Mass Transfer Characteristic for Solid–Liquid Mixing with Floating Particles

In solid–liquid mixing processes involving floating particles, the minimum impeller rotational speed for particle drawdown has been inconsistently defined, limiting its usefulness in process design and scale-up. In this study, the solid–liquid mass transfer coefficient, kL was measured using an electrical conductivity method based on ion exchange in a stirred tank over a wide range of volumetric power consumption, PV. Segmented regression analysis of the kL data revealed a distinct change point at which the rate of increase in kL with respect to PV decreased significantly. Independent observations of particle behavior indicated that this transition corresponds to the condition where no particles remain stationary at the liquid surface for more than 1–2 s, although some particles may still transiently appear at the surface. This result provides a practical criterion for particle drawdown, organized by PV, enabling scale-up and contributing to reliable operation and design of stirred vessels.