JP2005206460A - Easily sinterable alumina particle - Google Patents
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本発明はバイヤー法による水酸化アルミニウムを原料とする易焼結性アルミナ、特に高い焼結密度の焼結体を得ることが可能な易焼結性アルミナ粒子に関するものである。 The present invention relates to easily sinterable alumina using aluminum hydroxide as a raw material by the Bayer method, and particularly to easily sinterable alumina particles capable of obtaining a sintered body having a high sintering density.
従来よりアルミナは、各種セラミックスの原料、研磨剤、耐火物原料として用いられてきた。例えば焼結体用アルミナ に関する品質要求としては、高純度化、一次粒径の微粒化、形状の均一化および低温焼結性の向上などが挙げられる。アルミナ のかかる品質特性が向上すれば、得られる焼結体の嵩密度、機械的強度、硬度および耐摩耗性が改良されるとともに、より低温での焼結が可能となることによる焼結コスト及び焼結用設備の建設費の低減等の利点を有する。 Conventionally, alumina has been used as a raw material for various ceramics, an abrasive, and a refractory material. For example, quality requirements for alumina for sintered bodies include high purity, fine primary particle size, uniform shape, and improved low-temperature sinterability. If such quality characteristics of alumina are improved, the bulk density, mechanical strength, hardness and wear resistance of the resulting sintered body will be improved, and sintering costs and the ability to sinter at lower temperatures will be improved. It has the advantage of reducing the construction cost of the sintering equipment.
従来易焼結性アルミナ粒子の製造方法としては各種の方法が提案されている。例えばアルミナの９０％以上が灼熱減量１５％以下であって、完全にはα化していないアルミナを原料とするアルミナ粉末の製造方法が開示されている（特許文献１参照）。しかしながら、該製造方法においては中間アルミナ由来の高い表面活性により、スラリー化した際にチクソトロピックな流動特性を有するために成形性に問題があり、このアルミナから高い密度の焼結体は得られない。
特殊な製造方法としては、ゾルゲル法によって易焼結性アルミナ粉末を製造する方法が開示されている（特許文献２参照）が、該製造方法においてはコスト増を招く。また、合成される粒子の液相中における体積分率が低いために問題がある。
Conventionally, various methods have been proposed for producing easily sinterable alumina particles. For example, a method for producing alumina powder using alumina which is 90% or more of alumina and whose loss on ignition is 15% or less and which is not completely pregelatinized is disclosed (see Patent Document 1). However, in this production method, due to the high surface activity derived from the intermediate alumina, it has a thixotropic flow characteristic when slurried, so that there is a problem in formability, and a high-density sintered body cannot be obtained from this alumina. .
As a special production method, a method for producing a readily sinterable alumina powder by a sol-gel method is disclosed (see Patent Document 2), but this production method causes an increase in cost. In addition, there is a problem because the volume fraction of the synthesized particles in the liquid phase is low.
その他、アルミナ表面にスピネル層を形成することによって易焼結アルミナ粉末を製造する方法が開示されているが（特許文献３参照）、該製造方法においては、焼結嵩密度を３．９０g/cm３以上とするのに最低で１５６０℃を必要としている。
特殊な雰囲気中で焼成する方法としては、塩化水素ガスを含有する雰囲気で水酸化アルミニウムまたは遷移アルミナを焼成するアルミナ粉末の製造方法が開示されているが（特許文献４参照）、該製造方法においては、塩化水素ガスが各種製造装置を腐食するために問題がある。
同様に特殊な雰囲気中で焼成する方法としては、種晶を含む混合物を塩化水素ガスを含む雰囲気中で焼成するαアルミナ粉末の製造方法が開示されているが（特許文献５参照）、該製造方法においても、塩化水素ガスが各種製造装置を腐食するために問題がある。
In addition, although a method for producing an easily sintered alumina powder by forming a spinel layer on the alumina surface is disclosed (see Patent Document 3), the sintered bulk density is 3.90 g / cm. At least 1560 ° C. is required to achieve 3 or more.
As a method for firing in a special atmosphere, an alumina powder production method for firing aluminum hydroxide or transition alumina in an atmosphere containing hydrogen chloride gas has been disclosed (see Patent Document 4). Is problematic because hydrogen chloride gas corrodes various manufacturing equipment.
Similarly, as a method for firing in a special atmosphere, a method for producing α-alumina powder in which a mixture containing seed crystals is fired in an atmosphere containing hydrogen chloride gas is disclosed (see Patent Document 5). Even in the method, there is a problem because hydrogen chloride gas corrodes various manufacturing apparatuses.
いずれの方法においても工程が複雑化するために製造コストが高くなり、汎用アルミナの製造方法としては問題があった。また上記の方法で特許文献２の方法以外はバイヤー法による水酸化アルミニウムを焼成したものであるが、これらの方法で得られたアルミナからは相対密度が９９％以上のような高密度の焼結体は得られない。
本発明は高い焼結密度で、かつ高純度の焼結体を得ることが可能なアルミナを特殊な製造方法や複雑な工程を用いることなく、バイヤー法を基本にして製造することにある。 An object of the present invention is to manufacture alumina capable of obtaining a sintered body having a high sintering density and high purity on the basis of the Bayer method without using a special manufacturing method or a complicated process.
本発明は上記課題を解決するためになされたもので以下の請求項からなる。
（１）バイヤー法による水酸化アルミニウムを焼成することで得られるアルミナ粒子であって、ＢＥＴ比表面積が５〜９ｍ２／ｇ、粒度分布におけるＤ１０＝０．１〜０．２μｍ、Ｄ５０＝０．３〜０．５μｍ、Ｄ９０＝０．７〜２μｍで、かつ１μｍ以下が８０質量％以上、Ｍｇ含有量１００〜〜３５０ｐｐｍ、Ｎａ含有量４００ｐｐｍ以下、Ｓｉ含有量１００ｐｐｍ以下、Ｃａ含有量１００ｐｐｍ以下である易焼結性アルミナ粒子。
（２）水酸化アルミニウムの一次粒子の平均径が６〜８μｍ、二次粒子の平均径が３５〜４５μｍである上記（１）に記載の易焼結性アルミナ。
（３）水酸化アルミニウムの焼成温度が１１００〜１４００℃である上記（１）または（２）に記載の易焼結性アルミナ粒子。
（４）上記（１）〜（３）のいずれかに記載のアルミナ粒子を焼結した焼結体。
（５）焼結体の相対密度が９９％以上である上記（４）に記載の焼結体。
The present invention has been made to solve the above-mentioned problems, and comprises the following claims.
(1) Alumina particles obtained by firing aluminum hydroxide by the Bayer method, having a BET specific surface area of 5 to 9 m 2 / g, a particle size distribution of D10 = 0.1 to 0.2 μm, D50 = 0. 3 to 0.5 μm, D90 = 0.7 to 2 μm, and 1 μm or less is 80 mass% or more, Mg content is 100 to 350 ppm, Na content is 400 ppm or less, Si content is 100 ppm or less, and Ca content is 100 ppm or less. Some sinterable alumina particles.
(2) The easily sinterable alumina according to the above (1), wherein the average diameter of primary particles of aluminum hydroxide is 6 to 8 μm and the average diameter of secondary particles is 35 to 45 μm.
(3) The easily sinterable alumina particles according to (1) or (2) above, wherein the firing temperature of aluminum hydroxide is 1100 to 1400 ° C.
(4) The sintered compact which sintered the alumina particle in any one of said (1)-(3).
(5) The sintered body according to (4), wherein the relative density of the sintered body is 99% or more.
本発明によれば、特定の粒子径を有する水酸化アルミニウム粒子を原料に用いることにより、極めて焼結性に優れたアルミナを製造することができる。そしてこのアルミナから相対焼結密度が９９％以上のような極めて高密度にして、高純度の焼結体を得ることが可能である。これによって近年のアルミナの高品質の市場要求に一段と応えることが可能である。 According to the present invention, by using aluminum hydroxide particles having a specific particle size as a raw material, it is possible to produce alumina with extremely excellent sinterability. Then, it is possible to obtain a high-purity sintered body by making the relative sintering density from this alumina very high such as 99% or more. This makes it possible to further meet the recent high-quality market demand for alumina.
本発明のアルミナ粒子は基本的にはバイヤー法による水酸化アルミニウムから製造される。通常のバイヤー法はボーキサイトから得られたアルミン酸ソーダに水酸化アルミニウムの種子を加えて水酸化アルミニウムを析出させている。その他アルミン酸ソーダに鉱酸や硫酸アルミニウムのような鉱酸の塩を加え、水酸化アルミニウムの非晶質ゲルを析出させ、このゲルを種子にしてアルミン酸ソーダに添加し、水酸化アルミニウムを析出させる方法も知られている。本発明ではこれらの方法を含めてアルミン酸ソーダから水酸化アルミニウムを得る方法をバイヤー法と呼ぶ。
バイヤー法によれば、ゾルゲル法等の複雑な方法をとることなく、水酸化アルミニウムを得ることができる。
The alumina particles of the present invention are basically produced from aluminum hydroxide by the Bayer method. In the usual buyer method, aluminum hydroxide seeds are added to sodium aluminate obtained from bauxite to precipitate aluminum hydroxide. Other salts of mineral acid such as mineral acid and aluminum sulfate are added to sodium aluminate, and an amorphous aluminum hydroxide gel is precipitated. This gel is added to sodium aluminate as a seed to precipitate aluminum hydroxide. The method of making it known is also known. In the present invention, a method for obtaining aluminum hydroxide from sodium aluminate, including these methods, is referred to as a buyer method.
According to the Bayer method, aluminum hydroxide can be obtained without taking a complicated method such as a sol-gel method.
本発明のアルミナ粒子はＢＥＴ比表面積は５〜９ｍ２／ｇである。比表面積が５ｍ２／ｇ未満では粒子が全体的に大きく、易焼結性になりにくい。また比表面積が９ｍ２／ｇを超えると表面活性が高く、成形する際のスラリーのチクソトロピック性が増し、成形が難しくなり、その結果成形体を焼結した際の焼結密度が上がらない。
アルミナ粒子を易焼結性とするためには全体としての比表面積のみでなく、粒子の粒度分布も重要である。本発明のアルミナ粒子は特定の狭い粒度分布をもつものである。即ち、粒子の累積粒度分布曲線において、Ｄ１０の粒度（該曲線の１０質量％にあたる点の粒度、同様にＤ５０は５０質量％にあたる点の粒度）が０．１〜０．２μｍ、Ｄ５０の粒度が０．３〜０．５μｍ、Ｄ９０が０．７〜２μｍである。そして全体として１μｍ以下が８０質量％以上、好ましくは９５質量％以上である。粒度分布は狭いほど、即ちＤ１０やＤ９０の値はＤ５０に近い値の方が好ましいが、製法上等の制限もあり、上記の範囲としたが、これによって十分易焼結性とすることが可能である。なお、アルミナ粒子の粒径は二次粒子である。
The alumina particles of the present invention have a BET specific surface area of 5 to 9 m 2 / g. When the specific surface area is less than 5 m 2 / g, the particles are generally large and are not easily sintered. On the other hand, when the specific surface area exceeds 9 m 2 / g, the surface activity is high, the thixotropic property of the slurry at the time of molding increases, and the molding becomes difficult. As a result, the sintered density when the molded body is sintered does not increase.
In order to make the alumina particles easily sinterable, not only the specific surface area as a whole but also the particle size distribution of the particles is important. The alumina particles of the present invention have a specific narrow particle size distribution. That is, in the cumulative particle size distribution curve of the particles, the particle size of D10 (the particle size corresponding to 10% by mass of the curve, similarly D50 is the particle size corresponding to 50% by mass) is 0.1 to 0.2 μm, and the particle size of D50 is 0.3 to 0.5 μm and D90 is 0.7 to 2 μm. And as a whole, 1 micrometer or less is 80 mass% or more, Preferably it is 95 mass% or more. As the particle size distribution is narrower, that is, the values of D10 and D90 are preferably closer to D50, but there are also restrictions on the manufacturing method, etc., and the above range is set, but this makes it easy to sinter easily It is. In addition, the particle diameter of an alumina particle is a secondary particle.
さらにアルミナ粒子の焼結性は添加物や不純物にも影響される。特に効果のある添加物は酸化マグネシウムである。少量の酸化マグネシウムは焼成中にアルミナとスピネルを形成し、これがアルミナの焼結性に寄与する。酸化マグネシウムはマグネシウム元素として１００ｐｐｍ未満では効果が少なく、また３５０ｐｐｍを超える量では、スピネル形成以外に遊離の酸化マグネシウムが不純物として残り、かえって焼結性を阻害する。したがって酸化マグネシウムはマグネシウム元素として１００〜３５０ｐｐｍが適する。酸化マグネシウムは水酸化アルミニウムの焼成後、得られたアルミナ粒子に添加し、混合粉砕することが好ましい。酸化マグネシウムは水酸化マグネシウム等で添加し、焼成中に酸化マグネシウムにしてもよい。 Furthermore, the sinterability of alumina particles is also affected by additives and impurities. A particularly effective additive is magnesium oxide. A small amount of magnesium oxide forms alumina and spinel during firing, which contributes to the sinterability of alumina. Magnesium oxide is less effective as a magnesium element if it is less than 100 ppm, and if it exceeds 350 ppm, free magnesium oxide remains as an impurity in addition to spinel formation, which rather inhibits sinterability. Accordingly, 100 to 350 ppm of magnesium oxide is suitable as the magnesium element. Magnesium oxide is preferably added to the alumina particles obtained after firing aluminum hydroxide and mixed and ground. Magnesium oxide may be added with magnesium hydroxide or the like, and may be converted to magnesium oxide during firing.
アルミナ粒子中のその他の不純物はできるだけ少なくする。不純物のＮａ、Ｓｉ、Ｃaはアルミナ中に主として酸化物として存在するが、夫々元素分としてＮａは４００ｐｐｍ以下、Ｓｉは１００ｐｐｍ以下、Ｃａは１００ｐｐｍ以下である。アルミナ焼結体にＮａが多いとそれを使用する装置の腐食等の問題やその他トラブルの原因となる。またＮａ、Ｃａ、Ｓｉの酸化物は水酸化アルミニウムの焼成中に低融点のガラス成分が生成し、これが気孔の発生につながり、焼結体の密度低下の原因となる。またＳｉ、Ｃａが高いとアルミナの異常粒成長の原因となり、これも焼結体の密度低下につながる。そのために上記の範囲に限定する。
不純物のＦｅは酸化物の形で含まれるが、アルミナ焼結体中に存在すると着色し、商品価値を下げるので好ましくはＦｅ元素として１００ｐｐｍ以下とする。
Minimize other impurities in the alumina particles as much as possible. Impurities Na, Si, and Ca are mainly present as oxides in alumina, but as elements, Na is 400 ppm or less, Si is 100 ppm or less, and Ca is 100 ppm or less. If the alumina sintered body contains a large amount of Na, it may cause problems such as corrosion of the apparatus using the alumina sintered body and other troubles. Further, Na, Ca, and Si oxides generate a low melting point glass component during the firing of aluminum hydroxide, which leads to the generation of pores and causes a decrease in the density of the sintered body. Moreover, when Si and Ca are high, it causes abnormal grain growth of alumina, which also leads to a decrease in the density of the sintered body. Therefore, it limits to said range.
The impurity Fe is contained in the form of an oxide, but when it is present in the alumina sintered body, it is colored and the commercial value is lowered, so the Fe element is preferably 100 ppm or less.
次にアルミナ粒子の製造法について説明する。
アルミナ粒子の原料はバイヤー法による水酸化アルミニウムである。バイヤー法により先ずアルミン酸ソーダを得る。このアルミン酸ソーダ水溶液に通常の水酸化アルミニウム種子を加えて水酸化アルミニウムを析出させる方法でも、それから得られたアルミナの粉砕、分級等の粒度等の調整により、本発明のアルミナ粒子を得ることが可能であるが、以後の工程が容易な以下に示す方法が好ましい。
即ち、アルミン酸ソーダ水溶液に塩酸、硫酸等やその他酸性塩、例えば硫酸アルミニウムを加えて中和し、非晶質でゲル状の水酸化アルミニウムを析出させる。そしてこのゲル状の水酸化アルミニウムを種子にして、アルミン酸ソーダに加え、水酸化アルミニウムを析出させる公知の方法を利用する。ゲル状の水酸化アルミニウムは微粒子であり、これを種子とすれば析出する水酸化アルミニウムは一次粒子の平均径が６〜８μｍ、二次粒子の平均径が３５〜４５μｍのものを容易に得ることができる。この二次粒子の粒度分布の標準偏差は１５μｍ以下である。そしてこの水酸化アルミニウムを焼成すれば、アルミナ粒子の一次粒子は成長するが、二次粒子はほぼ同じ３５〜４５μｍのものが得られる。このアルミナ粒子を粉砕、さらに必要により分級すれば上記した本発明のアルミナ粒子の粒度にすることができる。しかも、この方法で得られたアルミナ粒子は粉砕後もチッピング粉や板状粉とならないので、高密度の成形体を得ることができ、したがって焼結体の密度も上げることができる。
Next, a method for producing alumina particles will be described.
The raw material for the alumina particles is aluminum hydroxide by the Bayer method. First, sodium aluminate is obtained by the buyer method. Even in the method of adding ordinary aluminum hydroxide seeds to this sodium aluminate aqueous solution to precipitate aluminum hydroxide, the alumina particles of the present invention can be obtained by adjusting the particle size such as pulverization and classification of the alumina obtained therefrom. Although possible, the following method is preferable because the subsequent steps are easy.
That is, hydrochloric acid, sulfuric acid, and other acidic salts such as aluminum sulfate are added to neutralize sodium aluminate aqueous solution to neutralize it to precipitate amorphous gel aluminum hydroxide. The gelled aluminum hydroxide is used as a seed and added to sodium aluminate, and a known method for precipitating aluminum hydroxide is used. Gelled aluminum hydroxide is a fine particle, and if it is used as a seed, aluminum hydroxide that precipitates can easily be obtained with an average primary particle size of 6-8 μm and an average secondary particle size of 35-45 μm. Can do. The standard deviation of the particle size distribution of the secondary particles is 15 μm or less. When this aluminum hydroxide is fired, primary particles of alumina particles grow, but secondary particles having substantially the same 35 to 45 μm are obtained. If the alumina particles are pulverized and further classified if necessary, the particle size of the alumina particles of the present invention described above can be obtained. Moreover, since the alumina particles obtained by this method do not become a chipping powder or a plate-like powder even after pulverization, a high-density molded body can be obtained, and therefore the density of the sintered body can be increased.
アルミナ中の不純物はアルミナを温水で洗浄することによってもかなり下げることができるが好ましくは水酸化アルミニウム中の不純物を下げることである。
アルミン酸ソーダ溶液を濃縮燃焼し、固形アルミン酸ソーダを得る。この固形アルミン酸ソーダを水および苛性ソーダに溶解し、所定の濃度にすることにより高純度アルミン酸ソーダ水溶液とする。この方法をＬＢＰ法（Liquir Burning Process)と云う。このＬＢＰ法によるアルミン酸ソーダを原料に用いる。
即ち、前記の硫酸アルミニウム等による中和法であれば、アルミン酸ソーダ水溶液に硫酸アルミニウム等を加えて水酸化アルミニウムのゲルを得、このゲルを種晶にして前記アルミン酸ソーダ水溶液に加え、水酸化アルミニウムを析出させる。この析出は長時間かけて穏やかに成長させることが望ましい。これによって種子はアルミン酸ソーダにより成長し、かつアルミン酸ソーダの過飽和による核発生を防止できるため粒子は不純物の少ない単粒状となる。
Impurities in the alumina can be reduced considerably by washing the alumina with warm water, but preferably the impurities in the aluminum hydroxide are reduced.
The sodium aluminate solution is concentrated and burned to obtain solid sodium aluminate. This solid sodium aluminate is dissolved in water and caustic soda to obtain a predetermined concentration to obtain a high-purity sodium aluminate aqueous solution. This method is called LBP method (Liquir Burning Process). Sodium aluminate by this LBP method is used as a raw material.
That is, in the case of the neutralization method using aluminum sulfate or the like, aluminum sulfate or the like is added to a sodium aluminate aqueous solution to obtain an aluminum hydroxide gel, and the gel is seeded and added to the sodium aluminate aqueous solution. Aluminum oxide is deposited. This precipitation is desirably grown gently over a long period of time. As a result, seeds grow with sodium aluminate, and nucleation due to supersaturation of sodium aluminate can be prevented, so that the particles become a single particle with few impurities.
上述したような特性を有する水酸化アルミニウムを1100〜1400℃にて、１〜10時間、好ましくは４〜７時間焼成しα−アルミナとする。焼成方法としては公知の焼成方法、例えば電気炉、ロータリーキルン、トンネル式焼成炉、ローラーハース式焼成炉、流動式焼成炉等が使用可能である。
焼成により生成したα−アルミナのBET比表面積は２〜６ｍ２／ｇ、加圧（９８ＭＰａ）嵩密度は１．５〜１．８ｇ／ｃｍ３である。
生成したα−アルミナはその後粉砕されるが、粉砕方法については、特に条件が指定されるものではないが、ボールミル、振動ミル、ビーズミル、ジェットミル等の公知の装置が用いられる。また、分級工程が含まれていても良く、この場合の分級方法は、気流分級、水ひ分級のいずれでも構わない。また、これら粉砕と分級を同時に行えるジェットミル等も有効な手段の一つとして考えられる。
粉砕工程において、ＭｇＯや水酸化マグネシウム及びエチレングリコール等を０．５質量％程度添加する。ＭｇＯや水酸化マグネシウムの添加量はＭｇ換算で１００〜３５０ｐｐｍとする。
以下に、本発明で得られたアルミナ の特性評価法を説明する。焼結特性評価は、調製された各粉末アルミナ６．５ｇを秤量し金型にいれ成形圧力９８ＭＰａで成形し、４００℃／ｈｒで昇温し、１５８０℃で４時間保持した後、焼結嵩密度を測定して比較した。焼結嵩密度の測定方法は、アルキメデス法を用いた。粒径は、分散剤にヘキサメタリン酸ソーダを用いた水溶液に充分に分散した後、日機装株式会社製マイクロトラック粒度分析計Models7995-30 を用いて測定した。
次に実施例により具体的に説明する。
Aluminum hydroxide having the above-described characteristics is baked at 1100 to 1400 ° C. for 1 to 10 hours, preferably 4 to 7 hours to obtain α-alumina. As the firing method, a known firing method such as an electric furnace, a rotary kiln, a tunnel-type firing furnace, a roller hearth-type firing furnace, a fluid-type firing furnace, or the like can be used.
Α-alumina produced by firing has a BET specific surface area of 2 to 6 m 2 / g and a pressurized (98 MPa) bulk density of 1.5 to 1.8 g / cm 3 .
The produced α-alumina is then pulverized, but the conditions for the pulverization method are not particularly specified, but known devices such as a ball mill, a vibration mill, a bead mill, and a jet mill are used. In addition, a classification step may be included, and the classification method in this case may be either airflow classification or hydration classification. In addition, a jet mill capable of simultaneously performing pulverization and classification is also considered as one of effective means.
In the pulverization step, about 0.5% by mass of MgO, magnesium hydroxide, ethylene glycol or the like is added. The amount of MgO or magnesium hydroxide added is 100 to 350 ppm in terms of Mg.
Below, the characteristic evaluation method of the alumina obtained by this invention is demonstrated. For the evaluation of sintering characteristics, 6.5 g of each powdered alumina prepared was weighed and placed in a mold, molded at a molding pressure of 98 MPa, heated at 400 ° C./hr, held at 1580 ° C. for 4 hours, The density was measured and compared. The Archimedes method was used as a method for measuring the sintered bulk density. The particle size was measured using a Microtrac particle size analyzer Model 7995-30 manufactured by Nikkiso Co., Ltd. after sufficiently dispersing in an aqueous solution using sodium hexametaphosphate as a dispersant.
Next, specific examples will be described.
（実施例１）
ＬＢＰ法により作製したアルミン酸ソーダ水溶液にほぼ反応等量の硫酸アルミニウムを用いて中和し、ゲル状の水酸化アルミニウムを析出し、これを種子として用いた。この種子を別にＬＢＰ法により作製されたアルミン酸ソーダ水溶液にＮａＡｌＯ２に対する量として３質量％添加し、５０℃で４０日穏やかに析出させることにより、二次平均粒子径３７μｍ、一次平均粒子径が７μｍの水酸化アルミニウムを作製した。ＬＢＰ法により作製したアルミン酸ソーダを用い、かつ穏やかに析出させることから不純物は中和等を行わずとも、通常のバイヤー法よりも低い値となり、定量的にはＮａ２６４ppｍ、Ｓｉ２１ｐｐｍ、Ｃａ５０ｐｐｍ、Ｆｅ４３ｐｐｍであった。この水酸化アルミニウムを電気炉にて昇温速度１８０℃／ｈｒ．のもと１３００℃で４時間保持することにより、α−アルミナを作製した。このα−アルミナは平均粒子径４０μｍ、ＢＥＴ比表面積３．０ｍ２／ｇ、９８ＭＰａの加圧条件における嵩密度１．７０ｇ／ｃｍ３ であった。このα−アルミナ６００ｇに対し、アルミナボール４８００ｇを用いエチレングリコール０．５質量％、水酸化マグネシウム３００ppｍ添加し５リットルポットで９０ｒｐｍのもと、３６時間粉砕した。粉砕後の平均粒子径Ｄ５０は０．４８μｍであり、その時のＤ１０は０．１８５μｍ、Ｄ９０は０．８７μｍ、１μｍ下は９４質量％であり、ＢＥＴ比表面積５．７９ｍ２／ｇ、９８ＭＰａの条件における加圧嵩密度２．２８ｇ／ｃｍ３ であった。また不純物レベルはＮａ３５５ｐｐｍ，Ｓｉ６４ｐｐｍ，Ｃａ９２ｐｐｍ，Ｆｅ７２ｐｐｍであった。この粉砕粉を９８ＭＰａで成形し、大気雰囲気で１５８０℃、４時間保持して焼結したところ 焼結嵩密度は３．９６２ｇ／ｃｍ３ であり相対密度に換算すると９９．５％であった。相対密度は理論密度３．９８ｇ／ｃｍ３ に対する嵩密度の比である。
(Example 1)
A sodium aluminate aqueous solution prepared by the LBP method was neutralized with an approximately equivalent amount of aluminum sulfate to precipitate gelled aluminum hydroxide, which was used as a seed. This seed was added to a sodium aluminate aqueous solution prepared separately by the LBP method in an amount of 3% by mass with respect to NaAlO 2 and precipitated gently at 50 ° C. for 40 days, so that the secondary average particle size was 37 μm and the primary average particle size was 7 μm aluminum hydroxide was produced. Since the sodium aluminate produced by the LBP method is used and it is precipitated gently, the impurities are lower than the normal buyer method without neutralization and the like, and quantitatively, Na264ppm, Si21ppm, Ca50ppm, Fe43ppm there were. This aluminum hydroxide was heated in an electric furnace at a heating rate of 180 ° C./hr. Was maintained at 1300 ° C. for 4 hours to prepare α-alumina. This α-alumina had an average particle diameter of 40 μm, a BET specific surface area of 3.0 m 2 / g, and a bulk density of 1.70 g / cm 3 under a pressurized condition of 98 MPa. To 600 g of this α-alumina, 4800 g of alumina balls were added, 0.5% by mass of ethylene glycol and 300 ppm of magnesium hydroxide were added, and the mixture was pulverized for 36 hours at 90 rpm in a 5 liter pot. The average particle diameter D50 after pulverization is 0.48 μm, D10 at that time is 0.185 μm, D90 is 0.87 μm, 1 μm is 94% by mass, BET specific surface area 5.79 m 2 / g, 98 MPa The pressed bulk density at 2.28 g / cm 3 . The impurity levels were Na 355 ppm, Si 64 ppm, Ca 92 ppm, and Fe 72 ppm. The pulverized powder was molded at 98 MPa, sintered at 1580 ° C. for 4 hours in an air atmosphere, and the sintered bulk density was 3.962 g / cm 3 , which was 99.5% when converted to relative density. The relative density is the ratio of the bulk density to the theoretical density of 3.98 g / cm 3 .
（比較例１）
実施例と同様の原料、焼成方法で得た α−アルミナに水酸化マグネシウムを添加せずに、粉砕した。水酸化マグネシウム添加以外の粉砕方法は、実施例と同様である。この粉砕粉を成形し焼結したところ、焼結嵩密度は３．７３６ｇ／ｃｍ３であり相対密度に換算すると９３．９％であった。
(Comparative Example 1)
The α-alumina obtained by the same raw material and firing method as in Examples was pulverized without adding magnesium hydroxide. The pulverization method other than the addition of magnesium hydroxide is the same as in the examples. When this pulverized powder was molded and sintered, the sintered bulk density was 3.736 g / cm 3 , which was 93.9% in terms of relative density.
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