JPH059520B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to an electrolytic cell for alkali chloride solution, and provides an electrolytic cell that completely prevents corrosion occurring in the cathode chamber of a unit electrolytic cell. As a method for producing caustic alkali and chlorine by electrolyzing an aqueous alkali chloride solution, for example, an anode and an anode are provided by sandwiching an ion exchange membrane to form a cathode and anode chamber.
Obtaining caustic alkaline solution by electrolyzing alkali chloride,
There is a so-called ion exchange membrane method and an alkali chloride electrolysis method. An electrolytic cell plant using such a method is formed by electrically connecting monopolar electrolytic cells in series, or by electrically connecting a plurality of bipolar electrolytic cells in parallel and/or series. . An aqueous caustic alkali solution produced by electrolyzing an alkali chloride solution using such an electrolytic cell plant is collected in a tank or the like through a common passage provided between a plurality of unit electrolytic cells. Furthermore, liquid is supplied to each of the plurality of unit electrolytic cells through a common passage. However, since the unit electrolytic cells are electrically connected in series, the potential in each unit cell is different, and during electrolysis, the liquid supply to the unit electrolytic cell cathode chamber and the generated caustic soda aqueous solution are carried out through a common path. During extraction, a part of the electrolytic current leaks through the liquid, and due to the leakage of this current, the electrolytic cell body, especially the vicinity of the liquid supply nozzle and the produced liquid extraction nozzle, act as so-called electrodes, and this part acts as an electrode. forms a battery, and the parts that act as anodes are corroded due to anodic oxidation. In other words, electrolytic corrosion occurs. Conventionally, as a method of preventing electrolytic corrosion occurring in the nozzle portion of an electrolytic cell, an attempt has been made to insert an insulating pipe into the nozzle (Japanese Patent Laid-Open No. 174479/1983). In addition, when extracting the electrolytically produced liquid and the gas generated by electrolysis in a gas-liquid mixed phase from the discharge nozzle at the top of the electrolytic cell in a unit electrolytic cell, the gas generated by electrolysis forms a gas phase part at the top of the electrolytic chamber, and At a certain moment, only the generated gas is discharged from the discharge nozzle, and as the gas is discharged, the liquid level rises.
A phenomenon occurs in which the liquid is drained at a certain moment.
Such pressure fluctuations within the electrolytic chamber induce vibrations in the ion exchange membrane, electrodes, electrolytic cell, etc., causing membrane tears and electrode damage, which may require stopping electrolyzing operations and requiring disassembly and repair. be. As a method of preventing such pressure fluctuations within the electrolytic cell, an attempt has been made to install an insertion tube in the nozzle to suppress liquid level fluctuations within the electrolytic cell as much as possible (Japanese Patent Laid-Open No. 56-5988). ). There have also been attempts to guide a liquid supply pipe into the cathode chamber for the purpose of stirring the liquid within the cathode chamber. The method of installing such an insertion tube in the electrolytic cell, especially in the cathode chamber nozzle, as described above, prevents electrolytic corrosion that occurs inside the electrolytic cell, prevents membrane rupture and electrode breakage due to pressure fluctuations, and reduces the concentration of liquid that occurs in the cathode chamber. This is an effective method in terms of preventing non-uniformity of the color. However, if such a method is adopted,
It will give rise to new problems. That is, there is a problem of natural corrosion inside the nozzle and around the nozzle, and a problem of electrolytic corrosion near the opening of the insertion tube inside the electrolytic cell. If a high-quality material is used as the material for the cathode chamber, there will be relatively no problems, but there are issues with economical efficiency, manufacturability,
When the cathode chamber material and its attached nozzles are made of iron or iron-based metals in consideration of operability, etc., the nozzle and the cathode chamber body around the nozzle and insertion tube opening may be damaged for a certain period of time due to corrosion damage. After electrolytic operation, it is necessary to replace it or repair it by welding. In recent years, so-called active cathodes have been developed that reduce the hydrogen overvoltage of the cathode in order to reduce the bath voltage of the electrolytic cell, and some of these have been reported (Soda and Chlorine, No. 8, 1981, p. 427-449). According to this, if eluted ions such as heavy metals are present in the catholyte, they will electrochemically precipitate on the active cathode, resulting in the loss of the original low hydrogen overvoltage characteristics. In other words, in order to install an active cathode in a unit electrolytic cell and maintain the low hydrogen overvoltage characteristics of the cathode for a long period of time, it is necessary to completely prevent corrosion that occurs in the cathode chamber body and the nozzles attached to the cathode chamber body. be. Similarly, in the case of ion exchange membranes used in ion exchange membrane electrolysis, if metal ions eluted from the catholyte are present, they will be adsorbed or precipitated on the surface of the ion exchange membrane, reducing the membrane overvoltage characteristics. Therefore, this also creates the need to completely prevent corrosion of the cathode chamber body. As a result of repeated research by the inventors from the above points,
We have arrived at the present invention. That is, the present invention provides a unit electrolytic cell in which an insulating insertion tube is provided in a liquid supply nozzle and/or a produced liquid extraction nozzle installed in a cathode chamber made of an iron or iron-based metal substrate, in which the nozzle material is made of Ni. Alternatively, the present invention provides an electrolytic cell for an alkali chloride solution, characterized in that it is made of a Ni alloy. "In other words, the present invention provides an insulating insertion tube in a liquid supply nozzle and/or a produced liquid extraction nozzle installed in a cathode chamber made of an iron or iron-based metal base, and The present invention provides an electrolytic cell for an alkali chloride solution, characterized in that the product liquid extraction nozzle is made of Ni or a Ni alloy, and an auxiliary electrode is installed near the opening of the insertion tube. The present invention prevents electrolytic corrosion, pressure fluctuations, and uneven liquid concentration by providing an insulating insertion tube in the electrolytic cell, and protects the nozzle portion, the vicinity of the nozzle portion, and the insertion tube opening that occur at this time. This provides an electrolytic cell that completely prevents corrosion in the vicinity. In this case, the corrosion in the nozzle part and the vicinity of the nozzle part is caused by the gap structure between the insulating insertion tube and the nozzle. Iron or iron is difficult to prevent corrosion by cathodic polarization because it is difficult for the cathode current to flow in the area, which causes natural corrosion, concentration of alkali, and concentration of eluted metal salts. As a result of extensive research, they discovered that the corrosion of metal nozzles is severe, and further revealed that by changing the nozzle material to Ni or Ni alloy, it is possible to completely prevent corrosion that occurs in the nozzle and the vicinity of the nozzle. In the present invention, the electrolytic cell main body is a member constituting the cell, and for example, in the case of a polar chamber pool type electrolytic cell, the box body corresponds to this; in the case of a filter press type electrolytic cell, the chamber frame corresponds to this. The material of the anode chamber and the nozzle attached to the anode chamber, which are the members that make up the tank, are generally titanium, titanium alloy, tantalum, zirconium,
Niobium etc. are used. As the cathode chamber material used in the present invention, iron, mild steel, cast iron, stainless steel, etc. are used in consideration of economic efficiency, manufacturability, operability, etc. In the present invention, the nozzle material attached to the cathode chamber must be made of a material that will not be corroded at all even in natural corrosion conditions without cathodic protection or in conditions where alkali is concentrated.
Ni alloy is preferred, and Ni is more preferred.
The Ni alloy in the present invention has a Ni content of 30% or more, and the Ni content in the Ni alloy is determined by conditions such as the temperature and concentration inside the cathode chamber. The nozzles attached to the anode and cathode chambers are generally connected by welding or the like. When welding the cathode chamber nozzle, it is desirable to use a Ni welding rod from the viewpoint of corrosion resistance. It goes without saying that the nozzle of the electrolytic cell according to the present invention can be of any type as long as it has excellent corrosion resistance under the extremely harsh environment of the crevice. For example, coating the inside of the nozzle using a plating method, thermal spraying method, vapor deposition method, etc., or lining the inside of the nozzle with Ni or Ni alloy using an explosion bonding method, a pressure bonding method, etc. are also preferable methods in consideration of economic efficiency. Although pipe-shaped nozzles are often used as liquid supply and liquid extraction nozzles, various other shapes are also used. In the present invention, it is essential that the insertion tube installed inside the liquid supply/discharge nozzle be electrically insulating. This is because if the insertion tube is electrically conductive, a corrosive current may flow through the insertion tube. The material for the insertion tube may be any material as long as it can withstand caustic soda solution, hydrogen gas, etc. at electrolytic temperatures and is electrically insulating. Examples include Teflon, asbestos, vinyl chloride, acrylic resin, EPDM rubber, and other rubbers. The insertion tube is often pipe-shaped and has at least one opening. In the electrolytic cell of the present invention, it is essential to install an auxiliary electrode near the opening of the insertion tube. The auxiliary electrode is made of Ni, Zr, Ag, etc., and by installing such an auxiliary electrode, corrosion inside the cathode chamber can be completely prevented. According to the present invention, in a unit electrolytic cell in which an insulating insertion tube is provided in a liquid supply nozzle and/or a produced liquid extraction nozzle installed in an electrolytic cell cathode chamber made of an iron or iron-based metal substrate, the nozzle material Ni
By using alloy or Ni, corrosion of the electrolytic cell body and the metal nozzle attached to the electrolytic cell body can be completely prevented. For this reason, the life of the electrolytic cell is significantly extended, and the electrolytic cell is extremely economical in that it can maintain a sufficient life without using expensive metals as the electrolytic cell main body material. The present invention is an extremely effective electrolytic cell from the viewpoint of preventing membrane rupture and electrode breakdown due to pressure fluctuations within the cathode chamber, as well as preventing uneven liquid concentration occurring within the cathode chamber. In the electrolytic cell of the present invention, the amount of eluted metal ions in the cathode chamber supply liquid and the cathode chamber effluent is almost zero, so even when an active cathode with low hydrogen overvoltage characteristics is used, metal ions eluted on the active cathode. It is possible to sustain stable electrolytic operation for a long period of time without causing deterioration due to adsorption and precipitation reactions. In addition, since there are no eluted metal ions, when a cation exchange membrane is used as a diaphragm, an amazing lifespan can be maintained without causing any adverse effects on the ion exchange membrane, that is, deterioration in the performance of the ion exchange membrane. Next, examples of the present invention will be described. Example 1 The anode has a bipolar electrode made of expanded titanium metal coated with ruthenium oxide and titanium oxide, and the cathode is made of expanded metal of Fe.The main body has a current carrying area of 200 dm, with the anode chamber made of titanium and the cathode chamber made of mild steel. ²
A cell unit (2 m wide x 1 m high) was used. At this time, both the liquid supply nozzle installed in the cathode chamber and the produced liquid extraction nozzle were made of Ni. A schematic diagram of a unit electrolytic cell cathode chamber is shown in FIG. A liquid supply pipe 3 and a produced liquid extraction pipe 4 made of fluororesin were installed in the nozzle 2 of the electrolytic cell cathode chamber main body 1, and the produced liquid extraction pipe 4 was connected to a dropper 5. Further, passages 6 and 7 common to the cell unit were connected to the liquid supply pipe 3 and the dropper 5, and an insertion tube 8 made of fluororesin was provided in the liquid supply nozzle and the produced liquid extraction nozzle. Also, near the insertion tube opening or inside the insertion tube.
An auxiliary electrode 9 made of Ni was installed. A cation exchange membrane is installed in the 25 pairs of cell units mentioned above.
Nafion 901 (trade name, manufactured by DuPont) was inserted as a diaphragm to create a bipolar electrolytic cell. Next, the salt was electrolyzed at a current density of 30 A/dm ² and an electrolysis temperature of 90°C. The concentration of Fe in the produced liquid at that time was measured every month. As a result, even after two years of operation, the Fe concentration in the produced caustic soda was all below 0.3 ppm. After two years of electrolytic operation, the electrolytic cell was dismantled and the corrosion situation was investigated, but the inside of the electrolytic cell, In particular, no corrosion was observed in or around the nozzle. Example 2 Salt electrolysis operation was carried out under the same conditions as in Example 1 except that the cathode in Example 1 was replaced with an expanded metal plated with activated nickel and having low hydrogen overvoltage characteristics under the following plating conditions. (Electroplating conditions) 1 Plating bath composition Nickel sulfate 1.0M / Thiourea 0.2M / Boric acid 0.3M / 2 Plating conditions Bath temperature 40℃ Current density 0.5A/dm ² Plating time 1 hour As a result, the cell voltage was As shown in the table, it showed a constant value of about 3.13V with almost no change over time. After one year of electrolytic operation, the electrolytic cell was dismantled and the corrosion situation was investigated, but no corrosion was observed inside the electrolytic cell, and no deposits were detected on the cathode.

[Table] Comparative Example 1 In Example 2, both the material of the liquid supply nozzle installed in the cathode chamber and the material of the produced liquid nozzle were STPG.
Salt was electrolyzed under the same conditions as in Example 2 except that (mild steel) was used. As a result, the initial bath voltage during electrolysis operation was 3.11V. However, after several days passed, an increase in bath voltage was observed, and after about two months, the bath voltage reached about 3.50 V, which is the same as when using an iron cathode. After 3 months of electrolysis operation, when the electrolytic cell was disassembled, severe corrosion was observed in and around the liquid supply nozzle and liquid extraction nozzle, and a considerable amount of corrosion was also observed on the cathode and ion exchange membrane. Precipitation of Fe had occurred. From this, it has become clear that the electrolytic cell of the present invention not only has complete corrosion prevention means, but also can maintain excellent electrolytic performance from the viewpoint of energy saving. Comparative Example 2 In Example 2, salt electrolysis was carried out under the same conditions as in Example 2, except that the material of the liquid supply nozzle installed in the cathode chamber was STPG. As a result, the bath voltage at the beginning of electrolysis operation was 3.11V, but after several days, the bath voltage increased to about 3.11V.
After a month had passed, the bath voltage was approximately 3.38V. After 4 months of electrolytic operation, I disassembled the electrolytic cell and found that
Severe corrosion was observed in and around the liquid supply nozzle, and a considerable amount of Fe was also deposited on the cathode and ion exchange membrane. Comparative Example 3 In Example 2, the material of the liquid extraction nozzle was STPG.
Salt electrolysis was carried out under the same conditions as in Example 2, except that the conditions were the same as in Example 2. As a result, the bath voltage at the beginning of electrolysis operation was 3.11V, but after several days, the bath voltage increased to about 3.11V.
After a month had passed, the bath voltage was approximately 3.42V. After 4 months of electrolytic operation, I disassembled the electrolytic cell and found that
Severe corrosion was observed in and around the liquid extraction nozzle, and a considerable amount of Fe was also deposited on the cathode and ion exchange membrane. Comparative Example 4 In Example 2, salt electrolysis was carried out under the same conditions as in Example 2, except that no Ni auxiliary electrodes were installed near the insertion tube opening or inside the insertion tube. As a result, the bath voltage at the beginning of electrolysis operation was 3.11V, but after a few days, the bath voltage increased to about 2V.
The bath voltage after a month had passed was approximately 3.35V. After 3 months of electrolytic operation, I disassembled the electrolytic cell and found that
Corrosion was observed near the opening of the insertion tube in the cathode chamber of 10 of the 25 unit electrolytic cells, and a considerable amount of Fe was also deposited on the cathode and ion exchange membrane.

[Brief explanation of the drawing]

FIG. 1 is an explanatory diagram schematically showing a unit electrolytic cell for an alkali chloride solution in the present invention. 1 is a unit electrolytic cell, 2 is a nozzle, 3 is a cathode chamber liquid supply pipe, 4 is a cathode chamber produced liquid extraction pipe, 5 is a dropper, 6
7 is a common passage for cathode chamber liquid supply pipes, 7 is a common passage for cathode chamber produced liquid extraction pipes, 8 is an insertion pipe, and 9 is an auxiliary electrode.

Claims (1)

[Claims] 1. An insulating insertion tube is provided in the liquid supply nozzle and/or the produced liquid extraction nozzle installed in the cathode chamber made of iron or iron-based metal substrate, and An electrolytic cell for alkali chloride solution electrolysis, characterized in that the material is made of Ni or a Ni alloy, and an auxiliary electrode is installed near the opening of the insertion tube.