Environmental Engineering Reference
In-Depth Information
Figure 19.35.
Principle of belt filtration (Willis, 1984).
19.5.7.4
Belt filtration
Belt filters are used for dewatering of some tailings to such a consistency that they can be
carried by truck for disposal in mine overburden dumps, either separately, or mixed with
say coarse rejects. Figure 19.35 shows the principle of belt filtration.
The tailings are spread on a filter cloth supported on a drainage deck and are dewatered
by gravity and the application of a vacuum to the underside of the drainage deck. The
process is often aided by large doses of flocculants.
In general, the capital and operating costs of such operations are high and the success
depends on having a uniform quality of tailings. To the authors' knowledge it has only
been used where space or environmental constraints preclude other methods.
19.5.7.5
Disposal into open cut and underground mine workings
An apparently attractive method of tailings disposal is to discharge the tailings into old open
cut or underground mine workings. This presents a low cost method of disposal. However,
there are several potential problems. In most open pits the area of the pit is relatively small
and results in a subaqueous discharge environment with resultant low densities. This in turn
leads to high compressibility and low strength. When the area is no longer in use, settlement
occurs, leaving a water pond. Settlement can total several metres and take tens of years to
complete. It may take some years before the tailings dry sufficiently on the surface to allow
rehabilitation. Tailings discharged into underground mine workings will only settle to low
densities and very low strength and will be susceptible to flowing out of the workings if for
example another seam or ore body is removed from below the workings.
19.5.7.6
Discharge into rivers or the sea
There are some notable examples of this e.g. Bougainville Copper and OK Tedi Mining in
Papua New Guinea and the Rosebery Mine, Tasmania. In the latter case, tailings dis-
charge has resulted in devastation of the river due to the chemical content of the tailings
leachate water. In Bougainville and in the lower reaches of the OK Tedi River, consider-
able sedimentation has occurred resulting in encroachment of the river into adjacent land.
As a result of these types of effects, generally speaking, disposal into a river is environ-
mentally unacceptable. The issues are discussed in Vick (1983) and Ritcey (1989).
