JPH0211521B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a continuous production method of boron nitride,
In particular, the present invention relates to a continuous method for producing hexagonal boron nitride having excellent physical and chemical properties. Hexagonal boron nitride has high heat resistance, thermal shock resistance,
It has excellent chemical stability, high thermal conductivity, high electrical insulation, and lubrication properties, and due to its properties, it is widely used as high-temperature structural materials, electrical insulation materials, heat sinks for transistors, ICs, microwaves, and wear-resistant materials. ing. The present invention relates to a new industrially advantageous method for producing hexagonal boron nitride, which has great utility value. Conventionally, a method using the following reaction formula is known as a typical method for producing hexagonal boron nitride. (1) 2B+N ₂ →2BN (2) B ₂ O ₃ +NH ₃ + high melting point substance (Ca ₃ (PO ₄ ) ₂ ) → BN (3) BCl ₃ +NH ₃ →BN+3HCl (4) Na ₂ B ₄ O ₇ +2CO (NH ₂ ) ₂ NH ₃ ---→ 4BN+Na ₂ O+4H ₂ O+2CO ₂ (5) B ₂ O ₃ +3C+N ₂ →2BN+3CO Among these production methods, the methods of reaction formulas (1) and (3) require expensive raw materials. However, the method for producing boron nitride according to the above reaction formulas (2) and (4) requires cheap raw materials and can be produced in large quantities. Although it is carried out industrially, the manufacturing method requires many steps, and in particular requires two or more stages of treatment in a nitriding furnace in an ammonia gas atmosphere, which requires high airtightness, making continuous production extremely difficult. It is difficult, and in fact, only the batch method is used for these manufacturing methods. When these many processing steps are carried out in batches, the disadvantage of high product costs due to increased equipment and manufacturing costs is unavoidable. In addition, the production of boron nitride based on reaction formula (5) is
Although it has the advantage of being a one-stage reaction and the price of raw materials is low, the reaction yield is generally low, there is a lot of residual carbon, and it needs to be separated.
Due to the high-temperature reaction of This was considered disadvantageous, and only a small-scale batch method using a fixed furnace or a small rotary furnace has been put into practice, and in particular, a continuous production method using the reaction formula (5) above has never been considered. However, there is a great need in society to provide high-quality hexagonal boron nitride in large quantities and at low cost, which has excellent properties and whose demand is rapidly increasing in a wide range of fields. Therefore, the object of the present invention is to use inexpensive raw materials,
An object of the present invention is to provide an industrial method for continuously producing hexagonal boron nitride. Another object of the present invention is to provide hexagonal boron nitride with high purity and high crystallinity at a low cost. Still another object is to provide a method for efficiently carrying out the reaction of the above reaction formula (5) and producing high-quality hexagonal boron nitride with small variations with high efficiency. Other objects and excellent effects of the present invention will become clearer from the following description. In order to achieve the above object, the present inventors, in particular,
As a result of repeated research on the method of the above reaction formula (5), which allows hexagonal boron nitride to be formed in a one-step reaction using inexpensive starting materials, we have found that it is extremely efficient, and also allows for a continuous process that was previously thought to be almost impossible. We found that high purity hexagonal boron nitride can be produced by this method. That is, the present invention provides a heating furnace equipped with an inlet and an outlet, in which a mixture of boric acid, carbon, and a nitriding catalyst having an elemental component weight ratio (B/C) of boron to carbon in a range of 0.59 to 1.22/1 is heated. It is continuously inserted from the inlet of the furnace and heated to a temperature of 1,650 to 2,300°C under countercurrent contact conditions of nitrogen gas or a non-oxidizing gas containing nitrogen gas to cause a reduction and nitriding reaction, and then from the outlet of the furnace. Provided is a method for continuously producing boron nitride, which is characterized in that a reaction product is continuously taken out. In the method of the present invention, the carbon used in combination with boric acid includes, for example, ordinary acetylene black, channel black, graphite, or carbon materials such as charcoal and wood flour. Acetylene black is preferably used because of its properties. In the method of the present invention, in order to make the nitriding reaction more efficient, catalysts used in ordinary nitriding reactions, such as metals such as iron, cobalt, nickel, calcium, magnesium, manganese, and molybdenum, or oxides thereof, A suitable amount of a metal compound such as a carbonate can be mixed with the above raw material compound. Further, the mixture of boric acid, carbon and nitriding catalyst may be granulated in advance, but there is no problem in adding and mixing a granulating binder such as poval, molasses, methyl cellulose, etc. at that time. For mixing these components, a conventional mixer such as a Henschel mixer, a ribbon mixer, an extruder, etc. can be used, and if necessary, water can be added before mixing and drying. or,
The mixing ratio of the main components, boric acid and carbon, is in the range of 0.59 to 1.22 in terms of element weight ratio B/C. If the weight of B to C is less than 0.59, the amount of carbon remaining after the reaction will increase;
If it is larger than 1.22, the ring element reaction of the boron compound before the nitriding reaction will be insufficient, and the nitriding will not proceed sufficiently, resulting in a decrease in yield. Therefore, the above weight range ratio is preferable, and the more preferable ratio is 0.7~
It is 1.0. In the method of the present invention, a nitriding furnace is used as the nitriding heating reactor, which is equipped with an inlet for supplying a raw material and an outlet for taking out a reaction product heated and reacted through the furnace. As such a heating furnace,
Although there are no restrictions on the shape, various commonly known vertical furnaces, fluidized bed furnaces, or tunnel furnaces can be conveniently used.
As the tunnel furnace, a shelf type pusher furnace is particularly preferred. These heating furnaces are constructed of a furnace material that can withstand the reaction temperatures in the method of the present invention, such as heat-resistant materials such as carbon, silicon carbide, boron nitride, and the like. In the present invention, a raw material mixture is moved in a fixed direction in a heating furnace, and at the same time, nitrogen gas or a nitrogen-containing non-oxidizing gas is flowed in a direction opposite to the moving direction, and the mixture is passed through the countercurrent nitrogen gas. It is extremely important to carry out the heating reaction under contact conditions.
In combination with this requirement, it is important to carry out the heating reaction at a temperature range of 1650 to 2300° C., which is higher than the reaction temperature conventionally known. Such a heat source for high temperature heating is not particularly limited, but a carbon heater is preferably used. If the heating temperature is less than 1650℃, the nitriding reaction will not proceed sufficiently, and
If the temperature exceeds 2300°C, undesirable substances such as boron carbide other than boron nitride will be produced as by-products, and the purity and yield of the target product will decrease, which is not preferable. On the other hand, when nitrogen or a nitrogen-containing non-oxidizing gas is caused to flow in the same direction as the moving direction of the raw material mixture, that is, in parallel flow, to cause a contact reaction, impurities in the mixture, especially oxygen, moisture, or carbon monoxide generated during the reaction, This is not preferable because the partial pressure of gases such as other metal impurities increases, and as a result, the nitrogen partial pressure decreases, making it difficult for the nitriding reaction to proceed quickly or entraining metal impurities into the product. Therefore, in the method of the present invention,
It is important that the nitriding gas travels countercurrently to the moving direction of the raw material mixture to bring them into countercurrent contact, which not only effectively avoids the above-mentioned disadvantages, but also has the additional effect of effectively preheating the introduced raw material mixture. can get. Examples of the non-oxidizing gas containing nitrogen gas include inert gases such as argon and helium, but it is desirable that the nitrogen gas concentration is as high as possible. Furthermore, in the method of the present invention, nitrogen gas includes, for example, ammonia, which can be easily converted into nitrogen gas in the reaction system. In the method of the present invention, the reaction time of the raw material mixture heated and reacted under countercurrent contact conditions of nitrogen gas varies depending on the temperature, but is usually 1 to 10 hours.
Further, for the countercurrent flow of nitrogen gas, etc., a relatively low flow rate is advantageously adopted, which does not cause the above-mentioned disadvantages and does not cause disadvantages such as delay in the nitriding reaction. As mentioned above, the method of the present invention can advantageously utilize a vertical heating furnace or a tunnel furnace, especially a stacked pusher furnace. To explain the case of each heating furnace, in a vertical heating furnace, raw materials are fed at a constant rate from the top of the furnace, non-oxidizing nitrogen-containing gas is introduced from the bottom in a countercurrent flow, and the inside of the furnace is heated by a heater. Heat to 1650-2300℃ to react. The reaction products are drawn off continuously at a constant rate from the bottom, for example via a rotary valve. On the other hand, the non-oxidizing gas introduced from the gas inlet cools the reaction products while heating itself, rises in the furnace, advances the reaction, recovers heat from the upper exhaust port, and then is discharged. Ru. In addition, a shelf-type pusher furnace (tunnel furnace) is equipped with a rail at the bottom of the tunnel-shaped furnace, a base plate is placed on the rail, and a sagger filled with the above mixture is placed on each base plate. This method transports the plate from the furnace inlet to the outlet. The shelf type pusher furnace does not have a driving part in the furnace main body, and the bed plates are sequentially extruded and transferred on the rail by the pusher, so mechanical strength is not required and carbon material, which is mainly used in high-temperature reactions like the present invention, is used. This is extremely convenient as it allows you to According to the method of the present invention, high purity, highly crystalline hexagonal boron nitride can be produced continuously at a high reaction rate from inexpensive raw materials through a one-step reaction. It has value and practicality. Next, the present invention will be explained in more detail using specific examples. Note that % in the examples is by weight unless otherwise specified. Example 1 A conventional vertical heating furnace with a capacity of 300 was equipped with a carbon heater. First, as raw materials, 500 kg of boric acid, acetylene,
110 kg of black and 50 kg of calcium oxide were kneaded and homogenized with 50 kg of a 1.0% polyvinyl alcohol aqueous solution, granulated into pellets with a diameter of 5 mm and a length of 30 mm, and heated to a temperature of 150°C to completely dehydrate and dry. This is supplied from the upper inlet of the furnace, and while nitrogen gas is introduced at a rate of 120/min from the gas supply port at the lower part of the furnace, the peripheral wall in the center of the furnace is heated to approximately 2000℃ with a carbon heater to react. started. Three hours after the start of the reaction, the reaction product boron nitride was taken out at a rate of 1 kg/hr while continuously operating the rotary valve at the outlet at the bottom of the furnace. The reaction was carried out for 24 hours and was extremely smooth. In addition, the obtained product was washed with 5% hydrochloric acid water,
After washing with hot water until the filtrate becomes neutral, incubate at 150℃ for 10
Dry for an hour. The average analysis result of this material is that it is hexagonal boron nitride according to X-ray diffraction, and has excellent crystallinity with a graphitization index (G・I value) of 1.65, and the result of chemical analysis shows that it is a hexagonal boron nitride. 0.04%,
It showed high purity with B ₂ O ₃ 0.08% and nitrogen content 56.1%. In addition, when we investigated the dispersion of the analysis values of the product every 3 hours, it was found that carbon was 0.008%,
The B ₂ O ₃ content was 0.005% and the nitrogen content was within 0.15%, which was within the analytical error range, and the variation in quality among the products was almost negligible. In addition, in the conventional batch method, there was usually a variation of about 5 to 20% in all analytical values. In addition, the overall yield was high and excellent at 97.8% of the theoretical value. Productivity was approximately 15 times higher than that of a batch furnace with the same volume. In addition, the above graphitization index (G・I value) is
The peak area of line diffraction was determined and calculated using the following formula. The lower the G.I value, the more advanced the crystallization is. G・I value = [100] area + [101] area / [102]
Area example 2 Shelving type pusher furnace (external size: 1500×
1500 x 8000 mm), flow nitrogen gas at 60/min from the furnace exit direction in countercurrent to the raw material mixture, and reduce the furnace pressure to 100 mm.
Adjusted to become Aq. 1000mm in the center of the furnace
A carbon heater is controlled to create a soaking zone of 2000℃, and a sagger (200×
A mixture of 500 g of boric acid, 110 g of acetylene black, and 100 g of magnesium oxide was charged per pot into a sagger (200 x 150 mm), and the saggers were sequentially transferred to the furnace using a pusher in a cycle of 1 pot/30 minutes, and this operation was continued for 240 hours. Boron nitride was produced continuously. The obtained product was washed with 5% hydrochloric acid and then with hot water until the wax liquid became neutral.
It was dried for 10 hours at a temperature of 150°C. The result is X
As a result of line diffraction, it is hexagonal boron nitride,
It was confirmed that the material had excellent crystallinity with a graphitization index of 1.50. Further, the results of chemical analysis showed average values of residual carbon 0.03%, B ₂ O ₃ 0.09%, and nitrogen content 56.27%. Incidentally, the variations in chemical analysis values among the saggers were 0.005%, 0.003%, and 0.10% or less, respectively, which were within the analysis error, and the variations in product quality were almost negligible. In the conventional batch method, the analysis values for each of the above components are usually 5 to 5.
A variation of about 20% can be seen. Moreover, the overall yield was as high as 98.5% of the theoretical value, which was excellent. This method was approximately 10 times more productive than producing boron nitride in batches using a furnace with the same volume. Examples 3 to 7 and Comparative Examples 1 to 5 Using the pusher furnace used in Example 2, the reaction temperature conditions or the weight ratio of boric acid and carbon material were changed as shown in Table 1. A series of experiments were conducted and the analysis results of each reaction product are summarized in Table 1 below. The analysis was performed on the reaction product treated with hydrochloric acid water, washed with hot water, and dried. Also, for reference, the above Example 2
are also listed. 【table】

Claims (1)

[Claims] 1. The elemental component weight ratio (B/C) of boron and carbon is
A mixture of boric acid, carbon and nitriding catalyst in a ratio ranging from 0.59 to 1.22/1 is continuously introduced through the inlet of a heating furnace equipped with an inlet and an outlet, and nitrogen gas or a non-oxidizing gas containing nitrogen gas is added. Under countercurrent contact conditions,
1. A method for continuously producing boron nitride, which comprises heating to a temperature of 1,650 to 2,300°C to cause a reduction and nitriding reaction, and continuously taking out the reaction product from an outlet of the furnace. 2. The method according to claim 1, wherein the heating furnace is a vertical furnace. 3. The method according to claim 1, wherein the heating furnace is a tunnel-type shelf-type pusher furnace.