JPS6124475B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a method for aqueous electrolysis of alkali metal halides, alkali metal hydroxides, etc. More specifically, the present invention provides an electrolysis method using a new cathode that can significantly reduce hydrogen overvoltage compared to conventional electrolysis. When electrolyzing an aqueous sodium chloride solution to produce hydrogen, chlorine, and caustic soda, or electrolyzing an alkali metal hydroxide aqueous solution to produce hydrogen and oxygen using an electrolytic cell, the power efficiency caused by hydrogen overvoltage at the cathode The loss of
This is a serious problem. It is known that the hydrogen overvoltage at the cathode varies significantly depending on the material, surface material, surface condition, etc. of the cathode. Based on these findings, a cathode for aqueous electrolysis of alkali metal halides or alkali metal hydroxides has been proposed in which the surface of the cathode substrate is treated with a metal having a low hydrogen overvoltage or a compound thereof. For example, a cathode in which the cathode base material is coated with an alloy of sacrificial metal and nickel etc. (JP-A-51-54877), a cathode in which rhenium is coated (JP-A-51-55782, 51-
83083), a cathode in which powdered metals such as nickel, cobalt, platinum, and iron are adhered to the cathode base material using a metal powder spraying method (Japanese Patent Application Laid-Open No. 52-32832), cobalt and zirconia powders using a metal powder spraying method Cathode coated with a mixture (Japanese Unexamined Patent Publication No. 52-36582), nickel,
A cathode whose cathode base material is coated with cobalt powder or a mixture of these and aluminum powder (Japanese Patent Laid-Open No. 52-36583), a cathode whose cathode base material is coated with nickel using a chemical plating method (Japanese Patent Laid-Open No. 52-36583).
110282), similarly, a cathode chemically plated with a nickel, cobalt, and tungsten alloy (Japanese Unexamined Patent Publication No. 110282)
-133100), or coating a nickel, vanadium, or molybdenum alloy by electroplating (Japanese Patent Application Laid-Open No. 52-102888), or coating a cathode base material with an alloy of nickel and molybdenum or vanadium in the coexistence of hydrosulfite. cathode (patent no.
No. 203635) Various cathodes in which ground steel powder is coated on a base material by a sintering method (Japanese Patent Application Laid-open No. 147479/1983) have been disclosed. However, each of these cathodes has weak coatings, low hydrogen overvoltage reduction effects, expensive metals or powders, and difficulty in homogeneous application to complex-shaped cathode substrates (e.g., mesh). There are many drawbacks such as difficulty in Therefore, the present inventor investigated a method for electrolyzing aqueous solutions of alkali metal halides, alkali metal hydroxides, etc. by electroplating iron on a specific substrate under various conditions and using this as a cathode. They discovered that it is effective to use the iron-plated material obtained in the above as a cathode, and completed the present invention. That is, the gist of the present invention is to coat at least one surface of a substrate made of titanium or a titanium alloy with one selected from the group consisting of starch, dextrin, poly-2-diethylaminoethyl methacrylate, polyaluminum chloride, and chromium sulfate. It is an aqueous electrolysis method in which iron is electroplated by adding it to an iron plating bath with an ion concentration of 0.1 to 1.7 mol/l and a pH of 6.0, and this is used as a cathode. Next, the method for manufacturing the cathode used in the present invention will be explained in detail. The specific substrate in the present invention is titanium or a titanium alloy, which has electrical conductivity, mechanical properties, chemical resistance to electrolytes, plating layer, coating layer, etc., as well as iron, nickel, stainless steel, platinum group metals, etc. Although it is suitable in terms of adhesive properties, it is equally suitable as a material for electrolytic cells and a base material for anodes, and can be used for both anodes and cathodes by applying appropriate plating or coating. This shows that it is promising not only as a substrate for monopolar electrodes but also as a substrate for bipolar electrodes. In addition to titanium, titanium alloys may also be used, but alloys of titanium with zirconium, tantalum, niobium, molybdenum, chromium, iron, vanadium, manganese, etc. are particularly suitable. In general, a suitable composition contains components other than titanium in an amount of several percent by weight. When titanium or titanium alloy is used as a cathode, it is generally in the form of a flat plate, a box, or a mesh, and at least one side of the titanium is subjected to pretreatment for plating, such as degreasing, wire foil polishing, sandblasting, etc., to mechanically roughen the surface. In addition to chemical surface roughening such as chemical roughening and etching, it is preferable to carry out appropriate treatments such as metal coating alone or in combination, from the viewpoint of adhesion of the plating layer which is a component of the present invention. It has been found that suitable additives for the electroplating bath are those that are generally used as flocculants for suspended substances in water in so-called water treatment technology, such as starch, dextrin, poly-2-diethylaluminoethyl methacrylate, Polyaluminum chloride or chromium sulfate can be used. Usually one type of these additives is sufficient, but there is no adverse effect when two or more types are used in combination. On the other hand, there is no particular limit to the amount of these added, but if too large, it will have a negative effect on the peeling resistance of the plating layer and the mechanical strength of the plating surface, so the amount added depends on the conditions of use as a cathode. must be determined. Next, in the present invention, iron is selected as the metal component to be plated, and the plating bath may be a conventional bath such as a ferrous chloride bath, a ferrous sulfate bath, a borofluoride bath, or a mixed bath thereof. Although there are plating baths, plating baths containing ammonium ions such as ammonium sulfate and ammonium chloride are completely unsuitable for this purpose. Among the plating baths mentioned above, the ferrous chloride bath is practically the most suitable. Furthermore, the plating bath pH has a significant effect on the electrical activity of the plating film. The pH must be maintained between 2.5 and 6.0. If you choose PH<2.5, you will only get a normal, inactive iron plating surface, and on the other hand, if you choose PH>6
Under these conditions, ferrous iron precipitates and the plating effect is poor and the plating work is likely to be hindered. Further, even if ferric iron precipitates and is present in the plating bath, it does not help in forming the plating layer and does not affect the cathode performance in any way. The ferrous ion concentration in the bath is 0.1~
1.7 mol/l, preferably 0.5-1.5 mol/l is chosen. If the concentration is made larger than this range, the activity as a cathode will drop and the hydrogen overvoltage reducing effect will be drastically reduced.
On the other hand, if the concentration is lower than this range, problems will arise in terms of peeling resistance of the plating film, current efficiency, etc. Japanese Patent No. 206743 discloses a method in which a ferric salt is allowed to coexist in a dissolved state with a ferrous salt, and iron oxide is strongly electroplated together with iron. Therefore, the pH range is narrow, PH < 1.5 to 2.0, and it is difficult to maintain conditions for forming a plating layer. However, in the plating of the present invention, there is no need for the presence or coexistence of a ferric iron component, and even if the ferric component is generated or becomes a precipitate, it does not cause any harm, and the range of possible pH conditions is wide. The benefits are great. In addition, in order to increase the electrical conductivity of the plating bath,
The addition of conventionally used inorganic salts such as calcium chloride and potassium chloride can also be used appropriately in the present invention. However, salts containing ammonium ions must be avoided since they interfere with the formation of the plating as described above. In the case of a plating bath with a relatively low ferrous ion concentration, the addition of the above-mentioned inorganic salt is useful because the plating solution has a low electrical conductivity. However, if it is added in an extremely large amount, the effect of reducing the hydrogen overvoltage of the plating cathode will be reduced and it cannot be used for electrolysis. The temperature of the plating bath is the customary plating temperature (25 to 90
70°C or higher, although not particularly limited. The current density during plating is the conventional iron plating condition (1 to 5 A/dm ² ).
is appropriate. Even higher current densities are possible, but the surface condition becomes rough and the peeling resistance deteriorates. Although it is possible to use a current density lower than the above, the peeling resistance of the plating film deteriorates. Further, it is preferable that the plating bath be stirred to the extent that it is used in conventional electroplating operations. However, since air agitation oxidizes ferrous ions, it must be avoided as much as possible. Non-oxidizing gas such as nitrogen gas stirring is preferred from the above point of view. As the stirring method, gas aeration stirring, liquid circulation stirring, or mechanical stirring can be applied. The present invention will be explained in detail below using Examples. Examples 1 to 12 A cathode plate plated under the conditions shown in Table 1 was placed opposite an anode made of a titanium plate coated with ruthenium oxide, and 100 g/l of sodium chlorate, 240 g/l of sodium chloride, and 4 g/l of sodium hydroxide were applied. Electrolysis is carried out at a temperature of 45°C by immersing it in an aqueous solution of
The lower column of the same table shows the reduction in hydrogen overvoltage compared to a conventional mild steel plate cathode (polished with abrasive paper) under each condition of 10 A/dm ² , 20 A/dm ² , and 30 A/dm ² . The cathode potential was measured using a saturated calomel electrode through a Luggin capillary. Examples 13 to 24 The lower column of the table shows the results of measuring the reduction in hydrogen overvoltage of the cathodes prepared under the conditions shown in Table 2 relative to the mild steel plate cathode in water electrolysis in response to current density. The cathode potential was measured by immersing the plated mild steel plate in a 10% by weight aqueous sodium hydroxide solution at 45°C, opposite to a pure nickel plate anode, and using a saturated calomel electrode through a Luggin capillary tube. Further, as the mild steel plate cathode, a mild steel plate polished with #80 abrasive paper was used. Example 25 Asbestos was deposited on a cathode wire mesh plated under the conditions shown in Table 3 to serve as a cathode for an asbestos diaphragm, and placed opposite to an anode coated with ruthenium oxide on a titanium wire mesh at a temperature of 70°C and a current Density 20A/
Asbestos diaphragm electrolysis of saturated saline solution for the production of sodium hydroxide was carried out at ^dm2 . Conventional mild steel wire mesh cathode (polished with a wire brush)
The lower column of the same table shows the effect of reducing the electrolytic voltage of the plating-treated cathode. Example 26 A cathode plate prepared under the conditions shown in Table 4 was immersed in a 30% by weight potassium hydroxide aqueous solution at 65°C opposite a pure nickel anode, and water electrolysis was performed at a current density of 20 A/dm ² . . As a result of electrolysis, the electrolytic voltage decreased considerably as shown in the same table. Comparative Examples 1 to 6 Comparisons were made with Examples 1 to 12 using cathode plates plated under the conditions shown in Table 5. Comparative Examples 1 to 12 The lower column of the table shows the results of measuring the amount of decrease in hydrogen overvoltage against a mild steel plate in water electrolysis using cathodes prepared under the plating bath conditions shown in Table 6, corresponding to each current density condition. . The cathode potential measuring method, the mild steel plate cathode, and its surface treatment were the same as in Examples 13-24.

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Claims (1)

[Claims] 1 Starch, dextrin, poly-2-
One selected from the group consisting of diethylaminoethyl methacrylate, polyaluminum chloride, and chromium sulfate at a ferrous ion concentration of 0.1 to 1.7 mol/l,
An aqueous solution electrolysis method in which iron is electroplated by adding it to an iron plating bath with a pH of 2.5PH6.0, and this is used as a cathode.