JPS633957B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to the structure of an anode for electrolysis. More specifically, in a perforated flat plate anode used in an alkali metal chloride aqueous solution electrolyzer that is divided into an anode chamber and a cathode chamber by a cation exchange membrane, the thickness of the anode active coating on the front surface and the side surface of the hole is the same as that of the anode active coating on the back surface. The present invention relates to a structure of an electrolytic anode characterized in that the thickness is larger than the thickness of the anode. In the present invention, the surface refers to the surface facing the cathode and close to the cation exchange membrane, the pore side refers to the surface corresponding to the thickness of the perforated part, and the back surface refers to the surface opposite to the front. Refers to the side surface. The main type of anodes used in aqueous alkali metal chloride electrolysis are so-called metal anodes in which the surface of a base material such as titanium is coated with a noble metal oxide such as ruthenium oxide. In addition, in electrolysis of aqueous alkali metal chloride solutions, the voltage increases due to current shielding by chlorine gas generated at the anode, so a perforated anode is used to place the chlorine gas generated at the anode behind the cathode. I'm making it easy. Typical structures include a structure in which round rods with a diameter of 2 to 6 mm are arranged at intervals of 1 to 3 mm, and an expanded metal structure made from thin plates with a thickness of 1 to 2 mm. The anode having the typical structure described above is also used in the alkali metal chloride aqueous solution electrolysis method using a cation exchange membrane.

In Japanese Patent Application No. 55-48634, the present inventors proposed that in the case of aqueous electrolysis of alkali metal chloride using a cation exchange membrane, openings in the shape of a circle, an oval, a square, a rectangle, a triangle, a rhombus, a cross, etc. are formed in a flat plate. By using a perforated flat plate anode made by forming an expanded metal into a flat plate by pressing etc., the electrolytic voltage can be lowered by 0.15 to 0.2V compared to the case of using an expanded metal anode, which is commonly used in the past. It was shown that the difference was not in the anodic potential, but in the voltage drop across the cation exchange membrane. It was also shown that the larger the total cut length of the perforated flat plate anode, the more uniform the current distribution within the cation exchange membrane and the lower the voltage. Furthermore, in the case of cation-exchange membrane method alkaline chloride electrolysis, it has long been known that the anode surface in contact with the cation-exchange membrane wears out quickly, and as a countermeasure to this problem, Japanese Patent Publication No. 53-1063 proposed an anode in contact with the cation-exchange membrane. It is proposed to eliminate the anode active coating on the front side and to coat the back side of the porous anode, ie the side not covered by the membrane, with a noble metal, noble metal alloy or noble metal oxide. However, it has been found that when this method is applied to a perforated flat plate anode, the surface that plays the most important role in making the current distribution uniform within the cation exchange membrane does not function as an anode, resulting in a drawback that the electrolytic voltage is high.

The present invention provides a new anode that solves these problems. As a result of intensive study of the current distribution of a perforated flat plate anode, the present inventors found that the current was concentrated on two surfaces of the perforated flat plate anode, the front surface and the side surface of the hole, and that the wear on these two surfaces was caused by the wear on the back surface. We found that it is faster than wear and tear. This phenomenon is particularly large when electrolysis is performed by making the cathode chamber pressure higher than the anode chamber pressure and pressing the cation exchange membrane toward the anode.

The present invention was made based on these findings, and the anodes on the surface and the side of the holes share a large amount of current in the perforated flat plate anode and play a major role in making the current distribution uniform within the cation exchange membrane. The thickness of the active material is made larger than the thickness of the anode active material on the back side, thereby providing an anode with a low electrolysis voltage and a long life.

In order to investigate the contribution of each of the front surface, the hole side surface, and the back surface of the perforated flat plate anode to the uniformity of the current distribution within the cation exchange membrane, the anode active coating was left on only one of each surface. Electrolysis was carried out using a perforated flat plate anode with the anode active coating shaved off on the other two sides. For the perforated flat plate anode, we used a 1.2 mm thick titanium plate with 2 mm diameter holes punched out at 3.5 mm pitch in a 60 degree staggered pattern so that each side had the same area, and was coated with ruthenium oxide as the anode active coating. did. The current carrying area is 10cm x 10cm, the cation exchange membrane is Nafion 315 with embedded Teflon fabric, the cathode is expanded metal made from a 1.5mm thick mild steel plate, and the anode chamber is a 3N saline solution with a pH of 2. , 5N caustic soda aqueous solution is supplied to the cathode chamber, and the pressure in the cathode chamber is lowered by 1 m than the pressure in the anode chamber.
Electrolysis was carried out at a current density of 50 A/dm ² and 90° C. while keeping H ₂ O high. At the same time, electrolysis was performed under the same conditions using an expanded metal anode made from a 1.5 mm thick titanium plate with a short axis of 7 mm and a long axis of 12.7 mm, and the electrolytic voltage was taken as a reference value, and the voltage when using each anode was calculated. The difference in voltage when using an expanded metal anode was compared. The voltage drop value is 0.11V when the surface with the anode active coating left is 0.11V, and 0.06V when it is on the side of the hole.
In the case of the back side, it was 0.03V. It is surprising that the presence of an anode active coating only on the back side of a perforated flat plate anode equalizes the current distribution within the cation exchange membrane and lowers the voltage compared to when an expanded metal anode is used. However, there is a larger voltage drop in the order of the pore side and the surface, and in this order, the current distribution within the cation exchange membrane becomes more uniform, and at the same time, when each surface has an anode active coating, This seems to indicate that a large amount of current is being shared. In addition, when we measured the thickness of the anode active coating on each surface after energizing a perforated flat plate anode whose entire surface was anode active coated for 6 months under the above-mentioned electrolytic conditions, we found that the ratio of front surface:hole side surface:back surface was 2:
The ratio was 1.4:1. The measurement method is Shimadzu X
Ray microanalyzer ARL-EMX-SM-2
Using a mold, the characteristic X-rays of the Ru component of the anode active coating and the Ti component of the base material were recorded on a chart, and the ratio of the area of the diagram was determined using a sample whose thickness of the anode active coating was known in advance. The thickness of the residual anode active coating was determined from the calibration curve, and the consumed thickness was calculated. The reason why the wear on the front and pore sides is greater than the wear on the back side may be due to the higher current density and the closer proximity to the alkaline cation exchange membrane.

In this way, the perforated flat plate anode, which has a large voltage reduction effect and wears out quickly, has a thicker anode active coating on the front and hole sides than the back side, and is extremely effective as an anode that has a long life and a long-lasting voltage reduction effect. It brings big profits.

The ratio of the thickness of the anode active coating on the front surface and the side surfaces of the hole to the back surface is determined based on the conditions selected so that the anodic active coating on each side is removed at the same time, since the wear on each side differs depending on various electrolytic conditions. It is also desirable to select the thickness of each surface. Preferably
It is better to increase the thickness of the surface and the side surfaces of the hole by at least 1.5 times. Further, as mentioned above, since the electrolytic voltage reduction effect on the back side is small, the anode active coating on the back side may be eliminated.

A perforated flat plate is generally made by flat plate punching, and the flat plate may be made into expanded metal and then flattened by pressing or the like. The opening may have any shape as long as it can be processed, but a circular shape that can be easily punched is particularly preferred. In the case of a circular shape, the openings are preferably located in a so-called 60-degree staggered shape or a 45-degree staggered shape, in which the center of the circular opening is located at the apex of an equilateral triangle or a right triangle. Also, the smaller the diameter, the better, from 0.5 mm to 6 mm, preferably from 1 mm to 5 mm. It is also effective to appropriately roughen the anode surface adjacent to the cation exchange membrane by sandblasting, chemical etching, cutting grooves, or the like.

The thickness of the perforated plate needs to be strong enough not to be significantly deformed when the cation exchange membrane is pressed toward the anode, and a suitable thickness is 0.8 to 3 mm.

The material of the perforated flat plate anode may be one commonly used for electrolysis of alkali metal chloride solutions. That is, titanium, zirconium, tantalum, niobium, and their alloys are used as the base material, and as the anode active coating material, materials mainly composed of noble metal oxides such as ruthenium oxide, noble metals, noble metal alloys, etc. that generally exhibit anodic activity are used. things are used. In addition, degreasing, polishing, acid treatment, etc. for enhancing the adhesion between the base material and the anode active coating may be carried out by conventional methods. The anode active coating can be obtained by dissolving precious metal chloride in water, hydrochloric acid, organic solvent, etc., applying it to the base material, and then thermally decomposing it, electroplating method, electroless plating method, and plating method. Heat treatment methods such as plasma spraying and ion plating are used. Any method may be used to obtain a thick anode active coating only on the surface and the side surfaces of the pores, but in the case of a method in which the coating is thermally decomposed after coating, it is sufficient to apply the coating only on the surface and the side surfaces of the pores and then thermally decompose the coating. In the case of the plating method, it is preferable to place a counter electrode only on the front side, or to apply an anti-plating paint after adding the necessary thickness to the back side, and then continue plating.

An electrolytic cell in which the anode of the present invention is used is disclosed in Japanese Unexamined Patent Publication No. 51
As described in No. 68477, it is preferable to provide a space behind the anode and cathode to improve gas release. A common cathode for hydrogen generation is used as the cathode. Further, the cation exchange membrane is not particularly limited, and generally any membrane used in aqueous alkali metal chloride solution electrolysis can be used. The ion exchange group may be of the sulfonic acid type, carboxylic acid type, or sulfonic acid amide type, but the carboxylic acid type with a good alkali metal transfer number or the combination type of carboxylic acid and sulfonic acid is optimal. In this case, it is most preferable to use the side where the sulfonic acid group is present as the anode side and the side where the carboxylic acid group is present as the cathode side. As the resin matrix, fluorocarbon resin is excellent in terms of chlorine resistance. In addition, it may be lined with cloth, net, etc. to improve strength.

The perforated flat plate anode of the present invention, in which the thickness of the anode active coating on the front surface and the side surface of the hole is larger than that on the back surface, has an electrolytic voltage of 0.15 to 0.2V compared to the case of using an expanded metal anode, which is commonly used in the past. Compared to a perforated flat plate anode in which the coating thickness on each surface is the same, the voltage reduction effect lasts longer even when the total coating amount on each surface is the same, resulting in a long-life anode chamber. EXAMPLES Hereinafter, the present invention will be explained in detail with reference to Examples, but the present invention is not limited thereto.

Examples 1 and 2 Perforated flat plate anodes were manufactured as follows. Thickness 1mm
Circular holes with a diameter of 2 mm were punched in a titanium plate with a pitch of 3 mm in a 60-degree staggered pattern using the punching method. It was degreased with commercially available polishing powder, and further immersed in a 20% by weight aqueous sulfuric acid solution to roughen the surface at 85°C for 3 hours. On top of that, a solution of ruthenium trichloride dissolved in a 10% hydrochloric acid aqueous solution to give 40 g of ruthenium was applied with a brush.
The operation of baking at 450°C for 5 minutes in the air was repeated. In Example 1, the coating was repeated 7 times and the back side was not coated in all 7 times. The coating thickness on the surface and side of the hole is approx.
It was 1.9μ. Example 2 was repeated 5 times, the first two times the entire surface was coated, and the remaining three times only the front surface and hole side surfaces were coated, and the back surface was not coated. The thickness of the front surface and the side surface of the hole was about 1.6 μm, and the thickness of the back surface was about 0.6 μm, and the coating amount was the same in both Examples 1 and 2, which was about 190 mg.
If not applying to the back side, apply 1% rapeseed oil to the back side.
It was wiped with gauze impregnated with melted carbon tetrachloride and then applied to the surface and sides of the hole. Examples 1 and 2
Finally, they were heat treated at 500°C for 3 hours in the air.

The cation exchange membrane was prepared as follows. Polymerize perfluoropropionyl peroxide with tetrafluoroethylene and perfluoro-3,6-dioxy-4-methyl-7-octensulfonyl fluoride in 1,1,2-trichloro-1,2,2-trifluoroethane. As an initiator, copolymerize a polymer with an equivalent weight of 1350 (polymer 1) and an equivalent weight of 1090
A polymer (Polymer 2) was obtained. These equivalent weights were determined by washing a portion of the polymer with water, saponifying it, and then titration.

These polymers were heat-molded to form a two-layer laminate with a thickness of 35 μm (polymer 1) and 100 μm (polymer 2), and then a Teflon woven fabric was applied from the side of polymer 2 using a vacuum lamination method. Embedded by. Only the surface of the polymer 1 of the sulfonic acid type cation exchange membrane obtained by saponifying the laminate was subjected to reduction treatment to convert it into carboxylic acid groups. (Side A) The electrolytic cell has a current-carrying area of 10 cm x 10 cm, the anode chamber frame is made of titanium, the cathode chamber frame is made of stainless steel, and there is a space of 3 cm behind each of the facing anode and cathode. was used.

A cation exchange membrane was installed in this electrolytic cell so that polymer 1 (side A) was on the cathode side, and the anode chamber was
A 3N saline solution with a pH of 2 and a 5N caustic soda aqueous solution were supplied to the cathode surface, and the current distribution was 50A/d while maintaining the cathode chamber pressure 1 m higher in water column than the anode chamber pressure.
m ² and electrolyzed at a temperature of 90 °C. The electrolysis voltage in Example 1 is
3.88 to 3.92V, Example 2 was operated stably at 3.85 to 3.90V, but Example 1 was operated stably at 3.85 to 3.90V, but Example 2
After 1.6 months, the voltage started to rise, and at the same time, the anode potential also started to rise, and the anode life was reached.

Comparative Examples 1 and 2 Perforated flat plate anodes were manufactured using the same base material and in the same manner as in Example 1. In Comparative Example 1, only the back side was coated four times to give a thickness of about 4.5μ. In Comparative Example 2, the coating was repeated four times so that the entire surface had the same thickness, and the total coating amount was about 190 mg, the same as in Examples 1 and 2. Finally in the atmosphere
It was heat treated at 500°C for 3 hours and used as an anode.

When electrolysis was performed in the same manner as in Example 1 using the same cation exchange membrane and the same apparatus as in Example 1, the electrolysis voltage in Comparative Example 1 was extremely high at 4.02V. In Comparative Example 2, the electrolytic voltage was stable at 3.85 V to 3.90 V, but after 13 months, the electrolytic voltage and anode potential began to rise and the life span was reached.

Example 3 As a perforated flat plate anode, circular holes with a diameter of 2 mm were punched in a titanium plate with a thickness of 1 mm at a pitch of 4 mm in a staggered pattern at a 45 degree angle. After pretreatment in the same manner as in Example 1, a solution containing ruthenium trichloride dissolved in ethyl alcohol to give 40 g of ruthenium and 10% of commercially available ethyl cellulose as a thickener was applied with a brush and heated at 450°C. Baked in air for 5 minutes. The surface and hole sides are coated 5 times, the back side is coated only once, and the coating thickness on the front and hole sides is
The coating thickness on the back side was 0.35μ. Moreover, the total coating amount was about 190 mg/dm ² . Finally in the atmosphere
Heat treatment was performed at 500°C for 3 hours.

The cation exchange membrane was prepared as follows. A copolymer of tetrafluoroethylene and CF ₂ = CFO (CF ₂ ) ₃ COOCH ₃ , with an equivalent weight of 650 and a thickness of 250μ, a Teflon woven fabric is embedded in a film using a hot press lamination method, and then hydrolyzed to form carbon dioxide. An acid type ion exchange membrane was obtained.

Electrolysis was carried out using the same apparatus and method as in Example 1 using these cation exchange membranes and anodes. In this example, the current density was 20 A/dm ² , the pH of the saline solution was 3, and the concentration of caustic soda was 13N. The electrolysis voltage was stable at 3.60 to 3.65V, but after 23 months, the electrolysis voltage and anode potential began to rise.

Comparative Example 3 As a perforated flat plate anode, the same punched metal as in Example 3 was used, and after the same pretreatment, the same coating solution was applied twice to each surface, and the total coating amount was approximately 190 mg/dm ² and Example 3. I made it the same as The coating thickness is on each side.
It was 1.35μ. When electrolysis was carried out under the same conditions using the same cation exchange membrane as in Example 3, the electrolysis voltage was 3.60.
The voltage remained the same at ~3.65V, but after 18 months, both the electrolytic voltage and the anode potential began to rise.

Claims (1)

[Scope of Claims] 1. In a perforated flat plate anode used in an alkali metal chloride aqueous solution electrolytic cell that is divided into an anode chamber and a cathode chamber by a cation exchange membrane, the thickness of the anode active coating on the front surface and the hole side surface is equal to that of the back surface. An anode for electrolysis, characterized in that the thickness is greater than the thickness of the anode active coating. 2. The electrolytic anode according to claim 1, which is used in an electrolytic cell in which the cathode chamber pressure is higher than the anode chamber pressure.