JPS61171533A - Preparation of minute spherical particle - Google Patents

Abstract

PURPOSE:To improve the shape and density of a minute spherical particle, by performing two-stage spray drying treatment for drying a sol, gel or suspension of inorg oxide or hydroxide at a low drying speed before drying the same at a high speed. CONSTITUTION:A sol, gel or suspension of inorg oxide or hydroxide with a viscosity of about 500cp or less, pref., 50cp or less is sprayed by using a gas- liquid two-fluid mixture and slowly dried at 10-100 deg.C at drying speed of 0.02k moisture/sec.kg solid or less, pref., 0.016kg moisture/sec.kg solid to obtain a powder with moisture content of about 12% or more. This powder is contacted with gas at 110-400 deg.C, pref., 150-300 deg.C and dried at a drying speed of 0.04kg moisture/sec.kg solid or more, pref., 0.05kg moisture/sec.kg solid to obtain a spherical particle with an average particle size of 0.5-30mum.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for producing microspherical inorganic oxides. More specifically, when producing microspherical inorganic oxides by spray drying, drying is carried out in two stages under different conditions. It is related to.

Obtaining spherical particles by the eruption drying method has been used for food products and other products since ancient times.
It is used in fields such as pharmaceuticals and catalyst industry, and spherical particles with a wide width of several tens of microns to several hundred microns are obtained.

These drying methods are methods for obtaining spherical powder by bringing sprayed droplets into contact with directly or indirectly heated hot air at 100 to 500°C, and rapidly evaporating the solvent from the droplet surface to dry it. Further, the above-mentioned droplet manufacturing method can be roughly divided into the following two types of methods. One method is to make droplets by passing the supplied liquid through fine holes under high pressure (nozzle type), and the other is to use a high-speed rotating plate to form droplets using centrifugal force (atomizer type, disk type). (Formula), and both of these are droplet production methods in which force is directly applied to the liquid to break it into small pieces.

However, both methods have problems in obtaining microspherical particles of 30 μm or less. The first is a continuous generation method of micro droplets, and the second is the shape of the powder obtained by drying. In order to obtain microspherical particles using a nozzle, it is naturally necessary to significantly increase the spray pressure or make the diameter of the micropores extremely small. Many problems occur, such as wear due to suspended matter and nozzle clogging due to lumps and foreign matter, and long-term stable production cannot be expected. Droplet production by rotation in an atomizer format involves increasing the number of rotations, but there are many problems, such as wear and tear on the rotation transmission mechanism and the elimination of heat generation.

After all, long-term stable operation cannot be expected.

The present inventors solved the first problem, that is, a method for continuously generating microdroplets, by using a two-fluid nozzle atomization method (supplying liquid and high-speed gas into the same nozzle, and using the shear force of the gas to turn the liquid into a mist). It was discovered that the objective could be achieved by adopting a method of finely spraying (an application of the Kirifuki principle). According to this method.

Gas pressure 1-10kg/a#, flow rate from nozzle 3
By operating at QOm/see or higher, it is possible to obtain fine particles of 30μ or less.

The second and bigger problem is that although it is possible to produce particles of 30μ or less using this two-fluid nozzle atomization method,
It is difficult to obtain solid particles with high sphericity.

First of all, the powder or colloid particles in the sprayed micro droplets have fluidity and freedom of movement even inside the droplet, and secondly, the initial water evaporation from the droplet occurs only from the droplet surface. be. Micro droplets have a large specific surface area, with a typical surface area of 100~
Droplets sprayed into a gas flow atmosphere at 500°C undergo rapid evaporation from the surface, and only the surface portion is likely to solidify, resulting in particles that not only lose their sphericity, but also have hollow spots and bulges. In extreme cases, the particles become miscellaneous non-spherical particles, such as cone-shaped particles. Furthermore, since the nozzle is directly exposed to a high temperature range, the nozzle is likely to become clogged and the resulting flow drifts, making stable long-term operation difficult.

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As a result of extensive research into 91, we have developed a technology that uses a two-fluid nozzle atomization method to dry droplets at a slow drying speed as the first step.
It has been found that microspherical particles with satisfactory shape and density can be obtained by drying at a high drying speed in the second step.

The present invention is based on this knowledge.

Hereinafter, the present invention will be explained step by step.

The raw materials used in the present invention include sol, hydrogel, oxide fine powder suspension, oxide xerogel suspension, etc., but for the purpose of obtaining microspherical particles, the viscosity of the sol or suspension is 5.
00 cp or less, preferably 50 cp or less, and raw materials include, for example, colloidal liquids with high concentration and low viscosity such as silica sol, alumina sol, iron sol, titania sol, and zirconia sol, organic silicon, organic titanium, organic aluminum, and organic Compound solutions such as zirconium or hydrolyzed suspensions thereof, silicic acid solutions, low molecular weight hydrolyzed solutions such as vitrogel, etc. can be used, and among these, high concentration and low viscosity sol solutions are most suitable. Furthermore, depending on the purpose, it is also possible to provide a mixture of these raw materials.

A two-fluid nozzle spray method is suitable for the micro-liquid manufacturing equipment used in the present invention. This is a general term for droplet production methods called "atomization," but compared to producing droplets by cutting a single liquid with high pressure or high-speed rotation, this method involves shearing the liquid with high-speed gas and turning it into droplets. This is because it is easy to produce fine droplets. When spraying with Honzuke, the gas pressure is 1 to 10
kg/cJ, preferably 2.5 to 8.0 kg/a#, and the linear velocity of the gas-liquid mixture ejected from the nozzle is 300 m/sec
c or more, preferably 400 to 700 m/see.

The first stage of drying of the present invention consists in drying the minute droplets sprayed by the gas-liquid two-fluid mixing nozzle at a low drying speed.

The drying air flow needs to be parallel or perpendicular to the spray flow.

In the first stage of drying, the drying gas is heated in advance,
The temperature is 10-100°C, preferably 30-60°C, and the flow rate ratio with the atomizing gas supplied from the nozzle is 11,000-500°C. Preferably s, ooo~900
It is necessary that Drying gas is LNG, LPG
, gas obtained by burning oily fuel may be used as is, or air that has been indirectly heated in advance by combustion or electricity) may be used.

The purpose of the present invention is to obtain microspherical particles that are neither hollow nor irregularly shaped, and for this purpose it is necessary to perform the first step of drying slowly. According to the present invention, the drying rate in the low temperature drying region is 0.02 kg moisture/sec, kg solids or less, preferably 0.
．． It must be less than 0.016 kg water/sec, kg solid content.

For convenience, this drying rate is expressed by the following formula.

When the diameter of the obtained powder is 30 μm or more, a powder with a very high degree of hollowness and a large number of hollow holes can be obtained even under these conditions. Also,
When the drying rate is higher than that of the present invention, not only a powder with many hollow holes and navels is obtained, but also light non-spherical particles in the shape of lobes and pieces are obtained.

The powder thus obtained after the first stage drying has a moisture content of 12% or more, indicating that it has not been exposed to an excessive drying atmosphere.

In the first stage of drying of the present invention, the drying gas may be humidified in advance to adjust its humidity. Further, it is preferable to recycle a part of the gas after powder has been removed from the gas that has already been used.

The second stage of drying of the present invention consists in drying the powder obtained in the first stage of drying at high speed.

The powder obtained in the first stage of drying contains a large amount of water and needs to be dehydrated as soon as possible. Otherwise, problems such as agglomerated powder due to contact between powders, wetting caused by droplets due to solidification of evaporated water, non-spherical shape and scaling due to collision with pipe walls and drying chamber walls may occur. Become.

According to the present invention, the obtained in the first stage drying
1 powder at 110-400℃, preferably 150-3
By contacting with gas at 00°C and setting the drying rate to 0.04 kg moisture/sec, kg solid content or more, preferably 0.05 kg moisture/sec 0 kg solid content, agglomerated particles and irregularly shaped particles due to collision are removed. This enabled us to obtain a product that was almost never available, and to enable continuous production.

The powder obtained in the second stage of drying can be collected by any method such as wet or dry collection, but a method using a dry bag filter is the simplest and preferred method. Furthermore, the obtained microsphere powder is subjected to a process such as classification or firing as required, and then used for the intended purpose.

The microspherical particles obtained by the present invention have the following properties.

Sphericity: 0.85 or more Bulk density: 0.7 to 1.5 g/al Size range: 0.5 to 30 μ Next, examples of the present invention will be shown together with comparative examples.

Examples and Comparative Examples Using commercially available silica sol, titania sol, and alumina sol as raw materials, dried fine powder products were obtained using a two-fluid nozzle jetting method. Samples A, B, and C are examples of the present invention using a method that involves two drying stages: a low drying speed region and a high drying degree region.

For sample E, a direct spray method was used in the high drying speed range.

Table 1 shows the manufacturing conditions for each sample.

The properties of the powder obtained by firing the spray-dried powder at 500°C for 1 hour were as shown in Table 2.

Note that the sphericity mentioned in the table below refers to the sphericity measured using a scanning electron microscope (SEM).
) to take an electron micrograph with 2,000 times magnification and dispersed single particles so that they do not overlap, and analyze this image with a Shimadzu image analyzer to calculate the area of the projected surface of each single particle. Measure the circumference, and let HD be the equivalent diameter obtained from the area assuming it is a perfect circle, and let Hd be the equivalent diameter obtained from the circumference assuming it is a perfect circle. It is.

A ball having a sphericity value of 0.850 to 1.00 was defined as a true sphere. Among the sampled particles, those in which 90% or more of the particles were true spheres were named true spherical fine particles.

In addition, particles that are observed to have small adhesion or depression on the surface are not considered to be true spheres.

Table 2 (Note) Figures 1, 2, and 3 are examples A and B of the present invention.
, is a microscopic photograph of material C, showing that it is a spherical object with uniform particles. On the other hand, FIGS. 4 and 5 are micrographs of materials according to comparative examples, and both show that the grain shape is not spherical or irregular.

[Brief explanation of drawings]

1 to 5 are microscopic photographs of particles obtained by the method of the present invention and particles obtained by the method of the comparative example. Elephant 1 Figure I... 311゛ Figure 2 b゛, :? Figure 4

Claims (1)

[Claims] 1. When spray drying a sol, gel, or suspension of an inorganic oxide or hydroxide to obtain spherical particles with an average particle size of 0.5 to 30 μm, drying is performed at a low drying rate. A method for producing microspherical inorganic oxide particles, comprising two drying steps: drying at a high drying rate, followed by drying at a high drying rate. 2. A patent in which the drying speed in the low drying speed range is 0.02 kg moisture/sec, kg solids or less, and the drying rate in the high drying speed range is 0.04 kg moisture/sec, kg solids or more The method according to claim 1.