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