A population balance (PB) model is used to describe the aggregation/breakage kinetics of calcite flocculation in turbulent pipe flow. The model describes simultaneous aggregation and breakage, and accounts for likely, process variables: fluid shear, flocculant dosage, primary particle size and solid fraction, using four fitted parameters. Particle collision is described by the turbulent collision kernel of Saffman and Turner,(1) however, the capture efficiency (a) is initially set to equal zero until the point of flocculant addition, then increases rapidly as described by an equation accounting for flocculant mixing/adsorption. Particles aggregated by high-molecular weight polymer flocculant typically do not achieve a steady-state aggregate size, where the rates of aggregation and breakage are equal. Instead, the aggregate size decreases gradually due to polymer chain scission or rearrangement on repeated aggregation/breakage. This phenomenon is accounted for in the model by a flocculant strength degradation term. (c) 2006 American Institute of Chemical Engineers.