Effective flocculation and dewatering of slurry streams containing clays (particularly kaolin and smectites which are often < 200nm in lateral dimension) is becoming increasingly urgent. Release of water (often > 75 wt%), for recycle and reduced make-up water consumption, from slurries in tailings streams and dam beds is usually slow and incomplete. Changes in settling rates and solids density of tailings containing clays can be achieved in practice by changes in pH, dissolved cations and flocculant additions. To achieve fast separation of clays from water and minimization of retained water, individual particles need to be bound in the initial stages of thickening into large, high-density aggregates, which may sediment more rapidly with lower intra-aggregate water content. Quantitative STIMAN (STructural IMage ANalysis) cryo-SEM imaging, shows that the structure of aggregates formed before flocculant addition has a determinative effect on these outcomes. Without flocculant addition, 4 stages occur in the mechanism of primary dewatering of kaolinite: initially separate particles and aggregates are dispersed (< 0.3GP, where GP is the gel point); a partially-gelled chain network then forms linking particles edge-edge (EE) still well below the GP but remains largely suspended; after an induction time, this network structure rearranges from EE to faceface (FF) contacts densifying, compacting and dewatering the aggregates with settling rates well above Stoke's Law estimates for > 20 mu m particles or aggregates; the GP is rapidly reached during settling producing a network spanning the vessel with compressive strength. This sponge-like structure with EE oriented particles limits slurry dewatering because the steric effect in the resulting. partially-gelled structure is much more dominant than the electrostatic effects predicted by DLVO theory. Van der Waal's forces at very close distances, as particles slide across; each other, may be responsible for the EE to FF change in structure. In this way, the previously extended network contracts, and when the solid volume fraction is low, the continuous network breaks apart forming aggregates and aggregate associations as flocs which are able to freely settle. These densified aggregates settle at increased rate, give lower bed density and retain less water after drainage. With flocculant addition, the internal structure and networking of the pre-aggregates is largely preserved but they are rapidly and effectively bound together by the bridging action of the flocculant(s). The STIMAN analysis shows that there is a inverse correlation of intra-aggegate porosity (and void pertneability) with Darcian permeability (and extent of dewatering) whereas there is a strong positive correlation of Darcian interaggregate permeability with settling and primary dewatering rate.
