Freeze drying, which is an excellent build-up method for preparing fine ceramics powders, is not widely applied to industrial-scale production. One of the reasons is that the drying process takes much time to produce fine powders, and in some cases frozen solutions melt during the operation. In this study, some basic data of drying characteristics of frozen aqueous metal salt solutions in freeze-dry processing were obtained. Drying times were measured in a thermal balance unit. Various physicochemical factors such as qualities of salt, pH of the solutions and formation of cluster ions, and operational conditions such as freezing temperature, thermal energy feeding and operational pressure were related to the drying times. The results qualitatively showed that the drying time depends on the amount of unfreezable water solidified as non-crystalline state in a frozen solution, the water consisting of hydration water, and immovable water trapped in a concentrated amorphous solution.
