When particle diameter falls below a few microns, particles tend to ag
glomerate and hence to degrade the performance of air classifiers. The
dispersion of agglomerated particles is a key technology to improve c
lassification accuracy. The dispersion effect of a ring nozzle jet on
dry powder was examined. Fine alumina and toner were used as the sampl
e powders. The performance can be expressed by a dispersability index,
based on the size distributions of the powder before and after disper
sion. Both powder concentration and air velocity at the nozzle throat
were found to affect the dispersion performance, the dispersing bounda
ry being determinable from these values. Dispersing boundary curves fo
r fine alumina (D50 = 1.0 mum) and toner (D50 = 6.4 mum) were obtained
by experiments. Classification performance, classification sharpness
and division indices, for fine alumina and toner powders were improved
by mounting the disperser immediately in front of the classifier.