JPH0329724B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for producing highly bleached powder, and more specifically, the present invention relates to a method for producing highly bleached powder, and more specifically, it is a process for producing neutral calcium hypochlorite dihydrate [Ca
(ClO) _{2.2H 2} O] (hereinafter simply referred to as "neutral salt"). This invention relates to an improved method for producing highly bleached powder for obtaining crystals.

Advanced bleaching powder is known as a strong oxidizing agent and is widely used for disinfecting and sterilizing pool water, bath water, septic tank drainage, wastewater treatment equipment drainage, etc.
Its main component is calcium hypochlorite.

Many methods have been proposed for the production of calcium hypochlorite for a long time, but they are basically based on the reactions shown in the following reaction formulas (1), (2), and (3).

2Ca(OH) ₂ +2Cl ₂ →Ca(ClO) ₂ +CaCl ₂ +2H ₂ O (1) 2NaOH+Cl ₂ →NaClO+NaCl+H ₂ O ……(2) 2NaClO+CaCl ₂ →Ca(ClO) ₂ +2NaCl ……(3) Actually, A two-stage chlorination method in which an aqueous sodium hydroxide solution is firstly chlorinated and then an aqueous suspension of calcium hydroxide is added for secondary chlorination.A mixed aqueous suspension of calcium hydroxide and sodium hydroxide is chlorinated at once. a one-stage chlorination method in which a calcium hydroxide aqueous suspension and a sodium hydroxide aqueous solution are chlorinated separately and then mixed with these chlorinated solutions to cause double decomposition; Combined methods have been adopted industrially. In either method, in the chlorination step, dibasic calcium hypochlorite [Ca(ClO) _2.2Ca
(OH) ₂ ] (hereinafter simply referred to as "basic salt") crystals are crystallized, and when chlorination is further progressed, the basic salt crystals collapse and neutral salt crystals crystallize instead.
Highly bleached powder separates the neutral salt crystals from the mother liquor,
It is a calcium hypochlorate composition with an available chlorine concentration of 60% or more that is produced by drying.

In general, the neutral salt crystals obtained by the above-mentioned methods are square plate-shaped crystals with a thickness of 1 to 3 μm and a side of 20 to 30 μm, so they easily contain a large amount of mother liquor between the crystal faces and have extremely high solid-liquid separation. bad. Furthermore, in post-processes such as drying and granulation of the crystals, the crystals are very easily crushed, producing fine powder and scattering, which reduces the product consumption rate and causes a deterioration of the working environment. Various proposals have been made to improve these drawbacks, especially as a method for obtaining neutral salt crystals with good solid-liquid separation, but each method has its advantages and disadvantages, and it is difficult to know which method can be adopted industrially. It's hard to say.

For example, Tokkosho's work involves chlorinating a calcium hydroxide aqueous suspension in the presence of zinc ions to form grown crystals with good solid-liquid separation without changing the crystal habit.
Although the method described in Publication No. 42-26593 does yield neutral salt crystals that do not have twin crystallinity, the crystal growth rate is small, and the water content after centrifugation is just under 40%, which is solid-liquid. Separability has not been significantly improved.

In Japanese Patent Application Laid-open No. 48-46597, calcium hydroxide is added to the reaction mother liquor, and 0.003
~0.015 molar ratio of tin ion, strontium ion or barium ion is added to 30~50
A method is described in which, after chlorination at ℃, an aqueous sodium hypochlorite solution with a concentration of 25% or more is added to the reaction solution to cause a double decomposition reaction to produce neutral salt crystals with good solid-liquid separation. has been done. However, although some improvement in solid-liquid separation is observed in this method, it is insufficient for industrial application.

In the method described in South African Patent No. 707296, ions selected from barium ions, strontium ions, cadmium ions, zinc ions, and silver ions are added to an aqueous suspension of calcium hydroxide to perform chlorination to form basic crystals. It is said that the floating suspension is removed during generation and chlorination is continued to obtain neutral salt crystals with good hyperactivity, but in reality, the resulting neutral salt crystals have twinning, and therefore are solid-liquid. Separability has hardly been improved. Furthermore, some additives contain substances that cause environmental pollution and pollution problems, and it is nearly impossible to separate these components from the product.

JP-A-Sho obtains columnar neutral salt crystals with good dehydration properties by chlorinating an aqueous suspension of calcium hydroxide in the presence of one or more crystal modifiers selected from carboxylic acids, carboxylic acid salts, and carbohydrates. The method described in Publication No. 54-117396 produces columnar crystals with a different shape from those of the above-mentioned methods, and since the crystals do not have twin crystallinity, the solid-liquid separability of the crystals is significantly reduced. It has been improved. However, the columnar crystals have poor thermal stability and may be completely decomposed during post-processing steps such as drying, and the decomposition rate of the dried product is also extremely high and unsafe. Therefore, it cannot be adopted as an industrial method. The constant decomposition rate of calcium hypochlorite is expressed by the following formula (4), and the constant decomposition rate of calcium hypochlorite obtained by a conventional method is about 2 to 3% per year.

Constant decomposition rate = Initial effective chlorine content - Effective chlorine content after a certain period of time / Initial effective chlorine content x 100 (%)...(4)
An object of the present invention is to solve the above-mentioned problems, that is, to provide an improved method for producing highly bleached powder that can yield neutral salt crystals with excellent thermal stability and good solid-liquid separation properties. Another object of the present invention is to provide a crystal growth agent that produces neutral salt crystals with good solid-liquid separation properties.

As a result of intensive research to achieve the above object, the present inventors have found that growing neutral salt crystals in the presence of iron ions, which were conventionally thought to be an obstacle to the production of calcium hypochlorite, is difficult to achieve in the conventional method. 1~3μ,
The present invention was completed by discovering that neutral salt crystals, which had grown only 20 to 30 microns on one side, grew to a thickness of 3 to 20 microns and 30 to 70 microns on each side, and that solid-liquid separation was extremely good.

The present invention is a process for producing highly bleached powder in which neutral calcium hypochlorite crystals are obtained by chlorinating an aqueous suspension of calcium hydroxide or an aqueous suspension of calcium hydroxide and sodium hydroxide. After crystallization of calcium chlorite and before crystallization of neutral calcium hypochlorite, add 0.1 to 1.0% in molar ratio to calcium to crystallize neutral calcium hypochlorite dihydrate crystals. Neutral calcium hypochlorite with good solid-liquid separation characteristics.
This is a method for producing highly bleached powder to obtain dihydrate.

In the present invention, iron ions act as a crystal growth agent for neutral salt crystals. As a raw material for iron ions, any compound containing iron ions can be used regardless of whether it is divalent, trivalent, inorganic or organic, but in consideration of product stability, inorganic compounds are preferable, and sulfates are amorphous. It is recommended to avoid using it as it may form crystals.

Examples of iron compounds that can be used include iron chloride, iron nitrate, iron carbonate, iron oxide, iron hydroxide, iron orthophosphate, iron hexacyanoferrate, and iron formate. The valence may be either divalent or trivalent.

The method of the present invention can be applied to any of the known methods for producing calcium hypochlorite by crystallizing neutral salt crystals, that is, two-stage chlorination method, one-stage chlorination method, double decomposition method, but calcium hydroxide Preferably, a method of direct chlorination of an aqueous suspension or a mixed aqueous suspension of calcium hydroxide and sodium hydroxide is employed.
The amount of iron ions added is 0.1 to 1.0% based on the molar ratio of calcium to the raw material calcium hydroxide, and the addition time may be any time before the crystallization of neutral salt crystals, but preferably before the crystallization of basic salt crystals. After the initial stage, the most preferred stage is the collapse stage of basic salt crystals, that is, the stage of nucleation of neutral salt crystals.

If the amount of iron ions added is less than 0.1%, the growth of neutral salt crystals will be insufficient, and if it exceeds 1.0%, there will be adverse effects such as coloring, which is not preferable. Addition of iron ions before the formation of basic salt crystals will form floating iron hydroxide, which will prevent the normal crystallization of basic salt crystals, and addition after formation of neutral salt crystals will result in the formation of floating iron hydroxide. Almost no effect was observed.

In the direct chlorination method of an aqueous suspension of calcium hydroxide or a mixed aqueous suspension of calcium hydroxide and sodium hydroxide, chlorination is carried out at a reaction temperature of 10 to 50°C.
Preferably, the reaction is carried out in a temperature range of 20 to 35°C, and the reaction is controlled by oxidation-reduction potential difference (ORP) measurement, PH measurement, or residual alkali analysis in the slurry.
By performing ORP measurement and adding iron ions at a stage of chlorination rate (=amount of chlorine consumed/amount of total reacted chlorine x 100) of 50 to 90%, preferably 75 to 85%, a thickness of 3 to 20μ,
A neutral salt crystal with extremely good solid-liquid separation properties is crystallized, which can be easily dehydrated to a water content of 35% or less using a normal centrifugation operation in the form of a rectangular plate with sides of 30 to 70 μm.

As the reaction temperature increases, the amount of decomposition reaction increases,
This is not preferable because it lowers the basic unit. Reaction management
ORP measurement is preferred because it is easy to operate and has little measurement error. Timing of addition of iron ions,
The point at which the chlorination rate is 50 to 90% corresponds to the above-mentioned stage of collapse of the basic salt crystals.

In the present invention, the crystallized neutral salt crystals are separated from the mother liquor and then dried to produce a calcium hypochlorite composition, that is, a highly bleached powder. To separate neutral salt crystals from the mother liquor, centrifugation, pressurization, constant pressure, and other methods can be used; however, considering the energy load in the drying process, centrifugation or pressurization has a higher dehydration rate. is obtained, which is preferable. For drying, hot air drying, infrared drying, etc. can be used, but hot air drying is preferably used because of its low equipment cost.

The neutral salt crystals obtained in the present invention have good solid-liquid separation properties, so thermal energy in the drying process can be reduced, and since they have grown larger, their mechanical strength has improved. This reduces the amount of crushing and scattering of particles in subsequent processes, improves the product consumption rate, and also improves the working environment. In addition, it has excellent thermal stability and has a low decomposition rate during drying, and also has an extremely low decomposition rate at room temperature when dried.

The iron ions used in the present invention have conventionally been considered to be a hindrance to the production of calcium hypochlorite, and it has been proposed to remove them as much as possible. For example, South African Patent No. 707296 proposes a method for removing iron in raw calcium hydroxide by converting it into ferrate or ferrite using an additive. The cause of crystal growth due to the addition of iron ions in the present invention is not clear, but it is possible that fine iron compounds produced by the reaction serve as the nuclei of the neutral salt crystals, or that iron ions are involved in the nucleation of the neutral salt crystals. In either case, it is presumed that the number of crystal nuclei to be generated is controlled.

The present invention uses a simple method of adding iron ions,
To provide an improved method for producing highly bleached powder, which yields neutral salt crystals with good solid-liquid separation properties, excellent thermal stability and mechanical strength, and in which the decomposition rate of the dry product is extremely low. Therefore, its industrial significance is extremely large.

The present invention will be explained in more detail below using Examples. However, the scope of the present invention is not limited by the following examples.

In addition, the units described in the examples represent weight units unless otherwise specified.

Example 1 Calcium hydroxide with a purity of 97% (Fe content
28 parts of water was added to 10 parts (350 ppm), and chlorine gas was passed through the mixture at a rate of 50 g/hr while maintaining the temperature at 25°C to carry out a chlorination reaction. At the point when the chlorination rate is 80%, when the base raw salt crystals formed as the chlorination reaction progresses and begin to disintegrate and disappear, 0.1 part of ferric chloride (FeCl ₃ ) is added to water.
Add the solution dissolved in 0.5 parts to the reaction solution to determine the chlorination rate.
The reaction was completed at 98%. After aging this reaction-completed solution for 1 hour, a neutral calcium hypochlorite dihydrate cake was separated by centrifugation. The crystals in the cake are square plate-shaped crystals with a side of 30 to 60μ and a thickness of 3 to 10μ, and the effective chlorine content of the separated cake is
The moisture content was 33.5%. The effective chlorine content was determined by infrared drying the same cake.
A highly bleached powder of 83.70% was obtained. The effective chlorine decomposition rate during drying was 2.5%. Radius of this product
A commercially available tin can measuring 25.9 mm and height 50 mm was filled with 80 g, and the thermal decomposition temperature (hereinafter referred to as "TC") under constant temperature heating was measured in 2°C increments.There was no thermal decomposition at 138°C but decomposition at 140°C, so the Tc is 140℃
It was determined that

Example 2 Add water to 10 parts of calcium hydroxide used in Example 1.
24 parts were added, and the chlorination reaction was carried out while maintaining the temperature at 30°C. When the chlorination rate was 82%, a solution of 0.1 part of potassium ferrocyanide (K ₄ Fe (CN) ₆ ) dissolved in 0.5 part of water was added to the reaction solution, and the reaction was completed at a chlorination rate of 98%. After the reaction-completed solution was aged for 1 hour, neutral calcium hypochlorite dihydrate crystals were separated by centrifugation. The generated neutral calcium hypochlorite crystals are square plate-shaped crystals with a side of 30 to 70 μm and a thickness of 5 to 10 μm. The available chlorine content of the separated cake is 59.51
%, and the moisture content was 33.0%. By infrared drying the same cake, the effective chlorine content was 82.27%.
A highly bleached powder was obtained. The effective chlorine decomposition rate during drying was 2.3%, and Tc was 140°C.

Comparative Example 1 A chlorination reaction was carried out in the same manner as in Example 1 without using any additives. The generated neutral calcium hypochlorite dihydrate crystals have a side of 10 to 30 μm and a thickness of 1 to 3 μm.
It is a twinned large square plate-shaped crystal, and the effective chlorine content of the separated cake is 51.25% and the water content is 40.5.
It was %. By drying the same cake with infrared rays, a highly bleached powder with an effective chlorine content of 79.28% was obtained.
It was found that the effective chlorine decomposition rate during drying was 2.9% and Tc was 140°C, the same as in Examples 1 and 2.

Comparative Example 2 10 parts of calcium hydroxide used in Example 1, 24 parts of water
1 part and 0.1 part of sodium citrate (C ₆ H ₅ O ₇ Na ₃ ) were mixed, and the chlorination reaction was carried out while maintaining the temperature at 25°C. After the reaction was completed, the generated neutral calcium hypochlorite dihydrate crystals were separated by centrifugation.

This crystal is a square plate-shaped crystal with a side of 30 to 50μ and a thickness of 3 to 10μ, and the available chlorine content of the cake is 61.63.
%, and the moisture content was 34.4%. By infrared drying the same cake, the effective chlorine content was 84.83%.
A highly bleached powder was obtained. The effective chlorine decomposition rate during drying of the same sample was 4.2%. It can be seen that the Tc of this highly bleached powder is 120°C, which is significantly lower than that of Example 1.

Claims (1)

[Claims] 1. In a method for producing a highly bleached powder in which neutral calcium hypochlorite crystals are obtained by chlorinating an aqueous suspension of calcium hydroxide or an aqueous suspension of calcium hydroxide and sodium hydroxide, iron ions are used. After basic calcium hypochlorite crystal crystallization, and before neutral calcium hypochlorite crystal crystallization, the molar ratio to calcium is 0.1.
~1.0% added neutral calcium hypochlorite 2
A method for producing highly bleached powder to obtain neutral calcium hypochlorite dihydrate with good solid-liquid separation, which is characterized by crystallizing hydrate crystals. 2. The method according to claim 1, wherein the iron ion is a divalent or trivalent iron ion. 3. The method according to claim 1, wherein the addition of iron ions is carried out at a stage where the chlorination rate is 50 to 90%.