Geology Reference
In-Depth Information
Even at this rate several cells must be permitted and in varying stages of
operation at any one time.
A variation of slurry cells is dewatered fine refuse cells. Mechanically
dewatered fine refuse can be placed in bermed cells. However, the combined
costs of dewatering and cell construction prevent this method from frequent use
in the coal industry.
Combined refuse disposal is another option. Combined refuse refers to fine
refuse from the static thickener that has been mechanically dewatered and
combined with coarse refuse for disposal on the surface. Depending on the
percentage of coarse and fine refuse and the moisture content of each, the
moisture content of combined refuse will range from 15 to 20 percent. This
material must be transported mechanically, since it is too dry to be pumped. It is
deposited in lifts approximately 2 feet thick, then graded with a bulldozer, and
compacted to create a stable surface. Once the fill reaches the designed level, it
is covered with 4 feet of soil and revegetated. Combined refuse fills require
underdrains to capture any water that leaches from the fill, and perimeter drains
to prevent runoff from entering it.
Problems associated with this method of disposal include the frequent
inability of mechanical dewatering to produce the moisture level required for
disposal of a combined refuse material (David Carris, J.T.Boyd Company,
personal communication, 2001). Because the moisture and clay content are still
fairly high, trucks or conveyors cannot handle the material easily, and
compaction is difficult. Davies and others (1998) report using small amounts of
cement to create a more stable refuse material. They found that the addition of 2
to 4 percent cement, by weight, results in a material that can be compacted
using standard procedures. Small amounts of fly ash and lime have also been
used to stabilize combined refuse. In addition, this method is fairly expensive
because of the high cost of mechanical dewatering and the potential need for
stabilizing chemical additives. The need for additives may be seasonal and
depends on climatic conditions. Finally, the method is best suited to flat land.
Co-disposal, developed and practiced primarily in Australia, involves the
combination of fine refuse from the static thickener with coarse refuse
(Williams et al., 1995). Since the fine refuse is not mechanically dewatered, the
combination has a fairly high moisture content that allows the combined
material to be pumped to the disposal area. The main difference between this
material and conventional slurry is the presence of coarse refuse in the mix. The
advantages are that it requires less total storage volume than separate fine and
coarse disposal methods, and the refuse stabilizes more quickly than typical
slurry.
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