JPS6038973B2 - Method for regenerating fluorine-based cation exchange membrane - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for regenerating a fluorinated intestinal ion exchange membrane used as a diaphragm for saline electrolysis.

Specifically, when the saline solution in the anode chamber is electrolyzed in an electrolytic cell divided into an anode chamber and a cathode chamber by a fluorine-based cation exchange membrane, and a caustic soda aqueous solution is produced from the cathode chamber, the amount of calcium contained in the saline solution is , relates to a method for regenerating a fluorine-based cation exchange membrane having carboxylic acid groups whose electrolytic performance has been degraded due to contamination with impurities such as magnesium.

In recent years, the ion exchange membrane method has attracted attention as an industrial method for producing caustic soda by electrolyzing salt water, compared to the conventional mercury method and asbestos diaphragm method.

As the ion exchange membrane, a fluorine-based ion exchange membrane is generally used because it has excellent heat resistance and chemical resistance so as to withstand the harsh conditions during electrolysis. However, in this ion exchange membrane method, calcium in the raw material saline solution,
The membrane is contaminated by impurities such as magnesium during fermentation, resulting in an increase in electrolytic voltage and, in some cases, a decrease in electrolytic performance, such as a decrease in current efficiency.

This is extremely disadvantageous industrially as it increases the power consumption rate. Conventionally proposed methods for regenerating fluorine-based cation exchange membranes with degraded electrolytic performance include acidifying m-saline solution and using caustic soda, for example, as described in JP-A-51-67292. A method of diluting an aqueous solution, performing electrolysis by reducing the current density, and maintaining the above conditions for a sufficient period of time to clean the ion exchange membrane. Examples include a method of removing contaminants by immersing the ion exchange membrane in an acid having a pH lower by about 1.5 to 3.5, and then reinstalling the ion exchange membrane in the electrolytic cell.

However, when these conventional methods were applied to fluorine-based cation exchange membranes having carboxylic acid groups and regenerated,
There were almost no effects of electrolysis performance recovery, such as a reduction in electrolysis voltage or recovery of current efficiency, and in some cases, phenomena were observed where the electrolysis voltage became higher than before regeneration or current efficiency decreased on the contrary. .

For example, an X-ray microanalyzer was used to measure the state of calcium distribution in the membrane cross section of a fluorinated intestinal ion exchange membrane that had been contaminated with impurities in saline and had decreased electrolytic performance. In an ion exchange membrane consisting of two layers, ie, a layer of a fluorine-based copolymer having a sulfonic acid group, and a layer of a fluorine-based copolymer having a sulfonic acid group, the amount of calcium deposited in the layer having a sulfonic acid group is small and the thickness of the deposited layer is thin. A large amount of calcium is deposited in the layer having acid groups, and the deposited layer is also thick.

After regenerating this membrane using the conventional method, we again examined the state of calcium deposition on the cross section of the membrane using an X-ray microanalyzer, and found that although the layer containing sulfonic acid groups had been sufficiently removed. On the other hand, removal of the layer containing carboxylic acid groups was insufficient. As described above, fluorine-based cation exchange membranes having carboxylic acid groups are easily contaminated by impurities such as calcium, and it is difficult to remove impurities due to severe contamination inside the membrane. The membrane was regenerated using a conventional method, used again for electrolysis of saline water, and then removed from the electrolytic bath to observe the surface and cross section of the membrane. It was found that peeling occurred everywhere between the layers.

For this reason, although fluorine-based cation exchange membranes with carboxylic acid groups have excellent electrolytic performance, the membranes are contaminated by impurities in the saline solution, causing problems such as an increase in electrolytic voltage and a decrease in current efficiency. If the electrolytic performance of the electrolyte deteriorates, conventional techniques cannot sufficiently regenerate it.

This is a serious problem in the long-term use of fluorinated intestinal ion exchange membranes having carboxylic acid groups, and there has been a demand for the development of such a method for regenerating the waist. Therefore, the present inventors have intensively investigated a regeneration method that removes the impurities deposited on the fluorine-based cation exchange membrane having such carboxylic acid groups and restores the electrolytic performance of the membrane. We have discovered a method characterized by immersing the material in an alkaline solution (hereinafter referred to as the alkali treatment step) and then immersing it in an alkaline solution (hereinafter referred to as the alkali treatment step).

The method of the present invention is used when the membrane is contaminated by impurities in the saline solution and the electrolytic performance is reduced, and is particularly useful in the case of contamination by polyvalent metal ions such as calcium and magnesium and/or their salts. Both the acid treatment step and the alkali treatment step are indispensable steps in the present invention.

Also, depending on the degree of contamination of the membrane and electrolytic conditions (for example, the concentration of caustic soda produced or the current density),
1 in the acid treatment step and/or alkali treatment step
It is highly preferred to add more than % by weight of a water-soluble organic solvent.

The cation exchange membrane that can be applied to the method of the present invention is not particularly limited, but preferred is a fluorine-based intestinal ion exchange membrane having carboxylic acid groups, for example, having repeating units as shown in (1) and (b) below. It is something.

fCF2-CX×'Na (1) (Here, X is F, CI, or CF3, X' is F, CI
, day, or CF3,R is a side chain with an ion exchange group.

) More preferred is one in which a carboxylic acid group and a sulfonic acid group coexist as ion exchange groups.

Carboxylic acid groups and sulfonic acid groups may coexist, but a layer of a fluorine-based copolymer having a carboxylic acid group (hereinafter referred to as a carboxylic acid layer) and a fluorine-based copolymer having a carboxylic acid group Coalescence layer (hereinafter referred to as sulfonic acid layer)
A membrane consisting of two layers (hereinafter referred to as a sulfonic acid/carboxylic acid type ion exchange membrane) is preferred. For such a film, for example, the structure of the side chain of the sulfonic acid layer is (n is 0
-3, n'' and 1-4, M are hydrogen, an alkali metal or quaternary ammonium.

) The structure of the side chain of the carboxylic acid layer is (m is 0 to 3, m'' is 1 to 3, and M is hydrogen, an alkali metal, or quaternary ammonium).

). The equivalent weight (grams of dry resin containing 1 equivalent of ion exchange group) of such a sulfonic acid/carboxylic acid type ion exchange membrane is 900 to 2000, preferably 1000 for both the sulfonic acid layer and the carboxylic acid layer.
~1600.

The equivalent weights of the sulfonic acid layer and the carboxylic acid layer may be the same or different. Furthermore, the equivalent weights may also differ within the sulfonic acid layer. Such sulfonic acids/
Carboxylic acid type ion exchange membranes generally have a film thickness of 50 to 50
0 micron, preferably 100-250 micron.

Further, the ratio of carboxylic acid groups in the sulfonic acid layer of the ion exchange membrane to the total exchange groups is 10 to 10 hol%, preferably 40 to 10 hol%. The thickness of the carboxylic acid layer can be selected arbitrarily, but is usually 1.5 to 20.
Microns are preferred.

Additionally, to increase the mechanical strength of the ion exchange membrane, it can be lined with a reinforcing material such as a polytetrafluoroethylene network. An optimal example of a sulfonic acid/carboxylic acid type ion exchange membrane that can carry out the method of the present invention is one in which the side chain or a part of the side chain having a sulfonic acid group is -OCF2CF2S03M.
(M is hydrogen, alkali metal or quaternary ammonium), and the side chain or part of the side chain having a carboxylic acid group is
-OCF2C02M (M is the same as above) ion exchange membrane.

As a method for manufacturing such an ion exchange membrane, for example,
JP-A-52-24176 and JP-A-52-241
As described in No. 77, there is a method of obtaining a sulfonic acid/carboxylic acid type ion exchange membrane by changing some of the sulfonic acid groups to carboxylic acid groups.

The fluorine-based cation exchange membrane having carboxylic acid groups produced in this way is used as a diaphragm for salt water electrolysis,
Although it exhibits excellent electrolytic performance, its electrolytic performance gradually deteriorates due to trace amounts of impurities such as calcium and magnesium contained in the raw material saline solution. The contaminated membrane whose electrolytic performance has been degraded in this way is regenerated by the method of the present invention. As the acid used in the acid treatment step in the method of the present invention, both organic acids and inorganic acids can be used. Organic acids include acetic acid, oxalic acid, formic acid, etc., and inorganic acids include hydrochloric acid, sulfuric acid, etc. , nitric acid, IJ acid, hydrofluoric acid, etc., but inorganic acids with strong cleaning ability are usually used, and hydrochloric acid is most preferred. Furthermore, two or more types of acids may be used simultaneously, or a neutral salt such as sodium chloride or potassium chloride may be added at the same time as the acid. When only water is used as a solvent for the acidic solution, the concentration of the acid used is preferably 1N or more, and if it is less than 1N, the removal of impurities will be insufficient.

Further, the treatment temperature is preferably 45 qo or higher; impurities cannot be removed sufficiently if the treatment temperature is 45 qo or lower. In order to remove impurities deposited on a fluorine-based cation exchange membrane having carboxylic acid groups, quite severe conditions are required.

Furthermore, if the membrane is severely contaminated, impurities within the membrane must be removed by immersion in a higher concentration acid at a higher temperature.

However, a film subjected to acid treatment in this manner exhibits a phenomenon in which the electrical resistance of the film increases despite sufficient removal of impurities within the film. Therefore, if the membrane is heavily contaminated, a method is required to completely remove impurities under mild conditions.

For this purpose, it is preferable to further add a water-soluble organic solvent to the acidic solution. Examples of organic solvents used include lower alcohols such as methanol, ethanol, and isopropanol, ethylene glycol, glycerin,
Polyhydric alcohols such as dimethylformamide, dimethyl sulfoxide, acetone, methyl ethyl chloride, etc. Examples include ketones such as methane, ethers such as tetrahydrofuran and dioxane, and lower alcohols such as methanol and ethanol are most preferred.

The content of the organic solvent in the acidic solution may be at least 1% by weight, preferably at least 1% by weight. If it is less than 1% by weight, the effect of removing impurities and the effect of lowering the electrolytic voltage will be small.

In the case of an acidic solution containing 1% by weight or more of an organic solvent, the acid concentration may be 0.1N or more and the treatment temperature may also be 20oo or more.

In this way, the film from which impurities are almost completely removed is
Next, an alkali treatment step is performed.

The alkaline treatment step is accomplished by immersion in an alkaline solution.

Examples of the alkali used include sodium hydroxide, potassium hydroxide, ammonium hydroxide, etc., but sodium hydroxide is usually used.

In this case, a neutral salt such as sodium chloride, potassium chloride, ammonium chloride, etc. may be used simultaneously with the alkali.

When only water is used as a solvent, the treatment conditions are that the concentration of alkali is 0.1N or more, and the treatment temperature is 45
It may be in the range of 00 or more. However, when using the regenerated membrane under electrolytic conditions such as producing caustic soda with a high concentration of 25% by weight or more, the electrolytic performance cannot be recovered by simply immersing it in an alkaline solution using only water as the solvent. It turned out to be enough. In such a case, it is preferable to further add a water-soluble organic solvent to the alkaline solution.

Examples of organic solvents used include methanol, ethanol, isopro/benol, ethylene glycol, glycerin, dimethyl sulfoxide, dimethyl formamide, acetone, methyl ethyl ketone, tetrahydrofuran, dioxane, etc.
Particularly preferred is dimethyl sulfoxide.

The content of the organic solvent in the alkaline solution may be at least 1% by weight, preferably at least 1% by weight.

The concentration of the alkali used in the alkaline aqueous solution containing 1% by weight or more of an organic solvent may be 0.01N or more, and the treatment temperature may also be 2000 or more.

Although the treatment time is not particularly limited, it is usually preferably 3 hours or more. In both the acid treatment step and the alkali treatment step, it is preferable to drain the liquid during the treatment time in order to uniformly treat the membrane.

The membrane regenerated in this way is loaded into the electrolytic cell again, but the membrane soaked in an alkaline solution containing an organic solvent is
It is preferable to wash with water or an alkaline aqueous solution to remove the organic solvent.

As described above, the electrolytic performance of a fluorine-based cation exchange membrane having carboxylic acid groups is restored to its initial state by the method of the present invention. In the method of the present invention, the mechanisms of action of the acid treatment step and the alkali treatment step are not necessarily clear, but are estimated as follows.

The acid treatment step is thought to have the effect of dissolving and removing impurities such as calcium and magnesium that have adhered to the membrane, and the effect of removing multivalent ions substituted with ion exchange groups.

Additionally, organic solvents added to acidic solutions swell the membrane;
This is thought to have the effect of promoting dissolution and removal of impurities. For example, when an alcohol is added, it may have the effect of binding to an ion exchange group and making it a hydrophobic ion exchange group. In addition, in the alkali treatment step, the effect of completely replacing the ion exchange groups of the membrane with alkali salt type and at the same time adjusting the membrane by appropriate swelling and contraction is considered.

The method of the present invention can be carried out at any time regardless of the degree of deterioration of the electrolytic performance of the intestinal ion exchange membrane, but in normal saline electrolysis, practical electrolytic conditions, such as a current density of 10 to 60 A, can be carried out. /dm2, in the range of caustic soda concentration from 2 normal to la normal, and the electrolytic voltage is 0.
It is effective to carry out when the current efficiency increases by 0.5V to 0.5V and/or the current efficiency decreases by 1 to 5%.

Fluorine-based cation exchange membranes with carboxylic acid groups that were contaminated by impurities in the saline solution during electrolysis and had degraded electrolytic performance were previously discarded as unusable, but with the method of the present invention, the initial Restore electrolytic performance,
Since it can be used repeatedly, it is extremely advantageous industrially.

Furthermore, the regeneration method of the present invention can be applied not only to the electrolysis of common salt but also to the electrolysis of other alkali metal halides. The present invention will be specifically explained below with reference to Examples, but the present invention is not limited thereto.

Examples 1 to 7 A film made from a copolymer of CF2=CF2 was
According to the method described in No. 77, one side was treated with a reducing agent to obtain a sulfonic acid/carboxylic acid type ion exchange membrane.

The equivalent weight of this membrane was 1200 and the membrane thickness was 125 microns. The carboxylic acid layer of this film has a side chain structure of 10 on one side, and the remaining sulfonic acid layer has a side chain structure of .

After ion-exchanging the membrane to Na type, the carboxylic acid layer was placed in an electrolytic cell with the carboxylic acid layer facing the cathode chamber, and the current density was 40A/
Electrolysis of saline water was carried out at dm2 and electrolysis temperature of 90°C. The electrolytic cell consists of an anode chamber and a cathode chamber via an ion exchange membrane, the anode being a dimensionally stable metal anode and the cathode being an iron wire mesh. The anolyte is a 3N saline solution with a pH of 3.5 and contains 1ppm of calcium and 1ppm of magnesium as impurities. The concentration of caustic soda in the catholyte is 2% by weight. The electrolysis voltage at the beginning of electrolysis was 3.8
V, the current efficiency was 95%, but during continuous operation, the membrane was contaminated by impurities in the salt water, and after 3000 hours, the electrolytic voltage was 4.1 V and the current efficiency was 93%. .

This contaminated membrane was removed from the electrolytic cell, regenerated by the method shown in Table 1, and the electrolytic performance after regeneration was measured. The results are shown in Table 1. Table 1 Comparative Examples 1 to 5 The same sulfonic acid/carboxylic acid type ion exchange membranes used in Examples 1 to 7 were used for electrolysis of saline water under the same conditions as Examples 1 to 7. The contaminated membrane whose performance had deteriorated was regenerated by the method shown in Table 2, and the electrolytic performance after regeneration was measured. Table 2 shows the results.

From this result, as seen in Comparative Examples 4 and 5, those without the alkali treatment step had worse electrolytic performance than before regeneration.

Further, as seen in Comparisons 1, 2, and 3, when the acid treatment step or the alkali treatment step is insufficient, the recovery of electrolytic performance is not sufficient. Table 2 Examples 8 to 14 Sulfonic acid/carboxylic acid type ion exchange membranes obtained in the same manner as in Examples 1 to 7 were subjected to electrolysis under the same conditions as in Examples 1 to 7.

However, in this case, the calcium contained in the saline solution is 3 ppm.
, magnesium was 2.5 ppm. At the beginning of electrolysis, the electrolysis voltage was 3. The current efficiency was 95%, but after 3000 hours, the electrolysis voltage decreased to 4.3V and the current efficiency decreased to 91%. This contaminated membrane was removed from the electrolytic cell, regenerated by the method shown in Table 3, and the electrolytic performance after regeneration was measured. The results are shown in Table 3. Table 3 Comparative Examples 6 to 10 The same sulfonic acid/carboxylic acid type ion exchange membranes used in Examples 8 to 14 were used for saline water electrolysis under the same conditions as Examples 8 to 14, and 300 field hours were used. Afterwards, the electrolytic voltage is 4
．． A contaminated membrane whose electrolytic performance had decreased to 3V and a current efficiency of 91% was regenerated* using the method shown in Table 4, and the electrolytic performance after regeneration was measured by installing it in the electrolytic cell again.Table 4 shows the results. Shown below.

From this result, when no organic solvent is used in the acid treatment process, there is almost no recovery in electrolytic performance, and when the acid concentration is 0.01N or the organic solvent is 0.1%, the electrolytic performance does not recover. is not enough.

Table 4 Examples 15 to 22 The sulfonic acid/carboxylic acid type ion exchange membranes obtained in Examples 1 to 14 were incorporated into the electrolytic cells shown in Examples 1 to 8, and the current density was 40 A/dm2 and the electrolysis temperature was 90°C. Electrolysis of saline water was carried out.

At this time, the anolyte is a 3N saline solution with a pH of 3.5, and this solution contains 1 ppm of calcium and 1 ppm of magnesium.
Contains ppm.

Further, the concentration of caustic soda in the catholyte was 40%.

The electrolysis voltage at the beginning of electrolysis was 3.95V, and the current efficiency was
However, it was contaminated by impurities in the saline solution during operation, and after 300 hours, the electrolytic voltage was 4.3V.
The current efficiency was 90%.

This contaminated film was regenerated by the method shown in Table 5.

The electrolytic performance after regeneration was measured. The results are shown in Table 5. Table 5 Comparative Examples 11-14 Same sulfonic acids/as used in Examples 15-22
A carboxylic acid type ion exchange membrane was used for electrolysis of saline water under the same conditions as in Examples 15 to 22, and after 300 feeding hours, the contaminated membrane had an electrolysis voltage of 4.3 V and a current efficiency of 90%. , and the electrolytic performance was measured after the regeneration treatment.

The results are shown in Table 6. From this result, it is clear that recovery of electrolytic performance is extremely insufficient when no organic solvent is included in the alkali treatment step.

Table 6

Claims (1)

[Scope of Claims] 1. A method for regenerating a fluorine-based cation exchange membrane having a carboxylic acid group, which is used for the electrolysis of common salt and whose electrolytic performance has deteriorated due to contamination with impurities in the salt water. A method for regenerating a fluorine-based cation exchange membrane, which comprises immersing it in an acidic solution and then immersing it in an alkaline solution. 2. The method according to claim 1, wherein the acidic solution contains 1% by weight or more of a water-soluble organic solvent. 3. The method according to claim 1 or 2, wherein the alkaline solution contains 1% by weight or more of a water-soluble organic solvent.