How the mining sector is
responding to change in
disposal of tailings and waste
rock
By Frank Palkovits
Originally published in
Engineering and Mining Journal
December 2007
THE LAYOUT OF THIS ARTICLE HAS BEEN CHANGED FOR THE PURPOSES OF THIS REPRINT.
How the mining sector is responding to change in
disposal of tailings and waste rock
ENGINEERING AND MINING JOUNRAL/ DECEMBER 2007
I
f you’re feeling squeezed by
environmental and social pressures to
find better ways to deal with tailings
and waste rock, you’re not alone.
Consider:
Today, it is difficult if not impossible
to get permission to use a lake or wetland
for tailings disposal. Even if it is permitted,
there usually must be compensation.
Marine disposal is rarely even considered,
because of international opposition.
In
some
jurisdictions,
valley
deposition is discouraged, particularly if
there are potential impacts on downstream
watercourses. Don’t think this is a problemmore site selection study…Don will know.
“Sustainability” has moved from
being seen as an impractical dream into the
mainstream. Local communities have a
bigger say in amine’s development, and
governments are keenly interested in
whether the local environmental, social and
economic needs will be met long-term. The
Equator Principles, to which many lending
agencies now adhere, link financing to
sustainable development.
Water has become one of the major
sustainability issues. Recent mining in one
of the drier parts of South America has
been hampered by a water shortage and the
government’s prohibition on additional
permits to take water, due to impacts on the
local population. The taking of water is not
the only issue – there are also higher
standards for water treatment prior to
disposal.
Partly because of the increasing
scrutiny of financial institutions, nongovernmental organizations (NGOs) and
governments, mining companies are
looking for better ways to manage risk.
This includes any risks that stem from
conventional tailings disposal facilities.
Failures of conventional tailings facilities
tend to be caused by dam overtopping,
embankment failure and foundation failure.
Because “risk” is the probability of an
occurrence multiplied by the consequences,
one of the best ways to manage risk in
tailings facilities is to minimize (or
eliminate) ponded water on top of the
tailings. Reduce the consequence by
eliminating water ponds.
Waste management is becoming a
bigger factor in mining companies’
considerations, right from the start of the
planning
process.
Traditionally,
maximizing productivity and recovery in
order to grow the top line – gross revenue –
in a company’s income statement has been
the major consideration. However, it is
increasingly important to control the factors
that influence the bottom line, which is the
after-cost net revenue. Waste management,
closure costs and ongoing obligations, risks
and liabilities increasingly influence
planning and operational decisions. So, we
see that the “tail” of post-production issues
is starting to wag the “dog” of exploration
and production.
The result of the combination of these
pressures is that mining company
executives feel like the toothpaste in a tube
that is being squeezed by several people at
once. And as it happens, one possible
solution involves a substance that can have
the consistency of toothpaste.
This is “paste,” a homogeneous nonsegregating material defined as tailings with
minimal water content that typically
sufficient water to allow pumping and
pipeline transport. Paste travels as a plug
flow and exits the pipe looking and
flowing much like toothpaste.
However, paste is only one subset of a
larger range of formulations of “thickened
tailings,” and the right form for each
situation depends on a variety of factors, as
we’ll see. For hard rock tailings, the typical
slrry densities are shown below:
Conventional slurry is the most
dilute form, typically 30 to 50 percent
solids, and can be moved with a centrifugal
pump. Given a water-constrained world,
slurry may see less application in future
than before.
Thickened non-segregating slurry is
typically 60 to 65 percent solids, and can
also be moved with a centrifugal pump.
Filtered tailings, generally over 80
percent solids, is too thick to pump – so
must be moved on a conveyor belt or by
truck.
Choosing between thickened tailings and
the alternatives
As with any technology, there are situations
in which thickened tailings may be the best
solution, and those where other methods
may be best..
There is the financial cost of the
equipment to be considered, and this cost
must be borne before production begins and
revenue flows. However, the capital cost is
often less than that for the dam needed for
conventional slurry deposition. The
operating cost may be higher than it is for
conventional deposition, partly because of
the more complex machinery involved.
There also may be some difficulty
predicting the stability of the thickened
tailings beaches at the design stage.
Experience has shown that some ores,
such as kimberlites, tend to not “thicken”
well. have large variability in material
characteristics and can flash-set when
mixed with cementituous products in
backfill.
Technical solutions for these issues
will likely develop sooner rather than later,
given the need to deal with issues such as
those listed at the start of this article.
Another factor is that not all mines
have tailings suitable for thickened
applications. Some of this is because a
major factor in the behaviour of the
thickened tailings is the grain size. Mine
executives will generally choose a grain
size that maximizes recovery, and this is
not necessarily the optimal size for
thickening purposes. However, as mining
companies become increasingly concerned
with closure and ongoing costs, this picture
may change.
One of the most important benefits to
tailings thickening is that it can reduce or
eliminate the risk posed by ponded water on
top of conventional tailings sites. This
significantly reduces ongoing post-closure
costs and liabilities. While thickened
requiring require sub-water deposition, are
placed upstream from the containment dam,
while the unsaturated free-draining sand
underflow is placed downstream of the
dam. Typically 65 percent solids, the
cyclone sand underflow can be moved to its
storage site by a positive displacement
pump.
tailings cannot for the foreseeable future
promise complete “walk-away closure,” it
is a big step towards that elusive but
desirable goal.
Transporting tailings in paste form to
the disposal site uses less water than does
slurry, so that water-management issues are
much reduced. In drier parts of the world
we see increasing pressures to reduce water
use, and these concerns are also appearing
in areas with more plentiful rainfall.
Because there is no particle segregation
with thickened tailings, the tailings mass is
denser, requiring less geographic area that
must be permitted. The density also allows
better control of wind and water erosion,
further reducing environmental risks.
In many cases thickened tailings can be
stacked, again reducing the surface area
needed. In situations where disposal space
is limited, having a practical tailingsdisposal option may mean the difference
between being able to mine a promising ore
body, or not.
Thickened tailings deposits can generally
be driven-over sooner than can other forms
of disposal, so that they can be vegetated.
This facilitates progressive closure – one
part of the facility can be growing
vegetation while another is being filled.
This helps tie closure costs more closely to
the revenue associated with those tailings. It
also has a positive public-relations aspect in
that the company can point to the grasses,
shrubs and trees on the tailings site as
evidence of its good corporate behavior.
Choice of disposal options depends on
circumstances
There is a wide range of disposal options
for thickened tailings, and the right choice
depends on mineralogy, the degree of
weathering of the parent ore, mineral
extraction process, the degree of grinding,
the reagents used in the milling process,
acid generating potential, the specific
gravity of the tailings particles and other
factors.
Surface disposal
While some surface disposal of thickened
tailings involves a significant containment
berm, one advantage of paste is that a major
containment structure may be unnecessary.
These higher-density formulations of
thickened tailings have only minor
propensity to bleed water, so that just a
small berm around the deposit is needed to
catch the bleed and precipitation runoff.
It is important to lay down the
deposits in thin layers, and that these layers
be allowed to dessicate between
applications. When deposition is complete,
vegetation can be planted to stabilize the
surface and reduce the risk of wind and
water erosion.
Disposal in mined-out open pits
While disposing of tailings in an existing
open pit has great advantages in that it does
not require additional area to be permitted,
it has its challenges. The first is that
companies are naturally reluctant to deposit
tailings in a pit until they are sure they have
extracted all of the economic ore – and the
definition of “economic” is partly a
function of the commodity price. Also, an
open pit is sometimes just the start of a
mining process that continues underground.
The viability of open-pit disposal depends
partly on the groundwater regime and the
permeability of the surrounding rock. It has
been a solution with uranium tailings in
Saskatchewan, Canada with special
measures taken to understand the hydrogeology and seepage issues.
Cyclone sand dam
Tailings ponds can be smaller if there is
less mine waste that must be covered with
water. A cyclone station can separate the
tailings. The cyclone’s over-flow of fines,
Co-disposal of thickened tailings and
waste rock
One of the recent developments in the
search for better mine-waste disposal issues
is “paste rock.” This emerging technology
involves converting tailings to paste form,
and this thickened material is mixed with
chunks of waste rock, filling in the voids
between the rock pieces. This makes for a
more efficient use of geographic area, and
puts the tailings into an inert, inaccessible
form that uses little water and produces
even less.
In an open pit operation, it may be
possible and practical to fit all of the
tailings into the voids in the rock –
rendering a traditional tailings pond
unnecessary. Studies indicate that this may
not be as possible for underground mining,
largely because there is less availability of
waste rock. Some additional storage for
tailings may be necessary – possibly as a
layer of paste rock as cover material over
the surface paste deposit.
Paste rock, as with other aspects of
thickened tailings, involves tried-and-true
technologies but is also an area of rapid
technological growth. These developments
hold significant promise for the mining
sector in its drive to manage its
environmental and social impacts.
ENGINEERING AND MINING JOUNRAL/ DECEMBER 2007
Underground disposal
Underground deposition of tailings can
reduce the amount of surface area required.
However,
surface disposal cannot be
eliminated. At best, about 60% of the
tailings can be placed underground, in part
because the density of the tailings is about
half that of the ore removed. As well as
reducing the surface-disposal footprint,
underground disposal of cemented tailings
can provide ground support to minimize or
prevent subsistence or collapses to surface.
This is becoming increasingly important as
a way to limit post-closure liabilities.
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ABOUT AUTHOR
Frank Palkovits, P.Eng. ([email protected]: +1.705.524.5533) is
senior project engineer with Golder Paste Technology, based in
Sudbury, Canada.