JP2542797B2 - Method for producing high-purity silica - Google Patents

Description

Detailed Description of the Invention

[0001]

FIELD OF THE INVENTION The present invention relates to a method for producing high-purity silica made from alkali silicate. For more details,
The present invention relates to a method for producing low-emission and extremely high-purity silica suitable for applications such as fillers for resins for IC sealants, substrates, high-purity silica glass and quartz glass for electronic materials and semiconductor manufacturing equipment, and raw materials for optical glass.

[0002]

2. Description of the Related Art In recent years, high-purity silica has come to be used for electronic materials, semiconductor manufacturing, etc. with the rapid development of the electronic industry. It is getting stronger. For example, LS
Although silica powder of high purity is used as a filler of epoxy resin for I or VLSI encapsulant, IC
U in the encapsulant with higher performance of
The problem of malfunction of IC, that is, soft error caused by α-rays emitted from (uranium) and Th (thorium) has come to be emphasized. In order to avoid this trouble, it is indispensable to reduce radioactive elements, particularly U and Th, which are α-radiation sources in silica as a filler mixed in the epoxy resin composition at a ratio of 50 to 90%. Becomes

Conventionally, silica of this type of filler for epoxy resin is one obtained by chemically treating high-quality natural silica sand having a low content of radioactive elements such as U and Th, and one obtained by melting and crushing high-quality natural quartz. Was mainly used, but in natural silica sand and quartz, even after acid treatment or purification treatment, U
And Th, respectively, are contained in the range of several tens to 100 ppb, and due to soft error, such silica is completely unsuitable as a filler for an IC encapsulant targeted for high integration of 256 kilobits or more. Become.

Among the natural crystals, rare ones containing particularly low contents of U and Th are rarely produced, but their availability is becoming difficult year by year.

On the other hand, as a method for producing extremely high-purity silica having U or Th of 1 ppb or less, a method of hydrolyzing and firing a silica source such as purified silicon tetrachloride or tetraethyl silicate and a method of vapor phase decomposition are particularly preferable. However, in both cases, the raw material itself is expensive, and since it has corrosiveness and flammability, special handling is required for handling and it is extremely expensive.

The high-purity silica glass widely used for the semiconductor industry has been mainly made of natural quartz until now, and is chemically treated to increase the purity of the rough stone, and a special crushing method is used to prevent contamination of impurities. It is made from crystal powder purified while maintaining its purity. However, it becomes difficult to obtain such high-quality natural crystals year by year, and it is expected that an alternative raw material will appear. Table 1 shows an example of the purity of natural quartz as a raw material of high-purity silica glass for the semiconductor industry.
Impurities other than 1 are 5 ppm or less, and each element is 5 pp
If it is m or less, it can be used as a raw material of high-purity silica glass.

[0007]

[Table 1]

Further, as a raw material silica for high-purity quartz glass or optical glass, it is required that the content of transition metals such as iron, chromium and manganese is 1 ppm or less.

[0009]

However, such high-purity silica has not been obtained by the conventional method by the reaction of alkali silicate and acid. Conventionally, as a method for producing high-purity silica using an alkali silicate as a silica source, an aqueous solution of an alkali silicate is ion-exchanged to obtain a silica silicate sol, and a salt or a surfactant is added to the silica sol to precipitate the silica in a precipitation state and the silica is recovered. Method (Japanese Patent Publication No. 36-18315, Japanese Patent Publication No. 37-4304), the alkali silicate aqueous solution is ion-exchanged to form a silica sol, and ammonia is added to this to adjust the pH.
Is adjusted, then cooled and frozen, and then heated and melted to precipitate silica for recovery (Japanese Patent Publication No. 36-9415). However, in all cases, the water content of the precipitated silica is as high as 80% or more, filtration and washing are difficult, the SiO ₂ purity is about 99.3 to 99.9%, and the impurity content is Na: 150. It is set to ~ 300 ppm. In fact, according to the examination results of the present inventors, Fe: 50 to 150 pp
m, Th: about 100 to 250 ppb, Fe: 5 ppm or less, Th: 10 even if a treatment with an acid is further added.
It was difficult to obtain silica of ppb or less. Then, recently, a method for producing quartz glass of U: 1 ppb or less from a silicate of an alkali metal or an alkaline earth metal and a mineral acid under the condition of a hydrogen ion concentration of 1.5 or less has been proposed (Japanese Patent Laid-Open No. 59-54632). Issue). However, this invention does not disclose any means for removing other heavy metals such as Th, which is the most difficult to remove.

By the way, a method of precipitating silica gel by adding an aqueous solution of alkali silicate to mineral acid is an advantageous method as a means for producing high-purity silica gel, in which impurities are remarkably reduced as compared with the reverse addition method. A significant difference in the precipitation properties of silica gel was observed due to subtle differences in the reaction conditions, which had a great effect on the operation during separation and recovery. At the same time, when the impurity content was discussed in terms of ppm or ppb, depending on the reaction conditions. Remarkable variation occurs, impurities that cannot be separated by the washing operation remain, and reproducibility is poor.

On the other hand, the idea of reducing impurities in silica or silica gel by using a complex forming agent is disclosed in JP-A-55-42294 and JP-A-58-41713. In the silica obtained by the method (1), 50 ppm or more of Fe remains as an example, and high-purity silica intended by the present inventors has not been obtained.

As described above, the impurity metal elements in silica are all 1 ppm or less, and the radioactive impurities are 1 p or less.
The reproducible production of extremely high-purity silica of pb or less from an aqueous alkali silicate solution has not been disclosed at all in the conventional method.

[0013]

The present invention relates to a method for producing silica by reacting an aqueous sodium silicate solution with a mineral acid, wherein the silica is present in an acidic region where the chelating agent and hydrogen peroxide are present and the acid concentration is 1 N or more. And then washing the separated and recovered silica with a mineral acid containing a chelating agent and hydrogen peroxide.
B, Ba, Ca, Co, Cr, Cu, Fe, Mg, M
n, Na, Ni, Pb, Sr, Ti, Zn and Zr impurity contents are all 1 ppm or less, and U and T
This is a method for producing high-purity silica having an h impurity content of 1 ppb or less.

That is, the method for producing high-purity silica according to the present invention is an improvement of the invention of Japanese Patent Application No. 60-15279 (Japanese Patent Application Laid-Open No. 61-178414) filed by the present inventors. . The high-purity silica obtained by the method of the present invention exceeds the purity of high-quality natural silica sand and quartz that have been conventionally used as raw materials for electronic materials and high-purity silica glass, and therefore can be used in place of them. Not only
A high-quality silica resource because it can be stably supplied for high-performance electronic materials such as encapsulants and fillers for highly integrated ICs that require higher purity, or for quartz glass and optical glass. It is of great significance to our poor country.

[0015]

As the sodium silicate used in the method of the present invention, a commercially available sodium silicate solution (water glass) having a molar ratio of SiO ₂ / Na ₂ O of 1 to 4 can be used. The relatively large ones are economical because the amount of mineral acid required for the reaction is small. The sodium silicate solution may be appropriately diluted with water or an aqueous sodium salt solution of a mineral acid before use. Used concentration is 20% by weight as SiO _2.
Above, preferably 25% by weight or more is suitable.

On the other hand, examples of the mineral acid used in the method of the present invention include hydrochloric acid, nitric acid and sulfuric acid. The mineral acids can be used alone or as a mixed acid of two or more kinds. Further, the mineral acid can be used after being appropriately diluted.

In the method of the present invention, in producing high-purity silica using the above-mentioned raw materials, a sodium silicate aqueous solution and a mineral acid are reacted in an acidic region containing a chelating agent and hydrogen peroxide and having an acid concentration of 1 N or more. One of the characteristics is that it causes the precipitation of silica. Examples of chelating agents include dicarboxylic acids such as oxalic acid, malonic acid, succinic acid, glutaric acid, maleic acid and fumaric acid; tricarballylic acid, propane-1,1,2,3-tetracarboxylic acid, butane-1,
Polycarboxylic acids such as 2,3,4-tetracarboxylic acid; oxycarboxylic acids such as glycolic acid, β-hydroxypropionic acid, citric acid, malic acid, tartaric acid, pyruvic acid, diglycolic acid; nitrile triacetic acid (NTA), Examples thereof include aminopolycarboxylic acids such as nitrilopropionic acid and ethylenediaminetetraacetic acid, or salts thereof. As the chelating agent, oxalic acid, citric acid, tartaric acid or their soluble salts are particularly preferable. The amount of chelating agent and hydrogen peroxide added is silica (SiO ₂ ) in the reaction system.
0.1 to 5% by weight, preferably 0.1 to 2% by weight
Is. If the amount of the chelating agent added is less than 0.1% by weight, the effect of addition will be insufficient, and if it exceeds 2% by weight, the effect of addition will tend to be saturated. Due to the presence of the chelating agent and hydrogen peroxide, it is possible to selectively remove impurities such as Zr and Ti, which are difficult to remove, from the silica.

In such a reaction, a method of adding an aqueous solution of sodium silicate to a mineral acid, or a method of simultaneously adding an aqueous solution of sodium silicate and a mineral acid can be considered. In either case, the acid concentration in the reaction system is always 1N. It is important to keep above. In the region where the acid concentration is less than 1 N, a large amount of impurities are strongly contained and a precipitate of silica having poor solid-liquid separation is formed, and it is difficult to sufficiently remove impurities even if the subsequent washing operation with an acid is performed. Is.

The temperature during the reaction is not so important,
The desired temperature may be set within the range of any temperature from normal temperature to about 100 ° C., and it is desirable to continue the aging stirring for a while after the reaction is completed.

Next, it is an essential step in the present invention to separate the precipitate of silica produced by the reaction by a conventional method, and to wash the separated silica with a mineral acid containing hydrogen peroxide and a chelating agent. Yes, this is another feature. In this case, the type of acid and the type of chelating agent are the same as above, and the concentration of acid during treatment is 0.5 to 3
Regulation is desirable. If it is less than 0.5 N, impurities adhering to silica are insufficiently removed, and if a strong acid exceeding 3 N is used, problems occur in neutralization or effective utilization of waste acid after acid treatment. The amount of the chelating agent and hydrogen peroxide added to the acid is 0.1 to SiO _{2 with} respect to each of the above.
It is preferable that the content is 5% by weight.

As described above, in the present invention, both the reaction step for forming a silica precipitate and the subsequent washing step are treated with an acid, but a chelating agent (a salt thereof may be used, if necessary) and a peroxide are used. It is necessary to include hydrogen oxide, and if these are not included in any of the steps,
The desired high-purity silica cannot be obtained.

That is, in the method of the present invention, the impurity metal elements in silica, such as Al, B, Ba, Ca, Co, Cr, C.
u, Fe, Mg, Mn, Na, Ni, Pb, Sr, T
The purpose is to reduce i, Zn, and Zr to 1 ppm or less and U and Th to 1 ppb or less, but among them, Zr and Ti exhibit similar behavior and are difficult to remove. Although it is an impurity, according to the method of the present invention, highly pure silica can be recovered with good reproducibility.

Needless to say, the acid treatment step by acid washing is not limited to one time, and may be performed twice or more depending on the nature, if desired, and the treatment temperature may be arbitrarily set.

The purified high-purity silica thus obtained is filtered,
After thoroughly washing and removing by centrifugation or other methods,
It is dried and then calcined or melted and recovered as high purity silica.

[0025]

EXAMPLES The method of the present invention will be described more specifically below with reference to examples. Example 1 4000 g of an aqueous nitric acid solution (HNO ₃ = 19.3% by weight) was placed in a reaction vessel equipped with a stirrer, and 6 g of oxalic acid (dihydrate: commercially available product) and 35% by weight hydrogen peroxide solution (commercially available product) ) 17 g was added and dissolved. In this nitric acid solution, JIS No. 3 sodium silicate
(Na ₂ O = 9.2 wt%, SiO ₂ = 28.5 wt%) 21
00g was added over a period of about 30 minutes to form a silica precipitate. During this period, stir the reaction tank well and keep the liquid temperature at 7
Hold at 0-80 ° C. After completion of the reaction, add 8 to the reaction slurry.
The mixture was aged at 0 ° C. for 2 hours for aging. The nitric acid concentration in the reaction mother liquor at this time was 1.1 normal. After repeating filtration and washing of the silica precipitate from this reaction-completed slurry, the silica precipitate was separated and collected. The separated and recovered silica was placed in an acid treatment tank equipped with a stirrer, and water and nitric acid were added to this to adjust the total amount of the slurry to 5 liters and the concentration of nitric acid in the liquid to 1 N.
Oxalic acid (6 g) and 35 wt% hydrogen peroxide solution (17 g) were added, and the mixture was heated at 90 ° C. for 3 hours with stirring for acid treatment. Silica is separated from this slurry by filtration, washed with repulp at room temperature at room temperature, solid-liquid separated, and dried.
It was calcined at 0 ° C. for 2 hours. Table 2 shows the content of impurities in the obtained silica.

Examples 2 to 3 High-purity silica was produced according to the procedure of Example 1 using hydrochloric acid and sulfuric acid as acids and citric acid and EDTA as chelating agents. Table 2 shows the reaction conditions at this time and the analysis results of the obtained high-purity silica.
Also described in.

Comparative Examples 1 to 3 Silicas were produced according to the procedure of Example 1 using hydrochloric acid. The reaction conditions at this time and the analysis results of the obtained silica are also shown in Table 2 as Comparative Examples 1, 2 and 3. As is clear from Table 2, in the case of these comparative examples which do not satisfy the production conditions according to the present invention, it is understood that the desired high-purity silica cannot be obtained.

Example 4 The aqueous nitric acid solution and sodium silicate solution used in Example 1 were added to a reactor at the same time while stirring continuously so that the addition of both solutions was completed at the same time, and the silica was precipitated at 75 ° C. Was generated. An aqueous solution of nitric acid having a concentration of 1.1 N was added to the reactor in advance such that the blade of the stirrer was immersed. In this example, the nitric acid concentration in the reaction mother liquor was kept at 1.1 N during the reaction. Thereafter, high-purity silica was obtained by the same operation as in Example 1. The reaction conditions and the analysis results of the obtained silica are also shown in Table 2.

[0029]

[Table 2]

Example 5 After the acid treatment obtained in Example 1, a high-purity silica slurry dispersed in water was spray-dried (manufactured by Okawara Seisakusho,
Spray dried with OC-20). The resulting silica gel had good fluidity and an average particle size of 14.5 μm.
Next, this powder was continuously flown down into a flame melting furnace using oxygen-propane gas and subjected to a melting treatment to obtain fused spherical silica. This silica was a high-purity silica spherical melt having an average particle size of 14.3 μm.

[0031]

As is apparent from the above description, according to the method for producing high-purity silica of the present invention, metal element impurities such as Al, B,
Ba, Ca, Co, Cr, Cu, Fe, Mg, Mn, N
High-purity silica having a content of a, Ni, Pb, Sr, Ti, Zn, and Zr of 1 ppm or less, and U and Th content of 1 ppb or less is relatively inexpensive from relatively inexpensive raw materials. It is possible to surely manufacture by a simple process. The high-purity silica produced by the method of the present invention is suitable for use as a filler for IC encapsulant resins, substrates, raw materials for high-purity silica glass for electronic materials and semiconductor manufacturing equipment, and is of depleting quality. It is particularly significant in that it enables a stable supply in place of resources such as natural silica sand and crystals.

Claims (3)

(57) [Claims] 1. A method for producing silica by the reaction of an aqueous sodium silicate solution with a mineral acid, wherein a silica precipitate is produced in an acidic region where an acid concentration is 1 N or more in the presence of a chelating agent and hydrogen peroxide, and then the silica is separated. Al, B, Ba, Ca, Co, C characterized in that the recovered silica is washed with a mineral acid containing a chelating agent and hydrogen peroxide.
r, Cu, Fe, Mg, Mn, Na, Ni, Pb, S
r, Ti, Zn, and Zr impurity contents are all 1 pp
A method for producing high-purity silica having a content of m or less and U and Th impurity contents of 1 ppb or less. 2. The method for producing high-purity silica according to claim 1, wherein the chelating agent is a carboxylic acid, a polycarboxylic acid, an oxycarboxylic acid, an aminopolycarboxylic acid or a salt thereof. 3. The method for producing high-purity silica according to claim 1, wherein the chelating agent and hydrogen peroxide present in the reaction system are each 0.1 to 5% by weight with respect to SiO ₂ .