Influence of anions on the rheological properties of clay mineral dispersions

The flow behavior of sodium montmorillonite dispersions and kaolin slurries can have different responses to anionic dispersants. Viscosity, pH, and dynamic mobility measurements using a variety of salts help elucidate this important observation. The flow behavior is determined not only by the counterions but also by the coions (anions) surrounding the clay mineral particles. The viscosity of sodium montmorillonite dispersions with increasing salt or acid concentration decreased to a minimum, then increased sharply. The concentration at this point was identified as the critical coagulation concentration, cK. Salts with multivalent anions such as sulphates and phosphates increased the critical coagulation concentration. Na4P2O7 actually impeded coagulation. The liquefying effect of these anions was counterbalanced by the protons of acidic salts such as NaHSO4, Na2HPO4, and NaH2PO4 and by acids. The critical coagulation concentration for sodium montmorillonite was a function of the montmorillonite concentration and the type of ion. In 2% sodium montmorillonite dispersions, cK for sodium chloride and sulphate was higher than cK in dilute dispersions (0.025%). For the strongly liquefying agents Na2HPO4, NaH2PO4, and H3PO4, the opposite was true. The cK values for the dilute dispersion were 1100, 460, and 32 mmol/l respectively, and for the 2% dispersion 80, 40, and 10 mmol/l. Kaolinite dispersions in contrast to sodium montmorillonite showed the pronounced liquefying effect of multivalent anions up to high salt concentrations. The data underscored the complexity of dispersed systems. For example, the dynamic mobility of the particles and the viscosity of the system were not always directly related. An interesting aspect concerns the dynamic mobility of the montmorillonite and kaolinite particles which in different ways changed with concentration of salts and acids.