JPH05310425A - Production of metal oxide fine particle - Google Patents

Abstract

PURPOSE:To produce metal oxide fine particles at a reduced production cost by introducing small droplets of an aqueous solution containing a metal salt in a state of a gas-liquid mixture into a reaction furnace having a specific inlet temperature and a specific temperature gradient, and subjecting the metal salt in the droplet to thermal decomposition. CONSTITUTION:A Zn or Ti metal salt such as Zn(NO3)2.6H2O is dissolved in water to obtain an aqueous solution having a concentration of 1X10<-5> to 20mol/L. The aqueous solution of the metal salt is charged into a liquid tank 1 and continuously supplied to a droplet-supplying apparatus 3 with a transfer circulation pump 2. The generated small droplets having diameter of 0.1-100mum are introduced into a reaction tube 6 provided with a temperature-controllable high-temperature heater 5 together with He gas, etc., supplied from a carrier gas supplying apparatus 4. The droplet is thermally decomposed in the reaction tube 6 having an inlet temperature of 80-300 deg.C and a temperature gradient of 0-50 deg.C/cm to form fine particles of an oxide such as Zn0 having particle diameter of 0.05-5mum in a state of a mixture of gas and solid. The formed particles are deposited on a collection plate in an electrostatic collector 7 provided with a corona discharging member 8 to obtain the objective spherical fine particles of metal oxide.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for producing metal oxide fine particles, and more particularly to a method for producing spherical metal oxide particles.

[0002]

2. Description of the Related Art Generally, metal oxide fine particles are used in various industrial fields by utilizing their properties such as photoconductivity, piezoelectricity, fluorescence, and catalytic effect. Among them, zinc oxide fine particles are Various industrial products, pharmaceuticals, vulcanization accelerators for rubber,
It is used in catalysts, varistors (variable resistors), paints, etc., and is recently used in UV cosmetics as an ultraviolet shielding material.

Further, fine particles of titanium oxide are also used for white pigments, magnetic raw materials, abrasives, pharmaceuticals, etc., and recently, they are also used as an ultraviolet shielding agent like fine particles of zinc oxide.
Other metal oxide fine particles are also used for various purposes.

As described above, the metal oxide fine particles have an extremely great industrial value, and it is important to make them fine in order to maximize their functions. That is, by making the particles finer, the specific surface area increases, and the ratio of the number of molecules located on the surface of the fine particles in the total number of molecules forming the fine particles increases, so that the surface energy of the fine particles increases, so Is extremely greatly expressed. If you want to increase the function by using a uniform shape,
Uniform spherical particles are important.

The above-mentioned methods for producing metal oxide fine particles having extremely important industrial value are roughly classified into a liquid phase method and a gas phase method.

In the liquid phase method, for example, in the production of zinc oxide fine particles, there is a method of hydrolyzing the metal alkoxide to obtain zinc oxide ultrafine particles (Japanese Patent Laid-Open No. 2-59425). In general, there has been a method for obtaining a desired metal oxide by adding an acid / alkali solution to a metal salt and causing a reaction in a liquid phase.

In the gas phase method, a metal is generally vaporized, and the vapor and a gas containing oxygen are mixed to obtain metal oxide fine particles (for example, in the production of zinc oxide fine particles, JP-A-1-286919). Japanese Patent Laid-Open No. 2083/1990
No. 69).

In addition to the liquid phase method and the gas phase method, there is a spray pyrolysis method, which atomizes an aqueous solution or an organic solvent solution containing an inorganic acid salt or an organic acid salt of a metal, and atomizes the atomized liquid particles. A method of obtaining oxide-based fine particles by carrying out a thermal decomposition reaction by transporting to a heating furnace (for example, in the production of oxide-based superconductors,
JP-A-2-196023).

In consideration of the above-mentioned conventional techniques, the liquid phase method has a limit in selecting the starting material thereof and synthesizing the metal alkoxide containing the metal with respect to the desired metal oxide fine particles. In other words, some metals cannot be converted into metal alkoxides, and it is difficult to say that a wide range of metal oxide fine particles can be produced. Moreover, in the method using metal salts, which has been used for a long time, there are almost no restrictions on the selection of starting materials, but in order to react them in the liquid phase, various acid / alkali solutions must be empirically selected. It has to be synthesized, and determining the synthesis process is complicated and takes a long time. And since the manufacturing process by these liquid phase methods is basically a batch system, it is difficult to automate, and the fine particles produced are obtained in a solid-liquid mixed phase state. Therefore, the whole manufacturing process becomes complicated and it is difficult to reduce the cost.

On the other hand, in the vapor phase method, it is necessary to use a high-purity metal as a raw material in order to improve the purity of the product, but the cost is increased accordingly. In addition, since the melting point of metal is generally high, it is necessary to raise the temperature to a considerably high temperature in order to vaporize the metal, and it is necessary to select a material that can maintain the high temperature even in the design of the equipment, and the safety of the manufacturing process. It is not preferable in terms of sex.

In the spray pyrolysis method, in order to recover the oxide-based fine particles obtained in the gas-solid mixed phase, after the particles of the atomized raw material liquid are charged by means such as radiation irradiation,
A method is used in which the particles are conveyed to a heating furnace to be charged into oxide-based charged fine particles by a thermal decomposition reaction, and the charged fine particles are deposited on a substrate heated at a predetermined temperature. However, in that method, since the liquid droplets enter the charging device, the charge generation section becomes wet with the passage of operating time, malfunction of the charging device occurs, and the substrate is installed in the heating furnace. The size of the substrate is limited, and the device system becomes complicated. Therefore, continuous operation is difficult and not suitable for mass production.

In the production of fine particles by the spray pyrolysis method, there is no regulation on the temperature distribution in the heating furnace, and the influence of the temperature distribution in the furnace on the shape of the produced fine particles is not clear. Therefore, when fine particles are produced by using a sharp temperature distribution in the furnace, many crushed particles are generated, and coarse particles having an extremely wide particle size distribution do not form a uniform shape.

[0013]

Therefore, according to the present invention, a wide variety of uniform metal oxide spherical fine particles can be continuously produced at a low cost by using a simple process in which the temperature distribution in the pipe length direction is defined. It is intended to provide a manufacturing method.

[0014]

Means for Solving the Problems In the present invention, an aqueous solution containing a metal salt is made into fine droplets having a droplet diameter of 0.1 μm to 100 μm, and the droplets are mixed in a gas-liquid phase using a carrier gas. characterized in feed to the high temperature reaction furnace, thereby generating a metal salt is thermally decomposed metal oxide fine particles contained in the reactor inside the droplet temperature T _in is 80 to 300 ° C. of the reactor inlet The present invention provides a method for producing fine metal oxide particles.

The present invention will be described in detail below with reference to the accompanying drawings. FIG. 1 is a schematic view showing one specific example of the apparatus used in the present invention. In the method of the present invention, the aqueous solution of the metal salt in the liquid tank 1 is continuously supplied to the droplet supply device 3 for generating fine droplets by using the liquid circulation pump 2, and the generated droplets are used as a carrier. The carrier gas sent from the gas supply device 4 is sent along with the carrier gas to a reaction tube 6 having a high-temperature heating body 5 capable of controlling temperature distribution, and a thermal decomposition reaction of droplets is performed in the reaction tube 6 to solidify gas. The metal oxide fine particles are generated in the mixed phase, and the fine particles are used for the corona discharge body 8.
Is electrostatically deposited on the collecting plate in the electrostatic collector 7 having. The gas containing water vapor, which flows out of the electrostatic collector 7, has its water content removed by passing through the cold trap 9 and the filter 10, and is forcibly exhausted by the pump 11. The collection of the fine particles is not limited to the method using the collector 7, and may be performed by other collection methods such as a method using a filter.

The metal element used as the metal salt is specifically an alkali metal, an alkaline earth metal or a transition metal. For example, as the alkali metal, Li, Na, K,
Examples of alkaline earth metals such as Rb, Cs and Fr include Be and M
The transition metals such as g, Ca, Sr, Ba, Ra, etc. are Sc, Ti, V, Cr, Mn, Fe, Co, Ni of the 4th period of the periodic table,
Cu, Zn, Y, Zr, Nb, Mo, Tc, Ru in the fifth cycle,
Rh, Pd, Ag, Cd, La, Hf, Ta, W of the sixth period,
Examples thereof include Re, Os, Ir, Pt, Au and Hg.

Examples of the type of salt include a hydrochloride, a nitrate, a sulfate, a phosphate, a carbonate, an acetate, a double salt composed of two or more kinds of salts, a complex salt containing a complex ion, and the like. These may be either anhydrous salts or hydrous salts. In particular,
Metal salts include Ti (SO ₄ ) ₂ , CuSO ₄ .5H ₂ O, Z
_{n (NO 3) 2 · 6H} 2 O, Ca (NO 3) 2 · 4H 2 O, CaC
l ₂ , MgCO ₃ , Fe ₃ (PO ₄ ) ₂ , Cu (CH ₃ COO) _2, etc.,
Examples of the double salt include KMGCl ₃ , AlK (SO ₄ ) _{2 and the} like, and examples of the complex salt include K ₃ [Fe (CN) ₆ ] and [CoCl (NH ₃ ) ₅ ] Cl ₂ . These metal salts may be used alone or as a mixture. As the mixture, for example, when using a mixture of titanium salt and zinc salt, depending on the thermal decomposition temperature,
Zinc titanate (Zn ₂ TiO ₄ ) is obtained, which is a mixture or composite of zinc oxide and titanium oxide.

As the solvent for the metal salt solution, water or an organic solvent may be used alone or in combination, but it is preferable to use water from the viewpoint of safety during operation. Examples of the organic solvent include alcohols such as methanol and ethanol, N, N-dimethylformamide,
Examples include polar solvents such as dimethyl sulfoxide and hexamethylphosphoramide. The concentration of the metal salt solution is in the range of 10 ^-5 mol / liter to 20 mol / liter, preferably 10 ^-4 mol / liter to 10 mol / liter. When the solution concentration is lower than 10 ^-5 mol / liter, the amount of metal oxide fine particles produced is extremely small, and when the solution concentration is higher than 20 mol / liter, the solution viscosity increases excessively to form fine droplets. Becomes difficult.

The carrier gas refers to an inert gas or a gas that does not hinder the progress of the thermal decomposition reaction, and examples thereof include helium, air and nitrogen. The flow rate of the carrier gas is adjusted so that the residence time of the droplet containing the metal salt in the reaction tube 6 does not become shorter than 1 second.

As a method for forming a solution into fine droplets, there are a method using ultrasonic vibration, a method using a spray nozzle, and the like. In order to obtain a fine droplet having a narrow droplet diameter distribution, ultrasonic vibration is preferable. Method is better.

The droplet diameter is preferably in the range of 0.1 μm to 100 μm, and the droplet diameter distribution is preferably as narrow as possible. When the droplet diameter is smaller than 0.1 μm, the diameter of the produced metal oxide fine particles is about 0.01 μm at the maximum, and the fine particles in that area are called ultrafine particles. Because of the large Brownian diffusion, deposition on the inner wall of the reaction tube The amount becomes very large, and the yield of the produced fine particles deteriorates. When the droplet diameter is larger than 100 μm, even if the diameter of the produced metal oxide fine particles is small, it is about several tens of μm, and it becomes difficult to atomize the particles. The droplet diameter is preferably measured in a gas-liquid mixed phase state, and can be measured, for example, by a light scattering type particle size distribution measuring instrument.

The metal oxide fine particles obtained by the present invention have good monodispersibility, and the concentration of the spray solution can be adjusted to 0.
The range of 01 μm to several tens of μm can be obtained, but the range of 0.05 μm to 5 μm is preferable in view of the yield of the produced fine particles and the improvement in function due to atomization.
The diameter of the metal oxide fine particles can be measured by various methods, for example, using a scanning electron microscope.

In the reaction furnace, in the reaction tube 6, the heating element 5 is arranged so that the isothermal portion is kept as wide as possible in the radial direction.
Control the temperature. In addition, the reactor inlet temperature and the reactor temperature in the present invention refer to ambient gas temperature. The temperature in the reaction furnace is preferably 8 when water is used as the solvent.
The range of 0 ° C to 2000 ° C is preferable. If the temperature is lower than 80 ° C, the water content of the droplets is hard to evaporate, and if it is higher than 2000 ° C, there is a possibility of steam explosion. When an organic solvent is used as the solvent, it is preferably in the range of 50 ° C to 400 ° C. If the temperature is lower than 50 ° C, the organic solvent component of the liquid droplets is hard to evaporate, and if it is higher than 400 ° C, soot is generated.

When the solvent is water, the reactor inlet temperature is 80 ° C.
As described above, the temperature of the heating element 5 is controlled so as to be 300 ° C. or lower. When the reactor inlet temperature is lower than 80 ° C, the water content in the droplets is hard to evaporate, and when it exceeds 300 ° C, the evaporation of the solvent and the thermal decomposition of the solute which is the thermal decomposition reaction substance occur simultaneously and rapidly. The fine particles are not spherical but crushed. The reactor length at which the reaction rate at the reactor outlet is 90% is L _0, and the distance from the reactor inlet is L (L = 0.1 ×
When the temperature at L ₀ ) is _TL , the temperature gradient α [α = ( _TL −T _in ) / L], 0 _in the region from the reactor inlet to L
When ≦ α ≦ 50 ° C./cm, crushed particles are not generated and uniform spherical fine particles are generated. Therefore, the temperature of the heating element 5 is controlled so as to satisfy the above conditions of T _in and α. More preferably, the temperature gradient is 0 ≦ α ≦ 40 ° C./cm. When the solvent is water and the temperature gradient is less than 0 ° C./cm, the solvent is less likely to evaporate, and the solute thermal decomposition reaction is less likely to occur. If it is higher than 50 ° C./cm, the evaporation of the solvent and the thermal decomposition of the solute, which is a thermal decomposition reaction substance, are
Moreover, since it occurs rapidly, the produced fine particles are not spherical but crushed.

When the solvent is an organic solvent, the temperature of the heating element 5 is controlled so that the reaction furnace inlet temperature is 50 ° C. or higher and 100 ° C. or lower. When the reactor inlet temperature is lower than 50 ° C., the organic solvent in the droplets is difficult to evaporate, and when it exceeds 100 ° C., the evaporation of the solvent and the thermal decomposition of the solute which is the thermal decomposition reaction substance occur simultaneously and rapidly. The produced fine particles are not spherical but crushed. Further, in the region from the reaction tube inlet to L, when 0 ≦ α ≦ 25 ° C./cm, uniform spherical fine particles are formed without forming crushed particles, so that the above conditions of T _in and α are satisfied. The temperature of the heating element 5 is controlled so as to do so. When the solvent is an organic solvent, if the temperature gradient is less than 0 ° C./cm, evaporation of the solvent is less likely to occur, and thermal decomposition reaction of the solute is less likely to occur. On the other hand, if it is higher than 25 ° C./cm, evaporation of the solvent and thermal decomposition of the solute as a thermal decomposition reaction substance occur rapidly at the same time, so that the produced fine particles are not spherical but crushed.

In the temperature distribution in the pipe length direction from the position L to the outlet of the reaction furnace, when the solvent is water, any temperature distribution within the range of 80 ° C. to 2000 ° C. may be used. When is an organic solvent, the above 5
Any temperature distribution may be used as long as it has a temperature distribution within the range of 0 ° C. to 400 ° C., but is preferably higher than T _L in order to improve the crystallinity of the produced fine particles.

In the present invention, in order to accelerate the thermal decomposition reaction of the metal salt, it is desirable to provide a temperature range of 200 ° C. or higher, preferably 300 ° C. or higher in the reaction furnace. As the thermal decomposition reaction, for example, 1) Ti (SO ₄ ) ₂ → TiO ₂ + SO _x 2) CuSO ₄ → CuO + SO _x 3) Zn (NO ₃ ) ₂ → ZnO + NO _x 4) Ca (NO ₃ ) ₂ → CaO + NO _x 5) MgCO ₃ → MgO + CO _{2 and the} like.

[0028]

EXAMPLES Next, the present invention will be described in more detail by way of examples, which should not be construed as limiting. Example 1 Zn (NO ₃ ) ₂ .6H ₂ O (zinc nitrate hexahydrate) and pure water were used to prepare a zinc nitrate aqueous solution adjusted to 0.1 mol / liter, and nitrogen was used as a carrier gas. Fine particles of zinc oxide were prepared by using a gas and an apparatus shown in FIG. The average droplet diameter was 5 μm. The average droplet diameter was measured using a light scattering type particle size distribution measuring device (Particlesizer, manufactured by Nippon Laser Co., Ltd.). Hereinafter, the same method was used in Comparative Example 1 and Example 2. More specifically, the reaction tube has a magnetic tube (inner diameter 35 mm, length 6).
0 cm) and the carrier gas flow rate is 1.0 liter /
It was constant in minutes. The temperature distribution of the reaction furnace is convex as shown in FIG. 2, the reaction furnace inlet temperature is 200 ° C., the temperature at the position 6 cm from the reaction furnace inlet in the pipe length direction is 360 ° C., and the temperature gradient α is 26.7 ° C. / Cm. However, the temperature distribution of the reaction furnace was measured using a temperature measuring device while moving the thermocouple in the tube length direction along the axis of the reaction tube. Hereinafter, also in Comparative Example 1 and Example 2, the temperature distribution in the reaction furnace was measured by the same method.

The zinc oxide fine particles produced under the above conditions had a hexagonal wurtzite type crystal form. The zinc oxide particle size is about 0.97 μm on average (number basis), and the particle size distribution (number basis) is 0.5% or less 10%, 0.5-1.0 μm 50%, 1.0 .About.1.5 .mu.m was 30%, and 1.5 .mu.m or more was 10%. The crystal form of the produced zinc oxide fine particles was measured by an X-ray diffractometer, and the fine particle diameter was measured by a scanning electron microscope. Hereinafter, the same method was used in Comparative Example 1 and Example 2.

Next, a scanning electron micrograph of the produced zinc oxide fine particles is shown in the drawing-substitute photograph of FIG. As is clear from FIG. 3, the produced fine particles had a substantially spherical shape.

[0031] Using Comparative Example _{1 Zn (NO 3) 2 ·} 6H 2 O and pure water was used after adjusting the aqueous solution of zinc nitrate of 0.1 mol / liter, using nitrogen gas in the carrier gas, Zinc oxide fine particles were prepared using the apparatus shown in FIG. The average droplet diameter of the metal salt solution was 5 μm. The carrier gas flow rate was constant at 1.0 liter / min, and the same reaction tube as in Example 1 was used. As shown in FIG. 2, the temperature distribution of the reactor is 400 ° C. at the inlet of the reactor, 620 ° C. at a position of 6 cm in the pipe length direction from the inlet of the reactor, and temperature gradient α is 36.7 ° C./c.
m, and after the position of 14 cm in the pipe length direction from the reactor inlet, the temperature was kept constant at 700 ° C.

The zinc oxide fine particles produced under the above conditions had a hexagonal wurtzite crystal form. The average particle size (number basis) is about 0.90 μm, and the particle size distribution (number basis)
Is less than 0.5 μm is 10%, 0.5 to 1.0 μm is 5
0%, 1.0-1.5 μm is 30%, 1.5 μm or more is 1
It was 0%.

Next, a scanning electron micrograph of the produced zinc oxide fine particles is shown in the drawing-substitute photograph of FIG. As is clear from FIG. 4, crushed fine particles were generated. This is because the reactor inlet temperature is as high as 400 ° C. as compared with Example 1.

Example 2 A mixture of Zn (NO ₃ ) ₂ .6H ₂ O and TiCl ₄ (molar ratio 1: 1) and pure water were used to adjust the solution concentration to 0.1 mol / liter. Then, air was used as the carrier gas, and a mixture of zinc oxide and titanium dioxide was prepared using the apparatus shown in FIG. The average droplet diameter of the metal salt solution was 4 μm. The carrier gas flow rate was constant at 1.0 liter / min, and the same reaction tube as in Example 1 was used. As shown in FIG. 2, the temperature distribution in the reaction furnace has a reactor entrance temperature of 1
The temperature was 00 ° C., the temperature at a position 6 cm from the reactor inlet in the pipe length direction was 230 ° C., the temperature gradient α was 21.7 ° C./cm, and 650 ° C. was constant after the position 30 cm from the reactor inlet in the pipe length direction.

The crystal forms of the mixed fine particles of zinc oxide and titanium dioxide produced under the above conditions were a mixture of hexagonal wurtzite type and anatase type. The average particle size (number basis) is about 0.67μm, and the particle size distribution (number basis) is 0.5μm.
The following is 30%, 0.5-1.0 μm is 50%, 1.0-1.
5 μm was 10%, and 1.5 μm or more was 10%.

Next, a scanning electron micrograph of the produced mixed fine particles is shown in the drawing-substitute photograph of FIG. As is clear from FIG. 5, the produced mixed fine particles had a substantially spherical shape, and the crushed fine particles as in Comparative Example 1 were not present.

[0037]

As described above, according to the present invention, a wide variety of metal salts are used as a starting material to form fine droplets from an aqueous solution thereof, and the droplets are subjected to a high temperature reaction in which the temperature distribution is controlled. By a simple manufacturing process in which a thermal decomposition reaction is performed in a furnace, a wide variety of uniform metal oxide spherical fine particles can be continuously produced at low cost. Further, since the obtained metal oxide fine particles have semiconductivity, in addition to the ultraviolet shielding ability,
It has functions peculiar to metal oxides such as photoconductivity, fluorescence, and piezoelectricity.

[Brief description of drawings]

FIG. 1 is a schematic diagram showing a specific example of an apparatus used in the present invention.

FIG. 2 is a graph showing temperature distributions of a reactor in Example 1, Comparative Example 1 and Example 2.

3 is a drawing-substituting photograph showing the particle shape of the zinc oxide particles of Example 1. FIG.

4 is a drawing-substituting photograph showing the particle shape of zinc oxide particles of Comparative Example 1. FIG.

FIG. 5 is a drawing-substituting photograph showing the particle shape of zinc oxide-titanium dioxide mixed particles of Example 2.

[Explanation of symbols]

1: Liquid tank, 2: Circulation pump for sending liquid, 3: Droplet supply device,
4: Carrier gas supply device, 5: High temperature heating body, 6: Reaction tube, 7: Electrostatic collector, 8: Corona discharge body, 9: Cold trap, 10: Filter, 11: Pump

Claims (3)

[Claims] 1. A droplet size of an aqueous solution containing a metal salt is 0.1 μm.
To 100 μm as fine droplets, and the droplets are sent to a high temperature reactor in a gas-liquid mixed state using a carrier gas, and the temperature T _in at the reactor inlet is 80 to 300 ° C. inside the reactor. A method for producing fine metal oxide particles, which comprises thermally decomposing the metal salt contained in the droplets to produce fine metal oxide particles. 2. The length L _{0 of the} reactor at which the reaction rate is 90% at the reactor outlet, and the distance L (L = L
Assuming that the temperature at 0.1 × L ₀ ) is T _L , the temperature gradient α [α = (T _L −T _in )
/ L] is a temperature distribution condition of 0 to 50 ° C./cm, and the production method according to claim 1, wherein the metal oxide fine particles are generated. 3. The production method according to claim 1, wherein the metal salt is a salt of zinc, titanium or a mixture thereof, and the obtained metal oxide is zinc oxide, titanium dioxide or a mixture or complex thereof. ..