A method for reducing the content of metal foreign matters in lithium iron phosphate precursor slurry

Abstract

The application discloses a method for reducing metal foreign matters in lithium iron phosphate precursor slurry, and relates to the technical field of lithium battery materials.The method is characterized by the following steps: performing multi-stage grinding in a grinding section, performing segmented filtering, and controlling the particle size of the lithium iron phosphate precursor slurry after each grinding process, so that the particle size of the final slurry meets the particle size requirement of the lithium iron phosphate precursor slurry, the metal foreign matters in the lithium iron phosphate precursor slurry are efficiently intercepted, and the content of the metal foreign matters in the slurry is reduced.The content of the magnetic metal foreign matter particles with a size of less than 50 microns in the lithium iron phosphate precursor slurry obtained by the method is less than 20 per kilogram, the content of the non-magnetic metal foreign matter elements Cu / Zn is less than 0.7 ppb, and the lithium iron phosphate precursor slurry is especially suitable for lithium battery materials.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of lithium battery materials, and more particularly to a method for reducing metal foreign matters in lithium iron phosphate precursor slurry. BACKGROUND

[0002] With the continuous popularization of new energy vehicles, the safety performance of new energy vehicles has attracted more and more attention. Lithium iron phosphate batteries are widely used in the field of new energy vehicle power batteries due to their excellent safety performance, cycle performance and cost performance. The size and content of metal foreign matters in the positive electrode material lithium iron phosphate are one of the important factors affecting the safety performance and self-discharge of the battery. The smaller the particle size and the lower the content of metal foreign matters, the more beneficial to the safety performance of the battery.

[0003] Currently, the main methods for removing metal foreign matters in the production process are sieving and magnetic removal. Sieving is mainly used in the powder section. If the mesh size of the sieve is too large, the material will be blocked on the sieve, so the mesh size is generally ≤400 mesh (pore size is about 35-45 μm). This method can only intercept larger particles of metal foreign matters. The current interception technology in the industry cannot completely remove 25-50 μm magnetic metal foreign matters in lithium iron phosphate slurry. Magnetic removal is a combination of powder magnetic removal and slurry magnetic removal. However, this method has low removal efficiency and is unstable, and cannot remove non-magnetic metal foreign matters.

[0004] The prior art discloses a method for removing magnetic foreign matters in lithium battery positive electrode material high-iron material by wet method. The lithium iron phosphate positive electrode material containing magnetic foreign matters is contacted with water to prepare slurry, which can disperse the positive electrode material particles and increase the contact area between the material and the impurity removal rod, thereby better removing the magnetic metal impurities. However, the removal effect of this method is unstable, there are many influencing factors, and the removal effect still needs to be improved. Moreover, it is still impossible to remove small particle magnetic metal foreign matters. SUMMARY

[0005] In order to overcome the problems of poor removal effect of magnetic and non-magnetic metal foreign matters in lithium iron phosphate precursor slurry and the inability to consider the efficient interception of small particle (particle size ≤50 μm) magnetic metal foreign matters in the prior art, the present application provides a method for reducing metal foreign matters in lithium iron phosphate precursor slurry. By performing multi-stage grinding and filtering in the grinding section and controlling the particle size of the slurry after each stage of grinding, efficient interception of non-magnetic metal foreign matters and small particle magnetic metal foreign matters in lithium iron phosphate precursor slurry is achieved.

[0006] Another object of the present application is to provide a lithium iron phosphate precursor slurry.

[0007] Still another object of the present application is to provide an application of the above-mentioned lithium iron phosphate precursor slurry in the preparation of lithium battery materials.

[0008] The above object of the present application is achieved by the following technical solutions.

[0009] A method for reducing metal foreign matters in lithium iron phosphate precursor slurry, comprising the following steps:

[0010] S1. The lithium iron phosphate precursor slurry is subjected to first grinding and filtration; the particle size D100 of the slurry after grinding is 15-25 μm; and the filtration medium precision is 25-30 μm.

[0011] S2. The slurry obtained in S1 is subjected to second grinding and filtration; the particle size D100 of the slurry after grinding is <5 μm; and the filtration medium precision is 5-15 μm.

[0012] It should be noted that:

[0013] The present application can reduce the content of metal foreign matters in lithium iron phosphate precursor slurry by segmental grinding and grading precision filtration of the lithium iron phosphate precursor slurry, and grading interception of metal foreign matters that cannot be effectively intercepted by subsequent processes (such as sieving and magnetic removal). In this process, the control of the particle size of the lithium iron phosphate precursor slurry after each grinding needs to meet the requirement of the particle size of the slurry after grinding, and at the same time, the metal foreign matters are as far as possible not broken and ground into fine particles. In combination with the subsequent filtration medium precision, the slurry is ensured to pass through the filtration medium, and the metal foreign matters with particles larger than the filtration medium precision are intercepted during filtration. In addition, the grading interception in the grinding section can avoid the subsequent slurry drying into powder with poor flowability, which cannot be filtered but only sieved for some large particles in the powder. The particles in the slurry section are uniformly dispersed and have good flowability, and the particles can pass through the filtration medium together with the liquid for filtration.

[0014] The metal foreign matters are metal crystals, and the main solid material components in the precursor slurry are ionic crystals. The metal crystals are combined by metal bonds and are not easy to be broken and ground into fine particles. The ionic crystals are combined by ionic bonds and are hard and brittle and easy to be broken. By controlling the particle size of the slurry after each grinding, the particle size of the metal foreign matters can be ensured to be larger than that of the slurry after the same grinding time, so that the metal foreign matters can be removed by subsequent filtration and interception.

[0015] The present application grinds the slurry particle size to a specified particle size range, so that the slurry particle size meets the process requirement and smoothly passes through the filter core, while preventing the metal foreign matters from being worn or broken.

[0016] Specifically, the particle size D100 of the slurry after S1 grinding is 16-22 μm, for example, it can be 16.2 μm, 19.6 μm, 21.5 μm, 16.8 μm, 17 μm or 22 μm, and more preferably 16-17 μm.

[0017] Preferably, the filtering medium accuracy in S1 is 25-26 microns, and more preferably 25 microns.

[0018] Preferably, the slurry particle size D100 after S2 grinding is 1-2 microns.

[0019] Preferably, the filtering medium accuracy in S2 is 5-8 microns.

[0020] Specifically, the grinding time in S1 is 1-2 hours, and more specifically 1.5-2 hours.

[0021] Specifically, the grinding time in S2 is 4-6 hours, and more specifically 5-6 hours.

[0022] By controlling the grinding time in S1 and S2, the slurry particle size can be better guaranteed, and the metal foreign matter is further prevented from being ground into small particles.

[0023] Specifically, the filtering medium is a non-metal material, and specifically a PP material.

[0024] Specifically, the lithium iron phosphate precursor slurry in S1 is subjected to homogenization and dispersion treatment.

[0025] After homogenization and dispersion treatment, the various particles in the slurry can be uniformly dispersed, which is beneficial to subsequent grinding.

[0026] The lithium iron phosphate precursor slurry is directly subjected to mixing and homogenization and dispersion after batching, and the specific steps are as follows:

[0027] Iron phosphate, lithium carbonate, glucose, a dopant, and pure water are batched according to actual requirements, and then are put into a dispersion tank for mixing and homogenization and dispersion to obtain the lithium iron phosphate precursor slurry.

[0028] The time for homogenization and dispersion is 2.5 hours.

[0029] The present application specifically protects a lithium iron phosphate precursor slurry, and the preparation steps of the lithium iron phosphate precursor slurry include the above method.

[0030] The lithium iron phosphate precursor slurry obtained by the present application has low metal foreign matter content and small metal foreign matter particle size, thereby reducing the metal foreign matter content and particle size in the finished lithium iron phosphate product and improving the performance of the lithium iron phosphate battery.

[0031] Specifically, the number of magnetic metal foreign matter particles of ≤50 microns in the lithium iron phosphate precursor slurry is <20 per kg.

[0032] Specifically, the content of non-magnetic metal foreign matter in the lithium iron phosphate precursor slurry is <0.7 ppb.

[0033] Specifically, the non-magnetic metal foreign matter is elemental Cu and / or Zn.

[0034] Specifically, the lithium iron phosphate precursor slurry of the present application can be obtained after spray drying, sintering, crushing, batch mixing and magnetic removal.

[0035] Specifically, the total content of non-magnetic metal foreign matter elemental Cu and Zn in the lithium iron phosphate product is not more than 1.3 ppb, and the particle number of magnetic metal foreign matter particles ≤50 μm is <40 particles / kg.

[0036] Due to the manual operation process in the subsequent process treatment after spray drying, sintering, crushing, batch mixing, etc., a small amount of metal foreign matter may be introduced again, or there may be a small amount of metal foreign matter remaining in the equipment, so the metal foreign matter content in the finished product may increase relative to the lithium iron phosphate precursor slurry.

[0037] The steps not described in detail in the present application can be obtained by selecting conventional methods in the art.

[0038] The present application particularly protects the use of the above-mentioned lithium iron phosphate precursor slurry in the preparation of lithium battery materials.

[0039] The lithium iron phosphate precursor slurry obtained by the present application can be applied to lithium batteries, which can improve the safety performance and self-discharge performance of the battery.

[0040] Compared with the prior art, the present application has the following beneficial effects:

[0041] The present application controls the particle size of the lithium iron phosphate precursor slurry after each grinding by multi-stage grinding and segmented filtration in the grinding section, efficiently intercepts the metal foreign matter in the lithium iron phosphate precursor slurry, and reduces the content of metal foreign matter in the slurry while ensuring that the particle size of the slurry meets the particle size requirements of the lithium iron phosphate precursor slurry.

[0042] The total content of non-magnetic metal foreign matter elemental Cu and Zn in the lithium iron phosphate precursor slurry obtained by the present application is not more than 0.7 ppb, the removal rate is more than 60%, the particle number of magnetic metal foreign matter particles ≤50 μm is <20 particles / kg, the total content of non-magnetic metal foreign matter elemental Cu and Zn in the lithium iron phosphate product is not more than 1.3 ppb, and the particle number of magnetic metal foreign matter particles ≤50 μm is <40 particles / kg, which is particularly suitable for use in lithium battery materials. DETAILED DESCRIPTION

[0043] The horizontal sand mill used in the specific embodiments of the present application is purchased from Changzhou Longxin Intelligence, and the microporous precision filter is purchased from Zhejiang Dongou, model DO-PGNFX-6L.

[0044] The application will be further described in conjunction with specific embodiments, but the embodiments do not limit the application in any form. Unless otherwise specified, the raw materials used in the embodiments of the application are commercially available raw materials.

[0045] Example 1

[0046] A method for reducing metal foreign matter in lithium iron phosphate precursor slurry, comprising the following steps:

[0047] S1. The lithium iron phosphate precursor slurry ① is ground by a horizontal sand mill for the first time for 1.7 h to obtain lithium iron phosphate precursor slurry ②, and the ground slurry is filtered by a microporous precision filter to obtain lithium iron phosphate precursor slurry ③; wherein the particle size of the ground slurry is D100 = 16.2 μm; and the precision of the filter medium is 25 μm.

[0048] S2. The lithium iron phosphate precursor slurry ③ is ground by a horizontal sand mill for the second time for 6 h to obtain lithium iron phosphate precursor slurry ④, and the ground slurry is filtered by a microporous precision filter to obtain lithium iron phosphate precursor slurry ⑤; wherein the particle size of the ground slurry is D100 = 1.16 μm; and the precision of the filter medium is 5 μm.

[0049] The above lithium iron phosphate precursor slurry ① is obtained by batching and homogenizing dispersion, and the specific steps are as follows:

[0050] 200 kg of iron phosphate, 50 kg of lithium carbonate, 20 kg of glucose, and 280 kg of pure water are added to a dispersion kettle for continuous stirring, and are circulated and homogenized by a homogenizing pump. After 2.5 h of homogenizing dispersion, the lithium iron phosphate precursor slurry ① is obtained.

[0051] The above lithium iron phosphate precursor slurry ⑤ is subjected to post-process treatment to obtain lithium iron phosphate finished product, and the post-process treatment is: spray drying, sintering, crushing, batch mixing, and screening and removing magnetism.

[0052] The content of magnetic metal foreign matter in the lithium iron phosphate precursor slurries ①-⑤ and the finished product is tested by a magnetic separation method combined with ICP-OES; the magnetic separation method is that the magnetic substances in the slurry to be tested are adsorbed by a plastic magnetic rod, and then the adsorbed magnetic substances are digested, and after digestion, the solution is diluted, and then ICP-OES is used for testing to obtain the content of the magnetic substances.

[0053] The lithium iron phosphate precursor slurries ①-⑤ and the finished product are sieved through an 800-mesh screen to intercept large-particle foreign matter in the slurry and enrich the metal foreign matter, and then reagents are added to complex elemental Cu and Zn, and ICP-OES is used to test the content of Cu and Zn in the complex solution.

[0054] The results are shown in Table 1.

[0055] The number of magnetic metal foreign particles in the lithium iron phosphate precursor slurries ①-⑤ and the finished product was tested by the above magnetic separation method and cleanliness analyzer; the results are shown in Table 2.

[0056] Table 1. Metal foreign matter content in the lithium iron phosphate precursor slurry of Example 1

[0057]

[0058] Table 2. Number of metal foreign particles in the lithium iron phosphate precursor slurry of Example 1

[0059]

[0060] It can be seen from Tables 1-2 that in the lithium iron phosphate precursor slurry obtained by Example 1, the number of magnetic metal foreign particles of 5-50 μm is 5 particles / kg, and the content of non-magnetic metal foreign matters Cu and Zn is 0.388 ppb; in the lithium iron phosphate finished product, the number of magnetic metal foreign particles of 5-50 μm is 23 particles / kg, the content of magnetic metal foreign matters is 0.005 ppm (5 ppb), and the content of non-magnetic metal foreign matters Cu and Zn is 1.04 ppb; since a small amount of metal foreign matters can be introduced again during manual operation in the post-processes such as spray drying, sintering, crushing and batch mixing, or a small amount of metal foreign matters can be left in the equipment, the content of metal foreign matters in the finished product of Tables 1-2 can increase compared with the precursor slurry ⑤.

[0061] Example 2

[0062] A method for reducing metal foreign matters in a lithium iron phosphate precursor slurry, comprising the following steps:

[0063] S1. The lithium iron phosphate precursor slurry ① is ground by a horizontal sand mill for the first time for 1.5 h to obtain a lithium iron phosphate precursor slurry ②, and the ground slurry is filtered by a microporous precision filter to obtain a lithium iron phosphate precursor slurry ③; wherein the particle size of the ground slurry is D100=19.6 μm; and the precision of the filter medium is 25 μm.

[0064] S2. The lithium iron phosphate precursor slurry ③ is ground by a horizontal sand mill for the second time for 5 h to obtain a lithium iron phosphate precursor slurry ④, and the ground slurry is filtered by a microporous precision filter to obtain a lithium iron phosphate precursor slurry ⑤; wherein the particle size of the ground slurry is D100=4.8 μm; and the precision of the filter medium is 10 μm.

[0065] The ingredients, homogenization and dispersion of the lithium iron phosphate precursor slurry ①, and the post-process of the lithium iron phosphate precursor slurry ⑤ are the same as those of Example 1.

[0066] The test method of the content and particle number of metal foreign matter is the same as that of Example 1, and the results are shown in Tables 3 and 4.

[0067] Table 3. Content of metal foreign matter in lithium iron phosphate precursor slurry of Example 2

[0068]

[0069] Table 4. Particle number of metal foreign matter in lithium iron phosphate precursor slurry of Example 2

[0070]

[0071] It can be seen from Tables 3 and 4 that in the lithium iron phosphate precursor slurry obtained in Example 2, the magnetic metal foreign matter particles of 5-50 μm are 17 per kg, and the content of non-magnetic metal foreign matter elements Cu and Zn is 0.595 ppb; in the lithium iron phosphate product, the magnetic metal foreign matter particles of 5-50 μm are 38 per kg, the content of magnetic metal foreign matter is 0.009 ppm (9 ppb), and the content of non-magnetic metal foreign matter Cu and Zn is 1.29 ppb; the reason for the fluctuation of the content of metal foreign matter in the product relative to the precursor slurry ⑤ is the same as that of Example 1.

[0072] Example 3

[0073] A method for reducing metal foreign matter in lithium iron phosphate precursor slurry, comprising the following steps:

[0074] S1. The lithium iron phosphate precursor slurry ① is ground by a horizontal sand mill for the first time for 1.5 h to obtain a lithium iron phosphate precursor slurry ②, and the ground slurry is filtered by a microporous precision filter to obtain a lithium iron phosphate precursor slurry ③; wherein the particle size of the ground slurry is D100 = 21.5 μm; and the precision of the filter medium is 30 μm;

[0075] S2. The lithium iron phosphate precursor slurry ③ is ground by a horizontal sand mill for the second time for 6 h to obtain a lithium iron phosphate precursor slurry ④, and the ground slurry is filtered by a microporous precision filter to obtain a lithium iron phosphate precursor slurry ⑤; wherein the particle size of the ground slurry is D100 = 1.2 μm; and the precision of the filter medium is 5 μm.

[0076] The batching, homogenization and dispersion of the lithium iron phosphate precursor slurry ①, and the post-process of the lithium iron phosphate precursor slurry ⑤ are the same as those of Example 1.

[0077] The test method of the content and particle number of metal foreign matter is the same as that of Example 1, and the results are shown in Tables 5 and 6.

[0078] Table 5. Content of metal foreign matter in lithium iron phosphate precursor slurry of Example 3

[0079]

[0080]

[0081] Table 6. Metal foreign particle number in lithium iron phosphate precursor slurry of Example 3

[0082]

[0083] As can be seen from Tables 5-6, in the lithium iron phosphate precursor slurry obtained by Example 3, the number of magnetic metal foreign particles of 5-50 μm is 9 per kg, and the content of non-magnetic metal foreign elements Cu and Zn is 0.416 ppb; in the lithium iron phosphate product, the number of magnetic metal foreign particles of 5-50 μm is 24 per kg, the content of magnetic metal foreign particles is 0.007 ppm (7 ppb), and the content of non-magnetic metal foreign elements Cu and Zn is 0.976 ppb; the fluctuation of the content of metal foreign particles in the product relative to the precursor slurry ⑤ is the same as that of Example 1.

[0084] Example 4

[0085] A method for reducing metal foreign particles in lithium iron phosphate precursor slurry, comprising the following steps:

[0086] S1. The lithium iron phosphate precursor slurry ① is ground by a horizontal sand mill for the first time for 1.8 h to obtain a lithium iron phosphate precursor slurry ②, which is filtered by a microporous precision filter after grinding to obtain a lithium iron phosphate precursor slurry ③; wherein the particle size of the slurry after grinding is D100 = 16.8 μm; and the precision of the filter medium is 25 μm;

[0087] S2. The lithium iron phosphate precursor slurry ③ is ground by a horizontal sand mill for the second time for 5 h to obtain a lithium iron phosphate precursor slurry ④, which is filtered by a microporous precision filter after grinding to obtain a lithium iron phosphate precursor slurry ⑤; wherein the particle size of the slurry after grinding is D100 = 3.6 μm; and the precision of the filter medium is 8 μm.

[0088] The ingredients, homogenization and dispersion of the lithium iron phosphate precursor slurry ①, and the post-process of the lithium iron phosphate precursor slurry ⑤ are the same as those of Example 1.

[0089] The content and particle number of metal foreign particles are tested by the method of Example 1, and the results are shown in Tables 7 and 8.

[0090] Table 7. Content of metal foreign particles in lithium iron phosphate precursor slurry of Example 4

[0091]

[0092] Table 8. Metal foreign particle number in lithium iron phosphate precursor slurry of Example 4

[0093]

[0094] As can be seen in combination with Tables 7-8, the magnetic metal foreign particle of 5-50 μm in the lithium iron phosphate precursor slurry obtained in Example 4 is 11 particles / kg, and the content of non-magnetic metal foreign particles of elemental Cu and Zn is 0.415 ppb; in the lithium iron phosphate product, the magnetic metal foreign particle of 5-50 μm is 30 particles / kg, the content of magnetic metal foreign particles is 0.012 ppm (12 ppb), and the content of non-magnetic metal foreign particles of Cu and Zn is 1.06 ppb; the fluctuation of the content of metal foreign particles in the product relative to the precursor slurry ⑤ is the same as that in Example 1.

[0095] Comparative Example 1

[0096] A method for reducing metal foreign particles in a lithium iron phosphate precursor slurry, comprising the following steps:

[0097] S1. The lithium iron phosphate precursor slurry ① is ground by a horizontal sand mill for the first time for 1.8 h to obtain a lithium iron phosphate precursor slurry ②; wherein the particle size of the slurry after grinding is D100 = 15.8 μm.

[0098] S2. The lithium iron phosphate precursor slurry ② is ground by a horizontal sand mill for the second time for 6 h to obtain a lithium iron phosphate precursor slurry ③; wherein the particle size of the slurry after grinding is D100 = 1.3 μm.

[0099] The batching, homogenization and dispersion of the lithium iron phosphate precursor slurry ①, and the post-process of the lithium iron phosphate precursor slurry ③ are the same as those in Example 1.

[0100] The content and particle number of metal foreign particles are tested by the same method as in Example 1, and the results are shown in Tables 9 and 10.

[0101] Table 9. Content of metal foreign particles in the lithium iron phosphate precursor slurry of Comparative Example 1

[0102]

[0103] Table 10. Particle number of metal foreign particles in the lithium iron phosphate precursor slurry of Comparative Example 1

[0104]

[0105] Comparative Example 2

[0106] A method for reducing metal foreign particles in a lithium iron phosphate precursor slurry, comprising the following steps:

[0107] S1. The lithium iron phosphate precursor slurry 1 is ground by a horizontal sand mill for the first time, ground for 3h to obtain lithium iron phosphate precursor slurry 2, and then filtered by a microporous precision filter to obtain lithium iron phosphate precursor slurry 3; wherein the particle size of the ground slurry is D100=9.7μm; and the precision of the filter medium is 25μm;

[0108] S2. The lithium iron phosphate precursor slurry 3 is ground by a horizontal sand mill for the second time, ground for 4.5h to obtain lithium iron phosphate precursor slurry 4, and then filtered by a microporous precision filter to obtain lithium iron phosphate precursor slurry 5; wherein the particle size of the ground slurry is D100=1.5μm; and the precision of the filter medium is 5μm.

[0109] The ingredients of the lithium iron phosphate precursor slurry 1, the homogenization and dispersion, and the post-process of the lithium iron phosphate precursor slurry 5 are the same as in Example 1.

[0110] The test method of the content and particle number of metal foreign matters is the same as in Example 1, and the results are shown in Tables 11 and 12.

[0111] Table 11. Content of metal foreign matters in the lithium iron phosphate precursor slurry of Comparative Example 2

[0112]

[0113] Table 12. Particle number of metal foreign matters in the lithium iron phosphate precursor slurry of Comparative Example 2

[0114]

[0115] Comparative Example 3

[0116] A method for reducing the content of metal foreign matters in a lithium iron phosphate precursor slurry, comprising the following steps:

[0117] S1. The lithium iron phosphate precursor slurry 1 is ground by a horizontal sand mill for the first time, ground for 1h to obtain lithium iron phosphate precursor slurry 2, and then filtered by a microporous precision filter to obtain lithium iron phosphate precursor slurry 3; wherein the particle size of the ground slurry is D100=26.8μm; and the precision of the filter medium is 30μm;

[0118] S2. The lithium iron phosphate precursor slurry 3 is ground by a horizontal sand mill for the second time, ground for 7h to obtain lithium iron phosphate precursor slurry 4, and then filtered by a microporous precision filter to obtain lithium iron phosphate precursor slurry 5; wherein the particle size of the ground slurry is D100=1.25μm; and the precision of the filter medium is 5μm.

[0119] The ingredients of the lithium iron phosphate precursor slurry 1, the homogenization and dispersion, and the post-process of the lithium iron phosphate precursor slurry 5 are the same as in Example 1.

[0120] The content and particle number of metal foreign matter were tested according to the method of Example 1, and the results are shown in Table 13 and Table 14 below.

[0121] Table 13. Content of metal foreign matter in lithium iron phosphate precursor slurry of Comparative Example 3

[0122]

[0123] Table 14. Particle number of metal foreign matter in lithium iron phosphate precursor slurry of Comparative Example 3

[0124]

[0125] Comparative Example 4

[0126] A method for reducing metal foreign matter in a lithium iron phosphate precursor slurry, comprising the following steps:

[0127] The lithium iron phosphate precursor slurry ① was ground by a horizontal sand mill for 7 h to obtain a lithium iron phosphate precursor slurry ②, and the ground slurry was filtered by a microporous precision filter to obtain a lithium iron phosphate precursor slurry ③; wherein the particle size of the ground slurry was D100 = 2.6 μm; and the precision of the filter medium was 5 μm.

[0128] The batching, homogenization and dispersion of the lithium iron phosphate precursor slurry ①, and the post-process of the lithium iron phosphate precursor slurry ③ were the same as in Example 1.

[0129] The content and particle number of metal foreign matter were tested according to the method of Example 1, and the results are shown in Table 15 and Table 16 below.

[0130] Table 15. Content of metal foreign matter in lithium iron phosphate precursor slurry of Comparative Example 4

[0131]

[0132] Table 16. Particle number of metal foreign matter in lithium iron phosphate precursor slurry of Comparative Example 4

[0133]

[0134] Results of detection

[0135] Removal rate of magnetic substance particles:

[0136] Examples 1-4 and Comparative Examples 2-3: (total number of magnetic substance particles in slurry ① - total number of magnetic substance particles in slurry ⑤) ÷ total number of magnetic substance particles in slurry ①

[0137] Comparative Example 1 and Comparative Example 4: (total number of magnetic substance particles in slurry ① - total number of magnetic substance particles in slurry ③) ÷ total number of magnetic substance particles in slurry ①

[0138] Magnetic substance content reduction rate:

[0139] Examples 1 to 4 and Comparative Examples 2 to 3: (Magnetic substance content of slurry 1 - Magnetic substance content of slurry 5) ÷ Magnetic substance content of slurry 1

[0140] Comparative Example 1 and Comparative Example 4: (Magnetic substance content of slurry 1 - Magnetic substance content of slurry 3) ÷ Magnetic substance content of slurry 1

[0141] Non-magnetic metal substance Cu removal rate:

[0142] Examples 1 to 4 and Comparative Examples 2 to 3: (Non-magnetic metal substance Cu content of slurry 1 - Non-magnetic metal substance Cu content of slurry 5) ÷ Non-magnetic metal substance Cu content of slurry 1

[0143] Comparative Example 1 and Comparative Example 4: (Total number of magnetic substance particles of slurry 1 - Total number of magnetic substance particles of slurry 3) ÷ Total number of magnetic substance particles of slurry 1

[0144] Non-magnetic metal substance Zn removal rate:

[0145] Examples 1 to 5 and Comparative Examples 2 to 3: (Non-magnetic metal substance Zn content of slurry 1 - Non-magnetic metal substance Zn content of slurry 5) ÷ Non-magnetic metal substance Zn content of slurry 1

[0146] Comparative Example 1 and Comparative Example 4: (Non-magnetic metal substance Zn content of slurry 1 - Non-magnetic metal substance Zn content of slurry 3) ÷ Non-magnetic metal substance Zn content of slurry 1

[0147] The results are shown in Table 17.

[0148] Table 17. Metal foreign matter reduction rate in lithium iron phosphate precursor slurry

[0149]

[0150] As can be seen from Table 17, the magnetic substance reduction amount is all above 90%, and the non-magnetic metal substance Cu and Zn content reduction is above 60%; the metal foreign matter removal rate in Example 2 is lower, because in step S2, the second grinding is to a slurry particle size D100 of 4.8 μm, which needs to be matched with a larger filtering precision, at this time, the particle size of the metal foreign matter particles is smaller than the slurry particle size, and the metal foreign matter particles are easy to be filtered together with the slurry; Comparative Example 1 does not perform the grading interception, and only removes the magnetic metal substance through the screening and magnetic removal in the subsequent process, so the metal foreign matter removal rate is low; in Comparative Example 2, the slurry particle size after the step S1 grinding is too small, and the metal foreign matter removal effect is also poor, because the first grinding time is increased, leading to the breakage of the metal foreign matter, and the increase of the small particle metal foreign matter, under the same interception condition, the metal particle number in the slurry obtained after the filtration is increased; in Comparative Example 3, the slurry particle size after the step S1 grinding is too large, and the metal foreign matter removal effect is also poor; in Comparative Example 4, the lithium iron phosphate precursor slurry is directly ground to 2.6 μm and then filtered, without grading interception, and the metal foreign matter removal effect is also poor, and the magnetic substance particle number reduction rate is only 55.8%, because in the process of one-time grinding to a small particle size, part of the metal particles are broken into small particles, and at this time, the interception effect is poor.

[0151] Obviously, the above embodiments of the present application are only examples for clearly illustrating the present application, and are not intended to limit the implementation modes of the present application. Based on the above description, other different forms of changes or variations can be made by those skilled in the art. Here, all the implementation modes are not required or can not be exhausted. Any modification, equivalent replacement and improvement within the spirit and principle of the present application should be included in the protection scope of the claims of the present application.

Claims

1. A method of reducing metal foreign matter in a lithium iron phosphate precursor slurry, characterized by, It comprises the following steps: S1. The lithium iron phosphate precursor slurry is subjected to first grinding and filtration; the particle size D100 of the ground slurry is 15-25 μm; the filtration medium precision is 25-30 μm; S2. The slurry obtained in S1 is subjected to second grinding and filtration; the particle size D100 of the ground slurry is <5 μm; the filtration medium precision is 5-15 μm.

2. The method of claim 1, wherein, The particle size D100 of the ground slurry in S1 is 16-22 μm.

3. The method of claim 1, wherein, The filtration medium precision in S1 is 25-26 μm.

4. The method of claim 1, wherein, The particle size D100 of the ground slurry in S2 is 1-2 μm.

5. The method of claim 1, wherein, The filtration medium precision in S2 is 5-8 μm.

6. A lithium iron phosphate precursor slurry, characterized in that, The preparation steps of the lithium iron phosphate precursor slurry comprise the method of any one of claims 1-5.

7. The lithium iron phosphate precursor slurry as described in claim 6, characterized in that, The number of magnetic metal foreign particles ≤50 μm in the lithium iron phosphate precursor slurry is <20 per kg.

8. The lithium iron phosphate precursor slurry of claim 6, wherein, The content of non-magnetic metal foreign particles in the lithium iron phosphate precursor slurry is <0.7 ppb.

9. The lithium iron phosphate precursor slurry of claim 8, wherein, The non-magnetic metal foreign particles are elemental Cu and / or Zn.

10. Use of the lithium iron phosphate precursor slurry of any one of claims 6-9 in the preparation of lithium battery materials.

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