263513 Particle Segregation in a Liquid Fluidized Bed Incorporating Inclined Channels Subjected to Centrifugal Forces

Tuesday, October 30, 2012: 9:50 AM
Conference C (Omni )
Kevin P. Galvin, Centre for Advanced Particle Processing and Transport, Newcastle Institute for Energy and Resources, The University of Newcastle, Callaghan, Australia and James E. Dickinson, Centre for Advanced Particle Processing and Transport, Newcastle Institute for Energy and Resources, The University of Newcastle

A liquid fluidized bed was used to elutriate and hence separate particles on the basis of size and also density, with the ratio of the superficial fluidization velocity, U, to particle terminal velocity, ut, (terminal velocity at g=9.8 ms-2) exceeding 1000 fold. This ratio is extraordinary because the centrifugal acceleration involved was much lower at 73g. The fluidized bed system incorporated a system of parallel channels inclined at an angle of 20o to the direction of the centrifugal force. Inclined channels have been deployed previously in a centrifugal field. However, this is the first study to also introduce fluidization, allowing efficient separation to be achieved. Moreover, it was shown that the combination of the inclined channels and the centrifugal force helped to almost fully suppress the effects of particle size over a wide size range.

The processing of ultrafine particles less than 0.1 mm is often limited by their exceedingly low terminal settling velocity. This is especially true when the particles need to remain dispersed in order to facilitate their classification, either by size or by density. Centrifugal forces, with an acceleration G times the normal gravitational value, offer one approach for increasing the particle settling velocity by a significant factor, up to a maximum of G. The particle Reynolds number then increases significantly, ultimately leading to a change in the settling regime from the Stokes' to Intermediate, and hence dependence of the terminal velocity on the particle diameter, from d2 to d1. This reduction in the dependence on the particle size also leads to improved separation on the basis of the particle density, hence centrifugal separations in mineral processing are referred to by the term, Enhanced Gravity.

Inclined channels have also been used to achieve a significant increase in the segregation rate of particles, exploiting the so-called “Boycott Effect” (Boycott, 1920) by presenting an increased settling area to the suspension of particles. Galvin et al (2009) and later Galvin and Liu (2011) investigated the segregation achieved using closely spaced inclined channels designed to promote a laminar flow profile and a high shear rate at the planar surfaces. They applied an elutriation condition that equated the local fluid velocity acting on the particle at the wall with the terminal velocity of the particle. In classifying particles with a terminal velocity dependent on dn, the superficial velocity was found to scale with dn-1, hence there was a full decade reduction in the dependence on the particle diameter. Further, the ratio of the superficial fluidization velocity, U`, through the channel relative to the terminal velocity, utT, in the tangent direction of the channel, was found to be approximately U`/utT=z/(3d).

This study was concerned with the combined effect of applying a centrifugal force to particles fluidized and then conveyed into planar channels inclined to the centrifugal force. The key question was whether the benefits of the inclined channels and the centrifugal force would multiply, delivering a significant throughput advantage. The Coriolis Force is known to impact negatively on the benefits of the inclined channels rotating in a centrifuge (Schaflinger, 1990), however, by utilizing a confined fluidization housing, it was possible to overcome this problem. A further objective of the study was to examine, for the first time, whether the benefits of the inclined channels and the centrifugal force could combine to eliminate the effect of particle size on the separations, potentially from d2 to almost d0, thereby fully suppressing the effect of particle size, and hence amplifying the significance of the particle density.

Indeed, both of these effects were observed, with U`/utT=gz/(3d). The superficial fluidization velocity within the inclined channels was as high as 0.34 m/s in some experiments involving separations of silica in water as fine as 7.5 microns. Here the value of U`/utT reached 3300 at g=73. Moreover, relatively low density particles were observed to elutriate, with the dependence of the superficial fluid velocity on the particle diameter scaling with d0.3. A powerful mechanism for separating particles on the basis of density was established, utilising the combined effects of inclined channels and a centrifugal force.

Acknowledgement
The authors acknowledge the technical support of Mark Mason in the experimental work, the workshop support of David Roberts and William Grant, and financial support of the Australian Research Council and the Australian Coal Association Research Program. The provision of the centrifuge by Ludowici Australia is also greatfully acknowledged.

References

Boycott, A.E., 1920, Sedimentation of Blood Corpuscles. Nature, 104, 532

Galvin, K.P., Walton, K., and Zhou, J., 2009, How to Elutriate Particles According to their Density, Chemical Engineering Science, 64, 2003-2010

Galvin, K.P., Walton, K., Zhou, J., 2010, Application of Closely Spaced Inclined Channels in Gravity Separation of Fine Particles, Minerals Engineering, 23 326-338

Galvin, K.P., and Liu, H., 2011, Role of Inertial Lift in Elutriating Particles According to their Density, Chemical Engineering Science, 66 3686-3691

King, M.R., and Leighton, D.T., 1997, Measurement of the inertial lift on a moving sphere in contact with a plane wall in shear flow, Physics of Fluids 9(5), 1248-1255

Nakamura H. and Kuroda K., 1937. La Cause de l'acceleration de la Vitesse de Sedimentation des Suspensions dans les Recipients Inclines. Keijo Journal of Medicine, 8, 256– 296

Ponder P., 1925. On sedimentation and Rouleaux formation. Quarterly Journal of Experimental Physiology. 15, 235–252

Schaflinger, Uwe, 1990, Review Article. Centrifugal Separation of a Mixture, Fluid Dynamics Research, 6, 213-249


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