JPS6358768B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an improvement in a method for producing boron carbide by reducing and carbonizing boric acid, boron oxide, or a mixture thereof with magnesium or calcium and carbon.

The present inventors previously developed a method in which boric acid, boron oxide, or a mixture thereof is reduced and carbonized using magnesium, calcium, or a mixture thereof, and carbon or a carbon source substance to obtain a boron carbide. Preheat the powder of the preheated raw material mixture or its green compact.
After reacting at 700-1200℃, remove the magnesium oxide or calcium oxide produced at that stage, and heat the intermediate reaction mixture from which magnesium oxide or calcium oxide has been removed to 1300-2000℃ to complete the carbonization reaction. A method for producing high-purity boron carbide powder is provided (Japanese Patent Application No. 56-034613, JP-A-57-175717).

In the invention, graphite powder, coke powder, carbon black, sugars or mixtures thereof are used as carbon source materials, and in this method up to 84.5
High-purity boron carbide can be obtained with a boron yield of 1.5%.

The boron yield referred to here is: Y: Boron yield (%) B ₁ : Weight of the boron carbide powder produced x boron content of the powder B ₂ : Weight of boric acid or boron oxide used as a raw material x the compound The boron content is defined as follows: Y=B ₁ /B ₂ ×100.

The present inventors tried this method again, but the boron yield remained at 80-85%, and there were some unsatisfactory points such as free carbon being observed in the product in some cases. The following findings were obtained regarding carbon source materials.

(1) Particle size for artificial graphite, petroleum coke powder, pitch coke powder, and carbon black powder
Since particles smaller than 1 μm tend to float, they escape out of the reaction system during the reduction reaction, resulting in a decrease in boron yield and deviation of the product composition from the stoichiometric ratio.

(2) Similarly, if the particle size is 50 μm or more, the intermediate product magnesium boride will volatilize and escape during the carbonization process, before the carbonization reaction is completed.
This results in a decrease in boron yield and residual unreacted carbon.

(3) In saccharide powders such as satucarose, the saccharides themselves are not completely carbonized at the reduction reaction initiation temperature (a little less than 700°C), so hydrogen and oxygen on the order of % remain. These rapidly vaporize due to the rapid temperature rise associated with the reduction reaction, causing a large amount of mechanical scattering of reactants, making it difficult to obtain a high boron yield.

(4) Artificial graphite with a particle size of 1 to 50 μm, petroleum coke,
As a result of comparing pitch coke, the degree of mechanical scattering was found to be higher than that of artificial graphite < pitch coke <
When we collected as much of the scattered matter as possible and compared the boron yields, we found that it was petroleum coke, or artificial graphite was less than pitch coke or petroleum coke.

Generally, the chemical reactivity of amorphous carbon such as coke is greater than that of graphite, and the tendency regarding boron yield can be confirmed with common sense. On the other hand, as a result of comparing various coke powders, it has become clear that the cause of scattering is not due to chemical activity derived from the carbon structure, but rather due to volatile impurities contained in the coke. Summer. That is, volatile impurities are rapidly vaporized by the heat generated by the reduction reaction, promoting mechanical scattering. Furthermore, as a result of observing the behavior at the start of the reduction reaction, when a volatile component is present, the temperature of the reactant increases rapidly due to heat generation, and it is assumed that there is a catalytic action that accelerates the reaction rate. This excessive temperature increase causes poor contact between reactants due to mechanical dispersion and accelerates evaporation of unreacted boron oxide and magnesium.

Based on the knowledge obtained above, we were able to remove volatile impurities from amorphous carbon powder of less than 50 μm under various conditions and obtained good results.

That is, according to the present invention, boric acid, boron oxide,
or a method of reducing and carbonizing a mixture thereof using magnesium, calcium or a mixture thereof and carbon or a carbon source substance to obtain a boron carbide, the method comprising: preheating a starting material mixture;
An intermediate reaction mixture obtained by reacting a preheated raw material mixture powder or its green compact at 700 to 1200°C, then removing the magnesium oxide or calcium oxide produced at that stage, and removing the magnesium oxide or calcium oxide. 1300~2000℃
A method for producing boron carbide powder comprising heating to complete a carbonization reaction: a method characterized in that amorphous carbon with a particle size of 50 μm or less from which volatile impurities have been removed is used as a carbon source material. provided.

In the present invention, amorphous carbon refers to hexagonal (002) carbon in which the distance between the hexagonal ring planes in the graphite crystal is
Any material with a thickness of around 3.34 Å may be used, but specifically, natural or synthetic resins, cellulose, carbides of sugars; petroleum coke or pitch coke can be used. Although black carbon is crystallographically amorphous carbon, it is generally undesirable because its particle size is much smaller than 1 μm. Petroleum coke or pitch coke is preferred from the viewpoint of cost and availability.

In the present invention, a preferred method for removing volatile impurities is heat treatment in an inert atmosphere, a reducing atmosphere, or a vacuum, at a temperature of 700 to 1800°C, and a holding time at the above temperature for 30 minutes or more.

The inert atmosphere is an atmosphere such as _N2 , Ar, etc., and the reducing atmosphere is usually formed by _H2 .

Specifically describing the method of the present invention, first, amorphous carbon is adjusted to a particle size of 50 μm or less.
If it exceeds 50 μm, magnesium boride (intermediate product) will volatilize faster than the carbonization reaction during the carbonization reaction, and unreacted residual coke will be contained in the product as free carbon, and at the same time, the boron yield will decrease. becomes. When the particle size is less than 1 μm, the particles themselves become extremely buoyant, and when magnesium or boron oxide, etc. volatilize as the temperature rises during the reduction reaction, the carbon source substance itself is scattered out of the reaction system by riding on the volatile gas. , resulting in decreased boron yield and fluctuations in product composition.

Therefore, it is desirable that the carbon source material has a particle size of 1 μm or more, but rather than taking the trouble of thoroughly removing particles smaller than 1 μm that are generated during pulverization, particles smaller than 1 μm can be removed as appropriate after a simple removal method. It is practical to use it as it is mixed. In this case, the amount of preparation should be increased in consideration of scattering loss.

Before mixing the raw materials, heat treatment is performed in an inert, reducing, or vacuum atmosphere, preferably at 700 to 1800°C for 30 minutes or more. At this time, since carbon is oxidized in air at temperatures above about 500°C, an oxidizing atmosphere is excluded. From the viewpoint of removing volatile impurities, a reducing atmosphere using H ₂ etc. or in a vacuum is preferable, but from the viewpoint of cost, an inert atmosphere using N ₂ etc. is advantageous.
And a sufficient effect can be obtained. If the temperature is below 700°C, sufficient removal effect of volatile impurities cannot be obtained, and if the temperature is above 1800°C, graphitization of carbon begins and chemical reactivity decreases. 800 considering the cost aspect.
The preferred range is ~1000°C.

Naturally, the minimum heating time will vary depending on conditions such as the content of evaporable impurities, pore conditions on the surface of coke particles, heating rate, heating temperature, etc., but the minimum heating time required will vary depending on conditions such as the content of evaporable impurities, the state of pores on the surface of coke particles, the rate of heating, and the heating temperature. For particles whose particle size has been adjusted by classification: ● In _N2 atmosphere (flow rate 1/min) min ● Temperature increase rate 150 to 300℃/hr ● Holding temperature 800 to 1000℃, holding time 30 minutes That's enough.

In order to obtain a high boron yield, it is sufficient to use amorphous carbon that does not contain volatile impurities and has a particle size of 50 μm or less, so it is necessary to newly manufacture powder with these properties. You can expect the same results even if you do it.

Example 1 Preparation of Γ amorphous carbon: Commercially available granular pitch coke was ground in a stainless steel rotary ball mill, and the particle size was adjusted to 1 to 44 μm by sieve classification using a 325 mesh sieve and air classification. This was placed in a graphite crucible, inserted into an electric furnace, replaced with _N2 gas, and then
Volatile impurities were removed by increasing the temperature at a temperature increase rate of 185°C/hr in a N ₂ gas stream of min., and performing heat treatment at 850°C for 60 minutes.

Synthesis of Γ boron carbide: Boric acid (particle size 50 to 300 μm) as raw materials, amorphous carbon prepared as above, magnesium (particle size
100 to 200 μm), and the molar mixing ratio H ₃ BO ₃ :C:
1040g of Mg mixed at a ratio of 4:1:6.5
The powder ^compact (40φ×40Hmm
This was heated in vacuum at 500°C for 60 minutes to dehydrate it. This mixture was then heated at 1040 °C for 90 min in an argon atmosphere, and after cooling, the product was
Leaching with 10vol% hydrochloric acid at 80℃ for 3 hours,
filtered, washed with water, and dried.

Furthermore, this is applied to a ^compacted powder (35
× 12 × 10 mm) at 1800℃ in an argon atmosphere.
Hold for 60 minutes. The resulting product contained 77.8% by weight of boron and 21.9% by weight of carbon, and X-ray diffraction revealed that no free carbon was observed and it was pure boron carbide. Impurity is Fe: 0.1
The main content was 0.09% by weight, and the average particle size was 5 μm. The weight of the boron carbide powder produced was 126.0 g, corresponding to a boron yield of 90.9%.

Example 2 Commercially available petroleum coke was subjected to the same pretreatment as in Example 1. However, the holding temperature is 1200℃ and the holding time is
It was set as 30 minutes. Using this as a raw material, a boron carbide powder was experimentally produced under exactly the same conditions as in Example 1. The amount of processed raw material compact was 1080 g, and the weight of the obtained boron carbide powder was 128.3 g. According to the analysis results,
It contains 77.6% by weight of boron and 22.0% by weight of carbon, which corresponds to a boron yield of 89.0%. Furthermore, no free carbon was observed by X-ray diffraction.

Comparative Example 1 When a test was conducted under the same conditions as in Example 1 except that only the heat treatment was omitted, the amount of powder produced was
The amount decreased to 119.5 g, with a boron content of 76.7% by weight and a carbon content of 23.2% by weight. This is the boron yield
This corresponds to 85.1%.

Example 3 As a carbon source material, vinyl chloride was carbonized, pulverized, and classified. 500 g of vinyl chloride was placed in a graphite crucible, heated to 1400°C in an argon stream, and held for 3 hours. The temperature increase rate was 20°C/hr. After heating, the mixture was allowed to cool to room temperature, taken out from the crucible, pulverized using a rotary ball mill, and the particle size was adjusted to 1 to 44 μm by sieve classification and air classification. Amorphous carbon powder obtained in this way
120 g was subjected to the same heat treatment as in Example 1.

Thereafter, under the same conditions as in Example 1, everything from mixing with boric acid and magnesium to carbonization was carried out. However, the total weight of the treated raw powder compact was 1210 g.

The resulting product contains 78.1% by weight of boron, carbon
According to the X-ray diffraction results, it was pure boron carbide containing no free carbon. The weight of the product is 144.9g, which is the boron yield.
This corresponds to 90.3%.

Comparative Example 2 Using acetylene black, which is amorphous carbon with a particle size of 0.02 to 0.2 μm, as the carbon source material, Example 1
A sample of boron carbide powder was produced under the same conditions as above. However, the weight of the charged powder is 875g, and the amount of powder produced is
The weight is 78.4g, and the composition is 79.1% by weight of boron and carbon.
It was 20.7% by weight. This corresponds to a boron yield of 68.4%. In addition, after the reduction treatment, a considerable amount of soot was observed adhering to the reduction furnace core tube and exhaust gas system piping.

Comparative Example 3 As a carbon source material, the powder remaining on the sieve during the sieve classification in Example 1 was heat-treated. The heat treatment conditions were a temperature increase rate of 300°C/hr, a holding temperature of 1500°C, a holding time of 30 minutes, and an Ar atmosphere (Ar flow rate of 1/min). The particle size distribution of this powder is
It was 44 to 105 μm. Using this, 1120 g of green compact was treated under the same conditions as in Example 1, and the results were as follows:
133.1g of powder was obtained. However, the composition was 76.0% by weight of boron and 23.6% by weight of carbon, and as a result of X-ray diffraction, a diffraction peak of free carbon (d value of 351 Å) estimated to be on the order of percent was observed.

To enumerate the effects of the present invention, (1) Conventional method, that is, Japanese Patent Application No. 56-34613 (Japanese Patent Application No. 34613)
As a result of research on the properties of carbon source materials, we clarified the conditions that should be met in order to increase the boron yield and obtain boron carbide containing no free carbon in implementing the method of 57-). .

(2) The boron yield has stably improved from the maximum of 84.5% obtained by the above method to 89-91%.

(3) Since the reduction reaction is also more gentle,
This is smaller than the increase in the internal pressure of the reduction furnace, and it is now possible to use a smaller buffer space for flying debris.

Claims (1)

[Claims] 1. A method for obtaining boron carbide by reducing and carbonizing boric acid, boron oxide, or a mixture thereof using magnesium, calcium, or a mixture thereof, and carbon or a carbon source substance, the method comprising: starting material; After preheating the mixture and reacting the preheated raw material mixture powder or green compact at 700 to 1200°C, the magnesium oxide or calcium oxide produced at that stage is removed, and the magnesium oxide or calcium oxide is In the method for producing boron carbide powder, which comprises heating an intermediate reaction mixture from which the A method characterized in that carbon is used. 2. The method according to claim 1, comprising:
A method characterized in that magnesium oxide or calcium oxide is removed by acid dissolution. 3. The method according to any one of claims 1 to 2, wherein the raw material mixed powder or its green compact is held at 1000 to 1200°C for 15 to 90 minutes, and then the generated oxidation A method characterized by removing magnesium or calcium oxide. 4. Any method according to claims 1 to 3, wherein the intermediate reaction mixture is heated at 1700 to 1900°C.
A method characterized by heating for 10 to 60 minutes to complete the carbonization reaction. 5. A method according to any one of claims 1 to 4, in which the amount of carbon source added is 85% of the reaction equivalent.
100%, and the amount of magnesium or calcium added is 100 to 140% of the reaction equivalent. 6. The method according to claim 5, in which the amount of carbon source added is 95 to 100% of the reaction equivalent, and the amount of magnesium or calcium added is 95 to 100% of the reaction equivalent.
105-110%. 7. The method according to any one of claims 1 to 6, which includes heat treatment at 700 to 1800°C for 30 minutes or more in a reducing atmosphere, inert atmosphere, or vacuum. , a method characterized in that volatile impurities are removed. 8. The method according to any one of claims 1 to 7, characterized in that amorphous carbon having a particle size of 1 μm or more is used.