JP2899080B2 - Method for producing anhydrous silica - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION (Industrial application field) The present invention removes silanol groups contained in synthetic silica produced by a sol-gel method or the like, has a very small water content, and has excellent heat resistance. The present invention relates to a method for producing anhydrous silica suitable for producing anhydrous silica.

(Prior Art) Natural quartz has been used as a raw material for quartz glass. However, in recent years, this quartz glass has been used as a material for electronic parts and the like. Because of the necessity of obtaining glass, the use of high-purity synthetic silica as a raw material has been studied (JP-A-51-77612, JP-A-61-186232).

However, generally synthetically produced silica has pores of several to several tens of millimeters depending on its production conditions,
Heating causes dehydration, and further firing at 1100 ° C. to 1300 ° C. for several hours results in sealing, and during the firing process up to the sealing, adsorbed water and condensed water are released, and dehydration is caused by a condensation reaction of silanol groups. Occur. However, the silanol groups isolated inside the silica cannot cause a condensation reaction in the firing process, and are sealed while remaining inside as moisture. When silica having the silanol group remaining as such is used as a raw material of quartz glass, there arises a problem that quartz glass having excellent heat resistance cannot be obtained.

Therefore, as a method of removing the silanol group, a method of firing silica gel in a mixed gas stream of a halogen-based deOH group and a carrier gas and using a reaction of replacing the silanol group with a halogen atom has been proposed. It is known (Japanese Patent Application Laid-Open No. 61-186232) that, even in this method, a halogen group remains in place of the silanol group. Problems such as corrosiveness occur.

(Problems to be Solved by the Invention) Therefore, an object of the present invention is to provide a method for removing as much silanol groups in silica as possible to obtain anhydrous silica having a low water content suitable as a raw material for producing quartz glass. It is here.

(Means for Solving the Problems) As a result of intensive studies conducted by the present inventors to solve the above problems, it has been found that water remaining in silica particles can be removed by firing silica in a dry airflow. And completed the present invention.

That is, the present invention relates to a silica having a silanol group obtained by hydrolyzing an alkoxysilane,
This is a method for producing anhydrous silica, characterized by firing at a temperature of 1100 ° C. or more in a dry gas atmosphere of 30 ppm or less and a flow rate of 3 l / hr or more per 1 kg of silica.

In the present invention, a silanol group in silica is regarded as water contained in silica.

In the present invention, the silica to be calcined is not particularly limited as long as it has a silanol group, but in particular, in order to obtain high-purity silica, the alkoxysilane is mixed with water, a catalyst, a solvent, and the like added as necessary. It is good to produce by hydrolysis in a solution. Here, the alkoxysilanes that can be used to produce such high-purity silica are not particularly limited, but are preferably those having an alkoxy group having 1 to 4 carbon atoms, such as tetramethoxysilane and tetramethoxysilane. Ethoxysilane, tetrapropoxysilane, tetrabutoxysilane and the like can be mentioned, and particularly, tetramethoxysilane and tetraethoxysilane having a high hydrolysis reaction rate are preferable.

Further, as a catalyst that can be used in this method, from the viewpoint of easy removal from silica after the reaction, an inorganic acid such as hydrochloric acid or nitric acid, an organic acid such as acetic acid, or an inorganic base such as ammonia or amines may be mentioned. Can be. Examples of the solvent that can be used in this method include alcohols such as methanol and ethanol.

The type of the dry gas that can be used in the present invention is not particularly limited as long as the gas has a water content of 30 ppm or less, such as dry air, pure air, and pure hydrogen. Also,
The flow rate of the drying gas is preferably 3 l / hr or more per kg of silica to be calcined. If the moisture content of the drying gas exceeds 30 ppm, or the flow rate of the drying gas is 3 l /
If it is less than hr, the silanol groups cannot be sufficiently reduced as shown in Comparative Examples.

In the present invention, it is preferable to perform preliminary firing before firing in order to reduce the water content in the silica to some extent. The pre-firing atmosphere and pre-firing temperature at this time are not particularly limited, but generally, the pre-firing is performed in the air at a temperature in the range of 1000 to 1100 ° C.

In the firing in a dry gas according to the present invention,
In order to improve the contact efficiency between the drying gas and the silica, a drying gas introduction pipe is inserted near the bottom of the crucible containing the silica to flow the drying gas so that the silica particles and the drying gas are efficiently contacted.

(Action) In the present invention, since silica is calcined in a dry gas, the dry gas is supplied to the silica surface, so that the silanol groups condense on the silica surface at a high temperature and the equilibrium is large in the direction of releasing water. Uneven moisture is removed. Further, after the silanol group on the silica surface is removed, a concentration gradient of the silanol group is formed between the surface and the inside of the silica. Water is removed for the same reason as described above.

It is presumed that water is removed from the entire silica by this repetition to obtain anhydrous silica.

(Examples) Examples of the present invention will be shown below, and the present invention will be described in more detail.

Example 1 A solution of 3.7 parts by weight of ammonium bicarbonate and 2.6 parts by weight of ammonia gas dissolved in 36 parts by weight of pure water was mixed with 60 parts by weight of methanol while stirring.
0 parts by weight was added dropwise over 3 hours to carry out a hydrolysis reaction to obtain precipitated silica gel having an average particle size of 120 μm. After drying the silica gel, the silica gel was preliminarily calcined at 1000 ° C. in the air to dehydrate, thereby obtaining a silica powder.

When this silica powder was measured by the FT-IR transmission method,
It contained a pm silanol group.

Next, put the obtained silica powder in a quartz glass crucible,
Cover and insert the quartz glass tube into the silica powder through the hole of the lid so that the dry gas outlet is near the bottom of the crucible, and dry pure air gas with a moisture content of 3 ppm is passed through the quartz glass tube. The temperature was raised while blowing into silica powder at a rate of 10 l / hr per kg, and at a temperature of 1150 ° C
Fired for 10 hours. The temperature was raised at a rate of 200 ° C./hr, and cooling was performed by natural cooling.

The silanol group content of the silica powder calcined in this manner was reduced to 59 ppm.

Example 2 The same procedure as in Example 1 was carried out except that the firing temperature was 1200 ° C., the silanol group content was reduced to 38 ppm.

Example 3 The sintering temperature was 1300 ° C., and the moisture content was as a dry gas.
When the same procedure as in Example 1 was carried out except that 10 ppm of nitrogen gas was used and the flow rate of the dry explosion gas was 5 l / hr per 1 kg of silica, the silanol group content was reduced to 42 ppm.

Comparative Example 1 The same procedure as in Example 1 was carried out except that the firing temperature was 1050 ° C., and the content of the silanol group was 270 ppm because the firing temperature was low.

Comparative Example 2 The same procedure as in Example 2 was carried out except that calcination was performed in the air without using a dry gas, and the content of the silanol group was 320 p.
pm.

Comparative Example 3 The same procedure as in Example 2 was carried out except that the flow rate of the drying gas was 2 l / hr per 1 kg of silica, and the silanol group content was not sufficiently reduced to 110 ppm.

Comparative Example 4 The same procedure as in Example 1 was carried out except that the gas having a water content of 38 ppm was used, and the content of the silanol group was 102 ppm.
And did not decrease enough.

Comparative Example 5 The same procedure as in Example 2 was carried out except that the silica glass tube into which the dry gas was introduced was not inserted into the silica powder, and the dry gas was blown onto the surface of the silica powder. 126 ppm was not enough.

Comparative Example 6 The same procedure as in Example 2 was carried out except that the pre-firing temperature was suppressed to 900 ° C. and the obtained silica powder was fired using silica powder having a silanol group content of 2120 ppm, and the silanol group content was sufficiently 137 ppm. Was not.

(Effects of the Invention) According to the present invention, the water content in silica can be reduced. Therefore, by using this silica as a raw material, it is possible to produce quartz glass having excellent performance in heat resistance and optical properties. it can.

Claims (2)

(57) [Claims] 1. Silica having silanol groups obtained by hydrolyzing alkoxysilane is calcined at a temperature of 1100 ° C. or more in a dry gas atmosphere having a water content of 30 ppm or less and a flow rate of 3 l / hr or more per 1 kg of silica. A method for producing anhydrous silica. 2. Before firing at 1100 ° C. or more in a dry gas atmosphere, silica is pre-fired to reduce the water content of the silica to 1000 pp.
2. The method for producing anhydrous silica according to claim 1, wherein the value is not more than m.