Tailings Disposal - Mining and the Environment: From Ore to Metal - page 724

tailings storage facilities were used during operations, with tailings later transferred to a
nearby Pit as part of mine closure. At Nabarlek, wick drains were installed to expedite
dewatering of the deposited tailings. Wick drains, usually installed in a close pattern,
are commonly used to expedite consolidation of weak soils. They would normally be too
expensive for dewatering of tailings. A more cost-effective approach would be to install
underdrains and recovery sumps in the Pit prior to tailings discharge.
In underground mines, tailings have long been used as backi ll in previously worked out
voids. Generally, coarse grained tailings are preferred for this purpose and, commonly, cyclones
are used to separate the coarse fraction from slimes, the latter being discharged to a TSF. The
tailings are commonly mixed at the surface with a binder, usually cement, and then pumped
to i ll voids. Using cemented backi ll, a room and pillar underground mining operation is able
to extract the in situ pillars containing ore, thereby increasing overall resource recovery; the
cemented backi ll provides support, preventing roof collapse and problems with subsidence.

In underground mines, tailings
have long been used as backfi ll in
previously worked out voids.

Sub-aqueous Tailings Placement

Typically, the storage of mine tailings in a surface disposal facility is required or preferred.
If the tailings are potentially acid-generating, then the tailings solids may need to be pro-
tected from exposure to the air. One option is to release tailings in a water-i lled storage
providing water coverage which effectively prevents oxidation (Tremblay 1998). This is
referred to as sub-aqueous placement. The management of sub-aqueous tailings place-
ment in an engineered TSF is similar to conventional on-land disposal, and the interested
reader is referred to the references for further discussion.

Lake Disposal
Lake Disposal has previously been mentioned as a means of avoiding acid generation from
mine tailings, and accordingly some tailings storages are maintained as lakes following
mine closure. There have only been a few examples of tailings disposal in natural lakes,
most of them in Canada. Conditions required, in particular, the presence of steep lake bed
gradients at the discharge point and deep water where the tailings solids accumulate, are
similar to those for marine disposal. Such conditions are uncommon in nature. However,
they do occur in some deep glacial lakes in North America and northern Europe, some
volcanic caldera lakes in Southeast Asia and deep lakes formed by recent tectonic activity
(also found in Southeast Asia and elsewhere).
A particular phenomenon experienced by many lakes at high latitudes and/or high alti-
tudes (but not in the sea) is that of 'seasonal turnover'. In most parts of the sea, the pres-
ence of permanent density stratii cation in which each layer is denser than the one above,
precludes deep water from rising to the surface. However, in lakes subject to seasonal
turnover, seasonal temperature changes reverse the density difference between the surface
layer and the bottom layer of the water, causing the surface layer to sink to the base of the
lake as the bottom water rises. In such circumstances, any contaminated water discharged
as part of the tailings stream would be brought to the surface.
In most cases, lakes are not considered as repositories for tailings, even in situations
where suitable conditions occur. This may be due to the fact that lakes are generally scenic;
they are usually relatively uncommon (Canada is a notable exception) and therefore par-
ticularly highly valued as aquatic habitats and also for their intrinsic values; and they often
provide water supplies and recreational opportunities. Just the thought of using lakes for
disposal of tailings is therefore anathema in most situations, even if a scientii c evaluation
was to conclude that an environmentally acceptable disposal scheme was feasible.

There have only been a few
examples of tailings disposal in
natural lakes, most of them in
Canada.

Just the thought of using lakes for
disposal of tailings is anathema in
most situations, even if a scientifi c
evaluation was to conclude that
an environmentally acceptable
disposal scheme was feasible.