JP2670935B2 - Electrolysis method - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an electrolysis method for obtaining an aqueous caustic alkali solution with high electrolysis efficiency when an aqueous alkali metal chloride solution is electrolyzed by an ion exchange membrane method using an oxygen cathode.

[0002]

2. Description of the Related Art Alkali metal chloride aqueous solution (hereinafter "salt water")
It is known that the caustic alkali is produced by electrolyzing the above) by an ion exchange membrane method using an oxygen cathode. In this manufacturing method, the outline is that an anode chamber having an anode and containing salt water and a cathode chamber having a cathode and containing water or an aqueous solution of caustic alkali are generally a cation exchange membrane. When a cell is divided by a membrane and electrolysis is performed by energizing between both electrodes, the cathode chamber is electrolyzed using a gas diffusion electrode, a so-called oxygen electrode, which is made of a porous material and is supplied with an oxygen-containing gas. To get caustic,
At the cathode, a hydrogen-oxygen reaction takes place, which has the advantage that the cathode potential drops and the electrolysis voltage drops considerably. Patent documents disclosing this production method include, for example, JP-A-54-97600, JP-A-56-44784, and JP-A-56-47484.
-130482, 57-152479, 59-
No. 133386, No. 61-266591, and Japanese Patent Publication 58
-44156, 58-4969, 60-95.
95 and 61-20634, etc. are mentioned.

In the patent documents of the ion exchange membrane method electrolysis using these oxygen cathodes, attention is paid only to the production method and performance improvement of the oxygen cathode, and the operation method of the electrolytic cell is almost considered. Did not. In the ion exchange membrane method electrolysis using an oxygen cathode, it is necessary to achieve high electrolysis efficiency, and therefore it is important to obtain high current efficiency or low voltage, and high current efficiency is mainly due to ion exchange membrane, The low voltage depends largely on the performance of the oxygen cathode, but heretofore, few methods have been proposed for operating an electrolytic cell to achieve the high electrolysis efficiency.

[0004]

In this ion-exchange membrane electrolysis method, in the conventional electrolysis method using a cathode that generates hydrogen gas without using an oxygen cathode, hydrogen gas rises in the catholyte. , The distance between the cathode and the ion exchange membrane cannot be made too narrow, the distance between the electrodes becomes large, and the hydrogen gas bubbles exist between the electrodes, so that the electrolyte that can be energized between the electrodes is cut off. Since the area is reduced, there is a drawback that the resistance between the electrodes is increased and the electrolysis voltage is increased. The occurrence of a hydrogen overvoltage when the hydrogen gas is generated also contributes to increasing the electrolysis voltage as described above. Since the new electrolysis method using the oxygen cathode does not generate hydrogen gas, it does not have the drawbacks described above and has a great advantage that electrolysis can be performed at a low electrolysis voltage.

However, while the ion-exchange membrane electrolysis method using the oxygen cathode brings about the above-mentioned advantages, its current efficiency is lower than that of the conventional electrolysis method using the cathode that generates hydrogen gas. I found that there is a drawback. Since the electrolysis efficiency of caustic is generally proportional to the current efficiency and inversely proportional to the electrolysis voltage, even if the electrolysis voltage can be lowered by using the bent oxygen cathode, the current efficiency will be lowered as a whole. The effect will not be improved.

Further, in the ion exchange membrane method electrolysis using an oxygen cathode, there occurs a phenomenon that the voltage rises during the electrolysis and the oxygen-containing gas leaks into the caustic aqueous solution. Since the voltage rises when this phenomenon occurs, the caustic aqueous solution and the oxygen-containing gas must be present in the oxygen cathode so that the reaction can be carried out most efficiently. An object of the present invention is to obtain an electrolysis method in which the current efficiency is improved or the electrolysis voltage is further reduced in the method of electrolyzing by the ion exchange membrane method using the oxygen cathode. It is another object of the present invention to provide an operating method for achieving a high electrolysis efficiency which has not yet been clarified in the method of electrolyzing by the ion exchange membrane method using the oxygen cathode.

[0007]

In view of the above circumstances, the present inventors have earnestly tried to establish an electrolysis method or an operation method for obtaining high electrolysis efficiency in ion exchange membrane electrolysis using an oxygen cathode. As a result of repeated research, the present invention has been completed. The present inventor, in the ion exchange membrane method electrolysis using an oxygen cathode, examined the cause of the phenomenon that the current efficiency is slightly lower than the conventional hydrogen generation type ion exchange membrane method electrolysis, and the reason is as follows. It became clear by the. In the conventional hydrogen generation type ion exchange membrane electrolysis cell, the dispersion of the caustic alkali concentration in the caustic alkali aqueous solution in the cathode chamber is extremely small due to stirring due to the rise of hydrogen gas, while the ion exchange membrane electrolysis using an oxygen cathode is used. In, since there is no generation of hydrogen gas, the caustic alkali concentration can be distributed in the vertical direction of the cathode chamber,
The optimum caustic concentration was not reached. For this reason,
It was conceived that the caustic alkali solution in the cathode chamber of the electrolytic cell should be made as uniform as possible in caustic alkali concentration, and the means for homogenizing the caustic alkali was investigated, and the range in which the homogenization was allowed was examined . Furthermore, further reduce the electrolysis voltage
In order to do so,
The relationship between pressure and pressure on the oxygen-containing gas side was examined
As a result, it is preferable to increase the pressure on the caustic solution side.
We found that it was better, and examined the conditions.

That is, the present invention was able to achieve the above object by the following means. (1) In the method of electrolyzing an alkali metal chloride aqueous solution by an ion exchange membrane method using an oxygen cathode , the oxygen cathode is
It has a gas chamber on the back side, to which oxygen-containing gas is supplied.
The front side is in contact with the caustic aqueous solution of catholyte,
Pressure on the caustic solution side of the oxygen cathode in the cathode chamber
Is 5,000 Pa.s from the pressure on the oxygen-containing gas side. More than 10 ⁶
Pa. An electrolysis method characterized by increasing the temperature below . (2) The oxygen cathode has a water repellent portion having a pore diameter of 0.1 μm or less.
It is below, The electrolysis method of said (1) term .

The present invention relates to a caustic aqueous solution from a cathode chamber.
By introducing it to the outside of the room and then introducing it again into the cathode room.
The aqueous caustic solution is circulated in the cathode chamber, and the maximum amount of caustic
By setting the difference between the concentration and the minimum concentration to 30 g / liter or less, preferably 20 g / liter or less, and more preferably 10 g / liter or less, the caustic aqueous solution in the cathode chamber is made uniform. When the difference in the concentration of caustic is larger than 30 g / liter, high current efficiency cannot be obtained, which is not preferable.

In the present invention, various means can be considered as means for circulating the caustic aqueous solution in the cathode chamber, but the caustic aqueous solution of the catholyte is taken out from the outlet of the cathode chamber and introduced from the inlet of the cathode chamber. The most common method is that the caustic alkali aqueous solution taken out from the cathode chamber outlet has the highest caustic alkali concentration and the caustic alkaline aqueous solution introduced from the cathode chamber inlet has the lowest caustic alkali concentration. The alkali concentration difference should be within the above range. Further, in the ion exchange membrane method, since the cathode chamber is thin, it is difficult to circulate the caustic aqueous solution inside the cathode chamber.Therefore, a liquid passage is provided inside the electrolytic cell outside the cathode chamber, It may be a circulation path. Further, as is known in the conventional diaphragm type electrolytic cell, a liquid passage may be provided in a portion that does not participate in electrolysis in the outer peripheral portion of the cathode chamber.

According to the study by the present inventors, for example, a method of circulating a caustic alkali aqueous solution from the cathode chamber outlet to the cathode chamber inlet is adopted to determine the difference in caustic alkali concentration between the cathode chamber outlet and the cathode chamber inlet. It has been clarified that it is preferable to change the circulating flow rate of the caustic aqueous solution depending on the current efficiency, the current density, the distance between the electrodes, and the vertical dimension of the cathode chamber when the value is set to a certain value. That is, the caustic aqueous solution circulation flow rate and the current efficiency are directly proportional, the caustic aqueous solution circulation flow rate and the current density are directly proportional, the caustic aqueous solution circulation flow rate and the inter-electrode distance are inversely proportional, and the caustic aqueous solution circulation flow rate is The vertical dimension is directly proportional. Based on these findings, it has become possible to obtain the best ion exchange membrane performance under any specifications and electrolysis conditions of the ion exchange membrane method electrolytic cell using an oxygen cathode.

In the ion exchange membrane method electrolysis using the oxygen cathode of the present invention, those conventionally known as the ion exchange membrane or the oxygen cathode can be used. As the ion exchange membrane, a perfluoro cation exchange membrane, which is known to be very excellent in chemical resistance, is suitable, and is composed of only a perfluoro resin having a carboxylic acid group as a polar group. A film may be used, or a two-layer film including a perfluoro resin layer having a carboxylic acid group and a perfluoro resin layer having a sulfonic acid group may be used. A perfluoro resin film having a sulfonic acid group is also known, but since the ion selectivity is inferior to the above, it is preferable to use the above two.

When electrolyzing salt water using these ion exchange membranes to produce caustic, it is known that there is a caustic concentration that maximizes current efficiency due to the properties of each ion exchange membrane. Thus, the current efficiency will be reduced if the caustic concentration of the catholyte is too high or too low. Therefore, the caustic alkali concentration of the caustic alkali aqueous solution in the cathode chamber is best adjusted so that the average concentration of the caustic alkaline aqueous solution becomes an optimum concentration which is determined by the ion exchange membrane species used.

According to the above-described method, high current efficiency can be obtained in the production of caustic alkali by electrolysis of salt water. In this method, the oxygen cathode conventionally used as the oxygen cathode can be used as it is. This oxygen cathode, for example, metal powder such as nickel, or carbon powder is sintered, oxygen, oxygen-enriched air, oxygen-containing gas such as air permeate the porous body holding the catalyst metal such as platinum, A so-called gas diffusion electrode that diffuses is used as a cathode. In this case, it was partitioned by the porous body
An oxygen cathode is constructed by supplying an oxygen-containing gas to the gas chamber.
To achieve.

Further, the present invention can achieve high electrolysis efficiency by reducing the electrolysis voltage, not by the current efficiency. It has been found that, in the ion exchange membrane method electrolysis using an oxygen cathode, the phenomenon that the voltage rises during the electrolysis and the oxygen-containing gas leaks into the caustic aqueous solution occurs due to the following reasons. At the oxygen cathode, water repellent treatment is performed to prevent the caustic aqueous solution from leaking to the oxygen-containing gas side during electrolysis, but due to this treatment, the supply of caustic aqueous solution to the cathode is insufficient, resulting in an increase in voltage. Occurs, and the oxygen-containing gas can quickly pass through the water-repellent portion, and it leaks into the caustic aqueous solution side. Therefore, the present inventors promoted the supply of the caustic aqueous solution to the oxygen cathode,
Further, the oxygen-containing gas was pushed back.

In the present invention, the pressure on the aqueous caustic alkali solution side is set to 5000 Pa. 10 ⁶ Pa. Below (Kg /
When expressed in units of cm ² , it is about 0.051 Kg / cm ² or more and about 10.20 Kg / cm ² or less). By increasing the pressure on the side of the caustic aqueous solution in this manner, the supply of the caustic aqueous solution to the oxygen cathode is increased, and further, the oxygen-containing gas can be prevented from entering the side of the caustic aqueous solution. The pressure difference between the caustic aqueous solution side and the oxygen-containing gas side is 5000 Pa.s. When it is less than 1, the solution of the phenomenon becomes insufficient, which is not preferable, and the pressure difference is 10 ⁶ Pa.s. If it is higher, it is not preferable because a special device for pressure resistance is required for the electrolytic cell and further improvement of the phenomenon cannot be expected.

In the present invention, various means for increasing the pressure on the side of the caustic aqueous solution to the pressure on the side of the oxygen-containing gas are conceivable and are not particularly limited. For example, relief is provided at the outlet of the cathode chamber of the caustic aqueous solution circulation system of the electrolytic cell. The method of installing the valve is adopted. In the present invention, the pressure on the oxygen-containing gas side is not particularly limited and may be any, but it is preferable to operate at atmospheric pressure without any special measures.

The present invention can be applied to any known electrolysis using an ion exchange membrane method using an oxygen cathode.
It is particularly effective when using an oxygen cathode, which is often 1 μm or less. Further, the anode and salt water used in the present invention may be the same as those used in the conventional hydrogen generating ion exchange membrane method electrolysis. As the anode, for example, a titanium substrate coated with platinum or a metal oxide is used, and in particular, an expanded metal surface made of titanium coated with a solid solution of ruthenium oxide and titanium oxide is preferably used. .

Next, the present invention will be described with reference to the drawings.
FIG. 1 shows an example of an embodiment of the present invention. An electrolytic cell 1 is divided into an anode chamber 2 and a cathode chamber 3 by an ion exchange membrane 4, and the anode chamber 2 has an anode 5 and salt water (saline). The cathode chamber 3 has an oxygen cathode 6 and is filled with an aqueous caustic soda solution 8. The oxygen cathode 6 is a porous metal body and has a gas chamber 9 on the back side, to which an oxygen-containing gas is supplied from an oxygen-containing gas inlet 10 and discharged from an oxygen-containing gas outlet 11.

When the power source 12 is connected between the anode 5 and the oxygen cathode 6 and an electric current is passed between the both electrodes, electrolysis is started and the anode chamber 2
Sodium ions and water enter the cathode chamber 3 through the ion exchange membrane 4, and the concentration of the caustic soda aqueous solution 8 starts to increase. A circulating caustic soda aqueous solution is sent by a pump (not shown) from the cathode chamber inlet 13, and an outlet caustic soda aqueous solution is discharged from the cathode chamber outlet 14 through a relief valve 15 and a caustic soda aqueous solution receiving tank 16
to go into. A part of the aqueous solution of caustic soda is taken out from the caustic soda aqueous solution receiving tank 16 as a caustic soda aqueous solution 18 of the product, and the other part thereof is mixed with hydration 17 and circulated to the cathode chamber 3 through a liquid pipe 19. The pressure on the side of the aqueous caustic soda solution at the oxygen cathode 6 is maintained at a predetermined value by a relief valve 15.

At that time, by maintaining the flow rate of the circulating caustic soda aqueous solution so that the difference in concentration between the outlet caustic soda aqueous solution coming out of the cathode chamber 3 and the circulating caustic soda aqueous solution entering the cathode chamber 3 becomes 30 g / liter or less, high current efficiency is obtained. Is obtained. Pressure sodium hydroxide aqueous solution side of the oxygen cathode 6 negative electrode chamber 3, the difference between the pressure of the pressure and the oxygen-containing gas side 5000 Pa. 10 ⁶ Pa. As described below, a lower electrolysis voltage can be obtained by making it higher than the oxygen-containing gas side. The circulation condition of the aqueous caustic soda solution and the pressure condition of the aqueous caustic soda solution may be set simultaneously.

[0022]

In the present invention, by circulating the caustic aqueous solution in the cathode chamber, the concentration of the caustic aqueous solution in the cathode chamber can be made as uniform as possible, and the difference in concentration can be reduced. Thus, it is possible to prevent a decrease in current efficiency due to the presence of a portion where the concentration of the aqueous caustic alkali solution in the cathode chamber is low or high. In particular, when the thin or dense concentration at that portion deviates from the optimum concentration for the ion exchange membrane, it is possible to improve the current efficiency by making the concentration uniform. If there is a portion where the concentration is too low, the current efficiency is lowered, and if the concentration is too high, the membrane is damaged and the current efficiency is lowered. Ma
Also, in the case of oxygen cathode, oxygen gas and caustic
Hydrogen-oxygen reaction due to the presence of alkaline aqueous solution
However, the oxygen-containing gas was conventionally used for the water repellent treatment of the cathode.
Oxygen in the porous part of the cathode so that
Easy to enter gas, difficult to enter caustic aqueous solution
The oxygen-containing gas occupies a large portion and becomes excessive.
In the present invention, the pressure on the aqueous caustic alkali side is
The pressure difference between the caustic solution side and the oxygen-containing gas side is
5000 Pa. 10 ⁶ Pa. Oxygen as below
By setting the pressure higher than that of the contained gas, caustic
Makes it easier for the potash solution to enter, lowers the cathode potential, and
Thereby, the electrolysis voltage can be reduced.

[0023]

【Example】 Hereinafter, the present invention will be specifically described with reference to examples.
You. However, the present invention is limited to only these examples.
Not something. Example Install the following anode and oxygen cathode, and use it as an ion exchange membrane.
Nafion Membrane from TU-PON (perf with carboxylic acid groups
Perfluoro-based resin layer with luoro-based resin layer and sulfonic acid group
Electrolysis in which an anode chamber and a cathode chamber are partitioned by a two-layer film including a resin layer)
Saline was electrolyzed using the bath under the following conditions. (Electrolysis conditions) Energization : 30A Current density : 30 A / dm ^Two anode : RuO on titanium base _Two And TiO _Two Coating Electrode (registered trademark, DSA) Oxygen cathode : Copper mesh inside, carbon powder and poly 4 Molded with ethylene oxide powder and sintered, 0.5% Pt 6 mg / cm ^Two Contained gas diffusion electrode Distance between poles : 15 mm Average temperature : 90 ℃ NaCl (anolyte): 200 g / liter Oxygen-containing gas : Oxygen (1.2 times the theoretical amount) Oxygen-containing gas pressure : 1ata At that time, the caustic soda aqueous solution at the cathode chamber outlet and cathode chamber inlet
Difference in the caustic soda concentration of the
Current effect at various pressure differences from the oxygen-containing gas side
The rate and voltage are shown in Table 1.

[0024]

[Table 1]

According to Table 1, if the difference in the caustic soda concentration of the caustic soda aqueous solution between the cathode chamber outlet and the cathode chamber inlet is 30 g / liter or less, the pressure on the caustic soda aqueous solution side at the oxygen cathode is set to the caustic soda aqueous solution side and the oxygen-containing gas. The pressure difference with the side is 5000 Pa. 10 ⁶ Pa. As described below, high current efficiency is obtained when the pressure is kept higher than the oxygen-containing gas side, or otherwise. But,
As seen in Comparative Examples 1 and 2 , when the pressure on the aqueous caustic soda solution side is not increased by the pressure difference, the voltage is increased. On the other hand, as seen in Examples 9 to 10, even if the caustic soda concentration difference exceeds the above-mentioned 30 g / liter,
When the pressure on the side of the aqueous caustic soda solution at the oxygen cathode is kept high in the above range, the voltage can be lowered. In Examples 9 to 10, the concentration of caustic soda at the outlet of the cathode chamber was considerably higher than that in Examples 1 to 8, so the current efficiency was poor, but the caustic soda concentration could be suppressed to about 435 g / liter. High current efficiency. From these points, in implementing the present invention, if both conditions are satisfied, high current efficiency and low voltage can be obtained together.

[0026]

According to the present invention, a high current efficiency can be obtained by reducing the concentration difference in the caustic aqueous solution in the cathode chamber and reducing the degree of caustic distribution. Furthermore, by increasing the pressure on the side of the caustic aqueous solution of the oxygen cathode within the above range than on the side of the oxygen-containing gas, the electrolysis voltage is lowered and the electrolysis efficiency is increased. If both these conditions are set at the same time, the electrolysis efficiency can be further improved. These effects make it possible to perform the ion exchange membrane method electrolysis using an oxygen cathode in which electrolysis is performed at a low electrolysis voltage with higher electrolysis efficiency.

[Brief description of the drawings]

FIG. 1 shows a schematic diagram of an electrolysis apparatus for carrying out the electrolysis method of the present invention.

[Explanation of symbols]

1 Electrolyzer 11 Oxygen-containing gas outlet 2 Anode chamber 12 Power supply 3 Cathode chamber 13 Inlet 4 Ion exchange membrane 14 Outlet 5 Anode 15 Relief valve 6 Oxygen cathode 16 Caustic soda solution receiving tank 7 Salt water 17 Pouring water 8 Caustic soda solution 18 Product Caustic soda solution 9 Gas Chamber 19 Liquid tube 10 Oxygen-containing gas inlet

 ─────────────────────────────────────────────────── ─── Continuation of front page (73) Patent holder 999999999 Kanefuchi Chemical Co., Ltd. 3-2-4 Nakanoshima, Kita-ku, Osaka-shi, Osaka (72) Inventor Choichi Furuya 2-14 Nakamura-cho, Kofu-shi, Yamanashi Prefecture No. (56) References JP-A-55-41986 (JP, A) JP-B 1-15593 (JP, B2)

Claims (2)

(57) [Claims] 1. A method of electrolyzing an alkali metal chloride aqueous solution by an ion exchange membrane method using an oxygen cathode, wherein the acid
The cathode has a gas chamber on the back side, and there is an oxygen-containing gas there.
Is supplied and the front side contacts the catholyte caustic solution.
And, caustic solution of the oxygen cathode in the cathode chamber
The pressure on the side of the oxygen-containing gas is 5000 Pa. Less than
Top 10 ⁶ Pa. An electrolysis method characterized by increasing the temperature below . 2. The oxygen cathode has a water repellent portion having a pore diameter of 0.
The electrolysis method according to claim 1, wherein the thickness is 1 μm or less .