Method for operating electrodialysis device provided with hydrocarbon anion exchange membrane having ion selective permeation characteristics

Abstract

This method for operating an electrodialysis device comprises a step for electrodialyzing an iodide-containing solution containing an iodide salt and a solvent using an electrodialysis device provided with an anion exchange membrane having a monovalent ion permselective layer. In the anion exchange membrane electrodialysis step, a current-voltage curve indicating the relationship between a direct current and a voltage is obtained by a direct current resistance measurement method, and a minimum value appears in a quadratic differential value curve obtained by performing quadratic differentiation on the current-voltage curve. The current density (A / dm2) is set to a1 / d or more, where d is the effective area (dm2) of the anion exchange membrane and a1 is the value of the direct current (A) corresponding to the minimum value appearing in the quadratic differential value curve.

Description

Technical Field

[0001] This invention relates to the operation method of an electrodialysis apparatus and anion exchange membranes. Background Technology

[0002] Various technologies for recovering iodine from waste liquids have been developed to date. One such technology is, for example, the one described in Patent Document 1. In the waste liquid, besides iodide ions (I... - In addition to iodine components such as iodine, sulfate ions (SO4) sometimes also coexist. 2- Such as divalent ions. For example, in waste liquid, sulfate ions are usually contained at a concentration of more than 1 g / L and below the saturation solubility of sulfate, and more generally at around 20 to 50 g / L.

[0003] Patent Document 1 describes a method for electrodialysis of a stock solution containing inorganic anions with iodine and inorganic anions with fluorine, which is contained in a desalination chamber, using a monovalent selective anion exchange membrane or the like (claim 1 of Patent Document 1, examples, etc.).

[0004] Existing technical documents

[0005] Patent documents

[0006] Patent document 1: Japanese Patent Application Publication No. 2021-079318. Summary of the Invention

[0007] The problem that the invention aims to solve

[0008] However, the results of the inventors' research show that there is room for improvement in the electrodialysis method described in the aforementioned Patent Document 1 regarding the productivity of iodide ions.

[0009] That is, so far, the IV curve (current-voltage curve), which represents the current-voltage characteristics of anion exchange membranes, is known to be approximately linear in the case of direct current. Therefore, even if the current density is increased in the electrodialysis method, the membrane voltage applied to the anion exchange membrane increases proportionally, and thus it can be considered that the productivity of iodide ions cannot be improved.

[0010] Methods for solving problems

[0011] The inventors studied the current-voltage characteristics of anion exchange membranes and found that by appropriately controlling the monovalent selectivity by changing the density of the monovalent anion selective permeability layer in the anion exchange membrane, the IV curve of the anion exchange membrane in an aqueous solution containing iodide salts showed a new current-voltage characteristic in which the membrane voltage did not increase proportionally even when the current density was increased.

[0012] Based on this understanding, in-depth research was conducted, and the results showed that, as an indicator representing the aforementioned current-voltage characteristics, the effective area d of the anion exchange membrane and the value of the DC current a1 corresponding to the minimum value appearing in the curve obtained by second-differentiation of the IV curve are used, specifically "a1 / d". This is achieved by considering the current density (A / dm³) in the electrodialysis process. 2 Setting the value to "a1 / d" or higher can suppress the increase in membrane voltage even under high current density conditions. Therefore, the movement of iodide ions becomes easier, and the productivity (energy efficiency) of iodide ions can be improved compared to the case where the IV curve is linear.

[0013] According to one aspect of the present invention, the following method of operating an electrodialysis apparatus is provided.

[0014] 1. A method for operating an electrodialysis device, wherein, The method of operating the electrodialysis apparatus includes the step of performing electrodialysis on an iodide-containing solution containing iodide salt and solvent using the electrodialysis apparatus. The electrodialysis device includes an anion exchange membrane with a monovalent ion-selective permeability layer. An IV curve representing the relationship between direct current and voltage is obtained from the anion exchange membrane using a direct current resistance measurement method. A minimum value appears in the second derivative curve obtained by taking the second derivative of this IV curve. In the electrodialysis process, the effective area (dm²) of the anion exchange membrane is... 2 When d is set as d and the value of the DC current (A) corresponding to the minimum value appearing in the second derivative curve is set as a1, the current density (A / dm) is... 2 Set to a1 / d or higher.

[0015] 2. The operation method of the electrodialysis device as described in 1, wherein, The lower limit of a1 / d is 0.5A / dm. 2 above.

[0016] 3. The operation method of the electrodialysis apparatus as described in 1. or 2, wherein, The upper limit of a1 / d is 10A / dm 2 the following.

[0017] 4. The method of operating the electrodialysis apparatus as described in any one of 1. to 3, wherein, Let the value of the DC current (A) corresponding to the maximum value in the curve obtained by second differentiation of the IV curve be a2 (where a2 > a1). The current density (A / dm) in the electrodialysis process 2 () is set to a2 / d or less.

[0018] 5. The operation method of the electrodialysis apparatus as described in 4, wherein, a2 / d - a1 / d is 0.5A / dm 2 Above and 10A / dm 2 the following.

[0019] 6. The method of operating the electrodialysis apparatus as described in any one of 1. to 5, wherein, The SO4 index in the anion exchange membrane, as determined by the following seawater concentration test, is 3.0 × 10⁻⁶. -3 N or higher and 50.0 × 10 -3 Below N.

[0020] <Seawater Concentration Experiment>

[0021] Using a small-scale electrodialysis apparatus, seawater at 25°C was flowed into the desalination chamber at a flow rate of 6 cm / s, filling the concentration chamber with a 3.5 mol / L NaCl aqueous solution. Measurements were taken at a current density of 3 A / dm³. 2 The SO4 in the concentrate obtained by electrodialysis until the concentration change in the concentration chamber disappears 2- The concentration is used as the SO4 index mentioned above, and the membrane resistance of this small electrodialysis device is 1.5Ω. cm 2 Above and 2.5Ω cm 2 The following cation exchange membrane was used as a pair membrane, and the anion exchange membrane, which was the object of evaluation, was assembled with its monovalent selective surface facing the desalination chamber. The electrodialysis device has a membrane area of ​​100 cm². 2 .

[0022] 7. The method of operating the electrodialysis apparatus as described in any one of 1. to 6, wherein, The anion exchange membrane comprises a copolymer of styrene and divinylbenzene having quaternary ammonium groups.

[0023] 8. An anion exchange membrane having a monovalent ion-selective permeability layer for electrodialysis of iodide-containing solutions containing iodide salts and solvents only, wherein, When obtaining the IV curve representing the relationship between the DC current and voltage of the anion exchange membrane, as measured by the DC resistance measurement method, a minimum value appears in the second derivative curve obtained by taking the second derivative of the IV curve.

[0024] The effects of the invention

[0025] According to the present invention, an operating method for an electrodialysis apparatus with excellent iodide ion production rate and an anion exchange membrane for the operating method are provided. Attached Figure Description

[0026] Figure 1 This is a cross-sectional view schematically illustrating an example of the structure of the electrodialysis apparatus of this embodiment.

[0027] Figure 2 This is a diagram schematically illustrating an example of the configuration of the iodine recovery system of this embodiment.

[0028] Figure 3 This is a flowchart illustrating an example of the iodine recovery process in this embodiment.

[0029] Figure 4 This represents the IV curve of Comparative Example 1.

[0030] Figure 5 This represents the curve of the second derivative obtained by taking the second derivative of the IV curve of Comparative Example 1.

[0031] Figure 6 This represents the IV curve of Example 1.

[0032] Figure 7 This represents the second derivative curve obtained by taking the second derivative of the IV curve of Example 1.

[0033] Figure 8 This represents the IV curve of Example 2.

[0034] Figure 9 This represents the second derivative curve obtained by taking the second derivative of the IV curve of Example 2.

[0035] Figure 10 This represents the IV curve of Example 3.

[0036] Figure 11 This represents the second derivative curve obtained by taking the second derivative of the IV curve of Example 3.

[0037] Figure 12 This represents the IV curve of Comparative Example 2.

[0038] Figure 13 This represents the second derivative curve obtained by taking the second derivative of the IV curve of Comparative Example 2. Detailed Implementation

[0039] The embodiments of the present invention will now be described using the accompanying drawings. It should be noted that in all the drawings, the same symbols are used to denote the same constituent elements, and descriptions are omitted where appropriate. Furthermore, the drawings are schematic diagrams and do not conform to actual scale.

[0040] This section provides an overview of the operation method of the electrodialysis apparatus according to this embodiment.

[0041] The operation method of the electrodialysis apparatus in this embodiment includes a step of performing electrodialysis on an iodide-containing solution containing iodide salt and solvent using the electrodialysis apparatus. The electrodialysis device includes an anion exchange membrane with a monovalent ion-selective permeability layer. An IV curve representing the relationship between direct current and voltage is obtained from the anion exchange membrane using a direct current resistance measurement method. A minimum value appears in the second derivative curve obtained by taking the second derivative of this IV curve. In the electrodialysis process, the effective area (dm²) of the anion exchange membrane is... 2 Let d be the DC current (A) corresponding to the minimum value appearing in the second derivative curve, and let a1 be the value of the DC current (A / dm). 2 Set to a1 / d or higher.

[0042] According to the inventors' understanding, a new current-voltage characteristic of anion exchange membranes has been identified: a minimum value appears in the second derivative curve obtained by taking the second derivative of the IV curve. The presence of a minimum value in the second derivative curve implies that the first derivative curve represents a decreasing function, and the membrane resistance decreases as the current increases.

[0043] Furthermore, the aforementioned current-voltage characteristics were observed in current-voltage measurement tests using aqueous solutions containing iodide salts. Although the detailed mechanism is not yet clear, experiments have shown that these characteristics are not observed when using other halides such as chloride or bromide salts.

[0044] When using an anion exchange membrane with the current-voltage characteristics observed when using such iodide salts, the current density (A / dm³) during the electrodialysis process of an iodide-containing solution containing iodide salts is... 2 The current density is set to a1 / d or higher, so that the increase in membrane voltage can be suppressed even under high current density conditions. Therefore, the movement of iodide ions becomes easier, and the productivity (energy efficiency) of iodide ions can be improved compared with the case where the IV curve is linear.

[0045] Furthermore, preferably, when the value of the DC current (A) corresponding to the maximum value appearing in the curve obtained by second differentiation of the IV curve is set as a2 (where a2 > a1), the current density (A / dm³) in the electrodialysis process is set as follows: 2 () is set to a2 / d or less.

[0046] The fact that the second derivative curve has a maximum value after the minimum value indicates that the first derivative curve has changed to an increasing function, which means that the membrane resistance increases with the increase of current.

[0047] Therefore, by adjusting the current density (A / dm³) in the electrodialysis process 2Setting the value to below a2 / d can suppress the increase in membrane voltage under higher current density conditions, thus suppressing the decrease in iodide ion productivity.

[0048] Furthermore, according to the present inventors' understanding, by using the SO4 index measured under the following specified conditions as an indicator, although the mechanism is uncertain, it has been clarified that the presence or absence of a minimum value in the second derivative curve obtained from the IV curve measured by the DC resistance measurement method can be stably evaluated. This is presumably related to structural factors such as the density of the monovalent anion-selective layer on the surface of the anion exchange membrane.

[0049] In this specification, the SO4 index in a monovalent selective anion exchange membrane can be determined according to the seawater concentration test described below.

[0050] <Seawater Concentration Experiment>

[0051] In a small electrodialysis device (electrolyte membrane area 100 cm²) 2 In this process, a film resistance of 1.5Ω is used. cm 2 Above and 2.5Ω cm 2 The following cation exchange membranes were used as a pair membrane, and the anion exchange membranes to be evaluated were assembled with the monovalent selective surface facing the desalination chamber.

[0052] The operating conditions are as follows: seawater at 25°C flows into the desalination chamber at a flow rate of 6 cm / s; the concentration chamber is filled with a 3.5 mol / L NaCl aqueous solution; and the current density is 3 A / dm³. 2 Electrodialysis was performed for at least 5 hours (until the concentration change in the concentration chamber disappeared) to determine the chloride ion concentration (Cl) in the concentration chamber. - ) concentration (N) and sulfate ions (SO4) 2- ) concentration (10 -3 N). The obtained sulfate ion concentration is used as the SO4 index mentioned above.

[0053] Here, regarding the selectivity of membrane permeation between monovalent and divalent anions in a monovalent selective anion exchange membrane, Cl is typically chosen. - As a monovalent anion, on the other hand, SO4 is chosen. 2- As divalent anions, the selective permeability (P0) of these two ions during electrodialysis Cl SO4 This is because monovalent selective anion exchange membranes are mainly used for seawater concentration in the salt production industry. In fact, in this application, the selective permeability coefficient (P) mentioned above is used. Cl SO4 Small membranes can effectively implement Cl -Selective concentration. Therefore, even in the case of iodine recovery, it is necessary to avoid the concentration of sulfate ions mentioned above. If a monovalent selective membrane is used as the anion exchange membrane, then the concentration of sulfate ions from the above Cl... - and SO4 2- In the selection of permeability as specified above, the material with the smallest possible property value is usually chosen.

[0054] However, according to the research of the inventors, it has been found that in iodine recovery, the selective transmission coefficient (P0) commonly used in the salt industry is used in a certain way. Cl SO4 When the membrane is small, the iodide ions and SO42- involved are... 2- The permeation behavior and the above Cl - and SO4 2- The permeation behavior exhibits unexpected differences, failing to concentrate iodide ions at a sufficiently high permeation rate. Furthermore, further research revealed that if the "SO4 index" of the monovalent selective anion exchange membrane is effectively utilized as an indicator of iodide ion permeability, it unexpectedly shows a good correlation with the permeation rate, despite iodide ions being monovalent anions.

[0055] In this embodiment, the SO4 index can be controlled, for example, by appropriately selecting the types and amounts of each component contained in the monovalent selective anion exchange membrane, and the preparation method of the monovalent ion selective permeability layer. Details will be described later; for example, as factors for setting the SO4 index to a desired numerical range, appropriately controlling the density of the surface dense layer of the anion exchange membrane can be cited.

[0056] Here, the monovalent selective anion exchange membranes used so far are those used to selectively permeate monovalent anions from a treated solution containing a mixture of monovalent or polyvalent anions. Therefore, membranes with the highest possible monovalent selectivity have always been used. As a result, in the monovalent selective anion exchange membranes used so far, when measuring the SO4 index, membranes with the lowest possible permeation are used; specifically, most use membranes with permeation rates below 3.0 × 10⁻⁶. -3 N membrane.

[0057] In this case, as described above, it is preferable to use an SO4 index of 3.0 × 10⁻⁶. -3 N or higher and 50.0 × 10 -3 Monovalent selective anion exchange membranes with N below 1.

[0058] The lower limit of the SO4 index in monovalent selective anion exchange membranes is, for example, 3.0 × 10⁻⁶. -3 N or higher, preferably 5.0 × 10 N. -3N or more, preferably 6.0 × 10 N. -3 N or higher. Therefore, the current density corresponding to the minimum value appearing in the second derivative curve can be controlled to be low, which can improve the productivity of iodide ions during electrodialysis.

[0059] Additionally, by setting it to 6.0×10 -3 Above N, even during repeated electrodialysis, the reduction in iodide ion productivity can be suppressed. The reason for this is not certain, but it can be attributed to the interaction between iodide ions and the ion permeation path in the membrane.

[0060] On the other hand, the upper limit of the SO4 index in monovalent selective anion exchange membranes is, for example, 50.0 × 10⁻⁶. -3 The preferred value is 30.0 × 10. -3 N or less, more preferably 20.0 × 10 N. -3 Below N. Therefore, the concentration of divalent ions such as sulfate ions can be suppressed to a low level. In other words, the recovery rate of iodide ions relative to divalent and other polyvalent ions can be increased.

[0061] The following is a detailed description of the configuration of the operation method of the electrodialysis apparatus according to this embodiment.

[0062] Figure 1 This is a cross-sectional view schematically showing an example of the structure of an electrodialysis device 1. Figure 2 This is a diagram illustrating an example of the structure of an iodine recovery system 100. Figure 3 This is a flowchart illustrating an example of an iodine recovery process.

[0063] An example of an iodine recovery method using the operation method of the electrodialysis apparatus of this embodiment includes using Figure 1 The electrodialysis apparatus 1 is an electrodialysis process that separates an iodide-containing solution (waste liquid 10) containing iodide salt and solvent into an iodine-containing concentrate (concentrate liquid 20) and a desalination liquid (desalination liquid 30).

[0064] Figure 2 The iodine recovery system 100 only needs to have equipment for supplying waste liquid 10 to the electrodialysis unit 1, and also needs to have equipment for treating the concentrate 20 and / or desalination liquid 30 generated in the electrodialysis unit 1.

[0065] Waste liquid 10 (stock liquid) is any liquid that contains at least iodine containing iodide salts, without any particular limitation. For example, it includes waste liquid discharged from the manufacturing process of products containing iodine, waste liquid when disposing of products containing iodine, waste liquid discharged from the synthesis process that uses iodine as a reaction catalyst, and waste liquid used for washing in manufacturing equipment that uses iodine.

[0066] Examples of specific raw materials for waste liquid 10 include waste liquids, waste powders, and waste solids from the manufacturing processes, manufacturing equipment, or waste products containing iodine, such as polarizing films, contrast agents, disinfectants, and radiation-related materials; or waste liquids or waste solids containing iodine catalysts (organic or inorganic iodine compounds) used in chemical synthesis such as the synthesis of pharmaceuticals containing antibiotics / antivirals. These can be used individually or in combination of two or more.

[0067] For iodine recovery solutions, in cases where organic matter and / or organic solvents are contained in the waste liquid, known organic matter decomposition treatments can be performed in advance. One such organic matter decomposition treatment is, for example, shown as an aqueous solution of iodine recovered by combustion of the organic components (organic matter, organic solvents), adsorption treatment of sublimed iodine using an alkaline agent (e.g., sodium hydroxide and sodium bisulfite) and a reducing agent.

[0068] Iodine components, for example, those containing iodide ions (I... - It can be defined as a substance that has been reduced by one or more of the following groups: iodine (I2), iodic acid (HIO3), periodic acid (HIO4), and iodides (including inorganic or organic iodine compounds).

[0069] Here, in the waste liquid from the aforementioned raw materials, the iodine component typically contains 3% or more, preferably about 3% to 30% of iodide ions. Furthermore, in these waste liquids, sulfate ions are typically present at a concentration of 1 g / L or more, below the saturation solubility of sulfate, and more generally about 20 to 50 g / L. Additionally, the solid containing iodine typically contains about 30% to 99.8% elemental iodine by mass.

[0070] It should be noted that waste liquid 10 can be used directly in its untreated state, but chemical or physical pretreatment such as pH adjustment, dilution, and organic matter decomposition / removal can be carried out as needed.

[0071] like Figure 1 As shown, the electrodialysis apparatus 1 includes a concentration chamber 2, a desalination chamber 3, an ion exchange membrane separating the concentration chamber 2 and the desalination chamber 3, an anode 4, a cathode 5, and a power supply 6. The electrodialysis apparatus 1 is not limited to... Figure 1 It possesses a known device structure.

[0072] Electrode solution is supplied from an electrode solution tank (not shown) to an electrode chamber containing anode 4 (anode chamber) and an electrode chamber containing cathode 5 (cathode chamber). The anode chamber and cathode chamber are respectively located on opposite sides of an electrodialysis tank consisting of a concentration chamber 2 and a desalination chamber 3. Electrodialysis begins in the electrodialysis tank when current is applied to anode 4 and cathode 5 using a power source 6.

[0073] It should be noted that known electrode solutions can be used, such as aqueous solutions of sodium hydroxide, potassium hydroxide, sodium bisulfate, and potassium sulfate. However, in cases where bipolar membrane electrodialysis is performed after electrodialysis, the electrode solution used in the electrodialysis apparatus 1 is preferably sulfate-free.

[0074] In the electrodialysis cell, the concentration chamber 2 and the desalination chamber 3 are alternately arranged and separated by ion exchange membranes (either anion exchange membrane 7 or cation exchange membrane 8).

[0075] The electrodialysis apparatus 1 may have at least a desalination chamber 3, anion exchange membrane 7, concentration chamber 2, cation exchange membrane 8, and a desalination chamber 3 arranged in this order. From the viewpoint of improving processing capacity, it may also have one or more, preferably two or more, concentration chambers 2.

[0076] It should be noted that an electrolyte solution can also be contained in the concentration chamber 2 before electrodialysis. The electrolyte solution can be, for example, any aqueous solution of an inorganic salt having conductivity. For example, ion-exchanged water can be used initially when using the electrodialysis apparatus 1, and the previous concentrate can be used when resuming continuous use after stopping operation. Besides such an electrolyte solution, aqueous solutions of sodium chloride, potassium iodide, or sodium iodide can also be used. However, the electrolyte solution is preferably sulfate-free.

[0077] A specific example of an electrodialysis apparatus 1 used for electrodialysis as described above is that a plurality of spacer elements (also called chamber frames) are stacked between a pair of electrodes, and anion exchange membranes 7 or cation exchange membranes 8 are sandwiched between adjacent chamber frames. As a whole, the anion exchange membranes 7 and cation exchange membranes 8 are arranged alternately in principle, and anion exchange chambers (electrified sections) separated by anion exchange membranes 7 and cation exchange membranes 8 are formed in each spacer.

[0078] Furthermore, the ion exchange chamber is structured as follows: the chamber with the cation exchange membrane 8 located on the cathode 5 side and the anion exchange membrane 7 located on the anode 4 side is the desalination chamber 3, and the chamber with the anion exchange membrane 7 located on the cathode 5 side and the cation exchange membrane 8 located on the anode 4 side is the concentration chamber 2. The desalination chamber 3 and the concentration chamber 2 are arranged alternately. That is, if a treatment solution (electrolyte solution) is circulated to the desalination chamber 3 while an electric current is applied, the cations in the treatment solution supplied to the desalination chamber 3 move through the cation exchange membrane 8 to the adjacent concentration chamber 2 on the cathode 5 side, and the anions in the treatment solution are transferred through the anion exchange membrane to the adjacent concentration chamber 2 on the anode 4 side. As described above, if a brine solution is circulated to the concentration chamber 2 while the treatment solution is circulated to the desalination chamber 3, a concentrated solution 20 with an increased salt concentration can be obtained while the treatment solution is being desalinated.

[0079] As a method for fixing the stack of these ion exchange chambers between the electrodes, a method using a filter press is preferred, but it is not limited to this.

[0080] In this embodiment, at least one or all of the anion exchange membranes 7 included in the electrodialysis apparatus 1 are composed of monovalent selective anion exchange membranes whose SO4 index meets the above-mentioned numerical range.

[0081] Here, a monovalent selective anion exchange membrane refers to a membrane that, among anions, reacts with SO42-. 2- Compared to divalent ions, Cl- is selectively used. - Anion exchange membranes, typically used for monovalent ions like SO42-, allow monovalent ions to pass through. 2- and Cl - Selective transmittance (P) Cl SO4 Membranes with an SO4 index of 0.01 to 1.0 are readily available, and membranes that meet the specified SO4 index values ​​are also easily obtained. Therefore, in monovalent selective anion exchange membranes used for anion exchange membrane 7, this selective permeation coefficient (P) is... Cl SO4 The preferred value is 0.06 to 0.90, and more preferably 0.11 to 0.37.

[0082] Such monovalent selectivity of anions can be imparted by forming a monovalent ion-selective permeability layer on at least one side of the anion exchange membrane 7. The specific composition of the monovalent ion-selective permeability layer is not particularly limited; for example, it is preferably at least one layer selected from the group consisting of a surface-dense layer, an electrically neutral layer, and an oppositely charged layer. A "surface-dense layer" refers to a dense structure formed on the surface of the anion exchange membrane (e.g., a layer with a high degree of cross-linking or a layer with a high fixed ion concentration formed on the surface). An "electrically neutral layer" refers to an electrically neutral thin layer formed on the surface of the anion exchange membrane that does not contain anion exchange groups. Furthermore, an "oppositely charged layer" refers to a thin layer with cation exchange groups formed on the surface of the anion exchange membrane.

[0083] As a monovalent selective anion exchange membrane, for example, it can be used on one or both sides of an anion exchange membrane serving as a substrate, with an SO4 index of 3.0 × 10⁻⁶. -3 N or higher and 50.0 × 10 -3 A membrane with a formation amount of N or less forms a monovalent ion-selective permeable layer. This monovalent ion-selective permeable layer can be at least one layer selected from the group consisting of a surface-dense layer, an electrically neutral layer, and an oppositely charged layer. From the viewpoint of easily obtaining an anion exchange membrane with a specified SO4 index, a surface-dense layer is preferred. In particular, as this surface-dense layer, a highly cross-linked resin layer formed using a diamine compound is preferred. As the substrate anion exchange membrane, from the viewpoint of structural robustness and ease of forming a monovalent ion-selective permeable layer, anion exchange membranes in which quaternary ammonium groups as strongly basic anion exchange groups are introduced into the basic framework of a styrene-divinylbenzene copolymer are preferred.

[0084] These monovalent selective anion exchange membranes with SO4 indices meeting the above-mentioned numerical range can be manufactured as follows.

[0085] The method involves introducing anion exchange groups obtained by polymerizing a polymerizable composition containing a styrene-based aromatic polymerizable monomer with a haloalkyl group and a crosslinkable polymerizable monomer such as divinylbenzene into one or both surfaces of a primary membrane, contacting them with a diamine compound, wherein the SO4 index on the membrane surface is 3.0 × 10⁻⁶. -3 N or higher and 50.0 × 10 -3 A higher cross-linking density surface layer is formed when the amount of N is less than that of the interior. Then, a trialkylamine is contacted with the residual haloalkyl group in the membrane, converting it into a quaternary ammonium group. According to this method, in the original membrane for introducing anion exchange groups, the haloalkyl group present on one side of the membrane surface is consumed in a predetermined proportion through contact with a diamine compound during cross-linking. Therefore, since a dense structure is formed on the membrane surface through cross-linking, SO42-... 2- The permeability of ions is reduced, exhibiting selectivity for monovalent anions.

[0086] In this method, the aforementioned styrene-based aromatic polymerizable monomers having haloalkyl groups can be any known monomers without limitation. The alkyl group preferably has 1 to 8 carbon atoms, and examples of halogen atoms that can substitute for it include chlorine, bromine, and iodine. Examples of such haloalkyl groups include chloromethyl, bromomethyl, iodomethyl, chloroethyl, bromoethyl, iodoethyl, chloropropyl, bromopropyl, iodopropyl, chlorobutyl, bromobutyl, iodobutyl, chloropentyl, bromopentyl, iodopentyl, chlorohexyl, bromohexyl, and iodohexyl. Specific examples of such aromatic polymerizable monomers having halogenated alkyl groups include chloromethylstyrene, bromomethylstyrene, iodomethylstyrene, chloroethylstyrene, bromoethylstyrene, iodoethylstyrene, chloropropylstyrene, bromopropylstyrene, iodopropylstyrene, chlorobutylstyrene, bromobutylstyrene, iodobutylstyrene, chloropentylstyrene, bromopentylstyrene, iodopentylstyrene, chlorohexylstyrene, bromohexylstyrene, and iodohexylstyrene. Among these, chloromethylstyrene, bromomethylstyrene, iodomethylstyrene, chloroethylstyrene, bromoethylstyrene, iodoethylstyrene, chloropropylstyrene, bromopropylstyrene, iodopropylstyrene, chlorobutylstyrene, bromobutylstyrene, and iodobutylstyrene are particularly preferred.

[0087] In the original membrane for introducing anion exchange groups obtained by polymerizing the above-described polymeric composition, the aromatic polymeric monomer having a haloalkyl group in the membrane is converted into anion exchange group, namely a quaternary ammonium group, as described later. This quaternary ammonium group is a strongly basic group and is an excellent anion exchange group. However, if it is desired that anion exchange groups other than this group also exist, in the polymeric composition, in addition to the aromatic polymeric monomer having a haloalkyl group, polymeric monomers having other anion exchange groups or polymeric monomers having functional groups capable of introducing other anion exchange groups may also be used. As such anion exchange groups other than quaternary ammonium groups, there are no particular limitations as long as the functional group can be positively charged in aqueous solution; examples include primary amino, secondary amino, tertiary amino, pyridyl, imidazolyl, and quaternary pyridinium groups. In the polymeric composition, there is no particular limitation on the amount of polymeric monomers having other anion exchange groups or polymeric monomers having functional groups capable of introducing other anion exchange groups. It is preferably 100 parts by mass or less, more preferably 50 parts by mass or less, relative to 100 parts by mass of the above-mentioned aromatic polymeric monomers having halogenated alkyl groups.

[0088] Furthermore, in the polymerizable composition, in addition to the polymerizable monomer used to introduce anion exchange groups into these anion exchange membranes, a polymerizable monomer that is not directly related to the introduction of such anion exchange membranes may be incorporated in an amount of 150 parts by mass or less relative to 100 parts by mass of the aforementioned aromatic polymerizable monomer having a haloalkyl group. Examples of polymerizable monomers unrelated to the introduction of such anion exchange membranes include, for example, styrene, acrylonitrile, methylstyrene, ethyl vinylbenzene, acrolein, methyl vinyl ketone, and vinyl biphenyl.

[0089] In the polymeric composition, the amount of these polymeric monomers that are not directly related to the introduction of the anion exchange membrane is not particularly limited, but is preferably 400 parts by mass or less, more preferably 150 parts by mass or less, relative to 100 parts by mass of the above-mentioned aromatic polymeric monomers having halogenated alkyl groups.

[0090] To increase the density and thus improve the strength of the resulting anion exchange membrane, a crosslinking polymerizable monomer is used in the polymerizable composition. Such a crosslinking polymerizable monomer can also be any monomer conventionally known for the manufacture of ion exchange membranes, without particular limitation. Specifically, examples include, for instance, m-divinylbenzene, p-divinylbenzene or o-divinylbenzene, divinylbiphenyl, divinyl sulfone, butadiene, chloroprene, isoprene, trivinylbenzene, divinylnaphthalene, diallylamine, triallylamine, divinylpyridine, or other functional vinylbenzyl compounds having three or more vinylbenzyl groups disclosed in Japanese Patent Application Publication No. 62-205153.

[0091] When too much of these crosslinking polymeric monomers are combined with styrene-based aromatic polymeric monomers containing halogenated alkyl groups, the ion exchange capacity of the anion exchange membrane decreases, and the degree of crosslinking becomes too high, increasing membrane resistance and potentially reducing iodine concentration efficiency. Conversely, even if the proportion of crosslinking polymeric monomers is too low, not only may the membrane strength decrease, but the density of the highly crosslinked resin layer on the membrane surface may not increase, resulting in insufficient monovalent selectivity. Therefore, relative to 100 parts by mass of the polymeric monomer component consisting of aromatic polymeric monomers containing halogenated alkyl groups and other polymeric monomers as described above and used as needed, the crosslinking polymeric monomer should be 2 to 20 parts by mass, preferably 3 to 15 parts by mass.

[0092] In polymerizable compositions, polymerization initiators are typically incorporated. Conventionally known polymerization initiators can be used without particular limitations, provided that appropriate selection is made considering the substrate, molding conditions, and other factors. As specific examples, preferred peroxides include menthol hydroperoxide, diisopropylbenzene hydroperoxide, α,α'-bis(tert-butylperoxide-m-isopropyl)benzene, di-tert-butyl peroxide, tert-butyl hydroperoxide, di-tert-pentyl peroxide, tert-butylcumyl peroxide, dicumyl peroxide, 2,5-dimethyl-2,5-di(tert-butylperoxide)hexane, 2,5-dimethyl-2,5-di(tert-butylperoxide)hexyne-3, isopropylbenzene hydroperoxide, 1,1,3,3-tetramethylbutyl hydroperoxide, 2,5-dimethyl-2,5-dihydroperoxyhexane, 2,5-dimethyl-2,5-dihydroperoxyhexyne-3, benzoyl peroxide, methyl ethyl ketone peroxide, methyl isobutyl ketone peroxide, cyclohexane peroxide, methyl cyclohexane peroxide, isobutyl peroxide, and 2,4-dichlorobenzoyl peroxide. Peroxides, o-methylbenzoyl peroxide, bis-3,5,5-trimethylhexanoyl peroxide, lauroyl peroxide, p-chlorobenzoyl peroxide, 1,1-di-tert-butylperoxy-trimethylcyclohexane, 1,1-di-tert-butylperoxy-cyclohexane, 2,2-di(tert-butylperoxy)-butane, 4,4-di-tert-butylperoxyvalerate, 2,4,4-trimethylpentylperoxy-phenoxyacetic acid ester, α-cumyl peroxyneodecanate, tert-butyl peroxyneodecanate, tert-butyl peroxy-tert-valerate, tert-butyl peroxy-2-ethylhexanoate, tert-butyl peroxy-isobutyrate, di-tert-butyl peroxy-hexahydroterephthalate, di-tert-butyl peroxy-azelate, tert-butyl peroxy-3,5,5-trimethylhexanoate, tert-butyl peroxyacetic acid ester, tert-butyl peroxybenzoate, etc. They can be added individually or in combination of two or more to the monomer paste for mixing.

[0093] Relative to 100 parts by mass of the polymerizable monomer component described above, the amount of polymerization initiator used as described above is generally preferably 0.1 to 30 parts by mass, more preferably in the range of 1 to 10 parts by mass.

[0094] In addition, a matrix resin can be incorporated into the polymeric composition as a viscosity modifier. By incorporating such a matrix resin, the coatability of the polymeric composition can be improved, preventing sagging when applied to a substrate.

[0095] Examples of suitable matrix resins include polyvinyl chloride (PVC), chlorinated PVC, ethylene-vinyl chloride copolymers, vinyl chloride elastomers, chlorinated polyethylene, chlorosulfonated polyethylene, ethylene-propylene copolymers, saturated aliphatic hydrocarbon polymers such as polybutene, styrene-butadiene copolymers, and substances copolymerized with styrene monomers such as vinyltoluene, vinylxylene, chlorostyrene, chloromethylstyrene, α-methylstyrene, α-halostyrene, and α,β,β'-trihalostyrene, monoolefins such as ethylene and butene, and conjugated dienes such as butadiene and isoprene. Additionally, styrene-butadiene rubber or its hydrogenated rubber, nitrile rubber or its hydrogenated nitrile rubber, pyridine rubber or its hydrogenated rubber, and styrene-based thermoplastic elastomers are also preferred.

[0096] Here, styrene-based thermoplastic elastomers refer to alternating copolymers of polystyrene polymers with styrene and polybutadiene, polyisoprene, vinyl polyisoprene, ethylene-butene, or ethylene-propylene. Examples include polystyrene-hydrogenated polybutadiene-polystyrene copolymers, polystyrene-(polyethylene / butene rubber)-polystyrene copolymers, polystyrene-hydrogenated polyisoprene rubber-polystyrene copolymers, polystyrene-(polyethylene / propylene rubber)-polystyrene copolymers, polystyrene-polyethylene-(polyethylene / propylene rubber)-polystyrene copolymers, and polystyrene-vinyl polyisoprene-polystyrene copolymers. The molecular weight of the matrix resin is not particularly limited, but is generally preferred to be in the range of 1,000 to 1,000,000, and particularly preferred to be in the range of 50,000 to 500,000.

[0097] In addition, the matrix resin is incorporated into the polymeric composition in an amount that ensures adequate viscosity, based on its molecular weight. For example, the amount is preferably 1 to 50 parts by mass relative to 100 parts by mass of the polymeric monomer component, and more preferably 3 to 15 parts by mass.

[0098] It should be noted that, in addition to the above-mentioned components, plasticizers such as dioctyl phthalate, dibutyl phthalate, tributyl phosphate, styrene oxide, or alcohol esters of fatty acids and aromatic acids, as well as organic solvents, may also be added to the polymeric composition as needed.

[0099] In addition, in order to capture halogen gas and hydrogen halide gas generated by the thermal decomposition of the above-mentioned styrene-based aromatic polymerizable monomers having halogenated alkyl groups, it is also preferable to add compounds having one or more epoxy groups, such as styrene oxide and diethylene glycol diglycidyl ether.

[0100] A polymeric composition consisting of the above-mentioned components is filled into the voids of a substrate serving as a reinforcing material and then polymerized to obtain a primary membrane for introducing anion exchange groups. As such a substrate, any substrate known as a substrate for ion exchange membranes can be used, typically a support material with a porosity of 20–90%, more preferably 40–80%, and even more preferably 45–55%. Examples include woven fabrics, nonwoven fabrics, porous membranes, and meshes formed from polyvinyl chloride, polyolefins, etc. From the viewpoint of affinity or chemical resistance to ion exchange resins, substrates made of polyolefins such as polypropylene or polyethylene are particularly preferred, especially polyethylene substrates. Furthermore, regarding the shape of the substrate, from the viewpoint of improving the iodine concentration efficiency after imparting monovalent selectivity, a porous membrane is preferred. The thickness of the substrate is generally selected from the range of 50–300 μm, and from the viewpoint of maintaining membrane resistance and strength, 70–250 μm is preferred.

[0101] There are no particular limitations on the method of filling such a substrate with a polymeric composition. For example, methods such as coating or spraying the polymeric composition onto the substrate or impregnating the substrate with the polymeric composition can be cited. When the polymeric composition is in paste form, coating is preferred. Filling the substrate by coating can be carried out by known methods such as roller coaters, flow coaters, blade coaters, comma coaters, spraying, and impregnation.

[0102] As described above, after introducing the polymerizable composition into the substrate, it is laminated together with a release material having release properties to prevent adhesion between the substrates, wound onto a roller, and heated for polymerization.

[0103] As the aforementioned release material, a material with heat resistance capable of withstanding polymerization and easy to release after polymerization is used. Examples include films formed from any of the following resins: polypropylene, poly(1-butene), poly(4-methyl-1-pentene), or α-olefin random or block copolymers such as ethylene, propylene, 1-butene, and 4-methyl-1-pentene; ethylene-vinyl acetate copolymers, ethylene-vinyl alcohol copolymers, ethylene-vinyl chloride copolymers, polymethyl methacrylate, polyvinyl compounds such as polymethyl methacrylate; polyamides such as nylon 6, nylon 6-6, nylon 6-10, nylon 11, and nylon 12; thermoplastic polyesters such as polyethylene terephthalate, polybutylene terephthalate, and polyethylene naphthalate; polycarbonate; polyphenylene ether; biodegradable resins such as polylactic acid; or mixtures thereof. These films can also be biaxially stretched.

[0104] That is, as long as an appropriate film is selected from the above-mentioned films according to the type of monomer component in the polymeric composition, it can be used as a release material. In particular, from the viewpoint of heat resistance and mold release properties, polyester films such as polyethylene terephthalate (PET) are most preferred.

[0105] The pressure during polymerization can be either atmospheric pressure or pressurized pressure, typically around 0.1 to 1.0 MPa.

[0106] The polymerization temperature can be any temperature lower than the melting point of the substrate, typically preferably in the range of 40 to 130°C. That is, by heating to this temperature range for polymerization, a portion of the substrate dissolves in the polymerizable composition. Polymerization under these conditions results in improved bonding strength between the ion exchange resin and the substrate, further enhancing membrane strength and concentration efficiency.

[0107] It should be noted that the polymerization time varies depending on factors such as polymerization temperature, and is usually around 3 to 20 hours.

[0108] As described above, after coating the polymerizable composition onto a substrate, heating is performed to polymerize, resulting in a primary membrane for introducing anion exchange groups composed of a membrane-like polymer. To obtain the monovalent selective anion exchange membrane of the present invention, after obtaining the primary membrane for introducing anion exchange groups, an operation is performed to form a monovalent ion-selective permeable layer composed of a highly cross-linked resin layer on the surface of the membrane before performing the anion exchange group introduction operation. For example, this is performed by contacting one side of the membrane surface of the primary membrane for introducing anion exchange groups with a diamine compound.

[0109] That is, if the purpose is to form a monovalent anion selective layer but only one side of the original membrane for introducing anion exchange groups, the membrane surface on the other side is covered with a membrane or the like, so that the membrane surface on the side not covered by the membrane comes into contact with the diamine compound, and a highly cross-linked resin layer with a higher cross-linking content than the interior is formed on the membrane surface.

[0110] The amino group of the diamine compound reacts with the alkyl halogroups present in the original membrane to remove hydrogen chloride and form a CN bond. This CN bond then reacts with two alkyl halogroups in the diamine compound to form a cross-linked structure. The denser the cross-linked structure, the higher the membrane resistance; conversely, the higher the monovalent anion selectivity and the lower the SO4 index. Furthermore, the cross-linking reaction induced by the diamine compound proceeds from the membrane surface inwards; however, the deeper the cross-linking, the higher the monovalent anion selectivity, but the membrane resistance also increases.

[0111] Examples of secondary amine compounds include dimethylamine, diethylamine, dipropylamine, dibutylamine, and diethanolamine. Considering the balance between electrical resistance and monovalent anion selectivity, shorter substituents are better, with dimethylamine being the preferred choice.

[0112] When contacting a secondary amine compound solution to introduce anion exchange groups into the halogenated alkyl crosslinks on the surface of the original membrane, if the diffusion of the diamine compound into the interior of the original membrane for introducing anion exchange groups is too slow or the solubility is poor, a portion of the water can be replaced with an organic solvent to adjust the diffusion rate and improve solubility. In this case, hydrophilic solvents such as methanol, ethanol, 1-propanol, 2-propanol, and acetone can be used as the organic solvent for replacing the water. Its content relative to water is preferably 30% by mass or less, and particularly preferably 5 to 15% by mass.

[0113] In order to obtain the SO4 index of 3.0 × 10⁻⁶ -3 N or higher and 50.0 × 10 -3 For anion exchange membranes with a nitrogen content below N, a moderately dense surface layer is crucial, making the conditions for contacting and reacting with secondary amine compounds particularly important. For example, the concentration at which the secondary amine compound dissolves is preferably 0.01 mol / L to 2 mol / L, and particularly preferably 0.03 mol / L to 1 mol / L. At excessively low concentrations, the crosslinking reactivity between the haloalkyl group and the diamine compound decreases, and the monovalent ion selectivity decreases, making it difficult to set the SO4 index below the aforementioned upper limit. On the other hand, at excessively high concentrations, diffusion into the membrane interior of the original membrane for introducing anion exchange groups is enhanced, resulting in excessively deep crosslinking, and the SO4 index remains below the specified value.

[0114] Furthermore, the reaction temperature for contact with the diamine compound is 20–50°C, and the reaction time is 1–24 hours. When the reaction temperature is below 20°C and / or the reaction time is less than 1 hour, the crosslinking reaction of the haloalkyl group cannot proceed sufficiently, making it difficult to set the SO4 index below the aforementioned upper limit. When the reaction temperature is above 50°C and / or the reaction time is greater than 24 hours, diffusion into the membrane interior of the original membrane for introducing the anion exchange group is enhanced, the crosslinking depth becomes too deep, and the SO4 index remains below the specified value. It should be noted that after reacting the original membrane for introducing the anion exchange group with the secondary amine compound, washing can also be performed to remove excess secondary amine compounds that were not supplied for the reaction.

[0115] As described above, the reaction between diamine compounds and haloalkyl groups sometimes stops at the reaction between the diamine and one haloalkyl group, failing to achieve cross-linking between the two haloalkyl groups. In this case, it is preferable to perform a treatment step with an alkaline aqueous solution (alkaline aqueous solution treatment step) after the contact step with the diamine compound. Although the detailed reaction is not understood, it is speculated that the reaction with the second haloalkyl group is promoted by neutralizing the hydrogen chloride generated during the reaction of the diamine compound with one haloalkyl group using an alkaline compound.

[0116] As an alkaline aqueous solution, known alkaline aqueous solutions such as sodium hydroxide solution, potassium hydroxide solution, barium hydroxide solution, and ammonia solution can be used. Since the treatment can be performed more efficiently, strongly alkaline sodium hydroxide solution or potassium hydroxide solution is preferred.

[0117] The alkaline aqueous solution described above only needs to have a pH of 8 to 14 at 25°C, but for more efficient processing, a pH of 10 to 14 at 25°C is preferred. It should be noted that the processing temperature is preferably 50 to 70°C, more preferably 55 to 65°C. Below 50°C, the processing time is longer, thus reducing productivity. Above 70°C, the processing becomes harsh, and the reacted amino compounds may become free.

[0118] The treatment time of the alkaline aqueous solution in this process only needs to be 4 to 24 hours, preferably 6 to 12 hours, and more preferably 6 to 10 hours. If the treatment time is less than 4 hours, the treatment may be insufficient. If the treatment time is greater than 24 hours, the treatment becomes harsh, and the resistance increases.

[0119] Then, a step is performed to introduce quaternary ammonium groups into the residual haloalkyl groups in the original membrane after the alkaline aqueous solution treatment step using a tertiary amine compound (quaternary ammonium group introduction step). Examples of tertiary amine compounds include trimethylamine, triethylamine, N,N-dimethylpropylamine, and N-ethyl-N-methylbutylamine. From the viewpoint of resistance, trimethylamine is preferred.

[0120] By contacting the membrane with an aqueous solution of a tertiary amine compound, quaternary ammonium groups are introduced, thereby replacing residual haloalkyl groups present in the membrane further inward than the surface, without affecting the monovalent anion-selective layer composed of a highly cross-linked resin layer. This method can be performed using conventional methods for introducing quaternary ammonium groups in the manufacture of anion exchange membranes.

[0121] The SO4 index can be obtained as 3.0 × 10⁻⁶ using the above method. -3 N or higher and 50.0 × 10 -3 Monovalent selective anion exchange membranes with N below 1.

[0122] Furthermore, in order to exhibit moderate permeability of iodide ions, the anion exchange capacity of the monovalent selective anion exchange membrane used in this invention is preferably 0.5 to 4.0 meq / g. - The dry film is particularly preferably 0.8–3.0 meq / g. - The dry film has a moisture content of 10-40%, preferably 25-35%, and a resistivity of 1.2Ω. cm 2 Above and 3.0Ω cm 2The preferred value is 1.7Ω. cm 2 Above and 2.5Ω cm 2 The following is more preferably 1.8Ω cm 2 Above and 2.4Ω cm 2 The following range. Adjust the thickness to a range of 50–320 μm, particularly 70–280 μm. That is, to achieve such physical properties, appropriately set the composition of the polymerizable composition (monomer components, amount or type of crosslinking agent), the thickness of the substrate, and the crosslinking reaction conditions using the diamine compound. Additionally, in the seawater concentration test, chloride ions (Cl...) - The permeability is typically above 3.2 mol / L, preferably above 3.5 mol / L.

[0123] When the anion exchange membrane 7 includes other anion exchange membranes besides the monovalent selective anion exchange membrane whose SO4 index meets the above-mentioned numerical range, known anion exchange membranes can be used as other anion exchange membranes, such as monovalent ion selective permeable anion exchange membranes, fully permeable anion exchange membranes, and high-strength alkali-resistant anion exchange membranes, preferably monovalent ion selective permeable anion exchange membranes.

[0124] There are no particular limitations on the cation exchange membrane 8; strongly acidic cation exchange membranes, high-strength alkali-resistant cation exchange membranes, etc., can be used. Furthermore, the cation exchange membrane 8 can be a monovalent ion-selective cation exchange membrane; more specifically, a cation exchange membrane in which sulfonic acid groups, which are strongly acidic cation exchange groups, are introduced into the styrene-divinylbenzene basic framework can be used.

[0125] As commercially available cation exchange membranes, products such as Neosepta (registered trademark) CSE and Neosepta (registered trademark) CMB manufactured by ASTOM Corporation are available.

[0126] Furthermore, an anion exchange membrane having an SO4 index within the aforementioned numerical range, and an electrodialysis apparatus equipped with such an anion exchange membrane, can be applied to the iodine recovery process of this embodiment.

[0127] That is, the aforementioned electrodialysis apparatus is an electrodialysis apparatus that separates waste liquid containing iodide salts into an iodine-containing concentrate and a desalination solution. It is configured to include a concentration chamber for generating the iodine-containing concentrate, a desalination chamber for supplying the waste liquid, and an ion exchange membrane separating the concentration chamber and the desalination chamber. This ion exchange membrane is an anion exchange membrane that exhibits a minimum value in the quadratic derivative curve obtained by second-order differentiation of the IV curve, which represents the relationship between DC current and voltage measured according to the DC resistance measurement method. A maximum value may appear after the minimum value in this quadratic derivative curve. Furthermore, this anion exchange membrane is configured to have an SO4 index of 3.0 × 10⁻⁶. -3 N or higher and 50.0 × 10 -3 Monovalent selective anion exchange membranes with N below 1.

[0128] Furthermore, the aforementioned anion exchange membrane is an anion exchange membrane used in electrodialysis to separate iodide-containing waste liquid into iodine-containing concentrate and desalination solution. It is an anion exchange membrane for electrodialysis of iodine-containing waste liquid where a minimum value appears in the quadratic derivative curve obtained by second-differentiation of the IV curve representing the relationship between DC current and voltage as measured by the DC resistance method. A maximum value may appear after the minimum value in this quadratic derivative curve. This anion exchange membrane is a monovalent selective anion exchange membrane, and its SO4 index can be 3.0 × 10⁻⁶. -3 N or higher and 50.0 × 10 -3 Below N. Additionally, the anion exchange membrane may also contain a copolymer of styrene and divinylbenzene having quaternary ammonium groups.

[0129] An example of the operation of electrodialysis device 1 during electrodialysis is as follows.

[0130] When a direct current is applied between the anode 4 and the cathode 5 using power supply 6, iodide ions (I-) in the desalination chamber 3 on one side... - The cations (including at least one of monovalent cations such as alkali metal ions and divalent cations such as alkaline earth metal ions) in the desalting chamber 3 on the other side move through the cation exchange membrane 8 into the same concentration chamber 2. In this concentration chamber 2, iodide salts such as potassium iodide (KI) are generated, for example. A concentrated solution 20 containing iodide salts (iodine-containing concentrate) is obtained from the concentration chamber 2. Additionally, a desalted solution 30 is obtained from the desalting chamber 3.

[0131] Here, use Figure 1 , 2 An example of the process flow preceding the electrodialysis step in the iodine recovery method is illustrated.

[0132] First, the waste liquid 10 is stored in the waste liquid tank 11 via the production line 12 (piping).

[0133] The waste liquid 10 in waste liquid tank 11 can also be subjected to the above-mentioned chemical pretreatment or physical pretreatment.

[0134] When waste liquid 10 contains boron, a pH adjustment step can be performed before the electrodialysis step to adjust the pH of waste liquid 10 to 9.5 or below, preferably 8 or below, and more preferably less than 7. When the pH of waste liquid 10 is adjusted to 9.5 or below, most of the boric acid (H3BO3) does not dissociate and exists as molecules. Therefore, even if electrodialysis is performed, the boric acid hardly moves and is discharged into the desalination liquid 30 in its original state. As a result, the boron and iodine components in waste liquid 10 can be separated efficiently.

[0135] When waste liquid 10 is acidic (pH less than 7), it is preferable to adjust the pH of waste liquid 10 to 3 or higher. Since the generation of free iodine can sometimes degrade the ion exchange membrane and reduce the efficiency of electrodialysis, this method can suppress the generation of free iodine caused by the air oxidation of iodide ions and prevent the degradation of the ion exchange membrane caused by free iodine.

[0136] The lower limit of the temperature of the waste liquid 10 in the waste liquid tank 11 can be, for example, 25°C or higher, preferably 30°C or higher, and more preferably 35°C or higher. By increasing the temperature of the waste liquid 10 supplied to the desalination chamber 3 in the electrodialysis apparatus 1, the efficiency of electrodialysis can be improved.

[0137] Regarding the upper limit of the liquid temperature of waste liquid 10 in waste liquid tank 11, if there are constraints on the device side, it is preferable to be as high as possible within the range of its upper limit of resistance temperature. If the electrodialysis device 1 is equipped with PVC piping, it can be, for example, below 40°C.

[0138] Then, the waste liquid 10 in the waste liquid tank 11 is supplied to the desalination chamber 3 of the electrodialysis unit 1 via the production line 13. Voltage is applied to the anode 4 and the cathode 5 to perform electrodialysis (electrodialysis process).

[0139] At this time, in the electrodialysis process of this embodiment, as described above, the current density (A / dm³) is... 2 Set to a1 / d or higher.

[0140] On the other hand, in the electrodialysis process, when the second derivative curve of the anion exchange membrane 7 reaches a maximum value, it is preferable to set the current density to a2 / d or less.

[0141] Alternatively, when the current near the zero second derivative value after the minimum is set as a3, the current density can also be set to a3 / d or less. If no maximum value appears on the second derivative curve of the anion exchange membrane 7, it is preferable to set the current density to a3 / d or less.

[0142] In the electrodialysis process, the current density can be gradually increased and maintained within a range of a1 / d above and a2 / d below, or a1 / d above and a3 / d below, for a specified time.

[0143] It should be noted that the upper limit of the minimum value in the second derivative curve obtained from the IV curve, which is measured with the voltage unit set to mV and the current unit set to mA, is less than -0.005, preferably less than -0.01, and more preferably less than -0.02. The lower limit of this minimum value is not particularly limited and can be greater than -0.5, greater than -0.3, or greater than -0.2.

[0144] Furthermore, the lower limit of the maximum value in the above-mentioned quadratic differential curve is greater than 0.005, preferably greater than 0.007, and more preferably greater than 0.01. The upper limit of this maximum value is not particularly limited and can be less than 0.3, less than 0.2, or less than 0.1.

[0145] The lower limit of a1 / d mentioned above is, for example, 0.5A / dm. 2 The preferred value is 3.1 A / dm. 2 The above is preferred, with 4.0 A / dm. 2 That's all. Therefore, the productivity of iodide ions can be further improved.

[0146] In addition, the upper limit of a1 / d mentioned above is, for example, 10A / dm. 2 The preferred value is 9.5 A / dm. 2 Below, 9A / dm is preferred. 2 Therefore, it is possible to suppress the decline in the productivity of iodide ions.

[0147] The lower limit of a2 / d-a1 / d is, for example, 0.5A / dm. 2 The preferred value is 1.0 A / dm. 2 The above is preferred, with 1.5 A / dm. 2 That's all. Therefore, it's possible to consistently increase the productivity of iodide ions.

[0148] There is no specific upper limit for a2 / d - a1 / d, which can be 10A / dm. 2 The following can also be 6A / dm 2 the following.

[0149] The desalinated liquid 30 generated in the desalination chamber 3 via electrodialysis is recovered via production line 31. On the other hand, the concentrated liquid 20 generated in the concentration chamber 2 via electrodialysis is recovered via production line 22.

[0150] At this time, at least a portion of the desalination liquid 30 can also be supplied to the waste liquid tank 11 via production line 32, which branches off from production line 31, and the mixed desalination liquid formed by mixing the desalination liquid 30 and the waste liquid 10 can be supplied to the desalination chamber 3 again. That is, the waste liquid 10 supplied to the desalination chamber 3 may also contain the desalination liquid 30.

[0151] By repeatedly performing electrodialysis, the concentration of iodide ions in the desalination solution 30 can be reduced to the desired value.

[0152] Alternatively, at least a portion of the concentrate 20 can be supplied to the concentrate tank 21 via the production line 23 branching off from the production line 22, and the concentrate 20 in the concentrate tank 21 can be supplied to the concentration chamber 2 again via the production line 24.

[0153] By repeatedly performing electrodialysis, the concentration of iodide ions in concentrate 20 can be concentrated to the desired value.

[0154] Alternatively, when there are two or more electrodialysis units, the first concentrate generated by the first electrodialysis unit can be supplied to the desalination chamber of the second electrodialysis unit. Furthermore, the second desalination solution generated in the desalination chamber of the second electrodialysis unit can be mixed with the first concentrate and then supplied to the desalination chamber of the second electrodialysis unit again.

[0155] Alternatively, an electrodialysis apparatus 1 equipped with a continuous circulation electrodialysis device can also be used.

[0156] Such an electrodialysis device 1 can continuously supply waste liquid 10 or mixed desalination liquid to the desalination liquid 30 while continuously discharging the desalination liquid 30.

[0157] Then, use Figure 3 An example of the process flow for the regeneration step in an iodine recovery method is provided.

[0158] Then, as Figure 3 As shown, the iodine recovery method, after the electrodialysis process, can include a regeneration process using the obtained iodine concentrate (concentrate 20) to produce one or more regeneration processes selected from the group consisting of iodine (I2), iodide salts and hydroiodic acid.

[0159] The iodine, iodide salts, and hydroiodic acid regenerated (recycled) by the above iodine recovery method can be aqueous solutions, and the iodine and iodide salts can be powders (including granules).

[0160] The iodine recovery method can use the resulting concentrate 20 to recover hydroiodic acid. For example, as... Figure 3 As shown, the iodine recovery method may also include a step of separating the iodine-containing concentrate (concentrate 20) into hydroiodic acid and hydroxide salt aqueous solution using bipolar membrane electrodialysis (bipolar membrane electrodialysis step).

[0161] Here, bipolar membrane electrodialysis is a method in which, in addition to cation exchange membranes and / or anion exchange membranes, bipolar membranes that generate hydrogen ions and hydroxide ions are arranged sequentially between the anode and cathode. Treatment solutions are supplied to each chamber separated by the membranes and electricity is passed through them to obtain acids and bases from neutral salts.

[0162] Specifically, the concentrate 20 (e.g., KI concentrate) discharged from the electrodialysis unit 1 is introduced into the bipolar membrane electrodialysis unit. If necessary, the concentrate 20 can be acidified by adjusting its pH to less than 7. When a direct current is applied to the bipolar membrane electrodialysis unit, the iodide salts in the concentrate 20 are electrolyzed, resulting in the discharge of an aqueous solution of hydroiodic acid (HI solution) and an aqueous solution of hydroxide (e.g., KOH). The HI solution is then distilled and purified to obtain hydroiodic acid.

[0163] In addition, the iodine recovery method can use the obtained concentrate 20 to recover iodine (I2).

[0164] like Figure 3 As shown, iodine (I2) is obtained by oxidizing the concentrated solution 20 containing iodide ions.

[0165] The iodine recovery method can use the obtained concentrate 20 to recover iodide salts.

[0166] like Figure 3 As shown, neutralizing the hydroiodic acid obtained above yields an iodide salt. Additionally, reducing / neutralizing the iodine (I₂) obtained above yields an iodide salt.

[0167] In the iodine recovery method, the oxidation, neutralization and reduction methods mentioned above are not particularly limited, and well-known methods can be used.

[0168] It should be noted that iodide salts include, for example, alkali metal iodides and alkaline earth metal iodides.

[0169] Specific examples of iodide salts include potassium iodide, sodium iodide, calcium iodide, magnesium iodide, and cesium iodide. These can be used alone or in combination of two or more.

[0170] On the other hand, the iodine recovery method can use the obtained desalination solution 30 to recover iodine (I2).

[0171] like Figure 3 As shown, the iodine recovery method may also include the step of passing the obtained desalination solution 30 into a strongly basic anion exchange resin, so that the iodide ions remaining in the desalination solution 30 are adsorbed onto the strongly basic anion exchange resin, thereby recovering iodine from the strongly basic anion exchange resin.

[0172] Specifically, the desalination solution 30 discharged from the electrodialysis unit 1 is passed through a strongly basic ion exchange resin, which adsorbs iodide ions. At this time, the pH of the desalination solution 30 introduced into the strongly basic anion exchange resin is adjusted to less than 7, similar to the electrodialysis process described above, preferably to 3 or higher and less than 7. This suppresses the adsorption of borate ions formed from the dissociation of boric acid onto the strongly basic anion exchange resin, selectively separating iodide salt ions. Iodine (I₂) can then be recovered from the strongly basic anion exchange resin using known methods.

[0173] In addition, if the waste liquid 10 contains boron, boron (B2) can be recovered from the desalination liquid 30 containing boron.

[0174] The iodine recovery method may include a boron recovery step of separating and recovering a boron-containing concentrate (boric acid concentrate) from the desalination solution 30 using a separate electrodialysis device after adjusting the pH of the boron-containing desalination solution 30 to, for example, 7 or higher, preferably 8 or higher, and more preferably 11 or higher. By making the boron-containing concentrate acidic and performing crystallization, boric acid (H3BO3) can be recovered.

[0175] Alternatively, a desalination solution 30 containing boron with a pH of 7 or higher, or the desalination solution 30 that is passed into a strongly basic anion exchange resin as described above, is passed into a boron-selective chelating resin to adsorb borate ions. Boric acid can then be recovered from the boron-selective chelating resin using known methods.

[0176] The embodiments of the present invention have been described above, but these are merely examples, and various structures other than those described can be employed. Furthermore, the present invention is not limited to the embodiments described above; modifications and improvements that achieve the objectives of the present invention are also included in the present invention.

[0177] Example

[0178] The present invention will now be described in detail with reference to the embodiments, but the present invention is not limited to the description of these embodiments.

[0179] <Manufacturing of Monovalent Selective Anion Exchange Membranes>

[0180] The physical properties of the anion exchange membranes in the examples and comparative examples were determined by the following methods.

[0181] (Example 1)

[0182] 91.2 parts by weight of chloromethylstyrene, 5.0 parts by weight of divinylbenzene, 3.8 parts by weight of ethylvinylbenzene, 4.0 parts by weight of di-tert-butyl peroxide as a free radical polymerization initiator, and 4.0 parts by weight of styrene oxide as a hydrogen chloride gas scavenger were added to obtain a paste-like polymerizable composition.

[0183] Then, the above-mentioned polymeric composition is coated onto a biaxially extended porous polyethylene membrane (100 μm thick, 46% porosity, and 0.13 μm average pore size) as a substrate, thus introducing the polymeric composition into the substrate.

[0184] Then, both sides of the substrate incorporating the polymerizable composition were covered with a release material made of polyethylene terephthalate film, and the substrate was wound onto a roller for polymerization. The polymerization temperature was set as follows: the temperature was increased from 20°C to 50°C over 30 minutes, held at 50°C for 20 minutes, then increased to 130°C over 80 minutes, and held at 130°C for 4 hours to produce a primary membrane for introducing anion exchange groups.

[0185] Then, the release film is peeled off from the original anion exchange group introduction membrane. At this point, the release material is not completely peeled off from both sides of the original membrane, but rather left adhering to only one side. This original anion exchange group introduction membrane, with only one side covered by the release material, is immersed in a 0.05 mol / L dimethylamine aqueous solution at 34°C for 8.5 hours, allowing the chloromethyl groups on the surface not covered by the release material to undergo a cross-linking reaction with the dimethylamine. The resulting original membrane is then washed with a 1.0 mol / L hydrochloric acid aqueous solution and pure water.

[0186] Then, after removing the single-sided stripping agent film, anion exchange groups are introduced into the original membrane, which is then immersed in a 0.01 mol / L sodium hydroxide aqueous solution at 60°C for 16 hours, performing a process of treatment with an alkaline aqueous solution. The original membrane treated with the alkaline aqueous solution is then washed with pure water.

[0187] Then, a quaternization reaction was carried out at 30°C for 16 hours using an aqueous solution containing 5% by mass trimethylamine and 25% by mass acetone, replacing the residual chloromethyl chloride in the membrane with trimethylamine to implement the quaternization group introduction process. The original membrane after quaternization group introduction was then washed with a 1.0 mol / L aqueous solution of hydrochloric acid and pure water to obtain a monovalent selective anion exchange membrane.

[0188] (Example 2)

[0189] Except that the reaction time of the dimethylamine aqueous solution was changed to 5 hours in Example 1, the same procedure was followed as in Example 1 to obtain a monovalent selective anion exchange membrane.

[0190] (Example 3)

[0191] Except that the treatment time using sodium hydroxide aqueous solution was changed to 8 hours in Example 1, the same procedure as in Example 1 was performed to obtain a monovalent selective anion exchange membrane.

[0192] (Comparative Example 1)

[0193] The commercially available ACS-8T (manufactured by Aston Corporation) was evaluated as a monovalent selective anion exchange membrane.

[0194] (Comparative Example 2)

[0195] Similar to Example 1, a primary membrane for introducing anion exchange groups was obtained. After peeling off the stripping agent membrane from both sides, the process of introducing quaternary ammonium groups using trimethylamine was performed without treatment with dimethylamine aqueous solution and alkaline aqueous solution, just like in Example 1, to obtain an anion exchange membrane without selective treatment.

[0196] <Method for Determining the DC Resistance of Anion Exchange Membranes>

[0197] The DC resistance of anion exchange membrane was measured in a 0.5 mol / L potassium iodide (KI) aqueous solution using a DC membrane resistance measuring device equipped with a silver / silver chloride electrode.

[0198] First, a four-chamber acrylic battery cell DC film resistance measuring device is prepared. The measuring device has: two electrode units, which have a silver electrode (anode) and a silver chloride electrode (cathode); and two measuring units, which are arranged in the center between the two electrode units and are equipped with a salt bridge.

[0199] A 0.5 mol / L potassium chloride (KCl) aqueous solution was introduced into the electrode unit as the electrode solution, and a 0.5 mol / L potassium iodide (KI) aqueous solution was introduced into the measuring unit as the electrolyte solution. The potential between the salt bridges was measured using a potentiometer via a glass electrode and taken as the membrane voltage. It should be noted that the temperatures of the electrode solution and electrolyte solution were maintained at 25°C (±0.5°C) using a water bath.

[0200] Then, the anion exchange membrane obtained above (effective area d: 0.04dm) 2 Soak in a 0.5 mol / L KI aqueous solution for 1 hour.

[0201] Then, the impregnated anion exchange membrane is sandwiched between the two measurement units and placed in the aforementioned DC membrane resistance measuring device. At this time, the monovalent selective surface (ion-selective permeability layer) of the anion exchange membrane is positioned facing the cathode side.

[0202] Then, the initial current value was set to 40 mA, and the DC current was increased by 40 mA every 20 seconds. At this time, the potential difference (V) was recorded every 20 seconds using a data logger to create an IV curve of the anion exchange membrane. The measurement ended at 400 mA after 180 seconds.

[0203] In addition, using the aforementioned DC membrane resistance measuring device without an anion exchange membrane, the voltage was calculated in the same manner as above, and a blank IV curve was generated.

[0204] Table 1 shows the IV curves of the anion exchange membranes in each figure, which are corrected curves after subtracting the blank IV curve. Here, the blank refers to the use of ion-exchanged water as the electrolyte solution instead of 0.5 mol / L potassium iodide (KI) aqueous solution in the DC resistance measurement method of the anion exchange membrane described above.

[0205] It should be noted that the IV curve is plotted with voltage (mV) on the vertical axis and current (mV) on the horizontal axis. The IV curves for Comparative Example 1, Examples 1-3, and Comparative Example 2 are as follows: Figure 4 , 6 8, 10, 12.

[0206] In the corrected IV curve of the anion exchange membrane, the current values ​​are set as X1, X2...Xn, and the voltage values ​​are set as Y1, Y2...Yn. n When n is 4 or more, The first derivative of the IV curve of anion exchange membrane (dY / dX) = (Y n-1 -Y n ) / [(X n-1 -X n ) / 2】, Defined as the second derivative (d) of the IV curve of anion exchange membrane. 2 Y / dX 2 ) = [(Y n-3 -Y n-2 ) / [(X n-3 -X n-2 ) / 2]-(Y n-1 -Y n ) / [(X n-1 -X n ) / 2]〕 / [(X n-1 -X n ) / 2] and calculated.

[0207] Here, a second derivative curve is constructed based on the calculated value of the second derivative. If a minimum value exists on the second derivative curve, the current value (a1) corresponding to the minimum value is shown in Table 1. If a maximum value exists, the current value (a2) corresponding to the maximum value is shown in Table 1.

[0208] In this second-order differential curve, variations within ±0.005 are not considered variations in the IV characteristic. That is, in the second-order differential curve obtained by measuring the IV curve with the voltage unit set to mV and the current unit set to mA, points existing within ±0.005 are excluded from the minimum or maximum values.

[0209] It should be noted that the quadratic differential curves of Comparative Example 1, Examples 1-3, and Comparative Example 2 are as follows: Figure 5 , 7 9, 11, 13.

[0210] <According to the Cl test of seawater concentration> - And SO4 index >

[0211] Small-scale electrodialysis device (electrode membrane area 100cm²) 2 In this study, a monovalent selective cation exchange membrane CIMS (manufactured by Aston Corporation, membrane resistance 2.0Ω) was used. cm 2 As a pair membrane, it is assembled together with the anion exchange membrane obtained above in such a way that the monovalent selective surface faces the desalination chamber.

[0212] The operating conditions are as follows: seawater at 25°C flows into the desalination chamber at a flow rate of 6 cm / s; the concentration chamber is filled with a 3.5 mol / L NaCl aqueous solution; and the current density is 3 A / dm³. 2 Electrodialysis was performed for more than 5 hours (until the concentration change in the concentration chamber disappeared) to determine the chloride ion concentration (Cl) in the concentration chamber. - ) and sulfate ions (SO4) 2- The concentration of chloride ions (Cl) in the concentration chamber was measured. - ) concentration (N) and sulfate ions (SO4) 2- ) concentration (10 -3 N). The obtained chloride ions (Cl) - ) concentration (N) and sulfate ions (SO4) 2- ) concentration as Cl - Transmission rate and SO4 index. The results are shown in Table 1.

[0213] <Select transmittance factor (P) Cl SO4 >

[0214] The effective current-carrying area is 4.0 cm². 2 A two-chamber glass unit, separated by an anion exchange membrane, was used. Silver / silver chloride electrodes were placed in each chamber. 100 ml of 0.5 mol / L NaCl aqueous solution was supplied to the anode chamber, and 100 ml of a mixed aqueous solution of 0.25 mol / L NaCl and 0.125 mol / L Na₂SO₄ was supplied to the cathode chamber. After applying a 40 mA DC current at 25°C for 1 hour, the amounts of chloride and sulfate ions in the cathode chamber were quantified. Based on the obtained chloride and sulfate ion amounts, the selective permeability coefficient P was calculated using the following formula. Cl SO4 .

[0215] P Cl SO4 = (t) SO4 / t Cl ) / (C SO4 / C Cl )

[0216] t SO4 It is the equivalent number of sulfate ions that permeate the membrane, t Cl It is the chloride ion equivalent number that permeates the membrane, C SO4 It is the equivalent concentration of sulfate ions in the cathode chamber after measurement, C Cl It is the equivalent concentration of chloride ions in the cathode chamber after measurement.

[0217] Compare P in Example 1 Cl SO4 It is 0.05.

[0218] Table 1

[0219] (Simulation of production efficiency in electrodialysis)

[0220] In a model where the IV curve of anion exchange membrane is linear, in an electrodialysis process containing iodide solution, when the current density is set to twice the normal value, the estimated processing time is 1 / 2 and the processing capacity is twice the normal value.

[0221] Processing capacity, or production per unit time (P), is proportional to current density (not voltage).

[0222] Here, the indicator used in the salt production process, namely "power consumption rate", is defined as current (I) × voltage (V) / production per unit time (P).

[0223] When the IV curve is a straight line, as above, if the current density is increased, the production per unit time (P) increases proportionally to the current (I).

[0224] On the other hand, as shown in Examples 1 to 3 above, when the IV curve is convex upward (there is a minimum value in the second derivative curve of the IV curve), the voltage V2 in the high current region (high current density region) of this minimum value becomes a smaller value than V1 when the IV curve is a straight line at the same current. That is, in the high current density region, V2 < V1.

[0225] Therefore, in the IV curves of Examples 1-3, even if the current density is increased to increase the production per unit time (P), the increase in "power consumption rate" can be suppressed in the high current density region because V2 < V1. In other words, in Examples 1-3, by setting the current density to "DC current (a1) in the high current density region" / "effective area (d) of the anion exchange membrane" or higher, it is expected to improve the energy efficiency (productivity) in the high current density region compared to the case where the IV curve is a straight line.

[0226] It should be noted that in the IV curves of Comparative Examples 1 and 2, the voltage V3 in the high current density region is larger or the same as that in the case where the IV curve is a straight line. Therefore, it is difficult to improve productivity compared with Examples 1 to 3.

[0227] This application claims priority based on Japanese Patent Application No. 2023-174426, filed on October 6, 2023, the entire disclosure of which is incorporated herein by reference.

[0228] Explanation of reference numerals in the attached figures

[0229] 1: Electrodialysis device.

[0230] 2: Concentration chamber.

[0231] 3: Desalination chamber.

[0232] 4: Anode.

[0233] 5: Cathode.

[0234] 6: Power supply.

[0235] 7: Anion exchange membrane.

[0236] 8: Cation exchange membrane.

[0237] 10: Waste liquid.

[0238] 11: Waste liquid tank.

[0239] 12, 13: Production line.

[0240] 20: Concentrate.

[0241] 21: Concentrate tank.

[0242] 22, 23, 24: Production lines.

[0243] 30: Desalination solution.

[0244] 31, 32: Production line.

[0245] 100: Iodine recovery system.

Claims

1. A method for operating an electrodialysis device, wherein, The method of operating the electrodialysis apparatus includes the step of performing electrodialysis on an iodide-containing solution containing iodide salt and solvent using the electrodialysis apparatus. The electrodialysis device includes an anion exchange membrane with a monovalent ion-selective permeability layer. A current-voltage curve representing the relationship between direct current and voltage is obtained from the anion exchange membrane using a direct current resistance measurement method. A minimum value appears in the second derivative curve obtained by taking the second derivative of the current-voltage curve. In the electrodialysis process, when the effective area of ​​the anion exchange membrane is set as d and the value of the DC current corresponding to the minimum value appearing in the second derivative curve is set as a1, the current density is set to a1 / d or higher. The unit of the effective area is dm. 2 The unit of direct current is ampere-ampere (A), and the unit of current density is ampere-ampere (A / dm³). 2 .

2. The method of operating the electrodialysis apparatus as described in claim 1, wherein, The lower limit of a1 / d is 0.5A / dm. 2 above.

3. The method of operating the electrodialysis apparatus as described in claim 1 or 2, wherein, The upper limit of a1 / d is 10A / dm 2 the following.

4. The method of operating the electrodialysis apparatus as described in claim 1 or 2, wherein, Let a2 be the value of the DC current corresponding to the maximum value in the curve obtained by second-differentiation of the current-voltage curve, and a2 > a1. The current density in the electrodialysis process is set to be below a² / d, where the unit of direct current is A and the unit of current density is A / dm. 2 .

5. The method of operating the electrodialysis apparatus as described in claim 4, wherein, a2 / d - a1 / d is 0.5A / dm 2 Above and 10A / dm 2 the following.

6. The method of operating the electrodialysis apparatus as described in claim 1 or 2, wherein, The SO4 index in the anion exchange membrane, as determined by the following seawater concentration test, is 3.0 × 10⁻⁶. -3 N or higher and 50.0 × 10 -3 Below N, Seawater concentration test: Using a small-scale electrodialysis apparatus, seawater at 25°C was flowed into the desalination chamber at a flow rate of 6 cm / s, filling the concentration chamber with a 3.5 mol / L NaCl aqueous solution. Measurements were taken at a current density of 3 A / dm³. 2 The SO4 in the concentrate obtained by electrodialysis until the concentration change in the concentration chamber disappears 2- The concentration is used as the SO4 index mentioned above, and the membrane resistance of this small electrodialysis device is 1.5Ω. cm 2 Above and 2.5Ω cm 2 The following cation exchange membrane was used as a pair membrane, and the anion exchange membrane, which was the object of evaluation, was assembled with its monovalent selective surface facing the desalination chamber. The electrodialysis device has a membrane area of ​​100 cm². 2 .

7. The method of operating the electrodialysis apparatus as described in claim 1 or 2, wherein, The anion exchange membrane comprises a copolymer of styrene and divinylbenzene having quaternary ammonium groups.

8. An anion exchange membrane having a monovalent ion-selective permeability layer for electrodialysis of iodide-containing solutions containing iodide salts and solvents only, wherein, When obtaining the current-voltage curve representing the relationship between DC current and voltage of the anion exchange membrane, as measured by the DC resistance measurement method, a minimum value appears in the second derivative curve obtained by second differentiation of the current-voltage curve.

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