Potassium-sodium coexistence type high-salinity wastewater zero-discharge treatment process method and system

Abstract

The invention belongs to the technical field of advanced treatment and resource utilization of industrial wastewater, and particularly relates to a potassium-sodium coexistence type high-salinity wastewater zero-discharge treatment process method and system. The treatment process method comprises the following steps: performing pretreatment, first-stage reverse osmosis, second-stage reverse osmosis, fourth-stage reverse osmosis and the like on the potassium-sodium coexisting type high-salinity wastewater to obtain fourth-stage reverse osmosis effluent; sequentially carrying out four-stage high-density precipitation, four-stage medium filtration, four-stage ultrafiltration and evaporative crystallization treatment on the four-stage reverse osmosis effluent to obtain sodium chloride and evaporative crystallization effluent; carrying out cooling flash evaporation on evaporative crystallization effluent to obtain potassium chloride and potassium-containing mother liquor, introducing the potassium-containing mother liquor into an evaporative crystallizer, and carrying out evaporative crystallization and cooling flash evaporation again to obtain sodium chloride and potassium chloride. According to the method, advanced treatment of the potassium-sodium coexisting type high-salinity wastewater is achieved, potassium chloride, sodium chloride and gypsum are prepared, environmental pollution and water recycling are reduced, and meanwhile efficiently-extracted potassium chloride and other precious resources are used as agricultural fertilizer.

Description

Technical Field

[0001] The present invention belongs to the technical field of deep treatment and resource utilization of industrial wastewater, and specifically relates to a zero-discharge treatment process method and system for potassium-sodium coexistence high-salt wastewater. Background Art

[0002] High-salinity wastewater refers to industrial wastewater with a salt content exceeding a certain concentration. It comes from a wide range of sources and is often accompanied by heavy metals (such as lead, cadmium, and mercury) and organic pollutants. Direct discharge of high-salinity wastewater poses multiple risks to the environment and human health. Most high-salinity wastewater is primarily composed of sodium ions (divalent calcium and magnesium cations are also replaced by sodium ions through chemical softening or ion exchange processes), such as in the coal chemical and petrochemical industries. The zero-discharge process for sodium-type high-salinity wastewater generally involves "pretreatment + membrane concentration + salt separation and crystallization," ultimately producing sodium chloride and sodium sulfate as byproducts. Salt separation methods for sodium-type high-salinity wastewater primarily include thermal separation and nanofiltration. Thermal separation exploits differences in the concentration and solubility of different inorganic salts, separating them by controlling the operating temperature and concentration factor during the crystallization process. Membrane separation utilizes differences in the ionic radius or charge characteristics of chloride and sulfate ions to separate or enrich the different salts prior to crystallization through membrane separation, followed by thermal crystallization to produce a solid.

[0003] Thermal salt separation and crystallization processes include direct evaporation crystallization processes, salt-nitrate co-production and salt separation crystallization processes, and low-temperature crystallization processes. When the content of a certain salt in high-salt wastewater is relatively dominant, the direct evaporation crystallization process is often used to separate and recover the dominant salt component. When there is no dominant salt component in the wastewater, the salt recovery rate of the direct evaporation crystallization process is low. Sodium sulfate and sodium chloride can be crystallized in steps, crystallizing at a higher temperature to obtain sodium sulfate and at a lower temperature to obtain sodium chloride. This is called the salt-nitrate co-production process. The low-temperature crystallization process involves concentrating high-salt wastewater containing a mixture of sodium sulfate and sodium chloride at a high temperature to a certain concentration, followed by rapid cooling. This allows the crystallization of large quantities of solid sodium sulfate decahydrate (glauber's salt). Because the market price of Glauber's salt obtained through low-temperature crystallization is low and transportation costs are high, a hot melt evaporation crystallization unit is often added to produce anhydrous sodium sulfate (sodium sulfate) to increase product value. Since the low-temperature crystallization process only produces solid sodium sulfate, it must be combined with a high-temperature crystallization process to obtain sodium chloride. Due to the large solubility variation, the low-temperature crystallization process can achieve higher sodium sulfate and sodium chloride recovery rates. Furthermore, the purity of the crystallized salt is easier to control than in the salt and nitrate co-production process. The impact of organic matter on the whiteness of the crystallized salt is also less pronounced during the low-temperature crystallization process. The nanofiltration salt separation process mainly utilizes the selective retention characteristics of nanofiltration membranes for divalent salts to achieve the separation of monovalent salt sodium chloride and divalent salt sodium sulfate in the liquid phase. Sodium chloride mainly enters the nanofiltration permeate, while sodium sulfate is concentrated in the nanofiltration concentrated water. By crystallizing the nanofiltration permeate and the concentrated liquid respectively, the recovery of sodium chloride and sodium sulfate crystals is finally achieved.

[0004] However, some high-salinity wastewaters, such as landfill leachate, contain a large amount of potassium ions in addition to sodium ions. Traditional sodium-type high-salinity wastewater mainly contains three ions: sodium ions, chloride ions, and sulfate ions. After thermal or nanofiltration salt separation, two by-products, sodium chloride and sodium sulfate, can be generated. However, potassium-sodium coexistence type high-salinity wastewater mainly contains four ions: potassium ions, sodium ions, chloride ions, and sulfate ions. The combination of anions and cations can form four by-products: sodium chloride, sodium sulfate, potassium chloride, and potassium sulfate. At this time, thermal salt separation is difficult to obtain the four by-products separately through the difference in solubility of the four substances at different temperatures. Even if the nanofiltration salt separation process is used, the sodium sulfate and potassium sulfate on the nanofiltration concentrate side cannot be crystallized by thermal salt separation. Therefore, the zero discharge of potassium-sodium coexistence type high-salinity wastewater faces a huge challenge. If salt separation cannot be carried out, a large amount of mixed salt hazardous waste will be generated, and the disposal cost is high. Summary of the Invention

[0005] The purpose of the present invention is to provide a zero-discharge treatment process method and system for potassium-sodium coexistence type high-salt wastewater, thereby overcoming the shortcomings of the existing technology, realizing deep treatment of potassium-sodium coexistence type high-salt wastewater, and realizing salt separation, crystallization and resource utilization.

[0006] In order to achieve the above object, the technical solution of the present invention is: In a first aspect, the present invention provides a zero-discharge treatment process for potassium-sodium coexistence high-salt wastewater, comprising the following steps: The pretreated potassium-sodium coexistence high-salt wastewater is subjected to a first-stage reverse osmosis treatment to produce fresh water and a first-stage reverse osmosis effluent; The first stage reverse osmosis effluent is sequentially subjected to a first stage high density sedimentation, a first stage medium filtration, a first stage ultrafiltration and a second stage reverse osmosis treatment to obtain fresh water and the second stage reverse osmosis effluent; The second-stage reverse osmosis effluent is subjected to second-stage calcium sulfate crystallization to obtain calcium sulfate and second-stage calcium sulfate crystallization effluent; The effluent from the second stage calcium sulfate crystallization is sequentially subjected to second stage high-density precipitation, second stage medium filtration, second stage ultrafiltration, and third stage nanofiltration to obtain a third stage nanofiltration concentrate and a third stage nanofiltration permeate. The three-stage nanofiltration concentrate is subjected to three-stage calcium sulfate crystallization treatment to obtain calcium sulfate and three-stage calcium sulfate crystallization effluent; The three-stage calcium sulfate crystallization effluent is sequentially subjected to three-stage high-density precipitation, three-stage medium filtration and three-stage ultrafiltration to obtain three-stage ultrafiltration effluent; The effluent from the third stage ultrafiltration is refluxed and mixed with the effluent from the second stage ultrafiltration before entering the third stage nanofiltration treatment again.

[0007] In some other embodiments, the process further includes sequentially subjecting the three-stage nanofiltration permeate to ion exchange resin treatment, decarbonization, and four-stage reverse osmosis treatment to produce fresh water and four-stage reverse osmosis effluent; The effluent from the four-stage reverse osmosis is sequentially subjected to four-stage high-density precipitation, four-stage medium filtration, four-stage ultrafiltration and evaporation crystallization treatment to produce sodium chloride and evaporation crystallization effluent; The evaporation and crystallization water is cooled and flash-evaporated to produce potassium chloride, and the cooled and flash-evaporated water is returned to the evaporation crystallizer and evaporated and crystallized again to produce sodium chloride.

[0008] In some other embodiments, the pretreatment is to perform hardness removal, silicon removal, biochemical treatment, ozone catalytic oxidation, media filtration and hollow fiber ultrafiltration membrane filtration on the potassium and sodium coexistence high-salt wastewater; The agent used for the hardness and silicon removal treatment is lime milk, with an addition amount of 150-250 mg / L and a concentration of 8-12%; After pretreatment, the total silicon content of the wastewater must be less than 30 mg / L, COD less than 20 mg / L, SDI less than 3, fluoride less than 3 mg / L, and calcium ion 100-200 mg / L.

[0009] In some other embodiments, the recovery rate of the reverse osmosis treatment is 70-80%, and the membrane flux is 15-18 L / (m 2 ·h), operating pressure is 1.4-1.6MPa; Alternatively, the reagents added in the first stage of high-density precipitation are sodium hydroxide and sodium carbonate, the addition point is the mixing area of the high-density precipitation tank, the addition amount of sodium hydroxide is 75-85 mg / L, the addition concentration is 25-35%, and the addition amount of sodium carbonate is 400-450 mg / L, and the addition concentration is 18-25%; The silicon content in the effluent from the first stage high-density sedimentation tank is less than 30 mg / L, and the calcium ion content is 200-500 mg / L.

[0010] In some other embodiments, the recovery rate of the second-stage reverse osmosis treatment is 60%-80%, and the membrane flux is 15-18 L / (m 2 ·h), operating pressure is 1.4-1.6MPa; Alternatively, the effluent from the second stage of calcium sulfate crystallization has a sulfate concentration of 5000-6000 mg / L and a calcium ion concentration of 1200-1600 mg / L; Alternatively, the reagent added to the second-stage high-density precipitation is sodium carbonate, the dosage of sodium carbonate is 1800-1900 mg / L, the addition concentration is 18-25%, and the silicon content in the effluent of the second-stage high-density precipitation is less than 30 mg / L and the calcium ion content is 400-800 mg / L.

[0011] In some other embodiments, the recovery rate of the three-stage nanofiltration is 50-75%, and the membrane flux is 15-18 L / (m 2 ·h), operating pressure is 2.8-3.2MPa; Alternatively, the reagent added to the three-stage calcium sulfate crystallization is calcium chloride, the calcium chloride addition amount is 16000-16500 mg / L, the addition concentration is 18-22%, and the sulfate concentration in the effluent of the three-stage calcium sulfate crystallization is 6000-8000 mg / L and the calcium ion concentration is 1400-1800 mg / L; Alternatively, the reagent added to the three-stage high-density precipitation is sodium carbonate, the dosage of sodium carbonate is 1500-1600 mg / L, the addition concentration is 18-22%, and the calcium ion content in the effluent of the three-stage high-density precipitation is 800-1200 mg / L.

[0012] In some other embodiments, the hardness of the effluent from the ion exchange resin is less than 1 mg / L, and the bicarbonate in the effluent from the decarbonization is less than 20 mg / L. Or, the recovery rate of the four-stage reverse osmosis is 40%-60%, the TDS of the effluent is 70000-80000 mg / L, and the membrane flux is 15-18 L / (m 2 ·h), operating pressure is 6.8-7.2MPa; Alternatively, the reagents added to the four-stage high-density sedimentation tank are sodium hydroxide and sodium aluminate, with a sodium hydroxide dosage of 35-45 mg / L and a dosage concentration of 25-35%, and a sodium aluminate dosage of 160-170 mg / L and a dosage concentration of 8-12%, and the silicon content in the effluent is less than 20 mg / L.

[0013] In some other embodiments, the evaporation crystallization temperature is 100-110° C., and the sodium chloride content is greater than 96%; The cooling flash temperature is 40-50°C and the potassium oxide content is greater than 60%; Alternatively, part of the evaporated crystallization mother liquor is discharged for drying.

[0014] In a second aspect, the present invention provides a zero-discharge treatment system for potassium-sodium coexistence high-salt wastewater, which is used to implement the zero-discharge treatment process method for potassium-sodium coexistence high-salt wastewater described in the first aspect, comprising a pretreatment unit, a first-stage reverse osmosis unit, a first-stage high-density sedimentation tank, a first-stage medium filtration tank, a first-stage ultrafiltration tank, a second-stage reverse osmosis unit, a second-stage calcium sulfate crystallization tank, a second-stage high-density sedimentation tank, a second-stage medium filtration tank, a second-stage ultrafiltration tank, and a third-stage nanofiltration unit connected in sequence; The three-stage nanofiltration unit is provided with a permeate outlet of the three-stage nanofiltration unit and a concentrate outlet of the three-stage nanofiltration unit; It also includes three sections of calcium sulfate crystallization tanks, three sections of high-density sedimentation tanks, three sections of medium filtration tanks, and three sections of ultrafiltration tanks, which are sequentially connected to the concentrated liquid outlet of the three-section nanofiltration unit; The water outlet of the three-stage ultrafiltration pool is connected to the water inlet of the three-stage nanofiltration unit.

[0015] In some other embodiments, the permeate outlet of the three-stage nanofiltration unit is sequentially connected to an ion exchange resin membrane, a decarbonization tank, a four-stage reverse osmosis unit, a four-stage high-density sedimentation tank, a four-stage medium filtration tank, an evaporation crystallizer, and a cooler; The outlet of the cooler is connected to the inlet of the evaporation crystallizer.

[0016] Beneficial effects of the present invention: (1) The first-stage reverse osmosis, second-stage reverse osmosis, fourth-stage reverse osmosis and condensed water from evaporation and crystallization used in the present invention can be recycled as high-quality recycled water. Wastewater is no longer discharged into natural water bodies, thus avoiding the damage to the water ecological environment caused by high-concentration wastewater and the waste of potassium resources.

[0017] (2) The calcium sulfate (gypsum) by-product recovered by the present invention can also be reused as a building decoration material, and the sodium chloride by-product can be reused as a snow melting agent or for producing acids and alkalis.

[0018] (3) The present invention can be widely used in industries such as landfill leachate, waste incineration fly ash, new energy battery industry, and semiconductor industry. Their high-salt wastewater contains four ions, namely potassium ions, sodium ions, chloride ions, and sulfate ions, and the concentration of sulfate in anions is greater than 50%; the concentration of potassium ions in cations is also greater than 20%. The zero-discharge treatment of high-salt wastewater in these industries will be conducive to the resource recovery and utilization of industrial salt.

[0019] In summary, this invention, through innovative treatment methods and systems, not only achieves advanced wastewater treatment but also partially recovers byproducts such as calcium sulfate (gypsum), potassium chloride, and sodium chloride during the treatment process, thus avoiding the difficult disposal of large amounts of mixed salt waste generated by traditional evaporation and crystallization. The recovery of valuable potassium chloride from high-salinity wastewater containing both potassium and sodium has important strategic value as a fertilizer. BRIEF DESCRIPTION OF THE DRAWINGS

[0020] The accompanying drawings, which constitute a part of the present invention, are used to provide a further understanding of the present invention. The exemplary embodiments of the present invention and their descriptions are used to explain the present invention and do not constitute improper limitations on the present invention.

[0021] Figure 1 This is a flow chart of the zero-discharge treatment process for potassium-sodium coexistence high-salt wastewater in Example 1 of the present invention; Figure 2 This is a flow chart of the zero-discharge treatment process for potassium-sodium coexistence high-salt wastewater in Comparative Example 1 of the present invention. DETAILED DESCRIPTION

[0022] Those skilled in the art will appreciate that the following examples are intended to illustrate the present invention only and should not be construed as limiting the scope of the invention. Unless otherwise specified, the examples were performed under conventional conditions or manufacturer's recommended conditions. Components used without manufacturer's indication are commercially available conventional products.

[0023] The TDS of potassium-sodium coexisting high-salt wastewater is 2000-6000 mg / L. The anions are mainly chloride ions and sulfate ions, and the sulfate concentration accounts for more than 50%; the cations are mainly potassium ions and sodium ions, and the potassium ion concentration accounts for more than 20%. The treatment process includes the following steps: First, pretreatment is performed on high-salinity wastewater containing both potassium and sodium to remove hardness and silicon to reduce scaling risk in subsequent membranes and evaporation crystallization reactors. The pretreatment unit utilizes a chemical method for synergistic silicon and hardness removal, using lime milk (Ca(OH)2) as a reagent. This removes only temporary hardness from the wastewater while retaining some calcium ions, thereby reducing the amount of calcium chloride required for the subsequent calcium sulfate crystallization unit. The pretreatment unit effluent contains 100-200 mg / L of calcium ion, and the calcium sulfate saturation is controlled below 40% by adjusting the lime milk dosage. Calcium silicate and calcium fluoride are also partially removed during the hardness removal process. The total silicon in the pretreatment unit effluent must be less than 30 mg / L (as SiO2), and the fluorine must be less than 3 mg / L to prevent scaling of SiO2 and calcium fluoride in the first-stage reverse osmosis concentrate. Furthermore, the pretreatment unit utilizes biochemical treatment or advanced oxidation methods to control COD to less than 20 mg / L. Media filtration and ultrafiltration membrane filtration are also used to control the SDI to less than 3, meeting the influent requirements of the first-stage reverse osmosis process and preventing fouling. The effluent from the pretreatment unit first enters a reverse osmosis unit for preliminary concentration. The recovery rate of the reverse osmosis unit is 70%-80%. The concentrations of various ions in the wastewater are enriched, and scaling substances such as calcium ions, silicon, and fluorine are also concentrated. At this time, the calcium sulfate saturation in the wastewater is less than 200%, and conventional scale inhibitors can be used to prevent calcium sulfate scaling. In order to avoid subsequent reverse osmosis membrane scaling, the concentrated water from the reverse osmosis unit enters a high-density sedimentation tank for synergistic hardness removal and silicon removal. The agents used are sodium hydroxide and sodium carbonate, which not only remove temporary hardness in the wastewater but also remove part of the permanent hardness. However, some calcium ions in the wastewater are retained to reduce the dosage of calcium chloride in the subsequent calcium sulfate crystallization unit. The calcium ion is 200-50 0mg / L, the first stage high-density sedimentation tank unit controls the calcium sulfate saturation to less than 100% by adjusting the dosage of sodium hydroxide and sodium carbonate to avoid calcium sulfate scaling before the wastewater enters the second stage reverse osmosis. Calcium silicate and calcium fluoride will also be partially removed during the hardness removal process, controlling the effluent silicon to less than 30mg / L (in terms of SiO2). The effluent from the first stage high-density sedimentation tank enters the first stage medium filtration unit and the first stage ultrafiltration unit to reduce SDI and slow down the subsequent reverse osmosis membrane fouling. The effluent from the first stage ultrafiltration unit enters the second stage reverse osmosis unit for re-concentration. The recovery rate of the second stage reverse osmosis unit is 60%-80%. The calcium sulfate saturation of the second stage reverse osmosis unit is controlled to 400%-500% by adjusting the recovery rate and adding high-efficiency scale inhibitors; The concentrated water from the second-stage reverse osmosis unit enters the second-stage calcium sulfate crystallization unit. As the scale inhibitor becomes ineffective, the supersaturated calcium sulfate will destabilize and crystallize. At the same time, due to insufficient calcium ions and a large number of sulfate ions, calcium chloride must be added to forcibly remove some sulfate ions and obtain a by-product of calcium sulfate (gypsum). The sulfate concentration of the effluent from the second-stage calcium sulfate crystallization unit is controlled to be 5000-6000 mg / L and the calcium ion concentration is 1200-1600 mg / L. The supersaturation of calcium sulfate in the effluent from the second-stage calcium sulfate crystallization unit is controlled to be 120%-150% by adjusting the dosage of calcium chloride. In order to prevent supersaturated calcium sulfate from entering the subsequent nanofiltration unit and causing scaling and clogging, the effluent from the second-stage calcium sulfate crystallization unit enters the second-stage high-density sedimentation tank unit to continue removing hardness and silicon. The added agent is sodium carbonate, and the calcium ion is 400-800 mg / L. The calcium sulfate saturation is controlled to be less than 100% by adjusting the sodium carbonate dosage to avoid calcium sulfate scaling before the wastewater enters the third-stage nanofiltration. In the process of hardness removal, calcium silicate and calcium fluoride will also be partially removed to control the effluent silicon to be less than 30 mg / L (in terms of SiO2).

[0024] The effluent from the second-stage high-density sedimentation tank unit enters the second-stage medium filtration unit and the second-stage ultrafiltration unit to reduce SDI and slow down the subsequent nanofiltration membrane fouling. The effluent from the second-stage ultrafiltration unit enters the third-stage nanofiltration unit. The third-stage nanofiltration unit concentrates sulfate while also separating monovalent and divalent ions. The third-stage nanofiltration unit uses high-pressure nanofiltration, and its sulfate retention rate is greater than 99%. Chloride ions are not retained or negatively retained. The recovery rate of the third-stage nanofiltration unit is 50-75%. The third-stage nanofiltration unit controls the calcium sulfate saturation to 400%-500% by adjusting the recovery rate and adding high-efficiency scale inhibitors. The concentrate from the three-stage nanofiltration unit enters the three-stage calcium sulfate crystallization unit. As the scale inhibitor expires, the supersaturated calcium sulfate destabilizes and crystallizes. Simultaneously, due to insufficient calcium ions and a high sulfate ion concentration, calcium chloride is added to forcibly remove some sulfate, resulting in the byproduct calcium sulfate (gypsum). The effluent from the three-stage calcium sulfate crystallization unit is controlled to have a sulfate concentration of 6000-8000 mg / L and a calcium ion concentration of 1400-1800 mg / L. The calcium sulfate supersaturation in the effluent is controlled at 120%-150% by adjusting the calcium chloride dosage. To prevent scaling and blockage caused by supersaturated calcium sulfate in the subsequent nanofiltration units, the effluent from the three-stage calcium sulfate crystallization unit enters the three-stage high-density sedimentation tank unit for further hardness removal. Sodium carbonate is added to the effluent, resulting in a calcium ion concentration of 800-1200 mg / L. The sodium carbonate dosage is adjusted to maintain the calcium sulfate saturation below 100% to prevent calcium sulfate scaling before the wastewater enters the nanofiltration membrane. The effluent from the three-stage high-density sedimentation tank unit enters the three-stage medium filtration unit and the three-stage ultrafiltration unit to reduce SDI and slow down the subsequent nanofiltration membrane fouling. The effluent from the three-stage ultrafiltration unit flows back and is mixed with the effluent from the two-stage ultrafiltration unit before entering the three-stage nanofiltration unit again.

[0025] The main components of fresh water in the three-stage nanofiltration unit are sodium chloride and potassium chloride. It enters the ion exchange resin for deep hardness removal, and the water hardness is less than 1 mg / L. The resin effluent enters the decarbonizer, and the bicarbonate in the decarbonizer effluent is less than 20 mg / L. It then enters the four-stage reverse osmosis unit for deep concentration. The recovery rate of the four-stage reverse osmosis unit is 40%-60%. The TDS of the concentrated brine is controlled at 70,000-80,000 mg / L. The brine enters the four-stage high-density sedimentation tank for deep silicon removal, and the silicon content of the effluent is less than 20 mg / L to prevent scaling of the evaporation and crystallization reactor. After deep silicon removal, it enters the four-stage media filtration unit and the four-stage ultrafiltration unit. The effluent of the four-stage ultrafiltration unit is first evaporated and crystallized at 100-110℃ to obtain sodium chloride. The potassium-rich mother liquor is then cooled to 40-50℃ and flash evaporated to obtain potassium chloride. The resulting mother liquor returns to the sodium chloride evaporator for cyclic heating and concentration treatment; a small amount of mother liquor is discharged for drying.

[0026] The technical solution and effects of the present application are further illustrated by the following examples: Example 1 The wastewater discharge of a new energy battery company is 40,000m 3 / d. The (texturing) cleaning process of the production process requires the addition of raw materials such as HF, HCL, H2O2, KOH, additives, etc., and produces acidic wastewater (HF, HCL) and alkaline wastewater (KOH, organic matter); the etching process requires the addition of raw materials such as HNO3, HF, HCL, KOH, H2SO4, etc., and produces alkaline wastewater (KOH, organic matter) and acidic nitrogen wastewater (F - 、NO3 - 、SO4 2- ), using KOH instead of NaOH in the cleaning and etching process can help improve product quality, but at the same time the discharged wastewater also contains a large amount of potassium resources that can be recovered.

[0027] The final wastewater discharged by the enterprise has a COD of 160 mg / L, a fluoride concentration of 4 mg / L, a total silicon content (as SiO2) of 100 mg / L, a hardness of 450 mg / L (as CaCO3), and a total salt content of 2600 mg / L (including chloride ions of 350 mg / L, sulfate ions of 1000 mg / L, bicarbonate ions of 120 mg / L, sodium ions of 230 mg / L, and potassium ions of 480 mg / L). The water quality is characterized by a high total salt content, primarily containing four ions: potassium, sodium, chloride, and sulfate. Sulfate accounts for more than 70% of the anions, and potassium ions account for more than 60% of the cations, making it a typical high-salinity wastewater characterized by the coexistence of potassium and sodium.

[0028] Discharge requirements for high-salinity wastewater are becoming increasingly stringent. Nationally, the primary standard of the Integrated Wastewater Discharge Standard (GB 8978-1996) requires chloride ions ≤ 1000 mg / L and sulfate ≤ 600 mg / L. Agricultural irrigation waters must also comply with the Farmland Irrigation Water Quality Standard (GB 5084-2021), which mandates total salt content ≤ 1000 mg / L and chloride ions ≤ 350 mg / L. Industry standards include the Petrochemical Industry Pollutant Discharge Standard (GB 31571-2015) requiring chloride ions ≤ 500 mg / L. The coal chemical and coking industries typically require that the TDS of discharged high-salinity wastewater after treatment be ≤ 1000 mg / L. Local standards include Shandong Province's "Integrated Discharge Standard for River Basin Water Pollutants" (DB 37 / 3416-2023), which mandates a total salinity of less than 3,000 mg / L. Shanxi Province's "Integrated Wastewater Discharge Standard" (revised in 2019) mandates a total salinity of ≤1,000 mg / L in the Fenhe River Basin, ≤2,000 mg / L in general areas, and ≤1,500 mg / L for wastewater discharged into the Yellow River. Industrial parks and water-intensive industries (such as coal chemical and power generation) are even required to achieve zero wastewater discharge.

[0029] Since the cations contain a large amount of potassium ions, traditional thermal salt separation or nanofiltration salt separation cannot achieve the purification and separation of various crystalline salts under the coexistence of four ions. In order to deeply treat the wastewater of this enterprise and recover the beneficial resources in the wastewater, this embodiment proposes a zero-discharge treatment process for high-salt wastewater with potassium and sodium coexistence. The water volume and water quality of each main unit are shown in Table 1, and the treatment process is as follows: Figure 1 As shown, the specific steps include: The wastewater undergoes pretreatment, including hardness and silicon removal, to reduce scaling risks in subsequent membrane and evaporation crystallization reactors. This hardness and silicon removal process utilizes a high-density sedimentation tank process, employing a chemically synergistic approach to remove silicon and hardness. The agent used is lime milk (Ca(OH)2), added at a dosage of 200 mg / L and a 10% concentration in the mixing zone of the high-density sedimentation tank. This removes temporary hardness from the wastewater, resulting in effluent calcium ion concentrations of 147 mg / L and bicarbonate ion concentrations of 20 mg / L. The calcium sulfate saturation is controlled at 29% by adjusting the lime milk dosage. Calcium silicate and calcium fluoride are also partially removed during the hardness removal process, resulting in effluent total silicon concentrations of 28 mg / L (as SiO2) and fluorine concentrations of 2 mg / L, to prevent scaling of SiO2 and calcium fluoride in the first-stage reverse osmosis concentrate.

[0030] In addition, the pretreatment unit must also use a combination of AAO biochemical treatment and ozone catalytic oxidation to control COD to less than 20 mg / L, and at the same time control SDI to less than 3 through media filtration and hollow fiber ultrafiltration membrane filtration to meet the first-stage reverse osmosis water inlet requirements.

[0031] The effluent from the pretreatment unit first enters a reverse osmosis unit (abbreviated as I-RO) for preliminary concentration. The recovery rate of the reverse osmosis unit is 75%, and the membrane flux is 16L / (m 2 ·h), the operating pressure is 1.5MPa, at this time, the concentrations of various ions in the wastewater are enriched, and scaling substances such as calcium ions, silicon, and fluorine are also concentrated; in order to avoid subsequent reverse osmosis membrane scaling, the concentrated water from the first reverse osmosis unit enters a high-density sedimentation tank (abbreviated as I-high-density tank) for synergistic hardness removal and silicon removal. The reagents used are sodium hydroxide and sodium carbonate, and the addition point is the mixing area of the high-density sedimentation tank. It not only removes temporary hardness in the wastewater but also removes part of the permanent hardness, but retains some calcium ions in the wastewater to reduce the dosage of calcium chloride in the subsequent calcium sulfate crystallization unit. The calcium ion content of the effluent from the first high-density sedimentation tank is 432mg / L, and the calcium sulfate saturation of the first high-density sedimentation tank is controlled at 91%. At this time, the dosage of sodium hydroxide is 80mg / L, the dosage concentration is 30%, and the dosage of sodium carbonate is 430mg / L, the dosage concentration is 20%.

[0032] During the hardness removal process, calcium silicate and calcium fluoride are also partially removed, and the silicon content of the effluent is controlled at 28 mg / L (in terms of SiO2). The effluent from the first high-density sedimentation tank enters the first medium filtration unit and the first hollow fiber ultrafiltration unit to reduce SDI and slow down the subsequent reverse osmosis membrane fouling. The effluent from the first ultrafiltration unit enters the second reverse osmosis unit (abbreviated as II-RO) for re-concentration. The recovery rate of the second reverse osmosis unit is 75%, and the membrane flux is 16 L / (m 2 h), the operating pressure was 3.5 MPa, and the calcium sulfate saturation of the second-stage reverse osmosis unit was controlled at 426% by adjusting the recovery rate and adding a high-efficiency scale inhibitor (compound phosphonic acid-polycarboxylic acid type calcium sulfate high-efficiency scale inhibitor CalTreat® SC-200); The concentrated water from the second-stage reverse osmosis unit enters the second-stage calcium sulfate crystallization unit (abbreviated as II-calcium sulfate crystallization). As the scale inhibitor fails, the supersaturated calcium sulfate will destabilize and crystallize. At the same time, due to insufficient calcium ions and a large number of sulfate ions, calcium chloride must be added to forcibly remove part of the sulfate and obtain a by-product of calcium sulfate (gypsum). The sulfate concentration of the effluent from the second-stage calcium sulfate crystallization unit is controlled to be 5500 mg / L and the calcium ion concentration is 1400 mg / L. The supersaturation of calcium sulfate in the effluent of the second-stage calcium sulfate crystallization unit is controlled to be 146% by adjusting the dosage of calcium chloride. At this time, the dosage of calcium chloride is 10800 mg / L, the dosage concentration is 20%, and the addition point is the mixing zone of the second-stage calcium sulfate crystallization unit.

[0033] In order to prevent supersaturated calcium sulfate from entering the subsequent nanofiltration unit and causing scaling and clogging, the effluent from the second-stage calcium sulfate crystallization unit enters the second-stage high-density sedimentation tank unit (abbreviated as II-high-density tank) to continue removing hardness and silicon. The added agent is sodium carbonate, and the effluent calcium ion is controlled at 700 mg / L. The calcium sulfate saturation is controlled at 75% by adjusting the sodium carbonate dosage. At this time, the sodium carbonate dosage is 1850 mg / L, the dosage concentration is 20%, and the addition point is the mixing area of the second-stage high-density sedimentation tank. In the process of hardness removal, calcium silicate and calcium fluoride will also be partially removed, and the effluent silicon is controlled to 28 mg / L (calculated as SiO2).

[0034] The effluent from the second-stage high-density sedimentation tank unit enters the second-stage medium filtration unit and the second-stage ultrafiltration unit to reduce SDI and slow down the subsequent reverse osmosis membrane fouling. The effluent from the second-stage ultrafiltration unit enters the third-stage nanofiltration unit (abbreviated as III-NF). The third-stage nanofiltration unit concentrates sulfate while also separating monovalent and divalent ions. The third-stage nanofiltration unit uses high-pressure nanofiltration, and its sulfate retention rate is greater than 99%. Chloride ions are not retained or negatively retained. The recovery rate of the third-stage nanofiltration unit is 71%, and the membrane flux is 16L / (m 2 ·h), the operating pressure was 3.0MPa, and the three-stage nanofiltration unit controlled the calcium sulfate saturation to 408% by adjusting the recovery rate and adding high-efficiency scale inhibitors.

[0035] The concentrated water from the three-stage nanofiltration unit enters the three-stage calcium sulfate crystallization unit (abbreviated as III-calcium sulfate crystallization). As the scale inhibitor fails, the supersaturated calcium sulfate will destabilize and crystallize. At the same time, due to insufficient calcium ions and a large number of sulfate ions, calcium chloride must be added to forcibly remove part of the sulfate and obtain a by-product of calcium sulfate (gypsum). The sulfate concentration of the effluent from the three-stage calcium sulfate crystallization unit is controlled to be 7500 mg / L and the calcium ion concentration is 1600 mg / L. The three-stage calcium sulfate crystallization unit controls the supersaturation of calcium sulfate in the effluent to 134% by adjusting the dosage of calcium chloride. At this time, the calcium chloride dosage is 16400 mg / L, the dosage concentration is 20%, and the addition point is the mixing area of the three-stage calcium sulfate crystallization unit.

[0036] To prevent supersaturated calcium sulfate from entering the subsequent nanofiltration units and causing scaling and clogging, the effluent from the three-stage calcium sulfate crystallization unit enters the three-stage high-density sedimentation tank unit (abbreviated as III-high-density tank) for further hardness removal. Sodium carbonate is added to control the effluent calcium ion content at 1000 mg / L. The calcium sulfate saturation is controlled at 84% by adjusting the sodium carbonate dosage. At this time, the sodium carbonate dosage is 1590 mg / L, the addition concentration is 20%, and the addition point is the mixing zone of the three-stage high-density sedimentation tank. The effluent from the three-stage high-density sedimentation tank unit enters the three-stage media filtration unit and the three-stage ultrafiltration unit to reduce SDI and mitigate subsequent reverse osmosis membrane fouling. The effluent from the three-stage ultrafiltration unit is returned and mixed with the effluent from the second-stage ultrafiltration unit before entering the three-stage nanofiltration unit again.

[0037] The main components of fresh water in the three-stage nanofiltration unit are sodium chloride and potassium chloride. It first enters the ion exchange resin for deep hardness removal, and the water hardness is 0.8 mg / L. The resin effluent enters the decarbonizer, and the bicarbonate of the decarbonizer effluent is 15 mg / L. Then it enters the four-stage reverse osmosis unit (abbreviated as IV-RO) for deep concentration. The recovery rate of the four-stage reverse osmosis unit is 45%, and the membrane flux is 16L / (m 2 h), the operating pressure is 7.0 MPa, the TDS of the brine after deep concentration is controlled at 74277 mg / L, the brine enters the four-stage high-density sedimentation tank for deep silicon removal, sodium hydroxide and sodium metaaluminate are used as silicon removal agents, the sodium hydroxide dosage is 40 mg / L, the dosage concentration is 30%, the addition point is the mixing zone of the four-stage high-density sedimentation tank, the sodium metaaluminate dosage is 165 mg / L, the dosage concentration is 10%, the effluent silicon is controlled at 15 mg / L, the addition point is the mixing zone of the four-stage high-density sedimentation tank, and after deep silicon removal, it enters the four-stage medium filtration unit and the four-stage ultrafiltration unit.

[0038] The effluent from the four-stage ultrafiltration unit enters the evaporation and crystallization unit, where it is first evaporated and crystallized at 100-110°C to obtain sodium chloride with a sodium chloride content greater than 96%. The potassium-rich mother liquor is then cooled to 40-50°C and flash evaporated to obtain potassium chloride with a converted potassium oxide content greater than 60%. The resulting mother liquor is returned to the sodium chloride evaporator for cyclic heating and concentration treatment; a small portion of the mother liquor is discharged for drying.

[0039] Table 1 shows the water volume and water quality of each major unit (flow unit is m 3 / d, concentration unit is mg / L)

[0040] As can be seen from Table 1, the water supply scale of photovoltaic enterprises is 40,000m 3 / d, the main cations in water are K + 、Na + Mainly, the anion is mainly SO4 2- and Cl - The main component that easily causes scaling pollution in membrane separation system is Ca 2+ , all silicon, F - 、COD、NH4-N、NO3 - 、HCO3 - Before the wastewater enters the membrane separation system, it must be pre-treated. After pre-treatment (Ⅰ-before RO), Ca 2+ Reduced to 147mg / L, total silicon reduced to 28mg / L, F - Reduced to 2mg / L, COD reduced to 20mg / L, NH4-N reduced to 2mg / L, NO3 - Reduced to 5mg / L, HCO3 - Reduced to 20mg / L, thus ensuring the water quality requirements of the membrane separation system, and introducing Na + Causes Na + After entering the membrane separation system, the concentrated water volume is concentrated to 10000m3 after I-RO concentration. 3 / d, K in concentrated water + Reaching 1877mg / L, Na + Reaching 1045mg / L, SO4 2- Reaching 3940mg / L, Cl - Reaching 1552 mg / L, at the same time, other major components that easily cause scaling and pollution of membrane separation systems, such as Ca 2+ , all silicon, F - , COD, HCO3 -The concentrated water volume is further concentrated to 2500m3 and the concentrated water volume is further concentrated to 35909mg / L. 3 / d. Ⅱ-RO concentrated water enters Ⅱ-calcium sulfate crystallization unit to precipitate calcium sulfate (gypsum), and SO4 in the effluent of Ⅱ-calcium sulfate crystallization unit 2- From 15524mg / L to 5500mg / L, Ca 2+ Reduced from 1700mg / L to 1400mg / L, the effluent from the II-calcium sulfate crystallization unit enters the II-high density pool for effluent hardness removal, Ca 2+ The effluent from the II-high density tank enters the III-NF unit. The TDS of the permeate (fresh water) of the III-NF unit is 41513 mg / L. After deep hardness removal and decarbonization, it enters the IV-RO unit for deep concentration. The TDS of the concentrated water of the IV-RO unit reaches 74277 mg / L, and the water volume is reduced to 2067m 3 / d, then enters the evaporation crystallization unit and cooling unit to produce sodium chloride and potassium chloride. The concentrated liquid TDS of the III-NF unit reaches 74610mg / L, of which SO4 2- Reaching 21962mg / L, Ca 2+ Reaching 1802mg / L, the concentrated liquid of the III-NF unit enters the III-calcium sulfate crystallization to precipitate calcium sulfate (gypsum), and the SO4 in the water of the III-calcium sulfate crystallization unit is 2- From 21962mg / L to 7500mg / L, Ca 2+ The effluent from the III-calcium sulfate crystallization unit enters the III-high density pool to remove the hardness of the effluent. 2+ The concentration of water in the III-NF unit is reduced from 1600 mg / L to 1000 mg / L to avoid scaling in the subsequent III-NF unit. The effluent from the III-high-density pool is filtered through media and ultrafiltration before returning to the III-NF unit for recycling treatment.

[0041] Ⅲ-NF unit with SO4 2- Further concentration and SO4 2- and Cl - The main component of III-NF concentrated water is sodium sulfate, and the TDS reaches 82771 mg / L, which can enter the subsequent evaporation crystallization unit to produce sodium sulfate; the main component of III-NF fresh water is sodium chloride, and the TDS is reduced to 34962 mg / L. It needs to be further concentrated by IV-RO before entering the subsequent evaporation crystallization unit. After IV-RO concentration, the water volume is concentrated to 945m 3 / d, TDS reaches 76149 mg / L, and can enter the subsequent evaporation crystallization unit to produce sodium chloride.

[0042] After treatment using the present invention, the mixed TDS of the first-stage, second-stage, and fourth-stage reverse osmosis freshwater has a value of less than 200 mg / L. This high-quality desalinated water can be used as a substitute for conventional water resources and reused in production processes, reducing the company's fresh water consumption. Furthermore, the potassium chloride produced using the present invention meets the Class I, first-grade requirements of "Potassium Chloride" (GB / T6549-2011) (potassium oxide content greater than 60%), and the sodium chloride meets the premium grade standard for refined industrial wet salt (sodium chloride content greater than 96%) specified in "Industrial Salt" (GB / 5462-2015).

[0043] Example 2 A potassium-sodium coexistence type high-salt wastewater zero-discharge treatment system comprises a pretreatment unit, a first-stage reverse osmosis unit, a first-stage high-density sedimentation tank, a first-stage medium filtration tank, a first-stage ultrafiltration tank, a second-stage reverse osmosis unit, a second-stage calcium sulfate crystallization tank, a second-stage high-density sedimentation tank, a second-stage medium filtration tank, a second-stage ultrafiltration tank, and a third-stage nanofiltration unit connected in sequence; The three-stage nanofiltration unit is provided with a permeate outlet of the three-stage nanofiltration unit and a concentrate outlet of the three-stage nanofiltration unit; The permeate outlet of the three-stage nanofiltration unit is connected to the ion exchange resin membrane, the decarbonization tank, the four-stage reverse osmosis unit, the four-stage high-density sedimentation tank, the four-stage medium filtration tank, the evaporation crystallizer, and the cooler in sequence; The outlet of the cooler is connected to the inlet of the evaporation crystallizer.

[0044] It also includes three sections of calcium sulfate crystallization tanks, three sections of high-density sedimentation tanks, three sections of medium filtration tanks, and three sections of ultrafiltration tanks, which are sequentially connected to the concentrated liquid outlet of the three-section nanofiltration unit; The water outlet of the three-stage ultrafiltration pool is connected to the water inlet of the three-stage nanofiltration unit.

[0045] The various treatment units and components in the potassium-sodium coexistence type high-salt wastewater zero-discharge treatment system used are all existing structures and can use equipment known to those skilled in the art.

[0046] Comparative Example 1 The treatment process of a certain industrial wastewater treatment plant specializing in treating photovoltaic solar cell wastewater is different from that of Example 1 in that calcium sulfate crystallization is not used to convert SO4 2- From the wastewater, only SO4 2- and Cl -The nanofiltration concentrate obtained by separation is mainly composed of potassium sulfate and sodium sulfate. It is impossible to use the difference in solubility of potassium sulfate and sodium sulfate to perform evaporation and crystallization at different temperatures to achieve salt separation and purification. Therefore, a large amount of potassium sulfate and sodium sulfate salts will eventually be produced. The salts need to be disposed of as hazardous waste, which is very expensive. The specific treatment process used is as follows: Figure 2 As shown: The treatment process is as follows: potassium-sodium coexistence high-salt wastewater is sequentially treated by a pretreatment unit, a first-stage reverse osmosis unit, a first-stage high-density sedimentation tank unit, a first-stage medium filtration unit, a first-stage ultrafiltration unit, and a second-stage reverse osmosis unit (the above process is the same as that in Example 1). The concentrated water from the second-stage reverse osmosis enters the second-stage high-density sedimentation tank for hardness removal, and the effluent from the second-stage high-density sedimentation tank enters the second-stage medium filtration and the second-stage ultrafiltration for further removal of pollutants to reduce SDI. The effluent from the second-stage ultrafiltration enters the third-stage nanofiltration for salt separation. The treatment process of the permeate from the third-stage nanofiltration unit is the same as that in Example 1; the concentrated liquid from the third-stage nanofiltration unit enters the third-stage high-density sedimentation tank for hardness removal, and then undergoes third-stage medium filtration and third-stage ultrafiltration to remove pollutants again to reduce SDI. The effluent from the third-stage ultrafiltration enters the ion exchange resin II for deep hardness removal. After deep hardness removal, the effluent enters the evaporation crystallization II for evaporation to obtain potassium sulfate and sodium sulfate salts, which are disposed of as hazardous waste.

[0047] The main difference from the process in Example 1 is that the second stage reverse osmosis concentrated water does not use calcium sulfate crystallization to convert SO4 2- It is removed from the wastewater by only using a high-density sedimentation tank to remove the hardness before entering the subsequent three-stage nanofiltration unit. The concentrate of the three-stage nanofiltration unit does not only use calcium sulfate crystallization to remove SO4 2- It is removed from the wastewater by only taking a high-density sedimentation tank to remove the hardness and then entering the subsequent evaporation crystallization. The resulting nanofiltration concentrate is mainly composed of potassium sulfate and sodium sulfate. It is impossible to use the difference in solubility of potassium sulfate and sodium sulfate to perform evaporation crystallization at different temperatures to achieve salt separation and purification. Therefore, a large amount of potassium sulfate and sodium sulfate salts will eventually be produced, and the salts need to be disposed of as hazardous waste. Compared with Example 1, the salt recovery rate in Example 2 is less than 30%. A large amount of salt is separated from the system in the form of salts. The precious salt resources are not recycled. At the same time, the salts need to be disposed of in accordance with the relevant hazardous waste disposal standards. The disposal cost is high, resulting in excessively high costs for sewage treatment plants.

[0048] The foregoing description is merely a preferred embodiment of the present invention and is not intended to limit the present invention. Those skilled in the art will readily appreciate that various modifications and variations of the present invention are possible. Any modifications, equivalent substitutions, or improvements made within the spirit and principles of the present invention are intended to be within the scope of protection of the present invention.

Claims

1. A zero-discharge treatment process for potassium-sodium coexistence high-salt wastewater, characterized in that: The following steps are involved: The pretreated potassium-sodium coexistence high-salt wastewater is subjected to a first-stage reverse osmosis treatment to produce fresh water and a first-stage reverse osmosis effluent; The first stage reverse osmosis effluent is sequentially subjected to a first stage high density sedimentation, a first stage medium filtration, a first stage ultrafiltration and a second stage reverse osmosis treatment to obtain fresh water and the second stage reverse osmosis effluent; The second-stage reverse osmosis effluent is subjected to second-stage calcium sulfate crystallization to obtain calcium sulfate and second-stage calcium sulfate crystallization effluent; The effluent from the second stage calcium sulfate crystallization is sequentially subjected to second stage high-density precipitation, second stage medium filtration, second stage ultrafiltration, and third stage nanofiltration to obtain a third stage nanofiltration concentrate and a third stage nanofiltration permeate. The three-stage nanofiltration concentrate is subjected to three-stage calcium sulfate crystallization treatment to obtain calcium sulfate and three-stage calcium sulfate crystallization effluent; The three-stage calcium sulfate crystallization effluent is sequentially subjected to three-stage high-density precipitation, three-stage medium filtration and three-stage ultrafiltration to obtain three-stage ultrafiltration effluent; The effluent from the third stage ultrafiltration is refluxed and mixed with the effluent from the second stage ultrafiltration before entering the third stage nanofiltration treatment again.

2. The potassium-sodium coexistence type high-salt wastewater zero-discharge treatment process according to claim 1, characterized in that: The invention also includes sequentially subjecting the three-stage nanofiltration permeate to ion exchange resin treatment, decarbonization and four-stage reverse osmosis treatment to produce fresh water and four-stage reverse osmosis effluent; The effluent from the four-stage reverse osmosis is sequentially subjected to four-stage high-density precipitation, four-stage medium filtration, four-stage ultrafiltration and evaporation crystallization treatment to produce sodium chloride and evaporation crystallization effluent; The evaporation and crystallization water is cooled and flash-evaporated to produce potassium chloride, and the cooled and flash-evaporated water is returned to the evaporation crystallizer and evaporated and crystallized again to produce sodium chloride.

3. The potassium-sodium coexistence type high-salt wastewater zero-discharge treatment process according to claim 1, characterized in that: The pretreatment is to perform hardness removal, silicon removal, biochemical treatment, ozone catalytic oxidation, medium filtration and hollow fiber ultrafiltration membrane filtration on the potassium and sodium coexistence high-salt wastewater; The agent used for the hardness and silicon removal treatment is lime milk, with an addition amount of 150-250 mg / L and a concentration of 8-12%; After pretreatment, the total silicon content of the wastewater must be less than 30 mg / L, COD less than 20 mg / L, SDI less than 3, fluoride less than 3 mg / L, and calcium ion 100-200 mg / L.

4. The potassium-sodium coexistence type high-salt wastewater zero-discharge treatment process according to claim 1, characterized in that: The recovery rate of the reverse osmosis treatment is 70-80%, and the membrane flux is 15-18 L / (m 2 ·h), operating pressure is 1.4-1.6MPa; Alternatively, the reagents added in the first stage of high-density precipitation are sodium hydroxide and sodium carbonate, the addition point is the mixing zone of the high-density precipitation tank, the addition amount of sodium hydroxide is 75-85 mg / L, the addition concentration is 25-35%, and the addition amount of sodium carbonate is 400-450 mg / L, the addition concentration is 18-25%; The silicon content in the effluent from the first stage high-density sedimentation tank is less than 30 mg / L, and the calcium ion content is 200-500 mg / L.

5. The potassium-sodium coexistence type high-salt wastewater zero-discharge treatment process according to claim 1, characterized in that: The recovery rate of the second-stage reverse osmosis treatment is 60%-80%, and the membrane flux is 15-18 L / (m 2 ·h), operating pressure is 1.4-1.6MPa; Alternatively, the effluent from the second stage of calcium sulfate crystallization has a sulfate concentration of 5000-6000 mg / L and a calcium ion concentration of 1200-1600 mg / L; Alternatively, the reagent added to the second-stage high-density precipitation is sodium carbonate, the dosage of sodium carbonate is 1800-1900 mg / L, the addition concentration is 18-25%, and the silicon content in the effluent of the second-stage high-density precipitation is less than 30 mg / L and the calcium ion content is 400-800 mg / L.

6. The potassium-sodium coexistence type high-salt wastewater zero-discharge treatment process according to claim 1, characterized in that: The recovery rate of the three-stage nanofiltration is 50-75%, and the membrane flux is 15-18 L / (m 2 ·h), operating pressure is 2.8-3.2MPa; Alternatively, the reagent added to the three-stage calcium sulfate crystallization is calcium chloride, the calcium chloride addition amount is 16000-16500 mg / L, the addition concentration is 18-22%, and the sulfate concentration in the effluent of the three-stage calcium sulfate crystallization is 6000-8000 mg / L and the calcium ion concentration is 1400-1800 mg / L; Alternatively, the reagent added to the three-stage high-density precipitation is sodium carbonate, the dosage of sodium carbonate is 1500-1600 mg / L, the addition concentration is 18-22%, and the calcium ion content in the effluent of the three-stage high-density precipitation is 800-1200 mg / L.

7. The potassium-sodium coexistence type high-salt wastewater zero-discharge treatment process according to claim 2, characterized in that: The hardness of the effluent from the ion exchange resin is less than 1 mg / L, and the bicarbonate in the effluent from the decarbonization is less than 20 mg / L. Or, the recovery rate of the four-stage reverse osmosis is 40%-60%, the TDS of the effluent is 70000-80000 mg / L, and the membrane flux is 15-18 L / (m 2 ·h), operating pressure is 6.8-7.2MPa; Alternatively, the reagents added to the four-stage high-density sedimentation tank are sodium hydroxide and sodium aluminate, with a sodium hydroxide dosage of 35-45 mg / L and a dosage concentration of 25-35%, and a sodium aluminate dosage of 160-170 mg / L and a dosage concentration of 8-12%, and the silicon content in the effluent is less than 20 mg / L.

8. The potassium-sodium coexistence type high-salt wastewater zero-discharge treatment process according to claim 2, characterized in that: The evaporation crystallization temperature is 100-110°C, and the sodium chloride content is greater than 96%; The cooling flash temperature is 40-50°C and the potassium oxide content is greater than 60%; Alternatively, part of the evaporated crystallization mother liquor is discharged for drying.

9. A potassium and sodium coexistence type high-salt wastewater zero discharge treatment system, characterized in that: Used to implement the zero-discharge treatment process for potassium-sodium coexistence high-salt wastewater according to claim 1, comprising a pretreatment unit, a first-stage reverse osmosis unit, a first-stage high-density sedimentation tank, a first-stage medium filtration tank, a first-stage ultrafiltration tank, a second-stage reverse osmosis unit, a second-stage calcium sulfate crystallization tank, a second-stage high-density sedimentation tank, a second-stage medium filtration tank, a second-stage ultrafiltration tank, and a third-stage nanofiltration unit connected in sequence; The three-stage nanofiltration unit is provided with a permeate outlet of the three-stage nanofiltration unit and a concentrate outlet of the three-stage nanofiltration unit; It also includes three sections of calcium sulfate crystallization tanks, three sections of high-density sedimentation tanks, three sections of medium filtration tanks, and three sections of ultrafiltration tanks, which are sequentially connected to the concentrated liquid outlet of the three-section nanofiltration unit; The water outlet of the three-stage ultrafiltration pool is connected to the water inlet of the three-stage nanofiltration unit.

10. The potassium-sodium coexistence type high-salt wastewater zero-discharge treatment system according to claim 9, characterized in that: The permeate outlet of the three-stage nanofiltration unit is connected in sequence to the ion exchange resin membrane, the decarbonization tank, the four-stage reverse osmosis unit, the four-stage high-density sedimentation tank, the four-stage medium filtration tank, the evaporation crystallizer, and the cooler; The outlet of the cooler is connected to the inlet of the evaporation crystallizer.

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