Effect of shear on the yield stress and aggregate structure of flocculant-dosed, concentrated kaolinite suspensions

The rheology of paste or thickened tailings, which are mineral tailings treated with polymers and subsequently dewatered, is complex and time-dependent. The complex rheological behavior is exacerbated by the permanent change in rheology that occurs when the mixture is exposed to shearing (e.g. in pumps and pipes), primarily through breakdown and restructuring of the aggregates of particles that comprise the dispersed phase. While the effects of shearing on structural changes of aggregates in a dilute suspension are reasonably well characterized, the relationship in more concentrated suspensions treated with polymer flocculants is not well understood. In the present study, a custom-built, concentric cylinder shearing apparatus is used to shear a large volume of flocculant-dosed (thickened) kaolinite suspension. The evolution of the particle size distribution (PSD) and vane yield stress were monitored as a function of shear energy input. Size distributions were obtained using the Focused Beam Reflectance Measurement (FBRM) technique. Concentrated suspensions were prepared using two different anionic polymers and at two suspension pH values to evaluate the effect of polymer structure and water chemistry on changes in the vane yield stress and aggregate size. Changes in both measured parameters correlated directly with shear energy input. During shearing, a large population of smaller aggregates, in the 10-100 mu m size range, is generated. Aggregate size reduction occurs in concert with structural changes; together, they dictate the value of the equilibrium (fully-sheared) yield stress. At pH 7, the aggregates were able to restructure due to edge-edge associations between kaolinite particles. This phenomenon was not observed at pH 8.5, resulting in an equilibrium yield stress similar to 1/3 the value of that measured at pH 7. The chemical composition of the polymer flocculants did not affect the yield stress or aggregate size over the shear energy input range studied here.