In various powder processes, a control of a particle size distribution is very important. The control method can be classified in two categories in general: the buildup and the classification. Among these, a sharp distribution can be obtained by the buildup technique but in most cases, its cost is high. On the other hand, although a sharpness of distribution is not as good as the buildup technique, the classification technique has good cost-performance. The classification method also can be classified in two categories in general: a dry classification and a wet one. In these, the wet classification technology is suitable for micro to sub-micro fine particles because an interaction between particles can be controlled easily in wet condition. There are many technologies in wet classification such as gravity settling, hydro-cyclone, filtration etc....

Then, we had been tried to develop a new filtration system, which improved the conventional cross flow filtration system, using a ceramic tube filter wound around with a spiral flow pass on the outside. The schematic illustration of the filtration unit was shown in Fig.1. In this system, particles in the slurry will move to outside of flow pass by centrifugal force, a filtrate flow out to the inside by filtration pressure. It seems to be that this system can be used for a wet classification because only small particles in the slurry can be obtained with filtrate by controlling centrifugal force and filtration pressure. Thus, in this study, we investigated the effect of the operating conditions on the classification accuracy. Fig.2 shows the results of the classification of alumina slurry. From this result, the cut-off diameter, x_c, could be controlled using this system