JPH08246178A - Electrochemical recovering method of salts and device therefor - Google Patents

Abstract

PURPOSE: To provide a method and device for electrochemically recovering salts containing no impurities more economically than a conventional salts recovering. CONSTITUTION: A gas diffusion electrode 27 is used as an anode of an electrolytic cell 21 separated into an anode chamber 23, an intermediate chamber 24 and a cathode chamber 26 by an anion exchange membrane 22 and a cation exchange membrane 25 and a salt solution supplied to the intermediate chamber 24 is electrolyzed and recovered while supplying gaseous hydrogen and steam to the anode. Since the anode chamber 23 different from an electrolytic cell using a conventional gas diffusion anode is constituted only of a gas chamber, the device is made compact.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method and an apparatus for electrochemically separating and recovering an acid and an alkali from a solution containing an inorganic salt or an organic salt such as industrial water and waste water.

[0002]

[Prior art and its problems] The water quality of factory effluent is strictly regulated as an environmental conservation measure, and the level of water quality control for inorganic salts is also increasing year by year to prevent it from causing eutrophication and pollution of rivers. It is becoming stricter, and dumping into the ocean is prohibited. Conventionally, as a method for recovering and removing inorganic salts and organic salts, biological treatment, physicochemical treatment (chelate adsorption, precipitation separation), ion exchange treatment, sludge concentration recovery, incineration, etc. have been carried out.

However, in consideration of future environmental problems, it is desirable to actively work on the realization of a grown-up system in which salt is recovered and a drug is reused. From this point of view, a technique of electrochemically separating a salt by using an ion exchange membrane and an electrode and recovering it as an acid and an alkali has been noted as a useful recovery method. FIG. 2 is a schematic view of a three-chamber type electrolytic cell used for recovering an acid and an alkali from a salt by a conventional electrodialysis. In this electrolytic system, the electrolytic cell 1 is divided into an anode chamber 4, an intermediate chamber 5 and a cathode chamber 6 by an anion exchange membrane 2 and a cation exchange membrane 3, and an inorganic or organic salt such as an aqueous solution of sodium sulfate is supplied to the intermediate chamber 5. Then, electrolysis proceeds. By the electrolysis, sulfate ions reach the anode chamber 4 through the anion exchange membrane 2 and react with hydrogen ions to be recovered as sulfuric acid. Further, sodium ions reach the cathode chamber 6 through the cation exchange membrane 3, react with hydroxyl ions and are recovered as sodium hydroxide. This electrolytic system has a disadvantage that the power consumption becomes large and an economical recovery operation cannot be performed.

FIG. 3 relates to the improvement of the electrolytic system of FIG. 2, in which the electrolytic cell 10 is composed of the bipolar membrane 7, the anion exchange membrane 8 and the cation exchange membrane 9 to form the anode chamber 11, the intermediate chamber 12 and the cathode chamber.
When electrolysis is carried out while being divided into 13 and supplying a sodium sulfate solution or the like to the intermediate chamber 12, sulfuric acid and sodium hydroxide are obtained as in the case of FIG. 2, but this is not electrolysis by electrodes, but a bipolar membrane. The splitting of water in the water produces protons and hydroxide ions, which results in the recovery of acids and alkalis. Although this method has a smaller power consumption than the electrolytic system shown in FIG. 2, it is difficult to recover at a high concentration and has a drawback that the current density is less than half that of the system shown in FIG. FIG. 4 shows a hydrogen anode as an anode in the electrolytic cell of FIG.
By using 14, an electrolytic cell in which the cell voltage can be further reduced by using the gas diffusion electrode as compared with the electrolytic cell using the gas generating electrode in FIG. 2 is shown. In this electrolytic cell, the anode chamber 4'is divided into a solution chamber 16 and a gas chamber 17 by the ion exchange membrane 15. Although this electrolyzer can be improved in terms of electric power consumption, it has a complicated structure and cannot be said to have a sufficiently low electric power consumption due to the fact that there are three liquid chambers. Further, there is a drawback that an acid having a high concentration cannot always be obtained due to the characteristics of the anion exchange membrane.

[0005]

SUMMARY OF THE INVENTION The present invention solves the above-mentioned problems of the prior art, that is, the drawbacks of high power consumption and easy generation of impurities, and recovers salts without producing impurities at a low cell voltage. It is an object of the present invention to provide a method and device.

[0006]

The present invention provides an intermediate chamber of a three-chamber type electrolytic cell in which an anion exchange membrane defines an anode chamber and an intermediate chamber, and a cation exchange membrane defines an intermediate chamber and a cathode chamber. In a method of supplying a solution containing an inorganic salt and / or an organic salt to the above and electrochemically recovering the salt as an acid and an alkali, a gas diffusion electrode is used as an anode and electrolysis is performed while supplying hydrogen gas to the anode chamber. The method is characterized in that the gas diffusion electrode is brought into close contact with the anion exchange membrane for electrolysis.

The present invention will be described in detail below. In the present invention, in acid / alkali recovery from inorganic salts and / or organic salts, a gas diffusion electrode is used as an anode and electrolysis is performed while supplying hydrogen gas to the anode to recover the acid and alkali. The electrolytic cell used in the present invention is of a three-chamber type, the anode chamber is partitioned from the intermediate chamber by the anion exchange membrane, and the cathode chamber is partitioned from the intermediate chamber by the cation exchange membrane. Only the gas chamber is used as the anode chamber, and the conventional solution chamber is not provided.
The gas diffusion electrode may be brought into direct contact with the anion exchange membrane that defines the anode chamber and the intermediate chamber, or may be placed at a short distance, but it is desirable to make contact with it for efficiency.

When electrolysis is carried out while supplying an inorganic salt such as sodium sulfate, sodium nitrate, sodium perchlorate or ammonium chloride or an organic salt such as sodium acetate or sodium citrate to the intermediate chamber, sulfate ion or acetate ion is obtained. Anions such as permeate through the anion exchange membrane to reach the anode chamber, react with the supplied hydrogen gas on the surface of the gas diffusion electrode, and are converted into corresponding acids, that is, sulfuric acid and acetic acid, and are recovered. In a normal anodic reaction, oxygen is generated and a by-product is generated by further oxidizing the generated acid, but in the present invention, the supplied hydrogen becomes a hydrogen ion and reacts with the anion to generate a corresponding acid. Therefore, the target acid can be recovered without producing a by-product. Note that it is generally preferable to supply water vapor together with the hydrogen gas to the anode chamber and recover the acid produced as an aqueous solution. Since the gas diffusion electrode has a porous structure, the acid dissolved in water vapor to form an aqueous solution is It can be easily taken out to the back side of the electrode, thereby suppressing an unnecessary rise in cell voltage.

This gas diffusion electrode preferably carries a noble metal catalyst such as silver, platinum and gold as it is or carries the noble metal on a conductive powder such as carbon or nickel to secure a gas passage. And a porous material having electrical conductivity, which is obtained by mixing and kneading a fluororesin such as polytetrafluoroethylene (PTFE) powder which is a binder for ensuring durability and water repellency as a gas diffusion electrode,
It is manufactured by coating, firing, spreading and supporting on a fibrous or mesh-like electrode substrate. Further, in the present invention, it is desirable to sufficiently form a hydrophilic region on the back surface of which the generated acid can easily escape. Porous materials such as carbon, zirconium, stainless steel, titanium, and ceramics are used as the material of the electrode substrate, and commercially available products such as fine wire mesh, fiber sintered body and celmet can be used.

It is preferable that the supported catalyst amount is 10 to 500 g / m ² , the gas diffusion electrode thickness is 0.1 to 5 mm, and the porosity is 30 to 70%. As the cathode catalyst, it is desirable to use a highly active substance such as silver, ruthenium oxide or platinum, and to form these on a porous material such as nickel or stainless steel to form the cathode. Power can be supplied to the gas diffusion electrode through a current collector having excellent acid resistance such as stainless steel, zirconium and carbon, and to the cathode directly or through a cathode current collector such as stainless steel or nickel.

As the ion exchange membrane for partitioning the electrode chamber, a membrane having various polymer resins as a skeleton and various ion exchange groups can be used. As the anion exchange membrane for partitioning the anode chamber and the intermediate chamber, a membrane such as a hydrocarbon resin or a fluororesin having a quaternary ammonium group as an exchange group can be used. Asahi Glass Co., Ltd. AAV, Tokuyama Co., Ltd. Commercially available products such as AMH and SF-34 manufactured by Tosoh Corporation can be used.
As a cation exchange membrane that partitions the cathode chamber and the intermediate chamber, a hydrocarbon-based or fluororesin-based membrane having a sulfonic acid group as an exchange group is available, and a fluororesin-based membrane is preferable from the viewpoint of durability, and is commercially available. Nafion (registered trademark) # 324, # 350, and # 427 manufactured by DuPont are excellent as products.

As described above, the anode and the anion exchange membrane and the cathode and the cation exchange membrane are preferably placed as close to each other as possible in order to reduce the cell voltage. The distance between the electrode and the ion exchange membrane is preferably adjusted in the range of 0 to 3 mm in consideration of the increase in resistance loss due to the generated bubbles.
In order to maintain the anion exchange membrane and the anode, and the cation exchange membrane and the cathode in close contact with each other, the water pressure in the intermediate chamber may be kept higher than the water pressure in the both electrode chambers. Preferred water pressure difference is 1-10 mH
(0.1 to 1 kg / cm ² ).

The intermediate chamber formed between the anion and cation ion exchange membranes should have a capacity such that the supplied aqueous solution of sodium sulfate or the like can smoothly flow. A spacer may be arranged between the two ion exchange membranes in order to prevent the flow of the aqueous solution from becoming non-uniform due to an excessively small capacity due to bending. The spacer is also effective when a water pressure difference cannot be generated between the intermediate chamber and the both electrode chambers, and pressure is applied between both current collectors in order to make the water flow in the intermediate chamber uniform and perform sufficient tightening. It is desirable (5 to 50 kgf / cm ² ).

The spacer is preferably a mesh structure having a resistance to weak acids and weak alkalis, such as polypropylene, polyethylene, or polyvinyl chloride, having an opening ratio of 30 to 80% and a mesh size of 10 to 200 mesh. It is used to maintain the intermembrane distance at about 0.5-10 mm. The electrolytic cell of the present invention is not limited to a mode in which only one pair of cation exchange membrane and anion exchange membrane is used. For example, as shown in FIG. 3, a bipolar membrane is used to form a plurality of anode chambers and intermediate chambers. And a mode of forming the cathode chamber.

An aqueous solution of sodium sulfate or the like is supplied to the intermediate chamber of the electrolytic cell thus constructed to recover the acid and alkali by electrolysis. The aqueous solution may show some acidity or alkalinity, and a concentration of about 0.5 to 5M is desirable. The electrolysis temperature may be determined depending on the heat resistance of the film or spacer, but is usually 40 to 60 ° C. The inorganic salt and organic salt aqueous solutions to be electrolyzed according to the present invention are often factory wastewater or the like, and in some cases, they are highly alkaline solutions or contain many polyvalent impurity cations. In these cases, it is desirable to carry out a pretreatment to reduce the cation content to about 1 ppm or less in advance to protect the ion exchange membrane.
The energization is carried out to such an extent that the current density becomes 10 to 40 A / dm ^2, and the energization causes 10 to 10 A / dm ² depending on the kind of salt supplied in the anode chamber.
About 25% sulfuric acid, hydrochloric acid, acetic acid, etc.
30% metal hydroxide, ammonium hydroxide, etc. are obtained,
The current efficiency is 50-90%. It is possible to control the concentration of the acid to 25% or more, but the current efficiency is significantly reduced, so it is desirable to recover the acid within the above range.

In the present invention, inorganic salts and organic salts in wastewater and the like can be recovered as acid and metal hydroxide or ammonium hydroxide, and the recovered material can be reused to indirectly reduce the wastewater treatment cost. Can be planned. In the conventional electrolysis method using a gas diffusion electrode as the anode, two chambers for gas and solution were required on both sides of the anode, which complicates the electrolytic cell, but in the present invention, the gas diffusion anode is adhered to the anion exchange membrane. Since it is installed in a single anode chamber, the electrolysis system can be simplified.

Next, an example of the electrochemical recovery device for salts according to the present invention will be described with reference to the accompanying drawings. FIG. 1 is a schematic cross-sectional view showing an example of the electrochemical recovery device for salts according to the present invention. The electrolytic cell 21 for recovering salts is a three-chamber type electrolytic cell in which the anode chamber 23 is partitioned from the intermediate chamber 24 by the anion exchange membrane 22 and the cathode chamber 26 is partitioned from the intermediate chamber 24 by the cation exchange membrane 25. It is configured. On the anode chamber 23 side of the anion exchange membrane 22, a gas diffusion cathode 27 prepared by kneading and firing carbon and PTFE is closely attached, and the gas diffusion electrode 27 is provided with a porous current collector 28. Are connected. On the cathode chamber 26 side of the anion exchange membrane 25, a porous cathode 29 made of nickel or stainless is closely attached.

When the intermediate chamber 24 is supplied with, for example, an aqueous sodium sulfate solution, the cathode chamber 26 is filled with a dilute sodium hydroxide aqueous solution, and the vapor and hydrogen gas are supplied to the anode chamber 23 in the vapor phase, electrolysis is performed. Sulfate ions in the intermediate chamber 24 permeate the anode chamber 23 through the anion exchange membrane 22 to form sulfuric acid molecules, which are dissolved in water vapor and taken out of the system as sulfuric acid to be recovered, while the intermediate chamber is The sodium ions of 24 permeate the cation exchange membrane 25, reach the cathode chamber 26, and react with hydroxide ions to produce sodium hydroxide, which is dissolved in the dilute aqueous solution of sodium hydroxide and becomes high. It is recovered as concentrated sodium hydroxide.

[0019]

EXAMPLES Next, examples of the electrochemical recovery method of salts according to the present invention will be described, but the examples do not limit the present invention.

Example 1 Graphite-treated carbon powder of 2 μm (manufactured by Tokai Carbon Co., Ltd.) was added to a PTFE aqueous dispersion to prepare C: PTF.
The mixture was mixed so that E = 1: 0.3 (weight ratio) and ultrasonically dispersed to prepare a coating liquid. Titanium micromesh material with a diameter of 49 mm and a plate thickness of 0.5 mm is used as the base metal, and this is 20%.
Etching was performed in boiling hydrochloric acid for 10 minutes, and pretreatment for increasing the surface area and roughening the surface was performed.

About 10 mg of the above-mentioned coating solution was applied to the entire surface of this substrate, then pre-dried at 60 ° C. for 30 minutes and then baked at 370 ° C. for 90 minutes to obtain a gas diffusion electrode substrate. Platinum was carried on both sides of this substrate by a thermal decomposition method to prepare a mesh-shaped gas diffusion electrode. Two of these gas diffusion electrodes were stacked to form a hydrogen anode, which was brought into close contact with an anion exchange membrane (A-200 manufactured by Asahi Kasei Corporation) as shown in FIG. 1, and hydrogen gas and water vapor were supplied from the back surface of the electrode. A nickel mesh with Raney nickel on the surface as a cathode, Nafion 32 manufactured by DuPont as a cation exchange membrane.
Using 4 each, the middle chamber liquid was 25% sodium sulfate aqueous solution, the catholyte was 20% sodium hydroxide aqueous solution, and the bath temperature was 70%.
An electrolytic test was conducted under conditions of a temperature of 30 ° C. and a current density of 30 A / dm ² . As a result, stable electrolysis was possible at a cell voltage of 3.8 V, and after 2 hours, 17 g of sulfuric acid (80% yield as pure sulfuric acid) was obtained, and the current efficiency of the catholyte sodium hydroxide aqueous solution was about 80%. There was a balance.

[0021]

[Comparative Example 1] A cation exchange membrane (Nafion 117) was brought into close contact with the surface of a gas diffusion electrode obtained by applying a platinum catalyst to a carbon cloth, and a hydrogen anode was constituted by using a platinum-plated zirconium micromesh as a current collector. Using this hydrogen anode, the conventional electrolytic cell shown in FIG. 4 was constructed in the same manner as in Example 1 except for the above. When the same test was performed using 15% sulfuric acid as the anolyte, 25% sodium sulfate aqueous solution as the intermediate chamber liquid, and 20% sodium hydroxide aqueous solution as the catholyte solution, the current density was 30 A / dm ²
The cell voltage at 4.2V was 4.2V. The current efficiency was 80% on both the cathode side and the anode side.

[0022]

Comparative Example 2 A cell voltage was 5.5 V when tested in the same manner as in Example 1 except that a dimensionally stable electrode was used as the anode and 15% sulfuric acid was used as the anolyte.

[0023]

According to the method of the present invention, an inorganic salt and an inorganic salt are contained in the intermediate chamber of a three-chamber type electrolytic cell in which the anode chamber and the intermediate chamber are partitioned by the anion exchange membrane, and the intermediate chamber and the cathode chamber are partitioned by the cation exchange membrane. In a method of supplying a solution containing an organic salt and / or electrochemically recovering the salt as an acid and an alkali, it is possible to use a gas diffusion electrode as an anode and perform electrolysis while supplying hydrogen gas to the anode chamber. This is a characteristic method.

In the method of the present invention, the gas diffusion electrode is used as the anode and the electrolytic recovery of the salts is performed while supplying the hydrogen gas and the water vapor. Therefore, the anions permeating from the intermediate chamber through the anion exchange membrane are used. Reacts with the supplied hydrogen gas to generate the corresponding acid and dissolves in the supplied steam to be recovered as a high-purity acid. In addition, compared with the case where an oxygen generating anode is used as the anode, the power consumption is reduced, economical salt recovery becomes possible, and the electrolytic cost can be further reduced indirectly by reusing the recovered material.

The apparatus for recovering salts according to the present invention is constructed by adhering a gas diffusion electrode as an anode to an anion exchange membrane in the electrolytic cell used in the above method. In conventional electrolysis using a gas diffusion electrode as an anode, two chambers, a gas chamber and a solution chamber, are required on both sides of the electrolysis cell, which complicates the electrolytic cell. A single anode chamber can be constructed by installing the device in close contact with the anion exchange membrane, and the device becomes compact. Further, as in the conventional case, the anode chamber, the intermediate chamber and the cathode chamber are surely partitioned by using the ion exchange membrane, so that high-purity acid, metal hydroxide and the like can be recovered.

[Brief description of drawings]

FIG. 1 is a schematic cross-sectional view showing an example of a device for electrochemically recovering salts according to the present invention.

FIG. 2 is a schematic cross-sectional view of a conventionally used three-chamber type electrolytic cell for recovering salt.

3 is a schematic cross-sectional view of an electrolytic cell using a conventional bipolar membrane which is an improvement of the electrolytic cell of FIG.

FIG. 4 is a schematic cross-sectional view of a conventional electrolytic cell obtained by improving the electrolytic cell of FIG. 2 using a gas diffusion electrode as an anode.

[Explanation of symbols]

21 ... Electrolyzer 22 ... Anion exchange membrane 23 ... Anode chamber 24 ... Intermediate chamber 25 ... Cation exchange membrane 26 ... Cathode chamber 27 ... Gas diffusion anode 28 ...・ Current collector 29 ・ ・ ・ Cathode

Claims (2)

[Claims] 1. An inorganic salt and / or an organic salt is contained in an intermediate chamber of a three-chamber type electrolytic cell in which an anion exchange membrane defines an anode chamber and an intermediate chamber, and a cation exchange membrane defines an intermediate chamber and a cathode chamber. A method of electrochemically recovering the salt as an acid and an alkali by supplying a solution containing the gas, using a gas diffusion electrode as an anode, and performing electrolysis while supplying hydrogen gas to the anode chamber. 2. An inorganic salt and / or an organic salt is added to an intermediate chamber of a three-chamber type electrolytic cell in which the anode chamber and the intermediate chamber are partitioned by an anion exchange membrane, and the intermediate chamber and the cathode chamber are partitioned by a cation exchange membrane. In an apparatus for electrochemically recovering the salt as an acid and an alkali by supplying a solution containing the gas diffusion electrode in the anode chamber in close contact with the anion exchange membrane, and supplying hydrogen gas in the anode chamber. While conducting electrolysis, an acid generated by the anodic reaction is recovered in the gas chamber side of the gas diffusion electrode.