Chemical and Hydrodynamic Effects in Polymer-Clay Flocculation: Anisotropic particulate size and surface morphology effects in varied and controlled hydrodynamic fields

Abstract

Polymer-driven flocculation of suspended particles is a critical process for many applications, including composite materials synthesis, paper manufacturing, and water treatment. However, the role of solution physicochemical properties on the polymer-particle assembly dynamics is nontrivial, particularly for non-spherical, polydisperse particulates such as natural clays. Properties including ionic strength and pH affect both the individual particulate aggregates themselves, as well as the polymer – particle flocculation event. In this work, we study the effects of ionic strength and aggregate size and structure on the polymer behavior and flocculation performance with anisotropic bentonite clay particles using traditional jar tests. The final floc structure is largely informed by ionic-strength driven changes to the initial clay aggregate size and surface structure. With increasing bentonite aggregate size, a transition from a networked to a patched polymer − aggregate floc structure is observed, independent of ionic strength during flocculation. Additionally, the clay’s aggregate morphology is a more direct control parameter of optimal polymer dose and final turbidity (turbidity after 5 min sedimentation) than zeta potential for aqueous bentonite systems. Flocculation performance is the same when bentonite aggregate morphology is the same, regardless of a change in zeta potential. Likewise, when bentonite aggregate morphology changes, flocculation performance also changes, regardless of the identical zeta potential. Therefore, initial clay aggregate morphology controls the extent of polymer adsorption and optimal polymer dose, while initial clay aggregate size controls the internal floc structure. While traditional jar tests offer the advantages of experimental simplicity, speed, and mimic treatment geometries, there is limited homogeneity and control over hydrodynamics within the system. Taylor-Couette cells offer a much higher degree of hydrodynamic control and have been shown to improve several industrial processes due to the wide variety of hydrodynamic flow states accessible. Traditional designs, however, limit the ability to introduce new fluids into the annulus during device operation due to geometric confinement and complexity. As a key part of this thesis effort, a co- and counter-rotating Taylor-Couette cell with radial fluid injection has been constructed. The new inner cylinder design does not modify the critical Re for flow instabilities and can precisely inject a desired mass at a desired flow rate. Using the newly designed, modified Taylor-Couette cell, axial mass transport behavior is experimentally determined over two orders of magnitude of Reynolds number. Four different flow states, including laminar and turbulent Taylor vortex flows and laminar and turbulent wavy vortex flows, were studied. Using flow visualization techniques, the estimated dispersion coefficient was found to increase with increasing Re, and a single, unified regression is found for all vortices studied. In addition to mass transport, the vortex structures’ stability to radial injection is also quantified. A new dimensionless stability criterion, the ratio of injection to diffusion timescales, was utilized to capture the conditions under which vortex structures are stable to injection. Using the stability criterion, global and transitional stability regions are identified as a function of Reynolds number, Re. Overall, this thesis examines chemical and hydrodynamic effects in polymer flocculation with natural clays, and shows the importance of initial contaminant properties on flocculation performance. The initial contaminant properties affect both flocculation efficiency and resultant floc structure and are often not considered at treatment plants. Consideration of these properties potentially can improve process predictive capabilities, which improves process performance.