PASTE KOVIT_proof 25/05/2016 11:18 Page 3
Paste Supplement
risk and not as forgiving as gravity distribution.
At 340 t/h (185 m3/h), the Tahoe system is one
of the largest backfill plants in the Americas,
requiring the largest paste (concrete) pumps
available on the market to meet throughput and
high pressure to cover 2 km strike and reaching
200 m above the paste plant.
Kovit’s review identified critical plant design
issues, relying on its extensive experience with
identical systems. This type of modular plant
can be effective for continuous free-flowing
sand, classified tailings or aggregate, but was
definitely incompatible for this operation.
Incomplete mixing and inability to control
rheology and strength have dramatic and
potential fatal flaws for most operations.
Coupled with a tailings dewatering method and
variable feed rates, the plant was not geared to
any of the main challenges. Bins and feeders
could not handle the incoming feed rate, while a
grenade explosion-like noise coming from the
mixer was in large packed clay-like clumps of
pressure filter cake ponding the inside. Clumps
and muddy water into the paste pump h opper
could not be termed an “engineered fill.”
Subtle rheological control of ≤0.5% moisture
is challenging in complex large batch plants so
is impossible within small continuous mixers
found in the constrained sea container. Even
slight variations in moisture can readily double
pipe friction losses, potentially arresting
P14 International Mining | JUNE 2016 Supplement
pumping and resulting in 2 km of blocked
pipeline and lost production. On the other hand,
slight increases in moisture from 175 to 250 mm
slump diminishes strength by up to 30%.
Additional design challenges of the modular
plant included:
n Non-sequenced discharge of 30 t batches
from each of four pressure filters resulting in
large peak surges and zero flows creating
havoc for feed regulation to the mixer
n Main 850 m conveyor belt resulted in
overload stoppages and difficult restart with
surges
n Lack of feed buffer created highly variable
mixer residence time
n Surges in flow into exceedingly small paste
hopper was ineffective for smooth control of
paste pump speed
Kovit initially focused on low cost solutions
with minimal shut-down impact to resolve the
above issues. The small compartmentalised
containers and inadequate seismic
consideration in the structural design precluded
modifications.
A fast-track concept-to-erection was
completed in 12 months. This was achieved even
though a bold and innovative approach was
needed due to existing infrastructure and
topography, with inability to excavate deeper.
Fitting into the main conveyor required fitting a
2.5 m reduction in height from a typical batch
plant. Stuck between a rock and a hard place, a
radical and innovative design ensued:
n large pressure filter cake receiving bin to
smoothen feed rate, required 2.5 m more
height
n integrated filter cake disintegration/feed
system
n innovative proprietary continuous-to-batch
paste preparation technology, with 10 m3
mixing (compared to modular plant with <2
m3) while reducing plant height by 2.5 m
n Novel 20 m3 paste pump hopper (compared
to <1.5 m3), reducing plant height by 1.5 m
n Innovative proprietary binder batch system
improving engineered paste mix
n Robust design with two mixers and feeder
with 450 kW versus the original single 75 kW
mixer.
Commissioning required two weeks for the
new customised plant, and has since continued
to effectively meet the customer’s needs. Final
scope for Kovit included the design/supply of
the plant and detailed design of the UDS.
Kovit Engineering Ltd is now part of Materials
Management within Outotec (Canada) Ltd,
bringing together the most significant global
experience from Canada, Australia and Sweden,
specialising in backfill, tailings management
and water treatment solutions. IM