A note on particle size analyses of kaolins and clays

Based on paradigmatic experimental results for one selected type of kaolin the systematic differences in the measurement of particle size distributions of kaolins and clays by the sedimentation method and by the low angle laser light scattering (LALLS) method are studied. A theoretically sound shape transformation procedure for sedimentation results is proposed, based on a physically justified modification of the Stokes law, which takes into account the oblate (plate-like) shape of kaolin and clay particles. It is found that, with realistic estimates of the shape factor (aspect ratio), the corrected sedimentation data come very close to the light scattering data. This indicates at the same time a way to extract shape information from the comparison of two independent size measurements. Scanning electron microscopy of the 2-μm-undersize sedimentation fraction shows kaolin particles with a disc diameter of 3–5 μm and is thus in full agreement with the interpretation of the size measure as an “equivalent disc diameter”.