JPS6299487A - Separation of acid and alkali from aqueous salt solution - Google Patents

Abstract

PURPOSE:To separate and recover efficiently high concn. acid and alkali from an aqueous salt soln. by electrolyzing the soln. with an anion exchange membrane contg. fluorine and having a special structure and a cation exchange membrane contg. fluorine. CONSTITUTION:An aqueous salt soln. such as industrial waste liquor is electrolyzed in an electrolytic cell provided with an anion exchange membrane and a cation exchange membrane as diaphragms to produce an aqueous acid soln. in the anode chamber and an aqueous alkali soln. in the cathode chamber. When the acid and alkali are separated from the aqueous salt soln. as mentioned above, a cation exchange membrane contg. fluorine is used as the cation exchange membrane and an anion exchange membrane contg. fluorine and made of a copolymer having repeating units represented by the formula is used as the anion exchange membrane. In the formula, X is F or CF3, l is 0 or an integer of 1-5, m is 0 or 1, n is an integer of 1-5, each of p and q is a positive number, the ratio of p/q is 2-16 and Y is a quat. ammonium group. The durability of the anion exchange membrane is increased and restrictions on the kind of salt, the concn. of acid and the electrolytic temp. are eliminated.

Description

[Detailed Description of the Invention] [Industrial Application Field] The present invention relates to a method for separating acids and alkalis from an aqueous salt solution using an ion exchange membrane 'wL solution method. system anion exchange membrane,
The present invention relates to a method for separating acids and alkalis from an aqueous salt solution by an ion exchange membrane electrolysis method using a fluorine-based cation exchange membrane as a diaphragm.

Although the method of the present invention can be used in various fields, it is particularly effective in the field of waste liquid treatment.

A wide variety of plants, including chemical factories, plating factories, semiconductor factories, etc.

There are countless neutralization processes in various factories, and the salt discharged from these processes is huge in both type and quantity. Disposing of these salts without treatment poses environmental problems. In particular, when treating salt wastewater containing radioactive substances and other harmful substances, such as in nuclear power-related facilities, it is necessary to completely treat the wastewater on the premises of the facility, concentrating the radioactive substances and other harmful substances in the wastewater. At the same time, there is a strong desire to develop a treatment system that can separate and recover acids and alkalis from salt waste liquid and reuse them.

[Prior art]

Separation of acids and alkalis from salts is not possible through normal chemical reactions and requires the use of ion exchange resins or ion exchange membranes. Compared to the ion exchange resin method, the ion exchange membrane electrolysis method is attracting attention as a process that, in principle, allows a large amount of salt to be efficiently separated into acids and alkalis through simple operations.

For example, in recent years, sulfur dioxide gas (so
,) 7) The removal method is So using caustic soda.

A wet method that absorbs gas is common, but a method has been proposed in which the Nal OAX produced in this process is separated and recovered into caustic soda and sulfuric acid using ion exchange membrane electrolysis. A fluorine-based cation exchange membrane (for example, DuPont's NafiOn membrane) is used between the intermediate chamber that supplies the aqueous solution of sulfur salt and the cathode chamber, and a microporous diaphragm is used between the intermediate chamber and the anode chamber. ,or,
Using a microporous diaphragm and a hydrocarbon-based anion exchange membrane,
The process produces caustic soda in the cathode chamber and sulfuric acid in the anode chamber. Although this process is an effective method for separating acids and alkalis from salts, it has several problems. That is, if a microporous diaphragm is used between the intermediate chamber and the anode chamber, it is impossible to avoid mixing of sodium sulfate into the sulfuric acid produced in the anode chamber, and the microporous diaphragm and Even when a hydrocarbon-based anion exchange membrane is combined, the durability (heat resistance, acid resistance, oxidation resistance) of the hydrocarbon-based anion exchange membrane becomes a problem.

The durability of this hydrocarbon-based anion exchange membrane is determined by high-temperature electrolysis at 60°C or higher, or by acids with stronger oxidizing power than sulfuric acid, such as nitric acid, hydrochloric acid, and hydrofluoric acid.

This becomes a more serious problem in systems where acids such as chromic acid are separated, and the economic efficiency of the process of separating acids and alkalis from salts using ion exchange membrane electrolysis is lost.

For example, Japanese Patent Application Laid-Open No. 58-57596 proposes a process using ion exchange membrane electrolysis with respect to a method for concentrating nitrate-containing radioactive waste liquid produced in nuclear facilities.

This process also supplies a nitrate solution to the intermediate chamber, producing nitric acid in the anode chamber and alkali hydroxide or ammonium hydroxide in the cathode chamber, but between the intermediate chamber and the anode chamber hydrocarbon-based anions are produced. Exchange membrane, fluorine-based cation exchange membrane (for example, DupOnt) between the intermediate chamber and the cation exchange membrane
A five-chamber electrolytic cell equipped with a Nafion membrane (manufactured by Nafion, Inc.) is used.

In this process, as an economical electrolysis method, it is stated that the concentration in the anode and cathode chambers should be 15M/or less, and the reason for this is to maintain economical current efficiency. However, it is presumed that there is also a problem with the durability of the hydrocarbon-based anion exchange membrane itself.

When considering the efficiency of the entire process including electrolytic efficiency, it is desirable to separate and recover the salt with the highest concentration possible into acids and alkalis in the method of separating acid and alkali from salt. Naturally, the durability of hydrocarbon-based anion exchange membranes in high-temperature, high-concentration nitric acid solutions is by no means satisfactory.

In a high-temperature, high-concentration acid solution, a hydrocarbon-based anion exchange membrane causes a significant increase in the electrical resistance of the membrane within a short period of one month, resulting in an increase in electrolytic voltage.

Therefore, if you try to carry out such a process using a conventional hydrocarbon-based anion exchange membrane in a high temperature, high concentration acid solution, you will be forced to frequently replace the anion exchange membrane. As a result, the economic efficiency of the process itself becomes poor.

To further mention, if the salt is a chloride salt, at the anode,
Hydrocarbon-based anion exchange membranes do not show any durability against strongly oxidizing gases such as chlorine gas, and the membranes will fail in a short period of about two weeks. may collapse. Therefore, it is considered that a method for separating acids and alkalis from chloride salts by ion exchange membrane electrolysis will not be realized in practice.

As mentioned above, although the method for separating acids and alkalis from aqueous salt solutions using ion exchange membrane electrolysis is a well-known technology and there is an extremely high demand for its practical application as an industrial process, First, process constraints,
Due to the many problems that need to be solved, it has not yet been established as a satisfactory industrial process.

[Problems to be solved by the present invention]

The purpose of the present invention is to solve the problem of salt acid of conventional ion exchange membrane electrolysis method,
The ion-exchange membrane electrolysis method eliminates the limitations of salt type, acid concentration, electrolysis temperature, etc., which were shortcomings of alkali separation methods, and efficiently separates and recovers aqueous salt solutions into high-concentration acids and alkalis. It provides:

[Means for solving problems]

The inventors of the present invention have conducted extensive studies regarding the method of separating acids and alkalis from salts using ion-exchange membrane electrolysis, especially regarding anion-exchange membranes, which had been a problem in the past. Sarak discovered that anion exchange membranes exhibit extremely excellent properties. Through electrolytic operation using fluorine-based anion exchange membranes and fluorine-based cation exchange membranes with this special structure, aqueous solutions of a wide variety of salts can be produced. efficiently from
It was discovered that highly concentrated acids and alkalis can be separated and recovered, leading to the completion of the present invention.

The fluorine-based anion exchange membrane having a special structure used in the present invention has the following general formula: CF.

■ Mo C.F.

>Furthermore, the fluorine-based anion exchange membrane used in the present invention has a fourth
As the group containing a quaternary ammonium group, it is desirable to use a fluorine-based anion exchange membrane having the following general formula or a group containing a quaternary ammonium group of the following general formula.

Specific examples of these fluorine-based anion exchange membranes include polymer membranes having the following structures.

OF.

F, 0-CF ■ Oh.

OF.

F, 0-OF 0H.

Ko at.

■ 0F-OF.

■ sauce (3F.

■ Oh.

C.F.

F, 0-OF 0H.

C.F.

F, 0-OF OF.

F, 0-CF ■ Oh.

■ OF.

c'p-aF.

OF.

0H8 OF.

3O-CF 0H.

CF! F, O-(!F ■ 0M3 CHs OF.

F, 0-OF ■ 0F.

CHs　　　　C! H3 ■ C.F.

F, O- (!
! A material in the range of LOmeq/9/dry resin can be used, but preferably a material in the range of α5 meq/9/dry resin to 2.8 meq/9/dry resin is used.

If the exchange capacity is less than the above range, the resistance of the membrane will be high and the electrolytic voltage will rise, leading to an increase in electricity costs.If the exchange capacity exceeds the above range, problems such as swelling and collapse of the membrane will occur. This may disturb stable electrolysis operation.

The thickness of the fluorine-based anion exchange membrane used in the present invention is usually 4
It can be used in the range of 0μ to 500μ, but preferably 1
Those having a diameter in the range of 00μ to 300μ are used. moreover,
A reinforcing material can also be introduced into the fluorine-based anion exchange membrane used in the present invention in order to increase the strength of the membrane.

The fluorine-based anion exchange membrane with the above-mentioned special structure exhibits excellent heat resistance, acid resistance, and oxidation resistance. A wide variety of products can be produced by electrolysis using membranes and fluorine-based cation exchange membranes, which will be described later.

It becomes possible to efficiently separate (highly concentrated acids and alkalis) from aqueous solutions of various salts.

The fluorine-based cation exchange membrane used in the present invention is a conventionally known cation exchange membrane (for example, DuPont's Nafion
membrane).

Conventionally known electrode materials can be used as the anode and cathode of the electrolytic cell used in the present invention, but electrode materials that are inexpensive, exhibit low overvoltage, and have excellent corrosion resistance for the electrode reaction of the intended electrolytic process can be used. is selected as appropriate.

Such an electrode material is, for example, Ti as a cathode.

pt+ on the surface of a corrosion-resistant base material such as Ta, Zn, Nb, etc.
An anode coated with a platinum group metal and/or an oxide of a platinum group metal such as Fθ, Rh, etc. is used as a cathode.
Metals such as N1゜Cu, or their alloys, or substances that exhibit low overvoltage on their surfaces (e.g. Raney nickel, etc.)
A cathode coated with can be used.

In the ion exchange membrane electrolysis method of the present invention, the electrolytic cell usually consists of three chambers: an anode chamber, an intermediate chamber, and a cathode chamber. The structure is such that a cation exchange membrane is placed between the chambers. Furthermore, when processing a large amount of salt solution, it is also possible to implement an efficient electrolysis method using a laminated cell.

An aqueous solution of salt is supplied to the intermediate chamber, and this concentration depends on the degree of concentration of salt in the previous step, for example in waste liquid treatment, but it is usually supplied in the range of Q, 1 rno4/l to saturation concentration. I can do it.

Furthermore, the concentration of acid and alkali in the anode chamber and cathode chamber is reduced to 1 due to the excellent durability of the anion exchange membrane used in the present invention.
It is possible to increase the concentration to more than 6 moν, and usually 1 mo! It can be a high concentration region of 71 or more.

Furthermore, in the ion exchange membrane electrolysis method of the present invention, the electrolysis temperature can range from room temperature to 100°C, and the current density is
Usually, it can be carried out in the range of 5A/υ to 50A/υ.

[Effects of the present invention]

As mentioned above, the ion-exchange membrane electrolysis method using a fluorine-porous ion-exchange membrane with a special structure and a fluorine-based cation exchange membrane efficiently extracts high-concentration acid from aqueous solutions of various salts. , it becomes possible to separate alkali.

Although the method of the present invention can be used in various fields, it has extremely high industrial value, particularly in the field of waste liquid treatment.

〔Example〕

Examples will be described below, but the present invention is not limited thereto.

Example 1. Comparative Example 1 An electrolysis process was carried out to produce nitric acid and caustic soda from an aqueous sodium nitrate solution using an ion exchange membrane electrolysis method.

The anion exchange membrane has the following structure. (-ay, cFd-(aF, cFu- OF.

■ F, 0-OF 0H.

Fluorinated anion exchange membrane (exchange capacity 1.10 m)
eq/9 dry resin, film thickness 140μ), and the cation exchange membrane used was a Nafion membrane manufactured by DupOnt.

This electrolytic process is shown in FIG. The electrolytic cell has an anode chamber 1.
Intermediate room 3. This is a three-chamber electrolytic cell consisting of two cathode chambers.

The anode chamber 1 and the intermediate chamber 3 are separated by an anion exchange membrane 16 and an on-exchange membrane 17.

In the anode chamber liquid, H ions are generated by the oxygen gas generation reaction that is the anode reaction, and NO passes through the anion exchange membrane; nitric acid is generated by the ions, but water is supplied from the water supply line 5. This allows the concentration of nitric acid in the anolyte to be maintained constant. Similarly, in the catholyte, OH- ions generated by the hydrogen gas generation reaction, which is the cathode reaction, pass through the cation exchange membrane. Caustic soda is generated by Na+ ions, and by supplying water from the water supply line 10, the concentration of caustic soda in the catholyte can be maintained constant.

As an anode, an electrode coated with noble metal oxide on a T1 Expanded Metal base material was used, and as a cathode, an ExpandθdMθtal of 1 was used.The electrode area was a12mm, and the distance between the electrodes was 2cps. .

5 in the intermediate chamber with a current density of 50 A/υ and an electrolytic temperature of 70°C.
Supply an aqueous sodium nitrate solution of rno4/L to raise the nitric acid concentration in the anode chamber to 2 mo! An electrolytic test was conducted for one month with the concentration of the caustic soda aqueous solution in the 7's cathode chamber set to 3 mo171.

The electrolysis voltage is approximately 7.5V, and the current efficiency for nitric acid generation is approximately 63V.
The current efficiency for generating caustic soda was approximately 81 h.

On the other hand, as Comparative Example 1, a hydrocarbon-based anion exchange membrane was used instead of the fluorine-based anion exchange membrane of Example 1,
The rest was as in Example 1 (a similar electrolytic test was carried out, and the electrolytic voltage was 7 V initially, but as the electrolysis time progressed, the electrolytic voltage gradually increased to 12 V or more, and the current efficiency of nitric acid production was The current efficiency for caustic soda production was about 82%.

Example 2 As an anion exchange membrane, the following structure Moc F, a F, Mc Ft c F-)-CF.

F, C! -OF C.I.F.

an.

Fluorine-based anion exchange membrane (exchange capacity 1.00 m
An electrolytic process was carried out to produce sulfuric acid and caustic soda from an aqueous sodium sulfate solution using eq/'9 dry resin, film thickness 150μ.

An electrolytic test was conducted for one month under the same conditions as in Example 1, except that the fluorine-based anion exchange membrane and the type of salt were changed, and the electrolytic voltage was approximately 7 V, the current efficiency for sulfuric acid generation was approximately 62 cm, The current efficiency for caustic soda production was approximately 86 cm.

Reference Example Table 1 shows the results of a durability test of a fluorine-based anion exchange membrane and a hydrocarbon-based anion exchange membrane having a special structure used in the present invention.

The durability was evaluated by immersing the membrane in each solution for a certain period of time, and then measuring the electrical resistance in a 0.1 mol/l sodium chloride aqueous solution. And so.

As is clear from the table, Table 1 shows the special structure used in the present invention.

[Brief explanation of drawings]

FIG. 1 is a conceptual diagram showing an example of the electrolysis process of the present invention. 1. Anode chamber λ Cathode chamber 5 Middle open chamber East Nitric acid recovery tank & water supply line & oxygen gas extraction line 2 Oxygen gas tank a Nitric acid recovery line 9 Sodium hydroxide recovery tank 1r1. Water supply line 11, hydrogen gas extraction line 12, hydrogen gas tank 1 sodium pentahydroxide recovery line 14 sodium nitrate supply line 15, sodium nitrate discharge line 1 & anion exchange membrane 1z cation exchange membrane 1a circulation of sodium nitrate Tank patent applicant Toyo Soda Kogyo Co., Ltd. Figure 1

Claims (4)

[Claims] (1) An acid in the anode chamber and an alkaline aqueous solution in the cathode chamber are generated from an aqueous salt solution by an ion exchange membrane electrolysis method using an anion exchange membrane and a cation exchange membrane as diaphragms. In the method for separating acids and alkalis from aqueous salt solutions, a fluorine-based cation exchange membrane is used as the cation exchange membrane, and as the anion exchange membrane, the following general formula▲Mathematical formula, chemical formula, table, etc.▼ [X =F or CF_3 l=0 or an integer from 1 to 5 m=0 or 1 n=an integer from 1 to 5 p and q are positive numbers, and the ratio p/q is from 2 to 16. Y is a quaternary ammonium group] A method for separating acids and alkalis from an aqueous salt solution, characterized by using a fluorine-based anion exchange membrane made of a copolymer of repeating units represented by the following. (2) Groups containing quaternary ammonium groups in fluorine-based anion exchange membranes include the following general formula ▲ Numerical formulas, chemical formulas, tables, etc. ▼ [R^1, R^2, R^3 are lower alkyl groups (but ,
The method according to claim 1, which uses an anion exchange membrane represented by the following formula: Z is a halogen anion. (3) Groups containing quaternary ammonium groups in fluorine-based anion exchange membranes include the following general formula ▲ Numerical formulas, chemical formulas, tables, etc. ▼ [R^1, R^2, R^3 are lower alkyl groups (but ,
R^1 and R^2 may be combined to form a tetramethylene chain or a pentamethylene chain. ) R^4 is a hydrogen atom or a lower alkyl group Z is a halogen ion] The method according to claim 1, using an anion exchange membrane represented by the following formula. (4) As a group containing a quaternary ammonium group in a fluorine-based anion exchange membrane, there are the following general formula ▲ mathematical formula, chemical formula, table, etc. ▼ [R^1, R^2, R^3 are lower alkyl groups ( however,
R^1 and R^2 may be combined to form a tetramethylene chain or a pentamethylene chain. ) R^4 and R^5 are hydrogen atoms or lower alkyl group Z is
Halogen anion a is an integer of 3 to 7.] The method according to claim 1, using an anion exchange membrane represented by: