Method for treating desulfurized gypsum slurry by limestone-gypsum method

Abstract

The invention provides a limestone-gypsum method desulfurized gypsum slurry treatment method, which comprises: adding ammonia water into desulfurized gypsum slurry, introducing desulfurized flue gas, carrying out a double decomposition reaction, and carrying out solid-liquid separation on the obtained reaction liquid to respectively obtain a calcium carbonate product and a first solution, sequentially performing heavy metal removal treatment and hardness ion removal treatment on the first solution to obtain a second solution, concentrating the second solution, performing evaporative crystallization to obtain an evaporation mother solution and a crystallization product, performing solid-solid separation on the crystallization product to respectively obtain an ammonium sulfate product and an ammonium chloride product, and performing freezing crystallization on the evaporation mother solution to obtain a sodium sulfate product. The treatment method provided by the invention can cooperatively treat the desulfurized gypsum, the desulfurized wastewater and part of the desulfurized flue gas, so that the problem of destination of the desulfurized gypsum can be solved, carbon emission can be reduced, zero emission of the desulfurized wastewater can be realized, various high-added-value products can be recovered, the comprehensive treatment cost is remarkably reduced, and the treatment efficiency is also remarkably improved.

Description

Technical Field

[0001] The invention relates to the field of industrial waste treatment, and in particular to a method for treating desulfurized gypsum slurry of a limestone-gypsum process. Background Art

[0002] In order to meet the SO2 emission standards, domestic coal-fired thermal power units generally use lime-limestone slurry to desulfurize boiler flue gas. In the process of absorbing SO2, the slurry generates a mixture with calcium sulfate dihydrate as the main component. The solid product after dehydration is desulfurized gypsum. At the same time, the desulfurization process uses limestone as an absorbent. For every ton of sulfur dioxide removed, about 0.7 tons of carbon dioxide need to be emitted, resulting in an increase in carbon emissions from power plants. According to data jointly compiled by the Ministry of Ecology and Environment and the China Economic Industry Research Institute, the amount of desulfurized gypsum produced in my country in 2019 was 110 million to 130 million tons. As a solid industrial waste, desulfurized gypsum is difficult to use directly in thermal power plants and requires a suitable place for storage and placement. This not only requires the occupation of a large area of ​​land, but also easily causes secondary pollution to the environment during the stacking process, especially in economically underdeveloped areas, where gypsum consumption is low, desulfurized gypsum cannot be exported, and the problem of storage and abandonment is prominent. Large-scale, high-value disposal and utilization of desulfurized gypsum produced by thermal power units is one of the problems that urgently need to be solved in the field of environmental protection in the current power industry.

[0003] For some power companies that cannot effectively digest desulfurized gypsum, the large amount of gypsum produced as a byproduct has become a heavy economic and environmental governance burden for the company, and the pressure to reduce carbon emissions is relatively greater, which directly affects the sustainable, high-quality and healthy development of the company. Therefore, developing new technologies and methods to offset the amount of desulfurized gypsum produced has become an urgent problem for some companies to solve.

[0004] Chinese patent CN 101284676B discloses a low-cost ammonium sulfate manufacturing process, which comprises adding gypsum powder with a particle size of 150 to 200 meshes to ammonia water containing CO2 with an ammonia concentration of 180 to 200 titers and an ammonia temperature of 40°C to 50°C, heating the carbonized ammonia water to react with the gypsum, wherein the pH value of the mixed liquid during the reaction is 7.2 to 7.5, the reaction temperature is controlled at 50 to 55°C, and the reaction time is 1 to 2 hours; filtering the mixed liquid after the reaction, cooling and crystallizing the ammonium sulfate liquid from which CaCO3 is separated, and separating water from the crystallized ammonium sulfate liquid to obtain an ammonium sulfate product and simultaneously producing calcium carbonate as a byproduct.

[0005] Chinese patent CN 110697731A discloses a method for preparing ammonium sulfate and calcium carbonate using desulfurized gypsum as raw material, using solid ammonium bicarbonate and high-concentration desulfurized gypsum slurry as reaction materials, and a solid phase product with a high calcium carbonate content and ammonium sulfate crystals can be obtained through a two-step reaction.

[0006] It can be seen from the above patents that although these methods can solve the problem of desulfurization gypsum treatment to a certain extent, they are not combined with the existing limestone-gypsum flue gas desulfurization system and the overall process of the power plant, and there is no coordinated treatment of desulfurization wastewater. In addition, the purity of the recovered ammonium sulfate is low, and the energy consumption of evaporation and crystallization is high. Therefore, it is urgent to develop a treatment method to achieve the coordinated treatment of desulfurization gypsum and desulfurization wastewater, solve the problem of the destination of desulfurization gypsum and the problem of low quality of additional products, and achieve zero discharge of desulfurization wastewater. Summary of the invention

[0007] In order to make up for the deficiencies in the prior art, the purpose of the present invention is to provide a method for treating desulfurized gypsum slurry by a limestone-gypsum method, which can achieve the coordinated treatment of desulfurized gypsum, desulfurized wastewater and partially desulfurized flue gas, while fully recovering high-quality by-products and achieving zero discharge of desulfurized wastewater, thereby effectively alleviating the economic and environmental pressures of power companies.

[0008] The method for treating desulfurized gypsum slurry of the limestone-gypsum process provided by the present invention comprises the following steps:

[0009] S1: adding ammonia water into the desulfurized gypsum slurry and introducing the desulfurized flue gas to perform double decomposition reaction, and performing solid-liquid separation on the obtained reaction liquid to obtain a calcium carbonate product and a first solution respectively;

[0010] S2: subjecting the first solution to a heavy metal removal treatment and a hardness ion removal treatment in sequence to obtain a second solution;

[0011] S3: concentrating the second solution and then evaporating and crystallizing it to obtain an evaporation mother liquor and a crystal product; and

[0012] S4: The crystallized product is subjected to solid-solid separation to obtain an ammonium sulfate product and an ammonium chloride product respectively, and the evaporated mother liquor is subjected to freeze crystallization to obtain a sodium sulfate product.

[0013] The usual desulfurization slurry treatment is to separate the desulfurized gypsum from it first, and produce desulfurized wastewater at the same time, and then treat the desulfurized gypsum and desulfurized wastewater separately. This treatment method first requires hydrocyclone, gypsum dehydration and other treatment units to separate the desulfurized gypsum, and also requires the establishment of a separate desulfurized wastewater treatment system. The treatment process is cumbersome, the system occupies a large area, and the treatment cost is high.

[0014] The inventors of the present invention have found that the desulfurized gypsum slurry treatment method provided by the present invention can be used to synergistically treat desulfurized gypsum and desulfurized wastewater. First, ammonia water and CO2 in the desulfurized flue gas are used to convert calcium sulfate into calcium carbonate and ammonium sulfate through a double decomposition reaction. The reaction formula is as follows:

[0015] CaSO4·2H2O+CO2+2NH3·H2O=CaCO3+(NH4)2SO4+3H2O

[0016] The above-mentioned double decomposition reaction can effectively utilize CO2 in the desulfurized flue gas, thereby reducing carbon emissions. The calcium carbonate separated after the reaction can be returned to the desulfurization system as a desulfurizer for the limestone-gypsum method, thereby reducing the use of fresh limestone and reducing the operating cost of the desulfurization system. The liquid phase solution after the double decomposition reaction contains ammonium sulfate and sodium chloride, as well as trace heavy metals and hardness ions (mainly calcium and magnesium ions). The liquid phase solution undergoes a series of subsequent treatments such as heavy metal removal, hardness ion removal, concentration, evaporation crystallization, and frozen crystallization to obtain high-purity ammonium sulfate and ammonium chloride products, which can be sold as a first-class standard fertilizer-grade product. In addition, sodium sulfate products can also be co-produced. The condensed water, concentrated fresh water, etc. generated during the treatment process can be reused in the desulfurization system as process water, thereby achieving zero discharge of desulfurization wastewater, which is not only beneficial to environmental protection, but also greatly reduces the cost of governance.

[0017] The liquid phase of the desulfurized gypsum slurry usually contains 2 to 4 wt.% of sodium chloride, which can not only recover the product, but also, compared with industrial water as the reaction medium (as shown in the comparative example), when this liquid phase solution is used as the reaction medium for the double decomposition reaction, due to the salt effect, the amount of calcium sulfate dissolved in the reaction system can be significantly increased. Therefore, the conversion rate of calcium sulfate in the desulfurized gypsum can be significantly improved, thereby significantly increasing the output of calcium carbonate and ammonium salt products, and further improving the economy of the treatment process.

[0018] In some preferred embodiments, in step S1, the dosage of the aqueous ammonia, measured in terms of the molar amount of NH3, can be 2 to 3 times, preferably 2 to 2.5 times, the molar amount of CaSO4 in the desulfurized gypsum slurry.

[0019] In some preferred embodiments, the amount of the desulfurized flue gas introduced, measured in terms of the molar amount of CO2, can be 1 to 2 times, preferably 1 to 1.5 times, the molar amount of CaSO4 in the desulfurized gypsum slurry.

[0020] In some preferred embodiments, in step S1, the reaction temperature of the metathesis reaction may be 20-60° C., and the reaction time may be 40-120 min. In some more preferred embodiments, the reaction temperature of the metathesis reaction may be 30-50° C., and the reaction time may be 80-100 min.

[0021] In some preferred embodiments, in the step S1, the metathesis reaction may be carried out under stirring, and the stirring rate may be 60 to 400 r / min.

[0022] In some preferred embodiments, in step S1, the solid-liquid separation may use one or a combination of several processes of a sedimentation tank, a clarifier, a filter, a cyclone separator, and a filter press. In some more preferred embodiments, the solid-liquid separation uses a process combination of "precipitation + filter press", and the obtained solid product calcium carbonate is recycled to the desulfurization system for flue gas desulfurization.

[0023] In some preferred embodiments, in step S2, the heavy metal removal treatment can be adding a sulfide agent to the first solution, and removing the generated precipitate after reacting for 0.5 to 3 hours, wherein the sulfide agent used can be a common agent for heavy metal removal in the art, such as organic or inorganic sulfide. In some more preferred embodiments, the sulfide agent can be sodium sulfide, and the amount of the sulfide agent can be adjusted according to different heavy metal contents. The method for removing the precipitate can be a conventional solid-liquid separation method, for example, those described in step S1.

[0024] In some preferred embodiments, in step S2, the hardness ion removal treatment can be to subject the first solution treated with heavy metal removal to ion adsorption by a weakly acidic cation exchange resin or a chelating resin. The hardness ions of the desulfurized gypsum slurry are mainly calcium and magnesium ions, so a weakly acidic cation exchange resin or a chelating resin that can adsorb and remove calcium and magnesium ions commonly used in the art can be selected. The flow rate of the solution flowing through the resin can be adjusted according to different hardness ion contents, resin types and other conditions.

[0025] In some preferred embodiments, in step S3, the concentration may be membrane concentration or steam thermal concentration, and the salt content of the concentrated solution is 15-20wt.%, and the energy consumption of subsequent evaporation and crystallization can be reduced by concentration. In some more preferred embodiments, the concentration method may be a combination of reverse osmosis concentration and electrodialysis concentration, that is, firstly reaching a certain salt content (e.g., 8-12wt.%) by reverse osmosis concentration, and then reaching the target salt content by electrodialysis concentration.

[0026] In some preferred embodiments, in step S3, the temperature of the evaporation crystallization may be 70-90° C. In some more preferred embodiments, the volume of the solution after the evaporation crystallization may be 6-10% (eg, 7-9%) of the volume of the solution before the evaporation crystallization.

[0027] In some preferred embodiments, in step S4, the crystallized product can be heated to 340-380° C. for solid-solid separation. At this heating temperature, ammonium chloride is decomposed into gas by heat to achieve separation from ammonium sulfate, and ammonium chloride is recovered after cooling. In addition, other solid-solid separation methods can also be used as long as the separation of the two substances can be achieved.

[0028] In some preferred embodiments, in step S4, the temperature of the freezing crystallization is -5 to 15°C, for example, 0 to 6°C.

[0029] In some preferred embodiments, in step S4, the mother liquor obtained after the freeze crystallization can be refluxed into the evaporative crystallization in step S3 to continue the evaporative crystallization.

[0030] In the processing method provided by the present invention, each of the above processing steps can be carried out in a common device or equipment in the art.

[0031] In the treatment method provided by the present invention, the desulfurized gypsum slurry can be a desulfurized gypsum slurry discharged from a common power plant limestone-gypsum desulfurization system, which usually contains a large amount of desulfurized gypsum and a small amount of sodium chloride, and also contains trace heavy metals and a small amount of calcium ions and magnesium ions. In some preferred embodiments, in the desulfurized gypsum slurry, the content of desulfurized gypsum can be 5-15wt.%, the content of sodium chloride can be 2-4wt.%, the content of heavy metals can be 1-10mg / L, the content of calcium ions can be 500-1500mg / L, and the content of magnesium ions can be 1000-2000mg / L. In some more preferred embodiments, in the desulfurized gypsum slurry, the content of desulfurized gypsum can be 8-12wt.%, the content of sodium chloride can be 2-4wt.%, the content of heavy metals can be 3-8mg / L, the content of calcium ions can be 600-1200mg / L, and the content of magnesium ions can be 1000-1500mg / L.

[0032] The method for treating desulfurized gypsum slurry by the limestone-gypsum process provided by the present invention has the following advantages:

[0033] (1) The treatment method of the present invention can directly treat the gypsum slurry discharged from the desulfurization system, and simultaneously coordinately treat the desulfurization gypsum, desulfurization wastewater and part of the desulfurization flue gas, thereby solving the problem of the destination of the desulfurization gypsum, reducing carbon emissions and achieving zero discharge of desulfurization wastewater. There is no need to set up a separate desulfurization wastewater treatment system, thereby reducing the equipment footprint and reducing the wastewater treatment cost.

[0034] (2) The treatment method of the present invention can convert desulfurized gypsum, desulfurized wastewater and desulfurized flue gas into desulfurizing agent calcium carbonate and high value-added products such as ammonium sulfate and ammonium chloride. The desulfurizing agent can be recycled back to the desulfurization system for flue gas desulfurization, thereby reducing ore mining and lowering desulfurization costs. Products such as ammonium sulfate and ammonium chloride have high purity and can be directly sold as high-quality products.

[0035] (3) The treatment method of the present invention can also significantly improve the conversion rate of calcium sulfate, thereby increasing the yield of products such as calcium carbonate and ammonium sulfate.

[0036] In summary, compared with the common process routes of treating desulfurization gypsum and desulfurization wastewater separately, the treatment method provided by the present invention can greatly reduce the process steps and save equipment and materials, and can also recover a variety of high value-added products. Therefore, the comprehensive treatment cost can be significantly reduced, and the treatment efficiency can also be significantly improved. It provides a desulfurization slurry treatment method with great economic and social value for power companies, and therefore has good industrial practicality and is suitable for large-scale promotion and application. BRIEF DESCRIPTION OF THE DRAWINGS

[0037] Figure 1 It is a schematic diagram of the treatment process of the desulfurized gypsum slurry of the embodiment;

[0038] Figure 2 Schematic diagram of the treatment process of desulfurized gypsum slurry in comparative example. DETAILED DESCRIPTION

[0039] The technical solution of the present invention is further described in detail below in conjunction with specific embodiments.

[0040] Unless otherwise specified, the raw materials or reagents used in the examples and comparative examples of the present invention are commercially available products.

[0041] The percentages used in the embodiments and comparative examples of the present invention are all mass percentages unless otherwise specified.

[0042] Example

[0043] like Figure 1 As shown, the treatment process of desulfurized gypsum slurry includes the following steps:

[0044] (1) The limestone-gypsum desulfurization system of the power plant discharges desulfurized gypsum slurry at a flow rate of 10 t / h, wherein the desulfurized gypsum content is 10 wt.%, the sodium chloride concentration in the slurry is 2 wt.%, the heavy metal content is 5 mg / L, the calcium ion content is 820 mg / L, and the magnesium ion concentration is 1200 mg / L. It is sent to a double decomposition reaction tank, ammonia water is added, a certain amount of flue gas is introduced, and the desulfurized gypsum, ammonia water and carbon dioxide in the flue gas undergo double decomposition reaction, wherein the ratio of the amount of NH3 added to the CaSO4 in the desulfurized gypsum is n(NH3):n(CaSO4)=2.4:1, the ratio of the amount of CO2 in the flue gas to the CaSO4 in the gypsum is 1.5:1, the reaction temperature of the double decomposition reaction is controlled at 40°C, the reaction time is 90 min, and the stirring rate is 300 r / min.

[0045] (2) The solid and liquid products generated after the treatment in step (1) are precipitated and filtered, and the obtained solid product mainly contains calcium carbonate, which is returned to the desulfurization system as a desulfurization agent for flue gas desulfurization.

[0046] (3) After the filter press in step (2), the main components of the liquid solution are ammonium sulfate and sodium chloride, and it also contains trace amounts of heavy metal impurities and calcium and magnesium ions. The liquid solution enters a heavy metal removal reaction tank, and sodium sulfide is added. After reacting for 1 hour, precipitation is performed to remove the heavy metal ions therein. The amount of sodium sulfide added is 30 mg / L.

[0047] (4) The solution after heavy metal removal enters the resin adsorption system to remove residual calcium and magnesium ions. The resin adsorption material uses chelating resin (model: Dusheng CH-93), and the flow rate of the solution flowing through the resin is 15m / h.

[0048] (5) The solution treated in step (4) is concentrated by a combination of seawater desalination reverse osmosis and electrodialysis, wherein the reverse osmosis uses a HydraNer PRO-LF1 reverse osmosis membrane and the electrodialysis uses a Lanran SED-4016-250 electrodialyzer. The solution is first concentrated by reverse osmosis to a salt content of 10 wt.%, and then concentrated by electrodialysis to a salt content of 16 wt.%.

[0049] (6) The mother liquor obtained after concentration in step (5) is evaporated and crystallized at 80° C., and the volume after evaporation is about 8% of that before evaporation. The solid phase obtained by separation is a mixture of ammonium sulfate and ammonium chloride. The condensate and concentrated fresh water are recovered as process water for the desulfurization system. The mixture of ammonium sulfate and ammonium chloride is heated and separated at 350° C. The ammonium chloride is decomposed into gas by heat. After cooling, the ammonium sulfate and ammonium chloride are separated to obtain ammonium sulfate and ammonium chloride products, respectively.

[0050] (7) The mother liquor produced by the evaporation and crystallization in step (6) is subjected to freeze crystallization at 3° C. to recover the sodium sulfate product, and the frozen crystallization mother liquor is refluxed to the evaporation and crystallization step to continue evaporation and crystallization.

[0051] The yield of calcium carbonate in desulfurized gypsum slurry is 95.2% (calculated as the conversion rate of calcium in desulfurized gypsum calcium sulfate into calcium carbonate), and the yield of ammonium salt product is 96.3% (calculated as the conversion rate of ammonia in added ammonia water into ammonium sulfate and ammonium chloride). The ammonia nitrogen content in the ammonium sulfate product is 20.8%, which meets the Class I standard of fertilizer-grade ammonium sulfate (GB / T 535-2020), and the heavy metal content is lower than the limit requirement of harmful substances; the ammonia nitrogen content in the ammonium chloride product is 24.8%, which meets the first-class standard of fertilizer-grade ammonium chloride (GB / T 2946-2018), and the heavy metal content is lower than the limit requirement of harmful substances.

[0052] Comparative Example

[0053] like Figure 2 As shown, the desulfurized gypsum slurry is the same as that in the embodiment, except that a conventional treatment method is adopted, that is, desulfurized gypsum and desulfurized wastewater are first obtained by solid-liquid separation, the amount of desulfurized gypsum is 1 t / h, and the amount of desulfurized wastewater is 9 t / h.

[0054] For desulfurized gypsum, industrial water is added thereto to make its content 10wt.%, that is, the amount of desulfurized gypsum slurry is 10t / h, and subsequent treatment is carried out in the same manner as in the embodiment, except that the impurity content of the liquid phase after solid-liquid separation after the double decomposition reaction is different, as shown in Table 1. Although the impurity content in the liquid phase of the comparative example is lower than that in the embodiment, it is still necessary to set up a heavy metal removal and resin adsorption system, and the treatment costs of the two are slightly different. The liquid phase of the comparative example does not contain sodium chloride, so there is no need to heat and separate ammonium sulfate and ammonium chloride during the treatment process, and the ammonium sulfate product is directly obtained after evaporation and crystallization.

[0055] In the comparative example, the yield of calcium carbonate was 89.5% (calculated as the conversion rate of calcium in desulfurized gypsum calcium sulfate into calcium carbonate), and the yield of ammonium sulfate product was 90.6% (calculated as the conversion rate of ammonia in added ammonia water into ammonium sulfate). Among them, the ammonia nitrogen content in the ammonium sulfate product was 20.7%, which met the Class I standard of fertilizer-grade ammonium sulfate (GB / T535-2020), and the heavy metal content was lower than the limit requirement of harmful substances.

[0056] For desulfurization wastewater, conventional treatment methods include pretreatment, impurity removal, concentration and solidification, and the general treatment cost is 40-80 yuan / ton of water (for example, Xiong Rihua. Room temperature crystallization and salt separation zero-emission desulfurization wastewater treatment technology [J]. Water Treatment Technology, 2019, 45(06): 10-14.).

[0057] The processing costs of the two methods of the embodiment and the comparative example (excluding the raw material cost, the raw material costs of the two methods are equivalent) are shown in Table 2.

[0058] Table 1 Comparison of mother liquor impurity content between the embodiment and the comparative example (unit: mg / L)

[0059]

[0060]

[0061] Table 2 Comparison of processing costs between the embodiment and the comparative example (unit: yuan)

[0062]

[0063] It can be seen that the comparative example increases the process cost by 281 yuan / ton compared with the embodiment due to the addition of separate treatment of desulfurized wastewater, even at the lowest cost, and a new desulfurized wastewater treatment project is also required, plus the cost of separating desulfurized gypsum, so the treatment cost and investment are both high. On the other hand, the embodiment uses desulfurized wastewater as the medium for the double decomposition reaction, wherein the sodium chloride content is 2%. Due to the salt effect, under the same conditions, the yield of desulfurized gypsum calcium carbonate can reach 95.2%, and the yield of ammonium salt can reach 96.3%, which is significantly higher than the comparative example.

[0064] Unless otherwise defined, the terms used in the present invention have the meanings commonly understood by those skilled in the art.

[0065] The embodiments described in the present invention are for illustrative purposes only and are not intended to limit the scope of protection of the present invention. Those skilled in the art may make various other substitutions, changes and improvements within the scope of the present invention. Therefore, the present invention is not limited to the above embodiments, but is only limited by the claims.

Claims

1. A method for treating desulfurized gypsum slurry by a limestone-gypsum process, characterized in that: The following steps are involved: S1: adding ammonia water into the desulfurized gypsum slurry and introducing the desulfurized flue gas to perform double decomposition reaction, and performing solid-liquid separation on the obtained reaction liquid to obtain a calcium carbonate product and a first solution respectively; S2: subjecting the first solution to a heavy metal removal treatment and a hardness ion removal treatment in sequence to obtain a second solution; S3: concentrating the second solution and then evaporating and crystallizing it to obtain an evaporated mother liquor and a crystal product; as well as S4: performing solid-solid separation on the crystallized product to obtain an ammonium sulfate product and an ammonium chloride product respectively, and performing freeze crystallization on the evaporated mother liquor to obtain a sodium sulfate product.

2. The processing method according to claim 1, characterized in that: In step S1, the amount of ammonia water added is 2 to 3 times the molar amount of CaSO4 in the desulfurized gypsum slurry, calculated on the molar amount of NH3; the amount of desulfurized flue gas introduced is 1 to 2 times the molar amount of CaSO4 in the desulfurized gypsum slurry, calculated on the molar amount of CO2.

3. The processing method according to claim 2, characterized in that: In the step S1, the reaction temperature of the metathesis reaction is 20 to 60° C., and the reaction time is 40 to 120 min.

4. The processing method according to any one of claims 1 to 3, characterized in that: In step S2, the heavy metal removal treatment is to add a sulfide agent to the first solution, react for 0.5 to 3 hours, and remove the generated precipitate; Preferably, the sulfide agent is sodium sulfide.

5. The processing method according to any one of claims 1 to 4, characterized in that: In the step S2, the hardness ion removal treatment is to subject the first solution that has been treated to remove heavy metals to ion adsorption through a weakly acidic cation exchange resin or a chelating resin.

6. The processing method according to any one of claims 1 to 5, characterized in that: In step S3, the concentration is membrane concentration or steam thermal concentration, and the salt content of the concentrated solution is 15-20wt.%.

7. The processing method according to any one of claims 1 to 6, characterized in that: In step S3, the temperature of the evaporation crystallization is 70-90°C; Preferably, the volume of the solution after evaporation and crystallization is 6-10% of the volume of the solution before evaporation and crystallization.

8. The processing method according to any one of claims 1 to 7, characterized in that: In the step S4, the crystallized product is heated to 340-380°C for solid-solid separation.

9. The processing method according to any one of claims 1 to 8, characterized in that: In step S4, the temperature of the freezing crystallization is -5 to 15°C, preferably 0 to 6°C; Preferably, the mother liquor obtained after the freeze crystallization is refluxed into the evaporative crystallization in step S3.

10. The processing method according to any one of claims 1 to 9, characterized in that: In the desulfurized gypsum slurry, the content of desulfurized gypsum is 5-15wt.%, the content of sodium chloride is 2-4wt.%, the content of heavy metals is 1-10mg / L, the content of calcium ions is 500-1500mg / L, and the content of magnesium ions is 1000-2000mg / L.

Images