High-power concentration method of seawater

Abstract

The invention discloses a high-power concentration method of seawater, and belongs to the technical field of comprehensive utilization of seawater. The method comprises the following steps: sequentially carrying out electrodialysis concentration and nanofiltration separation on pretreated seawater to realize high-power concentration of the seawater; an anion exchange membrane used in the electrodialysis concentration is a monovalent / divalent selective anion exchange membrane, and a cation exchange membrane is a conventional cation exchange membrane. By adopting the method disclosed by the invention, high-power concentration of the seawater can be realized without relying on monovalent / divalent selective cation exchange membranes with high preparation difficulty, sodium ions in the nanofiltration produced water are concentrated by more than 5.5 times relative to the original seawater, the content of calcium and magnesium ions in total salt is less than 2%, and the nanofiltration produced water can be directly evaporated to prepare salt.

Description

Technical Field

[0001] The invention relates to the technical field of comprehensive utilization of seawater, in particular to a method for high-multiple concentration of seawater. Background Art

[0002] Conventional industrial salt production technology uses an ion exchange membrane with monovalent / divalent selectivity for both anions and cations to perform electrodialysis concentration on seawater. The concentrate is mainly monovalent ions, while divalent ions and high-valent ions with scaling tendency are retained in the desalted water without synchronous concentration. Therefore, high-fold concentration of seawater can be achieved, avoiding the scaling of scale ions during the concentration process that affects the concentration rate. However, due to the great difficulty in preparing monovalent / divalent selective cation exchange membranes, the large-scale application of seawater concentration and salt production has been limited. Therefore, it is very important to develop a process that can achieve high-fold concentration of seawater without using monovalent / divalent selective cation exchange membranes. Summary of the invention

[0003] The purpose of the present invention is to provide a method for high-multiple concentration of seawater to solve the problems existing in the above-mentioned prior art.

[0004] To achieve the above object, the present invention provides the following solutions:

[0005] The technical solution of the present invention is a method for high-multiple concentration of seawater, comprising the following steps: sequentially performing electrodialysis concentration and nanofiltration separation on the pretreated seawater to achieve high-multiple concentration of the seawater;

[0006] The anion exchange membrane used in the electrodialysis concentration is a monovalent / divalent selective anion exchange membrane.

[0007] Furthermore, the cation exchange membrane used in the electrodialysis concentration is a conventional cation exchange membrane.

[0008] The high-concentration method of seawater of the present invention firstly performs electrodialysis concentration on the pretreated seawater with an electrodialysis membrane composed of a monovalent / divalent selective anion exchange membrane and a conventional cation exchange membrane with low manufacturing difficulty. Due to the presence of the monovalent / divalent selective anion exchange membrane, the selective retention of sulfate is achieved during the concentration process, and the sulfate ions are not highly concentrated in the concentrate, while cations such as calcium and magnesium are concentrated in the concentrate together with sodium ions and chloride ions; then the electrodialysis concentrate is subjected to nanofiltration separation to remove calcium and magnesium, and the nanofiltration water is a highly concentrated material with calcium, magnesium and sulfate ions removed, which can be used to evaporate the salt making process to make salt (NaCl). The method of the present invention avoids the dependence of the high-concentration process of seawater on the monovalent / divalent selective cation exchange membrane by combining electrodialysis concentration and nanofiltration separation, and does not require the pretreatment softening and hardness removal process of conventional concentration, avoids the generation of solid wastes such as calcium and magnesium salt mud, and is environmentally friendly.

[0009] Furthermore, the product of the calcium ion concentration (molar concentration) and the sulfate ion concentration (molar concentration) in the concentrated solution obtained by the electrodialysis concentration is less than the critical solubility product of calcium sulfate.

[0010] Furthermore, the parameters of the electrodialysis concentration include: current density of 300-400A / m 2 .

[0011] Furthermore, the concentration ratio is controlled to be 5.5-6.0 during the electrodialysis concentration process.

[0012] Furthermore, during the electrodialysis concentration process, the TDS of the desalted water obtained by the electrodialysis concentration needs to be controlled to be 15000-20000 mg / L.

[0013] Furthermore, the current efficiency needs to be controlled to be ≥80% during the electrodialysis concentration process.

[0014] The electrodialysis concentration process involves ion concentration and water electrolysis. In order to reduce water electrolysis, the TDS of the desalted water needs to be controlled at 15000-20000 mg / L and the current density needs to be controlled at 300-400 A / m 2 To ensure that the current efficiency of the electrodialysis process is ≥80% (if the TDS of the desalted water is lower than 15000 mg / L, the solution conductivity is poor and the current efficiency cannot be guaranteed to be ≥80%; if the TDS of the desalted water is higher than 20000 mg / L, the salt loss in the desalted water is large, resulting in waste; if the current density is lower than 300 A / m 2 , low current efficiency; current density is higher than 400A / m 2 , there is a risk of water being electrolyzed); at the same time, because calcium ions are concentrated, when the electrodialysis concentration ratio is too high, there will be a risk of scaling when the concentration product of calcium ions and sulfate ions exceeds the critical solubility product of the calcium sulfate solution state. Therefore, the concentration ratio of the electrodialysis process needs to be controlled so that the product of the calcium ion concentration and the sulfate ion concentration in the concentrated solution obtained by electrodialysis concentration is less than the critical solubility product of calcium sulfate. In summary, in the electrodialysis concentration process, the control of parameters such as current density, concentration ratio, and desalted water TDS can achieve efficient and high-multiple concentration of seawater without scaling.

[0015] The critical solubility product is not a fixed value, but is related to the solution temperature and the ion composition (TDS) of the solution. When the solution temperature is 25°C and the total TDS in the solution is 180,000-192,000 mg / L, the critical solubility product of calcium sulfate is 3.20×10 -3 -3.24×10 -3 between.

[0016] Furthermore, the product of the calcium ion concentration (molar concentration) and the sulfate ion concentration (molar concentration) in the concentrated water obtained by the nanofiltration separation is less than the critical solubility product of calcium sulfate.

[0017] Furthermore, the parameters of the nanofiltration separation include: a water recovery rate of 15-25%.

[0018] Sulfate ions and calcium ions in nanofiltration concentrated water (after nanofiltration separation, two products, produced water and concentrated water, can be obtained) have a tendency to scale, so the nanofiltration separation process needs to control the produced water recovery rate to ensure that the product of the calcium ion concentration and the sulfate ion concentration in the concentrated water obtained by nanofiltration separation is less than the critical solubility product of calcium sulfate. That is, by controlling the nanofiltration produced water recovery rate during the nanofiltration separation process, calcium and magnesium can be removed to avoid scaling of the nanofiltration concentrated water.

[0019] Furthermore, the pretreatment includes: performing multi-media filtration, ultrafiltration and magnetization treatment on the seawater in sequence.

[0020] The purpose of multi-media filtration and ultrafiltration is to remove insoluble matter and macromolecular organic matter from seawater. The purpose of magnetizing the seawater after ultrafiltration is to reduce the risk of scaling in the subsequent concentration process. The principle of magnetization to prevent scaling is that the calcium and magnesium ions in the water are inactivated by the magnetic field, thereby preventing the formation of scale. Multi-media filtration, ultrafiltration and magnetization processes do not change the ion composition of seawater.

[0021] Furthermore, the filter medium used in the multi-media filtration is quartz sand.

[0022] Furthermore, the filtration accuracy of the ultrafiltration membrane used in the ultrafiltration is 0.1 μm.

[0023] Furthermore, the magnetization treatment includes: magnetizing the ultrafiltered seawater through the NS inter-pole gap of two permanent magnets, and the flow rate of the seawater is 0.05-0.3 m / s.

[0024] The present invention discloses the following technical effects:

[0025] The method of the present invention can achieve high-fold concentration of seawater without relying on a monovalent / divalent selective cation exchange membrane that is difficult to prepare. The sodium ions in the nanofiltration water are concentrated by more than 5.5 times relative to the original seawater, and the calcium and magnesium ions account for less than 2% of the total salt content. The nanofiltration water can be directly evaporated to produce salt.

[0026] Compared with the traditional chemical precipitation method for removing calcium and magnesium ions, the present invention does not produce salt mud solid waste; compared with the resin method for removing calcium and magnesium ions, the present invention does not produce wastewater for resin regeneration; in summary, the present invention only produces by-product light salt water during the high-fold concentration of seawater, which can be directly discharged into the sea, and the process is environmentally friendly. BRIEF DESCRIPTION OF THE DRAWINGS

[0027] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings required for use in the embodiments will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying creative work.

[0028] Figure 1 The figure is a process flow diagram of the high-multiple concentration method of seawater in the present invention. DETAILED DESCRIPTION

[0029] Various exemplary embodiments of the present invention will now be described in detail. This detailed description should not be considered as limiting the present invention, but should be understood as a more detailed description of certain aspects, features, and embodiments of the present invention.

[0030] It should be understood that the terms described in the present invention are only for describing a particular embodiment and are not intended to limit the present invention. In addition, for the numerical range in the present invention, it should be understood that each intermediate value between the upper and lower limits of the scope is also specifically disclosed. The intermediate value in any stated value or stated range, and each smaller range between any other stated value or intermediate value in the described range is also included in the present invention. The upper and lower limits of these smaller ranges can be independently included or excluded in the scope.

[0031] Unless otherwise indicated, all technical and scientific terms used herein have the same meanings as those generally understood by those skilled in the art. Although the present invention describes only preferred methods and materials, any methods and materials similar or equivalent to those described herein may also be used in the implementation or testing of the present invention. All documents mentioned in this specification are incorporated by reference to disclose and describe the methods and / or materials associated with the documents. In the event of a conflict with any incorporated document, the content of this specification shall prevail.

[0032] It will be apparent to those skilled in the art that various modifications and variations may be made to the specific embodiments of the present invention description without departing from the scope or spirit of the present invention. Other embodiments derived from the present invention description will be apparent to the skilled artisan. The present invention description and examples are exemplary only.

[0033] The words “include,” “including,” “have,” “contain,” etc. used in this document are open-ended terms, meaning including but not limited to.

[0034] It should be pointed out that the matters not described in detail in the present invention are conventional operating means in the art and are not the focus of the present invention.

[0035] The present invention provides a method for high-concentration of seawater, comprising the following steps: performing electrodialysis concentration and nanofiltration separation on the pretreated seawater in sequence to achieve high-concentration of the seawater (the process flow diagram is shown in FIG. Figure 1 shown);

[0036] The anion exchange membrane used in the electrodialysis concentration is a monovalent / divalent selective anion exchange membrane.

[0037] As an embodiment of the present invention, the cation exchange membrane used in the electrodialysis concentration is a conventional cation exchange membrane.

[0038] As a preferred embodiment of the present invention, the product of the calcium ion concentration and the sulfate ion concentration in the concentrated solution obtained by electrodialysis concentration is less than the critical solubility product of calcium sulfate.

[0039] As a preferred embodiment of the present invention, the parameters of the electrodialysis concentration include: a current density of 300-400A / m 2 .

[0040] As a preferred embodiment of the present invention, the concentration ratio of the electrodialysis concentration is 5.5-6.0.

[0041] As a preferred embodiment of the present invention, the TDS of the desalted water obtained by electrodialysis concentration is 15000-20000 mg / L.

[0042] As a preferred embodiment of the present invention, the current efficiency of the electrodialysis concentration is ≥80%.

[0043] As a preferred embodiment of the present invention, the electrodialysis concentration can be operated intermittently (i.e., the feed is intermittent) or continuously (i.e., the feed is continuous). In intermittent operation, when the current density is fixed, the concentration ratio and the TDS of the desalted water can be controlled by adjusting the voltage, thereby controlling the current efficiency; in continuous operation, when the current density is fixed, the concentration ratio and the TDS of the desalted water can be controlled by adjusting the voltage and the feed flow rate (one of the two can be fixed and the other can be changed, or both can be changed), thereby controlling the current efficiency.

[0044] As a preferred embodiment of the present invention, the product of the calcium ion concentration and the sulfate ion concentration in the concentrated water obtained by the nanofiltration separation is less than the critical solubility product of calcium sulfate.

[0045] As a preferred embodiment of the present invention, the water recovery rate of the nanofiltration separation is 15-25%.

[0046] As a preferred embodiment of the present invention, the pretreatment comprises: performing multi-media filtration, ultrafiltration and magnetization treatment on the seawater in sequence.

[0047] As a preferred embodiment of the present invention, the filter medium used in the multimedia filtration is quartz sand.

[0048] As a preferred embodiment of the present invention, the filtration accuracy of the ultrafiltration membrane used in the ultrafiltration is 0.1 μm.

[0049] As a preferred embodiment of the present invention, the magnetization treatment includes: magnetizing the ultrafiltered seawater through the NS inter-pole gap of two permanent magnets, and the flow rate of the seawater is 0.05-0.3 m / s.

[0050] The method for high-multiple concentration of seawater of the present invention is further described below in conjunction with specific embodiments.

[0051] The model of the mono / divalent selective anion exchange membrane used in the following examples and comparative examples is AC (purchased from Shanghai Sanji New Material Technology Co., Ltd.); the model of the mono / divalent cation exchange membrane is CSO (purchased from AGC Co., Ltd. of Japan); the model of the conventional cation exchange membrane is CC (purchased from Shanghai Sanji New Material Technology Co., Ltd.); the model of the nanofiltration membrane is DK1812 (purchased from Suez Environment Group).

[0052] The electrodialysis concentration process and nanofiltration separation process in the following examples and comparative examples were all carried out at 25°C.

[0053] The critical solubility product of calcium sulfate under specific conditions involved in the following examples and comparative examples is obtained by consulting the "Relationship between the solubility product of calcium sulfate and ionic strength at 25° C." in the "Wastewater Treatment Engineering Technical Manual".

[0054] Example 1

[0055] A method for high concentration of seawater, comprising the following steps:

[0056] S1. Pretreatment of seawater

[0057] Take 50.5L of surface seawater, whose TDS (total dissolved solids) is 32000mg / L, and its main ion composition is: sodium ion concentration 9750mg / L, magnesium ion concentration 1190mg / L, calcium ion concentration 355mg / L, chloride ion concentration 17630mg / L, sulfate ion concentration 2140mg / L (some ions with very little content or basically no effect on electrodialysis and nanofiltration are not listed). The above surface seawater is subjected to multi-media filtration and ultrafiltration treatment to obtain 50L of ultrafiltration water. The filter medium in the multi-media filter is quartz sand, the ultrafiltration membrane material is anti-pollution PVDF material, and the ultrafiltration membrane filtration accuracy is 0.1μm. The ultrafiltered seawater is magnetized through the NS interpolar gap of two permanent magnets, and the seawater flow rate is 0.2m / s. The ion composition of the seawater does not change after multi-media, ultrafiltration and magnetization treatment.

[0058] S2. Electrodialysis concentration

[0059] The seawater (50 L) magnetized in step S1 was subjected to electrodialysis concentration (intermittent operation, i.e., 50 L of seawater was added at one time). The positive membrane in the electrodialysis concentration process was a conventional cation exchange membrane, and the negative membrane was a monovalent / divalent selective anion exchange membrane. The current density of the electrodialysis concentration process was set to 300 A / m 2 , by adjusting the voltage, the concentration ratio (the ratio of the TDS of the concentrate obtained by electrodialysis to the TDS of the electrodialysis inlet water) was 5.78, the TDS of the electrodialysis desalted water was 18621 mg / L, and the current efficiency was 90.5%. After electrodialysis concentration, 3.72L of concentrate was obtained, and the TDS of the concentrate was 185000 mg / L. Its main ion composition was: sodium ion concentration 59200 mg / L, magnesium ion concentration 5860 mg / L, calcium ion concentration 2300 mg / L, chloride ion concentration 103600 mg / L, and sulfate ion concentration 3980 mg / L. The salt recovery rate of the electrodialysis concentration process ((TDS of the concentrate × volume of the concentrate) / (TDS of the electrodialysis inlet water × volume of the electrodialysis inlet water)) was 43.0%. The product of the molar concentrations of calcium ions (2300 mg / L, i.e. 0.0575 mol / L) and sulfate ions (3980 mg / L, i.e. 0.0415 mol / L) in the concentrate is 2.38×10 -3 , which is less than the critical solubility product of calcium sulfate under this condition (under the condition that the solution TDS is 185000 mg / L) 3.20×10 -3 , the electrodialysis process does not scale. After 20 experiments, it was verified that the electrodialysis membrane did not scale.

[0060] S3, Nanofiltration Separation

[0061] The 3.72L electrodialysis concentrate obtained in step S2 was subjected to nanofiltration separation. The operating pressure of the nanofiltration separation process was 0.25MPa. To prevent the precipitation of calcium sulfate during the nanofiltration process, the nanofiltration water recovery rate was controlled to 20%. The TDS of the concentrated water obtained by nanofiltration separation was 189390mg / L. The product of the molar concentrations of calcium ions (2400mg / L) and sulfate ions (4859mg / L) in the concentrated water (i.e., (2400 / 40 / 1000)×(4859 / 96 / 1000)) was 3.03×10 -3 , which is less than the critical solubility product of calcium sulfate under this condition, 3.20×10 -3, the nanofiltration process does not scale. After the nanofiltration separation, 0.74L of nanofiltration water was obtained, and its TDS was 167436mg / L. Its main ion composition was: sodium ion concentration 55609mg / L, magnesium ion concentration 1119mg / L, calcium ion concentration 1410mg / L, chloride ion concentration 95759mg / L, and sulfate ion concentration 61.7mg / L. Due to the interception effect of nanofiltration on divalent ions, the concentrations of sulfate ions, calcium ions and magnesium ions in the nanofiltration water were reduced, and the concentrations of monovalent substances such as chloride ions and sodium ions in the nanofiltration water were high, achieving high concentration of seawater. The sodium ions in the nanofiltration water were concentrated 5.70 times relative to the original seawater, and the impurity calcium and magnesium ions accounted for 1.51% of the total salt content (calcium and magnesium ion content / TDS), and the impurity sulfate ions accounted for 0.04% of the total salt content. The nanofiltration water can be directly evaporated to make salt. After 20 experiments, it was verified that the nanofiltration membrane did not scale.

[0062] Example 2

[0063] A method for high concentration of seawater, comprising the following steps:

[0064] S1. Pretreatment of seawater

[0065] Take 50.5L of surface seawater, whose TDS (total dissolved solids) is 32000mg / L, and its main ion composition is: sodium ion concentration 9750mg / L, magnesium ion concentration 1190mg / L, calcium ion concentration 355mg / L, chloride ion concentration 17630mg / L, sulfate ion concentration 2140mg / L. The above surface seawater is subjected to multi-media filtration and ultrafiltration treatment to obtain 50L of ultrafiltration water. The filter medium in the multi-media filter is quartz sand, the ultrafiltration membrane material is anti-pollution PVDF material, and the ultrafiltration membrane filtration accuracy is 0.1μm. The ultrafiltered seawater is magnetized through the NS inter-pole gap of two permanent magnets, and the seawater flow rate is 0.1m / s. The ion composition of the seawater does not change after multi-media, ultrafiltration and magnetization treatment.

[0066] S2. Electrodialysis concentration

[0067] The seawater (50 L) magnetized in step S1 was subjected to electrodialysis concentration (intermittent operation), the positive membrane of the electrodialysis concentration process was a conventional cation exchange membrane, and the negative membrane was a monovalent / divalent selective anion exchange membrane. The current density of the electrodialysis concentration process was set to 320 A / m 2By adjusting the voltage, the concentration ratio was controlled to be 5.63, the TDS of the electrodialysis desalted water was 14002 mg / L, and the current efficiency was 85.5%. After electrodialysis concentration, 5.12 L of concentrated solution was obtained, and the TDS of the concentrated solution was 180000 mg / L. Its main ion composition was: sodium ion concentration 57100 mg / L, magnesium ion concentration 5830 mg / L, calcium ion concentration 2280 mg / L, chloride ion concentration 108600 mg / L, and sulfate ion concentration 3970 mg / L. The salt recovery rate of the electrodialysis concentration process was 57.6%. The product of the molar concentration of calcium ions and sulfate ions in the concentrated solution was 2.36×10 -3 , less than the critical solubility product of calcium sulfate under the condition that the solution TDS is 180000 mg / L) 3.20×10 -3 , the electrodialysis process will not scale. After 20 experiments, it was verified that the electrodialysis membrane did not scale.

[0068] S3, Nanofiltration Separation

[0069] The 5.12L electrodialysis concentrate obtained in step S2 was subjected to nanofiltration separation. The operating pressure of the nanofiltration separation process was 0.25MPa. To prevent the precipitation of calcium sulfate during the nanofiltration process, the nanofiltration water recovery rate was controlled to 18%. The TDS of the concentrated water obtained by nanofiltration separation was 183732mg / L, and the product of the molar concentrations of calcium ions (2473mg / L) and sulfate ions (4833mg / L) in the concentrated water was 3.11×10 -3 , which is less than the critical solubility product of calcium sulfate under this condition, 3.20×10 -3 , the nanofiltration process will not scale. After the nanofiltration separation, 0.92L of nanofiltration water was obtained, and its TDS was 163000mg / L. Its main ion composition was: sodium ion concentration 58349mg / L, magnesium ion concentration 1114mg / L, calcium ion concentration 1397mg / L, chloride ion concentration 100380mg / L, sulfate ion concentration 36.1mg / L. The sodium ions in the nanofiltration water were concentrated 5.98 times relative to the original seawater, the impurity calcium and magnesium ions accounted for 1.54% of the total salt content, and the impurity sulfate ions accounted for 0.02% of the total salt content. The nanofiltration water can be directly evaporated to make salt. After 20 experiments, it was verified that the nanofiltration membrane did not scale.

[0070] Comparative Example 1

[0071] A method for high concentration of seawater, comprising the following steps:

[0072] S1. Pretreatment of seawater

[0073] Take 50.5L of surface seawater, whose TDS (total dissolved solids) is 32000mg / L, and its main ion composition is: sodium ion concentration 9750mg / L, magnesium ion concentration 1190mg / L, calcium ion concentration 355mg / L, chloride ion concentration 17630mg / L, sulfate ion concentration 2140mg / L. The above surface seawater is subjected to multi-media filtration and ultrafiltration treatment to obtain 50L of ultrafiltration water. The filter medium in the multi-media filter is quartz sand, the ultrafiltration membrane material is anti-pollution PVDF material, and the ultrafiltration membrane filtration accuracy is 0.1μm. The ultrafiltered seawater is magnetized through the NS inter-pole gap of two permanent magnets, and the seawater flow rate is 0.1m / s. The ion composition of the seawater does not change after multi-media, ultrafiltration and magnetization treatment.

[0074] S2. Electrodialysis concentration

[0075] The seawater (50 L) magnetized in step S1 was subjected to electrodialysis concentration (intermittent operation), the positive membrane of the electrodialysis concentration process was a conventional cation exchange membrane, and the negative membrane was a monovalent / divalent selective anion exchange membrane. The current density of the electrodialysis concentration process was set to 320 A / m 2 By adjusting the voltage, the concentration ratio was controlled to be 5.63, the TDS of the electrodialysis desalted water was 14002 mg / L, and the current efficiency was 85.5%. After electrodialysis concentration, 5.12 L of concentrated solution was obtained, and the TDS of the concentrated solution was 180000 mg / L. Its main ion composition was: sodium ion concentration 57100 mg / L, magnesium ion concentration 5830 mg / L, calcium ion concentration 2280 mg / L, chloride ion concentration 108600 mg / L, and sulfate ion concentration 3970 mg / L. The salt recovery rate of the electrodialysis concentration process was 57.6%.

[0076] S3, Nanofiltration Separation

[0077] The 5.12L electrodialysis concentrate obtained in step S2 was subjected to nanofiltration separation. The operating pressure of the nanofiltration separation process was 0.25MPa, and the nanofiltration water recovery rate was controlled to be 40%. 2.05L of nanofiltration water was obtained. The TDS of the concentrated water obtained by nanofiltration separation was 191333mg / L, and the product of the molar concentration of calcium ions (2868mg / L) and sulfate ions (6592mg / L) in the concentrated water was 4.92×10 -3 , which is greater than the critical solubility product of calcium sulfate under this condition, 3.24×10 -3 ,There is a risk of scaling in the nanofiltration process. After three consecutive similar tests, the nanofiltration membrane was clogged and could no longer be used.

[0078] Comparative Example 2

[0079] In this comparative example, an ion exchange membrane having monovalent / divalent selectivity for both anions and cations is used to conduct electrodialysis concentration on seawater.

[0080] S1. Pretreatment of seawater

[0081] Take 50.5L of surface seawater, whose TDS (total dissolved solids) is 32000mg / L, and its main ion composition is: sodium ion concentration 9750mg / L, magnesium ion concentration 1190mg / L, calcium ion concentration 355mg / L, chloride ion concentration 17630mg / L, sulfate ion concentration 2140mg / L. The above surface seawater is subjected to multi-media filtration and ultrafiltration treatment to obtain 50L of ultrafiltration water. The filter medium in the multi-media filter is quartz sand, the ultrafiltration membrane material is anti-pollution PVDF material, and the ultrafiltration membrane filtration accuracy is 0.1μm. The ultrafiltered seawater is magnetized through the NS inter-pole gap of two permanent magnets, and the seawater flow rate is 0.2m / s. The ion composition of the seawater does not change after multi-media, ultrafiltration and magnetization treatment.

[0082] S2. Electrodialysis concentration

[0083] The seawater (50 L) magnetized in step S1 was subjected to electrodialysis concentration (intermittent operation), and the positive membrane of the electrodialysis concentration process was a monovalent / divalent selective cation exchange membrane, and the negative membrane was a monovalent / divalent selective anion exchange membrane. The current density of the electrodialysis concentration process was set to 300 A / m 2 By adjusting the voltage, the concentration ratio (the ratio of the TDS of the concentrate obtained by electrodialysis to the TDS of the electrodialysis inlet water) was controlled to be 5.80, the TDS of the electrodialysis desalted water was 18600 mg / L, and the current efficiency was 90.7%. After electrodialysis concentration, 5L of concentrate was obtained, and the TDS of the concentrate was 185500 mg / L. Its main ion composition was: sodium ion concentration 63650 mg / L, magnesium ion concentration 1200 mg / L, calcium ion concentration 1200 mg / L, chloride ion concentration 110400 mg / L, and sulfate ion concentration 240 mg / L. The salt recovery rate (in terms of TDS) of the electrodialysis concentration process was 58.0%. The product of the molar concentrations of calcium ions (1200 mg / L) and sulfate ions (240 mg / L) in the concentrate is 0.075×10 -3 , which is less than the critical solubility product of calcium sulfate under this condition, 3.20×10 -3, the electrodialysis process does not scale. After 20 experiments, it was verified that the electrodialysis membrane did not scale. The electrodialysis concentrate can be directly evaporated to make salt. Under the same concentration multiple (the sodium ions in the electrodialysis concentrate are concentrated 6.53 times relative to the original seawater), the impurity calcium and magnesium ions in the concentrate account for 1.29% of the total salt content, which is slightly better than the data in Example 1 (1.51%) and Example 2 (1.54%); the impurity sulfate ions account for 0.13% of the total salt content, which is slightly worse than the data in Example 1 (0.04%) and Example 2 (0.02%); the concentrate obtained in Example 1 is generally close to the concentrate obtained in this comparative example in terms of ion composition.

[0084] Comparative Example 3

[0085] A method for concentrating seawater, comprising the following steps:

[0086] S1. Pretreatment of seawater

[0087] Take 50.5L of surface seawater, whose TDS (total dissolved solids) is 32000mg / L, and its main ion composition is: sodium ion concentration 9750mg / L, magnesium ion concentration 1190mg / L, calcium ion concentration 355mg / L, chloride ion concentration 17630mg / L, sulfate ion concentration 2140mg / L. The above surface seawater is subjected to multi-media filtration and ultrafiltration treatment to obtain 50L of ultrafiltration water. The filter medium in the multi-media filter is quartz sand, the ultrafiltration membrane material is anti-pollution PVDF material, and the ultrafiltration membrane filtration accuracy is 0.1μm. The ultrafiltered seawater is magnetized through the NS inter-pole gap of two permanent magnets, and the seawater flow rate is 0.2m / s. The ion composition of the seawater does not change after multi-media, ultrafiltration and magnetization treatment.

[0088] S2. Electrodialysis concentration

[0089] The seawater (50 L) magnetized in step S1 was subjected to electrodialysis concentration (intermittent operation), the positive membrane of the electrodialysis concentration process was a conventional cation exchange membrane, and the negative membrane was a monovalent / divalent selective anion exchange membrane. The current density of the electrodialysis concentration process was set to 300 A / m 2, by adjusting the voltage, the concentration ratio (the ratio of the TDS of the concentrate obtained by electrodialysis to the TDS of the electrodialysis inlet water) was controlled to be 3.47, the TDS of the electrodialysis desalted water was 19397 mg / L, and the current efficiency was 90.5%. After electrodialysis concentration, 6.32 L of concentrate was obtained, and the TDS of the concentrate was 111190 mg / L. Its main ion composition was: sodium ion concentration 37220 mg / L, magnesium ion concentration 3784 mg / L, calcium ion concentration 1435 mg / L, chloride ion concentration 65389 mg / L, and sulfate ion concentration 3162 mg / L. The salt recovery rate of the electrodialysis concentration process ((TDS of the concentrate × volume of the concentrate) / (TDS of the electrodialysis inlet water × volume of the electrodialysis inlet water)) was 43.9%. The product of the molar concentrations of calcium ions (1435 mg / L, i.e. 0.0359 mol / L) and sulfate ions (3162 mg / L, i.e. 0.0329 mol / L) in the concentrate is 1.18×10 -3 , which is less than the critical solubility product of calcium sulfate under this condition (under the condition that the solution TDS is 111190 mg / L) 3.10×10 -3 , the electrodialysis process does not scale. After 20 experiments, it was verified that the electrodialysis membrane did not scale.

[0090] S3, Nanofiltration Separation

[0091] The 6.32L electrodialysis concentrate obtained in step S2 was subjected to nanofiltration separation. The operating pressure of the nanofiltration separation process was 0.25MPa. To prevent the precipitation of calcium sulfate during the nanofiltration process, the nanofiltration water recovery rate was controlled to 20%. The TDS of the concentrated water obtained by nanofiltration separation was 113815mg / L, and the product of the molar concentrations of calcium ions (1573mg / L) and sulfate ions (3945mg / L) in the concentrated water (i.e., (1573 / 40 / 1000)×(3945 / 96 / 1000)) was 1.62×10 -3 , which is less than the critical solubility product of calcium sulfate under this condition, 3.10×10 -3 , the nanofiltration process does not scale. After the nanofiltration separation, 1.264L of nanofiltration water was obtained, and its TDS was 100688mg / L. Its main ion composition was: sodium ion concentration 38034mg / L, magnesium ion concentration 723mg / L, calcium ion concentration 880mg / L, chloride ion concentration 60440mg / L, sulfate ion concentration 28.8mg / L. The sodium ions in the nanofiltration water were concentrated 3.90 times relative to the original seawater, and the impurity calcium and magnesium ions accounted for 1.59% of the total salt content (calcium and magnesium ion content / TDS), and the impurity sulfate ions accounted for 0.03% of the total salt content. After 20 experimental verifications, the nanofiltration membrane did not scale. The nanofiltration water can be directly evaporated to make salt. The concentration ratio of the electrodialysis process is reduced, and the total salt content of the nanofiltration water is 100688mg / L, which is a low concentration. The energy consumption of the subsequent evaporation process is greatly increased, and the economy is poor.

[0092] The embodiments described above are only descriptions of the preferred modes of the present invention, and are not intended to limit the scope of the present invention. Without departing from the design spirit of the present invention, various modifications and improvements made to the technical solutions of the present invention by ordinary technicians in this field should all fall within the protection scope determined by the claims of the present invention.

Claims

1. A method for high concentration of seawater, characterized in that: The following steps are involved: The pretreated seawater is sequentially subjected to electrodialysis concentration and nanofiltration separation to achieve high-fold concentration of the seawater; The anion exchange membrane used in the electrodialysis concentration is a monovalent / divalent selective anion exchange membrane.

2. The method for high concentration of seawater according to claim 1, characterized in that: The cation exchange membrane used in the electrodialysis concentration is a conventional cation exchange membrane.

3. The method for high concentration of seawater according to claim 1, characterized in that: The product of the calcium ion concentration and the sulfate ion concentration in the concentrated solution obtained by the electrodialysis concentration is less than the critical solubility product of calcium sulfate.

4. The method for high concentration of seawater according to claim 1, characterized in that: The product of the calcium ion concentration and the sulfate ion concentration in the concentrated water obtained by the nanofiltration separation is less than the critical solubility product of calcium sulfate.

5. The method for high concentration of seawater according to claim 1, characterized in that: The pretreatment comprises: performing multi-media filtration, ultrafiltration and magnetization treatment on the seawater in sequence.

6. The method for high concentration of seawater according to claim 5, characterized in that: The filter medium used in the multi-media filtration is quartz sand.

7. The method for high concentration of seawater according to claim 5, characterized in that: The filtration accuracy of the ultrafiltration membrane used in the ultrafiltration is 0.1 μm.

8. The method for high concentration of seawater according to claim 5, characterized in that: The magnetization treatment includes: magnetizing the ultrafiltered seawater through the NS inter-pole gap of two permanent magnets, and the flow rate of the seawater is 0.05-0.3 m / s.

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