JPH0978279A - Hydrochloric acid electrolysis device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain a hydrochloric acid electrolysis device eliminated in the defects of the conventional hydrochloric acid electrolysis device using a gas diffusion electrode short in service life and impossible to attain high decomposition rate, having profitability and capable of attaining high decomposition rate. SOLUTION: The hydrochloric acid electrolysis device is for electrolyzing by mounting a gaseous oxygen diffusion electrode in close contact state in a cathode chamber side and an anode in an anode chamber side across a cation exchange membrane and supplying a hydrochloric acid aq. solution in the anode chamber side and an oxygen-containing gas in the cathode chamber side. The use of the cation exchange membrane leads to prevent the practical transfer of chloride ion, which easily degrades the gas diffusion electrode, from the anode chamber to the cathode chamber and the poisoning of the electrode by chloride ion to prolong the service life of the gas diffusion electrode and to attain low electrolytic pressure.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an electrolytic cell for electrolyzing hydrochloric acid to obtain chlorine, and more particularly to an electrolytic cell for electrolyzing waste hydrochloric acid or byproduct hydrochloric acid to obtain chlorine at a high decomposition rate. Regarding

[0002]

BACKGROUND OF THE INVENTION Chemical synthesis processes often use chlorine as an intermediate. For example, a fluororesin is synthesized by first chlorinating an organic resin such as polyethylene and then replacing the chlorine with fluorine. Also, a large amount of chlorine is used as an intermediate for dyes.
These chlorines are eventually taken out as hydrochloric acid and often do not remain in the product. The hydrochloric acid thus produced is usually called a by-product and is used for water treatment and other purposes. However, since the by-product hydrochloric acid has a lower purity than ordinary hydrochloric acid, it is cheaper in price, and since it is produced regardless of the market demand, it may only lead to an increase in inventory.

If chlorine can be regenerated from this by-product hydrochloric acid, such a problem can be solved. In addition, the problem of imbalance in the production of caustic soda and chlorine in salt electrolysis currently being carried out in Japan, that is, the actual demand for caustic soda and chlorine produced in salt electrolysis at a ratio of 1: 1 is far greater with chlorine. It is possible to avoid the phenomenon that a large amount of caustic soda is left over, and various attempts have been made for that purpose. A typical example is chlorine production by hydrochloric acid electrolysis. The electrolysis has been tried for a long time, and an electrolytic cell used for the electrolysis has been proposed and put into practical use, but it is not suitable for industrial use and is practically not used on an industrial scale at present. is there. One of the causes is that there are few materials that have sufficient durability against a highly corrosive electrolytic solution such as hydrochloric acid. I mean
Titanium can be used as the anode material if it is used as the anode in a hydrochloric acid aqueous solution of 20% at about 60 ° C.
There are few materials that have durability as a cathode, and even if carbon can be used, metallic materials cannot be used.

Therefore, among the electrolytic cells proposed in the past, there was one in which the entire electrolytic cell was made of a carbon material. However, the electrolysis itself does not have sufficient economic efficiency, and the equipment becomes extremely expensive, and the depreciation thereof is required. There was a drawback that it was difficult to do. The present inventors have also proposed, as a cathode material that can be used in hydrochloric acid, a conductive core material made of tungsten and having its periphery coated with carbon (JP-A-62-240778 and JP-A-62-240779).
No.), good results were obtained. Other so-called SPE
Method, that is, using an ion exchange membrane as a substantial electrolytic solution, and forming an electrolytic cell by closely forming an anode and a cathode on both sides thereof, avoiding the use of corrosive metal as in the case of a metal electrode, and A method of minimizing the electrolysis voltage has been devised (Abstracts of the 4th Soda Electrolysis Industry Conference). However, even with this method, although there are certainly few problems in terms of materials, there is a problem in that hydrogen is generated from the cathode side, which increases the electrolysis voltage and cannot achieve sufficient economic efficiency. That is, in order to obtain chlorine normally, the latest method is to carry out a salt electrolysis. In the salt electrolysis, an electrolysis voltage is about 3 V and an equivalent amount of caustic soda can be obtained together with chlorine. On the other hand, in hydrochloric acid electrolysis by the SPE method, the theoretical electrolysis voltage is about 1.4 V, which is about 0.8 V lower, but the electrolysis temperature is lower than that of salt electrolysis, so the electrolysis voltage is actually 2 V or more, and caustic soda is not generated. There is an economic problem when considering.

In order to suppress the production of hydrogen gas, it is possible to use an oxygen gas diffusion electrode as the cathode.
When the oxygen gas diffusion electrode is used, the gas generated by electrolysis is only chlorine, so there is no need to use the above-described electrolytic cell in which the cathode chamber and the anode chamber are separated, and a simple so-called 1
Enables use of chamber electrolyzers [Electrochemistry 57 , 332 198
9)]. It seems that the use of this type of electrolytic cell certainly lowers the electrolysis voltage and results in economy, but in reality, the overvoltage of the gas diffusion electrode is increased by exposing the gas diffusion electrode to a strongly acidic liquid. It rises more than expected, and the theoretical electrolysis voltage of 0.13 V at a current density of 30 A / dm ² is actually
There is a problem of reaching 1.4 V or more. Usually, hydrochloric acid is an aqueous solution with a concentration of 35% or less, and by-product hydrochloric acid is a dilute aqueous solution. When electrolysis is performed using this dilute by-produced hydrochloric acid as the anolyte, hydrochloric acid is removed from the anolyte as the electrolysis continues, and the concentration of the anolyte decreases, thus reducing its conductivity and diluting the anolyte, resulting in stable electrolysis. May not be able to continue. In order to reuse the diluted hydrochloric acid, it is necessary to remove the water content, extra energy is required, and the economy is lacking. Disposal of diluted hydrochloric acid is expensive and the utilization rate of hydrochloric acid is high. There was a drawback that it became worse.

[0006]

SUMMARY OF THE INVENTION The present invention overcomes the above-mentioned problems of the prior art, namely, the economical efficiency and poor decomposition rate in hydrochloric acid electrolysis,
An object of the present invention is to provide a hydrochloric acid electrolysis device that can obtain chlorine under stable operating conditions.

[0007]

According to the present invention, an oxygen gas diffusion electrode is installed in close contact with the cathode chamber side of the ion exchange membrane of an electrolytic cell divided into an anode chamber and a cathode chamber by a cation exchange membrane. A hydrochloric acid electrolysis device is characterized in that an anode is installed on the side of the anode chamber, an aqueous hydrochloric acid solution is supplied to the side of the anode chamber, and an oxygen-containing gas is supplied to the side of the cathode chamber to carry out electrolysis.

Hereinafter, the present invention will be described in detail. A feature of the present invention is that a cation exchange membrane, which has not been used in conventional hydrochloric acid electrolysis using a gas diffusion electrode, is used and the electrolytic cell is divided into an anode chamber and a cathode chamber by the ion exchange membrane. Due to the presence of the ion-exchange membrane, the hydrochloric acid aqueous solution supplied to the anode chamber stays in the anode chamber and does not move to the cathode chamber side, so that the gas diffusion electrode existing in the cathode chamber is not deteriorated by highly corrosive hydrochloric acid and is long-term. Driving for a period is possible. Also, during the electrolysis, hydrogen ions permeate the cation exchange membrane and move to the cathode chamber side, and along with that, the water in the anolyte moves as so-called entrained water, and the concentration of the anolyte decreases due to the water in the aqueous hydrochloric acid supply. Is prevented and a high decomposition rate can be achieved. For example, the highest concentration of hydrochloric acid in commercial hydrochloric acid is about 35% by weight,
Hydrogen chloride in the hydrochloric acid contains about 4 molecules of water per molecule. When using a perfluorosulphonic acid type cation exchange membrane, the amount of transferred water is about 4 with respect to 1 hydrogen ion, and if the evaporation of water is ignored, a decomposition rate of 100% can be achieved. In practice, a considerable amount of water is expected to evaporate with the produced gas, so even if about 20 to 30% by weight of by-produced hydrochloric acid is used as a raw material, a decomposition rate of almost 100% can be achieved.

Further, by bringing the gas diffusion electrode into close contact with the cation exchange membrane, the acidic groups of the cation exchange membrane function as an electrolyte to obtain a stable electrolysis voltage. Further, as described above, since the amount of hydrochloric acid that permeates the cation exchange membrane is zero or very small, the water that migrates to the cathode chamber side is neutral or weakly acidic and can be discarded as it is or by simple treatment. Since it permeates the ion exchange membrane, it can be reused. Moreover, since the gas diffusion electrode in conventional hydrochloric acid electrolysis uses a substance dissolved in the catholyte as an electrolyte, the gas diffusion electrode is deteriorated by being immersed in the catholyte or the power consumption increases by the resistance of the catholyte. However, in the present invention, since a gas diffusion electrode, that is, an oxygen gas diffusion cathode is brought into close contact with the ion exchange membrane and the ion exchange membrane is used as an electrolyte, electrolysis can proceed without a catholyte and therefore a gas diffusion electrode. It is possible to avoid deterioration of the resistance and increase of resistance. It is desirable that the cathodic reaction is acidic and the potential becomes the most noble, and that the part of the gas diffusion electrode that contributes to the reaction is also placed in a strongly acidic atmosphere as much as possible. This condition is satisfied by the close contact.

As the anode of the present invention, an ordinary insoluble metal electrode for generating chlorine or a carbon electrode such as graphite can be used. For example, a titanium electrode coated with a ruthenium oxide-titanium oxide composite oxide can be preferably used as the former metal electrode. The electrode is extremely stable as an anode for chlorine generation and has a hydrochloric acid concentration of 25% as long as it is used as an anode.
If the temperature is 60 ° C or lower, a life of 2 years or more can be achieved. However, it may corrode within a short period of time if it is immersed without energization. When a carbon electrode is used, it is necessary to consider the structure and material of the catalyst itself or the current collector because the resistance of carbon itself is large. As described above, it is desirable that the anode is brought into close contact with the cation exchange membrane in order to reduce the ohmic loss due to the reduction of the electrolysis voltage.

On the other hand, as the oxygen gas diffusion electrode, the conventional gas diffusion electrode can be used as it is, but since it is used in close contact with the cation exchange membrane as described above, the water that migrates from the anode side reacts with the gas diffusion electrode. It is desirable to have a structure in which water generated by the above is extracted to the back side. That is, in the conventional semihydrophobic gas diffusion electrode, there is no permeation of the electrolytic solution into the gas chamber side, which causes a reverse pressure and may impede the permeation of water in the anode chamber side into the ion exchange membrane. Therefore, as the gas diffusion electrode of the present invention, a liquid-permeable electrode which is not semihydrophobic and is used in a polymer electrolyte fuel cell is desirable. Since it is desirable that the permeated liquid be removed from the electrode part sufficiently quickly, it has an electrode structure with a mixture of hydrophilic and hydrophobic properties. In addition, a combination of through holes to facilitate liquid permeation and pores for gas diffusion is combined. It is desirable to have

[0012] As such an electrode, there is an electrode such as ELAT, which is a product name of E-TEK Co., which is mainly formed of carbon fiber, graphite-based carbon and fluorocarbon resin. There is an electrode that has an electrode catalyst substance supported on the surface of a sintered body of fiber or a porous body made by another method.As long as it meets the above purpose, it is possible to use an electrode composed of a combination of metal and carbon. Is. An oxygen-containing gas such as air is supplied to the electrode from the back surface, but it is not necessary in the present invention to attach a fluororesin sheet on the back surface side for maintaining hydrophobicity as seen in a conventional semihydrophobic electrode. A current collector is arranged on the back surface of the electrode, and the electrode is energized through the current collector.

The catalyst of this gas diffusion electrode is that it is slightly poisoned by chlorine ions penetrating through the ion exchange membrane, and that the reaction between oxygen and hydrogen ions must be carried out at a sufficiently low overvoltage. It is necessary to select in consideration of, for example, platinum group metal oxides, typically ruthenium oxide and iridium oxide, and other combinations of these oxides or tantalum oxides of these oxides. , Oxides to which other metal oxides such as titanium oxide and tin oxide are added are used. Further, a platinum group metal represented by platinum which has been conventionally used as a catalyst can also be used. Carbon, which causes hydrogen peroxide generation by the two-electron reaction of water under other conditions, hardly causes hydrogen peroxide generation under the conditions of the present invention, and can be used. The method of supporting these catalysts is not particularly limited and may be appropriately selected depending on the substrate to be supported.

In addition to this, the metal material or the like when a metal is used for the current collector or the electrode is not particularly limited, but the potential of the cathode is +1.22 V as the equilibrium potential, which is positive even if an overvoltage is added. Therefore, it is preferable to select a material that is usually used as an anode, unlike a usual cathode material. Materials such as titanium with a conductive oxide coating on the surface are desirable. As the oxygen-containing gas supplied to the cathode, pure oxygen, oxygen-enriched air, air, etc. can be used,
When pure oxygen is used, electrolysis proceeds with a minimum overvoltage. However, considering the economical efficiency, it is necessary to consider the price, and in addition to this, in the present invention, it is desirable to consider the concentration of hydrochloric acid supplied. That is, there is an aqueous solution having a low concentration in the by-produced hydrochloric acid or the like supplied to the anode chamber, and in this case, it is preferable to give heat to evaporate a part of the aqueous solution to increase the concentration. In that case, it becomes desirable to carry out electrolysis while slightly increasing the electrolysis voltage to generate heat, and in consideration of this fact and the above-mentioned economic efficiency, the supply gas to be used is determined.

For example, the electric power required to obtain a gas from which impurities have been removed to nearly nearly pure oxygen by the PSA method corresponds to an electrolysis voltage of about 0.2 V. When air is used and when pure oxygen is used, the electrolysis voltage is lower by about 0.2 V in the latter case. Therefore, comparison of this value is not necessary when using pure oxygen by the PSA method and when using air as it is. There is no difference. However, when electrolysis is carried out under the condition of high 0.2 V overvoltage using air, 0.2 V overvoltage is generated as heat, and it is preferable to use air when this heat quantity can be utilized for water evaporation. These conditions differ depending on the type and purity of the gas used, the type of electrode, etc., so it is preferable to determine them according to the actual operating conditions. Even when air is used, since the electrode portion is weakly acidic or close to neutral, it is not necessary to remove carbon dioxide gas in the air. Even when air is used, the supply air need not be purified.

[0016]

EXAMPLES Next, examples of hydrochloric acid electrolysis cell according to the present invention and hydrochloric acid electrolysis using the electrolysis cell will be described, but the examples do not limit the present invention.

[0017]

Example 1 As a cation exchange membrane, Nafion 350 (trade name, manufactured by DuPont) was used as a diaphragm, and a 0.5 mm-thick porous expanded mesh having a surface coated with an anode material composed of ruthenium dioxide and titanium dioxide on one surface thereof. The insoluble metal electrode composed of was placed on the other surface so as to face a gas diffusion electrode (trade name ELAT manufactured by E-TEK) carrying an electrode catalyst. On the outside of each of the insoluble metal electrode and the gas diffusion electrode, a current collector having a titanium oxide layer formed by firing the surface of an expanded mesh having a thickness of 1.5 mm is installed, and the insoluble metal electrode and the gas diffusion electrode are mutually The electrolytic cell was formed so as to be in close contact.

The electrode catalyst of the gas diffusion electrode is
Powder of ruthenium oxide powder, platinum, complex oxide of iridium oxide and tantalum oxide, complex oxide of ruthenium oxide and titanium oxide, or complex oxide of ruthenium oxide and tin oxide was prepared by thermal decomposition from each salt solution. On the ion exchange membrane side of the gas diffusion electrode, a fluororesin liquid was used as a binder and baked and carried at 250 ° C. Hydrochloric acid having a concentration of 20% by weight was circulated as an anolyte, and an amount of hydrochloric acid corresponding to the consumption of hydrochloric acid was added as an aqueous solution of hydrochloric acid having a concentration of 30% by weight outside the electrolytic cell. As the cathode supply gas, the air from which solids have been removed by the filter should be double the theoretical amount.
It was supplied while applying a pressure of 0.5 atm.

Table 1 shows the electrolysis voltage and the cathode overvoltage when electrolysis was performed under the conditions of a temperature of 60 ° C. and a current density of 30 A / dm ² . Regardless of which cathode material was used, the concentration of the anolyte tended to increase little by little, so electrolysis was allowed to proceed while adding deionized water to maintain the concentration of the electrolyte. After continuous operation for one week, no abnormalities were found in the electrolysis performance, and stable electrolysis could be continued without the need to dispose of the diluted hydrochloric acid aqueous solution.

[0020]

[Table 1]

[0021]

[Comparative Example] Hydrochloric acid electrolysis was carried out under the same conditions as in Example 1 except that the ion-exchange membrane was not used and the distance between the anode and the cathode was 5 mm, and a normal gas diffusion electrode was used. As the cathode catalyst, the composite oxide of iridium oxide and tantalum oxide of Example 1 was used. Since the gas diffusion electrode cannot be used as it is in the electrolytic solution, a porous PTFE membrane was attached to the gas supply side to form a semihydrophobic electrode. When electrolysis was carried out under these conditions, in order to maintain the same electrolytic solution concentration of 20% by weight as in Example 1, about twice the hydrochloric acid of Example 1 was added, and a dilute solution equivalent to about 50% was obtained from the electrolysis chamber. Stable operation could not be achieved without removing hydrochloric acid. Under this condition, although relatively stable electrolysis was possible,
The electrolytic voltage was higher than in Example 1.

[0022]

Example 2 Graphite powder having an average particle size of 20μ and having ruthenium oxide as a catalyst supported on the surface was used as an anode,
The powder was baked on the surface of a cation exchange membrane, which is a diaphragm, using Nafion liquid (trade name) as a binder. As the cathode, loose sintered titanium powder having an average particle size of 15μ on the surface of a porous titanium substrate obtained by sintering titanium fibers, and ruthenium oxide was coated on this by thermal decomposition to produce a porous plate, Fluorine resin was thinly coated on this and the surface was made to have some water repellency. Both electrodes were installed on both sides of the ion exchange membrane so as to be in close contact with the ion exchange membrane, and an electrolytic cell similar to that of Example 1 was assembled.

35% hydrochloric acid was supplied to the anode chamber and PS was supplied to the cathode side.
Electrolysis was carried out at a temperature of 65 ° C and a current density of 30 A / dm ² while feeding air enriched with oxygen by A 1.2 times the theoretical amount and adding deionized water to maintain the hydrochloric acid concentration in the anode chamber at 25%. Was done. The electrolysis voltage was 1.15 V, and the performance did not change even after continuous operation for one month. The concentration of evolved chlorine gas was 99.3-99.7%. The concentration increased without deionized water being added to the anolyte, and a decomposition rate of hydrochloric acid close to 100% was not obtained.

[0024]

According to the present invention, the oxygen gas diffusion electrode is closely attached to the cathode chamber side of the ion exchange membrane of the electrolytic cell divided into the anode chamber and the cathode chamber by the cation exchange membrane, and the anode chamber side is provided. In the hydrochloric acid electrolysis device, an anode is installed on the anode and electrolysis is performed while supplying an aqueous hydrochloric acid solution to the anode chamber side and supplying an oxygen-containing gas to the cathode chamber side. Unlike hydrochloric acid electrolysis using a conventional gas diffusion electrode, in the present invention, the electrolytic cell is divided into an anode chamber and a cathode chamber by a cation exchange membrane. As a result, it is possible to extend the life of the gas diffusion electrode by preventing chlorine ion, which easily deteriorates the gas diffusion electrode, from being substantially transferred from the anode chamber to the cathode chamber and poisoning the electrode by the chlorine ion. Further, by bringing the oxygen gas diffusion electrode on the cathode side into close contact with the ion exchange membrane, the acidic groups of the cation exchange membrane function as an electrolyte on the cathode side, and a stable electrolysis voltage without resistance loss due to the catholyte can be obtained.

Further, the migration of chlorine ions and water from the anode chamber to the cathode chamber of the electrolysis cell is constant at a ratio of about 1: 4, which is almost equal to the ratio of chlorine ions and its hydrated water, due to the presence of the ion exchange membrane, and electrolysis is performed. The balance between the hydrochloric acid content and the water content in the liquid is kept constant, and the decomposition rate of hydrochloric acid can be maintained at almost 100%. Further, both the anode and the cathode are in the anodic polarization state, and it becomes possible to use a metal material such as titanium for both electrodes, the problem of conductivity is solved, and the hydrochloric acid electrolysis apparatus of the present invention can be obtained at a relatively low cost. it can.

Claims (2)

[Claims] 1. An oxygen gas diffusion electrode is installed in close contact on the cathode chamber side of the ion exchange membrane of an electrolytic cell divided into an anode chamber and a cathode chamber by a cation exchange membrane, and an anode is placed on the anode chamber side. Then, the hydrochloric acid electrolysis device is characterized in that electrolysis is performed while supplying an aqueous hydrochloric acid solution to the anode chamber side and supplying an oxygen-containing gas to the cathode chamber side. 2. The hydrochloric acid electrolysis apparatus according to claim 1, wherein the oxygen gas diffusion electrode has a liquid permeation function.