JP3538271B2 - Hydrochloric acid electrolyzer - Google Patents

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 by-product hydrochloric acid to obtain chlorine with a high decomposition rate. About.

[0002]

BACKGROUND OF THE INVENTION Chemical synthesis processes often use chlorine as an intermediate. For example, fluororesins are synthesized by first chlorinating an organic resin such as polyethylene and then replacing the chlorine with fluorine. Many chlorines are also used as dye intermediates.
These chlorines are ultimately extracted as hydrochloric acid and often do not remain in the product. The hydrochloric acid thus produced is usually called by-product and is used for water treatment and the like. However, by-product hydrochloric acid is inexpensive due to its lower purity than ordinary hydrochloric acid, and is produced irrespective of market demand, which 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 of caustic soda and chlorine produced at a ratio of 1: 1 by salt electrolysis is much higher for chlorine. The phenomenon of a large excess of caustic soda can also be avoided, and various attempts have been made for that purpose. A representative example is chlorine production by hydrochloric acid electrolysis. The electrolysis has been tried for a long time, and an electrolyzer used for the electrolysis has been proposed and put into practical use. However, it is not suitable for industrial use and is practically hardly used on an industrial scale. is there. One of the causes is that there are few materials having 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 at about 20% and 60 ° C.
Few materials have durability as a cathode, and even if carbon can be used, metal materials cannot be used.

[0004] For this reason, among the electrolyzers proposed in the past, there has been one in which the entire electrolyzer is made of a carbon material. However, the electrolysis itself does not have sufficient economical efficiency, and the equipment becomes extremely expensive, and the amortization of the equipment is extremely expensive. There was a drawback that it was difficult to do. The present inventors have also proposed, as a cathode material usable in hydrochloric acid, a material in which tungsten is used as a conductive core material and the periphery thereof is coated with carbon (JP-A-62-240778, JP-A-62-240779).
No.), good results were obtained. Other so-called SPE
In other words, by using the ion exchange membrane as a substantial electrolytic solution and forming an anode and a cathode in close contact on both surfaces to form an electrolytic cell, the use of corrosive metals as in the case of metal electrodes is avoided, and A method for minimizing the electrolysis voltage has been devised (Abstracts of the 4th Soda Electrolysis Industry Symposium). However, even with this method, although the problem is reduced in terms of material, there is a problem that hydrogen is generated from the cathode side, so that the electrolytic voltage is increased and sufficient economic efficiency cannot be obtained. That is, usually, the most recent method for obtaining chlorine is to perform salt electrolysis. In the salt electrolysis, an electrolysis voltage is about 3 volts and an equivalent amount of caustic soda is obtained together with chlorine. In contrast, in the hydrochloric acid electrolysis by the SPE method, although the theoretical electrolysis voltage is about 1.4 volts and about 0.8 volts lower, the electrolysis temperature is lower than that of salt electrolysis. Considering this, there is an economic problem.

In order to suppress the generation of hydrogen gas, an oxygen gas diffusion electrode can be used as a cathode.
When the oxygen gas diffusion electrode is used, the gas generated by electrolysis is only chlorine, so that it is not necessary to use the electrolytic cell in which the above-mentioned cathode chamber and anode chamber are separated, and a simple so-called 1-cell is used.
It is possible to use a chamber electrolyzer (Electrochemistry 57 , 332 198
9)]. Although it seems that the use of this type of electrolytic cell certainly lowers the electrolysis voltage and raises the economy, in practice, the gas diffusion electrode is exposed to a strongly acidic solution, so that the overvoltage of the gas diffusion electrode is reduced. The theoretical electrolysis voltage of 0.13 V at a current density of 30 A / dm ² is actually higher than expected.
There is a problem that the voltage reaches 1.4 V or more. Usually, hydrochloric acid is an aqueous solution having a concentration of 35% or less, and by-product hydrochloric acid is a dilute aqueous solution. When electrolysis is carried out using the diluted by-product hydrochloric acid as an anolyte, hydrochloric acid is removed from the anolyte as the electrolysis is continued, the anolyte concentration is reduced, the conductivity of the anolyte is reduced, and the anolyte is diluted. May not be able to continue. In order to reuse the diluted hydrochloric acid, water must be removed, which requires extra energy and lacks economy, and disposal of diluted hydrochloric acid increases the cost and the utilization rate of hydrochloric acid. However, 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 poor economy and poor decomposition rate of hydrochloric acid electrolysis.
An object of the present invention is to provide a hydrochloric acid electrolyzer capable of obtaining chlorine under stable operating conditions.

[0007]

According to the present invention, an oxygen gas diffusion electrode is provided in close contact with a cathode compartment side of the ion exchange membrane of an electrolytic cell partitioned by a cation exchange membrane into an anode compartment and a cathode compartment. A hydrochloric acid electrolysis apparatus characterized in that an anode is installed on the anode chamber side, and electrolysis is performed while supplying an aqueous hydrochloric acid solution to the anode chamber side and an oxygen-containing gas to the cathode chamber side.

Hereinafter, the present invention will be described in detail. A feature of the present invention is that a cation exchange membrane that has not been used in hydrochloric acid electrolysis using a conventional 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 aqueous hydrochloric acid solution supplied to the anode chamber stays in the anode chamber and does not migrate 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. Operation for a period is enabled. Also, during the electrolysis, hydrogen ions move through the cation exchange membrane to the cathode chamber side, and along with this, the water in the anolyte moves as so-called entrained water, and the anolyte concentration decreases due to the water in the aqueous hydrochloric acid solution And a high decomposition rate can be achieved. For example, the highest concentration of hydrochloric acid on the market is about 35% by weight,
Hydrogen chloride in the hydrochloric acid contains about 4 molecules of water per molecule. When a perfluorosulfonate-based cation exchange membrane is used, the amount of transferred water is about 4 per hydrogen ion, and a decomposition rate of 100% can be achieved if the evaporation of water is ignored. In practice, a considerable amount of water is expected to evaporate with the produced gas, so that a decomposition rate of about 100% can be achieved even when about 20 to 30% by weight of by-product hydrochloric acid is used as a raw material.

Further, by bringing the gas diffusion electrode into close contact with the cation exchange membrane, the acidic group of the cation exchange membrane functions as an electrolyte, and a stable electrolysis voltage can be obtained. Also, as described above, the amount of hydrochloric acid that permeates the cation exchange membrane is zero or very small, so the water that migrates to the cathode compartment is neutral or weakly acidic, and can be discarded as it is or by simple treatment. Can be reused because it has passed through the ion exchange membrane. The gas diffusion electrode in conventional hydrochloric acid electrolysis uses a substance dissolved in the catholyte as the electrolyte, so the gas diffusion electrode is immersed in the catholyte and deteriorates, or the power consumption increases by the resistance of the catholyte. However, in the present invention, a gas diffusion electrode, that is, an oxygen gas diffusion cathode is adhered to the ion exchange membrane and the ion exchange membrane is used as an electrolyte. Degradation and increase in resistance can be avoided. In the cathode reaction, the potential is the most noble in an acidic state, and it is desirable that the part contributing to the reaction of the gas diffusion electrode 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, as the former metal electrode, a titanium electrode coated with a composite oxide of ruthenium oxide-titanium oxide can be preferably used. 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 below 60 ° C., a life of 2 years or more can be achieved. However, if immersed without energization, it may corrode in a short time. When a carbon electrode is used, it is necessary to pay attention to 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 be in close contact with the cation exchange membrane in order to reduce resistance loss due to a decrease in electrolysis voltage.

On the other hand, as the oxygen gas diffusion electrode, a conventional gas diffusion electrode can be used as it is. However, since the oxygen gas diffusion electrode is used in close contact with the cation exchange membrane as described above, the reaction between moisture moving from the anode side and the gas diffusion electrode. It is desirable to have a structure in which moisture generated by the above is extracted to the back side. That is, in the conventional semi-hydrophobic gas diffusion electrode, there is no permeation of the electrolytic solution into the gas chamber side, and the reverse pressure is generated, which may hinder the permeation of the 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 semi-hydrophobic and is used in a solid polymer electrolyte fuel cell is desirable. Since it is desirable that the permeated liquid be removed from the electrode part sufficiently quickly, the electrode structure is a mixture of hydrophilicity and hydrophobicity, and a combination of through-holes to facilitate liquid permeation and pores for gas diffusion. It is desirable to have

As such an electrode, there is an electrode such as ELAT (trade name of E-TEK) mainly formed of carbon fiber, graphite-based carbon, and fluororesin. There are electrodes such as a sintered body of fiber or an electrode with an electrocatalyst substance carried on the surface of a porous body produced by other methods, and an electrode composed of a combination of metal and carbon can be used as long as the above purpose is met. It is. Although an oxygen-containing gas such as air is supplied to the electrode from the back surface, it is unnecessary in the present invention to attach a fluororesin sheet to the back surface side to maintain hydrophobicity as seen with a conventional semi-hydrophobic electrode. And a current collector is arranged on the back surface of the electrode, and electricity is supplied to the electrode through the current collector.

[0013] The catalyst of this gas diffusion electrode is, to a small extent, poisoned by chlorine ions penetrating through the ion exchange membrane and requires that the reaction between oxygen and hydrogen ions 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, in addition to these oxides in combination, or tantalum oxide An oxide obtained by adding another metal oxide such as titanium oxide, tin oxide, or the like is used. Further, a platinum group metal represented by platinum conventionally used as a catalyst can also be used. Further, under the conditions of the present invention, carbon which generates hydrogen peroxide by a two-electron reaction under other conditions hardly causes the generation of hydrogen peroxide, and can be used. The method for supporting these catalysts is not particularly limited, and may be appropriately selected depending on the substrate to be supported.

In addition, the metal material and the like when using a metal for the current collector and the electrode are not particularly limited, but the potential of the cathode is +1.22 V as an equilibrium potential, and is positive even when an overvoltage is added. Therefore, it is preferable to select from materials normally used as anodes, unlike ordinary cathode materials. For example, a material such as titanium having a surface coated with a conductive oxide is desirable. As the oxygen-containing gas to be supplied to the cathode, pure oxygen, oxygen-enriched air and air can be used,
When pure oxygen is used, electrolysis proceeds with a minimum overvoltage. However, in consideration of economy, it is necessary to consider the price. In addition, in the present invention, it is desirable to consider the concentration of the supplied hydrochloric acid. That is, there is an aqueous solution having a low concentration in by-product hydrochloric acid or the like supplied to the anode chamber. In this case, it is preferable to increase the concentration by applying heat to partially evaporate the aqueous solution. In this case, it is desirable to perform electrolysis while generating heat by slightly increasing the electrolysis voltage. In consideration of this and the above-described 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 almost pure oxygen by the PSA method is equivalent to about 0.2 V in electrolytic voltage. In the case of using air and the case of using pure oxygen, the latter has a lower electrolysis voltage of about 0.2 V. Therefore, comparing this value alone means the case of using pure oxygen by the PSA method and the case of using air as it is. There is no difference. However, when electrolysis is performed using air at a high overvoltage of 0.2 V, the overvoltage of 0.2 V is generated as heat, and if this amount of heat can be used for evaporating water, it is preferable to use air. Since these conditions vary depending on the type and purity of the gas used, the type of electrode, and the like, it is preferable to determine them according to actual operating conditions. When air is used, the electrode portion is weakly acidic or nearly neutral, so it is not necessary to remove carbon dioxide gas from the air. Even when air is used, the supply air need not be purified.

[0016]

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

[0017]

Example 1 A 0.5 mm thick porous expanded mesh in which Nafion 350 (trade name, manufactured by DuPont) was used as a cation exchange membrane and an anode material composed of ruthenium dioxide and titanium dioxide was coated on one surface of the membrane. An insoluble metal electrode made of was placed on the other surface with a gas diffusion electrode (trade name: ELAT, manufactured by E-TEK) facing the electrode catalyst. A current collector having a titanium oxide layer formed by sintering the surface of a 1.5 mm-thick expanded mesh on the outside of each of the insoluble metal electrode and the gas diffusion electrode is provided, and the insoluble metal electrode and the gas diffusion electrode are separated from each other. An electrolytic cell was formed so as to be in close contact.

As the electrode catalyst of the gas diffusion electrode,
Ruthenium oxide powder, platinum, a composite oxide of iridium oxide and tantalum oxide, a composite oxide of ruthenium oxide and titanium oxide, or a composite oxide of ruthenium oxide and tin oxide, prepared by pyrolysis of a salt solution from each salt solution. The fluororesin solution was baked at 250 ° C. as a binder and supported on the ion exchange membrane side of the gas diffusion electrode. Hydrochloric acid having a concentration of 20% by weight was circulated as an anolyte, and an amount of hydrochloric acid corresponding to consumption of hydrochloric acid was added as a 30% by weight aqueous hydrochloric acid solution outside the electrolytic cell. As the cathode supply gas, the air from which solid matter has been removed by a filter should be twice 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 at 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. Therefore, electrolysis was advanced while adding deionized water to maintain the concentration of the electrolyte. After one week of continuous operation, no abnormality was observed in the electrolysis performance, and stable electrolysis could be continued without requiring any disposal by diluting the aqueous hydrochloric acid solution.

[0020]

[Table 1]

[0021]

[Comparative Example] Hydrochloric acid electrolysis was performed under the same conditions as in Example 1 except that an ion exchange membrane was not used, the distance between the anode and the cathode was 5 mm, and a hydrochloric acid electrolytic cell using 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 film was attached to the gas supply side so as to be a semi-hydrophobic electrode. When electrolysis was performed under these conditions, in order to maintain the same electrolytic solution concentration of 20% by weight as in Example 1, about twice as much hydrochloric acid as in Example 1 was added, and a dilute solution corresponding to about 50% was obtained from the electrolytic chamber. Unless hydrochloric acid was removed, stable operation was not possible. Under these conditions, although relatively stable electrolysis was performed,
The electrolysis voltage was higher than in Example 1.

[0022]

Example 2 A graphite powder having an average particle size of 20 μm with ruthenium oxide supported on the surface as a catalyst was used as an anode.
The powder was baked on the surface of a cation exchange membrane as a diaphragm using Nafion liquid as a binder. As the cathode, a porous titanium plate was prepared by loosely sintering titanium powder having an average particle size of 15 μm on the surface of porous titanium obtained by sintering titanium fibers, and then covering this with ruthenium oxide by thermal decomposition to produce a porous plate. This was used by thinly coating a fluororesin to give the surface a certain degree of water repellency. Both electrodes were installed on both surfaces 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.

A 35% hydrochloric acid is supplied to the anode chamber, and PS is supplied to the cathode side.
Electrolysis at a temperature of 65 ° C. and a current density of 30 A / dm ² while feeding oxygen-enriched air by A 1.2 times the theoretical amount and dropping deionized water to maintain the hydrochloric acid concentration in the anode chamber at 25%. Was performed. The electrolysis voltage was 1.15 V, and the performance did not change at all even after continuous operation for one month. The concentration of generated chlorine gas was 99.3 to 99.7%. Unless deionized water was added to the anolyte, the concentration increased, and a decomposition rate of hydrochloric acid close to 100% was not obtained.

[0024]

According to the present invention, an oxygen gas diffusion electrode is provided in close contact with a cathode compartment side of the ion exchange membrane of an electrolytic cell partitioned into an anode compartment and a cathode compartment by a cation exchange membrane, and Is provided with an anode, and an electrolysis is performed while supplying an aqueous hydrochloric acid solution to the anode chamber side and 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. This can substantially prevent chlorine ions, which easily deteriorate the gas diffusion electrode, from moving from the anode chamber to the cathode chamber and prevent the electrode from being poisoned by the chlorine ions, thereby extending the life of the gas diffusion electrode. Further, by bringing the oxygen gas diffusion electrode on the cathode side into close contact with the ion exchange membrane, the acidic group of the cation exchange membrane functions as an electrolyte on the cathode side, and a stable electrolysis voltage without resistance loss due to the catholyte can be obtained.

Further, the transfer of chlorine ions and water from the anode compartment to the cathode compartment of the electrolytic cell is kept constant at a ratio of about 1: 4, which is substantially equal to the ratio of chlorine ions and the water of hydration due to the presence of the ion exchange membrane. The balance between hydrochloric acid content and water in the liquid is constant, and the decomposition rate of hydrochloric acid can be maintained at almost 100%. Furthermore, both the anode and the cathode are in a state of positive polarization, and both electrodes can use a metal material such as titanium, thereby solving the problem of conductivity and obtaining the hydrochloric acid electrolysis apparatus of the present invention at a relatively low cost. it can.

──────────────────────────────────────────────────続 き Continued on the front page (58) Field surveyed (Int. Cl. ⁷ , DB name) C25B 9/00 C25B 1/26

Claims (2)

(57) [Claims] 1. An oxygen gas diffusion electrode is installed in close contact with a cathode chamber side of an ion exchange membrane of an electrolytic cell partitioned into an anode chamber and a cathode chamber by a cation exchange membrane, and an anode is installed in an anode chamber side. An electrolysis apparatus for hydrochloric acid, wherein electrolysis is performed while supplying an aqueous hydrochloric acid solution to the anode chamber side and 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.