JPH1129890A - Electrolytic method - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method by which caustic alkali is removed in a simpler and surer manner than a conventional measure from a surface of a cathode, while, in the production of the caustic alkali using a liquid transmission type gas diffusion cathode, the supply of a gas is obstructed and the electrolytic efficiency is lowered because the surface of the cathode is covered by the produced caustic alkali. SOLUTION: Elecrolysis is executed, while intermittently supplying diluted caustic alkali or water on the cathode side surface of the gas diffusion cathode, to dilute the produced caustic alkali with a high concentration and high viscosity to easily remove the alkali from the cathode surface, thereby preventing the clogging of the cathode surface in the case of producing caustic alkali using a vertical electrolyzer having the liquid transmission type gas diffusion cathode. It is desirable to efficiently remove the produced caustic alkali by using a branch pipe or shower, and uniformly supplying the diluted caustic alkali, etc., over the entire surface of the cathode.

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 producing a caustic alkali by electrolyzing salt or the like using a liquid-permeable gas diffusion cathode, and more particularly, to a method of forming pores of the gas diffusion cathode during the electrolysis. The present invention relates to an electrolysis method for efficiently producing caustic alkali by preventing clogging with generated caustic alkali.

[0002]

2. Description of the Related Art Electrolysis industry represented by chloralkali electrolysis plays an important role as a material industry. Although having such an important role, the energy consumption required for chloralkali electrolysis is large, and energy saving is a major problem in countries with high energy costs such as Japan. For example, in chlor-alkali electrolysis, in order to solve environmental problems and achieve energy saving, the mercury method was switched to the ion exchange membrane method via the diaphragm method, and about 25%.
Annual energy savings of about 40% have been achieved. However, even this energy saving is not enough, and the power cost, which is energy, accounts for 50% of the total manufacturing cost. However, no further power saving is possible if the current method is used. In order to achieve more energy savings, drastic changes must be made, such as correcting the electrode reaction. As an example, the use of a gas diffusion electrode employed in a fuel cell or the like is the most likely and possible means of saving electric power.

[0003] The electrolytic reaction using a conventional metal electrode is
When a gas diffusion electrode is used as a cathode, it is converted into an electrolytic reaction. _{2NaCl + 2H 2 0 → Cl 2} + 2NaOH + H 2
E _O = 2.21V 2NaCl + 1 / 2O ₂ + H ₂ O → Cl ₂ + 2NaO
The H E _O = 0.96 V, that the metal electrodes by converting the gas diffusion electrode,
The theoretical decomposition voltage is reduced from 2.21V to 0.96V, and theoretically about 65% energy saving is possible. Therefore, various studies have been made toward the practical use of chloralkali electrolysis by using this gas diffusion electrode. The structure of the gas diffusion electrode is generally called a semi-hydrophobic (water-repellent) type, and has a structure in which a hydrophilic reaction layer carrying a catalyst such as platinum on the surface and a water-repellent gas diffusion layer are joined. I have. A water-repellent polytetrafluoroethylene (PTFE) resin is used as a binder for both the reaction layer and the gas diffusion layer, and the ratio is increased in the gas diffusion layer and reduced in the reaction layer by utilizing the characteristics of the PTFE resin. Make up the layers.

[0004] When such a gas diffusion electrode is used for chloralkali electrolysis, several problems arise. For example, in high-concentration caustic soda, PTFE resin as a water-repellent material becomes hydrophilic and easily loses water repellency. In order to prevent this, it has been attempted to attach a thin porous PTFE sheet to the gas diffusion layer on the gas chamber side. Electrolysis proceeds while supplying oxygen and air to the gas diffusion electrode. However, hydrogen peroxide is partially generated as a side reaction, which may corrode carbon as a constituent material to generate sodium carbonate. In an alkaline solution, the sodium carbonate precipitates and may block the gas diffusion layer or make the surface hydrophilic, thereby deteriorating the function of the gas diffusion electrode. It has also been observed that even if this sodium carbonate is not generated, only the catalyst is supported on the carbon surface, and that the catalyst causes carbon corrosion.

In order to solve the problem of blockage of the gas diffusion electrode, conventionally, selection of carbon to be used, a method of producing carbon, and adjustment of a mixing ratio of carbon resin have been performed. It did not solve the problem, but only stopped the apparent rate of carbon corrosion, and could not stop it. Since the problem of corrosion can be solved without using carbon, a method using silver instead of carbon has been proposed. Although the problem of corrosion can be solved by using silver, the gas diffusion electrode using silver, unlike the gas diffusion electrode using carbon, is manufactured by a sintering method, the manufacturing method is extremely complicated, and the water-repellent part and the hydrophilic part are hydrophilic. There is a problem that control of the sex part is extremely difficult.

In addition to this problem, in a caustic alkaline electrolysis using a conventional gas diffusion electrode, particularly in a practical tank, there is a pressure difference in the height direction of the electrolytic cell in the gas chamber, and a constant amount of gas is supplied at a constant pressure. However, there is a problem that it is extremely difficult to supply gas uniformly over the entire electrode surface. For this reason, on a laboratory scale, several types of electrodes are arranged in the height direction so that gas is supplied uniformly. However, applying this to a practical tank is too complicated and there is no practical example yet. Attempts have been made to supply the electrolytic solution together with the supply gas to the gas chamber side to perform electrolysis in order to alleviate the pressure difference in the height direction and facilitate the size increase. However, although this method solves the problem of pressure, the presence of the electrolyte causes insufficient gas supply to the gas diffusion layer of the gas diffusion electrode, and the insufficient gas amount reduces the current density accordingly. Therefore, even if a current density of about 30 A / dm ² is possible in a laboratory scale, there is a problem that the current density must be suppressed to about half of that in a large tank.

In order to solve the disadvantage that the gas supply becomes insufficient, it has been attempted to collect the supply gas in the vicinity of the gas diffusion tank of the gas diffusion electrode by forming irregularities on the electrode surface or making the electrode water-repellent. Have been. However, this method also does not serve as a driving force for the gas to reach the reaction layer through the gas diffusion electrode, and the problem of insufficient gas supply has not been solved. In order to solve this problem, the present inventors have attached a liquid-permeable gas diffusion electrode to an ion exchange membrane, and applied a liquid pressure to the ion exchange membrane so that no pressure was applied to the gas diffusion electrode. Was. However, in this method, there is a problem that caustic alkali, which is a product permeating the gas diffusion electrode, blocks the gas diffusion electrode to some extent and the gas supply is sometimes stopped. This can be prevented to some extent by adjusting the degree of hydrophobicity / hydrophilicity of the electrode surface, but it is not perfect. Particularly, when the size of the electrolytic cell is increased, clogging of the electrode becomes a serious problem.

[0008]

SUMMARY OF THE INVENTION It is an object of the present invention to solve the above-mentioned problems of the prior art, that is, the generation of caustic on the gas chamber side surface where the product caustic permeates and reaches in the electrolysis using the gas diffusion electrode. In particular, an object of the present invention is to provide an electrolysis method that can easily and reliably prevent a decrease in electrolysis efficiency due to inhibition of gas supply.

[0009]

The electrolysis method according to the present invention is directed to a caustic alkali generation electrolysis method using a vertical electrolytic cell having a liquid-permeable gas diffusion cathode, wherein an intermittent surface is provided on the cathode side of the gas diffusion cathode. This is an electrolysis method characterized by performing electrolysis while supplying a diluted caustic alkali or water.

Hereinafter, the present invention will be described in detail. The liquid-permeable gas diffusion cathode is usually installed in close contact with an ion-exchange membrane that is a diaphragm, and is divided into an anode chamber that is a solution chamber and a cathode chamber that is a gas chamber by the ion-exchange membrane. Used as a negative electrode. In the caustic alkali generation electrolysis using such an electrolytic cell, gas (oxygen) is supplied from the surface opposite to the ion exchange membrane of the gas diffusion cathode, and the supplied gas passes through the gas diffusion cathode and passes through the ion exchange membrane. And hydroxyl ions are generated by an electrolytic reaction on the surface of the ion exchange membrane. The hydroxyl ions combine with alkali metal ions from the anode side to generate caustic alkali, and the caustic flows in the gas diffusion cathode in a direction opposite to the flow of the supply gas, and the gas chamber, It reaches the gas diffusion cathode surface. This caustic alkali has a high concentration and becomes droplets on the surface or flows down the surface and is taken out from the lower part of the electrolytic cell. Drops of caustic as well as caustic flowing down partially cover the gas diffusion cathode surface,
A smooth gas supply will be hindered. If the gas supply is not sufficient, the efficiency of the production of hydroxyl ions on the surface of the ion-exchange membrane described above is reduced, and the efficiency of the production of caustic alkali as a product is also reduced.

How to remove the caustic covering the gas diffusion cathode surface is a problem to be solved in order to maintain high efficiency in caustic electrolysis using the gas diffusion cathode. Conventionally, in order to solve this problem, a special device is applied to the electrode structure, for example, an eaves-shaped guide plate is attached to remove droplets from the electrode surface, or as described above, the degree of hydrophilicity / water repellency of the electrode surface. In other words, the pore size of the porous portion in the electrode should be adjusted so that the pore size distribution can be divided for gas and liquid, and at the same time, the contact angle with the electrolyte and the degree of water repellency of the electrode surface should be adjusted. To a desired value, and droplets and the like are similarly removed from the electrode surface. However, of these methods, the former requires processing the gas diffusion cathode surface which is relatively susceptible to damage, and the latter requires operations that are relatively difficult to predict the results and require time-consuming processing. Sufficient and satisfactory results are not always achieved.

The present invention is an electrolysis method which can solve these problems of the prior art. It is an object of the present invention to efficiently perform caustic alkali generation electrolysis using a liquid-permeable gas diffusion cathode, that is, at a high current density and a low cell voltage. to enable. A portion of the caustic alkali that has reached the gas diffusion cathode surface flows down the cathode surface by its own weight due to the water repellency of the surface, but the concentration of the generated caustic alkali is as high as about 30 to 35%, and therefore the viscosity is high. Cannot flow down the surface, and at least a part thereof stays on the surface of the cathode to cover the cathode and obstruct gas supply. In other words, when the concentration of the caustic alkali is reduced to be a diluted caustic, the diluted caustic has a reduced viscosity, so that the diluted caustic can easily be removed by flowing down the cathode surface.

The present invention has been made based on such an idea. In an electrolysis method for producing caustic alkali by electrolysis of an alkali halide using a gas diffusion cathode, the gas diffusion cathode stays on the surface of the gas diffusion cathode. Dilute caustic or water is intermittently supplied to high-concentration caustic which may hinder smooth gas supply, and the low-concentration caustic is diluted to dilute the high-concentration caustic and flow down the cathode surface relatively smoothly. It is intended to be caustic. When a normal liquid-permeable gas diffusion cathode is used, empirically, at a current density of 30 to 40 A / dm ² , the cell voltage does not increase for 30 minutes to 1 hour, and thereafter increases. In the present invention, in order to suppress the voltage rise, dilute caustic or water is intermittently supplied to the cathode surface as described above.

It is desirable that the liquid to be supplied is water, especially pure water, since the dilution effect is maximum, that is, the supply amount can be minimized. However, a diluted caustic aqueous solution, for example, a caustic aqueous solution of 25% or less can also be used. With this concentration of caustic aqueous solution, the viscosity is sufficiently low, and the function of diluting and removing the high-concentration caustic existing on the cathode surface is provided. However, a particularly desirable concentration is 20% or less. Caustic alkali having a concentration of more than 25% has a function of removing high-concentration caustic on the cathode surface, but the amount of the supplied liquid needs to be increased, and the effect is slightly weakened. The dilute caustic or water can be simply sprayed from above.However, in this case, the diluting water or the like is sufficiently supplied to the upper part of the electrolytic cell, and the removing effect becomes remarkable. There is a possibility that the diluted water is not sufficiently supplied, and a high concentration of caustic remains, which may cause a decrease in cell voltage. Therefore, when supplying the dilution water or the like to the cathode surface, it is desirable to uniformly disperse the diluted water over the entire cathode surface by using a branch pipe, a shower, or the like. In the height direction of the cathode, since the lower portion is more likely to be covered with high-concentration caustic, it is also effective to slightly increase the amount of dilution water supplied to the lower portion than the amount supplied to the upper portion.

[0015]

EXAMPLES Next, examples of the electrolysis method according to the present invention will be described, but the examples do not limit the present invention.

[0016]

Example 1 A 0.6 mm-thick plate-shaped base material having a bulk density of 25% of pure metal by pressing a nickel foam was pressed, and nickel particles having an average particle size of 10 μm were coated on the surface of the base material with a water-repellent fluororesin. Was sintered as a binder, and a silver powder having an average particle size of about 1 μm was supported as a catalyst on one side of the sintered body to obtain a liquid-permeable gas diffusion cathode. This gas diffusion cathode was attached to an electrolytic cell having a width of 5 cm and a height of 25 cm so that the catalyst-carrying surface of the gas diffusion cathode was in close contact with the ion-exchange membrane side to constitute a two-chamber electrolytic cell. The ion-exchange membrane is Nafion 961 manufactured by DuPont. On the opposite side of the ion-exchange membrane, an insoluble electrode (so-called DSE) in which a 0.5 mm thick expanded mesh made of titanium is coated with a composite oxide of iridium oxide and ruthenium oxide is used. It was installed closely.

A liquid nozzle is provided obliquely downward at the upper part of the cathode chamber so that dilution water can be supplied to the cathode surface. In this state, a saline solution was circulated at 200 g / liter to the anode chamber of the electrolytic cell, and about 1.5 times the theoretical amount of pure oxygen was supplied to the cathode chamber.
In addition, about 4 ml of pure water was ejected from the nozzle once every 5 minutes to intermittently supply the water to the cathode surface. When the electrolysis was continued under these conditions, the cell voltage was stabilized at 2.25 V. When the current density difference between the upper half and the lower half of the electrolytic cell was measured, the lower half was about 2% lower than the upper half, but it was judged that this current density difference was within the practical range. The total caustic soda concentration to which the supplied water was added was 32%, which was the same as the caustic soda concentration obtained by a conventional method.

[0018]

[Comparative Example 1] Electrolysis was performed under the same conditions as in Example 1 except that sufficient water was added to pure oxygen without intermittent cleaning, and the cell voltage was initially 2.2 V. After a lapse of time, the voltage became 2.4 V, and the difference between the upper and lower current densities was lower by 20 to 25% in the lower half than in the upper half, and there was a large imbalance. In this comparative example, since there is no intermittent cleaning, the surface of the gas diffusion cathode is covered with the caustic soda solution, and the gas supply becomes insufficient.

[0019]

EXAMPLE 2 Instead of the pure water of Example 1, a diluted caustic soda solution having a concentration of about 15% prepared by mixing pure water and the obtained high-concentration caustic soda was used. Further, pure oxygen was used instead of pure oxygen. A moist gas to which water was added was used. This was intended to further homogenize the generated caustic soda concentration in addition to the dilution effect. As described above, electrolysis was performed under the same conditions as in Example 1 except that the diluted caustic soda solution was used instead of pure water and wet oxygen was used instead of pure oxygen, and the cell voltage was stable at 2.15 V from the beginning. The obtained caustic soda concentration was 32%, the same as in Example 1. It is considered that the reason why the cell voltage was lower than that in Example 1 was that the concentration of caustic soda generated in the vicinity of the ion exchange membrane due to the wet gas was lowered, and the electric resistance of the ion exchange membrane was lowered.

[0020]

According to the electrolysis method of the present invention, there is provided a method for producing caustic alkali using a vertical electrolytic cell having a liquid-permeable gas diffusion cathode, wherein the cathode side surface of the gas diffusion cathode is intermittently diluted with caustic or alkaline. This is an electrolysis method characterized by performing electrolysis while supplying water.

According to the method of the present invention, when electrolysis is carried out while intermittently supplying dilute caustic or water to the cathode side surface of the gas diffusion cathode, the high concentration caustic existing on the gas diffusion cathode surface is diluted and the low concentration caustic is diluted. Since it becomes alkali and is removed by flowing down the surface of the cathode, and the cathode is not clogged and the gas supply becomes smooth, the supplied gas easily reaches the entire surface of the gas diffusion cathode, and therefore reaches the surface of the ion exchange membrane where the reaction occurs. Thus, the reaction can be sufficiently advanced only by the amount of the supply gas slightly exceeding the stoichiometric amount. Therefore, large energy savings can be achieved because the efficiency of the reaction proceeds sufficiently, the cost of the supplied gas itself is reduced, and the means for supplying the gas is simplified. Moreover, since it is a simple method of supplying diluted caustic or the like to the cathode surface, the required cost is small,
The removal effect surely occurs, and a great merit can be obtained.

The above-mentioned diluted caustic or water is preferably sprayed not only from above but substantially over the entire cathode surface through a branch pipe or a shower.
As a result, high-concentration caustic alkali is removed from the entire surface of the cathode, and the current distribution over the entire cathode can be made substantially uniform. In the electrolytic cell, high-concentration caustic is more likely to accumulate on the lower surface than on the upper surface of the cathode. Therefore, when the supply amount to the lower surface is increased, the efficiency is further improved. Even if the method of the present invention is adopted, the drawback that the concentration of the formed caustic alkali is reduced does not occur, and stable operation is possible.

Continuation of the front page (72) Inventor Shuhei Wakita 5-5-9, Tsujido Motomachi, Fujisawa-shi, Kanagawa Prefecture II-3-3 (72) Inventor Yoshinori Nishiki 1-31-1 304 Fujisawa, Fujisawa-shi, Kanagawa

Claims (3)

[Claims] In a caustic alkali generating electrolysis method using a vertical electrolytic cell having a liquid-permeable gas diffusion cathode, electrolysis is performed while intermittently supplying dilute caustic or water to the cathode side surface of the gas diffusion cathode. An electrolysis method characterized by performing. 2. The electrolytic method according to claim 1, wherein the supply of the diluted caustic alkali or water is performed through a branch pipe substantially over the entire cathode surface. 3. The electrolysis method according to claim 1, wherein the amount of dilute caustic or water supplied to the lower surface of the cathode is larger than that of the lower surface.