A Spheroid Model for the Role of Shape in Particle Size Analysis

Abstract

Standard procedure in particle size analysis is to express size as an equivalent sphere diameter. As a consequence, results obtained by procedures based on different kinds of response give different results. An alternative approach is presented based on a spheroidal shape which, depending on elongation can represent shapes ranging from rods, through spheres to discs. Particle size and shape are then presented as volume diameter and elongation. Applications to commonly used procedures are presented. For particles of similar shape, i.e., elongation independent of size, the results of procedures such as sieving, microscopy and sedimentation show a simple shift in apparent size with the form of the distribution unaffected. In the case of methods based on radiation scattering, orientation effects lead to an apparent size distribution even for identical particles. Experimental results on glass spheres, crushed quartz particles and fine kaolin are shown to be consistent with the spheroid model. Analysis of the quartz data by a variety of techniques gave consistent values for the volume diameter and elongation. Light scattering measurements on a series of narrow sieve fractions of the quartz particles showed no variation in elongation with size.