Electrodialysis method

Abstract

To provide an electrodialysis method capable of efficient separation and recovery of components to be acquired from an aqueous solution containing the components to be acquired and components to be removed both.SOLUTION: In an electrodialysis method of the present invention, by using an electrodialysis tank where a plurality of sets with a four-membrane-four-chamber combination as a set, between a cathode and an anode, from the cathode side, and in sequence from a cathode chamber, a first cation-exchange membrane, a first intermediate chamber, a second cation-exchange membrane, a concentration chamber, a first anion-exchange membrane, a second intermediate chamber, a second anion-exchange membrane, and a desalination chamber in this order, are arranged and successively a cation-exchange membrane and an anode chamber are arranged, an aqueous solution is allowed to pass through the first intermediate chamber, a concentrated liquid is allowed to circulate in the concentration chamber, a liquid having been allowed to pass through the first intermediate chamber is allowed to pass through the second intermediate chamber, and a liquid having been allowed to pass through the second intermediate chamber is allowed to pass through the desalination chamber.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to an electrodialysis method for separating and obtaining a component to be obtained from an aqueous solution containing both the component to be obtained and the component to be removed by electrodialysis. [Background technology]

[0002] Industrial methods for separating and extracting iodine from brine associated with natural gas have been established. As of 2021, the main methods in use include the blowing-out method and the ion exchange resin method (see, for example, Patent Document 1). Methods for recovering iodine from industrial wastewater containing iodine, such as combustion decomposition and electrodialysis, have been put into practical use. In either method, a concentrated solution with an iodine concentration of several percent is produced as the raw material. This concentrated solution is called various names depending on the method or manufacturer, such as absorption solution, desorption solution, concentrated solution, or crude HI aqueous solution. All of these are acidic aqueous solutions with an iodide ion concentration of 1% or higher (hereinafter referred to as absorption solution). The absorption solution undergoes a purification process known as a crystallization process, which produces metallic iodine (hereinafter simply referred to as iodine or product iodine).

[0003] In the crystallization process, chlorine gas or a sodium hypochlorite solution is added to the absorption liquid to generate iodine precipitate (hereinafter referred to as iodine mud). The iodine mud is separated from the aqueous solution using a liquid cyclone or similar device and collected in a melting can. The collected iodine mud is heated in the melting can at a temperature above 108°C, the melting point of iodine, and below 184°C, the boiling point, and impurities such as remaining water, iodide salts, hydrochloric acid, chloride salts, sulfuric acid, and sulfate salts are separated due to differences in specific gravity. The iodine is then removed from the melting can while still in the molten state and rapidly cooled to solidify and produce the product.

[0004] On the other hand, the aqueous solution separated from the sludgy iodine (hereinafter referred to as crystallization wastewater) generally contains iodine at a concentration of 200 mg / L to 1,500 mg / L. By spraying the crystallization wastewater in an air stream, the iodine present in the form of iodine is extracted and recovered in the air stream, but most of it is in the form of iodide ions (I- Because the crystallization wastewater is a strongly acidic solution, it remains in the form of iodate ions (IO3 - The iodine in the crystallization wastewater is returned to the iodine acquisition step and recovered (see, for example, Patent Document 2).

[0005] However, when the iodine extraction process uses the blowing-out method or the ion exchange resin method, the recovery rate of iodine returned to the iodine extraction process is low. Figure 19 shows the iodine recovery status using the blowing-out method. The horizontal axis shows the iodide ion concentration in the brine after the crystallization wastewater has been added, and the vertical axis shows the iodine concentration in the brine after iodine extraction (discharged brine).

[0006] The slope of the fitted line is 0.33, so the increase in iodine recovery rate due to the addition of crystallization wastewater is estimated to be approximately 67%. However, the correlation coefficient is only 0.5, so it cannot be said that the increase in recovery corresponds to the amount of addition. In the blowing-out method and ion-exchange resin method, the amount of oxidizing agent added must be precisely controlled according to the mass of iodide ions, etc., to be oxidized. On the other hand, the iodide ion concentration in crystallization wastewater is constantly fluctuating. For this reason, it is unreasonable to return crystallization wastewater to iodine extraction processes such as the blowing-out method and ion-exchange resin method.

[0007] In particular, in the blowing-out method, when an oxidizing agent is added to brine to generate free iodine, chlorine gas or a high-concentration sodium hypochlorite solution is dissolved in the brine after an appropriate amount of iodine has been extracted (hereinafter referred to as waste brine) to produce chlorinated water, which is then added to the brine. When chlorinated water is produced, the iodide ions remaining in the waste brine come into contact with the high-concentration oxidizing agent and are almost all oxidized to iodic acid. In other words, when chlorinated water is produced, six times the equivalent amount of oxidizing agent is consumed as compared to the iodide ions remaining in the waste brine.

[0008] If the amount of iodide ions in the brine increases for any reason, such as when crystallization wastewater is added to the brine, not only will the yield not increase unless the amount of oxidizing agent is increased by the same amount, but the concentration of available oxidizing agent in the chlorinated water will decrease, leading to a further oxidizing agent shortage, which will result in a decrease in yield and an increase in iodide ions in the waste brine. This will further reduce the amount of available oxidizing agent in the chlorinated water. This vicious cycle will result in a decrease in yield over time.

[0009] It would be best to keep the iodide ion concentration in the brine after adding the waste brine constant and control the amount of oxidizing agent added accordingly, but because the iodide ion concentration in the crystallization wastewater fluctuates greatly, it is difficult to keep the iodide ion concentration in the brine after adding the waste brine constant.

[0010] For example, if crystallization wastewater for one day or several hours is collected, stirred to make it uniform, and the iodine concentration is measured before quantitatively adding it to the brine, the iodide ion concentration in the brine can be kept constant, and the amount of oxidizing agent to be added can be adjusted. However, this method is difficult because it requires large-scale equipment and requires a lot of effort for measurement and adjustment.

[0011] The same applies to the ion exchange resin method. The combustion recovery method can deal with changes in iodine concentration, but in the case of crystallization wastewater, the iodine concentration is low and there are no components that can be used as fuel, so the cost is high and it is not economically viable.

[0012] In addition, direct return of the crystallization wastewater to the crystallization process is not feasible in terms of volume, and simple concentration by reverse osmosis membrane concentration or distillation is difficult because it leads to an increase in non-volatile content (impurities) in the product in the crystallization process, as shown in Patent Document 3, for example.

[0013] Specific examples of components that lead to an increase in the nonvolatile content include sulfate ions, calcium ions, magnesium ions, and iron ions. Since these components are contained in the absorption solution, their presence is presumed to pose no problem as long as their molar ratio with iodide ions does not exceed the molar ratio contained in the absorption solution. However, they are present in the crystallization wastewater at a higher molar ratio than in the absorption solution. Therefore, the crystallization wastewater cannot be returned to the crystallization step unless it is not only concentrated but also the components that lead to an increase in the nonvolatile content are removed.

[0014] Electrodialysis can deal with fluctuations in the iodide ion concentration in the crystallization wastewater.

[0015] In the case of electrodialysis, sulfate ions, calcium ions, iron ions, etc. can be reduced by using special exchange membranes such as monovalent anion selectively permeable membranes and monovalent cation selectively permeable membranes.

[0016] However, sulfate ions cannot be removed sufficiently by using only a monovalent anion permselective membrane.

[0017] In electrodialysis, when using a commonly used electrodialysis cell with alternating anion-exchange membranes and cation-exchange membranes, the migration rate of iodide ions from the deionization compartment decreases over time. The migration rate of iodide ions from the deionization compartment to the concentration compartment is the sum of the rate of electrical migration toward the positive electrode through the anion-exchange membrane (concentration compartment ← deionization compartment) and the rate of diffusion through the anion-exchange membrane driven by the concentration difference (concentration compartment → deionization compartment). Furthermore, in two-compartment electrodialysis, the deionization compartment is located on the positive electrode side of the concentration compartment via a cation-exchange membrane, so the rate of electrical migration from the concentration compartment to the deionization compartment through the cation-exchange membrane (demineralization compartment ← concentration compartment) is added.

[0018] In particular, towards the end of the dialysis run, the difference in concentration between the deionization compartment and the concentration compartment increases, and as a result, the overall (apparent) migration rate decreases due to increased diffusion from the concentration compartment to the deionization compartment and increased migration through the cation exchange membrane. This means that a certain amount of iodide ions remains in the deionization compartment solution, making it impossible to achieve a high yield. In other words, the yield decreases. Either the yield or the concentration ratio must be sacrificed.

[0019] Among these, ion exchange membranes that more strongly suppress the migration of anions through cation exchange membranes, which is a negative factor that reduces the migration rate of iodide ions, are being studied on a daily basis, and improvements are being made, as reported in Non-Patent Document 1 and elsewhere. However, no fully satisfactory membrane has been obtained. Furthermore, reverse migration due to diffusion is not caused by the properties of the membrane, so it is not expected to be solved by developing a new membrane.

[0020] As described above, the use of electrodialysis for separating and concentrating iodide ions from crystallization wastewater is considered to be rational, but there are still problems that need to be solved. [Prior art documents] [Patent documents]

[0021] [Patent Document 1] Japanese Patent Publication No. 48-018187 [Patent Document 2] Japanese Patent Application Publication No. 02-208201 [Patent Document 3] Japanese Patent Application Laid-Open No. 2009-173462 [Non-patent literature]

[0022] [Non-Patent Document 1] Journal of the Society of Seawater Science, Vol. 66, No. 5 (2012) 248-256 "Research and Development of Next-Generation Ion Exchange Membranes for Salt Production": Naoto Yoshikawa Summary of the Invention [Problem to be solved by the invention]

[0023] An object of the present invention is to provide an electrodialysis method capable of efficiently separating and obtaining a component to be obtained from a raw material solution that is an aqueous solution containing both the component to be obtained and the component to be removed. [Means for solving the problem]

[0024] Such an object can be achieved by the present invention described below. The electrodialysis method of the present invention is an electrodialysis method for separating and obtaining a component to be obtained by electrodialysis from a raw material solution that is an aqueous solution containing both the component to be obtained and the component to be removed, Between the positive electrode and the negative electrode, from the positive electrode side to the positive electrode chamber, An electrodialysis cell is used in which multiple sets of four membranes and four compartments are arranged in the following order: a first cation exchange membrane which is a cation exchange membrane, a first intermediate compartment, a second cation exchange membrane which is a cation exchange membrane, a concentration compartment, a first anion exchange membrane which is an anion exchange membrane, a second intermediate compartment, a second anion exchange membrane which is an anion exchange membrane, and a deionization compartment, followed by a cation exchange membrane and a negative electrode compartment; The raw material liquid is passed through the first intermediate compartment in a single pass or by circulation, the concentrated liquid is passed through the concentration compartment in a single pass or by circulation, the liquid that has been passed through the first intermediate compartment in a single pass or by circulation is passed through the second intermediate compartment in a single pass or by circulation, and the liquid that has been passed through the second intermediate compartment in a single pass or by circulation is passed through the deionization compartment in a single pass or by circulation.

[0025] In other words, the present invention is a method in which an aqueous solution containing both the component to be obtained and the component to be removed is used as a raw material solution, and the component to be removed is left in the raw material solution by electrodialysis, while the component to be obtained is separated and recovered in a concentrated solution. In particular, an electrodialysis cell is used in which, between the positive electrode and the negative electrode, multiple sets of four membranes and four compartments are arranged in the following order from the positive electrode side: a first cation exchange membrane which is a cation exchange membrane, a first intermediate compartment, a second cation exchange membrane which is a cation exchange membrane, a concentration compartment, a first anion exchange membrane which is an anion exchange membrane, a second intermediate compartment, a second anion exchange membrane which is an anion exchange membrane, and a desalination compartment, followed by a cation exchange membrane and a negative electrode compartment, and the raw material liquid is passed through the first intermediate compartment in a single pass or in a circulation manner, the concentrated liquid is passed through the concentration compartment in a circulation manner, the liquid that has passed through the first intermediate compartment in a single pass or in a circulation manner is passed through the second intermediate compartment in a single pass or in a circulation manner, and the liquid that has passed through the second intermediate compartment in a single pass or in a circulation manner is passed through the desalination compartment in a single pass or in a circulation manner, thereby efficiently separating and recovering the components to be obtained from the raw material liquid in a concentrated liquid.

[0026] Another aspect of the present invention is an electrodialysis method for separating and obtaining a component to be obtained from a raw material solution that is an aqueous solution containing both the component to be obtained and the component to be removed by electrodialysis, comprising the steps of: Between the positive electrode and the negative electrode, from the positive electrode side to the positive electrode chamber, An electrodialysis cell is used in which multiple sets of three membranes and three compartments are arranged in the following order: a first cation exchange membrane, which is a cation exchange membrane, an intermediate compartment, a second cation exchange membrane, which is a cation exchange membrane, a concentration compartment, an anion exchange membrane, and a deionization compartment, followed by a cation exchange membrane and a negative electrode compartment. This method involves passing the raw material liquid through the intermediate compartment in a single pass or in a circulating manner, passing the concentrated liquid through the concentration compartment in a circulating manner, and passing the liquid that has passed through the intermediate compartment in a single pass or in a circulating manner through the deionization compartment in a single pass or in a circulating manner.

[0027] Another aspect of the present invention is an electrodialysis method for separating and obtaining a component to be obtained from a raw material solution that is an aqueous solution containing both the component to be obtained and the component to be removed by electrodialysis, comprising the steps of: Between the positive electrode and the negative electrode, from the positive electrode side, a positive electrode chamber, a cation exchange membrane, and a deionization chamber are successively arranged, An electrodialysis cell is used in which multiple sets of three membranes and three compartments are arranged in the following order: a cation exchange membrane, a concentration compartment, a first anion exchange membrane which is an anion exchange membrane, an intermediate compartment, a second anion exchange membrane which is an anion exchange membrane, and a demineralization compartment, followed by a cation exchange membrane and a negative electrode compartment. This method involves passing the raw material liquid through the intermediate compartment in a single pass or in a circulating manner, passing the concentrated liquid through the concentration compartment in a circulating manner, and passing the liquid that has passed through the intermediate compartment in a single pass or in a circulating manner through the deionization compartment in a single pass or in a circulating manner.

[0028] In the electrodialysis method of the present invention, a part or all of the anion exchange membrane is a monovalent anion selective permeable membrane, It is preferable that a part or all of the cation exchange membrane is a monovalent cation permselective membrane.

[0029] In the electrodialysis method of the present invention, the raw material solution is preferably adjusted to a pH of 4.0 or higher.

[0030] In the electrodialysis method of the present invention, the component to be obtained is preferably iodide ion.

[0031] In the electrodialysis method of the present invention, it is preferable that the iodide ion concentration in the raw material solution is 1,500 mg / L or less and the chloride ion concentration is 3,000 mg / L or more.

[0032] In the electrodialysis method of the present invention, the raw material liquid is preferably wastewater from an iodine purification step. [Effects of the Invention]

[0033] According to the present invention, it is possible to provide an electrodialysis method capable of efficiently separating and recovering a component to be obtained from a raw material solution that is an aqueous solution containing both the component to be obtained and the component to be removed. In particular, according to the present invention, it is possible to obtain an iodine concentrated solution in which substances that cause an increase in non-volatile content in the iodine purification step have been reduced, and the iodine concentrated solution can be easily returned to the iodine purification step. [Brief explanation of the drawings]

[0034] [Figure 1] FIG. 1 is a schematic diagram showing an electrodialysis cell used in the electrodialysis method according to the first embodiment of the present invention. [Figure 2] FIG. 2 is a diagram showing the overall flow of an example of the electrodialysis method of the present invention. [Figure 3] FIG. 3 is a graph showing the relationship between the pH of the raw material solution and the abundance of HSO4 −. [Figure 4] FIG. 4 is a schematic diagram showing an electrodialysis cell used in the electrodialysis method according to the second embodiment of the present invention. [Figure 5] FIG. 5 is a schematic diagram showing an electrodialysis cell used in the electrodialysis method according to the third embodiment of the present invention. [Figure 6] FIG. 6 is a graph showing the change in ion concentration over time in the concentrating compartment in Example 1. [Figure 7] FIG. 7 is a graph showing the change over time in the residual rate (%) of ions in the deionization compartments in Example 1. [Figure 8] FIG. 8 is a graph showing the change over time in the remaining rate (%) of ions in the second intermediate chamber in Example 1. [Figure 9] FIG. 9 is a graph showing the change in ion concentration over time in the concentrating compartment in Example 2. [Figure 10] FIG. 10 is a graph showing the change over time in the remaining rate (%) of ions in the deionization compartments in Example 2. [Figure 11] FIG. 11 is a graph showing the change over time in the remaining rate (%) of ions in the second intermediate chamber in Example 2. [Figure 12] FIG. 12 is a graph showing the change in ion concentration over time in the concentrating compartment in Example 3. [Figure 13] FIG. 13 is a graph showing the change over time in the remaining rate (%) of ions in the deionization compartments in Example 3. [Figure 14]FIG. 14 is a graph showing the change over time in the remaining rate (%) of ions in the second intermediate chamber in Example 3. [Figure 15] FIG. 15 is a graph showing the change in ion concentration over time in the concentrating compartments in Comparative Example 1. [Figure 16] FIG. 16 is a graph showing the change over time in the remaining rate (%) of ions in the deionization compartments in Comparative Example 1. [Figure 17] FIG. 17 is a graph showing the change in ion concentration over time in the concentrating compartments in Comparative Example 2. [Figure 18] FIG. 18 is a graph showing the change over time in the remaining rate (%) of ions in the dilution compartments in Comparative Example 2. [Figure 19] FIG. 19 is a graph showing the iodine recovery status in the blowing out method. DETAILED DESCRIPTION OF THE INVENTION

[0035] Preferred embodiments of the present invention will be described in detail below. [1] First embodiment First, the electrodialysis method according to the first embodiment of the present invention will be described. FIG. 1 is a schematic diagram showing an electrodialysis tank used in the electrodialysis method according to the first embodiment of the present invention. FIG. 2 is a diagram showing the overall flow of an example of the electrodialysis method according to the present invention. FIG. 3 is a diagram showing the relationship between the pH of the raw material solution and the concentration of HSO4 - 10 is a graph showing the relationship between the presence rate and

[0036] The electrodialysis method according to this embodiment is an electrodialysis method for separating and obtaining, by electrodialysis, a component to be obtained from a raw material solution 21, which is an aqueous solution containing both the component to be obtained and the component to be removed. The method uses an electrodialysis cell 1 in which, between a positive electrode 2 and a negative electrode 3, multiple sets of four membranes and four chambers are arranged in the following order from the positive electrode side: a first cation exchange membrane 5c which is a cation exchange membrane, a first intermediate chamber 12, a second cation exchange membrane 6c which is a cation exchange membrane, a concentration chamber 13, a first anion exchange membrane 7a which is an anion exchange membrane, a second intermediate chamber 14, a second anion exchange membrane 8a which is an anion exchange membrane, and a deionization chamber 15, followed by a cation exchange membrane 9c and an anode chamber 16.

[0037] In the electrodialysis cell 1 having the configuration shown in FIG. 1, a cation exchange membrane 4c and a deionization chamber 11 are further disposed between the positive electrode chamber 10 and the first cation exchange membrane 5c from the positive electrode side.

[0038] In the electrodialysis method of this embodiment, raw material liquid 21 is passed through first intermediate chamber 12 in a single pass or by circulation, concentrated liquid 23 is passed through concentration chamber 13 in a single pass or by circulation, treated liquid 22, which is a liquid that has been passed through first intermediate chamber 12 in a single pass or by circulation, is passed through second intermediate chamber 14 in a single pass or by circulation, and treated liquid 24, which is a liquid that has been passed through second intermediate chamber 14 in a single pass or by circulation, is passed through deionization chambers 11 and 15 in a single pass or by circulation.

[0039] This provides an electrodialysis method that can efficiently separate and recover the component to be obtained from a raw material solution that is an aqueous solution containing both the component to be obtained and the component to be removed. In particular, this embodiment can achieve a particularly superior yield of the component to be obtained compared to the second and third embodiments described in detail below.

[0040] The component to be obtained is not particularly limited, and examples thereof include iodide ions, bromide ions, phosphate ions, nitrate ions, nitrite ions, arsenite ions, and chromate ions. Among these, the component to be obtained is preferably iodide ions. In other words, the electrodialysis method of the present invention is preferably a method for separating iodide ions. The following description will focus on the case where the component to be obtained is iodide ions.

[0041] In this embodiment, the electrodialysis cell 1 used has a first intermediate chamber (iodide ion capture chamber, which is an ion capture chamber that must be captured) 12 provided on the positive electrode side of the concentration chamber 13 via a second cation exchange membrane 6c, and by passing the raw material liquid 21 through the first intermediate chamber 12, iodide ions that have leaked from the concentration chamber 13 via the second cation exchange membrane 6c can be recovered in the raw material liquid 21.

[0042] In addition, in this embodiment, a second intermediate chamber (pre-demineralization chamber) 14, separated on both sides by a first anion exchange membrane 7a and a second anion exchange membrane 8a, is provided between the negative electrode side of the concentration chamber 13 and the demineralization chamber 15, and the treatment liquid 22 that has passed through the first intermediate chamber 12 is passed through in a single pass or in circulation.

[0043] In the second intermediate chamber 14, iodide ions are supplied from the deionization chamber 15, so the iodide ion concentration does not fluctuate significantly. A significant concentration of iodide ions remains even at the end of dialysis (near the outlet of the dialysis tank in single-pass operation). This makes the chamber less susceptible to reverse migration of iodide ions, i.e., diffusion driven by concentration differences. In other words, by leaving a significant amount of iodide ions in the second intermediate chamber 14 until the end of dialysis, the time required for dialysis can be shortened and a high concentration ratio can be achieved. Furthermore, the iodide ions remaining in the second intermediate chamber 14 at the end of dialysis are separated and recovered in the deionization chamber 15 in the next batch, so they are not lost.

[0044] Furthermore, since the iodide ion concentration in the second intermediate compartment 14 is significantly lower than that in the concentrating compartment 13, the migration of iodide ions from the deionizing compartment 15 to the second intermediate compartment 14 is less susceptible to reverse migration due to the concentration difference.

[0045] As described above, according to the method of the present embodiment, by providing the first intermediate chamber 12 and the second intermediate chamber 14 in the electrodialysis cell 1, iodide ions can be efficiently separated and recovered from the raw material solution 21 into the concentrated solution 23.

[0046] In this specification, "efficient" means that the iodine recovery rate is high and the iodine concentration rate is high.

[0047] In particular, as will be described later, even when an aqueous solution having an iodine ion concentration of 1,500 mg / L or less, such as wastewater (crystallization wastewater) 121 from an iodine purification process, which has a lower iodine ion concentration than other iodine-containing industrial wastewaters and a greater fluctuation in iodide ion concentration, is used as the raw material liquid 21, iodide ions can be recovered with a high yield.

[0048] Furthermore, according to this embodiment, it is possible to produce a concentrated solution 23 that does not adversely affect the crystallization process, and it is possible to return the concentrated solution 23 to the crystallization process.

[0049] In this specification, "adversely affect" means that the quality of the product iodine-115 is deteriorated in the crystallization process.

[0050] Examples of components that have adverse effects include polyvalent metal ions such as iron ions and calcium ions, and sulfate ions contained in the crystallization wastewater 121.

[0051] [1-1] About the electrodialysis cell The electrodialysis device 104 includes, for example, an electrodialysis cell 1 having the configuration shown in FIG.

[0052] A pair of electrodes is arranged on both sides of the electrodialysis cell 1, one of which is a positive electrode 2 (anode) and the other is a negative electrode 3 (cathode). Examples of materials that can be used to form the positive electrode 2 include platinum (Pt), titanium (Ti) / platinum, carbon (C), nickel (Ni), ruthenium (Ru) / titanium, and iridium (Ir) / titanium.

[0053] Examples of materials that can be used to form the negative electrode 3 include iron (Fe), nickel, platinum, titanium / platinum, carbon, and chromium (Cr) steel as stainless steel.

[0054] The positive electrode 2 and the negative electrode 3 are formed in the shape of, for example, a thin rectangular flat plate, such as a sheet, a mesh, or a lattice.

[0055] The electrodialysis cell 1 is provided with a chamber frame serving as a frame body having a rectangular shape in plan view and a notched portion (not shown).

[0056] A positive electrode 2 and a negative electrode 3 are attached to the inside of both ends along the longitudinal direction of this chamber frame, and a positive electrode chamber (positive electrode chamber) 10 is formed by a cation exchange membrane 4c arranged on the negative electrode side of the positive electrode 2 of this electrodialysis cell 1, and a negative electrode chamber (negative electrode chamber) 16 is formed by a cation exchange membrane 9c arranged on the positive electrode side of the negative electrode 3.

[0057] Between the positive electrode chamber 10 and the negative electrode chamber 16, a cation exchange membrane 4c, a first cation exchange membrane 5c, a second cation exchange membrane 6c, a first anion exchange membrane 7a, a second anion exchange membrane 8a, and a cation exchange membrane 9c are sequentially arranged from the positive electrode side to the negative electrode side. These ion exchange membranes partition the positive electrode chamber 10 toward the negative electrode chamber 16 into a deionization compartment 11, a first intermediate compartment 12, a concentration compartment 13, a second intermediate compartment 14, and a deionization compartment 15.

[0058] In other words, the compartment frame separated by the cation exchange membrane 4c and the first cation exchange membrane 5c forms a deionization compartment 11. The compartment frame separated by the first cation exchange membrane 5c and the second cation exchange membrane 6c forms a first intermediate compartment 12. The compartment frame separated by the second cation exchange membrane 6c and the first anion exchange membrane 7a forms a concentration compartment 13. The compartment frame separated by the first anion exchange membrane 7a and the second anion exchange membrane 8a forms a second intermediate compartment 14. And the compartment frame separated by the second anion exchange membrane 8a and the cation exchange membrane 9c forms a deionization compartment 15.

[0059] The four compartments and four membranes from the first cation exchange membrane 5c to the deionization compartment 15, in other words, the first cation exchange membrane 5c, the first intermediate compartment 12, the second cation exchange membrane 6c, the concentration compartment 13, the first anion exchange membrane 7a, the second intermediate compartment 14, the second anion exchange membrane 8a, and the deionization compartment 15, are arranged repeatedly in a number ranging from several to several hundred sets (n sets). The number of repetitions, n, is not particularly limited, but is preferably 4 sets or more and 1800 sets or less, for example.

[0060] To provide tension to each ion exchange membrane, both ends are fastened and fixed to both sides of the chamber frame in a stretched and tensed state along both the vertical direction (up and down in Figure 1) and the horizontal direction (depth direction in Figure 1).

[0061] In the following description, the cation exchange membrane 4c, the first cation exchange membrane 5c, the second cation exchange membrane 6c, and the cation exchange membrane 9c may be collectively referred to as "cation exchange membranes."

[0062] In the following description, the first anion exchange membrane 7a and the second anion exchange membrane 8a may be collectively referred to as "anion exchange membrane."

[0063] In the following description, the positive electrode chamber 10, the dilution chamber 11, the first intermediate chamber 12, the concentration chamber 13, the second intermediate chamber 14, the dilution chamber 15, and the negative electrode chamber 16 may be collectively referred to as the "liquid chamber."

[0064] The inner surface of the chamber frame of each liquid chamber separated by each ion exchange membrane is provided with a liquid supply port and a liquid discharge port (not shown) that communicate with the interior of the chamber frame. Also, a spacer (not shown) is provided within the chamber frame to distribute the flow and make the thickness of the chamber frame uniform.

[0065] (anion exchange membrane) Examples of the first anion exchange membrane 7a and the second anion exchange membrane 8a include strongly basic styrene-divinylbenzene homogeneous anion exchange membranes.

[0066] It is preferable that part or all of the first anion exchange membrane 7a and the second anion exchange membrane 8a are monovalent anion permselective membranes.

[0067] When the iodide ion concentrate obtained in the present invention is returned to the raw material solution for the iodine purification step, as described in Patent Document 3, an increase in the sulfate ion concentration in the raw material solution for the iodine purification step causes an increase in the non-volatile content in the product iodine, so it is preferable to keep the sulfate ion concentration in the iodide ion concentrate below a certain level. Specifically, since the absorption solution originally contains sulfate ions at a sulfate ion:iodide ion molar ratio of about 0.5, it is preferable to keep the amount of sulfate ions in the iodide ion concentrate at a molar ratio to iodide ions of 0.5 or less. On the other hand, since the crystallization wastewater is the liquid remaining after iodide ions have been separated from the absorption solution, it has a high sulfate ion:iodide ion molar ratio of 20 or more. To obtain an iodide ion concentrated solution with a molar ratio of 0.5 or less, it is necessary to selectively separate iodide ions while leaving sulfate ions.

[0068] Therefore, it is preferable to use a monovalent anion permselective membrane having monovalent ion selectivity as the anion exchange membrane.

[0069] This makes it possible to suppress the migration of sulfate ions into the concentrated liquid 23 and improve the quality of the product iodine 115 obtained in the crystallization step. In addition, suppressing the migration of sulfate ions is expected to also speed up the migration of iodide ions.

[0070] Examples of such monovalent anion permselective membranes include Selemion ASV membrane (manufactured by AGC Corporation) and Neocepta ACS membrane (manufactured by Astom Corporation).

[0071] (cation exchange membrane) As the cation exchange membrane 4c, the first cation exchange membrane 5c, the second cation exchange membrane 6c and the cation exchange membrane 9c, for example, a strongly acidic styrene-divinylbenzene homogeneous cation exchange membrane or the like is used.

[0072] It is preferable that some or all of the cation exchange membrane 4c, the first cation exchange membrane 5c, the second cation exchange membrane 6c, and the cation exchange membrane 9c are monovalent cation permselective membranes.

[0073] When a cation exchange membrane without monovalent ion selectivity is used as the cation exchange membrane, polyvalent ions such as iron ions, calcium ions, and magnesium ions move preferentially over monovalent ions such as sodium ions into the concentrated liquid 23. If even a small amount of these components is contained in the auxiliary raw materials or service water used in producing the absorption liquid 111, the concentration of these polyvalent ions increases as the number of batches of iodine recovery from the crystallization wastewater 121 increases, which may eventually lead to an increase in nonvolatile content (impurities), etc.

[0074] Therefore, it is preferable to use a monovalent cation permselective membrane having monovalent ion selectivity as the cation exchange membrane.

[0075] This makes it possible to more effectively prevent substances that adversely affect the crystallization step, in other words, polyvalent cations that become impurities in the product iodine 115, from migrating into the concentrated liquid 23, thereby further improving the quality of the product iodine 115 obtained in the crystallization step.

[0076] Examples of such monovalent cation permselective membranes include Selemion CSO membranes (manufactured by AGC Corporation) and Neocepta CIMS membranes (manufactured by Astom Corporation).

[0077] (Demineralization room) In the electrodialysis cell 1 having the configuration shown in FIG. 1, the chamber located on the negative electrode side of the positive electrode chamber 10 is the deionization chamber 11.

[0078] This is to minimize the amount of iodide ions that migrate from the deionization chamber 11, which is a chamber located next to the positive electrode chamber 10, to the positive electrode chamber 10 via the cation exchange membrane 4c. The iodide ions that migrate to the positive electrode chamber 10 are not only lost, but may also corrode the positive electrode 2 depending on the material of the positive electrode 2 (or both electrodes, if the electrode chamber solution is circulated between the positive electrode chamber 10 and the negative electrode chamber 16). Alternatively, the iodide ions may be oxidized at the positive electrode 2 to form I2, which may corrode the surrounding area.

[0079] Because the migration rate of iodide ions through the cation exchange membrane 4c is proportional to the iodide ion concentration, the compartment adjacent to the positive electrode chamber 10 is preferably the compartment with the lowest iodide ion concentration. The average iodide ion concentration is lowest in the following order: deionization compartment 11 < second intermediate compartment 14 < first intermediate compartment 12 < concentrating compartment 13. The inlet concentration during single-pass operation is lowest in the following order: deionization compartment 11 < first intermediate compartment 12 < second intermediate compartment 14 < concentrating compartment 13. To minimize the migration of iodide ions to the positive electrode chamber, the deionization compartment 11, which has the lowest iodide ion concentration, is most preferably located adjacent to the positive electrode chamber 10. However, this location reduces efficiency because iodide ions cannot be obtained. This results in a loss of one compartment, particularly during single-pass operation. To reduce migration to the positive electrode chamber 10 without reducing efficiency, the first intermediate chamber 12, which allows iodide ions to be separated later and has a lower iodide ion concentration at the inlet than the second intermediate chamber 14, is most preferred, and the concentration chamber 13 is least preferred.

[0080] By using the chamber located on the negative electrode side of the positive electrode chamber 10 as the deionization chamber 11, it is possible to suitably suppress the effects of iodide ions migrating to the positive electrode chamber 10 via the cation exchange membrane 4c, as described above. Furthermore, by using the chamber located on the negative electrode side of the positive electrode chamber 10 as the deionization chamber 11, it is possible to suppress the effects of iodide ions migrating to the positive electrode chamber 10 without reducing efficiency.

[0081] (First intermediate room) The first intermediate chamber 12 is a chamber installed to recover iodide ions that migrate from the concentration chamber 13 to the positive electrode side through the second cation exchange membrane 6c into the raw material liquid 21 before they reach the deionization chamber 11.

[0082] This allows iodide ions to be efficiently separated and recovered from the raw material liquid 21 into the concentrated liquid 23.

[0083] (concentration chamber) In the concentration chamber 13, the concentrated liquid 23 is circulated, whereby iodide ions are separated from the raw material liquid 21 and concentrated in the concentrated liquid 23.

[0084] (Second intermediate room) The second intermediate chamber 14 is a chamber installed to recover iodide ions that migrate from the concentration chamber 13 to the negative electrode side through the first anion exchange membrane 7a using the concentration difference as a driving force before they reach the deionization chamber 15.

[0085] The (apparent) migration rate of iodide ions from the compartment adjacent to the concentrating compartment via an anion exchange membrane to the negative electrode of the concentrating compartment is affected by the iodide ion concentration in the concentrating compartment. Towards the end of dialysis (near the outlet in single-pass operation), the iodide ion concentration in the compartment adjacent to the concentrating compartment decreases, and as the difference in concentration between the two compartments increases, the (apparent) migration rate of iodide ions slows down and eventually stops. In other words, iodide ions remain in the compartment adjacent to the concentrating compartment in proportion to the iodide ion concentration in the concentrating compartment. Therefore, if the compartment adjacent to the concentrating compartment is used as a deionization compartment, it is not possible to expect both a high concentration ratio and a high yield.

[0086] To solve this problem, a second intermediate chamber 14 is provided on the positive electrode side of the deionization chamber 15, with both sides partitioned by a monovalent anion permselective membrane, and the liquid that has passed through the second intermediate chamber 14 is passed through the deionization chamber 15. At the end of dialysis (the outlet in one-pass operation), iodide ions remain in the second intermediate chamber 14, but they are separated and collected in the deionization chamber 15, so they are not lost. By leaving iodide ions in the second intermediate chamber 14, it is possible to increase the iodine concentration in the concentration chamber 13 (increase the concentration ratio).

[0087] The iodide ion concentration in the second intermediate chamber 14 is higher than that in the feed solution 21 at the start of dialysis (at the inlet in single-pass operation) because the first intermediate chamber 12 captures iodide ions coming from the concentrating chamber 13 through the second cation exchange membrane 6c. However, it becomes lower than that in the feed solution 21 at the end of dialysis. In either case, it is significantly lower than that in the concentrating chamber 13. Therefore, the (apparent) migration rate of iodide ions from the deionization chamber 15 is higher than when the second intermediate chamber 14 is not provided. This shortens the dialysis time (increases the feed solution processing rate in single-pass operation). Furthermore, the iodide ion concentration remaining in the deionization chamber 15 at the end of dialysis (at the outlet in single-pass operation) can be reduced. This ensures a high yield. Furthermore, because the liquid that has passed through the second intermediate chamber 14 has a lower iodide ion concentration than the feed solution, separation in the deionization chamber 15 is easier, which also leads to an improvement in yield.

[0088] Furthermore, when the concentration of monovalent anions (iodide ions and chloride ions) in the deionization compartment decreases toward the end of the dialysis operation, the rate at which sulfate ions, which are divalent ions, move through the monovalent anion permselective membrane to the positive electrode increases. Therefore, the use of the monovalent anion permselective membrane alone is not sufficient to prevent sulfate ions from moving to the concentration compartment 13.

[0089] A second intermediate chamber 14, partitioned on both sides by a monovalent anion permselective membrane, is provided on the positive electrode side of the deionization chamber 15. When the treated liquid 22 that has passed through the first intermediate chamber 12 is passed through the second intermediate chamber 14, approximately the same amount of monovalent anions enters the second intermediate chamber 14 from the deionization chamber 15 as exits the concentration chamber 13, maintaining a high monovalent anion concentration until the end of dialysis. Therefore, even if a large amount of sulfate ions migrate from the deionization chamber 15 toward the concentration chamber 13 toward the end of dialysis, the migration rate of sulfate ions from the second intermediate chamber 14 to the concentration chamber 13 does not increase. In other words, the sulfate ions that migrate in large amounts from the deionization chamber 15 toward the concentration chamber 13 toward the end of dialysis are captured by the second intermediate chamber 14.

[0090] This effectively prevents substances that adversely affect the crystallization step, in other words, sulfate ions that cause an increase in the nonvolatile content in the product iodine 115, from migrating into the concentrated liquid 23, thereby improving the quality of the product iodine 115 obtained in the crystallization step.

[0091] (Demineralization room) In the electrodialysis cell 1 having the configuration shown in FIG. 1, the chamber located on the positive electrode side of the negative electrode chamber 16 is the deionization chamber 15 .

[0092] This is to make the chamber next to the anode chamber 16 a chamber that is least affected by sulfate ions migrating from the anode chamber 16 through the cation exchange membrane 9c. The chamber that is most affected by the migration of sulfate ions from the anode chamber 16 is the concentration chamber 13, followed by the first intermediate chamber 12 and the second intermediate chamber 14. The migration of sulfate ions to the deionization chamber 15 has almost no effect.

[0093] By using the chamber located on the positive electrode side of the negative electrode chamber 16 as the deionization chamber 15, it is possible to minimize the influence of sulfate ions that migrate from the negative electrode chamber 16 through the cation exchange membrane 9c.

[0094] The cathode chamber 10, deionization chamber 11, first intermediate chamber 12, concentration chamber 13, second intermediate chamber 14, deionization chamber 15, and anode chamber 16 are each supplied with a chamber liquid adjusted to a predetermined concentration and volume according to the purpose, and are passed through each chamber individually in a single pass or in a circulation manner.

[0095] Alternatively, an external tank (not shown) may be provided to supply these chamber liquids, and these chamber liquids may be circulated between the liquid chambers and the external tank.

[0096] [1-2] Room fluid and pretreatment (raw material liquid) The raw material liquid 21 is an aqueous solution containing iodide ions. An example of an aqueous solution containing iodide ions is an aqueous solution of sodium iodide.

[0097] The raw material liquid 21 may be, for example, wastewater (crystallization wastewater) from an iodine purification process.

[0098] The wastewater from the iodine purification process (crystallization wastewater) contains 500–1,500 mg / L of iodide ions, a higher concentration than natural gas-associated brine. Therefore, it is returned to the iodine recovery process to recover the iodide ions. However, with the blowing-out method and ion-exchange resin method, which are currently used to recover iodine from natural gas-associated brine, it is difficult to adjust the oxidant dosage and to deal with changes in the concentration of oxidizable substances, including iodide ions, in the brine. Meanwhile, the iodide ion concentration in crystallization wastewater is not constant and fluctuates greatly. Mixing crystallization wastewater with brine naturally leads to large fluctuations in the concentration of oxidizable substances, including iodide ions, in the brine. These two methods have difficulty keeping up with these fluctuations, and thus do not promise high yields. In reality, they simply disregard yield and simply dilute the highly acidic wastewater with brine and discard it.

[0099] However, the electrodialysis method of the present invention is a method in which fluctuations in iodide ion concentration do not affect the yield, and therefore the present invention is suitable as a method for separating and recovering iodide from crystallization wastewater.

[0100] In addition, it is not possible to return the crystallization wastewater to the iodine purification step after concentrating it by distillation or reverse osmosis, because this would return a large amount of sulfate ions, etc., which would increase the nonvolatile content in the iodine product. The present invention is also suitable for obtaining an iodide ion concentrate from which sulfate ions, etc. have been removed.

[0101] In the following description, a case where wastewater (crystallization wastewater) 121 from the iodine purification step is used as the aqueous solution containing iodide ions, which is the raw material liquid 21, will be mainly described.

[0102] FIG. 2 is a diagram showing the overall flow of an example of an electrodialysis method (a method for separating iodide ions) when wastewater from an iodine purification step is used as a raw material liquid.

[0103] First, the absorption liquid (raw material liquid for the crystallization step) 111 is transported to the iodine purification device 101, where the iodine purification step is carried out.

[0104] In the iodine purification apparatus 101, product iodine 115 is obtained from the absorption liquid 111 through an iodine purification step (crystallization step). Wastewater (crystallization wastewater) 121 remaining after the product iodine 115 is obtained in the iodine purification apparatus 101 is subjected to a predetermined pretreatment and used as the raw material liquid 21 in this embodiment. By reusing the crystallization wastewater 121, valuable resources can be used effectively.

[0105] The crystallization wastewater 121 may be obtained from any manufacturing process as long as it contains iodide ions produced in a process using at least one of iodine and an iodine compound, and the concentration of iodide ions and the types and contents of other impurities are not particularly limited.

[0106] The crystallization wastewater 121 contains, for example, sulfate ions, chloride ions, iron ions, sodium ions, calcium ions, and the like, in addition to iodide ions.

[0107] As the pretreatment, first, the crystallization wastewater 121 from the iodine purification device 101 is conveyed to the oxidation-reduction potential adjusting tank 102, where the oxidation-reduction potential (ORP) is adjusted.

[0108] In the oxidation-reduction potential adjusting tank 102, the oxidation-reduction potential of the crystallization wastewater 121 is monitored to set an upper limit, and a reducing agent 112 is added to adjust the oxidation-reduction potential so that it is below the upper limit.

[0109] The reducing agent 112 is not particularly limited as long as it can reduce iodine in an acidic solution. Examples of such reducing agents 112 include sulfur dioxide gas, sodium bisulfite solution, and ascorbic acid solution. From an economical viewpoint, sodium bisulfite solution is preferred.

[0110] It is also preferable to monitor the conductivity of the crystallization wastewater 121, set a lower limit, and use the liquid crystal material 21 if the conductivity exceeds the lower limit. This allows for more efficient separation of iodide ions by electrodialysis.

[0111] The crystallization wastewater (pretreated liquid) 122, the oxidation-reduction potential of which has been adjusted in the oxidation-reduction potential adjusting tank 102, is transported to the pH adjusting tank 103, where the pH is adjusted.

[0112] In the pH adjusting tank 103, an alkaline component 113 is added to the pretreatment liquid 122 to adjust the pH of the pretreatment liquid 122 to a predetermined value.

[0113] The alkaline component 113 may be, for example, sodium hydroxide, potassium hydroxide, sodium carbonate, or the like.

[0114] Here, the raw material liquid 21 is preferably adjusted to a pH of 3.5 or higher, and more preferably adjusted to a pH of 4.0 or higher.

[0115] When crystallization wastewater 121 is used as the raw material liquid 21, the crystallization wastewater 121 is strongly acidic with a pH of about 1, and most of the sulfuric acid present therein is converted into hydrogen sulfate ions (SO3H - ), it is difficult for the monovalent anion permselective membrane to restrict the movement of hydrogen sulfate ions, and they cannot be sufficiently separated from iodide ions in the raw material solution 21.

[0116] In the 5th edition of the Dictionary of Physics and Chemistry (edited by Saburo Nagakura, Iwanami Shoten, 1998), the entry for sulfuric acid states that H2SO4 ⇔ H + +HSO4 - is complete and HSO4 - ⇔ H + +SO4 2- The pK2 is 1.99 (25℃).

[0117] From this, pH and HSO4 can be calculated. - The relationship between the abundance rate and the

[0118]

number

[0119] From the above formula, pH and HSO4 - Figure 3 shows a graph of the relationship between the abundance rate and As can be seen from FIG. 3, by adjusting the pH of the raw material solution 21 to 3.5 or more, preferably 4.0 or more, HSO4 - In other words, by adjusting the pH of the raw material solution 21 to a predetermined value or higher, most of the sulfuric acid in the raw material solution 21 is converted into sulfate ions (SO4), which are divalent anions, rather than hydrogen sulfate ions, which are monovalent anions. 2- This allows the monovalent anion permselective membrane to leave sulfuric acid in the raw material solution 21 and more suitably separate iodide ions.

[0120] The pretreatment liquid 123, the pH of which has been adjusted in the pH adjusting tank 103, is transported to an electrodialysis device 104 equipped with an electrodialysis tank 1. In this embodiment, for example, the pretreatment liquid 123 that has been subjected to the series of pretreatments is used as a raw material liquid 21, and iodide ions are separated by electrodialysis using the electrodialysis device 104.

[0121] The raw material solution 21 preferably has an iodide ion concentration of 1,500 mg / L or less and a chloride ion concentration of 3,000 mg / L or more.

[0122] By setting the iodide ion concentration and chloride ion concentration in the raw material liquid 21 within the above ranges, efficient treatment can be achieved by one-pass operation, and sulfate ions can be removed more efficiently.

[0123] The iodide ion concentration in the raw material solution 21 is more preferably 2,000 mg / L or less, and even more preferably 1,500 mg / L or less. This makes it possible to make the above-mentioned effects more pronounced.

[0124] The chloride ion concentration in raw material solution 21 is more preferably 50,000 mg / L or less, and even more preferably 30,000 mg / L or less, and is preferably 3,000 mg / L or more. This makes it possible to make the above-mentioned effects more pronounced.

[0125] (Concentrate) The concentrated liquid 23 is prepared from an aqueous solution having a pH of 4.0 or higher that is conductive and does not adversely affect the crystallization step, such as a concentrated liquid from a previous batch, saline, an aqueous sodium iodide solution, or a neutralized and diluted raw liquid from the iodine purification step.

[0126] (polar chamber liquid) The cathode liquid 31 and the anode liquid 32, which are the electrode chamber liquids, are, for example, an aqueous sodium sulfate solution.

[0127] [1-3] Separation of iodide ions by electrodialysis In the method for separating iodide ions of this embodiment, first, a cathode solution 31 is supplied to the cathode chamber 10 of the electrodialysis cell 1, and an anode solution 32 is supplied to the anode chamber 16. Furthermore, a raw material solution 21 is supplied to the first intermediate chamber 12 in a single pass or by circulation, and a concentrated solution 23 is supplied to the concentrating chamber 13. Furthermore, a treated solution 22 that has passed through the first intermediate chamber 12 is supplied to the second intermediate chamber 14 in a single pass or by circulation, and a treated solution 24 that has passed through the second intermediate chamber 14 is supplied to the deionization chambers 11 and 15 in a single pass or by circulation.

[0128] The electrode chamber liquid may be supplied in a circulating manner between the positive electrode chamber 10 and the negative electrode chamber 16. During operation of the electrodialysis cell 1, the temperature in each liquid chamber is kept below the heat-resistant temperature of the ion exchange membrane used.

[0129] Then, a current is supplied between the positive electrode 2 (anode) and the negative electrode 3 (cathode) at a voltage equal to or lower than the maximum operating voltage specified by the membrane manufacturer.

[0130] The voltage of the current supplied between the positive electrode 2 and the negative electrode 3 is not particularly limited, but is determined individually taking into consideration, for example, the composition of the raw material solution 21 (e.g., the crystallization wastewater 121) and the target composition, as well as the size (flow path length) of the electrodialysis cell 1 used, the chamber thickness, the flow rate, etc., and is set to a technically and economically advantageous voltage. By setting the voltage to a preferred value, the migration rate of iodide ions, which have high selectivity in the anion exchange membrane, can be further increased. The electrodialysis cell 1 may be operated at a constant current or at a constant voltage.

[0131] Constant current operation is suitable for research and testing because the voltage applied to each membrane remains roughly constant even if the number of membranes in the chamber is increased or decreased. On the other hand, constant voltage operation reduces the current when the ion concentration in the feed solution decreases and increases the current when the concentration increases, making it convenient when using crystallization wastewater, which has drastic changes in concentration, as the feed solution.

[0132] When a current is supplied between the positive electrode 2 and the negative electrode 3, anions contained in each chamber liquid are electrically attracted to the positive electrode side, and cations are electrically attracted to the negative electrode side.

[0133] In this case, anions can migrate through the anion exchange membrane but cannot pass through the cation exchange membrane, and similarly, cations can migrate through the cation exchange membrane but cannot pass through the anion exchange membrane.

[0134] In Fig. 1, solid arrows attached to anions indicate the movement of each anion through the anion exchange membrane, and dotted arrows attached to anions indicate the non-independent movement of each anion. Solid arrows attached to cations indicate the movement of each cation through the cation exchange membrane. The same applies to Figs. 4 and 5, which are also shown in the reverse direction.

[0135] Therefore, the sodium ions (Na + ) passes through the second cation exchange membrane 6c to the negative electrode side and moves to the concentration chamber 13, but cannot move further from the concentration chamber 13 through the first anion exchange membrane 7a to the second intermediate chamber 14, and therefore remains in the concentrated liquid 23 in the concentration chamber 13.

[0136] On the other hand, the iodide ions (I - ), sulfate ions (SO4 2- ) and chloride ions (Cl - ) and other anions cannot pass through the second cation exchange membrane 6c and remain in the raw material solution 21 in the first intermediate chamber 12.

[0137] The treatment liquid 22 that has passed through the first intermediate chamber 12 is supplied to the second intermediate chamber 14, and passes through the second intermediate chamber 14 in one pass or in circulation.

[0138] In the second intermediate chamber 14, anions such as iodide ions and chloride ions in the treatment liquid 22 permeate the first anion exchange membrane 7a and move to the concentration chamber 13, but cannot move further from the concentration chamber 13 through the second cation exchange membrane 6c to the first intermediate chamber 12, and therefore remain in the concentrate 23 in the concentration chamber 13. At this time, since the first anion exchange membrane 7a is a monovalent anion-selective permeable membrane, sulfate ions cannot permeate the first anion exchange membrane 7a and remain in the treatment liquid 22.

[0139] Moreover, sodium ions remain in the treatment liquid 22 without being able to permeate the first anion exchange membrane 7a.

[0140] The treated liquid 24 that has passed through the second intermediate chamber 14 is supplied to the dilution chamber 11 and the dilution chamber 15, respectively, and is passed through the dilution chamber 11 and the dilution chamber 15 in one pass or in circulation.

[0141] In the deionization compartment 11, sodium ions in the treatment liquid 24 permeate through the first cation exchange membrane 5c and move to the first intermediate compartment 12.

[0142] In the deionization compartment 15, anions such as iodide ions, chloride ions, and sulfate ions in the treatment liquid 24 permeate the second anion exchange membrane 8a and move to the second intermediate compartment .

[0143] Here, in the second intermediate chamber 14, as long as the iodide ion concentration of the treatment liquid 24 in the deionization chamber 15 is sufficient, the iodide ions that migrated from the second intermediate chamber 14 to the concentration chamber 13 migrate back from the deionization chamber 15, so the iodide ion concentration does not fluctuate significantly. Therefore, the second intermediate chamber 14 is less susceptible to the reverse migration of iodide ions driven by the concentration difference. In addition, because the iodide ion concentration in the second intermediate chamber 14 is lower than that in the concentration chamber 13, the migration of iodide ions from the deionization chamber 15 to the positive electrode side is also less susceptible to the reverse migration caused by the concentration difference.

[0144] Towards the end of dialysis, when the iodide ions and chloride ions in deionizing compartment 15 are almost gone, a large amount of sulfate ions in deionizing compartment 15 migrate to second intermediate compartment 14. However, even at the end of dialysis, sufficient amounts of chloride ions and iodide ions are still present in second intermediate compartment 14, so most of the sulfate ions do not migrate to concentrating compartment 13 but remain in second intermediate compartment 14. Furthermore, after passing through second intermediate compartment 14, the iodide ion concentration is lower than that of the original raw material solution 21, which has the advantage of improving the iodide ion recovery rate.

[0145] The treated liquid 25 that has passed through the deionization compartments 11 and 15 is discharged as dialysis wastewater 116 in FIG.

[0146] A concentrated liquid 23 is circulated and supplied to the concentration compartment 13 . In the concentration compartment 13, sodium ions in the concentrate 23 cannot permeate the first anion exchange membrane 7a and remain in the concentrate 23. Furthermore, anions such as iodide ions, sulfate ions, and chloride ions cannot permeate the second cation exchange membrane 6c and remain in the concentrate 23.

[0147] On the other hand, as described above, sodium ions in the first intermediate chamber 12 pass through the second cation exchange membrane 6c to the negative electrode side and move to the concentrating chamber 13. In addition, anions such as iodide ions and chloride ions in the second intermediate chamber 14 permeate the first anion exchange membrane 7a and move to the concentrating chamber 13. At this time, because the first anion exchange membrane 7a is a monovalent anion-selective permeable membrane, sulfate ions in the second intermediate chamber 14 cannot permeate the first anion exchange membrane 7a.

[0148] As a result, the concentrations of sodium ions, iodide ions, and chloride ions in the concentrated solution 23 in the concentrating chamber 13 increase, in other words, the ions are concentrated. Thereafter, by withdrawing the treated solution with increased ion concentrations in the concentrating chamber 13, i.e., the concentrated solution 23, from the concentrating chamber 13, iodine can be recovered at a high concentration.

[0149] In addition, the reverse movement of iodide ions is driven by the concentration difference, in other words, iodide ions leak from the concentration chamber 13 through the second cation exchange membrane 6c into the first intermediate chamber 12, but the iodide ions leaked into the first intermediate chamber 12 are recovered in the raw material liquid 21.

[0150] In this manner, in this embodiment, a first intermediate chamber 12 is provided on the positive electrode side of the concentration chamber 13 via the second cation exchange membrane 6c, and the raw material liquid 21 is passed through the first intermediate chamber 12, whereby iodide ions that have migrated back from the concentration chamber 13 to the first intermediate chamber 12 via the second cation exchange membrane 6c can be recovered in the raw material liquid 21.

[0151] The iodide ions recovered in the raw liquid 21 are then separated from the second intermediate compartment 14 into the concentrated liquid 23 in the concentrating compartment 13 by the cycle described above.

[0152] This allows iodide ions to be recovered in the concentrated solution 23 in the concentrating chamber 13 at a higher yield and a higher concentration ratio.

[0153] The volume of the concentrate 23 increases with the passage of operating time, so the increased volume is separated and returned to the crystallization step.

[0154] 2, a portion 114 of the concentrated solution 23 obtained by electrodialysis in the electrodialysis device 104 is mixed with an absorption liquid 111, or the like, and then transported to the iodine purification device 101 and reused in the crystallization step. In addition, the treated solution 25 that has passed through the deionization chambers 11 and 15 is discharged as dialysis wastewater 116.

[0155] When the concentrated liquid 23 produced by the method of this embodiment is reused by adding it to the absorption liquid 111, the molar ratio of the components that adversely affect the crystallization process to iodine must be reduced to the level of the original absorption liquid 111.

[0156] Specifically, for example, the concentration ratio of sulfate ions to iodide ions in the produced concentrated liquid 23 is preferably equal to or less than that of the absorption liquid 111, the concentration ratio of iron ions to iodide ions is preferably equal to or less than that of the absorption liquid 111, and the concentration ratio of calcium ions to iodide ions is preferably equal to or less than that of the absorption liquid 111.

[0157] Since the concentrations and concentration ratios of the above components in the absorbent 111 and the produced concentrated solution 23 vary, the ion concentration ratio is not particularly limited. For example, the molar ratio of sulfate ion to iodide ion concentration in the obtained concentrated solution 23 (SO4 2- / I - ) is preferably 0.5 or less, more preferably 0.3 or less, and even more preferably 0.2 or less.

[0158] By returning such a concentrated liquid 23 to the crystallization step, it is possible to more suitably obtain product iodine 115 of excellent quality without adversely affecting the crystallization step.

[0159] According to the method of the present embodiment as described above, it is possible to obtain a concentrated solution in which iodide ions are separated and concentrated at a high concentration while the migration of components that adversely affect the crystallization step, such as sulfate ions, is suppressed.

[0160] In this embodiment, if the size of the electrodialysis cell 1 is large and sufficient residence time can be secured within the electrodialysis cell 1, it is efficient to pass the solution through the chambers other than the concentration chamber 13, the positive electrode chamber 10, and the negative electrode chamber 16, in other words, the deionization chamber 11, the first intermediate chamber 12, the second intermediate chamber 14, and the deionization chamber 15, in a single pass.

[0161] Furthermore, in this embodiment, an example has been described in which an electrodialysis cell 1 is used in which the chamber located on the negative electrode side of the positive electrode chamber 10 is the deionization chamber 11, but the chamber located on the negative electrode side of the positive electrode chamber 10 may also be the first intermediate chamber 12. As described above, by using the chamber located on the negative electrode side of the positive electrode chamber 10 as the first intermediate chamber 12, a decrease in efficiency can be suppressed.

[0162] [2] Second embodiment Next, an electrodialysis method according to a second embodiment of the present invention (particularly, a method for separating iodide ions) will be described.

[0163] FIG. 4 is a schematic diagram showing an electrodialysis cell used in the electrodialysis method according to the second embodiment of the present invention.

[0164] Hereinafter, the electrodialysis method of the present invention according to the second embodiment will be described with reference to this figure, but the differences from the above-described embodiment will be mainly described, and a description of similar points will be omitted.

[0165] The electrodialysis method of this embodiment is an electrodialysis method for separating and obtaining, by electrodialysis, a component to be obtained from a raw material solution 21, which is an aqueous solution containing both the component to be obtained and the component to be removed. The method uses an electrodialysis cell 1' in which, between the positive electrode 2 and the negative electrode 3, multiple sets of three membranes and three chambers are arranged in the following order from the positive electrode side: a first cation exchange membrane 5c which is a cation exchange membrane, an intermediate chamber 12', a second cation exchange membrane 6c which is also a cation exchange membrane, a concentration chamber 13, an anion exchange membrane 8s', and a deionization chamber 15, followed by a cation exchange membrane 9c and an anode chamber 16.

[0166] In the electrodialysis method of this embodiment, raw material liquid 21 is passed through intermediate chamber 12' in one pass or by circulation, concentrated liquid 23 is passed through concentration chamber 13 in one pass or by circulation, and treated liquid 26, which is the liquid that has passed through intermediate chamber 12' in one pass or by circulation, is passed through deionization chamber 15 in one pass or by circulation.

[0167] The electrodialysis cell 1′ used in the electrodialysis method according to this embodiment is the electrodialysis cell 1 according to the first embodiment, in which the second intermediate chamber 14 and the first anion exchange membrane 7a on the positive electrode side of the second intermediate chamber 14 are omitted, making the set three chambers and three membranes.

[0168] In this way, even when the second intermediate chamber 14 is omitted from the electrodialysis cell 1 described in the first embodiment, the component to be obtained can be efficiently separated and recovered from the raw material solution, which is an aqueous solution containing both the component to be obtained and the component to be removed, as in the first embodiment described above.

[0169] The sodium ions (Na + ) passes through the second cation exchange membrane 6c to the negative electrode side and moves to the concentration compartment 13, but cannot move further from the concentration compartment 13 through the anion exchange membrane 8s' to the deionization compartment 15, and is therefore separated into the concentrated liquid 23 in the concentration compartment 13.

[0170] On the other hand, the iodide ions (I - ), sulfate ions (SO4 2- ) and chloride ions (Cl - ) cannot permeate the second cation exchange membrane 6c and remain in the raw material solution 21 in the intermediate chamber 12'.

[0171] The treated liquid 26 that has passed through the intermediate chamber 12' is supplied to the deionization chamber 15, and is passed through the deionization chamber 15 in a single pass or in a circulation manner.

[0172] In the deionization compartment 15, anions such as iodide ions and chloride ions in the treatment liquid 26 permeate through the anion exchange membrane 8s' and move to the concentration compartment 13. At this time, since the anion exchange membrane 8s' is a monovalent anion selective permeable membrane, sulfate ions cannot permeate through the anion exchange membrane 8s' and remain in the treatment liquid 26. The treated liquid 27 that has passed through the deionization chamber 15 is discharged as dialysis wastewater 116.

[0173] A concentrated liquid 23 is circulated and supplied to the concentration compartment 13 . In the concentration compartment 13, sodium ions in the concentrate 23 cannot permeate the anion exchange membrane 8s' and remain in the concentrate 23. In addition, anions such as iodide ions, sulfate ions, and chloride ions cannot permeate the second cation exchange membrane 6c and remain in the concentrate 23.

[0174] On the other hand, as described above, sodium ions in the intermediate chamber 12' pass through the second cation exchange membrane 6c to the negative electrode side and move to the concentration chamber 13.

[0175] Additionally, anions such as iodide ions and chloride ions in the deionization compartment 15 permeate the anion exchange membrane 8s' and move to the concentration compartment 13. At this time, since the anion exchange membrane 8s' is a monovalent anion selective permeable membrane, the sulfate ions in the deionization compartment 15 cannot permeate the anion exchange membrane 8s'.

[0176] As a result, the concentrations of sodium ions, iodide ions, and chloride ions in the concentrated solution 23 in the concentrating chamber 13 increase, in other words, the ions are concentrated. Thereafter, by withdrawing the treated solution with increased ion concentrations in the concentrating chamber 13, i.e., the concentrated solution 23, from the concentrating chamber 13, iodine can be recovered at a high concentration.

[0177] In addition, the reverse migration of iodide ions occurs due to the concentration difference, in other words, iodide ions leak from the concentration chamber 13 through the second cation exchange membrane 6c into the intermediate chamber 12', but the iodide ions leaked into the intermediate chamber 12' are recovered in the raw material liquid 21.

[0178] In this manner, in this embodiment, an intermediate chamber 12' is provided on the positive electrode side of the concentration chamber 13 via the second cation exchange membrane 6c, and the raw material liquid 21 is passed through the intermediate chamber 12', whereby iodide ions that have migrated back from the concentration chamber 13 to the intermediate chamber 12' via the second cation exchange membrane 6c can be recovered in the raw material liquid 21.

[0179] The iodide ions recovered in the raw material solution 21 are then separated from the deionization compartment 15 into the concentrate 23 in the concentration compartment 13 by the cycle described above.

[0180] This allows iodide ions to be recovered in the concentrated solution 23 in the concentrating chamber 13 at a higher yield and a higher concentration ratio.

[0181] Furthermore, the electrodialysis method according to this embodiment is particularly advantageous in terms of miniaturization of the electrodialysis tank 1' and power saving compared to the electrodialysis method according to the first embodiment described above.

[0182] In addition, in the electrodialysis cell 1 according to the first embodiment, the chamber located on the negative electrode side of the positive electrode chamber 10 was the deionization chamber 11, whereas in the electrodialysis cell 1' according to this embodiment, the chamber located on the negative electrode side of the positive electrode chamber 10 is the intermediate chamber 12'.

[0183] This makes it possible to suitably reduce the amount of iodide ions that migrate from the intermediate chamber 12', which is a chamber located adjacent to the positive electrode chamber 10, to the positive electrode chamber 10 via the first cation exchange membrane 5c, thereby preventing a decrease in efficiency.

[0184] In the configuration shown in FIG. 4, the chamber located on the negative electrode side of the positive electrode chamber is the intermediate chamber, but a deionization chamber may also be provided as a chamber located on the negative electrode side of the positive electrode chamber.

[0185] As described above, by using the chamber located on the negative electrode side of the positive electrode chamber as the deionization chamber, the effects of iodide ions migrating from the deionization chamber to the positive electrode chamber through the cation exchange membrane can be suitably suppressed.

[0186] [3] Third embodiment Next, an electrodialysis method according to a third embodiment of the present invention (particularly, a method for separating iodide ions) will be described.

[0187] FIG. 5 is a schematic diagram showing an electrodialysis cell used in the electrodialysis method according to the third embodiment of the present invention.

[0188] Hereinafter, the third embodiment of the present invention (particularly, the method for separating iodide ions) will be described with reference to this figure, but the differences from the previously described embodiments will be mainly described, and explanations of similar matters will be omitted.

[0189] The present embodiment (particularly, the method for separating iodide ions) is an electrodialysis method for separating and obtaining, by electrodialysis, a component to be obtained from a raw material solution 21, which is an aqueous solution containing both a component to be obtained and a component to be removed. The method uses an electrodialysis cell 1" in which, between the positive electrode 2 and the negative electrode 3, multiple sets of three membranes and three chambers are arranged in the following order from the positive electrode side: a positive electrode chamber 10, a cation exchange membrane 4c, a deionization chamber 11, a cation exchange membrane 6c', a concentration chamber 13, a first anion exchange membrane 7a which is an anion exchange membrane, an intermediate chamber 14', a second anion exchange membrane 8a which is an anion exchange membrane, and a deionization chamber 15, followed by a cation exchange membrane 9c and a negative electrode chamber 16.

[0190] In the electrodialysis method of this embodiment, raw material liquid 21 is passed through intermediate chamber 14' in a single pass or by circulation, concentrated liquid 23 is passed through concentration chamber 13 in a single pass or by circulation, and treated liquid 28, which is a liquid that has been passed through intermediate chamber 14' in a single pass or by circulation, is passed through deionization chambers 11 and 15 in a single pass or by circulation.

[0191] The electrodialysis cell 1″ used in the electrodialysis method according to this embodiment is the electrodialysis cell 1 according to the first embodiment, except that the first intermediate chamber 12 and the first cation exchange membrane 5c on the positive electrode side of the first intermediate chamber 12 are omitted, resulting in a set of three chambers and three membranes.

[0192] In this way, even when the first intermediate chamber 12 is omitted from the electrodialysis cell 1 described in the first embodiment, the component to be obtained can be efficiently separated and recovered from the raw material solution, which is an aqueous solution containing both the component to be obtained and the component to be removed, as in the first embodiment described above.

[0193] Anions such as iodide ions and chloride ions in the raw material liquid 21 supplied to the intermediate chamber 14' permeate the first anion exchange membrane 7a and move to the concentration chamber 13, but cannot move further from the concentration chamber 13 through the cation exchange membrane 6c' to the deionization chamber 11, and therefore remain in the concentrated liquid 23 in the concentration chamber 13. At this time, since the first anion exchange membrane 7a is a monovalent anion-selective permeable membrane, sulfate ions cannot permeate the first anion exchange membrane 7a and remain in the raw material liquid 21.

[0194] Moreover, sodium ions remain in the raw material solution 21 without being able to permeate the first anion exchange membrane 7a.

[0195] The treated liquid 28 that has passed through the intermediate chamber 14' is supplied to the dilution chamber 11 and the dilution chamber 15, respectively, and is passed through the dilution chamber 11 and the dilution chamber 15 in one pass or in circulation.

[0196] In the deionization compartment 11, sodium ions in the treatment liquid 28 permeate through the cation exchange membrane 6c' and move to the concentration compartment 13.

[0197] In the deionization compartment 15, anions such as iodide ions, chloride ions, and sulfate ions in the treatment liquid 28 permeate the second anion exchange membrane 8a and move to the intermediate compartment 14'.

[0198] Here, in the intermediate chamber 14', as long as the iodide ion concentration of the treatment solution 28 in the deionization compartment 15 is sufficient, the iodide ions that migrated from the intermediate chamber 14' to the concentration compartment 13 migrate back from the deionization compartment 15, so the iodide ion concentration does not fluctuate significantly. Therefore, the intermediate chamber 14' is less susceptible to the reverse migration of iodide ions driven by the concentration difference. In addition, because the iodide ion concentration in the intermediate chamber 14' is lower than that in the concentration compartment 13, the migration of iodide ions from the deionization compartment 15 to the positive electrode side is also less susceptible to the reverse migration caused by the concentration difference.

[0199] By providing the intermediate chamber 14', iodide ions can be recovered in the concentrated solution 23 in the concentrating chamber 13 with a high yield and a high concentration ratio.

[0200] Towards the end of dialysis, when the iodide ions and chloride ions in deionizing compartment 15 are almost gone, a large amount of sulfate ions in deionizing compartment 15 migrates to intermediate compartment 14'. However, even at the end of dialysis, sufficient amounts of chloride ions and iodide ions are still present in intermediate compartment 14', so most of the sulfate ions remain in intermediate compartment 14' without migrating to concentrating compartment 13. Furthermore, after passing through intermediate compartment 14', the iodide ion concentration is lower than in the original feed solution 21, which has the advantage of improving the iodide ion recovery rate.

[0201] The treated liquid 29 that has passed through the deionization compartments 11 and 15 is discharged as dialysis wastewater 116.

[0202] A concentrated liquid 23 is circulated and supplied to the concentration compartment 13 . In the concentration compartment 13, sodium ions in the concentrate 23 cannot permeate the first anion exchange membrane 7a and remain in the concentrate 23. In addition, anions such as iodide ions, sulfate ions, and chloride ions cannot permeate the cation exchange membrane 6c' and remain in the concentrate 23.

[0203] On the other hand, as described above, anions such as iodide ions and chloride ions in the intermediate chamber 14' permeate the first anion exchange membrane 7a and move to the concentration chamber 13. At this time, since the first anion exchange membrane 7a is a monovalent anion permselective membrane, sulfate ions in the intermediate chamber 14' cannot permeate the first anion exchange membrane 7a.

[0204] As a result, the concentrations of sodium ions, iodide ions, and chloride ions in the concentrated solution 23 in the concentrating chamber 13 increase, in other words, the ions are concentrated. Thereafter, by withdrawing the treated solution with increased ion concentrations in the concentrating chamber 13, i.e., the concentrated solution 23, from the concentrating chamber 13, iodine can be recovered at a high concentration.

[0205] Furthermore, the electrodialysis method according to this embodiment is particularly advantageous in terms of miniaturization of the electrodialysis tank 1' and power saving compared to the electrodialysis method according to the first embodiment described above.

[0206] Although the preferred embodiments of the present invention have been described above, the present invention is not limited to these. For example, modifications such as changing conditions or adding other steps may be made within the scope of the spirit of the present invention.

[0207] Finally, the features of the present invention will be described. First, among the target anions that have migrated to the concentrating compartment, some pass through the cation exchange membrane and return to the deionizing compartment, while other anions return via the anion exchange membrane to the deionizing compartment driven by the concentration difference. This method solves this problem by noting that the amounts of these anions cannot be ignored when the concentration of the target anions is relatively low and a high yield is desired. Furthermore, for example, in the present invention, the yield is increased by capturing iodide ions that pass through the cation exchange membrane from the concentrating compartment in the first intermediate compartment. Furthermore, the yield and the concentration ratio are increased by capturing iodide ions that migrate back from the concentrating compartment through the anion exchange membrane in the second intermediate compartment.

[0208] Second, the electrodialysis cell used in the present invention does not have cation exchange membranes and anion exchange membranes arranged alternately as in conventional electrodialysis cells. In the present invention, the object of the invention is achieved by using a yon-yon-yin-yin arrangement.

[0209] Third, the feedstock solution can be used multiple times for purposes other than as a feedstock. For example, in the present invention, the feedstock solution is first sent to the first intermediate compartment, where it is used to capture iodide ions that pass through the cation exchange membrane from the concentration compartment. It is then sent to the second intermediate compartment, where it captures iodide ions that migrate back from the concentration compartment through the anion exchange membrane. The second intermediate compartment is also used to capture sulfate ions that migrate in large quantities toward the concentration compartment through the monovalent anion permselective membrane when the monovalent anions in the deionization compartment decrease. Finally, it is sent to the deionization compartment, where it is used as a feedstock. [Example]

[0210] The present invention will be described in more detail below using examples. Example 1 <Configuration of electrodialysis cell> Electrodialysis was carried out using an electrodialysis device including an electrodialysis cell (manufactured by Asahi Kasei Corporation, G4 type).

[0211] A pair of electrodes was placed on both sides of the electrodialysis cell, one electrode was a positive electrode (anode) and the other electrode was a negative electrode (cathode). Between these positive and negative electrodes, a cation exchange membrane, a first cation exchange membrane, a second cation exchange membrane, a first anion exchange membrane, a second anion exchange membrane, and a cation exchange membrane were placed in this order from the positive electrode side to the negative electrode side.

[0212] The cation exchange membrane used was a monovalent cation selective permeable membrane, Selemion CSO (manufactured by AGC Corporation). The anion exchange membrane used was a monovalent anion selective permeable membrane, Selemion ASV-N (manufactured by AGC Corporation). The effective area of ​​the ion exchange membrane was 0.02 m per membrane. 2 It was decided.

[0213] These ion exchange membranes partitioned the electrodialysis cell into a positive electrode chamber, a deionization chamber, a first intermediate chamber, a concentration chamber, a second intermediate chamber, a deionization chamber, and a negative electrode chamber, from the positive electrode side to the negative electrode side. Four sets of four chambers and four membranes were arranged, each set consisting of a first cation exchange membrane, a first intermediate chamber, a second cation exchange membrane, a concentration chamber, a first anion exchange membrane, a second intermediate chamber, a second anion exchange membrane, and a deionization chamber.

[0214] <Preparation of the stock solution> The feed liquid for each chamber was prepared as follows: A 5% aqueous solution of sodium sulfate was prepared and used as the electrode chamber liquid for the positive and negative electrodes. A 6,000 mL aqueous solution containing 6 g of sodium iodide, 89 g of sodium chloride, and 108 g of sodium sulfate was prepared as a raw material solution. The pH of the raw material solution was 7.

[0215] In addition, a 10,000 mg / L aqueous solution of sodium iodide was prepared and used as a concentrated solution. All the feed solutions were prepared using neutral salts and distilled water.

[0216] The reagents used were sodium iodide (manufactured by Fujifilm Wako Pure Chemical Industries, Ltd.), sodium hydrogen sulfate monohydrate (manufactured by Fujifilm Wako Pure Chemical Industries, Ltd.), anhydrous sodium sulfate (manufactured by Fujifilm Wako Pure Chemical Industries, Ltd.), and 75% sulfuric acid (manufactured by Toshin Corporation). Furthermore, dilute hydroiodic acid produced by ourselves using electrodialysis was used to prepare the raw material solution.

[0217] <Electrodialysis> Between the negative and positive electrodes, 4.0 A (current density 2 A / dm 2 ) was applied, and 2,000 mL of electrode chamber solution was passed through each of the electrode chambers (positive and negative electrode chambers) at a rate of 0.2 L / min., 6,000 mL of raw material solution was passed through the first intermediate chamber, 6,000 mL of raw material solution was passed through the second intermediate chamber, and 6,000 mL of raw material solution was passed through the deionization chamber at a rate of 0.2 L / min. In addition, 500 mL of concentrated solution was passed through the concentration chamber at a rate of 0.2 L / min. The operation was carried out by circulating the liquids discharged from each chamber.

[0218] <Ion concentration analysis> The operation was carried out for 3.0 hours, and the discharged liquids from the first intermediate compartment, the second intermediate compartment, the deionization compartment, and the concentration of iodide ions (I - ), sulfate ions (SO4 2- ) and chloride ions (Cl - ) were analyzed for ion concentration.

[0219] For the analysis, the sample was diluted with pure water and measured using an ion chromatograph (Metrohm "883 Professional" with chemical ion suppressor, carbon dioxide gas suppressor, Asap5 column 150 mm, 3.2 mM Na2CO3 1.0 mM NaHCO3 mixed eluent).

[0220] The iodide ion (I - ), sulfate ions (SO4 2- ) and chloride ions (Cl - The ion concentrations of ) are shown in Tables 1 and 2, respectively.

[0221] Table 3 also shows the yield calculated from the amount of ions in the liquid charged to the first intermediate compartment and the liquid obtained from the deionization compartment.

[0222] The change in the ion concentration of each of the above ions over time in the concentration compartment is shown in Figure 6. The change in the residual rate (%) of each of the above ions over time in the deionization compartment is shown in Figure 7, and the change in the residual rate (%) of each of the above ions over time in the second intermediate compartment is shown in Figure 8.

[0223] [Table 1]

[0224] [Table 2]

[0225] [Table 3]

[0226] From Figures 6 to 8 and Tables 1 to 3, it can be seen that the residual rates of iodide ions and chloride ions in the deionization compartment and the second intermediate compartment are decreasing, while the ion concentrations of iodide ions and chloride ions in the concentration compartment are increasing, which indicates that iodide ions and chloride ions have migrated from the deionization compartment to the second intermediate compartment and then from the second intermediate compartment to the concentration compartment.

[0227] Furthermore, the sulfate ion residual rate in the dilution compartment decreased and the sulfate ion residual rate in the second intermediate compartment increased, indicating that sulfate ions migrated from the dilution compartment to the second intermediate compartment. Furthermore, the suppression of an increase in sulfate ion concentration in the concentration compartment indicates that sulfate ions that migrated from the dilution compartment to the second intermediate compartment remained in the second intermediate compartment, with almost no migration to the concentration compartment.

[0228] Furthermore, the amount of iodide ions in the first intermediate compartment increases, which indicates that the iodide ions that have come out of the concentration compartment through the cation exchange membrane are captured in the first intermediate compartment.

[0229] Example 2 <Configuration of electrodialysis cell> An electrodialysis cell having the same configuration as that used in Example 1 was used.

[0230] <Electrodialysis> Between the negative and positive electrodes, 4.0 A (current density 2 A / dm 2 ) was applied, and 2,000 mL of electrode chamber solution prepared in the same manner as in Example 1 was passed through the electrode chambers (positive electrode chamber and negative electrode chamber) at a rate of 0.2 L / min. 6,000 mL of raw material solution prepared in the same manner as in Example 1 was passed through the first intermediate chamber, 5,960 mL of the residual solution from the first intermediate chamber in Example 1 was passed through the second intermediate chamber at a rate of 0.2 L / min. 5,930 mL of the residual solution from the second intermediate chamber in Example 1 was passed through the deionization compartment at a rate of 0.2 L / min. 750 mL of the concentrated solution obtained in Example 1 was passed through the concentration compartment at a rate of 0.2 L / min. The operation was carried out in such a way that the liquids discharged from each compartment were circulated.

[0231] <Ion concentration analysis> The operation was carried out for 3.0 hours, and the discharged liquids from the first intermediate compartment, the second intermediate compartment, the deionization compartment, and the concentration of iodide ions (I - ), sulfate ions (SO4 2- ) and chloride ions (Cl - ) were analyzed for ion concentration in the same manner as in Example 1.

[0232] The iodide ion (I - ), sulfate ions (SO4 2- ) and chloride ions (Cl - ) are shown in Tables 4 and 5, respectively.

[0233] Table 6 shows the yield calculated from the amount of ions in the liquid charged to the first intermediate compartment and the liquid obtained from the deionization compartment.

[0234] The change in the ion concentration of each of the above ions over time in the concentration compartment is shown in Figure 9. The change in the residual rate (%) of each of the above ions over time in the deionization compartment is shown in Figure 10, and the change in the residual rate (%) of each of the above ions over time in the second intermediate compartment is shown in Figure 11.

[0235] [Table 4]

[0236] [Table 5]

[0237] [Table 6]

[0238] 9 to 11 and Tables 4 to 6 show that the yield of iodide ions was improved compared to Example 1. This is thought to be largely due to the fact that the amount of iodide ions in the feed liquid to the desalting compartment (the residual liquid in the second intermediate compartment in Example 1) was lower than that in the feed liquid to the desalting compartment in Example 1 (the model crystallization wastewater).

[0239] Furthermore, the migration start time and migration rate of sulfate ions in the desalting compartments were also faster than those in Example 1. This is also presumably due to the fact that the proportions of chloride ions and iodide ions in the feed solution in the desalting compartments were lower than those in Example 1.

[0240] However, sulfate ions that migrated from the dilution compartment to the positive electrode remained in the second intermediate compartment and barely migrated to the concentration compartment. Even though chloride ions and iodide ions migrated from the second intermediate compartment to the concentration compartment, new chloride ions and iodide ions migrated from the dilution compartment to the second intermediate compartment. Therefore, it is thought that a sufficient amount of chloride ions and iodide ions remained in the second intermediate compartment, limiting the migration of sulfate ions.

[0241] Example 3 <Configuration of electrodialysis cell> An electrodialysis cell having the same configuration as that used in Example 1 was used.

[0242] <Electrodialysis> Between the negative and positive electrodes, 4.0 A (current density 2 A / dm 2 ) was applied, and 2,000 mL of electrode chamber solution prepared in the same manner as in Example 1 was passed through the electrode chambers (positive electrode chamber and negative electrode chamber) at a rate of 0.2 L / min. 6,000 mL of raw material solution prepared in the same manner as in Example 1 was passed through the first intermediate chamber, 5,820 mL of the residual solution from the first intermediate chamber in Example 2 was passed through the second intermediate chamber at a rate of 0.2 L / min. 5,860 mL of the residual solution from the second intermediate chamber in Example 2 was passed through the deionization compartment at a rate of 0.2 L / min. 1,000 mL of the concentrated solution obtained in Example 2 was passed through the concentration compartment at a rate of 0.2 L / min. The operation was carried out in such a way that the liquids discharged from each compartment were circulated.

[0243] <Ion concentration analysis> The operation was carried out for 3.0 hours, and the discharged liquids from the first intermediate compartment, the second intermediate compartment, the deionization compartment, and the concentration of iodide ions (I - ), sulfate ions (SO4 2- ) and chloride ions (Cl - ) were analyzed for ion concentration in the same manner as in Example 1.

[0244] The iodide ion (I - ), sulfate ions (SO4 2- ) and chloride ions (Cl - ) are shown in Tables 7 and 8, respectively.

[0245] Table 9 shows the yield calculated from the amount of ions in the liquid charged to the first intermediate compartment and the liquid obtained from the deionization compartment.

[0246] The change in the ion concentration of each of the above ions over time in the concentration compartment is shown in Figure 12. The change in the residual rate (%) of each of the above ions over time in the deionization compartment is shown in Figure 13, and the change in the residual rate (%) of each of the above ions over time in the second intermediate compartment is shown in Figure 14.

[0247] [Table 7]

[0248] [Table 8]

[0249] [Table 9]

[0250] 12 to 14 and Tables 7 to 9 show that even after repeated dialysis, sufficient amounts of chloride ions and iodide ions are still present in the second intermediate compartment, so the movement of sulfate ions from the second intermediate compartment to the concentration compartment is limited.

[0251] The sulfuric acid:iodide ion molar ratio of the final concentrated solution was 0.14, which was significantly lower than the target of 0.5. The iodine yield calculated from the amount of iodide ions in the solution fed to the first intermediate compartment and the solution obtained from the desalting compartment was 98%.

[0252] (Comparative Example 1) <Configuration of electrodialysis cell> An electrodialysis cell having the following configuration was fabricated. The main body of the electrodialysis cell, the cation exchange membrane and the anion exchange membrane were the same as those in Example 1.

[0253] A pair of electrodes was placed on both sides of the electrodialysis cell, one electrode was a positive electrode (anode) and the other electrode was a negative electrode (cathode). Between these electrodes, a cation exchange membrane, a cation exchange membrane, an anion exchange membrane, and another cation exchange membrane were placed in this order from the positive electrode side to the negative electrode side.

[0254] These ion exchange membranes partitioned the electrodialysis cell into a positive electrode chamber, a deionization chamber, a concentration chamber, a deionization chamber, and a negative electrode chamber, from the positive electrode side to the negative electrode side. Four sets of two chambers and two membranes, each consisting of a deionization chamber, a cation exchange membrane, a concentration chamber, and an anion exchange membrane, were arranged in a repeated fashion.

[0255] <Preparation of the stock solution> The feed liquid for each chamber was prepared as follows: A 5% aqueous solution of sodium sulfate was prepared and used as the electrode chamber liquid for the positive and negative electrodes. In addition, a 790 mg / L aqueous solution of sodium iodide was prepared and used as a concentrated solution. A 6,000 mL aqueous solution containing 4,000 mg of HI, 53,000 mg of HCl, and 100,000 mg of NaHSO4 was prepared as the desalting compartment solution. The pH of the desalting compartment solution was 1.

[0256] <Electrodialysis> Between the negative and positive electrodes, 4.0 A (current density 2 A / dm 2 ) was applied, 2,000 mL of electrode chamber liquid was passed through each of the electrode chambers (positive and negative electrode chambers) at a rate of 0.2 L / min, 500 mL of concentrate was passed through the concentration chamber at a rate of 0.2 L / min, and 6,000 mL of deionization chamber liquid was passed through the deionization chamber at a rate of 0.2 L / min, and the liquid discharged from each chamber was circulated.

[0257] <Ion concentration analysis> The operation was carried out for 5.5 hours, and the discharged liquids from the deionization compartment and the concentration compartment were collected. The iodide ion (I - ), sulfate ions (SO4 2- ) and chloride ions (Cl - ) were analyzed for ion concentration in the same manner as in Example 1.

[0258] The iodide ion (I - ), sulfate ions (SO4 2- ) and chloride ions (Cl - ) are shown in Tables 10 and 11, respectively.

[0259] Table 12 shows the transfer rate in the deionization compartment calculated from the amount of ions in the deionization compartment before and after electrodialysis.

[0260] The change over time in the ion concentration of each of the above ions in the concentration compartment is shown in Figure 15. The change over time in the residual rate (%) of each of the above ions in the deionization compartment is shown in Figure 16.

[0261] [Table 10]

[0262] [Table 11]

[0263] [Table 12]

[0264] 15, 16 and Tables 10 to 12, approximately 20% of the iodide ions remained in the deionization compartment. In addition, the migration rate of sulfate ions was high, and a concentrated solution with a sulfate ion:iodide ion molar ratio of 0.5 or less was not obtained.

[0265] (Comparative Example 2) <Configuration of electrodialysis cell> An electrodialysis cell having the same configuration as that used in Comparative Example 1 was used.

[0266] <Preparation of the stock solution> The feed liquid for each chamber was prepared as follows. A 5% aqueous solution of sodium sulfate was prepared and used as the electrode chamber liquid for the positive and negative electrodes. In addition, a 10,000 mg / L aqueous solution of sodium iodide was prepared and used as a concentrated solution. A 6,000 mL aqueous solution containing 6 g of sodium iodide, 89 g of sodium chloride, and 108 g of sodium sulfate was prepared as the desalting compartment solution. The pH of the desalting compartment solution was 6.8.

[0267] <Electrodialysis> Between the negative and positive electrodes, 4.0 A (current density 2 A / dm 2) was applied, 2,000 mL of electrode chamber liquid was passed through each of the electrode chambers (positive and negative electrode chambers) at a rate of 0.2 L / min, 500 mL of concentrate was passed through the concentration chamber at a rate of 0.2 L / min, and 6,000 mL of deionization chamber liquid was passed through the deionization chamber at a rate of 0.2 L / min, and the liquid discharged from each chamber was circulated.

[0268] <Ion concentration analysis> The operation was carried out for 4.5 hours, and the discharged liquids from the deionization compartment and the concentration compartment were collected. The iodide ion (I - ), sulfate ions (SO4 2- ) and chloride ions (Cl - ) were analyzed for ion concentration in the same manner as in Example 1.

[0269] The iodide ion (I - ), sulfate ions (SO4 2- ) and chloride ions (Cl - ) are shown in Tables 13 and 14, respectively. Table 15 shows the transfer rate of the deionization compartment calculated from the amount of ions in the deionization compartment before and after electrodialysis.

[0270] The change over time in the ion concentration of each of the above ions in the concentration compartment is shown in Figure 17. The change over time in the residual rate (%) of each of the above ions in the deionization compartment is shown in Figure 18.

[0271] [Table 13]

[0272] [Table 14]

[0273] [Table 15]

[0274] 17, 18 and Tables 13 to 15 show that the movement of ions in the deionization compartments was almost the same as in Example 1.

[0275] When the pH of the raw material solution was neutralized, the sulfate ion transfer rate was low in the early stages of dialysis (up to 1 hour of operation). However, after 1 hour, the sulfate ion transfer rate increased rapidly.

[0276] It is believed that neutralizing the pH of the raw solution reduced the amount of hydrogen sulfate ions, which are monovalent ions, and that the migration of sulfate ions was suppressed by the monovalent anion-selective membrane. However, in the latter half of dialysis (after 1.5 hours), the proportion of sulfate ions among the anions in the raw solution increased, and it is believed that the migration of sulfate ions could not be suppressed despite the use of the monovalent anion-selective membrane.

[0277] From the above results, in Examples 1 to 3, in which the first and second intermediate compartments were provided, the migration of sulfate ions from the deionization compartment to the concentration compartment was more effectively suppressed than in Comparative Examples 1 and 2, in which no intermediate compartments were provided, and iodide ions were obtained in the concentrate in the concentration compartment at a high yield and a high concentration ratio. [Industrial Applicability]

[0278] The electrodialysis method of the present invention is an electrodialysis method for separating and obtaining a component to be obtained by electrodialysis from a raw material solution, which is an aqueous solution containing both the component to be obtained and the component to be removed. The method uses an electrodialysis cell in which, between a positive electrode and a negative electrode, multiple sets of four membranes and four compartments are arranged in the following order from the positive electrode side: a first cation exchange membrane which is a cation exchange membrane, a first intermediate compartment, a second cation exchange membrane which is a cation exchange membrane, a concentration compartment, a first anion exchange membrane which is an anion exchange membrane, a second intermediate compartment, a second anion exchange membrane which is an anion exchange membrane, and a desalination compartment, followed by a cation exchange membrane and a negative electrode compartment. The raw material solution is passed through the first intermediate compartment in a single pass or by circulation, the concentrated solution is passed through the concentration compartment in a circulation manner, the liquid that has been passed through the first intermediate compartment in a single pass or by circulation is passed through the second intermediate compartment in a single pass or by circulation, and the liquid that has been passed through the second intermediate compartment in a single pass or by circulation is passed through the desalination compartment in a single pass or by circulation.

[0279] Another embodiment of the electrodialysis method of the present invention is an electrodialysis method for separating and obtaining a component to be obtained by electrodialysis from a raw material solution, which is an aqueous solution containing both the component to be obtained and the component to be removed. The method uses an electrodialysis cell in which, between a positive electrode and a negative electrode, multiple sets of three membranes and three compartments are arranged, in this order from the positive electrode side, following the positive electrode compartment, a first cation exchange membrane which is a cation exchange membrane, an intermediate compartment, a second cation exchange membrane which is a cation exchange membrane, a concentration compartment, an anion exchange membrane, and a deionization compartment, followed by a cation exchange membrane and a negative electrode compartment, and the raw material solution is passed through the intermediate compartment in a single pass or by circulation, the concentrated solution is passed through the concentration compartment in a circulation, and the solution that has passed through the intermediate compartment in a single pass or by circulation is passed through the deionization compartment in a single pass or by circulation.

[0280] Another embodiment of the electrodialysis method of the present invention is an electrodialysis method for separating and obtaining a component to be obtained by electrodialysis from a raw material solution, which is an aqueous solution containing both the component to be obtained and the component to be removed. The method uses an electrodialysis cell in which, between a positive electrode and a negative electrode, multiple sets of three membranes and three compartments are arranged, in this order from the positive electrode side, a positive electrode chamber, a cation exchange membrane, and a deionization compartment, followed by a cation exchange membrane, a concentration compartment, a first anion exchange membrane which is an anion exchange membrane, an intermediate compartment, a second anion exchange membrane which is an anion exchange membrane, and a deionization compartment, followed by a cation exchange membrane and an anode compartment, and the raw material solution is passed through the intermediate compartment in a single pass or by circulation, the concentrated solution is passed through the concentration compartment in a circulation, and the solution that has passed through the intermediate compartment in a single pass or by circulation is passed through the deionization compartment in a single pass or by circulation.

[0281] According to the electrodialysis method of the present invention, it is possible to efficiently separate and recover a component to be obtained from a raw material solution, which is an aqueous solution containing both the component to be obtained and the component to be removed. In particular, by using crystallization wastewater, which has a low concentration and a large fluctuation in iodide ion concentration, as the raw material solution, it is possible to effectively utilize valuable resources. Therefore, the electrodialysis method of the present invention has industrial applicability.

[0282] For example, iodide ions separated and obtained by the present invention can be purified into iodine, an important industrial product. [Explanation of symbols]

[0283] 1, 1', 1" electrodialysis tank 2 Positive electrode 3 Negative electrode 4c Cation exchange membrane 5c First cation exchange membrane 6c Second cation exchange membrane 6c' Cation exchange membrane 7a First anion exchange membrane 8a Second anion exchange membrane 8s' anion exchange membrane 9c Cation exchange membrane 10 Positive electrode chamber 11 Desalination room 12 First intermediate chamber (iodide ion capture chamber) 12' Intermediate Room 13 Concentration chamber 14 Second intermediate chamber (pre-desalination chamber) 14' Intermediate Room 15 Desalination room 16 Anode chamber 21 Raw material liquid 22 Processing liquid 23 Concentrate 24 Processing liquid 25 Processing liquid 26 Processing liquid 27 Processing liquid 28 Processing liquid 29 Processing liquid 31 Positive electrolyte 32 Negative electrolyte 101 Iodine purification equipment 102 Oxidation-reduction potential adjustment tank 103 pH adjustment tank 104 Electrodialysis equipment 111 Absorbent 112 Reducing Agent 113 Alkaline ingredients 114 Part of the concentrate 115 Product Iodine 116 Dialysis wastewater 121 Drainage (crystallization drainage) 122 Pretreatment liquid 123 Pretreatment liquid

Claims

1. 1. An electrodialysis method for separating and obtaining a component to be obtained from a raw material solution that is an aqueous solution containing both a component to be obtained and a component to be removed by electrodialysis, comprising: Between the positive electrode and the negative electrode, from the positive electrode side to the positive electrode chamber, an electrodialysis cell in which a plurality of sets of four membranes and four compartments are arranged in the following order: a first cation exchange membrane which is a cation exchange membrane, a first intermediate compartment, a second cation exchange membrane which is a cation exchange membrane, a concentration compartment, a first anion exchange membrane which is an anion exchange membrane, a second intermediate compartment, a second anion exchange membrane which is an anion exchange membrane, and a deionization compartment, and a cation exchange membrane and an anode compartment are subsequently arranged; The raw material liquid is passed through the first intermediate compartment in a single pass or by circulation, the concentrated liquid is passed through the concentration compartment in a single pass or by circulation, the liquid which has been passed through the first intermediate compartment in a single pass or by circulation is passed through the second intermediate compartment in a single pass or by circulation, and the liquid which has been passed through the second intermediate compartment in a single pass or by circulation is passed through the deionization compartment in a single pass or by circulation. Electrodialysis method characterized by:

2. 1. An electrodialysis method for separating and obtaining a component to be obtained from a raw material solution that is an aqueous solution containing both a component to be obtained and a component to be removed by electrodialysis, comprising: Between the positive electrode and the negative electrode, from the positive electrode side to the positive electrode chamber, An electrodialysis cell is used in which a plurality of sets of three membranes and three compartments are arranged in the following order: a first cation exchange membrane which is a cation exchange membrane, an intermediate compartment, a second cation exchange membrane which is a cation exchange membrane, a concentration compartment, an anion exchange membrane, and a deionization compartment, followed by a cation exchange membrane and a negative electrode compartment; The raw material liquid is passed through the intermediate compartment in a single pass or in a circulating manner, the concentrated liquid is passed through the concentration compartment in a circulating manner, and the liquid that has passed through the intermediate compartment in a single pass or in a circulating manner is passed through the deionization compartment in a single pass or in a circulating manner. Electrodialysis method characterized by:

3. 1. An electrodialysis method for separating and obtaining a component to be obtained from a raw material solution that is an aqueous solution containing both a component to be obtained and a component to be removed by electrodialysis, comprising: Between the positive electrode and the negative electrode, from the positive electrode side, a positive electrode chamber, a cation exchange membrane, and a deionization chamber are successively arranged, An electrodialysis cell is used in which a plurality of sets of three membranes and three compartments are arranged in the following order: a cation exchange membrane, a concentration compartment, a first anion exchange membrane which is an anion exchange membrane, an intermediate compartment, a second anion exchange membrane which is an anion exchange membrane, and a demineralization compartment, followed by a cation exchange membrane and a negative electrode compartment; The raw material liquid is passed through the intermediate compartment in a single pass or in a circulating manner, the concentrated liquid is passed through the concentration compartment in a circulating manner, and the liquid that has passed through the intermediate compartment in a single pass or in a circulating manner is passed through the deionization compartment in a single pass or in a circulating manner. Electrodialysis method characterized by:

4. 4. The electrodialysis method according to claim 2, wherein the component to be obtained is iodide ion.

5. 5. The electrodialysis method according to claim 4, wherein the iodide ion concentration in the raw material solution is 1,500 mg / L or less and the chloride ion concentration is 3,000 mg / L or more.

6. 6. The electrodialysis method according to claim 4, wherein the raw material liquid is wastewater from an iodine purification step.

7. A part or all of the anion exchange membrane is a monovalent anion selective permeable membrane, 7. The electrodialysis method according to claim 1, wherein a part or all of the cation exchange membranes are monovalent cation permselective membranes.

8. 8. The electrodialysis method according to claim 1, wherein the raw material solution is adjusted to a pH of 4.0 or higher.

Images