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Sustainable coal waste disposal practices 261

technologies use massive quantities of water requiring additional dewatering technol-
ogy, without which efforts to recover the fine coal fraction are essentially wasted [18].
Dewatering coal is a simpler problem than dewatering FCPW due to the angular char-
acteristics of fine coal particles. Furthermore, dewatered coal generates revenue while
dewatering FCPW only adds to costs. Hence, FCPW is often looked at as the red-
headed stepchild of coal preparation.
As previously described, in a typical coal preparation plant, FCPW consists of
reject streams from spirals, flotation columns or cells, desliming cyclones, and efflu-
ent streams from filter presses, screenbowl centrifuges, and other dewatering equip-
ment, all of which report to a thickener where they are concentrated into a waste slurry.
While concentrated, the underflow slurry from a conventional thickener is still too low
in percent solids to be able to handle it like CCPW. Addressing this issue, conven-
tional thickener technology has been modified to create the paste thickener, which
has proved effective at increasing thickener underflow solid content to 50% solids.
At this concentration, FCPW material has the consistency of paste with rheological
properties that allow surface stacking.
In one paste thickening study [19], thickener underflow slurry from a central Appa-
lachia coal preparation plant was tested in a laboratory-scale T-Floc apparatus to opti-
mize flocculant dosages for obtaining maximum settling flux and underflow solid
concentration. A maximum solid concentration of 35% by weight was achieved.
Pilot-scale tests were then conducted using a Dorr-Oliver Eimco Deepcone thickener,
which concentrated the same thickener underflow slurry solids from 10% to 50% by
weight. Thickened paste had a yield stress of about 165Pa, which is sufficiently low to
allow transport to a disposal area using a conventional positive displacement pump.
Clarity of the paste thickener overflow stream was similar to that currently achieved
with a conventional thickener.
In a comparison of thickener and centrifuge technologies [20], it was shown that the
most obvious benefit of paste thickening technology is a reduction in surface
impoundment area. Other benefits include increased recovery and recycling of plant
process water, less potential for groundwater contamination, and easier site reclama-
tion. When the product from a paste thickener is mixed with appropriate chemical
agents, it can be pumped back into the mine as backfill, an opportunity that will be
examined later in this chapter. Some disadvantages of paste thickening are that it does
not replace the existing thickener, but requires installation of a second thickener and
the need for additional chemicals. Furthermore, transporting the paste over any dis-
tance beyond what can be achieved with free gravity flow requires expensive positive
displacement pumps and wear resistant piping, which increase capital and operating
expenses significantly.

13.2.2.2 Osmotic dehydration [21]

Virtually all filtration-type devices used in FCPW dewatering can be considered
active approaches in that they force water through the porous filter medium. However,
there is a passive technology used in municipal water systems that has been in-
vestigated for its applicability to dewatering FCPW. This technology utilizes the