JPS6230126B2 - - Google Patents

Description

[Detailed description of the invention]

TECHNICAL FIELD OF THE INVENTION The present invention relates to a method for continuously producing silicon nitride (hereinafter referred to as Si ₃ N ₄ ) powder or granules using a rotary kiln. Prior art Si ₃ N ₄ -based sintered bodies containing Si ₃ N ₄ as the main component have excellent high-temperature strength, thermal shock resistance, and corrosion resistance, so they are often used as heat-resistant structural materials, such as in gas turbines, engines, and heat exchanger materials. There is a lot of interest. However, since the Si ₃ N ₄ powder, which is the raw material for the Si ₃ N ₄ sintered body mentioned above, is produced by heating it to high temperatures in a non-oxidizing atmosphere, advanced technology is required to ensure the airtightness of the manufacturing equipment and control the atmosphere. It is difficult to say that it is required and manufactured on an industrial scale yet. In general, a method for producing Si ₃ N ₄ powder by heating metallic silicon (hereinafter referred to as Si) powder in N ₂ gas or a mixed gas of N ₂ and H ₂ is to heat Si powder or compacts in an electric furnace where the atmosphere can be controlled. is allowed to stand still and heated in a temperature range of 1200°C to 1500°C in a nitriding atmosphere to produce Si ₃ N ₄ . However, since nitriding of Si is an exothermic reaction, the temperature rises excessively and it sinters or melts into a solid mass, making it difficult for N ₂ to diffuse and stopping the reaction, resulting in a high reaction rate.
Si ₃ N ₄ cannot be obtained. Furthermore, due to the abnormally high temperature caused by heat generation, phase control becomes difficult, local grain growth of Si ₃ N ₄ occurs, and the uniformity of Si ₃ N ₄ is significantly reduced. For this reason, exothermic reactions have conventionally been controlled by various methods. For example, by inserting a thermocouple for temperature measurement into the Si powder to be nitrided or its compact, and controlling the heating according to the heat generation state, or by controlling the nitrogen partial pressure in the atmosphere with H ₂ , Ar or He gas, etc. Controls abnormal fever. Other methods include increasing the reaction time and gradually nitriding over several tens of hours. However, in reality, these methods require nitriding the raw Si powder into a layer or plate shape of several mm to several cm, and each manufacturing operation takes several days, making it inefficient as an industrial-scale manufacturing method. I can't say it's on target. In addition, the conventional method of nitriding by leaving Si powder or compacts in an electric furnace is essentially a batch operation, which makes loading and unloading raw materials and products difficult, and requires a large amount of energy for nitriding. becomes. In view of the drawbacks of the conventional method, the present inventors attempted to synthesize Si ₃ N ₄ using a rotary kiln. Conventionally, as a method for obtaining nitride using a rotary kiln, a method disclosed in Japanese Patent Publication No. 50882/1982 has been proposed. The method described in Japanese Patent Publication No. 55-50882 is a method in which the surface of the metal powder is nitrided by a primary nitriding reaction using a graphite rotary kiln, and then the reaction rate is increased by a secondary nitriding reaction at a higher temperature. The key point of the invention is the manufacturing method according to the method. The basic structure of the rotary kiln used in the invention is a rotating cylinder made of graphite and a casing covering the graphite cylinder supporting the rotating cylinder. For this reason, it is difficult to set the temperature distribution in the horizontal direction of the furnace appropriately, and it is considered unsuitable for nitriding Si, which is an exothermic reaction, due to its heat dissipation properties. Therefore, it is necessary to divide the nitriding into two stages or to limit the thickness of the powder layer flowing in the rotary kiln to 5 mm or less. That is, the two-stage nitriding has the disadvantage that the process becomes complicated and the production amount is limited due to the limitation of the thickness of the powder layer. Summary of the Invention As a result of extensive research into a method for continuously producing Si ₃ N ₄ powder with a substantially sufficient nitriding rate through one-stage nitriding, the present inventors have developed a method using a rotary kiln with an appropriate temperature distribution. It has been discovered that Si ₃ N ₄ powder with a sufficient reaction rate can be continuously produced by enabling sufficient heat exchange and further controlling the amount of oxygen in the raw Si powder. The present invention was completed based on this knowledge. According to the study conducted by the present inventors, Si powder was ground with a ball mill to have a BET specific surface area of 3.5 m ² /g, and when it was continuously put into a rotary kiln in an NH ₃ atmosphere and heated, it rotated over time. A layer of deposits formed on the inner wall of the furnace. In other words, the inner diameter of the kiln wall is 30 cm.
After only about 5 hours, the inner diameter of the φ one became 10 cm φ, making continuous operation impossible. The present inventors investigated the cause of deposit formation and found that oxygen in the raw Si powder was the cause. This deposit mainly consists of whiskers that are produced around 1000°C to 1300°C and aggregates that are mainly created below around 1000°C. Whiskers that form around 1000°C to 1300°C are more of a hindrance to continuous operation. As a result of investigation, these whiskers were found to be a mixture of silicon nitride and a small amount of silicon oxynitride whiskers. Incidentally, it was found that the amount of oxygen contained in the raw material Si powder was about 2 to 4%. This is because oxygen on the surface of the raw Si powder is heated.
It is thought that it was generated as SiO vapor and reacted with N ₂ gas to become silicon nitrogen or silicon oxynitride.
Furthermore, the aggregates below around 1000°C are thought to be the result of SiO vapor and impurities in Si condensing at low temperatures.
This aggregate also causes gas piping to become clogged. However, the raw material Si used in the present invention must have a higher reactivity than the Si used in static nitriding, and as a result, the specific surface area is large, resulting in a large amount of oxygen. The method of the present invention involves heating Si powder to 1200℃ to 1600℃ in a nitriding atmosphere using a rotary kiln to achieve a sufficient reaction rate.
This continuous Si ₃ N ₄ production method is characterized in that the amount of oxygen in the raw material is 1.5% by weight or less for granules and less than _1.0 % by weight for _powder . The nitriding atmosphere in the present invention is N ₂ or NH ₃ gas, or a mixed gas containing these with a non-oxidizing gas. The non-oxidizing gases mentioned here are H ₂ , Ar, He
Since nitriding is an exothermic reaction, it has high thermal conductivity.
A mixed gas with _H2 , He, etc. is desirable. [A] Methods for controlling the amount of oxygen in raw Si are (1) pulverizing without increasing the surface oxygen of Si; (2) removing oxygen from Si powder; (3) pulverizing There are methods to remove fine Si powder by classification; (4) methods to adsorb substances that prevent Si surface oxidation. In addition, as a method for sufficiently advancing the [B] reaction while passing through a rotary kiln, there is a method of adding one or more types of iron, iron compounds, calcium compounds, etc. to Si as a nitriding promoting catalyst. ], [B]
This method may be used in combination. (1) Methods of pulverizing metal Si without increasing surface oxygen are: (i) pulverizing in a non-oxidizing atmosphere (ii) pulverizing by adding a substance that adsorbs to the Si surface and prevents oxidation of the Si surface There is a way. The non-oxidizing atmosphere here refers to hydrocarbon gas, _H2 gas, Ar
gas, CO gas, etc. The hydrocarbon gas is gaseous at room temperature, such as methane, ethane, propane, and butane gas, and one or more of these are used. The crusher is a ball mill,
Either a vibration mill or a jet mill may be used, and it is preferable to use a type such as a dry attrition mill that allows the pulverized Si powder to be charged into the hopper without coming into contact with air.
After pulverizing, either put it into a hopper without exposing it to air, or inject a substance into the mill that adsorbs to the Si surface and prevents oxidation of the Si surface, re-pulverize it for a few minutes, and then take it out from the mill after it cools down. Also, if you are exposed to air, try to shorten the time as much as possible. (ii) Substances that adsorb to the Si surface and prevent oxidation of the Si surface include alcohols, alkylchlorosilanes, various monomers such as vinyl chloride monomer, vinyl acetate monomer, silicon tetrafluoride, methyl fluoride, fluorobenzene, silicon chloride,
It is one or more substances consisting of various halides such as methyl chloride and carbon tetrachloride. (2) 10 to 40 as a method to remove oxygen from Si powder
There is a method of treatment with hydrofluoric acid of % by weight, and in this case, the higher the concentration of hydrofluoric acid, the better, and the higher the treatment temperature. In this method, it is possible to carry out filtration drying in air after treatment. In addition, Si powder is heated at 1000℃ or higher at 1400℃ in a non-oxidizing atmosphere such as argon (Ar), helium (He), or hydrogen (H ₂ ).
There are methods such as heating to below ℃. When we attempted to nitride various Si powders using a rotary kiln with an inner diameter of 30 cm and a length of 400 cm that could control the atmosphere, we found that powders with a large specific surface area generally had deposits on the inner wall of the kiln, making continuous production impossible. As shown in Experimental Example 1, nitriding experiments were conducted on Si powders obtained by various grinding and processing methods, and the amount of oxygen that could be continuously produced was 1.5% by weight or less for granules and less than 1.0% by weight for powders. In addition,
Oxygen content was measured by activation analysis. In addition, since oxygen in the pulverized Si powder is mainly present in fine particles, the powder of sample No. 4 of Example 1 was used to classify and remove fine particles in the raw Si powder (with an average particle size (6μ) of the product to be classified). (2μ or less was cut), and the amount of oxygen in the Si powder was set to be less than the weight % listed in the table of Experimental Example 1. This made continuous operation possible. However, in general, Si powder with a small specific surface area has a slow nitriding rate, and Si ₃ N ₄ powder with a sufficient nitriding rate cannot be obtained. Therefore, we investigated catalysts for promoting nitriding and found that a satisfactory nitriding rate was achieved by adding one or more types of iron, iron compounds, calcium compounds, Si ₃ N ₄ powder, etc. to the silicon raw material. As mentioned above, the Si powder in which the amount of oxygen in the Si powder has been reduced to below the specified weight % is added with a calcium compound, iron or iron compound, or Si ₃ N ₄ powder as necessary, and then placed in an airtight popper and exposed to air. Continuously feed to the rotary kiln. At this time, the supplied Si powder may be in a powder form or in a well-granulated form. If granulated, the amount of oxygen can be up to 1.5% by weight. The raw material Si is rotated while moving in a rotary kiln at 1100° C. to 1400° C. in which the horizontal gradient of the kiln is made sufficiently small, and is nitrided with nitrogen and/or ammonia. In addition, the heat generated during the nitriding reaction is not accumulated but is sufficiently dissipated, so that Si ₃ N ₄ powder with a sufficiently high nitrogen content can be obtained without melting. Effects of the Invention According to the present invention, by preparing raw material Si powder using an appropriate method and controlling its oxygen content, it is possible to produce high-purity silicon nitride with high efficiency without causing sticking, which is an obstacle to continuous production. It is.
This has made it possible to produce highly pure silicon nitride at a lower cost than inefficient production methods such as batch production and two-stage firing using a rotary kiln. This has made it possible to produce large quantities of silicon nitride at low cost, which is suitable as a raw material for heat-resistant structural materials such as ceramic engines, which have recently been attracting attention. The present invention will be explained below with reference to Examples and Comparative Examples. Experimental Example 1 Various silicones shown in the following table were nitrided under the operating conditions of Example 1 (Samples Nos. 1 to 9 were powders, and Samples Nos. 10 and 11 were granules obtained by normal extrusion granulation). The results for problems caused by attachment are shown in the same table.

[Table] Example 1 -10% silicon (impurity Fe 1000ppm,
1100ppm of Ca, 1600ppm of Al, and 500ppm of others) were ground in a dry ball mill under an _N2 gas atmosphere. At this time, the specific surface area of the Si powder is 3 m ² /g and the amount of oxygen is 0.59.
It was in weight%. The conditions for the nitriding furnace in which this Si powder was charged into a hopper without exposure to air and nitrided using a rotary nitriding furnace were as follows. Rotating cylinder inner diameter 30cmφ Total length of rotating part 400cm Burning point temperature 1500℃ Hydrogen flow rate 40/min Nitrogen flow rate 400/min The supply amount of Si powder is 20Kg/hour, the residence time of Si powder is 3 hours, and 85mm thick Si powder is After continuous operation for several hours, no deposits were deposited on the inner wall of the cylinder, and the Si powder flowed smoothly, yielding silicon nitride. At this time, silicon nitride is obtained as grains of 1 to 5 mm, and the nitriding rate is
1500 kg of Si ₃ N ₄ was obtained with a gelatinization rate of 98% and a gelatinization rate of 80%. Example 2 Si ₃ N ₄ was added to 10 meshes of Si in an amount of 10% by weight based on Si, and the mixture was ground in a normal horizontal ball mill, and after the grinding, it was left in the air. The specific surface area of this Si powder is 2
The oxygen content was 3.0% by weight in m ² /g. This powder
When 50 kg was treated with 20% hydrofluoric acid 60 at 80°C and over-dried in an oven in air, the oxygen content was 0.6% by weight. Inject this Si powder into the hopper and adjust the supply amount.
Si powder with a thickness of 20 mm was operated continuously for 4 days at a rate of 0.5 kg/hour and a residence time of Si powder of 3 hours to obtain Si ₃ N ₄ powder. Conditions regarding the nitriding kiln were as follows. Inner diameter of rotating cylinder: 10cmφ Total length of rotating part: 150cm Burning point temperature: 1400℃ Dry ammonia gas, flow rate: 15/min The obtained Si ₃ N ₄ is a white granule with a size of 1 to 5 mm, with a nitriding rate of 97% and a gelatinization rate of 90 It was %. At this time, no deposits were attached to the inner wall of the cylinder. Example 3 Calcium hydroxide (hardening agent) was added to silicon of 15-15 mesh in an amount of 0.15% based on the silicon, and the mixture was ground in air using a dry ball mill. At this time, the specific surface area of the Si powder was 0.6 m ² /g, and the amount of oxygen was 0.95% by weight.
This raw material was nitrided in an N ₂ gas atmosphere using the same rotary kiln as in Example 2. At this time, the burning point temperature is 1500
°C, and the _N2 gas flow rate was 15/min. After continuous operation for 24 hours and checking for deposits on the inner wall of the cylinder, the temperature was 1250℃.
A deposit approximately 5 cm long and 1 cm thick was observed in the vicinity. The obtained Si ₃ N ₄ powder was a brown granule. This color was caused by flocculent aggregates adhering to the low-temperature area, and could be easily removed by washing with water. At this time, the nitriding rate and α-izing rate are respectively
It was 96% and 85%. Comparative Example 1 The raw material obtained in Example 3 was further pulverized to give a specific surface area of 3.5 m ² /g and an oxygen content of 3.6% by weight, and when nitriding was attempted under the same conditions as Example 3, a layered material appeared at around 1250°C. After about 5 hours, the inner diameter became about 10cmφ and the Si powder stopped flowing. Example 4 -10 meshes of silicon were ground in air using a dry ball mill. At this time, the specific surface area of Si powder is 2
m ² /g, and the oxygen content was 3.0% by weight. This powder was separated into coarse particles and fine particles using an air classifier, and the oxygen content was reduced to 0.96% by weight. When this was nitrided under the same conditions as in Example 1, continuous operation was possible. When the inside of the kiln was examined after 48 hours, almost no deposits were observed around 1250°C. The nitriding rate and gelatinization rate at this time were 96% and 74%, respectively. Comparative Example 3 Specific surface area not classified in Example 4: 2 m ² /g,
When Si powder with an oxygen content of 3.0% by weight was nitrided under the same conditions as in Example 4, the inner diameter of the kiln was approximately 15% after approximately 10 hours.
The powder stopped flowing when it became about cmφ. Example 5 10% by weight of Si ₃ N ₄ was added to a -10 mesh of silicon, and the mixture was ground in a dry ball mill under an N ₂ gas atmosphere for 4 hours. The specific surface area of the Si powder at this time is 5 m ² /g
The oxygen content was 0.8% by weight. This Si powder was charged into a hopper without being exposed to air, and nitrided using the same rotary kiln as in Example 1. The burning point temperature at this time was 1400°C, the argon gas flow rate was 40/min, and the nitrogen flow rate was 100/min. Increase the supply amount of Si powder to 5
Kg/hour, and after 48 hours of continuous operation, no deposits were deposited on the inner wall of the cylinder, and the Si powder flowed smoothly. Silicon nitride with a nitriding rate of 99.5% and a gelatinizing rate of 98% is 400
Kg obtained.

Claims (1)

[Claims] 1. When producing silicon nitride by heating and nitriding metallic silicon in a non-oxidizing atmosphere containing nitrogen, the oxygen content of the raw material silicon is less than 1.0% by weight in the case of powder and 1.5% by weight or less in the case of granules. A continuous silicon nitride manufacturing method using a rotary kiln.