Impinging jet studies of the kinetics of deposition and dissolution of calcium carbonate particles

Abstract

The deposition onto and detachment of calcium carbonate particles from cellophane and glass with and without retention aids, polyethylenimine and polyacrylamide, were studied under well-defined hydrodynamic conditions in a stagnation point flow field created in an impinging jet apparatus. With one exception, all deposition efficiencies were found to be in agreement with expectations based on electrostatic attractive or repulsive forces. The efficiency of deposition onto polyacrylamide-treated cellophane was found, however, not to be reduced by electrostatic repulsion, implying that polyacrylamide functions as a bridging agent. The bond strength between polyethylenimine and cellophane is shown to be much higher than the bond strength between polyethylenimine and pulp fibers. The calcium carbonate particles deposited onto cellophane using either of the retention aids were able to withstand shear rates of the order of 8000 s −1. These shear rates are comparable to the shear rates prevailing in a paper machine headbox. The lack of time for the establishment of an equilibrium ion concentration close to the surface of a dissolving calcium carbonate particle is shown to be responsible for the two to three orders of magnitude difference between experimentally observed dissolution rates and the much higher rates calculated for diffusion-controlled dissolution.