Environmental Engineering Reference
In-Depth Information
- Rapid and enhanced dewatering with up to 40% more water recovery;
- Denser fill, with potentially up to 40% storage volume saved (because the tailings fill
the voids in the coarse reject;
- Ready rehabilitation because of the denser, stronger fill, which can be trafficked soon
after placement;
- Acid generation and spontaneous combustion of the coarse reject is less likely.
Disadvantages claimed:
- Blockage of pipelines;
- Pipe wear and replacement costs;
- Segregation of fines to form a wet tailings beach at the bottom of the co-disposal
beach.
It would seem to the authors that co-disposal could be attractive where there is limited
space for more conventional tailings storages and/or where acid generation and sponta-
neous combustion from the coarse reject is a problem.
However to date it seems it has been widely trialled but seldom used for on-going
operations.
19.5.7.3
Paste disposal
Paste disposal involves dewatering the tailings to a solids content of 70-85% (water con-
tent 30-15%) using high efficiency cone thickeners or conventional thickening followed
by partial vacuum filtration and paste mixing.
It falls between thickened tailings and belt filtration (see
Section 19.5.7.4
)
in the
amount of dewatering. For successful application, the tailings must have at least 15%
finer than 0.020 mm so the water does not drain. The paste is often used as a underground
mine backfill with cement added to increase the strength.
The paste behaves as a Bingham fluid and flow is initiated only after the pressure in the
pipeline is sufficient to overcome the initial shear stress. The pressures to pump the paste
are high (Brackenbush and Shillakeer, 1998, quote 2.25 kPa/m). Pumps are generally pos-
itive displacement, hydraulically operated. If it is stacked, its angle of repose is three to ten
degrees. Claimed advantages and disadvantages are:
Advantages:
- Most process liquor is recovered by the thickening/filtration process, so less is likely to
be lost to evaporation;
- Low permeability and capillary action limit acid generation in sulphidic tailings;
- Reduced groundwater pollution;
- The potential to eliminate containing “dykes” or embankments;
- Possible elimination of liners because of the low permeability;
- Useful for underground mine backfill, particularly if cement is added.
Disadvantages:
- High capital and operating costs for dewatering of the tailings to paste consistency;
-Problems with performance of thickeners to give paste consistency;
- High pressures limit the distances paste can be pumped and costs are high.
Overall it is hard to see paste technology being attractive unless the paste is required for
backfill in underground mines. Most of the advantages are available with thickened tail-
ings (i.e. to say 60% solids content).
