Suspension of Mixtures of Solids in Stirred Tanks: Problem Definition and Model Identification

Solids suspension in stirred tanks has many applications in industry. The contributions of this thesis are particularly interesting for applications in mineral processing and nuclear waste clean up. The main issue in design of mixing tanks for such applications is the gap between industry and research. These applications involve mixtures of solids at high solids loadings; however, research has been limited to unimodal slurries at low solids loadings. This limitation is a result of the complexity of solids suspension. The fundamental understanding of solids suspension has not been fully established, and the effect of numerous parameters is not fully understood. The objective of this thesis is to contribute to the fundamental understanding of solids suspension, and provide a bridge between research and industry with improved design methods.;To provide this bridge, first a thorough experimental study was conducted and the behaviour of mixtures of solids at high solids loadings was investigated. The effect of the ratio of the particle size, particle density and solids loadings of the two solid phases in binary mixtures was analyzed. The total solids loading of the mixtures was increased up to that of industrial cases. The findings were discussed and the validity of general design heuristics was tested. Based on these findings additional data was collected and a deeper analysis was done to obtain a model for predicting mixture just suspended speed. The analysis showed that the current design heuristic that is used in industry is incapable of predicting physics behind solids suspension. A new model, based on the power required to suspend each solids fraction, showed accurate predictions up to high solids loadings. This study also revealed necessary improvements to the Zwietering correlation for predicting unimodal slurry just suspended speed.;While these studies targeted solving an industrial problem, another study was carried out to enhance the fundamental understanding of solids suspension. Solids suspension mechanisms in stirred tanks were investigated. An analogy between slurry pipeline flow and river sediment transport suggested that similar mechanisms apply in all three geometries. In this study the active solid suspension mechanisms in a stirred tank were defined, and the effect of geometry on the dominant solid suspension mechanism was investigated.