Process and system for recycling desulfurized gypsum of coal-fired power plant

Abstract

The invention relates to the field of desulfurized gypsum, and provides a process and a system for recycling desulfurized gypsum in a coal-fired power plant. The process comprises the following steps: 1) adding water into desulfurized gypsum to prepare slurry, reacting with ammonium bicarbonate to obtain a solid-liquid mixture containing calcium carbonate and ammonium sulfate, and separating into a liquid phase and a solid phase; 2) carrying out three-stage countercurrent washing on the solid phase to obtain calcium carbonate, directly adding water into the calcium carbonate to prepare slurry without drying, and recycling the slurry for gypsum process desulfurization; the washed water is reused in the slurry in the step 1); 3) adjusting the pH value of the liquid phase to 4.5-5.5, performing evaporative crystallization to obtain pure ammonium sulfate crystals and ammonium sulfate mother liquor, and returning the ammonium sulfate mother liquor to the liquid phase in the step 1); steam condensate water generated in the evaporation process is used for three-stage countercurrent washing, and steam generated in the evaporation process is used for drying pure ammonium sulfate crystals. The invention further provides a corresponding system. The whole process / system realizes continuous production operation of resource utilization of the desulfurized gypsum, and is simple to operate and good in production stability.

Description

Technical Field

[0001] The invention relates to the field of desulfurized gypsum, and in particular to a process and system for recycling desulfurized gypsum in a coal-fired power plant. Background Art

[0002] Sulfur dioxide in the flue gas of thermal power plants is considered to be a harmful substance and needs to be removed. As a commonly used desulfurization method, wet desulfurization mainly includes two processes: limestone gypsum method and ammonia method. The limestone gypsum method is a major method of wet desulfurization. The process flow is as follows: 1. Absorption stage: After pretreatment, the flue gas is contacted with limestone slurry to absorb sulfur dioxide into the liquid phase to form calcium sulfite; 2. Oxidation stage: Calcium sulfite is oxidized into gypsum (calcium sulfate) in the presence of oxygen; 3. Recycling: Gypsum is mixed with fresh lime slurry to form a recycled desulfurizer, which continues to be recycled.

[0003] Desulfurized gypsum is the main byproduct of the wet desulfurization process of coal-fired power plants. At present, the main utilization of desulfurized gypsum is to produce cement retarders and paper-faced gypsum boards, which limits the economic value of desulfurized gypsum. For example, patent CN101172796A discloses a full dry desulfurized gypsum building material production process, which uses desulfurized gypsum, phosphogypsum and natural gypsum as raw materials, calcined, sieved, added with inorganic silicon waterproofing agent, and then laid bricks with a brick laying machine, and high-pressure pressing to obtain a method for high-density building material products. Moreover, the wet desulfurization process of coal-fired power plants needs to consume a large amount of calcium carbonate raw materials. Therefore, on the one hand, due to the increase in the output of desulfurized gypsum year by year, there is a lack of efficient resource-based approaches, and on the other hand, the calcium carbonate raw materials for desulfurization are becoming increasingly tight, so it is of great significance to develop a diversified desulfurized gypsum comprehensive utilization process, produce high value-added resource products, and solve the huge amount of solid waste gypsum and the tight calcium carbonate raw materials at the same time. Summary of the invention

[0004] The invention provides a process and system for recycling desulfurized gypsum in a coal-fired power plant, which realizes continuous production and operation of resource utilization of desulfurized gypsum, has simple operation and good production stability.

[0005] In order to achieve the above object, the present invention adopts the following technical solutions: A process for recycling desulfurized gypsum from a coal-fired power plant comprises the following steps: 1) Add water to the desulfurized gypsum to form a slurry, react with ammonium bicarbonate to obtain a solid-liquid mixture containing calcium carbonate and ammonium sulfate, and separate into a liquid phase and a solid phase; 2) The solid phase is washed with three-stage countercurrent water to obtain calcium carbonate, which is directly added with water to prepare a slurry without drying and reused in gypsum desulfurization; the washed water is reused in the slurry of step 1); 3) The pH of the liquid phase is adjusted to 4.5-5.5, and pure ammonium sulfate crystals and ammonium sulfate mother liquor are obtained by evaporation and crystallization, and the ammonium sulfate mother liquor is returned to the liquid phase of step 1); the steam condensed water generated in the evaporation process is used for three-stage countercurrent washing, and the steam generated in the evaporation process is used for drying the pure ammonium sulfate crystals.

[0006] Preferably, the mass ratio of desulfurized gypsum, ammonium bicarbonate and water is 1:1.1-1.2:1; the reaction stirring rate is 80-120 rpm, and the reaction temperature is 40-50 o C, reaction time is 10~20 minutes.

[0007] Preferably, the separation in step 1) is carried out in a filter press with a separation pore size of 1-3 μm.

[0008] Preferably, in step 2), the mass ratio of calcium carbonate to water during slurry preparation is 1:2-4.

[0009] Preferably, the evaporation crystallization in step 3) adopts an MVR evaporation system with a negative pressure of 700-800 mbar, and the temperature is raised to 85-90°C for evaporation, and the 85-90°C water vapor is compressed into 100°C water vapor for circulating heat supply.

[0010] Preferably, the desulfurized gypsum is treated by impregnation dechlorination, and the impregnation liquid contains fly ash loaded with ferric oxyhydroxide. The preparation method of the fly ash loaded with ferric oxyhydroxide is as follows: the fly ash is soaked in acid and then taken out and dried, mixed and dispersed evenly with a sodium hydroxide solution with a concentration of 1.5-2 mol / L, the mixed solution is quickly added to a ferric chloride solution and stirred vigorously, aged at 50-70°C for 60-80 h, and the solid is taken out, washed and dried.

[0011] Preferably, the mass ratio of fly ash to ferric chloride is 10-15: 1. The particle size of fly ash is 100-200 μm.

[0012] Preferably, the molar ratio of sodium hydroxide to ferric chloride is 4-4.5:1, and the concentration of ferric chloride is 0.2-0.3 mol / L.

[0013] The present invention also provides a system of the process, comprising a reactor, a filter press, a three-stage countercurrent water washing device, an MVR evaporation system and a crystallizer, wherein the reactor outlet is connected to the filter press inlet, the liquid phase outlet of the filter press is connected to the three-stage countercurrent water washing device and an ammonium sulfate concentrate receiving device, the solid phase outlet of the filter press is connected to a calcium carbonate receiving device, the outlet of the three-stage countercurrent water washing device is connected to the reactor inlet and the calcium carbonate receiving device, the ammonium sulfate concentrate receiving device is connected to the MVR evaporation system inlet, the evaporation condensed water outlet of the MVR evaporation system is connected to the three-stage countercurrent water washing device inlet, the liquid phase outlet of the MVR evaporation system is connected to the crystallizer inlet, the crystallizer outlet is connected to the solid-liquid separation device, the solid phase outlet of the solid-liquid separation device is connected to the ammonium sulfate receiving device, and the liquid phase outlet of the solid-liquid separation device is connected to the ammonium sulfate concentrate receiving device.

[0014] Preferably, an acid regulating tank is provided between the ammonium sulfate concentrate receiving device and the MVR evaporation system; a drying device is provided between the solid-liquid separation device and the ammonium sulfate receiving device, and the steam of the MVR evaporation system provides heat for the drying device.

[0015] Therefore, the beneficial effects of the present invention are: (1) The entire process / system realizes the continuous production operation of desulfurized gypsum resource utilization, with simple operation and good production stability.

[0016] (2) The complete set of technology for the efficient recycling of desulfurized gypsum into calcium carbonate and the co-production of high value-added ammonium sulfate has been realized. Desulfurized gypsum and calcium carbonate are internally digested and replenished, and the co-production of high value-added ammonium sulfate products improves economic benefits. It can effectively solve the difficulties of desulfurized gypsum solid waste treatment in power plants and the increasingly tight source of calcium carbonate. The high conversion efficiency and considerable economic benefits will help promote the green circular economy. BRIEF DESCRIPTION OF THE DRAWINGS

[0017] Figure 1 This is a flow chart of the recycling of desulfurization gypsum in coal-fired power plants.

[0018] Figure 2 It is a schematic diagram of recycling of the present invention. DETAILED DESCRIPTION

[0019] The technical solution of the present invention is further described below through specific embodiments.

[0020] In the present invention, unless otherwise specified, the raw materials and equipment used can be purchased from the market or are commonly used in the art. The methods in the embodiments, unless otherwise specified, are all conventional methods in the art. Unless otherwise specified, parts are by weight, temperatures are expressed in ° C or at ambient temperature, and pressures are atmospheric pressure or near atmospheric pressure. There are many variations and combinations of reaction conditions (such as component concentrations, desired solvents, solvent mixtures, temperatures, pressures, and other reaction ranges) and conditions that can be used to optimize the purity and yield of the product obtained by the method, and only reasonable routine experiments will be required to optimize such method conditions.

[0021] Example 1. A process for recycling desulfurized gypsum from coal-fired power plants The following steps are involved: 1) Desulfurized gypsum is added with water to form a slurry, which reacts with ammonium bicarbonate to obtain a solid-liquid mixture containing calcium carbonate and ammonium sulfate. The reaction principle is as follows: Figure 2 As shown, it is separated into liquid and solid phases. The mass ratio of desulfurized gypsum, ammonium bicarbonate and water is 1:1.1-1.2:1; the reaction stirring rate is 80-120 rpm, and the reaction temperature is 40-50 o C, the reaction time is 10-20 minutes. The separation is carried out in a filter press, and the separation pore size of the filter press is 1-3 μm.

[0022] 2) The solid phase obtained in step 1) is subjected to three-stage countercurrent water washing to obtain calcium carbonate, and the calcium carbonate is directly added with water to prepare a slurry without drying, and the mass ratio of calcium carbonate to water is 1:2-4. The slurry is sent to the calcium carbonate slurry pipe of the power plant and reused in gypsum desulfurization; the washed water is reused in the slurry of step 1).

[0023] 3) The liquid phase obtained in step 1) is adjusted to pH 4.5-5.5, and pure ammonium sulfate crystals and ammonium sulfate mother liquor are obtained by evaporation and crystallization. A portion of the ammonium sulfate mother liquor is returned to the liquid phase of step 1) and a portion is sent to the waste liquid pool of the power plant. The pure ammonium sulfate crystals are dried to a water content of ≤0.5% and then packaged. The evaporation and crystallization adopts an MVR evaporation system with a negative pressure of 700-800 mbar, and the temperature is raised to 85-90 °C for evaporation. The 85-90 °C water vapor is compressed into 100 °C water vapor for circulation and heating. The steam condensed water generated in the evaporation process is circulated for three-stage countercurrent washing, and the steam generated in the evaporation process is used for drying pure ammonium sulfate crystals.

[0024] Preferably, the desulfurized gypsum is treated by impregnation dechlorination, and the impregnation liquid contains fly ash loaded with ferric oxyhydroxide. The preparation method of fly ash loaded with ferric oxyhydroxide is as follows: the fly ash is soaked in acid and then taken out and dried, and mixed and dispersed evenly with a sodium hydroxide solution with a concentration of 1.5-2 mol / L, the mixed solution is quickly added to a ferric chloride solution and stirred vigorously, aged at 50-70 ° C for 60-80 h, and the solid is taken out for cleaning and drying. The mass ratio of fly ash to ferric chloride is 10-15:1. The particle size of fly ash is 100-200 μm. The molar ratio of sodium hydroxide to ferric chloride is 4-4.5:1, and the concentration of ferric chloride is 0.2-0.3 mol / L.

[0025] 2. A system for recycling desulfurized gypsum from coal-fired power plants The invention comprises a reactor, a filter press, a three-stage countercurrent water washing device, an MVR evaporation system and a crystallizer, wherein the reactor outlet is connected to the filter press inlet, the liquid phase outlet of the filter press is connected to the three-stage countercurrent water washing device and an ammonium sulfate concentrate receiving device, the solid phase outlet of the filter press is connected to a calcium carbonate receiving device, the outlet of the three-stage countercurrent water washing device is connected to the reactor inlet and the calcium carbonate receiving device, the outlet of the ammonium sulfate concentrate receiving device is connected to the inlet of an acid regulating tank, the outlet of the acid regulating tank is connected to the inlet of an MVR evaporation system, the evaporation condensate outlet of the MVR evaporation system is connected to the inlet of the three-stage countercurrent water washing device, the liquid phase outlet of the MVR evaporation system is connected to the inlet of a crystallizer, the outlet of the crystallizer is connected to a solid-liquid separation device, the solid phase outlet of the solid-liquid separation device is connected to a drying device, the steam of the MVR evaporation system supplies heat to the drying device, the drying device is connected to an ammonium sulfate receiving device, and the liquid phase outlet of the solid-liquid separation device is connected to the ammonium sulfate concentrate receiving device.

[0026] Example 1 A system for recycling desulfurized gypsum from a coal-fired power plant, such as Figure 1 As shown, it includes: a reactor, a filter press, a calcium carbonate slurry tank, a first washing water tank, a second washing water tank, a clear water tank, an ammonium sulfate concentrate tank, a sulfuric acid tank, an acid adjustment tank, an ammonium sulfate feed tank, a gas-liquid separator, a steam compressor, an evaporative heat exchanger, a crystallizer, an ammonium sulfate centrifuge, an ammonium sulfate mother liquor tank, a fluidized bed dryer, an ammonium sulfate baler, and a condensate storage tank.

[0027] Specifically, the input end of the reactor is used for the input of desulfurized gypsum, ammonium bicarbonate, and pure water in the power plant, and the output end of the reactor is connected to the input end of the filter press, with a slurry pump provided in between for conveying slurry. The solid phase output end of the filter press is connected to the input end of the calcium carbonate slurry tank, with a belt conveyor provided in between for conveying calcium carbonate; the output end of the calcium carbonate slurry tank goes to the calcium carbonate slurry tank of the power plant to output calcium carbonate slurry for desulfurization; the liquid phase output end of the filter press is respectively connected to the first washing water tank, the second washing water tank, and the ammonium sulfate concentrate tank; the output ends of the first washing water tank, the second washing water tank, and the clean water tank are all connected to the washing input end of the filter press, with a water pump provided in between for conveying washing liquid; the clean water tank is also provided with two output ends respectively connected to the reactor input end and the input end of the calcium carbonate slurry tank, with a water pump provided in between for conveying clean water.

[0028] The output end of the ammonium sulfate concentrate pool is connected to the input end of the acid adjustment pool, and a water pump is provided in between to transport the ammonium sulfate concentrate; the output end of the sulfuric acid tank is connected to the input end of the acid adjustment pool, and a pump is provided in between to transport concentrated sulfuric acid; the output end of the acid adjustment pool is connected to the input end of the ammonium sulfate feed tank, and a pump is provided in between to transport ammonium sulfate; the output end of the ammonium sulfate feed tank is connected to the input end of the gas-liquid separator, and a pump is provided in between to transport ammonium sulfate.

[0029] The liquid in the gas-liquid separator is connected to the internal heat exchange cycle of the evaporative heat exchanger, and the water vapor output end of the gas-liquid separator is connected to the steam compressor for compression and temperature increase output for use as the heat source of the evaporative heat exchanger; the steam output end of the evaporative heat exchanger is connected to the input end of the condensate storage tank; the output end of the condensate storage tank is connected to the input end of the clean water tank, and a water pump is provided in between for clean water transportation; the output end of the gas-liquid separator is connected to the input end of the crystallizer, and a slurry pump is provided in between for ammonium sulfate slurry transportation.

[0030] The output end of the crystallizer is connected to the input end of the ammonium sulfate centrifuge, and a slurry pump is provided in between for transporting ammonium sulfate slurry; the liquid phase output end of the ammonium sulfate centrifuge is connected to the input end of the ammonium sulfate mother liquor tank; one end of the output end of the ammonium sulfate mother liquor tank is connected to the ammonium sulfate concentrate pool, and a water pump is provided in between for transporting the ammonium sulfate mother liquor circulation, and the other end is sent to the power plant waste liquid pool for the discharge of enriched ions. The solid phase output end of the ammonium sulfate centrifuge is connected to the input end of the fluidized bed dryer; the steam from the evaporative heat exchanger provides heat for the fluidized bed dryer, and the output end of the fluidized bed dryer is connected to the input end of the ammonium sulfate packing machine to output the ammonium sulfate product.

[0031] A process for recycling desulfurized gypsum from a coal-fired power plant is carried out in the above system, and the specific steps are as follows: Step 1: In a reactor, the desulfurization gypsum from the power plant is mixed with pure water to form a gypsum slurry, and ammonium bicarbonate is added to the reactor for mixing and stirring. The feed mass ratio is desulfurization gypsum: ammonium bicarbonate: pure water = 1:1.1:1, the stirring rate is 100 rpm, steam is introduced for heating, the heating reaction temperature is 45°C, the reaction lasts for 20 minutes, and the reaction is completed to obtain a solid-liquid mixture of calcium carbonate and ammonium sulfate.

[0032] The chloride ion content in the original gypsum of the power plant desulfurization gypsum is 1800 mg / kg. After two flushing and washing with water, the chloride ion content is reduced to 400 mg / kg, which meets the use requirements.

[0033] Step 2: Separate and wash the solid-liquid mixture obtained in step 1 by a filter press, wherein the filter cloth pore size of the filter press is 2 microns to ensure efficient separation. After the mother liquor is separated, three-stage countercurrent water washing is performed. Three-stage countercurrent water washing is a prior art. Specifically, the first washing is rinsed with the liquid in the first washing water tank, and the effluent is directly mixed with the ammonium sulfate concentrate; the second washing is rinsed with the liquid in the second washing water tank, and the effluent enters the first washing water tank; the third washing is rinsed with clean water from the clean water tank, and the effluent enters the second washing water tank; the washing is completed when the pH of the effluent in the last washing is ≈ 7. Compared with conventional washing, this process requires less water to achieve the same washing effect. After washing, solid calcium carbonate and liquid ammonium sulfate concentrate are obtained.

[0034] Step 3: The solid calcium carbonate separated in step 2 does not need to be dried, but is directly mixed with water at a mass ratio of calcium carbonate: water = 1:3 to prepare a slurry, and transported to the calcium carbonate slurry tank of the power plant for circulation of desulfurization absorption liquid. The liquid ammonium sulfate concentrate separated in step 2 is pH-adjusted with concentrated sulfuric acid, and the pH is controlled to be 4.5-5.5.

[0035] Step 4: Evaporate, concentrate and crystallize the ammonium sulfate concentrate whose pH has been adjusted in step 3. The ammonium sulfate is concentrated by the MVR evaporation system, and the system negative pressure is 800 mbar. At first, a small amount of power plant steam is introduced to heat it to 85°C for evaporation. Subsequently, the 85°C water vapor is compressed into 100°C water vapor by a steam compressor for circulating heat supply, and the generated steam condensate is circulated to the clear water tank for recycling. The concentrated liquid enters the crystallizer and is separated by an ammonium sulfate centrifuge. The separated liquid saturated liquid enters the ammonium sulfate mother liquor tank and circulates to the front-end ammonium sulfate concentrate pool. In addition, after the evaporator continuously concentrates and crystallizes for a period of time, the impurity components in the mother liquor are gradually enriched, and a part of the mother liquor needs to be discharged regularly or continuously to maintain the system balance; the separated solid ammonium sulfate is fully dried by a fluidized bed dryer, and the water content of ammonium sulfate after drying is ≤0.5%, and then enters the packaging machine for automatic packaging to obtain ammonium sulfate products. The ammonium sulfate product meets the technical index requirements of "GB∕T 535-2020 Fertilizer Grade Ammonium Sulfate", especially focusing on the chlorine content, chloride ion ≤1.0%.

[0036] Example 2 The difference from Example 1 is that the method for removing chlorine from desulfurization gypsum in the power plant is different.

[0037] The details are as follows: A process for recycling desulfurized gypsum from a coal-fired power plant is carried out in the above system, and the specific steps are as follows: Step 1: In a reactor, the desulfurization gypsum from the power plant is mixed with pure water to form a gypsum slurry, and ammonium bicarbonate is added to the reactor for mixing and stirring. The feed mass ratio is desulfurization gypsum: ammonium bicarbonate: pure water = 1:1.1:1, the stirring rate is 100 rpm, steam is introduced for heating, the heating reaction temperature is 45°C, the reaction lasts for 20 minutes, and the reaction is completed to obtain a solid-liquid mixture of calcium carbonate and ammonium sulfate.

[0038] The chloride ion content of the original gypsum of the power plant desulfurization gypsum is 1800 mg / kg. It is first rinsed with water and then placed in an impregnation solution for impregnation dechlorination treatment for 5 hours. The impregnation solution is a water suspension of fly ash loaded with iron oxyhydroxide at 10wt%, and the amount of fly ash loaded with iron oxyhydroxide is 20% of the mass of the power plant desulfurization gypsum.

[0039] The preparation method of fly ash loaded with iron oxyhydroxide is as follows: 400 g of fly ash with a particle size of 100-200 μm is soaked in 50wt% concentrated sulfuric acid overnight, then taken out and dried, mixed and dispersed evenly with 500mL of 2M sodium hydroxide solution to obtain a mixed solution, the mixed solution is quickly added into 1111mL of 0.2M ferric chloride solution and stirred vigorously, aged at 70°C for 80 h, and the solid is taken out, washed and dried.

[0040] After the dechlorination treatment, the chlorine content of the desulfurization gypsum in the power plant is reduced to about 220 mg / kg. More importantly, the amount of water used for dechlorination is reduced by about 40% compared with Example 1.

[0041] Step 2: Separate and wash the solid-liquid mixture obtained in step 1 by a filter press, wherein the filter cloth pore size of the filter press is 2 microns to ensure efficient separation. After the mother liquor is separated, three-stage countercurrent water washing is performed. Three-stage countercurrent water washing is a prior art. Specifically, the first washing is rinsed with the liquid in the first washing water tank, and the effluent is directly mixed with the ammonium sulfate concentrate; the second washing is rinsed with the liquid in the second washing water tank, and the effluent enters the first washing water tank; the third washing is rinsed with clean water from the clean water tank, and the effluent enters the second washing water tank; the washing is completed when the pH of the effluent in the last washing is ≈ 7. Compared with conventional washing, this process requires less water to achieve the same washing effect. After washing, solid calcium carbonate and liquid ammonium sulfate concentrate are obtained.

[0042] Step 3: The solid calcium carbonate separated in step 2 does not need to be dried, but is directly mixed with water at a mass ratio of calcium carbonate: water = 1:3 to prepare a slurry, and transported to the calcium carbonate slurry tank of the power plant for circulation of desulfurization absorption liquid. The liquid ammonium sulfate concentrate separated in step 2 is pH-adjusted with concentrated sulfuric acid, and the pH is controlled to be 4.5-5.5.

[0043] Step 4: Evaporate, concentrate and crystallize the ammonium sulfate concentrate whose pH has been adjusted in step 3. The ammonium sulfate is concentrated by the MVR evaporation system, and the system negative pressure is 800 mbar. At first, a small amount of power plant steam is introduced to heat it to 85°C for evaporation. Subsequently, the 85°C water vapor is compressed into 100°C water vapor by a steam compressor for circulating heat supply, and the generated steam condensate is circulated to the clear water tank for recycling. The concentrated liquid enters the crystallizer and is separated by an ammonium sulfate centrifuge. The separated liquid saturated liquid enters the ammonium sulfate mother liquor tank and circulates to the front-end ammonium sulfate concentrate pool. In addition, after the evaporator continuously concentrates and crystallizes for a period of time, the impurity components in the mother liquor are gradually enriched, and a part of the mother liquor needs to be discharged regularly or continuously to maintain the system balance; the separated solid ammonium sulfate is fully dried by a fluidized bed dryer, and the water content of ammonium sulfate after drying is ≤0.5%, and then enters the packaging machine for automatic packaging to obtain ammonium sulfate products. The ammonium sulfate product meets the technical index requirements of "GB∕T 535-2020 Fertilizer Grade Ammonium Sulfate", especially focusing on the chlorine content, chloride ion ≤1.0%.

[0044] Example 3 The difference from Example 2 is that the desulfurization gypsum of the power plant is first washed with water for dechlorination, and then placed in an impregnation liquid for impregnation dechlorination treatment for 5 hours. The impregnation liquid is a 10wt% aqueous suspension of 400g fly ash and 19.5g iron oxyhydroxide FeOOH.

[0045] After the dechlorination treatment, the chlorine content of the desulfurization gypsum in the power plant dropped to about 310 mg / kg, and the amount of water used for dechlorination was reduced by about 40% compared with Example 1.

[0046] Example 4 The difference from Example 2 is that the desulfurization gypsum of the power plant is first washed with water for dechlorination, and then placed in an impregnation liquid for impregnation dechlorination treatment for 5 hours. The impregnation liquid is a 10wt% aqueous suspension of 400g of fly ash.

[0047] After the dechlorination treatment, the chlorine content of the desulfurization gypsum in the power plant dropped to about 360 mg / kg, and the amount of water used for dechlorination was reduced by about 40% compared with Example 1.

[0048] Results Analysis The original chlorine content of the desulfurization gypsum in the power plant is relatively high. In order to produce ammonium sulfate that meets the technical requirements of "GB∕T 535-2020 Fertilizer Grade Ammonium Sulfate", it is necessary to reduce the chlorine content of the desulfurization gypsum in the power plant. Example 1 Using the existing technology, power plants usually directly use a method of multiple water flushing to remove chlorine. Although it is simple, it has the disadvantages of low efficiency and large water consumption.

[0049] Example 4: The desulfurization gypsum of the power plant is first flushed with water to remove part of the chlorine, and then the desulfurization gypsum of the power plant is immersed in a water suspension of fly ash to remove chlorine by adsorption. Fly ash is the main solid waste discharged from coal-fired power plants. Because the fly ash particles have a porous honeycomb structure, a large specific surface area, and a high adsorption activity, and the bead wall has a porous structure with a porosity of up to 50-80%, it can be used to adsorb chloride ions and realize resource utilization.

[0050] Example 3: Fly ash and ferric oxyhydroxide are directly mixed for adsorption of chloride ions. Ferric oxyhydroxide can remove chlorine by precipitation, ion exchange and adsorption, and cooperates with fly ash to achieve better chlorine removal effect.

[0051] In Example 2, fly ash is first impregnated with acid, and then sodium hydroxide and ferric chloride are added to react in the presence of the acid-treated fly ash to prepare ferric hydroxide, thereby obtaining fly ash loaded with ferric hydroxide. Compared with Example 3, fly ash and ferric hydroxide are directly mixed, and the dechlorination effect is significantly improved. The reasons are as follows: on the one hand, the particle size of fly ash is much larger than ferric hydroxide, and at least part of ferric hydroxide will be generated in the pores of fly ash. Compared with Example 3, the two directly exist independently, which has the following advantages: the pore size of fly ash can be dredged, and when it is used for adsorption and dechlorination, ferric hydroxide is chemically adsorbed in the pores of fly ash, thereby improving the adsorption activity in the pore size. On the other hand, fly ash is mainly composed of a variety of oxides composed of elements such as silicon, aluminum, iron, and calcium. The added alkali part will react with fly ash (therefore, the alkali dosage of the present invention is higher than the alkali dosage of conventional ferric hydroxide), and a large number of porous structures and surface hydroxyl structures are generated on fly ash. The formation of such porous structure and surface hydroxyl groups can increase the porosity and specific surface area of ​​fly ash, thereby improving the adsorption performance and reaction activity of fly ash. Alkali modification can dissolve some active components in fly ash, such as Ca(OH)2, and the dissolution of these components can enhance the activity of fly ash.

[0052] The above description is only a preferred embodiment of the present invention and does not limit the present invention in any form. Although the present invention has been disclosed as a preferred embodiment as above, it is not used to limit the present invention. Any technician familiar with this profession can make some changes or modifications to equivalent embodiments of equivalent changes using the technical contents disclosed above without departing from the scope of the technical solution of the present invention. However, any simple modification, equivalent change and modification made to the above embodiments based on the technical essence of the present invention without departing from the content of the technical solution of the present invention still fall within the scope of the technical solution of the present invention.

Claims

1. A process for recycling desulfurized gypsum from a coal-fired power plant, characterized in that: The following steps are involved: 1) Add water to the desulfurized gypsum to form a slurry, react with ammonium bicarbonate to obtain a solid-liquid mixture containing calcium carbonate and ammonium sulfate, and separate into a liquid phase and a solid phase; 2) The solid phase is washed with three-stage countercurrent water to obtain calcium carbonate, which is directly added with water to prepare a slurry without drying and returned to the gypsum desulfurization process; the washed water is returned to the slurry of step 1); 3) The liquid phase is adjusted to pH 4.5-5.5, and pure ammonium sulfate crystals and ammonium sulfate mother liquor are obtained by evaporation and crystallization, and the ammonium sulfate mother liquor is returned to the liquid phase of step 1); the steam condensed water generated in the evaporation process is returned to the third-stage countercurrent water washing, and the steam generated in the evaporation process is dried to obtain pure ammonium sulfate crystals.

2. A process for recycling desulfurized gypsum from a coal-fired power plant according to claim 1, characterized in that: Step 1) The mass ratio of desulfurized gypsum, ammonium bicarbonate and water is 1:1.1-1.2:1; the reaction stirring rate is 80-120 rpm, and the reaction temperature is 40-50 o C, reaction time is 10~20 minutes.

3. A process for recycling desulfurized gypsum from a coal-fired power plant according to claim 1 or 2, characterized in that: Step 1) The separation is carried out in a filter press with a separation pore size of 1-3 μm.

4. A process for recycling desulfurized gypsum from a coal-fired power plant according to claim 1, characterized in that: Step 2) When preparing the slurry, the mass ratio of calcium carbonate to water is 1:2-4.

5. A process for recycling desulfurized gypsum from a coal-fired power plant according to claim 1 or 4, characterized in that: Step 3) Evaporation and crystallization adopts MVR evaporation system with negative pressure of 700-800 mbar, and the temperature is raised to 85-90°C for evaporation. The 85-90°C water vapor is compressed into 100°C water vapor for circulation heating.

6. The process for recycling desulfurized gypsum from a coal-fired power plant according to claim 1, characterized in that: The desulfurized gypsum is treated by impregnation dechlorination, and the impregnation liquid contains fly ash loaded with ferric oxyhydroxide. The preparation method of the fly ash loaded with ferric oxyhydroxide is as follows: the fly ash is soaked in acid and then taken out and dried, mixed and dispersed evenly with a sodium hydroxide solution with a concentration of 1.5-2 mol / L, the mixed solution is quickly added into a ferric chloride solution and stirred vigorously, aged at 50-70 ℃ for 60-80 h, and the solid is taken out, washed and dried.

7. A process for recycling desulfurized gypsum from a coal-fired power plant according to claim 6, characterized in that: The mass ratio of fly ash to ferric chloride is 10-15:

1.

8. A process for recycling desulfurized gypsum from a coal-fired power plant according to claim 6 or 7, characterized in that: The molar ratio of sodium hydroxide to ferric chloride is 4-4.5:1, and the concentration of ferric chloride is 0.2-0.3 mol / L.

9. The process matching system according to any one of claims 1 to 8, characterized in that: The invention comprises a reactor, a filter press, a three-stage countercurrent water washing device, an MVR evaporation system and a crystallizer. The reactor is connected to the filter press. The liquid phase outlet of the filter press is connected to the three-stage countercurrent water washing device and an ammonium sulfate concentrate receiving device. The solid phase outlet of the filter press is connected to a calcium carbonate receiving device. The three-stage countercurrent water washing device is connected to the reactor and the calcium carbonate receiving device. The ammonium sulfate concentrate receiving device is connected to the MVR evaporation system. The evaporation condensate water outlet of the MVR evaporation system is connected to the three-stage countercurrent water washing device. The liquid phase outlet of the MVR evaporation system is connected to the crystallizer. The crystallizer is connected to a solid-liquid separation device. The solid phase outlet of the solid-liquid separation device is connected to the ammonium sulfate receiving device, and the liquid phase outlet is connected to the ammonium sulfate concentrate receiving device.

10. The system according to claim 9, characterized in that An acid regulating tank is also provided between the ammonium sulfate concentrate receiving device and the MVR evaporation system; a drying device is also provided between the solid-liquid separation device and the ammonium sulfate receiving device, and the steam of the MVR evaporation system provides heat for the drying device.

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