Scales, PJ, Crust, AH & Usher, S 2015, 'Thickener modelling – from laboratory experiments to full-scale prediction of what comes out the bottom and how fast', in R Jewell & AB Fourie (eds), Paste 2015: Proceedings of the 18th International Seminar on Paste and Thickened Tailings
, Australian Centre for Geomechanics, Perth, pp. 3-12, https://doi.org/10.36487/ACG_rep/1504_0.1_Scales
Predicting full-scale thickener performance, including the solids flux and concentration delivered from a thickener underflow based solely on laboratory-scale experiments, has long been the holy grail of thickener design and operation. A number of researchers have developed both thickener models and laboratory characterisation techniques to measure sedimentation and compressional properties of flocculated suspensions. Combining these to produce predictions of actual performance generally results in an
under-estimation of the thickener solids flux, often by a factor of between 10 and 20. Consequently, a range of empirical methods and industry scalars has been developed to get around this discrepancy.
Analysis of the reasons for the discrepancy shows that changes in aggregate structure in shear, due to
inter-aggregate buffeting and shear induced by collisions with surfaces and rakes, causes the flocculated aggregate to change from a fractal to a denser non-fractal object as it progresses through the thickener. These changes are shear rate and solids concentration dependent and as such, very difficult to reproduce in the laboratory and then incorporate into thickener models.
A method to quantify the time and shear rate dependent changes in aggregate structure is now available and a model has been developed that allows incorporation of this effect into modelling. The change in aggregate behaviour is incorporated through a shear rate dependent densification rate and final extent of aggregate densification. The latter parameter helps to define an upper limit in solids flux behaviour for a given solids underflow concentration. Using the new information, thickener models now predict a range of underflow solids flux outcomes between the upper (densified aggregate) and lower (undensified aggregate) limit for a particular underflow solids concentration, depending on the operational conditions. The difference in underflow solids flux between these two limits is significant and the actual outcome depends on the shear rate and time of shear, as well as total solids residence time in the thickener.
The data indicate that for non-segregating flocculated suspensions, operational conditions that produce the optimum thickener underflow solids flux for a given flocculation condition can now be explored quantitatively without resorting to extensive pilot trials.
Auzerais, FM, Jackson, R & Russel, WB 1990, ‘The transient settling of stable and flocculated dispersions’, Journal of Fluid Mechanics, vol. 221, pp. 613-639.
Bergstrom, L, Schilling, CH & Aksay, IA 1992, ‘Consolidation behaviour of flocculated alumina suspensions’, Journal of the American Ceramic Society, vol. 75, pp. 3305-3314.
Bürger, R, Concha, F & Karlsen, KH 2001, ‘Phenomenological model of filtration processes: 1. Cake formation and expression’, Chemical Engineering Science, vol. 56, pp. 4537-4553.
Bürger, R & Wedland, WL 1998, ‘Entropy boundary and jump conditions in the theory of sedimentation with compression’, Mathematical Methods in the Applied Sciences, vol. 21, pp. 865-882.
Buscall, R & White, LR 1987, ‘The consolidation of concentrated suspensions’, Journal of the Chemical Society, Faraday Transactions 1, vol. 83, pp. 873-891.
Bustos, MC, Concha, F, Burger, R & Tory, EM 1999, Sedimentation and Thickening, Kluwer Academic Publishers, Dordrecht.
Coe, HS & Clevenger, GH 1916, ‘Methods for determining the capacities of slime settling tanks’, AIME Transactions, vol. 55,
Comings, EW, Pruiss, CE & DeBord, C 1954, ‘Continuous settling and thickening’, Industrial and Engineering Chemistry, vol. 46,
de Kretser, RG, Usher, SP, Scales, PJ, Boger, DV & Landman, KA 2001, ‘Rapid filtration measurement of dewatering design and optimization parameters’, AIChE Journal, vol. 47, no. 8, pp. 1758-1769.
Diehl, S 2000, ‘On boundary conditions and solutions for ideal clarifier-thickener units’, Chemical Engineering Journal, vol. 80, pp. 119-133.
Farrow, J, Johnston, R, Simic, K & Swift, J 2000, ‘Consolidation and aggregate densification during gravity thickening’, Chemical Engineering Journal, vol. 80, pp. 141-148.
Fawell, PD, Farrow, JB, Heath, AR, Nguyen, TV, Owen, AT, Paterson, D, Rudman, M, Scales, PJ, Simic, K, Stephens, DW, Swift, JD, & Usher, SP 2009, ‘20 years of AMIRA P266 “Improving Thickener Technology” - how has it changed the understanding of thickener performance?’, in RJ Jewell, AB Fourie, S Barrera & J Wiertz, Proceedings of the 12th International Seminar on Paste and Thickened Tailings, Australian Centre for Geomechanics, Perth, pp. 59-68..
Fitch, B 1966, ‘Current theory and thickener design’, Industrial and Engineering Chemistry, vol. 58, pp. 18-28.
Fitch, B 1971, ‘Batch tests predict thickener performance, Chemical Engineering, 23 August, pp. 83-88.
Gibson, RE, England, GL & Hussey, MJL 1967, ‘The theory of one dimensional consolidation of saturated clays’, Géotechnique, vol. 17, pp. 261-273.
Gladman, B, de Kretser, RG, Rudman, M & Scales, PJ 2005, ‘Effect of shear on particulate suspension dewatering’, Chemical Engineering Research & Design, vol. 83 (A7), pp. 933-936.
Gladman, BR, Rudman, M & Scales, PJ 2010, ‘The effect of shear on gravity thickening: pilot scale modelling’, Chemical Engineering Science, vol. 65, issue 14, pp. 4293-4301.
Grassia, P, Zhang, Y, Martin, AD, Usher, SP, Scales, PJ, Crust, A & Spehar, R 2014, ‘Effects of aggregate densification upon thickening of Kynchian suspensions’, Chemical Engineering Science, vol. 111, pp. 56-72.
Heath, AR, Bahri, PA, Fawell, PD & Farrow, JB 2006a, ‘Polymer flocculation of calcite: sxperimental results from turbulent pipe flow’, AIChE Journal, vol. 52, pp. 1284-1293.
Heath, AR, Bahri, PA, Fawell, PD & Farrow, JB 2006b, ‘Polymer flocculation of calcite: Population balance model’, AIChE Journal, vol. 52, pp. 1641-16531.
Howells, I, Landman, KA, Panjkov, A, Sirakoff, C & White, LR 1990, ‘Time dependent batch settling of flocculated suspensions’, Applied Mathematical Modelling, vol. 14, pp. 77-86.
Karl, JR 1999, ‘Numerical model of sedimentation/thickening with inertial effects’, Journal of Environmental Engineering, vol. 125, pp. 792-806.
Kynch, GJ 1952, ‘A theory of sedimentation’, Transactions of the Faraday Society, vol. 48, pp. 166-176.
Landman, KA, White, LR & Buscall, R 1988, ‘The continuous flow gravity thickener: Steady state behaviour’, AIChE Journal, vol. 34, pp. 239-252.
Landman, KA, White, LR & Eberl, M 1995, ‘Pressure filtration of flocculated suspensions’, AIChE Journal, vol. 41, pp. 1687-1699.
Lester, DR, Rudman, M & Scales, PJ 2010, ‘Macroscopic dynamics of flocculated colloidal suspensions’. Chemical Engineering Science, vol. 65, no. 24, pp. 6362-6378.
Lester, DR, Usher, SP & Scales, PJ 2005, ‘Estimation of the hindered settling function R(phi) from batch-settling tests’. AICicheE Journal, vol. 51, no. 4, pp. 1158-1168.
Loan, C & Arbuthnot, I 2010, ‘Innovative technology for optimised thickening sedimentation’, Proceedings of the XXV International Mineral Processing Congress, Australasian Institute of Mining and Metallurgy, Melbourne.
Michaels, AS & Bolger, JC 1962, ‘Settling rates and sediment volumes of flocculated kaolin suspensions’, I&EC Fundamentals, vol. 1, no. 1, pp. 24-33.
Owen, AT, Fawell, PD, Swift, JD, Labbett, DM, Benn, FA & Farrow, JB 2008, ‘Using turbulent pipe flow to study the factors affecting polymer-bridging flocculation of mineral systems’, International Journal of Mineral Processing, vol. 87, no. 3-4, pp. 90-99.
Rudman, M, Simic, K, Paterson, DA, Strode, P, Brent, A & Sutalo, ID 2008, ‘Raking in gravity thickeners’, International Journal of Mineral Processing, vol. 86, no. 1-4, pp. 114-130.
Scott, KJ 1970, ‘Continuous thickening of flocculated suspensions. Comparison with batch settling tests and effects of floc compression using pyrophyllite pulp’, Industrial and Engineering Chemistry Fundamentals, vol. 9, pp. 422-427.
Sofra, F & Boger, DV 2002, ‘Environmental rheology for waste minimisation in the minerals industry’, Chemical Engineering Journal, vol. 86, no. 3, pp. 319-330.
Spehar, R 2014, ‘Modelling the role of shear in compressional dewatering’, PhD thesis, University of Melbourne, Melbourne.
Spehar, R, Kiviti-Manor, A, Fawell, PD, Usher, SP, Rudman, M & Scales, PJ 2015, ‘Aggregate densification in the thickening of flocculated suspensions in an un-networked bed’, Chemical Engineering Science, vol. 122, 585-595.
Stickland, AD, de Kretser, RG, Kilcullen, AR, Scales, PJ, Hillis, P & Tillotson, MR 2008, ‘Numerical Modeling of flexible-membrane plate-and-frame filtration’, AIChE Journal, vol. 54, no. 2, pp. 464-474.
Usher, SP, de Kretser, RG & Scales, PJ 2001, ‘Validation of a new filtration technique for dewaterability characterization’, AIChE Journal, vol. 47, no. 7, pp. 1561-1570.
Usher, SP & Scales, PJ 2005, ‘Steady state thickener modelling from the compressive yield stress and hindered settling function’, Chemical Engineering Journal, vol. 111, no. 2-3, pp. 253-261.
Usher, SP & Scales, PJ 2009, ‘Predicting settler/clarifier behaviour: the role of shear effects,’ Filtration, vol. 9, pp. 308-314.
Usher, SP, Spehar, R & Scales, PJ 2009, ‘Theoretical analysis of aggregate densification: impact on thickener performance’, Chemical Engineering Journal, vol. 151, pp. 202-208.
van Deventer, BBG, Usher, SP, Kumar, A, Rudman, M & Scales, PJ 2011, ‘Aggregate densification and batch settling’, Chemical Engineering Journal, vol. 171, no. 1, pp. 141-151.
Vesilind, PA & Jones, GN 1993, ‘Channelling in batch thickening’, Water Science and Technology, vol. 28, pp. 59-65.
Zhang, Y, Grassia, P & Martin, AD 2013, ‘Prediction of thickener performance with aggregate densification’, Chemical Engineering Science, vol. 101, pp. 346-358.