Sedimentation of suspensions exhibiting various regimes of settlement

This work reports an investigation of colloidal suspensions during sedimentation in what is known as the intermediate concentration range; one notoriously difficult to study. Two materials were examined: titanium dioxide P25 nanoparticles and magnesium hydroxide. The characterisation of these materials highlighted their high tendency to form aggregates under certain conditions. By the use of laser diffraction, measurements of the cluster size were conducted at under various conditions of shear. The use of sonication energy suggested the identification and classification of two classes of clusters related to their strength: high strength , and low strength clusters. A mathematical model which considered cluster formation and the occurrence of breakage predicted the aggregation and disaggregation kinetics. The approach was based on cluster-cluster interactions rather than particle-particle interactions. The results obtained by particle size analysis were compared with the size obtained using permeability analysis and settling velocity. In both cases the calculated sizes were comparable with the data obtained from size analysis; however, for magnesium hydroxide the predicted cluster size was a little higher and this may be due to the formation of channels giving faster settling. For the titanium dioxide it was noticeable that different mixing strategies exhibited different cluster size, network formation and settling behaviour. Finally, a novel modelling approach based on the presence of clusters rather than particles is presented. It is a combined model which considers the settling curve divided into two zones: a zone below the gel point modelled by a Kynch type approach and a zone above the gel point modelled by consolidation theory. This combined approach was only applicable to the settling data of titanium dioxide, where the settling data encompassed the gel point and the gel point could be identified by consideration of the Richardson and Zaki plots. For magnesium hydroxide, only the consolidation model was needed as all the initial settling concentrations were above the gel point.