JP2018150228A - Manufacturing method of graphite powder - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method for producing graphite powder, which can obtain high-purity graphite powder at low cost by efficiently discharging impurities in the graphite powder raw material with simple equipment. SOLUTION: This is a method for producing graphite powder using a furnace in which a vertical upper end surface is open to the atmosphere and an electrode is provided on an inner wall surface, and is embedded inside a filler in which inorganic siliceous particles and carbonaceous particles are mixed. , The step of arranging the graphite powder raw material connecting the electrodes in the furnace and the step of energizing and heating the graphite powder raw material, the bulk density of the filler is 0.50 × 103 kg / m3 or more 1.2 × It is 103 kg / m3 or less. By heating with a filler having such a bulk density, the SiO gas generated from the filler is rapidly diffused inside the filler while maintaining the shape of the graphite powder raw material to reduce the metal oxide, and the metal oxide is reduced with high efficiency. Metal can be discharged into the filler. [Selection diagram] Fig. 1

Description

  The present invention relates to a method for producing graphite powder using a furnace in which a vertical upper end surface is opened to the atmosphere and an electrode is provided on an inner wall surface.

  High-purity graphite powder is used as a secondary battery negative electrode material, a powder such as a mechanical seal material, or a member used for semiconductor manufacturing. In particular, a product used in a semiconductor manufacturing process having an ash content of the order of several tens to several ppm is manufactured by removing impurities at a high temperature for a long time and is expensive. Conventionally, a method for producing high-purity graphite powder using a halogen gas or a halide is known.

  Patent Document 1 describes an apparatus that continuously purifies by placing a cylindrical heating vessel made of graphite in a heating furnace filled with a heat insulating material, and heating the current while supplying graphite. Patent Document 2 describes a method in which a fused polycyclic hydrocarbon is polymerized to form a mesophase pitch, and then a crystalline graphite powder having an average particle size of 10 μm is obtained by heat treatment and pulverization. Patent Document 3 describes a method of obtaining a high-purity graphite member by firing a carbon raw material powder together with a binder, then impregnating an organic substance containing halogen, and heat-treating the impregnated body.

  All the methods described in Patent Documents 1 to 3 use halides, reduce metal oxides in graphite to metal halides, lower the boiling point, and volatilize them. Similarly to these methods, it is also common to reduce the ash content by applying heat in the presence of halogen gas or halide after being placed in a heating device by firing with a binder or being embedded in carbon. Has been done. However, if it is intended to produce high-purity graphite powder having an ash content of several tens of ppm or less using a halogen gas or halide, it is necessary to provide a gas leakage prevention measure and an exhaust gas treatment facility, resulting in high costs.

  On the other hand, a method for producing graphite using an Atchison furnace (see Non-Patent Document 1 and Patent Document 4) is known. In the method described in Patent Document 5, graphite is easily produced by filling a vessel in a furnace with a raw material to be graphitized and passing a current through a thin carbon rod. In the method described in Patent Document 6, a mixture of coke and silica is filled in a furnace, a carbon material is energized to generate silicon carbide, and silicon atoms are thermally dissociated by heating to produce graphite.

JP-A-8-198612 JP 10-121054 A JP 2006-232565 A Japanese Patent Laying-Open No. 2015-157737 1945 Patent Application Publication No. 525 Japanese Patent Laid-Open No. 9-157022

New edition industrial furnace handbook, issued by the Energy Conservation Center, Japan, November 28, 1997, P485-488

  As described above, conventionally, when a graphite powder is produced by a simple method, a high-purity graphite powder having an ash content of several tens of ppm or less cannot be produced. On the other hand, if an attempt is made to produce high-purity graphite powder, expensive equipment or the like is required, and the production cost increases.

  The present invention has been made in view of such circumstances, and a method for producing a graphite powder that discharges impurities in the graphite powder raw material with high efficiency and with a simple facility, thereby obtaining a low-cost and high-purity graphite powder. The purpose is to provide.

(1) In order to achieve the above object, a method for producing a graphite powder according to the present invention is a method for producing a graphite powder using a furnace having a vertical upper end face open to the atmosphere and having an electrode on the inner wall surface. Including a step of placing a graphite powder raw material embedded in a filler in which carbonaceous particles and carbonaceous particles are mixed and connecting electrodes in a furnace, and a step of electrically heating the graphite powder raw material, The bulk density of the filler is 0.50 × 10 ³ kg / m ³ or more and 1.2 × 10 ³ kg / m ³ or less.

  By heating with the bulk density filler as described above, the SiO gas generated from the filler is quickly diffused into the graphite powder raw material while maintaining the shape of the graphite powder raw material, thereby reducing the metal oxide and reducing it with high efficiency. Metal can be discharged into the filler. As a result, impurities in the graphite powder raw material are discharged with high efficiency with simple equipment, and high-purity graphite powder can be obtained at low cost. In addition to direct heating of the graphite powder raw material and direct heating, the filler is a mixture of inorganic siliceous particles and carbonaceous particles, and it is difficult for excess current to occur in the surroundings, so energy efficiency can be improved. .

  (2) Further, in the method for producing graphite powder of the present invention, the graphite powder raw material is a representative thick of the graphite powder raw material for both the inner wall surface and the vertical upper end surface where the furnace electrode is not installed. It is characterized by being embedded in the filler with an interval of not less than 2 times and not more than 10 times. Thereby, the yield of high purity graphite powder can be improved, maintaining a SiO gas in a furnace and removing a metal impurity efficiently.

  (3) Moreover, the method for producing graphite powder of the present invention is characterized in that the representative thickness of the graphite powder raw material is in the range of 50 mm or more and 500 mm or less. Thereby, metal impurities can be removed to the center of the graphite powder raw material while maintaining energization sufficient for heating.

  (4) Moreover, the method for producing graphite powder of the present invention is characterized in that the ash content excluding Si of the filler is 1000 ppm or less by weight with respect to the filler. Thereby, impurities flowing from the filler to the graphite powder material can be reduced, and the graphite powder material can be highly purified.

  (5) Further, in the method for producing graphite powder of the present invention, the weight fraction of the carbonaceous particles contained in the filler is 30 wt% or more and 60 wt% with respect to the total weight of the carbonaceous particles and the inorganic siliceous particles. It is characterized by the following. Thereby, SiO gas can be generated with high efficiency.

  According to the present invention, impurities in the graphite powder raw material are discharged with high efficiency with simple equipment, and high-purity graphite powder can be obtained at low cost.

(A), (b) is a side sectional view and a front sectional view showing a furnace, a filler, and a graphite powder raw material, respectively. It is a table | surface which shows the experimental condition of each Example and a comparative example, and the measurement result of the ash content after baking for 16 hours. It is a graph which shows the relationship between baking time and the amount of ash.

  As a result of diligent research, the inventors of the present invention filled a graphite powder raw material in a filler containing an inorganic siliceous raw material and a carbonaceous raw material, and heated the graphite powder raw material with current, so that a halide was not used and the cost was low Invented a method for producing high-purity graphite. Hereinafter, embodiments of the present invention will be described.

[Principle for obtaining high-purity graphite]
Generally, graphite is highly purified by heat treatment. This is because the volatile substance is volatilized and the metal oxide contained in the graphite is reduced to a single metal and evaporated. The reducing atmosphere can be maintained by placing graphite to be highly purified in a place kept with halogen gas or by burying it in carbon.

  Since the method of heating in the halogen gas is indirect heating through the gas, the energy efficiency is deteriorated. Moreover, it is necessary to use a large amount of highly reactive halogen gas, and it is necessary to take safety measures against the gas. There is a direct heating method in which a voltage is applied to carbon in which an object is buried and heating is performed with Joule heat. However, current flows also to other carbon, and current efficiency is inferior. Further, since there is no flow of particles in the furnace, even if the metal becomes a simple substance, it is difficult to be removed from the carbon raw material, and impurities are likely to remain.

  In contrast, if the graphite powder raw material is embedded in a filler composed of an inorganic siliceous raw material and a carbonaceous raw material and heated by energization, the graphite powder raw material can be directly heated, and energy efficiency is higher than when heated in gas. high. Further, since the filler is a mixture of an inorganic siliceous raw material and a carbonaceous raw material, an extra current is less likely to flow around than when carbon is used as the filler.

  Further, when the temperature inside the furnace becomes high, the inorganic siliceous raw material and the carbonaceous raw material react to generate highly active SiO gas. A part of the SiO gas enters between the graphite powder raw materials, but the SiO gas is highly reducible and can reduce the metal oxide contained in the graphite powder raw materials. The graphite powder raw material is heated to 2500 ° C. or more by heating, and at this temperature, at least a part of most of the metal is vaporized. The vaporized metal moves from the graphite powder raw material to the filler by the flow of SiO gas.

  At low temperatures, the metal becomes oxide and carbide again. Since the reducing atmosphere is weaker and the temperature is lower in the filler than in the graphite powder raw material, the impurities move to the filler. Similar migration of impurities occurs even when graphite is highly purified in carbon. In the above manufacturing method, by generating reducing SiO gas, the impurity element contained in the graphite powder raw material can be quickly reduced and discharged to the filler side. As a result, highly purified graphite powder can be obtained from the graphite powder raw material.

[Method for producing graphite powder]
Next, a method for producing graphite powder based on the above principle will be described.

(Configuration of furnace)
First, the structure of the furnace used for a manufacturing method is demonstrated. 1A and 1B are a side sectional view and a front sectional view showing a furnace 10, a filler 20, and a graphite powder raw material 30, respectively. The method for producing graphite powder of the present invention is performed using a furnace 10 which is a reaction vessel with electrodes 15a and 15b. The furnace 10 has a vertical upper end opened to the atmosphere and has an electrode on the inner wall.

  The shape of the container forming the furnace body 11 is not particularly limited, but it is necessary to have electrodes 15 a and 15 b for energizing the graphite powder raw material 30. It is preferable that the electrodes 15a and 15b are provided on opposite end surfaces on the inner side of the container, and the furnace body 11 preferably has two parallel opposing surfaces. For the furnace body 11, it is convenient and preferable to use a rectangular container. The furnace body 11 may have a slit as a gas escape gap for maintaining an appropriate concentration when the reaction gas is excessively generated.

  The material of the furnace body 11 is not particularly limited, but the wall surface becomes hot due to heat transfer from the graphite powder raw material when energized, so it is desirable to use a material with high fire resistance for the portion that comes into contact with the filler. For example, a high-alumina refractory brick or a calcium silicate board is suitable.

  As the electrodes 15a and 15b held in the furnace body 11, a material containing no metal is preferable from the viewpoint of high purity. Since the electrodes 15a and 15b are affected by heat transfer from the graphite powder raw material, a graphite molded body resistant to high temperatures is suitable.

(The whole procedure)
The graphite powder raw material 30 is embedded so that the electrodes 15a and 15b in the furnace 10 are connected to the inside of the filler 20 in which inorganic siliceous particles and carbonaceous particles are mixed.

As the inorganic siliceous raw material, a substance generally represented by the chemical formula SiO ₂ can be used. Examples of the substance represented by SiO ₂ include silica sand, quartz powder, crystalline silica powder, amorphous silica powder, and silica gel. Since generation of the SiO gas is more likely to occur when amorphous SiO ₂ is used, amorphous silica powder and silica gel are particularly suitable.

  As the carbonaceous raw material, both crystalline graphite and amorphous carbon black can be used. Any of them may be in any form, and may be, for example, earthy or scale-like. Since it is desirable in terms of energy cost to pass a larger amount of current through the graphite to be purified, amorphous carbon black powder with low conductivity is particularly suitable for the carbonaceous raw material.

  When the graphite powder raw material 30 is embedded in the filler 20, the electrodes 15a and 15b are energized. As a result, the filled graphite powder raw material 30 generates heat when energized. Gradually, heat is transferred from the graphite powder raw material 30 to the surrounding filler 20 by heat transfer, gradually generating SiO gas and accompanyingly increasing the purity of graphite.

  As the reaction proceeds, the carbonaceous raw material and the inorganic siliceous raw material around the graphite powder raw material 30 are gradually melted or reacted to produce a vitreous structure or silicon carbide crystals generated by the reaction. Thus, highly purified graphite powder is obtained from the graphite powder raw material 30. In addition to heating the graphite powder raw material directly by passing an electric current, the filler is a mixture of inorganic siliceous particles and carbonaceous particles, and an excess electric current hardly occurs in the surroundings, so that energy efficiency can be improved.

  For energization, it is preferable to adjust the current etc. so that the temperature around the graphite powder raw material 30 is 1500 ° C. or higher. Thereby, SiO gas is generated. In order to facilitate the isolation described later, it is particularly preferable to set the temperature to 2200 ° C. or more at which hard silicon carbide crystals are easily generated.

  After energization for a predetermined time, the obtained graphite powder is taken out of the furnace body 11 after being cooled. The graphite powder is in a state of being encased in a glassy structure or silicon carbide crystal shell formed during the above reaction, and can be easily separated from the filler and the shell. For example, the above shell can be taken out, the shell can be crushed with a hammer or the like, the graphite powder inside can be scraped out, and the mixed shell can be removed by sieving.

  Thus, the graphite powder containing about 0.1% of ash can be purified at a low cost, and the ash can be reduced to 50 ppm or less. The manufactured high-purity graphite powder can be used for high-purity graphite crucibles, semiconductor manufacturing members, and the like.

(Details of the filler in the furnace body and the graphite powder raw material)
The bulk density of the filler filled in the furnace body is adjusted to 0.50 × 10 ³ kg / m ³ or more and 1.2 × 10 ³ kg / m ³ or less. By setting the bulk density to 1.2 × 10 ³ kg / m ³ or less, the diffusion rate of SiO gas generated in the filler can be maintained, and the reduction reaction of the impurity metal can be promoted. Further, by setting the bulk density to 0.50 × 10 ³ kg / m ³ or more, a part of the filler reacts to form silicon carbide (true density 3.21 × 10 ³ kg / m ³ ). The volume of the filler can be maintained by suppressing the progress, and the collapse of the graphite powder raw material filled in the columnar shape can be prevented. The bulk density of the filler is more preferably 0.55 × 10 ³ kg / m ³ or more and 1.15 × 10 ³ kg / m ³ or less. As a result, the effect of discharging the impurity metal is enhanced and the handling becomes easier.

  The ash content excluding Si in the filler is preferably 1000 ppm or less by weight with respect to the filler. Thereby, impurities flowing from the filler to the graphite powder material can be reduced, and the graphite powder material can be highly purified. Since the ash content is 1000 ppm or less, the migration of the impurities of the filler to the graphite powder raw material is suppressed. As a result, it can be highly purified effectively and an ash content of 50 ppm or less can be achieved.

Moreover, it is preferable that the weight fraction of the carbonaceous particle contained in a filler is 30 wt% or more and 60 wt% or less with respect to the total weight of a carbonaceous particle and an inorganic siliceous particle. Thereby, SiO gas can be generated with high efficiency. Since the weight fraction of the carbonaceous particles is 30 wt% or more, a reduction reaction of SiO ₂ of the inorganic siliceous raw material is likely to occur, and the generation amount of SiO gas can be ensured. Further, since the weight fraction of the carbonaceous particles is 60 wt% or less, it is possible to secure the abundance of SiO ₂ and maintain a sufficient generation amount of SiO gas.

  The graphite powder raw material is only required to be electrically connected between the electrodes, but is preferably formed in a columnar shape. And as shown to Fig.1 (a), it is 2 times or more and 10 times or less of the typical thickness X of the graphite powder raw material 30 with respect to both the inner wall surface and the vertical upper end surface in which the electrode of the furnace 10 is not installed. It is preferable that it is embedded in the inside of the filler 20 with an interval of. Thereby, the yield of high purity graphite powder can be improved, maintaining a SiO gas in a furnace and removing a metal impurity efficiently.

  That is, since both the inner wall surface where the electrode of the furnace 10 is not installed and the vertical upper end surface are filled with the filler 20 at intervals of twice or more the representative thickness of the graphite powder raw material 30, It is possible to prevent the SiO gas, which is the key to purification, from diffusing outside the furnace, and the metal impurity removal efficiency is improved. In addition, heat transfer from the graphite powder is buffered, and damage to the furnace wall can be reduced. It is possible to prevent the graphite powder from being contaminated by metal impurities eluted from the furnace wall.

  On the other hand, since both the inner wall surface where the electrode of the furnace 10 is not installed and the vertical upper end surface are filled with the filler 20 at intervals of 10 times or less the representative thickness of the graphite powder raw material 30, The amount of material 20 used can be reduced. Moreover, since the pillar of the graphite powder raw material 30 is not too thin, sufficient energization can be ensured during the purification process.

  Here, the representative thickness X is the representative thickness of the graphite powder raw material 30 formed in a columnar shape, and indicates the length of one side when the column has a square cross section. Further, when the length of each side is a rectangle of A and B, X = (A + B) / 2, and when it is a columnar shape, the diameter is indicated. In other cases, when the length of the outer periphery of the cross section is L, it indicates the diameter of a circle having the same outer periphery length (ie, X = L / π). When the representative thickness X of the graphite powder raw material 30 changes in the furnace, X at the position where the cross-sectional area of the column of the graphite powder raw material 30 is the smallest is adopted.

  The representative thickness X of the graphite powder raw material 30 is preferably in the range of 50 mm or more and 500 mm or less. Since the representative thickness X of the graphite powder raw material 30 is 50 mm or more, sufficient energization can be ensured during the purification process. On the other hand, since it is 500 mm or less, the SiO gas is sufficiently diffused into the graphite powder raw material 30, and the metal impurities can be removed to the inside.

[Experimental conditions for Examples and Comparative Examples]
As Examples 1 to 10 and Comparative Examples 1 to 8, graphite powders were produced and experiments for measuring purity were performed.

Example 1
Using a high-alumina refractory brick, a rectangular parallelepiped furnace body having an internal size of 1050 mm in width, 2530 mm in length, and 1020 mm in height was produced. Then, on the two sides of the furnace body in the longitudinal direction, 480 mm in height, a hole in the brick at an equal position from the short side wall, and a cylindrical graphite molded body having a diameter of 100 mm and a length of 320 mm as an electrode Inserted.

Carbon black (ash content: 550 ppm, bulk density: 0.74 × 10 ³ kg / m ³ ) as a carbonaceous raw material, amorphous silica (ash content: 670 ppm, bulk density: 0.53 × 10 ³ kg / m ³ ) as an inorganic siliceous raw material Were mixed at a mass ratio of 4: 6 to prepare a filler having an ash content of 622 ppm and a bulk density of 0.66 × 10 ³ kg / m ³ . The bulk density of the raw material is described in “JIS K 1474 activated carbon test method packing density” for the carbonaceous raw material, and “JIS K 1150 silica gel test method bulk density” for the inorganic siliceous raw material. Measured by the method.

Further, a graphite powder raw material having an ash content of 420 ppm and a bulk density of 0.35 × 10 ³ kg / m ³ was prepared. However, when calculated from the volume and weight at the time of filling, the bulk density of the graphite powder raw material at the time of filling is 0.65 × 10 ³ kg / m ³ . The bulk density of the raw material was measured by the method described in “JIS K 1474 activated carbon test method packing density”.

  The furnace body was filled with 1610 kg of filler and 23.7 kg of graphite powder raw material. At this time, the graphite powder raw material was filled in a rectangular parallelepiped shape having a width of 120 mm, a height of 120 mm, and a length of 2530 mm so as to connect the electrodes at both ends. The filling height was 960 mm. After filling the filler and the graphite powder raw material, the graphite powder raw material was energized through the electrodes at both ends.

  After firing at 2400 ° C. for 16 hours by energization heating, air cooling was performed until the temperature reached room temperature, and the graphite powder included in the vitreous and silicon carbide crystals was taken out from the furnace body. The entire recovered graphite powder was sampled after being sufficiently mixed, and the ash content of the graphite powder was measured according to the ash content calculation method described below. Similarly, the ash content of the graphite powder when firing for 8 hours and 24 hours was also measured. In addition, baking for 8 hours and 24 hours was performed only about Example 1 and Comparative Examples 1 and 3.

(Example 2)
The filling process of the graphite powder raw material was a rectangular parallelepiped having a width of 180 mm, a height of 180 mm, and a length of 2530 mm, and the other steps were performed in the same manner as in Example 1.

(Example 3)
The filling process of the graphite powder raw material was a rectangular parallelepiped having a width of 80 mm, a height of 80 mm, and a length of 2530 mm. The other conditions were the same as in Example 1.

(Example 4)
A silica gel having a bulk density of 1.40 × 10 ³ kg / m ³ and an ash content of 1070 ppm was used as the inorganic siliceous material, and the other steps were performed in the same manner as in Example 1.

(Example 5)
An amorphous silica powder having a bulk density of 0.38 × 10 ³ kg / m ³ and an ash content of 540 ppm was used as the inorganic siliceous material, and the other steps were performed in the same manner as in Example 1.

(Example 6)
The shape of the graphite powder raw material of Example 2 and the inorganic siliceous raw material of Example 4 were adopted, and the other steps were performed in the same manner as in Example 1.

(Example 7)
The process was performed in the same manner as in Example 1 except that the mixing ratio of the carbonaceous material and the inorganic siliceous raw material was 5: 5.

(Example 8)
The process was performed in the same manner as in Example 1 except that the mixing ratio of the carbonaceous material and the inorganic siliceous raw material was 6: 4.

Example 9
The furnace body was configured with an internal size of length 2100 mm, width 2530 mm, height 2040 mm (filling height 2000 mm), and the electrode was disposed at a height 960 mm from the bottom. The filled shape of the graphite powder raw material was a rectangular parallelepiped having a width of 240 mm, a height of 240 mm, and a length of 2530 mm. The process was performed in the same manner as in Example 1 under other conditions.

(Example 10)
The furnace body was configured with the same dimensions as in Example 9, and the graphite powder raw material was filled in a rectangular parallelepiped having a width of 360 mm, a height of 360 mm, and a length of 2530 mm. The process was performed in the same manner as in Example 1 under other conditions.

(Comparative Example 1)
The filled shape of the graphite powder raw material was a rectangular parallelepiped having a width of 240 mm, a height of 240 mm, and a length of 2530 mm. The process was performed in the same manner as in Example 1 under other conditions.

(Comparative Example 2)
The filled shape of the graphite powder raw material was a rectangular parallelepiped having a width of 40 mm, a height of 40 mm, and a length of 2530 mm. The process was performed in the same manner as in Example 1 under other conditions.

(Comparative Example 3)
As an inorganic siliceous material, quartz sand having a bulk density of 1.66 × 10 ³ kg / m ³ and an ash content of 430 ppm was used. The process was performed in the same manner as in Example 1 under other conditions.

(Comparative Example 4)
As the inorganic siliceous material, amorphous silica having a bulk density of 0.16 × 10 ³ kg / m ³ and an ash content of 250 ppm was used. The process was performed in the same manner as in Example 1 under other conditions.

(Comparative Example 5)
Silica gel having a bulk density of 1.40 × 10 ³ kg / m ³ and an ash content of 1070 ppm is used as the inorganic siliceous material, and carbon black having a bulk density of 0.91 × 10 ³ kg / m ³ and an ash content of 380 ppm is used as the carbonaceous material. It was used. The process was performed in the same manner as in Example 1 under other conditions.

(Comparative Example 6)
The mixing ratio of the carbonaceous material and the inorganic siliceous raw material was 7: 3. The process was performed in the same manner as in Example 1 under other conditions.

(Comparative Example 7)
The dimensions of the furnace body were the same as in Example 9, and the graphite powder raw material was filled in a rectangular parallelepiped having a width of 480 mm, a height of 480 mm, and a length of 2530 mm. The process was performed in the same manner as in Example 1 under other conditions.

(Comparative Example 8)
As a carbonaceous raw material, carbon black having a bulk density of 0.68 × 10 ³ kg / m ³ and an ash content of 4200 ppm was used. The process was performed in the same manner as in Example 1 under other conditions.

(Calculation method of ash content)
The ash content of the carbonaceous raw material in the graphite powder and the filler can be analyzed in accordance with the ash content analysis method described in “JIS M 8511 natural graphite industrial analysis and test method”. The same analysis method is used when an amorphous carbonaceous raw material such as carbon black is used.

  For the ash content of the inorganic siliceous raw material, a method for analyzing silica sand described in “JIS R 2212-2: 2006 Chemical Analysis Method for Refractory Products—Part 2: Silicic Refractory” can be used. That is, quantitative determination of aluminum oxide, iron (III) oxide, titanium (IV) oxide, manganese (II) oxide, calcium oxide, magnesium oxide, phosphorus oxide (V), and boron oxide (III) (calculated as mass fraction) And the sum is calculated. The same analysis method is used when an amorphous inorganic siliceous raw material such as amorphous silica is used. The ash content of the entire filler can be calculated by measuring the ash content of each raw material constituting the filler and taking a weighted average according to the mixing ratio.

[Experimental results of examples and comparative examples]
(Amount of ash after baking for 16 hours)
FIG. 2 is a table showing the experimental conditions of each example and comparative example and the measurement results of ash content after baking for 16 hours. In each of Examples 1 to 10, the ash content was less than 50 ppm. In all of Comparative Examples 1 to 8, the ash content greatly exceeded 50 ppm.

  In Comparative Examples 2 and 4, a sudden increase in resistance value was observed during energization, making it impossible to apply power. In Comparative Example 2, electric power could not be applied after 1 hour from the start of energization, and in Comparative Example 4 after 3 hours from the start of energization. This is considered to be because the graphite powder raw material was disconnected, and indicates that the column of the graphite powder raw material tends to collapse when the column of the graphite powder raw material to be processed is too thin or the bulk density of the filler is too small.

  The reason why the ash content was not reduced in Comparative Examples 1, 3, 5, and 7 is considered that the SiO gas did not penetrate into the graphite powder raw material to be treated. The reason why the ash content was not reduced in Comparative Example 6 is considered to be because the carbonaceous raw material was excessive and the generation amount of SiO gas was small (or immediately changed to silicon carbide). In Comparative Example 8, it is considered that the purity of the carbon black is low, and some of the impurities in the carbon black diffuse into the graphite powder raw material, so that the ash content of the graphite powder raw material is also relatively high.

  In addition, the experiment which made the mixing ratio of carbonaceous material and inorganic siliceous raw material 3: 7 was not implemented. This is because it was predicted that the reduction of the siliceous raw material would not proceed and the generation of SiO gas would decrease, and that it would be difficult to separate the filler from the graphite powder after firing because no silicon carbide was formed. .

(Relationship between firing time and ash content)
For Example 1 and Comparative Examples 1 and 3, the ash content of the graphite powder was confirmed according to the firing time. FIG. 3 is a graph showing the relationship between firing time and ash content. In all cases, the value became almost constant after 16 hours of firing. This is presumably because the reaction of the filler progressed and the production of SiC became active, so that the supply of SiO to the graphite powder raw material was not performed. In Comparative Example 1 in which the amount of the graphite powder raw material is large and Comparative Example 3 in which the bulk density of the filler is large, the reaction is completed without sufficiently supplying SiO gas to the graphite powder raw material.

  From the above, when the graphite powder raw material is embedded in a filler composed of carbonaceous and inorganic siliceous materials, the bulk density of the filler and the size ratio of the graphite powder raw material to the filler are made to be in an appropriate range and are energized. It was proved that graphite powder can be sufficiently purified.

DESCRIPTION OF SYMBOLS 10 Furnace 11 Furnace main body 15a, 15b Electrode 20 Filler 30 Graphite powder raw material

Claims (5)

A method for producing graphite powder using a furnace whose vertical upper end surface is open to the atmosphere and has an electrode on the inner wall surface,
A step of placing a graphite powder raw material embedded in a filler in which inorganic siliceous particles and carbonaceous particles are mixed and connecting electrodes in a furnace;
A step of energizing and heating the graphite powder raw material,
The bulk density of the filler is 0.50 × 10 ³ kg / m ³ or more and 1.2 × 10 ³ kg / m ³ or less.   The graphite powder raw material is filled at an interval of 2 to 10 times the representative thickness of the graphite powder raw material with respect to both the inner wall surface where the furnace electrode is not installed and the vertical upper end surface. 2. The method for producing graphite powder according to claim 1, wherein the method is embedded in a material.   The method for producing a graphite powder according to claim 1 or 2, wherein a representative thickness of the graphite powder raw material is in a range of 50 mm to 500 mm.   The method for producing graphite powder according to any one of claims 1 to 3, wherein an ash content excluding Si in the filler is 1000 ppm or less by weight with respect to the filler.   The weight fraction of the carbonaceous particles contained in the filler is 30 wt% or more and 60 wt% or less with respect to the total weight of the carbonaceous particles and inorganic siliceous particles. A method for producing a graphite powder according to any one of the above.

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