JPH0314860B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial Application Field] The present invention relates to a method for regenerating a multilayer fluorine-containing ion exchange membrane used in electrolysis for producing chlorine and caustic alkali.

Fluorine-containing ion exchange membranes are currently used as diaphragms in electrolytic cells when alkali chloride is electrolyzed to produce chlorine and caustic alkali, and due to their excellent performance, they are expected to be used more and more in the future. .
As such fluorine-containing ion-exchange membranes, multilayer ion-exchange membranes, which are made by laminating two or more fluorine-containing polymer films having carboxylic acid groups or sulfonic acid groups, have recently been used due to their performance. There is a tendency.

On the other hand, the above-mentioned fluorine-containing ion exchange membrane has a finite lifespan due to performance degradation due to mechanical damage or accumulation of impurities within the membrane during use. Currently, fluorine-containing ion exchange membranes that are no longer suitable for use due to their lifespans are usually discarded despite their expensive raw materials.

The present invention provides a method for regenerating fluorine-containing ion-exchange membranes that are no longer suitable for use, particularly multilayer fluorine-containing ion-exchange membranes that have been widely used in recent years.

[Prior Art] Conventionally, a method for recovering fluorine-containing polymers made from used fluorine-containing ion exchange membranes has been disclosed in Japanese Patent Publication No. 56-
Known as No. 501394. In this method, when regenerating a fluorine-containing ion exchange membrane made of a single fluorine-containing polymer having a sulfonic acid group or a carboxylic acid group, the ion exchange membrane made of the fluorine-containing polymer is first After being converted into an ester type state, the fluorine-containing polymer is dissolved in a fluorine-based solvent having a boiling point of 200 to 350°C, such as a low molecular weight polymer of a specific perfluoro compound or polyfluorokerosine.

After impurities are separated from the dissolved fluorine-containing polymer, it is re-solidified into a desired shape and reused.

[Problems to be Solved by the Invention] In the above conventional method, when eluting the fluorine-containing polymer that forms the membrane into a solvent, first the ion exchange groups of the fluorine-containing polymer are converted into acid, which is a troublesome conversion process. It must be converted to the fluoride or ester form. Furthermore, the solvent used is not only an expensive fluorine-based solvent such as the above-mentioned low molecular weight polymer of the specific perfluorinated compound or perfluorokerosine, but also a temperature of 200°C or higher for dissolution. It requires a process at high temperature.

Thus, the conventional method has practical problems in terms of process and cost, but in addition, the target of the conventional method is an ion exchange membrane made of a single fluorine-containing polymer, and the present invention It cannot be fully applied as is to the target multilayer ion exchange membrane. That is, with conventional methods, it is difficult to recover the several types of fluorine-containing polymers that make up the multilayer membrane as individual polymers, and these fluorine-containing polymers cannot be recovered as a mixed solution.
If this is to be reused, its uses will be limited and it is not advantageous.

[Means for Solving the Problems] The present invention provides a regeneration method that is suitable for multilayer ion exchange membranes and does not have the problems of the conventional methods described above. After converting the ion exchange group of each fluorine-containing polymer into an acid type or an alkali metal salt type, The fluorine-containing polymer forming each layer is preferably immersed in a water-soluble organic solvent to be peeled off, and then eluted with a water-soluble organic solvent, and the fluorine-containing polymer is recovered from the eluate and shaped into a desired shape. It is to re-solidify.

According to the present invention, the water-soluble organic solvent used is inexpensive, the ion exchange group of the fluorine-containing polymer can be easily converted, and the fluorine-containing polymer is usually dissolved at 80°C or lower. It is practically advantageous because it can be carried out at relatively low temperatures. Furthermore,
According to the present invention, more preferably, the solubility of the fluorine-containing polymer forming the multilayer film in the organic solvent is changed depending on the type of ion exchange group to be converted in the fluorine-containing polymer and the type of water-soluble organic solvent. Preliminary immersion in a water-soluble organic solvent is extremely advantageous because the fluorine-containing polymers forming the multilayer film can be recovered independently by peeling off each fluorine-containing polymer.

To explain the present invention in more detail below, the multi-layered fluorine-containing ion exchange membrane targeted by the present invention includes:
It is formed from two or more types of fluorine-containing polymer layers having carboxylic acid groups and/or sulfonic acid groups.

Such a fluorine-containing polymer is composed of a polymer of at least two types of monomers, and is preferably a copolymer having the following polymerized units in use.

(a) (CF ₂ −CXX′) Here, X, X' are -F, -Cl, -H or _-CF3 , A is _-SO3M or -COOM (M is sodium or potassium), and Y is selected from the following: , where Z and Z' are -F or a perfluoroalkyl group having 1 to 10 carbon atoms, and x, y, and z each represent an integer of 1 to 10.

( _CF2 )x, -O-( _CF2 )x, In addition, the composition ratio (molar ratio) of A/B forming the above-mentioned fluorine-containing polymer is 0.8 to 2.0 meq/g when the exchange capacity is a carboxylic acid group, with respect to the exchange capacity of the ion exchange membrane to be formed. In the case of dry resin and sulfonic acid group, it is preferable to select 0.6 to 1.8 meq/g dry resin.

The above-mentioned fluorine-containing polymer is preferably a perfluoropolymer, and preferred examples thereof include:
CF ₂ = CF ₂ and CF ₂ = CFOCF ₂ CF (CF ₃ ) OCF ₂ CF ₂
Copolymers with SO ₂ F, CF ₂ = CF ₂ and CF ₂ = CFO
( _CF2 ) _3~5 Copolymer with _{SO2F CF2} = _CF2 and _CF2 =
CFO (CF ₂ ) _1-5 Copolymer with COOCH ₃ , CF ₂ =
CF ₂ and CF ₂ = CFOCF ₂ CF (CF ₃ ) OCF ₂ CF ₂
A copolymer with COOCH ₃ is exemplified.

A multilayer fluorine ion exchange membrane is usually obtained by using a combination of two or more ion exchange membranes of different types and/or exchange capacities, which are the fluorine-containing polymers mentioned above. That is, a combination of two or more types of polymers having the same type of ion exchange group but different capacities, or a combination of two or more types of polymers having a carboxylic acid group and a polymer having a sulfonic acid group,
It can be obtained by laminating films of each polymer, preferably by heating and pressing.
It is also produced by converting ion exchange groups, such as sulfonic acid groups, on one side of one type of polymer film into carboxylic acid groups.

Preferred examples of multilayer films having two or more types of fluorine-containing polymer layers include a film consisting of a first carboxylic acid polymer layer/second carboxylic acid polymer layer;
A membrane consisting of a first sulfonic acid polymer layer/a second sulfonic acid polymer layer, a sulfonic acid polymer layer/
A membrane consisting of a carboxylic acid polymer layer, a sulfonic acid polymer layer/a first carboxylic acid polymer layer/a membrane consisting of a second carboxylic acid polymer layer, a first sulfonic acid polymer layer/a second sulfonic acid polymer layer/
Mention may be made of a membrane consisting of a carboxylic acid polymer layer.
The thickness of each polymer layer is usually 5 to 500 μm
The thickness of the sulfonic acid polymer layer with low electrical resistance or the polymer layer with high ion exchange capacity is preferably made larger than other layers.

These multilayer ion-exchange membranes can be reinforced, if necessary, with a cloth, a woven fabric such as a net, a nonwoven fabric, a fibril or metal mesh, a porous body, etc., preferably made of a fluorine-containing polymer such as tetrafluoroethylene. . Additionally, the ion exchange membrane may optionally have a porous layer containing particles with gas- and liquid-permeable electrode activity (US Pat. No. 4,224,124) or a gas- and liquid-permeable electrode activity on one or both surfaces. A porous layer containing particles that do not have
2064586) or roughening the film surface. In such a multilayer ion exchange membrane, the ion exchange groups in each polymer layer are converted to acid type or alkali metal salt type.

Conversion to the acid form is carried out by immersing the membrane in an aqueous solution of an inorganic or organic acid, preferably in the presence of a polar organic compound. Here, as the inorganic acid, hydrochloric acid, sulfuric acid, nitric acid, phosphoric acid, etc. are preferable, and as the organic acid, acetic acid, propionic acid, perfluoroacetic acid, p-toluenesulfonic acid, etc. are preferable. These acids are preferably used as 0.5-90% by weight aqueous solutions. Further, as the polar organic compound used as necessary, methanol, ethanol, butanol, ethylene glycol, dimethyl sulfoxide, acetic acid, phenol, etc. are used. The polar organic compound is preferably added in an amount of 5 to 90% by weight in the acid aqueous solution. The membrane is immersed in the acid aqueous solution preferably at a temperature of 10 to 120°C for 30 minutes to 20 hours.

For conversion of ion exchange groups to alkali metal salt type,
For example, the conversion to a lithium salt is carried out by a known method, but preferably the ion exchange group converted to the acid form is contacted with an aqueous solution of a soluble lithium salt compound such as lithium hydroxide or lithium carbonate. This is done by The lithium salt compound aqueous solution is used at a concentration of 5 to 90% by weight, and the membrane is preferably placed in the aqueous solution at a temperature of 10 to 120°C for 30 to 20 minutes.
This is done by means such as soaking for a period of time.

After the ion exchange groups of the ion exchange membrane are converted into the acid type or alkali metal salt type, the membrane is immersed in a water-soluble organic solvent in the present invention. The fluorine-containing polymer of the membrane that is not converted into the ion exchange group is hardly dissolved.

Examples of the organic solvent used in the present invention include solvents that are highly polar, that is, have a large dipole moment, and have the ability to bond with protons. The solvent is at least 5% by weight, preferably at least 50% soluble in water, but in most cases is compatible with water in all proportions. As such organic solvents, solvents containing hydroxyl groups in the molecule, such as alcohol, polyhydric alcohol, and cellosolve, are preferred because they exhibit particularly excellent solubility. The alcohol preferably includes a hydrocarbon alcohol having 4 or less carbon atoms, such as methanol, ethanol, n- or iso-propanol, ^n- , iso- or t-butanol;
Preferred examples include ethylene glycol, propylene glycol, glycerin, and cellosolve. In addition, in the present invention, organic amides such as dimethylformamide and dimethylacetamide,
Dimethyl sulfoxide, acetone or hexafluoroacetone can also be used. Preferably,
Those with a boiling point of 150°C or lower are suitable for recovering polymers from solutions. Further, two or more organic solvents may be used in combination, if necessary, or a mixed medium with water may be used, if necessary.

Dissolving the membrane in an organic solvent is carried out by immersing a predetermined amount of the membrane in the solvent and, if necessary, promoting dissolution by stirring or the like.
The fluorine-containing polymer used may be in the form of a film or a sheet, but in order to promote dissolution, it may preferably be in the form of granules of 20 or more meshes. The higher the temperature during dissolution, the better it is for promoting dissolution, and is usually 25 to 150℃.
However, it can also be carried out under an increased pressure of 1 to 10 atmospheres to promote dissolution.

By doing so, the fluorine-containing polymer layer constituting the multilayer ion exchange membrane is dissolved in the organic solvent,
It is eluted. The characteristic feature of the present invention is that
As mentioned above, since the fluorine-containing polymer layers constituting the multilayer membrane differ in the type and/or exchange capacity of ion exchange groups, even if the same water-soluble organic solvent is used, each fluorine-containing polymer layer will be different. Since they have different solubility, they can be almost selectively eluted and recovered in organic solvents. That is, polymers with sulfonic acid groups and polymers with carboxylic acid groups have different solubility, and even with the same ion exchange group, a polymer with a large exchange capacity has a higher solubility.

In order to more selectively elute and recover each fluorine-containing polymer layer constituting the membrane in an organic solvent, multiple organic solvents each having different solubility are used, and each fluorine-containing polymer is gradually dissolved in a different organic solvent. It can be eluted in a solvent. This can also be achieved by changing the conditions under which the membrane is immersed in an organic solvent, such as temperature and pressure, and this can be applied not only when the same organic solvent is used, but also when different solvents are used. can. Further, the ion exchange group form of only a part of the fluorine-containing polymer forming the membrane is converted into the acid type or lithium salt type of the present invention, and after dissolving this in an organic solvent, another fluorine-containing polymer is added. The ion exchange groups of can be sequentially converted and dissolved. This can easily be done for the fluoropolymer layer on only one side of the membrane, and then for the polymer layers on many sides.

As another method for selectively eluting and recovering each fluorine-containing polymer layer, each polymer layer forming the multilayer membrane can be peeled off in advance. In the present invention, it has been found that each fluorine-containing polymer layer can be easily peeled off by immersing the multilayer membrane in a water-soluble organic solvent after preferably converting the ion exchange groups as described above. This is thought to be due to the fact that each polymer layer has a different degree of lubricity with respect to the water-soluble organic solvent due to the different types of ion exchange groups, exchange capacities, or polymer structures.

Here, the water-soluble organic solvent used is:
The same organic solvent can be used, and the same concentration and dipping conditions can be used, but the dipping time is sufficient as long as each layer peels off, and usually 1
~100 minutes.

As mentioned above, when a porous layer of particles with electrode activity or a porous layer of particles without electrode activity is attached or bonded to the surface of the ionic membrane targeted by the present invention, these particles are usually organic Since they do not dissolve in solvents, these particles can be separated from organic solvents even when applied as is. However, in some cases, the porous layer can be removed from the membrane surface by an appropriate method before being applied to the present invention.

As described above, a solution of an organic solvent in which each fluorine-containing polymer forming the film is dissolved is obtained.
Any suitable method can be used to recover the polymer from such a solution. That is, the polymer solution can be re-solidified and used as it is for various purposes as a solution of a fluorine-containing polymer having an ion exchange group.
For example, by casting the solution or impregnating asbestos or polytetrafluor porous material with the solution, a film of any shape for use in electrolysis, dialysis, fuel cells, etc. can be made. Furthermore, it is used for repairing pinholes in existing fluorine-containing polymer films, coating appropriate articles with fluorine-containing polymers, and the like.

Another method for recovering a polymer from a fluorine-containing polymer solution is to add a poor solvent for the polymer to the solution to precipitate and separate the polymer. Preferably, a halogenated hydrocarbon solvent such as carbon tetrachloride or trichloromonofluoromethane, or a hydrocarbon solvent such as hexane or heptane is used as the poor solvent for the fluoropolymer. A known method of recovering the polymer by precipitation can be employed. In this way, the precipitated and separated fluorine-containing polymer can be used not only for membranes for electrolysis, dialysis, fuel cells, etc., but also for all kinds of fluorine-containing polymers with ion exchange groups, as well as new polymers. Can be used for any purpose.

Examples are given below to illustrate the present invention more specifically, but the present invention is not limited to these Examples, and various changes can be made within the scope of the present invention.

Example 1 Two types of perfluorocarboxylic acid type polymers with ion exchange capacities _{of 1.8meq} /g and 1.25meq/g were synthesized by copolymerizing _C2F4 and _CF2 =CFO( _CF2 ) _3COOCH3 . , hot pressed at 230℃, each thickness
It was molded into 200μ and 30μ films. Next, after stacking these films and press laminating them at 200℃,
It was immersed in a 25% NaOH aqueous solution at 90°C for 16 hours for hydrolysis.

The cation exchange membrane was installed in a chloralkali electrolytic cell equipped with an anode made of RuO ₂ /Ti expanded metal and a cathode made of activated Ni/Fe expanded metal, with a 5N NaCl aqueous solution in the anode chamber and water in the cathode chamber. While supplying current density 30A/dm ² , liquid temperature
At 90℃. Electrolysis was performed for 30 days. As a result, 35%
NaOH was obtained at a current efficiency of 94.6% and a cell voltage of 3.06V.

After the electrolysis was completed, the membrane removed from the tank was stirred in a 12% HCl aqueous solution, filtered, and washed with water to convert the ion exchange groups into acid forms. Next, when this was immersed in acetone, it was separated into two films. The thicker one of these, that is, the ion exchange capacity is 1.8meq/
After stirring for 3 hours, leaving only the membrane of g, the membrane was completely dissolved and a clear and active solution was obtained.

Next, the thinner one, i.e. ion exchange capacity
A 1.25 meq/g membrane was immersed in a LiOH aqueous solution to form a Li salt type, and after filtering and washing with water, the membrane was completely dissolved when the solution was stirred in boiling methanol.

Next, each solution was added to CCl ₄ to precipitate and collect the polymer. After re-esterifying the recovered polymer using methanol, it was 1.8 meq/g.
1.25meq/g polymer was pressed again at 230℃,
After molding into 200μ and 30μ films and stirring at 200°C, they were similarly hydrolyzed. When electrolysis was performed again using the regenerated membrane using the regenerated polymer, it was confirmed that the performance was reproducible with a current efficiency of 94.5% and a cell voltage of 3.06V.

Example 2 C ₂ F ₄ and A perfluorosulfonic acid type polymer layer with an ion exchange capacity of 1.1 meq/g was synthesized by copolymerizing. After hot press molding this into a film with a thickness of 200μ, the ion exchange capacity is 1.25meq/
It was laminated with the perfluorocarboxylic acid type polymer layer of g, and was press-laminated at 260°C to integrate. This film was hydrolyzed to form an ion exchange group, and Example-
As a result of electrolysis under the same conditions as 1, 35%
NaOH was obtained at a current efficiency of 94.8% and a cell voltage of 3.15V.

Next, when the ion exchange groups of the electrolytically used membranes were converted into acid form and immersed in ethanol, each membrane was separated into two sheets. When the mixture was stirred at 60°C, leaving only the sulfonic acid type membrane, the membrane completely dissolved and became a viscous solution. The remaining carboxylic acid type film was dissolved and recovered by the method of Example-1.

Example 3 A perfluorocarboxylic acid type polymer layer having an ion exchange capacity of 1.44 meq/g was newly synthesized, and the following three types of films were prepared.

A: A 20μ thick film made of carboxylic acid type polymer with an ion exchange capacity of 1.25meq/g.

B; A film made of a carboxylic acid type polymer with an ion exchange capacity of 1.44 meq/g and a thickness of 180 μ; C; A film made of a carboxylic acid type polymer with an ion exchange capacity of 1.1 meq/g and a thickness of 30 μ. A film B was placed on top of the film B, and a film C was placed on top of the film C, and the two were laminated by hot press to form a single film. This film was hydrolyzed to obtain an ion exchange membrane, and electrolysis was performed under the same conditions as in Example-1. As a result, 35% NaOH, current efficiency 96.1%,
Obtained at a cell voltage of 3.19V.

After the electrolysis was completed, the ion exchange membrane was immersed in ethanol at 60°C, and the sulfonic acid membrane peeled off. When the two layers of carboxylic acid films were removed, leaving only the sulfonic acid film, and this was stirred overnight in ethanol, the film completely dissolved and became a viscous solution.

Next, the two-layer carboxylic acid film was converted into an acid form, immersed in acetone, and separated into two films. Of these, the thicker membrane with an ion exchange capacity of 1.44 meq/g was left behind, and when stirred overnight at 50°C, the membrane was completely dissolved. The final remaining ion exchange capacity
The 1.25 meq/g carboxylic acid film was dissolved in the same manner as in Example-1.

Example 4 A; SiC porous body B with a thickness of 20μ; Film C made of a carboxylic acid type polymer with an ion exchange capacity of 1.25 meq/g and a thickness of 20μ; Carboxylic acid with an ion exchange capacity of 1.44 meq/g Film D, which is made of type polymer and has a thickness of 180 μ; Film E, which is made of sulfonic acid type polymer and has a thickness of 30 μ, and has an ion exchange capacity of 1.1 meq/g; A porous body of ZrO ₂ with a thickness of 20 μ. , B, C, D, and E in the order of hot press lamination, and were integrally formed into a single film having a five-layer structure. This was hydrolyzed to obtain an ion exchange membrane, and electrolysis was performed under the same conditions as in Example-1. As a result, 35%
NaOH was obtained at a current efficiency of 96.0% and a cell voltage of 2.99V.

After the electrolysis was completed, the inorganic porosity on the membrane surface was removed using a resin brush or the like, and then the recovery operation of Example 3 was performed to recover each polymer component as three types of solutions. Among these solutions, a methanol solution of carboxylic acid type polymer with an ion exchange capacity of 1.25 meq/g and a methanol solution with an ion exchange capacity of 1.1 meq/g.
Inorganic particles that could not be completely removed were floating in the ethanol solution of the sulfonic acid type polymer in g. By centrifuging these solutions at 8000 rpm for about 1 hour, inorganic particles were almost completely removed and clear solutions could be obtained.

Claims (1)

[Scope of Claims] 1. A multi-layered fluorine-containing ion-exchange membrane consisting of two or more fluorine-containing polymer layers having carboxylic acid groups and/or sulfonic acid groups, wherein each fluorine-containing polymer layer has an ion exchange group. After converting into acid type or alkali metal salt type, each fluorine-containing polymer is eluted by immersion in a water-soluble organic solvent, and the fluorine-containing polymer is recovered from the eluate and resolidified into the desired shape. A method for regenerating a multilayer fluorine-containing ion exchange membrane, characterized by: 2. The method according to claim 1, wherein the multilayer fluorine-containing ion exchange membrane is immersed in a water-soluble organic solvent to separate each fluorine-containing polymer layer in advance. 3. The method according to claim 1 or 2, wherein the ion exchange capacity of the fluorine-containing polymer is 0.8 meq/g dry resin or more in the case of a carboxylic acid group, and 0.6 meq/g dry resin or more in the case of a sulfonic acid group. 4. The method according to any one of claims 1, 2, or 3, wherein the water-soluble organic solvent is an organic solvent having a boiling point of 150° C. or lower and soluble in water at least 5% by weight.