COMPARISON BETWEEN AND EVALUATION OF SOME METHODS FOR THE ASSESSMENT OF THE SPHERICITY OF PELLETS

The shape of pellets, prepared by extrusion-spheronisation, has been a ssessed by the analysis of the two-dimensional images in the form of a n elongation ratio (direct microscopic measurement), aspect ratio (ima ge analysis), one-plane critical stability (OPCS) and a shape factor e (R) (Podczeck and Newton, 1994) plus 3-dimensional characterisation by the Heywood shape coefficient and a permeametry shape factor (Eriksso n 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 meth ods of distinguishing between the variations in shape of pellets withi n a batch nor particularly sensitive to measure differences between ba tches. OPCS and the shape factor, e(R), were more distinguishing in te rms of detecting batch differences and inter-batch differences. The tw o-dimensional image methods ranked the batches in equivalent order of ranking. The Heywood shape coefficient gave the same ranking but the p ermeametry shape factor gave a significantly different ranking which c ould be associated with the surface texture of the spheres. (C) 1997 E lsevier Science B.V.