Zeolite-like molecular sieve membrane for removing fluorine and chlorine from contaminated acid and preparation method

Abstract

The present application relates to the technical field of contaminated acid treatment, and specifically discloses a zeolite-like molecular sieve membrane for removing fluorine and chlorine from contaminated acid and a preparation method. The molecular sieve membrane comprises a dispersed phase and a continuous phase; the dispersed phase consists of zeolite-like molecular sieve nanoparticles, and the continuous phase is formed by dissolving PVDF monomer particles and then polymerizing same; and the zeolite-like molecular sieve nanoparticles are prepared by dissolving a metal compound in a first solvent in an ultrasonic environment, then dissolving the solution in a second solvent to form a mixed solution I, and then drying the mixed solution I. The preparation method comprises the steps of preparing a PVDF molecular sieve blend membrane by a blending method and performing surface coating modification on the PVDF molecular sieve blended membrane. In the present application, a zeolite-like molecular sieve membrane for a contaminated acid system is prepared by a blending method, such that fluorine and chloride ions in a contaminated acid can be allowed to pass through and sulfate ions are intercepted, thereby removing fluorine and chloride and reducing the loss rate of sulfate ions, and realizing the recycling of sulfuric acid resources. The present application has the characteristics of a high fluorine and chloride removal rate, a high sulfate retention rate, good corrosion resistance, and high water permeation flux.

Description

A zeolite-like molecular sieve membrane for the removal of fluoride and chlorine from waste acid and its preparation method

[0001] This application claims priority to Chinese Patent Application No. 202510057982.6, filed on January 14, 2025, entitled "A Zeolite-like Molecular Sieve Membrane for Removing Fluorine and Chlorine from Sewage Acid and its Preparation Method", the entire contents of which are incorporated herein by reference. Technical Field

[0002] This application relates to the field of waste acid treatment technology, specifically to a zeolite-like molecular sieve membrane for the removal of fluoride and chlorine from waste acid, which has high fluoride and chlorine removal rate, high sulfate retention rate, good corrosion resistance, and high water flux, and its preparation method. Background Technology

[0003] The main component of the waste acid produced from metal smelting and the sulfuric acid production from smelting flue gas is sulfuric acid. If the sulfuric acid in the waste acid could be recycled during production, it would not only reduce production costs but also reduce pollutant emissions. However, due to the high concentration of fluoride and chloride ions in the waste acid, the equipment, pipeline valves, and other components require high corrosion resistance. Therefore, fluoride and chloride-containing waste acid must undergo fluoride and chloride ion removal before sulfate ions can be recycled.

[0004] Currently, the main methods for removing fluoride and chloride ions from waste acid are: physical removal methods, which involve hot air stripping and filtration using nanofiltration membranes or PVDF flat-panel ultrafiltration membranes. However, hot air stripping has low efficiency and high heat consumption, and membrane filtration has the problem of high fluoride ion rejection rate. Chemical precipitation methods, which involve adding bismuth salts, calcium salts, rare earth removal agents, etc., to precipitate fluoride and chloride impurities. However, these methods have problems such as introducing new impurity elements, high cost, and failure to achieve resource utilization of fluoride and chloride.

[0005] In existing technologies, neutralizing agents such as calcium carbonate, calcium oxide, and calcium hydroxide are added to fluoride-chloride waste acid to neutralize the waste acid; anionic polyacrylamide is added as a flocculant to promote the precipitation of calcium fluoride and calcium chloride; and then the removal of fluoride and chloride ions from the waste acid is achieved by preparing calcium fluoride and calcium chloride (such as patent CN111634934A). However, this also neutralizes the sulfuric acid in the waste acid, thus making it impossible to achieve the resource recycling of sulfuric acid.

[0006] To this end, by using lanthanum chloride as a defluorination agent and sodium sulfate as a lanthanum removal agent, other impurity ions are not introduced while effectively removing fluorine ions in the waste acid; moreover, the lanthanum fluoride and sodium lanthanum sulfate obtained in the defluorination and lanthanum removal processes can be converted by alkali dissolution and regenerated by acid dissolution to obtain lanthanum chloride, thereby realizing the regeneration and recycling of the defluorination agent, and the sodium sulfate obtained by alkali dissolution can also be recycled as a lanthanum removal agent; and silver sulfate is selected as a chlorine removal agent, and after removing chlorine ions in the waste acid, high-value nano silver powder can be prepared to realize the high-value recovery of the chlorine removal agent (such as patent CN114890526B). However, due to the introduction of the defluorination agent and the chlorine removal agent, the alkali dissolution conversion and acid dissolution regeneration processes are lengthy and complex to operate, and are not suitable for handling a large amount of waste acid wastewater.

[0007] According to the deficiencies in the prior art for removing fluorine and chlorine from waste acid, the inventors propose a concept: whether the molecular sieve membrane separation technology can be used to treat waste acid containing fluorine and chlorine. However, since waste acid is a complex acidic liquid with multiple components and high corrosiveness, the traditional molecular sieve membrane is not suitable for the waste acid containing fluorine and chlorine. For example, the silicon-aluminum compound molecular sieve membrane, the silicon-aluminum skeleton of which is easily damaged by the waste acid containing fluorine and chlorine and basically loses the molecular sieve function, becoming a one-time chemical precipitation consumable that cannot be regenerated and recycled, resulting in not only high cost but also a far lower defluorination rate than expected, and even adding new silicon-aluminum impurities to the waste acid and seriously interfering with the determination of fluorine.

[0008] Therefore, it is of great practical significance to study a molecular sieve membrane that can improve the removal rate of fluorine and chlorine ions in waste acid and reduce the loss rate of sulfate ions, and also provides a new idea for removing fluorine and chlorine ions from various waste acids. SUMMARY

[0009] In view of the deficiencies in the prior art, the present application provides a zeolite-like molecular sieve membrane for removing fluorine and chlorine from waste acid, which has high defluorination and dechlorination rates, high sulfate retention rate, good corrosion resistance, and high water flux, and a preparation method of the zeolite-like molecular sieve membrane for removing fluorine and chlorine from waste acid.

[0010] The zeolite-like molecular sieve membrane for removing fluorine and chlorine from waste acid is realized as follows: a dispersion phase and a continuous phase, the dispersion phase is zeolite-like molecular sieve nanoparticles, and the continuous phase is formed after the PVDF monomer particles are dissolved; the zeolite-like molecular sieve nanoparticles are prepared by dissolving a metal compound in a first solvent, then dissolving it in a second solvent to form a mixed solution I, and finally drying the mixed solution I.

[0011] The preparation principle of the zeolite-like molecular sieve nanoparticles is to react a specific metal compound with a specific solvent to obtain porous zeolite-like molecular sieve nanoparticles with metal ions as the center and organic matter as the ligand.

[0012] Further, the metal compound is ZrCl4 or Zr(OH)4, the first solvent is trimesic acid or diphenic acid, and the second solvent is deionized water solution of one or any combination of ethanol, isopentyl glycol and isohexyl glycol.

[0013] Further, the metal compound is any one of ZnSO4, ZnCl2, CoCl2, Co(NO3)2, CuSO4 and Cu(NO3)2, the first solvent is one or any combination of imidazole, dimethyl imidazole and 2-methyl imidazole, and the second solvent is deionized water solution of one or any combination of ethanol, isopentyl glycol and isohexyl glycol.

[0014] Further, the metal compound is ZrCl4, Zr(OH)4 or Cr2(SO4)3, the first solvent is succinic acid, and the second solvent is deionized water solution of one or any combination of ethanol, isopentyl glycol and isohexyl glycol.

[0015] Further, the mass fraction of the metal compound in the mixed solution is 2wt%-20wt%.

[0016] Further, the space structure of the zeolite-like molecular sieve nanoparticles is cage-shaped, and the interspace formed between the cages becomes a channel with a pore size of 1-6nm for ions to pass through.

[0017] The preparation method of the zeolite-like molecular sieve membrane for removing fluorine and chlorine from waste acid is achieved as follows: including a blending method for preparing a PVDF molecular sieve blended membrane and a surface coating modification step of the PVDF molecular sieve blended membrane, and the specific content is as follows:

[0018] A. Blending method for preparing a PVDF molecular sieve blended membrane: PVDF monomer particles are dissolved in an organic solvent DMF with a concentration of 8wt%-12wt%, then the zeolite-like molecular sieve nanoparticles in the above technical solution are added, and then monomer polymerization initiators VBC and BPO are added to prepare a mixed solvent II; then the mixed solvent II is reacted at 60-70℃ under nitrogen protection for 7-9h to prepare a casting solution; then the casting solution is vacuum degassed and left for 7-9h, then the casting solution is injected into a casting mold and scraped into a film by a scraper, then the scraped film is evaporated to remove the organic solvent, and finally the scraped film is immersed in a deionized water coagulation bath for solidification to prepare a PVDF molecular sieve blended membrane;

[0019] B. Surface coating modification of the PVDF molecular sieve blended membrane: the foregoing PVDF molecular sieve blended membrane is sequentially immersed in ethanol and deionized water for 20-40min each, then immersed in a dopamine hydrochloride buffer solution and stirred in an ultrasonic environment for 16-24h to prepare a zeolite-like molecular sieve membrane for removing fluorine and chlorine from waste acid.

[0020] Further, in the mixed solvent II of the A step, the mass fraction of PVDF is 6wt%-12wt%, the mass fraction of the zeolite-like molecular sieve nanoparticles is 0.5wt%-2wt%, the mass fraction of VBC is 2wt%, and the mass fraction of BPO is 0.02wt%.

[0021] Further, the preparation method of the dopamine hydrochloric acid buffer in the B step is as follows: first, trimethylolamine and hydrochloric acid are prepared into a buffer solution with pH 8.5, and then DA and PEI are dissolved in the buffer solution to prepare the dopamine hydrochloric acid buffer.

[0022] Further, the mass fraction of DA in the dopamine hydrochloric acid buffer is 0.2wt%-1.6wt%.

[0023] The preparation principle of the PVDF molecular sieve blend membrane is as follows: the polymer and the zeolite-like molecular sieve nanoparticles are blended into a membrane, the polymer becomes a continuous phase, the zeolite-like molecular sieve nanoparticles become a dispersed phase, and all the zeolite-like molecular sieve nanoparticles form a zeolite-like molecular sieve, so that the problem of poor compatibility between the conventional molecular sieve and the organic polymer matrix can be solved, and the influence of the membrane surface clusters and interface voids on separation is avoided; moreover, the space structure of the zeolite-like molecular sieve nanoparticles is mostly cage-shaped, different voids can be formed between the molecular cages, the voids become channels for ions to pass through, and a selective screening effect is achieved. In the formed blend membrane, the polymer is a continuous phase, the effective pore size is equivalent to that of a conventional nanofiltration membrane, the voids between the molecular cages in the dispersed phase are less than 6nm (preferably 1-6nm), and the fluorine and chlorine ions can pass through.

[0024] The modification principle of the surface coating of the PVDF molecular sieve blend membrane is as follows: the PVDF material is corrosion-resistant, however, the existence of C-F bonds leads to strong hydrophobicity of the PVDF material, which leads to easy pollution and low flux of the membrane material. Although the hydrophobicity of the PVDF molecular sieve blend membrane has been reduced to a certain extent, the water production flux of the continuous phase PVDF is still low, so the continuous phase needs to be modified to improve the hydrophilicity.

[0025] The beneficial effects of the application are as follows:

[0026] 1. The zeolite-like molecular sieve nanoparticles prepared in the ultrasonic environment can be applied to the high-corrosive fluorine and chlorine-containing waste acid system, and the zeolite-like molecular sieve nanoparticles are blended with the polymer into a membrane and the surface is coated with a hydrophilic modification through a blending method, so that the fluorine and chlorine ions in the waste acid can pass through and the sulfate ions are intercepted, so that the efficient removal of the fluorine and chlorine ions in the waste acid can be realized without introducing new impurity elements and consuming heat, and the loss rate of the sulfate ions can be significantly reduced, and the recycling of the sulfuric acid resource is realized.

[0027] 2. Compared with conventional nanofiltration membranes and PVDF flat-sheet ultrafiltration membranes, the zeolite-like molecular sieve membrane of this application can not only effectively remove fluoride and chlorine from waste acid, but also reduce the loss rate of sulfate ions in waste acid, realize the recycling of sulfuric acid resources, and provide a new process idea for the removal of fluoride and chlorine ions from various wastewaters.

[0028] 3. In the zeolite-like molecular sieve membrane preparation method of this application, the PVDF molecular sieve blend membrane is hydrophilically modified by pre-preparing dopamine hydrochloride buffer, so that the PVDF molecular sieve blend membrane has corrosion resistance while effectively reducing its hydrophobicity, thereby increasing the water flux when filtering waste acid.

[0029] In summary, this application features high fluoride and chlorine removal rate, high sulfate retention rate, good corrosion resistance, and high water production throughput. Attached Figure Description

[0030] Figure 1 is a partial magnified view of the zeolite-like molecular sieve membrane prepared in Example 1 of this application. Detailed Implementation

[0031] To make the objectives, technical solutions, and advantages of this application clearer, the following detailed description is provided in conjunction with the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are for illustrative purposes only and are not intended to limit the scope of this application.

[0032] This application discloses a zeolite-like molecular sieve membrane for the removal of fluorine and chlorine from waste acid, comprising a dispersed phase and a continuous phase. The dispersed phase consists of zeolite-like molecular sieve nanoparticles, and the continuous phase is formed by dissolving PVDF monomer particles. The zeolite-like molecular sieve nanoparticles are prepared by dissolving a metal compound in a first solvent under ultrasonic conditions, then dissolving it in a second solvent to form a mixed solution I, and finally drying the mixed solution I.

[0033] The metal compound is ZrCl4 or Zr(OH)4, the first solvent is pyromellitic acid or biphenyl dicarboxylic acid, and the second solvent is a deionized aqueous solution of one or any combination of ethanol, isopentylene glycol and isohexyl glycol.

[0034] The metal compound is any one of ZnSO4, ZnCl2, CoCl2, Co(NO3)2, CuSO4, and Cu(NO3)2, the first solvent is one or any combination of imidazole, dimethylimidazolium, and 2-methylimidazolium, and the second solvent is a deionized aqueous solution of one or any combination of ethanol, isopentylene glycol, and isohexylene glycol.

[0035] The metal compound is ZrCl4, Zr(OH)4 or Cr2(SO4)3, the first solvent is succinic acid, and the second solvent is a deionized aqueous solution of one or any combination of ethanol, isopentylene glycol and isohexylene glycol.

[0036] The mass fraction of the metal compound in the mixed solution is 2 wt% to 20 wt%.

[0037] The spatial structure of the zeolite-like molecular sieve nanoparticles is cage-like, and the gaps formed between the molecular cages become channels through which ions can pass with a pore size of 1 to 6 nm.

[0038] The mixed solution I was prepared by conventional low-temperature drying to obtain zeolite-like molecular sieve nanoparticles.

[0039] This application describes a method for preparing a zeolite-like molecular sieve membrane for the removal of fluoride and chlorine from waste acid, including a blending method for preparing a PVDF molecular sieve blend membrane and a surface coating modification step for the PVDF molecular sieve blend membrane. The specific details are as follows:

[0040] A. Preparation of PVDF molecular sieve blend membrane by blending method: PVDF (polyvinylidene fluoride) monomer particles are dissolved in an organic solvent DMF (dimethylformamide) with a concentration of 8wt% to 12wt%. Then, zeolite-like molecular sieve nanoparticles as described in the above technical solution are added, followed by monomer polymerization initiators VBC (p-chloromethylstyrene) and BPO (benzoyl peroxide) to obtain mixed solvent II. Subsequently, mixed solvent II is reacted at 60-70℃ for 7-9 hours under nitrogen protection to obtain casting solution. Then, the casting solution is degassed under vacuum and allowed to stand for 7-9 hours. The casting solution is then injected into a casting template and scraped into a membrane with a doctor blade. The scraped membrane is then evaporated to remove the organic solvent. Finally, the scraped membrane is immersed in a deionized water coagulation bath for curing to obtain PVDF molecular sieve blend membrane.

[0041] B. Surface modification of PVDF molecular sieve blend membrane: The aforementioned PVDF molecular sieve blend membrane is immersed in ethanol and deionized water for 20-40 min each, and then immersed in dopamine hydrochloride buffer solution and stirred under ultrasonic conditions for 16-24 h to obtain a zeolite-like molecular sieve membrane for the removal of fluorine and chlorine from polluted acid.

[0042] In the mixed solvent II of step A, the mass fraction of PVDF is 6wt% to 12wt%, the mass fraction of zeolite-like molecular sieve nanoparticles is 0.5wt% to 2wt%, the mass fraction of VBC is 2wt%, and the mass fraction of BPO is 0.02wt%.

[0043] The preparation method of dopamine hydrochloride buffer in step B is as follows: first, tris(hydroxymethyl)aminomethane and hydrochloric acid are prepared into a buffer solution with pH 8.5, and then DA (dopamine) and PEI (polyethyleneimine) are dissolved in the buffer solution to prepare dopamine hydrochloride buffer solution.

[0044] The mass fraction of DA in the dopamine hydrochloride buffer is 0.2 wt% to 1.6 wt%.

[0045] In step B, the sample is immersed in dopamine hydrochloride buffer solution and stirred conventionally under ultrasonic conditions for 16–24 hours.

[0046] Example 1

[0047] S100: Under ultrasonic conditions, based on equal mass, 6 parts of ZrCl4 are first dissolved in 44 parts of biphenyl dicarboxylic acid. Then, the obtained solution is dissolved in a second solvent of 30 parts of deionized water, 10 parts of ethanol, and 10 parts of isohexyl glycol to form mixed solution I (where the mass fraction of ZrCl4 is 6 wt%). Finally, mixed solution I is dried to obtain zeolite-like molecular sieve nanoparticles.

[0048] S200: PVDF (polyvinylidene fluoride) monomer particles are dissolved in 10% DMF (dimethylformamide) organic solvent, then the previously prepared zeolite-like molecular sieve nanoparticles are added, followed by VBC (p-chloromethylstyrene) and BPO (benzoyl peroxide) to prepare mixed solvent II, wherein the mass fraction of PVDF monomer particles is 6%, the mass fraction of zeolite-like molecular sieve nanoparticles is 0.5%, the mass fraction of VBC is 2%, and the mass fraction of BPO is 0.02%. Subsequently, mixed solvent II is reacted at 65°C for 8 hours under nitrogen protection to obtain casting solution. The casting solution is degassed under vacuum and allowed to stand for 8 hours, then the casting solution is injected into a casting template and scraped into a film using a doctor blade. The scraped film is then evaporated to remove the organic solvent, and finally the scraped film is immersed in a deionized water coagulation bath for curing to obtain a PVDF molecular sieve blend membrane.

[0049] S300: First, prepare a buffer solution with pH 8.5 by mixing tris(hydroxymethyl)aminomethane and hydrochloric acid. Then, dissolve DA (dopamine) and PEI (polyethyleneimine) in the buffer solution to obtain a dopamine hydrochloride buffer solution with a mass concentration of 0.2%. Then, place the PVDF molecular sieve blend membrane obtained above in ethanol and deionized water for 30 min each, and then place it in the dopamine hydrochloride buffer solution and stir it under ultrasonic conditions for 20 h to obtain a dopamine modified molecular sieve membrane, which is a zeolite-like molecular sieve membrane for the removal of fluorine and chlorine from waste acid.

[0050] In this embodiment, a fluoride and chlorine removal test was conducted on the prepared zeolite-like molecular sieve membrane. The purpose of the test was to study the volume of fresh water and concentrated water obtained after fluoride and chlorine removal, as well as the concentrations of fluoride and chloride ions and sulfate ions in the fresh water and concentrated water, and then to obtain the fluoride removal rate, chlorine removal rate, and sulfate retention rate of the molecular sieve membrane.

[0051] Different batches of waste acid were sampled and tested, then mixed for fluoride and chlorine removal tests. The main indicators are shown in Table 1.

[0052] Table 1. Detection results of different batches of waste acid samples

[0053] The average value in Table 1 was used as the final sampling data of the waste acid. The batch of mixed waste acid was subjected to two consecutive waste acid fluoride and chlorine removal tests using the prepared zeolite-like molecular sieve membrane. The test results are shown in Table 2.

[0054] Table 2 Results of two consecutive acid and fluoride removal tests

[0055] Primary fluoride and chlorine removal: 10L of mixed waste acid is removed using a zeolite-like molecular sieve membrane to obtain primary concentrate and primary desalination water; Secondary fluoride and chlorine removal: 10L of the obtained primary concentrate is subjected to secondary fluoride and chlorine removal to obtain secondary concentrate and secondary desalination water.

[0056] After processing the data in Table 2, the defluorination rate in the first-stage defluorination and chlorination process reached 65.87%, and the chlorination rate was 72.63%. The defluorination rate in the second-stage defluorination and chlorination process reached 40.63%, and the chlorination rate was 78.36%. After two removal processes, the combined defluorination rate of the first and second stages reached 79.96%, the chlorination rate reached 94.08%, and the sulfate retention rate reached 79.14%. In this embodiment, the concentrated water is considered as defluorinated and chlorinated water, and the recovery rate of the defluorinated and chlorinated water reaches 49%. The desalinated water is high-fluoride and chlorine waste liquid and is used for other purposes.

[0057] Example 2

[0058] S100: Under ultrasonic conditions, based on equal mass per part, first dissolve 5 parts of Zr(OH)4 in 40 parts of trimesic acid, then dissolve the aforementioned solution in a second solvent of 25 parts of deionized water, 15 parts of ethanol, 10 parts of isopentyl glycol and 5 parts of isohexyl glycol to form mixed solution I (where the mass fraction of Zr(OH)4 is 5wt%), and finally dry mixed solution I to obtain zeolite-like molecular sieve nanoparticles.

[0059] S200: The content is the same as the S200 step in Example 1, except that: PVDF monomer particles are dissolved in organic solvent DMF with a mass concentration of 8%, and the mass fraction of PVDF in the mixed solvent II is 12% and the mass fraction of zeolite nanoparticles is 2%; the mixed solvent II is reacted at 70°C for 7 hours under nitrogen protection to obtain casting solution; then the casting solution is degassed under vacuum and allowed to stand for 9 hours.

[0060] S300: The content is the same as the S300 step in Example 1, except that: a dopamine hydrochloride buffer with a mass concentration of 1.6% is prepared, and the PVDF molecular sieve blend membrane is immersed in ethanol and deionized water for 20 min each, and then immersed in dopamine hydrochloride buffer and stirred under ultrasonic conditions for 24 h, finally obtaining a zeolite-like molecular sieve membrane for the removal of fluorine and chlorine from polluted acid.

[0061] The mixed acid waste treated in this embodiment is the same as in Example 1. Two consecutive fluoride and chlorine removal tests were conducted using the prepared zeolite-like molecular sieve membrane. The test results are shown in Table 3.

[0062] Table 3. Results of two consecutive acid and fluoride removal tests.

[0063] After processing the data in Table 3, the defluorination rate in the first-stage defluorination and chlorination process reached 43.64%, and the chlorination rate was 63.33%. The defluorination rate in the second-stage defluorination and chlorination process reached 36.47%, and the chlorination rate was 65.32%. After two removal processes, the combined defluorination rate (first and second stages) reached 58.46%, the chlorination rate reached 85.24%, and the sulfate retention rate reached 90.95%. In this embodiment, the concentrated wastewater was considered the defluorinated and chlorinated water, and the recovery rate of the defluorinated and chlorinated water reached 52.57%.

[0064] Example 3

[0065] S100: Under ultrasonic conditions, based on the same mass per part, 14 parts of ZnSO4 are first dissolved in 30 parts of dimethylimidazole, and then the aforementioned solution is dissolved in a second solvent of 50 parts of deionized water and 6 parts of ethanol to form mixed solution I (where the mass fraction of ZnSO4 is 14wt%). Finally, mixed solution I is dried to obtain zeolite-like molecular sieve nanoparticles.

[0066] S200: The content is the same as the S200 step in Example 1, except that: PVDF monomer particles are dissolved in organic solvent DMF with a mass concentration of 12%, and the mass fraction of PVDF in the mixed solvent II is 8%, and the mass fraction of zeolite nanoparticles is 1%; the mixed solvent II is reacted at 60°C for 8 hours under nitrogen protection to obtain casting solution; then the casting solution is vacuum degassed and allowed to stand for 8 hours.

[0067] S300: The content is the same as the S300 step in Example 1, except that a dopamine hydrochloride buffer with a mass concentration of 1.0% is prepared, and the PVDF molecular sieve blend membrane is immersed in ethanol and deionized water for 40 min each, and then immersed in dopamine hydrochloride buffer and stirred under ultrasonic conditions for 16 h, finally obtaining a zeolite-like molecular sieve membrane for the removal of fluorine and chlorine from polluted acid.

[0068] The mixed acid waste treated in this embodiment is the same as that in Example 1. Two consecutive fluoride and chlorine removal tests were conducted using the prepared zeolite-like molecular sieve membrane. The test results are shown in Table 4.

[0069] Table 4. Results of two consecutive acid and fluoride removal tests.

[0070] After processing the data in Table 4, the defluorination rate in the first-stage defluorination and chlorination process reached 54.88%, and the chlorination rate was 70.62%. The defluorination rate in the second-stage defluorination and chlorination process reached 51.45%, and the chlorination rate was 68.32%. After two removal processes, the combined defluorination rate of the first and second stages reached 78.09%, the chlorination rate reached 90.69%, and the sulfate retention rate reached 78%. In this embodiment, the concentrated water is considered as defluorinated and chlorinated water, and the recovery rate of the defluorinated and chlorinated water reaches 53.15%.

[0071] Example 4

[0072] S100: Under ultrasonic conditions, based on equal mass, 4 parts CuSO4 are first dissolved in 35 parts imidazole. Then, the aforementioned solution is dissolved in a second solvent consisting of 25 parts deionized water, 15 parts ethanol, 11 parts isopentyl glycol, and 10 parts isohexyl glycol to form mixed solution I (where the mass fraction of CuSO4 is 4 wt%). Finally, mixed solution I is dried to obtain zeolite-like molecular sieve nanoparticles.

[0073] S200: The content is the same as the S200 step in Example 1, except that the mass fraction of PVDF in the prepared mixed solvent II is 6% and the mass fraction of zeolite nanoparticles is 2%; the mixed solvent II is reacted at 60°C for 8 hours under nitrogen protection to obtain the casting solution; then the casting solution is degassed under vacuum and allowed to stand for 9 hours.

[0074] S300: The content is the same as the S300 step in Example 1, except that a dopamine hydrochloride buffer with a mass concentration of 1.2% is prepared, and the PVDF molecular sieve blend membrane is immersed in ethanol for 20 min and deionized water for 40 min in sequence, and then immersed in dopamine hydrochloride buffer and stirred under ultrasonic conditions for 22 h to finally obtain a zeolite-like molecular sieve membrane for the removal of fluorine and chlorine from polluted acid.

[0075] The mixed acid waste treated in this embodiment is the same as that in Example 1. Two consecutive fluoride and chlorine removal tests were conducted using the prepared zeolite-like molecular sieve membrane. The test results are shown in Table 5.

[0076] Table 5 Results of two consecutive acid and fluoride removal tests

[0077] After processing the data in Table 5, the defluorination rate in the first-stage defluorination and chlorination process reached 57.17%, and the chlorination rate was 71.43%. The defluorination rate in the second-stage defluorination and chlorination process reached 53.64%, and the chlorination rate was 67.49%. After two removal processes, the combined defluorination rate of the first and second stages reached 80.14%, the chlorination rate reached 92.88%, and the sulfate retention rate reached 68.90%. In this embodiment, the concentrated water is considered as defluorinated and chlorinated water, and the recovery rate of the defluorinated and chlorinated water reaches 45.09%.

[0078] Example 5

[0079] S100: Under ultrasonic conditions, based on equal mass per part, first dissolve 8 parts of Cr2(SO4)3 in 51 parts of succinic acid, then dissolve the aforementioned solution in a second solvent of 30 parts of deionized water and 11 parts of ethanol to form mixed solution I (where the mass fraction of Cr2(SO4)3 is 8 wt%), and finally dry mixed solution I to obtain zeolite-like molecular sieve nanoparticles.

[0080] S200: The content of step S200 in Example 1 is the same, except that the mass fraction of PVDF in the prepared mixed solvent II is 12% and the mass fraction of zeolite nanoparticles is 0.5%; the mixed solvent II is reacted at 70°C for 6 hours under nitrogen protection to obtain the casting solution; then the casting solution is degassed under vacuum and allowed to stand for 8 hours.

[0081] S300: The content is the same as the S300 step in Example 1, except that a dopamine hydrochloride buffer with a mass concentration of 0.8% is prepared, and the PVDF molecular sieve blend membrane is immersed in ethanol for 30 min and deionized water for 20 min in sequence, and then immersed in dopamine hydrochloride buffer and stirred under ultrasonic conditions for 19 h, finally obtaining a zeolite-like molecular sieve membrane for the removal of fluorine and chlorine from polluted acid.

[0082] The mixed acid waste treated in this embodiment is the same as that in Example 1. Two consecutive fluoride and chlorine removal tests were conducted using the prepared zeolite-like molecular sieve membrane. The test results are shown in Table 6.

[0083] Table 6 Results of two consecutive acid and fluoride removal tests.

[0084] After processing the data in Table 6, the defluorination rate in the first-stage defluorination and chlorination process reached 60.39%, and the chlorination rate was 68.44%. The defluorination rate in the second-stage defluorination and chlorination process reached 53.27%, and the chlorination rate was 62.16%. After two removal processes, the combined defluorination rate of the first and second stages reached 81.45%, the chlorination rate reached 88.06%, and the sulfate retention rate reached 70.62%. In this embodiment, the concentrated water is considered as defluorinated and chlorinated water, and the recovery rate of the defluorinated and chlorinated water reaches 51.77%.

[0085] Comparative Example 1

[0086] The experimental steps were exactly the same as in Example 1, and the batches of treated waste acid were the same, except that the S300 step was not performed. The effect of surface coating modification of the blended membrane on the fluoride and chlorine removal rate and sulfate retention rate of the prepared molecular sieve membrane was studied. The experimental results are shown in Table 7.

[0087] Table 7 Results of the fluoride and chlorine removal test of Comparative Example 1

[0088] After processing the data in Table 7, it can be seen that the defluorination rate of the uncoated molecular sieve membrane obtained by blending the membrane reached 62.63%, the dechlorination rate reached 70.66%, and the sulfate retention rate reached 88.66%. In this comparative example, the concentrate was the product water, and the permeate flux through the membrane reached 24.35%.

[0089] In Example 1, the zeolite-like molecular sieve membrane achieved a comprehensive defluorination rate of 79.96%, a dechlorination rate of 94.08%, a sulfate retention rate of 79.14%, and a defluorinated chlorine water recovery rate of 49%.

[0090] Comparing Example 1 with Comparative Example 1, it can be concluded that the coating modification of the blended membrane can improve the removal rate of fluoride and chloride ions, significantly improve the recovery rate of defluorinated chlorine water, and make the PVDF molecular sieve blended membrane corrosion resistant while reducing its hydrophobicity and increasing the water production flux.

[0091] Comparative Example 2

[0092] Similar to the batch of waste acid treated in Example 1, this comparative example uses a PVDF flat sheet ultrafiltration membrane to conduct a fluoride and chlorine removal test on the waste acid. The purpose of the test is to study the concentrations of fluoride and chlorine ions and sulfate ions in the fresh water and concentrated water after fluoride and chlorine removal, and then to obtain the fluoride removal rate, chlorine removal rate and sulfate retention rate of the ultrafiltration membrane. The test results are compared with those of Example 1 and are shown in Table 8.

[0093] Table 8 Results of the acid and fluoride removal test in Comparative Example 2

[0094] After processing the data in Table 8, it can be seen that the defluorination rate of the ultrafiltration membrane is 0.4%, the dechlorination rate is 0.2%, and the sulfate retention rate reaches 0%.

[0095] Conclusion: Ultrafiltration membranes are not suitable for removing fluoride and chloride ions from polluted acid and for retaining sulfate ions.

[0096] Comparative Example 3

[0097] Similar to the batch of waste acid treated in Example 1, this comparative example uses a completely unmodified conventional nanofiltration membrane to conduct a fluoride and chlorine removal test on the waste acid. The purpose of the test is to study the concentrations of fluoride and chlorine ions and sulfate ions in the fresh water and concentrated water after fluoride and chlorine removal, and then to obtain the fluoride removal rate, chlorine removal rate and sulfate retention rate of the conventional nanofiltration membrane. The test results are compared with those of Example 1 and are shown in Table 9.

[0098] Table 9 Results of the fluoride and chlorine removal test of Comparative Example 3

[0099] After processing the data in Table 9, it can be seen that the defluorination rate of conventional nanofiltration membrane is 3.19%, the dechlorination rate is 9.07%, and the sulfate retention rate reaches 99.25%.

[0100] Conclusion: Compared with Example 1, conventional nanofiltration membranes have a lower removal rate of chloride ions, and both chloride ions and sulfate ions are difficult to separate from the pores of conventional nanofiltration membranes.

[0101] The above description is merely a preferred embodiment of this application, but the scope of protection of this application is not limited thereto. Any variations or substitutions that can be easily conceived by those skilled in the art within the scope of the technology disclosed in this application should be included within the scope of protection of this application. Therefore, the scope of protection of this application should be determined by the scope of the claims.

Claims

1. A zeolite-like molecular sieve membrane for the removal of fluoride and chlorine from waste acid, characterized in that: The zeolite-like molecular sieve membrane for removing fluoride and chlorine from waste acid is a surface-modified PVDF molecular sieve blend membrane. The hydrophilic modification is dopamine modification. The PVDF molecular sieve blend membrane includes a dispersed phase and a continuous phase. The dispersed phase is zeolite-like molecular sieve nanoparticles, which are porous zeolite-like molecular sieve nanoparticles with metal ions as centers and organic matter as ligands. The continuous phase is a polymer, which is polyvinylidene fluoride.

2. The zeolite-like molecular sieve membrane for fluoride and chlorine removal from waste acid according to claim 1, characterized in that: The spatial structure of the zeolite-like molecular sieve nanoparticles is cage-like, and the gaps formed between the molecular cages become channels through which ions can pass with a pore size of 1 to 6 nm.

3. The zeolite-like molecular sieve membrane for fluoride and chlorine removal from waste acid according to claim 1, characterized in that: The metal ions are provided by a metal compound, which is ZrCl4, Zr(OH)4, ZnSO4, ZnCl2, CoCl2, Co(NO3)2, CuSO4, Cu(NO3)2 or Cr2(SO4)3.

4. A zeolite-like molecular sieve membrane for the removal of fluoride and chlorine from waste acid, characterized in that: It includes a dispersed phase and a continuous phase. The dispersed phase is zeolite-like molecular sieve nanoparticles, and the continuous phase is formed by dissolving PVDF monomer particles and then polymerizing them. The zeolite-like molecular sieve nanoparticles are prepared by dissolving a metal compound in a first solvent under ultrasonic conditions, then dissolving it in a second solvent to form a mixed solution I, and finally drying the mixed solution I.

5. The zeolite-like molecular sieve membrane for fluoride and chlorine removal from waste acid according to claim 4, characterized in that: The metal compound is ZrCl4 or Zr(OH)4, the first solvent is pyromellitic acid or biphenyl dicarboxylic acid, and the second solvent is a deionized aqueous solution of one or any combination of ethanol, isopentylene glycol and isohexyl glycol.

6. The zeolite-like molecular sieve membrane for fluoride and chlorine removal from waste acid according to claim 4, characterized in that: The metal compound is any one of ZnSO4, ZnCl2, CoCl2, Co(NO3)2, CuSO4, and Cu(NO3)2, the first solvent is one or any combination of imidazole, dimethylimidazolium, and 2-methylimidazolium, and the second solvent is a deionized aqueous solution of one or any combination of ethanol, isopentylene glycol, and isohexylene glycol.

7. The zeolite-like molecular sieve membrane for fluoride and chlorine removal from waste acid according to claim 4, characterized in that: The metal compound is ZrCl4, Zr(OH)4 or Cr2(SO4)3, the first solvent is succinic acid, and the second solvent is a deionized aqueous solution of one or any combination of ethanol, isopentylene glycol and isohexylene glycol.

8. The zeolite-like molecular sieve membrane for fluoride and chlorine removal from waste acid according to any one of claims 4 to 7, characterized in that: The mass fraction of the metal compound in the mixed solution I is 2 wt% to 20 wt%.

9. The zeolite-like molecular sieve membrane for fluoride and chlorine removal from waste acid according to claim 8, characterized in that: The spatial structure of the zeolite-like molecular sieve nanoparticles is cage-like, and the gaps formed between the molecular cages become channels through which ions can pass with a pore size of 1 to 6 nm.

10. The method for preparing the zeolite-like molecular sieve membrane for fluoride and chlorine removal from waste acid according to any one of claims 1 to 9, characterized in that: The process includes steps such as preparing PVDF molecular sieve blend membranes via blending and surface coating modification of the PVDF molecular sieve blend membranes. Specific details are as follows: A. Preparation of PVDF molecular sieve blend membrane by blending method: PVDF monomer particles are dissolved in dimethylformamide, an organic solvent with a concentration of 8wt% to 12wt%, and then zeolite-like molecular sieve nanoparticles are added. Next, the monomer polymerization initiator p-chloromethylstyrene and benzoyl peroxide are added to obtain mixed solvent II. Then, mixed solvent II is reacted at 60–70°C for 7–9 h under nitrogen protection to obtain a casting solution. The casting solution is then degassed under vacuum and allowed to stand for 7–9 h. The degassed and stood casting solution is then injected into a casting template and scraped into a membrane using a doctor blade. The scraped membrane is then evaporated to remove the organic solvent. Finally, the scraped membrane, after removing the organic solvent, is immersed in a deionized water coagulation bath for curing to obtain the PVDF molecular sieve blend membrane. B. Surface coating modification of PVDF molecular sieve blend membrane: The PVDF molecular sieve blend membrane is immersed in ethanol and deionized water for 20-40 min each, and then immersed in dopamine hydrochloride buffer solution and stirred under ultrasonic conditions for 16-24 h to obtain a zeolite-like molecular sieve membrane for the removal of fluoride and chlorine from waste acid.

11. The method for preparing the zeolite-like molecular sieve membrane for fluoride and chlorine removal from waste acid according to claim 10, characterized in that: In the mixed solvent II, the mass fraction of PVDF monomer particles is 6wt% to 12wt%, the mass fraction of zeolite-like molecular sieve nanoparticles is 0.5wt% to 2wt%, the mass fraction of p-chloromethylstyrene is 2wt%, and the mass fraction of benzoyl peroxide is 0.02wt%.

12. The method for preparing the zeolite-like molecular sieve membrane for fluoride and chlorine removal from waste acid according to claim 10, characterized in that: The preparation method of the dopamine hydrochloride buffer is as follows: first, tris(hydroxymethyl)aminomethane and hydrochloric acid are prepared into a buffer solution with a pH value of 8.5, and then dopamine and polyethyleneimine are dissolved in the buffer solution to prepare the dopamine hydrochloride buffer solution.

13. The method for preparing a zeolite-like molecular sieve membrane for the removal of fluoride and chlorine from waste acid according to claim 10 or 12, characterized in that: The dopamine hydrochloride buffer solution contains 0.2 wt% to 1.6 wt% dopamine.

14. The application of the zeolite-like molecular sieve membrane for fluoride and chlorine removal from waste acid as described in any one of claims 1 to 9 in the removal of fluoride and chlorine from waste acid.

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