A method for preparing lithium hexafluorophosphate solution with reduced reaction by-product content

Abstract

This invention provides a method for preparing a lithium hexafluorophosphate solution with reduced reaction byproduct content, belonging to the field of lithium-ion electrolyte manufacturing technology. It solves the problems of excessively high reaction byproduct content and difficulty in removing impurities in existing preparation methods. The invention includes the following steps: S1 organic solvent preparation; S2 purification of phosphorus pentafluoride gas preparation; S3 suspension mixture preparation; S4 initial stage lithium hexafluorophosphate solution preparation; S5 impurity adsorption; S6 preliminary filtration; S7 deacidification; S8 precision filtration. Through high-purity raw material preparation, strict reaction control, effective impurity removal steps, and the application of precision filtration technology, this invention successfully prepares a high-purity lithium hexafluorophosphate solution with low impurity content.

Description

Technical Field

[0001] This invention belongs to the field of lithium-ion electrolyte manufacturing technology, and relates to a lithium hexafluorophosphate solution, particularly a method for preparing a lithium hexafluorophosphate solution that reduces the content of reaction byproducts. Background Technology

[0002] Lithium hexafluorophosphate, a key material in lithium-ion batteries, is typically used in battery electrolytes by forming liquid lithium salts with organic solvents. The organic solvent method offers good controllability; the reaction process and product properties can be controlled by adjusting reaction conditions and solvent types to meet the needs of different applications. The organic solvent method usually achieves high yields. However, some challenges still need to be overcome:

[0003] High purity requirement: Lithium hexafluorophosphate used in battery electrolytes needs to have extremely high purity. When prepared using organic solvent methods, the reactants may undergo adverse reactions such as polymerization and decomposition with some organic solvents, resulting in the production of reaction byproducts. It is difficult to obtain high-purity lithium hexafluorophosphate, which seriously affects the performance and lifespan of the battery. It is not easy to reduce the generation and removal of reaction byproducts during the production process.

[0004] Water sensitivity: Lithium hexafluorophosphate is sensitive to water and readily absorbs moisture to form hydrates, thus affecting its performance in electrolytes. Effectively controlling the introduction of moisture and preventing hydrate formation is a problem that needs to be solved during the preparation process.

[0005] Reaction condition control: In organic solvent methods, changes in reaction conditions may lead to different side reaction pathways, thereby affecting the quality of the product. It is crucial to determine and precisely control the reaction conditions to ensure the consistency and purity of the product.

[0006] Inadequate solvent performance: The performance of a single organic solvent is often insufficient, leading to poor battery performance. Therefore, when selecting an organic solvent, its performance needs to be comprehensively considered to improve the solubility and stability of lithium hexafluorophosphate.

[0007] Solving these challenges is crucial for improving the preparation efficiency, product quality, and battery performance of lithium hexafluorophosphate. With the widespread application of lithium-ion batteries in new energy vehicles and rechargeable batteries, addressing these challenges is both urgent and critical. Summary of the Invention

[0008] The purpose of this invention is to address the aforementioned problems in existing technologies by proposing a method for preparing a lithium hexafluorophosphate solution that reduces the content of reaction byproducts.

[0009] The objective of this invention can be achieved through the following technical solution: a method for preparing a lithium hexafluorophosphate solution with reduced reaction byproduct content, the preparation method comprising the following steps:

[0010] S1 Organic Solvent Preparation: Purification: Dimethyl carbonate and ethyl methyl carbonate are purified separately to obtain purified dimethyl carbonate and purified ethyl methyl carbonate; Mixing: The purified dimethyl carbonate and purified ethyl methyl carbonate are mixed uniformly using a mixing device to obtain a solvent mixture; Drying: The solvent mixture is dried using a molecular sieve to obtain an organic solvent; the purity of the organic solvent is greater than 99.9%;

[0011] The purposes of purification are as follows: Commercially available dimethyl carbonate and ethyl methyl carbonate products contain numerous impurities. In preparing high-purity lithium hexafluorophosphate solutions, the raw materials used need to be as pure as possible. Purification of dimethyl carbonate and ethyl methyl carbonate removes potential impurities such as moisture, other organic impurities, or unreacted raw materials, which can affect the quality of the final product. It also improves reaction efficiency: purified solvents can increase the efficiency of chemical reactions, reduce side reactions, and thus reduce the formation of byproducts. Finally, it increases the purity of the final product: using high-purity solvents directly affects the purity of the final product, which is particularly important for the battery industry.

[0012] The reasons for choosing a mixture of purified dimethyl carbonate and purified methyl ethyl carbonate as organic solvents are as follows: Boiling point: The boiling point of dimethyl carbonate is about 90°C, while that of methyl ethyl carbonate is about 107°C. This means that they can evaporate at lower temperatures. The difference in their boiling points is beneficial for controlling the evaporation and recovery of the solvent at different temperature stages. Both are considered relatively safe and environmentally friendly solvents.

[0013] The functions and advantages of dimethyl carbonate: Good solubility: It has good lithium salt solubility, which helps maintain the high solubility of lithium hexafluorophosphate in the electrolyte; High conductivity: Its relatively high conductivity helps improve the conductivity of the electrolyte, reduce the internal resistance of the battery, and thus improve the power performance of the battery; Low temperature performance: It can maintain good performance in low temperature environments, allowing the battery to work normally in cold regions; Improved battery energy density: The use of dimethyl carbonate helps to improve the energy density of the battery because it can provide a relatively high capacity.

[0014] The functions and advantages of ethyl methyl carbonate: Solubility and stability: Ethyl methyl carbonate has good lithium salt solubility, which helps maintain the stability of the electrolyte. It can form a stable electrolyte film and improve the cycle life of the battery; Temperature adaptability: It performs relatively well at low temperatures, which helps improve the performance of the battery in cold environments; Reduced crystallization risk: The use of ethyl methyl carbonate can reduce the risk of lithium hexafluorophosphate crystallization in the electrolyte, which helps maintain the stability of the battery; Increased electrolyte viscosity: Ethyl methyl carbonate has a certain viscosity, which can increase the viscosity of the electrolyte and help prevent leakage problems in the battery.

[0015] By properly mixing them, the advantages of dimethyl carbonate and ethyl methyl carbonate can be fully utilized, while balancing their different properties to meet the requirements of battery design. Such mixed organic solvents help improve battery performance, safety, and cycle life.

[0016] Reasons for choosing the ratio of 19.744-29.616:17.856-26.784: Balancing polarity and solubility: This ratio was determined through extensive experiments in laboratory and industrial practice, balancing the polarity and solubility of the solvents; the high polarity of dimethyl carbonate combined with the stable solvent environment of ethyl methyl carbonate provides optimal lithium salt dissolution; Thermal stability and evaporation control: This ratio considers both thermal stability and solvent evaporation characteristics, allowing for more effective control of solvent volatilization and recovery; Optimizing reaction conditions: This ratio optimizes chemical reaction conditions, reduces the possibility of side reactions, and increases the yield of the target product.

[0017] The purpose of drying is to remove moisture because even trace amounts of water can undergo hydrolysis with lithium hexafluorophosphate.

[0018] Preparation of S2 purified phosphorus pentafluoride gas: Mixing: Phosphorus trichloride and fluorine gas are mixed in a corrosion-resistant reactor; the reaction is initiated under controlled conditions to obtain a mixed gas; the mixed gas is then cooled;

[0019] Preliminary purification: Chlorine gas is removed from the cooled mixed gas using an alkaline solution; Fractional purification: Further purification is carried out using a fractionation column to obtain purified phosphorus pentafluoride gas, which is collected using a gas collection device and stored at low temperature.

[0020] The purpose of preparing purified phosphorus pentafluoride gas is as follows: Phosphorus pentafluoride is a key raw material for the preparation of lithium hexafluorophosphate; high-purity phosphorus pentafluoride is crucial to the quality of the final product. Commercially available phosphorus pentafluoride contains many impurities, such as P2O5 and PF3, which are detrimental to subsequent reactions. High purity of phosphorus pentafluoride ensures the selectivity of the reaction and the purity of the product. The pure state of phosphorus pentafluoride helps to reduce side reactions and improve reaction efficiency.

[0021] S3 Suspension Mixture Preparation: Organic solvent is added to the reactor, lithium fluoride is fed into the reactor through a powder feeding device, and the mixture is stirred for the first time using a stirring device to form a suspension mixture. The reactor is equipped with a temperature control device and a pressure monitoring device. The organic solvent to lithium fluoride ratio in the suspension mixture is 5-10:1 by weight.

[0022] The reasons for choosing an organic solvent-lithium fluoride ratio of 5-10:1 are as follows: Solubility of lithium hexafluorophosphate: The solubility of lithium hexafluorophosphate in organic solvents is crucial. When phosphorus pentafluoride gas reacts with suspended lithium fluoride, the resulting lithium hexafluorophosphate forms a dense protective film on the surface of the lithium fluoride. This film hinders the reaction. The role of the organic solvent here is to quickly dissolve the generated lithium hexafluorophosphate, thereby breaking this protective film and ensuring the reaction can continue and be as complete as possible. Reaction kinetics: The amount of organic solvent affects the overall reaction kinetics. In an appropriate ratio of organic solvent, the reaction mixture has better fluidity, which facilitates effective contact between the gas and solid, promoting the reaction. Prevention of supersaturation and precipitation: Excessively high solid concentration may lead to supersaturation of the solution, causing lithium hexafluorophosphate to precipitate instead of dissolve. Maintaining an appropriate ratio of organic solvent to lithium fluoride ensures effective dissolution of lithium hexafluorophosphate, rather than the formation of solid precipitates. Improved product quality: By controlling the amount of organic solvent, the concentration and quality of the generated lithium hexafluorophosphate solution can be controlled to a certain extent, avoiding the formation of impurities due to incomplete reaction or supersaturation.

[0023] Preparation of lithium hexafluorophosphate solution in the initial stage of S4: Under normal pressure, purified phosphorus pentafluoride gas is introduced into the suspension mixture, and the stirring device is started for a second stirring at a speed of 200-500 rpm. The time for introducing the purified phosphorus pentafluoride gas is the same as the time for the second stirring. After the introduction of the purified phosphorus pentafluoride gas is completed, a third stirring is performed using a stirring device at a speed of 100-300 rpm. When the pressure in the reactor no longer changes, the third stirring is stopped, and the initial stage lithium hexafluorophosphate solution is obtained. The molar ratio of purified phosphorus pentafluoride gas to lithium fluoride is 1.1-1.2:1. The remaining purified phosphorus pentafluoride gas is sent to the tail gas treatment system for treatment by water-alkali washing.

[0024] The reason for choosing a molar ratio of 1.1-1.2:1 is that a slight excess of phosphorus pentafluoride gas can ensure that lithium fluoride reacts completely to form lithium hexafluorophosphate; at the same time, it can prevent the product from being of low purity due to insufficient phosphorus pentafluoride gas.

[0025] S5 adsorption of impurities: A purifying agent is added to the lithium hexafluorophosphate solution in the initial stage, and a stirring device is used to perform the fourth stirring operation. After the fourth stirring operation is completed, the solution is allowed to stand to obtain the lithium hexafluorophosphate solution in the intermediate stage.

[0026] S6 Preliminary Filtration: The intermediate lithium hexafluorophosphate solution is preliminarily filtered through a membrane filter device to obtain a filtered lithium hexafluorophosphate solution.

[0027] The reasons for using membrane filter equipment are as follows: Highly efficient separation capability: Membrane filters, through the principle of physical size exclusion, can effectively separate suspended particles and larger molecular impurities in the solution, which is crucial for improving the purity of lithium hexafluorophosphate solution; Simple operation: Compared with traditional filtration methods, membrane filtration equipment is more efficient and easier to operate, suitable for continuous or batch production processes; Controllable separation process: By selecting filter membranes with different pore sizes, the filtration effect can be precisely controlled to meet the specific purity requirements of lithium hexafluorophosphate solution.

[0028] S7 Deacidification: The filtered lithium hexafluorophosphate solution undergoes a deacidification process to obtain a deacidified lithium hexafluorophosphate solution. The purpose of the deacidification process is as follows: Improving product purity: As a key component of the battery electrolyte, the purity of lithium hexafluorophosphate directly affects battery performance. The presence of acidic substances reduces the purity of lithium hexafluorophosphate, thus affecting battery performance and lifespan; Enhancing battery safety: Acidic substances may react with other components in the battery, producing harmful substances or gases, increasing safety risks during battery use; Preventing corrosion: Acidic substances may corrode internal battery materials, such as electrodes and separators, affecting the structural integrity and performance of the battery; Improving electrolyte stability: Acidic substances may accelerate electrolyte degradation, reducing battery cycle life and energy storage efficiency; Meeting standard requirements: The battery manufacturing industry has strict standards for electrolyte purity. The deacidification step ensures that the lithium hexafluorophosphate solution meets these standards.

[0029] S8 Precision Filtration: Precision filtration equipment is used to precisely filter the deacidified lithium hexafluorophosphate solution to obtain a final lithium hexafluorophosphate solution. Precision filtration helps improve the purity of the final product, ensuring that the lithium hexafluorophosphate solution does not contain any unremoved minute impurities.

[0030] In the above-mentioned method for preparing lithium hexafluorophosphate solution with reduced reaction by-product content, in step S1, the purification process involves distilling the pre-purified product through a fractionation column, collecting the top product of the fractionation column, and using gas chromatography to detect the top product of the fractionation column during the purification process to ensure that it meets the purity requirements.

[0031] After purification, the purity of purified dimethyl carbonate is greater than 99.9%; the purity of purified methyl ethyl carbonate is greater than 99.9%.

[0032] In the purification of dimethyl carbonate, the distillation temperature is 90-120℃ and the distillation pressure is 1-1.5 atm; in the purification of methyl ethyl carbonate, the distillation temperature is 107-135℃ and the distillation pressure is 1-1.5 atm. The selection of distillation temperature and pressure is to obtain the best separation effect within an effective energy consumption. These parameters are determined based on the boiling point, vapor pressure, and chemical stability of dimethyl carbonate and methyl ethyl carbonate. These temperature and pressure parameters are sufficiently efficient to achieve effective distillation and can effectively avoid thermal decomposition or unnecessary chemical reactions.

[0033] During the drying process, the drying temperature is 40-60℃, and the drying duration is 6-12 hours. The moisture content of the organic solvent is determined using the Karl Fischer method, and the moisture content in the organic solvent is not higher than 35 ppm. The molecular sieve type is one of 3A, 4A, and 5A molecular sieves. The pore size of 3A, 4A, and 5A molecular sieves can effectively adsorb and remove moisture and other small molecule impurities in the solvent, while avoiding the adsorption of large molecules, which helps to maintain the chemical structure and purity of the solvent.

[0034] Steps for gas chromatography detection of the top product of a fractionation column: Sampling: Collect the sample from the top of the fractionation column;

[0035] Injection: Injecting the sample into a gas chromatographic apparatus; Separation: The sample is separated in the chromatographic column according to its molecular characteristics; Detection: Quantitative analysis of each component is performed using a detector; Data analysis: The purity of the components is determined by comparing the chromatographic peaks with known standard substances.

[0036] The steps for determining moisture content using the Karl Fischer method are as follows: Sample preparation: Prepare the sample and ensure its properties and state are suitable for the Karl Fischer method; Solvent preparation: Prepare an appropriate Karl Fischer solvent, using methanol or isopropanol as the solvent, containing Karl Fischer reagent; Calibration: Calibrate the instrument before starting the actual measurement, including calibrating the electrodes and magnetic stirrer; Predictive titration: Place a certain amount of sample into the instrument and purge with dry nitrogen gas to remove moisture from the sample; Start titration: Start the Karl Fischer titrator and titrate the Karl Fischer reagent into the sample. The Karl Fischer reagent reacts with water to generate hydrogen gas. This titration process is automatically monitored by the instrument; Endpoint detection: The titration terminates when the moisture in the sample has completely reacted. The endpoint is determined by detecting changes in electrode potential or current; Calculation of moisture content: The moisture content in the sample can be calculated using the known concentration of Karl Fischer reagent and the volume of Karl Fischer reagent used in the titration process.

[0037] In the above-mentioned method for preparing lithium hexafluorophosphate solution with reduced reaction byproduct content, in step S2, the molar ratio of phosphorus trichloride to fluorine is 1:5; the controlled environment includes controlling the reaction temperature at 200-300℃ and the reaction pressure at 2-5 atmospheres.

[0038] Cooling temperature is -40℃ to 0℃;

[0039] The alkaline solution is any one of sodium hydroxide solution, potassium hydroxide solution, and ammonia solution, wherein the concentration of the sodium hydroxide solution is 15% to 30%, the concentration of the potassium hydroxide solution is 15% to 30%, and the concentration of the ammonia solution is 25% to 30%.

[0040] During the fractionation purification process, the working temperature inside the fractionation tower is controlled at -85.5℃ to 75.5℃, the working pressure of the fractionation tower is 0.6 to 0.9 atmospheres, and the number of layers in the fractionation tower is 10 to 20. During the fractionation operation, gas chromatography is used to detect the purity to ensure that the purity of purified phosphorus fluoride gas reaches 99.9% or higher.

[0041] Temperature has a significant impact on the synthesis of phosphorus pentafluoride. At temperatures of 200-300℃, the reaction between phosphorus trichloride and fluorine tends to be complete. If the temperature is too low, the reaction is incomplete and the yield is low; if the temperature is too high, it will lead to unnecessary side reactions and equipment damage.

[0042] Maintaining the pressure at 2-5 atmospheres helps maintain the stability of the reaction and increase the reaction rate; excessive pressure may lead to equipment exceeding pressure limits and safety risks, while excessively low pressure will result in an incomplete reaction.

[0043] Cooling temperature: -40℃ to 0℃; Cooling is to condense the generated phosphorus pentafluoride gas, which facilitates subsequent purification. This temperature range can effectively condense phosphorus pentafluoride while avoiding the condensation of other low-boiling-point impurities.

[0044] Temperature control is crucial during fractionation. The temperature range of -85.5℃ to -75.5℃ is optimized based on the boiling point of phosphorus pentafluoride gas and the boiling points of any impurities that may be present.

[0045] Operating pressure 0.6-0.9 atm: Fractionation at lower pressures helps improve separation efficiency and reduce energy consumption. This pressure range is suitable for effective fractionation of phosphorus pentafluoride and also ensures safe operation of the equipment.

[0046] The number of fractionation tower layers (10-20 layers) affects the efficiency and purity of fractionation. 10-20 layers can improve separation efficiency while controlling the size and cost of the equipment.

[0047] In the above-mentioned method for preparing lithium hexafluorophosphate solution with reduced reaction by-product content, in step S3, during a single stirring operation, the operating temperature of the temperature control device is set to 10-15°C, the operating pressure of the pressure monitoring device is set to 1-2 atmospheres, the stirring speed is 100-300 rpm, and the stirring time is 30-60 minutes. During the first stirring process, samples are taken and tested using a suspension particle size analyzer to ensure that the suspension mixture is uniform.

[0048] Stirring speed of 100-300 rpm: This speed range helps ensure uniform mixing of the suspension and avoids particle breakage or excessive shear force caused by excessive stirring.

[0049] Operating steps for a suspension particle size analyzer:

[0050] Sample preparation: Take a representative sample from the reaction vessel;

[0051] Equipment calibration: Ensure the suspension particle size analyzer is calibrated accurately to obtain reliable data;

[0052] Sample loading: Place the sample in the sample chamber of the analyzer;

[0053] Test run: Start the device and perform granularity analysis.

[0054] In the above-described method for preparing lithium hexafluorophosphate solution with reduced reaction byproduct content, in step S4, the operating temperature of the temperature control device is set to 10-15°C; the rate of introduction of purified phosphorus pentafluoride gas is 0.05-0.2 L / min.

[0055] The purpose of temperature control: Under low temperature conditions of 10-15℃, the reaction rate can be slowed down to avoid heat accumulation and side reactions caused by excessively fast reaction; The effect of temperature on product quality: Low temperature helps to improve the purity and quality of lithium hexafluorophosphate, while reducing the formation of by-products;

[0056] The infeed rate of 0.05-0.2 liters per minute is based on the consideration of ensuring sufficient reaction between the gas and the solid. Too fast an infeed rate may lead to difficulty in reaction control, while too slow an infeed rate will reduce production efficiency.

[0057] In the above-described method for preparing lithium hexafluorophosphate solution with reduced reaction byproduct content, in step S5, the stirring speed for the four stirring operations is 150-300 rpm, the stirring time is 20-40 minutes, and the standing time is 1-2 hours. The purifying agent is succinic acid exchange resin; the amount of succinic acid exchange resin used is 5-10 grams per liter of initial lithium hexafluorophosphate solution.

[0058] Succinic acid exchange resin has good chemical stability and will not react with other components in the solution, ensuring the stability and reliability of the adsorption effect. Succinic acid exchange resin has high adsorption capacity and can effectively remove impurities in lithium hexafluorophosphate solution, such as unreacted raw materials or by-products. The adsorption operation using succinic acid exchange resin is simple, and the resin can be reused through the regeneration process, improving the economic efficiency of the entire process.

[0059] Adsorption capacity: The adsorption capacity of succinic acid exchange resin is limited. The amount of 5-10 grams of lithium hexafluorophosphate solution used per liter of initial stage is determined based on experiments and production practice, which can ensure that impurities are effectively removed.

[0060] In the above method for preparing lithium hexafluorophosphate solution with reduced reaction byproduct content, the preliminary filtration step in step S6 is as follows:

[0061] B1 transfers the intermediate stage lithium hexafluorophosphate solution from the reactor to the membrane filter equipment. The membrane filter equipment operates at a flow rate of 0.5-1 liters / minute and a pressure of 1.5-3 atmospheres.

[0062] B2 uses a filter membrane with a pore size of 5-10 micrometers for the first stage of filtration;

[0063] B3 uses a filter membrane with a pore size of 0.45-1 micrometer for the second stage of filtration;

[0064] B4 monitors the pressure and flow rate during the filtration process to ensure filtration efficiency.

[0065] Membrane filter equipment operating flow rate 0.5-1 liters / minute: This flow rate range is based on the balance between obtaining the best filtration effect and maintaining high processing efficiency. Too fast a flow rate may lead to a decrease in filtration efficiency, while too slow a flow rate will reduce production efficiency. Controlling the appropriate flow rate helps to prevent membrane clogging and damage and extend the membrane's service life.

[0066] The operating pressure of membrane filter equipment is 1.5-3 atmospheres: This pressure range ensures that the solution passes through the filter membrane effectively, while avoiding excessive mechanical pressure on the membrane and reducing the risk of membrane wear and damage. The appropriate operating pressure is crucial for maintaining filtration efficiency and quality. Too high a pressure may cause the membrane to rupture or excessive material permeation, while too low a pressure may result in insufficient treatment efficiency.

[0067] The selection of pore size is as follows: 5-10 microns for the first stage and 0.45-1 micron for the second stage. By using filter membranes with different pore sizes in stages, impurities of different sizes can be removed more effectively. The first stage removes larger particles, and the second stage further refines them. This combination of pore sizes helps to balance filtration efficiency and purity requirements, ensuring that the final product meets high-quality standards.

[0068] In the above method for preparing lithium hexafluorophosphate solution with reduced reaction byproduct content, the deacidification operation in step S7 is as follows:

[0069] C1 deacidification environment setup: Ensure the entire deacidification process is carried out in a nitrogen atmosphere to eliminate interference from air and moisture;

[0070] C2 Deacidifying Agent Addition: The deacidifying agent is added gradually to neutralize the acidic substances in the filtered lithium hexafluorophosphate solution. The deacidifying agent is either anhydrous sodium hydroxide or anhydrous sodium carbonate. During the addition of the deacidifying agent, the concentration of acidic substances in the deacidified lithium hexafluorophosphate solution is detected by ion chromatography to ensure that the acidic substances are completely neutralized.

[0071] The steps for detecting the concentration of acidic substances using an ion chromatograph are as follows: Sampling: Periodically take small samples from the reaction system at different stages of adding the deacidifying agent; Sample preparation: Dilute the samples appropriately to meet the detection requirements of the ion chromatograph; Chromatographic analysis: Analyze the samples using an ion chromatograph to determine the concentration of acidic substances in the solution; Result evaluation: Evaluate the removal of acidic substances based on the chromatographic analysis results to guide whether further addition of deacidifying agent is needed; Repeat detection: Repeat sampling and analysis as needed throughout the deacidification process until it is confirmed that all acidic substances have been completely neutralized.

[0072] In the above-described method for preparing a lithium hexafluorophosphate solution with reduced reaction byproduct content, in step S8, the precision filtration equipment includes: a feed pump for conveying the solution to be filtered to the filtration system; a pre-filter for removing larger particles from the solution; a main filter, including a precision microporous membrane and a depth filter material; a flow meter and a pressure gauge for real-time monitoring of the flow rate and pressure during the filtration process to ensure the equipment operates in optimal condition; and a filtration control system for adjusting the filtration speed and pressure to ensure filtration efficiency and product quality.

[0073] The precision filtration steps for the deacidified lithium hexafluorophosphate solution are as follows:

[0074] D1 Preparation Stage: Ensure the precision filtration equipment is clean to avoid introducing new impurities; conduct integrity tests on the pre-filter and main filter to ensure there is no damage or leakage;

[0075] D2 Filter Start-up: Start the feed pump to slowly deliver the deacidified lithium hexafluorophosphate solution to the pre-filter; during the pre-filtration stage, particles larger than 10 micrometers are removed; adjust the flow rate and pressure to ensure the solution passes smoothly through the pre-filter, with the flow rate controlled at 0.1-0.5 liters / minute and the filtration pressure at 0.2-0.5 atmospheres;

[0076] D3 Main Filtration Process: After pre-filtration, the solution first passes through a precision microporous membrane to remove fine particles. The flow rate of the solution through the precision microporous membrane is 0.05-0.2 L / min, and the filtration pressure is 0.5-1.5 atm. The pore size of the precision microporous membrane is 4-10 micrometers.

[0077] The solution filtered through a precision microporous membrane then flows through a depth filter material, which is composed of porous, highly absorbent ion exchange resin to capture smaller particles and dissolved impurities. The depth filter material has 3-6 layers and a pore size range that gradually transitions from 1-5 micrometers to 0.1-1 micrometers. The flow rate and pressure during the main filtration process are monitored in real time using flow meters and pressure gauges.

[0078] D4 Filtration Testing: Periodic sampling is conducted, and atomic absorption spectroscopy is used to check the purity of the solution. If the test fails, the filtration speed and pressure are adjusted through the filtration control system; or the precision microporous filter membrane and deep filtration material are replaced.

[0079] The flow rate during the pre-filtration stage is controlled at 0.1-0.5 liters / minute and the pressure is 0.2-0.5 atmospheres. The purpose of this stage is to remove larger particles. Appropriate flow rate and pressure can ensure that the solution passes smoothly through the filter and avoid damage to the filter membrane or reduction in efficiency due to excessive flow rate or pressure.

[0080] The flow rate of the precision microporous filter membrane is controlled at 0.05-0.2 liters / minute and the pressure is 0.5-1.5 atmospheres: at this stage, fine particles are removed; the lower flow rate helps to improve filtration efficiency, allowing more particles to be captured by the filter membrane; at the same time, the appropriate pressure ensures that the solution can pass through the filter membrane effectively without causing clogging.

[0081] Precision microporous filter membranes have a pore size of 4-10 micrometers: this pore size can effectively remove fine particles in the solution while preventing the filter membrane from clogging too quickly.

[0082] The pore size range of depth filtration materials gradually transitions from 1-5 micrometers to 0.1-1 micrometers: this progressive pore size setting helps to capture particles of different sizes in stages, thereby improving filtration efficiency and purity; the large-pore layer first captures larger particles, while the finer pores are responsible for removing smaller particles.

[0083] Porous, highly adsorbent ion exchange resins: These materials are chosen because they can effectively capture tiny particles and dissolved impurities in the solution. The porous structure of ion exchange resins provides a large surface area, giving them high adsorption capacity. In addition, these resins are selective and adsorbent for specific ions such as metal ions and other pollutants, which helps to further improve the purity of the solution.

[0084] Compared with the prior art, the present invention has the following beneficial effects:

[0085] 1. Reduced content of reaction byproducts: By preparing purified pentafluoride and purified organic solvent, high-purity raw materials are provided. By selecting appropriate organic solvent ratios and precisely controlling reaction conditions, the content of reaction byproducts is effectively reduced, the purity of lithium hexafluorophosphate is improved, and the problem of obtaining high-purity products during the preparation process is solved.

[0086] 2. Water sensitivity control: This preparation method effectively controls the introduction of water, which helps prevent lithium hexafluorophosphate from absorbing water and forming hydrates, thereby maintaining its performance in the electrolyte.

[0087] 3. Precise control of reaction conditions: By monitoring temperature, stirring and pressure at different stages of the preparation process, the reaction conditions are accurately controlled. This helps to avoid the occurrence of different side reaction pathways, ensures the consistency and purity of the product, and solves the challenge of controlling reaction conditions.

[0088] 4. Organic solvent performance optimization: Comprehensive consideration was given to the selection and ratio of organic solvents to improve the solubility and stability of lithium hexafluorophosphate. This helps to solve the problem of unsatisfactory performance of a single organic solvent and improves the battery's performance during use.

[0089] 5. Staged filtration and deacidification operations: By introducing purifying agents, performing preliminary filtration, and deacidification operations during the preparation process, reaction byproducts and other impurities are effectively removed, which helps to obtain high-quality lithium hexafluorophosphate solution.

[0090] In summary, this invention improves the preparation efficiency and product purity of lithium hexafluorophosphate, solves problems such as water sensitivity and reaction condition control, and improves the performance and lifespan of batteries. Detailed Implementation

[0091] The following are specific embodiments of the present invention, which further describe the technical solution of the present invention, but the present invention is not limited to these embodiments.

[0092] A method for preparing a lithium hexafluorophosphate solution with reduced reaction byproduct content includes the following steps:

[0093] S1 Organic Solvent Preparation: Purification Treatment: Dimethyl carbonate and ethyl methyl carbonate were purified separately to obtain purified dimethyl carbonate and purified ethyl methyl carbonate;

[0094] Mixing: Purified dimethyl carbonate and purified methyl ethyl carbonate are mixed until homogeneous using a mixing device to obtain a solvent mixture; in the solvent mixture, the weight ratio of purified dimethyl carbonate to purified methyl ethyl carbonate is 19.744:17.856.

[0095] Drying: The solvent mixture is dried using molecular sieve drying to obtain an organic solvent; the purity of the organic solvent is greater than 99.9%;

[0096] The purification process involves distilling the pre-purified product through a fractionation column, collecting the top product from the fractionation column, and using gas chromatography to detect the top product during the purification process to ensure it meets purity requirements.

[0097] After purification, the purity of purified dimethyl carbonate is greater than 99.9%; the purity of purified methyl ethyl carbonate is greater than 99.9%.

[0098] During the purification of dimethyl carbonate, the distillation temperature is 90-120℃ and the distillation pressure is 1-1.5 atm; during the purification of methyl ethyl carbonate, the distillation temperature is 107-135℃ and the distillation pressure is 1-1.5 atm.

[0099] During the drying process, the drying temperature was 40-60℃ and the drying duration was 8 hours. The moisture content of the organic solvent was determined using the Karl Fischer method, and the moisture content of the organic solvent was 25 ppm. The molecular sieve type was 4A molecular sieve.

[0100] Preparation of S2 purified phosphorus pentafluoride gas: Mixing: Phosphorus trichloride and fluorine gas are mixed in a corrosion-resistant reactor; the reaction is initiated under controlled conditions to obtain a mixed gas; the mixed gas is then cooled;

[0101] Preliminary purification: Use an alkaline solution to remove reaction byproducts from the cooled gas mixture;

[0102] Fractional purification: Further purification is carried out using a fractionation column to obtain purified phosphorus pentafluoride gas, which is collected using a gas collection device and stored at low temperature.

[0103] The molar ratio of phosphorus trichloride to fluorine is 1:5; the controlled environment includes controlling the reaction temperature at 200-300℃, controlling the reaction pressure at 2-5 atmospheres; and the cooling temperature is -40℃ to 0℃.

[0104] The alkaline solution is a 20% sodium hydroxide solution;

[0105] During the fractionation and purification process, the operating temperature of the fractionation tower is controlled at -85.5℃ to 75.5℃, the operating pressure of the fractionation tower is 0.6 to 0.9 atmospheres, and the number of layers in the fractionation tower is 15. During the fractionation operation, gas chromatography is used to detect the purity, and the purity of purified phosphorus pentafluoride gas reaches more than 99.9%.

[0106] Preparation of S3 suspension mixture: An organic solvent is added to the reactor. Lithium fluoride is then fed into the reactor using a powder feeding device. The mixture is first stirred using a stirring device to form a suspension mixture. The reactor is equipped with a temperature control device and a pressure monitoring device. The weight ratio of organic solvent to lithium fluoride in the suspension mixture is 7:1. During the first stirring operation, the operating temperature of the temperature control device is set to 10–15°C, and the operating pressure of the pressure monitoring device is set to 1–2 atmospheres. The stirring speed is 100–300 rpm, and the stirring time is 30–60 minutes.

[0107] Preparation of lithium hexafluorophosphate solution in the initial stage of S4: Under normal pressure, purified phosphorus pentafluoride gas is introduced into the suspension mixture, and the stirring device is started for a second stirring at a speed of 200-500 rpm. The time for introducing the purified phosphorus pentafluoride gas is the same as the time for the second stirring. After the introduction of the purified phosphorus pentafluoride gas is completed, a third stirring is performed using a stirring device at a speed of 100-300 rpm. When the pressure in the reactor no longer changes, the third stirring is stopped, and the initial stage lithium hexafluorophosphate solution is obtained. The molar ratio of purified phosphorus pentafluoride gas to lithium fluoride is 1.15:1. The residual purified phosphorus pentafluoride gas is sent to the tail gas treatment system for treatment by water-alkali washing. The operating temperature of the temperature control device is set to 10-15℃. The introduction rate of purified phosphorus pentafluoride gas is 0.05-0.2 L / min.

[0108] S5 adsorption of impurities: A purifying agent is added to the lithium hexafluorophosphate solution in the initial stage;

[0109] S6 Preliminary Filtration: The intermediate-stage lithium hexafluorophosphate solution undergoes preliminary filtration using a membrane filter device to obtain a filtered lithium hexafluorophosphate solution; the preliminary filtration steps are as follows:

[0110] B1 transfers the intermediate stage lithium hexafluorophosphate solution from the reactor to the membrane filter equipment. The membrane filter equipment operates at a flow rate of 0.5-1 liters / minute and a pressure of 1.5-2 atmospheres.

[0111] B2 uses a filter membrane with a pore size of 8 micrometers for the first stage of filtration;

[0112] B3 uses a filter membrane with a pore size of 0.7 micrometers for the second stage of filtration;

[0113] B4 monitors the pressure and flow rate during the filtration process to ensure filtration efficiency.

[0114] S7 Deacidification: The filtered lithium hexafluorophosphate solution is deacidified to obtain a deacidified lithium hexafluorophosphate solution. The deacidification operation steps are as follows: C1 Deacidification Environment Setup: Ensure that the entire deacidification process is carried out in a nitrogen atmosphere to eliminate interference from air and moisture.

[0115] C2 Deacidifying Agent Addition: The deacidifying agent is added gradually to neutralize the acidic substances in the filtered lithium hexafluorophosphate solution. The deacidifying agent is anhydrous sodium hydroxide.

[0116] S8 Precision Filtration: A precision filtration system is used to precisely filter the deacidified lithium hexafluorophosphate solution to obtain a final lithium hexafluorophosphate solution. The precision filtration steps for the deacidified lithium hexafluorophosphate solution are as follows:

[0117] D1 Preparation Stage: Ensure the precision filtration equipment is clean to avoid introducing new impurities; conduct integrity tests on the pre-filter and main filter to ensure there is no damage or leakage;

[0118] D2 Filter Start-up: Start the feed pump to slowly deliver the deacidified lithium hexafluorophosphate solution to the pre-filter; during the pre-filtration stage, particles larger than 10 micrometers are removed; adjust the flow rate and pressure to ensure the solution passes smoothly through the pre-filter, with the flow rate controlled at 0.1-0.3 liters / minute and the filtration pressure at 0.2-0.5 atmospheres;

[0119] D3 Main Filtration Process: After pre-filtration, the solution first passes through a precision microporous membrane to remove fine particles. The flow rate of the solution through the precision microporous membrane is 0.05-0.2 L / min, and the filtration pressure is 0.5-1.5 atm. The pore size of the precision microporous membrane is 4-10 micrometers.

[0120] The solution filtered through the precision microporous membrane then flows through a deep filtration medium, which is composed of porous, highly absorbent ion exchange resin to capture smaller particles and dissolved impurities. The deep filtration medium consists of five layers, with pore sizes ranging from 5 micrometers to 0.2 micrometers. The flow rate and pressure during the main filtration process are monitored in real time using flow meters and pressure gauges.

[0121] Purity testing: The purity of the prepared lithium hexafluorophosphate solution was tested using atomic absorption spectrometry, as follows:

[0122] 1. Sample preparation: Take a sample from the prepared lithium hexafluorophosphate solution, ensuring that no external impurities are introduced during the sampling process, and place the obtained sample in an appropriate container for further spectral analysis.

[0123] 2. Instrument Calibration: Calibrate the atomic absorption spectrometer to ensure an accurate understanding of the absorption characteristics of the target element. Use standards for calibration to establish a benchmark for the absorption spectrum.

[0124] 3. Sample injection: The obtained sample solution is gradually injected into the injection cell of the atomic absorption spectrometer; the injection volume is controlled to ensure that the analysis is carried out within the range specified by the instrument.

[0125] 4. Spectral Scan: Start the atomic absorption spectrometer to perform a spectral scan and record the absorption spectrum curve, including the position and intensity of the absorption peaks.

[0126] 5. Data Analysis: Analyze the recorded spectral data using instrument software or relevant analysis tools.

[0127] 6. Identify the elements in the solution and calculate their concentrations: Quality Control: Perform quality control procedures and check parameters such as signal-to-noise ratio, peak shape, and peak width of the spectrum; Result Interpretation: Based on the results of the spectral analysis, assess the purity of the lithium hexafluorophosphate solution; detect the presence of different elements or impurities and their concentrations.

[0128] Testing revealed that the lithium hexafluorophosphate solution contained 0.0023% reaction byproducts, 0.0036% moisture, 0.001% inorganic impurities (Na+), and 0.001% phosphorus pentafluoride.

[0129] The following conclusions can be drawn from the analysis of the above data:

[0130] 1. Use of high-purity raw materials: In steps S1 and S2, the initial content of impurities is greatly reduced by using organic solvents (dimethyl carbonate and ethyl methyl carbonate) and phosphorus pentafluoride gas with a purity of over 99.9%, which lays a solid foundation for the production of high-purity lithium hexafluorophosphate solution.

[0131] 2. Precise reaction control: The strict setting of temperature and pressure control (10-15℃, 1-2 atmospheres) and stirring speed (100-300rpm) in step S3 ensures the uniformity and stability of the reaction, thereby reducing the occurrence of side reactions.

[0132] 3. In step S4, the reaction efficiency was further optimized and the formation of byproducts was reduced by controlling the rate of phosphorus pentafluoride gas introduction (0.05-0.2 L / min) and the molar ratio (1.15:1).

[0133] 4. Highly efficient impurity removal: The adsorption of impurities and preliminary filtration in steps S5 and S6, using filter membranes with specific pore sizes (8 microns initially, then 0.7 microns), effectively remove larger particles and some fine impurities.

[0134] The deacidification process of S7 is carried out under a nitrogen atmosphere, which effectively eliminates the interference of air and moisture, and further reduces the acidity of the solution.

[0135] 5. Precision filtration technology: The precision filtration equipment in step S8 effectively removes tiny particles and dissolved impurities from the solution through multi-stage deep filtration, ensuring the high purity of the final product.

[0136] 6. Significant impurity reduction effect: Tests have shown that the content of reaction byproducts in lithium hexafluorophosphate solution is significantly reduced.

Claims

1. A method for preparing a lithium hexafluorophosphate solution with reduced reaction byproduct content, characterized in that, The preparation method includes the following steps: Preparation of S1 organic solvent: Purification: Dimethyl carbonate and methyl ethyl carbonate were purified separately to obtain purified dimethyl carbonate and purified methyl ethyl carbonate. Mixing: Purified dimethyl carbonate and purified methyl ethyl carbonate are mixed until homogeneous using a mixing device to obtain a solvent mixture; in the solvent mixture, the weight ratio of purified dimethyl carbonate to purified methyl ethyl carbonate is 19.744-29.616:17.856-26.

784. Drying: The solvent mixture is dried using molecular sieve drying to obtain an organic solvent; the purity of the organic solvent is greater than 99.9%. Preparation of phosphorus pentafluoride gas by S2 purification: Mixing: Phosphorus trichloride and fluorine gas are mixed in a corrosion-resistant reactor; the reaction is initiated under controlled conditions to obtain a mixed gas; the mixed gas is then cooled. Preliminary purification: Use an alkaline solution to remove reaction byproducts from the cooled gas mixture; Fractional purification: Further purification is carried out using a fractionation column to obtain purified phosphorus pentafluoride gas, which is collected using a gas collection device and stored at low temperature. S3 Suspension Mixture Preparation: Organic solvent is added to the reactor, lithium fluoride is fed into the reactor through a powder feeding device, and the mixture is stirred for the first time using a stirring device to form a suspension mixture. The reactor is equipped with a temperature control device and a pressure monitoring device. The organic solvent to lithium fluoride ratio in the suspension mixture is 5-10:1 by weight. Preparation of lithium hexafluorophosphate solution in the initial stage of S4: Under normal pressure, purified phosphorus pentafluoride gas is introduced into the suspension mixture, and the stirring device is started for a second stirring at a speed of 200-500 rpm. The time for introducing the purified phosphorus pentafluoride gas is the same as the time for the second stirring. After the introduction of the purified phosphorus pentafluoride gas is completed, a third stirring is performed using a stirring device at a speed of 100-300 rpm. When the pressure in the reactor no longer changes, the third stirring is stopped, and the initial stage lithium hexafluorophosphate solution is obtained. The molar ratio of purified phosphorus pentafluoride gas to lithium fluoride is 1.1-1.2:

1. The remaining purified phosphorus pentafluoride gas is sent to the tail gas treatment system for treatment by water-alkali washing. S5 adsorption of impurities: A purifying agent is added to the lithium hexafluorophosphate solution in the initial stage, and a stirring device is used to perform the fourth stirring operation. After the fourth stirring operation is completed, the solution is allowed to stand to obtain the lithium hexafluorophosphate solution in the intermediate stage. S6 Preliminary Filtration: The intermediate lithium hexafluorophosphate solution is preliminarily filtered through a membrane filter device to obtain a filtered lithium hexafluorophosphate solution. S7 Deacidification: The filtered lithium hexafluorophosphate solution is deacidified to obtain a deacidified lithium hexafluorophosphate solution; S8 Precision Filtration: Precision filtration equipment is used to perform precision filtration on the deacidified lithium hexafluorophosphate solution to obtain lithium hexafluorophosphate solution.

2. The method for preparing a lithium hexafluorophosphate solution with reduced reaction byproduct content as described in claim 1, characterized in that, In step S1, the purification process involves distilling the pre-purified product through a fractionation column, collecting the top product from the fractionation column, and using gas chromatography to detect the top product during the purification process to ensure it meets purity requirements. After purification, the purity of purified dimethyl carbonate is greater than 99.9%; the purity of purified methyl ethyl carbonate is greater than 99.9%. During the purification of dimethyl carbonate, the distillation temperature is 90-120℃ and the distillation pressure is 1-1.5 atm; during the purification of methyl ethyl carbonate, the distillation temperature is 107-135℃ and the distillation pressure is 1-1.5 atm. During the drying process, the drying temperature is 40-60℃, and the drying duration is 6-12 hours; the moisture content of the organic solvent is determined by the Karl Fischer method, and the moisture content of the organic solvent is not higher than 35 ppm; the molecular sieve type is one of 3A, 4A and 5A molecular sieves.

3. The method for preparing a lithium hexafluorophosphate solution with reduced reaction byproduct content as described in claim 1, characterized in that, In step S2, the molar ratio of phosphorus trichloride to fluorine is 1:5; the controlled environment includes controlling the reaction temperature at 200-300℃, controlling the reaction pressure at 2-5 atmospheres; and the cooling temperature is -40℃ to 0℃. The alkaline solution is any one of sodium hydroxide solution, potassium hydroxide solution, and ammonia solution, wherein the concentration of the sodium hydroxide solution is 15% to 30%, the concentration of the potassium hydroxide solution is 15% to 30%, and the concentration of the ammonia solution is 25% to 30%. During the fractionation purification process, the operating temperature of the fractionation tower is controlled at -85.5℃ to 75.5℃, the operating pressure of the fractionation tower is 0.6 to 0.9 atmospheres, and the number of layers in the fractionation tower is 10 to 20. During the fractionation operation, gas chromatography is used to detect the purity to ensure that the purity of purified phosphorus pentafluoride gas reaches 99.9% or higher.

4. The method for preparing a lithium hexafluorophosphate solution with reduced reaction byproduct content as described in claim 1, characterized in that, In step S3, during the first stirring operation, the operating temperature of the temperature control device is set to 10-15℃, the operating pressure of the pressure monitoring device is set to 1-2 atmospheres, the stirring speed is 100-300 rpm, and the stirring time is 30-60 minutes. During the first stirring process, samples are taken and tested using a suspension particle size analyzer to ensure that the suspended mixture is uniform.

5. The method for preparing a lithium hexafluorophosphate solution with reduced reaction byproduct content as described in claim 1, characterized in that, In step S4, the operating temperature of the temperature control device is set to 10-15°C; the rate of introduction of purified phosphorus pentafluoride gas is 0.05-0.2 liters / minute.

6. The method for preparing a lithium hexafluorophosphate solution with reduced reaction byproduct content as described in claim 1, characterized in that, In step S5, the stirring speed of the fourth stirring operation is 150-300 rpm, the stirring time is 20-40 minutes, and the standing time is 1-2 hours. The purifying agent is succinic acid exchange resin. The amount of succinic acid exchange resin used is 5-10 grams per liter of lithium hexafluorophosphate solution in the initial stage.

7. The method for preparing a lithium hexafluorophosphate solution with reduced reaction byproduct content as described in claim 1, characterized in that, In step S6, the preliminary filtering steps are as follows: B1 transfers the intermediate stage lithium hexafluorophosphate solution from the reactor to the membrane filter equipment. The membrane filter equipment operates at a flow rate of 0.5-1 liters / minute and a pressure of 1.5-3 atmospheres. B2 uses a filter membrane with a pore size of 5-10 micrometers for the first stage of filtration; B3 uses a filter membrane with a pore size of 0.45-1 micrometer for the second stage of filtration; B4 monitors the pressure and flow rate during the filtration process to ensure filtration efficiency.

8. The method for preparing a lithium hexafluorophosphate solution with reduced reaction byproduct content as described in claim 1, characterized in that, In step S7, the deacidification operation steps are as follows: C1 deacidification environment setup: Ensure the entire deacidification process is carried out in a nitrogen atmosphere to eliminate interference from air and moisture; C2 Deacidifying Agent Addition: The deacidifying agent is added gradually to neutralize the acidic substances in the filtered lithium hexafluorophosphate solution. The deacidifying agent is either anhydrous sodium hydroxide or anhydrous sodium carbonate. During the addition of the deacidifying agent, the concentration of acidic substances in the deacidified lithium hexafluorophosphate solution is detected by ion chromatography to ensure that the acidic substances are completely neutralized.

9. The method for preparing a lithium hexafluorophosphate solution with reduced reaction byproduct content as described in claim 1, characterized in that, In step S8, the precision filtration equipment includes: a feed pump for delivering the solution to be filtered to the filtration system; a pre-filter and a main filter, including a precision microporous filter membrane and a depth filter material; a flow meter and a pressure gauge for real-time monitoring of the flow rate and pressure during the filtration process to ensure that the equipment operates in the optimal state; and a filtration control system for adjusting the filtration speed and filtration pressure to ensure filtration efficiency and product quality. The precision filtration steps for the deacidified lithium hexafluorophosphate solution are as follows: D1 Preparation Stage: Ensure the precision filtration equipment is clean to avoid introducing new impurities; conduct integrity tests on the pre-filter and main filter to ensure there is no damage or leakage; D2 Filter Start-up: Start the feed pump to slowly deliver the deacidified lithium hexafluorophosphate solution to the pre-filter; during the pre-filtration stage, particles larger than 10 micrometers are removed; adjust the flow rate and pressure to ensure the solution passes smoothly through the pre-filter, with the flow rate controlled at 0.1-0.5 liters / minute and the filtration pressure at 0.2-0.5 atmospheres; D3 Main Filtration Process: After pre-filtration, the solution first passes through a precision microporous membrane to remove fine particles. The flow rate of the solution through the precision microporous membrane is 0.05-0.2 L / min, and the filtration pressure is 0.5-1.5 atm. The pore size of the precision microporous membrane is 4-10 micrometers. The solution filtered through a precision microporous membrane then flows through a depth filter material, which is composed of porous, highly absorbent ion exchange resin to capture smaller particles and dissolved impurities. The depth filter material has 3-6 layers and a pore size range that gradually transitions from 1-5 micrometers to 0.1-1 micrometers. The flow rate and pressure during the main filtration process are monitored in real time using flow meters and pressure gauges. D4 Filtration Testing: Periodic sampling is conducted, and atomic absorption spectroscopy is used to check the purity of the solution. If the test fails, the filtration speed and pressure are adjusted through the filtration control system; or the precision microporous filter membrane and deep filtration material are replaced.