A model that makes it possible to evaluate the secondary nucleation rate in a pilot-scale industrial crystallizer is presented. It relates the secondary nucleation rate directly to the volume rate of attrition of large parent crystals. Two other terms are included in the equality, one that expresses the distribution formed by the attrition fragments, and the other that expresses a survival efficiency accounting for the percentage of attrition fragments that grow out after the attrition step. The model is further tested on a 970-L draft-tube-baffled evaporative crystallizer with ammonium sulfate as the model material used for crystallization and having an on-line crystal size-distribution measuring device.