A method for preparing calcium sulfate hemihydrate whiskers using a low water activity system

Abstract

The application provides a method for preparing calcium sulfate hemihydrate whiskers by using a low water activity system, and belongs to the technical field of calcium sulfate hemihydrate whiskers. The method comprises the following steps: aging, washing and drying treatment are performed on phosphogypsum raw materials to obtain modified phosphogypsum; inorganic salt, alcohol for reducing water activity and deionized water are mixed to obtain a mixed solution; the mixed solution is heated to a crystallization temperature, and then the modified phosphogypsum and a polyhydroxy alcohol crystallization regulator are added to perform a crystallization reaction; after the crystallization reaction is completed, solid-liquid separation, washing and drying are performed to obtain calcium sulfate hemihydrate whiskers. The method has simple process conditions and a controllable crystallization process, and can stably prepare calcium sulfate hemihydrate whiskers with a high length-diameter ratio.

Description

Technical Field

[0001] This invention relates to the field of calcium sulfate hemihydrate whisker technology, and specifically to a method for preparing calcium sulfate hemihydrate whiskers using a low water activity system. Background Technology

[0002] Phosphogypsum is a bulk industrial solid byproduct generated during phosphoric acid production, with calcium sulfate as its main component. With the development of the phosphorus chemical industry, the production of phosphogypsum has increased year by year. Long-term, large-scale stockpiling not only occupies land resources but also easily leads to environmental problems such as acid leaching, heavy metal migration, and the release of fluorine and phosphorus-containing substances. Phosphogypsum has certain application potential in building materials, chemicals, and agriculture, but due to its irregular crystal morphology, insufficient structural stability, and the influence of impurities, its current utilization is mainly low-value-added, making it difficult to achieve efficient and large-scale application. Therefore, exploring ways to improve the performance and added value of phosphogypsum for high-value utilization is of great significance for its resource utilization.

[0003] Calcium sulfate hemihydrate whiskers, as needle-like crystals with a high aspect ratio, have attracted attention in the fields of building materials and composite materials due to their excellent mechanical properties and structural reinforcing effects. Compared with ordinary blocky or short columnar calcium sulfate crystals, calcium sulfate hemihydrate whiskers can form effective bridging and framework structures within materials, thereby improving the mechanical properties and structural stability of materials. Therefore, converting phosphogypsum into calcium sulfate hemihydrate whiskers is a promising high-value utilization approach.

[0004] Currently, the main methods for preparing calcium sulfate hemihydrate whiskers include hydrothermal methods, acidification methods, salt solution methods, and alcohol-water system methods. Hydrothermal methods are typically carried out under closed, high-temperature, and high-pressure conditions, requiring sophisticated equipment, consuming large amounts of energy, and operating under stringent process conditions. Acidification methods involve highly corrosive systems and are difficult to control. Salt solution methods utilize inorganic salts to adjust the solubility and crystallization behavior of calcium sulfate in solution, enabling whisker preparation under normal pressure; however, the nucleation process is difficult to control precisely, resulting in limited crystal morphology stability. Alcohol-water methods introduce organic solvents to alter the water activity and crystallization environment, achieving whisker growth under relatively mild conditions; however, the system parameters are sensitive, requiring precise control over solvent composition and reaction conditions.

[0005] The literature (Rao Jiahuan, Wei Jiazhan, Wen Jiawei, Tian Yao. Study on Preparation of Calcium Sulfate Hemihydrate Whiskers from Flue Gas Desulfurization Gypsum by Atmospheric Pressure Alcohol-Salt Method. Inorganic Salts Industry) discloses a study on the preparation of calcium sulfate hemihydrate whiskers from flue gas desulfurization gypsum by an atmospheric pressure alcohol-salt method. The whiskers obtained by this method have uneven tips and exhibit polycrystalline aggregation characteristics, which is not conducive to stable whisker growth. The method indicates that the optimal gypsum content is 8%; if the content is too high, it will promote nucleation, hinder lattice ion migration, and reduce the size and efficiency of crystal formation. However, this study does not address how to increase the phosphogypsum content in the reaction system while stably obtaining calcium sulfate hemihydrate whiskers with smoother sides, more complete and flatter top crystal faces, and a concentrated aspect ratio distribution, and improving the whisker production efficiency per unit volume of the reaction system.

[0006] Chinese patent document CN118343820A discloses a method for promoting the crystallization of phosphogypsum and recycling the solution. This method uses sodium nitrate as the salt medium, which requires a large amount and is costly. Furthermore, the amount of dry-based phosphogypsum added is in the range of 6-15 wt%, with the specific embodiment in this document using 12 wt%. The document does not further explain how to stably obtain hemihydrate calcium sulfate whiskers with relatively smooth sides, relatively complete and flat top crystal faces, and a concentrated aspect ratio distribution under higher phosphogypsum addition conditions.

[0007] Chinese patent document CN120138772A discloses a method for controlling the aspect ratio of calcium sulfate hemihydrate whiskers prepared from phosphogypsum. This method uses substances containing hydroxyl groups, such as sugars and alcohols, as crystallization agents to control the crystallization process of phosphogypsum under normal pressure hydrothermal conditions, thereby adjusting the aspect ratio of calcium sulfate hemihydrate whiskers within a certain range. However, this method uses 16.5g of xylitol (or fructose, glucose, etc.) and 97.4g of deionized water to obtain a mixed slurry. Adding 1.1g of pretreated phosphogypsum to the mixed slurry for whisker preparation results in a low solid content in the reaction system, limiting the whisker yield per unit volume and making it difficult to meet the requirements for large-scale preparation. Increasing the solid content (phosphogypsum content) in the reaction system often leads to uncontrolled nucleation and crystal agglomeration, thus limiting the controllability of whisker morphology and yield efficiency.

[0008] Chinese patent document CN110747503A discloses a method for preparing hemihydrate calcium sulfate whiskers by microwave irradiation of phosphogypsum under normal pressure. This method utilizes the good absorption characteristics of microwave radiation by phosphogypsum and alcohol-water-sulfate solution system, so that the crystallization process can be carried out under normal pressure. This patent mainly relies on microwave irradiation to achieve reaction enhancement, and pays insufficient attention to crystal nucleation and axial growth of whiskers. It is also highly dependent on specific heating equipment (microwave irradiation equipment), and has limitations in terms of process versatility and scalability.

[0009] Chinese patent document CN103523814B discloses an integrated method for preparing and modifying gypsum shaped powder in an open system under normal pressure. The method involves preparing a slurry of pretreated gypsum and water at room temperature; after thorough stirring, adding glycerol and water to form an alcohol-water solution; continuously stirring at 90-140℃ for 20-180 min; adding 0.1%-5% of a modifier; stopping heating and continuing stirring for 5-60 min; naturally cooling to room temperature; filtering; washing with water 3-5 times to remove residual glycerol from the surface; and drying the filter residue at 80-200℃ to obtain the modified gypsum shaped powder product. The initial slurry concentration can reach 3%-35%, the reaction temperature is low, and the obtained modified calcium sulfate hemihydrate whiskers have uniform morphology, with a length of 45-200 μm and a diameter of 0.1-4 μm, with a yield of 100%. However, this method produces whiskers with a wide range of aspect ratios, resulting in uncontrollable processes and poor product consistency.

[0010] In summary, existing methods still have shortcomings in terms of scalability, process controllability, and stable preparation of whiskers. Therefore, a new preparation method is urgently needed to achieve stable, controllable, and large-scale preparation of calcium sulfate hemihydrate whiskers. Summary of the Invention

[0011] In view of this, the present invention provides a method for preparing calcium sulfate hemihydrate whiskers using a low water activity system. This method has simple process conditions, controllable crystallization process, and can stably and scalably prepare calcium sulfate hemihydrate whiskers with a high aspect ratio.

[0012] To achieve the above objectives, the present invention provides a method for preparing calcium sulfate hemihydrate whiskers using a low water activity system, comprising the following steps: (1) The phosphogypsum raw material is aged, washed and dried to obtain modified phosphogypsum; (2) Mix inorganic salt, alcohol with reduced water activity and deionized water to obtain a mixed solution; heat the mixed solution to the crystallization temperature, and then add modified phosphogypsum and polyhydroxy alcohol crystallizing agent to carry out the crystallization reaction. After the crystallization reaction is completed, separate the solid and liquid, wash and dry to obtain calcium sulfate hemihydrate whiskers.

[0013] Introducing inorganic salts, alcohols that regulate water activity, and polyhydroxy alcohols into the same crystallization system helps reduce the number of ineffective nuclei and promotes axial crystal growth, thereby improving the structural stability of calcium sulfate hemihydrate whiskers and resulting in whiskers with uniform morphology, high aspect ratio, and good repeatability.

[0014] Furthermore, in step (2), the alcohol that reduces water activity and the deionized water form an alcohol-water solution, and the mass ratio of modified phosphogypsum to the alcohol-water solution is 1:(2.5-6). That is, this mass ratio is the ratio of the mass of modified phosphogypsum to the total mass of the alcohol and deionized water that reduce water activity.

[0015] The reaction system of this invention has a high solid content and a high whisker yield per unit volume of the reaction system, which can meet the requirements of large-scale preparation.

[0016] Further, in step (2), the mass ratio of inorganic salt to deionized water is 1:(5-10); the mass ratio of alcohol to deionized water that reduces water activity is 1:(0.5-2).

[0017] The mass ratio of inorganic salt to deionized water is 1:(5-10). This range is mainly used to adjust the concentration of inorganic salt ions and supersaturation in the system, thereby controlling the crystal nucleation and whisker growth process. When the amount of inorganic salt is below this range (i.e., below 0.1), the concentration of sulfate ions in the solution is low, the supersaturation is low, the crystallization driving force is insufficient, the axial growth of crystals is difficult to sustain, and short columnar or irregular crystals are easily formed, with a low whisker aspect ratio. When the amount of inorganic salt is above this range (i.e., above 0.2), the excessively high ion concentration in the solution will lead to a significant increase in the nucleation rate, the generation of a large number of crystal nuclei in the system, the dispersion and consumption of solute, the axial growth of whiskers is inhibited, and crystal agglomeration or short crystals are easily formed, thus affecting the whisker morphology and aspect ratio. Therefore, controlling the mass ratio of inorganic salt to deionized water within the range of 1:(5–10) is beneficial to inhibit excessive nucleation while ensuring a suitable crystallization driving force, thereby promoting stable whisker growth.

[0018] The mass ratio of alcohol to deionized water is 1:(0.5–2). This parameter is mainly used to regulate the water activity of the system, thereby affecting crystal nucleation behavior and growth rate. When the amount of alcohol is below this range (i.e., below 0.5), the water activity of the system is high, the ion migration and nucleation process are accelerated, and a large number of disordered crystal nuclei are easily generated, causing the solute supply to be dispersed and consumed, which is not conducive to the continuous axial growth of whiskers. When the amount of alcohol is above this range (i.e., above 2), the water activity of the system is too low, the solute diffusion rate is reduced, the crystallization process is inhibited, the crystal growth rate decreases, and it may lead to insufficient whisker development or unstable morphology. Therefore, controlling the mass ratio of alcohol to deionized water within the range of 1:(0.5–2) can maintain appropriate solute migration and crystal growth rate while inhibiting disordered nucleation, thus facilitating the stable formation of whisker morphology.

[0019] Furthermore, in step (2), the mass ratio of the polyhydroxy alcohol crystallizing agent to the modified phosphogypsum is (1-10):100.

[0020] In the reaction system of this invention, a smaller amount of crystallizing agent is needed to process high-quality phosphogypsum, which helps to save costs and enable large-scale production.

[0021] Further, in step (2), the inorganic salt is one or more of sodium sulfate, potassium sulfate, and ammonium sulfate, and the alcohol that reduces water activity is one or more of glycerol, ethylene glycol, and propylene glycol.

[0022] Glycerin, ethylene glycol, and propylene glycol are all liquid alcohols at room temperature and are completely miscible with water. They can be used as organic solvents in water-alcohol mixed solvent systems.

[0023] Furthermore, in step (2), the polyhydroxy alcohol crystallizing agent is one or more of xylitol, erythritol, and sorbitol.

[0024] Further, in step (1), the aging method includes: adding carbide slag and water to the phosphogypsum raw material, mixing well, and then placing it. Based on the mass of phosphogypsum in the phosphogypsum raw material, the amount of carbide slag added is 1-3% of the mass of phosphogypsum, and the amount of water added is 10% of the mass of phosphogypsum.

[0025] Furthermore, in step (1), the aged gypsum is washed with deionized water to remove soluble impurities; then it is washed with an ethanol aqueous solution to enhance the removal of organic impurities and surface adsorbates.

[0026] Further, in step (1), the washed phosphogypsum is dried at a temperature of 40-60℃. After drying, the phosphogypsum is sieved to obtain modified phosphogypsum with a particle size of 100-200 mesh.

[0027] Furthermore, in step (2), the temperature of the crystallization reaction is 95-100 ℃, the crystallization reaction is carried out under stirring conditions, the stirring speed is 250-350 rpm, and the reaction time is 150-210 min.

[0028] The crystallization reaction temperature is 95-100 ℃ to ensure a relative balance between nucleation and axial growth of whiskers in the system. When the temperature is below 95 ℃, the supersaturation of the system is established more slowly, the crystal growth rate decreases, and short columnar crystals are easily formed. When the temperature is above 100 ℃, the nucleation rate may increase, easily producing a large number of crystal nuclei, thus affecting the whisker aspect ratio and morphological stability.

[0029] Furthermore, in step (2), solid-liquid separation is performed by vacuum filtration.

[0030] Furthermore, in step (2), the product is first washed with boiling water, and then washed with an alcohol solvent. Calcium sulfate has low solubility at around 100°C.

[0031] Furthermore, in step (2), the drying temperature is 40-60℃.

[0032] The above-described technical solution of the present invention has at least the following beneficial effects: This invention uses phosphogypsum as raw material and achieves effective removal of impurities and controllable transformation of crystal morphology in phosphogypsum through processes such as aging, washing, and crystallization control, providing a feasible technical approach for the resource utilization and high-value utilization of phosphogypsum.

[0033] This invention is not a simple combination of the salt solution method and the alcohol-water method, but rather a synergistic regulation of the crystallization environment from multiple levels, such as ion concentration, water activity, and crystal growth kinetics, by synergistically introducing inorganic salts, regulating alcohols in the solvent system, and polyhydroxy alcohol crystallization agents into the same crystallization system.

[0034] In this process, the salt solution increases the ion concentration of the system, altering the dissolution-precipitation behavior of calcium sulfate and making the system more prone to supersaturation, thus providing the necessary driving force for crystal growth. Alcohols, by reducing the water activity of the system, inhibit disordered nucleation. Polyhydroxy alcohol crystallizing agents, through selective adsorption on lateral growth facets during crystal growth, promote preferential growth of the crystal along the axial direction. The synergistic effect of these three agents helps reduce the number of ineffective nuclei and promotes axial crystal growth, thereby improving the structural stability of hemihydrate calcium sulfate whiskers and resulting in whiskers with uniform morphology, high aspect ratio, and good reproducibility.

[0035] If the salt solution method and alcohol-water method are not introduced (i.e., in step (2), the inorganic salt and the alcohol that reduces water activity are omitted and directly replaced with deionized water), and only polyhydroxy alcohol crystallization agents are used for regulation, the system lacks synergistic regulation of ion concentration and water activity, which easily leads to an increase in the number of nuclei, dispersion of solute supply and disordered crystal growth. Although the resulting product may form hemihydrate calcium sulfate crystals, its aspect ratio and morphological consistency are usually poor, and it is more likely to agglomerate and grow under conditions of higher solid content.

[0036] In traditional crystallization systems, increasing the solid content of the reaction system often leads to problems such as uncontrolled nucleation and crystal agglomeration, thus limiting the controllability of whisker morphology and yield efficiency. This invention reduces the water activity of the crystallization system, introduces sulfate ions to regulate the crystallization environment, and further adds polyhydroxy alcohol crystallization agents to finely control the crystal growth process. Stable growth and morphology control of hemihydrate calcium sulfate whiskers can still be achieved under high solid content conditions, effectively increasing the whisker yield per unit volume of the reaction system. Simultaneously, the reaction process can be carried out under normal pressure, avoiding the investment in high-temperature and high-pressure equipment. The inorganic salts, alcohol solvents, and polyhydroxy alcohol crystallization agents used are widely available and low in cost. The process parameters are highly adjustable, exhibiting good process adaptability and potential for scale-up applications. Attached Figure Description

[0037] Figure 1 This is a scanning electron microscope image of the calcium sulfate hemihydrate whiskers obtained in Example 2; Figure 2 This is a scanning electron microscope image of the top crystal face of the calcium sulfate hemihydrate whisker obtained in Example 2; Figure 3 The image shows a scanning electron microscope (SEM) image of the calcium sulfate hemihydrate whiskers obtained in Comparative Example 1. Figure 4 The image shows a scanning electron microscope (SEM) image of the top crystal face of the calcium sulfate hemihydrate whisker obtained in Comparative Example 1. Figure 5 The image shows a scanning electron microscope (SEM) image of the calcium sulfate hemihydrate whiskers obtained in Comparative Example 2. Figure 6 The image shows a scanning electron microscope (SEM) image of the top crystal face of the calcium sulfate hemihydrate whisker obtained in Comparative Example 2. Figure 7 The image shows a scanning electron microscope (SEM) image of the calcium sulfate hemihydrate whiskers obtained in Comparative Example 3. Figure 8 The image shows a scanning electron microscope (SEM) image of the top crystal face of the calcium sulfate hemihydrate whisker obtained in Comparative Example 3. Detailed Implementation

[0038] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some, not all, of the embodiments of the present invention. All other embodiments obtained by those skilled in the art based on the described embodiments of the present invention are within the scope of protection of the present invention.

[0039] Example 1 A method for preparing calcium sulfate hemihydrate whiskers includes the following steps: (1) Pretreatment of phosphogypsum raw materials includes aging, grading and washing, drying and screening. First, based on the mass of phosphogypsum, carbide slag is added to phosphogypsum at a rate of 2% of the phosphogypsum mass, and water is added at a rate of 10% of the phosphogypsum mass. After thorough mixing, the mixture is left to stand for 24 hours. After aging, the phosphogypsum is cleaned using a graded washing method: first, deionized water is used to wash the aged phosphogypsum to remove soluble impurities from its surface; then, 50% ethanol solution is used for washing; finally, deionized water is used for washing and solid-liquid separation is performed. After washing, the obtained phosphogypsum was dried at 45℃. After drying, the phosphogypsum was sieved to obtain modified phosphogypsum raw material with a particle size of 100-200 mesh. (2) Add 20 g of sodium sulfate and 100 g of glycerol to 100 g of deionized water and stir to mix evenly to obtain a mixed solution; heat the mixed solution in an oil bath to 97 °C, then add 40 g of modified phosphogypsum and 2 g of sorbitol to obtain a mixed slurry for preparing whiskers; After reacting the mixed slurry at 97 ℃ and 300 rpm for 180 min, it was directly filtered, and the resulting filter cake was filtered and washed with boiling water, then filtered and washed with anhydrous ethanol. Finally, the filtered and washed filter cake was dried at 60 ℃ for 24 hours to obtain hemihydrate calcium sulfate whiskers, whose aspect ratio was mainly distributed in the range of 20-30.

[0040] Example 2 A method for preparing calcium sulfate hemihydrate whiskers includes the following steps: (1) The steps for treating phosphogypsum are the same as in Example 1; (2) Add 10 g of sodium sulfate and 100 g of glycerol to 100 g of deionized water and stir to mix evenly to obtain a mixed solution; heat the mixed solution in an oil bath to 97 °C, then add 40 g of modified phosphogypsum and 2 g of sorbitol to obtain a mixed slurry for preparing whiskers; The mixed slurry was reacted at 97 °C and 300 rpm for 180 min, then directly filtered. The resulting filter cake was washed with boiling water, followed by filtration with anhydrous ethanol. Finally, the filtered and washed filter cake was dried at 60 °C for 24 hours to obtain hemihydrate calcium sulfate whiskers, as shown below. Figure 1 and Figure 2 As shown, its aspect ratio is mainly distributed in the range of 30-40.

[0041] Example 3 A method for preparing calcium sulfate hemihydrate whiskers includes the following steps: (1) The steps for treating phosphogypsum are the same as in Example 1; (2) 10 g of sodium sulfate and 100 g of glycerol were added to 100 g of deionized water and stirred until homogeneous. The mixture was heated to 97 °C in an oil bath, and then 40 g of modified phosphogypsum and 2 g of erythritol were added to obtain a whisker mixture slurry. The reaction slurry was reacted at 97 °C and 300 rpm for 180 min and then directly filtered. The resulting filter cake was filtered and washed with boiling water, and then filtered and washed with anhydrous ethanol. Finally, the filtered and washed filter cake was dried at 60 °C for 24 hours to obtain hemihydrate calcium sulfate whiskers, whose aspect ratio was mainly distributed in the range of 25-35.

[0042] Example 4 A method for preparing calcium sulfate hemihydrate whiskers includes the following steps: (1) The steps for treating phosphogypsum are the same as in Example 1; (2) 10 g of sodium sulfate and 100 g of glycerol were added to 100 g of deionized water and stirred until homogeneous. The mixture was then heated to 97 °C in an oil bath. 40 g of modified phosphogypsum and 2 g of xylitol were added to obtain a whisker mixture slurry. The reaction slurry was reacted at 97 °C and 300 rpm for 180 min and then directly filtered. The resulting filter cake was filtered and washed with boiling water, then filtered and washed with anhydrous ethanol. Finally, the filtered and washed filter cake was dried at 60 °C for 24 hours to obtain hemihydrate calcium sulfate whiskers, whose aspect ratio was mainly distributed in the range of 25-35.

[0043] Example 5 This embodiment is basically the same as embodiment 2, except that: in step (2), 10 g of potassium sulfate and 50 g of propylene glycol are added to 100 g of deionized water and stirred until homogeneous to obtain a mixed solution; the mixed solution is heated to 95 °C in an oil bath, and then 25 g of modified phosphogypsum and 0.25 g of sorbitol are added to obtain a mixed slurry for preparing whiskers; the mixed slurry is reacted at 95 °C and 250 rpm for 150 min and then directly filtered.

[0044] Example 6 This embodiment is basically the same as embodiment 2, except that: in step (2), 15 g of ammonium sulfate and 200 g of ethylene glycol are added to 100 g of deionized water and stirred evenly to obtain a mixed solution; the mixed solution is heated to 97 °C in an oil bath, and then 120 g of modified phosphogypsum and 12 g of sorbitol are added to obtain a mixed slurry for preparing whiskers; the mixed slurry is reacted at 97 °C and 350 rpm for 210 min and then directly filtered.

[0045] Comparative Example 1 A method for preparing calcium sulfate hemihydrate whiskers includes the following steps: (1) The steps for treating phosphogypsum are the same as in Example 1; (2) 10 g of sodium sulfate was added to 200 g of deionized water and stirred until homogeneous to obtain a mixed solution. The mixed solution was heated to 97 °C in an oil bath, and then 40 g of modified phosphogypsum and 2 g of sorbitol were added to obtain a whisker slurry. The reaction slurry was reacted at 97 °C and 300 rpm for 180 min, and then directly filtered. The resulting filter cake was filtered and washed with boiling water, and then filtered and washed with anhydrous ethanol. Finally, the filtered and washed filter cake was dried at 60 °C for 24 hours to obtain hemihydrate calcium sulfate whiskers, as shown in the figure. Figure 3 and Figure 4 As shown, its aspect ratio is usually less than 10.

[0046] Comparative Example 2 A method for preparing calcium sulfate hemihydrate whiskers includes the following steps: (1) The steps for treating phosphogypsum are the same as in Example 1; (2) 100 g of glycerol was added to 110 g of deionized water and stirred until homogeneous. The mixture was then heated in an oil bath to 97 °C. 40 g of modified phosphogypsum and 2 g of sorbitol were added to obtain a whisker slurry. The slurry was reacted at 97 °C and 300 rpm for 180 min, and then directly filtered. The resulting filter cake was rinsed with boiling water and then with anhydrous ethanol. Finally, the filtered and washed filter cake was dried at 60 °C for 24 hours to obtain hemihydrate calcium sulfate whiskers, as shown below. Figure 5 and Figure 6 As shown, its aspect ratio is usually less than 15.

[0047] Comparative Example 3 A method for preparing calcium sulfate hemihydrate whiskers includes the following steps: (1) The steps for treating phosphogypsum are the same as in Example 1; (2) 10 g of sodium sulfate and 100 g of glycerol were added to 100 g of deionized water and stirred until homogeneous. The mixture was then heated to 97 °C in an oil bath, and 40 g of modified phosphogypsum was added to form a slurry. The slurry was reacted at 97 °C and 300 rpm for 180 min, and then directly filtered. The resulting filter cake was rinsed with boiling water and then with anhydrous ethanol. Finally, the filtered and washed filter cake was dried at 60 °C for 24 hours to obtain hemihydrate calcium sulfate whiskers, as shown in the figure. Figure 7 and Figure 8 As shown, its aspect ratio is usually less than 15.

[0048] A comparison of the results of Examples 1-4 with Comparative Examples 1, 2, and 3 shows that different crystallization control methods have a significant impact on the nucleation behavior and morphological evolution of calcium sulfate hemihydrate whiskers. In Comparative Example 1, when no alcoholic substances reducing solvent water activity were introduced into the crystallization system, the system had a high water activity, which was conducive to ion migration and crystal nucleus formation. This led to an increase in the number of crystal nuclei and their dispersed consumption of solute supply, making it difficult for individual crystals to obtain continuous axial growth conditions. The final crystals were predominantly short crystals or short columnar morphologies, with a low whisker aspect ratio. The top crystal faces of the calcium sulfate hemihydrate whiskers exhibited passivation, fragmentation, or polycrystalline aggregation characteristics. Figure 3 and Figure 4 As shown.

[0049] In Comparative Example 2, no sulfate was introduced into the crystallization system to adjust the supersaturation of calcium sulfate. The supersaturation of calcium sulfate in the system was low, resulting in insufficient crystallization driving force. Axial crystal growth was difficult to sustain, and the whisker tips exhibited irregular, broken, or incomplete morphological characteristics. Figure 5 and Figure 6 As shown.

[0050] In Comparative Example 3, no polyhydroxy alcohol crystallizing agent was introduced into the crystallization system to further regulate the crystal growth process. Although the inorganic salt and alcohol solvent system can achieve basic control over nucleation and growth to some extent, the lack of fine-tuning of the crystal growth direction and morphological evolution during crystal growth resulted in insufficient stability and consistency of whisker axial growth. The obtained whiskers had a wide aspect ratio distribution, and the surface of the obtained calcium sulfate hemihydrate whiskers exhibited obvious irregular undulations. The crystal faces at the whisker tips showed passivation or truncation characteristics, such as... Figure 7 and Figure 8 As shown.

[0051] Meanwhile, under the high solids content crystallization conditions adopted in this invention, Comparative Examples 1-3 generally suffer from problems such as whisker agglomeration and limited growth due to the lack of synergistic control of the crystallization environment, making it difficult to form a stable and continuous whisker growth process, resulting in low whisker production efficiency per unit volume of reaction system. In contrast, the embodiments, through the synergistic introduction of inorganic salts, alcohols that regulate the solvent system, and polyhydroxy alcohol crystallization agents, effectively suppress disordered nucleation while ensuring suitable crystallization driving force, and finely control the crystal growth direction and morphological evolution, making the crystal growth evolution a controllable process dominated by axial growth, thereby improving the whisker aspect ratio and improving whisker production efficiency. Simultaneously, the resulting calcium sulfate hemihydrate whiskers have complete and flat top crystal faces, such as... Figure 1 and Figure 2 As shown.

[0052] In summary, this invention achieves effective control over the nucleation and growth process of calcium sulfate hemihydrate whiskers under normal pressure by aging and graded water washing pretreatment of phosphogypsum raw materials, and by synergistically introducing inorganic salts, alcohol solvents, and polyhydroxy alcohol crystallizing agents during crystallization. This method can stably prepare calcium sulfate hemihydrate whiskers with high aspect ratios and good morphological consistency under high solids content conditions, significantly improving the whisker yield efficiency per unit volume of the reaction system. Furthermore, this invention features mild process conditions, flexible parameter control, and a wide range of raw material sources, exhibiting good process adaptability and potential for scale-up applications, providing a practical and feasible technical approach for the high-value utilization of phosphogypsum.

[0053] The above description is merely a preferred embodiment of the present invention and is not intended to limit the implementation of the invention. Those skilled in the art can make other variations or modifications based on the above description. It is neither necessary nor possible to exhaustively describe all possible implementations. 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 calcium sulfate hemihydrate whiskers using a low water activity system, characterized in that, Includes the following steps: (1) The phosphogypsum raw material is aged, washed and dried to obtain modified phosphogypsum; (2) Mix inorganic salt, alcohol with reduced water activity and deionized water to obtain a mixed solution; heat the mixed solution to the crystallization temperature, and then add modified phosphogypsum and polyhydroxy alcohol crystallizing agent to carry out the crystallization reaction. After the crystallization reaction is completed, separate the solid and liquid, wash and dry to obtain calcium sulfate hemihydrate whiskers. In step (2), the alcohol with reduced water activity and deionized water form an alcohol-water solution, and the mass ratio of modified phosphogypsum to alcohol-water solution is 1:2.5-6. The mass ratio of inorganic salts to deionized water is 1:5-10; the mass ratio of alcohols that reduce water activity to deionized water is 1:0.5-2. The mass ratio of polyhydroxy alcohol crystallizing agent to modified phosphogypsum is (1-10):100; The inorganic salt is one or more of sodium sulfate, potassium sulfate, and ammonium sulfate; The alcohol that reduces water activity is one or more of glycerol, ethylene glycol, and propylene glycol.

2. The method for preparing calcium sulfate hemihydrate whiskers using a low water activity system according to claim 1, characterized in that, In step (2), the polyhydroxy alcohol crystallizing agent is one or more of xylitol, erythritol, and sorbitol.

3. A method for preparing calcium sulfate hemihydrate whiskers using a low water activity system according to claim 1 or 2, characterized in that, In step (1), the aging method includes: adding carbide slag and water to the phosphogypsum raw material, mixing them well and then placing them in the soil.

4. A method for preparing calcium sulfate hemihydrate whiskers using a low water activity system according to claim 1 or 2, characterized in that, In step (1), the dried phosphogypsum is sieved to obtain modified phosphogypsum with a particle size of 100-200 mesh.

5. A method for preparing calcium sulfate hemihydrate whiskers using a low water activity system according to claim 1 or 2, characterized in that, In step (2), the crystallization reaction is carried out at a temperature of 95-100 ℃, under stirring conditions, with a stirring speed of 250-350 rpm and a reaction time of 150-210 min.

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