Metal oxide powder and its manufacturing method

Abstract

A method for producing a metal oxide powder according to the present invention includes a step for heating in the presence of halogen, in which a metal oxide is heated in the presence of halogen. A metal oxide powder according to the present invention is a metal oxide powder of a metal other than uranium, iron, and titanium, the content of uranium in the metal oxide powder is at most 1.0 mass ppb, the content of iron is at most 100 mass ppm in terms of Fe2O3, and the content of titanium is at most 8 mass ppm in terms of TiO2. According to the present invention, it is possible to provide: a method which is for producing a metal oxide powder and with which a high-purity metal oxide powder can be produced by removing impurities from the metal oxide powder; and a metal oxide powder produced by the production method.

Description

[Technical field]

[0001] The present invention relates to a method for producing a metal oxide powder, which is capable of producing a high-purity metal oxide powder, and to a metal oxide powder obtained by the method. [Background technology]

[0002] Conventionally, impurities have been removed from metal oxide powders by various methods. For example, magnetizable foreign matter, which is an impurity, has been removed from the metal oxide powder by passing the metal oxide powder through a foreign matter removal device incorporating a magnet with a magnetic flux density of 4,000 gauss or more (see, for example, Patent Document 1). [Prior art documents] [Patent documents]

[0003] [Patent Document 1] JP 2010-159168 A Summary of the Invention [Problem to be solved by the invention]

[0004] However, users' requirements for the purity of metal oxide powders are becoming increasingly higher, and there is a problem in that the impurity removal method using magnets as described in Patent Document 1 cannot meet the user's requirements for the purity of metal oxide powders. Impurities that are difficult to remove include uranium, titanium, and iron. Therefore, an object of the present invention is to provide a technique required for removing all of the impurities, uranium, titanium and iron, from a metal oxide powder to obtain a high-purity metal oxide powder. [Means for solving the problem]

[0005] As a result of extensive research, the present inventors have found that the above-mentioned problems can be solved by heating a metal oxide in the presence of a halogen. The present invention is based on the above findings and has the following gist. [1] A method for producing a metal oxide powder, comprising a heating step in the presence of a halogen, of heating a metal oxide in the presence of a halogen. [2] The method for producing a metal oxide powder according to the above [1], wherein the halogen is chlorine. [3] The method for producing a metal oxide powder according to the above [1] or [2], further comprising a first reduced pressure heating step of heating the metal oxide under reduced pressure after the heating step in the presence of a halogen. [4] The method for producing a metal oxide powder according to the above [3], wherein a cycle consisting of the heating step in the presence of a halogen and the first heating step under reduced pressure is repeated two or more times. [5] The method for producing a metal oxide powder according to any one of the above [1] to [4], further comprising a heating step in the presence of an inert gas, in which the metal oxide is heated in the presence of an inert gas. [6] The method for producing a metal oxide powder according to the above [5], wherein the heating temperature in the heating step in the presence of an inert gas is higher than the heating temperature in the heating step in the presence of a halogen. [7] The method for producing a metal oxide powder according to the above [5] or [6], further comprising a second reduced pressure heating step of heating the metal oxide under reduced pressure after the heating step in the presence of an inert gas. [8] The method for producing a metal oxide powder according to the above [7], wherein a cycle consisting of the heating step in the presence of an inert gas and the second heating step under reduced pressure is repeated two or more times. [9] The method for producing a metal oxide powder according to any one of the above [1] to [8], wherein the heating temperature in the heating step in the presence of a halogen is a temperature at which a desired impurity reacts with the halogen to become a halide.

[10] The method for producing a metal oxide powder according to any one of the above [1] to [9], wherein the metal oxide is at least one metal oxide selected from the group consisting of silicon dioxide and aluminum oxide.

[11] Metal oxide powders of metals other than uranium, iron and titanium, The metal oxide powder has a uranium content of 1.0 ppb by mass or less, an iron content of 100 ppm by mass or less, calculated as Fe2O3, and a titanium content of 10 ppm by mass or less, calculated as TiO2. Effect of the Invention

[0006] According to the present invention, it is possible to provide a method for producing a metal oxide powder that can produce a high-purity metal oxide powder by removing impurities from a metal oxide powder, and a metal oxide powder produced by the method. [Brief description of the drawings]

[0007] [Figure 1] FIG. 1 is a diagram showing an outline of the temperature and pressure conditions of the chlorine treatment in the examples. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0008] Hereinafter, the method for producing the metal oxide powder of the present invention will be described using the method for producing the silicon dioxide powder of one embodiment of the present invention as an example. Note that the method for producing the metal oxide powder of the present invention is not limited to the method for producing the silicon dioxide powder of one embodiment of the present invention. A method for producing silicon dioxide powder according to one embodiment of the present invention includes the steps of: preparing a silicon dioxide powder raw material (A); melting and spheroidizing the silicon dioxide powder raw material to produce an unclassified molten spherical silicon dioxide powder (B); classifying the unclassified molten spherical silicon dioxide powder to produce molten spherical silicon dioxide powder of different particle sizes (C); blending the molten spherical silicon dioxide powders of different particle sizes to produce a blend of molten spherical silicon dioxide powder (D); chlorinating the blend of molten spherical silicon dioxide powder (E); and manufacturing the blend (F).

[0009] (Process (A)) In step (A), a silicon dioxide powder raw material is prepared. Examples of silicon dioxide powder raw materials include silica powder obtained by crushing natural silica, silica gel synthesized by a wet reaction between an alkali silicate and a mineral acid, crushed gel obtained from an alkoxysilane by a sol-gel method, and metal silicon particles. Among these silicon dioxide powder raw materials, crushed powder of natural silica is preferred in terms of production cost and ease of particle size adjustment of raw material powder. In addition, in order to produce high-purity silicon dioxide powder using silica powder, silica powder of natural high-purity silica having a SiO2 purity of 99.5 mass% or more is usually used. However, since natural high-purity silica is rare, raw material costs are high and raw material supply may become unstable. In the method for producing silicon dioxide powder according to one embodiment of the present invention, high-purity silicon dioxide powder can be produced even using natural silica having a low SiO2 purity by chlorine treatment described later, so that raw material costs can be reduced.

[0010] Silica powder, which is a raw material for silicon dioxide powder, is produced, for example, as follows: After washing the silica stone with water, it is pulverized using a pulverizer such as a vibration mill or a ball mill to produce silica powder. Then, the particle size distribution of the produced silica powder is adjusted using a vibration sieve or a classifier.

[0011] The particle size of the silicon dioxide powder raw material can be freely changed depending on the desired product particle size. In general, the average particle size is, for example, 100 μm or less, and it may contain a large amount of fine powder of 1 μm or less, which tends to aggregate with other powders.

[0012] (Process (B)) In step (B), the silicon dioxide powder raw material is melted and spheroidized to produce unclassified molten spherical silicon dioxide powder. For example, the silicon dioxide powder raw material is sprayed into a flame. As a result, the silicon dioxide powder raw material melts and at the same time becomes spheroidized due to surface tension. The droplets of the silicon dioxide powder raw material that pass through the flame are rapidly cooled and become unclassified molten spherical silicon dioxide powder, which is amorphous spherical silicon dioxide. The flame is formed. As the fuel gas for the combustion, for example, propane, butane, propylene, acetylene, hydrogen, etc. are used, and as the combustion supporting gas, for example, air, oxygen, etc. are used. The flame temperature is, for example, 2000 to 2400°C.

[0013] The sphericity of the unclassified molten spherical silicon dioxide powder can be adjusted, for example, by the heat of the flame and the injection amount of silicon dioxide powder raw material. If the heat of the flame is insufficient, the melting and spheroidization is insufficient, and the sphericity of the unclassified molten spherical silicon dioxide powder is low. If the heat of the flame is too high, the production cost of the unclassified molten spherical silicon dioxide powder increases. In addition, if the injection amount of the silicon dioxide powder raw material is too large, the particles are fused together after melting, and the sphericity of the unclassified molten spherical silicon dioxide powder decreases. If the injection amount of the silicon dioxide powder raw material is too small, the production cost of the unclassified molten spherical silicon dioxide powder increases.

[0014] (Process (C)) In step (C), the unclassified molten spherical silicon dioxide powder is classified to produce molten spherical silicon dioxide powder by particle size. For example, when collecting the unclassified molten spherical silicon dioxide powder obtained by passing the silicon dioxide powder raw material through a flame, it is classified according to particle size before collection. Coarse particles may be collected using a gravity settling chamber, a cyclone, etc., and fine particles may be collected using a bag filter, an electric dust collector, etc. The classified molten spherical silicon dioxide powder may then be further classified according to particle size using a classifier such as an air flow classifier, a vibrating sieve, or a cylindrical sieve.

[0015] (Process (D)) In step (D), molten spherical silicon dioxide powders of different particle sizes are blended to produce a blend of molten spherical silicon dioxide powders. For example, molten spherical silicon dioxide powders of different particle sizes are mixed in a ratio such that the molten spherical silicon dioxide powders of different particle sizes are combined to form the closest packing. The mixed molten spherical silicon dioxide powders are then mixed using a mixer such as a V-type blender, a double cone blender, or an air blender. This produces a blend of molten spherical silicon dioxide powders with good packing properties.

[0016] (Process (E)) In step (E), the blend of fused spherical silicon dioxide powder is chlorinated. The chlorine treatment of the blend of fused spherical silicon dioxide powder is described in detail below.

[0017] The chlorination treatment in step (E) includes a heating step in the presence of chlorine in which the blend is heated in the presence of chlorine. This causes impurities in the blend to be chlorinated into chlorides, which are then sublimated. As a result, the impurities are separated from the blend, and the purity of the blend is increased.

[0018] The heating temperature in the heating step in the presence of chlorine is not particularly limited as long as the impurities are chlorinated in the presence of chlorine to form chlorides and silicon dioxide is not chlorinated as much as possible. In addition, by adjusting the heating temperature in the heating step in the presence of chlorine to a temperature at which the desired impurities are chlorinated to form chlorides, the desired impurities can be removed from the blend. From the viewpoint of efficiently converting the impurities in the blend to chlorides and sublimating them in the presence of chlorine and from the viewpoint of suppressing the chlorination of silicon dioxide, the heating temperature in the heating step in the presence of chlorine is preferably 400 to 1200°C, more preferably 430 to 1100°C, and even more preferably 450 to 1050°C. In addition, from the viewpoint of converting the impurities in the blend to chlorides as much as possible and from the viewpoint of the efficiency of the chlorination treatment, the heating time in the heating step in the presence of chlorine is preferably 1 to 120 minutes, more preferably 1 to 60 minutes, and even more preferably 3 to 45 minutes. In addition, when the cycle described later is repeated, the above heating time is the heating time in the heating step in the presence of chlorine in each cycle. From the viewpoint of sublimating the chlorides generated by the chlorination of the impurities and from the viewpoint of the amount of chlorine in the atmosphere, the pressure around the blend in the heating step in the presence of chlorine is preferably 0.0001 to 100 kPa, more preferably 0.0005 to 50 kPa, and even more preferably 0.01 to 20 kPa. Since chlorine is corrosive, it is preferable to supply chlorine by mixing it with an inert gas such as nitrogen or argon. From the viewpoint of converting the impurities in the blend into chlorides and from the viewpoint of the corrosiveness of chlorine, the content of chlorine in the atmosphere around the blend is preferably 30% by volume or more, more preferably 50% by volume or more, and even more preferably 80% by volume or more.

[0019] From the viewpoint that impurities in the center of silicon dioxide particles can be removed while maintaining the shape of silicon dioxide powder by heating in the presence of chlorine, the average particle size of silicon dioxide powder from which impurities are removed by heating in the presence of chlorine is preferably 0.002 to 100 μm, more preferably 0.005 to 50 μm, even more preferably 0.01 to 11 μm, and most preferably 0.01 to 10 μm. The average particle size of silicon dioxide powder is the median size of the particle size distribution measured by a laser diffraction scattering method.

[0020] The chlorine treatment may further include a first reduced pressure heating step in which the blend is heated under reduced pressure after the heating step in the presence of chlorine. This allows the chlorides generated by the chlorination of the impurities to be more completely sublimated. As a result, the purity of the blend can be further increased. In addition, the chlorine remaining in the blend can also be removed from the blend. If the chlorine content in the blend is high, HCl may be generated and corrode the lead wire when the blend is used as a semiconductor encapsulation material. From the viewpoint of more completely sublimating the chlorides generated by the chlorination of the impurities and from the viewpoint of the cost required for heating, the heating temperature in the first reduced pressure heating step is preferably 600 to 1250°C, more preferably 700 to 1200°C, and even more preferably 800 to 1150°C. From the viewpoint of more completely sublimating the chlorides generated by the chlorination of the impurities and from the viewpoint of the cost required for reducing the pressure, the pressure around the blend in the first reduced pressure heating step is preferably 0.0001 to 100 kPa, more preferably 0.0005 to 50 kPa, and even more preferably 0.01 to 20 kPa, in absolute pressure. From the viewpoint of more completely sublimating the chlorides generated by the chlorination of the impurities and from the viewpoint of productivity, the time for maintaining the reduced pressure in the first reduced pressure heating step is preferably 5 to 180 minutes, more preferably 10 to 120 minutes, and even more preferably 15 to 60 minutes. In addition, when the cycle described later is repeated, the time for maintaining the above-mentioned predetermined pressure is the time for maintaining the predetermined pressure in the first reduced pressure heating step in each cycle.

[0021] It is preferable to repeat the cycle consisting of the heating step in the presence of chlorine and the first heating step under reduced pressure two or more times. This allows more impurities to be separated from the blend, and the purity of the blend is further increased. From the viewpoint of increasing the purity of the blend and the viewpoint of productivity, it is preferable to repeat the cycle consisting of the heating step in the presence of halogen and the first heating step under reduced pressure 2 to 20 times, more preferably 3 to 15 times, and even more preferably 4 to 9 times.

[0022] The chlorine treatment preferably further includes a heating step in the presence of an inert gas in which the blend is heated in the presence of an inert gas. This makes it possible to more completely remove the chlorine remaining in the blend. As a result, the purity of the blend is further increased. Examples of the inert gas include nitrogen gas and argon gas. Of these inert gases, nitrogen gas is preferred from the viewpoint of cost.

[0023] The heating temperature in the heating step in the presence of an inert gas is preferably higher than the heating temperature in the heating step in the presence of chlorine, but is preferably 200° C. or less higher, more preferably 100° C. or less higher, and even more preferably 50° C. or less higher than the heating temperature in the heating step in the presence of a halogen.

[0024] The chlorine treatment may further include a second reduced pressure heating step in which the blend is heated under reduced pressure after the heating step in the presence of an inert gas. This allows the chlorine remaining in the blend to be more completely removed. As a result, the purity of the blend is further increased. From the viewpoint of removing chlorine from the blend and the cost required for heating, the heating temperature in the second reduced pressure heating step is preferably 600 to 1250°C, more preferably 700 to 1200°C, and even more preferably 800 to 1150°C. From the viewpoint of removing chlorine from the blend and the cost required for reducing pressure, the pressure around the blend in the second reduced pressure heating step is preferably 0.0001 to 100 kPa, more preferably 0.005 to 50 kPa, and even more preferably 0.001 to 1 kPa, in absolute pressure. From the viewpoint of removing chlorine from the blend and from the viewpoint of productivity, the time for maintaining the reduced pressure in the second reduced pressure heating step is preferably 5 to 180 minutes, more preferably 10 to 120 minutes, and further preferably 15 to 60 minutes. When the cycle described below is repeated, the time for maintaining the above-mentioned predetermined pressure is the time for maintaining the predetermined pressure in the second reduced pressure heating step in each cycle.

[0025] It is preferable to repeat the cycle consisting of the heating step in the presence of an inert gas and the second heating step under reduced pressure two or more times. This makes it possible to more completely remove chlorine remaining in the blend product, and to further increase the purity of the blend product. From the viewpoint of increasing the purity of the blend product and the viewpoint of productivity, it is preferable to repeat the cycle consisting of the heating step in the presence of an inert gas and the second heating step under reduced pressure 2 to 20 times, more preferably 3 to 15 times, and even more preferably 5 to 12 times.

[0026] (Process (F)) In step (F), the blend is commercialized. Specifically, the blend may be subjected to a surface treatment using a silane coupling agent, or may be subjected to a classification treatment to precisely remove coarse particles, if desired. The blend that has undergone step (F) is shipped as a product.

[0027] (Silicon dioxide powder produced by the method for producing silicon dioxide powder according to one embodiment of the present invention) By carrying out the treatment under the above-mentioned preferred conditions, a silicon dioxide powder with a significantly reduced amount of impurities can be obtained. When silicon dioxide powder is produced by the method for producing silicon dioxide powder according to one embodiment of the present invention, the content of impurities in the silicon dioxide powder can be reduced. For example, by the method for producing silicon dioxide powder according to one embodiment of the present invention, silicon dioxide powder can be produced in which the uranium content is 2.0 mass ppb or less, the iron content is 100 mass ppm or less in terms of Fe2O3, and the titanium content is 10 mass ppm or less in terms of TiO2 oxide. When silicon dioxide powder is used as a semiconductor encapsulant, if the uranium content in the silicon dioxide powder is high, memory errors may occur in semiconductor elements due to alpha rays emitted from the silicon dioxide powder. In addition, if the iron and titanium content in the silicon dioxide powder is high, short circuits may occur between wirings of semiconductors, especially semiconductors for automobiles.

[0028] By adjusting the heating temperature in the heating step in the presence of chlorine to a temperature at which the desired impurities react with chlorine to form chlorides, the content of impurities other than uranium, iron, and titanium can also be reduced. For example, the content of sodium, aluminum, potassium, calcium, and magnesium in the silicon dioxide powder can be reduced. In addition, the magnetic foreign matter in the silicon dioxide powder can also be reduced. When the silicon dioxide powder is used as a filler for a resin composition, if the content of sodium in the silicon dioxide powder is high, sodium may become a catalytic poison for the curing catalyst, and the curing of the resin composition may be delayed. In addition, if the content of aluminum in the silicon dioxide powder is high, the flowability of the resin composition may decrease. Furthermore, if the content of potassium, calcium, and magnesium in the silicon dioxide powder is high, the moldability of the resin composition may decrease. When silicon dioxide powder is used as a semiconductor encapsulant, if the content of sodium in the silicon dioxide powder is high, it may cause wiring corrosion or the performance of the semiconductor element to deteriorate. Also, if the number of magnetizable foreign matter in the silicon dioxide powder is high, it may cause short circuit defects between the leads of semiconductors, especially semiconductors for automobiles. If the content of iron, potassium, calcium and magnesium in the silicon dioxide powder is high, the whiteness of the silicon dioxide powder may be reduced.

[0029] The number of magnetizable foreign matters having a particle diameter of 20 to 45 μm in 50 g of silicon dioxide powder is preferably 200 pieces / 50 g or less, 150 pieces / 50 g or less, 100 pieces / 50 g or less, 50 pieces / 50 g or less, 25 pieces / 50 g or less, or 10 pieces / 50 g or less. When the number of magnetizable foreign matters having a particle diameter of 20 to 45 μm in 50 g of silicon dioxide powder is 600 pieces / 50 g or less, when the silicon dioxide powder is used in a semiconductor encapsulation material, it is possible to suppress the occurrence of short circuit defects between wiring of semiconductors, particularly semiconductors for automobiles. In addition, the decrease in whiteness of the silicon dioxide powder can also be suppressed. The number of magnetizable foreign matters having a particle diameter of 20 to 45 μm in 50 g of silicon dioxide powder can be measured by the method described in the examples below. The number of magnetizable foreign matters having a particle diameter of 45 to 100 μm in 50 g of silicon dioxide powder is preferably 50 pieces / 50 g or less, 40 pieces / 50 g or less, 30 pieces / 50 g or less, 20 pieces / 50 g or less, 10 pieces / 50 g or less, 5 pieces / 50 g or less, or 2 pieces / 50 g or less. The number of magnetizable foreign matters having a particle diameter of 100 to 200 μm in 50 g of silicon dioxide powder is preferably 10 pieces / 50 g or less, 5 pieces / 50 g or less, 2 pieces / 50 g or less, 1 piece / 50 g or less, or 0 pieces / 50 g. The number of magnetizable foreign matters having a particle diameter of 200 to 275 μm in 50 g of silicon dioxide powder is preferably 1 piece / 50 g or less, or 0 pieces / 50 g.

[0030] In the silicon dioxide powder, the aluminum content is preferably 550 mass ppm or less or 500 mass ppm or less, calculated as Al2O3. When the aluminum content is 550 mass ppm or less, calculated as Al2O3, the silicon dioxide powder can be used in a semiconductor encapsulant to suppress a decrease in the fluidity of the semiconductor encapsulant. From this viewpoint, the aluminum content is more preferably 400 mass ppm or less, and even more preferably 350 mass ppm or less, calculated as Al2O3. The lower limit of the aluminum content is 0 mass ppm. The aluminum content can be measured by the measurement method described in the examples below.

[0031] In the silicon dioxide powder, the potassium content is preferably 30 mass ppm or less, 20 mass ppm or less, or 10 mass ppm or less, calculated as K2O. When the potassium content is 30 mass ppm or less, calculated as K2O, the silicon dioxide powder can be used in a semiconductor encapsulant to suppress a decrease in moldability of the semiconductor encapsulant. In addition, the decrease in whiteness of the silicon dioxide powder can also be suppressed. The lower limit of the potassium content is 0 mass ppm. The potassium content can be measured by the measurement method described in the examples below.

[0032] In the silicon dioxide powder, the sodium content is preferably 15 mass ppm or less in terms of Na2O. When the sodium content is 15 mass ppm or less in terms of Na2O, when the silicon dioxide powder is used in a semiconductor encapsulant, the hardening of the semiconductor encapsulant can be suppressed from being delayed. In addition, the occurrence of wiring corrosion and the deterioration of the performance of the semiconductor element can be suppressed. The lower limit of the sodium content is 0 mass ppm. The sodium content can be measured by the measurement method described in the examples below.

[0033] In the silicon dioxide powder, the calcium content is preferably 20 ppm or less or 15 ppm by mass or less, calculated as CaO. When the calcium content is 15 ppm by mass or less, calculated as CaO, the silicon dioxide powder can be used in a semiconductor encapsulant to suppress a decrease in moldability of the semiconductor encapsulant. In addition, the decrease in whiteness of the silicon dioxide powder can also be suppressed. The lower limit of the calcium content is 0 ppm by mass. The calcium content can be measured by the measurement method described in the examples below.

[0034] In the silicon dioxide powder, the magnesium content is preferably 10 mass ppm or less or 5 mass ppm or less in terms of MgO. When the magnesium content is 10 mass ppm or less in terms of MgO, the silicon dioxide powder can be used in a semiconductor encapsulant to suppress a decrease in moldability of the semiconductor encapsulant. In addition, the decrease in whiteness of the silicon dioxide powder can also be suppressed. The lower limit of the magnesium content is 0 mass ppm. The magnesium content can be measured by the measurement method described in the examples below.

[0035] (Modification of the method for producing silicon dioxide powder according to one embodiment of the present invention) (Variation 1) The method for producing silicon dioxide powder according to one embodiment of the present invention is not particularly limited as long as it includes a heating step in the presence of chlorine in which silicon dioxide is heated in the presence of chlorine. That is, the method for producing silicon dioxide powder according to one embodiment of the present invention is not particularly limited as long as it includes a step of chlorinating silicon dioxide, and further, the chlorination is not particularly limited as long as it includes a heating step in the presence of chlorine in which silicon dioxide is heated in the presence of chlorine. For example, the silicon dioxide powder divided by particle size before blending may be chlorinated, and the chlorinated silicon dioxide powder divided by particle size may be blended to produce a high-purity silicon dioxide powder. Also, commercially available silicon dioxide powder may be chlorinated to produce a high-purity silicon dioxide powder. The chlorine treatment may be performed only by a heating step in the presence of chlorine, if the impurities and chlorine in the silicon dioxide powder can be sufficiently reduced by the heating step in the presence of chlorine alone.

[0036] (Variation 2) In the method for producing silicon dioxide powder according to one embodiment of the present invention, impurities in silicon dioxide powder are removed using chlorine.However, since halides are more easily sublimated than oxides, the element used to remove impurities in silicon dioxide powder is not limited to chlorine as long as it is a halogen element.For example, impurities may be removed from silicon dioxide powder using fluorine, bromine, etc. in addition to chlorine.

[0037] (Variation 3) In one embodiment of the present invention, the method for producing silicon dioxide powder removes impurities from silicon dioxide powder. However, if impurities can be removed by heating in the presence of a halogen, the metal oxide from which the impurities are removed is not limited to silicon dioxide. For example, impurities in aluminum oxide can be removed by heating in the presence of a halogen. Sodium, silicon, iron, titanium, and other impurities in alumina are converted into halides by heating the alumina in the presence of a halogen, and are sublimated and separated from the alumina. This allows the production of high-purity alumina.

[0038] The method for producing a silicon dioxide powder according to one embodiment of the present invention and its modified example are merely examples of the method for producing a metal oxide of the present invention, and do not limit the method for producing a metal oxide of the present invention. EXAMPLES

[0039] The present invention will be described in detail below with reference to examples and comparative examples, but the present invention is not limited to the following examples.

[0040] The silicon dioxide powders of the examples and comparative examples were evaluated as follows. (Particle size) Using a laser scattering diffraction type particle size distribution analyzer (product name "CILAS920", manufactured by CILAS) or a particle size distribution analyzer (product name "LS-230", manufactured by Beckman Coulter, Inc.), the particle size (d10) at which the cumulative frequency of the silicon dioxide powder is 10%, the particle size (d50) at which the cumulative frequency is 50%, and the particle size (d90) at which the cumulative frequency is 90% were measured.

[0041] (specific surface area) The specific surface area of ​​the silicon dioxide powder was measured by the BET method using a specific surface area measuring device (product name "Macsorb HM model-1208", manufactured by MACSORB).

[0042] (sphericity) Measurements were performed using a wet flow particle size and shape analyzer (product name "FPIA-3000", manufactured by Malvern Panalytical). After dispersing the sample in pure water, the liquid was poured into a planar extensional flow cell, and images of 100 or more amorphous silica powder particles moving in the cell were recorded using an objective lens. The average circularity was calculated from the recorded images and the following formula (1). In formula (1), HD represents the circular equivalent diameter, which is calculated from the ratio of the projected area of ​​the target particle to the area of ​​a perfect circle. PM represents the projected perimeter of the target particle. The average value of 200 amorphous silica powder particles calculated in this way was taken as the average circularity. Equation (1): Average circularity = π HD / PM From this average circularity, the formula: average sphericity = (average circularity) 2 The average sphericity was calculated by

[0043] (U (uranium) content) In the present invention, the uranium content was measured using a spectrofluorophotometer (manufactured by Hitachi Instruments, Ltd., measurement limit: 0.01 ppb).

[0044] (Number of magnetic particles in 50g of silicon dioxide powder) A slurry was prepared by adding 50 g of silicon dioxide powder and 800 g of ion-exchanged water to a 1000 mL beaker. A bar magnet with a length of 150 mm, a diameter of 25 mm, and a magnetic force of 12000 Gauss, covered with a rubber cover with a thickness of 20 μm, was immersed in the slurry for 1 minute while rotating the slurry at 550 rpm with a stirring device at 5 second intervals to capture the magnetizable particles. The bar magnet capturing the magnetizable particles was removed from the slurry, and the rubber cover was removed over an empty beaker. The rubber cover was washed with ion-exchanged water to release the magnetizable particles, and the magnetizable particles were dispersed in water. The obtained dispersion was passed through a suction filtration device equipped with a nylon filter with a diameter of 25 mm (mesh size 13 μm) to collect the magnetizable particles on the nylon filter. The nylon filter on which the magnetizable particles had been collected was set in a microscope, and the number of magnetizable particles of 20 μm or more among the magnetizable particles collected on the nylon filter was counted at 100x magnification while moving the filter over its entire area.

[0045] (Aluminum, iron, potassium, sodium, calcium, magnesium and titanium content) A sample solution was prepared by thermal decomposition of silicon dioxide powder using hydrofluoric acid and nitric acid. The sample solution was then quantitatively analyzed using an inductively coupled plasma mass spectrometer to measure the aluminum content converted to Al2O3, the iron content converted to Fe2O3, the potassium content converted to K2O, the sodium content converted to Na2O, the calcium content converted to CaO, the magnesium content converted to MgO, and the titanium content converted to TiO2 in the silicon dioxide powder.

[0046] The following silicon dioxide powders were prepared: (Silicon dioxide powder A) Silicon dioxide powder A was prepared by treating the silicon dioxide powder shown in Table 1 with chlorine. A chlorine atmosphere batch furnace (product name "Model 480", manufactured by Noritake Co., Ltd.) was used for the chlorination. The temperature and pressure conditions for the chlorination are outlined in Figure 1. The operating conditions for the chlorination are as follows:

[0047] (Chlorination operating conditions) (Gas used) Purity of chlorine gas supplied to chlorine atmosphere batch furnace: 100% Purity of nitrogen gas supplied to chlorine atmosphere batch furnace: 99.998% (Heating process in the presence of chlorine) Flow rate of chlorine gas supplied to the chlorine atmosphere batch furnace: 5L / min Flow rate of nitrogen gas supplied to the chlorine atmosphere batch furnace: 20 L / min Chlorine gas content in the total gas supplied to the chlorine atmosphere batch furnace: 20% by volume Heating temperature: 900℃ Heating time in the heating step in the presence of chlorine in one cycle: 5 minutes Pressure: 91.3kPa Note: First, the inside of the batch furnace was depressurized to 0.01 kPa before chlorine gas and nitrogen gas were supplied. Also, the heating rate from room temperature to 900°C was 5°C / min. (First reduced pressure heating step) Heating temperature: 900℃ Pressure: 0.13kPa Time to maintain the above pressure in the first reduced pressure heating step in one cycle: 2 minutes (Cycle consisting of a heating step in the presence of chlorine and a first heating step under reduced pressure) Number of cycles: 6

[0048] (Heating process in the presence of inert gas) Flow rate of nitrogen gas supplied to the chlorine atmosphere batch furnace: 20 L / min Heating temperature: 1000℃ Heating time in the heating step in the presence of inert gas in one cycle: 5 minutes Pressure: 91kPa Note: When moving from the first depressurization step to the heating step in the presence of an inert gas, the pressure inside the batch furnace was reduced to 0.01 kPa to remove the chlorine gas inside the batch furnace, and then nitrogen gas was supplied. (Second reduced pressure heating step) Heating temperature: 1000℃ Pressure: 0.13kPa Time to maintain the above pressure in the second reduced pressure heating step in one cycle: 2 minutes (Cycle consisting of a heating step in the presence of an inert gas and a second heating step under reduced pressure) Number of cycles: 4 Note: After completing the 4 cycles, the pressure inside the batch furnace was reduced to 0.01 kPa and then returned to atmospheric pressure.

[0049] (Silicon dioxide powder B) Silicon dioxide powder B was prepared in the same manner as silicon dioxide powder A, except that the silicon dioxide powder used was changed to the silicon dioxide powder shown in Table 1.

[0050] (Silicon dioxide powder C) Silicon dioxide powder C was prepared in the same manner as silicon dioxide powder A, except that the silicon dioxide powder used was changed to the silicon dioxide powder shown in Table 1.

[0051] (Silicon dioxide powder D) Silicon dioxide powder D was prepared in the same manner as silicon dioxide powder A, except that the silicon dioxide powder used was the silicon dioxide powder shown in Table 1.

[0052] (Silicon dioxide powder E) Silicon dioxide powder E was prepared in the same manner as silicon dioxide powder A, except that the silicon dioxide powder used was changed to the silicon dioxide powder shown in Table 1.

[0053] (Silicon dioxide powder F) Silicon dioxide powder F was prepared in the same manner as silicon dioxide powder A, except that the silicon dioxide powder used was changed to the silicon dioxide powder shown in Table 1.

[0054] (Silicon dioxide powder G) Silicon dioxide powder G was prepared in the same manner as silicon dioxide powder A, except that the silicon dioxide powder used was changed to the silicon dioxide powder shown in Table 1.

[0055] The results are shown in Table 1. [Table 1]

[0056] By comparing the examples and comparative examples, it is found that impurities in silicon dioxide powder can be removed by heating silicon dioxide powder in the presence of chlorine.It is also found that the particle size, specific surface area and sphericity of silicon dioxide powder do not change much even if silicon dioxide is heated in the presence of chlorine.

Claims

[Claim 1] The method includes a heating step in the presence of a halogen in which a metal oxide is heated in the presence of a halogen, and a first heating step under reduced pressure in which the metal oxide is heated under reduced pressure after the heating step in the presence of a halogen, the halogen is chlorine; repeating the cycle consisting of the heating step in the presence of a halogen and the first heating step under reduced pressure two or more times; the method further comprises, after the cycle consisting of the heating step in the presence of a halogen and the first heating step under reduced pressure, a heating step in the presence of an inert gas in which the metal oxide is heated in the presence of an inert gas, and a second heating step under reduced pressure in which the metal oxide is heated under reduced pressure after the heating step in the presence of an inert gas, Repeating the cycle consisting of the heating step in the presence of an inert gas and the second heating step under reduced pressure two or more times; the heating temperature in the heating step in the presence of a halogen is a temperature at which a desired impurity reacts with the halogen to form a halide; The metal oxide is at least one metal oxide selected from the group consisting of silicon dioxide and aluminum oxide, The heating temperature in the heating step in the presence of a halogen is 400 to 1200° C., the heating time is 1 to 120 minutes, and the pressure is 0.0001 to 100 kPa; The heating temperature in the first reduced pressure heating step is 600 to 1200° C., the heating time is 5 to 180 minutes, and the pressure is 0.0001 to 100 kPa; The heating temperature in the heating step in the presence of an inert gas is higher than the heating temperature in the heating step in the presence of a halogen by 200° C. or less, The method for producing a metal oxide powder, wherein the heating temperature in the second reduced pressure heating step is 600 to 1250° C., the heating time is 5 to 180 minutes, and the pressure is 0.0001 to 100 kPa.

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