Design rules for suspending concentrated mixtures of solids in stirred tanks

This study examines complete off-bottom suspension conditions for slurries containing mixtures of solids at a wide range of concentrations. Binary combinations of five different particles; bronze, two sizes of glass beads, ion exchange resin, and nickel were tested with two impellers: the Lightnin A310 and the down pumping 45 degrees PBT. The effect of particle size ratio and density ratio of the two solid phases on N-js was investigated. Solids loadings were varied from 3 to 56 wt% (weight percent) with water as the liquid phase. The results showed that the highest particle N-js, in a mixture of solids is not a sufficient design specification to ensure suspension of the mixture. For one of the cases studied, the particle-particle interactions became significant at high loadings and resulted in a decrease in N-js. Different behaviours were observed for the other mixtures. The performance of the two impellers was also compared. The A310 impeller consumes significantly less power at N-js than the PBT. Both impellers showed an effect of off-bottom clearance on N-js, but the effect of off-bottom clearance depended on both the solids concentration and on the solids used. (C) 2011 The Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.