Comparison between and evaluation of some methods for the assessment of the sphericity of pellets

Abstract

The shape of pellets, prepared by extrusion-spheronisation, has been assessed by the analysis of the two-dimensional images in the form of an elongation ratio (direct microscopic measurement), aspect ratio (image analysis), one-plane critical stability (OPCS) and a shape factor ρ R,(Podczeck and Newton, 1994) plus 3-dimensional characterisation by the Heywood shape coefficient and a permeametry shape factor (Eriksson et al., 1993). Steel ball bearings were used as a standard for each of the methods. The elongation and aspect ratio proved to be poor methods of distinguishing between the variations in shape of pellets within a batch nor particularly sensitive to measure differences between batches. OPCS and the shape factor, ρ R, were more distinguishing in terms of detecting batch differences and inter-batch differences. The two-dimensional image methods ranked the batches in equivalent order of ranking. The Heywood shape coefficient gave the same ranking but the permeametry shape factor gave a significantly different ranking which could be associated with the surface texture of the spheres.