COLLOIDAL STABILITY OF CALCITE DISPERSION TREATED WITH SODIUM POLYACRYLATE

The stabilising action of sodium polyacrylate on colloidal dispersions of calcite has been investigated through measurement of viscosity, ion concentration and electrophoretic mobility. The dose of sodium polyacrylate was in the range 0 to 28 mg per g of calcite and the dispersions were prepared at a solids content of 70% (by weight). The ionic strength of the dispersions, ca. 0.005 to 0.5, increased with dose. An increase in divalent ion concentration with dose was attributed to sodium polyacrylate-ion exchange. The stabilising action of sodium polyacrylate was evident from the sharp fall in viscosity observed at low levels of addition, and the invariance of this low viscosity throughout the remainder of the dose range. The stability of the dispersions at low doses was quantified by DLVO theory and attributed to electric double layer (EDL) repulsion. However, at higher doses, and with the resultant EDL compression, DLVO theory was found inadequate. Instead, recourse was made to steric stabilisation theories in order to explain the observed stability. A model was formulated to characterise the observed multilayer uptake of polyacrylate at higher doses. The steric repulsion evaluated using this model increased with dose and explained the observed higher dose stability. The stability over the dose ranges < 2, 2 to 6, and > 6 mg per g is best described as arising from, respectively, electrostatic, electrosteric and steric repulsions.