JPH075289B2 - Method for producing low-thorium high-purity silica - Google Patents

Description

TECHNICAL FIELD The present invention relates to a method for producing high-purity silica produced from alkali silicate. More specifically, high-purity silica with extremely low thorium content, which is suitable for resin fillers for IC encapsulants, substrates, high-purity silica glass and quartz glass for electronic materials and semiconductor manufacturing equipment, optical glass raw material applications, etc. To a method of manufacturing.

[Conventional technology]

In recent years, with the rapid development of the electronic industry, high-purity silica has come to be used for electronic materials, semiconductor manufacturing, etc., but with the sophistication of products, the demand for high-purity silica has further increased. . For example, silica powder, which is used as a filler for IC encapsulant resin, is required to be highly purified to avoid soft error problems as the degree of integration of ICs increases. Is becoming. In the case of an IC having a high degree of integration of 1 megabit or more, the filler silica for the encapsulant is required to have a purity of 0.1 ppb or less in the content of uranium (U) and thorium (Th).

As a method for producing such high-purity silica, a method of selecting white silicate and quartz with a small amount of uranium and thorium, melting and pulverizing the refined high-purity rough stone, and a silica source such as silicon tetrachloride and tetraethyl silicate are used. A method of vapor-phase decomposition, a method of hydrolyzing these and firing, and the like are known. However, with the former method of using natural raw materials such as white silica stone and quartz, it is becoming increasingly difficult to obtain raw materials of high quality, while on the other hand, the raw materials such as silicon tetrachloride and tetraethyl silicate used in the latter method are becoming more difficult. Is corrosive and flammable, so special handling is required.
It has the drawback of being extremely expensive in terms of equipment and operation.

[Problems to be Solved by the Invention]

So far, such high-purity silica has not been obtained by the wet method by the reaction of alkali silicate and acid. Further, there are very few literatures or patents that mention thorium in silica regarding a method for producing silica using alkali silicate as a starting material. This is α emitted from thorium
It is considered that the malfunction of the IC caused by the line (soft error) has been regarded as a problem relatively recently, and the difficulty of the trace analysis of thorium in silica is difficult. For example, in activation analysis, the lower limit of quantification of thorium in silica is 1 ppb.
It is said that the behavior of thorium below 0.1 ppb is a completely unknown field. Recently, ICP-MS
With the progress of the analytical technique using, etc., it became possible to quantify thorium of 0.1 ppb or less, but no method for removing thorium from silica using alkali silicate as a starting material was disclosed at all.

[Means for Solving the Problems]

The present invention is a silica produced by a wet method of an alkali silicate and an acid, and a method for producing high-purity silica capable of purifying the content of a thorium impurity harmful as an α-ray emitting element to 0.05 ppb or less. It is provided. The present invention corresponds to the improved invention of the invention of JP-A-62-12608 filed by the applicant.

That is, the method for producing low-thorium high-purity silica according to the present invention is a method in which a hydrous silica precipitate is produced by a reaction between sodium silicate and a mineral acid. The silica-containing precipitate is generated in the above, and the hydrous silica precipitate separated and recovered is then aged in an aqueous sulfuric acid solution having an acid concentration of more than 3 N and in the presence of a chelating agent and hydrogen peroxide, so that the specific surface area of the silica is higher than that before the aging. The characteristic feature is to make it low.

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

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, the aqueous solution of sodium silicate and the 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 to obtain silica. One of the characteristics is that a precipitate of

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-1,2,3, -tetracarboxylic acid, butane-1,2,3 Polycarboxylic acids such as 4,4-tetracarboxylic acid; glycolic acid, β-hydroxypropionic acid,
Oxycarboxylic acids such as citric acid, malic acid, tartaric acid, pyruvic acid, diglycolic acid; nitrile triacetic acid (NT
A), aminopolycarboxylic acids such as nitrilolipropionic 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 the chelating agent and the hydrogen peroxide added is 0.1 to 5% by weight, preferably 0.1 to 5% by weight, based on the silica (SiO ₂ ) in the reaction system.
It is 0.1 to 2% by weight. 0.1% by weight of chelating agent
If it is less than the above range, the effect of addition is not sufficient. Due to the presence of the chelating agent and hydrogen peroxide, it is possible to selectively remove impurities such as zirconium and titanium, 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 any case, the acid concentration in the reaction system is always maintained at 1 N or more. It is important to. In a region where the acid concentration is less than 1 N, a large amount of impurities are strongly contained and a precipitate of silica having a poor solid-liquid separation property is generated, and the impurities can be sufficiently removed even if a subsequent washing operation with an acid is performed. Have difficulty.

Regarding the temperature during the reaction, the range of normal temperature to 50 ℃ is good, 50 ℃
If it exceeds, the surface of the obtained silica precipitate is rapidly hardened, and the impurities are not sufficiently diffused into the liquid phase. After completion of the reaction, it is desirable to continue the aging stirring for a while while keeping the temperature as it is or raising the temperature. The silica precipitate thus obtained is separated by a conventional method.

In the silica separated and recovered by the reaction, uranium usually has a content of 0.5 ppb or less, while thorium does not, although it varies depending on the coverage of sodium silicate and the processing conditions.
It is included as much as around 1 ppb. The difference in the amount of impurities between uranium and thorium is considered to be mainly due to the difference in solubility in acid.

Next, in the present invention, it is an essential step to age the separated and recovered hydrous silica precipitate in an aqueous sulfuric acid solution having an acid concentration of more than 3 N to make the specific surface area of silica lower than that before aging. In this process, the specific surface area of silica is 10% more than that before aging.
It is preferable to lower the above, and by this operation, it becomes possible to reduce the thorium in the silica to 0.05 ppb or less.

The concentration of the sulfuric acid aqueous solution used for aging needs to be set to an acid concentration exceeding 3 N. If it is 3 N or less, the effect of aging becomes insufficient, and it becomes difficult to reduce thorium to 0.05 ppb or less. The temperature during aging is not particularly limited, but is preferably 70 to 100 ° C., and is preferably the same as or higher than the temperature during reaction. Before and after aging, the water content of the silica precipitate, the sedimentation volume, etc. change with the specific surface area, and the degree thereof tends to increase as the difference between the acid concentration at the end of the reaction and the sulfuric acid concentration at aging increases.

When the specific surface area of silica is 10% or more lower than that before aging by such aging, thorium in the silica becomes 0.05 ppb or less, and at the same time, other metal ions are substantially removed,
High-purity silica containing almost no impurity metals. In addition, it is necessary to add a chelating agent and hydrogen peroxide during aging with sulfuric acid, and the type and concentration in this case are the same as in the reaction. Although such a mechanism of action is not clear in detail, it is considered that thorium that could not be completely removed in the reaction step is not attached or adsorbed on the silica surface, but is occluded inside the precipitate, The only way to remove the thorium inside the precipitate is to remove impurities by rearrangement of the bonds inside the precipitate by heat aging with sulfuric acid at a concentration exceeding 3 N, and to reduce the water content of the silica precipitate by aging. , Changes such as decrease in sedimentation volume and decrease in specific surface area appear. In the case of aging with acids other than sulfuric acid, the removal effect of thorium is small and thorium cannot be reduced to less than 0.05 ppb.

Needless to say, this aging with sulfuric acid is not limited to one time, but may be performed twice or more as necessary due to its nature.

The silica thus aged is solid-liquid separated,
The water-containing silica cake is washed with water and then dried and recovered, or if necessary, then calcined and / or melted to be recovered as high-purity silica. After the aging treatment, the sulfuric acid recovered by solid-liquid separation can be circulated for the next reaction to effectively utilize the sulfuric acid.

[Action]

According to the method of the present invention, first, a step of reacting an aqueous solution of sodium silicate with a mineral acid in an acidic region containing a chelating agent and hydrogen peroxide and having an acid concentration of 1 N or more is performed to remove impurities easily by a cleaning treatment. Precipitated silica with good liquid separation can be produced. However, in the silica separated and recovered through this reaction, although there are differences depending on the history of sodium silicate as a raw material and the processing conditions, uranium is usually reduced to 0.5 ppb or less, whereas thorium is reduced. The content of 1 remains as high as around 1 ppb. The reason for the difference in the contents of uranium and thorium is considered to be due to the difference in the solubility of both substances in acid.

Thorium remaining in the above reaction step is effectively removed by lowering the specific surface area of silica than before aging through the subsequent aging treatment with a sulfuric acid aqueous solution, and by reducing the reduction rate of the silica specific surface area to 10% or more. Thorium content of 0.
It can be removed to less than 05ppb.

Although the mechanism of this action has not been elucidated in detail, the thorium that could not be completely removed in the reaction step is not attached or adsorbed on the surface of silica but is occluded inside the precipitate. It is considered that the removal is extremely effectively performed by rearranging the bonds inside the precipitate through heat aging with a sulfuric acid aqueous solution of a constant concentration, and reducing the water content, sediment volume, specific surface area, etc. of the silica precipitate. Therefore, it is impossible to selectively and effectively remove thorium up to 0.05 ppb or less by aging treatment with an acid other than sulfuric acid.

〔Example〕

 Hereinafter, the present invention will be described based on examples.

Example 1 2125 g of water and 875 g of 98% sulfuric acid were placed in a reaction vessel (5 l) equipped with a stirrer, and 5 g of oxalic acid (dihydrate: commercially available product) and 15 ml of 35% hydrogen peroxide (commercially available product) were added and dissolved. The liquid temperature was kept at 25 ° C.
While stirring this sulfuric acid aqueous solution, 1750 g of JIS No. 3 sodium silicate (Na ₂ O 9.2 wt%, SiO ₂ 28.5 wt%) was added thereto over a period of about 45 minutes to generate a silica precipitate. The liquid temperature at the end of the addition rose to 33 ° C. After the addition is complete, continue stirring at the same temperature for 30 minutes, then raise the temperature to 80 ° C, and
The reaction was completed by stirring at ℃ for 2 hours. The sulfuric acid concentration of the slurry at the end of the reaction was about 3.2N. The silica precipitate from this reaction-completed slurry was repeatedly washed, and then separated and collected. Put the separated and recovered silica in the aging tank with a stirrer, and add 600 ml of water and 98% sulfuric acid to make a total volume of 4800 ml.
Further, add 5 g of oxalic acid and 15 ml of 35% hydrogen peroxide solution, and stir at 85 ℃.
Aged for 2 hours. After completion of the aging, the slurry was allowed to stand and the sulfuric acid was collected by a siphon. Water was added to the hydrous silica cake for repulp washing, followed by solid-liquid separation, and further drying to recover high-purity silica. Table 1 shows the analytical values of the obtained silica.
Shown in.

By aging with sulfuric acid, the specific surface area of silica is reduced by 13%,
It can be seen that thorium has dropped below 0.05 ppb.

Examples 2, 3 and Comparative Example The reaction was terminated under the same composition and conditions as in Example 1, and silica was separated and recovered. The silica was aged by changing the sulfuric acid aqueous solution concentration, and aged at 85 ° C. for 2 hours with a sulfuric acid concentration of 1 N (comparative example) and 5 N (example 2). In the same manner as in Example 1, 1 wt% of oxalic acid and hydrogen peroxide were added to the aged slurry with respect to SiO ₂ . After aging, high-purity silica was recovered in the same manner as in Example 1. Table 2 shows the analytical values of the obtained silica.

When the sulfuric acid concentration during aging exceeds 3 N, the specific surface area is 10% or more lower than that before aging, and the thorium content is reduced to 0.05 ppb or less. The decrease was small, and the thorium content exceeded 0.05 ppb.

Example 4 To 3000 g of the sulfuric acid aqueous solution (H ₂ SO ₄ 22.9 wt%) recovered from the sulfuric acid aging completed chiller in Example 1, 231 g of 98 wt% sulfuric acid was added, and further 10 ml of hydrogen peroxide solution was added to keep the liquid temperature at 25 ° C. It was As in Example 1, JIS No. 3 sodium silicate Na ₂ O 9.2 wt
%, SiO ₂ 28.5 wt%) 1750 g was added over about 45 minutes,
High-purity silica was recovered after the reaction and aging with sulfuric acid in the same manner as in Example 1. Table 3 shows the analytical values of the obtained silica.

There is no problem when the reaction is carried out using sulfuric acid recovered from the aged slurry, and it is clear that sulfuric acid can be effectively used.

In each of the examples and comparative examples, uranium was analyzed by a fluorescence method and thorium was analyzed by a concentration-ICP analysis method, and a specific surface area was determined by a BET method.

〔The invention's effect〕

According to the method for producing high-purity silica according to the present invention, using inexpensive sodium silicate as a raw material, the high-purity silica in which the contents of uranium and thorium which are α-ray emitting elements are reduced to 0.05 ppb or less is reproducibly industrially produced. Can be advantageously manufactured.

And the obtained high-purity silica 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, so it can be used instead of them. In addition, it is possible to provide a stable supply 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 has epoch-making significance for Japan, which is poor in silica resources.

Claims (2)

[Claims] 1. A method for producing a hydrous silica precipitate by reacting an aqueous solution of sodium silicate with a mineral acid, wherein the silica precipitate is produced in an acidic region having an acid concentration of 1 N or more in which a chelating agent and hydrogen peroxide are present, Next, the separated and recovered hydrous silica precipitate is aged in an aqueous solution of sulfuric acid having an acid concentration of more than 3N in the presence of a chelating agent and hydrogen peroxide to make the specific surface area of silica lower than that before aging. Method for producing high-purity silica. 2. The method for producing low-thorium high-purity silica according to claim 1, wherein the sulfuric acid separated and recovered from the aged slurry is reused in the reaction.