Process for separating and crystallizing salt in salt-containing wastewater

Abstract

The present application relates to the technical field of high-salinity wastewater treatment, in particular to a salt separation and crystallization resourceization process for salt-containing wastewater. The present application overcomes the problems of low salt purity, membrane assembly easy to be blocked and system easy to be scaled in the prior art of high-salinity nitrogen-containing wastewater salt recovery. The present application realizes the front-end cooperative interception of macromolecular organic matter and scaling ions by adding porous composite materials and targeted complexing agents. Then, through double-membrane separation, the concentrated water side is frozen to recover sodium sulfate, a small part of the permeate is converted into acid and alkali through a bipolar membrane for reuse, and most of the permeate enters a one-stage evaporation crystallization and a two-stage flash cooling crystallization, and is matched with an anti-fouling dispersant and a crystal form inducer, respectively, to recover high-purity sodium chloride and spherical ammonium chloride. The treatment mother liquor is introduced into a heat-coupled advanced oxidation bypass purification and then recycled, realizing a bidirectional closed loop of mass and energy, and being suitable for resourceization recovery of high-salinity nitrogen-containing wastewater.

Description

Technical Field

[0001] This invention relates to the field of high-salinity wastewater treatment technology, specifically a process for the separation and crystallization of salts in saline wastewater for resource recovery. Background Technology

[0002] In the deep processing and production of fine chemicals, high-end new materials and special chemical auxiliaries, a large amount of high-salt nitrogen-containing wastewater is inevitably generated. However, such wastewater has a complex composition, containing not only high concentrations of inorganic salts such as sodium chloride and sodium sulfate and high concentrations of ammonia nitrogen, but also often enriched with unreacted resin monomers, lipophilic macromolecular organic matter of benzene rings, and trace amounts of free heavy metal ions, silicate ions and fluoride ions.

[0003] Currently, most industrial technologies for zero-discharge and resource recovery of high-salt, nitrogen-containing wastewater employ a conventional series process involving pretreatment, membrane concentration, and evaporation crystallization. However, this existing process faces serious limitations in practical high-salt environments. Firstly, there are issues with membrane fouling and system scaling. Existing pretreatment methods struggle to completely remove lipophilic macromolecular organic matter and cannot target and remove easily scaling soluble silicates and fluoride complexes. When these macromolecular organic matter and trace impurities enter the membrane system, they cause irreversible fouling of ultrafiltration and nanofiltration membranes, leading to a sharp decline in membrane flux. Furthermore, once scaling factors penetrate the membrane system and enter the high-temperature evaporation crystallization section, they rapidly deposit on the heat exchange tube walls, forming dense, hard scale. This results in a sharp drop in system heat exchange efficiency, frequent equipment cleaning, and poor overall process stability. Secondly, there is the problem of low purity of recycled salt. Due to the lack of effective organic matter interception and mother liquor discharge channels in existing processes, polar small molecule organic matter will accumulate in the closed evaporation system. At high temperatures, these organic matter will not only cause violent vapor-liquid azeotropic foaming, but also coke and adhere to the surface of the precipitated crystals. In addition, when processing ammonium chloride, traditional processes are prone to capillary entrainment effect, which will wrap the impurity-rich mother liquor inside the crystals, further causing the final recovered sodium chloride, ammonium chloride and sodium sulfate to turn yellow and smelly, and the purity is difficult to reach the industrial grade standard.

[0004] In summary, while existing treatment processes for high-salt, nitrogen-containing wastewater can achieve effective wastewater treatment, the recycling of waste salt still suffers from yellowing, foul odor, and decreased purity. In actual treatment processes, problems such as low purity of recovered salt, easy fouling of membrane modules, and easy scaling of the system still exist.

[0005] Therefore, a process for the separation and crystallization of salt in saline wastewater is proposed. Summary of the Invention

[0006] The purpose of this invention is to provide a process for the separation and crystallization of salts from saline wastewater for resource recovery. This invention overcomes the problems of low salt purity, easy fouling of membrane modules, and easy scaling in existing technologies for high-salt, nitrogen-containing wastewater. This invention achieves front-end synergistic interception of macromolecular organic matter and scaling ions by adding porous composite materials and targeted complexing agents; subsequently, through dual-membrane separation, sodium sulfate is recovered by freezing on the concentrate side; a small portion of the permeate is converted into acid and alkali for reuse via a bipolar membrane, while the majority enters a first-stage evaporation crystallization and a second-stage flash evaporation cooling crystallization, respectively, with the addition of antifouling dispersants and crystal form inducers, to recover high-purity sodium chloride and spherical ammonium chloride; the treated mother liquor is purified by a heat-coupled advanced oxidation bypass and then recycled; achieving a two-way closed loop of quality and energy, suitable for the resource recovery of high-salt, nitrogen-containing wastewater.

[0007] This invention targets high-salt nitrogen-containing wastewater with an influent pH of 4.0-6.0, a chemical oxygen demand (COD) of 3000-5000 mg / L, ammonia nitrogen of 1000-2000 mg / L, and total dissolved solids (TDS) of 30000-50000 mg / L. The wastewater contains monovalent inorganic metal salts such as sodium chloride at 15000-25000 mg / L and divalent inorganic salts such as sodium sulfate at 5000-10000 mg / L. The wastewater also contains trace amounts of free heavy metal ions, silicate ions, and fluoride ions, as well as unreacted resin monomers and lipophilic macromolecular organic impurities such as benzene rings.

[0008] To achieve the above objectives, the present invention provides the following technical solution: the number of parts in the present invention is expressed as parts by mass; This invention provides a process for the separation and crystallization of salts in saline wastewater, comprising the following preparation steps: In the co-treatment and closed-loop resource utilization stage of high-salt nitrogen-containing wastewater, the high-nitrogen-containing raw water is introduced into the pretreatment adsorption tank, and 2.0-5.0 g / L of composite porous material is added. Under normal temperature conditions, the water is mechanically stirred at 300-500 rpm for 30-45 min, and the large molecular organic matter in the wastewater is physically intercepted. The pH value of the water is adjusted to 7.0-7.5 to obtain adsorbed effluent. The adsorbed effluent is then pumped into the complexation reaction tank after passing through a 10 μm filter. 15-30 mg / L of targeted complexing agent aqueous solution (equivalent to effective solid content) is added to the complexation reaction tank, and the reaction is carried out at pH 7.5-8.0 and 200-400 rpm for 20-30 min to obtain complexed wastewater. High-salinity wastewater pretreatment involves a wide variety of organic compounds. Activated carbon alone lacks sufficient hydrophilicity and has inconsistent pore sizes; silica gel alone lacks affinity for hydrophobic aromatic hydrocarbons. This invention introduces carbon-doped mesoporous silicon. The carbon framework endows the material with specific hydrophobic targeting capabilities, while the uniform mesopores (5-15 nm) accommodate large-molecule resin monomers. Furthermore, grafted amino groups not only provide electrostatic adsorption sites but also endow the material with the ability to spontaneously regulate the pH of high-salinity wastewater, avoiding the need for large-scale alkali additions in traditional processes and preventing additional increases in system salinity. In addition, traditional precipitants not only introduce impurities but also fail to effectively remove complexed heavy metals and soluble fluorine and silicon, which are major contributors to evaporator scaling. By covalently coupling chitosan (providing abundant primary amino and hydroxyl groups for chelating heavy metals) and polyaspartic acid (providing carboxyl groups for dispersing and capturing calcium, magnesium, and fluorine-silicon ions), a dual-effect complexing agent can be formed. The resulting large-molecule complex clusters possess good stability and membrane repulsion, perfectly suited for subsequent nanofiltration membrane physical retention.

[0009] Complexed wastewater is sequentially pressurized and passed through an ultrafiltration membrane system with a molecular weight cutoff of 50,000 Da and a nanofiltration membrane system with an effective pore size of 1-2 nm. The concentrate side of the nanofiltration membrane system is enriched with divalent sulfate and complex clusters, which are then introduced into a complex-breaking reactor. A mixture of sodium sulfide and polyferric sulfate at a concentration of 500-800 mg / L is added. After stirring and reaction, solid-liquid separation is performed to remove heavy metal sludge. The resulting clear liquid enters a cryogenic crystallizer to precipitate sodium sulfate decahydrate at -5-5℃. After dehydration, anhydrous sodium sulfate product is obtained. 20% of the permeate from the nanofiltration membrane system is diverted to the bipolar membrane electrodialysis unit, where it is converted into a 3%-5% hydrochloric acid solution and a mixed alkali solution under a DC current density of 20-40 mA / cm². The resulting acid-alkali solution is returned to the front end for internal pH adjustment. The remaining 80% of the permeate is pumped into a first-stage MVR evaporator crystallizer, where 10-25 mg / L of polyepoxysuccinic acid is added to the feed line. The evaporator crystallization system is kept at a constant temperature and negative pressure at an absolute operating pressure of 0.05-0.08 MPa and a boiling point of 95-105 °C for concentration. When the ammonium chloride concentration in the mother liquor reaches 60%, centrifugation is used to obtain industrial-grade sodium chloride crystals with a purity greater than 98.5%. The high-temperature crystallization solution discharged from the first centrifuge is introduced into the second-stage vacuum flash cooling crystallizer, which is rapidly cooled to 30-35℃ at 1-2℃ / min. When the solution is cooled to 60℃, 5-15mg / L of sodium polyacrylate is added to induce the solution to precipitate dense spherical ammonium chloride particles under low-temperature supersaturation. The solution is then centrifuged and dehydrated to obtain high-purity ammonium chloride product. During the high-temperature evaporation stage, PESA exhibits excellent anti-fouling and dispersing properties, inhibiting scaling caused by localized overheating. In the second stage, during the low-temperature cooling and precipitation of ammonium chloride, PAAS precisely adsorbs onto the dominant growth surface of ammonium chloride, blocking its natural branching development. The purity of the upstream material flow ensures that the inducer will not be affected or rendered ineffective by impurities, and the successful action of the crystal inducer makes the ammonium chloride crystal particles larger and rounder, reducing the encapsulation of organic matter in the mother liquor. The overall solution forms a holistic organic technology system.

[0010] After two stages of salt extraction, 20% of the total mother liquor mass flow rate is diverted to form a bypass tributary. This bypass mother liquor undergoes cross-flow heat exchange with high-temperature sodium chloride crystals and secondary steam condensate discharged from the first-stage evaporator crystallizer via a plate heat exchanger. Preheated to 60-70°C, it is then introduced into an advanced oxidation reaction tank. Ozone gas is introduced into the reaction tank, with a liquid ozone dosage of 30-50 mg / L, and a 30% hydrogen peroxide solution is added dropwise, equivalent to a pure hydrogen peroxide dosage of 1.0-2.0 mg / L. g / L; generates a large number of hydroxyl radicals, mineralizing the enriched small molecule organic matter into water and carbon dioxide and venting it; after the oxidation reaction has been carried out for 60-90 min, sodium diethyldithiocarbamate 2.0-5.0 mg / L is added to the system; react at a stirring speed of 150-300 r / min for 60-90 min to complete the mineralization of small molecule organic matter and the chelation and solidification of trace metals to obtain purified mother liquor; the purified mother liquor and the remaining 85% of the treatment mother liquor are combined at the feed main pipe and enter the first stage of MVR evaporation crystallizer.

[0011] This process follows a synergistic approach involving macromolecular stripping, impurity localization, membrane-based ion separation, thermal crystal form induction, and mother liquor bypass self-cleaning. Pre-physical adsorption reduces the load on the subsequent nanofiltration membrane, preventing organic fouling. Targeted complexation follows to remove scale-forming ions before thermal concentration. Bipolar membrane electrodialysis utilizes pure monovalent salts to produce acids and alkalis, which are then recycled back to the front end, achieving chemical self-regulation. Finally, a crystal form inducing agent is introduced to separate ammonium salts. The preceding purification processes provide a favorable environment for crystal growth, making crystal form alteration possible. Reversing the process, such as oxidation followed by adsorption, would result in the oxidant being consumed by a large number of useless macromolecules, leading to system collapse. This scheme establishes a complementary process of adsorption load reduction and alkali adjustment - macromolecular complexation and retention - bipolar membrane closed loop - stepwise crystal form regulation - bypass heat source coupled with advanced oxidation. If advanced oxidation is performed first, not only will a large amount of oxidant be consumed due to the high concentration of organic load in the water, but the small molecules after oxidation and breakdown can easily penetrate the nanofiltration membrane and enter the crystallization system, causing the product salt to be colored and smelly. This invention postpones the oxidation stage and sets it in the bypass of the mother liquor treatment. At this time, the water has already undergone dual purification by adsorption and nanofiltration, and mineralization can be achieved with a smaller amount of reagent. In addition, the reaction activation heat required for bypass oxidation absorbs the waste heat of evaporation to form high-temperature sodium chloride, forming an internal circulation of energy flow and mass flow, avoiding excessive energy use.

[0012] Preferably, the preparation of the composite porous material includes the following steps: Dissolve 20-30 parts of block copolymer P123 in 800-1000 parts of 2.0 mol / L hydrochloric acid aqueous solution, and mechanically stir at 400-600 rpm for 60-90 min in a water bath at 35-45℃ until the solution becomes clear. Then, slowly add 40-50 parts of tetraethyl orthosilicate dissolved in 800-1000 parts of 50% ethanol aqueous solution to the solution, and simultaneously add 4-10 parts of sucrose. Continue to copolymerize under normal pressure at 400-600 rpm for 20-24 h to obtain the reaction product. Transfer the reaction product to a polytetrafluoroethylene-lined reactor and hydrothermally age it at 95-105℃. After 40-48 hours, the product was centrifuged, washed, and vacuum dried at 80℃. It was then placed in a tube furnace and heated to 600-700℃ at 2-5℃ / min under a nitrogen atmosphere. The temperature was maintained for 3-5 hours to obtain a carbonized framework. This process achieved template removal and in-situ carbonization of sucrose. 100 parts of the carbonized framework were dispersed in anhydrous toluene solvent, and 10-15 parts of 3-aminopropyltriethoxysilane were added. The mixture was refluxed at 300-500 rpm in an oil bath at 85-95℃ for 10-14 hours. The mixture was then centrifuged at 3000-5000 rpm, washed three times with anhydrous ethanol, and vacuum dried at 60℃ to obtain a composite porous material.

[0013] Preferably, the preparation of the targeted complexing agent includes the following steps: Weigh 15-25 parts of chitosan powder and dissolve it in 800-1000 parts of a 1.5%-2.5% dilute acetic acid aqueous solution, stirring until dissolved. At a constant temperature of 45-55℃, vigorously stir at 500-800 rpm, slowly adding a polyaspartic acid aqueous solution, stirring until homogeneous to obtain a mixed solution. The polyaspartic acid aqueous solution includes 30-75 parts of polyaspartic acid and 200-500 parts of deionized water. Add 0.2-1.5 parts of EDC-HCl crosslinking agent and N-hydroxysuccinimide to the mixed solution as... The composite crosslinking system has a molar ratio of EDC-HCl to NHS of 1:1 and undergoes an amidation coupling reaction for 4-6 hours. After the reaction is complete, the pH of the system is slowly adjusted to 7.0-7.5 using 0.5-1.0 mol / L sodium hydroxide solution at a rate of 1-2 mL / min to induce partial crosslinking and precipitation of the polymer. The precipitated product is then placed in a dialysis bag with a molecular weight cutoff of 3500 and dialyzed with deionized water for 48 hours. Finally, it is freeze-dried at -40℃ to obtain the targeted complexing agent. For application, it is prepared as a 5% aqueous solution.

[0014] In addition, performance data tests were conducted on the processing procedures of this invention, specifically including resource recovery purity testing and process stability testing: The purity test for resource recovery begins with the testing of sodium chloride crystal purity and whiteness. This is primarily achieved by using argentometric titration to determine the chloride ion concentration in the solution, combined with complexometric titration to deduct trace impurities such as calcium and magnesium, thus accurately calculating the mass fraction of pure sodium chloride. For the sodium chloride whiteness test, a dried sodium chloride sample is pressed into a flat surface. Following GB / T13025.2-2008, a whiteness meter is used to measure the blue light diffuse reflectance of the surface under a light source, and the result is compared with a standard white plate. This aims to visually verify the organic matter stripping in the pretreatment stage and the synergistic effect of the antifouling dispersant in the first-stage evaporation, addressing the problem of unusable recovered salt turning yellow or black due to organic matter entrainment and localized high-temperature coking in practical engineering. The second step involves testing the purity of ammonium chloride and sodium sulfate. Formaldehyde is reacted with ammonium salt using a formaldehyde method to release an equal amount of acid, which is then titrated with a standard alkaline solution to calculate the nitrogen mass fraction. The purity was then converted to ammonium chloride purity. The classic barium sulfate gravimetric method was used, where barium chloride was added to form a precipitate, and the precipitate was weighed at constant weight to calculate the purity. This verified whether the freeze crystallization and stepwise cooling flash evaporation process of this invention could break the eutectic point in complex water-salt systems and solve the technical problem of generating economically worthless complex salts and mixed salts in high-salt wastewater treatment. Finally, the crystal morphology of ammonium chloride and the entrainment rate of the mother liquor were tested. The microscopic observation of the crystal morphology was mainly achieved by obtaining high-resolution morphology images using a scanning electron microscope, while the entrainment rate of the mother liquor was tested by washing the crystals and measuring the concentration of trace trace ions (such as sodium ions) inside them to infer the entrainment rate. This examined whether the crystal form inducer added during the second-stage low-temperature crystallization successfully targeted and adsorbed and masked the dominant growth surface of ammonium chloride, solving the problem that ammonium chloride tends to grow in needle-like or dendritic shapes under conventional processes, thus generating a capillary effect that encapsulates the impurity-rich mother liquor inside the crystals. In the process stability test, the first step was to test the nanofiltration membrane flux decay rate and the decarbonization and mineralization rate of the bypass mother liquor. The initial pure water flux of the membrane module was recorded during the initial operation under constant pressure and compared with the stable flux after a certain period of continuous treatment of complexed wastewater. The percentage decrease was calculated as the nanofiltration membrane flux decay rate. The difference in total organic carbon concentration of the treated mother liquor before and after entering the advanced oxidation tank was measured to calculate the decarbonization and mineralization rate of the bypass mother liquor. The membrane flux test was used to verify whether the composite porous material and targeted complexing agent effectively removed large molecules and scale ions that caused membrane fouling at the front end. The decarbonization and mineralization rate test aimed to evaluate the oxidation efficiency of the ozone-hydrogen peroxide system and address system foaming and abnormal boiling points caused by the enrichment of polar small molecule organic matter in closed-loop processes. The tests included two main aspects: First, the oxygen demand of the second-stage condensate discharge water and the scaling rate of the evaporator crystallizer. The determination of chemical oxygen demand (COD) and biological oxygen demand (BOD) primarily involved high-temperature digestion titration and 20°C constant-temperature incubation to measure dissolved oxygen consumption. Second, the scaling rate of the evaporator crystallizer heat exchange surface was assessed using a suspended metal test piece method. Standard metal test pieces were suspended inside the boiling evaporator, and the average annual scaling thickness was calculated through precise weighing and surface area calculations. Third, the water quality test aimed to verify whether the condensate discharged from the system met stringent industrial reuse or ecological discharge requirements. Finally, the scaling rate test verified that the overall process intercepted divalent sulfate and heavy metals at the front end, preventing them from entering the high-temperature section and addressing the pain points of frequent scaling on heat exchanger walls and short equipment lifespan in industrial applications.

[0015] Compared with the prior art, the beneficial effects of the present invention are as follows: 1. This invention solves the problem of recycled salt becoming hazardous waste due to organic matter entrainment in traditional high-salt wastewater treatment; through the synergistic pretreatment of front-end composite materials and targeted complexing agents, lipophilic macromolecules that cause crystal yellowing are intercepted; at the same time, during the high-temperature evaporation and cooling crystallization stages, polyepoxysuccinic acid antifouling dispersant and sodium polyacrylate crystal form inducer are precisely introduced respectively, transforming ammonium chloride from natural needle-like to dense spherical, eliminating capillary entrainment effect, so that the purity of sodium chloride and ammonium chloride both reach industrial-grade standards, realizing high-purity resource utilization. 2. To address the industrial limitations of high-salt systems that are prone to scaling and system collapse, this invention constructs a multi-dimensional anti-scaling and flow-stabilizing system. The targeted complexing agent utilizes the synergistic chelating effect of polyaspartic acid and chitosan to lock silicate ions, fluoride ions, and heavy metals into large molecular complex clusters and physically intercept them before membrane separation, eliminating inorganic hard scale factors. In addition, through the bypass advanced oxidation impurity removal mechanism of the mother liquor, the infinite accumulation of polar small molecules in the evaporator is avoided, eliminating high-temperature foaming and abnormal boiling point phenomena, ensuring high flux of nanofiltration membrane and long-term clean and stable operation of heat exchange surface. 3. This invention achieves tiered coordination of mass flow and energy flow within the system. In the bypass section for treating mother liquor, no traditional energy-consuming external heating source is used. Instead, the sensible heat from the 95°C high-temperature sodium chloride crystals discharged by a section of evaporation centrifugation and the condensate from the secondary steam is used to preheat the bypass mother liquor to the activation temperature required for advanced oxidation through cross-flow heat exchange. At the same time, the permeate from nanofiltration is diverted to the bipolar membrane electrodialysis unit, and the converted acids and alkalis are directly recycled for front-end pH adjustment. This design of heat energy recycling and self-sufficiency of chemical reagents reduces the overall process's operating energy consumption and external purchase costs. 4. Existing conventional processes often only transfer pollutants from the aqueous phase to a solid-phase mixed salt, failing to achieve harmless treatment. This invention implements stepwise phase change control for complex water quality with high nitrogen content, ensuring that both the produced water and salt reach extremely high purity. The hydroxyl radicals generated in the bypass oxidation section mineralize residual organic matter into water and carbon dioxide, which are then released into the air. Heavy metals, after sequential solidification, are discharged as trace amounts of sludge. The COD and biochemical indicators of the final secondary condensate leaving the system meet the requirements, satisfying the high-standard reuse needs of the plant's cooling tower makeup water, and cutting off the pollution of the natural ecosystem by wastewater. Attached Figure Description

[0016] Figure 1 This is a process flow diagram of the resource utilization treatment of saline wastewater in Embodiment 1 of the present invention. Detailed Implementation

[0017] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0018] In this invention, the block copolymer P123 has an average molecular weight of 5800 and a hydrophilic-lipophilic balance (HLB) of 8; the chitosan has a degree of deacetylation ≥85%, a molecular weight of 100,000-300,000, and a viscosity of 200-400 mPa·s; the polyaspartic acid has a weight-average molecular weight of 2000-5000; the EDC-HCl crosslinking agent is 1-(3-dimethylaminopropyl)-3-ethylcarbodiimide hydrochloride; the polyepoxysuccinic acid has a weight-average molecular weight of 400-1500; the sodium polyacrylate has a weight-average molecular weight of 1000-3000; and the mixed agent is obtained by mixing sodium sulfide and polyferric sulfate in a mass ratio of 1:1.

[0019] Please see Figure 1 This invention provides a process for the separation and crystallization of salt in saline wastewater for resource recovery. The technical solution is as follows: Example 1

[0020] 25 parts of block copolymer P123 were dissolved in 900 parts of 2.0 mol / L hydrochloric acid aqueous solution. The solution was mechanically stirred at 500 rpm for 60 min in a 40℃ water bath until the solution became clear. Then, 45 parts of tetraethyl orthosilicate dissolved in 800 parts of 50% ethanol aqueous solution were slowly added dropwise to the solution, and 7 parts of sucrose were added simultaneously. The copolymerization reaction was continued at 500 rpm under normal pressure for 20 h to obtain the reaction product. The reaction product was transferred to a polytetrafluoroethylene-lined reactor and hydrothermally aged at 100℃. After 45 hours, the product was centrifuged, washed, and vacuum dried at 80°C. It was then placed in a tube furnace and heated to 650°C at 3.5°C / min under a nitrogen atmosphere, and held at this temperature for 4 hours to obtain a carbonized framework. 100 parts of the carbonized framework were dispersed in 900 parts of anhydrous toluene solvent, and 12 parts of 3-aminopropyltriethoxysilane were added. The mixture was refluxed at 400 rpm in an oil bath at 90°C for 12 hours. After centrifugation at 3000 rpm, the mixture was washed three times with anhydrous ethanol and vacuum dried at 60°C to obtain a composite porous material. 20 parts of chitosan powder were dissolved in 800 parts of 2% dilute acetic acid aqueous solution and stirred until dissolved. At a constant temperature of 50℃, the solution was vigorously stirred at 500-800 rpm, and polyaspartic acid aqueous solution was slowly added dropwise until homogeneous, resulting in a mixed solution. The polyaspartic acid aqueous solution contained 50 parts of polyaspartic acid and 350 parts of deionized water. 0.8 parts of EDC-HCl crosslinking agent and N-hydroxysuccinimide were added to the mixed solution as a composite crosslinking system, with a molar ratio of EDC-HCl to NHS of 1:1. An amidation coupling reaction was carried out for 5 hours. After the reaction was completed, the pH of the system was slowly adjusted to 7.0 using 1 mol / L sodium hydroxide solution at 1.5 mL / min to induce partial crosslinking and precipitation of the polymer, resulting in a precipitated product. The precipitated product was placed in a dialysis bag with a molecular weight cutoff of 3500 and dialyzed with deionized water for 48 hours. Finally, it was freeze-dried at -40℃ to obtain a targeted complexing agent, which was prepared as a 5% aqueous solution for application. High-nitrogen, saline raw water was introduced into a pretreatment adsorption tank, and 3.5 g / L of composite porous material was added. The mixture was mechanically stirred at 400 rpm for 40 min at room temperature, where large molecular organic matter in the wastewater was physically retained. The pH of the water was adjusted to 7.5 to obtain adsorbed effluent. The adsorbed effluent was then pumped into a complexing reaction tank after passing through a 10 μm filter. 22 mg / L of a targeted complexing agent aqueous solution (equivalent to effective solid content) was added to the complexing reaction tank, and the mixture was reacted at pH 7.5-8.0 and 300 rpm for 20 min to obtain complexed wastewater. Complexed wastewater is sequentially pressurized and passed through an ultrafiltration membrane system with a molecular weight cutoff of 50,000 Da and a nanofiltration membrane system with an effective pore size of 1-2 nm. The concentrate side of the nanofiltration membrane system is enriched with divalent sulfate and complex clusters, which are then introduced into a complex-breaking reactor. A mixed reagent of sodium sulfide and polyferric sulfate at 650 mg / L is added, and after stirring and reaction, solid-liquid separation is performed to remove heavy metal sludge. The resulting clear liquid enters a cryogenic crystallizer to precipitate sodium sulfate decahydrate at 0°C. After dehydration, anhydrous sodium sulfate product is obtained. 20% of the permeate from the nanofiltration membrane system is diverted to the bipolar membrane electrodialysis unit, where it is converted into a 5% hydrochloric acid solution and a mixed alkali solution under a DC current density of 30 mA / cm². The resulting acid-alkali solution is returned to the front end for internal pH adjustment. The remaining 80% of the permeate is pumped into a first-stage MVR evaporator crystallizer, and 18 mg / L of polyepoxysuccinic acid is added to the feed line. The evaporator crystallization system is kept at a constant temperature and negative pressure at an absolute operating pressure of 0.065 MPa and a boiling point of 100 °C for concentration. When the concentration of ammonium chloride in the mother liquor reaches 60%, industrial-grade sodium chloride crystals are obtained by centrifugation. The 100°C crystallized solution discharged from the first centrifuge is fed into the second-stage vacuum flash cooling crystallizer, where it is rapidly cooled to 35°C at a rate of 1.5°C / min. In the initial stage when the solution cools to 60°C, 10 mg / L of sodium polyacrylate is added to induce the precipitation of dense, spherical ammonium chloride particles in a low-temperature supersaturated state. The solution is then centrifuged and dehydrated to obtain a high-purity ammonium chloride product. After two stages of salt extraction, 20% of the treated mother liquor by mass flow rate is diverted to form a bypass tributary. This bypass mother liquor passes through a plate heat exchanger and undergoes cross-flow heat exchange with 95°C high-temperature sodium chloride crystals and secondary steam condensate discharged from the first-stage evaporator crystallizer. After being preheated to 65°C, it is introduced into an advanced oxidation reaction tank. Ozone gas is introduced into the reaction tank, with a liquid ozone dosage of 40 mg / L, and a 30% hydrogen peroxide aqueous solution is added dropwise, equivalent to a pure hydrogen peroxide dosage of 1.5 g / L. After the oxidation reaction has proceeded for 90 minutes, 3.5 mg / L of sodium diethyldithiocarbamate is added to the system. The reaction is carried out at a stirring speed of 200 r / min for 60 minutes to complete the mineralization of small molecule organic matter and the chelation and solidification of trace metals, resulting in purified mother liquor. The purified mother liquor and the remaining 85% of the treated mother liquor are combined at the feed main and then fed into a first-stage MVR evaporator crystallizer for repeated treatment.

[0021] Examples 2-3 follow the same preparation method and parameter conditions as Example 1, with differences shown in Table 1.

[0022]

[0023] Comparative Example 1 is the same as Example 1, except that the complexed wastewater is directly diverted to the bipolar membrane electrodialysis unit and a first-stage MVR evaporator crystallizer without separation by ultrafiltration membrane module and nanofiltration membrane system, while the rest of the process remains unchanged.

[0024] Comparative Example 2 is the same as Example 1, except that polyepoxysuccinic acid is not added to a section of the MVR evaporator crystallizer feed line, while the rest of the process remains unchanged.

[0025] Comparative Example 3 is the same as Example 1, except that sodium polyacrylate is not added in the two-stage vacuum flash cooling crystallizer, while the rest of the process remains the same.

[0026] Comparative Example 4 is the same as Example 1, except that sodium diethyldithiocarbamate is not added in the bypass advanced oxidation section, while the rest of the process remains unchanged.

[0027] Comparative Example 5 is the same as Example 1, except that sucrose is not added during the preparation of the composite porous material, while the rest of the process remains unchanged.

[0028] Comparative Example 6 is the same as Example 1, except that 3-aminopropyltriethoxysilane is not added during the preparation of the composite porous material, while the rest of the process remains unchanged.

[0029] Comparative Example 7 is the same as Example 1, except that the mother liquor is not diverted from the bypass branch into the advanced oxidation reaction tank. Instead, all the mother liquor after the two-stage salt extraction is directly merged into the first-stage MVR evaporation crystallizer. The rest of the process remains unchanged.

[0030] Comparative Example 8 is the same as Example 1, except that polyaspartic acid aqueous solution is not added during the preparation of the targeted complexing agent, and only chitosan is used for cross-linking, while the rest of the process remains unchanged.

[0031] Experimental Example 1: Yield and Purity Test The purity of the recovered metal salts obtained from Examples 1-3 and Comparative Examples 1-4 was tested; the test results are shown in Table 2.

[0032]

[0033] As shown in Table 2, the comparative examples, through adjustments to the components and processes, exhibited significant differences in the purity and crystal morphology of the recovered salts compared to the examples. In the examples, composite porous materials and targeted complexing agents were introduced in the pretreatment stage. Through the synergistic effect of physical interception and chemical targeted complexation, macromolecular organic matter, divalent sulfate, and heavy metal ions were effectively enriched and intercepted before membrane separation, providing an extremely pure mother liquor environment for subsequent thermal crystallization. In addition, polyepoxysuccinic acid was introduced in the first-stage high-temperature evaporation stage to play an anti-fouling and dispersing role, inhibiting scaling and organic matter coking caused by local overheating. In the second-stage low-temperature cooling crystallization stage, sodium polyacrylate was introduced to precisely adsorb onto the dominant growth surface of ammonium chloride, changing its natural needle-like growth habit and inducing the precipitation of high-density spherical large particles, reducing the mother liquor entrainment rate, and enabling the purity of sodium chloride, ammonium chloride, and sodium sulfate to reach industrial-grade standards.

[0034] In Comparative Example 1, the lack of physical retention and phase separation barriers in the ultrafiltration and nanofiltration membrane systems prevented the early separation of divalent sulfate and heavy metal macromolecular complexes. This led to the influx of impurity ions into the subsequent high-temperature evaporation system, forming complex double salts and hard scale during high-temperature concentration. The severe physical mixing interfered with the crystallization process of sodium chloride and ammonium chloride, resulting in a decrease in purity to the point where industrial-grade standards could not be met, and even sodium sulfate could not be effectively recovered independently. In Comparative Example 2, the system lost its chemical defenses against organic contamination and scale inhibition, causing residual trace organic matter to undergo localized overheating and coking on the surface of the high-temperature heat exchange tubes. The adhesion and co-precipitation of coke directly led to a decrease in the whiteness of sodium chloride crystals. At the same time, the crystal surface was coated with impurities, weakening the separation of sodium chloride. In Comparative Example 3, sodium polyacrylate was not added as a crystal form inducer. When ammonium chloride precipitated under supersaturation, it reverted to its natural needle-like and dendritic crystallization habits, which easily generated a capillary effect, resulting in a significant increase in the mother liquor entrainment rate. This encapsulated the extreme mother liquor rich in trace organic matter and impurities inside the crystals, leading to a decrease in the crystal purity of ammonium chloride. In Comparative Example 4, sodium diethyldithiocarbamate metal chelating curing agent was missing. Although ozone and hydrogen peroxide could mineralize organic matter, the free trace heavy metal ions released by oxidation could not be precipitated and removed. As the purified mother liquor was continuously refluxed and enriched in the evaporation system, it was incorporated into the recovered monovalent inorganic salts through lattice defects and other forms, resulting in damage to the quality of the recovered salts and a decline in overall purity.

[0035] Experiment Example 2: Stability Performance Test The resource recovery processes of Examples 1-3 and Comparative Examples 1-8 were subjected to stability tests; the test results are shown in Table 3.

[0036]

[0037] As shown in Table 3, the stability performance of the high-salt nitrogen-containing wastewater resource recovery systems obtained by adjusting the components and processes in the comparative examples is significantly different from that of the examples. In the examples, the introduction of composite porous materials and targeted complexing agents in the pretreatment stage can effectively enrich and intercept large molecular organic matter and scale ions before membrane separation through the synergistic effect of physical interception and chemical targeted complexation, ensuring the long-term high-flux operation of the nanofiltration membrane. At the same time, the bypass advanced oxidation system and the main circulation evaporation crystallization system form a closed-loop synergy of mass flow and energy flow, using the action of ozone, hydrogen peroxide and chelating agents to mineralize organic matter and solidify heavy metals, avoiding the accumulation of impurities in the system, resulting in excellent condensate discharge water quality and good cleanliness of the heat exchange surface of the evaporator crystallizer. In Comparative Example 1, the absence of ultrafiltration and nanofiltration membrane systems resulted in front-end interception failure, allowing macromolecular complexes and scaling ions to directly enter the high-temperature evaporation section. As previously mentioned, this not only reduced salt purity but also significantly increased the scaling rate on the heat exchange surface, leading to deterioration of condensate water quality. In Comparative Example 2, the lack of polyepoxysuccinic acid resulted in insufficient anti-fouling and dispersion protection on the high-temperature pipe walls, leading to a significant increase in the scaling rate due to organic matter coking. In Comparative Example 3, the absence of sodium polyacrylate primarily resulted in an increased entrainment rate of ammonium chloride crystals in the mother liquor. In Comparative Example 4, the absence of sodium diethyldithiocarbamate prevented the effective solidification of heavy metal ions in the bypass oxidation section. Precipitation and mineralization reactions were interfered with by hetero ion catalysis, resulting in a slight decrease in decarbonization and mineralization rates. In Comparative Example 5, the lack of a hydrophobic carbon framework formed by in-situ sucrose carbonization meant that mesoporous silica could not effectively target and adsorb lipophilic macromolecular organic compounds such as benzene rings in the wastewater. The high organic load directly impacted subsequent processes, leading not only to organic fouling of the nanofiltration membrane but also to volatile organic compounds entering the condenser with the steam, resulting in a significant increase in COD in the final condensate discharge. In Comparative Example 6, the lack of surface amination modification with 3-aminopropyltriethoxysilane resulted in the material surface losing its ability to provide electrostatic adsorption. The presence of alkaline groups at the site of the complexation site and in-situ buffering of the water's pH prevents the initial pH of the wastewater from spontaneously adjusting to the optimal range for complexation reactions. This leads to a decrease in the subsequent large molecule complexation interception efficiency, and the impurities that are not effectively intercepted exacerbate the pore blockage of the nanofiltration membrane, resulting in an increase in both BOD and COD levels in the effluent. In Comparative Example 7, without a bypass advanced oxidation stage, under a high-salt system with continuous influent, polar small-molecule organic matter that was not pretreated and intercepted accumulates in the MVR evaporator crystallizer. The concentrated high-concentration organic matter triggers violent boiling and foaming phenomena, causing pollutants to directly cross the gas... When secondary steam enters the liquid interface, the COD of the second-stage condensate discharge water increases abnormally, the decarbonization rate approaches zero, and the long-term operational stability of the system is severely reduced. In Comparative Example 8, due to the lack of strong dispersion and chelation sites provided by the abundant carboxyl groups on the polyaspartic acid side chain, it is unable to target and lock the easily scaled silicate and fluoride ions in complex water quality. They can easily penetrate the primary physical barrier, causing inorganic salt scaling in the nanofiltration concentration stage, and further depositing as dense silicate hard scale on the high-temperature heat exchange tube wall of the evaporator crystallizer, resulting in an increased scaling rate and shortening the cleaning cycle and service life of the equipment.

[0038] Although embodiments of the invention have been shown and described, it will be understood by those skilled in the art that various changes, modifications, substitutions and alterations can be made to these embodiments without departing from the principles and spirit of the invention, the scope of which is defined by the appended claims and their equivalents.

Claims

1. A process for the separation and crystallization of salt in saline wastewater for resource recovery, characterized in that, The preparation process includes the following steps: High-nitrogen, saline raw water is introduced into a pretreatment adsorption tank, a composite porous material is added, and the mixture is stirred to obtain adsorbed effluent; then, the effluent is filtered, pumped into a complexation reaction tank, and a targeted complexing agent aqueous solution is added to react and obtain complexed wastewater; the complexed wastewater is filtered through a membrane system, a mixed reagent is added to the concentrated water side, and 20% of the permeate is diverted and electrolyzed to obtain hydrochloric acid solution and mixed alkali solution; the remaining permeate is pumped into a first-stage evaporator crystallizer, where polyepoxysuccinic acid is added for concentration to obtain a crystallization solution; then, it enters a second-stage vacuum flash cooling crystallizer, where sodium polyacrylate induces the formation of a treated mother liquor; 20% of the treated mother liquor is diverted through a plate heat exchanger and introduced into an oxidation reaction tank containing ozone and hydrogen peroxide, where sodium diethyldithiocarbamate is added to obtain a purified mother liquor; the purified mother liquor and the remaining treated mother liquor enter the first-stage evaporator crystallizer for further treatment.

2. The process for separating and crystallizing salt from saline wastewater according to claim 1, characterized in that, The process for the complexed wastewater includes the following steps: introducing the high-nitrogen, saline raw water into the pretreatment adsorption tank, adding 2.0-5.0 g / L of the composite porous material, and mechanically stirring to obtain the adsorbed effluent; then passing it through a 10 μm filter and pumping it into a complexation reaction tank, adding 15-30 mg / L of the targeted complexing agent aqueous solution, and reacting to obtain the complexed wastewater.

3. The process for salt separation and crystallization resource utilization in saline wastewater according to claim 2, characterized in that, The preparation of the composite porous material includes the following steps: dissolving 20-30 parts of block copolymer P123 in hydrochloric acid aqueous solution and mechanically stirring to obtain a solution; then adding an ethanol aqueous solution of tetraethyl orthosilicate dropwise to the solution, and simultaneously adding 4-10 parts of sucrose, and co-condensing to obtain a reaction product; then transferring to a reactor for hydrothermal aging, washing the product by centrifugation, vacuum drying, and heating to 600-700℃ at 2-5℃ / min under a nitrogen protective atmosphere to obtain a carbonized framework; then dispersing it in anhydrous toluene, adding 10-15 parts of 3-aminopropyltriethoxysilane for reflux reaction; centrifuging, washing and drying to obtain the composite porous material.

4. The process for separating and crystallizing salt from saline wastewater according to claim 2, characterized in that, The preparation of the targeted complexing agent aqueous solution includes the following steps: weighing 15-25 parts of chitosan powder and dissolving it in dilute acetic acid aqueous solution with stirring; adding polyaspartic acid aqueous solution dropwise at a constant temperature of 45-55℃ with stirring until uniform; then adding EDC-HCl crosslinking agent for amidation coupling reaction; adjusting the pH value of the system with sodium hydroxide solution; dialysis with deionized water; freeze drying to obtain the targeted complexing agent; and adding deionized water to prepare the targeted complexing agent aqueous solution with a mass concentration of 5%.

5. The process for separating and crystallizing salt from saline wastewater according to claim 1, characterized in that, The preparation of the permeate includes the following steps: the complexed wastewater is sequentially pressurized and passed through a 50000Da ultrafiltration membrane group and a nanofiltration membrane system to obtain the concentrate side and the permeate; 500-800 mg / L of the mixed reagent is added to the concentrate side, the mixture is stirred and reacted, and then frozen and crystallized to obtain anhydrous sodium sulfate; wherein the mixed reagent is obtained by mixing sodium sulfide and polyferric sulfate in a certain mass ratio.

6. The process for separating and crystallizing salt in saline wastewater according to claim 5, characterized in that, The permeate treatment includes the following steps: 20% of the permeate volumetric flow rate is diverted to a bipolar membrane electrodialysis unit, converted into hydrochloric acid solution and mixed alkali solution, and refluxed to the front end for internal pH adjustment; the remaining 80% of the permeate is pumped into a first-stage evaporator crystallizer, 10-25 mg / L of polyepoxysuccinic acid is added, and the solution is concentrated at a constant temperature of 0.05-0.08 MPa to collect sodium chloride crystals; the resulting crystallized solution enters a second-stage vacuum flash cooling crystallizer, is cooled at 1-2 °C / min, 5-15 mg / L of sodium polyacrylate is added, and the solution is centrifuged and dehydrated to obtain ammonium chloride.

7. The process for separating and crystallizing salt from saline wastewater according to claim 1, characterized in that, The treatment of the mother liquor includes the following steps: 15% of the mother liquor is used as a bypass tributary, which is then heat-exchanged through a plate heat exchanger and preheated before being introduced into an advanced oxidation reaction tank; ozone gas is introduced into the reaction tank and hydrogen peroxide solution is added dropwise; after the oxidation reaction is completed, 2.0-5.0 mg / L of sodium diethyldithiocarbamate is added to the system and stirred to obtain the purified mother liquor; the purified mother liquor and the remaining mother liquor are combined at the feed main and enter the first-stage evaporator crystallizer.

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