JPS6145653B2 - - Google Patents

Abstract

PURPOSE:To improve current efficiency in electrolytic separation in an OH<->-containing aq. solution by use of homhgeneous cation exchange membranes, by subjecting the membranes to swelling treatment with a water-miscible organic solvent to reduce their OH<-> permeability.

Description

[Detailed description of the invention]

The present invention relates to a method for improving the current efficiency of a homogeneous cation exchange membrane. More specifically, a homogeneous strong acid type cation exchange membrane is swollen with an organic solvent that is miscible with water, the solvent is then dried and removed from the membrane, and the hydroxide ion permeability of the membrane is reduced by using the membrane as it is. The present invention relates to a method for improving current efficiency when separating electrolytes using a cation exchange membrane in an aqueous solution containing acid ions. In general, cation exchange membranes have high resistance to anion permeation, but exceptionally they have high permeability to hydroxyl ions compared to other anions. is well known, and this is thought to be due to the fact that the mobility of hydroxyl ions in aqueous solutions is significantly greater than that of other anions. The fact that hydroxyl ions have relatively high permeability to the cation exchange membrane is useful in some respects. It becomes possible to recover alkali hydroxide by diffusion dialysis using a cation exchange membrane. However, on the other hand, these properties often bring about undesirable results, especially when performing operations such as electrically separating and concentrating an electrolyte solution containing hydroxide ions using a cation exchange membrane. Since hydroxyl ions diffuse through the exchange membrane and the current efficiency decreases, it is desirable that the cation exchange membrane used for these purposes is one that is difficult for hydroxide ions to permeate. According to the method of the present invention, the hydroxide ion permeability of a conventionally commonly used homogeneous cation exchange membrane can be lowered without significantly impairing other properties of the membrane. Current efficiency in various operations can be improved.
This is the first feature of the present invention. An example to which the method of the present invention is suitably applied is the treatment of a cation exchange membrane used as a diaphragm in a saline electrolyzer. For example, in a method for producing caustic soda using a diaphragm electrolytic cell using a cation exchange membrane as a diaphragm, in which electrolysis is performed while supplying salt water to the anode chamber to obtain a 20% by weight aqueous caustic soda solution in the cathode chamber, the caustic soda produced is It is also possible to improve the standard current efficiency by 5 to 10% compared to the case of using a cation exchange membrane not subjected to the treatment of the present invention. Of course, the applications of the present invention are not limited to these examples, but include the permeation of hydroxide ions through cation exchange membranes, such as concentration of alkali hydroxide by electrodialysis, production of caustic soda and sulfuric acid by mirabilite electrolysis, etc. It can be applied as a treatment method for cation exchange membranes in various fields where cation exchange membranes are undesirable.
The cation exchange groups contained in the cation exchange membrane exist in either acid form or salt form depending on the conditions under which the membrane is used, and the method of the present invention is effective for both types. In particular, it has been found that when the exchanger is of the salt type, the effect is even greater than when it is of the acid type. This means that the present invention is not only effective as a method for improving the current efficiency of cation exchange membranes, but also when applied to cation exchange membranes whose current efficiency has decreased after being used in an electrolyte aqueous solution for a long period of time. It is shown that this method is also effective as a recycling treatment method that can improve the performance of the membrane to the same level or even higher than that before use. The second feature of the present invention is the greater effect obtained by applying the treatment of the present invention to a cation exchange membrane having a salt-type cation exchange group. Cation exchange membranes used as salt electrolytic diaphragms are generally expensive, and membrane-related costs account for a significant portion of the cost of producing caustic soda using the ion exchange membrane method. However, the treatment of the present invention allows for repeated use of the membrane. Therefore, it is economically advantageous. The implementation of the present invention is extremely simple; a cation exchange membrane is immersed in a suitable water-miscible organic solvent as described below for a suitable period of time, the membrane is swollen with the solvent, and then the membrane is removed. It is only necessary to remove the solvent by drying. The immersion time varies depending on the swellability of the membrane to the solvent used, but the longer the better. In a solvent with good swelling properties, immersion for several minutes is effective. At this time, the solvent may be heated to promote immersion. The degree of swelling of a cation exchange membrane by an organic solvent is effective even when the percentage increase in membrane weight during swelling to the untreated dry membrane weight (hereinafter referred to as the swelling ratio) is about 3%.
% or more is more preferable. The solvent contained in the membrane due to the swelling treatment may be removed by natural drying or vacuum drying after the treatment. Many methods have been proposed in the past to reduce the hydroxyl ion permeability of cation exchange membranes and improve the current efficiency of the membranes in the various applications mentioned above. 66488,
There are various methods described in JP-A-50-105581, JP-A-50-108182, and JP-A-50-120492. However, all of these methods involve changing the chemical structure of part or all of the membrane, intervening other polymeric compounds, or using special exchange groups. However, it is fundamentally different from the method of the present invention. Furthermore, with these methods, it is thought to be extremely difficult to regenerate a membrane whose performance has deteriorated once, but with the present invention, not only can this be easily done, but it is also possible to improve the membrane beyond its initial performance. be. Alternatively, a method has been proposed in Japanese Patent Publication No. 4637-1983 to improve the ion-selective permeability of the ion-exchange membrane by evaporating part of the water contained in the membrane, thereby improving the current efficiency of the membrane. . This document states that, for example, by applying this method to a cation exchange membrane, the diffusion coefficient of chloride ions can be reduced.
However, with regard to the permeation of hydroxide ions, as will be shown later as a comparative example with respect to a specific example of the present invention, simply evaporating and removing the water contained in the membrane will not allow the permeation of hydroxyl ions as in the present invention. A good blocking effect on the permeation of is not obtained. That is, in the present invention, it is an essential requirement to swell the cation exchange membrane with an organic solvent that is miscible with water.
This method is completely different from the method described in . Regarding the swelling treatment of ion exchange membranes with organic solvents, organic solvent swelling treatment of the exchange membrane binder on heterogeneous ion exchange membranes causes a decrease in membrane function mainly due to membrane clogging, such as an increase in electrical resistance, and selective ion permeation. A method for preventing a decrease in the properties of the film is described in JP-A-50-158590. This is based on the so-called cleaning effect, which is easily permeated and removed by swelling with a solvent.
This treatment does not improve the performance of the membrane beyond the initial performance except for the permeability of ions constituting water. On the other hand, in the present invention, the hydroxyl ion permeability of the cation exchange membrane is further reduced compared to that of the original membrane, so it is clearly different from the invention of JP-A-50-158590 in its concept and effect. It is. In the present invention, it is essential to swell the ion exchanger part bonded with the cation exchange group in the cation exchange membrane with an organic solvent, in order to prevent the electrically inactive binding material and ion exchanger from being washed away. The effects of the present invention cannot be sufficiently exhibited in so-called heterogeneous cation exchange membranes having a network-structured polymeric substance coexisting in the membrane. That is, the present invention is expected to be effective mainly for homogeneous cation exchange membranes. Here, the homogeneous cation exchange membrane refers to the heterogeneous cation exchange membrane described above, and refers to a membrane other than a woven structure contained in the membrane for the purpose of reinforcing the membrane to improve its mechanical strength. It does not contain a binder unrelated to the ion exchanger or a polymer substance having an ionically inactive microreticular structure. As such a homogeneous cation exchange membrane, 1) a vinyl aromatic compound suitable for introducing an ion exchange group such as styrene, alkyl styrene, or halogenated styrene may be used as necessary, such as an initial polymer of these compounds or other A vinyl monomer is also added and polymerized together with a crosslinking agent such as divinylbenzene in the presence of a polymerization initiator and a suitable plasticizer, and the resulting bulk polymer is cut into a membrane, and then a cation exchange membrane is introduced. A cation exchange membrane manufactured by a so-called cutting method, 2) A reinforcing base material such as glass woven cloth is immersed in a latex such as a styrene-butadiene copolymer, pulled up and dried, and then the copolymer adhering to the base material is removed. The ion exchanger contained in the membrane has a crosslinked structure, such as a cation exchange membrane manufactured by the so-called dipping method, in which a crosslinked structure is introduced into a polymer by cyclization treatment, etc., and then a cation exchange group is introduced. , so-called cross-linked homogeneous cation exchange membrane,
This includes so-called non-crosslinked homogeneous cation exchange membranes in which the ion exchanger constituting the membrane does not have a crosslinked structure due to chemical bonds. More preferably, it is an ion exchange membrane. Such non-crosslinked homogeneous cation exchange membranes include those having the following structure. 1) Directly into a sheet made of linear polyolefin that does not have an ion exchange group, or after introducing a functional group into which a cation exchange group can be introduced into the membrane prior to the introduction of a cation exchange group, a cation exchange group can be introduced into the membrane. A cation exchange membrane obtained by introducing an exchange group, 2) A cation exchange group, a group that can become a cation exchange group, a cation exchange group, or a cation exchange group on a sheet made of linear polyolefin that does not have an ion exchange group. A polymerizable monomer having a functional group into which a potential group can be introduced, alone or together with other monovinyl monomers that can be copolymerized with it, is infiltrated into the polymerization initiator using an appropriate solvent if necessary. The sheet is graft-polymerized by adding, heating, light irradiation, X-ray irradiation, radiation irradiation, etc., and then the group contained in the monomer is a group that can become a cation exchange group, a cation exchange group, or a cation exchange group. If it is a functional group into which a group that can become a radical can be introduced, it is converted into a cation exchange group, or a cation exchange group is introduced, or a group that can become a cation exchange group is introduced and then converted into a cation exchange group. 3) A polymerizable monomer having a cation exchange group, a group capable of becoming a cation exchange group, or a functional group into which a cation exchange group or a group capable of becoming a cation exchange group can be introduced. alone or together with a monovinyl monomer copolymerizable with it, using an appropriate solvent as necessary, a polymerization initiator, heating, light irradiation,
By means of X-ray irradiation, radiation irradiation, etc., bulk polymerization, solution polymerization, suspension polymerization, emulsion polymerization,
Other methods include compression molding, extrusion molding, blow molding of a polymer obtained by polymerization using a conventionally known method, or methods of impregnating a reinforcing base material with the polymer latex and drying or further melting treatment, and other conventionally known molding methods. If the group contained in the polymer is a group that can become a cation exchange group or a functional group that can introduce a cation exchange group or a group that can become a cation exchange group, , each converted to a cation exchange group or introduced a cation exchange group,
Alternatively, a cation exchange membrane obtained by introducing a group that can become a cation exchange group and then converting it into a cation exchange group; A monovinyl monomer having an ion exchange group or a group that can become a cation exchange group, or a functional group that can become a cation exchange group or a cation exchange group, alone or together with other monovinyl monomers that can be copolymerized with it, If necessary, a suitable solvent is used to permeate the monomer to perform a polymerization reaction, and then the group contained in the monomer is a group that can become a cation exchange group or a cation exchange group or a group that can become a cation exchange group. In the case of a functional group capable of introducing a cation exchange group, the functional group is converted into a cation exchange group, or a cation exchange group is introduced, or a group capable of becoming a cation exchange group is introduced and then converted into a cation exchange group. The resulting cation exchange membrane, 5) The cation exchange groups present on or near the surface of the cation exchange membrane shown in 1) to 4) are electrically neutralized, as if an electrically neutral layer was formed. A structure that appears to be formed on or near the surface of a membrane. Alternatively, a functional group such as a nitro group or an amino group is introduced on or near the membrane surface. Further, when the cation exchange group contained in the cation exchange membrane is a sulfonic acid group, a portion thereof is converted into a sulfonamide group or an N-monoalkyl-substituted sulfonamide group. These cation exchange membranes mentioned herein may be appropriately reinforced with Teflon cloth or the like for the purpose of improving the mechanical properties of the membrane. Various types of cation exchange groups are used in the cation exchange membrane, but the most effective ones are so-called strong acid type cation exchange groups such as sulfonic acid groups. Of course, two or more types of cation exchange groups may coexist, and the exchange groups on or near the surface of the membrane may be appropriately modified. As a cation exchange membrane having a structure suitable for carrying out the present invention, for example, a monomer represented by the following general formula (1) is used. (In the formula, R represents fluorine or a trifluoromethyl group. n is an integer of 1 to 5, and m is 0 or 1.) and tetrafluoromethylene. There is a perfluorosulfonic acid type cation exchange membrane obtained by forming the membrane into a membrane and then hydrolyzing it, which is advantageous in that the membrane can be easily handled before and after solvent treatment. Such a cation exchange membrane may also be suitably reinforced with Teflon cloth or the like in order to improve the mechanical properties of the membrane. When these cation exchange membranes are subjected to the treatment of the present invention, the exchange group may be in the form of a free acid, or in the form of a monovalent metal salt or ammonium salt, but the latter It is more effective when The organic solvent that is miscible with water used in carrying out the present invention is one that swells the membrane to be treated, and has a solubility in water of 0.1 g/100 g at room temperature.
Anything that has a concentration of H ₂ O or higher is acceptable, but from the viewpoint of shortening treatment time and ease of handling after treatment, it is preferable to use one that swells the membrane quickly and can be easily removed from the membrane by drying after treatment. preferable. Examples of such solvents include aliphatic monohydric alcohols such as methanol, ethanol, propanol, and butanol; ketones such as acetone, methyl ethyl ketone, and diethyl ketone; esters such as methyl acetate, ethyl acetate, propyl acetate, and butyl acetate; Examples include ethers such as ethyl ether, prolether, tetrahydrofuran, and dioxane, and chloroform. Of course, some of these solvents may be used in combination, and water and electrolytes may also coexist. Therefore, it is also possible to regenerate the treated film in its used state without removing it. EXAMPLES The effects of the present invention will be explained in more detail with reference to Examples below, but it goes without saying that the scope of the present invention is not limited only to these Examples. Example 1 A non-crosslinked cation exchange membrane (manufactured by Dupont, trade name:
Nafion Membrane 110) was immersed in ethanol at room temperature overnight to swell. The swelling rate at this time is 47
It was %. The membrane was taken out and vacuum dried at 25°C to remove ethanol in the membrane. The cation transfer number of this membrane was determined to be 0.92 by the Hittorf method in a 1N aqueous sodium hydroxide solution. In addition, this film was tested in a 2% caustic soda aqueous solution at 25°C.
The electrical resistance was measured by the AC bridge method at 1000 C/S and was 2.5 Ωcm ² . On the other hand, when the cation transfer number and electrical resistance of the material not subjected to the swelling treatment with ethanol were similarly determined, they were 0.80 and 2.4 Ωcm ² , respectively. Comparative Example 1 A styrene-divinylbenzene-based strong acid type cation exchange resin (manufactured by Rohm and Haas, trade name: Amberlite 1R120B) was ground into a fine powder of about 300 mesh. This ion exchange resin powder and finely powdered polyvinyl fluoride were kneaded together with a small amount of dimethylformamide at a weight ratio of 7:3, and heated and pressed to form a membrane to form a heterogeneous cation exchange membrane. The electrical resistance and cation transfer number of this membrane were determined in the same manner as in Example 1, and were each 5.2.
It was Ωcm ² and 0.78. The membrane was then swollen with methanol overnight. At this time, the swelling rate is 1.2%
It was hot. Methanol was removed from the membrane by vacuum drying at 25°C, and the electrical resistance and cation transfer number of the membrane were determined in the same manner, and were found to be 5.0 Ωcm ² and 0.77, respectively. Example 2 The same cation exchange membrane as in Example 1 was swollen in acetone overnight. At this time, the swelling rate was 28%. After removing acetone by vacuum drying at 25° C., the cation transfer number and electrical resistance were measured in the same manner as in Example 1, and found to be 0.88 and 2.4 Ωcm ² , respectively. Example 3 The same cation exchange membrane as in Example 1 was immersed in methanol at 60°C for 2 hours. The swelling rate at this time is 42
It was %. Methanol was removed by vacuum drying at 25°C, and the cation transfer number and air resistance were determined in the same manner as in Example 1. They were 0.93 and 2.5, respectively.
It was Ωcm ² . Comparative Example 2 The same cation exchange membrane used in Example 1 was immersed in benzene for 7 days. At this time, the swelling ratio is 0.94
It was %. After vacuum drying at 25°C to remove benzene contained in the membrane, the cation transfer number and electrical resistance were determined in the same manner as in Example 1.
The resistance values were 0.80 and 2.4 Ωcm ² , respectively, and no effect of the treatment was observed. Comparative Example 3 The same cation exchange membrane as in Example 1 was vacuum dried at 25°C to remove most of the water contained in the membrane, and the cation transfer number and electrical resistance were determined in the same manner as in Example 1. However, they were 0.80 and 2.5 Ωcm ² , respectively. Examples 4 to 10 and Comparative Examples 4 and 5 The same cation exchange membranes used in Example 1 were immersed and swollen in various organic solvents overnight,
The solvent was then removed by vacuum drying at 25°C, and the cation transfer number and electrical resistance were determined in the same manner as in Example 1. The results are shown in Table-1.

[Table] Example 11 The same perfluorosulfonic acid type cation exchange membrane as in Example 1 was immersed in a caustic soda aqueous solution for equilibration, and the sulfonic acid groups contained in the membrane were converted to sodium salts. The membrane was dried and then swollen by soaking in ethanol overnight. The swelling rate at this time is 45%
It was hot. The cation transfer number and electrical resistance of this product after vacuum drying at 25°C were determined in the same manner as in Example 1, and were 0.95 and 2.7 Ωcm ² , respectively, indicating that the membrane performance was further improved compared to the one treated with swelling with sulfonic acid groups. was seen. Example 12 A perfluoroether sulfonic acid type cation exchange membrane (manufactured by Dupont, trade name: Nafion Membrane 390) reinforced with Teflon cloth and rayon cloth was immersed in methanol at room temperature overnight. The swelling rate at this time was 8%. After methanol was removed by vacuum drying at 25° C., an electrolytic cell with an effective area of 100 cm ² was constructed in which the anode, anode chamber, diaphragm, cathode chamber, and cathode were arranged in this order using this membrane as a diaphragm. Current density 20A/dm ² while supplying saturated saline to the anode chamber
Electrification was applied to electrolyze the saline solution. During the electrolysis operation, water was continuously poured into the cathode chamber so that the caustic soda concentration in the cathode chamber was always 20%. At this time, the current efficiency with respect to the produced caustic soda was 93%. On the other hand, when the current efficiency of a membrane that was not subjected to methanol swelling treatment was determined using the same electrolytic cell under the same conditions, it was 87%. Example 13 When the cation exchange membrane of Example 12 was continuously energized under the same conditions as in Example 12 without being subjected to swelling treatment with a solvent, the current efficiency decreased to 80% after 1000 hours. This membrane was removed from the container and immersed in methanol at room temperature overnight. At this time, the swelling rate was 8.2%. When this membrane was vacuum dried at 25°C, placed in the electrolytic cell again, and energized under the same conditions, the current efficiency increased to 97%, which is
It remained unchanged even after 100 hours. Comparative Example 6 After being treated with ethanol in the same manner as in Example 1,
It was immersed in water at 25°C overnight. The cation transfer number and electrical resistance were determined in the same manner as in Example 1 and found to be 0.68 and 1.7 Ωcm ² , respectively.

Claims (1)

[Claims] 1. A homogeneous strong acid type cation exchange membrane is swollen with an organic solvent that is miscible with water and has a water solubility of 0.1 g/100 g water or more at room temperature, and then the solvent is removed from the membrane by air drying or under reduced pressure. A method for improving the current efficiency of a cation exchange membrane, which is characterized in that it is used after being removed by drying. 2. The method according to claim 1, wherein the homogeneous cation exchange membrane is a non-crosslinked homogeneous cation exchange membrane. 3. The method according to claim 1, wherein the homogeneous cation exchange membrane is a non-crosslinked homogeneous cation exchange membrane containing sulfonic acid groups. 4. Claim 1, in which the swelling treatment of a homogeneous cation exchange membrane with an organic solvent is carried out while the cation exchange groups contained in the cation exchange membrane are in the state of free acid groups.
The method described in item 2 or 3. 5. Claim 1, 2 or 3 in which the swelling treatment of a homogeneous cation exchange membrane with an organic solvent is carried out while the cation exchange group contained in the cation exchange membrane is in the form of a monovalent metal salt or an ammonium salt. the method of.