Attrition is the general term that describes the breakdown of crystals due to collisions with each other or some other body, or by a shearing fluid. These interactions produce new crystals from the parent crystals and are likely to be a major source of secondary nucleation in industrial crystallizers. However, these collision interactions are difficult to devolve because they occur simultaneously. This work studies lactose monohydrate attrition within the small sample presentation unit of a laser diffraction particle sizer (Malvern Mastersizer, 2000, UK). Stirrer speed, particle concentration and initial crystal size were varied and particle size distributions (PSDs) were recorded every 2 min for 1 h. From these, three basic attrition mechanisms were recognizable: shattering, chipping and abrasion. The degree of attrition was quantified using indices: the shattering index (SI) and attrition index (AI), which are both typical of literature indices; and the differential attrition index (DAI) proposed here. All indices show similar trends although the DAI is more sensitive to subtle changes in PSDs. Of the three variables studied, particle concentration least affects attrition. Experiments with low initial crystal size and low impeller speed produce fines in the <10-mu m range, indicating that abrasion is the dominant attrition mechanism. In contrast, when the initial crystal size is large and impeller speed is high, crystal fragments are produced across all size ranges indicating that all three attrition mechanisms occur. This variation across the results indicates that the three mechanisms are differentiated by the collision energy intensity, which is a function of both crystal size and impact velocity. These experimental observations were found to closely follow theoretical predictions for crystal-impeller impacts (Synowiec et al. Chem. Eng. Sci. 48 (1993) 3485-3495 and Mersmann et al. Chem Eng & Technol.11 (1988) 80-88). When applied to industrial crystallizers, this work demonstrates that careful selection of pumps and impellers is needed to control secondary nucleation. (C) 2011 Elsevier B.V. All rights reserved.