THE EFFECT OF IMPACT ENERGY AND THE SHAPE OF CRYSTALS ON THEIR ATTRITION RATE

In an industrial crystallizer, contact between crystals and crystal-cr ystallizer parts increases the rate of production of potential nuclei. When brittle substances are crystallized at a low relative supersatur ation and a high specific power input, attrition is the dominant nucle ation mechanism. In order to predict nucleation rates caused by attrit ion, the following must be known: the volume of fragments produced, th eir particle size distribution as well as their growth behaviour. This paper considers the volume of attrition fragments produced from britt le crystals and demonstrates its relation to impact energy and particl e shape. Two different types of impact experiments were performed, one by letting the crystals fall through an evacuated tube onto a grass p late, the second by impacting the crystals in a stirred vessel. The im pact experiments demonstrate. that the volume removed from the particl es is almost proportional to the impact energy. However, repeated cont act of the same crystals does not give the same reduction of volume, w hich decreases significantly. The reason for this is the change of the crystals shape. which tend to round off after repeated impact. Crysta l growth, on the other hand, leads to the redevelopment of edges and c orners. From the experimental data presented it can he concluded that growth rates and attrition rates in industrial crystallizers are relat ed. The nucleation rate due to attrition increases with increasing gro wth rate.