【発明の詳細な説明】[Detailed description of the invention]
［産業上の利用分野］
本発明は窒化珪素（以下Si3N4という）粉末を
回転窯により製造する方法に関する。
［従来技術］
Si3N4を主成分としたSi3N4系焼結体は高温強
度、耐熱衝撃性及び耐食性等がすぐれているため
例えばガスタービン、エンジン及び熱交換器材料
など耐熱構造材料として多くの関心を寄せられて
いる。
しかしながら、上記Si3N4系焼結体の原料であ
るSi3N4粉末は非酸化性雰囲気下で高温に加熱し
て造られるため、製造装置の気密性や雰囲気制御
等に高度な技術が要求され、また工業的規模で製
造されているとは言い難い。
一般には金属珪素（以下Siという）粉末をN2
ガス又はN2とH2の混合ガス中で加熱しSi3N4粉
末を製造する方法としては、雰囲気制御可能な電
気炉中にSi粉末又は成形体を静置させ、窒化性雰
囲気下で1200℃〜1500℃の温度範囲で加熱し
Si3N4を製造する。しかしながら、Siの窒化は発
熱反応であるため温度が過度に上昇し焼結もしく
は溶融して固まりとなつてしまうため、N2の拡
散が困難となり反応の進行が停止してしまい、高
い反応率でSi3N4が得られない。また、発熱によ
る異常高温のため相の制御が難しくなつたり局部
的なSi3N4の粒成長等が起こり、Si3N4の均一性
が著しく低下するなどという欠点がある。このた
め従来から種々の方法により発熱反応が制御され
ている。例えば窒化されるSi粉末又はその成形体
中に測温用の熱電対を入れ発熱状態に応じて加熱
を制御したり、雰囲気中の窒素分圧をH2、Ar又
はHeガスなどで制御し、異常発熱を制御してい
る。また反応時間を長くし、数10時間かけ徐々に
窒化させるなどの方法がとられている。しかしな
がら、実際にはこれらの方法で原料Si粉末を数mm
から数cmの層状又は板状にし窒化せねばならない
とか、１回の製造操作により数日を要するために
工業的規模の製造方法としては効率的とは言えな
い。また、従来の電気炉中にSi粉末又は成形体を
静置させて窒化させる方法は本質的にはバツチ式
操業であるため、原料と製品の出し入れが厄介で
あり、窒化に必要なエネルギーも大きいものとな
る。
本発明者らは従来法の欠点に鑑み回転窯を用い
てSi3N4の合成を試みた。従来、回転窯を用いて
窒化物を得る方法としては特公昭55−50882号の
方法が提案されている。この特公昭55−50882号
公報記載の方法は黒鉛製の回転窯を用いて金属粉
末表面を一次窒化反応により窒化され、次いでよ
り高温で２次窒化反応により反応率を高める方法
であり、２段窒化による製造方法が該発明の要点
である。該発明で使用されている回転窯の基本構
造は黒鉛製の回転円筒と回転円筒を支持する黒鉛
製円筒をケーシングで覆う構造となつている。こ
のため、炉の水平方向の温度分布を適切に設定し
難いとか、熱の放散性などのため発熱反応である
Siの窒化には適さないと考えられる。
そのため窒化を２段階に分けたり回転窯内を流
れる粉末層の厚みを実質的に５mm以下に制限しな
くてはならない。すなわち、２段窒化により工程
が複雑となり粉末層の厚みの制限により生産量も
限られたものとなる欠点を有する。
本発明者らは一段の窒化により実質的に充分な
窒化率のSi3N4粉末を製造する方法について鋭意
研究を重ねた結果、適切な温度分布を持つ回転窯
を用い充分な熱交換を可能とし、更には原料Si粉
末を造粒することによりα化率の低下を防止し、
安定してSi3N4を製造可能であることを見出し、
この知見に基づいて本発明を完成した。
本発明者らの検討によれば回転窯を用いSi粉粒
体を窒化しSi3N4を製造する方法においては、回
転窯入口側の1000〜1300℃付近の温度域でレンガ
内壁に付着物が生成し、これが連続製造の障害と
なることを見出した。また、該温度域においては
粉粒体の流れが該温度域以外の流れと比較して悪
化し、Si3N4の窒化率及びα化率が低下する現象
を見出した。
例えばα化率が低下する現象としてSiをボール
ミルで粉砕し、BET非表面積で6.0m2／ｇとした
原料Si粉末をN2雰囲気下回転窯に連続的に投入
し加熱した場合、入口側の1000〜1300℃付近の温
度域で原料Si粉末が粘調となり、原料Si粉末層内
の運動が停止し、粉末層が固まりとして内壁レン
ガ上を円周方向にずれる運動をした。また、粉末
が固まりとなることにより熱が内部にこもり、内
部が異常に高温となることが観察された。この場
合、合成されたSi3N4のα化率、窒化率は低下
し、生産されたSi3N4の窒化率及びα化率は大き
くばらついた。
本発明者らの検討では、粉粒体の動きが悪化す
る現象は炉内温度分布、原料Si粉末の種類及び炉
の運転条件に拘わらず、該入口側温度域のみで観
察されることから、Si粉末の粉流体の流動特性に
起因するものではなく、Siを回転窯で窒化する場
合に起こる特異な現象であることを見出した。ま
た、該現象はH2ガス又はNH3ガスを含んだ窒化
性雰囲気でより顕著に観察し得る。
［発明が解決しようとする問題点］
本発明は回転窯を用いて窒素を含む非酸化性雰
囲気中でSiを加熱窒化してSi3N4を製造するに際
し、回転窯入口側の1000〜1300℃付近の温度域で
の原料Si粉粒体の流動性の悪化に起因するSi3N4
の窒化率との化率の低下及び製品の該特性のばら
つきを解決することを目的とする。
［問題点を解決するための手段］
即ち本発明は、窒素を含む非酸化性雰囲気中で
金属珪素を加熱窒化して窒化珪素を製造するに際
し、金属珪素粉末を粒径0.5mm以上の粒状物に造
粒し、これを回転窯中で窒化することを特徴とす
る、回転窯による窒化珪素の製造方法である。そ
のポイントは原料Si粉末を0.5mm以上の造流物と
し、回転窯入口側の1000〜1300℃付近の温度域で
の原料Siの流動性を良くすることにある。
［作用］
本発明方法に於ける窒化性雰囲気とはN2又は
NH3ガス又はこれらを含む非酸化性ガスとの混
合ガス雰囲気であり、ここで言う非酸化性ガスと
は、H2、Ar、He等であり、窒化が発熱反応であ
るため、熱伝導率の高いH2やHeとN2又はNH3
との混合ガスが望ましい。
本発明において使用する原料Si粉末は、比較的
短時間で反応が終了する程度細かいものから成る
ことが好ましく、最大径は20μｍ以下の粉末がよ
い。
造流物の径は0.5〜30mmであり、好ましくは１
〜20mm、さらに好ましくは３〜10mmである。径を
30mmより大きくすると徐々にα化率は低下する。
これは窒化反応による反応熱が、造粒物が大きく
なるに従い内部にこもる程度が大きくなり、α化
率が低下したものと思われる。従つて、上記理由
や取り扱いの容易さ等から、造粒物の径は実質的
には30mm程度以下が好ましい。造粒物の形状とし
ては円柱又は球状がよく、ここでの径としてはこ
れらの長径を意味する。
Si粉末の造粒方法としては、窯の転動で造粒物
が粉砕されない程度の強度が得られるものなら転
動、押し出し、及び圧縮等いずれの造粒方法を用
いてもよい。
［実施例］
以下実施例、実施例及び比較例に基づき本発明
を説明する。
実験例
塊状のSiのボールミルで粉砕し、最大粒径5μ
ｍ、平均粒径2μｍの原料Si粉末を造り、これをパ
ンペレタイザーで水を散布しながら造粒した。次
いで造粒物を各種の径に分級した。造粒物を後記
実施例の操業条件で窒化し、回転窯入口側の1000
〜1300℃付近での流動状態の観察結果とα化率を
下表に示す。
[Industrial Application Field] The present invention relates to a method for producing silicon nitride (hereinafter referred to as Si 3 N 4 ) powder 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, corrosion resistance, etc., so they are used as heat-resistant structural materials such as gas turbine, engine, and heat exchanger materials. There is a lot of interest in this. 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. Generally, metallic silicon (hereinafter referred to as Si) powder is heated with N2
A method for producing Si 3 N 4 powder by heating in a gas or a mixed gas of N 2 and H 2 is to leave the Si powder or compact in an electric furnace where the atmosphere can be controlled, and heat it in a nitriding atmosphere for 1200 min. Heating in the temperature range from ℃ to 1500℃
Manufacture Si 3 N 4 . However, since nitriding of Si is an exothermic reaction, the temperature rises excessively and it sinters or melts into a lump, 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, these methods actually reduce the raw material Si powder to several mm.
It cannot be said to be efficient as an industrial-scale manufacturing method because it must be formed into a layer or plate shape of several centimeters and then nitrided, and one manufacturing operation takes several days. 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 a hassle, and requires a large amount of energy for nitriding. Become something. 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 two-stage 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 manufacturing method by nitriding is the gist of the invention. 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 horizontal temperature distribution of the furnace appropriately, and the reaction is exothermic due to heat dissipation.
It is considered that it is not suitable for nitriding Si. 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 volume is limited due to the limitation on the thickness of the powder layer. The inventors of the present invention have conducted extensive research on a method for producing Si 3 N 4 powder with a substantially sufficient nitridation rate through one-stage nitriding, and have found that sufficient heat exchange can be achieved using a rotary kiln with an appropriate temperature distribution. In addition, by granulating the raw material Si powder, a decrease in the gelatinization rate is prevented,
We discovered that it is possible to stably produce Si 3 N 4 ,
The present invention was completed based on this knowledge. According to the study by the present inventors, in the method of producing Si 3 N 4 by nitriding Si powder using a rotary kiln, deposits on the inner wall of the brick occur in the temperature range of around 1000 to 1300°C on the entrance side of the rotary kiln. was found to be an obstacle to continuous production. Furthermore, we have found that the flow of powder particles in this temperature range is worse than that outside the temperature range, and the nitriding rate and gelatinization rate of Si 3 N 4 are reduced. For example, as a phenomenon in which the gelatinization rate decreases, when raw Si powder is ground in a ball mill and has a BET non-surface area of 6.0 m 2 /g and is continuously put into a rotary kiln in an N 2 atmosphere and heated, the inlet side In the temperature range of around 1000 to 1300°C, the raw Si powder became viscous, the movement within the raw Si powder layer stopped, and the powder layer became a lump and moved circumferentially on the inner wall brick. It was also observed that heat was trapped inside the powder as it became agglomerated, resulting in an abnormally high temperature inside. In this case, the gelatinization rate and nitridation rate of the synthesized Si 3 N 4 decreased, and the nitridation rate and gelatinization rate of the produced Si 3 N 4 varied widely. According to the study conducted by the present inventors, the phenomenon of deterioration of the movement of powder particles is observed only in the inlet side temperature range, regardless of the temperature distribution in the furnace, the type of raw Si powder, and the operating conditions of the furnace. It was discovered that this phenomenon was not caused by the flow characteristics of the Si powder fluid, but was a unique phenomenon that occurred when Si was nitrided in a rotary kiln. Moreover, this phenomenon can be observed more markedly in a nitriding atmosphere containing H 2 gas or NH 3 gas. [Problems to be Solved by the Invention] The present invention uses a rotary kiln to produce Si 3 N 4 by heating and nitriding Si in a non-oxidizing atmosphere containing nitrogen. Si 3 N 4 due to deterioration of fluidity of raw Si powder in the temperature range around ℃
The purpose of the present invention is to solve the problem of a decrease in the nitridation rate and the variation in the characteristics of the product. [Means for Solving the Problems] That is, the present invention provides a method for producing silicon nitride by heating and nitriding metallic silicon in a non-oxidizing atmosphere containing nitrogen, by converting metallic silicon powder into granules with a particle size of 0.5 mm or more. This is a method for producing silicon nitride using a rotary kiln, which is characterized by granulating the silicon nitride into granules and nitriding it in a rotary kiln. The key is to make the raw material Si powder into a granulated material with a thickness of 0.5 mm or more, and to improve the fluidity of the raw material Si in the temperature range of around 1000 to 1300°C on the inlet side of the rotary kiln. [Function] The nitriding atmosphere in the method of the present invention is N2 or
This is a mixed gas atmosphere with NH 3 gas or non-oxidizing gases containing these. Non-oxidizing gases here include H 2 , Ar, He, etc. Since nitriding is an exothermic reaction, the thermal conductivity is low. High H 2 or He and N 2 or NH 3
A mixed gas with The raw material Si powder used in the present invention is preferably fine enough to complete the reaction in a relatively short time, and preferably has a maximum diameter of 20 μm or less. The diameter of the sludge is 0.5 to 30 mm, preferably 1
-20 mm, more preferably 3-10 mm. diameter
When the diameter is made larger than 30 mm, the gelatinization rate gradually decreases.
This is thought to be because the reaction heat due to the nitriding reaction becomes more trapped inside as the granules become larger, and the gelatinization rate decreases. Therefore, for the above reasons and ease of handling, the diameter of the granules is preferably approximately 30 mm or less. The shape of the granules is preferably cylindrical or spherical, and the diameter here means the long axis of these. As a granulation method for the Si powder, any granulation method such as rolling, extrusion, and compression may be used as long as the granules are strong enough not to be crushed by rolling in a kiln. [Examples] The present invention will be described below based on Examples, Examples, and Comparative Examples. Experimental example: Grinding bulk Si in a ball mill to obtain a maximum particle size of 5μ
A raw material Si powder with an average particle size of 2 μm was prepared, and this was granulated using a pan pelletizer while sprinkling water. The granules were then classified into various sizes. The granulated material was nitrided under the operating conditions described in the example below, and
The table below shows the observation results of the fluid state and the gelatinization rate at temperatures around ~1300°C.
【表】【table】
【表】
実施例
実験例で使用したSi粉末をパンペレタイザーに
て水を散布しながら造粒し、径４〜５mmに整粒し
た。これを下表の操業条件で窒化した。この時の
回転窯入口側の1000〜1300℃付近の流動状態は良
好であつた。得られたSi3N4のα化率は80重量％
であつた。又、得られたSi3N4は径４〜５mmの白
色の粒状物でたがいに固結することはなかつた。
回転円筒内径 20cmφ
回転部全長 200cm
焼点温度 1520℃
窒素流量 20／分
投入量 1000ｇ／時間
比較例
実施例での造粒物を破砕し、篩で分級して0.2
〜0.3mmの径に整粒した。これを実施例と同一条
件で窒化したところ回転窯入口側の1000〜1300℃
付近の流動状態は図の如くであつた。この時造粒
物は固まりとして動き、内部が表面に比較し高温
となる現象が観察された。この固まりはより高温
域に進むにつれ粉砕され、高温域では室温での流
動状態と同様に良好となつた。また、製品中には
該造粒物がたがいに固着した大きな粒が混在（最
大径45mm、径10mm以上が製品に対して30重量％）。
この時平均のα化率は67重、径45mmの粒の中心の
α化率は60重量％であつた。実施例と比較しα化
率が低下し、大きな粒が混在することからα化率
にばらつきが生じた。
［発明の効果］
以上のように本発明によれば、原料Si粉末を適
切な方法により造粒し、その径を0.5mm以上とす
ることにより、回転窯入口側の1000〜1300℃付近
での流動性の悪化が防ぎ、結果として製品のα化
率の低下を防止し、α化率と窒化率のばらつきを
減少せしめることが可能となつた。[Table] Example The Si powder used in the experimental example was granulated using a pan pelletizer while sprinkling water, and the particles were sized to a diameter of 4 to 5 mm. This was nitrided under the operating conditions shown in the table below. At this time, the flow condition at around 1000 to 1300°C on the inlet side of the rotary kiln was good. The gelatinization rate of the obtained Si 3 N 4 was 80% by weight
It was hot. Moreover, the obtained Si 3 N 4 was white particles with a diameter of 4 to 5 mm and did not solidify with each other. Rotating cylinder inner diameter 20cmφ Full length of rotating part 200cm Burning point temperature 1520℃ Nitrogen flow rate 20/min Input amount 1000g/hour Comparative example The granulated material in the example was crushed and classified with a sieve to yield 0.2
The particles were sized to a diameter of ~0.3 mm. When this was nitrided under the same conditions as in the example, the temperature at the entrance of the rotary kiln was 1000-1300℃.
The flow conditions in the vicinity were as shown in the figure. At this time, the granules moved as a mass, and a phenomenon was observed in which the interior became hotter than the surface. This mass was crushed as it progressed to a higher temperature range, and the flow state in the high temperature range was as good as that at room temperature. In addition, large particles of the granules stuck to each other are mixed in the product (maximum diameter is 45 mm, and particles with a diameter of 10 mm or more account for 30% by weight of the product).
At this time, the average gelatinization rate was 67 weight%, and the gelatinization rate at the center of the grains with a diameter of 45 mm was 60% by weight. Compared to Examples, the gelatinization rate was lower, and the presence of large grains caused variations in the gelatinization rate. [Effects of the Invention] As described above, according to the present invention, the raw Si powder is granulated by an appropriate method and the diameter is set to 0.5 mm or more, so that it can This prevents deterioration of fluidity, and as a result, prevents a decrease in the gelatinization rate of the product, making it possible to reduce variations in the gelatinization rate and nitriding rate.
【図面の簡単な説明】[Brief explanation of the drawing]
図は比較例における回転窯入口側の1000〜1300
℃付近での流動状態を示す概略断面図である。図
中、
１……回転窯の回転方向、２……造粒物の固ま
り、３……２の造粒物の固まりのレンガ内壁をず
る運動の方向。
The figure shows 1000 to 1300 on the rotary kiln inlet side in a comparative example.
FIG. 2 is a schematic cross-sectional view showing a flow state near °C. In the figure, 1...The direction of rotation of the rotary kiln, 2...The mass of granules, 3...The direction of movement of the mass of 2 granules across the brick inner wall.