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One-dimensional computational dewatering models which account for the influence of networked particulate
bed compression, have enabled accurate predictions of dewatering in sedimentation, centrifugation and
filtration processes in both the laboratory and industry. However, in certain processes, such as gravity
thickening, the models have been observed to under predict the performance (in terms of solids throughput)
by up to a factor of 100 for a given underflow solids concentration. This discrepancy can be credited to
shear and channelling effects on the fundamental dewatering material properties which are unaccounted for
in current models. The expectation when shear is applied is that local pressure gradients will be produced
resulting in the expulsion of water from the aggregates and subsequent densification. This aggregate
densification phenomenon has been shown to be very significant for flocculated aggregates in the presence
of shear and/or a solid surface. Shear effects can involve raking, flow near sloped walls and also buffeting of
aggregates in un-networked fluidised regions of the thickener.
Experiments that look to characterise this behaviour have shown that there is an optimum shear rate beyond
which a further increase is detrimental to performance. This optimum is a trade off between the densification
of aggregates which dominates at low shear rates and the disintegration of aggregates which becomes more
significant as shear rate and time of shear increases. Changes to the extent of aggregate densification can be
inferred from changes in the material properties with shear and residence time in raked settling tests,
Couette-fluidisation tests and pilot thickener operations. Analysis of data from such tests enables the
development of more sophisticated dewatering models that can predict the rate of densification as a function
of the local shear rate. This type of analysis has the potential to identify whether aspects of full scale
thickener design and operation actually enhance or diminish dewatering performance.
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