The influence of shear on the dewaterability of flocculated particulate systems
AffiliationEngineering Collected Works
Document TypePhD thesis
Access StatusOpen Access
© 2017 Dr. Ashish Kumar
Solid-liquid separation is used in a wide range of industries, including wastewater treatment, biomass recovery and the mining industry. The use of thickening agents such as polymeric flocculants helps to capture fine suspended particles to form flocs, eventually settling to form a sediment bed at the base of the thickener or clarifier. The bed undergoes a combination of shearing (from rakes or pickets) and compressional forces (arising from the weight of solid material in the bed) which serves to densify the settled material while further increasing the concentration of the underflow. Shear forces will cause fractal aggregates, or flocs, to become more compact - a process called aggregate densification. However, excessive shear is detrimental causing aggregate breaking, resulting in increased fines, higher resistance to separation, longer residence times, and poorer overflow clarity. The optimum shear for a thickener is difficult to ascertain, due to the complexities of picket and rake design, thickener sizes, and material properties. Rheology is expected to provide a sensitive, controlled method to probe these densification processes – the expectation is that the failure of networked sediment beds depends on the critical strain, γc, rather than a shear stress or shear rate. This parameter is well known in the rheological characterisation of polymer melts, but has not been applied to flocculated mineral suspensions. This work presents the novel use of a vane-in-large cup configuration to characterise flocculated suspensions using stress- and strain controlled rheometers. The cup size plays an important role for suspensions with large particles, as these tend to span the annulus and affect data accuracy. As such, a sensitivity analysis was performed to compare the creep rheology of coagulated (at iso-electric point) and polymer flocculated alumina suspensions using different cup-to-vane diameter ratios, dc/dv. Measurements with dc/dv > 3 demonstrated consistent behaviour, greater reproducibility, and fewer fluctuations in the strain rate data. Configurations below this ratio may be suspect, and contain artefacts of wall contributions, particle migration, bridging effects and force chain jamming. The optimum dc/dv ratio was then used to perform creep, recovery, stressrelaxation, small and large amplitude oscillatory shear (SAOS/LAOS) rheology on coagulated and polymer flocculated alumina at different solids volume fractions, ϕ. LAOS data was interpreted using recent non-linear viscoelastic measures, Fourier Transform (FT) rheology and Lissajous-Bowditch plots to successfully identify a common critical strain value γc = 0.01 for non-linear failure. This value was consistent for both coagulated and flocculated samples, independent of solids volume fraction and applied stress, pointing to the idea that a flocculated particulate network must shift by a critical distance in order to undergo non-linear deformation. This value of γc is very small, and highlights the sensitive nature of polymer flocculated suspensions, and how susceptible they are to shear effects. This information is invaluable in promoting aggregate densification and mitigating aggregate breakage while optimising energy delivery to raking systems. A characterisation suite comprising stepped pressure filtration, batch settling tests, and vane rheology was applied to two model flocculated calcite systems, and one industrial flocculated bauxite residue sample to develop constitutive models describing Herschel-Bulkley fitting parameters, and to relate compressive yield stress, Py(ϕ), to the vane yield stress, ty(ϕ). These models provide on-site operators with powerful tools to predict and size processes such as paste backfilling, dry stacking and filtration belts.
Keywordsdewatering; vane yield stress; rheology; critical strain; nonlinear viscoelasticity; mineral tailings
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