JP6202608B2 - Fluoride ion removal method - Google Patents

Description

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

  Conventionally, in order to remove fluorine ions in recovered water such as cleaning water for semiconductors, lenses, liquid crystals, etc., a plurality of anion exchange membranes and cation exchange membranes are alternately arranged between electrodes, and a concentration chamber and a desalination chamber are provided. Alternating electrode demineralizers are often used. The electric desalination equipment is capable of efficient desalination treatment, does not require regeneration like an ion exchange resin, is capable of complete continuous water collection, and has the excellent effect of obtaining extremely high purity water. Play. In addition, in the electric desalination apparatus, there are a desalting chamber in which an anion exchange resin and a cation exchange resin are mixed and filled, and a desalting chamber in which an ion exchange resin is not filled, In terms of improving the quality of treated water, it is more effective to have a desalting chamber filled with an ion exchange resin.

  In the electric desalination apparatus, the fluorine ions in the raw water that flowed into the desalting chamber move in the direction of the electrode to which the potential is applied based on the affinity, concentration and mobility (perpendicular to the flow of water to be treated) Furthermore, it moves across the cation exchange membrane or anion exchange membrane that separates the desalting chamber and the concentration chamber, so that neutralization of charges is maintained in all the chambers. Then, due to the semi-permeation characteristics and potential of the ion exchange membrane, the fluoride ions in the raw water are reduced in the desalting chamber and concentrated in the adjacent concentration chamber. For this reason, desalted water is recovered from the desalting chamber.

  In the electric desalination system, the increase in the water flow differential pressure in the concentration chamber can be secured to a certain level by increasing the operating pressure by operating the valve without chemical cleaning to some extent. The flow distribution of water becomes non-uniform, leading to the precipitation of poorly soluble substances, the generation of microorganisms and the like, and further increases in pressure. In addition, when the pressure balance between the desalting chamber and the concentrating chamber is lost, the concentrated water leaks through the ion exchange membrane to the desalting chamber, which causes a change in the quality of the treated water. In order to prevent such troubles in advance, it is customary to clean the electric demineralizer with chemicals periodically or when the differential pressure rise exceeds a certain limit. This is inconvenient both in terms of operation management and cost.

  Patent Document 1 includes an example in which “NEPSEPTA CM-2” is used as a cation exchange membrane and “NEOSEPTTA AM-1” is used as an anion exchange membrane in removing fluorine ions from an aqueous solution containing ions by electrodialysis. (Example 1).

JP-A-7-265864

  However, the ion exchange membrane disclosed in Patent Document 1 has a problem that the effect of preventing slime generation due to microbial growth in the concentration chamber of the electric desalting apparatus is not sufficient.

  In order to efficiently remove fluorine ions with an electrodesalination apparatus, the present inventors performed electrodialysis defluorination ion treatment using a cation exchange membrane excellent in organic contamination resistance as a cation exchange membrane. We thought it would be effective to do this and set it as a problem to be solved.

  As a result of intensive studies on the above problems, the inventors of the present invention include, as a cation exchange membrane, an anionic polymer segment having an anionic group and a vinyl alcohol copolymer having a vinyl alcohol polymer segment, and a domain size. By using a cation exchange membrane having a microphase separation structure in which (X) is in the range of 0 nm <X ≦ 150 nm, the electrodialysis apparatus has excellent fluorine ion removability, excellent organic contamination resistance, and long-term use. As a result, the present invention has been reached.

  The present invention provides a method for removing fluorine ions from an aqueous solution containing fluorine ions by an electrodialysis apparatus in which a cation exchange membrane and an anion exchange membrane are alternately arranged between a cathode and an anode. A microphase separation in which an anionic polymer segment having an anionic group and a vinyl alcohol copolymer having a vinyl alcohol polymer segment are contained, and the domain size (X) is in the range of 0 nm <X ≦ 150 nm A method for removing fluorine ions, comprising using a cation exchange membrane having a structure.

  The vinyl alcohol polymer segment is a segment formed from a vinyl alcohol polymer not containing an anionic group, and electrodialysis treatment is performed using a cation exchange membrane containing a vinyl alcohol copolymer having the segment. It is preferred to do so.

  It is preferable that a cross-linked structure is introduced into the vinyl alcohol copolymer.

  The cross-linked structure is preferably introduced by reacting the vinyl alcohol copolymer with a dialdehyde compound.

  The vinyl alcohol copolymer may be an anionic block copolymer having a vinyl alcohol polymer block and an anionic polymer block having an anionic group.

  The vinyl alcohol copolymer may be an anionic graft copolymer having a vinyl alcohol polymer block and an anionic polymer block having an anionic group.

  The ion exchange capacity of the cation exchange membrane is preferably 0.30 meq / g or more.

The membrane resistance of the cation exchange membrane is preferably 50 Ωcm ² or less.

  It is preferable that the pre-electrodialysis apparatus is an electrodesalting apparatus in which an ion exchanger is accommodated in a desalting chamber.

  According to the present invention, in the method for removing fluorine ions from an aqueous solution containing fluorine ions by an electrodialysis apparatus in which a cation exchange membrane and an anion exchange membrane are alternately arranged between a cathode and an anode, the cation The exchange membrane contains an anionic polymer segment having an anionic group and a vinyl alcohol copolymer having a vinyl alcohol polymer segment, and the domain size (X) is in the range of 0 nm <X ≦ 150 nm. By using a cation exchange membrane having a phase separation structure, the fluorine ion removal property is excellent, and the ion exchange membrane is excellent in organic contamination resistance, so that the electrodialysis apparatus can be operated for a long time. .

It is a transmission electron microscope (TEM) photograph of the ion exchange membrane (CEM-5) used in Example 5, which is an example of a cation exchange membrane used in the method of the present invention. It is a TEM photograph of the ion exchange membrane (CEM-7) used in Example 7, which is an example of a cation exchange membrane used in the method of the present invention. 4 is a TEM photograph of an ion exchange membrane (CEM-8) used in Comparative Example 1. 4 is a TEM photograph of an ion exchange membrane (CEM-9) used in Comparative Example 2. 10 is a TEM photograph of an ion exchange membrane (CEM-10) used in Comparative Example 3. It is explanatory drawing of the apparatus used for the measurement of the membrane resistance of the cation exchange membrane used in this invention.

(Removal of fluorine ions)
The present invention relates to an anionic polymer segment having an anionic group as a cation exchange membrane and a vinyl alcohol polymer segment when electrodialysis is used for removing fluorine ions in recovered water such as washing water for semiconductors, lenses, liquid crystals and the like. And a cation exchange membrane having a microphase separation structure having a domain size (X) in the range of 0 nm <X ≦ 150 nm. Therefore, the cation exchange membrane used in the present invention will be described in detail below.

(Cation exchange membrane)
The cation exchange membrane used in the present invention is composed of an anionic polymer segment having an anionic group and a vinyl alcohol copolymer having a vinyl alcohol polymer segment. Usually, an anionic polymer segment and a vinyl alcohol polymer segment are bonded by a covalent bond to constitute a vinyl alcohol copolymer. In the present invention, the polymer segment means a polymer chain containing the same repeating unit in which two or more of the same monomer units are linked, and is a polymer block in a block copolymer, a trunk chain or a branch in a graft polymer. It is used as a term encompassing polymer blocks corresponding to chains. Further, in the anionic polymer segment having an anionic group, the anionic group may be contained at the end of the polymer, and therefore the monomer having the anionic group may not be a repeating unit.
As described above, the cation exchange membrane used in the present invention is composed of an anionic polymer segment having an anionic group and a vinyl alcohol copolymer having a vinyl alcohol polymer segment. In addition to this copolymer, a phase separation of a vinyl alcohol polymer that does not have an anionic group that is not bonded to an anionic polymer segment, and an anionic polymer that is not bonded to a vinyl alcohol polymer. It may be included to the extent that it does not affect the structure.

  The cation exchange membrane used in the present invention is formed by reducing the content of salts contained in the vinyl alcohol copolymer, so that the vinyl alcohol copolymer constituting the membrane exhibits a microphase separation structure. The inventors have found that the domain size can be made 150 nm or less. The cation exchange membrane used in the present invention is usually provided for practical use by crosslinking the vinyl alcohol polymer segment, but the domain size (X) is specified in the range of 0 nm <X ≦ 150 nm, It is possible to obtain a cation exchange membrane useful for electrodialysis having no change in ion path structure, stable membrane structure, and excellent electrical characteristics such as charge density and membrane resistance. The domain size (X) becomes smaller as the salt content is lowered, and can be set to 0 nm <X ≦ 130 nm, and further 0 nm <X ≦ 100 nm.

  The cation exchange membrane used in the present invention is a hydrophilic ion exchange membrane because it is composed of a vinyl alcohol copolymer having a vinyl alcohol polymer segment as described above. This has the advantage that contamination due to adhesion of organic substances contained in the liquid to be treated can be suppressed. Here, that the constituent polymer is hydrophilic means that the polymer has hydrophilicity in a structure having no functional group (anionic group) necessary for the anionic polymer. As described above, since the constituent polymer is a hydrophilic polymer, a cation exchange membrane having a high degree of hydrophilicity is obtained, and organic contaminants in the liquid to be treated adhere to the cation exchange membrane and the performance of the membrane. The problem of lowering can be reduced. Moreover, it has the advantage that film | membrane intensity | strength becomes high.

(Anionic polymer)
The anionic polymer constituting the anionic polymer segment used in the present invention is a polymer containing an anionic group in the molecular chain. The anionic group may be contained in any of the main chain, side chain, and terminal. Examples of the anionic group include a sulfonate group, a carboxylate group, and a phosphonate group. In addition, functional groups that can be converted into sulfonate groups, carboxylate groups, and phosphonate groups at least partially in water, such as sulfonic acid groups, carboxyl groups, and phosphonic acid groups are also included in the anionic groups. Of these, a sulfonate group is preferred because of its large ion dissociation constant. The anionic polymer may contain only one type of anionic group or may contain a plurality of types of anionic groups. Moreover, the counter cation of an anionic group is not specifically limited, A hydrogen ion, an alkali metal ion, etc. are illustrated. Of these, alkali metal ions are preferred from the viewpoint of less equipment corrosion problems. The anionic polymer may contain only one type of counter cation or may contain a plurality of types of counter cation.

  The anionic polymer used in the present invention may be a polymer composed only of a structural unit containing the anionic group, or may be a polymer further containing a structural unit not containing the anionic group. Good. Moreover, it is preferable that these polymers have a crosslinking property. An anionic polymer may consist of only one type of polymer, or may include a plurality of types of anionic polymers. Moreover, you may be a mixture of these anionic polymers and another polymer. Here, the polymer other than the anionic polymer is preferably not a cationic polymer.

  As an anionic polymer, what has the structural unit of the following general formula (1) and (2) is illustrated.

[Wherein R ⁵ represents a hydrogen atom or a methyl group. G represents _{^{-SO 3 H, -SO 3 -M +}} , -PO 3 H, -PO 3 -M +, a -CO ₂ H or _-CO 2 -M ^+. M ⁺ represents an ammonium ion or an alkali metal ion. ]

  Examples of the anionic polymer containing the structural unit represented by the general formula (1) include 2-acrylamido-2-methylpropanesulfonic acid homopolymer or copolymer.

  [Wherein, R5 represents a hydrogen atom or a methyl group, and T represents a phenylene group or a naphthylene group in which the hydrogen atom may be substituted with a methyl group. G is synonymous with the general formula (1). ]

  Examples of the anionic polymer containing the structural unit represented by the general formula (2) include homopolymers or copolymers of p-styrene sulfonate such as sodium p-styrene sulfonate.

  Examples of the anionic polymer include homopolymers or copolymers of sulfonic acids such as vinyl sulfonic acid and (meth) allyl sulfonic acid or salts thereof, fumaric acid, maleic acid, itaconic acid, maleic anhydride, itaconic anhydride. Examples include dicarboxylic acids such as acids, homopolymers or copolymers of derivatives or salts thereof.

In the general formula (1) or (2), G is preferably a sulfonate group, a sulfonic acid group, a phosphonate group, or a phosphonic acid group that gives a higher charge density. In the general formulas (1) and (2), examples of the alkali metal ion represented by M ⁺ include sodium ion, potassium ion, and lithium ion.

(Vinyl alcohol polymer segment)
In the present invention, as the polyvinyl alcohol constituting the vinyl alcohol polymer segment, a vinyl ester polymer obtained by polymerizing a vinyl ester monomer is saponified and the vinyl ester unit is used as a vinyl alcohol unit. . Examples of the vinyl ester monomers include vinyl formate, vinyl acetate, vinyl propionate, vinyl valelate, vinyl laurate, vinyl stearate, vinyl benzoate, vinyl pivalate, vinyl versatate, and the like. Of these, vinyl acetate is preferably used.
When the vinyl ester monomer is copolymerized, a monomer that can be copolymerized, if necessary, within a range that does not impair the effects of the invention (preferably 50 mol% or less, more preferably 30 mol% or less). It may be copolymerized.
Examples of the monomer copolymerizable with the vinyl ester monomer include olefins having 3 to 30 carbon atoms such as ethylene, propylene, 1-butene and isobutene; acrylic acid and salts thereof; methyl acrylate and acrylic acid. Ethyl, n-propyl acrylate, i-propyl acrylate, n-butyl acrylate, i-butyl acrylate, t-acrylate
Acrylic acid esters such as butyl, 2-ethylhexyl acrylate, dodecyl acrylate, octadecyl acrylate; methacrylic acid and its salts; methyl methacrylate, ethyl methacrylate, n-propyl methacrylate, i-propyl methacrylate, Methacrylic acid esters such as n-butyl methacrylate, i-butyl methacrylate, t-butyl methacrylate, 2-ethylhexyl methacrylate, dodecyl methacrylate, octadecyl methacrylate; acrylamide, N-methylacrylamide, N-ethyl Acrylamide, N, N-dimethylacrylamide, diacetone acrylamide, acrylamide propanesulfonic acid and its salt, acrylamidopropyldimethylamine and its salt, N-methylolacrylamide and its derivatives Methacrylamide such as methacrylamide, N-methylmethacrylamide, N-ethylmethacrylamide, methacrylamidepropanesulfonic acid and its salt, methacrylamidepropyldimethylamine and its salt, N-methylolmethacrylamide and its derivatives Derivatives; N-vinylamides such as N-vinylformamide, N-vinylacetamide, N-vinylpyrrolidone; methyl vinyl ether, ethyl vinyl ether, n-propyl vinyl ether, i-propyl vinyl ether, n-butyl vinyl ether, i-butyl vinyl ether, t -Vinyl ethers such as butyl vinyl ether, dodecyl vinyl ether and stearyl vinyl ether; Nitriles such as acrylonitrile and methacrylonitrile Vinyl halides such as vinyl chloride, vinylidene chloride, vinyl fluoride and vinylidene fluoride; allyl compounds such as allyl acetate and allyl chloride; maleic acid and its salts or esters thereof; itaconic acid and its salts or esters thereof; Examples thereof include vinylsilyl compounds such as methoxysilane; isopropenyl acetate and the like.

  The structural units other than the anionic group in the vinyl alcohol copolymer can be selected independently, but the copolymer is preferably composed of monomers having the same structural unit. . Thereby, since the affinity between domains becomes high, the mechanical strength of a cation exchange membrane increases. The vinyl alcohol copolymer preferably has 50 mol% or more of the same structural unit, more preferably 70 mol% or more.

Moreover, since it is desirable that it is hydrophilic, it is particularly preferable that the same structural unit is a vinyl alcohol unit. By having a vinyl alcohol unit, the domains can be chemically crosslinked with a crosslinking agent such as glutaraldehyde, so that the mechanical strength of the cation exchange membrane can be further increased.

As described above, in the present invention, the vinyl alcohol copolymer having an anionic polymer segment and a vinyl alcohol polymer segment has a structure in which the anionic polymer segment and the vinyl alcohol polymer segment are bonded. preferable.

  The cation exchange membrane used in the present invention is composed of a vinyl alcohol copolymer having a vinyl alcohol polymer segment in the anionic polymer segment as described above. The vinyl alcohol copolymer is anionic. You may comprise from the mixture containing the vinyl alcohol polymer which does not contain group.

(Block or graft copolymer)
In the present invention, the vinyl alcohol copolymer is preferably such that the anionic polymer segment and the vinyl alcohol polymer segment form a block copolymer or a graft copolymer. Among these, a block copolymer is more preferably used. By doing this, the vinyl alcohol polymer block responsible for the function of improving the strength of the entire cation exchange membrane, suppressing the degree of swelling of the membrane, and maintaining the shape, and the amount of anionic group responsible for the permeation of the cation The anionic polymer block formed by polymerizing the polymer can share the role, and both the ion permeation function and the dimensional stability of the cation exchange membrane can be achieved. The structural unit of the polymer block obtained by polymerizing the anionic group monomer is not particularly limited, and examples thereof include those represented by the general formulas (1) to (2). Among these, from the viewpoint of easy availability, as the monomer constituting the anionic polymer, p-styrene sulfonate or 2-acrylamido-2-methylpropane sulfonate is used. Anionic block copolymer containing an anionic polymer block obtained by polymerizing an acid salt and a vinyl alcohol polymer block, or an anionic polymer obtained by polymerizing 2-acrylamido-2-methylpropanesulfonate A block copolymer comprising a block and a vinyl alcohol polymer block is preferably used.
The graft copolymer includes an anionic polymer segment as a backbone and a vinyl alcohol polymer segment as a branched chain, and a vinyl alcohol polymer segment as a backbone and an anionic polymer segment as a branch. Sometimes it is a chain. Although not particularly limited in the present invention, from the viewpoint of easily obtaining strength properties, vinyl alcohol as a backbone,
A graft copolymer having an anionic polymer segment as a branched chain is preferred. A known method is applied as the graft copolymerization method.

(Method for producing block copolymer)
The method for producing a block copolymer containing an anionic polymer block obtained by polymerizing an anionic monomer used in the present invention and a vinyl alcohol polymer block is mainly classified into the following two methods. . (1) a method in which a desired block copolymer is produced and then an anionic group is bonded to a specific block; and (2) at least one anionic group monomer is polymerized to form a desired block copolymer. It is a method for producing a polymer. Among these, for (1), one or more types of monomers are block copolymerized in the presence of polyvinyl alcohol having a mercapto group at the terminal, and then one or more types of polymer in the block copolymer are copolymerized. For the method of introducing an ionic group into the coalescing component, (2), a block copolymer is produced by radical polymerization of at least one anionic group monomer in the presence of polyvinyl alcohol having a mercapto group at the terminal. However, these methods are preferable because of industrial ease. In particular, the type and amount of constituent monomers in each block of an anionic polymer block obtained by polymerizing a vinyl alcohol block and an anionic group monomer in the block copolymer can be easily controlled. A method of producing a block copolymer by radical polymerization of at least one anionic group monomer in the presence of a group-containing polyvinyl alcohol is preferred. The block copolymer containing a polymer block obtained by polymerizing the anionic group monomer thus obtained and a vinyl alcohol polymer block contains unreacted polyvinyl alcohol having a mercapto group at the terminal. It doesn't matter.

The vinyl alcohol polymer having a mercapto group at the end used for the production of these block copolymers can be obtained, for example, by the method described in JP-A-59-187003. That is, a vinyl ester monomer in the presence of thiolic acid,
For example, a method of saponifying a vinyl ester polymer obtained by radical polymerization of vinyl acetate can be mentioned. A method for obtaining a block copolymer using a polyvinyl alcohol having a mercapto group at the terminal thus obtained and an anionic group monomer is described in, for example, JP-A-59-189113. Method. That is, a block copolymer can be obtained by radical polymerization of an anionic group monomer in the presence of polyvinyl alcohol having a mercapto group at the terminal. This radical polymerization can be carried out by a known method such as bulk polymerization, solution polymerization, pearl polymerization, emulsion polymerization, etc., but mainly contains a solvent capable of dissolving polyvinyl alcohol containing a mercapto group at the terminal, such as water or dimethyl sulfoxide. It is preferable to carry out in the medium. As the polymerization process, any of a batch method, a semi-batch method, and a continuous method can be employed.

The above radical polymerization may be performed by using a normal radical polymerization initiator such as 2,2′-azobisisobutyronitrile, benzoyl peroxide, lauroyl peroxide, diisopropyl peroxycarbonate, 4,4′-azobis- (4-cyano Pentanoic sodium), 4,4'-azobis- (4-cyanopentanoic ammonium), 4,4'-azobis- (4-cyanopentanoic potassium), 4,4'-azobis- (4- Cyanopentanoic lithium), 2,2'-azobis {2-methyl-N- [1,1'-bis (hydroxymethyl) -2-hydroxyethyl] propionamide}, potassium persulfate, ammonium persulfate, etc. Can be used by using one that is suitable for the polymerization system, but in the case of polymerization in an aqueous system, vinyl alcohol Initiation of redox by a mercapto group at the end of a polymer and an oxidizing agent such as potassium bromate, potassium persulfate, ammonium persulfate, hydrogen peroxide, 2'-
Azobis [2-methyl-N (2-hydroxyethyl) propionamide] and the like are also possible. In particular, those that do not generate ionic residues after decomposition are particularly preferred.

As a catalyst for the saponification reaction of a vinyl ester polymer, an alkaline substance is usually used.
Examples thereof include alkali metal hydroxides such as potassium hydroxide and sodium hydroxide, and alkali metal alkoxides such as sodium methoxide. The saponification catalyst is
It may be added all at once at the beginning of the saponification reaction, or a part thereof may be added at the beginning of the saponification reaction, and the rest may be added during the saponification reaction. Examples of the solvent used for the saponification reaction include methanol, methyl acetate, dimethyl sulfoxide, diethyl sulfoxide, dimethylformamide and the like. Of these solvents, methanol is preferred. The saponification reaction can be carried out by either a batch method or a continuous method. After completion of the saponification reaction, the remaining saponification catalyst may be neutralized as necessary, and usable neutralizing agents include organic acids such as acetic acid and lactic acid, and ester compounds such as methyl acetate. .

(Degree of saponification of vinyl alcohol polymer)
Although the saponification degree of a vinyl alcohol polymer is not specifically limited, It is preferable that it is 40-99.9 mol%. If the degree of saponification is less than 40 mol%, the crystallinity is lowered and the strength of the cation exchange membrane may be insufficient. More preferably, the saponification degree is 60 mol% or more, and 8
More preferably, it is 0 mol% or more. Usually, the saponification degree is 99.9 mol% or less. At this time, the saponification degree when the polyvinyl alcohol is a mixture of plural kinds of polyvinyl alcohols refers to the average saponification degree of the whole mixture. The saponification degree of polyvinyl alcohol is a value measured according to JIS K6726.

(Polyvinyl alcohol polymerization degree)
The viscosity average degree of polymerization of the polyvinyl alcohol constituting the vinyl alcohol polymer segment (hereinafter sometimes simply referred to as the degree of polymerization) is not particularly limited, but is preferably 50 to 10,000. When the degree of polymerization is less than 50, there is a possibility that the cation exchange membrane cannot maintain sufficient strength in practical use. More preferably, the degree of polymerization is 100 or more. The degree of polymerization is 10,000
If it exceeds 1, the viscosity of the polymer aqueous solution will be too high, it will be difficult to apply, and the resulting film may be prone to defects. The degree of polymerization is more preferably 8000 or less. At this time,
The degree of polymerization when the polyvinyl alcohol is a mixture of a plurality of types of polyvinyl alcohol refers to the average degree of polymerization of the entire mixture. In addition, the viscosity average polymerization degree of polyvinyl alcohol is a value measured according to JIS K6726. The polymerization degree of polyvinyl alcohol containing no ionic group used in the present invention is also preferably within the above range.

(Content of anionic group monomer unit)
The content of the anionic group monomer unit in the vinyl alcohol copolymer constituting the cation exchange membrane is not particularly limited, but the content of the anionic group monomer unit of the copolymer, that is, the above The ratio of the number of anionic group monomer units to the total number of monomer units in the copolymer is preferably 0.1 mol% or more. When the content of the anionic group monomer unit is less than 0.1 mol%, the effective charge density in the cation exchange membrane is lowered, and the electrolyte permselectivity may be lowered. The content of the anionic group monomer unit is more preferably 0.5 mol% or more, and further preferably 1 mol% or more. Moreover, it is preferable that content of an anionic group monomer unit is 50 mol% or less. When the content of the anionic group monomer unit exceeds 50 mol%, the degree of swelling of the cation exchange membrane increases, the effective charge density in the cation exchange membrane decreases, and the electrolyte permselectivity may decrease. There is. The content of the anionic group monomer unit is more preferably 30 mol% or less, and further preferably 20 mol% or less. When the vinyl alcohol copolymer is a mixture of a polymer containing an anionic group and a polymer containing no anionic group, or a mixture of a plurality of polymers containing an anionic group The content of anionic group monomer units refers to the ratio of the number of anionic group monomer units to the total number of monomer units in the mixture.

(Method for producing cation exchange membrane)
A feature of the method for producing a cation exchange membrane used in the present invention is that a vinyl alcohol polymer segment and a vinyl alcohol copolymer (preferably a block copolymer) having an anionic polymer segment having an anionic group as constituent components. The phase separation domain size is reduced by forming a film by reducing the salt in the copolymer and forming a film. The salts referred to herein have an anionic group that constitutes an anionic polymer segment having an anionic group, such as sulfate and acetate which are impurities contained in the polyvinyl alcohol constituting the vinyl alcohol polymer segment. Examples thereof include metal salts such as bromide salts, chloride salts, nitrates, and phosphates contained as impurities in the monomer. These salts are inevitably mixed as impurities in the vinyl alcohol copolymer, and the present inventors have mixed the impurities into the anionic heavy weight in the vinyl alcohol copolymer during film formation. It has been found that the microphase separation of the coalesced segment is increased to adversely affect the membrane characteristics. In the present invention, the phase separation domain size of the membrane can be reduced by reducing the content of salts contained in the vinyl alcohol copolymer to form a membrane. At this time, the ratio (weight ratio) (C) / (P) of the weight (C) of the salt to the weight (P) of the vinyl alcohol copolymer is required to be 4.5 / 95.5 or less. In order to reduce the separation domain size, it is 4.0 / 96.0 or less, more preferably 3.5 / 96.5 or less. When the weight ratio (C) / (P) exceeds 4.5 / 95.5, the microphase separation domain size of the anionic group polymer segment increases, and when used as a cation exchange membrane, the ion path structure changes. As a result, a durable cation exchange membrane cannot be obtained.

Although there is no particular limitation on reducing the salt content contained in the vinyl alcohol copolymer below a predetermined value, for example, impurities contained in the polyvinyl alcohol constituting the vinyl alcohol polymer segment may be polymer flakes. Can be reduced by washing with water.
In addition, impurities contained in the polymer constituting the anionic polymer segment can be reduced by reprecipitation purification in a poor solvent of a polymer solution obtained by dissolving the polymer in an appropriate solvent, thereby purifying the polymer. Can do.
In the present invention, the salt content only needs to be reduced as described above. Therefore, one or both of polyvinyl alcohol and anionic polymer are purified to reduce the content to the above-described content. Just do it.

(Film formation)
The vinyl alcohol copolymer having a salt content adjusted as described above is dissolved in a solvent composed of water, a lower alcohol such as methanol, ethanol, 1-propanol, 2-propanol, or a mixed solvent thereof, A film having a predetermined thickness can be formed by extruding from a die, forming into a film, and removing the solvent by volatilization. The film forming temperature when the film is formed on a plate or a roller is not particularly limited, but a temperature range of about room temperature to about 100 ° C. is usually appropriate. Solvent removal can be performed by heating as appropriate.

(Film thickness)
The cation exchange membrane used in the present invention preferably has a thickness of about 30 to 1000 μm from the viewpoints of performance, mechanical strength, handling properties, etc. required as an electrolyte membrane for electrodialysis. When the film thickness is less than 30 μm, the mechanical strength of the film tends to be insufficient. On the other hand, when the film thickness exceeds 1000 μm, the membrane resistance increases and sufficient ion exchange properties are not exhibited, so that the electrodialysis efficiency tends to decrease. Preferably it is 40-500 micrometers, More preferably, it is 50-300 micrometers.

(Crosslinking treatment)
The cation exchange membrane used in the present invention is preferably subjected to a crosslinking treatment after film formation. By performing the crosslinking treatment, the mechanical strength of the resulting cation exchange membrane is increased. The method for the crosslinking treatment is not particularly limited, and may be a method of chemically bonding the molecular chains of the polymer or introducing physical bonds by heat treatment or the like.

When chemically bonding, a method of immersing in a solution containing a crosslinking agent is usually used. Examples of the crosslinking agent include polyvinyl alcohol acetalizing agents such as glutaraldehyde, formaldehyde, and glyoxal. Among them, dialdehyde crosslinking agents such as glutaraldehyde and gliogital are preferable. The concentration of the crosslinking agent is usually 0.001 to 10% by volume of the crosslinking agent relative to the solution. The crosslinking reaction can be performed by treating the polyvinyl alcohol copolymer with the above aldehyde in water, alcohol or a mixed solvent thereof under acidic conditions to chemically introduce a crosslinking bond. . After the crosslinking reaction, it is preferable to remove unreacted aldehyde, acid and the like by washing with water.

Moreover, as a method for the crosslinking treatment, physical crosslinking may be introduced between the molecular chains by performing a heat treatment. By performing the heat treatment, physical crosslinking occurs, and the mechanical strength of the resulting ion exchange membrane is increased. The method of heat treatment is not particularly limited, and a hot air dryer or the like is generally used. Although the temperature of heat processing is not specifically limited, In the case of polyvinyl alcohol, it is preferable that it is 50-250 degreeC. If the temperature of the heat treatment is less than 50 ° C., the mechanical strength of the obtained ion exchange membrane may be insufficient. The temperature is more preferably 80 ° C. or higher, and further preferably 100 ° C. or higher. When the temperature of the heat treatment exceeds 250 ° C., the crystalline polymer may be melted. The temperature is more preferably 230 ° C. or less, and further preferably 200 ° C. or less.

In the manufacturing method, both heat treatment and chemical crosslinking treatment may be performed, or only one of them may be performed. When both the heat treatment and the crosslinking treatment are performed, the crosslinking treatment may be performed after the heat treatment, the heat treatment may be performed after the crosslinking treatment, or both may be performed simultaneously.
After the heat treatment, a crosslinking treatment is performed. In particular, a film obtained by forming a film from a solution in which a vinyl alcohol copolymer solution is dissolved is heat-treated at a temperature of 100 ° C. or higher, and then water, alcohol, or a mixed solvent thereof. Among them, it is preferable from the viewpoint of mechanical strength of an ion exchange membrane obtained by performing a crosslinking treatment with a dialdehyde compound under acidic conditions.

(Ion exchange capacity)
In order to express sufficient ion exchange properties for use as a cation exchange membrane for electrodialysis, the ion exchange capacity of the resulting vinyl alcohol copolymer is preferably 0.30 meq / g or more, More preferably, it is 0.50 meq / g or more. The upper limit of the ion exchange capacity of the vinyl alcohol copolymer is preferably 3.0 meq / g or less because if the ion exchange capacity becomes too large, hydrophilicity increases and it becomes difficult to suppress the degree of swelling.

(Anion exchange membrane used in electrodialysis treatment)
In the electrodialysis treatment of the present invention, the anion exchange membrane used together with the cation exchange membrane is not particularly limited, and is a membrane made of a polymer having a strongly basic group such as a quaternary ammonium group, a primary grade. A film made of a polymer having a weakly basic functional group such as an amino group, a secondary amino group, or a tertiary amino group can be appropriately selected and used.

(Electrodialysis)
In the method for removing fluorine ions in the present invention, the electrodialysis tank is constituted by arranging a cation exchange membrane and an anion exchange membrane at least one of the cation membrane in the present invention between the anode and the cathode. A known electrodialysis tank can be used without particular limitation as long as it has a basic structure. For example, an anion exchange membrane and a cation exchange membrane are alternately arranged, such as a filter press type or unit cell type having a basic structure in which a desalination chamber and a concentration chamber are formed by the ion exchange membrane and a chamber frame. Such an electrodialysis tank can be preferably used. Note that the number of membranes used in the electrodialysis tank or the channel interval (membrane interval) between the desalting chamber and the concentrating chamber is appropriately selected depending on the type of liquid to be processed and the processing amount.

(Ion exchanger)
In the present invention, in the desalting chamber, a gap formed by the cation exchange membrane and the anion exchange membrane is filled with a cation exchange resin, an anion exchange resin, or a mixture thereof, preferably a mixture, as an ion exchanger. doing. As a result, when a DC voltage is applied between the electrodes, in the desalting chamber, fluorine ions in the water to be treated are adsorbed and removed by the ion exchanger filled in the desalting chamber, and then ionized by an electric propulsion force. It is desorbed from the exchanger, passes through the ion exchange membrane, moves to the concentration chamber, and is discharged. On the other hand, the treated water from which the fluorine ions have been removed flows out from the desalting chamber.
The cation exchange resin or anion exchange resin used may be in the form of a granule, or may be a porous body formed by forming the ion exchange resin into a plate shape using a binder polymer. In any case, when a mixture of cation exchange resin and anion exchange resin is used, the ratio of the total amount of cation exchange resin particles and the total amount of anion exchange resin particles accommodated in one desalting chamber is the total ion It is preferable that cation exchange resin / anion exchange resin = 30/70 to 80/20 in terms of exchange capacity ratio. If the total ion exchange capacity ratio is outside this range, the purity of the treated water may be lowered, which is not preferable.

  Hereinafter, examples will be given to describe the present invention in more detail, but the present invention is not limited to these examples. In the examples, “%” and “parts” are based on weight unless otherwise specified.

  The characteristics of the cation exchange membranes shown in Examples and Comparative Examples were measured by the following methods.

1) Membrane moisture content (H)
The dry weight of the ion exchange membrane was measured in advance, and then the wet weight was measured when it was immersed in deionized water and reached a swelling equilibrium. The membrane water content (H) was calculated by the following equation. _{H = <(W w -D w} ) / 1.0> / <(W w -D w) / 1.0+ (D w /1.3)>
Here, 1.0 and 1.3 indicate the specific gravity of water and polymer, respectively.
-H: membrane water content [-]
_Dw : dry weight of membrane [g]
W _w : wet weight of the film [g]

2) Measurement of cation exchange capacity A cation exchange membrane is immersed in a 1 mol / L aqueous HCl solution for 10 hours or more. Thereafter, the hydrogen ion type was replaced with the sodium ion type with 1 mol / L NaNO ₃ aqueous solution, and the liberated hydrogen ions were quantified by acid-base titration (Amol).

Next, the same cation exchange membrane is immersed in a 1 mol / L NaCl aqueous solution for 4 hours or more, washed thoroughly with ion exchange water, dried in a hot air drier at 105 ° C. for 8 hours, and the weight when dried W (G) was measured.
The ion exchange capacity was calculated by the following formula.
・ Ion exchange capacity = A × 1000 / W [mmol / g-dry membrane]

3) Measurement of the salt in the polyvinyl alcohol copolymer The amount of the salt in the polyvinyl alcohol copolymer was determined by Soxhlet extraction of the film before crosslinking of the polyvinyl alcohol copolymer with a methanol solution for 48 hours. The product was dried and then measured by ion chromatography ICS-5000 (manufactured by DIONEX).

4) Measurement of membrane resistance The membrane resistance was measured by sandwiching a cation exchange membrane in a two-chamber cell having a platinum black electrode plate as shown in Fig. 2, and filling a 0.5 mol / L-NaCl solution on both sides of the membrane. The resistance between the electrodes at 25 ° C. was measured by (frequency 10 cycles / second), and was determined by the difference between the resistance between the electrodes and the resistance between the electrodes when no cation exchange membrane was installed. The membrane used for the above measurement was previously equilibrated in a 0.5 mol / L-NaCl solution.

5) Measurement of domain size A cation exchange membrane immersed in distilled water was cut into a 1 cm square to prepare a measurement sample. This measurement sample was stained with lead acetate (II) and then observed using a TEM (transmission electron microscope) to obtain an image of a particle group in the measurement sample. The obtained image was subjected to image processing using image processing software “WINROOF” manufactured by Mitani Corporation, and the maximum particle size of each particle was determined. The maximum particle size was determined for about 400 particles, and the particle size at which the cumulative frequency of the maximum particle size was 50% was defined as the domain size of the anionic group polymer segment of the cation exchange membrane.

<Preparation of PVA-1 (Synthesis of polyvinyl alcohol copolymer having a mercapto group at the molecular terminal)>
Polyvinyl alcohol (PVA-1) having a mercapto group at the molecular end shown in Table 1 was synthesized by the method described in JP-A-59-187003. Table 1 shows the polymerization degree and saponification degree of PVA-1.

<NaSS-1 (anionic monomer)>
The polystyrene sulfonate sodium monomer (NaSS: manufactured by Tosoh Corporation) shown in Table 2 was used as it was. The salt content was measured by ion chromatography ICS-5000 (manufactured by DIONEX). The impurities in the monomer other than the total salt amount shown in Table 2 were moisture.

<Preparation of NaSS-2>
1000 g of sodium polystyrene sulfonate monomer (NaSS: manufactured by Tosoh Corporation), 950 g of pure water, 40 g of sodium hydroxide, and 1 g of sodium nitrate were dissolved at 60 ° C. for 1 hour, and cooled to 20 ° C. for recrystallization. Thereafter, the polystyrenesulfonic acid monomer crystals were separated by centrifugal filtration, and the crystals were dried to obtain NaSS-2 (purified polystyrenesulfonic acid monomer) shown in Table 2. The salt content was measured by ion chromatography ICS-5000 (manufactured by DIONEX).

<Synthesis of P-1 (anionic block copolymer)>
In a 1 L four-necked separable flask equipped with a reflux condenser and a stirring blade, 660 g of water, 80 g of PVA-1 shown in Table 1 as a vinyl alcohol polymer having a mercapto group at the end, and 46. 6 g was charged and heated to 95 ° C. with stirring to dissolve the vinyl alcohol polymer and NaSS-1. The system was purged with nitrogen for 30 minutes while bubbling nitrogen into the aqueous solution. After nitrogen substitution, the solution was cooled to 90 ° C., and 13 ml of a 5.4% solution of 2,2′-azobis [2-methyl-N (2-hydroxyethyl) propionamide] was sequentially added to the above aqueous solution over 1.5 hours. After the addition and the block copolymerization was started and proceeded, the system temperature was maintained at 90 ° C. for 1 hour to further proceed the polymerization, followed by cooling to a solid content concentration of 15% PVA- (b) -p -A sodium styrenesulfonate aqueous solution was obtained. A part of the obtained aqueous solution was dried, dissolved in heavy water, and subjected to ¹ H-NMR measurement at 400 MHz. As a result, the amount of modification of the sodium parastyrenesulfonate unit was 10 mol%. Table 3 shows the properties of the obtained anionic block copolymer.

<Synthesis of P-2>
The type and amount of the anionic group-containing monomer were changed to those shown in Table 3. Except for this, a PVA- (b) -p-sodium styrenesulfonate aqueous solution having a solid concentration of 15% was obtained in the same manner as P-1. Table 3 shows the properties of the obtained anionic block copolymer.

<Synthesis of P-3>
In a 1 L four-necked separable flask equipped with a reflux condenser and a stirring blade, 616 g of water, 80 g of PVA-1 shown in Table 1 as a vinyl alcohol polymer having a mercapto group at the end, and 46. NaSS-1 were obtained. 6 g was charged and heated to 95 ° C. with stirring to dissolve the vinyl alcohol polymer and NaSS-1, and then cooled to room temperature. 1/2 N sulfuric acid was added to the aqueous solution to adjust the pH to 3.0. The system was heated to 90 ° C., and the system was purged with nitrogen for 30 minutes while bubbling nitrogen into the aqueous solution. After nitrogen substitution, 63 mL of a 2.5% aqueous solution of potassium persulfate was sequentially added to the aqueous solution over 1.5 hours to start and proceed with block copolymerization, and then the system temperature was increased to 90 ° C. for 1 hour. The polymerization was further continued to proceed, followed by cooling to obtain a PVA- (b) -p-sodium styrenesulfonate block copolymer aqueous solution having a solid concentration of 15%. A part of the obtained aqueous solution was dried, dissolved in heavy water, and subjected to ¹ H-NMR measurement at 400 MHz. As a result, the amount of modification of the sodium p-styrenesulfonate unit was 10 mol%. Table 3 shows the properties of the obtained anionic block copolymer.

<Synthesis of P-4 to 5>
The type and amount of the anionic group-containing monomer were changed to those shown in Table 3. Except for this, a PVA- (b) -p-sodium styrenesulfonate aqueous solution having a solid concentration of 15% was obtained in the same manner as P-3. Table 3 shows the properties of the obtained anionic block copolymer.

<Synthesis of P-6>
A 6 L separable flask equipped with a stirrer, temperature sensor, dropping funnel and reflux condenser was charged with 2450 g of vinyl acetate, 762 g of methanol, and 27 g of sodium 2-acrylamido-2-methylpropanesulfonate (AMPS) under stirring. After the system was purged with nitrogen, the internal temperature was raised to 60 ° C. To this system, 20 g of methanol containing 0.8 g of 2,2′-azobisisobutyronitrile (AIBN) was added to initiate the polymerization reaction. While adding 567 g of a methanol solution containing 25% by mass of sodium 2-acrylamido-2-methylpropanesulfonate (AMPS) from the start of polymerization, the polymerization reaction was stopped for 4 hours, and then the polymerization reaction was stopped. The solid content concentration in the system when the polymerization reaction was stopped, that is, the solid content with respect to the entire polymerization reaction slurry was 30% by mass. Subsequently, methanol vapor was introduced into the system to drive out unreacted vinyl acetate monomer, thereby obtaining a methanol solution containing 30% by mass of a vinyl ester copolymer.

  In a methanol solution containing 30% by mass of this vinyl ester copolymer, the molar ratio of sodium hydroxide to vinyl acetate units in the copolymer is 0.02, and the solid content concentration of the vinyl ester copolymer is 30% by mass. Then, a methanol solution containing 10% by mass of methanol and sodium hydroxide was added in this order with stirring, and the saponification reaction was started at 40 ° C.

Immediately after the saponification reaction has occurred, a gelled product is formed and taken out from the reaction system and pulverized. Then, when 1 hour has passed since the gelated product was formed, methyl acetate was added to the pulverized product. By carrying out neutralization, an aqueous solution of a random copolymer of polyvinyl alcohol-poly (sodium 2-acrylamido-2-methylpropanesulfonate) in a swollen state was obtained. To this swollen anionic polymer, 6 times the amount of methanol (6 times the bath ratio) of methanol was added and washed under reflux for 1 hour, and the polymer was collected by filtration. The polymer was dried at 65 ° C. for 16 hours. When the obtained polymer was dissolved in heavy water and subjected to ¹ H-NMR measurement at 400 MHz, the content of the anionic monomer in the anionic polymer, that is, the monomer in the polymer The ratio of the number of sodium 2-acrylamido-2-methylpropanesulfonate monomer units to the total number of units was 5 mol%. Table 4 shows the properties of the obtained anionic random copolymer.

<Production of CEM-1>
The resin of P-3 was diluted to a concentration of 10 wt%, and the resin from which salts were removed by reprecipitation with 1 volume of methanol of the resin solution was taken out. At this time, the salt content (C) was 4.5 wt%. Next, a necessary amount of distilled water was added to prepare an aqueous solution having a concentration of 15 wt%. This aqueous solution was poured into a cast board made of acrylic having a length of 270 mm and a width of 210 mm to remove excess liquid and bubbles, and then dried on a hot plate at 50 ° C. for 24 hours to prepare a film.

  The film thus obtained was heat treated at 160 ° C. for 30 minutes to cause physical crosslinking. Subsequently, the film was immersed in an aqueous electrolyte solution of 2 mol / L sodium sulfate for 24 hours. Concentrated sulfuric acid was added to the aqueous solution so that the pH was 1, and then the film was immersed in a 1.0% by volume glutaraldehyde aqueous solution and stirred with a stirrer at 25 ° C. for 24 hours to carry out a crosslinking treatment. Here, as the glutaraldehyde aqueous solution, a product obtained by diluting “glutaraldehyde” (25% by volume) manufactured by Ishizu Pharmaceutical Co., Ltd. with water was used. After the cross-linking treatment, the film was immersed in deionized water, and the film was immersed until the film reached a swelling equilibrium while exchanging the deionized water several times in the middle to obtain a cation exchange membrane.

(Evaluation of ion exchange membrane)
The cation exchange membrane thus prepared was cut into a desired size to prepare a measurement sample. Using the obtained measurement sample, the membrane water content, the cation exchange capacity, the membrane resistance, and the phase separation domain size were measured according to the above methods. The results obtained are shown in Table 5.

<Production of CEM-2>
The resin of P-3 was diluted to a concentration of 10 wt%, and the resin from which salts were removed by reprecipitation with twice the volume of methanol as the resin solution was taken out. Next, a necessary amount of distilled water was added to prepare an aqueous solution having a concentration of 15 wt%. This aqueous solution was poured onto an acrylic cast plate having a length of 270 mm and a width of 210 mm to remove excess liquid and bubbles, and then dried on a hot plate at 50 ° C. for 24 hours to prepare a film. At this time, the salt content (C) was 4.0 wt%. Except for this, the membrane characteristics of the cation exchange membrane were measured in the same manner as CEM-1. The obtained measurement results are shown in Table 5.

<Preparation of CEM-3>
An aqueous solution of P-2 was poured onto an acrylic cast plate having a length of 270 mm and a width of 210 mm to remove excess liquid and bubbles, and then dried on a hot plate at 50 ° C. for 24 hours to prepare a film. At this time, the salt content (C) was 2.8 wt%. Except for this, the membrane characteristics of the cation exchange membrane were measured in the same manner as CEM-1. The obtained measurement results are shown in Table 5.

<CEM-4 production>
In CEM-3, the membrane characteristics of the cation exchange membrane were measured in the same manner as in CEM-3 except that the heat treatment temperature was changed as shown in Table 5. The obtained measurement results are shown in Table 5.

<CEM-5 production>
The resin of P-3 was diluted to a concentration of 10 wt%, and the resin from which salts were removed by reprecipitation with 5 times the volume of methanol of the resin solution was taken out. Next, a necessary amount of distilled water was added to prepare an aqueous solution having a concentration of 15 wt%. This aqueous solution was poured onto an acrylic cast plate having a length of 270 mm and a width of 210 mm to remove excess liquid and bubbles, and then dried on a hot plate at 50 ° C. for 24 hours to prepare a film. At this time, the salt content (C) was 2.0 wt%. Except for this, the membrane characteristics of the cation exchange membrane were measured in the same manner as CEM-1. The obtained measurement results are shown in Table 5.

<CEM-6 production>
The P-3 resin was diluted to a concentration of 10 wt%, and the resin from which salts were removed by reprecipitation with 10 times the volume of methanol of the resin solution was taken out. Next, a necessary amount of distilled water was added to prepare an aqueous solution having a concentration of 15 wt%. This aqueous solution was poured into a cast board made of acrylic having a length of 270 mm and a width of 210 mm to remove excess liquid and bubbles, and then dried on a hot plate at 50 ° C. for 24 hours to prepare a film. At this time, the salt content (C) was 1.5 wt%. Except for this, the membrane characteristics of the cation exchange membrane were measured in the same manner as CEM-1. The obtained measurement results are shown in Table 5.

<Production of CEM-7 to 11>
The membrane characteristics of the cation exchange membrane were measured in the same manner as CEM-1 except that the cation exchange resin was changed to the contents shown in Table 5. The obtained measurement results are shown in Table 5.

  1 (a), 1 (b), 1 (c), 1 (d) and 1 (e) are TEM photographs of cation exchange membranes using block copolymers having different salt contents. (See Table 5 for salt content). From the TEM photographs of FIGS. 1 (a) to 1 (e), the phase separation structure varies depending on the salt content, and the domain size decreases with decreasing salt-containing weight (C) / block copolymer weight (P). I understand that In particular, in FIG. 1 (b) [CEM-7] in which the weight (C) of the contained salt is the smallest at a modification amount of 10 mol%, the compatibility of the film is improved and the domain size is very small, 4 nm. . On the other hand, in FIG. 1 (c) [CEM-8] having the largest salt content (C), phase separation was severe and voids were generated.

From the results in Table 5, the cation exchange membrane using the block copolymer (P) having a salt content weight (C) of 4.5 % or less is a membrane having a domain size of 150 nm or less and a low membrane resistance. It can be seen that it is excellent as a cation exchange membrane (CEM-1 to 7). Furthermore, it can be seen that a film having a salt-containing weight (C) of 4.0 % or less has a domain size of 130 nm or less and a low film resistance (CEM-2 to 7). On the other hand, the size torquecontrol ion exchange membrane than the domain size is 150nm has a high film resistance, characteristics of the cation exchange membrane was not expressed (CEM-8~10). In particular, when the salt content (C) is high, the phase separation of the polymer segment of the membrane is severe, voids are generated in the entire ion exchange membrane, and satisfactory characteristics as an ion exchange membrane are not exhibited (CEM-8). . In CEM9, purified NaSS-2 is used, but since KPS is used as a polymerization initiator, the salt content in the obtained cation exchange resin is high.
On the other hand, the membrane (CEM-11) using the random block copolymer P-6, which was not confirmed to be completely compatible with the microphase separation, is a block copolymer P having a small phase separation domain size. Compared with -5 (CEM-7), the film resistance is high (Table 5). From this, it can be seen that a block copolymer having a phase separation domain and having a phase separation domain size (X) exceeding 0, that is, X> 0 is important for the expression of membrane performance.

<Example 1>
Using CEM-1 as the cation exchange membrane and AMV (manufactured by AGC Engineering Co., Ltd.) as the anion exchange membrane, 40/60 H-type strongly acidic cation exchange resin and OH-type strongly basic anion exchange resin are used in the desalination chamber. An electric desalting apparatus comprising an electrodialysis tank containing a mixture of (weight ratio) was assembled. Supplying the collected water of semiconductor factory and city water (fluorine ion concentration: 3mg / L, BOD: 10mg / L) as raw water, applying 8V voltage per unit cell and recovering 90% for 3 consecutive months Drove. After three months of operation, the deionizer was disassembled, and the occurrence of slime (organic adhesive) on the cation exchange membrane in the concentration chamber was visually determined. The results are shown in Table 6.

(Determination of slime occurrence status)
1: No slime was generated in the cation exchange membrane.
2: Slime was generated in part of the cation exchange membrane.
3: Slime was generated in the entire cation exchange membrane.

<Examples 2 to 7>
The electrodeionization test was conducted in the same manner as in Example 1 except that the cation exchange membrane used was changed to the contents shown in Table 6. The results obtained are shown in Table 6.

<Comparative Examples 1-4>
The electrodeionization test was conducted in the same manner as in Example 1 except that the cation exchange membrane used was changed to the contents shown in Table 6. The results obtained are shown in Table 6.

<Comparative Example 5>
An electrodesalting test was conducted in the same manner as in Example 1 using a commercial product CMV (manufactured by AGC Engineering Co., Ltd.) as the cation exchange membrane. The results obtained are shown in Table 6.

  As is clear from the results of Table 6, in the production method according to the present invention, the electrodialysis using a cation exchange membrane having a microphase separation structure with a domain size of 150 nm or less has a high fluorine ion removal rate, And it turns out that there is little contamination of a film | membrane (Examples 1-7). In particular, it has been shown that the removal rate is higher when the domain size is 100 nm or less (Examples 3 to 7). On the other hand, cation exchange membranes with strong phase separation (Comparative Examples 1 and 2), cation exchange membranes having a microphase separation structure with a domain size of 180 nm (Comparative Example 3), and cation exchange membranes with no confirmed phase separation structure ( When Comparative Example 4) is used, it can be seen that the fluorine ion removal rate is lower than in Examples 1-7. It can also be seen that electrodialysis using a commercially available cation exchange membrane is susceptible to membrane contamination (Comparative Example 5). From the above, when electrodialysis is performed using a cation exchange membrane made of a vinyl alcohol copolymer and the phase separation of the polymer segment within the range of 0 nm <X ≦ 150 nm, It can be seen that the removal rate is excellent and the organic contamination resistance is also excellent.

  The cation exchange membrane composed of a vinyl alcohol copolymer having an anionic polymer segment and a vinyl alcohol polymer segment used in the present invention is excellent in fluorine removal and organic contamination resistance, The fluorine ion removal method according to the present invention has industrial applicability.

Although the preferred embodiments of the present invention have been described above by way of example, those skilled in the art can make various modifications, additions and substitutions without departing from the scope and spirit of the present invention disclosed in the claims. It will be understandable.

A ion exchange membrane B platinum electrode C NaCl aqueous solution D water bath E LCR meter

Claims (9)

In a method of removing fluorine ions from an aqueous solution containing fluorine ions by an electrodialysis apparatus in which a cation exchange membrane and an anion exchange membrane are alternately arranged between a cathode and an anode,
The cation exchange membrane contains an anionic polymer segment having an anionic group and a vinyl alcohol copolymer having a vinyl alcohol polymer segment, and the domain size (X) is in the range of 4 nm ≦ X ≦ 150 nm. And the domain size (X) is a domain size of the anionic polymer segment of the vinyl alcohol copolymer, wherein the anion polymer segment has a microphase separation structure , The fluorine ion removal method which is a particle diameter whose accumulation frequency of the maximum particle diameter of the particle shape which the domain of a photopolymer segment shows is 50% .   The vinyl alcohol polymer segment is a segment formed from a vinyl alcohol polymer not containing an anionic group, and electrodialysis treatment is performed using a cation exchange membrane containing a vinyl alcohol copolymer having the segment. The method for removing fluorine ions according to claim 1, wherein the method is performed.   The method for removing fluorine ions according to claim 1 or 2, wherein a crosslinked structure is introduced into the vinyl alcohol copolymer.   The method for removing fluorine ions according to claim 3, wherein the crosslinked structure is introduced by reacting the vinyl alcohol copolymer with a dialdehyde compound.   The said vinyl alcohol-type copolymer is an anionic block copolymer which has a vinyl alcohol polymer block and an anionic polymer block which has an anionic group, The any one of Claims 1-4 characterized by the above-mentioned. The method for removing fluorine ions according to 1.   The said vinyl alcohol-type copolymer is an anionic graft copolymer which has a vinyl alcohol polymer block and an anionic polymer block which has an anionic group, The any one of Claims 1-4 characterized by the above-mentioned. The method for removing fluorine ions according to 1.   The method for removing fluorine ions according to any one of claims 1 to 6, wherein the ion exchange capacity of the cation exchange membrane is 0.30 meq / g or more. The membrane resistance of the said cation exchange membrane is 50 ohm-cm < ² > or less, The removal method of the fluorine ion of any one of Claims 1-7 characterized by the above-mentioned. Method for removing fluorine ions according to any one of claims 1 to 8, wherein the pre-Symbol electrodialysis apparatus is electrodeionization device containing the ion exchanger in the desalting compartment.

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