JPH02213488A - Method for electrolyzing alkali chloride - Google Patents

Abstract

PURPOSE:To electrolyze an alkali chloride aq. soln. without causing increase of electrolytic energy during electrolysis by utilizing a fluorine-contg. cation exchange membrane and electrolyzing the alkali chloride aq. soln. in which the concn. of organic substance is held at prescribed value or below. CONSTITUTION:Alkali hydroxide is produced by utilizing a fluorine-contg. ion exchange membrane wherein all parts of the membrane or the face opposed to at least a cathode is made of a polymer of perfluorocarboxylic acid and electrolyzing an alkali chloride aq. soln. In this case, the concn. of organic substance such as alcohols and glucoses contained in the alkali chloride aq. soln. is held at <=100ppm especially <=30ppm and electrolysis is performed. Thereby current efficiency is stably maintained for a long period and also rising of voltage of a cell can be prevented. It is considered that the effect resulting from lowering of the concn. of organic substance in the alkali chloride aq. soln. depends on such the phenomena that the velocity of infiltration of organic substance into the membrane is made low and the membrane is unexpanded until the degree affecting current density and electrolytic voltage even when electrolysis is performed for a long period.

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial Field of Application] The present invention relates to a method for electrolyzing alkali chloride using a fluorine-containing cation exchange membrane.

[Prior art] As a method for producing caustic soda and chlorine by electrolyzing sodium chloride, the ion exchange membrane method, which uses a fluororesin cation exchange membrane as a diaphragm, is less polluting than the conventional mercury method and asbestos diaphragm method. It has attracted attention in recent years because it is advantageous from the viewpoint of prevention and energy saving, and also because it allows the production of high-quality caustic sorters with extremely low sodium chloride content. As the fluororesin cation exchange membrane used in the ion exchange membrane method, a carboxylic acid type membrane is more advantageous than a sulfonic acid type membrane because it can produce highly concentrated caustic soda with high current efficiency. It is said that We also compared carboxylic acid type fluororesin membranes and sulfonic acid type fluororesin membranes.

In order to maintain high current efficiency and low electrolysis voltage for a long period of time using these cation exchange membranes, it is necessary to remove calcium and magnesium from the supplied brine using a chelate resin and maintain it at a low concentration. It is known that it is important. (Unexamined Japanese Patent Publication No. 51-86100. No. 54-754
97) However, according to the present inventor, even when electrolysis is performed using a cation exchange membrane using salt water with low calcium and magnesium concentrations as described above, current efficiency still decreases and electrolysis occurs over a long period of time. It was found that an increase in voltage occurred. This has been found to occur particularly when acid is added to the brine to lower the pH of the anolyte, thereby reducing the amount of oxygen in the chlorine generated at the anode.

[Problems to be Solved by the Invention] The present inventor has investigated and researched the causes of the decrease in current efficiency and increase in electrolysis voltage that occur during the electrolysis of the above-mentioned salt water. It is an object of the present invention to provide a method for electrolyzing salt water that does not cause an increase in electrolytic energy.

It has been found that the above-mentioned problems occur not only in salt water but also in the production of potassium hydroxide by electrolyzing potassium chloride, so a general electrolysis method for alkali chloride that does not cause these problems has been proposed. The purpose is to provide

[Means for Solving the Problems] According to the research of the present inventor, it was found that the decrease in current efficiency and increase in electrolysis voltage that occur during electrolysis are caused by organic substances contained in the aqueous alkali chloride solution. . In particular, it has been found that when hydrochloric acid is added to an aqueous alkali chloride solution, organic substances such as alcohol and glycol contained in the hydrochloric acid cause this, resulting in significant adverse effects such as a decrease in current efficiency and an increase in electrolytic voltage.

stop. According to the test of the present invention, the concentration of organic matter in the aqueous alkali chloride solution is 11% regardless of the source.
It has been found that it is necessary to maintain it at 00 pp or less.

When the concentration of organic matter in the aqueous alkali chloride solution exceeds 1oOppi+, it is not necessarily clear what mechanism causes a decrease in current efficiency and an increase in electrolytic voltage, and this should not limit the scope of the present invention. No, but it can be assumed as follows. The organic matter contained in the aqueous alkali chloride solution enters the membrane together with Ha and ions, and the membrane is wetted by 1 liv, reducing the current efficiency.At the same time, blisters are generated on the anode side surface of the membrane, and the blisters block the current. It is assumed that the electrolytic voltage increases because of this phenomenon.

Hereinafter, the present invention will be described in detail. The fluorine-containing cation exchange membrane as used in the present invention means a membrane in which the entire membrane or at least the surface facing the cathode is made of perfluorocarboxylic acid polybutylene. A membrane having perfluorocarboxylic acid groups on the cathode side can obtain highly concentrated caustic soda with high current efficiency, and is particularly affected by the organic substance concentration in the present invention. low resistance,
In order to obtain alkali hydroxide with high current efficiency and provide a practically large film strength, a perfluorocarboxylic acid polymer with a larger ion exchange capacity than the cathode polymer or a perfluorosulfonic acid with a higher water content is used. A so-called asymmetric structure can be adopted in which a porous body made of a polymer, or even a fluorine-containing polymer, is used on the anode side, and it can also be reinforced with cloth, microfibrils made of a corrosion-resistant fluororesin, nonwoven fabric, or the like.

In the present invention, various carboxylic acid type perfluorocarbon polymers and sulfonic acid type perfluorocarbon polymers may be used without particular limitation, including conventionally known ones. In a preferred embodiment, the following (a).

It is preferable to use a polymer having the structure (0).

(()ncFi-CFX→-, (0) +cF2-cx
+wherein X is F or -CF3, preferably F;
Y is selected from the following:

-+(:F, h A, -0÷F2←A, -Go
-CF2-CF Thank you A.

-CFz-0+CF278.

-0((:F2-CF-0+-y +CF2S-19 Rf A is selected from -5O, M, -COOM, or -303F which can be converted into these groups by hydrolysis.

-CN, -COF or -GOOR, M represents hydrogen or an alkali metal, and R represents an alkyl group having 1 to 10 carbon atoms.

The membrane used in the present invention has a total thickness of 60 to 350 microns, preferably 100 to 300 microns. Furthermore, the membrane of the present invention may have a roughened surface or a gas release layer formed on the membrane surface to prevent gas from adhering to the membrane surface, such as a porous thin layer made of metal oxide particles and having no electrode activity. etc. are also possible.

In the present invention, each layer can be formed by various conventionally known methods. For example,
Wet mixing using an aqueous dispersion, an organic solution, an organic disversion, etc. of a perfluorocarbon polymer containing an ion exchange group, or forming a film using a casting method etc. from such an organic solution or an organic disversion. etc. are also possible. Of course, the film can also be formed by employing a tri-blend method or by heating and melting molding. When forming each layer by heating and melt molding, the raw material polymer should be in the form of an appropriate ion exchange group that does not cause decomposition of the ion exchange group it has, for example, in the case of a carboxylic acid group, it should be in the acid or ester type. is preferable, and in the case of a sulfonic acid group, it is preferable to use the -3O,F type.Furthermore, the raw material polymer may be heated and melt-molded in advance to form a pellet, and then formed into a film by extrusion molding, press molding, etc. You can also do it.

The multilayer membrane used in the present invention usually has a main carboxylic acid membrane layer, a sulfonic acid membrane surface layer, a carboxylic acid membrane surface layer, and, if necessary, a coexisting membrane layer or a carboxylic acid membrane intermediate layer, each separately. It can be manufactured by forming into a predetermined film shape and laminating and integrating these layers. Examples of methods for laminating and integrating each layer include flat plate pressing, roll pressing, and the like. The lamination press temperature is 60-280℃, the pressure is 0.1-100kg/cm" for flat plate press, 0 for roll press.
．． 1 = -100 kg/cm".

Either type of electrode may be used in the present invention. for example,
Porous electrodes such as perforated plates, mesh or expanded metal are used.

As the anode, platinum group metals, conductive oxides thereof, or conductive reduced oxides thereof, etc. are usually used, while as the cathodes, platinum group metals, conductive oxides thereof, iron group metals, etc. are used.

When disposing the electrodes, the electrodes may be disposed in contact with the fluorine-containing cation exchange membrane, or may be disposed at appropriate intervals. Rather than firmly pressing the electrode against the ion exchange membrane surface, the electrode should be pressed onto the ion exchange membrane surface with a weight of, for example, 0 to 2.0 kg.
/c@2, preferably gently pressed.

The electrolytic layer used in the present invention may be of a monopolar type or a bipolar type, and the material constituting the electrolytic cell may be, for example, in the case of electrolysis of an aqueous alkali chloride solution, or in the case of an anode chamber, an aqueous alkali chloride solution or a bipolar cell. Materials that are resistant to chlorine, such as valve metal, titanium, are used, and in the case of the cathode chamber, iron, stainless steel, or nickel that are resistant to alkali hydroxide and hydrogen are used.

In the present invention, known conditions can be employed as process conditions for electrolyzing an aqueous alkali chloride solution. For example, an aqueous alkali chloride solution of 2.5 to 5.0 normal (N) is preferably supplied to the anode chamber, and water or diluted alkali hydroxide is supplied to the cathode chamber, preferably at a temperature of 80 to 120°C and a current density of 10. ~100 A/da''. In such a case, heavy metal ions such as calcium and magnesium in the aqueous alkali chloride solution cause deterioration of the ion exchange membrane, so they are preferably 1 ppm+ or less, especially 0.
It is preferable to make it as small as possible to 5 ppm or less,
Furthermore, an acid such as hydrochloric acid can be added to the aqueous alkali chloride solution in order to prevent the generation of oxygen at the anode as much as possible.

In the present invention, the current efficiency can be stabilized for a long period of time and an increase in sliding voltage can be prevented by maintaining the organic matter concentration in the supplied aqueous alkali chloride solution to 1100 pp or less, preferably 50 pp or less, and especially 30 pp or less. can.

In the present invention, various methods including known methods can be adopted as means for reducing the concentration of organic matter contained in the aqueous alkali chloride solution. It is preferable to remove the organic substances in the acid by, for example, a distillation method, an adsorption method, an electrodialysis method, or a diffusion dialysis method. Among these, the distillation method is particularly preferable. The types of organic substances to be removed include alcohols such as methanol and ethanol. Typical examples include glycols such as , propylene glycol, and ethylene glycol.

[Function] In the present invention, the maintenance effect of reducing the concentration of organic matter in the supplied brine is not necessarily clear, but the rate at which organic matter penetrates into the film is low, and even long-term electrolysis will affect current efficiency and electrolysis voltage. This is thought to be due to the fact that the membrane was not swollen to some extent.

[Example] Example 1 Tetrafluoroethylene and CFz-CFO(CFz)s
(OOCH: + is polymerized, ion exchange capacity 1.44
A comonopolymer with taec4/g and 1.25+*eq/g was obtained. The former copolymer is referred to as A, and the latter copolymer is referred to as B. Copolymer A was extrusion molded to obtain a film with a thickness of 200°. This film is designated as A-1.

Copolymer B was extruded to a thickness of 20. obtained the film. This film is designated as 8-1. Film A-1 and film B-1 were laminated by hot roll pressing to obtain a composite film.

On the other hand, 1 part of zirconium oxide powder with a particle size of 1 l, methylcellulose (viscosity of 2% aqueous solution 1500 centivoise)
A paste was obtained by kneading a mixture containing 0.4 parts of water, 19 parts of water, 2 parts of cyclohexanol, and 1 part of cyclohexanone. The A polymer 200fiL side of the ion exchange membrane was created by laminating the paste using a Tetron screen with a mesh number of 200 and a thickness of 75JL, a printing plate with a screen mask of 30IL thick underneath, and a polyurethane squeegee. screen printed on the surface.

The adhesive layer obtained on the membrane surface was dried in air.

On the other hand, β-silicon carbide particles having an average particle size of 0.3 IL■ were similarly deposited on the other surface of the membrane having the porous layer thus obtained. Thereafter, the particle layer on each membrane surface was pressed onto the ion exchange membrane surface under conditions of a temperature of 140 cm and a C5 pressure of 30 kg/am.

An ion exchange membrane was prepared in which 1.0 mg of zirconium oxide particles and 1.0 mg of silicon carbide particles were adhered to the anode and cathode sides of the membrane, respectively, per 1 cm of the membrane surface.

The membrane was hydrolyzed with a 25% caustic soda aqueous solution at 70° C. for 16 hours to obtain a Na-type ion exchange membrane.

The A-1 layer side of the membrane thus obtained was coated with a solid solution of ruthenium oxide, iridium oxide, and titanium oxide on punched titanium metal (21 layers in short axis, 5 μm in diameter), which had a low chlorine overvoltage. anode, and SO on the B-1 layer side.
3304 punched metal (minor diameter 2114, diameter 5m)
) with ruthenium in Raney nickel (ruthenium 5%,
A cathode electrodeposited with nickel (50% nickel, aluminum 45%) to have a low hydrogen overvoltage is separated by 0.5 m from the anode.
While supplying an aqueous sodium chloride solution to the anode chamber and water to the cathode chamber, the sodium chloride concentration in the anode chamber was adjusted to 200 g/l, and the caustic soda concentration in the cathode chamber was adjusted to 3 g/l.
Electrolysis was carried out at 90°C and 30 A/dm'' while maintaining the concentration at 5% by weight.The effective membrane area was 0.25 layer m''. After 5 days of electrolysis, the initial current efficiency was 96±0. 3
%, the supplied brine was switched to brine containing methanol and having a Ca and Mg concentration of xopppb or less, respectively, and electrolysis was performed.

To control the methanol concentration in the salt water, methanol was dissolved in the salt water and adjusted to the desired concentration. At that time, the concentration of methanol in the sodium chloride aqueous solution supplied to the anode chamber was adjusted to 0.30°50.100,200,300.
PP was kept at six different levels of haziness. As a result, the electrolytic performance under each condition was as shown in Table 1 below.

From Table 1, it was found that if the methanol concentration was kept below the xoopp layer, the current efficiency would not decrease and the sliding voltage would not increase.

Example 2 Electrolysis was carried out using the same ion exchange membrane as in Example 1 under the same conditions, and propylene glycol was added to the aqueous sodium chloride solution supplied to the anode chamber.

The concentration of propylene glycol in the aqueous sodium chloride solution fed to the anode chamber was maintained at four different levels: 0.50, 100 and 300 ppm.

As a result, the electrolytic performance under each condition was as shown in Table 2 below.

From Table 2, it was found that when the propylene glycol concentration was kept at 1100 pp or less, the current efficiency did not decrease and the sliding voltage did not increase.

Example 3゜Tetrafluoroethylene and CFz-CFOCFzCF(CF:+)O(CF2)i
(By catalytic polymerization of :00CH+, the ion exchange capacity is 0.
Copolymer A having a weight of 90 eq/g was obtained. Tetrafluoroethylene and (:F2-CFOCF, CF(CF3)O
Copolymer B with an ion exchange capacity of 0.91 meq/g was obtained by catalytic polymerization of CF2CF2SO□F0,
Polymer A and Polymer B were each extruded to form a film of 50 ml consisting of Polymer A and 200 ml consisting of Polymer B.
I got a film of Le.

Both films were laminated using an embossing roll to obtain a composite film with fine irregularities on both surfaces.

The membrane was treated with dimethyl sulfoxide-KOH mixed aqueous solution, 90%
Hydrolysis was carried out at °C for 20 minutes to obtain an ion exchange membrane. Sodium chloride was electrolyzed in the same manner as in Example 1, with the A polymer side of the laminated film facing the cathode. The effective membrane area was 0-25 d2.

After 5 days of electrolysis, the initial current efficiency was 95.7±0.3.
%, the supplied brine was switched to brine containing methanol and having Ca and Mg concentrations of 10 ppb or less, respectively, and electrolysis was performed.

To control the methanol concentration in the salt water, methanol was dissolved in the salt water and adjusted to the desired concentration. At that time, the concentration of methanol in the sodium chloride aqueous solution supplied to the anode chamber was adjusted to O, SO.

100, zoopp■ were held at four different levels.

As a result, the electrolytic performance under each condition was as shown in Table 3 below.

From Table 3, it was found that if the methanol concentration was kept at 100 pp or less, the current efficiency did not decrease and the sliding voltage did not increase.

Claims (8)

[Claims] (1) Electrolysis of alkali chloride using a fluorine-containing cation exchange membrane to produce alkali hydroxide by electrolyzing alkali chloride, which is characterized by maintaining the concentration of organic matter in the supplied aqueous alkali chloride solution to 100 ppm or less. Method. (2) Claims in which the organic substance is alcohol (1)
) electrolysis method. (3) The electrolysis method according to claim (1), wherein the fluorine-containing cation exchange membrane is made of a perfluorocarbon polymer having a carboxylic acid group as an ion exchange group. (4) Claim (1) wherein the fluorine-containing cation exchange membrane is an asymmetric membrane made of a perfluorocarbon polymer in which the ion exchange capacity on the side facing the anode is larger than the ion exchange capacity on the side facing the cathode. electrolysis method. (5) The fluorine-containing cation exchange membrane is made of a perfluorocarbon polymer having a carboxylic acid group on the side facing the cathode, and an asymmetric membrane consisting of a perfluorocarbon polymer having a sulfonic acid group on the side facing the anode. An electrolysis method according to claim (1). (6) The fluorine-containing cation exchange membrane has a roughened surface, or has a porous layer made of metal oxide or carbide particles with no electrode activity formed on its surface. Certain claims (1), (3), (4)
Or the electrolysis method of (5). (7) Claims (1), (3), (4), (
5) or (6) electrolysis method. (8) The electrolysis method according to claim (1), (3), (4), (S), (6) or (7), wherein the concentration of alkali hydroxide is 30% by weight.