The kinetics of aggregation/breakage of calcite particles flocculated with a high-molecular-weight polymer flocculant has been studied in turbulent pipe flow. The mean flocculation residence time was varied by changing the length of pipe between the flocculant injection point and the in-stream particle-sizing probe (Lasentec FBRM). A variety of pipe sizes andflow rates were used to produce a range of mean fluid shear rates. The mean shear rate was calculated from the pressure drop along the pipe reactor, as measured by manometer, and was found to vary markedly as a function of both the solid fraction and aggregate size. Increased fluid shear increased the initial mixing and aggregation rates, but ultimately lead to a reduced final aggregate size due to increased aggregate breakage. Several other process variables were also studied, with the aggregate size increased with flocculant dosage and primary particle size, but reduced at higher solid fraction. (c) 2005 American Institute of Chemical Engineers