Rheology investigations on the influence of addition sodium polyacrylate to calcium carbonate suspensions

The stability of concentrated CaCO3 suspensions (40 wt%) on addition of sodium polyacrylate (NaPAA, PA20, with molar mass of 2100) has been investigated using rheological measurements. On addition of NaPAA to the suspension, there is a selective adsorption of PA fraction with molar mass of 2000–5000. This selective adsorption is due to entropic effects and heterogeneous charge distribution. The smaller sized polyelectrolyte chains can more easily find local patches at the interface, devoted of previously adsorbed molecules, which are characterized by more favourable interaction profile. Steady-state shear stress-shear rate curves were obtained as a function of PA20 concentration (0–0.7 wt%). All suspensions showed pseudoplastic flow curves (shear thinning behavior) with some thixotropy. In the absence of PA20, and in the presence of 0.1% PA20 the flow curves show significant hysteresis indicating weak flocculation. This was confirmed by measuring the particle size of the suspension on dilution of system. On further addition of PA20 (0.2–0.7 wt%), the suspensions showed much less thixotropy and the weak flocs produced were broken down on dilution giving a mean diameter of about 1 μm. The flow curves could be fitted to the Herschel Bulkley model and values of the yield value σβ, consistency index k and shear thinning index n were obtained as a function of PA20 concentration. Initial addition of PA20 (0.1 wt%) caused an increase in σβ and k indicating more flocculation at this PA20 concentration. The data showed a reduction in σβ from 7.6 Pa at 0.1 wt% PA20 to 1.7 Pa at 0.7% PA20. The rheological results also showed a continuous reduction in k and increase in the value n (less shear thinning behavior). However, all suspensions showed weak flocculation at such high CaCO3 concentrations (40 wt%). Further insights into the flocculation behavior of CaCO3 suspensions with added NaPAA were gained in proton TD-NMR studies of the water phase of the suspension and sediment phases. This weak flocculation could be significantly reduced on dilution of the suspension from 40 to 20 wt%. A plot of σβ versus ζ2 showed non-linear behavior and this clearly indicated that the stability of CaCO3 suspensions in the presence of NaPAA could not be accounted for in terms of the DLVO theory. The presence of adsorbed loops and tails of NaPAA molecules on CaCO3 suspensions could play a major role in the stability of the suspensions.

Research highlights▶ By combining rheological tests, TD-NMR relaxation and particle size distribution measurements, we were able to obtain a clear picture on the effect of sodium polyacrylate on the stability of CaCO3 suspensions. ▶ Flocculation effects were reduced by increasing NaPAA concentration on CaCO3 suspensions. ▶ Selective Adsorption of Polyacrylic acid fraction with low molar mass on CaCO3 and consequentelly better suspension stability.