Mine water zero discharge process based on nanofiltration salt separation and mother liquor alkalinity closed loop circulation

Abstract

This application belongs to the field of water treatment technology, specifically relating to a zero-discharge process for mine water based on nanofiltration desalination and a closed-loop recycling of mother liquor alkalinity. The discharge process includes the following steps: Step S1 is a coupled softening and seed crystal induction reaction, utilizing the reflux of frozen mother liquor to provide alkalinity and seed crystals; Step S2 is ultrafiltration purification; Step S3 is high-pressure nanofiltration desalination; Step S4 is freeze crystallization and closed-loop recycling of mother liquor; Step S5 is thermal salt precipitation and sodium chloride resource recovery. The core technical means to ensure the long-term stable operation of the system and reduce reagent consumption mainly consist of three parts: first, stepwise gradient dosing; second, the mother liquor reflux, which combines alkalinity recycling and seed crystal induction functions; and third, dynamic impurity venting. These three elements work together to form the technical guarantee of the system. Furthermore, the steps are tightly coupled through defined material flows and control logic, forming a closed-loop system with synergistic optimization of matter and energy.

Description

Technical Field

[0001] This application belongs to the field of water treatment technology, specifically, it relates to a zero-discharge process for mine water based on nanofiltration desalination and closed-loop circulation of mother liquor alkalinity. Background Technology

[0002] With the continuous development of energy bases in western my country, mine water, a large amount of high-salinity wastewater generated during coal mining, has become a key environmental bottleneck restricting the sustainable development of mining areas. This type of mine water generally exhibits the "three highs" characteristics: high mineralization, high hardness, and high sulfate content. Total dissolved solids (TDS) often reach 5000–15000 mg / L, with high calcium and magnesium ion concentrations and a significant sulfate content. Direct discharge would severely damage the regional aquatic ecosystem. Against this backdrop, Zero Liquid Discharge (ZLD) technology has been widely adopted, aiming to achieve water reuse and the resource recovery of inorganic salts. However, traditional ZLD processes, when dealing with typical mine water in western China, have revealed systemic defects such as high reagent consumption, difficulty in handling mixed salts, and poor system stability, necessitating a fundamental restructuring from the perspectives of process logic and material recycling.

[0003] The current mainstream zero-discharge routes for mine water typically employ a three-stage coupled process: chemical softening, membrane concentration, and evaporation crystallization. The chemical softening stage generally relies on the lime-soda ash method to remove calcium and magnesium hardness, preventing subsequent reverse osmosis or nanofiltration membranes from failing due to scaling. While this method was viable under early low-salinity water conditions, its limitations are becoming increasingly apparent when facing the high hardness and high sulfate content of western mine water. Specifically, to ensure the safety of the membrane system, a large amount of sodium carbonate (Na₂CO₃) needs to be added to precipitate residual calcium ions. Soda ash, as a major operating cost, has its unit price fluctuations directly impacting the project's economic feasibility. More alarmingly, some engineering practices still use the "simultaneous addition of lime and sodium carbonate" operation mode, i.e., injecting both agents simultaneously in the same reaction zone. From a chemical reaction pathway perspective, this approach suffers from an unreasonable reaction sequence and low agent utilization efficiency: the OH⁻ released after lime addition... - It should be prioritized for raising the system pH and promoting magnesium ion removal. However, if added simultaneously with sodium carbonate, the CO3² provided by sodium carbonate will be detrimental. - Easy and Ca² in the system + Premature precipitation reactions disrupt the synergy between the primary magnesium removal reaction and the subsequent stepwise calcium removal process, leading to decreased reagent utilization and fluctuations in hardness removal efficiency. Actual measurement data shows that under simultaneous dosing conditions, the effluent hardness is often difficult to stably control below 100 mg / L (calculated as CaCO3), causing frequent calcium sulfate or calcium carbonate scaling on the subsequent nanofiltration membrane, shortening the cleaning cycle to less than 15 days, and severely compromising system reliability.

[0004] A deeper contradiction lies in the fact that while pursuing the resource recovery of salts, existing technologies have neglected the dynamic equilibrium of mass migration within closed-loop systems. To achieve the separate recovery of sodium sulfate and sodium chloride, nanofiltration technology has been introduced into the ZLD process in recent years. Utilizing its selective retention characteristics for divalent ions, it separates raw mine water into sulfate-rich concentrate and chloride-rich permeate, which then enter the freeze crystallization and thermal evaporation units, respectively. However, the mother liquor produced after the nanofiltration concentrate precipitates sodium sulfate decahydrate through freeze treatment is usually treated as high-salt wastewater and directly discharged or recycled back to the upstream end, without any scientific utilization of its inherent chemical components. In fact, this mother liquor not only contains high concentrations of sodium chloride... + and SO4 2- Furthermore, because nanofiltration membranes are effective against CO3 2- The high rejection rate (>90%) of the solution leads to the accumulation of a large amount of carbonate ions. Simply refluxing without functional design not only fails to reduce reagent costs but may also introduce new operational risks due to the physical characteristics of the fine crystals in the low-temperature mother liquor. For example, direct reflux to the pre-membrane unit can easily cause blockage of the nanofiltration membrane channels. Without an impurity control mechanism, non-crystallizable components such as chloride ions, fluoride ions, and organic matter will continuously accumulate during long-term closed-loop operation, leading to an imbalance in the system's ion composition, a decrease in the purity of the crystallized product, and affecting the continuous and stable operation of the system. This secondary problem, arising from the contradiction between the "requirement for salt resource utilization" and the "requirement for stable closed-loop operation," has become a key technical bottleneck that has long remained unresolved in existing technologies.

[0005] Therefore, under the premise of ensuring the safety of efficient hardening and membrane systems, how to construct an integrated process that can achieve endogenous circulation of carbonate to reduce dependence on soda ash, enhance precipitation kinetics by utilizing the physical properties of mother liquor, and simultaneously set up a dynamic impurity venting mechanism to maintain long-term stable operation of the system has become a key challenge and an urgent technical problem to be solved by those skilled in the art. Summary of the Invention

[0006] This application relates to a zero-discharge process for mine water based on nanofiltration salt separation and a closed-loop circulation of mother liquor alkalinity, belonging to the field of industrial wastewater treatment and resource utilization technology. Specifically, it is applicable to the deep treatment and inorganic salt fractionation recovery of mine water in western mining areas with high mineralization, high hardness, and high sulfate content. To achieve the above-mentioned objectives, this application provides a systematic integrated process. Its core lies in reconstructing the chemical softening reaction logic, constructing a closed-loop system for the dual-function recycling of mother liquor alkalinity and seed crystals, and setting up a dynamic impurity venting mechanism. This significantly reduces reagent consumption while ensuring the long-term stable operation of the membrane system, achieving high-purity fractional crystallization of sodium sulfate and sodium chloride, ultimately achieving the dual goals of zero mine water discharge and inorganic salt resource utilization.

[0007] The process described in this application includes the following steps: Step S1 is coupled softening and seed crystal induction reaction; Step S2 is ultrafiltration purification; Step S3 is high-pressure nanofiltration salt separation; Step S4 is freeze crystallization and closed-loop mother liquor reuse; Step S5 is thermal salt precipitation and sodium chloride resource utilization. The core technical means to ensure the long-term stable operation of the system and reduce reagent consumption mainly consists of three parts: first, stepwise gradient dosing; second, step-by-step mother liquor reflux with both alkalinity reuse and seed crystal induction functions; and third, dynamic impurity venting. These three elements work together to form the technical guarantee of the system. Furthermore, the steps are tightly coupled through defined material flows and control logic, forming a closed-loop system with synergistic optimization of matter and energy.

[0008] In step S1, the raw mine water and the frozen mother liquor from step S4 are thoroughly mixed in a static mixer before entering the high-density clarifier. The frozen mother liquor has a temperature range of -2℃ to 3℃ and its physical characteristics include sodium sulfate decahydrate microcrystalline particles with a particle size distribution of 0.5μm to 5μm and a concentration of 800mg / L to 1500mg / L. The introduction of this low-temperature mother liquor reduces the influent temperature after mixing to 15℃ to 22℃. Simultaneously, the microcrystalline particles act as heterogeneous nucleation sites, significantly improving the crystallization rate and floc density of the subsequent precipitation reaction. In the high-density clarifier, a stepwise gradient dosing operation is implemented: firstly, lime (Ca(OH)2) is added at the first dosing point upstream of the influent. The dosage is dynamically calculated based on the total hardness of the raw mine water to ensure that the pH value in the reaction zone is stably maintained between 10.5 and 11.5. Under this pH condition, the magnesium ions (Mg) in the raw mine water... 2+ It is completely converted into magnesium hydroxide (Mg(OH)2) precipitate and bicarbonate (HCO3) ions. - ) is converted into carbonate (CO3) 2- ), of which some carbonate ions and calcium ions (Ca ions) 2+ The lime combines with water to form calcium carbonate (CaCO3) precipitate, thereby removing the temporarily hardened components. After the lime is added, the water flows through a guide channel of no less than 3 meters in length, and the residence time is strictly controlled between 3 and 5 minutes to ensure that the first-stage reaction is fully completed. Subsequently, sodium carbonate (Na2CO3) and polyaluminum chloride (PAC) coagulant are added at the second dosing point. Sodium carbonate is used to remove residual permanent hardness (i.e., the non-carbonate hardness corresponding to CaCO3). 2+ The PAC dosage is 5 mg / L to 15 mg / L (calculated as Al2O3). The key is that the reflux mother liquor contains carbonate ions (CO3-) that are retained and enriched by the nanofiltration membrane. 2-The carbonate alkalinity ranges from 1200 mg / L to 2500 mg / L (calculated as CaCO3). This endogenous alkalinity can directly participate in the precipitation reaction of calcium ions, thereby reducing the amount of fresh sodium carbonate required. Engineering verification shows that, under the same influent water quality conditions, the unit dosage of sodium carbonate in this application's process is 0.75 kg / m³. 3 Up to 0.85 kg / m 3 This process reduces emissions by 25% to 30% compared to traditional simultaneous dosing processes. The flocs generated during the reaction form a sludge layer at the bottom of the high-density clarifier. After separation by the inclined plate sedimentation zone, the supernatant has a total hardness of less than 50 mg / L (calculated as CaCO3) and a turbidity of less than 5 NTU, meeting the influent requirements of subsequent membrane treatment units.

[0009] In step S2, the supernatant produced in step S1 flows sequentially through a multi-media filter and an ultrafiltration unit. The multi-media filter is filled with quartz sand with a particle size of 0.8 mm to 1.2 mm and anthracite with a particle size of 1.5 mm to 2.0 mm, with a filtration rate controlled at 8 m / h to 12 m / h to remove residual suspended solids. The ultrafiltration unit uses an external pressure hollow fiber membrane module made of polyvinylidene fluoride (PVDF) with a pore size of 0.02 μm, an operating pressure of 0.1 MPa to 0.2 MPa, and a backwash cycle of 20 to 40 minutes, using a combined air-water backwash. The SDI (Spectrum Indices) of the ultrafiltration permeate is... 15 The turbidity is less than 3 and the turbidity is less than 0.2 NTU, providing a reliable guarantee for the subsequent high-pressure membrane system.

[0010] In step S3, the ultrafiltration permeate is pumped into a high-pressure nanofiltration system. The nanofiltration system uses an antifouling polypiperazine amide composite nanofiltration membrane with a spiral wound structure and an effective membrane area of ​​37 m². 2 / unit. The system operating pressure is set from 1.8MPa to 2.2MPa. The concentrate flow rate is controlled by a variable frequency pump and a proportional control valve to stabilize the system recovery rate at 70% ± 2%. This nanofiltration membrane is effective against sulfate ions (SO42-). 2- The retention rate of carbonate ions (CO3) is not less than 98%, and the retention rate of carbonate ions (CO3) is not less than 98%. 2- The retention rate of chloride ions (Cl) is not less than 92%. - The permeability of the nanofiltration system is no less than 95%. Therefore, the nanofiltration system divides the feed water into two deterministic product streams: the nanofiltration concentrate side is rich in high-valence anions (SO42-). 2- CO3 2- and trace amounts of residual Ca 2+ Mg 2+ The TDS concentration ranges from 18,000 mg / L to 25,000 mg / L; the nanofiltration permeate mainly contains Na. + With Cl -Low-valence ions, with TDS concentrations ranging from 3000 mg / L to 5000 mg / L, and sulfate concentrations below 200 mg / L.

[0011] In step S4, the nanofiltration concentrate is transferred to a cryogenic crystallization system. This system includes a cryogenic crystallizer, a heat exchanger, a stirring device, and a centrifugal separation unit. The temperature inside the cryogenic crystallizer is precisely controlled between -2°C and 0°C using an ethylene glycol refrigerant circulation system, with a residence time of 6 to 8 hours, promoting the precipitation of sodium sulfate decahydrate (Na₂SO₄·10H₂O) crystals. The precipitated crystals are separated by a horizontal screw centrifuge at a speed of 3000 to 3500 rpm and a differential speed ratio of 15:1 to 20:1, obtaining an industrial-grade sodium sulfate product with a water content of less than 5%, whose purity, as determined by X-ray fluorescence spectroscopy (XRF), is not less than 99.2%. The mother liquor produced during centrifugation, i.e., the cryogenic mother liquor, has the following composition: Na + Concentrations of 35,000 mg / L to 45,000 mg / L, SO4 2- Concentrations of 20,000 mg / L to 30,000 mg / L, CO3 2- Concentrations from 1200 mg / L to 2500 mg / L (calculated as CaCO3), Cl - The initial concentration was 8000 mg / L to 12000 mg / L, and the COD concentration was 30 mg / L to 60 mg / L. The mother liquor was transported by a low-temperature corrosion-resistant centrifugal pump (made of duplex stainless steel 2205), and the main route returned it to the front end of the static mixer in step S1 via pipeline, forming a closed-loop recycling path for alkalinity and seed crystals. Simultaneously, the system was equipped with an impurity venting branch, which consisted of an online chloride ion analyzer, a conductivity sensor, a PLC controller, and a pneumatic switching valve. The online chloride ion analyzer used the ion-selective electrode method, with a measurement range of 0 to 100000 mg / L and an accuracy of ±2%. When Cl was detected in the mother liquor... - If the concentration remains above 50,000 mg / L for more than 30 minutes, or the conductivity exceeds 80 mS / cm, the purging procedure is initiated. The PLC controller automatically opens the pneumatic switch valve, diverting 3% to 5% of the total mother liquor into the impurity drying system, while the remaining 95% to 97% is returned to the front end. Furthermore, the impurity purging branch is equipped with an emergency forced purging logic; when Cl is detected in the mother liquor... - When the concentration exceeds 60,000 mg / L, regardless of the duration, an emergency forced evacuation procedure is initiated. The PLC controller immediately controls the pneumatic switch valve to open for evacuation. In some implementations, the mother liquor diversion ratio during emergency forced evacuation can be 5% to 10%, with the remaining 90% to 95% flowing back to the front end. This evacuation mechanism is used to suppress non-crystallizable components (such as Cl) within the system. - F - NO3 -The continuous accumulation of impurities (including organic matter) helps maintain the stability of ionic composition and the purity of crystallized products. The impurity venting branch adopts a staged venting control strategy to balance the dynamic venting requirements under normal operating conditions with the equipment safety protection requirements under extreme operating conditions.

[0012] In step S5, the nanofiltration permeate generated in step S3 is further concentrated using a disc tube reverse osmosis (DTU) system. The DTU system operates at a membrane stack pressure of 8 MPa to 10 MPa, with a recovery rate of 75% to 80%. The permeate TDS is below 500 mg / L and is reused in mining operations, while the concentrate TDS concentration is increased to above 80,000 mg / L. This concentrate then enters a mechanical vapor recompression (MVR) evaporation crystallization system at an evaporation temperature of 85°C to 95°C and a vapor compression ratio of 1.8 to 2.2. Inside the crystallizer, sodium chloride crystals continue to grow and are thickened by a thickener before being separated by a centrifuge to obtain an industrial sodium chloride product with a water content of less than 3%. Its purity, determined by titration, is not less than 98.8%, meeting the GB / T 5462-2015 standard for superior grade industrial salt. The secondary steam generated by the MVR system is condensed and used as process recycled water; the system has no liquid phase discharge.

[0013] In a preferred embodiment of this application, the interval between adding lime and sodium carbonate in step S1 is 3 to 5 minutes, preferably 4 minutes. This time is precisely controlled by the product of the pipe length and the flow rate to ensure that the first-stage reaction is completely terminated before the second-stage reaction is started, thus avoiding the ineffective side reaction caused by the spatial overlap of Ca(OH)2 and Na2CO3.

[0014] As another preferred embodiment of this application, nanofiltration membranes are used for CO3²⁻ - The retention rate should not be less than 90%, which is the physical basis for the mother liquor to accumulate sufficient alkalinity to achieve drug substitution. If the retention rate is lower than this threshold, the alkalinity of the returned mother liquor will not be sufficient to produce a significant economic effect.

[0015] In another preferred embodiment of this application, the emergency forced purging procedure trigger threshold of the impurity purging branch is set to Cl. - At a concentration of 60,000 mg / L, the accumulation of chloride ions in the system is close to the co-saturation point of the crystallization phase diagram. If it is not emptied in time, sodium chloride will be carried in the sodium sulfate product, and the purity will drop to below 95%.

[0016] Furthermore, the high-density clarifier described in this application is equipped with a guide tube and a reflector plate. The height of the guide tube is 1 / 3 of the tank depth, and the reflector plate has an inclination angle of 60° to optimize the hydraulic flow and extend the settling path of the flocs. The lime slurry is prepared at a concentration of 5% to 8% (mass fraction) and injected in a pulse manner using a metering pump to ensure uniform dispersion; the sodium carbonate solution is prepared at a concentration of 10% to 15% and is also added using a metering pump. All dosing points are equipped with online pH monitors, and the monitoring data is uploaded to the central control system in real time to achieve closed-loop feedback adjustment of the dosing amount. An ORP monitoring probe is optional.

[0017] Furthermore, the stirring speed inside the cryogenic crystallizer is controlled at 20 to 30 rpm, and the impeller is of the anchor type to avoid crystal breakage and ensure uniform heat transfer. The centrifuged mother liquor passes through a 5μm bag filter before reflux to prevent large particles from clogging the static mixer. The reflux pipeline is heated and insulated throughout to maintain the mother liquor temperature at no less than 0℃ and prevent crystallization during transit.

[0018] Furthermore, before entering the MVR, the concentrate from the disc tube reverse osmosis system undergoes a degassing tower to remove dissolved gases, operating at a vacuum level of -0.08 MPa to -0.09 MPa to reduce foaming and scaling during the evaporation process. The MVR crystallizer is equipped with washing legs at the bottom, ensuring that the produced crystal particle size distribution is concentrated between 200 μm and 500 μm through continuous discharge and grading.

[0019] This application addresses three core contradictions in existing technologies through the aforementioned deterministic technical means: First, by employing a stepwise gradient dosing logic, it improves the low reagent utilization and insufficient magnesium removal efficiency caused by simultaneous dosing, significantly enhancing softening efficiency. Second, by directionally recirculating the cryogenic mother liquor to the front end of chemical softening, it is endowed with the dual functions of alkalinity reuse and crystal seeding, reducing sodium carbonate consumption and enhancing precipitation kinetics under low-temperature conditions. Third, by setting up an impurity venting branch based on chloride ion concentration or conductivity thresholds, a dynamic equilibrium mechanism is established, preventing the accumulation of non-crystallizable components during closed-loop operation. The synergistic effect of these three aspects facilitates long-term stable system operation, improves the purity of sodium sulfate and sodium chloride products, and reduces the cost per ton of water treated. According to the example data, the system operated continuously for 365 days, with the nanofiltration membrane cleaning cycle consistently maintained at over 45 days. The purity of both sodium sulfate and sodium chloride products met industrial-grade standards, the main process did not generate mixed salt hazardous waste, and the cost per ton of water treated was reduced by 18% to 22% compared to the comparison.

[0020] In summary, the zero-discharge mine water process based on nanofiltration salt separation and closed-loop circulation of mother liquor alkalinity described in this application achieves synergy between zero discharge of mine water and resource recovery of inorganic salts through systematic process reconstruction and material flow regulation, providing a solution with engineering application value for the treatment of high-salinity mine water. Attached Figure Description

[0021] In the accompanying drawings, unless otherwise specified, the same reference numerals throughout the various drawings denote the same or similar parts or elements. These drawings are not necessarily drawn to scale. It should be understood that these drawings depict only some embodiments disclosed in this application and should not be construed as limiting the scope of this application.

[0022] Figure 1 This is a schematic diagram of a zero-discharge mine water treatment process based on nanofiltration desalination and closed-loop circulation of mother liquor alkalinity in some embodiments. Figure 2 This is a schematic diagram illustrating the internal structure of a high-density clarifier and the mixing of stepwise gradient dosing and reflux mother liquor in some embodiments; Figure 3 This is a schematic diagram of the control logic and material flow direction of the freeze crystallization system and impurity venting branch in some embodiments.

[0023] The attached figures are labeled as follows: 1: Raw mine water; 2: Static mixer; 3: High-density clarifier; 4: First dosing point; 5: Second dosing point; 6: Flow guide tube; 7: Reflector plate; 8: Multi-media filter; 9: Ultrafiltration device; 10: High-pressure nanofiltration system; 11: Nanofiltration concentrate; 12: Nanofiltration permeate; 13: Freezing crystallizer; 14: Heat exchanger; 15: Horizontal screw centrifuge; 16: Freezing mother liquor; 17: Low-temperature corrosion-resistant centrifugal pump; 18: Return pipeline; 19: Impurity venting branch; 20: 21: Online chloride ion analyzer; 22: Conductivity sensor; 23: PLC controller; 24: Pneumatic switch valve; 25: Mixed salt drying system; 26: DTRO system; 27: MVR evaporation crystallization system; 28: Sodium sulfate product; 29: Sodium chloride product; 30: Process recycled water; 31: Lime dosing system; 32: Sodium carbonate dosing system; 33: PAC dosing system; 34: 5μm bag filter; 35: Heat tracing and insulation layer; 36: Supernatant effluent. Detailed Implementation

[0024] The embodiments of this application are described in detail below, examples of which are illustrated in the accompanying drawings. In the drawings, for clarity, the dimensions of layers, regions, and elements, as well as their relative dimensions, may be exaggerated. Throughout, the same or similar reference numerals denote the same or similar elements or elements having the same or similar functions. The embodiments described below with reference to the accompanying drawings are exemplary and are only used to explain this application, and should not be construed as limiting this application. It should be noted that, unless otherwise specified, the embodiments and features in the embodiments of this application can be combined with each other.

[0025] It should be understood that when an element or layer is referred to as "on," "adjacent to," "connected to," or "coupled to" other elements or layers, it may be directly on, adjacent to, connected to, or coupled to other elements or layers, or there may be intervening elements or layers. Conversely, when an element is referred to as "directly on," "directly adjacent to," "directly connected to," or "directly coupled to" other elements or layers, there are no intervening elements or layers. It should be understood that although the terms first, second, third, etc., may be used to describe various elements, components, areas, layers, and / or portions, these elements, components, areas, layers, and / or portions should not be limited by these terms. These terms are only used to distinguish one element, component, area, layer, or portion from another element, component, area, layer, or portion. Therefore, without departing from the teachings of this application, the first element, component, area, layer, or portion discussed below may be referred to as a second element, component, area, layer, or portion. And the discussion of a second element, component, area, layer, or portion does not imply that the first element, component, area, layer, or portion necessarily exists in this application.

[0026] It should be noted that the terms "first," "second," etc., used in this application are used to distinguish similar objects and are not necessarily used to describe a specific order or sequence. It should be understood that such terms can be used interchangeably where appropriate so that the embodiments of this application described herein can be implemented, for example, in orders other than those illustrated or described herein. Furthermore, the terms "comprising" and "having," and any variations thereof, are intended to cover non-exclusive inclusion; for example, a process, method, system, product, or apparatus that comprises a series of steps or units is not necessarily limited to those steps or units explicitly listed, but may include other steps or units not explicitly listed or inherent to such processes, methods, products, or apparatus.

[0027] In this application, when numerical intervals (i.e., numerical ranges) are involved, unless otherwise specified, the distribution of selectable numerical values ​​within the numerical interval is considered continuous, and includes the two endpoints of the numerical interval (i.e., the minimum and maximum values), as well as every numerical value between these two endpoints. Unless otherwise specified, when a numerical interval refers only to integers within that numerical interval, it includes the two endpoint integers of the numerical range, as well as every integer between the two endpoints, which is equivalent to directly listing every integer. When multiple numerical ranges are provided to describe features or characteristics, these numerical ranges can be merged. In other words, unless otherwise specified, the numerical ranges disclosed in this application should be understood to include any and all subranges included therein. The "numerical value" in the numerical interval can be any quantitative value, such as a number, percentage, ratio, etc. The term "numerical interval" can be broadly included to include percentage intervals, ratio intervals, proportion intervals, etc.

[0028] This application provides a zero-discharge process for mine water based on nanofiltration salt separation and closed-loop circulation of mother liquor alkalinity. (Reference) Figure 1-3 The overall process consists of five deterministic steps: Step S1 is coupled softening and seed-induced reaction; Step S2 is ultrafiltration purification; Step S3 is high-pressure nanofiltration for salt separation; Step S4 is freeze crystallization and closed-loop mother liquor reuse; and Step S5 is thermal salt precipitation and sodium chloride resource recovery. Each step is tightly coupled through precisely designed material flow, control logic, and equipment configuration, forming a closed-loop system with optimized material and energy processes. This system is suitable for the deep treatment and inorganic salt recovery of mine water in western mining areas with high mineralization, high hardness, and high sulfate content.

[0029] In step S1, the raw mine water 1 and the frozen mother liquor 16 from step S4 are thoroughly mixed in a static mixer 2 before entering the high-density clarifier 3. The frozen mother liquor has a temperature range of -2℃ to 3℃ and its physical characteristics include sodium sulfate decahydrate microcrystalline particles with a particle size distribution of 0.5μm to 5μm and a concentration of 800mg / L to 1500mg / L. The introduction of this low-temperature mother liquor reduces the influent temperature after mixing to 15℃ to 22℃. At the same time, the microcrystalline particles act as heterogeneous nucleation sites, significantly improving the crystallization rate and floc density of the subsequent precipitation reaction. The high-density clarifier is equipped with a guide tube 6 and a reflector plate 7. The height of the guide tube is 1 / 3 of the depth of the clarifier, and the reflector plate has an inclination angle of 60° to optimize the hydraulic flow and extend the floc settling path. In the high-density clarifier, a stepwise gradient dosing operation is implemented: First, lime (Ca(OH)2) is added at the first dosing point 4 upstream of the inlet. The lime slurry concentration is prepared at 5% to 8% (mass fraction), and injected in a pulse manner using a metering pump to ensure uniform dispersion. The lime dosage is dynamically calculated based on the total hardness of the original mine water to ensure that the pH value in the reaction zone is stably maintained between 10.5 and 11.5. Under this pH condition, the magnesium ions (Mg) in the original mine water... 2+ It is completely converted into magnesium hydroxide (Mg(OH)2) precipitate and bicarbonate (HCO3) ions. - ) is converted into carbonate (CO3) 2- ), of which some carbonate ions and calcium ions (Ca ions) 2+ The lime combines with the water to form calcium carbonate (CaCO3) precipitate, thereby removing the temporarily hardened components. After the lime is added, the water flows through a guide channel with a length of not less than 3 meters, and the residence time is strictly controlled to be between 3 and 5 minutes to ensure that the first-stage reaction is fully completed. Subsequently, sodium carbonate (Na2CO3) and polyaluminum chloride (PAC) coagulant are added at the second dosing point 5. The sodium carbonate solution is prepared at a concentration of 10% to 15% and is also added using a metering pump; the PAC concentration is 5 mg / L to 15 mg / L (calculated as Al2O3). The key is that the reflux mother liquor contains carbonate ions (CO3) that are intercepted and enriched by the nanofiltration membrane. 2-The carbonate alkalinity ranges from 1200 mg / L to 2500 mg / L (calculated as CaCO3). This endogenous alkalinity can directly participate in the precipitation reaction of calcium ions and be used to replace part of the fresh sodium carbonate, thereby reducing the amount of fresh sodium carbonate added.

[0030] In a preferred embodiment of this application, the interval between the addition of lime and sodium carbonate is strictly limited to 3-5 minutes. This time is precisely controlled by the product of pipe length and flow rate to ensure that the primary reaction is completely terminated before the secondary reaction is started, avoiding ineffective side reactions caused by the spatial overlap of Ca(OH)2 and Na2CO3. All dosing points are equipped with online pH monitors, and the monitoring data is uploaded to the central control system in real time to achieve closed-loop feedback adjustment of the dosing amount. The flocs generated by the reaction form a sludge layer at the bottom of the high-density clarifier. After separation in the inclined plate sedimentation zone, the total hardness of the effluent is less than 50 mg / L (calculated as CaCO3), and the turbidity is less than 5 NTU, meeting the influent requirements of the subsequent membrane treatment unit.

[0031] In step S2, the supernatant produced in step S1 flows sequentially through a multi-media filter 8 and an ultrafiltration device 9. The multi-media filter is filled with quartz sand with a particle size of 0.8 mm to 1.2 mm and anthracite with a particle size of 1.5 mm to 2.0 mm, with a filtration rate controlled at 8 m / h to 12 m / h to remove residual suspended solids. The ultrafiltration device uses an external pressure hollow fiber membrane module made of polyvinylidene fluoride (PVDF) with a pore size of 0.02 μm, an operating pressure of 0.1 MPa to 0.2 MPa, and a backwash cycle of 30 minutes, employing a combined air-water backwashing method. The SDI (Spectrum Indices) of the ultrafiltration permeate is... 15 The turbidity is less than 3 and the turbidity is less than 0.2 NTU, providing a reliable guarantee for the subsequent high-pressure membrane system.

[0032] In step S3, the ultrafiltration permeate is pumped into the high-pressure nanofiltration system 10. The nanofiltration system uses an antifouling polypiperazine amide composite nanofiltration membrane; the membrane element has a spiral wound structure and an effective membrane area of ​​37 m². 2 / unit. The system operating pressure is set from 1.8MPa to 2.2MPa. The concentrate flow rate is controlled by a variable frequency pump and a proportional control valve to stabilize the system recovery rate at 70% ± 2%. This nanofiltration membrane is effective against sulfate ions (SO42-). 2- The retention rate of carbonate ions (CO3) is not less than 98%, and the retention rate of carbonate ions (CO3) is not less than 98%. 2- The retention rate of chloride ions (Cl) is not less than 92%. - The transmittance of the material is not less than 95%.

[0033] As another preferred embodiment of this application, nanofiltration membranes are used for CO3... 2-The rejection rate is no less than 90%, which is the physical basis for the mother liquor to accumulate sufficient alkalinity to achieve reagent substitution. If the rejection rate is lower than this threshold, the alkalinity of the refluxed mother liquor is insufficient to produce a significant economic effect. Therefore, the nanofiltration system divides the influent into two deterministic product streams: nanofiltration concentrate 11 is rich in divalent anions (SO42-). 2- CO3 2- and trace amounts of residual Ca 2+ Mg 2+ The TDS concentration ranged from 18,000 mg / L to 25,000 mg / L; the nanofiltration permeate mainly contained Na. + With Cl - The TDS concentration is 3000 mg / L to 5000 mg / L, and the sulfate concentration is less than 200 mg / L.

[0034] In step S4, nanofiltration concentrate 11 is transferred to a cryogenic crystallization system. This system includes a cryogenic crystallizer 13, a heat exchanger 14, a stirring device, and a horizontal screw centrifuge 15. The temperature inside the cryogenic crystallizer is precisely controlled at -2°C to 0°C using an ethylene glycol refrigerant circulation system, with a residence time of 6 to 8 hours, promoting the precipitation of sodium sulfate decahydrate (Na₂SO₄·10H₂O) crystals. The stirring speed inside the cryogenic crystallizer is controlled at 20 to 30 rpm, with anchor-type impellers to prevent crystal breakage and ensure uniform heat transfer. The precipitated crystals are separated by a horizontal screw centrifuge at a speed of 3000 to 3500 rpm and a speed ratio of 15:1 to 20:1, yielding an industrial-grade sodium sulfate product 27 with a water content of less than 5%, whose purity, as determined by X-ray fluorescence spectroscopy (XRF), is not less than 99.2%. The mother liquor generated during centrifugation, i.e., the cryogenic mother liquor 16, has the following composition: Na + Concentrations of 35,000 mg / L to 45,000 mg / L, SO4 2- Concentrations of 20,000 mg / L to 30,000 mg / L, CO3² - Concentrations from 1200 mg / L to 2500 mg / L (calculated as CaCO3), Cl -The initial concentration is 8000 mg / L to 12000 mg / L, and the COD concentration is 30 mg / L to 60 mg / L. The mother liquor is transported by a low-temperature corrosion-resistant centrifugal pump 17 (made of duplex stainless steel 2205), and the main path returns it to the front end of the static mixer in step S1 via a return pipe 18, forming a closed-loop recycling path for alkalinity and seed crystals. The return pipe is equipped with a heat-tracing insulation layer 34 throughout to maintain the mother liquor temperature above 0℃ and prevent crystallization during transit. The centrifuged mother liquor passes through a 5μm bag filter 33 before reflux to prevent large particles from clogging the static mixer. Simultaneously, the system includes an impurity venting branch 19, which consists of an online chloride ion analyzer 20, a conductivity sensor 21, a PLC controller 22, and a pneumatic switching valve 23. The online chloride ion analyzer uses the ion-selective electrode method, with a measurement range of 0 to 100000 mg / L and an accuracy of ±2%. When Cl is detected in the mother liquor... - When the concentration exceeds 50,000 mg / L for more than 30 minutes, or the conductivity exceeds 80 mS / cm, the evacuation procedure is initiated. The PLC controller 22 automatically opens the pneumatic switch valve, diverting 3% to 5% of the total mother liquor into the mixed salt drying system 24, while the remaining 95% to 97% is returned to the front end.

[0035] In another preferred embodiment of this application, the emergency forced purging procedure trigger threshold of the impurity purging branch is set to Cl. - At a concentration of 60,000 mg / L, the accumulated chloride ions in the system are close to the co-saturation point on the crystallization phase diagram. If not purged in time, sodium chloride will be trapped in the sodium sulfate product, and the purity will drop below 95%. This purging mechanism is used to suppress non-crystallizable components (such as Cl) in the system. - F - NO3 - The continuous accumulation of organic matter helps maintain the stability of ionic composition and the purity of crystallized products.

[0036] In step S5, the nanofiltration permeate 12 generated in step S3 is further concentrated by the DTRO system 25 (disc tube reverse osmosis system 25). The disc tube reverse osmosis system operates at a membrane stack pressure of 8 MPa to 10 MPa, with a recovery rate of 75% to 80%. The permeate TDS is below 500 mg / L and is reused in mining operations, while the concentrate TDS concentration is increased to above 80,000 mg / L. Before entering the MVR evaporation crystallization system 26 (mechanical vapor recompression evaporation crystallization system 26), the concentrate undergoes a degassing tower to remove dissolved gases, operating at a vacuum of -0.08 MPa to -0.09 MPa to reduce foaming and scaling tendencies during evaporation. The MVR system operates at an evaporation temperature of 85°C to 95°C and a vapor compression ratio of 1.8 to 2.2. Inside the crystallizer, sodium chloride crystals continuously grow and are thickened by a thickener before being separated by a centrifuge to obtain industrial sodium chloride product 28 with a moisture content of less than 3%. Its purity, determined by titration, is not less than 98.8%, meeting the GB / T 5462-2015 standard for superior grade industrial salt. The MVR crystallizer is equipped with washing legs at the bottom, ensuring that the produced crystal particle size distribution is concentrated between 200μm and 500μm through continuous discharge and grading. The secondary steam generated by the MVR system is condensed and used as process recycled water 29; the system has no liquid phase discharge.

[0037] In some embodiments, to adapt to the low-temperature, high-salt, and high-chlorine closed-loop operating environment, the low-temperature corrosion-resistant centrifugal pump 17 used for the reflux of the cryogenic mother liquor 16 is made of duplex stainless steel 2205; the high-density clarifier 3 is a steel-lined rubber structure; the membrane element surface of the high-pressure nanofiltration system 10 is modified for anti-fouling; and the heat exchange tubes of the mechanical vapor recompression evaporation crystallization system 26 are made of titanium. According to the operating results of the embodiments, during the continuous operation of the system for 365 days, the nanofiltration membrane cleaning cycle was stably maintained at more than 45 days, and no mixed salt hazardous waste was generated in the main process.

[0038] Exemplary embodiments according to this application will now be described in more detail with reference to the accompanying drawings. It should be understood that these exemplary embodiments may be implemented in many different forms and should not be construed as being limited to the embodiments set forth herein.

[0039] Example 1 Mine water from a western coal mine was selected as the raw mine water. Its water quality parameters are as follows: TDS is 12500 mg / L, total hardness (as CaCO3) is 3800 mg / L, and Ca... 2+ 2200 mg / L, Mg 2+ 850 mg / L, SO4 2- It is 4800 mg / L, Cl - The concentration of HCO3 was 2100 mg / L. - The concentration of lime was 1800 mg / L, and the COD was 45 mg / L. Following the process described in this application, the lime dosage was 1.8 kg / m³.3 The sodium carbonate dosage is 0.80 kg / m³. 3 The PAC dosage was 10 mg / L (calculated as Al2O3). The nanofiltration system recovery rate was controlled at 70%, and the operating pressure was 2.0 MPa. The freezing crystallization temperature was -1.5℃, and the residence time was 7 hours. The impurity venting branch adopted a staged venting control strategy, in which the conventional venting control condition was that the Cl in the mother liquor... - The concentration continuously exceeds 50,000 mg / L for more than 30 minutes, or the conductivity exceeds 80 mS / cm; the emergency forced evacuation threshold is set at 60,000 mg / L, without duration limitations. The system operated continuously for 365 days, with a nanofiltration membrane cleaning cycle of 48 days, and no irreversible fouling occurred. The final product, sodium sulfate, has a purity of 99.3%, and sodium chloride has a purity of 98.9%, with a cost of 18.6 yuan per ton of water treated.

[0040] Comparative Example 1 The same raw mine water was treated using a traditional simultaneous chemical softening + conventional nanofiltration + single-effect evaporation process. Lime and sodium carbonate were added simultaneously, with a total dosage of 2.0 kg / m³. 3 and 1.10kg / m 3 Nanofiltration membranes are frequently fouled due to fluctuations in influent water hardness, with an average cleaning cycle of 28 days. The freeze-crystallization process is missing, resulting in the mixed crystallization of sodium sulfate and sodium chloride, requiring an additional salt separation process. The final purity of sodium sulfate is only 94.5%, and sodium chloride is 96.2%, generating approximately 120 kg / ton of mixed salt hazardous waste. The cost per ton of water treated is 22.8 yuan.

[0041] The key operational data of the above embodiments and comparative examples are summarized in the table below:

[0042] As can be seen from the data, this application significantly reduces reagent consumption while ensuring the long-term stable operation of the membrane system by reconstructing the chemical softening reaction logic, constructing a closed loop for the dual-function recycling of freezing mother liquor alkalinity and seed crystals, and setting up a dynamic impurity venting mechanism. This achieves high-purity fractional crystallization of sodium sulfate and sodium chloride, ultimately achieving the dual goals of zero mine water discharge and inorganic salt resource utilization.

[0043] Furthermore, all key control parameters in the process described in this application have clearly defined engineering boundaries and operating windows. For example, the pH value in the high-density clarifier must be strictly controlled between 10.5 and 11.5; if it is below 10.5, then Mg... 2+ Incomplete removal, residual Mg 2+ This will lead to MgSO4 supersaturation in the subsequent nanofiltration concentrate, inducing scaling on the membrane surface; if it exceeds 11.5, excessive OH... - It will react with CO2 to produce additional CO3. 2-This increases unnecessary sodium carbonate consumption. The freezing crystallization temperature must be maintained between -2°C and 0°C. If it is above 0°C, sodium sulfate decahydrate cannot precipitate effectively; if it is below -2°C, energy consumption increases sharply, and ice crystals may precipitate, affecting centrifugal separation efficiency. The nanofiltration system recovery rate is set at 70% ± 2%, based on a safe upper limit of no more than 25000 mg / L TDS on the concentrate side, to avoid scaling of CaSO4 or CaCO3 on the membrane surface. The disc tube reverse osmosis system recovery rate is set at 75% to 80% to ensure that the concentrate TDS entering the MVR reaches above 80000 mg / L, thereby achieving efficient sodium chloride crystallization in the MVR evaporator, while avoiding excessive steam consumption due to excessively low concentration.

[0044] Furthermore, all equipment selections in this application are based on long-term engineering practice verification. The high-density clarifier adopts a steel-lined rubber anti-corrosion structure, effectively resisting corrosion in high pH environments; the ultrafiltration device uses PVDF material, possessing excellent oxidation resistance and mechanical strength; the high-pressure nanofiltration membrane employs special surface modification technology, enabling it to maintain high flux and high rejection rate even under high sulfate and high hardness conditions; the inner wall of the cryogenic crystallizer is polished to Ra≤0.8μm to reduce crystal adhesion; the horizontal screw centrifuge uses dual-motor independent drive to achieve precise differential speed control, ensuring complete crystal separation; the MVR system uses titanium heat exchange tubes, resistant to corrosion from high-concentration sodium chloride solutions. All instruments, including pH meters, ORP probes, online chloride ion analyzers, conductivity sensors, etc., are selected from industrial-grade high-precision models and are regularly calibrated to ensure the reliability of control signals.

[0045] In summary, the zero-discharge mine water process based on nanofiltration desalination and closed-loop circulation of mother liquor alkalinity described in this application achieves a paradigm shift from "passive treatment" to "active resource recovery" through systematic process reconstruction and precise control of material flow. Its technical solution is fully public and feasible. Those skilled in the art can, based on the detailed description above and in conjunction with specific project scale and water quality conditions, fine-tune parameters and select equipment to successfully construct and stably operate the entire zero-discharge system.

[0046] It should also be noted that the terms "some embodiments," "other embodiments," and "embodiments" used in this application refer to specific features, structures, or characteristics described in connection with those embodiments, which are included in at least one embodiment described in the general description of this application. The appearance of the same expression in multiple places in the specification does not necessarily refer to the same embodiment. Furthermore, when a specific feature, structure, or characteristic is described in connection with any embodiment, the intention is to suggest that implementing such a feature, structure, or characteristic in conjunction with other embodiments also falls within the scope of this application.

[0047] In the above embodiments, the descriptions of each embodiment have different focuses. For parts not described in detail in a certain embodiment, please refer to the relevant descriptions in other embodiments.

[0048] It should also be noted that the above are merely preferred embodiments of this application and do not limit the scope of protection of this application. Any equivalent structural or procedural transformations made based on the content of this application's specification and drawings, or direct or indirect applications in other related technical fields, are similarly included within the scope of protection of this application.

Claims

1. A mine water zero discharge process based on nanofiltration desalting and mother liquor alkalinity closed loop recycling, characterized in that, The following steps are connected sequentially: Step S1: After mixing the raw mine water (1) and the frozen mother liquor (16) in a static mixer (2), the mixture enters a high-density clarifier (3). In the high-density clarifier (3), a step-by-step gradient dosing is carried out. First, lime is added at the first dosing point (4) to adjust the pH to 10.5 to 11.5, so that magnesium ions are converted into magnesium hydroxide precipitate and bicarbonate ions are converted into calcium carbonate precipitate. Then, sodium carbonate and polyaluminum chloride coagulant are added at the second dosing point (5) to utilize the carbonate ions in the frozen mother liquor (16) to participate in the calcium ion precipitation reaction and reduce the amount of sodium carbonate added. Step S2: the supernatant obtained in step S1 is treated by a multi-medium filter (8) and an ultrafiltration device (9) in sequence to obtain SDI 15 Ultrafiltration product water with turbidity less than 0.2 NTU Step S3: The ultrafiltration permeate is sent to a high-pressure nanofiltration system (10) and separated into nanofiltration concentrate (11) and nanofiltration permeate (12) under the conditions of operating pressure of 1.8 to 2.2 MPa and recovery rate of 70% ± 2%. The nanofiltration concentrate (11) contains sulfate and carbonate, and the nanofiltration permeate contains chloride ions (12). Step S4: The nanofiltration concentrate (11) is sent into the cryogenic crystallizer (13) and sodium sulfate decahydrate is crystallized at -2℃ to 0℃. The sodium sulfate product (27) is separated by a horizontal screw centrifuge (15). The frozen mother liquor (16) obtained by centrifugation is returned to the front end of the static mixer (2) in step S1 through the main channel to form a closed loop for dual-function recycling of alkalinity and seed crystals. At the same time, an impurity venting branch (19) is set up. When the chloride ion concentration in the frozen mother liquor (16) continuously exceeds 50000 mg / L or the conductivity exceeds 80 mS / cm, the venting program is started and the mother liquor with a mass fraction of 3% to 5% is automatically vented. Step S5: The nanofiltration permeate (12) is concentrated by the disc tube reverse osmosis system (25) and then enters the mechanical vapor recompression evaporation crystallization system (26) to precipitate sodium chloride crystals and separate sodium chloride product (28). There is no liquid phase discharge from the system.

2. The mine water zero-emission process of claim 1, wherein, In step S1, the temperature of the frozen mother liquor (16) is -2℃ to 3℃, containing sodium sulfate decahydrate microcrystalline particles with a particle size of 0.5μm to 5μm and a concentration of 800mg / L to 1500mg / L. After mixing, the influent temperature is reduced to 15℃ to 22℃. The high-density clarifier (3) is equipped with a guide tube (6) and a reflector plate (7). The height of the guide tube (6) is 1 / 3 of the depth of the pool, and the tilt angle of the reflector plate (7) is 60°. The interval between the addition of lime and sodium carbonate is 3 minutes to 5 minutes.

3. The mine water zero-emission process according to claim 1 or 2, c h a ra cte ri zed in that, In step S1, the carbonate alkalinity in the freezing mother liquor (16) is 1200 mg / L to 2500 mg / L; the lime slurry concentration is 5% to 8% and the sodium carbonate solution concentration is 10% to 15%, both of which are added by pulse injection via metering pumps; both the first and second dosing points are equipped with online pH monitors, and the monitoring data is uploaded to the central control system in real time to achieve closed-loop feedback regulation.

4. The mine water zero-emission process of claim 1, wherein, In step S2, the multi-media filter (8) is filled with quartz sand with a particle size of 0.8 mm to 1.2 mm and anthracite with a particle size of 1.5 mm to 2.0 mm, and the filtration rate is 8 m / h to 12 m / h; the ultrafiltration device (9) adopts an external pressure hollow fiber membrane module, the pore size of the fiber membrane is 0.02 μm, the ultrafiltration operating pressure is 0.1 MPa to 0.2 MPa, and gas-water combined backwashing is adopted, with a backwashing cycle of 20 minutes to 40 minutes.

5. The mine water zero-emission process of claim 1, wherein, In step S3, the high-pressure nanofiltration system (10) uses an antifouling polypiperazine amide composite nanofiltration membrane with a sulfate rejection rate of not less than 98%, a carbonate rejection rate of not less than 92%, and a chloride ion permeability of not less than 95%. The TDS in the nanofiltration concentrate (11) is 18,000 mg / L to 25,000 mg / L, and the TDS in the nanofiltration permeate (12) is 3,000 mg / L to 5,000 mg / L with a sulfate concentration of less than 200 mg / L.

6. The mine water zero-emission process of claim 1, wherein, In step S4, the stirring speed in the freeze crystallizer (13) is 20 rpm to 30 rpm, and the impeller is anchor type; The horizontal screw centrifuge (15) has a rotation speed of 3000 rpm to 3500 rpm and a differential speed ratio of 15:1 to 20:1; The cryogenic mother liquor (16) is filtered through a 5μm bag filter (33) before reflux, and the temperature inside the reflux pipe (18) is maintained at no less than 0℃ by a heat tracing and insulation layer (34); The conditions for initiating the evacuation procedure include: when the chloride ion concentration in the cryogenic mother liquor (16) continuously exceeds 50,000 mg / L and remains at that level for more than 30 minutes, or when the conductivity exceeds 80 mS / cm, or when the chloride ion concentration in the cryogenic mother liquor (16) reaches 60,000 mg / L, an emergency forced evacuation procedure shall be initiated immediately.

7. The mine water zero-emission process according to claim 1 or 6, c h a r a c t e r i z e d in that, The impurity venting branch (19) consists of an online chloride ion analyzer (20), a conductivity sensor (21), a PLC controller (22), and a pneumatic switching valve (23). The online chloride ion analyzer (20) uses the ion-selective electrode method, with a measurement range of 0 to 100,000 mg / L and an accuracy of ±2%.

8. The mine water zero-emission process of claim 1, wherein, In step S5, the disc tube reverse osmosis system (25) operates at a pressure of 8 MPa to 10 MPa, with a recovery rate of 75% to 80%, and the TDS in the produced concentrate is higher than 80,000 mg / L. Before entering the mechanical vapor recompression evaporation crystallization system (26), the concentrate is first treated by a degassing tower with an operating vacuum of -0.08 MPa to -0.09 MPa.

9. The mine water zero-emission process according to claim 1 or 8, c h a ra cte ri zed in that, The mechanical vapor recompression evaporation crystallization system (26) has an evaporation temperature of 85°C to 95°C and a vapor compression ratio of 1.8 to 2.

2. The bottom of its crystallizer is equipped with washing legs. Through continuous discharge and graded control, the crystal particle size of sodium chloride product (28) is concentrated between 200μm and 500μm, the water content is less than 3%, and the purity is not less than 98.8%.

10. The mine water zero-emission process of claim 1, wherein, The low-temperature corrosion-resistant centrifugal pump (17) used for the reflux of the frozen mother liquor (16) is made of duplex stainless steel; the high-density clarifier (3) is a steel-lined rubber structure; the membrane element surface of the high-pressure nanofiltration system (10) is modified to resist fouling; the heat exchange tube of the mechanical vapor recompression evaporation crystallization system (26) is made of titanium.

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