Method, system and application for treating mixed salt wastewater

Abstract

The application relates to the field of water treatment and discloses a method, a system and an application for treating mixed salt wastewater. The method comprises the following steps: (1) performing nanofiltration on the mixed salt wastewater to obtain nanofiltration product water and nanofiltration concentrated water; (2) performing first membrane concentration on the nanofiltration product water to obtain first product water and first concentrated water; (3) performing electrodialysis crystallization on the first concentrated water and saturated salt solution to obtain first salt products, system concentrated drainage and electrodialysis fresh water; (4) performing second membrane concentration on the nanofiltration concentrated water to obtain second product water and second concentrated water; and (5) performing low-temperature crystallization on the second concentrated water and optional electrodialysis fresh water to obtain second salt products and low-temperature crystallization mother liquor, and mixing the low-temperature crystallization mother liquor with the mixed salt wastewater. The content of the first salt products in the electrodialysis fresh water is greater than 3 wt%, and the electrodialysis crystallization is performed; the content of the first salt products in the electrodialysis fresh water is less than or equal to 3 wt%, and the low-temperature crystallization is performed. The method has a simple technological process, can simultaneously obtain two kinds of salt products, has high resource utilization degree, and has high product recovery rate and purity.

Description

Technical Field

[0001] This invention relates to the field of water treatment, and more specifically to a method, system, and application for treating mixed saline wastewater. Background Technology

[0002] With the continuous improvement of social development, large amounts of saline wastewater are generated during the production processes of petrochemicals, coal chemicals, power generation, steel, and seawater desalination. The solids in this saline wastewater are mainly crystalline salts. In order to improve environmental protection and further enhance the utilization value of crystalline salts in saline wastewater, it is necessary to perform salt separation treatment.

[0003] CN112811687A discloses a method for treating saline wastewater. The method involves subjecting nanofiltration-treated saline wastewater to bipolar membrane electrodialysis to obtain acidic products, alkaline products, and intermediate saline wastewater. The intermediate saline wastewater is then subjected to electrodialysis to obtain a desalinated solution and a concentrated solution. The concentrated solution is then returned to the bipolar membrane electrodialysis process. However, this process fails to concentrate chloride in the nanofiltration permeate, making it impossible to extract monovalent salts into the system. While nanofiltration filters out polyvalent ions entering the subsequent bipolar membrane system, it fails to further utilize sulfate and other ions for resource recovery, resulting in a low overall value of the system's byproducts.

[0004] CN208898568U discloses an electrodialysis salt separation device, including a membrane stack, a chamber frame, electrode regions, and a clamping device. The membrane stack includes monovalent anion exchange membranes, cation exchange membranes, anion exchange membranes, and monovalent cation exchange membranes. The chamber frame is configured to support the membrane stack and is arranged side-by-side to divide the interior of the device into five compartments. The electrode regions include electrode plates and electrode frames located on both sides of the chamber frame. The electrode frames are configured to support the electrode plates. The clamping device includes pressure plates on the outside of the electrode frames and chamber frames, and is configured to fix and support the electrode frames and chamber frames. A high-salt wastewater treatment system includes a pretreatment device, an electrodialysis salt separation device, an evaporation device, and an RO concentration device. For the selective electrodialysis desalination process, the ordinary anion exchange membrane in traditional electrodialysis is replaced with a selective monovalent anion exchange membrane. Electrodialysis treatment of mixed salt wastewater can also separate chloride and sulfate. However, the cost of this type of monovalent anion exchange membrane is more than 50% higher than that of ordinary anion exchange membrane, which significantly increases the equipment cost. In addition, the high energy consumption of evaporation and crystallization operation increases the overall operating cost of the process. Summary of the Invention

[0005] The purpose of this invention is to overcome the problems of complex processes and low resource utilization in existing mixed-salt wastewater treatment processes, and to provide a method, system and application for treating mixed-salt wastewater. This method has a simple process flow, can obtain two salt products simultaneously, has a high degree of resource utilization, and has a high product recovery rate and purity.

[0006] To achieve the above objectives, the present invention provides a method for treating mixed saline wastewater, characterized in that the method includes the following steps:

[0007] (1) Nanofiltration treatment of mixed saline wastewater to obtain nanofiltration permeate and nanofiltration concentrate;

[0008] (2) The nanofiltration permeate is subjected to a first membrane concentration treatment to obtain a first permeate and a first concentrate.

[0009] (3) The first concentrated water is mixed with a saturated salt solution and subjected to electrodialysis crystallization to obtain the first salt product, system concentrated water, and electrodialysis desalinated water;

[0010] (4) The nanofiltration concentrate is subjected to a second membrane concentration treatment to obtain a second product water and a second concentrate;

[0011] (5) The second concentrated water and optionally the electrodialysis desalinated water are subjected to low-temperature crystallization to obtain a second salt product and a low-temperature crystallization mother liquor. The low-temperature crystallization mother liquor is returned to step (1) and mixed with mixed salt wastewater.

[0012] Wherein, when the content of the first salt product in the electrodialysis desalination water is >3wt%, the electrodialysis desalination water is returned to step (3) for electrodialysis crystallization;

[0013] When the content of the first salt product in the electrodialysis desalination water is ≤3wt%, the electrodialysis desalination water proceeds to step (5) for low-temperature crystallization.

[0014] A second aspect of the present invention provides a system for treating mixed saline wastewater, characterized in that the system comprises: a nanofiltration unit, a first membrane concentration unit, a second membrane concentration unit, an electrodialysis crystallization unit, and a low-temperature crystallization unit;

[0015] The nanofiltration unit is connected to the first membrane concentration unit and the second membrane concentration unit, and is used to perform nanofiltration treatment on mixed wastewater to obtain nanofiltration permeate and nanofiltration concentrate.

[0016] The first membrane concentration unit is connected to the electrodialysis crystallization unit and is used to perform a first membrane concentration treatment on the nanofiltration permeate to obtain a first permeate and a first concentrate.

[0017] The electrodialysis crystallization unit is connected to the low-temperature crystallization unit and is used to perform electrodialysis crystallization treatment on the first concentrated water to obtain the first salt product, system concentrated water and electrodialysis desalinated water.

[0018] The second membrane concentration unit is connected to the low-temperature crystallization unit and is used to perform a second membrane concentration treatment on the nanofiltration concentrate to obtain a second concentrate and a second product water.

[0019] The low-temperature crystallization unit is connected to the second membrane concentration unit, the electrodialysis crystallization unit, and the nanofiltration unit, and is used to perform low-temperature crystallization treatment on the electrodialysis desalinated water and / or the second concentrated water to obtain the second salt product and the low-temperature crystallization mother liquor; the low-temperature crystallization mother liquor is returned to the nanofiltration unit for circulating nanofiltration.

[0020] A third aspect of the present invention provides an application of the above-described method or system in the treatment of mixed saline wastewater.

[0021] Through the above technical solutions, the method, system, and application for treating mixed saline wastewater provided by the present invention achieve the following beneficial effects:

[0022] In this invention, the method for treating mixed salt wastewater provided by this invention reduces the system complexity of mixed salt wastewater treatment and improves the recovery rate of dissolved salts in the system.

[0023] In this invention, the system for treating mixed saline wastewater provided by this invention can efficiently treat mixed saline wastewater, obtain industrial products during the treatment process, realize the resource utilization of mixed saline wastewater, and improve the energy utilization efficiency of the system. Attached Figure Description

[0024] Figure 1 This is a schematic flowchart of the mixed salt wastewater treatment method according to Embodiment 1 of the present invention. Detailed Implementation

[0025] The endpoints and any values ​​of the ranges disclosed herein are not limited to the precise ranges or values, and these ranges or values ​​should be understood to include values ​​close to these ranges or values. For numerical ranges, the endpoint values ​​of the various ranges, the endpoint values ​​of the various ranges and individual point values, and individual point values ​​can be combined with each other to obtain one or more new numerical ranges, which should be considered as specifically disclosed herein.

[0026] The first aspect of this invention provides a method for treating mixed saline wastewater, characterized in that the method includes the following steps:

[0027] (1) Nanofiltration treatment of mixed saline wastewater to obtain nanofiltration permeate and nanofiltration concentrate;

[0028] (2) The nanofiltration permeate is subjected to a first membrane concentration treatment to obtain a first permeate and a first concentrate.

[0029] (3) The first concentrated water is mixed with a saturated salt solution and subjected to electrodialysis crystallization to obtain the first salt product, system concentrated water, and electrodialysis desalinated water;

[0030] (4) The nanofiltration concentrate is subjected to a second membrane concentration treatment to obtain a second product water and a second concentrate;

[0031] (5) The second concentrated water and optionally the electrodialysis desalinated water are subjected to low-temperature crystallization to obtain a second salt product and a low-temperature crystallization mother liquor. The low-temperature crystallization mother liquor is returned to step (1) and mixed with mixed salt wastewater.

[0032] Wherein, when the content of the first salt product in the electrodialysis desalination water is >3wt%, the electrodialysis desalination water is returned to step (3) for electrodialysis crystallization;

[0033] When the content of the first salt product in the electrodialysis desalination water is ≤3wt%, the electrodialysis desalination water proceeds to step (5) for low-temperature crystallization.

[0034] In this invention, the above-mentioned method for treating mixed salt wastewater can efficiently treat the wastewater and obtain industrial products, thereby improving the economic benefits of treating mixed salt wastewater, reducing the economic cost of treating mixed salt wastewater, and realizing the resource utilization of mixed salt wastewater. It has significant advantages over the commonly used evaporation and crystallization treatment process.

[0035] In this invention, the impurity in the first salt product is sodium sulfate, and the impurity in the second salt product is sodium chloride.

[0036] In this invention, prior to nanofiltration, the mixed saline wastewater to be treated also includes chemical precipitation, ion exchange, and advanced oxidation treatment steps.

[0037] In this invention, the mixed salt wastewater includes monovalent anionic salts, divalent anionic salts, and recalcitrant organic matter.

[0038] According to the present invention, in step (1), the pH of the mixed salt wastewater is ≤12.

[0039] In this invention, the pH of the mixed salt wastewater is ≤12, which enables the subsequent membrane concentration treatment to perform better.

[0040] According to the present invention, the total salt content of the mixed salt wastewater is ≥1wt%, preferably ≥3wt%.

[0041] Furthermore, the organic matter COD content in the mixed saline wastewater is <20 mg / L.

[0042] Furthermore, the total concentration of calcium and magnesium ions in the mixed saline wastewater is <20 mg / L, and Fe... 3+ The concentration of ions is <1 mg / L.

[0043] According to the present invention, in step (1), the nanofiltration treatment results in a desalination rate of <60% for the first salt in the mixed salt wastewater and a desalination rate of ≥95% for the first salt in the mixed salt wastewater.

[0044] According to the present invention, when the desalination rate of the first salt and the desalination rate of the second salt respectively meet the above-mentioned ranges, the two salts can be separated separately, thereby improving their respective recovery rates and purity.

[0045] According to the present invention, the nanofiltration recovery rate is 50-80%.

[0046] According to the present invention, the first membrane concentration treatment is selected from at least one of reverse osmosis, high-pressure reverse osmosis and forward osmosis.

[0047] Furthermore, the salt content of the first concentrated water is ≥3wt%, preferably ≥10wt%.

[0048] Furthermore, the salinity of the first produced water is ≤0.1wt%, preferably ≤0.05wt%.

[0049] According to the present invention, the recovery rate of the first membrane concentration treatment is 30-90%.

[0050] In this invention, when the recovery rate of the first membrane concentration treatment meets the above-mentioned range, the purity and recovery rate of the first salt product can be further improved.

[0051] Furthermore, the recovery rate of the first membrane concentration treatment is 50-80%.

[0052] According to the present invention, the temperature of the first membrane concentration treatment is 20-50°C, preferably 25-35°C.

[0053] According to the present invention, the conditions for the electrodialysis crystallization treatment include: a current density of 20-50 mA / cm². 2 Preferably 30-40 mA / cm 2 The DC voltage is 0.1-1V, preferably 0.3-0.7V.

[0054] According to the present invention, the flow rate ratio of the saturated salt solution to the first concentrated water is 1:0.3-5.

[0055] In this invention, when the flow rate ratio of the saturated salt solution to the first concentrated water meets the above-mentioned range, the service life of the ion exchange membrane in the electrodialysis system can be extended.

[0056] According to the present invention, the flow rate ratio of the saturated salt solution to the first concentrated water is 1:1-2.5.

[0057] In this invention, there is no particular limitation on the type of saturated salt solution; for example, it can be a saturated sodium chloride solution.

[0058] Furthermore, the salt content of the electrodialysis desalinated water is ≤5wt%.

[0059] Furthermore, the salinity of the electrodialysis desalinated water is ≤3.5wt%.

[0060] According to the present invention, the salt content of the second concentrated water is ≥4wt%.

[0061] In this invention, when the salt content of the second concentrated water meets the above-mentioned range, the recovery rate and purity of the second salt product can be higher.

[0062] According to the present invention, the salt content of the second concentrated water is ≥8 wt%.

[0063] Furthermore, the salinity of the second produced water is ≤0.1wt%, preferably ≤0.05wt%.

[0064] Furthermore, the recovery rate of the second membrane concentration treatment is 30-90%, preferably 50-80%; the treatment temperature is 20-50℃, preferably 25-35℃.

[0065] According to the present invention, the conditions for the low-temperature crystallization include a crystallization temperature of -10°C to 10°C.

[0066] In this invention, the crystallization is carried out at low temperature. When the crystallization temperature meets the above-mentioned range, it can achieve efficient treatment of mixed salt wastewater and improve treatment efficiency.

[0067] According to the present invention, the conditions for the low-temperature crystallization include a crystallization temperature of 0°C to 5°C.

[0068] According to the present invention, the salt content of the low-temperature crystallization mother liquor is 3-8 wt%, preferably 4-6 wt%.

[0069] According to the present invention, the first salt product is a monovalent anionic salt, and the second salt product is a divalent anionic salt.

[0070] In this invention, the monovalent anionic salt is sodium chloride, and the divalent anionic salt is sodium sulfate.

[0071] According to the present invention, the recovery rate of the monovalent anionic salt is 74-95%, and the purity is 95-100%.

[0072] According to the present invention, the second salt product is separated to obtain a divalent anionic salt, wherein the recovery rate of the divalent anionic salt is 85-95% and the purity is 90-100%.

[0073] According to the present invention, the separation method includes at least one of centrifugal separation, filtration separation and precipitation separation.

[0074] According to the present invention, the separation conditions include a residence time of 0.1-3h, preferably 0.3-0.5h.

[0075] A second aspect of the present invention provides a system for treating mixed saline wastewater, characterized in that the system comprises: a nanofiltration unit, a first membrane concentration unit, a second membrane concentration unit, an electrodialysis crystallization unit, and a low-temperature crystallization unit;

[0076] The nanofiltration unit is connected to the first membrane concentration unit and the second membrane concentration unit, and is used to perform nanofiltration treatment on mixed wastewater to obtain nanofiltration permeate and nanofiltration concentrate.

[0077] The first membrane concentration unit is connected to the electrodialysis crystallization unit and is used to perform a first membrane concentration treatment on the nanofiltration permeate to obtain a first permeate and a first concentrate.

[0078] The electrodialysis crystallization unit is connected to the low-temperature crystallization unit and is used to perform electrodialysis crystallization treatment on the first concentrated water to obtain the first salt product, system concentrated water and electrodialysis desalinated water.

[0079] The second membrane concentration unit is connected to the low-temperature crystallization unit and is used to perform a second membrane concentration treatment on the nanofiltration concentrate to obtain a second concentrate and a second product water.

[0080] The low-temperature crystallization unit is connected to the second membrane concentration unit, the electrodialysis crystallization unit, and the nanofiltration unit, and is used to perform low-temperature crystallization treatment on the electrodialysis desalinated water and / or the second concentrated water to obtain the second salt product and the low-temperature crystallization mother liquor; the low-temperature crystallization mother liquor is returned to the nanofiltration unit for circulating nanofiltration.

[0081] In this invention, the first membrane concentration unit consists of a membrane concentration membrane element, a membrane shell, a water pump, and a high-pressure pump, and can be a single stage or multiple stages.

[0082] In this invention, the electrodialysis crystallization unit includes a freshwater tank, a concentrated water tank, an electrode water tank, an electrodialysis membrane stack, a DC power supply, a control cabinet, a pump, and detection instruments.

[0083] In this invention, the low-temperature crystallization unit consists of a heat exchanger and a crystallizer.

[0084] In this invention, there are no requirements on the form of the crystallizer; it can be an OSLO crystallizer, a DTB crystallizer, or an FC crystallizer.

[0085] A third aspect of the present invention provides an application of the above-described method or system in the treatment of mixed brine.

[0086] like Figure 1As shown, in one specific embodiment of the present invention, the nanofiltration unit is connected to a first membrane concentration unit and a second membrane concentration unit for nanofiltration treatment of mixed wastewater to obtain nanofiltration permeate and nanofiltration concentrate; wherein, the pH of the mixed saline wastewater is ≤12, the total salt content of the mixed saline wastewater is ≥1wt%, the COD content of the mixed saline wastewater is <20mg / L, the total concentration of calcium and magnesium ions in the mixed saline wastewater is <20mg / L, and the Fe content is <20mg / L. 3+ The concentration of ions is <1 mg / L;

[0087] The first membrane concentration unit is connected to the electrodialysis crystallization unit and is used to perform a first membrane concentration treatment on the nanofiltration permeate to obtain a first permeate and a first concentrate; wherein, the first membrane concentration treatment is selected from at least one of reverse osmosis, high-pressure reverse osmosis and forward osmosis, the salt content of the first concentrate is ≥3wt%, the salt content of the first permeate is ≤0.1wt%, the recovery rate of the first membrane concentration treatment is 30%-90%, and the treatment temperature is 20-50℃;

[0088] The electrodialysis crystallization unit is connected to the low-temperature crystallization unit and is used to perform electrodialysis crystallization treatment on the first concentrated water to obtain a first salt product, system concentrated wastewater, and electrodialysis distillate. The conditions for the electrodialysis crystallization treatment include a current density of 20-50 mA / cm². 2 The DC voltage is 0.1-1V, and the flow rate ratio of the saturated salt solution to the first concentrated water is 1:0.3-5;

[0089] The second membrane concentration unit is connected to the low-temperature crystallization unit and is used to perform a second membrane concentration treatment on the nanofiltration concentrate at 20-50°C to obtain a second concentrate and a second product water. The second concentrate has a salt content ≥4wt%, the second product water has a salt content ≤0.1wt%, and the recovery rate of the second membrane concentration treatment is 30%-90%.

[0090] The low-temperature crystallization unit is connected to the second membrane concentration unit, the electrodialysis crystallization unit, and the nanofiltration unit, respectively, and is used to perform low-temperature crystallization treatment on the second concentrated water and optionally the electrodialysis desalinated water to obtain a second salt product and a low-temperature crystallization mother liquor. The low-temperature crystallization mother liquor is returned to the nanofiltration unit for circulating nanofiltration; wherein, the low-temperature crystallization temperature is -10℃ to 10℃;

[0091] When the content of the first salt product in the electrodialysis desalination water is >3wt%, the electrodialysis desalination water is returned to the electrodialysis crystallization unit for electrodialysis crystallization treatment.

[0092] When the content of the first salt product in the electrodialysis desalination water is ≤3wt%, the electrodialysis desalination water enters the low-temperature crystallization unit for low-temperature crystallization.

[0093] The present invention will be described in detail below through embodiments. In the following embodiments,

[0094] The concentration of ions in the solution was determined by ion chromatography.

[0095] Salt content is calculated using the formula t×V / (ρ) 水 The result is calculated as (×V)×100%;

[0096] Where t is the total dissolved solids content in water as measured by TDS, and ρ 水 Let V be the density of water, and V be the volume of the mixed saline wastewater.

[0097] In this invention, taking the content of the first salt as an example, calculated as sodium chloride, the content m of the first salt is determined according to the formula: c / M Cl - ×M NaCl Calculated by ×V×100%;

[0098] Where c is the mass concentration of chloride ions, M Cl - M is the relative atomic mass of the chloride ion. NaCl V is the relative molecular mass of sodium chloride, and V is the volume of the mixed salt wastewater.

[0099] In this invention, the desalination rate of the first salt is calculated using the formula: (mass of the first salt in the mixed salt wastewater - mass of the first salt in the nanofiltration permeate) / mass of the first salt in the mixed salt wastewater × 100%.

[0100] The desalination rate of the second salt is calculated using the formula: (mass of the second salt in the mixed salt wastewater - mass of the second salt in the nanofiltration product) / mass of the second salt in the mixed salt wastewater × 100%.

[0101] The recoveries of the first and second salt products were calculated using the following method:

[0102] Recovery rate of the first salt product = (mass of first salt ion concentration / mass of first salt in mixed salt wastewater) × 100%;

[0103] Second salt product recovery rate = (mass of second salt ion concentration / mass of second salt in mixed salt wastewater) × 100%.

[0104] The first salt is the form in which the first salt product exists before treatment, and the second salt is the form in which the second salt product exists before treatment.

[0105] The purity of the first salt product and the second salt product is calculated using the following formula:

[0106] Purity of the first salt product = Mass of first salt ion concentration / Mass of first salt product × 100%;

[0107] Purity of the second salt product = Mass of second salt ion concentration / Mass of second salt product × 100%.

[0108] The mass of the first salt ion concentration is the total mass of chloride and sodium ions. The test method is as follows: dissolve the first salt product, determine the chloride ion concentration using ion chromatography, and calculate the concentration according to the formula c / M. Cl - ×M NaCl The total mass of the first salt ion concentration is calculated by multiplying V by 100%.

[0109] Example 1

[0110] The main water quality characteristics of a certain industrial mixed saline wastewater after pretreatment (neutralization + flocculation + clarification) and electrocatalytic oxidation are shown in Table 1:

[0111] Table 1

[0112]

[0113] The specific processing method is as follows:

[0114] S1. The above 5L mixed salt wastewater (total salt content of 6wt%, of which the first salt content is 90634mg and the second salt content is 86911mg) is subjected to nanofiltration treatment by a nanofiltration unit to obtain nanofiltration permeate and nanofiltration concentrate. The nanofiltration recovery rate is 73%. The first salt content in the nanofiltration permeate is 75426mg and the second salt content is 3168mg. - The ion concentration is 12540 mg / L, SO4 2- The ion concentration was 587 mg / L, and the Cl- ion concentration in the nanofiltration concentrate was 6836 mg / L, SO42- . 2- The ion concentration was 41923 mg / L; the desalination rate of the first salt was 16.78%, and the desalination rate of the second salt was 96.4%.

[0115] S2. The nanofiltration permeate enters the first membrane concentration unit and is treated by reverse osmosis at 26°C with a recovery rate of 80%, yielding a first concentrate and a first permeate. The first concentrate has a salt content of 14 wt%, of which Cl... - The ion concentration is 62700 mg / L, SO4 2- The ion concentration is 2935 mg / L; the salinity of the first product water is 0.05 wt%, which meets the reuse standard.

[0116] S3. The first concentrated solution obtained in step S2, along with another stream of saturated sodium chloride solution, is introduced into the electrodialysis crystallization unit, with an operating current density of 35 mA / cm². 2The voltage was 0.7V, and the flow rate ratio of saturated sodium chloride solution to the first concentrated water was 1:0.67. The product was centrifuged and dried to obtain 79450 mg of the first salt product (total mass of the first salt ion concentration was 78258 mg), electrodialysis desalination water, and concentrated wastewater from the system. The sodium chloride content in the electrodialysis desalination water was found to be 1.5 wt%, which was discharged into the low-temperature crystallization system. The sodium chloride recovery rate was 86.3%, and the purity was 98.5%.

[0117] S4. The nanofiltration concentrate enters the second membrane concentration unit and is treated by reverse osmosis at 26°C with a recovery rate of 60%, yielding a second concentrate and a second permeate. The second concentrate has a salt content of 17 wt%, with a Cl- ion concentration of 11347 mg / L and SO42- content of 11347 mg / L. 2- The ion concentration is 69592 mg / L; the salinity of the second product water is 0.05 wt%, which meets the reuse standard.

[0118] S5. The second concentrated water enters the low-temperature crystallization unit and is subjected to low-temperature crystallization in an intermittent operation mode at 0°C. After running for 30 minutes, the low-temperature crystallization mother liquor and crystallized salt are obtained. 80090 mg of the second salt product is separated (the total mass of the second salt ion concentration is 78968 mg). The recovery rate of sodium sulfate is 92.2%, the purity is 98.6%, and the salt content in the low-temperature crystallization mother liquor is 6 wt%. It is then subjected to nanofiltration again.

[0119] Example 2

[0120] Similar to Example 1, except that in step S2, the flow rate ratio of saturated sodium chloride solution to the first concentrated water was 1:0.85. The product was centrifuged to obtain 74185 mg of the first salt product (total mass of the first salt ion concentration was 71440 mg), electrodialysis desalination water, and system concentrated wastewater. The sodium chloride content in the electrodialysis desalination water was detected to be 5.2 wt%, and it was returned to the electrodialysis crystallization unit for cyclic electrodialysis. At this point, the sodium chloride content in the electrodialysis desalination water was detected to be 1.8 wt%, and it was discharged into the low-temperature crystallization system. The sodium chloride recovery rate was 78.8%, and the purity was 96.3%.

[0121] Example 3

[0122] Similar to Example 1, except that in step S2, the nanofiltration permeate enters the first membrane concentration unit and is treated by reverse osmosis at 30°C with a recovery rate of 60%, yielding a first concentrate and a first permeate. The first concentrate has a salt content of 10 wt%, with a Cl- ion concentration of 44785 mg / L and an SO42- concentration of 10 wt%. 2- The ion concentration is 2100 mg / L;

[0123] In step S3, the first concentrated water and another stream of saturated sodium chloride solution enter the electrodialysis crystallization unit, with an operating current density of 35 mA / cm². 2 The voltage was 0.7V, and the flow rate ratio of saturated sodium chloride solution to the first concentrated water was 1:0.67. The product was centrifuged to obtain 70823 mg of the first salt product (total mass of the first salt ion concentration was 67494 mg), electrodialysis desalination water, and concentrated wastewater from the system. The sodium chloride content in the electrodialysis desalination water was found to be 1.5 wt%, which was discharged into the low-temperature crystallization system. The sodium chloride recovery rate was 74.5%, and the purity was 95.3%.

[0124] Example 4

[0125] Similar to Example 1, except that in step S2, the nanofiltration permeate enters the first membrane concentration unit and is treated by reverse osmosis at 50°C with a recovery rate of 20%, yielding a first concentrate and a first permeate. The first concentrate has a salt content of 2 wt%, wherein Cl... - The ion concentration was 8957 mg / L, SO4 2- The ion concentration is 420 mg / L;

[0126] In step S3, the first concentrated water and another stream of saturated sodium chloride solution enter the electrodialysis crystallization unit, with an operating current density of 35 mA / cm². 2 The voltage was 0.7V, and the flow rate ratio of saturated sodium chloride solution to the first concentrated water was 1:0.67. The product was centrifuged to obtain 17814 mg of the first salt product (total mass of the first salt ion concentration was 13467 mg), electrodialysis desalination water, and concentrated wastewater from the system. The sodium chloride content in the electrodialysis desalination water was found to be 0.3% by mass, and this was discharged into the low-temperature crystallization system. The sodium chloride recovery rate was 14.9%, and the purity was 75.6%.

[0127] Example 5

[0128] Similar to Example 1, except that in step S2, the flow rate ratio of saturated sodium chloride solution to the first concentrated water was 1:0.2. The product was centrifuged to obtain 59815 mg of the first salt product (total mass of the first salt ion concentration was 50842 mg), electrodialysis desalination water, and system concentrated wastewater. The sodium chloride content in the electrodialysis desalination water was detected to be 4.5 wt%, and it was returned to the electrodialysis crystallization unit for cyclic electrodialysis. At this point, the sodium chloride content in the electrodialysis desalination water was detected to be 2.8%, and it was discharged into the low-temperature crystallization system. The sodium chloride recovery rate was 56.1%, and the purity was 85%.

[0129] Example 6

[0130] Similar to Example 1, except that in step 4, the nanofiltration concentrate enters the second membrane concentration unit and is treated by reverse osmosis at 10°C with a recovery rate of 15%, yielding a second concentrate and a second permeate. The second concentrate has a salt content of 3 wt%, wherein Cl... - The ion concentration was 1896 mg / L, SO42- 2- The ion concentration was 12280 mg / L;

[0131] In step S5, the second concentrated water enters the low-temperature crystallization unit and is subjected to low-temperature crystallization in an intermittent operation mode at 0°C. After running for 30 minutes, the low-temperature crystallization mother liquor and crystallized salt are obtained, and 56988 mg of the second salt product (the total mass of the second salt ion concentration is 44792 mg) is separated. The recovery rate of sodium sulfate is 51.5%, and the purity is 78.6%.

[0132] Example 7

[0133] Similar to Example 1, except that in step S3, the flow rate ratio of saturated sodium chloride solution to the first concentrated water was 1:1.2. The product was centrifuged to obtain 82024 mg of the first salt product (total mass of the first salt ion concentration was 81203 mg), electrodialysis desalination water, and system concentrated wastewater. The sodium chloride content in the electrodialysis desalination water was measured to be 1.4 wt%, which was discharged into the low-temperature crystallization system. The sodium chloride recovery rate was 89.6%, and the purity was 99%.

[0134] Comparative Example 1

[0135] Similar to Example 1, except that electrodialysis crystallization was not performed, and the first and second concentrated waters entered the low-temperature crystallization unit simultaneously. Otherwise, it was the same as Example 1, with a sodium chloride recovery rate of 0%, a sodium sulfate recovery rate of 84%, and a purity of 51%.

[0136] Comparative Example 2

[0137] Similar to Example 2, except that the mass percentage of sodium chloride in the electrodialysis desalination water was detected to be 5.2 wt%, and it entered the electrodialysis crystallization unit. At this time, the mass percentage of sodium chloride in the electrodialysis desalination water was detected to be 4.5 wt%, and it was discharged into the low-temperature crystallization system. The first salt product was 68878 mg (the total mass of the first salt ion concentration was 61301 mg), the recovery rate of sodium chloride was 67.6%, and the purity was 89%.

[0138] Comparative Example 3

[0139] Similar to Example 1, except that the first concentrated water obtained in step S2, another saturated sodium chloride solution, and the second concentrated water were all fed into the low-temperature crystallization unit. Low-temperature crystallization was carried out at 0°C in intermittent operation mode. After running for 30 minutes, the low-temperature crystallization mother liquor and crystallized salt were obtained. The first salt product was 0 mg, and the second salt product was 13467 mg (the total mass of the second salt ion concentration was 4538 mg). The sodium chloride recovery rate was 0%, the sodium sulfate recovery rate was 5.2%, and the purity was 33.7%.

[0140] The results from the examples and comparative examples show that the technical solution of the present invention, which uses electrodialysis crystallization to extract the first salt product and low-temperature crystallization unit to extract the second salt product, has a significantly higher recovery rate and purity, and can treat mixed salt wastewater in a more resource-efficient manner.

[0141] The preferred embodiments of the present invention have been described in detail above; however, the present invention is not limited thereto. Within the scope of the inventive concept, various simple modifications can be made to the technical solutions of the present invention, including combinations of various technical features in any other suitable manner. These simple modifications and combinations should also be considered as the content disclosed in the present invention and are all within the protection scope of the present invention.

Claims

1. A method for treating mixed saline wastewater, characterized in that, The method includes the following steps: (1) Nanofiltration treatment of mixed saline wastewater to obtain nanofiltration permeate and nanofiltration concentrate; (2) The nanofiltration permeate is subjected to a first membrane concentration treatment to obtain a first permeate and a first concentrate; (3) The first concentrated water is mixed with a saturated salt solution and subjected to electrodialysis crystallization to obtain the first salt product, the concentrated water from the system and the distilled water from electrodialysis; (4) The nanofiltration concentrate is subjected to a second membrane concentration treatment to obtain a second product water and a second concentrate; (5) The second concentrated water and the electrodialysis desalinated water are subjected to low-temperature crystallization to obtain the second salt product and the low-temperature crystallization mother liquor. The low-temperature crystallization mother liquor is returned to step (1) and mixed with the mixed salt wastewater. Wherein, when the content of the first salt product in the electrodialysis desalination water is >3wt%, the electrodialysis desalination water is returned to step (3) for electrodialysis crystallization; When the content of the first salt product in the electrodialysis desalination water is ≤3wt%, the electrodialysis desalination water proceeds to step (5) for low-temperature crystallization. The flow rate ratio of the saturated salt solution to the first concentrated water is 1:0.3-5; The conditions for the low-temperature crystallization include: a crystallization temperature of -10°C to 10°C; The first salt product is a monovalent anionic salt, and the second salt product is a divalent anionic salt.

2. The method according to claim 1, characterized in that, In step (1), the pH of the mixed salt wastewater is ≤12 and the total salt content of the mixed salt wastewater is ≥1wt%.

3. The method according to claim 2, characterized in that, In step (1), the total salt content of the mixed salt wastewater is ≥3wt%.

4. The method according to claim 1, characterized in that, In step (1), the content of organic matter COD in the mixed salt wastewater is <20mg / L.

5. The method according to claim 1, characterized in that, In step (1), the total concentration of calcium and magnesium ions in the mixed saline wastewater is <20 mg / L, and Fe... 3+ The concentration of ions is <1 mg / L.

6. The method according to claim 1, characterized in that, In step (1), the nanofiltration treatment results in a desalination rate of <60% for the first salt in the mixed salt wastewater and a desalination rate of ≥95% for the second salt in the mixed salt wastewater.

7. The method according to claim 1, characterized in that, In step (1), the recovery rate of nanofiltration is 50-80%.

8. The method according to claim 1, characterized in that, The first membrane concentration process is selected from at least one of reverse osmosis, high-pressure reverse osmosis, and forward osmosis.

9. The method according to claim 1, characterized in that, The salt content of the first concentrated water is ≥3wt%.

10. The method according to claim 9, characterized in that, The salt content of the first concentrated water is ≥10wt%.

11. The method according to claim 1, characterized in that, The salinity of the first produced water is ≤0.1wt%.

12. The method according to claim 11, characterized in that, The salinity of the first produced water is ≤0.05wt%.

13. The method according to claim 1, characterized in that, The recovery rate of the first membrane concentration treatment is 30-90%, and the treatment temperature is 20-50℃.

14. The method according to claim 13, characterized in that, The recovery rate of the first membrane concentration treatment is 50-80%, and the treatment temperature is 25-35℃.

15. The method according to claim 1, characterized in that, The conditions for the electrodialysis crystallization treatment include: a current density of 20-50 mA / cm². 2 The DC voltage is 0.1-1V.

16. The method according to claim 15, characterized in that, The conditions for the electrodialysis crystallization treatment include: a current density of 30-40 mA / cm². 2 The DC voltage is 0.3-0.7V.

17. The method according to claim 1, characterized in that, The flow rate ratio of the saturated salt solution to the first concentrated water is 1:1-2.

5.

18. The method according to claim 1, characterized in that, The salinity of the electrodialysis desalinated water is ≤5wt%.

19. The method according to claim 18, characterized in that, The salinity of the electrodialysis desalinated water is ≤3.5wt%.

20. The method according to claim 1, characterized in that, The salt content of the second concentrate is ≥4wt%.

21. The method according to claim 20, characterized in that, The salt content of the second concentrate is ≥8wt%.

22. The method according to claim 1, characterized in that, The salinity of the second produced water is ≤0.1wt%.

23. The method according to claim 22, characterized in that, The salinity of the second product water is ≤0.05wt%.

24. The method according to claim 1, characterized in that, The recovery rate of the second membrane concentration treatment is 30-90%, and the treatment temperature is 20-50℃.

25. The method according to claim 24, characterized in that, The recovery rate of the second membrane concentration treatment is 50-80%, and the treatment temperature is 25-35℃.

26. The method according to claim 1, characterized in that, The conditions for low-temperature crystallization include a crystallization temperature of 0°C to 5°C.

27. The method according to claim 1, characterized in that, The salt content of the low-temperature crystallization mother liquor is 3-8 wt%.

28. The method according to claim 27, characterized in that, The salt content of the low-temperature crystallization mother liquor is 4-6 wt%.

29. The method according to claim 1, characterized in that, The recovery rate of the monovalent anionic salt is 74-95%, and the purity is 95-100%.

30. The method according to claim 1, characterized in that, The second salt product is separated to obtain a divalent anionic salt with a recovery rate of 85-95% and a purity of 90-100%.

31. The method according to claim 30, characterized in that, The separation method includes at least one of centrifugal separation, filtration separation, and precipitation separation.

32. The method according to claim 30, characterized in that, The separation conditions include a residence time of 0.1-3 hours.

33. The method according to claim 32, characterized in that, The separation conditions include a residence time of 0.3-0.5 hours.

34. An apparatus for treating mixed saline wastewater, characterized in that, The device includes: a nanofiltration unit, a first membrane concentration unit, a second membrane concentration unit, an electrodialysis crystallization unit, and a low-temperature crystallization unit; The nanofiltration unit is connected to the first membrane concentration unit and the second membrane concentration unit, and is used to perform nanofiltration treatment on mixed wastewater to obtain nanofiltration permeate and nanofiltration concentrate. The first membrane concentration unit is connected to the electrodialysis crystallization unit and is used to perform a first membrane concentration treatment on the nanofiltration permeate to obtain a first permeate and a first concentrate. The electrodialysis crystallization unit is connected to the low-temperature crystallization unit and is used to perform electrodialysis crystallization treatment on the first concentrated water to obtain the first salt product, system concentrated water and electrodialysis desalinated water. The second membrane concentration unit is connected to the low-temperature crystallization unit and is used to perform a second membrane concentration treatment on the nanofiltration concentrate to obtain a second concentrate and a second product water. The low-temperature crystallization unit is connected to the second membrane concentration unit, the electrodialysis crystallization unit, and the nanofiltration unit, respectively, and is used to perform low-temperature crystallization treatment on the second concentrated water and the electrodialysis desalinated water to obtain the second salt product and the low-temperature crystallization mother liquor; the low-temperature crystallization mother liquor is returned to the nanofiltration unit for circulating nanofiltration. When the content of the first salt product in the electrodialysis desalination water is >3wt%, the electrodialysis desalination water is returned to the electrodialysis crystallization unit for electrodialysis crystallization treatment. When the content of the first salt product in the electrodialysis desalination water is ≤3wt%, the electrodialysis desalination water enters the low-temperature crystallization unit for low-temperature crystallization.

35. The application of the method of claim 1 or the apparatus of claim 34 in the treatment of mixed saline wastewater.

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