Adsorbed phosphate additives for interrogating the nature of interparticles forces in kaolin clay slurries via rheological yield stress

Unimin’s kaolin clay slurry displayed a maximum yield stress that is not located at the point of zero charge or at low zeta potential. Adsorbed phosphate additives at sufficient concentration caused this yield stress to reduce to zero. The clear implication of these two results is that the particle interactions contributing to the maximum yield stress must be due to unlike charged or positive–negative charge attraction. The adsorbed negatively charged phosphate additives neutralised the positively charge sites of the clay particles causing the suspension to be completely dispersed and the yield stress to disappear. The monophosphate additive is less effective in dispersing the slurries. Based on charge concentration of additives, the pyrophosphate, triphosphate and polyphosphate appeared to be equally as effective in dispersing the clay slurries. The yield stress-DLVO force model that predicts a linear relationship between yield stress and square of zeta potential with a negative slope, is obeyed by the kaolin slurries with adsorbed phosphate additives. However, with the slurry without phosphate additive it displayed three linear relationships with negative and positive slopes. The negative slope relationships were located in the low and high zeta potential regions while the positive slope relationship was found in between these regions.