Characterizing slurry hydrodynamic transport in a large overflow tubular ball mill by an improved mixing cell model based on tracer response data

Abstract

Overflow tubular ball mills are depicted by an intrinsic pool of slurry at the toe of the ball charge, which facilitates the discharge of fine progenies (finished product) from the mill. To ensure that adequate slurry removal capacity is achieved while keeping the ball charge well saturated with slurry at all times (for efficient grinding), good knowledge of slurry transport inside the mill is essential. Most existing ball mill simulation models either inadequately account for the continuous exchange of slurry between the ball charge and the slurry pool or do not consider the realities such as back-mixing or bypasses in the system thereby leading to inaccurate prediction of slurry transport. To close that gap, a model is proposed here, in which both axial and radial flow parameters as well as a short-circuit component are incorporated as an improvement to the simple mixing cell model to correctly describe the inherent in-mill slurry transport dynamics. The model fits adequately to the experimental data for all conditions tested. Further, the model results show that the slurry axial back-mixing and the radial exchange rate are both affected by slurry solids concentration and ball load volume. Also, the model reveals that under conditions of low slurry solids concentration (< 37 vol.%), a part of the feed slurry may simply short-circuit to the exit. These results just serve as a priori indication that, having correct knowledge of in-mill slurry transport dynamics would enable mill operational variables to be appropriately tuned to achieve desirable mill behavior that corresponds to maximum milling and energy efficiencies.