The effect of large particles on the flow properties of powders

Abstract

This paper is presented to fill a gap in the knowledge of the effect of larger particles on the flow behaviour of finer powders. Very little has appeared in the literature on the matter and the study is justified in that it is common practice to remove coarse particles before testing on a shear tester. The assumption is made in removing larger particles that they do not affect behaviour and that it is the fines which cause the binding and hence the flowability problems. Four different dry fine powders, a fine white sand, an electrostatic precipitator dust, a mixture of zircon and pyrophyllite (ZAP75) and a local steaming coal were investigated. No generalisation can be made about the addition of various volumetric percentages of spheres of varying sizes or of irregular particles. Thus, with fine sand, balls had little effect on the shear strength whereas irregular coal particles produced marked effect increasing consistently with the relative amount added. On the other hand, with the ZAP75, an increase in strength at low normal loads was exhibited but a decrease (over strength of powder alone) was found at higher normal loads. Experiments were conducted on a whole coal with varying limits of upper size (and also with varying limits of lower size) and the shear strength was found to be strongly dependent on those size limits. It is concluded that whereas much more experimental work is necessary before generalisation can be made of the effect of the presence (or absence, depending on the viewpoint) of large particles on the flowability of a powder, such an effect cannot be ignored.