Drop Production in Coltette and Media Mill Devices

The media mill is a device used in the paint and pigment insustry for mixing and dispersing solid/liquid mixtures. The media consists of spheres which are agitated in a vessel and the hydrodynamic interactions between these media spheres cause the fine-scale dispersion. The details of the flow are very complex. A technique to characterize the dispersion efficiency of a media mill is presented which involves monitoring the maximum drop diameters of a dispersed organic phase in an aqueous continuous phase. The largest surviving drops reflect the maximum shear and elongation fields that exist in the flow. The maximum drop size scales as impeller speed to the -0.82 power. This dependence is much lower than the dependence for a Rushton turbine in a tank, which would be -1.38. The mechanism of breakup is clarified by experiments in a Couette geometry for large drop size to gap ratios, where the dependence of drop size on shear rate also scales to the -0.82 power. An analysis of the hydrodynamics involved in two approaching spheres explains the similarity between drop formation in a media mill and the narrow gap Couette cell; drop-solid surface interactions strongly influence breakup in both geometries.