JP2013203641A - Synthetic amorphous silica powder and production method therefor - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a synthetic amorphous silica powder suited as a raw material for synthetic silica glass articles which undergoes very little bubble formation or expansion in use under an environment of a high temperature and a diminished pressure, and its production method.SOLUTION: There is provided a synthetic amorphous silica powder which is a synthetic amorphous silica powder obtained by subjecting a granulated silica powder to spherization treatment and performing washing and drying, wherein the mode Dof the volume-based particle size distribution is 50-200 μm, its frequency is 2-5%, the number-based arithmetic mean particle diameter Dis 0.4×D±10 μm, the mode Dof a number-based particle size distribution is 5-10 μm, its frequency is 3-10%, the tapped bulk density is ≥1.38 g/cmand ≤1.50 g/cm, the circularity is ≥0.85 and ≤1.00, the value resulting from the division of the BET specific surface area by the theoretical specific surface area is 1.00-1.35, the true density is 2.10-2.20 g/cm, and the intraparticle porosity is 0.00-0.05.

Description

  The present invention relates to a high-purity synthetic amorphous silica powder suitable as a raw material for producing synthetic silica glass products such as jigs and crucibles used under high temperature and reduced pressure environments in the semiconductor industry and the like, and a method for producing the same.

  Conventionally, crucibles and jigs used in the production of single crystals for semiconductor applications have been produced using quartz powder obtained by pulverizing and refining natural quartz or silica sand as a raw material. However, natural quartz and silica sand contain various metal impurities, and even if the above purification treatment is performed, the metal impurities cannot be completely removed, so that the purity is not satisfactory. In addition, as semiconductors are highly integrated, quality requirements for single crystals as materials have increased, and high-purity products are required for crucibles and jigs used in the production of single crystals. Therefore, synthetic silica glass products using high-purity synthetic amorphous silica powder as a raw material instead of natural quartz or silica sand have attracted attention.

  As a method for producing this high-purity synthetic amorphous silica powder, high-purity silicon tetrachloride is hydrolyzed with water, and the resulting silica gel is dried, sized and fired to obtain a synthetic amorphous silica powder. A method is disclosed (for example, refer to Patent Document 1). Also disclosed is a method for obtaining a synthetic amorphous silica powder by hydrolyzing an alkoxysilane such as a silicate ester in the presence of an acid and an alkali to form a gel, and drying, pulverizing and firing the resulting gel. (For example, refer to Patent Documents 2 and 3.) The synthetic amorphous silica powder produced by the method described in the above Patent Documents 1 to 3 has a higher purity than natural quartz and silica sand, and is used to synthesize crucibles and jigs produced from these raw materials. Impurity contamination from silica glass products can be reduced and performance can be improved.

Japanese Patent Publication No. 4-75848 (Claim 1) Japanese Patent Laid-Open No. 62-176929 (Claim 1) JP-A-3-275527 (page 2, lower left column, line 7 to page 3, upper left column, line 6)

  However, the synthetic silica glass product produced using the synthetic amorphous silica powder produced by the method described in the above Patent Documents 1 to 3 is used in an environment where the synthetic silica glass product is used at high temperature and reduced pressure. In this case, bubbles were generated or expanded, and the performance of the synthetic silica glass product was greatly reduced.

  For example, a crucible for pulling up a silicon single crystal is used in a high temperature and reduced pressure environment at around 1500 ° C. and around 7000 Pa, and the drastic reduction in the performance of the crucible due to the generation or expansion of the aforementioned bubbles is a It has become a problem that affects the quality of. In addition, due to the increase in the diameter of silicon single crystals, the crucible for pulling silicon single crystals is used at higher temperatures.

  With respect to the problems that occur in the use under such high temperature and reduced pressure environment, the synthetic amorphous silica powder obtained by hydrolysis of silicon tetrachloride is subjected to heat treatment, By reducing the concentration of hydroxyl and carbon in the synthetic amorphous silica powder by reducing the concentration of hydroxyl and chlorine, respectively, and by subjecting the synthetic amorphous silica powder obtained by the sol-gel method of alkoxysilane to heat treatment It is conceivable to reduce the concentration of impurities that can be gas components in the synthetic amorphous silica powder.

  However, even with the above measures, the generation or expansion of bubbles in a synthetic silica glass product used in an environment of high temperature and reduced pressure cannot be sufficiently suppressed.

The present inventor adsorbs on the surface of the raw material powder used for the production of the glass product for the purpose of sufficiently suppressing the generation or expansion of bubbles in the synthetic silica glass product used under high temperature and reduced pressure environment. We focused on reducing the gas components and the like. And the invention which concerns on the synthetic amorphous silica powder which reduced the said gas component by giving the raw material powder the spheroidization process on predetermined conditions was discovered. Specifically, after performing spheroidizing treatment granulated silica powder, average particle size D ₅₀ obtained by washing drying a synthetic amorphous silica powder 10～2000Myuemu, BET specific The value obtained by dividing the surface area by the theoretical specific surface area calculated from the average particle diameter is more than 1.35 and not more than 1.75, the true density is 2.10 to 2.20 g / cm ³ , and the intra-particle space ratio is 0 to 0.05. The circularity is 0.75 or more and 1.00 or less, and the undissolved rate is greater than 0.00 and 0.25 or less.

  Although this synthetic amorphous silica powder has very little gas component adsorbed on the powder surface and gas component existing inside the powder, it has a very high effect of suppressing the generation or expansion of bubbles, but these are more stable. However, it was not always sufficient in terms of restraint, and there was room for further improvement.

  Therefore, an object of the present invention is to overcome the above-described conventional problems and to provide a synthetic silica glass product that can more stably suppress the generation or expansion of bubbles when used in an environment of high temperature and reduced pressure. To provide a synthetic amorphous silica powder suitable for a raw material and a method for producing the same.

  As a result of intensive studies by the present inventors, in synthetic silica glass products used under high temperature and reduced pressure environments, in order to more stably suppress the generation or expansion of bubbles, adsorption to the powder surface of the raw powder The present inventors have found that it is important to further reduce the amount of gas present between the powders in addition to reducing the gas components present and the gas components present inside the powder. In order to reduce the amount of gas existing between the powders, it is necessary to control the powder bulk density to a desired high density, that is, to reduce the volume of voids between the powders as stably as possible. For that purpose, it is important to precisely control the particle size and distribution of the powder after the spheroidizing treatment, and further the powder shape and the like within an appropriate range.

A first aspect of the present invention is a synthetic amorphous silica powder obtained by subjecting a granulated silica powder to spheronization treatment, followed by washing and drying. Value D _VM is 50 to 200 μm, its frequency is 2 to 5%, number-based arithmetic average particle diameter D _NAV is 0.4 × D _VM ± 10 μm, and number-based particle size distribution mode value D _NM is 5 to 5 and that a frequency of 3-10% in 10 [mu] m, compacted bulk density of 1.38 g / cm ³ or more 1.50 g / cm ³ or less, and a circularity of 0.85 to 1.00, theory BET specific surface area A synthetic amorphous silica powder having a value divided by a specific surface area of 1.00 to 1.35, a true density of 2.10 to 2.20 g / cm ³ , and a particle space ratio of 0.00 to 0.05. is there.

According to a second aspect of the present invention, silicon tetrachloride is hydrolyzed to produce a siliceous gel, the siliceous gel is dried to obtain a dry powder, the dry powder is pulverized, and then classified. A granulation step of obtaining a first silica powder having a volume-based particle size distribution mode value D _VM of 60 to 300 μm and a number-based particle size distribution mode value D _{NM of 5} to 20 μm; Powder mixing in which the first silica powder and the second silica powder are uniformly mixed at a mass ratio (first silica powder: second silica powder) at a ratio of 1: 0.005-0.05. And the third silica powder is introduced at a predetermined supply rate into a plasma torch in which argon is introduced at a predetermined flow rate and plasma is generated at a predetermined high-frequency output, from 2000 ° C. to the boiling point of silicon dioxide Thermal plasma heated and melted at temperature Including a spheroidizing step, a cleaning step for removing fine particles adhering to the surface of the spheroidized silica powder after the spheronizing step, and a drying step for drying the silica powder after the cleaning step, in this order. When the high frequency output (W) in the process is A and the supply rate (kg / hr) of the silica powder is B, the value of A / B (W · hr / kg) is 1.0 × 10 ⁴ or more. The volume-based particle size distribution mode D _VM is 50-200 μm, the frequency is 2-5%, the number-based arithmetic mean particle size D _NAV is 0.4 × D _VM ± 10 μm, and the mode D _NM of the particle size distribution of number basis is the frequency of 3-10% in 5 to 10 [mu] m, compacted bulk density of 1.38 g / cm ³ or more 1.50 g / cm ³ or less, roundness 0. 85 or more and 1.00 or less, and the value obtained by dividing the BET specific surface area by the theoretical specific surface area is .00～1.35, true density of 2.10~2.20g / cm ^3, a method for producing a synthetic amorphous silica powder particles in the space ratio is from 0.00 to 0.05.

The third aspect of the present invention is to hydrolyze the organic silicon compound to produce a siliceous gel, dry the siliceous gel to dry powder, pulverize the dry powder, and then classify To obtain a first silica powder having a volume-based particle size distribution mode value D _VM of 60 to 300 μm and a number-based particle size distribution mode value D _{NM of 5} to 20 μm; Powder that obtains third silica powder by uniformly mixing first silica powder and second silica powder in a mass ratio (first silica powder: second silica powder) at a ratio of 1: 0.005-0.05. The third silica powder is charged at a predetermined supply rate into a plasma torch in which argon is introduced at a predetermined flow rate and plasma is generated at a predetermined high-frequency output by introducing argon at a predetermined flow rate, from 2000 ° C. to the boiling point of silicon dioxide. Heat and melt at a temperature of Including a spheroidizing step by thermal plasma, a cleaning step of removing fine powder adhering to the surface of the spheroidized silica powder after the spheronizing step, and a drying step of drying the silica powder after the cleaning step, When the high frequency output (W) in the spheroidizing step is A and the supply rate (kg / hr) of the silica powder is B, the value of A / B (W · hr / kg) is 1.0 × 10 ⁴ or more. adjusted performed, the frequency is 2-5%, the arithmetic average particle diameter D _NAV is 0.4 × D _VM ± 10μm number reference in the mode D _VM is 50~200μm of volume-based particle size distribution as and the mode D _NM of the particle size distribution of number basis is the frequency of 3-10% in 5 to 10 [mu] m, compacted bulk density of 1.38 g / cm ³ or more 1.50 g / cm ³ or less, roundness 0.85 or more and 1.00 or less, and the BET specific surface area is the theoretical specific surface area. Values Tsu is 1.00 to 1.35, the true density of 2.10~2.20g / cm ^3, in the manufacturing method of synthetic amorphous silica powder is a particle in space ratio 0.00 to 0.05 is there.

According to a fourth aspect of the present invention, fumed silica is hydrolyzed to produce a siliceous gel, the siliceous gel is dried to obtain a dry powder, the dry powder is pulverized, and then classified. A granulation step of obtaining a first silica powder having a volume-based particle size distribution mode value D _VM of 60 to 300 μm and a number-based particle size distribution mode value D _{NM of 5} to 20 μm; Powder mixing in which the first silica powder and the second silica powder are uniformly mixed at a mass ratio (first silica powder: second silica powder) at a ratio of 1: 0.005-0.05. And the third silica powder is introduced at a predetermined supply rate into a plasma torch in which argon is introduced at a predetermined flow rate and plasma is generated at a predetermined high-frequency output, from 2000 ° C. to the boiling point of silicon dioxide Heating process to heat and melt at temperature Including a spheroidizing step with a laser, a cleaning step for removing fine powder adhering to the surface of the spheroidized silica powder after the spheronizing step, and a drying step for drying the silica powder after the cleaning step in this order. When the high frequency output (W) in the conversion step is A and the supply rate (kg / hr) of the silica powder is B, the value of A / B (W · hr / kg) is 1.0 × 10 ⁴ or more. adjusted done, volume-based particle size distribution. the frequency of 2-5% mode value D _VM is in 50~200μm of number-based arithmetic average particle diameter D _NAV is 0.4 × D _VM ± 10μm in the, and a mode value D _NM is the frequency from 3 to 10 percent 5~10μm of the particle size distribution of number-based, compacted bulk density of 1.38 g / cm ³ or more 1.50 g / cm ³ or less, roundness 0 .85 or more and 1.00 or less, and the BET specific surface area is divided by the theoretical specific surface area. Values from 1.00 to 1.35, the true density of 2.10~2.20g / cm ^3, a method for producing a synthetic amorphous silica powder particles in the space ratio is from 0.00 to 0.05.

  A fifth aspect of the present invention is an invention based on the second to fourth aspects, and in the granulation step, the first silica powder pulverizes the dry powder, and then the pulverized dry powder is used. The second silica powder is obtained by classification using a sieve, and the second silica powder is obtained by further classifying the dried powder under the sieve at the time of classification to obtain the first silica powder.

The synthetic amorphous silica powder according to the first aspect of the present invention is a synthetic amorphous silica powder obtained by subjecting the granulated silica powder to spheroidization treatment, washing and drying, and having a volume The standard value D _VM of the standard particle size distribution is 50 to 200 μm, the frequency is 2 to 5%, the number average arithmetic average particle size D _NAV is 0.4 × D _VM ± 10 μm, and the number standard particle size distribution Shikichi D _NM is the frequency of 3-10% in 5 to 10 [mu] m, compacted bulk density of 1.38 g / cm ³ or more 1.50 g / cm ³ or less, in roundness 0.85 to 1.00 The value obtained by dividing the BET specific surface area by the theoretical specific surface area is 1.00 to 1.35, the true density is 2.10 to 2.20 g / cm ³ , and the intra-particle space ratio is 0.00 to 0.05. . This synthetic amorphous silica powder has a desired bulk density by precisely controlling the particle size, its distribution, and the powder shape within an appropriate range. Therefore, in addition to the gas component adsorbed on the powder surface and the gas component present inside the powder, the amount of gas present between the powders is extremely small, and a synthetic silica glass product manufactured using this synthetic amorphous silica powder is manufactured. In this case, the generation or expansion of bubbles can be more stably reduced than when the conventional raw material powder is used.

The method for producing a synthetic amorphous silica powder according to the second to fourth aspects of the present invention includes, for example, hydrolyzing silicon tetrachloride to produce a siliceous gel, or using an organic silicon compound such as tetramethoxysilane. Hydrolysis produces a siliceous gel or fumed silica is used to produce a siliceous gel. Then, the siliceous gel is dried to obtain a dry powder, and the dry powder is pulverized and then classified, whereby the mode value D _VM of the volume-based particle size distribution is 60 to 300 μm and the first silica powder and the number standard The granulation step of obtaining a second silica powder having a mode value D _{NM of 5} to 20 μm and a mass ratio (first silica powder: second silica powder) ) And a powder mixing step of uniformly mixing at a ratio of 1: 0.005 to 0.05 to obtain a third silica powder, and plasma in which argon is introduced at a predetermined flow rate and plasma is generated at a predetermined high frequency output. The third silica powder is charged into the torch at a predetermined supply rate, heated at a temperature from 2000 ° C. to the boiling point of silicon dioxide and melted, and a spheronization step by the thermal plasma, and the spheronization after the spheronization step Adhering to the silica powder surface A washing step for removing the fine powder and a drying step for drying the silica powder after the washing step in this order, the high frequency output (W) in the spheroidizing step is A, and the silica powder supply rate (kg / hr) And B is adjusted so that the value of A / B (W · hr / kg) is 1.0 × 10 ⁴ or more, and the mode value D _{VM of} the volume-based particle size distribution is 50 to 50 The frequency is 2 to 5% at 200 μm, the arithmetic average particle diameter D _{NAV based on the number} is 0.4 × D _VM ± 10 μm, and the frequency value D _{NM of the} number based particle size distribution is 5 to 10 μm and the frequency is 3 was 10%, and characterized in that compacted bulk density 1.38 g / cm ³ or more 1.50 g / cm ³ or less, roundness obtain synthetic amorphous silica powder is 0.85 to 1.00 To do. By passing through the above steps, the inevitable gas adsorption amount can be reduced, and the gas components in the powder can be reduced. Furthermore, it is possible to obtain a powder having a desired compacted bulk density by precisely controlling the particle size, its distribution, and the powder shape within an appropriate range. As a result, the amount of gas present between the powders is extremely reduced, and a synthetic amorphous silica powder that can be suitably used as a raw material for a synthetic silica glass product can be easily produced.

  In the method for producing a synthetic amorphous silica powder according to the fifth aspect of the present invention, in the granulation step, the first silica powder pulverizes the dry powder, and then classifies the pulverized dry powder using a sieve. The second silica powder can be obtained by further classifying the dried powder under the sieve for obtaining the first silica powder. Therefore, the specific surface area of the silica powder is reduced by, for example, dividing the pulverized dry powder into two and reducing the fine powder having a particle size of less than submicron compared to the method obtained by directly classifying one of the pulverized dry powders. Therefore, the generation or expansion of bubbles can be more stably suppressed.

It is a photograph figure which shows the typical powder particle of the synthetic | combination amorphous silica powder of this invention. It is a process flow figure showing a manufacturing process of synthetic amorphous silica powder of the present invention. It is a schematic sectional drawing of the spheroidization apparatus by a thermal plasma. It is a photograph figure which shows the typical silica powder particle which has not performed the spheroidization process.

  Next, an embodiment for carrying out the present invention will be described based on the drawings.

The synthetic amorphous silica powder of the present invention is obtained by subjecting the granulated silica powder to spheroidization treatment, washing and drying, and the mode value D _{VM of the} volume-based particle size distribution. Is 50 to 200 μm, the frequency is 2 to 5%, the number-based arithmetic average particle diameter D _NAV is 0.4 × D _VM ± 10 μm, and the number-based particle size distribution mode value D _NM is 5 to 10 μm. the frequency is 3-10%, a packed bulk density of 1.38 g / cm ³ or more 1.50 g / cm ³ or less, roundness, characterized in that 0.85 to 1.00.

When the mode value D _{VM of the} volume-based particle size distribution is less than 50 μm, the particle size of the silica powder becomes small, so the density of the bulk becomes low and the generation or expansion of bubbles cannot be stably suppressed. On the other hand, if it exceeds 200 μm, the space between the silica powders becomes large, so that the bulk density is lowered and the generation or expansion of bubbles cannot be stably suppressed. Further, even if the mode value D _VM is 50 to 200 μm, if the frequency is less than 2%, silica powder with a small particle size increases, so the density of the bulk becomes low and the generation or expansion of bubbles is stable. Can not be suppressed. On the other hand, if it exceeds 5%, silica powder having a large particle diameter increases and the space between the silica powders increases, so the density of the bulk becomes low and the generation or expansion of bubbles cannot be stably suppressed. Among these, it is preferred that the mode value D _VM of volume-based particle size distribution in the range of 75-150, the frequency is preferably in the range of 3-4%.

Further, when the number average arithmetic average particle diameter D _NAV exceeds the upper limit (0.4 × D _VM +10 μm),
Since it becomes difficult for a powder with a small particle size to enter a gap between powders with a large particle size, the density of the bulk becomes low and the generation or expansion of bubbles cannot be stably suppressed. If it is less than the arithmetic mean particle diameter D _NAV is the lower limit of the number-based (0.4 × D _VM -10μm), the gap between particle size larger powder, leaving a gap even contain small particle size powder For this reason, the bulk density is lowered, and the generation or expansion of bubbles cannot be stably suppressed. Further, when the mode value D _{NM of} the number-based particle size distribution is less than 5 μm, the silica powder having a small particle size increases, the specific surface area increases, and the generation or expansion of bubbles cannot be stably suppressed. On the other hand, if it exceeds 10 μm, the powder with a smaller particle size cannot enter the gap between the powders with a larger particle size after the powder with a smaller particle size has entered. Therefore, the generation or expansion of bubbles cannot be stably suppressed. Further, even if the mode value D _NM is in the range of 5 to 10 μm, if the frequency is less than 3%, the silica powder having a small particle diameter increases too much, so the density of the bulk becomes low and the generation or expansion of bubbles. Cannot be suppressed stably. On the other hand, if it exceeds 10%, the powder with a small particle size that can enter the gap between the powders with a large particle size decreases, so the density of the bulk becomes low and the generation or expansion of bubbles cannot be stably suppressed. . Among these, the number-based arithmetic average particle diameter D _NAV is preferably in the range of 0.4 × D _VM ± 5 μm. The mode value D _NM of the number-based particle size distribution is preferably in the range of 5 to 7.5 μm, and the frequency is preferably in the range of 5 to 8%.

  Further, the circularity is a value calculated from the following equation (1), and the circularity means that the closer to 1.00, the closer the powder particle is to a true sphere.

Circularity per particle = π × area equivalent diameter / perimeter (1)
When the circularity is less than 0.85, the bulk density is low, and the generation or expansion of bubbles cannot be stably suppressed. On the other hand, the circularity does not exceed 1.00 regardless of the spheroidizing process. Of these, the circularity is preferably in the range of 0.90 to 1.00.

As described above, the synthetic amorphous silica powder of the present invention is a spheroidized silica powder, and the particle size and distribution thereof, and further, the powder shape is controlled within an appropriate range and precisely. Therefore, the volume of the voids between the particles is very small and has the desired solid density. Therefore, the amount of gas present between the powders is extremely small. If the particle size etc. is not controlled within the above appropriate range, and if the bulk density is less than 1.38 g / cm ³ , the gap between the powders becomes large and the amount of gas existing between the powders increases, so that bubbles are generated. Or expansion cannot be suppressed stably. On the other hand, if it exceeds 1.50 g / cm ³ , when melting the powder in the manufacture of glass products, the gas component present between the powders, the gas component adsorbed on the powder surface, or the gas component present in the powder However, since it becomes impossible to escape from the molten powder or melt, there is a problem that bubbles remain in the glass product. Of these, the bulk density is preferably in the range of 1.40 to 1.47 g / cm ³ .

  Moreover, the gas etc. which are adsorb | sucking to the surface of the raw material powder used for manufacture of a product as a cause of generation | occurrence | production or expansion | swelling of a bubble in a synthetic silica glass product are considered. That is, when the synthetic silica glass product is manufactured, the gas component adsorbed on the surface of the raw material powder is released during melting, which is one step. And this gas component remains in a synthetic silica glass product, and this causes a bubble generation or expansion.

  Since the silica powder used as the raw material for the synthetic silica glass product usually undergoes a pulverization step, it contains a large number of particles having an irregular shape (pulverized powder shape) as shown in FIG. Therefore, it is considered that the specific surface area increases and the inevitable gas adsorption amount increases.

Therefore, the synthetic amorphous silica powder of the present invention has a value obtained by dividing the BET specific surface area by the theoretical specific surface area by subjecting the powder to spheroidization treatment, and the true density is 2.10. 2.20 g / cm < ³ >, The particle | grain space ratio is 0.00-0.05, It is characterized by the above-mentioned. Here, the theoretical specific surface area is a value calculated from the average particle size D _L50 , and the average particle size D _L50 is obtained by using a laser diffraction scattering type particle distribution measuring device (model name: LA-950, manufactured by HORIBA). The median value of the volume-based particle size distribution measured in the above is measured three times, and the average value is measured. The BET specific surface area is a value measured by the BET three-point method. Further, the theoretical specific surface area of the particle is calculated from the following equation (2) when it is assumed that the particle is a true sphere and the surface is smooth. In the formula (2), D represents the particle diameter, and ρ represents the true density.

Theoretical specific surface area = 6 / (D × ρ) (2)
In the present specification, the theoretical specific surface area of the powder is a value calculated from the theoretical true density assuming that D is the average particle diameter D _L50 of the powder and ρ is the true density of 2.20 g / cm ³ in the above formula (2). It is. That is, the theoretical specific surface area of the powder is calculated from the following equation (3).

Theoretical specific surface area of the powder = 2.73 / D _L50 (3)
When the value obtained by dividing the BET specific surface area by the theoretical specific surface area calculated from the average particle diameter D _L50 increases, the specific surface area increases and the inevitable gas adsorption amount increases. The theoretical specific surface area of the powder is 1.00 when the particles are spherical and the surface is smooth. The theory that the BET specific surface area is calculated from the average particle size D _L50 when the measurement of the average particle size D _L50 is small because there is a deviation in the particle size distribution and the peak of the particle size distribution on the side where the average particle size D _L50 is small. The value divided by the specific surface area may be less than 1.00. In that case, a value obtained by dividing the BET specific surface area by the theoretical specific surface area calculated from the average particle diameter D _L50 is defined as 1.00. If it exceeds 1.35, the effect of reducing the generation or expansion of bubbles due to gas components adsorbed on the powder surface is small. Of these, the value obtained by dividing the BET specific surface area by the theoretical specific surface area calculated from the average particle diameter D _L50 is particularly preferably in the range of 1.00 to 1.10.

When attention is paid to one particle of the synthetic amorphous silica powder, it is preferable that no internal space such as a void or a closed crack exists in the particle. That is, if there is a space inside the synthetic amorphous silica powder, it may cause generation or expansion of bubbles in the synthetic silica glass product. Therefore, the true density is preferably at 2.10 g / cm ³ or more, and particularly preferably 2.15~2.20g / cm ^3. The true density is an average value obtained by measuring the true density three times in accordance with the measurement method (d) true specific gravity measurement of JIS R7212 carbon block. Further, the intra-particle space ratio is preferably 0.05 or less, and particularly preferably 0.01 or less. The intra-particle space ratio is a measurement of the cross-sectional area of a particle when the cross-section of the 50 powder particles is observed with an SEM (scanning electron microscope), and the area of the space if there is a space in the particle. This is the average value of the values calculated from the equation (4).

Intraparticle space ratio = total area of intraparticle space / total area of particle breakage (4)
Moreover, in order to make the meltability of the powder uniform, a powder having a broad diffraction peak and no crystalline silica powder observed when measured by a powder X-ray diffraction method using CuKα rays is preferable. Amorphous and crystalline silica have different behaviors in melting, and the melting of crystalline silica tends to start later. For this reason, if a synthetic silica glass product or the like is manufactured using a synthetic amorphous silica powder in which amorphous and crystalline silica are mixed, bubbles are likely to remain in the synthetic silica glass product. It is.

  In addition, in order to reduce the contamination of synthetic silica glass products and to improve performance, the synthetic amorphous silica powder has an impurity concentration of 1A group, 2A-8 group, 1B-3B group, carbon and silicon excluding hydrogen atoms. It is preferable that the concentration of the 4B group except 5B group, the 6B group excluding oxygen, and the 7B group excluding chlorine is less than 1 ppm. Of these, the impurity concentration is particularly preferably less than 0.05 ppm. Moreover, in order to suppress generation | occurrence | production or expansion | swelling of the bubble in the synthetic silica glass product under high temperature and pressure reduction, it is preferable that the hydroxyl group which can become a gas component is 60 ppm or less, the chlorine concentration is 5 ppm or less, and the carbon concentration is 5 ppm or less.

  The synthetic amorphous silica powder of the present invention contains many particles in which a plurality of fused and spheroidized particles as shown in FIG. 1 are aggregated by washing and drying after spheroidizing treatment. Thus, since the synthetic amorphous silica powder of the present invention contains many aggregated particles, the powder surface becomes smooth by melting, so that the inevitable gas adsorption amount is reduced. In addition, in this specification, about the particle | grains which the particle | grains fuse | melted and spheroidized aggregated, one aggregated particle comprises 1 particle | grain.

  Then, the manufacturing method of the synthetic | combination amorphous silica powder of this invention is demonstrated. As shown in FIG. 2, the synthetic amorphous silica powder of the present invention is obtained by subjecting the silica powder as a raw material to a spheroidizing treatment, and then washing and drying. Each step will be described in detail below.

The silica powder used as the raw material for the synthetic amorphous silica powder of the present invention is obtained, for example, by the following method. As a first method, first, ultrapure water in an amount corresponding to 45 to 80 mol is prepared with respect to 1 mol of silicon tetrachloride. The prepared ultrapure water is put in a container and hydrolyzed by adding silicon tetrachloride while stirring at a temperature of 20 to 45 ° C. in an atmosphere of nitrogen, argon or the like. After the addition of silicon tetrachloride, stirring is continued for 0.5 to 6 hours to produce a siliceous gel. At this time, the stirring speed is preferably in the range of 100 to 300 rpm. Next, the siliceous gel is transferred to a drying container, and this is put into a drier. While flowing nitrogen, argon or the like at a flow rate of preferably 10 to 20 L / min in the drier, a temperature of 200 ° C. to 300 ° C. Dry for 12 to 48 hours to obtain a dry powder. Next, the dry powder is taken out from the dryer and pulverized using a pulverizer such as a roll crusher. When using a roll crusher, it adjusts suitably to roll clearance 0.2-2.0mm and roll rotation speed 3-200rpm. Finally, by classifying the pulverized dry powder using a sieve or the like, the mode value D _VM of the volume-based particle size distribution is the first silica powder having a _VM of 60 to 300 μm, and the mode value D of the particle size distribution based on the number. _A second silica powder having an _{NM of 5} to 20 μm is obtained. As a sieve used for classification, in addition to a vibrating sieve, for example, a gyro-type or air-flow classification-type sieve can be used. The second silica powder may be obtained, for example, by dividing the pulverized dry powder into two and directly classifying from one of the pulverized dry powders. However, the second silica powder may be dried under sieving at the time of classification to obtain the first silica powder. A method of further classifying the powder is preferable because fine powder less than submicron is reduced and the specific surface area of the silica powder is reduced, so that generation or expansion of bubbles can be more stably suppressed.

As a 2nd method, 0.5-3 mol of ultrapure water and 0.5-3 mol of ethanol are prepared with respect to 1 mol of tetramethoxysilane as an organic silicon compound. The prepared ultrapure water and ethanol are put in a container, and tetramethoxysilane is added and hydrolyzed in an atmosphere of nitrogen, argon or the like while maintaining the temperature at 60 ° C. and stirring. After stirring for 5 to 120 minutes after adding tetramethoxysilane, 1 to 50 mol of ultrapure water is further added to 1 mol of tetramethoxylane, and stirring is continued for 1 to 12 hours. Generate. At this time, the stirring speed is preferably in the range of 100 to 300 rpm. Next, the siliceous gel is transferred to a drying container, and this is put into a drier. While flowing nitrogen, argon or the like at a flow rate of preferably 10 to 20 L / min in the drier, a temperature of 200 ° C. to 300 ° C. Dry for 6 to 48 hours to obtain a dry powder. Next, the dry powder is taken out from the dryer and pulverized using a pulverizer such as a roll crusher. When using a roll crusher, it adjusts suitably to roll clearance 0.2-2.0mm and roll rotation speed 3-200rpm. Finally, the pulverized dry powder is classified using a vibrating sieve or the like, as in the first method, so that the first silica powder having a volume-based particle size distribution mode value D _VM of 60 to 300 μm. , and the mode D _NM of the particle size distribution of number-based second silica powder of 5~20μm is obtained.

As a third method, first, 3.0 to 35.0 mol of ultrapure water is prepared with respect to 1 mol of fumed silica having a specific surface area of 50 to 380 m ² / g. The prepared ultrapure water is put in a container, and fumed silica is added while stirring at a temperature of 10 to 30 ° C. in an atmosphere of nitrogen, argon or the like. In addition, the specific surface area said here means the BET specific surface area measured by the BET three-point method. Next, after adding fumed silica, stirring is continued for 0.5 to 6 hours to produce a siliceous gel. At this time, the stirring speed is preferably in the range of 10 to 50 rpm. Next, the siliceous gel is transferred to a drying container and placed in a dryer, and preferably at a temperature of 200 ° C. to 300 ° C. while flowing nitrogen, argon, etc. at a flow rate of 1 to 20 L / min. Dry for 12 to 48 hours to obtain a dry powder. Next, the dry powder is taken out from the dryer and pulverized using a pulverizer such as a roll crusher. When using a roll crusher, it adjusts suitably to roll clearance 0.2-2.0mm and roll rotation speed 3-200rpm. Finally, the pulverized dry powder is classified using a vibrating sieve or the like, as in the first method, so that the first silica powder having a volume-based particle size distribution mode value D _VM of 60 to 300 μm. , and the mode D _NM of the particle size distribution of number-based second silica powder of 5~20μm is obtained.

Here, when the mode value D _VM of the first silica powder is less than 60 μm, the silica powder having a small particle size increases, so that the bulk density decreases and the generation or expansion of bubbles cannot be stably suppressed. . On the other hand, when the mode value D _VM of the first silica powder is larger than 300 μm, the gap between the powders becomes large, the density of the bulk becomes low, and the generation or expansion of bubbles cannot be stably suppressed. Further, when the mode value D _NM of the second silica powder is less than 5 μm, the silica powder having a small particle size increases, so that the bulk density is reduced and the generation or expansion of bubbles can be stably suppressed. I can't. On the other hand, if the mode value D _NM of the second silica powder is larger than 20 μm, the second silica powder cannot enter into the gaps between the first silica powders. Cannot be suppressed stably.

  The first silica powder and the second silica powder obtained by the first to third methods are uniformly in a mass ratio (first silica powder: second silica powder) at a ratio of 1: 0.005-0.05. To obtain a third silica powder. The reason why the mixing ratio is limited to the above range is that if the second silica powder is less than 0.005, the second silica powder entering the gaps between the first silica powders is reduced, so that the desired solid density cannot be obtained. Generation or expansion of bubbles cannot be stably suppressed. On the other hand, if it exceeds 0.05, the second silica powder having a small particle size is excessively increased, so that the bulk density becomes low and the generation or expansion of bubbles cannot be stably suppressed. The mixing of the first silica powder and the second silica powder is not particularly limited, but can be performed by putting the silica powder in a plastic drum and tilting and rotating and swinging the plastic drum. In addition, the reason why this powder mixing step is performed before the spheroidizing treatment described later is that the first silica powder and the second silica powder are mixed after the spheroidizing treatment described later, respectively. This is because a small silica powder having a particle size of less than 10 μm causes a problem that a large amount evaporates.

The spheroidization of the third silica powder obtained through the granulation step and the powder mixing step is performed by a spheronization treatment using thermal plasma. In the spheroidization process using thermal plasma, for example, an apparatus shown in FIG. 3 can be used. The apparatus 30 includes a plasma torch 31 that generates plasma, a chamber 32 that is a reaction cylinder provided below the plasma torch 31, and a collection unit that collects the processed powder provided below the chamber 32. 33. The plasma torch 31 has a quartz tube 34 sealed at the top communicating with the chamber 32 and a high-frequency induction coil 36 around which the quartz tube 34 is wound. A raw material supply pipe 37 is provided through the quartz tube 34 and a gas introduction pipe 38 is connected thereto. A gas exhaust port 39 is provided on the side of the chamber 32. In the plasma torch 31, when the high-frequency induction coil 36 is energized, plasma 40 is generated, and a gas such as argon or oxygen is supplied from the gas introduction tube 38 to the quartz tube 34. The raw material powder, that is, the third silica powder is supplied into the plasma 40 through the raw material supply pipe 37. Further, the gas in the chamber 32 is exhausted from a gas exhaust port 39 provided on the side of the chamber 32. First, the working gas argon is introduced from the gas introduction pipe 38 of the apparatus 30 at a flow rate of 50 L / min or more, preferably 65 to 110 L / min, and a high frequency with a frequency of 4 to 5 MHz and an output of 90 to 180 kW is applied to the plasma torch 31. To generate plasma. After the plasma is stabilized, oxygen is gradually introduced at a flow rate of 35 to 130 L / min to generate an argon-oxygen plasma. Next, the silica powder obtained by the above first to third methods is charged into the argon-oxygen plasma from the raw material supply pipe 37 at a supply rate of 4.0 to 18.0 kg / hr, By melting and dropping the melted particles, the recovered particles are recovered by the recovery unit 33, whereby the spheroidized silica powder 41 can be obtained. The particle size distribution, the solid bulk density, the circularity, etc. of the synthetic amorphous silica powder can be adjusted by adjusting the argon flow rate, high frequency output, silica powder supply rate, and the like. For example, when the flow rate of argon is adjusted to 50 L / min or more and the high frequency output (W) is A and the supply rate of silica powder (kg / hr) is B within the above range, the high frequency output A and the silica powder The feed rate B is adjusted so that the value of A / B (W · hr / kg) is in the range of 1.0 × 10 ⁴ or more. A powder having a density and circularity is obtained.

  Since the silica powder after the spheroidization treatment has fine silica powder evaporated in the argon-oxygen plasma on its surface, put the spheroidized silica powder and ultrapure water after the spheroid process into the cleaning container. Perform ultrasonic cleaning. After the ultrasonic cleaning, the silica powder is filtered through a coarse filter in order to move into the ultrapure water. This operation is repeated until there is no more fine silica powder.

  In the drying of the silica powder after the washing step, first, the powder is put into a drying container, and this drying container is put into a dryer. And it is preferable to carry out by hold | maintaining at the temperature of 100 to 400 degreeC for 3 to 48 hours, flowing nitrogen, argon, etc. in the dryer at the flow volume of 1-20 L / min.

  Through the above steps, the synthetic amorphous silica powder of the present invention is obtained. This synthetic amorphous silica powder is subjected to a spheroidizing treatment under the predetermined conditions, so that there are few gas components adsorbed on the powder surface and gas components existing inside the powder. In addition, the particle size after the spheroidizing treatment, its distribution, and further the powder shape and the like are precisely controlled within a proper range, thereby achieving a desired compacted bulk density. As a result, the amount of gas present between the powders is very small, and the effect of suppressing the generation or expansion of bubbles is extremely high, so that it can be suitably used as a raw material for synthetic silica glass products.

  Next, examples of the present invention will be described in detail together with comparative examples.

<Example 1>
First, 13 mol of ultrapure water was prepared for 1 mol of fumed silica having a specific surface area of 50 m ² / g. The prepared ultrapure water was put in a container, and fumed silica was added while stirring at a temperature of 25 ° C. in a nitrogen atmosphere. After the fumed silica was added, stirring was continued for 3 hours to produce a siliceous gel. At this time, the stirring speed was 30 rpm. Next, the siliceous gel was transferred to a drying container, put in a dryer, and dried at a temperature of 300 ° C. for 12 hours while flowing nitrogen at a flow rate of 10 L / min to obtain a dried powder. . The dried powder was taken out from the dryer and pulverized using a roll crusher. At this time, the roll gap was adjusted to 0.2 mm, and the roll rotation speed was adjusted to 30 rpm. The pulverized dry powder was classified using a vibration sieve having openings of 45 μm and 85 μm to obtain a first silica powder. Further, the dried powder under the sieve for obtaining the first silica powder was used as the second silica powder. The mode value D _VM of the volume-based particle size distribution of the obtained first silica powder and the mode value D _NM of the number-based particle size distribution of the second silica powder are shown in Table 1 below. In addition, the specific surface area of the fumed silica which is a raw material is a BET specific surface area measured by the BET three-point method.

  Next, the obtained first silica powder and second silica powder are put into a plastic drum so as to have a mass ratio shown in Table 1 below, and the plastic drum is tilted and rotated to be uniformly mixed. A third silica powder was obtained.

  Subsequently, using the apparatus 30 shown in FIG. 3, the obtained third silica powder was spheroidized under the conditions shown in Table 1 below. Specifically, first, argon as a working gas was introduced from the gas introduction tube 38 of the apparatus 30, and a high frequency was applied to the plasma torch 31 to generate plasma. After the plasma was stabilized, oxygen was gradually introduced to generate an argon-oxygen plasma. The obtained silica powder is put into an argon-oxygen plasma from the raw material supply pipe 37, the silica powder is melted, the melted particles are dropped, and the dropped particles are recovered by the recovery unit 33. Thereby, the spheroidized silica powder 41 was obtained.

  After the spheronization treatment, the powder and ultrapure water were put in a cleaning container, and ultrasonic cleaning was performed. After ultrasonic cleaning, it was filtered with a filter having an opening of 35 μm. This operation was repeated until there was no fine powder adhering to the surface of the silica powder particles.

  Finally, the washed powder was put into a drying container, and kept at a temperature of 200 ° C. for 36 hours while flowing nitrogen at a flow rate of 20 L / min in the dryer to obtain a synthetic amorphous silica powder.

<Example 2>
First, 30 mol of ultrapure water was prepared for 1 mol of fumed silica having a specific surface area of 90 m ² / g. The prepared ultrapure water was put in a container, and fumed silica was added while stirring at a temperature of 30 ° C. in a nitrogen atmosphere. Stirring was continued for 6 hours after the fumed silica was added to produce a siliceous gel. At this time, the stirring speed was 45 rpm. Next, the siliceous gel was transferred to a drying container, put into a dryer, and dried at a temperature of 300 ° C. for 12 hours while flowing nitrogen at a flow rate of 15 L / min into the dryer to obtain a dry powder. . The dried powder was taken out from the dryer and pulverized using a roll crusher. At this time, the roll gap was adjusted to 0.2 mm, and the roll rotation speed was adjusted to 200 rpm. The pulverized dry powder was classified using a vibration sieve having an opening of 90 μm and 180 μm to obtain a first silica powder. Further, the dried powder under the sieve for obtaining the first silica powder was used as the second silica powder.

  Next, the obtained first silica powder and second silica powder are put into a plastic drum so as to have a mass ratio shown in Table 1 below, and the plastic drum is tilted and rotated to be uniformly mixed. A third silica powder was obtained.

  Subsequently, using the apparatus 30 shown in FIG. 3, the third silica powder obtained by performing the same steps as in Example 1 under the conditions shown in Table 1 below is subjected to spheronization treatment, and spheronization is performed. Thus obtained silica powder 41 was obtained.

  After the spheronization treatment, the powder and ultrapure water were put in a cleaning container, and ultrasonic cleaning was performed. After ultrasonic cleaning, it was filtered with a filter having an opening of 45 μm. This operation was repeated until there was no fine powder adhering to the surface of the silica powder particles.

  Finally, the powder after washing was put in a drying container, and kept at a temperature of 400 ° C. for 12 hours while flowing argon at a flow rate of 10 L / min in the dryer to obtain a synthetic amorphous silica powder.

<Example 3>
First, 20 mol of ultrapure water was prepared with respect to 1 mol of fumed silica having a specific surface area of 130 m ² / g. The prepared ultrapure water was put in a container, and fumed silica was added while stirring while maintaining the temperature at 10 ° C. in an argon atmosphere. After the fumed silica was added, stirring was continued for 4 hours to produce a siliceous gel. At this time, the stirring speed was 25 rpm. Next, the siliceous gel was transferred to a drying container, put into a dryer, and dried at a temperature of 300 ° C. for 24 hours while flowing argon at a flow rate of 20 L / min to obtain a dry powder. . The dried powder was taken out from the dryer and pulverized using a roll crusher. At this time, the roll gap was adjusted to 0.4 mm, and the roll rotation speed was adjusted to 30 rpm. The pulverized dry powder was classified using a vibrating sieve having an opening of 200 μm and 400 μm to obtain a first silica powder. Moreover, the dry powder under the sieve at the time of classification to obtain the first silica powder was further classified using a vibrating sieve having an opening of 40 μm, and the powder on the sieve at this time was used as the second silica powder.

  Next, the obtained first silica powder and second silica powder are put into a plastic drum so as to have a mass ratio shown in Table 1 below, and the plastic drum is tilted and rotated to be uniformly mixed. A third silica powder was obtained.

  Subsequently, using the apparatus 30 shown in FIG. 3, the third silica powder obtained by performing the same steps as in Example 1 under the conditions shown in Table 1 below is subjected to spheronization treatment, and spheronization is performed. Thus obtained silica powder 41 was obtained.

  After the spheronization treatment, the powder and ultrapure water were put in a cleaning container, and ultrasonic cleaning was performed. After ultrasonic cleaning, the mixture was filtered with a filter having an opening of 125 μm. This operation was repeated until there was no fine powder adhering to the surface of the silica powder particles.

  Finally, the powder after washing was placed in a drying container, and kept at a temperature of 300 ° C. for 24 hours while flowing nitrogen at a flow rate of 10 L / min in the dryer, thereby obtaining a synthetic amorphous silica powder.

<Example 4>
First, 25 mol of ultrapure water was prepared for 1 mol of fumed silica having a specific surface area of 300 m ² / g. The prepared ultrapure water was put in a container, and fumed silica was added while stirring at a temperature of 15 ° C. in a nitrogen atmosphere. After the fumed silica was added, stirring was continued for 3 hours to produce a siliceous gel. At this time, the stirring speed was 25 rpm. Next, the siliceous gel was transferred to a drying container, put in a dryer, and dried at a temperature of 200 ° C. for 28 hours while flowing nitrogen at a flow rate of 10 L / min into the dryer to obtain a dry powder. . The dried powder was taken out from the dryer and pulverized using a roll crusher. At this time, the roll gap was adjusted to 0.2 mm, and the roll rotation speed was adjusted to 200 rpm. The pulverized dry powder was classified using a vibration sieve having openings of 45 μm and 85 μm to obtain a first silica powder. Moreover, the dry powder under the sieve at the time of the classification which obtains the said 1st silica powder was made into the 2nd silica powder.

  Next, the obtained first silica powder and second silica powder are put into a plastic drum so as to have a mass ratio shown in Table 1 below, and the plastic drum is tilted and rotated to be uniformly mixed. A third silica powder was obtained.

  Subsequently, using the apparatus 30 shown in FIG. 3, the third silica powder obtained by performing the same steps as in Example 1 under the conditions shown in Table 1 below is subjected to spheronization treatment, and spheronization is performed. Thus obtained silica powder 41 was obtained.

  After the spheronization treatment, the powder and ultrapure water were put in a cleaning container, and ultrasonic cleaning was performed. After ultrasonic cleaning, it was filtered with a filter having an opening of 35 μm. This operation was repeated until there was no fine powder adhering to the surface of the silica powder particles.

  Finally, the washed powder was put into a drying container, and kept at a temperature of 200 ° C. for 36 hours while flowing nitrogen at a flow rate of 10 L / min in the dryer to obtain a synthetic amorphous silica powder.

<Example 5>
First, 25 mol of ultrapure water was prepared for 1 mol of fumed silica having a specific surface area of 90 m ² / g. The prepared ultrapure water was put in a container, and fumed silica was added while stirring at a temperature of 15 ° C. in a nitrogen atmosphere. Stirring was continued for 6 hours after the fumed silica was added to produce a siliceous gel. At this time, the stirring speed was 25 rpm. Next, the siliceous gel was transferred to a drying container, put in a dryer, and dried at a temperature of 250 ° C. for 36 hours while flowing nitrogen at a flow rate of 5 L / min in the dryer to obtain a dry powder. . The dried powder was taken out from the dryer and pulverized using a roll crusher. At this time, the roll gap was adjusted to 0.4 mm, and the roll rotation speed was adjusted to 30 rpm. The pulverized dry powder was classified using a vibrating sieve having an opening of 200 μm and 400 μm to obtain a first silica powder. Moreover, the dry powder under the sieve at the time of classification to obtain the first silica powder was further classified using a vibration sieve having an opening of 25 μm, and the powder on the sieve at this time was used as the second silica powder.

  Next, the obtained first silica powder and second silica powder are put into a plastic drum so as to have a mass ratio shown in Table 1 below, and the plastic drum is tilted and rotated to be uniformly mixed. A third silica powder was obtained.

  Subsequently, using the apparatus 30 shown in FIG. 3, the third silica powder obtained by performing the same steps as in Example 1 under the conditions shown in Table 1 below is subjected to spheronization treatment, and spheronization is performed. Thus obtained silica powder 41 was obtained.

  After the spheronization treatment, the powder and ultrapure water were put in a cleaning container, and ultrasonic cleaning was performed. After ultrasonic cleaning, the mixture was filtered with a filter having an opening of 125 μm. This operation was repeated until there was no fine powder adhering to the surface of the silica powder particles.

  Finally, the powder after washing was placed in a drying container, and kept at a temperature of 300 ° C. for 24 hours while flowing nitrogen at a flow rate of 10 L / min in the dryer, thereby obtaining a synthetic amorphous silica powder.

<Example 6>
First, an amount of ultrapure water corresponding to 55.6 mol was prepared with respect to 1 mol of silicon tetrachloride. This ultrapure water was put in a container, and hydrolyzed by adding silicon tetrachloride while stirring at a temperature of 25 ° C. in a nitrogen atmosphere. After the addition of silicon tetrachloride, stirring was continued for 3 hours to produce a siliceous gel. At this time, the stirring speed was 150 rpm. Next, the siliceous gel was transferred to a drying container, put into a dryer, and dried at a temperature of 250 ° C. for 18 hours while flowing nitrogen at a flow rate of 15 L / min into the dryer to obtain a dry powder. . The dried powder was taken out from the dryer and pulverized using a roll crusher. At this time, the roll gap was adjusted to 0.2 mm, and the roll rotation speed was adjusted to 200 rpm. The pulverized dry powder was classified using a vibration sieve having an opening of 90 μm and an opening of 180 μm, thereby obtaining a first silica powder. Moreover, the dry powder under the sieve at the time of the classification which obtains the said 1st silica powder was made into the 2nd silica powder.

  Next, the obtained first silica powder and second silica powder are put into a plastic drum so as to have a mass ratio shown in Table 1 below, and the plastic drum is tilted and rotated to be uniformly mixed. A third silica powder was obtained.

  Subsequently, using the apparatus 30 shown in FIG. 3, the third silica powder obtained by performing the same steps as in Example 1 under the conditions shown in Table 1 below is subjected to spheronization treatment, and spheronization is performed. Thus obtained silica powder 41 was obtained.

  After the spheronization treatment, the powder and ultrapure water were put in a cleaning container, and ultrasonic cleaning was performed. After ultrasonic cleaning, it was filtered with a filter having an opening of 45 μm. This operation was repeated until there was no fine powder adhering to the surface of the silica powder particles.

  Finally, the powder after washing was put in a drying container, and kept at a temperature of 400 ° C. for 12 hours while flowing argon at a flow rate of 10 L / min in the dryer to obtain a synthetic amorphous silica powder.

<Example 7>
First, 1.5 mol of ultrapure water and 1.5 mol of ethanol were prepared with respect to 1 mol of tetramethoxysilane. The prepared ultrapure water and ethanol were put in a container, and tetramethoxysilane was added and hydrolyzed while stirring at a temperature of 60 ° C. in a nitrogen atmosphere. After stirring for 60 minutes after the addition of tetramethoxysilane, 25 mol of ultrapure water was further added to 1 mol of tetramethoxylane, and stirring was continued for 6 hours to produce a siliceous gel. At this time, the stirring speed was 100 rpm. Next, the siliceous gel was transferred to a drying container, put into a dryer, and dried at a temperature of 200 ° C. for 24 hours while flowing nitrogen at a flow rate of 20 L / min into the dryer to obtain a dry powder. . The dried powder was taken out from the dryer and pulverized using a roll crusher. At this time, the roll gap was adjusted to 0.25 mm, and the roll rotation speed was adjusted to 50 rpm. The pulverized dry powder was classified using a vibration sieve having an opening of 110 μm and an opening of 250 μm, thereby obtaining a first silica powder. Moreover, the second silica powder was obtained by classifying the dry powder under the sieve for obtaining the first silica powder using a vibrating sieve having an opening of 25 and an opening of 40 μm.

  Next, the obtained first silica powder and second silica powder are put into a plastic drum so as to have a mass ratio shown in Table 1 below, and the plastic drum is tilted and rotated to be uniformly mixed. A third silica powder was obtained.

  Subsequently, using the apparatus 30 shown in FIG. 3, the third silica powder obtained by performing the same steps as in Example 1 under the conditions shown in Table 1 below is subjected to spheronization treatment, and spheronization is performed. Thus obtained silica powder 41 was obtained.

  After the spheronization treatment, the powder and ultrapure water were put in a cleaning container, and ultrasonic cleaning was performed. After ultrasonic cleaning, the mixture was filtered with a filter having an opening of 75 μm. This operation was repeated until there was no fine powder adhering to the surface of the silica powder particles.

  Finally, the powder after washing was put in a drying container, and kept at a temperature of 400 ° C. for 18 hours while flowing argon at a flow rate of 10 L / min in the dryer to obtain a synthetic amorphous silica powder.

<Comparative Example 1>
The first silica powder was obtained by classification using a vibration sieve having an opening of 35 μm and an opening of 75 μm, and the dried powder under the sieve for obtaining the first silica powder was used as the second silica powder. A synthetic amorphous silica powder was obtained in the same manner as in Example 1 except that the spheroidizing treatment conditions were changed to the conditions shown in Table 2 below.

<Comparative example 2>
The first silica powder was obtained by classification using a vibrating sieve having an opening of 275 μm and an opening of 475 μm, and the dried powder under the sieve for obtaining the first silica powder was further divided into a vibrating sieve having an opening of 35 μm. As in Example 5, except that the powder on the sieve was changed to the second silica powder and the spheroidizing conditions were changed to the conditions shown in Table 2 below. Amorphous silica powder was obtained.

<Comparative Example 3>
The first silica powder was obtained by classification using a vibration sieve having an opening of 60 μm and an opening of 180 μm, and the dried powder under the sieve for obtaining the first silica powder was used as the second silica powder. A synthetic amorphous silica powder was obtained in the same manner as in Example 2 except that the spheroidizing treatment conditions were changed to the conditions shown in Table 2 below.

<Comparative example 4>
The dry powder under the sieve for obtaining the first silica powder was further classified using a vibrating sieve having an opening of 60 μm, and the powder on the sieve at this time was used as the second silica powder, and the spheroidizing treatment conditions A synthetic amorphous silica powder was obtained in the same manner as in Example 2 except that was changed to the conditions shown in Table 2 below.

<Comparative Examples 5 and 6>
A synthetic amorphous silica powder was obtained in the same manner as in Example 2 except that the first silica powder and the second silica powder were mixed at a mass ratio shown in Table 2 below to obtain a third silica powder. .

<Comparative Example 7>
The first silica powder was obtained by classification using a vibration sieve having an opening of 95 μm and an opening of 185 μm, and the dried powder under the sieve for obtaining the first silica powder was used as the second silica powder. A synthetic amorphous silica powder was obtained in the same manner as in Example 2 except that the spheroidizing treatment conditions were changed to the conditions shown in Table 2 below.

<Comparative Example 8>
The first silica powder was obtained by classification using a vibration sieve having an opening of 30 μm and an opening of 75 μm, and the dried powder under the sieve for obtaining the first silica powder was used as the second silica powder. A synthetic amorphous silica powder was obtained in the same manner as in Example 1 except that the spheroidizing treatment conditions were changed to the conditions shown in Table 2 below.

<Comparative Example 9>
A synthetic amorphous silica powder was obtained in the same manner as in Example 3 except that the spheroidizing treatment conditions were changed to the conditions shown in Table 2 below.

<Comparative Example 10>
The first silica powder was obtained by classification using a vibration sieve having an opening of 75 μm and an opening of 175 μm, and the dried powder under the sieve for obtaining the first silica powder was used as the second silica powder. The first silica powder and the second silica powder were mixed at a mass ratio shown in Table 2 below to obtain a third silica powder, and the spheroidizing treatment conditions were changed to the conditions shown in Table 2 below. Except for the above, a synthetic amorphous silica powder was obtained in the same manner as in Example 2.

<Comparative Example 11>
The first silica powder was obtained by classification using a vibration sieve having an opening of 90 μm and an opening of 185 μm, and the dried powder under the sieve for obtaining the first silica powder was further divided into a vibration sieve having an opening of 40 μm. The powder on the sieve at this time was used as the second silica powder, and the first silica powder and the second silica powder were mixed at a mass ratio shown in Table 2 below to obtain the third silica powder. A synthetic amorphous silica powder was obtained in the same manner as in Example 2 except that the obtained conditions were changed to the conditions shown in Table 2 below.

<Comparative Example 12>
The second silica powder was obtained by classifying the dried powder under the sieve for obtaining the first silica powder using a vibrating sieve having openings of 25 μm and 40 μm, and the first silica powder and the second silica. The powder was mixed at a mass ratio shown in Table 2 below to obtain a third silica powder, and the spheroidizing treatment conditions were changed to the conditions shown in Table 2 below, as in Example 2. A synthetic amorphous silica powder was obtained.

<Comparative Example 13>
The dried powder under the sieve at the time of classification to obtain the first silica powder was further classified using a vibrating sieve having an opening of 50 μm, and the powder on the sieve at this time was used as the second silica powder, the first silica powder And the second silica powder were mixed at a mass ratio shown in Table 2 below to obtain a third silica powder, and the spheroidizing treatment conditions were changed to the conditions shown in Table 2 below. As in Example 2, a synthetic amorphous silica powder was obtained.

<Comparative example 14>
The dried powder under the sieve at the time of classification to obtain the first silica powder was further classified using a vibrating sieve having an opening of 35 μm, and the powder on the sieve at this time was used as the second silica powder, the first silica powder And the second silica powder were mixed at a mass ratio shown in Table 2 below to obtain a third silica powder, and the spheroidizing treatment conditions were changed to the conditions shown in Table 2 below. As in Example 2, a synthetic amorphous silica powder was obtained.

<Comparative Example 15>
The first silica powder was obtained by classification using a vibration sieve having an opening of 90 μm and an opening of 190 μm, and the dried powder under the sieve for obtaining the first silica powder was further divided into a vibration sieve having an opening of 50 μm. The powder on the sieve at this time was used as the second silica powder, and the first silica powder and the second silica powder were mixed at a mass ratio shown in Table 2 below to obtain the third silica powder. A synthetic amorphous silica powder was obtained in the same manner as in Example 2 except that the obtained conditions were changed to the conditions shown in Table 2 below.

<Comparative Example 16>
The dried powder under the sieve at the time of classification to obtain the first silica powder was further classified using a vibrating sieve having an opening of 35 μm, and the powder on the sieve at this time was used as the second silica powder, and the spheronization treatment A synthetic amorphous silica powder was obtained in the same manner as in Example 5 except that the conditions were changed to the conditions shown in Table 3 below.

<Comparative Example 17>
The second silica powder was obtained by classifying the dried powder under the sieve for obtaining the first silica powder using a vibrating sieve having openings of 25 μm and 40 μm, and the first silica powder and the second silica. The powder was mixed at a mass ratio shown in Table 3 below to obtain a third silica powder, and the spheroidizing treatment conditions were changed to the conditions shown in Table 3 below, as in Example 5. A synthetic amorphous silica powder was obtained.

<Comparative Example 18>
Except that the first silica powder and the second silica powder were mixed at a mass ratio shown in Table 3 below to obtain a third silica powder, and the spheroidizing treatment conditions were changed to the conditions shown in Table 3 below. Obtained a synthetic amorphous silica powder in the same manner as in Example 4.

<Comparative Example 19>
Except that the first silica powder and the second silica powder were mixed at a mass ratio shown in Table 3 below to obtain a third silica powder, and the spheroidizing treatment conditions were changed to the conditions shown in Table 3 below. As in Example 1, a synthetic amorphous silica powder was obtained.

<Comparative Example 20>
The first silica powder was obtained by classification using a vibrating sieve having an opening of 200 μm and an opening of 400 μm, and the dried powder under the sieve for obtaining the first silica powder was further divided into a vibrating sieve having an opening of 50 μm. The powder on the sieve at this time was used as the second silica powder, and the first silica powder and the second silica powder were mixed at a mass ratio shown in Table 3 below to obtain the third silica powder. A spheroidized silica powder 41 was obtained in the same manner as in Example 6 except that the obtained conditions were changed to the conditions shown in Table 3 below.

  After the spheronization treatment, a synthetic amorphous silica powder was obtained by washing and drying under the same conditions as in Example 5.

<Comparative Example 21>
The first silica powder was obtained by classification using a vibrating sieve having an opening of 200 μm and an opening of 400 μm, and the dried powder under the sieve for obtaining the first silica powder was further divided into a vibrating sieve having an opening of 50 μm. The powder on the sieve at this time was used as the second silica powder, and the first silica powder and the second silica powder were mixed at a mass ratio shown in Table 3 below to obtain the third silica powder. A spheroidized silica powder 41 was obtained in the same manner as in Example 7 except that the obtained conditions and the spheroidizing treatment conditions were changed to the conditions shown in Table 3 below.

  After the spheronization treatment, a synthetic amorphous silica powder was obtained by washing and drying under the same conditions as in Example 5.

For the powders obtained in Examples 1 to 7 and Comparative Examples 1 to 21, the arithmetic average particle size, particle size distribution, solid bulk density, circularity, BET specific surface area, theoretical specific surface area, BET specific surface area / theoretical specific surface area, true The density and the intraparticle space ratio were measured or calculated. These results are shown in the following Tables 4-6. These values shown in Tables 4 to 6 are values obtained by producing synthetic silica powder under the same conditions five times for each example or comparative example and averaging the above measured values and the like.

  (i) Arithmetic mean particle size, particle size distribution: The area of a particle image taken with a dry particle image analyzer (Spectris Corp., model name: Morphologi G3) is measured, and the diameter of the circle having the same area, that is, the area The equivalent circle diameter (CE Diameter) was obtained and used as the “particle diameter” of the photographed particle image.

The arithmetic average particle diameter D _{NAV based on the} number was the arithmetic average of the above “particle diameter” for all the photographed particles (about 6000 or more per sample). Also, classify the “particle size” of these particles into 2.5 μm sections, measure the number of particles in each section, calculate the percentage of the total number of particles in each section, and calculate this for each section. The number-based particle size distribution was obtained. In this number-based particle size distribution, the most frequent particle size value was defined as the mode value D _NM of the number-based particle size distribution.

Further, assuming that the silica density is 2.2 g / cm ³ , the mass of the particles in each section is calculated from the number of particles in each section classified by the 2.5 μm section, and the mass of the particles in each section is The percentage (frequency) in the mass was calculated and the volume-based particle size distribution was determined.

  (ii) Circularity: The circumference of a particle image taken using the dry particle image analyzer is measured, and the circumference and the equivalent circle diameter are substituted into the following equation (1) to obtain the particle 1 The circularity of each particle was calculated, and the arithmetic average of the total number of particles was defined as the circularity of the powder.

Circularity per particle = π × area equivalent diameter / perimeter (1)
(iii) Hard bulk density: Measured using a powder tester PT-X manufactured by Hosokawa Micron.

  (iV) BET specific surface area: Measured by the BET three-point method using a measuring device (QUANTACHROME AUTOSORB-1 MP). In the BET three-point method, the slope A was obtained from the nitrogen adsorption amount with respect to three relative pressure points, and the specific surface area value was obtained from the BET equation. The nitrogen adsorption amount was measured under conditions of 150 ° C. and 60 minutes.

(v) Theoretical specific surface area: In the following equation (2), D was calculated from the following equation (3), assuming that D is the average particle diameter D _L50 of the powder and ρ is the true density of 2.2 g / cm ³ .

Theoretical specific surface area = 6 / (D × ρ) (2)
Theoretical specific surface area of the powder = 2.73 / D _L50 (3)
The average particle size D _L50 is obtained by measuring the median value of the volume-based particle size distribution measured using a laser diffraction / scattering particle distribution measuring apparatus (model name: LA-950, manufactured by HORIBA) three times. The average value.

  (vi) BET specific surface area / theoretical specific surface area: calculated from the measured specific surface area and theoretical specific surface area.

  (vii) True density: According to JIS R7212 carbon block measurement method (d) True specific gravity measurement, true density measurement was performed three times, and this average value was calculated.

  (viii) In-particle space ratio: The obtained powder is embedded in a resin and polished to obtain a powder cross section. The powder cross section was observed by SEM (scanning electron microscope). For the 50 powder particles, the cross-sectional area and the area of the space, if any, were measured and calculated from the following equation (4).

          Intraparticle space ratio = total area in the particle / total area of the particle break (4)

<Evaluation 1>
The impurity concentrations of the powders obtained in Examples 1 to 7 and Comparative Examples 1 to 21 were measured. The results are shown in the following Tables 7 to 9. In addition, these values shown in Table 7 to Table 9 are values obtained by producing synthetic silica powder under the same conditions five times for each example or comparative example, and averaging the measured values for them.

  (i) Na, K, Ca, Fe, Al, P: The powder was thermally decomposed with hydrofluoric acid and sulfuric acid, and after heating and condensing, a constant volume liquid was prepared using dilute nitric acid. This constant volume liquid was analyzed with a high frequency inductively coupled plasma mass spectrometer (model name: SII Nanotechnology SPQ9000).

  (ii) B: The powder was thermally decomposed with hydrofluoric acid, and after heating and condensing, a constant volume liquid was prepared using ultrapure water. This constant volume liquid was analyzed with a high frequency inductively coupled plasma mass spectrometer (model name: SII Nanotechnology SPQ9000).

  (iii) C: Iron, tungsten, and tin were added to the powder as auxiliary combustors, and analysis was performed in an oxygen atmosphere using a high-frequency furnace combustion-infrared absorption method (model name: HORIBA EMIA-920V).

  (iv) Cl: Ultra pure water is mixed with synthetic amorphous silica powder, and Cl is leached under ultrasonic waves. The synthetic amorphous silica powder and the leachate were separated by a centrifuge, and the leachate was analyzed by ion chromatography (model name: Dionex DX-500).

(v) OH: Measured at a peak height near 3660 cm ⁻¹ with a Fourier transform infrared spectrometer (model name: Thermo Fisher Nicolet 4700FT-IR).

<Comparison test and evaluation 2>
Using the powders obtained in Examples 1 to 7 and Comparative Examples 1 to 25, a rectangular parallelepiped block material having a length of 20 mm, a width of 20 mm, and a height of 40 mm was produced, and the number of bubbles generated in the block material was evaluated. . The results are shown in the following Tables 7 to 9. Specifically, powder was put into a carbon crucible, and this was heated to 2200 ° C. with a carbon heater in a 2.0 × 10 ⁴ Pa vacuum atmosphere, and held for 48 hours to produce a block material. This block material was heat-treated at a temperature of 1600 ° C. for 48 hours in a 5.0 × 10 ² Pa vacuum atmosphere. After the heat treatment, the block material is cut into a 20 mm × 20 mm square cross section at a height of 20 mm, polished, and the number of bubbles observed in a 2 mm depth and 2 mm width region is measured from the surface (cross section) of the block material. These average values were defined as the average number of bubbles.

As is apparent from Tables 1 to 9, the powders of Examples 1 to 7 have a lower concentration of K than Comparative Examples 1 to 19. In addition, in the powders of Examples 1 to 5, in the synthetic silica glass product at high temperature and reduced pressure, compared to the powders of Comparative Examples 5, 8, 9 and the like which were subjected to spheroidizing treatment and were made from the same fumed silica. It can be seen that both the carbon concentration and the chlorine concentration that cause the generation or expansion of bubbles are extremely low. Further, the powder of Example 6 using silicon tetrachloride as a raw material has a low carbon concentration, and the powder of Example 7 using tetramethoxysilane as a raw material has a low chlorine concentration, while fumed silica is used as a raw material. In the powders of Examples 1 to 5, it can be seen that both the carbon concentration and the chlorine concentration are extremely low.

  Moreover, when the block manufactured using the powder of Examples 1-7 is compared with the block manufactured using the powder of Comparative Examples 1-21, it turns out that the number of produced | generated bubbles is reduced significantly. From this, it was confirmed that the use of the synthetic amorphous silica powder of the present invention makes it possible to produce a synthetic silica glass product with very little generation or expansion of bubbles when used in a high temperature and reduced pressure environment.

  The synthetic amorphous silica powder of the present invention is used as a raw material for producing synthetic silica glass products such as crucibles and jigs used for producing single crystals for semiconductor applications.

Claims (5)

A synthetic amorphous silica powder obtained by subjecting the granulated silica powder to spheroidization treatment, washing and drying,
Volume mode particle size distribution mode D _VM is 50 to 200 μm, frequency is 2 to 5%, number based arithmetic average particle size D _NAV is 0.4 × D _VM ± 10 μm, and number based particle size distribution is the mode D _NM is the frequency of 3-10% in 5 to 10 [mu] m, compacted bulk density of 1.38 g / cm ³ or more 1.50 g / cm ³ or less, roundness of 0.85 to 1.00 The value obtained by dividing the BET specific surface area by the theoretical specific surface area is 1.00 to 1.35, the true density is 2.10 to 2.20 g / cm ³ , and the intra-particle space ratio is 0.00 to 0.05. A synthetic amorphous silica powder. Silicon tetrachloride is hydrolyzed to form a siliceous gel, and the siliceous gel is dried to form a dry powder. After pulverizing the dry powder, classification is performed to determine the mode of the volume-based particle size distribution. A granulation step of obtaining a first silica powder having a D _VM of 60 to 300 μm and a second silica powder having a mode number D _{NM of 5} to 20 μm and a number-based particle size distribution;
The first silica powder and the second silica powder are uniformly mixed at a mass ratio (first silica powder: second silica powder) at a ratio of 1: 0.005-0.05 to obtain a third silica powder. A powder mixing step;
The third silica powder is introduced at a predetermined supply rate into a plasma torch in which argon is introduced at a predetermined flow rate and plasma is generated at a predetermined high-frequency output, and heated at a temperature from 2000 ° C. to the boiling point of silicon dioxide. And spheroidizing process by thermal plasma to be melted,
A washing step for removing fine particles adhering to the surface of the spheroidized silica powder after the spheronization step;
A drying step of drying the silica powder after the washing step in this order,
When the high frequency output (W) in the spheroidizing step is A and the supply rate (kg / hr) of silica powder is B, the value of A / B (W · hr / kg) is 1.0 × 10 ⁴ or more. Adjusted to be
The frequency of 2-5% mode value D _VM is in 50~200μm of volume-based particle size distribution, the arithmetic mean particle diameter D _NAV number criterion 0.4 × D _VM ± 10μm and the number-based particle size distribution, the mode D _NM is the frequency of 3-10% in 5 to 10 [mu] m, compacted bulk density of 1.38 g / cm ³ or more 1.50 g / cm ³ or less, roundness of 0.85 to 1.00 The value obtained by dividing the BET specific surface area by the theoretical specific surface area is 1.00 to 1.35, the true density is 2.10 to 2.20 g / cm ³ , and the intra-particle space ratio is 0.00 to 0.05. A method for producing a synthetic amorphous silica powder. Hydrolysis of the organic silicon compound produces a siliceous gel, the siliceous gel is dried to form a dry powder, and the dried powder is pulverized and classified to determine the mode of particle size distribution on a volume basis. A granulation step of obtaining a first silica powder having a value D _VM of 60 to 300 μm and a second silica powder having a mode value D _{NM of 5} to 20 μm in a number-based particle size distribution;
The first silica powder and the second silica powder are uniformly mixed at a mass ratio (first silica powder: second silica powder) at a ratio of 1: 0.005-0.05 to obtain a third silica powder. A powder mixing step;
The third silica powder is introduced at a predetermined supply rate into a plasma torch in which argon is introduced at a predetermined flow rate and plasma is generated at a predetermined high-frequency output, and heated at a temperature from 2000 ° C. to the boiling point of silicon dioxide. And spheroidizing process by thermal plasma to be melted,
A washing step for removing fine particles adhering to the surface of the spheroidized silica powder after the spheronization step;
A drying step of drying the silica powder after the washing step in this order,
When the high frequency output (W) in the spheroidizing step is A and the supply rate (kg / hr) of silica powder is B, the value of A / B (W · hr / kg) is 1.0 × 10 ⁴ or more. Adjusted to be
The frequency of 2-5% mode value D _VM is in 50~200μm of volume-based particle size distribution, the arithmetic mean particle diameter D _NAV number criterion 0.4 × D _VM ± 10μm and the number-based particle size distribution, the mode D _NM is the frequency of 3-10% in 5 to 10 [mu] m, compacted bulk density of 1.38 g / cm ³ or more 1.50 g / cm ³ or less, roundness of 0.85 to 1.00 The value obtained by dividing the BET specific surface area by the theoretical specific surface area is 1.00 to 1.35, the true density is 2.10 to 2.20 g / cm ³ , and the intra-particle space ratio is 0.00 to 0.05. A method for producing a synthetic amorphous silica powder. The fumed silica is hydrolyzed to form a siliceous gel, the siliceous gel is dried to form a dry powder, and the dry powder is pulverized and classified to determine the mode of the volume-based particle size distribution. A granulation step of obtaining a first silica powder having a D _VM of 60 to 300 μm and a second silica powder having a mode number D _{NM of 5} to 20 μm and a number-based particle size distribution;
The first silica powder and the second silica powder are uniformly mixed at a mass ratio (first silica powder: second silica powder) at a ratio of 1: 0.005-0.05 to obtain a third silica powder. A powder mixing step;
The third silica powder is introduced at a predetermined supply rate into a plasma torch in which argon is introduced at a predetermined flow rate and plasma is generated at a predetermined high-frequency output, and heated at a temperature from 2000 ° C. to the boiling point of silicon dioxide. And spheroidizing process by thermal plasma to be melted,
A washing step for removing fine particles adhering to the surface of the spheroidized silica powder after the spheronization step;
A drying step of drying the silica powder after the washing step in this order,
When the high frequency output (W) in the spheroidizing step is A and the supply rate (kg / hr) of silica powder is B, the value of A / B (W · hr / kg) is 1.0 × 10 ⁴ or more. Adjusted to be
The frequency of 2-5% mode value D _VM is in 50~200μm of volume-based particle size distribution, the arithmetic mean particle diameter D _NAV number criterion 0.4 × D _VM ± 10μm and the number-based particle size distribution, the mode D _NM is the frequency of 3-10% in 5 to 10 [mu] m, compacted bulk density of 1.38 g / cm ³ or more 1.50 g / cm ³ or less, roundness of 0.85 to 1.00 The value obtained by dividing the BET specific surface area by the theoretical specific surface area is 1.00 to 1.35, the true density is 2.10 to 2.20 g / cm ³ , and the intra-particle space ratio is 0.00 to 0.05. A method for producing a synthetic amorphous silica powder.   In the granulation step, the first silica powder is obtained by pulverizing the dry powder and then classifying the pulverized dry powder using a sieve, and the second silica powder is obtained by classifying the first silica powder. The method for producing a synthetic amorphous silica powder according to any one of claims 2 to 4, which is obtained by further classifying the obtained dry powder under a sieve during classification.