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Geotechnical News • December 2014
www.geotechnicalnews.com
WASTE GEOTECHNICS
Supporting the assessment of water recovery for mines in
Northern Chile
Eduardo Salfate
Northern Chile is one the driest areas
in the world. Evaporation largely
exceeds precipitation, in some cases
by as much as 2,000 times, and avail-
ability of surface water is limited. The
area holds some of the largest copper
mines, an activity with high water con-
sumption. A single mine in this area
typically requires 60,000 to 70,000
m3/day of make-up water for min-
eral processing. As can be expected,
sourcing these flows in the context of
an extremely dry climate constitutes a
significant challenge. The challenge is
so substantial that the associated costs
for obtaining this water could become
the key driver of project feasibility.
The use of seawater has gained
significant ground in recent years as
it ensures a constant water supply
for the mines and reduces the impact
on limited fresh water supplies. The
downside is that seawater often needs
to be pumped over 100 km and over
2,000 m in elevation, which translates
to high capital and operational costs.
Mining companies have focused
their efforts on reducing the amount
of water that needs to be sourced
(pumped) from outside of the mine
through the recovery of process water.
If properly implemented, experience
has shown that recirculation rates can
range from 40% to as much as 80% of
the water used in the process. In mines
where copper is recovered through
flotation, most processing water is
discharged along with the tailings,
and as such, they generally constitute
the primary source for this recovery.
Although thickening provides oppor-
tunities for recovering between 50%
and 80% of the water in the tailings,
experience shows that the lowest
water make-ups are achieved in mines
that also manage tailings deposition
to maximize water recoveries from
the impoundment. Losses due to
evaporation and rewetting of dry tail-
ings (through infiltration) are crucial
for estimating this recovery and are
typically predicted with unsaturated
numerical models, which in the
absence of proper calibration may
have limited accuracy.
This article provides an overview of
laboratory testing procedures that have
been used to validate the results of
these numerical models and increase
the confidence in water balance calcu-
lations developed for the estimation of
potential water recoveries from tail-
ings impoundment in dry climates.
Tailings deposition planning and
its role in water recovery
Tailings deposition is often planned
and managed to meet the design objec-
tives set for the tailings impoundment,
which can include:
• Optimizing storage capacity;
• Optimizing water recovery from
the impoundment;
• Minimizing operational costs and
energy consumption;
• Minimizing capital investments
required for the construction of
start-up infrastructure; and
• Minimizing land use due to envi-
ronmental or space constraints.
Deposition strategies can vary signifi-
cantly depending on which of these
objectives are to be prioritized, and
in some cases, some of them need to
be sacrificed for the benefit of others.
For example, in mines with limited
storage area for tailings placement, the
design objective of the facility may
be set towards maximizing storage
volume for a given impoundment area.
In such cases, tailings are deposited so
that densities can increase rapidly in
the impoundment. The sequencing of
a large number of deposition points to
control discharge rates, promote thin
layer deposition and enhance evapora-
tion and drying of the tailings could
be the desired strategy under these
circumstances. The outcome of such a
deposition strategy (enhanced evapo-
rative drying) would not be desired
in a facility designed for maximizing
water recovery, as is the case in north-
ern Chile.
In a region with high evaporation rates
and large areas of dry tailings beaches
(as illustrated in Figure 1), designing
a deposition strategy that enhances
water recovery may require:
• Depositing the tailings to achieve
impoundment geometries that
result in tailings ponds with small
surface areas to reduce evaporation
losses from the pond;
• Controlling tailings deposition rates
so that surface runoff is enhanced
and travel times of tailings and wa-
ter to the pond are reduced;
• Selecting deposition points that are
close to the pond to reduce water
losses through evaporation and
rewetting as the tailings travel over
dry areas; and