Operating method of an electrodialysis apparatus comprising a hydrocarbon-based anion exchange membrane having ion selective permeability

Abstract

To provide a method of operating an electrodialyzer with excellent productivity of iodide ions.SOLUTION: A method of operating an electrodialyzer includes a process of electrodialyzing an iodide-containing solution containing an iodide salt and a solvent using the electrodialyzer. The electrodialyzer is provided with an anion exchange membrane with a monovalent ion selective permeable layer, in which when an IV curve representing a relationship between DC current and voltage is obtained by following a DC resistance measurement method, a minimum appears in a second derivative curve obtained by second derivative of the IV curve. In the process of electrodialysis, when the effective area (dm2) of the anion exchange membrane is d and the value of the DC current (A) corresponding to the minimum that appears in the second derivative curve is a1, the current density (A / dm2) is set to a1 / d or higher.SELECTED DRAWING: Figure 1

Description

Technical Field

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

Background Art

[0002] Various developments have been made on technologies for recovering iodine from waste liquid. As this type of technology, for example, the technology described in Patent Document 1 is known. In addition to iodine components such as iodide ions (I - ), divalent ions such as sulfate ions (SO4 2- ) may also coexist. For example, sulfate ions are usually contained in the waste liquid at a concentration of 1 g / L or more and below the saturation solubility of sulfate, and more generally, may be contained at about 20 to 50 g / L. Patent Document 1 describes a method of performing electrodialysis on a stock solution containing inorganic anions having iodine and inorganic anions having fluorine and accommodated in a desalting chamber using a monovalent-selective anion exchange membrane or the like (Claim 1, Examples, etc. of Patent Document 1).

Prior Art Documents

Patent Documents

[0003]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0004] However, as a result of investigations by the present inventor, it has been found that there is room for improvement in terms of the productivity of iodide ions in the electrodialysis method described in Patent Document 1 above. That is, heretofore, it has been known that the IV curve (current-voltage curve) representing the current-voltage characteristics of an anion exchange membrane shows a substantially straight line in the case of direct current. Therefore, even if the current density is increased during the electrodialysis method, the membrane voltage applied to the anion exchange membrane also increases proportionally, so it has been considered that the productivity of iodide ions cannot be enhanced.

Means for Solving the Problems

[0005] As a result of studying the current-voltage characteristics of an anion exchange membrane, the present inventor has appropriately controlled the monovalent selectivity by changing the density of the monovalent anion selective permeable layer in the anion exchange membrane, etc., and in an aqueous solution containing an iodide salt, It has been found that the IV curve of the anion exchange membrane shows new current-voltage characteristics such that the membrane voltage does not increase proportionally even when the current density is increased. As a result of intensive studies based on such findings, as an index representing the above current-voltage characteristics, 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 value curve obtained by second differentiating the IV curve are used. By setting the current density (A / dm 2 ) during the electrodialysis process to be equal to or greater than "a1 / d", it has been found that even at a relatively high current density, the increase in the membrane voltage can be suppressed, so that the movement of iodide ions becomes easier, and the productivity (energy efficiency) of iodide ions can be enhanced as compared with the case where the IV curve is a straight line.

[0006] According to one aspect of the present invention, an operation method of the following electrodialysis device is provided.

[0007] 1. A step of electrodialyzing an iodide-containing solution containing an iodide salt and a solvent using an electrodialysis device, The electrodialysis device includes an anion exchange membrane having a monovalent ion selective permeable layer that acquires an IV curve representing the relationship between a direct current and a voltage according to the direct current resistance measurement method, and a minimum appears in the second derivative value curve obtained by second differentiating the IV curve. In the step of electrodialyzing, the effective area of the anion exchange membrane (dm2 ) is denoted as d, and when the value of the direct current (A) corresponding to the minimum appearing in the second derivative value curve is a1, the current density (A / dm 2 ) is set to be a1 / d or more, a method for operating an electrodialysis device. 2. The method for operating an electrodialysis device according to 1., wherein the lower limit of a1 / d is 0.5 A / dm 2 or more, a method for operating an electrodialysis device. 3. The method for operating an electrodialysis device according to 1. or 2., wherein the upper limit of a1 / d is 10 A / dm 2 or less, a method for operating an electrodialysis device. 4. The method for operating an electrodialysis device according to any one of 1. to 3., when the value of the direct current (A) corresponding to the maximum appearing in the second derivative value curve obtained by second differentiating the IV curve is a2 (where a2 > a1), the current density (A / dm 2 ) during the electrodialysis step is set to be a2 / d or less, a method for operating an electrodialysis device. 5. The method for operating an electrodialysis device according to 4., wherein a2 / d - a1 / d is 0.5 A / dm 2 or more and 10 A / dm 2 or less, a method for operating an electrodialysis device. 6. The method for operating an electrodialysis device according to any one of 1. to 5., wherein the SO4 index in the anion exchange membrane, measured according to the following seawater concentration test, is 3.0×10 -3 N or more and 50.0×10 -3 N or less, a method for operating an electrodialysis device. <Seawater Concentration Test> A cation exchange membrane having a membrane resistance of 1.5 Ω·cm 2 or more and 2.5 Ω·cm 2 or less is used in pair, and a small electrodialysis device in which the anion exchange membrane to be evaluated is incorporated such that the monovalent selection surface faces the desalination chamber (the energized membrane area is 100 cm 2) is used to flow seawater at 25 °C into the desalination chamber at a flow rate of 6 cm / s, fill the concentration chamber with a 3.5 mol / L aqueous NaCl solution, and perform electrodialysis until the concentration change in the concentration chamber disappears at a current density of 3 A / dm 2 The concentration of SO4 in the concentrated solution obtained when performing electrodialysis until the concentration change in the concentration chamber disappears is measured as the above SO4 index. 2- 7. An operation method of the electrodialysis device according to any one of 1. to 6., wherein the anion exchange membrane contains a copolymer of styrene divinylbenzene having a quaternary ammonium group, and an operation method of the electrodialysis device. 8. An anion exchange membrane having a monovalent ion selective permeable layer, which is used only for the electrodialysis of an iodide-containing solution containing an iodide salt and a solvent, when an IV curve representing the relationship between the direct current and voltage of the anion exchange membrane measured according to the direct current resistance measurement method is obtained, a minimum appears in the second derivative value curve obtained by second differentiating the IV curve, Anion exchange membrane.

Effect of the Invention

[0008] According to the present invention, there are provided an operation method of an electrodialysis device excellent in the productivity of iodide ions, and an anion exchange membrane used therefor.

Brief Description of the Drawings

[0009]

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Mode for Carrying Out the Invention

[0010] Hereinafter, embodiments of the present invention will be described with reference to the drawings. In all the drawings, the same components are denoted by the same reference numerals, and the description will be omitted as appropriate. Also, the figures are schematic diagrams and do not match the actual dimensional ratios.

[0011] The outline of the operation method of the electrodialysis apparatus of this embodiment will be described.

[0012] The operation method of the electrodialysis apparatus of this embodiment includes a step of electrodialyzing an iodide-containing solution containing an iodide salt and a solvent using an electrodialysis apparatus, the electrodialysis apparatus is provided with an anion exchange membrane having a monovalent ion selective permeable layer in which a minimum appears in the second derivative value curve obtained by second differentiating the IV curve obtained by measuring the relationship between the direct current and the voltage according to the direct current resistance measurement method, in the step of electrodialysis, when the effective area (dm 2 ) of the anion exchange membrane is d and the value of the direct current (A) corresponding to the minimum appearing in the second derivative value curve is a1, the current density (A / dm 2 ) is set to a value of a1 / d or more.

[0013] According to the findings of the present inventors, as new current-voltage characteristics of an anion exchange membrane, it has been found that a minimum appears in the second derivative value curve obtained by second-differentiating the IV curve. Having a minimum in the second derivative value curve means that the first derivative value curve represents a decreasing function and the membrane resistance decreases as the current increases. Further, the above current-voltage characteristics are manifested in a current-voltage measurement test using an aqueous solution containing an iodide salt. Although the detailed mechanism is unknown, it has also been found from experiments that they are not manifested when other halides such as chloride salts and bromide salts are used.

[0014] When electro-dialyzing an iodide-containing solution containing an iodide salt using an anion exchange membrane having the current-voltage characteristics that appear when such an iodide salt is used, by setting the current density (A / dm 2 ) during the electro-dialysis process to a1 / d or more, even at a relatively high current density, an increase in the membrane voltage can be suppressed. Therefore, the migration of iodide ions becomes easier, and it becomes possible to increase the productivity (energy efficiency) of iodide ions as compared with the case where the IV curve is linear.

[0015] Also, when the value of the direct current (A) corresponding to the maximum appearing in the second derivative value curve obtained by second-differentiating the IV curve is a2 (where a2 > a1), it is preferable to set the current density (A / dm 2 ) during the electro-dialysis process to a2 / d or less.

[0016] The fact that the second derivative value curve has a maximum after a minimum means that the first derivative value curve changes to an increasing function, which means that the membrane resistance increases as the current increases. Therefore, by setting the current density (A / dm 2 ) during the electro-dialysis process to a2 / d or less, an increase in the membrane voltage at an even higher current density can be suppressed, so that it becomes possible to suppress a decrease in the productivity of iodide ions.

[0017] Further, according to the findings of the present inventors, by using the SO4 index measured under the following predetermined conditions as an index, although the mechanism is not clear, in the IV curve measured according to the direct current resistance measurement method, it has been found that the presence or absence of a minimum in the second derivative value curve obtained from the IV curve can be stably evaluated. This is presumably related to structural factors such as the density of the monovalent anion selectivity layer on the surface of the anion exchange membrane.

[0018] In this specification, the SO4 index in the monovalent selectivity anion exchange membrane can be measured in accordance with the following seawater concentration test. <Seawater Concentration Test> A small electrodialysis device (with an energized membrane area of 100 cm 2 ) is used in pair with a cation exchange membrane having a membrane resistance of 1.5 Ω·cm 2 or more and 2.5 Ω·cm 2 or less, and the monovalent selective surface of the anion exchange membrane to be evaluated is assembled so as to face the desalination chamber. The operating conditions are as follows: seawater at 25°C is flowed through the desalination chamber at a flow rate of 6 cm / second, the concentration chamber is filled with a 3.5 mol / L NaCl aqueous solution, and electrodialysis is performed at a current density of 3 A / dm 2 for 5 hours or more (until the concentration change in the concentration chamber disappears), and the chloride ion (Cl - ) concentration (N) and sulfate ion (SO4 2- ) concentration (10 -3 N) in the concentration chamber are measured. The obtained sulfate ion concentration is used as the above SO4 index.

[0019] Here, the selectivity of membrane permeation between monovalent anions and divalent anions in the monovalent selectivity anion exchange membrane is generally defined by the selective permeation coefficient (P - 2- ) of these two ions during electrodialysis, where Cl Cl SO4 is adopted as the monovalent anion and SO4 Cl SO4 is adopted as the divalent anion. This is because the monovalent selectivity anion exchange membrane is mainly used in seawater concentration in the salt manufacturing industry. In fact, in this application, by using a membrane with a small selective permeation coefficient (P) of these two ions, Cl -The selective concentration is carried out well. Therefore, even in the case of iodine recovery, in order to avoid the concentration of the sulfate ion, if a monovalent selectivity anion exchange membrane is used, this is the same as the above Cl - and SO4 2- In the selective permeability defined in relation to, it is normal to select one having as small a physical property value as possible. However, according to the study by the present inventors, in iodine recovery, if a membrane having a small selective permeability coefficient (P Cl SO4 ) is used, the permeation behavior of the iodide ion and SO4 2- is unexpectedly different from that of the permeation of the above Cl - and SO4 2- , and it was found that the iodide ion could not be concentrated at a sufficiently high permeation rate. Then, when further research was carried out, it was unexpectedly found that if the "SO4 index" of the monovalent selectivity anion exchange membrane was used as an index of the permeability of the iodide ion, the iodide ion, despite being a monovalent anion, was found to have a good correlation with this permeation amount.

[0020] In the present embodiment, for example, by appropriately selecting the types and blending amounts of the respective components contained in the monovalent selectivity anion exchange membrane, the preparation method of the monovalent ion selective permeation layer, etc., it is possible to control the above SO4 index. Although details will be described later, for example, appropriately controlling the density in the surface dense layer of the anion exchange membrane is cited as an element for setting the above SO4 index within a desired numerical range.

[0021] Here, the monovalent selectivity anion exchange membranes heretofore have been used when it is desired to selectively permeate the monovalent anion from a liquid to be treated in which a monovalent anion and a polyvalent anion are mixed. Therefore, those having as high monovalent selectivity as possible have been used. As a result, in the monovalent selectivity anion exchange membranes heretofore, when the above SO4 index was measured, those having as small a permeation amount as possible were used. Specifically, 3.0×10 -3Those below N were often used.

[0022] Under such circumstances, in the present invention, as described above, the SO4 index is 3.0×10 -3 N or more and 50.0×10 -3 N or less, and it is preferable to use a monovalent-selective anion exchange membrane. The lower limit of the SO4 index in the monovalent-selective anion exchange membrane is, for example, 3.0×10 -3 N or more, preferably 5.0×10 -3 N or more, more preferably 6.0×10 -3 N or more. Thereby, it becomes possible to control the current density corresponding to the minimum appearing in the second derivative value curve to be low, and the productivity of iodide ions during electrodialysis can be increased. Also, by setting it to 6.0×10 -3 N or more, even when repeated electrodialysis is performed, a reduction in the productivity of iodide ions can be suppressed. The reason is not necessarily clear, but it is considered that the interaction between iodide ions and the ion permeation channels in the membrane has an influence. On the other hand, the upper limit of the SO4 index in the monovalent-selective anion exchange membrane is, for example, 50.0×10 -3 or less, preferably 30.0×10 -3 N or less, more preferably 20.0×10 -3 N or less. Thereby, it becomes possible to suppress the concentration of divalent ions such as sulfate ions to be low. In other words, the recovery ratio of iodide ions with respect to polyvalent ions such as divalent ions can be increased.

[0023] Hereinafter, the configuration of the operation method of the electrodialysis apparatus of the present embodiment will be described in detail.

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

[0025] An example of the iodine recovery method using the operation method of the electrodialysis apparatus of the present embodiment includes an electrodialysis step of separating an iodide-containing solution (waste liquid 10) containing an iodide salt and a solvent into an iodine-containing concentrated solution (concentrated solution 20) and a desalted solution (desalted solution 30) using the electrodialysis apparatus 1 of FIG. 1.

[0026] The iodine recovery system 100 of FIG. 2 only needs to include at least equipment for supplying the waste liquid 10 to the electrodialysis apparatus 1, and may further include equipment for treating the concentrated solution 20 and / or the desalted solution 30 generated by the electrodialysis apparatus 1.

[0027] The waste liquid 10 (stock solution) is not particularly limited as long as it is a liquid containing at least an iodine component containing an iodide salt. For example, it includes waste liquid discharged from the manufacturing process of products containing an iodine component, waste liquid when products containing an iodine component are discarded, waste liquid discharged from a synthesis process using an iodine component as a reaction catalyst, and waste liquid of a cleaning liquid used for cleaning a manufacturing apparatus using an iodine component.

[0028] Specific raw materials for the waste liquid 10 include waste liquid, waste powder, and waste solids from the manufacturing process, manufacturing apparatus, or waste of products containing an iodine component of any of a polarizing film, a contrast agent, a disinfectant, and a radiation-related material, or waste liquid or waste solids containing an iodine catalyst (organic iodine compound or inorganic iodine compound) used in chemical synthesis such as pharmaceutical synthesis of antibiotics and antiviral agents. These may be used alone or in combination of two or more.

[0029] When the iodine recovery solution contains an organic substance and / or an organic solvent in the waste liquid, a known organic substance decomposition treatment may be performed in advance. As one of this organic substance decomposition treatments, for example, the organic components (organic substances and organic solvents) are burned by a combustion method, and the sublimated iodine is adsorbed by an alkaline agent (such as sodium hydroxide and sodium bisulfite) and a reducing agent, and an aqueous solution showing that the iodine component is recovered is shown. The iodine component is, for example, iodide ion (I -) as long as it contains one or more reduced products selected from the group consisting of iodine (I2), iodic acid (HIO3), periodic acid (HIO4), and iodides (including inorganic iodine compounds or organic iodine compounds). Here, the waste liquid derived from the raw materials, etc. usually contains iodide ions among the above iodine components in an amount of 3% or more, preferably about 3% to 30%. Further, these waste liquids usually contain sulfate ions at a concentration of 1 g / L or more and below the saturation solubility of sulfate, more generally about 20 to 50 g / L. In addition, the solid matter containing the iodine component usually contains iodine element in an amount of about 30% to 99.8% in terms of mass.

[0030] Note that the waste liquid 10 may be used as it is without treatment, or if necessary, chemical pretreatment or physical pretreatment such as pH adjustment, dilution, and organic matter decomposition / removal treatment may be performed.

[0031] As shown in FIG. 1, the electrodialysis device 1 has a concentration chamber 2, a desalting chamber 3, an ion exchange membrane separating the concentration chamber 2 and the desalting chamber 3, an anode 4, a cathode 5, and a power source 6. The electrodialysis device 1 is not limited to FIG. 1 and can have a 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 respectively arranged on both sides of the electrodialysis cell composed of the concentration chamber 2 and the desalting chamber 3. When a current is applied to the anode 4 and the cathode 5 using the power source 6, electrodialysis starts in the electrodialysis cell. Note that a known one can be used as the electrode solution. For example, an aqueous sodium hydroxide solution, an aqueous potassium hydroxide solution, an aqueous sodium hydrogen sulfate solution, an aqueous potassium sulfate solution, etc. are used. However, when bipolar electrodialysis or the like is performed after electrodialysis, the electrode solution used in the electrodialysis device 1 preferably does not contain sulfate.

[0033] In the electrodialysis cell, the concentration chamber 2 and the desalting chamber 3 are each partitioned by an ion exchange membrane (either an anion exchange membrane 7 or a cation exchange membrane 8) and arranged alternately. The electrodialysis apparatus 1 may at least have an arrangement configuration in which the desalting chamber 3, the anion exchange membrane 7, the concentration chamber 2, the cation exchange membrane 8, and the desalting chamber 3 are arranged in this order. From the viewpoint of enhancing the processing capacity, one or more, preferably two or more, concentration chambers 2 may be provided. Note that an electrolyte solution may be accommodated in the concentration chamber 2 before electrodialysis. As the electrolyte solution, for example, an aqueous inorganic salt solution having electrical conductivity may be included. For example, when the electrodialysis apparatus 1 is used for the first time, ion-exchanged water may be used, and when restarting after stopping the operation, the previous concentrated solution may be used. In addition to such an electrolyte solution, an aqueous sodium chloride solution, an aqueous potassium iodide solution, an aqueous sodium iodide solution, etc. may be used. However, it is preferable that the electrolyte solution does not contain sulfates.

[0034] A specific example of the electrodialysis apparatus 1 used for electrodialysis as described above is such that a large number of gasket spacers (also called cell frames) are stacked and arranged between a pair of electrodes. An anion exchange membrane 7 or a cation exchange membrane 8 is sandwiched between adjacent cell frames, and as a whole, the anion exchange membrane 7 and the cation exchange membrane 8 are arranged so as to be positioned alternately in principle. Ion exchange chambers (electrified parts) partitioned by the anion exchange membrane 7 and the cation exchange membrane 8 are formed in each gasket. And the ion exchange chamber where the cation exchange membrane 8 is located on the cathode 5 side and the anion exchange membrane 7 is located on the anode 4 side becomes the desalination chamber 3, and the chamber where the anion exchange membrane 7 is located on the cathode 5 side and the cation exchange membrane 8 is located on the anode 4 side becomes the concentration chamber 2. The desalination chamber 3 and the concentration chamber 2 are alternately arranged. That is, when energization is performed while circulating and supplying the treatment liquid (electrolyte solution) to the desalination chamber 3, the cations in the treatment liquid supplied to the desalination chamber 3 pass through the cation exchange membrane 8 and migrate to the adjacent concentration chamber 2 on the cathode 5 side, and the anions in the treatment liquid pass through the anion exchange membrane and migrate to the adjacent concentration chamber 2 on the anode 4 side. In this way, when the treatment liquid is circulated and supplied to the desalination chamber 3 and at the same time the aqueous salt solution is circulated to the concentration chamber 2, the treatment liquid is desalted and at the same time, the concentrated liquid 20 with an increased salt concentration can be obtained. As a method for fixing the laminate of these ion exchange chambers between the electrodes, the method using a filter press is preferably used, but it is not limited thereto.

[0035] In the present embodiment, at least one or all of the anion exchange membranes 7 included in the electrodialysis apparatus 1 are composed of a monovalent selective anion exchange membrane in which the SO4 index satisfies the above numerical range.

[0036] Here, the monovalent selective anion exchange membrane refers to an anion exchange membrane that selectively permeates monovalent ions represented by Cl 2- rather than divalent ions represented by SO4 - . Generally, the selective permeation coefficient (P 2- - ) of SO4 - Cl SO4 and Cl Cl SO4 is in the range of 0.01 to 1.0. Among them, those that satisfy the specified value of the SO4 index are easy to obtain. Therefore, in the monovalent selective anion exchange membrane used for the anion exchange membrane 7, the selective permeation coefficient (P Cl SO4 ) is preferably in the range of 0.06 to 0.90, and particularly preferably in the range of 0.11 to 0.37.

[0037] Such monovalent selectivity of anions can be imparted by forming a monovalent ion selective permeable layer on at least one surface of the anion exchange membrane 7. The specific configuration of the monovalent ion selective permeable layer is not particularly limited, but 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. The "surface dense layer" is formed by forming a dense structure on the surface portion of the anion exchange membrane (for example, a layer with a high degree of crosslinking or a layer with a high fixed ion concentration in the surface layer portion), the "electrically neutral layer" is formed by forming an electrically neutral thin layer containing no anion exchange groups on the surface of the anion exchange membrane, and the "oppositely charged layer" is formed by forming a thin layer with cation exchange groups on the surface of the anion exchange membrane.

[0038] As such a monovalent selective anion exchange membrane, for example, on one or both sides of the base anion exchange membrane, the SO4 index is 3.0×10 -3 N or more and 50.0×10 -3 N or less, and a monovalent ion selective permeable layer is formed in such a formation amount, and it can be used. Such a monovalent ion selective permeable layer may be at least one layer selected from the group consisting of a surface dense layer, an electrically neutral layer, and an oppositely charged layer. However, since it is easy to obtain an anion exchange membrane having a specified SO4 index, it is preferably a surface dense layer. In particular, as the surface dense layer, a highly crosslinked resin layer formed using a divalent amine compound is preferable. As the base anion exchange membrane, a styrene divinylbenzene-based copolymer having a basic skeleton and having a quaternary ammonium group, which is a strong basic anion exchange group, introduced therein is preferably used because of its strong structure and ease of forming a monovalent ion selective permeable layer.

[0039] These monovalent selective anion exchange membranes having an SO4 index satisfying the above numerical range can be produced as follows. That is, on one or both membrane surfaces of a raw membrane for introducing an anion exchange group obtained by polymerizing a polymerizable composition containing a styrenic aromatic polymerizable monomer having a halogenoalkyl group and a crosslinkable polymerizable monomer such as divinylbenzene, a divalent amine compound is brought into contact so that the SO4 index on the membrane surface is 3.0×10 -3 N or more and 50.0×10 -3 N or less, and a surface dense layer with higher crosslinking than the inside is formed. Then, a trialkylamine is brought into contact with the remaining halogenoalkyl groups in the membrane and converted into a quaternary ammonium group. According to this method, in the raw membrane for introducing an anion exchange group, a certain proportion of the halogenoalkyl groups present in the membrane surface portion on one side thereof will be consumed for crosslinking by contact with the divalent amine compound. Therefore, since the membrane surface portion has a dense structure due to crosslinking, the permeability of SO4 2- ions etc. decreases and the selectivity for monovalent anions is exhibited.

[0040] In this method, known styrenic aromatic polymerizable monomers 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 substituting for this include chlorine, bromine, iodine, etc. Examples of such halogenoalkyl groups include chloromethyl group, bromomethyl group, iodomethyl group, chloroethyl group, bromoethyl group, iodoethyl group, chloropropyl group, bromopropyl group, iodopropyl group, chlorobutyl group, bromobutyl group, iodobutyl group, chloropentyl group, bromopentyl group, iodopentyl group, chlorohexyl group, bromohexyl group, iodohexyl group, etc. 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, iodohexylstyrene, etc. Among these, it is particularly preferable to use chloromethylstyrene, bromomethylstyrene, iodomethylstyrene, chloroethylstyrene, bromoethylstyrene, iodoethylstyrene, chloropropylstyrene, bromopropylstyrene, iodopropylstyrene, chlorobutylstyrene, bromobutylstyrene, iodobutylstyrene.

[0041] In the original film for introducing an anion exchange group obtained by polymerizing the above-mentioned polymerization composition, it has a halogenoalkyl group in the film derived from the aromatic polymerizable monomer having the above-mentioned halogenoalkyl group, and as will be described later, this is converted into an anion exchange group, that is, a quaternary ammonium group. Such a quaternary ammonium group is a strongly basic group and is very excellent as an anion exchange group. However, when the presence of other anion exchange groups besides this is also desired, in the polymerization composition, 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 into which another anion exchange group can be introduced may be used in combination. Such anion exchange groups other than the quaternary ammonium group are not particularly limited as long as they are functional groups that can become positively charged in an aqueous solution. Examples include primary to tertiary amino groups, pyridyl groups, imidazole groups, quaternary pyridinium groups, and the like. In the polymerization composition, the blending amount of the polymerizable monomer having these other anion exchange groups or the polymerizable monomer having a functional group into which another anion exchange group can be introduced is not particularly limited, but it is preferably 100 parts by mass or less, more preferably 50 parts by mass or less, based on 100 parts by mass of the aromatic polymerizable monomer having the halogenoalkyl group.

[0042] Furthermore, in the polymerization composition, in addition to the polymerizable monomer for introducing an anion exchange group into these anion exchange membranes, a polymerizable monomer that is not directly involved in the introduction of such anion exchange membranes may be blended in a blending amount of 150 parts by mass or less based on 100 parts by mass of the aromatic polymerizable monomer having the halogenoalkyl group. Examples of the polymerizable monomer that is not involved in the introduction of such anion exchange membranes include styrene, acrylonitrile, methylstyrene, ethylvinylbenzene, acrolein, methyl vinyl ketone, vinyl biphenyl, and the like.

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

[0044] In the polymerizable composition, a crosslinkable polymerizable monomer is used in order to increase the denseness of the obtained anion exchange membrane and improve the membrane strength. Such crosslinkable polymerizable monomers can be used without particular limitation as the monomers used in the production of conventionally known ion exchange membranes. Specifically, for example, m-, p- or o-divinylbenzene, divinylbiphenyl, divinylsulfone, butadiene, chloroprene, isoprene, trivinylbenzene, divinylnaphthalene, diallylamine, triallylamine, divinylpyridine, or other functional vinylbenzyl compounds having three or more vinylbenzyl groups disclosed in JP-A-62-205153 are used.

[0045] If these crosslinkable polymerizable monomers are blended too much with respect 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, resulting in an increase in membrane resistance and a possible decrease in iodine concentration efficiency. On the contrary, if the blending ratio of the crosslinkable polymerizable monomer is too small, not only will the strength of the membrane decrease, but the denseness of the highly crosslinked resin layer provided on the membrane surface will not increase, and sufficient monovalent selectivity may not be obtained. From these, the crosslinkable polymerizable monomer is preferably blended in an amount of 2 to 20 parts by mass, preferably 3 to 15 parts by mass, based on 100 parts by mass of the polymerizable monomer component composed of the aromatic polymerizable monomer having a halogenoalkyl group and other polymerizable monomers used in combination as described above as necessary.

[0046] A polymerization initiator is usually compounded in the coincidence composition. Conventionally known polymerization initiators can be used without particular limitation, and can be appropriately selected in consideration of the base material to be used, molding conditions, etc. Specific examples thereof 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-butyl cumyl peroxide, dicumyl peroxide, 2,5-dimethyl-2,5-di(tert-butylperoxy)hexane, 2,5-dimethyl-2,5-di(tert-butylperoxy)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, 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-butylperoxy valeric acid-n-butyl ester, 2,4,4-trimethylpentyl peroxy-phenoxyacetate, α-cumyl peroxyneodecanoate, tert-butyl peroxyneodecanoate, tert-butyl peroxypivalate, tert-butyl peroxy-2-ethylhexanoate, tert-butyl peroxy-isobutyrate, di-tert-butyl peroxy-hexahydroterephthalate, di-tert-butyl peroxyazelate, tert-butyl peroxy-3,5,5-trimethylhexanoate, tert-butyl peroxyacetate, tert-butyl peroxybenzoate, etc. are suitable.These are added and mixed into the monomer paste alone or in combination of two or more kinds. The amount of the polymerization initiator used as described above is usually preferably 0.1 to 30 parts by mass, more preferably 1 to 10 parts by mass, based on 100 parts by mass of the polymerizable monomer component described above.

[0047] In addition, a matrix resin may be blended as a viscosity modifier in the polymerizable composition. By blending such a matrix resin, the coatability of the polymerizable composition can be enhanced, and dripping or the like when this is applied to a substrate can be prevented. Examples of such matrix resins include saturated aliphatic hydrocarbon polymers such as polyvinyl chloride, chlorinated polyvinyl chloride, ethylene-vinyl chloride copolymer, vinyl chloride-based elastomer, chlorinated polyethylene, chlorosulfonated polyethylene, ethylene-propylene copolymer, polybutylene, styrene-based polymers such as styrene-butadiene copolymer, and those obtained by copolymerizing these with styrene-based monomers such as vinyltoluene, vinylxylene, chlorostyrene, chloromethylstyrene, α-methylstyrene, α-halogenated styrene, α,β,β'-trihalogenated styrene, monoolefins such as ethylene and butylene, and conjugated diolefins such as butadiene and isoprene; etc. can be used. 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 can also be preferably used. Here, the styrenic thermoplastic elastomer refers to a copolymer of a polystyrene polymer and styrene with polybutadiene, polyisoprene, vinyl polyisoprene, an alternating copolymer of ethylene-butylene, or an alternating copolymer of ethylene-propylene. For example, polystyrene-hydrogenated polybutadiene-polystyrene copolymer, polystyrene-(ethylene / butylene rubber)-polystyrene copolymer, polystyrene-hydrogenated polyisoprene rubber-polystyrene copolymer, polystyrene-(ethylene / propylene rubber)-polystyrene copolymer, polystyrene-ethylene-(ethylene / propylene rubber)-polystyrene copolymer, polystyrene-vinyl polyisoprene-polystyrene copolymer, etc. are exemplified. The molecular weight of such a matrix resin is not particularly limited, but usually, it is preferably in the range of 1,000 to 1,000,000, particularly preferably in the range of 50,000 to 500,000. Further, such a matrix resin is blended in the polymerizable composition according to the molecular weight in an amount sufficient to ensure appropriate viscosity. For example, the amount is preferably 1 to 50 parts by mass, more preferably 3 to 15 parts by mass, based on 100 parts by mass of the polymerizable monomer component described above.

[0048] In addition, in the polymerizable composition, in addition to the various components described above, if necessary, plasticizers such as dioctyl phthalate, dibutyl phthalate, tributyl phosphate, styrene oxide, or alcohol esters of fatty acids and aromatic acids, and organic solvents may be blended.

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

[0050] The polymerizable composition having the above component composition is filled into the voids of the base material serving as a reinforcing material and then polymerized to obtain a raw membrane for introducing an anion exchange group. As such a base material, any known material as the base material of an ion exchange membrane may be used. Generally, a support material having a porosity of 20 to 90%, more preferably 40 to 80%, and even more preferably 45 to 55% is used. Examples include woven fabrics, non-woven fabrics, porous films, mesh-like materials, etc. formed from polyvinyl chloride, polyolefin, etc. Among these, in particular, those made of polyolefin such as polypropylene and polyethylene are preferred in terms of affinity with the ion exchange resin and chemical resistance, and those made of polyethylene are even more preferred. Also, as the shape of the base material, it is preferable to use a porous film in terms of enhancing the iodine concentration efficiency after imparting monovalent selectivity. The thickness of the base material is generally selected from the range of 50 to 300 μm, and is preferably 70 to 250 μm from the viewpoints of membrane resistance and maintaining strength.

[0051] The method for filling the polymerizable composition into such a base material is not particularly limited. For example, methods such as applying or spraying the polymerizable composition onto the base material, or immersing the base material in the polymerizable composition are exemplified. When the polymerizable composition is in a paste form, it is preferably carried out by coating. Filling the base material by coating can be performed by known means such as a roll coater, a flow coater, a knife coater, a comma coater, spraying, dipping, etc.

[0052] After introducing the polymerizable composition into the base material as described above, it is laminated together with a release material having releasability so that the base materials do not adhere to each other, wound around a roller, and heated to perform polymerization.

[0053] As the release material, a material having heat resistance capable of withstanding polymerization and being easily peelable after polymerization is used. For example, polypropylene, poly-1-butene, poly-4-methyl-1-pentene, or random or block copolymers of α-olefins such as ethylene, propylene, 1-butene, 4-methyl-1-pentene, etc., i.e., polyolefins, ethylene-vinyl acetate copolymers, ethylene-vinyl alcohol copolymers, ethylene-vinyl chloride copolymers, etc., i.e., ethylene-vinyl compound copolymers, polyvinyl compounds such as polymethyl acrylate, polymethyl methacrylate, polyamides such as nylon 6, nylon 6-6, nylon 6-10, nylon 11, nylon 12, etc., thermoplastic polyesters such as polyethylene terephthalate, polybutylene terephthalate, polyethylene naphthalate, etc., polycarbonate, polyphenylene oxide, etc., biodegradable resins such as polylactic acid, or films made of any of these resins can be mentioned, and such films may be biaxially stretched. That is, from among the above-mentioned films, an appropriate one may be selected according to the type of monomer component in the polymerizable composition and used as the release material. In particular, from the viewpoints of heat resistance and releasability, polyester films such as polyethylene terephthalate (PET) are most suitable.

[0054] The pressure during polymerization may be either normal pressure or pressurized, and generally, it may be about 0.1 to 1.0 MPa. The polymerization temperature may be a temperature lower than the melting point of the base material, but generally, a range of 40 to 130°C is suitable. That is, by heating to such a temperature range for polymerization, a part of the base material is dissolved 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 base material can be increased, and the film strength and concentration efficiency can be further improved. In addition, the polymerization time varies depending on the polymerization temperature and the like, but generally, it is about 3 to 20 hours.

[0055] After applying the polymerizable composition to the substrate as described above, heating is performed to carry out polymerization, and a raw film for introducing an anion exchange group composed of a film-shaped polymer is obtained. In order to obtain the monovalent selectivity anion exchange membrane of the present invention, after obtaining the raw film for introducing an anion exchange group in this manner, before performing the operation of introducing the anion exchange group, an operation of forming a monovalent ion selective permeation layer composed of a crosslinked resin layer on the surface of the film is performed. This is carried out, for example, by bringing a divalent amine compound into contact with one side surface of the raw film for introducing an anion exchange group. That is, in the case where the monovalent anion selective layer is formed only on one side surface of the raw film for introducing an anion exchange group, the other side surface of the film is covered with a film or the like, and the non-covered one side surface of the film is brought into contact with a divalent amine compound, and a crosslinked resin layer is formed on the film surface from the inside.

[0056] The amino group of the divalent amine compound can react with the haloalkyl group contained in the raw film for introducing an anion exchange group to eliminate hydrogen chloride and form a C-N bond. In the case of the divalent amine compound, this reacts with two haloalkyl groups to form a crosslinked structure. The denser the formation of this crosslinked structure, the higher the membrane resistance, while the higher the monovalent anion selectivity and the smaller the SO4 index. Further, the crosslinking reaction by the divalent amine compound proceeds from the film surface toward the inside, and the deeper the crosslinking depth, the higher the monovalent anion selectivity while the membrane resistance increases. Examples of the secondary amine compound include dimethylamine, diethylamine, dipropylamine, dibutylamine, diethanolamine, etc. From the balance between resistance and monovalent anion selectivity, a shorter substituent is better, and dimethylamine is preferred.

[0057] When the halogenoalkyl group on the surface of the anion-exchange-group-introduced raw membrane is crosslinked by contacting with a secondary amine compound solution, if the divalent amine compound diffuses too slowly into the anion-exchange-group-introduced raw membrane in an aqueous solution or if the solubility is poor, a part of the water may be replaced with an organic solvent to adjust the diffusion rate and improve the 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 SO4 index is 3.0 x 10 -3 N or more 50.0×10 -3 In order to obtain an anion exchange membrane having an appropriate surface density layer for N or less, the conditions for contacting and reacting the secondary amine compound are particularly important, and 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, the monovalent ion selectivity decreases, and it becomes difficult to make the SO4 index below the upper limit value. On the other hand, if the concentration is too high, the diffusion of the anion exchange group introduction raw membrane into the membrane is strengthened, the crosslinking depth becomes too deep, and the SO4 index becomes less than the predetermined value.

[0059] In addition, 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 less than 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 it becomes difficult to make the SO4 index equal to or less than the upper limit value. If the reaction temperature exceeds 50°C and / or the reaction time exceeds 24 hours, the diffusion of the anion exchange group introduction raw membrane into the membrane is strengthened, the crosslinking depth becomes too deep, and the SO4 index also becomes less than the predetermined value. After reacting the anion exchange group introduction raw membrane with the secondary amine compound, washing may be performed to remove excess secondary amine compound that was not used in the reaction.

[0060] The reaction between the diamine compound and the halogenoalkyl group as described above may stop at the reaction between the diamine and one halogenoalkyl group and may not lead to crosslinking between two halogenoalkyl groups. In such a case, it is preferable to perform a step of treating with an alkaline aqueous solution (alkaline aqueous solution treatment step) after the step of contacting with the diamine compound. Although the detailed reaction is not understood, it is presumed that the reaction with the second halogenoalkyl group is promoted by the alkaline compound neutralizing hydrogen chloride generated by elimination when the diamine compound reacts with one halogenoalkyl group. As the alkaline aqueous solution, known ones such as an aqueous sodium hydroxide solution, an aqueous potassium hydroxide solution, an aqueous barium hydroxide solution, and aqueous ammonia can be used. Since the treatment can be performed more efficiently, an aqueous sodium hydroxide solution or an aqueous potassium hydroxide solution having strong basicity is preferable. The alkaline aqueous solution may have a pH of 8 to 14 at 25°C, but since the treatment can be performed more efficiently, it is preferably 10 to 14 at 25°C. The treatment temperature is preferably 50 to 70°C, more preferably 55 to 65°C. When it is less than 50°C, the treatment takes a long time, resulting in low productivity. When it exceeds 70°C, the treatment becomes severe and there is a possibility that the free amino compound that has once reacted may be generated. The treatment time of the alkaline aqueous solution in this step may be 4 to 24 hours, preferably 6 to 12 hours, more preferably 6 to 10 hours. When it is less than 4 hours, the treatment may be insufficient. When it exceeds 24 hours, the treatment becomes severe and the electrical resistance increases.

[0061] Next, a step of introducing a quaternary ammonium group (quaternary ammonium group introduction step) into the residual halogenoalkyl group in the membrane of the ion exchange group-introduced original membrane after the alkaline aqueous solution treatment step is performed using a tertiary amine compound. Examples of the tertiary amine compound include trimethylamine, triethylamine, N,N-dimethylpropylamine, N-ethyl-N-methylbutylamine, and the like. From the viewpoint of resistance, trimethylamine is preferred. By bringing it into contact with an aqueous solution of a tertiary amine compound and introducing a quaternary ammonium group, the residual halogenoalkyl group in the membrane present from the surface to the inside can be substituted without affecting the monovalent anion selective layer composed of the crosslinked resin layer. The method may follow the conventional method for introducing a quaternary ammonium group in the production of an anion exchange membrane.

[0062] By the above method, a monovalent selectivity anion exchange membrane with an SO4 index of 3.0×10 -3 N or more and 50.0×10 -3 N or less can be obtained. Furthermore, the monovalent selectivity 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, in order to exhibit appropriate permeability to iodide ions. The water content is preferably in the range of 10 to 40%, more preferably 25 to 35%. The electrical resistance is 1.2 Ω·cm 2 or more and 3.0 Ω·cm 2 or less, preferably 1.7 Ω·cm 2 or more and 2.5 Ω·cm 2 or less, more preferably 1.8 Ω·cm 2 or more and 2.4 Ω·cm 2 or less. The thickness is adjusted to be in the range of 50 to 320 μm, particularly 70 to 280 μm. That is, in order to have such physical properties, the composition of the polymerizable composition (the amount or type of monomer components and crosslinking agents), the thickness of the base material, and the crosslinking reaction conditions with the divalent amine compound are appropriately set. Also, the chlorine ion (Cl - ) permeation amount in the seawater concentration test is usually 3.2 mol / L or more, preferably 3.5 mol / L or more.

[0063] When the anion exchange membrane 7 includes an anion exchange membrane other than the monovalent selective anion exchange membrane whose SO4 index satisfies the above numerical range, known anion exchange membranes may be used as the other anion exchange membranes. For example, a monovalent ion selective permeable anion exchange membrane, a completely permeable anion exchange membrane, or a high-strength alkali-resistant anion exchange membrane may be used, and it is preferably a monovalent ion selective permeable anion exchange membrane.

[0064] There is no particular limitation on the cation exchange membrane 8, and a strongly acidic cation exchange membrane, a high-strength alkali-resistant cation exchange membrane, etc. can be used. Further, the cation exchange membrane 8 may be a monovalent ion selective cation exchange membrane. More specifically, a cation exchange membrane in which a sulfonic acid group, which is a strongly acidic cation exchange group, is introduced into the basic skeleton of styrene-divinylbenzene can be used. As commercially available products of the cation exchange membrane, NEOSEPTA (registered trademark) CSE, NEOSEPTA (registered trademark) CMB, etc. manufactured by Asahi Kasei Corporation can be used.

[0065] In addition, the following anion exchange membranes whose SO4 index satisfies the above numerical range and electrodialysis devices equipped with the same can be suitably used in the iodine recovery step of the present embodiment.

[0066] That is, the above electrodialysis device is an electrodialysis device that separates a waste liquid containing an iodide salt into an iodine-containing concentrated liquid and a desalted liquid, and includes a concentration chamber that generates the iodine-containing concentrated liquid, a desalting chamber that supplies the waste liquid, and an ion exchange membrane that partitions the concentration chamber and the desalting chamber. This ion exchange membrane is configured to include an anion exchange membrane in which a minimum appears in the second derivative value curve obtained by second differentiating the IV curve representing the relationship between the direct current and the voltage measured according to the direct current resistance measurement method. A maximum may appear after the minimum in such a second derivative value curve. Further, this anion exchange membrane may be configured to include a monovalent selective anion exchange membrane whose SO4 index is 3.0×10 -3 N or more and 50.0×10 -3 N or less.

[0067] Further, the above anion exchange membrane is an anion exchange membrane for use in electrodialysis for separating a waste liquid containing an iodide salt into an iodine-containing concentrated liquid and a desalted liquid. In an IV curve representing the relationship between a direct current and a voltage measured according to the direct current resistance measurement method, it is an anion exchange membrane for electrodialysis of a waste liquid containing an iodine component in which a minimum appears in a second derivative value curve obtained by second differentiating the IV curve. In such a second derivative value curve, a maximum may appear after the minimum. This anion exchange membrane is a monovalent selectivity anion exchange membrane, and the SO4 index is 3.0×10 -3 N or more and 50.0×10 -3 N or less. Further, this anion exchange membrane may further contain a copolymer of styrene divinylbenzene having a quaternary ammonium group.

[0068] An example of the operation of the electrodialysis apparatus 1 during electrodialysis is as follows. When a direct current is applied using a power source 6 between the anode 4 and the cathode 5, iodide ions (I - ) in the desalting chamber 3 on one side pass through the anion exchange membrane 7 and move to the concentration chamber 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 the desalting chamber 3 on the other side pass through the cation exchange membrane 8 and move to the same concentration chamber 2. In this concentration chamber 2, for example, an iodide salt such as potassium iodide (KI) is generated. From the concentration chamber 2, a concentrated liquid 20 (iodine-containing concentrated liquid) containing an iodide salt is obtained. Note that a desalted liquid 30 is obtained from the desalting chamber 3.

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

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

[0071] The above-described chemical pretreatment or physical pretreatment may be performed on the waste liquid 10 in the waste liquid tank 11. When the waste liquid 10 contains a boron component, before the electrodialysis step, a pH adjustment step of adjusting the pH of the waste liquid 10 to 9.5 or less, preferably 8 or less, more preferably less than 7 may be performed. When the pH of the waste liquid 10 is adjusted to 9.5 or less, most of boric acid (H3BO3) exists as molecules without dissociation. Therefore, even if electrodialysis is performed, boric acid hardly moves and is discharged into the desalted liquid 30 as it is. Thereby, the boron component and the iodine component in the waste liquid 10 can be efficiently separated.

[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 and the ion exchange membrane may deteriorate, resulting in a decrease in 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 caused by free iodine.

[0073] The lower limit of the liquid temperature of the waste liquid 10 in the waste liquid tank 11 may be, for example, 25°C or higher, preferably 30°C or higher, more preferably 35°C or higher. By increasing the liquid temperature of the waste liquid 10 supplied to the desalting chamber 3 in the electrodialysis apparatus 1, the efficiency of electrodialysis can be increased. When there are restrictions on the device side for the upper limit of the liquid temperature of the waste liquid 10 in the waste liquid tank 11, it is better to make it as high as possible within the range of the upper limit temperature of its resistance. When the electrodialysis apparatus 1 is equipped with a PVC pipe, for example, it may be set to 40°C or lower.

[0074] Subsequently, the waste liquid 10 in the waste liquid tank 11 is supplied to the desalting chamber 3 of the electrodialysis apparatus 1 via the line 13. A voltage is applied to the anode 4 and the cathode 5 to perform electrodialysis (electrodialysis step). At this time, in the electrodialysis step of the present 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 maximum appears in the second derivative value curve of the anion exchange membrane 7, it is preferable to set the current density to a2 / d or less. Also, when the current at the point where the second derivative value after the minimum approaches near zero is defined as a3, the current density may be set to a3 / d or less. When no maximum appears in the second derivative value 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 minimum value in the second derivative value curve obtained from the IV curve measured with the unit of voltage being mV and the unit of current being mA is less 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. Also, the lower limit of the maximum value in the second derivative value 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 above a1 / d is, for example, 0.5 A / dm 2 or more, preferably 3.1 A / dm 2 or more, more preferably 4.0 A / dm 2 or more. Thereby, the productivity of iodide ions can be further enhanced.

[0076] Also, the upper limit of the above a1 / d is, for example, 10 A / dm 2 or less, preferably 9.5 A / dm 2 or less, more preferably 9 A / dm 2 or less. Thereby, a decrease in the productivity of iodide ions can be suppressed.

[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 or more, more preferably 1.5 A / dm 2The above is the case. By this, the productivity of iodide ions can be stably improved. The upper limit of a2 / d - a1 / d is not particularly limited, but it may be 10 A / dm 2 or less, and may be 6 A / dm 2 or less.

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

[0079] At this time, at least a part of the desalted liquid 30 may be supplied to the waste liquid tank 11 via line 32 branched from line 31, and the mixed desalted liquid in which the desalted liquid 30 and the waste liquid 10 are mixed may be supplied to the desalting chamber 3 again. That is, the waste liquid 10 supplied to the desalting chamber 3 may contain the desalted liquid 30. By repeatedly performing electrodialysis in this way, the iodide ion concentration in the desalted liquid 30 can be reduced to a desired value.

[0080] On the other hand, at least a part of the concentrated liquid 20 may be supplied to the concentrated liquid tank 21 via line 23 branched from line 22, and the concentrated liquid 20 in the concentrated liquid tank 21 may be supplied to the concentration chamber 2 again via line 24. By repeatedly performing electrodialysis in this way, the iodide ion concentration in the concentrated liquid 20 can be concentrated to a desired value.

[0081] When two or more electrodialysis devices are provided, the first concentrated liquid generated from the first electrodialysis device may be supplied to the desalting chamber of the second electrodialysis device. Further, the second desalted liquid generated in the desalting chamber of the second electrodialysis device may be mixed with the first concentrated liquid and supplied to the desalting chamber of the second electrodialysis device again.

[0082] Also, the electrodialysis device 1 provided with a continuous circulation type electrodialysis facility may be used. Such an electrodialysis device 1 enables continuous operation in which the waste liquid 10 or the mixed desalted liquid is continuously supplied to the desalted liquid 30 while the desalted liquid 30 is continuously discharged.

[0083] Next, an example of the process flow of the regeneration step of the iodine recovery method will be described with reference to FIG. 3.

[0084] Subsequently, the iodine recovery method can include a regeneration step of producing one or more selected from the group consisting of iodine (I2), iodide salts, and hydroiodic acid, as shown in FIG. 3, using the obtained iodine-containing concentrated solution (concentrated solution 20) after the electrodialysis step. The iodine, iodide salt, and hydroiodic acid regenerated (recycled) by the above iodine recovery method may each be in an aqueous solution, and the iodine and iodide salt may also be in powder form (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 the iodine-containing concentrated solution (concentrated solution 20) into hydroiodic acid and an aqueous solution of a hydroxide salt by bipolar membrane electrodialysis, as shown in FIG. 3. Here, the bipolar membrane electrodialysis method is a method in which bipolar membranes that generate hydrogen ions and hydroxide ions are sequentially arranged between an anode and a cathode in addition to a cation exchange membrane and / or an anion exchange membrane, a treatment liquid is supplied to each chamber partitioned by the membranes, and electric current is applied to obtain an acid and an alkali from a neutral salt.

[0086] Specifically, the concentrated solution 20 (for example, KI concentrated solution) discharged from the electrodialysis device 1 is introduced into a bipolar membrane electrodialysis device. If necessary, the pH of the concentrated solution 20 may be adjusted to less than 7 to be acidified. 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 a hydroxide (for example, KOH) are discharged. Thereafter, the HI solution is distilled and purified to obtain hydroiodic acid.

[0087] Also, the iodine recovery method can recover iodine (I2) using the obtained concentrated solution 20. As shown in FIG. 3, when the concentrated solution 20 containing iodide ions is oxidized, iodine (I2) is obtained.

[0088] The iodine recovery method can recover iodide salts using the obtained concentrated solution 20. As shown in FIG. 3, when the hydroiodic acid obtained above is neutralized, an iodide salt is obtained. Also, when the iodine (I2) obtained above is reduced and neutralized, an iodide salt is obtained. In the iodine recovery method, the above oxidation, neutralization, and reduction methods are not particularly limited, and known means can be used. Note that the iodide salts include, for example, alkali metal iodide salts and alkaline earth metal iodide salts. Specific examples of the iodide salts include, for example, potassium iodide, sodium iodide, calcium iodide, magnesium iodide, cesium iodide, etc. These may be used alone or in combination of two or more.

[0089] On the other hand, the iodine recovery method can recover iodine (I2) using the obtained desalted solution 30. As shown in FIG. 3, the iodine recovery method may include a step of passing the obtained desalted solution 30 through a strongly basic anion exchange resin to adsorb the iodide ions remaining in the desalted solution 30 onto the strongly basic anion exchange resin, and recovering iodine from the strongly basic anion exchange resin.

[0090] Specifically, the desalted solution 30 discharged from the electrodialysis device 1 is passed through a strongly basic ion exchange resin, and iodide ions are adsorbed onto this strongly basic anion exchange resin. At this time, the pH of the desalted solution 30 when passing through the strongly basic anion exchange resin is less than 7, preferably adjusted to 3 or more and less than 7, as in the case of the above-mentioned electrodialysis. Thereby, it is possible to suppress the adsorption of borate ions formed by the dissociation of boric acid onto the strongly basic anion exchange resin, and selectively separate iodide salt ions. Thereafter, iodine (I2) can be recovered from the strongly basic anion exchange resin by known means.

[0091] Also, when the waste liquid 10 contains a boron component, boron (B2) can be recovered from the obtained desalted liquid 30 containing the boron component.

[0092] The iodine recovery method may include a boron recovery step of adjusting the desalted liquid 30 containing a boron component so that the pH is, for example, 7 or more, preferably 8 or more, more preferably 11 or more, and then separating and recovering a boron-containing concentrated liquid (boric acid concentrated liquid) from the desalted liquid 30 using another electrodialysis device. When the boron-containing concentrated liquid is acidified and crystallized, boric acid (H3BO3) can be recovered.

[0093] Also, the desalted liquid 30 containing a boron component with a pH of 7 or more, or the desalted liquid 30 when passing through the strongly basic anion exchange resin described above, is passed through a boron-selective chelating resin to adsorb borate ions. Thereafter, boric acid can be recovered from the boron-selective chelating resin by known means.

[0094] As described above, the embodiments of the present invention have been described, but these are examples of the present invention, and various configurations other than the above can be adopted. Further, the present invention is not limited to the above-described embodiments, and modifications, improvements, etc. within the range capable of achieving the object of the present invention are included in the present invention.

Example

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

[0096] <Manufacture 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 method.

[0097] (Example 1) 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 were added to obtain a paste-like polymerizable composition.

[0098] Next, the above-described polymerizable composition was applied to a biaxially stretched porous polyethylene film (thickness: 100 μm, porosity: 46%, average pore diameter: 0.13 μm) serving as a base material, and the polymerizable composition was introduced into the base material.

[0099] Next, both surfaces of the base material into which the polymerizable composition had been introduced were coated with a release material made of a polyethylene terephthalate film and wound around a roller, and then polymerization was carried out. The pattern of the polymerization temperature was such that the temperature was raised from 20°C to 50°C over 30 minutes, held at 50°C for 20 minutes, then raised from 50°C to 130°C over 80 minutes, held at 130°C for 4 hours, to produce a raw film for introducing an anion exchange group.

[0100] Next, the release film was peeled off from the raw film for introducing an anion exchange group. At this time, instead of completely peeling off the release materials on both surfaces of the raw film, the release material was left on one side. The raw film for introducing an anion exchange group covered only on one side with the release material was immersed in a 0.05 mol / L dimethylamine aqueous solution at 34°C for 8.5 hours, and the surface chloromethyl groups on the surface not covered with the release material were subjected to a crosslinking reaction with dimethylamine. Thereafter, the obtained raw film was 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 raw film for introducing an anion exchange group was immersed in a 0.01 mol / L sodium hydroxide aqueous solution at 60°C for 16 hours, and a step of treating with an alkaline aqueous solution was carried out. Thereafter, the raw film after the treatment with the alkaline aqueous solution was washed with pure water.

[0102] Next, a quaternization reaction was carried out at 30°C for 16 hours using an aqueous solution containing 5% by mass of trimethylamine and 25% by mass of acetone to substitute the chlorine of the remaining chloromethyl groups in the film with trimethylamine, thereby introducing a quaternary ammonium group. Thereafter, the raw film after the introduction of the quaternary ammonium group was washed with a 1.0 mol / L hydrochloric acid aqueous 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 of the dimethylamine aqueous 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 sodium hydroxide aqueous solution was changed to 8 hours.

[0105] (Comparative Example 1) A commercially available ACS-8T (manufactured by Asahi Kasei Corporation) was evaluated as the monovalent-selective anion exchange membrane. (Comparative Example 2) A raw membrane for introducing an anion exchange group was obtained in the same manner as in Example 1. After peeling off the release agent film from both sides, the step of introducing a quaternary ammonium group with trimethylamine was carried out in the same manner as in Example 1 without treating with a dimethylamine aqueous solution and an alkali aqueous solution, and an anion exchange membrane without selectivity treatment was obtained.

[0106] ><Method for Measuring DC Resistance of Anion Exchange Membrane> Using a DC membrane resistance measuring device equipped with the following silver / silver chloride electrodes, the DC resistance of the 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 measuring cells arranged in the center between the two electrode cells and incorporating a salt bridge. Into this electrode cell, a 0.5 mol / L potassium chloride (KCl) aqueous solution was introduced as the electrode solution, and into the measuring cell, a 0.5 mol / L potassium iodide (KI) aqueous solution was introduced as the electrolyte solution. The potential between the salt bridges was detected with a potentiometer via a glass electrode and used as the membrane voltage. The liquid temperatures of the electrode solution and the electrolyte solution were maintained at 25 °C (±0.5 °C) using a water bath. Subsequently, the anion exchange membrane obtained above (effective area d: 0.04 dm 2) was immersed in an aqueous solution of 0.5 mol / L KI for 1 hour. Subsequently, the anion exchange membrane after immersion was sandwiched between two measurement cells and installed in the above DC membrane resistance measurement device. At this time, the monovalent selective surface (ion selective permeable layer) of the anion exchange membrane was installed facing the cathode side. 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 by a data logger, and an IV curve of the anion exchange membrane was created. The measurement was terminated at 400 mA after 180 seconds. Also, using the above DC membrane resistance measurement device without installing the anion exchange membrane, 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 the corrected ones obtained by subtracting the blank IV curve. Here, the blank means that in the DC resistance measurement method of the above anion exchange membrane, ion-exchanged water was used instead of an aqueous solution of 0.5 mol / L potassium iodide (KI) as the electrolyte solution. The IV curve was created by plotting voltage (mV) on the vertical axis and current (mA) on the horizontal axis. The IV curves of Comparative Example 1, Examples 1 to 3, and Comparative Example 2 are shown in FIGS. 4, 6, 8, 10, and 12, respectively. In the IV curve of the corrected anion exchange membrane, when the current values are X1, X2 ··· X n and the voltage values are Y1, Y2 ··· Y n (n is 4 or more), The first derivative (dY / dX) of the IV curve of the anion exchange membrane is = (Y n-1 - Y n ) / [(X n-1 - X n ) / 2], and The second derivative (d 2 Y / dX 2 ) of the IV curve of the anion exchange membrane is = 〔(Y n-2 - Y n-1 ) / [(X n-2 - X n-1 ) / 2] - (Y n-1 - Y n ) / [(X n-1 - X n) / 2]) / [(X n-1 -X n ) / 2] was defined and calculated. Here, a second derivative value curve was created from the calculated second derivative values. If a minimum exists in the second derivative value curve, the current value (a1) corresponding to the minimum and, if a maximum exists, the current value (a2) corresponding to the maximum are shown in Table 1. In this second derivative value curve, fluctuations within the range of ±0.005 were not regarded as fluctuations in the IV characteristics. That is, in the second derivative value curve obtained from the IV curve measured with the voltage unit in mV and the current unit in mA, plots existing within the range of ±0.005 are excluded from the minimum or maximum. Note that the second derivative value curves of Comparative Example 1, Examples 1 to 3, and Comparative Example 2 are shown in Figures 5, 7, 9, 11, and 13.

[0107] <Cl index by seawater concentration test - and SO4 index> In a small electrodialysis device (with an energized membrane area of 100 cm 2 ), a monovalent-selective cation exchange membrane CIMS (manufactured by Asahi Kasei Corporation, membrane resistance 2.0 Ω·cm 2 ) was used in pairs and assembled together with the anion exchange membrane obtained above so that the monovalent-selective surface faced the desalination chamber. The operating conditions were as follows: seawater at 25°C was flowed through the desalination chamber at a flow rate of 6 cm / sec, the concentration chamber was filled with a 3.5 mol / L aqueous NaCl solution, and electrodialysis was performed at a current density of 3 A / dm 2 for 5 hours or more (until the concentration change in the concentration chamber ceased). The chlorine ions (Cl - ) and sulfate ions (SO4 2- ) in the concentration chamber were measured. The concentration of chlorine ions (Cl - ) (N) and the concentration of sulfate ions (SO4 2- ) (10 -3 N) in the concentration chamber were measured. The obtained concentration of chlorine ions (Cl - ) (N) and the concentration of sulfate ions (SO4 2- ) were used as the Cl - permeation amount and the SO4 index. The results are shown in Table 1.

[0108] <Selective Permeation Coefficient (P Cl SO4 )> With an effective energized area of 4.0 cm 2 Silver-silver chloride electrodes were provided in both chambers of a two-chamber glass cell separated by an anion exchange membrane. 100 ml of a 0.5 mol / L NaCl aqueous solution was placed on the anode chamber side, and 100 ml of a mixed aqueous solution of 0.25 mol / L NaCl and 0.125 mol / L Na2SO4 was placed on the cathode chamber side. After passing a direct current of 40 mA for 1 hour at 25°C, the amounts of chloride ions and sulfate ions in the cathode chamber were quantified. From the obtained amounts of chloride ions and sulfate ions, the selective permeation coefficient P Cl SO4 was determined by the following formula. P Cl SO4 =(t SO4 / t Cl ) / (C SO4 / C Cl ) t SO4 is the number of equivalents of sulfate ions permeated through the membrane, t Cl is the number of equivalents of chloride ions permeated through the membrane, C SO4 is the equivalent concentration of sulfate ions in the cathode chamber after measurement, C Cl is the equivalent concentration of chloride ions in the cathode chamber after measurement. The P Cl SO4 of Comparative Example 1 was 0.05.

[0109]

Table 1

[0110] (Simulation of Production Efficiency in Electrodialysis) In a model case where the IV curve of the anion exchange membrane is linear, in the electrodialysis process of an iodide-containing solution, it is estimated that when the current density is doubled, the treatment time becomes 1 / 2 and the treatment capacity becomes 2 times. The treatment capacity, that is, the production amount per unit time (P), is proportional to the current density (not the voltage). Here, the "power unit", which is an index adopted in the salt production process, is defined as current (I) × voltage (V) / production amount (P) per unit time. When the IV curve is linear, as described above, increasing the current density causes the production amount (P) per unit time to increase in proportion to the current (I). On the other hand, as in Examples 1 to 3 above, when the IV curve is convex upward (there is a minimum in the second derivative value curve of the IV curve), 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 amount (P) per unit time, in the high current density region, since V2 < V1, an increase in the "power unit" 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 (a1) in the high current density region" / "effective area (d) of the anion exchange membrane", it is expected that the energy efficiency (productivity) in the high current density region can be increased compared to the case where the IV curve is linear. Note that in the IV curves of Comparative Example 1 and Comparative Example 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, so it is difficult to increase the productivity compared to Examples 1 to 3.

Explanation of Symbols

[0111] 1 Electrodialysis device 2 Concentrating chamber 3 Desalting chamber 4 Anode 5 Cathode 6 Power supply 7 Anion exchange membrane 8 Cation exchange membrane 10 Waste liquid 11 Waste liquid tank 12, 13 Lines 20 Concentrate 21 Concentrate tank 22, 23, 24 Lines 30 Desalted liquid 31, 32 Lines 100 Iodine Recovery System

Claims

1. The method includes the step of electrodialyzing an iodide-containing solution, the iodide-containing solution including an iodide salt and a solvent, and having an iodide ion concentration of 3% or more, using an electrodialysis device; the electrodialysis apparatus 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 a direct current and a voltage according to a direct current resistance measurement method and second-order differentiating the IV curve in accordance with the following definition: In the electrodialysis step, the effective area (dm 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. (definition) In the IV curve, the current value is X １ , X ２ ...X ｎ Let the voltage value be Y １ , Y ２ ...Y ｎ (n is 4 or more). The first differential (dY / dX) of the IV curve of the anion exchange membrane is ｎ－１ -Y ｎ ) / [(X ｎ－１ -X ｎ ) / 2], The second derivative (d ２ Y / dX ２ ) is [(Y ｎ－２ -Y ｎ－１ ) / [(X ｎ－２ -X ｎ－１ ) / 2] - (Y ｎ－１ -Y ｎ ) / [(X ｎ－１ -X ｎ ) / 2]] / [(X ｎ－１ -X ｎ ) / 2] is defined and calculated. The second derivative curve is created from the calculated second derivative values.

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

3. A method for operating the electrodialysis apparatus according to claim 1 or 2, comprising the steps of: 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 the electrodialysis apparatus according to claim 1 or 2, comprising the steps of: 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. A method for operating an electrodialysis apparatus according to claim 4, comprising the steps of: a2 / d-a1 / d is 0.5A / dm ２ 10A / dm or more ２ The method of operating an electrodialysis apparatus is as follows:

6. A method for operating the electrodialysis apparatus according to claim 1 or 2, comprising the steps of: The SO 2 concentration in the anion exchange membrane is measured according to the seawater concentration test described below. ４ The index is 3.0 x 10 －３ N or more 50.0×10 －３ A method for operating an electrodialysis apparatus, <Seawater concentration test> Film resistance is 1.5 Ω cm 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 SO in the concentrated solution obtained by performing electrodialysis until the concentration in the concentration compartment stops changing ４ ２－ The concentration is ４ Measured as an index.

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

8. A method for operating the electrodialysis apparatus according to claim 1 or 2, comprising the steps of: The method of claim 1, wherein the iodide-containing solution comprises sulfate ions.

9. A method for operating the electrodialysis apparatus according to claim 1 or 2, comprising the steps of:

13. A method for operating an electrodialysis apparatus, wherein the iodide-containing solution excludes marine extracts.

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