Method for preparing alpha-hemihydrate gypsum using phosphogypsum

Abstract

This invention belongs to the field of solid waste recycling technology, specifically relating to a method for preparing α-hemihydrate gypsum using phosphogypsum. The method includes the following steps: adding pretreated phosphogypsum to a chloride-free composite salt solution, carrying out a gradient temperature conversion reaction, and after the reaction, separating, washing, and drying to obtain α-hemihydrate gypsum; the chloride-free composite salt solution is a mixed solution of sodium nitrate and sodium sulfate. This invention uses phosphogypsum as raw material and replaces the chloride salt system with a nitrate system, solving the problems of performance degradation and equipment corrosion caused by chloride ion residue. By adding a small amount of sodium sulfate, the concentration of the main reaction salt solution is effectively reduced, thereby reducing raw material costs and reducing sodium ion residue in the product. The gradient temperature technology solves the problem of particle size refinement caused by the short induction period of the nitrate system, and a simple ion compensation method is used to achieve mother liquor recycling. The method is simple and low-cost.

Description

Technical Field

[0001] This invention belongs to the field of solid waste recycling technology, specifically relating to a method for preparing α-hemihydrate gypsum using phosphogypsum. Background Technology

[0002] Phosphogypsum is a major industrial byproduct emitted during the wet-process phosphoric acid production, and its main component is calcium sulfate dihydrate (CaSO4·2H2O). With the continuous growth in demand for phosphoric acid and phosphate fertilizers, its stockpiles have increased dramatically. Currently, the stockpiles of phosphogypsum are enormous and increasing significantly annually. The land occupation caused by its open-air storage, as well as the pollution of water and soil environments by acids and heavy metal ions in the leachate, are becoming increasingly serious problems. Although the comprehensive utilization rate of phosphogypsum has improved in recent years, it still cannot meet application demands.

[0003] Utilizing phosphogypsum to prepare high-value-added α-hemihydrate gypsum is one of the important ways to realize its high-value resource utilization. α-Hemihydrate gypsum, due to its regular crystal structure, low water requirement, high strength of the hardened body, and excellent performance, has broad application prospects in many fields such as biomedicine, precision manufacturing, building materials, and 3D printing. Currently, the main methods for preparing α-hemihydrate gypsum are the autoclaving method and the atmospheric pressure salt solution method. The autoclaving method is technically mature, but it has inherent disadvantages such as high energy consumption, large equipment investment, high operational risks, and difficulty in continuous production. The atmospheric pressure salt solution method, by controlling the dissolution-recrystallization process of gypsum under mild conditions through a salt solution medium, has advantages such as lower energy consumption, continuous production capability, and easy control of process parameters, making it a key research and application direction. However, the atmospheric pressure salt solution method still faces the following problems in its path towards large-scale industrial application: Currently, the more mature atmospheric pressure salt solution process mostly uses chloride salt systems such as magnesium chloride and calcium chloride. Although these systems have good conversion efficiency, residual chloride ions are easily incorporated into or attached to α-hemihydrate gypsum crystals, leading to defects such as moisture absorption and efflorescence, and poor water resistance, which seriously affects the long-term performance stability of the product. At the same time, chloride ions are highly corrosive to metal equipment such as reaction vessels and pipelines, significantly increasing equipment maintenance costs and production safety risks. To avoid chloride ion problems, chloride-free systems have become a research hotspot, but existing systems each have their shortcomings. For example, sulfate systems (such as MgSO4 and Na2SO4) have fast reaction rates, but the resulting α-hemihydrate gypsum crystals often have excessively large aspect ratios, rough surfaces, and poor crystal integrity, resulting in poor performance of the final product; nitrate systems (such as Mg(NO3)2 and Ca(NO3)2) can obtain crystals with good morphology, but usually require near-saturated high-concentration salt solutions, resulting in high raw material costs and poor economic efficiency. Meanwhile, in existing processes, the mother liquor after the reaction is often difficult to recycle directly due to the continuous enrichment of impurity ions such as phosphorus and fluoride, as well as the imbalance of salt concentration. It usually requires complex treatment or a large amount of fresh salt to be added before it can be reused. This not only leads to high salt consumption and complex process flow, but also generates a large amount of wastewater that needs to be treated, which seriously restricts the overall economic efficiency and environmental friendliness of the process. Summary of the Invention

[0004] The purpose of this invention is to provide a method for preparing α-hemihydrate gypsum using phosphogypsum, thereby overcoming the shortcomings of existing technologies. By neutralizing and aging the phosphogypsum with wet-mixed lime to eliminate the influence of soluble fluorine and phosphorus, a low-concentration chlorine-free compound salt system (NaNO3-Na2SO4) is constructed, and combined with a "gradient temperature rise" crystallization control technology, high-performance α-hemihydrate gypsum is efficiently prepared. Furthermore, the salt solution is efficiently recycled through the ion compensation method (Na2SO4), with no waste liquid discharge throughout the process, significantly reducing production costs.

[0005] To achieve the above objectives, the technical solution of the present invention is as follows: This invention provides a method for preparing α-hemihydrate gypsum using phosphogypsum, comprising the following steps: The pretreated phosphogypsum was added to a chlorine-free composite salt solution and subjected to a gradient temperature conversion reaction. After the reaction was completed, the α-hemihydrate gypsum was obtained by separation, washing and drying. The chlorine-free composite salt solution is a mixed solution of sodium nitrate and sodium sulfate, wherein the mass ratio of sodium nitrate to sodium sulfate is (0.85-0.95):(0.05-0.15).

[0006] In some other embodiments, the mass concentration of the chlorine-free composite salt solution is 20-25%; The mass ratio of sodium nitrate to sodium sulfate is 0.95:0.05.

[0007] In some other embodiments, each gram of pretreated phosphogypsum is added to 3-5 mL of a chlorine-free compound salt solution.

[0008] In some other embodiments, the gradient temperature conversion reaction is as follows: first, the chlorine-free composite salt solution is heated to 85-92°C, and pretreated phosphogypsum is added, and the reaction is carried out for 0.2-0.4 h under stirring conditions; then, the temperature is increased to 95-98°C, and the reaction is continued for 1.0-1.5 h under stirring conditions.

[0009] In some other embodiments, the gradient temperature conversion reaction is as follows: first, the chlorine-free composite salt solution is heated to 90-92°C, pretreated phosphogypsum is added, and the reaction is carried out under stirring for 0.3 h; then, the temperature is increased to 96-98°C, and the reaction is continued under stirring for 1.5 h.

[0010] Specifically, controlling the temperature below 92℃ for half an hour before conversion and raising it to above 96℃ after half an hour helps to obtain α-hemihydrate gypsum products with uniform particle size and large dimensions.

[0011] The inventors discovered in their research that, due to NO3 - It has a "planar triangular structure" and can be used in Ca 2+ The surrounding area accumulates and generates strong electrostatic forces, increasing supersaturation and resulting in a short induction period and rapid nucleation in the nitrate reaction system. This leads to finer crystals in the final hemihydrate gypsum product, reducing its flowability and strength. This invention employs a "gradient heating" method: lowering the temperature in the early stages of the reaction prolongs the induction period, reducing the number of hemihydrate gypsum crystal nuclei and increasing the product particle size to improve application performance; then raising the temperature in the middle and later stages of the reaction accelerates the conversion rate and improves production efficiency. This measure effectively solves the problem of excessively short induction periods and finer product particle size in nitrate reaction systems.

[0012] In some other embodiments, the method for pretreating phosphogypsum is as follows: mix phosphogypsum, water and quicklime evenly, and age them to obtain pretreated phosphogypsum. The mass ratio of the phosphogypsum, water, and quicklime is 100:(18-22):(0.6-0.7). The aging time is 20-30 hours.

[0013] The pretreatment technology of this invention is simple to operate and does not generate any pollution or waste. Wet lime neutralization and aging can effectively solve the adverse effects of soluble fluoride and phosphorus ions on the conversion process of hemihydrate gypsum and the recycling of salt solutions. This invention uses sodium nitrate as the main salt medium to construct a chloride-free salt reaction system, which eliminates the adverse effects of chloride ions in the chloride salt system. At the same time, the addition of a small amount of sodium sulfate significantly reduces the salt solution concentration, reduces costs and reduces the residue of sodium ions in hemihydrate gypsum.

[0014] In some other embodiments, the chlorine-free composite salt solution includes a fresh chlorine-free composite salt solution or a recovered chlorine-free composite salt solution after ion compensation treatment.

[0015] In some other embodiments, the ion compensation process includes adding sodium sulfate solution to the recovered chloride-free complex salt solution to compensate for the sulfate ions consumed in the solution and adjusting the solution concentration.

[0016] The inventors discovered during their research that harmful impurities contained in phosphogypsum, when used as a raw material, significantly impact the performance of the recovered solution. The salt solution recovery and reuse technology in this invention is based on phosphogypsum pretreatment, eliminating the harmful effects of soluble fluoride and phosphorus in the recovered solution. The performance of the recovered solution can be restored simply by adding salt media. Compared with existing reaction solution recovery and reuse technologies, the method of this invention is simple to operate, has low processing costs, and allows for multiple recycling cycles.

[0017] In some other embodiments, the concentration of the sodium sulfate solution is 1-3 wt%, and the number of times the chlorine-free complex salt solution is recovered is 1-3 times.

[0018] In some other embodiments, the α-hemihydrate gypsum has an average particle size of 90-92 μm, a conversion rate of 100%, an induction period of >25 min, and an oven-dry compressive strength of >40 MPa after 24 hours.

[0019] The beneficial effects of this invention are: This invention uses industrial by-product phosphogypsum as raw material to prepare high-value-added α-hemihydrate gypsum, which significantly promotes the large-scale high-value utilization of phosphogypsum and achieves significant resource, environmental and economic benefits. By using wet-mixed lime for neutralization and aging pretreatment, soluble phosphorus and fluoride impurities in the raw materials are efficiently and thoroughly fixed without additional wastewater discharge, eliminating their fundamental interference with the conversion process and mother liquor circulation. By constructing a chloride-free reaction system based on nitrates and supplemented with a small amount of sulfates, the problems of product performance degradation and equipment corrosion caused by chloride ions are completely avoided. The working concentration of the main salt solution is also significantly reduced from the traditional 40% or more to about 20%, greatly saving raw material costs and improving product purity. By introducing a "gradient heating" crystallization control strategy, the temperature conditions for crystal nucleation and crystal growth are precisely coordinated, successfully solving the problem of crystal refinement caused by a short induction period. The average particle size of α-hemihydrate gypsum is increased from 56.3 μm to 91.1 μm, an increase of 70%, effectively optimizing product performance. Finally, through a simple ion compensation method, the chemical balance and conversion activity of the circulating mother liquor are effectively restored, achieving efficient recycling of the salt solution more than three times. There is no process wastewater discharge throughout the process, and the overall economic efficiency, environmental protection, and sustainability of the process are significantly enhanced. Attached Figure Description

[0020] The accompanying drawings, which form part of this invention, are used to provide a further understanding of the invention. The illustrative embodiments of the invention and their descriptions are used to explain the invention and do not constitute an improper limitation of the invention.

[0021] Figure 1 The conversion rates of hemihydrate gypsum in different reaction systems in Examples 1-2 and Comparative Example 1 of this invention are shown, wherein the concentration of the salt solution in the composite system is 20 wt%. Figure 2 The XRD patterns of the products after multiple cycles of salt solution reaction for 2 hours in Examples 1 and 3 of this invention are shown. Detailed Implementation

[0022] Those skilled in the art will understand that the following embodiments are for illustrative purposes only and should not be construed as limiting the scope of the invention. Specific conditions not specified in the embodiments are performed under conventional conditions or conditions recommended by the manufacturer. Components whose manufacturers are not specified are all commercially available conventional products.

[0023] As mentioned earlier, while existing chlorine-free nitrate systems can yield good products, they suffer from the problem of requiring high salt solution concentrations (usually above 40%) and the tendency for product particle size to become fine due to a short induction period. At the same time, soluble fluoride and phosphorus impurities in phosphogypsum enter the liquid phase during the reaction, severely degrading the conversion performance of the mother liquor and restricting the effective recycling of the salt solution.

[0024] To address the aforementioned problems, this invention provides a method for preparing α-hemihydrate gypsum using phosphogypsum. The method includes: firstly, pretreating the phosphogypsum using a wet-mixing lime neutralization and aging process to convert soluble fluoride and phosphorus ions into insoluble substances, eliminating their interference with the conversion process and mother liquor recovery at the source, without generating additional wastewater; secondly, constructing a low-concentration, chloride-free composite salt reaction system (NaNO3-Na2SO4), and through optimized salt compounding, significantly reducing the salt solution concentration to approximately 20%, achieving efficient conversion of α-hemihydrate gypsum under relatively mild conditions; for the conversion process, employing a "gradient heating" crystallization control strategy to effectively coordinate the different temperature requirements of nucleation induction and crystal growth, solving the problem of excessively fine product particle size caused by the short induction period in traditional nitrate systems; finally, using an ion compensation method, the conversion performance of the circulating mother liquor is easily and efficiently restored, achieving stable recycling of the salt solution more than three times. The entire process generates no process wastewater discharge, significantly reducing production costs and environmental burden.

[0025] The solution of the present invention will be further described below with reference to specific embodiments: Example 1 This embodiment provides a method for preparing α-hemihydrate gypsum using phosphogypsum, specifically including the following steps: (1) Pretreatment of phosphogypsum: The phosphogypsum raw material is ground through a 150-mesh sieve. The ground phosphogypsum, water and quicklime powder are mixed evenly in a mass ratio of 100:20:0.65. After aging for 24 hours, it is dried (drying temperature is 60℃) to obtain the pretreated phosphogypsum.

[0026] (2) Preparation of α-hemihydrate gypsum: Sodium nitrate and sodium sulfate were mixed at a mass ratio of 0.95:0.05 and dissolved in water to prepare a composite salt solution with a mass concentration of 20%. The prepared composite salt solution (sodium nitrate and sodium sulfate = 0.95:0.05) was poured into a three-necked flask and placed in an oil bath for heating. When the temperature of the composite salt solution reached 90°C, pretreated phosphogypsum was added at a solid-liquid ratio of 1:4. The mixture was stirred at a speed of 200 r / min for 30 min. Then the temperature was raised to 96°C and the reaction was continued for 1.5 h. The reaction product and the salt solution were separated by filtration and washed twice with deionized water at a temperature higher than 90°C. The washed product was then placed in an oven at 100°C to obtain the α-hemihydrate gypsum product.

[0027] α-Hemihydrate gypsum conversion process and product parameters: 100% conversion rate of hemihydrate gypsum in 2 hours, induction period of 26 minutes, average particle size of hemihydrate gypsum of 91.1 μm, standard consistency water consumption of 39%, and oven-dry compressive strength of 41.24 MPa in 24 hours.

[0028] Example 2 Unlike Example 1, in the preparation of α-hemihydrate gypsum, sodium nitrate and sodium sulfate were mixed and dissolved in water at mass ratios of 0.9:0.1 and 0.85:0.15, respectively, to prepare a composite salt solution with a mass concentration of 20%. Then, the 20% composite salt solution (sodium nitrate:sodium sulfate = 0.95:0.05) in Example 1 was replaced, while other preparation methods remained unchanged.

[0029] Comparative Example 1 Unlike Example 1, in the preparation of α-hemihydrate gypsum, 30wt% and 40wt% sodium nitrate solutions were used to replace 20% of the composite salt solution (sodium nitrate: sodium sulfate = 0.95: 0.05) in Example 1, while other preparation methods remained unchanged.

[0030] Depend on Figure 1It can be seen that in the pure sodium nitrate (NaNO3) system of Comparative Example 1, the complete conversion of hemihydrate gypsum was not achieved after 6 hours of reaction at a concentration of 30 wt% (conversion rate <100%). While a complete conversion was achieved at a concentration of 40 wt%, the high concentration of salt solution had a significant impact on process costs and the environment. In Examples 1-2, when sodium sulfate (Na2SO4) was introduced to form a composite system at a concentration of 20 wt%, all three ratios (NaNO3:Na2SO4 = 0.95:0.05, 0.9:0.1, 0.85:0.15) achieved complete conversion of hemihydrate gypsum within 6 hours (conversion rate ≥100%). This phenomenon originates from SO4. 2- Homo ion effect: small amount of SO4 2- The introduction of [the substance] significantly reduced the effective concentration requirement of sodium nitrate solution, enabling the composite system to achieve the conversion effect of the pure NaNO3 system at a low concentration (20wt%).

[0031] Comparing three composite systems with a concentration of 20 wt%, it was found that the optimal dosage of 5% (NaNO3:Na2SO4 = 0.95:0.05) resulted in a moderate conversion rate, ensuring complete conversion within 6 hours while avoiding structural defects caused by excessively rapid crystallization. Excess sodium sulfate (dosages of 10% and 15%, i.e., ratios of 0.9:0.1 and 0.85:0.15) significantly accelerated the conversion rate (increased curve slope), but reduced crystal integrity (excessively rapid nucleation / growth rates easily lead to crystal defects, grain refinement, or agglomeration), adversely affecting the mechanical properties and purity of the hemihydrate gypsum product. Therefore, a 20 wt% composite system with a 5% sodium sulfate dosage (NaNO3:Na2SO4 = 0.95:0.05) was selected as the optimal ratio—meeting both conversion efficiency and crystal quality requirements.

[0032] Comparative Example 2 A method for preparing α-hemihydrate gypsum using phosphogypsum, differing from Example 1 in that a "gradient temperature increase" is not used in the preparation process; the entire reaction temperature is kept constant at 96°C. Specifically, the method includes the following steps: (1) Pretreatment of phosphogypsum: The phosphogypsum raw material is ground through a 150-mesh sieve. The ground phosphogypsum, water and quicklime powder are mixed evenly in a mass ratio of 100:20:0.65. After aging for 24 hours, it is dried (drying temperature is 60℃) to obtain the pretreated phosphogypsum.

[0033] (2) Preparation of α-hemihydrate gypsum: Sodium nitrate and sodium sulfate were prepared in a mass ratio of 0.95:0.05 to form a 20% composite salt solution. The prepared composite salt solution (sodium nitrate, sodium sulfate = 0.95:0.05) was thoroughly mixed and poured into a three-necked flask. The flask was then placed in an oil bath for heating. When the temperature of the composite salt solution reached 96℃, pretreated phosphogypsum was added at a solid-liquid ratio of 1:4. The mixture was stirred at 200 r / min for 2 hours. The reaction product was separated from the salt solution by filtration and washed twice with deionized water at a temperature higher than 90℃. The washed product was then placed in a 100℃ oven to obtain α-hemihydrate gypsum.

[0034] The conversion process and product parameters of α-hemihydrate gypsum: 100% conversion rate of hemihydrate gypsum in 2 hours, induction period of 11 minutes, average particle size of hemihydrate gypsum of 56.3 μm, standard consistency water requirement of 51%, and 24-hour oven-dry compressive strength of 29.11 MPa. The significantly shortened induction period of the reaction process led to excessive nucleation kinetics, resulting in the formation of a large number of crystal nuclei in a short time, leading to finer grains and reduced flow and mechanical properties in the product.

[0035] The conversion process and product parameters of the α-hemihydrate gypsum prepared in Example 1 and Comparative Example 2 are shown in Table 1.

[0036] Table 1. Effects of temperature setting on the conversion process and results

[0037] As shown in Table 1, Example 1 solved the problem of product particle size refinement due to the short induction period of the reaction system by using the "gradient heating" technology, increasing the average particle size of hemihydrate gypsum from 56.3 μm to 91.1 μm, an increase of 70%.

[0038] Example 3 This embodiment provides a method for preparing α-hemihydrate gypsum using phosphogypsum. Unlike Example 1, the composite salt solution used in the preparation of α-hemihydrate gypsum is a recycled salt solution. The method for preparing the recycled salt solution is as follows: the composite salt solution (sodium nitrate: sodium sulfate = 0.95:0.05) from Example 1, which has undergone 1, 2, and 3 conversion reactions respectively, is recycled, and 2wt% NaSO4 is added and stirred until homogeneous. Specifically, the method includes the following steps: (1) Pretreatment of phosphogypsum: The phosphogypsum raw material is ground through a 150-mesh sieve. The ground phosphogypsum, water and quicklime powder are mixed evenly in a mass ratio of 100:20:0.65. After aging for 24 hours, it is dried (drying temperature is 60℃) to obtain the pretreated phosphogypsum.

[0039] (2) Preparation of α-hemihydrate gypsum: The salt solutions that have undergone the first, second, and third conversion reactions were recovered and treated with 2wt% NaSO4, then poured into a three-necked flask and heated in an oil bath. When the temperature of the salt solution reached 90℃, the pretreated phosphogypsum was added at a solid-liquid ratio of 1:4. The reaction was carried out at a stirring speed of 200 r / min for 30 min, and then the temperature was raised to 96℃ and the reaction was continued for 1.5 h. The reaction product was separated from the salt solution by a vacuum filter and washed twice with deionized water above 90℃. The washed product was then placed in a 100℃ oven and α-hemihydrate gypsum was prepared by circulating the salt solution once, twice, and three times.

[0040] The XRD patterns of α-hemihydrate gypsum prepared from salt solutions after 1, 2, and 3 cycles are shown below. Figure 2 As shown.

[0041] Depend on Figure 2 It can be seen that the XRD pattern of α-hemihydrate gypsum prepared by the salt solution after 1 to 3 cycles is comparable to that of Example 1 (original salt solution reaction system).

[0042] The conversion process and product parameters of α-hemihydrate gypsum prepared from a salt solution that underwent three cycles of conversion were as follows: 100% conversion rate of hemihydrate gypsum in 2 hours, induction period of 23 minutes, average particle size of hemihydrate gypsum of 90.2 μm, standard consistency water content of 40%, and oven-dry compressive strength of 40.10 MPa in 24 hours. The reaction system was a recovered salt solution. After ion replenishment, the conversion performance of the recovered solution was restored, achieving 100% conversion in 2 hours, and the performance of the conversion product was comparable to that of Example 1 (original salt solution reaction system).

[0043] The above are merely preferred embodiments of the present invention and are not intended to limit the present invention. Various modifications and variations can be made to the present invention by those skilled in the art. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. A method for preparing α-hemihydrate gypsum using phosphogypsum, characterized in that, Includes the following steps: The pretreated phosphogypsum was added to a chlorine-free composite salt solution and subjected to a gradient temperature conversion reaction. After the reaction was completed, the α-hemihydrate gypsum was obtained by separation, washing and drying. The chlorine-free composite salt solution is a mixed solution of sodium nitrate and sodium sulfate, wherein the mass ratio of sodium nitrate to sodium sulfate is (0.85-0.95):(0.05-0.15).

2. The method for preparing α-hemihydrate gypsum using phosphogypsum according to claim 1, characterized in that, The mass concentration of the chlorine-free composite salt solution is 20-25%; The mass ratio of sodium nitrate to sodium sulfate is 0.95:0.

05.

3. The method for preparing α-hemihydrate gypsum using phosphogypsum according to claim 1, characterized in that, Add 3-5 mL of chlorine-free compound salt solution to each gram of pretreated phosphogypsum.

4. The method for preparing α-hemihydrate gypsum using phosphogypsum according to claim 1, characterized in that, The gradient temperature conversion reaction is as follows: first, the chlorine-free composite salt solution is heated to 85-92℃, and the pretreated phosphogypsum is added. The reaction is carried out for 0.2-0.4h under stirring conditions. Then, the temperature is increased to 95-98℃, and the reaction is continued for 1.0-1.5h under stirring conditions.

5. The method for preparing α-hemihydrate gypsum using phosphogypsum according to claim 4, characterized in that, The gradient temperature conversion reaction is as follows: first, the chlorine-free composite salt solution is heated to 90-92℃, and the pretreated phosphogypsum is added. The reaction is carried out under stirring for 0.3h. Then, the temperature is increased to 96-98℃, and the reaction is continued under stirring for 1.5h.

6. The method for preparing α-hemihydrate gypsum using phosphogypsum according to claim 1, characterized in that, The method for pretreating phosphogypsum is as follows: mix phosphogypsum, water and quicklime evenly, age and then dry to obtain pretreated phosphogypsum; The mass ratio of the phosphogypsum, water, and quicklime is 100:(18-22):(0.6-0.7). The aging time is 20-30 hours.

7. The method for preparing α-hemihydrate gypsum using phosphogypsum according to claim 1, characterized in that, The chlorine-free composite salt solution includes a fresh chlorine-free composite salt solution or a recovered chlorine-free composite salt solution after ion compensation treatment.

8. The method for preparing α-hemihydrate gypsum using phosphogypsum according to claim 7, characterized in that, The ion compensation method includes adding sodium sulfate solution to the recovered chlorine-free compound salt solution to compensate for the sulfate ions consumed in the solution and to adjust the solution concentration.

9. The method for preparing α-hemihydrate gypsum using phosphogypsum according to claim 8, characterized in that, The concentration of the sodium sulfate solution is 1-3 wt%, and the number of times the chlorine-free compound salt solution is recovered is 1-3 times.

10. The method for preparing α-hemihydrate gypsum using phosphogypsum according to claim 1, characterized in that, The α-hemihydrate gypsum has an average particle size of 90-92 μm, a conversion rate of 100%, an induction period of >25 min, and an oven-dry compressive strength of >40 MPa after 24 hours.

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