Crystallization fluorine removal method for fluorine-containing wastewater in presence of high-concentration sulfate radicals

Abstract

The invention provides a crystallization defluorination method for fluorine-containing wastewater in the presence of high-concentration sulfate radicals, and belongs to the technical field of water treatment.The crystallization defluorination method comprises the following steps that before defluorination is conducted on fluorine-containing water containing the high-concentration sulfate radicals, lanthanum fluoride crystal seeds, a composite regulator and a defluorination agent lanthanum hydroxide are added into the water, crystallization reaction conditions are controlled, and the fluorine-containing water containing the high-concentration sulfate radicals is obtained. Enabling fluorine ions and lanthanum hydroxide to form a stable fluorine lanthanum compound crystal through directional crystallization; fluorine lanthanum compound crystals are removed through solid-liquid separation, and deep fluorine removal of the water body is achieved. According to the present invention, with the lanthanum hydroxide crystallization method, by using the extremely low solubility product of the fluorine lanthanum compound, the efficient fixation of the fluorine ions can still be achieved under the interference of the high-concentration sulfate radical, the fluorine concentration of the treated water body is stably reduced to less than 10 mg / L, the secondary pollution is avoided, and the defects of poor selectivity and easy saturation in the high sulfate radical environment of the traditional adsorption method are overcome; the method is suitable for treating high-salt fluorine-containing water bodies such as mining area wastewater and metallurgical fluorine-containing wastewater.

Description

Technical Field

[0001] This application belongs to the field of water treatment technology, and more specifically, it relates to a method for deep defluorination of water by utilizing the crystallization of lanthanum hydroxide under high-concentration sulfate interference environment. Background Technology

[0002] With the advancement of industrial production (such as coal chemical industry, non-ferrous metal smelting, and photovoltaic industry) and mineral development, the discharge of fluoride-containing wastewater has increased significantly, and this type of wastewater often contains high concentrations of sulfate (SO4). 2- Fluoride concentrations can reach 10-100 g / L, forming complex high-salt, fluoride-containing water bodies. Excessive fluoride intake can lead to dental fluorosis and skeletal fluorosis in humans. High-fluoride water bodies can also inhibit crop growth and corrode industrial equipment, thus requiring intensive treatment.

[0003] Among existing defluorination technologies, adsorption is widely used due to its simple operation, and lanthanum hydroxide is one of the commonly used adsorbents. However, in high-concentration sulfate environments, SO42-... 2- Will with F - Competition for adsorption sites causes the adsorption capacity of lanthanum hydroxide to drop from over 100 mg / g to below 50 mg / g, and regeneration is difficult after adsorption saturation. Precipitation methods (such as lime precipitation) require the addition of large amounts of reagents, and the resulting fluoride precipitates are small and difficult to settle. In addition, sulfate ions will form calcium sulfate precipitates with calcium ions to encapsulate fluorides, reducing the defluorination efficiency. Although membrane separation can achieve deep defluorination, high concentrations of sulfate ions can easily cause membrane fouling and clogging, resulting in high maintenance costs and making it unsuitable for large-scale treatment.

[0004] Crystallization is a separation technology that controls reaction conditions to induce the formation of stable crystals of target ions, offering advantages such as high selectivity and easy product recovery. However, traditional crystallization for fluoride removal often relies on the formation of crystals such as fluoroaluminates and fluoromagnesiaates. In the presence of high sulfate concentrations, sulfate ions readily react with aluminum and magnesium ions to form stable sulfates, inhibiting fluoride crystallization. Furthermore, the crystallization reaction of lanthanum hydroxide with fluoride ions is easily affected by the pH and ionic strength of the water, making directional crystallization difficult to achieve. Therefore, developing a method for deep fluoride removal through directional crystallization of lanthanum hydroxide in a high-sulfate environment is crucial for solving the complex challenges of treating fluoride-containing water bodies. Summary of the Invention

[0005] The purpose of this application is to overcome the shortcomings of existing technologies in removing fluoride in high-concentration sulfate environments, which are characterized by low efficiency and poor selectivity. This application provides a crystallization method for removing fluoride from fluoride-containing wastewater in the presence of high-concentration sulfate. By optimizing crystallization conditions and adding composite regulators, the method achieves directional crystallization of fluoride ions and lanthanum hydroxide, while suppressing sulfate interference, thus achieving the dual effects of deep fluoride removal and resource recovery.

[0006] To achieve the above objectives, the technical solution adopted in this application is as follows: A method for defluoridation of fluoride-containing wastewater in the presence of high-concentration sulfate ions by crystallization includes the following steps: (1) Filter the fluoride-containing water containing high concentrations of sulfate to remove suspended impurities and adjust the pH of the water to 5.0-7.0; (2) Add lanthanum fluoride crystal seeds, composite regulator and lanthanum hydroxide defluorinating agent to the pretreated fluoride-containing water, control the reaction temperature at 40-60℃ and the stirring rate at 200-300r / min, and carry out the crystallization reaction fully; the composite regulator is a mixture of calcium chloride and boric acid; (3) After the crystallization reaction is completed, the solid and liquid are separated, the precipitated lanthanum fluoride crystals are collected, and the effluent is the water body after deep defluorination.

[0007] Furthermore, in step (1), before filtration, the initial pH value, sulfate concentration, fluoride concentration, and metal ion concentration of the fluoride-containing water are measured.

[0008] Furthermore, in step (1), the fluoride-containing water is filtered through a quartz sand filter column with a pore size of 1.0-2.0 μm to remove suspended impurities, and the filtration flow rate is 3-5 m / h.

[0009] Furthermore, the lanthanum fluoride crystal seeds are prepared by the following method: a 1.0-1.5 mol / L lanthanum nitrate solution and a 2.0-2.5 mol / L sodium fluoride solution are mixed according to the La... 3+ With F - The mixture is stirred at a molar ratio of 1:3.0-3.2 for 1-2 hours at 30-40℃ to form lanthanum fluoride precipitate. The precipitate is then aged in a constant temperature water bath at 50-60℃ for 8-12 hours, centrifuged, and washed with deionized water until neutral. Finally, it is vacuum dried at 70-80℃ for 6-8 hours and ground through a 200-300 mesh sieve to obtain lanthanum fluoride crystal seeds.

[0010] Furthermore, in step (2), the amount of lanthanum fluoride crystal seeds added is 0.5-1.2 g / L based on the volume of fluoride-containing water.

[0011] Furthermore, the composite regulator mentioned in step (2) is a mixture of calcium chloride and boric acid in a mass ratio of 2-3:1. The dosage of the composite regulator is 0.3-0.8 g / L based on the volume of fluoride-containing water.

[0012] Furthermore, in step (2), the dosage of the defluorinating agent lanthanum hydroxide is based on the fluoride concentration in the fluoride-containing water body according to La 3+ With F - Add the product at a molar ratio of 1:3.

[0013] Furthermore, during the crystallization reaction described in step (2), the pH value of the water is monitored in real time using an online pH meter. When the pH value deviates from the range of 6.5-8.0, it is adjusted by adding 0.1-0.5 mol / L hydrochloric acid or sodium hydroxide solution.

[0014] Furthermore, the solid-liquid separation in step (3) is carried out by plate and frame filtration or centrifugation. The centrifugation speed is 8000-10000 r / min and the separation time is 10-15 min.

[0015] Furthermore, a portion of the lanthanum fluoride crystals collected in step (3) can be used as lanthanum fluoride crystal seeds for recycling; or a portion of the collected lanthanum fluoride crystals can be prepared into lanthanum hydroxide defluorinating agent for recycling. The preparation steps are as follows: Lanthanum fluoride crystals are mixed with nitric acid solution with a concentration of 1.5-2.0 mol / L at a solid-liquid ratio of 1:5-1:8 (g / ml), stirred and dissolved at 50-60℃ for 2-3 hours, filtered to remove impurities, and ammonia water is added to adjust the pH value to 8.0-9.0, precipitating lanthanum hydroxide. The precipitate is collected and dried to obtain lanthanum hydroxide defluorinating agent.

[0016] Compared with the prior art, this application has the following technical effects: (1) Breakthrough in both efficiency and stability of fluoride removal under high salinity: For fluoride-containing water with sulfate concentration as high as 10-100 g / L, this application uses lanthanum hydroxide directional crystallization process, combined with lanthanum fluoride crystal seed guidance and composite regulator synergistic effect, to stabilize the fluoride concentration of the treated water to below 10 mg / L, with a fluoride removal rate of over 90%.

[0017] (2) Significantly improved resistance to sulfate interference and crystallization selectivity: Through customized composite regulators (calcium chloride-boric acid system) and precise ratio design, adaptability control for water bodies with different sulfate concentrations is achieved. For different sulfate concentrations of 10-30 g / L, 30-50 g / L, and 50-100 g / L, calcium-boron mass ratios of 2:1, 2.5:1, and 3:1 are used respectively. This weakens the interaction between boric acid and sulfate by forming a stable complex with La. 3+ The binding ability of fluoride ions is enhanced by calcium chloride, which increases the ionic strength of the water and promotes the diffusion of fluoride ions to the crystal seeds. Ultimately, the interference rate of sulfate ions on the crystallization of lanthanum hydroxide is reduced from more than 50% in the traditional process to less than 10%, and the purity of the generated LaF3·xH2O crystals reaches more than 95%. This provides a high-purity raw material basis for subsequent resource recovery and avoids the problem of impurities affecting the value of the product in the traditional process.

[0018] (3) Efficient resource recycling pathway, balancing economic and environmental benefits: The “pretreatment-inert gas protected stepped calcination” process is adopted to directly convert lanthanum fluoride crystals into high-value lanthanum fluoride products. After vacuum drying and impurity removal at 80-100℃ and stepped calcination at 800-900℃, high-purity LaF3 products can be obtained, forming a closed-loop resource cycle of “fluorine removal-recycling-reuse”. Attached Figure Description

[0019] To more clearly illustrate the technical solutions in the embodiments of this application, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of this application. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0020] Figure 1 The X-ray diffraction pattern of the lanthanum fluoride crystal, the crystallization product in Example 1; Figure 2 This is a comparison chart of fluoride removal efficiency at different sulfate concentrations in Example 2; Figure 3 This is a graph showing the effect of different reaction temperatures on the transmittance of ultraviolet light during the crystallization of lanthanum fluoride compounds in Example 2. Detailed Implementation

[0021] The present invention will be further described below with reference to embodiments. Example

[0022] Step 1: Preparation of Lanthanum Fluoride Crystal Seeds: Lanthanum fluoride crystal seeds provide growth sites for the directional crystallization of fluoride ions and lanthanum hydroxide. Their preparation requires strict control of reaction conditions to ensure stable crystal form and uniform particle size. The specific steps are as follows: (1.1) Preparation of lanthanum nitrate solution: Accurately weigh 58.6 g of lanthanum nitrate hexahydrate (La(NO3)3·6H2O) using an electronic analytical balance (accuracy 0.001 g), slowly add it to 100 mL of deionized water, place it on a magnetic stirrer, adjust the stirring speed to 300 r / min, and continue stirring until the lanthanum nitrate hexahydrate is completely dissolved to obtain a clear and transparent lanthanum nitrate solution with a concentration of 1.2 mol / L.

[0023] (1.2) Preparation of lanthanum fluoride precipitate: While maintaining a magnetic stirrer speed of 350 r / min, slowly add a 2.2 mol / L sodium fluoride solution dropwise to the above lanthanum nitrate solution. Control the dropping rate to approximately 1 mL / min to ensure the La precipitate is precipitated. 3+ With F -The molar ratio was maintained at 1:3.1. After the addition was complete, the reaction system was transferred to a constant temperature water bath, the temperature was set at 35°C, and the reaction was stirred for another 1.5 hours, resulting in the formation of a white flocculent lanthanum fluoride precipitate.

[0024] (1.3) Crystal Seed Purification: The generated lanthanum fluoride precipitate, along with the reaction solution, was transferred to a constant temperature water bath and aged at 55℃ for 10 hours to promote crystal growth and crystal form stability. After aging, the mixture was poured into centrifuge tubes and centrifuged at 9000 r / min for 10 minutes using a high-speed centrifuge. The bottom precipitate was collected. The precipitate was repeatedly washed with deionized water, centrifuged (4000 r / min, 5 minutes) after each wash, until the pH of the supernatant reached 7.0 ± 0.2. The washed precipitate was placed in a vacuum drying oven and dried at 80℃ for 7 hours. After drying, the precipitate was ground in a mortar and pestle and then screened through a 250-mesh vibrating sieve to obtain lanthanum fluoride crystal seeds with uniform particle size (200-300 mesh), which were then sealed and stored in a desiccator for later use.

[0025] Step 2: Preparation of the compound regulator The composite regulator consists of calcium chloride and boric acid, which can enhance the ionic strength of water and complex high-concentration sulfate ions to suppress their interference. In this embodiment, the sulfate concentration in the wastewater is 15 g / L (within the 10-30 g / L range), and the mass ratio of calcium chloride to boric acid is selected as 2:1. The specific preparation steps are as follows: (2.1) Reagent weighing: Use an electronic analytical balance to accurately weigh 66.67g of calcium chloride and 33.33g of boric acid to ensure that the mass error of the two components is ≤±0.1% and avoid affecting the sulfate complexation effect due to the ratio deviation.

[0026] (2.2) Stepwise dissolution: First, add the weighed calcium chloride to 500 mL of deionized water, place it on a magnetic stirrer, adjust the speed to 350 r / min, and stir until the calcium chloride is completely dissolved (the solution is clear and transparent, with no white flocculent residue). After the calcium chloride solution has cooled naturally to room temperature (25±2℃), slowly add the weighed boric acid and continue stirring for 25 min. During this period, the water bath temperature can be raised to 32℃ to accelerate the dissolution of boric acid and avoid local clumping, until the boric acid is completely dissolved and a homogeneous and stable colorless solution is formed.

[0027] (2.3) Volumetric storage: Transfer the dissolved compound regulator solution to a 1000mL volumetric flask, dilute to the mark with deionized water, and shake well by inverting 3-5 times. Transfer the solution to a polyethylene container, seal it, and store it at a constant temperature of 20-25℃. The shelf life shall not exceed 7 days to prevent the hydrolysis of boric acid or deterioration of the solution from affecting its effectiveness.

[0028] Step 3: Pretreatment of fluoride-containing water Pretreatment aims to remove suspended impurities from the water and adjust the pH value to create suitable conditions for the subsequent crystallization reaction. The specific steps are as follows: (3.1) Water quality analysis: The concentration of sulfate (15 g / L) and fluoride (80 mg / L) in the wastewater were determined by ion chromatography. The initial pH value was determined to be 6.5 by an online pH meter. The concentration of metal ions was determined by atomic absorption spectrometry. The basic water quality data were recorded.

[0029] (3.2) Filtration and impurity removal: Fluoride-containing wastewater is passed through a quartz sand filter column (inner diameter 50mm, height 500mm, filled with quartz sand with a particle size of 0.8mm), and the filtration flow rate is controlled at 4m / h by a peristaltic pump. During the filtration process, the pressure difference between the inlet and outlet of the filter column is recorded every 30 minutes. When the pressure difference exceeds 0.1MPa, backwashing is used (backwash flow rate 8m / h, backwash time 10min) to restore the permeability of the filter column, ensuring that the removal rate of suspended impurities (such as silt and flocs) is ≥90%, and avoiding impurities adhering to the surface of the crystal seeds and affecting the crystallization reaction.

[0030] (3.3) Initial pH adjustment: Since the initial pH of the wastewater is 6.5, no further pH adjustment is required.

[0031] Step 4: Defluorination and solid-liquid separation of lanthanum hydroxide crystallization By adding crystal seeds, a composite regulator, and lanthanum hydroxide (a defluorinating agent), and controlling the reaction conditions, fluoride ions are directionally crystallized. The treated water is then obtained through solid-liquid separation. The specific steps are as follows: (4.1) Reagent addition: Take 1L of pretreated fluoride-containing wastewater and put it into a 5000mL three-necked flask. Add 0.8g of lanthanum hydroxide crystal seeds and 0.5g of composite regulator to it. Stir gently with a glass rod to disperse the reagents evenly. Prepare a 0.1mol / L solution of lanthanum hydroxide and add it slowly.

[0032] (4.2) Adjustment of reaction conditions: Place the three-necked flask in a constant temperature water bath, turn on the magnetic stirrer and adjust the stirring speed to 250 r / min; set the water bath temperature to 50℃, and after the water temperature stabilizes, adjust the pH value of the wastewater to 7.2 with 0.3 mol / L sodium hydroxide solution, and monitor the parameters in real time through an online pH meter and temperature sensor. Adjust the pH value in time when it deviates from the range of 6.5-8.0.

[0033] (4.3) Directional crystallization reaction: The stirring rate was maintained at 250 r / min, the temperature at 50 ± 1℃, and the pH at 7.2 ± 0.1 for 3 h. During the reaction, fluoride ions and lanthanum hydroxide grew directionally on the surface of the crystal seed, forming LaF3·xH2O crystals. The fluoride ion concentration was measured every 30 min to ensure the stability of the reaction process. The extremely low solubility product of lanthanum fluoride compounds (Ksp≈10) was utilized. -29 This enables efficient fixation of fluoride ions.

[0034] (4.4) Solid-liquid separation: After the reaction is complete, turn off the stirring and water bath heating, and centrifuge the reaction mixture at a speed of 4000-5000 r / min for 10-15 min. Collect the filtrate, and determine the fluoride concentration in the filtrate using ion chromatography. The concentration is 5.23 mg / L, which meets the national water discharge standard. The fluoride removal rate is calculated to be 93.46%. Collect the filter cake (lanthanum fluoride compound crystals) for subsequent recovery.

[0035] Step 5: Recycling of Lanthanum Fluoride Compound Crystals Lanthanum fluoride compound crystals are converted into recyclable lanthanum hydroxide or high-value lanthanum fluoride products through washing and high-temperature calcination, forming a closed-loop resource recycling system. The specific steps are as follows: (5.1) Crystal pretreatment: The lanthanum fluoride compound crystals obtained by centrifugation were taken out and washed three times with deionized water, centrifuged each time, and then placed in a vacuum drying oven at 90℃ for 5 hours. After drying, the crystals were crushed with a grinder and screened through a 150-mesh vibrating sieve to obtain dry and pure lanthanum fluoride compound crystal powder. A portion of the powder was reserved as lanthanum fluoride crystal seeds for recycling.

[0036] (5.2) Lanthanum hydroxide recycling preparation: Take 10g of lanthanum fluoride compound crystal powder, add 60mL of 1.8mol / L nitric acid solution (solid-liquid ratio 1:6), place in a 55℃ constant temperature water bath, and stir to dissolve for 2.5h at a stirring rate of 200r / min. After dissolution, filter with qualitative filter paper to remove insoluble impurities, slowly add ammonia water to the filtrate while monitoring the pH value. When the pH value reaches 8.5, stop adding ammonia water, and lanthanum hydroxide precipitate. Centrifuge the precipitate (5000r / min, 8min), wash with deionized water until neutral (pH=7.0±0.2), and dry to obtain lanthanum hydroxide as a defluorinating agent for the next defluorination.

[0037] (5.3) Purification of Lanthanum Fluoride: Take 20g of lanthanum fluoride compound crystal powder and add it to a crucible. Place the crucible in a muffle furnace with a nitrogen protection device. Turn on the muffle furnace and set the heating program: In the first stage, heat from room temperature to 300℃ at a heating rate of 8℃ / min and hold for 1h to remove residual moisture and volatile impurities; in the second stage, continue heating to 850℃ at a heating rate of 12℃ / min and hold for 2.5h to completely decompose the lanthanum fluoride compound (LaF3·xH2O) into pure lanthanum fluoride (LaF3) (reaction formula: LaF3·xH2O → LaF3 + xH2O↑). After calcination, turn off the muffle furnace and allow it to cool naturally to room temperature under nitrogen protection. Take out the pure lanthanum fluoride for X-ray diffraction analysis. The XRD pattern is shown below. Figure 1 As shown, X-ray diffraction (XRD) analysis confirmed that the purity reached 96.8%, which can be used as a rare earth resource for subsequent industrial production, achieving resource recycling without secondary pollution.

[0038] In summary, this embodiment uses fluoride-containing wastewater from a non-ferrous metal smelter as the treatment target. The wastewater has a sulfate concentration of 15 g / L, an initial fluoride ion concentration of 80 mg / L, and a pH value of 6.0-7.2. A deep defluorination process based on lanthanum hydroxide crystallization is adopted to achieve deep defluorination of the water and recovery of lanthanum fluoride compound resources. The comparison of water quality indicators and key parameters before and after treatment in this embodiment is shown in Table 1: Table 1 Comparison of water quality indicators and key parameters before and after treatment

[0039] As shown in Table 1, the process in this embodiment effectively weakens the interference of sulfate on lanthanum hydroxide crystallization under an environment of sulfate concentration of 15 g / L, achieving deep removal of fluoride ions (the fluoride concentration after treatment is far below the national emission standard), and the lanthanum fluoride compound can be efficiently recovered and recycled without secondary pollution. It overcomes the defects of poor selectivity and easy saturation of traditional adsorption methods in this type of water body, and is suitable for the treatment of high-salt fluoride-containing water bodies. Example

[0040] In this embodiment, fluoride-containing wastewater with different sulfate concentrations was simulated to demonstrate that the fluoride removal method provided in this application has good fluoride removal effect under different sulfate concentrations. Two comparative examples were also set up: Comparative Example 1 added only lanthanum fluoride crystal seeds and the fluoride removal agent lanthanum hydroxide, without adding the composite regulator; Comparative Example 2 added only the composite regulator and the fluoride removal agent lanthanum hydroxide, without adding lanthanum fluoride crystal seeds.

[0041] Test results are as follows Figure 2 As shown, Figure 2As can be seen, the combination of lanthanum fluoride crystal seeds and composite regulators exhibits excellent defluorination effects at different sulfate concentrations, with defluorination rates exceeding 98% (it should be noted that in this embodiment, the treated object is simulated fluoride-containing wastewater prepared in the laboratory, which has a higher defluorination rate than the highly complex industrial fluoride-containing wastewater in Example 1). Furthermore, a horizontal comparison between Comparative Example 1 and Comparative Example 2 shows that the combined use of lanthanum fluoride crystal seeds and composite regulators in this application significantly improves defluorination efficiency compared to using either alone.

[0042] This embodiment also included an experiment to investigate the effect of different reaction temperatures on the transmittance of ultraviolet light during the crystallization of lanthanum fluoride compounds. The results are as follows: Figure 3 As shown, from Figure 3 As can be seen, the higher the temperature, the faster the crystallization rate, but the final result is roughly the same.

Claims

1. A method for removing fluorine by crystallization from fluorine-containing wastewater in the presence of high concentrations of sulfate ions, characterized in that, The method comprises the following steps: (1) filtering the fluorine-containing water body with high concentration of sulfate to remove suspended impurities, and adjusting the pH value of the water body to 5.0-7.0; (2) adding lanthanum fluoride crystal seeds, a composite regulator and a defluorination agent lanthanum hydroxide into the pretreated fluorine-containing water body, controlling the reaction temperature to be 40-60℃, the stirring rate to be 200-300r / min, and fully performing the crystallization reaction; the composite regulator is a mixture of calcium chloride and boric acid; (3) after the crystallization reaction is completed, solid-liquid separation is performed, and the precipitated lanthanum fluoride crystals are collected, and the effluent is the water body after deep defluorination.

2. The method for removing fluorine by crystallization in the presence of high-concentration sulfate radicals according to claim 1, characterized in that: In step (1), the initial pH value, the concentration of sulfate, the concentration of fluorine and the concentration of metal ions of the fluorine-containing water body are measured before filtration.

3. The method for removing fluorine by crystallization in the presence of high-concentration sulfate radicals according to claim 1, characterized in that: In step (1), the fluorine-containing water body is filtered through a quartz sand filter column with a pore size of 1.0-2.0μm to remove suspended impurities, and the filtration flow rate is 3-5m / h.

4. The method for removing fluorine by crystallization in the presence of high-concentration sulfate radicals according to claim 1, characterized in that: The lanthanum fluoride crystal seed is prepared by mixing a lanthanum nitrate solution with a concentration of 1.0-1.5 mol / L and a sodium fluoride solution with a concentration of 2.0-2.5 mol / L according to La 3+ F - at a molar ratio of 1:3.0-3.2, stirring and reacting at 30-40°C for 1-2 h to form a lanthanum fluoride precipitate; then placing the precipitate in a constant-temperature water bath at 50-60°C for aging for 8-12 h, washing with deionized water until neutral after centrifugal separation; and finally vacuum drying at 70-80°C for 6-8 h and grinding through a 200-300 mesh sieve to obtain the lanthanum fluoride crystal seed.

5. The method for removing fluorine by crystallization in the presence of high-concentration sulfate radicals according to claim 1, characterized in that: In step (2), the adding amount of the lanthanum fluoride crystal seeds is 0.5-1.2g / L based on the volume of the fluorine-containing water body.

6. The method for removing fluorine by crystallization in the presence of high-concentration sulfate radicals according to claim 1, characterized in that: In step (2), the composite regulator is prepared by mixing calcium chloride and boric acid at a mass ratio of 2-3:1, and the adding amount of the composite regulator is 0.3-0.8g / L based on the volume of the fluorine-containing water body.

7. The method for removing fluorine by crystallization in the presence of high-concentration sulfate radicals according to claim 1, characterized in that: The dosage of the defluorination agent lanthanum hydroxide in step (2) is La 3+ F - The dosage is carried out according to the molar ratio 1:

3.

8. The method for removing fluorine by crystallization in the presence of high-concentration sulfate radicals according to claim 1, characterized in that: In step (2), the pH value of the water body is monitored in real time by using an online pH meter during the crystallization reaction process, and when the pH value deviates from the range of 6.5-8.0, 0.1-0.5mol / L hydrochloric acid or sodium hydroxide solution is added dropwise for adjustment.

9. The method for removing fluorine by crystallization in the presence of high-concentration sulfate radicals according to claim 1, characterized in that: In step (3), the solid-liquid separation is performed by using a plate and frame filter press or centrifugal separation, and when the centrifugal separation is performed, the rotating speed is 8000-10000r / min, and the separation time is 10-15min.

10. The method for removing fluorine by crystallization in the presence of high-concentration sulfate radicals according to claim 1, characterized in that: Part of the collected lanthanum fluoride crystals is used as the lanthanum fluoride crystal seeds for recycling; or part of the collected lanthanum fluoride crystals is prepared into the defluorination agent lanthanum hydroxide for recycling, and the preparation steps are as follows: the lanthanum fluoride crystals are mixed with a nitric acid solution with a concentration of 1.5-2.0mol / L at a solid-liquid ratio of 1:5-1:8 (g / ml), stirred and dissolved at 50-60℃ for 2-3h, after the impurities are removed, ammonia water is added dropwise to adjust the pH value to 8.0-9.0, and the lanthanum hydroxide precipitate is separated and dried to obtain the defluorination agent lanthanum hydroxide.

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