solution and insoluble calcium disilicate. The limestone sintering method is the earliest proposed method for extracting aluminum from fly ash at home and abroad, and it is also the only industrialized application process reported in China. The basic process flow of limestone sintering method is shown in Figure 1.
Figure 1 Basic process of limestone sintering process
Liu Elin and Zhao Jianguo have made improvements on the basis of this process: direct carbonation and filtration of sodium aluminate crude solution, and the obtained high silicon aluminum hydroxide solids are dissolved by low temperature Bayer process, and the obtained sodium aluminate semen is passed. The seeding and calcination obtain alumina, and the carbon mother liquor returns to the clinker dissolution process. At present, the process has been put into operation in Inner Mongolia. Although the limestone sintering method has been industrialized at present, its own defects have limited its popularization and application: high energy consumption (sintering at 1200-1400 °C), complicated process, adding a large amount of limestone due to sintering, so that the amount of slag is 7~ of alumina products. 10 times, for this reason, only silicon-calcium slag can be used to co-produce cement. However, due to the small effective radius of the mud market, the demand for local cement is increased, and the market risk is high.
In order to solve the defects of high energy consumption and large slag content in the limestone sintering method, a sodium salt such as Na 2 CO 3 may be used in part or in whole instead of the calcium salt as a sintering aid to reduce the sintering temperature, save energy and reduce the amount of slag. However, when the calcium salt is replaced by a sodium salt such as Na 2 CO 3 , since the silicon aluminum in the fly ash is relatively high, when the clinker is leached with the alkali solution, some aluminum and alkali are taken away due to the precipitation of the hydrated aluminosilicate sodium salt. , reducing the recovery rate of aluminum, the alkali consumption is increased, so only the clinker can be leached with acid. For example, Ma Hongwen proposed using Na 2 CO 3 as a fluxing agent to decompose high-aluminum fly ash at 750-880 ° C to form acid-soluble aluminosilicate materials, which were then leached with sulfuric acid to make alumina in fly ash. The silica is separated and further produced alumina and silica, and when leached with 98% concentrated sulfuric acid, the alumina leaching rate is greater than 90%. The calcium salt is partially replaced by a sodium salt such as Na 2 CO 3 , and the clinker is leached with a sodium carbonate solution, which not only reduces the sintering temperature, but also saves energy, and also avoids the problems of strict material requirements and high cost caused by acid leaching. For example, Zheng Guohui sinters fly ash and lime and sodium carbonate at high temperature into soluble sodium aluminate and insoluble calcium disilicate. After separation, the alumina is prepared. The lye is returned to the clinker dissolution process, and the residue is used as the Portland cement raw material. The dissolution rate of alumina is above 90%, and the energy consumption is lower than that of limestone sintering, but CO 2 needs to be additionally provided.
At present, many scholars at home and abroad are conducting in-depth research on the alkali-sintering fly ash aluminum extraction technology. Considering the use of waste residue, waste gas and waste liquid, and promoting clean production, we should also choose a suitable flux to reduce sintering temperature, clinker self-pulverization, aluminum-silicon separation, high-quality aluminum products, and utilization of silicon-calcium slag. In terms of technology, we will increase research efforts to further reduce energy consumption and product costs, improve product quality, enhance market competitiveness, and strive to move toward large-scale industrial applications as soon as possible.
(2) Acid leaching
There are many researches on acid leaching fly ash aluminum extraction technology. The DAL method designed by Oak Ridge National Laboratory (Direct Acid Leaching) is a method that has a great influence on the development of acid leaching method. . Features DAL method is to fly as far as possible the entire resources into a variety of products, regardless obtain the highest extraction rate for certain metals, which emphasizes the DAL method is the overall efficiency of the process. The basic reaction of direct acid leaching of fly ash to aluminum is as follows:
3H 2 SO 4 +Al 2 O 3 =Al 2 (SO 4 ) 3 +3H 2 O
or
6HCl+Al 2 O 3 =2AlCl 3 +3H 2 O
For example, Sun Yazhen and other 60% sulfuric acid and fly ash mixed and heated, so that the activated alumina in the fly ash and sulfuric acid fully reacted, through filtration, cooling, crystallization, suction filtration and other processes to prepare aluminum salt (aluminum sulfate), The alumina extraction rate is 60% to 65%.
In view of the disadvantage of low aluminum leaching rate in direct acid leaching, fluoride (such as ammonium fluoride, sodium fluoride, potassium fluoride, etc.) can be added as a cosolvent to destroy the aluminosilicate glass body and mullite, thereby increasing Al 2 . The dissolution effect of O 3 . The basic reaction is as follows:
3H 2 SO 4 +6NH 4 F+SiO 2 (-Al 2 O 3 )=H 2 SiF 6 +3(NH 4 ) 2 SO 4 +2H 2 O
3H 2 SO 4 +Al 2 O 3 =Al 2 (SO 4 ) 3 +3H 2 O
or
6HCl+6NH 4 F+SiO 2 (-Al 2 O 3 )=H 2 SiF 6 +6NH 4 Cl+2H 2 O
6HCl+Al 2 O 3 =2AlCl 3 +3H 2 O
For example, Zhao Jianyu used aluminum fluoride to assist in the extraction of aluminum from fly ash, and the dissolution rate of alumina was as high as 97%. The addition of a fluoride co-solvent can improve the activity of aluminum in fly ash and increase the leaching rate, but the fluoride is easy to cause secondary pollution to the environment, and the operation is also dangerous. Therefore, some scholars have studied the use of a certain means to activate aluminum in fly ash before acid extraction to improve the leaching rate. For example, Qin Jinguo proposed to use the calcination activation-sulfuric acid leaching process at 300-760 °C to extract aluminum from fly ash. Under normal pressure without any additives, the dissolution rate of alumina in fly ash can be made with sulfuric acid. Up to 85% or more, and on this basis, the fly ash mixed concentrated sulfuric acid roasting-hot water leaching process is proposed, eliminating the previous acid slag separation process, simplifying the process and increasing the effective dissolution rate of alumina to over 90%. . Although the high-temperature roasting-sulfuric acid leaching method and related processes can make the aluminum leaching rate as high as 85% or more, the concentration of residual acid in the leaching solution is high due to the leaching of concentrated sulfuric acid, which not only causes the acid loss of the slag to be taken away, but also leaching and filtering. The material of the material conveying equipment is difficult to solve and the operation is difficult. Therefore, acid leaching has not yet been reported for industrial applications.
(3) Other methods
Many scholars have tried other methods around how to improve the leaching activity of aluminum in fly ash. For example, when Li Laishi waits for the fine grinding of fly ash, it is sintered with ammonium sulfate at 400 ° C, leaching with sulfuric acid, and the extraction rate of alumina can reach 95.6%. After recrystallization from ammonium aluminum sulfate, the purity can be higher than 99.9%. Pure alumina. Compared with the limestone sintering method, the sintering temperature of the process is obviously reduced, and the alumina extraction rate is high and the slag amount is small, so it has certain positive significance and deserves further attention. Zhao Jianyu et al. studied the alumina extraction method based on microwave fluxing. Although the dissolution rate of alumina can be increased to over 95%, the technology still needs to realize the activation of fly ash by means of sintering, and energy consumption and microwave technology. Problems such as magnifying applications have yet to be further resolved, and it is currently difficult to scale up to industrial production.
Third, the outlook
With the increasingly strict environmental protection requirements and the depletion of high-grade bauxite resources, it is foreseeable that fly ash has a good utilization prospect as a non-traditional aluminum resource. At present, there are many factors limiting the large-scale industrial application of fly ash and aluminum extraction technology. In addition to the encouragement and support of national and local policies, and the market demand, the above analysis shows that there are also many technical deficiencies. Therefore, further research should be carried out to improve and perfect the existing fly ash aluminum extraction technology. At the same time, we should actively explore new fly ash aluminum extraction technology, and strive to improve its comprehensive economic benefits while meeting environmental protection requirements. The organic unity of the environment and economy. In this sense, achieving high efficiency, energy saving, low consumption, reduction (waste residue, waste gas) and avoiding secondary pollution is the development trend of fly ash aluminum extraction technology.