Microstructure, rheology and dewatering behaviour of smectite dispersions during orthokinetic flocculation

Abstract

The influence of high molecular weight (>7.5 × 10⁶ Da) polyacrylamide (PAM) flocculant type (anionic vs non-ionic), solution speciation (KNO₃ + CaCl₂ vs KNO₃) and shear on microstructure, rheology and the dewatering behaviour of Na-exchanged smectite clay dispersions has been investigated under orthokinetic flocculation conditions at pH 7.5. Massive osmotic swelling, accompanied by high shear yield stress and non-settling behaviour, were displayed by dispersions prepared in 10⁻³ M KNO₃ background electrolyte and to which 0.05 M CaCl₂ was subsequently added. Prior clay dispersion in CaCl2-based solution however led to rapid exchange of Ca²⁺ with Na⁺ in the clay and suppression of swelling which reflected in low yield stress, dramatic reduction in particle zeta potential and marginally improved dewaterability. Cryo-SEM image analysis indicated the presence of highly networked “honeycomb” structure in the swelling clay pulps and loosely networked, chain-like structure in non-swelling dispersions. Dewatering of suppressed swelling pulps indicated optimum conditions for orthokinetic flocculation, dependent upon flocculant structure type and shear intensity, which strongly influenced the settling rate but not the low pulp consolidation (~21 wt% solid) behaviour. Under optimum conditions, anionic PAM produced significantly faster settling flocs than non-ionic PAM at up to 500 g polymer t⁻¹ solid. Post-sedimentation shear action markedly improved pulp solid loading by 5–7 wt%, independent of PAM type. Anionic PAM flocculated pulps displayed 25% higher yield stresses than non-ionic PAM-based pulps. Upon shear however, the yield stress of the former decreased dramatically to values lower than those of the latter, indicating that anionic PAM-based flocs structures were less robust, a finding which is consistent with cryo-SEM imaging.