The effect of impact energy and the shape of crystals on their attrition rate

Abstract

In an industrial crystallizer, contact between crystals and crystal-crystallizer parts increases the rate of production of potential nuclei. When brittle substances are crystallized at a low relative supersaturation and a high specific power input, attrition is the dominant nucleation mechanism. In order to predict nucleation rates caused by attrition, the following must be known: the volume of fragments produced, their particle size distribution as well as their growth behaviour. This paper considers the volume of attrition fragments produced from brittle crystals and demonstrates its relation to impact energy and particle shape. Two different types of impact experiments were performed, one by letting the crystals fall through an evacuated tube onto a glass plate, the second by impacting the crystals in a stirred vessel. The impact experiments demonstrate, that the volume removed from the particles is almost proportional to the impact energy. However, repeated contact of the same crystals does not give the same reduction of volume, which decreases significantly. The reason for this is the change of the crystals shape, which tend to round off after repeated impact. Crystal growth, on the other hand, leads to the redevelopment of edges and corners. From the experimental data presented it can be concluded that growth rates and attrition rates in industrial crystallizers are related. The nucleation rate due to attrition increases with increasing growth rate.