(212e) Particle Segregation in a Liquid Fluidized Bed Incorporating Inclined Channels Subjected to Centrifugal Forces

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, u_t, (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 20^o 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 d^n-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, u<sub>tT</sub>,
in the tangent direction of the channel, was found to be approximately U`/u_tT=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`/u<sub>tT</sub>=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`/u_tT 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 d^0.3.
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.

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