Circulating freezing-hot method salt separation treatment process and device for highly-mineralized mine water

Abstract

The invention discloses a circulating freezing-hot method salt separation treatment process and device for hypersalinity mine water, and belongs to the technical field of hypersalinity mine water treatment and recovery. According to the process, an improved sodium sulfate decahydrate freezing crystallization device is adopted, the bottom of a tank body is of a W-shaped structure, crystal suspension and circulation are facilitated, a filter screen is arranged at the upper part of the tank body and used for filtering out fine crystals, freezing crystallization is performed at about-5 DEG C, and obtained sodium sulfate decahydrate is subjected to hot melting, thermal crystallization and centrifugal drying to prepare high-purity anhydrous sodium sulfate. And the two mother solutions are combined and enter a carnallite crystallizer, and based on carnallite rate judgment (when the concentration is greater than 5%), the temperature and the pH value are regulated and controlled, and potassium, magnesium and calcium salts are sequentially recovered. According to the process, efficient recovery of sodium sulfate and sodium chloride (the recovery rates are greater than or equal to 98% and 99% respectively, and the particle sizes reach 0.4 mm and 0.5 mm) is realized, carnallite recycling is completed, and the mine water recovery efficiency is remarkably improved.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of high salinity mine water treatment and recovery, in particular to a circulating freezing-thermal method salt separation treatment process and device for high salinity mine water. BACKGROUND

[0002] At present, domestic research mainly focuses on membrane separation technology (double membrane method) or evaporation crystallization technology for treating high salinity wastewater. Among them, the double membrane method refers to the combination of ultrafiltration and reverse osmosis to remove suspended solids and large molecular organic matter by ultrafiltration, and further desalination by reverse osmosis. Although the double membrane method can significantly reduce the salinity of water, the salt or organic matter in the reverse osmosis concentrate is enriched, which causes subsequent disposal difficulties, and the salt in the concentrate is complex, with low resource utilization value; at the same time, membrane pollution is easy to occur during the treatment process, resulting in frequent chemical cleaning, high operation and maintenance cost, and the inability to realize salt separation and recovery. The evaporation crystallization technology refers to the concentration of wastewater through multi-stage evaporation, and finally the salt is crystallized and separated. Although the evaporation crystallization technology can avoid the problem of salt enrichment, it also has limitations such as poor quality of crystallized salt, which cannot meet the industrial salt standard; the crystallization process is easily disturbed by high-concentration organic matter (COD>1000mg / L); and high energy consumption. Therefore, these two traditional high salinity mine water treatment technologies can achieve partial desalination, but have problems such as disposal difficulty of concentrate, high energy consumption, and low salt resource utilization rate. SUMMARY

[0003] The purpose of the present application is to provide a circulating freezing-thermal method salt separation treatment process and device for high salinity mine water, which selectively separates single salts such as sodium sulfate and sodium chloride by precisely controlling the crystallization conditions (temperature, pH, additives), and realizes salt resource recovery and wastewater "near-zero discharge" by coupling the freezing-thermal method circulation process, thereby breaking through the limitations of traditional high salinity mine water treatment technology.

[0004] To achieve the above purpose, the present application provides a circulating freezing-thermal method salt separation treatment process for high salinity mine water, comprising the following steps: (1) pretreating the high salinity mine water concentrate to reduce its COD and TDS content; sending the pretreated concentrate into an improved sodium sulfate decahydrate freezing crystallization device for freezing crystallization, and then centrifugally separating to obtain sodium sulfate decahydrate; (2) introducing the sodium sulfate decahydrate obtained in step (1) into a molten salt device for thermal melting to form a sodium sulfate saturated solution, and then performing thermal crystallization to obtain anhydrous sodium sulfate salt by centrifugal drying; (3) evaporating and crystallizing the mother liquor containing sodium chloride discharged after centrifugal separation in step (1) to obtain sodium chloride by centrifugal drying; (4) combining the mother liquor produced in steps (2) and (3), and passing it into a miscellaneous salt crystallizer, and determining the subsequent flow direction according to the calculated miscellaneous salt rate: When the impurity salt content is ≤5%, the solution is refluxed to the freeze crystallization step for reprocessing; When the impurity salt content is >5%, potassium salt, magnesium salt and calcium salt are sequentially precipitated and recovered by adjusting the temperature and pH value.

[0005] Preferably, in step (1), the pretreatment includes ozone oxidation, flocculation sedimentation, filtration and two-stage reverse osmosis, and the COD content of the effluent after pretreatment is less than 100 mg / L and the TDS content is 2500~3000 mg / L.

[0006] Preferably, in step (1), the operating temperature of the modified sodium sulfate decahydrate cryogenic crystallizer is -5°C, ethylene glycol is used as the refrigerant, and centrifugal separation is performed when the solid-liquid ratio of the crystal slurry in the crystallizer reaches 20%~25%.

[0007] Preferably, in step (2), the hot melting is carried out under stirring conditions of 200 r / min, the crystal growth time is 90 min, and the hot crystallization temperature is 32.4℃.

[0008] Preferably, the specific operation of step (3) is as follows: under stirring conditions of 200r / min for 60min, sodium chloride concentrate is obtained, which is then introduced into a sodium chloride evaporator crystallizer. The evaporation and crystallization temperature is 162℃, and sodium chloride is precipitated when the concentration is reduced to 40%.

[0009] Preferably, in step (4), the conditions for recovering potassium salt, magnesium salt and calcium salt are: the temperature is controlled at 50°C, and the salt is precipitated sequentially by adjusting the pH value, wherein the pH is 7~8 when recovering potassium salt, the pH is 8~9 when recovering magnesium salt, and the pH is 10.5 when recovering calcium salt.

[0010] Preferably, in step (4), when the impurity salt rate is ≤5%, steps (1) to (4) are repeated until the impurity salt rate is >5%. Potassium chloride and sodium citrate are used as additives for evaporation and crystallization. Then, potassium salt, magnesium salt and calcium salt are recovered in sequence at a temperature of 50°C and pH values ​​of 7~8, 8~9 and 10.5, respectively.

[0011] Preferably, in step (4), the impurity salt rate is determined by a sensor inside the impurity salt crystallizer, and the formula for calculating the impurity salt rate is as follows: ; Wherein, C (impurities) represents the concentration of dissolved solids that are identified as impurities after the concentrate has been evaporated and concentrated, and TDS represents the total dissolved solids concentration in the concentrate.

[0012] The present invention also provides an improved sodium sulfate decahydrate cryo-crystallization apparatus for implementing the above-mentioned processing technology, comprising: The crystallization tank has a W-shaped bottom structure. A freezing converter, which is installed around the inner wall of the crystallization tank, is used to freeze the liquid material and lower its temperature; A guide tube, coaxially disposed inside the crystallization tank, is used to guide the liquid material to form a circulating flow path; A temperature sensor, which is installed on the side wall of the guide tube, is used to determine whether the liquid material after freezing treatment is below -5℃; A stirring assembly, wherein the stirring paddle extends into the guide tube to drive the movement of the liquid material; The clarification zone is a diameter-enlarged section located at the upper part of the crystallization tank. A fine crystal filter assembly, disposed within the clarification zone, is used to separate and discharge fine crystals; The feed inlet, located at the top of the crystallization tank, is used to introduce new concentrate to mix with the circulating mother liquor inside the tank. The discharge port, located at the bottom of the guide tube, is used to discharge sodium sulfate decahydrate obtained from the freeze crystallization process.

[0013] Preferably, the fine-crystal filter assembly includes: A fine-grained filter screen is horizontally positioned above the clarification zone; A fine crystal outlet is provided on the side wall or top of the clarification zone and is located above or connected to the fine crystal filter screen, for discharging the trapped fine crystals and impurities.

[0014] Therefore, the circulating freeze-thermal desalination process and apparatus for high-salinity mine water provided by the present invention have the following beneficial effects: (1) In the field of "zero discharge" of industrial wastewater, thermal salt separation technology can effectively separate Na2SO4, NaCl and K. + Mg 2+ Ca 2+ Resource-based separation of inorganic salts effectively solves problems such as pollution from mixed salts and high energy consumption.

[0015] (2) The cryogenic separation technology, through the combination of low-temperature phase change and solubility difference, is conducive to achieving wastewater reduction, salt resource utilization, and process greening. The multi-dimensional innovation framework of cryogenic separation technology will help to better integrate with other technologies in the future and promote the sustainable development of high-salinity wastewater treatment.

[0016] (3) In the salt separation crystallization process, both cryogenic and thermal salt separation can not only solve the problem of separating mixed salts, but also transform the treatment of high-salt wastewater from a "cost center" to a "resource-benefit center". The salt separation crystallization process, through the innovative combination of thermal and cryogenic processes, can effectively overcome the limitations of traditional methods, improve desalination efficiency, enhance resource utilization, and reduce energy consumption, providing methodological experience for the treatment of high-salt mine water. The salt separation crystallization technology has demonstrated great application value in the treatment of high-salt mine water. In the future, with the continuous development of technology, the treatment technology can be better optimized, thereby promoting the green treatment and sustainable development of high-salt mine water.

[0017] The technical solution of the present invention will be further described in detail below with reference to the accompanying drawings and embodiments. Attached Figure Description

[0018] Figure 1 This is a flowchart of the cyclic freeze-thermal crystallization process of the present invention; Figure 2 This is a structural diagram of a modified sodium sulfate decahydrate cryo-crystallization apparatus; Figure Labels 1. Crystallization tank; 2. Flow guide tube; 3. Refrigeration heat exchanger; 4. Temperature sensor; 5. Agitator; 6. Agitator blades; 7. Clarification zone; 81. Fine crystal filter screen; 82. Fine crystal outlet; 9. Feed inlet; 10. Discharge outlet. Detailed Implementation

[0019] This invention provides a method based on Figure 2 The improved sodium sulfate decahydrate cryogenic crystallization unit is used for the cyclic cryogenic-thermal separation of highly mineralized mine water concentrate containing potassium, magnesium, and calcium salts, with dominant ions being sulfate, chloride, and sodium ions. The process route is as follows: Figure 1 As shown, by combining multiple technologies such as freeze-melting, high-temperature evaporation crystallization, and reflux of mixed salt solids, along with a modified novel sodium sulfate decahydrate freeze-crystallization device, the goal of recovering salts from high-mineralization mine water concentrate is ultimately achieved.

[0020] The improved sodium sulfate decahydrate cryogenic crystallization apparatus mainly includes a crystallization tank 1, a flow guide tube 2, a stirring device, and a fine crystal treatment system. The crystallization tank 1 features a W-shaped bottom design, which facilitates crystal suspension and the circulating mixing of the liquid. A flow guide tube 2 is coaxially arranged inside the tank, forming the main flow channel internally and an annular flow channel between its exterior and the tank wall. Except for the top cover, the crystallization tank 1 is entirely insulated, and four cryogenic heat exchangers 3 are installed around the tank. New feed liquid enters the crystallization tank 1 through the feed inlet 9 and mixes with the circulating mother liquor before undergoing circulating cryogenic treatment. Temperature sensors 4 on the side wall of the flow guide tube 2 detect the temperature changes. The insulation material ensures that the entire circulating cryogenic process is maintained at a low temperature, with heat exchange occurring only at the top of the tank. Once the liquid reaches the specified cryogenic temperature, it enters the flow guide tube 2 and flows downwards under the impeller 5, then flows upwards along the outside of the flow guide tube, forming a circulation. The stirring blades 6 of the stirring device are located inside the guide tube 2, driving the liquid to flow downwards after entering from the top of the guide tube 2, and then turning from the bottom to flow upwards along the annular channel, forming an internal circulation within the tank. The upper part of the crystallization tank 1 has an enlarged clarification zone 7, whose starting position is higher than the upper inlet of the guide tube 2, reducing the upward flow velocity and making it difficult for large crystal particles to enter the guide tube 2, thus reducing collisions with the stirring blades 5. A fine crystal filter assembly is installed in the clarification zone 7 to separate and discharge fine crystals. The fine crystal filter assembly includes a fine crystal filter screen 81 and a fine crystal outlet 82. A fine crystal filter screen 81 with a pore size of 0.2 mm is horizontally installed in the upper layer of the clarification zone 7 to trap fine crystals; the fine crystal outlet 82 is located on the upper side wall of the clarification zone 7 to discharge the trapped fine crystals to an external fine crystal removal system, where they are mixed and dissolved with the raw material liquid and then returned. A discharge port 10 is provided at the bottom of the guide tube 2 to discharge sodium sulfate decahydrate crystals precipitated during freezing. The structure of the novel crystallization device fully considers the influence of secondary nucleation in continuous crystallizers. By optimizing the structure of traditional guide tube baffle crystallizers, the occurrence of secondary nucleation is reduced, thereby increasing the crystal growth rate. Simultaneously, a fine crystal discharge structure is introduced, which improves the average particle size of the crystals. Fine crystal recovery employs a mixing method with the raw material, which both pre-cools the raw material and eliminates excess crystal nuclei by dissolving the fine crystals, thus promoting crystal growth.

[0021] The circulating freezing-thermal desalination process for high-mineralization mine water specifically includes the following steps: (1) The pre-concentrated wastewater solution enters the improved freeze crystallization device, and after freeze crystallization, it is centrifuged to obtain sodium sulfate decahydrate.

[0022] (2) The sodium sulfate decahydrate obtained in step (1) is introduced into the salt molten salt vessel, and the water of crystallization is released by steam heating. After forming a saturated sodium sulfate solution with a stirring speed of 200 r / min and a crystal growth time of 90 min, it enters the sodium sulfate crystallizer and the temperature is set at 32.4℃. Finally, anhydrous sodium sulfate salt is obtained by centrifugation and drying.

[0023] (3) The mother liquor rich in sodium chloride discharged after the freezing crystallization and centrifugation operation in step (1) is treated with a stirring speed of 200 r / min and a crystal growth time of 60 min to obtain sodium chloride concentrate. The concentrate is then introduced into a sodium chloride evaporator crystallizer and the temperature is increased to 162℃. When the concentrate is concentrated to 40%, the sodium chloride saturation precipitation point is reached. When the solid-liquid ratio of the crystal slurry reaches 20%~25%, the product is obtained by centrifugation and drying.

[0024] (4) The mother liquor from the sodium chloride evaporator crystallizer in step (3) and the mother liquor from the sodium sulfate crystallizer in step (2) are fed together into the impurity salt crystallizer. The impurity salt rate is determined by the sensor inside the crystallizer. The formula for calculating the impurity salt rate is as follows: ; Wherein, C (impurities) represents the concentration of dissolved solids that are identified as impurities after the concentrate has been evaporated and concentrated, and TDS represents the total dissolved solids concentration in the concentrate.

[0025] A portion of the mixed salt solution meeting the requirement of a mixed salt ratio greater than 5% will be recovered by sequentially precipitating various salt solids under specified conditions such as temperature and pH adjustment. When the temperature is adjusted to 50℃ and the pH to 7-8, potassium salts will precipitate first for recovery; when the temperature is adjusted to 50℃ and the pH to 8-9, magnesium salts will precipitate for recovery; and when the temperature is adjusted to 50℃ and the pH to 10.5, calcium salts will precipitate last for recovery. Simultaneously, the recovered waste liquid, after adding an appropriate amount of hydrochloric acid solution to adjust the pH, will be reintroduced into the salt melter and returned to the inlet of the freeze crystallization unit, completing the feed liquid circulation process. The remaining mixed salt solution, not meeting the requirement of a mixed salt ratio greater than 5%, will be directly introduced into the salt melter and returned to the inlet of the freeze crystallization unit via a reflux pipe, completing the feed liquid circulation process.

[0026] (5) When the impurity salt rate is ≤5%, repeat steps (1) to (4) until the impurity salt rate is greater than 5%. The impurity salt solution is evaporated and crystallized at 50°C with potassium chloride and sodium citrate as additives. The impurity salt solution is recovered in sequence at 50°C with pH values ​​of 7-8, 8-9 and 10.5.

[0027] (6) After completing all the material circulation and recovering the five salts of sodium sulfate, sodium chloride, potassium chloride, magnesium sulfate and calcium carbonate in sequence, ensure that the recovery rate of the five salts meets the following requirements: sodium sulfate 98%, sodium chloride 99%, potassium chloride 86%, magnesium sulfate 83%, calcium carbonate 92%, and the impurity salt rate is less than 5%. Then, the remaining circulating liquid in the system is evaporated at a high temperature of 200°C through a sodium chloride evaporator crystallizer to obtain 0.3% impurity salt, the main component of which is sodium citrate, which can be used as an additive in the next mine water recycling cycle.

[0028] The technical solution of the present invention will be further described below with reference to the accompanying drawings and embodiments. It should be understood that these embodiments are for illustrative purposes only and are not intended to limit the scope of the invention. Any changes, modifications, substitutions, combinations, or simplifications made without departing from the spirit and principle of the present invention should be considered equivalent substitutions and are included within the protection scope of the present invention. Furthermore, it should be understood that after reading the contents of this invention, those skilled in the art can make various alterations or modifications to the invention, and these equivalent forms also fall within the scope defined by the appended claims and are all within the protection scope of the present invention.

[0029] In this document, the term "embodiment" means that a specific feature, structure, or characteristic described in connection with an embodiment may be included in at least one embodiment of this application. The term "embodiment" appearing in various places throughout the specification does not necessarily refer to the same embodiment, nor does it specifically limit its independence or connection with other embodiments. In principle, in this application, as long as there are no technical contradictions or conflicts, the technical features mentioned in each embodiment can be combined in any way to form corresponding implementable technical solutions.

[0030] Unless otherwise defined, the technical terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this application pertains; the use of related terms herein is merely for the purpose of describing particular embodiments and is not intended to limit this application.

[0031] Unless otherwise specified, the reagents, instruments, and equipment used in this invention are all commonly used by those skilled in the art, and the testing standards all use national or international standards commonly used in the field, without further explanation.

[0032] Example A high-mineralization mine water concentrate was prepared, with a TDS content of 6000 mg / L, a sulfate / chloride ion ratio of 3.3, sodium chloride and sodium sulfate accounting for 72% of the total salt content, and a COD content of 2350 mg / L. The concentrate underwent pretreatment processes including ozone oxidation, flocculation sedimentation filtration, and two-stage reverse osmosis. An external sensor at the outlet was used to ensure that the COD content was controlled at 85 mg / L and the TDS content at 2550 mg / L. The total dissolved sodium, chloride, and sulfate ions in the high-mineralization mine water account for over 90% of the total dissolved content, with sodium sulfate and sodium chloride being the dominant salts. High-temperature evaporation crystallization can be used for fractional salt extraction. A high-efficiency vertical tube falling film evaporator was employed, with high-efficiency heat transfer treatment on both the heat exchange and crystallization surfaces. The heat exchange tubes adopted a special tube structure. The evaporator included both sodium chloride and sodium sulfate evaporators.

[0033] Inject the pretreated concentrate Figure 2 In the improved sodium sulfate decahydrate cryogenic crystallization apparatus, the flow direction of the fluid in the guide tube 2 is first changed to downward. The newly fed liquid mixes with the circulating mother liquor and enters the heat exchanger. After heat exchange, the specified freezing temperature is reached, resulting in supersaturation. The fluid then directly enters the guide tube 2 from the top and flows downward under the impeller 5, subsequently flowing upward along the outside of the guide tube 2, forming a circulation. The tank bottom adopts a W-bottom design, which is beneficial for crystal suspension and liquid circulation. The uniform stirring of the agitator keeps the crystal grains in suspension, causing them to continuously grow. The design of the guide tube 2 and the tank diameter, as well as the selection of the impeller speed, are all aimed at ensuring good suspension of the crystals in the straight section of the tank. Ethylene glycol is selected as the refrigerant, the process temperature is controlled at -5℃, and the liquid circulation concentration is controlled to not exceed 1g / L. When sodium sulfate decahydrate continuously precipitates in the crystallization apparatus and the solid-liquid ratio of the crystal slurry reaches 25%, sodium sulfate decahydrate is separated by a centrifugal separator.

[0034] The improved novel sodium sulfate decahydrate cryo-crystallization apparatus fully considers factors that reduce secondary nucleation during the crystallization process and promote crystal growth. Inside the crystallization apparatus, crystal particles move at different flow rates depending on their size. Smaller crystal particles move synchronously with the circulating liquid, completing a full cycle and continuously growing. For larger crystals, the upward flow velocity of the liquid outside the guide tube 2 is only sufficient to suspend them. Due to the addition of a diameter-enlarging section at the top of the crystallization apparatus, the upward flow velocity is reduced, making it difficult for larger crystal particles to cross the upper end of the guide tube 2 and enter its interior, thus reducing the chance of large particles colliding with the agitator 5. Collisions between crystals and the agitator 5 are the main cause of secondary nucleation during crystallization. According to the empirical formula for secondary nucleation, the nucleation rate is proportional to the energy transfer rate to the crystal during collision; for larger crystals, the energy transferred during collision is greater. Therefore, reducing the probability of larger particles colliding with the agitator 5 can reduce the nucleation rate. The new structure reduces the breakage of large particles, maintaining a better crystal shape, while the reduction of secondary nucleation promotes crystal growth throughout the entire crystallization apparatus. The upper section of the crystallization device is the clarification zone 7, with a 0.2mm fine crystal filter 81 installed in the upper layer. Fine crystal particles smaller than 0.2mm will be suspended in the filter layer and output to the treatment liquid to participate in the circulation. The fine crystal filter component 8 can ensure the crystal quality of the frozen crystallization product sodium sulfate decahydrate, and also ensure that the number of particles entrained in the overflow circulating liquid is small and the particle size is very small. This can reduce the secondary nucleation when the circulating liquid comes into contact with the impeller of the circulating pump. At the same time, an axial flow pump is selected for the circulating pump, and the impeller speed is reduced as much as possible while ensuring the flow rate and head. These measures all ensure that the circulating liquid has as few opportunities for secondary nucleation as possible during the circulation process.

[0035] The improved novel sodium sulfate decahydrate cryogenic crystallization apparatus features a specially designed fine crystal discharge outlet, thereby increasing the average particle size of the sodium sulfate decahydrate crystals. Crystals in the crystallizer are formed from crystal nuclei. Within a given crystal slurry volume, the fewer crystal nuclei generated, the larger the product crystals will grow. Conversely, if too many crystal nuclei are generated, the solute in the solution will deposit on the surfaces of too many nuclei, resulting in a very small product crystal particle size. In actual production, the nucleation process is difficult to control, and the number of crystal nuclei is often excessive. Therefore, it is necessary to remove the excess crystal nuclei to increase the average particle size. In the clarification zone 7 at the top of the crystallization apparatus, the crystal slurry flows upward at a low speed, allowing crystals larger than a certain "fine crystal cutting size" to settle out of the solution and return to the main body of the crystallization apparatus to re-participate in the internal circulation and grow. Fine crystals smaller than this size will enter the fine crystal elimination circulation system from the fine crystal discharge outlet 82 located in the clarification zone 7. The fine crystal elimination method involves mixing this portion of the fine crystal solution with the uncooled raw material. This lowers the temperature of the raw material, achieving precooling, and dissolves the fine crystals, eliminating the overall amount of crystal nuclei within the crystallization device. Simultaneously, this portion of fine crystal solute returns to the raw material for reuse. Ultimately, this improves the particle size and quality of sodium sulfate decahydrate crystals and ensures that the sulfate ion content of the separated mother liquor fully meets the standards. The novel crystallization device structure considers the influence of secondary nucleation in continuous crystallizers and introduces a fine crystal discharge structure, achieving an average sodium sulfate decahydrate particle size of 0.4 mm and increasing the precipitation rate by 13.25%.

[0036] Sodium sulfate decahydrate was introduced into a molten salt vessel equipped with a stirrer and heated with steam to release the water of crystallization. A saturated sodium sulfate solution was formed by stirring at 200 rpm for 90 minutes. This solution was then introduced into a sodium sulfate evaporator crystallizer, where the temperature was adjusted to 32.4℃. When the concentration reached approximately 30%, anhydrous sodium sulfate precipitated. After centrifugation and drying, anhydrous sodium sulfate product was obtained with a crystal particle size of 0.4 mm, a recovery rate of 98%, and a purity of 99%.

[0037] The concentrated mother liquor from the freeze crystallization process is heated in a preheater and then treated with a stirring speed of 200 r / min and a crystal growth time of 60 min to obtain a concentrated sodium chloride solution. This solution is then fed into a sodium chloride evaporator crystallizer and concentrated to 40%, at which point the sodium chloride reaches its saturation precipitation point. The temperature is then adjusted to 162℃, and the solid-liquid ratio of the crystal slurry reaches 25%. After centrifugation and washing with a saturated salt solution, the solution is dried to obtain the sodium chloride salt product with a crystal particle size of 0.5 mm, a recovery rate of 99%, and a purity of 99%.

[0038] In addition, the concentrated mother liquor from the sodium chloride and sodium sulfate evaporators is combined and enters the impurity salt crystallizer. The impurity salt rate is determined by sensors inside the crystallizer, and the formula for calculating the impurity salt rate is as follows: ; Where C (impurities) represents the concentration of dissolved solids identified as impurities after evaporation and concentration of the concentrate, and TDS represents the total dissolved solids concentration in the concentrate. The salt content of the evaporation concentrate mother liquor, also known as saline wastewater, is calculated to determine whether further solids recovery is necessary. Current evaporation and concentration technologies produce wastewater with a salt content of 3-20%. When the concentration of salt ions is too low, it is difficult to remove them efficiently and completely using chemical precipitation. Slower reaction kinetics, difficult crystal growth, and low solid-liquid separation efficiency may lead to substandard purity of the recovered product, forming "inferior impurities." Therefore, concentrates with a salt content of less than 5% are recirculated directly into the salt melter and then returned to the inlet of the cryogenic crystallization unit via a reflux pipe, completing the feed-liquid circulation process. Impurity solutions with a salt content greater than 5% are recovered by sequentially precipitating various salt solids under specified conditions such as temperature and pH adjustment. When the temperature is adjusted to 50℃ and the pH to 7.7, potassium chloride is first precipitated and recovered, with a recovery rate of 86% and a purity of 93.7%. When the temperature is adjusted to 50℃ and the pH to 8.1, magnesium sulfate is then precipitated and recovered, with a recovery rate of 83% and a purity of 97.1%. When the temperature is adjusted to 50℃ and the pH to 10.5, calcium carbonate is finally precipitated and recovered, with a recovery rate of 92% and a purity of 97.5%. After completing all the feed liquid circulation and sequentially recovering the five salts—sodium sulfate, sodium chloride, potassium chloride, magnesium sulfate, and calcium carbonate—ensuring that the recovery rates of the five salts meet the following requirements—sodium sulfate 98%, sodium chloride 99%, potassium chloride 86%, magnesium sulfate 83%, and calcium carbonate 92%, and the impurity salt rate is below 5%—the remaining circulating liquid in the system is evaporated at 200℃ using a sodium chloride evaporator crystallizer to obtain 0.3% impurity salt, mainly sodium citrate, which can be used as an additive in the next mine water recovery cycle.

[0039] In summary, the cyclic freeze-thermal crystallization process of this invention reduces the enrichment effect of impurity ions and organic matter in wastewater, which is conducive to the stable and long-term operation of the entire fractional crystallization system, and the quality of the recovered product salt is stable and controllable. Ultimately, it achieves effective fractional recovery of salt from saline wastewater, realizes the goal of resource utilization, and achieves near-zero wastewater discharge.

[0040] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and not to limit them. Although the present invention has been described in detail with reference to preferred embodiments, those skilled in the art should understand that modifications or equivalent substitutions can still be made to the technical solutions of the present invention, and these modifications or equivalent substitutions cannot cause the modified technical solutions to deviate from the spirit and scope of the technical solutions of the present invention.

Claims

1. A circulating freeze-thermal desalination process for high-mineralization mine water, characterized in that, Includes the following steps: (1) Pre-treat the high-mineralization mine water concentrate to reduce its COD and TDS content; send the pre-treated concentrate into a modified sodium sulfate decahydrate freeze crystallization device for freeze crystallization, and then centrifuge to obtain sodium sulfate decahydrate. (2) The sodium sulfate decahydrate obtained in step (1) is introduced into a salt melting vessel for hot melting to form a saturated sodium sulfate solution. Then, it is subjected to hot crystallization and centrifugation to dry anhydrous sodium sulfate. (3) Evaporate and crystallize the mother liquor containing sodium chloride that was discharged after centrifugation in step (1), and then centrifuge and dry it to obtain sodium chloride; (4) Combine the mother liquors produced in steps (2) and (3) and pass them into the mixed salt crystallizer. Determine the subsequent flow direction based on the calculated mixed salt ratio: When the impurity salt content is ≤5%, the solution is refluxed to the freeze crystallization step for reprocessing; When the impurity salt content is >5%, potassium salt, magnesium salt and calcium salt are sequentially precipitated and recovered by adjusting the temperature and pH value.

2. The circulating freeze-heat desalination process for high-salinity mine water according to claim 1, characterized in that: In step (1), the pretreatment includes ozone oxidation, flocculation sedimentation, filtration and two-stage reverse osmosis. After pretreatment, the COD content of the effluent is less than 100 mg / L and the TDS content is 2500~3000 mg / L.

3. The circulating freeze-heat desalination process for high-salinity mine water according to claim 1, characterized in that: In step (1), the operating temperature of the modified sodium sulfate decahydrate cryogenic crystallizer is -5℃, ethylene glycol is used as the refrigerant, and centrifugal separation is performed when the solid-liquid ratio of the crystal slurry in the crystallizer reaches 20%~25%.

4. The circulating freeze-heat desalination process for high-salinity mine water according to claim 1, characterized in that: In step (2), the hot melting is carried out under stirring at 200 r / min, the crystal growth time is 90 min, and the hot crystallization temperature is 32.4℃.

5. The circulating freeze-heat desalination process for high-salinity mine water according to claim 1, characterized in that: The specific operation of step (3) is as follows: under the stirring condition of 200r / min, crystallize for 60min to obtain sodium chloride concentrate, enter sodium chloride evaporator crystallizer, the evaporation crystallization temperature is 162℃, and sodium chloride precipitates when concentrated to 40%.

6. The circulating freeze-heat desalination process for high-salinity mine water according to claim 1, characterized in that: In step (4), the conditions for recovering potassium salt, magnesium salt and calcium salt are: temperature controlled at 50℃, precipitation in sequence by adjusting pH value, wherein the pH is 7~8 when recovering potassium salt, pH is 8~9 when recovering magnesium salt and pH is 10.5 when recovering calcium salt.

7. The circulating freeze-heat desalination process for high-salinity mine water according to claim 1, characterized in that: In step (4), when the impurity salt rate is ≤5%, steps (1) to (4) are repeated until the impurity salt rate is >5%. Potassium chloride and sodium citrate are used as additives for evaporation and crystallization. Then, potassium salt, magnesium salt and calcium salt are recovered in sequence at a temperature of 50°C and pH values ​​of 7~8, 8~9 and 10.

5.

8. The circulating freeze-heat desalination process for high-salinity mine water according to claim 1, characterized in that: In step (4), the impurity salt rate is determined by a sensor inside the impurity salt crystallizer. The formula for calculating the impurity salt rate is as follows: ； Wherein, C (impurities) represents the concentration of dissolved solids that are identified as impurities after the concentrate has been evaporated and concentrated, and TDS represents the total dissolved solids concentration in the concentrate.

9. A modified sodium sulfate decahydrate cryo-crystallization apparatus, characterized in that: To implement the processing technology according to any one of claims 1-8, comprising: The crystallization tank has a W-shaped bottom structure. A freezing converter, which is installed around the inner wall of the crystallization tank, is used to freeze the liquid material and lower its temperature; A guide tube, coaxially disposed inside the crystallization tank, is used to guide the liquid material to form a circulating flow path; A temperature sensor, which is installed on the side wall of the guide tube, is used to determine whether the liquid material after freezing treatment is below -5℃; A stirring assembly, wherein the stirring paddle extends into the guide tube to drive the movement of the liquid material; The clarification zone is a diameter-enlarged section located at the upper part of the crystallization tank. A fine crystal filter assembly, disposed within the clarification zone, is used to separate and discharge fine crystals; The feed inlet, located at the top of the crystallization tank, is used to introduce new concentrate to mix with the circulating mother liquor inside the tank. The discharge port, located at the bottom of the guide tube, is used to discharge sodium sulfate decahydrate obtained from the freeze crystallization process.

10. The improved sodium sulfate decahydrate cryo-crystallization apparatus according to claim 9, characterized in that, The fine-crystal filter assembly includes: A fine-grained filter screen is horizontally positioned above the clarification zone; A fine crystal outlet is provided on the side wall or top of the clarification zone and is located above or in communication with the fine crystal filter screen, for discharging the trapped fine crystals.

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