Preparation method of low-impurity iron phosphate with high iron-phosphorus ratio

Abstract

The invention relates to the technical field of iron phosphate preparation, in particular to a preparation method of low-impurity iron phosphate with a high iron-phosphorus ratio, which at least comprises the following steps: preparing an iron source solution and a phosphorus source solution; mixing an oxidizing agent and a phosphorus source solution as a phosphorus source mixed solution; adding the phosphorus source mixed solution into the iron source solution, and controlling the molar ratio of iron to phosphorus to be 1: (1.1-1.3) to prepare slurry; dividing the slurry into two parts, respectively adding an impurity removal agent, and then carrying out segmented aging at 90 DEG C to prepare crystallized slurry; and filtering, washing, drying and calcining the crystallized slurry to obtain a target product iron phosphate finished product. The impurity removal agent is selected from one or more of ferrous sulfate, ferrous chloride, ferrous nitrate and ferrous oxalate. Seed crystals are formed through segmented aging crystal transformation to promote crystal growth, and autonomous grain size distribution is achieved; an impurity removing agent is added in the aging stage, impurities are reduced by using the same ion effect, the iron-phosphorus ratio is increased, and the compaction density and the electrical property of the prepared lithium iron phosphate are remarkably improved.

Description

TECHNICAL FIELD

[0001] The application relates to the technical field of iron phosphate preparation, in particular to a preparation method of low-impurity high-iron-phosphorus ratio iron phosphate. BACKGROUND

[0002] The core driving force of an electric vehicle comes from an electric vehicle battery, and the service life, safety and capacity of the electric vehicle battery are always the focus of attention in the industry. Existing power lithium batteries are mainly divided into two categories: lithium iron phosphate batteries and ternary batteries. Among them, the application proportion of lithium iron phosphate batteries in the market gradually increases due to the significant advantages in cost control and safety, and the lithium iron phosphate battery has become a current hot choice.

[0003] The performance of the lithium iron phosphate is closely related to the performance of the raw material iron phosphate, and the performance of the iron phosphate determines the performance level of the lithium iron phosphate to a great extent. Specifically, the compaction performance and the electrical performance of the lithium iron phosphate are closely related to the primary particle size and the particle size distribution of the iron phosphate.

[0004] In order to improve the performance of the iron phosphate, many studies have been carried out in the related technical field. For example, the patent application CN118324108 A provides a preparation method of iron phosphate, which can obtain an iron phosphate product with mismatched particle size distribution, mismatched primary particle size and high tap density. The specific steps include: S1, reacting a phosphorus source, a divalent iron source and an oxidizing agent to obtain a product A; S2, filtering and washing the product A to obtain a slurry B; S3, taking 33-67wt% of the slurry B, adding phosphoric acid, adjusting the pH to 0.85-1.0, and performing a first-stage crystallization treatment to obtain a slurry C; S4, adding the remaining slurry B to the slurry C, and performing a second-stage crystallization treatment to obtain a slurry D; S5, filtering and washing the slurry D, and drying and calcining the obtained solid phase to obtain the iron phosphate product.

[0005] In the crystallization treatment process of the prior art, a part of the slurry is first subjected to a high-temperature crystallization to obtain a monoclinic iron phosphate dihydrate which is used as a crystal seed, and then the slurry which has not been subjected to a high-temperature reaction is added for a second high-temperature crystallization treatment, so that the crystal continues to grow on the original crystal seed. Due to the difference in the growth time of the iron phosphate dihydrate crystal in the two high-temperature reactions, the particle size distribution of the obtained iron phosphate dihydrate is bimodal, realizing the compounding effect of iron phosphates with different particle sizes, and thus improving the compaction performance and the electrical performance of the lithium iron phosphate prepared by using the iron phosphate as a raw material.

[0006] However, the prepared iron phosphate in the prior art still has obvious defects: the porosity of the micro-morphology is high, and the content of impurities such as magnesium and manganese is high. These defects directly limit the further improvement of the electrical performance of the lithium iron phosphate prepared by taking the iron phosphate as a raw material. Based on this, it is of great significance to develop a preparation method of low-impurity high-iron-phosphorus ratio iron phosphate for promoting the optimization of the performance of lithium iron phosphate, and it is a technical problem to be solved in the current field. SUMMARY

[0007] In view of the technical problem that the high impurity content in the iron phosphate affects the electrical performance of the lithium iron phosphate prepared by taking it as a raw material, the present application provides a preparation method of low-impurity high-iron-phosphorus ratio iron phosphate, which promotes crystal growth by segmenting aging and forming crystal seeds to realize independent particle size grading; impurities are removed and the iron-phosphorus ratio is increased by adding a decontamination agent during the aging stage, so that the tap density and electrical performance of the lithium iron phosphate prepared in this way are significantly improved, the subsequent grading process is omitted, and the production efficiency is improved.

[0008] The technical scheme of the present application is as follows: A preparation method of low-impurity high-iron-phosphorus ratio iron phosphate, at least comprising the following steps: S1, preparing iron source solution and phosphorus source solution as raw materials; S2, mixing an oxidizing agent and the phosphorus source solution as a phosphorus source mixed solution; S3, adding the phosphorus source mixed solution to the iron source solution, controlling the iron-phosphorus molar ratio of the iron source solution and the phosphorus source mixed solution to be 1:1.1~1.3, and preparing a slurry; S4, dividing the prepared slurry into two parts, adding a decontamination agent to each part, and then performing segmental aging at 90℃ to prepare a crystallized slurry; S5, filtering and washing the crystallized slurry, and drying and calcining to prepare the target product iron phosphate finished product; In step S4, the decontamination agent is selected from one or more of ferrous sulfate, ferrous chloride, ferrous nitrate and ferrous oxalate.

[0009] Further, in step S1, the iron source is selected from one or more of ferrous sulfate, ferrous oxalate, ferrous chloride, ferrous nitrate, iron powder and iron scale.

[0010] Further, in step S1, the phosphorus source is selected from one or more of industrial-grade monoammonium phosphate, agricultural-grade monoammonium phosphate, diammonium phosphate, ammonium phosphate and phosphoric acid.

[0011] Further, in step S2, the oxidizing agent is used to oxidize divalent iron ions to trivalent iron ions, and can be selected from one or more of hydrogen peroxide, ammonium persulfate, sodium persulfate and potassium permanganate.

[0012] Furthermore, in step S3, the phosphorus source mixed solution is added to the iron source solution while stirring, so that the phosphorus source mixed solution and the iron source solution are quickly and evenly mixed to prevent problems such as agglomeration caused by excessive local phosphorus ion concentration; Controlling the addition time of the phosphorus source mixed solution to 30-60 minutes can avoid excessive reaction caused by too rapid addition, making the reaction proceed more smoothly and facilitating the formation of structurally stable iron phosphate particles. Controlling the stirring speed to 100-200 rpm can ensure the mixing effect without causing solution splashing or excessive energy consumption due to stirring too fast.

[0013] This operation mode can promote the full progress of the reaction, reduce the residual unreacted raw materials, and help improve the purity and quality of ferric phosphate.

[0014] Furthermore, step S4 is specifically as follows: The prepared slurry is divided into slurry A and slurry B, wherein slurry A accounts for 30% to 70% of the total volume of the slurry. Impurity removers are added to slurry A and slurry B respectively, and then slurry A is placed at 90°C for aging and crystallization. After the crystallization is completed, slurry B is added to continue aging and crystallization to obtain a crystallized slurry.

[0015] The slurry is divided into slurry A and slurry B for staged aging and crystallization. Slurry A is aged first to form seed crystals, providing a growth foundation for slurry B. A volume ratio of 30%-70% slurry A to 70%-30% slurry B achieves a reasonable balance between the number of seed crystals and the amount of subsequently grown crystals, promoting orderly crystal growth. The optimal aging and crystallization temperature of 90°C ensures normal crystal growth without excessive growth or structural damage due to high temperatures.

[0016] This segmented processing method enables iron phosphate to form smaller crystal sizes and autonomously form particle size grading, without the need for subsequent additional particle grading work, thereby improving production efficiency and at the same time helping to improve the compaction performance and electrical properties of lithium iron phosphate.

[0017] Furthermore, in step S5, the crystallization slurry is filtered and washed with 5 to 20 times the volume of water to fully dissolve and remove soluble impurities in the crystallization slurry.

[0018] Furthermore, in step S5, the drying and calcining temperature is controlled to be 530-580° C. At this temperature, the ferric phosphate can complete the dehydration process and form a stable anhydrous ferric phosphate structure. If the temperature is too low, the dehydration may be incomplete, affecting the crystal structure of the ferric phosphate. If the temperature is too high, the ferric phosphate may decompose or change its structure. The drying and calcining time is controlled to be 2 to 4 hours, which can ensure that the reaction proceeds fully and make the crystal structure of iron phosphate more stable.

[0019] Appropriate drying and calcining conditions can improve the stability and performance of iron phosphate, which in turn is beneficial to improving the relevant properties of lithium iron phosphate using it as raw material.

[0020] Furthermore, the preparation method preferably comprises the following steps: (1) Prepare 3000 g of ferrous sulfate solution, the iron content of which is 6.83 wt%; prepare 1917.89 g of monoammonium phosphate solution, the phosphorus content of which is 7.12 wt%; (2) adding 253.84 g of 27.5 wt% hydrogen peroxide to the monoammonium phosphate solution of step (1) as a phosphorus source mixed solution; (3) The ferrous sulfate solution prepared in step (1) was added to the reactor as a base solution, and then the phosphorus source mixed solution prepared in step (2) was added to the reactor, the stirring speed was set to 150 rpm, and the addition time of the phosphorus source mixed solution was controlled to be 50 min. After the addition was completed, the mixture was reacted for 30 min to obtain a slurry; (4) The slurry prepared in step (3) was divided into slurry A and slurry B, with the volume ratio of slurry A to slurry B being 0.5:0.5. 75 g of ferrous sulfate solution was added to each of slurry A and slurry B. Slurry A was added to a reactor, the temperature was raised to 90°C, and the temperature was kept for 2 h. Then, slurry B was added and the temperature was kept for another 2 h to obtain a crystallized slurry. (5) The crystallized slurry obtained in step (4) was filtered and washed with 10 times the volume of water until the conductivity of the washing water was less than 200 μs / cm to obtain an iron phosphate filter cake, which was calcined at 550° C. for 4 h to obtain anhydrous iron phosphate.

[0021] The principle of segmented crystal transformation of the present invention is as follows: During the reaction process, slurry A is first added to carry out the first stage of aging and crystallization. After the crystallization is completed, slurry B is added to carry out the second stage of aging and crystallization. At this time, the iron phosphate crystals that first completed aging and crystallization in the first stage will exist as seeds, further promoting the crystallization of the iron phosphate crystals in the second stage, thereby resulting in smaller primary particles. At the same time, the growth time of the iron phosphate crystals in the second stage is inconsistent with that of the iron phosphate crystals in the second stage, so the particle size classification of iron phosphate can be achieved.

[0022] The impurity removal principle of the present invention is as follows: The cations contained in the impurity removing agent added in step S4 can form a common ion effect with magnesium, manganese, zinc and other ions in the solution, thereby reducing the amount of magnesium, manganese, zinc and other impurity ions in the iron phosphate, and increasing the iron-phosphorus ratio to a certain extent. At the same time, the timing of adding the impurity removing agent is very critical. If it is added too early, it may react with the oxidizing agent, causing the ferrous ions in the impurity removing agent to be oxidized to iron ions. Since the valence of iron ions is different from that of magnesium, manganese, zinc and other impurity ions, iron ions cannot compete with divalent magnesium, manganese, zinc and other impurity ions, and the impurity removing effect is poor.

[0023] The beneficial effects of the present application are: The present application reduces the content of impurity elements in the iron phosphate by adding an impurity removing agent during the aging stage, and divides the iron phosphate slurry into two steps for aging, so that the iron phosphate itself forms smaller crystal sizes and independently forms a particle size distribution, without the need for subsequent particle size distribution work. This not only improves the compaction performance of the lithium iron phosphate, but also further improves the electrical performance of the lithium iron phosphate. BRIEF DESCRIPTION OF DRAWINGS

[0024] In order to more clearly illustrate the technical solutions in the embodiments of the present application or the prior art, the following will briefly introduce the drawings needed to be used in the embodiments or prior art description. Obviously, for those skilled in the art, other drawings can also be obtained without creative labor based on these drawings.

[0025] Figure 1 is the SEM diagram of the anhydrous iron phosphate prepared in Example 1.

[0026] Figure 2 is the particle size distribution diagram of the anhydrous iron phosphate prepared in Example 1.

[0027] Figure 3 is the XRD diagram of the anhydrous iron phosphate prepared in Example 1.

[0028] Figure 4 is the electrical performance test result diagram of the lithium iron phosphate prepared by using the anhydrous iron phosphate prepared in Example 1 as raw material.

[0029] Figure 5 is the SEM diagram of the anhydrous iron phosphate prepared in Comparative Example 1. DETAILED DESCRIPTION

[0030] In order to make the person skilled in the art better understand the technical solutions in the present application, the following will combine the drawings in the embodiments of the present application to clearly and completely describe the technical solutions in the embodiments of the present application. Obviously, the described embodiments are only a part of the embodiments of the present application, not all the embodiments. Based on the embodiments in the present application, all other embodiments obtained by those skilled in the art without creative labor should belong to the protection scope of the present application.

[0031] Example 1 A low-impurity high-iron-to-phosphorus ratio iron phosphate is obtained by the following preparation method: (1) 3000 g of ferrous sulfate solution is prepared, the iron content in the ferrous sulfate solution is 6.83wt%, 1917.89 g of monoammonium phosphate solution is prepared, the phosphorus content in the monoammonium phosphate solution is 7.12wt%; (2) 253.84 g of hydrogen peroxide solution with a concentration of 27.5wt% is added to the monoammonium phosphate solution of step (1) as a phosphorus source mixed solution; (3) The ferrous sulfate solution prepared in step (1) is added to the reaction kettle as the bottom liquid, and then the phosphorus source mixed solution prepared in step (2) is added to the reaction kettle, the stirring speed is set to 150 rpm, the addition time of the phosphorus source mixed solution is controlled to 50 min, and after addition is completed, reaction is carried out for 30 min to obtain a slurry; (4) The slurry prepared in step (3) is divided into slurry A and slurry B, the volume ratio of slurry A to slurry B is 0.5:0.5, 75 g of ferrous sulfate solution is added to each of slurry A and slurry B, slurry A is added to the reaction kettle, heated to 90℃, and kept for 2h, then slurry B is added, and kept for another 2h to obtain a crystallized slurry; (5) The crystallized slurry obtained in step (4) is filtered and washed with 10 times the volume of water until the conductivity of the washing water is less than 200 μs / cm to obtain an iron phosphate filter cake, and the iron phosphate filter cake is calcined at 550℃ for 4h to obtain anhydrous iron phosphate.

[0032] As shown in Figure 1 , Figure 2 , the anhydrous iron phosphate prepared in Example 1 has the characteristics of particle size grading and small primary particles. The special morphology is beneficial to the improvement of the compaction density and electrical properties of lithium iron phosphate.

[0033] As shown in Figure 3 , the X-ray diffraction spectrum of the anhydrous iron phosphate has fewer impurity peaks, indicating that the anhydrous iron phosphate prepared in Example 1 is a low-impurity iron phosphate.

[0034] Example 2 A low-impurity high-iron-to-phosphorus ratio iron phosphate is obtained by the following preparation method: (1) 3500 g of ferrous chloride solution is prepared, the iron content in the ferrous chloride solution is 6.0wt%, 2500 g of diammonium phosphate solution is prepared, the phosphorus content in the diammonium phosphate solution is 5.35wt%; (2) 239 g of hydrogen peroxide solution with a concentration of 30wt% is added to the diammonium phosphate solution of step (1) as a phosphorus source mixed solution; (3) The ferrous chloride solution prepared in step (1) is added to the reaction kettle as a bottom solution, and then the phosphorus source mixed solution prepared in step (2) is added to the reaction kettle, the stirring speed is set to 100 rpm, the addition time of the phosphorus source mixed solution is controlled to 30 min, and after the addition is completed, the reaction is carried out for 60 min to obtain a slurry; (4) The slurry prepared in step (3) is divided into slurry A and slurry B, the volume ratio of slurry A to slurry B is 0.7:0.3, 80 g of ferrous chloride solution is added to each of slurry A and slurry B, slurry A is added to the reaction kettle, the temperature is raised to 90°C, and the temperature is maintained for 2 h, then slurry B is added, and the temperature is maintained for another 2 h to obtain a crystallized slurry; (5) The crystallized slurry obtained in step (4) is washed with 5 times the volume of water until the conductivity of the washing water is less than 200 μs / cm to obtain a ferric phosphate filter cake, and the ferric phosphate filter cake is calcined at 580°C for 2 h to obtain anhydrous ferric phosphate.

[0035] Example 3 A low-impurity high-iron-phosphorus-ratio ferric phosphate is obtained by the following preparation method: (1) A ferrous nitrate solution 2700 g is prepared, the iron content in the ferrous nitrate solution is 7.6wt%, and an ammonium phosphate solution 2000 g is prepared, the phosphorus content in the ammonium phosphate solution is 7.10wt%; (2) 142 g of hydrogen peroxide with a concentration of 50wt% is added to the ammonium phosphate solution of step (1) as a phosphorus source mixed solution; (3) The ferrous nitrate solution prepared in step (1) is added to the reaction kettle as a bottom solution, and then the phosphorus source mixed solution prepared in step (2) is added to the reaction kettle, the stirring speed is set to 200 rpm, the addition time of the phosphorus source mixed solution is controlled to 60 min, and after the addition is completed, the reaction is carried out for 30 min to obtain a slurry; (4) The slurry prepared in step (3) is divided into slurry A and slurry B, the volume ratio of slurry A to slurry B is 0.3:0.7, 100 g of ferrous oxalate solution is added to each of slurry A and slurry B, slurry A is added to the reaction kettle, the temperature is raised to 90°C, and the temperature is maintained for 2 h, then slurry B is added, and the temperature is maintained for another 2 h to obtain a crystallized slurry; (5) The crystallized slurry obtained in step (4) is washed with 20 times the volume of water until the conductivity of the washing water is less than 200 μs / cm to obtain a ferric phosphate filter cake, and the ferric phosphate filter cake is calcined at 530°C for 4 h to obtain anhydrous ferric phosphate.

[0036] Comparative Example 1 A ferric phosphate is obtained by the following preparation method: (1) 3000 g of ferrous sulfate solution was prepared, the iron content in the ferrous sulfate solution was 6.83 wt%, 1917.89 g of monoammonium phosphate solution was prepared, the phosphorus content in the monoammonium phosphate solution was 7.12 wt%; (2) 253.84 g of hydrogen peroxide solution with a concentration of 27.5 wt% was added to the monoammonium phosphate solution of step (1) as a phosphorus source mixed solution; (3) The ferrous sulfate solution prepared in step (1) was added to the reaction kettle as the bottom liquid, then the phosphorus source mixed solution prepared in step (2) was added to the reaction kettle, the stirring speed was set to 150 rpm, the addition time of the phosphorus source mixed solution was controlled to be 50 min, and after addition was completed, the reaction was carried out for 30 min to obtain a slurry; (4) The slurry prepared in step (3) was heated to 90℃ and kept for 4 h to obtain a crystallized slurry; (5) The crystallized slurry obtained in step (4) was washed with 10 times the volume of water until the conductivity of the washing water was less than 200 μs / cm to obtain a phosphorus iron filter cake, and the phosphorus iron filter cake was calcined at 550℃ for 4 h to obtain anhydrous iron phosphate.

[0037] Comparative Example 2 An iron phosphate was obtained by the following preparation method: (1) 3000 g of ferrous sulfate solution was prepared, the iron content in the ferrous sulfate solution was 6.83 wt%, 1917.89 g of monoammonium phosphate solution was prepared, the phosphorus content in the monoammonium phosphate solution was 7.12 wt%; (2) 253.84 g of hydrogen peroxide solution with a concentration of 27.5 wt% was added to the monoammonium phosphate solution of step (1) as a phosphorus source mixed solution; (3) The ferrous sulfate solution prepared in step (1) was added to the reaction kettle as the bottom liquid, then the phosphorus source mixed solution prepared in step (2) was added to the reaction kettle, the stirring speed was set to 150 rpm, the addition time of the phosphorus source mixed solution was controlled to be 50 min, and after addition was completed, the reaction was carried out for 30 min to obtain a slurry; (4) The slurry prepared in step (3) was divided into slurry A and slurry B, the volume ratio of slurry A to slurry B was 0.5:0.5, slurry A was added to the reaction kettle, heated to 90℃, and kept for 2 h, then slurry B was added, and kept for another 2 h to obtain a crystallized slurry; (5) The crystallized slurry obtained in step (4) was washed with 10 times the volume of water until the conductivity of the washing water was less than 200 μs / cm to obtain a phosphorus iron filter cake, and the phosphorus iron filter cake was calcined at 550℃ for 4 h to obtain anhydrous iron phosphate.

[0038] The iron phosphate prepared in Examples 1-3 and Comparative Examples 1-2 was prepared into lithium iron phosphate under the same conditions, and the compaction performance was evaluated. Then the lithium iron phosphate was prepared into 2032 button cells, and the electrical performance was evaluated. The results are shown in Table 1. Compared with Comparative Examples 1-2, the process of Examples 1-3 can improve the compaction density of lithium iron phosphate and the electrical performance of the prepared button cells.

[0039] Table 1: Comparison of compaction density and electrical performance

[0040] The impurity elements of the iron phosphate prepared in Examples 1-3 and Comparative Examples 1-2 were detected, and the results are shown in Table 2. It can be seen that due to the common ion effect, the metal cations in the reaction solution of Examples 1-3 and the cations of the impurity removing agent form a competitive relationship. The cations of the impurity removing agent are more easily adsorbed on the surface of the iron phosphate than the metal impurity cations in the solution, thereby reducing the content of metal impurities such as Zn, Mg, Mn, Na, and K in the finished iron phosphate.

[0041] Table 2: Comparison of impurity element test results

[0042] Although the present application has been described in detail with reference to the preferred embodiments, the application is not limited to the preferred embodiments. Those skilled in the art can make various equivalent modifications or replacements to the embodiments of the present application without departing from the spirit and essence of the present application, and these modifications or replacements should be within the scope of the present application. Any person skilled in the art can easily think of changes or replacements within the technical range disclosed by the present application, and these changes or replacements should be within the protection scope of the present application.

Claims

1. A method for preparing ferric phosphate with low impurities and high iron-phosphorus ratio, characterized in that: At least the following steps are included: S1. Prepare iron source solution and phosphorus source solution as raw materials; S2. mixing an oxidant and a phosphorus source solution to form a phosphorus source mixed solution; S3, adding the phosphorus source mixed solution to the iron source solution, controlling the iron-phosphorus molar ratio of the iron source solution to the phosphorus source mixed solution to be 1:1.1-1.3, to prepare a slurry; S4, dividing the prepared slurry into two parts, adding an impurity remover to each part, and then aging the two parts in stages at 90° C. to obtain a crystallized slurry; S5, filtering and washing the crystallized slurry, drying and calcining to obtain the target product, ferric phosphate; In step S4, the impurity remover is selected from one or more of ferrous sulfate, ferrous chloride, ferrous nitrate, and ferrous oxalate.

2. The method for preparing low-impurity and high iron-to-phosphorus ratio ferric phosphate according to claim 1, wherein: In step S1, the iron source is selected from one or more of ferrous sulfate, ferrous oxalate, ferrous chloride, ferrous nitrate, iron powder, and iron sheet.

3. The method for preparing ferric phosphate with low impurities and high iron-phosphorus ratio according to claim 1, wherein: In step S1, the phosphorus source is selected from one or more of industrial-grade monoammonium phosphate, agricultural-grade monoammonium phosphate, diammonium phosphate, ammonium phosphate, and phosphoric acid.

4. The method for preparing ferric phosphate with low impurities and high iron-phosphorus ratio according to claim 1, wherein: In step S2, the oxidant is selected from one or more of hydrogen peroxide, ammonium persulfate, sodium persulfate, and potassium permanganate.

5. The method for preparing low-impurity and high iron-to-phosphorus ratio ferric phosphate according to claim 1, wherein: In step S3, the phosphorus source mixed solution is added to the iron source solution while stirring, the addition time of the phosphorus source mixed solution is controlled to be 30-60 minutes, and the stirring speed is controlled to be 100-200 rpm.

6. The method for preparing ferric phosphate with low impurities and high iron-phosphorus ratio according to claim 1, wherein: Step S4 is specifically as follows: The prepared slurry is divided into slurry A and slurry B, wherein slurry A accounts for 30% to 70% of the total volume of the slurry. Impurity removers are added to slurry A and slurry B respectively, and then slurry A is placed at 90°C for aging and crystallization. After the crystallization is completed, slurry B is added to continue aging and crystallization to obtain a crystallized slurry.

7. The method for preparing ferric phosphate with low impurities and high iron-phosphorus ratio according to claim 1, wherein: In step S5, the crystallized slurry is filtered and washed using 5 to 20 times the volume of water.

8. The method for preparing low-impurity and high iron-to-phosphorus ratio ferric phosphate according to claim 1, wherein: In step S5, the temperature of drying and calcining is controlled to be 530-580° C., and the time of drying and calcining is controlled to be 2-4 h.

9. The method for preparing ferric phosphate with low impurities and high iron-phosphorus ratio according to claim 1, wherein: The preparation method comprises the following steps: (1) Prepare 3000 g of ferrous sulfate solution, the iron content of which is 6.83 wt%; prepare 1917.89 g of monoammonium phosphate solution, the phosphorus content of which is 7.12 wt%; (2) adding 253.84 g of 27.5 wt% hydrogen peroxide to the monoammonium phosphate solution of step (1) as a phosphorus source mixed solution; (3) The ferrous sulfate solution prepared in step (1) was added to the reactor as a base solution, and then the phosphorus source mixed solution prepared in step (2) was added to the reactor, the stirring speed was set to 150 rpm, and the addition time of the phosphorus source mixed solution was controlled to be 50 min. After the addition was completed, the mixture was reacted for 30 min to obtain a slurry; (4) The slurry prepared in step (3) was divided into slurry A and slurry B, with the volume ratio of slurry A to slurry B being 0.5:0.

5. 75 g of ferrous sulfate solution was added to each of slurry A and slurry B. Slurry A was added to the reactor, the temperature was raised to 90°C, and the temperature was kept for 2 h. Then, slurry B was added and the temperature was kept for another 2 h to obtain a crystallized slurry. (5) The crystallized slurry obtained in step (4) was filtered and washed with 10 times the volume of water until the conductivity of the washing water was less than 200 μs / cm to obtain an iron phosphate filter cake, which was calcined at 550° C. for 4 h to obtain anhydrous iron phosphate.

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