Shear effects in thickening

Authors: Usher, SP; Spehar, R; Scales, PJ

DOI https://doi.org/10.36487/ACG_rep/1063_32_Usher

Cite As:
Usher, SP, Spehar, R & Scales, PJ 2010, 'Shear effects in thickening', in R Jewell & AB Fourie (eds), Paste 2010: Proceedings of the Thirteenth International Seminar on Paste and Thickened Tailings, Australian Centre for Geomechanics, Perth, pp. 375-384, https://doi.org/10.36487/ACG_rep/1063_32_Usher

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Abstract:
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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