Chemistry of inorganic compounds – Silicon or compound thereof – Oxygen containing
Reexamination Certificate
2000-02-14
2001-12-25
Hendrickson, Stuart L. (Department: 1754)
Chemistry of inorganic compounds
Silicon or compound thereof
Oxygen containing
C423S335000
Reexamination Certificate
active
06333013
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a method for producing a high purity ultra fine silica powder.
2. Discussion of Background
Heretofore, an ultra fine silica powder having a large specific surface area has been packed for e.g. rubbers and resins for reinforcement thereof. The ultra fine silica powder may, for example, be silica obtained by flame thermal decomposition of silicon tetrachloride (fumed silica) or precipitated silica made of sodium silicate solution (precipitated silica), and it has an extremely large specific surface area of a level of from 50 to 500 m
2
/g.
However, the fumed silica is high-priced since an expensive silicon tetrachloride gas is employed as a raw material, and the precipitated silica has a low purity of a level of 95%, and its use is limited, although it is available at a relatively low cost. Further, these methods are not always adequate as a mass production process, and it has thereby been desired to develop a mass production process to produce a high purity ultra fine silica powder at a low cost.
Accordingly, as a production method by using a silica raw material of low price, a method of spraying a slurry of a silica raw material and an inflammable liquid into a flame (JP-A-10-297915) or a method of quenching vapor formed by heating a silica raw material at a temperature of at least the boiling point of the silica (JP-A-2-233515) may, for example, be proposed.
However, the technique as disclosed in JP-A-10-297915 is basically a melting process by a high temperature flame, and the average particle size of the obtained silica particles is a level of several &mgr;m, and does not reach the ultra fine powder level of submicron. On the other hand, since the method as disclosed in JP-A-2-233515, comprises a vapor phase reaction, ultra fine powder can be obtained. However, heat treatment at a temperature of at least the boiling point of the silica (2230° C.) is required, and accordingly, the product will be contaminated by impurities in the furnace material and will have a low purity, or it may be necessary to employ an extremely expensive furnace material improper for an industrial process.
SUMMARY OF THE INVENTION
The present invention has been made under these circumstances, and it is an object of the present invention to provide a method for producing a high purity ultra fine silica powder, by heat treatment at a relatively low temperature by using a silica raw material for mass production.
Namely, the present invention provides a method for producing an ultra fine silica powder, which comprises subjecting a raw material mixture containing a silica powder, a reducing agent comprising a metal silicon powder and/or a carbon powder, and water, to heat treatment at a high temperature in a reducing atmosphere with an oxygen concentration of less than 1 wt % to generate a SiO-containing gas, immediately cooling said gas in an atmosphere containing oxygen, and collecting fine particles, wherein the raw material mixture is an aqueous slurry having a solid content concentration of from 20 to 60 wt %, and the heat treatment is carried out in a flame, and further, the cooling is a forced cooling by supplying a gas containing oxygen.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
Now, the present invention will be explained in further detail.
The type of the silica powder to be used in the present invention is not particularly limited, and a silica powder obtained by grinding silica stone is preferred in view of cost. The grain size is preferably from submicron to 100 &mgr;m, particularly preferably from 1 to 30 &mgr;m, since the production method of the present invention comprises a SiO gas generating reaction. Beyond this range, it is difficult to gasify coarse particles into SiO at a heat treatment temperature of the present invention. Further, with respect to fine particles, not only handling efficiency will deteriorate, but also the particles will aggregate to similarly impair SiO gasification. The purity is preferably as high as possible.
The present invention is characterized by using a raw material mixture containing both reducing agent comprising a metal silicon powder and/or a carbon powder, and water, blended with a silica powder. The SiO gasification may be inadequate by using only one of the reducing agent and the water, and no ultra fine silica powder may be produced. Here, the ultra fine powder is defined as a powder consisting mainly of submicron or smaller particles, having a specific surface area of at least 35 m
2
/g, preferably at least 50 m
2
/g, and having a content of coarse particles having sizes of at least 1 &mgr;m of not more than 5 wt %, preferably not more than 2 wt %.
It is not clean why a surprisingly ultra fine powder can be obtained even by heat treatment at a temperature of at most the boiling point of the silica, by employing both reducing agent and water as in the present invention. However, it is considered that due to the synergistic effect of the reducing agent and the water, the bonds of Si atoms and O atoms on the surface of the silica particles are weakened by water vapor, and then the reducing agent reacts therewith, whereby the gasification into SiO is significantly accelerated.
Further, the presence of the water not only increases the specific surface area, but also suppresses the reducing agent remaining.
The reducing agent to be used in the present invention comprises a metal silicon powder and/or a carbon powder. The higher the purity, the better, and a metal silicon is preferred in view of acceleration of SiO gasification due to reaction heat. Although the amount of the reducing agent depends upon the reaction temperature and may not be limited, approximately it is preferably from 0.25 to 4 mol, particularly preferably from 0.7 to 1.5 mol, per mol of the SiO
2
content in the silica powder.
The amount of the water is not particularly limited so long as the water content is at least 5 wt % in the mixture with the raw material silica powder and the reducing agent, although it should not be too high. Further, the water may be substituted by an alcohol such as ethanol in an amount of at most a level of 30 wt %.
The raw material mixture to be used in the present invention may be in the form of a slurry or a powder. In the case of a slurry, it will be easy to inject the droplets into a flame from a nozzle, whereby the productivity will further be increased. In such a case, as the slurry concentration, the solid content concentration is preferably from about 20 to about 60 wt %. If it is less than 20 wt %, the productivity will be low, and the amount of the heat of vaporization of the water will be large, whereby the gasification into SiO will be inhibited. Further, if it exceeds 60 wt %, it will be difficult to inject the droplets into a flame, and further, the gasification into SiO will be inhibited also. As a method of injecting the slurry, preferred is an air atomizing nozzle which can reduce the droplet size as much as possible, and particularly preferred is one having a structure which can reduce the droplet size to a level of several &mgr;m.
In the present invention, the raw material mixture is subjected to heat treatment at a high temperature in a reducing atmosphere with an oxygen concentration of less than 1 wt %, to generate a SiO-containing gas. The temperature for the heat treatment is preferably at least 1700° C., particularly preferably from 1800 to 2100° C. If the heat treatment temperature is significantly low, the gasification reaction into SiO will be inadequate. Although the upper limit of the heat treatment temperature is not particularly limited, if it exceeds the boiling point of the silica (2230° C.), the above-described inconvenience may be caused, and accordingly it is preferably at most 2230° C.
The site of high temperature in the heat treatment in the present invention may be obtained in e.g. an electric furnace or a combustion furnace by a flame, and a combustion furnace is preferred in view of e.g. mass productivity, easiness in adju
Kobayashi Akira
Nagasaka Hideaki
Yoshida Akio
Denki Kagaku Kogyo Kabushiki Kaisha
Hendrickson Stuart L.
Oblon & Spivak, McClelland, Maier & Neustadt P.C.
LandOfFree
Method for producing an ultra fine silica powder does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Method for producing an ultra fine silica powder, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Method for producing an ultra fine silica powder will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-2598731