JPH0387393A - Production of aqueous alkali hypochlorite solution - Google Patents

Abstract

PURPOSE:To stably produce an aq. alkali hypochlorite soln. at a low electrolytic voltage over a long period by separating an electrolytic cell with an electrode obtained by coating the surface of a solid polymer electrolyte with an electrode active material, electrolyzing an alkali chloride soln. and mixing the products. CONSTITUTION:The electrolytic cell 1 is separated by an electrode obtained by coating the surface of a solid polymer electrolyte 2 with electrode active materials 3 and 4 into an anode chamber 5 and a cathode chamber 6. A fluorine- based cation-exchange membrane is appropriately used as the electrolyte 2. Carbon, graphite, PbO2, platinum-group metals or their oxides, etc., are used as the anode active material 5, and carbon, graphite, platinum-group metals, etc., as the cathode active material 6. An alkali chloride soln. is supplied to the anode chamber 5 and water to the cathode chamber 6, and electrolysis is carried out. The generated gaseous chlorine, the anolyte and catholyte freed of gaseous hydrogen are mixed in a reaction vessel 13 to cause a reaction, and an aq. alkali hypochlorite soln. is obtained at a low electrolytic voltage.

Description

DETAILED DESCRIPTION OF THE INVENTION (Industrial Field of Application) The present invention relates to a method for producing an aqueous alkali hypochlorite solution useful as a water treatment agent.

(Prior art) Conventionally, chlorine treatment has been used to sterilize water in both water and sewerage systems, but the chlorine source is liquid chlorine, which is liquefied at high pressure and filled into a cylinder, or hypochlorite. Sodium chlorate is used. Handling of such high-pressure cylinders requires skilled professionals, as the cylinders themselves are quite heavy and contain highly pressurized toxic substances. Furthermore, when using a sodium hypochlorite solution, the chemical is transported from a manufacturing factory as an aqueous solution of a certain concentration, and it requires a lot of effort and additional equipment to adjust the concentration and amount to be added to the area where it will be used. Furthermore, the concentration of sodium hypochlorite products is usually about 12%, and transporting such an aqueous solution requires transporting a large amount of water, which is also uneconomical.

Therefore, recently, a small diaphragm-type alkaline chloride electrolyzer is installed at the place of use, and saturated brine is made from salt and water.This is electrolyzed to obtain chlorine from the anode chamber and a caustic soda solution from the cathode chamber, and a separate reaction chamber is installed. A method of reacting these in a chamber to obtain a sodium hypochlorite solution is being used. According to this method, not only can the production amount be adjusted by adjusting the amount of electricity, but also the cost of transporting a large amount of water can be reduced.

This type of small diaphragm electrolytic cell is equipped with a synthetic resin diaphragm to prevent a decrease in current efficiency caused by non-diaphragm electrolytic cells, and a communication tube is provided between the anode chamber and the cathode chamber to transfer the anolyte to the catholyte (
The common method is to mix hypochlorite with alkaline solution (caustic alkaline solution) and add chlorine gas generated in the anode chamber in a separate chamber attached to the electrolytic cell to produce an alkaline hypochlorite solution, which is used as is as a water treatment agent. Yes (Japanese Patent Publication No. 57-53436, Japanese Patent Publication No. 57-94579).

However, in this type of electrolytic cell, good current efficiency can be maintained when the concentration of available chlorine in the alkaline hypochlorite solution produced is 1% by weight or less, but when the concentration of available chlorine is increased to 3 to 4% by weight, If an attempt is made to increase the decomposition rate of salt water, there is a drawback that the current efficiency will drop significantly. Another method uses a cation exchange membrane as a diaphragm to increase the caustic alkali concentration in the catholyte and reacts it with chlorine gas to produce an alkaline hypochlorite solution.After electrolysis, the anolyte is resaturated with alkali chloride. Because the electrolysis equipment is circulated through the electrolytic tank, the electrolysis equipment becomes large and the work process becomes complicated, so there are many problems when installing it adjacent to a water treatment facility. Furthermore, although the method of using an asbestos diaphragm as a diaphragm has the advantage of not circulating the anolyte, it has the drawbacks of lower overall current efficiency and a shorter service life of the asbestos diaphragm than the cation exchange membrane method.

In order to improve these shortcomings, electrolysis is carried out at a decomposition rate of 50 to 70% of alkali chloride while adding alkali chloride to the anode chamber and water to the cathode chamber using a cation exchange membrane as a diaphragm. A method has been proposed in which alkali hypochlorite is produced by mixing chlorine gas and used as a water treatment agent (Japanese Patent Publication No. 1-14830). However, when commercially available sodium chloride is used, scale adheres to the cation exchange membrane and the cathode, increasing the sliding voltage, so there is a drawback that the cathode chamber must be periodically cleaned with acid.

(Problems to be Solved by the Invention) The present invention has been made to solve the above-mentioned problems. An object of the present invention is to provide a method for producing an aqueous alkali chlorate solution.

(Means for Solving the Problem) That is, the present invention covers the surface of a solid polymer electrolyte with an electrode active substance to form an electrode, and charges an alkali chloride solution into the anode chamber of an electrolytic cell divided by the electrode. In this method, chlorine gas is generated by electrolysis, and the anolyte, catholyte, and chlorine gas are mixed to produce an alkaline hypochlorite aqueous solution.

The present invention will be explained in detail below with reference to the drawings.

In FIG. 1, an electrolytic cell 1 is fitted with an electrode consisting of a solid polymer electrolyte 2 having an anode active material 3 and a cathode active material 4 as electrode active materials. A cation exchange membrane can be used as the solid polymer electrolyte used in the present invention, and a fluororesin-based cation exchange membrane is suitable from the viewpoint of ion selectivity and corrosion resistance. The anode active material applied to the surface of the solid polymer electrolyte is one of carbon, graphite, lead dioxide, platinum group metals, platinum group metal oxides, or a mixture thereof. Furthermore, carbon, graphite, and platinum group metals are used as the cathode active material.

In order to coat a solid polymer electrolyte with the above-mentioned active substance, it is first necessary to roughen its surface, and there are two methods: etching using a high-frequency low-temperature plasma device under a stream of oxygen, argon, etc., and glass beads, etc. There is a method of sandblasting using blasting material particles. Both methods can form an etching layer consisting of numerous uniform three-dimensional micropores without impairing the function of the solid polymer electrolyte.

The thus roughened solid polymer electrolyte is coated with an electrode active material. Although many coating methods are known, some methods are more suitable depending on the electrode active material to be coated. In other words, for carbon and graphite, CVD or vacuum sputtering is better, for platinum group metals, wet chemical plating is better, and for platinum group metal oxides and lead dioxide, hot pressing using a binder such as polytetrafluoroethylene is better. is suitable. The electrode active coating according to the present invention does not necessarily have to directly cover the surface of the solid polymer electrolyte; for example, the coated titanium mesh may be pressed against the anode surface of the solid polymer electrolyte.

As mentioned above, the electrolytic cell 1 is divided into an anode chamber 5 and a cathode chamber 6 by the solid polymer electrolyte 2 coated with an electrode active material, and the alkaline chloride solution is supplied from the conduit 7 to the chlorine gas separator 8. It is introduced into the lower part of the anode chamber 5 from the conduit 18 through the . As the alkali chloride, sodium chloride, lithium chloride, and potassium chloride are used. The water is introduced into the lower part of the cathode chamber 6 from the water injection pipe 9 through the hydrogen gas separator 10 and from the conduit 17. After electrolysis, the electrolyte is circulated to the chlorine gas separator 8 by the gas lift effect of the generated chlorine gas, and the separated chlorine gas is led to a reaction tank 13 through a conduit 11 and the anolyte through a conduit 12. The caustic alkaline solution (catholyte) generated in the cathode chamber 6 is circulated to the hydrogen gas separator IO by the gas lift effect of the generated hydrogen gas, the separated hydrogen gas is discharged from the conduit 18, and the catholyte is transferred to the reaction tank through the conduit 14. 13
guided by. In the reaction tank 13, the chlorine gas, the anolyte, and the catholyte react to produce an alkaline hypochlorite solution of a predetermined concentration, which is discharged through the conduit 15 and made available for use. In the cathode chamber 6, water accompanying the alkali ions that have moved within the solid polymer electrolyte 2 from the anode chamber 5 is reduced to generate hydrogen gas and caustic alkali. Since hydrogen ions move backward within the solid polymer electrolyte 2 and the chlorine generation current efficiency decreases, the hydroxide ion concentration in the cathode chamber 6 should be made as low as possible.

In conventional diaphragm electrolysis, water is placed in the cathode chamber to keep this ion concentration low, and the caustic alkali concentration is several tens of grams.
/l or less, there was a problem that the resistance of the electrolyte increases and electrolysis becomes impossible, but in the method of the present invention, since the solid polymer electrolyte 2 is used, the amount of water injected from the water injection pipe 9 is increased and the caustic alkali is Even if the concentration is made extremely low, the electrolytic voltage will not increase. The hydrogen produced is 99.9% or more and can be recovered after being discharged from the conduit 18, but if it is to be disposed of, it is diluted with air to a concentration that is at the lower explosive limit, or it is combusted using a catalyst or the like.

(Example) Next, the present invention will be explained with reference to Examples, but the present invention is not limited thereto.

An aqueous sodium hypochlorite solution was continuously produced using the apparatus shown in FIG.

A fluorocarbon-based cation exchange membrane (Nafion NX-90209 manufactured by DuPont) was used as the solid polymer electrolyte, and 100 mesh glass powder was used as the blasting material, and sandblasting was performed for 10 seconds per ICm2 at an air pressure of 4 kg/cm2. provided.

Seal one side of this cation exchange membrane and immerse it in boiling water for 30 minutes.
The sample was immersed for 1 minute, then acid-treated in dilute hydrochloric acid for 1 hour, and then immersed in a solution of chloroplatinic acid dissolved in aqueous ammonia. This was reduced with sodium borohydride. A cathode active coating was then applied by a wet chemical plating method in which the film was immersed in a chloroplatinic acid solution to grow thick.

As an anode active coating, platinum (Example 1) or platinum-iridium alloy (Example 2) was coated on the cation exchange membrane by the same method as above.

The thus prepared electrode was assembled in an electrolytic cell and chlorinated.) An anolyte containing 300 g/l of IJium was supplied to the anode chamber, and tap water was supplied to the cathode chamber using a metering pump for electrolysis. Electrolyzed at an anode current density of 50 A/dm2,
The caustic soda concentration in the cathode chamber was controlled to be 3 g/1 or less during operation. The sliding voltage and current efficiency during operation are shown in Table 1. When the cathode was observed after 500 hours of operation, a small amount of scale was observed, but there was almost no change in the sliding voltage. The average concentration of the produced sodium hypochlorite was 5.0% by weight.

Comparative Example 1 A normal ion exchange membrane electrolytic cell was assembled using a fluorocarbon cation exchange membrane manufactured by DuPont (Nafion NX-90209) as a diaphragm, a platinum-plated titanium electrode for the anode, and a titanium plate for the cathode, and the catholyte The operation was carried out under the same electrolytic conditions as in the example, with the concentration of caustic soda being 50 to 60 g/l.

The average concentration of the produced sodium hypochlorite was 4.5% by weight. Table 1 shows the sliding voltage and current efficiency during operation.

When the cathode was observed after 500 hours of operation, white scale was observed all over the surface.

Table 1 *2 A platinum-plated titanium plate is used as an anode and a titanium plate is used as a cathode (effects of the invention) By using an electrode coated with an electrode active material on the surface of a solid polymer electrolyte, the electrolytic voltage is considerably higher than that of conventional electrolytic voltage. Alkaline hypochlorite can now be produced at low voltage. Furthermore, even after long-term operation, no scale is generated, and a low electrolytic voltage can be maintained, eliminating the need for acid cleaning.

[Brief explanation of drawings]

FIG. 1 is a flow sheet illustrating a specific example of the method of the present invention. 1. Electrolytic cell 2. Solid polymer electrolyte 3. Anode active material 4. Cathode active material 5. Anode chamber 6.
・Cathode chamber 8...Chlorine gas separator 10...Hydrogen gas separator 13...Reaction tank

Claims (1)

[Claims] 1. In a method for producing an aqueous alkali hypochlorite solution by electrolyzing an alkali chloride solution, an electrolytic cell is connected to an anode chamber by an electrode in which the surface of a solid polymer electrolyte is coated with an electrode active material. and a cathode chamber, an alkali chloride solution is supplied to the anode chamber, water is supplied to the cathode chamber for electrolysis, and the generated chlorine gas, anolyte and catholyte are mixed. Method for producing acid-alkali aqueous solution. 2. The method for producing an alkaline hypochlorite aqueous solution according to claim 1, wherein the solid polymer electrolyte is a cation exchange membrane.