JPH0516372B2 - - Google Patents

Description

[Detailed description of the invention]

[Industrial Application Field] The present invention relates to high-purity silica made from alkali silicate and a method for producing the same. In more detail, 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, and raw materials for optical glass. Regarding the manufacturing method. [Prior art] In recent years, with the rapid development of the electronic industry, high-purity silica has come to be used for electronic materials such as semiconductor manufacturing, but as products become more sophisticated, there is a demand for higher purity silica. is becoming even stronger. For example, high-purity silica powder is used as a filler in epoxy resins used as encapsulants for LSIs or VLSIs, but as the performance of ICs increases, or the degree of integration increases, U( The issue of IC malfunctions, or soft errors, caused by α-rays emitted from uranium (Uranium) and Th (Thorium) has become important. To avoid this trouble, 50 to 90%
It is essential to reduce α-radiation elements, especially U and Th, in the filler silica, which is mixed in a proportion of 1. Conventionally, the main silica fillers for this type of epoxy resin have been chemically treated high-quality natural silica sand with a low content of radioactive elements such as U and Th, and high-quality natural quartz crystals that have been melted and crushed. However, in natural silica sand and quartz, even after acid treatment and refining treatment, U and Th each remain in the range of 1 to 1.
It contains about 100 ppb, and such silica is completely unsuitable as a filler for IC encapsulant intended for high integration density of 256 kilobits or more due to soft errors. 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 and Th of 1 ppb or less 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 care when handling, making them extremely expensive. [Problems 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 to ion-exchange an aqueous solution of alkali silicate to form acidic silica gel, then add salts and surfactants to this to precipitate silica and collect it. (Japanese Patent Publication No. 36-18315, Japanese Patent Publication No. 37-4304), an aqueous alkali silicate solution is ion-exchanged to form a silica sol, ammonia is added to this to adjust the pH, and then cooled and frozen.
Furthermore, methods are known in which silica is precipitated and recovered by heating and dissolving it (Japanese Patent Publication No. 36-415). , cleaning etc. becomes difficult.
Moreover, the SiO ₂ purity is said to be about 99.3-99.9% and the impurity content is Na150-300ppm, but the inventors' study results show that Fe50-150ppm and Th100-300ppm.
It was about 250 ppb, and it was difficult to obtain silica with Fe of less than 5 ppm and Th of less than 10 ppb even if purification treatment with acid was added. In addition, recently, hydrogen ion concentration
A method has been proposed for producing silica glass with U1 ppb or less from alkali metal or alkaline earth metal silicates and mineral acids under conditions of U1.
Publication No. 54632). However, this invention does not give any consideration 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 Patent Application Laid-open No. 12608/1983). However, this wet method does not achieve a high enough purity to reduce α-radioactive elements such as U and Th 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 effects for solving the problem] That is, the high-purity silica provided by the present invention is a silica gel obtained by acid decomposition of an alkali silicate (wet method) or a molten synthetic silica originating from this. Therefore, U and α-radioactive
Each is characterized by a Th content of 0.02 ppb or less. 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 significantly achieved the removal of Th, which was traditionally considered the most difficult process in a wet reaction.
In particular, the α-radioactive elements U and Th are
It has a surprisingly high purity of 0.02 ppb, and the purity level of other metal elements is substantially below 1 ppm. This synthetic silica means silica gel or fused silica derived from silica gel, and one of the characteristics of the former is that it is porous with a BET specific surface area of 300 m ² /g or more. . The reason for this is that silica gel with a BET specific surface area of less than approximately 300 m ² /g cannot have the above-mentioned high purity ^; This is because there is a correlation that must be higher than that. 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 gel 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. The synthetic silica according to the present invention becomes suitable as a filler for IC resin sealing when the silica gel is melted and spheroidized. A particularly suitable structure has a BET specific surface area of 0.2~
Examples include fused spherical silica having a density of 3 m ² /g and a tap density of 1.36 g/cm ³ or more. The above-mentioned high-purity silica according to the present invention is produced by reacting sodium silicate and mineral acid in a mineral acid containing a chelating agent and hydrogen peroxide at an acid concentration of 1N or higher to form a silica precipitate, and then separating and recovering the silica. In the manufacturing method of high-purity silica, which consists of washing silica with a chelating agent and a mineral acid containing hydrogen peroxide, the silica precipitation reaction is carried out at a temperature of 40°C or less using flowing acid, and then the reaction is formed. Manufactured by a process of ripening substances at temperatures above 70°C. This production method is an improvement on the prior art disclosed in Japanese Patent Application Laid-open No. 12608/1983, and under the selective conditions, an unexpectedly significant increase in purity and favorable properties were found. As the sodium silicate used in the method of the present invention, a commercially available sodium silicate solution (water glass) with a molar ratio SIO ₂ /Na ₂ O of 1 to 4 can be used, but the value of the molar ratio is relatively large. 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 SiO ₂ . 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-mentioned 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 1 standard or more. Forms a silica precipitate. 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; glycolic acid, β-hydroxypropionic acid, citric acid, malic acid, tartaric acid, pyruvic acid,
Oxacarboxylic acids such as diglycolic acid; nitrile triacetic acid (NTA), nitrilolipropionic acid,
Examples include aminopolycarboxylic acids such as ethylenediaminetetraacetic acid and salts thereof. Among these, oxalic acid, citric acid, tartaric acid, or their soluble salts are particularly preferred. The amount of the chelating agent and hydrogen peroxide added is 0.1 to 5% by weight, preferably 0.1 to 2% by weight, based on the silica (SiO ₂ ) in the reaction system. If the amount of the chelating agent added is less than 0.1% by weight, the addition effect will not be sufficient;
Conversely, if it exceeds 2% by weight, the effect of addition tends to be saturated. Due to the presence of this chelating agent and hydrogen peroxide, in particular, it is a tetravalent impurity similar to Th.
Impurity components that are difficult to remove, such as Zr and Ti, can be selectively removed from silica. 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.
In either case, it is important to always maintain the acid concentration in the reaction system at 1N or higher. In the region where the acid concentration is less than 1 degree, a silica precipitate containing a large amount of impurities and having poor solid-liquid separation is formed.
Even if a subsequent acid washing operation is performed, it is difficult to sufficiently remove impurities. In the manufacturing method of the present invention, the following two points are particularly important. Using sulfuric acid as the mineral acid for the reaction. Keep the reaction temperature during silica precipitate formation below 40°C, and then ripen the resulting precipitate at a temperature of 70°C or above. When the silica precipitation reaction is carried out at about 40°C or higher,
Although dehydration and hardening of the silica precipitate occurs quickly and a slurry of the reaction product is obtained that is easy to separate into solid and liquid, impurities such as U and Th are removed, probably because the impurities inside the precipitate are difficult to elute into the liquid phase. There are limits to this. On the other hand, if the silica precipitation reaction is carried out at a temperature below 40°C, a soft precipitate with a high water content is produced, and a slurry of the reaction product becomes viscous and difficult to separate into solid and liquid. In common sense, such reaction conditions that make workability difficult should be avoided, but in the method of the present invention, sulfuric acid is used to carry out the reaction under such temperature conditions, and then the temperature is increased to 70°C. This was based on the completely unexpected fact that by heating and aging to the above-mentioned extent, a silica precipitate with extremely low impurity content and low water content could be obtained. That is, after completing the reaction at 40°C or lower, preferably near room temperature, the reaction is maintained for 15 to 60 minutes, and then the temperature is raised to 70°C or higher, preferably 75°C to 30°C near the boiling point of the system.
It is aged for ~180 minutes, but the holding time, aging time, etc. are not particularly limited. The synthetic silica obtained in this way has a BET specific surface area of 300
It is porous with a density of m ² /g or more. The silica precipitation reaction and aging treatment affect the separation of impure metal components with respect to the mutual structure of silica particles.
Anything less than m ² /g cannot substantially reach the purity targeted by the present invention. Next, the silica precipitate produced by the reaction is separated by a conventional method, and the separated silica is acid-washed with hydrogen peroxide and a mineral acid containing a chelating agent. In this case, the type of acid and type of chelating agent are the same as above, and the concentration of acid during treatment is
A regulation of 0.5 to 4 is desirable. If it is less than 0.5N, the removal of impurities attached to the silica will be insufficient, and if a strong acid exceeding 4N is used, problems will arise in neutralizing or effectively utilizing the waste acid after acid treatment. Note that the amounts of the chelating agent and hydrogen peroxide added to the acid are preferably in the range of 0.1 to 5% by weight relative to _SiO2 , as described above. As described above, in the present invention, both the reaction step to form silica precipitate and the subsequent washing step are treated with acid, but the chelating agent (or its salt may be used as necessary) and hydrogen peroxide are always used. It is necessary to carry out the process by including them, and if they 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. After the purified high-purity silica is thoroughly washed and removed by filtration, salt separation, or other methods,
Dry and collect. 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 m ² /g or more is used in an amount of 50 m ² /g or less, preferably 30 m 2 /g or less.
The method is to sinter the particles until they have a specific surface area of m ² /g or less, and then spheroidize the sintered and pulverized particles by flame melting. The particles of the fired product are suitably adjusted to have an average particle diameter of 2 to 50 μm, preferably 3 to 35 μm. Thus, according to the method according to the invention:
The tap density (TD) is 1.36 g/cm ³ or more, preferably 1.39 to 1.46 g/cm ³ , and the BET specific surface area is 0.2.
Obtained as high-purity, high-density fused spherical silica of ~3 m ² /g. This fused spherical silica is
It has properties suitable as a filler for resin sealing. [Examples] The present invention will be described below based on Examples and Comparative Examples. Example 1 A sulfuric acid aqueous solution (H ₂ SO ₄ −
23.7% by weight) was taken, and 2 g of oxalic acid (dihydrate: commercially available) and 5 g of 35% hydrogen peroxide (commercially available) were added and dissolved therein. In this sulfuric acid aqueous solution,
JIS No. 3 Sodium silicate (Na ₂ O = 9.2% by weight, SiO ₂ −
28.5% by weight) was continuously added over a period of about 20 minutes from the tip of a 1 mm diameter 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
After stirring at 30°C for 30 minutes, the temperature rose to 80°C, and the temperature rose to 80°C.
The mixture was aged by stirring for 2 hours. The silica precipitate from this matured slurry was filtered and washed repeatedly, and then separated and recovered. The separated and recovered silica was placed in an acid treatment tank equipped with a stirrer, and water and sulfuric acid were added thereto to adjust the slurry total volume to 1.7% and the sulfuric acid concentration in the liquid to be 16.6% by weight.Additionally, 2g of oxalic acid and 35% hydrogen peroxide were added. 5 g was added and heated at 85° C. for 2 hours with stirring for acid treatment. Silica was over-separated from this slurry, followed by repulping washing with water at room temperature and solid-liquid separation.
It was dried at 105°C for 2 hours. Further, a portion of the powder was calcined at 1100°C for 2 hours, and then pulverized to obtain calcined silica powder with an average particle size of 22.3 μm. The impurity content and physical properties of the fired product are shown in Table 1. Example 2 Following the operating procedure of Example 1, using 0.5 g of EDTA instead of 2 g of oxalic acid as the chelating agent,
High purity silica was produced under all other conditions similar to those in Example 1. The content of impurities in the obtained silica is also shown in Table-1. 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 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 contents of U and Th are large, and the desired high-purity silica cannot be obtained. Comparative example 2 35% by weight hydrochloric acid, 250g, water in a reaction tank with a stirrer
350 g and 0.25 g of EDTA were taken and dissolved. To this hydrochloric acid aqueous solution, 350 g of JIS No. 3 sodium silicate (92% by weight of Na ₂ O, 28.5% by weight of SiO ₂ ) was continuously dropped from a 1 mm diameter 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 45-50°C. After the reaction was completed, the slurry was aged at 50°C for 2 hours, and then 35%
After adding 3 ml of hydrogen peroxide 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 is added to it.
Add 100ml and water to make a total volume of 900ml, and
0.25 g of EDTA was added and the mixture was heated at 80° C. for 2 hours for acid treatment. After cooling to 70° C., 3 ml of 35% hydrogen peroxide was added and stirred for 10 minutes, followed by solid-liquid separation, drying, and calcination in the same manner as in Example 1 to obtain high-purity silica. The amount of impurities in the obtained silica is also shown in Table 1. As is clear from Table 1, when the reaction is carried out at 40°C or higher, the desired high purity silica cannot be obtained due to the high content of U and Th.

【table】

〔Effect 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 content is reduced by U (uranium) through the wet reaction with alium silicate and acid.
It becomes possible to reliably produce high-purity silica containing 0.02 ppb or less of thorium and Th (thorium) from relatively inexpensive raw materials through a relatively simple process. The high-purity silica of the present invention can be used as a filler for resins for IC sealants,
It is suitable for use as a raw material for high-purity silica glass for substrates, electronic materials, and semiconductor manufacturing equipment, and is particularly meaningful in that it can provide a stable supply in place of depleting resources such as high-quality natural silica sand and crystal. It is something.

Claims (1)

[Scope of Claims] 1. Silica gel obtained by acid decomposition of alkali silicate (wet method) or molten synthetic silica derived from this, U (uranium) exhibiting α-radiation.
and Th (thorium) content is 0.02ppb 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 fused spherical silica having a BET specific surface area of 0.2 to 3 m ² /g and a tap density of 1.36 g/cm ³ or more. 4 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 mixed with a chelating agent and hydrogen peroxide. In the manufacturing method of high-purity silica, which consists of washing with mineral acid, 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: 5. The method for producing high-purity silica according to claim 4, wherein the chelating agent is dicarboxylic acid, polycarboxylic acid, oxycarboxylic acid, aminopolycarboxylic acid, or a salt thereof. 6. The method for producing high-purity silica according to claim 4, wherein the 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. 7. A method for producing high-purity silica, which comprises flame-melting the synthetic silica obtained by the production method according to claim 4 through a firing step such that the BET specific surface area becomes 50 m ² /g or less.