ABRASION AND BREAKAGE PHENOMENA IN MECHANICALLY STIRRED CRYSTALLIZERS

Attrition experiments lasting several hours were carried out for low c oncentration crystal suspensions (3 kg m(-3)) at two different stirrin g rates (960 and 1154 rev min(-1)). The size distributions were measur ed 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 muc h lower one below 32 mu m: the amount of fine fragments increased cont inuously throughout the run. The size distributions of the particles p resent 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 respon sible for more than 98% of the fracture, but its importance rapidly de clines with time and at the end of the run (8-12 h) the contributions of abrasion and breakage mechanisms to attrition appear comparable.