Page 269 - Advances In Productive, Safe, and Responsible Coal Mining

248 Advances in Productive, Safe, and Responsible Coal Mining

depth, surface mining is the preferred option for coal production. Strip or open-cast
mining is the most common method for extracting these coal seams. With that method,
overburden waste is cleared to expose the underlying coal and placed in an adjacent
excavation where the coal has already been excavated.
The overall economic goal in surface mining is to move the least amount of over-
burden waste necessary to mine the greatest amount of marketable product. Two pri-
mary economic parameters are the mining rate, which is used to determine how long
the coal reserve will last (i.e., mine life), and the stripping ratio, which is used to define
economic limits for surface mining when the coal seam is dipping or the surface
topography varies such that overburden depth is increasing.
Overburden waste consists of unconsolidated material (soil) at the surface and con-
solidated material (rock) overlying the coal seam. Removal is a cyclical process of
drilling, blasting, loading, and hauling. Generally, unconsolidated material is easily
removed without the need for drilling and blasting. Loading and hauling of unconsol-
idated material are performed by scrapers, dozers, backhoes, and dump trucks at most
operations with bucket wheel excavators used at large mines. Loading and hauling of
consolidated material are performed by dozers, front-end loaders, and off-highway
trucks at smaller mines and by drag lines and/or truck-and-shovel combinations at
larger operations.
Before current reclamation laws were enacted, overburden waste was moved in the
most economical fashion and left in “spoil” piles or rows without any grading or sep-
arate recovery of top soil to use in restoring land to its premining productiveness.
However, since such laws have been enacted, overburden waste must now be carefully
managed with grading to original contour and restoration of stratigraphic columns
with a top layer of topsoil. In addition to being managed in ongoing reclamation pro-
grams, removed overburden can be used for building mine haul roads and constructing
stable coal refuse structures. Some overburden waste may have nonmining commer-
cial value and be sold as fill dirt.

13.1.3.2 Coal refuse

As previously stated, full extraction results in out-of-seam material in the ROM prod-
uct, which usually must be removed to satisfy customer specifications. This requires a
complex arrangement of coal storage, coal processing, and waste handling facilities
(see Fig. 13.1), which are located on the surface in close proximity to the actual mine.
These facilities typically include a closed-loop water management system where
on-site water resources are continuously recycled and off-site water discharges are
carefully regulated. Such discharges typically occur from sedimentation basins, but
only during larger precipitation events when less stringent regulations are applied
and/or there is a considerable amount of dilution water available. Coal refuse includes
all waste generated from the overall facility but primarily by the cleaning process used
in the coal preparation (wash) plant.
The conventional approach to coal refuse disposal is to construct stable embank-
ments with CCPW that are used to contain FCPW as concentrated slurry of 15%
solids or as thickened paste of 50%–70% solids. With increased scrutiny being given