Crystallization of microscopic Y2O3 powders by different techniques of fluidization at high temperature
Résumé
Ahigh temperature fluidized bed reactor (HTFBR)working at 900 to 1200 ◦Chas been developed to crystallize microscopic yttria (Y2O3) powders synthesized by spray pyrolysis. Such crystallization is classically performed in crucible or in moving belt furnaces. In order to demonstrate the advantages of the fluidized bed process over the conventional static mode treatments, a comparative study of the main characteristics of particles after heat treatment in a crucible and in the HTFBR has been performed. The high interparticle forces existing in such Geldart group C powders made it necessary to activate their fluidization. Following previous results, two activated fluidization processes were studied: addition of coarse powders to fine particles and vibrated fluidization. The hydrodynamic behavior of these fluidized beds was analyzed through pressure drop measurements. Convenient fluidization conditions were obtained for the two activated fluidization processes, leading to isothermal beds. The size distribution, the crystallinity and the outer morphology of particles before and after thermal treatments were analyzed and compared for the three processes tested. Some pre-sintering phenomena occurred at 1200 ◦C, which were clearly more intense in crucible than in activated fluidization. The crystallinity of the samples treated was equivalent for the three methods of thermal treatment. The interest of fluidization processes to post-treat microscopic particles is thus fully demonstrated.