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


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

Download citation as:   ris   bibtex   endnote   text   Zotero


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.

References:
Aziz, A.A.A., de Kretser, R.G., Dixon, D.R. and Scales, P.J. (2000) The Characterisation of Slurry Dewatering, Water
Science and Technology, Vol. 41(8), pp. 9–16.
Bürger, R., Concha, F., Fjelde, K.K. and Karlsen, K.H. (2000a) Numerical simulation of the settling of polydisperse
suspensions of spheres, Powder Technology, Vol. 113(1–2), pp. 30–54.
Bürger, R., Evje, S., Hvistendahl Karlsen, K. and Lie, K.A. (2000b) Numerical methods for the simulation of the
settling of flocculated suspensions, Chemical Engineering Journal, Vol. 80, pp. 91–104.
Bürger, R., García, A., Karlsen, K.H. and Towers, J.D. (2008) A kinematic model of continuous separation and
classification of polydisperse suspensions, Computers and Chemical Engineering, Vol. 32(6), pp. 1173–1194.
Buscall, R. and White, L.R. (1987) The Consolidation of Concentrated Suspensions, J. Chem. Soc. Faraday Trans. I 83,
pp. 873–891.
Channell, G.M., Miller, K.T. and Zukoski, C.F. (2000) Effects of microstructure on the compressive yield stress, AIChE
Journal, Vol. 46(1), pp. 72–78.
Comings, E.W., Pruiss, C.E. and DeBord, C. (1954) Continuous settling and thickening, Ind. Eng. Chem., Vol. 46(6),
pp. 1164–1172.
de Kretser, R.G., Usher, S.P. and Scales, P.J. (2005) Comprehensive Dewatering Behaviour Analysis for Fine and
Flocculated Materials, American Filtration and Separations Society, 18th Annual Conference, Atlanta GA USA,
April 10–13 2005, American Filtration and Separations Society.
de Kretser, R.G., Usher, S.P., Scales, P.J., Boger, D.V. and Landman, K.A. (2001) Rapid Filtration Measurement of
Dewatering Design and Optimization Parameters, AIChE J., Vol. 47(8), pp. 1758–1769.
Farrow, J.B., Johnston, R.R.M., Simic, K. and Swift, J.D. (2000) Consolidation and aggregate densification during
gravity thickening, Chemical Engineering Journal, Vol. 80(1–3), pp. 141–148.
Garrido, P., Burgos, R., Concha, F. and Bürger, R. (2003) Software for the design and simulation of gravity thickeners,
Minerals Engineering, Vol. 16, pp. 85–92.
Gladman, B.R. (2004) The Effect of Shear on Dewatering of Flocculated Suspensions, PhD in Department of Chemical
and Biomolecular Engineering, Melbourne, Australia, The University of Melbourne, 332 p.
Gladman, B.R., de Kretser, R.G., Rudman, M. and Scales, P.J. (2005) Effect of shear on particulate suspension
dewatering, Trans IChemE, Part A, Chemical Engineering Research and Design, Vol. 83(A7), pp. 933–936.
Gladman, B.R., Usher, S.P. and Scales, P.J. (2006a) Compressive rheology of aggregated particulate suspensions,
Korea Australia Rheology Journal, Vol. 18(4), pp. 191–197.
Gladman, B.R., Usher, S.P. and Scales, P.J. (2006b) Understanding the thickening process, in Proceedings of the Ninth
International Seminar on Paste and Thickened Tailings, Australian Centre for Geomechanics, Perth, Australia,
pp. 5–12.
Grassia, P., Usher, S.P. and Scales, P.J. (2008) A simplified parameter extraction technique using batch settling data to
estimate suspension material properties in dewatering applications, Chem. Eng. Sci., Vol. 63, pp. 1971–1986.
Green, M.D. (1997) Characterisation of Suspensions in Settling and Compression, PhD in Department of Chemical
Engineering, Melbourne, Australia, The University of Melbourne, 246 p.
Shear effects in thickening S.P. Usher et al.
384 Paste 2010, Toronto, Canada
Johnson, S.B., Scales, P.J., Dixon, D.R. and Pascoe, M. (2000) Use of a superthickener device to concentrate potable
water sludge, Water Research, Vol. 34(1), pp. 288–294.
Landman, K.A. and White, L.R. (1994) Solid/Liquid Separation of Flocculated Suspensions, Advances in Colloid and
Interface Science, Vol. 51, pp. 175–246.
Landman, K.A., White, L.R. and Buscall, R. (1988) The continuous flow gravity thickener: Steady state behaviour,
AIChE J., Vol. 34, pp. 239–252.
Lester, D.R. (2003) Colloidal Suspension Dewatering Analysis, PhD in Department of Chemical and Biomolecular
Engineering, Melbourne, Australia, The University of Melbourne, 227 p.
Lester, D.R., Usher, S.P. and Scales, P.J. (2005) Estimation of the Hindered Settling Function R(φ) from Batch-Settling
Tests, AIChE J., Vol. 51(4), pp. 1158–1168.
Martin, A.D. (2004) Optimisation of clarifier-thickeners processing stable suspensions for turn-up/turn-down, Water
Research, Vol. 38, pp. 1568–1578.
Novak, J.T. and Bandak, N. (1994) The effect of shear on the dewatering of water treatment residuals, AWWA,
Vol. 86(11), pp. 84–91.
Stickland, A.D. (2005) Solid-Liquid Separation in the Water and Wastewater Industries, PhD in Department of
Chemical and Biomolecular Engineering, The University of Melbourne, 461 p.
Stickland, A.D., de Kretser, R.G., Scales, P.J., Usher, S.P., Hillis, P. and Tillotson, M.R. (2006) Numerical modelling of
fixed-cavity plate-and-frame filtration: Formulation, validation and optimisation, Chemical Engineering Science,
Vol. 61, pp. 3818–3829.
Usher, S.P. (2002) Suspension Dewatering: Characterisation and Optimisation, PhD in Department of Chemical
Engineering. Melbourne, Australia, The University of Melbourne, 347 p.
Usher, S.P., de Kretser, R.G. and Scales, P.J. (2001) Validation of a New Filtration Technique for Dewaterability
Characterization, AIChE J., Vol. 47(7), pp. 1561–1570.
Usher, S.P. and Scales, P.J. (2005) Steady state thickener modelling from the compressive yield stress and hindered
settling function, Chemical Engineering Journal, Vol. 111(2–3), pp. 253–261.
Usher, S.P. and Scales, P.J. (2009) Predicting settler/clarifier behaviour: The role of shear effects, Filtration, Vol. 9(4),
pp. 308–314.
Usher, S.P., Spehar, R. and Scales, P.J. (2009) Theoretical analysis of aggregate densification: Impact on thickener
performance, Chemical Engineering Journal, Vol. 151(1–3), pp. 202–208.
Vesilind, P.A. and Jones, G.N. (1993) Channelling in batch thickening, Water Science and Technology, Vol. 28(1),
pp. 59–65.




© Copyright 2019, Australian Centre for Geomechanics (ACG), The University of Western Australia. All rights reserved.
Please direct any queries or error reports to repository-acg@uwa.edu.au