PASTE KOVIT_proof 25/05/2016 11:18 Page 1
Paste Supplement
Boxed in or not – is modular better in backfill
Frank Palkovits, President of Kovit Engineering, now part of
Outotec, asks whether modular backfill plants are better
and the considerations before stepping into the box
Kovit supplied a customised solution to replace
a modular plant at Tahoe Resources’ Escobal
mine, located in southeast Guatemala with fasttrack concept-to-erection in 12 months
hether in a protracted mining drought
or in a surging economy, responsible
mine builders will always seek ways to
improve economic performance of their project
with lower capex and opex, recovery or higher
throughput and economies of scale. An
integrated approach at any mining operation is
important to meet those goals. Mine backfill is a
key component in the mix. Studies covering a
range of backfill options work towards project
optimisation, but using available resources to
enhance mining productivity, recovery and
dilution, and not just a product to fill the void. A
positive impact by reducing tailings and process
water management is a synergistic part of the
package. A recently custom-built hydraulic fill
plant in northern Ontario reportedly cost over
C$60 million, yet hydraulic fill plants handling
the same material are successfully built and
operated at ≤C$10 million. But even at that
considerable capital cost, it cannot achieve the
rated placement density promised. Nearby
applications of low-cost modular systems have
also worked well. Hydraulic fill is easy to mix
and control within agitated tanks. So how can
C$60 million be justified.
While an integrated approach seems obvious,
it doesn’t always occur. Care in selecting
consultants, EPCM firms, and operators whose
core business is in backfill and tailings will help
to ensure a robust design ensues and realise
the economic model. Relying on vendors for
equipment selection and/or configuration may
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P12 International Mining | JUNE 2016 Supplement
be well-intended but a sal es organisation is not
typically a solution-qualified team.
Development and selection based on
first principles
A quick-fix often sought is the use of modular
systems. Modularisation and productisation can
be done well. Once the purchase order is
completed, the modularised plant arrives in a
number of boxes for fast erection, and plug-andplay in theory. Simple, as it minimises the
project staffing, as detailed design,
manufacturing, fabrication, installation, QA/QC,
and ‘productisation’ takes place off site,
assuming the appropriate process equipment
can fit into modular components and be
maintained.
As mines come in all shapes and sizes, so do
site specific needs, and they are in all climatic
regions, some reaching 40°C above and below
zero. Standards differ widely by country and
company, ranging from safety systems and
devices, fire protection, motor winding and
instrumentation/control systems, in addition to
mining and tailings specifics.
In particular for paste backfill, designing a
single plant applicable to all mines with
extremely wide variability in tailings
characterisation is questionable. No two tailings
are the same, nor behave the same. Mineral
formations that are mined cannot be
represented by one sample, though vendors and
engineers plead foul without it. A far too
simplistic approach within a single mine let
alone a single design for all mines is a standard
product. A set of representative samples at each
site should be carefully considered spanning the
upper and lower bounds. But upper and lower in
what context. Good for grinding may be poor for
flotation. Optimisation of one step may
compromise the next. The entire mineral
process steps from comminution, flotation,
leaching, to reagent consumption all affect the
tailings and may result in more than one stream,
from which preferred tailings disposal methods
(conventional or alternative disposal methods
using dewatering methods) and backfill
(hydraulic fill, paste) need sorting out. Tangible
solutions are often hidden in the mineralogy,
and notions of solutions need to be subjected to
physical testing in developing solutions that are
robust and provide expected return on
investment.
Tailings behaviour determined by broader
testing programs as opposed to specific vendor
application will provide much greater insight. As
a frame of reference, mixing concrete in a
wheelbarrow is a fairly easy task. For large
building structures, QA/QC of aggregate and
strength is critical. Aggregates and recipes are
defined and controlled (mineralogy, hardness,
particle size and dryness, cement types,
admixtures).
Constant variability, much like throwing a few
shovels of clay, silt, sand all in slightly varying
ratios and mineral types is not what concrete
producers would like to work with, but that is
exactly what we expect to receive in a paste
plant, and from which an “engineered fill” is
expected. We can control blended ore only
slightly better than the weather, it seems.
Vendors (salespeople) are typically far
removed from the reality of mining and geology,
with a narrow view of or little direct inside
knowledge or experience of what makes it work
and work well.
Modular designs typically fit into global
freight handling systems, defined by the
ubiquitous standardised sea container.
Underground mines vary from 500 t/d to 15,000
t/d with all sorts of geology, and grinds of
P80=250 μm or a fine gold or ultra-fine
polymetallic of P80<20 μm. Tailings from either
may consist of clay-sized actual clay. Some
individual orebodies such as the complex
Sudbury copper-nickel mines consist of an
inordinate number of rock and mineral types,
from felsic to ultramafic and numerous
sulphides. Do the perceived cost savings
outweigh the risks?
Variability requires robustness, though
optimisation is generally the enemy of
robustness.
Preparation of tailings via one or more
dewatering methods can simplify the entire
operation and lower costs; continuous and/or