JPS5948665B2 - Method for regenerating cation exchange membrane for alkali chloride electrolysis - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for regenerating a cation exchange membrane for alkali chloride electrolysis.

That is, when a cation exchange membrane is used as a diaphragm for electrolysis of an aqueous alkali chloride solution for a long period of time, the performance of the cation exchange membrane gradually decreases, resulting in undesirable phenomena such as a decrease in current efficiency and an increase in voltage between electrodes. The present invention relates to a method for restoring a cation exchange membrane whose performance has deteriorated to substantially its original performance state. Conventionally, electrolysis methods for aqueous alkali chloride solutions using a cation exchange membrane as a diaphragm include a two-chamber electrolysis method in which a single cation exchange membrane is placed between the negative and anode electrodes, and a two-chamber electrolysis method in which a single cation exchange membrane is placed between the negative and anode electrodes. Three-chamber electrolysis method with a porous diaphragm, three-chamber electrolysis method with a porous diaphragm between the cation exchange membrane and cathode, and a two-chamber electrolysis method with both electrodes and membranes arranged substantially horizontally. There are electrolysis methods.

The concentration of aqueous alkali chloride solution in the anode chamber where the anode exists is usually between 1 normal and saturation, and water may or may not be supplied to the cathode chamber where the cathode exists depending on the target concentration of alkali hydroxide. There are cases. Despite these various electrolysis methods, in all cases, if electrolysis is carried out continuously to some extent, the current efficiency of the membrane gradually decreases.
Furthermore, the voltage between electrodes gradually increases. For example, when electrolyzing saline water using a cation exchange membrane in which sulfonic acid groups are bonded pendantly to the perfluorocarbon skeleton through ether bonds, the current density is 30 A/dT.
rl) Produced Caustic Concentration 6 When salt water with the composition shown in the table below is continuously electrolyzed under the specified conditions, the current efficiency (ηNaOH) decreases over time, the voltage between the electrodes increases, etc. as illustrated in Figure 1. Unfavorable phenomena occur. According to FIG. 1, in just three months, the current efficiency decreases by about 10% and the inter-electrode voltage increases by about 0.7 volts.

It is not necessarily clear why such an undesirable phenomenon occurs, but when a cation exchange membrane that has been subjected to electrolysis for a long time is removed, precipitates adhere to the surface and inside the membrane. It is presumed that the influence of sediment is also a contributing factor. Since these precipitates contain hydroxides of calcium and magnesium, it is conceivable to bring these into contact with an acid and dissolve and remove them as easily soluble salts. In fact, this precipitate can be substantially removed by immersing such a membrane in an aqueous acid solution such as hydrochloric acid. However, according to the experience of the present inventors, even with a cation exchange membrane from which precipitates have been removed in this way, the membrane resistance recovers almost completely, but the current efficiency, once reduced, hardly recovers. Decrease in current efficiency and
Needless to say, an increase in inter-electrode voltage due to an increase in membrane resistance is disadvantageous in terms of cost. The present inventors have conducted repeated research on methods for regenerating cation exchange membranes whose performance has deteriorated in terms of current efficiency and interelectrode voltage. The present invention is based on the discovery of a method for recovering to a state close to that of the original state, that is, to substantially the original state. That is, the present invention
This is a method for regenerating a cation exchange membrane, which is characterized in that a perfluorocarbon cation exchange membrane that has been subjected to electrolysis of an aqueous alkali chloride solution is brought into contact with an acid solution having oxidizing power and having a pH below PHL. By this regeneration method, the current efficiency and voltage between electrodes can be restored to substantially their original state. Conventionally, it has been common practice to treat ion exchange membranes after electrolysis with a simple mineral acid, etc., which is effective not only for ion exchange membranes used in alkaline electrolysis methods but also for ion exchange membranes used in ion exchange membrane salt production methods. It has often been carried out for the purpose of removing precipitates such as calcium and magnesium, lowering the electrical resistance of the membrane, and lowering the interelectrode voltage.
However, it has never been expected for any type of ion exchange membrane to recover the current efficiency once it has decreased. However, among cation exchange membranes, the present inventors targeted perfluorocarbon cation exchange membranes in particular, and by contacting them with an acid solution having oxidizing power, they improved not only the interelectrode voltage but also the current efficiency. This also makes it possible to substantially restore the original state.

As described above, it is not clear why the effects of the present invention are obtained only when a perfluorocarbon-based cation exchange membrane is brought into contact with an acid solution having oxidizing power.

However, since recovery occurred only after contact with an acid solution having oxidizing power, it is presumed that some kind of organic matter may be a contributing factor to the decrease in membrane performance. Furthermore, it is surprising that among cation exchange membranes, only perfluorocarbon membranes recover well in both current efficiency and interelectrode voltage. This is presumed to be related to the presence or absence of crosslinking, since normally perfluorocarbon cation exchange membranes do not have crosslinks and such membranes are regenerated well. In general, ion exchange membranes used for electrolysis of aqueous alkali chloride solutions are broadly divided into hydrocarbon-based ion-exchange membranes based on a copolymer of styrene and divinylbenzene, and perfluorocarbon-based ion-exchange membranes. .

However, as mentioned above, the regeneration method of the present invention is effectively applied to perfluorocarbon ion exchange membranes. For sulfonic acid-based or carboxylic acid-based ion exchange membranes based on copolymers of styrene and divinylbenzene, the membrane resistance decreases to some extent even after these treatments, but current efficiency does not recover. It can't be achieved. Perfluorocarbon membranes used in the present invention include those having sulfonic acid groups obtained by copolymerizing and hydrolyzing tetrafluoroethylene and perfluoro(alkyl vinyl ether sulfonyl fluoride) as exchange groups, and those having tetrafluoroethylene and perfluoroacrylic acid groups as exchange groups. The one having a carboxylic acid group obtained by polymerizing an acid, or the sulfonic acid type ion exchange membrane with the structural formula CF2=CF-0÷C→NCO
Any film based on perfluorocarbon, such as one that has a combination of sulfonic acid groups and carboxylic acid groups, is polymerized and hydrolyzed after being impregnated with a monomer called OCH3 (where n is an integer from 1 to 6). It can be applied to a wide range of things.

Further, the acid solution having oxidizing power used in the present invention is not limited in any way as long as it is an acid solution and has oxidizing properties.

Examples include a solution of an acid with oxidizing power, a mixed solution of an oxidizing agent and an acid, and the like. Specifically, a mixed solution of nitric acid: aqua regia; chromic acid; a mixed solution of hydrogen peroxide and a mineral acid or organic acid such as hydrochloric acid or acetic acid are preferably used; nitric acid is particularly effective and easy to handle. ing. Also,
If the concentration of the acid is too low, it will take a long time to contact the membrane to recover the performance of the membrane, and the recovery rate of performance will decrease, so it is preferable to use an acid solution with a PHL or lower. An acid concentration of 0.5 normal or more is preferably used. Further, when an oxidizing agent is used in combination, the amount of the oxidizing agent varies depending on the concentration of the acid, but is generally effective at about 1 to 30 mol percent relative to the acid. Further, in the present invention, the method of bringing the cation exchange membrane into contact with the acid solution having oxidizing power is not limited at all.

One of the simplest ways to make contact is to introduce an aqueous solution of an oxidizing acid into the anode chamber of the electrolytic cell, fill the anode chamber with the solution, or preferably, slowly drain the solution into the anode chamber. The solution is to renew the solution continuously or intermittently. Furthermore, if a high concentration of caustic alkali is present in the cathode chamber during these treatments, it may be undesirable that it takes a long time to achieve a desired effect or that the effect is degraded. Therefore, during treatment, it is preferable to lower the alkali concentration in the cathode chamber, and in particular, it is desirable to substantially introduce water into the cathode chamber. Of course, normal electrolysis cannot be performed during the treatment of the present invention. Generally, the current will be cut off, but a small amount of current may be passed for corrosion protection of the electrolytic cell and for other maintenance purposes. Yet another treatment method is to remove the cation exchange membrane that has been subjected to electrolysis and whose performance has deteriorated from the electrolytic cell, and immerse it in an acid solution with oxidizing power or wash it with an acid solution that has oxidizing power. .

Alternatively, the acid solution can be applied to the membrane, left for an appropriate period of time, and then washed with water. These contact methods and contact times may be appropriately selected empirically. In general, the contact time should be 3 to 3 to achieve almost complete recovery of the cation exchange membrane.
It is preferable to set it as 0 minutes or more, and it is sufficient if it carries out for 1 to 24 hours. Further, the contact temperature is not particularly limited, but it is usually carried out in a temperature range of room temperature to about 90°C. An example of the effect of the regeneration method of the present invention is shown in FIG. Figure 2 shows the relationship between the recovery rate of membrane performance and acid concentration when a cation exchange membrane used under the same conditions as in Figure 1 was immersed in a nitric acid aqueous solution at room temperature for 10 hours. This is what is shown. According to this, performance is recovered by 50 to 60% even at a dilute acid concentration of about PH4, but the current efficiency recovers to almost its original state when the concentration is below PH1. For voltage, PHL
．． It is understood that the recovery rate increases rapidly below 5.
The present invention is effective for any perfluorocarbon cation exchange membrane used for electrolysis of an aqueous alkali chloride solution, generally an aqueous solution of sodium chloride or potassium chloride.

However, it is desirable to regenerate the membrane before its performance deteriorates excessively, and in general, it is a perfluorocarbon-based cation exchange membrane that has been subjected to electrolysis of an aqueous alkali chloride solution at a current density of 10 to 50 A/DWI. The current efficiency decreases by several percent to several tens of percent, and the voltage between electrodes reaches 0.
, 1 to 1.0 volts higher than the value at the initial stage of electrolysis can be regenerated by the method of the present invention. As explained above, the present invention relates to a method for regenerating perfluorocarbon-based cation exchange membranes, and is capable of almost completely recovering not only the interelectrode voltage but also the current efficiency, which was considered impossible with conventional techniques. The present invention provides a recycling method that can be used to reuse the product, making it possible to reuse it in the same way as a new product, which has great economic significance.

EXAMPLES Hereinafter, the effects of the present invention will be illustrated more specifically by Examples.

Example 1 RUO2 and TiO2 were placed on a lath-shaped titanium base material as an anode.
A two-chamber electrolytic cell with a current-carrying area of 1 d was assembled using a lath-shaped iron coating as a cathode and Nafion 427 (trade name: manufactured by DuPont, USA) as a cation exchange membrane.

Note that Nafion is a perfluorocarbon-based cation exchange membrane that uses sulfenic acid groups as exchange groups. Using this electrolytic cell, saline solution was supplied so that the salt concentration in the anode chamber was 1.7 normal, and continuous operation was performed at a current density of 30 A/DTI without supplying water to the cathode chamber. Under these conditions, approximately 10.5N of caustic soda was generated in the cathode chamber, but at the beginning of energization, the current efficiency was 78% and the voltage between electrodes was 3.93V, but on the 20th day after energization started, the current Efficiency decreased to 69%,
The interelectrode voltage rose to 4.08V. At this point, disassemble the battery case, take out the membrane, and add it to 3N nitric acid aqueous solution at room temperature for 2 hours.
PH of the treatment solution before and after the 0-hour immersion treatment
was less than 0.5. After the treatment, the membrane was washed with water, put back into the original electrolytic cell, and electrolyzed under the same conditions as before. As a result, the current efficiency was 79% and the interelectrode voltage was 3.95V. Example 2 The electrodes and electrolytic cell were the same as in Example 1, Nafion 324 (trade name, manufactured by DuPont, USA) was used as the ion exchange membrane, and saline solution was supplied so that the salt concentration in the anode room was 4.6 normal. However, steady operation at 30 A/Dd was performed under conditions such that water was added to the cathode chamber to obtain 6N caustic soda.

The initial current efficiency was 84% and the voltage between electrodes was 3.57V, but on the 30th day of energization, the performance decreased to 78.5% and the voltage between electrodes was 3.85V. At this point, the membrane was taken out and immersed in dilute aqua regia (a mixture of 1 volume of concentrated nitric acid and 3 volumes of concentrated hydrochloric acid, diluted twice with water) for 16 hours at room temperature. Before and after the treatment, the pH of the treatment solution was , 0.5 or less. After the treatment, the membrane was subjected to electrolysis under the same conditions as before treatment, and the current efficiency was 83% and the voltage between electrodes was 3.59V. Example 3 Using the same electrode and electrolytic cell as in Example 1, Nafion 315
(Product name; manufactured by DuPont, USA) as a diaphragm, salt water was supplied to the anode chamber so that the salt concentration was 4.0N, and water was supplied to the cathode chamber so that the caustic soda concentration was 3N.
Electrolysis was performed at a current density of 30 A/DTrl.

At the beginning of electrolysis, the current efficiency was 91% and the sliding voltage was 3.72V, but after 30 days, the current efficiency was 84% and the voltage was 3.
．． It became 97V. At this point, the electricity was turned off, and while the electrolytic cell remained assembled, 1N nitric acid was continuously supplied to the anode chamber, and the caustic soda in the cathode chamber was replaced with water. The pH of the acid solution discharged from the anode chamber was 0.5 or less. Further, water was continuously supplied and the temperature was kept at 60° C. for 2 hours. Next, after cleaning the inside of the anode chamber, electrolysis was carried out under the same conditions as before, and the current efficiency was 90% and the sliding voltage was 3.78V. Example 4 A mixture of 10y of tetrafluoroethylene and 89% of perfluoroacrylic acid was applied to a polytetrafluoroethylene cloth and polymerized. Thickness: 0.15"l) Exchange capacity: 1.
5meq/9 ・DryResin film resistance 4.8Ω-
The same electrolytic cell as in Example 1 was assembled using a c-d (80°C, 6N caustic medium) membrane as a diaphragm.

A 3.5N potassium chloride aqueous solution was supplied to the anode chamber, and water was added to the cathode chamber to obtain 4N caustic potassium.
When electrolysis was carried out at a current density of 30 A/d, the current efficiency was 78% and the voltage between electrodes was 4.15 V at the initial stage of energization, but the current density was 3.
After 0 days of continuous operation, the current efficiency was 72% and the voltage between electrodes was 4.
．． It became 55V. At this point, the membrane was taken out and immersed for 10 hours at room temperature in a mixed solution of 3N hydrochloric acid and 20% hydrogen peroxide solution added. The pH of this treatment liquid is 0.
It was 5 or less. After the treatment, the membrane was washed with water and put back into the original electrolytic cell for electrolysis. As a result, the current efficiency was 77% and the interelectrode voltage was 4.13V. Comparative Example 1 Nafion 3 was used as the same electrode, electrolytic cell, and diaphragm as in Example 1.
30A/D using 24 (trade name; manufactured by DuPont, USA)
When electrolyzed (6 N caustic soda) for 30 consecutive days with wl, the current efficiency decreased from 84% to 78%, and the voltage was 3.
．． The voltage rose from 61V to 3.88V.

At this point, the membrane was taken out and immersed in a 3N aqueous hydrochloric acid solution at room temperature for 16 hours. Before and after the treatment, the pH of the treatment liquid is 0.
．． It was 5 or less. After the treatment, the membrane was electrolyzed again under the same conditions as before, and the current efficiency was 79% and the voltage was 3.68V. Comparative Example 2 Styrene 20y) A mixture of 109% divinylbenzene, 259% methacrylic acid, and 159% stearyl methacrylate was applied to a polypropylene cloth and polymerized. Thickness: 0.21, exchange capacity: 3.5 meq/Dry-R
An ion exchange membrane with a membrane resistance of 4.2Ω-Cd (80°C, 6N caustic) was placed between the same electrodes as in Example 1, and a porous polytetrafluoroethylene membrane was placed between the anode and the ion exchange membrane. Hydrophilic treated (carbon black filled) membrane resistance 0.8 ohm, water permeability 0.02 CC/Cd-Hr
An electrolytic cell with a current-carrying area of 1 dw1 in which a -CnLH2O film was arranged was assembled.

Saline solution was supplied so that the salt concentration in the anode chamber was 4.5N, and the 4.5N saline solution was circulated in the intermediate chamber at a linear velocity of 2 cm/sec. At this time, the intermediate chamber has a water column of approximately 8
There was a pressure of 0 cm. Water was added to the cathode chamber to obtain 6N caustic soda. When electrolyzed for 20 days at a current density of 30A/Dwl, the current efficiency increased from 92% to 8.
5%, the voltage between electrodes decreased from 4.05V to 4.25V.

This membrane was taken out and treated with 3N nitric acid at room temperature for 20 days. The pH of the treatment solution was 0.5 or less before and after the treatment.
When the membrane was put back into the original electrolytic cell and electrolyzed under the same conditions, the current efficiency was 85% and the interelectrode voltage was 4.15.

[Brief explanation of the drawing]

FIG. 1 is an example of a graph showing how the performance of a cation exchange membrane deteriorates over time in the electrolysis of common salt, and FIG. 2 is an example of a graph showing how the performance is recovered by the treatment of the present invention.

Claims (1)

[Scope of Claims] 1. A method for regenerating a cation exchange membrane, which comprises bringing a perfluorocarbon cation exchange membrane subjected to electrolysis of an aqueous alkali chloride solution into contact with an acid solution having oxidizing power and having a pH of 1 or less. 2. The method according to claim 1, in which nitric acid is used as the acid solution having oxidizing power. 3. The method according to claim 1, in which the perfluorocarbon cation exchange membrane has an exchange group having a sulfonic acid group and/or a carboxylic acid group.