A model of the fluidized bed calcination process based on solids flow and mixing has been developed to calculate the change in the bed particle size distribution as a function of time and calciner operating conditions. The model is based on a mechanistic approach for determining the particle size distributions resulting from feed deposition, particle attrition, elutriation, seed addition and product removal. Comparison between the model results and data from a pilot plant calciner show good agreement. Because the model is based on mechanisms occurring in the fluidized bed calciner, it should be able to accurately scale up pilot plant results to predict operations in a full-scale calciner.