JPS61264187A - Method for electrolyzing aqueous solution - Google Patents

Abstract

PURPOSE:To drop the hydrogen overvoltage in an electrolytic cell and to reduce the quantity of electric power used when an aqueous soln. of an alkali metallic halide is electrolyzed, by using an electrode obtd. by plating the surface of Ti or a Ti alloy with an Fe-Co alloy as the cathode. CONSTITUTION:An aqueous soln. of an alkali metallic halide such as NaCl is electrolyzed in an electrolytic cell to produce NaOH on the cathode and gaseous Cl on the anode. At this time, an electrode having an Fe-Co alloy film formed by plating on the surface of the substrate of Ti or a Ti alloy contg. Zr, Ta, Nb, Mo, Cr, Fe, V, Mn or the like is used as the cathode. The Fe-Co alloy film is formed by plating in a plating soln. of 2.5-6.0pH at 20-90 deg.C and 1-5A/dm<2> current density. The plating soln. contains 0.1-2mol/l in total of Fe ions from an Fe salt such as ferrous chloride and Co ions from a Co salt such as cobalt chloride in 0.01-0.5 ratio of Co/Fe ions and further contains starch, dextrin, poly-2-diethylaminoethyl methacrylate or polyaluminum chloride.

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 is large and is a serious problem. It is known that the hydrogen overvoltage at the cathode varies significantly depending on the surface material or surface condition of the cathode base 2. 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 or its compound having a low hydrogen overvoltage. For example, a cathode in which the cathode base material is coated with an alloy of sacrificial metal and nickel, etc.
7), Cathode coated with rhenium (Japanese Patent Application Laid-open No. 1983-
55782.5l-83083), a cathode in which powdered metals such as nickel, cobalt, platinum, iron, etc. are adhered to the cathode base material using a metal powder spraying method (Japanese Patent Laid-Open No. 52-32832), a cathode made by a metal powder spraying method. A cathode coated with a mixture of cobalt and zirconia powder (Japanese Unexamined Patent Publication No. 52-36582), a cathode whose cathode base material is coated with nickel, cobalt powder, or a mixture of these and aluminum powder using the same method (Japanese Unexamined Patent Publication No. 52-36582) -36583), a cathode in which the cathode base material is coated with nickel using a chemical plating method (JP-A-52-110282), a cathode in which a nickel, cobalt, and tungsten alloy is similarly plated chemically (JP-A-52-133100) Alternatively, a cathode in which a nickel, vanadium, or molybdenum alloy is coated by electroplating (Japanese Patent Application Laid-Open No. 52-102888), or a cathode base material is coated with an alloy of nickel and molybdenum or vanadium in the coexistence of hydrosulfide (patented patent application) No. 203635) Various cathodes have been disclosed, including a cathode in which ground steel powder is coated on a base material by a sintering method (Japanese Unexamined Patent Publication No. 51-147479).

However, these cathodes each have weak coatings, low hydrogen overvoltage reduction effects, expensive metals or powders, and difficulty in homogeneous application to cathode substrates with complex shapes (e.g., mesh bodies). There are many drawbacks such as difficulty.

Therefore, the present inventor has investigated a method for electrolyzing aqueous solutions of alkali metal halides, alkali metal hydroxides, etc. by electroplating an iron-cobalt alloy on a specific substrate under various conditions and using this as a cathode. They discovered that it is effective to use an iron-cobalt alloy plated product obtained under these conditions 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, and polyaluminum chloride with ferrous ions and cobalt. □Total on concentration is 0.1
~2 mol/j! , an aqueous solution electrolysis method in which an iron-cobalt alloy is electroplated by adding it to an iron-cobalt alloy plating bath where 2.5≦9H≦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, as well as iron, nickel, stainless steel, platinum group metals, etc., in terms of electrical conductivity, mechanical properties, chemical resistance to electrolytes, and compatibility with plating layers and coating layers. Although it is suitable in terms of adhesive properties, it is equally suitable as a material for electrolytic cells and a substrate 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.At least one surface of the titanium is subjected to pre-treatment for plating, i.e. mechanical roughening such as degreasing, wire foil polishing, sandblasting, etc. In addition to chemical roughening such as surface roughening and etching, it is preferable to perform appropriate treatments such as metal coating alone or in combination from the viewpoint of adhesion of the plated layer, which is a component of the present invention. .

It has been found that additives to the electroplating bath are preferably those that are generally used as flocculants for suspended substances in water in so-called water treatment technology, such as starch, dextrin, poly-2-diethylaminoethyl methacrylate, or polyester. Aluminum chloride 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 are no particular limitations on the amount of these additives, but if they are 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 of addition should be determined depending on the conditions of use as a cathode. must be decided.

Next, in the present invention, an iron-cobalt alloy is selected as the component to be plated, and the plating bath consists of a ferrous salt such as ferrous chloride or ferrous sulfate, and a cobalt salt such as cobalt chloride or cobalt sulfate. A bath consisting of a mixed solution of

The pH of the plating bath has a significant effect on the plating film. The preferred pH range of the plating bath is 2.5 to 6.0, although it is influenced by the strength of bath agitation.

If the stirring is weak, it is possible to lower the pH, but
Problems arise with the uniformity of the plated surface.

The concentration of ferrous ions and cobalt ions in the plating bath is such that the content of both is 0.1 mol/R~2 mol/1
is good, more preferably 0.4 n+ol/1 to 1.
7 mol/! is good.

The ratio of ferrous ions and cobalt ions is not particularly limited, but practically Co/Fe=0.0
1 to 0.5 is preferred.

If the total concentration of ferrous ions and cobalt ions exceeds the above range, an inactive plated surface is likely to be formed, and the effect of reducing hydrogen overvoltage as a cathode is significantly reduced. If the concentration is lower than this range, the peeling resistance of the plating film,
This causes problems in terms of current efficiency, etc. Also, Patent No. 2067
No. 43 requires the presence of ferric salt in the plating bath,
I am trying to include iron oxide in the generated plating layer, but
In the present invention, one of the constituent requirements is the coexistence of the above-mentioned additives in the iron-cobalt alloy plating bath, and there is no need to add ferric salts or iron oxides. Although iron oxide is easily produced, it does not cause any harm even if it is produced or separated by precipitation, so there is a great advantage that the pH conditions can be set as wide as possible.

In order to increase the electrical conductivity of the plating bath, an inorganic salt such as calcium chloride, which is conventionally added, may be added as necessary. However, salts containing ammonium ions must be avoided because they interfere with the formation of the plating.

In the case of a plating bath having a relatively low concentration of metal ions, the conductivity of the plating solution is low, so the addition of the above-mentioned inorganic salt is useful. However, if a large amount is added, the effect of reducing the hydrogen overvoltage of the plating cathode will be reduced.

The temperature of the plating bath is the conventional plating temperature (approximately 20 to 90 degrees Celsius).
) may be adopted, but is not particularly limited to 70°C.
The above is preferable.

The current density during plating is under the conventional conditions (1 to 5 A/di"
) 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, if air agitation is used, ferrous ions are easily oxidized and converted to unnecessary ferric iron (this must be avoided. Therefore, agitation with a non-oxidizing gas such as nitrogen gas agitation is preferred from the above point of view).

The present invention will be described in detail below using Examples.

Examples 1 to 5 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 sodium chlorate was 100 g/l and sodium chloride was 240 g/l.
, immersed in an aqueous solution of 4 g/l of sodium hydroxide at a temperature of 4.
Electrolysis was carried out at 5°C with a current density of 10 A/dm,
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 conditions of 20 A/dm" and 30 A/dm". The cathode potential was measured using a saturated calomel electrode through a Luggin capillary.

Examples 6 to 14 The lower column of the table shows the results of measuring the decrease 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 15 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. Asbestos diaphragm electrolysis of saturated saline solution for producing sodium hydroxide was carried out at 20 A/dm''.

The lower column of the same table shows the electrolysis voltage reduction effect of the plated cathode compared to the conventional mild steel wire mesh cathode (polished with a wire brush).

Example 16 A cathode plate prepared under the conditions shown in Table 4 was immersed opposite a pure nickel anode in a 30% by weight potassium hydroxide aqueous solution at 65° C., 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 5 Comparisons were made with Examples 1 to 5 using cathode plates plated under the conditions shown in Table 5.

Comparative Examples 6 to 11 The lower column of the table shows the results of measuring the reduction in hydrogen overvoltage for mild steel plates in water electrolysis using cathodes prepared under the plating bath conditions shown in Table 6, corresponding to each current density condition. . Cathode potential measurement method, mild steel plate cathode and its surface treatment are shown in Example 6
-14.

Table 3 Table 4

Claims (1)

[Claims] At least one surface of a substrate made of titanium or a titanium alloy is coated with one selected from the group consisting of starch, dextrin, poly-2-diethylaminoethyl methacrylate, and polyaluminum chloride so that the total concentration of ferrous ions and cobalt ions is 0.1. ~2mol/l, 2.5≦pH≦6
．． An aqueous solution electrolysis method in which an iron-cobalt alloy is electroplated by adding it to an iron-cobalt alloy plating bath, and this is used as a cathode.