Preparation method of iron phosphate with high magnification and high grinding efficiency

Abstract

The invention relates to the technical field of lithium battery materials, in particular to a preparation method of iron phosphate with high magnification and high grinding efficiency, which comprises the following steps: (1) dissolving an iron source in phosphoric acid with the temperature of 50-100 DEG C to obtain an iron phosphorus solution; (2) simultaneously adding an oxidizing agent and the iron-phosphorus solution into a continuous reactor for reaction, and generating a yellow material after the reaction is finished; (3) filtering and washing the yellow material by adopting a continuous membrane, and then adding water to prepare yellow slurry, wherein the iron concentration in the yellow slurry is 1.2-1.7 mol / L; (4) adding the yellow slurry into a continuous reactor, heating to 90-100 DEG C, and reacting for 5-10 minutes to generate powder white slurry; and (5) filtering and washing the prepared powdery white slurry by adopting a continuous membrane to obtain a filter cake, and drying and calcining the filter cake to obtain iron phosphate. The iron phosphate prepared by the preparation method provided by the invention has high magnification and high grinding efficiency.

Description

Technical Field

[0001] The invention relates to the technical field of lithium battery materials, and in particular to a method for preparing iron phosphate with high rate and high grinding efficiency. Background Art

[0002] Compared with nickel-cobalt-manganese-oxide ternary materials, lithium iron phosphate has become the core battery material for current new energy vehicle power batteries and large-scale energy storage systems due to its outstanding cost advantages, excellent thermal stability, and long cycle life. However, its low intrinsic conductivity and lithium ion migration efficiency lead to significant performance degradation of the battery during high-rate charge and discharge, which has become a key bottleneck restricting its further application. At present, the mainstream preparation process of industrial-grade lithium iron phosphate still uses precursor iron phosphate as the core raw material, with lithium carbonate as the lithium source and carbon source (such as glucose or sucrose), and through multiple processes such as sand milling premixing, spray drying granulation, high-temperature solid-phase reaction sintering, and air flow crushing and grading, a positive electrode material with a specific particle size distribution and carbon coating structure is finally obtained.

[0003] The grinding efficiency of iron phosphate in the sand grinding process plays a vital role in the production efficiency of lithium iron phosphate. The low grinding efficiency of iron phosphate and the long grinding time will directly affect the production efficiency and production capacity of the product, and will also affect the rate performance of lithium iron phosphate products. For example, CN117509583A uses a two-step intermittent reaction to prepare iron phosphate, and a high-grinding-efficiency iron phosphate is obtained by controlling the solid content of the yellow slurry in the second step, and adding phosphoric acid and phosphate at the same time. This high-grinding-efficiency iron phosphate has a high charge and discharge capacity after being prepared into lithium iron phosphate, but the rate performance of the positive electrode material has not been studied, and the reactor used is still an intermittent reaction, and the reaction time is long. The existing mainstream preparation method of iron phosphate mainly adopts an intermittent preparation method, that is, a reactor is used as a reaction vessel, and a filter press is used as a filtering and washing equipment. Due to the intermittent reaction, there are differences in the materials between the kettles, and the stability and batch stability of the materials are poor. In addition, the time for material reaction and insulation is long, generally more than 2 hours, resulting in high energy consumption and low production capacity. For example, in CN117509582A, intermittent reaction is used to prepare ferric phosphate, which needs to be kept warm for 3-5 hours after the material turns white, with long insulation time, high energy consumption and low production efficiency. In CN117623257A, a micro-dynamic reactor is used to continuously react and prepare ferric phosphate, but the impurity content of the ferric phosphate prepared is relatively large, and the primary particles are relatively large, which is 100-200nm, which is not conducive to the improvement of electrical properties and rate performance, and the secondary particles are dense large agglomerates with low grinding efficiency. In CN107473196A, the first step is to use a microchannel mixer for continuous mixing reaction, and the second step is to use microchannel secondary continuous hydrothermal preparation of ferric phosphate, but the hydrothermal reaction temperature of the second step is 110-180°C, the reaction temperature is high, the energy consumption is high, and the reaction is under high pressure, there is a certain safety risk, and the hydrothermal reaction is not conducive to the mass production of the production line. Summary of the invention

[0004] In view of the technical problems of long reaction time and low production efficiency in preparing iron phosphate by intermittent reaction, and low grinding efficiency and high reaction temperature in conventional continuous reaction, the present invention provides a method for preparing iron phosphate with high rate and high grinding efficiency, adopting a continuous reactor, continuous inlet and outlet of raw materials, which can greatly improve production efficiency, reduce production energy consumption and production cost. The primary particle size of iron phosphate prepared by the continuous method is 10-20nm, and the primary particles are evenly dispersed and not agglomerated, so it has a very high grinding efficiency, can improve the production efficiency and production capacity of lithium iron phosphate, thereby reducing the production cost of lithium iron phosphate, and the prepared lithium iron phosphate has excellent rate performance.

[0005] The technical solution of the present invention is as follows: A method for preparing iron phosphate with high magnification and high grinding efficiency comprises the following steps: (1) dissolving an iron source in phosphoric acid at a temperature of 50-100° C. to obtain an iron-phosphorus solution, wherein the molar ratio of iron to phosphorus in the iron-phosphorus solution is 3:3.6-3.8; (2) adding an oxidant and an iron-phosphorus solution to a continuous reactor for reaction at the same time, wherein the molar ratio of the oxidant to iron is 0.6-0.7:1, the reaction temperature is room temperature, the reaction time is 5-10 min, and a yellow material is generated after the reaction is completed; (3) The yellow material is washed by continuous membrane filtration and then water is added to prepare a yellow slurry, wherein the iron concentration in the yellow slurry is 1.2-1.7 mol / L; (4) Add the yellow slurry to a continuous reactor, raise the temperature to 90-100°C, and react for 5-10 minutes to generate a pink-white slurry; (5) The obtained powdery white slurry is washed by continuous membrane filtration to obtain a filter cake, and the filter cake is dried and calcined to obtain iron phosphate.

[0006] Furthermore, in step (1), the iron source is selected from scraps of iron products, including iron sheets, iron blocks and iron bars, and the concentration of phosphoric acid is 35%-85%.

[0007] Furthermore, in step (2), the oxidant is selected from at least one of hydrogen peroxide, potassium permanganate, peracetic acid and ammonium persulfate; and the reaction time of the oxidant and iron is 5-6 minutes.

[0008] Furthermore, after step (2) is completed, the yellow material is tested to confirm that there is no ferrous ion; the detection reagent is potassium ferrocyanide solution.

[0009] Furthermore, in step (3), the pressure of the yellow material during continuous membrane washing is 0.2-1.2 MPa, the temperature during washing is 40-80°C, the conductivity of the washing water is 5000-10000 us / cm, the solid content of the prepared yellow slurry is 4%-20%, and the yellow slurry is in a stirring state at a stirring speed of 250-350 rpm.

[0010] Furthermore, in step (3), the pressure of the yellow material during continuous membrane washing is 0.2 MPa, the temperature during washing is 50° C., and the conductivity of the washing water is 6000 us / cm.

[0011] Furthermore, the solid content of the yellow slurry prepared in step (3) is 15%-20%.

[0012] Furthermore, in step (5), the white slurry is washed with a continuous membrane at a pressure of 0.2-1.2 MPa and a temperature of 40-80° C., and is filtered and washed until the conductivity is ≤1000 us / cm.

[0013] Furthermore, in step (5), the white slurry is washed with a continuous membrane at a pressure of 0.2 MPa and a temperature of 50° C., and is filtered and washed until the conductivity is less than 1000 us / cm.

[0014] Furthermore, in step (6), the drying temperature of the filter cake is 100-110°C, the drying time is 3-5h, the sintering temperature is 550-650°C, and the sintering time is 3-5h.

[0015] The beneficial effects of the present invention are: Traditional iron phosphate is produced in an intermittent manner, with a long reaction time, a long heat preservation time, low production efficiency, and large differences between reactors, resulting in poor stability and consistency of material batches. The preparation method of iron phosphate with high rate and high grinding efficiency provided by the present invention adopts continuous production, which can greatly improve the reaction speed, shorten the reaction time, does not require insulation, has high production efficiency, and because the material is continuously fed in and out, there is no difference between reactors, and the stability and consistency between material batches are good, while reducing production costs, the quality of the product can be improved. The prepared iron phosphate has a high grinding efficiency as a raw material in the process of preparing lithium iron phosphate, which can improve the production efficiency of lithium iron phosphate and reduce the production cost of lithium iron phosphate. The prepared lithium iron phosphate has the characteristics of high compaction density, high discharge capacity and good rate performance. BRIEF DESCRIPTION OF THE DRAWINGS

[0016] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings required for use in the embodiments or the description of the prior art will be briefly introduced below. Obviously, for ordinary technicians in this field, other drawings can be obtained based on these drawings without paying any creative work.

[0017] Figure 1 This is the XRD pattern of the iron phosphate prepared in Example 1.

[0018] Figure 2 This is the SEM image (5000 times) of the iron phosphate prepared in Example 1.

[0019] Figure 3 This is the SEM image (50,000 times) of the iron phosphate prepared in Example 1.

[0020] Figure 4 This is the XRD pattern of the iron phosphate prepared in Example 2.

[0021] Figure 5 This is the SEM image (5000 times) of the iron phosphate prepared in Example 2.

[0022] Figure 6 This is the SEM image of the iron phosphate prepared in Example 2 (50,000 times).

[0023] Figure 7 This is the XRD pattern of the iron phosphate prepared in Example 3.

[0024] Figure 8 This is the SEM image (5000 times) of the iron phosphate prepared in Example 3.

[0025] Fig. 9 This is the SEM image (50,000 times) of the iron phosphate prepared in Example 3.

[0026] Fig.10 This is the XRD pattern of the iron phosphate prepared in Comparative Example 1.

[0027] Fig.11 This is the SEM image (5000 times) of the iron phosphate prepared in Comparative Example 1.

[0028] Fig.12 This is the SEM image (50,000 times) of the iron phosphate prepared in Comparative Example 1.

[0029] Fig.13 This is the XRD pattern of the iron phosphate prepared in Comparative Example 2.

[0030] Fig.14 This is the SEM image (5000 times) of the iron phosphate prepared in Comparative Example 2.

[0031] Fig.15 This is the SEM image (50,000 times) of the iron phosphate prepared in Comparative Example 2.

[0032] Fig.16 The figure is a comparison chart of the grinding efficiency of the iron phosphate prepared in Examples 1, 2, 3 and Comparative Examples 1 and 2.

[0033] Fig.17 It is a comparison chart of the rate performance of Examples 1-3 and Comparative Examples 1-2. DETAILED DESCRIPTION

[0034] In order to enable those skilled in the art to better understand the technical solutions in the present invention, the technical solutions in the embodiments of the present invention will be clearly and completely described below in conjunction with the drawings in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work should fall within the scope of protection of the present invention.

[0035] Example 1 A method for preparing iron phosphate with high magnification and high grinding efficiency comprises the following steps: (1) Weigh 1.7 kg of scrap iron flakes and dissolve them in 85% hot phosphoric acid (constant temperature 50°C) to obtain a green iron-phosphorus solution for later use; the molar ratio of iron to phosphoric acid in the iron-phosphorus solution is 3:3.6; (2) adding 15% hydrogen peroxide and iron-phosphorus solution into a continuous reactor at the same time, wherein the molar ratio of hydrogen peroxide to iron is 0.6:1, and the reaction temperature is room temperature. During the reaction, hydrogen peroxide and iron-phosphorus solution are continuously fed and discharged in the continuous reactor. The residence time of the materials in the continuous reactor is 5 min (i.e., the reaction time). After the reaction is completed, a yellow material is generated, and ferrous ions are detected with potassium ferrocyanide solution to confirm that there are no ferrous ions. (3) The yellow material was washed by continuous membrane filtration at a pressure of 0.2 MPa and a temperature of 50°C until the conductivity of the washing water reached 6000 us / cm, and then water was added to prepare 25 L of yellow slurry; the iron concentration in the yellow slurry was 1.2 mol / L, the solid content was 15%, and the yellow slurry was in a continuous stirring state at a stirring speed of 250 rpm; (4) Add the yellow slurry into a continuous reactor and heat it to 90°C. After reacting for 5 minutes, a pink-white slurry is generated. (5) washing the obtained powdery white slurry by continuous membrane filtration at a pressure of 0.2 MPa and a temperature of 50° C. until the conductivity is less than 1000 us / cm, thereby obtaining a filter cake; (6) The filter cake was dried at 100°C for 3 h and then calcined at 550°C for 5 h to obtain 4.50 kg of iron phosphate.

[0036] The samples prepared from the obtained iron phosphate were subjected to XRD and SEM tests, and the test results were shown in Figure 1-3 In. By Figure 1 The XRD results show that the obtained iron phosphate is a pure phase without other impurities. From the peak diffraction intensity, the crystallinity is high. Figure 2 The low-magnification SEM results show that the secondary particles are dispersed and no agglomerates are formed, which is beneficial to the grinding during the preparation of lithium iron phosphate and can improve the grinding efficiency. Figure 3 The high-magnification SEM results show that the primary particles of iron phosphate are small, 10-20nm, which is conducive to the entry of lithium to form lithium iron phosphate when synthesizing lithium iron phosphate. When the slurry is in an intermittent reactor, it is easy to form a horizontal flow due to the large volume of the reactor and the stirring method, and the material mixing and reaction speed are slow. However, in a continuous reactor, the material is easy to form turbulence due to the small reactor volume, fast flow rate, and stirring, so the reaction speed is fast and the nucleation speed is also fast. A large number of nuclei limit the growth of primary particles, resulting in small primary particles of the material produced by the continuous reaction. Small primary particles are conducive to shortening the diffusion distance of lithium ions during charging and discharging, and can improve the rate performance of the positive electrode material.

[0037] Example 2 A method for preparing iron phosphate with high magnification and high grinding efficiency comprises the following steps: (1) Weigh 4.5 kg of scrap iron flakes and dissolve them in 35% hot phosphoric acid (constant temperature 50°C) to obtain a green iron-phosphorus solution for later use; the molar ratio of iron to phosphoric acid in the iron-phosphorus solution is 3:3.7; (2) adding 20% ​​hydrogen peroxide and iron-phosphorus solution into a continuous reactor at the same time, wherein the molar ratio of hydrogen peroxide to iron is 0.65:1, and the reaction temperature is room temperature. During the reaction, hydrogen peroxide and iron-phosphorus solution are continuously fed and discharged in the continuous reactor. The residence time of the materials in the continuous reactor is 5 min (i.e., the reaction time). After the reaction is completed, a yellow material is generated, and ferrous ions are detected with potassium ferrocyanide solution to confirm that there are no ferrous ions. (3) The yellow material was washed by continuous membrane filtration at a pressure of 0.2 MPa and a temperature of 50°C until the conductivity of the washing water reached 6000 us / cm, and then water was added to prepare 50 L of yellow slurry; the iron concentration in the yellow slurry was 1.5 mol / L, the solid content was 17%, and the yellow slurry was in a continuous stirring state at a stirring speed of 350 rpm; (4) Add the yellow slurry into a continuous reactor and heat it to 95°C. After reacting for 5 minutes, a pink-white slurry is generated. (5) washing the obtained powdery white slurry by continuous membrane filtration at a pressure of 0.2 MPa and a temperature of 50° C. until the conductivity is less than 1000 us / cm, thereby obtaining a filter cake; (6) The filter cake was dried at 100°C for 3 h and then calcined at 550°C for 5 h to obtain 9.12 kg of iron phosphate.

[0038] The samples prepared from the obtained iron phosphate were subjected to XRD and SEM tests, and the test results were shown in Figure 4-6 In. By Figure 4 The XRD results show that the obtained iron phosphate is a pure phase without other impurities. From the peak diffraction intensity, the crystallinity is high. Figure 5 The low-magnification SEM results show that the secondary particles are dispersed and no agglomerates are formed, which is beneficial to the grinding during the preparation of lithium iron phosphate and can improve the grinding efficiency. Figure 6 From the high-magnification SEM results, it can be seen that the primary particles of iron phosphate are small, 10-20nm, which is conducive to the entry of lithium into the formation of lithium iron phosphate when synthesizing lithium iron phosphate. Small primary particles are also conducive to shortening the diffusion distance of lithium ions during charging and discharging, and improving the rate performance of positive electrode materials.

[0039] Example 3 A method for preparing iron phosphate with high magnification and high grinding efficiency comprises the following steps: (1) Weigh 98 kg of scrap iron flakes and dissolve them in 85% hot phosphoric acid (constant temperature 50°C) to obtain a green iron-phosphorus solution for later use; the molar ratio of iron to phosphoric acid in the iron-phosphorus solution is 3:3.8; (2) adding 15% hydrogen peroxide and iron-phosphorus solution into a continuous reactor at the same time, wherein the molar ratio of hydrogen peroxide to iron is 0.7:1, and the reaction temperature is room temperature. During the reaction, hydrogen peroxide and iron-phosphorus solution are continuously fed and discharged in the continuous reactor. The residence time of the materials in the continuous reactor is 6 minutes (i.e., the reaction time). After the reaction is completed, a yellow material is generated. The ferrous ions are detected with a potassium ferrocyanide solution to confirm that there are no ferrous ions. (3) The yellow material was washed by continuous membrane filtration. The pressure during continuous membrane washing was 0.2 MPa and the temperature during washing was 50°C. The washing water conductivity was 6000 us / cm. Then water was added to prepare 1000 L of yellow slurry. The iron concentration in the yellow slurry was 1.7 mol / L and the solid content was 20%. The yellow slurry was in a continuous stirring state at a stirring speed of 350 rpm. (4) Add the yellow slurry into a continuous reactor and heat it to 95°C. After reacting for 5 minutes, a pink-white slurry is generated. (5) washing the obtained powdery white slurry by continuous membrane filtration at a pressure of 0.2 MPa and a temperature of 50° C. until the conductivity is less than 1000 us / cm, thereby obtaining a filter cake; (6) The filter cake was dried at 100°C for 3 h and then calcined at 550°C for 5 h to obtain 197.6 kg of iron phosphate.

[0040] The samples prepared from the obtained iron phosphate were subjected to XRD and SEM tests, and the test results were shown in Figure 7-9 In. By Figure 7 The XRD results show that the obtained iron phosphate is a pure phase without other impurities. From the peak diffraction intensity, the crystallinity is high. Figure 8 The low-magnification SEM results show that the secondary particles are dispersed and no agglomerates are formed, which is beneficial to the grinding during the preparation of lithium iron phosphate and can improve the grinding efficiency. Fig. 9 From the high-magnification SEM results, it can be seen that the primary particles of iron phosphate are small, 10-20nm, which is conducive to the entry of lithium into the formation of lithium iron phosphate when synthesizing lithium iron phosphate. Small primary particles are also conducive to shortening the diffusion distance of lithium ions during charging and discharging, and improving the rate performance of positive electrode materials.

[0041] In addition, from the XRD and SEM results, the repeatability, stability and consistency of the iron phosphate samples of Examples 1-3 are also very good, indicating that the iron phosphate prepared by the preparation method provided by the present invention does have advantages in stability and consistency.

[0042] Example 4 A method for preparing iron phosphate with high magnification and high grinding efficiency comprises the following steps: (1) Weigh 2.4 kg of scrap iron flakes and dissolve them in 85% hot phosphoric acid (constant temperature 50°C) to obtain a green iron-phosphorus solution for later use; the molar ratio of iron to phosphoric acid in the iron-phosphorus solution is 3:3.6; (2) adding 15% hydrogen peroxide and iron-phosphorus solution into a continuous reactor at the same time, wherein the molar ratio of hydrogen peroxide to iron is 0.6:1, and the reaction temperature is room temperature. During the reaction, hydrogen peroxide and iron-phosphorus solution are continuously fed and discharged in the continuous reactor. The residence time of the materials in the continuous reactor is 10 min (i.e., the reaction time). After the reaction is completed, a yellow material is generated, and ferrous ions are detected with potassium ferrocyanide solution to confirm that there are no ferrous ions. (3) The yellow material was washed by continuous membrane filtration at a pressure of 1.2 MPa and a temperature of 40°C until the conductivity of the washing water reached 6000 us / cm, and then water was added to prepare 30 L of yellow slurry; the iron concentration in the yellow slurry was 1.4 mol / L, the solid content was 17%, and the yellow slurry was in a continuous stirring state at a stirring speed of 300 rpm; (4) Add the yellow slurry into a continuous reactor and heat it to 100°C. After reacting for 10 minutes, a pink-white slurry is generated. (5) washing the obtained powdery white slurry by continuous membrane filtration at a pressure of 0.2 MPa and a temperature of 50° C. until the conductivity is less than 1000 us / cm, thereby obtaining a filter cake; (6) The filter cake was dried at 110°C for 5 h and then calcined at 650°C for 3 h to obtain 2.35 kg of iron phosphate.

[0043] Example 5 A method for preparing iron phosphate with high magnification and high grinding efficiency comprises the following steps: (1) Weigh 1.8 kg of scrap iron pieces and dissolve them in 85% hot phosphoric acid (constant temperature 50°C) to obtain a green iron-phosphorus solution for later use; the molar ratio of iron to phosphoric acid in the iron-phosphorus solution is 3:3.8; (2) adding 15% hydrogen peroxide and iron-phosphorus solution into a continuous reactor at the same time, wherein the molar ratio of hydrogen peroxide to iron is 0.6:1, and the reaction temperature is room temperature. During the reaction, hydrogen peroxide and iron-phosphorus solution are continuously fed and discharged in the continuous reactor. The residence time of the materials in the continuous reactor is 8 minutes (i.e., the reaction time). After the reaction is completed, a yellow material is generated, and ferrous ions are detected with potassium ferrocyanide solution to confirm that there are no ferrous ions. (3) The yellow material was washed by continuous membrane filtration at a pressure of 1.0 MPa and a temperature of 80°C until the conductivity of the washing water reached 5000 us / cm, and then water was added to prepare 24 L of yellow slurry; the iron concentration in the yellow slurry was 1.3 mol / L, the solid content was 17%, and the yellow slurry was in a continuous stirring state at a stirring speed of 250 rpm; (4) Add the yellow slurry into a continuous reactor and heat it to 100°C. After reacting for 8 minutes, a pink-white slurry is generated. (5) washing the obtained powdery white slurry by continuous membrane filtration at a pressure of 1.2 MPa and a temperature of 40° C. until the conductivity is less than 1000 us / cm, thereby obtaining a filter cake; (6) The filter cake was dried at 100°C for 3 h and then calcined at 550°C for 5 h to obtain 1.75 kg of iron phosphate.

[0044] Comparative Example 1 A method for preparing ferric phosphate (adopting intermittent reaction), comprising the following steps: (1) Weigh 98 kg of scrap iron flakes and dissolve them in 45% hot phosphoric acid (constant temperature 60°C) to obtain a green iron-phosphorus solution for later use; the molar ratio of iron to phosphoric acid in the iron-phosphorus solution is 3:3.8; (2) Add 20% hydrogen peroxide and iron-phosphorus solution into the reactor at the same time, wherein the molar ratio of hydrogen peroxide to iron is 0.7:1, the reaction temperature is room temperature, the addition time is 2 hours, the reaction time is 1 hour, and a yellow material is generated after the reaction is completed. Use potassium ferrocyanide solution to detect ferrous ions to confirm that there are no ferrous ions; (3) The yellow material was washed with a filter press until the conductivity of the washing water reached 6000us / cm, and then water was added to prepare 1000L of yellow slurry; the iron concentration in the yellow slurry was 1.7mol / L, the solid content was 19.5%, and the yellow slurry was in a continuous stirring state at a stirring speed of 350rpm; (4) Add the yellow slurry into a 2000L reactor, heat it to 95°C, react for 1.5 hours and turn it into a white slurry, and keep it warm for another 2 hours; (5) The obtained white slurry is washed again by a filter press until the conductivity is less than 6000 us / cm to obtain a filter cake; (6) The filter cake was dried at 100°C for 3 h and then calcined at 550°C for 5 h to obtain 197.6 kg of iron phosphate.

[0045] The samples prepared from the obtained iron phosphate were subjected to XRD and SEM tests, and the test results were shown in Figure 10-12 In. By Fig.10 The XRD results show that the obtained iron phosphate is a pure phase without other impurities, but from the peak diffraction intensity, the crystallinity is slightly low, and the intensity of the diffraction peak is much lower than that of the embodiment. Fig.11 From the low-magnification SEM, it can be seen that the secondary agglomerates are relatively large, spherical, and 10-20um in size. There are few gaps in the agglomerated particles, which is not conducive to the grinding during the preparation of lithium iron phosphate and will reduce the grinding efficiency. Fig.12 From the high-magnification SEM results, it can be seen that the primary particles of iron phosphate are large, 100-200nm, which will not be conducive to the entry of lithium into the formation of lithium iron phosphate during the synthesis of lithium iron phosphate, and the reaction may not be sufficient. Large primary particles are also not conducive to shortening the diffusion distance of lithium ions during charging and discharging, and are not conducive to improving the rate performance of positive electrode materials.

[0046] Comparative Example 2 A method for preparing iron phosphate with high magnification and high grinding efficiency (using a one-step continuous method) comprises the following steps: (1) Weigh 98 kg of scrap iron flakes and dissolve them in 45% hot phosphoric acid (constant temperature 60°C) to obtain a green iron-phosphorus solution for later use; the molar ratio of iron to phosphoric acid in the iron-phosphorus solution is 3:3.8; (2) adding 20% ​​hydrogen peroxide and iron-phosphorus solution into a continuous reactor at the same time, wherein the molar ratio of hydrogen peroxide to iron is 0.7:1, and the reaction temperature is room temperature. During the reaction, hydrogen peroxide and iron-phosphorus solution are continuously fed and discharged in the continuous reactor. The residence time of the materials in the continuous reactor is 6 minutes (i.e., the reaction time). After the reaction is completed, a yellow material is generated, and ferrous ions are detected with potassium ferrocyanide solution to confirm that there are no ferrous ions. (3) Add the yellow material into the continuous reactor, raise the temperature to 95°C, and react for 5 minutes to generate a pink-white slurry; (5) washing the obtained powdery white slurry by continuous membrane filtration at a pressure of 0.2 MPa and a temperature of 50° C. until the conductivity is less than 1000 us / cm, thereby obtaining a filter cake; (6) The filter cake was dried at 100°C for 3 h and then calcined at 550°C for 5 h to obtain 197.6 kg of iron phosphate.

[0047] The samples prepared from the obtained iron phosphate were subjected to XRD and SEM tests, and the test results were shown in Figure 13-15 In. By Fig.13 From the XRD results, it can be seen that the obtained iron phosphate is a pure phase without other impurities, but from the peak diffraction intensity, the intensity of the diffraction peak is much lower than that of the embodiment, and the crystallinity is low. Fig.14 The low-magnification SEM results show that the secondary particles are agglomerated to a certain extent, with a size of 5-10um, and there are few gaps in the agglomerated particles, which is not conducive to the grinding during the preparation of lithium iron phosphate and will reduce the grinding efficiency. Fig.15 From the high-magnification SEM results, it can be seen that the primary particles of iron phosphate are large, 100-200nm, and the primary particles are blocky, relatively compact and strong, which is not conducive to grinding, nor is it conducive to the entry of lithium to form lithium iron phosphate during the synthesis of lithium iron phosphate, and the reaction may not be sufficient. Large primary particles are also not conducive to shortening the diffusion distance of lithium ions during charging and discharging, and are not conducive to improving the rate performance of positive electrode materials.

[0048] Experimental Example 1: Impurity Detection The iron phosphate obtained in Examples 1-3 and Comparative Examples 1-2 was tested for impurities by ICP, and the test results are shown in Table 1 below. As can be seen from Table 1, the impurity content of Examples 1-3 is significantly lower than that of Comparative Examples 1-2, probably because the continuous reaction process is more fully reacted, the product has better stability and consistency, and the crystallization is more sufficient, compared with the intermittent reaction in the comparative example, and the crystallization process is a process of removing impurities. Although Comparative Example 2 is also a continuous method, it adopts a one-step continuous method and is only washed once, so the impurities are higher than the two washes in the embodiment.

[0049] Table 1 ICP impurity detection results of examples and comparative examples

[0050] Experimental Example 2 Grinding Efficiency Test The iron phosphate prepared in Example 1-3 and Comparative Example 1-2 was used to prepare lithium iron phosphate under the same conditions, and was named 1-5# lithium iron phosphate, respectively. The specific operation was as follows: The iron phosphate prepared in Examples 1-3 and Comparative Examples 1-2 was mixed with deionized water, lithium carbonate and glucose under the same conditions, and the mixture was sand-milled to obtain a slurry. Samples were taken at different time periods to detect the particle size, and the grinding efficiency of the iron phosphate in Examples 1-3 and Comparative Examples 1-2 was obtained. The results are shown in FIG. Fig.16 The sand-milled slurry was spray-dried, and then the material was sintered at 720° C. for 20 h in a nitrogen atmosphere to obtain 1-5# lithium iron phosphates, respectively.

[0051] Depend on Fig.16It can be seen that the iron phosphate prepared in Example 1-3 has a very high grinding efficiency, and can be ground to a target particle size of less than 400 nm in 20 minutes, while the iron phosphate in Comparative Example 1-2 is ground to 100 minutes, but still does not reach the target particle size of less than 400 nm, and is very difficult to grind. The grinding efficiency of the iron phosphate prepared in Example 1-3 is significantly higher than the grinding efficiency of the iron phosphate prepared in Comparative Example 1-2.

[0052] Experimental Example 3 Electrical Performance Test The 1-5# lithium iron phosphate prepared in Experimental Example 2 was prepared into button batteries under the same conditions according to conventional methods, and were named as button batteries 1-5# respectively. Then, the electrical properties of the button batteries 1-5# were tested under the same conditions according to conventional methods in the art. The electrical performance test results of the button batteries 1-5# are shown in Table 2, and the comparison of the battery rate performance is shown in Table 3.

[0053] Table 2 Comparison of electrical performance of button batteries #1-5

[0054] From Table 2, it can be seen that compared with button batteries 4# and 5# (the source of raw material iron phosphate is comparative example 1 and comparative example 2), the charge and discharge performance and compaction density of button batteries 1-3# (the source of raw material iron phosphate is embodiment 1-3) have been significantly improved. Compared with button batteries 4# and 5#, the charge capacity of button batteries 1-3# has increased by 4-5 mAh / g, the discharge capacity has increased by 6-7 mAh / g, and the compaction density has increased by 0.2 g / cm 3 This indicates that the iron phosphate prepared by the continuous method has more advantages than the iron phosphate prepared by the intermittent method and the one-step continuous method.

[0055] Table 3 Comparison of rate performance of button batteries #1-5 (mAh / g)

[0056] Table 3 and Fig.17The data comparison results of the battery rate performance of the button batteries prepared by the same method for the iron phosphate of Examples 1-3 and Comparative Examples 1-2 are shown. As can be seen from Table 3, the rate performance of the button batteries (1-3# button batteries) prepared by the two-step continuous method is significantly better than the rate performance of the button batteries (4#, 5# button batteries) prepared by the two-step intermittent method and the one-step continuous method. This is mainly because the precursor prepared by the two-step continuous method has small primary particles and small and dispersed secondary agglomerates. Small primary particles are conducive to lithium entering the reaction to generate positive electrode materials when synthesizing lithium iron phosphate. In addition, small primary particles are conducive to shortening the path of lithium ion extraction and embedding, thereby improving its rate performance. Small and dispersed secondary particles are conducive to the depolymerization of the grinding process section when synthesizing lithium iron phosphate, making it easy to grind iron phosphate to form small particles, thereby improving the rate performance of lithium iron phosphate.

[0057] Although the present invention has been described in detail with reference to the accompanying drawings and in combination with the preferred embodiments, the present invention is not limited thereto. Without departing from the spirit and essence of the present invention, a person of ordinary skill in the art may make various equivalent modifications or substitutions to the embodiments of the present invention, and these modifications or substitutions shall be within the scope of the present invention. Any person of ordinary skill in the art may easily think of changes or substitutions within the technical scope disclosed by the present invention, and these shall be within the scope of protection of the present invention.

Claims

1. A method for preparing iron phosphate with high magnification and high grinding efficiency, characterized in that: The steps include: (1) dissolving an iron source in phosphoric acid at a temperature of 50-100° C. to obtain an iron-phosphorus solution, wherein the molar ratio of iron to phosphorus in the iron-phosphorus solution is 3:3.6-3.8; (2) adding an oxidant and an iron-phosphorus solution to a continuous reactor for reaction at the same time, wherein the molar ratio of the oxidant to iron is 0.6-0.7:1, the reaction temperature is room temperature, the reaction time is 5-10 min, and a yellow material is generated after the reaction is completed; (3) The yellow material is washed by continuous membrane filtration and then water is added to prepare a yellow slurry, wherein the iron concentration in the yellow slurry is 1.2-1.7 mol / L; (4) Add the yellow slurry to a continuous reactor, raise the temperature to 90-100°C, and react for 5-10 minutes to generate a pink-white slurry; (5) The obtained powdery white slurry is washed by continuous membrane filtration to obtain a filter cake, and the filter cake is dried and calcined to obtain iron phosphate.

2. A method for preparing iron phosphate with high magnification and high grinding efficiency as claimed in claim 1, characterized in that: In step (1), the iron source is selected from the scraps of iron products, including iron sheets, iron blocks and iron bars, and the concentration of phosphoric acid is 35%-85%.

3. The method for preparing iron phosphate with high magnification and high grinding efficiency as claimed in claim 1, characterized in that: In step (2), the oxidant is selected from at least one of hydrogen peroxide, potassium permanganate, peracetic acid and ammonium persulfate; and the reaction time of the oxidant and iron is 5-6 minutes.

4. The method for preparing iron phosphate with high magnification and high grinding efficiency as claimed in claim 1, characterized in that: After step (2) is completed, the yellow material is tested to confirm that there is no ferrous ion; the detection reagent is potassium ferrocyanide solution.

5. The method for preparing iron phosphate with high magnification and high grinding efficiency as claimed in claim 1, characterized in that: In step (3), the pressure of the yellow material during continuous membrane washing is 0.2-1.2 MPa, the temperature during washing is 40-80°C, the conductivity of the washing water is 5000-10000 us / cm, and the solid content of the prepared yellow slurry is 4%-20%. The yellow slurry is in a stirring state at a stirring speed of 250-350 rpm.

6. The method for preparing iron phosphate with high magnification and high grinding efficiency as claimed in claim 1, characterized in that: In step (3), the pressure of the yellow material when washed by continuous membrane is 0.2 MPa, the temperature during washing is 50°C, and the conductivity of the washing water is 6000 us / cm.

7. The method for preparing iron phosphate with high magnification and high grinding efficiency as claimed in claim 1, characterized in that: The yellow slurry prepared in step (3) has a solid content of 15%-20%.

8. The method for preparing iron phosphate with high magnification and high grinding efficiency as claimed in claim 1, characterized in that: In step (5), the white slurry is washed by continuous membrane at a pressure of 0.2-1.2 MPa and a temperature of 40-80° C. The slurry is filtered and washed until the conductivity is ≤1000 us / cm.

9. The method for preparing iron phosphate with high magnification and high grinding efficiency as claimed in claim 1, characterized in that: In step (5), the white slurry is washed with a continuous membrane at a pressure of 0.2 MPa and a temperature of 50° C. The slurry is filtered and washed until the conductivity is less than 1000 us / cm.

10. The method for preparing iron phosphate with high magnification and high grinding efficiency as claimed in claim 1, characterized in that: In step (6), the drying temperature of the filter cake is 100-110°C, the drying time is 3-5h, the sintering temperature is 550-650°C, and the sintering time is 3-5h.

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