Mixtures of cationic starch and anionic polyacrylate used for flocculation of calcium carbonate—influence of electrolytes

Abstract

Mixtures of high molecular mass cationic starch and low molecular mass anionic sodium polyacrylate were characterized (isothermal titration calorimetry, turbidity, polyelectrolyte titration, viscosity, and electrophoretic mobility) and used for controlled flocculation of semi-dilute calcium carbonate dispersions. The polyelectrolytes were mixed at ratios ranging from pure sodium polyacrylate to pure starch. For certain conditions strong correlation was found between the properties of the polyelectrolyte mixtures, foremost the amount of complexes formed, and the flocculation behavior of the calcium carbonate dispersions. The correlation was strong at low total amount of polyelectrolytes added, where the properties (or the amount) of the individual complexes determined the flocculation efficiency of the mixtures. At high amount of polyelectrolyte the calcium carbonate particles were likely, on the other hand, completely covered by complexes and the effects of the individual complexes were screened. On the basis of our attempt to correlate the properties of the polyelectrolyte mixtures, used as flocculating agents, to the flocculation behavior of the calcium carbonate dispersions, further support was found for our previously proposed mechanisms of the complex-induced flocculation observed in the calcium carbonate−starch−polyacrylate system. In particular, further support was found for the relation between the amount of complexes and the degree of flocculation of the calcium carbonate dispersions. Additions of NaCl and CaCl 2 showed that both the ionic strength and the ion specificity are important tools when tuning the properties of the polyelectrolyte mixtures as well as the calcium carbonate dispersions.