JPS6347386A - Separation of acid and alkali from aqueous salt solution - Google Patents

Abstract

PURPOSE:To efficiently separate acid and alkali at high concn. from an aq. salt soln. independently of the kind of salt by carrying out electrolysis with an anion exchange membrane contg. fluorine and made of a polymer represented by a specified general formula and a cation exchange membrane contg. fluorine. CONSTITUTION:An electrolytic cell is divided into chambers with an anion exchange membrane and a cation exchange membrane as diaphragms and an aq. salt soln. is fed to the middle chamber and electrolyzed to produce an aq. acid soln. in the anode chamber and an aq. alkali soln. in the cathode chamber. When acid and alkali are separated by electrolysis with the ion exchange membranes 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 general 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, p/q=2-16, each of R1-R3 is lower alkyl, R4 is H or lower alkyl, Z(-) is a halogen anion and a is an integer of 2-10. Thus, restrictions on the kind of salt, electrolytic conditions, etc., can be 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 by ion exchange membrane electrolysis. The present invention relates to a method for separating acids and alkalis from an aqueous salt solution using an ion exchange membrane electrolysis method using an anion exchange membrane and a fluorine-based cation exchange membrane as a diaphragm.

Although the method of the present invention can be used in various fields, it presents an extremely effective method particularly 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 amount of salt discharged from these processes is huge in both types. Disposing of these salts without treatment poses environmental problems. In particular, when treating salt waste liquid that contains harmful substances such as radioactive substances, such as in nuclear power-related facilities, it is necessary to completely treat it on the premises of the facility, and it is necessary to concentrate the harmful substances such as radioactive substances in the waste liquid. 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 impossible by ordinary 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 for the efficient separation of major salts into acids and alkalis through simple operations.

For example, in recent years, nitrite gas (S0
The common method for removing 2) is a wet method that uses caustic soda to absorb SO2 gas, but the nitrate (Na5O4) produced in this process is separated into caustic soda and sulfuric acid using ion exchange membrane electrolysis. A method of recovery has been proposed. In this process, a fluorine-based cation exchange membrane (for example, DLlp
A microporous diaphragm or a microporous diaphragm and a hydrocarbon-based anion exchange membrane are used between the intermediate chamber and the anode chamber, and caustic soda, This process produces sulfuric acid in the FjA electrode 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 combined with a hydrocarbon-based anion exchange membrane, the durability of the hydrocarbon-based anion exchange membrane (heat resistance, acid resistance, oxidation resistance)
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 in which acids such as chromic acid are separated, and the economic efficiency of the process for separating acids and alkalis from salts using ion exchange S electrolysis is lost.

For example, in Japanese Patent Application Laid-Open No. 58-37596, nuclear power-related M
A process using ion exchange membrane electrolysis has been proposed for concentrating nitrate-containing radioactive waste generated in 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 (e.g., overpont
A three-chamber electrolytic cell equipped with a Narion membrane (manufactured by Co., Ltd.) is used.

In this process, it is stated that the concentration in the anode and cathode chambers should be 0.5M71 or less as an economical electrolysis method, 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 salt with a high concentration of acid and alkali as much as possible 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-I1 acid solution, a hydrocarbon-based anion exchange membrane will significantly increase its electrical resistance in a short period of one month or less, 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 chloride salt, in Yanggui,
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 of separating acids and alkalis from chloride salts using the ion exchange WAN method will not be realized in practice.

As mentioned above, the method for separating acids and alkalis from aqueous solutions of salts using ion-exchange membrane electrolysis is a well-known technique.
Furthermore, although there is an extremely high demand for its practical application as an industrial process, it has not yet been established as a satisfactory industrial process due to process constraints and many problems that need to be resolved. In the current situation.

[Problems to be Solved by the Present Invention] The purpose of the present invention is to solve the problem of conventional ion exchange wA! Type of salt, W1, which was a drawback of the method of separating acid and alkali from salt
The present invention provides an ion-exchange membrane electrolysis method that removes constraints such as concentration and electrolysis temperature, and efficiently separates and recovers salt aqueous solutions into highly concentrated acids and alkalis.

[Means for Solving the Problems] The present inventors have made extensive studies regarding the method for separating acids and alkalis from salts using ion exchange VA electrolysis, especially regarding anion exchange membranes, which have been a problem in the past. As a result, it was discovered that a fluorine-based anion exchange membrane with a special structure exhibits extremely excellent properties, and electrolytic operations using fluorine-based anion exchange membranes and fluorine-based cation exchange membranes with this special structure were performed. The inventors have discovered that highly concentrated acids and alkalis can be efficiently separated and recovered from aqueous solutions of a wide variety of salts, and have completed the present invention.

The fluorine-based anion exchange membrane having a special structure used in the present invention is expressed by the following general formula % [where X is F or CF, 1 is O or an integer from 1 to 5, 1m is O or 1. n is an integer from 1 to 5.

p and q are positive numbers, and their ratio is 2 to 16°R, R,
R3 is a lower alkyl group (However, R1 and R2 may be combined to form a tetramethylene chain or a pentamethylene chain.) R2 is a hydrogen atom or a lower alkyl group (However, R and R
4 may be combined to form an ethylene chain or a trimethylene chain. ) Z○=halogen anion, a is an integer from 2 to 10] refers to a fluorine-based anion exchange membrane made of a copolymer of repeating units represented by the following formula.

The anion exchange group of these fluorine-based anion exchange membranes can be exemplified by the following structural formula.

0 C)It-CHt (J13OCH,-C
H, -CH2CH3 OCH, -CH, CH, -CH.

The exchange capacity of the fluorine-based anion exchange membrane with a special structure used in the present invention is 0.16 + neQ/ri - dry resin ~
3. Qmeq/g - A range of dry resin can be used, but preferably 0.5ffleQ/! :
1 - Dry resin to 2.81eQ/III - 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 10
Those having a diameter in the range of 0μ to 300μ are used. Furthermore, a reinforcing material may be introduced into the fluorine-based anion exchange membrane used in the present invention in order to increase the strength of the membrane.

As the fluorine-based anion exchange membrane used in the present invention, an anion exchange membrane in which exchange groups are uniformly present can also be used.
Anion exchange membranes having different exchange membranes ω on one side and the other side can also be used.

Such an anion exchange membrane is effective in suppressing the permeation of H+ ions through the anion exchange membrane.

Suppression of the permeation of H + ions through the anion exchange membrane brings about an increase in current efficiency in the method for separating acids and alkalis from aqueous solutions of salts according to the present invention.

That is, in the ion-exchange WA electrolysis method of the present invention, the electrolytic cell usually consists of three chambers: an anode chamber, two intermediate chambers, and a cathode chamber. An anion exchange membrane is disposed between the anode chamber and the intermediate chamber, and A cation exchange membrane is placed between the anode chamber and the cathode chamber, and acid is placed in the anode chamber.
It is an electrolytic process that produces an alkaline aqueous solution in the cathode chamber.

In general, in the ion exchange membrane, the higher the acid concentration in the liquid in contact with the membrane, that is, in the ion exchange membrane electrolysis method of the present invention, the higher the acid concentration in the anode chamber, the more H +
It becomes easier for ions to pass through, and therefore, in such an electrolytic process, the transport number of anions in the anion exchange membrane decreases, resulting in a decrease in current efficiency.

However, by using a fluorine-based anion exchange membrane with different exchange membranes on one side and the other side, H+
Suppresses ion transmission and increases current efficiency.

For example, an anion exchange membrane with one side and the other side having different ratios of exchange vessels on one side and the other side is 1.1 to 6.0.
, preferably in the range of 1.3 to 4.0.

When the exchange capacity ratio is less than the above range, the effect of suppressing H+ ion permeation is insufficient, and when the exchange membrane amount ratio exceeds the above range, the electrical resistance of the membrane tends to increase.

Note that the direction of the fluorine-based anion exchange membrane, which has different exchange capacities on one side and the other side, is the strong acid side, that is, in the ion exchange membrane electrolysis method of the present invention, the side with a smaller exchange membrane amount is placed on the anode chamber side. It is preferable to face the side with a large amount of exchange membrane toward the intermediate chamber side, and a large increase in current efficiency can be expected.

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
II) can be used.

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 electrode materials include, for example, ■+, Ta as an anode.
, Zn, Nd, etc. on the surface of a corrosion-resistant base material, pt, ir,
An anode coated with a platinum group metal such as Rh and/or an oxide of a platinum group metal is used, and the cathode is made of Fe, Ni
A cathode made of a metal such as , Cu, or an alloy thereof, or whose surface is coated with a substance exhibiting a low overvoltage (eg, Raney nickel, etc.) can be used.

In the ion-exchange membrane electrolysis method of the present invention, as mentioned above, it can be configured with a three-chamber electrolytic cell in principle, but
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 salt solution is supplied to the intermediate chamber, and its concentration, for example in waste liquid treatment, depends on the degree of salt concentration in the previous step, but is usually 0.111101/l to a saturation concentration vA@
can be supplied with

Furthermore, the concentration of acid and alkali in the anode chamber and cathode chamber can be reduced to 0 due to the excellent durability of the anion exchange membrane used in the present invention.
．． It is possible to increase the concentration up to 6 mol/I or more, and it is usually possible to achieve a high concentration region of 1 mol/1 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 can usually be carried out at a range of 5 A/(jyd to 50 A/dyd)2i.

[Effects of the present invention] As described above, by the ion exchange membrane electrolysis method using a fluorine-based anion exchange membrane and a fluorine-based cation exchange membrane with a special structure, aqueous solutions of various kinds of salts can be It becomes possible to efficiently separate highly concentrated acids and alkalis.

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.

As the anion exchange membrane, the following gt construction +CF, -CF
2←CF2-CF) SOF.

F, C-CF " II 1 0 CH.

Fluorinated anion exchange III (exchange capacity 0, 9
1 meq/g dry resin, film thickness 175μ) was used, and the cation exchanger was Nafion from DuPont.
WA was used.

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

The cathode chamber 1 and the intermediate chamber 3 are separated by an anion exchange membrane 16, and the intermediate chamber 3 and the cathode chamber 2 are separated by a cation exchange membrane 17.
It is separated by

In the cathode chamber liquid, nitric acid is produced by H ions generated by the oxygen gas generation reaction, which is an anode reaction, and No''S ions passing through the anion exchange membrane, but water is supplied from the water supply line 5. By doing so, the nitric acid concentration in the anolyte can be maintained constant.

Similarly, in the catholyte, caustic soda is produced by OH- ions produced by the hydrogen gas generation reaction, which is the cathode reaction, and Na+ ions passing through the cation exchange membrane. By supplying the solution, the concentration of caustic soda in the catholyte can be maintained constant.

An electrode made of a Ti expanded metal base coated with a noble metal oxide was used as the anode, and a Ni expanded metal was used as the cathode. The electrode area is 0.126 mm each, and the distance between positive and negative electrodes is 2 era.
And so.

Current density 30A/d TIt, electrolysis temperature 70℃ in the middle chamber! Supplying a Mol/I sodium nitrate aqueous solution,
An electrolytic test was carried out for one month with the concentration of nitric acid in the cathode chamber being 2 mol/I and the concentration of the caustic soda aqueous solution in the cathode chamber being 3 mol/I.

The electrolysis voltage is approximately 6. OV, the current efficiency for nitric acid production is approximately 58
%, the current efficiency for caustic soda production was about 80%.

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,
An electrolytic test was conducted in the same manner as in Example 1, and the electrolytic voltage was initially 5.8 V, 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 , about 53%, and the current efficiency for caustic soda production was about 82%.

Example 2 As an anion exchange membrane, the exchange group a on one side is 0,
62 meqlo - Dry resin, film thickness 50μ, exchange capacity on the other side 0.91111eq/g ・Dry resin, film thickness 125μ, using a fluorine-based anion exchange membrane having the same structure as Example 1. An electrolytic process was carried out to produce nitric acid and caustic soda from an aqueous sodium nitrate solution under the same electrolytic conditions as in Example 1.

The anion exchange membrane was oriented such that the side with a smaller exchange capacity faced the anode chamber and the side with a larger exchange capacity faced the intermediate chamber.

As a result of the same electrolytic test as in Example 1, the electrolytic voltage was 6.
7V, the current efficiency for nitric acid production was about 81%, and the current efficiency for caustic soda production was about 86%.

Example 3 As an anion exchange membrane, one of the membranes has an exchange capacity of 0,
67 meq/c+ ・Dry resin, film thickness 40μ, exchange capacity on the other side is 1, OOmeqlo ・Dry resin,
f! An electrolytic process for producing sulfuric acid and caustic soda from an aqueous sodium sulfate solution was carried out using a fluorine-based anion exchange membrane having a J thickness of 130 μm and having the following structure: +CF2- CF2+-4CF2- CF +rt 3C-CF.

The anion exchange membrane was oriented such that the side with a smaller exchange capacity faced the anode air conditioning, and the side with a larger exchange capacity faced the intermediate chamber side.

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 6.8 V, and the current efficiency for sulfuric acid generation was approximately 8.
0%, and the current efficiency for caustic soda production was about 85%.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a conceptual diagram showing an example of the electrolysis process of the present invention. 1, Li electrode chamber 2, cathode chamber 3, intermediate chamber 4, nitric acid recovery tank 5, water supply line 6, oxygen gas handling line 7, oxygen gas tank 8, nitric acid recovery line 9, sodium hydroxide recovery Tank 10, water supply line 11, hydrogen gas extraction line 12, hydrogen gas tank 13, sodium hydroxide recovery line 14, sodium nitrate supply line 15, sodium nitrate discharge line 16, anion exchange membrane 17, l! i ion exchange membrane

Claims (3)

[Claims] (1) Using an ion exchange membrane electrolysis method using an anion exchange membrane and a cation exchange membrane as diaphragms, an acid and alkali aqueous solution are generated from an aqueous salt solution in an anode chamber and an alkali aqueous solution in a cathode chamber. In the separation method, a fluorine-based cation exchange membrane is used as the cation exchange membrane, and as the anion exchange membrane, the following general formula ▲ Numerical formula, chemical formula, table, etc. ▼ [However, X is F or CF_3, l is 0 or 1-5
m is an integer of 0 or 1, n is an integer of 1 to 5, p and q are positive numbers, and the ratio is 2 to 16, R_1, R_2,
R_3 is a lower alkyl group (However, 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 (However, R_
3 and R_4 may be combined to form an ethylene chain or a trimethylene chain. ) Z^■ = halogen anion, a is an integer of 2 to 10] Separation method. (2) The method according to claim 1, characterized in that an anion exchange membrane exhibiting a uniform exchange capacity is used as the fluorine-based anion exchange membrane. (3) As a fluorine-based anion exchange membrane, use an anion exchange membrane with different exchange capacities on one side and the other side, with the side with the smaller exchange capacity facing the anode chamber and the side with the larger exchange capacity facing the intermediate chamber. The method according to claim 1, characterized in that the method is arranged so as to face downward.