Alternatives for Future Coal Waste Disposal - Coal Waste Impoundments: Risks, Responses, and Alternatives - page 123

personal communication, 2001). Deep cone thickeners produce dewatered
paste in the range of 65 to 75 percent solids by weight (Osborne, 1988). While
this material still requires disposal, its volume is less than that of unthickened
slurry.
Four types of filtration devices are used in coal preparation: gravity,
vacuum, pressure, and centrifugal. Although gravity and centrifugal methods
are used extensively, vacuum and pressure filtration methods offer the greatest
potential for dewatering material smaller than 100 mesh (150 micrometer) that
typically leaves the conventional thickener as underflow.
Three basic types of vacuum filtration devices are rotary drum, rotary disc,
and horizontal belt or disc, depending on their filter configuration type. With all
of these devices, the feed is applied to the filter medium under vacuum. This
draws the water through the filter medium while retaining the solids on the
surface. The filter cake is then removed either by a burst of air pressure,
mechanical scraping, or both. Vacuum filters produce a filter cake with a
moisture content of 20 to 30 percent (Leonard, 1991).
Pressure filtration devices are classified as either batch or continuous
press. Batch devices, such as the plate and frame filter press, have not been
widely accepted in the U.S. coal industry because they operate discontinuously
(Osborne, 1988). However, solids recovery is high and effluent water is clear.
Belt filter presses operate continuously and are more widely used than
batch methods ( Figure 7.3 ). Belt presses produce a cake with a moisture content
reportedly in the range of 20 to 30 percent (Osborne, 1988); however, in
practice, moisture content ranging from 35 to 40 percent is more common. Belt
filter presses currently produce a dewatered product that still must be disposed
of behind a retaining structure.
Hyperbaric pressure filters, a fairly recent development, combine the
filtration technology of disc or drum filters and the low moisture levels of
discontinuous plate and frame presses. The specific solids throughput of a
hyperbaric filter is several times that of a vacuum filter or other batch-operation
pressure filters (B.K.Parekh, University of Kentucky, personal communication,
2001).
Chemical additives are almost always used in conjunction with any
dewatering mechanism. Most of the mechanical methods (thickeners, filters,
and presses) rely on chemical additives (usually flocculants) to expedite and
enhance the separation process. The additives represent a significant operating
cost, but dewatering would be largely ineffective without them.
Thermal drying is an effective way of removing moisture, but it is very
expensive and energy intensive and is currently used only for drying fine coal.
In addition, it entails the environmental cost of air pollutants produced