JPH03131511A - High-purity silica and production thereof - Google Patents

Abstract

PURPOSE:To reduce metal impurities by reacting an alkali silicate in a specific concentration with a mineral acid at a prescribed temperature for a prescribed time. CONSTITUTION:An aqueous solution of an alkali silicate in <=5wt.% SiO2 concentration is added and mixed with a mineral acid such as sulfuric acid so as to provide >=15wt.% mineral acid concentration. Reaction is then carried out at a reaction temperature for a reaction time expressed by the formula [T( deg.C) is the temperature for producing SiO2; t (hr) is the time required for the production]. After completing the reaction, the reaction product is then filtered, washed with water and burned to afford high-purity SiO2 in >=90% yield. The formed high-purity SiO2 has respective impurity contents as follows. <=0.1wt.ppb U and Th and <=1wt.ppm Fe, Al, Ti, Na and K.

Description

[Detailed Description of the Invention] [Industrial Application Field] The present invention relates to a technology for producing high-purity silica produced from alkali silicate, particularly high-purity silica with extremely low levels of metal impurities including harmful impurities such as U and Th. It is something.

[Prior Art] High-purity silica is in high demand in the field of electronic components, and silica with higher purity is increasingly required.

Elements contained in silica whose impurity levels are particularly required to be reduced include U and Th, and the reason for this is that these elements emit alpha rays and cause memory errors. Therefore, it is desirable to completely eliminate U and Th in silica used as a sealant for highly integrated ICs (it is required that the content of these elements be 0.1 wtppb or less. Silica can be roughly divided into natural silica made directly from crystals, silica, etc., and synthetic silica made from silicon halides, alkoxysilanes, or alkali silicate.Natural silica contains U and Th contained in the raw materials. Since there are many
Zero synthetic silica, which remains around vtppb, is more suitable for reducing the content of these impurities. When synthesizing using silicon halide or alkoxysilane as raw materials,
After hydrolyzing these yKfl, silica is obtained by calcining or by vapor phase decomposition. However, silicon halides and alkoxysilanes are expensive, corrosive and flammable, and require special care when handling, resulting in extremely high product prices. The cost of raw materials for alkali silicate is low, and by devising the manufacturing process, inexpensive products can be expected. In the manufacturing method using an alkali silicate as a raw material, there is also a method of preventing the precipitation of these elements using a reagent such as a chelating agent in order to avoid contamination of U Th during the production of silica. However, since the reagents used in this method are expensive, the manufacturing cost increases and it is difficult to put it into practical use industrially. For example, there is a proposal for a manufacturing method that takes practical aspects into consideration, as found in Japanese Patent Application Laid-Open No. 62-283809. In this proposal, sodium silicate with a concentration of 30 wt% in terms of 5i02 is used as a raw material, and this is spun in an acidic solution to form a fibrous silica gel, and impurities are removed by acid treatment and water washing to obtain a high-purity product.

[Problems to be Solved by the Invention] This method does not require any special reagents and can be expected to have reasonable manufacturing costs. However, U, Th in the obtained product
The content of PPI is reduced to only a few tenths of wtPPI), and a purity high enough to meet the requirements cannot be obtained.

This invention was made to solve such problems, and it can produce U, Th at low manufacturing cost without using special chemicals.
The purpose of this invention is to provide high-purity silica containing extremely few impurities such as.

[Means for Solving the Problem] The means for achieving this object is to reduce the impurity content of uranium and thorium to 0 and 1 wLppb or less, respectively,
Moreover, the impurity content of iron, aluminum, titanium, sodium, and potassium is each 1 wtppII! The following are high-purity silica and a method for producing such high-purity silica. In this production method, when producing high-purity silica by producing silica through a reaction between an alkali silicate and a mineral acid, the concentration of the alkali silicate used as a raw material is 8102.
The concentration is 5wt% or less, and the mineral acid concentration to be reacted is 15%.
This is a method for producing high-purity silica, in which silica is produced by mixing both of them so that the amount is at least % wt%.Furthermore, when producing silica, the production temperature is T ('C), and the time required for production is t (hours). ), it is more desirable to generate it under the following conditions: tT"'≧1150...[1]

[Working] When an alkali silicate is added to a mineral acid, the silicate ions are dehydrated by condensation to form silica and precipitate. In this process, when we investigate the behavior of U and Th, which are most desired to be removed, we find that [U 02 (C0
s)s]'-1 and [Th(Co31s]'
It exists as soluble carbonate complex ions such as -, and in acid solutions as soluble ions such as UO22+ and Th'+.

However, in the transitional neutral region where the alkali changes to the acid, it becomes an insoluble compound such as hydroxide. Most of these insoluble compounds become the above-mentioned soluble ions under strong acidity, but a small portion remains insoluble and is incorporated into the silica network, which remains without being filtered out.
It is the main cause of impurity contamination. This invention was made based on this idea. That is, the lower the silicic acid concentration (hereinafter referred to as 5i02 concentration) at the time of silica production, the lower the rate of U and Th incorporated. When this was confirmed quantitatively, the results shown in FIG. 1 were obtained. U in the silica produced by keeping the concentration of a 1:1 mixed acid of sulfuric acid and nitric acid constant at 20 wL% and changing the concentration of sodium silicate as a raw material.

Figure 1 shows the results of examining changes in the Th content, where the vertical axis is U, the Th content, and the horizontal axis is the sodium silicate concentration.The graph represents Th and the circle represents U. It is. Th is more difficult to remove than U, but as the silicic acid concentration decreases, U,
The content rate of both Th decreases, and when the sodium silicate concentration reaches about 5 wt%, the Th concentration also decreases to 0.1 vtppb. Furthermore, even if the alkali silicate concentration is lowered to 1 wt% or less, the effect gradually decreases.As for the mineral acid concentration,
The higher the concentration, the lower the U and Th impurities should be, and this was actually confirmed. This result is shown in Figure 2, where the vertical axis is tJ, the content of Th, the horizontal axis is the mineral acid concentration, the graph represents Th, and the circle represents U. . When the acid concentration becomes about 15 wt%, the content of Th, which is difficult to remove, also becomes lower than 0.1 vtppb. Even if the acid concentration is further increased to 40 wt% or more, the effect does not change much.

In this way, when the alkali silicate and mineral acid are reacted, if the concentration of the alkali silicate is 5 wt% or less, and the concentration of the mineral acid to be reacted is 15W 7% or more, the content of U and Th is 0, 1. wtppb or less. When the alkali silicate concentration is 5 wt% or more, the Th content is Q
, lwtppb, and even if the mineral acid concentration is 15wt%, it is still
There is a risk that the a degree may exceed O, L wtppb. Furthermore, since the acid concentration is as high as 15 wt% or more, Fe, AJ
, Ti, Na, etc. are also sufficiently dissolved and separated by filtration from the silica precipitate, so these impurities are also l vt
ppm or less.

Regarding the temperature at the time of silica production as well as the concentration, the higher the temperature, the higher the ability of the mineral acid to elute impurities, and the longer the production time, the more impurities are removed. FIG. 4 shows the results of examining the U content by varying the formation temperature and time. A comparison is made between a case where the production temperature is go'c and a case where the production temperature is 40°C, and the higher the temperature, the faster the impurities are removed. Furthermore, the generation time affects the yield of silica;
In order to increase the yield of high-purity silica, the lower the production temperature, the longer the time required. Figure 5 shows the results of investigating the yield by varying the production temperature and production time. When the production temperature is 80°C, the yield in 2 hours exceeds 90%, but when the production temperature is 20°C, it takes 15 hours to obtain a 90% yield.

That is, in order to increase the yield, it is necessary to increase the production temperature and prolong the production time. FIG. 3 shows the range of conditions for ensuring a yield of 90% or more. The figure shows the production temperature on the vertical axis and the production time on the horizontal axis, with ○ indicating a yield of 90% or more.
If it is less than 90%, it is indicated by a 3Δ mark.

The boundary of the area where the ○ marks are distributed is approximately represented by a curve A, and when this curve A is approximated by a mathematical formula to indicate this area, the formation temperature is T ('C), the time required for formation is t (hour), tTO7≧130...[1], that is,
In this range of conditions, high purity silica can be produced with a yield of 90% or more.

[Example] (Example 1) 20wt% sulfuric acid 25j was placed in a reaction vessel equipped with a stirrer, and No. 3 sodium silicate specified in JIS-1408 was diluted with distilled water to form SiO□. Concentration: 15wt%
51 was added with stirring. After the addition, the reaction vessel was maintained at 60°C to 70°C to ensure the reaction was fully completed, and the perilla precipitate was separated by filtration, washed with distilled water, and then calcined at 1200°C for 1 hour. The silica thus obtained was analyzed for impurities. Regarding impurity analysis, U and Th were determined to be Fe by ICP mass spectrometry.

Aρ and Ti were measured by TCP luminescence method, and Na was measured by atomic absorption method.

The measurement results are shown in Table 1 together with the results of other Examples and Comparative Examples in which the acid and alkali silicate conditions were changed.

Example N [L2 to Example No. 6 are examples manufactured by the same method as Example 1 by changing the acid concentration and the type and concentration of alkali silicate, and Example Nα7 to Example Na
Examples up to No. 12 are examples manufactured in the same manner as Example NIL1 to Example No. 6 using still another mineral acid.

Comparative examples are examples manufactured under conditions outside the range of conditions of the present invention with regard to mineral acid concentration or alkali silicate concentration.

In all examples in Table 1, both U and Th are 0.1wtpp.
Silica with a content of less than b is obtained, and Fe, A! J
Regarding other impurities such as , Ti, Na, etc.
The content is less than pp. However, in the comparative examples, the silica has a Th content of 0.1 wtppb or more, and other impurities have a content of 1 wtppm or more. Further, the impurity content obtained in the examples is the impurity content U, that is, the impurity content of conventional silica obtained by allowing acid to act on alkali silicate.

Th is a tenth of a number wtppb - other impurities are a few vtppm
This is an epoch-making decrease compared to the previous year.

The purity of the silica produced in this way is extremely high;
In particular, since both U and Th are below 0.1 wtppb, the adverse effects of α rays can be eliminated, and Fe, Aj
Since there are very few impurities such as L, Ti, and Na,
It also has good physical properties such as thermal strain, thermal conductivity, and electrical conductivity.

(Example 2) High purity silica was produced in the same manner as in Example 1 and impurities were measured, but the yield was examined by changing the production temperature and production time. Note that the generation time includes the time required for adding the alkali silicate and the time for maintaining the temperature at a constant temperature while stirring only after that, so to speak, the aging time. The results are shown in Table 2 together with comparative examples.

The yield is 90% or more in all of the Examples in Table 2, but it is less than 90% in the Comparative Examples.Also, the impurity content is low in the Examples with an average of 0.04wtppb of U and 0.07% of Th. , 0,06.1 and Owtppb in the comparative example, respectively.
It has increased slightly.

In addition, in order to secure a yield of 90% or more when the production temperature is less than 20°C, the production time must be extended beyond 15 hours.The higher the production temperature, the faster the impurities can be reduced and the yield increased. However, in actual operation, at temperatures exceeding 80°C, acid fume generation,
Changes in concentration due to water evaporation increased, making operation difficult.

[Effects of the Invention] As described above, according to the present invention, the alkali silicate concentration of the raw material is regulated and the silica production reaction is performed in a state of high acid concentration, so the silica production reaction can be performed without using special reagents or methods. The amount of undissolved impurities is extremely small, so U and Th are 0.
, 1 wLppb or less, for other impurities l wLpp
High purity silica with a purity of less than m can be obtained in high yield and at low cost. In the field of electronic components, there is an increasing demand for high-purity silica.
The effects of this highly practical invention are very large.

[Brief explanation of the drawing]

Figure 1 is a diagram showing the relationship between raw material concentration and impurity content for detailed explanation of the invention, Figure 2 is a diagram showing the relationship between acid concentration and impurities, and Figure 3 is a diagram showing the relationship between production conditions and yield. FIG. 4 is a diagram showing the relationship between production time and U content, and FIG. 5 is a diagram showing the relationship between production time and yield.

Claims (3)

[Claims] (1) Impurity content of uranium and thorium is each 0.1
wtppb or less, and iron, aluminum, titanium,
Impurity content of sodium and potassium is 1wtp each
High purity silica characterized by having a content of pm or less. (2) In a method for producing high-purity silica in which silica is produced by a reaction between an alkali silicate and a mineral acid, silica is produced by setting the alkali silicate concentration to 5 wt% or less in SiO_2 concentration and the mineral acid concentration to 15 wt% or more. A method for producing high-purity silica, characterized by: (3) When generating silica, the generation temperature is T (℃),
Letting the time required for generation be t (time), tT^1^. ^
The method for producing high-purity silica according to claim 2, wherein the production is performed under the following conditions: 4^5≧1150...[1].