System and process for recycling low-temperature salt separation coupled plate nanofiltration carnallite

Abstract

The invention discloses a low-temperature salt separation coupled flat plate nanofiltration carnallite recycling system and process, which adopts a low-temperature salt separation process to directly perform low-energy-consumption and high-efficiency separation and purification on solid carnallite, thereby simplifying the process flow. By integrating key processes such as re-dissolution, filtration, separation and concentration of the carnallite, intermediate links are reduced, and the equipment cost and the operation energy consumption are reduced. A low-temperature salt separation coupling flat plate nanofiltration technology is adopted, multi-element mixed salt such as sodium chloride, sodium sulfate, sodium nitrate and potassium nitrate is separated into a simple substance salt solution, the quality of recycled crystalline salt reaches the industrial grade standard, harmless and resourceful treatment of carnallite is achieved, the resource utilization rate is increased, and the economic value of a product is increased. The method is not limited by the quality of the impurity salt, and the impurity salt with different sources and different component proportions can be effectively treated.

Description

Technical Field

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

[0002] The crystalline salts produced by the traditional high-salt wastewater treatment process mainly include 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, they are temporarily classified as hazardous wastes. The cost of treating such hazardous wastes is high, and if they are not properly handled, they will also cause environmental pollution problems.

[0003] In the current field of miscellaneous salt resource utilization, evaporation crystallization and freezing crystallization are widely used for the separation of sodium sulfate and sodium chloride. These methods are based on the phase diagram of the water-salt system, and by means of the concentration difference and solubility characteristics of different inorganic salts in aqueous solution, the separation and purification of salts are achieved by accurately regulating the operating temperature, concentration multiple and other parameters during the crystallization process. For example, the efficient single substance salt separation recovery technology announced by CN105036222A adopts an evaporation-thermal crystallization integrated process, and in the temperature range of 50°C to 150°C, evaporation crystallization is used to realize the separation of sodium sulfate and sodium chloride crystals, and the selective precipitation of sodium nitrate is further realized by cooling crystallization technology (-15~0°C). However, in practical applications, when water quality conditions fluctuate frequently, the salt separation system in the thermal salt-nitrate co-production process is difficult to operate stably for a long time, and the stability of the quality of the crystallized salt product cannot be guaranteed, and there is also a problem of high energy consumption. Summary of the invention

[0004] In view of the deficiencies in the prior art, the present invention provides a low-temperature salt separation coupled flat plate nanofiltration mixed salt resource utilization system and process, which solves the problems of the traditional technology that it is difficult to operate stably for a long time, the stability of the quality of the crystalline salt product cannot be guaranteed, and there is also the problem of high energy consumption.

[0005] The technical solution of the present invention to solve the above technical problems is as follows: a low-temperature salt separation coupled flat plate nanofiltration miscellaneous salt resource 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 pool is used to dissolve the raw material impurity salt particles into a solid-liquid mixture; the refrigeration device provides 0°C cold water for the first salt dissolving pool to dissolve the impurity salt and provides washing liquid for the low-temperature circulation washing device of the first vacuum belt filter; the first vacuum belt filter is used to separate and enrich sodium chloride in combination with its low-temperature circulation washing device;

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

[0008] The electrocatalytic oxidation unit and the activated carbon filter sequentially purify the filtrate treated by the first pretreatment unit; the first evaporation water tank evaporates the filtrate treated 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 pool is used to dissolve the filter cake obtained by the first vacuum belt filter into a solid-liquid mixture, and the heating device provides warm water of a set temperature to the second salt dissolving pool to dissolve the filter cake;

[0010] The second vacuum belt filter is used to filter out solid impurities in the re-dissolved solid-liquid mixture; the second liquid storage tank temporarily stores the filtrate treated by the second vacuum belt filter, and the second pretreatment unit is used to remove silicon and heavy metal impurities in the filtrate;

[0011] The CDNF unit utilizes the separation characteristics of monovalent and divalent ions to make NO3 - Through entering the produced water side, SO4 2- The concentrated water tank stores the SO4 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 water 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 the low-temperature salt separation coupled flat plate nanofiltration impure salt resource recovery system, during the filtration process of the first vacuum belt filter:

[0014] The filter disc runs synchronously with the filter cloth under vacuum state, and runs relative to the filter cloth under non-vacuum state; the material is evenly distributed on the surface of the filter belt through the slurry distributor, and the filter cloth is used as the filter medium. The filter is filtered under the action of vacuum suction, and the undissolved solid salt particles are trapped on the surface of the filter belt to form a filter cake; the low-temperature circulation washing device uses 0℃ cold water as the washing liquid, which is evenly sprayed on the surface of the filter cake through the nozzle. Under the action of vacuum suction, the washing liquid penetrates the filter cake and the filter cloth to dissolve the residual sodium chloride soluble substances in the filter cake. The washing liquid adopts a circulation mode. When the Cl in the washing liquid - After the concentration reaches the preset value, it is discharged into the first liquid storage tank, and low-temperature water is reintroduced to continue washing.

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

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

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

[0018] As a preferred solution for the low-temperature salt separation coupled flat plate nanofiltration impure salt resource recovery system, the control strategy of the first vacuum belt filter and the low-temperature circulation washing device is:

[0019] When the start button is pressed, the filter state is entered, and the filter plate vacuum sensor is checked. If the filter plate vacuum sensor is normal, the filter plate and filter belt motor are started, the filter belt motor speed is set, and the slurry distribution valve is opened. If the filter plate vacuum sensor is abnormal, the vacuum valve is opened;

[0020] When the value of the filter belt position sensor reaches the set value, the washing state is entered and the valve of the washing liquid nozzle is opened; during the washing process, the value of the chloride ion concentration sensor is checked. When the maximum chloride ion concentration is reached, or the value of the washing liquid level sensor reaches the minimum washing liquid level, the drain valve is opened, the replenishing valve is closed, and the number of washing cycles is increased by 1;

[0021] When the number of washing cycles reaches the maximum number of washing liquids, it enters the unloading state and starts the scraper machine; if the equipment fault sensor is true, it alarms and enters the stop state; when the stop button is pressed, a processing cycle ends and the equipment stops.

[0022] The present invention also provides a low-temperature salt separation coupled with flat plate nanofiltration impure salt resource recovery process, comprising the following steps:

[0023] Dissolution and preliminary separation of foreign salts: The foreign salt particles of 1-3 mm are transported to the first salt dissolving pool by the lifting device, and 0°C water prepared by the refrigeration device is injected to dissolve the foreign salt into a solid-liquid mixture, which 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 foreign salt particles remain on the filter belt to form a filter cake.

[0024] The filter cake is washed with 0℃ cold water produced by the refrigeration device. The vacuum suction allows the cold water to penetrate the filter cake and the filter belt. The solubility difference of salts at low temperatures is used to dissolve the soluble salt in the washing liquid to achieve preliminary separation of the salt. The washing liquid circulation is controlled, and when Cl- is enriched to a preset concentration, it is discharged into the first liquid storage tank. At the same time, low-temperature water is reintroduced into the washing system to achieve the enrichment of the sodium chloride solution. The filter cake moves with the filter belt to the unloading device and is transported to the second salt dissolving tank.

[0025] Sodium chloride purification: The filtrate treated by the first vacuum belt filter enters the first pretreatment unit to remove silicon elements and heavy metal impurities, and reduce the solution hardness caused by calcium and magnesium ions. It then flows through the electrocatalytic oxidation unit and activated carbon filter in sequence to remove organic matter and improve water quality. After reaching the water inlet standard of the evaporation system, it enters the first evaporation water tank and is then introduced into the sodium chloride crystallization unit by the lifting pump. Sodium chloride is evaporated and crystallized, and the sodium chloride product salt is obtained after drying, and the mother liquor is refluxed to the CDNF unit;

[0026] Salt separation and purification: The filter cake in the second salt dissolving pool includes sodium sulfate, nitrate and solid insoluble impurities; the water is heated to the set temperature by a heating device, and the warm water dissolves the filter cake into a solid-liquid mixture and transports it to the second vacuum belt filter; during the warm water circulation washing process, the sodium sulfate and nitrate in the slurry are dissolved, and the solid impurities are filtered out by the second vacuum belt filter; when SO4 2- After the accumulated concentration reaches the preset concentration, it is discharged into the second liquid storage tank, and the warm water is replaced to continue washing; the filtrate enters the second pretreatment unit to remove impurities and reduce hardness, and then flows into the CDNF unit; in the CDNF unit, NO3 - Through entering the produced water side, SO4 2- The concentrated water is retained at the concentrate end and enters the concentrate tank, and is subsequently pumped into the sodium sulfate evaporation unit for evaporation to obtain sodium sulfate product salt. The produced water passes through the second evaporation water tank, is concentrated by thermal method, and is cooled and crystallized in the cooling crystallization unit to obtain sodium nitrate and potassium nitrate products respectively.

[0027] As the optimal solution for low-temperature salt separation coupled with flat plate nanofiltration impure salt resource recovery process, in the impure salt dissolution and preliminary separation steps, the water-salt ratio of impure salt particles to 0°C water is 2.5:1;

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

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

[0030] As a preferred solution for low-temperature salt separation coupled with flat-plate nanofiltration and salt resource utilization, when preparing sodium nitrate and potassium nitrate products in the cooling crystallization unit, the produced 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.

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

[0032] First, the low-temperature salt separation process is used to directly separate and purify solid impure salts with low energy consumption and high efficiency, simplifying the process flow. By integrating key processes such as re-dissolution, filtration, separation and concentration of impure salts, the intermediate links are reduced, and the equipment cost and operating energy consumption are reduced.

[0033] Second, compared with the traditional thermal process which is constrained by the co-saturated system of multiple salts, the low-temperature salt separation process separates a single salt solution with a single component, which can enrich the target salt to a near-saturated state, greatly improving the salt separation efficiency. Its circulating washing system can accurately control the amount of dissolved salt water, reduce the amount of water processed by the system, and further reduce the system's energy consumption and operating costs.

[0034] Third, by using low-temperature salt separation coupled with flat-plate nanofiltration technology, we successfully separated multi-component mixed salts such as sodium chloride, sodium sulfate, sodium nitrate, and potassium nitrate into single salt solutions, and the quality of the recovered crystalline salts met industrial-grade standards, achieving harmless and resource-based treatment of impure salts, improving resource utilization, and increasing the economic value of the products.

[0035] Fourth, the treatment effect of this technology is not limited by the quality of mixed salts. It can effectively treat mixed salts from different sources and with different composition ratios, showing excellent adaptability and stability. It provides new ideas and methods for the comprehensive utilization of mixed salt resources, and has broad application prospects and significant social significance. BRIEF DESCRIPTION OF THE DRAWINGS

[0036] In order to more clearly illustrate the implementation methods of the present invention or the technical solutions in the prior art, the drawings required for the implementation methods or the prior art descriptions are briefly introduced below. Obviously, the drawings in the following description are only exemplary, and for ordinary technicians in this field, other implementation drawings can be derived from the provided drawings without creative work.

[0037] The structures, proportions, sizes, etc. illustrated in this specification are only used to match the contents disclosed in the specification so as to facilitate understanding and reading by persons familiar with the technology. They are not used to limit the conditions under which the present invention can be implemented, and therefore have no substantial technical significance. Any structural modification, change in proportion or adjustment of size shall still fall within the scope of the technical contents disclosed in the present invention without affecting the effects and purposes that can be achieved by the present invention.

[0038] Figure 1 A schematic diagram of a low-temperature salt separation coupled flat plate nanofiltration and miscellaneous salt resource recovery system provided in an embodiment of the present invention;

[0039] Figure 2 A control strategy for a vacuum belt filter combined with a low-temperature circulating washing device provided in an embodiment of the present invention.

[0040] In the figure, 1, the first salt dissolving tank; 2, the refrigeration device; 3, the first vacuum belt filter; 4, the first liquid storage tank; 5, the first pretreatment unit; 6, the electrocatalytic oxidation unit; 7, the activated carbon filter; 8, the first evaporation water tank; 9, the sodium chloride crystallization unit; 10, the second salt dissolving tank; 11, the heating device; 12, the second vacuum belt filter; 13, the second liquid storage tank; 14, the second pretreatment unit; 15, the buffer water tank; 16, the CDNF unit; 17, the second evaporation water tank; 18, the cooling crystallization unit; 19, the concentrated water tank; 20, the sodium sulfate evaporation unit. DETAILED DESCRIPTION

[0041] In order to make the above-mentioned objects, features and advantages of the present invention more obvious and easy to understand, the specific embodiments of the present invention are described in detail below in conjunction with the accompanying drawings. In the following description, many specific details are set forth to facilitate a full understanding of the present invention. However, the present invention can be implemented in many other ways different from those described herein, and those skilled in the art can make similar improvements without violating the connotation of the present invention, so the present invention is not limited by the specific embodiments disclosed below.

[0042] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as those commonly understood by those skilled in the art of the present invention. The terms used herein in the specification of the present invention are only for the purpose of describing specific embodiments and are not intended to limit the present invention. The term "and / or" used herein includes any and all combinations of one or more related listed items.

[0043] See also Figure 1 The embodiment of the present invention provides a low-temperature salt separation coupled flat nanofiltration miscellaneous salt resource system, including a first salt dissolving tank 1, a refrigeration device 2, a first vacuum belt filter 3, a first liquid storage tank 4, a first pretreatment unit 5, an electrocatalytic oxidation unit 6, an activated carbon filter 7, a first evaporation water 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 liquid storage tank 13, a second pretreatment unit 14, a CDNF unit 16, a second evaporation water tank 17, a cooling crystallization unit 18, a concentrated water tank 19 and a sodium sulfate evaporation unit 20;

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

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

[0046] The electrocatalytic oxidation unit 6 and the activated carbon filter 7 sequentially purify the filtrate after being treated by the first pretreatment unit 5; the first evaporation water tank 8 evaporates the filtrate after being 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 pool 10 is used to dissolve the filter cake obtained by the first vacuum belt filter 3 into a solid-liquid mixture, and the heating device 11 provides warm water of a set temperature to the second salt dissolving pool 10 to dissolve the filter cake;

[0048] The second vacuum belt filter 12 is used to filter out solid impurities in the re-dissolved solid-liquid mixture; the second liquid storage tank 13 temporarily stores the filtrate treated by the second vacuum belt filter 12, and the second pretreatment unit 14 is used to remove silicon and heavy metal impurities in the filtrate;

[0049] Among them, CDNF unit 16 uses the separation characteristics of monovalent and divalent ions to make NO3 -Through entering the produced water side, SO4 2- The concentrated water tank 19 stores the SO4 separated by the CDNF unit 16. 2- The concentrated water in the concentrated water tank 19 is evaporated by the sodium sulfate evaporation unit 20 to obtain sodium sulfate product salt;

[0050] The second evaporation water tank 17 performs thermal concentration on 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.

[0051] In this embodiment, the function of the first salt dissolving pool 1 is to dissolve the raw material salt particles into a solid-liquid mixture. The principle is to use the dissolving effect of water on the salt to disperse the various components in the salt in the water to form a material state that is convenient for subsequent processing. This step provides the initial processing object for the entire system to ensure that the subsequent separation operation can proceed smoothly. On the one hand, the refrigeration device 2 provides 0°C cold water for the first salt dissolving pool 1 for dissolving the salt, and on the other hand, it supplies washing liquid for the low-temperature circulating washing device of the first vacuum belt filter 3. Under the low temperature environment of 0°C, the solubility difference of different components in the salt will be more obvious, which is conducive to the subsequent separation of sodium chloride. At the same time, the low-temperature washing liquid can more effectively dissolve the sodium chloride in the filter cake, while dissolving less other salts, thereby realizing the efficient enrichment of sodium chloride. This technical means of utilizing the difference in solubility of salts at low temperatures greatly improves the accuracy and efficiency of separation compared to the traditional normal temperature treatment method.

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

[0053] Specifically, the first liquid storage tank 4 is used to temporarily store the filtrate processed by the first vacuum belt filter 3. It plays a buffering role, making the operation of the entire system more stable and avoiding system failures caused by mismatched filtrate processing speeds. The first pretreatment unit 5 is responsible for removing silicon, fluorine and heavy metal impurities in the filtrate. The 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 silicon and fluorine may be removed through precipitation reactions. This step effectively reduces the impurity content in the filtrate, provides purer raw materials for subsequent processing, reduces the interference of impurities on subsequent processes, and improves the quality of the final product.

[0054] Specifically, the electrocatalytic oxidation unit 6 and the activated carbon filter 7 sequentially purify the filtrate after being treated 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 the organic matter and other impurities in the filtrate, thereby converting them into harmless substances. The activated carbon filter 7 utilizes the porous structure and strong adsorption capacity of the activated carbon to further adsorb the residual organic matter, pigments, and some ions and other impurities that have not been completely removed. After being treated by these two units, the water quality of the filtrate has been greatly improved, meeting the strict requirements of the subsequent evaporation system for the inlet water quality, ensuring the smooth progress of the evaporation process, reducing the occurrence of problems such as scaling and corrosion of the evaporator, and extending the service life of the equipment.

[0055] Specifically, the first evaporation water tank 8 performs an evaporation operation on the purified filtrate, and then the sodium chloride crystallization unit 9 crystallizes the sodium chloride. In the first evaporation water tank 8, the water in the filtrate is gradually evaporated by heating, and the solution concentration is continuously increased. When the saturated concentration of sodium chloride is reached, in the sodium chloride crystallization unit 9, the sodium chloride crystallization is promoted by controlling the temperature, stirring speed and other conditions. This evaporation and crystallization process is based on the characteristic that the solubility of sodium chloride in water changes with temperature and concentration, and realizes the conversion from solution to solid sodium chloride product. The salt purity of the obtained sodium chloride product meets the secondary standard of industrial dry salt and the refined dry salt standard for ion membrane caustic soda, which improves the economic value of sodium chloride.

[0056] Specifically, the second salt dissolving pool 10 is used to dissolve the filter cake produced by the first vacuum belt filter 3 into a solid-liquid mixture, and the heating device 11 provides warm water of a set temperature (such as 40°C) to the second salt dissolving pool 10 to promote the dissolution of the filter cake. As the temperature rises, the solubility of the remaining salts (such as sodium sulfate, nitrates, etc.) in the filter cake in warm water increases, so that it can be more fully dissolved in water, creating conditions for subsequent separation operations. Compared with cold water dissolution, warm water dissolution can speed up the dissolution rate and improve the processing efficiency.

[0057] Specifically, the second vacuum belt filter 12 is used to filter out solid impurities in the re-dissolved solid-liquid mixture. It plays a filtering role again to ensure that the solution entering the subsequent processing link is purer and reduce the impact of solid impurities on subsequent processes.

[0058] Specifically, the second liquid 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 in the filtrate. This is similar to the function of the first pretreatment unit 5, but according to the characteristics of the filtrate after dissolution in the second salt dissolving pool 10, impurities are removed again to further improve the purity of the solution, preparing for subsequent more accurate ion separation.

[0059] Specifically, the CDNF unit uses the separation characteristics of monovalent and divalent ions to make NO3 - Passing into the produced water side, while SO4 2- The membrane material of the CDNF unit has special pore size and surface charge characteristics, and has selective permeability to ions of different valence states. - , can smoothly pass through the membrane into the water production side, while the divalent SO4 2- The membrane is retained, thus achieving NO3 - With SO4 2- This ion separation technology has the advantages of high efficiency, energy saving, and no phase change. Compared with traditional separation technologies such as chemical precipitation, it can separate ions more accurately, reduce the use of chemical reagents, reduce production costs, and improve separation effects and product quality.

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

[0061] Specifically, the second evaporation water tank 17 performs thermal concentration on the produced water of the CDNF unit, and the cooling crystallization unit 18 performs cooling crystallization operation by controlling the temperature to prepare sodium nitrate and potassium nitrate products. In the second evaporation water tank 17, part of the water in the produced water is removed by heating evaporation to increase the concentration of sodium nitrate and potassium nitrate. Then in the cooling crystallization unit 18, according to the difference in solubility of sodium nitrate and potassium nitrate at different temperatures, the produced water after thermal concentration is first cooled to 30-40°C, at which time the solubility of sodium nitrate is reduced, and crystals are precipitated first; then the remaining solution is cooled to 0-10°C, and the solubility of potassium nitrate is further reduced, so that crystals are precipitated. This cooling crystallization method with precise temperature control can effectively separate sodium nitrate and potassium nitrate to obtain high-purity products, increase the added value of the products, and realize the comprehensive resource utilization of multiple components in miscellaneous salts.

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

[0063] The filter disc runs synchronously with the filter cloth under vacuum state, and runs relative to the filter cloth under non-vacuum state; the material is evenly distributed on the surface of the filter belt through the slurry distributor, and the filter cloth is used as the filter medium. The filter is filtered under the action of vacuum suction, and the undissolved solid salt particles are trapped on the surface of the filter belt to form a filter cake; the low-temperature circulation washing device uses 0℃ cold water as the washing liquid, which is evenly sprayed on the surface of the filter cake through the nozzle. Under the action of vacuum suction, the washing liquid penetrates the filter cake and the filter cloth to dissolve the residual sodium chloride soluble substances in the filter cake. The washing liquid adopts a circulation mode. When the Cl - After the concentration reaches the preset value, it is discharged into the first liquid storage tank 4, and low-temperature water is reintroduced to continue washing.

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

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

[0066] See also Figure 2 In a possible embodiment, the control strategy of the first vacuum belt filter 3 and the low-temperature circulation washing device is:

[0067] When the start button is pressed, the filter state is entered, and the filter plate vacuum sensor is checked. If the filter plate vacuum sensor is normal, the filter plate and filter belt motor are started, the filter belt motor speed is set, and the slurry distribution valve is opened. If the filter plate vacuum sensor is abnormal, the vacuum valve is opened;

[0068] When the value of the filter belt position sensor reaches the set value, the washing state is entered and the valve of the washing liquid nozzle is opened; during the washing process, the value of the chloride ion concentration sensor is checked. When the maximum chloride ion concentration is reached, or the value of the washing liquid level sensor reaches the minimum washing liquid level, the drain valve is opened, the replenishing valve is closed, and the number of washing cycles is increased by 1;

[0069] When the number of washing cycles reaches the maximum number of washing liquids, it enters the unloading state and starts the scraper machine; if the equipment fault sensor is true, it alarms and enters the stop state; when the stop button is pressed, a processing cycle ends and the equipment stops.

[0070] In this embodiment, a low-temperature salt separation coupled with flat plate nanofiltration and impure salt resource recovery process is also provided, comprising the following steps:

[0071] S1, dissolution and preliminary separation of foreign salts: 1-3 mm foreign salt particles are transported to the first salt dissolving pool 1 by the lifting device, and 0°C water prepared by the refrigeration device 2 is injected to dissolve the foreign salt into a solid-liquid mixture, which 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, and the liquid is sucked into the vacuum box. The undissolved foreign salt particles remain on the filter belt to form a filter cake;

[0072] Specifically, the solubility of water for impurity salts is used to convert solid impurity salts into a solid-liquid mixed state for subsequent separation operations. On the one hand, cold water at 0°C can dissolve impurity salts, and on the other hand, it creates conditions for subsequent separation using the solubility differences of salts at low temperatures. The vacuum belt filter uses gravity to initially settle the material, and then uses vacuum suction to strengthen the solid-liquid separation process to achieve initial separation of solids and liquids. This design can quickly separate impurity salts into solid filter cakes and liquid filtrate, laying the foundation for subsequent precise separation of different salts. Moreover, the low temperature environment helps to reduce the dissolution loss of certain salts during the dissolution and filtration process, and improve the overall separation efficiency. For example, compared with dissolution at room temperature, the amount of salts such as sodium sulfate dissolved at low temperatures is relatively small, which can keep more target salts in the filter cake, which is convenient for subsequent targeted separation. At the same time, the vacuum filtration method is more efficient and has better separation effect than ordinary filtration. It can effectively remove most of the liquid and obtain a relatively compact filter cake, which is convenient for subsequent processing.

[0073] S2, using 0°C cold water produced by the refrigeration device 2 to wash the filter cake, vacuum suction allows the cold water to penetrate the filter cake and the filter belt, and using the difference in solubility of salts at low temperatures to dissolve soluble salts in the washing liquid to achieve preliminary salt separation; controlling the circulation of the washing liquid, and when Cl- is enriched to a preset concentration, discharge it into the first liquid storage tank 4, and at the same time reintroduce low-temperature water into the washing system to achieve the enrichment of the sodium chloride solution; the filter cake moves with the filter belt to the unloading device and is transported to the second salt dissolving pool 10;

[0074] Specifically, based on the different solubility characteristics of different salts at low temperatures, cold water at 0°C has good solubility for some salts such as sodium chloride, but poor solubility for other salts. Through cyclic washing, sodium chloride is continuously dissolved in the washing liquid to achieve preliminary separation from other salts. At the same time, the enrichment degree of sodium chloride is judged by monitoring the concentration of Cl- in the washing liquid. When the preset concentration is reached, the washing liquid is discharged and collected, and new low-temperature water is added to continue washing to continuously increase the concentration of sodium chloride. Thus, the difference in solubility of salts is utilized to achieve efficient enrichment of sodium chloride and improve the purity of sodium chloride. The cyclic washing method not only improves resource utilization, reduces the loss of sodium chloride, but also reduces the difficulty and cost of subsequent purification. The washing liquid rich in sodium chloride is discharged into the first liquid storage tank 4, providing high-quality raw materials for the subsequent preparation of high-purity sodium chloride products. In addition, the sodium chloride content in the washed filter cake is greatly reduced, which is conducive to the subsequent separation and purification of other salts in the second salt dissolving pool 10, reduces impurity interference, and improves the treatment effect of the entire system.

[0075] S3, sodium chloride purification: the filtrate treated by the first vacuum belt filter 3 enters the first pretreatment unit 5 to remove silicon elements and heavy metal impurities, and reduce the solution hardness caused by calcium and magnesium ions, and then flows through the electrocatalytic oxidation unit 6 and the activated carbon filter 7 in sequence to remove organic matter and improve water quality. After reaching the water inlet standard of the evaporation system, it enters the first evaporation water tank 8, and then is introduced into the sodium chloride crystallization unit 9 by the lifting pump; sodium chloride is evaporated and crystallized, and sodium chloride product salt is obtained after drying, and the mother liquor is refluxed to the CDNF unit;

[0076] Specifically, the first pretreatment unit 5 removes silicon elements, heavy metal impurities and reduces the hardness of calcium and magnesium ions by chemical reactions and physical adsorption. For example, ion exchange resin is used to exchange with calcium and magnesium ions, and silicon elements and heavy metal ions are removed by precipitation reaction. The electrocatalytic oxidation unit 6 uses the strong oxidizing substances generated by the electrode reaction 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 impurities such as residual organic matter, some ions that are not completely removed, and pigments. Evaporative crystallization utilizes the characteristics that the solubility of sodium chloride in water changes with temperature and concentration. The solution is concentrated by heating and evaporation. When the saturated concentration of sodium chloride is reached, the crystallization conditions are controlled to crystallize sodium chloride. Thereby, various impurities in the filtrate are effectively removed, and a high-purity sodium chloride product salt is obtained, which meets the secondary standard of industrial dry salt and the refined dry salt standard for ion membrane caustic soda, and greatly improves the economic value of sodium chloride. The mother liquor is refluxed to the CDNF unit, realizing the recycling of resources, reducing waste emissions, and improving the resource utilization and economic benefits of the entire system. At the same time, this purification process ensures that the water quality entering the evaporation system meets the requirements, reduces scaling and corrosion problems of 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 pool 10 includes sodium sulfate, nitrate and solid insoluble impurities; the water is heated to a set temperature by a heating device 11, and the warm water dissolves the filter cake into a solid-liquid mixture and transports it to the second vacuum belt filter 12; during the warm water circulation washing process, the sodium sulfate and nitrate in the slurry are dissolved, and the solid impurities are filtered out by the second vacuum belt filter 12; when SO4 2- The accumulated concentration is discharged into the second liquid storage tank 13, and the warm water is replaced to continue washing; the filtrate enters the second pretreatment unit 14 to remove impurities and reduce hardness, and then flows into the CDNF unit; in the CDNF unit, NO3 - Through entering the produced water side, SO4 2- The concentrated water is retained at the concentrated water end and enters the concentrated water tank 19, and is subsequently pumped into the sodium sulfate evaporation unit 20 for evaporation to obtain sodium sulfate product salt. The produced water passes through the second evaporation water tank 17, is concentrated by thermal method, and is cooled and crystallized in the cooling crystallization unit 18 to obtain sodium nitrate and potassium nitrate products respectively.

[0078] Specifically, the heating device 11 heats the water to a set temperature (e.g., 40°C) and makes the sodium sulfate, nitrate, etc. in the filter cake more soluble by using the principle that the solubility of salts increases with increasing temperature. The second vacuum belt filter 12 performs solid-liquid separation again to remove solid impurities. 2-The concentration of the washing liquid is controlled, and the discharge and replacement of the washing liquid are controlled to achieve the enrichment of sodium sulfate. The second pretreatment unit 14 further removes impurities in the filtrate and reduces the hardness to ensure the quality of the solution entering the CDNF unit. The CDNF unit uses a special membrane material that has selective permeability to monovalent and divalent ions, monovalent NO3 - It can pass through the membrane into the water production side, while the divalent SO4 2- is intercepted, thus achieving ion separation. For the concentrated water obtained, sodium sulfate product salt is prepared by evaporation and crystallization; the produced water is concentrated by thermal method, and then the cooling crystallization temperature is controlled according to the difference in solubility of sodium nitrate and potassium nitrate at different temperatures, so that they are crystallized separately. Thus, the effective separation and purification of sodium sulfate, sodium nitrate and potassium nitrate are achieved, and products that meet industrial standards are obtained, which improves the resource utilization value of impurities. Temperature control and ion separation improve the separation accuracy and product purity. The cyclic washing and impurity removal process reduces the impact of impurities on product quality and improves product quality. The entire process makes full use of the physical and chemical properties of different salts, realizes the continuous and efficient separation of multiple salts, improves the system's processing capacity and economic benefits, and reduces the impact on the environment.

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

[0080] Specifically, the salt particles are dissolved in 0°C water at a water-salt ratio of 2.5:1, which is an optimized ratio based on experiments and theoretical calculations. At this ratio, it can not only ensure that the salts are fully dissolved to form a suitable solid-liquid mixture, which is convenient for subsequent filtration and separation operations, but also maintain a good dissolution and separation effect in a low temperature environment. 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 form precipitation during the subsequent evaporation and crystallization process, blocking the pipeline, affecting the heat transfer efficiency of the equipment, and reducing the purity of the sodium chloride product. These problems can be effectively avoided by reducing its content to a certain extent.

[0081] In a possible embodiment, in the salt separation and purification step, SO4 in the washing liquid of the second vacuum belt filter 12 2- When the accumulated amount reaches 160000 mg / L, it is discharged into the second liquid storage tank 13. Set the SO4 content in the washing liquid to 2-The discharge when the accumulated concentration reaches 160,000 mg / L is determined based on the solubility of sodium sulfate in the washing liquid, the requirements of the subsequent treatment process, and the performance of the equipment. When this concentration is reached, the washing liquid's solubility for sodium sulfate is close to saturation, and continued washing does not significantly improve the enrichment effect of sodium sulfate. At this time, discharge and replacement of new warm water can ensure the efficiency and economy of the washing process.

[0082] In a possible embodiment, when preparing sodium nitrate and potassium nitrate products in the cooling crystallization unit 18, the produced 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, there are significant differences in the solubility of sodium nitrate and potassium nitrate at different temperatures. At 30-40°C, the solubility of sodium nitrate decreases significantly as the temperature decreases, and it will crystallize first; at 0-10°C, the solubility of potassium nitrate decreases more significantly as the temperature decreases, and potassium nitrate crystallizes at this time. By precisely controlling the cooling temperature range, this solubility difference is used to separate the two salts. In this way, sodium nitrate and potassium nitrate can be separated efficiently and accurately to obtain high-purity sodium nitrate and potassium nitrate products, which increases the added value and market competitiveness of the products. The mixed crystallization of the two salts is avoided, the subsequent purification steps are reduced, the production cost is reduced, and the resource utilization rate and economic benefits of the entire miscellaneous salt resource system are improved.

[0084] In a possible embodiment, 0°C water prepared by a refrigeration device 2 is injected into the first salt dissolving pool 1, and a solid-liquid mixture of 1-3mm salt particles is dissolved at a water-salt ratio of 2.5:1. The salt particle components are shown in Table 1, and pumped into the first vacuum belt filter 3, and the liquid is sucked into the vacuum box under the action of vacuum. After the remaining undissolved salt particles on the filter belt are washed by low-temperature circulation, soluble salts such as NaCl are dissolved into the washing liquid, and its liquid phase composition is 22% sodium chloride, 2.81% sodium sulfate, and 0.76% nitrate. Enter the first pretreatment unit 5 to remove impurities such as silicon elements and heavy metals, and reduce the content of calcium and magnesium ions to less than 20 mg / L. Then enter the electrocatalytic oxidation unit 6 and the activated carbon filter 7 in turn to remove organic matter so that the water quality meets the water inlet standard of the evaporation system. Enter the first evaporation water tank 8, and then introduce the sodium chloride crystallization unit 9 by the lifting pump. Sodium chloride is evaporated and crystallized, and sodium chloride quality product salt is obtained after drying, and the mother liquor is refluxed to the CDNF unit.

[0085] Table 1 Miscellaneous salt components

[0086] <![CDATA[SO4 2- ]]> <![CDATA[Cl - ]]> <![CDATA[Na + ]]> <![CDATA[NO3 - ]]> <![CDATA[K + ]]> <![CDATA[Ca 2+ ]]> <![CDATA[Mg 2+ ]]> Si F heavy metal Solid impurities content / % 46.3 27.1 23.9 1.15 0.47 0.48 0.2 0.03 0.07 0.03 0.27

[0087] The filter cake is heated to 40°C in the second salt dissolving pool 10 by means of a heating device 11, and is redissolved into a solid-liquid mixture with a solid content of about 40%, and then transported to the second vacuum belt filter 12. During the warm water circulation washing process, Na2SO4 and nitrate in the slurry are dissolved, and solid impurities are filtered out by the second vacuum belt filter 12. 2- When the accumulated concentration reaches 160,000 mg / L, it is discharged into the second liquid storage tank 13 and replaced with new warm water for further washing. The filtrate enters the second pretreatment unit 14 to remove impurities and reduce hardness, and then flows into the CDNF unit. 3- Entering the water production side through the CDNF unit, SO4 2- The concentrated water is retained at the concentrated water end, and the concentrated water enters the concentrated water tank 19, and is subsequently pumped into the sodium sulfate evaporation unit 20 for evaporation to produce sodium sulfate product salt. The produced water passes through the second evaporation water tank 17, and is concentrated by thermal method to obtain sodium nitrate and potassium nitrate products in the cooling crystallization unit 18. The final product salt index is shown in Table 2.

[0088] Table 2 Product salt index

[0089]

[0090] The technical features of the above-described embodiments may be arbitrarily combined. To make the description concise, not all possible combinations of the technical features in the above-described 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 above-mentioned embodiments only express several implementation methods of the present invention, and the description thereof is relatively specific and detailed, but it cannot be understood as limiting the scope of the invention patent. It should be pointed out that for ordinary technicians in this field, several modifications and improvements can be made without departing from the concept of the present invention, which all belong to the protection scope of the present invention. Therefore, the protection scope of the patent of the present invention shall be based on the attached claims.

Claims

1. A low-temperature salt separation coupled flat plate nanofiltration mixed salt resource system, characterized in that: It comprises a first salt dissolving tank (1), a refrigeration device (2), a first vacuum belt filter (3), a first liquid storage tank (4), a first pretreatment unit (5), an electrocatalytic oxidation unit (6), an activated carbon filter (7), a first evaporation water 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 liquid storage tank (13), a second pretreatment unit (14), a CDNF unit (16), a second evaporation water tank (17), a cooling crystallization unit (18), a concentrated water tank (19) and a sodium sulfate evaporation unit (20); The first salt dissolving pool (1) is used to dissolve raw material impure salt particles into a solid-liquid mixture; the refrigeration device (2) provides cold water at 0°C for the first salt dissolving pool (1) to dissolve impure salt and provides washing liquid for the low-temperature circulation washing device of the first vacuum belt filter (3); the first vacuum belt filter (3) is used to separate and enrich sodium chloride in combination with the low-temperature circulation washing device of the first vacuum belt filter (3); The first liquid storage tank (4) is used to temporarily store the filtrate processed by the first vacuum belt filter (3), and the first pretreatment unit (5) is used to remove silicon, fluorine and heavy metal impurities in the filtrate; The electrocatalytic oxidation unit (6) and the activated carbon filter (7) sequentially purify the filtrate treated by the first pretreatment unit (5); the first evaporation water tank (8) evaporates the filtrate treated by the electrocatalytic oxidation unit (6) and the activated carbon filter (7); the sodium chloride crystallization unit (9) is used to crystallize sodium chloride; The second salt dissolving pool (10) is used to dissolve the filter cake obtained by the first vacuum belt filter (3) into a solid-liquid mixture, and the heating device (11) provides warm water of a set temperature to the second salt dissolving pool (10) to dissolve the filter cake; The second vacuum belt filter (12) is used to filter out solid impurities in the re-dissolved solid-liquid mixture; the second liquid storage tank (13) temporarily stores the filtrate treated by the second vacuum belt filter (12); the second pretreatment unit (14) is used to remove silicon and heavy metal impurities in the filtrate; The CDNF unit (16) utilizes the separation characteristics of monovalent and divalent ions to make NO3 - Through entering the produced water side, SO4 2- The concentrated water tank (19) stores the SO4 separated by the 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; The second evaporation water tank (17) performs thermal concentration on 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.

2. The low-temperature salt separation coupled flat plate nanofiltration mixed salt resource recovery system according to claim 1 is characterized in that: During the filtration process of the first vacuum belt filter (3): The filter disc and the filter cloth run synchronously under vacuum state, and under non-vacuum state, the filter disc and the filter cloth run relative to each other; the material is evenly distributed on the surface of the filter belt through the slurry distributor, and the filter cloth is used as the filter medium. The material is filtered under the action of vacuum suction, and the 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 a washing liquid, which is evenly sprayed on the surface of the filter cake through a nozzle. Under the action of vacuum suction, the washing liquid penetrates the filter cake and the filter belt to dissolve the residual sodium chloride soluble substances in the filter cake. The washing liquid adopts a circulating mode. When the Cl- concentration in the washing liquid reaches a preset value, it is discharged into the first liquid storage tank (4), and low-temperature water is reintroduced to continue washing.

3. The low-temperature salt separation coupled flat plate nanofiltration mixed salt resource recovery system according to claim 1 is characterized in that: During the cycle washing process of the second vacuum belt filter (12), when SO4 2- After the concentration reaches the preset value, it is discharged into the second liquid storage tank (13), and 40°C warm water is reintroduced to continue washing until the sodium sulfate and nitrate are fully dissolved in the washing liquid.

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

5. The low-temperature salt separation coupled flat plate nanofiltration mixed salt resource recovery system according to claim 1 is characterized in that: The system further comprises a buffer water tank (15), wherein the buffer water tank (15) is arranged between the second pretreatment unit (14) and the CDNF unit (16), and the buffer water tank (15) is used for buffering the solution.

6. The low-temperature salt separation coupled flat plate nanofiltration mixed salt resource recovery system according to claim 1 is characterized in that: The control strategy of the first vacuum belt filter (3) and the low-temperature circulation washing device is: When the start button is pressed, the filter state is entered, and the filter plate vacuum sensor is checked. If the filter plate vacuum sensor is normal, the filter plate and filter belt motor are started, the filter belt motor speed is set, and the slurry distribution valve is opened. If the filter plate vacuum sensor is abnormal, the vacuum valve is opened; When it is checked that the value of the filter belt position sensor reaches the set value, it enters the washing state and opens the washing liquid nozzle valve; During the washing process, check the value of the chloride ion concentration sensor. When the maximum chloride ion concentration is reached, or the value of the detergent level sensor reaches the minimum detergent level, open the drain valve, close the replenishing valve, and increase the number of washing cycles by 1. When the number of washing cycles reaches the maximum number of washing liquid, it enters the unloading state and starts the scraper machine; If the equipment fault sensor is true, an alarm will be sounded and the system will enter the stop state; when the stop button is pressed, a processing cycle will end and the equipment will stop.

7. A low-temperature salt separation coupled with flat plate nanofiltration and salt recycling process, characterized in that: The steps include: Dissolution and preliminary separation of foreign salts: The foreign salt particles with a size of 1-3 mm are transported to the first salt dissolving pool (1) by the lifting device, and 0°C water prepared by the refrigeration device (2) is injected to dissolve the foreign salts into a solid-liquid mixture, which 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, and the liquid is sucked into the vacuum box. The undissolved foreign salt particles remain on the filter belt to form a filter cake. The filter cake is washed with cold water at 0°C produced by the refrigeration device (2). The cold water is made to penetrate the filter cake and the filter belt by vacuum suction. The soluble salt is dissolved in the washing liquid by utilizing the difference in solubility of salts at low temperature, thereby achieving preliminary separation of the salt. The washing liquid circulation is controlled, and when Cl- is enriched to a preset concentration, it is discharged into the first liquid storage tank (4). At the same time, low-temperature water is reintroduced into the washing system to achieve enrichment of the 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 treated by the first vacuum belt filter (3) enters the first pretreatment unit (5) to remove silicon elements and heavy metal impurities and reduce the solution hardness caused by calcium and magnesium ions. It then flows through the electrocatalytic oxidation unit (6) and the activated carbon filter (7) in sequence to remove organic matter and improve water quality. After reaching the water inlet 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 lifting pump. The sodium chloride is evaporated and crystallized, and the sodium chloride product salt is obtained after drying. The mother liquor is refluxed to the CDNF unit (16); Salt separation and purification: The filter cake in the second salt dissolving pool (10) includes sodium sulfate, nitrate and solid insoluble impurities; the water is heated to a set temperature by a heating device (11), and the warm water dissolves 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, the sodium sulfate and nitrate in the slurry are dissolved, and the solid impurities are filtered out by the second vacuum belt filter (12); when SO4 2- After the accumulated concentration reaches a preset concentration, it is discharged into the second liquid storage tank (13), and the warm water is replaced to continue washing; the filtrate enters the second pretreatment unit (14) to remove impurities and reduce hardness, and then flows into the CDNF unit (16); in the CDNF unit (16), NO3 - Through entering the produced water side, SO4 2- The concentrated water is retained at the concentrated water end and enters the concentrated water tank (19), and is subsequently pumped into the sodium sulfate evaporation unit (20) for evaporation to obtain sodium sulfate product salt. The produced water passes through the second evaporation water tank (17), is concentrated by thermal method, and is cooled and crystallized in the cooling crystallization unit (18) to obtain sodium nitrate and potassium nitrate products respectively.

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

9. The low-temperature salt separation coupled flat plate nanofiltration mixed salt resource recovery process according to claim 6, characterized in that: In the salt separation and purification step, the SO4 2- When the accumulated concentration reaches 160,000 mg / L, it is discharged into the second liquid storage tank (13).

10. The low-temperature salt separation coupled flat plate nanofiltration mixed salt resource recovery process according to claim 6, characterized in that: When preparing sodium nitrate and potassium nitrate products in the cooling crystallization unit (18), the produced 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.

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