JPH0138874B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a method for producing caustic alkali using a cation exchange membrane, particularly to a method for producing caustic alkali at low voltage. Electrolysis of an aqueous alkali metal chloride solution using a cation exchange membrane has overcome the drawbacks of the conventional mercury method and diaphragm method, and has developed as a new energy-saving technology. There is no risk of pollution, product purity is excellent, and the total energy cost (combined electricity and steam) is said to already exceed the mercury method and diaphragm method. However, although they are energy-saving types, electricity costs account for a large proportion of manufacturing costs, and the need for technological development to reduce power consumption is increasing as energy costs rise. Several attempts have been made to lower the electrolysis voltage in electrolysis methods using cation exchange membranes. Reducing the distance between the anode and cathode and reducing the amount of liquid and gas present in that area is considered an effective method for lowering the voltage. It is disclosed in Publication No. 50-109899, etc. However, in these methods, although the distance between the anode and the cation exchange membrane is shortened, the distance between the cathode and the cation exchange membrane is still large and the voltage drop is insufficient. JP-A No. 54-47877 proposes mechanically bringing both the negative and positive electrodes into close contact with the cation exchange membrane using a force such as a spring, but this is not recommended from the viewpoint of manufacturing accuracy of the electrodes and protection of the membrane. The distance between the poles cannot be made very small, and the voltage drop is insufficient. In addition, as electrolyzers in which the inter-electrode distance is further shortened, there are Japanese Patent Laid-Open Nos. 52-78788 and 53-52297. These have an anode on one side of the membrane,
A device with a cathode embedded in the other side has been proposed.
In this method, the distance between the electrodes is only the thickness of the cation exchange membrane, so if this method is applied to the electrolysis of alkali metal chloride salts, although a voltage drop can certainly be expected, there are the following disadvantages: be. 1 The electrolytic voltage decreases in the range of low current density, but as the current density increases, the IR of the electrode itself decreases.
Conversely, it tends to increase due to losses. 2 For this reason, it is necessary to use an expensive device as a current supply device to both electrodes, a so-called current collector. 3. If the contact between the current collector and the electrode is not ensured, the contact resistance will increase, so it is necessary to bring the current collector and the electrode into contact using a mechanical force such as a spring. This mechanical force tends to damage the membrane. 4. Difficult to apply to large industrial electrolytic cells. 5. If either the electrode or membrane is damaged, it will be impossible to continue electrolysis. 6. Current efficiency tends to decrease as the current density increases, and at the anode, water decomposition tends to occur due to insufficient supply of alkali metal chloride, and the oxygen gas content in the chlorine gas increases compared to the conventional method. In other words, although this method has great promise in reducing voltage drops, there are many mechanical and equipment problems that need to be solved as mentioned above, and it is difficult to improve the stability of operation and equipment, which is the most important industrially. is problematic and very difficult in practice. Further, JP-A-55-164086 discloses a method in which a cathode is fixed to the cathode side surface of a cation exchange membrane and electricity is applied to the cathode to perform electrolysis. This method is improved in several respects compared to the method in which electrodes are fixed to the cathode and anode surfaces, but in order to supply current to the cathode, a porous support and an elastic laminate are required. This requires a device to ensure contact between the current collector and the cathode and the current collector, and the above-mentioned industrially important problem has hardly been solved. That is, with a cation exchange membrane in between,
In electrolysis using a conventional electrolytic cell with a cathode and anode, the cell structure is simple, so operating conditions, operation,
Although maintenance management is easy and operation safety is excellent,
The disadvantage is that the voltage is high due to the liquid and bubbles present between the electrode and the membrane. On the other hand, when the positive and negative electrodes are fixed to the membrane and electrolysis is carried out by passing current through the fixed electrodes, a large reduction in voltage can be expected. Even if the above-mentioned problems are solved, because it requires an electrolytic cell equipped with a device with high
Compared to the conventional method, the cost of the container, operation, and maintenance costs are higher, and it is difficult to say that it is an economically advantageous method. As a result of intensive studies to achieve an industrially and economically superior electrolysis method that eliminates the disadvantages of both of these and has only the advantages of both, the inventors of the present invention have found that metals are added to the cathode side of the cation exchange membrane. The present invention was completed based on the discovery that a significant reduction in electrolytic voltage could be achieved by using a membrane to which the electrolytic cell was fixed in a conventional electrolytic cell. In the present invention, a cathode and an anode are installed so that a cation exchange membrane with a metal fixed to the cathode side surface of the cation exchange membrane by chemical plating is placed between them, and the pressure difference between the anode chamber and the cathode chamber is A method for electrolyzing an aqueous alkali metal chloride solution, which is characterized by bringing the membrane and the cathode into contact with each other and applying electricity to the cathode and anode, the purpose of which is to provide an electrolysis method that requires extremely low power. Another object of the present invention is to provide an electrolytic cell that does not require special devices or devices such as current collectors, springs, or elastic laminates. This practical and industrially excellent method discovers new knowledge that was not known until now, as detailed below, and based on this knowledge, develops technical ideas that are completely different from conventional ones. It was invented by. The present invention will be specifically explained below. Figure 1 shows a conventional cation exchange membrane, a porous anode coated with an electrochemically active catalyst, and two iron expanders placed independently in internal and external relation to the membrane. Current supply ratio to each cathode when current is supplied to the membrane at a current density of 30 A/dm ² using an independent current supply device for each cathode in an electrolytic cell with a metal cathode. This figure shows the relationship between the electrolytic voltage and the electrolytic voltage. As is clear from Figure 1, when a small amount of current flows through the inner cathode, the electrolytic voltage decreases rapidly, and when more than 20% of the total current flows through the inner cathode, the It is almost the same as the electrolytic voltage. Table 1 shows the same electrolytic cell as in Figure 1, with two cathodes connected so that they have the same potential, and 30A/d with respect to the membrane.
Current is supplied at a current density of ^m2 , and the relationship between the current supply ratio to each cathode and cell voltage is shown. The current supply ratio was varied using cathodes with different porosity. This results in a total current of 20
% or more, even if the ratio becomes larger, the cell voltage is almost the same, and the same results as in FIG. 1 were obtained. In other words, the inventor found that when two electrodes are used, the cell voltage does not change linearly depending on the current supply ratio, but when 20% or more of the total current flows through the inner electrode, the cell voltage changes. We discovered a previously unexpected phenomenon in which the cell voltage becomes almost the same as the cell voltage when the full current is supplied to the inner electrode, and by applying this knowledge to the method of fixing the electrode to the cation exchange membrane, we were able to achieve the voltage as described above. I thought that I could achieve my goal. In other words, in a conventional electrolytic cell, 20% of the total current is
We believed that if the above voltage could be applied to a membrane-fixed electrode, we would be able to achieve a sufficient voltage drop without the need for special devices such as current collectors, springs, or elastic laminates, and after repeated studies, we achieved our goal. The electrolytic cell used in the present invention has a normal structure in which a cation exchange membrane is provided between the cathode and the anode to form a cathode chamber and an anode chamber, and the electrolytic cell has a current collector and a spring or elastic laminate. No special equipment is required. In other words, a cation exchange membrane with a metal capable of acting as a cathode fixed to the electrode and cathode surface, which is normally used for the electrolysis of aqueous solutions of alkali metal chlorides, is used, and the pressure difference between the anode chamber and the cathode chamber causes the membrane and cathode to separate. By bringing the membrane into contact with the membrane and energizing the anode and cathode, it is possible to achieve a voltage drop of 0.3 to 1.0 volts compared to when using a cation exchange membrane that does not have metal attached to it. A large voltage reduction can be achieved by fixing an electrode to the membrane and supplying current to the electrode for electrolysis;
Furthermore, the effect of fixing the electrode to the membrane is greater on the cathode surface than on the anode surface, as mentioned in the above-mentioned JP-A-55-
This is disclosed in Publication No. 164086. According to the report, the reason why the voltage decreases significantly when the electrode is fixed to the membrane is mainly due to the reduction of liquid and air bubbles existing between the electrode and the membrane, and also due to gas adhesion to the membrane surface. and electrode overvoltage. In order to fully obtain these effects, it is necessary to uniformly supply current to the electrodes and to advance the electrode reactions smoothly, as described in this publication. However, since the IR loss of the fixed electrode itself is extremely large in practice, an external current supply device, that is, a current collector is required. Supplying current from the current collector to the electrodes must rely on mechanical pressure contact between the current collector and the electrodes, thus requiring the various special devices and devices mentioned above. In other words, various improvements have been proposed based on the technical idea that in order to achieve the goal of a wide voltage drop, the electrode reaction must substantially occur on the electrode fixed to the membrane. . However, in the method of the present invention, unlike the conventional methods that require close contact, only a portion of the cathodic reaction occurs at the membrane-fixed electrode. More specifically, the surprising result is that the amount of voltage reduction is large even though much more reaction is occurring with a cathode that is not fixed to a conventional membrane. Known methods for fixing cathode materials to cation exchange membranes include embedding them in the membrane under high temperature and pressure (crimping method) and chemical plating. In the compression bonding method, powder of a substance normally used as a cathode in caustic alkali, such as platinum black, nickel powder, or iron powder, is mixed with carbon, fluorine resin, or the like. The present inventor manufactured 2 by the crimping method and the chemical plating method.
As a result of examining various types of membranes, we found that membranes manufactured using the chemical plating method can provide a much larger voltage drop than membranes manufactured using the compression bonding method. The reason why there is a large difference depending on the method of fixing the electrode is that almost no current is supplied to the membrane-fixed electrode manufactured by the pressure bonding method, whereas when manufactured by the chemical plating method, there is no contact between the cathode and the membrane-fixed electrode. This is because current can be more effectively supplied to the membrane-fixed electrode even under the same conditions. Table 2 shows an electrolysis method that has an anode, a cation exchange membrane with a fixed electrode on the cathode surface, two cathodes connected by a good electrical conductor, and pressurizes the anode chamber so that only the membrane and the inner cathode are in contact. 30A/d against membrane in tank
This figure shows the relationship between the current values flowing through each cathode when electrolyzing at ^m2 . As is clear from Table 2, there is almost no change in the current supply rate to the inner cathode when using a film that is fixed by the pressure bonding method compared to the current supply rate to the inner cathode using a film that does not have an electrode fixed to it. When a fixed membrane is used, the rate of current delivery to the cathode in contact with the inner membrane increases, clearly providing current to the fixed electrode. The reason for this difference in the current supplied to fixed cathodes is that electrodes fixed using the pressure bonding method have a high electrical resistance due to the addition of fluororesin powder, which is an electrical insulator, whereas chemical We believe that this is because the cathode part fixed using the plating method is made only of metals that are good electrical conductors, and has low electrical resistance. Furthermore, the present invention is characterized in that the voltage reduction due to contact between the electrode and the membrane-fixed electrode is greater at the cathode than at the anode, and while the maximum voltage drop can be obtained by simultaneously applying the anode and cathode, the current flow to the membrane-fixed electrode is Since this requires special equipment and ingenuity, it is far more economical to carry out the method of the present invention from an industrial perspective.
I found it to be excellent. In addition, in order to increase the contact area and contact pressure between the cathode and the membrane and achieve a voltage drop more efficiently, we used a stainless steel wire mesh demister welded to an ordinary cathode as the cathode, resulting in a crimp bond. Not only was there no improvement in the effect of electrodes fixed using the chemical plating method, but the voltage reduction effect of the chemical plating method was significantly reduced. The reason for this is thought to be that the generated gas tends to accumulate in the wire mesh demister, affecting the escape of the gas generated at the fixed electrode, and as a result, overvoltage of the fixed membrane electrode or the adhesion of air bubbles to the membrane may occur. ing. As is clear from the above description, in the present invention, an independent cathode is used in addition to the membrane-fixed cathode, they are brought into contact only by the pressure difference between the anode chamber and the cathode chamber, and the membrane-fixed cathode is It is essential to manufacture by chemical plating method. The cathode used in the present invention may be any cathode that can withstand the environment in which it is used, has a sufficient catalytic effect on the reaction, and does not prevent the generated gas from escaping from the fixed electrode. If so, the objective can be achieved. Examples include porous materials such as iron, mild steel, nickel, and styrene steel, such as wire mesh, expanded metal, lattice, or punched metal. The metal fixed to the cathode side of the cation exchange membrane is not particularly limited as long as it has sufficient catalytic activity for the cathode reaction, but platinum such as platinum, palladium, ruthenium, and iridium is usually used. Group metals are used. These metals are fixed to the film by chemical plating. However, the chemical plating method itself is a normal method and there are no particular limitations. For example, a metal salt solution to be plated is placed on the cathode side of a cation exchange membrane, and a reducing agent solution is placed on the anode side, so that the permeation rate into the membrane is adjusted. Method of plating using the difference between
-38934 publication), a method in which a membrane is immersed in a metal salt solution to impregnate the membrane with the metal salt, and then immersed in a reducing agent to deposit metal on the membrane surface, using an electroless plating solution. The method can be selected as appropriate, and the above methods may be combined. The anode used in the present invention can withstand the usage environment,
A conventional anode having a sufficient catalytic effect for the desired reaction can be used without any limitations. For example, platinum on the surface of graphite or valve metals such as titanium, tantalum, tungsten, and zirconium.
A porous anode coated with a platinum group metal such as palladium, ruthenium, or iridium, an oxide of a platinum group metal, or a mixture of an oxide of a platinum group metal and an oxide of a valve metal is used. There are no particular restrictions on the cation exchange membrane, and any membrane generally used for electrolysis of an aqueous alkali metal chloride solution can be used. The ion exchange group may be of the sulfonic acid, carboxylic acid or sulfonic acid amide type, but
The carboxylic acid type or the combination type of carboxylic acid and sulfonic acid is most suitable. In this case, it is preferable to use the side where the sulfonic acid group is present as the anode surface and the side where the carboxylic acid group is present as the cathode surface. The resin matrix of the membrane is preferably fluorocarbon-based, and may be lined with cloth, netting, etc. to improve strength. There are no particular limitations on the conditions for carrying out the present invention, such as current density, alkali metal chloride concentration and pH in the anode chamber, and caustic alkali concentration in the cathode chamber.
The effects of the present invention are particularly noticeable when operating at a high current density of 20 A/dm ² or higher. Contact between the cathode and the membrane requires a contact resistance that is low enough to achieve a voltage drop, but no internal device is required to ensure contact, which is easily achieved by the pressure difference between the cathode and anode chambers. It can be achieved. For example, a method is used to slightly reduce the pressure in the cathode chamber or to pressurize the anode chamber. In particular, it is possible to pressurize the anode chamber by increasing the level of fresh salt water extracted from the anode chamber and bring the cathode and membrane into contact. This is a preferred form. In this case, the level at which the fresh salt water is extracted may be 0.2 m or more and 5 m or less relative to the liquid level in the cathode chamber. In addition, the electrical conductivity of the fixed electrode is
A conductivity of 1Ω ⁻¹ Cm ⁻¹ or more is sufficient to achieve the purpose, and this conductivity can be easily achieved by a chemical plating method. As described above, the method of the present invention does not require special devices such as current collectors, springs, or elastic laminates, so 1) the electrolytic cell is simple and the operating conditions, operation, and maintenance are easy; be. 2) Since the contact pressure between the electrode and the membrane is small, there is no damage to the membrane. 3) Easy to apply to large battery containers. 4) Electrolysis can continue as long as the membrane itself is not damaged. 5) Since the load on the electrode fixed to the membrane is small, the durability of the cathode is very high. 6) The chemical plating method is easy to apply to larger sizes. 7) Applicable to existing ion exchange membrane electrolyzers. 8) It is an extremely economical and industrially excellent method that not only eliminates all the drawbacks of the conventional electrolytic method in which electrodes are fixed to a membrane, but also has new advantages. An example of the effect will be shown below using a specific example. Note that the present invention is not limited to these specific examples. Example 1 After impregnating the surface of a cation exchange membrane Nafion 315 manufactured by DuPont with an equivalent weight of 1500 with platinum salt,
It was reduced using NaBH ₄ and then chemically plated with a platinum electroless plating solution containing DMAB. Titanium expanded metal coated with ruthenium oxide was used as the anode, and expanded iron metal was used as the cathode. The distance between the anode and cathode was set to 3 mm, and the level at which the fresh salt water was extracted from the anode chamber was set 20 cm higher than the liquid level in the cathode chamber so that the platinum-fixed surface of the membrane was in contact with the cathode. While supplying saturated saline solution to the anode chamber and a 30% by weight caustic soda aqueous solution to the cathode chamber, the temperature was 80℃ and the current density was 30A.
When electrolyzed at ^dm2 , the voltage was 3.1V and the current efficiency was 82%. Comparative Example 1 An electrolytic cell was assembled and electrolysis was carried out in the same manner as in Example 1, except that chemical plating of platinum was not performed and the membrane and cathode were not brought into contact. The voltage was 3.8 V, the current efficiency was 82%, and Ta. Further, when the membrane and the cathode were brought into contact by the method of Example 1, the voltage was 3.9V. Comparative Example 2 50 mg of platinum black powder and 10 mg of polytetrafluoroethylene dispersion were mixed and applied thinly to an area of 10 cm ² on aluminum foil, and after sintering at 360°C,
The aluminum foil was dissolved and removed to obtain a thin film. An electrolytic cell was assembled and electrolysis was carried out in the same manner as in Example 1, except that this thin film was pressed and embedded on the surface of the membrane of Example 1 with an equivalent weight of 1500. The voltage was 3.6 V and the current efficiency was 81%. It was hot. Next, electrolysis was carried out in the same manner as above except that three SUS304 wire mesh demisters with a wire diameter of 0.3 mm were welded to the iron expanded metal cathode of Example 1, and the voltage was It was 3.6V.

【table】

【table】 [Brief explanation of drawings]

FIG. 1 shows the relationship between the current supply ratio to each cathode and the electrolysis voltage.

Claims (1)

[Claims] 1. Using a membrane in which a conductive porous material is fixed to the cathode side surface of a cation exchange membrane, the porous layer is brought into contact with the cathode by a pressure difference between the cathode chamber and the anode chamber. A method for electrolyzing an aqueous alkali metal chloride solution, characterized in that a cation exchange membrane having a metal having a catalytic effect on the cathode reaction fixed to the cathode side surface by a chemical plating method is used. Method of electrolysis of salt aqueous solution. 2. The electrolysis method according to claim 1, wherein the metal is selected from platinum metals. 3. The electrolysis method according to claim 1, wherein the cathode is selected from iron, mild steel, nickel, and stainless steel.