JP3167756B2 - Hollow fine particles of zinc oxide, method for producing the same and apparatus therefor - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION The present invention relates to zinc oxide hollow fine particles,
The present invention relates to a method and an apparatus for manufacturing the same.

[0002]

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

[0003] Therefore, zinc oxide has an extremely high industrial value, and it is important to make it porous and finely divided in order to maximize its function. In other words, by making porous and fine particles, the specific surface area is extremely increased, and the ratio of the number of molecules located on the surface of the fine particles to the total number of molecules constituting the fine particles is increased, so that the surface energy of the fine particles is increased. , Its function is extremely greatly expressed. In addition, by hollowing out the fine structure of the zinc oxide fine particles, the conventional function can be more greatly expressed or a new function can be expressed by utilizing the hollow portion. Furthermore, since the functions of the single crystal particles are gathered by making the zinc oxide fine particles into the aggregate of the primary particles, the functions are amplified.

As described above, methods for producing zinc 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, there is a method of hydrolyzing zinc alkoxide or zinc alkoxide to obtain ultrafine zinc oxide particles (JP-A-2-59425). In general, there has been a method for obtaining zinc oxide fine particles by adding an acid / alkali solution to a zinc salt and causing a reaction in a liquid phase.

In the gas phase method, generally, metal zinc is vaporized,
There is a method of mixing the vapor with a gas containing oxygen to obtain zinc oxide fine particles (Japanese Patent Application Laid-Open Nos. 1-286919 and 2-208369).

[0007] In addition to the liquid phase method and the gas phase method, there is a spray pyrolysis method in which an aqueous solution or an organic solvent solution containing an inorganic acid salt or an organic acid salt of a metal is atomized, and the atomized liquid particles are formed. This is a method in which the particles are transported to a heating furnace to obtain two or more kinds of metal oxide-based composite fine particles by a thermal decomposition reaction (for example, in the production of an oxide-based superconductor, JP-A-2-19623).

[0008] The structure of the zinc oxide fine particles obtained by the above-mentioned conventional liquid phase method is a single crystal, and a porous or hollow shape cannot be obtained. The production process using these liquid-phase methods is difficult to automate because it is based on a batch process, and the produced fine particles are obtained in a solid-liquid mixed phase. A process is required. Therefore, the entire manufacturing process becomes complicated, and it is difficult to reduce the cost.

Further, in the above-mentioned vapor phase method, similarly to the liquid phase method, porous or hollow zinc oxide fine particles cannot be obtained. Generally, in the gas phase method, it is necessary to use high-purity metallic zinc as a raw material in order to increase the purity of a product, but the cost increases accordingly. Further, in order to vaporize metallic zinc, it is necessary to raise the temperature considerably, and it is necessary to select a material that can maintain the high temperature even in the design of the apparatus, which is not preferable in terms of safety of the manufacturing process. .

In the above-mentioned spray pyrolysis method, a method for producing two or more kinds of metal oxide composite fine particles is disclosed, but no description is made on the production of porous or hollow zinc oxide fine particles. As a production method, in order to collect the oxide-based fine particles obtained in a gas-solid mixed phase, the particles of the atomized raw material liquid are charged by means of irradiation or the like, and then transported to a heating furnace to carry out a thermal decomposition reaction. To form oxide-based charged fine particles, and deposit the charged fine particles on a predetermined heated substrate. However, in that method, since the droplets enter the charging device, the charge generation unit becomes wet with the operation time, and the charging device malfunctions, or the base is placed in the heating furnace. The size of the base is limited, and the apparatus system becomes complicated. Therefore,
Continuous operation is difficult and unsuitable for mass production.

[0011]

DISCLOSURE OF THE INVENTION The present inventors have conducted intensive studies to solve these problems in the production of hollow zinc oxide particles. As a result, a simple manufacturing process of forming minute droplets from an aqueous solution using zinc nitrate as a starting material, thermally decomposing the droplets in a high-temperature reactor, and collecting the generated fine particles by an electrostatic collector. When used,
The inventors have found that the object can be achieved, and have completed the present invention.

[0012]

That is, the present invention is a fine particle comprising an aggregate of primary particles of zinc oxide, wherein the fine zinc oxide particles are hollow spheres;

A zinc nitrate aqueous solution having a droplet diameter of 0.1 μm to 1
The droplets are sent into a high-temperature reactor in a gas-liquid mixed phase using a carrier gas of nitrogen or air, and the zinc nitrate contained in the droplets is thermally decomposed inside the reactor. A method for producing zinc oxide hollow microparticles, wherein the method produces zinc oxide hollow microparticles;

A continuous supply apparatus for an aqueous zinc nitrate solution, a liquid droplet supply apparatus for forming the liquid droplets into fine droplets and mixing the liquid droplets with a carrier gas of nitrogen or air, and a heat source for supplying zinc oxide contained in the supplied liquid droplets A high-temperature reactor for performing a decomposition reaction, and a zinc oxide hollow fine particle, which is provided outside the reactor and comprises an electrostatic collector that collects zinc oxide hollow fine particles obtained by the reaction. A manufacturing apparatus is provided.

Hereinafter, the present invention will be described in detail with reference to the accompanying drawings. FIG. 1 is a schematic diagram showing one specific example of the apparatus of the present invention. In the method of the present invention, an aqueous solution of zinc nitrate in a liquid tank 1 is continuously supplied to a droplet supply device 3 for generating fine droplets by using a circulating pump 2 for liquid transport, and the generated droplets are transferred to a carrier. The carrier gas sent from the gas supply device 4 is sent to a reaction tube 6 having a high-temperature heating body 5 accompanying the carrier gas, and a thermal decomposition reaction of droplets is performed in the reaction tube 6 to form a zinc oxide hollow in a gas-solid mixed phase state. Fine particles are generated, and the fine particles are electrostatically deposited on a collecting plate in an electrostatic collector 7 having a corona discharger 8. The gas containing water vapor that exits the electrostatic trap 7 is passed through the cold trap 9 and the filter 10 to remove moisture, and is forcibly exhausted by the pump 11. The gas flow rate is monitored by the gas flow meter 12. The temperature of the reaction tube 6 and the heat retaining section 14 after leaving the reaction tube are controlled by the temperature controller 13.

The concentration of the aqueous zinc nitrate solution used in the present invention is 1
0 in the range of ⁵ mol / l of 20 mol / l, preferably, be in the range of 10 ^-4 mol / l to 10 mol / liter. When the solution concentration is lower than 10 ^-5 mol / l, the amount of generated zinc oxide fine particles becomes extremely small. When the solution concentration is higher than 20 mol / l, the solution viscosity is excessively increased and fine droplets are formed. It will be difficult.

Nitrogen or air is used as the carrier gas, and the flow rate of the carrier gas must be adjusted so that the residence time of the droplet containing zinc nitrate in the reaction tube 6 does not become shorter than 1 second. Must.

As a method for forming a fine droplet of an aqueous solution of zinc nitrate, there are a method using ultrasonic vibration, a method using a spray nozzle, and the like. A method using sound wave vibration is preferable.

The droplet diameter is in the range of 0.1 μm to 100 μm, and the droplet diameter distribution is preferably as narrow as possible. If the droplet diameter is smaller than 0.1 μm, the diameter of the generated fine particles is at most about 0.01 μm, and the fine particles in that region are called ultrafine particles, and the amount of deposition on the inner wall of the reaction tube is extremely large due to large Brownian diffusion. As a result, the yield of the produced fine particles deteriorates. When the diameter of the droplet is larger than 100 μm, the diameter of the generated fine particles is about several tens μm even if it is small, making it difficult to atomize the particles. The droplet diameter is preferably measured in a gas-liquid mixed phase state, for example, by a light scattering type particle size distribution analyzer.

The zinc oxide hollow fine particles obtained by the present invention have good monodispersibility, and can be adjusted to a concentration of 0.1 by adjusting the concentration of the spray solution.
A range of 001 μm to several tens μm can be obtained. However, in consideration of the yield of the produced fine particles and improvement of the function due to atomization, the range of 0.01 μm to 1 μm is preferable. The zinc oxide fine particle diameter can be measured by various methods, for example, by a scanning electron microscope.

In the reaction furnace, the temperature of the heating element 5 must be controlled in the reaction tube 6 so that the isothermal portion is kept as wide as possible in the tube axis direction and the radial direction. The temperature of the reactor is preferably between 200 ° C and 200 ° C.
A range of 0 ° C. is good. If the temperature is lower than 200 ° C., thermal decomposition of zinc nitrate hardly proceeds, and if the temperature is higher than 2000 ° C., zinc oxide fine particles are not melted to form hollow fine particles, which is not preferable. Further, the thermal decomposition reaction in the present invention is Zn (NO ₃ ) ₂ → ZnO + NOx.

There are a filter type and an electrostatic collection type in the fine particle collector. However, in the filter type, when used for a long time, clogging occurs and the balance between the inflow and outflow of gas in the present system is lost. Therefore, the device may not operate normally for a long time. However, in the electrostatic collection type,
The fine particles are charged by the electric charge generated from the corona discharger 8, the fine particles are collected on the collecting plate by the electrostatic attraction between the grounded collecting plate and the charged fine particles, and the gas is supplied to a hole provided in the collecting plate. Through the system. In the apparatus of the present invention, since the electrostatic collector is provided outside the reaction furnace, the area of the collecting plate can be increased. Therefore, it can sufficiently cope with long-term use. Further, by providing a plurality of electrostatic collectors and switching between them, a further continuous long-time operation is possible. A method of installing an electrostatic collector in the reaction furnace is also conceivable, but in that case, the area of the collecting plate cannot be increased or a plurality of collectors cannot be provided as described above. In the apparatus of the present invention, since the electrostatic collector is provided outside the reaction tube, the collection efficiency of the fine particles is dramatically improved.

[0023]

EXAMPLES Next, the present invention will be described in more detail with reference to Examples, but it should not be construed that the invention is limited thereto.

Example 1 An aqueous solution of zinc nitrate was adjusted to 10 ^-3 mol / l using Zn (NO ₃ ) ₂ .6H ₂ O (zinc nitrate hexahydrate) and pure water to prepare a carrier. Nitrogen gas was used as the gas, and zinc oxide fine particles were prepared according to the above method.
The average droplet diameter was 5 μm. The average droplet diameter was measured using a light scattering particle size distribution analyzer (Particle Sizer, manufactured by Nippon Laser Co., Ltd.). The measurement was performed in the same manner in Example 2 described later. The reaction tube is made of ceramic having an inner diameter of 30 mm and a length of 1000 mm, and is installed in a vertical electric heating furnace (heating length: 600 mm). The carrier gas flow rate was constant at 1 liter / min, and the reactor temperature was constant at 900 ° C.

The crystal form of the hollow zinc oxide fine particles produced under the above conditions was a hexagonal wurtzite type. The average particle diameter (number basis) of the zinc oxide particles is about 0.32 μm, and the particle diameter distribution (number basis) is 0.2 μm or less, 10%, 0.2 to 0.2 μm.
0.3 μm is 40%, 0.3 to 0.4 μm is 40%, 0.4
It was 20% for μm or more. Next, a transmission electron micrograph of a cross section of the particles showing the particle structure of the zinc oxide fine particles is shown in FIG. As apparent from this figure, the produced fine particles are porous hollow spherical fine particles composed of aggregates of primary particles of zinc oxide, and the size of the primary particles is about 20 to 40 nm. The crystal form of the generated zinc oxide fine particles was measured with an X-ray diffractometer, and the fine particle diameter was measured with a scanning electron microscope. In Example 2 described later, the measurement was performed in the same manner.

Example 2 Zinc oxide was prepared in the same manner as in Example 1 by using an aqueous solution of zinc nitrate adjusted to 10 ⁻² mol / L using Zn (NO ₃ ) ₂ .6H ₂ O and pure water. Hollow fine particles were produced. The average droplet diameter of the aqueous zinc nitrate solution was 5 μm. The carrier gas flow rate was constant at 1 liter / min, the reactor temperature was constant at 900 ° C., and the same reaction tube as in Example 1 was used.

The crystal form of the zinc oxide hollow fine particles produced under the above conditions was a hexagonal wurtzite type. The average particle diameter (number basis) is about 0.45 μm, and the particle diameter distribution (number basis) is 0.3 μm or less, 10%, 0.3 to 0.4 μm.
Was 30%, 0.4 to 0.5 μm was 50%, and 0.5% or more was 10%.

Next, a transmission electron micrograph showing the particle structure of the zinc oxide fine particles is shown in FIG. As is apparent from this figure, the produced fine particles are porous hollow spherical fine particles composed of aggregates of primary particles of zinc oxide. In preparing a sample for a microscope, zinc oxide fine particles are dispersed in a resin, and an ultra-thin section of the resin is cut out. is there.

[0029]

As described above, according to the present invention, microdroplets are formed from an aqueous solution of zinc nitrate as a starting material,
The droplets are subjected to a thermal decomposition reaction in a high-temperature reactor, and the produced fine particles are collected by an electrostatic collector. Can be produced continuously, and products can be obtained at low cost.

[Brief description of the drawings]

FIG. 1 is a schematic view showing a specific example of the device of the present invention.

FIG. 2 is a drawing substitute photograph of a particle cross section showing the particle structure of the zinc oxide fine particles of Example 1.

FIG. 3 is a drawing substitute photograph of a particle cross section showing the particle structure of the zinc oxide fine particles of Example 2.

[Explanation of symbols]

Reference Signs List 1; liquid tank 2: liquid circulation pump 3; droplet supply device
4; Carrier gas supply device 5; High temperature heater 6; Reaction tube 7; Electrostatic collector 8; Corona discharger 9; Cold trap 10; Filter 11; Pump 12; Gas flow meter 13; Temperature controller 14; Department

Continued on the front page (72) Noboru Toge Inventor 547-12 Koizumi-cho, Yamatokoriyama-shi, Nara Prefecture (58) Field surveyed (Int. Cl. ⁷ , DB name) C01G 9/03

Claims (3)

(57) [Claims] 1. Fine particles of hollow zinc oxide, wherein the fine particles are composed of aggregates of primary particles of zinc oxide and have a hollow spherical shape. 2. An aqueous zinc nitrate solution is formed into fine droplets having a droplet diameter of 0.1 μm to 100 μm, and the droplets are fed into a high-temperature reactor in a gas-liquid mixed phase using a carrier gas of nitrogen or air. A method for producing zinc oxide hollow fine particles by thermally decomposing zinc nitrate contained in droplets inside the reaction furnace. 3. A continuous supply device for an aqueous solution of zinc nitrate, a droplet supply device for making the solution into fine droplets and mixing the droplets with a carrier gas of nitrogen or air, and zinc nitrate contained in the supplied droplets. A high-temperature reactor for performing a thermal decomposition reaction of
An apparatus for producing hollow zinc oxide particles, comprising: an electrostatic collector provided outside the reactor for collecting hollow zinc oxide particles obtained by the reaction.