Abrasion and breakage phenomena in mechanically stirred crystallizers

Attrition experiments lasting several hours were carried out for low concentration crystal suspensions (3 kg m(-3)) at two different stirring rates (960 and 1154 rev min(-1)). The size distributions were measured at fixed time intervals by continuously circulating the suspension through the cell of a laser particle sizer. Monosized sodium chloride crystals (500-560 mu m), after attrition, gave rise to a bimodal size distribution with a higher peak in the parent crystal region and a much lower one below 32 mu m: the amount of fine fragments increased continuously throughout the run. The size distributions of the particles present in the crystallizer at each time were successfully modelled by superimposing the effects of abrasion and breakage fracture mechanisms. At the beginning of the run, abrasion largely prevails, being responsible for more than 98% of the fracture, but its importance rapidly declines with time and at the end of the run (8-12 h) the contributions of abrasion and breakage mechanisms to attrition appear comparable.