Desulfurization gypsum learning summary

 Desulfurization gypsum learning summary
1、 Basic knowledge of gypsum:
Gypsum mainly exists in nature as dihydrate gypsum (CaSO4 • 2H2O) and anhydrite (CaSO4 • 2H2O). The important reason why gypsum is a useful industrial raw material is that when it is heated, it can lose part or all of the crystal water and become burnt gypsum. When it meets water, the burnt gypsum will coagulate and harden to form dihydrate gypsum with original chemical composition. These phenomena are called dehydration and hydration respectively, which are the technological basis of gypsum industry. Dihydrate gypsum is a stable phase at room temperature, but with the increase of temperature and different external conditions, various varieties of hemihydrate gypsum and anhydrite can be obtained. Scholars all over the world have done a lot of research work on the existence conditions and mutual transformation of gypsum phases and varieties, and their views are different.
There are five phases in the CaSO4.H2O system, four of which can exist under normal temperature and pressure, namely dihydrate gypsum, hemihydrate gypsum, anhydrite III and anhydrite II, while the fifth phase anhydrite I can only exist above 1180 ℃. All kinds of gypsum materials cemented after mixing water are made of dihydrate gypsum. Due to different heating temperatures and environmental conditions, water containing and anhydrous calcium sulfate variants can be obtained. During the industrial dehydration of gypsum, it is always expected to use the lowest energy consumption and complete in the shortest possible time. Therefore, the dehydration temperature of gypsum industry is always much higher than the experimental conversion temperature of the desired gypsum phase or variant, which means that it is impossible to produce products composed of a single phase, usually the mixture of various phase variants of CaSO4.H2O system, collectively referred to as gypsum or calcined gypsum.
2、 Main processes of flue gas desulfurization and characteristics of desulfurization products:
There are many kinds of flue gas desulfurization methods, including dry method, semi dry method and wet method. According to the survey, the flue gas desulfurization technology adopted by the major coal-fired power plants in China in the future will mainly be the wet limestone gypsum method (FGD), which is due to the high desulfurization efficiency of the "limestone gypsum" method, mature technology and high utilization value of desulfurization by-products. This method is the leading flue gas desulfurization technology adopted by thermal power plants in the world at present. Desulfo Gypsum (DSG for short) is the by-product obtained after forced oxidation and dehydration of desulfurization by using the "limestone gypsum wet process" process. Compared with natural gypsum, desulfurization gypsum has the characteristics of high purity, stable composition, small particle size, less harmful impurities, and is a kind of gypsum with good quality. all countries
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Practice has proved that desulfurization gypsum can better replace natural gypsum, which can not only achieve comprehensive utilization of resources, but also bring economic benefits to enterprises.
3、 Reasons for quality difference of desulfurization gypsum:
1. In order to obtain high-quality desulfurization gypsum, it is necessary to improve the grinding fineness and quality of limestone as desulfurization absorbent. In the United States, Japan and Germany, the fineness of limestone is generally 90%~95% through 325 mesh sieve; At present, the actual operation of limestone powder in China is that every increase in the fineness of limestone powder will increase the reactivity to limestone, which will lead to greater wear on the mill, more power consumption and lower relative productivity. This will have a greater impact on the profits of limestone powder suppliers. The fineness of limestone produced by manufacturers to reduce costs may not be able to ensure that more than 80% of the limestone powder passes through the 280 mesh screen, so there will be more CaCO3 in desulfurization gypsum formed by crystallization. In addition, in the production process of some limestone powder suppliers, some of them produce stones with different grain sizes in the mining area, so that the small particles screened from the production stones are grinded with Raymond mill, so that there will be many impurities in the limestone powder.
2. Limestone can be divided into high calcium limestone (CaCO3 content is more than 95%); Magnesium limestone (CaCO3 content is 80%~90%, MgCO3 content is 5%~15%); Dolomite (the content of CaCO3 is 50%~80%, and the content of MgCO3 is 15%~45%) can only use high calcium limestone as the desulfurization absorbent for the production of desulfurization gypsum to meet the application requirements of desulfurization gypsum. As the limestone with a purity of 90% is used, the desulfurization gypsum with a purity of 90% can be obtained. If the gypsum purity is to be increased to 93% by increasing the limestone purity under the same operating conditions, the limestone with a purity of 95.1% is required. If limestone with high magnesium content is used, the performance of desulfurized gypsum will become worse, the strength will become lower, the setting time will become longer, and the gypsum products will also be frosted, which will seriously affect the product quality.
3. For desulfurization gypsum containing more magnesium, the drying temperature should not exceed 40 ℃ only to remove free water in gypsum, because the calcination temperature of magnesium limestone and dolomite is much lower than that of limestone. Gypsum containing more magnesium can be dehydrated at 66 ℃. When the temperature is higher than 180 ℃, most of the crystal water will be separated. Gypsum is converted into CaCO3, which is also the reason for low strength of mature gypsum.
4. The crystal of lime is fine, and its average particle size is 15~19um. When the desulfurization slurry contains high concentration of soluble MgSO4 magnesium sulfate, it plays a good buffer role in SO2 absorption, and also prevents the possibility of scaling of operating equipment. These are willing to be used in the desulfurization operation of the power plant. Some power plants sometimes need to add some soluble magnesium for SO2 oxidation during desulfurization operation, which not only has a bad impact on desulfurization gypsum, but also the increase of magnesium content will directly affect the crystal form, size, viscosity and dehydration of desulfurization gypsum in the later stage. In this respect, the desulfurization operation is contradictory to the production of gypsum products, so we must
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The impurity composition of limestone shall be solved in a proper range during desulfurization operation.
5. Calcium sulfite is produced by the neutralization reaction of sulfur dioxide through lime milk. It is slightly soluble in water, slowly oxidized into calcium sulfate in air, and decomposed in acid to release SO2100 ℃, lose crystal water, and decompose at 650 ℃; The product reacted during the operation of hybrid desulfurization is a mixture of MgSO3 and CaCO3. The content of MgSO3 in desulfurization gypsum shall be less than 0.35%.
6. To ensure the quality of desulfurized gypsum, it is necessary to start from two aspects: design and operation management. When determining the purity of desulfurized gypsum products, priority is often given to the utilization rate of limestone. If the utilization rate of limestone is required to reach 95%~97%, the lime grade should be greater than 93%, and the CaCO3 content in limestone should be close to 95%.
7. During desulfurization operation, when the oxidation rate decreases, the concentration of soluble sulfite in circulating slurry increases, and in severe cases, a higher content of solid CaSO3 · 1/2H2O will appear in gypsum. The increase of soluble sulfite concentration in the slurry will inhibit the dissolution of CaCO3 and increase the concentration of reactive CaCO3 in the slurry. Therefore, complete oxidation is not only beneficial to improve the desulfurization rate, but also an important factor to ensure the quality of gypsum.
4、 Selection of drying and calcining process and equipment for desulfurization gypsum:
The key to the advancement and success of desulfurized gypsum production line lies in the progressiveness and rationality of drying and calcining technology and equipment.
According to the selection principle of gypsum calcination process and equipment, the one-step production process is adopted when the moisture content of desulfurized gypsum is low, that is, drying and calcination are completed at the same time. However, in actual production, the moisture content of desulfurized gypsum is 10%~12%, so it is reasonable to choose the process of drying and calcination separately. The flash air flow drying equipment is ideal. In addition, the steam drying equipment developed by Zhejiang Ningbo Lianda Building Materials Industry Co., Ltd. is also suitable for power plants (the investment is 4 million yuan, 100000 tons/year).
The main calcination equipment includes continuous cooking pot, indirect rotary kiln, fluidized bed furnace, FC chamber furnace, fluidized bed furnace, direct heating rotary kiln, Shashi base grinding, DELTA mill, Stoddar furnace and Peter mill. Among them, indirect rotary kiln is used (investment is about 8~10 million yuan, 0.2~2.4 million tons/year). Suitable for hot flue gas are continuous boiler (investment: 3 million yuan, 50000 tons/year), FC chamber furnace (investment: 3 million yuan~5 million yuan, 50000~100000 tons/year), direct heated rotary kiln (investment: 4 million yuan, 100000 tons/year), Peter mill (investment: 12 million yuan, 300000 tons/year). The coal consumption of these drying and calcining equipment is generally above 35kg/t, and the power consumption is generally above 8kWh/t.
5、 Main utilization ways of desulfurization gypsum at home and abroad:
At present, the main utilization ways of desulfurized gypsum include: production of building gypsum, plastering gypsum, gypsum blocks, gypsum plasterboards, gypsum hollow boards, natural leveling gypsum slurry, cement retarders, etc. in the construction and building materials industries,
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Agriculture has been reported to be used for chemical fertilizer production and saline alkali soil improvement.
In foreign countries, the industrial production and use of FGD gypsum have been more than 20 years. Germany is the most developed country in the research, development and application of FGD gypsum. Almost all German gypsum enterprises use FGD gypsum, mainly for the production of building products and cement retarders. The comprehensive treatment and application of FGD gypsum in China has started, For example, Luosulfong Power Plant in Sichuan and Chongqing Power Plant process flue gas desulfurization gypsum into gypsum balls and hemihydrated gypsum, which are used as raw materials for making cement and building materials and transported to gypsum product plants, cement plants and relevant construction units for use; Hangzhou Banshan Power Plant supplies desulfurization gypsum to small and medium-sized gypsum board plants and gypsum hollow block manufacturers nearby; Beijing No. 1 Thermal Power Plant will produce gypsum blocks (with an annual output of 300000 m3) from desulfurization products. However, in general, the disposal and utilization of desulfurization gypsum in China are relatively slow, and industrialization, large-scale and specialized production has not yet been formed. In agriculture, the improvement of saline alkali land by desulfurization gypsum is still in the research stage. It was found that FGD gypsum could reduce pH value, ESP and exchangeable Na+in soil and increase crop yield.
At present, I understand that most power plants still sell desulfurization gypsum to cement plants as a retarder, but due to regional differences, the price of desulfurization gypsum fluctuates greatly. The price of desulfurized gypsum is relatively low compared with that in places where natural gypsum is abundant. For example, Tokto Yunfa Power Co., Ltd. directly sells desulfurized gypsum to cement plants at a price of 14 yuan/ton. Compared with Tianjin, where natural gypsum is relatively scarce, the dried desulfurized gypsum is generally sold to cement plants at a cost of more than 100 yuan/ton, while the cost is about 60 yuan. The cost price of making granular FGD gypsum is 65 yuan, and the factory price is generally 100 yuan/ton.