Breakage Behavior of NaCl Crystals in a Stirred Vessel

The presence of the small particles resulting from breakage during crystallization may cause unnecessary filter plugging downstream and, often, changes the crystal size distribution. The affect of a particle's shape and size on particle size distribution resulting from breakage can be attributed to two phenomenons – attrition and fragmentation. It is well known that crystals can break due to impact with the crystallizer impeller. Some research has indicated that almost all of this breakage is by attrition rather than fragmentation. However, it is not clear if this is a factor of crystal size or of other operating parameters such as agitation rate.

This breakage was investigated using cubic NaCl crystals, which are usually considered to be more prone to attrition than fragmentation. Studies were performed with the crystals suspended in a saturated solution and a nonsolvent. Due to the highly regular shape of the NaCl crystals, it is more apparent when there is fragmentation rather than attrition. Particle size distributions and weight distributions for each experiment was recorded and quantified. A wide range of initial particle sizes were used. One set was with crystals grown in the lab where the initial size range was ~3500-7500 µm, while other experiments were performed with commercially available crystals in smaller size ranges such as ~350 – 700 µm. In all cases experiments were performed across a range of agitation rates, residence times, and magma densities.

Results are presented which show that there may be significant fragmentation for the larger crystals. This research indicates that for some cases it is necessary to account for fragmentation as well as attrition. This is demonstrated using population balance modeling.