Ultrafine spherical alumina has excellent properties such as corrosion resistance,high temperature resistance,high hardness,high strength,wear resistance,oxidation resistance,good insulation,and large surface area.It is widely used in high-tech fields such as metallurgy,chemical industry,electronics,national defense,aerospace,and nuclear industry.At present,the annual growth rate of demand for ultrafine spherical alumina in domestic and foreign markets is constantly increasing.Therefore,further exploring new methods for preparing ultrafine spherical alumina powder is of great significance.
　　1、Preparation method of spherical alumina
　　The preparation methods of spherical alumina mainly include sol lotion gel method,dropping ball method,homogeneous precipitation method,template method,aerosol decomposition method,spray method,etc.
　　1.Sol lotion gel method
　　The sol lotion gel method is developed on the basis of the sol gel method.The main process is to use alcoholic hydrolysis to prepare spherical alumina powder through the sol gel process.The whole hydrolysis system is relatively complex,in which 50%of the octanol dissolved in alcoholic aluminum,40%of the acetonitrile solvent,9%and 1%of the octanol and butanol dispersed in water,respectively.In addition,hydroxypropyl cellulose is used as the dispersant to obtain spherical alumina powder with very good sphericity.
　　The sol lotion gel method is not conducive to the separation and drying of alumina powder due to the use of organic solvents and surfactants.
　　2.Drop ball method
　　The ball dropping method is to drop the alumina sol into the oil layer(usually paraffin,mineral oil,etc.),form spherical sol particles by the effect of surface tension,then gel the sol particles in the ammonia solution,finally dry the gel particles,and calcine them to form spherical alumina.The spherical alumina prepared by the drop ball method is mainly used as adsorbent or catalyst carrier.
　　Dropping ball method is an improvement on the process of sol lotion gel method,which has the advantage of eliminating the separation of powder and oily reagent.The disadvantage is that the particle size of spherical alumina prepared is relatively large.
　　3.Homogeneous precipitation method
　　Homogeneous precipitation method refers to the precipitation process in homogeneous solutions of A2(SO4)3 or NH4AI(SO4)2,which includes nucleation,aggregation and growth,and precipitation.Under the action of a precipitant,the concentration in a homogeneous solution decreases,and a large number of small crystal nuclei are uniformly generated.The resulting small precipitate particles are uniformly dispersed throughout the entire solution,resulting in the preparation of spherical alumina.
　　It should be noted that spherical alumina powder particles can only be obtained in A2(S04)3 or NH4A(S04)2 solutions,and cannot be obtained in A(NO3)3 or AICI3 solutions,indicating that SO42-plays a crucial role in the formation of spherical particles.
　　Homogeneous precipitation method has the advantage of being able to prepare highly spherical alumina powder with uniform morphology and narrow particle size distribution.The disadvantage is that this method has significant limitations and the mechanism of morphology formation is not yet clear.
　　4.Template method
　　The template method uses spherical raw materials as reagents for controlling the morphology during the process,and the product is usually hollow or core-shell structure.The main process involves using polystyrene microspheres as templates,encapsulating alumina nanoparticles functionalized with carbonate,and then washing them with toluene to prepare hollow alumina spheres.