Aggregate densification behaviour in sheared suspensions

© 2012 Dr. Ben Barend Gert van DeventerAnalysis of batch settling tests incorporating shear indicates that the aggregate densification rate increases in time as the test proceeds, nominally as the concentration of solids in the settled bed increases. The concentration dependent behaviour of the rate of densification has been investigated and a semi-empirical relation proposed that describes the rate of densification as a function of solids concentration. This semi-empirical relation is an application of collision rate theory. Modelling and analysis were performed by further generalisation of the Extended Kynch Method to include the effect of concentration on the densification term. While the Extended Kynch Method provides a macroscopic view of aggregate densification, a study of micro-scale behaviour provided insight into the fundamental cause of aggregate densification. To achieve this, simulations of aggregate-aggregate collisions were performed using the Discrete Element Method (DEM). DEM simulations provide some insight into such factors as bonding stiffness, collision velocity, and the effect of multiple impacts on colliding aggregates. Finally, consideration of the effect of raking a networked suspension shows that whilst aggregate densification may be achieved at higher rates, ultimately shear enhanced dewatering is less likely to be effective at high concentrations, despite the concentration dependence of the rate. It was found that for the systems studied, the material properties reveal that the rate of consolidation is flux limited. Through the incorporation of densification functional forms into multi-dimensional simulations, future work would bridge the gap between laboratory scale measurements and predictions of full-scale thickener behaviour