Small Particle Migration Mechanisms in Consolidating Pigment Coating Layers

The displacements of small-sized coating particles (D <= 300nm) in consolidating coating layers were studied by numerical simulation. Particle displacement distributions were compared as a function of time and position of the coating layer. The difference between vertical (z-directional) and lateral mobility was also studied. The particle size distribution of the systems simulated resembled fine grade ground calcium carbonate (GCC) and the post-metered coating layer thickness was set between 10 and 30{ extmu}m. Both theoretical model cases and dewatering strategies based on pilot trials were used for investigating the influence of different drying conditions. A 3D particle model based on modified Stokesian dynamics was employed for simulating the consolidation process. The model included hydrodynamic and colloidal particle interactions, a free surface model and the Brownian motion. Fundamental particle mobility mechanisms are reported at different points in time and coating layer positions. Particle trajectories are utilised in tracking the movement of particles within the layer. The drying strategy was found to have a clear influence on particle mobility. Distinct differences could also be found between the vertical and lateral directions. While hindering z-direction movement, the increased dry solids content was beneficial to lateral movement both in a filter cake and during skinning at the top of the coating. This work contributes to the understanding of fundamental particle migration mechanisms in consolidating coating layers. The results aid in explaining the causes for differences in particle distribution that result from the drying strategy applied.