JP2015020138A - Method for recovering acid - Google Patents

Abstract

PROBLEM TO BE SOLVED: To prevent deterioration in electrolysis performance caused by membrane contamination owing to organic matter upon acid recovery from an acid solution by electrolysis.SOLUTION: In the method for recovering acid by electrolysis using a cation-exchange membrane and an anion-exchange membrane, a cation-exchange membrane is used. The cation-exchange membrane includes a vinyl alcohol-based copolymer having an anionic polymer segment which has an anionic group and a vinyl alcohol copolymer having a vinyl alcohol polymer segment, and has a micro phase separation structure in which domain size (X) lies in the range of 0 nm<X≤150 nm.

Description

  The present invention relates to an acid solution (for example, a metal pickling solution, an etching solution, or a metal used for surface treatment for removing rust, oxide, etc. adhering to the surface of steel, stainless steel, aluminum, metal silicon, etc. The present invention relates to an acid recovery method for reusing an acid in an acid solution in which an eluate or the like has deteriorated due to use.

  For metal materials such as steel and stainless steel, remove oxides (rust, scale) and foreign matters (iron, chromium, aluminum), etc. attached to the metal material before surface treatment such as plating after chemical conversion treatment. The surface treatment is performed after the surface of the metal material is cleaned. An acid solution such as sulfuric acid, phosphoric acid, nitric acid or nitric hydrofluoric acid is used to remove the oxides and foreign matters.

  When surface treatment is performed using this acid solution, the free acid in the acid solution is consumed, and at the same time, the oxidizing substances and foreign substances accumulate in the solution, and the acid solution deteriorates as the treatment amount increases. End up. This deteriorated acid solution (hereinafter referred to as a waste acid solution) has a reduced ability to dissolve rust, scale, etc. and becomes unusable, so it must be replaced with a new acid solution.

  Therefore, the treatment of the waste acid solution after exchange becomes a problem, but conventionally, diffusion dialysis, ion exchange resin, electrodialysis, cooling crystallization, or pressure dialysis is used for this treatment. ing. In these methods, the foreign component in the waste acid solution is removed to recover the acid solution for reuse.

  Patent Document 1 discloses a deoxidation chamber of an electrodialysis tank in which a hydrogen ion selective permeable membrane and an anion exchange membrane are alternately arranged between an anode and a cathode, and a deoxidation chamber and an acid concentration chamber are alternately formed. Discloses a method of recovering concentrated acid from an acid concentration chamber by supplying a waste liquid containing acid (hereinafter referred to as waste acid) and energizing between an anode and a cathode.

  In Patent Document 2, in the X-ray intensity distribution of the sulfur element in the thickness direction of the cation exchange membrane measured using an X-ray element analyzer, it appears in the vicinity of the central portion in the thickness direction of the cation exchange membrane. The average value of the minimum value (Smin), the first maximum value (Smax1) appearing near one surface of the cation exchange membrane, and the second maximum value (Smax2) appearing near the other surface of the cation exchange membrane ( An acid recovery method by electrodialysis using a cation exchange membrane and a hydrogen ion permselective membrane having a ratio (Smin / Smax) to Smax) of 0.2 to 0.8 is disclosed.

Japanese Patent No. 3491352 JP 2011-189223 A

  However, in Patent Document 1, as a cation exchange membrane, an ion exchange group such as a sulfonic acid group, a carboxylic acid group, and a phosphonic acid group, and a main chain of a hydrocarbon type or a perfluoro type are used. In addition, in Patent Document 2, as the cation exchange membrane, polyolefin, fluororesin, polyvinylidene chloride, polysulfone, nylon, polyester and the like are mentioned. Polyolefin and fluororesin are preferred because water molecule permeability is suppressed. Since both use a cation exchange membrane formed from a hydrophobic polymer, it prevents fouling (fouling) from adhering to the membrane surface when the acid of an aged acid solution is recovered by electrodialysis. There was a problem that was not enough.

  The inventors of the present invention are used for surface treatment of rust and oxides of steel, stainless steel, aluminum, metal silicon, etc. by electrodialysis using a cation exchange membrane and an anion exchange membrane. However, when sulfuric acid is used in the semiconductor manufacturing process, there is a problem that organic impurities (such as novolac resin and naphthoquinoazide photosensitizers) are mixed in the waste acid and contaminate the cation exchange membrane. Therefore, in order to efficiently recover the acid from the acid solution, it is considered highly necessary to improve the organic contamination resistance of the cation exchange membrane, and a cation exchange membrane excellent in organic contamination resistance is considered. It was set to be a problem to be solved by using electrodialysis treatment.

  As a result of intensive studies on the above problems, the present inventors have reached the present invention described below. That is, the present invention contains an anionic polymer segment having an anionic group and a vinyl alcohol copolymer having a vinyl alcohol polymer segment as a cation exchange membrane in an acid recovery method by electrodialysis from an acid solution. And a method of recovering an acid, wherein a cation exchange membrane having a microphase separation structure having a domain size (X) in a range of 0 nm <X ≦ 150 nm is used.

  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. Preferably it is done.

  It is preferable that a cross-linked structure is introduced into the vinyl alcohol copolymer. The crosslinked structure is preferably introduced by reacting a 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.

  According to the electrodialysis treatment of the waste acid solution according to the present invention, the cation exchange membrane used for the electrodialysis treatment is composed of a vinyl alcohol copolymer having an anionic polymer segment and a vinyl alcohol polymer segment. However, since the ion exchange membrane is formed by reducing the content of salts contained as impurities in the vinyl alcohol-based copolymer, the anionic polymer segment and the vinyl alcohol polymer segment Phase separation is suppressed, and a film having a domain size larger than 0 nm and 150 nm or less is formed. For this reason, as the cation exchange membrane, since the vinyl alcohol polymer segment has high hydrophilicity, the membrane resistance is low, and the microphase separation is suppressed as described above, so that the ion path structure is densely formed and durable. The electrodialysis can be performed stably and efficiently over a long period of time. By performing electrodialysis treatment on the waste acid solution using such a cation exchange membrane, the cation exchange membrane can be maintained at a low electric resistance without being contaminated by various organic substances contained in the waste acid solution. However, electrodialysis can be stably performed over a long period of time, and acid can be efficiently recovered.

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. It is the schematic which shows an example of the electrodialysis apparatus used in the acid recovery method of this invention.

(Acid recovery method from waste acid solution)
A feature of the present invention is that when an acid is recovered from a waste acid solution containing an organic impurity by electrodialysis, an anionic polymer segment having an anionic group and a vinyl alcohol polymer segment having a vinyl alcohol polymer segment are used as a cation exchange membrane. A cation exchange membrane having a microphase separation structure containing a polymer and having a domain size (X) in the range of 0 nm <X ≦ 150 nm is used. 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. Acrylics such as ethyl, n-propyl acrylate, i-propyl acrylate, n-butyl acrylate, i-butyl acrylate, t-butyl acrylate, 2-ethylhexyl acrylate, dodecyl acrylate, octadecyl acrylate, etc. Acid esters; methacrylic acid and salts thereof; methyl methacrylate, ethyl methacrylate, n-propyl methacrylate, i-propyl methacrylate, n-butyl methacrylate, i-butyl methacrylate, t-butyl methacrylate, methacrylic acid 2-ethylhexyl, dodecyl methacrylate Methacrylic acid esters such as octadecyl methacrylate; acrylamide, N-methylacrylamide, N-ethylacrylamide, N, N-dimethylacrylamide, diacetoneacrylamide, acrylamidepropanesulfonic acid and its salt, acrylamidopropyldimethylamine and its salt, N Acrylamide derivatives such as methylolacrylamide and its derivatives; methacrylamide, N-methylmethacrylamide, N-ethylmethacrylamide, methacrylamidepropanesulfonic acid and its salts, methacrylamidepropyldimethylamine and its salts, N-methylolmethacrylamide and Methacrylamide derivatives such as derivatives thereof; N-vinylformamide, N-vinylacetamide, N-vinylpyrrolidone, etc. -Vinyl amides; Vinyl ethers such as methyl vinyl ether, ethyl vinyl ether, n-propyl vinyl ether, i-propyl vinyl ether, n-butyl vinyl ether, i-butyl vinyl ether, t-butyl vinyl ether, dodecyl vinyl ether, stearyl vinyl ether; acrylonitrile, methacrylonitrile Nitriles such as vinyl chloride, vinylidene chloride, vinyl fluoride, vinylidene fluoride, etc .; allyl compounds such as allyl acetate and allyl chloride; maleic acid and its salts or esters; itaconic acid and its salts or Examples thereof include vinyl silyl compounds such as vinyltrimethoxysilane; 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 to a predetermined value or less, for example, for impurities contained in polyvinyl alcohol constituting the vinyl alcohol polymer segment, polymer flakes may be used. 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 evaporation. 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. The crosslinking treatment agent, glutaraldehyde, formaldehyde, Polyvinyl alcohol acetalizing agent, such as glyoxal can be exemplified, among others glutaraldehyde, dialdehyde crosslinking agents such as glyoxal key Saar is preferred. 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 vinyl aldehyde 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 acid recovery method according to the present invention, the electrodialysis tank is configured by arranging a cation exchange membrane and an anion exchange membrane at least one of the cation membrane according to 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.

FIG. 3 is a schematic view showing an example of an electrodialysis apparatus used in the acid recovery method of the present invention. The electrodialysis tank 1 has a cation exchange membrane 4 disposed between the anode 2 and the cathode 3 on the anode 2 side, an anion exchange membrane 5 disposed on the cathode 3 side, and the anode 2 and the cation exchange membrane. 4, a deoxidation chamber 6 is formed between the cathode 3 and the anion exchange membrane 5, and an acid concentration chamber 7 is formed between the cation exchange membrane 4 and the anion exchange membrane 5. .
Hydrogen that selectively passed through the cation exchange membrane 4 from the deoxidation chamber 6 to the acid concentration chamber 7 by supplying the waste acid solution 8 to the deoxidation chamber 6 and energizing between the anode 2 and the cathode 3. The ions and sulfate ions selectively permeated through the anion exchange membrane 5 from the deoxidation chamber 6 to the acid concentration chamber 7 gather in the acid concentration chamber 7, and the concentrated acid 9 is recovered from the acid concentration chamber 7. 6 from which deoxidizing liquid 10 is recovered.

  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 salts in vinyl alcohol copolymer The amount of salts in the vinyl 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.

<Production of PVA-1 (Synthesis of vinyl 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 Corp.), 950 g of pure water, 40 g of sodium hydroxide, 1 g of sodium nitrate were dissolved at 60 ° C. for 1 hour, cooled to 20 ° C. and recrystallized. 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 vinyl alcohol- (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 crosslinking 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.

<Preparation of CEM-4>
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.

<Production of CEM-5>
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.

<Production of CEM-6>
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 4.0% 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 3.5% or less has a domain size of 130 nm or less and a low film resistance (CEM-2 to 7). On the other hand, a cation exchange membrane using a block copolymer having a salt content of more than 4.0% has a domain size larger than 150 nm, a high membrane resistance, and exhibits properties as a cation exchange membrane. Not (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 CEM-9, purified NaSS-2 is used, but since KPS is used as the 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>
The acid concentration performance was evaluated using the electrodialysis tank shown in FIG.
The electrodialyzer is a small electrodialyzer CH-0 type (manufactured by AGC Engineering Co., Ltd.), CEM-1 as a cation exchange membrane, and hydrogen ion selective permeable membrane Selemion HSF (AGC Engineering Co., Ltd.) as an anion exchange membrane. The stack was assembled by alternately arranging between the cathode and anode electrodes. Model waste acid (sulfuric acid: 100 g / L, aluminum: 7 g / L, benzalkonium chloride: 100 mg / L) was supplied as raw water, and the system was operated for 1 hour at a recovery rate of 80% and a current density of 10 mA / cm ² . Table 6 shows the results of measuring the sulfuric acid concentration and aluminum concentration of the concentrated acid after 1 hour, and the degree of membrane contamination (voltage increase rate after 1 hour operation from the voltage at the start).

<Examples 2 to 7>
An acid concentration 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>
An acid concentration 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 acid concentration 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 in Table 6, in the production method according to the present invention, in the electrodialysis using a cation exchange membrane having a microphase separation structure with a domain size of 150 nm or less, the sulfuric acid concentration of the concentrated acid solution is It can be seen that the concentration is high, the aluminum concentration is low, and the increase in film resistance is low (contamination of the film is small) (Examples 1 to 7). In particular, it is shown that the sulfuric acid concentration of the concentrated acid solution is higher in a membrane having a domain size of 100 nm or less and a high ion exchange capacity (Examples 3 to 6). 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 sulfuric acid concentration of the concentrated acid solution is lower than that of Examples 1 to 7, and the aluminum concentration is high. It can also be seen that electrodialysis using a commercially available cation exchange membrane has a remarkably high rate of increase in electricity and is easily contaminated with the membrane (Comparative Example 5). From the above, when electrodialysis is carried out using a cation exchange membrane made of a vinyl alcohol copolymer and suppressing the phase separation of the polymer segment in the range of 0 nm <X ≦ 150 nm, It can be seen that it has excellent recoverability and excellent resistance to contamination.

The cation exchange membrane composed of a vinyl alcohol copolymer having an anionic polymer segment and a vinyl alcohol polymer segment according to the present invention is excellent in acid recovery and organic contamination resistance. Since it can be used suitably for the recovery of the acid in the acid solution, it 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 1: Electrodialysis tank 2: Anode 3: Cathode 4: Cation exchange membrane 5: Anion exchange membrane 6: Deoxidation chamber 7: Acid concentration chamber 8: Waste acid solution 9: Concentrated acid solution 10: Deacidification solution

Claims (8)

In the acid recovery method by electrodialysis using a cation exchange membrane and an anion exchange membrane,
As a cation exchange membrane, 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 within a range of 0 nm <X ≦ 150 nm. A method for recovering an acid, comprising using a cation exchange membrane having a certain microphase separation 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. The method for recovering an acid according to claim 1, wherein the method is performed.   The acid recovery method according to claim 1 or 2, wherein a crosslinked structure is introduced into the vinyl alcohol copolymer.   The acid recovery method according to claim 3, wherein the crosslinked structure is introduced by reacting a 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 recovering an acid described in 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 recovering an acid described in 1.   The method for recovering an acid 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 acid recovery method according to claim 1, wherein the cation exchange membrane has a membrane resistance of 50 Ωcm ² or less.

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