JP3304221B2 - Method for removing chlorate from aqueous alkali chloride solution - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to the purification of an aqueous alkali chloride solution (hereinafter also referred to as "brine water") for electrolysis of an aqueous alkali chloride solution by an ion exchange membrane method, and particularly to chloric acid accumulated in circulating saline water. It relates to a method for removing salts in an effective and economical manner.

[0002]

2. Description of the Related Art When electrolyzing salt water in an electrolytic cell partitioned into an anode chamber and a cathode chamber by an ion exchange membrane, hydroxide ions in the cathode chamber permeate the ion exchange membrane and become chlorinated in the anolyte. It is known that chlorate reacts with chlorate. Chlorate accumulates in salt water with the progress of electrolysis, permeates through the cation exchange membrane, diffuses into the cathode compartment, and impairs the quality of alkali hydroxide. Removal has been performed to maintain the concentration below a certain level, and various removal methods have been proposed.

For example, there is known a method in which hydrochloric acid is excessively added to salt water, and chlorate in the salt water is decomposed and removed by the following reaction. However, this reaction does not proceed rapidly unless hydrochloric acid several times the theoretically required amount is added. Therefore, a large amount of the hydrochloric acid is used for neutralizing excess hydrochloric acid after the reaction is completed because ClO ₃ ⁻ +6 HCl → 3Cl ₂ + Cl ⁻ + 3H ₂ O. There was a disadvantage that alkali hydroxide was required.

As an improved method, in the electrolysis of an alkali chloride aqueous solution using an ion exchange membrane method, a part of the anolyte containing chlorate is extracted, excess hydrochloric acid is added, and the chlorate in the salt water is decomposed. A method for preventing the accumulation of chlorate by recovering chlorine in the dechlorination step of (1) to save the alkali for neutralization (JP-A-53-18498 and JP-A-54-1984)
-28294) has been proposed. However, even in this method, the amount of hydrochloric acid added is still excessive, and in order to reduce the amount of alkali for neutralization, the amount of treated salt water must be kept low. As a result, the chlorate concentration in the salt water is reduced to 10 g / l. It has drawbacks such as being difficult to maintain below.

Further, a method of introducing circulating salt water supplied to an electrolytic cell of an aqueous alkali chloride solution by an ion exchange membrane method into a chlorate decomposition catalyst layer provided in a circulation path in the presence of hydrogen or a gas containing hydrogen. (JP-A-56-163286)
) Or a method using a chlorate decomposition catalyst that does not require hydrogen gas (Japanese Patent Application Laid-Open No. 3-65507). However, in these methods, nickel or cobalt is required to achieve a high decomposition rate. It is necessary to use, as a catalyst, a metal that causes a decrease in the performance of an ion exchange membrane installed in an alkali chloride electrolytic cell such as an ion exchange membrane method, and there is a risk of elution into salt water. Furthermore, the method of extracting and cooling chlorate by extracting a part of the circulating salt water and cooling it (Japanese Patent Laid-Open No. 51-144399) complicates the process, and the cooling and crystallization separation are costly. And it is difficult to employ it industrially. Also, a method of adding a reducing agent such as sodium sulfite, hydrogen sulfide, formaldehyde or the like to salt water (JP-A-53-1233396, JP-A-60-77982, and JP-A-3-153890).
However, in these methods, the cost of chemicals to be added is required, and in the removal step, sulfate that causes a decrease in the performance of the ion exchange membrane is generated. In addition, since the chlorine gas generated by the reaction with formaldehyde contains carbon dioxide, it cannot be treated as it is in the same manner as the chlorine gas generated in the electrolytic cell. And there is a problem that storage and handling of chemicals are dangerous.

[0006]

SUMMARY OF THE INVENTION According to the present invention, the amount of hydrochloric acid to be added can be extremely low, and the chlorate in the salt water can be efficiently removed without accumulating impurities and the like in the treated salt water. It is an object of the present invention to provide a method for performing the above.

[0007]

Means for Solving the Problems In order to solve the above-mentioned problems, the present inventors have conducted intensive studies on a method for decomposing chlorate in salt water. As a result, hydrochloric acid was added to salt water containing chlorate to obtain an acidic solution. After that, it is introduced into the cathode compartment of the ion exchange membrane electrolytic cell partitioned by the cation exchange membrane, and into the anode compartment, an electrolytic solution in which a proton generation reaction occurs at the anode is introduced, and electrolysis is performed. The inventors have found that chlorate can be removed, and have completed the present invention. Further, it is a method for removing chlorate in salt water which is electrolyzed using sulfuric acid as an anolyte.

That is, according to the method of the present invention, chlorate is decomposed and removed in the cathode chamber of the ion exchange membrane electrolytic cell, and a proton generation reaction is caused in the anode chamber. This is a method for removing chlorate, which is capable of efficiently decomposing chlorate by preventing an increase in pH in the liquid. The cation exchange membrane used in the ion exchange membrane electrolytic cell used in the method of the present invention can be a fluororesin type and a hydrocarbon type,
In particular, a perfluorosulfonic acid-based cation exchange membrane having excellent corrosion resistance is desirable. Hydrocarbon-based cation exchange membranes are not preferable for long-term use because the temperature of the catholyte compartment becomes high and the anode compartment becomes an extremely oxidizing atmosphere. The electrolytic cell may have any structure such as a filter press type and a box type.

The salt water to be introduced into the cathode chamber is suitably one having a concentration of an alkali chloride such as sodium chloride or potassium chloride of about 150 to 300 g / l, which is used for ordinary alkali chloride electrolysis. Prior to introduction into the cathode compartment, hydrochloric acid is added to the salt water to make the solution acidic, which is necessary for efficient reduction of chlorate on the cathode surface of the cathode compartment. Cathodic reduction of chlorate proceeds when the salt water is in the acidic region, but the pH of the salt water is desirably about 1 to 2 for the reaction to proceed with high efficiency.

When the concentration of the chlorate in the circulating salt water in the alkali chloride electrolysis by the ion exchange membrane method is about 10 g / l, it is sufficient if the pH of the salt water is about 2, and immediately after the dechlorination step. When the salt water is extracted and supplied to the electrolytic cell of the present invention, almost no new hydrochloric acid needs to be added. Also, the concentration of chlorate in the circulating saline was 5 g /
When it is set to about 1, it is desirable to set the pH of the salt water to about 1. In order to keep the chlorate decomposition efficiency in the cathode chamber high, the temperature of the salt water should be at least 6 ° C.
The temperature is set to 0 ° C. or higher, preferably 75 ° C. or higher.

At the cathode, both the reduction of chlorate and the hydrogen generation reaction occur. However, when the chlorate concentration in the salt water is high and the degree of negative polarization is small, no generation of hydrogen is observed on the cathode. . This is because the cathode reduction potential of chlorate is more noble than the hydrogen generation potential. When the degree of negative polarization increases and generation of hydrogen is confirmed, the current efficiency is significantly reduced. For the cathode, an electrode using a noble metal such as palladium oxide, platinum, ruthenium oxide or an oxide thereof as an electrode catalyst material, titanium, zirconium, or the like can be used. In particular, palladium oxide, platinum, ruthenium oxide, or the like is used as an electrode catalyst material. The preferred electrode is palladium oxide.

In the anode chamber of the ion exchange membrane electrolytic cell of the present invention, the anode chamber liquid is preferably an inorganic acid such as sulfuric acid or nitric acid which generates protons by an anodic reaction. When sulfuric acid is circulated as the anolyte, protons are generated in the anode chamber at the same time as oxygen is generated, and the generated protons move into the cathode chamber. On the other hand, in the cathode chamber, the salt water introduced
Although hydroxide ions are generated and the pH in the cathode chamber increases, the transferred protons react with hydroxide ions generated in the cathode chamber and play a role in suppressing the increase in pH of the salt water.

In the anode compartment, protons are generated together with oxygen, and hydration water is transferred to the cathode compartment together with the protons on the cation exchange membrane. Therefore, water consumed by the anodic reaction to maintain the concentration of the anode compartment liquid constant. It is only necessary to replenish the anolyte with hydration water. The higher the concentration of the anolyte, the lower the electric resistance of the solution and the lower the electrolytic cell voltage. However, in consideration of the corrosion resistance of the anode material and the like and the efficiency of proton generation, about 0.5 to 2 normal is desirable. As the anode, an electrode using a noble metal such as platinum or iridium oxide, a noble metal oxide as an electrode catalyst material, a lead dioxide electrode, or the like can be used.In particular, iridium oxide is used for a base of a thin film-forming metal such as titanium or tantalum. It is preferable to use an electrode on which a coating is contained.

[0014]

According to the method for removing chlorate of the present invention, protons generated in the anode compartment are transferred to the cathode compartment in the electrolytic cell in which the anode compartment and the cathode compartment are partitioned by a cation exchange membrane, and the pH of the cathode compartment is adjusted.
Is a method for efficiently removing chlorate in the cathode chamber by constantly maintaining acidity, and the amount of hydrochloric acid added to the salt water can be greatly reduced.

[0015]

The present invention will be described in more detail with reference to the following examples and comparative examples. Example 1 An anode having an iridium oxide coating formed on a titanium substrate was provided in an anode chamber of an electrolytic cell partitioned by a cation exchange membrane (Nafion 550 manufactured by DuPont), and a palladium oxide was formed on a titanium substrate in a cathode chamber. And a platinum coated with platinum at a ratio of 4: 1 were provided, 300 ml of 1N sulfuric acid (reagent grade, manufactured by Wako Pure Chemical Industries, Ltd.) was circulated in the anode chamber, and 5.071 g / l chlorine was passed through the cathode chamber. Sodium acid (manufactured by Wako Pure Chemical Industries, Ltd.)
A saline solution containing a reagent (special grade) having a concentration of 200 g / l was adjusted to pH 1 with hydrochloric acid, and 300 ml was circulated. The electrolyte temperature is 75 to 85 for both the anode chamber circulating liquid and the cathode chamber circulating liquid.
While the temperature was adjusted to be ℃, the anode and the cation exchange membrane were in close contact with each other, and the distance between the cathode and the cation exchange membrane was 13 mm.
And electrolysis was performed at a current density of 6.29 A / dm ² . The electrolysis was continued for 3 hours, and the sodium chlorate concentration in the salt water after 1, 2 and 3 hours had passed was measured using an ion chromatography analyzer (IC-7 manufactured by Yokogawa Analytical Systems).
000E column ICS-A13). Table 1 shows the results.

[0016]

[Table 1]

Here, the current efficiency indicates the average current efficiency for one hour before the measurement time, and the average current efficiency indicates the average current efficiency from the start of operation to the measurement time. From Table 1, it was found that if the concentration of sodium chlorate was up to about 3 g / l, decomposition was possible with almost 100% efficiency. Further, one hour after the start of the operation, the generation of hydrogen began to be observed on the cathode, and at the same time, the tendency of the reduction in current efficiency became remarkable.

Example 2 Using the same apparatus as in Example 1, a test was conducted under the following conditions only.

Cathode: Changed to a titanium substrate coated with iridium oxide. Cathode chamber circulating fluid: the concentration of sodium chlorate is 4.070 g
/ L 200g / l saline (pH = 1) changed to 300ml. Current density: changed to 3.14 A / dm ² . Table 2 shows the results.

[0020]

[Table 2]

Here, the current efficiency indicates the average current efficiency for one hour before the measurement time, and the average current efficiency indicates the average current efficiency from the start of operation to the measurement time. In the case of the iridium oxide-coated cathode, when the current density is further increased, generation of hydrogen becomes remarkable, and it has been difficult to maintain high current efficiency.

Example 3 Using the same apparatus as in Example 1, a test was conducted under the following conditions only. Cathode: Changed to platinum-coated titanium Cathode chamber circulating fluid: The concentration of sodium chlorate is 5.562 g
/ L 200g / l saline (pH = 1) changed to 300ml. Table 3 shows the results.

[0023]

[Table 3]

Here, the current efficiency indicates the average current efficiency for one hour before the measurement time, and the average current efficiency indicates the average current efficiency from the start of operation to the measurement time. Example 4 A test was performed using the same apparatus as in Example 1 except for changing only the following conditions.

Cathode: changed to titanium Cathode chamber circulating fluid: sodium chlorate concentration of 5.048 g
/ L 200g / l saline (pH = 1) changed to 300ml. Table 4 shows the results.

[0026]

[Table 4]

Here, the current efficiency indicates the average current efficiency for one hour before the measurement time, and the average current efficiency indicates the average current efficiency from the start of operation to the measurement time.

[0028]

According to the method of the present invention, chlorate in salt water can be easily reduced and removed by adding a small amount of hydrochloric acid. In particular, in the case of ion exchange membrane method salt electrolysis, the hydrochloric acid used in the present invention can be recovered by returning the salt water after chlorate removal to the dechlorination step of returning salt water from the electrolytic cell, and it is difficult with the conventional hydrochloric acid decomposition method. Sodium chlorate management concentration 5
g / l or less can be easily achieved, and is of high value as an industrial process.

Continuation of front page (56) References JP-A-53-18498 (JP, A) JP-A-54-28294 (JP, A) JP-A-56-163286 (JP, A) JP-A-3-65507 (JP) JP-A-51-144399 (JP, A) JP-A-53-123396 (JP, A) JP-A-60-77982 (JP, A) JP-A-3-153890 (JP, A) 1-219185 (JP, A) (58) Fields investigated (Int. Cl. ⁷ , DB name) C25B 1/00-15/08

Claims (2)

(57) [Claims] In the method for removing chlorate from an aqueous alkali chloride solution, an aqueous chlorate-containing aqueous alkali chloride solution is introduced into a cathode compartment of an electrolytic cell partitioned by a cation exchange membrane, and a chlorate-containing aqueous alkali chloride solution is introduced into an anode compartment. A method for removing chlorate from an aqueous alkali chloride solution, comprising introducing an electrolytic solution that undergoes a proton generation reaction at the anode and performing electrolysis. 2. The method according to claim 1, wherein the anolyte is an inorganic acid.