JP2012176860A - Method of manufacturing flaky zinc oxide powder - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method of manufacturing a flaky zinc oxide powder that can stably manufacture flaky zinc oxide particles excellent in optical characteristics even in the large scale of at least 20 L of a raw material capacity.SOLUTION: The method of manufacturing the flaky zinc oxide powder includes a process in which alkali of 2.0-3.0 of a molar ratio to the zinc salt is added to a mixture including a zinc salt and water as an alkali solution under the agitation condition in which (a) of the following formula (1) becomes at least 0.45 for 30-70 seconds of an adding time. I(θ)=eθ (1). In the formula (1), I(θ) denotes separation strength, (e) denotes the base of natural logarithm, and θ denotes the mixing time (unit: second) from the beginning of the mixing.

Description

  The present invention relates to a method for producing flaky zinc oxide powder.

  Conventionally, zinc oxide has been widely used as a blending component for cosmetics and paints. In these fields, it is desired to be transparent in the visible region and to have a strong ultraviolet absorbing ability because of its use. As such a material, finely divided zinc oxide having a particle size of less than 0.1 μm is known. However, since this zinc oxide easily aggregates, there is a problem that its dispersibility is poor and it is difficult to mix it with cosmetics and paints.

As a solution to such problems, flakes can be prevented by making zinc oxide into flaky particles, and it is excellent in transparency and ultraviolet absorption, and is suitable as a blending component for cosmetics and paints. Zinc oxide powder has been proposed.
For example, Patent Document 1 discloses flaky zinc oxide powder having a predetermined average particle diameter, average particle thickness, and average plate ratio for the purpose of providing a zinc oxide powder having an unprecedented form and particle size. An ultraviolet absorber containing is disclosed.
In addition, Patent Document 2 is specified for the purpose of providing an ultraviolet-absorbing powder having a flaky zinc oxide powder as a main component, which is excellent in transparency and ultraviolet-absorbing property and is suitable as a blending component for cosmetics and paints. An ultraviolet absorbing powder containing a zinc oxide powder having a shape and a trace element integrated with the zinc oxide powder is disclosed.

In Patent Documents 1 and 2, as a method for producing the above flaky zinc oxide powder, an aqueous solution in which a zinc salt and a water-soluble alkali metal salt are dissolved in water is added with high stirring, and an alkaline agent is added to a pH of 11 or more. A method is disclosed in which a precipitate is generated, and the precipitate is filtered and dried to obtain a flaky zinc oxide powder. Regarding the stirring conditions at this time, Patent Document 1 discloses that stirring Reynolds number = (diameter of stirring blade) ² × (stirring rotation speed) × (density of solution) / (viscosity coefficient of solution). As a result, the range of sufficient stirring is 30 or more, and 100 to 10 ⁵ is preferred.

JP-A-9-137152 JP 7-330334 A

  In producing the flaky zinc oxide powder, it is desirable to stir at high speed. However, on a large scale used for production (for example, the raw material volume is 20 L or more), there is a problem that an apparatus for obtaining a high stirring Reynolds number is expensive and the manufacturing cost becomes high. Moreover, even if the stirring Reynolds number is set within the range shown in Patent Document 1, there is a problem in that flaky zinc oxide powder having target optical characteristics may not be obtained.

  This invention makes it a subject to provide the manufacturing method of flaky zinc oxide powder which can manufacture stably the flaky zinc oxide particle which was excellent in the optical characteristic also in the large scale whose raw material capacity | capacitance is 20L or more.

By adjusting the stirring conditions at the time of adding the alkali and the addition time of the alkali, the present inventors can stabilize the flaky zinc oxide powder having excellent optical properties even on a large scale having a raw material capacity of 20 L or more. It has been found that it can be manufactured.
That is, the present invention provides a mixture of 20 L or more containing a zinc salt and water under a stirring condition where a in the following formula (1) is 0.45 or more and a molar ratio of 2.0 to 3 with respect to the zinc salt. A method for producing flaky zinc oxide powder, which comprises a step of adding 0.0 alkali as an alkaline solution at an addition time of 30 to 70 seconds.
I _s (θ) = e ^−a θ (1)
(In formula (1), I _s (θ) is the separation strength, e is the base of the natural logarithm, and θ is the stirring time from the start of mixing (unit: seconds).)

  According to the production method of the present invention, it is possible to stably produce flaky zinc oxide particles having excellent optical characteristics even on a large scale having a raw material capacity of 20 L or more.

It is a perspective view which shows the internal structure of the stirring apparatus used by the Example and the comparative example. It is the perspective view which took out only the stirring tank of the stirring apparatus shown in FIG. It is the perspective view which took out only the homomixer of the stirring apparatus shown in FIG. It is a perspective view from another viewpoint of the homomixer shown in FIG. It is the perspective view which took out only the paddle blade of the stirring apparatus shown in FIG.

[Stirring conditions]
In the method for producing the flaky zinc oxide powder of the present invention, the stirring conditions for adding and reacting an alkali solution containing an alkali to a mixture of 20 L or more containing a zinc salt and water include a in the following formula (1): Is performed under the condition of 0.45 or more.
I _s (θ) = e ^−a θ (1)
In the above formula (1), I _s (θ) is the separation strength, e is the base of the natural logarithm, and θ is the stirring time from the start of mixing (unit: seconds).
In the following description, a is also referred to as “mixability index”. This mixability index is defined by using the separation intensity I _s (θ) (Intensity of Segregation), and is an index described in the Revised Sixth Edition Chemical Engineering Handbook.

The separation strength represented by I _s (θ) is an evaluation index of stirring and mixing states. For example, “Danckwerts, PV, 1952. The definition and measurement of some characteristics of mixtures. Applied Science Research A3, p279-296 (Reference 1) ”. This mixing property evaluation by separation strength is widely known. Besides, “Revised Sixth Edition, Chemical Engineering Handbook, p425-426 (Reference 2)”, “Basics and Applications of Latest Mixing Technology, p23-24 (Reference) 3) ”and the like.
The definition of the separation intensity I _s (θ) is as shown in the following formula (3).

In the above formula (3), θ represents the stirring time (unit: seconds) from the start of mixing, as in the above formula (1). C (θ, x) indicates the stirring time θ and the concentration of the solute or dispersion in the mixing tank position x, and C ₀ indicates the average concentration (volume ratio of the solute or dispersion) in the mixing tank. Furthermore, V is a mixing volume (unit: m ³ ).
At the start of mixing (θ = 0), the separation intensity I _s (0) is 1, and mixing is performed, and the value of I _s (θ) decreases. Therefore, it is shown that the smaller the value of I _s (θ), the more uniformly mixed.

However, as pointed out in Reference 3, the separation intensity I _s (θ) is an important index that defines the mixed state in detail, but C (θ, x) is calculated by experiment. It is difficult. However, the value of C (θ, x) can be obtained by numerical simulation. Therefore, the separation intensity I _s (θ) can be obtained by calculating the value of C (θ, x) in Expression (3) by the numerical simulation. Then, by using the separation intensity I _s (θ) as an indicator of the mixed state, the mixing characteristics between different devices and different device scales can be compared and arranged.
That is, conventionally, even if the stirring Reynolds number is set in a specific range, the reason why the flaky zinc oxide powder having preferable optical characteristics is not necessarily obtained is that the mixing characteristics are the same even if the stirring Reynolds number is the same. This is because the separation strength I _s (θ) shown is different. On the other hand, in the production method of the present invention, flaky zinc oxide powder having favorable optical characteristics can be obtained reliably by arranging the stirring conditions and the stirring apparatus according to the separation strength I _s (θ).

In the above formula (3), when the average concentration C ₀ and the mixing volume V are constant, the separation strength I _s (θ) is a function of the stirring time θ. The concentration distribution C (θ, x) is obtained by numerical simulation and numerically integrated. Since the separation intensity I _s (θ) at time 0 (at the start of mixing) is normalized by the denominator C ₀ (1−C ₀ ), I _s (0) = 1. Further, the separation intensity I _s (θ) monotonously decreases with time.

The separation strength I _s (θ) obtained by the calculation is plotted on the y-axis (vertical axis), the stirring time θ is plotted on the x-axis (horizontal axis), and the relationship between the stirring time θ and the separation strength I _s (θ) is plotted. When the plotted data was approximated by an exponential function by regression analysis, it was found that there was a correlation represented by the following formula (1).
I _s (θ) = e ^−a θ (1)

The change with time of the separation intensity I _s (θ) represents how much mixing has progressed from the start of mixing. The mixing state at the start of stirring depends on, for example, the method of charging the mixed solution into the stirring tank and does not depend on the stirring device. That is, what truly represents the performance of the stirrer and the stirring conditions is the slope of the segregation strength change curve represented by the above formula (1), and the coefficient (mixability index) in the above formula (1). a corresponds to it. That is, the mixability index a can be said to be a mixing speed.
In the present invention, the mixing index a is 0.45 or more, preferably 0.50 or more, more preferably 0.55 or more. If it is less than 0.45, the time for uniform mixing becomes longer, the thickness of the flaky zinc oxide crystals to be produced becomes thicker, and the optical properties are lowered.
Moreover, as an upper limit of the mixability parameter | index a, Preferably it is 1.8 or less, More preferably, it is 1.1 or less, More preferably, it is 0.7 or less. If it is 1.8 or less, it can be adjusted without using an expensive stirrer, so that the manufacturing cost can be suppressed, and the generated crystal can be prevented from being crushed and the crystal size becoming too small.

In the present invention, the mixing index a can be obtained by fluid analysis software. The fluid analysis software to be used is not particularly limited, and a multi-rotation reference coordinate system, a sliding mesh, a high-order discretization method, etc. are modeled, and fluid analysis suitable for calculation of stirring tanks and mixing tanks Any software can be used.
Further, since the separation strength I _s (θ) is a concept (index) unique to chemical engineering, the standard function of general-purpose (commercial) fluid analysis software does not include a separation strength data output function. Therefore, in order to obtain the mixing index a using general-purpose fluid analysis software, it is preferable that a customization function for an analyst to extract necessary data is installed.
As general-purpose fluid analysis software that satisfies these requirements, for example, FLUENT (manufactured by Ansys Japan) can be cited.

<Simulation procedure for calculating the mixability index>
Hereinafter, an example of a simulation procedure for calculating a mixing index using general-purpose fluid analysis software will be described. First, a modeling diagram of the stirring device is drawn using modeling software. Since the mixability index is affected by the configuration, size, form, and the like of the stirrer used for stirring and mixing, it is necessary to model the stirrer. The modeling software to be used is not particularly limited. For example, when FULLENT is used as general-purpose fluid analysis software, “GAMBIT”, “Design Modeler” (both manufactured by Ansys Japan) and the like can be mentioned.

Various types of stirring blades such as a multistage blade, an anchor type, a horseshoe type, a screw type, a double ribbon, a turbine type, a propeller type, a max blend, and a bister device can be used as the stirring device used in the stirring and mixing. . Alternatively, for example, a static mixer or a line mixer may be used alone or in combination. Although the shape of the stirring tank used for stirring and mixing is not particularly limited, a cylindrical shape or the like can be used.
When drawing the model diagram of the stirring device, these homomixer, turbine blade, paddle blade, stirring tank, parts supporting each stirring blade, baffle plate, etc. are drawn.
After drawing, create a calculation grid (mesh) of the fluid part. The homomixer modeling can be simplified by a method of setting a boundary condition for the discharge flow rate of the homomixer on the simplified homomixer discharge surface. From the viewpoint of obtaining an appropriate calculation result, it is preferable to reduce the calculation lattice size and also reduce the calculation lattice volume distortion (Skew).

When actually performing the simulation, the number of rotations of the homomixer, paddle mixer, turbine mixer, and the like used for stirring, the blending ratio and physical properties of the raw materials used, and the like are also set. Moreover, it is preferable to perform the calculation in two stages of steady fluid calculation and unsteady separation strength calculation. The flow state under the condition of stirring at a constant rotational speed can be obtained by the steady fluid calculation, and the advection diffusion state of the solute (or dispersion) can be obtained by the unsteady separation strength calculation. This makes it possible to obtain a mixing index at a constant stirring speed. It can be said that the unsteady separation strength calculation is a kind of tracer analysis. The calculation time can be shortened by dividing the calculation into two stages.
In addition, it is necessary to use the result of steady fluid calculation as the initial condition for unsteady calculation, and fluid calculation and separation strength calculation are not coupled to each other. Therefore, fluid calculation and separation strength calculation are combined to perform unsteady calculation. There is no difference in the results.

In the unsteady analysis, the actual analysis time (not the time required for calculation, but the time from the start of stirring corresponding to θ in Equation (1)) is preferably set to at least 5 seconds. If the analysis is performed in an analysis real time of 5 seconds or more, the approximate curve slope (mixability index) of Equation (1) can be calculated appropriately. It should be noted that since the approximate curve slope does not change greatly even if analysis is performed for a long time, the actual analysis time should be 20 seconds or less, and preferably around 10 seconds.
The separation intensity data for 10 seconds obtained by the simulation is preferably output with a sampling interval of 1 second or less. If it is 1 second or less, the size of the sampling interval is appropriate, the number of data points in the approximation is not too coarse, and an appropriate approximate curve slope can be obtained.

The purpose of calculating the mixability index is to appropriately express the performance of the stirring device and the stirring conditions as described above. In the reaction between a mixture containing a zinc salt and water and an alkaline solution, the viscosity of the mixed solution during the reaction reaches a maximum of 200 mmPa · s. In general, a high viscosity liquid is more difficult to stir and mix than a low viscosity liquid.
Therefore, when performing fluid calculation for obtaining the mixing index, it is necessary to perform simulation in a state where the viscosity is the highest in the process. That is, it is preferable to perform a simulation under an environment having a viscosity of 200 mmPa · s, and the stirring Reynolds number is on the order of 10 ^{2 at the} viscosity. The flow is preferably analyzed in a laminar flow state.

[Method for producing flaky zinc oxide]
In the method for producing the flaky zinc oxide powder of the present invention, a mixture of 20 L or more containing a zinc salt and water has a molar ratio of 2.0 to 3.0 with respect to the zinc salt under the specific stirring conditions described above. It has the process of adding an alkali as an alkaline solution with an addition time of 30 to 70 seconds.
As one preferable embodiment, the method for producing a flaky zinc oxide powder of the present invention may include the following steps 1 and 2.
Step 1: A step of preparing a mixture of 20 L or more containing a zinc salt and water.
Step 2: An alkali having a molar ratio of 2.0 to 3.0 with respect to a zinc salt is added to the mixture prepared in Step 1 as an alkaline solution with an addition time of 30 to 70 seconds under the specific stirring conditions described above. Process.
Moreover, it is preferable to further have the following process 3 as needed.
Step 3: A step of aging treatment after Step 2, collecting a precipitate and drying by heating.

<Step 1>
In Step 1, a mixture of 20 L or more containing a zinc salt and water can be prepared.
Examples of the zinc salt used include zinc sulfate, nitrate, acetate, phosphate, carbonate, oxalate, chloride, and hydrates thereof.
As water to be mixed with the zinc salt, water may be blended as it is, but instead of water, an aqueous acid solution such as an aqueous sulfuric acid solution, an aqueous nitric acid solution or an aqueous acetic acid solution may be used.

The mixture preferably further contains a water-soluble alkali metal salt. By containing the water-soluble alkali metal salt, it is possible to stably obtain a flaky zinc oxide powder that suppresses aggregation and has excellent optical properties.
The water-soluble alkali metal salt is water-soluble and, when ionized, SO ₄ ²⁻ , NO ₃ ⁻ , CH ₃ COO ⁻ , PO ₄ ³⁻ , CO ₃ ²⁻ , C ₂ O ₄ ²⁻ , Cl−, etc. However, from the viewpoint of the effect of inhibiting aggregation, sodium sulfate, potassium sulfate, sodium chloride, potassium chloride and the like are preferable. These water-soluble alkali metal salts can be used alone or in combination of two or more.
The content of the water-soluble alkali metal salt is preferably 5 to 40 mol, more preferably 10 to 35 mol, and still more preferably 15 to 30 mol with respect to 100 mol of the zinc salt, from the viewpoint of the effect of suppressing aggregation. .

In addition, it is preferable that the above mixture is further mixed with a salt containing a trace element. By blending a salt containing a trace element, the trace element is bound and held on the surface of the obtained flaky zinc oxide powder or inside thereof, thereby improving the ultraviolet absorptivity.
Examples of the salt containing a trace element include sulfate, nitrate, acetate, phosphate, carbonate, oxalate, and chloride of iron, zirconium, calcium, germanium, manganese, magnesium, or yttrium.
The content of the salt containing these trace elements is preferably 0.005 to 1.0 mol, more preferably 0.01 to 0, with respect to 100 mol of the zinc salt, from the viewpoint of the effect of improving ultraviolet absorption. .5 moles.

  Although there is no restriction | limiting in particular as an apparatus used when these components are mix | blended after mixing each component and a mixture of 20L or more is obtained, It is preferable to use the stirring apparatus used at the next process. Moreover, the mixing conditions in this process are not specifically limited, You may mix on the conditions similar to the stirring conditions in the following process.

<Process 2>
In step 2, an alkali having a molar ratio of 2.0 to 3.0 with respect to the zinc salt is added to the mixture obtained in step 1 as an alkaline solution under the above-mentioned specific stirring conditions in an addition time of 30 to 70 seconds. This is a step of adding, and after the completion of this step, a precipitate of zinc oxide is obtained.
As the stirring device to be used, various types of stirring blades such as a multistage blade, an anchor type, a horseshoe type, a screw type, a double ribbon, a turbine type, a propeller type, a max blend, a bister device and the like can be used. Alternatively, for example, a static mixer or a line mixer may be used alone or in combination. Although the shape of the tank used for stirring and mixing is not particularly limited, a cylindrical shape or the like can be used.
In this step, since the mixing index a obtained by the above simulation is stirred and mixed under a stirring condition of 0.45 or more, the value of the separation strength I _s (θ) can be maintained at a suitable value. Even on a large scale of 20 L or more, flaky zinc oxide powder having excellent optical properties can be stably produced.
In addition, as a volume of a mixture, although it is 20L or more, from the viewpoint of acquiring the effect of this invention more, Preferably it is 30L or more, More preferably, it is 35L or more.

Examples of the alkali used in this step include sodium hydroxide, potassium hydroxide, lithium hydroxide, and ammonium hydroxide. Among these, sodium hydroxide is preferable.
The molar ratio of the alkali to be added to 1 mol of zinc salt is 2.0 to 3.0, preferably 2.2 to 2.8, more preferably 2.3 to 2.7. If the molar ratio is less than 2.0, the crystal thickness of zinc oxide becomes thick and the transparency is lowered, and if it exceeds 3.0, the generated zinc oxide crystal dissolves, which is not preferable.
These alkalis are dissolved in water or the like and added to the mixture as an alkaline solution. By adding, a precipitate of zinc oxide is generated. The alkali solution is preferably added dropwise from the viewpoint of obtaining crystals of zinc oxide having an appropriate size and thickness.

The addition time of the alkaline solution is 30 to 70 seconds, preferably 35 to 65 seconds, and more preferably 40 to 60 seconds. If it is less than 30 seconds, the average plate ratio (average particle diameter / average particle thickness) of zinc oxide obtained by reducing the crystal size is reduced, and the optical properties are deteriorated. Further, if it exceeds 70 seconds, the average plate ratio of zinc oxide obtained by increasing the crystal thickness becomes small, or when a salt containing a trace element is blended, zinc is a precipitation pH in the mixture. Since it is difficult to precipitate trace elements having different concentrations together with zinc and to keep the trace elements uniformly in zinc, the optical characteristics are deteriorated.
As temperature at the time of addition, 30 degrees C or less is preferable. If it is 30 degrees C or less, it will be easy to make the zinc oxide obtained into a flake shape, and it will be easy to prevent becoming spherical or a lump particle, and an optical characteristic can be improved.

<Step 3>
In step 3, after step 2, an aging treatment is performed, and a precipitate is collected and dried by heating. This step is an optional step.
The aging treatment is preferably carried out by holding the precipitate while it is heated as necessary after the precipitate is formed. As the treatment conditions for the aging treatment, it is preferable to treat at about 60 to 100 ° C. for 30 minutes to 5 hours from the viewpoint of obtaining flaky zinc oxide with better crystallinity.
After the aging treatment, it is preferable to wash with water if necessary, collect the precipitate by means such as filtration, and heat dry. The drying temperature is desirably not too high, preferably about 200 to 300 ° C., more preferably 220 to 250 ° C., and the drying time is preferably 1 to 20 hours, more preferably 5 to 15 hours. It's time.

[Flamed zinc oxide powder]
The flaky zinc oxide powder obtained by the production method of the present invention preferably has a specific range of average particle diameter, average particle thickness and average plate ratio.
As an average particle diameter, Preferably it is 0.1-1 micrometer, More preferably, it is 0.2-0.7 micrometer, More preferably, it is 0.3-0.5 micrometer. If it is 0.1 micrometer or more, the fall of the dispersibility by aggregation is prevented, and if it is 1 micrometer or less, the outstanding transparency and ultraviolet-ray absorptivity are obtained.
The average particle thickness is preferably 0.005 to 0.2 μm, more preferably 0.01 to 0.1 μm, and still more preferably 0.01 to 0.05 μm. If it is 0.005 μm or more, the flaky shape is difficult to collapse, and if it is 0.2 μm or less, there is no discomfort when blended in cosmetics, and the practicality is excellent.
The average plate ratio is preferably 7 or more, more preferably 10 or more, and still more preferably 11 or more. If it is 3 or more, it has excellent transparency. The average plate ratio means the ratio of (average particle diameter) / (average particle thickness) between the average particle diameter and the average particle thickness.

In the flaky zinc oxide powder obtained by the production method of the present invention, it is preferable that trace elements are bound and held on the surface of the flaky zinc oxide powder or inside thereof.
The trace element is one or more elements selected from the group consisting of iron, zirconium, calcium, germanium, manganese, magnesium, and yttrium. These can be used singly or in combination of two or more. Examples of combinations include zirconium and iron, zirconium and magnesium, iron and magnesium, iron and calcium, magnesium and germanium and the like.
The content of the trace element is preferably 0.005 to 1.0 mol, more preferably 0.01 to 0.5 mol, per 100 mol of the zinc salt. If it is the said range, ultraviolet-ray absorptivity can fully be improved.

In the present invention, the optical properties of the flaky zinc oxide powder can be evaluated by the value of the optical property P represented by the following formula (2).
P = log (100 / _T370 ) / log (100 / _T500 ) (2)
(In formula (2), T ₃₇₀ and T ₃₇₀ represent absorbance at 370 nm and 500 nm, respectively.)
The optical property P of the flaky zinc oxide powder obtained by the production method of the present invention is preferably 6 or more, more preferably 6.2 or more, and even more preferably 6.4 or more. The larger the value of the optical characteristic, the higher the ultraviolet shielding property as compared with the visible light transparency. If the optical characteristic P is 6 or more, the ultraviolet shielding performance is higher than the visible light transmission property, for example, the obtained flaky zinc oxide powder. Cosmetics containing can hardly be whitened when applied to the skin.

The optical properties of the flaky zinc oxide powders obtained in the following Examples and Comparative Examples, and the average particle diameter and average particle thickness were measured by the following methods.
(Measurement of optical properties)
The obtained flaky zinc oxide powder was dispersed in glycerin / water = 9/1 (weight ratio) so as to be 0.03% by weight, and this dispersion was put in a cell having an optical path length of 1 mm, and a spectrophotometer (Shimadzu) With a product name “Solid-Spec3700” manufactured by Seisakusho, a transmission spectrum of 250 nm to 700 nm was measured under the condition without an integrating sphere, and absorbances at 370 nm and 500 nm were obtained. Based on the absorbance value, the optical property value P was calculated from the above equation (2). The larger the value of the optical property P, the higher the ultraviolet shielding property compared to the visible light transparency.
[Measurement of average particle diameter and average particle thickness]
The average particle diameter is a particle of 20 arbitrary particles in an arbitrary field of view in a photograph in which zinc oxide particles are photographed and developed using a transmission electron microscope (manufactured by JEOL Ltd., product name “JEM-2100”). It was determined by repeatedly measuring the average diameter. For the oval zinc oxide particles, the arithmetic average of the major axis and the minor axis was regarded as the particle diameter.
The average particle thickness was determined by measuring the plate thickness of all the particles that can be read in the photograph used in the measurement of the average particle diameter, and calculating the average particle thickness by the arithmetic average.
Further, based on the value obtained by the above measurement, the value of (average particle diameter) / (average particle thickness) was defined as the average plate ratio.

Example 1
(Agitator)
In FIG. 1, the internal structure of the stirring apparatus used in the present Example is shown, and each structure of the stirring apparatus is shown in FIGS. In addition, the unit of the numerical value shown by FIGS. 2-5 is mm. The stirring device 10 used in the present example includes a homomixer 12 having a turbine blade and a paddle blade 13 in a stirring tank 11, and the homomixer, the turbine blade, and the paddle blade are independently motor-driven. A stirring device having wings and capable of adjusting each rotation speed was used.

(Calculation of mixability index (a))
The above stirrer is modeled using modeling software “GAMBIT” (manufactured by Ansys Japan), and the calculation lattice size is about 520,000 lattices mainly with a hexahedral lattice, and the maximum volume strain is 0.79. did. This model is set, and the homomixer rotation speed: 3000 rpm, the flow rate discharged from the homomixer stirring section: 110 L / min, the paddle mixer rotation speed: 75 rpm, the turbine mixer rotation speed: 75 rpm, the rotation of the paddle mixer and the turbine mixer The conditions of the agitator were set so that the direction was the reverse direction, and the steady flow calculation was performed using the general-purpose fluid analysis software “FLUENT” (manufactured by Ansys Japan Co., Ltd.), and the unsteady flow calculation was performed based on the data. . The actual analysis time of unsteady flow calculation was 10 seconds, and data was output at 1 second intervals. The time was plotted on the x-axis, the separation intensity I _s (θ) was plotted on the y-axis, the data obtained by the above calculation was plotted, and the least square approximation was performed to obtain the mixing index. The value of a of the obtained mixability index was 0.61.

(Preparation of flaky zinc oxide powder)
1.69 kg of zinc sulfate heptahydrate, 0.520 kg of sodium sulfate, 4.53 g of iron sulfate, and 31.5 kg of water were placed in the stirring apparatus shown in FIG. The mixture was stirred at 110 L / min, paddle mixer rotation speed: 75 rpm, turbine mixer rotation speed: 75 rpm, and 34.6 L of the mixture was obtained. Also, under this stirring condition (mixability index; a = 0.61), the addition time was 40 seconds, and 19.3 kg of 2N aqueous sodium hydroxide solution (2.45 mol based on zinc sulfate heptahydrate) was added. Added at 21 ° C. Further, after stirring for 10 minutes under the same stirring conditions, the mixture was heated and stirred at 85 ° C. to 95 ° C. for 90 minutes with homomixer 2000 rpm, paddle mixer 35 rpm, and turbine mixer 35 rpm. Then, after cooling to 40 degreeC, washing with water and filtration were repeated twice, and pH of the filtrate was 10 or less. After drying at 100 ° C. overnight, drying was performed at 230 ° C. for 10 hours to obtain flaky zinc oxide powder. The obtained powder was added to a 10 wt% glycerin aqueous solution and dispersed for 30 minutes with an ultrasonic cleaner. The absorbance of this dispersion was measured in a cell having an optical path length of 1 mm, and a value P of optical characteristics was calculated.

Examples 2-4, Comparative Examples 1-4
A flaky zinc oxide powder was obtained in the same manner as in Example 1 except that the mixing index a and the addition time of the 2N sodium hydroxide aqueous solution were changed as shown in Table 1. Further, the optical properties of the obtained flaky zinc oxide powder were calculated in the same manner as in Example 1. The results are shown in Table 1.

Example 5
Raw material weight: 2345 g of zinc sulfate heptahydrate, 260 g of sodium sulfate, 2.27 g of iron sulfate, 15.8 kg of water, 9.7 kg of 2N aqueous sodium hydroxide solution to be added (2.45 against zinc sulfate heptahydrate) Except for the above, a flaky zinc oxide powder was obtained in the same manner as in Example 1. Further, optical properties of the obtained flaky zinc oxide powder were calculated in the same manner as in Example 1. The results are shown in Table 1.

  From Table 1, it turns out that the flaky zinc oxide powder obtained by the method of Examples 1-5 has the outstanding optical characteristic. Further, when the addition time of the 2N sodium hydroxide aqueous solution is out of the range of 30 to 70 seconds as in Comparative Examples 1 to 3, or when the mixing index a is less than 0.45 as in Comparative Example 4, It can be seen that the optical properties of the zinc oxide powder are inferior to those of the examples.

  The flaky zinc oxide powder obtained by the production method of the present invention is suitable as a compounding component for applications such as cosmetics and paints that are desired to be transparent in the visible region and have a strong ultraviolet absorbing ability.

10 Stirrer 11 Stirrer 12 Homomixer 13 Paddle Blade

Claims (4)

In a mixture of 20 L or more containing a zinc salt and water, an alkali having a molar ratio of 2.0 to 3.0 with respect to the zinc salt under stirring conditions in which a in the following formula (1) is 0.45 or more, A method for producing flaky zinc oxide powder, comprising a step of adding an alkaline solution at an addition time of 30 to 70 seconds.
I _s (θ) = e ^−a θ (1)
(In formula (1), I _s (θ) is the separation strength, e is the base of the natural logarithm, and θ is the stirring time from the start of mixing (unit: seconds).)   The method for producing flaky zinc oxide powder according to claim 1, wherein the mixture further contains at least one water-soluble alkali metal salt selected from sodium sulfate, potassium sulfate, sodium chloride, and potassium chloride.   The method for producing a flaky zinc oxide powder according to claim 1 or 2, wherein the flaky zinc oxide powder has an average particle size of 0.1 to 1 µm and an average particle thickness of 0.005 to 0.2 µm. The manufacturing method of the flaky zinc oxide powder in any one of Claims 1-3 whose optical characteristic P represented by following formula (2) of flaky zinc oxide powder is 6 or more.
P = log (100 / _T370 ) / log (100 / _T500 ) (2)
(In formula (2), T ₃₇₀ and T ₃₇₀ represent absorbance at 370 nm and 500 nm, respectively.)

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