Zero-discharge treatment process for realizing high-value recovery of salt in industrial wastewater

Abstract

The invention belongs to the technical field of advanced treatment and resource utilization of industrial wastewater, and particularly relates to a zero-emission treatment process for realizing high-value recovery of salt in industrial wastewater. The treatment method comprises the following steps: performing high-power concentration on sodium sulfate in the wastewater by adopting nanofiltration I and nanofiltration II, and performing a combined process of freezing crystallization I, evaporation concentration I, freezing crystallization II and evaporation crystallization I on a nanofiltration II concentrated solution to realize high-purity recovery of sodium sulfate. The anhydrous sodium sulfate meets the requirements of I-class first-grade products (the purity is greater than 99%) in Industrial Anhydrous Sodium Sulfate (GB / T 6009-2014). Meanwhile, mother liquor of freezing crystallization II is subjected to nanofiltration III concentration again, nanofiltration III permeate is discharged out of a sodium sulfate crystallization system to guarantee the purity of sodium sulfate, nanofiltration III concentrated liquor is subjected to ozone catalytic oxidation to remove COD and then returns to evaporation concentration I, and the mode is beneficial to reducing the chromaticity of sodium sulfate. According to the invention, high-value recovery of the salt in the industrial wastewater is realized, and the reuse value of the salt is improved.

Description

Technical Field

[0001] This invention belongs to the field of deep treatment of industrial wastewater and resource utilization of salt, specifically relating to a zero-discharge treatment process for high-value recovery of salt from industrial wastewater. Background Technology

[0002] The information disclosed in this background section is intended only to enhance understanding of the overall background of the invention and is not necessarily to be construed as an admission or in any way implying that such information constitutes prior art known to those skilled in the art.

[0003] Industrial production processes often require the addition of large amounts of chemical reagents, and the resulting industrial wastewater often contains high levels of salts, such as sodium. + SO4 2- Cl - Ca 2+ Mg 2+ SiO2, F - Direct discharge of salts into natural water bodies inevitably has a serious impact on aquatic ecological security. Currently, more and more industries and regions are beginning to restrict the total salt content in industrial wastewater discharge standards. For example, the "Comprehensive Discharge Standard for Water Pollutants in the South Four Lakes Basin" jointly issued by Shandong, Jiangsu, Anhui, and Henan provinces requires total salt content to be less than 2500 mg / L and sulfate to be less than 650 mg / L. Therefore, resource recovery of salts in industrial wastewater is unavoidable. At the same time, some water-scarce areas have strictly limited the fresh water consumption of industrial enterprises, forcing them to treat wastewater before recycling to reduce fresh water consumption. Zero discharge of industrial wastewater can help industrial enterprises achieve both high-quality wastewater reuse and salt resource utilization. The core process of zero discharge of industrial wastewater is pretreatment + membrane treatment + salt separation and crystallization. Currently, the main problems with zero discharge of industrial wastewater projects include the following three categories: First, incomplete pretreatment leads to high levels of calcium in the wastewater. 2+ Mg 2+ SiO2, F - First, COD and other pollutants cause membrane scaling and fouling after being concentrated through the membrane, affecting the stable operation of the system. Second, the wastewater recovery rate is limited by the pressure-bearing capacity of the membrane material, and the salt concentration at the concentration endpoint is low, resulting in a large scale and high energy consumption for subsequent crystallization units. Third, regardless of whether the salt crystallization unit adopts direct evaporation, salt-nitrate separation, freeze crystallization, or nanofiltration, it suffers from low salt quality and low regeneration value.

[0004] A study has disclosed a zero-discharge treatment process for high-salt wastewater from photovoltaic solar cells. However, this process involves evaporating and crystallizing the nanofiltration concentrate first. This process involves a large volume of water and a large temperature difference, resulting in high energy consumption. In addition, the anhydrous sodium sulfate obtained by the evaporation and crystallization method has low purity.

[0005] A study has disclosed a zero-discharge treatment process for high-salinity wastewater containing both sodium sulfate and potassium chloride. This process targets wastewater with high proportions of sodium sulfate, sodium chloride, potassium sulfate, and potassium chloride. By removing sulfate ions from the wastewater as calcium sulfate, the mixture of sodium sulfate, sodium chloride, potassium sulfate, and potassium chloride is converted into a mixture of potassium chloride and sodium chloride. The difference in solubility between potassium chloride and sodium chloride due to temperature is then used to separate them. However, this study does not address the zero-discharge treatment of wastewater primarily composed of sodium sulfate and sodium chloride (with very low levels of potassium sulfate and potassium chloride). Furthermore, because this process employs a single-stage nanofiltration process, its SO42- content is limited. 2- and Cl - The separation effect still needs to be improved. Summary of the Invention

[0006] To address the aforementioned problems, this invention provides a zero-discharge treatment process for high-value salt recovery from industrial wastewater. This invention aims to recycle industrial wastewater after deep treatment and to achieve high-value salt recovery from industrial wastewater, thereby reducing industrial enterprises' dependence on fresh water resources and promoting the resource utilization of salt.

[0007] To achieve the above objectives, the present invention adopts the following technical solution: The first aspect of the present invention provides a zero-discharge treatment process for achieving high-value salt recovery from industrial wastewater, comprising: S1: Pre-treat industrial wastewater to achieve COD ≤ 20 mg / L and F - ≤2mg / L, Ca 2+ ≤100mg / L, SDI≤3; S2: The pretreated effluent first enters the reverse osmosis I for preliminary concentration. The reverse osmosis I uses anti-fouling brackish water to desalinate the reverse osmosis membrane. S3: The concentrate from reverse osmosis I sequentially enters high-density sedimentation tank I, mixed circulation tank I, and tubular membrane I for combined silica removal, hardness removal, and COD removal. Tubular membrane I controls the effluent SiO2 to ≤20mg / L and Ca to ≤20mg / L. 2+ ≤50mg / L, COD≤50mg / L; the effluent from the tubular membrane I sequentially enters the ion exchange resin and decarbonator for deep hardening and carbon removal, controlling the effluent Ca content. 2+ ≤1mg / L, HCO3 - ≤15mg / L; Freshwater from reverse osmosis I is reused in a freshwater tank; S4: The water effluent from the decarbonizer enters the reverse osmosis II for further concentration. The reverse osmosis II uses a fouling-resistant seawater desalination reverse osmosis membrane. The freshwater from the reverse osmosis II is then recycled into a freshwater tank. S5: The concentrate from reverse osmosis II sequentially enters nanofiltration I and nanofiltration II to remove SO4. 2-Two concentration processes were performed; S6: The concentrate from nanofiltration II sequentially enters high-density sedimentation tank II, mixed circulation tank II, and tubular membrane II for combined defluorination and desilicon removal. Tubular membrane II controls the effluent SiO2 to ≤15mg / L and F... - ≤2mg / L; S7: Water from tubular membrane II enters freeze crystallization I to precipitate sodium sulfate decahydrate. Water from freeze crystallization I enters evaporation concentration I for concentration and volume reduction, and then enters freeze crystallization II to precipitate sodium sulfate decahydrate again. After melting, sodium sulfate decahydrate enters evaporation crystallization I to obtain anhydrous sodium sulfate. Part of the mother liquor from freeze crystallization II is periodically discharged for drying treatment. S8: The permeate from nanofiltration I, nanofiltration II, and nanofiltration III is mixed and then enters reverse osmosis III for deep concentration. The concentrate from reverse osmosis III enters high-density sedimentation tank III, mixed circulation tank III, and tubular membrane III for combined defluoridation, desiliconization, and COD removal. The tubular membrane III controls the effluent SiO2 to ≤15mg / L and F... - ≤2mg / L, COD removal rate ≥50%; the freshwater from reverse osmosis III is reused in the freshwater tank; S9: Water from tubular membrane III enters evaporation and crystallization II to produce sodium chloride. Evaporation and crystallization II periodically discharges a portion of the mother liquor for drying treatment.

[0008] The overall idea of ​​this invention is to concentrate SO4 in industrial wastewater through two nanofiltration processes. 2 High-concentration and Cl - Separation is performed to separate sodium sulfate and sodium chloride components. The nanofiltration concentrate contains a high concentration of sodium sulfate, which is directly introduced into freeze crystallization I (temperature reduced from 25℃ to -5℃) to precipitate most of the sodium sulfate decahydrate, instead of first undergoing high-temperature evaporation (temperature increased from 25℃ to 98℃) followed by freeze crystallization (temperature reduced from 98℃ to -5℃). The mother liquor from freeze crystallization I is then evaporated and concentrated (temperature increased from -5℃ to 90℃) until the sodium sulfate and sodium chloride are co-saturated but do not precipitate. The remaining small portion of the co-saturated solution is then introduced into freeze crystallization II (temperature reduced from 90℃ to -5℃) for re-freezing to precipitate a small amount of sodium sulfate decahydrate. The two portions of sodium sulfate decahydrate are diluted to a concentration of 28% with distilled water produced during the evaporation process and then evaporated and crystallized again (temperature is 108℃) to obtain high-purity anhydrous sodium sulfate, meeting the Class I Grade 1 requirements (purity greater than 99%) in "Industrial Anhydrous Sodium Sulfate" (GB / T 6009-2014). The nanofiltration permeate is highly concentrated using ultra-high pressure spiral wound reverse osmosis to minimize the amount of concentrated sodium chloride brine. The concentrated water from the ultra-high pressure spiral wound reverse osmosis then enters a sodium chloride evaporation and crystallization reactor for evaporation and crystallization to obtain high-purity sodium chloride. The sodium chloride meets the first-grade standard for industrial dry salt (purity requirement greater than 98.5%) in the "Industrial Salt" standard (GBT 5462-2015). To ensure the purity of anhydrous sodium sulfate, the mother liquor from freeze crystallization II is further treated with nanofiltration III to remove SO42-.2 Concentrate and separate Cl - The concentrate from nanofiltration III is returned to evaporation concentration I after ozone catalytic oxidation to reduce COD, which helps reduce the color of sodium sulfate crystals. Simultaneously, to prevent reactor scaling in evaporation concentration I and evaporation crystallization II, both nanofiltration concentrate and ultra-high pressure reverse osmosis concentrate undergo enhanced pretreatment using a combination of high-density sedimentation tanks, mixing circulation tanks, and tubular membranes to achieve deep removal of silica, fluoride, and recalcitrant COD before the wastewater enters the evaporation system. To minimize the size of the nanofiltration system and reduce system investment, wastewater undergoes two high-concentration processes via reverse osmosis before entering the nanofiltration system. Before reverse osmosis concentration, a similar enhanced pretreatment process using high-density sedimentation tanks, mixing circulation tanks, and tubular membranes is employed to achieve deep removal of silica, fluoride, and recalcitrant COD before the wastewater enters the membrane, preventing membrane scaling.

[0009] A second aspect of the present invention provides a zero-emission treatment system for high-value salt recovery from industrial wastewater, comprising: The following components are connected in sequence: pretreatment unit, reverse osmosis I, high-density sedimentation tank I, mixed circulation tank I, tubular membrane I, ion exchange resin, decarbonizer, reverse osmosis II, nanofiltration I, and nanofiltration II. The concentrate outlet of nanofiltration II is sequentially connected to high-density sedimentation tank II, mixing circulation tank II, tubular membrane II, cryogenic crystallizer I, evaporator concentrator I, and cryogenic crystallizer II; the material outlets of cryogenic crystallizer I and cryogenic crystallizer II are respectively connected to evaporator crystallizer I. The outlet of the cryogenic crystallizer II is connected in sequence to nanofiltration III, ozone catalytic oxidation device, and evaporator I; The permeate outlets of nanofiltration I, nanofiltration II, and nanofiltration III are sequentially connected to reverse osmosis III, high-density sedimentation tank III, mixed circulation tank III, tubular membrane III, and evaporation crystallization II.

[0010] Beneficial effects of the present invention (1) This invention achieves high-value recovery of sodium sulfate in wastewater through two nanofiltration processes. The concentrated solution is then subjected to a combination of freezing crystallization I + evaporation concentration I + freezing crystallization II + evaporation crystallization I. At the same time, the mother liquor from freezing crystallization II is subjected to nanofiltration III again, and the nanofiltration permeate is discharged from the sodium sulfate crystallization system, which also helps to ensure the purity of sodium sulfate. In addition, the concentrated solution from nanofiltration III is subjected to ozone catalytic oxidation to remove COD, which can effectively reduce the color of sodium sulfate crystal salt.

[0011] (2) The present invention uses ultra-high pressure spiral reverse osmosis to highly concentrate the nanofiltration permeate, which reduces the amount of sodium chloride wastewater entering the evaporation crystallization II process and reduces the engineering investment.

[0012] (3) This invention achieves deep removal of silicon, hardness, fluoride and COD by using a combination process of high-density sedimentation tank + mixed circulation tank + tubular membrane, which strongly guarantees the safe and stable operation of evaporation concentration I, evaporation crystallization II and membrane.

[0013] (4) The process of this invention is simple, practical and easy to promote. Attached Figure Description

[0014] The accompanying drawings, which form part of this invention, are used to provide a further understanding of the invention. Exemplary embodiments of the invention and their descriptions are used to explain the invention and do not constitute an improper limitation of the invention.

[0015] Figure 1 This is a flowchart of a zero-emission treatment process for high-value salt recovery from industrial wastewater according to Embodiment 1 of the present invention; Figure 2 This is a flowchart of a zero-emission treatment process for salt recovery from industrial wastewater, as described in Comparative Example 1 of the present invention. Detailed Implementation

[0016] It should be noted that the following detailed descriptions are exemplary and intended to provide further illustration of the invention. Unless otherwise specified, all technical and scientific terms used in this invention have the same meaning as commonly understood by one of ordinary skill in the art to which this invention pertains.

[0017] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of skill in the art. The reagents and raw materials used in this invention are readily available through conventional means, and unless otherwise specified, they are used in accordance with conventional methods in the art or product instructions. Similarly, unless otherwise specified, the test methods of this invention are performed in accordance with conventional methods in the art or industry-standard methods or practices. Furthermore, any methods and materials similar to or equivalent to those described herein may be applied to the methods of this invention. The preferred embodiments and materials described herein are for illustrative purposes only.

[0018] As described in the background section, industrial wastewater zero-discharge projects suffer from problems such as film formation and scaling pollution, low salt concentration at the concentration endpoint, and low salt quality. This invention provides a zero-discharge treatment process for achieving high-value salt recovery from industrial wastewater, comprising: S1: Pre-treat industrial wastewater to achieve COD ≤ 20 mg / L and F - ≤2mg / L, Ca 2+ ≤100mg / L, SDI≤3; S2: The pretreated effluent first enters the reverse osmosis I for preliminary concentration. The reverse osmosis I uses anti-fouling brackish water to desalinate the reverse osmosis membrane. S3: The concentrate from reverse osmosis I sequentially enters high-density sedimentation tank I, mixed circulation tank I, and tubular membrane I for combined silica removal, hardness removal, and COD removal. Tubular membrane I controls the effluent SiO2 to ≤20mg / L and Ca to ≤20mg / L. 2+ ≤50mg / L, COD≤50mg / L; the effluent from the tubular membrane I sequentially enters the ion exchange resin and decarbonator for deep hardening and carbon removal, controlling the effluent Ca content. 2+ ≤1mg / L, HCO3 - ≤15mg / L; Freshwater from reverse osmosis I is reused in a freshwater tank; S4: The water effluent from the decarbonizer enters the reverse osmosis II for further concentration. The reverse osmosis II uses a fouling-resistant seawater desalination reverse osmosis membrane. The freshwater from the reverse osmosis II is then recycled into a freshwater tank. S5: The concentrate from reverse osmosis II sequentially enters nanofiltration I and nanofiltration II to remove SO4. 2- Two concentration processes were performed; S6: The concentrate from nanofiltration II sequentially enters high-density sedimentation tank II, mixed circulation tank II, and tubular membrane II for combined defluorination and desilicon removal. Tubular membrane II controls the effluent SiO2 to ≤15mg / L and F... - ≤2mg / L; S7: Water from tubular membrane II enters freeze crystallization I to precipitate sodium sulfate decahydrate. Water from freeze crystallization I enters evaporation concentration I for concentration and volume reduction, and then enters freeze crystallization II to precipitate sodium sulfate decahydrate again. After melting, sodium sulfate decahydrate enters evaporation crystallization I to obtain anhydrous sodium sulfate. Part of the mother liquor from freeze crystallization II is periodically discharged for drying treatment. S8: The permeate from nanofiltration I, nanofiltration II, and nanofiltration III is mixed and then enters reverse osmosis III for deep concentration. The concentrate from reverse osmosis III enters high-density sedimentation tank III, mixed circulation tank III, and tubular membrane III for combined defluoridation, desiliconization, and COD removal. The tubular membrane III controls the effluent SiO2 to ≤15mg / L and F... - ≤2mg / L, COD removal rate ≥50%; the freshwater from reverse osmosis III is reused in the freshwater tank; S9: Water from tubular membrane III enters evaporation and crystallization II to produce sodium chloride. Evaporation and crystallization II periodically discharges a portion of the mother liquor for drying treatment.

[0019] Preferably, the high-density sedimentation tank I comprises three mixing zones, one flocculation zone, and one sedimentation zone. In mixing zone one, sodium hydroxide is added to control the pH at 10.1-10.5; in mixing zone two, magnesium chloride is added at a dosage of 250-500 mg / L; in mixing zone three, polyferric sulfate is added at a dosage of 10-15 mg / L; in the flocculation zone, polyacrylamide is added at a dosage of 1-3 mg / L; and the surface loading of the sedimentation zone is 10-15 m³ / L. 3 / (m 2 •h); More preferably, the high-density sedimentation tank I includes three mixing zones (each with a reaction time of 3-5 min), one flocculation zone (with a residence time of 8-10 min), and one sedimentation zone.

[0020] Preferably, the mixing and circulating tank I includes mixing zone four, mixing zone five, and circulating zone one. Sodium hydroxide is added to mixing zone four to control the pH at 10.5-11.5, and sodium carbonate is added at 800-1000 mg / L. Polyferric sulfate is added to mixing zone five at 10-15 mg / L. Powdered activated carbon is added to circulating zone one at 250-500 mg / L. The effluent from circulating zone one enters tubular membrane I through a circulating pump. The operating pressure of the circulating pump is 30-50 m, and the circulating water volume is 5-8 times the influent volume. More preferably, the mixing and circulating pool I includes mixing zone four and mixing zone five (both with a residence time of 10-20 min) and circulating zone one (with a residence time of 60-120 min).

[0021] Preferably, the tubular membrane I is made of polyvinylidene fluoride, alumina, or zirconium oxide, with a pore size ≤50nm and a membrane flux of 150-200L / (m²). 2 •h).

[0022] Preferably, the high-density sedimentation tank II includes two mixing zones, one flocculation zone, and one sedimentation zone. In mixing zone one, the dosage of defluorinating agent is 600-1200 mg / L; in mixing zone two, sodium hydroxide is added to control the pH at 6-7; in the flocculation zone, polyacrylamide is added at 1-2 mg / L; and the surface loading of the sedimentation zone is 6-10 m³ / L. 3 / (m 2 ·h); More preferably, the high-density sedimentation tank II includes two mixing zones (mixing zone one with a residence time of 8-12 min and mixing zone two with a residence time of 4-6 min), one flocculation zone (with a residence time of 8-10 min), and one sedimentation zone.

[0023] Preferably, the mixing and circulating tank II includes mixing zone III, mixing zone IV, and circulating zone I. Sodium hydroxide is added to mixing zone III to control the pH at 10.5-12, and sodium aluminate is added at 150-300 mg / L. Polyferric sulfate is added to mixing zone IV at 10-15 mg / L. The effluent from circulating zone I enters tubular membrane II via a circulating pump. The operating pressure of the circulating pump is 30-50 m, and the circulating water volume is 5-8 times the influent volume. More preferably, the mixing circulation tank II includes mixing zone three and mixing zone four (both with a residence time of 10-20 min) and circulation zone one (with a residence time of 60-120 min).

[0024] Preferably, the tubular membrane II is made of polyvinylidene fluoride, alumina, or zirconium oxide, with a pore size ≤50nm and a membrane flux of 150-200L / (m²). 2 ·h).

[0025] Preferably, the high-density sedimentation tank III includes two mixing zones, one flocculation zone, and one sedimentation zone. In mixing zone one, the dosage of defluorinating agent is 2500-5000 mg / L; in mixing zone two, sodium hydroxide is added to control the pH at 6-7; in the flocculation zone, the dosage of polyacrylamide is 1-2 mg / L; and the surface loading of the sedimentation zone is 6-10 m³ / L. 3 / (m 2 ·h); More preferably, the high-density sedimentation tank III includes two mixing zones (mixing zone one with a residence time of 8-12 min and mixing zone two with a residence time of 4-6 min), one flocculation zone (with a residence time of 8-10 min), and one sedimentation zone.

[0026] Preferably, the mixed circulation tank III includes a mixing zone three, a mixing zone four, and a circulation zone one. Sodium hydroxide is added to the mixing zone three to control the pH at 10.5-12, and sodium aluminate 400-800 mg / L is added at the same time. Polyferric sulfate is added to the mixing zone four at a dosage of 10-15 mg / L. Powdered activated carbon is added to the circulation zone one at a dosage of 800-1600 mg / L. The effluent from the circulation zone one enters the tubular membrane III through a circulation pump. The circulation pump operates at a pressure of 30-50 m, and the circulation water volume is 5-8 times the influent volume. More preferably, the mixing and circulating pool III includes mixing zone III and mixing zone IV (both with a residence time of 10-20 min) and circulating zone I (with a residence time of 60-120 min).

[0027] Preferably, the tubular membrane III is made of polyvinylidene fluoride, alumina, or zirconium oxide, with a pore size ≤50nm and a membrane flux of 150-200L / (m²). 2 ·h).

[0028] Compared to single-stage nanofiltration for salt separation, this invention employs a two-stage nanofiltration separation process (i.e., the concentrate from nanofiltration is further separated using nanofiltration), thus achieving a more thorough removal of SO4. 2- and Cl - The separation of Cl in the concentrate - Lower concentrations result in higher purity sodium sulfate crystals. Preferably, both nanofiltration I and nanofiltration II utilize anti-fouling high-pressure nanofiltration membranes. Preferably, the nanofiltration I flux is 18-22 L / (m²). 2 •h), with a recovery rate of 45%-55%; Preferably, the flux of nanofiltration II is 16-20 L / (m²). 2The recovery rate is 25%-35%, and the TDS of the nanofiltration II concentrate is controlled to reach 150,000-180,000 mg / L, thereby reducing the scale of the sodium sulfate crystallization system.

[0029] Unlike traditional zero-emission processes, this invention directly freezes and crystallizes the nanofiltration concentrate. This process involves a small temperature difference, low energy consumption, and yields high-purity sodium sulfate decahydrate. The mother liquor from the freeze crystallization is then evaporated and concentrated. Due to the significantly reduced concentration in the mother liquor, the volume of water after evaporation and concentration is very low. The small amount of mother liquor remaining after evaporation is then freeze-crystallized again, resulting in low energy consumption and high-purity sodium sulfate decahydrate. The anhydrous sodium sulfate obtained by melting and evaporating the two portions of sodium sulfate decahydrate meets the Class I Grade 1 requirements (purity greater than 99%) of "Industrial Anhydrous Sodium Sulfate" (GB / T 6009-2014), exceeding the purity of the product obtained from patent CN120553895A, thus achieving high-value salt recovery. Furthermore, the sulfate concentration in the freeze crystallization mother liquor is significantly reduced while the chloride ion concentration is significantly increased. At this point, nanofiltration is used to further reduce SO4 in the freeze crystallization mother liquor. 2- and Cl - The separation of sodium sulfate and the introduction of chloride ions into the sodium chloride evaporation system are beneficial to improving the purity of sodium sulfate. Furthermore, the nanofiltration concentrate has a high COD concentration; ozone catalytic oxidation treatment of the nanofiltration concentrate degrades COD, preventing COD enrichment during evaporation and concentration, thus ensuring the whiteness of the sodium sulfate. Preferably, the temperatures for freeze crystallization I and freeze crystallization II are -5℃, and the temperature for evaporation concentration I is 98℃. The concentration endpoint is when sodium sulfate and sodium chloride are co-saturated without the precipitation of sodium chloride or sodium sulfate. The sodium sulfate decahydrate precipitated from freeze crystallization I and freeze crystallization II is mixed and redissolved in evaporation condensate to a concentration of 28%, and then subjected to evaporation crystallization I at a temperature of 108℃ to obtain anhydrous sodium sulfate. The obtained anhydrous sodium sulfate meets the Class I Grade 1 requirements (purity greater than 99%) in "Industrial Anhydrous Sodium Sulfate" (GB / T 6009-2014), achieving high-value recovery of sodium sulfate.

[0030] Unlike traditional zero-emission processes, the nanofiltration concentrate of this invention achieves high-value salt recovery through a two-stage freeze-crystallization + evaporation-crystallization method. Preferably, the effluent from freeze-crystallization II enters nanofiltration III for SO4 removal. 2- For further concentration, Nanofiltration III employs a fouling-resistant high-pressure nanofiltration membrane with a flux of 16-20 L / (m³). 2 ·h), with a recovery rate of 30%-50%, the nanofiltration III concentrate enters ozone catalytic oxidation to remove recalcitrant COD, with a COD removal rate ≥50%, and the ozone catalytic oxidation effluent is returned to evaporation concentration I to reduce the color of anhydrous sodium sulfate in order to achieve high-value recovery of sodium sulfate. The freeze crystallization II periodically discharges part of the mother liquor for drying treatment.

[0031] Preferably, the reverse osmosis III uses an ultra-high pressure spiral wound reverse osmosis membrane with a flux of 16-20 L / (m²). 2 The operating pressure is 8.0-10.0 MPa, the recovery rate is 70%-80%, the TDS of the concentrate is controlled to reach 100,000-110,000 mg / L, and the scale of the sodium chloride crystallization system is reduced.

[0032] Preferably, the flux of the reverse osmosis I is 16-20 L / (m³). 2 (·h), operating pressure 1.0-2.0 MPa, recovery rate 75%-85%; Preferably, the flux of the reverse osmosis II is 16-20 L / (m²). 2 The operating pressure is 3.0-4.0 MPa, and the recovery rate is 70%-80%.

[0033] More specifically, including: S1: Pre-treatment of industrial wastewater including hardness removal, silicon removal, fluoride removal, and COD removal. After pre-treatment, total silicon must be ≤30mg / L (as SiO2), COD must be ≤20mg / L, and F... - ≤2mg / L, Ca 2+ ≤100mg / L, SDI ≤3; S2: The pretreated effluent first enters Reverse Osmosis I for preliminary concentration. Reverse Osmosis I uses a fouling-resistant brackish water desalination membrane with a flux of 16-20 L / (m²). 2 (·h), operating pressure 1.0-2.0 MPa, recovery rate 75%-85%; S3: The concentrate from reverse osmosis I sequentially enters high-density sedimentation tank I, mixed circulation tank I, and tubular membrane I for combined silica removal, hardness removal, and COD removal. Tubular membrane I controls the effluent SiO2 to ≤20mg / L and Ca to ≤20mg / L. 2+ ≤50mg / L, COD≤50mg / L; the effluent from the tubular membrane I sequentially enters the ion exchange resin and decarbonator for deep hardening and carbon removal, controlling the effluent Ca content. 2+ ≤1mg / L, HCO3 - ≤15mg / L; Freshwater from reverse osmosis I is reused in a freshwater tank; S4: The effluent from the decarbonizer enters the reverse osmosis II process for further concentration. Reverse osmosis II uses a fouling-resistant seawater desalination reverse osmosis membrane with a flux of 16-20 L / (m²). 2 •h), operating pressure is 3.0-4.0MPa, recovery rate is 70%-80%; the freshwater from reverse osmosis II enters the freshwater tank for reuse; S5: The concentrate from reverse osmosis II sequentially enters nanofiltration I and nanofiltration II to remove SO4. 2- Two concentration processes were performed; S6: The concentrate from nanofiltration II sequentially enters high-density sedimentation tank II, mixed circulation tank II, and tubular membrane II for combined defluorination and desilicon removal. Tubular membrane II controls the effluent SiO2 to ≤15mg / L and F... - ≤2mg / L; S7: The effluent from tubular membrane II enters freeze crystallization I to precipitate sodium sulfate decahydrate. The effluent from freeze crystallization I enters evaporation concentration I for concentration and volume reduction, and then enters freeze crystallization II to precipitate sodium sulfate decahydrate again. After melting, the sodium sulfate decahydrate enters evaporation crystallization I to obtain anhydrous sodium sulfate. The anhydrous sodium sulfate meets the Class I Grade 1 requirements (purity greater than 99%) in "Industrial Anhydrous Sodium Sulfate" (GB / T 6009-2014), realizing high-value recovery of sodium sulfate. Freeze crystallization II periodically discharges part of the mother liquor for drying treatment. S8: The permeate from nanofiltration I, nanofiltration II, and nanofiltration III is mixed and then enters reverse osmosis III for deep concentration. The concentrate from reverse osmosis III enters high-density sedimentation tank III, mixed circulation tank III, and tubular membrane III for combined defluoridation, desiliconization, and COD removal. The tubular membrane III controls the effluent SiO2 to ≤15mg / L and F... - ≤2mg / L, COD removal rate ≥50%; the freshwater from reverse osmosis III is reused in the freshwater tank; S9: The effluent from the tubular membrane III enters the evaporation crystallization II to produce sodium chloride. The sodium chloride meets the first-class standard of industrial dry salt in "Industrial Salt" (GBT 5462-2015) (purity requirement greater than 98.5%), realizing high-value recovery of sodium chloride. The evaporation crystallization II periodically discharges part of the mother liquor for drying treatment.

[0034] The present invention will be further described in detail below with reference to specific embodiments. It should be noted that the specific embodiments are explanations of the present invention and not limitations thereof.

[0035] Example 1 An industrial enterprise discharges 40,000 m³ of wastewater. 3 / d, after pretreatment including fluoride removal, silicon removal, hardening removal, and biochemical treatment, the wastewater has COD=120mg / L, NH3-N=1.5mg / L, TN=5mg / L, and F - =2mg / L, Total silicon (as SiO2) =30mg / L, Hardness (as CaCO3) =250mg / L, TDS =4189mg / L (Cl - =700 mg / L, SO4 2- =2000mg / L, HCO3 - =30mg / L, Na + =1300 mg / L, Ca 2+=100mg / L), SDI≤3. According to the requirements, the wastewater discharged by the enterprise must be deeply treated before reuse (TDS<200mg / L), and the salt in the wastewater must be recovered and reused. Among them, sodium sulfate must meet the Class I Grade 1 standard (purity greater than 99%) in "Industrial Anhydrous Sodium Sulfate" (GB / T6009-2014), and sodium chloride must meet the Grade 1 standard (purity greater than 98.5%) in "Industrial Salt" (GBT 5462-2015).

[0036] A zero-discharge treatment process for achieving high-value salt recovery from industrial wastewater, the specific process flow is as follows: Figure 1 As shown, it includes the following steps: The pretreated wastewater first enters the reverse osmosis I system for preliminary concentration. The reverse osmosis I system uses a fouling-resistant brackish water desalination membrane (commercially available product) with a flux of 18L / (m²). 2 The operating pressure is 1.6 MPa, and the recovery rate is 80%. The concentrate from reverse osmosis I sequentially enters high-density sedimentation tank I, mixing circulation tank I, and tubular membrane I for combined silica removal, hardening removal, and COD removal. High-density sedimentation tank I includes three mixing zones (reaction time 4 min each), one flocculation zone (retention time 9 min), and one sedimentation zone. In mixing zone I, sodium hydroxide is added to control the pH at 10.1-10.5. In mixing zone II, magnesium chloride is added at a dosage of 400 mg / L. In mixing zone III, polyferric sulfate is added at a dosage of 12 mg / L. In the flocculation zone, polyacrylamide is added at a dosage of 2 mg / L. The surface loading of the sedimentation zone is 12 m³ / h. 3 / (m 2 The SiO2 concentration in the effluent from high-density sedimentation tank I is ≤20 mg / L. The effluent from high-density sedimentation tank I enters mixed circulation tank I, which includes mixing zone four and mixing zone five (both with a residence time of 15 min) and circulation zone one (with a residence time of 100 min). In mixing zone four, sodium hydroxide is added to control the pH at 10.5-11.5, and sodium carbonate (900 mg / L) is added simultaneously. In mixing zone five, polyferric sulfate is added at a concentration of 12 mg / L. In circulation zone one, powdered activated carbon is added at a concentration of 400 mg / L. The effluent from circulation zone one is pumped into tubular membrane I, with a circulation volume six times the influent volume and an operating pressure of 40 m³ / h. Tubular membrane I is made of alumina and can operate stably for a long period within a pH range of 1-12. The membrane pore size is 50 nm, and the membrane flux is 180 L / (m³). 2 ·h), Ca in the effluent of tubular membrane I 2+ ≤50mg / L, COD≤50mg / L; after adjusting the pH of the tubular membrane I effluent to 7-8, it sequentially enters the aminophosphonic acid chelating resin and decarbonizer for deep hardening and carbon removal. The Ca in the ion exchange resin effluent... 2+ ≤1mg / L, HCO3 in decarbonator effluent- ≤15mg / L.

[0037] The effluent from the decarbonizer enters the reverse osmosis II stage for further concentration. The reverse osmosis II stage uses a fouling-resistant seawater desalination reverse osmosis membrane (commercially available product) with a flux of 18 L / (m³). 2 •h), operating pressure 3.5MPa, recovery rate 75%; the concentrate from reverse osmosis II sequentially enters nanofiltration I and nanofiltration II for SO4 removal. 2- Two concentration processes are performed; both nanofiltration I and nanofiltration II use anti-fouling high-pressure nanofiltration membranes (commercially available products), with nanofiltration I having a flux of 20 L / (m³). 2 The recovery rate was 50%; the flux of nanofiltration II was 16 L / (m³). 2 The recovery rate was 30%, and the TDS of the nanofiltration II concentrate reached 172052 mg / L, which significantly reduced the scale of the sodium sulfate crystallization system compared with conventional reverse osmosis concentration (the highest TDS of the concentrate is about 120000 mg / L).

[0038] The concentrate from nanofiltration II sequentially enters high-density sedimentation tank II, mixing circulation tank II, and tubular membrane II for combined defluorination and desiliconization. High-density sedimentation tank II includes two mixing zones (mixing zone one with a residence time of 10 min and mixing zone two with a residence time of 5 min), one flocculation zone (residence time of 9 min), and one sedimentation zone. In mixing zone one, the defluorinating agent (commercially available product) is added at a dosage of 900 mg / L; in mixing zone two, sodium hydroxide is added to control the pH at 6-7; in the flocculation zone, polyacrylamide is added at a dosage of 1.5 mg / L, and the surface loading of the sedimentation zone is 8 m³ / s. 3 / (m 2 ·h), F in the effluent of high-density sedimentation tank II - ≤2mg / L. The effluent from high-density sedimentation tank II enters mixing and circulating tank II, which includes mixing zone three and mixing zone four (both with a residence time of 15 min) and circulating zone one (with a residence time of 100 min). Sodium hydroxide is added to mixing zone three to control the pH at 10.5-12, and sodium aluminate 250 mg / L is added simultaneously. Polyferric sulfate is added to mixing zone four at a dosage of 12 mg / L. The effluent from circulating zone one enters tubular membrane II via a circulating pump. The circulating water volume is 7 times the influent volume, and the operating pressure is 45 m. The tubular membrane II is made of alumina and can operate stably for a long time within a pH range of 1-12. The membrane pore size is 50 nm, and the membrane flux is 180 L / (m²). 2 ·h), SiO2 in the effluent of tubular membrane II is ≤15mg / L (it is generally difficult to remove silicon by traditional coagulation, sedimentation and filtration with a concentration of less than 20mg / L).

[0039] The effluent from tubular membrane II (temperature approximately 25℃) enters freeze crystallization I (temperature reduced to -5℃) to precipitate sodium sulfate decahydrate (at this point, the sodium sulfate concentration decreases while the sodium chloride concentration remains almost unchanged). The effluent from freeze crystallization I enters evaporation concentration I (temperature 98℃) for concentration and volume reduction (both sodium sulfate and sodium chloride concentrations increase). The concentration endpoint is when sodium sulfate and sodium chloride are co-saturated and no further precipitation occurs. The effluent from evaporation concentration I enters freeze crystallization II (temperature -5℃) again to precipitate sodium sulfate decahydrate (at this point, the sodium sulfate concentration decreases while the sodium chloride concentration remains almost unchanged). The sodium sulfate decahydrate precipitated from freeze crystallization I and freeze crystallization II are mixed and redissolved in the condensate generated during the evaporation stage to a concentration of 28%, and then enter evaporation crystallization I (temperature 108℃) for evaporation crystallization to obtain anhydrous sodium sulfate. The obtained anhydrous sodium sulfate meets the Class I Grade 1 requirements (purity greater than 99%) in "Industrial Anhydrous Sodium Sulfate" (GB / T 6009-2014), achieving high-value recovery of sodium sulfate.

[0040] The effluent from freeze crystallization II is heated to 25°C and then enters nanofiltration III to remove SO4. 2- Further concentration was carried out while Cl was... - For separation, Nanofiltration III uses a fouling-resistant high-pressure nanofiltration membrane (commercially available product) with a flux of 16 L / (m³). 2 •h), with a recovery rate of 40%, the nanofiltration III concentrate (rich in sodium sulfate) enters the ozone catalytic oxidation tank to remove recalcitrant COD, achieving a COD removal rate of ≥50%. The effluent from the ozone catalytic oxidation tank is returned to evaporation concentration I for further concentration and volume reduction. COD removal reduces the color of anhydrous sodium sulfate, enabling high-value recovery of sodium sulfate. Freeze crystallization II periodically discharges a portion of the mother liquor for drying treatment.

[0041] The permeates from nanofiltration I, nanofiltration II, and nanofiltration III are mixed and then fed into reverse osmosis III for deep concentration. Reverse osmosis III uses an ultra-high pressure spiral wound reverse osmosis membrane with a flux of 18 L / (m²). 2 The operating pressure was 9 MPa, the recovery rate was 70%, and the concentrated water TDS reached 107,166 mg / L (the concentration endpoint of conventional high-pressure reverse osmosis sodium chloride is about 70,000 mg / L), significantly reducing the scale of the sodium chloride crystallization system. The concentrated water from reverse osmosis III entered high-density sedimentation tank III, mixing circulation tank III, and tubular membrane III for combined defluorination, desiliconization, and COD removal. High-density sedimentation tank III included two mixing zones (mixing zone one with a residence time of 10 min and mixing zone two with a residence time of 5 min), one flocculation zone (residence time of 9 min), and one sedimentation zone. The defluorinating agent dosage in mixing zone one was 4000 mg / L; sodium hydroxide was added in mixing zone two to control the pH at 6-7; polyacrylamide was added in the flocculation zone at 1.5 mg / L, and the surface loading of the sedimentation zone was 8 m³ / s. 3 / (m 2 ·h), F in the effluent of high-density sedimentation tank III- ≤2mg / L. The effluent from high-density sedimentation tank III enters mixing and circulating tank III, which includes mixing zone three and mixing zone four (both with a residence time of 15 min) and circulating zone one (with a residence time of 100 min). Sodium hydroxide is added to mixing zone three to control the pH at 10.5-12, along with 600 mg / L of sodium aluminate. Polyferric sulfate is added to mixing zone four at 12 mg / L. Powdered activated carbon is added to circulating zone one at 1200 mg / L. The effluent from circulating zone one is pumped into tubular membrane II, with a circulating flow rate six times the influent flow rate and an operating pressure of 40 m³ / h. The tubular membrane III is made of alumina and can operate stably for a long time within a pH range of 1-12. The membrane pore size is 50 nm, and the membrane flux is 180 L / (m³). 2 •h). The SiO2 in the effluent of the tubular membrane III is ≤15mg / L (it is generally difficult to remove less than 20mg / L using traditional coagulation, sedimentation and filtration for silicon removal), and the COD removal rate is ≥50%.

[0042] The effluent from tubular membrane III enters evaporation crystallization II (temperature 60℃) to produce sodium chloride. The sodium chloride meets the first-class standard of industrial dry salt in "Industrial Salt" (GBT5462-2015) (purity requirement greater than 98.5%), realizing high-value recovery of sodium chloride. Evaporation crystallization II periodically discharges part of the mother liquor for drying treatment.

[0043] The freshwater generated during the reverse osmosis I, reverse osmosis II, and reverse osmosis III membrane separation processes, along with the condensate generated during evaporation and crystallization (partially recycled for sodium sulfate decahydrate redissolution), after mixing and blending, has a TDS < 200 mg / L, meeting the requirements for industrial water reuse. The water quantity and quality of each major unit are shown in Table 1.

[0044] Table 1. Water quantity and quality of each major unit (water quantity unit: m³) 3 / d, concentration unit mg / L)

[0045] As can be seen from Table 1, the industrial wastewater treatment capacity is 40,000 m³. 3 / d, the TDS after pretreatment was 4189 mg / L. After concentration by reverse osmosis I, the TDS reached 20481 mg / L. After further concentration by reverse osmosis II, the TDS reached 81357 mg / L. The concentrated water rich in sodium sulfate and sodium chloride entered nanofiltration I for sodium sulfate concentration and sodium chloride separation. The TDS of the nanofiltration I concentrate reached 127296 mg / L, of which sulfate reached 77152 mg / L. The nanofiltration I concentrate entered nanofiltration II for sodium sulfate further concentration and sodium chloride separation. The TDS of the nanofiltration II concentrate reached 172052 mg / L, of which sulfate reached 77152 mg / L. The concentration of sodium sulfate in the first stage reaches 109,886 mg / L. After treatment, the concentrated water from nanofiltration II enters freeze crystallization I to precipitate sodium sulfate decahydrate. The TDS of the effluent from freeze crystallization I is reduced to 41,942 mg / L, with sulfate concentration reduced to 22,000 mg / L. The effluent from freeze crystallization I then enters evaporation concentration I for further concentration and volume reduction. The TDS of the effluent from evaporation concentration I reaches 366,991 mg / L, with sulfate concentration reaching 192,500 mg / L. It then enters freeze crystallization II to precipitate sodium sulfate decahydrate, with the TDS of the effluent from freeze crystallization II reduced to 114,793 mg / L, with sulfate concentration reduced to 22,000 mg / L. The effluent from freeze crystallization II enters nanofiltration III for further concentration of sodium sulfate and separation of sodium chloride. The concentrated water from nanofiltration III reaches a TDS of 154,481 mg / L, with sulfate concentration reaching 36,520 mg / L. After ozone catalytic oxidation, it is returned to freeze crystallization II. The TDS of the freshwater from nanofiltration I, nanofiltration II, and nanofiltration III was 32983 mg / L, with 19494 mg / L of chloride ions and 470 mg / L of sulfate ions. After high concentration by reverse osmosis III, the TDS reached 107166 mg / L, and then entered evaporation and crystallization II to obtain high-purity sodium chloride.

[0046] The treatment process of this invention is applicable to industrial wastewater rich in sodium ions, sulfate ions, and chloride ions, and is suitable for applications requiring high-value recovery and utilization of sodium sulfate and sodium chloride from industrial wastewater.

[0047] Comparative Example 1 A certain industrial wastewater treatment plant differs from Example 1 in that... (1) The pretreatment process before membrane treatment all adopts a high-density sedimentation tank + media filtration + organic ultrafiltration process, and the effluent SiO2 and Ca are reduced. 2+ F - These methods cannot achieve deep removal and easily cause scaling on subsequent membranes and evaporation reactors; (2) Sodium sulfate was concentrated and sodium chloride was separated by nanofiltration only once. The TDS of the nanofiltration concentrate was still low, the water volume was large, and the scale of subsequent evaporation and crystallization was large. (3) After pretreatment, the nanofiltration concentrate is first evaporated and crystallized to precipitate anhydrous sodium sulfate. The water volume is large and the temperature difference is large (25℃ to 98℃). Then, it is frozen and crystallized (the temperature is then lowered from 98℃ to -5℃) to precipitate sodium sulfate decahydrate, so the energy consumption is large. In addition, the mother liquor of the frozen crystallization has a high chloride ion content, which is not separated by nanofiltration, affecting the purity of sulfate. In addition, a large amount of wastewater is generated in the evaporation and crystallization section, and the sodium sulfate has a low purity, about 96%. Furthermore, the fresh water from the first nanofiltration is then evaporated and crystallized to precipitate sodium chloride, which has a low purity, about 96%.

[0048] (4) Nanofiltration desalination uses conventional high-pressure reverse osmosis for concentration instead of ultra-high-pressure reverse osmosis. The TDS of the concentrate can only reach about 70,000 mg / L. Therefore, the scale of sodium chloride evaporation and crystallization is large and the energy consumption is high. The specific treatment process used is as follows: Figure 2 As shown: The treatment process is as follows: Industrial wastewater first enters a first-stage reverse osmosis system for concentration. The concentrated wastewater then enters a first high-density sedimentation tank for hardening removal, followed by a second high-density sedimentation tank for silicon removal. After media filtration, ultrafiltration, ion exchange, and decarbonization, it enters a second-stage reverse osmosis system for concentration. The concentrated wastewater from the second-stage reverse osmosis system enters a third-stage nanofiltration system for salt separation. The concentrated wastewater from the third-stage nanofiltration system enters a third high-density sedimentation tank for fluoride removal, followed by a fourth high-density sedimentation tank for silicon removal. After media filtration and ultrafiltration, it enters a first-stage evaporation crystallization system to produce anhydrous sodium sulfate. The effluent from the first evaporation crystallization system enters a freeze crystallization system to obtain sodium sulfate decahydrate, which is then melted and returned to the first evaporation crystallization system to produce anhydrous sodium sulfate. The effluent from the freeze crystallization system is treated with ozone catalytic oxidation and then returned to the first evaporation crystallization system for recycling. The nanofiltration desalinated water, after four stages of reverse osmosis concentration, enters a fifth high-density sedimentation tank for fluoride removal, followed by a sixth high-density sedimentation tank for silicon removal. After media filtration and ultrafiltration, it enters a second-stage evaporation crystallization system to produce sodium chloride.

[0049] The above description is merely a preferred embodiment of the present invention and is not intended to limit the invention. Various modifications and variations can be made to the present invention by those skilled in the art. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. A zero-discharge treatment process for achieving high-value salt recovery from industrial wastewater, characterized in that, include: S1: Pre-treat industrial wastewater to achieve COD ≤ 20 mg / L and F - ≤2mg / L, Ca 2+ ≤100mg / L, SDI≤3; S2: The pretreated effluent first enters the reverse osmosis I for preliminary concentration. The reverse osmosis I uses anti-fouling brackish water to desalinate the reverse osmosis membrane. S3: The concentrate from reverse osmosis I sequentially enters high-density sedimentation tank I, mixed circulation tank I, and tubular membrane I for combined silica removal, hardness removal, and COD removal. Tubular membrane I controls the effluent SiO2 to ≤20mg / L and Ca to ≤20mg / L. 2+ ≤50mg / L, COD≤50mg / L; the effluent from the tubular membrane I sequentially enters the ion exchange resin and decarbonator for deep hardening and carbon removal, controlling the effluent Ca content. 2+ ≤1mg / L, HCO3 - ≤15mg / L; Freshwater from reverse osmosis I is reused in a freshwater tank; S4: The water effluent from the decarbonizer enters the reverse osmosis II for further concentration. The reverse osmosis II uses a fouling-resistant seawater desalination reverse osmosis membrane. The freshwater from the reverse osmosis II is then recycled into a freshwater tank. S5: The concentrate from reverse osmosis II sequentially enters nanofiltration I and nanofiltration II to remove SO4. 2- Two concentration processes were performed; S6: The concentrate from nanofiltration II sequentially enters high-density sedimentation tank II, mixed circulation tank II, and tubular membrane II for combined defluorination and desilicon removal. Tubular membrane II controls the effluent SiO2 to ≤15mg / L and F... - ≤2mg / L; S7: Water from tubular membrane II enters freeze crystallization I to precipitate sodium sulfate decahydrate. Water from freeze crystallization I enters evaporation concentration I for concentration and volume reduction, and then enters freeze crystallization II to precipitate sodium sulfate decahydrate again. After melting, sodium sulfate decahydrate enters evaporation crystallization I to obtain anhydrous sodium sulfate. During the freeze-crystallization process II, a portion of the mother liquor is periodically discharged for drying. S8: The permeate from nanofiltration I, nanofiltration II, and nanofiltration III is mixed and then enters reverse osmosis III for deep concentration. The concentrate from reverse osmosis III enters high-density sedimentation tank III, mixed circulation tank III, and tubular membrane III for combined defluoridation, desiliconization, and COD removal. The tubular membrane III controls the effluent SiO2 to ≤15mg / L and F... - ≤2mg / L, COD removal rate ≥50%; the freshwater from reverse osmosis III is reused in the freshwater tank; S9: Water from tubular membrane III enters evaporation and crystallization II to produce sodium chloride. Evaporation and crystallization II periodically discharges a portion of the mother liquor for drying treatment.

2. The zero-discharge treatment process for high-value salt recovery from industrial wastewater as described in claim 1, characterized in that, High-density sedimentation tank I comprises three mixing zones, one flocculation zone, and one sedimentation zone. In mixing zone one, sodium hydroxide is added to control the pH at 10.1-10.

5. In mixing zone two, magnesium chloride is added at a dosage of 250-500 mg / L. In mixing zone three, polyferric sulfate is added at a dosage of 10-15 mg / L. In the flocculation zone, polyacrylamide is added at a dosage of 1-3 mg / L. The surface loading rate in the sedimentation zone is 10-15 m³ / L. 3 / (m 2 •h); Alternatively, the mixed circulation tank I includes mixing zone four and mixing zone five, as well as circulation zone one. Sodium hydroxide is added to mixing zone four to control the pH at 10.5-11.5, and sodium carbonate is added at 800-1000 mg / L. Polyferric sulfate is added to mixing zone five at 10-15 mg / L. Powdered activated carbon is added to circulation zone one at 250-500 mg / L. The effluent from circulation zone one enters tubular membrane I through a circulation pump. The circulation pump operates at a pressure of 30-50 m, and the circulation volume is 5-8 times the influent volume. Alternatively, the tubular membrane I is made of polyvinylidene fluoride, alumina, or zirconium oxide, with a pore size ≤50nm and a membrane flux of 150-200L / (m²). 2 •h).

3. The zero-discharge treatment process for high-value salt recovery from industrial wastewater as described in claim 1, characterized in that, High-density sedimentation tank II comprises two mixing zones, one flocculation zone, and one sedimentation zone. In mixing zone one, the dosage of defluorinating agent is 600-1200 mg / L; in mixing zone two, sodium hydroxide is added to control the pH at 6-7; in the flocculation zone, polyacrylamide is added at 1-2 mg / L; and the surface loading rate in the sedimentation zone is 6-10 m³ / L. 3 / (m 2 ·h); Alternatively, the mixed circulation tank II includes mixing zone III and mixing zone IV, as well as circulation zone I. Sodium hydroxide is added to mixing zone III to control the pH at 10.5-12, and sodium aluminate is added at 150-300 mg / L. Polyferric sulfate is added to mixing zone IV at 10-15 mg / L. The effluent from circulation zone I enters tubular membrane II via a circulation pump. The circulation pump operates at a pressure of 30-50 m, and the circulation volume is 5-8 times the influent volume. Alternatively, the tubular membrane II is made of polyvinylidene fluoride, alumina, or zirconium oxide, with a pore size ≤50nm and a membrane flux of 150-200L / (m²). 2 ·h).

4. The zero-discharge treatment process for high-value salt recovery from industrial wastewater as described in claim 1, characterized in that, High-density sedimentation tank III comprises two mixing zones, one flocculation zone, and one sedimentation zone. In mixing zone one, the dosage of defluorinating agent is 2500-5000 mg / L; in mixing zone two, sodium hydroxide is added to control the pH at 6-7; in the flocculation zone, the dosage of polyacrylamide is 1-2 mg / L; and the surface loading rate in the sedimentation zone is 6-10 m³ / L. 3 / (m 2 ·h); Alternatively, the mixed circulation tank III includes mixing zone III, mixing zone IV, and circulation zone I. Sodium hydroxide is added to mixing zone III to control the pH at 10.5-12, and sodium aluminate is added at 400-800 mg / L. Polyferric sulfate is added to mixing zone IV at 10-15 mg / L. Powdered activated carbon is added to circulation zone I at 800-1600 mg / L. The effluent from circulation zone I enters tubular membrane III via a circulation pump. The circulation pump operates at a pressure of 30-50 m, and the circulation volume is 5-8 times the influent volume. Alternatively, the tubular membrane III is made of polyvinylidene fluoride, alumina, or zirconium oxide, with a pore size ≤50nm and a membrane flux of 150-200L / (m²). 2 ·h).

5. The zero-discharge treatment process for high-value salt recovery from industrial wastewater as described in claim 1, characterized in that, Both nanofiltration I and nanofiltration II use fouling-resistant high-pressure nanofiltration membranes; Alternatively, the flux of nanofiltration I is 18-22 L / (m²). 2 •h), with a recovery rate of 45%-55%; Alternatively, the flux of nanofiltration II is 16-20 L / (m²). 2 The recovery rate is 25%-35%, and the TDS of the nanofiltration II concentrate is controlled to reach 150,000-180,000 mg / L.

6. The zero-discharge treatment process for high-value salt recovery from industrial wastewater as described in claim 1, characterized in that, The freezing crystallization I and freezing crystallization II were performed at -5℃, and the evaporation concentration I was performed at 98℃. The concentration endpoint was when sodium sulfate and sodium chloride were co-saturated but no sodium chloride or sodium sulfate was precipitated. The sodium sulfate decahydrate precipitated from freezing crystallization I and freezing crystallization II was mixed and diluted to a concentration of 28% by adding evaporation condensate. Then it was evaporated and crystallized in evaporation crystallization I to obtain anhydrous sodium sulfate. The temperature of evaporation crystallization I was 108℃.

7. The zero-discharge treatment process for high-value salt recovery from industrial wastewater as described in claim 1, characterized in that, The effluent from freeze crystallization II enters nanofiltration III for SO4 removal. 2- For further concentration, Nanofiltration III employs a fouling-resistant high-pressure nanofiltration membrane with a flux of 16-20 L / (m³). 2 ·h), with a recovery rate of 30%-50%, the nanofiltration III concentrate enters ozone catalytic oxidation to remove recalcitrant COD, with a COD removal rate ≥50%, the ozone catalytic oxidation effluent is returned to evaporation concentration I, and the freeze crystallization II periodically discharges part of the mother liquor for drying treatment.

8. The zero-discharge treatment process for high-value salt recovery from industrial wastewater as described in claim 1, characterized in that, Reverse osmosis III uses ultra-high pressure spiral wound reverse osmosis membranes with a flux of 16-20 L / (m²). 2 The operating pressure is 8.0-10.0 MPa, the recovery rate is 70%-80%, and the TDS of the concentrate is controlled to reach 100,000-110,000 mg / L.

9. The zero-discharge treatment process for high-value salt recovery from industrial wastewater as described in claim 1, characterized in that, The flux of the reverse osmosis I is 16-20 L / (m³). 2 (·h), operating pressure 1.0-2.0 MPa, recovery rate 75%-85%; Alternatively, the flux of the reverse osmosis II is 16-20 L / (m²). 2 The operating pressure is 3.0-4.0 MPa, and the recovery rate is 70%-80%.

10. A zero-discharge treatment system for achieving high-value salt recovery from industrial wastewater, characterized in that, include: The following components are connected in sequence: pretreatment unit, reverse osmosis I, high-density sedimentation tank I, mixed circulation tank I, tubular membrane I, ion exchange resin, decarbonizer, reverse osmosis II, nanofiltration I, and nanofiltration II. The concentrate outlet of nanofiltration II is sequentially connected to high-density sedimentation tank II, mixing circulation tank II, tubular membrane II, cryogenic crystallizer I, evaporator concentrator I, and cryogenic crystallizer II; the material outlets of cryogenic crystallizer I and cryogenic crystallizer II are respectively connected to evaporator crystallizer I. The outlet of the cryogenic crystallizer II is connected in sequence to nanofiltration III, ozone catalytic oxidation device, and evaporator I; The permeate outlets of nanofiltration I, nanofiltration II, and nanofiltration III are sequentially connected to reverse osmosis III, high-density sedimentation tank III, mixed circulation tank III, tubular membrane III, and evaporation crystallization II.

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