JP4975402B2 - Electrolysis method - Google Patents

Description

  The present invention relates to a method for electrolyzing an electrolytic solution containing impurities, and more particularly to a method for electrolyzing an electrolytic solution containing impurities using a three-dimensional electrode.

In an industrial electrolytic cell such as a salt electrolytic cell, a leaf spring or a metal mesh may be used to smoothly supply power from the electrode current collector to the electrode.
However, since the leaf spring and the metal net are rigid bodies, the ion exchange membrane may be damaged, the deformation rate may be small, and sufficient electrical connection may not be obtained.
In order to eliminate such drawbacks, an electrolytic cell is disclosed in which a metallic coil is mounted between a cathode and a cathode end plate in place of a metal mesh, and the cathode is uniformly pressed in the direction of the diaphragm and the members are brought into close contact with each other. (Patent Document 1).
Japanese Patent Publication No. 63-53272 (FIGS. 1 to 8)

This technique is characterized in that the metallic coil is not used in such a manner that the electrode is pressed in the direction of the ion exchange membrane, but is used as the electrode itself. This electrode is maintained in its form for a long time due to its high strength and toughness, so that the ion exchange membrane or the like is not mechanically damaged, and may be excessively deformed to cause insufficient power supply. And has the advantage of being able to produce caustic soda and the like with high efficiency. However, although the electrode has such great advantages, there is a disadvantage that it takes time to manufacture.
The present applicant has proposed a three-dimensional electrode for electrolysis that has solved the drawbacks of the prior art (Japanese Patent Application No. 2005-278198). This three-dimensional electrode is manufactured by bending a plurality of cuts formed in a plate-like metal electrode substrate in the same direction with respect to the substrate to form an elastic conductor.

By the way, as an effective use of wood resources, increasing the yield of chemical pulp is an important issue, and there is a polysulfide cooking process as a technique for increasing the yield of kraft pulp which is the mainstream of this chemical pulp. The cooking chemical in this polysulfide cooking process is produced by oxidizing an alkaline aqueous solution containing sodium sulfide, so-called white liquor, with molecular oxygen such as air in the presence of a catalyst such as activated carbon.
By this method, a polysulfide cooked product having a conversion rate of about 60% and a selectivity of about 60% on a sulfide ion basis and a sulfur polysulfide concentration of about 5 g / L can be obtained. However, in this method, thiosulfate ions that do not contribute to cooking at all are produced as a by-product due to side reactions, so that it is difficult to produce a cooking solution containing a high concentration of polysulfide ions with high selectivity.

Here, the polysulfide ion is also referred to as polysulfide sulfide (PS-S), for example, sulfur having a valence of 0 in sodium polysulfide Na ₂ S _X , that is, sulfur corresponding to (X-1) atoms. In the present specification, sulfur corresponding to sulfur having an oxidation number of −2 in polysulfide ions (sulfur for one atom per S _X ²⁻ ) and sulfide sulfur (S ²⁻ ) are collectively referred to as Na ₂ S. Sometimes referred to as state sulfur.

On the other hand, PCT International Publication No. WO95 / 00701 discloses an electrolytic production method for polysulfide cooking liquor. In this method, an anode in which an oxide such as ruthenium, iridium, platinum or palladium is coated on a support is used. Specifically, a carrier three-dimensional mesh electrode in which a large number of expanded metals are combined is disclosed.
Similarly, Japanese Patent Publication No. 2000-515106 discloses a method for electrolytic production of polysulfide cooking liquor, using a porous anode made of carbon, in particular, an aggregate of carbon fibers having a diameter of 1 to 300 μm is used as the anode. Yes.

Whether these electrodes are used for white liquor electrolysis (electrolysis production of polysulfide cooking liquor) or for other types of electrolysis, if impurities are included in the raw electrolyte, these impurities are being electrolyzed. In order to avoid this, the cell voltage increases due to adhesion to the electrode surface, and it is necessary to clean the electrode and, in the worst case, periodically replace the electrode.
In particular, impurities deposited inside the porous body cannot be sufficiently removed by physical cleaning, and chemical cleaning using an acid, a chelate, or the like is required, resulting in high equipment costs and complicated handling.

When electrolyzing an electrolytic solution containing impurities in this manner using a conventional electrode for electrolysis, it not only deposits on the electrode surface, but also has an adverse effect on the film, which is an obstacle to realizing stable operation for a long time. ing.
Therefore, the present invention provides an electrolysis method that allows stable electrolysis operation over a longer period of time, even when used for electrolysis of an electrolyte solution containing impurities, with less precipitation of impurities on the electrode surface. Objective.

The present invention provides a plate-like metal electrode substrate in which at least one of the anode and cathode of an ion exchange membrane electrolytic cell partitioned into an anode chamber containing an anode by an ion exchange membrane and a cathode chamber containing a cathode carries an electrode catalyst. A three-dimensional electrode in which an elastic conductor is formed by bending a plurality of cut portions formed in the same direction with respect to the electrode base, and the metal electrode base of the three-dimensional electrode is in close contact with the ion exchange membrane, and the elastic conductive Between the ion exchange membrane of the anode chamber and / or the cathode chamber and the electrode current collector, elastically filling only the three-dimensional electrode as a member having electrode activity so that the body contacts the electrode current collector, An electrolysis method characterized in that the solid electrode is elastically held between an ion exchange membrane and an electrode current collector, and a white liquor that is an electrolyte containing impurities is electrolyzed using the solid electrode. is there.

The present invention will be described in detail below.
In the three-dimensional electrode used in the method of the present invention, an elastic conductor is formed by bending a plurality of cut portions formed in a plate-shaped metal electrode substrate in the same direction with respect to the electrode substrate. The bending angle (θ) can be set in an arbitrary range of 0 ° <θ <180 °, preferably 10 ° to 90 °, more preferably 30 ° to 80 °.
When an elastic conductor formed by bending the cut portion is installed so as to be pressed inward between, for example, an ion exchange membrane and an electrode current collector, the elastic conductor obtains elasticity, and the ion exchange membrane And the electrode current collector.

This eliminates the need to install an elastic member other than the electrode in the electrolytic cell, and the electrode can be elastically pressed against the ion exchange membrane or the like in addition to the electrode function. Thus, for example, an effect that the electrode and the ion exchange membrane are in close contact with each other occurs. Moreover, since the elastic conductor that generates elasticity does not contact the ion exchange membrane, the ion exchange membrane is not damaged.
Further, when the bent tip portions of the plurality of elastic conductors that are present are brought into contact with or welded to the electrode current collector, the same power supply paths as the number of the elastic conductors can be secured.
In addition, unlike the case where a normal perforated plate is used as the electrode substrate, the elastic conductor itself has an electrode function, so that the effective electrode area does not decrease.

The electrode base of the three-dimensional electrode of the method of the present invention is preferably composed of a metal having a low specific resistance obtained by electroless nickel plating on the entire surface of nickel, nickel alloy, stainless steel, or copper alloy exhibiting good corrosion resistance. The electrode substrate may be a non-porous sheet or may be porous such as expanded metal.
An electrode catalyst can be supported on the electrode substrate by dispersing and plating Raney nickel catalyst with nickel.

The cut portion is preferably formed in a rectangular (strip) shape, but can be any shape such as a square, semi-circular shape, a tapered trapezoidal shape, or a tapered trapezoidal shape. The plurality of cut portions may be randomly formed on the electrode substrate, but are preferably formed by being aligned vertically and horizontally.
The formation ratio of the cut portion to the total surface area of the electrode substrate is preferably 5 to 60%, and more preferably 15 to 30%. If it is less than 5%, elasticity and conductivity may be insufficient. If it exceeds 60%, the strength of the entire electrode is insufficient, or the ratio of elastic conductors that are separated from the ion exchange membrane increases so that the resistance value increases. Energy loss may occur.
The surface of the electrode substrate after the formation of the elastic conductor may be smooth, but knurling, louvering, corrugated (corrugated) processing or the like can also be performed.

The electrolytic reaction in the ion exchange membrane electrolytic cell used in the method of the present invention includes the production of polysulfide ions by white liquor electrolysis containing the aforementioned impurities, particularly the electrolytic production of polysulfide cooking liquor , and the recovery of waste acid and seawater. Applies to electrolytic reactions.
When the three-dimensional electrode used in the method of the present invention is accommodated in the ion exchange membrane electrolytic cell, it is pressed inwardly between the ion exchange membrane and the electrode current collector as described above (usually including an elastic conductor by the electrode current collector). If the three-dimensional electrode is pressed against the ion exchange membrane, elasticity is imparted to the three-dimensional electrode, and for example, the three-dimensional electrode is brought into close contact with the ion-exchange membrane.
As the ion exchange membrane, a perfluoro cation exchange membrane having an ion exchange group of carboxylic acid or sulfonic acid used in current electrolysis or a complex acid of both can be used.

  In order to perform white liquor electrolysis using an ion exchange membrane electrolytic cell having such a structure, for example, while supplying a white liquor containing impurities in the anode chamber or a diluted solution thereof, a diluted caustic soda aqueous solution is supplied to the cathode chamber, Energize between both poles. Due to the high strength and toughness of the three-dimensional electrode, the positional relationship between the members is stabilized, so that the ion exchange membrane or the like is not mechanically damaged or excessively deformed, resulting in insufficient power supply. No caustic soda can be produced with high efficiency. Furthermore, since the solid electrode to be used has a large surface area, even if impurities are deposited on the electrode surface, the amount deposited per unit area is reduced, the cell voltage is increased less than the conventional electrode, and the surface is not porous. Impurities precipitated in can be easily removed.

The three-dimensional electrode used in the method of the present invention can be manufactured simply by forming a plurality of cut portions in a plate-like metal electrode substrate, bending the cut portions in the same direction, and forming an elastic conductor. Elasticity is imparted to the entire electrode by the conductor, and it functions as a high-strength and tough electrode.
The ion exchange membrane electrolytic cell equipped with this three-dimensional electrode can perform smooth electrolysis with the positional relationship between the members stabilized by the high strength and toughness of the three-dimensional electrode.
When this three-dimensional electrode is used for electrolysis of an electrolytic solution containing impurities, particularly white liquor electrolysis, electrolysis can be continued stably for a relatively long period of time.

Next, an ion exchange membrane electrolytic cell equipped with a three-dimensional electrode that can be used in the method of the present invention will be described based on an example shown in the accompanying drawings.
FIG. 1a is a partially broken perspective view showing an electrode substrate in which a cut portion is formed, FIG. 1b is a partially broken perspective view of a three-dimensional electrode in which an elastic conductor is formed by bending the cut portion of FIG. 1a, and FIG. FIG. 3 is a perspective view showing the flow of electricity in the cathode chamber of the ion exchange membrane electrolytic cell shown in FIG. 2.

As shown in FIG. 1a, in a non-porous plate-like metal electrode base 11, in the illustrated example, three rectangular cut portions 12 facing the same direction are formed in a row, and a total of 15 examples of 5 in a row. The notches 12 in adjacent rows are formed so as to face in opposite directions.
Next, each cut portion 12 is bent in the same direction with respect to the electrode substrate 11, and in the illustrated example, is bent toward the lower side of the electrode substrate 11 to form the elastic conductor 13, and the tip portion of each elastic conductor 13 is A connection piece 14 is formed by bending in parallel with the electrode base 11 to form a solid electrode unit 15 having a total of 15 elastic conductors 13 (FIG. 1b).

The ion exchange membrane electrolytic cell 16 shown in FIG. 2 shows an example in which the three-dimensional electrode unit 15 shown in FIG. 1B is used as an anode 17 and a cathode 18 as a set of three units. The three-dimensional electrode units 15 functioning as the anode 17 and the cathode 18 have their respective surface sides (sides where no elastic conductor is present) in close contact with the ion exchange membrane 19, and their respective short sides are adjacent to the three-dimensional electrode unit. A three-dimensional electrode is formed in contact with the short side of 15.
The ion exchange membrane electrolytic cell 16 has an anode current collector 22 and a cathode current collector 23 in an anode chamber 20 and a cathode chamber 21, respectively. A contact portion between the adjacent solid electrode unit 15 on the anode 17 side and the anode current collector 22 are connected by a first anode power supply plate 24, and a contact portion between the adjacent solid electrode unit 15 on the cathode 18 side and the cathode The current collectors 23 are connected by a first cathode power supply plate 25.

  Furthermore, the first anode power supply plates 24 are electrically connected to each other by a second anode power supply plate 26, and the connection pieces 14 of all the anode-side three-dimensional electrodes 15 are electrically connected to the second anode power supply plate 26 to be elastic. An external force is applied to the conductor 13 in the direction of the ion exchange membrane 19. Further, the first cathode power supply plates 25 are electrically connected to each other by a second cathode power supply plate 27, and the connection pieces 14 of all the cathode side three-dimensional electrodes 15 are electrically connected to the second cathode power supply plate 27, and are elastic. An external force is applied to the conductor 13 in the direction of the ion exchange membrane 19.

When a white liquor containing impurities is supplied to the anode chamber 20 of the ion exchange membrane electrolytic cell 16 and the dilute caustic soda aqueous solution is supplied to the cathode chamber 21, polysulfide ions are generated in the anode chamber, and the anode side Impurities and the like on the surface of the three-dimensional electrode 15 are deposited. However, since the surface area of the three-dimensional electrode 15 is large and the surface thereof is smooth, the voltage rise is slight and the deposited impurities can be easily removed. When saline is supplied to the anode chamber instead of white liquor, a concentrated aqueous solution of caustic soda is obtained in the cathode chamber.
At this time, the elastic conductor 13 of each three-dimensional electrode unit 15 imparts elasticity to the entire electrode and functions as a high-strength and tough electrode, thus enabling stable operation for a long period of time. Moreover, as shown in FIG. 3, on the cathode side (the anode side is omitted, power is supplied in the same manner), power is directly supplied from the cathode current collector 23 to the contact portion of the adjacent three-dimensional electrode unit 15 through the first cathode power supply plate 25. In addition, the current supplied to the first cathode power supply plate 25 branches to the second cathode power supply plate 27 and passes through the connection piece 14 and the elastic conductor 13 connected to the second cathode power supply plate 27. Power is supplied to the surface of the electrode 15. Therefore, there are many power supply paths, and reliable power supply can be achieved.

  Next, although the Example of the electrolysis driving | operation by this invention method is described, this Example does not limit this invention.

[Example 1]
A unit ion exchange membrane electrolytic cell was assembled as follows.
The cathode was a Raney nickel catalyst dispersedly plated with nickel on a nickel expanded metal having an effective area of 20 cm ² (width 4 cm × height 5 cm) to carry the catalyst.
This cathode was attached to the cathode compartment partition by welding using a cathode rib.
In the anode chamber, an anode rib made of flat nickel was used, and an expanded metal anode current collector in which an electroless nickel plating was applied to a copper alloy and a Raney nickel catalyst was dispersed and plated was attached to the anode chamber partition wall. .

A copper alloy plate having a length of 50 mm, a width of 40 mm, and a thickness of 0.2 mm was used as the electrode base of the three-dimensional electrode unit. After this copper alloy plate was formed into an expanded metal mold, pressing was performed to form 10 notches with 5 mm pitch, 4 mm in width and 9 mm in length in one row. Thereafter, electroless nickel plating was applied to the entire surface of the copper alloy, and Raney nickel catalyst was dispersedly plated with nickel to carry the electrode catalyst.
Next, each of the cut portions was bent at an angle of about 45 ° in the same direction to obtain an elastic conductor, and the tip thereof was bent so as to be parallel to the electrode substrate to obtain a three-dimensional electrode unit.

This three-dimensional electrode unit was disposed on the anode current collector.
An ion exchange membrane electrolytic cell was assembled by placing a fluororesin cation exchange membrane (Flemion manufactured by Asahi Glass Co., Ltd.) between the anode and the cathode.
A pseudo white liquor was prepared by adding 20 ppm of suspended substances as impurities to an aqueous sodium sulfide solution having a concentration of 30 g / liter.
The pseudo white liquor is accommodated in the anode chamber, the 10 wt% aqueous sodium hydroxide solution is accommodated in the cathode chamber, and electrolysis is performed at a temperature of 84 to 86 ° C. while changing the current density in the range of 0.5 to 6 KA / m ^2. went. The relationship between the current density and the cell voltage is shown by “A” in the graph (current-voltage curve) of FIG.

[Comparative example]
An ion exchange membrane electrolytic cell was assembled as follows using an electrode having no three-dimensional structure. The same electrode as in Example 1 was used as the cathode.
Instead of the three-dimensional electrode of Example 1, a nickel foam having a length of 50 mm, a width of 40 mm, a thickness of 2.0 mm, a surface area of 2500 m ² / m ³ and an average pore diameter of 0.8 mm was used.
The relationship between the current density and the cell voltage when electrolyzed under the same conditions as in Example 1 is shown in “B” of the graph of FIG.
As can be seen from the graph of FIG. 4, at each current density, the cell voltage of the three-dimensional electrode of Example 1 was 0.2 to 0.7 V lower than the foamed nickel of the comparative example.

FIG. 1a is a partially broken perspective view showing an electrode substrate in which a cut portion is formed, and FIG. 1b is a partially broken perspective view of a three-dimensional electrode in which an elastic conductor is formed by bending the cut portion of FIG. 1a. It is a partial cross-sectional top view of the ion exchange membrane electrolyzer equipped with the three-dimensional electrode of FIG. 1b. It is a perspective view which shows the flow of the electricity in the cathode chamber of the ion exchange membrane electrolytic cell of FIG. It is a graph which shows the relationship (current voltage curve) of the current density and cell voltage in Example 1 and a comparative example.

Explanation of symbols

  DESCRIPTION OF SYMBOLS 11 ... Electrode base 12 ... Notch part 13 ... Elastic conductor 14 ... Connection piece 15 ... Solid electrode (unit) 16 ... Ion exchange membrane electrolytic cell 17 ... Anode 18 ... Cathode 19 ... Ion exchange Membrane 20... Anode chamber 21... Cathode chamber 22... Anode current collector 23... Cathode current collector 24... First anode power supply plate 25. ... Second cathode power supply plate

Claims (2)

A plurality of the anode and cathode of the ion exchange membrane electrolytic cell partitioned into an anode chamber containing an anode by an ion exchange membrane and a cathode chamber containing a cathode are formed on a plate-like metal electrode substrate carrying an electrode catalyst. A three-dimensional electrode in which an elastic conductor is formed by bending the cut portion of the electrode in the same direction with respect to the electrode base, the metal electrode base of the three-dimensional electrode is in close contact with the ion exchange membrane, and the elastic conductor is the electrode collector. Only the three-dimensional electrode is elastically filled as an electrode active member between the ion exchange membrane and the electrode current collector in the anode chamber and / or the cathode chamber so as to contact the electric body, An electrolysis method characterized by electrolyzing a white liquor that is an electrolytic solution containing impurities , elastically held between an ion exchange membrane and an electrode current collector, and using the three-dimensional electrode. The electrolysis method according to claim 1 , wherein polysulfide is generated by electrolysis of white liquor.

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