PASTE SUPPLEMENT 2019
Considering alternative tailings discharge
Bryan Ulrich, Vice President of Geotechnical Engineering
of the Mine Site Infrastructure group at Stantec, argues
that central tailings discharge can be especially
advantageous at closure
ining companies have opportunities to
reduce long-term risk and increase the
resiliency of their tailing storage
facilities (TSFs) by evaluating and implementing
new tailing management strategies, such as
central tailings discharge (CTD).
Most conventional perimeter-discharge TSFs
lack water efficiency and their dish-shaped
geometry has significant disadvantages for
closure, especially if the closure requires
shedding precipitation runoff. CTD creates a
conical geometry that offers three key
advantages for TSF closure: improved water
management, less land disturbance and
expedited TSF closure.
M
Improved water management
CTD methods require thickened tailings.
Thickening the tailings reduces the amount of
water in the tailings impoundment and allows
for more mine water to be reclaimed. Recovering
water in a thickener is more efficient than
and release covers (or a combination of the two)
that take advantage of the conical post-closure
landform.
Less land disturbance and
earthmoving
CTDs typically require smaller containment
structures during the operational period of the
TSF. Thickened tailings have the potential for
higher beach slopes and higher shear and cyclic
resistance than conventional tailings – this
creates the opportunity for smaller, less robust
embankments.
More importantly, because the tailings can
be used for fill material, the need for borrow
materials – which potentially cause additional
land disturbance in the form of a borrow pit – is
also reduced. Lower containment structures and
use of active tailings deposition as backfill
reduce the capital expenditures, construction
costs and required closure funds. Less land
disturbance generally provides an opportunity
Bryan Ulrich
Converting to CTD for closure
A CTD (orange) and a conventional tailings ring
dyke (black) of similar capacity.
Source: Fitton 2017 for faster permitting due to reduced
environmental impacts.
There can be challenges in switching from
conventional deposition to CTD for closure –
primarily because a new approach to water
management will be required. Thickened
tailings displace the centralised pond and water
will migrate to areas near the perimeter
embankments requiring most operations to
excavate sumps in the tailings beaches near the
embankments, add ditches to move the water to
those sumps, as well as pumps and pipes to move
the water to an external reclaim water pond.
sending the water to an impoundment where it
is lost through evaporation, infiltration and
within the tailings voids.
Reducing water in the TSF also reduces
overall cost of tailings storage because there is
less water to manage and monitor. In addition,
less water poses fewer downstream risks in the
event of an impoundment failure. Water
conservation that leads to reduced fresh water
requirements is a positive environmental and
social feature for any project or mine.
The CTD geometry improves the ability to
manage the effects of precipitation and prevents
infiltration at closure. If the TSF is designed to
shed water, the domed shape of tailings allows
for designing water shedding covers or store Faster TSF closure When to Consider CTD
With early planning, the use of CTD for closure
can bring the closure process forward by using
active tailings deposition, avoiding the need for
borrow materials and a permitted borrow pit.
The mine owner also benefits because the
closure work is initiated during the active
mining process, when revenue is still being
generated.
An earlier, lower cost approach to closure has
financial benefits too. It can reduce bonds
needed to allow post-mining land to be returned
to its intended post-mining state. If CTD is
implemented early and progressively during
operations, the need to defer closure costs and
the liability at closure are reduced. Factors that might motivate a mine to evaluate
the switch to CTD for closure include:
n High cost of water
n Constraints on land use
n Accelerated TSF closure time
n Closure concept is to shed precipitation
runoff
n Significant re-grading and earthmoving
needed to close the conventional TSF
The closure planning team should include a
tailings management professional, and should
consider conventional and alternative disposal
methods, and TSF closure strategies that take
an integrated approach to develop effective,
long-term solutions.
By beginning the TSF closure planning
process early, mining companies can evaluate
and benefit from a move to CTD. Yet it is
important to note that switching to CTD is a
commitment. Converting from CTD to another
method or back to conventional TSF would be
very difficult. IM
Conversion to Central Thickened Discharge for Closure
P12 International Mining | APRIL 2019 Supplement