JPS6358767B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to improvements 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. There is already a well-known technology for producing boron carbide by reducing and carbonizing boric acid or boron oxide using reducing metals such as magnesium and calcium and carbon source materials such as graphite, coke, carbon black, or sugars, or mixtures thereof. Are known.
Among these raw reactants, boric acid and sugars are usually preheated at 300-600℃ before or after mixing with other raw reactants and before the reduction carbonization reaction, and are converted to boron oxide and carbon, respectively. Therefore, the basic reaction formula for reduction carbonization can be expressed as follows. 2B ₂ O ₃ +C+6Mg→B ₄ C+6MgO (a) or 2B ₂ O ₃ +C+6Ca→B ₄ C+6CaO (b) In the conventional method, the raw material mixture (preheated) is heated all at once to 1300 to 1900℃ and the above After completing the reaction, by-products such as magnesium oxide or calcium oxide are removed by acid dissolution or other methods to recover boron carbide. However, this method has the following drawbacks. (b) The produced boron carbide contains a considerable amount of free carbon, making it difficult to obtain a composition with a stoichiometric ratio. In order to reduce this free carbon content, attempts have been made to drastically reduce the amount of raw carbon added or to make the raw carbon powder into ultra-fine powder.
In that case, the recovery rate of boron will drop significantly. (b) The ratio of magnesium oxide or calcium oxide to the total volume of the production system is 75~
It reaches 85 vol%, and furthermore, even if the raw material is used as a green compact, it becomes bulky as a collection of porous particles due to mechanical shock during the rapid reduction reaction and evaporation of some reactants and produced substances. Due to the reduced density, the soaking zone volume of the reactor cannot be used as efficiently as possible. (c) At temperatures above 1500°C, at least with respect to MgO, the reaction B ₄ C + 5MgO → 4BO + CO + 5Mg (c) occurs and the produced boron carbide is decomposed, resulting in a decrease in yield. The present inventors have discovered that the reduction carbonization reaction represented by (a) or (b) is actually divided into two stages, and have focused on this fact. In other words, the rapid reaction in the first stage as shown in the following equation occurs at 700 to 1200℃.
It was found that this phenomenon occurs in the temperature range of . B ₂ O ₃ +C+Mg→B ₄ C+B+C +MgB ₂ +MgO+CO↑ (d) or B ₂ O ₃ +C+Ca→B ₄ C+B+C +CaB ₆ +CaO+CO↑ (e) (Since the abundance ratio of products is not constant, the coefficients are omitted and are simply qualitative. (d)
As a result of X-ray diffraction and chemical analysis of the product B ₄ C shown in (e), it was found that magnesium or calcium atoms were present in the crystal lattice. In the high temperature range of 1300 to 2000℃, the magnesium or calcium present in the crystal lattice of boron carbide produced in the first stage reaction is swept away and becomes highly purified, while the remaining boron, magnesium or calcium is removed. The boride and carbon react to form boron carbide according to the following equation. B+C+MgB ₂ →B ₄ C+Mg↑ (f) B+C+CaB ₆ →B ₄ C+Ca↑ (g) (Coefficients are omitted for the reason mentioned above) Furthermore, above 1500℃, the reaction shown in (c) also occurs, of course. It is. The cause of the incorporation of free carbon into the produced boron carbide described in (a) above was estimated as follows. That is, a large amount of magnesium oxide or calcium oxide is produced at the time of reaction (d) or (e), and the mechanical shock caused by the rapid exothermic reaction causes the product to become porous particles. The reaction is a solid phase reaction that requires close contact of the reactants for two reasons:
The progression of (f) or (g) is severely inhibited. Furthermore, since MgB ₂ and CaB ₆ in the reaction system are substances that are relatively easily volatile, they evaporate and scatter at high temperatures, and the relative amount of boron to carbon in the reaction system decreases, resulting in an excess of carbon. It is thought that this excess raw material carbon remains as free carbon in the produced boron carbide. Based on the above findings and considerations, the present inventors removed only magnesium oxide or calcium oxide from the intermediate product (formation system mixture of reaction formulas (d) and (e)) after the completion of the first stage reaction, and further By completing the second stage reaction, high purity boron carbide containing no free carbon can be obtained, and at the same time, the reverse reaction shown in (c) can be avoided and a high recovery rate can be obtained. The present invention was completed by discovering that it can also be done. According to the present invention, in the method of 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 to obtain a boron carbide, a starting material mixture is pre-prepared. Heat-treated and preheated raw material mixture powder 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. Provided is a method for producing high-purity boron carbide powder characterized by the following. In a preferred embodiment of the present invention, the raw material reactants are boric acid, boron oxide, or mixtures thereof, magnesium, calcium, or mixtures thereof, carbon source substances such as graphite, coke, carbon black, or sugars, or their Mix the mixture. The mixing ratio is determined during heating of the reactants;
Considering evaporation and escape during the reaction, the carbon source material should be 85 to 100% of the reaction equivalent shown in formula (a) or (b), and magnesium or calcium should be 85 to 100% of the reaction equivalent.
Adjust the amount to 100-140%. Especially for the former 95-100%, for the latter 105-105%
A range of 110% is desirable. The particle size of the raw material powder is preferably as fine as possible within a range that does not easily scatter during handling or adjusting the atmosphere in the reactor.Although the mixed powder may be used as it is, it is preferable to increase the reactivity and
In order to suppress scattering due to impact during reaction, it is preferable to compress the powder into a green compact. If water vapor is present in the reaction system, the boron carbide produced will be easily oxidized, and at around 600°C, which is just below the starting temperature of the first-stage reduction reaction, moisture will be present in the boron oxide, which is in the form of molten glass. It is desirable to carry out sufficient dehydration as a pretreatment for two reasons: if it remains, it becomes water vapor bubbles and expands due to the heat generated during the reaction, causing scattering of the reactants. Although dehydration may be carried out before mixing the raw material powders, it is more reliable to carry out the dehydration after mixing or after forming a green compact, considering that moisture control during handling up to the main reaction becomes strict. In that case, the dehydration temperature is below the starting temperature of the reduction reaction.
Vacuum dehydration is preferably carried out at a temperature in the range of 300 to 600°C, preferably 450 to 600°C. The conditions for carrying out the first stage reaction are an inert or reducing atmosphere at a temperature of 700 to 1200°C, preferably a temperature of 1000°C or higher. The holding time depends on the processing temperature, but it is 15 minutes or more, and 90 minutes or more is not necessary. The removal of magnesium oxide or calcium oxide from the intermediate reaction products formed in the first stage reaction is sufficient to ensure that their removal is sufficient and that the other products, namely boron carbide, boron, carbon, and magnesium or calcium oxide, are borides (these are collectively called intermediate boron carbides, etc.)
Any method may be used as long as it can be recovered with a sufficient yield. However, an easy method is to dissolve the oxide by leaching with hydrochloric acid or sulfuric acid and recover the intermediate boron carbide by filtration. Since nitric acid also dissolves boron and borides, it is not preferred from the perspective of the present invention. After removal of the oxide, the intermediate boron carbide is subjected to a second stage reaction. At this time as well, it is desirable to use a green compact so that the contact area is increased and the reaction proceeds quickly. The reaction conditions for the second stage are an inert or reducing atmosphere, a treatment temperature of 1300 to 2000°C, and a holding time of 10 minutes or more, depending on the treatment temperature, but not more than 60 minutes. The purity of the produced boron carbide improves as the treatment temperature increases, but when the temperature exceeds 2000°C, sintering between powder particles becomes significant and the product cannot be recovered as a powder. A preferred temperature range is 1700-1900°C. According to the present invention, highly pure boron carbide powder containing no free carbon can be obtained in high yield. Moreover, since the first stage reaction is carried out at a temperature of around 1000°C, it is possible to use an inexpensive electric furnace. Since the compacted powder is processed after calcium has been removed, it is possible to use a heating furnace with a small soaking zone volume, and economical effects are also expected. In particular, the volume reduction effect due to removal and compression of oxides reaches a volume ratio of approximately 10, and the second stage reaction is extremely mild, so buffering is needed to cope with the rapid reaction of the first stage. Since no space is required, in other words, by combining a low-cost electric furnace with a high-temperature furnace of the same capacity, production volume can be increased by more than 10 times compared to conventional methods. . Since the operation of dissolving and removing magnesium oxide or calcium oxide must ultimately be carried out in conventional methods, there is no disadvantage in this regard. Rather, since it is an oxide produced at a low temperature, it has the advantage of being easy to dissolve and remove with acid. Examples of the present invention will be shown below. Example 1 (Reference example) As raw materials, boric acid (particle size 100 to 700 μm), artificial graphite (particle size 35 μm or less), magnesium (particle size 100 to 700 μm),
200 μm), and the molar mixing ratio H ₃ BO ₃ :C:Mg=
500Kg of 90g of the mixture in the ratio of 4:0.9:8
A ^compacted powder (35 x 12 x 30 mm 7
), and this was heated in vacuo at 500°C for 30 minutes to dehydrate it. This mixture was then heated for 30 min at various temperatures in an argon atmosphere, and after cooling, the product was
Leaching for 3 hours using 30vol% hot sulfuric acid 3, filtering,
Washed with water and dried. The results are shown in FIG. The yield is calculated by dividing the weight of the product after the reaction by-product MgO is removed by the sum of the weights of the starting materials boron and carbon. 100 because it contains impurity Mg.
It may exceed %. It has been shown that the first stage reduction reaction starts at around 700°C during heating. Example 2 Using the raw materials described in Example 1, the molar mixing ratio
1030g of H ₃ BO ₃ :C:Mg mixed at a ratio of 4:1:7 was made into a compact (40φ x 30Hmm, 22 pieces) at a pressure of 500Kgw/cm ² and heated at 480°C in vacuum.
Dehydrated by heating for 90 min in an argon atmosphere.
Hold at 1000℃ for 90 minutes, and after cooling, remove the product with dilute hydrochloric acid.
30 for 4 hours, filtered, washed with water, and dried. Further, this was made into a green compact (35 x 12 x 10 mm, 24 pieces) under a pressure of 1000 Kgw/cm ² , divided into 8 equal parts, and treated under the temperature and time conditions shown in Table 1 in an argon atmosphere. The final product was analyzed by X-ray diffraction and the results are also shown in Table 1.

【table】

[Table] Example 3 Using the raw materials described in Example 1, molar mixing ratio
90 g of H ₃ BO ₃ :C:Mg=4:x:7 (x is a variable) mixed in the ratio is made into powder compacts (35 x 12 x 30 mm, 7 pieces) at a pressure of 500 Kgw/cm ² , This was heated in vacuo at 490°C for 90 minutes to dehydrate it. The mixture was then held at 1000° C. for 30 minutes in an argon atmosphere, and after cooling, the product was leached with dilute hydrochloric acid 3 for 3 hours, filtered, washed with water, and dried. Furthermore, this
Green compact (35×12×12mm, ²
) and heat treated at 1650°C for 45 minutes in an argon atmosphere. The results of chemical analysis of boron and carbon in the final product are shown in FIG. 2 as a function of the mixing ratio of raw material artificial graphite. Note that the purity was 98% or higher in all cases. Example 4 Using the raw materials described in Example 1, the molar mixing ratio
90 g of H ₃ BO ₃ :C:Mg=4:1:x (x is a variable) mixed in the ratio is made into powder compacts (35 x 12 x 30 mm, 7 pieces) at a pressure of 500 Kgw/cm ² , This was heated in vacuo at 460°C for 90 minutes to dehydrate it. The mixture was then held at 1020°C for 30 minutes in argon and after cooling the product was leached with dilute hydrochloric acid 3, filtered and
Washed with water and dried. Furthermore, this was made into powder compacts (35×12×12 mm, 2 pieces) under a pressure of 1000 Kgw/cm ² and held at 1650° C. for 45 minutes in an argon atmosphere. The boron yield obtained as a result of analysis of the final product is shown in FIG. 3 as a relationship with the mixing ratio of raw material magnesium. The highest yield was obtained with a magnesium content of approximately 108%. Example 5 Using the raw materials described in Example 1, the molar mixing ratio
1030g of H ₃ BO ₃ :C:Mg mixed at a ratio of 4:1:6.5 was made into a powder compact (22 pieces of 40φ x 30Hmm) at a pressure of 500Kgw/ ^cm2 , and this was heated at 480℃ in vacuum.
Dehydrated by heating for 90 minutes, then in an argon atmosphere.
It was held at 1050°C for 90 minutes. After cooling, the product was leached with dilute hydrochloric acid 20 for 4 hours, filtered, washed with water and dried. Furthermore, this was made into powder compacts (35×12×12 mm, 20 pieces) under a pressure of 1000 Kgw/cm ² and held at 1800° C. for 45 minutes in an argon atmosphere. This results in purity
Boron carbide powder was obtained with a boron yield of 99%, a B/C molar ratio of 4.0, and a boron yield of 84.5%. The average particle size of the powder was 8.5 μm as measured by the sedimentation method. Furthermore, no free carbon was observed by X-ray diffraction. Example 6 Using the raw materials described in Example 1, the molar mixing ratio
90g of H ₃ BO ₃ :C:Mg=4:0.85:7 was mixed into a ^green compact (35
x 12 x 30 mm, 7 pieces) and heated in vacuum at 500°C for 45 minutes to dehydrate. This mixture was then held at 1000°C for 30 minutes in a nitrogen atmosphere, and after cooling, the product was leached using 30 vol% hot sulfuric acid 3 for 3 hours.
The mixture was filtered, washed with water, dried, and then made into green compacts (35 x 12 x 12 mm, 2 pieces) under a pressure of 1000 Kgw/cm ² . next,
This green compact was held at 1650° C. for 30 minutes in an argon atmosphere. As a result, purity 98%, B/C molar ratio
4.2, boron carbide powder was obtained with a boron yield of 82%. No free carbon was observed by X-ray diffraction. Comparative example Using the raw materials described in Example 1, the molar mixing ratio
H ₃ BO ₃ :C:Mg = 4:1:7 mixture 95g was powder ^compacted (35×12
x 30mm, 7 pieces), and heated it in vacuum at 500℃ for 45 minutes.
It was dehydrated by heating for a minute. Next, this mixture was heated to 1650° C. in an argon atmosphere, held for 30 minutes, and after cooling, the product was leached using diluted hydrochloric acid 3 for 3 hours, filtered, washed with water, and dried. The resulting boron carbide powder has a purity of 98.5%, a B/C molar ratio of 3.5,
The boron yield was 79%, and X-ray diffraction clearly showed graphite peaks in the product.

[Brief explanation of the drawing]

FIG. 1 shows the yield of the product of the preliminary reaction of the process of the invention. FIG. 2 shows the relationship between the mixing ratio of artificial graphite to boric acid and the composition of the product. FIG. 3 shows the relationship between the mixing ratio of magnesium to boric acid and the boron yield.

Claims (1)

[Claims] 1. A method for obtaining a 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, in which a starting material mixture is After preheating and reacting the powder of the preheated raw material mixture or its green compact at 700 to 1200℃, the magnesium oxide or calcium oxide produced at that stage is removed; A method for producing high-purity boron carbide powder, which comprises heating the intermediate reaction mixture to 1300 to 2000°C to complete the carbonization reaction. 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 produced magnesium oxide or A method characterized by removing 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%.