Talc from Puebla de Lillo, Spain .2. Effect of dry grinding on particle size and shape

The effect of dry grinding on the structure, particle size and shape of talc from Puebla de Lillo (Leon, Spain), was studied by means of nitro en adsorption, XRD, DTA-TG and by SEM-EDS methods. Grinding of talc produces an increase of the starting BET surface area value progressively up to a maximum of 110 m(2)/g at 30 min. A subsequent decrease of surface area with increasing grinding time was observed. The ground, activated particles with high surface energy start a reaggregation process recovering the original surface area value at 210 min. The values of coherent scattering thickness (crystallite size) indicated a reduction of thickness due to crystalline degradation along [00l], with initial delamination accompanied by a lesser degree of crystal structure breakdown and amorphization of the structure. The latter increases with increasing grinding time. A profound structural alteration, with progressive smearing of 00l basal reflections, have been observed. Structural changes have been correlated with particle's size, shape and coefficient of texture. The tendency of particles to preferred orientate decreases with grinding. A breakdown of the original particles due to mechanochemical effect produced more rounded particles, aggregates and agglomerates, as proved by SEM. With prolonged grinding, the rate of size reduction decreases with time, but reduction continues up to about 30 min. At this stage, particle size was reduced to about 0.02 mu m in diameter and 0.009 mu m in thickness, with aspect ratio 2.6, producing an increased degree of amorphism and a decrease in the surface area by reaggregation of the ground powder. After 30 min, the mechanical reduction of the original particles appears to have a limit. The thermal behavior is strongly dependent on the structural state of the material. Grinding produces a slight shift in the main endothermal DTA peak associated to talc dehydroxylation, as indicated also by the TG and DTG curves. An increase of weight loss on heating at lower temperatures than those observed in unground talc have also been detected. The exothermic DTA effect appears on grinding of talc. It has been detected from 10 min grinding time. This effect increases markedly in intensity during grinding, especially when structural breakdown is very important and the amorphous reactive solid is produced. (C) 1997 Elsevier Science B.V.