In this paper, novel techniques have been assessed to quantify the extent and time dependence of shear enhancement within a pilot thickener using both data from the thickener and a laboratory based fluidisation rig. Work was conducted in the absence of mechanical shear in the case of the pilot-thickener work and both with and without mechanical shear in the fluidisation rig study. The level of solids permeability enhancement inferred as a consequence of aggregate densification was measured and the data were surprisingly consistent between both devices. Even in the absence of mechanical shear and at concentrations below the gel point, aggregate densification was observed to result in permeability enhancements of order 2 to 9 times those predicted from laboratory batch settling tests on the same material. These improvements were predicted to have resulted from only modest densifications, in which the aggregates were calculated to have densified to around 86 to 88% of their original size. The time taken to achieve such densification was of the order of 4000 s.
Laboratory data typically underestimates thickener performance, even at the pilot scale, and the agreement between laboratory and pilot results presented herein represents a novel outcome for the characterisation of material dewatering properties. Operation of the pilot thickener and fluidisation rig in a flux regime such that all suspension concentrations were less than the gel point ensured that results were free of effects due to networked bed consolidation. The results indicate that mechanical shear (e.g. raking) in the hindered zone in a thickener may be just as beneficial as in the consolidated thickener bed. This is consistent with in-field observations (Abbott, 1979 and Loan and Arbuthnot, 2010). To our knowledge, this marks the first time in which permeability enhancement has been quantified solely due to buffeting of aggregates in steady state thickening. The shear rates to achieve the observed outcomes in the absence of mechanical shear are unquantified and assumed to be quite low, since even a small amount of mechanical shear imposed in the fluidisation rig significantly increased the rate and extent of densification over and above fluidisation alone.
Despite the increased rate and extent of densification with mechanical shear, the calculated hindered settling function data overlap for all scenarios, indicating that concentration is an important parameter in relation to changes in aggregate structure. The observations herein are an important step towards full-scale thickener modelling incorporating aggregate densification effects due to shear.