Effect of polyelectrolyte charge density and molecular weight on the flotation of oil in water emulsions

Abstract

The effect of polyelectrolyte charge density and molecular weight on the flotation of oil emulsified in aqueous electrolyte was investigated. Flotation performance improved with increasing polyelectrolyte charge density, while polyelectrolyte intrinsic viscosity (molecular weight) had no effect on flotation over the range 6–12 dl g −1. Turbidity removal improved as floc size increased, indicating that polyelectrolyte enhanced flotation by increasing the floc size. The trends in the flotation and flocculation behaviour of the emulsion with changing polyelectrolyte intrinsic viscosity (molecular weight) were consistent with electrostatic patch flocculation. Raising the charge density of the oil increased the amount of polyelectrolyte required for optimal flotation and led to improved performance, while raising the background KNO 3 concentration from 10 −3 to 10 −1 M decreased flotation performance. The effect of electrolyte concentration was greatest at low oil charge density (pH 5) and low polyelectrolyte charge density. It is proposed that the reduction in flotation performance with rising electrolyte concentration was due to increased electrostatic shielding leading to smaller floc sizes, as electrostatic attraction between charged patches on the oil was thought to be the flocculation mechanism. The flotation process was more robust towards changes in polyelectrolyte dose when polyelectrolytes of lower charge density were used.