JPH0715152B2 - Oxygen cathode protection method - Google Patents

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for protecting an oxygen cathode used in electrolysis using an ion exchange membrane method.

[0002]

2. Description of the Related Art Various proposals have been made to use a gas diffusion electrode, for example, an oxygen cathode, as a cathode when electrolyzing an aqueous sodium chloride solution by an ion exchange membrane method to obtain caustic soda. This ion exchange membrane electrolysis is generally carried out in an electrolytic cell divided into an anode chamber and a cathode chamber by an ion exchange membrane which is a cation membrane, and the outline of the electrolytic cell is that it has an anode and contains an aqueous sodium chloride solution. The cathode chamber is divided into an anode chamber, an electrolyte chamber containing water or a caustic soda aqueous solution by a cathode, a gas supply chamber containing an oxygen-containing gas, and an ion exchange membrane partitioning the cathode chamber. When electrolyzing by energizing between both electrodes of the electrolysis tank, by electrolyzing using a gas diffusion electrode (so-called oxygen cathode in which the oxygen-containing gas is supplied from the gas supply chamber made of a porous material) as the cathode To obtain caustic soda in the electrolyte chamber of the cathode,
The use of an oxygen gas diffusion electrode has the advantage that the hydrogen-oxygen reaction takes place at the cathode and the cathode potential drops, so that the electrolysis voltage is significantly reduced.

A typical structure of a sodium chloride aqueous solution ion exchange membrane electrolytic cell equipped with an oxygen cathode, which has been conventionally proposed to obtain high-purity and high-concentration caustic soda, will be described with reference to FIG. As shown in FIG. 4, the electrolytic cell 23 is partitioned by a cation exchange membrane 25 into an anode chamber 26 having an anode 24 and a cathode chamber 28 having an oxygen cathode 27 by a conventional method.
The cathode chamber 28 is an oxygen-containing gas supply chamber 2 by the oxygen cathode 27.
9 and the electrolytic solution chamber 30. Into the anode chamber 26, an aqueous solution of sodium chloride, which is an electrolytic solution, is supplied to the electrolytic solution supply port 31
The liquid to be electrolyzed, which has been supplied from the electrolyzed liquid outlet 32 and flows out from the electrolyzed liquid outlet 32, is mixed with a new liquid to be electrolyzed, is supplied again from the liquid to be electrolyzed supply port 31 to the anode chamber 26, and is circulated. Further, the anode chamber 26 is provided with a discharge port 33 for discharging the generated chlorine gas. The electrolytic solution chamber 30 is filled with the generated caustic soda aqueous solution, and the caustic soda aqueous solution having a constant concentration always flows out from the outlet 34 provided in the electrolytic solution chamber 30.
To obtain a constant concentration of caustic soda solution, the outlet 34
The caustic soda aqueous solution flowing out from the tank is circulated again in the electrolytic solution chamber 30 and circulated, and fresh water is supplied in the middle of the piping for circulating the caustic soda aqueous solution to prepare a diluted caustic soda aqueous solution having a constant concentration, which is electrolyzed. Will be realized. Oxygen-containing gas supply chamber 29 partitioned by oxygen cathode 27
An oxygen-containing gas is supplied from the gas supply port 34.

There are various oxygen cathodes which can be used for the ion exchange membrane method electrolysis of an aqueous sodium chloride solution to reduce the electrolysis voltage. The general structure of the oxygen cathode will be described below. The oxygen cathode is a cathode that is permeable to air and allows the electrolyte to permeate, and the side surface of the electrode that contacts the electrolyte (caustic soda solution) is supported by a current collector made of a wire mesh made of a conductor such as sodium. It is composed of a porous thin layer formed mainly of a porous conductor carrying a catalyst for oxidizing water in the presence of ions and oxygen. Activated carbon is usually used for the porous conductor, and the catalyst (made of a noble metal such as platinum) is carried in the fine pores. The oxygen-containing gas supply side surface is composed of a water-repellent porous thin layer that prevents leakage of the electrolytic solution. The water-repellent porous thin layer is usually formed mainly of fine particles of a fluororesin-based polymer having resistance to a redox reaction.

Carbon, fluororesin fine particles and other fine particle resins are mixed and molded and integrated so that the porous thin layer having the above-mentioned catalytic activity is changed stepwise or continuously from the water repellent porous thin layer. The produced porous oxygen cathode can efficiently supply the oxygen-containing gas from the oxygen-containing gas supply side to the side in contact with the electrolytic solution, and the electrolytic solution can be easily introduced into the electrode from the side in contact with the electrolytic solution. Penetrate and diffuse into. Thus, in the presence of the above-mentioned catalyst and sodium ions supplied from the side surface in contact with the electrolytic solution in the oxygen cathode, water is oxidized to form a hydroxyl group and caustic soda is produced. Therefore, hydrogen generated at the cathode in the previous electrolysis of sodium chloride solution was not generated inside the oxygen cathode, and therefore the electrolytic voltage could be lowered.

As described above, various methods for obtaining high purity caustic soda by ion exchange membrane electrolysis using an oxygen cathode capable of lowering the electrolysis voltage are being tested.
Regarding the oxygen cathode, it has a porous structure for efficiently supplying the oxygen-containing gas, and has a complicated structure in which the conductive surface is changed to the water-repellent surface stepwise from the electrolyte surface to the oxygen-containing gas supply chamber. Many improvements have been made in the past to achieve this.

That is, JP-A-54-97600 and JP-A-56-44784 propose electrode structures and constituent materials that can effectively lower the electrolytic voltage. JP-B-58-49639 discloses a device for uniformly dispersing a catalyst in an electrode, and JP-B-60-9.
Japanese Patent No. 595 describes a device for enhancing the porosity in the electrode substrate so that the caustic soda aqueous solution and oxygen can more easily penetrate into the electrode. A proposal for a method for improving the strength of the porous structure of an electrode is disclosed in Japanese Patent Application Laid-Open No. 57-152479.
And JP-A-59-133386. Further, Japanese Patent Application Laid-Open No. 59-133386 discloses a method of preventing leakage of an electrolytic solution by enhancing water repellency of a water repellent surface. Further, JP-A-56-130482 describes a method of starting electrolysis by maintaining the temperature of the electrolytic cell at 70 ° C. or higher to maintain the temperature for stable electrolysis for a long period of time and at a low electrolysis voltage.

Although many other proposals have been made regarding the production method and the performance improvement of the gas-permeable oxygen cathode, attention has been paid only to the production method and the performance improvement of the oxygen cathode, and in the electrolysis of the oxygen cathode. No proposal has been made regarding performance degradation. In order to use the oxygen cathode with high performance for a long period of time, it is necessary to prevent the deterioration of the oxygen cathode during electrolysis and during electrolysis stop, but a method for preventing deterioration during electrolysis has not been heretofore known.

On the other hand, the deterioration of the oxygen cathode while the electrolysis is stopped is described only in the following two publications. That is, JP-A-60-221595 discloses that the oxygen cathode is deteriorated due to carbon corrosion during electrolysis stoppage due to a local cell phenomenon caused by non-uniformity of oxygen adsorbed on activated carbon in the oxygen cathode. There is a description. In this publication, in order to prevent the deterioration of the oxygen cathode, it is proposed to dilute the electrolytic solution in the electrolytic cell during electrolysis or replace it with water. In addition, Japanese Patent Publication No. 61-323
No. 97 discloses that sodium carbonate is generated in the oxygen cathode during electrolysis, and the damage of the oxygen cathode due to the precipitation of the sodium carbonate causes deterioration of the oxygen cathode. To prevent this, the electrolytic solution in the electrolytic cell is It has been proposed to replace the electrolyte solution with an electrolyte solution, dilute the electrolyte solution with water, or replace it with water. However, according to the study by the present inventors, it was found that these methods are insufficient to prevent the deterioration of the oxygen cathode while the electrolysis is stopped.

[0010]

DISCLOSURE OF THE INVENTION In the present invention, in the ion-exchange membrane method electrolysis using an oxygen cathode, the present invention investigates and prevents the deterioration of the oxygen cathode during electrolysis and during electrolysis termination. The challenge is to provide a method.

[0011]

In order to solve the above-mentioned problems, the inventors of the present invention have earnestly aimed at the establishment of prevention of deterioration of the cathode when electrolyzing an aqueous alkali metal chloride solution by an ion exchange membrane method using an oxygen cathode. As a result of repeated research, the present invention has been completed.

That is, (1) in a method of electrolyzing an aqueous alkali metal chloride solution by ion exchange membrane electrolysis using an oxygen cathode, a chemical treatment for decomposing hydrogen peroxide contained in the aqueous caustic alkali solution during electrolysis is performed. Characteristic method of protecting oxygen cathode. (2) The method for protecting an oxygen cathode according to (1) above, wherein the chemical treatment is activated carbon treatment. (3) The method for protecting an oxygen cathode according to (1) above, wherein the chemical treatment is hydrazine treatment. (4) In the method of electrolyzing an aqueous solution of an alkali metal chloride by ion exchange membrane electrolysis using an oxygen cathode, the cathode chamber is filled with water saturated with hydrogen gas or an aqueous solution of caustic alkali while the electrolysis is stopped. Is a method of protection.

Chemically treating the caustic aqueous solution to decompose hydrogen peroxide in the caustic aqueous solution is to prevent deterioration during electrolysis, and filling the cathode chamber with hydrogen gas-saturated water or caustic aqueous solution is to prevent electrolysis. Preventing deterioration during stoppage.

First, prevention of deterioration during electrolysis will be described. In the ion exchange membrane method electrolysis using an oxygen cathode of an aqueous alkali metal chloride solution, a phenomenon was observed in which the oxygen cathode deteriorates during electrolysis and the voltage rises. As a result of examining the cause of deterioration of the oxygen cathode, the following conclusion was reached. That is,
The fact that hydrogen peroxide, which is partially generated at the oxygen cathode during electrolysis, is always present in the catholyte and oxidizes the activated carbon and precious metal-based catalysts that are components of the oxygen cathode, resulting in deterioration of the active part of the oxygen cathode. Is. Therefore, it was thought that hydrogen peroxide should be removed from the catholyte during electrolysis, and as a result of various studies, the present invention was completed.

In the present invention, a chemical treatment is carried out in order to decompose the hydrogen peroxide in the catholyte, but the activated carbon treatment or hydrazine treatment is preferable as the chemical treatment. The activated carbon treatment method may be a method in which activated carbon is packed in a suitable column and the catholyte is passed therethrough, or a method in which activated carbon and catholyte are mixed in a slurry state and then the activated carbon and catholyte are separated by an appropriate separator.

The shape of the activated carbon used is not particularly limited,
Any shape such as granular or powdery can be used. Depending on the type of activated carbon and the manufacturing method, it may contain impurities such as heavy metals that deteriorate the activity of the oxygen cathode. In that case, it is preferable to use an appropriate pretreatment such as acid washing to remove impurities to the extent that there is no problem.

The hydrazine treatment is preferably carried out by injecting hydrazine or an aqueous hydrazine solution into an aqueous caustic alkali solution using an appropriate amount of a pump or the like. As the hydrazine to be used, salts such as hydrazine sulfate and hydrazine hydrochloride can also be used. However, since the use of these increases the anionic impurities in the caustic product, those not salts are preferred. The hydrazine or hydrazine aqueous solution used should be free of impurities such as heavy metals that reduce the activity of the oxygen cathode.

The amount of hydrazine added to the caustic aqueous solution is preferably equivalent to or slightly larger than the equivalent amount of hydrogen peroxide produced at the cathode. If the amount of hydrazine added is small, the deterioration of the oxygen cathode cannot be sufficiently prevented, which is not preferable.

It goes without saying that the activated carbon treatment and the hydrazine treatment may be used in combination. In this case, hydrogen peroxide that could not be completely decomposed by activated carbon treatment is later treated with hydrazine, or a certain amount of hydrazine is added a little less than the equivalent amount of hydrogen peroxide, and then treated with activated carbon. Various methods are conceivable.

As a method of decomposing hydrogen peroxide in caustic alkali, it is possible to add various reducing agents other than those of the present invention, but those methods lead to deterioration of the quality of caustic products and the activity of oxygen cathode. Is lowered, which is not preferable. In the present invention, the temperature of the caustic soda aqueous solution to be chemically treated is not particularly limited, but the higher the temperature, the higher the decomposition rate of hydrogen peroxide.
C. or less is preferable.

Next, a method of preventing deterioration while electrolysis is stopped will be described. In the ion exchange membrane method electrolysis using an oxygen cathode,
A phenomenon was observed in which the oxygen cathode deteriorated and the voltage increased while the electrolysis was stopped. It is not yet fully understood why the oxygen cathode deteriorates during electrolysis. As mentioned above, JP-A-60-221595 describes that the cause is the corrosion of activated carbon due to the local cell phenomenon caused by the non-uniformity of oxygen adsorbed on the activated carbon in the catalyst layer in the oxygen cathode. However, the present inventors believe that the mechanism is as follows. That is, when electrolysis is stopped, the oxygen cathode has a portion that adsorbs oxygen to various extents, and a battery is formed between a portion having a large amount of oxygen and a portion having a small amount of oxygen. The active sites (including the active sites of the catalyst as well as the active carbon in the catalyst layer) are oxidized, resulting in deterioration of the oxygen cathode.

According to the deterioration mechanism of the oxygen cathode when the electrolysis is stopped, the method of preventing the deterioration can be considered to prevent the oxygen cathode from being oxidized by the local cell phenomenon even during the electrolysis stop, and the present invention can be completed. It was That is, the deterioration of the oxygen cathode can be prevented by filling the cathode chamber with water saturated with hydrogen gas or a caustic aqueous solution while the electrolysis is stopped. As a method of filling the cathode chamber with water or a caustic aqueous solution saturated with hydrogen gas, a method of filling the cathode chamber with water or a caustic aqueous solution, and blowing hydrogen gas into it, or water saturated with hydrogen gas in advance or A method in which a caustic aqueous solution is prepared externally and the solution is continuously or intermittently added to the cathode chamber of the electrolytic cell may be used.

It has not been sufficiently clarified why the deterioration of the oxygen cathode can be prevented by putting water saturated with hydrogen gas or aqueous caustic solution in the cathode chamber while the electrolysis is stopped. I think that the mechanism is as follows. That is, the hydrogen gas dissolved in the liquid acts as a depolarizer and is oxidized and consumed in the local cell reaction, but on the other hand, since hydrogen is oxidized, the oxidation of the active site of the oxygen cathode is prevented, and as a result, the oxygen is The deterioration of the cathode can be prevented.

In the present invention, the anode chamber is preferably emptied while the electrolysis is stopped or is flushed with pure water. In particular, when the cathode chamber is filled with a caustic aqueous solution saturated with hydrogen gas, it is necessary to flush pure water to prevent deterioration of the anode coating due to caustic alkali diffusing from the cathode chamber through the ion exchange membrane. Good.

In the present invention, the temperature of the liquid while the electrolysis is stopped is not particularly limited, but if there is a risk that the oxygen cathode will be deteriorated due to thermal contraction or the like due to too low temperature,
It is preferable to heat the liquid.

The ion exchange membrane used in the present invention is not particularly limited, and any of perfluoro sulfonic acid alone, carboxylic acid alone, and a two-layer membrane of sulfonic acid and carboxylic acid can be used.

In the present invention, the anode and the aqueous solution of alkali metal chloride used are preferably the same as those used in the conventional hydrogen generating ion exchange membrane method electrolysis. Further, the present invention can be applied to all currently known oxygen cathodes, but an oxygen cathode whose active portion is composed of activated carbon carrying a gold or platinum-based noble metal catalyst is particularly effective.

The present invention will be described with reference to the drawings. FIG. 1 shows the deterioration of the oxygen cathode during electrolysis by the electrolysis of the aqueous solution of alkali metal chloride by the ion exchange membrane method electrolysis equipped with the oxygen cathode. It shows an example in which the caustic aqueous solution passing through the cathode chamber is treated with activated carbon to prevent deterioration. In FIG. 1, an electrolytic cell 1 is partitioned by a cation exchange membrane 3 into an anode chamber 4 having an anode 2 and a cathode chamber 6 having an oxygen cathode 5, and the cathode chamber 6 is made of a caustic aqueous solution by an oxygen cathode 5. It is partitioned into a filled electrolyte chamber 7 and an oxygen-containing gas supply chamber 8.

An alkali metal chloride aqueous solution as an electrolyzed solution is supplied to the anode chamber 4 from the electrolyzed solution supply port 9 and flows out from the electrolyzed solution outlet 10. The outflowed electrolyzed solution is stored in the electrolyzed solution storage tank 20. It is stored, mixed there with a new electrolyzed solution, supplied again from the electrolyzed solution supply port 9 to the anode chamber 4, and circulated. Further, the anode chamber 4 is provided with a chlorine gas discharge port 11 for discharging the generated chlorine gas. The electrolytic solution chamber 7 is filled with the generated caustic alkaline aqueous solution, and the caustic alkaline aqueous solution having a constant concentration always flows out from the caustic alkaline water outlet 12 provided in the electrolytic solution chamber 7, passes through the liquid sending pipe 13, and is caustic alkaline. It is sent to the receiving tank 15.

Caustic alkali receiving tank 1 in the middle of the liquid sending pipe 13
5 is provided with an activated carbon packed tower 16 and the caustic aqueous solution sent through the liquid supply pipe 13 is treated with activated carbon when passing through this tower, and hydrogen peroxide contained in the caustic aqueous solution is adsorbed on the activated carbon. To be removed. The caustic aqueous solution from which hydrogen peroxide has been removed enters the caustic receiving tank 15 as a product. The product caustic aqueous solution is discharged from the caustic receiving tank 15 to the outside of the system through the product outlet. A part of the product caustic aqueous solution is returned to the electrolytic solution chamber 7 again through the reflux pipe 14, and the water injection port 1
Inject water from 7 to give an appropriate concentration. Reference numeral 18 is a (oxygen-containing) gas supply port, and 19 is a (oxygen-containing) gas discharge port.

FIG. 2 is a diagram showing a step of producing an aqueous caustic alkali solution by electrolyzing an aqueous alkali metal chloride solution in an ion-exchange membrane electrolytic cell equipped with an oxygen cathode similar to that shown in FIG. The process shown in FIG. 2 is an example of preventing deterioration of the oxygen cathode during electrolysis by another deterioration prevention method of the present invention. That is, it shows an example in which the caustic aqueous solution passing through the cathode chamber is treated with hydrazine to prevent deterioration of the oxygen cathode.

In FIG. 2, the electrolytic cell 1 and the auxiliary equipment are the same as those described in FIG. 1 except the auxiliary equipment for treating the caustic aqueous solution with hydrazine, and the reference numerals of the main parts of the equipment are the same as those in FIG. The same code was used.

In FIG. 2, the hydrazine treatment of the caustic aqueous solution is performed by the product outlet 12 provided in the electrolyte chamber 7.
The caustic soda aqueous solution flowing out of the tank is sent to the caustic alkali receiving tank 15 through the liquid sending pipe 13, but before the caustic alkali receiving tank 15 in the middle of the liquid sending pipe 13, the hydrazine injection port 2
Place 1 and inject hydrazine. Hydrogen peroxide contained in the caustic soda aqueous solution containing hydrazine is decomposed while being stored in the caustic soda receiving tank 15. The product caustic soda aqueous solution free of hydrogen peroxide is discharged from the product outlet of the caustic alkali receiving tank 15 to the outside of the system. A part of the product caustic aqueous solution is returned to the electrolyte chamber 7 through the reflux pipe 14 again, and pouring water is injected from the pouring water injection port 17 to an appropriate concentration.

FIG. 3 is an explanatory view for explaining the prevention of deterioration of the oxygen cathode while the electrolysis is stopped. In FIG. 3, the electrolytic cell 1
The auxiliary equipment is the same as that described in FIG. 1 except for the auxiliary equipment for the hydrogen peroxide removal treatment performed in the activated carbon tower, and the reference numerals of the main parts of the equipment are the same as those in FIG. In FIG. 3, the method for preventing the deterioration of the oxygen cathode while the electrolysis is stopped is as follows. In the caustic receiving tank 15, hydrogen gas is blown from the hydrogen gas blowing port 22 to prepare water saturated with hydrogen gas or a caustic alkaline aqueous solution. This water or caustic aqueous solution is sent to the cathode chamber 6 by a pump, and the cathode chamber 6 is filled with the saturated hydrogen gas while the electrolysis is stopped. When stopping, the caustic alkali in the cathode chamber 6 is returned to the caustic alkali receiving tank 10.

[0035]

EXAMPLE A Nafion membrane manufactured by DuPont was used as an ion exchange membrane, and the electrolytic cell shown in FIG. 1 was used to perform electrolysis under the following conditions. Energization amount: 30 A Current density: 30 A / dm ² Oxygen cathode: Gas diffusion electrode (Pt 0.56
mg / cm ² ) Anode: DSA (RuO ₂ + Ti
O ₂ ) Distance between electrodes: 15 mm Average caustic concentration: 32% Average temperature: 90 ° C. NaCl / Anolyte: 200 g / liter Oxygen-containing gas: Oxygen (1.2 times the theoretical amount) Oxygen The cathode gas diffusion electrode uses a platinum catalyst as the oxidation catalyst, the anode uses the metal oxide electrode shown in parentheses, and the electrolyte solution is 200
An aqueous solution of sodium chloride having a concentration of g / liter was used.

(Example 1) A circulating caustic soda solution was treated in a caustic soda solution by a method of installing an activated carbon packed tower 16 in the middle of a liquid sending pipe 13 as an auxiliary equipment in the ion exchange membrane electrolytic cell 1 shown in FIG. Hydrogen peroxide was decomposed and removed. The activated carbon used was granular activated carbon (Shirasagi W, trade name, manufactured by Takeda Pharmaceutical Co., Ltd.) that was previously washed with hydrochloric acid to remove metals. Granular activated carbon 301 with P of 300 mm inside diameter
A TEF treatment tank was filled with the activated carbon packed tower 16, and a 32% caustic soda aqueous solution was passed through the activated carbon packed tower 16 at a flow rate of 400 liters / hour. By this treatment, hydrogen peroxide in the circulating caustic soda aqueous solution could be maintained at 0.1 ppm or less. When electrolysis was continued for 1 month by this treatment method, no increase in electrolysis voltage was observed.

(Example 2) A method for injecting hydrazine from a hydrazine injection port 19 provided in the middle of a liquid sending pipe 13 as an auxiliary equipment to the ion exchange membrane electrolytic cell 1 shown in FIG. Hydrogen oxide was decomposed and removed. The hydrazine used was a reagent grade hydrated hydrazine, and the hydrazine was dissolved by pouring and added to the circulating caustic soda aqueous solution. Addition amount of hydrazine is 0.02mmo
1 / hour. By this treatment, hydrogen peroxide in the circulating caustic soda aqueous solution could be maintained at 0.1 ppm or less. When electrolysis was continued for 1 month by this treatment method, no increase in electrolysis voltage was observed.

(Comparative Example 1) When electrolysis was carried out in the electrolytic cell 1 for ion exchange membrane method shown in FIG. 1 without treatment with activated carbon, the hydrogen peroxide concentration in the circulating caustic soda solution was 1
It became 0 mg / liter. When electrolysis was continued for 1 month under these conditions, the electrolysis voltage increased by 15 mV.

Next, Table 1 shows the change in electrolysis voltage when the electrolysis was stopped, the electrolytic cell was stored under the conditions shown in the following Examples 3, 4 and Comparative Example 2, and the electrolysis was restarted. Table 1 Examples of electrolysis stopping conditions Voltage before electrolysis is stopped Voltage after electrolysis is stopped ------------------------- Example 3 2.285V 2.284V Example 4 2.294V 2.290V Comparative Example 2 2,279V 2.397V

(Example 3) In the electrolytic equipment shown in FIG. 3, hydrogen gas was blown into pure water in the caustic soda receiving tank 15 to saturate it.
The pure water saturated with hydrogen was pumped to the cathode chamber 6,
Electrolysis was stopped for 2 days while the cathode chamber 6 was filled with water.
The anode chamber was flushed with pure water, and the electrolytic cell 1 was maintained at 80 ° C.

(Embodiment 4) In the electrolysis equipment shown in FIG. 3, hydrogen gas is saturated in a caustic soda aqueous solution having a concentration of 32% in the caustic soda receiving tank 15, and the caustic soda aqueous solution is pumped to the cathode chamber 6 to feed the cathode chamber 6. Electrolysis was stopped for 2 days by a method in which the hydrogen gas was filled with a saturated liquid. The anode chamber was flushed with pure water, and the electrolytic cell 1 was maintained at 80 ° C.

(Comparative Example 2) Electrolysis was stopped for 2 days by a method in which the cathode chamber 6 was filled with pure water in the electrolytic equipment shown in FIG. The anode chamber is flushed with pure water, and the electrolytic cell 1 is at 80 ° C.
Maintained at.

From Table 1, it can be seen that in the method of storing the oxygen electrode under the conditions of Example 3 and Example 4, the electrolytic voltage at the time of restart is rather lower than that before the stop. On the other hand, under the conditions of Comparative Example 2, the electrolytic voltage at the time of restart is higher than that before the stop.

[0044]

The method of the present invention prevents deterioration of the oxygen cathode during electrolysis and during electrolysis, and can maintain a low electrolysis voltage for a long period of time. it can.

[Brief description of drawings]

FIG. 1 is a process diagram showing the method of the present invention in which a caustic aqueous solution is treated with activated carbon during electrolysis.

FIG. 2 is a process chart showing the method of the present invention in which a caustic aqueous solution is treated with hydrazine during electrolysis.

FIG. 3 is a process diagram showing the method of the present invention for supplying a hydrogen-saturated aqueous caustic alkali solution to the cathode chamber of the treatment tank while the electrolysis is stopped.

FIG. 4 is a process diagram showing a conventional method of electrolyzing an aqueous alkali metal chloride solution by ion exchange membrane electrolysis using an oxygen cathode.

[Explanation of symbols]

1 Electrolyzer 21 Hydrazine inlet 2 Anode 22 Hydrogen gas inlet 3 Cation exchange membrane 23 Electrolyzer 4 Anode chamber 24 Anode 5 Oxygen cathode 25 Cation exchange membrane 6 Cathode chamber 26 Anode chamber 7 Electrolyte chamber 27 Oxygen cathode 8 Gas supply chamber 28 Cathode chamber 9 Electrolyte solution supply port 29 Gas supply chamber 10 Electrolyte solution outlet 30 Electrolyte chamber 11 Chlorine gas outlet 31 Electrolyte solution supply port 12 Caustic alkaline water outlet 32 Electrolyte solution outlet 13 Sending Liquid pipe 33 Chlorine gas discharge port 14 Reflux pipe 34 Outlet port 15 Caustic alkali receiving tank 35 Supply port 16 Activated carbon filling tower 36 Gas supply port 17 Pouring supply port 18 Oxygen-containing gas supply port 19 Oxygen-containing gas outlet 20 Electrolyte storage Tank

Claims (4)

[Claims] 1. A method of electrolyzing an aqueous alkali metal chloride solution by ion exchange membrane electrolysis using an oxygen cathode, characterized in that a chemical treatment for decomposing hydrogen peroxide contained in the aqueous caustic alkali solution is performed during the electrolysis. Oxygen cathode protection method. 2. The method for protecting an oxygen cathode according to claim 1, wherein the chemical treatment is activated carbon treatment. 3. The method for protecting an oxygen cathode according to claim 1, wherein the chemical treatment is hydrazine treatment. 4. A method of electrolyzing an aqueous solution of an alkali metal chloride by ion exchange membrane electrolysis using an oxygen cathode, characterized in that the cathode chamber is filled with water saturated with hydrogen gas or an aqueous solution of caustic alkali while the electrolysis is stopped. Oxygen cathode protection method.