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

Abstract

This invention belongs to the field of waste acid treatment technology, specifically disclosing a zeolite-like molecular sieve membrane for removing fluoride and chlorine from waste acid and its preparation method. The molecular sieve membrane comprises 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. 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, followed by drying. The preparation method includes a blending method to prepare a PVDF molecular sieve blend membrane, and a surface coating modification step for the PVDF molecular sieve blend membrane. This invention prepares a zeolite-like molecular sieve membrane for waste acid systems through a blending method, allowing fluoride and chlorine ions to permeate while retaining sulfate ions, thereby removing fluoride and chlorine and reducing the loss rate of sulfate ions, achieving the recycling of sulfuric acid resources. It features high fluoride and chlorine removal rate, high sulfate retention rate, good corrosion resistance, and high water flux.

Description

Technical Field

[0001] This invention 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, and its preparation method. The membrane has high fluoride and chlorine removal rate, high sulfate retention rate, good corrosion resistance, and high water flux. Background Technology

[0002] 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.

[0003] 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-plate 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.

[0004] 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 calcium fluoride and calcium chloride are prepared to remove fluoride and chloride ions from the waste acid (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.

[0005] To address this, other methods employ lanthanum chloride as a defluorinating agent and sodium sulfate as a lanthanum removal agent. These methods effectively remove fluoride ions from waste acid without introducing other impurity ions. Furthermore, the lanthanum fluoride and sodium lanthanum sulfate obtained from the defluorination and lanthanum removal processes can be converted to lanthanum chloride through alkaline dissolution and acid regeneration processes, achieving the regeneration and recycling of the defluorinating agent. The sodium sulfate obtained from alkaline dissolution can also be recycled as a lanthanum removal agent. Additionally, silver sulfate is used as a dechlorinating agent. After removing chloride ions from the waste acid, high-value nano-silver powder can be prepared, achieving high-value recovery of the dechlorinating agent (e.g., patent CN114890526B). However, because this method introduces defluorinating and dechlorinating agents, the alkaline dissolution and acid regeneration processes are lengthy and complex, making it unsuitable for treating large quantities of waste acid wastewater.

[0006] To address the shortcomings of existing technologies for removing fluoride and chlorine from waste acid, the inventors proposed a concept: could molecular sieve membrane separation technology be used to treat fluoride- and chlorine-containing waste acid? However, because waste acid is a complex, multi-component, and highly corrosive acidic liquid, traditional molecular sieve membranes are unsuitable for systems containing fluoride and chlorine. For example, silicon-aluminum compound molecular sieve membranes are easily destroyed by the fluoride- and chlorine-containing waste acid, essentially losing their molecular sieve function and becoming non-recyclable, disposable chemical precipitates. This results in not only high costs but also a fluoride removal rate far below expectations, and may even introduce new silicon-aluminum impurities into the waste acid and severely interfere with fluoride measurement.

[0007] Therefore, researching a molecular sieve membrane that can improve the removal rate of fluoride and chloride ions in waste acid and reduce the loss rate of sulfate ions is not only of great practical significance, but also provides a new approach for the removal of fluoride and chloride ions from various waste acids. Summary of the Invention

[0008] To address the shortcomings of existing technologies, this invention provides a zeolite-like molecular sieve membrane for removing 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. It also provides a method for preparing the zeolite-like molecular sieve membrane for removing fluoride and chlorine from waste acid.

[0009] The zeolite-like molecular sieve membrane for the removal of fluorine and chlorine from waste acid of the present invention is realized as follows: it includes a dispersed phase and a continuous phase, wherein the dispersed phase is zeolite-like molecular sieve nanoparticles, and the continuous phase is formed by dissolving PVDF 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, and finally drying the mixed solution.

[0010] The preparation principle of zeolite-like molecular sieve nanoparticles: reacting specific metal compounds with specific solvents to obtain porous zeolite-like molecular sieve nanoparticles with metal ions as the center and organic matter as ligands.

[0011] Further, 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.

[0012] 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, 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.

[0013] Further, 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.

[0014] Furthermore, the mass fraction of the metal compound in the mixed solution is 2wt% to 20wt%.

[0015] Furthermore, the spatial structure of the zeolite-like molecular sieve nanoparticles is cage-like, and the gaps formed between the cages become channels through which ions can pass with a pore size of 1 to 6 nm.

[0016] The method for preparing the zeolite-like molecular sieve membrane for the removal of fluoride and chlorine from waste acid according to this invention is as follows: it includes the steps of preparing a PVDF molecular sieve blend membrane by blending and surface coating modification of the PVDF molecular sieve blend membrane. The specific details are as follows:

[0017] A. Preparation of PVDF molecular sieve blend membrane by blending method: PVDF particles are dissolved in an organic solvent DMF with a concentration of 8wt% to 12wt%, and then the aforementioned zeolite-like molecular sieve membrane nanoparticles used for the removal of fluorine and chlorine from polluted acid are added. Then, monomer polymerization initiators VBC and BPO are added to obtain mixed solvent II. Subsequently, mixed solvent II is reacted at 60 to 70°C for 7 to 9 hours under nitrogen protection to obtain casting solution. Then, the casting solution is degassed under vacuum and allowed to stand for 7 to 9 hours. Then, the casting solution is injected into a casting template and a film is formed by scraping with a doctor blade. The scraped film is then evaporated to remove the organic solvent. Finally, the scraped film is immersed in a deionized water coagulation bath for curing to obtain PVDF molecular sieve blend membrane.

[0018] 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.

[0019] Furthermore, 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%.

[0020] Furthermore, 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 and PEI are dissolved in the buffer solution to prepare dopamine hydrochloride buffer solution.

[0021] Furthermore, the mass fraction of DA in the dopamine hydrochloride buffer is 0.2wt% to 1.6wt%.

[0022] The preparation principle of PVDF molecular sieve blend membranes is as follows: A polymer is blended with a zeolite-like molecular sieve to form a membrane. The polymer forms the continuous phase, while the zeolite-like molecular sieve nanoparticles form the dispersed phase. All the zeolite-like molecular sieve nanoparticles constitute the zeolite-like molecular sieve, thus solving the problem of poor compatibility between conventional molecular sieves and organic polymer matrices. This avoids the formation of clusters and interfacial voids on the membrane surface, which would affect separation. Furthermore, the spatial structure of zeolite-like molecular sieves is mostly cage-like, and different voids can form between the molecular cages, providing channels for ion passage and achieving selective sieving. In the resulting blend membrane, the polymer is the continuous phase, with an effective pore size equivalent to that of a conventional nanofiltration membrane. The voids between the molecular cages in the dispersed phase are <6 nm, allowing fluoride and chloride ions to pass through.

[0023] Principle of PVDF molecular sieve blend membrane surface coating modification: PVDF material is corrosion resistant, however, the presence of CF bonds makes PVDF material highly hydrophobic, which makes the membrane material susceptible to fouling and affects flux. Although the hydrophobicity of PVDF molecular sieve blend membrane has been reduced to a certain extent, the permeate flux is still low for continuous phase PVDF. Therefore, modification is required to improve the hydrophilicity of the continuous phase.

[0024] The beneficial effects of this invention are as follows:

[0025] 1. This invention prepares zeolite-like molecular sieve nanoparticles under ultrasonic conditions, making the newly prepared zeolite-like molecular sieve nanoparticles suitable for highly corrosive fluoride- and chlorine-containing waste acid systems. Furthermore, it cleverly uses a blending method to blend the zeolite-like molecular sieve nanoparticles with polymers to form a film and then coats and modifies the surface to obtain a zeolite-like molecular sieve membrane specifically for waste acid systems. This membrane allows fluoride and chloride ions in the waste acid to pass through while retaining sulfate ions. Thus, without introducing new impurity elements or consuming heat, it can achieve efficient removal of fluoride and chloride ions from waste acid and significantly reduce the loss rate of sulfate ions, thereby realizing the recycling of sulfuric acid resources.

[0026] 2. Compared with conventional nanofiltration membranes and PVDF flat-panel ultrafiltration membranes, the zeolite-like molecular sieve membrane of the present invention 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 chloride ions from various wastewaters.

[0027] 3. In the zeolite-like molecular sieve membrane preparation method of the present invention, 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.

[0028] In summary, the present invention has the characteristics of high fluoride and chlorine removal rate, high sulfate retention rate, good corrosion resistance, and high water production throughput. Attached Figure Description

[0029] Figure 1 This is a partial magnified view of the zeolite-like molecular sieve membrane prepared in Example 1 of the present invention. Detailed Implementation

[0030] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to 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 invention.

[0031] The present invention relates to 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 is zeolite-like molecular sieve nanoparticles, and the continuous phase is formed by dissolving PVDF 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, and finally drying the mixed solution.

[0032] 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.

[0033] 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.

[0034] 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 isohexyl glycol.

[0035] The mass fraction of the metal compound in the mixed solution is 2wt% to 20wt%.

[0036] The spatial structure of the zeolite-like molecular sieve nanoparticles is cage-like, and the gaps formed between the cages become channels through which ions can pass with a pore size of 1 to 6 nm.

[0037] The mixed solution was prepared by conventional low-temperature drying to obtain zeolite-like molecular sieve nanoparticles.

[0038] This invention relates to a method for preparing a zeolite-like molecular sieve membrane for the removal of fluoride and chlorine from waste acid, comprising the steps of preparing a PVDF molecular sieve blend membrane by blending and surface coating modification of the PVDF molecular sieve blend membrane. The specific details are as follows:

[0039] A. Preparation of PVDF molecular sieve blend membrane by blending method: PVDF (polyvinylidene fluoride) particles are dissolved in an organic solvent DMF (dimethylformamide) with a concentration of 8wt% to 12wt%. Then, the aforementioned zeolite-like molecular sieve membrane nanoparticles used for the removal of fluorine and chlorine from waste acid are added, followed by the monomer polymerization initiator VBC (p-chloromethylstyrene) and BPO (benzoyl peroxide) to obtain mixed solvent II. Subsequently, mixed solvent II is reacted at 60 to 70°C for 7 to 9 hours under nitrogen protection to obtain casting solution. Then, the casting solution is degassed under vacuum and allowed to stand for 7 to 9 hours. The 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 is immersed in a deionized water coagulation bath for curing to obtain PVDF molecular sieve blend membrane.

[0040] 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.

[0041] 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%.

[0042] 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.

[0043] The mass fraction of DA in the dopamine hydrochloride buffer is 0.2wt% to 1.6wt%.

[0044] In step B, the sample is immersed in dopamine hydrochloride buffer solution and stirred conventionally under ultrasonic conditions for 16–24 hours.

[0045] Example 1

[0046] S100: Under ultrasonic conditions, based on equal mass per part, 6 parts of ZrCl4 are first dissolved in 44 parts of biphenyl dicarboxylic acid. Then, the aforementioned 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.

[0047] S200: PVDF (polyvinylidene fluoride) particles are dissolved in DMF (dimethylformamide) organic solvent with a mass concentration of 10%, then the zeolite-like molecular sieve nanoparticles prepared above are added, followed by VBC (p-chloromethylstyrene) and BPO (benzoyl peroxide) to prepare mixed solvent II, wherein the mass fraction of PVDF is 6%, the mass fraction of zeolite-like nanoparticles is 0.5%, the mass fraction of VBC is 2%, and the mass fraction of BPO is 0.02%.

[0048] Subsequently, mixed solvent II was reacted at 65°C for 8 hours under nitrogen protection to obtain casting solution; the casting solution was degassed under vacuum and allowed to stand for 8 hours, then the casting solution was injected into the casting template and scraped into a film with a scraper, then the scraped film was evaporated to remove organic solvent, and finally the scraped film was immersed in a deionized water coagulation bath for curing to obtain 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%.

[0050] The PVDF molecular sieve blend membrane prepared above was then placed in ethanol and deionized water for 30 min each, and then placed in dopamine hydrochloride buffer solution and stirred under ultrasonic conditions for 20 h to prepare a dopamine modified molecular sieve membrane, which is a zeolite-like molecular sieve membrane for the removal of fluorine and chlorine from polluted acid.

[0051] 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.

[0052] 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.

[0053] Table 1. Detection results of different batches of waste acid samples

[0054]

[0055] 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.

[0056] Table 2 Results of two consecutive acid and fluoride removal tests

[0057]

[0058] 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. 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.

[0059] 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 considered as high-fluoride and chlorine waste liquid and is used for other purposes.

[0060] Example 2

[0061] 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.

[0062] S200: The content is the same as the S200 step in Example 1, except that: PVDF particles are dissolved in an 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 the casting solution; then the casting solution is vacuum degassed and allowed to stand for 9 hours.

[0063] 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.

[0064] 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.

[0065] Table 3. Results of two consecutive acid and fluoride removal tests.

[0066]

[0067] After processing the data in Table 3, the defluorination rate in the first-stage defluorination process reached 43.64%, and the dechlorination rate was 63.33%. The defluorination rate in the second-stage defluorination process reached 36.47%, and the dechlorination rate was 65.32%. After two removal processes, the combined defluorination rate (first and second stages) reached 58.46%, the dechlorination rate reached 85.24%, and the sulfate retention rate reached 90.95%. In this embodiment, the concentrated wastewater was considered the defluorinated water, and the recovery rate of the defluorinated water reached 52.57%.

[0068] Example 3

[0069] 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, the mixed solution I is dried to obtain zeolite-like molecular sieve nanoparticles.

[0070] S200: The content is the same as the S200 step in Example 1, except that: PVDF particles are dissolved in DMF organic solvent 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.

[0071] 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.

[0072] 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.

[0073] Table 4. Results of two consecutive acid and fluoride removal tests.

[0074]

[0075] 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%.

[0076] Example 4

[0077] 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.

[0078] 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 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.

[0079] 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 environment for 22 h, finally obtaining a zeolite-like molecular sieve membrane for the removal of fluorine and chlorine from polluted acid.

[0080] 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.

[0081] Table 5 Results of two consecutive acid and fluoride removal tests

[0082]

[0083] 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%.

[0084] Example 5

[0085] S100: Under ultrasonic conditions, based on the same 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 8wt%), and finally dry mixed solution I to obtain zeolite-like molecular sieve nanoparticles.

[0086] 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 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.

[0087] 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.

[0088] 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.

[0089] Table 6 Results of two consecutive acid and fluoride removal tests.

[0090]

[0091] 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 reached 51.77%.

[0092] Comparative Example 1

[0093] 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.

[0094] Table 7 Results of the fluoride and chlorine removal test of Comparative Example 1

[0095]

[0096] 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%.

[0097] 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%.

[0098] 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.

[0099] Comparative Example 2

[0100] 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.

[0101] Table 8 Results of the acid and fluoride removal test in Comparative Example 2

[0102]

[0103] 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%.

[0104] Conclusion: Ultrafiltration membranes are not suitable for removing fluoride and chloride ions from polluted acid and for retaining sulfate ions.

[0105] Comparative Example 3

[0106] 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.

[0107] Table 9 Results of the fluoride and chlorine removal test of Comparative Example 3

[0108]

[0109] 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%.

[0110] 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.

[0111] The above description is merely a preferred embodiment of the present invention, but the scope of protection of the present invention 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 the present invention should be included within the scope of protection of the present invention. Therefore, the scope of protection of the present invention should be determined by the scope of the claims.

Claims

1. A zeolite-like molecular sieve membrane for the removal of fluorine 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 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, and finally drying the mixed solution. The mass fraction of the metal compound in the mixed solution is 2wt% to 20wt%. The spatial structure of the zeolite-like molecular sieve nanoparticles is cage-like, and the gaps formed between the cages become channels through which ions can pass with a pore size of 1 to 6 nm.

2. The zeolite-like molecular sieve membrane for removing fluoride and chlorine from waste acid according to claim 1, 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.

3. The zeolite-like molecular sieve membrane for removing fluoride and chlorine from waste acid according to claim 1, 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.

4. The zeolite-like molecular sieve membrane for removing fluoride and chlorine from waste acid according to claim 1, 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.

5. A method for preparing a zeolite-like molecular sieve membrane for the removal of fluorine and chlorine from waste acid, 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 particles are dissolved in an organic solvent DMF with a concentration of 8wt% to 12wt%, and then zeolite molecular sieve nanoparticles for the removal of fluorine and chlorine from waste acid as described in any one of claims 1 to 4 are added. Then, monomer polymerization initiators VBC and BPO are added to obtain mixed solvent II. Subsequently, mixed solvent II is reacted at 60 to 70°C for 7 to 9 hours under nitrogen protection to obtain casting solution. Then, the casting solution is vacuum degassed and allowed to stand for 7 to 9 hours. Then, the casting solution is injected into a casting template and a film is formed by scraping with a doctor blade. The scraped film is then evaporated to remove the organic solvent. Finally, the scraped film is immersed in a deionized water coagulation bath for curing to obtain PVDF molecular sieve blend membrane. 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.

6. The method for preparing the zeolite-like molecular sieve membrane for the removal of fluorine and chlorine from waste acid according to claim 5, characterized in that: 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%.

7. The method for preparing the zeolite-like molecular sieve membrane for the removal of fluoride and chlorine from waste acid according to claim 5, characterized in that: 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 and PEI are dissolved in the buffer solution to prepare dopamine hydrochloride buffer solution.

8. The method for preparing the zeolite-like molecular sieve membrane for the removal of fluorine and chlorine from waste acid according to claim 7, characterized in that: The mass fraction of DA in the dopamine hydrochloride buffer is 0.2wt% to 1.6wt%.

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