JPH0816275B2 - Method for separating acid and alkali from aqueous salt solution - Google Patents

Description

TECHNICAL FIELD The present invention relates to a method for separating an acid and an alkali from an aqueous salt solution by an ion exchange membrane electrolysis method,
More specifically, using a special fluorine-based anion exchange membrane and a three-chamber electrolytic cell using a fluorine-based cation exchange membrane as a diaphragm, a cation exchange membrane was placed on the anode side and sandwiched between both membranes. The present invention relates to a method for separating an acid and an alkali from an aqueous salt solution by an ion exchange membrane electrolysis method in which an aqueous salt solution is supplied to an intermediate chamber to generate an acid in the anode chamber and an alkali is generated in the cathode chamber.

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.

There are innumerable neutralization processes in various types of factories such as chemical factories, plating factories, and semiconductor factories, and the amount of salt discharged from them is huge. Discarding these salts untreated is an environmental problem. In particular, in the treatment of salt waste liquid containing harmful substances such as radioactive substances, such as nuclear power related facilities, it is necessary to completely treat it within the facility premises, and the harmful substances such as radioactive substances in the waste liquid are concentrated. At the same time, it is strongly desired to develop a treatment system capable of separating and recovering acid and alkali from the salt waste liquid and reusing them.

[Prior Art] Separation of salt from acid and alkali is impossible by a normal chemical reaction, and it is necessary to use an ion exchange resin or an ion exchange membrane. Compared to the ion exchange resin method, the ion exchange membrane electrolysis method, in principle, uses a large amount of salt with a simple operation.
It is attracting attention as a process that can be efficiently separated into acid and alkali. For example, in recent years, as a method for removing sulfur dioxide (SO ₂ ) in the flue gas of a thermal power plant, a wet method in which SO ₂ gas is absorbed using caustic soda is generally used. A method of separating and recovering sodium (Na ₂ SO ₄ ) into caustic soda and sulfuric acid by ion exchange membrane electrolysis has been proposed. This process uses a fluorinated cation exchange membrane (eg, Dupont's Nafion membrane) between the cathode chamber and the intermediate chamber that supplies the aqueous solution of Glauber's salt. It is a process of producing caustic soda in the cathode chamber and sulfuric acid in the anode chamber using a porous diaphragm, or a microporous diaphragm and a hydrocarbon-based anion exchange membrane. Although this process is an effective method for separating salt acid and alkali, it has some problems.

That is, when a microporous diaphragm is used between the intermediate chamber and the anode chamber, it is not possible to prevent the mixture of sodium sulfate into the sulfuric acid generated in the anode chamber, and the microporous diaphragm is used. Even when a hydrocarbon-based anion exchange membrane is combined, 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 that high-temperature electrolysis at 60 ° C or higher, or acids that have a stronger oxidizing power than sulfuric acid, such as nitric acid, hydrochloric acid, hydrofluoric acid, and chromic acid, are separated. However, the ion exchange membrane electrolysis method loses the economical efficiency of the salt / acid separation process.

For example, Japanese Patent Application Laid-Open No. 58-37596 proposes a process using an ion exchange membrane electrolysis method for a method for concentrating a nitrate-containing radioactive waste liquid produced in nuclear facilities.

This process is also a process in which a nitrate solution is supplied to the intermediate chamber and nitric acid is generated in the anode chamber and alkali hydroxide or ammonium hydroxide is generated in the cathode chamber. An exchange membrane, a fluorinated cation exchange membrane between the intermediate chamber and the cation exchange membrane (for example, Dupont
A three-chamber type electrolytic cell in which a Nafion membrane) is arranged is used.

In this process, as an economical electrolysis method,
It is stated that the concentration in the anode chamber and cathode chamber should be 0.5 M / or less. The reason for this is to maintain economical current efficiency. It is speculated that the durability of the film itself is also problematic.

Considering the efficiency of the whole process including the electrolysis efficiency,
In the method of separating salts from acids and alkalis, it is natural that it is desirable to separate and recover as much salt as possible into high concentrations of acid and alkali, but in a high temperature, high concentration nitric acid solution. The durability of the hydrocarbon-based anion exchange membrane is not satisfactory.

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

Therefore, if an attempt is made to carry out such a process using a conventional hydrocarbon-based anion exchange membrane in a high temperature, high concentration acid solution, frequent replacement of the anion exchange membrane is unavoidable. As a result, the economic efficiency of the process itself becomes poor.

Furthermore, when the salt is a chloride salt, chlorine gas is generated at the anode, but a hydrocarbon-based anion exchange membrane can be said to be completely effective against a strong oxidizing gas such as chlorine gas. The film does not exhibit such durability, and the film may collapse in a short period of about 2 weeks. Therefore, it is considered that the method of separating the acid and the alkali from the chloride salt by the ion exchange membrane electrolysis method cannot be practically realized.

As described above, the method for separating an acid and an alkali from an aqueous solution of a salt by the ion exchange membrane electrolysis method is a known technique and is extremely demanded for practical use as an industrial process. However, due to process constraints and 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] An object of the present invention is to solve the problems such as salt type, acid concentration, electrolysis temperature, etc. The present invention provides an ion exchange membrane electrolysis method that removes constraint conditions and efficiently separates and recovers an aqueous salt solution into high-concentration acids and alkalis.

[Means for Solving the Problems] The inventors of the present invention have found that the salt acid
Regarding the method of separating Lucari, the shadow
As a result of extensive studies on the ion exchange membrane, a special structure
Fluorine-based anion exchange membrane, which has a structure, has extremely excellent characteristics
We have found that a foot with this special structure
Use an anion exchange membrane and a fluorine cation exchange membrane
Efficient from aqueous solutions of various salts by electrolysis
And found that high concentrations of acid and alkali can be separated and recovered.
However, the present invention has been completed. In the present invention
What is a fluorine-based anion exchange membrane with a special structure used?
The following general formula[However, X is F or CF₃, l is 0 or an integer of 1 to 5,
m is 0 or 1, n is an integer from 1 to 5, p and q are positive numbers,
The ratio is 2 ~ 16, R₁, R₂, R₃Is a lower alkyl group (provided that R
₁And R₂Integrated into a tetramethylene chain, pentamethyle
Chains may be formed. ) R_FourIs a hydrogen atom or a lower alkyl group (provided that R₃And R_Four
Unite to form an ethylene chain and a trimethylene chain
Good. ) Z = Halogen anion, a is an integer of 2 to 10]
It means an anion exchange membrane.

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

The exchange capacity of the fluorine-based anion exchange membrane having a special structure used in the present invention is 0.16 meq / g · dry resin to 3.0 meq / g ·
A dry resin in the range of 0.5 meq / g.dry resin to 2.8 meq / g.dry resin is preferably used.

If the exchange capacity is less than the above range, the resistance of the membrane is high,
If the electrolysis voltage rises and the electric power cost rises, and the exchange capacity exceeds the above range, problems such as swelling and collapsing of the membrane occur, which hinders stable electrolysis operation.

The film thickness of the fluorine-based anion exchange membrane used in the present invention is usually
It can be used in the range of 40μ ~ 500μ, but preferably 100μ ~
Those in the range of 300μ are used. Further, in the fluorine-based anion exchange membrane used in the present invention, a reinforcing material can be introduced in order to increase the strength of the membrane.

The fluorine-based anion exchange membrane used in the present invention may be an anion exchange membrane having exchange groups uniformly present, or an anion exchange membrane having different exchange capacities on one surface and the other surface may be used. it can.

Such an anion exchange membrane is effective in suppressing the permeation of H ⁺ ions in the anion exchange membrane. Suppression of H ⁺ ion permeation through the anion exchange membrane causes the cation described below to increase the current efficiency in the method for separating acid and alkali from the aqueous solution of the salt of the present invention.

That is, in the ion exchange membrane electrolysis method of the present invention, the electrolytic cell is usually composed of three chambers, an anode chamber, an intermediate chamber and a cathode chamber, and an anion exchange membrane is arranged between the cathode chamber and the intermediate chamber. And a cation exchange membrane between the cathode chamber and the acid chamber,
It is an electrolysis process that produces an alkaline aqueous solution in the cathode chamber.

Generally, the anion exchange membrane has an 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 ⁺ from the anode chamber to the intermediate chamber.
Ions are allowed to easily pass through, and therefore, in such an electrolysis process, the anion exchange ratio of the anions in the anion exchange membrane is reduced, resulting in a reduction in current efficiency.

However, by using the fluorine-based anion exchange membranes having different exchange capacities on the one surface and the other surface as described above, H ⁺
It suppresses the permeation of ions, resulting in increased current efficiency.

The anion exchange membrane having different one surface and the other surface is used, for example, in a ratio of the exchange capacities of the one surface and the other surface of 1.1 to 6.0, preferably 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 capacity ratio exceeds the above range, the electric resistance of the membrane tends to increase.

Incidentally, the orientation of the fluorine-based anion exchange membranes having different exchange capacities on one surface and the other surface is a strong acid side, that is, in the ion exchange membrane electrolysis method of the present invention, a surface having a small exchange capacity is placed on the anode chamber side. However, it is preferable to direct the surface having a large exchange capacity toward the intermediate chamber side, and a large increase in current efficiency can be expected.

The above-mentioned fluorine-based anion exchange membrane with a special structure exhibits excellent heat resistance, acid resistance, and oxidation resistance. Furthermore, the fluorine-based anion exchange membrane with this special structure By an electrolysis operation using an exchange membrane and a fluorinated cation exchange membrane which will be described later, it becomes possible to efficiently separate high-concentration acids and alkalis from aqueous solutions of various salts.

As the fluorine-based cation exchange membrane used in the present invention, a conventionally known cation exchange membrane (for example, Nafion membrane manufactured by Dupont) can be used.

As the anode and cathode of the electrolytic cell used in the present invention, a conventionally known electrode material can be used, but with respect to the electrode reaction in the intended electrolytic process, it is inexpensive and exhibits low overvoltage,
Moreover, an electrode material having excellent corrosion resistance is appropriately selected.

Such an electrode material is, for example, Ti, Ta, Z for the anode.
An anode coated with a platinum group metal such as Pt, Ir, or Rh and / or an oxide of a platinum group metal is used on the surface of a corrosion-resistant substrate such as n, Nd, etc., and a cathode such as Fe, Ni, Cu, etc. is used. It is possible to use a metal, an alloy thereof, or a cathode having a surface thereof coated with a substance exhibiting a low overvoltage (for example, Raney nickel).

In the ion-exchange membrane electrolysis method of the present invention, as described above, in principle, a three-chamber type electrolytic cell can be used. Therefore, when a large amount of salt solution is processed, a stacked cell is used. It is also possible to implement a good electrolysis method.

Although an aqueous solution of salt is supplied to the intermediate chamber, this concentration is usually 0.1 mol / to a saturated concentration range, for example, in waste liquid treatment, although it depends on the degree of concentration of salt in the previous step. .

Furthermore, the concentrations of acid and alkali in the anode chamber and cathode chamber are
Due to the excellent durability of the anion exchange membrane used in the present invention,
It is possible to increase the concentration up to 0.6mol /,
Usually, it can be set to a high concentration region of 1 mol / or more.

Further, in the ion exchange membrane electrolysis method of the present invention, the electrolysis temperature can be from room temperature to 100 ° C., and the current density can be usually carried out in the range of 5 A / dm ² to 50 A / dm ² .

[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 having a special structure, from various aqueous solutions of various salts, It is possible to efficiently separate high-concentration 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.

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

Example 1, Comparative Example 1 An electrolytic process of producing nitric acid and caustic soda from an aqueous sodium sulfate solution was carried out by an ion exchange membrane electrolysis method.

The anion exchange membrane has the following structure Fluorine-based anion exchange membrane (exchange capacity 0.91meq / g dry resin, film thickness 175μ) is used, and as a cation exchange membrane, D
Upont's Nafion membrane was used.

This electrolysis process is shown in FIG. The electrolytic cell is the anode chamber 1,
It is a three-chamber type electrolytic cell consisting of an intermediate chamber 3 and a cathode chamber 2.

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

The anode chamber liquid, coming through the H ⁺ ions and an anion exchange membrane produced by the oxygen gas generation reaction which is an anode reaction ▲ NO ^- ₃ ▼ Although nitrate is produced by ion over the supply line 5 of the water By supplying water, the nitric acid concentration in the anolyte can be kept constant.

Similarly, in the catholyte, caustic soda is produced by OH ⁻ ions generated by the hydrogen gas generation reaction, which is a cathodic reaction, and Na ⁺ ions that pass through the cation exchange membrane. By supplying the solution, the concentration of caustic soda in the catholyte can be maintained constant.

As an anode, an electrode coated with a noble metal oxide on an expanded metal substrate of Ti is used, and as an anode, an expanded metal of Ni is used.
etal was used. The electrode area was 0.12 dm ² , and the distance between the positive and negative electrodes was 2 cm. With a current density of 30 A / dm ² and an electrolysis temperature of 70 ° C, 5 mol / sodium nitrate aqueous solution was supplied to the intermediate chamber,
An electrolytic test was carried out for one month with the concentration of nitric acid in the anode chamber being 2 mol / and the concentration of the caustic soda solution in the cathode chamber being 3 mol /.

Electrolysis voltage is about 6.0V, nitric acid production current efficiency is about 58%,
The current efficiency of caustic soda production was about 80%.

On the other hand, as Comparative Example 1, a hydrocarbon-based anion exchange membrane was used in place of the fluorine-based anion exchange membrane of Example 1, and the same electrolytic test as in Example 1 was conducted except that the electrolytic voltage was changed. Was initially 5.8V, but the electrolysis voltage gradually increased with the passage of electrolysis time to 12V or more, and the current efficiency for 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 capacity on one side was 0.62.
meq / g / dry resin, film thickness 50μ, exchange capacity of the other side is 0.9
Nitric acid and caustic soda are produced from an aqueous solution of sodium nitrate under the same electrolytic conditions as in Example 1, using a fluorine-based anion exchange membrane having the same structure as in Example 1 with 1 meq / g · dry resin and a film thickness of 125 μm. An electrolytic process was carried out.

As for the orientation of the anion exchange membrane, the surface with a small exchange capacity was directed toward the anode chamber side, and the surface with a large exchange capacity was directed toward the intermediate chamber side.

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

Example 3 As an anion exchange membrane, the exchange capacity on one side was 0.67.
meq / g / dry resin, film thickness 40μ, exchange capacity of the other side is 1.0
0meq / g ・ dry resin, film thickness 130μ, the following structure An electrolytic process for producing sulfuric acid and caustic soda was carried out from an aqueous solution of sodium sulfate using the fluorine-based anion exchange membrane shown in.

Regarding the orientation of the anion exchange membrane, the surface with a small exchange capacity was directed toward the anode chamber side, and the surface with a large exchange capacity was directed toward the intermediate chamber side.

A one-month electrolysis test was carried out under the same conditions as in Example 1 except that the fluorine-based anion exchange membrane and the type of salt were changed. The electrolysis voltage was about 6.8 V, and the current efficiency for sulfuric acid generation was about 80.
%, The current efficiency of caustic soda production was about 85%.

[Brief description of drawings]

FIG. 1 is a conceptual diagram showing an example of the electrolysis process of the present invention. 1 ... Anode chamber 2 ... Cathode chamber 3 ... Intermediate chamber 4 ... Nitric acid recovery tank 5 ... Water supply line 6 ... Oxygen gas extraction 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 …… Cation exchange membrane 18 …… Sodium nitrate circulation tank

Claims (3)

[Claims] 1. A three-chamber electrolytic cell using an anion exchange membrane and a cation exchange membrane as a diaphragm is used, an anion exchange membrane is arranged on the anode side, and a cation exchange membrane is arranged on the cathode side. In the ion exchange membrane electrolysis method in which an aqueous salt solution is supplied to the intermediate chamber sandwiched between the membranes to generate an acid in the anode chamber and an alkali in the cathode chamber, a fluorine-based cation exchange membrane is used as the cation exchange membrane. Used and as an anion exchange membrane, the following general formula [Where X is F or CF ₃ , l is 0 or an integer of 1 to 5,
m is 0 or 1, n is an integer from 1 to 5, p and q are positive numbers,
The ratio is 2 to 16, R ₁ , R ₂ and R ₃ are lower alkyl groups (provided that R
₁ and R ₂ may be integrated to form a tetramethylene chain or a pentamethylene chain. ) R ₄ is a hydrogen atom or a lower alkyl group (provided that the ethylene chain is R ₃ and R ₄ are integrated, may form a trimethylene chain) Z ^-. = Halogen anion, a is 2 to 10 integer ] A method for separating an acid or an alkali from an aqueous solution of a salt, which comprises using a fluorine-based anion exchange membrane composed of a copolymer of repeating units represented by 2. The method according to claim 1, wherein an anion exchange membrane having a uniform exchange capacity is used as the fluorine-based anion exchange membrane. 3. An anion exchange membrane having different exchange capacities on one side and the other side is used as the fluorine-based anion exchange membrane,
The method according to claim 1, wherein the surface having a small exchange capacity is arranged toward the anode chamber side and the surface having a large exchange capacity is arranged toward the intermediate chamber side.