A method for preparing iron phosphate and lithium iron phosphate using an iron-based process.

Abstract

This invention relates to the field of electrode material technology, specifically to a method for preparing iron phosphate and lithium iron phosphate. The preparation method includes the following steps: (1) adding phosphoric acid with a phosphorus-to-iron ratio of 0.4 to 1.8 and an iron source into an iron melting reactor for iron dissolution reaction at a reaction temperature of 50 to 90°C for 2 to 8 hours, and filtering to obtain a ferrous dihydrogen phosphate solution; (2) synthesis oxidation: adding the ferrous dihydrogen phosphate solution into a synthesis reactor, stirring and heating, and adding hydrogen peroxide; (3) heating and aging: heating and aging, keeping the system at a temperature after it turns white, and obtaining a slurry after the reaction is completed; (4) washing, drying, and calcining: washing, drying to obtain ferric phosphate dihydrate, calcining, and cooling to obtain anhydrous ferric phosphate. This invention uses less phosphoric acid, has a high raw material utilization rate, and is cost-controllable, possessing industrial advantages; the obtained ferric phosphate has fine primary particles, good dispersibility, and high grinding efficiency, significantly improving processing performance and production efficiency.

Description

Technical Field

[0001] This invention relates to the field of electrode materials technology, specifically to a method for preparing iron phosphate and lithium iron phosphate. Background Technology

[0002] Iron phosphate (FePO4) is an important precursor for the preparation of lithium iron phosphate (LiFePO4), a cathode material for lithium-ion batteries. Its purity, crystal structure, and morphology directly affect the electrochemical performance and compaction density of the final lithium iron phosphate product. With the rapid development of the new energy vehicle industry, the market demand for lithium iron phosphate continues to surge, which in turn places more stringent requirements on the preparation technology of high-performance, low-cost iron phosphate.

[0003] Currently, the main industrial methods for preparing ferric phosphate include the ammonia method, the sodium method, and the iron method. Among them, the iron method has become a process with great development potential due to its low equipment investment cost, high product purity, and environmentally friendly process, which effectively avoids the problems of high raw material costs and heavy by-product disposal associated with the ammonia method and the sodium method.

[0004] However, existing iron-based ferric phosphate preparation technologies still have many drawbacks. For example, patent CN111377426A discloses a method for preparing anhydrous ferric phosphate nanoparticles, in which iron powder is dissolved in phosphoric acid, hydrogen peroxide is added, and the product is obtained through multiple steps of filtration, washing, drying, and sintering. The phosphorus-to-iron ratio is 2.2~2.5, with a large amount of excess phosphoric acid, which not only increases the raw material cost but also leads to a long washing time and low production efficiency. Patent CN115285959A discloses a low-cost continuous production method for ferric phosphate, in which iron powder is first mixed with phosphoric acid, and after the iron-to-phosphate reaction is completed, ferrous dihydrogen phosphate solution is obtained by filtration. A pH adjuster (phosphoric acid or iron oxide) is added, followed by hydrogen peroxide, aging, filtration, washing, drying, and sintering to obtain ferric phosphate. In this process, the phosphorus-to-iron ratio is further increased to 3.0~4.0, and the problem of excess phosphoric acid is more prominent. In addition, phosphoric acid or iron oxide needs to be added as a pH adjuster, which increases the production cost and is not conducive to ensuring the stability of production batches.

[0005] Furthermore, the traditional iron-based lithium iron phosphate (LiFePO4) production process lacks effective control over crystal growth, leading to the formation of dense agglomerates and extremely low grinding efficiency. This, in turn, causes uneven material mixing, decreased product performance, and blockages in the spray drying and calcination systems during subsequent LiFePO4 production, significantly reducing production efficiency and increasing the risk of equipment damage and production costs. While existing technologies have attempted to address these issues through equipment improvements, slurry preheating, or the addition of dispersants, none of these methods have fundamentally improved the grindability of LiFePO4, failing to meet the practical needs of industrial production. Summary of the Invention

[0006] To address the technical problems of existing iron-based iron phosphate preparation processes, such as complex subsequent washing or the need for pH adjusters due to excessive phosphoric acid usage, and the difficulty in grinding the obtained product leading to decreased product performance or material blockage during lithium iron phosphate preparation, this invention provides a method for preparing iron-based iron phosphate and lithium iron phosphate. By optimizing the reaction system ratio, the use of pH adjusters in the traditional iron-based process is avoided, and the amount of phosphoric acid used is reduced, effectively simplifying the process flow, lowering production costs, and resulting in smaller primary particle sizes with fewer large agglomerates. This fundamentally improves the particle morphology and agglomeration state of iron phosphate, significantly enhancing processing performance and production efficiency.

[0007] The technical solution of this invention is as follows: In a first aspect, the present invention provides a method for preparing ferric phosphate by means of iron, comprising the following steps: (1) Preparation of ferrous dihydrogen phosphate solution: Phosphoric acid with a phosphorus-to-iron ratio of 0.4 to 1.8 and an iron source are added to an iron leaching kettle to carry out an iron dissolution reaction. The reaction temperature is 50 to 90°C and the reaction time is 2 to 8 hours. The ferrous dihydrogen phosphate solution is obtained by filtration. When the phosphorus-to-iron ratio is 1.8 and the reaction temperature is 90°C, the reaction time is ≤2 hours. For every 0.1 decrease in the phosphorus-to-iron ratio, the reaction time is extended by 0.2 to 0.3 hours. For every 10°C decrease in the reaction temperature, the reaction time is extended by 0.5 to 1 hour. The iron source leaching rate is 15% to 17%. The phosphorus-to-iron ratio refers to the molar ratio between the effective phosphorus element in the added phosphoric acid and the iron element in the added iron source. (2) Synthesis oxidation: Add the ferrous dihydrogen phosphate solution obtained in step (1) into the synthesis vessel, stir and heat, and add hydrogen peroxide; oxidize all the ferrous ions in the solution into ferric ions, and the temperature of the reaction system is 40~65℃; (3) Heating and aging: The reaction system of step (2) is heated and aged. After the system turns white, it is kept at the temperature. After the reaction is completed, a white slurry is obtained. (4) Washing, drying and calcining: The white slurry obtained in step (3) is subjected to solid-liquid separation. The obtained filter cake is washed and dried to obtain ferric phosphate dihydrate. Then it is placed in a muffle furnace for calcination and cooled to obtain anhydrous ferric phosphate.

[0008] Furthermore, in step (1), the iron source is at least one of iron powder, iron bar, iron sheet or iron ingot, with an iron content of about 95%; the phosphoric acid is industrial grade, with a mass fraction of 30% to 60%; wherein, the mass fraction of phosphoric acid affects the phosphorus-iron ratio by changing the concentration of phosphorus in the reaction system. Under a determined amount of iron source, the phosphorus-iron ratio of the reaction system can be set by adjusting the mass fraction and amount of phosphoric acid. The mass fraction range of 30% to 60% not only ensures the acidity conditions required for the dissolution of the iron source, but also provides sufficient space for flexible adjustment of the phosphorus-iron ratio in the range of 0.4 to 1.8.

[0009] Furthermore, in step (2), the stirring frequency is 20~50Hz.

[0010] Furthermore, in step (2), the mass fraction of hydrogen peroxide is 5%~25%; the molar ratio of hydrogen peroxide to iron in the ferrous dihydrogen phosphate solution is (0.7~1.5):1; the hydrogen peroxide is added slowly over a period of 0.5~1h.

[0011] Furthermore, in step (3), the temperature for heating and aging is 86~95℃, and the aging time is 1~4h.

[0012] Furthermore, in step (4), the drying temperature is 80~110℃ and the drying time is 6~12h; the calcination temperature is 550~650℃ and the calcination time is 3~5h.

[0013] Secondly, the present invention also provides a method for preparing lithium iron phosphate, comprising the following steps: S1. Prepare iron phosphate according to the above preparation method; S2. After adding pure water, lithium source and carbon source to the iron phosphate prepared in S1, the mixture is ground and then spray-dried. S3. The dried mixture is calcined at high temperature in a protective atmosphere at a temperature of 750~850℃ for 10~15h to obtain lithium iron phosphate.

[0014] Furthermore, in step S2, the lithium source is at least one of lithium carbonate, lithium hydroxide, or lithium sulfate; and the carbon source is at least one of glucose, sucrose, carbon black, or polyethylene glycol.

[0015] Furthermore, in step S2, the particle size range of the mixture obtained by grinding is 0.15~0.25μm.

[0016] Furthermore, in step S3, the protective gas is a mixture of argon and nitrogen, wherein the volume percentage of argon in the mixture is 60% to 70%.

[0017] In the preparation method provided by this invention, step (1) is the iron dissolution reaction process. The theoretical phosphorus-iron ratio calculated according to the chemical equivalent of the reaction formula is 2. However, in reality, phosphoric acid is a moderately strong acid, and its aqueous solution cannot be completely ionized under heating conditions. Even if an excess of phosphoric acid is added, it cannot be completely dissolved, and the iron dissolution reaction tends to stagnate after reaching a certain extent. The iron phosphate preparation method provided by this invention has a phosphorus-iron ratio of 0.4 to 1.8, and the amount of phosphoric acid is less than 2 times the equivalent. The iron source leaching rate in the iron dissolution reaction is controlled within the range of 15% to 17%, which avoids the use of pH adjusters in the traditional iron process, effectively simplifies the process flow, reduces production costs, and controls the conversion conditions so that the resulting product has a small primary particle size and no large agglomerates. The undissolved iron source is recycled to participate in the next iron dissolution reaction, avoiding the ineffective feeding of excess phosphoric acid. The iron sheet leaching rate refers to the ratio of the mass of the iron source participating in the reaction to the total mass of the iron source added.

[0018] In the iron dissolution reaction, the phosphorus-to-iron ratio (i.e., the amount of phosphoric acid), the temperature during the reaction, and the reaction time all affect the iron leaching rate. The iron dissolution reaction between the iron source and phosphoric acid is a controllable chemical reaction at the solid-liquid interface. This invention allows for the control of the reaction rate by adjusting the aforementioned process parameters. When the iron leaching reaction is carried out under the conditions of a phosphorus-iron ratio of 1.8 and a reaction temperature of 90°C, the reaction rate is fast. By controlling the reaction time to ≤2h, the iron leaching reaction is terminated in advance, thereby limiting the total amount of iron source dissolved. When the iron leaching reaction is carried out under the conditions of low temperature and low phosphoric acid dosage, the reaction rate is slow. Therefore, by appropriately extending the reaction time, that is, for every 0.1 decrease in the phosphorus-iron ratio, the reaction time is extended by 0.2~0.3h; for every 10°C decrease in the reaction temperature, the reaction time is extended by 0.5~1h, thereby making up for the reaction process and increasing the total amount of iron source dissolved. Through the above adjustment method, this invention achieves a stable iron source leaching rate under different iron leaching reaction conditions, that is, maintaining the iron source leaching rate in the range of 15%~17%. With the conversion conditions, that is, the temperature of the reaction system for synthesis oxidation in step (2) is 40~65°C, the resulting iron phosphate particles are small in size and relatively dispersed, without large agglomerates, thus fundamentally improving the particle morphology and agglomeration state of traditional iron-based iron phosphate.

[0019] The beneficial effects of this invention are as follows: 1. This invention provides a simple and cost-controllable method for preparing iron phosphate using the iron-based process. Compared with the traditional iron-based iron phosphate preparation method, this method uses less phosphoric acid, avoiding the ineffective feeding of excess phosphoric acid in the prior art. Furthermore, the undissolved iron source can be recycled to participate in the next iron dissolution reaction, making the process more economical and the raw material utilization rate higher, thus possessing advantages for industrial application.

[0020] 2. This invention provides a method for preparing iron phosphate by the iron-based process. By controlling the amount of phosphoric acid, the temperature of iron dissolution, and the time of iron dissolution in the iron-dissolution reaction, the reaction rate can be controlled. Under different iron-dissolution reaction conditions, the iron source leaching rate can be maintained at 15%~17%. At the same time, the reaction conditions of synthesis oxidation can be controlled, so that the resulting product has a small primary particle size, more dispersed particles, and no large agglomerates. This fundamentally improves the particle morphology and agglomeration state of traditional iron-based iron phosphate, ensuring efficient grinding and uniform mixing in the subsequent preparation of lithium iron phosphate, avoiding material blockage, significantly improving processing performance, and greatly increasing production efficiency.

[0021] 3. This invention also provides a method for preparing lithium iron phosphate, using the aforementioned iron phosphate, lithium source, and carbon source as raw materials, and obtaining lithium iron phosphate through grinding, drying, and calcination; this method only requires 30 minutes of grinding to achieve a particle size of 0.15~0.25μm in the mixture, with simple process steps, short operation time, and strong parameter controllability; the obtained lithium iron phosphate product has a significantly improved compaction density, a specific discharge capacity of up to 159mAh / g at 0.1C rate, and a discharge efficiency of 99%, exhibiting excellent electrical performance, which can meet the application requirements of lithium iron phosphate battery cathode materials with stringent performance requirements. Attached Figure Description

[0022] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, for those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0023] Figure 1 This is a SEM image of ferric phosphate dihydrate prepared in Example 1 of this invention at 10000x magnification.

[0024] Figure 2 This is a SEM image of ferric phosphate dihydrate prepared in Example 1 of this invention at a magnification of 30,000.

[0025] Figure 3 This is a SEM image of anhydrous ferric phosphate prepared in Example 1 of the present invention at a magnification of 10,000.

[0026] Figure 4 This is a SEM image of anhydrous ferric phosphate prepared in Example 1 of this invention at a magnification of 30,000.

[0027] Figure 5 This is a SEM image of anhydrous ferric phosphate prepared in Comparative Example 1 of this invention at a magnification of 30,000.

[0028] Figure 6 This is a SEM image of ferric phosphate dihydrate prepared in Comparative Example 1 of this invention at a magnification of 30,000.

[0029] Figure 7 This is a SEM image of anhydrous ferric phosphate prepared in Comparative Example 1 of the present invention at a magnification of 10,000.

[0030] Figure 8 This is a SEM image of anhydrous ferric phosphate prepared in Comparative Example 1 of the present invention at a magnification of 30,000.

[0031] Figure 9 The image shows the XRD pattern of ferric phosphate dihydrate prepared in Example 1 of this invention.

[0032] Figure 10 The image shows the XRD pattern of anhydrous ferric phosphate prepared in Example 1 of this invention.

[0033] Figure 11 The graph shows a comparison of the grinding efficiency of ferric phosphate prepared in Examples 1, 2 and Comparative Example 1. In the graph, the vertical axis D50 value (μm) represents the particle size value corresponding to the cumulative particle size distribution of the sample reaching 50%, and the unit is μm.

[0034] Figure 12 The charge-discharge curves of lithium iron phosphate prepared using the iron phosphate prepared in Example 5 of this invention as a precursor are shown. Detailed Implementation

[0035] To enable those skilled in the art to better understand the technical solutions of this invention, the technical solutions of the embodiments of this invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of this invention, and not all embodiments. Based on the embodiments of this invention, all other embodiments obtained by those skilled in the art without creative effort should fall within the scope of protection of this invention.

[0036] Example 1 A method for preparing ferric phosphate by iron-based process includes the following steps: (1) Preparation of ferrous dihydrogen phosphate solution: 513g of pure water was added to the iron melting reactor beforehand. After stirring, 280g of industrial grade phosphoric acid with a mass fraction of 85% was slowly added. After mixing evenly, a phosphoric acid solution with a mass fraction of 30% was obtained. The temperature was raised to 50℃ for later use. According to the phosphorus-iron ratio of 0.4, 358g of iron sheet with an iron content of 95% was weighed and put into the iron melting reactor. The timing was started after the reaction was carried out. After 8.0h, the mixture was filtered immediately to obtain ferrous dihydrogen phosphate solution and 301g of undissolved iron sheet. The iron source leaching rate was 16%. The undissolved iron sheet can be used as the iron source for the next batch. (2) Synthesis oxidation: Add the above ferrous dihydrogen phosphate solution to the synthesis vessel, start stirring at a frequency of 50 Hz, heat the solution to 40°C, and slowly add 522 g of 25% hydrogen peroxide over a period of 1 hour. (3) Heating and aging: The reaction system of step (2) is heated to 95°C for aging. After the system turns white, it is kept at the temperature for 1 hour. After the reaction is completed, a white slurry is obtained. (4) Washing, drying and calcining: The above white slurry is filtered, and the white filter cake is washed with pure water. The washed filter cake is dried in an oven at 80°C for 12 hours to obtain ferric phosphate dihydrate. The dried material is then placed in a muffle furnace for sintering at 550°C for 5 hours to obtain anhydrous ferric phosphate.

[0037] Example 2 A method for preparing ferric phosphate by iron-based process includes the following steps: (1) Preparation of ferrous dihydrogen phosphate solution: 708g of pure water was added to the iron melting reactor beforehand. After stirring, 630g of industrial phosphoric acid with a mass fraction of 85% was slowly added. After mixing evenly, a phosphoric acid solution with a mass fraction of 40% was obtained. The temperature was raised to 70℃ for later use. According to the phosphorus-iron ratio of 0.9, 358g of iron sheet with an iron content of 95% was weighed and put into the iron melting reactor. The timer was started after the reaction was 5.5h. After filtration, ferrous dihydrogen phosphate solution and 300g of undissolved iron sheet were obtained. The iron source leaching rate was 16%. The undissolved iron sheet can be used as the iron source for the next batch. (2) Synthesis oxidation: Add the above ferrous dihydrogen phosphate solution to the synthesis vessel, start stirring at a frequency of 40 Hz, heat the solution to 50°C, and slowly add 1088 g of 18% hydrogen peroxide over a period of 0.8 h. (3) Heating and aging: The reaction system of step (2) is heated to 92°C for aging. After the system turns white, it is kept at the temperature for 2 hours. After the reaction is completed, a white slurry is obtained. (4) Washing, drying and calcining: The above white slurry is filtered, and the white filter cake is washed with pure water. The washed filter cake is dried in an oven at 90°C for 10 hours to obtain ferric phosphate dihydrate. The dried material is then placed in a muffle furnace for sintering at 560°C for 4 hours to obtain anhydrous ferric phosphate.

[0038] Example 3 A method for preparing ferric phosphate by iron-based process includes the following steps: (1) Preparation of ferrous dihydrogen phosphate solution: 637g of pure water was added to the iron melting reactor beforehand. After stirring, 910g of industrial phosphoric acid with a mass fraction of 85% was slowly added. After mixing evenly, a phosphoric acid solution with a mass fraction of 50% was obtained. The temperature was raised to 80℃ for later use. According to the phosphorus-iron ratio of 1.3, 358g of iron sheet with an iron content of 95% was weighed and put into the iron melting reactor. The timer was started after the reaction was carried out. After 3.5h, the mixture was filtered immediately to obtain ferrous dihydrogen phosphate solution and 305g of undissolved iron sheet. The iron source leaching rate was 15%. The undissolved iron sheet can be used as the iron source for the next batch. (2) Synthesis oxidation: Add the above ferrous dihydrogen phosphate solution to the synthesis vessel, start stirring at a frequency of 50 Hz, heat the solution to 65°C, and slowly add 1306 g of 25% hydrogen peroxide over a period of 1 hour. (3) Heating and aging: The reaction system of step (2) is heated to 95°C for aging. After the system turns white, it is kept at the temperature for 4 hours. After the reaction is completed, a white slurry is obtained. (4) Washing, drying and calcining: The above white slurry is filtered, and the white filter cake is washed with pure water. The washed filter cake is dried in an oven at 110°C for 12 hours to obtain ferric phosphate dihydrate. The dried material is then placed in a muffle furnace for sintering at 650°C for 5 hours to obtain anhydrous ferric phosphate.

[0039] Example 4 A method for preparing ferric phosphate by iron-based process includes the following steps: (1) Preparation of ferrous dihydrogen phosphate solution: 525g of pure water was added to the iron melting reactor beforehand. After stirring, 1260g of industrial phosphoric acid with a mass fraction of 85% was slowly added. After mixing evenly, a phosphoric acid solution with a mass fraction of 60% was obtained. The temperature was raised to 90℃ for later use. According to the phosphorus-iron ratio of 1.8, 358g of iron sheet with an iron content of 95% was weighed and put into the iron melting reactor. The timing was started after the reaction was carried out. After 2.0h, the mixture was filtered immediately to obtain ferrous dihydrogen phosphate solution and 298g of undissolved iron sheet. The iron source leaching rate was 17%. The undissolved iron sheet can be used as the iron source for the next batch. (2) Synthesis oxidation: Add the above ferrous dihydrogen phosphate solution to the synthesis vessel, start stirring at a frequency of 20 Hz, heat the solution to 40°C, and slowly add 3047 g of 5% hydrogen peroxide over a period of 0.5 h. (3) Heating and aging: The reaction system of step (2) is heated to 86°C for aging. After the system turns white, it is kept at the temperature for 1 hour. After the reaction is completed, a white slurry is obtained. (4) Washing, drying and calcining: The above white slurry is filtered, and the white filter cake is washed with pure water. The washed filter cake is dried in an oven at 80°C for 6 hours to obtain ferric phosphate dihydrate. The dried material is then placed in a muffle furnace for sintering at 550°C for 3 hours to obtain anhydrous ferric phosphate.

[0040] Example 5 A method for preparing lithium iron phosphate includes the following steps: S1. Prepare 146g of ferric phosphate according to Example 1 above; S2. Add pure water, lithium carbonate, and glucose to the ferric phosphate prepared in S1, and then perform grinding and spray drying. The mass ratio of ferric phosphate: lithium carbonate: glucose: water is 1:0.25:0.01:1.8, the grinding time is 25 min, and the particle size of the mixture after sand milling is 0.17~0.23 μm. S3. The dried mixture is calcined at high temperature in a mixed protective gas of argon and nitrogen at 750℃ for 10h to obtain lithium iron phosphate; the volume ratio of argon to nitrogen in the mixed protective gas is 0.6:0.4.

[0041] Example 6 A method for preparing lithium iron phosphate includes the following steps: S1. Prepare 148g of ferric phosphate according to Example 2 above; S2. Add pure water, lithium carbonate, lithium phosphate, and polyethylene glycol to the ferric phosphate prepared in S1, and then perform grinding and spray drying; wherein, by mass ratio, ferric phosphate: lithium carbonate: lithium phosphate: polyethylene glycol: water = 1:0.23:0.02:0.01:1.8, the grinding time is 27 min, and the particle size of the mixture after sand milling is 0.15~0.25 μm; S3. The dried mixture is calcined at high temperature in a mixed protective gas of argon and nitrogen at 850℃ for 15 hours to obtain lithium iron phosphate; the volume ratio of argon to nitrogen in the mixed protective gas is 0.6:0.4.

[0042] Example 7 A method for preparing lithium iron phosphate includes the following steps: S1. Prepare 153g of ferric phosphate according to Example 4 above; S2. Add pure water, lithium hydroxide, lithium phosphate, sucrose, and carbon black to the ferric phosphate prepared in S1, and then perform grinding and spray drying. The mass ratio of ferric phosphate: lithium hydroxide: lithium phosphate: sucrose: carbon black: water is 1:0.23:0.02:0.005:0.005:1.8, the grinding time is 30 min, and the particle size of the mixture after sand milling is 0.15~0.25 μm. S3. The dried mixture is calcined at high temperature in a protective gas mixture of argon and nitrogen at 850°C for 12 hours to obtain lithium iron phosphate. The volume ratio of argon to nitrogen in the protective gas mixture is 0.7:0.3.

[0043] Comparative Example 1 A method for preparing ferric phosphate by iron-based method, differing from Example 1 only in that, in step (1), the phosphorus-to-iron ratio in Example 1 is 0.4, while the phosphorus-to-iron ratio in Comparative Example 1 is 0.25, including the following steps: (1) Preparation of ferrous dihydrogen phosphate solution: 321g of pure water was added to the iron melting reactor beforehand. After stirring, 175g of industrial grade phosphoric acid with a mass fraction of 85% was slowly added and mixed to obtain a 30% phosphoric acid solution. The temperature was raised to 50℃ for later use. According to the phosphorus-iron ratio of 0.25, 358g of iron sheet with an iron content of 95% was weighed and put into the iron melting reactor. The timer was started after the reaction was carried out. After 8.0h, the mixture was filtered immediately to obtain ferrous dihydrogen phosphate solution and 315g of undissolved iron sheet. The iron source leaching rate was 12%. The undissolved iron sheet can be used as the iron source for the next batch. (2) Synthesis oxidation: Add the above ferrous dihydrogen phosphate solution to the synthesis vessel, start stirring at a frequency of 50 Hz, heat the solution to 40°C, and slowly add 522 g of 25% hydrogen peroxide over a period of 1 hour. (3) Heating and aging: The reaction system of step (2) is heated to 95°C for aging. After the system turns white, it is kept at the temperature for 1 hour. After the reaction is completed, a white slurry is obtained. (4) Washing, drying and calcining: The above white slurry is filtered, and the white filter cake is washed with pure water. The washed filter cake is dried in an oven at 80°C for 12 hours to obtain ferric phosphate dihydrate. The dried material is then placed in a muffle furnace for sintering at 550°C for 5 hours to obtain anhydrous ferric phosphate.

[0044] Comparative Example 2 A method for preparing iron phosphate by iron-based reaction differs from Example 3 only in that, in step (1), the reaction temperature in Example 3 is 80°C, while the reaction temperature in Comparative Example 2 is 30°C. The method includes the following steps: (1) Preparation of ferrous dihydrogen phosphate solution: 637g of pure water was added to the iron melting reactor beforehand. After stirring, 910g of industrial grade phosphoric acid with a mass fraction of 85% was slowly added and mixed to obtain a 50% phosphoric acid solution. The temperature was raised to 30℃ for later use. According to the phosphorus-iron ratio of 1.3, 358g of iron sheet with an iron content of 95% was weighed and put into the iron melting reactor. The timer was started after the reaction was carried out. After 3.5h, the mixture was filtered immediately to obtain ferrous dihydrogen phosphate solution and 315g of undissolved iron sheet. The iron source leaching rate was 12%. The undissolved iron sheet can be used as the iron source for the next batch. (2) Synthesis oxidation: Add the above ferrous dihydrogen phosphate solution to the synthesis vessel, start stirring at a frequency of 50 Hz, heat the solution to 65°C, and slowly add 522 g of 25% hydrogen peroxide over a period of 1 hour. (3) Heating and aging: The reaction system of step (2) is heated to 95°C for aging. After the system turns white, it is kept at the temperature for 1 hour. After the reaction is completed, a white slurry is obtained. (4) Washing, drying and calcining: The above white slurry is filtered, and the white filter cake is washed with pure water. The washed filter cake is dried in an oven at 80°C for 12 hours to obtain ferric phosphate dihydrate. The dried material is then placed in a muffle furnace for sintering at 550°C for 5 hours to obtain anhydrous ferric phosphate.

[0045] Comparative Example 3 A method for preparing iron phosphate by iron-based reaction differs from Example 4 only in that, in step (1), the reaction time in Example 4 is 2.0 h, while the reaction time in Comparative Example 3 is 4.0 h, and includes the following steps: (1) Preparation of ferrous dihydrogen phosphate solution: 525g of pure water was added to the iron melting reactor beforehand. After stirring, 1260g of industrial grade phosphoric acid with a mass fraction of 85% was slowly added and mixed to obtain a 60% phosphoric acid solution. The temperature was raised to 90℃ for later use. According to the phosphorus-iron ratio of 1.8, 358g of iron sheet with an iron content of 95% was weighed and put into the iron melting reactor. The timer was started after the reaction was carried out. After 4.0h, the mixture was filtered immediately to obtain ferrous dihydrogen phosphate solution and 282g of undissolved iron sheet. The iron source leaching rate was 21%. The undissolved iron sheet can be used as the iron source for the next batch. (2) Synthesis oxidation: Add the above ferrous dihydrogen phosphate solution to the synthesis vessel, start stirring at a frequency of 50 Hz, heat the solution to 40°C, and slowly add 522 g of 25% hydrogen peroxide over a period of 1 hour. (3) Heating and aging: The reaction system of step (2) is heated to 95°C for aging. After the system turns white, it is kept at the temperature for 1 hour. After the reaction is completed, a white slurry is obtained. (4) Washing, drying and calcining: The above white slurry is filtered, and the white filter cake is washed with pure water. The washed filter cake is dried in an oven at 80°C for 12 hours to obtain ferric phosphate dihydrate. The dried material is then placed in a muffle furnace for sintering at 550°C for 5 hours to obtain anhydrous ferric phosphate.

[0046] Comparative Example 4 A method for preparing iron phosphate by iron-based reaction differs from Example 1 only in that, in step (2), the reaction temperature in Example 1 is 40°C, while the reaction temperature in Comparative Example 4 is 75°C. The method includes the following steps: (1) Preparation of ferrous dihydrogen phosphate solution: 513g of pure water was added to the iron melting reactor beforehand. After stirring, 280g of industrial grade phosphoric acid with a mass fraction of 85% was slowly added. After mixing evenly, a phosphoric acid solution with a mass fraction of 30% was obtained. The temperature was raised to 50℃ for later use. According to the phosphorus-iron ratio of 0.4, 358g of iron sheet with an iron content of 95% was weighed and put into the iron melting reactor. The timing was started after the reaction was carried out. After 8.0h, the mixture was filtered immediately to obtain ferrous dihydrogen phosphate solution and 301g of undissolved iron sheet. The iron source leaching rate was 16%. The undissolved iron sheet can be used as the iron source for the next batch. (2) Synthesis oxidation: Add the above ferrous dihydrogen phosphate solution to the synthesis vessel, start stirring at a frequency of 50 Hz, heat the solution to 75°C, and slowly add 522 g of 25% hydrogen peroxide over a period of 1 hour. (3) Heating and aging: The reaction system of step (2) is heated to 95°C for aging. After the system turns white, it is kept at the temperature for 1 hour. After the reaction is completed, a white slurry is obtained. (4) Washing, drying and calcining: The above white slurry is filtered, and the white filter cake is washed with pure water. The washed filter cake is dried in an oven at 80°C for 12 hours to obtain ferric phosphate dihydrate. The dried material is then placed in a muffle furnace for sintering at 550°C for 5 hours to obtain anhydrous ferric phosphate.

[0047] Comparative Example 5 A method for preparing lithium iron phosphate includes the following steps: S1. Prepare 258g of ferric phosphate according to the above Comparative Example 1; S2. Add pure water, lithium carbonate and glucose to the ferric phosphate prepared in S1, and then perform grinding and spray drying; wherein, by mass ratio, ferric phosphate: lithium carbonate: glucose: water = 1:0.25:0.01:1.8, grinding time is 40 min, and the particle size of the mixture after sand milling is 0.75 μm. S3. The dried mixture is calcined at high temperature in a mixed protective gas of argon and nitrogen at 750°C for 10 hours to obtain lithium iron phosphate; the volume ratio of argon to nitrogen in the mixed protective gas is 0.6:0.4.

[0048] Test case 1. Scanning electron microscopy (SEM) detection The ferric phosphate dihydrate and anhydrous ferric phosphate prepared in Examples 1-3 were examined by scanning electron microscopy (SEM); the SEM images of ferric phosphate dihydrate from Example 1 at different magnifications are shown below. Figures 1-2 As shown, the SEM images of anhydrous ferric phosphate in Example 1 at different magnifications are as follows. Figures 3-4 As shown; the ferric phosphate dihydrate and anhydrous ferric phosphate prepared in Comparative Example 1 were examined by scanning electron microscopy (SEM). The SEM images of ferric phosphate dihydrate in Comparative Example 1 at different magnifications are shown below. Figures 5-6 SEM images of anhydrous ferric phosphate in Comparative Example 1 at different magnification ratios are shown below. Figures 7-8 As shown.

[0049] Depend on Figures 1-4 It can be seen that the primary particles of ferric phosphate prepared in Example 1 are spherical or irregularly shaped, with a particle size concentrated in the range of 0.1~0.5μm. There is no obvious agglomeration of large particles, exhibiting good macroscopic morphological uniformity. Similar results were also detected in other examples (not shown in the figures); Figures 5-8 It can be seen that the ferric phosphate prepared in Comparative Example 1 is mainly in the form of flakes, with obvious agglomeration between particles forming flower-like aggregates, and the aggregate particle size is distributed in the range of 3~15μm. Therefore, the ferric phosphate prepared by the method provided by the present invention has smaller primary particle size and more dispersed particles.

[0050] 2. X-ray diffraction test X-ray diffraction (XRD) tests were performed on the ferric phosphate dihydrate and anhydrous ferric phosphate prepared in Examples 1-3. The detection results for ferric phosphate dihydrate and anhydrous ferric phosphate in Example 1 are as follows: Figures 9-10 As shown, the positions of all characteristic diffraction peaks in the XRD pattern of ferric phosphate dihydrate in Example 1 completely match the standard card (JCPDS No. 04-011-0357), indicating that the prepared ferric phosphate is pure phase ferric phosphate dihydrate; the XRD pattern of ferric phosphate in Example 1 completely matches the standard card (JCPDS No. 04-010-9184), indicating that the prepared product is pure phase anhydrous ferric phosphate; similar results were also detected in other examples (not shown in the figures).

[0051] 3. Iron and phosphorus content detection The iron and phosphorus contents of the anhydrous ferric phosphate prepared in Examples 1-3 and Comparative Example 1 were tested, and the results are shown in Table 1. The iron-phosphorus ratio is the molar ratio of iron to phosphorus in the anhydrous ferric phosphate.

[0052] Table 1. Results of iron and phosphorus content detection

[0053] 4. Grinding efficiency test The anhydrous ferric phosphate prepared in Examples 1-4 and Comparative Examples 1-4 was subjected to grinding efficiency tests. The grinding efficiency test results of the anhydrous ferric phosphate prepared in Examples 1-2 and Comparative Example 1 are as follows: Figure 11 As shown. The anhydrous ferric phosphate prepared in Examples 1 and 2 had an initial D50 value of 2 μm. After grinding for 10 min, the D50 value reached below 1.0 μm. With the extension of grinding time, the final grinding D50 value could be as low as 0.25 μm. Similar results were also tested in Examples 3 and 4. The anhydrous ferric phosphate prepared in Comparative Example 1 had an initial D50 value of 2 μm. After grinding for 30 min, the D50 value approached 1.0 μm. With the extension of grinding time, the final grinding D50 value could be as low as 0.75 μm. Similar results were also tested in Comparative Examples 2 and 4.

[0054] Because the phosphorus-to-iron ratio in step (1) of Comparative Example 1 is different from that in Example 1, the iron leaching rate in the iron dissolving reaction of Comparative Example 1 is much lower than that in Example 1, resulting in a grinding efficiency of anhydrous ferric phosphate prepared in Comparative Example 1 that is much lower than that in Example 1; because the reaction temperature in step (1) of Comparative Example 2 is different from that in Example 3, the iron leaching rate is much lower than that in Example 4, resulting in a grinding efficiency of anhydrous ferric phosphate prepared in Comparative Example 3 that is much lower than that in Example 4; because the reaction time in step (1) of Comparative Example 3 is different from that in Example 4, the iron leaching rate is much higher than that in Example 4, resulting in a grinding efficiency of anhydrous ferric phosphate prepared in Comparative Example 3 that is much lower than that in Example 4; because the reaction temperature in step (2) of Comparative Example 4 is much higher than that in Example 1, resulting in a grinding efficiency of anhydrous ferric phosphate prepared in Comparative Example 4 that is much lower than that in Example 1.

[0055] 5. Performance Testing The lithium iron phosphate samples prepared in Examples 5-7 and Comparative Example 5 were used to prepare coin cell samples, and their compaction density and electrical performance were tested. The test results are shown in Table 2. The charge-discharge curves of the lithium iron phosphate coin cell sample prepared in Example 5 at 0.1C are shown in Table 2. Figure 12 As shown.

[0056] Table 2. Compacted density and electrical performance test results of button cell samples.

[0057] As can be seen from the test results in Table 2, the lithium iron phosphate prepared in Examples 5-7 of this invention has a significantly higher compaction density than that of Comparative Example 5, and also exhibits a higher first charge-discharge efficiency. In particular, Example 5 achieved a compaction density of 2.569 g / cm³. 3The 0.1C specific discharge capacity is as high as 159.6 mAh / g, and the discharge efficiency is above 99%. This indicates that the preparation method provided by the present invention has a significant effect on improving the compaction density and electrical properties of lithium iron phosphate.

[0058] Although the present invention has been described in detail with reference to the accompanying drawings and preferred embodiments, the present invention is not limited thereto. Various equivalent modifications or substitutions can be made to the embodiments of the present invention by those skilled in the art without departing from the spirit and essence of the invention, and such modifications or substitutions should all be within the scope of the present invention. Any variations or substitutions that can be easily conceived by those skilled in the art within the technical scope disclosed in the present invention should also be covered within the protection scope of the present invention.

Claims

1. A method for preparing ferric phosphate using the iron-based process, characterized in that, Includes the following steps: (1) Preparation of ferrous dihydrogen phosphate solution: Phosphoric acid with a phosphorus-to-iron ratio of 0.4 to 1.8 and an iron source are added to an iron melting kettle to carry out an iron dissolution reaction. The reaction temperature is 50 to 90°C and the reaction time is 2 to 8 hours. The ferrous dihydrogen phosphate solution is obtained by filtration. When the phosphorus-to-iron ratio is 1.8 and the reaction temperature is 90°C, the reaction time is 2 hours. For every 0.1 decrease in the phosphorus-to-iron ratio, the reaction time is extended by 0.2 to 0.3 hours. For every 10°C decrease in the reaction temperature, the reaction time is extended by 0.5 to 1 hour. The iron source leaching rate is 15% to 17%. (2) Synthesis oxidation: The ferrous dihydrogen phosphate solution obtained in step (1) is added to the synthesis vessel, stirred and heated, and hydrogen peroxide is added; the temperature of the reaction system is 40~65℃; (3) Heating and aging: The reaction system of step (2) is heated and aged. After the system turns white, it is kept at the temperature. After the reaction is completed, the slurry is obtained. (4) Washing, drying and calcining: The slurry obtained in step (3) is subjected to solid-liquid separation, the filter cake is washed and dried to obtain ferric phosphate dihydrate, which is then placed in a muffle furnace for calcination and cooled to obtain anhydrous ferric phosphate.

2. The method for preparing ferric phosphate by the iron-based method as described in claim 1, characterized in that, In step (1), the iron source is at least one of iron powder, iron bar, iron sheet or iron ingot; the mass fraction of the phosphoric acid is 30% to 60%.

3. The method for preparing ferric phosphate by the iron-based process as described in claim 1, characterized in that, In step (2), the stirring frequency is 20~50Hz.

4. The method for preparing ferric phosphate by the iron-based method as described in claim 1, characterized in that, In step (2), the mass fraction of hydrogen peroxide is 5%~25%; the molar ratio of hydrogen peroxide to iron in the ferrous dihydrogen phosphate solution is (0.7~1.5):1; the hydrogen peroxide is added slowly over a period of 0.5~1h.

5. The method for preparing ferric phosphate by the iron-based method as described in claim 1, characterized in that, In step (3), the temperature for heating and aging is 86~95℃, and the aging time is 1~4h.

6. The method for preparing ferric phosphate by the iron-based method as described in claim 1, characterized in that, In step (4), the drying temperature is 80~110℃ and the drying time is 6~12h; the calcination temperature is 550~650℃ and the calcination time is 3~5h.

7. A method for preparing lithium iron phosphate, characterized in that, Includes the following steps: S1. Ferric phosphate is prepared according to any one of claims 1 to 6; S2. After adding pure water, lithium source and carbon source to the iron phosphate prepared in S1, the mixture is ground and then spray-dried. S3. The dried mixture is calcined at high temperature in a protective atmosphere at a temperature of 750~850℃ for 10~15h to obtain lithium iron phosphate.

8. The method for preparing lithium iron phosphate as described in claim 7, characterized in that, In step S2, the lithium source is at least one of lithium carbonate, lithium hydroxide, or lithium sulfate; the carbon source is at least one of glucose, sucrose, carbon black, or polyethylene glycol.

9. The method for preparing lithium iron phosphate as described in claim 7, characterized in that, In step S2, the particle size range of the mixture obtained by grinding is 0.15~0.25μm.

10. The method for preparing lithium iron phosphate as described in claim 7, characterized in that, In step S3, the protective gas is a mixture of argon and nitrogen, wherein the volume percentage of argon in the mixture is 60% to 70%.

Images