JPH09255311A - Production of silicon nitride powder - Google Patents

Abstract

PROBLEM TO BE SOLVED: To efficiently obtain Si3 N4 powder by classifying intermediate powder by the difference in specific gravity correlated with the rate of nitriding and distributing it to processes in accordance with the rate of nitriding at the time of continuous multistage direct nitriding of metallic Si powder. SOLUTION: Metallic Si powder or granulated metallic Si powder is continuously fed into a fluidized bed, a moving bed or a rotary kiln as a nitriding apparatus, it is nitrided with N-contg. gas at about 1,150-1,400 deg.C to form Si3 N4 and this Si3 N4 is continuously discharged. Powder of an intermediate of Si3 N4 obtd. by the 1st stage reaction is classified by difference in specific gravity correlated with the rate of nitriding and it is distributed to processes in accordance with the rate of nitriding. For example, intermediate powder having such a high rate of nitriding as <=5wt.% metallic Si content is pulverized as it is so as to obtain a product, powder having a low rate of nitriding is returned to starting material and the remaining powder is fed to the 2nd nitriding reaction.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for producing silicon nitride powder, and in particular, a method for producing silicon nitride having a high nitriding rate by directly nitriding metal silicon with equipment such as fluidized bed, moving bed or rotary kiln. It is about how to do it.

[0002]

2. Description of the Related Art Conventionally, as a method for producing silicon nitride powder, a direct nitriding method of nitriding metal silicon, a silica reduction method, and a thermal decomposition method of imide have been known, but they are excellent in industrial scale and economical. The direct nitriding method is the most promising as a manufacturing method. The fluidized bed method, moving bed method, rotary kiln method, and tunnel furnace method have been proposed for this direct nitriding method. However, in order to continuously obtain silicon nitride powder by directly nitriding metal silicon powder with a fluidized bed equipment. Is N ₂ alone, N ₂ —H ₂ or N ₂ —N in the temperature range of 1,150 to 1,400 ° C.
Nitriding is performed in an atmosphere such as a mixed gas of H ₃ .

[0003]

However, in general, in order to obtain a high-strength and high-density silicon nitride sintered body,
It is desirable to produce silicon nitride with a high α conversion by using the crystal system of the raw material powder as a high α type, but since the α phase is easily generated at low temperature reaction and the β phase is generated at high temperature reaction, the reaction temperature should be increased to increase the nitriding rate. When the temperature is raised to accelerate the reaction, the temperature further rises due to the abrupt heat generation accompanying the reaction, and it becomes difficult to produce silicon nitride having a high α conversion rate. Further, in this case, due to a sudden temperature rise, unreacted metal silicon powders are fused to each other in the worst case and the specific surface area is reduced, so that the reaction is suppressed and the nitriding rate becomes stable at a low rate. In addition, there is a problem that the product is agglomerated due to the aggregation and adhesion of the particles, and the reaction cannot be continued.

In order to continuously produce silicon nitride in a reaction furnace while keeping the metal silicon in powder or granular form,
Fluidized bed, moving bed, and rotary kiln systems are known, but it is difficult to maintain a uniform residence time for each particle by continuous supply and continuous discharge methods of these, and since there is a residence time distribution, nitriding between particles Distribution of rates occurs. on the other hand,
In the direct nitriding reaction, diffusion of N ₂ into the metal silicon is rate-determining. Further, the nitride film itself generated by the nitriding reaction hinders the diffusion of N ₂ , and the reached nitriding rate does not reach 100% only by keeping it for a predetermined time, and it is saturated at a value depending on the reaction temperature. Therefore, in the method of continuously supplying the raw material and continuously discharging the intermediate, in order to obtain a silicon nitride powder having a high nitriding rate and a high α-converting rate, it is said that a multi-step reaction of repeating the reaction at a low temperature is essential. ing.

[0005]

SUMMARY OF THE INVENTION The present invention relates to a method for producing silicon nitride powder which solves the above disadvantages and problems, which is obtained by directly nitriding metal silicon powder continuously in multiple stages. It is characterized in that the intermediate powder is classified according to the difference in specific gravity having a correlation with the nitriding rate, and is distributed to the next step according to the nitriding rate.

[0006]

BEST MODE FOR CARRYING OUT THE INVENTION The present invention uses a reactor which continuously supplies metal silicon powder or a granulated product thereof and continuously discharges nitrides, and nitrides silicon nitride in a temperature range of 1,150 to 1,400 ° C. Is continuously produced, and the silicon nitride intermediate obtained in the one-step reaction is classified according to the nitriding rate.

In order to obtain silicon nitride having a high nitriding rate and a high α conversion rate in the above-described method for producing silicon nitride by the continuous method, the nitriding rate is gradually increased by a multi-step reaction divided into several times at low temperature. Is good. However, for example, the nitriding rate of the intermediate obtained by performing the nitriding at the first stage at 1,230 ° C. is as shown in FIG. 1 when the average residence time is 6 hours.
62% for 90-100%, 13% for 80-90% by weight,
7-80% for 70-80%, 5% for 60-70% ... 0-10
The% nitriding rate is gradually decreasing to 2% and the average nitriding rate is 80
It is ± 2%, but it is difficult to make the residence time of individual particles completely uniform because it is continuous supply and continuous discharge, and the difference in these residence times and the difference in particle size distribution (specific surface area) Difference), which causes a distribution of the nitriding rate among the particles.

The average nitriding rate is a nitriding rate measured by uniformly mixing an aggregate consisting of 1,000 or more particles, and the average residence time is the retention rate (kg) in the reactor and the discharge rate (kg / h).
It is the value divided by r). Therefore, the continuously discharged intermediate particles are aggregates having different residence times. For example, the average nitriding rate at 1,230 ° C. and the average residence time of 6 hours is stable at 80 ± 2%. The nitriding rate of individual particles forming the body varies depending on the residence time. for that reason,
In the present invention, the silicon nitride intermediates discharged by this method are classified according to the nitriding rate, and these are classified into high nitriding rate products, low nitriding rate products and intermediate products thereof, and are sorted according to the next process at each nitriding rate. The silicon nitride was obtained in good yield.

In the sorting of the silicon nitride intermediate products, the metal silicon becomes silicon nitride and the density increases according to the nitriding rate, and the density of 100% nitriding increases the density by 1.67 times.
Since the difference in density correlates with the difference in specific gravity, classification of the intermediate product can be easily performed by a known method, for example, a method using gravity sedimentation, a method using centrifugal force such as a centrifuge, or a cyclone.

Therefore, when the content of metal silicon after classification is, for example, 5% by weight or less, if it satisfies the specifications of the product, it is sufficient to proceed to the pulverization in the next step. Only one process is required. When a higher nitriding rate is required, it can be advanced to the second stage nitriding reaction. In this case, since a higher nitriding rate can be used by classification, the two-step reaction is performed unclassified. Since the reaction temperature of the second step can be lowered and the residence time can be shortened as compared with the above, the load of the second step reaction can be reduced.

Further, the amount of the nitriding rate of 60 to 90% is 25 to 30% of the whole amount, but since the product specifications cannot be satisfied as they are, it is sufficient to react them in two steps at a time. Those with a rate of 90% or more can be obtained in a considerable amount, and those with a low nitriding rate can be advanced to the reaction in the third step. However, the amount of low nitriding rate of 50% or less is as small as 15% or less of the whole, and it is difficult to increase the nitriding rate even if the two-step reaction is performed. It is preferable to mix with the metal silicon powder and carry out the first-step reaction again, which allows the nitriding step to proceed efficiently. The low nitriding rate product can be supplied at the same time by installing a line for mixing with the initial raw material metal silicon in front of the raw material feeder, or by providing a supply facility unique to the intermediate separately from the raw material metal silicon. Only one of the methods should be selected.

[0012]

Next, examples of the present invention and comparative examples will be described. Example 1 Fluidized bed reaction equipped with a reaction tube having an inner diameter of 80 mmφ, a dispersion plate which is a porous plate on the bottom surface, a supply tube for supplying raw material silicon metal powder into the bed, and a discharge tube for discharging generated nitride by overflow. Charge 700g of granular silicon nitride granulated product into a container,
The temperature was raised to 1,250 ° C and maintained. At this time, a mixed gas of N ₂ -20% H ₂ was flown as a reaction gas so that the filling material in the layer was kept in a sufficiently fluidized state. Then, a granular metal silicon granulated product prepared by granulating and calcining a metal silicon powder having a maximum particle size of 10 μm was supplied at a rate of 80 g / min. The nitrided product was continuously discharged from the discharge pipe at an average rate of 120 g / hr and an average nitriding rate of 80% (average residence time of about 6 hours), and this was combined with a centrifugal classifier and a cyclone as follows. Like A,
Classified into 3 components of B and C. Ingredient A Nitriding rate 38.7% Ratio 13.8% B 74.5% 33.2% C 94.8% 53.0%

Next, among these intermediates, the nitriding rate is 94.8.
%, The component C was roughly pulverized and finely pulverized by a usual method. In this case, the unreacted metal silicon remaining in the pulverization step may be oxidized, and the apparent nitriding rate is improved.
A silicon nitride powder having a nitriding rate of 96.5% could be obtained in one reaction step.

Comparative Example 1 The intermediate obtained in Example 1 was classified according to the nitriding ratio,
When the same crushing process as in the example was carried out to obtain a silicon nitride product as it was, the nitriding ratio was as low as 88.2%, which was not sufficient as a product.

Example 2 The component C obtained in Example 1 was subjected to a second nitriding reaction at 1,300 ° C. using the same apparatus as in Example 1, and silicon nitride having an average nitriding rate of 99.5% was obtained. was gotten.

Comparative Example 2 The intermediate product obtained in Example 1 was subjected to the second nitriding reaction in the same manner as in Example 2 without being divided by the nitriding rate, and the average nitriding of the obtained silicon nitride was performed. The rate was only 90.0%, which was not sufficient as a product. Example 3 Component A obtained in Example 1 is the usual raw material metal silicon 8
On the other hand, when mixed at a ratio of 2 (weight ratio) and subjected to the first stage nitriding reaction at 1,230 ° C. using the same apparatus as in Example 1, the average nitriding rate was 80.3%, which was the usual first stage. Almost the same silicon nitride as the reaction was obtained. Example 4 The component B obtained in Example 1 was subjected to a second nitriding reaction at 1,350 ° C. using the same apparatus as in Example 1 as it was. The average nitriding rate was 97.0%, which was a sufficient nitriding rate. Of silicon nitride was obtained.

[0017]

According to the present invention, the intermediate product of the silicon nitride powder obtained by the one-step nitriding reaction is classified according to the nitriding rate, and the one having the high nitriding rate can be directly used as a product.
Further, the load of the two-step reaction for obtaining a high nitriding rate is lightened, and the material having a low nitriding rate is returned to the raw material, which enables an efficient nitriding process.

[Brief description of drawings]

FIG. 1 is a graph showing the relationship between the nitriding ratio of a silicon nitride intermediate obtained by a nitriding reaction at 1,230 ° C. and an average residence time of 6 hours and its abundance ratio.

Claims (6)

[Claims] 1. When directly nitriding metal silicon powder continuously in multiple stages, the intermediate powder is classified according to the specific gravity difference correlated with the nitriding rate, and distributed to the next step according to the nitriding rate. A method for producing a silicon nitride powder, which is characterized. 2. The method for producing a silicon nitride powder according to claim 1, wherein primary particles or secondary particles obtained by granulating metallic silicon powder are used. 3. The method for producing silicon nitride powder according to claim 1, wherein the nitriding equipment is a fluidized bed, a moving bed, or a rotary kiln capable of continuously supplying, reacting, and discharging powdery material. 4. The production of silicon nitride powder according to claim 1, wherein among the intermediates classified by nitriding rate, those having a high nitriding rate with a metal silicon content of 5% by weight or less are pulverized as they are into a product. Method. 5. The method for producing silicon nitride powder according to claim 1, wherein the intermediate having a low nitriding rate among the intermediates divided by the nitriding rate is returned to the raw material. 6. Among the intermediates classified by the nitriding rate, the following nitriding reaction is performed by using the silicon nitride powder having a high nitriding rate of 5% by weight or less of the metal silicon content, and the metal silicon content is 0.5
% Of silicon nitride powder is obtained.