JP2004300510A - Protection method of ion-exchange membrane electrolytic cell using gas diffusion cathode - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To prevent corrosion of a cathode gas chamber while an electrolytic cell is out of operation and maintain a high electrolytic performance of the two-chambered alkali chloride electrolytic cell which is equipped with a gas diffusion cathode and is used for preparing chlorine and caustic alkali through electrolysis of an aqueous alkali chloride solution. <P>SOLUTION: When stopping the operation of the alkali chloride electrolytic cell 1 equipped with the gas diffusion cathode 8, the supply of an oxygen-containing gas to the cathode gas chamber 4 is stopped, and the oxygen-containing gas atmosphere in the cathode gas chamber is substantially replaced by an aqueous caustic alkali solution. <P>COPYRIGHT: (C)2005,JPO&amp;NCIPI

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a protection method and apparatus when an ion exchange membrane electrolytic cell is stopped, and more particularly to a protection method and device when an ion exchange membrane type alkaline chloride electrolytic cell using a gas diffusion cathode is stopped.
[0002]
[Prior art]
The electrolytic industry represented by alkali chloride electrolysis plays an important role as a material industry. Although it has such an important role, energy consumption required for alkali chloride electrolysis is large, and energy saving is a big problem in countries with high energy costs such as Japan. For example, in alkaline chloride electrolysis, in order to solve environmental problems and achieve energy savings, the mercury method has been switched to the ion exchange membrane method via the diaphragm method, and energy savings of about 40% have been achieved in about 25 years. However, even this energy saving is not enough, and the power cost of energy accounts for about half of the total manufacturing cost, but no further power savings are possible using the current method. In order to achieve further energy saving, fundamental changes such as conversion of electrode reactions must be made. As an example, the use of a gas diffusion cathode employed in a fuel cell or the like is the most probable and is a means for saving power. The conventional electrolytic reaction (1) using a metal cathode is converted into an electrolytic reaction (2) when oxygen is supplied using a gas diffusion cathode as a cathode.
[0003]
2NaCl + 2H ₂ O → Cl ₂ + 2NaOH + H ₂ Eo = 2.19V (1)
2NaCl + 1 / 2O ₂ + 2H ₂ O → Cl ₂ + 2NaOH Eo = 1.14V (2)
[0004]
In other words, by converting the metal cathode into a gas diffusion cathode and performing an electrolytic reaction for supplying oxygen, the theoretical decomposition potential is reduced from 2.19 V to 1.14 V, and theoretically it is possible to save 40% or more of energy. . Various studies have been made for the practical use of alkali chloride electrolysis by using this gas diffusion cathode. However, as a method for further reducing the electrolysis voltage, the gas diffusion cathode is installed in close contact with the ion exchange membrane, and the cathode is substantially reduced. A number of methods for eliminating the liquid chamber, in other words, forming the cathode chamber as a gas chamber (consisting of two chambers consisting of an anode chamber and a cathode gas chamber, collectively referred to as a two-chamber method) are proposed in JP-A-11-124698 Has been. When this method is adopted, the space such as the catholyte chamber is minimized between the ion exchange membrane and the cathode (hydrophilic layer), so that the electrical resistance of the catholyte is minimized and the electrolysis voltage can be kept to a minimum. Have
[0005]
As described above, although there have been many proposals for improving the performance of the two-chamber alkaline chloride electrolytic cell equipped with a gas diffusion cathode, there have been few proposals for a method for protecting the electrolytic cell so far. .
[0006]
[Patent Document 1]
Japanese Patent Laid-Open No. 2001-26893
[Problems to be solved by the invention]
In the case of an ion exchange membrane electrolytic cell using a gas diffusion cathode, the electrolytic operation conditions are important in order to operate over a long period of time while maintaining the original high performance, and particularly when the electrolytic cell is stopped. The way is also an important issue. This is because depending on the method of treatment, the performance of the electrolytic cell may be significantly reduced at the time of stoppage, or in some cases it may be unusable.
By the way, in the two-chamber alkali chloride electrolysis by the ion exchange membrane method provided with a gas diffusion cathode, the following process is performed. That is, an anode chamber including an anode and a cathode gas chamber including a cathode are partitioned by an ion exchange membrane, an aqueous alkali chloride solution is supplied to the anode chamber, chlorine gas is generated at the anode, and oxygen gas is supplied to the cathode gas chamber. The contained gas and moisture are supplied, and caustic is generated at the gas diffusion cathode.
[0008]
The anode has a noble potential corresponding to the chlorine overvoltage from the chlorine generation potential, and the gas diffusion cathode has a base potential corresponding to the overvoltage from the reduction potential of oxygen. If the electrolytic cell is stopped, the chlorine generation reaction and the oxygen reduction reaction are stopped. However, since chlorine is dissolved in the alkali chloride aqueous solution that is the anolyte, the voltage potential of the anode and the anode chamber becomes the chlorine generation potential. Retained. On the other hand, since the gas diffusion cathode and the cathode gas chamber are in contact with the caustic aqueous solution and the oxygen-containing gas, the voltage potential of the gas diffusion cathode and the cathode gas chamber is maintained at the oxygen reduction potential. Originally, a material that can withstand such a voltage potential condition is used for the electrolytic cell, and specifically, titanium is generally used as a material constituting the anode chamber.
[0009]
The anode is made of titanium as a base material and coated with a noble metal such as platinum, ruthenium, iridium, palladium, rhodium or their oxides, which are specific electrode catalysts for generating chlorine. Nickel or a nickel-containing alloy is used as the cathode gas chamber constituent material.
When an electrolytic reaction is performed in such an electrolytic cell, the voltage potential of the gas diffusion cathode is lower than the oxygen reduction potential by the amount of overvoltage. When the electrolysis is stopped, the overvoltage disappears and becomes equal to the oxygen reduction potential, so that the potential becomes noble compared to when electrolysis is performed. If an oxygen-containing gas is present under these conditions, corrosion of the cathode gas chamber, which is nickel or a nickel-containing alloy, tends to proceed.
[0010]
An anode chamber including an anode and a cathode gas chamber including a cathode are partitioned by an ion exchange membrane. When the operation of the two-chamber electrolytic cell is stopped, the aqueous alkali chloride solution remains in the anode chamber, but no caustic alkali is generated in the cathode gas chamber, so that the caustic aqueous solution is a slight amount of caustic alkali retained in the hydrophilic layer. There is only an aqueous solution. In this case, when the aqueous alkali chloride solution in the anode chamber penetrates the ion exchange membrane according to the concentration gradient and enters the cathode gas chamber, the aqueous caustic solution is immediately replaced with the aqueous alkali chloride solution. Originally, the cathode gas chamber has sufficient corrosion resistance against an alkaline caustic aqueous solution, but does not have corrosion resistance against a neutral alkaline chloride aqueous solution and easily corrodes.
[0011]
As a means for protecting the gas diffusion cathode of the alkali chloride electrolytic cell during such operation stop, a method of passing a minute current through the gas diffusion cathode has been proposed (Patent Document 1). In this method for protecting the gas diffusion cathode, the potential of the gas diffusion cathode can be maintained at substantially the same value as when the operation is continued by a minute current, so that corrosion of the gas diffusion cathode can be substantially avoided. However, since the current continues to flow even though it is a minute current, it cannot be said to be strictly when the operation is stopped.For example, when the operation is stopped and the electrolytic cell or the gas diffusion cathode is inspected and repaired, this method is Cannot be adopted. Furthermore, even if the operation is stopped for a long time although it is a minute current, the amount of power consumption cannot be ignored, and during that time, an electrolysis product such as caustic soda cannot be obtained, resulting in a large economic loss. Therefore, there is a demand for a method capable of protecting the electrolytic cell and the gas diffusion cathode when the operation is stopped without passing a minute current.
[0012]
Therefore, the present invention prevents corrosion of the stopped cathode gas chamber when the operation of electrolyzing an aqueous alkali chloride solution to produce chlorine and caustic alkali is stopped, particularly in a two-chamber electrolytic cell equipped with a gas diffusion cathode. Thus, an object of the present invention is to provide a protection method and a protection device for an alkaline chloride electrolytic cell capable of maintaining the original high performance of the electrolytic cell over a long period of time.
[0013]
[Means for Solving the Problems]
The present inventors have repeatedly studied a method for producing chlorine and caustic by electrolyzing an aqueous alkali chloride solution in a two-chamber electrolytic cell equipped with a gas diffusion cathode. In particular, the present invention has been completed as a result of intensive studies on means capable of preventing the corrosion of the cathode gas chamber generated while the electrolytic cell is stopped and maintaining the high electrolytic performance inherent in the electrolytic cell for a long period of time.
That is, the present invention can solve the above problems by the following means. (1) In the method for protecting an electrolytic cell when the operation of a two-chamber alkaline chloride electrolytic cell equipped with a gas diffusion cathode is stopped, the supply of the oxygen-containing gas to the cathode gas chamber is stopped, and the cathode gas chamber is A method for protecting an alkaline chloride electrolytic cell, characterized by substituting a caustic aqueous solution from an oxygen-containing gas atmosphere substantially.
(2) The method for protecting an alkaline chloride electrolytic cell according to claim 2 or 3, wherein a caustic aqueous solution is circulated in the cathode gas chamber as a method for replacing the cathode gas chamber. (3) When the two-chamber alkaline chloride electrolytic cell equipped with the gas diffusion cathode is stopped, the operation of stopping the supply of the oxygen-containing gas to the cathode gas chamber, and the cathode gas chamber in the cathode gas chamber after the supply is stopped A protection device for an alkaline chloride electrolysis tank, comprising a protection system for performing an operation of replacing the alkaline electrolyte.
[0014]
The electrolytic cell to be protected according to the present invention is an alkali chloride electrolytic cell having a gas diffusion cathode, and is a two-chamber electrolytic cell in which the gas diffusion cathode is in close contact with the diaphragm.
The operation of the electrolytic cell is stopped when an abnormality occurs or for periodic inspections. As described above, the alkaline aqueous chloride solution on the anode side permeates the cathode side through the diaphragm (2). In the case of a chamber electrolytic cell), residual oxygen corrodes the gas diffusion cathode and other electrolytic cell members, or the member is corroded due to a difference from the operating potential corresponding to the overvoltage.
In order to prevent this, in the present invention, after the operation of the electrolytic cell is stopped, that is, the energization is stopped, the oxygen-containing gas atmosphere in the cathode gas chamber of the electrolytic cell is replaced with the corrosion-inhibiting atmosphere of the electrolytic cell member. The corrosion-inhibiting atmosphere is a caustic aqueous solution.
[0015]
Since the aqueous caustic solution is often generated continuously during operation and stored in the vicinity of the electrolytic cell, there is no need for production or transportation, which is very convenient. When this caustic aqueous solution is supplied to the cathode gas chamber to replace the oxygen-containing gas atmosphere immediately after the operation is stopped, oxygen is discharged out of the electrolytic cell, and corrosion of the electrolytic cell member by oxygen is substantially prevented. Furthermore, even if a neutral alkali chloride aqueous solution permeates through the ion exchange membrane, which is a diaphragm, and reaches the cathode gas chamber due to the concentration gradient, it is maintained in an alkaline atmosphere as long as the alkaline caustic aqueous solution is filled in the cathode gas chamber. Therefore, the cathode gas chamber can be maintained without being corroded.
Caustic aqueous solution is an aqueous solution of the cathode chamber product before operation stop, and there is no problem even if it is introduced into the cathode chamber. Absent.
[0016]
Since corrosion of the electrolytic cell member occurs immediately after the operation is stopped, the atmosphere replacement should be performed as soon as possible after the energization is stopped. When the oxygen-containing gas remains in the cathode gas chamber for a certain time after energization, a minute current may be passed through the electrolytic cell as described above until the atmosphere replacement is completed.
[0017]
DETAILED DESCRIPTION OF THE INVENTION
An example of a two-chamber electrolytic cell that can be used in the present invention will be described with reference to FIG.
The electrolytic cell body 1 is partitioned into an anode chamber 3 and a cathode gas chamber 4 by an ion exchange membrane 2, and a mesh-like insoluble anode 6 is in close contact with the anode chamber 3 side of the ion exchange membrane 2. A gas diffusion cathode 8 is in close contact with the cathode gas chamber 4 side with a hydrophilic layer 7 made of carbon fiber fabric or metal fiber interposed therebetween. Between the gas diffusion cathode 8 and the gas chamber back plate 5, that is, inside the cathode gas chamber 4, there is a gas chamber filler 9 formed by superimposing one or more metal meshes. In addition, 10 is an anolyte inlet provided in the lower part of the anode chamber, 11 is an anolyte and gas outlet provided in the upper part of the anode chamber, 12 is an oxygen-containing gas inlet provided in the upper part of the gas chamber, and 13 is a gas. It is a caustic aqueous solution outlet provided in the lower part of the chamber.
[0018]
When an alkaline chloride aqueous solution is supplied to the anode chamber 3 of the electrolytic cell main body 1 and an oxygen-containing gas is supplied to the cathode gas chamber 4 while energization is performed between the electrodes 6 and 8, the gas diffusion cathode 8 starts from the hydrophilic layer 7 side. The moisture of the caustic aqueous solution is supplied with oxygen-containing gas from the opposite side of the cathode gas chamber 4, and the caustic generation reaction proceeds at the reaction point of the gas diffusion cathode 8. The high-concentration caustic aqueous solution generated at the reaction point of the gas diffusion cathode 8 diffuses into the hydrophilic layer 7 according to the concentration gradient and is immediately absorbed and held, and flows down in the gas chamber inside the hydrophilic layer 7 and the back surface of the gas diffusion cathode 8. And discharged from the caustic aqueous solution outlet 13.
As a material for forming the cathode gas chamber 4, the gas chamber back plate 5, and the gas chamber filler 9, nickel or a nickel-containing alloy is generally used. In the case of the two-chamber electrolytic cell shown in FIG. 1, caustic alkali generated in the gas diffusion cathode 8 is once held in the hydrophilic cell 7, but after that, most of the caustic is transferred to the back side of the gas diffusion cathode, that is, the cathode gas chamber 4. It moves, flows down and is discharged.
[0019]
When an electrolytic reaction is performed in such an electrolytic cell 1, the voltage potential of the gas diffusion cathode 8 is lower than the oxygen reduction potential by an amount corresponding to an overvoltage. When the electrolysis is stopped, the overvoltage disappears and becomes equal to the oxygen reduction potential, so that the potential becomes noble compared to when electrolysis is performed. If oxygen gas is present under the conditions, corrosion of the cathode gas chamber 4, the gas chamber back plate 5, the gas chamber filler 9 and the like proceeds. Further, when the aqueous alkali chloride solution reaches the cathode gas chamber 4 through the ion exchange membrane 2 from the anode chamber 3, the inside of the cathode gas chamber 4 shifts from alkaline to neutral, and the cathode gas chamber 4 and the gas chamber back plate 5 are transferred. And corrosion of the gas chamber filler 9 appears remarkably.
When the electrolytic cell 1 is stopped, the supply of the oxygen-containing gas to the cathode gas chamber 4 is not necessary, so the supply of the oxygen-containing gas is naturally stopped, but it is provided inside the cathode gas chamber 4 and above the gas chamber. The oxygen-containing gas remains inside the oxygen-containing gas inlet 12 and the caustic aqueous solution outlet 13 provided at the lower part of the gas chamber, and this state continues even during the stoppage. Accordingly, it has been found that it is difficult to prevent corrosion inside the cathode gas chamber 4 only by stopping the supply of the oxygen-containing gas. When the electrolytic cell is stopped, the inside of the cathode gas chamber 4 can be corroded because the aqueous alkali chloride solution reaches the cathode gas chamber 4 through the ion exchange membrane 2 from the anode chamber 3.
[0020]
Therefore, in order to prevent corrosion inside the cathode gas chamber 4, it is desirable to keep the inside of the cathode gas chamber 4 alkaline, which makes it possible to suppress corrosion. Several methods are conceivable as the method, and a caustic aqueous solution such as a caustic soda aqueous solution that is actually present during the electrolytic reaction is taken from the caustic aqueous solution outlet 13 to fill the gas chamber and replace it. Alternatively, it is simplest and most effective to keep the inside of the cathode gas chamber 4 alkaline by taking in from the caustic aqueous solution outlet 13 and taking out from the oxygen-containing gas inlet 12 and then taking in again from the caustic aqueous solution outlet 13 and circulating. Is.
Moreover, since this method can be replaced using a caustic aqueous solution generated in an electrolytic cell, it is not necessary to provide a new replacement facility. The caustic aqueous solution that replaces or circulates here has a greater corrosion resistance effect when the oxygen-containing gas present in the liquid is removed.
[0021]
If the concentration of the aqueous caustic solution is in the range of 25 to 35% by weight, the concentration gradient disappears and the aqueous alkali chloride solution in the anode chamber does not penetrate into the cathode gas chamber through the ion exchange membrane. A range of 30 to 35% by weight is preferred. The temperature may be in the range of 20 to 90 ° C. If the temperature is lower than this temperature range, an environment in which the caustic alkali in the aqueous solution is solidified and crystallized is obtained, and maintaining the temperature higher than this temperature range is unfavorable because it adversely affects members such as an ion exchange membrane that constitute the electrolytic cell.
Corrosion of the inside of the cathode gas chamber 4 and the like is accelerated from the moment when the electrolytic cell 1 is stopped, so that the replacement work of the caustic aqueous solution needs to be performed promptly after the electrolytic cell 1 is stopped. It should be automated. That is, the supply of oxygen-containing gas is stopped by a valve that is automatically controlled by a system that detects electrolysis stop, and a caustic aqueous solution is supplied from the caustic aqueous solution outlet 13 or taken from the caustic aqueous solution outlet 13 to contain oxygen. A method of automatically taking out the gas from the gas inlet 12 and recirculating it through the caustic aqueous solution outlet 13 is automatically performed.
According to this embodiment, corrosion of the inside of the cathode gas chamber can be prevented while the electrolysis of the two-chamber alkaline chloride electrolytic cell equipped with the gas diffusion cathode is stopped, and the electrolytic cell can be maintained with high performance over a long period of time.
[0022]
【Example】
Next, the present invention will be described specifically by way of examples. However, the present invention is not limited to these examples.
[0023]
〔Example〕
A two-chamber electrolytic cell having an effective area of 100 mm width and 600 mm height was used. A gas diffusion cathode manufactured by mixing and filling 4 parts of PTFE powder and 6 parts of carbon powder using a silver micromesh as a core material was used. This gas diffusion cathode has a two-layer structure of a gas diffusion layer and a reaction layer, and a silver fine particle catalyst was supported on the reaction layer.
Nickel gas chamber back plate, gas chamber filler, gas diffusion cathode, carbon fiber fabric (hydrophilic layer), Asahi Glass Flemion 8934 (diaphragm), insoluble anode coated with a mixture of ruthenium oxide and titanium oxide on a titanium mesh The electrolytic cell was constructed by stacking in order.
[0024]
Saline solution having a concentration of 220 g / liter and heated to 87 ° C. is supplied to the anode chamber, and then PSA concentrated oxygen (94 vol%) is supplied to the cathode gas chamber from the oxygen-containing gas supply port at 0.75 N liter / min based on oxygen. (1.2 times the required theoretical amount), heated to a temperature of 87 ° C. and supplied. A current of 180 A was supplied while adjusting the entire electrolytic cell to 87 ° C. The current density at this time was 3 kA / square meter. After reaching the steady state, the electrolysis voltage was 1.95 V, the anolyte outlet concentration was 145 g / liter, and the generated caustic soda concentration was 32.4% by weight. The current efficiency at this time was 97%.
The electrolysis test was conducted for 120 days. In the middle of the operation, the operation was stopped for 24 hours, 11 times, 10 days, 20 days, 30 days, 40 days, 50 days, 60 days, 70 days, 80 days, 90 days, 100 days, and 110 days. When the operation was stopped, the supply of the oxygen-containing gas was stopped, and the cathode gas chamber was filled with an aqueous 31.5 wt% sodium hydroxide solution at 55 ° C.
After the 120-day electrolysis test, the electrolytic cell was disassembled and the cathode gas chamber was observed, but no corrosion was observed.
[0025]
[Comparative Example]
An electrolysis test for 120 days was performed under the same conditions as in the examples. However, although the supply of the oxygen-containing gas was stopped at the time of stopping, the caustic soda aqueous solution was supplied to the cathode gas chamber and left as it was without being replaced.
After the 120-day electrolysis test, the electrolytic cell was disassembled and the cathode gas chamber was observed. Corrosion was observed on the entire cathode gas chamber.
[0026]
【The invention's effect】
Since the present invention is constituted as described above, when the electrolytic cell is stopped, the inside of the cathode gas chamber is prevented from being corroded, and thereby the alkaline chloride electrolytic cell capable of maintaining the original high performance of the electrolytic cell over a long period of time. A protection method and a protection device can be provided.
[Brief description of the drawings]
FIG. 1 is a diagram for explaining the structure of a two-chamber electrolytic cell of the present invention.
[Explanation of symbols]
DESCRIPTION OF SYMBOLS 1 Electrolyzer main body 2 Ion exchange membrane 3 Anode chamber 4 Cathode gas chamber 5 Gas chamber backplate 6 Anode 7 Hydrophilic layer 8 Gas diffusion cathode 9 Gas chamber filler 10 Anolyte inlet 11 Anolyte and gas outlet 12 Oxygen-containing gas Inlet 13 Caustic aqueous solution outlet

Claims (3)

In the method for protecting an electrolytic cell when the operation of a two-chamber alkaline chloride electrolytic cell equipped with a gas diffusion cathode is stopped, the supply of the oxygen-containing gas to the cathode gas chamber is stopped, and the cathode gas chamber is substantially A method for protecting an alkaline chloride electrolytic cell, characterized by substituting a caustic aqueous solution from an oxygen-containing gas atmosphere. 4. The method for protecting an alkaline chloride electrolytic cell according to claim 2, wherein a caustic aqueous solution is circulated in the cathode gas chamber as a method for replacing the cathode gas chamber. When the two-chamber alkaline chloride electrolytic cell equipped with the gas diffusion cathode is stopped, the operation of stopping the supply of the oxygen-containing gas to the cathode gas chamber, and the cathode gas chamber is replaced with a caustic aqueous solution after the supply is stopped. A protection device for an alkaline chloride electrolytic cell, comprising a protection system for operation.

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