JP7615448B2 - Method for separating iodide ions - Google Patents

Description

The present invention relates to a method for separating iodide ions from an aqueous solution containing iodide ions by electrodialysis.

Natural gas can be broadly divided into structural gas and water-soluble gas. Water-soluble natural gas is dissolved in underground brine buried in underground aquifers. Natural gas-associated brine, which is the underground brine that remains after it is pumped to the surface by underground pumping wells and the water-soluble natural gas is vaporized, may contain significant concentrations of iodide salts in addition to salt components similar to sea salt, such as sodium chloride, potassium chloride, calcium chloride, and magnesium sulfate.

Such a method for separating and obtaining iodine from natural gas associated brine is industrially established. As of 2021, for example, the blowing out method (e.g., Patent Document 1) and the ion exchange resin method (e.g., Patent Document 2) are being used. Among these, the blowing out method is a method in which an oxidizing agent such as chlorine water, chlorine gas, or sodium hypochlorite solution is added to underground brine to generate free iodine (I ₂ ), and the generated free iodine is extracted and obtained in an air stream. In addition, the ion exchange resin method is a method in which an oxidizing agent such as chlorine water, chlorine gas, or sodium hypochlorite solution is added to underground brine to partially oxidize iodide ions present in the underground brine, generate triiodide ions (I ₃ ⁻ ), and obtain the generated triiodide ions by adsorbing them on an anion exchange resin.

Both methods are excellent, but they involve the addition of oxidizing agents to the underground brine, which can cause various problems. Below are some of the problems that can arise from adding oxidizing agents to underground brine.

(1) Transition metal ions such as iron and manganese present in groundwater are oxidized to produce insoluble metal oxides. Insoluble metal oxides often clog flow paths. Therefore, in order to remove these insoluble metal oxides, a water production facility is required as a pretreatment facility before the addition of an oxidizing agent. This water production facility is often larger and more complex than the iodine extraction facility, and maintenance is not easy.

(2) Even if iron ions are removed using huge water production facilities, the small amount of manganese ions that remain turn into insoluble metal oxides when an oxidizing agent is added, and precipitate and accumulate in the diffusion equipment using the blowing out method and the adsorption equipment using the ion exchange resin method, leading to blockage of the flow paths in the worst case scenario. For this reason, regular maintenance is required to remove the precipitates.

(3) The discharged brine, which is the underground brine after iodine extraction, is high in temperature and is clean saltwater similar to seawater, but it contains relatively large amounts of oxidizing substances such as residual chlorine, making it difficult to use it for tertiary biological purposes such as aquaculture or cultivation.

(4) When discharged brine is injected into an underground brine aquifer to prevent land subsidence (for example, Patent Document 3), the residual oxidizing agent reacts with the ferrous ions and other substances in the brine aquifer to produce precipitates, making the injection difficult.

For these reasons, it was desirable to develop a method for obtaining iodine that did not involve adding an oxidizing agent directly to the underground brine.

In the blowing out method, iodide ions are converted to free iodine ( _I2 ) and then separated. However, because free iodine is located in the center of the chemical equilibrium between iodide ions ⇔ free iodine ⇔ iodate ions ( ^IO3- ), the total amount never becomes free iodine.

Therefore, the blowing out method results in a loss of iodine of 5 mg/L or more. In addition, due to the presence of organic matter in the groundwater, an unknown amount called bound iodine or organic iodine is produced at a level of 5 mg/L or more. As a result, regardless of the iodine concentration in the groundwater, iodine is lost at a level of 10 mg/L or more.

Furthermore, because the concentration of iodine in the groundwater is low, a large proportion of the oxidizing agent added is consumed not to oxidize iodide ions, but to increase the redox potential of the groundwater. The acid added to adjust the pH is also added in proportion to the volume of the groundwater, so the iodine ratio is very high.

For this reason, it is economically viable when the iodide ion concentration in groundwater exceeds 100 mg/L, but it becomes difficult to achieve this below 50 mg/L, and is not economically viable below 30 mg/L.

In the ion exchange resin method, due to the adsorption equilibrium between triiodide ions and chloride ions, a loss of 10 mg/L or more occurs, and a loss of 5 mg/L occurs as unknowns. For this reason, it is difficult to use at iodide ion concentrations of 50 mg/L or less, and is not viable at 30 mg/L or less.

For these reasons, there was a need to develop a method for concentrating iodine more economically.
As a method for concentrating iodine, a method for obtaining an iodine-concentrated solution from underground brine by electrodialysis has been proposed. By using the electrodialysis method, a concentrated solution that is substantially free of organic matter can be obtained. By using a monovalent cation selective permeable membrane that has monovalent cation selectivity as the cation exchange membrane, a concentrated solution that is substantially free of iron and manganese can be obtained.

However, in a two-chamber electrodialysis device (e.g., Patent Document 4) in which anion exchange membranes and cation exchange membranes are alternately arranged and desalting compartments and concentrating compartments are alternately arranged, it has not been possible to achieve a sufficiently high level of both yield and concentration ratio. This is because, when the iodide ion concentration in the concentrating compartment increases, the rate of movement in the opposite direction to the electrical movement from the concentrating compartment to the desalting compartment through the anion exchange membrane, which is driven by the concentration difference, increases, and the rate of movement of iodide ions passing through the cation exchange membrane from the concentrating compartment to the desalting compartment increases.

In other words, when separating iodide ions using a two-chamber electrodialysis device, if the yield is prioritized, the iodide ion concentration in the concentration chamber cannot be increased sufficiently, while if the concentration rate is prioritized, the yield is inevitably reduced.

Special Publication No. 28-006615 Japanese Unexamined Patent Publication No. 48-018187 Japanese Unexamined Patent Publication No. 129405/1983 JP 2005-58896 A

The object of the present invention is to provide an economical method for separating iodide ions, which can achieve both a sufficiently high level of yield and concentration ratio, in a method for separating iodide ions from an aqueous solution containing iodide ions by electrodialysis. It is also to provide a method for separating iodine from natural gas associated brine, which does not require the addition of an oxidizing agent to the natural gas associated brine.

Such an object can be achieved by the present invention described below.
A method for separating iodide ions according to a first aspect of the present invention is a method for separating iodide ions by electrodialysis from a raw material liquid that is an aqueous solution containing iodide ions, the method comprising the steps of:
Between the positive electrode and the negative electrode, from the positive electrode side to the positive electrode chamber,
When n is an integer of 2 or more, an electrodialysis cell is used in which a plurality of membrane chamber sets are arranged, each set being a (n+1) membrane (n+1) chamber including a cation exchange membrane, a product chamber, a first anion exchange membrane which is an anion exchange membrane, a first raw material chamber, a second anion exchange membrane which is an anion exchange membrane, a second raw material chamber, ... an nth anion exchange membrane which is an anion exchange membrane, and an nth raw material chamber, and a cation exchange membrane and a negative electrode chamber are successively arranged,
The raw material liquid is passed through all the chambers other than the positive electrode chamber and the negative electrode chamber in a single pass or in circulation, and the raw material liquid that has passed through the product chamber is obtained as a product liquid.

A second aspect of the present invention is a method for separating iodide ions by electrodialysis from a raw material solution that is an aqueous solution containing iodide ions, 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 a plurality of membrane chamber sets are arranged, each set consisting of a first cation exchange membrane which is a cation exchange membrane, a first intermediate chamber, a second cation exchange membrane which is a cation exchange membrane, a concentration chamber, a first anion exchange membrane which is an anion exchange membrane, a second intermediate chamber, a second anion exchange membrane which is an anion exchange membrane, and a desalting chamber, and in which a cation exchange membrane and a negative electrode chamber are successively arranged;
The method comprises passing a concentrated liquid through the concentration compartment by circulation, passing the raw material liquid or the product liquid obtained by subjecting the raw material liquid to the method of the first aspect by one pass or by circulation through the first intermediate compartment, passing the liquid that has been passed through the first intermediate compartment by one pass or by circulation through the second intermediate compartment by one pass or by circulation, and passing the liquid that has been passed through the second intermediate compartment by one pass or by circulation through the desalting compartment by one pass or by circulation.

A third aspect of the present invention is a method for separating iodide ions by electrodialysis from a raw material solution that is an aqueous solution containing iodide ions, the method comprising the steps of:
Between the positive electrode and the negative electrode, from the positive electrode side to the positive electrode chamber,
When n is an integer of 2 or more, an electrodialysis cell is used in which a plurality of membrane chamber sets are arranged, each set including a first cation exchange membrane which is a cation exchange membrane, a first intermediate chamber, a second cation exchange membrane which is a cation exchange membrane, a concentration chamber, a first anion exchange membrane which is an anion exchange membrane, a second intermediate chamber/product chamber, a second anion exchange membrane which is an anion exchange membrane, a desalting chamber/first raw material chamber, a third anion exchange membrane which is an anion exchange membrane, a second raw material chamber, ... a (n+1) th anion exchange membrane which is an anion exchange membrane, and a (n+3) membrane (n+3) chamber of the nth raw material chamber, and a cation exchange membrane and a negative electrode chamber are arranged in succession;
The raw material liquid or the product liquid obtained by subjecting the raw material liquid to the method of the first aspect is passed through the first intermediate chamber in a single pass or in a circulation manner, and a concentrated liquid is passed through the concentration chamber in a circulation manner;
The second intermediate chamber/product chamber is passed through the liquid that has passed through the first intermediate chamber in a single pass or in a circulation manner.
The liquid that has passed through the second intermediate chamber/product chamber is passed through the desalting chamber/first raw material chamber in a single pass or in circulation,
The second raw material chamber ... the nth raw material chamber are passed in a single pass or in circulation with the raw material liquid or the product liquid obtained by subjecting the raw material liquid to the method of the first aspect,
The increased amount of liquid circulated through the concentrating compartment is collected as the concentrated liquid.

In the iodide ion separation method of the present invention, it is particularly preferable that the raw material liquid is water-soluble natural gas associated brine.

In addition, in the iodide ion separation method of the present invention, it is preferable that all of the cation exchange membranes used in the electrodialysis cell are monovalent cation selective exchange membranes, and when the raw material liquid contains sulfate ions, the anion exchange membranes used in the electrodialysis cell are monovalent anion selective permeable membranes.

According to the present invention, in a method for separating iodide ions by electrodialysis from an aqueous solution containing iodide ions, it is possible to provide a method for separating iodide ions that can achieve both a sufficiently high level of yield and a high concentration ratio, and is also economically excellent.

FIG. 1 is a schematic diagram showing an example of the configuration of an electrodialysis cell used in a method for separating iodide ions according to a first embodiment of the present invention. FIG. 2 is a schematic diagram showing an example of the configuration of an electrodialysis cell used in the iodide ion separation method according to the second embodiment of the present invention. FIG. 3 is a schematic diagram showing an example of an entire flow path according to an embodiment of the present invention. FIG. 4 is a graph showing the change over time in the iodide ion concentration in each chamber in the positive/double negative method of Example 1. Figure 5 is a graph showing the change in iodide ion concentration over time in each chamber in the yon-yon-yon-yin method of Example 2, where Figure 5(A) is a graph showing the first intermediate chamber, the second intermediate chamber, and the desalting chamber, and Figure 5(B) is a graph showing the concentration chamber.

Preferred embodiments of the present invention will now be described in detail.
Fig. 1 is a schematic diagram showing a configuration example of an electrodialysis cell used in a method for separating iodide ions according to a first embodiment of the present invention. Fig. 2 is a schematic diagram showing a configuration example of an electrodialysis cell used in a method for separating iodide ions according to a second embodiment of the present invention. Fig. 3 is a schematic diagram showing an example of the entire flow path according to an embodiment of the present invention.

The method for separating iodide ions according to the first embodiment of the present invention is a method for separating iodide ions by electrodialysis from a raw material solution 21, which is an aqueous solution containing iodide ions. Between the positive electrode 2 and the negative electrode 3, from the positive electrode side, following the positive electrode chamber 10, there are a cation exchange membrane 4c, a product chamber 11, a first anion exchange membrane 5a which is an anion exchange membrane, a first raw material chamber 12, a second anion exchange membrane 5b which is an anion exchange membrane, and a second anion exchange membrane 5c which is an anion exchange membrane, where n is an integer of 2 or more. A plurality of membrane chamber sets, each consisting of a membrane 6a, a second raw material chamber 13, an anion exchange membrane, an nth anion exchange membrane, and an (n+1) membrane (n+1) chamber of the nth raw material chamber, are arranged, followed by a cation exchange membrane 8c and a negative electrode chamber 15. The first electrodialysis cell 1 is used, which is an electrodialysis cell in which the raw material liquid 21 is passed through all chambers except the positive electrode chamber 10 and the negative electrode chamber 15 in a single pass or in circulation, and the raw material liquid 21 that has passed through the product chamber 11 is obtained as the product liquid 22. In FIG. 2, an example of n=3 is shown.

In this specification, the iodide ion separation method according to the first embodiment will be referred to as the "positive-negative method."

In the double-yellow-yellow process, raw material solution 21, which is an aqueous solution containing iodide ions, can be concentrated to produce a product solution 22 with a reduced volume. Specifically, in the double-yellow-yellow-yellow process, when a set of membrane chambers is provided with n raw material chambers, raw material solution 21 can be reduced in volume to 1/(n+1) to produce product solution 22.

In this specification, "volume reduction" means reducing the volume by concentrating a large amount of dilute raw material liquid.

Iodine can be suitably obtained from the product liquid 22 obtained by the double positive/negative process by applying the blowing out method or the ion exchange resin method. In particular, the yield can be improved by increasing the iodide ion concentration in the product liquid 22.

In addition, according to this embodiment, the size of the electrodialysis cell and the power consumption can be reduced compared to a two-chamber electrodialysis method using a two-chamber electrodialysis device. For example, if the number of raw material chambers (n) in the first electrodialysis cell 1 is three, the size of the electrodialysis cell and the power consumption required to process the same amount of raw material liquid 21 are approximately half that of the two-chamber electrodialysis method. In addition, a product liquid 22 with a volume reduced to approximately one-quarter of the raw material liquid 21 can be obtained.

Therefore, according to this embodiment, even when the iodide ion concentration in the raw material solution is low, the yield can be made sufficiently high, and an economical method for separating iodide ions can be provided.

The method for separating iodide ions according to the second embodiment of the present invention is a method for separating iodide ions by electrodialysis from a raw material liquid, which is an aqueous solution containing iodide ions. Between the positive electrode 32 and the negative electrode 33, multiple membrane chamber sets are arranged, each set consisting of four membranes and four chambers, including, from the positive electrode side, a first cation exchange membrane 35c, a first intermediate chamber 42, a second cation exchange membrane 36c, a concentration chamber 43, a first anion exchange membrane 37a, a second intermediate chamber 44, a second anion exchange membrane 38a, and a desalting chamber 45, followed by a cation exchange membrane 39c, a negative electrode chamber 46, The second electrodialysis tank 31 is an electrodialysis tank in which the above-mentioned components are arranged, and the concentrated liquid 49 is passed through the concentration chamber 43 by circulation, the raw material liquid itself or the product liquid 22 obtained by subjecting the raw material liquid to the method according to the first embodiment is passed through the first intermediate chamber 42 by one pass or circulation, the treated liquid 48 that has been passed through the first intermediate chamber 42 by one pass or circulation is passed through the second intermediate chamber 44 by one pass or circulation, and the treated liquid 50 that has been passed through the second intermediate chamber 44 by one pass or circulation is passed through the desalting chamber 45 by one pass or circulation.

The iodide ion separation method according to this embodiment can achieve a sufficiently high level of both yield and concentration ratio, providing an economically superior method for separating iodide ions.

In this specification, the iodide ion separation method according to the second embodiment of the present invention will be referred to as the "yang-yang-yin method."

The positive-negative-positive method can produce a concentrated solution 49 that is substantially free of transition metal ions and organic matter.

The chamber liquid passed through the first intermediate chamber 42 can be the product liquid 22 obtained by the iodide ion separation method (positive/negative double cathode method) according to the first embodiment described above, or the raw material liquid 21, but in the illustrated configuration, the product liquid 22 is used.

By using the raw material liquid 21 as the chamber liquid passed through the first intermediate chamber 42, it is possible to eliminate losses that occur in the double-yin/yang process and to increase the yield. This is particularly advantageous in that it is possible to achieve a higher level of both the iodine concentration rate and the yield.

On the other hand, by using the product liquid 22 obtained by the yang-yang double-yin method as the chamber liquid to be passed through the first intermediate chamber 42, in other words, by reducing the volume of the raw material liquid 21 in advance using the yang-yang double-yin method before the yang-yang double-yin method, the size of the second electrodialysis cell 31 and the amount of power used can be significantly reduced compared to when the raw material liquid 21 is used as the chamber liquid to be passed through the first intermediate chamber 42. When raw material liquid that has not been reduced in volume is directly used, the size and power of the electrodialysis cell of the second electrodialysis cell 31 of the yang-yang double-yin method are approximately twice the size and power of the electrodialysis cell used in the two-chamber electrodialysis method.

For example, if the number of raw material chambers (n) in the first electrodialysis cell 1 is three, the yang double-yin method produces a product liquid 22 in which the volume of the raw material liquid 21 is reduced to approximately one-quarter. By performing electrodialysis using the yang-yang negative-yin method using this product liquid 22 whose volume has been reduced to one-quarter, the size and power consumption of the second electrodialysis cell 31 can be reduced to approximately one-quarter (one-half that of the two-chamber electrodialysis method) compared to when the volume of the raw material liquid 21 is not reduced.

When the number of raw material chambers (n) is three, the size and power of the first electrodialysis cell 1 is half that of the two-chamber electrodialysis method, so the combined size and power consumption of the first electrodialysis cell 1 and the second electrodialysis cell 31 is within the size and power consumption of one electrodialysis cell for the two-chamber electrodialysis method. In other words, by combining the yang double-yin method and the yang-yang negative-yin method, the size and power consumption of the electrodialysis cell can be reduced to approximately half that of when the raw material liquid 21 is directly treated in a single stage of the yang-yang negative-yin method.

In addition, the polyvalent metal ions and organic matter contained in the raw material liquid 21 remain in the product liquid 22 obtained by the double positive-negative method, but in this embodiment, by adopting the positive-positive-negative-negative method, it is possible to produce a concentrated liquid 49 that is substantially free of transition metal ions and organic matter.

In addition, by using the product liquid 22 that has been reduced in volume in advance using the double-yin/sun method, it is possible to achieve a higher level of both yield and concentration ratio, making it more economical.

In this embodiment, for example, water-soluble natural gas associated brine can be used as the raw material liquid 21, which is an aqueous solution containing iodide ions.

When iodide ions are separated and concentrated from natural gas associated brine using this embodiment, a high-concentration iodide ion concentrate that does not contain transition metal ions or organic matter can be obtained in high yield. Therefore, when iodine is subsequently separated and obtained using a method such as the blowing out method, the size of the equipment can be reduced, and the amount of chemicals used, electricity, etc. can all be reduced, and the yield is also high because the iodine concentration is significantly higher than that of brine. The obtained iodide ion concentrate does not contain organic matter, so there is no generation of organic iodine, further increasing the yield. In addition, because the obtained concentrate does not contain transition metal ions, the equipment is not contaminated by the precipitation of transition metal oxides.

In particular, this method does not require the addition of oxidizing agents to the natural gas associated brine, and iodine can be efficiently separated and concentrated from the natural gas associated brine, so the dialysis wastewater 61 discharged after iodide ion separation does not contain any oxides such as residual chlorine. This makes it easy to perform underground reduction, and furthermore, as a mild saltwater, it can easily be used for tertiary biological purposes such as cultivation and aquaculture.

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

In the following explanation, the iodide ion separation method will be described mainly in the case where brine associated with water-soluble natural gas is used as the raw material liquid, which is an aqueous solution containing iodide ions, and the aqueous solution containing iodide ions is electrodialyzed using an electrodialysis cell, in particular in the case where the brine associated with water-soluble natural gas is electrodialyzed using a first electrodialysis cell 1 and a second electrodialysis cell 31 connected to each other, but this does not exclude the use of an aqueous solution containing iodide ions other than brine, or electrodialysis using the first electrodialysis cell 1 or the second electrodialysis cell 31 alone.

[1] Overall Configuration The electrodialysis method combining the positive double-yin method and the positive-positive double-yin method will be described below with reference to Fig. 3. In addition, the following will be described using an example in which an electrodialysis cell with three raw material chambers is used as the positive double-yin method.

Figure 3 is a schematic diagram showing an example of the overall flow path of this embodiment. Figure 3 shows an electrodialysis device that includes two electrodialysis cells connected in series, i.e., a first electrodialysis cell 1 and a second electrodialysis cell 31.

Each electrodialysis cell is energized, and raw material liquid 21, which is an aqueous solution containing iodide ions, is passed through the product chamber 11 and the first raw material chamber 12 to the third raw material chamber 14 of the first electrodialysis cell 1 in the preceding stage. The product liquid 22 obtained from the product chamber 11 of the first electrodialysis cell 1 is passed through the first intermediate chamber 42 of the second electrodialysis cell 31 in the succeeding stage. The treated liquid 23 that has passed through the first raw material chamber 12 to the third raw material chamber 14 of the first electrodialysis cell 1 is used as dialysis wastewater 61 after iodine has been obtained.

The product liquid 22 passed through the first intermediate chamber 42 of the second electrodialysis tank 31 is passed through the first intermediate chamber 42, the second intermediate chamber 44, and the desalting chambers 41 and 45 in that order. The volume of the concentrated liquid 49 obtained from the concentration chamber 43 of the second electrodialysis tank 31 increases during operation. This increase is used as the obtained concentrated liquid 62, and the remaining concentrated liquid 49 is circulated to the concentration chamber 43 of the second electrodialysis tank 31. The treated liquid 51 that has passed through the desalting chambers 41 and 45 of the second electrodialysis tank 31 is used as the dialysis wastewater 61 after iodine has been obtained.

[2] Brine associated with water-soluble natural gas In this embodiment, for example, brine containing iodide ions is used as the raw material liquid as an aqueous solution containing iodide ions. Specifically, for example, brine associated with water-soluble natural gas, in other words, underground-derived brine remaining after extracting water-soluble natural gas from brine pumped from an underground brine aquifer, can be used. Furthermore, in this embodiment, underground brine with fluctuations in iodide ion concentration, oxidized matter concentration, and pH value, such as mixed brine collected from multiple wells at any time, can also be used.

The iodide ion content of the underground brine used is not particularly limited, but is preferably 10 mg/L or more, and more preferably 20 mg/L or more. Compared to the conventional method of directly adding an oxidizing agent to the brine, the advantages of the present invention are particularly evident for brine with a low iodide ion concentration of 80 mg/L or less, particularly 50 mg/L or less. The present invention also makes it possible to use brine with an iodide ion content of 30 mg/L or less, which has not been economically viable up until now.

[3] Pretreatment It is preferable to perform pretreatment on the groundwater before performing electrodialysis treatment as described later in detail, for example, to remove impurities such as insoluble matter and microorganisms from the groundwater in order to prevent clogging of the flow path in the electrodialysis tank. The pretreatment can be, for example, a coagulation and sedimentation process, a sand filtration process, a porous filtration membrane process, etc., either alone or in combination.

As the filtration material, for example, particles such as sand, coal, activated carbon, inorganic oxides or resins, or porous filtration membranes such as microfiltration membranes or ultrafiltration membranes can be used. The average pore size of the porous filtration membrane is not particularly limited, but is preferably 0.01 μm or more and 1 μm or less in terms of the balance between removal performance and the amount of permeated water.

In this embodiment, it is preferable to handle the underground brine pumped from the underground aquifer in a non-oxidizing state without exposing it to air. To achieve this, it is preferable to handle the underground-derived brine in a non-oxidizing atmosphere, such as a nitrogen gas atmosphere, without exposing it to light, and by not adding any oxidizing substances, in a non-oxidizing state. Note that natural gas may remain in the brine after natural gas separation, and may accumulate in the head space of the liquid tank beyond the explosion limit. The underground brine in a non-oxidizing state is a non-oxidizing brine, and has an oxidation-reduction potential of 0 mV or less, preferably 0 to -50 mV, on the platinum electrode potential. When the underground brine is handled in a non-oxidizing state, the transition metal components such as iron ions contained in the underground brine are not oxidized, so that the precipitation of insoluble oxides in the equipment and piping can be prevented.

In addition to the above-mentioned methods, other pretreatment methods can be used, such as aerating the underground brine in advance to precipitate transition metal oxides, and then removing the transition metal oxides by filtration or the like.

[4] First embodiment Hereinafter, electrodialysis using a positive-double-negative method as a method for separating iodide ions according to a first embodiment will be described in detail with reference to FIG.

Figure 1 is a schematic diagram showing an example of the configuration of a first electrodialysis cell used in the iodide ion separation method (positive/negative double negative method) according to the first embodiment.

[4-1] Regarding the first electrodialysis cell The first electrodialysis cell 1 is arranged between the positive electrode 2 and the negative electrode 3, and from the positive electrode side, a plurality of membrane chamber sets are arranged, each of which includes a positive electrode chamber 10, a cation exchange membrane 4c, a product chamber 11, a first anion exchange membrane 5a, a first raw material chamber 12, a second anion exchange membrane 6a, a second raw material chamber 13, ... an nth anion exchange membrane and an (n+1) membrane (n+1) chamber of the nth raw material chamber, followed by a cation exchange membrane 8c and an anode chamber 15 (n is an integer of 2 or more).

In the example shown in FIG. 1, the number of raw material chambers (n) is three. That is, four membranes and four chambers are grouped together: the cation exchange membrane 4c, the product chamber 11, the first anion exchange membrane 5a, the first raw material chamber 12, the second anion exchange membrane 6a, the second raw material chamber 13, the third anion exchange membrane 7a, and the third raw material chamber 14.

A pair of electrodes is arranged on both sides of the first electrodialysis cell 1, one of which is a positive electrode 2 (anode) and the other is a negative electrode 3 (cathode).

Examples of materials constituting the positive electrode 2 include platinum (Pt), carbon (C), nickel (Ni), and composite materials (e.g., alloys, plating, etc.) such as titanium (Ti)/platinum, ruthenium (Ru)/titanium, iridium (Ir)/titanium, and titanium/palladium (Pd).

Examples of materials constituting the negative electrode 3 include iron (Fe), nickel, platinum, titanium/platinum, carbon, and chromium (Cr) steel as stainless steel.

The positive electrode 2 and negative electrode 3 are formed in the shape of, for example, a long and narrow rectangular flat plate, a mesh, a lattice, or the like.

The first electrodialysis cell 1 has a chamber frame that is a rectangular frame in plan view and has a notch (not shown).

The positive electrode 2 and negative electrode 3 are attached to the inside of both ends along the longitudinal direction of this chamber frame, and the positive electrode chamber (positive electrode chamber 10) is formed by the cation exchange membrane 4c arranged on the negative electrode side of the positive electrode 2 of this first electrodialysis cell 1, and the negative electrode chamber (negative electrode chamber 15) is formed by the cation exchange membrane 8c arranged on the positive electrode side of the negative electrode 3.

Between the positive electrode chamber 10 and the negative electrode chamber 15, from the positive electrode side to the negative electrode side, a cation exchange membrane 4c, a first anion exchange membrane 5a, a second anion exchange membrane 6a, a third anion exchange membrane 7a, and a cation exchange membrane 8c are arranged in this order. These ion exchange membranes divide the positive electrode chamber 10 toward the negative electrode chamber 15 into a product chamber 11, a first raw material chamber 12, a second raw material chamber 13, and a third raw material chamber 14.

In other words, the chamber frame separated by the cation exchange membrane 4c and the first anion exchange membrane 5a forms the product chamber 11. The chamber frame separated by the first anion exchange membrane 5a and the second anion exchange membrane 6a forms the first raw material chamber 12. The chamber frame separated by the second anion exchange membrane 6a and the third anion exchange membrane 7a forms the second raw material chamber 13. The chamber frame separated by the third anion exchange membrane 7a and the cation exchange membrane 8c forms the third raw material chamber 14.

Then, the (n+1) membrane (n+1) chambers from the cation exchange membrane 4c to the nth raw material chamber, n=3 in the first electrodialysis cell 1 having the configuration shown in the figure, are arranged repeatedly, with the four membranes and four chambers from the cation exchange membrane 4c to the third raw material chamber 14, in other words, the membrane chamber set consisting of the cation exchange membrane 4c, the product chamber 11, the first anion exchange membrane 5a, the first raw material chamber 12, the second anion exchange membrane 6a, the second raw material chamber 13, the third anion exchange membrane 7a, and the third raw material chamber 14 as one set. The number of repetitions (m) of the membrane chamber set is not particularly limited, but is preferably, for example, several sets to several hundred sets, more specifically, 4 sets or more and 1800 sets or less.

To provide tension to each ion exchange membrane, both ends are fastened and fixed to both sides of the chamber frame while being pulled and taut in both the vertical direction (up and down in Fig. 1) and horizontal direction (depth direction in Fig. 1).

In the following description, the cation exchange membrane 4c and the cation exchange membrane 8c may be collectively referred to as the "cation exchange membrane."

In the following description, the first anion exchange membrane 5a, the second anion exchange membrane 6a, and the third anion exchange membrane 7a may be collectively referred to as the "anion exchange membranes."

In the following description, the positive electrode chamber 10, the product chamber 11, the first raw material chamber 12, the second raw material chamber 13, the third raw material chamber 14, and the negative electrode chamber 15 may be collectively referred to as the "liquid chambers."

In the following description, the first raw material chamber 12, the second raw material chamber 13, and the third raw material chamber 14 may be collectively referred to as the "raw material chambers."

The inner surface of the chamber frame in 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. In addition, a spacer (not shown) that has a flow distribution function to make the thickness inside the chamber frame uniform is provided inside the chamber frame.

The positive electrode chamber 10, product chamber 11, first raw material chamber 12, second raw material chamber 13, third raw material chamber 14, and negative electrode chamber 15 are each fed with a chamber liquid of a predetermined concentration and amount appropriate for the purpose, which is passed through each chamber individually in a single pass or in a circulation manner.

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

(Anion exchange membrane)
As the anion exchange membrane, a membrane having enhanced selective permeability for anions can be suitably used.

Anions such as chloride ions (Cl ⁻ ) and iodide ions (I ⁻ ) contained in the raw material liquid 21, for example, underground brine, selectively permeate the anion exchange membrane and move from the raw material chamber to the product chamber 11. Non-ionizable inorganic compounds and polymeric organic compounds such as fulvic acid do not permeate the anion exchange membrane and remain in the raw material chamber, and are contained in the treated liquid 23 and discharged as dialysis wastewater 61.

When the raw material liquid 21 used, for example underground brine, contains sulfate ions (SO ²⁻ ), it is preferable to use a monovalent anion permselective membrane having monovalent anion selectivity as the anion exchange membrane.

When an anion exchange membrane that does not have monovalent anion selectivity is used, sulfate ions pass through the anion exchange membrane and move to the product chamber 11, and the sulfate ion concentration in the product chamber 11 increases, which may cause the precipitation of poorly soluble calcium sulfate.

By using a monovalent anion selective permeable membrane that has monovalent anion selectivity, the occurrence of the above problems can be effectively prevented.

As such a monovalent anion selective permeable membrane, for example, a strongly acidic styrene-divinylbenzene homogeneous anion exchange membrane is used, and more specifically, for example, Selemion ASV-N membrane (manufactured by AGC Inc.), Selemion AMV-N membrane (manufactured by AGC Inc.), Neocepta ACS membrane (manufactured by Astom Co., Ltd.), etc. are used.

(Cation exchange membrane)
As the cation exchange membrane, a monovalent cation permselective membrane, which is a membrane having enhanced selectivity for monovalent cations, can be suitably used.
It is preferred that all of the cation exchange membranes are monovalent cation permselective membranes.

By using a monovalent cation selectively permeable membrane as the cation exchange membrane, a product solution is obtained in which the transition metal ions contained in the brine are not concentrated. If a cation exchange membrane without monovalent cation selectivity is used, the transition metal ions are concentrated in the product solution. If a monovalent cation selectively permeable membrane is used, no precipitation due to transition metal oxides occurs when extracting iodine from the product solution. In addition, divalent ions such as calcium ions and magnesium ions, which tend to form insoluble carbonates and sulfates, are not concentrated, so there is an effect that no blockage of flow paths in electrodialysis cells or valves due to insoluble salt precipitation occurs.

As a monovalent cation selective permeable membrane, for example, a strongly acidic styrene-divinylbenzene homogeneous cation exchange membrane can be used. More specifically, for example, a Selemion CSO membrane (manufactured by AGC Inc.) or a Neocepta CIMS membrane (manufactured by Astom Co., Ltd.) can be used.

[4-2] Method for separating iodide ions by the positive/double cathode method In the positive/double cathode method, first, a positive electrode solution 24 is supplied to the positive electrode chamber 10 of the first electrodialysis cell 1, and a negative electrode solution 25 is supplied to the negative electrode chamber 15.

The electrode solutions (positive electrode solution 24 and negative electrode solution 25) include, for example, an aqueous sodium sulfate solution.
The electrode solution may be supplied in a circulating manner between the positive electrode chamber 10 and the negative electrode chamber 15 .

Then, raw material liquid 21, which is an aqueous solution containing iodide ions, is passed in a single pass or in a circulation manner through all chambers other than the positive electrode chamber 10 and the negative electrode chamber 15, in other words, the product chamber 11, the first raw material chamber 12, the second raw material chamber 13, and the third raw material chamber 14.

During operation of the first electrodialysis cell 1, the temperature in each liquid chamber is kept below the heat resistance temperature of the ion exchange membrane being used.

A direct current equal to or lower than the maximum operating voltage specified by the ion exchange membrane manufacturer is then supplied between the positive electrode 2 (anode) and the negative electrode 3 (cathode).

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 and target composition of the raw material liquid 21, the size (flow path length), chamber thickness, flow rate, etc. of the first electrodialysis cell 1 to be used, and a technically and economically advantageous voltage is set. Increasing the voltage increases the migration speed of iodide ions and chloride ions, but decreases the selectivity of iodide ions. When the chloride ions are gone from the desalting chamber, no more current flows and electrodialysis ends, so increasing the voltage also shortens the time available for electrodialysis. In order to increase the yield, it is advantageous to decrease the voltage and increase the dialysis time, but the amount of iodide ions remaining in the first raw material chamber 12, which is affected by diffusion migration, also increases. In addition, the size of the electrodialysis cell becomes larger. 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.
In the case of constant current operation, the voltage applied to each membrane is approximately the same even if the number of membrane chambers is increased or decreased or some of the membranes are changed, so that it is suitable for preliminary tests such as the selection of dialysis membranes to be used. On the other hand, in the case of constant voltage operation, the current becomes low when the ion concentration in the raw material solution decreases, and becomes high when the concentration increases, so it is also convenient when the concentration changes.

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

In this case, anions can move through the anion exchange membrane but cannot move through the cation exchange membrane. Similarly, cations can move through the cation exchange membrane but cannot move through the anion exchange membrane.

In Figure 1, the solid arrows attached to the anions indicate the movement of each anion through the anion exchange membrane.

At the start of dialysis operation (near the inlet of the electrodialysis cell in one-pass operation), the iodide ions in the first raw material chamber 12 to the nth raw material chamber move toward the positive electrode at substantially the same speed. That is, in the first raw material chamber 12 to the (n-1)th raw material chamber, iodide ions move to the chamber on the positive electrode side, but iodide ions move from the chamber on the negative electrode side at substantially the same speed. Therefore, the concentration of iodide ions in the first raw material chamber 12 to the (n-1)th raw material chamber does not substantially change.

In the nth raw material chamber, iodide ions move to the chamber on the positive electrode side, i.e., the (n-1) raw material chamber, but iodide ions do not move from the chamber on the negative electrode side, i.e., the negative electrode chamber 15. Therefore, the concentration of iodide ions decreases only in the nth raw material chamber.

In the first electrodialysis cell 1 having the configuration shown in FIG. 1, n=3, so the concentrations of iodide ions in the first raw material chamber 12 and the second raw material chamber 13 do not substantially change, but the concentration of iodide ions decreases only in the third raw material chamber 14.

In addition, in the product chamber 11, iodide ions do not move to the positive electrode chamber, i.e., the positive electrode chamber 10, but iodide ions move from the negative electrode chamber, i.e., the first raw material chamber 12. Therefore, the concentration of iodide ions increases only in the product chamber 11.

As the dialysis operation progresses, the iodide ion concentration in the nth raw material chamber decreases significantly, so the rate at which iodide ions move from the nth raw material chamber to the (n-1)th raw material chamber decreases, and the iodide ion concentration in the (n-1)th raw material chamber decreases. In the first electrodialysis cell 1 configured as shown in FIG. 1, the iodide ion concentration in the second raw material chamber 13 decreases.

As the dialysis operation progresses further, the iodide ion concentration decreases in all of the raw material chambers, from the first raw material chamber 12 to the nth raw material chamber. In the first raw material chamber 12, the iodide ion concentration decreases last, and it is affected by the increase in iodine concentration in the product chamber 11, so the yield decreases slightly.

In the case of one-pass operation, the yield of the first raw material chamber 12 can be improved by passing the raw material 21 in a countercurrent manner only in the product chamber 11, in other words, by passing the raw material 21 supplied to the product chamber 11 and the raw material 21 supplied to the first raw material chamber 12 to the nth raw material chamber in a direction opposite each other across the first anion exchange membrane 5a.

In electrodialysis, ions adsorbed to the ion exchange groups on the ion exchange membrane move. The abundance ratio on the ion exchange membrane is the abundance ratio in the solution multiplied by the adsorption ratio, so ion species that are easily adsorbed to the ion exchange groups move fast, and ion species that are not easily adsorbed move slow. Iodide ions are more strongly adsorbed than chloride ions, so they move faster than chloride ions. However, when there is an overwhelmingly greater amount of chloride ions than iodide ions, the adsorption of iodide ions is inhibited, and the migration rate of iodide ions decreases.

When the number of raw material chambers in one membrane chamber set is n, in the positive double negative method, the concentration ratio of iodide ions is a maximum of (n+1) times. That is, when the number of raw material chambers n=3, the concentration ratio of iodide ions is a maximum of 4 times. On the other hand, the concentration ratio of chloride ions is a maximum of 2 times. Therefore, if 2≦n, the iodine:chlorine ratio of the product liquid 22 is higher than that of the raw material liquid 21, and this is suitable for concentrating again using electrodialysis in the next stage.

The number of raw material chambers n depends on the properties of the raw material liquid 21, for example, underground brine. If the number of raw material chambers is increased, for example, when further electrodialysis is performed in the next stage using the yang-yang-yin-yin method, the size and power consumption of the second electrodialysis cell 31 can be reduced. However, the electrodialysis time using the yang-double-yin method is extended (the processing speed decreases in one-pass operation), so the size of the first electrodialysis cell 1 must be increased. In addition, the concentration difference between the first raw material chamber 12 and the product chamber 11 at the end of dialysis increases, so the amount of iodide ions remaining in the first raw material chamber 12 increases, resulting in a decrease in yield.

Therefore, the number of raw material chambers n can be selected based on the underground brine properties, costs, and yield, and can be determined as most appropriate.

The number of raw material chambers (n) in one set is three in FIG. 1, but it is sufficient if it is two or more, and it is preferably two to five, and more preferably two to four.

This allows the size and power consumption of the first electrodialysis cell 1 to be appropriately reduced while achieving a higher concentration rate and yield in the product liquid 22.

Then, the raw material liquid 21 that has passed through the product chamber 11 is obtained as the product liquid 22. This product liquid 22 is obtained by concentrating the raw material liquid 21 and reducing its volume.

Iodine can be obtained by applying the blowing out method or the ion exchange resin method to the product liquid 22. In particular, the yield can be improved by increasing the iodide ion concentration in the product liquid 22. In addition, the equipment for the blowing out method or the ion exchange resin method can be reduced in size, which is economically advantageous.

The product liquid 22 obtained in the first electrodialysis cell 1 is supplied to the second electrodialysis cell 31 in the subsequent stage, and electrodialysis is performed using the yon-yon-yin-yin method, thereby effectively removing organic matter and polyvalent metal ions contained in the product liquid 22.

When the product liquid 22 comes into contact with air, it produces insoluble matter such as ferric hydroxide. Therefore, it is preferable to either isolate the product liquid 22 from the air before supplying it to the second electrodialysis cell 31, or to aerate the product liquid 22 in advance to precipitate transition metal oxides, which are then removed by filtration.

[5] Second embodiment Hereinafter, electrodialysis according to a yon-yon-yin-yin method will be described in detail as a method for separating iodide ions according to a second embodiment with reference to FIG.
FIG. 2 is a schematic diagram showing an example of the configuration of a second electrodialysis cell used in the method for separating iodide ions (yon-yon-yin method) according to the second embodiment.

In the following description of the iodide ion separation method according to the second embodiment, the differences from the iodide ion separation method according to the first embodiment will be mainly described, and descriptions of similar points will be omitted.

[5-1] Regarding the second electrodialysis cell The second electrodialysis cell 31 has, from the positive electrode to the negative electrode side, a positive electrode 32, a positive electrode chamber 40, and multiple membrane chamber sets, each of which includes a first cation exchange membrane 35c, a first intermediate chamber 42, a second cation exchange membrane 36c, a concentration chamber 43, a first anion exchange membrane 37a, a second intermediate chamber 44, a second anion exchange membrane 38a, and a desalting chamber 45, followed by a cation exchange membrane 39c and an anode chamber 46.

In the second electrodialysis cell 31 shown in FIG. 2, a cation exchange membrane 34c and a desalting chamber 41 are further arranged between the positive electrode chamber 40 and the first cation exchange membrane 35c from the positive electrode side.

In the second electrodialysis cell 31, a first intermediate chamber 42 is provided between the desalting chamber 41 and the first cation exchange membrane 35c and the second cation exchange membrane 36c. By passing the product liquid 22 through the first intermediate chamber 42, iodide ions that have leaked from the concentration chamber 43 through the second cation exchange membrane 36c, in other words, iodide ions that have moved to the first intermediate chamber 42 by diffusion through the second cation exchange membrane 36c driven by the concentration difference, can be captured in the product liquid 22 in the first intermediate chamber 42 and prevented from reaching the desalting chamber 41.

The (apparent) migration rate of iodide ions from the chamber adjacent to the negative electrode of the concentrating chamber through an anion exchange membrane to the concentrating chamber is affected by the iodide ion concentration in the concentrating chamber. At the end of dialysis (near the outlet in one-pass operation), the iodide ion concentration in the chamber adjacent to the concentrating chamber decreases, and when the concentration difference between the two chambers becomes large, the (apparent) migration rate of iodide ions slows down and eventually stops. In other words, iodide ions will always remain in the chamber adjacent to the concentrating chamber in proportion to the iodide ion concentration in the concentrating chamber. Therefore, if the chamber adjacent to the concentrating chamber is used as a deionization chamber, both a high concentration rate and a high yield cannot be expected.

To solve this problem, a second intermediate chamber 44 is provided on the positive electrode side of the desalting chamber 45, with both sides partitioned by a monovalent anion selective permeable membrane, and the liquid that has passed through the second intermediate chamber 44 is passed through the desalting chamber 41. At the end of dialysis (the outlet in one-pass operation), iodide ions remain in the second intermediate chamber 44, but they are separated and collected in the desalting chamber 41, so they are not lost. By leaving iodide ions in the second intermediate chamber 44, it is possible to increase the iodine concentration in the concentrating chamber 43 (increase the concentration ratio).

The iodide ion concentration in the second intermediate chamber 44 is higher than that of the raw liquid at the start of dialysis (at the inlet in one-pass operation) because the first intermediate chamber 42 captures iodide ions coming from the concentration chamber 43 through the cation exchange membrane, but is lower than that of the raw liquid at the end of dialysis. In any case, it is significantly lower than that of the concentration chamber 43. Therefore, the (apparent) movement speed of iodide ions from the desalting chamber 45 is higher than when the second intermediate chamber 44 is not provided. This makes it possible to shorten the dialysis time (increase the raw liquid processing speed in one-pass operation). In addition, the iodide ion concentration remaining in the desalting chamber 45 at the end of dialysis (at the outlet in one-pass operation) can be reduced. This makes it possible to ensure a high yield. Furthermore, since the liquid that has passed through the second intermediate chamber 44 has a lower iodide ion concentration than the raw liquid, separation in the desalting chamber 45 is also easier, which also leads to an improvement in yield.

In this way, by providing the first intermediate chamber 42 and the second intermediate chamber 44 on both sides of the concentration chamber 43 in the second electrodialysis tank 31, the migration speed of iodide ions from the deionization chambers 41, 45 is not affected by the iodide ion concentration in the concentration chamber 43. This makes it possible to achieve a sufficiently high level of both yield and concentration ratio, and to separate and recover iodide ions from the product liquid 22 into the concentrate 49. In addition, the electrodialysis time can be shortened.

A pair of electrodes is arranged on both sides of the second electrodialysis cell 31, one of which is a positive electrode 32 (anode) and the other is a negative electrode 33 (cathode).

The materials constituting the positive electrode 32 and the negative electrode 33 can be the same as those constituting the positive electrode 2 and the negative electrode 3 of the first electrodialysis cell 1 described above.

The positive electrode 32 and the negative electrode 33 are attached to the inside of both ends along the longitudinal direction of the chamber frame, and the positive electrode chamber (positive electrode chamber 40) is formed by the cation exchange membrane 34c arranged on the negative electrode side of the positive electrode 32 of this second electrodialysis cell 31, and the negative electrode chamber (negative electrode chamber 46) is formed by the cation exchange membrane 39c arranged on the positive electrode side of the negative electrode 33.

Between the positive electrode chamber 40 and the negative electrode chamber 46, from the positive electrode side to the negative electrode side, the cation exchange membrane 34c, the first cation exchange membrane 35c, the second cation exchange membrane 36c, the first anion exchange membrane 37a, the second anion exchange membrane 38a, and the cation exchange membrane 39c are arranged in this order. These ion exchange membranes divide the positive electrode chamber 40 toward the negative electrode chamber 46 into a desalting chamber 41, a first intermediate chamber 42, a concentrating chamber 43, a second intermediate chamber 44, and a desalting chamber 45.

In other words, the chamber frame divided by the cation exchange membrane 34c and the first cation exchange membrane 35c forms the desalting chamber 41. The chamber frame divided by the first cation exchange membrane 35c and the second cation exchange membrane 36c forms the first intermediate chamber 42. The chamber frame divided by the second cation exchange membrane 36c and the first anion exchange membrane 37a forms the concentration chamber 43. The chamber frame divided by the first anion exchange membrane 37a and the second anion exchange membrane 38a forms the second intermediate chamber 44. And the chamber frame divided by the second anion exchange membrane 38a and the cation exchange membrane 39c forms the desalting chamber 45.

The four membranes and four chambers from the first cation exchange membrane 35c to the desalting chamber 45, in other words, the first cation exchange membrane 35c, the first intermediate chamber 42, the second cation exchange membrane 36c, the concentration chamber 43, the first anion exchange membrane 37a, the second intermediate chamber 44, the second anion exchange membrane 38a and the desalting chamber 45 are arranged as one set, and are arranged repeatedly multiple times. The number of repetitions (m) is not particularly limited, but is preferably, for example, several sets to several hundred sets, more specifically, 4 sets or more and 1800 sets or less.

In the following description, the cation exchange membrane 34c, the first cation exchange membrane 35c, the second cation exchange membrane 36c, and the cation exchange membrane 39c may be collectively referred to as the "cation exchange membranes."

In the following description, the first anion exchange membrane 37a and the second anion exchange membrane 38a may be collectively referred to as the "anion exchange membrane."

In addition, in the following description, the positive electrode chamber 40, the deionization chamber 41, the first intermediate chamber 42, the concentration chamber 43, the second intermediate chamber 44, the deionization chamber 45 and the negative electrode chamber 46 may be collectively referred to as the "liquid chambers."

(Anion exchange membrane)
As the anion exchange membrane, a membrane having enhanced selective permeability for anions can be suitably used.

Anions such as chloride ions (Cl ⁻ ) and iodide ions (I ⁻ ) contained in the product liquid 22 selectively permeate the anion exchange membrane and move from the deionization chamber 45 through the second intermediate chamber 44 to the concentration chamber 43. Non-ionizable inorganic compounds and high molecular weight organic compounds such as fulvic acid do not permeate the anion exchange membrane and remain in the deionization chamber 45, and are contained in the treated liquid 51 and discharged as dialysis wastewater 61.

However, when the product liquid 22 contains sulfate ions (SO ²⁻ ), it is preferable to use a monovalent anion permselective membrane having monovalent anion selectivity.

When an anion exchange membrane that does not have monovalent anion selectivity is used, sulfate ions can permeate the anion exchange membrane and move to the concentration chamber 43, and the sulfate ion concentration in the concentration chamber 43 increases, which can cause the precipitation of poorly soluble sulfate salts.

By using a monovalent anion selective permeable membrane that has monovalent anion selectivity, the occurrence of the above problems can be effectively prevented.

As the anion exchange membrane, particularly the monovalent anion selective permeable membrane, the same anion exchange membrane as that of the first electrodialysis cell 1 described above can be used.

(Cation exchange membrane)
As the cation exchange membrane, a monovalent cation permselective membrane, which is a membrane having enhanced selectivity for monovalent cations, can be suitably used. It is preferable that all of the cation exchange membranes are monovalent cation permselective membranes.

By using the monovalent cation selectively permeable membrane, monovalent cations such as sodium ions (Na ⁺ ) contained in the product liquid 22 selectively permeate the cation exchange membrane and move from the deionization chamber 41 toward the concentration chamber 43. Polyvalent cations such as calcium ions (Ca ²⁺ ), magnesium ions (Mg ²⁺ ), and transition metal ions do not permeate the monovalent cation selectively permeable membrane and remain, and are included in the treatment liquid 51 from the deionization chamber 41 and discharged as dialysis effluent 61.

As the cation exchange membrane, particularly the monovalent cation selective permeable membrane, the same cation exchange membrane as that of the first electrodialysis cell 1 described above can be used.

By using a monovalent anion selectively permeable membrane and a monovalent cation selectively permeable membrane as the ion exchange membrane, a concentrated liquid 49 can be obtained that is substantially free of polyvalent metal ions, non-ionizable inorganic compounds, and polymeric organic compounds such as fulvic acid.

(Desalination room)
In the second electrodialysis cell 31, it is preferable that the chamber located on the negative electrode side of the positive electrode chamber 40 is the deionization chamber 41 or the first intermediate chamber 42. In the second electrodialysis cell 31 having the configuration shown in FIG. 2, the chamber located on the negative electrode side of the positive electrode chamber 40 is the deionization chamber 41.

This is to minimize the amount of iodide ions that migrate to the positive electrode chamber 40 through the cation exchange membrane 34c from the chamber adjacent to the positive electrode chamber 40. The iodide ions that migrate to the positive electrode chamber 40 are not only lost, but may also corrode the positive electrode 32 depending on the material of the positive electrode 32 (or both electrodes, in the case where the electrode chamber liquid is circulated between the positive electrode chamber 40 and the negative electrode chamber 46). Alternatively, the iodide ions may be oxidized to _I2 at the positive electrode 32 and corrode the surroundings.

The speed at which iodide ions pass through the cation exchange membrane 34c is proportional to the iodide ion concentration, so it is preferable that the chamber adjacent to the positive electrode chamber 40 is the chamber with the lowest iodide ion concentration. The average iodide ion concentrations are, from lowest to highest, deionization chamber 41 < second intermediate chamber 44 < first intermediate chamber 42 << concentration chamber 43, and the inlet concentrations in one-pass operation are, deionization chamber 41 < first intermediate chamber 42 < second intermediate chamber 44 << concentration chamber 43. Of the four chambers, deionization chamber 41 is the most preferable chamber adjacent to the positive electrode chamber 40, and concentration chamber 43 is the least preferable.

By making the chamber located on the negative electrode side of the positive electrode chamber 40 the deionization chamber 41, the effects of iodide ions migrating through the cation exchange membrane 4c as described above can be effectively suppressed.

However, in this electrodialysis method, if the chamber next to the positive electrode chamber 40 is the deionization chamber 41, anions do not move from this deionization chamber 41, resulting in poor efficiency. This is not a problem if the operation is circulated, but if one-pass is used, the entire chamber is lost. If one wishes to pass the deionization chamber 41 in one pass and eliminate this loss, it is preferable that the chamber next to the positive electrode chamber 40 be the first intermediate chamber 42, which has the second lowest iodine concentration at the inlet of the second electrodialysis cell 31 after the deionization chamber 41.

(First intermediate chamber)
As described above, the first intermediate chamber 42 is a chamber that is installed to capture iodide ions that move from the concentration chamber 43 to the positive electrode side through the second cation exchange membrane 36c using the concentration difference as a driving force, and to prevent them from reaching the deionization chamber 41.

This allows iodide ions to be efficiently separated and recovered from the product liquid 22 into the concentrated liquid 49.

(Concentration chamber)
In the concentration chamber 43 , the concentrated liquid 49 is circulated, whereby iodide ions are separated from the product liquid 22 and concentrated in the concentrated liquid 49 .

(Second intermediate chamber)
As described above, the second intermediate chamber 44 is a chamber that is installed to capture iodide ions that migrate from the concentration chamber 43 to the negative electrode side through the anion exchange membrane using the concentration difference as a driving force, and to prevent them from reaching the deionization chamber 45.

(Desalination room)
In the second electrodialysis cell 31 having the configuration shown in FIG. 2, the chamber located on the positive electrode side of the negative electrode chamber 46 serves as a deionization chamber 45 .

This is to make the chamber next to the negative electrode chamber 46 a chamber that is as little affected as possible by sulfate ions migrating from the negative electrode chamber 46 through the anion exchange membrane. The chamber that is most affected by the migration of sulfate ions from the negative electrode chamber 46 is the concentration chamber 43, followed by the first intermediate chamber 42 and the second intermediate chamber 44. The migration of sulfate ions to the deionization chamber 45 has almost no effect.

By making the chamber located on the positive electrode side of the negative electrode chamber 46 the desalting chamber 45, the effects of sulfate ions migrating from the negative electrode chamber 46 through the cation exchange membrane 39c, specifically, for example, the precipitation of poorly soluble sulfates, can be minimized.

The positive electrode chamber 40, the desalination chamber 41, the first intermediate chamber 42, the concentration chamber 43, the second intermediate chamber 44, the desalination chamber 45 and the negative electrode chamber 46 are each fed with a chamber liquid of a predetermined concentration and amount appropriate for the purpose, which is then passed through each chamber individually in a single pass or in a circulation manner.

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

[5-2] Method for separating iodide ions by cation-cation-cation-cation method In the method for separating iodide ions by the cation-cation-cation-cation method, first, a positive electrode liquid 52 is supplied to the positive electrode chamber 40 of the second electrodialysis cell 31, and a negative electrode liquid 53 is supplied to the negative electrode chamber 46. The raw material liquid 21 or the product liquid 22 obtained by the method for separating iodide ions according to the first embodiment (cation-cation-cation-cation method) is supplied to the first intermediate chamber 42 in a single pass or by circulation, and a concentrated liquid 49 is supplied to the concentrating chamber 43. The treated liquid 48 that has passed through the first intermediate chamber 42 is supplied to the second intermediate chamber 44 in a single pass or by circulation, and the treated liquid 50 that has passed through the second intermediate chamber 44 is supplied to the desalting chambers 41 and 45 in a single pass or by circulation.

The following explanation will focus on the case where the product liquid 22 obtained by the iodide ion separation method (positive and negative double cathode method) according to the first embodiment is used as the chamber liquid to be supplied to the first intermediate chamber 42, but this does not exclude the case where a raw material liquid 21 that is an aqueous solution containing iodide ions, such as underground brine, is supplied to the first intermediate chamber 42.

The electrode solutions (positive electrode solution 52 and negative electrode solution 53) include, for example, an aqueous sodium sulfate solution.
The electrode solution may be supplied in a circulating manner between the positive electrode chamber 40 and the negative electrode chamber 46 .

While the second electrodialysis tank 31 is in operation, the temperature in each liquid chamber is kept below the heat resistance temperature of the ion exchange membrane being used.

Then, a current equal to or lower than the maximum operating voltage specified by the membrane manufacturer is supplied between the positive electrode 32 (anode) and the negative electrode 33 (cathode).

The voltage of the current supplied between the positive electrode 32 and the negative electrode 33 is not particularly limited, but is determined individually taking into consideration, for example, the composition of the product liquid 22, the target composition, the size (flow path length), the chamber thickness, the flow rate, etc. of the second electrodialysis cell 31 used, and a technically and economically advantageous voltage is set. 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 second electrodialysis tank 31 may be operated at a constant current or at a constant voltage.

With constant current operation, the voltage applied to each membrane remains roughly the same even if the number of membranes in the chamber is increased or decreased, making it ideal for testing and research. On the other hand, with constant voltage operation, the current becomes low when the ion concentration in the raw liquid decreases, and becomes high when the concentration increases, making it convenient when using crystallization wastewater, which has drastic changes in concentration, as the raw liquid.

When a current is supplied between the positive electrode 32 and the negative electrode 33, the anions contained in each chamber liquid are electrically attracted to the positive electrode, and the cations are electrically attracted to the negative electrode.

In this case, anions can move through the anion exchange membrane but cannot move through the cation exchange membrane. Similarly, cations can move through the cation exchange membrane but cannot move through the anion exchange membrane.

In FIG. 2, the solid arrows attached to the anions indicate the movement of each anion through the anion exchange membrane, and the dotted arrows attached to the anions indicate the non-subjective movement of each anion through the cation exchange membrane. The solid arrows attached to the cations indicate the movement of each cation through the cation exchange membrane.

Therefore, sodium ions (Na ⁺ ) in the product liquid 22 supplied to the first intermediate chamber 42 permeate through the second cation exchange membrane 36c to the negative electrode side and move to the concentration chamber 43, but cannot further move from the concentration chamber 43 through the first anion exchange membrane 37a to the second intermediate chamber 44, and therefore remain in the concentrated liquid 49 in the concentration chamber 43.

In addition, polyvalent cations such as calcium ions (Ca ²⁺ ), magnesium ions (Mg ²⁺ ), transition metal ions, non-ionizable inorganic compounds, and high molecular weight organic compounds such as fulvic acid contained in the product liquid 22 are unable to pass through the second cation exchange membrane 36c, which is a monovalent anion selective permeable membrane, and remain in the product liquid 22 in the first intermediate chamber 42.

On the other hand, anions such as iodide ions (I ⁻ ), sulfate ions (SO ₄ ²⁻ ), and chloride ions (Cl ⁻ ) in the product liquid 22 cannot permeate the second cation exchange membrane 36 c and remain in the product liquid 22 in the first intermediate chamber 42 .

The treatment liquid 48 that has passed through the first intermediate chamber 42 is supplied to the second intermediate chamber 44 and passes through the second intermediate chamber 44 in one pass or in circulation.

In the second intermediate chamber 44, anions such as iodide ions and chloride ions in the treatment liquid 48 permeate the first anion exchange membrane 37a and move to the concentration chamber 43, but cannot move further from the concentration chamber 43 to the first intermediate chamber 42 through the second cation exchange membrane 36c, and therefore remain in the concentrated liquid 49 in the concentration chamber 43. At this time, since the first anion exchange membrane 37a is a monovalent anion selective permeable membrane, sulfate ions cannot permeate the first anion exchange membrane 37a and remain in the treatment liquid 48.

In addition, cations such as sodium ions, calcium ions, and magnesium ions, non-ionizable inorganic compounds, and polymeric organic compounds such as fulvic acid cannot permeate the first anion exchange membrane 37a and remain in the treatment liquid 48.

The treated liquid 50 that has passed through the second intermediate chamber 44 is supplied to the desalting chamber 41 and the desalting chamber 45, respectively, and passes through the desalting chamber 41 and the desalting chamber 45 in one pass or in circulation.

In the desalting chamber 41, sodium ions in the treatment liquid 50 permeate the first cation exchange membrane 35c and move to the first intermediate chamber 42.

In addition, polyvalent cations such as calcium ions and magnesium ions, non-ionizable inorganic compounds, and polymeric organic compounds such as fulvic acid cannot pass through the first cation exchange membrane 35c, which is a monovalent cation selective permeable membrane, and remain in the treatment liquid 50 in the desalting chamber 41.

In the desalting chamber 45, anions such as iodide ions, chloride ions, and sulfate ions in the treatment liquid 50 permeate the second anion exchange membrane 38a and move to the second intermediate chamber 44.

Here, in the second intermediate chamber 44, as long as the iodide ion concentration of the treatment liquid 50 in the deionization chamber 45 is sufficient, the iodide ions that moved from the second intermediate chamber 44 to the concentration chamber 43 move back from the deionization chamber 45, so the iodide ion concentration does not fluctuate significantly. Therefore, it is not easily affected by the reverse movement of iodide ions from the concentration chamber 43 to the second intermediate chamber 44, which is driven by the concentration difference. In addition, since the iodide ion concentration in the second intermediate chamber 44 is lower than that in the concentration chamber 43, the movement of iodide ions from the deionization chamber 45 to the positive electrode side is also not easily affected by the reverse movement from the second intermediate chamber 44 to the deionization chamber 45.

In addition, after passing through the second intermediate chamber 44, the iodide ion concentration is lower than that of the original product liquid 22, which has the advantage of improving the iodide ion recovery rate.

In addition, polyvalent cations such as calcium ions and magnesium ions, non-ionizable inorganic compounds, and polymeric organic compounds such as fulvic acid cannot permeate the second anion exchange membrane 38a and remain in the treatment liquid 50 in the desalting chamber 45.

The treated liquid 51 that has passed through the deionization chambers 41 and 45 is discharged as dialysis wastewater 61. Polyvalent cations such as calcium ions (Ca ²⁺ ), magnesium ions (Mg ²⁺ ), and transition metal ions, non-ionizable inorganic compounds, and high molecular weight organic compounds such as fulvic acid are contained in the treated liquid 51 and discharged.

Concentrated liquid 49 is circulated and supplied to the concentration chamber 43. If there is no concentrated liquid 49 to serve as a seed at the start of operation, a sodium chloride solution or the like with a concentration that does not cause problems when electricity is applied can be used.

In the concentration chamber 43, sodium ions in the concentrated liquid 49 cannot pass through the first anion exchange membrane 37a and remain in the concentrated liquid 49. Anions such as iodide ions and chloride ions cannot pass through the second cation exchange membrane 36c and remain in the concentrated liquid 49.

On the other hand, as described above, sodium ions in the first intermediate chamber 42 permeate through the second cation exchange membrane 36c to the negative electrode side and move to the concentration chamber 43. In addition, anions such as iodide ions and chloride ions in the second intermediate chamber 44 permeate through the first anion exchange membrane 37a and move to the concentration chamber 43. At this time, since the first anion exchange membrane 37a is a monovalent anion selective permeable membrane, sulfate ions in the second intermediate chamber 44 cannot permeate through the first anion exchange membrane 37a.

As a result, the concentrations of sodium ions, iodide ions, and chloride ions in the concentrated liquid 49 in the concentration chamber 43 increase, in other words, they become concentrated. After this, the treated liquid with an increased ion concentration in the concentration chamber 43, i.e., the concentrated liquid 49, is extracted from the concentration chamber 43, thereby obtaining iodine at a high concentration.

In addition, the concentration difference drives the reverse movement of iodide ions, in other words, iodide ions leak from the concentration chamber 43 through the second cation exchange membrane 36c into the first intermediate chamber 42, but the iodide ions that leak into the first intermediate chamber 42 are captured in the product liquid 22.

In this manner, in this embodiment, a first intermediate chamber 42 is provided on the positive electrode side of the concentration chamber 43 via the second cation exchange membrane 36c, and the product liquid 22 is passed through the first intermediate chamber 42, so that iodide ions that have migrated back from the concentration chamber 43 to the first intermediate chamber 42 via the second cation exchange membrane 36c can be captured and recovered in the product liquid 22.

The iodide ions recovered in the product liquid 22 are then separated from the second intermediate chamber 44 into the concentrated liquid 49 in the concentration chamber 43 by the cycle described above.

This allows iodide ions to be obtained in the concentrated liquid 49 in the concentration chamber 43 with a higher yield and a higher concentration ratio.

The volume of the concentrated liquid 49 increases over the course of operating time, so the increased volume is separated off and collected as the obtained concentrated liquid 62.

The obtained concentrated liquid 62, i.e., the iodide ion concentrated liquid, has a high iodide ion concentration, is substantially free of polyvalent metal ions such as iron, manganese, and calcium, and is substantially free of high molecular weight organic compounds such as fulvic acid, making it easy to separate and obtain iodine.

For example, when separating and obtaining iodine from the obtained concentrated liquid 62, the precipitation of insoluble oxides due to the oxidation of transition metal ions in the equipment and piping can be effectively prevented.

According to this embodiment, in a method for separating iodide ions from an aqueous solution containing iodide ions by electrodialysis, it is possible to achieve both a sufficiently high level of yield and concentration ratio, and also to obtain economically excellent results.

In particular, by reducing the volume of an aqueous solution containing iodide ions by electrodialysis (positive-negative double-negative method) to produce a product liquid, and then further subjecting this product liquid to electrodialysis (positive-negative-negative double-negative method), the size of the electrodialysis cell and the power consumption can be more effectively reduced, making the process even more economical.

In addition, in this embodiment, the dialysis wastewater generated by electrodialysis does not contain oxidizing agents such as chlorine and is not oxidizing, so when it is injected into an underground aquifer and comes into contact with groundwater, the transition metal ions dissolved in the groundwater are not oxidized to form insoluble oxides, and the flow of the groundwater is prevented from being hindered. Therefore, the dialysis wastewater can be effectively injected into an underground brine aquifer, particularly an underground brine aquifer from which groundwater has been pumped up, without causing flow hinderance. In addition, since the dialysis wastewater does not contain oxidizing agents, particularly residual chlorine, it can be easily used biologically as mild, clean saltwater for aquaculture, cultivation, etc.

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.

For example, in the above embodiment, the case where the chamber located on the negative electrode side of the positive electrode chamber in the second electrodialysis cell is the desalting chamber has been described as an example, but the chamber located on the negative electrode side of the positive electrode chamber may be the first intermediate chamber. As described above, by using the chamber located on the negative electrode side of the positive electrode chamber as the first intermediate chamber, it is possible to suppress a decrease in efficiency.

In addition, for example, in the second electrodialysis cell described above, it is also possible to omit either the first intermediate chamber or the second intermediate chamber from the electrodialysis cell having the configuration shown in FIG. 2.

Even if either the first or second intermediate chamber is omitted from the electrodialysis cell shown in FIG. 2, the method for separating iodide ions by electrodialysis can achieve both a sufficiently high level of yield and concentration ratio, as in the above-described embodiment, and also provide economically excellent results. In particular, this is even more advantageous in terms of miniaturization and power saving of the electrodialysis cell, and is even more economically excellent.

In addition, an electrodialysis cell having a configuration in which the first embodiment and the second embodiment described above are combined may be used. More specifically, the method for separating iodide ions of the present invention is a method for separating iodide ions by electrodialysis from a raw material solution that is an aqueous solution containing iodide ions, the method comprising the steps of:
Between the positive electrode and the negative electrode, from the positive electrode side to the positive electrode chamber,
When n is an integer of 2 or more, an electrodialysis cell is used in which a plurality of membrane chamber sets are arranged, each set including a first cation exchange membrane which is a cation exchange membrane, a first intermediate chamber, a second cation exchange membrane which is a cation exchange membrane, a concentration chamber, a first anion exchange membrane which is an anion exchange membrane, a second intermediate chamber/product chamber, a second anion exchange membrane which is an anion exchange membrane, a desalting chamber/first raw material chamber, a third anion exchange membrane which is an anion exchange membrane, a second raw material chamber, ... a (n+1) th anion exchange membrane which is an anion exchange membrane, and a (n+3) membrane (n+3) chamber of the nth raw material chamber, and a cation exchange membrane and a negative electrode chamber are arranged in succession;
The raw material liquid or the product liquid obtained by subjecting the raw material liquid to the first embodiment is passed through the first intermediate chamber in a single pass or in a circulation manner, and a concentrated liquid is passed through the concentration chamber in a circulation manner;
The second intermediate chamber/product chamber is passed through the liquid that has passed through the first intermediate chamber in a single pass or in a circulation manner.
The liquid that has passed through the second intermediate chamber/product chamber is passed through the desalting chamber/first raw material chamber in a single pass or in circulation,
The raw material liquid or the product liquid obtained by subjecting the raw material liquid to the method of the first embodiment is passed through the second raw material chamber to the nth raw material chamber in a single pass or in circulation,
The method may include obtaining an increased amount of the liquid circulated through the concentration compartment as the concentrated liquid.

With this configuration, the same effects as in the previously described embodiment can be obtained. In addition, by using an electrodialysis cell with six membranes and six chambers, the raw material liquid is passed through three of the six chambers, so the size and power of the electrodialysis cell can be reduced to that of a single two-chamber system.

Finally, the features of the present invention will be described.
First, among the anions to be obtained that have moved to the concentration compartment or the product compartment, some anions pass through the cation exchange membrane and return to the desalting compartment, and some anions return to the desalting compartment through the anion exchange membrane driven by the concentration difference. This method focuses on the fact that the amount of these anions cannot be ignored when the concentration of the target anions is relatively low and in order to increase the yield, and solves this problem. In addition, for example, in the second embodiment of the present invention, the yield is increased by capturing iodide ions that pass through the cation exchange membrane from the concentration compartment and come out in the first intermediate chamber. In addition, the yield and the concentration ratio are increased by capturing iodide ions that move backward from the concentration compartment through the anion exchange membrane in the second intermediate chamber. In addition, in the first embodiment, the yield is increased by reducing the concentration ratio to a maximum of 4 times.

Secondly, 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 yang-yin-yin... or yang-yang-yin-yin arrangement.

Thirdly, the raw material liquid can be used multiple times for purposes other than as a raw material. For example, in the second embodiment of the present invention, the raw material liquid is first sent to the first intermediate chamber, where it is used to capture iodide ions that pass through the cation exchange membrane from the concentration chamber. It is then sent to the second intermediate chamber, where it captures iodide ions that migrate back from the concentration chamber through the anion exchange membrane. Finally, it is sent to the deionization chamber, where it is used as a raw material.

Fourth, the volume of the raw material liquid is reduced to make it economically advantageous.
Fifth, in the present invention, since no chlorine-based oxidizing agent is added to the brine, the dialysis wastewater (discharged brine) after iodine extraction does not contain residual chlorine or organic chlorine compounds.

The present invention will now be described more specifically with reference to examples.
Example 1
<Configuration of electrodialysis cell>
Electrodialysis was carried out by a positive-double-negative method using an electrodialysis tank (G4 type, manufactured by Asahi Kasei Corporation).

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

The cation exchange membrane used was a monovalent cation selective permeable membrane, Selemion CSO (manufactured by AGC Inc.). The first to third anion exchange membranes used were monovalent anion selective permeable membranes, Selemion ASV-N (manufactured by AGC Inc.). The effective area of the ion exchange membrane was 0.02 ^m2 per sheet.

These ion exchange membranes divide the electrodialysis cell into a positive electrode chamber, a product chamber, a first raw material chamber, a second raw material chamber, a third raw material chamber, and a negative electrode chamber from the positive electrode side to the negative electrode side, and four membrane chambers are arranged, each consisting of a cation exchange membrane, a product chamber, a first anion exchange membrane, a first raw material chamber, a second anion exchange membrane, a second raw material chamber, a third anion exchange membrane, and a third raw material chamber.

The brine used was prepared by pumping up brine from a brine aquifer 1,300 to 600 meters below ground in Niigata City, Niigata Prefecture, aerating it, precipitating transition metal oxides, and removing them by filtration. The iodide ion concentration of this brine was 29 mg/L to 32 mg/L.

<Electrodialysis>
A constant current of 2.5 A (current density 1.25 A/dm ² ) was applied between the negative and positive electrodes, and 2,000 mL of a 3 mass % aqueous sodium sulfate solution was passed through the positive and negative electrode chambers at 1,000 mL/min as an electrode chamber liquid.

6.0 L of the above brine was circulated at 0.2 L/min through the product chamber, the first raw material chamber, the second raw material chamber, and the third raw material chamber. The iodide ion concentration of the underground brine at this time was 32.0 mg/L.

<Ion concentration analysis>
The operation was carried out for 5.5 hours, and the discharged liquids from the product chamber, the first raw material chamber, the second raw material chamber, and the third raw material chamber were obtained, and the ion concentration of iodide ion (I ⁻ ) in each of the obtained liquids was analyzed.

The analysis was performed using an ion chromatograph (Metrohm "883 Professional" equipped with a suppressor, Asap5 column 150 mm, eluent mixture of Na ₂ CO ₃ (3.2 mM) and NaHCO ₃ (3.2 mM)).

Table 1 shows the amount and concentration of iodide ions (I ⁻ ) in the charged solution and the collected solution in each chamber.
FIG. 4 shows the change in iodide ion concentration over time in each chamber solution.

As shown in Table 1 and Figure 4, the concentration of iodide ions decreased in the first to third ingredient chambers, while the concentration of iodide ions increased significantly in the product chamber, indicating that iodide ions moved from the ingredient chambers to the product chamber.

In particular, at the beginning of the dialysis operation, the iodide ion concentration in the first and second raw material chambers hardly changed, but the iodide ion concentration only decreased in the third raw material chamber. As the dialysis operation progressed, the iodide ion concentration in the third raw material chamber decreased significantly, and the iodide ion concentration in the second raw material chamber also decreased. As the dialysis operation progressed further, the iodide ion concentration decreased in all raw material chambers, from the first to the third raw material chambers.

In this example, electrodialysis using the positive-negative double-negative method resulted in a product solution with an iodide ion concentration of 104.0 mg/L. The iodine yield was 83%.

Example 2
<Configuration of electrodialysis cell>
Electrodialysis was carried out by the positive-negative-positive method using an electrodialysis tank (G4 type, manufactured by Asahi Kasei Corporation).

A pair of electrodes was placed on both sides of the electrodialysis cell, one of which was a positive electrode (anode) and the other a negative electrode (cathode). Between the positive and negative electrodes, 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.

These ion exchange membranes divide the electrodialysis cell into a positive electrode chamber, a first intermediate chamber, a concentration chamber, a second intermediate chamber, a desalting chamber, and a negative electrode chamber from the positive electrode side to the negative electrode side, and four membrane chamber sets are arranged, each set consisting of four membrane chambers: 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 desalting chamber.

<Electrodialysis>
(1st time)
A constant current of 4.0 A (current density 2 A/ ^dm2 ) was passed between the negative and positive electrodes, 2,000 mL of electrode chamber solution prepared in the same manner as in Example 1 was passed through the positive and negative electrode chambers at 1,000 mL/min, 500 mL of 3 mass% saline was passed through the concentration chamber at 0.2 L/min, 6.0 L of the product liquid obtained in Example 1 was passed through the first intermediate chamber at 0.2 L/min, 6.0 L of the product liquid obtained in Example 1 was passed through the second intermediate chamber at 0.2 L/min, and 6.0 L of the product liquid obtained in Example 1 was passed through the desalting chamber at 0.2 L/min, and the liquids discharged from each chamber were circulated for 3 hours.
The iodide ion concentration of the product solution obtained in Example 1 was 118.9 mg/L.

(2nd time)
A constant current of 4.0 A (current density 2 A/ ^dm2 ) was passed between the negative and positive electrodes, the residual liquid from the first concentration compartment was passed directly through the concentration compartment, 6.0 L of the product liquid obtained in Example 1 was passed directly through the first intermediate compartment, the residual liquid from the first first intermediate compartment was passed directly through the second intermediate compartment, and the residual liquid from the first second intermediate compartment was passed directly through the desalting compartment, and the liquid leaving each compartment was circulated for 3 hours.

(Third time)
A constant current of 4.0 A (current density 2 A/ ^dm2 ) was passed between the negative and positive electrodes, 600 mL of the residual liquid from the second concentration compartment was passed through the concentration compartment, 6.0 L of the product liquid obtained in Example 1 was passed through the first intermediate compartment, the residual liquid from the first intermediate compartment for the second time was passed directly through the second intermediate compartment, and the residual liquid from the second intermediate compartment for the second time was passed directly through the desalting compartment, and the liquid leaving each compartment was circulated for 3 hours.

<Ion concentration analysis>
The above first to third operations were performed, and the discharged liquids from the first intermediate compartment, the second intermediate compartment, the deionization compartment, and the concentration compartment were obtained. The iodide ion (I ⁻ ) concentration in each of the obtained liquids was analyzed in the same manner as in Example 1.

The iodide ion (I ⁻ ) concentrations in the feed solution and the obtained solution in each compartment in the first to third electrodialysis are shown in Tables 2 to 4, respectively. Also shown are the yields calculated from the amounts of ions in the feed solution to the first intermediate compartment and the obtained solution from the desalting compartment.

Figure 5 shows a graph showing the change over time in iodide ion concentration in each chamber in the yon-yon-yin-yin method of Example 2. In particular, Figure 5(A) shows the change over time in iodide ion concentration in the first intermediate chamber, second intermediate chamber, and desalting chamber during the third electrodialysis, and Figure 5(B) shows the change over time in iodide ion concentration in the concentration chamber.

From Tables 2 to 4 and Figure 5, it can be seen that while the iodide ion concentration in the deionization compartment and the second intermediate compartment is decreasing, the iodide ion concentration in the concentration compartment is increasing, which indicates that iodide ions have migrated from the deionization compartment to the second intermediate compartment and then from the second intermediate compartment to the concentration compartment.

In addition, the iodide ion concentration in the first intermediate chamber increased slightly, which is thought to be due to iodide ions leaking from the concentration chamber through the second cation exchange membrane and being captured in the product liquid in the first intermediate chamber.

In this example, the yield after the third run of electrodialysis using the positive-positive-negative-negative method was 94%. Combined with the yield in Example 1, the total yield was 77%.

Combined with Example 1, a concentrated solution with an iodide ion concentration of 1,400 mg/L was obtained from underground brine with an iodide ion concentration of 32.0 mg/L.

The final concentration ratio of iodide ions in the concentrated solution obtained in Example 2 from the underground brine used as the raw material solution in Example 1 was 44 times.

In addition, the electrodialysis cell is configured such that, between the positive and negative electrodes, multiple membrane chamber sets are arranged, each set consisting of six membranes and six chambers, including, from the positive electrode side, a first cation exchange membrane which is a cation exchange membrane, a first intermediate chamber, a second cation exchange membrane which is a cation exchange membrane, a concentration chamber, a first anion exchange membrane which is an anion exchange membrane, a second intermediate chamber/product chamber, a second anion exchange membrane which is an anion exchange membrane, a desalting chamber/first raw material chamber, a third anion exchange membrane which is an anion exchange membrane, a second raw material chamber, a fourth anion exchange membrane which is an anion exchange membrane, and a third raw material chamber, followed by a cation exchange membrane and a negative electrode chamber. The raw liquid was circulated through the first intermediate chamber, the concentrated liquid was circulated through the concentration chamber, the liquid that had passed through the first intermediate chamber was circulated through the second intermediate chamber/product chamber, the liquid that had passed through the second intermediate chamber/product chamber was circulated through the desalting chamber/first raw material chamber, the raw liquid was circulated through the second raw material chamber, and the raw liquid was circulated through the third raw material chamber. Except for this, the increase in the amount of liquid that had circulated through the concentration chamber was obtained as the concentrated liquid. Electrodialysis was performed in the same manner as in the previous example, and excellent results were obtained.

The iodide ion separation method of the present invention is a method for economically obtaining iodide ions with a high yield from underground brine associated with natural gas that contains iodide ions by using electrodialysis. In particular, this method makes it possible to obtain iodide ions with a high yield from brine that has a low iodide ion concentration and has not been used for iodine extraction until now.

In addition, because no chlorine-based oxidizing agents are used in the iodine separation, the discharged brine after iodine extraction does not contain residual chlorine or organic chlorine compounds. This is an excellent method for protecting the environment. In addition, because it does not contain oxidizing substances such as residual chlorine, it is also extremely advantageous when injecting the brine into a brine aquifer to prevent land subsidence. Furthermore, this discharged brine is a mild saltwater that can easily be used for aquaculture and culture.

Therefore, the method for separating iodide ions of the present invention has industrial applicability.
The iodide ion separated by this invention is purified as iodine, an important industrial product.

LIST OF SYMBOLS 1 First electrodialysis cell 2 Positive electrode 3 Negative electrode 4c Cation exchange membrane 5a First anion exchange membrane 6a Second anion exchange membrane 7a Third anion exchange membrane 8c Cation exchange membrane 10 Positive electrode chamber 11 Product chamber 12 First raw material chamber 13 Second raw material chamber 14 Third raw material chamber 15 Negative electrode chamber 21 Raw material liquid 22 Product liquid 23 Treatment liquid 24 Positive electrode liquid 25 Negative electrode liquid 31 Second electrodialysis cell 32 Positive electrode 33 Negative electrode 34c Cation exchange membrane 35c First cation exchange membrane 36c Second cation exchange membrane 37a First anion exchange membrane 38a Second anion exchange membrane 39c Cation exchange membrane 40 Positive electrode chamber 41 Deionization chamber 42 First intermediate chamber 43 Concentration chamber 44 Second intermediate chamber 45 Deionization chamber 46 Anode chamber 48 Treated liquid 49 Concentrated liquid 50 Treated liquid 51 Treated liquid 52 Positive electrode liquid 53 Negative electrode liquid 61 Dialysis wastewater 62 Obtained concentrated liquid

Claims (5)

A method for separating iodide ions by electrodialysis from a raw material solution that is an aqueous solution containing iodide ions, comprising the steps of:
Between the positive electrode and the negative electrode, from the positive electrode side to the positive electrode chamber,
When n is an integer of 2 or more, an electrodialysis cell is used in which a plurality of membrane chamber sets are arranged, each set being a (n+1) membrane (n+1) chamber including a cation exchange membrane, a product chamber, a first anion exchange membrane which is an anion exchange membrane, a first raw material chamber, a second anion exchange membrane which is an anion exchange membrane, a second raw material chamber, ... an nth anion exchange membrane which is an anion exchange membrane, and an nth raw material chamber, and a cation exchange membrane and a negative electrode chamber are successively arranged,
A method for separating iodide ions, comprising passing the raw material liquid through all of the chambers other than the positive electrode chamber and the negative electrode chamber in a single pass or in circulation, and obtaining the raw material liquid that has passed through the product chamber as a product liquid. A method for separating iodide ions by electrodialysis from a raw material solution that is an aqueous solution containing iodide ions, 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 a plurality of membrane chamber sets are arranged, each set consisting of a first cation exchange membrane which is a cation exchange membrane, a first intermediate chamber, a second cation exchange membrane which is a cation exchange membrane, a concentration chamber, a first anion exchange membrane which is an anion exchange membrane, a second intermediate chamber, a second anion exchange membrane which is an anion exchange membrane, and a desalting chamber, and in which a cation exchange membrane and a negative electrode chamber are successively arranged;
A method for separating iodide ions, characterized in that a concentrated liquid is passed through the concentration compartment by circulation, the raw material liquid or the product liquid obtained by subjecting the raw material liquid to the method described in claim 1 is passed through the first intermediate compartment by one pass or circulation, the liquid that has been passed through the first intermediate compartment by one pass or circulation is passed through the second intermediate compartment by one pass or circulation, and the liquid that has been passed through the second intermediate compartment by one pass or circulation is passed through the desalting compartment by one pass or circulation. A method for separating iodide ions by electrodialysis from a raw material solution that is an aqueous solution containing iodide ions, comprising the steps of:
Between the positive electrode and the negative electrode, from the positive electrode side to the positive electrode chamber,
When n is an integer of 2 or more, an electrodialysis cell is used in which a plurality of membrane chamber sets are arranged, each set including a first cation exchange membrane which is a cation exchange membrane, a first intermediate chamber, a second cation exchange membrane which is a cation exchange membrane, a concentration chamber, a first anion exchange membrane which is an anion exchange membrane, a second intermediate chamber/product chamber, a second anion exchange membrane which is an anion exchange membrane, a desalting chamber/first raw material chamber, a third anion exchange membrane which is an anion exchange membrane, a second raw material chamber, ... a (n+1) th anion exchange membrane which is an anion exchange membrane, and a (n+3) membrane (n+3) chamber of the nth raw material chamber, and a cation exchange membrane and a negative electrode chamber are arranged in succession;
The raw material liquid or the product liquid obtained by subjecting the raw material liquid to the method according to claim 1 is passed through the first intermediate chamber in a single pass or in a circulation manner, and a concentrated liquid is passed through the concentration chamber in a circulation manner;
The second intermediate chamber/product chamber is passed through the liquid that has passed through the first intermediate chamber in a single pass or in a circulation manner.
The liquid that has passed through the second intermediate chamber/product chamber is passed through the desalting chamber/first raw material chamber in a single pass or in circulation,
The raw material liquid or the product liquid obtained by subjecting the raw material liquid to the method according to claim 1 is passed through the second raw material chamber to the nth raw material chamber in a single pass or in a circulation manner,
A method for separating iodide ions, comprising recovering an increased amount of the liquid circulated through the concentrating compartment as the concentrated liquid. The method for separating iodide ions according to claims 1 to 3, wherein the raw material liquid is water-soluble natural gas associated brine. The method for separating iodide ions according to claims 1 to 4, wherein all of the cation exchange membranes used in the electrodialysis cell are monovalent cation selective exchange membranes, and when the raw material liquid contains sulfate ions, the anion exchange membranes used in the electrodialysis cell are monovalent anion selective permeable membranes.

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