Influence of polymer charge on the compressive yield stress of silica aggregated with adsorbed cationic polymers

Abstract

Flocs were produced by adding three cationic polymers (10% charge density, 3.0 × 10 5 g/mol molecular weight; 40% charge density, 1.1 × 10 5 g/mol molecular weight; and 100% charge density, 1.2 × 10 5 g/mol molecular weight) to 90 nm diameter silica particles. The compressive yield stresses of the consolidated sediment beds from settled and centrifuged flocs were determined using the volume fraction profile method. The polymer charge density plays an important role in influencing the compressive yield stresses of sediment beds. The compressive yield stresses of sediment beds from flocs induced by the 10% charged polymer were observed to increase with an increase in polymer dose, initial solids concentration, and background electrolyte concentration at all volume fraction. In comparison, polymer dose has marginal effect on the compressive yield stresses of sediment beds from flocs induced by 40% and 100% charged polymers. Initial solids concentration has no influence on the compressive yield stresses of sediment beds from flocs induced by either 40% or 100% charged polymers at the polymer doses used. The compressive yield stresses of sediments from flocs induced by the 40% charged polymer are independent of salt concentration whereas the addition of salt decreases the compressive yield stresses of sediments from flocs induced by the 100% charged polymer. It is found that the shear intensity during aggregation only affects the compressive yield stresses of sediments from flocs induced by the 10% charged polymer. When flocculated at the optimum dose for each polymer (12 mg/g silica for the 10% charged polymer at 0.03 M NaCl, 12 mg/g for 40% and 2 mg/g for 100%), compressive yield stress increases as polymer charge increases. The effects observed are related to the flocculation mechanism (bridging, patch attraction or charge neutralisation) and the magnitude of the attractive force.