JPH0764535B2 - Method for removing radioactive elements from metallic silicon - Google Patents

Description

TECHNICAL FIELD OF THE INVENTION The present invention relates to a metal silicon which is suitable as a raw material for directly synthesizing silicon dioxide (silica) from metal silicon and which can obtain inexpensive and highly pure metal silicon. The present invention relates to a method for removing radioactive elements.

2. Description of the Related Art In recent years, an epoxy resin composition in which a large amount of silicon dioxide (silica) powder is filled in an epoxy resin is used as an IC or LSI sealing material.

However, some silica powders may contain a large amount of radioactive elements such as uranium and thorium. Therefore, IC and LSI highly integrated with epoxy resin compositions manufactured using this type of silica powder There is a problem that the memory malfunctions due to alpha rays when the is sealed. Therefore, conventionally, in order to produce such a silica containing no radioactive element, for example, JP-A-60-81011 discloses a method for producing high-purity fused silica using natural high-purity quartz as a raw material. No. 61-190556 discloses a sol-gel method using a high-purity silicon compound as a raw material, or a method for producing high-purity silica by hydrolysis / thermal oxidation, and JP-A-58-168267 discloses natural high-purity silica. A method for highly purifying silica by chemical treatment, and further, JP-A-60-42217 discloses that high-purity silica is obtained by treating water glass as a raw material with an ion exchange resin, and then gelling and baking the raw material. A method of manufacturing has been proposed.

However, even in such a method, expensive high-purity metallic silicon must be used as a raw material in order to reduce the content of radioactive elements such as uranium in the silica powder. Silica powder was very expensive and had low versatility. Therefore, it has been desired to supply inexpensive high-purity metallic silicon.

The present invention has been made in view of the above circumstances, and it is possible to inexpensively obtain high-purity metallic silicon suitable as a raw material for directly synthesizing silica from metallic silicon. A radioactive element is removed from metallic silicon. The purpose is to provide a method.

Means and Actions for Solving the Problems As a result of extensive studies conducted by the present inventor to achieve the above object, a metal silicon powder, particularly a metal silicon powder pulverized to a particle size of 200 microns or less, an aqueous solution of a mineral acid, preferably 0.1 By washing with a combined use of a compound having an alcoholic hydroxyl group in an aqueous solution of a mineral acid of a prescribed amount or more, radioactive elements in the metal silicon powder can be easily removed, and the content of uranium and thorium is 1 ppb or less of high purity. It was found that metallic silicon can be obtained at a low cost, and that this high-purity metallic silicon is suitable as a raw material for directly synthesizing silica from metallic silicon, and has completed the present invention.

Therefore, the present invention is a method of removing radioactive elements from metallic silicon, characterized in that the metallic silicon powder is washed with an aqueous solution of mineral acid, and the contents of uranium and thorium in the metallic silicon powder are each ppb or less. Further, when the metallic silicon powder is washed with an aqueous solution of a mineral acid, the above method is used, in which an aqueous solution of a mineral acid of 0.1 N or more is used and a compound having an alcoholic hydroxyl group is used in combination.

Hereinafter, the present invention will be described in more detail.

The metallic silicon used in the method of the present invention has a silicon content of 99.5
Any material may be used as long as it is at least%. For example, those manufactured in Brazil, Norway, China, etc. can be used.

Further, the metallic silicon powder is previously pulverized by a ball mill, an impact pulverizer or the like in order to enhance the cleaning effect by the mineral acid aqueous solution, and has a particle size of 200 microns or less and an average particle size of 5 to 50 microns, especially a particle size of 150 microns or less. It is preferable to use a powder having an average particle size of 10 to 30 microns.

As the aqueous solution of mineral acid, for example, an aqueous solution of sulfuric acid, hydrochloric acid, nitric acid or the like is preferably used, and a mixed aqueous solution of these mineral acids may be used. Of these mineral acid aqueous solutions, nitric acid aqueous solutions are preferable. This is because even if nitric acid remains in metallic silicon, it is exposed to high temperatures in the next oxidation step, and is oxidized and volatilized.

Furthermore, the acid concentration of the mineral acid aqueous solution is preferably 0.1 normal or more and 5 normal or less, and if it is less than 0.1 normal, radioactive elements such as uranium may not be removed. The powder yield may decrease.

In this case, the washing of the metallic silicon powder with the aqueous solution of mineral acid may be carried out by mixing the metallic silicon powder and the aqueous solution of the mineral acid at room temperature for reaction removal, but in order to remove radioactive elements efficiently with a low concentration of mineral acid. It is better to mix and stir at 50 ° C or higher to react. Although the reaction time depends on the temperature and the concentration of the metal silicon powder, the radioactive element in the metal silicon powder can be reduced to 1 ppb or less in 1 to 20 hours.

In this case, radioactive elements such as uranium in metallic silicon are present in a large amount in the flow field of metallic silicon. Therefore, in order to efficiently remove radioactive elements such as uranium from metallic silicon, the aqueous solution of mineral acid penetrates into the grain boundaries. Preferably. Therefore, in the present invention, when the metallic silicon is washed with the aqueous solution of mineral acid, it is preferable to add a compound having an alcoholic hydroxide group.

Here, as the compound having an alcoholic hydroxyl group,
For example, glycol compounds such as ethylene glycol, diethylene glycol, triethylene glycol, tetraethylene glycol, propylene glycol, dipropylene glycol and tripropylene glycol, monomethyl ethers of these glycol compounds, monoalkyl ether compounds such as monoethyl ether, and these glycol compounds. It can be used for organic compounds that do not contain alkali metals such as ether type surfactants such as dialkyl ether compounds (so-called glymes), and water such as methanol, ethanol, isopropyl alcohol, n-propyl alcohol, butanol, erythritol and glycerin. Soluble lower alcohols and polyhydric alcohols are preferred.

Furthermore, the addition amount of the compound having an alcoholic hydroxyl group is
0.01 or more and 1 or less (weight ratio) relative to 1 aqueous mineral acid solution, especially
It is preferable to set it to 0.05 or more and 0.5 or less, and if it is less than 0.01, radioactive elements may not be sufficiently removed, and if it exceeds 1, it will be disadvantageous in terms of cost, and if the concentration of mineral acid is not increased, it will be diluted. In some cases, the effect of removing a large amount of radioactive elements may not be achieved. In general, the mineral acid concentration is lowered by adding a compound having an alcoholic hydroxyl group. Therefore, it is preferable to use an aqueous solution of mineral acid having a high acid concentration in advance.

The metallic silicon may be sufficiently wet with an alcohol such as methanol and then treated with an aqueous solution of mineral acid.

After treating the metallic silicon powder with the mineral acid aqueous solution, the mineral acid aqueous solution is removed by a centrifuge or filtration, and then repeatedly washed with ion-exchanged water and pure water until the pH of the washing solution becomes 6 or more. The metal silicon powder is dried with a dryer such as a dryer or a spray dryer. As a result, a high-purity metallic silicon powder having a uranium and thorium content of 1 ppb or less can be obtained.

Effects of the Invention According to the method of the present invention, radioactive elements can be easily removed from metallic silicon, and highly pure metallic silicon having a uranium and thorium content of 1 ppb or less can be obtained at low cost.

Therefore, by using the high-purity metallic silicon obtained by the method of the present invention, it is possible to inexpensively produce high-purity silica having a uranium and thorium content of 0.5 ppb or less, and the high-purity silica is an epoxy resin. It can be used as a filler for a composition or the like, and such a composition is suitable as a sealing material for highly integrated ICs.

Hereinafter, the present invention will be specifically described with reference to Examples and Comparative Examples, but the present invention is not limited to the following Examples.

Example 1 Metallic silicon crushed to a maximum particle size of 150 microns and an average particle size of 35 microns (silicon content 99.65%, uranium content 10 ppb, thorium content 4.9 ppb, iron content 0.15%, calcium content 0.02% , Aluminum content 0.2%), 1N aqueous nitric acid solution (500 ml) and methanol (100 ml) were added, and the mixture was stirred at 60 ° C. for 1 hour. After the reaction was completed, the aqueous solution was removed using a centrifuge and then washed with pure water. High-purity metallic silicon was obtained by drying after washing. As a result of measuring the uranium and thorium contents in this metallic silicon, the uranium content was 0.4 ppb,
The thorium content was 0.3 ppb.

[Examples 2 to 5, Comparative Example] Using 100 g of the metal silicon used in Example 1, the reaction was carried out under the reaction conditions shown in Table 1 to highly purify the metal silicon. The results are also shown in Table 1.

From the results of Table 1, it was confirmed that the method of the present invention can efficiently remove uranium and thorium from metallic silicon.

Claims (2)

[Claims] 1. The metallic silicon powder is washed with a mineral acid aqueous solution to adjust the contents of uranium and thorium in the metallic silicon powder.
A method for removing radioactive elements from metallic silicon, characterized in that the content is 1 ppb or less. 2. When the metallic silicon powder is washed with an aqueous solution of mineral acid,
The method for removing a radioactive element from metallic silicon according to claim 1, wherein a mineral acid aqueous solution having a normality of 0.1 N or more is used together with a compound having an alcoholic hydroxyl group.