2
readily captured by screenbowl centrifuges. An opportunity
for incremental plant efficiency gains at the Leer prepara-
tion plant was identified in the discarded deslime cyclone
overflow stream containing low-ash ultrafine coal.
Ultra-Fine Coal Recovery Challenges
Recovery of ultrafine coal typically discarded in a deslime
flotation circuit presents several challenges due to particle
size distribution and high flow volumes [4]. Conventional
mechanically agitated flotation cells, commonly used in
coal preparation, allow some portion of the ultrafine min-
eral slimes to be recovered with the water that reports to
the froth. This particle entrainment is the non- selective
hydraulic conveyance of gangue into the product launder.
The liquid phase that holds the froth together and main-
tains mobility also carries fine particles that have not been
collected through a bubble/particle attachment. Fine par-
ticles (0.045-mm) tend to report to the froth concentrate
in direct proportion to the amount of product water. As
a result, mechanically agitated flotation cells are often not
able to meet product ash requirements for ultrafine coal, or
only can do so at a substantial loss in product yield [5-6].
Column flotation technology is a proven alternative
to conventional mechanically agitated flotation cells for
fine coal recovery. A distinguishing aspect of column flo-
tation is the formation of a deep froth bed that facilitates
froth washing to remove unwanted impurities from the
float product. Wash water, added at the top of the column,
percolates through the froth zone displacing dirty pro-
cess water and non-selectively entrained particles trapped
between the bubbles. In addition, froth wash water serves
to stabilize and add mobility to the froth. Also, the use of
robust sparging systems in column flotation technology
ensures high combustible recoveries through the generation
of fine bubbles that maximize bubble surface area flux and
gas holdup [4].
The primary advantage of utilizing wash water is the
ability to provide superior separation performance com-
pared to conventional flotation processes. As reported by
Davis et al. [7], the separation performance for column
flotation cells utilizing wash water is far superior to that
obtained from conventional flotation cells. In fact, the data
for the column cells tend to fall just below the separation
curve predicted by release analysis, which is an indication
of the ultimate flotation performance for fine coal [8].
Columns provide a level of performance that would be dif-
ficult to achieve even after multiple stages of cleaning by
conventional mechanically agitated flotation cells.
Another economical constraint for ultrafine coal recov-
ery is associated with the dilute flotation feed, which typi-
cally ranges between 2.0% and 6.0% solids, by weight. As
a result, volume flows in these circuits can be quite large.
Consequently, significant cell volume must be available to
achieve the desired retention time to ensure adequate com-
bustible recovery. The ultimate volume is typically based
on kinetic data generated from laboratory testing, but is
generally constrained by practical limits as dictated by both
economic and site-specific engineering requirements [4].
Finally, the dewatering of ultrafine coal represents an
additional challenge to economic recovery. Traditional
dewatering methods, such as screen bowl centrifuges, are
less efficient for ultrafine particles due to high losses in
Figure 1. Deslime coal flotation circuit flowsheet
readily captured by screenbowl centrifuges. An opportunity
for incremental plant efficiency gains at the Leer prepara-
tion plant was identified in the discarded deslime cyclone
overflow stream containing low-ash ultrafine coal.
Ultra-Fine Coal Recovery Challenges
Recovery of ultrafine coal typically discarded in a deslime
flotation circuit presents several challenges due to particle
size distribution and high flow volumes [4]. Conventional
mechanically agitated flotation cells, commonly used in
coal preparation, allow some portion of the ultrafine min-
eral slimes to be recovered with the water that reports to
the froth. This particle entrainment is the non- selective
hydraulic conveyance of gangue into the product launder.
The liquid phase that holds the froth together and main-
tains mobility also carries fine particles that have not been
collected through a bubble/particle attachment. Fine par-
ticles (0.045-mm) tend to report to the froth concentrate
in direct proportion to the amount of product water. As
a result, mechanically agitated flotation cells are often not
able to meet product ash requirements for ultrafine coal, or
only can do so at a substantial loss in product yield [5-6].
Column flotation technology is a proven alternative
to conventional mechanically agitated flotation cells for
fine coal recovery. A distinguishing aspect of column flo-
tation is the formation of a deep froth bed that facilitates
froth washing to remove unwanted impurities from the
float product. Wash water, added at the top of the column,
percolates through the froth zone displacing dirty pro-
cess water and non-selectively entrained particles trapped
between the bubbles. In addition, froth wash water serves
to stabilize and add mobility to the froth. Also, the use of
robust sparging systems in column flotation technology
ensures high combustible recoveries through the generation
of fine bubbles that maximize bubble surface area flux and
gas holdup [4].
The primary advantage of utilizing wash water is the
ability to provide superior separation performance com-
pared to conventional flotation processes. As reported by
Davis et al. [7], the separation performance for column
flotation cells utilizing wash water is far superior to that
obtained from conventional flotation cells. In fact, the data
for the column cells tend to fall just below the separation
curve predicted by release analysis, which is an indication
of the ultimate flotation performance for fine coal [8].
Columns provide a level of performance that would be dif-
ficult to achieve even after multiple stages of cleaning by
conventional mechanically agitated flotation cells.
Another economical constraint for ultrafine coal recov-
ery is associated with the dilute flotation feed, which typi-
cally ranges between 2.0% and 6.0% solids, by weight. As
a result, volume flows in these circuits can be quite large.
Consequently, significant cell volume must be available to
achieve the desired retention time to ensure adequate com-
bustible recovery. The ultimate volume is typically based
on kinetic data generated from laboratory testing, but is
generally constrained by practical limits as dictated by both
economic and site-specific engineering requirements [4].
Finally, the dewatering of ultrafine coal represents an
additional challenge to economic recovery. Traditional
dewatering methods, such as screen bowl centrifuges, are
less efficient for ultrafine particles due to high losses in
Figure 1. Deslime coal flotation circuit flowsheet