Method for removing battery-grade ammonium dihydrogen phosphate magnetic particles

Abstract

The invention discloses a method for removing battery-grade ammonium dihydrogen phosphate magnetic particles, and relates to the technical field of lithium iron phosphate production.The method comprises the steps that fine particles of raw materials are intercepted through a microfilter, then the raw materials enter a pipeline type permanent magnet iron remover to remove large-particle magnetic impurities, and then the raw materials enter a neutralization reactor, a crystallizer for crystallization, centrifugal separation and fluidized bed drying; centrifugally separated liquid enters a mother liquor tank and enters a neutralization reactor for neutralization reaction through slurry electromagnetic iron removal, and reacted slurry enters a crystallizer for crystallization, centrifugal separation and fluidized bed drying to obtain powder; the powder enters a process sieve, then enters a powder section series connection rotary type grating iron remover, enters a plate cooler after magnetic particles are removed, and is collected in a finished product bin for storage; after the materials enter the electromagnetic iron remover and are adsorbed by the magnetic gathering medium and the magnetic particles, the vibration motor vibrates at high frequency to enable the materials to pass through, and the distribution valve automatically discharges iron. Efficient and stable control over the battery-grade ammonium dihydrogen phosphate magnetic particles is achieved, and the magnetic particles are controlled to be reduced from 1200 pcs / kg to 30 pcs / kg or below.

Description

Technical Field

[0001] This invention relates to the field of lithium iron phosphate production technology, and specifically to a method for removing magnetic particles of battery-grade ammonium dihydrogen phosphate. Background Technology

[0002] Ammonium dihydrogen phosphate, also known as monoammonium phosphate, is a white crystalline solid with the molecular formula NH₄H₂PO₄. It is the most stable of the ammonium phosphate salts, soluble in water and slightly soluble in ethanol. Its aqueous solution is acidic, with a pH of 4.3. At room temperature (20℃), its solubility in water is 37.4g, its relative density is 1.803, and its melting point is 190℃. Upon heating, it decomposes into ammonium metaphosphate (NH₄PO₃).

[0003] With the rapid development of new energy vehicles and the energy storage industry, the performance requirements for lithium-ion battery cathode materials (such as lithium iron phosphate) are becoming increasingly stringent. The purity of its core precursor, battery-grade ammonium dihydrogen phosphate (BMAP), has become a key factor affecting battery safety and energy density. Magnetic particles (such as elemental iron and its oxides) are key impurities in BMAP, which may puncture the separator during battery charging and discharging, causing internal short circuits, thermal runaway, and other safety hazards, seriously restricting the development of lithium-ion batteries towards higher energy density and larger sizes.

[0004] Battery-grade ammonium dihydrogen phosphate (BMAP) is a raw material for lithium iron phosphate (LFP) production. However, the number of magnetic particles has an increasingly significant impact on product quality. These magnetic particles, during their movement, can even puncture the battery separator, causing a short circuit and rapid discharge, posing a significant safety hazard. The safety of lithium-ion batteries is a major obstacle hindering their development towards larger, higher-energy batteries. However, traditional iron removal processes face significant bottlenecks: mainstream permanent magnet or electromagnetic separators rely on frequent manual cleaning to maintain efficiency, but residual iron adsorbed on the magnetic rod surface easily leads to magnetic field attenuation. After long-term operation, iron removal efficiency drops significantly or even fails, making it difficult to meet the demands of continuous production. Furthermore, insufficient control of magnetic particles at the BMAP front end directly exacerbates the difficulty of iron removal at the downstream LFP / LFP stage. Because LFP particles are fine, easily electrostatically adsorbed, and possess weak magnetism, traditional electromagnetic iron removal easily causes material agglomeration, making it difficult to consistently maintain a magnetic content below the industry standard of 1 ppm.

[0005] Current technologies primarily focus on controlling the magnetic particles in finished lithium iron phosphate (LiFePO4) or lithium iron phosphate products, neglecting the source purification of battery-grade ammonium dihydrogen phosphate (MDH) precursors. Equipment wear, pipeline corrosion, and elemental iron introduced from raw materials accumulate during the production of battery-grade MDH, further increasing the iron removal burden at the downstream stage. Therefore, reducing the magnetic content at the source is crucial for improving the safety performance of cathode materials and reducing downstream processing costs. Summary of the Invention

[0006] The purpose of this invention is to provide a method for removing magnetic particles of battery-grade ammonium dihydrogen phosphate, thereby solving the problem of demagnetization of lithium iron phosphate precursor materials.

[0007] To solve the above-mentioned technical problems, the present invention adopts the following technical solution:

[0008] A method for removing battery-grade ammonium dihydrogen phosphate magnetic particles includes the following steps:

[0009] S1 Raw Material Purification and Demagnetization: The raw material passes through a micro filter to intercept fine particles, then enters a pipeline permanent magnet separator to remove large magnetic impurities, and then enters a neutralization reactor. The neutralized slurry enters a crystallizer for crystallization, and after crystallization, it is separated by centrifugation and then enters a drying fluidized bed for drying.

[0010] S2 Slurry Electromagnetic Iron Removal: The centrifugally separated liquid enters the mother liquor tank, passes through the slurry electromagnetic iron removal process, and then enters the neutralization reactor for neutralization reaction. The reacted slurry enters the crystallizer for crystallization, and after crystallization, it is centrifuged and then enters the fluidized bed for drying, sieving, and cooling to obtain powder.

[0011] S3 Powder Grating Demagnetization: Powder enters the process screen, then enters the powder section series rotary grid iron separator. After demagnetization, the particles enter the plate cooler, are cooled, enter the bucket elevator, and then enter the buffer silo for collection and storage in the finished product silo.

[0012] S4 Dry Powder Electromagnetic Iron Separator: Particles stored in the silo enter the electromagnetic iron separator through the buffer silo. After being attracted by the magnetic medium, the magnetic particles are attracted by the magnetic medium. The high-frequency vibration of the vibrating motor causes the material to pass through, and the distribution valve automatically discharges the iron.

[0013] Furthermore: In step S1, during the neutralization reaction, the raw materials, liquid ammonia, and phosphoric acid undergo a single reaction in the reactor to obtain a primary reaction slurry;

[0014] The primary reaction slurry is then piped back into the neutralization reactor, where it undergoes a secondary reaction with circulating liquid ammonia to obtain the secondary reaction slurry.

[0015] Furthermore: the degree of neutralization of the first-order reaction is 0.5 to 0.8, the degree of neutralization of the second-order reaction is 0.98 to 1.05, the reaction temperature of both the first-order and second-order reactions is 100 to 110°C, the pH of the neutralization reaction is 4.4 to 4.7, and the density of the slurry in the second-order reaction is 1.35 to 1.40 g / mL.

[0016] Further: In step S1, the crystallization temperature in the crystallizer is 50-54℃, the vacuum degree of the crystallizer is -60 to -75kPa, the solid content volume ratio of the crystal slurry obtained after crystallization is 25-40%, the neutralization degree of the slurry in the crystallizer is 1.01-1.03, and the rotation speed of the agitator used in the crystallizer is 35-70% of the full speed.

[0017] Furthermore: In step S2, the mother liquor outlet is pumped into a plate and frame filter press for filtration, and the filtrate is filtered by a bag filter and then enters the slurry electromagnetic separator and the neutralization reactor.

[0018] Furthermore: In step S2, the magnetic particles of the slurry electromagnetic separator are captured by the medium and then subjected to dynamic flushing and demagnetization. Dynamic flushing is the automatic power-off demagnetization after the medium is saturated, and compressed air is used to flush and remove iron.

[0019] Further: In step S2, the crystallizer and thickener are connected by pipelines, the thickener outlet is connected by centrifuge pipelines, the slurry in the thickener is discharged when the solid content is 25-40%, and the two centrifuges are connected by the drying fluidized bed conveyor belt.

[0020] Furthermore: When the moisture content of the material entering the drying fluidized bed is greater than 5%, the material flows back to the mother liquor tank in a countercurrent manner. The upper outlet of the thickener is connected to the mother liquor tank pipeline, and the centrifugal mother liquor outlet of the centrifuge is connected to the mother liquor tank pipeline. Part of the mother liquor in the mother liquor tank flows back to the crystallizer, and the other part flows back to the neutralization filter after being filtered by the activated carbon decolorization filter. After treatment, the Mg ion content and Al ion content in the recycled mother liquor are <100 ppm and <100 ppm, respectively.

[0021] Furthermore: In step S3, after the demagnetized particles enter the plate cooler for 20 to 40 minutes and are cooled to below 35°C, they enter the bucket elevator.

[0022] Furthermore, in step S4, the proportion of particles with a diameter of 20-60 mesh in the finished product is greater than 85%.

[0023] Compared with the prior art, the present invention has the following beneficial effects:

[0024] This application achieves efficient and stable control of battery-grade ammonium dihydrogen phosphate magnetic particles by employing permanent magnet separators, slurry electromagnetic iron removal, powder section cascade rotary grid demagnetization, and dry powder electromagnetic iron removal, combined with permanent magnets, electromagnetics, and surface-modified media. The magnetic particle control is reduced from 1200 pcs / kg to less than 30 pcs / kg, achieving precise capture and efficient cleaning of magnetic particles to meet the large-scale demand of the new energy industry for high-purity battery-grade ammonium dihydrogen phosphate. Detailed Implementation

[0025] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to embodiments. It should be understood that the specific embodiments described herein are merely illustrative and not intended to limit the invention.

[0026] A method for removing battery-grade ammonium dihydrogen phosphate magnetic particles includes the following steps:

[0027] (1) Raw material purification and demagnetization:

[0028] Battery-grade ammonium dihydrogen phosphate raw materials contain phosphoric acid, demineralized water, liquid ammonia, and steam. Due to the lack of control over the metal magnetic particles in the upstream raw materials, the raw materials carry metal magnetic particles into the system, with the magnetic particles in the raw acid exceeding 1200 pcs / kg.

[0029] Ammonium dihydrogen phosphate (MDP) raw material is placed in an industrial purified phosphoric acid storage tank. Fine particles are intercepted by a microfilter in the tank, then the sample enters a pipeline permanent magnet separator to remove large magnetic impurities. It then enters a neutralization reactor. Liquid ammonia is filtered through a Y-type filter, and then neutralized with phosphoric acid. Demineralized water is filtered through a microfilter to intercept fine particles, then enters a pipeline permanent magnet separator to remove large magnetic impurities, and then enters the neutralization reactor for slurry preparation. Low-pressure steam (0.5 MPa) is filtered through a microfilter to intercept fine particles, then enters a pipeline permanent magnet separator to remove large magnetic impurities, and then enters a drying fluidized bed for drying. The magnetic particle count in the raw material is controlled from 1200 pcs / kg to below 300 pcs / kg. The raw material purification and demagnetization process mainly involves removing iron from raw acid, liquid ammonia, and demineralized water. The raw materials, liquid ammonia, and phosphoric acid undergo a neutralization reaction. The slurry that passes the reaction is pumped into a crystallizer for crystallization. Qualified materials are collected by a crystallization collection pump (unqualified fine crystals are collected from the top of the crystallizer, dissolved by steam, and then returned to the crystallizer for re-crystallization after passing through a pipeline iron separator). After crystallization, the product enters a centrifuge for centrifugal separation, and then enters a fluidized bed for drying, sieving, and cooling.

[0030] During the neutralization reaction, industrial-grade refined phosphoric acid and liquid ammonia are separately introduced into tubular reactors for a primary reaction, yielding a primary reaction slurry. This primary slurry is then piped back into the neutralization reactor for a secondary reaction with circulating liquid ammonia, producing a secondary reaction slurry.

[0031] The degree of neutralization for the first-order reaction is 0.5–0.8, and for the second-order reaction it is 0.98–1.05. The reaction temperatures for both the first and second-order reactions are 100–110℃, and the pH for the neutralization reaction is 4.4–4.7. When the density of the slurry in the second-order reaction is 1.35–1.40 g / mL, the slurry enters the crystallizer. The crystallization temperature is 50–54℃, and the vacuum degree of the crystallizer is -60–-75 kPa. The solids content of the crystal slurry obtained after crystallization is 25–40% by volume, the degree of neutralization of the slurry in the crystallizer is 1.01–1.03, and the speed of the stirrer used in the crystallizer is 35–70% of its full speed.

[0032] In the pipeline-type permanent magnet separator, the magnetic rods are arranged in a cross pattern. The rod parameters are: surface Gaussian peak value 6000–16000 GS, temperature resistance 60–350℃, demagnetization rate <5% over 3 years, and the rods are coated with PTFE / ETFE. The temperature range of the pipeline-type permanent magnet separator is 60–350℃, normally 75% acid at 70℃, and normal flow rate is sufficient. Liquid ammonia comes from a liquid ammonia storage tank, as the liquid ammonia pipeline is made of Q345E material. The filter cloth specification of the micro-filter is 20μm. A multi-stage filter is installed at the front end of the pipeline-type permanent magnet separator for demineralized water and steam treatment.

[0033] (2) Electromagnetic iron removal of slurry:

[0034] The liquid separated by the centrifuge (centrifugal mother liquor) enters the mother liquor tank, is pumped to a fully automatic slurry electromagnetic iron removal system, and then enters the neutralization reactor for neutralization. The slurry that passes the neutralization reaction is pumped into a crystallizer for crystallization. Qualified material is collected by the crystallization collection pump (unqualified fine crystals are collected from the top of the crystallizer, dissolved by steam, and then returned to the crystallizer for recrystallization after iron removal by a pipeline iron separator). The product then enters the centrifuge for centrifugal separation. The product then enters a fluidized bed for drying, screening, and cooling before proceeding to the next stage of processing. The liquid separated by the centrifuge (centrifugal mother liquor), after passing through the fully automatic slurry electromagnetic iron removal system, is returned to the neutralization reactor for neutralization.

[0035] The mother liquor from the mother liquor tank outlet is pumped into a plate and frame filter press for filtration (ammonia slag). After filtration through a bag filter, the filtrate enters a slurry electromagnetic separator before proceeding to a neutralization reactor. The slurry electromagnetic separator is installed on the outlet pipeline of the mother liquor tank delivery pump. Ammonium dihydrogen phosphate slurry flows through the separator at a velocity of 0.45 m / s, where magnetic particles are captured by the medium. Dynamic rinsing then demagnetizes the slurry; this process involves automatic demagnetization after medium saturation, followed by iron removal using compressed air. Utilizing a high-gradient magnetic field and media rinsing technology, the number of magnetic particles in the centrifuged mother liquor stabilizes at 150-2000 pcs / kg, achieving a mother liquor demagnetization efficiency of over 85%.

[0036] During operation, the mother liquor processing capacity is 25-200m³. 3 / h; Ammonium dihydrogen phosphate slurry passes through a slurry electromagnetic separator with a hollow field strength of 10,000 GS and a working field strength of 40,000 GS. Magnetic particles are captured using a stacked magnetic medium (316L stainless steel). Once the medium is saturated, power is automatically cut off and demagnetization is achieved, followed by iron removal via compressed air flushing. This operation reduces maintenance labor costs by 50%-70%. Constant current control prevents temperature fluctuations from affecting the magnetic field strength. The magnetic head's magnetic circuit design, positioned inside the slurry electromagnetic separator, reduces the flow rate to 0.45 m / s, enhancing magnetic field uniformity.

[0037] The material entering the thickener has a solid content of 40%. After sedimentation and thickening, the bottom crystallized slurry enters the centrifuge for centrifugal separation, and the clear liquid at the top of the thickener overflows into the mother liquor tank. The crystallizer and thickener are connected by pipelines. The thickener outlet is connected to the pipelines of the first centrifuge and the second centrifuge, respectively. The thickener discharges when the solid content of the slurry is 25-40%. The first and second centrifuges are connected to the conveyor belt of the drying fluidized bed. When the moisture content of the material entering the drying fluidized bed is found to be greater than 5%, the material flows back to the mother liquor tank. The upper outlet of the thickener is connected to the mother liquor tank pipeline. The centrifugal mother liquor outlets of the first and second centrifuges are connected to the mother liquor tank pipeline. Part of the mother liquor in the mother liquor tank flows back to the crystallizer, and the other part is filtered through an activated carbon decolorization filter and then flows back to the neutralization filter. After treatment, the Mg ion content and Al ion content in the reused mother liquor are <100 ppm and <100 ppm, respectively. When the feed rate of the thickener to the first and second centrifuges is 100%, the rotation speed of the centrifuge drums is...

[0038] 1200-2000 r / min; pushing frequency is 50-70 times / min.

[0039] (3) Demagnetization of powder section by cascade rotary grid:

[0040] After centrifugal separation, drying, and sieving, the powdered material enters the process sieve, then enters the powder section via a series rotary bar separator. After demagnetizing the particles, it enters a plate cooler for 20-40 minutes to cool to below 35°C before entering a bucket elevator. From there, it enters a buffer silo, undergoes dry powder electromagnetic iron removal, and finally enters the finished product silo for storage. The rotating magnetic bars of the series rotary bar separator prevent clogging, and magnetic particles are automatically discharged after adsorption. The magnetic particle count is controlled from 200 pcs / kg to below 100 pcs / kg. The iron separator uses a rotary bar design with 12000GS magnetic bars in a ring rotation (5-10 rpm, processing capacity 7-20 tons / hour), effectively solving the problem of powder agglomeration and clogging.

[0041] (4) Dry powder electromagnetic iron removal:

[0042] Before entering the finished product silo after cooling, the finished product particles have a particle size of 20-60 mesh accounting for more than 85%. The finished particles then enter the electromagnetic separator via a buffer silo, passing through the magnetic medium at a flow rate of 10 tons / hour. After the magnetic particles are adsorbed, a vibrating motor vibrates at high frequency (amplitude 0.5-1mm) to allow the material to pass through, and the distribution valve automatically discharges the iron. The magnetic particle count is controlled from 100 pcs / kg to within 30 pcs / kg, with a demagnetization efficiency ≥70%. The finished product then passes through a fully automatic dry powder electromagnetic separator (central magnetic field 20000GS, 22 layers of magnetic medium), combined with high-frequency vibration (2 counter-rotating motors) to achieve demagnetization of ultrafine materials. Processing capacity: 10 tons / hour; Demagnetization efficiency: ≥70% (single electromagnetic separator).

[0043] In the electromagnetic separator, the internal oil circulation cooling system ensures a coil lifespan of ≥10 years. The PTFE / ETFE coating material channel reduces the risk of secondary contamination from magnetic foreign matter. The intelligent control system dynamically adjusts the excitation current and vibration frequency based on the magnetic particle content; abnormal data triggers early warnings and automatically optimizes process parameters.

[0044] Although the invention has been described herein with reference to several illustrative embodiments, it should be understood that many other modifications and implementations can be devised by those skilled in the art, which will fall within the scope and spirit of the principles disclosed herein. More specifically, various variations and modifications can be made to the components and / or layout of the subject matter combination within the scope of the disclosure and claims. Besides variations and modifications to the components and / or layout, other uses will be apparent to those skilled in the art.

Claims

1. A method for removing battery-grade ammonium dihydrogen phosphate magnetic particles, characterized in that: Includes the following steps: S1 Raw Material Purification and Demagnetization: The raw material passes through a micro filter to intercept fine particles, then enters a pipeline permanent magnet separator to remove large magnetic impurities, and then enters a neutralization reactor. The neutralized slurry enters a crystallizer for crystallization, and after crystallization, it is separated by centrifugation and then enters a drying fluidized bed for drying. S2 Slurry Electromagnetic Iron Removal: The centrifugally separated liquid enters the mother liquor tank, passes through the slurry electromagnetic iron removal process, and then enters the neutralization reactor for neutralization reaction. The reacted slurry enters the crystallizer for crystallization, and after crystallization, it is centrifuged and then enters the fluidized bed for drying, sieving, and cooling to obtain powder. S3 Powder Grating Demagnetization: Powder enters the process screen, then enters the powder section series rotary grid iron separator. After demagnetization, the particles enter the plate cooler, are cooled, enter the bucket elevator, and then enter the buffer silo for collection and storage in the finished product silo. S4 Dry Powder Electromagnetic Iron Separator: Particles stored in the silo enter the electromagnetic iron separator through the buffer silo. After being attracted by the magnetic medium, the magnetic particles are attracted by the magnetic medium. The high-frequency vibration of the vibrating motor causes the material to pass through, and the distribution valve automatically discharges the iron.

2. The method according to claim 1, characterized in that: In step S1, during the neutralization reaction, the raw materials, liquid ammonia, and phosphoric acid undergo a single reaction in the reactor to obtain a primary reaction slurry. The primary reaction slurry is then piped back into the neutralization reactor, where it undergoes a secondary reaction with circulating liquid ammonia to obtain the secondary reaction slurry.

3. The method according to claim 2, characterized in that: The degree of neutralization of the first-order reaction is 0.5–0.8, the degree of neutralization of the second-order reaction is 0.98–1.05, the reaction temperature of both the first-order and second-order reactions is 100–110℃, the pH of the neutralization reaction is 4.4–4.7, and the density of the slurry in the second-order reaction is 1.35–1.40 g / mL.

4. The method according to claim 1, characterized in that: In step S1, the crystallization temperature in the crystallizer is 50-54℃, the vacuum degree of the crystallizer is -60--75kPa, the solid content volume ratio of the crystal slurry obtained after crystallization is 25-40%, the neutralization degree of the slurry in the crystallizer is 1.01-1.03, and the rotation speed of the agitator used in the crystallizer is 35-70% of the full speed.

5. The method according to claim 1, characterized in that: In step S2, the mother liquor outlet is pumped into a plate and frame filter press for filtration. The filtrate is filtered by a bag filter and then enters the slurry electromagnetic separator and the neutralization reactor.

6. The method according to claim 1, characterized in that: In step S2, the magnetic particles of the slurry electromagnetic separator are captured by the medium and then subjected to dynamic flushing and demagnetization. Dynamic flushing is the automatic power-off demagnetization after the medium is saturated, and compressed air is used to flush and remove iron.

7. The method according to claim 1, characterized in that: In step S2, the crystallizer and thickener are connected by pipelines, the thickener outlet is connected to the centrifuge pipeline, the slurry in the thickener is discharged when the solid content is 25-40%, and the two centrifuges are connected to the drying fluidized bed conveyor belt.

8. The method according to claim 7, characterized in that: When the moisture content of the material entering the drying fluidized bed is greater than 5%, the material flows back to the mother liquor tank in a countercurrent manner. The upper outlet of the thickener is connected to the mother liquor tank pipeline, and the centrifugal mother liquor outlet of the centrifuge is connected to the mother liquor tank pipeline. Part of the mother liquor in the mother liquor tank flows back to the crystallizer, and the other part flows back to the neutralization filter after being filtered by the activated carbon decolorization filter. After treatment, the Mg ion content and Al ion content in the recycled mother liquor are <100 ppm and <100 ppm, respectively.

9. The method according to claim 1, characterized in that: In step S3, after the demagnetized particles enter the plate cooler for 20 to 40 minutes and are cooled to below 35°C, they enter the bucket elevator.

10. The method according to claim 1, characterized in that: In step S4, the proportion of particles with a diameter of 20-60 mesh in the finished product is greater than 85%.