PROCESS FOR PREPARING LOW-SULFUR NANOMETRIC FERRIC PHOSPHATE

Abstract

Disclosed in the present invention is a preparation method for nano ferric phosphate with low sulphur content: first mixing a phosphorus source and an iron source to obtain a raw material solution, then adding an alkali and a surfactant, adjusting the pH, and stirring and reacting to obtain a ferric phosphate dihydrate slurry; adding phosphoric acid solution to the ferric phosphate dihydrate slurry, adjusting the pH, heating and stirring to implement ageing, and filtering to obtain ferric phosphate dihydrate; adding water to the ferric phosphate dihydrate to make slurry and grinding to obtain a ground slurry; adding the ground slurry to a washing solution for washing, performing solid-liquid separation, and taking the solid phase for calcining to obtain the nano ferric phosphate with low sulphur content. In the present invention, the grinding process in the lithium ferric phosphate synthesis process is preliminarily prepared, so that sulphate radicals wrapped therein can be better dissolved and removed in the washing water, greatly reducing the amount of washing water; when calcined directly without drying, the ferric phosphate dihydrate filter residue loses free water and crystallised water, leaving pores in the particles, which provides favourable conditions for the diffusion and removal of SO2.

Description

Preparation method of low-sulfur nano-ferric phosphate Technical Field

[0001] The present invention belongs to the technical field of lithium ion batteries, and in particular relates to a method for preparing low-sulfur nano-iron phosphate. Background Art

[0002] With the rapid development of the new energy industry, lithium-ion batteries, as a new, green energy source, are widely used in automotive power batteries, electrochemical energy storage, 3C product batteries, and other fields. Among them, lithium iron phosphate batteries occupy a large market share due to their excellent cycle performance, safety, low price, and environmental and pollution-free characteristics. With the widespread adoption of new energy vehicles, their demand is rapidly growing.

[0003] At present, the main methods for preparing lithium iron phosphate positive electrode materials include high-temperature solid-phase method, carbon thermal reduction method, sol-gel method, co-precipitation method, hydrothermal method, etc. Among them, the carbon thermal reduction method has a stable process, low cost, and easy control, and has become the mainstream industrial preparation method. As a key raw material, the structure, performance and quality of iron phosphate have a very large impact on the electrical properties of the finished lithium iron phosphate. At present, the main method for preparing iron phosphate is to use ferrous sulfate as raw material and prepare it through controlled crystallization method. The product contains a large amount of impurities, which are difficult to remove during the subsequent calcination to synthesize lithium iron phosphate, and have a great impact on the electrical performance of lithium iron phosphate batteries. This has greatly affected the application of iron phosphate in battery materials. Among them, the influence of impurity sulfur is the greatest. Luo Yanhua et al. found that when the sulfur mass fraction reaches a certain level, the influence on the particle morphology, discharge capacity and cycle performance of lithium iron phosphate gradually becomes obvious. When the sulfur mass fraction is lower than 0.22%, the lithium iron phosphate particle morphology is spherical, and the 1C first discharge capacity reaches 152mAh / g. After 150 cycles, the capacity can still be maintained at 140mAh / g, and the electrochemical performance is good; when the sulfur mass fraction is higher than 0.34%, the lithium iron phosphate particles agglomerate, and the 1C first discharge capacity is lower than 130mAh / g. After 150 cycles, the capacity is lower than 107mAh / g.

[0004] Under the existing production process, it is difficult to remove sulfate from iron phosphate, and a large amount of washing water is often required to remove the impurities. However, with the development of technology, the crystal structure of iron phosphate has gradually become nano-sized, and the surface and interior of the synthesized iron phosphate material particles contain a large amount of SO4 2- The current conventional washing process has a great impact on the surface adsorbed SO4 2- It has a certain effect, but it has a certain effect on the SO4 contained in the particles. 2- The removal effect is poor, which seriously affects the electrochemical properties of the lithium iron phosphate positive electrode material prepared therefrom on the one hand, and on the other hand requires the use of a large amount of washing water, greatly increasing the production cost and environmental burden.

[0005] Currently, the main methods used in industry to control the sulfur content in ferric phosphate include controlling the pH of the synthesis process, multi-stage washing, citric acid washing, and long calcination. These methods often affect the product's properties such as tap density, reactivity, and surface morphology, or use large amounts of wash water and high-cost citric acid, which is highly polluting and places great pressure on subsequent wastewater treatment. During the calcination and crystallization of ferric phosphate dihydrate into ferric phosphate, residual sulfate ions in the particles are often released as SO2 at high temperatures. The existing process has low calcination temperatures and long calcination times, which is not effective in removing the sulfur element and also causes the primary ferric phosphate particles to melt, resulting in a decrease in reactivity. Therefore, developing a method for reducing sulfur content with excellent results, low cost, and low environmental impact is of great significance for optimizing the synthesis process of battery-grade nano-ferric phosphate and improving its product performance.

[0006] Summary of the Invention

[0007] The present invention aims to solve at least one of the technical problems existing in the above-mentioned prior art. To this end, the present invention provides a method for preparing nano-ferric phosphate with low sulfur content.

[0008] According to one aspect of the present invention, a method for preparing low-sulfur nano-ferric phosphate is proposed, comprising the following steps:

[0009] S1: mixing a phosphorus source and an iron source to obtain a raw material solution, adding an alkali and a surfactant, adjusting the pH, and stirring the mixture to obtain a ferric phosphate dihydrate slurry;

[0010] S2: adding phosphoric acid solution to the ferric phosphate dihydrate slurry, adjusting the pH, heating and stirring for aging, and filtering to obtain ferric phosphate dihydrate;

[0011] S3: adding water to ferric phosphate dihydrate to prepare a slurry, and grinding the slurry to obtain a ground slurry;

[0012] S4: adding a washing liquid to the ground slurry for washing, separating the solid and the liquid, and calcining the solid phase to obtain nano-iron phosphate with low sulfur content.

[0013] In some embodiments of the present invention, in step S1, the iron source is ferrous sulfate, and an oxidant is further added to the raw material solution; the molar ratio of iron to phosphorus in the raw material solution is 1:(0.9-1.1).

[0014] In some embodiments of the present invention, in step S1, the phosphorus source is phosphoric acid.

[0015] In some preferred embodiments of the present invention, the oxidant is H2O2. 2+ Oxidized to Fe 3+ .

[0016] In some embodiments of the present invention, in step S1, the base is at least one of sodium hydroxide, potassium hydroxide, sodium carbonate, sodium bicarbonate, potassium carbonate, ammonia water or ammonium salt; preferably, the base is sodium hydroxide.

[0017] In some embodiments of the present invention, in step S1, the surfactant is at least one of sodium stearate, polyvinyl pyrrolidone, sodium lauryl sulfonate, dodecylphenol polyoxyethylene ether, cetyltrimethylammonium bromide or cetyltrimethylammonium chloride.

[0018] In some embodiments of the present invention, in step S1, the surfactant is a compound of polyvinyl pyrrolidone and sodium dodecyl sulfate in a mass ratio of approximately 1:1. During the synthesis of battery-grade nanoferric phosphate, a certain amount of surfactant is generally added to control the nanoparticle size. The present invention innovatively optimizes the surfactant ratio, effectively reducing sulfate attachment within the particles while ensuring the synthesis of ferric phosphate, thereby lowering the sulfur content of the final product.

[0019] In some embodiments of the present invention, in step S1, the pH is 1.0-2.5.

[0020] In some embodiments of the present invention, in step S1, the mass ratio of the raw material liquid to the alkali solution is 1:(0.1-0.3).

[0021] In some embodiments of the present invention, in step S1, the stirring speed is 100-800 rpm; the reaction temperature is 20-60° C., and the reaction time is 0.5-5 h.

[0022] In some embodiments of the present invention, in step S1, the surfactant and water are prepared into a surfactant mixture with a mass concentration of 10-40%, and the mass ratio of the raw material liquid to the surfactant mixture is 1:(0.004-0.04).

[0023] In some embodiments of the present invention, in step S2, the mass concentration of the phosphoric acid solution is 60-80%; and the pH is 1.0-2.0.

[0024] In some embodiments of the present invention, in step S2, the stirring speed of aging is 50-300 rpm, the aging temperature is 60-100° C., and the aging time is 1-5 h.

[0025] In some embodiments of the present invention, in step S2, the particle size of the obtained ferric phosphate dihydrate is 8-20 μm.

[0026] In some embodiments of the present invention, in step S3, the particle size of the dispersed phase of the ground slurry is 2.5-10 μm. The particle size of wet grinding needs to be adjusted according to the requirements of the subsequent lithium iron phosphate synthesis process.

[0027] In some embodiments of the present invention, in step S3, the mass ratio of ferric phosphate dihydrate to water is 1:(1-4).

[0028] In some embodiments of the present invention, in step S4, the washing is performed twice.

[0029] In some embodiments of the present invention, in step S4, the washing liquid is one of water or a 0.5-2% sodium carbonate solution; preferably, the water is hot water at 60-90°C. In the selection of the washing liquid, in addition to conventional deionized water, the present invention also innovatively uses hot pure water, which can reduce the viscosity of the slurry and improve the washing effect. Sodium carbonate solution is also innovatively selected. Sodium carbonate has a good reaction effect with sulfate and is easy to remove. At the same time, the wastewater is easy to treat, which can further reduce the content of impurity sulfur in iron phosphate. The choice of washing liquid can be selected according to production cost control and product performance requirements.

[0030] In some embodiments of the present invention, in step S4, the mass ratio of the washing liquid to ferric phosphate dihydrate is (5-20):1.

[0031] In some embodiments of the present invention, in step S4, the water content of the filter residue is 15-30%.

[0032] In some embodiments of the present invention, in step S4, the calcination temperature is 450-800° C., the calcination time is 0.5-5 h, and the heating rate is 2-10° C. / min.

[0033] In some preferred embodiments of the present invention, in step S4, the calcination temperature is 600-800°C, and the calcination time is 0.5-3 hours. A rapid, high-temperature, short-cycle calcination method, based on thermodynamic calculations and experimental results, is employed to raise the calcination temperature to above 600°C and control the calcination time to less than 3 hours. This effectively removes residual sulfate in the ferric phosphate particles while maintaining the chemical properties of the product.

[0034] In some embodiments of the present invention, in step S4, compressed air is introduced during the calcination. The introduction of pure compressed air can accelerate the removal of the S element.

[0035] According to a preferred embodiment of the present invention, there are at least the following beneficial effects:

[0036] 1. The present invention innovatively pre-processes the grinding process in the lithium iron phosphate synthesis process, adopts wet grinding to reduce the particle size of the dihydrated iron phosphate, and breaks up the agglomeration of its secondary particles, so that the sulfate ions contained therein can be better dissolved in the washing water and then removed, thereby greatly reducing the amount of washing water.

[0037] 2. This invention uses direct calcination without drying. As the ferric phosphate dihydrate residue loses free water and water of crystallization, pores are formed within the particles, creating favorable conditions for the diffusion and release of SO2. The resulting finished product has a sulfur content of less than 0.01%, meeting the national standard for battery-grade nano-ferric phosphate products.

[0038] 3. The process of the present invention is simple, low-cost, stable, and easy to industrialize on a large scale. The traditional carbothermal reduction method for preparing lithium iron phosphate requires wet grinding to reduce the particle size of the raw materials, improve the uniformity of the dispersion of the raw materials, and then spray drying. The present invention advances the subsequent wet grinding process to the preparation process of iron phosphate, combining the front and back steps and optimizing the process steps. This not only provides good guidance for the process optimization of battery-grade nano-iron phosphate, but also has a certain reference value for the preparation process of other related products. BRIEF DESCRIPTION OF THE DRAWINGS

[0039] The present invention will be further described below with reference to the accompanying drawings and embodiments, in which:

[0040] FIG1 is a process flow chart of Example 1 of the present invention;

[0041] FIG2 is a SEM image of nano-iron phosphate prepared in Example 1 of the present invention;

[0042] FIG3 is an XRD comparison diagram of the nano-iron phosphate prepared in Example 1 of the present invention and Comparative Example 1. DETAILED DESCRIPTION

[0043] The following will clearly and completely describe the concept and technical effects of the present invention in conjunction with the embodiments to fully understand the purpose, features and effects of the present invention. Obviously, the embodiments described are only part of the embodiments of the present invention, not all of them. Based on the embodiments of the present invention, other embodiments obtained by those skilled in the art without creative work are all within the scope of protection of the present invention.

[0044] Example 1

[0045] This embodiment prepares a low-sulfur nano-ferric phosphate. Referring to the process flow chart of FIG1 , the specific process is as follows:

[0046] (1) adding ferrous sulfate stock solution, excess oxidant H2O2 and phosphoric acid solution into a stirred tank in sequence, and stirring thoroughly to obtain a raw material solution having a P / Fe molar ratio of 1.05:1, and separately preparing a sodium hydroxide solution having a concentration of 15%, and separately preparing a surfactant mixture having a concentration of 25% and a polyvinyl pyrrolidone and sodium lauryl sulfate mass ratio of 1:1;

[0047] (2) slowly adding sodium hydroxide solution to the raw material solution, and simultaneously adding a surfactant mixture of 2% by weight of the raw material solution, strictly controlling the addition rate, adjusting the pH value to between 1.7 and 1.9, maintaining a speed of 200 rpm and stirring sufficiently to react, to obtain a ferric phosphate dihydrate slurry;

[0048] (3) adding a certain amount of 70% phosphoric acid solution to the ferric phosphate dihydrate slurry, adjusting the pH value to 1.3-1.6, heating to 85°C, maintaining a speed of 100 rpm, and stirring for about 5 hours to carry out an aging reaction. The reaction is terminated when the product particle size is controlled to be 8-20 μm, and filtering to obtain a ferric phosphate dihydrate residue;

[0049] (4) The ferric phosphate dihydrate filter residue and deionized water were mixed in a mass ratio of 1:1, and the slurry was ground using a sand mill to a D50 of 3 μm to obtain a ground slurry. The above slurry was added to deionized water with a mass of 20 times that of the ferric phosphate dihydrate filter residue, stirred and washed for 30 minutes, and filtered after washing to obtain a filter cake 1 with a water content of about 20%;

[0050] (5) The filter cake was added to deionized water with a mass 20 times that of the ferric phosphate dihydrate filter residue and stirred and washed again for 30 minutes. After washing, the filter cake was filtered again to obtain a filter cake 2 with a water content of about 20%;

[0051] (6) The filter cake 2 was directly placed in the pot and then calcined at 700℃ for 1h at a heating rate of 8℃ / min. 10Nm 3 / h of pure compressed air, the calcined material is ground, crushed and sieved to obtain a battery-grade nano-iron phosphate product with low impurity content and excellent performance.

[0052] The ICP test results show that the S content in the ferric phosphate dihydrate filter residue obtained in this example is 0.3564%.

[0053] Figure 2 is an SEM image of the nano-iron phosphate prepared in this embodiment. It can be clearly seen from the SEM image that the synthesized battery-grade nano-iron phosphate has changed from conventional secondary particle agglomerates to relatively loose primary particles with disordered distribution, which is closer to the state of iron phosphate in the subsequent wet grinding and coarse grinding process. The secondary particle agglomerates are opened to help the impurity sulfur element attached inside to escape.

[0054] Figure 3 is the XRD pattern of the nano-iron phosphate prepared in Example 1 of the present invention and Comparative Example 1. It can be clearly seen from the XRD pattern that the iron phosphate obtained in Comparative Example 1 using the conventional low-temperature, long-cycle calcination and dehydration method is in an amorphous state, while the iron phosphate obtained by the rapid, high-temperature, short-cycle calcination method used in Example 1 has good crystallinity, sharp characteristic peaks, and a pure crystal phase structure free of impurities.

[0055] Table 1 shows the particle size distribution of the ferric phosphate product obtained in this example.

[0056] Table 1

[0057] D10D50D90D99Particle size (μm)0.662.4712.7225.90

[0058] Example 2

[0059] This example prepares a low-sulfur nano-ferric phosphate, and the specific process is as follows:

[0060] (1) adding ferrous sulfate stock solution, excess oxidant H2O2 and phosphoric acid solution into a stirred tank in sequence, and stirring thoroughly to obtain a raw material solution having a P / Fe molar ratio of 1.05:1, and separately preparing a sodium hydroxide solution having a concentration of 15%, and separately preparing a surfactant mixture having a concentration of 25% and a mass ratio of polyvinyl pyrrolidone to sodium lauryl sulfate of 1:1;

[0061] (2) slowly adding sodium hydroxide solution to the raw material solution, and simultaneously adding a surfactant mixture of 2% by weight of the raw material solution, strictly controlling the addition rate, adjusting the pH value to between 1.7 and 1.9, maintaining a speed of 200 rpm and stirring sufficiently to react, to obtain a ferric phosphate dihydrate slurry;

[0062] (3) adding a certain amount of 70% phosphoric acid solution to the ferric phosphate dihydrate slurry, adjusting the pH value to 1.3-1.6, heating to 85°C, maintaining a speed of 100 rpm, and stirring for about 5 hours to carry out an aging reaction. The reaction is terminated when the product particle size is controlled to be 8-20 μm, and filtering to obtain a ferric phosphate dihydrate residue;

[0063] (4) The ferric phosphate dihydrate filter residue was mixed with deionized water in a mass ratio of 1:1, and the slurry was ground using a sand mill to a D50 of 5 μm to obtain a ground slurry. The above slurry was added into 70° C. pure water with a mass of 15 times that of the ferric phosphate dihydrate filter residue, stirred and washed for 30 minutes, and filtered after washing to obtain a filter cake 1 with a water content of about 20%;

[0064] (5) The filter cake was added into 70°C pure water with a mass 15 times that of the ferric phosphate dihydrate filter residue and stirred and washed again for 30 min. After washing, the filter cake was filtered again to obtain a filter cake 2 with a water content of about 20%;

[0065] (6) The filter cake 2 was directly placed in the pot and then calcined at 600℃ for 1.5h at a heating rate of 8℃ / min. 10Nm 3 / h of pure compressed air, the calcined material is ground, crushed and sieved to obtain a battery-grade nano-iron phosphate product with low impurity content and excellent performance.

[0066] Example 3

[0067] This example prepares a low-sulfur nano-ferric phosphate, and the specific process is as follows:

[0068] (1) adding ferrous sulfate stock solution, excess oxidant H2O2 and phosphoric acid solution into a stirred tank in sequence, and stirring thoroughly to obtain a raw material solution having a P / Fe molar ratio of 1.05:1, and separately preparing a sodium hydroxide solution having a concentration of 15%, and separately preparing a surfactant mixture having a concentration of 25% and a mass ratio of polyvinyl pyrrolidone to sodium lauryl sulfate of 1:1;

[0069] (2) slowly adding sodium hydroxide solution to the raw material solution, and simultaneously adding a surfactant mixture of 2% by weight of the raw material solution, strictly controlling the addition rate, adjusting the pH value to between 1.7 and 1.9, maintaining a speed of 200 rpm and stirring sufficiently to react, to obtain a ferric phosphate dihydrate slurry;

[0070] (3) adding a certain amount of 70% phosphoric acid solution to the ferric phosphate dihydrate slurry, adjusting the pH value to 1.3-1.6, heating to 85°C, maintaining a speed of 100 rpm, and stirring for about 5 hours to carry out an aging reaction. The reaction is terminated when the product particle size is controlled to be 8-20 μm, and filtering to obtain a ferric phosphate dihydrate residue;

[0071] (4) The ferric phosphate dihydrate filter residue was mixed with deionized water in a mass ratio of 1:1, and the slurry was ground using a sand mill to a D50 of 8 μm to obtain a ground slurry. The slurry was added to a 1% sodium carbonate solution with a mass 10 times that of the ferric phosphate dihydrate filter residue, stirred and washed for 30 minutes, and filtered after washing to obtain a filter cake 1 with a water content of approximately 20%;

[0072] (5) The filter cake was added to deionized water with a mass 20 times that of the ferric phosphate dihydrate filter residue and stirred and washed again for 30 minutes. After washing, the filter cake was filtered again to obtain a filter cake 2 with a water content of about 20%;

[0073] (6) The filter cake 2 was directly placed in the pot and then calcined at 450℃ for 3h with a heating rate of 8℃ / min. 10Nm 3 / h of pure compressed air, the calcined material is ground, crushed and sieved to obtain a battery-grade nano-iron phosphate product with low impurity content and excellent performance.

[0074] Comparative Example 1

[0075] This comparative example prepared a nano-iron phosphate, and the specific process was as follows:

[0076] According to the steps (1) to (3) of Example 1, the ferric phosphate dihydrate filter residue was prepared. The ferric phosphate dihydrate filter residue was directly washed 3 times with 50 times deionized water, each time for 30 minutes, filtered, dried at 120 ° C for 10 hours, and then put into a pot and calcined at 300 ° C for 5 hours with a heating rate of 5 ° C / min. 10Nm 3 / h of pure compressed air, the calcined material can be ground, crushed and sieved to produce battery-grade nano-iron phosphate products.

[0077] Comparative Example 2

[0078] This embodiment prepares ferric phosphate dihydrate, which differs from Example 1 in that the ratio of the surfactant solution is different. The specific process is as follows:

[0079] (1) adding ferrous sulfate stock solution, excess oxidant H2O2 and phosphoric acid solution into a stirred tank in sequence and stirring thoroughly to obtain a raw material solution having a P / Fe molar ratio of 1.05:1, and separately preparing a sodium hydroxide solution with a concentration of 15% and a surfactant solution of cetyltrimethylammonium bromide with a concentration of 25%;

[0080] (2) slowly adding sodium hydroxide solution to the raw material solution, and simultaneously adding 2% surfactant solution by weight of the raw material solution, strictly controlling the addition rate, adjusting the pH value to between 1.7 and 1.9, maintaining a speed of 200 rpm and stirring sufficiently to react, to obtain ferric phosphate dihydrate slurry;

[0081] (3) adding a certain amount of 70% phosphoric acid solution to the ferric phosphate dihydrate slurry, adjusting the pH value to 1.3-1.6, heating to 85°C, maintaining a speed of 100 rpm, and stirring for about 5 hours to carry out an aging reaction. The reaction is terminated when the product particle size is controlled to be 8-20 μm, and filtering to obtain a ferric phosphate dihydrate residue;

[0082] The ICP test results showed that the S content in the ferric phosphate dihydrate filter residue obtained in this comparative example was 0.8129%.

[0083] Finished product quality

[0084] Table 1 shows the impurity element content of the battery-grade nano-ferric phosphate products prepared in Examples 1-3 and Comparative Example 1. The specific data were obtained by ICP-AES testing.

[0085] Table 1 Impurity content of battery-grade nano-ferric phosphate products

[0086] Impurity element content (%) Example 1 Example 2 Example 3 Comparative Example 1 S 0.0093 0.0086 0.0097 0.0649 Mn 0.0012 0.0011 0.0004 0.0026 Na 0.0092 0.0065 0.0133 0.0064 Co 0.0003 0.0002 0.0008 0.0089 Al 0.0052 0.0049 0.0076 0.0213 Cr 0.0143 0.0074 0.0097 0.0155

[0087] As can be seen from Table 1, the sulfur impurity content of the battery-grade nano-ferric phosphate products prepared in the examples is much lower than that in comparative example 1.

[0088] Table 2 shows the main differences between the preparation processes of Examples 1-3 and Comparative Example 1.

[0089] Table 2 Differences in preparation process between Examples and Comparative Examples

[0090] Total water consumption of the experimental group Desulfurization time Example 1 40: 17h Example 2 30: 16.5h Example 3 30: 16h Comparative Example 1 150: 120h

[0091] As shown in Table 2, the water consumption and total time consumption of the process used in Example 1 are much lower than those of the process used in Comparative Example 1.

[0092] While the embodiments of the present invention have been described in detail above with reference to the accompanying drawings, the present invention is not limited to the embodiments described above. Various modifications may be made within the scope of knowledge possessed by a person skilled in the art without departing from the spirit of the present invention. Furthermore, the embodiments of the present invention and the features thereof may be combined with one another unless there is a conflict.

Claims

1. A method for preparing low-sulfur nano-ferric phosphate, characterized in that: The following steps are involved: S1: mixing a phosphorus source and an iron source to obtain a raw material solution, adding an alkali and a surfactant, adjusting the pH, and stirring the mixture to obtain a ferric phosphate dihydrate slurry; S2: adding phosphoric acid solution to the ferric phosphate dihydrate slurry, adjusting the pH, heating and stirring for aging, and filtering to obtain ferric phosphate dihydrate; S3: adding water to ferric phosphate dihydrate to prepare a slurry, and grinding the slurry to obtain a ground slurry; S4: adding a washing liquid to the ground slurry for washing, separating the solid and the liquid, and calcining the solid phase to obtain nano-iron phosphate with low sulfur content.

2. The preparation method according to claim 1, characterized in that In step S1, the iron source is ferrous sulfate, and an oxidant is added to the raw material solution; the molar ratio of iron to phosphorus in the raw material solution is 1:(0.9-1.1).

3. The preparation method according to claim 1, characterized in that In step S1, the surfactant is compounded by polyvinyl pyrrolidone and sodium lauryl sulfate in a mass ratio of about 1:

1.

4. The preparation method according to claim 1, characterized in that In step S1, the pH is 1.0-2.

5.

5. The preparation method according to claim 3, characterized in that In step S1, the surfactant and water are mixed to prepare a surfactant mixture with a mass concentration of 10-40%, and the mass ratio of the raw material liquid to the surfactant mixture is 1:(0.004-0.04).

6. The preparation method according to claim 1, characterized in that In step S2, the particle size of the obtained ferric phosphate dihydrate is 8-20 μm.

7. The preparation method according to claim 1, characterized in that In step S3, the particle size D50 of the dispersed phase of the ground slurry is 2.5-10 μm.

8. The preparation method according to claim 1, characterized in that In step S4, the washing liquid is water or a 0.5-2% sodium carbonate solution; preferably, the water is hot water at 60-90°C.

9. The preparation method according to claim 1, characterized in that In step S4, the calcination temperature is 450-800° C., and the calcination time is 0.5-5 h.

10. The preparation method according to claim 1, characterized in that In step S4, compressed air is required for the calcination.