Detection method of magnetic foreign matter in lithium iron phosphate

Abstract

The present invention relates to the technical field of battery material detection, and in particular to a method for detecting magnetic foreign matter in lithium iron phosphate. The method for detecting magnetic foreign matter in lithium iron phosphate comprises: using a first magnetic rod complex to collect magnetic foreign matter in lithium iron phosphate and enriching it on a first filter membrane, detecting the particle size and quantity of the magnetic foreign matter on the first filter membrane; mixing the first filter membrane after detection with an organic solvent in a container, dissolving the first filter membrane, and removing the liquid in the container; using an additive solution to rinse the magnetic foreign matter in the container into a sealed container to obtain a first mixed system, using a second magnetic rod complex to collect magnetic foreign matter in the first mixed system and enriching it on a second filter membrane; detecting the particle size and quantity of the magnetic foreign matter on the second filter membrane. The method can separate ferrous phosphide and other metallic magnetic foreign matter in lithium iron phosphate, and obtain the magnetic particle size and quantity of ferrous phosphide and other metallic magnetic foreign matter.

Description

Technical Field

[0001] The present invention relates to the technical field of battery material detection, and in particular to a method for detecting magnetic foreign matter in lithium iron phosphate. Background Art

[0002] There are many production processes for positive electrode materials. There is a risk of introducing metal foreign matter in every link of the manufacturing process, which puts higher requirements on the degree of equipment automation and on-site quality management. For battery factories, the introduced magnetic metal abnormalities will be oxidized into metal ions during the charging and discharging process, and after penetrating the diaphragm, they will accumulate and deposit into metal single substances on the surface of the negative electrode, causing battery self-discharge. What's worse, it will cause the battery to catch fire and explode, causing serious safety accidents. Therefore, the detection of metal foreign matter in positive electrode materials is particularly important.

[0003] At present, the lithium battery industry, including material factories and battery factories, uses magnetic rods to adsorb and screen battery materials, and then uses ICP-OES or cleanliness analyzers or SEM-EDS tests. The above three common methods in the industry all have certain defects. For example, ICP-OES can only test the element content of foreign matter, but cannot determine the composition, size, morphology, and quantity of foreign matter; although the cleanliness analyzer can distinguish between metals and non-metals, particle size and quantity, it cannot determine the elements and composition: SEM-EDS can observe the composition, quantity and size, but the detection efficiency is low and the equipment cost is high. More importantly, the production process of lithium iron phosphate materials is often accompanied by the production of ferrous phosphide (Fe2P). The effect of ferrous phosphide on self-discharge is lower than that of metallic iron. Severe excess will also cause self-discharge, but the content can be used normally within a certain range. Therefore, metallic iron and ferrous phosphide in lithium iron phosphate materials need to be treated differently. Ferrous phosphide has weak magnetism and can be adsorbed by a magnetic rod. If detected by ICP-OES, it will interfere with the detection results of the metallic iron content. If detected by a cleanliness analyzer, it is impossible to distinguish whether the magnetic foreign matter is metallic iron or ferrous phosphide. If tested by SEM-EDS, the detection efficiency is low and the equipment cost is extremely high, which is not suitable for large-scale industrial production detection.

[0004] Patent application document CN 110333168 A provides a solution for using a cleanliness microscope to detect magnetic metal particles in lithium iron phosphate or ternary materials, but it cannot distinguish between metal foreign matter and ferrous phosphide in lithium iron phosphate; patent application document CN113899608 A provides a method for observing the composition and size of magnetic foreign matter under an electron microscope, but it requires the additional addition of a conductive agent, and it is well known that electron microscope detection has low efficiency and high cost. If the number of magnetic foreign matter is large, the electron microscope detection will take a long time to complete.

[0005] In view of this, the present invention is proposed. Summary of the invention

[0006] An object of the present invention is to provide a method for detecting magnetic foreign matter in lithium iron phosphate, which can distinguish metallic foreign matter and ferrous phosphide in lithium iron phosphate separation, and obtain the particle size and quantity of metallic foreign matter and ferrous phosphide.

[0007] In order to achieve the above-mentioned purpose of the present invention, the following technical solutions are particularly adopted:

[0008] The method for detecting magnetic foreign matter in lithium iron phosphate comprises the following steps:

[0009] (a) using a first magnetic rod complex to collect magnetic foreign matter in lithium iron phosphate and enrich it on a first filter membrane; detecting the particle size and quantity of the magnetic foreign matter on the first filter membrane;

[0010] (b) mixing the first filter membrane detected in step (a) with an organic solvent in a container, dissolving the first filter membrane, and removing the liquid in the container;

[0011] (c) using an additive solution to flush the magnetic foreign matter in the container in step (b) into a sealed container to obtain a first mixed system, using a second magnetic rod complex to collect the magnetic foreign matter in the first mixed system and enrich it on a second filter membrane; and detecting the particle size and quantity of the magnetic foreign matter on the second filter membrane.

[0012] In one embodiment, the first magnetic bar complex and the second magnetic bar complex respectively include a magnetic bar body and a heat shrink tube sheath covering the magnetic bar body.

[0013] In one embodiment, the magnetic field strength of the magnetic bar body is 6000-12000 GS.

[0014] In one embodiment, the method of using the first magnetic rod complex to collect magnetic foreign matter in lithium iron phosphate and enriching it on the first filter membrane specifically includes the following steps:

[0015] The lithium iron phosphate, the first magnetic rod complex and water are first mixed in a sealed container, the first magnetic rod complex is taken out, the magnetic substance adsorbed on the first magnetic rod complex is rinsed into a first glass container with water, and then a first ultrasonic washing is performed; the magnetic substance after the first ultrasonic washing is then transferred to the first filter membrane and a first drying is performed.

[0016] In one embodiment, the first mixing time is 25 to 40 minutes.

[0017] In one embodiment, the first mixing is performed by a rolling device, and the rotation speed of the rolling device is 100-120 r / min.

[0018] In one embodiment, the first ultrasonic washing specifically includes: adding washing liquid into the first glass container and performing a first ultrasonic treatment, and then using a magnetic block to attach to the outer bottom of the first glass container to adsorb and aggregate the magnetic material, and then pouring out the washing liquid.

[0019] In one embodiment, the first ultrasonic treatment lasts for 1 to 3 minutes.

[0020] In one embodiment, the first ultrasonic washing is performed 4 to 6 times.

[0021] In one embodiment, the first drying temperature is 50-65° C., and the first drying time is 25-35 min.

[0022] In one embodiment, the material of the first filter membrane includes at least one of polyacetic acid, polyethersulfone and mixed cellulose.

[0023] In one embodiment, the pore size of the first filter membrane is 1 to 25 μm.

[0024] In one embodiment, the material of the second filter membrane includes at least one of polyethylene, polypropylene, polytetrafluoroethylene, polyvinylidene fluoride, nylon, polyacetic acid, mixed cellulose and polyethersulfone.

[0025] In one embodiment, the pore size of the second filter membrane is 1 to 25 μm.

[0026] In one embodiment, a cleanliness analyzer is used to detect the particle size and quantity of the magnetic foreign matter on the first filter membrane.

[0027] In one embodiment, a cleanliness analyzer is used to detect the particle size and quantity of the magnetic foreign matter on the second filter membrane.

[0028] In one embodiment, in step (b), ultrasonic treatment is used during the mixing process; the time of the ultrasonic treatment is 4 to 6 minutes.

[0029] In one embodiment, in step (b), removing the liquid in the container specifically includes: attaching a magnetic block to the outer bottom of the container to absorb magnetic foreign matter in the container, and then pouring out the liquid in the container.

[0030] In one embodiment, the organic solvent comprises N-methylpyrrolidone.

[0031] In one embodiment, in step (b), the magnetic foreign matter in the first mixed system is collected by a second magnetic rod complex and enriched on a second filter membrane, specifically comprising the following steps:

[0032] The first mixed system is mixed with the second magnetic rod complex for the second time, the second magnetic rod complex is taken out, the magnetic foreign matter on the second magnetic rod complex is rinsed into a second glass container with water, and then a second ultrasonic washing is performed; the magnetic foreign matter after the second ultrasonic washing is transferred to the second filter membrane and a second drying is performed.

[0033] In one embodiment, the first mixing system and the second magnetic rod complex are mixed using a rolling device, and the rotation speed of the rolling device is 100-120 r / min.

[0034] In one embodiment, the second mixing time is 8 to 12 minutes.

[0035] In one embodiment, the second ultrasonic washing specifically includes: adding washing liquid into the second glass container and performing a second ultrasonic treatment, and then using a magnetic block to attach to the outer bottom of the second glass container to adsorb and aggregate the magnetic material, and then pouring out the washing liquid.

[0036] In one embodiment, the second ultrasonic treatment lasts for 1 to 3 minutes.

[0037] In one embodiment, the second ultrasonic washing is performed 2 to 4 times.

[0038] In one embodiment, the second drying temperature is 50-65° C., and the second drying time is 25-35 min.

[0039] In one embodiment, the additives include at least one of sodium carboxymethyl cellulose, methyl cellulose, polyethylene glycol, polyacrylonitrile, polyacrylate, agar, pectin and starch.

[0040] In one embodiment, the mass content of the additive in the additive solution is 1% to 10%.

[0041] Compared with the prior art, the present invention has the following beneficial effects:

[0042] The method of the present invention can effectively separate weakly magnetic ferrous phosphide and other metallic magnetic foreign matter in lithium iron phosphate, and by detecting the particle size and quantity of the magnetic foreign matter on the filter membrane twice before and after, the particle size and quantity of the removed ferrous phosphide can be calculated; the operation process is simple, no other special equipment is required, and the method is suitable for the detection of magnetic foreign matter in lithium iron phosphate in material factories and battery factories in the lithium battery industry. DETAILED DESCRIPTION

[0043] The embodiments of the present invention will be described in detail below in conjunction with the examples, but it will be appreciated by those skilled in the art that the following examples are only used to illustrate the present invention and should not be considered as limiting the scope of the present invention. If no specific conditions are specified in the examples, the conditions are carried out according to normal conditions or the conditions recommended by the manufacturer. If the manufacturer of the reagents or instruments used is not specified, they are all conventional products that can be obtained commercially.

[0044] The method for detecting magnetic foreign matter in lithium iron phosphate comprises the following steps:

[0045] (a) using a first magnetic rod complex to collect magnetic foreign matter in lithium iron phosphate and enrich it on a first filter membrane; detecting the particle size and quantity of the magnetic foreign matter on the first filter membrane;

[0046] (b) mixing the first filter membrane detected in step (a) with an organic solvent in a container, dissolving the first filter membrane, and removing the liquid in the container;

[0047] (c) using an additive solution to flush the magnetic foreign matter in the container in step (b) into a sealed container to obtain a first mixed system, using a second magnetic rod complex to collect the magnetic foreign matter in the first mixed system and enrich it on a second filter membrane; and detecting the particle size and quantity of the magnetic foreign matter on the second filter membrane.

[0048] The method for detecting magnetic foreign matter in lithium iron phosphate of the present invention comprises the following steps: in step (a), the total number of metallic foreign matter and ferrous phosphide in different particle size ranges can be detected; in step (c), after the ferrous phosphide is removed by an additive, the magnetic foreign matter on the second filter membrane is metallic foreign matter; by detecting the particle size and number of the magnetic foreign matter on the first filter membrane and the second filter membrane, the particle size and number of the removed ferrous phosphide can be calculated, and the operation process is simple without using other special equipment.

[0049] Compared with the existing technology, the detection method of magnetic foreign matter in lithium iron phosphate of the present invention can not only detect the particle size and quantity of other metal magnetic foreign matter, but also detect the particle size and quantity of ferrous phosphide. Since the effect of ferrous phosphide on self-discharge is lower than that of metallic iron, it will also cause self-discharge when the content exceeds the standard seriously, but its content can be used normally within a certain range. Therefore, it is of great significance to distinguish between other metal foreign matter and ferrous phosphide in lithium iron phosphate material.

[0050] In one embodiment, the first magnetic rod complex and the second magnetic rod complex respectively include a magnetic rod body and a heat shrink tube covering the magnetic rod body. In one embodiment, the length of the magnetic rod body is 14 to 18 cm, and the cross-sectional diameter is 20 to 30 mm. In one embodiment, the specific preparation methods of the first magnetic rod complex and the second magnetic rod complex respectively include: sealing one end of the heat shrink tube under the mouth of a heat sealer, using water to confirm that the seal is completely watertight, and then putting it on the magnetic rod body, and then sealing the other end with a heat sealer. In one embodiment, the heat shrink tube is 18 cm long, 250 μm thick, and has a diameter of 1.5 to 2.5 mm. is 25mm.

[0051] In one embodiment, the magnetic field strength of the magnetic bar body is 6000-12000 GS. In one embodiment, the magnetic field strength of the magnetic bar body includes but is not limited to 7000 GS, 8000 GS, 9000 GS, 10000 GS, 11000 GS, 12000 GS, etc.

[0052] In one embodiment, the method of using the first magnetic rod complex to collect magnetic foreign matter in lithium iron phosphate and enriching it on the first filter membrane specifically includes the following steps:

[0053] The lithium iron phosphate, the first magnetic rod complex and water are first mixed in a sealed container, the first magnetic rod complex is taken out, the magnetic substance adsorbed on the first magnetic rod complex is rinsed into a first glass container with water, and then a first ultrasonic washing is performed; the magnetic substance after the first ultrasonic washing is then transferred to the first filter membrane and a first drying is performed.

[0054] The present invention can better adsorb the magnetic material in the lithium iron phosphate onto the first magnetic rod complex through the above method.

[0055] In one embodiment, the usage ratio of the lithium iron phosphate and the water is (0.8-1.2) kg: (5.5-6.5) L.

[0056] In one embodiment, the first mixing time is 25 to 40 minutes, such as 28 minutes, 30 minutes, 32 minutes, 35 minutes or 37 minutes. In one embodiment, the first mixing is performed by a rolling device, that is, the sealed container is placed flat on a rolling machine and rolled at a constant speed. In one embodiment, the rotation speed of the rolling device is 100 to 120 r / min, such as 105 r / min, 108 r / min, 110 r / min, 115 r / min or 118 r / min.

[0057] In one embodiment, the first ultrasonic washing specifically includes: adding washing liquid to the first glass container and performing a first ultrasonic treatment, then using a magnetic block to attach to the outer bottom of the first glass container to adsorb and aggregate the magnetic material, and pouring out the washing liquid. In one embodiment, the time of the first ultrasonic treatment is 1 to 3 minutes, such as 1.5 minutes, 2 minutes, 2.5 minutes, etc. In one embodiment, the number of times of the first ultrasonic washing is 4 to 6 times, such as 5 times. In one embodiment, the volume of water used in each first ultrasonic washing is 9 to 16 mL.

[0058] In one embodiment, the first drying temperature is 50-65°C, for example, 52°C, 55°C, 57°C, 60°C or 63°C; the first drying time is 25-35 min, for example, 28 min, 30 min, 32 min, 35 min, etc.

[0059] In one embodiment, the material of the first filter membrane includes at least one of polyacetic acid, polyethersulfone and mixed cellulose. In one embodiment, the pore size of the first filter membrane is 1 to 25 μm, such as 2 μm, 5 μm, 7 μm, 10 μm, 12 μm, 15 μm, 18 μm, 20 μm, 22 μm or 25 μm.

[0060] In one embodiment, the material of the second filter membrane is at least one of polyethylene, polypropylene, polytetrafluoroethylene, polyvinylidene fluoride, nylon, polyacetic acid, mixed cellulose and polyethersulfone. In one embodiment, the pore size of the second filter membrane is 1 to 25 μm, such as 2 μm, 4 μm, 5 μm, 7 μm, 10 μm, 15 μm, 18 μm, 20 μm, 22 μm, etc.

[0061] In one embodiment, a cleanliness analyzer is used to detect the particle size and quantity of the magnetic foreign matter on the first filter membrane. In one embodiment, a cleanliness analyzer is used to detect the particle size and quantity of the magnetic foreign matter on the second filter membrane.

[0062] In one embodiment, in step (b), ultrasonic treatment is used during the mixing process; the time of ultrasonic treatment is 4 to 6 minutes, such as 4.5 minutes, 5 minutes, 5.5 minutes, etc.

[0063] In one embodiment, in step (b), removing the liquid in the container specifically includes: attaching a magnetic block to the outer bottom of the container to absorb magnetic foreign matter in the container, and then pouring out the liquid in the container.

[0064] In one embodiment, the organic solvent comprises N-methylpyrrolidone.

[0065] In one embodiment, in step (b), the magnetic foreign matter in the first mixed system is collected by a second magnetic rod complex and enriched on a second filter membrane, specifically comprising the following steps:

[0066] The first mixed system is mixed with the second magnetic rod complex for the second time, the second magnetic rod complex is taken out, the magnetic foreign matter on the second magnetic rod complex is rinsed into a second glass container with water, and then a second ultrasonic washing is performed; the magnetic foreign matter after the second ultrasonic washing is transferred to the second filter membrane and a second drying is performed.

[0067] In one embodiment, the first mixed system and the second magnetic rod complex are mixed by a rolling device, that is, the first mixed system and the second magnetic rod and the sealed container are placed flat on a rolling machine and rolled at a constant speed; the speed of the rolling device is 100-120r / min, such as 105r / min, 110r / min, 115r / min, 118r / min, etc. In one embodiment, the magnetic foreign matter on the second magnetic rod complex is washed into the second glass container with water, specifically comprising: taking out the second magnetic rod complex, taking out the heat shrink tube in a clean beaker, and washing all the magnetic substances on the heat shrink tube into the beaker with deionized water.

[0068] In one embodiment, the second mixing time is 8 to 12 minutes, such as 9 minutes, 10 minutes, 11 minutes, etc.

[0069] In one embodiment, the second ultrasonic washing specifically includes: adding washing liquid to the second glass container and performing a second ultrasonic treatment, then using a magnetic block to attach to the outer bottom of the second glass container to adsorb and aggregate the magnetic material, and pouring out the washing liquid. In one embodiment, the time of the second ultrasonic treatment is 1 to 3 minutes, such as 1.5 minutes, 2 minutes, 3 minutes, etc. In one embodiment, the number of times of the second ultrasonic washing is 2 to 4 times. In one embodiment, the volume of water used in each second ultrasonic washing is 9 to 16 mL.

[0070] In one embodiment, the second drying temperature is 50-65°C, for example, 52°C, 55°C, 57°C, 60°C, 62°C, etc.; the second drying time is 25-35 min, for example, 28 min, 30 min, 33 min, etc.

[0071] In one embodiment, the additive solution includes at least one of sodium carboxymethylcellulose, methylcellulose, polyethylene glycol, polyacrylonitrile, polyacrylate, agar, pectin and starch. ...

[0072] In one embodiment, the sealed container of the present invention comprises a plastic bucket; the glass container comprises a beaker. In one embodiment, the water of the present invention comprises deionized water, etc.

[0073] The following is further explained in conjunction with specific embodiments and comparative examples.

[0074] Example 1

[0075] The method for detecting magnetic foreign matter in lithium iron phosphate comprises the following steps:

[0076] (1) Prepare two clean plastic buckets with a capacity of 10 L, two magnetic bars with a size of 25 mm*15 cm and a magnetic field strength of 6000 GS, and take a section of 18 cm long and 250 μm thick. For heat shrink tubing, seal it under the mouth of the heat sealer for about 5 seconds. After confirming that the seal is completely watertight with water, put it on the magnetic rod and seal the other end with the heat sealer.

[0077] (2) Add 6L of deionized water to the bucket, weigh 1.0kg of powder sample and add it to the bucket, add the prepared magnetic bar, cover the bucket, place the bucket flat on a roller and roll it at a constant speed of 110r / min for 30min; remove the magnetic bar, remove the heat shrink tube in a clean beaker, and rinse all the magnetic substances on the heat shrink tube into the beaker with deionized water.

[0078] (3) Pour 15 mL of deionized water into the beaker for extracting magnetic substances, place it in an ultrasonic instrument for 2 minutes, then use a magnetic block on the bottom of the beaker to adsorb and aggregate the magnetic substances, pour the solution in the beaker into a waste liquid bucket, add 10 mL of deionized water to the beaker, repeat the above steps 5 times, and finally add 10 mL of deionized water for suction filtration and rinse the inner wall of the beaker to ensure that the extracted magnetic substances are all dispersed on the polyacetic acid filter membrane with a pore size of 5 μm. After suction filtration, the filter membrane with magnetic substances on the surface is placed in a 60°C oven for drying for 30 minutes.

[0079] (4) The dried filter membrane is placed on a cleanliness microscope to test the particle size and quantity of the extracted magnetic material.

[0080] (5) Place the tested filter membrane in a beaker containing 15 mL of NMP and sonicate for 5 min to ensure that the filter membrane is completely dissolved.

[0081] (6) Use a magnetic block to absorb and gather the magnetic material at the bottom of the beaker, pour the NMP in the beaker into a recovery bucket, and then use 3L of an aqueous solution containing 6% sodium carboxymethyl cellulose to rinse the magnetic material in the beaker into a clean plastic bucket, add the prepared magnetic bar, cover the bucket, and place the bucket flat on a roller and roll it at a constant speed of 110 r / min for 10 minutes; remove the magnetic bar, remove the heat shrink tube in a clean beaker, and use deionized water to rinse all the magnetic material on the heat shrink tube into the beaker.

[0082] (7) Add 10 mL of deionized water to the beaker, place it in an ultrasonic device for 2 min, then use a magnetic block on the bottom of the beaker to absorb and gather the magnetic material, pour the solution in the beaker into a waste liquid bucket, add 10 mL of deionized water to the beaker, repeat the above steps 3 times, and finally add 10 mL of deionized water for suction filtration and rinse the inner wall of the beaker to ensure that the extracted magnetic material is completely dispersed on the polytetrafluoroethylene filter membrane with a pore size of 5 μm. After suction filtration, the filter membrane with magnetic material on the surface is placed in a 60°C oven to dry for 30 min.

[0083] (8) The dried filter membrane is placed on a cleanliness microscope to test the particle size and quantity of the extracted magnetic material.

[0084] Example 2

[0085] The method for detecting magnetic foreign matter in lithium iron phosphate is the same as that in Example 1 except that a polyacetic acid filter membrane with a pore size of 25 μm is used.

[0086] Example 3

[0087] The method for detecting magnetic foreign matter in lithium iron phosphate is the same as Example 1 except that the additive is polyethylene glycol.

[0088] Example 4

[0089] The method for detecting magnetic foreign matter in lithium iron phosphate comprises the following steps:

[0090] (1) Prepare two clean plastic buckets with a capacity of 10 L, two magnetic bars with a size of 25 mm*15 cm and a magnetic field strength of 6000 GS, and take a section of 18 cm long and 250 μm thick. For heat shrink tubing, seal it under the mouth of the heat sealer for about 5 seconds. After confirming that the seal is completely watertight with water, put it on the magnetic rod and seal the other end with the heat sealer.

[0091] (2) Add 6L of deionized water to the bucket, weigh 1.0kg of powder sample and add it to the bucket, add the prepared magnetic bar, cover the bucket, place the bucket flat on a roller and roll it at a constant speed of 120r / min for 25min; remove the magnetic bar, remove the heat shrink tube in a clean beaker, and rinse all the magnetic substances on the heat shrink tube into the beaker with deionized water.

[0092] (3) Pour 15 mL of deionized water into the beaker for extracting magnetic substances, place it in an ultrasonic instrument for 3 minutes, then use a magnetic block on the bottom of the beaker to adsorb and gather the magnetic substances, pour the solution in the beaker into a waste liquid bucket, add 10 mL of deionized water to the beaker, repeat the above steps 4 times, and finally add 10 mL of deionized water for suction filtration and rinse the inner wall of the beaker to ensure that the extracted magnetic substances are all dispersed on the polyethersulfone filter membrane with a pore size of 12 μm. After suction filtration, the filter membrane with magnetic substances on the surface is placed in a 55°C oven for drying for 35 minutes.

[0093] (4) The dried filter membrane is placed on a cleanliness microscope to test the particle size and quantity of the extracted magnetic material.

[0094] (5) Place the tested filter membrane in a beaker containing 15 mL of NMP and sonicate for 4 min to ensure that the filter membrane is completely dissolved.

[0095] (6) Use a magnetic block to absorb and gather the magnetic material at the bottom of the beaker, pour the NMP in the beaker into a recovery bucket, and then use 3L of an aqueous solution containing 10% polyacrylate to rinse the magnetic material in the beaker into a clean plastic bucket, add the prepared magnetic bar, cover the bucket, and place the bucket flat on a roller and roll it at a constant speed of 100 r / min for 12 minutes; remove the magnetic bar, remove the heat shrink tube in a clean beaker, and use deionized water to rinse all the magnetic material on the heat shrink tube into the beaker.

[0096] (7) Add 10 mL of deionized water to the beaker, place it in an ultrasonic device for 1 min, then use a magnetic block to absorb and gather the magnetic material at the bottom of the beaker, pour the solution in the beaker into a waste liquid bucket, add 10 mL of deionized water to the beaker, repeat the above steps 4 times, and finally add 10 mL of deionized water for suction filtration and rinse the inner wall of the beaker to ensure that the extracted magnetic material is completely dispersed on the polyvinylidene fluoride filter membrane with a pore size of 12 μm. After suction filtration, the filter membrane with magnetic material on the surface is placed in a 65°C oven and dried for 25 min.

[0097] (8) The dried filter membrane is placed on a cleanliness microscope to test the particle size and quantity of the extracted magnetic material.

[0098] Example 5

[0099] The method for detecting magnetic foreign matter in lithium iron phosphate comprises the following steps:

[0100] (1) Prepare two clean plastic buckets with a capacity of 10 L, two magnetic bars with a size of 25 mm*15 cm and a magnetic field strength of 6000 GS, and take a section of 18 cm long and 250 μm thick. For heat shrink tubing, seal it under the mouth of the heat sealer for about 5 seconds. After confirming that the seal is completely watertight with water, put it on the magnetic rod and seal the other end with the heat sealer.

[0101] (2) Add 6L of deionized water to the bucket, weigh 1.0kg of powder sample and add it to the bucket, add the prepared magnetic bar, cover the bucket, place the bucket flat on a roller and roll it at a constant speed of 100r / min for 35min; remove the magnetic bar, remove the heat shrink tube in a clean beaker, and rinse all the magnetic substances on the heat shrink tube into the beaker with deionized water.

[0102] (3) Pour 15 mL of deionized water into the beaker for extracting magnetic substances, place it in an ultrasonic instrument for 1 min, then use a magnetic block on the bottom of the beaker to adsorb and aggregate the magnetic substances, pour the solution in the beaker into a waste liquid bucket, add 10 mL of deionized water to the beaker, repeat the above steps 6 times, and finally add 10 mL of deionized water for suction filtration and rinse the inner wall of the beaker to ensure that the extracted magnetic substances are all dispersed on the polyethersulfone filter membrane with a pore size of 12 μm. After suction filtration, the filter membrane with magnetic substances on the surface is placed in a 65°C oven for drying for 25 min.

[0103] (4) The dried filter membrane is placed on a cleanliness microscope to test the particle size and quantity of the extracted magnetic material.

[0104] (5) Place the tested filter membrane in a beaker containing 15 mL of NMP and sonicate for 6 min to ensure that the filter membrane is completely dissolved.

[0105] (6) Use a magnetic block to absorb and gather the magnetic material at the bottom of the beaker, pour the NMP in the beaker into a recovery bucket, and then use 3L of an aqueous solution containing 4% methyl cellulose to rinse the magnetic material in the beaker into a clean plastic bucket, add the prepared magnetic rod, cover the bucket, and place the bucket flat on a roller and roll it at a constant speed of 120 r / min for 8 minutes; remove the magnetic rod, remove the heat shrink tube in a clean beaker, and use deionized water to rinse all the magnetic material on the heat shrink tube into the beaker.

[0106] (7) Add 10 mL of deionized water to the beaker, place it in an ultrasonic device for 3 min, then use a magnetic block on the bottom of the beaker to absorb and gather the magnetic material, pour the solution in the beaker into a waste liquid bucket, add 10 mL of deionized water to the beaker, repeat the above steps twice, and finally add 10 mL of deionized water for suction filtration and rinse the inner wall of the beaker to ensure that the extracted magnetic material is completely dispersed on the nylon membrane with a pore size of 12 μm. After suction filtration, the filter membrane with magnetic material on the surface is placed in a 55°C oven to dry for 35 min.

[0107] (8) The dried filter membrane is placed on a cleanliness microscope to test the particle size and quantity of the extracted magnetic material.

[0108] Comparative Example 1

[0109] The method for detecting magnetic foreign matter in lithium iron phosphate comprises the following steps:

[0110] (1) Prepare a clean plastic bucket with a capacity of 10 L, a magnetic bar with a size of 25 mm*15 cm and a magnetic field strength of 6000 GS, and take a section of 18 cm long and 250 μm thick. For heat shrink tubing, seal it under the mouth of the heat sealer for about 5 seconds. After confirming that the seal is completely watertight with water, put it on the magnetic rod and seal the other end with the heat sealer.

[0111] (2) Add 6L of deionized water to the bucket, weigh 1.0kg of powder sample and add it to the bucket, add the prepared magnetic bar, cover the bucket, place the bucket flat on a roller and roll it at a constant speed of 110r / min for 30min; remove the magnetic bar, remove the heat shrink tube in a clean beaker, and rinse all the magnetic substances on the heat shrink tube into the beaker with deionized water.

[0112] (3) Pour 15 mL of deionized water into the beaker for extracting magnetic substances, place it in an ultrasonic instrument for 2 minutes, then use a magnetic block at the bottom of the beaker to adsorb and gather the magnetic substances, pour the solution in the beaker into a waste liquid bucket, add 10 mL of deionized water to the beaker, repeat the above steps for 5 times, and finally add 10 mL of deionized water for suction filtration and rinse the inner wall of the beaker to ensure that the extracted magnetic substances are all dispersed on the polytetrafluoroethylene filter membrane with a pore size of 5 μm. After suction filtration, the filter membrane with magnetic substances on the surface is placed in a 60°C oven for drying for 30 minutes.

[0113] (4) The dried filter membrane is placed on a cleanliness microscope to test the particle size and quantity of the extracted magnetic material.

[0114] Comparative Example 2

[0115] The method for detecting magnetic foreign matter in lithium iron phosphate comprises the following steps:

[0116] (1) Prepare two clean plastic buckets with a capacity of 10 L, two magnetic bars with a size of 25 mm*15 cm and a magnetic field strength of 6000 GS, and take a section of 18 cm long and 250 μm thick. For heat shrink tubing, seal it under the mouth of the heat sealer for about 5 seconds. After confirming that the seal is completely watertight with water, put it on the magnetic rod and seal the other end with the heat sealer.

[0117] (2) Add 6L of deionized water to the bucket, weigh 1.0kg of powder sample and add it to the bucket, add the prepared magnetic bar, cover the bucket, place the bucket flat on a roller and roll it at a constant speed of 110r / min for 30min; remove the magnetic bar, remove the heat shrink tube in a clean beaker, and rinse all the magnetic substances on the heat shrink tube into the beaker with deionized water.

[0118] (3) Pour 15 mL of deionized water into the beaker for extracting magnetic substances, place it in an ultrasonic instrument for 2 minutes, then use a magnetic block on the bottom of the beaker to adsorb and aggregate the magnetic substances, pour the solution in the beaker into a waste liquid bucket, add 10 mL of deionized water to the beaker, repeat the above steps for 5 times, and finally add 10 mL of deionized water for suction filtration and rinse the inner wall of the beaker to ensure that the extracted magnetic substances are all dispersed on the polyacetic acid filter membrane with a pore size of 5 μm. After suction filtration, the filter membrane with magnetic substances on the surface is placed in a 60°C oven for drying for 30 minutes.

[0119] (4) The dried filter membrane is placed on a cleanliness microscope to test the particle size and quantity of the extracted magnetic material.

[0120] (5) Place the tested filter membrane in a beaker containing 15 mL of NMP and sonicate for 5 min to ensure that the filter membrane is completely dissolved.

[0121] (6) Use a magnetic block to absorb and gather the magnetic material at the bottom of the beaker, pour the NMP in the beaker into a recovery bucket, and then use 3L of aqueous solution to rinse the magnetic material in the beaker into a clean plastic bucket, add the prepared magnetic rod, cover the bucket, and place the bucket flat on a roller and roll it at a constant speed of 110 r / min for 10 minutes; remove the magnetic rod, remove the heat shrink tube in a clean beaker, and use deionized water to rinse all the magnetic material on the heat shrink tube into the beaker;

[0122] (7) Add 10 mL of deionized water to the beaker, place it in an ultrasonic device for 2 min, then use a magnetic block on the bottom of the beaker to absorb and gather the magnetic material, pour the solution in the beaker into a waste liquid bucket, add 10 mL of deionized water to the beaker, repeat the above steps 3 times, and finally add 10 mL of deionized water for suction filtration and rinse the inner wall of the beaker to ensure that the extracted magnetic material is completely dispersed on the polytetrafluoroethylene filter membrane with a pore size of 5 μm. After suction filtration, the filter membrane with magnetic material on the surface is placed in a 60°C oven to dry for 30 min.

[0123] (8) The dried filter membrane is placed on a cleanliness microscope to test the particle size and quantity of the extracted magnetic material.

[0124] Experimental example

[0125] The test results of the size and quantity of magnetic foreign matter particles in the examples and comparative examples are shown in Table 1.

[0126] Table 1 Detection results of magnetic foreign matter particle size and quantity

[0127]

[0128] As can be seen from Table 1, the method of the present invention can effectively separate weakly magnetic ferrous phosphide and other metallic magnetic foreign matter in lithium iron phosphate, and by detecting the particle size and quantity of magnetic foreign matter on the filter membrane twice before and after, the particle size and quantity of the removed ferrous phosphide can be calculated; the operation process is simple and does not require the use of other special equipment. It is suitable for the detection of magnetic foreign matter in lithium iron phosphate in material factories and battery factories in the lithium battery industry.

[0129] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, rather than to limit it. Although the present invention has been described in detail with reference to the aforementioned embodiments, those skilled in the art should understand that they can still modify the technical solutions described in the aforementioned embodiments, or replace some or all of the technical features therein by equivalents. However, these modifications or replacements do not make the essence of the corresponding technical solutions deviate from the scope of the technical solutions of the embodiments of the present invention.

Claims

1. A method for detecting magnetic foreign matter in lithium iron phosphate, characterized in that: The following steps are involved: (a) using a first magnetic rod complex to collect magnetic foreign matter in lithium iron phosphate and enrich it on a first filter membrane; detecting the particle size and quantity of the magnetic foreign matter on the first filter membrane; (b) mixing the first filter membrane detected in step (a) with an organic solvent in a container, dissolving the first filter membrane, and removing the liquid in the container; (c) using an additive solution to rinse the magnetic foreign matter in the container in step (b) into a sealed container, wherein the additive in the additive solution includes at least one of sodium carboxymethyl cellulose, methyl cellulose, polyethylene glycol, polyacrylonitrile, polyacrylate, agar, pectin and starch, to obtain a first mixed system, using a second magnetic rod complex to collect the magnetic foreign matter in the first mixed system and enrich it on a second filter membrane; and detecting the particle size and quantity of the magnetic foreign matter on the second filter membrane; By measuring the particle size and quantity of magnetic foreign matter on the filter membrane twice before and after, the particle size and quantity of ferrous phosphide removed can be calculated.

2. The method for detecting magnetic foreign matter in lithium iron phosphate according to claim 1, characterized in that: Contains at least one of the following features (1) to (2): (1) The first magnetic rod complex and the second magnetic rod complex respectively include a magnetic rod body and a heat shrink tube sheath covering the magnetic rod body; (2) The magnetic field strength of the magnetic bar body is 6000~12000GS.

3. The method for detecting magnetic foreign matter in lithium iron phosphate according to claim 1, characterized in that: The method of using the first magnetic rod complex to collect magnetic foreign matter in lithium iron phosphate and enriching it on the first filter membrane specifically includes the following steps: The lithium iron phosphate, the first magnetic rod complex and water are first mixed in a sealed container, the first magnetic rod complex is taken out, the magnetic substance adsorbed on the first magnetic rod complex is rinsed into a first glass container with water, and then a first ultrasonic washing is performed; the magnetic substance after the first ultrasonic washing is then transferred to the first filter membrane and a first drying is performed.

4. The method for detecting magnetic foreign matter in lithium iron phosphate according to claim 3, characterized in that: Contains at least one of the following features (1) to (6): (1) The first mixing time is 25 to 40 minutes; (2) The first mixing is performed by a rolling device, and the rotation speed of the rolling device is 100-120 r / min; (3) The first ultrasonic washing specifically comprises: adding a washing liquid into the first glass container and performing a first ultrasonic treatment, then attaching a magnetic block to the outer bottom of the first glass container to allow the magnetic substance to be adsorbed and aggregated, and then discarding the washing liquid; (4) The first ultrasonic treatment time is 1 to 3 minutes; (5) The first ultrasonic washing is performed 4 to 6 times; (6) The temperature of the first drying is 50-65°C, and the time of the first drying is 25-35 minutes.

5. The method for detecting magnetic foreign matter in lithium iron phosphate according to claim 1, characterized in that: Contains at least one of the following features (1) to (4): (1) The material of the first filter membrane includes at least one of polyacetic acid, polyethersulfone and mixed cellulose; (2) The pore size of the first filter membrane is 1-25 μm; (3) The material of the second filter membrane includes at least one of polyethylene, polypropylene, polytetrafluoroethylene, polyvinylidene fluoride, nylon, polyacetic acid, mixed cellulose and polyether sulfone; (4) The pore size of the second filter membrane is 1-25 μm.

6. The method for detecting magnetic foreign matter in lithium iron phosphate according to claim 1, characterized in that: Contains at least one of the following features (1) to (2): (1) Using a cleanliness analyzer to detect the particle size and quantity of magnetic foreign matter on the first filter membrane; (2) A cleanliness analyzer is used to detect the particle size and quantity of magnetic foreign matter on the second filter membrane.

7. The method for detecting magnetic foreign matter in lithium iron phosphate according to claim 1, characterized in that: Contains at least one of the following features (1) to (3): (1) In step (b), ultrasonic treatment is used during the mixing process; the ultrasonic treatment time is 4 to 6 minutes; (2) In step (b), the liquid in the container is removed, specifically comprising: attaching a magnetic block to the outer bottom of the container to absorb the magnetic foreign matter in the container, and then pouring out the liquid in the container; (3) The organic solvent includes N-methylpyrrolidone.

8. The method for detecting magnetic foreign matter in lithium iron phosphate according to claim 6, characterized in that: In step (b), the magnetic foreign matter in the first mixed system is collected by a second magnetic rod complex and enriched on a second filter membrane, which specifically includes the following steps: The first mixed system is mixed with the second magnetic rod complex for the second time, the second magnetic rod complex is taken out, the magnetic foreign matter on the second magnetic rod complex is rinsed into a second glass container with water, and then a second ultrasonic washing is performed; the magnetic foreign matter after the second ultrasonic washing is transferred to the second filter membrane and a second drying is performed.

9. The method for detecting magnetic foreign matter in lithium iron phosphate according to claim 8, characterized in that: Contains at least one of the following features (1) to (6): (1) The first mixing system and the second magnetic rod complex are mixed by a rolling device, and the rotation speed of the rolling device is 100-120 r / min; (2) The second mixing time is 8 to 12 minutes; (3) The second ultrasonic washing specifically comprises: adding a washing liquid into the second glass container and performing a second ultrasonic treatment, then attaching a magnetic block to the outer bottom of the second glass container to allow the magnetic material to be adsorbed and aggregated, and then discarding the washing liquid; (4) The second ultrasonic treatment time is 1 to 3 minutes; (5) The second ultrasonic washing is performed 2 to 4 times; (6) The temperature of the second drying is 50-65°C, and the time of the second drying is 25-35 minutes.

10. The method for detecting magnetic foreign matter in lithium iron phosphate according to claim 1, characterized in that: In the additive solution, the mass content of the additive is 1% to 10%.

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