JP2001020089A - Protective method of alkali chloride electrolytic cell and protective device therefor - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a protective method and a protective device for an alkali chloride electrolytic cell, which is capable of securing the high performance of the electrolytic cell of its own through a long period by preventing the degradation of a gas diffusion electrode and the corrosion of an cathode chamber in the period of stoppage. SOLUTION: The protective method of the alkali chloride electrolytic cell is by stopping the supply of an oxygen-containing gas to a gas chamber and substantially replacing the oxygen-containing gas atmosphere in the gas chamber by an inert gas atmosphere at the time of stopping the operation of the alkali chloride electrolytic cell provided with the gas diffusion cathode. As the inert gas for replacing the gas chamber, nitrogen is preferably used. The protective device includes an automated protective system performing the operation to stop the supply of the oxygen-containing gas to the gas chamber at the time of stopping the operation of the alkali chloride electrolytic cell provided with the gas diffusion negative electrode and the operation to replace by the inert gas in the gas chamber after the supply thereof is stopped.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method and an apparatus for protecting an alkaline chloride electrolytic cell, and more particularly, to an electrolytic cell provided with a gas diffusion cathode and electrolyzing an aqueous alkaline chloride solution to produce chlorine and caustic alkali. The present invention relates to a method and a device for protecting an alkaline chloride electrolytic cell when an electrolysis operation is stopped.

[0002]

2. Description of the Related Art It is known that a caustic alkali can be obtained by electrolyzing an aqueous alkali chloride solution by an ion exchange membrane method using a gas diffusion cathode. The outline of this manufacturing method is as follows. An anode chamber having an anode and containing an aqueous alkali chloride solution, and a cathode chamber having a gas diffusion cathode and containing water or caustic aqueous solution are partitioned by an ion exchange membrane, which is generally a cation exchange membrane, and a space between the two electrodes is provided. To electrolyze the aqueous alkali chloride solution. In this case, caustic is obtained in the cathode chamber by performing electrolysis using a gas diffusion cathode that is made of a porous material as a cathode and is electrolyzed while supplying an oxygen-containing gas, and hydrogen is generated at the cathode. Since no gas is generated, there is the advantage that the electrolysis voltage is significantly reduced. Patent documents disclosing this production method include, for example, JP-A-54-97600 and JP-A-56-44.
784, JP-A-56-130482, JP-A-57-152479, JP-A-59-13338.
No. 6, Japanese Patent Application Laid-Open No. Sho 61-266591, Japanese Patent Publication No. Sho 5
JP-A-8-44156, JP-B-58-49639, JP-B-60-9595, and JP-B-61-206.
No. 34, and the like.

[0003] Many proposals have been made regarding the production method and performance of gas diffusion cathodes. However, there are only a few proposals on how to prevent performance degradation during shutdown of the gas diffusion cathode. For example, JP
Japanese Patent Application No. 0-221595 proposes a method in which the electrolytic solution in the electrolytic cell is diluted with water or replaced with water during electrolysis stop. Japanese Patent Application Laid-Open No. 5-255882 proposes filling the cathode chamber with water or a caustic aqueous solution saturated with hydrogen gas while the electrolysis is stopped. The present inventors have also found that these methods have a certain effect in terms of protecting the gas diffusion cathode. On the other hand, it turned out that there were many problems. For example, Japanese Patent Laid-Open No. 60-221595 proposes that "during the electrolysis stop, the electrolytic solution in the electrolytic cell is diluted with water or replaced with water." This method may cause a decrease in the performance of the ion exchange membrane. JP-A-5-25588
No. 2 proposes "filling the cathode chamber with water or a caustic aqueous solution saturated with hydrogen gas while the electrolysis is stopped". In this method, the gas chamber is filled with an oxygen-containing gas,
Since the gas diffusion cathode is gas permeable, it was found that hydrogen gas remaining inside mixed with the gas diffusion cathode was highly likely to form a detonation gas, which was problematic in practical use.

[0004]

In the case of an ion-exchange membrane electrolytic cell using a gas diffusion cathode, in order to operate for a long time while maintaining its original high performance, the electrolytic operation conditions are important, and in particular, The method of treatment when the electrolytic cell is stopped is also an important problem. This is because, depending on the method of treatment, the performance of the electrolytic cell may be significantly reduced at the time of stoppage, or may become unusable in some cases. The ion exchange membrane method alkali chloride electrolysis without using a gas diffusion cathode by a technique already known from the past is performed as follows. When an anode chamber including an anode and a cathode chamber including a cathode are separated by an ion exchange membrane, and an alkaline chloride aqueous solution is supplied to the anode chamber to perform electrolysis, chlorine gas is generated on the anode side, and caustic or water is supplied to the cathode chamber. Is supplied to generate caustic alkali and hydrogen gas on the cathode side. Therefore, in normal operation, the anode has a noble potential from the chlorine generation potential by the chlorine overvoltage, and the cathode has a noble potential from the hydrogen generation potential by the hydrogen overvoltage.

When the electrolysis operation of the electrolytic cell is stopped, both the generation of chlorine and the generation of hydrogen are stopped. In the stopped state, chlorine is dissolved in the aqueous alkali chloride solution as the anolyte, and hydrogen gas is dissolved in the aqueous caustic alkali solution as the catholyte. Eventually, the anode and anode compartments are held at a chlorine generating potential, and the cathode and cathode compartment are held at a hydrogen generating potential. For the electrolytic cell, a material that can withstand such a voltage potential condition is used. Specifically, the anode chamber generally uses titanium. The anode includes titanium as a base material, which is coated with a noble metal such as platinum, ruthenium, iridium, palladium, and rhodium, which is a specific electrode catalyst for chlorine generation, and an oxide thereof. The cathode chamber uses nickel or high-grade stainless steel. The cathode uses nickel or high-grade stainless steel as a base material, on which various substances are coated as an electrode catalyst for hydrogen generation.

On the other hand, in an alkali chloride electrolytic cell using a gas diffusion cathode, as long as normal operation is performed, the anode has a noble potential corresponding to the chlorine overpotential from the chlorine generation potential.
This is the same as the ion exchange membrane method alkali chloride electrolysis without using a gas diffusion cathode. However, the gas diffusion cathode has a potential lower than the reduction potential of oxygen by an overvoltage. When the electrolytic cell stops, the chlorine generation reaction and oxygen reduction reaction stop, but since the chlorine is dissolved in the aqueous alkali chloride solution as the anolyte, the voltage potential of the anode and the anode chamber is maintained at the chlorine generation potential. Is done. On the other hand, since the gas diffusion cathode is in a condition of being brought into contact with the aqueous caustic alkali solution and the oxygen gas, the voltage potential between the gas diffusion cathode and the cathode chamber is maintained at the oxygen reduction potential.

[0007] When these two electrolysis methods are compared, the anodic reaction is exactly the same. Even in the electrolytic cell using the gas diffusion cathode, there is no problem in designing the material of the anode and the anode chamber as in the conventional ion exchange membrane method electrolysis without using the gas diffusion cathode. However, cathodic reactions are very different. When a normal metal material such as nickel or stainless steel used in conventional ion exchange membrane electrolysis without a gas diffusion cathode is used in the gas diffusion cathode or cathode chamber of an electrolytic cell using a gas diffusion cathode However, the susceptibility to corrosion increased, and there was a problem in durability when used for a long period of time. Silver is generally more resistant to corrosion than these materials and is not as expensive as other precious metals other than silver.
Therefore, it is notable to corrode even in an oxidizing environment, and is attracting attention as a material that can be used for a long time. Japanese Patent Publication No. 58-4
No. 9639 discloses a technique using silver obtained by thermal decomposition of a carboxylic acid as a catalyst.
Japanese Patent Laid-Open No. 0-158877 discloses a technique using silver or a silver alloy for joining a gas diffusion cathode to a conductive porous body of a gas chamber. There is disclosed a technique of coating sponge nickel, which is a conductive porous material, with silver.

However, according to the study of the present inventors, the following has been found. In an ion exchange membrane electrolysis using a gas diffusion cathode, when ordinary electrolysis is performed, silver is certainly a material having sufficient corrosion resistance as a material used for the gas diffusion cathode and the cathode chamber, and there is no problem. Even when nickel is used, the degree of appearance of corrosion is not remarkable. However, it was found that the corrosion resistance was poor when the electrolysis was stopped. In a gas diffusion cathode, silver may be used as a catalyst. When silver is used as a catalyst, its poor corrosion resistance can have a fatal effect.
In some cases, a noble metal such as platinum is used as a catalyst for the gas diffusion cathode. Even precious metals, such as platinum, may corrode at the oxygen reduction potential. The present invention, in an electrolytic cell equipped with a gas diffusion cathode, when stopping the operation of electrolyzing an aqueous alkali chloride solution to produce chlorine and caustic, prevents deterioration of the gas diffusion cathode during the shutdown and corrosion of the cathode chamber. Accordingly, it is an object of the present invention to provide a method and a device for protecting an alkaline chloride electrolytic cell which can maintain the original high performance of the electrolytic cell for a long period of time.

[0009]

Means for Solving the Problems The present inventors have repeatedly studied a method for producing chlorine and caustic alkali by electrolyzing an aqueous alkali chloride solution in an electrolytic cell provided with a gas diffusion cathode. In particular, while the electrolytic cell was stopped, the gas diffusion cathode and the cathode chamber were prevented from being corroded, and as a result of intensive studies on means for maintaining the high electrolytic performance inherent in the electrolytic cell for a long period of time, the present invention was completed.

That is, the present invention can solve the above problems by the following means. (1) When the operation of the alkali chloride electrolytic cell equipped with the gas diffusion cathode is stopped, the supply of the oxygen-containing gas to the gas chamber is stopped, and the gas chamber is substantially changed from the oxygen-containing gas atmosphere to the inert gas atmosphere. A method for protecting an alkaline chloride electrolytic cell, characterized by substituting. (2) The method for protecting an alkali chloride electrolytic cell according to the above (1), wherein nitrogen is used as an inert gas for replacing the gas chamber. (3) An operation for stopping the supply of the oxygen-containing gas to the gas chamber when the alkali chloride electrolytic cell provided with the gas diffusion cathode is stopped, and an operation for replacing the gas chamber with an inert gas after the supply is stopped. A protection device for an alkaline chloride electrolytic cell, comprising an automatic protection system for performing the following.

[0011]

DETAILED DESCRIPTION OF THE INVENTION The present invention will be described in more detail.
In the ion exchange membrane method alkali chloride electrolysis using a gas diffusion cathode, the following reaction occurs in the gas diffusion cathode. _{1 / 4O 2 + 1 / 2H} 2 O + e → OH - In this manner, oxygen and water participate in the reaction in the gas diffusion cathode. FIG. 1 shows a schematic view of an example of a three-chamber ion exchange membrane method electrolytic cell 1 using a gas diffusion cathode. An ion exchange membrane 2 is provided in an electrolytic cell 1, and the inside is partitioned into a cathode side and an anode side. The whole of the anode side partitioned by the ion exchange membrane 2 is an anode chamber 3, an anode 4 having a large number of openings is provided on the surface of the ion exchange membrane 2, and an anolyte supply port 5 is opened at the lower end face.
An anolyte outlet 6 is opened at the upper end surface. On the cathode side,
A gas diffusion cathode 7 is provided, a cathode chamber 8 is provided between the ion exchange membrane 2 and the gas diffusion cathode 7, and a gas chamber 9 is formed on the other side of the gas diffusion cathode 7. The cathode chamber 8 has a caustic liquid supply port 10 opened at the lower end face, and a caustic liquid outlet 11 at the upper end face.
Is open. The gas chamber 9 has a gas supply port 12 opened at an upper part and a gas discharge port 13 opened at a lower part.

The anode chamber 3 is the same as an ordinary ion-exchange membrane electrolytic cell that does not use the gas diffusion cathode 7, and is supplied with an aqueous alkali chloride solution from an anolyte supply port 5 and contacts the gas liquid permeable anode 4. And is electrolyzed. Chlorine gas and alkali metal ions are generated, and the concentration of the aqueous alkali chloride solution decreases.
The generated chlorine gas and the diluted aqueous solution of alkali chloride are discharged from the anolyte outlet 6. The alkali metal ions generated at the anode 4 move to the cathode chamber 8 through the ion exchange membrane 2. A caustic aqueous solution or water is supplied to the cathode chamber 8 from a caustic liquid supply port 10 and comes into contact with the gas diffusion cathode 7.
Further, an oxygen-containing gas is supplied from the gas chamber 9 to the gas diffusion cathode 7. As a result, an electrolytic reaction proceeds on the surface of the gas diffusion cathode 7 according to the above equation. The generated hydroxyl ions react with alkali metal ions that have moved through the ion exchange membrane 6 to generate caustic alkali. The aqueous caustic solution or water supplied from the caustic liquid supply port 10 is discharged from the caustic liquid discharge port 11 after increasing the alkali concentration.
On the opposite side of the gas diffusion cathode 7 from the cathode chamber 8, there is a gas chamber 9, to which an oxygen-containing gas is supplied from a gas supply port 12. Part of the oxygen is involved in the reaction, and the rest is
It is discharged from 3.

FIG. 2 is a schematic view of an example of a two-chamber type ion-exchange membrane electrolytic cell using a gas diffusion cathode 7. An ion exchange membrane 2 is provided in an electrolytic cell 1 and the interior is partitioned into two chambers, an anode chamber 3 and a cathode chamber 8. The ion exchange membrane 2 which partitions the inside is provided with an anode 4 having a large number of openings on the surface on the anode chamber 3 side, and the gas diffusion cathode 7 is provided on the cathode chamber 8 side with a spacer (not shown) interposed therebetween. It is. An anolyte supply port 5 is opened at the lower end face of the anode chamber 3, and an anolyte discharge port 6 is opened at the upper end face. A gas / water supply port 14 is opened at the upper end face of the cathode chamber 8, and a caustic liquid / exhaust gas discharge port 15 is opened at the lower end face. The principle of the anode chamber 3 is the same as that of the three-chamber type shown in FIG.

In the case of the two-chamber type shown in FIG.
Are disposed in contact with the ion exchange membrane 2, and the cathode chamber 8 is also used as a gas chamber. An oxygen-containing gas and water are supplied from the gas / water supply port 14, and a part of the water is electrolyzed on the surface of the gas diffusion cathode 7 according to the above formula. The generated hydroxyl ions react with the alkali metal ions that have passed through the ion exchange membrane 2 to generate caustic alkali, which is converted into a caustic aqueous solution and discharged from the caustic liquid / exhaust gas outlet 15 together with the exhaust gas. The concentration of the aqueous caustic solution
It is adjusted by the amount of water supplied from the water supply port 14.

As described above, there are several types of ion exchange membrane electrolysis using the gas diffusion cathode 7, and the present invention is not limited to any of these types as long as the oxygen reduction reaction is carried out in a caustic alkali. Can also be preferably applied. Various types of gas diffusion cathodes 7 have been proposed. One of typical ones is formed by hot pressing or the like from carbon powder and polytetrafluoroethylene (PTFE). It is a gas-permeable sheet having fine pores and supports a noble metal such as platinum or an alloy thereof, as a catalyst, as a catalyst. It may be reinforced with a metal mesh to increase strength and conductivity. The present invention can be applied to any gas diffusion cathode 7 regardless of the type of the material, as long as a metal material is used.

Nickel is generally used as a material for forming the cathode chamber 8. This may further increase the corrosion resistance by coating with silver or the like. In the case of the three-chamber type shown in FIG. 1, basically no liquid flows into the gas chamber 9. If the caustic solution leaks through the gas diffusion cathode 7, consideration must be given to corrosion. When an electrolytic reaction is performed in such an electrolytic cell 1, the voltage potential of the gas diffusion cathode 7 is lower than the oxygen reduction potential by an overvoltage. When the electrolysis is stopped, the overvoltage is eliminated and the potential becomes equal to the oxygen reduction potential. If oxygen gas is present under these conditions, the gas diffusion cathode 7 will deteriorate,
It is estimated that the corrosion of the cathode chamber 8 increases.

When the electrolytic cell 1 is stopped, the supply of the oxygen-containing gas to the gas chamber 9 is unnecessary, so that the supply of the oxygen-containing gas is naturally stopped. Is exposed to the remaining oxygen-containing gas, and this state will continue even during shutdown. Therefore, it was found that it was difficult to prevent the deterioration of the gas diffusion cathode and the corrosion of the cathode chamber only by stopping the supply of the oxygen-containing gas. Therefore, it is necessary to actively remove the oxygen-containing gas in order to prevent deterioration and corrosion. Although several methods are conceivable, the simplest and most effective method is to replace the remaining oxygen-containing gas with an inert gas. Helium, argon, nitrogen, and the like are preferable as the type of inert gas that can be used for replacement with an oxygen-containing gas. Among them, nitrogen is the cheapest and preferable. Carbon dioxide is not preferable because it produces alkali carbonate in the gas diffusion cathode 7.

Since the deterioration of the gas diffusion cathode 7 and the corrosion of the cathode chamber 8 are accelerated from the moment the electrolytic cell 1 is stopped, the replacement operation with the inert gas is performed after the electrolytic cell 1 is stopped. It needs to be implemented promptly. The gas replacement may be performed automatically. The electrolytic cell 1 is provided with the following devices.
That is, assuming that factors to stop the electrolytic cell 1 are factors A, B, C,..., A first sensor that senses these factors and emits a factor signal is provided. Further, an automatic stop device that receives the factor signal to stop the electrolysis operation of the electrolytic cell 1 and also closes the supply valve of the oxygen-containing gas, and a second device that confirms the closing of the oxygen-containing gas supply valve and issues a closing confirmation signal. Two sensors and an inert gas releasing device that receives the closing confirmation signal and releases the inert gas into the gas chamber are provided.

FIG. 3 shows an instrumentation block flow according to this embodiment. In FIG. 3, the electrolysis operation of the electrolytic cell 1 is stopped (22) by a signal based on the factor 21 to be stopped.
After the stop, the oxygen supply valve for supplying the oxygen-containing gas in the electrolytic cell gas supply pipe is closed according to the electrolysis stop signal (23), and at the same time, the nitrogen supply valve connected to the same pipe is opened (24).
These operations are automated. According to this embodiment, in the ion-exchange membrane method electrolysis using a gas diffusion cathode, it is possible to prevent the deterioration of the gas diffusion cathode even during shutdown, and also to prevent the corrosion of the cathode chamber, and to improve the performance of the electrolytic cell for a long time. Can be maintained.

[0020]

The present invention will be described in detail with reference to the following examples. However, the present invention is not limited to only this embodiment.

Example 1 An electrolytic test was carried out under the following conditions.
We went for 26 days. The progress of the electrolytic voltage and the gas diffusion cathode overvoltage from the start of the test were as shown in the table below. The electrolysis was stopped on the way, 9 days, 11, 12, 13, 13, 16, 18, 23, 27, and 97 days, a total of 9 times. At this time, after the oxygen gas was stopped, the gas was replaced with nitrogen. No abnormalities were found in the gas diffusion cathode even after 126 days of electrolysis. <Electrolysis conditions>

[0022]

[Table 1]

Note 1: Using titanium as a base material, RuO ₂ /
Electrodes coated with material mainly composed of TiO _2, Permelec electrode Ltd. Note 2: formed by stacking a gas diffusion layer and the reaction layer. The gas diffusion layer is made of hydrophobic carbon black (manufactured by Denki Kagaku Kogyo,
It is composed of 60% of acetylene black) and 40% of PTFE (D-1 manufactured by Daikin Industries, Ltd.). The reaction layer is composed of 20 parts of hydrophilic carbon black (AB-12, manufactured by Denki Kagaku Kogyo KK) and 10 parts of PTFE. The laminate was formed by integrally forming a silver mesh as a current collector by hot pressing, and supported 3 mg / cm ² of silver as a catalyst. <Electrolysis Performance> Table 2 shows changes in electrolysis voltage and the like in electrolysis under the above electrolysis conditions.

[0024]

[Table 2]

Comparative Example An electrolytic test was performed under the same conditions as in the example. The operation was stopped for 10 hours on the 7th day after the start of operation. During shutdown, oxygen was allowed to pass. Thereafter, the electrolysis was restarted. The electrolysis voltage after 6 days before stopping electrolysis was 2.
At 18 V, the gas diffusion cathode overvoltage was 0.54 V. Eight days after the electrolysis was stopped, the electrolysis voltage was 2.24 V, and the gas diffusion cathode overvoltage was 0.59 V. Thereafter, electrolysis was continued, but after 19 days, the electrolysis voltage was 2.25 V, and the gas diffusion cathode overvoltage was 0.60 V, and did not recover. Note that the caustic circulating solution during the suspension of electrolysis was colored slightly green, and it was estimated that nickel was eluted from the cathode chamber.

[0026]

Since the present invention has the above-mentioned structure, it prevents deterioration of the gas diffusion cathode and corrosion of the cathode chamber while the electrolytic cell is stopped, thereby maintaining the original high performance of the electrolytic cell for a long period of time. Thus, it is possible to provide a protection method and a protection device for an alkaline chloride electrolytic cell that can be performed.

[Brief description of the drawings]

FIG. 1 shows a schematic diagram of a three-chamber type ion exchange membrane electrolytic cell having a gas diffusion cathode.

FIG. 2 shows another schematic view of a two-chamber ion exchange membrane electrolytic cell having a gas diffusion cathode.

FIG. 3 shows an instrumentation block flow diagram during shutdown according to the present invention.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Electrolysis tank 2 Ion exchange membrane 3 Anode chamber 4 Anode 5 Anolyte supply port 6 Anolyte discharge port 7 Gas diffusion cathode 8 Cathode chamber 9 Gas chamber 10 Caustic liquid supply port 11 Caustic liquid discharge port 12 Gas supply port 13 Gas discharge port 14 Gas / water supply port 15 Caustic liquid / exhaust gas discharge port 21 Factor 22 Electrolysis stop 23 Oxygen supply valve closed 24 Nitrogen supply valve open

 ──────────────────────────────────────────────────続 き Continuing on the front page (72) Inventor Akihiro Sakata 1-14-1 Nishi-Shimbashi, Minato-ku, Tokyo Inside Toagoasei Co., Ltd. (72) Inventor Koji Saiki 4-6-1 Hojo-cho, Toyonaka-shi, Osaka No. 815 (72) Inventor Takeshi Watanabe 1-6 Takasago, Takaishi-shi, Osaka F-term (reference) 4K021 AA03 AB01 BA03 BC06 CA13 CA14 DB16 DB31 DB50 DB53

Claims (3)

[Claims] When the operation of an alkali chloride electrolytic cell provided with a gas diffusion cathode is stopped, the supply of an oxygen-containing gas to a gas chamber is stopped, and the gas chamber is substantially deactivated from an oxygen-containing gas atmosphere. A method for protecting an alkaline chloride electrolytic cell, wherein the method is replaced with an atmosphere. 2. As an inert gas that replaces the gas chamber,
2. The method for protecting an alkaline chloride electrolytic cell according to claim 1, wherein nitrogen is used. 3. An operation for stopping the supply of an oxygen-containing gas to a gas chamber when an alkali chloride electrolytic cell provided with a gas diffusion cathode is stopped, and replacing the gas chamber with an inert gas after the supply is stopped. A protection device for an alkaline chloride electrolytic cell, comprising an automatic protection system for performing an operation of performing an operation.