A method of treating brine

Abstract

The present disclosure relates to a method for treating brine, comprising the steps of: S1, making the mixed brine to be treated into a reverse osmosis device for reverse osmosis treatment to obtain first concentrated brine and fresh water; S2, making the first concentrated brine into an electrodialysis device for first electrodialysis treatment to obtain second concentrated brine; the conductivity of the second concentrated brine is 35-75 mS / cm; S3, making the second concentrated brine into the salt chamber of a selective bipolar membrane electrodialysis device for second electrodialysis treatment, the base chamber of the selective bipolar membrane electrodialysis device obtains a base solution, the acid chamber obtains a mixed acid solution, and the salt chamber obtains a mixed salt solution; S4, optionally, mixing the base solution with the mixed salt solution obtained from the salt chamber for precipitation reaction; and optionally, making the mixed acid solution obtained from the acid chamber to clean the reverse osmosis device. The present disclosure can obtain base and mixed acid with high purity, and the acid and base can be recycled and reused, which has significant environmental and economic benefits.

Description

Technical Field

[0001] This disclosure relates to the field of wastewater treatment technology, and more specifically, to a method for treating brine. Background Technology

[0002] Major brine desalination technologies include reverse osmosis (RO), nanofiltration (NF), electrodialysis (ED), multistage flash evaporation, and multi-effect distillation, with RO playing a dominant role. However, a key issue in desalination applications such as RO, NF, and ED is the treatment of concentrated brine byproducts. During desalination, a significant amount of feedwater is discharged as brine. Therefore, effective treatment of the concentrated brine byproducts is necessary to fully utilize water resources and minimize contamination from mixed brine discharge. Membrane desalination also presents with membrane fouling. For example, concentrated acids (such as HCl or H₂SO₄) and alkalis (such as NaOH) are commonly used in membrane systems to adjust the pH during operation or as cleaning agents in the cleaning cycle. However, due to cost and transportation issues, the acids and alkalis required for membrane systems are not readily available. Therefore, the ability to produce acids or alkalis on-site at the desalination plant is crucial for the normal operation of brine desalination in remote areas. Summary of the Invention

[0003] The purpose of this disclosure is to provide a method for treating brine to obtain alkali and mixed acid with high purity, and to achieve the recycling and reuse of acid and alkali, which has significant environmental and economic benefits.

[0004] To achieve the above objectives, this disclosure provides a method for treating brine, comprising the following steps: S1. The mixed brine to be treated is fed into the reverse osmosis unit for reverse osmosis treatment to obtain the first concentrated brine and fresh water. S2. The first concentrated brine is introduced into an electrodialysis device for first electrodialysis treatment to obtain a second concentrated brine; the conductivity of the second concentrated brine is 35~75 mS / cm. S3. The second concentrated brine is introduced into the salt chamber of a selective bipolar membrane electrodialysis device for a second electrodialysis treatment; wherein, in the direction from anode to cathode, the selective bipolar membrane electrodialysis device includes a first monovalent selective cation exchange membrane, a first bipolar membrane, a first anion exchange membrane and a second monovalent selective cation exchange membrane arranged in sequence at intervals, so that an alkali chamber, an acid chamber and the salt chamber are formed between two adjacent membranes in sequence. An alkaline solution is obtained from the alkaline chamber, a mixed acid solution is obtained from the acid chamber, and a mixed salt solution is obtained from the salt chamber; S4. Optionally, the alkaline solution is mixed with the mixed salt solution obtained in the salt chamber to carry out a precipitation reaction; and optionally, the mixed acid solution obtained in the acid chamber is used to clean the reverse osmosis device.

[0005] Optionally, in step S1, the total concentration of salts in the mixed saline solution to be treated is 10-30 mg / L, preferably 15-25 mg / L; Optionally, the sodium ion concentration in the mixed brine to be treated is 4000~12000 μg / L, the potassium ion concentration is 38~113 μg / L, the calcium ion concentration is 45~150 μg / L, and the magnesium ion concentration is 35~115 μg / L.

[0006] Optionally, in step S1, the reverse osmosis treatment conditions include: osmosis pressure of 0.3~2.5 MPa, osmosis temperature of 5~45℃, and feed flow rate of the mixed brine to be treated of 10~40 L / h; preferably, the osmosis pressure is 2~2.2 MPa, osmosis temperature is 10~30℃, and feed flow rate of the mixed brine to be treated of 20~30 L / h. Optionally, the thickness of the reverse osmosis membrane in the reverse osmosis device is 0.2~0.4mm, preferably 0.25~0.35mm; Optionally, the conductivity of the first concentrated brine obtained by the reverse osmosis treatment is 25~45 mS / cm, preferably 30~40 mS / cm; Optionally, step S1 further includes: allowing the mixed brine to be treated to first enter the ultrafiltration device for ultrafiltration treatment, and then enter the reverse osmosis device; and optionally, allowing the mixed acid solution obtained in the acid chamber to clean the ultrafiltration device.

[0007] Optionally, in step S2, the conditions for the first electrodialysis treatment include: a treatment temperature of 5~45℃ and a current density of 5~30mA / cm². 2 The liquid flow rate is 20~40 L / h; preferably, the processing temperature is 10~30℃ and the current density is 15~20 mA / cm². 2 The flow rate of the liquid is 15~25L / h; Preferably, the conductivity of the second concentrated brine obtained in step S2 is 40~60 mS / cm; Optionally, the sodium ion concentration in the second concentrated brine is 10,000-25,000 mg / L, preferably 15,000-20,000 mg / L; the potassium ion concentration is 135-285 mg / L, preferably 160-200 mg / L; the magnesium ion concentration is 105-265 mg / L, preferably 120-180 mg / L; and the calcium ion concentration is 130-330 mg / L, preferably 170-260 mg / L.

[0008] Optionally, in step S3, the conditions for the second electrodialysis include: a treatment temperature of 5~45℃, a treatment time of 30~180min, and a current density of 5~25mA / cm.2 The liquid flow rate is 20~40 L / h; preferably, the treatment temperature is 10~30℃, the treatment time is 90~120 min, and the current density is 15~20 mA / cm². 2 The liquid flow rate is 15~25L / h; Optionally, the first monovalent selective cation exchange membrane and the second monovalent selective cation exchange membrane may be the same or different; preferably, the materials of the first monovalent selective cation exchange membrane and the second monovalent selective cation exchange membrane are each independently selected from one or more of CSO and CIMS. Preferably, the thickness of the first monovalent selective cation exchange membrane and the second monovalent selective cation exchange membrane is independently 0.1~0.3 mm, more preferably 0.1~0.2 mm; the exchange capacity of the first monovalent selective cation exchange membrane and the second monovalent selective cation exchange membrane is 1.5~3.0 mmol / g, more preferably 2.0~2.5 mmol / g.

[0009] Optionally, step S3 further includes: introducing an externally added initial acid solution into the acid chamber, and introducing an externally added initial alkali solution into the alkali chamber; and, The second electrodialysis treatment includes: circulating the alkaline solution obtained in the alkaline chamber back to the alkaline chamber for reuse via external circulation; circulating the mixed acid solution obtained in the acid chamber back to the acid chamber for reuse via external circulation; and circulating the mixed salt solution obtained in the salt chamber back to the salt chamber for reuse via external circulation.

[0010] Optionally, in step S3, the concentration of the initial acid solution is 0.05~0.15 mol / L, preferably 0.08~0.12 mol / L; optionally, the initial acid solution is selected from one or more of hydrochloric acid, sulfuric acid or nitric acid. The concentration of the initial alkaline solution is 0.05~0.15 mol / L, preferably 0.08~0.12 mol / L; optionally, the initial alkaline solution is selected from one or more of sodium hydroxide solution and potassium hydroxide.

[0011] Optionally, an anode electrode chamber is formed between the anode and the first monovalent selective cation exchange membrane, and a cathode electrode chamber is formed between the second monovalent selective cation exchange membrane and the cathode; step S3 further includes: The externally added initial electrode solution is introduced into the anode electrode chamber and the cathode electrode chamber; and The second electrodialysis treatment includes: circulating the electrode solution in the anode electrode chamber and the cathode electrode chamber back to the anode electrode chamber and / or the cathode electrode chamber for reuse via external circulation; Optionally, the initial electrode solution includes a sodium sulfate solution; preferably, the concentration of the sodium sulfate solution is 0.1~0.5 mol / L, more preferably 0.2~0.4 mol / L.

[0012] Optionally, the first monovalent selective cation exchange membrane, the first bipolar membrane, the first anion exchange membrane, and the second monovalent selective cation exchange membrane are used as membrane stack units. The selective bipolar membrane electrodialysis device includes multiple membrane stack units connected in series; preferably, it includes three membrane stack units. Two adjacent membrane stack units share a monovalent selective cation exchange membrane, wherein the second monovalent selective cation exchange membrane in the membrane stack unit closer to the anode serves as the first monovalent selective cation exchange membrane in the membrane stack unit closer to the cathode.

[0013] Optionally, in step S2, the first electrodialysis treatment further yields a first desalinated brine, and the method further includes: The first desalinated brine obtained from the electrodialysis unit is returned to the reverse osmosis unit for further processing; Optionally, the conductivity of the first desalinated brine returned to the reverse osmosis unit is 35~75 mS / cm, preferably 50~60 mS / cm; preferably, the flow ratio of the first desalinated brine entering the electrodialysis unit to the mixed brine to be treated is 1~3:1, preferably 1~1.5:1.

[0014] Through the above technical solution, this disclosure provides a method for treating brine. This method involves first efficiently concentrating a low-concentration mixed brine solution through reverse osmosis and a first electrodialysis treatment to obtain a high-concentration second concentrated brine solution. This second concentrated brine solution has a certain range of conductivity, making it more suitable for subsequent second electrodialysis treatment. Introducing the second concentrated brine solution into a selective bipolar membrane electrodialysis device for second electrodialysis treatment yields high-concentration alkali, mixed acid, and mixed salt solutions, enabling the recycling and reuse of acids and alkalis. For example, the alkali solution can be directly used for the precipitation reaction of the mixed salt solution containing divalent ions obtained from the second electrodialysis treatment, and the mixed acid solution can be used to clean the equipment within the process system. The efficient impurity removal and salt separation process and green, harmless acid and alkali production process provided by this disclosure significantly reduces the discharge of high-salt wastewater and significantly improves the utilization rate of salt resources, realizing the resource utilization of concentrated mixed salt water. Furthermore, the entire process provided by this disclosure does not involve the addition of impurity removal agents or the discharge of saline wastewater. The process operation is simple, and it has significant environmental and economic benefits.

[0015] Other features and advantages of this disclosure will be described in detail in the following detailed description section. Attached Figure Description

[0016] The accompanying drawings are provided to further illustrate the present disclosure and form part of the specification. They are used together with the following detailed description to explain the present disclosure, but do not constitute a limitation thereof. In the drawings: Figure 1 This is a schematic diagram of the process flow of a desalination method provided in this disclosure; Figure 2 This is a schematic diagram of the selective bipolar membrane electrodialysis device provided in this disclosure.

[0017] Figure label: 1-Anode, 2-Cathode, 3-First monovalent selective cation exchange membrane, 4-First bipolar membrane, 5-First anion exchange membrane, 6-Second monovalent selective cation exchange membrane, 7-Second bipolar membrane, 8-Second anion exchange membrane, 9-Third monovalent selective cation exchange membrane, 10-First alkali chamber, 11-First acid chamber, 12-First salt chamber, 13-Second alkali chamber, 14-Second acid chamber, 15-Second salt chamber, 16-Anode electrode chamber, 17-Cathode electrode chamber, 18-Alkali collection device, 19-Mixed acid collection device, 20-Mixed salt collection device, 21-Electrode solution collection device. Detailed Implementation

[0018] The specific embodiments of this disclosure will be described in detail below with reference to the accompanying drawings. It should be understood that the specific embodiments described herein are for illustration and explanation only and are not intended to limit this disclosure.

[0019] This disclosure provides a method for treating brine, comprising the following steps: S1. The mixed brine to be treated is fed into the reverse osmosis unit for reverse osmosis treatment to obtain the first concentrated brine and fresh water. S2. The first concentrated brine is introduced into an electrodialysis device for first electrodialysis treatment to obtain a second concentrated brine; the conductivity of the second concentrated brine is 35~75 mS / cm. S3. The second concentrated brine is introduced into the salt chamber of a selective bipolar membrane electrodialysis device for a second electrodialysis treatment; wherein, in the direction from anode to cathode, the selective bipolar membrane electrodialysis device includes a first monovalent selective cation exchange membrane, a first bipolar membrane, a first anion exchange membrane and a second monovalent selective cation exchange membrane arranged in sequence at intervals, so that an alkali chamber, an acid chamber and the salt chamber are formed between two adjacent membranes in sequence. An alkaline solution is obtained from the alkaline chamber, a mixed acid solution is obtained from the acid chamber, and a mixed salt solution is obtained from the salt chamber; S4. Optionally, the alkaline solution is mixed with the mixed salt solution obtained in the salt chamber to carry out a precipitation reaction; and optionally, the mixed acid solution obtained in the acid chamber is used to clean the reverse osmosis device.

[0020] This disclosure provides a method for treating brine. The method involves first efficiently concentrating a low-concentration mixed brine solution through reverse osmosis and a first electrodialysis treatment to obtain a high-concentration second concentrated brine solution. This second concentrated brine solution has a certain range of conductivity, making it more suitable for subsequent second electrodialysis treatment. The second concentrated brine solution is then introduced into a selective bipolar membrane electrodialysis device for second electrodialysis treatment, yielding high-concentration alkali, mixed acid, and mixed salt solutions. This allows for the recycling and reuse of acids and alkalis. For example, the alkali solution can be directly used for the precipitation reaction of the mixed salt solution containing divalent ions obtained from the second electrodialysis treatment, and the mixed acid solution can be used to clean the equipment within the process system. The highly efficient impurity removal and salt separation process and green, harmless acid and alkali production provided by this disclosure significantly reduce the discharge of high-salt wastewater and significantly improve the utilization rate of salt resources, realizing the resource utilization of concentrated mixed salt water. Furthermore, the entire process provided by this disclosure does not involve the addition of impurity removal agents or the discharge of saline wastewater. The process operation is simple, and it has significant environmental and economic benefits.

[0021] According to this disclosure, the process principle of the brine treatment method includes: first, subjecting the low-concentration mixed brine to reverse osmosis treatment to concentrate and increase the salt concentration, obtaining a first concentrated brine and fresh water. The obtained fresh water can be used in the subsequent electrodialysis process of a selective bipolar membrane electrodialysis device, improving resource utilization; then, introducing the first concentrated brine into the electrodialysis device for a first electrodialysis treatment to further concentrate the brine, obtaining a salt solution with a higher salt content (second concentrated brine), and the fresh water obtained from the first electrodialysis treatment can be returned to the reverse osmosis device for further treatment, improving utilization efficiency; finally, introducing the second concentrated brine into the selective bipolar membrane electrodialysis device, the high-concentration brine... Concentrated brine enters the salt chamber of a selective bipolar membrane electrodialysis device. Using a monovalent selective cation exchange membrane, monovalent cations (sodium and potassium ions) and divalent cations (calcium and magnesium ions) in the second concentrated brine entering the salt chamber are separated. (Monovalent cations, such as sodium and potassium ions, can flow through the monovalent selective cation exchange membrane to the cathode to form an alkaline solution with hydroxide ions in the alkali chamber; while divalent cations, such as calcium and magnesium ions, are retained, resulting in a divalent mixed salt in the salt chamber, which can be used for subsequent precipitation reactions.) Through this process, mixed acid and high-purity alkali are obtained in the acid and alkali chambers of the selective bipolar membrane electrodialysis device, respectively.

[0022] In one embodiment, in step S1, the total salt concentration in the mixed brine to be treated is 10-30 mg / L, preferably 15-25 mg / L, and more preferably 20 mg / L; this disclosure can use saline wastewater with a low salt concentration as raw material. The salts in the mixed brine are the types commonly found in lightly salted water, such as sodium salts, potassium salts, calcium salts, and magnesium salts.

[0023] In one specific embodiment, the sodium ion concentration in the mixed saline solution to be treated is 4000~12000μg / L, the potassium ion concentration is 38~113μg / L, the calcium ion concentration is 45~150μg / L, and the magnesium ion concentration is 35~115μg / L.

[0024] In one embodiment, in step S1, the reverse osmosis treatment conditions include: an osmosis pressure of 0.3~2.5 MPa, an osmosis temperature of 5~45°C, and a feed flow rate of 10~40 L / h for the mixed brine to be treated; preferably, the osmosis pressure is 2~2.2 MPa, the osmosis temperature is 10~30°C, and the feed flow rate of the mixed brine to be treated is 20~30 L / h. Optionally, the conductivity of the first concentrated brine is 25~45 mS / cm, preferably 30~40 mS / cm, and more preferably 35 mS / cm. Processing according to the process conditions provided in this embodiment can achieve better reverse osmosis results; and the first concentrated brine obtained by processing according to the process conditions in this embodiment is more suitable for subsequent processing steps.

[0025] In this disclosure, the reverse osmosis apparatus can employ a conventional structure in the art.

[0026] In one specific embodiment, the thickness of the reverse osmosis membrane in the reverse osmosis device is 0.2~0.4 nm, preferably 0.25~0.35 mm, and more preferably 0.3 nm; the reverse osmosis membrane can be of a type conventionally selected in the art, which can be purchased through ordinary commercial channels or prepared by known methods.

[0027] In one specific embodiment, step S1 further includes: first, allowing the mixed brine to be treated to enter an ultrafiltration device for ultrafiltration treatment, and then entering the reverse osmosis device; and optionally, using the mixed acid solution obtained in the acid chamber to clean the ultrafiltration device; optionally, the ultrafiltration device uses a polyvinylidene fluoride ultrafiltration membrane with a pore size of 10~100nm. Ultrafiltration treatment can remove insoluble impurities such as colloids and suspended particles from the brine, thereby reducing the risk of contamination and clogging in the subsequent electrodialysis device and ensuring the purity of the subsequently prepared acids and alkalis.

[0028] In one embodiment, in step S2, the conditions for the first electrodialysis treatment include: a treatment temperature of 5~45℃ and a current density of 5~30mA / cm². 2 The liquid flow rate is 20~40 L / h; preferably, the processing temperature is 10~30℃ and the current density is 15~25 mA / cm². 2 Further preferred is 20mA / cm 2The liquid flow rate is 15-25 L / h, more preferably 20 L / h. Performing the first electrodialysis treatment according to the process conditions provided in this embodiment, especially according to the preferred process conditions, yields a salt solution (second concentrated brine) with a higher salt content for subsequent treatment, improving the treatment effect of the subsequent second electrodialysis treatment. For example, the resulting alkaline solution has a higher sodium hydroxide concentration and the mixed acid has a higher hydrogen ion concentration, resulting in higher electrodialysis efficiency and lower energy consumption in the second electrodialysis.

[0029] In one specific embodiment, the electrodialysis device is a two-compartment selective electrodialysis device. The membrane stack is composed of several unit electrodialysis cells connected in series. The assembly sequence of each unit electrodialysis cell from anode to cathode is cation exchange membrane, anion exchange membrane, and another cation exchange membrane, with adjacent membranes separated by a partition. The inventors of this disclosure have found that when the membrane stack is composed of 1 to 5 unit electrodialysis cells connected in series (preferably 3), the total effective area of ​​the ion exchange membrane is 187.2 to 1123.2 cm². 2 (Preferred size is 561.6cm) 2 The electrodialysis device concentrates the brine twice under constant pressure, ensuring that the flow rate in each compartment is the same, thus achieving better electrodialysis results.

[0030] In a preferred embodiment, the conductivity of the second concentrated brine obtained in step S2 is 40~60 mS / cm; Optionally, the sodium ion concentration in the second concentrated brine is 10,000-25,000 mg / L, preferably 15,000-20,000 mg / L; the potassium ion concentration is 135-285 mg / L, preferably 160-200 mg / L; the magnesium ion concentration is 105-265 mg / L, preferably 120-180 mg / L; and the calcium ion concentration is 130-330 mg / L, preferably 170-260 mg / L. The second concentrated brine obtained in step S2, having the conductivity and ion concentration of this embodiment, especially the second concentrated brine with the preferred conductivity and ion concentration, can achieve better electrodialysis results and improve acid-base recovery in subsequent selective bipolar membrane electrodialysis devices.

[0031] In a preferred embodiment, in step S3, the conditions for the second electrodialysis include: a treatment temperature of 5~45℃, a treatment time of 30~180min, and a current density of 5~25mA / cm². 2 The liquid flow rate is 20~40 L / h; preferably, the treatment temperature is 10~30℃, the treatment time is 90~120 min, and the current density is 15~20 mA / cm². 2The liquid flow rate is 15~25 L / h. Following the process conditions in this embodiment, especially the preferred process conditions, the second electrodialysis treatment can achieve better concentrated brine treatment results. For example, the concentration of sodium hydroxide in the treated alkaline solution and the hydrogen ion concentration in the mixed acid are higher, the electrical efficiency of the second electrodialysis is higher, and the energy consumption of the second electrodialysis is lower.

[0032] In one specific embodiment, the first monovalent selective cation exchange membrane and the second monovalent selective cation exchange membrane may be the same or different; preferably, the materials of the first and second monovalent selective cation exchange membranes are each independently selected from one or more of CSO and CIMS; the thickness of the first and second monovalent selective cation exchange membranes is each independently 0.1~0.3 nm, preferably 0.1~0.2 mm, more preferably 0.1 nm; the exchange capacity of the first and second monovalent selective cation exchange membranes is 1.5~3.0 mmol / g, preferably 2.0~2.5 mmol / g, more preferably 2.5 mmol / g. The first and second monovalent selective cation exchange membranes used in this disclosure can be purchased through ordinary commercial channels or prepared by known methods.

[0033] In one embodiment, step S3 further includes: introducing an initial acid solution into the acid chamber, and introducing an initial alkali solution into the alkali chamber; and / or, The second electrodialysis treatment includes: circulating the alkaline solution obtained in the alkali chamber back to the alkali chamber for reuse via external circulation; circulating the mixed acid solution obtained in the acid chamber back to the acid chamber for reuse via external circulation; and circulating the mixed salt solution obtained in the salt chamber back to the salt chamber for reuse via external circulation. In this disclosure, the initial acid solution and initial alkali solution can be introduced during the second electrodialysis process by introducing the second concentrated brine, or they can be introduced simultaneously; they can also be introduced sequentially, and it is not necessary to ensure that they are introduced into the device simultaneously with the second concentrated brine.

[0034] In a preferred embodiment, in step S3, the concentration of the initial acid solution is 0.05~0.15 mol / L, preferably 0.08~0.12 mol / L, and more preferably 0.10 mol / L; optionally, the initial acid solution is selected from one or more of hydrochloric acid, sulfuric acid, and nitric acid. The concentration of the initial alkaline solution is 0.05~0.15 mol / L, preferably 0.08~0.12 mol / L, and more preferably 0.10 mol / L; optionally, the initial alkaline solution is selected from one or more of sodium hydroxide solution and potassium hydroxide solution. Using the initial acid and alkaline solutions of the concentrations described in this embodiment can help improve current efficiency and is accompanied by lower energy consumption.

[0035] In one specific embodiment, an anode electrode chamber is formed between the anode and the first monovalent selective cation exchange membrane, and a cathode electrode chamber is formed between the second monovalent selective cation exchange membrane and the cathode; step S3 further includes: Allow the initial electrode solution to enter the anode electrode chamber and the cathode electrode chamber; and / or, The second electrodialysis treatment includes: circulating the electrode solution in the anode electrode chamber back to the anode electrode chamber for reuse via external circulation, and circulating the electrode solution in the cathode electrode chamber back to the cathode electrode chamber for reuse via external circulation. In this disclosure, the initial electrode solution can be introduced during the second electrodialysis process by introducing the second concentrated brine, and the order of introduction is not required; it is not necessary to ensure that the initial electrode solution is introduced into the device simultaneously with the second concentrated brine.

[0036] In a preferred embodiment, the initial electrode solution comprises a sodium sulfate solution; preferably, the concentration of the sodium sulfate solution is 0.1~0.5 mol / L, more preferably 0.2~0.4 mol / L, and even more preferably 0.3 mol / L. Using the initial electrode solution of this concentration in this embodiment can improve current efficiency while also having lower energy consumption.

[0037] In a preferred embodiment, the first monovalent selective cation exchange membrane, the first bipolar membrane, the first anion exchange membrane, and the second monovalent selective cation exchange membrane are used as membrane stack units. The selective bipolar membrane electrodialysis device includes multiple membrane stack units connected in series; preferably, it includes three membrane stack units. Two adjacent membrane stack units share one monovalent selective cation exchange membrane, wherein the second monovalent selective cation exchange membrane in the membrane stack unit closer to the anode serves as the first monovalent selective cation exchange membrane in the membrane stack unit closer to the cathode. By setting multiple membrane stacks connected in series, a better second electrodialysis treatment effect can be ensured.

[0038] In one specific implementation, such as Figure 1 In step S2, the first electrodialysis treatment further yields a first desalinated brine, and the method further includes: Returning the first desalinated brine obtained from the electrodialysis unit to the reverse osmosis unit for further processing can further increase the salt concentration.

[0039] In one specific embodiment, the conductivity of the first desalinated brine returned to the reverse osmosis unit is 35-75 mS / cm, preferably 50-60 mS / cm, and more preferably 55 mS / cm; preferably, the flow ratio of the first desalinated brine entering the electrodialysis unit to the mixed brine to be treated is 1-3:1, preferably 1-1.5:1, and more preferably 1:1. Introducing the desalinated brine and the first concentrated brine together for the first electrodialysis treatment according to the ratio in this embodiment can achieve a better treatment effect.

[0040] In one specific implementation, such as Figure 2 As shown, the selective bipolar membrane electrodialysis device used in this disclosure includes an anode 1 and a cathode 2. Along the direction from the anode 1 to the cathode 2, the selective bipolar membrane electrodialysis device includes a first monovalent selective cation exchange membrane 3, a first bipolar membrane 4, a first anion exchange membrane 5, a second monovalent selective cation exchange membrane 6, a second bipolar membrane 7, a second anion exchange membrane 8, and a third monovalent selective cation exchange membrane 9 arranged sequentially at intervals, such that a first alkaline chamber 10, a first acidic chamber 11, a first salt chamber 12, a second alkaline chamber 13, a second acidic chamber 14, and a second salt chamber 15 are formed between each pair of adjacent membranes; wherein the first alkaline chamber 10... The alkali outlet and alkali inlet of the second alkali chamber 13 are respectively connected to the alkali collection device 18, the mixed acid outlet and mixed acid inlet of the first acid chamber 11 and the second acid chamber 14 are respectively connected to the mixed acid collection device 19, and the mixed salt outlet and mixed salt inlet of the first salt chamber 12 and the second salt chamber 15 are respectively connected to the mixed salt collection device 20; and an anode electrode chamber 16 is formed between the anode 1 and the first monovalent selective cation exchange membrane 3, and a cathode electrode chamber 17 is formed between the third monovalent selective cation exchange membrane 9 and the cathode 2. The electrode liquid outlet and electrode liquid inlet of the anode electrode chamber 16 and the cathode electrode chamber 17 are respectively connected to the electrode liquid collection device 21.

[0041] In one specific implementation, such as Figures 1-2 As shown, the method for treating brine provided in this disclosure includes the following steps: The mixed brine (low-concentration brine) to be treated is first fed into an ultrafiltration unit for ultrafiltration treatment, and then into a reverse osmosis unit to obtain a first concentrated brine and fresh water; the fresh water can be introduced into a selective bipolar membrane electrodialysis unit to control the concentration of materials in each treatment chamber of the selective bipolar membrane electrodialysis unit; or it can be used for other water uses, such as cleaning the unit. The first concentrated brine is fed into an electrodialysis unit for first electrodialysis treatment to obtain a second concentrated brine and a first desalinated brine; the conductivity of the second concentrated brine is 35~75 mS / cm; the first desalinated brine can be returned to the reverse osmosis unit for further treatment; The second concentrated brine and the first and / or second salt chambers of the selective bipolar membrane electrodialysis device are introduced, the externally added initial acid solution is introduced into the first and / or second acid chambers, the externally added initial alkali solution is introduced into the first and / or second alkali chambers, and the initial electrode solution is introduced into the anode and cathode electrode chambers for the second electrodialysis treatment. The first and second alkali chambers produce an alkali solution, the first and second acid chambers produce a mixed acid solution, and the first and second salt chambers produce a mixed salt solution. The alkaline solutions in the first and second alkaline chambers are stored in the alkaline collection device; the alkaline solutions in the alkaline collection device are returned to the first and / or second alkaline chambers for further processing; and / or the mixed acid solutions in the first and second acid chambers are stored in the mixed acid collection device, and the acid solutions in the mixed acid collection device are returned to the first and / or second acid chambers for further processing; and / or the mixed salt solutions in the first and second salt chambers are stored in the mixed salt collection device, and the mixed salt solutions in the mixed salt collection device are returned to the first and / or second salt chambers for further processing; the electrode solution in the electrode solution collection device is returned to the anode electrode chamber and / or cathode electrode chamber for further processing.

[0042] The present disclosure is further described in detail below through examples. Unless otherwise specified, all raw materials, reagents, instruments and equipment used in the present disclosure are commercially available or can be prepared by existing methods.

[0043] In the following examples and comparative examples, the ultrafiltration device used a polyvinylidene fluoride ultrafiltration membrane with a filtration pore size of 10 nm, which was purchased from Drill Core Environmental Protection Technology Co., Ltd. (Guangdong, China). The reverse osmosis unit uses an RO membrane (cellulose acetate membrane) with a filtration precision of 0.1 nm, purchased from Times Waterton (Guizhou, China); and the reverse osmosis unit consists of three electrodialysis cells connected in series, with a total effective ion exchange membrane area of ​​561.6 cm². 2 ; In a selective bipolar membrane electrodialysis device, the membrane stack has polar liquid chambers on both sides, and the membrane stack is composed of several unit bipolar membrane electrodialysis cells connected in series (e.g., ...). Figure 2 As shown, Figure 2 The device shown can be considered as two membrane stacks connected in series to form a membrane unit. The assembly sequence of a single bipolar membrane electrodialysis cell from anode to cathode is: a monovalent selective cation exchange membrane, a bipolar membrane, an anion exchange membrane, and another monovalent selective cation exchange membrane. Adjacent membranes are separated by a partition (the partition can be made of a conventional material in the art, such as polytetrafluoroethylene). The monovalent selective cation exchange membrane is a type of CSO, and the bipolar membrane is an FBM. In the following examples and comparative examples, the membrane stack consists of four bipolar membrane electrodialysis cells connected in series, with a total effective membrane area of ​​748.8 cm².2 .

[0044] The following examples illustrate the overall process effect of the brine desalination method provided in this disclosure.

[0045] Example 1 Adopt this disclosure Figure 1 The process flow shown adopts Figure 2 The selective bipolar membrane electrodialysis device shown is used to desalinate a low-concentration brine (mixed brine to be treated), wherein the total salt concentration in the mixed brine to be treated is 20 mg / L, and the sodium ion concentration is 7982 μg / L, potassium ion concentration is 80.74 μg / L, calcium ion concentration is 142 μg / L, and magnesium ion concentration is 80 μg / L; the device includes the following steps: (1) The mixed salt water to be treated is first put into the ultrafiltration device for ultrafiltration treatment. After ultrafiltration treatment, the insoluble impurities (including flocculants, etc.) contained therein are intercepted and removed to reduce the risk of clogging of the electrodialysis unit and ensure the purity of the acid and base prepared in the subsequent process. The ultrafiltration-treated brine is then fed into a reverse osmosis unit for reverse osmosis treatment to obtain a first concentrated brine and fresh water. The reverse osmosis treatment conditions include: an osmotic pressure of 2.1 MPa, an osmotic temperature of 20°C, and a feed flow rate of 20 L / h for the brine to be treated. The conductivity of the resulting first concentrated brine is 35 mS / cm. The concentrations of sodium ions in the first concentrated brine are: 10807.87 mg / L, potassium ions: 94.16 mg / L, magnesium ions: 90 mg / L, and calcium ions: 120 mg / L. The reverse osmosis unit can also introduce a first desalinated brine (conductivity of 36 mS / cm) obtained from the electrodialysis unit, with a flow ratio of 1:1 between the first desalinated brine and the mixed brine to be treated. (2) The first concentrated brine obtained from reverse osmosis is fed into an electrodialysis unit for first electrodialysis treatment to obtain a second concentrated brine and a first desalinated brine; the conditions for the first electrodialysis treatment include: a treatment temperature of 20°C and a current density of 20 mA / cm². 2 The liquid flow rate is 20 L / h; the conductivity of the resulting second concentrated brine is 55 mS / cm; the concentration of sodium ions in the second concentrated brine is 17380.8 mg / L, the concentration of potassium ions is 181 mg / L, the concentration of magnesium ions is 177 mg / L, and the concentration of calcium ions is 234.9 mg / L; the first desalinated brine is returned to the reverse osmosis unit for further processing; (3) The second concentrated brine (conductivity 55 mS / cm) from the electrodialysis unit is introduced into the mixed salt collection device, and then into the first and second salt chambers of the selective bipolar membrane electrodialysis unit for the second electrodialysis treatment. The added initial acid solution (hydrochloric acid, concentration 0.1 mol / L) is introduced into the first acid chamber and / or the second acid chamber, the added initial alkali solution (sodium hydroxide solution, concentration 0.1 mol / L) is introduced into the first alkali chamber and / or the second alkali chamber, and the added initial electrode solution (sodium sulfate solution, concentration 0.3 mol / L) is introduced into the anode electrode chamber and the cathode electrode chamber for the second electrodialysis treatment. The flow rate of the liquid in each treatment chamber is kept the same. The conditions for the second electrodialysis include: a treatment temperature of 20°C and a current density of 20 mA / cm. 2 The liquid flow rate in each chamber is the same, 20 L / h; and during the second electrodialysis treatment, the solution in each treatment chamber is circulated back to the respective treatment chamber for reuse; after 120 min of reaction, the alkali solution in the alkali collection device includes NaOH, with a concentration of 0.68 mol / L and a purity of over 98%; the H+ of the mixed acid solution... + The ion concentration was 0.66 mol / L; in the mixed salt solution obtained in the salt chamber, the magnesium ion concentration was 169.65 mg / L and the calcium ion concentration was 214.15 mg / L; the current efficiency of the second electrodialysis treatment was 80.05%, and the energy consumption was 3.18 kWh / kg (calculated as sodium hydroxide, i.e., the energy consumption for producing 1 kg of sodium hydroxide). (4) The alkaline solution obtained by selective bipolar membrane electrodialysis can be used to precipitate calcium and magnesium ions in the mixed salt solution obtained by selective bipolar membrane electrodialysis; the mixed acid solution obtained by selective bipolar membrane electrodialysis can be used to clean ultrafiltration and reverse osmosis devices.

[0046] Example 2 This embodiment refers to the process and conditions in Embodiment 1, but differs from Embodiment 1 in the following aspects: The conditions for the first electrodialysis treatment in the electrodialysis unit were adjusted to: a treatment temperature of 40℃ and a current density of 25 mA / cm². 2 The flow rate of the liquid was 40 L / h; the conductivity of the resulting second concentrated brine was 75 mS / cm; the sodium ion concentration in the second concentrated brine was 25458.1 mg / L, the potassium ion concentration was 284.6 mg / L, the magnesium ion concentration was 264 mg / L, and the calcium ion concentration was 331.9 mg / L; the rest of the process was the same as in Example 1.

[0047] Example 3 This embodiment refers to the process and conditions in Embodiment 1, except that the concentrations of the initial acid solution and the initial alkali solution added to the selective bipolar membrane electrodialysis device are both 0.15 mol / L; the rest of the process is the same as in Embodiment 1.

[0048] Example 4 This embodiment refers to the process and conditions in Embodiment 1, but differs from Embodiment 1 in the following aspects: The concentrations of the initial acid solution and the initial alkali solution added in the selective bipolar membrane electrodialysis device were both 0.01 mol / L; the rest of the process was the same as in Example 1.

[0049] Example 5 This embodiment refers to the process and conditions in Embodiment 1, but differs from Embodiment 1 in the following aspects: The current density of the second electrodialysis treatment in the selective bipolar membrane electrodialysis device is 25 mA / cm². 2 The remaining process is the same as in Example 1.

[0050] Example 6 This embodiment refers to the process and conditions in Embodiment 1, but differs from Embodiment 1 in the following aspects: The current density of the second electrodialysis treatment in the selective bipolar membrane electrodialysis device is 2 mA / cm². 2 The remaining process is the same as in Example 1.

[0051] Example 7 This embodiment refers to the process and conditions in Embodiment 1, but differs from Embodiment 1 in the following aspects: The liquid flow rate in the selective bipolar membrane electrodialysis device is 40 L / h; the rest of the process is the same as in Example 1.

[0052] Example 8 This embodiment refers to the process and conditions in Embodiment 1, but differs from Embodiment 1 in the following aspects: The liquid flow rate in the selective bipolar membrane electrodialysis device is 5 L / h; the rest of the process is the same as in Example 1.

[0053] Example 9 This embodiment refers to the process and conditions in Embodiment 1, but differs from Embodiment 1 in the following aspects: The reverse osmosis treatment conditions were adjusted as follows: osmotic pressure of 2.5 MPa, osmotic temperature of 40°C, and feed flow rate of the mixed brine to be treated of 40 L / h; the conductivity of the resulting first concentrated brine was 45 mS / cm; the remaining process was the same as in Example 1.

[0054] Comparative Example 1 This comparative example follows the process and conditions of Example 1, but differs from Example 1 in that: no reverse osmosis treatment is performed; the rest of the process is the same as in Example 1.

[0055] Comparative Example 2 This comparative example follows the process and conditions of Example 1, but differs from Example 1 in that the first electrodialysis treatment is not performed; the remaining processes are the same as in Example 1.

[0056] Comparative Example 3 This comparative example uses the same process and conditions as in Example 1, but differs from Example 1 in the following ways: The monovalent selective cation exchange membrane in the selective bipolar membrane electrodialysis device is replaced with a conventional cation exchange membrane; the rest of the process is the same as in Example 1.

[0057] Comparative Example 4 This comparative example follows the process and conditions of Example 1, but differs from Example 1 in that the reverse osmosis treatment conditions are adjusted to pre-concentrate the second concentrated brine to a lower concentration, resulting in a conductivity of 20 mS / cm for the second concentrated brine introduced into the selective bipolar membrane electrodialysis device (where the sodium ion concentration is 7710 mg / L, the potassium ion concentration is 95.4 mg / L, the magnesium ion concentration is 81.1 mg / L, and the calcium ion concentration is 93.6 mg / L); the rest of the process is the same as in Example 1.

[0058] The contents, current efficiency, and energy consumption of the alkaline solutions, mixed acids, and mixed salts obtained from the above examples and comparative treatments are listed in Table 1 below.

[0059] Table 1

[0060] The data in Table 1 shows that: Comparative Example 1 did not undergo reverse osmosis treatment, Comparative Example 2 did not undergo first electrodialysis treatment, and Comparative Example 3 underwent selective bipolar membrane electrodialysis treatment. Compared with Comparative Examples 1-3, Examples 1-9, which were treated with brine according to the method provided in this disclosure, had higher NaOH content in the alkaline solutions and higher H2 content in the mixed acid. + The concentration is higher, and the second electrodialysis current efficiency is higher and the energy consumption of the second electrodialysis is lower in Examples 1-9; The conductivity of the second concentrated brine in Comparative Example 4, which incorporates a selective bipolar membrane electrodialysis device, is 20 mS / cm, outside the range provided in this disclosure. Comparing Comparative Example 4 with Example 2, it can be seen that, under the same second electrodialysis treatment conditions, the conductivity of the second concentrated brine in Example 2, which incorporates a selective bipolar membrane electrodialysis device, is within the optimized range provided in this disclosure. Furthermore, the NaOH concentration and content in the alkaline solution obtained after treatment in Example 2 are higher, and the H+ content of the mixed acid is also lower. +Higher concentration; Comparing Example 1 and Example 2, it can be seen that the process conditions of the first electrodialysis treatment in Example 1 are within the preferred range of this disclosure. The second electrodialysis current efficiency and the second electrodialysis energy consumption of the second concentrated brine obtained from the first electrodialysis treatment in Example 1 are higher. Comparing Example 3 and Example 4, it can be seen that the concentration of the initial alkaline solution added in Example 3 is within the optimized range provided in this disclosure. The alkaline solution obtained in Example 3 has a higher NaOH concentration and NaOH content, and the H+ content of the mixed acid is also higher. + The concentration is also higher, and the current efficiency of the second electrodialysis is higher, and the energy consumption of the second electrodialysis is also lower; Further comparison of Example 3 with Example 1 shows that the concentration of the initial alkaline solution added in Example 1 is within the further preferred range provided in this disclosure, and the second electrodialysis current efficiency and the second electrodialysis energy consumption are higher in Example 1. Comparing Example 5 and Example 6, it can be seen that the current density of the second electrodialysis treatment in Example 5 is within the optimized range provided in this disclosure. The NaOH concentration and content in the alkaline solution obtained in Example 5 are also higher, and the H+ content of the mixed acid is also higher. + The concentration is also higher, and in Example 5, the second electrodialysis current efficiency is higher and the second electrodialysis energy consumption is lower; Further comparison of Example 5 and Example 1 reveals that the current density of the second electrodialysis treatment in Example 1 is within the further preferred range provided in this disclosure. Additionally, the concentration and content of NaOH in the alkaline solution in Example 1 are higher, and the H+ content of the mixed acid is also higher. + The concentration is also higher; and the current efficiency of the second electrodialysis is higher, and the energy consumption of the second electrodialysis is also lower. Comparing Example 7 and Example 8, it can be seen that the liquid flow rate in the selective bipolar membrane electrodialysis device in Example 7 is within the optimized range provided in this disclosure. The NaOH concentration and content in the alkaline solution obtained in Example 7 are also higher, and the H+ content in the mixed acid is also higher. + The concentration is higher; and the current efficiency of the second electrodialysis is higher, and the energy consumption of the second electrodialysis is also lower. Further comparing Example 7 with Example 1, the liquid flow rate in the selective bipolar membrane electrodialysis device in Example 1 is within the further preferred range provided in this disclosure, and the second electrodialysis current efficiency and energy consumption in Example 1 are higher. Comparing Example 1 and Example 9, it can be seen that the reverse osmosis treatment conditions in Example 1 are within the preferred range provided in this disclosure. In Example 1, the concentration and content of NaOH in the alkaline solution are higher, and the H+ content of the mixed acid is also higher. + The concentration is also higher; and the current efficiency of the second electrodialysis is higher, and the energy consumption of the second electrodialysis is also lower.

[0061] The preferred embodiments of this disclosure have been described in detail above with reference to the accompanying drawings. However, this disclosure is not limited to the specific details of the above embodiments. Within the scope of the technical concept of this disclosure, various simple modifications can be made to the technical solutions of this disclosure, and these simple modifications all fall within the protection scope of this disclosure.

[0062] It should also be noted that the various specific technical features described in the above specific embodiments can be combined in any suitable manner without contradiction. In order to avoid unnecessary repetition, this disclosure will not describe the various possible combinations separately.

[0063] Furthermore, various different embodiments of this disclosure can be combined in any way, as long as they do not violate the spirit of this disclosure, they should also be regarded as the content disclosed in this disclosure.

Claims

1. A method for treating brine, characterized in that, Includes the following steps: S1. The mixed brine to be treated is fed into the reverse osmosis unit for reverse osmosis treatment to obtain the first concentrated brine and fresh water. S2. The first concentrated brine is introduced into an electrodialysis device for first electrodialysis treatment to obtain a second concentrated brine; the conductivity of the second concentrated brine is 35~75 mS / cm. S3. The second concentrated brine is introduced into the salt chamber of the selective bipolar membrane electrodialysis device for a second electrodialysis treatment. In the direction from anode to cathode, the selective bipolar membrane electrodialysis device includes a first monovalent selective cation exchange membrane, a first bipolar membrane, a first anion exchange membrane and a second monovalent selective cation exchange membrane arranged in sequence at intervals, so that an alkaline chamber, an acid chamber and a salt chamber are formed between two adjacent membranes in sequence. An alkaline solution is obtained from the alkaline chamber, a mixed acid solution is obtained from the acid chamber, and a mixed salt solution is obtained from the salt chamber; S4. Optionally, the alkaline solution is mixed with the mixed salt solution obtained in the salt chamber to carry out a precipitation reaction; and optionally, the mixed acid solution obtained in the acid chamber is used to clean the reverse osmosis device.

2. The method according to claim 1, characterized in that, In step S1, the total concentration of salts in the mixed saline solution to be treated is 10~30 mg / L, preferably 15~25 mg / L; Optionally, the sodium ion concentration in the mixed brine to be treated is 4000~12000 μg / L, the potassium ion concentration is 38~113 μg / L, the calcium ion concentration is 45~150 μg / L, and the magnesium ion concentration is 35~115 μg / L.

3. The method according to claim 1, characterized in that, In step S1, the reverse osmosis treatment conditions include: osmosis pressure of 0.3~2.5 MPa, osmosis temperature of 5~45℃, and feed flow rate of the mixed brine to be treated of 10~40 L / h; preferably, the osmosis pressure is 2~2.2 MPa, the osmosis temperature is 10~30℃, and feed flow rate of the mixed brine to be treated of 20~30 L / h. Optionally, the thickness of the reverse osmosis membrane in the reverse osmosis device is 0.2~0.4mm, preferably 0.25~0.35mm; Optionally, the conductivity of the first concentrated brine obtained by the reverse osmosis treatment is 25~45 mS / cm, preferably 30~40 mS / cm; Optionally, step S1 further includes: allowing the mixed brine to be treated to first enter the ultrafiltration device for ultrafiltration treatment, and then enter the reverse osmosis device; and optionally, allowing the mixed acid solution obtained in the acid chamber to clean the ultrafiltration device.

4. The method according to claim 1, characterized in that, In step S2, the conditions for the first electrodialysis treatment include: a treatment temperature of 5~45℃ and a current density of 5~30mA / cm². 2 The liquid flow rate is 20~40 L / h; preferably, the processing temperature is 10~30℃ and the current density is 15~20 mA / cm². 2 The flow rate of the liquid is 15~25L / h; Preferably, the conductivity of the second concentrated brine obtained in step S2 is 40~60 mS / cm; Optionally, the sodium ion concentration in the second concentrated brine is 10,000-25,000 mg / L, preferably 15,000-20,000 mg / L; the potassium ion concentration is 135-285 mg / L, preferably 160-200 mg / L; the magnesium ion concentration is 105-265 mg / L, preferably 120-180 mg / L; and the calcium ion concentration is 130-330 mg / L, preferably 170-260 mg / L.

5. The method according to claim 1, characterized in that, In step S3, the conditions for the second electrodialysis include: a treatment temperature of 5~45℃, a treatment time of 30~180min, and a current density of 5~25mA / cm². 2 The liquid flow rate is 20~40 L / h; preferably, the treatment temperature is 10~30℃, the treatment time is 90~120 min, and the current density is 15~20 mA / cm². 2 The liquid flow rate is 15~25L / h; Optionally, the first monovalent selective cation exchange membrane and the second monovalent selective cation exchange membrane may be the same or different; preferably, the materials of the first monovalent selective cation exchange membrane and the second monovalent selective cation exchange membrane are each independently selected from one or more of CSO and CIMS. Preferably, the thickness of the first monovalent selective cation exchange membrane and the second monovalent selective cation exchange membrane is independently 0.1~0.3 mm, more preferably 0.1~0.2 mm; the exchange capacity of the first monovalent selective cation exchange membrane and the second monovalent selective cation exchange membrane is 1.5~3.0 mmol / g, more preferably 2.0~2.5 mmol / g.

6. The method according to claim 1, characterized in that, Step S3 further includes: introducing an externally added initial acid solution into the acid chamber, introducing an externally added initial alkali solution into the alkali chamber; and / or, The second electrodialysis treatment includes: circulating the alkaline solution obtained in the alkaline chamber back to the alkaline chamber for reuse via external circulation; circulating the mixed acid solution obtained in the acid chamber back to the acid chamber for reuse via external circulation; and circulating the mixed salt solution obtained in the salt chamber back to the salt chamber for reuse via external circulation.

7. The method according to claim 6, characterized in that, In step S3, the concentration of the initial acid solution is 0.05~0.15 mol / L, preferably 0.08~0.12 mol / L; optionally, the initial acid solution is selected from one or more of hydrochloric acid, sulfuric acid and nitric acid. The concentration of the initial alkaline solution is 0.05~0.15 mol / L, preferably 0.08~0.12 mol / L; optionally, the initial alkaline solution is selected from one or more of sodium hydroxide solution and potassium hydroxide.

8. The method according to claim 1, characterized in that, An anode electrode chamber is formed between the anode and the first monovalent selective cation exchange membrane, and a cathode electrode chamber is formed between the second monovalent selective cation exchange membrane and the cathode; step S3 further includes: The externally added initial electrode solution is introduced into the anode electrode chamber and the cathode electrode chamber; and / or, The second electrodialysis treatment includes: circulating the electrode solution in the anode electrode chamber and the cathode electrode chamber back to the anode electrode chamber and / or the cathode electrode chamber for reuse via external circulation; Optionally, the initial electrode solution includes a sodium sulfate solution; preferably, the concentration of the sodium sulfate solution is 0.1~0.5 mol / L, more preferably 0.2~0.4 mol / L.

9. The method according to claim 1, characterized in that, Using the first monovalent selective cation exchange membrane, the first bipolar membrane, the first anion exchange membrane, and the second monovalent selective cation exchange membrane as membrane stack units, the selective bipolar membrane electrodialysis device includes multiple membrane stack units connected in series; preferably, it includes three membrane stack units; two adjacent membrane stack units share one monovalent selective cation exchange membrane, wherein the second monovalent selective cation exchange membrane in the membrane stack unit closer to the anode serves as the first monovalent selective cation exchange membrane in the membrane stack unit closer to the cathode.

10. The method according to claim 1, characterized in that, In step S2, the first electrodialysis treatment further yields a first desalinated brine, and the method further includes: The first desalinated brine obtained from the electrodialysis unit is returned to the reverse osmosis unit for further processing; Optionally, the conductivity of the first desalinated brine returned to the reverse osmosis unit is 35~75 mS / cm, preferably 50~60 mS / cm; preferably, the flow ratio of the first desalinated brine entering the electrodialysis unit to the mixed brine to be treated is 1~3:1, preferably 1~1.5:1.

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