Method for operating electrodialysis device provided with hydrocarbon-based anion exchange membrane having ion permselectivity characteristics

Abstract

This method for operating an electrodialysis device includes a step for electrodialyzing an iodide-containing solution which contains an iodide salt and a solvent with use of an electrodialysis device. The electrodialysis device includes an anion exchange membrane that has a monovalent ion permselective layer, in which a minimum appears in a secondary differential value curve that is obtained by acquiring, in accordance with a DC resistance measurement method, an IV curve that shows the relationship between the DC current and the voltage and secondarily differentiating the IV curve. In the step for electrodialysis, 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 DC current (A) corresponding to the minimum appeared in the secondary differential value curve.

Description

Method for operating an electrodialysis device equipped with a hydrocarbon-based anion exchange membrane having ion-selective permeability

[0001] The present invention relates to a method for operating an electrodialysis apparatus and an anion exchange membrane.

[0002] Various techniques for recovering iodine from wastewater have been developed. For example, the technique described in Patent Document 1 is known as one of these techniques. The wastewater contains iodide ions (I － In addition to iodine components such as sulfate ions (SO ４ ２－ ) may also be present. For example, the wastewater typically contains sulfate ions at a concentration of 1 g / L or more but less than the saturated solubility of sulfate, more typically about 20 to 50 g / L. Patent Document 1 describes a method for performing electrodialysis on a raw solution containing iodine-containing inorganic anions and fluorine-containing inorganic anions contained in a desalination compartment using a monovalent selective anion exchange membrane or the like (see, for example, Claim 1 and Examples of Patent Document 1).

[0003] Japanese Patent Application Laid-Open No. 2021-079318

[0004] However, as a result of investigations by the present inventors, it has been found that the electrodialysis method described in Patent Document 1 leaves room for improvement in terms of iodide ion productivity. Specifically, it has been known that the IV curve (current-voltage curve) representing the current-voltage characteristics of an anion exchange membrane is nearly linear in the case of direct current. Therefore, even if the current density is increased during electrodialysis, the membrane voltage applied to the anion exchange membrane also increases proportionally, and it has been thought that this does not allow for an increase in iodide ion productivity.

[0005] The present inventors have 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 permselective layer in the anion exchange membrane, the IV curve of the anion exchange membrane in an aqueous solution containing an iodide salt exhibits new current-voltage characteristics in which the membrane voltage does not increase proportionally even when the current density is increased. As a result of extensive research based on this finding, the inventors have adopted "a1 / d" as an index representing the above current-voltage characteristics, where "a1" is the effective area d of the anion exchange membrane and the value a1 of the direct current corresponding to the minimum appearing in the second derivative curve obtained by second-order differentiation of the IV curve, and have found that "a1 / d" is the current density (A / dm ２ ) to be equal to or greater than "a1 / d", it is possible to suppress an increase in membrane voltage even at a relatively high current density, which facilitates the movement of iodide ions and thereby improves the productivity (energy efficiency) of iodide ions compared to when the IV curve is linear.

[0006] According to one aspect of the present invention, there is provided the following method for operating an electrodialysis apparatus.

[0007] 1. A method for electrodialyzing an iodide-containing solution containing an iodide salt and a solvent using an electrodialysis device, wherein the electrodialysis device is provided with an anion exchange membrane having a monovalent ion permselective layer, which has a minimum in a second derivative curve obtained by obtaining an IV curve representing the relationship between direct current and voltage according to a DC resistance measurement method and second-order differentiating the IV curve, and wherein in the electrodialysis step, the effective area (dm ２ ) is d, and the value of the DC current (A) corresponding to the minimum appearing in the second derivative curve is a1, the current density (A / dm ２ 2. The method for operating an electrodialysis apparatus according to 1., wherein the lower limit of a1 / d is set to 0.5 A / dm ２ 3. The method for operating an electrodialysis apparatus according to 1. or 2., wherein the upper limit of a1 / d is 10 A / dm or more. ２4. A method for operating an electrodialysis apparatus according to any one of 1. to 3., wherein when the value of the direct current (A) corresponding to the maximum appearing in the second derivative curve obtained by second-order differentiation of the IV curve is a2 (where a2>a1), the current density (A / dm ２ 5. The method for operating an electrodialysis apparatus according to 4., wherein a2 / d - a1 / d is set to 0.5 A / dm ２ 10A / dm or more ２ 6. A method for operating an electrodialysis apparatus according to any one of 1. to 5., wherein the SO 2 concentration in the anion exchange membrane is measured according to the following seawater concentration test. ４ The index is 3.0 x 10 －３ N or more 50.0×10 －３ A method for operating an electrodialysis apparatus in which the membrane resistance is 1.5 Ω cm or less. ２ 2.5Ω・cm or more ２ A small electrodialysis device (with a current-carrying membrane area of ​​100 cm) was used in which the anion exchange membrane to be evaluated was incorporated so that the monovalent selective surface faced the deionization compartment. ２ ) was used, seawater at 25°C was passed through the desalination compartment at a flow rate of 6 cm / sec, and the concentration compartment was filled with a 3.5 mol / L NaCl aqueous solution, and the current density was 3 A / dm ２ The SO in the concentrated solution obtained when electrodialysis is performed until the concentration in the concentration compartment stops changing. ４ ２－ The concentration is ４The anion exchange membrane is measured as an index. 7. A method for operating the electrodialysis apparatus described in any one of 1. to 6., wherein the anion exchange membrane comprises a styrene-divinylbenzene copolymer having a quaternary ammonium group. 8. An anion exchange membrane having a monovalent ion permselective layer used only for electrodialysis of an iodide-containing solution containing an iodide salt and a solvent, wherein, when an IV curve representing the relationship between direct current and voltage of the anion exchange membrane measured according to a direct current resistance measurement method is obtained, a local minimum appears in a second derivative curve obtained by second-differentiating the IV curve.

[0008] According to the present invention, there are provided a method for operating an electrodialysis apparatus which is excellent in productivity of iodide ions, and an anion exchange membrane used therein.

[0009] FIG. 1 is a cross-sectional view schematically showing an example of the configuration of an electrodialysis apparatus of this embodiment. FIG. 2 is a diagram schematically showing an example of the configuration of an iodine recovery system of this embodiment. FIG. 3 is a flow chart showing an example of an iodine recovery process of this embodiment. FIG. 4 shows an IV curve of Comparative Example 1. FIG. 5 shows a second-order derivative curve obtained by second-order differentiation of the IV curve of Comparative Example 1. FIG. 6 shows an IV curve of Example 1. FIG. 7 shows a second-order derivative curve obtained by second-order differentiation of the IV curve of Example 1. FIG. 8 shows an IV curve of Example 2. FIG. 9 shows a second-order derivative curve obtained by second-order differentiation of the IV curve of Example 2. FIG. 10 shows an IV curve of Example 3. FIG. 11 shows a second-order derivative curve obtained by second-order differentiation of the IV curve of Example 3. FIG. 12 shows an IV curve of Comparative Example 2. FIG. 13 shows a second-order derivative curve obtained by second-order differentiation of the IV curve of Comparative Example 2.

[0010] Hereinafter, embodiments of the present invention will be described with reference to the drawings. In all drawings, similar components are designated by similar reference numerals, and descriptions thereof will be omitted as appropriate. Furthermore, the drawings are schematic diagrams and do not correspond to actual dimensional proportions.

[0011] An outline of the method of operating the electrodialysis apparatus of this embodiment will be described.

[0012] The method for operating an electrodialysis apparatus according to the present embodiment includes a step of electrodialyzing an iodide-containing solution containing an iodide salt and a solvent using the electrodialysis apparatus, wherein the electrodialysis apparatus is provided with an anion exchange membrane having a monovalent ion permselective layer, which has a minimum in a second-order derivative curve obtained by obtaining an IV curve representing the relationship between direct current and voltage according to a DC resistance measurement method and second-order differentiating the IV curve, and wherein the effective area (dm ２ ) is d, and the value of the DC current (A) corresponding to the minimum that appears on the second derivative curve is a1, the current density (A / dm ２ ) is set to be equal to or greater than a1 / d.

[0013] According to the findings of the present inventors, a new current-voltage characteristic of anion exchange membranes has been found, characterized by the appearance of a minimum in the second-order derivative curve obtained by second-order differentiation of the IV curve. The presence of a minimum in the second-order derivative curve means that the first-order derivative curve represents a decreasing function, and the membrane resistance decreases with increasing current. Furthermore, it has been experimentally found that the above current-voltage characteristic is exhibited in a current-voltage measurement test using an aqueous solution containing an iodide salt, but does not appear when other halides such as chloride salts or bromide salts are used, although the detailed mechanism is unknown.

[0014] When an anion exchange membrane having current-voltage characteristics that appear when such an iodide salt is used to electrodialyze an iodide-containing solution containing the iodide salt, the current density (A / dm ２ By setting the ratio a1 / d or more, an increase in membrane voltage can be suppressed even at a relatively high current density, which facilitates the movement of iodide ions and makes it possible to increase the productivity (energy efficiency) of iodide ions compared to when the IV curve is linear.

[0015] In addition, when the value of the direct current (A) corresponding to the maximum value appearing in the second derivative curve obtained by second-order differentiation of the IV curve is a2 (where a2>a1), the current density (A / dm ２ ) is preferably set to a2 / d or less.

[0016] The fact that the second derivative curve has a maximum after a minimum indicates that the first derivative curve changes to an increasing function, meaning that the membrane resistance increases with increasing current. Therefore, the current density (A / dm ２ By setting the ratio a2 / d to a2 / d or less, it is possible to suppress an increase in membrane voltage at a higher current density, and therefore it is possible to suppress a decrease in productivity of iodide ions.

[0017] According to the findings of the present inventors, the SO measured under the following predetermined conditions ４ Although the mechanism is unclear, it has been found that the presence or absence of a minimum in the second derivative curve obtained from the IV curve measured by the DC resistance measurement method can be stably evaluated by using the index as an indicator. 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.

[0018] In this specification, the SO 2 selective anion exchange membrane ４ The index can be measured in accordance with the following seawater concentration test. <Seawater concentration test> A small electrodialysis device (current-carrying membrane area 100 cm) was used. ２ ) with a membrane resistance of 1.5 Ω cm ２ 2.5Ω・cm or more ２ The following cation exchange membranes are used as a pair, and the monovalent selective surface of the anion exchange membrane to be evaluated is assembled so that it faces the deionization compartment. The operating conditions are as follows: seawater at 25°C flows through the deionization compartment at a flow rate of 6 cm / sec, the concentration compartment is filled with a 3.5 mol / L NaCl aqueous solution, and the current density is 3 A / dm ２ Electrodialysis was performed for 5 hours or more (until the concentration in the concentration compartment stopped changing) to measure the amount of chloride ions (Cl － ) concentration (N) and sulfate ions (SO ４ ２－ ) concentration (10 －３ The obtained sulfate ion concentration is measured using the above SO ４ Let it be an index.

[0019] Here, the selectivity of monovalent anions and divalent anions permeating the membrane in a monovalent selective anion exchange membrane is as follows: －On the other hand, SO is used as a divalent anion. ４ ２－ The selective permeability coefficient (P Ｃｌ ＳＯ４ This is because monovalent selective anion exchange membranes are mainly used for concentrating seawater in the salt manufacturing industry, and in this application, the permselectivity coefficient (P Ｃｌ ＳＯ４ ) by using a membrane with a small Cl － Therefore, even in the case of recovering iodine, if an anion exchange membrane with monovalent selectivity is used to avoid the concentration of sulfate ions, it is preferable to use an anion exchange membrane with monovalent selectivity. － and SO ４ ２－ However, according to the investigations of the present inventors, in the case of iodine recovery, it is common to use a material whose physical property value is as small as possible in the selective permeability defined in relation to the coefficient of selective permeability (P Ｃｌ ＳＯ４ ) is used, the iodide ions and SO ４ ２－ The permeation behavior of the Cl － and SO ４ ２－ It was discovered that there was an unexpected difference between the permeation of iodide ions and that iodide ions could not be concentrated at a sufficiently high permeation rate. Further research revealed that the "SO ４ When the "index" is used as an indicator of iodide ion permeability, it has been unexpectedly found to correlate well with the amount of iodide ion permeated, despite the fact that iodide ions are monovalent anions.

[0020] In this embodiment, for example, by appropriately selecting the type and amount of each component contained in the monovalent ion selective anion exchange membrane, the method for preparing the monovalent ion selective permeation layer, etc., the SO ４ As will be described in detail later, for example, the SO 2 index can be controlled by appropriately controlling the density of the surface dense layer of the anion exchange membrane. ４This is one factor for setting the index to a desired numerical range.

[0021] Here, the monovalent selective anion exchange membranes have been used to selectively pass monovalent anions from a liquid to be treated that contains a mixture of monovalent anions and polyvalent anions, and therefore, membranes with as high a monovalent selectivity as possible have been used. ４ When measuring the index, the transmittance is as small as possible. Specifically, 3.0 × 10 －３ In most cases, values ​​below N were used.

[0022] In this situation, as described above, the present invention ４ The index is 3.0 x 10 －３ N or more 50.0×10 －３ It is preferable to use a monovalent selective anion exchange membrane having a SO 2 concentration of 0.01% or less. ４ The lower limit of the index is, for example, 3.0 x 10 －３ N or more, preferably 5.0 × 10 －３ N or more, more preferably 6.0 × 10 －３ N or more. This makes it possible to control the current density corresponding to the minimum appearing in the second derivative curve to a low value, thereby increasing the productivity of iodide ions during electrodialysis. －３ By setting the SO 2 concentration at 1000 kJ / cm2 or more, it is possible to suppress the decrease in productivity of iodide ions even when electrodialysis is repeated. The reason for this is not entirely clear, but it is thought to be due to the interaction between iodide ions and the ion permeation channels in the membrane. On the other hand, the SO 2 concentration in the monovalent selective anion exchange membrane is ４ The upper limit of the index is, for example, 50.0 x 10 －３ or less, preferably 30.0 × 10 －３ N or less, more preferably 20.0 × 10 －３ This makes it possible to suppress the concentration of divalent ions such as sulfate ions to a low level. In other words, the recovery ratio of iodide ions to multivalent ions such as divalent ions can be increased.

[0023] The configuration of the method for operating the electrodialysis apparatus of this embodiment will be described in detail below.

[0024] Fig. 1 is a cross-sectional view schematically showing an example of the configuration of an electrodialysis apparatus 1. Fig. 2 is a diagram schematically showing an example of the configuration of an iodine recovery system 100. Fig. 3 is a flow chart showing an example of an iodine recovery process.

[0025] An example of a method for recovering iodine using the method for operating an electrodialysis apparatus according to the present embodiment includes an electrodialysis step in which an iodide-containing solution (liquid waste 10) containing iodide salts and a solvent is separated into an iodine-containing concentrate (concentrate 20) and a desalted solution (desalted solution 30) using the electrodialysis apparatus 1 shown in FIG. 1 .

[0026] The iodine recovery system 100 shown in FIG. 2 is only required to include at least a facility for supplying the waste liquid 10 to the electrodialysis apparatus 1, and further includes a facility for treating the concentrated solution 20 and / or the desalted solution 30 produced in the electrodialysis apparatus 1.

[0027] The waste liquid 10 (raw liquid) is not particularly limited as long as it is a liquid containing at least an iodine component including an iodide salt, but examples thereof include waste liquid discharged from the manufacturing process of a product containing an iodine component, waste liquid discharged when a product containing an iodine component is disposed of, waste liquid discharged from a synthesis process using an iodine component as a reaction catalyst, and waste liquid from a cleaning solution used to clean manufacturing equipment that uses an iodine component.

[0028] Specific examples of raw materials for the waste liquid 10 include waste liquids, waste powders, and waste solids from the manufacturing process, manufacturing equipment, or waste products of any of the iodine-containing products, such as polarizing films, contrast agents, disinfectants, and radiation-related materials, or waste liquids or waste solids containing iodine catalysts (organic iodine compounds or inorganic iodine compounds) used in chemical synthesis, such as the synthesis of pharmaceuticals, including antibiotics and antiviral agents. These may be used alone or in combination of two or more.

[0029] When organic matter and / or organic solvents are contained in the waste liquid, the iodine recovery solution may be subjected to a known organic matter decomposition treatment in advance. One example of such organic matter decomposition treatment is to burn organic components (organic matter and organic solvent) by a combustion method, and then adsorb the sublimated iodine with an alkali agent (e.g., sodium hydroxide and sodium hydrogen sulfite) and a reducing agent to recover the iodine component. The iodine component is, for example, iodide ions (I － ) is sufficient, and iodine (I ２ ), iodic acid (HIO ３ ), periodic acid (HIO ４ The term "iodine waste" is defined as including one or more reduced iodine compounds selected from the group consisting of iodine compounds, iodides (including inorganic iodine compounds and organic iodine compounds), and iodides (including inorganic iodine compounds and organic iodine compounds). Among the iodine components, waste liquids derived from the raw materials and the like typically contain iodide ions at a concentration of 3% or more, preferably about 3% to 30%. Furthermore, these waste liquids typically contain sulfate ions at a concentration of 1 g / L or more but not exceeding the saturated solubility of sulfate, more typically about 20 to 50 g / L. Furthermore, solids containing iodine components typically contain elemental iodine at a concentration of about 30% to 99.8% by mass.

[0030] The waste liquid 10 may be used as is, but if necessary, may be subjected to chemical or physical pretreatment such as pH adjustment, dilution, decomposition and removal of organic matter.

[0031] As shown in Fig. 1, electrodialysis device 1 has concentrating compartment 2, deionization compartment 3, an ion exchange membrane separating concentrating compartment 2 from deionization compartment 3, anode 4, cathode 5, and power supply 6. Electrodialysis device 1 is not limited to the configuration shown in Fig. 1 and may have any known device configuration.

[0032] An electrode solution is supplied from an electrode solution tank (not shown) to the electrode chamber (anode chamber) containing the anode 4 and the electrode chamber (cathode chamber) containing the cathode 5. The anode chamber and the cathode chamber are arranged on either side of an electrodialysis cell composed of a concentration compartment 2 and a deionization compartment 3. When a current is applied to the anode 4 and the cathode 5 using a power source 6, electrodialysis begins in the electrodialysis cell. Any known electrode solution can be used, such as an aqueous sodium hydroxide solution, an aqueous potassium hydroxide solution, an aqueous sodium hydrogen sulfate solution, or an aqueous potassium sulfate solution. However, when bipolar electrodialysis or the like is performed after electrodialysis, the electrode solution used in the electrodialysis device 1 is preferably one that does not contain sulfate.

[0033] In the electrodialysis tank, the concentration compartments 2 and the deionization compartments 3 are alternately arranged, each separated by an ion exchange membrane (either anion exchange membrane 7 or cation exchange membrane 8). The electrodialysis device 1 may have at least the following configuration: deionization compartment 3, anion exchange membrane 7, concentration compartment 2, cation exchange membrane 8, and deionization compartment 3 arranged in this order. To enhance processing capacity, the electrodialysis device 1 may have one or more, preferably two or more, concentration compartments 2. The concentration compartments 2 may contain an electrolyte solution prior to electrodialysis. The electrolyte solution may contain, for example, an aqueous solution of an inorganic salt having electrical conductivity. For example, ion-exchanged water may be used when the electrodialysis device 1 is first used, and the previous concentrate may be used when the device is subsequently restarted after being stopped. In addition to these electrolyte solutions, aqueous solutions of sodium chloride, potassium iodide, sodium iodide, etc. may also be used. However, it is preferable that the electrolyte solution does not contain sulfate.

[0034] A specific example of the electrodialysis apparatus 1 used for the above-described electrodialysis has a plurality of gasket spacers (also called chamber frames) stacked between a pair of electrodes, anion exchange membranes 7 or cation exchange membranes 8 sandwiched between adjacent chamber frames, and the anion exchange membranes 7 and cation exchange membranes 8 are arranged so that, as a rule, they are alternately positioned overall, and each gasket forms an ion exchange chamber (current-carrying section) partitioned by the anion exchange membranes 7 and the cation exchange membranes 8. The ion exchange chambers are structured such that the deionization chambers 3 are chambers in which the cation exchange membrane 8 is positioned on the cathode 5 side and the anion exchange membrane 7 is positioned on the anode 4 side, and the concentration chambers 2 are chambers in which the anion exchange membrane 7 is positioned on the cathode 5 side and the cation exchange membrane 8 is positioned on the anode 4 side, and the deionization chambers 3 and the concentration chambers 2 are arranged alternately. That is, when current is applied while circulating the treatment liquid (electrolyte liquid) through the deionization compartment 3, cations in the treatment liquid supplied to the deionization compartment 3 pass through the cation exchange membrane 8 and migrate to the concentrating compartment 2 on the adjacent cathode 5 side, and anions in the treatment liquid pass through the anion exchange membrane and migrate to the concentrating compartment 2 on the adjacent anode 4 side. In this way, when the treatment liquid is circulated through the deionization compartment 3 and at the same time a salt water solution is circulated through the concentrating compartment 2, the treatment liquid is desalted and a concentrated liquid 20 with an increased salt concentration is obtained. A filter press is preferably used as a method for fixing the stack of these ion exchange compartments between the electrodes, but this is not a limitation.

[0035] In this embodiment, at least one or all of the anion exchange membranes 7 included in the electrodialysis device 1 are SO ４ The anion exchange membrane is composed of a monovalent selective anion exchange membrane whose index satisfies the above numerical range.

[0036] Here, the monovalent selective anion exchange membrane is a membrane that selectively exchanges anions such as SO ４ ２－ Cl is preferred over divalent ions such as － Generally, the anion exchange membrane is an anion exchange membrane that selectively allows the permeation of monovalent ions, such as SO ４ ２－ and Cl － The selective permeability coefficient (P Ｃｌ ＳＯ４) is 0.01 to 1.0, but even among them, the SO ４ Since it is easy to obtain a monovalent selective anion exchange membrane used in the anion exchange membrane 7 that satisfies the specified value of the index, the selective permeability coefficient (P Ｃｌ ＳＯ４ ) is preferably 0.06 to 0.90, particularly preferably 0.11 to 0.37.

[0037] Such monovalent anion selectivity can be imparted by forming a monovalent ion permselective layer on at least one surface of the anion exchange membrane 7. The specific configuration of the monovalent ion permselective layer is not particularly limited, but it is preferably at least one layer selected from the group consisting of a dense surface layer, an electrically neutral layer, and an oppositely charged layer. The "dense surface layer" is a layer in which the surface of the anion exchange membrane is formed with a dense structure (e.g., a layer with a high degree of cross-linking or a layer with a high fixed ion concentration in the surface layer). The "electrically neutral layer" is a thin electrically neutral layer that does not contain anion exchange groups formed on the surface of the anion exchange membrane. The "oppositely charged layer" is a thin layer with cation exchange groups formed on the surface of the anion exchange membrane.

[0038] The monovalent selective anion exchange membrane may be, for example, a base anion exchange membrane having the SO 2 -containing anion exchange membrane on one or both sides thereof. ４ The index is 3.0 x 10 －３ N or more 50.0×10 －３ The monovalent ion permselective layer may be at least one layer selected from the group consisting of a dense surface layer, an electrically neutral layer, and an oppositely charged layer. ４ A dense surface layer is preferred because it is easy to obtain an anion exchange membrane with a high index, and a dense surface layer formed of a highly crosslinked resin layer using a divalent amine compound is particularly preferred. As the base anion exchange membrane, one in which a quaternary ammonium group, which is a strongly basic anion exchange group, is introduced into the basic skeleton of a styrene-divinylbenzene copolymer is preferably used because it has a strong structure and is easy to form a monovalent ion permselective layer.

[0039] These, SO ４ A monovalent selective anion exchange membrane having an index that satisfies the above numerical range can be produced as follows: That is, a divalent amine compound is brought into contact with one or both membrane surfaces of a raw membrane for introducing anion exchange groups, which is obtained by polymerizing a polymerizable composition containing a styrene-based aromatic polymerizable monomer having a halogenoalkyl group and a crosslinkable polymerizable monomer such as divinylbenzene, to form SO on the membrane surface. ４ The index is 3.0 x 10 －３ N or more 50.0×10 －３ N or less, a surface dense layer that is more highly crosslinked than the interior is formed, and then the remaining halogenoalkyl groups in the membrane are contacted with trialkylamine to convert them into quaternary ammonium groups. According to this method, in the raw membrane for introducing anion exchange groups, a certain proportion of the halogenoalkyl groups present in the membrane surface portion on one side is consumed for crosslinking by contact with a divalent amine compound. Therefore, the membrane surface portion becomes a dense structure due to crosslinking, and SO ４ ２－ The permeability of ions and the like decreases, and selectivity for monovalent anions is exhibited.

[0040] In this method, any known styrene-based aromatic polymerizable monomer having a halogenoalkyl group can be used without limitation. The alkyl group preferably has 1 to 8 carbon atoms, and examples of the halogen atom substituted thereon include chlorine, bromine, and iodine. Examples of such a halogenoalkyl group include a chloromethyl group, a bromomethyl group, an iodomethyl group, a chloroethyl group, a bromoethyl group, an iodoethyl group, a chloropropyl group, a bromopropyl group, an iodopropyl group, a chlorobutyl group, a bromobutyl group, an iodobutyl group, a chloropentyl group, a bromopentyl group, an iodopentyl group, a chlorohexyl group, a bromohexyl group, and an iodohexyl group. Specific examples of aromatic polymerizable monomers having such a halogenoalkyl group include chloromethylstyrene, bromomethylstyrene, iodomethylstyrene, chloroethylstyrene, bromoethylstyrene, iodoethylstyrene, chloropropylstyrene, bromopropylstyrene, iodopropylstyrene, chlorobutylstyrene, bromobutylstyrene, iodobutylstyrene, chloropentylstyrene, bromopentylstyrene, iodopentylstyrene, chlorohexylstyrene, bromohexylstyrene, and iodohexylstyrene. Of these, it is particularly preferable to use chloromethylstyrene, bromomethylstyrene, iodomethylstyrene, chloroethylstyrene, bromoethylstyrene, iodoethylstyrene, chloropropylstyrene, bromopropylstyrene, iodopropylstyrene, chlorobutylstyrene, bromobutylstyrene, and iodobutylstyrene.

[0041] The base membrane for introducing anion exchange groups obtained by polymerizing the polymerizable composition contains halogenoalkyl groups derived from the aromatic polymerizable monomer having the halogenoalkyl group. These halogenoalkyl groups are converted into anion exchange groups, i.e., quaternary ammonium groups, as described below. Such quaternary ammonium groups are strongly basic and are excellent anion exchange groups. However, if other anion exchange groups are desired, the polymerizable composition may contain, in addition to the aromatic polymerizable monomer having the halogenoalkyl group, a polymerizable monomer having another anion exchange group or a polymerizable monomer having a functional group capable of introducing other anion exchange groups. Such anion exchange groups other than quaternary ammonium groups are not particularly limited as long as they are functional groups capable of becoming positively charged in aqueous solution, and examples thereof include primary, secondary, and tertiary amino groups, pyridyl groups, imidazole groups, and quaternary pyridinium groups. In the polymerizable composition, the amount of these other polymerizable monomers having an anion exchange group or polymerizable monomers having a functional group into which other anion exchange groups can be introduced is not particularly limited, but is preferably 100 parts by mass or less, and more preferably 50 parts by mass or less, per 100 parts by mass of the aromatic polymerizable monomer having a halogenoalkyl group.

[0042] Furthermore, in addition to the polymerizable monomer for introducing anion exchange groups into the anion exchange membrane, the polymerizable composition may contain a polymerizable monomer that is not directly involved in the introduction of such an anion exchange membrane, in an amount of 150 parts by mass or less per 100 parts by mass of the aromatic polymerizable monomer having a halogenoalkyl group. Examples of such polymerizable monomers that are not directly involved in the introduction of anion exchange membranes include styrene, acrylonitrile, methylstyrene, ethylvinylbenzene, acrolein, methylvinylketone, and vinylbiphenyl.

[0043] In the polymerizable composition, the amount of the polymerizable monomer that is not directly involved in the introduction of these anion exchange membranes is not particularly limited, but is preferably 400 parts by mass or less, and more preferably 150 parts by mass or less, per 100 parts by mass of the aromatic polymerizable monomer having a halogenoalkyl group.

[0044] A crosslinkable polymerizable monomer is used in the polymerizable composition to increase the density of the resulting anion exchange membrane and thereby enhance its strength. As such a crosslinkable polymerizable monomer, any monomer known in the art for producing ion exchange membranes can be used without any particular limitation. Specific examples include m-, p-, or o-divinylbenzene, divinylbiphenyl, divinylsulfone, butadiene, chloroprene, isoprene, trivinylbenzene, divinylnaphthalene, diallylamine, triallylamine, divinylpyridine, and polyfunctional vinylbenzyl compounds having three or more vinylbenzyl groups, as disclosed in, for example, JP-A-62-205153.

[0045] If these crosslinkable polymerizable monomers are blended in too much relative to the styrene-based aromatic polymerizable monomer having a halogenoalkyl group, the ion exchange capacity of the anion exchange membrane will decrease and the degree of crosslinking will become too high, increasing the membrane resistance and potentially reducing the iodine concentration efficiency. Conversely, if the blending ratio of the crosslinkable polymerizable monomer is too low, not only will the strength of the membrane decrease, but the density of the highly crosslinked resin layer provided on the membrane surface will not increase, potentially preventing sufficient monovalent selectivity from being obtained. For these reasons, it is preferable to blend 2 to 20 parts by mass, preferably 3 to 15 parts by mass, of the crosslinkable polymerizable monomer relative to 100 parts by mass of the polymerizable monomer component consisting of the aromatic polymerizable monomer having a halogenoalkyl group and the other polymerizable monomers used in combination as necessary as described above.

[0046] A polymerization initiator is usually blended into the polymerizable composition. Any conventionally known polymerization initiator can be used without any particular limitation, and may be appropriately selected taking into consideration the substrate to be used, molding conditions, etc. Specific examples include p-menthane hydroperoxide, diisopropylbenzene hydroperoxide, α,α'-bis(tert-butylperoxy-m-isopropyl)benzene, di-tert-butyl peroxide, tert-butyl hydroperoxide, di-tert-amyl peroxide, tert-butylcumyl peroxide, dicumyl peroxide, 2,5-dimethyl-2,5-di(tert-butylperoxy)hexane ... ) Hexyne-3, cumene 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, methylcyclohexane peroxide, isobutyl peroxide, 2,4-dichlorobenzoyl peroxide, o-methylbenzoyl peroxide peroxide, bis-3,5,5-trimethylhexanoyl peroxide, lauroyl peroxide, p-chlorobenzoyl peroxide, 1,1-di-tert-butylperoxy-trimethylcyclohexane, 1,1-di-tert-butylperoxycyclohexane, 2,2-di-(tert-butylperoxy)-butane, 4,4-di-tert-butylperoxyvaleric acid-n-butyl ester, 2,4,4-trimethylpentylperoxy-phenoxyacetate, α-cumylperoxyneodene Preferred are tert-butylperoxybenzoate, tert-butylperoxyneodecanoate, tert-butylperoxypivalate, tert-butylperoxy-2-ethylhexanoate, tert-butylperoxyisobutyrate, di-tert-butylperoxyhexahydroterephthalate, di-tert-butylperoxyazelate, tert-butylperoxy-3,5,5-trimethylhexanoate, tert-butylperoxyacetate, and tert-butylperoxybenzoate.These may be added to the monomer paste alone or in combination of two or more. The amount of the polymerization initiator as described above is usually preferably 0.1 to 30 parts by mass, more preferably 1 to 10 parts by mass, per 100 parts by mass of the polymerizable monomer component.

[0047] The polymerizable composition may also contain a matrix resin as a viscosity modifier. The incorporation of such a matrix resin can improve the coatability of the polymerizable composition and prevent sagging and other problems when applied to a substrate. Examples of such matrix resins include saturated aliphatic hydrocarbon polymers such as polyvinyl chloride, chlorinated polyvinyl chloride, ethylene-vinyl chloride copolymers, vinyl chloride elastomers, chlorinated polyethylene, chlorosulfonated polyethylene, ethylene-propylene copolymers, and polybutylene; styrene polymers such as styrene-butadiene copolymers; and copolymers of these with styrene monomers such as vinyltoluene, vinylxylene, chlorostyrene, chloromethylstyrene, α-methylstyrene, α-halogenated styrene, and α,β,β'-trihalogenated styrene; monoolefins such as ethylene and butylene; and conjugated diolefins such as butadiene and isoprene. Also suitable for use are styrene-butadiene rubber or hydrogenated rubber thereof, nitrile rubber or hydrogenated nitrile rubber thereof, pyridine rubber or hydrogenated rubber thereof, and styrene-based thermoplastic elastomers. Here, the term "styrene-based thermoplastic elastomer" refers to polystyrene polymer, styrene and polybutadiene, polyisoprene, vinyl polyisoprene, ethylene-butylene alternating copolymer, and ethylene-propylene alternating copolymer. Examples include polystyrene-hydrogenated polybutadiene-polystyrene copolymer, polystyrene-(polyethylene / butylene rubber)-polystyrene copolymer, polystyrene-hydrogenated polyisoprene rubber-polystyrene copolymer, polystyrene-(polyethylene / propylene rubber)-polystyrene copolymer, polystyrene-polyethylene-(polyethylene / propylene rubber)-polystyrene copolymer, and polystyrene-vinyl polyisoprene-polystyrene copolymer. The molecular weight of such matrix resins is not particularly limited, but is preferably in the range of 1,000 to 1,000,000, and more preferably 50,000 to 500,000.The matrix resin is blended into the polymerizable composition in an amount that ensures an appropriate viscosity, depending on the molecular weight. For example, the amount is preferably 1 to 50 parts by mass, and more preferably 3 to 15 parts by mass, per 100 parts by mass of the polymerizable monomer component.

[0048] In addition to the various components described above, the polymerizable composition may contain, if necessary, a plasticizer such as dioctyl phthalate, dibutyl phthalate, tributyl phosphate, styrene oxide, or an alcohol ester of a fatty acid or an aromatic acid, an organic solvent, or the like.

[0049] It is also preferable to add a compound having one or more epoxy groups, such as styrene oxide or diethylene glycol diglycidyl ether, to the polymerizable composition so as to capture halogen gas or hydrogen halide gas generated by thermal decomposition of the styrene-based aromatic polymerizable monomer having a halogenoalkyl group.

[0050] The polymerizable composition comprising the above components is filled into the voids of a substrate serving as a reinforcing material and then polymerized to obtain a base membrane for introducing anion exchange groups. Any known substrate for ion exchange membranes may be used as the substrate. Generally, a support material with a porosity of 20 to 90%, more preferably 40 to 80%, and even more preferably 45 to 55% is used. Examples include woven fabrics, nonwoven fabrics, porous films, and net-like materials made from polyvinyl chloride, polyolefins, and the like. Among these, polyolefins such as polypropylene and polyethylene are preferred in terms of their affinity with ion exchange resins and chemical resistance, with polyethylene being particularly preferred. Furthermore, the use of a porous film as the substrate shape is preferred because it enhances the efficiency of iodine concentration after imparting monovalent selectivity. The thickness of the substrate is generally selected in the range of 50 to 300 μm, with 70 to 250 μm being preferred from the viewpoints of membrane resistance and strength retention.

[0051] The method for filling the substrate with the polymerizable composition is not particularly limited. For example, the polymerizable composition may be applied or sprayed onto the substrate, or the substrate may be immersed in the polymerizable composition. When the polymerizable composition is in a paste form, application is preferred. The application of the polymerizable composition to the substrate can be carried out by known means such as a roll coater, a flow coater, a knife coater, a comma coater, spraying, or dipping.

[0052] After the polymerizable composition is introduced into the substrate as described above, the substrates are laminated together with a release material having releasability so as to prevent the substrates from adhering to each other, and then wound up on a roller, and heated to carry out polymerization.

[0053] The release material used should have heat resistance sufficient to withstand polymerization and be easily peeled off after polymerization. Examples include polyolefins such as polypropylene, poly-1-butene, poly-4-methyl-1-pentene, or random or block copolymers of α-olefins such as ethylene, propylene, 1-butene, and 4-methyl-1-pentene; ethylene-vinyl compound copolymers such as ethylene-vinyl acetate copolymer, ethylene-vinyl alcohol copolymer, and ethylene-vinyl chloride copolymer; polyvinyl compounds such as polymethyl acrylate and 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 oxide; biodegradable resins such as polylactic acid; and mixtures thereof. These films may be biaxially stretched. In other words, an appropriate film can be selected from the above films depending on the type of monomer component in the polymerizable composition and used as the release material. In particular, polyester films such as polyethylene terephthalate (PET) are most suitable in terms of heat resistance and releasability.

[0054] The pressure during polymerization may be either normal pressure or pressure, and is generally about 0.1 to 1.0 MPa. The polymerization temperature may be any temperature lower than the melting point of the substrate, and is generally in the range of 40 to 130°C. That is, by carrying out polymerization by heating within this temperature range, a portion of the substrate dissolves in the polymerizable composition, and polymerization is carried out in this state. As a result, the bonding strength between the ion exchange resin and the substrate can be increased, and the membrane strength and concentration efficiency can be further improved. The polymerization time varies depending on the polymerization temperature, etc., but is generally about 3 to 20 hours.

[0055] After applying the polymerizable composition to the substrate as described above, the composition is heated to polymerize, thereby obtaining a base membrane for introducing anion exchange groups, which is made of a membranous polymer. To obtain the monovalent anion exchange membrane of the present invention, after the base membrane for introducing anion exchange groups is obtained in this manner, a monovalent ion permselective layer made of a highly cross-linked resin layer is formed on the membrane surface before the anion exchange groups are introduced. This is carried out, for example, by contacting a divalent amine compound with one side of the membrane surface for introducing anion exchange groups. That is, if the monovalent anion selective layer is formed only on one side of the base membrane for introducing anion exchange groups, the other membrane surface is covered with a film or the like, and the uncovered membrane surface is contacted with a divalent amine compound to form a highly cross-linked resin layer on the membrane surface, rather than the interior.

[0056] The amino group of the divalent amine compound can react with the halogenoalkyl group contained in the base membrane for introducing anion exchange groups, resulting in the elimination of hydrogen chloride and the formation of a C-N bond, which reacts with two halogenoalkyl groups in the divalent amine compound to form a cross-linked structure. The denser this cross-linked structure is, the higher the membrane resistance becomes, while the monovalent anion selectivity becomes higher, resulting in the SO ４The index becomes smaller. In addition, the crosslinking reaction by the divalent amine compound proceeds from the membrane surface toward the inside, and the deeper the crosslinking depth, the higher the monovalent anion selectivity but the greater the membrane resistance. Examples of secondary amine compounds include dimethylamine, diethylamine, dipropylamine, dibutylamine, and diethanolamine. In terms of the balance between resistance and monovalent anion selectivity, the shorter the substituent, the better, and dimethylamine is preferred.

[0057] When the divalent amine compound is brought into contact with a secondary amine compound solution to crosslink the halogenoalkyl groups on the surface of the anion-exchange-group-introduced base membrane, if the divalent amine compound diffuses too slowly into the anion-exchange-group-introduced base membrane in an aqueous solution or if its solubility is poor, a portion of the water may 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 to replace water. The content of the organic solvent is preferably within 30% by mass, particularly 5 to 15% by mass, relative to the water.

[0058] The SO ４ The index is 3.0 x 10 －３ N or more 50.0×10 －３ In order to obtain an anion exchange membrane having a surface density of N or less, the conditions for contacting and reacting the secondary amine compound are particularly important. For example, the concentration at which the secondary amine compound is dissolved is preferably 0.01 mol / L to 2 mol / L, and particularly preferably 0.03 mol / L to 1 mol / L. If the concentration is too low, the crosslinking reactivity of the halogenoalkyl group with the divalent amine compound decreases, resulting in a decrease in monovalent ion selectivity and an increase in SO ４ On the other hand, if the concentration is too high, the diffusion of the anion exchange group-introducing base membrane into the interior of the membrane will be intensified, the cross-linking depth will be too deep, and the SO ４ The index becomes less than the predetermined value.

[0059] The reaction temperature during contact with the divalent amine compound is 20 to 50°C, and the reaction time is 1 to 24 hours. If the reaction temperature is below 20°C and / or the reaction time is less than 1 hour, the crosslinking reaction of the halogenoalkyl group cannot be sufficiently carried out, and the SO ４ If the reaction temperature exceeds 50°C and / or the reaction time exceeds 24 hours, the anion exchange group introduction raw membrane will diffuse into the interior of the membrane, the crosslinking depth will become too deep, and the SO ４ After the raw membrane for introducing anion exchange groups is reacted with the secondary amine compound, washing may be carried out to remove excess secondary amine compound that has not been subjected to the reaction.

[0060] The reaction between the divalent amine compound and a halogenoalkyl group as described above may terminate at the reaction between the divalent amine and one halogenoalkyl group, failing to result in crosslinking between the two halogenoalkyl groups. In such cases, it is preferable to perform a treatment step with an alkaline aqueous solution (alkaline aqueous solution treatment step) after the contact step with the divalent amine compound. Although the details of the reaction are not fully understood, it is presumed that the alkaline compound neutralizes the hydrogen chloride produced when the divalent amine compound reacts with one halogenoalkyl group, thereby promoting the reaction with the second halogenoalkyl group. Known alkaline aqueous solutions, such as aqueous sodium hydroxide, aqueous potassium hydroxide, aqueous barium hydroxide, and aqueous ammonia, can be used. Strongly basic aqueous sodium hydroxide or aqueous potassium hydroxide solutions are preferred because they enable more efficient treatment. The alkaline aqueous solution may have a pH of 8 to 14 at 25°C, but a pH of 10 to 14 at 25°C is preferred because it enables more efficient treatment. The treatment temperature is preferably 50 to 70°C, more preferably 55 to 65°C. If the temperature is less than 50°C, the treatment takes a long time, resulting in low productivity. If the temperature is more than 70°C, the treatment becomes too harsh, and there is a possibility that the amino compound that has reacted once may be liberated. The treatment time with the alkaline aqueous solution in this step may be 4 to 24 hours, preferably 6 to 12 hours, and more preferably 6 to 10 hours. If the temperature is less than 4 hours, the treatment may be insufficient. If the temperature is more than 24 hours, the treatment becomes too harsh, resulting in an increase in electrical resistance.

[0061] Next, a step (quaternary ammonium group introduction step) is carried out in which quaternary ammonium groups are introduced using a tertiary amine compound to the residual halogenoalkyl groups in the raw membrane after the alkaline aqueous solution treatment 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. By contacting the membrane with an aqueous solution of a tertiary amine compound and introducing quaternary ammonium groups, the residual halogenoalkyl groups present in the membrane from the surface to the interior are substituted without affecting the monovalent anion-selective layer consisting of a highly crosslinked resin layer. This method may be performed in accordance with conventional methods for introducing quaternary ammonium groups in the production of anion exchange membranes.

[0062] By the above method, SO ４ The index is 3.0 x 10 －３ N or more 50.0×10 －３ Furthermore, in order to exhibit appropriate permeability to iodide ions, the monovalent selective anion exchange membrane used in the present invention preferably has an anion exchange capacity in the range of 0.5 to 4.0 meq / g-dry membrane, particularly 0.8 to 3.0 meq / g-dry membrane, a water content in the range of 10 to 40%, preferably 25 to 35%, and an electrical resistance of 1.2 Ω cm ２ 3.0Ω・cm or more ２ Preferably 1.7 Ω cm or less ２ 2.5Ω・cm or more ２ or less, more preferably 1.8 Ω cm ２ 2.4Ω・cm or more ２ The thickness is preferably in the following range. The thickness is adjusted to be in the range of 50 to 320 μm, particularly 70 to 280 μm. That is, in order to obtain such physical properties, the composition of the polymerizable composition (amount or type of monomer component and crosslinking agent), the thickness of the substrate, and the crosslinking reaction conditions by the divalent amine compound are appropriately set. In addition, the concentration of chloride ions (Cl ) in a seawater concentration test is － The permeation rate is usually 3.2 mol / L or more, preferably 3.5 mol / L or more.

[0063] SO４ When the anion exchange membrane 7 includes an anion exchange membrane other than a monovalent ion-selective anion exchange membrane having an index that satisfies the above-mentioned numerical range, the other anion exchange membrane may be a known one, for example, a monovalent ion-permselective anion exchange membrane, a fully permeable anion exchange membrane, or a high-strength alkali-resistant anion exchange membrane. A monovalent ion-permselective anion exchange membrane is preferred.

[0064] There are no particular limitations on the cation exchange membrane 8, and a strongly acidic cation exchange membrane, a high-strength alkali-resistant cation exchange membrane, or the like can be used. The cation exchange membrane 8 may also be a monovalent ion-selective cation exchange membrane, and more specifically, a cation exchange membrane in which sulfonic acid groups, which are strongly acidic cation exchange groups, have been introduced into a styrene-divinylbenzene basic skeleton can be used. Examples of commercially available cation exchange membranes that can be used include Neosepta (registered trademark) CSE and Neosepta (registered trademark) CMB manufactured by Astom Corporation.

[0065] Also, the following SO ４ An anion exchange membrane having an index that satisfies the above-mentioned numerical range and an electrodialysis apparatus including the same can be suitably used in the iodine recovery step of this embodiment.

[0066] That is, the electrodialysis apparatus described above is an electrodialysis apparatus for separating a waste liquid containing iodide salts into an iodine-containing concentrate and a desalted liquid, and includes a concentration compartment for producing the iodine-containing concentrate, a desalting compartment for supplying the waste liquid, and an ion exchange membrane separating the concentration compartment from the desalting compartment, and the ion exchange membrane is configured to include an anion exchange membrane that exhibits a minimum in a second derivative curve obtained by second-order differentiation of an IV curve that represents the relationship between direct current and voltage measured according to a direct current resistance measurement method. A maximum may appear after the minimum in the second derivative curve. The anion exchange membrane may also exhibit a minimum in an ion exchange membrane that exhibits a minimum in an ion exchange membrane that exhibits a maximum ... maximum in an ion exchange membrane that exhibits a minimum in an ion exchange membrane that exhibits a maximum in an ion exchange membrane that exhibits a maximum in an ion exchange membrane that exhibits a maximum in an ion exchange membrane that exhibits a minimum in an ion exchange membrane that exhibits a maximum in an ion exchange membrane that exhibits a maximum in an ion exchange membrane that exhibits a maximum in an ion exchange membrane that exhibits a maximum in an ion exchange membrane that exhibits a maximum in an ion exchange membrane that exhibits a maximum in an ion exchange membrane that exhibits a maximum in an ion exchange membrane that exhibits a maximum in an ion exchange membrane that exhibits a maximum in an ion exchange membrane that exhibits a maximum in an ion exchange membrane that exhibits a maximum ４ The index is 3.0 x 10 －３ N or more 50.0×10 －３ The anion exchange membrane may be configured to include a monovalent selective anion exchange membrane having a valence of N or less.

[0067] The anion exchange membrane is an anion exchange membrane for use in electrodialysis for separating a waste solution containing iodide salts into an iodine-containing concentrate and a desalted solution, and is an anion exchange membrane for electrodialysis of an iodine-containing waste solution, in which a minimum appears in a second-order derivative curve obtained by second-order differentiation of an IV curve representing the relationship between a direct current and a voltage measured according to a direct current resistance measurement method. A maximum may appear after the minimum in the second-order derivative curve. This anion exchange membrane is a monovalent selective anion exchange membrane, and has SO ４ The index is 3.0 x 10 －３ N or more 50.0×10 －３ The anion exchange membrane may further contain a styrene-divinylbenzene copolymer having a quaternary ammonium group.

[0068] An example of the operation of the electrodialysis device 1 during electrodialysis is as follows: When a direct current is applied between the anode 4 and the cathode 5 using the power supply 6, iodide ions (I － ) passes through anion exchange membrane 7 and moves to concentration compartment 2, and cations (including at least one of monovalent cations such as alkali metal ions and divalent cations such as alkaline earth metal ions) in deionization compartment 3 on the other side pass through cation exchange membrane 8 and move to the same concentration compartment 2. In this concentration compartment 2, iodide salts such as potassium iodide (KI) are produced. A concentrate 20 containing iodide salts (iodine-containing concentrate) is obtained from concentration compartment 2. A deionized solution 30 is obtained from deionization compartment 3.

[0069] Here, an example of a process flow up to the electrodialysis step in the iodine recovery method will be described with reference to FIGS. 1 and 2.

[0070] First, waste liquid 10 is stored in waste liquid tank 11 via line 12 (piping).

[0071] The waste liquid 10 in the waste liquid tank 11 may be subjected to the above-mentioned chemical pretreatment or physical pretreatment. When the waste liquid 10 contains a boron component, a pH adjustment step may be carried out before the electrodialysis step to adjust the pH of the waste liquid 10 to 9.5 or less, preferably 8 or less, more preferably less than 7. When the pH of the waste liquid 10 is adjusted to 9.5 or less, boric acid (H ３ BO ３ Since most of the boron and iodine components in the wastewater 10 are present as molecules without being dissociated, even when electrodialysis is performed, the boric acid hardly moves and is discharged as it is into the desalted liquid 30. This allows efficient separation of the boron component and the iodine component in the wastewater 10.

[0072] When the waste liquid 10 is acidic (pH less than 7), it is preferable to adjust the pH of the waste liquid 10 to 3 or more. Since free iodine may be generated, which may deteriorate the ion exchange membrane and reduce the efficiency of electrodialysis, this can suppress the generation of free iodine due to the air oxidation of iodide ions and prevent the deterioration of the ion exchange membrane due to free iodine.

[0073] The lower limit of the temperature of the waste liquid 10 in the waste liquid tank 11 may be set to, for example, 25° C. or higher, preferably 30° C. or higher, and more preferably 35° C. or higher. The efficiency of electrodialysis can be improved by increasing the temperature of the waste liquid 10 supplied to the desalting compartment 3 in the electrodialysis apparatus 1. If there are restrictions on the apparatus itself, the upper limit of the temperature of the waste liquid 10 in the waste liquid tank 11 should be as high as possible within the range of the upper limit temperature that the apparatus can tolerate. If the electrodialysis apparatus 1 is equipped with PVC piping, the upper limit may be set to, for example, 40° C. or lower.

[0074] Subsequently, the waste liquid 10 in the waste liquid tank 11 is supplied to the deionization chamber 3 of the electrodialysis device 1 via the line 13. A voltage is applied to the anode 4 and the cathode 5 to carry out electrodialysis (electrodialysis step). At this time, in the electrodialysis step of this embodiment, as described above, the current density (A / dm ２) is set to a1 / d or more. On the other hand, in the electrodialysis step, when a local maximum appears in the second derivative curve of the anion exchange membrane 7, it is preferable to set the current density to a2 / d or less. Furthermore, when the current at which the second derivative value after the local minimum becomes near zero is a3, the current density may be set to a3 / d or less. When a local maximum does not appear in the second derivative curve of the anion exchange membrane 7, it is preferable to set the current density to a3 / d or less. In the electrodialysis step, the current density may be gradually increased and maintained within the range of a1 / d or more and a2 / d or less, or within the range of a1 / d or more and a3 / d or less, for a predetermined time. Note that the upper limit of the local minimum in the second derivative curve obtained from the IV curve measured with the voltage unit being mV and the current unit being mA is smaller than -0.005, preferably -0.01 or less, and more preferably -0.02 or less. The lower limit of this minimum value is not particularly limited, but may be -0.5 or more, -0.3 or more, or -0.2 or more. The lower limit of the maximum value in the second derivative curve is greater than 0.005, preferably 0.007 or more, and more preferably 0.01 or more. The upper limit of this maximum value is not particularly limited, but may be 0.3 or less, 0.2 or less, or 0.1 or less.

[0075] The lower limit of the a1 / d is, for example, 0.5 A / dm ２ or more, preferably 3.1 A / dm ２ More preferably, 4.0 A / dm ２ This makes it possible to further increase the productivity of iodide ions.

[0076] The upper limit of the a1 / d is, for example, 10 A / dm ２ Less than or equal to 9.5 A / dm ２ or less, more preferably 9 A / dm ２ This makes it possible to suppress a decrease in productivity of iodide ions.

[0077] The lower limit of a2 / d-a1 / d is, for example, 0.5 A / dm ２ or more, preferably 1.0 A / dm ２ More preferably, 1.5 A / dm ２This allows the productivity of iodide ions to be stably improved. The upper limit of a2 / d-a1 / d is not particularly limited, but is preferably 10 A / dm ２ or less, 6 A / dm ２ The following is also acceptable.

[0078] The desalted liquid 30 produced in the desalting compartment 3 by electrodialysis is recovered via line 31. On the other hand, the concentrated liquid 20 produced in the concentrating compartment 2 by electrodialysis is recovered via line 22.

[0079] At this time, at least a portion of the desalted solution 30 may be supplied to the waste solution tank 11 via a line 32 branched from the line 31, and a mixed desalted solution obtained by mixing the desalted solution 30 and the waste solution 10 may be supplied again to the desalting compartment 3. In other words, the waste solution 10 supplied to the desalting compartment 3 may contain the desalted solution 30. By repeatedly performing electrodialysis in this manner, the iodide ion concentration in the desalted solution 30 can be reduced to a desired value.

[0080] Alternatively, at least a portion of the concentrate 20 may be supplied to the concentrate tank 21 via a line 23 branched from the line 22, and the concentrate 20 in the concentrate tank 21 may be supplied again to the concentrating compartment 2 via a line 24. By repeatedly performing electrodialysis in this manner, the iodide ion concentration in the concentrate 20 can be concentrated to a desired value.

[0081] In addition, when two or more electrodialysis devices are provided, a first concentrate produced in a first electrodialysis device may be supplied to a desalting compartment of a second electrodialysis device, and a second desalted solution produced in the desalting compartment of the second electrodialysis device may be mixed with the first concentrate and supplied again to the desalting compartment of the second electrodialysis device.

[0082] Alternatively, an electrodialysis device 1 equipped with a continuous circulation type electrodialysis facility may be used. Such an electrodialysis device 1 can be continuously operated by continuously supplying the waste liquid 10 or a mixed desalted solution to the desalted solution 30 while continuously discharging the desalted solution 30.

[0083] Next, an example of a process flow of the regeneration step in the iodine recovery method will be described with reference to FIG.

[0084] Subsequently, in the iodine recovery method, after the electrodialysis step, the obtained iodine-containing concentrated solution (concentrated solution 20) is used to extract iodine (I ２ The method may include a regeneration step of producing one or more selected from the group consisting of iodine, iodide salt, and hydroiodic acid. The iodine, iodide salt, and hydroiodic acid regenerated (recycled) by the above-mentioned iodine recovery method may each be in the form of an aqueous solution, and the iodine and iodide salt may be in the form of powder (including granules).

[0085] The iodine recovery method can recover hydroiodic acid using the obtained concentrated solution 20. For example, the iodine recovery method may include a step (bipolar electrodialysis step) of separating an iodine-containing concentrated solution (concentrated solution 20) into hydroiodic acid and an aqueous hydroxide salt solution by bipolar membrane electrodialysis, as shown in Fig. 3. Here, the bipolar membrane electrodialysis method is a method in which, between an anode and a cathode, bipolar membranes that generate hydrogen ions and hydroxide ions, in addition to cation exchange membranes and / or anion exchange membranes, are arranged in this order, and a treatment solution is supplied to each chamber partitioned by the membranes and an electric current is applied to obtain an acid and an alkali from a neutral salt.

[0086] Specifically, the concentrated solution 20 (e.g., KI concentrated solution) discharged from the electrodialysis device 1 is introduced into the bipolar membrane electrodialysis device. If necessary, the concentrated solution 20 may be acidified by adjusting the pH to less than 7. When a direct current is applied to the bipolar membrane electrodialysis device, the iodide salt in the concentrated solution 20 is electrolyzed, and an aqueous solution of hydroiodic acid (HI solution) and an aqueous solution of hydroxide (e.g., KOH) are discharged. The HI solution is then purified by distillation to obtain hydroiodic acid.

[0087] In addition, the iodine recovery method uses the obtained concentrated solution 20 to recover iodine (I ２ As shown in FIG. 3, when the concentrated solution 20 containing iodide ions is oxidized, iodine (I ２ ) is obtained.

[0088] In the iodine recovery method, iodide salts can be recovered using the obtained concentrated solution 20. As shown in FIG. 3, iodide salts can be obtained by neutralizing the hydroiodic acid obtained above. In addition, iodine (I ２) is reduced and neutralized to obtain an iodide salt. In the iodine recovery method, the oxidation, neutralization, and reduction methods are not particularly limited, and known means can be used. Examples of iodide salts include alkali metal iodide salts and alkaline earth metal iodide salts. Specific examples of iodide salts include potassium iodide, sodium iodide, calcium iodide, magnesium iodide, and cesium iodide. These may be used alone or in combination of two or more.

[0089] On the other hand, the iodine recovery method uses the obtained desalted liquid 30 to extract iodine (I ２ As shown in Fig. 3, the method for recovering iodine may include a step of passing the obtained desalted solution 30 through a strong basic anion exchange resin to adsorb iodide ions remaining in the desalted solution 30 onto the strong basic anion exchange resin, and recovering iodine from the strong basic anion exchange resin.

[0090] Specifically, the desalted solution 30 discharged from the electrodialysis device 1 is passed through a strong basic ion exchange resin, and iodide ions are adsorbed onto the strong basic anion exchange resin. At this time, the pH of the desalted solution 30 when passed through the strong basic anion exchange resin is adjusted to less than 7, preferably between 3 and 7, as in the electrodialysis described above. This prevents borate ions formed by dissociation of boric acid from being adsorbed onto the strong basic anion exchange resin, and allows selective separation of iodide ions. Thereafter, iodine (I) is removed from the strong basic anion exchange resin by a known means. ２ ) can be recovered.

[0091] In addition, when the waste liquid 10 contains a boron component, boron (B ２ ) can be recovered.

[0092] The iodine recovery method may include a boron recovery step in which the pH of the desalted solution 30 containing a boron component is adjusted to, for example, 7 or more, preferably 8 or more, more preferably 11 or more, and then a boron-containing concentrate (boric acid concentrate) is separated and recovered from the desalted solution 30 using another electrodialysis device. When the boron-containing concentrate is acidified and crystallized, boric acid (H ３ BO３ ) can be recovered.

[0093] Alternatively, the desalted solution 30 containing boron components and having a pH of 7 or higher, or the desalted solution 30 obtained when passing through the strongly basic anion exchange resin, may be passed through a boron-selective chelating resin to adsorb boric acid ions. Thereafter, boric acid can be recovered from the boron-selective chelating resin by known means.

[0094] Although the embodiments of the present invention have been described above, these are merely examples of the present invention, and various other configurations may be adopted. Furthermore, the present invention is not limited to the above-described embodiments, and modifications and improvements within the scope of achieving the object of the present invention are included in the present invention.

[0095] The present invention will be described in detail below with reference to examples, but the present invention is not limited to the descriptions of these examples.

[0096] <Production of Monovalent Selective Anion Exchange Membrane> The physical properties of the anion exchange membranes in the examples and comparative examples were measured by the following methods.

[0097] Example 1 A paste-like polymerizable composition was obtained by mixing 91.2 parts by mass of chloromethylstyrene, 5.0 parts by mass of divinylbenzene, 3.8 parts by mass of ethylvinylbenzene, 4.0 parts by mass of di-tert-butyl peroxide as a radical polymerization initiator, and 4.0 parts by mass of styrene oxide as a scavenger for hydrogen chloride gas.

[0098] Next, the polymerizable composition was applied to a biaxially stretched porous polyethylene film (thickness 100 μm, porosity 46%, average pore size 0.13 μm) as a substrate to incorporate the polymerizable composition into the substrate.

[0099] Next, both sides of the substrate into which the polymerizable composition had been introduced were covered with a release material made of polyethylene terephthalate film, and the substrate was wound up on a roller, followed by polymerization. The polymerization temperature pattern was as follows: the temperature was raised from 20°C to 50°C over 30 minutes, held at 50°C for 20 minutes, then raised to 130°C over 80 minutes, and held at 130°C for 4 hours, to produce a base membrane for introducing anion exchange groups.

[0100] Next, the release film was peeled off from the base membrane for introducing anion exchange groups. However, at this time, the release material on both sides of the base membrane was not completely peeled off, but the release material was left on one side. This base membrane for introducing anion exchange groups, covered on only one side with the release material, was immersed in a 0.05 mol / L-dimethylamine aqueous solution for 8.5 hours at 34°C, and the chloromethyl groups on the surface not covered with the release material were crosslinked with dimethylamine. The resulting base membrane was then washed with a 1.0 mol / L-hydrochloric acid aqueous solution and pure water.

[0101] Next, after removing the release agent film covering one side, the base membrane for introducing anion exchange groups was immersed in a 0.01 mol / L aqueous sodium hydroxide solution for 16 hours at 60° C. to carry out a step of treating with an alkaline aqueous solution. Thereafter, the base membrane after the alkaline aqueous solution treatment was washed with pure water.

[0102] Next, a quaternization reaction was carried out for 16 hours at 30°C using an aqueous solution containing 5% by mass of trimethylamine and 25% by mass of acetone, and the chlorine of the remaining chloromethyl groups in the membrane was replaced with trimethylamine, thereby introducing quaternary ammonium groups. Thereafter, the raw membrane after the introduction of the quaternary ammonium groups was washed using a 1.0 mol / L aqueous hydrochloric acid solution and pure water to obtain a monovalent selective anion exchange membrane.

[0103] Example 2 A monovalent selective anion exchange membrane was obtained in the same manner as in Example 1, except that the reaction time with the aqueous dimethylamine solution was changed to 5 hours.

[0104] Example 3 A monovalent selective anion exchange membrane was obtained in the same manner as in Example 1, except that the treatment time with the aqueous sodium hydroxide solution was changed to 8 hours.

[0105] (Comparative Example 1) A commercially available ACS-8T (manufactured by Astom Corporation) was evaluated as a monovalent selective anion exchange membrane. (Comparative Example 2) A base membrane for introducing anion exchange groups was obtained in the same manner as in Example 1, and the release agent films were peeled off from both sides of the base membrane. After that, the step of introducing quaternary ammonium groups using trimethylamine was carried out in the same manner as in Example 1, without carrying out the treatment with the dimethylamine aqueous solution and the alkaline aqueous solution, to obtain an anion exchange membrane that had not been subjected to the selectivity treatment.

[0106] <Method for Measuring DC Resistance of Anion Exchange Membranes> Using a DC membrane resistance measuring device equipped with the following silver / silver chloride electrodes, the DC resistance of an anion exchange membrane was measured in a 0.5 mol / L potassium iodide (KI) aqueous solution. First, a four-chamber acrylic cell DC membrane resistance measuring device was prepared, which included two electrode cells equipped with a silver electrode (anode) and a silver chloride electrode (cathode), and two measurement cells with salt bridges installed centrally between the two electrode cells. A 0.5 mol / L potassium chloride (KCl) aqueous solution was introduced into the electrode cells as the electrode solution, and a 0.5 mol / L potassium iodide (KI) aqueous solution was introduced into the measurement cells as the electrolyte solution. The potential across the salt bridge was detected with a potentiometer via a glass electrode and used as the membrane voltage. The temperatures of the electrode solution and the electrolyte solution were maintained at 25°C (±0.5°C) using a water bath. Next, the anion exchange membrane (effective area d: 0.04 dm ) obtained above was measured. ２) was immersed in a 0.5 mol / L KI aqueous solution for 1 hour. Subsequently, the immersed anion exchange membrane was sandwiched between two measurement cells and installed in the DC membrane resistance measuring device. At this time, the monovalent selective surface (ion permselective layer) of the anion exchange membrane was installed facing the cathode. Thereafter, 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) every 20 seconds was recorded using a data logger, and an IV curve of the anion exchange membrane was created. The measurement was completed at 400 mA after 180 seconds. In addition, using the DC membrane resistance measuring device without the anion exchange membrane installed, the voltage was calculated in the same manner as above, and a blank IV curve was created. The IV curves of the anion exchange membranes in Table 1 and each figure were corrected by subtracting the blank IV curve. Here, the blank refers to the above-mentioned anion exchange membrane DC resistance measurement method, in which ion-exchanged water was used as the electrolyte solution instead of a 0.5 mol / L potassium iodide (KI) aqueous solution. The IV curve was created by plotting voltage (mV) on the vertical axis and current (mV) on the horizontal axis. The IV curves of Comparative Example 1, Examples 1 to 3, and Comparative Example 2 are shown in Figures 4, 6, 8, 10, and 12, respectively. In the IV curves of the anion exchange membranes after correction, the current value is expressed as X １ , X ２ ..X ｎ Let the voltage be Y １ , Y ２ ...Y ｎ (n is 4 or more), the first derivative (dY / dX) of the IV curve of the anion exchange membrane is ｎ－１ -Y ｎ ) / [(X ｎ－１ -X ｎ ) / 2], and the second derivative (d ２ Y / dX ２ ) is = [(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] was defined and calculated. A second-order derivative curve was created from the calculated second-order derivative value, and if a minimum exists in the second-order derivative curve, the current value (a1) corresponding to the minimum is shown in Table 1. If a maximum exists, the current value (a2) corresponding to the maximum is shown in Table 1. In this second-order derivative curve, fluctuations within a range of ±0.005 are not considered to be fluctuations in the IV characteristics. That is, in a second-order derivative curve obtained from an IV curve measured with the voltage unit being mV and the current unit being mA, plots within a range of ±0.005 are excluded from the minimum or maximum. The second-order derivative curves for Comparative Example 1, Examples 1 to 3, and Comparative Example 2 are shown in Figures 5, 7, 9, 11, and 13, respectively.

[0107] <Cl concentration test in seawater － and S.O. ４ Index > Small electrodialysis device (electric membrane area 100 cm ２ ) was treated with a monovalent selective cation exchange membrane CIMS (manufactured by Astom Corporation, membrane resistance 2.0 Ω cm ２ The anion exchange membrane was used in pairs and assembled together with the anion exchange membrane obtained above so that the monovalent selective surface faced the deionization compartment. The operating conditions were as follows: seawater at 25°C was passed through the deionization compartment at a flow rate of 6 cm / sec, the concentration compartment was filled with a 3.5 mol / L NaCl aqueous solution, and the current density was 3 A / dm ２ Electrodialysis was performed for 5 hours or more (until the concentration in the concentration compartment stopped changing) to measure the amount of chloride ions (Cl － ) and sulfate ions (SO ４ ２－ The concentration of chloride ions (Cl) in the concentration compartment was measured. － ) concentration (N) and sulfate ions (SO ４ ２－ ) concentration (10 －３ The resulting chloride ions (Cl － ) concentration (N) and sulfate ions (SO ４ ２－ ) concentration, － Permeation rate and SO ４ The results are shown in Table 1.

[0108] <Selective permeability coefficient (P Ｃｌ ＳＯ４ )> Effective current area 4.0 cm２ A two-chamber glass cell separated by an anion exchange membrane was equipped with silver-silver chloride electrodes in both chambers, and 100 ml of 0.5 mol / L NaCl aqueous solution was placed in the anode chamber, and 0.25 mol / L NaCl and 0.125 mol / L NaCl aqueous solutions were placed in the cathode chamber. ２ SO ４ A direct current of 40 mA was applied at 25°C for 1 hour, and the amounts of chloride ions and sulfate ions in the cathode chamber were then quantified. The permselectivity coefficient P was calculated from the obtained amounts of chloride ions and sulfate ions using the following formula: Ｃｌ ＳＯ４ was sought. Ｃｌ ＳＯ４ = (t ＳＯ４ / t Ｃｌ ) / (C ＳＯ４ / C Ｃｌ ) t ＳＯ４ is the number of sulfate ion equivalents that permeate the membrane, t Ｃｌ is the number of chloride ion equivalents that permeate the membrane, C ＳＯ４ is the sulfate ion equivalent concentration in the cathode chamber after measurement, C Ｃｌ is the chloride ion equivalent concentration in the cathode chamber after measurement. Ｃｌ ＳＯ４ was 0.05.

[0109]

[0110] (Simulation of production efficiency in electrodialysis) In a model case in which the IV curve of an anion exchange membrane is linear, doubling the current density in the electrodialysis process of an iodide-containing solution is estimated to halve the treatment time and double the treatment capacity. Treatment capacity, i.e., production volume (P) per unit time, is proportional to the current density (not the voltage). Here, the "electricity consumption rate," an indicator used in salt production processes, is defined as current (I) × voltage (V) / production volume (P) per unit time. When the IV curve is linear, as described above, increasing the current density increases the production volume (P) per unit time in proportion to the current (I). On the other hand, when the IV curve is convex (there is a minimum in the second derivative curve of the IV curve), as in Examples 1 to 3 above, the voltage V2 in the high current region (high current density region) at this minimum is smaller than V1 when the IV curve is linear at the same current. That is, in the high current density region, V2<V1. Therefore, in the IV curves of Examples 1 to 3, even if the current density is increased to increase the production volume (P) per unit time, V2<V1 in the high current density region, so that an increase in the "power consumption rate" can be suppressed. In other words, in Examples 1 to 3, by setting the current density to be equal to or greater than "direct current in the high current density region (a1)" / "effective area of ​​the anion exchange membrane (d)," it is expected that the energy efficiency (productivity) in the high current density region can be improved compared to when the IV curve is linear. Note that in the IV curves of Comparative Examples 1 and 2, the voltage V3 in the high current density region is greater than or approximately the same as V1 when the IV curve is linear, making it difficult to improve productivity compared to Examples 1 to 3.

[0111] This application claims priority based on Japanese Patent Application No. 2023-174426, filed October 6, 2023, the disclosure of which is incorporated herein by reference in its entirety.

[0112] REFERENCE SIGNS LIST 1 Electrodialysis device 2 Concentration compartment 3 Demineralization compartment 4 Anode 5 Cathode 6 Power source 7 Anion exchange membrane 8 Cation exchange membrane 10 Waste liquid 11 Waste liquid tank 12, 13 Lines 20 Concentrated liquid 21 Concentrated liquid tank 22, 23, 24 Lines 30 Demineralized liquid 31, 32 Lines 100 Iodine recovery system

Claims

1. A method for electrodialyzing an iodide-containing solution containing an iodide salt and a solvent, comprising: using an electrodialysis device, said electrodialysis device being provided with an anion exchange membrane having a monovalent ion selective permeable layer, which has a minimum in a second derivative curve obtained by obtaining an IV curve representing the relationship between a direct current and a voltage according to a direct current resistance measurement method and second-order differentiation of said IV curve; and in said electrodialysis step, determining whether or not the effective area (dm2) of said anion exchange membrane is greater than or equal to 100 μm. ２ ) is d, and the value of the direct current (A) corresponding to the minimum appearing in the second derivative curve is a1, the current density (A / dm ２ ) is set to be equal to or greater than a1 / d.

2. The method for operating the electrodialysis apparatus according to claim 1, wherein the lower limit of a1 / d is 0.5 A / dm ２ The above is the method for operating an electrodialysis apparatus.

3. The method for operating the electrodialysis apparatus according to claim 1 or 2, wherein the upper limit of a1 / d is 10 A / dm ２ The method of operating an electrodialysis apparatus is as follows:

4. A method for operating an electrodialysis apparatus according to claim 1 or 2, wherein, when the value of the direct current (A) corresponding to the maximum value appearing in the second derivative curve obtained by second-order differentiation of the IV curve is a2 (where a2>a1), the current density (A / dm ２ ) is set to a2 / d or less.

5. The method for operating the electrodialysis apparatus according to claim 4, wherein a2 / d-a1 / d is 0.5 A / dm ２ 10A / dm or more ２ The method of operating an electrodialysis apparatus is as follows:

6. A method for operating an electrodialysis apparatus according to claim 1 or 2, comprising the steps of: determining whether or not the amount of SO in the anion exchange membrane is greater than the amount of SO in the anion exchange membrane, as measured according to the following seawater concentration test. ４ The index is 3.0 x 10 －３ N or more 50.0×10 －３ A method for operating an electrodialysis apparatus in which the membrane resistance is 1.5 Ω cm or less. ２ 2.5Ω・cm or more ２ A small electrodialysis device (current-carrying membrane area 100 cm) was used in which the anion exchange membrane to be evaluated was incorporated so that the monovalent selective surface faces the desalting compartment, and a cation exchange membrane having the following characteristics was used as a pair. ２ ) was used, seawater at 25°C was passed through the desalting compartment at a flow rate of 6 cm / sec, and the concentration compartment was filled with a 3.5 mol / L NaCl aqueous solution, and a current density of 3 A / dm ２ The concentration of SO in the concentrated solution obtained by performing electrodialysis until the concentration in the concentration compartment stops ４ ２－ The concentration is ４ Measured as an index.

7. A method for operating an electrodialysis apparatus according to claim 1 or 2, wherein the anion exchange membrane comprises a styrene-divinylbenzene copolymer having a quaternary ammonium group.

8. An anion exchange membrane having a layer selectively permeable to monovalent ions, used only for electrodialysis of an iodide-containing solution containing an iodide salt and a solvent, wherein, when an IV curve showing the relationship between a direct current and a voltage of the anion exchange membrane measured according to a direct current resistance measurement method is obtained, a minimum appears in a second derivative curve obtained by second-order differentiation of the IV curve.

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