JPH0739729A - Process for recovering acid and/or alkali from neutral salt - Google Patents

Abstract

PURPOSE:To recover acid and/or alkali from neutral salt at a low energy cost in a long period by using an electrodialysis tank in which a bipolar membrane, an anion exchange membrane and/or a cation exchange membrane are arranged and recovering acid and/or alkali from neutral salt. CONSTITUTION:A bipolar membrane comprising an inorganic ion exchanger preferably comprising the inorganic ion exchanger and a matrix polymer, interposed in the interface between a cation exchange layer and an anion exchange layer. Waste liquid is sent from a spinning bath 1 to a neutralization tank 3 through a line 2. In order to remove heavy metallic ions herein, sodium hydroxide recovered from an electrodialyzer 13 provided with the bipolar membrane is added to settle and separate heavy metal. Then purified waste liquid is sent to a electrodialyzer 8 and an acid sodium sulfate aqueous solution is added from the electrodialyzer 13. Waste liquid is desalted in a desalting chamber 9 and concentrated in a concentration chamber 10 and thereafter supplied to the salt chamber 14 of the electrodialyzer 13. DC is impressed and electrodialysis attended with water splitting is performed. Sulfuric acid and sodium hydroxide are recovered from liquid containing sodium sulfate being neutral salt.

Description

Detailed Description of the Invention

[0001]

FIELD OF THE INVENTION The present invention uses an electrodialysis water splitting method to convert neutral salts to acids and / or acids at low energy costs.
Or, it relates to a process for recovering alkali.

[0002]

2. Description of the Related Art When a current is applied with the anion exchange layer side of a bipolar membrane composed of an anion exchange layer and a cation exchange layer as the anode side and the cation exchange layer side as the cathode side, water splits. The dissociation of hydrogen ions and hydroxide ions is widely known since Frilette's report in 1956.

A bipolar membrane is useful because it has this ability, and it is known that a neutral salt can be recovered into an acid and an alkali by appropriately using an anion exchange membrane and / or a cation exchange membrane. . Considering the production costs of acids and alkalis, bipolar membranes must have a low voltage drop due to the bipolar membrane and at the same time have high water dissociation efficiency, and these performances must be exhibited for a long period of time. I have to.

Some reports have already been made on the recovery process of acids and alkalis from neutral salts by electrodialysis using a bipolar membrane. For example, a method for recovering an acid and an alkali from a used rayon spinning bath solution by an electrodialysis water split method using a bipolar membrane, a cation exchange membrane, and optionally an anion exchange membrane is disclosed in Japanese Examined Patent Publication No. 63-186.
No. 69 discloses a method for recovering an acid and an alkali from an SO ₂ absorbent, and US Pat. No. 4,552,635 discloses a method for recovering an acid and an alkali from an aqueous solution comprising a mixed acid containing hydrofluoric acid. It is disclosed in US Pat. No. 4,999,095.

However, in the conventional typical bipolar membrane, for example, a cation-exchange group is introduced into one side of a film based on a styrene-divinylbenzene copolymer by a treatment such as sulfonation, and the other side is introduced. It is a bipolar membrane produced by introducing an anion exchange group of a quaternary ammonium group (Japanese Patent Publication No. Sho 61-31860 and Japanese Patent Publication No. 63-95235), and also an anion exchange membrane and a cation produced in advance. It is a bipolar membrane manufactured by fusing an exchange membrane with heat and pressure (US Pat. No. 3,372,210).
1), and a bipolar film (JP-A-61-207444) joined by using polyvinylamine as an adhesive.

However, since these bipolar membranes have a cation exchange group and an anion exchange group in the same membrane, these groups having opposite charges intrude into each other with passage of time to pass an ionic bond. Form an electrically neutral layer,
There is a drawback that a large voltage drop occurs and the energy cost increases with time.

To avoid this, US Pat. No. 4,253
No. 900 and Japanese Patent Publication No. 60-35936 disclose a bipolar membrane in which an ion exchange resin having a high degree of crosslinking is introduced between a cation exchange layer and an anion exchange layer to prevent mutual penetration of groups having opposite charges. . Further, there is a method for producing a bipolar membrane by impregnating a water-soluble inorganic compound at the interface of a cation exchange membrane or an anion exchange membrane, or by impregnating it with an alkali and then pressing.
No. 35 and Japanese Patent Publication No. 3-505894.

However, these bipolar films also have the following drawbacks. That is, even when a highly cross-linked ion exchange resin is interposed between the cation exchange layer and the anion exchange layer, even if the cross-linking degree is high, the organic compound cannot prevent the mutual invasion of ion exchange groups, and the voltage over time after energization is increased. Causes an increase in descent. As a result, the energy cost for recovering the neutral salt will increase over time.

[0009]

DISCLOSURE OF THE INVENTION The present invention uses a novel bipolar film which has solved the above-mentioned increase in voltage over time, and thereby recovers neutral salts at a low energy cost for a long period of time. It is intended to provide a pulling process.

[0010]

The above object is characterized in that an inorganic ion exchanger, preferably a layer comprising an inorganic ion exchanger and a matrix polymer, is present at the interface between the cation exchange layer and the anion exchange layer. This is achieved by performing electrodialysis using a bipolar membrane that

First, the bipolar film used in the present invention will be described. As the inorganic ion exchanger, for example, aluminosilicate type inorganic ion exchanger, hydrous oxide type inorganic ion exchanger, acidic salt type inorganic ion exchanger, basic salt type inorganic ion exchanger, heteropolyacid type inorganic ion exchanger Etc. are used, and any of a cation exchanger, an anion exchanger or an amphoteric ion exchanger can be used. However, since the acid salt type inorganic ion exchanger has excellent acid resistance and alkali resistance as compared with other inorganic ion exchangers, it can exhibit stable performance over a long period of time and is preferably used.

Examples of the acidic salt type inorganic ion exchanger include zirconium phosphate, titanium phosphate, hydroxyapatite and the like. The metal ions of the acidic salt type inorganic ion exchanger include Zr, Ti, Sn, Ge, Hf, Ta,
Nb, Fe, Al, Ga, In, Th and the like are known. In addition to phosphoric acid, V, As, Nb,
Sb, Ta, Mo, Te, W, Se, Si, Cr and other oxygen acids are known and used.

Further, a crystalline acidic type inorganic ion exchanger is
Since it has a crystal structure, it has excellent acid resistance and alkali resistance, and is preferably used for the bipolar film used in the process of the present invention.

As the matrix polymer, hydrophilic and insoluble ones are preferable, and the water content in water at the bipolar membrane operating temperature: [{(wet weight-dry weight)
/ Dry weight}) × 100] is preferably 5% by weight or more. If the water content is smaller than this, the voltage drop of the bipolar membrane becomes large, which is not preferable. Preferably 40
It is preferably 0% by weight or less, more preferably 300% by weight or less. If the water content is excessively high, the adhesive strength is reduced and delamination is likely to occur, which is not preferable. Further, the matrix polymer preferably has no ion-exchangeable functional group. For example, polyvinyl acetate, polyvinyl alcohol,
Polyvinylpyrrolidone, polyacrylamide, polyvinyl methyl ether, polyethylene oxide, starch,
Cellulose or those obtained by insolubilizing them by a method such as heat treatment or crosslinking, a copolymer with another monomer, a blended polymer with another polymer, or the like can be used.

The weight ratio of the inorganic ion exchanger to the matrix polymer is from 10/90 to 90/10, preferably 3
It is used at 0/70 to 70/30, and the thickness of the interface layer composed of this is preferably 0.01 to 100 μm, particularly preferably 0.1 to 10 μm. If it is thinner than this, the inorganic ion exchanger will be eluted out of the membrane over time,
The potential drop of the bipolar film gradually increases, resulting in an increase in energy cost. On the other hand, if it is thinner than this, the adhesive strength between the anion exchange layer and the cation exchange layer is lowered, which is not preferable.

The particle size of the inorganic ion exchanger is preferably 1 μm or less, and if it is larger than this, the adhesive strength between the anion exchange layer and the cation exchange layer is lowered, and bubbles are apt to enter the bonding interface during bonding. It is not preferable. The particle size is particularly preferably 0.01 to 0.5 μm.

As the cation exchange layer constituting the bipolar membrane used in the process of the present invention, a cation exchange membrane which has a large hydrogen ion permeability generated in the bipolar membrane and is difficult to permeate anions can be used. Is a cation exchange membrane having a sulfonic acid group as an ion exchange group. As such a cation exchange membrane, an ion exchange membrane obtained by introducing a sulfonic acid group into a styrene / divinylbenzene copolymer, a styrene / butadiene copolymer,
Alternatively, a cation exchange membrane in which a sulfonic acid group is introduced into a material obtained by graft-polymerizing a monomer such as styrene onto a woven or non-woven fabric of polyolefin or a fluorine-containing polymer is used.

Further, the cation-exchange membrane formed of the perfluoro cation-exchangeable polymer composed of the repeating unit represented by the chemical formula 2 has an excellent selective permeation and is resistant to acid such as sulfuric acid, nitric acid and hydrofluoric acid. It was found that it is suitable for the cation exchange layer of the bipolar membrane used in the process of the present invention because of its excellent properties.

[0019]

[Chemical 2]

In Chemical formula 2, m is 0 or 1, and n is 1.
-5, x / y is 2-16, X is SO ₃ M or COOM,
M represents hydrogen, an alkali metal, an alkaline earth metal or an ammonium group.

Such a perfluoro cation exchange membrane has a high anion-excluding property and has a high water dissociation efficiency as a bipolar membrane. The thickness of the cation exchange membrane is usually in the range of 5 to 300 μm, but from the viewpoint of membrane resistance and strength, it is preferably in the range of 20 to 150 μm. The ion exchange capacity is 0.5 to 2.0 meq / g dry resin, particularly, 0.
Desirably, it is 8-1.5 meq / g dry resin.

The anion-exchange membrane constituting the bipolar membrane used in the process of the present invention has a high permeability of hydroxide ions generated in the bipolar membrane and a low permeability of cations. An anion exchange membrane is used.
For example, a film obtained by introducing a quaternary ammonium group as an anion exchange group into a copolymer of styrene and divinylbenzene, an anion exchange group or a monomer having a functional group capable of being converted into the group is an olefin-based or fluorine-containing polymer. The porous material, woven fabric, non-woven fabric, anion-exchange membrane obtained by graft polymerization of a film and the like can be used.

Of these, a woven fabric of polyolefin such as polypropylene or polyethylene is preferably used because of its excellent alkali resistance and chemical resistance, and the woven fabric is a copolymer of styrene and divinylbenzene, or Further, it is desirable to use an anion exchange membrane having a quaternary ammonium group in which a part of the copolymer obtained by adding vinylbenzyl chloride thereto is graft-polymerized with the above-mentioned polyolefin by high energy such as radiation.

The thickness of the anion exchange membrane is 5 to 300 μm.
In general, the range of 20 to 150 μm is preferably used from the viewpoint of film resistance and film strength. Ion exchange capacity is 0.5-4.0 meq
/ G dry resin, especially 0.8 to 3.0 meq / g dry resin is desirable.

The recovery process of the present invention will be described below with reference to the rayon spinning bath effluent, which is the preferred neutral salt of the present invention, as an example. The composition of the rayon spinning bath effluent generally has the following composition. Sulfuric acid: 8 to 12% by weight Sodium sulfate: 13 to 28% by weight Zinc sulfate: 0 to 2% by weight Additive: Surfactant

The rayon spinning bath effluent is sent from the spinning bath 1 through line 2 to the neutralization tank 3. In the neutralization tank 3, sodium hydroxide recovered in the bipolar membrane electrodialysis tank 13 is supplied from a line 18 in order to remove heavy metal ions (mainly zinc) contained in the waste liquid of the used rayon spinning bath. Heavy metals are precipitated and separated as metal hydroxides in the neutralization tank 3. The sludge is taken out of the system through the line 29, and the supernatant liquid is filtered by the filter 5 through the line 4 for further purification. Purified used spinning bath effluent is line 7
Then, for concentration, the solution is supplied to the electrodialysis tank 8, but the aqueous sodium acid sulfate solution drained from the bipolar membrane electrodialysis tank 13 is added in the midway line 23 to adjust the liquidity to the acidic side.

The electrodialysis tank 8 has a structure consisting of a desalting chamber 9 and a concentrating chamber 10 in which cation exchange membranes and anion exchange membranes are alternately combined. An electric current is applied and a dialysis operation is performed. The cation exchange membrane used is not particularly limited, but a general-purpose cation exchange membrane obtained by sulfonation of a copolymer of styrene and divinylbenzene is sufficient, but a perfluoro cation exchange membrane having no cross-linking structure is also usable. I do not care.

On the other hand, the anion exchange membrane is also not particularly limited, but a copolymer of styrene and vinylbenzyl halide may be used.
Although a quaternary ammonium-ized anion exchange membrane can be used, it is more preferable to use a weakly basic non-quaternary ammonium salt such as a pyridinium salt in which 4-vinylpyridine is used instead of vinylbenzyl halide as an anion. The anion exchange membrane used as the exchange group can obtain higher current efficiency. The purified rayon spinning solution supplied to the desalting chamber 9 of the electrodialysis tank 8 via the line 7 is concentrated in the concentrating chamber 10 up to 20% by weight to a saturated solubility, and then the salt chamber of the bipolar membrane electrodialysis tank 13 via the line 11. 14 are supplied. Part of the concentrated aqueous sodium sulfate solution is returned to the spinning bath via line 12.

The bipolar membrane electrodialysis tank 13 is composed of a salt chamber 14, a base chamber 15 and an acid chamber 16 by alternately combining a bipolar membrane, an anion exchange membrane and a cation exchange membrane, and at the end thereof. A direct current is applied by disposing an anode and a cathode, electrodialysis involving water splitting is performed, and sulfuric acid and sodium hydroxide are recovered from a rayon spinning solution containing sodium sulfate as a neutral salt.

As the anion exchange membrane and the cation exchange membrane constituting the bipolar membrane electrodialysis tank 13, those similar to the electrodialysis tank 8 are used. The rayon spinning bath effluent supplied to the salt chamber 14 is desalted, desalinated, recycled for concentration to the desalting chamber 9 of the electrodialysis tank 8 and concentrated. Water is supplied to the base chamber 15, and the hydroxide anion generated by the water splitting phenomenon at the interface between the anion exchange layer and the cation exchange layer forming the bipolar membrane and the sodium ion that has permeated from the salt chamber Are combined with each other to generate an aqueous solution of sodium hydroxide, which is discharged to the line 17. The concentration of the produced aqueous sodium hydroxide solution is approximately 5% by weight to 25% by weight.

A part of the drained sodium hydroxide aqueous solution is used as an alkali for neutralizing the neutralization layer through the line 18, and the other is supplied to the concentrator 19 and concentrated to 35% by weight.
Concentrated to ~ 48% by weight to become a product. Water is supplied to the acid chamber 16 as in the base chamber, but hydrogen ions generated by the water splitting phenomenon at the interface between the anion exchange layer and the cation exchange layer forming the bipolar membrane and permeate from the salt chamber. Sulfate ions are combined to form a sulfuric acid aqueous solution. The produced sulfuric acid is supplied to the concentrator 26 via the line 25. The sulfuric acid concentrated to the required concentration joins the purified concentrated sodium sulfate from the line 12 via the line 28 and is sent to the spinning bath 1.

[0032]

The mechanism by which the bipolar film of the present invention maintains a low voltage drop for a long period of time is presumed as follows.

Since the inorganic ion exchanger existing at the interface of the bipolar membrane of the present invention has a solid structure and, in some cases, a crystal structure, the ion exchange groups of the ion exchange membrane bonded to the inorganic ion exchanger layer cannot penetrate. , It is difficult to form a strong ionic bond and form a neutral layer. Further, since the ion exchange capacity per unit volume is larger than that of the ion exchange resin, it is presumed that the electrical resistance of the inorganic ion exchanger at the interface is small and the voltage drop of the bipolar membrane can be kept low for a long period of time. . Furthermore, by dispersing the inorganic ion exchanger in the matrix polymer, a sufficient amount of the inorganic ion exchanger can be retained in the interface region, and the inorganic ion exchanger can be introduced into the interface layer without impairing the lamination strength. That is why.

[0034]

EXAMPLES The present invention will be described below with reference to examples, but the present invention is not limited to the examples.

[Example 1] A viscose rayon spinning effluent having the following composition was neutralized with a 10% by weight aqueous sodium hydroxide solution, and the metal contained was made into a metal hydroxide for sedimentation separation. Then, the supernatant was filtered to obtain a 20 wt% sodium sulfate aqueous solution.

Viscose rayon spinning effluent composition Sulfuric acid: 10% by weight Sodium sulphate: 20% by weight Sulfuric acid sulphite: 1% by weight

Next, the filtered and purified aqueous sodium sulfate solution was concentrated to 25% by weight using an electrodialysis tank having the following specifications. Membrane effective energization area: 1.2 dm ² Number of units: 5 Liquid temperature: 40 ° C Current density: 4 A / dm ²

As the cation exchange membrane, styrene and divinylbenzene copolymer type sulfonic acid membrane (ion exchange capacity 3.3
meq / g dry resin, film thickness 140 μm), while styrene and divinylbenzene copolymer-based anion exchange membrane (ion exchange capacity 2.0 meq / g dry resin, film thickness 120 μm) were used as the anion exchange membrane. . The purified sodium sulfate aqueous solution concentrated to 25% by weight was then collected in a three-chamber bipolar membrane electrodialysis tank to collect sulfuric acid and sodium hydroxide.

The bipolar film used was one formed as follows. Crystalline phosphoric acid was formed on an anion exchange membrane (ion exchange capacity 3.0 meq / g dry resin, film thickness 120 μm) made of styrene / divinylbenzene copolymer and reinforced with polypropylene woven fabric and having quaternary ammonium groups. A dispersion of zirconium in a 10% by weight aqueous solution of polyvinyl alcohol was applied and dried to form a zirconium phosphate / polyvinyl alcohol layer (weight ratio 60/40) having a thickness of 5 μm. After that, CF ₂ = CF _{2
/ CF 2 = CFOCF 2 CF (} CF 3) OCF 2 CF 2
An ethanol solution of a perfluoro cation exchanger (ion exchange capacity: 1.1 meq / g dry resin) consisting of an O ₃ H copolymer was cast, dried at 150 ° C. for 15 minutes, and then a thickness of 30.
A cation exchange layer having a thickness of μm was formed to obtain a bipolar membrane.
In this example, the bipolar film thus formed was used.

On the other hand, as the anion exchange membrane used in the bipolar membrane electrodialysis tank, styrene and divinylbenzene copolymer system weak basic anion exchange membrane (ion exchange capacity 2.0 m
eq / g dry resin, film thickness 120 μm) was used. For the cation exchange membrane, CF ₂ = CF ₂ / CF ₂ CFOCF ₂ C
Perfluoro cation exchange membrane (ion exchange capacity of 0.9) composed of a copolymer of F (CF ₃ ) OCF ₂ CF ₂ SO ₃ H
1 meq / g dry resin, reinforced with PTFE woven cloth, thickness 140 μm) was used.

The bipolar membrane electrodialysis tank has a structure in which bipolar membranes, anion exchange membranes and cation exchange membranes are alternately arranged, and constitutes a three-chamber type electrodialysis tank having an acid chamber, a base chamber and a salt chamber. The specifications of this electrodialysis tank are as follows. Membrane effective energization area: 1.2 dm ² Number of units: 5 Liquid temperature: 55 ° C Current density: 10 A / dm ²

A concentrated 25 wt% sodium sulfate aqueous solution was supplied to the salt chamber of the bipolar membrane electrodialysis tank, and the acid chamber was supplied with pure water in an amount necessary to keep the generated sulfuric acid concentration at a required concentration. The base chamber is supplied with the necessary amount of pure water to keep the generated sodium hydroxide at the required concentration.
10% by weight of sulfuric acid and 20% by weight of sodium hydroxide were obtained. The bipolar membrane electrodialysis cell was operated continuously for 3 months. During this period, the voltage drop of the bipolar membrane was stable at 1.1 V, and the water dissociation efficiency was stable at 93%. The water dissociation efficiency here means the transport number of hydrogen ions and hydroxyl ions generated by water dissociation at the interface of the bipolar membrane. In addition, it showed a very low value of 1900 Kwt / t-NaOH for producing 1t of sodium hydroxide.

[Comparative Example 1] An operation was performed under the same conditions as in Example 1 except that the bipolar film produced as described below was used. Anion exchange membrane (ion exchange capacity 3.0, made of styrene / divinylbenzene copolymer, reinforced with polypropylene woven fabric and having quaternary ammonium groups)
meq / g dry resin, film thickness 120 μm), CF ₂ =
_{CF 2 / CF 2 = CFOCF 2} F (CF 3) OCF 2 C
A perfluoro cation exchange membrane (ion exchange capacity 1.1 meq / g dry resin, thickness 80 μm) composed of a F ₂ SO ₃ H copolymer was overlaid, and the temperature was set to 190 ° C. by a roll press machine equipped with a heating roll. A bipolar film was obtained by roll-pressing at a contact linear pressure of 70 kg / cm and heating and welding.

An electrodialysis operation was carried out in the same manner as in Example 1 using the above bipolar membrane in a bipolar membrane electrodialysis tank. As a result, the potential drop of the bipolar membrane increased with time, and the electric potential was reduced to 2 in a week. The voltage increased to 0.0 V and showed a further increasing tendency. The water dissociation efficiency was 93%. In addition, 2700 Kwt / t-NaO is required to generate 1t of sodium hydroxide.
The value was extremely high as H. This value tended to further increase after 1 week.

[0045]

Industrial Applicability The acid (sulfuric acid) and / or alkali (sodium hydroxide) can be recovered from a neutral salt, especially from a used spinning solution of viscose rayon at a low energy cost for a long period of time. .

[Brief description of drawings]

FIG. 1 is a representative example of the present invention, a process for recovering acids and alkalis from viscose rayon spinning effluent.

[Explanation of symbols]

 1: Spinning bath 3: Neutralization tank 5: Filtration device 8: Electrodialysis tank 9: Desalination room 10: Concentration room 14: Salt room 15: Basic room 16: Acid room 17: Bipolar membrane electrodialysis tank 19: Alkali concentration Vessel 26: Acid concentrator

Claims (12)

[Claims] 1. An electrodialysis tank comprising a bipolar membrane having an inorganic ion exchanger present at the interface between an anion exchange layer and a cation exchange layer, an anion exchange membrane, and / or a cation exchange membrane. Recovery process, which comprises recovering an acid and / or an alkali from a neutral salt. 2. The recovery process according to claim 1, wherein the inorganic ion exchanger is a crystalline inorganic ion exchanger. 3. The recovery process according to claim 1, wherein the inorganic ion exchanger is an acid salt type inorganic ion exchanger. 4. A bipolar membrane in which a layer composed of an inorganic ion exchanger and a matrix polymer is present at the interface between the anion exchange layer and the cation exchange layer is used.
2 or 3 recovery processes. 5. The recovery process according to claim 4, wherein the matrix polymer has a water content of 5% or more in water at the operating temperature of the bipolar membrane. 6. The recovery process according to claim 4, wherein the matrix polymer has no ion-exchangeable functional group. 7. The recovery process according to claim 1, wherein the cation exchange layer is composed of a polymer having a repeating unit represented by Chemical formula 1. [Chemical 1] In Chemical formula 1, m is 0 or 1, n is 1 to 5, x / y
Is 2 to 16, X is SO ₃ M or COOM, and M is hydrogen, an alkali metal, an alkaline earth metal or an ammonium group. 8. The anion-exchange layer of the bipolar membrane comprises styrene or a copolymer of styrene and divinylbenzene carried on a polyolefin film or cloth,
The recovery process according to any one of claims 1 to 7, which has a quaternary ammonium group as an anion exchange group. 9. The recovery process according to claim 1, wherein the liquid temperature in the electrodialysis tank is 50 ° C. or higher. 10. The recovery process according to claim 1, wherein the neutral salt is an alkali metal sulfate. 11. The recovery process of claim 10 wherein the alkali metal sulfate is included in the rayon spinning bath effluent. 12. The recovery process according to claim 11, wherein the rayon spinning effluent is neutralized, filtered, or pretreated with an ion exchange resin.