A low-temperature salt separation coupling flat plate nanofiltration brine resource system and process

Abstract

The application discloses a low-temperature salt separation and coupling flat-plate nanofiltration system and process for recycling of miscellaneous salt resources, which directly separates and purifies solid miscellaneous salt with low energy consumption and high efficiency by using a low-temperature salt separation process, and simplifies a process flow. By integrating key processes such as redissolution, filtration, separation and concentration of miscellaneous salt, intermediate links are reduced, and equipment cost and operation energy consumption are reduced. By using the low-temperature salt separation and coupling flat-plate nanofiltration process, multi-component mixed salts such as sodium chloride, sodium sulfate, sodium nitrate and potassium nitrate are separated into elemental salt solutions, and the quality of recovered crystalline salts all reaches an industrial-grade standard, harmless and resourceful treatment of miscellaneous salt is realized, resource utilization is improved, and economic value of products is increased. The application is not limited by the quality of miscellaneous salt, and can effectively treat miscellaneous salt with different sources and different component proportions.

Description

Technical Field

[0001] This invention relates to the field of mixed salt resource utilization, and in particular to a system and process for separating and purifying mixed salts by coupling low-temperature salt separation technology with flat plate nanofiltration process. Background Technology

[0002] The crystalline salts produced by traditional high-salinity wastewater treatment processes mainly contain sodium chloride, sodium sulfate, sodium nitrate, and potassium nitrate. According to the "Environmental Access Conditions for Modern Coal Chemical Construction Projects (Trial)" issued by relevant departments, these are temporarily classified as hazardous waste. Treating this type of hazardous waste is costly, and improper disposal can lead to environmental pollution problems.

[0003] In the current field of mixed salt resource utilization, evaporation crystallization and freeze crystallization methods are widely used for the separation of sodium sulfate and sodium chloride. These methods are based on the water-salt system phase diagram and utilize the concentration differences and solubility characteristics of different inorganic salts in aqueous solutions. By precisely controlling parameters such as operating temperature and concentration factor during the crystallization process, the separation and purification of salts can be achieved. For example, the high-efficiency elemental salt separation and recovery technology disclosed in CN105036222A adopts an integrated evaporation-thermal crystallization process. Within a temperature range of 50℃ to 150℃, evaporation crystallization is used to separate sodium sulfate and sodium chloride crystals, and further selective precipitation of sodium nitrate is achieved through cooling crystallization technology (-15~0℃). However, in practical applications, when water quality conditions fluctuate frequently, the salt separation system in this thermal salt-nitrate co-production process is difficult to operate stably for a long time, which cannot guarantee the stability of the quality of the crystallized salt product. It also suffers from high energy consumption. Summary of the Invention

[0004] This invention addresses the shortcomings of existing technologies by providing a low-temperature salt separation coupled with flat-plate nanofiltration system and process for the resource recovery of mixed salts. This solves the problems of traditional technologies, such as difficulty in long-term stable operation, inability to guarantee the stability of crystalline salt product quality, and high energy consumption.

[0005] The technical solution of the present invention to solve the above-mentioned technical problems is as follows: A low-temperature salt separation coupled flat plate nanofiltration mixed salt resource utilization system, comprising a first salt dissolving tank, a refrigeration device, a first vacuum belt filter, a first liquid storage tank, a first pretreatment unit, an electrocatalytic oxidation unit, an activated carbon filter, a first evaporation water tank, a sodium chloride crystallization unit, a second salt dissolving tank, a heating device, a second vacuum belt filter, a second liquid storage tank, a second pretreatment unit, a CDNF unit, a second evaporation water tank, a cooling crystallization unit, a concentrated water tank, and a sodium sulfate evaporation unit;

[0006] The first salt dissolving tank is used to dissolve the raw material miscellaneous salt particles into a solid-liquid mixture; the refrigeration device provides 0°C cold water to the first salt dissolving tank for dissolving the miscellaneous salt and provides washing liquid to the low-temperature circulating washing device of the first vacuum belt filter; the first vacuum belt filter is used in conjunction with its low-temperature circulating washing device for the separation and enrichment of sodium chloride.

[0007] The first storage tank is used to temporarily store the filtrate after being processed by the first vacuum belt filter, and the first pretreatment unit is used to remove silicon, fluorine and heavy metal impurities from the filtrate.

[0008] The electrocatalytic oxidation unit and the activated carbon filter sequentially purify the filtrate after treatment by the first pretreatment unit; the first evaporation tank evaporates the filtrate after treatment by the electrocatalytic oxidation unit and the activated carbon filter; and the sodium chloride crystallization unit is used to crystallize sodium chloride.

[0009] The second salt dissolving tank is used to redissolve the filter cake obtained from the first vacuum belt filter into a solid-liquid mixture, and the heating device provides warm water at a set temperature to the second salt dissolving tank to dissolve the filter cake.

[0010] The second vacuum belt filter is used to filter out solid impurities in the redissolved solid-liquid mixture; the second storage tank temporarily stores the filtrate after being processed by the second vacuum belt filter; and the second pretreatment unit is used to remove silicon and heavy metal impurities from the filtrate.

[0011] The CDNF unit utilizes its separation properties for monovalent and divalent ions to enable NO3- - SO4 enters the product water side 2- It is retained at the concentrate end; the concentrate tank stores the SO4-containing solution separated by the CDNF unit. 2- The sodium sulfate evaporation unit evaporates the concentrated water in the concentrated water tank to obtain sodium sulfate product salt.

[0012] The second evaporation tank performs thermal concentration on the water produced by the CDNF unit, and the cooling crystallization unit performs cooling crystallization operation by controlling the temperature to prepare sodium nitrate and potassium nitrate products.

[0013] As a preferred solution for a low-temperature salt separation coupled with flat-plate nanofiltration system for the resource recovery of mixed salts, the first vacuum belt filter, during its filtration process:

[0014] The filter disc operates synchronously with the filter cloth under vacuum conditions; under non-vacuum conditions, the filter disc and filter cloth operate relative to each other. Material is evenly distributed on the surface of the filter belt via a slurry distributor. Using the filter cloth as the filtration medium, filtration is performed under vacuum suction. Undissolved solid salt particles are trapped on the filter belt surface to form a filter cake. The low-temperature circulating washing device uses 0°C cold water as the washing liquid, which is evenly sprayed onto the filter cake surface through nozzles. Under vacuum suction, the washing liquid penetrates the filter cake and filter cloth, dissolving residual sodium chloride soluble substances in the filter cake. The washing liquid operates in a circulating mode. When the Cl in the washing liquid... - Once the concentration reaches the preset value, it is discharged into the first storage tank, and low-temperature water is introduced again to continue washing.

[0015] As a preferred solution for a low-temperature salt separation coupled with flat-plate nanofiltration system for the resource recovery of mixed salts, the second vacuum belt filter, during the circulating washing process, when SO4 in the washing liquid... 2- Once the concentration reaches the preset value, it is discharged into the second storage tank, and at the same time, 40°C warm water is introduced again to continue washing until sodium sulfate and nitrate are fully dissolved in the washing solution.

[0016] As a preferred solution for a low-temperature salt separation coupled with flat-plate nanofiltration mixed salt resource utilization system, the sodium chloride crystallization unit precipitates sodium chloride crystals, and the sodium chloride product obtained by drying treatment meets the secondary standard for industrial dry salt and the standard for refined dry salt for ion-exchange membrane caustic soda. The remaining mother liquor is returned to the CDNF unit.

[0017] As a preferred embodiment of the low-temperature salt separation coupled flat-plate nanofiltration mixed salt resource recovery system, it also includes a buffer water tank, which is located between the second pretreatment unit and the CDNF unit. The buffer water tank is used to buffer the solution and ensure stable operation of the system.

[0018] As a preferred embodiment of the low-temperature salt separation coupled with flat-plate nanofiltration system for resource recovery of mixed salts, the control strategy for the first vacuum belt filter and the low-temperature circulating washing device is as follows:

[0019] When the start button is pressed, the filter enters the filtration state. Check the filter disc vacuum sensor. If the filter disc vacuum sensor is normal, start the filter disc and filter belt motor, set the filter belt motor speed, and open the slurry distribution valve. If the filter disc vacuum sensor is not normal, open the vacuum valve.

[0020] When the filter belt position sensor value reaches the set value, the washing state is entered and the washing liquid nozzle valve is opened. During the washing process, the chloride ion concentration sensor value is checked. When the maximum chloride ion concentration is reached, or the washing liquid level sensor value reaches the minimum washing liquid level, the drain valve is opened and the replenishment valve is closed. At the same time, the washing cycle count is incremented by 1.

[0021] When the washing cycle reaches the maximum number of washing liquid cycles, the machine enters the unloading state and starts the scraper. If the equipment fault sensor is true, an alarm will sound and the machine will enter the stop state. When the stop button is pressed, one processing cycle ends and the equipment stops.

[0022] This invention also provides a low-temperature salt separation coupled with flat-plate nanofiltration process for the resource recovery of mixed salts, comprising the following steps:

[0023] Salt dissolution and preliminary separation: 1-3mm salt particles are conveyed to the first salt dissolution tank by the lifting device and 0℃ water prepared by the refrigeration device is injected to dissolve the salt into a solid-liquid mixture. The mixture is then pumped to the first vacuum belt filter. The material is separated into solid and liquid on the filter belt by gravity and vacuum suction. The liquid is sucked into the vacuum box, and the undissolved salt particles remain on the filter belt to form a filter cake.

[0024] The filter cake is washed with 0°C cold water produced by the refrigeration unit. Vacuum suction forces the cold water to penetrate the filter cake and filter belt. Taking advantage of the difference in solubility of salts at low temperatures, soluble salts are dissolved in the washing liquid, achieving preliminary salt separation. The washing liquid is circulated and discharged into the first storage tank when Cl- is enriched to a preset concentration. At the same time, low-temperature water is reintroduced into the washing system to enrich the sodium chloride solution. The filter cake moves with the filter belt to the unloading device and is then transported to the second salt dissolving tank.

[0025] Sodium chloride purification: The filtrate after being treated by the first vacuum belt filter enters the first pretreatment unit to remove silicon and heavy metal impurities and reduce the solution hardness caused by calcium and magnesium ions. Then, it flows sequentially through the electrocatalytic oxidation unit and the activated carbon filter to remove organic matter and improve water quality. After meeting the inlet water standard of the evaporation system, it enters the first evaporation tank and is then introduced into the sodium chloride crystallization unit by a booster pump. Sodium chloride evaporates and crystallizes, and after drying, sodium chloride product salt is obtained. The mother liquor is returned to the CDNF unit.

[0026] Salt separation and purification: The filter cake in the second salt dissolving tank contains sodium sulfate, nitrate, and solid insoluble impurities; water is heated to a set temperature using a heating device, and the warm water redissolves the filter cake into a solid-liquid mixture, which is then conveyed to the second vacuum belt filter; during the warm water circulation washing process, sodium sulfate and nitrate in the slurry dissolve, and solid impurities are filtered out by the second vacuum belt filter; when SO4... 2- Once the concentration reaches the preset level, it is discharged into the second storage tank, and the warm water is replaced for further washing. The filtrate enters the second pretreatment unit for impurity removal and hardness reduction before flowing into the CDNF unit. In the CDNF unit, NO3... - SO4 enters the product water side 2- The concentrated water is retained at the concentrate end and enters the concentrate tank. It is then pumped into the sodium sulfate evaporation unit to produce sodium sulfate product salt. The product water passes through the second evaporation tank and is concentrated by thermal method. It is then cooled and crystallized in the cooling crystallization unit to obtain sodium nitrate and potassium nitrate products respectively.

[0027] As a preferred scheme for the low-temperature salt separation coupled with flat plate nanofiltration process for the resource utilization of mixed salts, in the steps of mixed salt dissolution and preliminary separation, the water-salt ratio of mixed salt particles to 0℃ water is 2.5:1;

[0028] In the sodium chloride purification step, the first pretreatment unit reduces the calcium and magnesium ion content in the filtrate to below 20 mg / L.

[0029] As a preferred solution for the low-temperature salt separation coupled with flat-plate nanofiltration process for the resource recovery of mixed salts, in the salt separation and purification step, SO4 in the washing liquid of the second vacuum belt filter... 2- When the concentration reaches 160,000 mg / L, it is discharged into the second storage tank.

[0030] As a preferred solution for the low-temperature salt separation coupled with flat-plate nanofiltration process for the resource utilization of mixed salts, when preparing sodium nitrate and potassium nitrate products in the cooling crystallization unit, the product water after thermal concentration is first cooled to 30-40℃ to crystallize out sodium nitrate, and then the remaining solution is cooled to 0-10℃ to crystallize out potassium nitrate.

[0031] The beneficial effects of this invention are as follows:

[0032] First, a low-temperature salt separation process is adopted to directly separate and purify solid mixed salts with low energy consumption and high efficiency, simplifying the process flow. By integrating key processes such as resolution, filtration, separation, and concentration of mixed salts, intermediate steps are reduced, lowering equipment costs and operating energy consumption.

[0033] Secondly, compared to traditional thermal processes which are constrained by the co-saturated system of multiple mixed salts, the low-temperature salt separation process produces a single-component salt solution, which can enrich the target salt to near saturation, greatly improving the salt separation efficiency. Its circulating washing system can precisely control the amount of dissolved salt, reducing the amount of water processed and further lowering system energy consumption and operating costs.

[0034] Third, by employing a low-temperature salt separation coupled with flat-plate nanofiltration process, we successfully separated a mixture of multiple salts, such as sodium chloride, sodium sulfate, sodium nitrate, and potassium nitrate, into elemental salt solutions. The quality of the recovered crystalline salts all met industrial-grade standards, achieving the harmless and resource-based treatment of mixed salts, improving resource utilization, and increasing the economic value of the products.

[0035] Fourth, the technology's processing effect is not limited by the quality of the mixed salts. It can effectively process mixed salts from different sources and with different component ratios, demonstrating excellent adaptability and stability. It provides a brand-new idea and method for the comprehensive utilization of mixed salt resources, and has broad application prospects and significant social significance. Attached Figure Description

[0036] To more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings in the following description are merely exemplary, and those skilled in the art can derive other embodiments based on the provided drawings without creative effort.

[0037] The structures, proportions, sizes, etc. illustrated in this specification are only for the purpose of assisting those skilled in the art in understanding and reading the content disclosed herein, and are not intended to limit the conditions under which the present invention can be implemented. Therefore, they have no substantial technical significance. Any modifications to the structure, changes in the proportions, or adjustments to the size, without affecting the effects and objectives that the present invention can produce, should still fall within the scope of the technical content disclosed in the present invention.

[0038] Figure 1 This is a schematic diagram of a low-temperature salt separation coupled flat-plate nanofiltration system for resource recovery of mixed salts provided in an embodiment of the present invention;

[0039] Figure 2 This is the control strategy for the vacuum belt filter combined with the low-temperature circulating washing device provided in the embodiments of the present invention.

[0040] In the diagram, 1. First salt dissolving tank; 2. Refrigeration device; 3. First vacuum belt filter; 4. First storage tank; 5. First pretreatment unit; 6. Electrocatalytic oxidation unit; 7. Activated carbon filter; 8. First evaporation tank; 9. Sodium chloride crystallization unit; 10. Second salt dissolving tank; 11. Heating device; 12. Second vacuum belt filter; 13. Second storage tank; 14. Second pretreatment unit; 15. Buffer tank; 16. CDNF unit; 17. Second evaporation tank; 18. Cooling crystallization unit; 19. Concentrate tank; 20. Sodium sulfate evaporation unit. Detailed Implementation

[0041] To make the above-mentioned objects, features, and advantages of the present invention more apparent and understandable, specific embodiments of the present invention will be described in detail below with reference to the accompanying drawings. Many specific details are set forth in the following description to provide a thorough understanding of the present invention. However, the present invention can be practiced in many other ways different from those described herein, and those skilled in the art can make similar modifications without departing from the spirit of the present invention. Therefore, the present invention is not limited to the specific embodiments disclosed below.

[0042] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention pertains. The terminology used herein in the description of the invention is for the purpose of describing particular embodiments only and is not intended to be limiting of the invention. The term "and / or" as used herein includes any and all combinations of one or more of the associated listed items.

[0043] See Figure 1 This invention provides a low-temperature salt separation coupled flat-plate nanofiltration mixed salt resource recovery system, including a first salt dissolving tank 1, a refrigeration device 2, a first vacuum belt filter 3, a first storage tank 4, a first pretreatment unit 5, an electrocatalytic oxidation unit 6, an activated carbon filter 7, a first evaporation tank 8, a sodium chloride crystallization unit 9, a second salt dissolving tank 10, a heating device 11, a second vacuum belt filter 12, a second storage tank 13, a second pretreatment unit 14, a CDNF unit 16, a second evaporation tank 17, a cooling crystallization unit 18, a concentrate tank 19, and a sodium sulfate evaporation unit 20;

[0044] The first salt dissolving tank 1 is used to dissolve the raw material miscellaneous salt particles into a solid-liquid mixture; the refrigeration device 2 provides 0°C cold water to the first salt dissolving tank 1 for dissolving the miscellaneous salt and provides washing liquid to the low-temperature circulating washing device of the first vacuum belt filter 3; the first vacuum belt filter 3 is used in conjunction with its low-temperature circulating washing device to separate and enrich sodium chloride.

[0045] The first storage tank 4 is used to temporarily store the filtrate after being processed by the first vacuum belt filter 3, and the first pretreatment unit 5 is used to remove silicon, fluorine and heavy metal impurities from the filtrate.

[0046] The electrocatalytic oxidation unit 6 and the activated carbon filter 7 sequentially purify the filtrate after it has been treated by the first pretreatment unit 5; the first evaporation tank 8 evaporates the filtrate after it has been treated by the electrocatalytic oxidation unit 6 and the activated carbon filter 7; and the sodium chloride crystallization unit 9 is used to crystallize sodium chloride.

[0047] The second salt dissolving tank 10 is used to redissolve the filter cake obtained by the first vacuum belt filter 3 into a solid-liquid mixture, and the heating device 11 provides warm water at a set temperature to the second salt dissolving tank 10 to dissolve the filter cake.

[0048] The second vacuum belt filter 12 is used to filter out solid impurities in the redissolved solid-liquid mixture; the second storage tank 13 temporarily stores the filtrate after being processed by the second vacuum belt filter 12; and the second pretreatment unit 14 is used to remove silicon and heavy metal impurities from the filtrate.

[0049] Among them, CDNF unit 16 utilizes the separation characteristics of monovalent and divalent ions to enable NO3- -SO4 enters the product water side 2- It is trapped at the concentrate end; concentrate tank 19 stores the SO4-containing solution separated by CDNF unit 16. 2- The sodium sulfate evaporation unit 20 evaporates the concentrated water in the concentrated water tank 19 to obtain sodium sulfate product salt.

[0050] The second evaporation tank 17 thermally concentrates the water produced by the CDNF unit 16, and the cooling crystallization unit 18 performs cooling crystallization by controlling the temperature to produce sodium nitrate and potassium nitrate products.

[0051] In this embodiment, the first salt dissolving tank 1 serves to dissolve the raw material mixed salt particles into a solid-liquid mixture. The principle is to utilize the dissolving effect of water on the mixed salt, dispersing its various components in the water to form a material state suitable for subsequent processing. This step provides the initial processing target for the entire system, ensuring the smooth progress of subsequent separation operations. The refrigeration unit 2 provides 0°C cold water to the first salt dissolving tank 1 for mixed salt dissolution and supplies washing liquid to the low-temperature circulating washing device of the first vacuum belt filter 3. At 0°C, the solubility differences of different components in the mixed salt are more pronounced, which is beneficial for the subsequent separation of sodium chloride. Simultaneously, the low-temperature washing liquid can more effectively dissolve sodium chloride in the filter cake while dissolving other salts less, thus achieving efficient enrichment of sodium chloride. This technique, utilizing the differences in salt solubility at low temperatures, significantly improves the accuracy and efficiency of separation compared to traditional room-temperature processing methods.

[0052] Specifically, the first vacuum belt filter 3, combined with its low-temperature circulating washing device, achieves the separation and enrichment of sodium chloride. During filtration, under the action of vacuum suction, the liquid passes through the filter cloth and is drawn into the vacuum chamber, while solid impurities are trapped on the filter cloth to form a filter cake. The low-temperature circulating washing device uses 0°C cold water as the washing liquid, which is evenly sprayed onto the surface of the filter cake through nozzles. With the assistance of vacuum suction, the washing liquid penetrates the filter cake and filter cloth, dissolving residual sodium chloride and other soluble substances in the filter cake into the washing liquid. Because the washing liquid adopts a circulating mode, the Cl- concentration in the washing liquid gradually increases with the increase of the number of cycles. When the Cl- concentration reaches the preset value, the washing liquid is discharged into the first storage tank 4, and new low-temperature water is introduced to continue washing, thereby achieving the enrichment of sodium chloride solution. This separation and enrichment method not only improves the purity of sodium chloride but also reduces the mixing of other impurities, thus improving product quality.

[0053] Specifically, the first storage tank 4 is used to temporarily store the filtrate processed by the first vacuum belt filter 3. It acts as a buffer, making the entire system more stable and preventing system failures caused by mismatched filtrate processing speeds. The first pretreatment unit 5 is responsible for removing silicon, fluorine, and heavy metal impurities from the filtrate. Its principle is to separate these impurities from the filtrate through a series of chemical reactions and physical adsorption processes. For example, ion exchange resins may be used to remove metal ions, and precipitation reactions may be used to remove silicon and fluorine. This step effectively reduces the impurity content in the filtrate, providing purer raw materials for subsequent processing, reducing interference from impurities in subsequent processes, and improving the quality of the final product.

[0054] Specifically, the electrocatalytic oxidation unit 6 and the activated carbon filter 7 sequentially purify the filtrate after treatment by the first pretreatment unit 5. The electrocatalytic oxidation unit 6 utilizes the strong oxidizing substances generated by the electrode reaction to oxidize and decompose organic matter and other impurities in the filtrate, transforming them into harmless substances. The activated carbon filter 7 utilizes the porous structure and strong adsorption capacity of activated carbon to further adsorb residual organic matter, pigments, and some incompletely removed ions and other impurities. After treatment by these two units, the water quality of the filtrate is greatly improved, meeting the stringent requirements of the subsequent evaporation system for influent water quality, ensuring the smooth operation of the evaporation process, reducing scaling and corrosion in the evaporator, and extending the service life of the equipment.

[0055] Specifically, the first evaporation tank 8 evaporates the purified filtrate, followed by sodium chloride crystallization in the sodium chloride crystallization unit 9. In the first evaporation tank 8, water in the filtrate is gradually evaporated by heating, causing the solution concentration to continuously increase. When the saturation concentration of sodium chloride is reached, sodium chloride crystallizes in the sodium chloride crystallization unit 9 by controlling conditions such as temperature and stirring speed. This evaporation-crystallization process is based on the characteristic that the solubility of sodium chloride in water changes with temperature and concentration, realizing the conversion from solution to solid sodium chloride product. Furthermore, the purity of the obtained sodium chloride product meets the secondary standard for industrial dry salt and the standard for refined dry salt used in ion-exchange membrane caustic soda, thus improving the economic value of sodium chloride.

[0056] Specifically, the second salt dissolving tank 10 is used to redissolve the filter cake produced by the first vacuum belt filter 3 into a solid-liquid mixture. The heating device 11 provides warm water at a set temperature (e.g., 40°C) to the second salt dissolving tank 10 to promote filter cake dissolution. As the temperature rises, the solubility of remaining impurities (such as sodium sulfate, nitrates, etc.) in the filter cake increases in warm water, allowing them to dissolve more fully and creating conditions for subsequent separation operations. Compared to dissolving in cold water, dissolving in warm water can accelerate the dissolution rate and improve processing efficiency.

[0057] Specifically, the second vacuum belt filter 12 is used to filter out solid impurities from the redissolved solid-liquid mixture. It performs a filtering function again to ensure that the solution entering subsequent processing stages is purer and to reduce the impact of solid impurities on subsequent processes.

[0058] Specifically, the second storage tank 13 temporarily stores the filtrate processed by the second vacuum belt filter 12, and the second pretreatment unit 14 further removes silicon and heavy metal impurities from the filtrate. This is similar to the function of the first pretreatment unit 5, but it removes impurities again to further improve the purity of the solution, taking into account the characteristics of the filtrate dissolved in the second salt dissolving tank 10, thus preparing for more precise ion separation in the future.

[0059] Specifically, the CDNF unit utilizes its ability to separate monovalent and divalent ions to enable NO3- - Through the entry of SO4 into the product water side, 2- It is trapped at the concentrate end. The membrane material of the CDNF unit has special pore size and surface charge characteristics, exhibiting selective permeability to ions of different valence states. For monovalent NO3... - It can smoothly pass through the membrane into the product water side, while divalent SO4 2- The NO3 is then retained by the membrane, thus achieving NO3 retention. - With SO4 2- This highly efficient ion separation technology boasts advantages such as high efficiency, energy saving, and no phase change. Compared to traditional separation techniques like chemical precipitation, it can separate ions more precisely, reduce the use of chemical reagents, lower production costs, and simultaneously improve separation efficiency and product quality.

[0060] Specifically, the concentrate tank 19 is used to store the SO4-containing water separated from the CDNF unit. 2- The concentrated water in the concentrated water tank 19 is evaporated by the sodium sulfate evaporation unit 20, thereby producing sodium sulfate product salt. By evaporating the concentrated water, the concentration of sodium sulfate is continuously increased until it reaches saturation and crystallizes out, finally obtaining sodium sulfate product salt that meets the Class II qualified product standard in the "Industrial Anhydrous Sodium Sulfate" (GB / T6009-2014) standard, realizing the resource utilization of sodium sulfate and improving the resource utilization rate.

[0061] Specifically, the second evaporation tank 17 thermally concentrates the permeate from the CDNF unit, while the cooling crystallization unit 18 performs cooling crystallization by controlling the temperature to produce sodium nitrate and potassium nitrate products. In the second evaporation tank 17, some water is removed from the permeate by heating and evaporation, increasing the concentrations of sodium nitrate and potassium nitrate. Then, in the cooling crystallization unit 18, based on the difference in solubility of sodium nitrate and potassium nitrate at different temperatures, the thermally concentrated permeate is first cooled to 30-40°C. At this temperature, the solubility of sodium nitrate decreases, causing it to crystallize first. The remaining solution is then cooled to 0-10°C, further reducing the solubility of potassium nitrate, thus causing it to crystallize. This precisely temperature-controlled cooling crystallization method effectively separates sodium nitrate and potassium nitrate, yielding high-purity products, increasing the added value of the products, and realizing the comprehensive resource utilization of multiple components in miscellaneous salts.

[0062] In one possible embodiment, during the filtration process of the first vacuum belt filter 3:

[0063] The filter disc operates synchronously with the filter cloth under vacuum conditions; under non-vacuum conditions, the filter disc and filter cloth operate relative to each other. Material is evenly distributed onto the filter belt surface via a slurry distributor. Using the filter cloth as the filtration medium, filtration is performed under vacuum suction. Undissolved solid salt particles are trapped on the filter belt surface, forming a filter cake. A low-temperature circulating washing device uses 0°C cold water as the washing liquid, which is evenly sprayed onto the filter cake surface through nozzles. Under vacuum suction, the washing liquid penetrates the filter cake and filter cloth, dissolving residual sodium chloride-soluble substances in the filter cake. The washing liquid operates in a circulating mode. When the Cl in the washing liquid... - Once the concentration reaches the preset value, it is discharged into the first storage tank 4, and low-temperature water is introduced again to continue washing.

[0064] In the second vacuum belt filter 12, during the circulating washing process, when SO4 in the washing liquid... 2- After the concentration reaches the preset value, it is discharged into the second storage tank 13, and at the same time, 40°C warm water is introduced again to continue washing until sodium sulfate and nitrate are fully dissolved in the washing liquid; after sodium chloride crystallization unit 9 crystallizes out sodium chloride, the sodium chloride product obtained by drying has a salt purity that meets the secondary standard in industrial dry salt and the standard for refined dry salt for ion-exchange membrane caustic soda, and the remaining mother liquor is returned to CDNF unit 16.

[0065] In one possible embodiment, a buffer tank 15 is also included. The buffer tank 15 is disposed between the second pretreatment unit 14 and the CDNF unit 16. The buffer tank 15 is used to buffer the solution and ensure stable operation of the system.

[0066] See Figure 2 In one possible embodiment, the control strategy for the first vacuum belt filter 3 and the low-temperature circulating washing device is as follows:

[0067] When the start button is pressed, the filter enters the filtration state. Check the filter disc vacuum sensor. If the filter disc vacuum sensor is normal, start the filter disc and filter belt motor, set the filter belt motor speed, and open the slurry distribution valve. If the filter disc vacuum sensor is not normal, open the vacuum valve.

[0068] When the filter belt position sensor value reaches the set value, the washing state is entered and the washing liquid nozzle valve is opened. During the washing process, the chloride ion concentration sensor value is checked. When the maximum chloride ion concentration is reached, or the washing liquid level sensor value reaches the minimum washing liquid level, the drain valve is opened and the replenishment valve is closed. At the same time, the washing cycle count is incremented by 1.

[0069] When the washing cycle reaches the maximum number of washing liquid cycles, the machine enters the unloading state and starts the scraper. If the equipment fault sensor is true, an alarm will sound and the machine will enter the stop state. When the stop button is pressed, one processing cycle ends and the equipment stops.

[0070] In this embodiment, a low-temperature salt separation coupled with flat-plate nanofiltration process for resource recovery of mixed salts is also provided, including the following steps:

[0071] S1. Dissolution and preliminary separation of mixed salts: Mixed salt particles of 1-3mm are conveyed to the first salt dissolving pool 1 by the lifting device and 0℃ water prepared by the refrigeration device 2 is injected to dissolve the mixed salts into a solid-liquid mixture. The mixture is then pumped to the first vacuum belt filter 3. The material is separated into solid and liquid on the filter belt by gravity and vacuum suction. The liquid is sucked into the vacuum box, and the undissolved mixed salt particles remain on the filter belt to form a filter cake.

[0072] Specifically, water's ability to dissolve miscellaneous salts is utilized to transform solid miscellaneous salts into a solid-liquid mixture for subsequent separation operations. 0°C cold water not only dissolves the miscellaneous salts but also creates conditions for subsequent separation based on the differences in solubility of salts at low temperatures. The vacuum belt filter uses gravity to allow the material to initially settle, then uses vacuum suction to enhance the solid-liquid separation process, achieving preliminary separation. This design can quickly separate miscellaneous salts into a solid filter cake and a liquid filtrate, laying the foundation for subsequent precise separation of different salts. Furthermore, the low-temperature environment helps reduce the dissolution loss of certain salts during dissolution and filtration, improving overall separation efficiency. For example, compared to dissolution at room temperature, the amount of sodium sulfate and other salts that dissolve at low temperatures is relatively small, allowing more of the target salts to remain in the filter cake for subsequent targeted separation. At the same time, vacuum filtration is more efficient and has a better separation effect than ordinary filtration, effectively removing most of the liquid and obtaining a more compact filter cake for easier subsequent processing.

[0073] S2. The filter cake is washed with 0°C cold water produced by the refrigeration unit 2. The vacuum suction forces the cold water to penetrate the filter cake and filter belt. The soluble salts are dissolved in the washing liquid by utilizing the difference in solubility of salts at low temperatures, thus achieving preliminary salt separation. The washing liquid is circulated and discharged into the first storage tank 4 when Cl- is enriched to the preset concentration. At the same time, low-temperature water is reintroduced into the washing system to achieve the enrichment of sodium chloride solution. The filter cake moves with the filter belt to the unloading device and is transported to the second salt dissolving tank 10.

[0074] Specifically, based on the different solubility of various salts at low temperatures, 0°C cold water has good solubility for some salts, such as sodium chloride, but poor solubility for others. Through cyclic washing, sodium chloride continuously dissolves into the washing liquid, achieving initial separation from other salts. Simultaneously, the concentration of Cl- in the washing liquid is monitored to determine the enrichment level of sodium chloride. When a preset concentration is reached, the washing liquid is discharged and collected, while fresh low-temperature water is added to continue washing, continuously increasing the sodium chloride concentration. Thus, by utilizing the differences in salt solubility, efficient enrichment of sodium chloride is achieved, improving its purity. The cyclic washing method not only improves resource utilization and reduces sodium chloride loss but also reduces the difficulty and cost of subsequent purification. The sodium chloride-rich washing liquid is discharged into the first storage tank 4, providing high-quality raw materials for the subsequent preparation of high-purity sodium chloride products. Furthermore, the significantly reduced sodium chloride content in the washed filter cake facilitates the subsequent separation and purification of other salts in the second salt dissolving tank 10, reducing impurity interference and improving the overall system's processing efficiency.

[0075] S3, Sodium Chloride Purification: The filtrate after being treated by the first vacuum belt filter 3 enters the first pretreatment unit 5 to remove silicon and heavy metal impurities and reduce the solution hardness caused by calcium and magnesium ions. Then it flows sequentially through the electrocatalytic oxidation unit 6 and the activated carbon filter 7 to remove organic matter and improve water quality. After meeting the inlet water standard of the evaporation system, it enters the first evaporation water tank 8 and is then introduced into the sodium chloride crystallization unit 9 by a booster pump. Sodium chloride evaporates and crystallizes out, and after drying, sodium chloride product salt is obtained. The mother liquor is returned to the CDNF unit.

[0076] Specifically, the first pretreatment unit 5 removes silicon, heavy metal impurities, and reduces calcium and magnesium ion hardness through chemical reactions and physical adsorption. For example, ion exchange resin is used to exchange calcium and magnesium ions, removing silicon and heavy metal ions through precipitation. The electrocatalytic oxidation unit 6 utilizes the strong oxidizing substances generated by electrode reactions to oxidize and decompose organic matter into harmless substances. The activated carbon filter 7 relies on its porous structure and high specific surface area to adsorb residual organic matter, some incompletely removed ions, and impurities such as pigments. Evaporation crystallization utilizes the characteristic that the solubility of sodium chloride in water changes with temperature and concentration. The solution is concentrated by heating and evaporation. When the saturation concentration of sodium chloride is reached, crystallization conditions are controlled to allow sodium chloride to crystallize out. This effectively removes various impurities from the filtrate, yielding high-purity sodium chloride product salt that meets the secondary standard for industrial dry salt and the standard for refined dry salt for ion-exchange membrane caustic soda, greatly improving the economic value of sodium chloride. The mother liquor is returned to the CDNF unit, realizing resource recycling, reducing waste emissions, and improving the resource utilization rate and economic benefits of the entire system. At the same time, this purification process ensures that the water entering the evaporation system meets the requirements, reduces scaling and corrosion problems in the evaporator, extends the service life of the equipment, and reduces equipment maintenance costs.

[0077] S4. Salt Separation and Purification: The filter cake in the second salt dissolving tank 10 contains sodium sulfate, nitrate, and solid insoluble impurities. Water is heated to a set temperature using a heating device 11, and the warm water redissolves the filter cake into a solid-liquid mixture, which is then conveyed to the second vacuum belt filter 12. During the warm water circulation washing process, sodium sulfate and nitrate dissolve in the slurry, and solid impurities are filtered out by the second vacuum belt filter 12. When SO4... 2- Once the concentration reaches the preset level, it is discharged into the second storage tank 13, and the warm water is replaced for further washing. The filtrate enters the second pretreatment unit 14 for impurity removal and hardness reduction before flowing into the CDNF unit. In the CDNF unit, NO3... - SO4 enters the product water side 2- The concentrated water is retained at the concentrate end and enters the concentrate tank 19. It is then pumped into the sodium sulfate evaporation unit 20 to evaporate and produce sodium sulfate product salt. The product water passes through the second evaporation tank 17 and is concentrated by thermal method. It is then cooled and crystallized in the cooling crystallization unit 18 to obtain sodium nitrate and potassium nitrate products respectively.

[0078] Specifically, heating device 11 heats the water to a set temperature (e.g., 40°C), utilizing the principle that the solubility of salts increases with temperature, making sodium sulfate, nitrates, and other substances in the filter cake more easily soluble. The second vacuum belt filter 12 then performs solid-liquid separation again to remove solid impurities. SO4 levels in the washing liquid are monitored. 2-The concentration of sodium sulfate is controlled to regulate the discharge and replacement of the washing solution, thereby achieving sodium sulfate enrichment. The second pretreatment unit 14 further removes impurities and reduces hardness in the filtrate, ensuring the quality of the solution entering the CDNF unit. The CDNF unit utilizes a special membrane material with selective permeability to monovalent and divalent ions, allowing monovalent NO3- to pass through. - It can permeate through the membrane into the product water side, while divalent SO4 2- The ions are trapped, thus achieving ion separation. The concentrated water is used to prepare sodium sulfate product salt through evaporation and crystallization; the permeate is concentrated thermally, and the cooling crystallization temperature is controlled based on the difference in solubility of sodium nitrate and potassium nitrate at different temperatures, causing them to crystallize out separately. This achieves effective separation and purification of sodium sulfate, sodium nitrate, and potassium nitrate, yielding products that meet industrial standards and improving the resource utilization value of mixed salts. Temperature control and ion separation improve the accuracy of separation and product purity. The circulating washing and impurity removal process reduces the impact of impurities on product quality, improving product quality. The entire process fully utilizes the physicochemical properties of different salts, achieving continuous and efficient separation of multiple salts, improving the system's processing capacity and economic benefits, while reducing environmental impact.

[0079] In one possible embodiment, in the step of dissolving and initially separating the mixed salts, the water-salt ratio of the mixed salt particles to water at 0°C is 2.5:1; in the step of purifying sodium chloride, the first pretreatment unit 5 reduces the calcium and magnesium ion content in the filtrate to below 20 mg / L.

[0080] Specifically, the 2.5:1 water-to-salt ratio of mixed salt particles to 0℃ water is an optimized ratio derived from experiments and theoretical calculations. This ratio ensures that the mixed salts dissolve sufficiently to form a suitable solid-liquid mixture, facilitating subsequent filtration and separation operations, while maintaining good dissolution and separation performance at low temperatures. The calcium and magnesium ion content of the filtrate in the first pretreatment unit 5 is reduced to below 20 mg / L because calcium and magnesium ions may precipitate during subsequent evaporation and crystallization, clogging pipes, affecting equipment heat transfer efficiency, and reducing the purity of the sodium chloride product. Reducing their content to a certain level effectively avoids these problems.

[0081] In one possible embodiment, during the salt separation and purification step, SO4 in the washing liquid of the second vacuum belt filter 12 2- When the concentration reaches 160,000 mg / L, it is discharged into the second storage tank 13. The SO4 concentration in the washing solution is set. 2-The discharge point, reaching a cumulative concentration of 160,000 mg / L, is determined based on a comprehensive consideration of factors including the solubility of sodium sulfate in the washing solution, subsequent treatment process requirements, and equipment performance. When this concentration is reached, the washing solution's ability to dissolve sodium sulfate is nearly saturated, and continued washing does not significantly improve the enrichment effect of sodium sulfate. Discharging at this point and replacing the water with fresh warm water ensures the high efficiency and economy of the washing process.

[0082] In one possible embodiment, when preparing sodium nitrate and potassium nitrate products in the cooling crystallization unit 18, the product water after thermal concentration is first cooled to 30-40°C to crystallize sodium nitrate, and then the remaining solution is cooled to 0-10°C to crystallize potassium nitrate.

[0083] Specifically, the solubility of sodium nitrate and potassium nitrate differs significantly at different temperatures. At 30-40℃, the solubility of sodium nitrate decreases significantly with decreasing temperature, leading to preferential crystallization. However, at 0-10℃, the solubility of potassium nitrate decreases even more dramatically with decreasing temperature, resulting in crystallization. By precisely controlling the cooling temperature range, this solubility difference can be utilized to separate the two salts. This allows for efficient and precise separation of sodium nitrate and potassium nitrate, yielding high-purity products and increasing their added value and market competitiveness. It also avoids the mixed crystallization of the two salts, reducing subsequent purification steps, lowering production costs, and simultaneously improving the resource utilization rate and economic benefits of the entire mixed salt resource recovery system.

[0084] In one possible embodiment, 0°C water prepared by a refrigeration device 2 is injected into the first salt dissolving tank 1. A solid-liquid mixture containing 1-3mm impurity salt particles is dissolved at a water-to-salt ratio of 2.5:1 (see Table 1 for the composition of the impurity salt particles). This mixture is pumped into the first vacuum belt filter 3, where the liquid is drawn into a vacuum chamber under vacuum. Undissolved impurity salt particles remaining on the filter belt are washed by low-temperature circulation, dissolving soluble salts such as NaCl into the washing liquid. The liquid phase composition is 22% sodium chloride, 2.81% sodium sulfate, and 0.76% nitrate. The solution then enters the first pretreatment unit 5 to remove impurities such as silicon and heavy metals, and to reduce the calcium and magnesium ion content to below 20 mg / L. It then sequentially enters the electrocatalytic oxidation unit 6 and the activated carbon filter 7 to remove organic matter and bring the water quality up to the evaporation system inlet water standard. Finally, it enters the first evaporation tank 8 and is then introduced into the sodium chloride crystallization unit 9 by a booster pump. Sodium chloride evaporates and crystallizes, and after drying, sodium chloride product salt is obtained. The mother liquor is returned to the CDNF unit.

[0085] Table 1. Mixtures of heterosalts

[0086] content / % 46.3 27.1 23.9 1.15 0.47 0.48 0.2 0.03 0.07 0.03 0.27

[0087] In the second salt dissolving tank 10, the filter cake is heated to 40°C by a heating device 11, redissolved into a solid-liquid mixture with a solid content of approximately 40%, and then conveyed to the second vacuum belt filter 12. During the warm water circulation washing process, Na2SO4 and nitrates in the slurry dissolve, and solid impurities are filtered out by the second vacuum belt filter 12. When SO4... 2- When the concentration reaches 160,000 mg / L, it is discharged into the second storage tank 13, and fresh warm water is used for further washing. The filtrate enters the second pretreatment unit 14 for impurity removal and hardness reduction before flowing into the CDNF unit. 3- SO4 enters the product water side through the CDNF unit. 2- The concentrated water is then retained at the concentrate end, and the concentrate enters the concentrate tank 19. It is then pumped into the sodium sulfate evaporation unit 20 to produce sodium sulfate product salt. The product water passes through the second evaporation tank 17 and is concentrated by thermal method. Sodium nitrate and potassium nitrate products are obtained in the cooling crystallization unit 18, respectively. The final product salt indicators are shown in Table 2.

[0088] Table 2 shows the product salt index.

[0089]

[0090] The technical features of the above embodiments can be combined in any way. For the sake of brevity, not all possible combinations of the technical features in the above embodiments are described. However, as long as there is no contradiction in the combination of these technical features, they should be considered to be within the scope of this specification.

[0091] The embodiments described above are merely illustrative of several implementations of the present invention, and while the descriptions are relatively specific and detailed, they should not be construed as limiting the scope of the invention patent. It should be noted that those skilled in the art can make various modifications and improvements without departing from the concept of the present invention, and these all fall within the protection scope of the present invention. Therefore, the protection scope of this invention patent should be determined by the appended claims.

Claims

1. A low-temperature salt separation coupled with flat-plate nanofiltration system for resource recovery of mixed salts, characterized in that, It includes a first salt dissolving tank (1), a refrigeration device (2), a first vacuum belt filter (3), a first storage tank (4), a first pretreatment unit (5), an electrocatalytic oxidation unit (6), an activated carbon filter (7), a first evaporation tank (8), a sodium chloride crystallization unit (9), a second salt dissolving tank (10), a heating device (11), a second vacuum belt filter (12), a second storage tank (13), a second pretreatment unit (14), a CDNF unit (16), a second evaporation tank (17), a cooling crystallization unit (18), a concentrate tank (19), and a sodium sulfate evaporation unit (20). The first salt dissolving tank (1) is used to dissolve the raw material miscellaneous salt particles into a solid-liquid mixture; the refrigeration device (2) provides 0°C cold water to the first salt dissolving tank (1) for dissolving miscellaneous salt and provides washing liquid to the low-temperature circulating washing device of the first vacuum belt filter (3); the first vacuum belt filter (3) is used in conjunction with the low-temperature circulating washing device of the first vacuum belt filter (3) for the separation and enrichment of sodium chloride; The first storage tank (4) is used to temporarily store the filtrate after being processed by the first vacuum belt filter (3), and the first pretreatment unit (5) is used to remove silicon, fluorine and heavy metal impurities from the filtrate. The electrocatalytic oxidation unit (6) and the activated carbon filter (7) sequentially purify the filtrate after it has been treated by the first pretreatment unit (5); the first evaporation tank (8) evaporates the filtrate after it has been treated by the electrocatalytic oxidation unit (6) and the activated carbon filter (7); and the sodium chloride crystallization unit (9) is used to crystallize sodium chloride. The second salt dissolving tank (10) is used to redissolve the filter cake obtained by the first vacuum belt filter (3) into a solid-liquid mixture. The heating device (11) provides warm water at a set temperature to the second salt dissolving tank (10) to dissolve the filter cake. The second vacuum belt filter (12) is used to filter out solid impurities in the redissolved solid-liquid mixture; the second storage tank (13) temporarily stores the filtrate after being processed by the second vacuum belt filter (12); and the second pretreatment unit (14) is used to remove silicon and heavy metal impurities from the filtrate. The CDNF unit (16) utilizes the separation characteristics of divalent ions to enable... By entering the product water side, It is retained at the concentrate end; the concentrate tank (19) stores the concentrate separated by the CDNF unit (16) containing The sodium sulfate evaporation unit (20) evaporates the concentrated water in the concentrated water tank (19) to obtain sodium sulfate product salt; The second evaporation tank (17) thermally concentrates the water produced by the CDNF unit (16), and the cooling crystallization unit (18) performs cooling crystallization operation by controlling the temperature to prepare sodium nitrate and potassium nitrate products. During the filtration process of the first vacuum belt filter (3): The filter disc operates synchronously with the filter cloth under vacuum conditions, and in non-vacuum conditions, the filter disc and the filter cloth operate relative to each other. The material is evenly distributed on the surface of the filter belt through the slurry distributor. The filter cloth is used as the filter medium and is filtered under vacuum. Undissolved solid salt particles are trapped on the surface of the filter belt to form a filter cake. The low-temperature circulating washing device uses 0°C cold water as the washing liquid, which is evenly sprayed onto the surface of the filter cake through a nozzle. Under vacuum suction, the washing liquid penetrates the filter cake and filter belt, dissolving residual sodium chloride-soluble substances in the filter cake. The washing liquid adopts a circulating mode. Once the concentration reaches the preset value, it is discharged into the first storage tank (4), and low-temperature water is introduced again to continue washing. The control strategy for the first vacuum belt filter (3) and the low-temperature circulating washing device is as follows: When the start button is pressed, the filter enters the filtration state. Check the filter disc vacuum sensor. If the filter disc vacuum sensor is normal, start the filter disc and filter belt motor, set the filter belt motor speed, and open the slurry distribution valve. If the filter disc vacuum sensor is not normal, open the vacuum valve. When the filter belt position sensor value reaches the set value, the washing state is entered and the washing liquid nozzle valve is opened. During the washing process, check the chloride ion concentration sensor value. When the maximum chloride ion concentration is reached, or the washing liquid level sensor value reaches the minimum washing liquid level, open the drain valve and close the replenishment valve, and at the same time, increment the washing cycle number by 1. When the maximum number of washing cycles is reached, the machine enters the unloading state and starts the scraper. If the equipment fault sensor is active, an alarm will sound and the machine will stop. When the stop button is pressed, one processing cycle ends and the equipment stops. When preparing sodium nitrate and potassium nitrate products in the cooling crystallization unit (18), the product water after thermal concentration is first cooled to 30-40℃ to crystallize sodium nitrate, and then the remaining solution is cooled to 0-10℃ to crystallize potassium nitrate.

2. The low-temperature salt separation coupled flat-plate nanofiltration system for resource recovery of mixed salts according to claim 1, characterized in that, During the circulating washing process, when the washing liquid of the second vacuum belt filter (12) is in the washing liquid... Once the concentration reaches the preset value, it is discharged into the second storage tank (13), and at the same time, 40°C warm water is introduced again to continue washing until sodium sulfate and nitrate are fully dissolved in the washing liquid.

3. The low-temperature salt separation coupled flat-plate nanofiltration system for resource recovery of mixed salts according to claim 1, characterized in that, After sodium chloride crystallizes out in the sodium chloride crystallization unit (9), the sodium chloride product obtained by drying has a salt purity that meets the secondary standard for industrial dry salt and the standard for refined dry salt for ion-exchange membrane caustic soda. The remaining mother liquor is returned to the CDNF unit (16).

4. The low-temperature salt separation coupled flat-plate nanofiltration system for resource recovery of mixed salts according to claim 1, characterized in that, It also includes a buffer tank (15) disposed between the second pretreatment unit (14) and the CDNF unit (16), the buffer tank (15) being used to buffer the solution.

5. A low-temperature salt separation coupled with flat-plate nanofiltration process for resource recovery of mixed salts, employing the low-temperature salt separation coupled with flat-plate nanofiltration system as described in any one of claims 1 to 4, characterized in that, Includes the following steps; Salt dissolution and preliminary separation: 1-3 mm salt particles are transported to the first salt dissolution pool (1) by the lifting device and 0℃ water prepared by the refrigeration device (2) is injected to dissolve the salt into a solid-liquid mixture. The mixture is then pumped to the first vacuum belt filter (3). The material is separated into solid and liquid by gravity and vacuum suction on the filter belt. The liquid is sucked into the vacuum box and the undissolved salt particles remain on the filter belt to form a filter cake. The filter cake is washed with 0°C cold water produced by the refrigeration device (2). The vacuum suction forces the cold water through the filter cake and filter belt. By utilizing the difference in solubility of salts at low temperatures, soluble salts are dissolved in the washing liquid, achieving preliminary salt separation. The circulation of the washing liquid is controlled until... The solution is enriched to a preset concentration and discharged into the first storage tank (4). At the same time, low-temperature water is reintroduced into the washing system to achieve the enrichment of sodium chloride solution. The filter cake moves with the filter belt to the unloading device and is transported to the second salt dissolving tank (10). Sodium chloride purification: The filtrate after being treated by the first vacuum belt filter (3) enters the first pretreatment unit (5) to remove silicon and heavy metal impurities and reduce the solution hardness caused by calcium and magnesium ions. Then it flows through the electrocatalytic oxidation unit (6) and activated carbon filter (7) in sequence to remove organic matter and improve water quality. After meeting the inlet water standard of the evaporation system, it enters the first evaporation tank (8) and is then introduced into the sodium chloride crystallization unit (9) by the booster pump. Sodium chloride evaporates and crystallizes out, and after drying, sodium chloride product salt is obtained. The mother liquor is returned to the CDNF unit (16). Salt separation and purification: The filter cake in the second salt dissolving tank (10) contains sodium sulfate, nitrate, and solid insoluble impurities; the water is heated to a set temperature using a heating device (11), and the warm water redissolves the filter cake into a solid-liquid mixture, which is then transported to the second vacuum belt filter (12); during the warm water circulation washing process, sodium sulfate and nitrate in the slurry dissolve, and the solid impurities are filtered out by the second vacuum belt filter (12); when ⁻Accumulate to the preset concentration, discharge into the second storage tank (13), and replace with warm water to continue washing; the filtrate enters the second pretreatment unit (14) for impurity removal and hardness reduction, and then flows into the CDNF unit (16); in the CDNF unit (16), By entering the product water side, The concentrated water is retained at the concentrate end and enters the concentrate tank (19). It is then pumped into the sodium sulfate evaporation unit (20) to evaporate and produce sodium sulfate product salt. The product water passes through the second evaporation tank (17) and is concentrated by thermal method. It is then cooled and crystallized in the cooling crystallization unit (18) to obtain sodium nitrate and potassium nitrate products respectively.

6. The low-temperature salt separation coupled flat-plate nanofiltration process for resource recovery of mixed salts according to claim 5, characterized in that, In the steps of dissolving and initially separating the mixed salts, the water-salt ratio of the mixed salt particles to water at 0°C is 2.5:1; In the sodium chloride purification step, the first pretreatment unit (5) reduces the calcium and magnesium ion content in the filtrate to below 20 mg / L.

7. The low-temperature salt separation coupled flat-plate nanofiltration process for resource recovery of mixed salts according to claim 5, characterized in that, In the salt separation and purification step, the washing liquid of the second vacuum belt filter (12) When the concentration reaches 160,000 mg / L, it is discharged into the second storage tank (13).

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