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Study of suspensions of ground calcium carbonate : Characterization and dispersion processe
We study physical properties of suspensions of ground calcium carbonate in the presence or absence of polyelectrolyte dispersant. Suspensions of varying volume fractions are obtained by the dispersion of a highly concentrated paste in solutions of dispersant. The particle size distributions are measured by laser granulometer and show a bimodal population between 0.03 and 10 µm. The dispersions are not stable: the aggregation of particles in aqueous media is shown by rheological measurements, using a Couette geometry of non-standard type (6-bladed vane) and by measurements of sedimentation rates. The scanning electron microscopy shows mostly particles smaller than micron, forming larger aggregates. In order to obtain an optimal amount of dispersant and a good dispersion, we propose new methods based on centrifugation-redispersion cycles that improve considerably the stability of suspensions and the adsorption of the dispersant. Under these conditions, the sedimentation shows multiple fronts and the amount of submicron particles increases with the number of cycles. Finally, we propose the characterization of powders by impregnation by liquids that do not break the pellet or dissolve calcite particles. The powders were prepared from drying of suspensions obtained by various protocols. The contact angle measurements, the diameter of the base and volume of the drops show different regimes of impregnation. The impregnation of successive drops leads to the complete saturation of the pores of the pellet. The effect of compacting pressure was also studied. We tested various theoretical approaches in order to interpret the impregnation rate and the spreading of the liquid inside the pellets. The different kinetics of impregnation are related to the presence of dispersant.