High-salinity wastewater nanofiltration desalination and resource recovery treatment process

Abstract

The application relates to the technical field of wastewater treatment, and discloses a high-salinity wastewater nanofiltration desalination resource treatment process. ‑ Transmembrane migration simultaneously enhances SO4 2‑ Static repulsion realizes secondary purification, and the concentrated water of the primary nanofiltration enters a three-stage electric nanofiltration washing unit under the assistance of a pulse electric field. The application realizes passive interception to active regulation of nanofiltration separation through electric field assisted nanofiltration. The sodium chloride purity of the secondary electric nanofiltration water can reach more than 99.2%, the sodium sulfate purity of the three-stage electric nanofiltration concentrated water can reach more than 98.5%, the Cl ‑ content in the sodium sulfate product can be reduced to below 0.6%, the membrane single continuous operation cycle is prolonged to more than 14 months, and efficient and stable operation of the high-salinity wastewater nanofiltration desalination link is realized.

Description

Technical Field

[0001] This invention relates to the field of wastewater treatment technology, specifically to a nanofiltration desalination and resource recovery process for high-salinity wastewater. Background Technology

[0002] With rapid industrialization and increasingly stringent environmental standards, the treatment and resource utilization of high-salinity wastewater has become a research hotspot in the field of industrial wastewater treatment. Industries such as coal chemical engineering, rare earth smelting, pharmaceuticals, and dyeing generate large amounts of high-salinity wastewater during production processes. This wastewater typically contains high concentrations of sodium chloride and sodium sulfate. Direct discharge not only wastes water resources but also causes soil salinization and water pollution. Nanofiltration membranes, due to their high rejection rate for divalent ions and low rejection rate for monovalent ions, are widely used for the salt separation treatment of high-salinity wastewater.

[0003] However, the existing nanofiltration desalination process still faces the following technical bottlenecks: First, the selectivity of desalination is limited, with high residual sulfate in the first-stage nanofiltration permeate and severe chloride ion entrainment in the concentrate, resulting in the purity of sodium chloride and sodium sulfate products generally being lower than the industrial salt standard; Second, severe concentration polarization occurs on the membrane surface, and a concentration polarization layer easily forms on the membrane surface under high salinity conditions, accelerating membrane fouling and scaling, and the single continuous operation cycle of nanofiltration membranes is usually only 6-9 months; Third, the yield of mixed salt mother liquor is high, and incomplete desalination forces the subsequent evaporation and crystallization unit to discharge a large amount of mixed salt mother liquor, with a sodium chloride resource utilization rate of only 70-85%, increasing the cost of hazardous waste disposal.

[0004] In summary, there is an urgent need to develop a nanofiltration salt separation and resource recovery process that features high salt purity, high resource recovery rate, good operational stability, and controllable membrane fouling. Summary of the Invention

[0005] The purpose of this invention is to provide a nanofiltration desalination and resource recovery process for high-salinity wastewater to solve the problems mentioned in the background art.

[0006] To achieve the above objectives, the present invention provides a nanofiltration desalination and resource recovery process for high-salinity wastewater, comprising the following steps: First-stage nanofiltration desalination: High-salt wastewater is passed into a first-stage nanofiltration unit for nanofiltration separation to obtain first-stage nanofiltration permeate containing sodium chloride and first-stage nanofiltration concentrate containing sodium sulfate; Secondary electro-nanofiltration purification: The primary nanofiltration permeate is passed into the secondary electro-nanofiltration unit for secondary salt separation under the assistance of a DC electric field. The electric field strength of the DC electric field is 2-30 V / cm. The concentrate from the secondary electro-nanofiltration is returned to the inlet of the primary nanofiltration unit, and the permeate from the secondary electro-nanofiltration is output as sodium chloride product solution. Three-stage electro-nanofiltration washing and purification: The concentrate from the first-stage nanofiltration is mixed and diluted with dilution water and then passed into the third-stage electro-nanofiltration unit. Nanofiltration separation is performed under the assistance of a pulsed electric field. The frequency of the pulsed electric field is 10-200Hz, the duty cycle is 5-30%, and the electric field strength is 5-50V / cm. The permeate from the third-stage electro-nanofiltration is returned to the inlet of the first-stage nanofiltration unit, and the concentrate from the third-stage electro-nanofiltration is output as sodium sulfate product solution.

[0007] Preliminary separation of Cl by primary nanofiltration - and SO4 2- Building upon this foundation, a dual challenge of sulfate residue on the sodium chloride side and chloride ion entrainment on the sodium sulfate side was addressed by introducing a DC electric field-assisted two-stage electro-nanofiltration and a pulsed electric field-assisted three-stage electro-nanofiltration washing and purification process. The DC electric field enhances the Cl-retention capacity through the electric field force perpendicular to the membrane surface. - The driving force for transmembrane migration of Cl - The synergistic effect of electric field force and pressure difference allows for more efficient permeation through the nanofiltration membrane; the pulsed electric field generates electric convection vortices (ECVs) on the membrane surface by periodically changing the direction and intensity of the electric field. The electric field intensity in its core region can reach 2-3 times the conventional value, effectively destroying the concentration polarization layer on the membrane surface. This design increases the effective separation rate of sodium chloride in nanofiltration from 80-85% in traditional single-stage nanofiltration to over 97%, while reducing the yield of mixed salt mother liquor to below 6%.

[0008] Preferably, in the secondary electro-nanofiltration purification, the electric field strength of the DC electric field is 5-20 V / cm, the direction is perpendicular to the membrane surface, the anode is set on the feed side, and the cathode is set on the permeate side. - Under the influence of an electric field, additional driving force for transmembrane migration is obtained, significantly improving the migration rate; simultaneously, multivalent cations near the membrane surface (such as Ca) 2+ Mg 2+ Under the influence of an electric field, polyvalent cations migrate towards the cathode (the liquid-permeable side), but due to the high rejection rate of the nanofiltration membrane for polyvalent cations, they accumulate on the membrane surface, causing a positive shift in the zeta potential of the membrane surface. The original nanofiltration membrane is usually negatively charged, but during the electric field-assisted process, polyvalent cations migrate towards the membrane surface and undergo complexation under the drive of the electric field, causing a positive shift in the zeta potential of the membrane surface. This positive shift in the zeta potential creates an enhanced Donnan effect, further strengthening the rejection of SO42-. 2- Electrostatic repulsion reduces SO4 in the secondary nanofiltration product water. 2- The residual amount was reduced to below 0.3%.

[0009] Preferably, in the secondary electrofiltration purification, the nanofiltration membrane of the secondary electrofiltration unit is a positively charged nanofiltration membrane with a zeta potential of +15 mV to +35 mV and a pure water flux of not less than 60 L·m⁻¹. -2 ·h -1 ·bar-1 Under the assistance of a DC electric field, the surface of a positively charged nanofiltration membrane reacts with Cl... - The electrostatic attraction between them is stronger, Cl - The permeation rate is significantly higher than that of ordinary nanofiltration membranes; at the same time, the positively charged membrane surface faces SO4 2- The electrostatic repulsion is stronger, further reducing sulfate residue in the sodium chloride product solution. Nanofiltration membranes with a zeta potential of +15 mV to +35 mV can achieve SO42- under these process conditions. 2- Retention rate > 99% and Cl - Retention rate <15%, SO4 in sodium chloride product liquid 2- The content can be controlled below 0.3%.

[0010] Preferably, in the three-stage electrofiltration washing and purification process, the temperature of the dilution water is 5-20°C, and the volume mixing ratio of the dilution water to the first-stage nanofiltration concentrate is 1:(0.5-2.0). The dilution water is one or a combination of two or more of the following: cooling water from the bipolar membrane electrodialysis unit, condensate from the MVR evaporation and crystallization unit, or permeate from reverse osmosis. The introduction of dilution water reduces the Cl- content of the first-stage nanofiltration concentrate. - Concentration, reducing SO4 concentration on the membrane surface 2- With Cl - Competition through; on the other hand, the 5-25°C temperature gradient formed by the low-temperature dilution water (5-20°C) and the primary nanofiltration concentrate (20-35°C) utilizes the low-temperature Cl... - The synergistic effect of decreased hydration level, reduced hydration radius, and slight shrinkage of nanofiltration membrane pore size at low temperatures enhances Cl. - Transmembrane migration selectivity. The volumetric mixing ratio of dilution water to primary nanofiltration concentrate is controlled within the range of 1:(0.5-2.0), ensuring that the TDS of the tertiary electrofiltration nanofiltration feed water is 50,000-120,000 mg / L, thus ensuring the washing effect while avoiding excessive system circulation load due to over-dilution. Internal condensate (MVR condensate, bipolar membrane cooling water) is used as the dilution water source, with a conductivity ≤50 μS / cm, achieving cascaded utilization of internal water resources and a total system water recovery rate of 92-94%.

[0011] Preferably, in the three-stage electro-nanofiltration washing and purification, the pulsed electric field is a square wave pulse or a sine wave pulse, with a pulse duty cycle of 10-20% and an electric field strength of 10-40 V / cm. The pulsed electric field, by periodically changing the direction and intensity of the electric field, effectively breaks the steady-state concentration polarization layer formed on the membrane surface under traditional continuous current mode, causing a redistribution of the concentration gradient on the membrane surface. When the duty cycle is below 20%, the frequency of the electro-convection vortex and the concentration polarization relaxation period achieve optimal matching, and the ion migration efficiency reaches its peak. Experiments have shown that the sudden change in electric field strength at the beginning of the pulse induces electro-convection vortices, with the electric field strength in the core region reaching 2-3 times the conventional value. This local enhancement effect significantly promotes Cl... - Transmembrane migration. At a duty cycle of 10-20%, Cl... - The transmittance is increased by 25-40% compared to the continuous electric field mode. The application of pulsed electric field in the three-stage nanofiltration washing unit reduces the Cl- concentration in the sodium sulfate product solution. - The content decreased from 1.5-2.5% without an electric field to below 0.6%.

[0012] Preferably, in the three-stage electro-nanofiltration washing and purification process, the nanofiltration membrane of the three-stage electro-nanofiltration unit is a polyamide composite nanofiltration membrane with a molecular weight cutoff of 150-300 Da and a membrane surface zeta potential of -30 mV to -60 mV. This type of nanofiltration membrane has a negative surface charge characteristic (zeta potential -30 mV to -60 mV), and under the assistance of a pulsed electric field, the negatively charged membrane surface interacts with Cl. - The electrostatic repulsion between them weakens, Cl - It is easier to pass through; while it is more effective against SO4 2- The high electrostatic repulsion force of the chlorine effectively traps it. The three-stage electro-nanofiltration unit combines pulsed electric fields with dilution washing to achieve Cl... - With SO4 2- Highly efficient separation of Cl in sodium sulfate product solution - The content should be controlled below 0.6%.

[0013] Preferably, in the primary nanofiltration salt separation, the nanofiltration membrane of the primary nanofiltration unit is a polyamide composite nanofiltration membrane with a molecular weight cutoff of 200-400 Da, an operating pressure of 1.0-3.0 MPa, and an operating temperature of 20-35℃.

[0014] Preferably, the total dissolved solids of the high-salinity wastewater is 20,000-80,000 mg / L, and the Cl... - Concentrations of 5000-25000 mg / L, SO4 2- The concentration is 3000-20000 mg / L.

[0015] Preferably, the high-salt wastewater is further pretreated before the first-stage nanofiltration desalination. The pretreatment includes one or more of the following: coagulation sedimentation, multi-media filtration, ultrafiltration, and ion exchange softening. After pretreatment, the turbidity of the wastewater is not greater than 1 NTU, the hardness (calculated as CaCO3) is not greater than 50 mg / L, and the COD is not greater than 100 mg / L.

[0016] Preferably, the method further includes product output: passing the sodium sulfate product solution into a sodium sulfate evaporation and crystallization unit or a freeze crystallization unit to produce anhydrous sodium sulfate product, and passing the sodium chloride product solution into a sodium chloride evaporation and crystallization unit or a bipolar membrane electrodialysis unit to produce sodium chloride product or acid-base product.

[0017] The beneficial effects of this invention are as follows: (1) This invention introduces a DC electric field and a pulsed electric field, using electric field force as an additional driving mechanism, to maintain the high SO4 content of the nanofiltration membrane. 2- While significantly increasing the retention rate, Cl - The transmembrane migration rate. Electric field-assisted nanofiltration can actively regulate ion migration behavior while maintaining the advantages of membrane separation, reducing the ion transport activation energy difference of traditional nanofiltration from -0.4 kJ·mol⁻¹. -1 Increased to 8.0 kJ·mol -1 above.

[0018] (2) In this invention, a pulsed electric field is introduced into the three-stage electro-nanofiltration unit. The electric convection eddies generated by the pulse effectively destroy the concentration polarization layer on the membrane surface, promoting the accumulation of Cl near the membrane surface. - The membrane desorbs and migrates to the permeate side. This design extends the single continuous operation cycle of nanofiltration membranes from 6-9 months in conventional processes to more than 14 months, significantly reducing membrane replacement frequency and operation and maintenance costs.

[0019] (3) Based on primary nanofiltration for salt separation, this invention employs a dual-field synergistic scheme of DC electric field-assisted secondary electro-nanofiltration and pulsed electric field-assisted tertiary electro-nanofiltration washing, simultaneously achieving high-purity purification on both the sodium chloride and sodium sulfate sides. SO4 in the secondary electro-nanofiltration product water... 2- The residual amount was reduced to below 0.3%, and the sodium chloride purity reached over 99.2%; the Cl in the concentrated water from the three-stage electrofiltration was reduced to below 0.3%. - With the content reduced to below 0.6% and the sodium sulfate purity reaching over 98.8%, the purity of both products is significantly higher than that of traditional nanofiltration salt separation processes, and also superior to multi-stage nanofiltration schemes without electric fields.

[0020] (4) This invention enhances separation efficiency through electric field assistance. With the optimized design of dilution water temperature and ratio, the total water recovery rate of the system can reach over 93%, and the yield of mixed salt mother liquor is reduced to below 5%. Electric field assistance significantly improves the separation accuracy of nanofiltration salt separation. The downstream evaporation and crystallization unit does not need to maintain product purity by discharging a large amount of mother liquor, and the production of mixed salt is significantly reduced, thereby reducing the cost of hazardous waste disposal.

[0021] Other features and advantages of the present invention will be described in detail in the following detailed description section. Detailed Implementation

[0022] The following provides a detailed description of specific embodiments of the present invention. It should be understood that the specific embodiments described herein are for illustrative and explanatory purposes only and are not intended to limit the scope of the invention.

[0023] It should be noted that all reagents and raw materials used in this invention are commercially available, and the reagents are of analytical grade.

[0024] The nanofiltration membranes for both the primary and tertiary nanofiltration units are sourced from Entai Environmental Protection Technology (Changzhou) Co., Ltd., and are model number MASP300-8040. The nanofiltration membrane for the secondary nanofiltration unit is sourced from Suzhou Surui Membrane Nanotechnology Group Co., Ltd., and is model number NFEP-400.

[0025] Example 1 High-salinity wastewater quality: TDS 47650 mg / L, Cl - 14280 mg / L, SO4 2- 11670 mg / L, COD 72 mg / L, hardness (calculated as CaCO3) 38 mg / L.

[0026] First-stage nanofiltration: operating pressure 2.2 MPa, operating temperature 26℃, membrane module MASP300-8040; Secondary electro-nanofiltration: operating pressure 1.4MPa, operating temperature 26℃, membrane module NFEP-400, DC electric field strength 12V / cm, direction perpendicular to the membrane surface; Three-stage electro-nanofiltration: operating pressure 1.3MPa, membrane module MASP300-8040, pulsed electric field square wave, frequency 50Hz, duty cycle 15%, electric field strength 25V / cm; Dilution water: MVR condensate, temperature 10℃, mixed with first-stage nanofiltration concentrate at a volume ratio of 1:1.2.

[0027] A nanofiltration desalination and resource recovery process for high-salinity wastewater includes the following steps: First-stage nanofiltration for salt separation: High-salinity wastewater is separated at a flow rate of 25m³. 3The solution is fed into the primary nanofiltration unit at a flow rate of 16.2 m³ / h for nanofiltration separation, yielding primary nanofiltration permeate containing sodium chloride and primary nanofiltration concentrate containing sodium sulfate. 3 / h,Cl - 22000mg / L, SO4 2- 110 mg / L; First-stage nanofiltration concentrate flow rate 8.8 m³ / h 3 / h,Cl - 68mg / L, SO4 2- 32950 mg / L; Secondary electro-nanofiltration purification: The primary nanofiltration product water is purified at 16.2m³ / h. 3 The solution is fed into the secondary electrofiltration unit at a rate of / h, where secondary salt separation is performed under the assistance of a DC electric field of 12V / cm. The concentrate from the secondary electrofiltration unit (Cl) is... - 22570mg / L, SO4 2- 267 mg / L) at 4.2 m 3 The permeate from the secondary electrofiltration unit is returned at a rate of 12.0 m³ / h to the inlet of the primary nanofiltration unit. The permeate from the secondary electrofiltration unit is output as sodium chloride product solution. 3 / h,Cl - 21800mg / L, SO4 2- 55 mg / L; Three-stage electrofiltration washing and purification: 8.8 ml of first-stage nanofiltration concentrate. 3 Mix / h with dilution water at a volume ratio of 1:1.2 (dilution water 10.6m³). 3 The water (Cl) is introduced into a three-stage electro-nanofiltration unit at a frequency of 50 Hz, a duty cycle of 15%, and an electric field strength of 25 V / cm under the assistance of a pulsed electric field. The product water (Cl) from the three-stage electro-nanofiltration unit... - 16.7 mg / L, SO4 2- 1.3 mg / L) at 12.2 m 3 The water is returned to the inlet of the first-stage nanofiltration unit at a flow rate of 7.2 m³ / h. The concentrated water from the third-stage electrofiltration is output as the sodium sulfate product solution. 3 / h,Cl - 55mg / L, SO4 2- 40270mg / L; Product output: The concentrated water from the three-stage electrofiltration nanofiltration process was fed into an MVR evaporator crystallizer, where it was concentrated and crystallized at an evaporation temperature of 90℃ and an evaporation pressure of -0.07MPa, with a concentration ratio controlled at 3.5 times. After thickening and centrifugation, the wet crystals were dried in a fluidized bed at 125℃ to obtain approximately 0.82 t / h of anhydrous sodium sulfate product and approximately 0.5 m³ of mother liquor. 3 / h) Return to the inlet of the third-stage electro-nanofiltration system for reprocessing; The permeate from the secondary electrofiltration nanofiltration process is fed into a forced circulation evaporator crystallizer, where it is concentrated and crystallized at an evaporation temperature of 110℃ and an evaporation pressure of -0.05MPa, with a concentration ratio controlled at 4.0 times. After thickening and centrifugation, the wet crystals are dried in a fluidized bed at 135℃ to obtain approximately 0.23 t / h of sodium chloride product and approximately 0.6 m³ of mother liquor. 3 ( / h) Return to the first-stage nanofiltration inlet for reprocessing.

[0028] Example 2 High-salinity wastewater quality: TDS 52730 mg / L, Cl - 16740 mg / L, SO4 2- 13820 mg / L, COD 68 mg / L, hardness (calculated as CaCO3) 41 mg / L.

[0029] First-stage nanofiltration: operating pressure 2.5 MPa, operating temperature 26℃, membrane module MASP300-8040; Secondary electro-nanofiltration: operating pressure 1.4MPa, operating temperature 26℃, membrane module NFEP-400, DC electric field strength 15V / cm, direction perpendicular to the membrane surface; Three-stage electro-nanofiltration: operating pressure 1.3MPa, membrane module MASP300-8040, pulsed electric field square wave, frequency 80Hz, duty cycle 12%, electric field strength 30V / cm; Dilution water: MVR condensate, temperature 8℃, mixed with first-stage nanofiltration concentrate at a volume ratio of 1:1.5.

[0030] A nanofiltration desalination and resource recovery process for high-salinity wastewater includes the following steps: First-stage nanofiltration for salt separation: High-salinity wastewater is separated at a flow rate of 25m³. 3 The solution is fed into the primary nanofiltration unit at a flow rate of 16.5 m³ / h for nanofiltration separation, yielding primary nanofiltration permeate containing sodium chloride and primary nanofiltration concentrate containing sodium sulfate. 3 / h,Cl - 25360mg / L, SO4 2- 130 mg / L; First-stage nanofiltration concentrate flow rate 8.5 m³ / h 3 / h,Cl - 7mg / L, SO4 2- 40395mg / L; Secondary electro-nanofiltration purification: The primary nanofiltration product water is purified at 16.5m³ / h. 3 The solution is fed into the secondary electrofiltration unit at a rate of / h, where secondary salt separation is performed under the assistance of a DC electric field of 15V / cm. The concentrate from the secondary electrofiltration unit (Cl) is... - 24423 mg / L, SO4 2- 400 mg / L) at 4.2 m 3The permeate from the secondary electrofiltration unit is returned at a rate of 12.3 m³ / h to the inlet of the primary nanofiltration unit. The permeate from the secondary electrofiltration unit is output as sodium chloride product solution. 3 / h,Cl - 25680mg / L, SO4 2- 38 mg / L; Three-stage electrofiltration washing and purification: 8.5 ml of first-stage nanofiltration concentrate. 3 Mix / h with dilution water at a volume ratio of 1:1.5 (dilution water 12.8m³). 3 The water (Cl) is introduced into a three-stage electro-nanofiltration unit and nanofiltration separation is performed under the assistance of a pulsed electric field. The frequency of the pulsed electric field is 80Hz, the duty cycle is 12%, and the electric field strength is 30V / cm. The product water (Cl) from the three-stage electro-nanofiltration unit is... - 1.9 mg / L, SO4 2- 14.8 mg / L) at 13.7 mg 3 The water is returned at a rate of 7.6 m³ / h to the inlet of the first-stage nanofiltration unit. The concentrated water from the third-stage electrofiltration is output as the sodium sulfate product solution, with a flow rate of 7.6 m³ / h. 3 / h,Cl - 4.5 mg / L, SO4 2- 45152 mg / L; Product output: The concentrated water from the three-stage electrofiltration nanofiltration process was fed into an MVR evaporator crystallizer, where it was concentrated and crystallized at an evaporation temperature of 90℃ and an evaporation pressure of -0.07MPa, with a concentration ratio controlled at 4.0 times. After thickening and centrifugation, the wet crystals were dried in a fluidized bed at 125℃ to obtain approximately 0.73 t / h of anhydrous sodium sulfate product and approximately 0.4 m³ of mother liquor. 3 / h) Return to the inlet of the third-stage electro-nanofiltration system for reprocessing; The permeate from the secondary electrofiltration nanofiltration process is fed into a forced circulation evaporator crystallizer, where it is concentrated and crystallized at an evaporation temperature of 110℃ and an evaporation pressure of -0.05MPa, with a concentration ratio controlled at 4.2 times. After thickening and centrifugation, the wet crystals are dried in a fluidized bed at 135℃, yielding approximately 0.32 t / h of sodium chloride product and approximately 0.5 m³ of mother liquor. 3 ( / h) Return to the first-stage nanofiltration inlet for reprocessing.

[0031] Example 3 High-salinity wastewater quality: TDS 41820 mg / L, Cl - 11250 mg / L, SO4 2- 9460 mg / L, COD 75 mg / L, hardness (calculated as CaCO3) 35 mg / L.

[0032] First-stage nanofiltration: operating pressure 1.8MPa, operating temperature 23℃, membrane module MASP300-8040; Secondary electro-nanofiltration: operating pressure 1.4MPa, operating temperature 26℃, membrane module NFEP-400, DC electric field strength 8V / cm, direction perpendicular to the membrane surface; Three-stage electro-nanofiltration: operating pressure 1.3MPa, membrane module MASP300-8040, pulsed electric field sine wave, frequency 120Hz, duty cycle 20%, electric field strength 15V / cm; Dilution water: MVR condensate, temperature 15℃, mixed with first-stage nanofiltration concentrate at a volume ratio of 1:0.8.

[0033] A nanofiltration desalination and resource recovery process for high-salinity wastewater includes the following steps: First-stage nanofiltration for salt separation: high-salinity wastewater is separated at a concentration of 30m³. 3 The solution is fed into the primary nanofiltration unit at a flow rate of 19.8 m³ / h for nanofiltration separation, yielding primary nanofiltration permeate containing sodium chloride and primary nanofiltration concentrate containing sodium sulfate. 3 / h,Cl - 17020 mg / L, SO4 2- 90 mg / L; first-stage nanofiltration concentrate flow rate 10.2 m³ / h 3 / h,Cl - 49 mg / L, SO4 2- 27649 mg / L; Secondary electro-nanofiltration purification: The primary nanofiltration product water is purified at 19.8m³ / h. 3 The solution is fed into the secondary electrofiltration unit at a rate of / h, where secondary salt separation is performed under the assistance of a DC electric field of 8V / cm. The concentrate from the secondary electrofiltration unit (Cl) is... - 20370mg / L, SO4 2- 315 mg / L) at 3.3 m 3 The permeate from the secondary electrofiltration unit is returned at a rate of 16.5 m³ / h to the inlet of the primary nanofiltration unit. The permeate from the secondary electrofiltration unit is output as sodium chloride product solution. 3 / h,Cl - 16350mg / L, SO4 2- 45 mg / L; Three-stage electrofiltration washing and purification: 10.2 ml of first-stage nanofiltration concentrate. 3 Mix / h with dilution water at a volume ratio of 1:0.8 (8.2m³ of dilution water). 3 The water (Cl) is introduced into a three-stage electro-nanofiltration unit and nanofiltration separation is performed under the assistance of a pulsed electric field. The frequency of the pulsed electric field is 120Hz, the duty cycle is 20%, and the electric field strength is 15V / cm. The product water (Cl) from the three-stage electro-nanofiltration unit is... - 13.1 mg / L, SO4 2- 0.5 mg / L) at 9.9 m 3 The water is returned at a rate of 8.5 m³ / h to the inlet of the first-stage nanofiltration unit. The concentrated water from the third-stage electrofiltration is output as the sodium sulfate product solution, with a flow rate of 8.5 m³ / h.3 / h,Cl - 44 mg / L, SO4 2- 33178 mg / L; Product output: The concentrated water from the three-stage electrofiltration process is fed into a cryogenic crystallizer, where indirect cooling is achieved using an ethylene glycol aqueous solution (30 wt%) at an operating temperature of -3°C and a residence time of 12 hours. This allows sodium sulfate to precipitate as sodium sulfate. After thickening and centrifugation, the wet crystals are melt-dried at 95°C to convert them into anhydrous sodium sulfate, yielding approximately 0.61 t / h of product and approximately 0.8 m³ of mother liquor. 3 / h) Return to the inlet of the third-stage electro-nanofiltration system for reprocessing; The permeate from the secondary electro-nanofiltration process is fed into a triple-effect co-current evaporator crystallizer (first effect 110℃ / -0.02 MPa, second effect 100℃ / -0.05 MPa, third effect 80℃ / -0.07 MPa), achieving a concentration ratio of 4.5 times. After thickening and centrifugation, the wet crystals are dried in a fluidized bed at 135℃, yielding approximately 0.28 t / h of sodium chloride product and approximately 1.2 m³ of mother liquor. 3 ( / h) Return to the first-stage nanofiltration inlet for reprocessing.

[0034] Example 4 High-salinity wastewater quality: TDS 59400 mg / L, Cl - 19500 mg / L, SO4 2- 12450mg / L, COD 82mg / L, hardness (calculated as CaCO3) 45mg / L.

[0035] First-stage nanofiltration: operating pressure 2.8 MPa, operating temperature 32℃, membrane module MASP300-8040; Secondary electro-nanofiltration: operating pressure 1.4MPa, operating temperature 26℃, membrane module NFEP-400, DC electric field strength 18V / cm, direction perpendicular to the membrane surface; Three-stage electro-nanofiltration: operating pressure 1.3MPa, membrane module MASP300-8040, pulsed electric field sine wave, frequency 30Hz, duty cycle 10%, electric field strength 40V / cm; Dilution water: MVR condensate, temperature 6℃, mixed with first-stage nanofiltration concentrate at a volume ratio of 1:1.8.

[0036] A nanofiltration desalination and resource recovery process for high-salinity wastewater includes the following steps: First-stage nanofiltration for salt separation: high-salinity wastewater is separated at 22m... 3 The solution is fed into the primary nanofiltration unit at a flow rate of 14.5 m³ / h for nanofiltration separation, yielding primary nanofiltration permeate containing sodium chloride and primary nanofiltration concentrate containing sodium sulfate. 3 / h,Cl -19550 mg / L, SO4 2- 140 mg / L; First-stage nanofiltration concentrate flow rate 7.5 m³ / h 3 / h,Cl - 70mg / L, SO4 2- 36249 mg / L; Secondary electro-nanofiltration purification: The primary nanofiltration product water is purified at 14.5m³ / h. 3 The solution is fed into the secondary electrofiltration unit at a rate of / h, where secondary salt separation is performed under the assistance of a DC electric field of 18V / cm. The concentrate from the secondary electrofiltration unit (Cl) is... - 27650mg / L, SO4 2- 368 mg / L) at 3.7 m 3 The permeate from the secondary electrofiltration unit is returned at a rate of 10.8 m³ / h to the inlet of the primary nanofiltration unit. The permeate from the secondary electrofiltration unit is output as sodium chloride product solution. 3 / h,Cl - 30200mg / L, SO4 2- 62 mg / L; Three-stage electrofiltration washing and purification: 7.5ml of first-stage nanofiltration concentrate. 3 Mix / h with dilution water at a volume ratio of 1:1.8 (dilution water 13.5m³). 3 The water (Cl) is introduced into a three-stage electro-nanofiltration unit and nanofiltration separation is performed under the assistance of a pulsed electric field. The frequency of the pulsed electric field is 30Hz, the duty cycle is 10%, and the electric field strength is 40V / cm. The product water (Cl) from the three-stage electro-nanofiltration unit is... - 17.5 mg / L, SO4 2- 6.8 mg / L) at 14.6 m 3 The water is returned at a rate of 6.4 m³ / h to the inlet of the first-stage nanofiltration unit. The concentrated water from the third-stage electrofiltration is output as the sodium sulfate product solution, with a flow rate of 6.4 m³ / h. 3 / h,Cl - 42 mg / L, SO4 2- 42464 mg / L; Product output: Three-stage electrofiltration concentrate 6.4m³ 3 The sodium sulfate is fed into a cryogenic crystallizer at a rate of / h, and indirectly cooled using an ethylene glycol aqueous solution (30 wt%) at an operating temperature of -3°C for a residence time of 12h, causing sodium sulfate to precipitate as sodium sulfate. Approximately 60% of the sodium sulfate is melt-dried to obtain approximately 0.63 t / h of anhydrous sodium sulfate product; the remaining portion (approximately 40%) is prepared into a 30 wt% sodium sulfate solution and passed into a bipolar membrane electrodialysis system (current density 500 A / m). 2 (Operating voltage 60V), producing H2SO4 and NaOH. The effluent from the bipolar membrane desalination chamber (approximately 0.3 m³) 3 / h) Returns to the inlet of the third-stage electro-nanofiltration system; Two-stage electrofiltration produces 10.8m³ of water.3 The solution is fed into a forced circulation evaporator crystallizer at a rate of / h, and concentrated and crystallized at an evaporation temperature of 110℃ and an evaporation pressure of -0.05MPa, with a concentration ratio controlled at 4.0 times. After thickening and centrifugation, the wet crystals are dried in a fluidized bed at 135℃ to obtain approximately 0.34t / h of sodium chloride product and approximately 0.2m³ of crystallization mother liquor. 3 ( / h) Return to the first-stage nanofiltration inlet for reprocessing.

[0037] Comparative Example 1 Compared with Example 1, the difference is that the secondary electro-nanofiltration purification, tertiary electro-nanofiltration washing purification and product output steps are not included. In the primary nanofiltration desalination step, the primary nanofiltration permeate is directly output as sodium chloride product liquid, and the primary nanofiltration concentrate is directly output as sodium sulfate product liquid.

[0038] Comparative Example 2 The difference from Example 1 is that no electric field is applied in the secondary electro-nanofiltration purification and the tertiary electro-nanofiltration washing purification steps.

[0039] Comparative Example 3 The difference from Example 1 is that no electric field is applied during the three-stage electro-nanofiltration washing and purification step.

[0040] Comparative Example 4 The difference compared to Example 1 is that no electric field is applied in the secondary electro-nanofiltration purification step.

[0041] Comparative Example 5 Compared with Example 1, the difference is that the temperature of the dilution water in the three-stage electrofiltration washing and purification step is the same as the temperature of the first-stage nanofiltration concentrate, which is 26°C.

[0042] The intermediates and products obtained in Examples 1-4 and Comparative Examples 1-5 were subjected to relevant performance tests. The test indicators and methods are as follows: Sodium chloride purity: Refer to GB / T 5462-2015 "Industrial Salt" and GB / T 13025.5-2012 "General Test Methods for Salt Production Industry - Determination of Chloride Ions". Dissolve sodium chloride in water to prepare a neutral sample solution. Use potassium chromate as an indicator and titrate with silver nitrate standard solution to determine the chloride ion content. Calculate the chloride ion content based on the volume of silver nitrate solution consumed in the titration. The sodium chloride content is calculated sequentially according to the ion binding order (calcium sulfate → magnesium sulfate → sodium sulfate → calcium chloride → magnesium chloride → sodium chloride). Take the arithmetic mean of three parallel determinations as the final result.

[0043] Sodium sulfate purity: The purity shall be determined in accordance with GB / T 6009-2014 "Industrial Anhydrous Sodium Sulfate". After dissolving the anhydrous sodium sulfate product in water and filtering out the insoluble matter, barium chloride shall be added under acidic conditions to precipitate the sulfate ions in the test solution. The precipitate shall be filtered, ashed, and ignited to constant weight before weighing. The sodium sulfate content shall be calculated by multiplying the mass of barium sulfate by the conversion factor (Na2SO4 / BaSO4 molar mass ratio = 142.04 / 233.39 = 0.6086). The purity of sodium sulfate shall be expressed as mass fraction (%).

[0044] SO4 in sodium chloride products 2- Content: According to GB / T 13025.8-2012 "General Test Methods for Determination of Sulfate in Salt Industry", after dissolving the sodium chloride product sample, the total amount of calcium and magnesium in the sample solution is titrated with EDTA standard titration solution in an ammonia buffer solution with a pH of approximately 10, using Eriochrome Black T as an indicator (the volume consumed is recorded as V1); another portion of the same sample solution is taken, and excess barium chloride is added to precipitate the sulfate ions to form barium sulfate precipitate. After filtering to remove the precipitate, the remaining calcium and magnesium ions in the filtrate are titrated with EDTA standard titration solution (the volume consumed is recorded as V2); the sulfate content is calculated based on the volume difference of EDTA consumed in the two titrations (V1-V2).

[0045] Sodium sulfate products contain Cl - Content: The determination of chloride content shall be performed according to section 6.6 of GB / T 6009-2014 "Industrial Anhydrous Sodium Sulfate". Weigh 20g of the sodium sulfate solid sample to be tested and place it in a 400mL beaker. Add approximately 200mL of distilled water to dissolve it. Heat to boiling to remove carbon dioxide. After cooling to room temperature, transfer the solution to a 250mL volumetric flask, dilute to the mark with distilled water, and shake well to obtain the test solution. Transfer 25mL of the test solution to an Erlenmeyer flask, dilute with water to approximately 100mL, add 0.5mL of potassium chromate solution (100 g / L), and titrate with silver nitrate standard titration solution (concentration approximately 0.05 mol / L) until the solution forms a brick-red suspension that does not disappear. The chloride content is expressed as the mass fraction of chlorine (Cl) and calculated based on the volume of silver nitrate standard titration solution consumed in the titration. The arithmetic mean of two parallel determinations shall be taken as the final result.

[0046] Nanofiltration membrane continuous operation cycle: Timing begins from the start of the nanofiltration system's restart after cleaning. Inlet pressure, concentrate pressure, and permeate flow rate are continuously monitored. Chemical cleaning is deemed necessary when the transmembrane pressure difference increases by 15% from its initial value or the standardized permeate flux decreases by 15% from its initial value. This continuous operation time is recorded. The formula for calculating permeate flux is: Where Q is the permeate flow rate, A is the membrane area, and ΔP is the operating pressure difference.

[0047] The performance test results are shown in Table 1.

[0048] Table 1 Performance Test Results

[0049] Comparative Example 2 employed a three-stage nanofiltration cascade without applying an electric field. The purity of sodium chloride was 97.4%, and the purity of sodium sulfate was 96.2%. The Cl content in the sodium sulfate product solution was... - The content was as high as 1.18%. Compared with Comparative Example 2, after applying a DC electric field and a pulsed electric field in Example 1, the purity of sodium chloride increased to 98.9% (+1.5 percentage points), and the purity of sodium sulfate increased to 97.9% (+1.7 percentage points). The Cl content in the sodium sulfate product solution was... - The content decreased from 1.18% to 0.82% (a reduction of 30%), which verifies the significant effect of electric field assistance in improving the purity of salt separation.

[0050] (2) In Comparative Example 3, only the second stage was subjected to a DC electric field, and the third stage was not subjected to a pulsed electric field. The Cl in the sodium sulfate product solution was... - The content was as high as 2.02%; in Comparative Example 4, only a pulsed electric field was applied at the third stage, and no DC electric field was applied at the second stage, resulting in a lower SO4 content in the sodium chloride product solution. 2- The content was as high as 1.46%. Neither method achieved simultaneous high purification of the two products; only when the secondary DC electric field and the tertiary pulsed electric field worked synergistically (Example 1) did the SO4 content in the sodium chloride product solution decrease. 2- The content decreased to 0.37%, and the Cl in the sodium sulfate product solution... - The content decreased to 0.82%, indicating that the synergistic effect of the dual electric fields plays an important role in product purification.

[0051] (3) In Comparative Example 3, without applying a pulsed electric field to the third stage, the membrane operating cycle was only 10 months. In Example 1, after applying a pulsed electric field to the third stage, the Cl in the sodium sulfate product solution... - The concentration decreased from 2.02% to 0.82% (a 59% reduction), and the membrane operating cycle was extended from 10 months to 14 months. The electric convection eddies generated by the pulsed electric field effectively disrupted the concentration polarization layer on the membrane surface, promoting the activity of Cl near the membrane surface. - The desorption and transmembrane migration of the pulsed electric field significantly reduced the membrane fouling rate, indicating that the pulsed electric field has an improving effect on the concentration polarization of the membrane surface.

[0052] (4) The dilution water temperature of Comparative Example 5 was the same as that of the first-stage nanofiltration concentrate (26°C), with no temperature gradient. The Cl in the sodium sulfate product solution was... - The content was 1.08%. In Example 1, after diluting with water at 10°C and creating a 16°C temperature gradient, the Cl content in the sodium sulfate product solution was... - The content decreased to 0.82%. The mechanism of the temperature gradient is that at low temperatures, Cl... -The degree of hydration decreases and the hydration radius shrinks. Simultaneously, the nanofiltration membrane pore size slightly shrinks at low temperatures, creating a "molecular sieve concentration" effect. - The significantly enhanced selectivity indicates that the temperature gradient has a strengthening effect on washing.

[0053] (5) The membrane operation cycle of the three-stage nanofiltration cascade without electric field in Comparative Example 2 is 9 months; after applying electric field assistance in Example 1, the membrane operation cycle is extended to 14 months (extended by 56%), indicating that electric field assistance has an overall economic improvement on the system.

[0054] The preferred embodiments of the present invention have been described in detail above. However, the present invention is not limited to the specific details of the above embodiments. Within the scope of the technical concept of the present invention, various simple modifications can be made to the technical solutions of the present invention, and these simple modifications all fall within the protection scope of the present invention. Furthermore, it should be noted that the various specific technical features described in the above embodiments can be combined in any suitable manner without contradiction. To avoid unnecessary repetition, the present invention will not further describe the various possible combinations.

[0055] Furthermore, various different embodiments of the present invention can be combined in any way, as long as they do not violate the spirit of the present invention, they should also be regarded as the content disclosed by the present invention.

Claims

1. A nanofiltration desalination and resource recovery process for high-salinity wastewater, characterized in that, Includes the following steps: First-stage nanofiltration desalination: High-salt wastewater is passed into a first-stage nanofiltration unit for nanofiltration separation to obtain first-stage nanofiltration permeate containing sodium chloride and first-stage nanofiltration concentrate containing sodium sulfate; Secondary electro-nanofiltration purification: The primary nanofiltration permeate is passed into the secondary electro-nanofiltration unit for secondary salt separation under the assistance of a DC electric field. The electric field strength of the DC electric field is 2-30 V / cm. The concentrate from the secondary electro-nanofiltration is returned to the inlet of the primary nanofiltration unit, and the permeate from the secondary electro-nanofiltration is output as sodium chloride product solution. Three-stage electro-nanofiltration washing and purification: The concentrate from the first-stage nanofiltration is mixed and diluted with dilution water and then passed into the third-stage electro-nanofiltration unit. Nanofiltration separation is performed under the assistance of a pulsed electric field. The frequency of the pulsed electric field is 10-200Hz, the duty cycle is 5-30%, and the electric field strength is 5-50V / cm. The permeate from the third-stage electro-nanofiltration is returned to the inlet of the first-stage nanofiltration unit, and the concentrate from the third-stage electro-nanofiltration is output as sodium sulfate product solution.

2. The nanofiltration desalination and resource recovery process for high-salinity wastewater according to claim 1, characterized in that, In the secondary electrofiltration purification, the electric field strength of the DC electric field is 5-20V / cm, the direction is perpendicular to the membrane surface, the anode is set on the feed side, and the cathode is set on the permeate side.

3. The nanofiltration desalination and resource recovery process for high-salinity wastewater according to claim 1, characterized in that, In the secondary electrofiltration purification process, the nanofiltration membrane of the secondary electrofiltration unit is a positively charged nanofiltration membrane with a zeta potential of +15 mV to +35 mV and a pure water flux of not less than 60 L·m⁻¹. -2 ·h -1 ·bar -1 .

4. The nanofiltration desalination and resource recovery process for high-salinity wastewater according to claim 1, characterized in that, In the three-stage electrofiltration washing and purification process, the temperature of the dilution water is 5-20℃, the volume mixing ratio of the dilution water to the first-stage nanofiltration concentrate is 1:(0.5-2.0), and the dilution water is one or a combination of two or more of the cooling water of the bipolar membrane electrodialysis unit, the condensate of the MVR evaporation crystallization unit, or the permeate of reverse osmosis.

5. The nanofiltration desalination and resource recovery process for high-salinity wastewater according to claim 1, characterized in that, In the three-stage electro-nanofiltration washing and purification process, the pulsed electric field is a square wave pulse or a sine wave pulse, with a pulse duty cycle of 10-20% and an electric field strength of 10-40 V / cm.

6. The nanofiltration desalination and resource recovery process for high-salinity wastewater according to claim 1, characterized in that, In the three-stage electro-nanofiltration washing and purification process, the nanofiltration membrane of the three-stage electro-nanofiltration unit is a polyamide composite nanofiltration membrane with a molecular weight cutoff of 150-300 Da and a membrane surface zeta potential of -30 mV to -60 mV.

7. The nanofiltration desalination and resource recovery process for high-salinity wastewater according to claim 1, characterized in that, In the primary nanofiltration salt separation, the nanofiltration membrane of the primary nanofiltration unit is a polyamide composite nanofiltration membrane with a molecular weight cutoff of 200-400 Da, an operating pressure of 1.0-3.0 MPa, and an operating temperature of 20-35℃.

8. The nanofiltration desalination and resource recovery process for high-salinity wastewater according to claim 1, characterized in that, The total dissolved solids in the high-salinity wastewater are 20,000-80,000 mg / L, Cl... - Concentrations of 5000-25000 mg / L, SO4 2- The concentration is 3000-20000 mg / L.

9. The nanofiltration desalination and resource recovery process for high-salinity wastewater according to claim 1, characterized in that, Before the first-stage nanofiltration desalination, the high-salt wastewater is pretreated. The pretreatment includes one or more of the following: coagulation sedimentation, multi-media filtration, ultrafiltration, and ion exchange softening. After pretreatment, the turbidity of the wastewater is no greater than 1 NTU, the hardness (calculated as CaCO3) is no greater than 50 mg / L, and the COD is no greater than 100 mg / L.

10. The nanofiltration desalination and resource recovery process for high-salinity wastewater according to claim 1, characterized in that, It also includes product output: passing the sodium sulfate product solution into a sodium sulfate evaporation and crystallization unit or a freeze crystallization unit to produce anhydrous sodium sulfate product, and passing the sodium chloride product solution into a sodium chloride evaporation and crystallization unit or a bipolar membrane electrodialysis unit to produce sodium chloride product or acid-base product.

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