Geology Reference
In-Depth Information
has recently found application at power plants for disposal of ash; and has seen
limited use in the coal industry for the disposal of fine coal refuse.
Although rainfall does not reconstitute a properly formed paste, and
erosion is not excessive on typical stacks with 2 to 5 percent slopes, this method
is best suited to areas of low rainfall and high evaporation. Under these
conditions, the surface of the stack can be traversed by bulldozers within days
of final placement. Regardless of the amount of precipitation, perimeter drains
are necessary to catch runoff and divert it around the stacks. Underdrains are
also needed for stability to prevent an increase in the phreatic surface.
Three considerations—land availability, steep terrain, and cost—hamper
applying unsupported thickened high-density residue stacking to fine coal
refuse disposal. This method is best suited to areas where the slope of the land
is less than 5 percent. Secondly, although the technology is based on a
significant modification of the standard thickener, these thickeners are much
deeper and require a longer residence time. Therefore, the lower throughput rate
of deep cone thickeners compared with that of standard thickeners may
significantly affect the economic feasibility of the method.
Underground Disposal
In 1975, the National Research Council conducted a study entitled
Underground Disposal of Coal Mine Wastes, which evaluated the technical and
economic feasibility of several underground disposal methods; namely,
pneumatic backfilling, hydraulic backfilling, mechanical backfilling, hand
packing, controlled flushing, blind flushing, and pneumatic flushing. Much of
the study drew heavily on European experience, where backfilling has long
been used to control subsidence. The backfilling methods emphasized the use of
coarse refuse and did not specifically address the issue of fine refuse or slurry;
only the hydraulic methods dealt specifically with fine refuse. The report did
not discuss pneumatic flushing of fine coal waste, but it did indicate that this
method had been used to inject fly ash into underground mines. The two
primary methods for injecting fine coal refuse into underground mines are
controlled flushing, where the underground workings are accessible, and blind
or uncontrolled flushing, where the underground workings are abandoned or
have caved in.
The 1975 NRC report found that a number of underground disposal
methods were technologically feasible at that time; however, none was
universally economically feasible. The optimal solution varies from site to site.
In addition, the question of workers' health and safety was raised when
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