A method for converting phosphogypsum into high-aspect-ratio calcium sulfate whiskers under normal pressure in aqueous phase by using trace composite crystal transformation agent

Abstract

This invention discloses a method for converting phosphogypsum into anhydrous calcium sulfate whiskers with a high aspect ratio, belonging to the field of chemical materials. The purpose of this invention is to provide an effective method for converting phosphogypsum into anhydrous calcium sulfate whiskers with a high aspect ratio under normal pressure aqueous phase conditions. This method utilizes trace amounts of magnesium chloride and calcium nitrate in a specific ratio as a composite crystallization agent, and uses sulfuric acid to adjust the pH value to achieve efficient reuse of phosphogypsum while producing anhydrous calcium sulfate whiskers with a high aspect ratio and excellent performance. The synergistic effect of magnesium chloride and calcium nitrate significantly improves the crystallization efficiency and quality of calcium sulfate. This invention can improve the utilization rate of phosphogypsum, increase the purity, whiteness, and added value of the product; the prepared anhydrous phosphogypsum whiskers have smaller diameters, larger aspect ratios, and higher purity; it provides a possibility for the sustainable development of the phosphate fertilizer industry; moreover, the process conditions are easy to control, the reaction conditions are mild, and the operation steps are simple; it is beneficial to the resource utilization of phosphogypsum.

Description

Technical Field

[0001] This invention relates to the field of chemical materials, specifically to a method for converting phosphogypsum into anhydrous calcium sulfate whiskers with a high aspect ratio. Phosphogypsum is a solid waste of calcium sulfate discharged during the wet process of phosphoric acid production, usually existing in the form of dihydrate calcium sulfate. Large-scale stockpiling of phosphogypsum poses environmental problems. This invention aims to provide a method for converting it into anhydrous calcium sulfate whiskers with high added value and a high aspect ratio. Anhydrous calcium sulfate whiskers can be widely used in materials such as plastics, rubber, and coatings that have strict requirements for corrosion resistance and high-temperature resistance as reinforcing and toughening agents or functional fillers. Background Technology

[0002] Current status of phosphogypsum utilization: Producing 1 ton of phosphoric acid generates approximately 4.5 to 5.5 tons of phosphogypsum. Compared to natural gypsum, phosphogypsum typically contains phosphorus, fluorine, and organic matter, affecting its applicability and increasing its utilization cost. Currently, it is mainly disposed of through stockpiling. However, with the development of the phosphorus chemical industry, soil and water pollution caused by phosphogypsum stockpiling is becoming increasingly serious. Existing methods of utilizing phosphogypsum, such as calcination to produce β-building gypsum and pressurized steam preparation of α-high-strength gypsum, have drawbacks including high production costs and relatively poor product performance.

[0003] The Value and Current Status of Anhydrous Calcium Sulfate Whiskers: Anhydrous calcium sulfate whiskers are a novel inorganic material with comprehensive properties, including high strength, high modulus, high dielectric strength, wear resistance, high temperature resistance, corrosion resistance, good infrared reflectivity, ease of surface treatment, strong affinity with polymers, and non-toxicity. Existing methods for preparing anhydrous calcium sulfate whiskers include the hydrothermal method and the atmospheric pressure acidification method. The hydrothermal method requires a high-pressure reactor, resulting in high equipment costs; while the atmospheric pressure acidification method is relatively simple, the product performance may have certain limitations. Furthermore, there is currently no effective method for converting phosphogypsum into anhydrous calcium sulfate whiskers with an aspect ratio of 20:1 or higher using trace amounts of composite crystallizing agents and under stable conditions. Patent CN101792932A describes a method for preparing ultrafine calcium sulfate whiskers from phosphogypsum. This method uses phosphogypsum to prepare anhydrous calcium sulfate whiskers with a high aspect ratio. However, the added crystal growth promoter, magnesium sulfate, is 10% to 20% of the mass of phosphogypsum, and the added seed crystals are 10% to 20% of the mass of phosphogypsum.The required amount of additive is large, which will affect the purity and preparation cost of anhydrous calcium sulfate whiskers; Patent CN107190325B, a method for preparing anhydrous calcium sulfate whiskers using industrial by-product phosphogypsum, requires that the mass of the crystal form regulator added to the phosphogypsum powder should be 8-18% of the mass of the phosphogypsum powder to obtain anhydrous calcium sulfate whiskers with a high aspect ratio, which will also affect the purity and preparation cost of anhydrous calcium sulfate whiskers; Patent CN110205671A A method for preparing calcium sulfate dihydrate whiskers and calcium sulfate hemihydrate whiskers, and the products obtained therefrom. The crystal-transforming agent used in preparing high aspect ratio calcium sulfate dihydrate whiskers is any one or a combination of at least two of hexadecyltrimethylammonium bromide, hexadecyltrimethylammonium chloride, dodecyltrimethylammonium chloride, or dodecyltrimethylammonium bromide. It also requires an organic auxiliary agent, any one or a combination of at least two of methanol, ethanol, ethylene glycol, propanol, propylene glycol, glycerol, butanediol, glycerol, or pentanediol. The cost and environmental friendliness of the solvent need improvement. CN101311355A A method for preparing calcium sulfate whiskers from phosphogypsum. The crystal-transforming agent used in preparing high aspect ratio calcium sulfate dihydrate whiskers is also 15%–20% of the phosphogypsum mass, which similarly affects the purity and preparation cost of anhydrous calcium sulfate whiskers. CN106480507A A novel process for preparing low-cost hemihydrate calcium sulfate whiskers from phosphogypsum uses a crystal-transforming agent comprising one or more of the following inorganic salts: sodium chloride, potassium chloride, zinc chloride, magnesium chloride, sodium sulfate, potassium sulfate, zinc sulfate, magnesium sulfate, aluminum sulfate, sodium phosphate, potassium phosphate, zinc phosphate, magnesium phosphate, and aluminum phosphate. The organic additives are one or more of the following: hexadecyltrimethylammonium bromide, sodium dodecylbenzenesulfonate, sodium hexadecyl sulfate, oleic acid, and sodium oleate. The prepared hemihydrate calcium sulfate whiskers have a low aspect ratio. CN105603505A describes a hydrothermal method for preparing high aspect ratio calcium sulfate whiskers from phosphogypsum. This method first carbonizes the phosphogypsum with sodium carbonate solution, then uses organic acids and ferric chloride as crystal-transforming agents. Although high aspect ratio calcium sulfate whiskers are obtained, the carbonation and addition of organic acids affect the purity of the calcium sulfate whiskers. Summary of the Invention

[0004] Purpose of the invention: The purpose of this invention is to provide an effective method for converting phosphogypsum into anhydrous calcium sulfate whiskers with a high aspect ratio under normal pressure aqueous conditions. This method uses trace amounts of magnesium chloride and calcium nitrate in a specific ratio as a composite crystallization agent and uses sulfuric acid to adjust the pH value to achieve efficient reuse of phosphogypsum, while producing anhydrous calcium sulfate whiskers with a high aspect ratio and excellent performance.

[0005] Technical Solution: Raw Material Preparation: Phosphogypsum is selected as the initial raw material, and anhydrous calcium sulfate with an aspect ratio of 20:1 or higher is used as seed crystals. Reaction System Construction: Phosphogypsum is placed in an aqueous system under normal pressure. After adding the seed crystals, 1% magnesium chloride and 0.5% calcium nitrate by mass fraction are added to the system as a composite crystal-transforming agent. Utilizing the synergistic effect of magnesium chloride and calcium nitrate, the crystal-transforming efficiency and quality of calcium sulfate are significantly improved. This solution can increase the purity, whiteness, and added value of the product; improve the utilization rate of phosphogypsum; the prepared anhydrous phosphogypsum whiskers have smaller diameters and larger aspect ratios; provide the possibility for sustainable development of the phosphate compound fertilizer industry; moreover, the process conditions are easy to control, the reaction conditions are mild, and the operation steps are simple; it is conducive to the resource utilization of phosphogypsum; this preparation technology greatly improves the quality of the final product. Beneficial Effects: In terms of resource utilization, this invention effectively utilizes phosphogypsum waste, reducing environmental pollution from its storage and achieving high-value-added transformation of phosphogypsum resources, thus contributing to the sustainable development and environmental protection of the phosphate chemical industry. In terms of product performance, the obtained anhydrous calcium sulfate whiskers have an aspect ratio of over 20:1, exhibiting high purity and whiteness. This high aspect ratio helps to better improve the performance of composite materials when used as a reinforcing and toughening agent or functional filler. In terms of cost, the use of atmospheric pressure aqueous phase reaction conditions simplifies equipment requirements and reduces costs compared to traditional high-pressure hydrothermal methods. Furthermore, the amount of composite crystallizing agent used is less than that of traditional crystallizing agents, avoiding material waste while achieving the desired conversion effect, reducing production costs, and improving the purity of the final product. Attached Figure Description

[0006] Figure 1 This is a SEM image of the product of Example 1 of the present invention; Figure 2 This is a SEM image of the product of Example 2 of the present invention; Figure 3 This is a SEM image of the product of Example 3 of the present invention; Figure 4 This is a SEM image of the product of Example 4 of the present invention; Figure 5 This is a SEM image of the product of Example 5 of the present invention; Figure 6 This is a SEM image of the product of Example 6 of the present invention; Figure 7 This is a SEM image of the product of Example 7 of the present invention; Figure 8 This is a SEM image of the product of Comparative Example 1 of the present invention; Figure 9 The XRD patterns are of the product of Example 1 and Comparative Example 1 of the present invention;

[0007] The technical solutions in the embodiments of the present invention will be clearly and completely described below. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative effort are within the scope of protection of the present invention.

[0008] The term "embodiment" used herein, as an example, is not necessarily to be construed as superior to or better than other embodiments. Performance tests in the embodiments of this application, unless otherwise specified, employ conventional testing methods in the art.

[0009] Unless otherwise stated, the technical and scientific terms used herein have the same meanings as commonly understood by one of ordinary skill in the art to which this application pertains; other experimental methods and techniques not specifically mentioned herein refer to experimental methods and techniques commonly used by one of ordinary skill in the art. Unless otherwise specified, the methods and reagents used in the embodiments are conventional methods and reagents, respectively. It should be understood that the terminology used in this application is merely for describing particular implementation methods and is not intended to limit the content disclosed in this application.

[0010] To better illustrate the content of this application, specific details are provided in the following detailed embodiments. Those skilled in the art should understand that this application can be implemented even without certain specific details. In the embodiments, some methods, means, instruments, and devices well-known to those skilled in the art are not described in detail in order to highlight the main points of this application.

[0011] To better understand the present invention, the following embodiments are provided for further detailed description of the present invention, but they should not be construed as limiting the present invention. Any non-essential improvements and adjustments made by those skilled in the art based on the above-described invention are also considered to fall within the protection scope of the present invention.

[0012] The present invention will be further described in detail below through specific embodiments, but the scope of protection of the present invention is not limited to the following description. Example 1

[0013] A method for converting phosphogypsum into high aspect ratio anhydrous calcium sulfate whiskers using a trace amount of composite crystal-converting agent under ambient pressure comprises the following steps: 1) Purification and purification of phosphogypsum: Phosphogypsum was mixed with a calcium hydroxide solution at pH 11 to prepare a 10% slurry. The slurry was pumped into an FX75 (75mm diameter) hydrocyclone while being stirred in a mixing tank. The inlet pressure was set to 0.15-0.25 MPa. After purification and separation by the hydrocyclone, the overflow and underflow products were subjected to sieve particle size analysis and chemical composition analysis. The sieve mesh sizes were 65 / 100 / 200 / 325 / 400 / 500 mesh. The 500-mesh product was selected and fed into a secondary hydrocyclone. The underflow product of the secondary hydrocyclone was used as purified phosphogypsum A.

[0014] 2) Purification of phosphogypsum dissolution Weigh 0.1 L (100 g) of deionized water using a graduated cylinder. Weigh 0.2 g of purified phosphogypsum A and dissolve it in 0.1 L of deionized water. Add sulfuric acid, stir, and adjust the pH to 5-6 to remove impurities (calcium hydroxide, calcium oxide). Partially dissolve the purified phosphogypsum, filter, remove undissolved residue, and take the purified phosphogypsum aqueous solution. Add ammonia water dropwise to adjust the pH to 7 and remove unreacted sulfuric acid. This yields purified phosphogypsum aqueous solution B.

[0015] 3) Regulation of calcium sulfate crystal growth Weigh 0.00002 mol (0.002 g) of magnesium chloride (a crystal growth regulator) and 0.00001 mol (0.001641 g) of calcium nitrate using a balance. Add purified phosphogypsum aqueous solution B containing calcium sulfate whisker seeds with an aspect ratio greater than 20:1. Heat in an oil bath at 95°C with stirring until recrystallization yields anhydrous calcium sulfate whiskers with a high aspect ratio. Filter to obtain anhydrous calcium sulfate whiskers C with a high aspect ratio.

[0016] 4) Drying of high aspect ratio anhydrous calcium sulfate whiskers High aspect ratio anhydrous calcium sulfate whiskers C were vacuum dried in a vacuum drying oven at 80°C for 8 hours to obtain the final product. Example 2

[0017] A method for converting phosphogypsum into high aspect ratio anhydrous calcium sulfate whiskers using a trace amount of composite crystal-converting agent under ambient pressure comprises the following steps: 1) Purification and purification of phosphogypsum: Phosphogypsum was mixed with a calcium hydroxide solution at pH 11 to prepare a 10% slurry. The slurry was pumped into an FX75 (75mm diameter) hydrocyclone while being stirred in a mixing tank. The inlet pressure was set to 0.15-0.25 MPa. After purification and separation by the hydrocyclone, the overflow and underflow products were subjected to sieve particle size analysis and chemical composition analysis. The sieve mesh sizes were 65 / 100 / 200 / 325 / 400 / 500 mesh. The 500-mesh product was selected and fed into a secondary hydrocyclone. The underflow product of the secondary hydrocyclone was used as purified phosphogypsum A.

[0018] 2) Purification of phosphogypsum dissolution Weigh 0.1 L (100 g) of deionized water using a graduated cylinder. Weigh 0.2 g of purified phosphogypsum A and dissolve it in 0.1 L of deionized water. Add sulfuric acid, stir, and adjust the pH to 5-6 to remove impurities (calcium hydroxide, calcium oxide). Partially dissolve the purified phosphogypsum, filter, remove undissolved residue, and take the purified phosphogypsum aqueous solution. Add ammonia water dropwise to adjust the pH to 7 and remove unreacted sulfuric acid. This yields purified phosphogypsum aqueous solution B.

[0019] 3) Regulation of calcium sulfate crystal growth Weigh 0.00003 mol of magnesium chloride (a crystal growth regulator) and 0.000015 mol of calcium nitrate using a balance. Add purified phosphogypsum aqueous solution B containing calcium sulfate whisker seeds with an aspect ratio greater than 20:1. Heat in an oil bath at 95°C with stirring until recrystallization yields anhydrous calcium sulfate whiskers with a high aspect ratio. Filter to obtain anhydrous calcium sulfate whiskers C with a high aspect ratio.

[0020] 4) Drying of high aspect ratio anhydrous calcium sulfate whiskers High aspect ratio anhydrous calcium sulfate whiskers C were vacuum dried in a vacuum drying oven at 80°C for 8 hours to obtain the final product. Example 3

[0021] A method for converting phosphogypsum into high aspect ratio anhydrous calcium sulfate whiskers using a trace amount of composite crystal-converting agent under ambient pressure comprises the following steps: 1) Purification and purification of phosphogypsum: Phosphogypsum was mixed with a calcium hydroxide solution at pH 11 to prepare a 10% slurry. The slurry was pumped into an FX75 (75mm diameter) hydrocyclone while being stirred in a mixing tank. The inlet pressure was set to 0.15-0.25 MPa. After purification and separation by the hydrocyclone, the overflow and underflow products were subjected to sieve particle size analysis and chemical composition analysis. The sieve mesh sizes were 65 / 100 / 200 / 325 / 400 / 500 mesh. The 500-mesh product was selected and fed into a secondary hydrocyclone. The underflow product of the secondary hydrocyclone was used as purified phosphogypsum A.

[0022] 2) Purification of phosphogypsum dissolution Weigh 0.1 L (100 g) of deionized water using a graduated cylinder. Weigh 0.2 g of purified phosphogypsum A and dissolve it in 0.1 L of deionized water. Add sulfuric acid, stir, and adjust the pH to 5-6 to remove impurities (calcium hydroxide, calcium oxide). Partially dissolve the purified phosphogypsum, filter, remove undissolved residue, and take the purified phosphogypsum aqueous solution. Add ammonia water dropwise to adjust the pH to 7 and remove unreacted sulfuric acid. This yields purified phosphogypsum aqueous solution B.

[0023] 3) Regulation of calcium sulfate crystal growth Weigh 0.00004 mol of magnesium chloride (a crystal growth regulator) and 0.00002 mol of calcium nitrate using a balance. Add purified phosphogypsum aqueous solution B containing calcium sulfate whisker seeds with an aspect ratio greater than 20:1. Heat in an oil bath at 95°C and stir until recrystallization yields anhydrous calcium sulfate whiskers with a high aspect ratio. Filter to obtain anhydrous calcium sulfate whiskers C with a high aspect ratio.

[0024] 4) Drying of high aspect ratio anhydrous calcium sulfate whiskers High aspect ratio anhydrous calcium sulfate whiskers C were vacuum dried in a vacuum drying oven at 80°C for 8 hours to obtain the final product. Example 4

[0025] A method for converting phosphogypsum into high aspect ratio anhydrous calcium sulfate whiskers using a trace amount of composite crystal-converting agent under ambient pressure comprises the following steps: 1) Purification and purification of phosphogypsum: Phosphogypsum was mixed with a calcium hydroxide solution at pH 11 to prepare a 10% slurry. The slurry was pumped into an FX75 (75mm diameter) hydrocyclone while being stirred in a mixing tank. The inlet pressure was set to 0.15-0.25 MPa. After purification and separation by the hydrocyclone, the overflow and underflow products were subjected to sieve particle size analysis and chemical composition analysis. The sieve mesh sizes were 65 / 100 / 200 / 325 / 400 / 500 mesh. The 500-mesh product was selected and fed into a secondary hydrocyclone. The underflow product of the secondary hydrocyclone was used as purified phosphogypsum A.

[0026] 2) Purification of phosphogypsum dissolution Weigh 0.1 L (100 g) of deionized water using a graduated cylinder. Weigh 0.2 g of purified phosphogypsum A and dissolve it in 0.1 L of deionized water. Add sulfuric acid, stir, and adjust the pH to 5-6 to remove impurities (calcium hydroxide, calcium oxide). Partially dissolve the purified phosphogypsum, filter, remove undissolved residue, and take the purified phosphogypsum aqueous solution. Add ammonia water dropwise to adjust the pH to 7 and remove unreacted sulfuric acid. This yields purified phosphogypsum aqueous solution B.

[0027] 3) Regulation of calcium sulfate crystal growth Weigh 0.00005 mol of magnesium chloride (a crystal growth regulator) and 0.000025 mol of calcium nitrate using a balance. Add purified phosphogypsum aqueous solution B containing calcium sulfate whisker seeds with an aspect ratio greater than 20:1. Heat in an oil bath at 95°C with stirring until recrystallization yields anhydrous calcium sulfate whiskers with a high aspect ratio. Filter to obtain anhydrous calcium sulfate whiskers C with a high aspect ratio.

[0028] 4) Drying of high aspect ratio anhydrous calcium sulfate whiskers High aspect ratio anhydrous calcium sulfate whiskers C were vacuum dried in a vacuum drying oven at 80°C for 8 hours to obtain the final product. Example 5

[0029] A method for converting phosphogypsum into high aspect ratio anhydrous calcium sulfate whiskers using a trace amount of composite crystal-converting agent under ambient pressure comprises the following steps: 1) Purification and purification of phosphogypsum: Phosphogypsum was mixed with a calcium hydroxide solution at pH 11 to prepare a 10% slurry. The slurry was pumped into an FX75 (75mm diameter) hydrocyclone while being stirred in a mixing tank. The inlet pressure was set to 0.15-0.25 MPa. After purification and separation by the hydrocyclone, the overflow and underflow products were subjected to sieve particle size analysis and chemical composition analysis. The sieve mesh sizes were 65 / 100 / 200 / 325 / 400 / 500 mesh. The 500-mesh product was selected and fed into a secondary hydrocyclone. The underflow product of the secondary hydrocyclone was used as purified phosphogypsum A.

[0030] 2) Purification of phosphogypsum dissolution Weigh 0.1 L (100 g) of deionized water using a graduated cylinder. Weigh 0.2 g of purified phosphogypsum A and dissolve it in 0.1 L of deionized water. Add sulfuric acid, stir, and adjust the pH to 5-6 to remove impurities (calcium hydroxide, calcium oxide). Partially dissolve the purified phosphogypsum, filter, remove undissolved residue, and take the purified phosphogypsum aqueous solution. Add ammonia water dropwise to adjust the pH to 7 and remove unreacted sulfuric acid. This yields purified phosphogypsum aqueous solution B.

[0031] 3) Regulation of calcium sulfate crystal growth Weigh 0.00006 mol of magnesium chloride (a crystal growth regulator) and 0.00003 mol of calcium nitrate using a balance. Add purified phosphogypsum aqueous solution B containing calcium sulfate whisker seeds with an aspect ratio greater than 20:1. Heat in an oil bath at 95°C and stir until recrystallization yields anhydrous calcium sulfate whiskers with a high aspect ratio. Filter to obtain anhydrous calcium sulfate whiskers C with a high aspect ratio.

[0032] 4) Drying of high aspect ratio anhydrous calcium sulfate whiskers High aspect ratio anhydrous calcium sulfate whiskers C were vacuum dried in a vacuum drying oven at 80°C for 8 hours to obtain the final product. Example 6

[0033] A method for converting phosphogypsum into high aspect ratio anhydrous calcium sulfate whiskers using a trace amount of composite crystal-converting agent under ambient pressure comprises the following steps: 1) Purification and purification of phosphogypsum: Phosphogypsum was mixed with a calcium hydroxide solution at pH 11 to prepare a 10% slurry. The slurry was pumped into an FX75 (75mm diameter) hydrocyclone while being stirred in a mixing tank. The inlet pressure was set to 0.15-0.25 MPa. After purification and separation by the hydrocyclone, the overflow and underflow products were subjected to sieve particle size analysis and chemical composition analysis. The sieve mesh sizes were 65 / 100 / 200 / 325 / 400 / 500 mesh. The 500-mesh product was selected and fed into a secondary hydrocyclone. The underflow product of the secondary hydrocyclone was used as purified phosphogypsum A.

[0034] 2) Purification of phosphogypsum dissolution Weigh 0.1 L (100 g) of deionized water using a graduated cylinder. Weigh 0.2 g of purified phosphogypsum A and dissolve it in 0.1 L of deionized water. Add sulfuric acid, stir, and adjust the pH to 5-6 to remove impurities (calcium hydroxide, calcium oxide). Partially dissolve the purified phosphogypsum, filter, remove undissolved residue, and take the purified phosphogypsum aqueous solution. Add ammonia water dropwise to adjust the pH to 7 and remove unreacted sulfuric acid. This yields purified phosphogypsum aqueous solution B.

[0035] 3) Regulation of calcium sulfate crystal growth Weigh 0.00008 mol of magnesium chloride (a crystal growth regulator) and 0.00004 mol of calcium nitrate using a balance. Add purified phosphogypsum aqueous solution B containing calcium sulfate whisker seeds with an aspect ratio greater than 20:1. Heat in an oil bath at 95°C and stir until recrystallization yields anhydrous calcium sulfate whiskers with a high aspect ratio. Filter to obtain anhydrous calcium sulfate whiskers C with a high aspect ratio.

[0036] 4) Drying of high aspect ratio anhydrous calcium sulfate whiskers High aspect ratio anhydrous calcium sulfate whiskers C were vacuum dried in a vacuum drying oven at 80°C for 8 hours to obtain the final product. Example 7

[0037] A method for converting phosphogypsum into high aspect ratio anhydrous calcium sulfate whiskers using a trace amount of composite crystal-converting agent under ambient pressure comprises the following steps: 1) Purification and purification of phosphogypsum: Phosphogypsum was mixed with a calcium hydroxide solution at pH 11 to prepare a 10% slurry. The slurry was pumped into an FX75 (75mm diameter) hydrocyclone while being stirred in a mixing tank. The inlet pressure was set to 0.15-0.25 MPa. After purification and separation by the hydrocyclone, the overflow and underflow products were subjected to sieve particle size analysis and chemical composition analysis. The sieve mesh sizes were 65 / 100 / 200 / 325 / 400 / 500 mesh. The 500-mesh product was selected and fed into a secondary hydrocyclone. The underflow product of the secondary hydrocyclone was used as purified phosphogypsum A.

[0038] 2) Purification of phosphogypsum dissolution Weigh 0.1 L (100 g) of deionized water using a graduated cylinder. Weigh 0.2 g of purified phosphogypsum A and dissolve it in 0.1 L of deionized water. Add sulfuric acid, stir, and adjust the pH to 5-6 to remove impurities (calcium hydroxide, calcium oxide). Partially dissolve the purified phosphogypsum, filter, remove undissolved residue, and take the purified phosphogypsum aqueous solution. Add ammonia water dropwise to adjust the pH to 7 and remove unreacted sulfuric acid. This yields purified phosphogypsum aqueous solution B.

[0039] 3) Regulation of calcium sulfate crystal growth Weigh 0.0001 mol of magnesium chloride (a crystal growth regulator) and 0.00005 mol of calcium nitrate using a balance. Add purified phosphogypsum aqueous solution B containing calcium sulfate whisker seeds with an aspect ratio greater than 20:1. Heat in an oil bath at 95°C and stir until recrystallization yields anhydrous calcium sulfate whiskers with a high aspect ratio. Filter to obtain anhydrous calcium sulfate whiskers C with a high aspect ratio.

[0040] 4) Drying of high aspect ratio anhydrous calcium sulfate whiskers High aspect ratio anhydrous calcium sulfate whiskers C were vacuum dried in a vacuum drying oven at 80°C for 8 hours to obtain the final product.

[0041] Comparative Example 1 A method for converting phosphogypsum into high aspect ratio anhydrous calcium sulfate whiskers using a trace amount of composite crystal-converting agent under ambient pressure comprises the following steps: 1) Purification and purification of phosphogypsum: Phosphogypsum was mixed with a calcium hydroxide solution at pH 11 to prepare a 10% slurry. The slurry was pumped into an FX75 (75mm diameter) hydrocyclone while being stirred in a mixing tank. The inlet pressure was set to 0.15-0.25 MPa. After purification and separation by the hydrocyclone, the overflow and underflow products were subjected to sieve particle size analysis and chemical composition analysis. The sieve mesh sizes were 65 / 100 / 200 / 325 / 400 / 500 mesh. The 500-mesh product was selected and fed into a secondary hydrocyclone. The underflow product of the secondary hydrocyclone was used as purified phosphogypsum A.

[0042] 2) Purification of phosphogypsum dissolution Weigh 0.1 L (100 g) of deionized water using a graduated cylinder. Weigh 0.2 g of purified phosphogypsum A and dissolve it in 0.1 L of deionized water. Add sulfuric acid, stir, and adjust the pH to 5-6 to remove impurities (calcium hydroxide, calcium oxide). Partially dissolve the purified phosphogypsum, filter, remove undissolved residue, and take the purified phosphogypsum aqueous solution. Add ammonia water dropwise to adjust the pH to 7 and remove unreacted sulfuric acid. This yields purified phosphogypsum aqueous solution B.

[0043] 3) Regulation of calcium sulfate crystal growth Weigh 0.000021 mol (0.002 g) of magnesium chloride crystal growth regulator using a balance, add purified phosphogypsum aqueous solution B containing calcium sulfate whisker seeds with an aspect ratio greater than 20:1, heat in an oil bath at 95°C, stir, and recrystallize to obtain anhydrous calcium sulfate whiskers with a high aspect ratio. Filter to obtain anhydrous calcium sulfate whiskers C with a high aspect ratio.

[0044] 4) Drying of high aspect ratio anhydrous calcium sulfate whiskers High aspect ratio anhydrous calcium sulfate whiskers C were vacuum dried in a vacuum drying oven at 80°C for 8 hours to obtain the final product.

Claims

1. A method for converting phosphogypsum into high aspect ratio anhydrous calcium sulfate whiskers using a trace amount of composite crystal-converting agent in an aqueous phase under normal pressure, characterized in that, Includes the following steps: a) Purification of phosphogypsum: Phosphogypsum was mixed with calcium hydroxide solution at pH 11 to prepare a slurry with a concentration of 10%. The slurry was purified and separated by hydrocyclone under an inlet pressure of 0.15-0.25 MPa. The product with a sieve mesh size of 500 mesh was selected as purified phosphogypsum A. b) Dissolving purified phosphogypsum: Dissolve purified phosphogypsum A in deionized water, adjust the pH to 5-6 to remove impurities, filter to obtain purified phosphogypsum aqueous solution, then adjust the pH to 7 to remove unreacted sulfuric acid, to obtain purified phosphogypsum aqueous solution B. c) Calcium sulfate crystal growth regulation: Add magnesium chloride (0.0002-0.001 mol / L relative to the purified phosphogypsum aqueous solution B) and calcium nitrate (0.0001-0.0005 mol / L relative to the purified phosphogypsum aqueous solution B) to the purified phosphogypsum aqueous solution B, and heat and stir at 95°C to recrystallize and obtain anhydrous calcium sulfate whiskers C with a high aspect ratio; d) Drying of high aspect ratio anhydrous calcium sulfate whiskers: High aspect ratio anhydrous calcium sulfate whiskers C were vacuum dried in a vacuum drying oven at 80°C for 8 hours to obtain the final product.

2. The method according to claim 1, characterized in that, The hydrocyclone is an FX75 hydrocyclone with a diameter of 75mm.

3. The method according to claim 1, characterized in that, The sieve mesh sizes are 65 / 100 / 200 / 325 / 400 / 500 mesh.

4. The method according to claim 1, characterized in that, The purified phosphogypsum A is derived from the underflow product of the secondary hydrocyclone.

5. The method according to claim 1, characterized in that, The drying temperature in the vacuum drying oven is 80℃, and the drying time is 8 hours.

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