JPH07178319A - Method for producing aqueous acid solution and aqueous alkali solution - Google Patents

Abstract

(57) [Summary] [Objective] When an acid aqueous solution and an alkaline aqueous solution are produced from a salt aqueous solution by performing bipolar membrane electrodialysis by a method of circulatingly supplying the acid aqueous solution and the alkaline aqueous solution, the salt aqueous solution in the salt chamber is excessive. To develop a method capable of efficiently carrying out the electrodialysis, which is prevented from being acidic or alkaline. [Structure] A cation exchange membrane, a bipolar membrane and an anion exchange membrane are sequentially arranged between an anode and a cathode to form a salt chamber, an acid chamber and an alkali chamber, and an aqueous salt solution is supplied to the salt chamber for electrodialysis. Then, the acid aqueous solution and the alkaline aqueous solution are discharged from the acid chamber and the alkaline chamber, respectively, and the discharged acidic aqueous solution and the alkaline aqueous solution are circulated as in the feed-and-bleed method, and are supplied again to the acid chamber and the alkaline chamber, respectively, and circulated. In the method of partially acquiring an acid aqueous solution and an alkaline aqueous solution and adjusting the concentration, the current efficiencies of the acid and the alkali during electrodialysis are approximately the same for each concentration of the acid aqueous solution and the alkaline aqueous solution whose concentration is adjusted as described above. A method for producing an acid aqueous solution and an alkaline aqueous solution, which is characterized by having a concentration.

Description

Detailed Description of the Invention

[0001]

TECHNICAL FIELD The present invention relates to a method for producing an acid aqueous solution and an alkaline aqueous solution by electrodialysis using a bipolar membrane of a salt aqueous solution.

[0002]

2. Description of the Related Art It is known to produce an acid aqueous solution and an alkaline aqueous solution by electrodialysis of a salt aqueous solution using a bipolar membrane (hereinafter also referred to as bipolar membrane electrodialysis). For example, a cation exchange membrane between the anode and cathode,
A plurality of bipolar membranes and anion exchange membranes are arranged in order to form a salt chamber, an acid chamber and an alkaline chamber, and an aqueous salt solution is supplied to the salt chamber for electrodialysis to carry out an acid aqueous solution and an alkaline aqueous solution from the acid chamber and the alkaline chamber. The three-chamber cell system for discharging each of the three types is disclosed in Japanese Examined Patent Publication No. 32-3962 and Japanese Examined Patent Publication No.
It is known from JP-A-3-6963, JP-A-63-65912 and the like.

As one of the operating methods of the electrodialysis using the three-chamber cell system, an acid aqueous solution and an alkaline aqueous solution, which are respectively stretched in the tank, are circulated and supplied to the electrodialysis tank, which corresponds to the generated acid and alkali. To the above tanks, and overflow the acid aqueous solution and alkaline aqueous solution corresponding to the supplemented amounts from the tanks,
A so-called feed-and-bleed method is known in which the acid aqueous solution and the alkaline aqueous solution are sequentially obtained at a constant concentration. In this feed-and-bleed method, the acid and alkali concentrations of the obtained aqueous acid solution and aqueous alkali solution can be arbitrarily set by adjusting the amount of water to be replenished in each tank. Then, the acid and alkali concentrations of the obtained acid aqueous solution and alkali aqueous solution are generally set to be the same as each other, and in this case, they are circulated and supplied to the electrodialysis tank. The acid and alkali concentrations of the acid aqueous solution and the alkali aqueous solution are naturally the same.

[0004]

However, in such a feed-and-bleed system, an acid aqueous solution and an alkaline aqueous solution are circulated and supplied to an electrodialysis tank, and the acid aqueous solution and the alkaline aqueous solution are partially acquired during the circulation, And when the bipolar membrane electrodialysis is carried out by a method of adjusting the concentration, the salt aqueous solution in the salt chamber of the electrodialysis tank is more acidic than the acidity or alkalinity predicted from the type and concentration of the salt dissolved in the salt aqueous solution. It often happens that it slips. For example, an aqueous solution of sodium sulfate is neutral, and in the above bipolar membrane electrodialysis of the aqueous solution of sodium sulfate, the aqueous solution in the salt chamber should be consistently kept neutral regardless of the degree of desalting. However, in reality, it becomes acidic or alkaline with the progress of the electrodialysis.

When the aqueous salt solution in the salt chamber becomes excessively acidic or alkaline in this way, H ⁺ ions permeate the cation exchange membrane from the salt chamber to the adjacent alkaline chamber. In the acid chamber, and O
H ⁻ ions come to permeate through the anion exchange membrane and enter, which causes a problem of neutralizing the acid or alkali produced by electrodialysis. Further, when the desalted solution discharged from the salt chamber becomes acidic or alkaline as described above, it becomes necessary to further neutralize the desalted solution,
The operation becomes complicated.

From the above background, when the bipolar membrane electrodialysis is carried out by the feed-and-bleed method to produce the acid aqueous solution and the alkaline aqueous solution from the salt aqueous solution, the salt aqueous solution in the salt chamber becomes excessively acidic or alkaline. It has been desired to develop a method capable of efficiently carrying out the electrodialysis, which is prevented.

[0007]

[Means for Solving the Problems] The present inventors have conducted extensive studies to solve the above technical problems. As a result, during electrodialysis, it was found that the above problems can be solved by setting the respective concentrations of the acid aqueous solution and the alkaline aqueous solution that are circulated and supplied so that the current efficiencies of the acid and the alkali during electrodialysis are approximately equal. The invention has been provided.

That is, according to the present invention, a cation exchange membrane, a bipolar membrane and an anion exchange membrane are sequentially arranged between an anode and a cathode to form a salt chamber, an acid chamber and an alkaline chamber, and a salt aqueous solution is placed in the salt chamber. Is supplied to perform electrodialysis to discharge the acid aqueous solution and the alkaline aqueous solution from the acid chamber and the alkaline chamber, respectively,
In the method in which the discharged aqueous acid solution and alkaline solution are circulated and supplied again to the acidic chamber and the alkaline chamber, respectively, the acidic aqueous solution and the alkaline aqueous solution are partially acquired during the circulation, and the concentration is adjusted, the concentration is adjusted A method for producing an acid aqueous solution and an alkali aqueous solution, wherein each concentration of the acid aqueous solution and the alkali aqueous solution is set to a concentration at which current efficiencies of the acid and the alkali during electrodialysis are substantially equal to each other.

The electrodialysis cell used in the present invention comprises a bipolar membrane (B), an anion exchange membrane (A) and a cation exchange membrane (C) between an anode 11 and a cathode 12, as shown in FIG. 3 types are arranged in order, and the alkaline chamber 13 and the acid chamber 1
As long as it has a structure in which three chambers of 4 and the salt chamber 15 are formed, known ones can be adopted without any limitation.
Here, the chamber between the cation exchange membrane (C) and the bipolar membrane (B) is the alkali chamber 13, the chamber between the bipolar membrane (B) and the anion exchange membrane (A) is the acid chamber 14, and the anion exchange chamber. The chamber between the membrane (A) and the cation exchange membrane (C) is called the salt chamber 15.

A typical construction of an electrodialysis cell is an anode- (C
-BA-) nC-cathode. Here, the minimum repeating unit composed of a cation exchange membrane, a bipolar membrane, an anion exchange membrane and the like is called a cell, and an assembly of membranes, gaskets and spacers excluding electrodes is called a stack. n is the number of repeated stacks of cells. The bipolar membrane is usually used with the anion exchanger side facing the anode side and the cation exchanger side facing the cathode side.

In the present invention, an aqueous salt solution is supplied to the salt chamber of the bipolar membrane electrodialysis tank to perform electrodialysis to discharge the acid aqueous solution and the alkaline aqueous solution from the acid chamber and the alkaline chamber, respectively. Then, the discharged acidic aqueous solution and alkaline aqueous solution are circulated and supplied again to the acidic chamber and the alkaline chamber, respectively. Further, during this circulation, the acid aqueous solution and the alkaline aqueous solution are partially concentrated as a production liquid, and the concentration is adjusted so as to be circulated and supplied to the acid chamber and the alkaline chamber. In that case, the acquisition and concentration adjustment of the acid aqueous solution and the alkaline aqueous solution may be carried out by any means at any point in the circulating flow of the acid aqueous solution and the alkaline aqueous solution. Usually, an acid aqueous solution tank and an alkaline aqueous solution tank are installed at arbitrary points in a circulation line of the acid aqueous solution and the alkaline aqueous solution, and the acid aqueous solution and the alkaline aqueous solution which circulate in each of the tanks are stored, and the acid produced by electrodialysis is stored therein. And an amount of water corresponding to the alkali is replenished, and the acid aqueous solution and the alkaline aqueous solution corresponding to the replenished amount are overflowed from the tank to sequentially obtain the acid aqueous solution and the alkaline aqueous solution at a constant concentration. It is preferable to use a bleed method.

The greatest feature of the present invention is that when carrying out bipolar membrane electrodialysis for circulating and supplying the acid aqueous solution and the alkaline aqueous solution as described above, the concentration of each of the acid aqueous solution and the alkaline aqueous solution is adjusted during the circulating supply. Concentration
The point is that the concentration is such that the current efficiencies of the acid and alkali during electrodialysis are approximately equal. That is, in the bipolar membrane electrodialysis, the current efficiency of the generated acid and alkali changes in correlation with the respective concentrations of the acid aqueous solution and the alkaline aqueous solution supplied to the acid chamber or the alkaline chamber. As a general tendency, as the concentration of the supplied acid and alkaline aqueous solution becomes higher, the current efficiency of each becomes lower due to the influence of the Donan ion adsorbed on the membrane. However, the rate of decrease in current efficiency at that time is due to the difference in acid and alkali species, the types of cation exchange membrane and anion exchange membrane used, and operating conditions such as temperature and current density. The case and the case of alkali are different from each other. As a result, in such bipolar membrane electrodialysis, as described above, even if the acid and alkali concentrations of the acid aqueous solution and the alkaline aqueous solution supplied to the acid chamber and the alkaline chamber are equal to each other,
There is some difference between the current efficiencies of acid and alkali in electrodialysis at that concentration condition. However, in such a bipolar membrane electrodialysis, an acid or an alkali, which corresponds to the difference in the current efficiency, invades the salt, and as described above, the acidity or alkalinity of the liquid in the salt chamber is altered. Occurs.

On the other hand, in the present invention, when the bipolar membrane electrodialysis is operated, the acid concentration and the alkali concentration of each aqueous solution supplied to the acid chamber and the alkali chamber are determined as described above during the electrodialysis. It has been found that it is possible to prevent the acidity and alkalinity of the salt chamber from deteriorating by setting the concentration so that the current efficiency of the acid and the alkali are approximately equal to each other, so that the amount of the acid or the alkali that enters the salt chamber becomes approximately the same. It is a thing.

In the present invention, the concentration of the acid aqueous solution and the alkaline aqueous solution at which the current efficiencies of the acid and the alkali during electrodialysis are substantially equal to each other means that when the salt aqueous solution in the salt chamber is electrolyzed by bipolar membrane electrodialysis The current efficiency at which the constituent cations move to the alkali chamber to generate alkali and the current efficiency at which the anions forming the salt move to the acid chamber to generate acid are almost equal to each other. It means the concentration of each of the aqueous alkali solution and the aqueous acid solution in the chamber. The current efficiency refers to the amount of generated acid and alkali with respect to the amount of electricity supplied, which is calculated by the following formula.

[0015]

[Equation 1]

Here, in the present invention, the acid concentration and alkali concentration of each aqueous solution supplied to the acid chamber and the alkali chamber are
It is not always necessary that the current efficiencies of the acid and the alkali are completely equal to each other, and if the values of the two are substantially the same, a slight difference is acceptable. Preferably, the absolute value of the difference between the current efficiencies of the acid and the alkali during the electrodialysis is such that the absolute value of the acid and the alkali aqueous solution is not larger than the smaller one of the normalities of the acid and the alkali aqueous solution to be supplied. It is preferable that the concentrations of are the same. Further, it is preferable that the difference between the current efficiencies of the acid and the alkali is such that the pH of the salt chamber is maintained at 5 to 9 even if the electrodialysis is operated for a long time.

Further, in the present invention, the method for adjusting the concentrations of the acid aqueous solution and the alkaline aqueous solution which are circulated and supplied during the electrodialysis as described above so that the current efficiencies of the acid and the alkali are approximately equal is particularly limited. Any method may be used instead of the one. Usually, when performing electrodialysis under the same conditions as the bipolar membrane electrodialysis to be performed,
The relationship between the acid and alkali concentrations of each aqueous solution in the acid chamber or alkali chamber and the current efficiencies of the generated acid and alkali is measured in advance, and from that relation, the current efficiencies of the acid and alkali are equal and the acid aqueous solution and the alkali. It is preferable to determine the concentration of the aqueous solution and adjust the concentration of each aqueous solution in the actual electrodialysis based on it. The concentration of the acid aqueous solution and the alkaline aqueous solution at which the current efficiencies of the acid and the alkali during the electrodialysis are equal to each other are not limited to the cation exchange membrane, the bipolar membrane, and the anion exchange membrane. Even if it is not measured under the condition of simultaneously producing, in each of the cation exchange membrane or the anion exchange membrane, an electrode is placed through the membrane, and an aqueous salt solution and an acid are placed in the chambers on the anode side and the cathode side of the membrane, It may be determined by a simple method of supplying an alkaline aqueous solution and energizing it to generate an acid or an alkali, and measuring the relationship between each acid or alkali concentration and the current efficiency of the generated acid or alkali. Generally, the concentration of the aqueous acid solution and the aqueous alkali solution is 0.
It is selected from the range of 1 to 8.0 N, preferably 2.0 to 6.0 N. Further, for the measurement of each concentration of the acid aqueous solution and the alkaline aqueous solution, a method of measuring with an electric conductivity, a pycnometer, a refractometer, an ultrasonic meter, a gas chromatography or the like can be arbitrarily applied.

The cation exchange membrane used in the bipolar membrane electrodialysis of the present invention is not particularly limited, and a known cation exchange membrane can be used. For example, a cation exchange membrane having a sulfonic acid group, a carboxylic acid group, a phosphonic acid group, a sulfuric acid ester group and a phosphoric acid ester group, and a cation exchange membrane in which a plurality of these ion exchange groups are mixed can be used. In addition, the cation exchange membrane is a polymerization type, condensation type, uniform type,
Whatever the type, whether it is a non-uniform type or not, regardless of the presence or absence of reinforcing core material, hydrocarbon type, fluorine type, cation exchange membrane type and model derived from the material and manufacturing method, Good. Furthermore, 2N-saline solution was added at 5 A / d.
If electrodialyzed at a current density of m ² and having a current efficiency of 70% or more and substantially functioning as a cation exchange membrane,
Even what is generally called an amphoteric ion exchange membrane can be used as the cation exchange membrane of the present invention. Further, it is preferable to use a fluorine-based cation exchange membrane in contact with the anode chamber.

On the other hand, as the bipolar membrane, a composite ion exchange membrane having a structure in which a cation exchange membrane and an anion exchange membrane are adhered together can be used without particular limitation. The following is known as a manufacturing method thereof. For example, a method of laminating a cation exchange membrane and an anion exchange membrane with a mixture of polyethyleneimine-epichlorohydrin and curing and adhering them (Japanese Patent Publication No. 32-3962), anion exchange membrane and an anion exchange membrane. Method of adhering with an agent (Japanese Patent Publication No. 34-3961), a cation exchange membrane and an anion exchange membrane are coated with a fine powder of an ion exchange resin, or a paste-like mixture of an anion or cation exchange resin and a thermoplastic substance. A method of press-bonding (Japanese Patent Publication No. 35-14531), a method of coating a paste-like substance composed of vinylpyridine and an epoxy compound on the surface of a cation exchange membrane, and irradiating the paste-like substance (Japanese Patent Publication No. 38- 16633), after adhering a sulfonic acid type polymer electrolyte and allylamines on the surface of an anion exchange membrane, ionizing radiation Irradiation cross-linking method (Japanese Patent Publication No. 51-4113), and a method of depositing a mixture of a dispersion of an ion exchange resin having an opposite charge and a base polymer on the surface of an ion exchange membrane (Japanese Patent Laid-Open No. 53-37190). ), Polyethylene film impregnated with styrene and divinylbenzene for polymerization, sandwiched between sheets, sandwiched in a stainless steel frame, sulfonated on one side, and then removed the sheet to chlormethylate the rest, then amination (US Pat. No. 3,562,139), or a method in which the interface between the anion exchange membrane and the cation exchange membrane is treated with an inorganic compound and the two membranes are joined (JP-A-59-472).
35).

Further, the anion exchange membrane is not particularly limited, and a known anion exchange membrane can be used. For example, a quaternary ammonium group, a primary amino group, a secondary amino group, a tertiary amino group, or an anion exchange membrane in which a plurality of these ion exchange groups are mixed can be used. Further, the anion exchange membrane may be a polymerization type, a condensation type, a uniform type, or a non-uniform type,
Further, any material may be used regardless of the presence / absence of the reinforcing core material, the hydrocarbon-based material, the fluorine-based material, the type and model of the anion exchange membrane derived from the material and the manufacturing method. Further, if a 2N-salt solution is electrodialyzed at a current density of 5 A / dm ² and has a current efficiency of 70% or more and substantially functions as an anion exchange membrane, it is generally called an amphoteric ion exchange membrane. Even so, it can be used as the anion exchange membrane of the present invention. Since an anion exchange membrane tends to easily permeate an acid, it is preferable to use an anion exchange membrane that hardly permeates an acid.

As the anode and cathode used in the electrodialysis tank, electrodes used in the electrochemical industry such as water electrolysis and salt electrolysis are
It is used without any restrictions. For example, nickel, iron, lead, platinum, graphite or the like can be suitably used as the anode material, and nickel, iron, stainless steel, platinum or the like can be suitably used as the cathode material.

The type of anolyte supplied to the anode chamber can be appropriately selected according to the type of anode material. Examples of preferable combinations of these are as follows. Examples thereof include nickel or iron-sodium hydroxide aqueous solution, lead-sulfuric acid aqueous solution, platinum-sulfuric acid or sodium sulfate aqueous solution, and graphite-salt aqueous solution. Further, preferable combinations of the cathode material and the catholyte are as follows. Mention may be made of nickel, iron or stainless steel-sodium hydroxide, sodium sulphate or saline solution.

In the present invention, the salt used as the subject of electrodialysis is not limited to organic salts and inorganic salts, as long as the acid and alkali produced by salt decomposition by electrodialysis form an aqueous solution. Can be used without restrictions. Examples of the cations forming the salt include sodium, potassium, lithium and ammonium ions. Examples of anions constituting the salt include halogen ions such as fluorine, chlorine, bromine and iodine, sulfate ion, nitrate ion, acetate ion, lactate ion and the like. The aqueous salt solution is supplied to the salt chamber of the electrodialysis tank for electrodialysis, and then supplied again to the salt chamber through a salt aqueous solution tank in which a salt aqueous solution tank is installed in the same manner as the acid aqueous solution and the alkaline aqueous solution. However, it is preferable to repeat the desalting until a predetermined salt concentration is reached.

As operating conditions for the above bipolar membrane electrodialysis, the temperature is usually 5 to 70 ° C., preferably 20.
It is preferably in the range of -50 ° C. The current density is not particularly limited, but is generally 1 to 30 A / d.
It is suitable that it is m ² , preferably 3 to 20 A / dm ² .

[0025]

According to the present invention, bipolar membrane electrodialysis is carried out by a method of circulating and supplying an acidic aqueous solution and an alkaline aqueous solution to an electrodialysis tank at constant concentrations, such as a feed-and-bleed method, and a salt aqueous solution to an acidic aqueous solution. Also, it is possible to prevent the salt aqueous solution in the salt chamber from becoming excessively acidic or alkaline when producing the alkaline aqueous solution. Therefore,
The present invention is extremely useful because it is possible to produce an acid aqueous solution and an alkaline aqueous solution by performing electrodialysis extremely efficiently without neutralizing the desalted solution.

[0026]

EXAMPLES In order to more specifically describe the present invention, examples and comparative examples will be given below, but the present invention is not limited to these examples. The ion exchange membrane used in the examples is shown below.

Anion exchange membrane and cation exchange membrane

[0028]

[Table 1]

Bipolar film It was obtained as follows. That is, a viscous polymer solution containing 50 parts of vinylbenzyl chloride, 35 parts of styrene, 15 parts of divinylbenzene having a purity of 50%, 2 parts of benzoyl peroxide, 2 parts of styrene oxide and 5 parts of acrylonitrile-butadiene rubber was prepared. This polymer solution was subjected to heat polymerization between glass plates at 70 ° C. for 16 hours in a nitrogen atmosphere to obtain a polymer film. Next, this polymer film was added to 96% sulfuric acid at 60 ° C. for 1 hour.
It was immersed for 0 minutes to introduce a sulfonic acid group on the surface of the film. Furthermore, trimethylamine-acetone-water (1:
1: 8) Place in mixed solution, treat at 30 ° C for 1 day,
An anion exchange group was introduced into the membrane to obtain an anion exchange membrane. A mixture of 5% polyvinyl alcohol and 5% glutaraldehyde in an equal amount was applied between the surface-sulfonated anion exchange membrane and the Tokuyama Soda Co., Ltd. cation exchange membrane (trade name, CM-1), A hot press was carried out at 50 ° C. for 1 hour for adhesion to obtain a bipolar film.

Example 1 The relationship between the concentration of the aqueous sodium hydroxide solution in the alkaline chamber and the current efficiency of sodium hydroxide produced when sodium sulfate was electrodialyzed using the cation exchange membrane CM-1 was calculated as follows.
It was measured by the following method. In the case of a cation exchange membrane, an electrode was placed through the membrane, a 2N sodium sulfate aqueous solution was placed on the anode side, and various concentrations of sodium hydroxide aqueous solution were placed on the cathode side to conduct electricity. Both electrodes were platinum plates. Effective membrane area is 1 cm ² , current density is 10 A / dm ² ,
The temperature was 40 ° C. Since the anode side becomes acidic during energization, sodium hydroxide was added to maintain the neutrality when the pH became 8 or more. When the conductivity of the catholyte increased, water was added to keep the concentration constant. After 20 minutes, the current efficiency was determined from the amount of increase in the catholyte. Table 2 shows the relationship between the alkali concentration and the current efficiency.

[0031]

[Table 2]

Next, the relationship between the concentration of the sulfuric acid aqueous solution in the acid chamber and the current efficiency of the generated sulfuric acid during electrodialysis using the anion exchange membrane AMX was similarly measured. The results are shown in Table 3. In the measurement of current efficiency in the case of anion exchange membrane,
Sulfuric acid was placed on the anode side and sodium sulfate was placed on the cathode side to replenish water and sulfuric acid, respectively. The energization conditions are the same as for the cation exchange membrane.

[0033]

[Table 3]

Based on the above results, a bipolar membrane electrodialysis operated by a feed-and-bleed system as shown in FIG. 1 was used to produce an aqueous sulfuric acid solution and an aqueous sodium hydroxide solution by electrodialysis of sodium sulfate. .
The electrodialysis tank has a cation exchange membrane CM- between a pair of anion and anode.
1. Bipolar membrane and anion exchange membrane AMX are sequentially 11 sheets, 10 sheets, 10 sheets (the effective membrane area of the cation exchange membrane, the bipolar membrane and the anion exchange membrane is 1d, respectively).
m ² and the total membrane area were 11, 10 and 10 dm ² respectively, and a filter press type bipolar membrane electrodialysis tank in which an alkali chamber 13, an acid chamber 14 and a salt chamber 15 were formed was used.

First, in the acid chamber tank 21 and the alkaline chamber tank 23, which have a capacity of 3 L and overflow when more liquid enters, the concentrations are such that the current efficiencies of acid and alkali during electrodialysis are equal to 75%, respectively. A 0.0 N sulfuric acid aqueous solution and a 3.0 N sodium hydroxide aqueous solution were filled up to the full extent. This tank has a conductivity meter (Toa Denpa C
(M-40S) was attached, the concentration was monitored, and when the conductivity increased, water was automatically added to control the concentration to be constant. The salt room tank was filled with 3 L of a 3.0 N sodium sulfate aqueous solution. 1 cm / sec, 1% of these sulfuric acid aqueous solution, sodium hydroxide aqueous solution, and sodium sulfate aqueous solution
It was circulated and supplied with electricity at an in-membrane linear velocity of 10 cm / sec through the acid chamber 14, the alkali chamber 13, and the salt chamber 15, respectively. The current density during operation was 10 A / dm ² , and the temperature was 40 ° C. 180 minutes later, when the operation was stopped,
The salt solution has a sodium sulfate concentration of 0.24 N and the liquid volume is 2.
5 L, pH was 7.2. The acid aqueous solution was 4.2 L of 2.0 N sulfuric acid aqueous solution, and the alkaline aqueous solution was 2.8 L of 3.0 N sodium hydroxide aqueous solution. The current efficiency was calculated to be 75% for both acid and alkali, which was the same as the previously determined value of the current efficiency of each ion-exchange membrane alone.

Comparative Example 1 An aqueous sulfuric acid solution and an aqueous sodium hydroxide solution were produced under the same conditions as in Example 1 except that the concentration of sulfuric acid was 3N in the bipolar membrane electrodialysis. 18
After 0 minutes, the salt solution had a sodium sulfate concentration of 0.6 N, a liquid volume of 2.5 L, and a sulfuric acid concentration of 0.4 N. The produced acid aqueous solution was 3.0N sulfuric acid aqueous solution 2.2L, and the alkaline aqueous solution was 3.0N sodium hydroxide aqueous solution 2.6L. Calculated current efficiency is 59% for acid and 6 for alkali
At 9%, all were lower than the current efficiency of the membrane alone.

Example 2 In the same manner as in Example 1, when the sodium phosphate was electrodialyzed using an anion exchange membrane AMX, the relationship between the concentration of the aqueous phosphoric acid solution in the acid chamber and the current efficiency of the generated phosphoric acid. Table 4 shows the results of the measurement. Here, phosphoric acid was placed on the anode side and sodium phosphate was placed on the cathode side of the film, and electricity was applied.

[0038]

[Table 4]

Based on the results of Tables 2 and 4, a bipolar membrane electrodialysis operated by a feed and bleed system as shown in FIG. 1 was used, and an aqueous phosphoric acid solution and sodium hydroxide were obtained by electrodialysis of sodium phosphate. The production of an aqueous solution was carried out. The apparatus including the electrodialysis tank and tanks, and the operating method are the same as those in the first embodiment.

4 L of an aqueous solution of 4.0 N phosphoric acid was added to the acid solution, and 4 L of an aqueous solution of 5.0 N sodium hydroxide was added to the alkali solution. The salt room tank was filled with 3 L of a 3N sodium phosphate aqueous solution, and 180 ml of 85% phosphoric acid was added to adjust the pH to 7. When the operation was stopped after 180 minutes of energization, the salt solution had a sodium phosphate concentration of 0.90 N and a liquid amount of 2.5.
L and pH were 7.1. As for the acid aqueous solution, 1.3 L of 4.0 N phosphoric acid aqueous solution and 1.7 L of 5.0 N sodium hydroxide aqueous solution were obtained as alkaline aqueous solution. When the current efficiency was calculated, both the acid and the alkali were 60%, which was the same as the previously determined value of the current efficiency of each ion exchange membrane alone.

[Brief description of drawings]

FIG. 1 is a schematic diagram of the feed-and-bleed system three-chamber bipolar electrodialysis cell used in the present invention.

[Explanation of symbols]

 A Anion exchange membrane B Bipolar membrane C Cation exchange membrane 11 Anode 12 Cathode 13 Alkaline chamber 14 Acid chamber 15 Salt chamber 21 Acid aqueous solution tank 22 Salt aqueous solution tank 23 Alkaline aqueous solution tank 24 Acid aqueous solution circulation line 25 Salt aqueous solution circulation line 26 Alkaline Aqueous solution circulation line 27 Acid aqueous solution tank overflow line 28 Alkaline aqueous solution tank overflow line

Claims (1)

[Claims] 1. A cation exchange membrane, a bipolar membrane and an anion exchange membrane are sequentially arranged between an anode and a cathode to form a salt chamber, an acid chamber and an alkali chamber, and an aqueous salt solution is supplied to the salt chamber. By performing electrodialysis, the acid aqueous solution and the alkaline aqueous solution are discharged from the acid chamber and the alkaline chamber, respectively, and the discharged acid aqueous solution and the alkaline aqueous solution are circulated and supplied again to the acid chamber and the alkaline chamber, respectively. In the method of acquiring a part of the aqueous solution and adjusting the concentration,
A method for producing an acid aqueous solution and an alkali aqueous solution, characterized in that the respective concentrations of the acid aqueous solution and the alkali aqueous solution whose concentrations are adjusted are such that the current efficiencies of the acid and the alkali during electrodialysis are approximately equal.