Chelating membrane and purifier for removing metal ions

Abstract

The present application relates to the technical field of filter media, in particular to a chelating membrane for removing metal ions and a purifier. The chelating membrane comprises a porous body with continuous non-directional tortuous passages in the porous body, and the porous body has metal chelating groups. On the cross section of the porous body, the average pore area in the region corresponding to the first macropore zone and the second macropore zone is greater than the average pore area in the region corresponding to the micropore zone. The IPA bubble point of the chelating membrane is 60 kPa-320 kPa. The first macropore zone, the second macropore zone and the intermediate micropore zone in the cross section of the porous body form a large number of microchannel structures similar to Venturi tubes. The specific channel morphology and non-directional tortuous passages form a synergy, realizing a cascade mass transfer process of high-speed impact, residence reaction and multi-stage interception, promoting the collision between metal ions and chelating groups, and still being able to purify metal ion impurities to less than 10 ppt under the low Reynolds number condition of the submicron filter membrane.

Description

Technical Field

[0001] This invention relates to the field of filter media technology, and in particular to a chelating membrane and purifier for removing metal ions. Background Technology

[0002] The TMAH (Transient Metal Acrylonitrile) development process is one of the key photolithography processes. The developer is used in the final step of photolithography patterning to dissolve soluble areas of the photoresist caused by exposure, thus revealing the desired pattern on the silicon wafer. During TMAH production, the use of stainless steel pipes and equipment inevitably leads to the presence of various metal ions (such as Fe, Cr, Ni, Co, etc., which dissolve from the stainless steel) in the TMAH solution. The subsequent steps in photolithography are primarily etching processes, where the photoresist pattern serves as a mask. If metal impurities are present in the photoresist pattern, these impurities will impact the substrate film during etching. These metal contaminants can cause various semiconductor device malfunctions. The presence of metal impurities can also cause changes in the etching rate of the photoresist near the impurity. When a CVD film is deposited on this tiny defect in the substrate, the defect can grow abnormally; this type of defect is known in the industry as a conede defect. Conede defects are one of the main problems negatively impacting device yield.

[0003] It's important to note that while micro-contaminants in the developer solution aren't limited to metallic impurities and can also contribute to lithographic pattern defects, if these contaminants exist as insoluble particles in the developer solution, they can be easily detected by pattern defect inspection tools after the lithography process. A rework process can then eliminate defects caused by these insoluble contaminants. However, if these micro-contaminants exist in a soluble form in the developer solution, such as metal ions, they are difficult to detect using defect inspection tools, making defects caused by metal ions difficult to control. Therefore, reducing metallic contamination from TMAH developer solution is crucial. In fact, with the application of thicker coatings in products such as 3D-NAND, residual metallic impurities in chemicals are more likely to penetrate into the base material during etching and ashing, altering the material's electrical properties and causing a decrease in yield. Therefore, minimizing metallic impurities in lithography chemicals before the wafer manufacturing process begins can effectively reduce the generation of defects during the process. Because of the significant impact of these soluble metal impurities on the performance of semiconductor chips, the SEMI G5 standard of the International Semiconductor Equipment and Materials International (SEMI) currently requires that the metal ion content in microelectronic chemicals be less than 10 ppt (ppt is one part per trillion).

[0004] However, while some commercially available products can reduce metal ion contamination in TMAH solutions and photolithography chemicals to the ppb level, the purification requirement of below 10 ppt exponentially increases the difficulty. Metal ions typically exist as cations in solution, therefore, cation exchange media are commonly used in the industry to remove metal ion contamination. For example, some commercially available cation exchange resins and cation exchange depth filters, while effective for removing metal ions from general solutions and solvents, are not necessarily suitable for more complex situations such as TMAH developers, and often fail to meet purification requirements below 10 ppt. This is because metal ions in TMAH solutions do not always exist as cations. Due to the extremely strong alkalinity of TMAH (pH approximately 13), Fe... 3+ For example, its initial form in TMAH solution is a cation, but some may be converted into... Anions ultimately exist in complex ionic forms capable of interconversion, and some may even transform into anion-containing micelles. This makes cation-type ion exchange media less effective in treating TMAH solutions contaminated with metal ions. Furthermore, the high concentration of TMA in the TMAH solution further complicates the process. + Cation, high concentration of TMA + There is competition between cations and metal cations, and high concentrations of TMA + A charged layer may form on the surface of the ion exchange material, creating an electrostatic shielding effect, which will also affect the purification effect of cation-type ion exchange media. Anion-type ion exchange media have the same problem. Therefore, neither cation-type nor anion-type ion exchange media can effectively remove metal ions from TMAH solutions. This means that although commercially available products can remove metal contaminants from chemicals such as resins, photoresists, and solvents to single-digit ppb levels, this level is clearly insufficient to meet the high purity requirements of <10ppt for SEMI G4 or even SEMI G5 standards.

[0005] Compared to ion exchange mechanisms, chelation mechanisms are less affected by shielding effects and the form of ions, making them a better choice for purifying high-purity metal ions. For example, CN114904403A mentions that iminodiacetic acid (IDA) chelating resin can be mixed with polyisobutylene and polyhexafluoroethylene emulsions and vacuum degassed to form a coating solution. This coating solution is then coated onto a hydrophilic base membrane to create a chelating membrane, which is used for purifying metal ions from wet electronic chemicals. However, the embodiments of this method only demonstrate its application in purifying metal ion contamination in solvents such as methanol, rather than in the complex TMAH system. Furthermore, even after purifying methanol (initially 1-2 ppb), several hundred ppt (1 ppb = 1000 ppt) of metal ions remain, failing to meet the SEMI G5 standard's requirement of <10 ppt purity.

[0006] Furthermore, purifiers using granular-filled ion exchange resins or chelating resins are also ineffective at removing metal particles from TMAH solutions. Filters with large-pore flow channels, such as those using granular-filled ion exchange resins or chelating resins, are less demanding in terms of mass transfer and diffusion requirements when used for purifying high-concentration stock solutions. Even with some flow deviation (when multiple channels exist, fluid tends to flow towards adjacent channels with less resistance and less blockage, leading to flow deviation; the local flow path at the deviation point is fast, has low resistance, and poor purification effect), although the ion concentration of the fluid emanating from the deviation path is slightly higher, the overall ion concentration in the product solution remains very low after dilution, meeting general requirements (e.g., tens or single ppb). However, for systems with a purification target of <10 ppt purity, local filtration path defects and flow deviations can easily cause the metal ion concentration in the entire product solution to exceed 10 ppt. Moreover, the pore size of the flow channels formed by granular-filled ion exchange resins or chelating resins is inherently large, which can easily lead to weak mass transfer and diffusion capabilities along the filtration path.

[0007] Currently, there are many ppb-level metal purifiers on the market that can purify raw solutions containing tens to hundreds of ppb of metal ions to a level of a few ppb to hundreds of ppt. While these purifiers seem to have high purification intensity, on the one hand, this purification level does not meet the requirements of G4 or even G5. On the other hand, as the concentration of metal ions in the raw solution decreases to a level of a few to hundreds of ppt, further purification to below 10 ppt becomes exponentially more difficult, and the performance requirements of the purification medium also increase exponentially. For example, a certain purification system can purify metal impurities of 600+ ppb to less than 5 ppb with a purification rate of over 99% (close to 100%). However, if the goal is to purify 5 ppb to less than 10 ppt, the purification difficulty increases exponentially. Even if the purification system runs repeatedly, it is difficult to further reduce the metal impurities (and may even wash away captured metal ions). Especially in specific application scenarios such as TMAH solutions, conventional purification media are insufficient.

[0008] In summary, for the high purity requirement of ≤10ppt metal ion contamination based on the SEMI G5 standard, almost all existing purification media are unsuitable. Summary of the Invention

[0009] (a) Technical problems to be solved

[0010] In view of the aforementioned shortcomings and deficiencies of existing technologies, this invention provides a chelating membrane for removing metal ions. This chelating membrane is based on a conventional chelating membrane, incorporating a special filter pore structure design. This design not only prevents fluid deviation within the membrane but also enhances the mass transfer and diffusion efficiency between the chelating groups grafted onto the membrane and the metal ions. With these structural advantages, this purification membrane can purify metal ions in the original solution from concentrations of several hundred ppt to several ppb to below 10 ppt, thereby better meeting the stringent requirements of the semiconductor industry for controlling metal impurity contamination. Furthermore, this invention also relates to a purifier incorporating this chelating membrane.

[0011] (II) Technical Solution

[0012] This invention provides a chelating membrane for removing metal ions, comprising a porous body with continuous, non-directional tortuous pathways within it, and the porous body having metal chelating groups. The porous body sequentially comprises a first macropore region, a micropore region, and a second macropore region along its thickness direction. The first macropore region is located near the liquid inlet side of the porous body, the second macropore region is located near the liquid outlet side of the porous body, and the micropore region is located between the first and second macropore regions. In the cross-section of the porous body, the average pore area in the corresponding regions of the first and second macropore regions is greater than the average pore area in the corresponding region of the micropore region. The IPA bubble point of the chelating membrane is 60 kPa-320 kPa.

[0013] The chelating membrane of this application has an IPA bubble point of 60-320 kPa, which roughly corresponds to a filter membrane with a nominal pore size of 0.1-0.7 μm. That is, the nominal pore size of the chelating membrane in this application is at the submicron level. This pore size characteristic results in a smaller size of the internal pore channel structure of the chelating membrane. The smaller pore channel size can limit the distance between the pore wall and the center of the fluid inside the pore, thereby reducing the difficulty of mass transfer and diffusion of the fluid.

[0014] Of course, the IPA bubble point of the chelating membrane should not be too high (corresponding to a pore size that is not too small). If the IPA bubble point is too high and the pore size is too small, it will lead to a decrease in processing efficiency. On the other hand, the small pore size of the submicron-level porous membrane itself means that the fluid flow rate inside the membrane is slower, which will also result in a lower Reynolds number (Re=ρvd / μ, where Re is the Reynolds number, ρ is the fluid density, v is the flow rate, d is the characteristic length, and μ is the viscosity coefficient). As the pore size decreases, both v and d decrease, which will lead to a sharp drop in the Reynolds number. Under low Reynolds number conditions, the fluid is usually in a stable laminar flow state. At this time, the fluid velocity in the orifice channel is parabolic, with a higher velocity in the central region and a lower velocity near the orifice wall. This will further bring two problems: First, a mass transfer boundary layer with a low velocity will form near the orifice wall of the orifice channel structure. This boundary layer is relatively stable under laminar flow conditions. This means that if the substance in the fluid in the orifice wants to contact the chelation sites on the orifice wall, it must first pass through the mass transfer boundary layer through diffusion, which increases the difficulty of the substance contacting the chelation sites. Second, the high fluid velocity in the central region of the orifice will cause the solute (such as metal ions) in this region to easily "escape through the channel" - that is, the solute flows quickly through the center of the orifice with the high-speed fluid and is difficult to fully contact the chelation sites on the orifice wall, thus affecting the capture effect of metal ions.

[0015] The porous body of the chelating membrane of this application has non-directional tortuous pathways, which specifically refer to randomly oriented groove structures and / or discretely distributed pore structures, and the non-directional tortuous pathways are interconnected. This non-directional tortuous pathway belongs to a three-dimensional mass transfer network with high structural complexity, strong flow disturbance, multi-scale coupling and spatial random distribution characteristics. It can maximize the interaction opportunities between fluid and functional groups. Specifically, it is manifested as follows: (1) When the fluid passes through the chelating membrane, it will experience a highly disordered and unpredictable flow path, exhibiting a high degree of topological disorder in three-dimensional space: the pore direction has no obvious directional preference on any cross section, forming a maze-like connected structure. This characteristic significantly prolongs the residence time of the fluid and enhances the mass transfer efficiency; (2) The structural features of the pores, such as bending, branching and merging, are arranged non-periodically in space and lack long-range order, which further enhances the unpredictability of the flow path and strengthens the diffusion-dominated transport mechanism; (3) Non-directional flow can force the solution to penetrate into a wider area inside the membrane, effectively avoiding phenomena such as "short-circuit flow", "channel flow" and "deviation flow", thereby improving the overall utilization efficiency of the chelating sites.

[0016] The porous matrix of the chelating membrane in this application sequentially constructs a first macropore region, a micropore region, and a second macropore region along the fluid flow direction in the thickness direction, forming a pore structure with a significant pore size gradient. From a microscopic scale, this structure resembles a series of irregularly distributed micro Venturi-like channels: when the fluid passes through the pore size contraction region (i.e., the micropore region), it accelerates; upon entering the expansion region (i.e., the second macropore region), it gradually decelerates and achieves pressure recovery. This process triggers drastic changes in the local flow state. For submicron-level pores with low Reynolds numbers, this periodic contraction-expansion structure can significantly alter the fluid flow behavior. Specifically, in this structure, through the local acceleration and shear enhancement induced by the Venturi effect, combined with flow separation, secondary flow, and the formation of microvortices, the local Reynolds number is significantly increased. This not only suppresses the stability of typical laminar flow to a certain extent but also promotes the transformation of the fluid into a transitional flow or even a microturbulent state. This unsteady flow causes the fluid, which was originally concentrated in the center of the channel, to migrate continuously towards the orifice wall region, effectively enhancing radial mass transfer efficiency and reducing boundary layer formation. At the same time, the frequent changes in flow direction and velocity prolong the residence time of the fluid in the functional region, increasing the collision probability and binding opportunity of metal ions with immobilized chelating groups, and ultimately significantly reducing the phenomenon of uncaptured "short-circuit flow".

[0017] Specifically, when the fluid flows in the first large-pore region, the flow velocity is relatively slow. This allows sufficient time for metal ions in the fluid to contact and be captured by the chelating groups on the pore wall, thereby reducing the metal ion concentration in the vicinity of the pore wall and thus reducing the metal ion load in the subsequent small-pore region. As the fluid flows further from the first large-pore region to the small-pore region, the flow velocity increases due to the smaller pore size, and the Reynolds number also increases accordingly, causing the fluid to gradually de-laminarize. Simultaneously, the shear force of the fluid on the pore wall of the flow channel structure increases synchronously, creating a scouring effect on the pore wall. This process significantly reduces the thickness of the mass transfer boundary layer. Since the mass transfer coefficient is inversely proportional to the boundary layer thickness, the thinning of the boundary layer directly promotes a significant increase in the mass transfer rate of metal ions from the fluid center to the pore wall. Furthermore, in the environment of the smaller pore size in the small-pore region, the distance between the fluid center and the pore wall is already shortened, and the diffusion distance is correspondingly reduced. Under the synergistic effect of multiple factors such as "lower diffusion resistance, shorter diffusion distance, and more unstable flow state," the difficulty of metal ions diffusing to the pore wall is significantly reduced. They can be "pushed" to the vicinity of the chelating groups on the pore wall more quickly and efficiently, and the probability of the metal ions in the fluid center being captured by the chelating groups in the small pore region is also greatly increased. When the fluid continues to flow from the small pore region to the second large pore region, the fluid that maintains a high flow velocity in the small pore region experiences a sharp decrease in velocity after entering the second large pore region due to the sudden increase in the cross-sectional area of ​​the flow channel. This easily leads to flow separation—that is, the fluid tends to detach from the pore wall after entering the second large pore region, thus forming a backflow zone and eddies between the main flow and the pore wall. Although this phenomenon leads to a large pressure drop, it also avoids the formation of a boundary layer near the pore wall. Overall, the velocity distribution in the pore channel no longer exhibits a clear parabolic shape, but becomes flatter, with a sharp increase in velocity gradient only appearing in a very small area near the pore wall. This means that after the fluid flows from the small pore region into the second large pore region, the flow state tends to be chaotic and enters a turbulent state. Under the combined influence of factors such as "lower flow rate" and "more chaotic flow state", the small amount of metal ions remaining after initial capture in the first large pore region and concentrated capture in the small pore region will undergo violent movement and frequently collide with the pore wall of the second large pore region. This process greatly reduces the possibility of metal ions escaping.

[0018] In summary, the synergistic effect of the non-directional tortuous pathway and the Venturi tube morphology of the flow channel, combined with precise control of the IPA bubble point (60kPa-320kPa) of the chelating membrane, significantly increases the interaction opportunities between metal ions and chelating groups, thereby significantly improving the removal efficiency of metal ions. Thanks to this advantage, even when processing complex systems such as TMAH (tetramethylammonium hydroxide), the concentration of metal ions in the final fluid can be purified to below 10ppt. Of course, the chelating membrane of this application is not only applicable to TMAH systems; it can also be used for the purification of metal ions in general photoresists and other solvents, demonstrating a wide range of applications.

[0019] It is understood that the IPA bubble point test of the chelating membrane in this application can be performed in accordance with the relevant provisions of GB / T32361-2015 "Separation Membrane Pore Size Test Method: Bubble Point and Average Flow Rate Method". It should be specifically noted that the liquid used in the test is isopropanol (IPA), and the "IPA bubble point" here refers to the point at which IPA begins to bubble, not the point at which it completely bubbles. Furthermore, when it is only necessary to qualitatively determine the average pore area comparison of each region of the chelating membrane (first macropore region, micropore region, second macropore region), direct observation of the cross-sectional SEM image of the chelating membrane (any clearly observable magnified microscopic image is acceptable; an SEM image is not mandatory, but is used as an example, the same applies below) is sufficient. From the cross-sectional SEM image, the boundaries between the first macropore region, the micropore region, and the second macropore region can be clearly distinguished; however, it should be noted that since the pore size changes in each region are continuous, the boundaries between each region may fluctuate within a small thickness range and are not absolutely regular straight lines or planes.

[0020] When it is necessary to further quantitatively determine the average pore area of ​​the first macropore region, the micropore region, and the second macropore region, the cross-sectional SEM images of the corresponding regions of the chelating membrane can be imported into Image-J measurement software. The average pore area of ​​each region can then be directly measured using this software. The specific steps are as follows:

[0021] ① Open the image and adjust the pixels; Open the image in the image processing software ImageJ, and select Image>Type>8-bit to convert the electron microscope image into an 8-bit grayscale image;

[0022] ② Set the scale; Use the line tool from the toolbar to draw a straight line within the scale area, making the drawn line the same length as the scale on the electron microscope image. Select Analyze > Set Scale, enter the length of the scale in "KnownDistance" (for example, enter 10), enter the unit in "Unitoflength" (for example, enter um), and select OK.

[0023] ③ Image binarization; Select Image > Adjust > Threshold... to divide the electron microscope image into thresholds to obtain a binarized image;

[0024] ④ Set measurement parameters; In Analyze>SetMeasurements, ensure that “Area” is checked and select OK; When you need to measure parameters such as Feret’s diameter, you can also check “Feret’s diameter”;

[0025] ⑤ Data Measurement; In Analyze>AnalayzeParticles, set “Size(pixel^2 / μm^2)” to 0.01-Infinity, set “Show” to “Outlines”, and confirm that the boxes for “Display results”, “Exclude on edges”, and “Include holes” are checked. Select OK, then select Results>Summarize. In the Results screen, you will obtain the average area of ​​the holes, the Feret diameter (in the final results, “Feret” and “MinFeret” are the maximum and minimum Feret diameters, respectively).

[0026] When it is necessary to calculate the area equivalent diameter based on the measured area, the average area equivalent diameter D can be further calculated from the average area S of the hole measured in the above steps. The calculation formula is as follows:

[0027]

[0028] Of course, other feasible testing methods and testing software are also acceptable.

[0029] According to a preferred embodiment of the present invention, the thickness of the porous body is 50-200 μm; the thickness of the pore region accounts for 10-50% of the total thickness of the porous body, preferably 25-50%, and more preferably 30-45%.

[0030] Optionally, the thickness ratio of the first large hole region to the second large hole region is 0.8-2.

[0031] From the perspective of metal ion capture characteristics, the flow state in the first macropore region tends towards laminar flow, and the concentration difference of metal ions between the fluid interior and the pore wall is the core driving force for diffusion and mass transfer. Although the fluid in the first macropore region possesses a stronger driving force for diffusion and mass transfer due to the higher initial metal ion concentration, it is limited by two factors: firstly, the stable and relatively thick boundary layer formed near the pore wall hinders the migration of metal ions to the pore wall; secondly, the large pore size results in an excessively long mass transfer distance, further reducing mass transfer efficiency. However, the first macropore region has a strong ability to capture metal ions in the region near the pore wall, which can initially reduce the metal ion concentration in the fluid, thereby alleviating the metal ion load pressure in subsequent regions.

[0032] Compared to the first macropore region, the micropore region is characterized by its smaller pore size and greater number of pores, thus often possessing a higher specific surface area and consequently loading more chelating groups. Given a porous bulk thickness of 50-200 μm, the fluid flow velocity within the micropore region is relatively high. This dictates that the thickness percentage of the micropore region should not be less than 10%—even if the first macropore region has already reduced the loading of the micropore region by capturing metal ions, a sufficient thickness percentage is still needed to prevent the fluid from rapidly flowing through the micropore region and to prevent excessive metal ion escape. Although a second macropore region follows the micropore region, it is important to note that the specific surface area of ​​the second macropore region is lower than that of the micropore region. If the metal ion loading in the fluid entering the second macropore region is too high, it is difficult to avoid the escape of a small amount of metal ions. Specifically, if the purification target is single-digit ppb or tens to hundreds of ppt, the escape of a small amount of metal ions is acceptable; however, for high-purity purification targets of single-digit ppt, even the escape of extremely small amounts of metal ions must be strictly avoided.

[0033] Of course, the thickness of the pore area should not exceed 50%, mainly for the following reasons: Firstly, if the pore area proportion is too high, the filtration efficiency will inevitably decrease—the pore size is small and the flow resistance is relatively large. An excessively high proportion will prolong the flow path of the fluid within the membrane, increasing the overall resistance of the filtration process and thus affecting the processing rate. Secondly, increasing the thickness of the pore area does not necessarily lead to a further improvement in the purification effect of metal ions; on the contrary, it will exhibit a diminishing marginal effect: when the pore area proportion reaches a certain threshold, even if the thickness is further increased, the ability to capture metal ions will only slightly improve (as the thickness of the pore area increases, the flow state of the fluid within the pore area tends to stabilize), and the escape of a very small number of metal ions is still difficult to completely avoid. Based on this, sacrificing a significant decrease in filtration efficiency for a slight improvement in the purification effect of metal ions is clearly not in line with the cost-effectiveness requirements of practical applications and is not the preferred solution.

[0034] It is important to note that the pore size of the chelating membrane gradually decreases as it transitions from the first large pore region to the small pore region. This inevitably increases fluid resistance—the resistance barrier that the fluid must overcome as it flows through the porous channel structure rises accordingly. Setting up a first large pore region with a lower barrier before the small pore region allows the barrier to gradually increase, thus creating a more efficient fluid flow drag force transmission network and avoiding a precipitous rise in the barrier due to abrupt changes in pore size. If the barrier abruptly changes, a large amount of fluid flow drag force cannot be smoothly transmitted and can only act on the chelating membrane itself. This not only causes energy loss but also generates a greater pressure drop, ultimately making the chelating membrane (especially the small pore region) more susceptible to damage and defects, leading to a decrease in purification efficiency.

[0035] Based on this, the core functions and thickness design considerations for each pore zone are as follows: First Large Pore Zone: Besides initially reducing the metal ion load, its core function is to reduce fluid resistance and ensure high water flux by forming a continuous drag force transmission network, thus storing energy for subsequent fluid entry into the small pore zone (contraction section). It needs a certain thickness: too thin a pore will cause rapid changes in pore size, disrupting the continuity of the drag force transmission network, leading to energy dissipation and excessive impact on the small pore zone; too thick a pore will waste material and reduce the thickness ratio of the small pore zone, weakening the metal ion purification effect. Small Pore Zone: Based on Bernoulli's principle, a sudden decrease in the pore cross-sectional area will cause a sudden increase in fluid velocity and a sharp drop in pressure, causing the fluid to tend to expand towards the pore wall. This process effectively disrupts the laminar boundary layer. Its thickness ratio needs to be controlled between 10% and 50%: not less than 10% ensures the fluid has a sufficiently long path through the thin boundary layer region to fully contact the chelating groups; not more than 50% avoids excessive increase in filtration resistance, which would cause the purification effect to be offset by a decrease in efficiency. The second large pore region's core function is to restore the large pore size, reduce fluid velocity, increase pressure, and simultaneously promote turbulent flow. A certain thickness percentage maintains turbulent inertia, allowing the fluid center and edges to mix thoroughly and collide extensively with the pore walls, preventing metal ions from escaping due to excessively high velocity at the center.

[0036] It is evident that the thickness ratios of the first and second macropore regions must be controlled within a reasonable range (neither too high nor too low). Combined with the micropore regions of a specific thickness ratio, these regions not only achieve excellent metal ion purification but also ensure high purification efficiency (high water flux) while reducing the risk of chelate membrane damage. In summary, given a total porous body thickness of 50-200 μm, controlling the micropore region thickness ratio to 10%-50%, and further setting the thickness ratio of the first to second macropore regions to 0.8-2, achieves the following effects: First, it ensures balanced mechanical strength across regions, preventing micropore regions from cracking due to excessive thinness or insufficient ion purification capacity, while also preventing excessively thick micropore regions from leading to decreased overall membrane flexibility, reduced flux, and excessive flow resistance. Second, it avoids insufficient pressure changes and weak turbulence caused by excessively thick macropore regions, thereby reducing the risk of metal ion escape from the fluid center.

[0037] Understandably, data such as the thickness of each region of the chelating membrane can be obtained by importing the cross-sectional SEM image of the chelating membrane into measurement software such as Nano Measurer.

[0038] According to a preferred embodiment of the present invention, the effective pore size and pore diameter ratio of each pore region on the cross-section of the porous body are defined as follows:

[0039] The first effective hole is defined as the hole with an area ≥ 0.01 μm in the first large hole region cross-section; the second effective hole is defined as the hole with an area ≥ 0.01 μm in the small hole region cross-section; and the third effective hole is defined as the hole with an area ≥ 0.01 μm in the second large hole region cross-section.

[0040] The average area equivalent diameter of the second effective hole is 0.25-0.85 times that of the average area equivalent diameter of the first effective hole; and / or, the average area equivalent diameter of the second effective hole is 0.35-0.75 times that of the average area equivalent diameter of the third effective hole.

[0041] The definition of effective pores is based on the following: The nominal pore size corresponding to the IPA bubble point of the chelating membrane in this application is at least approximately 0.1 μm. However, due to the presence of non-directional tortuous pathways within the chelating membrane, the microscopically observable pore structures continuously accumulate and overlap in the thickness direction, resulting in a nominal pore size that is often lower than the actual pore size of the microscopically observable pore structures. For example, after a large number of microscopically measurable pore structures with a diameter of 0.1 μm are accumulated in the thickness direction, their nominal pore size is usually lower than 0.1 μm. Furthermore, a microscopically observable pore structure with an area of ​​0.01 μm has an equivalent diameter of approximately 0.11 μm; at the same time, smaller pore structures will generate greater flow resistance within the membrane and are not the preferred path for fluid flow. Based on this, this application defines microscopically observable pore structures with an area of ​​not less than 0.01 μm as effective pores.

[0042] In the aforementioned porous main structure, by maintaining a relatively loose structure between the first and second large pore regions and controlling the pore diameter of the small pore regions to meet the requirements of "0.25-0.85 times the pore diameter of the first large pore region" and / or "0.35-0.75 times the pore diameter of the second large pore region", a lower overall flow resistance is achieved. The pore diameter design of each pore region plays a key role: the large pore diameter of the first large pore region ensures high initial flux, which not only reduces membrane fouling but, more importantly, enables fluid pre-acceleration, constructs a more coherent drag force transmission network, and provides kinetic energy reserves for the subsequent rapid increase in fluid velocity in the small pore region; the large pore diameter of the second large pore region, through the increase in pore diameter, makes it easier for the fluid flow state to change from laminar to turbulent, thereby "filling in the gaps" for the last remaining metal ions and further reducing the risk of escape.

[0043] Since the average pore diameter of the effective pores in the small pore region is only 0.25-0.85 times that of the average pore diameter in the first large pore region, this proportional design makes the individual channels in the small pore region exhibit the characteristics of "smaller size, denser and more uniform distribution": the pore area and diameter are significantly reduced compared to the first large pore region (the maximum is no more than 0.85 times that of the first large pore region), while avoiding obvious abrupt changes in pore diameter (the minimum is no less than 0.25 times that of the first large pore region), ensuring the continuity of fluid flow and the continuity of the drag force transmission network, but without making it difficult to change the laminar flow state due to insufficient pore diameter variation, so as to obtain a good metal ion purification effect.

[0044] When fluid enters the orifice region (i.e., the contraction section), according to the continuity equation in fluid mechanics, the reduction in the flow cross-sectional area leads to a significant increase in flow velocity. This acceleration effect is a classic Venturi tube behavior, which has multiple effects at the microscale: forming a local high-velocity region, constructing a strong shear force field, and achieving streamline compression and kinetic energy concentration. These effects collectively enhance the momentum transfer of metal ions carried by the fluid to the orifice wall, increase the collision frequency between metal ions and chelating groups, and thus increase the probability of metal ions being captured. At the same time, this structural design can actively "perturb" the laminar flow state, significantly reduce the boundary layer thickness, effectively break the diffusion limitation, and "push" the central fluid and its carried metal ions, which were originally difficult to contact the orifice wall, to the chelating surface of the orifice wall, significantly improving mass transfer efficiency and ensuring that metal ions are stably chelated and fixed.

[0045] The pore structure of the second macropore region is similar to that of the first macropore region. When fluid enters the second macropore region from the small pore region, a series of flow phenomena occur due to the sudden expansion of the flow channel cross-sectional area: flow separation, backflow vortices, and delayed pressure recovery. These phenomena prolong the hydraulic residence time of the fluid within the membrane, and some fluid forms a "circulation path" in the local backflow vortex, repeatedly flowing through the chelating region. Given a fixed total thickness of the porous body, this process not only prolongs the effective mass transfer path but also creates opportunities to disrupt laminar flow again, providing more possibilities for metal ions to contact chelation sites, thereby reducing "short-circuit flow" and the escape of uncaptured metal ions. Similarly, the second macropore region should not have insufficient pore size variation, thus failing to alter the laminar flow state and affecting the metal ion purification effect, or excessive abrupt changes in pore size, causing excessive pressure drop and reducing filtration efficiency and metal ion purification effect.

[0046] It is understandable that the so-called effective pores with an area ≥ 0.01 μm means that when measuring data in software such as Image-J, in step ⑤, the lower limit of "Size (pixel^2 / μm^2)" is adjusted from 0 to 0.01, that is, only pore structures with an area greater than or equal to 0.01 μm are measured and counted.

[0047] According to a preferred embodiment of the present invention, the area ratio of the effective holes in each hole region must meet the following conditions: the difference between the area ratio of the first effective hole and the area ratio of the second effective hole is not greater than 10%; and / or, the difference between the area ratio of the third effective hole and the area ratio of the second effective hole is not greater than 10%.

[0048] The measurement of pore area ratio is based on the following: Since the three-dimensional porosity of each region is difficult to measure accurately, this application adopts a method of measuring the two-dimensional pore area ratio (i.e., pore area percentage) of each region in the cross-sectional microscopic image of the chelate membrane. Although the two-dimensional pore area ratio cannot completely restore the three-dimensional porosity of each region, the two are usually positively correlated. Therefore, the pore characteristics of each region can be indirectly reflected by the two-dimensional pore area ratio.

[0049] Observations from cross-sectional micrographs show that the pore area ratio of the small pore region is generally lower than that of the large pore region due to its smaller pore size and individual pore area. However, this application, through structural design, controls the difference between the pore area ratio of the small pore region and that of the first and second large pore regions to within 10%—meaning that the small pore region also possesses a relatively high pore area ratio. This design offers two core advantages: firstly, it effectively ensures the overall high permeation flux of the chelating membrane, avoiding a significant decrease in flux due to the small pore size of the small pore region; secondly, it increases the wall exposure rate and the exposure density of chelating sites, providing sufficient action sites for metal ion capture, thereby achieving a good balance between high flux and high capture rate.

[0050] Further analysis of the flow characteristics and capture effects of each pore region reveals that the fluid velocity is relatively slow in the first and second large pore regions, allowing sufficient time for metal ions in the fluid to undergo mass transfer with the chelating groups on the inner wall of the pores, thus achieving initial capture and filling gaps. In the middle small pore region, the fluid velocity is fast, the shear stress is significantly increased (the velocity gradient is larger, and the fluid near the wall is violently stretched), and the density of chelating groups is higher, enabling efficient capture of metal ions carried by the central fluid, forming a "slow-fast-slow" stepped capture mode.

[0051] Understandably, the percentage of the pore area in each region can be calculated based on the total pore area "area" data of each region measured above. Specifically, it is the ratio of the total pore area ΔS of the region to the total area S of the region, and ΔS / S*100% is the percentage of the pore area in that region. Of course, the "%Area" data can also be directly output through the Image-J software, which is the percentage of the pore area.

[0052] Optionally, the pore area of ​​the second effective pore accounts for 20-55%, and the average area equivalent diameter of the second effective pore is 0.2-1 μm. Even if the pore area of ​​the micropore region is low and the pore size is small, its pore area is not less than 20% and the pore size is not less than 0.2 μm. This design ensures that the micropore region has sufficient microchannels, thereby maintaining a high permeation flux. At the same time, the pore area is not higher than 55% and the pore size is not greater than 1 μm, which ensures that the micropore region has enough solid structures with chelating groups (i.e., pore wall structures) to achieve concentrated capture of metal ions, thereby reducing the metal ion load in the second macropore region and ensuring the final purification effect.

[0053] According to a preferred embodiment of the present invention, the maximum Fe-max diameter of the first effective aperture is 0.5-2.5 μm, and the ratio of the maximum Fe-max diameter to the minimum Fe-max diameter is 1.2-2.2; and / or, the maximum Fe-max diameter of the third effective aperture is 0.5-2.5 μm, and the ratio of the maximum Fe-max diameter to the minimum Fe-max diameter is 1.2-2.2.

[0054] As can be seen from the above parameters, in the thickness direction of the chelating membrane, the channels in both the first and second macropore regions are elongated (length-to-width ratio not less than 1.2), rather than nearly circular sponge-like pores. Since a spherical structure has the smallest surface area for the same volume, these elongated channels, deviating from a spherical shape, can significantly increase the specific surface area, improve the exposure of chelating groups, and thus enhance the membrane's static adsorption capacity and dynamic capture efficiency. Furthermore, the elongated channels also have the following advantages:

[0055] First, it generates stronger shear force, which helps to thin the mass transfer boundary layer and strips the solvation layer around the adsorbed metal ions, promoting the coordination reaction between metal ions and chelating groups and reducing the probability of ions being washed away. Second, it works synergistically with the micro Venturi tubes formed by the gradient pore size to further enhance convective mass transfer and increase the contact opportunities between metal ions and pore walls by utilizing the fluid self-excitation effect induced by the pore structure.

[0056] Based on the aforementioned pore size ratios (the ratios of the second effective pore to the first and third effective pores are 0.25-0.85 and 0.35-0.75, respectively), it can be seen that the XFe-max and XFe-min of the second effective pore only decrease moderately. This indicates that the narrow channels in the small pore region do not shrink excessively, achieving a balance between flux and capture efficiency, thus avoiding problems such as excessive pressure drop, easy clogging, and insufficient flux. This structure increases flow velocity and shear force without significantly sacrificing permeability flux, making it a cost-effective mass transfer enhancement scheme. It is understood that XFe-max and XFe-min are average values ​​and can be directly output using Image-J software.

[0057] According to a preferred embodiment of the present invention, at least one side surface of the porous body is further provided with a skin layer, the total thickness of which does not exceed 10% of the thickness of the porous body, and the pore area ratio of the skin layer is lower than that of the porous body. The function of the skin layer needs to be distinguished according to its position: When the skin layer is located at the inlet surface: the pore diameter of the skin layer is smaller than that of the first large pore area, which slightly increases the flow resistance, but when the fluid enters the large pore of the first large pore area from the small pores of the skin layer, the flow velocity changes from fast to slow, which can introduce a small amount of turbulence near the inlet surface, creating appropriate disturbance to the flow in the first large pore area; combined with the high metal ion concentration gradient in the first large pore area, it can improve the metal ion purification effect. When the skin layer is located at the outlet surface (between the second large pore area and the outlet surface): the pore diameter of the skin layer is smaller than that of the second large pore area, and the fluid gradually accelerates when flowing through this area, shortening the distance between the residual metal ions and the pore wall of the skin layer; at the same time, the turbulent state of the second large pore area can extend to the vicinity of the skin layer, keeping the fluid turbulent, thereby further capturing the very small amount of residual metal ions. Of course, this design will lead to a certain degree of decrease in membrane flux. It is understandable that the cortex and the first or second macropore region are an integral structure and are also part of the porous body.

[0058] For scenarios with higher purification requirements, a skin layer can be placed on both the inlet and outlet liquid surfaces. This approach achieves higher purification results at the cost of a slight decrease in flux—considering that the purification difficulty increases exponentially with lower metal ion concentrations, this method is practically feasible. Furthermore, controlling the total thickness of the skin layer to no more than 10% (even with skin layers on both sides, the total thickness should still be ≤10%) can prevent excessive flux reduction.

[0059] According to a preferred embodiment of the present invention, the porous substrate is made of a film-forming polymer, specifically selected from at least one of polyethylene (PE), polypropylene (PP), poly4-methyl-1-pentene (PMP), polysulfone (PSF), polyethersulfone (PES), regenerated cellulose (RC), nitrocellulose (CN), cellulose acetate (CA), polyvinylidene fluoride (PVDF), and polytetrafluoroethylene (PTFE). Alkali-resistant film-forming polymers are preferred, especially suitable for treating metal ion impurities in strongly alkaline solutions such as TMAH (tetramethylammonium hydroxide).

[0060] According to a preferred embodiment of the present invention, the chelating group is derived from at least one of polyol chelating agents, polycarboxyl chelating agents, small molecule amine chelating agents, sulfur-containing chelating agents, and phosphonic acid derivative chelating agents. Preferred chelating agents include at least one of IDA (iminodiacetic acid), IDE (ethyl iminodiacetate / iminodiethanol), EDTA (ethylenediaminetetraacetic acid), CDTA (1,2-cyclohexanediamine-N,N,N',N'-tetraacetic acid), IDS (iminodisuccinic acid), NMDG (N-methyldiethanolamine), NTA (nitrilotriacetic acid), TED (N,N,N-tris(carboxymethyl)ethylenediamine), TEPA (tetraethylenepentamine), CM-DASA (carboxymethyl α,β-diaminesuccinic acid), and EDDA (ethylenediamine N,N-diacetic acid). In practical applications, the corresponding chelating groups and proportions can be selected or combined according to the type of metal ion to be purified. Of course, chelating membranes with different or identical chelating groups can also be used in combination as needed.

[0061] According to a preferred embodiment of the present invention, there are two routes for preparing chelating membranes: one is to use chelating resin grafted with chelating agent to form a membrane by thermal phase separation, non-solvent phase separation or melt extrusion; the other is to first use resin material to form a membrane by the above methods, and then chelate modify the membrane.

[0062] According to another embodiment of the present invention, the porous body can be disposed on the surface of a substrate material, which may be a non-woven fabric or a porous membrane, etc., and its main function is to improve the mechanical strength of the chelating membrane.

[0063] On the other hand, the present invention also relates to a purifier comprising the chelating membrane described in any of the above embodiments. The purifier utilizes the chelating membrane, and according to actual purification needs, is equipped with corresponding flow channel designs and control components, such as being fabricated as a membrane package or a pleated filter element, to achieve efficient removal of metal ions from different systems (such as TMAH, photoresist, solvents, etc.).

[0064] (III) Beneficial Effects

[0065] The technical advantages of this invention are as follows:

[0066] 1. This invention solves the problem of metal ion escape caused by low Reynolds number laminar flow in traditional chelating membranes.

[0067] In submicron to micron-sized channels, the conventional flow state is low Reynolds number laminar flow, with a parabolic velocity distribution: high velocity at the center and slow velocity at the edges. In this state, mass transfer primarily relies on molecular diffusion, which is extremely inefficient—because the binding rate of chelating groups to metal ions is much faster than the mass transfer diffusion rate, mass transfer diffusion capacity becomes a critical step determining purification efficiency. Excessively low diffusion-mass transfer rates can lead to two problems: first, metal ions in the central region of the fluid cannot diffuse to the pore walls in time, resulting in "filtration escape"; second, a static boundary layer forms near the pore walls, further hindering mass transfer between metal ions and chelating groups.

[0068] To address this pain point, the chelating membrane of this invention constructs several venturi-like microchannel structures by sequentially setting a first macropore region, a micropore region, and a second macropore region along the thickness direction within a porous body. This enables effective control of fluid kinetic energy and active enhancement of the mass transfer process at the microscopic level. Even under low Reynolds number conditions, the structure induces local high-speed flow, shear enhancement, and microscale turbulent mixing effects, significantly improving the transport efficiency of metal ions to chelating sites. This "structure-driven performance" strategy overcomes the limitations of traditional porous materials that rely on static adsorption, providing a promising technical path for the purification of metal ions from ultra-high purity microelectronic semiconductor wet chemicals.

[0069] 2. The chelating membrane of this invention achieves synergy between Venturi-like channels and non-directional tortuous pathways.

[0070] The porous matrix, with its venturi-like microchannel structure, combined with the characteristics of non-directional tortuous pathways, achieves a combination of active fluid control and passive diffusion enhancement. The specific synergistic effects are as follows:

[0071] Complementary functions: The Venturi-like channel generates a local high-speed jet in the orifice area, "pushing" the fluid to the orifice wall, achieving short-term and efficient convection mass transfer; the non-directional tortuous path forces the fluid to constantly change direction, branch, and merge, forming a maze-like migration path, which significantly extends the hydraulic residence time.

[0072] Mass transfer enhancement: In the contraction section (small hole region), the Venturi-like channel destroys the static boundary layer near the wall through high shear force, "throwing" metal ions in the central region toward the wall. However, low-speed eddy current regions or stagnation regions may exist at some corners or branch ends of the channel. At this time, the long diffusion path and low flow rate environment brought about by the non-directional tortuous structure are actually conducive to slow but thorough mass transfer dominated by molecular diffusion.

[0073] Flow control: Venturi-like channels provide directional kinetic energy input, inducing a local increase in Reynolds number and generating micro-vortices; tortuous pathways lead to frequent deflection, separation, and reattachment of streamlines, forming three-dimensional asymmetric flows (such as Dean flow and secondary vortices). Among them, Venturi-like structures are concentrated in the small-aperture region and the second large-aperture region, forming a highly active reaction zone to enhance mass transfer; non-directional tortuous structures are distributed throughout the three regions, effectively preventing "short-circuit flow".

[0074] In summary, the two mechanisms form a synergistic effect: the Venturi-like channel actively breaks the laminar boundary layer and reduces mass transfer resistance by increasing local flow velocity and enhancing shear, driving metal ions to migrate towards the functional wall; the non-directional tortuous pathway extends fluid residence time through a highly tortuous three-dimensional network, promoting diffusion mass transfer and preventing short-circuit flow. The combination of these two mechanisms achieves a cascaded mass transfer process of "high-speed impact - retention reaction - multi-stage interception," significantly improving chelation efficiency even under low Reynolds number conditions, and possessing both high throughput and high removal rate advantages.

[0075] 3. This invention, through "structure-driven mass transfer," better solves the problem of flow deviation.

[0076] In the field of filtration technology, flow deviation is a key issue. The root cause is that when fluid migrates in porous media, it tends to preferentially follow paths with lower flow resistance, leading to uneven flow distribution—some areas experience overflow, while others become dead zones or low-velocity areas. This phenomenon not only reduces the effective filtration area but also exacerbates local leakage, concentration polarization, and flux decay, severely impacting membrane separation efficiency and lifespan. Traditional membrane fabrication processes, in order to suppress flow deviation, typically pursue highly uniform pore structures, aiming to homogenize flow resistance from the source and ensure the fluid flows through the membrane in a parallel and stable manner. This approach has long dominated porous membrane design paradigms.

[0077] This application breaks away from the traditional thinking that "uniformity equals excellence," proposing a novel strategy that actively utilizes asymmetric structures to induce controllable flow disturbances: the porous main layer is designed with a first large pore region, a small pore region, and a second large pore region distributed sequentially along the thickness direction, constructing a large number of microchannel structures similar to Venturi tubes. This structure exhibits macroscopic asymmetry, with gradient differences in local flow resistance, seemingly "non-uniform," but in reality, it achieves precise functional control.

[0078] When the fluid enters the orifice region (i.e., the "throat" of the Venturi tube), the flow cross-sectional area shrinks significantly, the shear force increases dramatically, effectively disrupting the laminar boundary layer near the orifice wall and weakening the mass transfer resistance. At the same time, the high-speed flow induces radial (lateral) convection, forcibly pushing the metal ions that were originally concentrated in the center of the flow channel and were easy to "escape through the filter" toward the orifice wall. The lateral convection also collides and superimposes with the main flow (transmembrane flow), exciting local vortices and microturbulence effects at the microscale, and enhancing fluid mixing.

[0079] When the fluid flows out of the small pore region and into the second large pore region (i.e., the "expansion section" of the Venturi tube), the flow channel suddenly expands, further triggering flow separation and recirculation. The fluid state tends towards turbulence, prolonging the residence time and increasing the probability of chelation reactions. Therefore, this invention does not suppress flow deviation by improving the uniformity of the flow channel, but rather actively guides the redistribution of fluid kinetic energy through microchannels similar to Venturi tubes, achieving "structure-driven mass transfer," improving the transport efficiency of metal ions to immobilized chelating groups, and overcoming the limitations of traditional porous membrane structure design.

[0080] 4. The unique multi-stage variable flow provides the foundation for high-speed mass transfer and efficient ion purification.

[0081] The flow process of fluid within a porous substrate can be summarized as "acceleration in the first large pore region - fastest flow in the small pore region - deceleration in the second large pore region." The flow characteristics and design considerations for each stage are as follows: First large pore region → Small pore region: When the fluid enters the small pore region, the pore wall suddenly narrows, causing shearing and stripping of the fluid boundary layer, initiating radial convection, laying the foundation for subsequent high-speed mass transfer and reducing flow deviation. Small pore region → Second large pore region: When the fluid enters the second large pore region, the flow channel suddenly expands, causing backflow and eddies, extending the residence time to enhance the chelation effect. However, it should be noted that if the expansion of the flow channel in the second large pore region is too large relative to the small pore region, flow separation (similar to jetting) can easily occur, causing the fluid to detach from the pore wall and lose the opportunity to contact or collide with the inner wall of the pore, thus causing metal ions to escape chelation. Therefore, it is necessary to control the expansion degree of the flow channel in the second large pore region to ensure that the fluid forms a decelerating flow and generates backflow and eddies within the second large pore region, avoiding jetting phenomena.

[0082] 5. Based on the chelating membrane (IPA bubble point approximately 160 kPa) provided by this invention, a specific purification experiment was conducted: a chelating membrane was used to purify a 2.38% TMAH solution spiked with 1 ppb metal ions at a flow rate of 100 mL / min. Experimental results show that the chelating membrane of this invention can effectively reduce the concentration of metal ion impurities in the 2.38% TMAH solution—even with only a single filtration, the concentration of some metal ions in the original solution can be reduced from 1 ppb before purification to <10 ppt. Compared with existing technologies, the purification effect on metal ions in TMAH solutions is superior. For different needs in practical applications, a scheme of stacking chelating membranes can also be adopted: for example, for chelating membranes with a low IPA bubble point and thinness, two chelating membranes with an IPA bubble point of 60 kPa can be stacked (stacking will increase the overall bubble point of the stacked membrane assembly). This method can also efficiently remove metal ions from the TMAH solution and is suitable for scenarios with higher purification requirements. It should be further explained that the IPA bubble point range of the chelating membrane of the present invention is set to 60-320 kPa. This range is designed based on the following considerations: if the IPA bubble point is too small, the membrane pore size will be too large, which may increase the risk of metal ions escaping from the solution; if the IPA bubble point is too large, the corresponding membrane pore size will be too small, which will increase the fluid flow resistance and may lead to a decrease in filtration efficiency. Therefore, the bubble point range of 60-320 kPa is the preferred range after balancing the "metal ion removal rate" and the "filtration efficiency". Attached Figure Description

[0083] Figure 1 The cross-sectional morphological structure of the chelating membrane of UPE modified with iminodiacetic acid (IDA) with a nominal pore size of 0.2 μm in Example 1 is shown in the 1000x SEM image.

[0084] Figure 2 The morphological and structural features of the cross-section of the chelating membrane of UPE modified with iminodiacetic acid (IDA) with a nominal pore size of 0.2 μm in Example 1 are shown in the 2000x SEM image.

[0085] Figure 3 The cross-sectional morphological structure of the chelating membrane of N-methyldiethanolamine (NMDG) modified UPE with a nominal pore size of 0.2 μm in Example 3 is shown in the 1000x SEM image.

[0086] Figure 4 The cross-sectional morphological structure of the chelating membrane of UPE modified with N-methyldiethanolamine (NMDG) with a nominal pore size of 0.2 μm in Example 3 is shown in the 2000x SEM image.

[0087] Figure 5This is a flowchart illustrating the treatment of TMAH stock solution containing metal ions using the UPE chelating membrane prepared in Examples 1-3.

[0088] Figure 6 The residual amount of metal ions in the filtrate produced after treating TMAH stock solution with the UPE chelating membrane of Examples 1-3. Detailed Implementation

[0089] To better explain and facilitate understanding of the present invention, the present invention will be described in detail below with reference to the accompanying drawings and specific embodiments.

[0090] For chelating membranes with micron-sized pores, due to their extremely small characteristic size (pore diameter), the Reynolds number is typically low even at high flow rates, and the flow state is mostly laminar. This makes it difficult for the fluid in the center to collide with the porous substrate structure, causing metal ions in the fluid center to easily escape the capture of the chelating membrane. To solve this problem, this invention incorporates a unique structure in the porous substrate of the chelating membrane, featuring large pore regions on both sides and a small pore region in the center to form Venturi tube channels. The accelerated flow in the small pore region increases the fluid inertia; simultaneously, combined with a non-directional tortuous pathway, the high-inertia fluid repeatedly collides and transfers mass with the porous substrate, continuously breaking the laminar flow state and transforming it into a turbulent state. This continuously pushes the liquid and metal ions in the fluid center to the edge of the channel, where they are further captured by the chelating groups, improving the purification effect on metal ions in the solution. The following description is based on specific embodiments of this invention.

[0091] Example 1

[0092] like Figure 1 The image shows the cross-sectional morphology of the chelating membrane of UPE modified with iminodiacetic acid (IDA) with a nominal pore size of 0.2 μm under a 1000x electron microscope. This UPE membrane is prepared using UPE as the substrate and iminodiacetic acid (IDA) as the chelating modifier. The two surfaces of the chelating membrane are skin layers 1, located on the inlet and outlet sides of the chelating membrane, respectively, with thicknesses of 3.8 μm and 2.2 μm. The total thickness of the porous body 2 is 90.6 μm. The porous body 2 includes a first large pore area 21, a small pore area 22, and a second large pore area 23 sequentially along its thickness direction between the skin layers 1 on both sides. The first large pore area 21 is closer to the liquid inlet side, the second large pore area 23 is closer to the liquid outlet side, and the small pore area 22 is located in the middle of the first large pore area 21 and the second large pore area 23. The skin layers 1 on both sides, the first large pore area 21, the small pore area 22, and the second large pore area 23 all have non-directional tortuous pathways, thereby forming a liquid flow path from the liquid inlet surface to the liquid outlet surface in the porous body 2.

[0093] As shown in the figure, the first macropore region 21 and the second macropore region 23 in the porous body 2 have a relatively loose structure, while the micropore region 22 is more dense. Specifically, on the cross-section of the chelating membrane, the average pore area in the regions of the first macropore region 21 and the second macropore region 23 of the porous body 2 is greater than the average pore area in the region of the micropore region 22. This conclusion can be clearly drawn by observing the microscopic images without any data measurement. There are relatively clear boundaries between the first macropore region 21 and the micropore region 22, and between the micropore region 22 and the second macropore region 23. This structural feature creates a Venturi effect on the solution permeating the porous body 2. The fluid initially flows at a slower speed in the first macropore region 21. Upon entering the micropore region 22, the fluid's flow state rapidly changes and it is redistributed within the channel, breaking the laminar flow. Liquid that was originally flowing in the middle of the channel is redistributed to the channel edge, where it collides and undergoes mass transfer with the chelating groups on the solid fibers. As the fluid enters the second macropore region 23 from the micropore region 22, the flow state changes again, breaking the laminar flow and flowing at a slower speed. This allows for a longer contact and mass transfer time between the metal ions in the fluid and the chelating groups on the solid fibers, further removing metal ions from the fluid.

[0094] Combination Figure 2 Under a 2000x electron microscope, the pore structure features of the porous body 2 in the thickness direction are more clearly visible in regions A (first large pore region 21), B (small pore region 22), and C (second large pore region 23). The thicknesses of the first large pore region 21, small pore region 22, and second large pore region 23 are 39.5 μm, 18.8 μm, and 26.3 μm, respectively. The thickness of small pore region 22 accounts for 20.8% of the total thickness of the porous body 2, and the thickness ratio of the first large pore region to the second large pore region is 1.5.

[0095] Furthermore, the pore structure characteristics of the porous body 2 of the chelating membrane in this embodiment were analyzed using Image-J software. The analysis results are as follows:

[0096] Porous body 2 as a whole: with a diameter >0.01μm 2 The pores are considered effective pores, with an effective pore area ratio of 44.2% and an average pore area of ​​0.689 μm. 2 The equivalent diameter is 0.936 μm; the average maximum Freret diameter is 0.782 μm, the average minimum Freret diameter is 0.433 μm, and the aspect ratio is 1.81.

[0097] First large pore region 21: with ≥0.01μm 2 The pore is the first effective pore, with a pore area ratio of 46.3% and an average pore area of ​​1.65 μm. 2The equivalent diameter is 1.45 μm; the maximum Ferrette diameter is 1.492 μm, the minimum Ferrette diameter is 0.900 μm, and the aspect ratio is 1.66.

[0098] Small pore region 22: with ≥0.01μm 2 The pores are the second effective pores, with a pore area ratio of 42.1% and an average pore area of ​​0.237 μm. 2 The equivalent diameter is 0.55 μm; the maximum Ferrette diameter is 0.622 μm, the minimum Ferrette diameter is 0.364 μm, and the aspect ratio is 1.71.

[0099] Second largest pore region 23: with ≥0.01μm 2 The pore is the third effective pore, with a pore area ratio of 45.1% and an average pore area of ​​0.768 μm. 2 The equivalent diameter is 0.99 μm; the maximum Ferrette diameter is 1.362 μm, the minimum Ferrette diameter is 0.782 μm, and the aspect ratio is 1.74.

[0100] The average area equivalent diameter of the second effective hole is 0.38 times that of the first effective hole; the average area equivalent diameter of the second effective hole is 0.56 times that of the third effective hole.

[0101] Analysis shows that, at a particle size of ≥0.01μm 2 When the holes are effective holes, the effective hole area ratio of the first large hole region 21, the small hole region 22, and the second large hole region 23 on the cross-section of the porous body 2 is the largest, at 46.3%, with a diameter ≥0.01μm. 2 When the holes are effective holes, the pore area ratio on the cross-section of the entire porous body 2 is 44.2%.

[0102] The chelating membrane described above is prepared by using UPE to obtain the base membrane via thermally induced phase separation and further modifying it with IDA. The preparation process is as follows:

[0103] Step 1: Preparation of casting solution. The casting solution includes UPE resin and mixed solvent. The solid content of UPE resin in the casting solution is 15 wt%, and the molecular weight of UPE resin is approximately 4 × 10⁻⁶. 6 g / mol, the mixed solvent is a mixture of solvent and non-solvent in a mass ratio of 1:3, and the solvent is specifically decahydronaphthalene, and the non-solvent is specifically dimethyl phthalate. The temperature of the casting solution needs to be controlled at 90-110℃ during mixing in order to initially swell the UPE resin. In this embodiment, the mixing temperature is 100℃±1℃.

[0104] Step 2: Casting. The casting liquid is melted and mixed at a temperature of about 230°C and extruded onto the carrier through a slit-shaped die to form a liquid film. The distance between the die and the carrier is about 10cm, so that the casting liquid passes through an air section between the die and the carrier. The temperature of the air section is controlled at 25°C±1°C. The ratio of the carrier's movement speed to the extrusion speed of the casting liquid is 1.3, that is, the carrier's movement speed is greater, so that the liquid film is stretched to a certain extent by the carrier.

[0105] Step 3, cooling and curing, specifically includes the following steps:

[0106] The liquid film on the carrier is continuously transported in the oven to make the temperature of the carrier side and the air side of the liquid film approximately the same. The temperature in the oven is controlled to be 50°C lower than the temperature when the liquid film is poured, that is, the oven temperature is set to about 180°C. The liquid film is transported in the oven for about 3 seconds, which promotes the slow phase separation on both sides of the liquid film.

[0107] The liquid film is transported from the oven to the cooling bath, which is a 25% v / v ethanol aqueous solution at a temperature of 25°C. The liquid film is rapidly and completely cooled, and the phases are separated and solidified to form a green film.

[0108] Step 4: Cleaning and extraction. Use anhydrous ethanol as the extraction bath to remove the residual solvent system in the membrane and wash with deionized water at least 3 times. After drying, the UPE base membrane is obtained.

[0109] Step 5: Chelation Modification. The obtained UPE base film is immersed in the modifier, maintaining a pressure of approximately 0.2 bar during immersion. This negative pressure environment promotes the modifier's penetration into the internal pore structure of the UPE base film. The immersion time is approximately 1 hour. After immersion, graft polymerization is initiated by irradiation with 365 nm ultraviolet light for approximately 6 hours. After the reaction is complete, the film is thoroughly washed with deionized water to remove any unreacted modifier, thus obtaining the chelated film. The modifier comprises, by mass ratio: 4.5% chelating agent (IDA), 3.5% monomer (glycidyl methacrylate), 0.5% photoinitiator (methyl benzoylformate), 5% sodium hydroxide, and the remainder being deionized water.

[0110] In this embodiment, the solid content of the casting solution in step one directly affects the nominal pore size of the final UPE base film. The chelated membrane obtained by this process has an IPA bubble point of approximately 160 kPa, which roughly corresponds to a nominal pore size of 0.2 μm. The nominal pore size can be adjusted by adjusting the solid content. By controlling the distance of the air section between the die and the carrier in step two, the casting solution is extruded from the die and both sides of the surface cool down relatively quickly in the air, thus generating a skin structure. The thickness of the skin can be controlled by controlling the length of the air section. When a skin is required to be introduced on one side, a heating device can be introduced on one side of the air section. The temperature of the heating device should be similar to the temperature of the casting solution to ensure that only one side of the extruded liquid film cools down rapidly in the air section, thereby introducing a skin on the rapidly cooling side. When a skin is not required, heating devices can be introduced on both sides, or the air section can be essentially removed to avoid rapid cooling of the liquid film surface. Furthermore, using hot airflow to purge the liquid film surface in the air section can induce solvent evaporation and change the local solid content of the liquid film, thus obtaining a skin structure. This will not be elaborated further in this embodiment, nor is it a limitation. The velocity ratio of the carrier to the casting liquid is controlled at 1.3, which stretches the casting liquid to help the molecular chains untangle and reorient. In the cooling and curing step three, a high oven temperature is used for pre-curing to control the slow phase separation in the areas near the two sides of the liquid film. This allows the molecular chains sufficient time to move, rearrange, and form larger polymer-rich and solvent-rich phases. Then, the liquid film inside is rapidly cooled and phase-separated by immersion in a lower-temperature cooling bath. Due to insufficient time for molecular chain movement and rearrangement, the polymer-rich and solvent-rich phases are relatively small. The final UPE base film has skins on both sides, with large pore areas inside the skins and small pore areas between the first and second large pore areas. Finally, IDA chelating groups are introduced into the UPE base film by irradiation-initiated polymerization, thus obtaining a UPE chelated film with IDA chelating groups.

[0111] Furthermore, the preparation method of metal chelate membranes is not limited to the specific methods described above. Those skilled in the art can prepare them based on the following known principles and in combination with appropriate control methods:

[0112] ①Using IDA chelating groups, UPE masterbatch is chelated and modified using a similar radiation graft polymerization method to obtain IDA-modified UPE masterbatch.

[0113] ②The IDA-modified UPE masterbatch was used to prepare a chelating membrane by the aforementioned thermally induced phase separation method.

[0114] It is important to note that in thermally induced phase separation, the polymer-rich phase forms a porous, solid structure after solidification, while the solvent-rich phase ultimately becomes a porous structure. Furthermore, the faster the temperature decreases, the faster the molecular chain mobility declines and the shorter the movement time, making it more difficult for them to aggregate into a large-sized polymer-rich phase. Therefore, treating the casting solution at low temperatures facilitates the formation of small-pore structures, while treating it at high temperatures facilitates the formation of large-pore structures. Additionally, the ratio of solvent to non-solvent in the solvent system can be appropriately adjusted. A higher proportion of non-solvent results in a less stable thermodynamic state of the casting solution, making it relatively easier for phase separation and solidification to form a small-pore structure.

[0115] Example 2

[0116] This embodiment provides a UPE chelating membrane modified with ethyl iminodiacetate (IDE) with a nominal pore size of 0.20 μm. The two surfaces of the chelating membrane are skin layers 1, located on the inlet and outlet sides respectively, with skin layer thicknesses of 1.2 μm and 1.5 μm respectively; the total thickness of the porous body 2 is 90.4 μm.

[0117] The porous body 2, between its two sides of the skin layer 1, consists of a first macropore region 21, a micropore region 22, and a second macropore region 23, arranged sequentially along the thickness direction. The first macropore region 21 is closer to the liquid inlet side, the second macropore region 23 is closer to the liquid outlet side, and the micropore region 22 is located between the first macropore region 21 and the second macropore region 23. The thicknesses of the micropore region are 29.5 μm, 30.7 μm, and 27.5 μm, respectively, accounting for 33.96% of the total thickness. The thickness ratio of the first macropore region to the second macropore region is 1.1. Electron microscopy reveals a relatively dense micropore region 22 formed between the first macropore region 21 and the second macropore region 23, with the average pore area of ​​the macropore region being greater than that of the micropore region.

[0118] Furthermore, the pore structure characteristics of the porous body 2 of the chelating membrane in this embodiment were analyzed using Image-J software. The analysis results are as follows (with ≥0.01μm as the effective pore size):

[0119] The porous body 2 as a whole has an effective pore area ratio of 42.2% and an average pore area of ​​0.657 μm. 2 The equivalent diameter is 0.915 μm; the average maximum Ferrette diameter is 0.767 μm, the average minimum Ferrette diameter is 0.411 μm, and the aspect ratio is 1.87.

[0120] The first large pore region 21 has a pore area ratio of 44.2%, an average pore area of ​​1.47 μm, an equivalent diameter of 1.37 μm, a maximum Ferrette diameter of 1.321 μm, a minimum Ferrette diameter of 0.792 μm, and an aspect ratio of 1.67.

[0121] Small hole region 22: The second effective hole has a pore area ratio of 40.5%, an average pore area of ​​0.19 μm, an equivalent diameter of 0.49 μm, a maximum Ferrette diameter of 0.492 μm, a minimum Ferrette diameter of 0.312 μm, and an aspect ratio of 1.58.

[0122] The second largest pore region 23: The pore area ratio of the third effective pore is 43.9%, the average pore area is 0.712μm, the equivalent diameter is 0.95μm; the maximum Ferrette diameter is 1.254μm, the minimum Ferrette diameter is 0.723μm, and the aspect ratio is 1.73.

[0123] The difference in the area ratio of the three groups of regions is ≤4%. The equivalent diameter of the second effective hole is 0.36 times the diameter of the first effective hole, and the equivalent diameter of the second effective hole is 0.52 times the diameter of the third effective hole.

[0124] The preparation method of the UPE base membrane in the above chelating membrane is roughly the same as that in Example 1. The main difference lies in the different modifiers used in the chelation modification process in step five, as detailed below:

[0125] Step 1: Preparation of casting solution. The casting solution includes UPE resin and mixed solvent. The solid content of UPE resin in the casting solution is 15 wt%, and the molecular weight of UPE resin is approximately 4 × 10⁻⁶. 6 g / mol, the mixed solvent is a mixture of solvent and non-solvent in a mass ratio of 1:3, and the solvent is specifically decahydronaphthalene, and the non-solvent is specifically dimethyl phthalate. The temperature of the casting solution needs to be controlled at 90-110℃ during mixing in order to initially swell the UPE resin. In this embodiment, the mixing temperature is 100℃±1℃.

[0126] Step 2: Casting. The casting liquid is melted and mixed at a temperature of about 230°C and extruded onto the carrier through a slit-shaped die to form a liquid film. The distance between the die and the carrier is about 5 cm, so that the casting liquid experiences a shorter air section between the die and the carrier compared to Example 1. The temperature of the air section is controlled at 25°C ± 1°C. The ratio of the carrier's movement speed to the extrusion speed of the casting liquid is 1.2, that is, the carrier's movement speed is greater, so as to stretch the liquid film to a certain extent through the carrier.

[0127] Step 3, cooling and curing, specifically includes the following steps:

[0128] The liquid film on the carrier is continuously transported in the oven to make the temperature of the carrier side and the air side of the liquid film approximately the same. The temperature in the oven is controlled to be 40°C lower than the temperature when the liquid film is poured, that is, the oven temperature is set to about 190°C. The liquid film is transported in the oven for about 2 seconds, which promotes the slow phase separation on both sides of the liquid film.

[0129] The liquid film is transported from the oven to the cooling bath, which is a 25% v / v ethanol aqueous solution at a temperature of 25°C. The liquid film is rapidly and completely cooled, and the phases are separated and solidified to form a green film.

[0130] Step 4: Cleaning and extraction. Use anhydrous ethanol as the extraction bath to remove the residual solvent system in the membrane and wash with deionized water at least 3 times. After drying, the UPE base membrane is obtained.

[0131] Step 5: Chelation Modification. The obtained UPE base film is immersed in the modifier, maintaining a pressure of approximately 0.2 bar during immersion. This negative pressure environment promotes the modifier's penetration into the internal pore structure of the UPE base film. The immersion time is approximately 1 hour. After immersion, graft polymerization is initiated by irradiation with 365 nm ultraviolet light for approximately 6 hours. After the reaction is complete, the film is thoroughly washed with deionized water to remove any unreacted modifier, thus obtaining the chelated film. The modifier comprises, by mass ratio: 4.5% chelating agent (IDE), 3.5% monomer (glycidyl methacrylate), 0.5% photoinitiator (methyl benzoylformate), 5% sodium hydroxide, and the remainder being deionized water.

[0132] Example 3

[0133] like Figure 3 The figure shows the morphological characteristics of the cross-section of the NMDG-modified UPE chelating membrane with a nominal pore size of 0.2 μm under a 1000x electron microscope in this embodiment. This UPE membrane is prepared using UPE as the substrate and N-methyldiethanolamine (NMDG) as the chelating modifier. The two surfaces of the chelating membrane are skin layers 1, located on the inlet and outlet sides of the chelating membrane, respectively, with thicknesses of 3.3 μm and 2.3 μm. The total thickness of the porous body 2 is 91 μm. As shown in the figure, in the cross-section of the chelating membrane, the average pore area in the first macropore region 21 and the second macropore region 23 of the porous body 2 is greater than the average pore area in the small pore region 22. There are relatively clear boundaries between the first macropore region 21 and the small pore region 22, and between the small pore region 22 and the second macropore region 23. By utilizing this special filtration pore structure, a Venturi effect can be formed on the solution passing through the porous body 2. This causes the fluid to undergo at least two significant changes in flow state as it passes through the porous body 2, flowing in a turbulent manner through the pores of the porous body 2. This avoids the problem of the fluid in the middle of the flow channel continuously flowing in a laminar state during the process of passing through the chelating membrane, which could lead to the escape of metal ions carried therein and their capture by the chelating groups.

[0134] Combination Figure 4Under a 2000x electron microscope, the pore structure features of the porous body 2 in the thickness direction are more clearly visible in regions A (first large pore region 21), B (small pore region 22), and C (second large pore region 23). The thicknesses of the first large pore region 21, small pore region 22, and second large pore region 23 are 27.8 μm, 30.3 μm, and 27.3 μm, respectively. The thickness of small pore region 22 accounts for 33.3% of the total thickness of the porous body 2, and the thickness ratio of the first large pore region to the second large pore region is 1.0.

[0135] Furthermore, the pore structure characteristics of the porous body 2 of the chelating membrane in this embodiment were analyzed using ImageJ software. The analysis results are as follows (with ≥0.01μm as the effective pore size):

[0136] Porous substrate 2: The effective pore area ratio is 34.8%, and the average pore area is 0.481 μm. 2 The equivalent diameter is 0.783 μm; the maximum Ferrette diameter is 0.742 μm, the minimum Ferrette diameter is 0.415 μm, and the aspect ratio is 1.79.

[0137] The first large pore region 21: The pore area ratio of the first effective pore is 35.65%, and the average pore area is 0.820 μm. 2 The equivalent diameter is 1.02 μm; the maximum Ferrette diameter is 1.280 μm, the minimum Ferrette diameter is 0.749 μm, and the aspect ratio is 1.71.

[0138] Small-hole region 22: The pore area ratio of the second effective pore is 33.49%, and the average pore area is 0.158 μm. 2 The equivalent diameter is 0.45 μm; the maximum Ferrette diameter is 0.554 μm, the minimum Ferrette diameter is 0.315 μm, and the aspect ratio is 1.76.

[0139] The second largest pore region 23: The pore area ratio of the third effective pore is 35.15%, and the average pore area is 1.069 μm. 2 The equivalent diameter is 1.17 μm; the maximum Ferrette diameter is 1.427 μm, the minimum Ferrette diameter is 0.787 μm, and the aspect ratio is 1.81.

[0140] The equivalent diameter of the second effective hole is 0.44 times the diameter of the first effective hole, and the equivalent diameter of the second effective hole is 0.38 times the diameter of the third effective hole.

[0141] In this embodiment, a process route of first chelating and modifying the masterbatch before film formation is adopted. Specifically, the preparation process is as follows:

[0142] Step 1: Masterbatch modification. Dissolve ultra-high molecular weight polyethylene resin (molecular weight approximately 5 × 10⁻⁶) in NMP. 6The modified masterbatch was prepared by adding 4 wt% NMDG (N-methyldiethanolamine) as a grafting modifier and 0.1 wt% BYK-333 to a solid content of 10 wt%. The mixture was heated to 180 °C and stirred for 8 hours to ensure complete dissolution. After degassing, drying, cleaning, and granulation, the modified masterbatch was obtained.

[0143] Step 2: Preparation of casting solution. The casting solution includes modified masterbatch and mixed solvent. The solid content of the modified masterbatch in the casting solution is 18 wt%. The mixed solvent is a mixture of solvent and non-solvent in a mass ratio of 1:3. Specifically, the solvent is decahydronaphthalene and the non-solvent is dimethyl phthalate. The temperature of the casting solution needs to be controlled at 90-110℃ during mixing to allow the UPE resin to initially swell. In this embodiment, the mixing temperature is 100℃±1℃.

[0144] Step 3: Casting. The casting liquid is melted and mixed at a temperature of about 230°C and extruded onto the carrier through a slit-shaped die to form a liquid film. The distance between the die and the carrier is about 10cm, so that the casting liquid passes through an air section between the die and the carrier. The temperature of the air section is controlled at 25°C±1°C. The ratio of the carrier's movement speed to the extrusion speed of the casting liquid is 1.3, that is, the carrier's movement speed is greater, so that the liquid film is stretched to a certain extent by the carrier.

[0145] Step 3, cooling and curing, specifically includes the following steps:

[0146] The liquid film on the carrier is continuously transported in the oven to make the temperature of the carrier side and the air side of the liquid film approximately the same. The temperature in the oven is controlled to be 50°C lower than the temperature when the liquid film is poured, that is, the oven temperature is set to about 180°C. The liquid film is transported in the oven for about 2 seconds, which promotes the slow phase separation on both sides of the liquid film.

[0147] Step 4: Cleaning and extraction. Use anhydrous ethanol as the extraction bath to remove the residual solvent system in the membrane and wash with deionized water at least 3 times. After drying, the chelated membrane is obtained.

[0148] It should be noted that the preparation methods mentioned in the above embodiments are merely examples of specific preparation methods for the chelating membrane based on the concept of the present invention, and are not intended to limit the chelating membrane of the present invention to preparation only according to the above methods. Besides solution casting and casting methods, those skilled in the art can use chelating resin solution coating, hot pressing, or melt casting to prepare the membrane, or use a matrix material to form a membrane material through solution casting, solution coating, hot pressing, or melt casting, and then graft chelating groups onto the membrane surface and pores of the membrane material through chemical or irradiation methods. It is easily understood that during the preparation process, the porous body of the chelating membrane, forming a "first macropore region, micropore region, second macropore region" with Venturi tube effect along the thickness sequence, can be prepared using the aforementioned basic raw materials for chelating membrane preparation via a one-step molding method or a multi-step molding method. Multi-step molding methods include coating chelating resin solutions with different formulations into a single unit in three separate steps.

[0149] 2 L of 1 ppb metal ions were treated using the UPE chelating membranes described in Examples 1-3 (the concentration of each metal was measured using an Agilent 7900 ICP-MS after spiking; test data are shown in [link to data]). Figure 6 The blue bar column contains 2.38% TMAH solution spiked with 2.38%. See the procedure below. Figure 5 The filtration flow rate was 100 mL / min, with a single filtration. The filtrate was collected, and the residual metal ions in the filtrate were tested using an Agilent 7900 ICP-MS. The results of single-pass filtration for each group of chelating membranes are shown below. Figure 6 As shown. Orange represents the filtration results of the IDA-modified UPE chelating membrane (abbreviated CAU, i.e., the chelating membrane prepared in Example 1), gray represents the filtration results of the IDA-modified UPE chelating membrane (abbreviated CEU, i.e., the chelating membrane prepared in Example 2), and yellow represents the filtration results of the NMDG-modified UPE chelating membrane (abbreviated CNU, i.e., the chelating membrane prepared in Example 3).

[0150] Depend on Figure 6It is known that the solution in the container is forced through the purifier at a flow rate of 100 mL / min by air pressure, and the filtrate is collected downstream. The concentration of metal ions in the filtrate is analyzed using an Agilent 7900 ICP-MS. The results show that the chelating membranes of Examples 1-3 can effectively reduce the concentration of metal ion impurities in the 2.38% TMAH solution. Even with a single filtration, the concentration of some metal ions can be reduced from 1 ppb before purification to <10 ppt. In particular, they show extremely high removal efficiency for Mg, Cr, Mn, Fe, Co, Ni, Cu, Ag, Cd, and Ba in the TMAH solution. These are the main leaching metal elements in, for example, stainless steel pipelines, and are the metal ions that most need to be removed. It should be noted that the chelating groups in Examples 1-3 of this application can effectively purify the main leaching metal elements in semiconductor stainless steel pipelines. If different metal ions need to be purified, different chelating groups can be used to modify the base membrane to obtain different chelating properties for different metal ions.

[0151] Example 4

[0152] This embodiment provides an EDTA-modified PVDF chelating membrane with a nominal pore size of 0.7 μm. The chelating membrane has a skin layer 1 only on the liquid inlet side, with a skin layer thickness of approximately 8 μm; the porous body 2 has a thickness of 88 μm. The thickness ratio of the first macropore region 21, the micropore region 22, and the second macropore region 23 in the porous body 2 is 1:2:1, with the micropore region 22 accounting for 45.5% of the thickness, and the thickness ratio of the first macropore region to the second macropore region being 1. Under electron microscopy, a dense micropore region 22 is formed between the first macropore region 21 and the second macropore region 23, and the average pore area of ​​the macropore region is greater than that of the micropore region.

[0153] Furthermore, the pore structure characteristics of the porous body 2 of the chelating membrane in this embodiment were analyzed using ImageJ software. The analysis results are as follows (with ≥0.01μm as the effective pore size):

[0154] First large pore region 21: The pore area ratio of the first effective pore is 45%, the average pore area is 1.13μm, the equivalent diameter is 1.2μm; the maximum Ferrette diameter is 1.8μm, the minimum Ferrette diameter is 1.2μm, and the aspect ratio is 1.5.

[0155] Small hole region 22: The second effective hole has a pore area ratio of 42%, an average pore area of ​​0.28 μm, an equivalent diameter of 0.6 μm, a maximum Ferrette diameter of 0.7 μm, a minimum Ferrette diameter of 0.4 μm, and an aspect ratio of 1.75.

[0156] The second largest pore region 23: The pore area ratio of the third effective pore is 43%, the average pore area is 0.50μm, the equivalent diameter is 0.8μm; the maximum Ferrette diameter is 1.6μm, the minimum Ferrette diameter is 0.9μm, and the aspect ratio is 1.78.

[0157] The difference in the area ratio of the three groups of regions is ≤3%. The equivalent diameter of the second effective hole is 0.5 times that of the first effective hole; the equivalent diameter of the second effective hole is 0.75 times that of the third effective hole.

[0158] The above-mentioned chelating membrane was prepared by melt extrusion, and the preparation process is as follows:

[0159] The first step is to prepare EDTA-modified PVDF. The PVDF has a molecular weight of approximately 5 × 10⁻⁶. 5 g / mol, modification rate 15%. PVDF and plasticizer DOP were mixed at a mass ratio of 45:55.

[0160] The second step involves melting and stirring at 180°C for 3 hours to cast a 100μm pre-film. The film is then placed at 25°C for 5 seconds, followed by holding at 80°C for 5 seconds, and finally at 40°C for 10 seconds. DOP is extracted with ethanol at 60°C for 2 hours.

[0161] The third step involves vacuum drying at 50°C for 4 hours to obtain the EDTA-modified PVDF chelating membrane, which has an IPA bubble point of 75 kPa. In this embodiment, two membranes are used in combination to increase the IPA bubble point when removing metal ions from the TMAH solution.

[0162] In the second step, at 25°C, the air side cools down rapidly to form a skin structure, while at 80°C, the interior slowly forms the first and second large pore areas, and finally at 40°C, the small pore area in the middle is formed.

[0163] Example 5

[0164] This embodiment provides an NTA-modified PES chelating membrane with a nominal pore size of 0.45 μm. Electron microscopy measurements show that the chelating membrane has a skin layer 1 on both sides, with thicknesses of 4.5 μm and 4.9 μm, respectively; the porous body 2 has a thickness of 95.6 μm, and the total thickness of the skin layer 1 accounts for 9.8% of the porous body. The thickness ratio of the first macropore region, the small pore region, and the second macropore region in the porous body is 2:2:1.5, with the small pore region accounting for 32.8% of the thickness, and the thickness ratio of the first macropore region to the second macropore region being 1.33.

[0165] Furthermore, the pore structure characteristics of the porous body 2 of the chelating membrane in this embodiment were analyzed using ImageJ software. The analysis results are as follows (with ≥0.01μm as the effective pore size):

[0166] First large pore region 21: The pore area ratio of the first effective pore is 38%, the average pore area is 0.78μm, the equivalent diameter is 1.0μm; the maximum Ferrette diameter is 1.5μm, the minimum Ferrette diameter is 0.8μm, and the aspect ratio is 1.88.

[0167] Small hole region 22: The second effective hole has a pore area ratio of 35%, an average pore area of ​​0.13 μm, an equivalent diameter of 0.41 μm, a maximum Ferrette diameter of 0.5 μm, a minimum Ferrette diameter of 0.3 μm, and an aspect ratio of 1.67.

[0168] The second largest pore region 23: The pore area ratio of the third effective pore is 36%, the average pore area is 0.57μm, the equivalent diameter is 0.85μm; the maximum Ferrette diameter is 1.4μm, the minimum Ferrette diameter is 0.75μm, and the aspect ratio is 1.87.

[0169] The difference in the area ratio of the three groups of regions is ≤3%. The equivalent diameter of the second effective hole is 0.41 times that of the first effective hole; the equivalent diameter of the second effective hole is 0.48 times that of the third effective hole.

[0170] The above-mentioned chelating membrane was prepared by a solvent-free phase separation method, and the preparation process is as follows:

[0171] Step 1: Preparation of casting solution. The casting solution includes polyethersulfone resin, mixed solvent and pore-forming agent. The solid content of polyethersulfone resin is 12wt%. The mixed solvent is a mixture of solvent and non-solvent in a mass ratio of 4:1. The solvent is dimethylformamide and the non-solvent is water. The pore-forming agent is PVP-K30.

[0172] Step 2: Extrusion. The casting liquid is extruded vertically downward through a slit-shaped die to form a liquid film.

[0173] Step 3: Phase separation and solidification. The liquid film is directly immersed in the phase separation bath, which is a 25 v / v% ethanol aqueous solution. Phase separation and solidification form a film.

[0174] Step 4: Cleaning and extraction. Use anhydrous ethanol as the extraction bath to remove the residual solvent system in the membrane and wash with deionized water at least 3 times. After drying, the base membrane is obtained.

[0175] Step 5: Place the base membrane in an ethanol solution containing 10wt% NTA at 50℃ and heat for 4 hours to modify it. After removing, cleaning and drying, the chelated membrane is obtained, with a bubble point of approximately 110 kPa.

[0176] Example 6

[0177] This embodiment provides an IDS (iminodisuccinic acid) modified CA (cellulose acetate) chelating membrane with a nominal pore size of 0.1 μm. The chelating membrane has a skin layer 1 on both sides of its cross-section in the thickness direction, each 10 μm thick; the porous body 2 has a thickness of 200 μm, and the total thickness of the skin layers accounts for 10% of the porous body. The thickness ratio of the porous body is 1.8:2:1.2, the thickness of the small pore region accounts for 36%, and the thickness ratio of the first large pore region 21 to the second large pore region 23 is 1.5.

[0178] Furthermore, the pore structure characteristics of the porous body 2 of the chelating membrane in this embodiment were analyzed using ImageJ software. The analysis results are as follows (with ≥0.01μm as the effective pore size):

[0179] First large pore region 21: The pore area ratio of the first effective pore is 37%, the average pore area is 1.77μm, the equivalent diameter is 1.5μm; the maximum Ferrette diameter is 2.0μm, the minimum Ferrette diameter is 1.0μm, and the aspect ratio is 2.0.

[0180] Small hole region 22: The second effective hole has a pore area ratio of 35%, an average pore area of ​​0.25 μm, and an equivalent diameter of 0.56 μm; the maximum Ferrette diameter is 1.0 μm, the minimum Ferrette diameter is 0.5 μm, and the aspect ratio is 2.0.

[0181] The second largest pore region 23: The third effective pore has a pore area ratio of 36%, an average pore area of ​​0.64 μm, an equivalent diameter of 0.9 μm, a maximum Ferrette diameter of 1.9 μm, a minimum Ferrette diameter of 0.95 μm, and an aspect ratio of 2.0.

[0182] The difference in pore area ratio among the three groups of regions is ≤2%, the second effective pore area accounts for 35%, and the equivalent diameter is 0.56μm. The equivalent diameter of the second effective pore is 0.37 times that of the first effective pore; the equivalent diameter of the second effective pore is 0.62 times that of the third effective pore.

[0183] The above-mentioned chelating membrane was prepared by casting with a casting solution and then by a solvent-free phase separation method, as follows:

[0184] The first step is to prepare IDS-modified CA. The degree of CA substitution is 2.5, and the modification rate is 14%.

[0185] The second step involves dissolving IDS-modified CA in acetone / water (85:15), adding 0.1 wt% leveling agent, stirring at 50°C for 6 hours, and degassing to obtain a casting solution with a solid content of 18 wt%.

[0186] The third step involves preparing the chelate membrane using a casting-phase transfer method. A glass plate was selected, cleaned with ethanol, coated with a release agent, and pre-cooled to 10°C. A wet film was cast and coated. First, 10°C cold air was used to evaporate the solvent on the surface of the wet film, causing a rapid increase in polymer concentration within 12 seconds, forming a dense skin layer on the film surface. After 20 seconds, the cold air was removed, allowing the area below the surface of the wet film to evaporate naturally in the air for 2 minutes. The first macroporous region 21 and the second macroporous region 23 were formed near the liquid inlet and liquid outlet sides of the wet film, respectively. Immediately afterwards, the film was briefly annealed with 52°C hot air for 10 seconds, and then quickly immersed in 10°C water containing 20v / v% ethanol to solidify the structure of the first macroporous region 21 and the second macroporous region 23. This also caused rapid phase transfer and freezing of the solvent in the small pores near the center, resulting in small pores 22. The film was cleaned with deionized water at 40°C for 1 hour and vacuum dried at 50°C for 3 hours to obtain the IDS-modified CA chelate membrane, with an IPA bubble point of 290 kPa.

[0187] Comparative Example 1

[0188] The cross-section of the IDA-modified UPE chelating membrane in this comparative example, with a nominal pore size of 0.2 μm, uses the same substrate (UPE), the same chelating agent (iminodiacetic acid IDA), and the same preparation route (thermally induced phase separation method) as Example 1. However, the cross-section of the chelating membrane exhibits a roughly uniform pore size in the thickness direction.

[0189] The two surfaces of the chelating membrane are skin layers 1, located on the inlet and outlet sides respectively, with skin layer thicknesses of 3.4 μm and 2.5 μm respectively; the porous body 2 has a thickness of 91.2 μm.

[0190] Under electron microscopy and analysis using ImageJ software (with ≥0.01μm as the effective pore size), the porous body 2 is divided into a liquid inlet region (region A) and a liquid outlet region (region B) along the thickness direction. The analysis results are as follows:

[0191] The porous body 2 as a whole has an effective pore area ratio of 43.87%, an average pore area of ​​0.672 μm, an equivalent diameter of 0.921 μm, a maximum Ferrette diameter of 0.765 μm, a minimum Ferrette diameter of 0.428 μm, and an aspect ratio of 1.79.

[0192] Liquid inlet side region (Area A): effective pore area ratio 44.52%, average pore area 0.681μm, equivalent diameter 0.928μm; maximum Ferrette diameter 0.773μm, minimum Ferrette diameter 0.430μm, aspect ratio 1.79.

[0193] Liquid outlet side region (zone B): effective pore area ratio 42.98%, average pore area 0.663μm, equivalent diameter 0.914μm; maximum Ferrette diameter 0.758μm, minimum Ferrette diameter 0.425μm, aspect ratio 1.78.

[0194] The average pore area difference between region A and region B is only 0.018 μm, the pore area ratio difference is only 1.54%, and there is no significant difference in the pore structure in the thickness direction.

[0195] Preparation process: Steps one, two, four, and five of Comparative Example 1 are the same, the only difference from Example 1 is that step three uses the following process:

[0196] Step 3, cooling and curing, specifically includes the following steps:

[0197] The liquid film is transported from the oven to the cooling bath, which is a 25% v / v ethanol aqueous solution at a temperature of 60°C. The liquid film cools down rapidly and completely, and solidifies to form a green film.

[0198] Comparative Example 2

[0199] This comparative example uses iminodiacetic acid (MDA) type chelating resin particles (CR-10 chelating resin from Mitsubishi Chemical Industries, Ltd., Japan) to treat a 1 ppb metal ion-spiked 2.38% TMAH solution. The MDA type chelating resin particles, with over 95% falling within the 400-500 μm range, were packed in a resin column to a thickness of 1.0 m. The Mitsubishi CR-10 resin was soaked in 5% HCl for 2 h, washed with deionized water until neutral, and then activated with 5% NaOH for 30 min. The empty column velocity was 10 m / h.

[0200] Comparative Example 3

[0201] This comparative example uses an electrospinning process to prepare a UPE nonwoven filter membrane (approximately 200 μm thick) with a basis weight of 85 g / m and a nominal pore size of 0.2 μm. The membrane is then modified with iminodiacetic acid (IDA) using the same method as in Example 1. The nonwoven filter membrane is stacked in three layers and subjected to dead-end filtration (0.1 MPa).

[0202] Prepare 31 L of 2.38% TMAH solution spiked with 1 ppb metal ions, and divide it into 7 equal portions of 3 L each. Use the chelating membranes from Examples 1, 4-6, and Comparative Examples 1-3 respectively... Figure 6 The filtration process was performed as shown, with a flow rate of 100 mL / min. After a single filtration, the filtrate was collected, and the concentration of metal ions in the filtrate was analyzed using an Agilent 7900 ICP-MS. The results are as follows:

[0203] Table 1 (Concentration unit is ppb):

[0204]

[0205] The test results of the above embodiments demonstrate that the metal ion removal rate tests prove that the chelating membrane of the present invention possesses high-efficiency mass transfer performance and metal ion purification capability. Compared with the comparative example, the chelating membrane prepared by the present invention, due to its "large→small→large" pore structure in the thickness direction, generates a Venturi effect, which can break laminar flow and prolong the contact time between metal ions and chelating groups. Even in a single filtration, it can purify Ag, Co, Cu, Cd, and Ba with a concentration of 1 ppb to below 10 ppt or even to almost zero. In contrast, in the purifier of the comparative example, the fluid is in a stable laminar flow state within the porous body. The fluid in the middle region has difficulty making sufficient contact with the chelating groups on the solid fibers, and it is also difficult to overcome the flow deviation problem, thus failing to effectively capture metal ions and achieve efficient metal ion removal.

[0206] Furthermore, different chelating membranes possess different chelating groups, resulting in varying capacities for different metal elements. For instance, the EDTA chelating group in the chelating membrane of Example 4 exhibits stronger chelating effects on Fe, Cu, Ni, Co, Cr, V, and Ti. Therefore, the chelating membrane of Example 4 demonstrates superior purification performance for these metal ions compared to Example 1. The NTA chelating group in the chelating membrane of Example 5 exhibits strong chelating effects on Ag and Cr, while the IDS chelating group in the chelating membrane of Example 6 demonstrates strong chelating effects on Ag, Cr, and V. In practical applications, chelating membranes with corresponding chelating groups can be prepared based on the types of metal ions contained in the solution to be treated.

[0207] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, and not to limit them. Although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some or all of the technical features. These modifications or substitutions, or combinations of technical features in the above embodiments that do not conflict with each other, can be made in accordance with the manner described in the embodiments. These modifications, substitutions or combinations do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the embodiments of the present invention.

Claims

1. A chelating membrane for removing metal ions, characterized in that, It comprises a porous body with continuous, non-directional tortuous pathways and metal chelating groups. The porous body, in its thickness direction, sequentially comprises a first macropore region, a micropore region, and a second macropore region. The first macropore region is located near the liquid inlet side of the porous body, and the second macropore region is located near the liquid outlet side. The micropore region is located between the first and second macropore regions. On the cross-section of the porous body, the average pore area in the corresponding regions of the first and second macropore regions is greater than the average pore area in the corresponding region of the micropore region. The IPA bubble point of the chelating membrane is 60 kPa-320 kPa. The chelating membrane can purify metal ions in the original solution with a concentration of several hundred ppt to several ppb to below 10 ppt.

2. The chelating membrane according to claim 1, characterized in that, The thickness of the porous body is 50-200 μm; the thickness of the pore region accounts for 10-50% of the thickness of the porous body.

3. The chelating membrane according to claim 1, characterized in that, The thickness ratio of the first large hole region to the second large hole region is 0.8-2.

4. The chelating membrane according to claim 1, characterized in that, In the cross-section of the porous body, the cross-sectional region corresponding to the first macropore region has a diameter of ≥0.01μm. 2 The hole is the first effective hole, and the cross-sectional region corresponding to the small hole area has a diameter of ≥0.01μm. 2 The hole is the second effective hole, and the cross-sectional region corresponding to the second large hole region has a diameter of ≥0.01μm. 2 The hole is the third effective hole, and the average area equivalent diameter of the second effective hole is 0.25-0.85 times the average area equivalent diameter of the first effective hole; and / or, the average area equivalent diameter of the second effective hole is 0.35-0.75 times the average area equivalent diameter of the third effective hole.

5. The chelating membrane according to claim 4, characterized in that, The difference between the area ratio of the first effective hole and the area ratio of the second effective hole is no greater than 10%; and / or, the difference between the area ratio of the third effective hole and the area ratio of the second effective hole is no greater than 10%.

6. The chelating membrane according to claim 5, characterized in that, The area ratio of the second effective pore is 20-55%, and the average area equivalent diameter of the second effective pore is 0.2-1μm.

7. The chelating membrane according to claim 4, characterized in that, The maximum Fe-max diameter of the first effective pore is 0.5-2.5 μm, and the ratio of the maximum Fe-max diameter to the minimum Fe-max diameter is 1.2-2.

2. And / or, the maximum Fe-max diameter of the third effective pore is 0.5-2.5 μm, and the ratio of the maximum Fe-max diameter to the minimum Fe-max diameter is 1.2-2.

2.

8. The chelating membrane according to claim 1, characterized in that, At least one side surface of the porous body also has a skin layer, the total thickness of the skin layer not exceeding 10% of the thickness of the porous body, and the pore area ratio of the skin layer is lower than the pore area ratio of the porous body.

9. The chelating membrane according to any one of claims 1-8, characterized in that, The porous substrate is made of a film-forming polymer, which is at least one of polyethylene (PE), polypropylene (PP), poly(4-methyl-1-pentene) (PMP), polysulfone (PSF), polyethersulfone (PES), regenerated cellulose (RC), nitrocellulose (CN), cellulose acetate (CA), polyvinylidene fluoride (PVDF), and polytetrafluoroethylene (PTFE).

10. The chelating membrane according to any one of claims 1-8, characterized in that, The chelating group is a chelating group of at least one of the following: polyol chelating agents, polycarboxylic acid chelating agents, small molecule amine chelating agents, sulfur-containing chelating agents, and phosphonic acid derivative chelating agents.

11. The chelating membrane according to claim 10, characterized in that, The chelating agent is selected from at least one of IDA, IDE, ethylenediaminetetraacetic acid, 1,2-cyclohexanediamine-N,N,N',N'-tetraacetic acid, iminodisuccinic acid, NMDG, NTA, TED, TEPA, CM-DASA, and EDDA.

12. The chelating membrane according to any one of claims 1-8, characterized in that, The chelating membrane is prepared by using a film-forming polymer modified with a chelating agent through a thermally induced phase separation method, a non-solvent-induced phase separation method, or a melt extrusion method; or the chelating membrane is prepared by using a film-forming polymer through a thermally induced phase separation method, a non-solvent-induced phase separation method, or a melt extrusion method to form a film, and then modified with a chelating agent.

13. The chelating membrane according to any one of claims 1-8, characterized in that, The porous body is disposed on the surface of a substrate material, which is a non-woven fabric or a porous membrane.

14. A purifier for removing metal ions, characterized in that, It comprises the chelating membrane according to any one of claims 1-13.

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