Method for preparing battery-grade iron phosphate from titanium dioxide byproduct ferrous sulfate
By combining phosphate buffer decontamination and gradient cooling with a non-magnetic oxidation step, the purity and impurity problems of the titanium dioxide by-product ferrous sulfate heptahydrate were solved, and high-purity ferric phosphate dihydrate was prepared to meet battery material requirements, reduce costs and minimize environmental pollution.
Patent Information
- Application Number
- CN202510767730.2
- Authority / Receiving Office
- CN · China
- Patent Type
- Applications(China)
- Current Assignee / Owner
- Filing Date
- 2025-06-10
- Publication Date
- 2025-09-05
AI Technical Summary
Ferrous sulfate heptahydrate, a by-product of titanium dioxide production, cannot directly meet high value-added applications due to its high impurity content. In addition, the existing purification process is costly and impurities introduce new problems, affecting battery performance and safety.
Phosphate buffer is used for impurity removal, combined with gradient cooling and non-magnetic oxidation steps, and hydrogen peroxide and surfactants are used to prepare high-purity iron phosphate dihydrate through multi-step precise control to avoid the introduction of impurities and magnetic defects.
The preparation of high-purity ferric phosphate dihydrate has been achieved, which reduces costs, improves battery performance and safety, reduces environmental pollution, and improves resource utilization efficiency.
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Abstract
Description
Technical Field
[0001] The invention relates to the field of chemical industry, in particular to a method for preparing battery-grade ferric phosphate from ferrous sulfate produced as a by-product of titanium dioxide. Background Art
[0002] In the chemical industry, the titanium dioxide production process produces a large amount of ferrous sulfate heptahydrate as a byproduct. Containing impurities such as titanyl sulfate, titanium sulfate, aluminum silicate, magnesium sulfate, and manganese sulfate, this byproduct cannot directly meet the needs of high-value-added applications. Instead, it is often used as a feed additive, water treatment agent, and other low-cost treatments. However, much of this byproduct is stockpiled or simply treated, which not only consumes significant land resources but also poses the risk of heavy metal ions escaping to the surface, causing environmental pollution and becoming a pressing challenge for the titanium dioxide industry.
[0003] With the rapid development of the new energy battery industry, the demand for battery-grade materials is growing. Ferric phosphate dihydrate, a key battery material precursor, has a purity and quality that directly impacts battery performance. Purifying ferrous sulfate, a byproduct of titanium dioxide production, and using it as an iron source for the preparation of ferric phosphate would be of great significance and potential application prospects for reducing lithium iron phosphate production costs, increasing the added value of ferrous sulfate products, and achieving comprehensive resource utilization and effective environmental protection.
[0004] On the one hand, in order to make the purity of ferric sulfate dihydrate meet the requirements of battery-grade materials, the traditional process for preparing ferric phosphate dihydrate relies on high-purity and expensive divalent or trivalent iron salts as chemical reagents. This not only increases production costs, but may also face problems with the stability of raw material supply.
[0005] Furthermore, when purifying the existing titanium dioxide byproduct FeSO4·7H2O to produce battery-grade FePO4·2H2O, the deslagging agent used during the impurity removal process introduces new impurity ions, making the resulting FePO4·2H2O purity insufficient to meet the stringent requirements for battery-grade materials, thereby impacting battery performance and safety. Furthermore, the existing production process produces magnetic iron oxide during the oxidation of ferrous sulfate heptahydrate to produce FePO4·2H2O. The lithium iron phosphate material produced using this magnetic iron oxide exhibits reduced electrochemical capacity and poorer conductivity. Summary of the Invention
[0006] The purpose of the present invention is to propose a method for preparing battery-grade ferric phosphate from ferrous sulfate, a by-product of titanium dioxide. The method converts ferrous sulfate heptahydrate, a by-product generated during the production of titanium dioxide, into high-value-added battery-grade ferric phosphate dihydrate. Through an innovative buffer purification process, the problems of inefficient utilization and environmental pollution of ferrous sulfate, a by-product of titanium dioxide, are solved. At the same time, high-purity, high-performance ferric phosphate dihydrate is provided for the field of battery materials, which has significant economic and environmental benefits and technological advancement.
[0007] To achieve the above object, the present invention provides a method for preparing battery-grade ferric phosphate from ferrous sulfate produced as a by-product of titanium dioxide, comprising the following steps: (1) Mixing the titanium dioxide by-product ferrous sulfate heptahydrate with a phosphate buffer solution of pH 2-4, heating and dissolving at 90-110°C for 1-3 hours, filtering to remove impurities, and cooling to 2°C for crystallization to obtain semi-crude ferrous sulfate heptahydrate; (2) dissolving the semi-crude ferrous sulfate heptahydrate obtained in step (1) in a phosphate buffer solution of pH 5.8-8, aging at 50-80° C. for 1.5-3.5 hours, filtering with a suction flask, cooling and crystallizing at 2-5° C. for 1.5-3.5 hours to obtain high-purity ferrous sulfate heptahydrate; (3) The high-purity ferrous sulfate heptahydrate product obtained in step (2) is dissolved in water and heated to 50°C, hydrogen peroxide is added, and the mixture is aged for 2 hours. A surfactant is then added to adjust the temperature to 70-90°C, and concentrated phosphoric acid is then added to adjust the pH to 1-3. The mixture is reacted at 70-90°C for 2-4 hours to generate a pink precipitate. The precipitate is filtered, washed twice with alcohol and twice with distilled water, and then placed in an oven at 160°C for drying for 1.5-3.5 hours to obtain a high-purity yellow-white ferric phosphate dihydrate solid.
[0008] The phosphate buffer in step (1) is prepared by mixing sodium dihydrogen phosphate and disodium hydrogen phosphate in a mass ratio of 1:5 to 1:30 and a pH value of 2.5-3.5.
[0009] In the step (1), the heating and dissolving temperature is 95-105° C., the dissolving time is 1.5-2.5 hours, and after filtering, the mixture is cooled to 2° C. and allowed to stand for crystallization for 2-3 hours.
[0010] The pH value of the phosphate buffer in step (2) is 6.0-7.5, the aging temperature is 60-75°C, the aging time is 2-3 hours, and after filtration, the mixture is gradually cooled to 2-5°C and maintained for 2-3 hours.
[0011] In step (2), the gradient cooling is carried out in two stages. In the first stage, the temperature is cooled to 10-15°C at a rate of 5-10°C / h. After standing for 1 hour, the temperature is cooled to 2-5°C at a rate of 1-3°C / h in the second stage.
[0012] In step (3), the concentration of hydrogen peroxide is 28-32%, the molar ratio of hydrogen peroxide to ferrous sulfate heptahydrate is 1.2:1 to 1.5:1, and the aging temperature is 55-65°C.
[0013] The surfactant in step (3) is cetyltrimethylammonium bromide, and its addition amount is 0.05-0.1% of the total mass of the reaction system. After adjusting the temperature to 80-85°C, concentrated phosphoric acid is added dropwise.
[0014] In step (3), the concentrated phosphoric acid is added at a rate of 1-3 mL / min, the pH is adjusted to 1.5-2.5, and the reaction time is 2.5-3.5 hours.
[0015] In the step (3), after filtering the precipitate, the precipitate is washed twice with ethanol having a volume concentration of 90-95% and deionized water at 60-70°C, respectively. The drying temperature is 160-165°C and the drying time is 2-3 hours. The alkaline solution for adjusting the solution pH to 1-3 is a 32% sodium hydroxide solution.
[0016] The final product, ferric phosphate dihydrate solid, has a purity of ≥99.8%, a magnetic material content of ≤1 ppm, an Fe content of 29.8-30.0%, a P content of 16.9-17.0%, and an Fe:P molar ratio of 0.95-1.0; a tap density of 0.70-0.71 g / cm³, and a particle size distribution D50 of 1-6 μm.
[0017] The preparation principle is as follows:
[0018] The method for preparing battery-grade ferric phosphate from ferrous sulfate produced as a by-product of titanium dioxide disclosed herein has the following beneficial effects: 1) phosphate buffer is used as an impurity removal solvent, and its unreacted phosphate can participate in the subsequent ferric phosphate synthesis, avoiding the introduction of sulfide or ammonia impurities. The content of impurities such as titanium and aluminum in the final product, ferric phosphate dihydrate (FePO4·2H2O), is reduced to <0.1 ppm (Table 8), and the purity is ≥99.5% (Example 1), which is significantly better than similar processes (residual impurities are reduced by more than 90%). 2) Gradient cooling optimizes crystal quality. Through two-stage gradient cooling (10-15℃→2-5℃), the crystallization dynamics is controlled to reduce the inclusion of impurities such as metatitanic acid and magnesium sulfate (Table 5-7). The morphology of ferrous sulfate heptahydrate crystals changes from "fine crystals" to "regular particles" ( Figure 1-3 ), the solution clarity reached 100%, and the crystallization yield was increased to 46.13 g (Example 1), providing high-purity raw materials for the subsequent oxidation reaction; 3) The oxidation process without magnetic defects uses hydrogen peroxide (H2O2) as an oxidant. Under the synergistic action of a surfactant (cetyltrimethylammonium bromide), the complete conversion of Fe²⁺ to Fe³⁺ is achieved. The content of magnetic substances (such as Fe3O4) in the product ferric phosphate dihydrate (FePO4·2H2O) is less than 1 ppm (Table 9-11). This avoids the battery capacity degradation caused by magnetic impurities in traditional processes (the electrochemical capacity is increased by 5-8%), and the conductive performance meets the HG / T4701-2021 standard.
[0019] 4) Precise parameter control and product performance optimization. The process parameters were determined by orthogonal experiments. The Fe:P molar ratio of the obtained FePO4·2H2O was stable at 0.95-1.0 (Example 1), the tap density was 0.70-0.71 g / cm³, and the particle size distribution D50 was 1-6μm ( Figure 14 ), meeting the stringent requirements of lithium battery positive electrode materials on composition uniformity and compaction density.
[0020] 5) Cost reduction: Replacing high-purity iron salts with titanium dioxide by-products reduces raw material costs by more than 30%; 6) Environmental protection and efficiency improvement: tens of thousands of tons of ferrous sulfate heptahydrate can be processed annually, reducing the risk of heavy metal pollution; 7) Simplification of process steps; This invention efficiently converts titanium dioxide byproducts into battery-grade iron phosphate through solvent selection, gradient cooling, and non-magnetic oxidation steps, overcoming the purity bottleneck (≥99.5%) and magnetic defects of traditional processes while reducing costs and environmental impact. The patent covers the core process, with clear technical barriers and significant industrialization prospects and market competitive advantages. The ferric phosphate dihydrate crystals prepared by the present invention have a regular morphology and a tap density of 0.705 g / cm 3 , meeting the compaction requirements of battery materials; particle size distribution D50 is 1-6 μm. BRIEF DESCRIPTION OF THE DRAWINGS
[0021] Figure 1 :a is pH=2, b is pH=3, c is pH=4 ferrous sulfate heptahydrate crystal diagram; Figure 2 : b is the crystallization diagram of ferrous sulfate heptahydrate at T=100℃, d is the crystallization diagram of ferrous sulfate heptahydrate at T=110℃, and e is the crystallization diagram of ferrous sulfate heptahydrate at T=90℃; Figure 3 :b is t=2h, f is t=1h, g is t=3h ferrous sulfate heptahydrate crystal diagram; Figure 4 :h is pH=5.8, i is pH=7, j is the crystal diagram of ferrous sulfate heptahydrate with pH=8; Figure 5 :h is T=50℃, k is T=65℃, L is T=80℃ ferrous sulfate heptahydrate crystal diagram; Figure 6 :L is t=2.5h, m is t=1.5h, n is t=3.5h ferrous sulfate heptahydrate crystal diagram; Figure 7 :o is T=90℃, p is T=80℃, q is T=70℃ iron phosphate dihydrate powder diagram; Figure 8 :o is t=2h, r is t=3h, s is t=4h iron phosphate dihydrate powder diagram; Figure 9 :s is pH=2.0, t is pH=3, u is pH=1.5, v is pH=1 iron phosphate dihydrate powder diagram; Figure 10 :Thermogravimetric diagram of FeSO4·7H2O; Figure 11 :XRD pattern of FeSO4·7H2O Figure 12 : Electron microscopy spectrum of FeSO4·7H2O; Figure 13 : Infrared spectrum of FePO4·2H2O; Figure 14 : FePO4·2H2O particle size test results; Figure 15 : Electron microscopy spectrum of FePO4·2H2O; Figure 16 :FePO4·2H2O EDS element scanning analysis diagram, where is the distribution diagram of each element, and is the total spectrum of the distribution diagram of each element. DETAILED DESCRIPTION Example
[0022] The method for preparing battery-grade ferric phosphate from ferrous sulfate produced as a by-product of titanium dioxide according to the present invention comprises the following steps: 1. Initial purification of titanium dioxide by-product ferrous sulfate heptahydrate FeSO4·7H2O (hydrolysis to remove titanium impurities): Take 70g of crude ferrous sulfate heptahydrate (impurity composition see Table 1, containing titanyl sulfate, titanium sulfate, aluminum silicate and other impurities), add it to a 250ml round-bottom flask, use deionized water as solvent, prepare 100ml of sodium phosphate hydroxide solution with pH=2, add the sodium phosphate hydroxide solution to the round-bottom flask, heat to 90℃ in an oil bath, and stir at constant temperature for 2 hours; after the reaction is completed, filter it while hot to remove the titanic acid precipitate generated by hydrolysis, which mainly contains titanyl sulfate (TiOSO4) and titanium sulfate.
[0023] The filtrate was transferred to a refrigerator at 2°C for cooling and crystallization to obtain semi-crude ferrous sulfate heptahydrate with a crystal mass of about 46.13 g.
[0024] 2. Phosphate buffer deep purification: 1.1 g of sodium dihydrogen phosphate and 0.117 g of sodium dihydrogen phosphate were weighed and dissolved in 80 ml of deionized water, and the volume was adjusted to 100 ml to prepare a phosphate buffer solution with a pH of 5.8. 30 g of the semi-crude ferrous sulfate heptahydrate purified in step 1 was dissolved in 50 ml of the above buffer solution and transferred to a 250 ml round-bottom flask. The solution was heated to 50°C in an oil bath and stirred at this constant temperature for 3.5 hours. After the reaction, the solution was filtered while hot and subjected to gradient cooling crystallization to remove residual impurities. The filtrate was rapidly cooled to 10°C, allowed to stand for 1 hour, and then further cooled to 2°C and maintained for 2 hours. Initially, fine ice crystals appeared, but after 2 hours, a thick layer of regular crystals formed at the bottom of the beaker, resulting in high-purity FeSO4·7H2O crystals.
[0025] 3. Preparation of battery-grade FePO4·2H2O: 30 g of the purified, cooled, and crystallized FeSO₄·7H₂O from step 2 was weighed and dissolved in 150 ml of distilled water to make a 200 g / L solution. The solution was poured into a round-bottom flask and the temperature was controlled at 50°C in an oil bath (to prevent excessive temperature, decomposition of hydrogen peroxide, and incomplete oxidation of ferrous ions). 20 ml of 30% hydrogen peroxide was added and the mixture was aged for 2 h. An appropriate amount of hexadecyltrimethylammonium bromide was added as a surfactant to reduce product agglomeration. The temperature was controlled at 90°C, 24 ml of concentrated phosphoric acid was added, and the pH of the solution was adjusted to 1 with 32% liquid caustic soda. The reaction was controlled at 90°C for 2 h to form a light pink precipitate. The precipitate was filtered, washed twice with alcohol and twice with distilled water, and then dried in an oven at 160°C for 2 h to remove free water from the molecules, yielding a high-purity yellow-white ferric phosphate dihydrate solid.
[0026] 4. Product performance testing: Iron and phosphorus content determination: The iron content (29.61%) was determined by potassium dichromate titration, and the phosphorus content (16.91%) was determined by ammonium phosphomolybdate volumetric method, and the Fe:P molar ratio (≈0.97) was calculated.
[0027] Tap density and particle size analysis: The tap density was measured to be 0.705 g / cm³, and the D50 measured by a laser particle size analyzer was 1-6 μm.
[0028] Impurity detection: ICP testing confirmed that the content of magnetic material is <1ppm, and impurities such as Al, Ti, and Mn are all below battery-grade standards (e.g., Ti <0.1ppm). Example
[0029] The method for preparing battery-grade ferric phosphate from ferrous sulfate produced as a by-product of titanium dioxide according to the present invention comprises the following steps: 1. Initial purification of titanium dioxide by-product ferrous sulfate heptahydrate FeSO4·7H2O (hydrolysis to remove titanium impurities): Take 70g of crude ferrous sulfate heptahydrate (impurity composition see Table 1, containing titanyl sulfate, titanium sulfate, aluminum silicate and other impurities), add it to a 250ml round-bottom flask, use deionized water as solvent, prepare 100ml of sodium phosphate hydroxide solution with pH=3, add the sodium phosphate hydroxide solution to the round-bottom flask, heat to 100℃ in an oil bath, and stir at constant temperature for 2 hours; after the reaction is completed, filter it while hot to remove the titanic acid precipitate generated by hydrolysis, which mainly contains titanyl sulfate (TiOSO4) and titanium sulfate.
[0030] The filtrate was transferred to a refrigerator at 2°C for cooling and crystallization to obtain semi-crude ferrous sulfate heptahydrate with a crystal mass of about 46.13 g.
[0031] 2. Phosphate buffer deep purification: Weigh 0.447 g of sodium dihydrogen phosphate and 0.89 g of sodium dihydrogen phosphate, dissolve them in 80 ml of deionized water, and dilute to 100 ml to prepare a phosphate buffer solution with a pH of 7. Dissolve 30 g of the semi-crude ferrous sulfate heptahydrate purified in step 1 in 50 ml of the above buffer solution and transfer the solution to a 250 ml round-bottom flask. Heat the solution to 50°C in an oil bath and stir at this constant temperature for 3.5 hours. After the reaction, filter the solution while hot and remove any impurities by gradient cooling crystallization. Rapidly cool the filtrate to 10°C, let it stand for 1 hour, and then further cool it to 2°C and maintain the temperature for 2 hours. Initially, fine ice crystals will form, but after 2 hours, a thick layer of regular crystals will form at the bottom of the beaker, resulting in high-purity FeSO4·7H2O crystals.
[0032] 3. Preparation of battery-grade FePO4·2H2O: 30 g of the purified, cooled, and crystallized FeSO₄·7H₂O from step 2 was weighed and dissolved in 150 ml of distilled water to make a 200 g / L solution. The solution was poured into a round-bottom flask and the temperature was controlled at 65°C in an oil bath (to prevent excessive temperature, decomposition of hydrogen peroxide, and incomplete oxidation of ferrous ions). 20 ml of 30% hydrogen peroxide was added and the mixture was aged for 2 h. An appropriate amount of hexadecyltrimethylammonium bromide was added as a surfactant to reduce product agglomeration. The temperature was controlled at 90°C, 24 ml of concentrated phosphoric acid was added, and the pH of the solution was adjusted to 1 with 32% liquid caustic soda. The reaction was maintained at 90°C for 2 h to form a light pink precipitate. The precipitate was filtered, washed twice with alcohol and twice with distilled water, and then dried in an oven at 160°C for 2 h to remove free water from the molecules, yielding a high-purity yellow-white ferric phosphate dihydrate solid. Example
[0033] The method for preparing battery-grade ferric phosphate from ferrous sulfate produced as a by-product of titanium dioxide according to the present invention comprises the following steps: 1. Initial purification of titanium dioxide by-product ferrous sulfate heptahydrate FeSO4·7H2O (hydrolysis to remove titanium impurities): Take 70g of crude ferrous sulfate heptahydrate (impurity composition see Table 1, containing titanyl sulfate, titanium sulfate, aluminum silicate and other impurities), add it to a 250ml round-bottom flask, use deionized water as solvent, prepare 100ml of sodium phosphate hydroxide solution with pH=4, add the sodium phosphate hydroxide solution to the round-bottom flask, heat to 110℃ in an oil bath, and stir at constant temperature for 3 hours; after the reaction is completed, filter it while hot to remove the titanic acid precipitate generated by hydrolysis, which mainly contains titanyl sulfate (TiOSO4) and titanium sulfate.
[0034] The filtrate was transferred to a refrigerator at 2°C for cooling and crystallization to obtain semi-crude ferrous sulfate heptahydrate with a crystal mass of about 46.13 g.
[0035] 2. Phosphate buffer deep purification: Weigh 0.063 g of sodium dihydrogen phosphate and 1.343 g of sodium dihydrogen phosphate in 80 ml of deionized water, and dilute to 100 ml to prepare a phosphate buffer solution with a pH of 8. Dissolve 30 g of the semi-crude ferrous sulfate heptahydrate purified in step 1 in 50 ml of the buffer solution and transfer the solution to a 250 ml round-bottom flask. Heat the solution to 80°C in an oil bath and stir at this constant temperature for 3.5 hours. After the reaction, filter the solution while hot and remove any impurities by gradient cooling crystallization. Rapidly cool the filtrate to 10°C, let it stand for 1 hour, and then further cool it to 2°C and maintain the temperature for 2 hours. Initially, fine ice crystals will form, but after 2 hours, a thick layer of regular crystals will form at the bottom of the beaker, resulting in high-purity FeSO4·7H2O crystals.
[0036] 3. Preparation of battery-grade FePO4·2H2O: 30 g of the purified, cooled, and crystallized FeSO₄·7H₂O from step 2 was weighed and dissolved in 150 ml of distilled water to make a 200 g / L solution. The solution was poured into a round-bottom flask and the temperature was controlled at 50°C in an oil bath (to prevent excessive temperature, decomposition of hydrogen peroxide, and incomplete oxidation of ferrous ions). 20 ml of 30% hydrogen peroxide was added and the mixture was aged for 2 h. An appropriate amount of hexadecyltrimethylammonium bromide was added as a surfactant to reduce product agglomeration. The temperature was controlled at 90°C, 24 ml of concentrated phosphoric acid was added, and the pH of the solution was adjusted to 1 with 32% liquid caustic soda. The reaction was controlled at 90°C for 2 h to form a light pink precipitate. The precipitate was filtered, washed twice with alcohol and twice with distilled water, and then dried in an oven at 160°C for 2 h to remove free water from the molecules, yielding a high-purity yellow-white ferric phosphate dihydrate solid.
[0037] Table 1 Impurity detection results of by-product ferrous sulfate heptahydrate From the test results of the by-product ferrous sulfate heptahydrate sample, it can be seen that the sample mainly contains a large amount of impurities such as titanyl sulfate, titanium sulfate, aluminum silicate, magnesium sulfate, and manganese sulfate.
[0038] Table 2 The results of impurity removal of ferrous sulfate heptahydrate in sodium phosphate and sodium hydroxide solution at the same reaction time and different pH values at the same temperature
[0039] Table 3 is the impurity removal results of ferrous sulfate heptahydrate in sodium phosphate and sodium hydroxide solution at different temperatures under the same reaction time and the same pH value.
[0040] Table 4 is the result of impurity removal of ferrous sulfate heptahydrate in sodium phosphate and sodium hydroxide solution at different reaction times under the same temperature and the same pH value.
[0041] Table 5 shows the results of impurity removal of ferrous sulfate heptahydrate in phosphate buffer solution at different pH values under the same temperature and the same reaction time.
[0042] Table 6 shows the results of impurity removal of ferrous sulfate heptahydrate in phosphate buffer solution at different temperatures under the same pH value and the same reaction time.
[0043] Table 7 is the result of impurity removal of ferrous sulfate heptahydrate in phosphate buffer solution at the same pH value and different reaction times at the same temperature.
[0044] Table 8 is the test results of ferrous sulfate heptahydrate after impurity removal
[0045] Table 9 shows the FePO4·2H2O generation results at different temperatures under the same pH value and the same reaction time.
[0046] Table 10 shows the FePO4·2H2O generation results at different reaction times under the same temperature and pH value.
[0047] Table 11 shows the FePO4·2H2O generation results at different pH values under the same temperature and reaction time.
Claims
1. A method for preparing battery-grade ferric phosphate from ferrous sulfate as a by-product of titanium dioxide, characterized in that: The following steps are involved: (1) Mixing the titanium dioxide by-product ferrous sulfate heptahydrate with a phosphate buffer solution of pH 2-4, heating and dissolving at 90-110°C for 1-3 hours, filtering to remove impurities, and cooling to 2°C for crystallization to obtain semi-crude ferrous sulfate heptahydrate; (2) dissolving the semi-crude ferrous sulfate heptahydrate obtained in step (1) in a phosphate buffer solution of pH 5.8-8, aging at 50-80° C. for 1.5-3.5 hours, filtering with a suction flask, cooling and crystallizing at 2-5° C. for 1.5-3.5 hours to obtain high-purity ferrous sulfate heptahydrate; (3) The high-purity ferrous sulfate heptahydrate product obtained in step (2) is dissolved in water and heated to 50°C, hydrogen peroxide is added, and the mixture is aged for 2 hours. A surfactant is then added to adjust the temperature to 70-90°C, and concentrated phosphoric acid is then added to adjust the pH to 1-3. The mixture is reacted at 70-90°C for 2-4 hours to generate a pink precipitate. The precipitate is filtered, washed twice with alcohol and twice with distilled water, and then placed in an oven at 160°C for drying for 1.5-3.5 hours to obtain a high-purity yellow-white ferric phosphate dihydrate solid.
2. The method according to claim 1, wherein: The phosphate buffer in step (1) is prepared by mixing sodium dihydrogen phosphate and disodium hydrogen phosphate in a mass ratio of 1:5 to 1:30 and a pH value of 2.5-3.
5.
3. The method according to claim 1 or 2, wherein: In the step (1), the heating and dissolving temperature is 95-105° C., the dissolving time is 1.5-2.5 hours, and after filtering, the mixture is cooled to 2° C. and allowed to stand for crystallization for 2-3 hours.
4. The method according to claim 1, wherein: The pH value of the phosphate buffer in step (2) is 6.0-7.5, the aging temperature is 60-75°C, the aging time is 2-3 hours, and after filtration, the mixture is gradually cooled to 2-5°C and maintained for 2-3 hours.
5. The method according to claim 1, wherein: In step (2), the gradient cooling is carried out in two stages. In the first stage, the temperature is cooled to 10-15°C at a rate of 5-10°C / h. After standing for 1 hour, the temperature is cooled to 2-5°C at a rate of 1-3°C / h in the second stage.
6. The method according to claim 1, wherein: In step (3), the concentration of hydrogen peroxide is 28-32%, the molar ratio of hydrogen peroxide to ferrous sulfate heptahydrate is 1.2:1 to 1.5:1, and the aging temperature is 55-65°C.
7. The method according to claim 1, wherein: The surfactant in step (3) is cetyltrimethylammonium bromide, and its addition amount is 0.05-0.1% of the total mass of the reaction system. After adjusting the temperature to 80-85°C, concentrated phosphoric acid is added dropwise.
8. The method according to claim 1, wherein: In step (3), the concentrated phosphoric acid is added at a rate of 1-3 mL / min, the pH is adjusted to 1.5-2.5, and the reaction time is 2.5-3.5 hours.
9. The method according to claim 1, wherein: In step (3), after filtering the precipitate, the precipitate is washed twice with ethanol having a volume concentration of 90-95% and deionized water at 60-70°C, respectively. The drying temperature is 160-165°C and the drying time is 2-3 hours. The alkaline solution for adjusting the solution pH to 1-3 is a 32% sodium hydroxide solution.
10. The method according to claim 1, wherein: The final product, ferric phosphate dihydrate solid, has a purity of ≥99.5%, a magnetic material content of ≤1 ppm, an Fe content of 29.8-30.0%, a P content of 16.9-17.0%, and an Fe:P molar ratio of 0.95-1.0; a tap density of 0.70-0.71 g / cm³, and a particle size distribution D50 of 1-6 μm.