WO2024018933A1

WO2024018933A1 - Method for producing boron nitride powder

Info

Publication number: WO2024018933A1
Application number: PCT/JP2023/025328
Authority: WO
Inventors: 駿歌川; 裕介和久田; 厚樹五十嵐; 聖治小橋
Original assignee: デンカ株式会社
Priority date: 2022-07-20
Filing date: 2023-07-07
Publication date: 2024-01-25

Abstract

Provided is a method for producing boron nitride powder that has a step that dehydrates boric acid while heating and stirring a first composition that contains boric acid and carbon-containing powder and obtains a second composition containing boron oxide and carbon-containing powder and a step that fires the second composition in an atmosphere containing at least one selected from the group consisting of nitrogen and nitrogen-containing compounds to generate boron nitride.

Description

Manufacturing method of boron nitride powder

The present disclosure relates to a method for producing boron nitride powder.

Boron nitride can be produced by (i) directly nitriding boron using nitrogen or ammonia, or (ii) reacting a boron compound such as boric acid or boron oxide with a nitrogen-containing compound such as melamine at high temperature. (iii) heating a boron compound and a carbon source to high temperature in a nitrogen atmosphere to reduce and nitride the boron compound. For example, Patent Document 1 proposes a technique for producing hexagonal boron nitride powder using a catalyst such as an oxygen-containing calcium compound in addition to a boron compound such as boric acid and a carbon source such as carbon black. .

Japanese Patent Application Publication No. 2015-212217

The method of producing boron nitride by reducing and nitriding boric acid is carried out in two steps: a step of generating boron oxide from boric acid by a dehydration reaction, and a step of reducing and nitriding boron oxide to generate boron nitride. When producing boron nitride on an industrial scale, it is first necessary to allow the dehydration reaction to proceed sufficiently in order for the reaction to proceed with high uniformity, but since dehydration takes time, it tends to take a long time to produce boron nitride. be.

Therefore, the present disclosure aims to provide a manufacturing method that can efficiently mass-produce boron nitride powder.

One aspect of the present disclosure provides the following method for producing boron nitride powder.

[1] Dehydrating the boric acid while heating and stirring the first composition containing boric acid and the carbon-containing powder to obtain a second composition containing boron oxide and the carbon-containing powder, and the second composition. and firing in an atmosphere containing at least one selected from the group consisting of nitrogen and nitrogen-containing compounds to produce boron nitride.

In [1] above, the second composition is obtained by dehydrating boric acid while heating and stirring the first composition. Therefore, the dehydration reaction of boric acid proceeds smoothly, and mixing and dehydration of boric acid can be performed in a short time. Since the second composition obtained by dehydrating the first composition is fired and subjected to reduction nitridation, the load on the firing apparatus can be reduced. Therefore, boron nitride powder can be efficiently mass-produced.

The manufacturing method of [1] above may be any of the following [2] to [4].

[2] The method for producing boron nitride powder according to [1], wherein microwaves are used as a heat source to heat the first composition.
[3] The method for producing boron nitride powder according to [1] or [2], wherein a heat medium is used as a heat source for heating the first composition.
[4] The method for producing boron nitride powder according to any one of [1] to [3], wherein the first composition is heated and stirred under reduced pressure to dehydrate the boric acid.

Since the first composition contains carbon-containing powder, in the manufacturing method of [2] above, the first composition being stirred can be efficiently heated with high uniformity. In the production method [3] above, the first composition being stirred can be efficiently heated in a short time. The above [2] and [3] may be combined to use a heating medium and microwave as a heat source. Thereby, the first composition being stirred can be heated more efficiently with high uniformity. Therefore, the time required for dehydration can be further shortened. In the production method [4] above, boric acid is dehydrated under reduced pressure, so the time required for dehydration can be further shortened.

A manufacturing method that can efficiently mass-produce boron nitride powder can be provided.

1 is a diagram schematically showing an example of manufacturing equipment used in a method for manufacturing boron nitride powder. 1 is a perspective view of a rotary tablet press, which is an example of a molding device used in a method for producing boron nitride powder. FIG. 3 is an exploded view of a part of the rotary tablet press of FIG. 2; (A) is a sectional view showing the positions of the upper rod and lower rod when the second composition is molded in the tableting cell. (B) is a sectional view showing the positions of the upper rod and the lower rod when the molded body is taken out from the tableting cell onto the rotary disk. It is a schematic diagram of the briquette roll which is another example of the shaping|molding apparatus used for the manufacturing method of boron nitride powder.

Hereinafter, embodiments of the present disclosure will be described with reference to the drawings as the case may be. However, the following embodiments are examples for explaining the present disclosure, and are not intended to limit the present disclosure to the following contents. In the description, the same reference numerals will be used for the same elements or elements having the same function, and redundant description will be omitted in some cases. In addition, the positional relationships such as vertical, horizontal, etc. are based on the directions of the symbols shown in the drawings, unless otherwise specified. Furthermore, the dimensional ratios in the drawings are not limited to the illustrated ratios. In this specification, the numerical range indicated by the symbol "~" includes a lower limit value and an upper limit value. That is, the numerical range indicated by "x to y" means greater than or equal to x and less than or equal to y.

The method for producing boron nitride powder includes the step of dehydrating boric acid while heating and stirring a first composition containing boric acid and carbon-containing powder to obtain a second composition containing boron oxide and carbon-containing powder ( (first step), and a step (second step) of firing the second composition in an atmosphere containing at least one selected from the group consisting of nitrogen and a nitrogen-containing compound to produce boron nitride.

In the first step, boric acid (orthoboric acid, H ₃ BO ₃ ) contained in the first composition as a boron source is dehydrated. The dehydration reaction at this time is represented by the following formula (1).
2H ₃ BO ₃ → B ₂ O ₃ + 3H ₂ O (1)

The boric acid (orthoboric acid, H ₃ BO ₃ ) contained in the first composition as a boron source may be in powder form (boric acid powder). Examples of the carbon-containing powder contained in the first composition as a carbon source include amorphous carbon such as carbon black, activated carbon, and carbon fiber, crystalline carbon such as diamond, graphite, and nanocarbon, and pyrolyzed monomers or polymers. Examples include pyrolytic carbon obtained by Examples of carbon black include acetylene black, thermal black, channel black, and furnace black. By including such a carbon-containing powder in the first composition, boron oxide (B ₂ O ₃ ) can be prevented from solidifying into lumps. Thereby, the second and subsequent steps can be performed smoothly.

The first composition may be in powder form. From the viewpoint of sufficiently progressing the production of boron nitride, the content of the carbon-containing powder relative to 100 parts by mass of boric acid may be 10 parts by mass or more, 15 parts by mass or more, and 20 parts by mass or more. You can. From the viewpoint of reducing carbon remaining in the boron nitride powder, the content of the carbon-containing powder relative to 100 parts by mass of boric acid may be 40 parts by mass or less, 35 parts by mass or less, and 30 parts by mass or less. There may be. An example of the content of the carbon-containing powder based on 100 parts by mass of boric acid may be 10 to 40 parts by mass.

The first composition may contain components other than boric acid and carbon-containing powder. The first composition may include a reaction accelerator as such component. The reaction accelerator may have the function of adjusting the melting point by reacting with boron oxide, for example. Such reaction promoters include sodium carbonate (Na ₂ CO ₃ ), potassium carbonate (K ₂ CO ₃ ), sodium hydroxide (NaOH), potassium hydroxide (KOH), sodium tetraborate (Na ₂ B ₄ O ₇ ), lithium tetraborate (Li ₂ B ₂ O ₇ ), and the like. Further, the reaction accelerator may include an oxygen-containing calcium compound such as calcium oxide.

From the viewpoint of reducing the amount of reaction accelerator remaining in the boron nitride powder while sufficiently promoting the production of boron nitride, the content of the reaction accelerator with respect to 100 parts by mass of boric acid is 0.5 to 10 parts by mass. The amount may be from 1 to 8 parts by weight, or from 2 to 5 parts by weight.

The first composition may contain components other than the above-mentioned components. Such components include components that serve as nucleating agents for boron nitride production. Examples of the nucleating agent include boron nitride. Note that components other than boric acid and carbon-containing powder in the first composition may be added to the second composition obtained in the first step. By using a nucleating agent, it is possible to improve the uniformity of the particle size of boron nitride particles contained in the boron nitride powder.

The first composition may be prepared by, for example, placing boric acid, carbon-containing powder, and other components added as necessary in a mixing device and mixing them. As the mixing device, a mixing device such as a Henschel mixer may be used, or a heating stirring device capable of mixing each component while heating may be used. If a heating stirring device is used for mixing, the first step can be performed without using a mixing device and a heating stirring device separately. The heating and stirring device is not particularly limited as long as it can perform heating and stirring in parallel.

The heating temperature of the first composition may be 150 to 300°C, or 200 to 280°C. If this heating temperature becomes too high, the boric acid will volatilize and the yield of the ultimately obtained boron nitride will tend to decrease. If this heating temperature becomes too low, the time required for dehydration of boric acid tends to increase, and the dehydration tends to become insufficient. The heating time at the above heating temperature may be 1 to 8 hours, or 2 to 6 hours. This heating time may be adjusted depending on the mass of the first composition. For example, the ratio of the heating time at the heating temperature to the mass (I) of the first composition may be 0.05 to 0.5 hours/kg, or 0.1 to 0.3 hours/kg. You can.

Heating and stirring of the first composition may be performed in air under atmospheric pressure or under reduced pressure. If carried out under reduced pressure, dehydration will proceed more smoothly. Moreover, aggregation of the second composition can be sufficiently suppressed. The pressure during heating and stirring may be, for example, 100 kPaA or less, or 95 kPaA or less, from the viewpoint of promoting dehydration and suppressing aggregation. There is no particular restriction on the lower limit of the pressure, and from the viewpoint of reducing equipment costs, it may be 5 kPaA or more. Note that "kPaA" in this specification indicates absolute pressure.

In the first step, since the first composition is heated and stirred in parallel, boron oxide (B ₂ O ₃ ) obtained by dehydrating boric acid can be prevented from solidifying into lumps. Stirring can be performed using a stirring blade, a stirring blade, a stirring bar, or the like. The first step may be performed using a ribbon blender, for example.

The heat source used to heat the first composition may include at least one selected from the group consisting of microwaves and heating medium. Since the first composition contains a carbon-containing powder, the first composition can be efficiently heated with high uniformity by direct heating using microwave irradiation. Examples of the heating medium include steam and heating oil. The first composition may be heated by indirect heating using such a heating medium. Thereby, the first composition being stirred can be efficiently heated in a short time. A heating medium and microwave may be used together as a heat source. By performing both direct heating and indirect heating in this way, the first composition being stirred can be heated more efficiently.

The second composition obtained in the first step contains granular boron oxide and carbon-containing powder. The second composition may be particulate and may include boric acid. However, from the viewpoint of increasing the production amount of boron nitride powder, it is preferable that the content of boric acid in the second composition is lower. The conversion rate of boric acid by the reaction of formula (1) above may be 80% by mass or more, 90% by mass or more, or 95% by mass or more. The second composition may contain the reaction accelerator and/or nucleating agent described above. Furthermore, a reaction accelerator and/or a nucleating agent may be added to the second composition before the second step.

After the first step, the second composition may be pulverized and/or particle size adjusted by pulverization and/or classification. For pulverization, a general pulverizer such as a hammer mill, a vibration mill, or a pulverizer may be used. Classification may be performed by general methods such as sieving and air classification. At this time, it is desirable to adjust the ratio on a 1 mm sieve to 10% by weight or less based on the entire second composition. Thereby, boron nitride powder can be manufactured with high yield.

In the second step, the second composition is fired in an atmosphere containing at least one selected from the group consisting of nitrogen and nitrogen-containing compounds. When the second composition is fired in an atmosphere containing nitrogen, boron oxide contained in the second composition is reduced and nitrided to produce boron nitride (BN) as shown in formula (2) below.
B ₂ O ₃ +3C+N ₂ → 2BN+3CO (2)

Examples of nitrogen-containing compounds include compounds that contain nitrogen as a constituent element and produce boron nitride when reacted with boron oxide and carbon-containing powder. The nitrogen-containing compound may be, for example, ammonia.

The firing temperature of the second composition may be 1850° C. or higher, or 1900° C. or higher from the viewpoint of promoting reductive nitridation of boron oxide. The firing temperature of the second composition may be 2100° C. or lower, 2050° C. or lower, or 2000° C. or lower, from the viewpoint of suppressing the produced boron nitride from yellowing. An example of the firing temperature of the second composition is 1850 to 2100°C.

The firing time of the second composition at the above firing temperature may be 0.5 to 30 hours, 2 to 20 hours, or 5 to 15 hours. This baking time may be adjusted depending on the mass of the second composition. For example, the ratio of the firing time at the above firing temperature to the mass (II) of the second composition may be 0.01 to 0.6 hours/kg, and may be 0.1 to 0.6 hours/kg. You can. Thereby, boron nitride powder with sufficiently high boron nitride purity can be produced with sufficient efficiency.

The firing of the second composition may be performed under pressure higher than atmospheric pressure. The pressure when firing the second composition at the above firing temperature may be 0.25 MPaG or more, 0.30 MPaG or more, or 0.50 MPaG or more, from the viewpoint of promoting the production of boron nitride. You can. The pressure when firing the second composition at the above firing temperature may be 5.0 MPaG or less, 3.0 MPaG or less, or 1.0 MPaG or less, from the viewpoint of reducing manufacturing costs. good. "MPaG" in this specification indicates gauge pressure.

The second step may be performed using a pressurized batch furnace capable of firing the second composition under pressure. In the second step, a fired product containing boron nitride (a boron nitride-containing composition) is obtained. In the second step, the second composition may be filled into a firing boat, for example, and the firing boat may be introduced into a pressurized batch furnace and fired. If there is a limit to the filling volume of the firing boat, the second composition, not the first composition, is nitrided and reduced in the second step, so that the production amount of boron nitride powder can be increased. Alternatively, the load on the pressurized batch furnace can be reduced.

The fired product containing boron nitride obtained in the second step may be agglomerated or lumpy. A powdering step of pulverizing or crushing such a fired product may be performed. For pulverization, general pulverizing equipment such as a hammer mill, a vibration mill, a pulverizer, etc. may be used. A crushing device such as a Henschel mixer may be used for crushing. In the powdering step, particle size may be adjusted by sieving using an ultrasonic sieve or the like. In this way, boron nitride powder whose particle size has been adjusted can be obtained. The boron nitride contained in the boron nitride powder may be hexagonal boron nitride. That is, it may be hexagonal boron nitride powder.

The boron nitride content (purity) in the boron nitride powder may be 90% by mass or more, 95% by mass or more, or 98% by mass or more. The boron nitride content can be calculated using the following formula (A) from the nitrogen content measured using an oxygen/nitrogen simultaneous analyzer. As the oxygen/nitrogen simultaneous analyzer, for example, an oxygen/nitrogen analyzer (trade name: EMGA-920) manufactured by Horiba, Ltd. can be used.
BN (mass%) = N (mass%) x 1.772 (A)

The content of carbon in the boron nitride powder may be 0.5% by mass or less, 0.1% by mass or less, and 0.05% by mass or less. Boron nitride powder with high boron nitride purity and low carbon content can be suitably used as a raw material for a sintered body. The carbon content in the boron nitride powder can be measured using a carbon analyzer manufactured by LECO (trade name: IR-412) or the like. The carbon content can be adjusted by changing the blending ratio of boric acid and carbon-containing powder in the first composition and the conditions of the reductive nitriding in the second step. For example, the content of boron nitride and carbon may be adjusted by varying the amount of the second composition filled into the firing boat.

The average particle diameter (median diameter, D50) of the boron nitride powder (boron nitride powder) may be 3 to 40 μm, 5 to 30 μm, or 10 to 20 μm. Such boron nitride powder can be suitably used as a raw material for a sintered body. This average particle size can be adjusted by, for example, changing the firing conditions of the second composition, the crushing or pulverizing conditions of the fired product obtained in the second step, or the opening of the sieve used for sieving. . The average particle size in this specification is determined based on the method described in JIS Z 8825:2013 "Particle size analysis - laser diffraction/scattering method". In the particle size distribution (cumulative distribution) measured based on the above method, where the horizontal axis is the particle size [μm] on a logarithmic scale and the vertical axis is the frequency [volume %], the integrated value from small particle sizes is the whole. The particle size when the particle size reaches 50% is defined as the average particle size.

The BET specific surface area of the boron nitride powder (boron nitride powder) may be 0.5 to 5 m ² /g, or 0.7 to 3 m ² /g. Such boron nitride powder can be suitably used as a raw material for a sintered body. This BET specific surface area can be adjusted, for example, by changing the composition of the second composition (ratio of boric acid, boron oxide, and carbon-containing powder) or the firing conditions of the second composition. The BET specific surface area in this specification is measured by the BET single point method using nitrogen gas in accordance with the method described in JIS Z 8830:2013 "Method for measuring specific surface area of powder (solid) by gas adsorption". It is a value.

The boron nitride powder production method described above may be performed using the boron nitride powder production equipment 100 shown in FIG. The manufacturing equipment 100 includes a heating and stirring device 10 that heats and stirs the first composition to obtain a second composition, and a firing device 30 that heats and stirs the first composition to obtain a boron nitride powder. The heating and stirring device 10 may perform the first step, and the baking device 30 may perform the second step. The description of each step can also be applied to the heating and stirring device 10 and the baking device 30.

The heating stirring device 10 includes a storage section 11 that stores a first composition containing boric acid and a carbon-containing powder, a stirring section 12 that stirs the first composition stored in the storage section 11, and a microwave oscillator 16 that irradiates the first composition with microwaves, and a heat medium supply section 15 that supplies a heat medium to the jacket section 11a for heating the first composition within the housing section 11. Be prepared. The heating medium may be, for example, heating oil, and the heating oil may be used in circulation. Note that the heat medium is not limited to heat medium oil, and may be, for example, steam (superheated steam) or the like.

As the housing section 11, a reaction tank equipped with a jacket section 11a can be used. The stirring section 12 includes stirring blades 12B (ribbon blades) and a motor 12A that rotationally drives the stirring blades 12B. The stirring blade 12B, which is rotationally driven by the motor 12A, mixes and stirs the first composition in the storage section 11. The first composition may be prepared by introducing boric acid, carbon-containing powder, and other components into the storage section 11, and then mixing them in the stirring section 12 (stirring blade 12B). Thereafter, the first composition may be heated in the storage section 11 while continuing stirring by the stirring section 12 (stirring blades 12B).

The microwave oscillator 16 directly heats the first composition by irradiating it with microwaves to the first composition being stirred within the storage section 11 . Further, the heating medium supply section 15 supplies a heating medium to the jacket section 11a to indirectly heat the first composition. Thereby, the temperature of the first composition being stirred can be quickly raised to the target temperature. Furthermore, variations in temperature of the first composition can be reduced and the first composition can be heated with high uniformity. As a result, dehydration of boric acid proceeds smoothly and uniformly, and boric acid can be sufficiently converted into boron oxide in a short time. Note that the components and heating conditions of the first composition are as described above.

The heating section of the heating and stirring device 10 in FIG. It has a heat medium supply section 15. The heating section is not limited to such a configuration, and may include only one of the microwave oscillator 16 and the heating medium supply section 15, or may include a device different from the microwave oscillator 16 and the heating medium supply section 15. may have. Further, the first composition may be heated by the heating medium not through the jacket portion 11a but through a tube disposed within the housing portion 11.

The heating and stirring device 10 connects the storage unit 11 and the cooling unit 18 to a cooling unit 18 that condenses moisture through heat exchange between a refrigerant and a gas containing moisture generated by dehydration of boric acid contained in the first composition. A bag filter 17 is provided in the flow path, and a gas suction part 19 that sucks gas is provided downstream of the bag filter 17 and the cooling part 18. The gas in the storage section 11 containing moisture generated by dehydration of boric acid is sucked by the gas suction section 19, passes through the bag filter 17, and is introduced into the cooling section 18. The bag filter 17 captures solids contained in the gas.

Cooling water is supplied to the cooling unit 18 as a refrigerant. In the cooling unit 18, the gas is cooled by heat exchange between the cooling water and the gas, and moisture contained in the gas is condensed. The condensed water obtained in the cooling section 18 is discharged from the cooling section 18. The gas whose moisture content has been reduced by the cooling unit 18 is sucked into the gas suction unit 19 and then discharged to the outside of the system. By suctioning with the gas suction unit 19 in this manner, the inside of the storage unit 11 can be brought below atmospheric pressure to promote dehydration. Therefore, the first step can be further shortened. The gas suction unit 19 may be, for example, a vacuum pump. Note that it is not essential to provide the bag filter 17 and the cooling section 18. Modified examples of the heating and stirring device 10 may not include one or both of these.

The firing device 30 may be any device in which the second composition is fired in an atmosphere containing at least one selected from the group consisting of nitrogen and a nitrogen-containing compound, so that the reaction as described in (2) above proceeds, for example. . The firing device 30 may be, for example, a normal batch furnace, or a pressurized batch furnace that can perform firing while pressurizing. Further, a continuous furnace such as a pusher type tunnel furnace may be used. In the firing device 30, the second composition obtained in the heating stirring device 10 is fired to obtain boron nitride powder containing boron nitride.

Since the boron nitride powder production facility 100 includes the heating and stirring device 10, boric acid is dehydrated while being heated and stirred. Therefore, the first step can be performed in a short time. Moreover, when the equipment capacity of the firing apparatus 30 is limited, the yield of boron nitride powder can be increased compared to the case where boron nitride powder is obtained by firing the first composition as it is. For example, when boric acid is reduced and nitrided without dehydration, boron nitride is generated by the reaction according to the following equation (3).
H ₃ BO ₃ +3/2C+1/2N ₂ →BN+3/2CO+3/2H ₂ O (3)

Table 1 shows the amount of boron nitride produced calculated from the stoichiometric ratio when 100 kg of the raw material is used in each of the above formulas (2) and (3).

As shown in Table 1, the reaction of formula (2) above can significantly increase the amount of boron nitride (boron nitride powder) obtained than the reaction of formula (3). Therefore, if the processing capacity of the firing device 30 is a bottleneck in the production of boron nitride powder, the production amount of boron nitride powder can be significantly improved by introducing the heating stirring device 10. Further, the load on the firing device 30 can be reduced. Therefore, the boron nitride powder production equipment 100 and the above-described boron nitride powder production method are particularly useful for production on an industrial scale. However, the boron nitride powder production equipment 100 and the above-described boron nitride powder production method are not limited to production on an industrial scale.

The boron nitride powder manufacturing equipment 100 may include a particle size adjustment device that adjusts the particle size by pulverizing or crushing the fired product obtained in the firing device 30. Examples of the particle size adjusting device include a crushing device, a crushing device, a sieving device, and the like. It may have at least one of these. Examples of the crushing device include a Henschel mixer. Examples of the crushing device include a hammer mill, a pulverizer, and the like. By providing such a particle size adjustment device, boron nitride powder with uniform particle size can be obtained.

Although the embodiments of the present disclosure have been described above, the present disclosure is not limited to the above embodiments. For example, in a modification of the method for producing boron nitride powder, between the first step and the second step, the second composition is molded to produce a molded body, and in the second step, the molded body of the second composition is molded. A sintered body containing boron nitride may be obtained by firing in an atmosphere containing at least one selected from the group consisting of nitrogen and nitrogen-containing compounds. By pulverizing the obtained sintered body, boron nitride powder (boron nitride-containing powder) can be obtained. The second composition obtained in the first step has better moldability than the first composition.

By including the molding step of producing a molded object of the second composition, handling properties after the second step can be improved. Furthermore, it is possible to reduce the size of the firing device 30 used in the second step, and to increase the packing density when filling the firing boat. Thereby, the amount of processing in the second step can be increased, and the yield of boron nitride powder can be further increased. From the viewpoint of sufficiently promoting reductive nitridation in the second step while improving handling properties, the volume of the compact may be 0.1 to 1.0 cm ³ , or 0.2 to 0.6 cm ³ . Good too. From the same viewpoint, the molding density of the molded body may be 1.0 to 1.7 g/cm ³ or 1.1 to 1.6 g/cm ³ . With such a compacting density, the fired product obtained in the second step can be smoothly pulverized while sufficiently improving productivity. Thereby, boron nitride powder can be smoothly manufactured.

The compacted density of the compact in this specification can be calculated from the measured values of the volume and mass of the compact. Specifically, in accordance with JIS Z 8807:2012 "Measurement method of density and specific gravity by geometric measurement", the volume calculated from the length of each side of the molded object (measured with calipers) and the electronic balance. It can be determined based on the measured mass of the nitride sintered body (see section 9 of JIS Z 8807:2012). As the molding device for producing the molded body, for example, a rotary tablet press or a briquette roll may be used.

The rotary tablet press 110 illustrated in FIG. 2 is arranged so as to sandwich the rotary disk 41 and a rotary disk 41 that rotates at a constant speed along the circumferential direction (in the direction of the arrow in the figure), and is synchronized with the rotary disk 41. A pair of holding

plates

43 and 46 are provided. The upper holding plate 43 holds a plurality of upper rods 45 (tableting rods) arranged at regular intervals along the circumferential direction, and the lower holding plate 46 holds a plurality of lower rods arranged at regular intervals along the circumferential direction. Holds the rod 47 (tableting rod). Although some of the upper rods 45 and lower rods 47 are omitted in FIG. 2, the upper rods 45 may be held in all the holding holes 45h, and the lower rods 47 may be held in all the holding holes 47h. It's good that it has been done.

A plurality of tableting cells 42 (mortars) are formed in the rotary disk 41 along the circumferential direction. Above each tableting cell 42, an upper rod 45 (punch) having a tip 45A having an outer diameter equivalent to the inner diameter of the tableting cell 42 is arranged. A lower rod 47 (punch) having a tip 47A having an outer diameter equivalent to the inner diameter of the tableting cell 42 is arranged below each tabletting cell 42.

As shown in FIG. 2, the upper rod 45 is held by a holding plate 43 so as to be movable up and down, and the lower rod 47 is held by a holding plate 46 so that it can be moved up and down. In FIG. 3, a part of the rotary tablet press 110 is shown expanded, with the holding

plates

43 and 46 omitted. In FIG. 3, the circumferential direction is expanded in the horizontal direction. As shown in FIG. 3, the upper rod 45 and the lower rod 47 are configured to be movable up and down.

The upper rod 45 reciprocates once between the most lowered position and the highest raised position by the elevating mechanism 44, depending on the rotational position, while the rotary disk 41 (holding disk 43) makes one rotation. Each lower rod 47 also moves back and forth between the lowest position and the highest position by an elevating mechanism (not shown) depending on the rotational position during one rotation of the rotary plate 41 (holding plate 46). do. The elevating mechanism for elevating and lowering the lower rod 47 may be the same as the elevating mechanism 44.

The rotary tabletting machine 110 includes a hopper 50 that supplies a powdered second composition 52 onto a rotary disc 41, and a hopper 50 that guides the second composition 52 supplied from the hopper 50 to the rotary disc 41 to a tableting cell 42. A feeder 57 is provided for filling the second composition 52 into the tableting cell 42. When the tableting cell 42 is filled with the second composition 52, the lower rod 47 is in the lowest position. A predetermined amount of the second composition 52 filled into the tableting cell 42 by the feeder 57 is pushed up by the tip 47A of the lower rod 47 inserted into the tableting cell 42 from below, and It is pushed down by the tip 45A of the upper rod 45 inserted into the tableting cell 42 from above the cell 42.

As shown in FIG. 4(A), the tip 45A of the upper rod 45 and the tip 47A of the lower rod 47 press the second composition 52 in the tableting cell 42 in directions facing each other. As a result, the second composition is compressed and molded within the tableting cell 42. After the second composition 52 is compressed to obtain the molded body 54, the upper rod 45 is retracted upward, as shown in FIG. 4(B). The molded body 54 is pushed out from the tableting cell 42 onto the rotary disk 41 by the lower rod 47. The tableting cells 42 on the rotary disk 41 are collected in a tray (not shown) provided around the outer periphery of the rotary disk 41.

As shown in FIG. 4(B), the end surface 45r of the distal end portion 45A of the upper rod 45 and the end surface 47r of the distal end portion 47A of the lower rod 47 each have a concave surface formed of a curved surface. Therefore, the upper surface 54r and lower surface 54s of the molded body 54 have convex curved surfaces. Therefore, when the distal end portion 47A of the lower rod 47 pushes out the molded body 54 from the tableting cell 42 onto the rotary disk 41, it is possible to suppress the molded body 54 from being damaged due to elastic recovery.

In the rotary tablet press 110, the second composition 52 is molded by biaxial compression to obtain a molded body 54. By using the rotary tablet press 110, the molded bodies 54 can be continuously manufactured in large quantities. Furthermore, the molded body 54 includes a circumferential surface and a pair of convex surfaces (upper surface 54r, lower surface 54s). The molded body 54 having such a shape is difficult to break and has excellent handling properties. Therefore, it is particularly suitable for production on an industrial scale. However, the method for forming the molded body is not limited to the method using such a rotary tablet press 110.

The briquette roll 120 illustrated in FIG. 5 includes a pair of

roll bodies

61 and 62 arranged side by side so that their rotation axes are parallel to each other. A plurality of recesses 64 are provided on the peripheral surfaces of the

roll bodies

61 and 62. The surface 64r of the recess 64 is formed of a curved surface. The roll body 61 is supported so as to be rotatable clockwise, and the roll body 62 is supported so as to be rotatable counterclockwise. A guide plate 65 is provided above the

roll bodies

61 and 62 to guide the powdered second composition 52 between the

roll bodies

61 and 62.

The second composition 52 passes through the guide plate 65 and is supplied between the

roll bodies

61 and 62. The second composition 52 supplied between the

roll bodies

61 and 62 fills the recess 64 and is compressed while being sandwiched between the

roll bodies

61 and 62. The compressed second composition 52 is molded into a molded body 55. The molded body 55 is compressed and molded within a pair of mutually opposing recesses 64 . Therefore, the surface 55r of the molded body 55 is a curved surface. In the briquette roll 120, the molded body 55 is guided out from the recess 64 by utilizing the elastic recovery of the molded body 55. Therefore, damage during demolding can be sufficiently suppressed.

When using a molded body as the second composition, the fired product obtained by firing in the second step may be pulverized using the above-mentioned pulverizer. In this way, boron nitride powder can be obtained.

The contents of the present disclosure will be described in more detail with reference to Examples and Comparative Examples, but the present disclosure is not limited to the Examples below.

(Example 1)
<First step (dehydration step)>
Boric acid powder, acetylene black (powder), and sodium carbonate powder were introduced into a housing section of a commercially available heating stirring device equipped with a jacket section. The blending ratio was 25 parts by mass of acetylene black and 3 parts by mass of sodium carbonate powder with respect to 100 parts by mass of boric acid powder. The total mass of the boric acid powder, acetylene black, and sodium carbonate powder (mass (I) of the first composition) was 25 kg. A vacuum pump installed downstream of the cooling section was started to adjust the pressure inside the storage section to 90 kPaA.

Thermal oil at 250° C. was supplied to the jacket portion while stirring the first composition in the storage portion containing the boric acid powder, acetylene black, and sodium carbonate powder with a ribbon blade. Further, microwaves were irradiated from two microwave oscillators toward the first composition in the housing section. The output of the microwave oscillators was 1.5 kW x 2, and the oscillation frequency was 2,450 MHz.

After 4 hours from the start of heating, heating was stopped. The heating time per 1 kg of the first composition was as shown in Table 1. A second composition containing boron oxide, acetylene black, and sodium carbonate was taken out from the container, and the mass (II) of the second composition was measured. The results were as shown in Table 2. The second composition was in powder form similar to the first composition. From mass (I) and mass (II), the conversion rate of H ₃ BO ₃ by the dehydration reaction shown in the above formula (1) was calculated. The results were as shown in Table 2. Moreover, the ratio of mass (I) to mass (II) was as shown in Table 2.

<Second step (reduction nitriding step)>
A box-shaped firing boat made of boron nitride (filling volume: 3000 cm ³ ) was filled with the second composition. The bulk density of the second composition was measured from the filling volume and filling mass of the second composition in the firing boat. The results were as shown in Table 2. A firing boat filled with the second composition was placed in a pressurized batch furnace. The second composition was fired at 1800 to 2000° C. for 10 hours in a nitrogen atmosphere pressurized to 0.8 MPaG. The fired product obtained by firing was an aggregate. Therefore, after crushing the fired product using a Henschel mixer, it was passed through a sieve with an opening of 63 μm to obtain boron nitride powder (under-sieve material).

The nitrogen content in the boron nitride powder was measured using an oxygen/nitrogen analyzer (trade name: EMGA-920). Then, the boron nitride content in the boron nitride powder was determined using the above formula (A). The results were as shown in Table 2. The boron nitride was confirmed to be hexagonal boron nitride from the results of XRD.

The average particle diameter (median diameter) of the boron nitride powder was measured using a particle size distribution analyzer (manufactured by Nikkiso Co., Ltd., MT3300EX). Using this measuring device, based on the method described in JIS Z 8825:2013 "Particle size analysis - laser diffraction/scattering method", the horizontal axis is the particle diameter [μm] on a logarithmic scale, and the vertical axis is the frequency [volume %] A particle size distribution (cumulative distribution) was obtained. In the particle size distribution (cumulative distribution), the particle size when the integrated value from the small particle size reached 50% of the total was defined as the average particle size. The results were as shown in Table 2.

The BET specific surface area of the boron nitride powder was measured using a measuring device compliant with JIS Z8830:2013. Using this measuring device and nitrogen gas, the BET specific surface area was measured by the BET single point method in accordance with the method described in JIS Z 8830:2013 "Method for measuring specific surface area of powder (solid) by gas adsorption". The results were as shown in Table 2.

(Example 2)
Boron nitride powder was obtained in the same manner as in Example 1, except that the first composition was heated at a pressure in the storage section of 10 kPaA in the first step, and that the heating time in the storage section was 3 hours. . Each evaluation was performed in the same manner as in Example 1. The results were as shown in Table 2.

(Comparative example 1)
The boric acid powder, acetylene black, and sodium carbonate powder used in Example 1 were blended in the same proportions as in Example 1, and mixed using a mixer to prepare a first composition. The total mass (mass (I)) of the boric acid powder, acetylene black, and sodium carbonate powder was 9 kg. This first composition was placed in a container and the container was placed in a rack dryer. The boric acid was dehydrated by holding it for 12 hours in a tray dryer whose temperature was adjusted to 250° C. under atmospheric pressure. In Comparative Example 1, the first composition (second composition) held in the tray dryer was not stirred. The obtained second composition was crushed for 10 minutes using a Henschel mixer. The heating time per 1 kg of the first composition was as shown in Table 2. The mass (II) of the second composition thus obtained and the conversion rate of H ₃ BO ₃ were as shown in Table 2. Using this second composition, a second step (reduction nitriding step) was performed in the same manner as in Example 1. Each evaluation was performed in the same manner as in Example 1. The results were as shown in Table 2.

In Comparative Example 1, although the mass (I) of the first composition was smaller than in Examples 1 and 2, dehydration took a longer time than in Examples 1 and 2. In the method of Comparative Example 1, if the heating time in the first step was shorter than 12 hours, the conversion rate of boric acid would decrease, so it could not be made shorter than 12 hours. In Examples 1 and 2, it was confirmed that boron nitride powder with a sufficiently high boron nitride content could be mass-produced in a shorter time than in Comparative Example 1 by dehydrating boric acid while heating and stirring.

DESCRIPTION OF SYMBOLS 10... Heating stirring device, 11... Storage part, 11a... Jacket part, 12... Stirring part, 12A... Motor, 12B... Stirring blade, 15... Heat medium supply part, 16... Microwave oscillator, 17... Bag filter, 18... Cooling unit, 19... Gas suction unit, 30... Baking device, 41... Rotating disk, 42... Tableting cell, 43, 46... Holding plate, 44... Lifting mechanism, 45... Upper rod, 45A, 47A... Tip part, 45h , 47h...holding hole, 45r, 47r...end surface, 47...lower rod, 50...hopper, 52...second composition, 54, 55...molded body, 54r...top surface, 54s...bottom surface, 55r...surface, 57...feeder , 61, 62...roll body, 64...recess, 64r...surface, 65...guide plate, 100...manufacturing equipment, 110...rotary tablet press, 120...briquette roll.

Claims

Dehydrating the boric acid while heating and stirring the first composition containing boric acid and the carbon-containing powder to obtain a second composition containing boron oxide and the carbon-containing powder;
A method for producing boron nitride powder, comprising the step of firing the second composition in an atmosphere containing at least one selected from the group consisting of nitrogen and a nitrogen-containing compound to produce boron nitride.
The method for producing boron nitride powder according to claim 1, wherein microwaves are used as a heat source to heat the first composition.
The method for producing boron nitride powder according to claim 1 or 2, wherein a heat medium is used as a heat source for heating the first composition.
The method for producing boron nitride powder according to claim 1 or 2, wherein the boric acid is dehydrated while heating and stirring the first composition under reduced pressure.