Method for preparing calcium sulfate whisker and fluorine recovery by deep defluorination of fluorogypsum and application of prepared product

Abstract

The present application belongs to the technical field of industrial solid waste resource utilization, and particularly relates to a method for deep defluorination of fluorogypsum to prepare calcium sulfate whiskers and fluorine recovery and application of the prepared products. The present application aims at the industry bottleneck problems of existing fluorogypsum resource utilization technology, such as resource waste, insufficient defluorination depth, low purity of prepared whiskers, fluorine content exceeding the standard, etc. A mechanical activation pretreatment coupled with a three-stage gradient defluorination process is adopted to efficiently remove soluble fluorine, insoluble fluoride and trace residual fluorine impurities in fluorogypsum, and simultaneously realize fluorine resource closed-loop recovery. Through seed induction and synergistic regulation of compounded surfactants, high-crystallinity high-purity calcium sulfate whiskers are prepared by directional whisker growth. The present application realizes low-temperature liquid-phase reaction throughout the whole process, has low energy consumption, no secondary pollution, and the process is suitable for industrial mass production. The obtained calcium sulfate whiskers have a purity of ≥99.3%, a fluorine content of ≤0.06%, a length-diameter ratio of 25-45, and a whiteness of ≥90%. The purity of recovered calcium fluoride is ≥99%, and the total fluorine recovery rate is ≥93%. The calcium sulfate whiskers can be widely applied in fields such as high-end composite materials, electronic packaging, fluorine chemical raw materials, etc., realize full-component high-value resource utilization of fluorogypsum, and have high industrialization promotion value.

Description

Technical Field

[0001] This invention belongs to the field of industrial solid waste resource utilization technology, specifically involving a method for deep defluorination of fluorogypsum to prepare calcium sulfate whiskers and fluorine recovery, as well as the application of the prepared products. It is particularly suitable for the deep removal of impurities from fluorogypsum, a by-product of the fluorochemical industry, the preparation of high-end new materials, and the closed-loop recovery of valuable fluorine resources. It can realize the efficient preparation of high-purity calcium sulfate whiskers and the simultaneous recovery and utilization of fluorine resources. Background Technology

[0002] Fluoropyroxene is a large-scale industrial solid waste generated during the acid hydrolysis of fluorite to produce hydrofluoric acid. Its main component is anhydrous calcium sulfate. Industry production data shows that approximately 3.6 tons of fluorogyroxene are produced as a byproduct for every ton of hydrofluoric acid produced. Currently, the comprehensive utilization rate of fluorogyroxene in China is less than 40%. A large amount of fluorogyroxene is disposed of through stockpiling and landfilling, which not only occupies land resources but also causes environmental problems such as soil and water pollution due to the material's strong acidity (pH=1.5-2.5) and the easy leaching of fluorine impurities. The core bottleneck restricting the high-value utilization of fluorogyroxene is the presence of 0.5%-2% complex fluorine impurities in the material, mainly including soluble fluorides, insoluble calcium fluoride, and fluorosilicates. These impurities are complex in composition and difficult to remove.

[0003] Current technologies have been used to prepare calcium sulfate whiskers from fluorogypsum, but these technologies generally suffer from many inherent defects, which severely limit the high-end application of the product and the maximization of resource utilization. First, there is a serious waste of fluorine resources and significant environmental risks. Existing fluoride gypsum-to-whisker production processes primarily focus on the resource utilization of calcium sulfate components, neglecting the recovery value of fluorine impurities. They simply discharge fluorine-containing wastewater or allow fluorine impurities to remain in the gypsum product, resulting in a massive waste of scarce fluorine chemical resources. Residual fluoride ions also lead to poor product stability and fluorine leaching during use, posing ongoing environmental risks. Even if existing co-production processes achieve the synergistic preparation of gypsum and whiskers, a comprehensive fluorine resource recovery system has not been established, and the full resource utilization of solid waste components has not been achieved.

[0004] Secondly, the defluorination effect is limited, and the product purity is relatively low. Traditional processes mostly use single acid leaching or water washing for defluorination, which has poor targeting and insufficient depth. It can only remove 60%-70% of the surface fluoride ions in the material. After treatment, the residual fluoride content in the calcium sulfate whiskers is still greater than 0.3%, which is difficult to meet the low-fluoride requirements of fields such as electronic packaging, high-end composite materials, and precision fillers. The products of conventional processes can only be used in low-end building materials and ordinary filler scenarios, and the added value of the products is low.

[0005] Third, the process is energy-intensive and generates secondary pollution. Some existing high-purity whisker preparation technologies rely on high-temperature calcination processes above 600 ℃ to decompose impurities and control crystal form, resulting in extremely high overall energy consumption and high production costs. At the same time, the high-temperature conditions will cause residual fluorides in the materials to volatilize, generating fluorine-containing waste gas, causing secondary environmental pollution, and the process is not green enough.

[0006] In summary, the industry currently lacks an integrated green process that can achieve deep defluorination of fluorogypsum, directional preparation of high-purity whiskers, and efficient recovery of fluorine resources. It is difficult to simultaneously achieve high-value utilization of solid waste, closed-loop resource utilization, and low-carbon and environmentally friendly production. This is a technical problem that urgently needs to be solved in the field of fluorogypsum resource utilization. Summary of the Invention

[0007] The purpose of this invention is to provide a method for deep defluorination of fluorogypsum to prepare high-purity calcium sulfate whiskers and for fluorine recovery. This invention utilizes a three-stage gradient defluorination system of physical-chemical-deep adsorption, combined with seed induction and compound additive-based directional crystallization control technology. Under low-temperature liquid phase conditions, it achieves deep removal of impurities from fluorogypsum and directional growth of high-purity calcium sulfate whiskers. Simultaneously, it completes the enrichment and recovery of fluorine-containing waste liquid throughout the entire process, producing high-purity, low-fluoride calcium sulfate whiskers and high-purity calcium fluoride products. This achieves the resource utilization of all components of fluorogypsum—calcium, sulfur, and fluorine—significantly increasing the added value of solid waste utilization and aligning with the industrial development needs of green, low-carbon, and solid waste resource utilization.

[0008] To achieve the above objectives, the technical solution adopted by the present invention is as follows: A method for deep defluorination of fluorogypsum to prepare high-purity calcium sulfate whiskers and recover fluorine resources includes the following steps: S1. Mechanical activation pretreatment Fluorogypsum raw materials are crushed and ground, and sieved to obtain fluorogypsum powder with a particle size ≤50 μm. Mechanical activation is then performed using any one or more combinations of ball mills, vibratory mills, and air jet mills, controlling the activation speed at 800-1200 rpm and the activation time at 10-20 min. Mechanical force disrupts the dense crystalline structure of fluorogypsum, dissociating complex fluorine impurities encapsulated within calcium sulfate crystals, significantly enhancing the activity of subsequent defluorination reactions and laying the foundation for gradient defluorination and whisker growth.

[0009] S2. Three-stage gradient defluorination treatment This invention employs a graded, progressive defluorination logic to precisely remove fluorine impurities in different forms, achieving comprehensive and deep defluorination. Specifically, it includes three stages: physical defluorination, chemical defluorination, and deep adsorption defluorination. a. Primary physical defluorination: The activated fluorogypsum powder is uniformly mixed with deionized water at a solid-liquid ratio of 1:3-6 and placed in a constant temperature stirred reactor. The reaction temperature is controlled at 40-50 ℃, the stirring speed is 200-300 rpm, and the reaction is carried out for 30-60 min to fully dissolve soluble fluorine impurities such as free fluorosilicic acid and hydrogen fluoride in the material. After the reaction is completed, solid-liquid separation is performed to obtain primary defluorination slurry and fluorine-containing washing solution, respectively.

[0010] b. Secondary chemical defluorination: The slurry after primary defluorination is redispersed in deionized water, and an inorganic dilute acid solution with a concentration of 0.5-1.0 mol / L is added. The inorganic dilute acid is selected from one or more of dilute hydrochloric acid, dilute sulfuric acid, and dilute nitric acid. The pH value of the system is precisely controlled to be stable at 1.5-2.0, the stirring speed is 150-250 rpm, the temperature is raised to 45-65 ℃, and the reaction is carried out at a constant temperature for 40-80 min. Through the weak acid activation reaction, the insoluble solid bound fluoride impurities such as calcium fluoride and fluorosilicate in the material are converted into soluble fluoride ions, further removing the bound fluoride impurities. After the reaction is completed, the solid and liquid are separated to obtain secondary defluorinated slurry and fluoride-containing acid solution.

[0011] c. Three-stage deep defluorination: The secondary defluorination slurry is uniformly dispersed in deionized water, and a composite fluoride ion trapping agent is added. The trapping agent is prepared by combining hydroxyapatite and alumina at a mass ratio of (1.5-2.5):1. The amount of trapping agent added is 4%-9% of the mass of fluorogypsum raw material. In order to achieve better results, the hydroxyapatite in the trapping agent can be modified by a silane coupling agent, and the amount of the modifier is 1%-3% of the mass of hydroxyapatite. The reaction temperature is controlled at 30-40 ℃, and the reaction is carried out with constant temperature stirring for 20-40 min. Through the dual action of physical adsorption and chemical complexation, the residual trace fluoride ions in the slurry are deeply trapped, and deep defluorination is achieved. After the reaction, solid and liquid are separated to obtain deep defluorination slurry and fluoride-containing trapping solution.

[0012] S3. Closed-loop recovery of fluorine resources The fluorinated washing liquid, fluorinated acid liquid, and fluorinated collection liquid generated in step S2 are all combined and collected into the waste liquid treatment system. The waste liquid is concentrated to 1 / 3-1 / 6 of its original volume by evaporation or membrane concentration. It is then cooled and crystallized at 5-15 ℃ for 2-4 h. After centrifugation and low-temperature drying, excess water and trace impurities are removed, and calcium fluoride product is recovered. This realizes the resource recovery of fluorine impurities and completely solves the problems of fluorine resource waste and fluorinated waste liquid pollution.

[0013] S4. Seed-induced directional whisker growth High aspect ratio and high purity calcium sulfate whiskers were prepared by using in-situ seeding technology combined with a compound surfactant to precisely control the growth orientation of calcium sulfate crystals. The specific steps are as follows: a. Preparation of nanocrystal seeds: Take 5%-10% of the total mass of deep defluorination slurry, disperse it thoroughly by ultrasonication, and slowly add a water-soluble sulfate solution with a concentration of 0.1-0.4 mol / L. The water-soluble sulfate is preferably at least one of sodium sulfate and potassium sulfate. Stir at a speed of 200-300 rpm, heat to 55-75 ℃, and react at a constant temperature for 1-2 h to prepare calcium sulfate nanocrystal seeds with a particle size of 50-100 nm in situ, providing a crystalline substrate for subsequent directional growth.

[0014] b. Directional crystallization growth: The remaining deep defluorination slurry is mixed with the prepared nanocrystal seeds at a mass ratio of 1~4:100. A compound surfactant is added, which is a mixture of cationic and nonionic surfactants at a mass ratio of 1:0.3~0.6, and the amount added is 0.05%-0.15% of the total mass of the slurry. The cationic surfactant is selected from one or more of dodecyltrimethylammonium chloride, hexadecyltrimethylammonium bromide, and hexadecyltrimethylammonium chloride, and the nonionic surfactant is selected from one or more of polyvinylpyrrolidone, polyethylene glycol, and sodium polyacrylate. The reaction system temperature is controlled at 85-95℃, the stirring speed is 100-150 rpm, and the isothermal crystallization reaction is carried out for 2-4 h to induce the preferential directional growth of calcium sulfate crystals along the c-axis, and the whisker morphology and aspect ratio are precisely controlled.

[0015] c. Constant temperature aging modification: After the crystallization reaction is completed, the temperature is slowly lowered to 50-60 ℃ and aged at a constant temperature for 1-2 h to fully improve the whisker crystal structure, ensure uniform grain growth, eliminate crystal defects, and improve the integrity and stability of the whiskers.

[0016] S5. Product separation and purification The aged whisker slurry was separated into solid and liquid components by centrifugation or vacuum filtration. The filter cake was washed repeatedly with deionized water 3-5 times to thoroughly remove residual impurities and additives from the surface. The washed product was placed in an oven and dried at a constant temperature of 100-110 ℃ for 2-3 h. After cooling, high-purity, low-fluoride calcium sulfate whisker products were obtained.

[0017] Another object of the present invention is to provide a calcium sulfate whisker product and a recovered calcium fluoride product prepared by the above preparation method. The calcium sulfate whisker has a purity of ≥99.3%, a fluorine content of ≤0.06%, an aspect ratio of 25-45, and a whiteness of ≥90%. The recovered calcium fluoride has a purity of ≥99% and a total fluorine recovery rate of ≥93%. The calcium sulfate whisker can be used in the preparation of polymer composite materials, electronic packaging materials, optical plastics, and paper reinforcement. The recovered calcium fluoride can be used in the fields of fluorochemical raw materials, lithium battery electrolytes, optical glass, and ceramic glazes.

[0018] The advantages of this invention are: 1. This invention significantly improves product quality through staged defluorination. It employs a three-stage gradient defluorination system—physical leaching, chemical conversion, and deep adsorption—to specifically remove soluble, insoluble, and trace residual fluorine impurities. The defluorination effect is far superior to traditional single-stage defluorination processes. The product standards obtained by this invention are: calcium sulfate whisker purity ≥99.3%, fluorine content ≤0.06%, aspect ratio 25-45, whiteness ≥90%, recovered calcium fluoride purity ≥99%, and total fluorine recovery rate ≥93%. By optimizing process parameters, high-end products with a purity of 99.5% and a fluorine content as low as 0.05% can be further prepared, balancing mass production practicality with the needs of high-end product preparation. This fully meets the requirements of high-end fields such as electronic packaging, high-end composite materials, and precision filling, breaking through the bottleneck of low added value in traditional products. 2. This invention realizes closed-loop recycling of fluorine resources and maximizes resource utilization. This invention uniformly enriches, purifies and crystallizes the defluorination waste liquid throughout the entire process, and transforms industrial harmful solid waste impurities into high-value calcium fluoride raw materials, realizing the resource utilization of all components of calcium, sulfur and fluorine, and completely solving the problems of fluorine resource waste and fluorine pollution in traditional processes. The process is green and environmentally friendly and has significant economic benefits. 3. The entire process uses a low-temperature liquid phase reaction, resulting in low energy consumption and no secondary pollution. This invention adopts a low-temperature liquid phase reaction at a temperature below 100°C throughout the process, abandoning the traditional high-temperature calcination process, which significantly reduces production energy consumption and production costs. At the same time, there is no problem of high-temperature fluoride volatilization throughout the process, which eliminates secondary pollution from fluorine-containing waste gas at the source, greatly improving the safety and greenness of the process. 4. Strong process synergy, capable of large-scale mass production. This invention couples and links defluorination and purification with whisker directional growth process. The process is simple and coherent, the process parameter control range is reasonable, and the repeatability is strong. There is no need to customize complex special equipment. It can be directly adapted to existing solid waste treatment and crystal preparation production lines, which have both environmental protection value and industrial promotion value. Attached Figure Description

[0019] Figure 1 This is the overall process flow diagram of the present invention.

[0020] Figure 2 A photograph of the calcium sulfate whiskers prepared in Example 1; Figure 3 The image shows the XRD pattern of the calcium sulfate whiskers prepared in Example 1. Figure 4 The image shows the XRD pattern of calcium fluoride prepared in Example 1. Detailed Implementation

[0021] Example 1 A method for deep defluorination of fluorogypsum to prepare high-purity calcium sulfate whiskers and for fluorine recovery includes the following steps: S1. Mechanical activation pretreatment: The fluorogypsum raw material was crushed and ground to a particle size of 45 μm, and mechanically activated for 15 min using a ball mill at a speed of 1000 rpm to obtain activated fluorogypsum powder.

[0022] S2. Three-stage gradient defluorination Primary physical defluorination: Activated fluoride gypsum and deionized water are mixed at a solid-liquid ratio of 1:4, reacted at 45 ℃, stirred at a constant temperature for 45 min, and filtered to separate the primary defluorination slurry and fluoride-containing washing solution. Secondary chemical defluorination: Add 0.75 mol / L dilute hydrochloric acid to the primary defluorination slurry, adjust the pH of the system to 1.8, stir at 200 rpm, heat to 55℃ and react at a constant temperature for 60 min, then filter to obtain secondary defluorination slurry and fluorinated acid solution; Three-stage deep defluorination: The secondary defluorination slurry is dispersed in water, and a fluoride ion trapping agent composed of hydroxyapatite and alumina in a mass ratio of 2:1 is added. The amount of trapping agent added is 6.5% of the mass of fluorogypsum. The reaction temperature is 35 ℃ and the reaction time is 30 min. The deep defluorination slurry and fluoride-containing trapping solution are obtained by filtration. The hydroxyapatite is pre-modified with a silane coupling agent, and the amount of the modifier is 2% of the mass of hydroxyapatite.

[0023] S3. Fluorine Resource Recovery: Combine various fluorine-containing waste liquids, concentrate them to 1 / 4 of their original volume by evaporation, cool and crystallize them at 10 ℃ for 3 h, and then dry them after solid-liquid separation to obtain high-purity calcium fluoride products.

[0024] S4. Directional whisker growth: Take 8% of the total mass of the deep defluorination slurry, disperse it ultrasonically, add a 0.2 mol / L sodium sulfate solution, and react at 250 rpm and 65 ℃ for 1.5 h to prepare calcium sulfate nanocrystals with a particle size of 60~80 nm; mix the remaining slurry and calcium sulfate seed crystals at a mass ratio of 100:2, add a surfactant of cetyltrimethylammonium bromide and polyvinylpyrrolidone at a mass ratio of 1:0.4, and add 0.10%; crystallize at 90 ℃ and 120 rpm for 3 h, and after the reaction is completed, cool to 55 ℃ and age at a constant temperature for 1.5 h to obtain a whisker suspension.

[0025] S5. Product separation and purification: Solid-liquid separation was performed by vacuum filtration, and the product was washed four times with deionized water. The filter cake was placed in an oven and dried at a constant temperature of 108 °C for 2.5 h. After cooling, high-purity calcium sulfate whiskers were obtained. The data of the finished product after testing are summarized in Table 1.

[0026] Example 2 A method for deep defluorination of fluorogypsum to prepare high-purity calcium sulfate whiskers and for fluorine recovery includes the following steps: S1. Mechanical activation pretreatment: Fluorogypsum was pulverized and ground to a particle size of 50 μm, and mechanically activated at 800 rpm for 20 min to obtain activated fluorogypsum.

[0027] S2. Three-stage gradient defluorination Primary physical defluorination: solid-liquid ratio 1:3, reaction temperature 40 ℃, stirring speed 150 rpm, reaction time 60 min; Secondary chemical defluorination: dilute hydrochloric acid concentration 0.5 mol / L, system pH adjusted to 1.5, reaction time 50 ℃, reaction time 80 min; Tertiary deep defluorination: composite fluoride ion trapping agent dosage is 5% of fluorogypsum mass, reaction time 30 ℃, reaction time 40 min.

[0028] S3. Fluorine resource recovery: The waste liquid is concentrated to 1 / 3 of its original volume, crystallized at 5 ℃ for 4 h, and calcium fluoride is recovered.

[0029] S4. Directional whisker growth: Take 5% of the total mass of deep defluorination slurry, add 0.1 mol / L sodium sulfate solution, and react at 60 ℃ for 2 h to prepare seed crystals; the mass ratio of seed crystals to slurry is 100:1, the amount of compound surfactant added is 0.05%, and the surfactant ratio is 1:0.3; the crystallization temperature is 85 ℃ for 4 h, and the reaction is carried out at 50 ℃ for 2 h.

[0030] The remaining process steps and post-processing methods are consistent with those in Example 1. The product test results are shown in Table 1.

[0031] Example 3 A method for deep defluorination of fluorogypsum to prepare high-purity calcium sulfate whiskers and for fluorine recovery includes the following steps: S1. Mechanical activation pretreatment: Fluorogypsum was pulverized and ground to a particle size of 40 μm, and mechanically activated at 1200 rpm for 10 min.

[0032] S2. Three-stage gradient defluorination Primary physical defluorination: solid-liquid ratio 1:5, reaction temperature 50℃, stirring speed 300 rpm for 30 min; Secondary chemical defluorination: dilute hydrochloric acid concentration 1.0 mol / L, system pH adjusted to 2.0, stirring at 60℃ for 250 rpm for 40 min; Tertiary deep defluorination: composite trapping agent dosage 8%, reaction at 40℃ for 20 min.

[0033] S3. Fluorine resource recovery: Concentrate the waste liquid to 1 / 5 of its original volume and crystallize at 15 ℃ for 2 h.

[0034] S4. Directional whisker growth: Take 10% of the total mass of deep defluorination slurry, add 0.3 mol / L sodium sulfate solution, and react at 70 ℃ for 1 h to prepare seed crystals; the mass ratio of seed crystals to slurry is 100:3, the amount of compound surfactant added is 0.15%, and the surfactant ratio is 1:0.5; the crystallization temperature is 95 ℃ for 2 h, and the reaction is carried out at 60 ℃ for 1 h.

[0035] The remaining process steps and post-processing methods are consistent with those in Example 1.

[0036] Example 4 A method for deep defluorination of fluorogypsum to prepare high-purity calcium sulfate whiskers and for fluorine recovery includes the following steps: The overall process flow and basic parameters are set according to Example 1, with only the following parameters slightly adjusted: mechanical activation time 12 min, secondary chemical defluorination reaction time 50 min, directional crystallization reaction time 2.5 h. All other material ratios, reaction temperatures, reagent types and dosages, and fluorine recovery processes are exactly the same as in Example 1.

[0037] Example 5 A method for deep defluorination of fluorogypsum to prepare high-purity calcium sulfate whiskers and for fluorine recovery includes the following steps: The overall process is the same as in Example 1, with only the following parameters slightly adjusted: the solid-liquid ratio of the first-stage physical defluorination is adjusted to 1:3.5, the reaction time of the third-stage deep defluorination is 25 min, the constant temperature aging time is 1.2 h, and the remaining process parameters, reagent system and product preparation process are consistent with those in Example 1.

[0038] Comparative Example 1 (only primary physical defluorination, omitting secondary and tertiary defluorination stages) The mechanical activation pretreatment of raw materials, fluorine resource recovery method, seed crystal preparation, directional whisker growth, and post-treatment process parameters are all consistent with those in Example 1; only the secondary chemical defluorination process and the tertiary deep defluorination process are cancelled, and the activated fluorogypsum is only defluorinated by primary water washing, while the rest of the operations remain completely unchanged.

[0039] Comparative Example 2 The mechanical activation pretreatment of raw materials, fluorine resource recovery method, seed preparation, directional whisker growth, and post-treatment process parameters are all consistent with those in Example 1; only the secondary chemical defluorination process is retained, the primary water washing and tertiary adsorption processes are omitted, and the activated fluorinated gypsum is directly added to dilute hydrochloric acid for acid leaching defluorination, and the rest of the operations remain completely unchanged.

[0040] Comparative Example 3 (first-stage + second-stage defluorination, omitting third-stage deep adsorption defluorination) The process parameters for the mechanical activation pretreatment of raw materials, primary water washing defluorination, and secondary chemical defluorination are all the same as in Example 1. Only the tertiary deep defluorination process is omitted. The slurry after secondary defluorination directly enters the subsequent whisker preparation and fluorine recovery process. All other operations remain completely unchanged.

[0041] Comparative Example 4 (Primary + Tertiary Defluorination, Secondary Chemical Conversion Defluorination omitted) All parameters of the raw material mechanical activation pretreatment, primary physical water washing defluorination, and tertiary deep adsorption defluorination are completely consistent with those of Example 1; only the secondary chemical defluorination process is omitted, and the slurry after primary physical water washing is directly added to the composite fluoride ion scavenger for tertiary adsorption treatment without acid leaching to convert bound fluoride, and the other operating conditions remain unchanged.

[0042] Table 1. Performance Comparison Data Between Example and Comparative Product Example 1 99.52 0.032 34.2 98.7 99.55 96.8 Example 2 99.41 0.048 26.8 97.9 99.51 95.2 Example 3 99.55 0.029 43.5 99.1 99.60 97.5 Example 4 99.52 0.036 31.5 98.3 99.53 96.1 Example 5 99.54 0.041 29.7 98.1 99.52 95.7 Comparative Example 1 97.25 0.356 22.1 68.2 92.30 72.6 Comparative Example 2 98.03 0.272 24.2 75.8 94.25 78.2 Comparative Example 3 99.12 0.118 28.6 92.4 98.15 87.3 Comparative Example 4 98.47 0.214 25.3 81.6 95.72 79.4 Comparative experiments show that a single defluorination process can only remove some forms of fluorine impurities, resulting in a relatively limited overall defluorination depth. The product has high fluorine residue and low purity, failing to meet the requirements of high-end applications. While combining any two-stage defluorination process can improve the purification effect to some extent, it still struggles to achieve deep removal of fluorine impurities, and its overall performance differs from the product specifications of the complete process of this invention. When a three-stage defluorination process—physical leaching, chemical conversion, and deep adsorption—is used in synergistic coordination, efficient removal of fluorine impurities can be achieved step-by-step, significantly improving both defluorination efficiency and product purity. The absence of any one stage prevents the achievement of deep purification. The various process steps of this invention work together synergistically to achieve the technical effects of deep defluorination of fluorogypsum and the directional preparation of high-purity whiskers.

Claims

1. A method for deep defluorination of fluorogypsum to prepare calcium sulfate whiskers and for fluorine recovery, characterized in that, Includes the following steps: S1. Mechanical activation pretreatment: Fluorogypsum is pulverized and then mechanically activated; S2. Three-stage gradient defluorination: Mechanically activated fluorogypsum is subjected to first-stage physical defluorination to obtain first-stage defluorinated slurry and fluorinated washing solution; the first-stage defluorinated slurry is mixed with inorganic acid for second-stage chemical defluorination to obtain second-stage defluorinated slurry and fluorinated acid solution; the second-stage defluorinated slurry is mixed with fluoride ion trapping agent for third-stage deep defluorination to obtain deep defluorinated slurry and fluorinated trapping solution; S3. Fluorine Resource Recovery: Combine the fluorine-containing washing solution, fluorine-containing acid solution and fluorine-containing collection solution produced in step S2, and obtain calcium fluoride product through concentration and cooling crystallization. S4. Directional whisker growth: Take 5-10% of the total mass of deep defluorination slurry to prepare calcium sulfate seed crystals, mix the remaining slurry with the seed crystals, add compound surfactant, and perform directional crystallization reaction at 85-95 ℃ for 2-4 h, then cool to 50-60 ℃ and age for 1-2 h to obtain whisker suspension; S5. Separation and purification: The whisker suspension is separated by centrifugation or vacuum filtration, washed with deionized water 3-5 times, and dried at 100-110 ℃ for 2-3 h to obtain calcium sulfate whiskers.

2. The method as described in claim 1, characterized in that: In step S1, the fluorogypsum is pulverized and ground to a particle size ≤50 μm, and then mechanically activated at 800-1200 rpm for 10-20 min.

3. The method as described in claim 1, characterized in that: The operating conditions for the primary physical defluorination in step S2 are as follows: fluorogypsum and deionized water are mixed at a solid-liquid ratio of 1:3~6, the reaction temperature is 40-50 ℃, the stirring speed is 200-300 rpm, the stirring time is 30-60 min, and the primary defluorination slurry and fluoride-containing washing solution are obtained by filtration.

4. The method as described in claim 1, characterized in that: The secondary chemical defluorination operation in step S2 is as follows: the pH of the system is adjusted to 1.5~2.0 with an inorganic acid of concentration of 0.5-1.0 mol / L, the reaction is carried out at 45-65 ℃ for 40-80 min, the stirring rate is 150-250 rpm, and the secondary defluorination slurry and fluorinated acid solution are obtained by filtration.

5. The method as described in claim 4, characterized in that: The inorganic acid is any one of hydrochloric acid, sulfuric acid, and nitric acid.

6. The method as described in claim 1, characterized in that: In step S3, the tertiary deep defluorination process, the fluoride ion trapping agent is a mixture of hydroxyapatite and alumina in a mass ratio of (1.5~2.5):

1. The amount of trapping agent added is 4%~9% of the mass of fluorogypsum. The reaction temperature is 30-40 ℃, the reaction time is 20-40 min, and the deep defluorination slurry and fluoride-containing trapping liquid are obtained by filtration.

7. The method as described in claim 1, characterized in that: In the preparation of the calcium sulfate seed crystals, a 0.1-0.4 mol / L water-soluble sulfate solution is added to the deep defluorination slurry, and the reaction is carried out at 55-75 ℃ for 1-2 h with a stirring rate of 200-300 rpm. After ultrasonic dispersion, calcium sulfate seed crystals with a particle size of 50-100 nm are obtained. The compound surfactant is composed of a cationic surfactant and a nonionic surfactant in a mass ratio of 1:0.3-0.

6. The amount of the compound surfactant is 0.05%-0.15% of the mass of the deep defluorination slurry. The mass ratio of the calcium sulfate seed crystals to the deep defluorination slurry is 1-4:

100.

8. The calcium sulfate whiskers prepared by the method according to any one of claims 1-7 and the recovered calcium fluoride, characterized in that: The calcium sulfate whiskers have a purity of ≥99.3%, a fluorine content of ≤0.06%, an aspect ratio of 25-45, and a whiteness of ≥90%; the recovered calcium fluoride has a purity of ≥99% and a total fluorine recovery rate of ≥93%.

9. The application of calcium sulfate whiskers as described in claim 8 in the preparation of polymer composite materials, electronic packaging materials, optical plastics, and papermaking.

10. The application of the recovered calcium fluoride as described in claim 8 in fluorochemical raw materials, lithium battery electrolytes, optical glass, and ceramic glazes.

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