A method for selectively digesting a separator base film and leaving an acid-resistant coating, a method for measuring an acid-resistant coating of a battery separator

Abstract

This invention relates to the field of battery testing technology, and more particularly to a method for selectively digesting the base film of a battery separator while retaining the acid-resistant coating, and a method for determining the acid-resistant coating of a battery separator. The method includes the following steps: nitric acid, sulfuric acid, and perchloric acid are added sequentially to the separator, followed by a gradient temperature increase to 280–300°C, and digestion under normal pressure until the base film is completely dissolved. This invention utilizes an appropriate acid system and reaction temperature to ensure that the base film of the battery separator is completely dissolved under normal pressure, while the acid-resistant coating material remains unreacted and intact, enabling accurate determination of its content and structure.

Description

Technical Field

[0001] This invention relates to the field of battery testing technology, and in particular to a method for selectively digesting the separator base film while retaining the acid-resistant coating, and a method for measuring the acid-resistant coating of a battery separator. Background Technology

[0002] Lithium-ion battery separators often use acid-resistant ceramic coatings such as α-Al2O3 (62% market share) and boehmite (35% market share) to improve thermal stability. However, current testing methods for the coating content of separators have certain shortcomings.

[0003] For example, the commonly used differential weighing method (arbitration method) includes the following steps: take a composite separator sample and weigh it to obtain m1; then completely peel off the coating with a solvent (such as NMP, ethanol), dry it, and weigh the substrate m2; finally, calculate the coating mass as m1-m2. This method is applicable to common coatings such as ceramics, PVDF, and aramid, but its disadvantage is that the coating and separator cannot be completely separated; for example, residual coating on the inner wall of the separator fibers affects the results. Similarly, patent document CN113293000A discloses a peeling reagent and a method for peeling off the coating of a secondary battery separator. It uses a peeling reagent composed of an organic solvent and an alkali to peel off the battery separator coating, thus separating the coating from the separator. However, it focuses more on the non-destructive preservation of the separator base film, which is not conducive to the accurate detection of coating content and structure.

[0004] For example, the method provided in GB / T 6672-2001 "Mechanical Contact Measurement Method for Determination of Thickness of Plastic Films and Sheets" indirectly estimates the coating thickness / content by testing the difference between the total thickness and the substrate thickness. This method is quick and simple, but it cannot quantify the coating layer, and the test accuracy is greatly affected by the uniformity of the substrate thickness.

[0005] Therefore, there is an urgent need to provide a method that can effectively separate the battery separator base film and coating while completely preserving the coating material, so as to accurately measure the coating. Summary of the Invention

[0006] The present invention aims to solve the above-mentioned problems by providing a method for selectively digesting the base film of a battery separator while completely preserving the acid-resistant coating material, and a method based on which the acid-resistant coating of the battery separator can be accurately measured.

[0007] The technical solution to the problem of this invention is, firstly, to provide a method for selectively digesting the base membrane of a diaphragm while retaining the acid-resistant coating, comprising the following steps: sequentially adding nitric acid, sulfuric acid, and perchloric acid to the diaphragm, then gradually heating to 280–300 °C, and digesting under normal pressure until the base membrane is completely dissolved.

[0008] As a preferred embodiment of the present invention, the base film is selected from at least one of PP, PE, and PVDF.

[0009] As a preferred embodiment of the present invention, the acid-resistant coating is selected from at least one of α-Al₂O₃ and boehmite. α-Al₂O₃ has stable crystal structure, high density, resistance to strong acids, high temperatures, and oxidation; it is insoluble in cold acids and also sparingly soluble in hot acids and strong acids below their boiling point, thus corroding the surface only extremely slowly. Boehmite (AlOOH) dehydrates to become γ-Al₂O₃, which is insoluble in strong acids.

[0010] In this invention, an appropriate acid system and temperature are determined so that the battery separator base film can be completely dissolved in the corresponding acid system under the corresponding high temperature and normal pressure, but the acid-resistant coating material does not react under these conditions, thus preserving the acid-resistant coating material completely and enabling accurate measurement.

[0011] As a preferred embodiment of the present invention, the mass recovery rate of the acid-resistant coating material is not less than 98%; preferably, the mass recovery rate of the acid-resistant coating material is not less than 99%.

[0012] For acidic systems, the order of acid addition needs to be restricted: first, add nitric acid for pre-oxidation, then add sulfuric acid, mix and cool, and finally add perchloric acid to the cooled system to form a digestion solution, and then start the temperature program. This order avoids direct contact between perchloric acid and the active organic substrate membrane and establishes a chemical gradient in which the oxidizing capacity is gradually released with temperature, ensuring safe digestion at atmospheric pressure.

[0013] The concentration of acid is preferably limited. As a preferred embodiment of the present invention, the mass concentration of nitric acid is 65% to 68%, for example, it can be 65%, 65.5%, 66%, 66.5%, 67%, 67.5%, or 68%. At this concentration, the oxidizing and volatile properties of nitric acid are moderate. If the concentration is too low, on the one hand, the system will boil before reaching the required temperature, causing the system to evaporate to dryness, and on the other hand, the oxidizing capacity will be insufficient.

[0014] As a preferred embodiment of the present invention, the mass concentration of the sulfuric acid is 95% to 98%, for example, it can be 95%, 95.5%, 96%, 96.5%, 97%, 97.5%, or 98%. By using sulfuric acid with a sufficiently high boiling point and high concentration as a heat medium, it is ensured that the reaction system can be heated to 280 to 300 °C under normal pressure without being evaporated and the base film can be completely dissolved.

[0015] As a preferred embodiment of the present invention, the mass concentration of perchloric acid is 70% to 72%, for example, it can be 70%, 70.5%, 71%, 71.5%, or 72%; in concentrated sulfuric acid medium, perchloric acid in this concentration range has strong oxidizing properties and can completely oxidize carbonized residues.

[0016] The proportion of acids is preferably limited. Preferably, the volume ratio of nitric acid, sulfuric acid, and perchloric acid is (1.5–2.5):1:(1.5–2.5). For example, when the volume fraction of sulfuric acid is 1 part, the volume fraction of perchloric acid can be 1.5 parts, 1.6 parts, 1.7 parts, 1.8 parts, 1.9 parts, 2 parts, 2.1 parts, 2.2 parts, 2.3 parts, 2.4 parts, or 2.5 parts, preferably 2 parts, to ensure the safe and maximized oxidizing effect of perchloric acid in concentrated sulfuric acid medium. When the volume fraction of sulfuric acid is 1 part, the volume fraction of nitric acid can be 1.5 parts, 1.6 parts, 1.7 parts, 1.8 parts, 1.9 parts, 2 parts, 2.1 parts, 2.2 parts, 2.3 parts, 2.4 parts, or 2.5 parts, preferably 2 parts, to dilute the reaction system, reduce viscosity, and sufficiently pre-oxidize the base film surface before the addition of perchloric acid.

[0017] As a preferred embodiment of the present invention, the volume ratio of nitric acid to perchloric acid is not less than 70% to ensure safety.

[0018] The ratio of the diaphragm to the acid is preferably limited. Preferably, the ratio of the diaphragm to the total amount of nitric acid, sulfuric acid, and perchloric acid is 0.004–0.02 g / mL. Examples include 0.004 g / mL, 0.005 g / mL, 0.0075 g / mL, 0.01 g / mL, 0.0125 g / mL, 0.015 g / mL, 0.0175 g / mL, and 0.02 g / mL. Excessive diaphragm usage can lead to incomplete digestion or excessively long digestion times, while insufficient diaphragm usage can result in an overly vigorous reaction and insufficient safety.

[0019] Regarding temperature, a gradient heating approach should be adopted to ensure a smooth and progressive increase in oxidation capacity throughout the digestion process. This transforms the vigorous base film digestion reaction into a controllable three-stage relay reaction, guaranteeing safety, facilitating the corresponding function of the acid components, and ensuring the complete preservation of the acid-resistant coating material. If the temperature is directly and rapidly increased to a high level, nitric acid will decompose and volatilize quickly, failing to fully participate in the pre-oxidation reaction. Simultaneously, an overly vigorous reaction will generate numerous bubbles and localized hot spots that will impact and damage the coating's microstructure, affecting the integrity of the coating material.

[0020] As a preferred embodiment of the present invention, the heating rate is 1 to 5 °C / min, for example, it can be 1 °C / min, 2 °C / min, 3 °C / min, 4 °C / min, or 5 °C / min.

[0021] As a preferred embodiment of the present invention, the gradient heating is as follows: first, the temperature is increased to 40-80 °C, then to 120-180 °C, and finally to 280-300 °C and maintained until the solution becomes clear.

[0022] The first stage of heating is used to wet the diaphragm and supply nitric acid for its action. For example, the temperature can be 40 °C, 50 °C, 60 °C, 70 °C, or 80 °C. The slower heating rate ensures that the heating time is sufficient for the nitric acid to fully exert its effect. To further ensure the pre-oxidation of the base film, as a preferred embodiment of the present invention, the temperature is raised to 40–80 °C and held for 10–30 min. For example, the time can be 10 min, 15 min, 20 min, 25 min, or 30 min.

[0023] The second stage of heating is used for dehydration and carbonization. For example, the temperature can be 120 ℃, 130 ℃, 140 ℃, 150 ℃, 160 ℃, or 170 ℃. Preferably, the temperature is raised to 120–180 ℃ and maintained until the reddish-brown NO2 fumes fade or white smoke appears.

[0024] The third stage of heating is used to allow perchloric acid to function in hot sulfuric acid and completely dissolve carbides. For example, the temperature can be 280 ℃, 285 ℃, 290 ℃, 295 ℃, or 300 ℃.

[0025] Secondly, another objective of this invention is to provide a method for determining the acid-resistant coating of a battery separator, including the method described above for selectively digesting the separator base film while retaining the acid-resistant coating. The determination can focus on the content, structural morphology, etc., of the acid-resistant coating material.

[0026] As a preferred embodiment of the present invention, the content determination method includes the following steps:

[0027] S1. Weigh the diaphragm; the weight is M1.

[0028] S2. Digest the diaphragm base membrane according to the method described above; after digestion, separate the solid and liquid, wash the obtained solid to neutral and dry it to constant weight, and weigh it as M2;

[0029] S3. The coating content is (M2 / M1)×100%.

[0030] As a preferred embodiment of the present invention, in step S2, solid-liquid separation is performed by filtration to obtain the acid-resistant coating material intact.

[0031] The filter membrane used for vacuum filtration is preferably a filter membrane that is resistant to strong acids, strong oxidation, and has low adsorption. For example, it can be at least one of polytetrafluoroethylene (PTFE) filter membrane and polyvinylidene fluoride (PVDF) filter membrane.

[0032] The pore size of the filter membrane should not be too large to ensure that the coating material is effectively retained. Preferably, the pore size of the filter membrane is 0.1 to 0.5 μm, and for example, it can be 0.1 μm, 0.2 μm, 0.3 μm, 0.4 μm, or 0.5 μm.

[0033] The negative pressure of the filtration should not be too high to avoid clogging of the filter membrane and loss of coating material. As a preferred embodiment of the present invention, the negative pressure of the filtration is -0.02 to -0.05 MPa, for example, it can be -0.02 MPa, -0.03 MPa, -0.04 MPa, or -0.05 MPa.

[0034] The beneficial effects of this invention are:

[0035] 1. This invention provides a method for selectively digesting the base film of a battery separator while completely preserving the acid-resistant coating material. By using an appropriate acid system and reaction temperature, the base film is completely dissolved under normal pressure, while the acid-resistant coating material does not react and is thus completely preserved, thereby effectively separating the acid-resistant coating material from the separator.

[0036] 2. The present invention provides a method for determining the acid-resistant coating of battery separators using the above method. Based on the intact preserved acid-resistant coating material, it is possible to accurately determine its content and structure. Attached Figure Description

[0037] Figure 1 This is a surface SEM image of the diaphragm after treatment in Comparative Example 1;

[0038] Figure 2 This is a surface SEM image of the diaphragm after treatment in Comparative Example 2. Detailed Implementation

[0039] The following are specific embodiments of the present invention, and the technical solution of the present invention will be further described in conjunction with the accompanying drawings. However, the present invention is not limited to these embodiments.

[0040] Example 1

[0041] Prepare a standard diaphragm with a known coating content: the diaphragm uses PE as the base membrane and boehmite as the acid-resistant coating, with an acid-resistant coating content of 40 wt%.

[0042] A method for selectively digesting the diaphragm base membrane while retaining the acid-resistant coating includes the following steps:

[0043] Take a standard diaphragm sample and accurately weigh it (M1 = 0.2003 g). Add 10 mL of 66% nitric acid and 5 mL of 97% sulfuric acid to the beaker containing the diaphragm. After cooling, add 10 mL of 71% perchloric acid. Heat to 50°C at a rate of 2°C / min and hold for 20 min. Then heat to 150°C at a rate of 2°C / min and hold until white fumes appear (hold for 30 min). Finally, heat to 275°C at a rate of 2°C / min and hold until the solution becomes clear (hold for 8 min).

[0044] A method for determining the acid-resistant coating of a battery separator includes the following steps:

[0045] After cooling the obtained solution to room temperature, it was filtered through a 0.3 μm PTFE membrane under vacuum at a pressure difference of -0.03 MPa. After filtration, the filter cake was washed with deionized water until neutral, and then vacuum dried at 60 °C to constant weight to obtain the boehmite coating material, with a weight M2 = 0.0796 g. Therefore, the boehmite coating content in the battery separator can be calculated to be 39.74%.

[0046] Compared with the theoretical content of 0.08012 g of boehmite coating material, the dissolution loss rate of the boehmite coating material obtained by this method is only 0.6%, which is within the allowable error range.

[0047] Example 2

[0048] This embodiment is basically the same as embodiment 1, except that the diaphragm is different. A standard diaphragm with a known coating content is prepared: the diaphragm is based on PP and has α-Al2O3 as an acid-resistant coating with an acid-resistant coating content of 50 wt%.

[0049] A method for selectively digesting the diaphragm base membrane while retaining the acid-resistant coating includes the following steps:

[0050] Take a standard diaphragm sample and accurately weigh it (M1 = 0.2005 g). Add 10 mL of 66% nitric acid and 5 mL of 97% sulfuric acid to the beaker containing the diaphragm. After cooling, add 10 mL of 71% perchloric acid. Heat to 50°C at a rate of 2°C / min and hold for 20 min. Then heat to 150°C at a rate of 2°C / min and hold until white fumes appear (hold for 30 min). Finally, heat to 290°C at a rate of 2°C / min and hold until the solution becomes clear (hold for 20 min).

[0051] A method for determining the acid-resistant coating of a battery separator includes the following steps:

[0052] After cooling the obtained solution to room temperature, it was filtered through a 0.3 μm PTFE membrane under vacuum at a pressure difference of -0.03 MPa. After filtration, the filter cake was washed with deionized water until neutral, and then vacuum dried at 60 °C to constant weight to obtain the alumina coating material, with a weight of M2 = 0.1001 g. Therefore, the coating content in the battery separator can be calculated to be 49.92%.

[0053] Compared with the theoretical content of 0.10025 g of coating material, the dissolution loss rate of boehmite coating material obtained by this method is only 0.15%, which is due to the higher acid resistance of α-Al2O3.

[0054] Example 3

[0055] This embodiment is basically the same as Embodiment 1, except that the heating procedure is different.

[0056] Take a standard boehmite-coated diaphragm sample and accurately weigh it (M1 = 0.2002 g). Add 10 mL of 66% nitric acid and 5 mL of 97% sulfuric acid to the beaker containing the diaphragm. After cooling, add 10 mL of 71% perchloric acid. Then heat to 50 °C at a rate of 1 °C / min, then directly heat to 150 °C at a rate of 1 °C / min, and finally directly heat to 275 °C at a rate of 1 °C / min. Hold at 275 °C until the solution becomes clear.

[0057] After cooling the obtained solution to room temperature, it was filtered through a 0.3 μm PTFE membrane under vacuum at a pressure difference of -0.03 MPa. After filtration, the filter cake was washed with deionized water until neutral, and then vacuum dried at 60 °C to constant weight to obtain the boehmite coating material, with a weight of M2 = 0.0787 g. Therefore, the boehmite coating content in the battery separator can be calculated to be 39.31%.

[0058] Compared with the theoretical content of 0.08008 g of boehmite coating material, the dissolution loss rate of boehmite coating material obtained by this method is 1.7%, which is higher than the loss rate in Example 1. This may be because, although the heating rate is reduced, direct heating and lack of heat preservation platform can easily lead to the accumulation of heat and oxidants, which in turn leads to a relatively violent reaction and impacts the coating material, causing damage to it.

[0059] Example 4

[0060] This embodiment is basically the same as Embodiment 1, except that the volume ratio of nitric acid, sulfuric acid and perchloric acid is different.

[0061] Take a standard boehmite-coated diaphragm sample and accurately weigh it (M1 = 0.2008 g). Add 12.5 mL of 66% nitric acid and 5 mL of 97% sulfuric acid to the beaker containing the diaphragm. After cooling, add 7.5 mL of 71% perchloric acid. Then heat to 50 °C at a rate of 2 °C / min and hold for 20 min; then heat to 150 °C at a rate of 2 °C / min and hold until white fumes appear; finally, heat to 275 °C at a rate of 2 °C / min and hold until the solution becomes clear.

[0062] After cooling the obtained solution to room temperature, it was filtered through a 0.3 μm PTFE membrane under vacuum at a pressure difference of -0.03 MPa. After filtration, the filter cake was washed with deionized water until neutral, and then vacuum dried at 60 °C to constant weight to obtain the boehmite coating material, weighing M2 = 0.0807 g. Therefore, the boehmite coating content in the battery separator can be calculated to be 40.19%, which is higher than the theoretical content. This increase may be due to incomplete digestion of the base membrane caused by a reduction in the amount of perchloric acid used.

[0063] Example 5

[0064] This embodiment is basically the same as Embodiment 1, except that the volume ratio of nitric acid, sulfuric acid and perchloric acid is different.

[0065] Take a standard boehmite-coated diaphragm sample and accurately weigh it (M1 = 0.2005 g). Add 10 mL of 66% nitric acid and 5 mL of 97% sulfuric acid to the beaker containing the diaphragm. After cooling, add 12.5 mL of 71% perchloric acid. Then heat to 50 °C at a rate of 2 °C / min and hold for 20 min; then heat to 150 °C at a rate of 2 °C / min and hold until white fumes appear; finally, heat to 275 °C at a rate of 2 °C / min and hold until the solution becomes clear.

[0066] After cooling the obtained solution to room temperature, it was filtered through a 0.3 μm PTFE membrane under vacuum at a pressure difference of -0.03 MPa. After filtration, the filter cake was washed with deionized water until neutral, and then vacuum dried at 60 °C to constant weight to obtain the boehmite coating material, weighing M2 = 0.0788 g. Therefore, the boehmite coating content in the battery separator can be calculated to be 39.30%. The increased loss rate compared to Example 1 may be due to the excessive and severe oxidation by perchloric acid, which reduces the controllability of the reaction and impacts the coating material, causing damage.

[0067] Comparative Example 1

[0068] The diaphragm was treated with alcohol in the same manner as in Example 1.

[0069] Five drops of anhydrous ethanol were added to the boehmite-coated diaphragm, and the temperature was increased according to the heating procedure of Example 1 to obtain the product as shown. Figure 1 As shown, the coating at the dripping point is partially dissolved, but fuzzy residue is visible at the edge, indicating that the coating is not completely dissolved.

[0070] Comparative Example 2

[0071] The diaphragm was treated with NMP in the same manner as in Example 1.

[0072] Five drops of NMP were added to the boehmite-coated diaphragm, and the temperature was increased according to the heating procedure of Example 1 to obtain the product as shown. Figure 2 As shown, the coating at the dripping point is partially dissolved, but coating residue is visible at the edges and inside, indicating that the coating is not completely dissolved.

[0073] Comparative Example 3

[0074] This comparative example is basically the same as Example 1, except that the volume ratio of nitric acid, sulfuric acid and perchloric acid is different.

[0075] A standard boehmite-coated diaphragm sample was taken, and 7.5 mL of 66% nitric acid and 5 mL of 97% sulfuric acid were added sequentially to the beaker containing the diaphragm. After cooling, 12.5 mL of 71% perchloric acid was added. The solution was then heated to 50 °C at a rate of 2 °C / min and held for 20 min; then heated to 150 °C at a rate of 2 °C / min and held until white fumes were observed; finally, it was heated at a rate of 2 °C / min. However, when the temperature reached above 200 °C, the exothermic reaction was intense, and continued heating posed a safety hazard. This was likely due to the reduced amount of nitric acid, resulting in a large amount of the base membrane remaining in an active state at high temperatures, which then underwent violent oxidation with the excess perchloric acid. Therefore, heating was discontinued, and the temperature was lowered to 200 °C and held at 200 °C. However, after 3 hours of holding, the solution remained dark in color, and the digestion endpoint could not be observed.

[0076] Comparative Example 4

[0077] This comparative example is basically the same as Example 1, except that the acid solution is different, with sulfuric acid replaced by phosphoric acid.

[0078] A standard diaphragm sample was taken, and 10 mL of 66% nitric acid and 5 mL of 85% phosphoric acid were added sequentially to the beaker containing the diaphragm. After cooling, 10 mL of 71% perchloric acid was added. The mixture was then heated to 50 °C at a rate of 2 °C / min and held for 20 min. It was then heated to 150 °C at a rate of 2 °C / min, but no carbonization was observed, so it was held for 30 min. Finally, heating was continued at a rate of 2 °C / min. However, when the temperature reached approximately 160 °C, the reaction system began to boil violently and faced the risk of evaporation to dryness. Further heating was not possible due to safety concerns. An attempt was made to cool to 155 °C and hold at 155 °C, but the reaction system continued to slowly evaporate to dryness, the solution remained turbid, and the reaction could not continue. This indicates that phosphoric acid has weak dehydration properties, a low boiling point, and cannot form a stable concentrated acid medium to activate the high-temperature oxidizing properties of perchloric acid.

[0079] The specific embodiments described herein are merely illustrative of the spirit of the invention. Those skilled in the art to which this invention pertains may make various modifications or additions to the described specific embodiments or use similar methods to substitute them, without departing from the spirit of the invention or exceeding the scope defined by the appended claims.

Claims

1. A method of selectively digesting a separator base film and leaving an acid resistant coating, characterized by: Includes the following steps: Nitric acid, sulfuric acid, and perchloric acid were added to the diaphragm in sequence, and then the temperature was gradually increased to 280–300 °C. The diaphragm was then digested under normal pressure until the base membrane was completely dissolved.

2. The method of selectively digesting a separator base film and leaving an acid resistant coating of claim 1, wherein: The base film is selected from at least one of PP, PE, and PVDF; the acid-resistant coating is selected from at least one of α-Al2O3 and boehmite.

3. A method of selectively digesting a separator base film and leaving an acid resistant coating according to claim 1 or 2, characterized in that: The mass concentration of the nitric acid is 65%–68%, the mass concentration of the sulfuric acid is 95%–98%, and the mass concentration of the perchloric acid is 70%–72%.

4. A method for selectively digesting the diaphragm base membrane and retaining the acid-resistant coating according to claim 1 or 2, characterized in that: The volume ratio of nitric acid, sulfuric acid and perchloric acid is (1.5-2.5):1:(1.5-2.5), and the volume ratio of nitric acid to perchloric acid is not less than 70%.

5. A method for selectively digesting the diaphragm base membrane and retaining the acid-resistant coating according to claim 1 or 2, characterized in that: The ratio of the amount of diaphragm used to the total amount of nitric acid, sulfuric acid and perchloric acid used is 0.004 to 0.02 g / mL.

6. A method for selectively digesting the diaphragm base membrane and retaining the acid-resistant coating according to claim 1 or 2, characterized in that: The heating rate is 1–5 °C / min.

7. The method for selectively digesting the diaphragm base membrane and retaining the acid-resistant coating according to claim 6, characterized in that: The gradient heating is as follows: first, the temperature is increased to 40–80 °C, then to 120–180 °C, and finally to 280–300 °C and held until the solution becomes clear.

8. A method for determining the acid-resistant coating of a battery separator, characterized in that: The method includes the selective digestion of the diaphragm base membrane and retention of the acid-resistant coating as described in any one of claims 1 to 7.

9. The method for determining the acid-resistant coating of a battery separator according to claim 8, characterized in that: Includes the following steps: S1. Weigh the diaphragm; the weight is M1. S2. Digest the diaphragm base membrane according to the method of selectively digesting the diaphragm base membrane while retaining the acid-resistant coating; after digestion, separate the solid and liquid, wash the obtained solid to neutral and dry it to constant weight, and weigh it as M2; S3. The coating content is (M2 / M1)×100%.

10. The method for determining the acid-resistant coating of a battery separator according to claim 9, characterized in that: In step S2, solid-liquid separation is achieved by filtration through a filter membrane with a pore size of 0.1 to 0.5 μm, and the filtration pressure difference is -0.02 to -0.05 MPa.

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