Götz Bickert

Characterisation of short-range structure of silica aggregates—implication to sediment compaction

Abstract In this study, both short-range and long-range structures of silica aggregates were studied by small-angle light scattering. The silica particles were aggregated by using different concentrations of KCl and MgCl 2 , with and without continuous shear. It was found from both small-angle light scattering and transmission electron microscopy (TEM) measurements that the aggregates had a compact short-range structure and a looser long-range structure. The floc sediments were studied by allowing the silica aggregates to settle under gravity, as well as having them consolidated by centrifugation. The results show that under gravity and lower centrifugation force (50× g ), the short-range structure of silica aggregates affects the compacted sediment structure, while under higher centrifugation force (453× g ), the sediment structure is independent of the short-range structure of the aggregates.

Separation of flocs in hydrocyclones—significance of floc breakage and floc hydrodynamics

Abstract Improved understanding of the behaviour of flocs within a hydrocyclone is necessary to progress the use of hydrocyclones for the clarification and thickening of fine particles. This paper describes experimental and modelling work to investigate the separation behaviour of flocculated particles in a hydrocyclone to better understand the different mechanisms influencing separation. Flocculated pseudo-monodisperse and polydisperse alumina trihydrate in a 1 wt.% water slurry was separated in a 22-mm Mozley hydrocyclone. Floc structure properties, floc size distribution and also primary particle composition within those flocs were measured experimentally for all flows (feed, overflow and underflow). For the case study system at 100-kPa hydrocyclone-operating pressure, there was an improvement in reduced efficiency from 0.75 to 0.84 with flocculation. Contrary to the assumption of literature models, breakage was limited, with the effect of floc hydrodynamics determining the separation behaviour. With density being the most important hydrodynamic effect, the reduced density of flocs compared to primary particles is the main reason for the limited improvement flocculation could achieve in hydrocyclone separation and not, as often suggested, floc breakup. A micro-scale semiempirical model was developed to predict the separation performance of flocculated particles in hydrocyclones. The model represents the relationships between floc hydrodynamics and hydrocyclone classification as well as between floc strength and hydrocyclone shear. Comparison with the experimental results is used to highlight the areas where further work is required to progress understanding.