Alternatives for Future Coal Waste Disposal - Coal Waste Impoundments: Risks, Responses, and Alternatives - page 121

account for 3 to 7 percent of the total plant feed, of which coal may comprise as
much as 50 percent (R.Honaker, University of Kentucky, personal
communication, 2001). These fine coal particles may be untreated, partially
treated, or fully treated.
With no treatment, the fine material, usually smaller than 100 mesh (150
micrometer), is sent to a thickener and then pumped to a slurry impoundment.
Alternatively, the fine coal and refuse could be blended with the coarse coal
product and sent to the power plant. This feed can be either burned directly or
further cleaned in the power plant after pulverization; advances in magnetic and
electrostatic separation hold promise for dry cleaning the coal at this stage. In
fact, pulverization liberates more of the mineral matter in coal. Dry cleaning
with magnetic and electrostatic separators has shown encouraging results (Oder
et al., forthcoming). However, this process still generates a waste stream (albeit
not a slurry) that requires disposal.
Partial treatment utilizes classifying cyclones to remove coal particles to
approximately 325 mesh (45-micrometer) size. In full treatment, which
theoretically captures all of the fine coal particles, the fine waste from the
cyclone is subjected to flotation. As air bubbles rise through the flotation tank,
the coal particles attach themselves to the bubbles and are carried to the top of a
column of water.
Although the recovery of all or most of the coal fines will not eliminate the
need for slurry impoundments, it will reduce the required disposal volume. At
the same time, by increasing the clay content, a slurry that is more difficult to
stabilize may be produced.
In contrast, dewatering the refuse stream could eliminate the need for
slurry impoundments by changing the strength properties of the waste material,
although disposal of the resulting dewatered waste raises other issues.
Dewatering employs either sedimentation or filtration or both. In sedimentation,
the liquid is constrained, and the solid particles move freely. This results in
clarification of the liquid and thickening of the remaining slurry. In filtration, a
medium constrains the particles while the liquid flows through. This is
accomplished by screening and centrifugation (Osborne, 1988).
In coal preparation plants, wet refuse is usually sent to a thickener
( Figure 7.2 ). The underflow from the thickener (30 to 35 percent solids by
weight), the fine waste stream, is sent to a slurry impoundment. Deep cone or
other paste thickeners produce an underflow with a higher solids content than a
conventional thickener. Their steep-sided deep cone construction takes
advantage of the high differential pressure applied by the depth of solids to
produce a paste (Steve Slottee, Eimco Process Equipment Company,