JPS644966B2 - - Google Patents

Description

[Detailed description of the invention]

(Technical Field) The present invention is a novel carbon-containing composition containing boron oxide and elemental carbon. More specifically, the present invention is a novel carbon-containing composition particularly suitable for producing fine boron carbide powder. (Background technology) Ceramic molded bodies obtained by sintering boron carbide (B ₄ C) powder have properties such as high hardness, chemical stability, and large neutron absorption capacity. Used in various wear-resistant parts and nuclear reactor control materials. In this case, the finer the boron carbide powder that is the raw material, the easier it is to sinter, and the greater the strength of the molded body obtained by sintering. Furthermore, the higher the purity of the boron carbide powder, the smaller the variation in strength of the molded body. Boron carbide powder is produced by mechanically pulverizing and mixing boron oxide as a boron source and elemental carbon such as coke or carbon black as a carbon source.
The resulting pulverized mixture is heated and fired at 2000℃ or higher using an electric furnace such as an arc furnace or an Acheson furnace to produce a boron carbide ingot, which is then pulverized for a long time using a ball mill, vibration mill, etc. It was commonly manufactured. However, such methods are batch-based;
In addition to the complexity of the work process, the generation of noise and dust, there are problems such as contamination and a decrease in purity due to the introduction of impurities during the mixing and charging of raw materials. Furthermore, it takes a long time to grind a highly hard boron carbide ingot into a fine powder, and requires a significantly large amount of grinding energy. Furthermore, a considerable amount of impurities are mixed in due to wear of the crusher itself. Therefore, in order to obtain highly pure boron carbide powder, the powder obtained by pulverization must be purified by washing and filtration several times, which is a large and economically undesirable process. There were flaws. (Object of the Invention) The object of the present invention is to provide a method of grinding without any mechanical grinding operation, which has many problems such as noise, wear, generation of dust, and contamination of impurities, as in grinding using a ball mill or the like. The object of the present invention is to provide a method for continuously producing a novel carbon-containing composition in which fine particles of boron oxide and elemental carbon are mixed extremely uniformly. Another object of the present invention is to provide a new boron carbide consisting of extremely fine particles by heating this new carbon-containing composition. Further objects of the invention will become apparent from the description below. (Disclosure of the Invention) As a result of thorough consideration of the advantages and disadvantages of the above-mentioned conventional techniques, the present inventors have discovered a conventional method for obtaining boron carbide powder from boron oxide and elemental carbon as a method for obtaining a boron carbide sintered body with excellent physical properties. A method fundamentally different from the method described above is used to chemically prepare a novel carbon-containing composition containing or substantially consisting of boron oxide and elemental carbon with a sufficiently high uniformity and constituent particles having a fine particle size. The present inventors have discovered that the desired high-purity and fine boron carbide powder can be produced by heating and calcining the powder without any mechanical grinding or washing, and have completed the present invention. . That is, in the present invention, a boric acid ester is charged into hot gas containing water vapor and decomposed to produce a mixed aerosol decomposition product containing aerosols of boron oxide and elemental carbon, and the resulting dispersoid is solidified. - This is an invention of a novel carbon-containing composition, characterized in that it is obtained by collection through a gas separation operation. (Detailed Disclosure of the Invention) The present invention will be described in detail below. The mixed aerosol as used in the present invention means a dispersoid in which boron oxide and elemental carbon are mixed as fine solid particles in a gas. In the present invention, a boric acid ester is charged into a hot gas containing water vapor and subjected to thermal decomposition, oxidative decomposition or hydrolysis, thereby immediately producing such a mixed aerosol containing boron oxide and elemental carbon aerosols. can be generated. The boric acid ester used in the present invention has the general formula B
(OR) _o (OH) _3-o (n is an integer from 1 to 3, R is
Methyl, ethyl, propyl, isopropyl, butyl, isobutyl, sec-butyl, tert-butyl,
Alkyl groups such as pentyl and hexyl; cycloalkyl groups such as cyclopropyl and cyclopentyl; aryl groups such as phenyl, tolyl, xylyl, mesityl, benzyl, and phenethyl), and specific examples include B. (OCH ₃ ) ₃ ,
B(OCH ₃ ) ₂ (OH), B(OCH ₂ CH ₃ ), B
(OC ₆ H ₅ ) ₃ , B (OC ₆ H ₅ ) ₂ (OH), B (OC ₆ H ₅ )
(OH) ₂ etc. By charging these boric acid esters into hot gas containing water vapor, the boric acid esters are decomposed into boron oxides and organic substances by hydrolysis or thermal decomposition, and the organic substances are further thermally decomposed. Therefore, it changes into elemental carbon. That is, by decomposing B(OR) _o (OH) _3-o ,
For example, B ₂ O ₃ and C are produced. A furnace is used to obtain the carbon-containing composition of the present invention. The heating device is preferably equipped with a combustion burner, an energized heating element, etc., and is also equipped with a borate ester charging nozzle, a hot gas charging duct, and a mixed aerosol discharge duct, and is surrounded by a refractory material. It will be done. In the present invention, a spatial region of at least 700° C. or higher must exist in the furnace as a decomposition reaction zone. If the temperature is above this temperature, boron oxide and elemental carbon are respectively produced as fine particles from the boric acid ester, and a mixed aerosol state which is a mixture of gas and these fine solid particles is generated. In the present invention, boron oxide is used to include boron hydroxide. In the present invention, as a method for obtaining hot gas containing water vapor, water vapor may be injected into hot gas obtained by an electric heating method, a high frequency heating method, a discharge method, etc., but hydrogen, methane, ethane, propane, etc. The method of using air to combust combustible materials that inevitably produce water vapor as a combustion product, such as hydrocarbons such as butane, light oil, kerosene, and heavy oil, is simple in terms of equipment, and is efficient in terms of thermal efficiency. Also desirable. The boric acid ester used in the practice of the present invention has the property of converting into boron oxide through a hydrolysis reaction with water vapor, as well as the property of changing into solid particles of elemental carbon through a thermal decomposition reaction in hot gas. has
Moreover, these reactions are completed extremely quickly in about 0.1 to 0.5 seconds, so the residence time in the furnace is 1 second to 10 seconds.
If the reaction time is about seconds, in an atmosphere where heat and water vapor coexist, the boric acid ester remains in an unreacted gas state and is hardly volatilized outside the reaction system. The mixed aerosol obtained as above is
After guiding the aerosol out of the furnace, the solid dispersoids contained in the aerosol are collected by performing a solid-gas separation operation using a collection device such as a back filter, cyclone, or electrostatic precipitator. In order to reduce the heat load on the equipment, it is desirable to cool it in advance. Although the cooling method is arbitrary, for example, cooling the zone after the reaction or injecting water can be adopted. The carbon-containing composition of the present invention thus collected is
Fine boron carbide powder can be obtained by heating and firing at 1800° C. or higher using a high-frequency heating furnace, current-carrying resistance furnace, arc furnace, or the like. for example,
2B ₂ O ₃ + 7C → B ₄ C + 6CO. In this sintering step, if oxygen is present in the heating atmosphere, elemental carbon will be burned and removed, so it is preferable to heat and sinter in a non-oxidizing atmosphere such as argon, helium, nitrogen, or hydrogen. However, as shown in the above formula, boron oxide and carbon react during the heating process, producing boron carbide and carbon monoxide at the same time, and the firing system naturally becomes a non-oxidizing atmosphere. There is no need to prepare a separate non-oxidizing atmosphere. (Operations and Effects of the Invention) As described in detail above, in the present invention, boric acid ester undergoes a chemical reaction in hot gas containing water vapor.
That is, since fine particles of boron oxide and elemental carbon are produced by hydrolysis, thermal decomposition, etc., and at the same time, mixing is carried out in the gas phase, a carbon-containing composition in which fine particles are mixed extremely uniformly can be obtained. Moreover, unlike the conventional batch method which involves mechanical crushing and mixing, it is possible to obtain a carbon-containing composition consisting of fine powders of boron oxide and elemental carbon continuously and in one step. , the complexity of the working process as in the conventional method is significantly reduced. In addition, all of the conventional problems such as noise, dust generation, and impurity contamination during mixing and charging of raw materials can be solved. Furthermore, since the boron carbide obtained by firing the carbon-containing composition of the present invention is itself an extremely fine powder compared to conventional powders, it is difficult to mechanically grind boron carbide ingots for a long time as in the conventional method. There is no need to grind the powder separately, and therefore, problems such as increase in costs, complexity of the work process, and contamination of impurities during the work process are all solved, which is a remarkable effect. At present, it is not completely clear why fine powdered boron carbide can be obtained so easily by using and sintering the carbon-containing composition of the present invention, but it is probably because the starting material is Because it is a single substance called boric acid ester, the mixed form of boron oxide and elemental carbon in the carbon-containing composition produced by decomposing it is extremely uniform and fine, which is unprecedented. It is presumed that this makes it possible to produce a boron carbide powder with high quality. (Example) The present invention will be specifically described below with reference to Examples. In addition, % represents weight % unless otherwise specified. Example 1 Using the furnace 1 (diameter 300 mm, length 3 m) shown in Fig. 1, air was supplied from the duct 2 and methane as hot air fuel was supplied from the combustion burner 3 at a rate of 80 Nm ³ /
h, charged at a flow rate of 8Nm ³ /h, and 19Kg/h of B(OCH ₂ CH ₃ ) ₃ as boric acid ester from nozzle 4.
It was charged into the furnace at a flow rate of . The temperature inside the furnace was maintained at 1150°C at position A in Figure 1. The aerosol generated in the furnace was extracted from the duct 5, and after cooling was collected by a back filter to obtain 10.7 kg/h (dry weight) of the fine carbon-containing composition of the present invention. As a result of chemical analysis, this carbon-containing composition contained 58.1% elemental carbon and 41.6% B ₂ O ₃ (the remainder was 0.2% bonded hydrogen).
%, others 0.1% or less). Examples 2 to 4 In addition to methane, propane, hydrogen,
A carbon-containing composition having the composition shown in Table 1 was obtained in the same manner as in Example 1 using butane and the boric acid esters shown in Table 1.

【table】

[Table] Reference Example 1 100g of the carbon-containing composition obtained in Example 1 was heated and fired at 1900°C for 1 hour using a high-frequency heating furnace, and once cooled, it was heated to 800°C in air to remove the remaining elemental carbon. Burning off yielded 13.5 g of powder. As a result of analysis using an X-ray diffraction device, the obtained powder was confirmed to be boron carbide with a composition of B ₄ C, and the yield of B ₄ C based on B ₂ O ₃ in the carbon-containing mixture was As a result of observation using an electron microscope image, which was 82%, only particles with a diameter of 1 μm or less were observed.
It was confirmed that it was a very fine powder. Further, the nitrogen adsorption specific surface area was 15.1 m ² /g. Reference Examples 2 to 4 Each of the carbon-containing compositions obtained in Examples 2 to 4 was heated and fired at the temperature and time shown in Table 2 in the same manner as in Reference Example 1.
B ₄ C powder was obtained in the amount shown in the table. As a result of observation using electron microscope images, only particles with a diameter of 1 μm or less were observed in each powder, and it was confirmed that the powders were extremely fine. Further, the nitrogen adsorption specific surface area was the value shown in Table 2. Comparative reference example 1 B ₂ O ₃ with an average particle size of 1 μm and carbon powder (nitrogen adsorption specific surface area 116 m ² /g) were mixed at a ratio of 41.6 to 58.1 so that the composition matched that of the carbon-containing composition obtained in Example 1.
100 g of the mixture obtained by mixing for 24 hours using a ball mill at a weight ratio of

[Table] was removed by combustion to obtain 9.9 g of powder. As a result of analysis of the obtained powder using an X-ray diffraction device, it was observed that it was boron carbide with a composition of B ₄ C, and the yield of B ₄ C based on B ₂ O ₃ was 60%.
It was hot. As a result of observation using an electron microscope image, it was observed that the particles contained particles with a diameter of 1 mm or more, and most of the particles were 100 μm or more, and the nitrogen adsorption specific surface area was 0.4 m ² /g.

[Brief explanation of drawings]

FIG. 1 is a sectional view showing one example of a furnace used in carrying out the present invention. In the drawings, 1...furnace, 2...duct, 3...
... combustion burner, 4 ... nozzle, 5 ... duct.

Claims (1)

[Claims] 1. A boric acid ester is charged into a hot gas containing water vapor and decomposed to produce a mixed aerosol decomposed product containing aerosols of boron oxide and elemental carbon, and the resulting dispersoid is subjected to a solid-gas separation operation. A novel carbon-containing composition characterized by being obtained by collection by.