The influence of different shapes and size distributions of coating pigments on packing and dewatering

Particle shape and particle-size distribution (PSD) are important factors for pigment packing and water retention in the pigment coating process, being closely associated with many runnability problems. Diverse experimental investigations on the packing and dewatering of pigment slurries have been made. However, theoretical approaches remain lacking. This paper presents a joint experimental and theoretical analysis of the influence of pigment shape and PSD on pigment packing and dewatering. The relative viscosities of very dilute pigment slurries were measured and used to determine the intrinsic viscosity, based on the Einstein equation. The deviation of pigment shape from spherical caused an increase in viscosity of the slurries. Acicular precipitated calcium carbonates gave lower shape factor values than platy clay particles, which indicated that the needle-like particles could rotate more easily around their long axes under shearing conditions. The packing of pigments, which determines porosity, permeability, light scattering, and mechanical properties, was examined. The packing volumes of nonspherical pigments were estimated based on their respective PSDs, using an algorithm developed for spherical particles, modified with a correction factor. This model is based on the close random-packing volume fraction (\(\emptyset_{\text{p}}^{ 2}\)) of particles of uniform size being an independent variable, regardless of their shape. Therefore, it was valid as long as the particles always achieve the same close random-packing volume (\(\emptyset_{\text{p}}^{ 1}\)) by sedimentation or tapping. Overall, the effect of the particle shape and PSD on the dewatering and filter-cake permeability (K_f) was analyzed. The dewatering rates were measured with a common pressure filtration method, and the permeability of the filter cakes was obtained by fitting the data to a filtration equation. The particle size and PSD were found to influence the permeability constant, but there was little correlation between the permeability and particle shape.