JPH0343352B2 - - Google Patents

Description

Detailed Description of the Invention (Technical Field of the Invention) The present invention is directed to the electrolysis of an aqueous solution of alkali metal salts (hereinafter abbreviated as salt water), in which contaminants such as barium sulfate, etc., adhere to the electrolysis process, resulting in a decrease in electrolysis performance such as an increase in overvoltage. A simple regeneration method is provided to recover the performance of the anode by contacting the anode with ethylenediaminetetraacetic acid (hereinafter referred to as EDTA).

(Prior Art and its Problems) In recent years, diaphragm electrolysis using an ion exchange membrane has become mainstream as a method for producing highly pure and highly concentrated alkali metal hydroxides by electrolyzing salt water. The important point of development and improvement in such electrolysis technology is to reduce the electrolysis voltage as much as possible and bring it close to the theoretical electrolysis voltage. For this reason, for example, the structure and material of the battery case, the material, composition and shape of the anode and cathode, the composition of the ion exchange membrane, the type of ion exchange group, etc. have been studied, and various means have been proposed.

On the other hand, in order to lower the electrolytic voltage of the salt water used for electrolysis, impurities in the salt water are actively removed. As a method for removing impurities such as Ca ²⁺ , Mg ²⁺ , SO ₄ ^2- , etc. present in the salt water, a coagulant is added to the salt water and the impurities are removed by coagulation and precipitation. Furthermore, if necessary, it is common to purify it by passing it through a chelate resin. Among these, in the desulfurization process to remove SO ₄ ^2- from salt water, a barium compound such as barium chloride or barium carbonate is generally added to increase the desulfurization effect, and the SO ₄ ^2- is removed as barium sulfate. It will be done. However, in such a desulfurization method, when purified salt water is subjected to electrolysis by adding an excess of barium compound due to an analysis error or an incorrect addition method, the surface of the substrate constituting the anode chamber or the anode surface may be damaged. It was confirmed that barium sulfate was attached. Since such a state is accompanied by an increase in the battery cell (electrolysis) voltage, it is necessary to interrupt the electrolysis operation and perform cleaning work or dismantle the battery cell, making it difficult to operate continuously for a long period of time. Therefore,
It is necessary to easily and efficiently remove contaminants such as barium sulfate adhering to the anode chamber and the anode to recover and regenerate the electrolytic performance, particularly the anode performance.

In general, methods for removing barium sulfate adhering to the base of a battery case include mechanical removal using a brush or the like, or removal by dissolving it with a chemical solution. The inventors investigated using these methods and found that, although contaminants such as barium sulfate attached to relatively smooth surfaces such as the partition walls of battery containers can be easily removed, contaminants attached to the surface of the anode can be easily removed. It has been found that when barium sulfate is completely removed, problems such as damage to the substrate arise, and therefore it cannot be removed efficiently. For example, when immersed in concentrated sulfuric acid as a chemical solution, the anode substrate made of titanium or other material used for electrolysis of salt water as mentioned above will be damaged.
In the method of immersing the anode in EDTA, the above-mentioned barium sulfate adhering to the anode was not sufficiently dissolved, so it was not possible to recover the performance of the anode.
In other words, barium sulfate that simply adheres to a smooth substrate surface and barium sulfate that adheres to the surface of an anode that is complex and has an uneven surface made of titanium or expanded metal with a platinum group metal coated on the titanium surface during electrolysis are different. The situation is different, and the barium sulfate attached to the anode has stronger adhesion to the substrate, so it is presumed that it cannot be easily removed by the conventional method described above.

(Means for Solving the Problems) The inventors of the present invention have conducted intensive studies on a simple method for efficiently removing barium sulfate attached to the anode and restoring the anode performance. Pre-clean the anode with substances attached to it with ammonia.
After contacting with EDTA, we discovered that, for example, by washing with water or performing a light mechanical treatment, barium sulfate, a contaminant, was surprisingly efficiently removed and the anode performance was restored, which led us to propose the present invention. Ivy. That is, the present invention is characterized in that an anode whose electrolytic performance has deteriorated due to adhesion of contaminants due to electrolysis of an aqueous alkali metal salt solution is brought into contact with ammoniacal EDTA to bring the contaminants into a swollen state, and then removed. This is a method for regenerating anodes.

In the present invention, for the salt water electrolysis method, conditions in known ion exchange membrane processes can be employed without particular limitation. That is, in an electrolytic cell in which an ion exchange membrane is installed as a diaphragm between the anode and cathode, the anode chamber is supplied with salt water such as 3 to 5 normal (N) sodium chloride or potassium chloride, and one cathode chamber is supplied with salt water such as 3 to 5 N (N) sodium chloride or potassium chloride. Water or a dilute alkali metal hydroxide solution is supplied, and electrolysis is generally carried out at a temperature of room temperature to 95° C. and a current density of 10 to 50 A/dm ² . The electrolytic cell in the present invention may be of a monopolar type or a bipolar type as long as it has the above configuration. The materials used to construct the electrolytic cell include titanium, which is resistant to salt water and chlorine, for the anode chamber, and iron, stainless steel, and other materials that are resistant to high concentrations of alkali metal hydroxide and hydrogen, for the cathode chamber. Or nickel etc. are used. Furthermore, the anode may be made of an expanded metal such as titanium coated with a platinum group metal such as ruthenium or palladium, its alloys, their alloys, or their oxides, or the above metals, their alloys, and their oxides. A perforated plate, a mesh body, or the like consisting of a metal oxide is used, and among these, an expanded metal of titanium coated with a mixture of ruthenium oxide and titanium oxide is particularly preferable because it allows the electrolysis voltage to be lowered. On the other hand, for the cathode, a perforated plate made of mild steel, stainless steel, nickel, etc., a wire mesh, an expanded metal, or a material coated with a cathode active material of these materials is used.

In the present invention, when salt water is electrolyzed by such an electrolytic method, it is necessary to contact the anode with ammoniacal EDTA to degrade the electrolytic performance mainly due to adhesion of barium sulfate. Next, in the present invention, the above-mentioned barium sulfate-attached anode is treated with ammonia.
It is extremely important to remove the barium sulfate by, for example, washing with water or applying a slight mechanical treatment to the state in which the barium sulfate swells upon contact with EDTA, and the barium sulfate can be easily and efficiently removed, thereby improving the anode performance. recovery will be achieved.
By the way, if the above-mentioned anode is simply immersed in ammoniacal EDTA and subjected to contact treatment, the barium sulfate attached to the anode cannot be easily dissolved even if the pH, temperature, flow rate, etc. are changed. Processing operations take a long time and are complicated.

In the present invention, the swollen state of barium sulfate attached to the anode by contacting it with ammoniacal EDTA can be determined by immersing the anode in an ammoniacal EDTA solution and observing the surface of the anode.
It can be easily confirmed. Also ammoniacal
The anode can be immersed in the EDTA solution either with the anode removed from the battery case or with it still installed in the battery case, but considering ease of workability, the latter is preferred. is preferred.
In the latter method, an insulating and liquid-impermeable sheet is inserted in place of the ion exchange membrane to prevent damage to the cathode chamber substrate, cathode, and ion exchange membrane caused by ammoniacal EDTA solution. An ammoniacal EDTA solution is introduced into the anode, allowed to stay there for a required time, or allowed to flow through at an appropriate flow rate, or alternatively, brought into contact with the anode, and then discharged. At this time, ammonia is removed due to contact between the anode and a liquid-impermeable sheet such as polyethylene.
To prevent poor contact between EDTA and the anode surface, the pressure in the anode chamber (PA) is changed to the pressure in the cathode chamber (PK).
It is preferable to set the differential pressure higher than PA−PK.
= 10 to 200 mmAq is suitable.

The concentration of EDTA used in the present invention ranges from 0.01 weight (wt)% to saturation concentration, preferably from 0.1 wt% to 10 wt.
%, the EDTA is adjusted to have a pH of 9 or higher, preferably 9.5 to 13, using an ammonium buffer such as ammonium chloride (NH ₄ Cl) or ammonium hydroxide (NH ₄ OH).
If the concentration of EDTA is too low or the pH is less than 9, the barium sulfate removal effect will decrease.
Even if it is too high, no noticeable difference in removal effect will be observed.

Generally, the higher the temperature of the ammoniacal EDTA solution, the more effective the reaction with barium sulfate will be. However, if the temperature is too high, it will have an adverse effect on the members constituting the anode chamber, so a temperature of about 10 to 80°C is appropriate. Also,
The removal effect can be further enhanced by rapidly changing the flow rate of the ammoniacal EDTA solution to provide a shock to the barium sulfate adhering to the anode, or by causing minute bubbles to flow along with the ammoniacal EDTA.

After contact treatment with the ammoniacal EDTA solution described above, the anode is subjected to a light mechanical treatment such as brush cleaning or washing with running water, and then washed with water to remove mainly barium sulfate that has adhered to the surface of the anode. Since the contaminants are in a swollen state, they are easily removed.

(Effects) As explained above, according to the present invention, the barium sulfate can be removed by a simple operation by bringing the anode, which has deteriorated electrolytic performance due to adhesion of contaminants mainly from barium sulfate, into contact with an ammoniacal EDTA solution. It is efficiently removed, and the regenerated anode has the same performance as before the barium sulfate was deposited. Also, use
Since almost no contaminants such as barium sulfate are dissolved or mixed into the EDTA solution, it can be reused many times as is.

(Examples) Hereinafter, the present invention will be explained based on Examples, but the present invention is not particularly limited to the following Examples.

Example 1 Bipolar electrolytic unit with current carrying area of 270 ^dm2
An ion-exchange membrane electrolyzer consisting of 46 pairs is
Electrolysis of saline solution was carried out at 25 A/dm ² . The electrolysis conditions were as follows.

Anode chamber salt concentration (g/): 230 Cathode chamber caustic concentration (wt%): 32 Electrolysis temperature (°C): 75 Initially, the average electrolysis voltage was 3.45 V/pair. Later, due to trouble in the desulfurization process during the salt water purification process,
Contaminants, mainly barium sulfate, adhered to the anode, and the voltage rose to an average of 3.75V/pair. We investigated the cause of the voltage increase by measuring membrane resistance and anode overvoltage, and found that the voltage increase due to anode overvoltage was 0.25V/pair.

Therefore, the ion exchange membrane was removed and 50 kg of EDTA was removed.
7 m ³ of an aqueous solution containing 70 kg of ammonium chloride and 100 kg of ammonia was prepared and circulated in the anode chamber using an anolyte circulation system, and the treatment liquid was brought into contact with the anode for 72 hours. At this time, the solution temperature was maintained at 50°C, the anode chamber and cathode chamber were separated by a polyethylene film, pure water was circulated in the cathode chamber, and the pressure in the anode chamber was maintained at about 200 mmAq higher than the pressure in the cathode chamber.

After the treatment was completed, the container was removed and the anode was observed. Most of the deposits, mainly consisting of barium sulfate, had peeled off from the anode and settled at the bottom of the anode chamber, but some remained attached to the anode. When I sprayed it with pure water, it was completely removed. When a part of the treated anode was cut out and subjected to fluorescent X-ray analysis, no Ba was detected. Also, when the overvoltage was measured, it showed the same value as the initial value.

Therefore, after thoroughly cleaning the electrolytic cell and circulation system with pure water, the ion exchange membrane was reset and electricity was applied under the same electrolytic conditions, and the voltage decreased to an average of 3.50 V/pair.

Claims (1)

[Claims] 1. A method characterized in that an anode to which contaminants have adhered due to electrolysis of an aqueous alkali metal salt solution is brought into contact with ammoniacal ethylenediaminetetraacetic acid to cause the contaminants to swell, and then the swollen contaminants are removed. A method for regenerating anodes. 2. The regeneration method according to claim 1, wherein the contaminant is mainly barium sulfate. 3. An insulating and liquid-impermeable sheet is inserted between the anode chamber and the cathode chamber, and the pressure in the anode chamber is made higher than the pressure in the cathode chamber to swell the contaminants. How to play. 4. The regeneration method according to claim 1, wherein the swollen contaminants are removed by washing with water.