Alternatives for Future Coal Waste Disposal - Coal Waste Impoundments: Risks, Responses, and Alternatives - page 139

However, co-disposal does not eliminate the need for a starter
impoundment structure. After an initial dam is built, the mixed beach
sediments, settling out of the co-disposal material can be used to construct the
embankment for above-ground storage. Ideally, the fine particles fill the voids
between the coarse refuse fragments. As the mixture is discharged, it forms a
steep beach sloped at approximately 10 percent. This slope decreases to
approximately 1 percent at the end of the mixed beach. At this point, the
sediment consists mainly of fine coal suspended in ponded water in a small
impoundment. The success of this method is closely linked to the ratio of coarse
to fine particles. It also depends on gradation of the refuse (large gaps in
particle size are not acceptable) and proper particle shape (angular or “platy”
particles cause problems). Since the impounding structure is raised by
deposition of mostly coarse material, it does not compact as the structure
increases in elevation.
This method has been used primarily in sparsely populated areas with low
annual rainfall. Questions remain about its suitability for steep hills with high
annual rainfall. Unlike a conventional slurry impoundment, which contains only
fine refuse, the co-disposal system places all refuse (both coarse and fine) in a
slurry and deposits it behind an impounding structure. Therefore, even though
the refuse dewaters more quickly and forms a stable bench, it requires more
impoundment storage volume than an impoundment designed only for fine
refuse. So, for steep terrains, this factor negates the advantage of less total
storage area by actually requiring more material (both coarse and fine) to be
placed in an impoundment. Its use would hinge on whether increased stability
of the refuse outweighs the additional volume of the impoundment.
If an effective dewatering approach, such as paste thickening, is used, the
resulting waste can be disposed of by thickened high-density residue stacking
(Lech Brzezinski, LSB Consulting Services, personal communication, 2001).
Deep cone paste thickeners produce a homogeneous, non-segregating paste with
a solids content of approximately 60 percent. The degree of dewatering is
determined by the pumping capabilities. Under controlled conditions, the paste
can be deposited in thin layers over the disposal site at uniform slopes of 2 to 5
percent and does not require an impoundment structure. This method is most
suitable for homogeneous residues of fine gradation, where the thickening
process prevents segregation of the coarse and fine particles during
transportation.
Thickened high-density residue stacking was developed more than 20
years ago to handle red mud tailings generated by alumina plants. It has been
used for approximately 10 years for disposal of gold and base metal tailings;