JPH07265864A - Fluorine-ion removing method - Google Patents

Abstract

PURPOSE:To selectively remove fluorine ions from aqueous liquid containing fluorine ions by increasing selective permeability of fluorine ions compared to a conventional anion exchange membrane. CONSTITUTION:When fluorine ions are removed from aqueous liquid containing fluorine ions by electric dialysis, a membrane having a negative charged thin film of 10Angstrom -5mum thickness on the membrane surface, e.g. a layer of polyvinyl sulfonic acid salt is used as the anion exchange membrane.

Description

Detailed Description of the Invention

[0001]

[Industrial field of application] In recent years, a large number of pharmaceuticals, agricultural chemicals, functional chemicals, polymers, and the like containing fluorine due to their properties have been manufactured, and are expected to be further expanded in the future. Hydrofluoric acid is used as one of the conventional metal surface treatment techniques. Thus, not only inorganic, organic, and polymeric compounds, but also fluorine-bonded compounds have been widely used. However, when synthesizing or using these compounds at the same time, a waste liquid containing fluorine ions in some form is naturally discharged in many cases. Fluorine ion is a harmful ion species, and it is necessary to treat it and discharge it in order to protect the environment.

[0002]

2. Description of the Related Art Various methods have been proposed as methods for removing harmful fluorine ions, but a typical example is insolubilizing the fluorine ions as calcium fluoride and precipitating it. , There is a way to remove it. Further, hydrofluoric acid as a waste acid used for the metal surface treatment is recovered by a diffusion dialysis method using an anion exchange membrane. In addition, a method of performing exchange adsorption on an anion exchange resin is also performed.

However, in the above method of insolubilizing calcium fluoride to precipitate it, the treatment of the precipitate must be considered even if it is desired to simply remove the fluorine ion. The diffusion dialysis method using an anion exchange membrane can be applied only when the acid concentration is high, and the driving force becomes small when the acid concentration becomes low. The removal by anion exchange resin is not limited to the case of fluorine ion alone, but in the case of a solution in which anions such as chlorine ion, nitrate ion, and sulfate ion coexist, the selectivity coefficient of fluorine ion to these other anions is remarkable. small. Therefore, a large amount of anion exchange resin must be used. In addition, when a salt solution containing a fluorine ion is treated by electrodialysis to remove the fluorine ion, the permeability of the fluorine ion to other anions is extremely low, and a large capacity electrodialysis tank must be used.
From this point of view, an anion exchange membrane that selectively permeates fluorine ions in electrodialysis or an electrodialysis method that selectively permeates fluorine ions is required.

[0004]

In view of the above, the present inventors have found that anions that selectively permeate fluorine ions do not significantly change their electrochemical properties as compared with conventional anion exchange membranes. As a result of intensive studies on the exchange membrane or the electrodialysis method, by forming a thin layer of negative charge on the surface layer portion of the anion exchange membrane, the electric resistance of the membrane hardly increased, and the current efficiency Surprisingly, it was found that fluorine ions selectively permeate through the membrane without causing a decrease, and the present invention has been completed.

That is, the present invention uses an anion exchange membrane having a thin layer of negative charge on the surface of the membrane as an anion exchange membrane when removing the fluorine ions from an aqueous solution containing fluorine ions by electrodialysis. This is a characteristic method for removing fluorine ions.

The aqueous solution to be treated in the present invention contains fluorine ions and other anions,
For example, it may contain an inorganic ion such as a sulfate ion, a nitrate ion, a chlorine ion, a bromine ion, a carbonate ion, and a bicarbonate ion and an organic ion depending on the case.

Particularly, a waste liquid generated from a step of synthesizing or using a fluorine compound is suitable as an aqueous solution to be treated. The salt concentration of the aqueous solution containing fluorine ions is not particularly limited, and may be in the range of several 100 ppm to several stipulated in terms of total salt concentration, and is selected according to the purpose. Permeability of fluorine ions is not particularly changed by the salt concentration of the aqueous solution.

The anion exchange membrane used in the present invention is an anion exchange membrane having a thin layer of negative charge on the membrane surface. The thin layer of negative charge may be present on both sides of the anion exchange membrane, or may be present on only one side.
If the thickness of the thin layer of negative charges is too thin, the effect of the present invention is difficult to obtain, and if it is too thick, it becomes a kind of bipolar ion exchange membrane, which increases the electric resistance of the membrane and causes hydrolysis of water. Therefore, the range is preferably 10 Å to 5 μm, and 20 to 3 is preferable.
More preferably, it is in the range of μm.

When the thin layer of negative charges existing on the surface of the anion exchange membrane used in the present invention has a certain thickness, a specific element existing in the thin layer with respect to the cross section of the anion exchange membrane by an X-ray microanalyzer. For example, it can be determined by observing the intensity of sulfur present, but it is difficult to measure the thickness when the thin layer is extremely thin. In such a case, the weight can be calculated by increasing the weight before and after forming a thin layer of negative charge on the surface of the anion exchange membrane, and the thickness can be estimated. Also, FT
Analytical means such as IR and ESCA can be used. Furthermore, the anion exchange membrane can be dipped in a solution of a substance having a cation exchange group that forms the membrane surface layer, and the concentration change of the substance having a cation exchange group in the solution before and after the immersion can be obtained. The thickness of the above-mentioned thin layer of negative charge is approximately in the range of 10 ⁻⁷ to 10 ⁻² g / cm ² when converted into the weight of the thin layer of negative charge per unit surface area of the anion exchange membrane.

The thin layer formed on the surface of the anion exchange membrane used in the present invention has a negative charge. The negative charge is usually imparted by the introduction of cation exchange groups.
Examples of the cation exchange group include a sulfonic acid group, a carboxylic acid group, a phosphoric acid group, a phosphorous acid group, a phenolic hydroxyl group, a sulfonic acid ester group, a phosphoric acid ester group, a thiol group, and a tertiary perfluoroalcohol group. Etc. can be mentioned.

The amount of the negative charges possessed by the thin layer is not particularly limited, but 5.0 × 10 ^{−10 to} 5 × 10 ⁻⁵ equivalents / equivalent to improve the permeability of fluorine ions. It is preferably in the range of cm ² .

The state of existence of the thin layer of negative charges on the surface of the anion exchange membrane is not particularly limited, but the following states (1) to (3) are preferable.

(1) Adhesion, ion exchange, or adsorption on the film surface.

(2) Although not chemically bonded to the membrane body,
It is entangled on the film surface and has a three-dimensional structure.

(3) It is chemically bonded to the film body.

The general method for producing the anion exchange membrane of the present invention will be described below. A conventionally known anion exchange membrane is used as the anion exchange membrane (hereinafter, also simply referred to as untreated anion exchange membrane) which is a base of the anion exchange membrane having a thin layer of negative charge on the membrane surface of the present invention. To be For example, various untreated anion exchange membranes are appropriately used depending on the type of polymerization system, condensation system, homogeneous system, heterogeneous system, etc., the manufacturing method, and the presence or absence of a reinforcing material. Further, as the untreated anion exchange membrane, a polymer membrane material into which an anion exchange group can be easily introduced is used, and an anion exchange group is introduced into the polymer membrane material.

In order to obtain an anion exchange membrane having a thin layer of negative charge on the surface of the membrane used in the present invention, a substance having a cation exchange group is present on the surface of the untreated anion exchange membrane. Examples of the substance having a cation exchange group include condensation products of phenolsulfonic acid and formalin, polystyrenesulfonic acid, polyvinylsulfonic acid, polyvinyl alcohol sulfate, polyacrylic acid, polymethacrylic acid and salts thereof. . It is preferable that these substances having a cation exchange group have a molecular weight of 300 or more from the viewpoint of easy presence on the surface of the untreated anion exchange membrane.

In order to adjust the ion exchange capacity of the substance having a cation exchange group, a substance which is copolymerized or cocondensed with a substance not having an ion exchange group may be used. A substance having a partially crosslinked structure formed by a crosslinkable substance such as divinylbenzene or phenol may be used.

The method of allowing the above-mentioned substance having a cation exchange group to exist on the substantial surface of the untreated anion exchange membrane will be described below according to the bonding forms (1) to (3). Become.

The substance having a cation exchange group may be attached, ion-exchanged or adsorbed on the surface of the untreated anion exchange membrane by dipping, coating or spraying, and further by assembling it in a dialysis tank and then attaching it by liquid passage. , Ion exchange, or adsorption may be performed. The substance having a cation exchange group may be used in the state of an aqueous solution or may be used by dissolving it in an organic solvent. In the case of an insoluble substance such as a crosslinked polymer, it may be used as a suspension. Further, in the solution or suspension of the substance having a cation exchange group, an inorganic electrolyte or other substance which does not harm may be present. The conditions of adhesion, ion exchange, and adsorption are temperature, time, concentration, solution,
The pH and the like may be appropriately selected according to the properties of the substance having a cation exchange group.

Next, although a substance having a cation exchange group is not chemically bonded to the untreated anion exchange membrane body, a method for forming a three-dimensional structure by entangled with the membrane body on the membrane surface is provided. It is as follows. Has a cation exchange group,
And a substance having a reactive functional group, for example, first, second, third
Immersion and application of substances such as amino groups such as primary amines, hydroxyl groups, halogen atoms, epoxy groups, carbonyl groups, ethylenic or acetylenic unsaturated bonds, phenol nuclei in which the condensable position is not substituted, and aldehyde groups After being attached to the surface of the untreated anion exchange membrane by the aforementioned method such as, a substance having a functional group capable of reacting with these, for example,
Aldehydes such as formaldehyde; polyhalogenated alkyls such as dibromobutane; haloepoxy compounds such as epichlorohydrin; polyepoxy compounds; polyamines such as polyethyleneimine; polycarboxylic acids such as polyacrylic acid. Further, the unsaturated bond attached to the surface of the untreated anion exchange membrane is subjected to radical polymerization, irradiation to initiate ionic polymerization, or
Alternatively, a three-dimensional structure can be formed on the film surface by acting a peroxide, a Lewis acid, or the like.

Further, a method of chemically bonding a substance having a cation exchange group to the untreated anion exchange membrane body is a method of reacting a substance having a cation exchange group and a reactive functional group with the membrane body, In addition, when forming the anion exchange membrane, a surface layer having a cation exchange group or capable of being converted into a cation exchange group is prepared, and after introducing the anion exchange group into the inside, the surface layer is annealed. There is a method of converting into an ion exchange group. For example, a method such as sulfonation of a styrene-divinylbenzene-based film only on the surface and then chloromethylation and amination of the inside thereof is an example.

By using such a special anion exchange membrane, for example, the permeability of fluorine ions to chlorine ions is significantly improved, and the permeability of coexisting anions, such as sulfate ions, is also significantly improved. In this case, sulfate ion becomes impermeable. At this time, in the electrodialysis, the fluorine ion selectively permeates through the membrane, so the concentration of the fluorine ion increases in the concentrated solution, and if calcium ion is contained in the concentrated solution, precipitation of calcium fluoride may occur. There is. In order to prevent the precipitation of calcium fluoride, a method of suppressing the permeation of calcium ions into the concentrated solution is suitable. Therefore, as a cation exchange membrane, sodium is used against polyvalent ions such as calcium ions. It is preferable to use a monovalent cation selective permeable membrane that selectively permeates monovalent ions such as ions.

The above-mentioned anion exchange membrane having a thin layer of negative charge is generally used by being arranged such that the side having the thin layer is the desalination chamber side.

In the present invention, in electrodialyzing an aqueous solution containing fluorine ions, a known electrodialysis cell in which a cation exchange membrane and the above-mentioned anion exchange membrane having a thin layer of negative charge are alternately arranged is used. It can be preferably used.
That is, either a clamp type or a water tank type electrodialysis tank can be used, and the electrodialysis tank having a limiting current density as high as possible is operated by a known method. It is also used for collecting and concentrating fluorine ions in the anode chamber by placing only an anion exchange membrane between the anode and cathode, which can be regarded as a type of electrodialysis, and flowing a solution containing fluorine into the cathode chamber. As the electrodialysis tank (electrolysis tank) in this case, a conventionally known one can be used without any limitation.

As the cation exchange membrane used for electrodialysis, a conventionally known cation exchange membrane is used. For example, various cation exchange membranes are appropriately used depending on the type of polymerization system, condensation system, homogeneous system, heterogeneous system, etc., the manufacturing method, and the presence or absence of a reinforcing material. Further, as the cation exchange membrane, a polymer membrane material into which a cation exchange group can be easily introduced is used, and the one having a cation exchange group introduced therein is also used. As the cation exchange membrane, it is preferable to use a monovalent cation selective permeable membrane as described above.

[0027]

[Function] Originally, since fluorine ions are extremely difficult to be ion-exchanged and adsorbed on anion-exchange resins and anion-exchange membranes, they are adsorbed and removed by anion-exchange resins and removed by electrodialysis after passing through anion-exchange membranes. Is extremely difficult. However, by forming a thin layer of negative charge on the surface of the anion-exchange membrane, fluorine ions are remarkably easily permeated through the membrane. Although this effect is still unclear, it is considered that the electrostatic repulsion of negative charges existing on the film surface to each anion species is different. For example, fluoride ion has a large hydration ion radius due to its strong hydration power with respect to chlorine ion, and the repulsion due to the negative charge on the membrane surface is smaller than that of chlorine ion, so that it easily permeates anion exchange membranes. It is supposed to be.

[0028]

[Effect] Compared with conventional anion exchange membranes, the permeability of fluorine ions is increased, and as a result, it becomes possible to treat a solution containing more fluorine ions in an electrodialysis tank having the same membrane area. The industrial significance is extremely large.

[0029]

EXAMPLES The contents of the present invention will be specifically described by the following examples, but the present invention is not limited to these examples.

The fluorine ion permeability was measured by a four-chamber electrodialysis cell having silver-silver chloride electrodes at both ends as shown in FIG. The anode chamber 1 and the cathode chamber 4 were filled with a sodium chloride solution so as to have the same sodium ion concentration as the measurement solution contained in the desalting chamber 2 and the concentrating chamber 3. That is, when the desalting chamber 2 and the concentrating chamber 3 are filled with a 1: 1 mixed aqueous solution of 0.04N sodium chloride and 0.04N sodium fluoride, both electrode chambers are filled with a 0.08N sodium chloride aqueous solution. Satisfied. The desalting chamber 2 and the concentrating chamber 3 were filled with a 1: 1 mixed aqueous solution of sodium chloride and sodium fluoride or a 1: 1 mixed aqueous solution of sodium fluoride and sodium sulfate.

C is a cation exchange membrane (4 cm × 5 cm) used as a diaphragm, and A is an anion exchange membrane (2 cm × 5 c).
m). The current density was changed in proportion to the concentration. After electrodialysis, the concentrations of fluorine ion, chlorine ion, and sulfate ion were analyzed by ion chromatography, and the permeation equivalent ratio of fluorine ion to chlorine ion permeating the anion-exchange membrane A and permeation equivalence ratio of fluorine ion to sulfate ion were determined. I asked.

The amount of the substance having a cation exchange group attached to the anion exchange membrane was determined from the change in concentration before and after the membrane was immersed in the solution of the substance. The quantification was carried out using an ultraviolet absorption spectrum.

Example 1 NEOSEPTA CM-2 (trade name, manufactured by Tokuyama Soda Co., Ltd.) was used as a cation exchange membrane. Anion exchange membrane NEOSEPTA AM-
1 (trade name, manufactured by Tokuyama Soda Co., Ltd.) has a molecular weight of about 10,0.
It was immersed in a 1000 ppm aqueous solution of polystyrene sulfone of 00 at 25 ° C. for 17 hours and then washed with water to prepare an anion exchange membrane having a thin layer of polystyrene sulfonic acid on both sides of the membrane. These ion-exchange membranes were installed in the four-chamber electrodialysis tank shown in FIG. 1, and a 1: 1 mixed aqueous solution of sodium chloride and sodium fluoride with a concentration (concentration is sodium ion concentration) shown in Table 1 was used as a measurement solution. The permeability of fluorine ions to chlorine ions was measured.

[0034]

[Table 1]

The permeation ratio indicates the number of permeation equivalents of fluorine ions when one equivalent of chlorine ions permeates the membrane.

Further, when the permeation ratio of fluorine ions to sulfate ions was measured at a concentration of 0.04N using this membrane, the permeation ratio of an ordinary anion exchange membrane was 0.38.
However, it was 1.8 in the electrodialysis method of the present invention.

The amount of polystyrene sulfonic acid adhering to the surface of this membrane was 0.05 × 10 ^-5 per unit area.
It was g / cm ² , which corresponds to a thin layer thickness of 12 Å. The charge amount of the thin layer of polystyrene sulfonic acid was 5.5 × 10 ^{-1 0} equivalents / cm ^2.

Next, the membrane was equilibrated with 0.5N saline and the electric resistance was measured at 25.0 ° C. by 1000 cycles of alternating current. As a result, it was 1.7 Ω · cm ² , and the untreated anion exchange membrane was It was 1.6 Ω · cm ² .

Example 2 Using the same electrodialysis tank as in Example 1, NEOSEPTA CM-2 (trade name, manufactured by Tokuyama Soda Co., Ltd.) was used as a cation exchange membrane. NEOSEPTA AM-2 (trade name, manufactured by Tokuyama Soda Co., Ltd.) was used as the anion exchange membrane, and this membrane was used at 10,000 p of sodium naphthalenesulfonate and formalin condensate.
It was immersed in a pm aqueous solution at 50 ° C. for 10 hours to adhere the condensate to both surfaces of the film. The amount of the above condensate attached to the film was 8.3.
× presence of sulfur was observed from 10 ^-5 g / cm ² a and X-ray was measured the distribution of sulfur microanalyzer film surface up to the about 3 [mu] m. The charge of the thin layer of this film was 3 × 10 ⁻⁷ equivalent / cm ² . 0.25 of this membrane
In electrodialysis of a mixed solution of normal sodium chloride and 0.25 normal sodium fluoride, the membrane permeation equivalent ratio of fluorine ion to chlorine ion was 0.35, while on the other hand, in the untreated anion exchange membrane not subjected to the above treatment. The membrane permeation equivalent ratio of fluorine ions to chlorine ions was 0.013. Further, the ratio of the permeation equivalent of sulfate ion to fluorine ion was 11.7 in the case of the electrodialysis method of the present invention, and was 0.33 in the case of the usual untreated anion exchange membrane.

Further, the electric resistance of the film was measured in 0.5N saline and found to be 1.7 Ω · cm ² . On the other hand, the untreated anion exchange membrane had a resistance of 1.8 Ω · cm ² .

Example 3 A vinylpyridine-based anion exchange membrane NEOSEPTA AF-4T (trade name, manufactured by Tokuyama Soda Co., Ltd.) was used as the anion exchange membrane. This membrane was made into a chlorine ion type and immersed in a 5% aqueous solution of sodium styrene sulfonate for 3 hours to allow styrene sulfonic acid to be ion-exchanged and adsorbed on the surface layers on both sides of the membrane, and then 2% sodium persulfate and 2% sodium sulfite were contained. The membrane surface was polymerized with styrene sulfonic acid by immersing it in an aqueous solution for 8 hours. Using this membrane, the membrane permeation equivalent ratio of fluorine ion to chlorine ion and the membrane permeation equivalent ratio of fluorine ion to sulfate ion were measured in the same manner as in Example 2, and were 0.42 and 12.5, respectively. . On the other hand, the respective values of the untreated anion exchange membrane are 0.021 and 0.32.
Met.

When the distribution of sulfur was measured on the cross section of the obtained film with an X-ray microanalyzer, the presence of sulfur was recognized at a thickness of about 4 μm from the film surface. Furthermore, the electric charge of the thin layer of this film is 1.6 × 10 ⁻⁶ equivalent / cm 2.
^{Was 2} .

[0043] Further, 2.4Ω · cm ² and 2.3Ω · cm ² met the surface treated membrane and untreated membrane was the electrical resistance of the film was measured at 25.0 ° C. in 0.5N saline It was

[Brief description of drawings]

FIG. 1 is a schematic diagram showing the structure of an electrodialysis tank used in an embodiment of the present invention.

[Explanation of symbols]

 C: Cation exchange membrane A: Anion exchange membrane 1: Anode chamber 2: Desalination chamber 3: Concentration chamber 4: Cathode chamber

─────────────────────────────────────────────────── ─── Continuation of the front page (51) Int.Cl. ⁶ Identification code Internal reference number FI technical display location C02F 1/58 ZAB M

Claims (1)

[Claims] 1. When removing fluorine ions from an aqueous solution containing fluorine ions by electrodialysis, an anion exchange membrane having a thin layer of negative charge on the membrane surface is used as the anion exchange membrane. Ion removal method.