(158c) The Effect of Particle Properties and Solids Loading On Suspension of Mixed Solids

Solids suspension is a complicated operation with many variables related to solid and liquid properties and impeller and tank geometry. The objective of most solid-liquid mixing operations is to utilize the entire surface area of the solids for mass transfer purposes. This requires operating at N_js, just suspended speed, where complete off bottom suspension condition is satisfied. N_js increases with increasing solid density and particle size. It is relatively easier to determine N_js and design when the slurry is composed of unimodal solids; however, this is hardly ever the case in industry. The slurries in industrial operations, such as mineral processing, are composed of many solid phases with varying density and particle sizes. The design correlations currently available in the literature are for unimodal slurries; therefore, it is harder to design for mixed slurries. In this study we aim to investigate the behaviour of the suspension of slurries with mixed solids at varying solids loadings. For this reason both unimodal and mixed slurry N_js data were observed. The mixed slurries were composed of two solid phases at specific solids loadings for each phase. The unimodal slurries were composed of each of these solid phases at the specific solids loadings in mixed slurry experiments. This allows us to predict the effect of presence of a second solid phase on N_js. Glass beads, bronze powder, ion exchange resin and nickel were used in the experiments. All these particles differ in particle size and density. The total solids loadings were varied from 1.5 to 55 wt% (weight percent). Two impellers were tested over a range of off-bottom clearances: Lightnin A310 and down pumping 45° PBT. The results showed that the dense particles dominate at low solids loadings; therefore, N_js is determined by the dense particles. At high solids loadings the particle-particle interactions become significant and a different trend is seen in the results.