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Paste Supplement
The Escobal mine installation from Kovit has
continued to effectively meet the operation’s
needs. Final scope for Kovit included the
design/supply of the plant and detailed design
of the UDS
batch mixing is then considered where
sensitivity to water or risk of the underground
distribution system is more critical. Designing in
a box also removes degrees of freedom in a
design that results in limitations on size and
selection of process equipment. Generally
undersized and under-powered, these systems
fall down on larger throughput and/or with
difficult tailings.
Integrated paste backfill and alternative
tailings approaches will typically include various
arrangements of thickeners, filters (vacuum or
pressure filters), conveyors, bins and control
systems, though it is important to explore
beyond these. Staffing at most consulting,
EPCM and vendors has not been sustained or
included in-house staff and there may not be a
proper succession plan to maintain the
knowledge, particularly during low protracted
economic cycles. Splitting the work into silos
often results in misaligned or different
interpretations, and where interpretation by
each successive user of “data” leads to a loss in
the fidelity of the intent and or knowledge.
However, often misplaced trust is placed into
designs. Perhaps a modular system is better?
Modularity is not new including early
examples in the 1800s of homes built offshore
and transported to Australia. Easy for control
rooms, labs, offices and whole construction
accommodations. But heavy industrial
equipment configured for a specific need is
quite different than for a bed, table or lab
bench.
Whether modular or custom “stick-built”, a
well-designed plant requires specific expertise
that includes mineralogy, mineral processing,
and materials handling, along with a
comprehensive understanding of the
operational needs. These plants are not simply
for dumpi ng fill down a hole; they not only
facilitate but enhance mining processes
achieving higher productivity and profitability
while tailings as underground fill reduces
surface placement. A “one-size fits all”
approach suggests that all tailings behave the
same and mining needs don’t really change.
Standardisation to reduce capital costs has
merit, only if the benefits are delivered.
The allure of a simple plant-in-a-box is well
understood: “just add water.” Manufacture in a
low cost country may provide savings. Modular
sea containers are simple and easy, so long as
the form meets seismicity, in-country codes,
road and bridge crossings, etc, HVAC, electrical
and control other preferences, and slowly the
standardisation is eroded.
In a strategy that is based on hire and dehire,
most vendors and large engineering firms have
shed senior and experienced staff in favour of
lower cost junior engineers. Often, this new
entourage lacks the depth of operational and
design experience and yet does not have the
guidance alongside it. Investing and maintaining
internal knowledge base and expertise through
continuous learning over many successive
projects is no longer in favour. Productisation is
one way to compete, so long as the site
specifics are not lost or betrayed. Sending out
sales agents is clearly not the solution, and will
not gain the client’s trust in protecting
shareholder value. Pressure to ‘sell’ replaces
value in listening and validating a solution
worthy across a mining project (geology, mining,
ore management, mineral processing,
concentrate and tailings streams, backfill
(tailings, sand/silt resources), environmental
site management (footprint, ARD/ML,
alternative tailings management) and synergies
that backfill incorporates within a holistic site
wide strategy.
What is in a box
The business of standardisation in boxes
attempts to contain costs, a key driver for
everyone. But standardisation means
compromises and may not be realistic within the
limited sea container. Will a small 100 t/h mixer
be applicable for 200 or 300 t/h, or vice versa.
Experience demonstrates that sand or classified
tailings mixes easily at 200 t/h in an agitated
tank, whereas 75 t/h of pressure filter cake
would be impossible. Concrete mixer vendors
wrongly assume mixing paste fill is much like
concrete. Some tailings are unforgiving,
requiring different mixer configurations, greater
mixing energy and retention time to obtain a
well-mixed and rheologically controlled
engineered paste. Designers relying on such
vendors having inadequate frames of reference
are bound to translate errors into designs.
End-users should challenge the promises of a
modular plant, like any plant, when there is no
choice but to make it work; money isn’t
available to start over, especially if production is
lost. Modular often compromises both
optimisation and robustness – is it value-added?
Loss of just one stope per year for a mid-size
mine is often worth more than the cost of a
backfill plant. On the other hand, a welldesigned system can easily facilitate additional
stopes and payback.
Plant design – robust and optimised
A recent project is a case in point. In 2013, Kovit
Engineering was requested by Tahoe Resources
Inc to review their modular paste plant and
provide detailed engineering of the
underground paste distribution system (UDS).
Tahoe’s Escobal mine, located in southeast
Guatemala is a high-grade silver mine with gold,
lead, and zinc. Local conditions include high
seismicity, rainfall, small footprint, local farming
villages, shared water, and economic, social and
political tensions. The tertiary andesite and
volcanoclastic host sedimentary formations run
over 2 km in strike, and include mud zones
requiring novel drifting development methods.
Adits and ramps enter the side of a mountain,
where the ore extends above and below the
surface plants. As a consequence, all cemented
paste must be pumped at very high pressure.
The modular backfill plant allowed
construction to focus on the concentrator and
other core facilities. Dry stack tailings storage
facility using pressure filters were selected in
the sustainable closure solution.
Some of the filtered tailings are transported
by conveyor to the paste backfill plant
approximately 800 m up the valley and 150 m
vertical. Mixed with water and cement, the
engineered pastefill is pumped to the stopes.
Rapid filling is needed to stabilise exposed
weak hangingwall rock formations in the 30,000 t
bulk stopes.
The critical design inputs that closely
integrate tailings production and characteristics,
materials handling, mixing and control and UDS
simply cannot be understated, particularly since
paste pumping systems are generally higher
JUNE 2016 Supplement | International Mining P13