JP3553781B2 - Electrolysis method using gas diffusion cathode - Google Patents

Description

[0001]
[Industrial applications]
The present invention relates to an electrolytic method for obtaining an alkali hydroxide at a low electrolysis voltage by electrolyzing an alkali chloride while smoothly supplying a gas to a gas diffusion cathode, and more particularly, to a gas supply to the gas diffusion cathode. The present invention relates to an alkali chloride electrolysis method using an oxygen gas diffusion cathode which can achieve a large energy saving effect in the production of alkali hydroxide by smoothing the process and prevent the ion exchange membrane from being contaminated due to elution of the electrode material.
[0002]
[Prior art and its problems]
The electrolysis industry represented by chloralkali electrolysis plays an important role as a material industry. Although it has such an important role, 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 the environmental problem and achieve energy saving, the mercury method was switched to the ion exchange membrane method via the diaphragm method, and about 25 years have achieved about 40% energy saving. However, this energy saving is not enough, and the power cost, which is energy, accounts for 50% of the total manufacturing cost. No further power savings are possible using current methods. In order to achieve further energy saving, drastic changes must be made, such as using an electrode reaction different from the conventional one. As an example, the use of a gas diffusion electrode employed in a fuel cell or the like is the most probable means at present, and is a means for greatly saving power.
[0003]
Gas diffusion electrodes are characterized by having a property of easily supplying a gas as a reactant to the electrode surface, and have been developed in consideration of applications such as fuel cells. Recently, the use of gas diffusion electrodes for industrial electrolysis has begun to be studied.For example, in the production of alkalis and various recovery processes, as an alternative to the generation of oxygen at the anode as the counter electrode reaction or the generation of hydrogen at the cathode, hydrogen oxidation at the anode or The oxygen reduction reaction at the cathode is performed using a gas diffusion electrode to reduce power consumption. It has also been reported that depolarization with a hydrogen anode is possible as a counter electrode for recovering metals such as collecting zinc or for galvanizing.
However, these industrial electrolysis systems have a problem that the electrode life and performance cannot be sufficiently obtained because the composition of the solution or gas or the operating conditions are not as simple as those of the fuel cell.
[0004]
An example of the sodium hydroxide production process by salt electrolysis will be described. Sodium hydroxide and chlorine, which are important as industrial raw materials, are mainly produced by salt electrolysis. This electrolysis process has undergone a transition as described above, and has shifted to an ion exchange membrane method using an ion exchange membrane as a diaphragm and using an activated cathode with a small overvoltage. During this time, the power consumption for the production of 1 ton of sodium hydroxide has been reduced to 2000 kWh. Furthermore, if an oxygen reduction reaction without hydrogen generation is performed instead of performing hydrogen generation at the cathode as in the conventional method, the theoretical decomposition voltage becomes 0.96 V from the conventional 2.19 V, which can be reduced by 1.23 V. And significant energy savings can be expected.
In order to industrially realize this new process, it is essential to develop an oxygen gas diffusion cathode (a gas diffusion cathode using oxygen as a supply gas) having high performance and sufficient stability in the electrolytic system.
FIG. 1 is a schematic diagram of a salt electrolysis cell using an oxygen gas diffusion cathode which is currently most commonly used.
[0005]
In the electrolytic cell 1, the electrolytic cell 1 is partitioned by a cation exchange membrane 2 into an anode chamber 3 and a cathode chamber 4, and the cathode chamber 4 is partitioned into a solution chamber 6 and a gas chamber 7 by an oxygen gas diffusion cathode 5. ing. Oxygen gas as a raw material is supplied from the gas chamber 7 side to the gas phase surface of the oxygen gas diffusion cathode 5, diffuses inside the oxygen gas diffusion cathode 5, and reacts with water in a catalyst layer in the cathode 5 to convert sodium hydroxide. Generate. Therefore, the cathode used in this electrolysis method must be a so-called gas-liquid separation type gas diffusion electrode that sufficiently permeates only oxygen and prevents permeation of sodium hydroxide from the solution chamber to the gas chamber. Oxygen gas diffusion cathodes currently proposed for salt electrolysis as electrodes meeting such demands are gas diffusion catalysts such as silver and platinum supported on an electrode substrate formed by mixing carbon powder and PTFE into a sheet shape. The electrodes are centered.
The anodic reaction and the cathodic reaction in the conventional salt electrolysis are as follows, and the theoretical decomposition voltage is 2.19V.
Anode reaction: 2Cl ⁻ → Cl ₂ + 2e (1.36 V)
Cathodic _{reaction: 2H 2 O + 2e → 4OH} - + H 2 (-0.83V)
[0006]
Here, when electrolysis is performed while supplying oxygen to the cathode, hydrogen is consumed by the supplied oxygen, and the cathode reaction is as follows.
Cathode reaction: 2H ₂ O + O ₂ + 4e → 4OH ⁻ (0.40 V)
Therefore, the power consumption can be reduced theoretically by 1.23 V, and about 0.8 V even in a practical current density range, and a saving of 700 kWh per ton of sodium hydroxide can be achieved. From the viewpoint of energy saving, practical application of salt electrolysis using a gas diffusion electrode has been studied since the 1980s.
The structure of the gas diffusion electrode used for chlor-alkali electrolysis is called a semi-hydrophobic type due to the specificity of the electrolysis conditions, and has a structure in which a hydrophilic reaction layer and a hydrophobic gas diffusion layer are laminated. . Both the reaction layer and the gas diffusion layer are made of carbon and PTFE resin is used as a binder. The PTFE resin is hydrophobic, and the properties of the two layers are determined by the amount of the PTFE resin used. That is, in the gas diffusion layer, the amount of the PTFE resin used is increased to improve the hydrophobicity of the gas diffusion layer. In JP-A No. 7-1980, the amount used is reduced to improve the hydrophilicity of the reaction layer.
[0007]
When a gas diffusion electrode containing carbon as a main component is used for salt electrolysis, the generated sodium hydroxide reacts with the carbon of the gas diffusion electrode and precipitates as sodium carbonate in the pores of the gas diffusion electrode, and the gas of the gas diffusion electrode The diffusivity may be reduced, and the skeleton of the gas diffusion electrode may be weakened. In order to solve this problem, a metal such as silver may be used instead of carbon, and although there are inconveniences such as difficulty in controlling the hydrophilic part and the hydrophobic part, its use is being studied. .
However, even if the problem of deterioration of carbon, which is the electrode material, is solved, the gas diffusion electrode cannot be applied to industrial electrolysis with another problem, that is, the supply gas cannot be uniformly supplied to the entire electrolytic surface due to the liquid pressure difference. The problem remains. In a normal electrolytic cell, there is a pressure difference due to the liquid in the height direction, and it is extremely difficult to uniformly supply the supply gas to the entire electrolysis surface against the pressure difference. Although a plurality of relatively short unit electrodes are arranged vertically to achieve a uniform gas supply, this method is too complicated in a practical tank and there is no practical example.
[0008]
In order to solve this drawback, the present inventors have proposed an electrolytic cell using a so-called zero-gap type gas diffusion electrode shown in FIG. In the electrolytic cell 8, the solution chamber of FIG. 1 is eliminated by bringing the oxygen gas diffusion cathode 9 into close contact with the ion-exchange membrane 10, and oxygen gas and water as raw materials are supplied. It is characterized by being collected from the side.
In this electrolytic cell, there is no solution chamber, and therefore there is no change in the liquid pressure in the height direction, so that there is no difference in liquid pressure due to the height.
As described above, attempts to adapt the gas diffusion electrode to an industrial electrolysis system have been continuously made, and various improvements have been made and results have been obtained. However, when an existing electrolytic cell having a height of even 1 m is used, the original electrolytic performance cannot be sufficiently obtained even with the gas diffusion electrode having the above structure. The reason is that, in addition to the alkali solution moving to the oxygen supply side, the liquid moved in the height direction due to gravity stays inside the electrode, so that the gas supply is hindered.
[0009]
[Object of the invention]
The present invention relates to the above-mentioned problems of the prior art, namely, gas diffusion electrode type electrolysis, in particular, zero-gap type alkali chloride electrolysis and hydrogen peroxide generation electrolysis in which electrolysis is performed by adhering an oxygen gas diffusion electrode to an ion exchange membrane. Solves the problem that the gas supply to the cathode surface is not smooth, can produce alkali hydroxide etc. under low electrolysis voltage, and is stable while maintaining low voltage even under high current density when enlarging the electrolytic cell It is an object of the present invention to provide an electrolysis method using a gas diffusion electrode capable of continuing the electrolysis.
[0010]
[Means for solving the problem]
In the electrolysis method according to the present invention, a gas diffusion cathode is disposed in an anode chamber and a cathode chamber of an electrolytic cell partitioned by an ion exchange membrane, and an alkali chloride solution is supplied to the anode chamber, and an oxygen-containing gas is supplied to the cathode chamber. In an electrolysis method for obtaining an alkali hydroxide by performing electrolysis while supplying, a hydrophilic liquid permeable layer is provided between the ion exchange membrane and the gas diffusion cathode, and all or a part of the generated alkali hydroxide is the solution. An electrolysis method characterized by extracting the ion-exchange membrane and the gas diffusion cathode to the periphery through a permeable layer.
[0011]
Hereinafter, the present invention will be described in detail.
Conventionally, application of an oxygen gas diffusion cathode to industrial electrolysis such as salt electrolysis has been studied and reported. In an electrolytic cell in which the cathode chamber is partitioned into a solution chamber and a gas chamber by an oxygen gas diffusion cathode, the liquid resistance between the ion exchange membrane and the cathode due to the liquid is not negligible.
The zero-gap type in which the ion exchange membrane and the cathode are in close contact with each other is a technique developed to reduce the liquid resistance, and the accompanying problems caused by the liquid pressure difference can be solved. For example, in the case of salt electrolysis, the above-mentioned cathodic reaction: 2H ₂ O + 2e → 4OH ⁻ + H ₂ occurs at the interface between the ion exchange membrane and the cathode, and the generated sodium hydroxide permeates through the oxygen gas diffusion cathode as a solution to form the cathode. It is taken out from the gas phase side. In this case, the flow direction of the sodium hydroxide and the flow direction of the oxygen-containing gas are opposite, so that the solution stays in the oxygen electrode or the gas supply speed decreases.
[0012]
For example, when the oxygen gas diffusion cathode is used for the salt electrolysis and when the gas generating electrode is used for the salt electrolysis, the increase in the electrolysis voltage with respect to the increase in the current density is about 1.5 to 2 times that of the latter. It is known. This is regarded as the characteristic of the oxygen gas diffusion cathode, and it is known that the main factor is not the type of reaction but the overvoltage other than the electrode reaction. One of the causes of the overvoltage rise is a shortage of supply gas to the oxygen gas diffusion cathode. For example, in the case of salt electrolysis, when the gas source is air or pure oxygen, the former is about 200 mV higher in overvoltage. It is known. In addition, the overvoltage becomes lower when the supply amount is increased, but it hinders the removal of the product, and as a result, the gas cannot be supplied smoothly.
[0013]
Another object of the present invention is to provide an electrolysis method capable of smoothly supplying both a solution containing this product and an oxygen-containing gas, thereby realizing the possibility of realizing an industrial electrolytic cell using an oxygen gas diffusion cathode. Get higher.
In the present invention, a liquid permeable layer is provided between the ion exchange membrane and the oxygen gas diffusion cathode of the zero gap type electrolytic cell in which the ion exchange membrane and the oxygen gas diffusion cathode are installed in close contact with each other, and the ion exchange membrane and the gas diffusion electrode are provided. Substantially and physically integrated, thereby suppressing the gas supply hindrance due to the clogging of the pores by the generated alkali hydroxide, which was a main factor of the voltage increase, and the disadvantage due to the liquid pressure difference applied to the gas diffusion cathode Can also be eliminated.
Further, in conventional salt electrolysis, particularly in zero gap type salt electrolysis, there is no problem in the case of normal operation, but when shutdown occurs, the electrode material of the gas diffusion cathode elutes, and the electrode material diffuses into the adhered ion exchange membrane. The ion exchange membrane may be poisoned, and this poisoning may cause a decrease in current efficiency or an increase in cell voltage. On the other hand, in the method of the present invention, the liquid permeable layer existing in the ion exchange membrane and the gas diffusion cathode is hardly extracted together with the alkali hydroxide solution generating the electrode substance eluted from the gas diffusion cathode and reaches the ion exchange membrane. Does not adversely affect the exchange membrane. Further, chlorine or hypochlorous acid which may permeate from the anode chamber is hardly diffused into the liquid permeable layer to corrode the gas diffusion cathode.
Furthermore, since the liquid is present on the entire surface between the ion exchange membrane and the gas diffusion cathode, the entire electrolytic surface is reliably used, and a reduction in the electrolytic voltage can be expected.
[0014]
In the liquid permeable layer used in the present invention, all or a part of the solution in which the alkali hydroxide generated by the ion exchange membrane is dissolved is drawn through the liquid permeable layer to the periphery of the cathode chamber, particularly to the lower portion, and the solution is subjected to ion exchange. The residence time between the membrane and the oxygen gas diffusion cathode is shortened, whereby the supply of the oxygen-containing gas from the back surface of the oxygen gas diffusion cathode is performed smoothly. The liquid permeable layer needs to be made of a material that is inert and does not contribute to the electrolytic reaction in addition to being hydrophilic, but may be conductive or insulating. In addition, it is necessary to have resistance to high-concentration alkali and, to a lesser extent, chlorine and hypochlorous acid permeating the ion exchange membrane. Furthermore, since it is installed between the ion exchange membrane and the gas diffusion cathode, it is flexible and can absorb the pressure by deforming when pressure unevenness occurs, and is physically connected to the ion exchange membrane and the gas diffusion cathode and substantially. It is desirable that the ion exchange membrane, the liquid permeable layer, and the gas diffusion cathode be integrated, and that the material and the structure be capable of always holding the catholyte.
[0015]
As a substance satisfying the requirement of hydrophilicity of the liquid permeable layer in the present invention , titanium oxide, oxides such as zirconium oxide and tin oxide, metals and alloys such as nickel and mesh plated with gold and silver, and fluororesins and the like There are resins having acid resistance and chemical resistance, carbonaceous materials such as graphite and carbon black, and ceramics such as silicon carbide. When the resin is used, since the resin is usually hydrophobic, it is necessary to hydrophilize the surface or use it in combination with a hydrophilic oxide. When silver is used as the material of the liquid permeable layer, since silver itself has an electrode activity, it is necessary to suppress the activity by setting the particle size to 10 μm or more. The same applies to the case where carbon is used, and it is necessary to suppress the activity by setting the particle size to 5 μm or more.
[0016]
The liquid permeable layer used in the method of the present invention is desirably formed by applying and baking a material powder kneaded with a resin or the like on the surface of a gas diffusion cathode or an ion exchange membrane, but separately forming a sheet-shaped liquid permeable layer. Then it may be sandwiched between them. The liquid permeable layer produced by firing becomes porous having a large number of pores. However, when a sheet-shaped liquid permeable layer is separately prepared, it may be nonporous or porous. However, in the method of the present invention, it is desirable that the liquid permeable layer is porous for smooth gas supply. The structure and pore size of the liquid-permeable layer are inevitably determined by the conditions in the case of the firing method, but the structure of the separately formed sheet-shaped liquid-permeable layer includes a mesh, a woven fabric, a nonwoven fabric, and a foam. In particular, a sintered plate obtained by forming a sheet from a powder as a raw material with a pore-forming agent and various binders and removing pore-forming particles with a solvent, or a laminate of the sintered plates is preferable.
[0017]
When a liquid permeable layer is formed on the surface of an ion exchange membrane using the former calcination method, an ion exchange membrane manufactured by Asahi Glass Co., Ltd., which is commercially available as an ion exchange membrane having a hydrophilic surface, is used. A surface such as Flemion F866) is coated with a material such as zirconium oxide, and a gas diffusion cathode can be used as it is on the zirconium oxide side of the ion exchange membrane. This material coating is performed by, for example, kneading a powder of zirconium oxide or the like with an aqueous dispersion of a fluororesin, for example, a Teflon (registered trademark) dispersion 30J solution manufactured by DuPont, and then coating the pretreated ion exchange membrane surface with hot water. What is necessary is just to bake at 150-250 degreeC with a press for about 10 to 30 minutes. The pressure may be about 1 to 30 kg / cm ² at which the liquid permeable layer is smoothly formed on the surface of the ion exchange membrane.
[0018]
When this liquid permeable layer is formed on the surface of the gas diffusion cathode, firing may be performed under the same conditions. In this case, in order to proceed the operation more efficiently, the electrode layer at the time of manufacturing the gas diffusion cathode is formed by hot pressing. Before carrying out the process, it is also possible to apply a dispersion of the material for the liquid permeable layer, and simultaneously perform the production of the gas diffusion cathode and the production of the liquid permeable layer by one hot press. In this case, the optimum conditions of the former and the latter may be different, but by controlling the amount of the fluororesin and the particle size of the material particles to adjust the porosity and the like, a suitable gas diffusion cathode and a liquid permeable layer are simultaneously produced. it can. When a fluororesin is used, a suitable particle size of the material of the liquid permeable layer is about 5 to 30 μm, and when the fluorinated resin is baked using 20 to 40%, a liquid permeable layer having a porosity of 50 to 89% is obtained. It is formed. If the amount of the fluororesin exceeds 40%, the hydrophobicity becomes strong and the alkali hydroxide solution hardly flows in the liquid permeable layer.
When using a liquid permeable layer such as a network, the network may be baked using a fluororesin on the gas diffusion cathode surface, or the network may be simply sandwiched between the gas diffusion cathode and the ion exchange membrane. .
[0019]
From the viewpoint of the liquid resistance, it is preferable that nothing exists between the ion-exchange membrane and the oxygen gas diffusion cathode. Therefore, it is better not to insert the liquid permeable layer of the present invention between them. If it does, the electrolysis voltage rises. However, there is no necessity that the ion exchange membrane and the cathode have to be in close contact with each other except when the ion exchange membrane such as pure water electrolysis is used as a solid electrolyte, and the effect of increasing the electrolytic voltage by the insertion of the liquid permeable layer is more than that. Appears, energy saving as a whole can be achieved.
From this side, the liquid resistance is preferably reduced as the liquid permeable layer becomes thinner, but if the liquid permeable layer is too thin, the function of taking out the alkali hydroxide solution, which is the original function of the liquid permeable layer, to the peripheral portion without passing through the gas diffusion cathode is preferred. Therefore, it is desirable to determine the thickness in consideration of the liquid resistance and the function, and the preferable thickness is about 0.1 to 1 mm.
[0020]
When the electrolytic cell of the present invention is used for salt electrolysis, a fluororesin-based membrane is preferable as the ion exchange membrane from the viewpoint of corrosion resistance. As the anode, it is desirable to use a titanium insoluble electrode called ordinary DSA, but other electrodes can be used.
The electrolysis conditions are preferably, for example, a temperature of 60 to 90 ° C. and a current density of 10 to 100 A / dm ^2, and the supplied oxygen-containing gas is humidified as necessary. As a humidification method, a humidification device humidified at 70 to 95 ° C. is provided at the inlet of the electrolytic cell, and the humidification is controlled by passing the oxygen-containing gas. According to the performance of currently commercially available membranes, if the concentration of the anolyte is maintained at 200 g / liter or less, particularly around 170 g / liter, humidification of the oxygen-containing gas becomes unnecessary. The obtained sodium hydroxide concentration is suitably about 25 to 40%, but is basically determined by the performance of the ion exchange membrane.
[0021]
When performing salt electrolysis using the electrolytic cell of the present invention, sodium hydroxide mainly generated near the ion exchange membrane side surface of the oxygen gas diffusion cathode is extracted through the liquid permeable layer, that is, without passing through the oxygen gas diffusion cathode. Can be. At this time, if the liquid permeable layer is in a seed form, the sodium hydroxide is not extracted unless it reaches the periphery, and it may take a relatively long time to extract. In order to solve this problem, in the present invention, for example, the sheet is divided into a plurality of sheets, and one end of each of the divided sheets is separated from the gaps of the oxygen gas diffusion cathode formed with, for example, a slit or guide having a width of 1 to 5 mm. If it is arranged so as to reach the back surface, the produced sodium hydroxide is extracted from between the ion exchange membrane and the oxygen gas diffusion cathode in a short time before reaching the periphery.
[0022]
FIG. 3 is a longitudinal sectional view showing an example of a saline electrolytic cell using an oxygen gas diffusion cathode usable in the method of the present invention.
The electrolytic cell body 11 is divided into an anode chamber 13 and a cathode chamber 14 by an ion exchange membrane 12, and a mesh-shaped insoluble anode 15 is in close contact with the ion exchange membrane 12 on the anode chamber 13 side. A liquid permeable layer 16 is in close contact with the cathode chamber 14, and a liquid permeable oxygen gas diffusion cathode 17 is in close contact with the liquid permeable layer 16 on the cathode chamber side. An electric body 18 is connected and supplied with power by the current collector 18.
Reference numeral 19 denotes an anolyte (saturated saline) inlet formed on the side wall near the bottom of the anode chamber. Reference numeral 20 denotes an anolyte (unreacted saline) and chlorine gas outlet formed on the side wall near the top of the anode chamber. Is a (humidified) oxygen-containing gas inlet formed on the side wall near the top of the cathode chamber, and 22 is an outlet for sodium hydroxide and excess oxygen formed on the side wall near the bottom of the cathode chamber.
[0023]
When a saturated saline solution as an anolyte solution is supplied to the anode chamber 13 of the electrolytic cell 11 and a humidified oxygen-containing gas such as pure oxygen or air is supplied to the cathode chamber 14, electricity is supplied between the electrodes 15 and 16 to perform ion exchange. Sodium hydroxide is generated on the surface of the membrane 12 on the cathode chamber 14 side. In a usual electrolytic cell, the sodium hydroxide permeates the oxygen gas diffusion cathode as an aqueous solution and reaches the cathode chamber side surface. However, in the illustrated electrolytic cell 11, since the liquid permeable layer 16 exists between the ion exchange membrane 12 and the oxygen gas diffusion cathode 17, the resistance of the aqueous sodium hydroxide solution becomes smaller than that permeating the inside of the cathode 17. The liquid is dispersed in the liquid permeable layer 16, in particular, descends due to gravity, reaches the lower end of the liquid permeable layer 16, drops as droplets, and is stored at the bottom of the cathode chamber 14.
When this electrolytic cell is compared with the conventional electrolytic cell shown in FIG. 2 or the like, in the conventional electrolytic cell shown in FIG. 2, the generated sodium hydroxide aqueous solution has to permeate through the dense oxygen gas diffusion cathode, and The residence time in the reactor becomes longer, the smooth permeation of the supplied oxygen-containing gas is hindered, and the gas supply that controls the reaction becomes insufficient. . In contrast, in the electrolytic cell shown in FIG. 3, the generated sodium hydroxide aqueous solution is taken out from the reaction site by dispersion in the liquid permeable layer having relatively low resistance, and hardly stays in the cathode. Is carried out smoothly, and thus the reaction efficiency is also kept high.
[0024]
FIG. 4 is a perspective view of an essential part of a part of the electrolytic cell of FIG. 3 in which a generated aqueous sodium hydroxide solution can be more smoothly taken out. FIG. 4A is an example in which a cathode is divided into a plurality of parts, and FIG. Shows an example in which a slit is formed.
In FIG. 4a, the oxygen gas diffusion cathode 17a is divided into a plurality of pieces 17b, and the hydrophilic liquid permeable layer 16a is also divided into a corresponding number of liquid permeable layer pieces 16b. The lower end of each liquid permeable layer piece b is bent in the direction of the cathode 17b, passes between vertically adjacent cathodes 17b, reaches the back surface of the cathode 17b, and forms a bent piece 16c.
[0025]
When electrolysis is performed using this electrolytic cell, an aqueous solution of sodium hydroxide generated on the surface of the ion exchange membrane on the side of the cathode chamber permeates through the hydrophilic liquid permeable layer piece 16b as in the electrolytic cell of FIG. I do. Since the liquid permeable layer 16b is divided, the sodium hydroxide aqueous solution moves in each liquid permeable layer piece 16b within a relatively short distance to the lower end thereof without moving to the peripheral portion, toward the cathode 17b. It falls as a droplet from the folded piece 16c. Therefore, the liquid can be drained more smoothly than the electrolytic cell shown in FIG.
FIG. 4B shows an example in which the cathode 17c is not divided into a plurality of parts, and the cathode 17c is formed with a slit 23 having a horizontally long rectangular shape. When the cathode is divided into a plurality of parts as shown in FIG. 4A, it is necessary to supply power for each divided piece, which is complicated. However, as shown in FIG. 4B, a slit 23 is formed in the cathode 17c, and the cathode 16b is bent through the slit 23. If the piece 16c is located on the back surface of the cathode, it is more convenient because power can be supplied to the cathode by a single current collector.
[0026]
【Example】
Next, examples of the electrolysis method according to the present invention will be described, but the examples do not limit the present invention.
[0027]
Embodiment 1
A nickel foam having a thickness of 4 mm was crushed to a thickness of 1 mm with a breath to form a substrate. Nickel particles having an average particle size of 5 μm were dispersed on the surface of the substrate in a 40% by weight aqueous dispersion of Teflon (30J manufactured by DuPont). ) Was added and kneaded, and the mixture was hot-pressed at a pressure of 10 kg / cm ² at 300 ° C. for 15 minutes. Submicron silver particles having an average particle size of 0.5 μm as an electrode catalyst were dispersed on one surface of the mixture using the same Teflon dispersion. It was applied together with the solution and baked at 250 ° C. to obtain a gas diffusion cathode.
To the silver particle side of this gas diffusion cathode, a mixture of zirconium oxide powder having an average particle diameter of 20 μm and the same Teflon dispersion liquid of 20% by weight was applied to a thickness of 200 μm, and baked at a temperature of 200 ° C. A transmission layer was formed.
[0028]
The gas diffusion cathode having the liquid permeable layer formed thereon was brought into close contact with an ion exchange membrane Nafion 961 manufactured by DuPont at a pressure of 0.1 kg / cm ² . Other electrolysis conditions were as follows: current density was 30 A / dm ² , temperature was 90 ° C., supply rate of anolyte salt was 170 g / liter, and a normal dimensionally stable electrode (dimensionally stable electrode) was used as the anode. Electrolysis was performed while supplying 1.5 times the theoretical amount of 88% pure PSA oxygen, which was enriched in air by the PSA method, to the cathode chamber, which was a gas chamber only, and was energized. The electrolysis voltage in the initial stage of electrolysis was 2.05 V, and a 32-33% aqueous sodium hydroxide solution (catholyte) was obtained with a stable current efficiency of 96-97%. When electrolysis was continued under these conditions for 10 hours, the increase in electrolysis voltage was only 10 mV. After 10 hours, the voltage was shut down to terminate the electrolysis, but no change was observed in the ion exchange membrane. This shutdown was repeated several times, but no decrease in current efficiency was observed.
About 70% of the sodium hydroxide contained in the catholyte flows through the zirconium oxide liquid permeable layer, is taken out from the lower side of the liquid permeable layer, passes through the gas diffusion cathode, and reaches the cathode back surface. Things were slight.
[0029]
Embodiment 2
The same experiment as in Example 1 was performed by setting the contact pressure of the gas diffusion cathode to the ion exchange membrane to 0.1, 0.2, 0.3, and 0.4 kg / cm ^2. The concentration of the obtained aqueous sodium hydroxide solution was the same as in Example 1.
[0030]
[Comparative Example 1]
Electrolysis was performed under the same conditions as in Example 1 except that the liquid permeable layer made of zirconium oxide was not formed. The initial electrolysis voltage and current efficiency were 2.05 V and 96 to 97%, respectively, which were the same as Example 1. However, after a lapse of 10 hours, the electrolytic voltage increased to 2.1 V, and thereafter a slight increase in the voltage was observed.
When the electrolysis was completely stopped and then started again, the electrolysis voltage was initially 2.05V. This is because the aqueous solution of sodium hydroxide that has passed through the gas diffusion cathode blocks the gas diffusion cathode and shields part of the above-described enriched oxygen supplied through the cathode. It is considered that it was removed.
When the electrolysis was stopped, the ion exchange membrane turned black. It was found that a small amount of nickel in the gas diffusion cathode was deposited in the ion exchange membrane.
When the above-described electrolysis stop (shutdown) was repeated, the current efficiency gradually decreased to 96 to 93% of the initial value.
[0031]
[Comparative Example 2]
The electrolysis was performed under the same conditions as in Example 2 except that the liquid permeable layer made of zirconium oxide was not formed. At 0.3 and 0.4 kg / cm ² , almost the same time-dependent change in voltage as in Comparative Example 1 was observed. Was observed, but the efficiency and the concentration of the obtained aqueous sodium hydroxide solution were the same as in Example 1. At 0.1 and 0.2 kg / cm ² , the fluctuation of the voltage with the passage of time was large, and the overall voltage drop was larger than that of Comparative Example 1.
[0032]
Embodiment 3
Example 2 was the same as Example 1 except that the liquid permeable layer was a silver net having an apparent thickness of 0.2 mm, and the net was sandwiched between the ion exchange membrane and the gas diffusion cathode at a pressure of 0.2 kg / cm ^2. When electrolysis was performed under the same conditions, the initial electrolysis voltage was 2.05 V, which was the same as in Example 1, and the electrolysis was stable even when the electrolysis was continued.
[0033]
【The invention's effect】
In the method of the present invention, a gas diffusion cathode is disposed in the cathode chamber of an electrolytic cell partitioned into an anode chamber and a cathode chamber by an ion exchange membrane, while supplying an alkali chloride solution to the anode chamber and an oxygen-containing gas to the cathode chamber. In the electrolysis method for obtaining an alkali hydroxide by electrolysis, a hydrophilic liquid permeable layer is provided between the ion exchange membrane and the gas diffusion cathode, and all or a part of the generated alkali hydroxide passes through the liquid permeable layer. An electrolysis method characterized by extracting the ion-exchange membrane and the gas diffusion cathode to the periphery.
In a conventional electrolytic cell using a gas diffusion cathode, particularly a zero-gap type electrolytic cell in which a gas diffusion electrode is in close contact with an ion exchange membrane, the target product generated on the cathode chamber side surface of the ion exchange membrane is a gas having a relatively high density. The gas must pass through the gas diffusion cathode while passing through the diffusion cathode, that is, in the direction opposite to the supply direction of the supplied reaction gas, in other words, while obstructing the supply of the reaction gas, the more the product increases, the more the reaction gas However, there is a problem that the supply to the reaction site is inhibited and the reaction efficiency is reduced.
[0034]
On the other hand, in the method of the present invention, since the hydrophilic liquid permeable layer is arranged between the oxygen gas diffusion cathode and the ion exchange membrane, almost all of the conventional method must be extracted through the oxygen gas diffusion cathode. At least a part of the sodium hydroxide solution can be extracted from the surface of the ion exchange membrane without passing through the oxygen gas diffusion cathode and passing through the liquid permeable layer and facing the supply direction of the reaction gas. Therefore, even if the amount of the product increases, the supply of the reaction gas is hardly affected, and the predetermined electrolytic reaction can be continued while maintaining the reaction efficiency at a high level.
In other words, the present invention performs the opposing operations in the different directions of the smooth extraction of the solution in which the product is dissolved and the smooth supply of the oxygen gas at the maximum efficiency, thereby reducing the electrolysis voltage more than before and reducing the oxygen gas diffusion cathode. It has made it possible to widen the road for application to industrial electrolysis.
[0035]
Furthermore, in the method of the present invention, impurities such as a cathode material and chlorine and hypochlorous acid which are easily mixed into the catholyte can be extracted from the liquid permeable layer between the ion exchange membrane and the gas diffusion cathode. It hardly reaches the gas diffusion cathode, and the ion exchange membrane and the gas diffusion cathode can be protected at the same time.
In the liquid permeable layer of the method of the present invention, it is desirable that the constituent material is applied to the surface of the gas exchange cathode on the ion exchange membrane side and then baked to be integrated with the gas diffusion cathode. Combined with the cathode, stable operation is possible.
[Brief description of the drawings]
FIG. 1 is a schematic view showing an example of a conventional salt electrolysis tank.
FIG. 2 is a schematic view showing another example of a conventional salt electrolysis tank.
FIG. 3 is a longitudinal sectional view showing an example of a saline electrolytic cell using an oxygen gas diffusion cathode usable in the method of the present invention.
4 is a longitudinal sectional view showing another example of a salt electrolysis cell using an oxygen gas diffusion cathode usable in the method of the present invention, FIG. 4a shows an example in which a plurality of cathodes are disassembled, and FIG. Is formed and an example is shown.
[Explanation of symbols]
DESCRIPTION OF SYMBOLS 11 ... Electrolyzer main body 12 ... Ion exchange membrane 13 ... Anode chamber 14 ... Cathode chamber 15 ... Insoluble anode 16 ... Liquid permeable layer 17 ... Oxygen gas diffusion cathode 18 ...・ Current collector

Claims (2)

A gas diffusion cathode is arranged in the cathode compartment of an electrolytic cell partitioned by an ion exchange membrane into an anode compartment and a cathode compartment, and an alkali chloride solution is supplied to the anode compartment and electrolysis is performed while supplying an oxygen-containing gas to the cathode compartment, and hydroxylation is performed. In the electrolysis method for obtaining an alkali, a hydrophilic liquid permeable layer is provided between the ion exchange membrane and the gas diffusion cathode, and all or a part of the generated alkali hydroxide is passed through the liquid permeable layer through the ion exchange membrane and An electrolysis method, wherein the gas is extracted to a peripheral portion of a gas diffusion cathode. 2. The electrolysis method according to claim 1, wherein the liquid permeable layer is formed on the side surface of the ion exchange membrane of the gas diffusion cathode.

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