Method for recovering molybdenum from acidic waste water in ammonium molybdate production process

Abstract

The present application relates to resource recycling technical field, especially to a kind of molybdate preparation process molybdenum in acid wastewater recovery method;The present application first adopts the composite conductive nanofiltration membrane with special surface positive charge, carries out primary separation to wastewater under acidic conditions, utilizes the positive charge of membrane surface and selectively intercepts impurity metal cation, while let molybdate anion pass through smoothly;Then, the filtrate enriched with molybdate is subjected to acid precipitation crystallization, and the main recovery of molybdenum is realized;Residual low-concentration molybdenum-containing precipitation mother liquor is further purified using ion exchange resin;Finally, the concentrated liquid containing impurity metal is subjected to precipitation treatment.The method solves the problem of poor selectivity of molybdenum and impurity metal separation in acid wastewater through the coupling of membrane separation, chemical precipitation and ion exchange, realizes high recovery rate, high purity recovery of molybdenum and cascade purification of wastewater.

Description

Technical Field

[0001] This invention relates to the field of resource recycling technology, and in particular to a method for recovering molybdenum from acidic wastewater in the preparation process of ammonium molybdate. Background Technology

[0002] The production of ammonium molybdate generates a large amount of acidic wastewater, which contains not only valuable molybdenum but also various metal impurity ions such as copper, iron, calcium, and magnesium. Direct discharge of this wastewater would lead to resource waste and environmental pollution. Therefore, it is necessary to recover the molybdenum from the wastewater and purify it.

[0003] Currently, common methods for recovering molybdenum from such wastewater include chemical precipitation, ion exchange, and solvent extraction. Chemical precipitation precipitates molybdenum as molybdic acid by adjusting the pH, but this method has poor selectivity, and impurity metals easily co-precipitate, resulting in low product purity and generating large amounts of sludge containing heavy metals. While ion exchange can obtain high-purity molybdenum, it is prone to resin poisoning and deactivation when treating high-concentration, multi-component acidic wastewater, requiring frequent regeneration and incurring high operating costs. Solvent extraction carries risks of organic solvent loss, emulsification, and secondary pollution.

[0004] Meanwhile, existing technologies generally share a common shortcoming: they struggle to efficiently and selectively separate molybdate ions from coexisting multiple metal cations under strongly acidic conditions. This results in lengthy recovery processes, high molybdenum loss rates, or final product purity that fails to meet requirements. Therefore, developing a molybdenum recovery process that can efficiently separate impurity metal ions from ammonium molybdate wastewater is a pressing technical problem that needs to be solved in this field. Summary of the Invention

[0005] The purpose of this invention is to address the shortcomings of existing technologies by proposing a method for recovering molybdenum from acidic wastewater during the preparation of ammonium molybdate. This invention first employs a specially designed composite conductive nanofiltration membrane with a positively charged surface to perform primary separation of the wastewater under acidic conditions. The positive charge on the membrane surface selectively traps impurity metal cations while allowing molybdate anions to pass through smoothly. Then, the filtrate enriched with molybdate is subjected to acid precipitation and crystallization to achieve primary molybdenum recovery. Next, ion exchange resin is used to further purify the residual low-concentration molybdenum-containing mother liquor. Finally, the concentrated solution containing impurity metals undergoes precipitation treatment. This method, through the coupling of membrane separation, chemical precipitation, and ion exchange, solves the problem of poor selectivity in separating molybdenum from impurity metals in acidic wastewater, achieving high molybdenum recovery rate, high purity recovery, and tiered purification of wastewater.

[0006] To achieve the above objectives, the present invention adopts the following technical solution: A method for recovering molybdenum from acidic wastewater during the preparation of ammonium molybdate, comprising the following steps: Step S1: Adjust the pH of the ammonium molybdate wastewater to 3.0, and then filter it through a composite conductive nanofiltration membrane system. The filtrate enters step S2, and the concentrate enters step S4. The operating pressure is 0.6 MPa, the applied voltage is 1.8 V, and it operates in cross-flow mode. Step S2: Stir a 25% nitric acid solution and add it to the filtrate obtained in step S1. Adjust the pH to 1.5, and then mature the precipitate for 1 hour at a stirring speed of 150 rpm and a temperature of 75°C. Filter the precipitate, wash it with deionized water at 5°C, and dry it to obtain molybdate particles, precipitate mother liquor, and washing liquid. The precipitate mother liquor enters step S3, and the washing liquid enters the ammonium molybdate wastewater in step S1. Step S3: Adjust the pH of the precipitate mother liquor obtained in step S2 to 3 and pass it into an adsorption column packed with ion exchange resin. Control the flow rate to 100 mL / L. When the effluent ρ(Mo) of the resin adsorption column is greater than 100 mg / L, desorption is carried out. Desorption is performed using a 12% ammonia solution to obtain the desorption solution, which is then introduced into the ammonium molybdate production process. Step S4: Discharge the concentrated liquid obtained after multiple runs in Step S1, adjust the pH value to 8.5 with a 10% ammonia solution, stir for 60 minutes at room temperature, allow to settle and separate the metal oxide sludge, and enter the wastewater treatment system with the supernatant.

[0007] Furthermore, the ion exchange resin is ZJ-32 macroporous weakly basic anion exchange resin, which is sourced from Zhejiang Zhenjing Environmental Technology Co., Ltd.

[0008] The composite conductive nanofiltration membrane system is prepared through the following steps: A titanium / titanium dioxide-ruthenium dioxide coated electrode with a diameter of 5 cm is used as the anode, and an austenitic stainless steel mesh with a diameter of 5 cm is used as the cathode. The anode is placed on the feed side of the composite conductive nanofiltration membrane, and the cathode is placed on the permeation side of the composite conductive nanofiltration membrane. The composite conductive nanofiltration membrane is attached to the cathode, and the distance between the anode and the cathode is 1 mm. The anode and the cathode are connected to a DC power supply device.

[0009] The composite conductive nanofiltration membrane is prepared by the following steps: Step A1: Add d-Ti3C2T xPowder, ammonia solution, ethanol, and deionized water were mixed and stirred for 30 min under nitrogen protection at a stirring rate of 500 rpm and a temperature of 25°C. Then, 3-aminopropyltriethoxysilane was added, and stirring was continued for 24 h. After centrifugation, filtration, washing with ethanol / deionized water, and drying, amino-modified MXene was obtained. Amino-modified MXene, iodomethane, potassium carbonate, and 18-crown ether-6 were mixed and reacted for 24 h under nitrogen protection at a stirring rate of 300 rpm and a temperature of 25°C. After centrifugation, filtration, washing with sodium thiosulfate / deionized water, and adding to a methanol / water solution of potassium chloride, the mixture was allowed to stand for 24 h, centrifuged, filtered, washed with deionized water, and dried to obtain quaternized MXene. Furthermore, in step A1: during the preparation of amino-modified MXene: d-Ti3C2T x The ratio of powder, ammonia solution, ethanol, deionized water, and 3-aminopropyltriethoxysilane is 1g:15-18mL:160mL:10mL:1-1.2mL, and the mass fraction of the ammonia solution is 10%. In the preparation of quaternized MXene, the ratio of amino-modified MXene, iodomethane, potassium carbonate, and 18-crown ether-6 is 1g:18-20mL:0.1g:0.05g, and the volume ratio of methanol to water in the methanol / water solution is 7:3.

[0010] Step A2: Dimethylacetamide, quaternized MXene and amino carbon nanotubes are mixed and ultrasonically dispersed at 5°C for 2 hours. Then, polyethersulfone and polyethylene glycol 20000 are added and stirred at 200 rpm and 70°C for 12 hours. Vacuum degassing is performed to obtain a casting solution. The casting solution is coated onto a glass plate and coated using a doctor blade with a gap of 200 μm. The coating is then transferred to deionized water and allowed to stand for 24 hours to obtain a conductive substrate film. Furthermore, in step A2, the ratio of dimethylacetamide, quaternized MXene, amino carbon nanotubes, polyethersulfone, and polyethylene glycol 20000 is 10g:0.3-0.4g:0.08g:2.5g:3g.

[0011] Step A3: Mix propanesulfonic acid and N,N-dimethylformamide, and stir at 300 rpm and 25°C. Add tert-butyl 2-(dimethylamino)ethylcarbamate and stir for 24 h. Rotary evaporate, wash with ethyl acetate, filter, and dry to obtain sulfobetaine intermediate. Mix sulfobetaine intermediate with methanol, and stir at 300 rpm and 0°C. Add hydrochloric acid / methanol solution and stir for 5 min. Then raise the temperature to room temperature and continue stirring for 12 h to obtain aminobetaine. Furthermore, in step A3: the ratio of propanesulfonic acid, N,N-dimethylformamide and tert-butyl 2-(dimethylamino)ethylcarbamate is 5.9g:30mL:6g; the ratio of sulfobetaine intermediate, methanol and hydrochloric acid / methanol solution is 7g:20mL:40mL, and the mass fraction of hydrochloric acid in the hydrochloric acid / methanol solution is 10%.

[0012] Step A4: Piperazine, triethylamine, and deionized water are mixed and stirred at 300 rpm and 25°C for 1 hour to obtain an aqueous solution. 1,3,5-Benzotrimyl chloride and n-hexane are mixed and stirred at 300 rpm and 25°C for 30 minutes to obtain an oil solution. The conductive substrate membrane is immersed in the aqueous solution at room temperature for 2 minutes, then immersed in the oil solution for 30 seconds, and then transferred to a mixture of polyethyleneimine, aminobetaine, and deionized water for 2 minutes. After drying, it is washed with deionized water to obtain a conductive nanofiltration membrane. Furthermore, in step A4: during the preparation of the aqueous phase solution, the ratio of piperazine, triethylamine, and deionized water is 1.8-2 g: 0.5 g: 95 mL; during the preparation of the oil phase solution, the ratio of 1,3,5-benzenetricarboxyl chloride and n-hexane is 0.5-0.6 g: 95 mL; in the mixture of polyethyleneimine, aminobetaine, and deionized water, the ratio of polyethyleneimine, aminobetaine, and deionized water is 1 g: 0.3-0.4 g: 95 mL, and the molecular weight of polyethyleneimine is 70,000.

[0013] Compared with existing technologies, the beneficial effects of this invention are as follows: This invention discloses a method for recovering molybdenum from acidic wastewater during the preparation of ammonium molybdate. Firstly, a specially designed composite conductive nanofiltration membrane with a positively charged surface is used to perform primary separation of the wastewater under acidic conditions. The positive charge on the membrane surface selectively traps impurity metal cations while allowing molybdate anions to pass through smoothly. Then, the filtrate enriched with molybdate is subjected to acid precipitation and crystallization to achieve primary molybdenum recovery. Next, ion exchange resin is used to deeply purify the residual low-concentration molybdenum-containing precipitate mother liquor. Finally, the concentrated solution containing impurity metals is subjected to precipitation treatment. This method, through the coupling of membrane separation, chemical precipitation, and ion exchange, solves the problem of poor selectivity in separating molybdenum from impurity metals in acidic wastewater, achieving high molybdenum recovery rate, high purity recovery, and tiered purification of wastewater.

[0014] The nanofiltration membrane prepared in this invention has a conductive substrate composed of quaternized MXene and amino carbon nanotubes. The quaternized MXene is covalently grafted with positively charged quaternary ammonium groups, providing stable and strong positive charge under acidic conditions. The amino carbon nanotubes further enhance the conductive network and positive charge density. On this substrate, a polyamide layer is formed by the interfacial polymerization of piperazine and 1,3,5-benzenetriacyl chloride, followed by secondary polymerization modification of polyethyleneimine / aminobetaine. Without changing the positive electric field advantage provided by the substrate, the hydrophilicity and antifouling properties of the membrane surface are improved by the zwitterionic structure of aminobetaine. This structure ensures that the membrane surface exhibits strong positive charge under acidic operating conditions, strongly retaining positively charged impurity metal ions through repulsion, while promoting the permeation of negatively charged molybdate ions through electrostatic attraction, fundamentally solving the problem of selective separation of anions and cations under acidic conditions.

[0015] This invention uses membrane separation as the core of pretreatment, which first efficiently removes the vast majority of impurity cations, creating conditions for subsequent acid precipitation to obtain high-purity molybdate products. The residual molybdenum in the acid precipitation mother liquor and low-concentration tailings is deeply recovered through ion exchange, and the desorbed liquid is returned to the main production line as a high-purity ammonium molybdate solution, realizing closed-loop recovery of molybdenum and improving the overall yield. At the same time, the concentrate containing impurity metals obtained from membrane concentration is subjected to centralized precipitation treatment, reducing the end-of-pipe treatment load. The entire process forms a highly efficient system of graded treatment and resource recycling.

[0016] In step S1, the pH is controlled to ensure that molybdate ions exist in an easily permeable anionic form while keeping impurity metal ions in a cationic state for easy retention. At the same time, it avoids excessively low pH, which could lead to acid corrosion of the membrane material or excessive energy consumption. In step S2, the pH of acid precipitation is controlled and the precipitation is aged at a certain temperature, which promotes the formation of molybdate crystals with large particle size that are easy to filter, thereby improving the ease of recovery and the purity of the product. Attached Figure Description

[0017] Figure 1 This is a schematic diagram of the recycling method proposed in this invention; Figure 2 This is a schematic diagram of the reaction in step A3 of the present invention; Figure 3 This is a schematic diagram of the reaction in step A4 of the present invention. Detailed Implementation

[0018] To further illustrate the technical means and effects of the present invention in achieving its intended purpose, the following detailed description of the specific implementation methods, structures, features, and effects of the present invention, in conjunction with the accompanying drawings and preferred embodiments, is provided below.

[0019] The composition of the acidic wastewater used in the ammonium molybdate preparation process in the embodiments and comparative examples of this invention is as follows: Preparation Example 1: The composite conductive nanofiltration membrane was prepared by the following steps: Step A1: Add d-Ti3C2T x Powder, ammonia solution, ethanol, and deionized water were mixed and stirred for 30 min under nitrogen protection at a stirring rate of 500 rpm and a temperature of 25°C. Then, 3-aminopropyltriethoxysilane was added, and stirring was continued for 24 h. After centrifugation, filtration, washing with ethanol / deionized water, and drying, amino-modified MXene was obtained. Amino-modified MXene, iodomethane, potassium carbonate, and 18-crown ether-6 were mixed and reacted for 24 h under nitrogen protection at a stirring rate of 300 rpm and a temperature of 25°C. After centrifugation, filtration, washing with sodium thiosulfate / deionized water, and adding to a methanol / water solution of potassium chloride, the mixture was allowed to stand for 24 h, centrifuged, filtered, washed with deionized water, and dried to obtain quaternized MXene. Furthermore, in step A1: during the preparation of amino-modified MXene: d-Ti3C2T x The ratio of powder, ammonia solution, ethanol, deionized water, and 3-aminopropyltriethoxysilane is 1g:15mL:160mL:10mL:1mL, and the mass fraction of the ammonia solution is 10%. In the preparation of quaternized MXene, the ratio of amino-modified MXene, iodomethane, potassium carbonate, and 18-crown ether-6 is 1g:18mL:0.1g:0.05g, and the volume ratio of methanol to water in the methanol / water solution is 7:3. Step A2: Dimethylacetamide, quaternized MXene and amino carbon nanotubes are mixed and ultrasonically dispersed at 5°C for 2 hours. Then, polyethersulfone and polyethylene glycol 20000 are added and stirred at 200 rpm and 70°C for 12 hours. Vacuum degassing is performed to obtain a casting solution. The casting solution is coated onto a glass plate and coated using a doctor blade with a gap of 200 μm. The coating is then transferred to deionized water and allowed to stand for 24 hours to obtain a conductive substrate film. Furthermore, in step A2: the ratio of dimethylacetamide, quaternized MXene, amino carbon nanotubes, polyethersulfone, and polyethylene glycol 20000 is 10g:0.3g:0.08g:2.5g:3g; Step A3: Mix propanesulfonic acid and N,N-dimethylformamide, and stir at 300 rpm and 25°C. Add tert-butyl 2-(dimethylamino)ethylcarbamate and stir for 24 h. Rotary evaporate, wash with ethyl acetate, filter, and dry to obtain sulfobetaine intermediate. Mix sulfobetaine intermediate with methanol, and stir at 300 rpm and 0°C. Add hydrochloric acid / methanol solution and stir for 5 min. Then raise the temperature to room temperature and continue stirring for 12 h to obtain aminobetaine. Furthermore, in step A3: the ratio of propanesulfonic acid, N,N-dimethylformamide, and tert-butyl 2-(dimethylamino)ethylcarbamate is 5.9 g: 30 mL: 6 g; the ratio of sulfobetaine intermediate, methanol, and hydrochloric acid / methanol solution is 7 g: 20 mL: 40 mL, and the mass fraction of hydrochloric acid in the hydrochloric acid / methanol solution is 10%. Step A4: Piperazine, triethylamine, and deionized water are mixed and stirred at 300 rpm and 25°C for 1 hour to obtain an aqueous solution. 1,3,5-Benzotrimyl chloride and n-hexane are mixed and stirred at 300 rpm and 25°C for 30 minutes to obtain an oil solution. The conductive substrate membrane is immersed in the aqueous solution at room temperature for 2 minutes, then immersed in the oil solution for 30 seconds, and then transferred to a mixture of polyethyleneimine, aminobetaine, and deionized water for 2 minutes. After drying, it is washed with deionized water to obtain a conductive nanofiltration membrane. Furthermore, in step A4: during the preparation of the aqueous phase solution, the ratio of piperazine, triethylamine, and deionized water is 1.8 g: 0.5 g: 95 mL; during the preparation of the oil phase solution, the ratio of 1,3,5-benzenetricarboxyl chloride and n-hexane is 0.5 g: 95 mL; in the mixture of polyethyleneimine, aminobetaine, and deionized water, the ratio of polyethyleneimine, aminobetaine, and deionized water is 1 g: 0.3 g: 95 mL, and the molecular weight of polyethyleneimine is 70,000.

[0020] Preparation Example 2: Compared to Preparation Example 1, Preparation Example 2 differs in that, in step A1, during the preparation of amino-modified MXene, d-Ti3C2T... x The ratio of powder, ammonia solution, ethanol, deionized water and 3-aminopropyltriethoxysilane was adjusted to 1g:18mL:160mL:10mL:1.2mL; in the preparation of quaternized MXene: the ratio of amino-modified MXene, iodomethane, potassium carbonate and 18-crown ether-6 was adjusted to 1g:20mL:0.1g:0.05g, and the other steps were the same.

[0021] Preparation Example 3: Compared with Preparation Example 1, in step A2, the ratio of dimethylacetamide, quaternized MXene, amino carbon nanotubes, polyethersulfone and polyethylene glycol 20000 was adjusted to 10g:0.4g:0.08g:2.5g:3g, and the other steps were the same.

[0022] Preparation Example 4: Compared to Preparation Example 1, in step A4, the following changes were made: in the preparation of the aqueous phase solution, the ratio of piperazine, triethylamine, and deionized water was adjusted to 2 g: 0.5 g: 95 mL; in the preparation of the oil phase solution, the ratio of 1,3,5-benzenetricarboxyl chloride and n-hexane was adjusted to 0.6 g: 95 mL; and in the mixture of polyethyleneimine, aminobetaine, and deionized water, the ratio of polyethyleneimine, aminobetaine, and deionized water was adjusted to 1 g: 0.4 g: 95 mL. All other steps remained the same.

[0023] Preparation Example 5: Compared with Preparation Example 1, in step A2, the ratio of dimethylacetamide, quaternized MXene, amino carbon nanotubes, polyethersulfone and polyethylene glycol 20000 was adjusted to 10g:0.1g:0.08g:2.5g:3g, and the other steps were the same.

[0024] Preparation Example 6: Compared to Preparation Example 1, in step A4, the following changes were made: in the preparation of the aqueous phase solution, the ratio of piperazine, triethylamine, and deionized water was adjusted to 1.8 g: 0.5 g: 95 mL; in the preparation of the oil phase solution, the ratio of 1,3,5-benzenetricarboxyl chloride and n-hexane was adjusted to 0.2 g: 95 mL; and in the mixture of polyethyleneimine, aminobetaine, and deionized water, the ratio of polyethyleneimine, aminobetaine, and deionized water was adjusted to 1 g: 0.3 g: 95 mL. All other steps remained the same.

[0025] Comparative Preparation Example 1: Compared to Preparation Example 1, 3-aminopropyltriethoxysilane was removed in step A1, while the other steps were the same.

[0026] Comparative Preparation Example 2: Compared to Preparation Example 1, Comparative Preparation Example 2 removes aminobetaine in step A4, while the other steps are the same.

[0027] Example 1: A method for recovering molybdenum from acidic wastewater during the preparation of ammonium molybdate, comprising the following steps: Step S1: Adjust the pH of the ammonium molybdate wastewater to 3.0, and then filter it through a composite conductive nanofiltration membrane system. The filtrate enters step S2, and the concentrate enters step S4. The operating pressure is 0.6 MPa, the applied voltage is 1.8 V, and it operates in cross-flow mode. Step S2: Stir a 25% nitric acid solution and add it to the filtrate obtained in step S1. Adjust the pH to 1.5, and then mature the precipitate for 1 hour at a stirring speed of 150 rpm and a temperature of 75°C. Filter the precipitate, wash it with deionized water at 5°C, and dry it to obtain molybdate particles, precipitate mother liquor, and washing liquid. The precipitate mother liquor enters step S3, and the washing liquid enters the ammonium molybdate wastewater in step S1. Step S3: Adjust the pH of the precipitate mother liquor obtained in step S2 to 3 and pass it into an adsorption column packed with ion exchange resin. Control the flow rate to 100 mL / L. When the effluent ρ(Mo) of the resin adsorption column is greater than 100 mg / L, desorption is carried out. Desorption is performed using a 12% ammonia solution to obtain the desorption solution, which is then introduced into the ammonium molybdate production process. Step S4: Discharge the concentrated liquid obtained after multiple runs in Step S1, adjust the pH value to 8.5 with a 10% ammonia solution, stir for 60 minutes at room temperature, allow to settle and separate the metal oxide sludge, and enter the wastewater treatment system with the supernatant.

[0028] The composite conductive nanofiltration membrane system is prepared through the following steps: A titanium / titanium dioxide-ruthenium dioxide coated electrode with a diameter of 5 cm is used as the anode, and an austenitic stainless steel mesh with a diameter of 5 cm is used as the cathode. The anode is placed on the feed side of the composite conductive nanofiltration membrane, and the cathode is placed on the permeation side of the composite conductive nanofiltration membrane. The composite conductive nanofiltration membrane is attached to the cathode, and the distance between the anode and the cathode is 1 mm. The anode and the cathode are connected to a DC power supply device.

[0029] Example 2: Compared with Example 1, Example 2 replaces the composite conductive nanofiltration membrane in step S1 with the one prepared in Example 2, while the other steps are the same.

[0030] Example 3: Compared with Example 1, Example 3 replaces the composite conductive nanofiltration membrane in step S1 with the one prepared in Example 3, while the other steps are the same.

[0031] Example 4: Compared with Example 1, Example 4 replaces the composite conductive nanofiltration membrane in step S1 with the one prepared in Example 4, while the other steps are the same.

[0032] Example 5: Compared with Example 1, Example 5 replaces the composite conductive nanofiltration membrane in step S1 with the one prepared in Example 5, while the other steps are the same.

[0033] Example 6: Compared with Example 1, Example 6 replaces the composite conductive nanofiltration membrane obtained in Preparation Example 6 in step S1, while the other steps are the same.

[0034] Example 7: Compared with Example 1, Example 7 adjusts the applied voltage in step S1 to 1.5V, while the other steps are the same.

[0035] Example 8: Compared with Example 1, Example 8 adjusts the applied voltage in step S1 to 2.1V, while the other steps are the same.

[0036] Comparative Example 1: Compared with Example 1, the composite conductive nanofiltration membrane in step S1 was replaced with the one prepared in Comparative Preparation Example 1, while the other steps were the same.

[0037] Comparative Example 2: Compared with Example 1, the composite conductive nanofiltration membrane in step S1 was replaced with the one prepared in Comparative Preparation Example 2, while the other steps were the same.

[0038] Comparative Example 3: Compared with Example 1, the pH value of the ammonium molybdate wastewater in step S1 was adjusted to 2, while the other steps were the same.

[0039] Comparative Example 4: Compared with Example 1, the pH value of the filtrate in step S2 was adjusted to 2, and the other steps were the same.

[0040] The composite conductive nanofiltration membranes prepared in Preparation Examples 1-6 and Comparative Preparation Examples 1-2 were tested: The potential of the sample under pH 3 was measured using a solid surface potential analyzer. The sample was freeze-dried for 12 h and attached to the sample stage. The zeta potential was measured using 0.001 mmol / L potassium chloride solution as the electrolyte solution. The pore size was determined using a method that restricts the concentration of neutral organic molecules. Polyethylene glycol with molecular weights of 200, 400, 600, 800, and 1000 was selected and prepared into feed solutions with a concentration of 1 g / L. The pore size was calculated using the following formula: ; In the formula, M is the relative molecular mass of polyethylene glycol when the sample membrane has a polyethylene glycol rejection rate of 90%; The single-salt separation performance of the composite conductive nanofiltration membrane system constructed according to Example 1 was tested using a 0.2 g / L magnesium chloride solution. The operating pressure was 0.6 MPa, the applied voltage was 1.8 V, and the system was operated in cross-flow mode. The rejection rate was calculated. The test results are shown in Table 1 below: Table 1 Performance test results of the preparation example The test results shown in the table indicate that preparation examples 1-6 and comparative preparation examples 1-2 are compared as follows: Preparation Example 1: As a baseline example, the membrane simultaneously possesses a stable and strong positive charge provided by quaternized MXene, a dense separation layer formed by interfacial polymerization, and an antifouling hydrophilic surface imparted by aminobetaine. It exhibits strong positive charge, has suitable nanoscale pore size, and shows a high rejection rate for magnesium ions.

[0041] Preparation Example 2: Compared with Preparation Example 1, the amount of aminosilanizing and quaternizing reagents was increased, which made the amino grafting density and quaternization degree of MXene surface higher, resulting in an increase in its Zeta potential, a stronger positive electric field, and a slight increase in magnesium ion rejection rate, but its pore size decreased.

[0042] Preparation Example 3: Compared to Preparation Example 1, the amount of quaternized MXene added to the casting solution was increased, which resulted in a higher density of positive charge carriers in the membrane, leading to a significant increase in its Zeta potential. At the same time, the increased number of MXene sheets made the membrane structure more compact and the pore size decreased again. The two factors worked together to further improve the magnesium ion rejection rate.

[0043] Preparation Example 4: Compared to Preparation Example 1, the concentration of monomers in the interfacial polymerization reaction was increased, which resulted in a thicker polyamide separation layer with a higher degree of cross-linking, leading to a significant reduction in membrane pore size. Therefore, the magnesium ion rejection rate was the highest. However, the thicker polymerization layer partially shielded the positive charge of the substrate, resulting in a slight decrease in the Zeta potential.

[0044] Preparation Example 5: Compared to Preparation Example 1, the amount of quaternized MXene added was significantly reduced, which resulted in a severe deficiency of the main source of positive charge in the membrane, leading to a significant decrease in the overall Zeta potential of the membrane. At the same time, the insufficient filler caused the membrane pore structure to become loose and the pore size to increase slightly. The combined effect of these two factors resulted in a decrease in the magnesium ion rejection rate.

[0045] Preparation Example 6: Compared to Preparation Example 1, the amount of crosslinking agent 1,3,5-benzenetricarboxyl chloride was significantly reduced, which resulted in insufficient interfacial polymerization reaction. The polyamide separation layer formed was very thin and defective, which in turn led to a significant increase in membrane pore size. Although the Zeta potential showed the highest value due to the weak thin-layer shielding effect, the magnesium ion rejection rate was lower than that of other preparation examples.

[0046] Comparative Preparation Example 1: Since 3-aminopropyltriethoxysilane was completely removed in step A1 compared to Preparation Example 1, MXene could not undergo aminosilanization and subsequent quaternization. It lacked the covalently grafted quaternary ammonium groups to impart a positive charge to the membrane, which in turn caused the membrane surface to be negatively charged under acidic conditions. The negatively charged surface attracts rather than repels cations, so the retention rate of magnesium ions is extremely low.

[0047] Comparative Preparation Example 2: Compared to Preparation Example 1, aminobetaine was removed in step A4, resulting in the absence of zwitterionic compounds in the final modified layer on the membrane surface. This lacks the mechanism by which aminobetaine forms an antifouling hydrophilic layer on the membrane surface through strong hydration. Although the zeta potential and pore size are similar to Preparation Example 1, and the initial magnesium ion rejection rate is also higher, the surface is more prone to adsorbing pollutants during long-term operation or when treating complex wastewater, leading to irreversible fouling and performance degradation.

[0048] The acidic wastewater from the ammonium molybdate preparation process was recovered using the recovery methods of Examples 1-8 and Comparative Examples 1-4. The filtrate obtained in step S1 and the precipitate mother liquor obtained in step S2 were measured, and the results are shown in Tables 2 and 3 below: Table 2. Concentration of major ions in the filtrate (g / L) Table 3. Concentration of major ions in the mother liquor of the precipitate (g / L) The test results shown in the table indicate that Examples 1-8 are compared with Comparative Examples 1-4: Example 1: As a baseline example, the composite conductive nanofiltration membrane prepared in Example 1 was used. This membrane has strong positive charge, suitable pore size and good antifouling properties. Under the synergistic effect of an electric field of 1.8V, it can efficiently retain impurity cations, while allowing most of the molybdate ions to permeate into S2. S2 is completely acid precipitated at pH=1.5, and the residual molybdenum concentration in the mother liquor is low and there are few impurities, which creates good conditions for the recovery of S3.

[0049] Example 2: Compared with Example 1, Example 2 uses a membrane with a higher Zeta potential and a greater charge density, which leads to a stronger electrostatic repulsion of impurity cations in step S1, further reducing the concentration of Cu and Fe in the filtrate. At the same time, the repulsion of molybdenum is not significantly enhanced, and the molybdenum permeability is comparable to that of Example 1. The molybdenum concentration in the mother liquor of S2 is slightly reduced.

[0050] Example 3: Compared to Example 1, Example 3 used a higher amount of quaternized MXene to prepare the membrane, resulting in improved overall positive electric field and density. This optimized the retention of multivalent cations in step S1, leading to the lowest concentration in the filtrate. However, the denser membrane structure slightly hindered the formation of molybdate, resulting in a slight decrease in molybdenum concentration in the S1 filtrate and the S2 precipitation mother liquor.

[0051] Example 4: Compared to Example 1, Example 4 used a membrane with smaller pore size and a denser cross-linked layer, resulting in the S1 step relying mainly on size sieving to trap impurities, leading to an extremely low impurity concentration in the filtrate. However, the excessively small pore size severely hindered the permeation of larger polymeric molybdate ions, resulting in a decrease in molybdenum concentration in S1 and a decrease in molybdenum concentration in the S2 precipitation mother liquor.

[0052] Example 5: Compared to Example 1, Example 5 used a membrane with insufficient quaternized MXene content, resulting in weak positive charge of the membrane and reduced electrostatic retention capacity for impurity cations. The concentrations of Cu and Fe in the S1 filtrate increased significantly. After these impurities entered S2, some of them underwent co-precipitation or adsorption, resulting in a high impurity content in the precipitate mother liquor, which affected the lifespan of the subsequent ion exchange resin and the purity of the final molybdenum product.

[0053] Example 6: Compared to Example 1, the membrane prepared in Example 6 was significantly enlarged due to insufficient crosslinking agent, resulting in a lower rejection rate for large-sized polymeric molybdate ions. However, the rejection rate for small-sized impurity cations mainly relied on charge repulsion. Although its zeta potential was high, some small-particle cations could still pass through due to the excessively large pore size. Therefore, the impurity concentration in the S1 filtrate was higher than that in Examples 1-4.

[0054] Example 7: Since the applied voltage is reduced from 1.8V to 1.5V compared to Example 1, the electric field driving force is reduced, which leads to a weakening of the electric effect. The electric field is insufficient to drive the cations to migrate to the cathode, which makes some cations more accessible and permeable to the membrane. Therefore, the impurity concentration in the S1 filtrate is higher than that in Example 1.

[0055] Example 8: Compared to Example 1, the applied voltage is increased from 1.8V to 2.1V, which enhances the driving force of the electric field. However, the excessively high voltage may slightly aggravate side reactions such as water electrolysis, causing local pH or temperature fluctuations. Although the retention effect of S1 on impurities is comparable to or even slightly better than that of Example 1, the excessively high voltage does not bring about a significant improvement in separation performance. Instead, it increases energy consumption and instability risk.

[0056] Comparative Example 1: Compared to Example 1, it uses a negatively charged membrane, which causes the membrane surface to electrostatically attract impurity cations. Under the action of the electric field, the cations migrate more easily to the cathode, resulting in the impurity concentration in the S1 filtrate being almost the same as that in the influent. At the same time, the negatively charged surface severely repels molybdate, causing a significant decrease in the molybdenum permeability. This directly leads to an increased load on the subsequent S2 step, poor product purity, and huge processing pressure in the S3 step.

[0057] Comparative Example 2: Compared to Example 1, it used a membrane prepared in Comparative Example 2 without aminobetaine surface modification. In short-term testing, its S1 separation performance was very close to that of Example 1, indicating that the initial charge and pore size characteristics were maintained. However, during long-term operation or when treating more complex actual wastewater, its membrane surface will be covered by pollutants such as organic matter and colloids more quickly, leading to accelerated flux decline and decreased separation performance, thereby affecting the stability of the entire process and operating costs.

[0058] Comparative Example 3: Compared to Example 1, the pH of the feed water in step S1 was adjusted from 3.0 to 2.0, which increased the acidity of the feed. Under stronger acidic conditions, on the one hand, the hydrolytic polymerization morphology of impurity metal ions changed, and their stability increased; on the other hand, the higher hydrogen ion concentration competed with cations for positive charge sites on the membrane surface. At the same time, the excessively low pH promoted the polymerization of molybdate into larger anionic groups, which increased their transport resistance in the membrane pores. The recovery rate of molybdenum in step S1 decreased, and the residual molybdenum in the mother liquor of step S2 increased. This further illustrates that controlling the pH of step S1 within a suitable range is beneficial to balancing the impurity ion morphology, membrane surface charge state, and the transport efficiency of the target molybdate.

[0059] Comparative Example 4: Compared to Example 1, the pH of S2 acid precipitation was relaxed from 1.5 to 2.0, which weakened the driving force of the precipitation reaction. It lacked a sufficiently low pH environment to achieve complete precipitation of molybdic acid, which led to the precipitation equilibrium shifting towards dissolution. Therefore, under the same aging precipitation conditions, the molybdenum precipitation recovery rate decreased, the residual molybdenum concentration in the S2 precipitation mother liquor increased, and the load on the S3 step and the overall risk of molybdenum loss increased.

[0060] The above description is merely a preferred embodiment of the present invention and is not intended to limit the present invention in any way. Although the present invention has been disclosed above with reference to preferred embodiments, it is not intended to limit the present invention. Any person skilled in the art can make some modifications or alterations to the above-disclosed technical content to create equivalent embodiments without departing from the scope of the present invention. Any simple modifications, equivalent changes and alterations made to the above embodiments based on the technical essence of the present invention without departing from the scope of the present invention shall still fall within the scope of the present invention.

Claims

1. A method for recovering molybdenum from acidic wastewater during the preparation of ammonium molybdate, characterized in that: Includes the following steps: Step S1: Adjust the pH of the ammonium molybdate wastewater to 3.0, and then filter it through a composite conductive nanofiltration membrane system. The filtrate enters step S2, and the concentrate enters step S4. The operating pressure is 0.6 MPa, the applied voltage is 1.8 V, and it operates in cross-flow mode. Step S2: Stir a 25% nitric acid solution and add it to the filtrate obtained in step S1. Adjust the pH to 1.5, and then mature the precipitate for 1 hour at a stirring speed of 150 rpm and a temperature of 75°C. Filter the precipitate, wash it with deionized water at 5°C, and dry it to obtain molybdate particles, precipitate mother liquor, and washing liquid. The precipitate mother liquor enters step S3, and the washing liquid enters the ammonium molybdate wastewater in step S1. Step S3: Adjust the pH of the precipitate mother liquor obtained in step S2 to 3 and pass it into an adsorption column packed with ion exchange resin. Control the flow rate to 100 mL / L. When the effluent ρ(Mo) of the resin adsorption column is greater than 100 mg / L, desorption is carried out. Desorption is performed using a 12% ammonia solution to obtain the desorption solution, which is then introduced into the ammonium molybdate production process. Step S4: Discharge the concentrated liquid obtained after multiple runs in Step S1, adjust the pH value to 8.5 with a 10% ammonia solution, stir for 60 minutes at room temperature, allow to settle and separate the metal oxide sludge, and enter the wastewater treatment system with the supernatant.

2. The method for recovering molybdenum from acidic wastewater during the preparation of ammonium molybdate according to claim 1, characterized in that: The composite conductive nanofiltration membrane system is prepared through the following steps: A titanium / titanium dioxide-ruthenium dioxide coated electrode with a diameter of 5 cm is used as the anode, and an austenitic stainless steel mesh with a diameter of 5 cm is used as the cathode. The anode is placed on the feed side of the composite conductive nanofiltration membrane, and the cathode is placed on the permeation side of the composite conductive nanofiltration membrane. The composite conductive nanofiltration membrane is attached to the cathode, and the distance between the anode and the cathode is 1 mm. The anode and the cathode are connected to a DC power supply device.

3. The method for recovering molybdenum from acidic wastewater during the preparation of ammonium molybdate according to claim 2, characterized in that: The composite conductive nanofiltration membrane is prepared by the following steps: Step A1: Mix d-Ti3C2Tx powder, ammonia solution, ethanol and deionized water. Stir for 30 min under nitrogen protection, stirring rate of 500 rpm and temperature of 25℃. Then add 3-aminopropyltriethoxysilane and continue stirring for 24 h. Centrifuge, filter, wash with ethanol / deionized water, and dry to obtain amino-modified MXene. Mix amino-modified MXene, iodomethane, potassium carbonate and 18-crown ether-6. React for 24 h under nitrogen protection, stirring rate of 300 rpm and temperature of 25℃. Centrifuge, filter, wash with sodium thiosulfate / deionized water, and then add to potassium chloride in methanol / water solution. Let stand for 24 h, centrifuge, filter, wash with deionized water and dry to obtain quaternized MXene. Step A2: Dimethylacetamide, quaternized MXene and amino carbon nanotubes are mixed and ultrasonically dispersed at 5°C for 2 hours. Then, polyethersulfone and polyethylene glycol 20000 are added and stirred at 200 rpm and 70°C for 12 hours. Vacuum degassing is performed to obtain a casting solution. The casting solution is coated onto a glass plate and coated using a doctor blade with a gap of 200 μm. The coating is then transferred to deionized water and allowed to stand for 24 hours to obtain a conductive substrate film. Step A3: Mix propanesulfonic acid and N,N-dimethylformamide, and stir at 300 rpm and 25°C. Add tert-butyl 2-(dimethylamino)ethylcarbamate and stir for 24 h. Rotary evaporate, wash with ethyl acetate, filter, and dry to obtain sulfobetaine intermediate. Mix sulfobetaine intermediate with methanol, and stir at 300 rpm and 0°C. Add hydrochloric acid / methanol solution and stir for 5 min. Then raise the temperature to room temperature and continue stirring for 12 h to obtain aminobetaine. Step A4: Piperazine, triethylamine, and deionized water are mixed and stirred for 1 hour at 300 rpm and 25°C to obtain an aqueous solution. 1,3,5-Benzotrimyl chloride and n-hexane are mixed and stirred for 30 minutes at 300 rpm and 25°C to obtain an oil solution. The conductive substrate membrane is immersed in the aqueous solution at room temperature for 2 minutes, then immersed in the oil solution for 30 seconds, and then transferred to a mixture of polyethyleneimine, aminobetaine, and deionized water for 2 minutes. After drying, it is washed with deionized water to obtain a conductive nanofiltration membrane.

4. The method for recovering molybdenum from acidic wastewater during the preparation of ammonium molybdate according to claim 3, characterized in that: In step A1: During the preparation of amino-modified MXene: the ratio of d-Ti3C2Tx powder, ammonia solution, ethanol, deionized water and 3-aminopropyltriethoxysilane is 1g:15-18mL:160mL:10mL:1-1.2mL, and the mass fraction of the ammonia solution is 10%; During the preparation of quaternized MXene: the ratio of amino-modified MXene, iodomethane, potassium carbonate and 18-crown ether-6 is 1g:18-20mL:0.1g:0.05g, and the volume ratio of methanol to water in the methanol / water solution is 7:

3.

5. The method for recovering molybdenum from acidic wastewater during the preparation of ammonium molybdate according to claim 3, characterized in that: In step A2: the ratio of dimethylacetamide, quaternized MXene, amino carbon nanotubes, polyethersulfone and polyethylene glycol 20000 is 10g:0.3-0.4g:0.08g:2.5g:3g.

6. The method for recovering molybdenum from acidic wastewater during the preparation of ammonium molybdate according to claim 3, characterized in that: In step A3: the ratio of propanesulfonic acid, N,N-dimethylformamide and tert-butyl 2-(dimethylamino)ethylcarbamate is 5.9 g: 30 mL: 6 g; the ratio of sulfobetaine intermediate, methanol and hydrochloric acid / methanol solution is 7 g: 20 mL: 40 mL, and the mass fraction of hydrochloric acid in the hydrochloric acid / methanol solution is 10%.

7. The method for recovering molybdenum from acidic wastewater during the preparation of ammonium molybdate according to claim 3, characterized in that: In step A4: during the preparation of the aqueous phase solution, the ratio of piperazine, triethylamine, and deionized water is 1.8-2 g: 0.5 g: 95 mL; during the preparation of the oil phase solution, the ratio of 1,3,5-benzenetricarboxyl chloride and n-hexane is 0.5-0.6 g: 95 mL; in the mixture of polyethyleneimine, aminobetaine, and deionized water, the ratio of polyethyleneimine, aminobetaine, and deionized water is 1 g: 0.3-0.4 g: 95 mL, and the molecular weight of polyethyleneimine is 70,000.

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