Dispersion of nano-particle clusters using mixed flow and high shear impellers in stirred tanks

When dry nano-particulate powders are first added into a liquid, clusters as large as hundreds of microns can be formed. These tend to float at the liquid surface, either due to their poor wettability, or because air is trapped inside. Many formulation processes require suspension of these particle clusters in the liquid, followed by breakage or de-agglomeration, ideally down to the primary nano-particle size. Stirred tanks are often used for this wetting and suspension stage and for the generation of a coarse dispersion. In this study, a variety of impeller types, with different power and flow characteristics, were used to suspend and break-up these agglomerates in a stirred vessel. Initially, the operating window of rotational speeds for each impeller type was determined from (1) the minimum speed for off-bottom suspension of nano-particle agglomerates and (2) the maximum speed, above which surface aeration occurred. The effects of different impeller types on the rate of particle draw-down and wetting was also investigated and subsequently, the break up process was studied. Impellers characterized by a high local energy dissipation rate, such as the Ekatomizer sawtooth impeller, or the in-tank rotor-stator, could break up nano-particle clusters to submicron sizes by an erosional mechanism. The rate of fines generation within the de-agglomeration process was found to be controlled by the maximum energy dissipation rates produced by the mixing device. However, the size of the fine aggregates produced was a constant and not a function of the energy dissipation rate.