JPH01230422A - High-purity silica and production thereof - Google Patents

Abstract

PURPOSE:To readily obtain high-purity silica with specified physical characteristics, little in impurities such as U or Th, by reaction between sodium silicate and a specified aqueous sulfuric acid containing a chelating agent and hydrogen peroxide to precipitate silica. CONSTITUTION:Firstly, a reaction is carried out at <=40 deg.C between (A) sodium silicate and (B) >=1N aqueous sulfuric acid containing a chelating agent and hydrogen peroxide to precipitate silica. Thence, the reaction product is aged at >=70 deg.C, thus obtaining the objective high-purity silica having the following characteristics: 1. content of U, Th...<=0.02ppb 2. BET specific surface area of uncalcined hydrous silica...>=30m<2>/g 3. BET specific surface area of calcined silica...<=50m<2>/g 4. BET specific surface area of fused spherical silica...0.2-3m<2>/g 5. tap density...>=1.36g/cm<3>.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to high-purity silica made from alkali silicate and a method for producing the same.

More specifically, we will introduce low-emissivity, extremely high-purity silica suitable for use as a filler for resins for IC encapsulants, substrates, high-purity silica glass and quartz glass for electronic materials and semiconductor manufacturing equipment, raw materials for optical glass, etc. Regarding the manufacturing method.

[Conventional technology]

In recent years, with the rapid development of the electronics industry, high-purity silica has come to be used for electronic materials and semiconductor manufacturing, but as products become more sophisticated, the demand for higher purity silica has become even stronger. .

For example, high-purity silica powder is used as a filler in epoxy resins used as encapsulants for LSIs or VLSIs. uranium) and Th (thorium)
The problem of IC malfunctions, that is, soft errors, caused by α-rays emitted from the IC has become important. In order to avoid this trouble, α
- Reduction of radioactive elements, especially U and Th, is an essential requirement.

Conventionally, the silica used as filler for this type of epoxy resin has mainly been made by chemically treating high-quality natural silica sand with a low content of radioactive elements such as U and Th, or by melting and pulverizing high-quality natural quartz. However, natural silica sand and crystal each contain about 1 to 100 ppb of U'PTh even after acid treatment and purification, and such silica contains more than 256 kb due to soft errors. It is completely unsuitable as a filler for an IC encapsulant intended for high integration.

Natural crystals with particularly low contents of U and Th are occasionally produced, but their acquisition is becoming more difficult year by year.

On the other hand, methods for producing extremely high-purity silica with U, Th, or less than 1 pl)b include a method in which a silica source such as particularly purified silicon tetrachloride or tetraethyl silicate is hydrolyzed and calcined, and a method in which gas phase decomposition is performed. However, the raw materials themselves are expensive, corrosive and flammable, and require special consideration when handling, making them extremely expensive.

[Problem to be solved by the invention]

However, such high-purity silica has not been obtained by the conventional method of reacting an alkali silicate with an acid.

Conventionally, the method for producing high-purity silica using alkali silicate as a silica source is: 1) ion-exchange an aqueous alkali silicate solution to form acidic silica sol, and add salts and surfactants to this to precipitate silica. Method of recovery (Special Publication No. 3G-18315, Publication No. 37-4304)
(No. Publication), ■ A method of ion-exchanging an aqueous silicate solution to obtain silica sol, adding ammonia to it to adjust the pH, cooling and freezing, and then heating and melting to precipitate and recover silica (Special Publication No. 3B) 9415), but in both cases, the water content of the precipitated silica reaches 80% or more, making filtration, washing, etc. difficult.

Furthermore, the impurity content is said to be 150 to 300 ppm Na at a SIO2 purity of 99.3 to 999%, but the inventors' study revealed that the impurity content is 50 to 150 ppm Fe.
m.

Th is about 100 to 250 ppb, and even if a purification treatment with an acid is applied, it is difficult to obtain silica with Fe of 3 ppm or less and Th of 1 Oppb or less. Recently, a method has been proposed for producing silica glass with U 1 ppb or less from silicates of alkali metals or alkaline earth metals and mineral acids under conditions where the hydrogen ion concentration is 1.5 or less (Japanese Patent Application Laid-Open No. 59-1971). -54832). However, in this invention, no consideration is given at all to the means for removing Th, which is the most difficult to remove.

By the way, the present inventors have already succeeded in developing high-purity silica containing all impurity metal elements of 1 ppm or less through a wet reaction between an alkali silicate and a mineral acid (Japanese Unexamined Patent Publication No. 62-12).
Publication No. 808). However, in this wet method, U,
The purity has not been reached to the extent that α-radioactive elements such as Th can be reduced to 0.02 ppb or less.

The object of the present invention is to provide high-purity silica by further improving the prior art disclosed in JP-A-62-12608.

[Means and actions for solving problems]

That is, the high-purity silica provided by the present invention is a synthetic silica obtained by acid decomposition of an alkali silicate (wet method), and the content of U and Th, which exhibit α-radiation, is each 0.02 ppb or less. It is characterized by

Acid decomposition of an alkali silicate (wet method) refers to a wet reaction between an alkali silicate and a mineral acid, and the high-purity silica of the present invention can be obtained selectively using sulfuric acid as the mineral acid under the conditions described below. be. This high-purity silica has achieved remarkable success in removing Th, which was conventionally considered to be the most difficult process in a wet reaction, and has surprisingly high purity of 0.02 ppb each of U and Th, which are α-radioactive elements. and other metal elements have a purity level of substantially 1 ppm or less.

One of the characteristics of this synthetic silica is that it is porous with a BET specific surface area of 300 r&/g or more. The reason for this is that the BET specific surface area is approximately 300 r.
Synthetic silica of less than rr/g can have the above-mentioned high purity. In order to have this purity,
This is because there is a correlation that it must be 300 ni/g or more due to its manufacturing history.

Therefore, the high-purity silica of the present invention includes uncalcined hydrated silica with a BET specific surface area of 300 r&/g or more and calcined silica with a BET specific surface area of 50 rrr/g or less.

Such high-purity silica has a purity higher than that of high-quality natural silica sand and crystal that have been conventionally used as raw materials for electronic materials and high-purity silica glass, so it can not only be used in place of them, but also We are blessed with high-quality silica resources in that we can stably supply it for use in high-performance electronic materials such as resin sealing fillers for highly integrated ICs that require higher purity, as well as for quartz glass and optical glass. This is of epoch-making significance for our country.

When the synthetic silica according to the present invention is molten and spheroidized, it becomes suitable as a filler for IC resin encapsulation.

A particularly suitable structure has a BET specific surface area of 0.2 to 3r.
Examples include fused spherical silica having r1''/g and a tap density of 1.36 g/c% or more.

The above-mentioned high-purity silica according to the present invention is produced by reacting sodium silicate with a mineral acid in a mineral acid having an acid concentration of 1N or more in the presence of a chelating agent and hydrogen peroxide to form a silica precipitate.
In a method for producing high-purity silica, which consists of washing the separated and recovered silica with a mineral acid containing a chelating agent and hydrogen peroxide, a silica precipitation reaction was carried out using sulfuric acid at a temperature of 40'C or less. Later, the reaction product was
It is produced by a process of ripening at temperatures above 'C.

This production method is an improvement on the prior art disclosed in Japanese Patent Application Laid-Open No. 12608/1982, and under the selective conditions, unexpectedly remarkable high purity and favorable properties were found.

As the sodium silicate used in the method of the present invention, commercially available sodium silicate solutions (water glass) with a molar ratio SiO/NaO of 1 to 4 can be used, but those with a relatively large molar ratio value are preferred. It is economical because the amount of mineral acid required for the reaction is small. The sodium silicate solution may be used after being appropriately diluted with water or an aqueous solution of a sodium salt of a mineral acid. The concentration used is preferably 20% by weight or more, preferably 25% by weight or more as SiO2.

On the other hand, it is essential to use sulfuric acid as the mineral acid used in the method of the present invention, and hydrochloric acid, nitric acid, etc. can be used in combination as necessary.

In the method of the present invention, when producing high-purity silica using the above raw materials, a sodium silicate aqueous solution and a mineral acid are reacted in a sulfuric acid acidic region containing a chelating agent and hydrogen peroxide and having an acid concentration of 1N or more. Forms a silica precipitate.

Chelating agents include dicarboxylic acids such as oxalic acid, malonic acid, succinic acid, glutaric acid, maleic acid, fumaric acid, nitricarballylic acid, propane-1,1,2,3-tetracarboxylic acid, butane-1,2,3 , 4-tetracarboxylic acid; oxycarboxylic acids such as glycolic acid, -10-β-hydroxypropionic acid, citric acid, malic acid, tartaric acid, pyruvic acid, diglycolic acid; nitrile triacetic acid (NTA) , nitrilolipropionic acid, aminopolycarboxylic acids such as ethylenediaminetetraacetic acid, or salts thereof. Among these, oxalic acid, citric acid, tartaric acid or their soluble salts are particularly preferred. The amounts of the chelating agent and hydrogen peroxide added are determined by the amount of silica (S i02
), preferably 0.1 to 2% by weight. If the amount of the chelating agent added is less than 0.1% by weight, the effect of the addition will not be sufficient, and if it exceeds 2% by weight, the effect of the addition will tend to be saturated. The presence of the chelating agent and hydrogen peroxide makes it possible to selectively remove impurity components from the silica that are difficult to remove, such as Zr and Ti, which are tetravalent impurities similar to Th.

In such a reaction, a method of adding an aqueous sodium silicate solution to a mineral acid, or a method of adding an aqueous sodium silicate solution and a mineral acid simultaneously can be considered, but in either case, the acid concentration in the reaction system must always be maintained at 1N or higher. What you do is important. In the region where the acid concentration is less than 1N, a large amount of impurities are strongly included, and li! A silica precipitate having poor separation properties from the i'I liquid is formed, making it difficult to sufficiently remove impurities even in the subsequent acid washing operation.

In the manufacturing method of the present invention, the following two points are particularly important.

■Use sulfuric acid as the mineral acid for the reaction.

(2) Maintaining the reaction temperature during silica precipitate formation at 40°C or lower, and then aging the resulting precipitate at a temperature of 70°C or higher.

When the silica precipitate formation reaction is carried out at a temperature of about 40°C or higher, the silica precipitate is rapidly dehydrated and hardened, and a slurry of the reaction product that is easily separated into solid and liquid is obtained, but on the other hand, impurities inside the precipitate are eluted into the liquid phase. Perhaps because of this difficulty, there is a limit to the removal of impurities such as U and Th.

On the other hand, if the silica precipitation reaction is carried out at 40°C or lower,
A soft precipitate with a high water content is formed, and a slurry of reaction products that is viscous and difficult to separate from solid and liquid is formed. In common sense, such reaction conditions that make workability difficult are avoided, but in the method of the present invention, sulfuric acid is used and the reaction is carried out under such temperature conditions.
This was based on the completely unexpected fact that silica precipitates with extremely low impurity content and low water content can be obtained by aging at temperatures above C. That is, after completing the reaction at 40°C or lower, preferably near room temperature, the reaction is held for 15 to 60 minutes, and then heated to 70°C.
As mentioned above, it is preferably aged at 75° C. to near the boiling point of the system for 30 to 180 minutes, but the holding time, aging time, etc. are not particularly limited.

The synthetic silica obtained in this manner is porous with a BET specific surface area of 300 rrr/g or more, although it is somewhat affected by the treatment described below. The silica precipitation reaction and aging treatment affect the mutual structure of silica particles and affect the separation of impure metal components, and anything less than 300 d/g is substantially the object of the present invention. Purity cannot be achieved.

Next, the silica precipitate produced by the reaction is separated by the Blue 4' method, and the separated silica is washed with mineral acid containing hydrogen peroxide and a chelating agent.

Regarding the type of acid and type of chelating agent in this case,
This is the same as above, and the acid concentration during treatment is preferably 0.5 to 4 normal. If it is less than 0.5N, impurities adhering to the silica will not be removed sufficiently, and if a strong acid exceeding 4N is used, problems will arise in neutralizing or effectively utilizing the waste acid after acid treatment.

The amounts of the chelating agent and hydrogen peroxide to be added to the acid are preferably within the range of 0.1 to 5% by weight relative to SiO2, as described above.

In this way, in the present invention, both the reaction step to form silica precipitate and the subsequent washing step are treated with acid, but a chelating agent (or its salt may be used if necessary) is always used.
It is necessary to contain hydrogen peroxide and hydrogen peroxide, and if these are not included in any of the steps, the desired high-purity silica cannot be obtained.

Further, it goes without saying that the acid treatment step by acid washing is not limited to one time, but may be performed two or more times depending on the nature of the process, and the treatment temperature can be set arbitrarily.

Thus, the purified high-purity silica is thoroughly washed and removed by purification, centrifugation, or other methods, and then dried and recovered. Furthermore, if necessary, it can be fired or melted to obtain a molten spheroidized body of high-purity silica.

The method for obtaining fused spherical silica is preferably carried out under the following conditions.

That is, porous synthetic silica having a specific surface area of 300 rrr1''/g or less is fired until it has a specific surface area of 50 rrr1''/g or less, preferably 30 rrr1''/g or less,
The fired and pulverized particles are flame-fused and spheroidized.

The particles of the fired product are suitably adjusted to have an average particle size in the range of 2 to 50 uyl, preferably 3 to 35 tErl. Thus, according to the method according to the invention, the tapped density (TD) is 1.38 g/c% or more, preferably 1.
．． It is obtained as high-purity, high-density fused spherical silica with a particle size of 39-1.46 g/c++1 and a BET specific surface area of 0.2 to 3 m2/g. This fused spherical silica has properties suitable as a filler for IC resin encapsulation.

〔Example〕

The present invention will be described below based on Examples and Comparative Examples.

Example-1 A sulfuric acid aqueous solution (H2S O4-
23.7% by weight) was taken, and 2 g of oxalic acid (dihydrate, commercial product) and 5 g of 35% hydrogen peroxide solution (commercial product) were added and dissolved therein. Add JI83 to this sulfuric acid aqueous solution.
No. Sodium silicate (Na O-9, 2% by weight, S 102-
28.5% by weight) 600g was heated in about 20 minutes.
It was added continuously from the tip of a mmφ nozzle to form a silica precipitate. During this time, the reaction tank was sufficiently stirred and the liquid temperature was maintained at 25 to 30°C. After the reaction is completed, the slurry is heated to 30°C.
After stirring for 30 minutes, the temperature was raised to 80℃, and the
Aging was performed by stirring for hours.

After repeated filtration and washing, the silica precipitate from this aged slurry was separated and recovered. The separated and recovered silica was placed in an acid treatment tank with a stirrer, and water and sulfuric acid were added to it to make a slurry with a total volume of 1.7ρ and a sulfuric acid concentration of 16.6% by weight.
Further, 2 g of sulfuric acid and 5 g of 35% hydrogen peroxide were added, and the mixture was heated at 85° C. for 2 hours with stirring for acid treatment. Silica was separated from this slurry by filtration, followed by repulping washing with water at room temperature, solid-liquid separation, and drying at 105° C. for 2 hours. Furthermore, a portion of the powder was calcined at 1100° C. for 2 hours and then ground to obtain calcined silica powder with an average particle size of 23.3 ml.

The impurity content and physical properties of fired products are shown in Table 1.
Shown below.

Example-2 Following the operating procedure of Example-1, using 5 g of EDTA O instead of 2 g of oxalic acid as the cleaning agent, and using high-purity silica under all the same conditions as Example-1. was manufactured. The impurity content in the obtained silica is also shown in Table-1.

116 - Comparative Example-1 According to the operating procedure of Example-1, the reaction was carried out at 80°C for 20 minutes, and then aged as it was at 80°C for 2 hours. High purity silica was produced under all other conditions similar to those in Example 1. The impurity content in the obtained silica is also shown in Table-1. As is clear from Table 1, when the reaction is carried out at a high temperature of 40°C or higher, the content of U and Th is high;
The desired high purity silica cannot be obtained.

Comparative Example-2 250g of 35w 1% hydrochloric acid and 350g of water in a reaction tank with a stirrer
g and 25 g of EDTAo were taken and dissolved. JIS No. 3 sodium silicate (N a 209.
2% by weight, SIO228, 5% by weight) was continuously dropped from a 1 mmφ nozzle tip over a period of about 12 minutes to form a silica precipitate. During this time, the reaction tank was sufficiently stirred and the liquid temperature was maintained at 45 to 50°C.

After the reaction was completed, the slurry was aged at 50°C for 2 hours, and then
After adding 3 ml of 35% hydrogen peroxide solution and stirring for 10 minutes, solid-liquid separation was performed in the same manner as in Example-1. The separated and recovered silica is placed in an acid treatment tank equipped with a stirrer, and 35% by weight hydrochloric acid (10%) is added to it.
Add 0 ml and water to make a total volume of 900 ml, and then add ED
25 g of TAo was added and heated at 80° C. for 2 hours for acid treatment. After cooling to 70℃, add 3m of 35% hydrogen peroxide solution.
After stirring for 10 minutes, solid-liquid separation, drying, and calcination were performed in the same manner as in Example-1 to obtain high-purity silica. The impurity content in the obtained silica is also shown in Table-1. Table-1
As is clearer, if the reaction is carried out at 40° C. or higher, the desired high purity silica cannot be obtained due to the high U and Th contents.

Table-I It was determined by analysis using the ICP method.

Note 2) All specific surface areas are based on the BET method.

-191 Example-3 The high-purity sintered silica product obtained in Example-1 was pulverized with a resin ball mill to an average particle size of 25 mm. Next, this powder was melted by flowing continuously into a flame melting furnace using oxygen-propane gas to obtain fused spherical silica. This silica was a high purity spherical molten silica having an average particle size of 31 mm, a specific surface area of 0.8 Bttf/g, and a TDl of 45 g/c.

[Effects of the Invention] As is clear from the above description, according to the method for producing high-purity silica of the present invention, the impurity contents are reduced by U (uranium) and Th (thorium) through wet reaction with alkali silicate and acid. High purity silica of 0.02 ppb or less can be reliably produced from relatively inexpensive raw materials through a relatively simple process. The high-purity silica of the present invention is suitable for use as a filler for resins for IC encapsulants, substrates, electronic materials, and raw materials for high-purity silica glass for semiconductor manufacturing equipment, etc. It is particularly significant in that it enables a stable supply in place of resources such as crystal.

Claims (1)

[Scope of Claims] 1. Synthetic silica obtained by acid decomposition of alkali silicate (wet method), wherein the content of U (uranium) and Th (thorium), which exhibit α-radiation, is 0.02 ppb or less each. High purity silica characterized by: 2. The high-purity silica according to claim 1, which is uncalcined hydrated silica having a BET specific surface area of 300 m^2/g or more. 3. The high-purity silica according to claim 1, which is calcined silica having a BET specific surface area of 50 m^2/g or less. 4. BET specific surface area is 0.2 to 3 m^2/g, and tap density is 1.36 g/g.
The high-purity silica according to claim 1, which is fused spherical silica with a diameter of cm^3 or more. 5. Sodium silicate and mineral acid are reacted in a mineral acid with an acid concentration of 1N or more in which a chelating agent and hydrogen peroxide are present to form a silica precipitate, and then the separated and recovered silica is treated with a chelating agent and hydrogen peroxide. In the manufacturing method of high-purity silica, which consists of washing with mineral acid contained in the product, the silica precipitation reaction is carried out using sulfuric acid at a temperature of 40°C or lower, and then the reaction product is aged at a temperature of 70°C or higher. A method for producing high purity silica, characterized by: 6. The method for producing high-purity silica according to claim 5, wherein the chelating agent is dicarboxylic acid, polycarboxylic acid, oxycarboxylic acid, aminopolycarboxylic acid, or a salt thereof. 7. Claim 5: chelating agent and hydrogen peroxide are each added in an amount of 0.1 to 5% by weight based on SiO_2 in the reaction system.
The method for producing high-purity silica described above. 8. A method for producing high-purity silica, which comprises flame-melting the synthetic silica obtained by the production method according to claim 5 through a firing step such that the BET specific surface area becomes 50 m^2/g or less.