Geology Reference
In-Depth Information
do not present a danger of sudden failure. Although underground mines
may be below these ponds, they are generally separated by a considerable
thickness, and the underground workings do not crop out above the level of
surface drainage.
Where incised ponds are used, slurry is pumped to the pond, and surface
and groundwater are prevented from filling the cut. The long cuts can be
divided into cells with plugs of spoil material. The cells are filled sequentially,
and subsequent cells can serve as overflow containment for the cells closest to
the discharge point. Clear water is decanted from the last cell for recycling
through the preparation plant. The pond is operated similarly to other slurry
impoundments. When the pond is full, it can either become a wetland or be
reclaimed by covering it with soil and revegetating the soil. Incised ponds do
not have the benefit of underdrains; consequently, they tend to dewater slowly
and may present problems in traversing the fill with earth-moving equipment if
the reclamation plan calls for covering the surface with soil. Permits require
groundwater monitoring and compliance with all surface water effluent
limitations.
Slurry cells are used to dispose of waste in a diked impoundment. This
type of impoundment reduces some of the problems and risks associated with
the more common cross-valley impounding structure. Berms that bound
individual cells are constructed of compacted coarse refuse. Cells may reach
depths of 12 feet, with 8 feet of fine refuse covered by 4 feet of compacted
coarse refuse; if they are designed to impound less than 20 acre-feet of slurry,
they do not have to be permitted as an impoundment (M.Day, Arch Coal
Company, personal communication, 2001).
Slurry from the thickener is pumped directly to each cell. After a cell fills,
the discharge point is moved to another cell. Water can be decanted from the
surface, or the cells can be allowed to dewater by evaporation. The time
required varies according to weather conditions, but the fine refuse is usually
dry enough in two or three months to allow coarse refuse to be placed on top
and compacted. Subsequent cells can be constructed on top of completed cells
to a final height of a few hundred feet. For stability, benches must be
constructed every 50 feet to reduce the overall slope angle.
The advantage of this method over conventional cross-valley slurry
impoundments is that each cell is small and self-contained and can be designed
according to the strength properties of the coarse refuse. The main disadvantage
in steep terrain is the limited availability of flat land to construct the cells.
Another disadvantage is that slurry cell operations are not compatible with a
high production rate at the coal preparation plant. The maximum plant capacity
for this type of disposal option is about 500 tons per hour (E.Kitts, Summit
Engineering, personal communication, 2001).
Search WWH ::
Custom Search