Size classification of industrial sub-micron powder by a virtual impactor

Abstract The size classification of sub-micron particles has been investigated both theoretically and experimentally by use of a large-scale virtual impactor. The newly constructed impactor is a two-dimensional type, and clean air is introduced in order to attain the sharp-cut in the classification of industrial powders such as the raw material for ceramics. It was found that the particles were successfully classified in the sub-micron range. Classification efficiencies, based on a centrifugal size analysis, were well predicted by numerical calculations. The classified particles were also examined by electron micrographs, showing that the classification was very sharp. The sharpness index (D p 75 /D p 25 ) attained in the experiment was as small as 1.69, which was adjustable by changing the flow rate ratio between the clean air flow and the total flow. It was confirmed that both the sharpness of separation and the cut size could be controlled by changing the classification parameters such as the flow rate ratios between the clean air and the aerosol flow, the jet width, the jet velocity, or the ratio of the slit gap to the jet width.