Breakthroughs in spherical alumina preparation by researchers at home and abroad
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Update time : 2019-08-20 10:38:10
The manufacture of alumina powder with good particle size and morphology has been a hot topic at home and abroad. A variety of morphological alumina powders were prepared by various synthetic methods, including fibrous, sand, plate, rod, and porous membranes. In the rapid development of the global industry, spherical alumina powder has been extensively researched for nearly 10 years. Among them, synthetic spherical alumina is mostly used in catalysts and catalyst carriers, ceramic powders and chemical mechanical polishing abrasives.
The different crystal forms and unique physical and chemical properties of alumina and its hydrates have determined their wide application in petrochemical, electronic refractory, ceramics, abrasives, pharmaceuticals and aerospace, and alumina products in many fields. The application performance has a great relationship with the morphology and size of the raw material powder particles. Among the powder particles of different shapes, the spherical particles have a regular morphology, a small specific surface area, a large bulk density and a good bulk density. The flow properties can greatly improve the application performance of the product.
At present, there are mainly reported methods for preparing ultrafine spherical alumina: ball milling, homogeneous precipitation, sol emulsion-gel method, dropping ball method, template method, aerosol decomposition method and spraying method. The spherical alumina produced by these methods has a particle size ranging from nanometers to millimeters.
Ball milling
The ball milling method is the most common method for preparing ultrafine alumina powder. Usually, the rotation or vibration of the ball mill is used, the material is impacted by abrasive, ball milled and stirred, and the powder of large particle size is refined into ultrafine powder. Lu Baiping studied the influencing factors of high-energy ball milling for the preparation of ultra-fine alumina powder, prolonged ball milling and increased ball milling speed to reduce the particle size; adding grinding aid during ball milling can improve the uniformity of the particle size of the powder. The ultrafine alumina powder is prepared by ball milling, which is simple in operation, low in cost and high in yield, but has limitations, such as uneven distribution of product size, minimum particle size is mechanically restricted, and it is difficult to obtain spherical particles.
Homogeneous precipitation
The precipitation process in a homogeneous solution is the formation of nucleation, then aggregation and growth, and finally the process of precipitation from the solution, usually non-equilibrium, but if the concentration of the precipitant in the homogeneous solution is reduced, even slow If it is formed, a large number of tiny crystal nuclei will be uniformly formed, and the finally formed fine precipitated particles will be uniformly dispersed throughout the solution, and will maintain an equilibrium state for a relatively long period of time. It is a homogeneous precipitation method.
Sol-emulsion-gel method
This method was developed on the basis of the sol-gel method. In the early stage, the sol-gel method was mostly used to prepare alumina sol, and more was to study the structure of the obtained colloid. Gradually, this method became a superfine powder. In order to obtain spherical powder particles, in order to obtain spherical powder particles, a small spherical droplet is produced by utilizing the interfacial tension between the oil phase and the water phase, so that the formation and gelation of the sol particles are limited to minute droplets. Finally, spherical precipitated particles are obtained. Takashi Ogihara et al. used the aluminum alkoxide hydrolysis to prepare spherical alumina powder through the sol-gel process. The whole hydrolysis system is complicated, in which the octanol dissolved in aluminum aluminum accounts for 50%, the acetonitrile solvent accounts for 40%, and the octanol and butyl water dispersed. The alcohols accounted for 9% and 1%, respectively, and hydroxypropylcellulose was used as a dispersing agent to obtain spherical γ-alumina powder having a very good sphericity.
Drip ball method
The dropping method is to drop the alumina sol into the oil layer (usually using paraffin, mineral oil, etc.), and form spherical sol particles by the action of surface tension, and then the sol particles are gelled in the aqueous ammonia solution, and finally the gel particles are obtained. A method of drying and calcining to form spherical alumina. This method is a further improvement of the sol-emulsion-gel method in the process, applying the emulsion technology to the aging stage of the sol, and keeping the oil phase stationary, eliminating the separation of the powder and the oily reagent. However, this method is generally used to prepare spherical alumina having a large particle size, and is mainly applied to an adsorbent or a catalyst carrier.
Template method
The templating method uses a spherical material as a reagent for controlling the morphology in the process, and the product is usually hollow or a core-shell structure. Jin Lu used a carbonaceous microsphere enriched in carboxylate to prepare hollow spherical alumina.
Aerosol decomposition
Aerosol decomposition usually takes aluminum alkoxide as raw material, utilizes the property of easy hydrolysis and high temperature pyrolysis of aluminum alkoxide, and uses the physical means of phase change to vaporize the aluminum alkoxide, and then contact with water vapor to hydrolyze and atomize. After high temperature drying or direct high temperature pyrolysis, the gas-liquid-solid or gas-solid phase transformation is achieved, and finally spherical alumina powder is formed. A complex experimental setup consisting of an atomized portion and a reactive portion is the key to this method.
Jet method
The essence of the preparation of spherical alumina by spraying method is to realize the phase transformation in a short time, and the product is spheroidized by the action of surface tension. According to the characteristics of phase transformation, it can be divided into spray pyrolysis method, spray drying method and spray melting method. M. Vallet-Regi et al. formed a spherical droplet by atomization by AIC1, Al(SO) and AI(NO3)3 solution, and formed a spherical powder after high-temperature pyrolysis. This process requires a thermal decomposition temperature of 900 °C and consumes a large amount of energy.
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