Abstract:
The South African coal industry produces a
large quantity of coal per annum. The rejects
from various unit operations, such as spirals,
consist of fine coal that joins the plants
tailings dam waste. As existing high quality
resources become depleted, the need to improve
recovery of this fine coal grows. This project
investigates the use of a teetered bed
separator (TBS); a hindered settling gravity
concentration device for fine coal recovery.
This device has proven successful in the
United Kingdom and in Australian collieries
for fine coal separation in the size range
between 2mm and 0.3mm. It has also been used
for decades as a classifying device for silica
sand and tin. The TBS operates in the size
range of water-only cyclones and spiral
concentrators, and could potentially be used
to separate a broader size range of coal fines
so as to offer a lower footprint device for
the fines recovery section of a plant. Spiral
concentrators cannot always be operated
efficiently at a separating specific gravity
of lower than 1.6; a TBS may also extend the
density range for separation and thus improve
recovery. The objective of this project was to
gain a full understanding of the TBS from
fundamental particle interaction and develop a
lab scale unit, which is capable of separation
to about 0.1mm at optimum conditions. This
involved the development of design parameters
based on the various distributor plates and
flow pattern modelling. The hydrodynamics of
the separator were investigated using the
Eulerian-Eulerian modelling approach of
commercial CFD package, Fluent 6.1. Seven
distributor plates of varying aperture size
and geometric arrangement were considered.
Coal and shale particles, sized between 2mm
and 0.038mm with a specific gravity (SG) range
of 1.2 to 2.0, were separated using the
laboratory scale unit. The results of both the
simulations and the laboratory tests were then
compared. The simulations revealed that Plate
3 was the best option for implementation. It
had an even upward velocity profile compared
to the other plates, with minimum wall effects
and disturbances. The upward water flow rate
(teeter water) was varied experimentally and
the composition of the teeter bed, underflow
and overflow were analysed using 1.5, 2 and
Smm cubic density tracers with an SG range of
1.2-2.0. Analysis of the partition curves of
the distributor plates revealed that Plate 3
had the lowest Ecart Probable (Ep) and cut-
point densities. The comparison of simulated
results and experimental results show that the
simulator could predict the distributor plate
design with the lowest Ep in practical tests.
The simulator could be beneficial when
optimising an industrial scale unit, by
allowing prediction of improved segregation
patterns and thus separation efficiency.
