New method for improving purity of calcium sulfate in process for preparing calcium sulfate from phosphogypsum

Abstract

The invention discloses a method for improving the purity of calcium sulfate in a process for preparing calcium sulfate from ardealite, which comprises the following steps: mixing ardealite with an organic solvent to obtain ardealite turbid liquid; adding a composite solvent into the phosphogypsum suspension to obtain a mixed solution; a main substance in the mixed solution is a chelate formed by an organic solvent and calcium ions. And heating and stirring the obtained mixed solution to react, and filtering to obtain a clear calcium complex solution. And adding a surfactant and a decomplexing, purifying and absorbing composite functional agent into the clear calcium complex solution, heating and stirring to react, and performing suction filtration and separation to obtain the high-purity calcium sulfate crystal. In the process of crystallizing and separating out calcium sulfate, impurities such as partial heavy metal ions, phosphorus and fluorine are attached to the surfaces of crystals, so that the purity of calcium sulfate is reduced. A surfactant and a decomplexing and purifying composite functional agent are added to improve the dispersing performance of the calcium sulfate crystal, and impurities such as heavy metal, phosphorus and fluorine in a solution are adsorbed to improve the purity of the calcium sulfate crystal.

Description

TECHNICAL FIELD

[0001] The present application relates to the field of separation science and technology, and relates to industrial solid waste treatment, and particularly relates to treatment and resource utilization of phosphogypsum. BACKGROUND

[0002] Phosphogypsum is a by-product in the production process of wet-process phosphoric acid. According to the General Industrial Solid Waste Storage, Disposal Site Pollution Control Standard (GB5086-2002) document standard, phosphogypsum belongs to the second general industrial solid waste. With the rapid development of China's phosphate chemical industry, the treatment of phosphogypsum has become an urgent environmental and resource recycling problem. At present, the annual output of phosphogypsum is about 70 million tons, and it is increasing at a rate of 10% per year. Most of the phosphogypsum is still treated by stacking, such as wet discharge, valley or flat stacking, etc.

[0003] Phosphogypsum disposal and comprehensive utilization is a worldwide problem. For example, Morocco, the United States, etc. mainly use long-term stacking and sea disposal, and there is no perfect utilization system. In the history of Florida, in Hillsborough, Mulberry, Tampa Bay and other areas, the leakage of phosphogypsum stacking plants has caused serious acute disasters to downstream water bodies. A scientific and promising method to stop this phenomenon is to convert phosphogypsum into reusable environmental resources. In recent years, several purification technologies have been widely explored for phosphogypsum purification, including chemical leaching, neutralization, water washing and flotation. However, there are at least three major limitations that hinder the widespread application of these well-studied technologies. 1) These methods usually produce low-quality gypsum with insufficient whiteness and purity, with a calcium sulfate content of less than 95% and a whiteness of about 70%. 2) The treatment cost is high, usually higher than the cost of the obtained gypsum product, which makes the industrialization of such technologies neither economical nor profitable for phosphorus chemical plants. 3) The impurities in phosphogypsum are different because they are extracted from phosphate rocks, and the composition of phosphate rocks varies from place to place. Therefore, the low-cost reduction and high-value utilization of phosphogypsum are the most critical factors in solving the problem of phosphogypsum stacking and accumulation. SUMMARY

[0004] In the current production process of purifying phosphogypsum to prepare high-purity calcium sulfate by using complexing agents to interact with calcium sulfate in phosphogypsum, the calcium complex is decomplexed in an acidic environment, and the calcium ions combine with the calcium sulfate ions in the solution to form calcium sulfate crystals, which are separated from the complexing agent dissolved in the solvent. However, during the crystallization of calcium sulfate, part of the heavy metal ions will precipitate with the calcium ions, resulting in the presence of a small amount of heavy metal impurities in the calcium sulfate crystals. In addition, because the soluble phosphorus and fluorine in the phosphogypsum are also dissolved in the solution, during the gradual precipitation of calcium sulfate and the growth of the crystals, part of the phosphorus and fluorine adhere to the surface of the crystals, resulting in a decrease in the purity of calcium sulfate.

[0005] To this end, the application provides a new method for improving the purity of calcium sulfate in the process of preparing calcium sulfate from phosphogypsum. The method increases the dispersibility of calcium sulfate after crystallization by adding a surfactant, reducing the attachment of phosphorus and fluorine on the surface of the crystal. At the same time, the addition of the compound functional agent of complex decomposition promotes the decomposition of the complex and increases the precipitation of calcium sulfate crystals. On the other hand, it also adsorbs heavy metal ions, phosphorus, fluorine and other impurities, reduces the impurity content in the calcium sulfate crystals and improves the purity of the calcium sulfate crystals. The method is simple, the purity of calcium sulfate is improved significantly, and it is easy to be industrialized.

[0006] To achieve the above purpose, the application provides a new method for improving the purity of calcium sulfate in the process of preparing calcium sulfate from phosphogypsum, comprising the following steps: C1, adding phosphogypsum into an organic solvent to obtain a phosphogypsum suspension; C2, adding the phosphogypsum suspension obtained in step C1 into a compound solvent to obtain a mixed solution; C3, reacting the mixed solution obtained in step C2 under heating and stirring, and then separating the clear calcium complex solution filtrate and phosphogypsum residue by filtration after the reaction is completed.

[0007] C4, adding a surfactant to the clear calcium complex solution filtrate obtained in step C3, and then mixing and reacting with the compound functional agent of complex decomposition, and then separating by filtration after the reaction is completed to obtain high-purity calcium sulfate crystals and an organic solvent mother liquor.

[0008] The phosphogypsum in step C1 includes calcium sulfate, silicon dioxide, water and impurities. In terms of mass fraction, the calcium sulfate is 70-75%, the silicon dioxide is 13-15%, the water is 10-15%, and the impurities are 1-2%.

[0009] The organic solvent in step C1 includes any one or a combination of the following: ethylene glycol bis-aminoethyl ether tetraacetic acid solution, ethylenediaminetetraacetic acid disodium solution, and citric acid solution. When it is ethylene glycol bis-aminoethyl ether tetraacetic acid solution, ethylenediaminetetraacetic acid disodium solution, and citric acid solution, the volume ratio of ethylene glycol bis-aminoethyl ether tetraacetic acid solution: ethylenediaminetetraacetic acid disodium solution: citric acid solution is 2:1:1~1:20:20 mL / mL / mL.

[0010] The concentration of ethylene glycol bis-aminoethyl ether tetraacetic acid solution in the organic solvent in step C1 is 0.5~20 g / L, the concentration of ethylenediaminetetraacetic acid disodium solution is 1~25 g / L, and the concentration of citric acid solution is 1~30 g / L.

[0011] In step C2, the volume ratio of the compound solvent: phosphogypsum suspension is 1:50~1:500 mL / mL.

[0012] In step C2, the composite solvent includes two or more of the following: alanine, valine, leucine, aspartic acid, glutamic acid, and sodium polyaspartate solution.

[0013] In step C2, the concentrations of alanine solution, valine solution, leucine solution, aspartic acid solution, glutamic acid solution, and sodium polyaspartate solution are 5-10 g / L, valine solution, leucine solution, aspartic acid solution, glutamic acid solution, and sodium polyaspartate solution are 5-50 g / L.

[0014] In step C3, the heating temperature is 20℃~60℃, and the reaction time is 0.5~2h.

[0015] In step C3, the stirring rate is 100 r / min to 2000 r / min.

[0016] In step C4, the surfactant includes one or more of the following: nonylphenol polyoxyethylene ether phosphate potassium salt, monoalkyl ether phosphate potassium salt, alkylphenol ether phosphate OPP-4, fatty alcohol polyoxyethylene ether AEO-9, and Pingpingjia OS.

[0017] The amount of nonylphenol polyoxyethylene ether phosphate potassium salt added to the surfactant is 0.01%~0.3% of the mass of phosphogypsum.

[0018] In some preferred embodiments, in step C4, the amount of nonylphenol polyoxyethylene ether phosphate potassium salt added to the surfactant is 0.01%~0.2% of the mass of phosphogypsum, the amount of monoalkyl ether phosphate potassium salt added is 0.01%~0.2% of the mass of phosphogypsum, the amount of alkylphenol ether phosphate OPP-4 added is 0.01%~0.1% of the mass of phosphogypsum, the amount of fatty alcohol polyoxyethylene ether AEO-9 added is 0.05%~0.2% of the mass of phosphogypsum, and the amount of Pingpingjia OS added is 0.1%~0.3% of the mass of phosphogypsum.

[0019] In step C4, the complex-resolving and adsorption-purifying composite functional agent includes one or more of chitosan, cyclodextrin, mannan, carrageenan, alginate, xanthan gum, gellan gum, kodana gum, and konjac glucomannan.

[0020] The amount of the compound functional agent for relieving phosphogypsum absorption is 0.1% to 5% of the mass of phosphogypsum.

[0021] In some preferred embodiments, in step C4, the amount of chitosan added to the complex-removing and astringent composite function is 1%~5% of the mass of phosphogypsum, the amount of cyclodextrin added is 0.5%~3% of the mass of phosphogypsum, the amount of mannan added is 0.1%~2% of the mass of phosphogypsum, the amount of carrageenan added is 0.5%~2% of the mass of phosphogypsum, the amount of alginate added is 0.1%~1% of the mass of phosphogypsum, the amount of xanthan gum added is 0.1%~1% of the mass of phosphogypsum, the amount of gellan gum added is 0.1%~2% of the mass of phosphogypsum, the amount of kodana gum added is 0.5%~2% of the mass of phosphogypsum, and the amount of konjac glucomannan added is 0.5%~2% of the mass of phosphogypsum.

[0022] In step C4, the reaction temperature is 20℃~80℃, the reaction time is 0.5~5h, and the stirring rate is 100 r / min~1000 r / min.

[0023] The high-purity calcium sulfate crystals in step C4 have a purity greater than 99.95%, a phosphorus content of less than 100 mg / kg, a fluorine content of less than 30 mg / kg, a lead content of less than 2 mg / kg, an arsenic content of less than 2 mg / kg, and a selenium content of less than 3 mg / kg.

[0024] Compared with the prior art, the present invention has the following advantages: (1) This invention improves the dispersibility of calcium sulfate crystals in solution by adding a surfactant, reduces the adhesion of impurities to the crystal surface, and increases the purity of calcium sulfate. Simultaneously, it indirectly increases the particle size of calcium sulfate, which is beneficial for expanding the product's application range. Furthermore, this invention also promotes the decomplexing and adsorption of complexes by adding a complexing and adsorbing functional agent, thereby increasing the yield of calcium sulfate crystals. The added complexing functional agent can also adsorb impurities in the solution, reducing the impurity content in the calcium sulfate crystals and improving their purity.

[0025] (2) This process has a series of advantages such as high efficiency, simple operation, low energy consumption, green environmental protection and easy industrialization. The high-purity calcium sulfate crystals obtained can be used in fields such as whisker fillers, separation materials, and food-grade materials, laying a solid foundation for the high-value utilization of phosphogypsum. Detailed Implementation

[0026] To make the present invention easier to understand, the present invention will be further described in detail below with reference to embodiments. These embodiments are for illustrative purposes only and are not intended to limit the scope of application of the present invention. Unless otherwise specified, the raw materials or components used in the present invention can be purchased commercially or obtained by conventional methods.

[0027] Example 1 Step 1: Add phosphogypsum to an organic solvent to obtain a phosphogypsum suspension; The phosphogypsum suspension contains phosphogypsum with a concentration of 300 g / L. Ethylene glycol diaminoethyl ether tetraacetic acid, disodium ethylenediaminetetraacetate, and citric acid are dissolved in water to obtain ethylene glycol diaminoethyl ether tetraacetic acid solution, ethylenediaminetetraacetate disodium solution, and citric acid solution with concentrations of 10 g / L, respectively.

[0028] The organic solvent is obtained by mixing ethylene glycol diaminoethyl ether tetraacetic acid solution, disodium ethylenediaminetetraacetic acid solution, and citric acid solution in a volume ratio of 2:10:10 (mL / mL).

[0029] Step 2: Add the phosphogypsum suspension obtained in Step 1 to a composite solvent to obtain a mixed solution; The composite solvent consists of 10 g / L alanine solution and 10 g / L valine solution, with a concentration of 10 g / L and an addition amount of 1:500 mL / mL for the composite solvent to phosphogypsum suspension.

[0030] Step 3: The mixed solution obtained in Step 2 is reacted at 40℃ with a stirring speed of 800 r / min for 1 h. After the reaction is completed, the solution is filtered at 0.1 MPa and 25℃ to obtain phosphogypsum residue and clear calcium complex solution. According to analysis and calculation, the water content of phosphogypsum residue is 19.51%.

[0031] Step 4: Add the clarified calcium complex solution obtained in Step 3 to the surfactant, and then mix and react with the complex-removing and adsorption-reducing composite functional agent. React at 20℃ with a stirring speed of 500 r / min for 1.5 h. After the reaction is completed, the solution is filtered and separated at 0.1 MPa and 25℃ to obtain high-purity calcium sulfate crystals. The amount of nonylphenol polyoxyethylene ether phosphate potassium salt added to the surfactant is 0.05% of the mass of phosphogypsum, and the amount of monoalkyl ether phosphate potassium salt added is 0.05% of the mass of phosphogypsum. The amount of chitosan added to the complex-resolving and absorption-removing composite functional agent is 2.5% of the mass of phosphogypsum, and the amount of alginate added is 0.5% of the mass of phosphogypsum.

[0032] Analysis, testing, and calculations revealed that the high-purity calcium sulfate crystals contained 99.96% calcium sulfate by mass, 84 mg / kg phosphorus, 25 mg / kg fluorine, 0.05 mg / kg lead, 0.02 mg / kg arsenic, and 0.05 mg / kg selenium.

[0033] Example 2 Step 1: Add phosphogypsum to an organic solvent to obtain a phosphogypsum suspension; The phosphogypsum suspension contains phosphogypsum with a concentration of 300 g / L. Ethylene glycol diaminoethyl ether tetraacetic acid, disodium ethylenediaminetetraacetate, and citric acid are dissolved in water to obtain ethylene glycol diaminoethyl ether tetraacetic acid solution, ethylenediaminetetraacetate disodium solution, and citric acid solution with concentrations of 10 g / L, respectively.

[0034] The organic solvent is obtained by mixing ethylene glycol diaminoethyl ether tetraacetic acid solution, disodium ethylenediaminetetraacetic acid solution, and citric acid solution in a volume ratio of 2:10:10 (mL / mL).

[0035] Step 2: Add the phosphogypsum suspension obtained in Step 1 to a composite solvent to obtain a mixed solution; The composite solvent consists of 10 g / L alanine solution and 10 g / L valine solution, with a concentration of 10 g / L and an addition amount of 1:500 mL / mL for the composite solvent to phosphogypsum suspension.

[0036] Step 3: The mixed solution obtained in Step 2 is reacted at 40℃ with a stirring speed of 800 r / min for 1 h. After the reaction is completed, the solution is filtered at 0.1 MPa and 25℃ to obtain phosphogypsum residue and clear calcium complex solution.

[0037] Step 4: Add the clarified calcium complex solution obtained in Step 3 to the surfactant, and then mix and react with the complex-removing and adsorption-reducing composite functional agent. React at 20℃ with a stirring speed of 500 r / min for 1.5 h. After the reaction is completed, the solution is filtered and separated at 0.1 MPa and 25℃ to obtain high-purity calcium sulfate crystals. The amount of nonylphenol polyoxyethylene ether phosphate potassium salt added to the surfactant is 0.05% of the mass of phosphogypsum, and the amount of Pingpingjia OS added is 0.15% of the mass of phosphogypsum. The amount of cyclodextrin added to the anti-colony and purifying composite functional agent is 2% of the mass of phosphogypsum, and the amount of mannan added is 1% of the mass of phosphogypsum.

[0038] Analysis, testing, and calculations revealed that the high-purity calcium sulfate crystals contained 99.98% calcium sulfate by mass, 80 mg / kg phosphorus, 21 mg / kg fluorine, 0.04 mg / kg lead, 0.02 mg / kg arsenic, and 0.05 mg / kg selenium.

[0039] Example 3 Step 1: Add phosphogypsum to an organic solvent to obtain a phosphogypsum suspension; The phosphogypsum suspension contains phosphogypsum with a concentration of 300 g / L. Ethylene glycol diaminoethyl ether tetraacetic acid, disodium ethylenediaminetetraacetate, and citric acid are dissolved in water to obtain ethylene glycol diaminoethyl ether tetraacetic acid solution, ethylenediaminetetraacetate disodium solution, and citric acid solution with concentrations of 10 g / L, respectively.

[0040] The organic solvent is obtained by mixing ethylene glycol diaminoethyl ether tetraacetic acid solution, disodium ethylenediaminetetraacetic acid solution, and citric acid solution in a volume ratio of 2:10:10 (mL / mL).

[0041] Step 2: Add the phosphogypsum suspension obtained in Step 1 to a composite solvent to obtain a mixed solution; The composite solvent consists of 10 g / L alanine solution and 10 g / L valine solution, with a concentration of 10 g / L and an addition amount of 1:500 mL / mL for the composite solvent to phosphogypsum suspension.

[0042] Step 3: The mixed solution obtained in Step 2 is reacted at 40℃ with a stirring speed of 800 r / min for 1 h. After the reaction is completed, the solution is filtered at 0.1 MPa and 25℃ to obtain phosphogypsum residue and clear calcium complex solution.

[0043] Step 4: Add the clarified calcium complex solution obtained in Step 3 to the surfactant, and then mix and react with the complex-removing and adsorption-reducing composite functional agent. React at 20℃ with a stirring speed of 500 r / min for 1.5 h. After the reaction is completed, the solution is filtered and separated at 0.1 MPa and 25℃ to obtain high-purity calcium sulfate crystals. The surfactants used include alkylphenol ether phosphate OPP-4 at 0.05% of the phosphogypsum mass and fatty alcohol polyoxyethylene ether AEO-9 at 0.1% of the phosphogypsum mass. The complex-resolving and absorbent composite functional agent contains cyclodextrin at 1.5% of the phosphogypsum mass, mannan at 1% of the phosphogypsum mass, and alginate at 0.5% of the phosphogypsum mass.

[0044] Analysis, testing, and calculations revealed that the high-purity calcium sulfate crystals contained 99.97% calcium sulfate by mass, 61 mg / kg phosphorus, 15 mg / kg fluorine, 0.03 mg / kg lead, 0.01 mg / kg arsenic, and 0.03 mg / kg selenium.

[0045] Example 4 Step 1: Add phosphogypsum to an organic solvent to obtain a phosphogypsum suspension; The phosphogypsum suspension contains phosphogypsum with a concentration of 300 g / L. Ethylene glycol diaminoethyl ether tetraacetic acid, disodium ethylenediaminetetraacetate, and citric acid are dissolved in water to obtain ethylene glycol diaminoethyl ether tetraacetic acid solution, ethylenediaminetetraacetate disodium solution, and citric acid solution with concentrations of 10 g / L, respectively.

[0046] The organic solvent is obtained by mixing ethylene glycol diaminoethyl ether tetraacetic acid solution, disodium ethylenediaminetetraacetic acid solution, and citric acid solution in a volume ratio of 2:10:10 (mL / mL).

[0047] Step 2: Add the phosphogypsum suspension obtained in Step 1 to a composite solvent to obtain a mixed solution; The composite solvent consists of 10 g / L alanine solution and 10 g / L valine solution, with a concentration of 10 g / L and an addition amount of 1:500 mL / mL for the composite solvent to phosphogypsum suspension.

[0048] Step 3: The mixed solution obtained in Step 2 is reacted at 40℃ with a stirring speed of 800 r / min for 1 h. After the reaction is completed, the solution is filtered at 0.1 MPa and 25℃ to obtain phosphogypsum residue and clear calcium complex solution.

[0049] Step 4: Add the clarified calcium complex solution obtained in Step 3 to the surfactant, and then mix and react with the complex-removing and adsorption-reducing composite functional agent. React at 20℃ with a stirring speed of 500 r / min for 1.5 h. After the reaction is completed, the solution is filtered and separated at 0.1 MPa and 25℃ to obtain high-purity calcium sulfate crystals. The surfactants used include nonylphenol polyoxyethylene ether phosphate potassium salt at a mass of 0.1% and alkylphenol ether phosphate OPP-4 at a mass of 0.05% of phosphogypsum. The compound functional agent for chelating and purifying contains alginate at a mass of 0.5% of phosphogypsum and xanthan gum at a mass of 0.5% of phosphogypsum.

[0050] Analysis and calculations revealed that the high-purity calcium sulfate crystals contained 99.96% calcium sulfate by mass, 75 mg / kg phosphorus, 20 mg / kg fluorine, 0.04 mg / kg lead, 0.03 mg / kg arsenic, and 0.04 mg / kg selenium.

[0051] Comparative Example 1 Step 1: Add phosphogypsum to an organic solvent to obtain a phosphogypsum suspension; The phosphogypsum suspension contains phosphogypsum with a concentration of 300 g / L. Ethylene glycol diaminoethyl ether tetraacetic acid, disodium ethylenediaminetetraacetate, and citric acid are dissolved in water to obtain ethylene glycol diaminoethyl ether tetraacetic acid solution, ethylenediaminetetraacetate disodium solution, and citric acid solution with concentrations of 10 g / L, respectively.

[0052] The organic solvent is obtained by mixing ethylene glycol diaminoethyl ether tetraacetic acid solution, disodium ethylenediaminetetraacetic acid solution, and citric acid solution in a volume ratio of 2:10:10 (mL / mL).

[0053] Step 2: Add the phosphogypsum suspension obtained in Step 1 to a composite solvent to obtain a mixed solution; The composite solvent consists of 10 g / L alanine solution and 10 g / L valine solution, with a concentration of 10 g / L and an addition amount of 1:500 mL / mL for the composite solvent to phosphogypsum suspension.

[0054] Step 3: The mixed solution obtained in Step 2 is reacted at 40℃ with a stirring speed of 800 r / min for 1 h. After the reaction is completed, the solution is filtered at 0.1 MPa and 25℃ to obtain phosphogypsum residue and clear calcium complex solution.

[0055] Step 4: Add formic acid to the clear calcium complex solution obtained in Step 3 to de-complex the complex. React at 20°C with a stirring speed of 500 r / min for 1.5 h. After the reaction is complete, filter the solution at 0.1 MPa and 25°C to obtain calcium sulfate crystals. Analysis and calculations revealed that the calcium sulfate crystals contained 99.01% calcium sulfate by mass, 142 mg / kg phosphorus, 106 mg / kg fluorine, 2.3 mg / kg lead, 2.1 mg / kg arsenic, and 5.5 mg / kg selenium.

[0056] It should be noted that the embodiments described above are only for explaining the present invention and do not constitute any limitation on the present invention. The present invention has been described with reference to typical embodiments, but it should be understood that the words used therein are descriptive and explanatory terms, not limiting terms. Modifications can be made to the present invention within the scope of the claims, and revisions can be made to the present invention without departing from the scope and spirit of the present invention. Although the present invention described herein relates to specific methods, materials, and embodiments, it does not mean that the present invention is limited to the specific examples disclosed herein; on the contrary, the present invention can be extended to all other methods and applications with the same function.

Claims

1. A method for improving the purity of calcium sulfate in the process of preparing calcium sulfate from phosphogypsum, characterized in that, Includes the following steps: C1. Add phosphogypsum to an organic solvent to obtain a phosphogypsum suspension; C2. Add the phosphogypsum suspension obtained in step C1 to a composite solvent to obtain a mixed solution; C3. The mixed solution obtained in step C2 is heated and stirred to react. After the reaction is completed, the solution is filtered to obtain a clear calcium complex solution filtrate and phosphogypsum residue. C4. Add a surfactant to the filtrate of the clarified calcium complex solution obtained in step C3, and then mix and react it with the complex-removing and adsorption-reducing composite functional agent. After the reaction is completed, filter and separate to obtain high-purity calcium sulfate crystals and organic solvent mother liquor.

2. The method for improving the purity of calcium sulfate in the process of preparing calcium sulfate from phosphogypsum according to claim 1, characterized in that, The phosphogypsum in step C1 comprises calcium sulfate, silicon dioxide, water, and impurities, with the following mass fractions: calcium sulfate 70%, silicon dioxide 13%, water 15%, and impurities 2%. The organic solvent includes any one or more combinations of ethylene glycol diaminoethyl ether tetraacetic acid solution, disodium ethylenediaminetetraacetic acid solution, and citric acid solution.

3. The method for improving the purity of calcium sulfate in the process of preparing calcium sulfate from phosphogypsum according to claim 1, characterized in that, In step C2, the composite solvent includes two or more of the following: alanine, valine, leucine, aspartic acid, glutamic acid, and sodium polyaspartate solution.

4. The method for improving the purity of calcium sulfate in the process of preparing calcium sulfate from phosphogypsum according to claim 1, characterized in that... In step C3, the heating temperature is 20℃~60℃, the reaction time is 0.5~2h, and the stirring rate is 100 r / min~2000 r / min.

5. The method for improving the purity of calcium sulfate in the process of preparing calcium sulfate from phosphogypsum according to claim 1, characterized in that, In step C4, the surfactant includes one or more of the following: nonylphenol polyoxyethylene ether phosphate potassium salt, monoalkyl ether phosphate potassium salt, alkylphenol ether phosphate OPP-4, fatty alcohol polyoxyethylene ether AEO-9, and Pingpingjia OS.

6. The method for improving the purity of calcium sulfate in the process of preparing calcium sulfate from phosphogypsum according to claim 1, characterized in that, In step C4, the amount of nonylphenol polyoxyethylene ether phosphate potassium salt added to the surfactant is 0.01% to 0.3% of the mass of phosphogypsum.

7. The method for improving the purity of calcium sulfate in the process of preparing calcium sulfate from phosphogypsum according to claim 1, characterized in that, In step C4, the complex-resolving and adsorption-purifying composite functional agent includes one or more of chitosan, cyclodextrin, mannan, carrageenan, alginate, xanthan gum, gellan gum, kodana gum, and konjac glucomannan.

8. The method for improving the purity of calcium sulfate in the process of preparing calcium sulfate from phosphogypsum according to claim 1, characterized in that, In step C4, the amount of the anti-corrosion and anti-absorption composite functional agent added is 0.1% to 5% of the mass of phosphogypsum.

9. A method for improving the purity of calcium sulfate in the process of preparing calcium sulfate from phosphogypsum according to claim 1, characterized in that, In step C4, the reaction temperature is 20℃~80℃, the reaction time is 0.5~5h, and the stirring rate is 100 r / min~1000 r / min.

10. The calcium sulfate obtained by the method according to any one of claims 1-9, characterized in that, In step C4, the high-purity calcium sulfate crystals have a purity greater than 99.95%, a phosphorus content less than 100 mg / kg, a fluorine content less than 30 mg / kg, a lead content less than 2 mg / kg, an arsenic content less than 2 mg / kg, and a selenium content less than 3 mg / kg.