JPH0790659A - Removing method of chlorate in production process of caustic soda and analysis of chlorate - Google Patents

Abstract

PURPOSE:To provide a removing method of chlorate capable of efficiently removing the chlorate in a solution in a production process of caustic soda and accurately measure the concentration of the chlorate in the solution. CONSTITUTION:In a method for producing caustic soda by diaphragm process, the chlorate in the solution is decomposed by reduction to turn into sodium chloride and removed by adding sodium sulfite of 1-2 equivalent to the chlorate contained in the solution into a sodium chloride solution after adjusting pH or into a caustic soda solution after electrolysis. And the conc. of chlorate is accurately measured by analyzing the conc. of the chlorate after adding sodium hypochlorite and/or hydrogen peroxide water into the solution after the chlorate is removed, and removing by oxidation unreacted sodium sulfite to avoid the reaction of chlorate with sodium sulfite in analyzing.

Description

Detailed Description of the Invention

[0001]

FIELD OF THE INVENTION The present invention relates to a method for electrolyzing a sodium chloride solution to produce caustic soda, a method for removing chlorate from a solution containing chlorate, and a concentration of chlorate in the solution. Regarding how to analyze.

[0002]

2. Description of the Related Art As a method for producing caustic soda (NaOH), for example, a diaphragm method has been conventionally practiced. In addition, the mercury method and the ion exchange membrane method are also known. Since the quality of caustic soda produced by these methods is superior to that produced by the diaphragm method, the mercury method has been the mainstream in the past. However, it is not currently implemented due to administrative guidance triggered by mercury pollution.

By the way, the diaphragm method is a method for producing caustic soda by electrolyzing a salt solution, and the production process is as follows.
A salt water purification step of dissolving the raw salt and removing impurities in the raw salt, an electrolysis step of electrolyzing the salt water in an electrolytic cell to obtain caustic soda, and about 50 weight of 10 to 12% by weight caustic soda obtained by the electrolytic cell. The process consists of an evaporative concentration step of concentrating in an evaporator until the content becomes%.

[0004]

However, in the above-mentioned diaphragm method, in order to reduce the wastewater from the factory, the wastewater is promoted to be closed, and a part of the treated water (dissolved salt solution of precipitated salt) dissolves the original salt. Therefore, chlorate exists in salt water (saline solution) in which the original salt is dissolved, and the content of this chlorate (NaClO ₃ ) increases in the electrolytic solution (caustic soda solution) after electrolysis. Are known.
If a large amount of chlorate is present in salt water, the current efficiency at the time of electrolysis will be reduced, and the life of the electrolytic cell or the diaphragm partitioning the electrolytic cell will be shortened, which will cause a reduction in power consumption.

Further, the presence of a large amount of chlorate in the electrolytic solution shortens the service life of the nickel (Ni) evaporator used in the evaporative concentration step, but the evaporative concentration step is an important step in this manufacturing method. However, if the evaporator causes a trouble and the concentration process is stopped, a large loss is caused, and since nickel, which is a material of the evaporator, is expensive, the manufacturing cost increases if the number of times of replacement is large.

Therefore, it is necessary to remove the chlorate from the salt water or the electrolytic solution. For this method, for example, an aqueous caustic soda solution containing chlorate is contacted with a strongly basic anion exchange resin to remove impurities, and then contacted with activated carbon. A method of purifying an aqueous solution of caustic soda, or a caustic alkali solution containing impurities such as chlorate, after adding a small amount of ferrous salt or ferric salt and sodium formaldehyde sulfoxylate, heat treating this solution. To remove impurities, or to decompose chlorate in the presence of ruthenium catalyst in the presence of at least one reducing agent such as formaldehyde, alcohol or formal compound in an aqueous solution having a pH of 4 or more, etc. Is proposed.

However, the method using an ion-exchange resin requires an apparatus for contacting an aqueous solution of caustic soda with an ion-exchange resin or activated carbon, which complicates the apparatus structure and causes a large amount of chlorate in the electrolytic solution for the diaphragm method. There is a problem that the exchange cost of the exchange resin or activated carbon increases and the operating cost rises, and the method of adding ferrous salt and sodium formaldehyde sulfoxylate is
There are problems that the operating cost rises and that a step of removing the added substance after the treatment is required. Furthermore, the method of adding a reducing agent such as formaldehyde under a ruthenium catalyst is
The solution must be adjusted to pH 4 or more, and the treatment is troublesome, and there are problems that a catalyst layer is required, the catalyst life is long, and harmful formaldehyde remains due to the use of caustic soda in foods. is there.

As described above, various methods have been tried in the past for the removal of chlorates such as chlorate, but the effective decisive factor has not yet been found.
Further, as additives to be added for removing chlorate, there are restrictions such as not being harmful and causing no significant deterioration in the quality of caustic soda.

The present invention has been made under the circumstances as described above, and its purpose is in the manufacturing process of caustic soda.
It is an object of the present invention to provide a method for removing chlorate which can efficiently remove chlorate from a salt solution and / or a caustic soda solution.

Another object of the present invention is to provide a method for analyzing chlorate which can accurately measure the concentration of chlorate in a solution containing a reducing substance.

[0011]

The invention according to claim 1 is a method for producing a caustic soda solution by electrolyzing a salt solution, wherein sodium sulfite is added to the salt solution and / or the caustic soda solution. It is characterized in that the chlorate contained in the salt solution and / or the caustic soda solution is decomposed and removed.

The invention of claim 2 is a method for quantitatively analyzing the remaining amount of chlorate after removing chlorate contained in a solution having a pH of 2 or more by adding sodium sulfite to the solution. , A step of adding excess sodium hypochlorite and / or hydrogen peroxide solution to the solution to oxidize unreacted sodium sulfite, and thereafter adding silver peroxide to this solution to add sodium hypochlorite and And / or a step of decomposing hydrogen peroxide water, and then a step of obtaining a filtrate by filtering this solution, and using the filtrate obtained in this pretreatment, It is characterized in that the residual amount is quantitatively analyzed.

According to the third aspect of the invention, in a method for producing a caustic soda solution by electrolyzing a salt solution, sodium sulfite is added to the salt solution, and the solution is pretreated.

[0014]

In the method for producing a caustic soda by electrolyzing a salt solution, sodium sulfite is added to the salt solution in which the original salt is dissolved and / or the electrolyzed caustic soda solution to add each solution. Hypochlorite and chlorate contained therein react with sodium sulfite, and although chlorate has a different decomposition rate depending on the pH range, it is decomposed and removed by sodium sulfite even if it is alkaline.

If excess sodium hypochlorite and / or hydrogen peroxide solution is added to the solution after removing the chlorate, unreacted sodium sulfite is oxidized. When silver peroxide is further added to the solution, unreacted sodium hypochlorite and / or hydrogen peroxide solution is decomposed, and unreacted silver peroxide is removed by filtration. When the residual amount of chlorate is analyzed using this filtrate, the reaction between chlorate and sodium sulfite at the time of analysis can be avoided, and the concentration of chlorate can be accurately analyzed.

[0016]

EXAMPLES Examples of the present invention will be described below. The present invention relates to a method of producing caustic soda by electrolyzing a salt solution, that is, a diaphragm method, in which sodium sulfite (Na ₂ SO ₃ ) is added to a solution containing a chlorate, for example, chlorate to add chlorate to sodium chloride (NaCl). It is decomposed and removed.

The process of actually adding sodium sulfite will be described with reference to the manufacturing process diagram of the diaphragm method shown in FIG. First, the raw salt as a raw material is dissolved in a dissolving tank (raw salt dissolving step), and sodium hydroxide (NaOH), sodium carbonate (Na ₂ CO ₃ ) or the like is added to the salt water in the refining tank. Impurities such as Ca, Mg and Fe contained in the salt that are harmful to the diaphragm are removed (primary purification step).

Next, in a pH adjusting tank, for example, hydrochloric acid (HCl)
Is added to neutralize the salt water, and then sodium sulfite is added. Here, sodium sulfite may be added in a large amount with respect to chlorate, but the presence of unreacted sodium sulfite deteriorates the quality of caustic soda and also increases the cost of sodium sulfite. It is preferably 1 to 2 equivalents relative to the chlorate present in the salt water. 10 after addition of sodium sulfite
The solution is stirred and reacted by, for example, pump circulation or a stirrer for 15 minutes or more. The temperature of the salt water supplied from the pH adjusting tank to the electrolytic cell 1 in which the next step is carried out is about 60-80.
Since it is at 0 ° C, it is not necessary to particularly heat it, but the higher the reaction temperature, the more preferable.

When chlorate is reacted with sodium sulfite, the chlorate is reductively decomposed into sodium chloride, and sodium sulfite itself is oxidized to sodium sulfate (Na ₂ SO ₄ ) as shown in the following formula. NaClO ₃ + 3Na ₂ SO ₃ → 3Na ₂ SO ₄ + NaCl (1) After the decomposition reaction of chlorate, salt water is divided into an anode chamber 13 and a cathode chamber 12 by a diaphragm 11 made of asbestos fiber and asbestos cloth, for example. 1 to perform electrolysis (electrolysis step). Cl ₂ gas from the anode chamber 13 by the electrolysis, sodium hydroxide solution and H ₂ gas is obtained from the cathode compartment 12.

When the chlorate concentration in the solution in each step is measured, it is about 0.3 to 0.6 g / L in both the primary refining step and the pH adjusting step, whereas the electrolytic solution after the electrolysis step, that is, caustic soda. The solution was about 1 to 3 g / L. From this result, it is considered that chlorate is mainly produced by the following mechanism in the electrolysis process. That is, the caustic soda generated in the cathode chamber 12 diffuses into the anode chamber 13 through the diaphragm, and Cl ₂ generated in the anode chamber 13
Reacting with gas produces sodium hypochlorite (NaClO) as shown in the following formula. Then, this sodium hypochlorite is stored in the anode chamber 13 by the following equation (3), (4)
As shown in the formula, it is decomposed or oxidized to form chlorate. 2NaOH + Cl ₂ → NaCl + NaClO (2) 3NaClO → 2NaCl + NaClO ₃ (3) NaClO + O ₂ → NaClO ₃ (4) Next, the 10-12 wt% caustic soda solution obtained by electrolysis was once stored in the electrolyte reservoir layer 2. After that, it is concentrated by a vaporizer 3 composed of, for example, an evaporator made of nickel (evaporative concentration step) to obtain a 48 wt% caustic soda solution, and the obtained caustic soda solution is stored in a caustic soda solution storage tank 4. It At this time, unreacted and sodium chloride produced by decomposition of chlorate are separated by precipitation utilizing the solubility in a 48 wt% caustic soda solution, and sent to the raw salt dissolving step for reuse. On the other hand, sodium sulfate produced by the oxidation of sodium sulfite is also separated by precipitation utilizing the solubility in a 48 wt% caustic soda solution.

Further, when solid caustic soda is obtained,
The 48 wt% caustic soda solution is sent to an evaporative simmer 5 made of, for example, nickel to be boiled down, and made into a flaky caustic soda solid by a seal pot and a flaker not shown. It is sent to the caustic soda receiving tank 6.

As shown by the broken line in FIG. 1, sodium sulfite may be added to the electrolytic solution before the pH adjusting step or after the electrolysis, or added to the caustic soda solution before being supplied to the evaporative boiling device 5. You may.

Next, a chlorate removal test conducted to confirm the effect of the present invention will be described together with a comparative example. <Experimental Example 1> 1 g / L of anhydrous sodium sulfite was added to salt water after pH adjustment to a chloride concentration of 0.21 g / L, and the solution was stirred at room temperature for 60 minutes to react, and then the chloride concentration in the solution was adjusted. The removal rate was measured. The same experiment was tried by adding sodium sulfite at 2 g / L. The results are shown in Table 1.

<Experimental Example 2> Chlorate concentration 2.40 g /
5 g of anhydrous sodium sulfite is added to the electrolytic solution after electrolysis of L.
/ L was added and the solution was stirred at room temperature for 60 minutes to cause a reaction, and then the chlorate concentration in the solution was measured to determine the removal rate. Further, the same experiment was tried by adding 10 g / L of sodium sulfite. The results are shown in Table 1.

Comparative Example 1 Chlorate concentration 2.40 g /
Lithium aluminum hydride (LiAlH
₄ ) was added at 3 g / L, and the solution was stirred for 60 minutes at room temperature to react, and then the chlorate concentration in the solution was measured. The chlorate concentration was almost unchanged, and there was no effect of removing chlorate.

<Comparative Example 2> Chlorate concentration 0.21 g /
0.35 g / L of sodium sulfide (Na ₂ S.7H ₂ O) was added to L salt water, the solution was stirred at room temperature for 60 minutes to react, and then the chlorate concentration in the solution was measured to remove the solution. I asked. The results are shown in Table 1.

[0027]

[Table 1] From the above experimental results, it was confirmed that the chlorate contained in the salt water and the electrolytic solution can be removed at a removal rate of 40 to 60% by adding sodium sulfite in a fixed amount of 1 or more with respect to the content of chlorate.

When sodium sulfite is added to the salt water and then electrolyzed, the chlorate in the salt water is reduced and unreacted sodium sulfite decomposes sodium hypochlorite produced in the electrolytic cell to produce chlorate. Since it is estimated that this can be suppressed, it is more effective to add sodium sulphite from the viewpoint of reaction efficiency depending on the pH range than to remove chloride from salt water compared to the case of removing chloride from electrolyte. Since the amount is small, the cost can be reduced, and the useful life of the electrolytic cell and the diaphragm can be extended, which is considered to be effective.

Furthermore, it was confirmed in Comparative Example 1 that the effect of removing chlorate by lithium aluminum hydride was almost nonexistent, and in Comparative Example 2, the effect of removing chlorate by sodium sulfide was slightly inferior to that of sodium sulfite, and hydrogen sulfide (H ₂ S) It was confirmed that actual application is difficult due to strong odor. The reaction formula of this reaction is as shown in the following formula (5). 4NaClO ₃
+ 3Na ₂ S → 3Na ₂ SO ₄ + 4NaCl (5) Next, the method for analyzing the chlorate concentration used to measure the chlorate concentration after reacting with sodium sulfite in the above-mentioned chlorate removal test will be described. In this method, the present inventors have understood that chlorate reacts very well with sodium sulfite in the pH region of 0, and therefore, in the analysis step of acidifying the solution, chlorate is decomposed by sodium sulfite. Concerned, what was found to accurately analyze the chlorate concentration by analyzing the chlorate concentration of the solution after removing unreacted sodium sulfite contained in the solution to be measured before analysis Specifically, it comprises a pretreatment step of a measurement sample and a chlorate analysis step described below.

<Pretreatment Step> Excessive oxidizing agents such as sodium hypochlorite and hydrogen peroxide (H ₂ O ₂ ) are added to the measurement solution to oxidize unreacted sodium sulfite to sodium sulfate. Then, silver peroxide is added to decompose the unreacted oxidizing agent, and then the silver peroxide is removed by filtration, and the filtrate is used as a measurement sample.

<Analysis step> An appropriate amount of the sample prepared in the pretreatment step was accurately sampled with a whole pipette, and sulfuric acid (H ₂ SO
₄ ) acidify 5% FeSO ₄ (NH ₄ ) ₂ SO ₄
Accurately collect 10 ml and add. Then, after boiling for about 5 to 10 minutes, the mixture is cooled, about 50 ml of water is added, and the mixture is titrated with a N / 20 potassium permanganate (KMnO ₄ ) solution. The amount of the potassium permanganate solution used at this time is Aml.

Next, a blank test is performed. That is, the same amount of sulfuric acid as that added to the sample was added to 50 ml of water, and 5% FeS was added.
Accurately collect and add 10 ml of O ₄ (NH ₄ ) ₂ SO _4, and then titrate with a N / 20 potassium permanganate solution. At this time, the amount of the potassium permanganate solution used is Bm.
Let l.

From the titration result, the chlorate concentration in the measurement sample can be calculated by the following formula.

NaClO ₃ (g / L) = 0.000887 × (B−A) ml × F × 1000 / sample amount (ml) (6) Chlorate and iron sulfate, sodium hypochlorite and iron sulfate in the analysis step The reaction between potassium permanganate and iron sulfate is as shown in the following formulas (7), (8), and (9), respectively.

NaClO ₃ + 6FeSO ₄ + 3H ₂ SO ₄ → NaCl + 3Fe ₂ (SO ₄ ) + 3H ₂ O (7) NaClO + 2FeSO ₄ + H ₂ SO ₄ → NaCl + Fe ₂ (SO ₄ ) + H ₂ O ... (8) 2KMnO ₄ + 10FeSO ₄ + 8H ₂ SO ₄ → K ₂ SO ₄ + 2MnSO ₄ + 5FeSO ₄ (SO ₄ ) ₃ + 8H ₂ O (9) The following experiment was attempted in order to confirm the effect of this chlorate analysis method. That is, three kinds of solutions having different pH (acid solution of pH 2.0, neutral solution of pH 7.0, alkaline solution of pH 12.1) were prepared from salt water having a chlorate concentration of 0.33 g / L, and sulfite was added to each solution. After adding 1 equivalent of sodium and stirring the solution at room temperature for 60 minutes,
The removal rate was determined by measuring the chlorate concentration in the solution using the above-described analysis method. The result shows that the acid solution has a removal rate of 60.
%, The neutral solution was 50%, and the alkaline solution was 20%.

Even if the amount of sodium sulfite added in this way was the same, since the removal rate of chlorate was different in the acidic solution, neutral solution and alkaline solution, unreacted sodium sulfite was oxidized in the pretreatment step. After removing
It was confirmed that by obtaining the concentration of chlorate in the measurement sample by titration in the analysis step, the reaction between chlorate and unreacted sodium sulfite in the analysis step can be suppressed and the concentration of chlorate can be accurately measured.

Further, since chlorate is very stable in an alkaline solution, it has been conventionally considered difficult to decompose chlorate with sodium sulfite in an alkaline solution.
The decomposition rate (removal rate) of about 20% is obtained even in the alkaline solution of about 50% in a neutral solution or an acidic solution.
It was confirmed that a decomposition rate of 60% was obtained.

As described above, in the method of this embodiment, the chlorate is reduced and decomposed into sodium chloride by a simple method in which 1 to 2 equivalents of sodium sulfite is added to the chlorate contained in the solution to react. By using the chlorate analysis method of this embodiment, the reaction between chlorate and unreacted sodium sulfite can be suppressed and the chlorate concentration can be accurately measured.

Even if the solution to which sodium sulfite is added is alkaline, a chlorate removal rate of about 20% can be obtained, and if it is neutral to acidic, a higher removal rate can be obtained.

Further, it is possible to deal with the removal of chlorates of both salt water and the electrolytic solution, and the same removal rate can be obtained. Therefore, when chlorate is removed from salt water,
It is possible to improve the current efficiency and prolong the service life of the electrolytic cell, the diaphragm, the evaporator and the like. In addition, when chlorate removal treatment is performed on the electrolytic solution, it is possible to prolong the service life of the evaporator, etc., and by prolonging the service life of such a device, the number of times of replacement of the device is reduced and the manufacturing cost is reduced. Can be lowered.

The sodium sulfite may be added at any time after the raw salt dissolving step and before the evaporative concentration step, but the removal rate of chlorate is higher in the neutral to acidic solution than in the alkaline solution, and before the electrolysis step. Considering that the required amount of sodium sulfite is smaller, it is preferable to add it to the salt water after the pH adjusting step. This makes it possible to reduce the cost of sodium sulfite and suppress the generation of chlorate during electrolysis.

[0042]

According to the method for removing chlorate of the present invention,
The simple method of adding sodium sulfite to a solution containing chlorate can decompose and remove chlorate, and the removal of chlorate is necessary for the production of caustic soda such as electrolytic cells and evaporators. The useful life of various devices can be extended.

According to the chlorate analysis method of the present invention, the concentration of chlorate in the solution is measured after removing unreacted sodium sulfite. Since it is possible to avoid the reaction with, it is possible to accurately analyze the concentration of chlorate.

[Brief description of drawings]

FIG. 1 is a manufacturing process diagram of caustic soda production by a diaphragm method.

[Explanation of symbols]

 1 Electrolysis tank 2 Electrolyte storage tank 3 Evaporator 4 Caustic soda solution storage tank 5 Evaporative boiler 6 Caustic soda receiving tank

Claims (3)

[Claims] 1. A method for producing a caustic soda solution by electrolyzing a salt solution, wherein sodium sulfite is added to the salt solution and / or the caustic soda solution so that the solution is contained in the salt solution and / or the caustic soda solution. A method for removing chlorate in a caustic soda manufacturing process, characterized by decomposing and removing chlorate. 2. A method for quantitatively analyzing the residual amount of chlorate after removing the chlorate contained in a solution having a pH of 2 or more by adding sodium sulfite to the solution, and adding the excess amount to the solution. Of sodium hypochlorite and / or hydrogen peroxide water to oxidize unreacted sodium sulfite, and then silver peroxide is added to this solution to add sodium hypochlorite and / or hydrogen peroxide. Quantitative analysis of the residual amount of chlorate using the filtrate obtained by pretreatment including the step of decomposing water, and then the step of filtering this solution to obtain a filtrate. A method for analyzing chlorate in a caustic soda production process, which comprises: 3. A method for producing a caustic soda solution by electrolyzing a salt solution, wherein sodium sulfite is added to the salt solution, and this solution is pretreated. Method for analyzing chlorate in manufacturing process.