TWI635043B - Production process of aqueous sodium hypochlorite solution - Google Patents

Abstract

The present invention provides a method for producing an aqueous solution of sodium hypochlorite having a small content of impurities such as chloric acid or sodium chloride in a high yield.

The method for producing a low-salt sodium hypochlorite aqueous solution of the present invention comprises the steps of: supplying a 30-60% by mass aqueous sodium hydroxide solution to a reaction tank (1), and introducing chlorine gas into the hydrogen peroxide supplied to the reaction tank. a step (3) of performing a chlorination reaction in an aqueous sodium solution at 20 ° C to 50 ° C, a step (3) of obtaining a sodium hypochlorite aqueous solution by separating and removing the by-produced sodium chloride precipitated in the step (2), and the step (2) The stirring is carried out under the following conditions: the power required for stirring per unit volume is 0.1 to 15 kW/m ³ , and the ratio of the number of power required to be stirred Np to the number of circulating flow rates Nq (Np/Nq) is 0.5~ 8.

Description

Method for producing sodium hypochlorite aqueous solution

The present invention relates to a method for efficiently producing an aqueous solution of sodium hypochlorite having a sodium chloride concentration and a low concentration of chloric acid.

Sodium hypochlorite (NaClO) is known to have excellent bactericidal action or bleaching effect, and is generally widely used as a general industrial drug in the form of an aqueous solution, as a sterilization application for swimming pools, water pipes, sewers, and households, or as a paper industry or a fiber industry. Such as bleaching or drainage treatment.

A sodium hypochlorite aqueous solution is generally commercially available, and a sodium hypochlorite aqueous solution widely used for sodium chloride containing about 10% by mass of a reaction by-product is used, and a sodium salt of sodium hypochlorite having a sodium chloride concentration of 4% by mass or less is used. Aqueous solution.

When sodium hypochlorite is obtained, a method of reacting sodium hydroxide with chlorine is generally employed. For example, in Patent Document 1, when an aqueous solution of sodium hydroxide is reacted with chlorine to produce an aqueous solution of sodium hypochlorite, the opening position of the chlorine introduction tube is set to be higher than the final liquid level of the reaction liquid in the reaction tank, and the reaction is sufficiently mixed. The liquid side is subjected to a chlorination reaction.

Further, in the lower right column of the second page of Patent Document 1, "when the agitation is small, the contact time of the reaction liquid with chlorine is prolonged, so that the decomposition reaction of the produced sodium hypochlorite is promoted, and the by-product sodium chlorate or the like is not Preferably, a certain degree of agitation is required. However, since the reaction proceeds rapidly, no vigorous stirring is required, and it is described that "specifically, the reaction tank of 10 m ³ is about 60 rpm, which is not much different from the stirring of the usual reaction. It is also stated that "the droplets of the reaction liquid adhere to the upper part of the reaction tank when it is too fast, and it requires no other power cost, so there is no benefit." Accordingly, in Patent Document 1, as a stirring condition for producing an aqueous solution of sodium hypochlorite, the stirring speed is described. However, the shape and size of the stirring blade and the balance with the stirring tank are only described as "the stirring speed is only based on the reaction liquid material or the stirring. The optimum conditions can be set by designing the shape of the wing, etc."

Further, in Examples 1 and 2 of Patent Document 1, the yields of sodium hypochlorite were 86% and 88%, both of which were limited to less than 90%, and in Example 3, although the yield was 94%, the obtained sodium hypochlorite aqueous solution was obtained. The sodium hypochlorite concentration was as low as 29%, and even a low salt sodium hypochlorite aqueous solution could not be obtained even if the sodium hypochlorite aqueous solution was diluted.

Here, the reaction of obtaining sodium hypochlorite from sodium hydroxide and chlorine is represented by the following reaction formula.

2NaOH+Cl ₂ → NaClO+NaCl+H ₂ O

However, this reaction is by-produced by sodium chloride and chloric acid by decomposition or by a side reaction represented by the following formula.

6NaOH+3Cl ₂ → NaClO ₃ +5NaCl+3H ₂ O

When a side reaction or the like represented by the above formula is caused, the sodium hypochlorite concentration is lowered and the concentration of sodium chloride or chloric acid is increased by the by-product sodium chloride or chloric acid. As a result, there is a problem that the salt concentration or the chloric acid concentration in the diluted thin sodium hypochlorite increases.

Further, chlorination is an exothermic reaction, and due to the crystallization of by-product sodium chloride, the higher the reaction temperature, the lower the energy applied by heat removal can be suppressed, and the scaling of the salt crystals to the cooling coil can be prevented. However, when the reaction temperature is high, the amount of decomposition of sodium hypochlorite is increased, and in particular, the decomposition is rapidly performed at 40 ° C or higher, so that the original unit is greatly deteriorated (see Non-Patent Document 1).

[Previous Technical Literature] [Patent Literature]

[Patent Document 1] Japanese Patent Laid-Open Publication No. 59-182204

[Non-patent literature]

[Non-Patent Document 1] Japanese soda industry association Soda manual editorial working group, "Soda Technical Handbook 2009", issued by Japan Soda Industry Association

An object of the present invention is to provide a method for producing an aqueous solution of sodium hypochlorite having a small content of impurities such as chloric acid or sodium chloride in a high yield.

As a result of the active review by the inventors of the present invention, it has been found that the above-described problem can be solved by performing a chlorination reaction while stirring with a specific stirring condition in a method for producing a sodium hypochlorite aqueous solution in which a chlorine gas is introduced into a sodium hydroxide solution, and thus the present invention has been completed. That is, the present invention relates to the following items.

[1] A method for producing a low-salt sodium hypochlorite aqueous solution, comprising the steps of: supplying a 30-60% by mass aqueous sodium hydroxide solution to a reaction tank (1), and introducing chlorine gas into the reaction tank; Step (2) of carrying out a chlorination reaction in an aqueous sodium hydroxide solution at a reaction temperature of 20 ° C to 50 ° C, separating from the reaction liquid, and removing the by-product sodium chloride precipitated in the above step (2) to obtain an aqueous solution of sodium hypochlorite (3), the above step (2) is that the power required for stirring per unit volume is 0.1 to 15 kW/m ³ , and the ratio of the number of power required to be stirred Np to the number of circulating flow rates Nq (Np/Nq) is 0.5 to 8 Under the conditions of stirring, proceed.

[2] The method for producing a low-salt sodium hypochlorite aqueous solution according to [1], wherein the chlorine gas is diluted with an inert gas and introduced.

[3] The method for producing a low-salt sodium hypochlorite aqueous solution according to [1] or [2], wherein a molar ratio of sodium hydroxide to chlorine (NaOH/Cl ₂ ) introduced in the chlorination reaction is 2.0. ~2.5.

[4] The method for producing a low-salt sodium hypochlorite aqueous solution according to any one of [1] to [3] wherein the reaction temperature in the step (2) is 30 to 50 °C.

[5] The method for producing a low-salt sodium hypochlorite aqueous solution according to any one of [1] to [4] wherein the sodium hypochlorite obtained in the above step (3) The sodium chloride concentration of the aqueous solution is 5.0% by mass or less.

[6] The method for producing a low-salt sodium hypochlorite aqueous solution according to any one of [1] to [5] wherein the chlorinated ion concentration of the sodium hypochlorite aqueous solution obtained in the step (3) is 1.5% by mass or less.

[7] The method for producing a low-salt sodium hypochlorite aqueous solution according to any one of [1] to [6] wherein the sodium hypochlorite aqueous solution obtained in the step (3) has a sodium hypochlorite concentration of 30 to 40% by mass.

[8] A method for producing a dilute sodium hypochlorite aqueous solution, which comprises the step of diluting the low-salt sodium hypochlorite aqueous solution obtained by the production method according to any one of [1] to [7] to a specific effective chlorine concentration.

[9] The method for producing a thin aqueous sodium hypochlorite solution according to [8], wherein the effective chlorine concentration is 1 to 20% by mass.

According to the present invention, since the complicated operation is not required, the low-salt sodium hypochlorite aqueous solution can be advantageously produced in a high yield on the cost side and the equipment preparation surface.

1‧‧‧Reaction tank

2‧‧‧Sodium hydroxide supply tube

3‧‧‧ chlorine gas introduction tube

4‧‧‧Extracted

5‧‧‧Agitator wing

Fig. 1 is a view schematically showing a reaction tank used in Examples and Comparative Examples.

Fig. 2 is a view schematically showing the stirring blade used in Example 1. Stir In the mixing wing, the upper and lower wings are crossed at 45 degrees, and the ends of the lower wing are bent to be opposite to the direction of rotation.

Fig. 3 is a view schematically showing the stirring blade used in Example 2. The agitating wing is a typical disk turbine wing, and the teeth are equally juxtaposed on one side (lower side) of the disk.

Fig. 4 is a view schematically showing the stirring blade used in Comparative Example 1. The agitating wing is a typical blade.

Hereinafter, the method for producing the sodium hypochlorite aqueous solution of the present invention will be described in detail.

The method for producing a low-salt sodium hypochlorite aqueous solution of the present invention comprises the steps of: supplying a 30-60% by mass aqueous sodium hydroxide solution to a reaction tank (1), and introducing chlorine gas into the hydrogen peroxide supplied to the reaction tank. a step (2) of performing a chlorination reaction in a sodium aqueous solution at a reaction temperature of 20 ° C to 50 ° C, and a step of removing the by-product sodium chloride precipitated in the above step (2) to obtain an aqueous sodium hypochlorite solution (3) The above step (2) is carried out by stirring under the following conditions: the power required for stirring per unit volume is 0.1 to 15 kW/m ³ , and the ratio of the number of power required to be stirred Np to the number of circulating flow rates Nq (Np/Nq) ) is 0.5~8.

The concentration of the aqueous sodium hydroxide solution supplied to the reaction tank in the step (1) is usually 30 to 60% by mass, preferably 35 to 55% by mass. %, preferably 40 to 48% by mass. When the concentration of the raw material aqueous sodium hydroxide solution is less than the above range, there is a tendency that it is difficult to produce a sodium hypochlorite aqueous solution having a desired low salt concentration. On the other hand, when the concentration of the raw material aqueous sodium hydroxide solution is higher than the above range, in order to adjust the sodium hydroxide aqueous solution of a specific concentration, complicated operation such as distillation is required.

The reaction temperature in the chlorination reaction in the step (2) is usually from 20 ° C to 50 ° C, preferably from 25 ° C to 50 ° C, more preferably from 25 ° C to 40 ° C. When the reaction temperature is lower than the above range, the cooling coil is likely to cause scale formation. On the other hand, when the reaction temperature is higher than the above range, the decomposition rate of sodium hypochlorite is accelerated, and the tendency of the original unit is lowered.

The reaction time in the chlorination reaction in the step (2) is preferably from 10 to 200 minutes, more preferably from 50 to 150 minutes, most preferably from 70 to 130 minutes.

In the chlorination reaction of the step (2), the introduced molar ratio of sodium hydroxide to chlorine (NaOH/Cl ₂ ) is preferably from 2.0 to 2.5, more preferably from 2.01 to 2.30, still more preferably from 2.02 to 2.20. When the molar ratio of sodium hydroxide to chlorine is less than the above range, perchlorination becomes easy. On the other hand, when the ratio is higher than the above range, the concentration of sodium hydroxide remaining in the aqueous sodium hypochlorite solution becomes high, so that the quality is higher. Not good.

In the step (2), a chlorine gas is introduced into an aqueous sodium hydroxide solution to carry out a reaction of the following formula to produce sodium hypochlorite.

2NaOH+Cl ₂ → NaClO+NaCl+H ₂ O

In the chlorination reaction, although sodium chloride (salt) such as sodium hypochlorite is formed, the sodium hydroxide aqueous solution having the above concentration is used as the original. In the case of the material, sodium chloride having a low solubility is crystallized. A high concentration of sodium hypochlorite aqueous solution having a low salt concentration is obtained by removing it.

Here, according to the tap water method, as the impurity of the sodium hypochlorite aqueous solution, the restriction on the above-mentioned sodium chloride and p-chlorous acid tends to be strong. In order to reduce the amount of the chloric acid, it is necessary to maintain the reaction temperature at 26 to 29 ° C as described in JP-A-2009-132583. This is because of the "natural decomposition" and "side reaction" described below due to the formation of chloric acid.

The "natural decomposition" is a phenomenon in which sodium hypochlorite is naturally decomposed, and is particularly rapidly decomposed at 40 ° C or higher (see Non-Patent Document 1). This decomposition is represented by the following reaction, whereby sodium chlorate (NaClO ₃ ) is produced.

NaClO → NaCl+O

2NaClO → NaCl+NaClO ₂

_{NaClO + NaClO 2 → NaCl + NaClO} 3

The "side reaction" is a side reaction caused by the reaction of sodium hydroxide with chlorine, and refers to the reaction by-product sodium chlorate shown below.

6NaOH+3Cl ₂ → NaClO ₃ +5NaCl+3H ₂ O

Such natural decomposition and side reactions reduce the original chlorine unit relative to sodium hypochlorite in any reaction system. That is, the formation of sodium chlorate reduces the original unit, and inhibition of the formation of sodium chlorate means an increase in the original unit.

These two phenomena can be described as unavoidable reactions, especially in the high temperature state which is advantageous in terms of cost surface and equipment preparation surface, However, since the decomposition proceeds rapidly, it is considered that it is very difficult to obtain a sodium hypochlorite aqueous solution in a good yield by suppressing the formation of chloric acid.

However, after the inventors of the present invention conducted a positive review, it was found that the method of blowing chlorine gas by stirring the sodium hydroxide aqueous solution with stirring agitation means that neither the "natural decomposition" nor the "side reaction" generates chloric acid and the original unit is lowered. main reason. In other words, it is considered that in addition to "natural decomposition" and "side reaction", there is a reaction that causes chloric acid to form and lower the original unit.

Therefore, the inventors of the present invention have focused on "perchlorination". In the above-mentioned "perchlorination", according to Non-Patent Document 1, when the chlorination reaction is completed and the caustic soda is lost, the following decomposition reaction occurs in the chain, and all the sodium hypochlorite is rapidly decomposed.

NaClO+Cl ₂ +H ₂ O → NaCl+2HClO

NaClO+2HClO → NaClO ₃ +2HCl

NaClO+2HCl → NaCl+H ₂ O+Cl ₂

The perchlorination is considered to be a violent reaction caused by the supply of the above-mentioned chlorine to the sodium hydroxide. However, the inventors of the present invention thought that local perchlorination may be caused not only in the above conditions but also in the vicinity of the chlorine gas inlet. That is, it is considered that the concentration of sodium hydroxide in the vicinity of the inlet of the chlorine gas is lowered to increase the concentration of sodium hypochlorite, thereby reacting the chlorine gas with sodium hypochlorite. If so, sodium hypochlorite is decomposed by the above reaction formula to form sodium chlorate, and chlorine is regenerated by chlorination. Further, in addition to the vicinity of the inlet of the chlorine gas, since the concentration of sodium hydroxide is sufficient, the regenerated chlorine is consumed. Therefore, all of the hypochlorite is not rapidly decomposed, but the sodium hypochlorite near the inlet is decomposed into chloric acid, resulting in a decrease in the original unit.

Based on the findings discovered by the inventors of the present invention, in order to suppress the above partial perchlorination, the present invention performs a chlorination reaction while stirring under the condition that the power required for stirring per unit volume is large. Here, the "power required for stirring per unit volume" is the stirring power [kW] required to rotate the stirring blade at an arbitrary speed divided by the volume of the reaction liquid [m ³ ], and the unit is [kW/m ³ ]. . The value of the agitation power is obtained, for example, by measuring the motor power during agitation.

The stirring force per unit volume of the stirring condition in the step (2) is usually 0.1 to 15 kW/m ³ , preferably 0.2 to 10 kW/m ³ , more preferably 0.3 to 5 kW/m ³ . When the power required for agitation per unit volume is large, considerable energy is required, which is not preferable in terms of cost. Further, even if the power required for stirring per unit volume is larger than the above range, the degree of improvement in yield obtained thereby is also low.

Further, the present inventors have found that even under the same stirring power, even if the shearing force is made by the action of breaking the blown chlorine bubbles, even the bubbles or reactions of chlorine blown by the discharge force are caused. The effect of liquid dispersion is very high in suppressing the effect of perchlorination. Therefore, the lower the ratio (Np/Nq) of the number of powers required to be stirred and the number of circulating flow rates Nq (Np/Nq), the more the local perchlorination reaction can be suppressed, and the aqueous sodium hypochlorite solution having a low perchloric acid concentration and a low salt concentration can be obtained in a high yield.

The stirring condition in the step (2) has a Np/Nq of usually 0.5 to 8, preferably 1 to 7.5, more preferably 1.5 to 6. By stirring under these conditions, an aqueous solution of sodium hypochlorite having a low salt and a low perchloric acid concentration can be obtained in a high yield. Under the condition that Np/Nq is higher than the above range, due to low ejection force Therefore, it is difficult to effectively suppress the tendency of the perchlorination reaction by stirring. On the other hand, when Np/Nq is less than the above range, the shearing force is too low, and a sufficient effect cannot be obtained.

Here, the number of powers required for stirring Np is a non-dimensional number represented by "Np ≡ P / (ρn ³ d ⁵ )", P is a stirring power [W], and ρ is a density of a stirring liquid [kg/m ³ ], n is the rotational speed [rps], and d is the agitating wing diameter [m]. And, the number of cycles in the flow line Nq "Nq≡q / (nd ^3)" indicates, q is full circulation rate [m ³ / s]. The number of circulating flow rates Nq is determined by the reaction tank type and the stirring airfoil type, and Np/Nq is a characteristic indicating the characteristics of the stirring blade. The lower Np/Nq means a discharge type agitating blade having a high discharge efficiency, and the higher Np/Nq is a shear-type agitating blade having a lower discharge efficiency. Further, the above-mentioned "conditions for which the power required for stirring per unit volume is large" is substantially the same as the value of Np in the above Np/Nq.

The stirring wing should be located near the chlorine inlet. Specifically, the ratio of the distance between the inlet and the stirring wing of the chlorine gas (B) relative to the height from the bottom of the reactor to the liquid surface (A) (B/ A) preferably from 0.1 to 0.8, more preferably from 0.2 to 0.6.

In the present invention, the chlorine gas introduced in the step (1) can also be diluted with an inert gas. Thereby, the chlorine concentration in the vicinity of the blowing inlet can be reduced, and local perchlorination can be suppressed. Moreover, the inert gas for dilution also has the effect of stirring in the reaction solution, so that the degree of dispersion in the system is improved, and the chlorination can be further suppressed.

The so-called inert gas system in the present invention is a gas which is less likely to cause a chemical reaction with chlorine or oxygen. Specifically, it is a gas of a rare gas element such as helium, neon or argon, or a nitrogen gas, etc. Further, the air or carbon dioxide in the present invention Gas is also considered an inert gas.

The method of diluting the raw material chlorine gas is, for example, a method of adjusting the diluted chlorine to a specific concentration in advance, or a method of simultaneously blowing 100% of chlorine gas and an inert gas from a separate line and combining them in a nozzle.

The concentration of chlorine gas diluted with an inert gas is preferably from 5 to 95% by volume, more preferably from 20 to 80% by volume, most preferably from 30 to 70% by volume, based on the chlorine concentration. When the concentration of the diluted chlorine gas is higher than the above range, a sufficient effect of suppressing perchlorination may not be obtained. On the other hand, when the concentration of the diluted chlorine gas is lower than the above range, the efficiency of the chlorination reaction tends to be lowered, and it is uneconomical. Further, the reaction liquid may be scattered in the reaction tank due to the blowing of the inert gas.

In the step (3), a by-product sodium chloride precipitated in the step (2) is separated from the reaction liquid by using a solid-liquid separation device such as a centrifugal separator or a filter. Thereby, an aqueous sodium hypochlorite solution having a sodium hypochlorite concentration of preferably 30 to 40% by mass, more preferably 32 to 38% by mass, is obtained.

The sodium chloride concentration of the sodium hypochlorite aqueous solution obtained in the step (3) is preferably 5.0% by mass or less, more preferably 1.0 to 5.0% by mass, most preferably 3.0 to 4.8% by mass.

Further, the chlorinated ion concentration of the sodium hypochlorite aqueous solution obtained in the step (3) is preferably 1.5% by mass or less, more preferably 0.01 to 1.2% by mass, most preferably 0.05 to 1.0% by mass. As described above, the low-salt sodium hypochlorite aqueous solution obtained by the production method of the present invention has a low impurity chloric acid concentration, and thus has a sufficient product value as a hypochlorite sodium hypochlorite aqueous solution.

Method for producing dilute sodium hypochlorite aqueous solution of the present invention The feature comprises a step of diluting the low-salt sodium hypochlorite aqueous solution obtained by the above-described method for producing a low-salt sodium hypochlorite aqueous solution of the present invention with water to obtain a specific effective chlorine concentration.

The effective chlorine concentration is preferably from 1 to 20% by mass, more preferably from 2 to 17% by mass, most preferably from 3 to 15% by mass.

[Examples]

Hereinafter, the present invention will be more specifically described based on examples, but the present invention is not limited by the examples.

Further, in the following examples and comparative examples, a chlorination reaction was carried out using a reaction tank as shown in Fig. 1 . The material of the reaction tank 1 and the stirring blade 5 is titanium, and the material of the sodium hydroxide supply pipe 2 and the chlorine gas introduction pipe 3 is vinyl chloride. The reactant slurry can be withdrawn from the withdrawal port 4. The curve shown near the center of the reaction vessel in the vertical direction of Fig. 1 indicates the liquid level at the time of operation. In the reaction tank of Fig. 1, the ratio (B/A) of "the distance between the introduction port of chlorine gas and the stirring blade (B)" with respect to the "height from the bottom surface of the reactor to the liquid surface (A)" is 0.54 [=1300/ (1050+1350)]. The stirring blades 5 are described later, and in each of Examples 1, 2 and Comparative Example 1, the different shapes are used, and the distance (B) between the inlet port of the chlorine gas and the stirring blade is determined from the lowest position of the stirring blade.

[Example 1]

In the reaction tank shown in Fig. 1 equipped with a stirrer, a coil cooler and an external circulation type cooler, the stirring blade shown in Fig. 2 was used, and the power required for stirring per unit volume was 0.41 kW/m ^{3 .} And the ratio of the ratio of the power required to be stirred, Np, and the number of circulating flow rates Nq (Np/Nq) was 2.5, and a 45 mass% aqueous sodium hydroxide solution was supplied as a raw material at 1491 kg/hr, and the sodium hydroxide was used as the raw material. The aqueous solution was maintained at 30 ° C while introducing 560 kg / hr of chlorine gas to carry out a chlorination reaction in such a manner that the average residence time was about 100 minutes. Moreover, the power required for stirring is P=2.22 kW, the density of the reaction liquid is ρ=1500 kg/m ³ , the rotation speed n=1.30 rps, the stirring wing diameter d=0.67 m, and the total circulation flow rate q=0.351 m ³ /s, so Np ≡ P / (ρn ³ d ⁵ ) and Nq ≡ q / (nd ³ ), and Np = 5 and Nq = 2 were calculated.

Next, the reactant slurry was taken out from the reaction tank at 2051 kg/hr, and subjected to solid-liquid separation by a centrifugal separator, thereby obtaining precipitated sodium chloride of 631 kg/hr, sodium hypochlorite concentration of 33.7% by mass, and sodium chloride concentration of 4.8% by mass. The low-salt sodium hypochlorite aqueous solution having a chloric acid ion concentration of 0.68% by mass was 1419 kg/hr. The yield at this time was 94.9%. Further, the yield is a value calculated from the number of moles of sodium hypochlorite obtained based on the introduced chlorine gas (the same applies hereinafter).

The sodium salt of sodium hypochlorite in an aqueous solution of dilute sodium hypochlorite having an effective chlorine concentration of 13% by mass was diluted with pure water to have a sodium chloride concentration of 1.9% by mass and a chlorate ion concentration of 0.27% by mass.

[Embodiment 2]

In the reaction tank shown in Fig. 1 equipped with a stirrer, a coil cooler and an external circulation type cooler, the stirring blade shown in Fig. 3 was used, and the power required for stirring per unit volume was 0.41 kW/m ^{3 .} And stirring the Np/Nq under the conditions of 7.1, and supplying a 45 mass% sodium hydroxide aqueous solution as a raw material at 1,442 kg/hr, and introducing 552 kg/hr of chlorine gas while maintaining the sodium hydroxide aqueous solution at 30 ° C, The chlorination reaction was carried out in such a manner that the average residence time was about 100 minutes. Further, the power required for stirring is P=2.22 kW, the density of the reaction liquid is ρ=1500 kg/m ³ , the rotation speed n=1.30 rps, the stirring wing diameter d=0.67 m, and the total circulation flow rate q=0.27 m ³ /s, so Np ≡P/(ρn ³ d ⁵ ) and Nq≡q/(nd ³ ), Np=5 and Nq=0.7 were calculated.

Next, the reactant slurry was withdrawn from the reaction vessel at 2018 kg/hr, and subjected to solid-liquid separation by a centrifugal separator, thereby obtaining precipitated sodium chloride of 655 kg/hr, sodium hypochlorite concentration of 32.1% by mass, and sodium chloride concentration of 4.4% by mass. The sodium hypochlorite aqueous solution having a chloric acid ion concentration of 1.63 mass% was 1360 kg/hr. The yield at this time was 90.0%.

The sodium salt of sodium hypochlorite aqueous solution diluted with pure water was adjusted to have an effective chlorine concentration of 13% by mass, and the sodium chloride concentration of the sodium hypochlorite aqueous solution was 1.9% by mass, and the chloric acid ion concentration was 0.69% by mass.

[Comparative Example 1]

In the reaction tank shown in Fig. 1 equipped with a stirrer, a coil cooler and an external circulation type cooler, the stirring blade shown in Fig. 4 was used, and the power required for stirring per unit volume was 0.41 kW/m ^{3 .} While stirring the condition that Np/Nq was 12, a 45 mass% sodium hydroxide aqueous solution was supplied as a raw material at 1520 kg/hr, and while maintaining the sodium hydroxide aqueous solution at 30 ° C, 560 kg/hr of chlorine gas was introduced thereto. The chlorination reaction was carried out in such a manner that the average residence time was about 100 minutes. Moreover, the power required for stirring is P=2.22 kW, the density of the reaction liquid is ρ=1500 kg/m ³ , the rotation speed is n=0.85 rps, the stirring wing diameter d is 0.67 m, and the total circulation flow rate q=0.204 m ³ /s, so Np ≡P/(ρn ³ d ⁵ ) and Nq≡q/(nd ³ ), Np=17.8 and Nq=1.48 were calculated.

Next, the reactant slurry was withdrawn from the reaction vessel at 2080 kg/hr, and subjected to solid-liquid separation by a centrifugal separator, thereby obtaining precipitated sodium chloride of 624 kg/hr, sodium hypochlorite concentration of 28.9% by mass, and sodium chloride concentration of 6.3% by mass. The low-salt sodium hypochlorite aqueous solution having a chloric acid ion concentration of 2.04% by mass was 1456 kg/hr. The yield at this time was 81.5%.

The sodium salt of sodium hypochlorite aqueous solution diluted with pure water was adjusted to have an effective chlorine concentration of 13% by mass, and the sodium chloride concentration of the sodium hypochlorite aqueous solution was 3.0% by mass, and the chloric acid ion concentration was 0.95% by mass.

The results of the above examples and comparative examples are shown in Table 1 below.

As shown in Table 1, although the power required for stirring is the same, the reason for the decrease in the yield of sodium hypochlorite, the concentration of salt and the concentration of sodium chlorate is considered to cause local perchlorination of chlorine near the inlet due to the low discharge force. Decomposed by sodium hypochlorite.

Claims (9)

A method for producing a low-salt sodium hypochlorite aqueous solution, comprising the steps of: supplying a 30-60% by mass aqueous sodium hydroxide solution to a reaction tank (1), and introducing chlorine gas into the sodium hydroxide supplied to the reaction tank Step (2) of carrying out a chlorination reaction in an aqueous solution at a reaction temperature of 20 ° C to 50 ° C, separating and removing the by-produced sodium chloride precipitated in the above step (2) to obtain a sodium hypochlorite aqueous solution (3) The foregoing step (2) is performed under the condition that the power required for stirring per unit volume is 0.1 to 15 kW/m ³ and the ratio of the number of powers required to be stirred Np to the number of circulating flow rates Nq (Np/Nq) is 0.5 to 8. Stirring is carried out. A method for producing a low-salt sodium hypochlorite aqueous solution according to claim 1, wherein the chlorine gas is diluted with an inert gas and introduced. The method for producing a low-salt sodium hypochlorite aqueous solution according to claim 1, wherein the chlorination reaction has a molar ratio of sodium hydroxide to chlorine (NaOH/Cl ₂ ) of 2.0 to 2.5. The method for producing a low-salt sodium hypochlorite aqueous solution according to any one of claims 1 to 3, wherein the reaction temperature in the step (2) is 30 to 50 °C. The method for producing a low-salt sodium hypochlorite aqueous solution according to any one of claims 1 to 3, wherein the sodium hypochlorite aqueous solution obtained in the step (3) has a sodium chloride concentration of 5.0% by mass or less. A low-salt sodium hypochlorite aqueous solution according to any one of claims 1 to 3 In the production method, the chlorinated ion concentration of the sodium hypochlorite aqueous solution obtained in the above step (3) is 1.5% by mass or less. The method for producing a low-salt sodium hypochlorite aqueous solution according to any one of claims 1 to 3, wherein the sodium hypochlorite aqueous solution obtained in the step (3) has a sodium hypochlorite concentration of 30 to 40% by mass. A method for producing a thin aqueous sodium hypochlorite solution, which comprises the step of diluting a low-salt sodium hypochlorite aqueous solution obtained by the production method according to any one of claims 1 to 7 to a specific effective chlorine concentration. The method for producing a thin aqueous solution of sodium hypochlorite according to claim 8, wherein the effective chlorine concentration is 1 to 20% by mass.