Positron emission particle tracking (PEPT) was used to investigate the grinding media dynamics in a laboratory-scale attritor mill in the absence of powder. The grinding media motion was analysed as a function of the equipment’s typical operating parameters: impeller speed, impeller clearance and bead fill level. It was observed that the impeller speed had the strongest influence on the media motion. An increase of the impeller speed from 300 rpm to 600 rpm led to a change in the bead recirculation patterns with the increasing formation of well segregated upper and lower recirculation loops that fully developed at the maximum speed of 600 rpm. For a constant impeller speed, an increase of the bead loading did not majorly affect the bead velocity as remarked by minor changes on the flow field. For all the impeller clearance values, the occupancy plots revealed an inefficient dead region at the bottom of the attritor where the beads were moving at very low velocity. In this region the beads were tightly packed under their own weight and, furthermore, there was an absence of direct contact with the impeller arms. The depth of this region increased proportionally to the distance between the bottom of the impeller and the vessel base indicating that a minimum value of clearance should be set to optimise the lower recirculation pattern. For two experimental conditions, the data generated by PEPT measurements were utilised to set-up a friction-adjusted discrete element method (DEM) model. Here, the simulation results were qualitatively and quantitatively compared against the PEPT data by assessing the averaged velocity flow fields and the average velocity profiles at different radial locations inside the vessel. Given the intrinsic uncertainty of the PEPT measurements, the DEM model results were in considerably good agreement with the experimental results. The major discrepancy was observed close to the vessel wall where the simulations overpredicted the velocity by about 10%.

Controlling film peeling by modifying the properties of the evaporating solution and consequently the length of the drying front.

This article describes the effect of vessel configurations on the drawdown and incorporation of floating solids to prepare concentrated alumina slurries in stirred tanks. The impeller speed and power draw required to incorporate all dry powder within four seconds, N_JI and P_JI, are used to evaluate incorporation performance. The effect of impeller type is assessed, with pitched blade impellers proving to be the most effective across the full range of solid contents considered. At higher solids content the energy demand is shown to increase dramatically, with a 100‐fold increase in energy required to add 1% w/w more solid at 50% by weight compared to 1% by weight. Analysis of impeller power numbers show this coincides with a transition from constant power number to a region where power number increases linearly with decreasing Reynolds number. Contrary to studies at low solids content, the presence of baffles is shown to inhibit drawdown. © 2018 The Authors AIChE Journal published by Wiley Periodicals, Inc. on behalf of American Institute of Chemical Engineers AIChE J, 64: 1885–1895, 2018