JPS6046384A - Preparation of alkali chlorate - Google Patents

Abstract

PURPOSE:To enhance not only safety but also NaClO3 forming efficiency by suppressing the generation of O2 gas, in preparing NaClO3 by the electrolysis of an aqueous NaCl solution, by adding HCl in the aqueous NaCl solution to keep the pH thereof to a specific range. CONSTITUTION:Salt is dissolved in water within a tank 1 and the resulting aqueous NaCl solution is stored in a storage tnak 3 after impurities are removed in a refining process 2. This solution is introduced into a plurality of electrolytic cells 6 arranged in series each equipped with an anode 7, a cathode 8 and a draft tube 9 by a pump 4. HCl is added to the aqueous NaCl solution from the lower part of the draft tube 9 to hold the pH thereof to 4.8-6.0 and electrolytic treatment is performed while the mol ratio of HClO and NaClO3 in the NaCl electrolyte is held to 2:1. This operation is repeated in plural stages to recover the treated solution as a flooded solution 10 increased in a NaClO3 concn. The gas generated from the electrolytic cell 6 is reduced in O2 content and held out of an explosion range while operation is made safe and high purity NaClO3 reduced in NaClO content is prepared.

Description

[Detailed description of the invention] The present invention is electrolysis of alkali chlorate I! 1 Decide how to make it.

More specifically, the present invention provides a method for efficiently producing alkali chlorate while improving the operating conditions of an electrolytic device and the method for treating chlorine in generated gas.

There are two main steps in producing alkali chlorate by electrolyzing an aqueous alkali chloride solution. First, due to an electrolytic reaction, n-type ions are discharged at the anode to generate hypochlorous acid, and at the cathode, hydrogen ions are reduced to generate hydrogen gas. Next, a self-oxidation reaction of hypochlorous acid occurs to produce chlorate.

On the other hand, as an undesirable side reaction that reduces current efficiency, hypochlorous acid is re-discharged at the anode, producing chlorate, and at the same time, a reaction accompanied by the generation of oxygen and hypochlorite. There is a reaction in which chlorine ions are reduced to the original chlorine ions at the cathode.

In order to suppress such undesirable side reactions, various improvement measures have been developed in the past.

Examples include improving the anode material, preventing cathode reduction by adding chromate, or improving operating conditions to promote the self-oxidation reaction of hypochlorous acid.

Hypochlorous acid undergoes a self-oxidation reaction according to the following formula and is converted to chlorate.

2HCJ! O+Na CAO- Na CJ2.03+2HCJ2 ・・・・・・41)(
According to measurements made by the present inventors, this reaction is most effective when the molar ratio of hypochlorous acid and hypochlorite is 2:1, and when the alkali metal is sodium. Hypochlorite M iR degree can be lowered. The pLl that maintains the above molar ratio varies depending on the temperature, but is generally 6.2 to 6.
．． 8, and efforts have been made to adjust and operate within this range. In order to promote the reaction of formula (1) and reduce the concentration of hypochlorous acid, there are other factors such as temperature and reaction volume. The influence of temperature is large, and the higher the temperature, the lower the temperature of hypochlorous acid, but the rate of electrode side reactions also increases, and the current efficiency decreases. Although the reaction volume is large and the hypochlorous acid concentration can be lowered, the equipment becomes relatively large and there are design constraints.

Despite the various improvements that have been made in the past, hypochlorous acid m degree IJ in the electrolyte solution is 1.5 to 3.5 g.
/ j2 , the oxygen content in the generated hydrogen gas is accepted to be 2 to 4%.

In view of the current situation, the inventors of the present invention have repeatedly investigated various methods for suppressing the generation of oxygen and improving current efficiency, and as a result, they have completed the present invention.

That is, the method for producing alkali chlorate according to the present invention is as follows:
When producing alkali chlorate by electrolyzing an electrolytic solution consisting of an aqueous alkali chloride solution, electrolysis is carried out by injecting hydrochloric acid into the electrolytic solution and maintaining the pl-1 of the electrolytic solution at 4.8 to 6.0. The oxygen content in the hydrogen gas generated by electrolysis is reduced, while the chlorine and/or chlorine dioxide gas generated by electrolysis is collected with a strong alkaline solution, and the collected liquid is neutralized with hydrochloric acid. to generate alkali chlorate in the collection liquid, and then unreacted M 1il in the collection liquid.
The present invention is characterized in that the llm sulfate is removed by a reducing agent, and the collected liquid containing alkali chlorate thus obtained is used for reprinting when preparing an aqueous aqueous solution of chloride.

In the present invention, by operating the electrolyzer in a pH range lower than conventional pH conditions, that is, in the range of pH 4.8 to 6.0, the oxygen content in the generated hydrogen gas can be effectively reduced. . When electrolysis is carried out under conditions lower than pH 4.8, the chlorine gas concentration in the generated gas increases, and C
j22-1-1° should be avoided due to the risk of generating explosive gas, and on the other hand, if the pH is higher than 6.0, no significant effect of suppressing oxygen generation can be expected.

On the other hand, although the oxygen content in the generated gas can be reduced during operation at low pHhfi as described above, the generation of chlorine C13 and/or chlorine dioxide increases. Therefore, if the generation of chlorine and chlorine dioxide is regarded as a loss, the apparent current efficiency will decrease. Therefore, in the present invention, the current efficiency can be substantially improved by collecting these chlorine and chlorine dioxide in the generated gas, converting them into alkali chlorate, and recovering them into the system.

In order to maintain the pH of the electrolytic solution at 4.8 to 6.0, in the present invention, hydrochloric acid is injected into the electrolytic solution, but when concentrated hydrochloric acid is added to the aqueous alkaline chlorate solution, it causes localized damage near the injection port. It becomes strongly acidic and there is a risk of explosion due to the generation of carbon dioxide. To avoid such dangers, 30-7
It is possible to add dilute hydrochloric acid of 0 to 1'A, but in this case, the electrolyte is diluted and it is not possible to use a large amount.
．． It is 7. Therefore, in order to safely inject concentrated hydrochloric acid into the electrolytic solution, in the present invention, hydrochloric acid is injected into the flow path of the electrolytic solution brought about by the buoyant force of the hydrogen gas generated in the electrolytic solution 4a. This is desirable. As a result, the injected hydrochloric acid is effectively mixed and diffused throughout the electrolyte in the electrolytic cell as the electrolyte rises, and it is possible to prevent the electrolyte from becoming locally strong acidic.

In addition, even in an electrolytic cell operated under conventional conditions, the generated hydrogen gas contains 0.1 to 0.3 vol. The methods used have been to use the wastewater in factory wastewater treatment or to neutralize it with a reducing agent and then discharge it. However, as in the present invention, 1) When electrolyzing in the H range, the generation of chlorine scum is 1
Since the current efficiency increases by 0 to 20 times and has no significant effect on current efficiency, conventional treatment methods cannot be applied.

Therefore, in the present invention, the hypochlorous acid generated in the collected Wb is converted into chlorate by continuously neutralizing the excess alkali of the collected water scum with an alkaline solution using hydrochloric acid. This chlorate is recovered into the electrolytic system so that it can be collected.At this time, carbonic acid should be included in the collection liquid in the range of 10 to 6C)o/β. or desirable. That is, the conversion of hypochlorous acid to chlorate proceeds according to the above equation (1), but unless the excess alkaline is neutralized and removed, the reaction proceeds to the right, producing HC, and pl.
-1 becomes low and the reaction stops progressing. Therefore, it is necessary to add alkali to return r pl-1. As described above, it is necessary to perform complicated control using both alkali and hydrochloric acid. However, by containing alkali carbonate in the collection alkaline solution, the alkali carbonate acts as a buffering agent due to the reaction (2), and the reaction caused by hydrochloric acid Only by neutralizing the excess alkali, it can be easily adjusted to 1G, 0 to 7.0, which is optimal for the chlorate conversion reaction.

The invention will be briefly described below with reference to preferred embodiments shown in the drawings.

FIG. 1 (J Preferred flowchart for carrying out the method of the present invention (- indicates, 1 is a salt dissolving tank, 2 is a salt water purification step,
3 is a storage tank for purified salt water. Dissolved water or unsaturated precipitated salt water for alkali chlorate crystallization process? 232 to the salt dissolving tank 1, passed through the salt layer, and taken out as a saturated solution of salt. In the purification step 2, sodium hydroxide, soda carbonate, etc. were added to remove impurities, and then stored in the purified salt water storage tank a.
The liquid is sent to the electrolytic cell 6 by the liquid sending pump 4. A plurality of electrolytic cells 6 are arranged in series and in a cascade, and the electrolytic cells 6 sequentially flow downstream to increase the concentration of alnochloric acid and are taken out as a product from the overflow 10 of the final electrolytic cell. 5 is a rectifier, and the electrolytic current is passed through the bus bar 33 to each electric current. The flow flows in series through each of the anode 7 and cathode 8 of the tank, performs a 21ffi solution, and returns to the rectifier 5.

In the illustrated embodiment, the electrolytic cell 6 is a tower-type electrolytic cell equipped with a draft tube 9 inside which serves as a flow path for the upward flow of the electrolytic solution so that the hydrochloric acid injected into the premises can be effectively mixed and diffused. However, external circulation electrolyzers with gas lifts or pumps can also be preferably used.

As shown in FIGS. 2 and 3, a plurality of approximately semicircular baffle plates 34 are arranged in the draft tube 9 at intervals in the longitudinal direction of the draft tube and arranged alternately in the 6° direction. Concentrated hydrochloric acid is injected from the nozzle 11 into the center of the pipe above the bottom baffle plate.The tip of the baffle plate 34 is located at
It is preferable to elongate it to 40 to 70% of the diameter of the draft tube, and it is preferable that the mounting interval (pitch) is 1 to 3 times the diameter of the draft tube.

In order to further improve the mixing and diffusion effect of the hydrochloric acid injected from the nozzle 11, it is desirable to provide a notch 35 in the center of the ridgeline at the tip of the baffle plate 34. In the example of FIG. 3, the notch 35 is semicircular, but the shape of the notch may be triangular or square. It is preferable that the middle phase of the notch 35 is 20 to 50% of the diameter of the draft tube 9 and the depth is 10 to 30%.

The above-mentioned hydrochloric acid mixing and diffusion device such as 1 is placed in the electrolytic cell 6 with Qi
! By injecting hydrochloric acid into the tank, electrolyte 1
) It is possible to maintain the H value in the range of 4.8 to G, 0, preferably 4.9 to 6.0, and operate without the risk of localized strong acidity. Thus, the proportion of chlorine and chlorine dioxide in the composition of the gas generated from electrolysis if 6 increases, but the proportion of oxygen decreases.

In other words, the explosive range of oxygen and chlorine in the generated hydrogen gas is 5-6% or more and 12-14% or more, respectively, and the self-decomposition rate of chlorine dioxide is 7-8% or more, making it safer. Can be secured.

The generated gas from each electrolytic cell 6 is connected to the generated gas piping 12 in FIG.
This process leads to the chlorine collection process and removal process. 1
3 is a collection tower, 17 (J abatement j C) and 21 are water washing towers.
It circulates to the collection tower 13 through 6, absorbs chlorine and chlorine dioxide, and returns to the receiving tank 14. The composition of the collection liquid is 5 to 40 q/β of alkali and 10 to 60 g of alkali carbonate.
j2 and 50 to 100 Q/fl of alkali hypochlorite are suitable. To maintain the alkaline level of the circulating fluid,
A highly concentrated alkali is injected from the nozzle 16, and the gas that has passed through the collection tower 13 contains chlorine 5 = 30 ppm.
, chlorine dioxide (100 to 5001) + 1111, it is guided to the abatement tower 17 and detoxified. For example, a sodium sulfite aqueous solution is supplied from the nozzle 18, and the receiver 19 and circulation pump 2
0 to the abatement tower 17, where it is brought into contact with the gas from the collection tower 13 to remove chlorine and chlorine dioxide until they are substantially undetectable. Elimination tower 17
Since the gas leaving the air is accompanied by mist, it is led to the water washing tower 21 and washed with washing water from the nozzle 22, and then blown out from the discharge nozzle 23, or used for other purposes.

The collected liquid is collected in an increased amount by the receiver 4u14 and sent to the neutralization tank 24. Hydrochloric acid is poured into the neutralization tank from nozzle 25.
t) The temperature is maintained at l-1 [3.0 to 7.0°C, and the temperature is controlled at 40 to 80°C by blowing steam through the nozzle 26. The neutralized slag collection liquid flows down the reaction tank 27, completing the conversion reaction of the alkali chlorate l\ according to the above equation (1), and flows to the next step. Since the hydrochloric acid produced in this reaction is consumed to decompose the alkali bicarbonate in the collection liquid, 13H in the solution is kept constant and the reaction proceeds smoothly. has unreacted free chlorine of 1000 to 2000
Since ppm exists, this is sent to the free chlorine elimination tank 28,
For example, an aqueous solution such as urea, formic acid, ammonia, or sodium sulfite, or a reducing agent such as activated carbon is injected through the injection nozzle 29 to completely eliminate free chlorine. 30 is the above reduction reaction (
1rl which is a pickpocket and collection liquid storage device and contains alkali chlorate
The i-collection is sent to the salt solution +11 by the liquid sending pump 31, passed through the salt water purification step 2, storage tank 3, and then circulated back to the electrolytic cell.

Next, the present invention will be explained by examples.

Examples 1 to B Electrolytic cell specifications: Basic; 0.5 + 11φ
4 notched holes at a pitch of 0.12 m downward from the 3 m position, 45 mm diameter semicircular electrolysis conditions: Temperature -89 to 94°C Injected hydrochloric acid temperature: 24% Electrolyte composition: Na Cj2 159 to IC 17 g/J.

N a CJ203404-4220/ pN a C
r Q 6.5-7 6(]/ f1 Collection tower conditions: Injectable alkali i 11 degrees; 25% temperature; 50-55 degrees Celsius Collection liquid composition: NaOH 13-21 g/J2 1Na2C0, 25-34 g/J2 Na cxo 73~81 CJ/j2N a CJ'
l 03 5-16 g/ ANaCβ 59-65Q
/A Neutralization tank conditions: Injected hydrochloric acid degree: 12% Temperature: 265-70°C pH: 6.3-6.6 Reduction tank column A'1: Added urea temperature: 5% Temperature: 45-52°C The electrolytic current was 10 KA (anode 15.5 A/dm) by varying the electrolytic solution TIH in the range of 4.8 to 6.0 depending on the conditions of 1 free chlorine; 1.4 to 2.50/dm or more. '
) and 6KA (anode 9.3 A/dm''1), the results are shown in the following table as Examples 1 to 5 and Examples 6 to 8, respectively.

For comparison, for electrolytic current 10KA, electrolytic solution 1) H6,6, for electrolytic current 6KA, electrolytic solution pH 6.
, 7 are compared to Comparative Example 1 and Comparative Example 2, respectively.
As shown in the table below.

As can be seen from the above table, all of the examples according to the present invention improve the actual current efficiency by 3 to 7% over the comparative examples, and the oxygen temperature in the generated hydrogen gas can also be extremely lowered. , chlorine and chlorine dioxide contamination levels are also substantially low in the pH range of the present invention;
Safety will be fully ensured.

As explained above, the method of the present invention not only improves energy efficiency, but also improves safety by lowering the oxygen content in the generated hydrogen gas, and at the same time enables the effective use of hydrogen residue. This is an extremely right-handed method.

[Brief explanation of drawings]

Figure 1 shows a preferred flowchart 1 for carrying out the present invention.
FIG. 2 is a longitudinal sectional view of a device for injecting and mixing hydrochloric acid into an electrolytic solution used in a preferred embodiment of the present invention;
FIG. 3 is a cross-sectional view taken along the line ■--■ in FIG. 2. 1... Salt dissolution (a, 2... Salt water purification process, 3...
・Purified salt water storage tank, 6... Electrolytic tank, 9... Draft tube, 12... Generated gas piping, 13... Collection tower,
17...Abatement tower, 21...Water washing tower, 24...Neutralization 1g, 27...Reaction tank, 28...'1iIllll
Chlorine elimination tank, 30... Reduction reaction tank and collection liquid storage tank. Patent Applicant: Hodogaya Chemical Industry Co., Ltd. Representative: Yuki Omata Shigemi Yudo (a) Tominosuke Araki (Figure 2, Figure 3, Figure 4)

Claims (1)

[Claims] 1. When producing alkali chlorate by electrolyzing an electrolyte consisting of an aqueous alkali chloride solution, hydrochloric acid is injected into the electrolyte to adjust the pH of the electrolyte to 4.8 to 6.0. On the other hand, chlorine and/or chlorine dioxide gas generated by electrolysis is collected with a strong alkaline solution, and the collected liquid is mixed with hydrochloric acid. to generate alkali chlorate in the collection liquid, then remove unreacted free chlorine in the collection liquid with a reducing agent, and chlorinate the collection liquid containing the alkali chlorate thus obtained. 1. A method for producing alkali chlorate, characterized in that it is reused in preparing an aqueous alkali solution. 2. The injection of hydrochloric acid into the electrolytic solution is carried out by injecting the hydrochloric acid into the flow path of the upward flow of the liquid generated by electrolysis.
The method described in section. 3. It becomes a flow path for the upward flow of electrolyte. : The rough 1 to tubes are arranged in an electrolytic cell, and a plurality of approximately semicircular baffle plates are installed in the draft tube at intervals and in alternating directions in the longitudinal direction of the tube. 3. The method according to claim 2, wherein hydrochloric acid is injected from a nozzle above the lowest baffle plate. 4. The method according to claim 3, wherein the baffle plate is a baffle plate having a notch in the center of the tip edge line. 5. The method according to claim 1, wherein the collection liquid contains 10 to 600/J2 of alkali carbonate.