JPS6367558B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to an electrolytic cell for electrolyzing an aqueous solution of an alkali metal salt using an ion exchange membrane method, and particularly to an electrolytic cell for electrolyzing an aqueous solution of an alkali metal chloride such as sodium or potassium. More specifically, it is a so-called two-chamber type vertical electrolytic cell, and is characterized by having a metal diaphragm between the cation exchange membrane and the cathode. Conventionally, an ion exchange membrane method has been known as a method for electrolyzing an aqueous alkali metal salt solution. Furthermore, with the development of a more durable cation exchange membrane, a so-called two-chamber electrolytic cell was proposed. This type of electrolyzer is generally positive,
The basic structure is that the cathode and both electrodes are porous, and are placed relatively close to each other, with a cation exchange membrane between them, and an anode chamber space and a cathode chamber space behind each electrode. Various design changes have been made. Examples of various improvement proposals include lowering the electrolytic voltage by bringing the ion exchange membrane closer to the anode side or in close contact with the anode surface, and reducing the electrolytic voltage by lowering the electrolytic voltage between the ion exchange membrane and the anode and/or cathode. By interposing a spacer, it is possible to prevent the accumulation of air bubbles, or by bringing the anode and cathode into close contact via a cation exchange membrane, the distance between the electrodes can be minimized and the electrical resistance of the solution can be minimized. There are suggestions to make it smaller. The present invention provides a structure in which the thickness between the cation exchange membrane and the porous cathode is 0.05 to 3, which has a hydrogen overvoltage greater than that of the cathode.
This is an electrolytic cell for aqueous alkali metal chloride solution electrolysis using an ion-exchange membrane method, characterized in that a porous metal membrane of mm in diameter is inserted therein. The most important feature of the present invention is that a porous metal membrane having a higher hydrogen overvoltage than the cathode is inserted between the cation exchange membrane and the cathode. As mentioned above, there have been proposals to interpose a spacer between the cation exchange membrane and the cathode. The purpose in this case is (1) to prevent the cation exchange membrane from coming into contact with the cathode and damage it, to push the cation exchange membrane toward the anode side and reduce the interelectrode voltage during electrolysis, and to It is thought that a sufficient gap is provided between the exchange membrane and the cathode so that the aqueous caustic alkali solution can flow into the gap, and hydrogen gas between the cation exchange membrane and the cathode is guided into the space behind the cathode. Therefore, the spacers used in the prior art are all made of non-conductive materials and have extremely large pores, for example, mesh-like spacers with openings of 10 mm or more. Another technique is to create a metal mesh between the cation exchange membrane and the cathode, which can also be called an auxiliary electrode.
There is also a method in which by applying a potential to this, the diffusion of hydroxyl ions generated on the cathode toward the cation exchange membrane is suppressed by electrical repulsion. In this case, a relatively large distance is required between the cathode and the auxiliary cathode, for example, about 5 to 10 mm, and the auxiliary cathode is composed of a relatively large mesh metal, and moreover, it is larger than the cathode. , a potential with a small absolute value is separately added.
This shape increases the thickness of the electrode chamber and is not suitable for use as a practical electrolytic cell such as a stacked type. In a preferred embodiment of the present invention, the distance between the anode and the cathode is 0.5
It is particularly important that the membrane be as narrow as 5 mm or even 2 to 4 mm, and a metal membrane is interposed so as to be in contact with both the cation exchange membrane and the cathode. The metal film of the present invention has the same potential as the cathode by coming into contact with the cathode. However, since the hydrogen overvoltage is larger than that of the cathode, it does not function as a cathode itself. In the present invention, the hydrogen gas generated on the cathode surface is released from the cathode surface in a very short period of time, and in caustic alkali, it does not associate with each other relatively, and becomes like an emulsified state. By elucidating the mechanism by which hydrogen gas adheres to the surface of the cation exchange membrane due to its good staining, reducing the current-carrying area and causing an increase in voltage during electrolysis. We investigated ways to prevent the hydrogen generated on the cathode from coming into contact with the cation exchange membrane as much as possible. As a result, in the case of spacer-like Thrilene, which is a non-conductive material, as in the case of cation-exchange membranes, hydrogen bubbles are noticeably attached to it in caustic alkali, and as a result, the cation-exchange membrane ends up being As in the previous case, I learned that this would reduce the effective current-carrying area. However, strangely, metals do not stain well with hydrogen in aqueous caustic alkali solutions, and there is very little tendency for hydrogen gas to adhere to their surfaces. Furthermore, when considering the state of the pores of a porous metal that is suitable for preventing hydrogen gas generated at the cathode from passing through the porous metal membrane, reaching the cation exchange membrane, and adhering to the membrane surface, it is generally found that the pore size It is natural that the smaller the diameter, the better the performance, but according to observation, a pore diameter of about 0.5 mm is sufficient, and even a pore diameter of about 1 mm can be observed to be sufficiently effective. Therefore,
Generally, pore sizes of 1000μ or less are preferred. In addition, in order to obtain a pore size that is too small, the shielding part that constitutes it also generally increases, so it is usually 50~
It would be better to use a mesh membrane with a pore size of 1000 microns. The thickness of the porous metal film is generally preferably in the range of 0.05 to 3 mm. Furthermore, if there is a risk that the membrane may vibrate or bend during electrolysis, it may be effective to fix it onto the cathode by welding or other means. Furthermore, the material of the porous metal membrane of the present invention is determined by its relative relationship with the cathode, and it is essential that the material has a larger hydrogen overvoltage than the cathode. It is preferable that the substance has a hydrogen overvoltage larger than the algebraic sum of the potential difference in the caustic aqueous solution and the cathode overvoltage, but generally the potential difference in the aqueous solution is only 20 to 40 mV/mm, and ignoring this, The material selection does not cause any significant problems in reality. Of course, if the thickness of the porous metal film is large, for example, 2 to 3 mm,
It is desirable to consider the potential difference in the solution depending on the thickness. Examples of such materials include nickel or nickel alloys such as stainless steel, copper, etc. for mild steel cathodes.
A cathode made of aluminum, etc. has a lower hydrogen overvoltage than mild steel, such as platinum, rhodium, tungsten, gold, or a cathode made of these plated on a base metal as a cathode, or a nickel-plated cathode made of a nickel layer containing sulfur. In the case of a cathode, mild steel, nickel, or the like is used as the porous metal membrane. In general, iron, nickel, or stainless steel wire mesh is used as the cathode, with or without copper plating on the cathode substrate, such as cathode having a lower hydrogen overvoltage than iron, such as iron, or plating containing rodan nickel. It is preferable to use one that has been electroplated using a bath. DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 1 is a sectional view of a part of a conventional ion-exchange membrane type vertical electrolytic cell, for example, a filter press type bipolar cell. The part constituted by the electrolytic cell frames 1, 1' and the partition walls 2, 2' constitutes one unit cell, which sandwiches a cation exchange membrane 3, with 4 being a cathode and 5 being an anode. Further, 6 and 7 are conductive ribs for supplying electricity to the cathode and the anode, respectively, and the spaces between the partition walls and the electrodes are a cathode chamber and an anode chamber, respectively, which serve as flow paths for the liquid and gas in the chambers. Of course, the present invention is not limited to bipolar electrolytic cells, but any electrolytic cell that can basically constitute the unit cell shown in FIG. 1 may be used. FIG. 2 is a partial sectional view of an example of the electrolytic cell of the present invention, and corresponds to FIG. 1. In this figure, 8 is a porous metal film. FIG. 3 shows this state more clearly. FIG. 3 is a diagram showing a part of a cross section taken in a direction perpendicular to the cross section of FIG. 2. FIG. In this way, the narrow gap between the anode and the cathode is preferably 0.5 to 5 mm, especially 1 mm.
The present invention is particularly effective in the case where the porous metal membrane is substantially completely present between the cation exchange membrane and the cathode with a width of 4 mm. That is, an additional function is added to press and fix the cation exchange membrane against the anode side. Furthermore, during operation of the electrolytic cell of the present invention, the cathode chamber pressure is lower than the anode chamber pressure by 200 to 600 mm of water column, and further by 350 to 450 mm.
mm It is more effective to keep the water column high. Examples of use of the electrolytic cell of the present invention are shown below. Example 1 An electrolytic cell with a current carrying area of 18 dm ² , the anode is made of titare lath coated with ruthenium oxide, the cathode is made of mild steel lath plated with copper by the usual method, and further electroplated with a rodan nickel bath. I used what I had done. (The presence of sulfur in this plating layer was confirmed using an X-ray microanalyzer.) The structure had a distance between electrodes of 3 mm and a space of 25 mm behind each electrode. As a cation exchange membrane, the sulfonic acid group of Nafion (trade name) manufactured by DuPont was converted to sulfonyl chloride by treating only one surface of the membrane with phosphorus pentoxide, which was then oxidized and replaced with a carboxylic acid group. (EW=1100) was used. A stainless steel plate is placed between the cathode and the cation exchange membrane.
Table 1 shows the data and electrolytic conditions for salt electrolysis in the presence and absence of a porous metal membrane of 80 mesh plain weave net (wire diameter 0.15 mm).

[Table] Example 2 Using the electrolytic cell of Example 1, similar experiments were conducted using mild steel plain-woven wire meshes with various openings. Table 2 shows the results.
Shown below.

【table】 [Brief explanation of drawings]

FIG. 1 is a sectional view of one unit of a conventional rigid ion exchange membrane electrolyzer, FIG. 2 is a sectional view of a portion of the electrolytic cell of the present invention corresponding to FIG. 1, and FIG. is a partial sectional view taken in a direction perpendicular to the cross section in FIG. 2; In the figure, 1 and 1' are electrolytic cell frames, 2 and 2' are partition walls, and 3
is a cation exchange membrane, 4 is a cathode, 5 is an anode, 6, 7
each represents a rib.

Claims (1)

[Scope of Claims] 1. An ion exchange membrane method characterized by interposing a metal porous membrane with a thickness of 0.05 to 3 mm, which has a larger hydrogen overvoltage than the cathode, between a cation exchange membrane and a porous cathode. Electrolytic cell for aqueous alkali metal chloride solution electrolysis. 2. Claim 1, wherein the metal porous membrane is made of one metal selected from iron, nickel, or an alloy containing these, and the cathode has a hydrogen overvoltage lower than that of iron. The electrolytic cell described. 3. The metal porous membrane is made of one metal selected from iron, nickel, or an alloy containing these, and the cathode has a surface plated with a sulfur-containing nickel compound on the base. An electrolytic cell according to claim 2. 4. The electrolytic cell according to claim 1, wherein the gap between the anode and the cathode is 0.5 to 5 mm.