Secondary nucleation studies on alpha lactose monohydrate under stirred conditions

Secondary nucleation is the dominant source of nuclei at the industrial scale. Traditionally, secondary nucleation has been expressed empirically as a function of stirrer speed and magma density with little or no understanding about the role of individual mechanisms (attrition and contact). This study considers the various mechanisms of secondary nucleation of alpha-lactose monohydrate crystals and link them to either crystal-crystal or crystal-impeller collisions. An agitated system was used to conduct trials at various levels of stirring speed (400 and 550 rpm), seed size (150, 250, 357 and 502 μm) and seed loading (2%, 5% and 10%, v/v) at a constant supersaturation. Crystal-impeller contacts are proposed to be the major source of secondary nucleation. It was found that the kinetic energy and not the collision frequency govern the rate of secondary nucleation with a threshold kinetic energy value below which no secondary nucleation occurs.