Evolution of pyrophyllite particle sizes during dry grinding

The Bertaut-Warren-Averbach (BWA) technique and high-resolution transmissio n electron microscopy (HRTEM) were used to characterize the products of dry -ground pyrophyllite. Mean crystallite thickness and crystallite thickness distributions were measured for each sample using the BWA technique. Mean c rystallite thickness decreases during the treatment with respect to grindin g time and energy applied per unit mass. The BWA data were checked by HRTEM measurements and good fits were obtained for samples having small mean par ticle thicknesses. Samples with thicker particles could not be measured pro perly by HRTEM because the number of particles counted from images is stati stically insufficient. The shape of the crystallite and the particle-size d istribution were used to determine the mechanism of pyrophyllite particle d egradation. Particles initially having a lognormal size distribution are fi rst delaminated randomly, then some are delaminated preferentially, thereby producing polymodal thickness distributions. Finally all particles undergo delamination yielding a lognormal thickness distribution.