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PROF. M. LE ROUX, DR N. NAUDÉ & DR S. BADA
DRY COAL BENEFICIATION
Introduction
In principle, dry technologies are similar to most wet
beneficiation techniques that are currently industrialised.
These operate based on the relative movement of a particle
within a medium in accordance to the difference in said
particle and medium density. In the simplest terms, particles
denser than the medium sink and those less dense, float.
One of the preferred method of dry dense medium separation
is proving to be the Air Dense Medium Fluidised Bed (ADMFB)
technology. ADMFB has been used extensively in China for dry
coal processing, however, more recently it has fallen out of
favour, mostly due to suspected difficulty in separation of the
medium and product/discards.
In ADMFB technology, air is blown through a particulate bed
of suitable dense medium at a specific velocity. This forms a
fluidised bed of particles with pseudo-fluid properties such as
(and most applicable to this study) density. Upon the addition
of coal to the suspended medium, stratification occurs and the
heavier coal particles (associated with high mineral content
and low calorific value) sink to the bottom of the bed, while the
lighter, better quality particles float. Experimentally, a mixture
of coal and medium is loaded into a column and fluidised
with air for a period of time. After completion the particulate
suspension is allowed to settle into a packed bed state which is
cut in layers of specific height. From here the coal is separated
from the dense medium and sampled for analyses. The extent
of beneficiation is observed from the quality of the coal in
terms of the ash yield, calorific value and density, reporting to
each layer.
This total project was split into five subsets and divided
between three South African universities as follow:
1. ADMFB operation of -6mm particles done at North-West
University, 2014-2015
2. ADMFB operation of -13mm +6mm particles done at North-
West University, 2016-2017
3. Dry medium recovery with the use of magnetic separation
done at the University of Pretoria, 2016-2017
4. The recovery of ilmenite (medium) froma dry densemedium
fluidized bed done at the University of Pretoria, 2016-2017
5. The cost evaluation of different medium solids for an air
dense medium fluidised bed coal separator done at the
University of the Witwatersrand, 2016-2017
North-West University: ADMFB of -6mm particles
Completed at the end of 2015 by Danie Langner when he
graduated with an M.Eng. degree in Chemical Engineering, the
findings of the project were reported to Coaltech in a previous
report. They are summarised below to provide a holistic view
on the total project.
The purpose of this sub-project was to determine whether
the ADMFB can successfully remove impurities from the duff
(small) coal particles (-5.6 +0.5mm). A Seam 4 run-of-mine
coal, from Witbank, was used during the project which had an
initial ash yield percentage of 22.95% and a CV of 24.16MJ/kg.
The fluidised bed was operated with and without vibration, and
the influence of different dense media is tested. Magnetite,
sand and a fine discard coal were used as fluidising medium.
It was found that the addition of magnetite was able to
segregate coal particles according to density, but did not
improve the destoning capabilities of the fluidised bed.
This was mainly attributed to the very fine nature of the
magnetite, which caused back mixing, plug-flow and turbulent
behaviour within the bed. Furthermore the separation of
magnetite from the coal was troublesome.
From all the different dense media, sand was found to give
the best separation efficiencies. The use of sand as fluidising
medium created a more stable bed, which aided in the density
separation. Therefore, the results indicated that sand is a
lucrative alternate to magnetite.
A fine discard coal, used as a fluidising medium, gave moderate
results. However the destoning capabilities of the fluidised bed,
with the use of this fine discard medium, were in most cases
better than when magnetite was used. Hence, the addition of a
fluidising medium did not improve the separation efficiencies
of the fluidised bed, and the fluidisation of coal without any
medium gave the best results.
The addition of vibration did not have a significant impact
on the destoning capabilities of the fluidised bed, when no
medium was added. However, when a medium was used to
beneficiate the coal, the addition of vibration improved the
sharpness of separation. It was concluded that the use of a
fluidised bed is a viable process in order to remove high ash
value material from a typical South Africa coal with a size range
between 0.5 and 5.6mm.
North-West University: ADMFB of -13mm +6mm particles
The project on the larger coal particle ranges in the fluidised
bed was done by Nikki Hughes and completed in 2017 when
she graduated cum laude with a Master’s degree in Chemical
Engineering. This project is summarised below.
The purpose of this study was to determine whether ADMFB
is suited for the effective beneficiation of South African small
(+5.6-13.2mm) coal particles. The separation efficiency of the
fluidised bed, (how effectively high-density gauge and low-
density coal are separated in the ADMFB), was evaluated by
considering the following objectified variables the effects
of differing particle size distributions, variance in medium-
to-coal ratios and the influence of activated vibration.
Auxiliary investigations toward the quality of the feed coal
were also conducted to further comprehend the significance
of this variable on the separation efficiency of the fluidised bed.
