Zero-discharge salt separation and recovery system for wastewater containing sodium sulfate and sodium chloride

Abstract

The utility model discloses a zero-discharge salt separation recovery system for wastewater containing sodium sulfate and sodium chloride, which is characterized in that the lower part of a falling film evaporator is connected with a falling film separator, the bottom outlets of the falling film evaporator and the falling film separator are connected with the inlet of a falling film circulating pump, and the outlet of the falling film circulating pump is connected with the top inlet of the falling film evaporator; an outlet of the raw water feeding pipeline G1 is connected with a falling film concentration circulating pipe; bottom outlets of the falling film evaporator and the falling film separator are further connected with a forced concentration circulating pipe through a falling film discharging pump, the upper end of the forced concentration circulating pipe is connected with a circulating outlet of the forced separator, and a lower end outlet of the forced concentration circulating pipe is connected with an inlet of a forced circulating pump. An outlet of the forced circulation pump is connected with a tube pass inlet of the forced evaporator, a tube pass outlet of the forced evaporator is connected with a circulation inlet of the forced separator, and a salt leg bottom outlet of the forced separator is connected with the freezing crystallization system through the forced discharging pump. The system is low in energy consumption, and the separated sodium sulfate and sodium chloride are high in quality.

Description

Technical Field

[0001] The utility model relates to a salt separation treatment system, in particular to a zero-discharge salt separation and recovery system for wastewater containing sodium sulfate and sodium chloride, belonging to the technical field of comprehensive resource utilization. Background Art

[0002] Since the structural, root and trend pressures on China's ecological environment protection have not been fundamentally alleviated, and the economic and social development has entered a high-quality development stage of accelerating greening and low-carbonization, the modern society has higher and higher requirements for environmental protection, and thus puts forward higher requirements for the sewage discharge of industries such as pharmaceuticals, petrochemicals, printing and dyeing, and textiles. High-salt wastewater must be treated more deeply before it can meet the discharge standards.

[0003] High-salt wastewater refers to wastewater with a total salt content of at least 3.5 wt%, which mainly comes from chemical industry and the collection of oil and natural gas. The sources of salt-containing wastewater are extensive and the water volume increases year by year. For such wastewater, common treatment methods include electrolysis method, membrane separation method, biological method, incineration method and evaporation crystallization method.

[0004] The Chinese patent application with the publication number of CN 11279450 discloses a method for separating sodium chloride and sodium sulfate in high-salt wastewater based on membrane treatment. By using a variety of membrane treatment methods, it can effectively remove the particles and salts in the wastewater, effectively reduce the salinity of the wastewater entering the reverse osmosis membrane module, and improve the water production recovery rate. Although the membrane treatment method can achieve the purpose of zero discharge, its disadvantages are that the treatment capacity is small, and when the concentration of high-salt wastewater is too high, it is extremely easy to block the membrane, and the membrane needs to be cleaned and replaced regularly, which has instability for production and low economic value.

[0005] The Chinese patent application with the publication number of CN 11270741 discloses a method for separating sodium sulfate and sodium chloride, which mainly uses the method of thermal crystallization to separate and treat high-salt wastewater, and uses the setting of a demister to separate the liquid foam in the mixed liquid to improve the salt separation purity. However, the process of this technical solution is relatively simple, the separation means is relatively single, the quality of the separated salt is poor, and the directly evaporated and crystallized miscellaneous salts have complex components and contain a large amount of toxic substances, which can only be treated as solid waste, not only polluting the environment twice but also increasing the enterprise's treatment costs.

[0006] In summary, for the production of high-salt wastewater, there is an urgent need to develop a salt separation treatment system to improve the productivity and quality of crystalline salts, realize the resource utilization of enterprise wastewater, and ensure a high system fault tolerance rate, which can not only ensure the normal production of enterprises but also reduce the operation costs of enterprises. Summary of the Utility Model

[0007] The purpose of this section is to outline some aspects of the embodiments of the present utility model and briefly introduce some preferred embodiments. Some simplifications or omissions may be made in this section, as well as in the abstract of the specification and the title of the utility model of this application, and such simplifications or omissions shall not be used to limit the scope of the present utility model.

[0008] In view of the above and / or problems existing in the prior art, the present utility model is proposed.

[0009] The purpose of the present utility model is to overcome the problems existing in the prior art, and provide a zero-discharge salt separation and recovery system for wastewater containing sodium sulfate and sodium chloride, so as to improve the evaporation heat transfer coefficient, reduce energy consumption, improve the quality of sodium sulfate and sodium chloride, increase the yield of crystalline salt, and reduce the influence of organic matter and nitrate on the system.

[0010] To solve the above technical problems, a zero-discharge salt separation and recovery system for wastewater containing sodium sulfate and sodium chloride of the present utility model includes a falling film concentration MVR system. The falling film concentration MVR system includes a falling film evaporator. The lower part of the falling film evaporator is connected to a falling film separator. The bottom outlets of the falling film evaporator and the falling film separator are both connected to the inlet of a falling film circulation pump. The outlet of the falling film circulation pump is connected to the top inlet of the falling film evaporator through a falling film concentration circulation pipe. The outlet of the raw water feed pipeline is connected to the falling film concentration circulation pipe.

[0011] The bottom outlets of the falling film evaporator and the falling film separator are also connected to the forced concentration circulation pipe of the forced concentration MVR system through a falling film discharge pump. The upper end of the forced concentration circulation pipe is connected to the circulation outlet of the forced separator. The lower end outlet of the forced concentration circulation pipe is connected to the inlet of a forced circulation pump. The outlet of the forced circulation pump is connected to the tube side inlet of the forced evaporator. The tube side outlet of the forced evaporator is connected to the circulation inlet of the forced separator. The bottom outlet of the salt leg of the forced separator is connected to the freezing crystallization system through a forced discharge pump.

[0012] Furthermore, the freezing crystallization system includes a freezing crystallizer. The outlet of the forced discharge pump is connected to the feed inlet of the freezing crystallizer. The circulation outlet of the freezing crystallizer is connected to the inlet of a freezing crystallization circulation pump. The outlet of the freezing crystallization circulation pump is connected to the tube side inlet of a freezing heat exchanger. The tube side outlet of the freezing heat exchanger is connected to the circulation inlet of the freezing crystallizer. The bottom salt slurry outlet of the freezing crystallizer is connected to the inlet of a freezing thickening tank through a freezing slurry pump. The bottom outlet of the freezing thickening tank is connected to the inlet of a freezing centrifuge. The solid phase outlet of the freezing centrifuge is connected to the sodium sulfate hot melting system through a chute.

[0013] Furthermore, the clear liquid overflow port of the freezing thickening tank is connected to the inlet of the freezing precipitation tank. The bottom outlet of the freezing precipitation tank and the liquid phase outlet of the freezing centrifuge are respectively connected to the inlet of the freezing mother liquor tank. The outlet of the freezing mother liquor tank is connected to the reflux port of the freezing thickening tank through a freezing mother liquor circulation pump;

[0014] The clear liquid outlet of the freezing precipitation tank is connected to the primary salt MVR system through a freezing mother liquor discharge pump.

[0015] Furthermore, the solid phase outlet of the freezing centrifuge is connected to the inlet of the nitrate dissolving tank in the sodium sulfate heat melting system through a chute. The sidewall circulation outlet of the nitrate dissolving tank is connected to the inlet of the nitrate dissolving circulation pump. The outlet of the nitrate dissolving circulation pump is connected to the tube side inlet of the nitrate dissolving heat exchanger. The tube side outlet of the nitrate dissolving heat exchanger is connected to the central feed pipe of the nitrate dissolving tank. The lower end of the central feed pipe extends to the lower part of the nitrate dissolving tank. The bottom outlet of the nitrate dissolving tank is connected to the nitrate recrystallization system through a nitrate discharging pump.

[0016] Furthermore, the outlet of the nitrate discharging pump is connected to the inlet of the nitrate thickening tank in the nitrate recrystallization system. The crystal slurry outlet of the nitrate thickening tank is connected to the inlet of the nitrate centrifuge. The mother liquor outlet of the nitrate centrifuge is connected to the inlet of the nitrate mother liquor tank. The bottom outlet of the nitrate mother liquor tank is connected to the middle part of the nitrate evaporator circulation pipe through a nitrate mother liquor pump. The upper end of the nitrate evaporator circulation pipe is connected to the circulation outlet of the nitrate crystallizer. The lower end of the nitrate evaporator circulation pipe is connected to the inlet of the nitrate circulation pump. The outlet of the nitrate circulation pump is connected to the tube side inlet of the nitrate evaporator. The tube side outlet of the nitrate evaporator is connected to the circulation inlet of the nitrate crystallizer; The bottom crystal slurry outlet of the nitrate crystallizer is connected to the inlet of the nitrate thickening tank through a nitrate crystal slurry pump.

[0017] Furthermore, the solid phase outlet of the nitrate centrifuge is connected to the inlet of the anhydrous sodium sulfate drying bed through a chute and a nitrate auger. The outlet of the anhydrous sodium sulfate drying bed is connected to the anhydrous sodium sulfate packing machine.

[0018] Furthermore, the freezing mother liquor discharge pump is connected to the middle part of the primary salt concentration circulation pipe in the primary salt MVR system. The upper end of the primary salt concentration circulation pipe is connected to the circulation outlet of the primary salt crystallizer. The lower end of the primary salt concentration circulation pipe is connected to the inlet of the primary salt circulation pump. The outlet of the primary salt circulation pump is connected to the tube side inlet of the primary salt heat exchanger. The tube side outlet of the primary salt heat exchanger is connected to the circulation inlet of the primary salt crystallizer;

[0019] The salt leg outlet of the primary salt crystallizer is connected to the inlet of the primary salt thickener through a primary salt slurry pump. The bottom outlet of the primary salt thickener is connected to the inlet of the primary salt centrifuge. The solid phase outlet of the primary salt centrifuge is connected to the inlet of the primary salt dissolution tank. The outlet of the primary salt dissolution tank is connected to the inlet of the nanofiltration membrane device through a primary salt dissolution and external delivery pump. The concentrated water outlet of the nanofiltration membrane device is connected to the inlet of the freezing crystallizer. The produced water outlet of the nanofiltration membrane device is connected to the secondary salt MVR system.

[0020] Furthermore, the liquid phase outlet of the primary salt centrifuge is connected to the inlet of the primary salt mother liquor tank. The outlet of the primary salt mother liquor tank is connected to the reflux port of the primary salt thickener through a primary salt mother liquor circulation pump. The top clear liquid outlet of the primary salt thickener is connected to the inlet of the primary salt precipitation tank. The bottom outlet of the primary salt precipitation tank is connected to the inlet of the primary salt mother liquor tank.

[0021] The upper side wall outlet of the primary salt precipitation tank is connected to the mixed salt evaporation and crystallization system through a primary salt mother liquor external discharge pump.

[0022] Furthermore, the produced water outlet of the nanofiltration membrane device is connected to the middle part of the secondary salt concentration circulation pipe in the secondary salt MVR system. The upper end of the secondary salt concentration circulation pipe is connected to the circulation outlet of the secondary salt crystallizer. The lower end of the secondary salt concentration circulation pipe is connected to the inlet of the secondary salt circulation pump. The outlet of the secondary salt circulation pump is connected to the tube side inlet of the secondary salt heat exchanger. The tube side outlet of the secondary salt heat exchanger is connected to the circulation inlet of the secondary salt crystallizer. The salt leg outlet of the secondary salt crystallizer is connected to the inlet of the secondary salt thickener through a secondary salt slurry pump. The bottom outlet of the secondary salt thickener is connected to the inlet of the secondary salt centrifuge. The solid phase outlet of the secondary salt centrifuge is connected to the sodium chloride drying system.

[0023] Furthermore, the mother liquor outlet of the secondary salt centrifuge is connected to the inlet of the secondary salt mother liquor tank. The outlet of the secondary salt mother liquor tank is connected to the reflux port of the secondary salt thickener through a secondary salt mother liquor circulation pump.

[0024] The clear liquid overflow port of the secondary salt thickener is connected to the inlet of the secondary salt precipitation tank. The bottom outlet of the secondary salt precipitation tank is connected to the inlet of the secondary salt mother liquor tank.

[0025] The middle clear liquid outlet of the secondary salt precipitation tank is connected to the inlet of the freezing crystallizer through a secondary salt mother liquor external discharge pump and a secondary salt mother liquor external discharge pipe.

[0026] Compared with the prior art, the utility model has achieved the following beneficial effects: 1. By combining the application of evaporation crystallization and membrane treatment technologies, the resource utilization of wastewater can be realized, which not only protects the environment from pollution, saves water resources, but also reduces the enterprise's water treatment cost and improves economic efficiency. The separated sodium sulfate salt meets the first-class product of Class I of "Sodium Sulfate in Industrial Sewage" (GB / T 6009-2003) and can be sold externally; the separated sodium chloride salt meets the first-class product standard of industrial dry salt in "Industrial Salt" (GB / T 5462-2015) and can also be sold externally; the proportion of miscellaneous salts shall not exceed 10%, and the moisture content ≤ 10%.

[0027] 2. For the falling film concentration and forced concentration systems, the MVR technology is used. The steam at a low temperature level is compressed by a compressor, with its temperature, pressure increased, and enthalpy increased, and then enters the heat exchanger for condensation, so as to make full use of the latent heat of the steam, reduce the system energy consumption, and save energy. By the way of MVR falling film concentration + MVR forced concentration, the material is concentrated to be close to saturation or supersaturation, and the concentration end point control range is wide. Only the discharge density needs to be controlled, and the control difficulty is low. Among them, the MVR falling film concentration adopts two sets of parallel designs, which are used as backups for each other, improving the safety production capacity and reducing the operation cost.

[0028] 3. By the way of freeze crystallization + heat melting + nitrate recrystallization, the sodium sulfate salt is purified. All the sodium sulfate is produced by the heat melting and recrystallization of mirabilite, and the quality of the crystalline salt is higher. Since the solubility of sodium sulfate is lower at a low temperature state, only by controlling the freezing temperature at -5 to 0 °C, the stable quality of the sodium chloride influent water can be ensured, so as to ensure the stable operation of the sodium chloride system; at the same time, the discharge of mirabilite centrifugal mother liquor adopts secondary sedimentation separation + precision heat preservation filter to ensure that the discharged mother liquor does not contain fine mirabilite crystals, improve the quality of sodium chloride, increase the recovery rate of sodium sulfate, and increase the added value.

[0029] 4. The sodium chloride washing salt system is adopted to further clean and purify the crude sodium chloride salt produced by the sodium chloride primary salt evaporation crystallization system, further ensuring the quality of the sodium chloride salt; at the same time, by the way of coupling thermal crystallization and nanofiltration membrane, the crude sodium chloride is redissolved and then sent to the nanofiltration system. Utilizing the selective retention characteristics of the nanofiltration membrane for divalent salts, the separation of monovalent salt sodium chloride and divalent salt sodium sulfate in the liquid phase is realized. The relative content of sodium chloride in the nanofiltration permeate is usually higher than 95%, greatly increasing the recovery rate of the sodium chloride crystalline salt. And the solution entering the nanofiltration membrane is the redissolved solution of crude sodium chloride, with a lower sulfate content and not easily blocking the nanofiltration membrane.

[0030] 5. The crystalline salt produced by single-effect evaporation crystallization is redissolved by adding water and then returned to the freeze crystallization system for the recovery of sodium sulfate, or discharged to the front end and then returned to the system after being treated again, which can increase the recovery rate of the crystalline salt and reduce the amount of miscellaneous salts.

[0031] 6. In the device, the MVR forced, nitrate recrystallization, primary salt evaporation, and secondary salt evaporation systems all use vacuum pumps to reduce the system vacuum and lower the system boiling point. The primary salt system and the secondary salt system perform thermal evaporation at a temperature of about 95°C, and the forced system and the nitrate recrystallization system perform evaporation at about 90°C, which not only ensures safety but also reduces energy consumption and saves operating costs. Moreover, the MVR forced, nitrate recrystallization, primary salt evaporation, and secondary salt evaporation systems all apply MVR technology. The low-temperature steam is compressed by a compressor, increasing its temperature, pressure, and enthalpy, and then enters the heat exchanger for condensation to fully utilize the latent heat of the steam, reducing system energy consumption and saving energy. BRIEF DESCRIPTION OF THE DRAWINGS

[0032] In order to more clearly illustrate the technical solutions of the embodiments of the present invention, the following will briefly introduce the drawings required for the description of the embodiments. Obviously, the following drawings are only some embodiments of the present invention. For those of ordinary skill in the art, without creative efforts, other drawings can also be obtained based on these drawings. The drawings are only for reference and illustration, and are not used to limit the present invention. Among them:

[0033] Figure 1 It is a flowchart of the zero-emission salt separation and recovery system for wastewater containing sodium sulfate and sodium chloride of the present invention;

[0034] In the figure: falling film concentration MVR system; 1a. falling film evaporator; 1b. falling film separator; 1c. falling film circulation pump; 1d. falling film discharge pump;

[0035] Forced concentration MVR system; 2a. forced evaporator; 2b. forced separator; 2c. forced circulation pump; 2d. forced discharge pump;

[0036] Freezing crystallization system; 3a. freezing heat exchanger; 3b. freezing crystallizer; 3c. freezing crystallization circulation pump; 3d. refrigerant circulation pump; 3e. freezing crystal slurry pump; 3f. freezing thickening tank; 3g. freezing centrifuge; 3h. freezing mother liquor tank; 3j. freezing mother liquor circulation pump; 3k. freezing precipitation tank; 3m. freezing mother liquor discharge pump;

[0037] Sodium sulfate hot melting system; 4a. nitrate dissolution heat exchanger; 4b. nitrate dissolution tank; 4c. nitrate dissolution circulation pump; 4d. nitrate dissolution discharge pump;

[0038] Nitrate recrystallization system; 5a. nitrate evaporator; 5b. nitrate crystallizer; 5c. nitrate circulation pump; 5d. nitrate crystal slurry pump; 5e. nitrate thickening tank; 5f. nitrate centrifuge; 5g. nitrate mother liquor tank; 5h. nitrate mother liquor pump;

[0039] Sodium sulfate drying system; 6a. nitrate auger; 6b. sodium sulfate drying bed; 6c. sodium sulfate packing machine;

[0040] Primary salt MVR system; 7a. Primary salt heat exchanger; 7b. Primary salt crystallizer; 7c. Primary salt circulation pump; 7d. Primary salt crystal slurry pump; 7e. Primary salt thickener; 7f. Primary salt centrifuge; 7g. Primary salt sedimentation tank; 7h. Primary salt mother liquor tank; 7j. Primary salt mother liquor circulation pump; 7k. Primary salt dissolution tank; 7m. Primary salt dissolution and external delivery pump; 7n. Primary salt mother liquor external discharge pump;

[0041] 8. Nanofiltration membrane device;

[0042] Secondary salt MVR system; 9a. Secondary salt heat exchanger; 9b. Secondary salt crystallizer; 9c. Secondary salt circulation pump; 9d. Secondary salt crystal slurry pump; 9e. Secondary salt thickener; 9f. Secondary salt centrifuge; 9g. Secondary salt sedimentation tank; 9h. Secondary salt mother liquor tank; 9j. Secondary salt mother liquor circulation pump; 9k. Secondary salt mother liquor external discharge pump;

[0043] Sodium chloride drying system; 10a. Sodium chloride auger; 10b. Sodium chloride drying bed; 10c. Sodium chloride packing machine;

[0044] Mixed salt evaporation and crystallization system; 11a. Mixed salt evaporator; 11b. Mixed salt crystallizer; 11c. Mixed salt circulation pump; 11d. Mixed salt crystal slurry pump; 11e. Mixed salt thickener; 11f. Mixed salt centrifuge; 11g. Mixed salt mother liquor tank; 11h. Mixed salt mother liquor pump; 11j. Mixed salt dissolution tank; 11k. Mixed salt dissolution and external discharge pump;

[0045] Waste salt mother liquor drying system; 12a. Drum dryer; 12b. Waste salt auger; 12c. Waste salt packing machine;

[0046] G1. Raw water feed pipeline; G2. Secondary salt mother liquor external discharge pipe. Detailed implementation manners

[0047] In order to make the technical means, creative features, achieved purposes and functions of the present utility model easy to understand, the present utility model will be further described below with reference to specific drawings. Obviously, the described embodiments are only a part of the embodiments of the present utility model, rather than all of the embodiments.

[0048] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by those skilled in the technical field to which the present utility model belongs. The terms used in the description of the present utility model herein are only for the purpose of describing specific embodiments, and are not intended to limit the present utility model.

[0049] As Figure 1As shown in the figure, the zero-emission salt separation and recovery system for wastewater containing sodium sulfate and sodium chloride of the present utility model includes a falling film evaporation MVR system, a forced evaporation MVR system, a freezing crystallization system, a sodium sulfate hot melting system, a nitrate recrystallization system, a sodium sulfate drying system, a primary salt MVR system, a nanofiltration membrane system, a secondary salt MVR system, a sodium chloride drying system, a mixed salt evaporation and crystallization system, and a miscellaneous salt mother liquor drying system.

[0050] The falling film evaporation MVR system includes a falling film evaporator 1a, a falling film separator 1b, a falling film circulation pump 1c and a circulation pipeline. The lower part of the falling film evaporator 1a is connected to the falling film separator 1b. The bottom outlets of the falling film evaporator 1a and the falling film separator 1b are both connected to the inlet of the falling film circulation pump 1c. The outlet of the falling film circulation pump 1c is connected to the top inlet of the falling film evaporator 1a through a falling film concentration circulation pipe. The outlet of the raw water feed pipeline G1 is connected to the falling film concentration circulation pipe.

[0051] The brine from the raw water feed pipeline G1 enters the falling film evaporator 1a for concentration. The lower part of the falling film evaporator 1a is connected and communicated with the falling film separator 1b. The concentrated liquid is subjected to circulating heat exchange through the falling film circulation pump 1c. The shell-side heat source of the falling film evaporator 1a is the secondary gas heated and pressurized by the compressor, that is, using MVR technology, the low-temperature steam is compressed by the compressor, the temperature and pressure are increased, the enthalpy is increased, and then it enters the shell side of the falling film evaporator 1a to condense, so as to make full use of the latent heat of the steam, reduce the system energy consumption, and save energy.

[0052] There are two sets of falling film evaporation MVR systems in parallel and used as spares for each other, ensuring the safety of the subsequent system while guaranteeing the evaporation capacity.

[0053] The bottom outlets of the falling film evaporator 1a and the falling film separator 1b are also connected to the forced concentration circulation pipe of the forced evaporation MVR system through a falling film discharge pump 1d. When the discharge density of the falling film concentration system reaches 1100 g / L, it is sent to the forced evaporation MVR system through the falling film discharge pump 1d.

[0054] The forced evaporation MVR system includes a forced evaporator 2a, a forced separator 2b, a forced circulation pump 2c and a circulation pipeline. The upper end of the forced concentration circulation pipe is connected to the circulation outlet of the forced separator 2b. The lower end outlet of the forced concentration circulation pipe is connected to the inlet of the forced circulation pump 2c. The outlet of the forced circulation pump 2c is connected to the tube-side inlet of the forced evaporator 2a. The tube-side outlet of the forced evaporator 2a is connected to the circulation inlet of the forced separator 2b. The bottom outlet of the salt leg of the forced separator 2b is connected to the freezing crystallization system through a forced discharge pump 2d.

[0055] The solution processed by the falling film concentration system is sent to the tube side of the forced evaporator 2a for heat exchange and temperature rise. The heat source for the shell side of the forced evaporator 2a is the secondary gas after being heated and pressurized by the compressor, and the MVR technology is also used. The solution circulates in the system for secondary concentration. When the discharge density of the forced concentration system reaches 1150 - 1160 g / L, it is sent to the freezing crystallization system through the forced discharge pump 2d.

[0056] The freezing crystallization system includes a freezing heat exchanger 3a, a freezing crystallizer 3b, a freezing crystallization circulation pump 3c, a refrigerant circulation pump 3d, a freezing crystal slurry pump 3e, a freezing thickening tank 3f, a freezing centrifuge 3g, a freezing mother liquor tank 3h, a freezing mother liquor circulation pump 3j, a freezing precipitation tank 3k, and a freezing mother liquor discharge pump 3m. The cold source for the freezing heat exchanger 3a is the refrigerant provided by the freezer through a pipeline, and the refrigerant circulation pump 3d sends the cold source into the shell side of the freezing heat exchanger 3a for circulation. The outlet of the forced discharge pump is connected to the inlet of the freezing crystallizer 3b. The circulation outlet of the freezing crystallizer 3b is connected to the inlet of the freezing crystallization circulation pump 3c. The outlet of the freezing crystallization circulation pump 3c is connected to the inlet of the tube side of the freezing heat exchanger 3a. The outlet of the tube side of the freezing heat exchanger 3a is connected to the circulation inlet of the freezing crystallizer 3b. The bottom salt slurry outlet of the freezing crystallizer 3b is connected to the inlet of the freezing thickening tank 3f through the freezing crystal slurry pump 3e. The bottom outlet of the freezing thickening tank 3f is connected to the inlet of the freezing centrifuge 3g. The solid phase outlet of the freezing centrifuge 3g is connected to the sodium sulfate hot melting system through a chute.

[0057] The overflow port of the clear liquid of the freezing thickening tank 3f is connected to the inlet of the freezing precipitation tank 3k. The bottom outlet of the freezing precipitation tank 3k and the liquid phase outlet of the freezing centrifuge 3g are respectively connected to the inlet of the freezing mother liquor tank 3h. The outlet of the freezing mother liquor tank 3h is connected to the reflux port of the freezing thickening tank 3f through the freezing mother liquor circulation pump 3j. The clear liquid outlet of the freezing precipitation tank 3k is connected to the primary salt MVR system through the freezing mother liquor discharge pump 3m.

[0058] After being concentrated nearly 4.5 times by the falling film concentration MVR system and the forced concentration MVR system, the brine enters the freezing crystallizer 3b. Sodium sulfate is nearly saturated while sodium chloride is unsaturated. Under the action of the cold source, the concentrated liquid is cooled and frozen. When the temperature in the freezing crystallizer 3b drops below 20°C, sodium sulfate decahydrate, i.e., mirabilite, begins to crystallize out in the system. When the temperature in the freezing crystallizer 3b drops to 0°C, a large amount of mirabilite precipitates, and the solubility of sodium sulfate in the system drops to 4.9 g / L. The precipitated salt slurry is pumped into the freezing thickening tank 3f by the freezing crystal slurry pump 3e for thickening, and then enters the freezing centrifuge 3g through the pipeline for solid-liquid separation. The separated mother liquor enters the freezing mother liquor tank 3h through the pipeline for temporary storage, and then is sent back to the freezing thickening tank 3f by the freezing mother liquor circulation pump 3j for re-thickening. The supernatant liquid on the upper layer of the freezing thickening tank 3f overflows to the freezing precipitation tank 3k through the connecting pipe. The supernatant liquid is a sodium chloride-rich solution. In the freezing precipitation tank 3k, some fine sodium sulfate crystals settle again, reducing the content of fine sodium sulfate crystals in the discharged mother liquor, increasing the crystallization salt productivity, and ensuring the quality of the subsequent sodium chloride salt. The clear liquid in the freezing precipitation tank 3k is sent into the primary salt MVR system by the freezing mother liquor discharge pump 3m. The solid mirabilite separated by the freezing centrifuge 3g is sent into the sodium sulfate hot melting system through the chute for treatment.

[0059] The sodium sulfate hot melting system consists of a nitrate dissolving heat exchanger 4a, a nitrate dissolving tank 4b, a nitrate dissolving circulation pump 4c, and a nitrate dissolving discharge pump 4d. The solid phase outlet of the freezing centrifuge 3g is connected to the inlet of the nitrate dissolving tank 4b in the sodium sulfate hot melting system through a chute. The side wall circulation outlet of the nitrate dissolving tank 4b is connected to the inlet of the nitrate dissolving circulation pump 4c. The outlet of the nitrate dissolving circulation pump 4c is connected to the tube side inlet of the nitrate dissolving heat exchanger 4a. The tube side outlet of the nitrate dissolving heat exchanger 4a is connected to the central feed pipe of the nitrate dissolving tank 4b. The lower end of the central feed pipe extends to the lower part of the nitrate dissolving tank 4b. The bottom outlet of the nitrate dissolving tank 4b is connected to the nitrate recrystallization system through the nitrate dissolving discharge pump 4d.

[0060] The mirabilite discharged from the solid phase of the freezing centrifuge 3g enters the nitrate dissolving tank 4b through the chute. Utilizing the characteristic that mirabilite melts when the temperature is above 32.8°C, that is, it dissolves in its own crystal water, and absorbs heat during melting, with a heat of fusion of 77 kJ / mol. In the nitrate dissolving tank 4b of this system, mirabilite dissolves into 44% sodium sulfate and 56% crystal water at a temperature above 40°C. To ensure its dissolution temperature, the brine in the nitrate dissolving tank 4b enters the tube side of the nitrate dissolving heat exchanger 4a through the nitrate dissolving circulation pump 4c for heating and then returns to the nitrate dissolving tank 4b for circulation. The heat source on the shell side is live steam. Subsequently, the solution is sent into the nitrate recrystallization system through the nitrate dissolving discharge pump 4d.

[0061] The nitrate recrystallization system includes a nitrate evaporator 5a, a nitrate crystallizer 5b, a nitrate circulation pump 5c, a nitrate crystal slurry pump 5d, a nitrate thickening tank 5e, a nitrate centrifuge 5f, a nitrate mother liquor tank 5g, and a nitrate mother liquor pump 5h. The outlet of the nitrate dissolution discharge pump is connected to the inlet of the nitrate thickening tank 5e in the nitrate recrystallization system. The crystal slurry outlet of the nitrate thickening tank 5e is connected to the inlet of the nitrate centrifuge. The mother liquor outlet of the nitrate centrifuge is connected to the inlet of the nitrate mother liquor tank 5g. The bottom outlet of the nitrate mother liquor tank 5g is connected to the middle of the nitrate evaporator circulation pipe through the nitrate mother liquor pump 5h. The upper end of the nitrate evaporator circulation pipe is connected to the circulation outlet of the nitrate crystallizer 5b. The lower end of the nitrate evaporator circulation pipe is connected to the inlet of the nitrate circulation pump 5c. The outlet of the nitrate circulation pump 5c is connected to the tube side inlet of the nitrate evaporator 5a. The tube side outlet of the nitrate evaporator 5a is connected to the circulation inlet of the nitrate crystallizer 5b. The bottom crystal slurry outlet of the nitrate crystallizer 5b is connected to the inlet of the nitrate thickening tank 5e through the nitrate crystal slurry pump 5d.

[0062] The re-dissolved sodium sulfate brine is sent into the nitrate thickening tank 5e by the nitrate dissolution discharge pump 4d. After the crystal slurry is separated by solid-liquid separation through the nitrate centrifuge 5f, high-purity sodium sulfate salt is obtained for the solid. The mother liquor of the solid-liquid separation enters the nitrate mother liquor tank 5g for buffering, and then is sent into the nitrate evaporator circulation pipe through the nitrate mother liquor pump 5h, and then enters the tube side of the nitrate evaporator 5a for heat exchange through the nitrate circulation pump 5c. The brine after heat exchange to 93 °C enters the nitrate crystallizer 5b for flashing. When the solid-liquid ratio of the bottom crystal slurry of the nitrate crystallizer 5b reaches about 10%, it is sent into the nitrate thickening tank 5e for thickening through the nitrate crystal slurry pump 5d, forming a cycle.

[0063] The sodium sulfate drying system includes a nitrate screw conveyor 6a, a mirabilite drying bed 6b, and a mirabilite packing machine 6c. The solid phase outlet of the nitrate centrifuge 5f is connected to the inlet of the mirabilite drying bed 6b through a chute and the nitrate screw conveyor 6a. The outlet of the mirabilite drying bed 6b is connected to the mirabilite packing machine 6c. The sodium sulfate discharged from the solid phase of the nitrate centrifuge 5f is sent into the mirabilite drying bed 6b for drying through the nitrate screw conveyor 6a. The drying temperature is about 110 °C, and then it enters the mirabilite packing machine 6c for packing through a chute, obtaining high-quality sodium sulfate.

[0064] The primary salt MVR system includes a primary salt heat exchanger 7a, a primary salt crystallizer 7b, a primary salt circulation pump 7c, a primary salt crystal slurry pump 7d, a primary salt thickening tank 7e, a primary salt centrifuge 7f, a primary salt precipitation tank 7g, a primary salt mother liquor tank 7h, a primary salt mother liquor circulation pump 7j, a primary salt dissolution tank 7k, a primary salt dissolution and external delivery pump 7m, and a primary salt mother liquor external discharge pump 7n.

[0065] The external discharge pump 3m of the frozen mother liquor is connected to the middle of the primary salt concentration circulation pipe in the primary salt MVR system. The upper end of the primary salt concentration circulation pipe is connected to the circulation outlet of the primary salt crystallizer 7b, and the lower end of the primary salt concentration circulation pipe is connected to the inlet of the primary salt circulation pump 7c. The outlet of the primary salt circulation pump 7c is connected to the tube-side inlet of the primary salt heat exchanger 7a, and the tube-side outlet of the primary salt heat exchanger 7a is connected to the circulation inlet of the primary salt crystallizer 7b.

[0066] The frozen sodium chloride-rich mother liquor is evaporated and crystallized through the primary salt MVR system. The external discharge pump 3m of the frozen mother liquor sends the clear liquid in the frozen precipitation tank 3k into the primary salt heat exchanger 7a for heat exchange to 102 °C, and then sends it into the primary salt crystallizer 7b for flash evaporation, and is driven by the primary salt circulation pump 7c for circulating concentration.

[0067] The salt leg outlet of the primary salt crystallizer 7b is connected to the inlet of the primary salt thickener 7e through the primary salt slurry pump 7d. The bottom outlet of the primary salt thickener 7e is connected to the inlet of the primary salt centrifuge 7f. The solid phase outlet of the primary salt centrifuge 7f is connected to the inlet of the primary salt dissolution tank 7k. The outlet of the primary salt dissolution tank 7k is connected to the inlet of the nanofiltration membrane device through the primary salt dissolution external delivery pump 7m. The concentrated water outlet of the nanofiltration membrane device is connected to the inlet of the freezing crystallizer 3b, and the produced water outlet of the nanofiltration membrane device is connected to the secondary salt MVR system.

[0068] When the solid-liquid ratio of the slurry in the salt leg of the primary salt crystallizer 7b reaches about 10%, it is sent into the primary salt thickener 7e through the primary salt slurry pump 7d for thickening. When the stirring current of the primary salt thickener 7e rises to 4 ~, the slurry can enter the primary salt centrifuge 7f through the pipeline for solid-liquid separation.

[0069] The solid separated by the primary salt centrifuge 7f is crude sodium chloride, which enters the primary salt dissolution tank 7k through the chute for dissolving and washing the salt to wash away the sodium sulfate attached to the surface of the salt. The dissolved sodium chloride solution is then sent into the nanofiltration membrane device 8 through the primary salt dissolution external delivery pump 7m. By utilizing the differences in the ionic radii or charge characteristics of chloride ions and sulfate ions, etc., and through the selective retention characteristics of the nanofiltration membrane for divalent salts, the separation of monovalent salt sodium chloride and divalent salt sodium sulfate in the liquid phase is achieved. Sodium chloride mainly enters the nanofiltration permeate, and sodium sulfate is concentrated in the nanofiltration concentrated water. Since divalent salts are retained by the nanofiltration membrane, the relative content of sodium chloride in the nanofiltration permeate is usually higher than 95%. Therefore, the recovery rate of this part of sodium chloride crystal salt is relatively high.

[0070] The nanofiltration permeate (produced water) and nanofiltration concentrated liquid (concentrated water) are separated through the nanofiltration membrane device 8. The nanofiltration concentrated water is a mixed solution of sodium chloride and sodium sulfate, which is returned to the freezing crystallizer 3b in the freezing crystallization system for re-separation.

[0071] The nanofiltration product water enters the secondary salt MVR system for re-evaporation, and refined sodium chloride salt is obtained through secondary MVR evaporation crystallization. To ensure the quality of the crystalline salt and improve the interception rate of sodium sulfate fine crystals, the primary and secondary salt MVR system is also designed with a secondary precipitation and supernatant reflux and external discharge circulation loop, which can effectively reduce the impact of organic matter on the chromaticity of the crystalline salt, while also ensuring the purity and recovery rate of sodium chloride and improving the efficiency of salt separation and crystallization.

[0072] The liquid phase outlet of the primary salt centrifuge 7f is connected to the inlet of the primary salt mother liquor tank 7h, and the outlet of the primary salt mother liquor tank 7h is connected to the reflux port of the primary salt thickening tank through the primary salt mother liquor circulation pump 7j; the supernatant outlet at the top of the primary salt thickening tank is connected to the inlet of the primary salt precipitation tank 7g, and the bottom outlet of the primary salt precipitation tank 7g is connected to the inlet of the primary salt mother liquor tank 7h; the upper side wall outlet of the primary salt precipitation tank 7g is connected to the mixed salt evaporation crystallization system through the primary salt mother liquor external discharge pump 7n.

[0073] The liquid separated by the primary salt centrifuge 7f enters the primary salt mother liquor tank 7h for temporary storage, and is sent back to the primary salt thickening tank 7e for circulation through the primary salt mother liquor circulation pump 7j. The supernatant at the top of the primary salt thickening tank 7e enters the primary salt precipitation tank 7g through a connecting pipe for secondary sedimentation, ensuring that the supernatant in the primary salt precipitation tank 7g is an unsaturated nitrate and COD solution. After the primary salt precipitation tank 7g reaches 80% of its liquid level, the nitrate-rich and organic matter solution in the precipitation tank is sent to the mixed salt evaporation crystallization system through the primary salt mother liquor external discharge pump 7n. The main purpose is to eliminate the influence of nitrate, COD and other ions on the primary salt MVR system, and prevent the boiling point of the primary salt system from rising due to the enrichment of organic matter and impurity ions, increasing the system energy consumption.

[0074] The secondary salt MVR system includes a secondary salt heat exchanger 9a, a secondary salt crystallizer 9b, a secondary salt circulation pump 9c, a secondary salt crystal slurry pump 9d, a secondary salt thickening tank 9e, a secondary salt centrifuge 9f, a secondary salt precipitation tank 9g, a secondary salt mother liquor tank 9h, a secondary salt mother liquor circulation pump 9j and a secondary salt mother liquor external discharge pump 9k. The water outlet of the nanofiltration membrane device is connected to the middle of the secondary salt concentration circulation pipe in the secondary salt MVR system. The upper end of the secondary salt concentration circulation pipe is connected to the circulation outlet of the secondary salt crystallizer 9b, and the lower end of the secondary salt concentration circulation pipe is connected to the inlet of the secondary salt circulation pump 9c. The outlet of the secondary salt circulation pump 9c is connected to the tube side inlet of the secondary salt heat exchanger 9a, and the tube side outlet of the secondary salt heat exchanger 9a is connected to the circulation inlet of the secondary salt crystallizer 9b; the salt leg outlet of the secondary salt crystallizer 9b is connected to the inlet of the secondary salt thickening tank 9e through the secondary salt crystal slurry pump 9d, the bottom outlet of the secondary salt thickening tank 9e is connected to the inlet of the secondary salt centrifuge 9f, and the solid phase outlet of the secondary salt centrifuge 9f is connected to the sodium chloride drying system.

[0075] The mother liquor outlet of the secondary salt centrifuge 9f is connected to the inlet of the secondary salt mother liquor tank 9h. The outlet of the secondary salt mother liquor tank 9h is connected to the reflux port of the secondary salt thickening tank 9e through the secondary salt mother liquor circulation pump 9j. The clear liquid overflow port of the secondary salt thickening tank 9e is connected to the inlet of the secondary salt precipitation tank 9g. The bottom outlet of the secondary salt precipitation tank 9g is connected to the inlet of the secondary salt mother liquor tank 9h. The middle clear liquid outlet of the secondary salt precipitation tank 9g is connected to the inlet of the freezing crystallizer 3b through the secondary salt mother liquor discharge pump 9k and the secondary salt mother liquor discharge pipe.

[0076] The produced water of the nanofiltration membrane device 8 is a sodium chloride-rich solution, which enters the secondary salt heat exchanger 9a for heat exchange to 102 °C, and then is sent to the secondary salt crystallizer 9b for flash evaporation. When the solid-liquid ratio of the crystal slurry in the salt leg of the secondary salt crystallizer 9b reaches about 10%, it is sent to the secondary salt thickening tank 9e for thickening through the secondary salt crystal slurry pump 9d. When the stirring current of the secondary salt thickening tank 9e rises to 4~, the crystal slurry can enter the secondary salt centrifuge 9f through the pipeline for solid-liquid separation. The solid separated by the secondary salt centrifuge 9f is refined sodium chloride and is sent to the sodium chloride drying system. The mother liquor separated by the secondary salt centrifuge 9f enters the secondary salt mother liquor tank 9h for temporary storage, and then is sent back to the secondary salt thickening tank 9e for circulation by the secondary salt mother liquor circulation pump 9j. Subsequently, the clear liquid of the secondary salt thickening tank 9e overflows into the secondary salt precipitation tank 9g. At this time, the clear liquid in the secondary salt precipitation tank 9g is an unsaturated sodium chloride and sodium sulfate solution, which is sent back to the freezing crystallizer 3b through the secondary salt mother liquor discharge pump 9k and the secondary salt mother liquor discharge pipe G2 to improve the salt recovery rate.

[0077] The sodium chloride drying system includes a sodium chloride auger 10a, a sodium chloride drying bed 10b, and a sodium chloride packing machine 10c. The sodium chloride salt separated from the solid phase of the secondary salt centrifuge 9f is sent to the sodium chloride drying bed 10b for drying through the sodium chloride auger 10a. The drying temperature is about 130 °C, and then it slides into the sodium chloride packing machine 10c for packing to obtain high-quality sodium chloride.

[0078] The mixed salt evaporation and crystallization system includes a mixed salt evaporator 11a, a mixed salt crystallizer 11b, a mixed salt circulation pump 11c, a mixed salt crystal slurry pump 11d, a mixed salt thickening tank 11e, a mixed salt centrifuge 11f, a mixed salt mother liquor tank 11g, a mixed salt mother liquor pump 11h, a mixed salt dissolution tank 11j, and a mixed salt dissolution and discharge pump 11k. The primary salt mother liquor discharge pump 7n is connected to the middle of the mixed salt concentration circulation pipe. The upper end of the mixed salt concentration circulation pipe is connected to the circulation outlet of the mixed salt crystallizer 11b, and the lower end is connected to the inlet of the mixed salt circulation pump 11c. The outlet of the mixed salt circulation pump 11c is connected to the tube side inlet of the mixed salt evaporator 11a, and the tube side outlet of the mixed salt evaporator 11a is connected to the circulation inlet of the mixed salt crystallizer 11b. The salt leg outlet of the mixed salt crystallizer 11b is connected to the inlet of the mixed salt thickening tank 11e through the mixed salt crystal slurry pump 11d. The bottom outlet of the mixed salt thickening tank 11e is connected to the inlet of the mixed salt centrifuge 11f. The solid phase outlet of the mixed salt centrifuge 11f is connected to the inlet of the mixed salt dissolution tank 11j. The bottom outlet of the mixed salt dissolution tank 11j is connected to the reflux port of the freezing crystallizer 3b through the mixed salt dissolution and discharge pump 11k.

[0079] The primary salt mother liquor discharge pump 7n sends the nitrate- and organic matter-rich solution into the tube side of the mixed salt evaporator 11a, heats it to 93°C, and then enters the mixed salt crystallizer 11b for flash evaporation. The circulation is maintained by the mixed salt circulation pump 11c. When the solid-liquid ratio of the crystal slurry in the salt leg of the mixed salt crystallizer 11b reaches about 10%, it is sent into the mixed salt thickening tank 11e through the mixed salt crystal slurry pump 11d for thickening. When the stirring current of the mixed salt thickening tank 11e rises to 4 - [the value is incomplete in the original text], the crystal slurry can enter the mixed salt centrifuge 11f through the pipeline for solid-liquid separation.

[0080] The solid separated by the mixed salt centrifuge 11f is a mixed salt of sodium sulfate and sodium chloride, which enters the mixed salt dissolution tank 11j through a chute pipe. After dissolution, it is sent back to the freezing crystallizer 3b through the mixed salt dissolution and discharge pump 11k for re-separation to improve the salt recovery rate.

[0081] The liquid phase outlet of the mixed salt centrifuge is connected to the inlet of the mixed salt mother liquor tank 11g. The outlet of the mixed salt mother liquor tank 11g is connected to the inlet of the drum dryer through the mixed salt mother liquor pump 11h. The waste salt outlet of the drum dryer is connected to the waste salt baler through a waste salt auger. The liquid separated by the mixed salt centrifuge 11f is the mixed salt mother liquor, whose main components are nitrate and organic matter. It enters the mixed salt mother liquor tank 11g for temporary storage and then is sent to the waste salt mother liquor drying system through the mixed salt mother liquor pump 11h.

[0082] The miscellaneous salt mother liquor drying system consists of a drum dryer 12a, a miscellaneous salt auger 12b, and a miscellaneous salt baler 12c. After the waste mother liquor is sent into the drum dryer 12a by the mixed salt mother liquor pump 11h, it is sprayed onto the drum. Subsequently, the dried powdered or flaky miscellaneous salt is scraped off by a scraper. The temperature of the drum roller is about 130°C. The miscellaneous salt is conveyed to the miscellaneous salt baler 12c through the miscellaneous salt auger 12b and packed in ton bags.

[0083] The above are only the preferred and feasible embodiments of the present invention, which show and describe the basic principles, main features and advantages of the present invention. The patent protection scope of the present invention is not limited thereby. Those skilled in the art should understand that the present invention is not limited by the above embodiments. Except for the above embodiments, without departing from the spirit and scope of the present invention, the present invention can also have other implementation manners. The present invention will also have various changes and improvements. Any technical solutions formed by equivalent substitution or equivalent transformation fall within the protection scope required by the present invention. The protection scope required by the present invention is defined by the appended claims and their equivalents. The technical features not described in the present invention can be realized by or adopted the prior art, and will not be elaborated herein.

Claims

1. A zero-discharge salt separation and recovery system for wastewater containing sodium sulfate and sodium chloride, including a falling film concentration MVR system, characterized in that, The falling film concentration MVR system includes a falling film evaporator. The lower part of the falling film evaporator is connected to a falling film separator. The bottom outlets of the falling film evaporator and the falling film separator are both connected to the inlet of a falling film circulation pump. The outlet of the falling film circulation pump is connected to the top inlet of the falling film evaporator through a falling film concentration circulation pipe. The outlet of the raw water feed pipe G1 is connected to the falling film concentration circulation pipe. The bottom outlets of the falling film evaporator and the falling film separator are also connected to the forced concentration circulation pipe of the forced concentration MVR system through a falling film discharge pump. The upper end of the forced concentration circulation pipe is connected to the circulation outlet of the forced separator. The lower end outlet of the forced concentration circulation pipe is connected to the inlet of a forced circulation pump. The outlet of the forced circulation pump is connected to the tube side inlet of the forced evaporator. The tube side outlet of the forced evaporator is connected to the circulation inlet of the forced separator. The bottom outlet of the salt leg of the forced separator is connected to the freezing crystallization system through a forced discharge pump.

2. The zero-discharge salt separation and recovery system for wastewater containing sodium sulfate and sodium chloride according to claim 1, characterized in that: The freezing crystallization system includes a freezing crystallizer. The outlet of the forced discharge pump is connected to the feed inlet of the freezing crystallizer. The circulation outlet of the freezing crystallizer is connected to the inlet of a freezing crystallization circulation pump. The outlet of the freezing crystallization circulation pump is connected to the tube side inlet of the freezing heat exchanger. The tube side outlet of the freezing heat exchanger is connected to the circulation inlet of the freezing crystallizer. The bottom salt slurry outlet of the freezing crystallizer is connected to the inlet of a freezing thickening tank through a freezing slurry pump. The bottom outlet of the freezing thickening tank is connected to the inlet of a freezing centrifuge. The solid phase outlet of the freezing centrifuge is connected to the sodium sulfate hot melting system through a chute.

3. The zero-emission salt separation and recovery system for wastewater containing sodium sulfate and sodium chloride according to claim 2, characterized in that: The clear liquid overflow port of the freezing thickening tank is connected to the inlet of a freezing sedimentation tank. The bottom outlet of the freezing sedimentation tank and the liquid phase outlet of the freezing centrifuge are respectively connected to the inlet of a freezing mother liquor tank. The outlet of the freezing mother liquor tank is connected to the reflux port of the freezing thickening tank through a freezing mother liquor circulation pump. The clear liquid outlet of the freezing sedimentation tank is connected to the primary salt MVR system through a freezing mother liquor discharge pump.

4. The zero-discharge salt separation and recovery system for wastewater containing sodium sulfate and sodium chloride according to claim 2, wherein: The solid phase outlet of the freezing centrifuge is connected to the inlet of the nitrate dissolving tank in the sodium sulfate hot melting system through a chute. The side wall circulation outlet of the nitrate dissolving tank is connected to the inlet of a nitrate dissolving circulation pump. The outlet of the nitrate dissolving circulation pump is connected to the tube side inlet of the nitrate dissolving heat exchanger. The tube side outlet of the nitrate dissolving heat exchanger is connected to the central feed pipe of the nitrate dissolving tank. The lower end of the central feed pipe extends to the lower part of the nitrate dissolving tank. The bottom outlet of the nitrate dissolving tank is connected to the nitrate recrystallization system through a nitrate dissolving discharge pump.

5. The zero-discharge salt separation and recovery system for wastewater containing sodium sulfate and sodium chloride according to claim 4, characterized in that: The outlet of the nitrate dissolution discharge pump is connected to the inlet of the nitrate thickening tank in the nitrate recrystallization system. The crystal slurry outlet of the nitrate thickening tank is connected to the inlet of the nitrate centrifuge. The mother liquor outlet of the nitrate centrifuge is connected to the inlet of the nitrate mother liquor tank. The bottom outlet of the nitrate mother liquor tank is connected to the middle of the nitrate evaporator circulation pipe through the nitrate mother liquor pump. The upper end of the nitrate evaporator circulation pipe is connected to the circulation outlet of the nitrate crystallizer. The lower end of the nitrate evaporator circulation pipe is connected to the inlet of the nitrate circulation pump. The outlet of the nitrate circulation pump is connected to the tube side inlet of the nitrate evaporator. The tube side outlet of the nitrate evaporator is connected to the circulation inlet of the nitrate crystallizer. The bottom crystal slurry outlet of the nitrate crystallizer is connected to the inlet of the nitrate thickening tank through the nitrate crystal slurry pump.

6. The zero-emission salt separation and recovery system for wastewater containing sodium sulfate and sodium chloride according to claim 5, wherein: The solid phase outlet of the nitrate centrifuge is connected to the inlet of the sodium sulfate dry bed through a chute and a nitrate auger. The outlet of the sodium sulfate dry bed is connected to the sodium sulfate packing machine.

7. The zero-emission salt separation and recovery system for wastewater containing sodium sulfate and sodium chloride according to claim 3, wherein: The refrigerated mother liquor discharge pump is connected to the middle of the primary salt concentration circulation pipe in the primary salt MVR system. The upper end of the primary salt concentration circulation pipe is connected to the circulation outlet of the primary salt crystallizer. The lower end of the primary salt concentration circulation pipe is connected to the inlet of the primary salt circulation pump. The outlet of the primary salt circulation pump is connected to the tube side inlet of the primary salt heat exchanger. The tube side outlet of the primary salt heat exchanger is connected to the circulation inlet of the primary salt crystallizer. The salt leg outlet of the primary salt crystallizer is connected to the inlet of the primary salt thickening tank through the primary salt crystal slurry pump. The bottom outlet of the primary salt thickening tank is connected to the inlet of the primary salt centrifuge. The solid phase outlet of the primary salt centrifuge is connected to the inlet of the primary salt dissolution tank. The outlet of the primary salt dissolution tank is connected to the inlet of the nanofiltration membrane device through the primary salt dissolution external delivery pump. The concentrated water outlet of the nanofiltration membrane device is connected to the inlet of the refrigerated crystallizer. The produced water outlet of the nanofiltration membrane device is connected to the secondary salt MVR system.

8. The zero-emission salt separation and recovery system for wastewater containing sodium sulfate and sodium chloride according to claim 7, characterized in that: The liquid phase outlet of the primary salt centrifuge is connected to the inlet of the primary salt mother liquor tank. The outlet of the primary salt mother liquor tank is connected to the return port of the primary salt thickening tank through the primary salt mother liquor circulation pump. The clear liquid outlet at the top of the primary salt thickening tank is connected to the inlet of the primary salt precipitation tank. The bottom outlet of the primary salt precipitation tank is connected to the inlet of the primary salt mother liquor tank. The upper side wall outlet of the primary salt precipitation tank is connected to the mixed salt evaporation and crystallization system through the primary salt mother liquor discharge pump.

9. The zero-emission salt separation and recovery system for wastewater containing sodium sulfate and sodium chloride according to claim 7, characterized in that: The produced water outlet of the nanofiltration membrane device is connected to the middle of the secondary salt concentration circulation pipe in the secondary salt MVR system. The upper end of the secondary salt concentration circulation pipe is connected to the circulation outlet of the secondary salt crystallizer. The lower end of the secondary salt concentration circulation pipe is connected to the inlet of the secondary salt circulation pump. The outlet of the secondary salt circulation pump is connected to the tube side inlet of the secondary salt heat exchanger. The tube side outlet of the secondary salt heat exchanger is connected to the circulation inlet of the secondary salt crystallizer. The salt leg outlet of the secondary salt crystallizer is connected to the inlet of the secondary salt thickening tank through the secondary salt crystal slurry pump. The bottom outlet of the secondary salt thickening tank is connected to the inlet of the secondary salt centrifuge. The solid phase outlet of the secondary salt centrifuge is connected to the sodium chloride drying system.

10. The zero-emission salt separation and recovery system for wastewater containing sodium sulfate and sodium chloride according to claim 9, wherein: The mother liquor outlet of the secondary salt centrifuge is connected to the inlet of the secondary salt mother liquor tank, and the outlet of the secondary salt mother liquor tank is connected to the reflux port of the secondary salt thickening tank through the secondary salt mother liquor circulation pump; The clear liquid overflow port of the secondary salt thickening tank is connected to the inlet of the secondary salt precipitation tank, and the bottom outlet of the secondary salt precipitation tank is connected to the inlet of the secondary salt mother liquor tank; The middle clear liquid outlet of the secondary salt precipitation tank is connected to the inlet of the freezing crystallizer through the secondary salt mother liquor discharge pump and the secondary salt mother liquor discharge pipe.

11. The zero-emission salt separation and recovery system for wastewater containing sodium sulfate and sodium chloride according to claim 8, characterized in that: The primary salt mother liquor discharge pump is connected to the middle of the mixed salt concentration circulation pipe in the mixed salt evaporation and crystallization system. The upper end of the mixed salt concentration circulation pipe is connected to the circulation outlet of the mixed salt crystallizer, and the lower end of the mixed salt concentration circulation pipe is connected to the inlet of the mixed salt circulation pump. The outlet of the mixed salt circulation pump is connected to the tube side inlet of the mixed salt evaporator, and the tube side outlet of the mixed salt evaporator is connected to the circulation inlet of the mixed salt crystallizer; The salt leg outlet of the mixed salt crystallizer is connected to the inlet of the mixed salt thickening tank through the mixed salt crystal slurry pump. The bottom outlet of the mixed salt thickening tank is connected to the inlet of the mixed salt centrifuge. The solid phase outlet of the mixed salt centrifuge is connected to the inlet of the mixed salt dissolution tank. The bottom outlet of the mixed salt dissolution tank is connected to the reflux port of the freezing crystallizer through the mixed salt dissolution discharge pump; The liquid phase outlet of the mixed salt centrifuge is connected to the inlet of the mixed salt mother liquor tank. The outlet of the mixed salt mother liquor tank is connected to the inlet of the drum dryer through the mixed salt mother liquor pump. The waste salt outlet of the drum dryer is connected to the waste salt baler through the waste salt auger.

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