The effect of polymer structure type, pH and shear on the interfacial chemistry, rheology and dewaterability of model iron oxide dispersions

The effect of high molecular weight (7.5–9 × 106 Da) polyacrylamide flocculant structure type (anionic polyacrylamide, PAM A and non-ionic polyacrylamide, PAM N) on the interfacial chemistry, rheology and dewaterability of a colloidal iron oxide dispersion has been investigated at three pH values where the particles are at the isoelectric point (iep) and either side of the iep. PAM A was found to adsorb with a lower affinity than PAM N at all pH values tested, however, PAM A had a more significant effect on the particle zeta potential and the shear yield stress. Highest settling rates (100 m/h) were achieved with PAM A at pH values at and below the iep, with flocculant dosage below 100 g polymer/t solid. Settling rates with PAM N at similar concentrations, however, only produced settling rates between 5 and 10 m/h. Pulp pH and the addition of the flocculants (up to 500 g polymer/t solid) had no noticeable effect on the extent of the pulp consolidation. Significantly, shear of pre-sedimented dispersions was found to increase the consolidation by up to 15 wt%. Polymer structure type was found to have a significant effect on the rheology of the dispersion, with PAM A-flocculated dispersions leading to a higher yield stress than dewatering with PAM N.