Chemistry of inorganic compounds – Silicon or compound thereof – Oxygen containing
Reexamination Certificate
2000-03-16
2001-07-31
Bos, Steven (Department: 1754)
Chemistry of inorganic compounds
Silicon or compound thereof
Oxygen containing
Reexamination Certificate
active
06267942
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Technical Field
The present invention relates to a process for producing spherical silica particles, particularly a process for producing solid spherical silica particles having no depression or crack on the surface.
2. Background Art
Spherical silica particles are widely used for a catalyst, a catalyst support, a pigment for cosmetics, a column packing for chromatography, a resin filler, an adsorbent or a drying agent, from the viewpoint of varieties in the particle size, the pore structure and the surface physical properties. For such applications, in view of e.g. particle strength or packing density, solid spherical silica particles having no depression or crack on the surface are preferred.
As a process for producing the spherical silica particles, a method has been known wherein an aqueous sodium silicate solution or an alkyl silicate is emulsified in a solvent having no miscibility therewith, followed by gelation with e.g. an acid, an alkali or water (JP-A-4-154605). Further, a method has been known wherein an aqueous sodium silicate solution or an alkyl silicate is gelated with e.g. an acid, an alkali or water, followed by emulsification in a solvent having no miscibility therewith, for granulation (JP-B-4-2525). By such so-called emulsification methods, solid spherical silica particles having no depression can relatively easily be obtained.
Further, as a method for making the shape spherical, a spray method has been widely known. JP-A-61-168520 discloses a method of spray-drying a silica sol. JP-A-4-68247 discloses a method of spray-drying an alkali silicate. By such methods, spherical silica particles having a pore volume of at most about 0.5 cm
3
/g can be obtained.
Further, JP-B-6-99135 discloses a method of carrying out wet grinding of a hydrogel, followed by spray drying. By the spray-drying method, hollow particles, depressions and cracks are likely to result, and to overcome these, JP-B-2-61407 proposes a two-stage drying method at a low drying rate and successively at a high drying rate. JP-B-2-61406 and JP-B-5-3412 disclose a spray-drying method by the wet air.
DISCLOSURE OF THE INVENTION
The present invention provides a process for producing spherical silica particles, which comprises dispersing silica gel particles having an average particle size of from 0.05 to 3.0 &mgr;m in a mixed solution of an alkali silicate and an acid, spraying the dispersion to obtain droplets, and heating the droplets in a gas and gelating the mixed solution portion of an alkali silicate and an acid in the droplets.
BEST MODE FOR CARRYING OUT THE INVENTION
The silica gel particles to be dispersed in a mixed solution of an alkali silicate and an acid are required to have an average particle size of from 0.05 to 3.0 &mgr;m. In a case where the average particle size of the silica gel particles is smaller than 0.05 &mgr;m, mechanical strength of the spherical silica particles to be obtained will be low, and irregular particles are likely to form, such being unsuitable. Similarly, in a case where the average particle size of the silica gel particles is larger than 3.0 &mgr;m, mechanical strength of the spherical silica particles to be obtained will be low, and irregular particles are likely to form, such being unsuitable. The more preferred range of the average particle size of the silica gel particles is from 0.1 to 1.0 &mgr;m.
In the production process of the present invention, with respect to the mixed solution of an alkali silicate and an acid, the alkali silicate will hydrolyze by heating, and polymerization of silicic acid as a hydrolysis product will proceed, for gelation. With respect to the spherical silica particles to be obtained by the production process of the present invention, the gel produced from the mixed solution of an alkali silicate and an acid, will be the matrix which bonds the silica gel particles in the dispersion. In the present specification, the dispersion having the silica gel particles dispersed in the mixed solution of an alkali silicate and an acid will be referred to as a composite dispersion.
In a case of obtaining droplets of an dispersion having the silica gel particles alone followed by drying, without using the mixed solution of an alkali silicate and an acid, or in a case of using a silica sol instead of the mixed solution of an alkali silicate and an acid, mixing the silica gel particles thereto to obtain droplets followed by drying, spherical silica gel particles can be obtained. By such methods, gelation is caused mainly by evaporation of the solvent, and accordingly, along with evaporation of the solvent, the silica gel particles in the dispersion will move towards the surface of the droplets, whereby the spherical silica particles are likely to have hollows, and depressions and cracks are likely to form, unless the conditions are well controlled.
On the other hand, in the present invention, gelation takes place by hydrolysis of the alkali silicate and polymerization of silicic acid. Although the solvent may evaporate in this stage, as the chemical reaction is accelerated mainly by heating for gelation, the volume will not significantly change, the spherical silica particles are less likely to have hollows, and formation of depressions and cracks will be suppressed. Accordingly, it is preferred to employ such a condition that the gelation rate by heating is relatively higher than the evaporation rate of the solvent. Namely, it is important to adjust the relative rate of the gelation rate by heating to the evaporation rate of the solvent. The evaporation rate of the solvent from the droplets will be high when the heating temperature becomes high, and will be low when the solvent vapor pressure surrounding the droplets becomes high.
As the temperature for heating the droplets of the composite dispersion in a gas, the temperature of the gas which is in contact with the spherical particles when the gelation has substantially completed, is preferably from 60 to 200° C. The time when the gelation has substantially completed, is the time when fluidity of the mixed solution of an alkali silicate and an acid in the composite dispersion has disappeared. The gas temperature lower than 60° C. is unfavorable since gelation by heating may be inadequate, and the strength of the spherical particles to be produced may be weak. The gas temperature higher than 200° C. is unfavorable from reasons such that utilization efficiency of energy will decrease.
In the production process of the present invention, if the solvent rapidly evaporates during the gelation of the droplets, there is a fear that the shape of the silica particles to be obtained may not be spherical. Accordingly, in a case where the solvent is water, it is preferred to increase the water vapor pressure in the gas during heating in the gas. The relative humidity of the gas which is in contact with the spherical particles is preferably at least 20% when the gelation has substantially completed. The relative humidity at this point is more preferably at least 30%. From the viewpoint of physical properties of the spherical silica particles, there is no upper limit of the relative humidity. However, if the humidity is high, there is a possibility of problems arising, such as undesirable dropwise condensation in a production apparatus. Accordingly, the relative humidity of the gas which is in contact with the spherical particles when the gelation has substantially completed, is preferably at most 80%.
Further, in order to control evaporation of the solvent from the droplets of the composite dispersion, a substance which decrease the vapor pressure of the solvent, may be contained in the dispersion. In the case where the solvent is water, it is preferred to increase the concentration of coexisting salts of the water, or to mix an organic substance having a boiling point lower than the water. In such cases, by changing the concentration of coexisting salts or the addition amount of the organic substance having a boiling point lower than the water, evaporation rate of moisture f
Hirano Hachiro
Kusaka Makoto
Mori Hiroo
Asahi Glass Company Limited
Bos Steven
Oblon & Spivak, McClelland, Maier & Neustadt P.C.
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