Effect of Water Cationic Content on flocculation, flocs resistance and reflocculation capacity of PCC induced by polyelectrolytes

Abstract

In papermaking, mill water closure may result in a significant increase of inorganic salts in the white water. The effect of these contaminants on the performance of retention aids was evaluated, in this study, through flocculation of Precipitated Calcium Carbonate (PCC) with three very high molecular weight cationic polyacrylamides (C-PAM) of medium charge density and with different degrees of branching. Furth rmore, flocs resistance and reflocculation capacity was also investigated when different types of shear forces were applied. Tests were carried out in distilled water and in industrial water containing a high concentration of cationic compounds. The flocculation, deflocculation and reflocculation processes were monitored by evaluating particle size distribution using a light diffraction spectroscopy technique. Additionally to the effect of the cationic content of the medium, the influence of the flocculant dosage and degree of polymer branching were also studied. The effect of these parameters on the flocs structure was estimated by determining both the mass fractal dimension and the scattering exponent of the aggregates. The results show that the presence of inorganic salts affects significantly the performance of the polyelectrolytes. The flocculation kinetics is faster but the required flocculant dosage is higher when the suspending medium is industrial water. The cationic entities affect also the flocs structure because they reduce the reconformation of the polymer during flocculation. Additionally, in industrial water, flocs become more resistant and this effect is more pronounced as the flocculant branching decreases. In the case of the linear polymer, this effect is not so obvious because reconformation is less pronounced due to its molecular structure. Reflocculation capacity of flocs is very reduced both in distilled and industrial water.