Method for Separating Anions or Cations

Abstract

To provide a method for separating, by electrodialysis, ionic species to be separated with higher resolution.SOLUTION: A method for separating, by electrodialysis using an anion exchange membrane, an iodide ion and a first inorganic anion that has a smaller absorption selectivity to the anion exchange membrane than the iodide ion, which is characterized in that a salt (excluding a hydroxide salt) of a second anion having an absorption selectivity for the anion exchange membrane between the iodide ion and the first inorganic anion is added to a stock solution containing the iodide ion and the first inorganic anion so as to obtain a liquid to be treated, and the liquid to be treated is subjected to electrodialysis.SELECTED DRAWING: None

Description

Technical Field

[0001] The present invention relates to a method for separating anions or cations.

Background Art

[0002] Electrodialysis is known as a technique for separating specific ions from an aqueous solution containing multiple types of ions such as industrial waste liquid and seawater (Patent Documents 1 to 3). Electrodialysis can continuously process a large amount of aqueous solution in a short time at once, and also has high separation ability. Therefore, it is widely used for purposes such as separating compounds containing boron, fluorine, etc. from an aqueous solution for wastewater treatment so that the waste liquid meets the drainage standards, concentrating electrolytes such as saline water, and extracting and recovering rare elements such as iodine.

Prior Art Documents

Patent Documents

[0003]

Patent Document 1

Patent Document 2

Patent Document 3

Non-Patent Documents

[0004]

Non-Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0005] Here, if the separation ability between target ions can be further enhanced, it may be possible to omit an additional purification step after electrodialysis, and regarding the treatment of waste liquid, etc., it may be possible to save the time and cost involved in the treatment. Therefore, it is desired to develop a separation method by electrodialysis with enhanced resolution in a more convenient manner.

[0006] An object of the present invention is to provide a method capable of separating ionic species to be separated with higher resolution by electrodialysis.

Means for Solving the Problems

[0007] The method for separating anions of the present invention is a method for separating iodide ions and a first inorganic anion having a lower adsorption selectivity for the anion exchange membrane than the iodide ions by an electrodialysis method using an anion exchange membrane. A step of adding a salt of a second anion (excluding hydroxide salts) having an adsorption selectivity for the anion exchange membrane between the iodide ions and the first inorganic anion to a stock solution containing iodide ions and the first inorganic anion to obtain a liquid to be treated, and performing electrodialysis on the liquid to be treated.

[0008] Preferably, the first inorganic anion is at least one of fluoride ions and ionic boric acid.

[0009] Preferably, the second anion is at least one of an anion containing chlorine, an anion containing sulfur, bicarbonate ions, and carbonate ions.

[0010] Preferably, the second anion is chloride ions.

[0011] The method for separating anions of the present invention is a method for separating anions by an electrodialysis method using an anion exchange membrane. A step of adding an anion having an adsorption selectivity for the anion exchange membrane between any two of the at least two kinds of anions to a stock solution containing at least two kinds of anions to obtain a liquid to be treated, and performing electrodialysis on the liquid to be treated.

[0012] The method for separating cations of the present invention is a method for separating cations by electrodialysis using a cation exchange membrane. The method includes obtaining a liquid to be treated by adding to a stock solution containing at least two kinds of cations a cation whose adsorption selectivity for the cation exchange membrane is between any two of the at least two kinds of cations, and subjecting the liquid to be treated to electrodialysis.

Advantages of the Invention

[0013] According to the present invention, it is possible to provide a method capable of separating ion species to be separated by electrodialysis with higher separation ability.

Brief Description of the Drawings

[0014]

Figure 1

Embodiments for Carrying Out the Invention

[0015] The method for separating anions according to this embodiment is a method for separating anions by electrodialysis using an anion exchange membrane. The method includes obtaining a liquid to be treated by adding to a stock solution containing at least two kinds of anions an anion whose adsorption selectivity for the anion exchange membrane is between any two of the at least two kinds of anions (hereinafter also referred to as an additional anion), and subjecting the liquid to be treated to electrodialysis. Further, the method for separating cations according to this embodiment is a method for separating cations by electrodialysis using a cation exchange membrane. The method includes obtaining a liquid to be treated by adding to a stock solution containing at least two kinds of cations a cation whose adsorption selectivity for the cation exchange membrane is between any two of the at least two kinds of cations (hereinafter also referred to as an additional cation), and subjecting the liquid to be treated to electrodialysis. According to such a method, when separating two kinds of anions or cations contained in the stock solution from each other by electrodialysis, an effect of obtaining high separation ability is achieved.

[0016] The above method for separating anions is particularly useful for a method of separating iodide ions from a first inorganic anion having a lower adsorption selectivity for the anion exchange membrane than the iodide ions by electrodialysis using an anion exchange membrane. Such a method includes subjecting a liquid to be treated, obtained by adding a salt of a second anion (excluding hydroxide salts) having an adsorption selectivity for the anion exchange membrane between the iodide ions and the first inorganic anion, to electrodialysis in a stock solution containing iodide ions and the first inorganic anion.

[0017] The liquid to be treated refers to a liquid obtained by adding the additional anion or additional cation to a stock solution containing the anion or cation to be separated, and is the liquid to be subjected to electrodialysis.

[0018] The present inventor believes that the reason for the above effect is based on the following mechanism. First, the separation of coions (cations or anions) by electrodialysis utilizes the difference in the permeation rate of the coions contained in the liquid to be treated with respect to the ion exchange membrane. That is, for each ion, the permeation rate with respect to the cation exchange membrane is different if it is a cation, and the permeation rate with respect to the anion exchange membrane is different if it is an anion. When separating coions from an aqueous solution containing a plurality of coions by electrodialysis, ions that are more likely to be adsorbed (higher adsorption selectivity) to the ion exchange membrane tend to have a better permeation rate. And since the number of sites where ions existing in the ion exchange membrane can be adsorbed is finite, among the ions to be separated first, ions with high adsorption selectivity are preferentially adsorbed, and when the sites existing in the ion exchange membrane are saturated, ions with low adsorption selectivity cannot be adsorbed to the ion exchange membrane. Thereafter, ions with high adsorptivity permeate through the ion exchange membrane, the concentration of ions with high adsorptivity in the liquid to be treated decreases, and the number of sites not occupied by ions with high adsorptivity in the ion exchange membrane increases, so that ions with low adsorption selectivity can permeate through the ion exchange membrane. In other words, ions with high adsorptivity have the effect of delaying the permeation of coions with low adsorption selectivity through the ion exchange membrane. In the method for separating anions according to this embodiment, electrodialysis is performed on a liquid to be treated prepared by adding an additional anion to the stock solution. The additional anion has an adsorption selectivity between two types of anions to be separated with respect to the anion exchange membrane used (that is, the adsorption selectivity is greater than that of one of the two types of anions to be separated and smaller than that of the other). Therefore, among the two types of anions to be separated, the anion with low adsorption selectivity is further retarded in permeation through the anion exchange membrane due to the additional anion, and thus is more easily separated from the anion with high adsorption selectivity. Similarly, when separating two types of cations contained in the stock solution, if electrodialysis is performed after adding the above additional cation, among the two types of cations to be separated, the cation with low adsorption selectivity is further retarded in permeation through the cation exchange membrane due to the additional cation, and thus is more easily separated from the cation with high adsorption selectivity.

[0019] In this specification, as the adsorption selectivity for the anion exchange membrane, the selective permeation coefficient when electrodialysis is performed using the anion exchange membrane as a diaphragm can be used. For two types of anions A and B, the selective permeation coefficient T A B is defined by the following formula. T A B =(J B / J A ) / (c B / c A )=(J B / c B ) / (J A / c A )···(I) In the above formula (I), J A and J B respectively represent the fluxes of anion A and anion B (the amount of ions permeating through the anion exchange membrane with an effective membrane area per unit area per unit time. The unit is, for example, mol / (m 2 ·s)), and c A and c B respectively represent the concentrations of anion A and anion B on the liquid to be treated side (desalination chamber side) (the unit is, for example, mol / l). As can be seen from formula (I), the selective permeability coefficient T A B is the flux per unit concentration of anion B with respect to anion A, and if T A B is greater than 1, it means that anion B permeates the anion exchange membrane in a larger amount per unit time than anion A, indicating high adsorption selectivity. For cation exchange membranes as well, the selective permeability coefficient can be used as an index of adsorption selectivity in the same way.

[0020] Measurements of the selective permeability coefficient have been carried out for various anions. For example, as the selective permeability coefficient T A B , the selective permeability coefficient described in Non-Patent Document 1 can be referred to. When measuring the selective permeability coefficient T A B for an anion with an unknown selective permeability coefficient, a test aqueous solution containing these two types of anions is prepared, the pH and temperature are adjusted, and electrodialysis is performed on the test aqueous solution using an electrodialysis device having a predetermined anion exchange membrane. It can be calculated by measuring the flux flowing out to the concentration chamber side and the ion concentration on the desalting chamber side. However, since the selective permeability coefficient changes when the ion concentration ratio on the desalting chamber side changes, it is preferable to make the solution volume on the desalting chamber side sufficiently large or to continuously replenish the desalting chamber with a solution of a constant composition during the measurement. As the anion exchange membrane used for measuring the selective permeability coefficient, an anion exchange membrane having a quaternary ammonium group as an ion exchange group may be used.

[0021] The principle of adsorption selection for ion exchange membranes utilizes the same principle as adsorption onto ion exchange resins. For example, in ion-exchange chromatography, when an aqueous solution containing multiple anions is passed through an anion exchange resin column, the higher the selective adsorption property with the anion exchange resin, the longer the retention time, and the lower the adsorption selectivity, the shorter the retention time. Therefore, when qualitatively determining the order of adsorption selectivity among multiple anions of the same valence simultaneously, it can be judged that the longer the retention time in ion exchange chromatography, the higher the adsorption selectivity. As the ion exchange resin for the anion exchange resin column, it may be an ion exchange resin containing a functional group having the same molecular structure as the ion exchange group of the anion exchange membrane used in electrodialysis.

[0022] When determining which of two anions of the same sign has higher adsorption selectivity (higher permeation rate), for example, a test aqueous solution containing the two anions of the same sign is prepared, the pH and temperature are adjusted, and electrodialysis is performed on the test aqueous solution using an electrodialysis device having a predetermined anion exchange membrane, and it can also be determined by confirming the selective permeation coefficient. In this method, the order of the selective permeation coefficient considering the multi-valent ion impermeability peculiar to the ion exchange membrane can be judged. As the anion exchange membrane used for measuring the selective permeation coefficient, it may be an anion exchange membrane having a quaternary ammonium group as the ion exchange group.

[0023] For example, the order of the selective permeation coefficients for some anions is typically as follows. I - >NO3 - >S2O3 2- >Br - >Cl - >SO4 2- >HPO4 2- >OH - >CH3COO - >F - >Ionic boric acid

[0024] The anions to be separated are not particularly limited because different ion species have different adsorption selectivities for the anion exchange membrane. Therefore, the anions may be a combination of organic anions, a combination of inorganic anions, or a combination of organic and inorganic anions.

[0025] Examples of inorganic anions include fluoride ion, chloride ion, bromide ion, iodide ion, oxoacid ions containing halogen, polyhalide ions, sulfur oxoacid ions such as sulfate ion, sulfite ion, thiosulfate ion, and tetrathionate ion, phosphorus oxoacid ions such as nitrate ion, phosphate ion, hydrogen phosphate ion, and dihydrogen phosphate ion, carbonate ion, bicarbonate ion, sulfide ion, hydrogen sulfide ion, and cyanide ion. Preferably, the inorganic anion does not contain a metal element (i.e., it is not a metal complex anion).

[0026] The halogen-containing oxoacid ion may contain any of fluorine, chlorine, bromine, and iodine as the halogen. More specifically, the oxoacid ion (IO 3- ), hypoiodite ion (IO - oxoacid ions containing iodine, such as chlorate ion (ClO 3- ), hypochlorite ion (ClO - oxoacid ions containing chlorine, such as bromate ion (BrO 3- ), hypobromite ion (BrO - oxoanions of bromine, such as fluorine ion (FO 3- ), hypofluorite ion (FO - ) and other fluorine oxo acid ions.

[0027] The polyhalide ion may be either one containing only a single halogen element or one containing two or more halogen elements, and specifically, I2Cl - , I3 - , ICl2 - , I2Br - , IBr2 - etc.

[0028] Examples of the organic anion include acetate ion, formate ion, methacrylate ion, oxalate ion, salicylate ion, benzoate ion, and the like.

[0029] The stock solution preferably contains iodide ion. When the stock solution contains iodide ion, the anion contained in the stock solution other than iodide ion is preferably an inorganic anion (hereinafter also referred to as the first inorganic anion) having a lower adsorption selectivity to the anion exchange membrane than iodide ion. The first inorganic anion is not particularly limited, but is preferably at least one of fluoride ion and ionic boric acid.

[0030] The concentration of the first inorganic anion in the stock solution is not particularly limited, but may be 0.05 mmol / l or more, may be 0.01 mol / l or more, may be 0.1 mol / l or more, may be 0.3 mol / l or more, or may be 1 mol / l or more. The concentration of iodide ion in the stock solution may be below the saturation concentration in the stock solution and may be 2.0 mol / l or less.

[0031] When a weakly dissociating electrolyte is contained in the aqueous solution, it exists as an equilibrium state between non-dissociated molecular state and dissociated ionic state in the aqueous solution. Since the selective permeation coefficient varies depending on these ionic forms, it is preferable that the existence ratio in each ionic form of the two anions to be separated and the additional anion in the liquid to be treated does not change during electrodialysis.

[0032] When the stock solution contains boric acid, boric acid is one of the weakly dissociating electrolytes and exists in the aqueous solution in a molecular state such as B(OH)3, metaboric acid (O=B-OH), polyboric acid, etc., and an ionic state in which they are ionized. For example, as the ionic boric acid, B(OH)4 - ; and B3O3(OH)4 - 、B5O6(OH)4 - 、B3O3(OH)5 2- 、B4O5(OH)4 2-Examples of polyborate anions include those such as. Also, polyborate ions with higher molecular weights formed by dehydration condensation of multiple boric acids are included in the ionic boric acid. In this specification, the term "boric acid" shall include all states of the above-mentioned boric acid in molecular and ionic states that can be taken in an aqueous solution.

[0033] The cations contained in the stock solution are not particularly limited, and examples include metal ions such as alkali metal ions, alkaline earth metal ions, transition metal ions, and aluminum ions; ammonium ions; hydrogen ions, etc. The ammonium ion may be any of primary to quaternary ammonium ions, and may be NH4 + or the like.

[0034] The additional anion is not particularly limited as long as its adsorption selectivity for the anion exchange membrane is between any two of the anions contained in the stock solution. In other words, the additional anion may be one having an adsorption selectivity greater than that of the anion having the maximum adsorption selectivity for the anion exchange membrane among the anions contained in the stock solution, and one having an adsorption selectivity smaller than that of the anion having the minimum adsorption selectivity for the anion exchange membrane among the anions contained in the stock solution. When the stock solution contains three or more types of anions, any combination of the above-mentioned two types of anions can be arbitrarily selected, and two types desired to be separated may be selected.

[0035] When the stock solution contains iodide ions and the first anion, the additional anion is not particularly limited as long as the adsorption selectivity for the anion exchange membrane is between the iodide ion and the first inorganic anion (hereinafter also referred to as the second anion). The second anion is preferably at least one of ions containing chlorine, nitrate ions, bromide ions, ions containing sulfur, bicarbonate ions, and carbonate ions, more preferably at least one of ions containing chlorine, ions containing sulfur, bicarbonate ions, and carbonate ions, and still more preferably ions containing chlorine. These are particularly preferable when the first anion is at least one of fluoride ions and ionic boric acid.

[0036] The ions containing chlorine are preferably ions that do not contain a metal element (that is, not metal complex ions). Specifically, chloride ions; oxoacid ions of chlorine such as hypochlorite ions, chlorite ions, chlorate ions, and perchlorate ions can be mentioned. The ions containing sulfur are preferably ions that do not contain a metal element (that is, not metal complex ions). For example, oxoacid ions of sulfur such as sulfate ions, sulfite ions, thiosulfate ions, and tetrathionate ions can be mentioned.

[0037] The additional anion preferably contains the anion and is added to the stock solution as a water-soluble compound. Among them, hydroxide ions are one of the anions having adsorption selectivity and may affect the adsorption selectivity of the two anions to be separated. Therefore, the additional anion is preferably contained in the normal salt and preferably not in the form of a hydroxide salt or an acid (the conjugate acid in which the additional anion is protonated). That is, it is preferable that the two anions to be separated do not become non-dissociated molecular states by adding the additional anion to the stock solution.

[0038] The additional anionic salt is not particularly limited as long as it is a water-soluble salt. For example, it may be a salt of a monovalent cation or a divalent cation and an additional anion. More specifically, for example, it may be a salt of chloride ion, nitrate ion, bromide ion, sulfate ion, or carbonate ion and an alkali metal. The cation contained in the salt may be one or more of lithium, sodium, potassium, rubidium, cesium, beryllium, magnesium, calcium, strontium, or barium. The salt of the additional anion preferably does not contain a transition metal element in order to include the additional anion as a free ion in the aqueous solution.

[0039] The state of the compound to be added is not particularly limited, and it may be an aqueous solution in which the compound has been previously dissolved. It is preferably completely dissolved in the liquid to be treated, and it is preferable that no solid compound precipitates after the addition.

[0040] From the viewpoint of sufficiently saturating the anion exchange membrane and more effectively inhibiting the permeation of anions with low adsorption selectivity, the molar concentration ratio of the anion with high adsorption characteristics to the anion exchange membrane among the two anions to be separated is 5% or more, preferably 20% or more, more preferably 50% or more. It is preferable to add the additional anion so that the concentration of the additional anion in the liquid to be treated is adjusted to the above range when the same ion as the additional anion is originally contained in the stock solution.

[0041] The stock solution is not particularly limited. For example, it may be factory wastewater, leachate, etc. Examples of factory wastewater include wastewater generated during the production of polarizing films. Also, it may be an aqueous solution naturally existing such as seawater.

[0042] The stock solution may be prepared by dissolving a salt containing an inorganic anion in an aqueous solution. When the stock solution contains iodide ions, the stock solution may be prepared by dissolving a salt of iodide ions, preferably a salt of iodide ions and metal ions, more preferably a salt of iodide ions and alkali metal ions. When the stock solution contains fluoride ions, the stock solution may be prepared by dissolving a salt of fluoride ions, preferably a salt of iodide ions and metal ions, more preferably a salt of iodide ions and alkali metal ions. Note that the concentration of the stock solution may be adjusted by concentrating or diluting it before adding additional anions to the stock solution (before preparing the liquid to be treated).

[0043] When the stock solution contains boric acid in an ionic state, the boric acid may be formulated by dissolving at least one of B(OH)3, metaboric acid, polyboric acid, and their salts. The cations in the salts of B(OH)3, metaboric acid, and polyboric acid are not particularly limited, and examples include metal ions such as alkali metal ions and alkaline earth metal ions. Examples of the salts of polyboric acid include borax.

[0044] The stock solution may contain nonionic impurities. Such impurities include, for example, water-soluble organic compounds such as water-soluble polymers and hydrophilic organic solvents. Since nonionic compounds do not respond to an electric field, they can be successfully separated by electrodialysis, but they may be removed from the stock solution or the liquid to be treated in advance before electrodialysis. Also, when the stock solution contains insoluble impurities, it may cause clogging of the pipes during electrodialysis, etc., so it is preferable to remove them from the stock solution or the liquid to be treated in advance by filtration or the like before electrodialysis. After preparing the liquid to be treated, the concentration of the liquid to be treated may be adjusted by concentrating or diluting it before subjecting it to electrodialysis.

[0045] In the method of this embodiment, first, the above-mentioned additional anions are added to the stock solution to prepare a liquid to be treated. Note that the pH of the stock solution may be adjusted before adding the additional anions, or the pH of the liquid to be treated may be adjusted after adding the additional anions. In order to separate anions from each other by utilizing the adsorption selectivity for the anion exchange membrane, the two kinds of anions to be separated and the additional anions need to be in an ionic state in the liquid to be treated or in the ion exchange membrane. For the largest pKa (max) among the first acid dissociation constants pKa of the respective anions, it is preferable that the pH is greater than pKa (max), more preferably pKa (max) + 1 or more, and particularly preferably 10 or more. Although there is no particular limitation on the upper limit of the pH of the liquid to be treated, from the viewpoint of the durability of the ion exchange membrane, the pH of the liquid to be treated may be 14 or less.

[0046] There is no particular limitation on the electrodialysis apparatus used to implement the method of this embodiment. For example, FIG. 1 is a schematic diagram showing an example of an electrodialysis apparatus. The electrodialysis apparatus 1 includes an electrode chamber (anode chamber) having an anode 8, an electrode chamber (cathode chamber) having a cathode 9, a concentration chamber 11 partitioned from the anode chamber by a cation exchange membrane 6, and a desalting chamber 10 partitioned from the cathode chamber by a cation exchange membrane 6, and has an electrodialysis cell 20. The desalting chamber 10 and the concentration chamber 11 are partitioned by an anion exchange membrane 7. A polar liquid is accommodated in the cathode chamber and the anode chamber. The polar liquid is not particularly limited, and examples thereof include an aqueous sodium hydrogen sulfate solution and an aqueous potassium sulfate solution.

[0047] The anion exchange membrane is not particularly limited, and a strongly basic anion exchange membrane or the like can be used. As the anion exchange membrane, a strongly basic anion exchange membrane is preferred. The strongly basic anion exchange membrane may have a quaternary ammonium group as an ion exchange group. Further, as the anion exchange membrane, 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 may be a monovalent ion selective permeable anion exchange membrane. More specifically, an anion exchange membrane in which a quaternary ammonium group, which is a strongly basic anion exchange group, is introduced into a base film having a styrene-divinylbenzene basic skeleton (styrene-divinylbenzene-based base film) can be used. As commercially available products of the anion exchange membrane, Selemion (registered trademark) AMV, Selemion (registered trademark) AMT manufactured by AGC Engineering Co., Ltd., and Selemion (registered trademark) ASV, which is a monovalent anion selective membrane, etc. can be used. In addition, Neocepta (registered trademark) ASE (completely permeable anion exchange membrane), monovalent anion selective membrane ACS, Neocepta (registered trademark) AXP-D, etc. manufactured by Asahi Kasei Corporation can also be used.

[0048] The cation exchange membrane is not particularly limited, and a strongly acidic cation exchange membrane, a high-strength alkali-resistant cation exchange membrane, or the like can be used. Further, the cation exchange membrane may be a monovalent ion selective permeable 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 a base film having a styrene-divinylbenzene basic skeleton (styrene-divinylbenzene-based base film) can be used. As commercially available products of the cation exchange membrane, Selemion (registered trademark) CMV, Selemion (registered trademark) CMB manufactured by AGC Engineering Co., Ltd., etc. can be used. In addition, Neocepta (registered trademark) CSE, Neocepta (registered trademark) CMB, etc. manufactured by Asahi Kasei Corporation can also be used.

[0049] The electrodialysis device 1 includes a storage tank 2 for storing the stock solution and the like transferred to the desalting chamber 10, and an adjustment tank 12 for adjusting the stock solution stored in the storage tank 2. For example, the adjustment tank 12 may contain an aqueous solution containing additional anions, and by transferring the aqueous solution to the stock solution in the storage tank 2, additional anions can be added to the stock solution to prepare the liquid to be treated. Further, the adjustment tank 12 may contain an aqueous solution for adding other additives other than the aqueous solution containing additional anions, for example, an alkaline or acidic aqueous solution for adjusting the pH of the stock solution stored in the storage tank 2. Also, an alkaline or acidic substance may be added to the aqueous solution containing additional anions in the storage tank 2 to simultaneously add additional anions and adjust the pH to the stock solution.

[0050] Note that the liquid to be treated may be prepared in the storage tank 2 by directly adding an aqueous solution containing additional anions to the stock solution stored in the storage tank 2, or adding a salt containing additional anions as a solid. Also, additional anions may be added in advance to the stock solution before storing it in the storage tank 2 to prepare the liquid to be treated, and the pre-prepared liquid to be treated may be put into the storage tank 2. In these cases, the adjustment tank 12 may not be provided in the electrodialysis device 1, and when directly inputting the liquid to be treated into the desalting chamber 10, the storage tank 2 may not be provided in the electrodialysis device 1.

[0051] The liquid to be treated stored in the storage tank 2 is transferred to the desalting chamber 10 through a pipe. Here, the aqueous solution accommodated or circulated in the desalting chamber is called the desalted liquid. A continuous operation may be performed in which the desalted liquid is continuously discharged while continuously supplying the liquid to be treated to the desalting chamber 10.

[0052] Before electrodialysis, the electrolysis chamber 11 contains an electrolytic solution. The electrolytic solution is not particularly limited, and examples include aqueous solutions of sodium chloride, potassium iodide, sodium iodide, and the like. Note that the aqueous solution accommodated or circulated in the concentration chamber 11 is called the concentrated liquid. When performing continuous operation, the concentrated liquid may be appropriately withdrawn and the electrolytic solution may be supplied to the concentration chamber.

[0053] The operating conditions of electrodialysis are not particularly limited. However, if the concentration of each ion in the liquid to be treated is measured in advance and the amount of electricity required for transfer to the concentration chamber side is calculated, it can be used as a reference for the operating conditions for fractionating and separating each component. It is more preferable to analyze the change over time in the concentration of ions moving to the concentration chamber side in advance through preliminary experiments or the like, while considering the types and concentrations of coexisting ions, the acceptance conditions on the iodine recovery device side, etc., and determine the operating conditions of the electrodialysis device.

[0054] During electrodialysis, the anions to be separated and the additional anions contained in the liquid to be treated flow out to the concentration chamber 11 side in order of high adsorption selectivity for the anion exchange membrane. The combination of the anions to be separated and the additional anions can be appropriately selected according to the purpose of the separation. For example, when the stock solution contains three or more types of anions and only two of them are to be separated, an anion with an adsorption selectivity for the anion exchange membrane between the two anions can be added as the additional anion.

[0055] In the concentration chamber 11, anions with high adsorption selectivity for the anion exchange membrane among the anions to be separated flow out to generate a first concentrated solution. The first concentrated solution is transferred from the concentration chamber 11 and stored in the first concentrated solution tank 3. The first concentrated solution may be recovered and further purified, or the first concentrated solution may be returned from the first concentrated solution tank 3 to the concentration chamber 11 for further electrodialysis to increase the concentration of the anions to be separated as the target.

[0056] During electrodialysis, it is preferable to monitor the concentration of the anions to be separated in the concentration chamber 11. Thereby, in the concentration chamber 11, it is possible to find the optimal time period for separation where the concentration of a specific anion among the anions to be separated increases and the concentration of other anions to be separated decreases.

[0057] Further, the electrodialysis device 1 further includes a second concentrated solution tank 4 and a third concentrated solution tank 5, and the concentrated solution can be fractionated separately from the first concentrated solution.

[0058] Note that the preparation of the liquid to be treated may be performed in the desalting chamber 10. That is, after the stock solution is introduced into the desalting chamber 10, additional anions may be added to the stock solution to prepare the liquid to be treated. In this case, electrodialysis may be started after the preparation of the liquid to be treated. However, since it takes some time until anions with low adsorption selectivity start to flow out, additional anions may be added after the start of electrodialysis to prepare the liquid to be treated.

[0059] The separation method of this embodiment can be appropriately changed according to the purpose, such as which anions contained in the stock solution are to be separated. For example, when separating only two kinds of anions (each referred to as anion A and anion B, and the adsorption selectivity for the anion exchange membrane is higher for anion A) among the anions contained in the stock solution, an anion with an adsorption selectivity between anion A and anion B may be added to the stock solution as an additional anion to prepare the liquid to be treated. In this example, anion A starts to flow out to the concentration chamber side first. When the outflow amount of anion A decreases, the outflow of anion B starts to increase. During electrodialysis, the concentration of anion B contained in the concentrated solution is monitored, and electrodialysis is terminated while the concentration of anion B (or the additional anion) in the concentration chamber is below the desired concentration, and the concentrated solution may be recovered. Alternatively, a threshold value of the concentration of anion B (or the additional anion) in the concentrated solution is set in advance. Until the threshold value is reached, the concentrated solution is stored in the first concentrated solution tank 3 as the first concentrated solution. When the threshold value is exceeded, the conveyance destination of the concentrated solution is switched and stored in the second concentrated solution tank 4 as the second concentrated solution. Since the second concentrated solution has a lower purity of anion A than the first concentrated solution, as shown in FIG. 1, it may be returned to the storage tank 2 and electrodialysis may be performed again. When the stock solution contains other anions other than anion A and anion B, if it is not necessary to separate the other anions from anion A and anion B, the other anions may be contained in either the desalted solution or the concentrated solution.

[0060] When separating three or more types of anions among the anions contained in the stock solution, if the total number of anions to be separated is n types, arrange the anions to be separated in order from the highest to the lowest in terms of adsorption selectivity. As an additional anion, an anion between the anion with the k-th highest adsorption selectivity (k is an integer from 1 to (n - 1)) and the anion with the (k + 1)-th highest adsorption selectivity may be added to the stock solution to prepare the liquid to be treated. That is, in the method of this embodiment, (n - 1) types of additional anions will be used. The (n - 1) types of additional anions may all be added to the stock solution before electrodialysis, or may be added sequentially according to the progress stage of electrodialysis. As a way of adding according to the progress stage, for example, to the liquid to be treated at the time of starting electrodialysis, an anion between the anion with the highest adsorption selectivity and the anion with the second highest adsorption selectivity among the anions contained in the stock solution is added as an additional anion. After observing that the anion with the highest adsorption selectivity has sufficiently flowed out into the concentration chamber as electrodialysis progresses, an anion between the anion with the second highest adsorption selectivity and the anion with the third highest adsorption selectivity may be added to the desalting solution as an additional anion. Similarly, thereafter, electrodialysis may be performed by sequentially adding additional anions with the (n - 1)-th highest adsorption selectivity.

[0061] When the stock solution contains a nonionic compound such as a water-soluble polymer, even for the anion with the lowest adsorption selectivity among the anions to be separated, electrodialysis may be performed and allowed to flow out into the concentration chamber. Thereby, an electrically neutral compound and an ionic compound can be separated.

[0062] In the method of this embodiment, electrodialysis may be performed again on the first concentrated solution to further purify the first concentrated solution. Specifically, the first concentrated solution may be returned to the concentration chamber 11 for electrodialysis again, or returned to the storage tank 2 and then transferred to the desalting chamber 10 again for electrodialysis. Alternatively, the first concentrated solution may be transferred from the concentration chamber 11 to the desalting chamber of another electrodialysis cell (the second electrodialysis cell), or via a separately prepared second stock solution chamber to the second electrodialysis cell, and electrodialysis may be performed in the other electrodialysis cell.

[0063] When the ion to be separated is an iodide ion and an aqueous solution containing the iodide ion is recovered after separation by electrodialysis, the iodide ion may be contained in the aqueous solution as a salt containing the iodide ion or hydroiodic acid. The cation contained in the stock solution may be a cation contained in the electrolyte used as the polar liquid or the electrolyte in the concentration chamber 11.

[0064] When producing hydroiodic acid by the method of the present embodiment, a bipolar membrane may be used. For example, in the electrodialysis chamber, bipolar membranes can be arranged on the anode side and the cathode side of the anion exchange membrane, respectively. In this case, when electrodialysis is performed using an electrodialysis device in which the region sandwiched between the bipolar membrane on the anode side and the anion exchange membrane is used as the concentration chamber, and the region sandwiched between the bipolar membrane on the cathode side and the anion exchange membrane is used as the desalting chamber, hydrogen ions are generated in the concentrated solution and hydroiodic acid is produced. Since hydroxide ions are released on the desalting chamber side, even if hydroxide ions move into the concentrated solution through the anion exchange membrane, the pH of the concentration chamber tends to be kept low and the pH of the desalting chamber tends to be kept high.

[0065] Regarding the method for separating cations as well, since the principle is the same as that of the method for separating anions, it can be implemented by using a similar device. Also, the method for separating anions and the method for separating cations of the present embodiment can be used in combination. That is, electrodialysis may be performed on the liquid to be treated obtained by adding both additional anions and additional cations to a stock solution containing a plurality of anions and a plurality of cations to perform both anion separation and cation separation.

[0066] In the method for separating anions of the present embodiment, the additional anion does not have to be directly added to the stock solution, and may be generated by a reaction in the stock solution. Examples of such a reaction include an acid-base reaction. For example, by adding chlorine molecules to a stock solution containing hydroxide ions and generating hypochlorite ions (ClO - ) by the reaction between the hydroxide ions and the chlorine molecules, a liquid to be treated containing hypochlorite ions as an additional anion may be prepared. Also, by adding carbon dioxide to a solution containing sodium hydroxide and generating carbonate ions (CO32- ) may be generated. Similarly, additional cations may also be generated by reaction in the stock solution. For example, ammonia may be added to an acidic stock solution to generate ammonium ions (NH4 + ), and a liquid to be treated containing ammonium ions may be prepared.

Example

[0067] (Preparation of stock solution and liquid to be treated) Potassium iodide and potassium fluoride were dissolved in water to prepare a stock solution. The concentrations of potassium iodide and potassium fluoride in the stock solution were each about 1.00 mol / l. Potassium chloride was further added to the above stock solution to prepare a liquid to be treated. The concentration of potassium chloride in the liquid to be treated was about 1.00 mol / l.

[0068] (Electrodialysis device) As the electrodialysis device, a Micro Analyzer EX3B manufactured by Astom Corporation was used. A pair of cation exchange membranes and anion exchange membranes was taken as one unit, and a two-chamber electrodialysis device with 10 units was used. As the cation exchange membrane, a strongly acidic cation exchange membrane (manufactured by Astom Corporation, trade name: Neosepta CSE) was used, and as the anion exchange membrane, a monovalent anion selective membrane (trade name: Neosepta AXP-D) was used. The effective membrane area was 550 cm 2 .

[0069] (Example) 700 ml of the above liquid to be treated was introduced into the desalting chamber. 700 ml of a 0.1 M NaCl aqueous solution was introduced into the concentration chamber as the electrolytic solution. Electrodialysis was carried out at a room temperature of 25 °C under operating conditions of an average current of 1.1 A and a voltage of 10 V. At 90 minutes, 120 minutes, and 150 minutes from the start of electrodialysis, the amounts of iodide ions and fluoride ions flowing out into the concentration chamber were measured by ion chromatography, respectively. For each ion, the ratio of the outflow amount to the concentration chamber side to the concentration in the liquid to be treated before electrodialysis (referred to as I transmittance and F transmittance, respectively) was calculated. Also, the molar ratio of fluoride ions to iodide ions (I / F) flowing out into the concentration chamber side was calculated. The results are shown in Table 1. Note that as the ion chromatography apparatus, measurement was performed by Dionex (registered trademark) ion chromatography (IC) (manufactured by Thermo Scientific), and the selective permeability coefficient was calculated.

[0070] (Comparative Example) Electrodialysis was performed in the same manner as in the example, except that 700 ml of the above stock solution was introduced into the desalting chamber instead of the above liquid to be treated. Similar to the example, the selective permeability coefficient was calculated by taking the ratio of the fluxes of fluoride ions to iodide ions at 90 minutes, 120 minutes, and 150 minutes from the start of electrodialysis. The results are shown in Table 1.

[0071] [Table 1]

[0072] As shown in Table 1, since iodide ions are sufficiently present until the 90-minute time point, both show a low selective permeability coefficient T I F In the example, at the time point of 150 minutes, almost all of the iodide ions moved to the concentration chamber side, but only about 6% of the fluoride ions moved to the concentration chamber side, and the selective permeability coefficient between iodide ions and fluoride ions was good. On the other hand, in the comparative example, at the time point of 150 minutes, almost all of the iodide ions moved to the concentration chamber side as in the example, but about 60% of the fluoride ions also moved to the concentration chamber side, and the selective permeability coefficient between iodide ions and fluoride ions increased significantly. [Explanation of Symbols]

[0073] 1... Electrodialysis device, 2... Storage tank, 3... First concentrated liquid tank, 4... Second concentrated liquid tank, 5... Third concentrated liquid tank, 6... Cation exchange membrane, 7... Anion exchange membrane, 8... Anode, 9... Cathode, 10... Desalination chamber, 11... Concentration chamber, 12... Adjustment tank, 20... Electrodialysis cell.

Claims

1. A method for separating iodide ions and a first inorganic anion by electrodialysis using an anion exchange membrane, comprising: preparing a liquid to be treated by adding a salt of a second anion (excluding hydroxide salts) to a stock solution containing iodide ions and the first inorganic anion; performing electrodialysis on the liquid to be treated; the liquid to be treated containing the first inorganic anion in an ionic state; the first inorganic anion being at least one of fluoride ions and ionic boric acid; the molar concentration ratio of the second anion to iodide ions being 5% or more; the second anion being chloride ions; when the first inorganic anion, the second anion, and the iodide ions are denoted as A1, A2, and B, respectively; The selectivity permeation coefficient T represented by the following formula (1) for the iodide ion and the first inorganic anion Ａ１ Ｂ is greater than 1, The selectivity permeation coefficient T represented by the following formula (2) for the iodide ion and the second anion Ａ２ Ｂ is greater than 1, The selection permeability coefficient T represented by the following formula (3) for the first inorganic anion and the second anion Ａ２ Ａ１ is less than 1, and a method for separating anions. T Ａ１ Ｂ = (J Ｂ / J Ａ１ ) / (c Ｂ / c Ａ１ ) = (J Ｂ / c Ｂ ) / (J Ａ１ / c Ａ１ ) (1) T Ａ２ Ｂ = (J Ｂ / J Ａ２ ) / (c Ｂ / c Ａ２ ) = (J Ｂ / c Ｂ ) / (J Ａ２ / c Ａ２ ) (2) T Ａ２ Ａ１ = (J Ａ１ / J Ａ２ ) / (c Ａ１ / c Ａ２ ) = (J Ａ１ / c Ａ１ ) / (J Ａ２ / c Ａ２ ) (3) [In formulas (1) to (3), J Ａ１ , J Ａ２ and J Ｂ respectively represent the fluxes (unit: mol / (m ２ ·s)) of the first inorganic anion, the second anion, and the iodide ion, c Ａ１ , c Ａ２ and c Ｂ respectively represent the concentrations (unit: mol / l) of the first inorganic anion, the second anion, and the iodide ion on the side of the liquid to be treated, and the flux represents the amount of ions passing through the anion exchange membrane with an effective membrane area per unit area per unit time.]

2. The method for separating anions according to claim 1, wherein the salt of the second anion contains one or more of lithium, sodium, potassium, rubidium, and cesium.

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