Tailings (also known as
slimes,
tailings pile,
tails,
leach residue, or
slickens) are
the materials left over after the process of separating the
valuable fraction from the worthless fraction (
gangue) of an
ore. The extraction
of minerals from ore requires that the ore be ground into fine
particles, so tailings are typically small and range from the size
of a grain of sand to a few microns. Mine tailings are usually
produced from the
mill (grinding) in
slurry form (a mixture of fine mineral particles and
water).
Tailings represent an
external cost of
mining. Tailings dams are often the most significant environmental
liability for a mining project.
In
coal and
oil
sands mining, the word 'tailings' refers specifically to fine
waste suspended in water.
Tailings composition
The composition of tailings is directly dependent on the
composition of the ore and the process of mineral extraction used
on the ore.
Certain types of extraction process, like
heap leaching for example, may result in
quantities of chemicals used to perform the leaching remaining in
the material once leaching has been completed. Older forms of
mineral extraction, such as those utilised during the early gold
boom years of Australian gold mining, resulted in large heaps of
fine tailings being left dotted around the landscape. These
tailings dumps would continue to leach residual chemicals into the
environment, and if weather conditions allowed it the finer
fraction would become windborne, blowing around the townships
surrounding the now-dormant mining areas.
Typically, the bulk quantity of a tailings product will be barren
rock, crushed and ground to a fine size ranging from coarse sands
down to a talcum powder consistency.
Tailings may contain trace quantities of metals found in the host
ore, and they may contain minute amounts of added compounds used in
the extraction process. Elements are rarely in elemental form, more
often as complex compounds.
Common minerals and elements found in tailings include
Common additives found in tailings
- Cyanide - as both sodium cyanide (NaCN)
and hydrogen cyanide (HCN).
Leaching agent in extremely dilute quantities which readily
volatize upon exposure to sunlight.
- SEX - Sodium Ethyl Xanthate. Flotation
agent.
- PAX - Potassium Amyl Xanthate. Flotation agent.
- MIBC - Methyl Isobutyl Carbinol. Frothing agent.
- Sulfamic acid - Cleaning /
descaling agent.
- Sulfuric acid - Used in large
quantities in the PAL process (Pressure Acid
Leaching).
- Activated Carbon - Used in CIP
(Carbon In Pulp) and CIL (Carbon In
Leach) processes.
- Calcium - Different compounds,
introduced as lime to aid in pH
control.
Tailings present a long term cost to the mining industry. If the
company leaves or goes bankrupt, the local government can find
itself with responsibility for the maintenance and monitoring of
tailings dumps essentially forever - this, and other costs of
cleanup, can impose liabilities that were estimated at up to 12
billion dollars in the US alone in 2005
Tailings are not an option for a mine that has mills it ore. The
option present is how to store those tailings for what needs to be
considered perpetuity and the article below describes some of
alternative storage methods available to the modern mining
industry.
Environmental and social considerations
In the past, non-environmentally friendly methods (see "Storage
Methods" below) were the method of the day. Some mining operations
use environmentally friendly storage options, others use less
environmentally friendly ones. The bulk of these operations tend to
exist in developing nations where legislative requirements are more
permissive than in industrialized countries, as these developing
countries want the same economic advantages the developed countries
have (and obtained without the stringent environmental legislation
that these developed countries want to impose on the developing
countries).
Reprocessing of old tailings impoundments and dams has assisted in
the cleaning up of legacy tailings dumps, with the reprocessed
tailings being stored in a more effective method.
Special consideration needs to be taken in the case of
uranium tailings, whose radioactivity makes
it easy to detect their diffusion into the environment.
Tailings storage methods
Tailings continuum
Summary of the range of tailings
products
Historically, tailings were disposed of however was convenient,
such as in downstream running water or down
drains. Because of concerns about these sediments in
the water and other issues, tailings ponds began to be constructed,
which were bounded by impoundments (an impoundment is a dam). These
dams typically use "local materials" including the tailings
themselves, and may be considered
embankment dams. Traditionally, the only
option for tailings storage was to deal with a tailings slurry.
This slurry was a dilute stream of the tailings solids within water
that was sent to the tailings storage area. The modern tailings
designer has a range of tailings products to choose from depending
upon how much water is removed from the slurry prior to discharge.
The removal of water not only can create a better storage system in
some cases (e.g. dry stacking, see below) but can also assist in
water recovery which is a major issue as many mines are in arid
regions. In a 1994 description of tailings impoundments, however,
the U.S. EPA stated that dewatering methods may be prohibitively
expensive except in special circumstances. Subaqueous storage of
tailings has also been used.
Pond storage
Tailing ponds are areas of refused
mining tailings where the water borne refuse material
is made into a
ponded to allow the
sedimentation (meaning separation) of solid
particles from the water. The pond is generally impounded with a
dam, and known as tailings impoundments or tailings dams. It was
estimated in 2000 that there were about 3,500 active tailings
impoundments in the world. The ponded water is of some benefit as
it minimizes fine tailings from being transported by wind into
populated areas where the toxic chemicals could be potentially
hazardous to human health; however, it is also harmful to the
environment. Tailing ponds are often somewhat dangerous because
they attract wildlife such as waterfowl or caribou as they appear
to be a natural pond, but they can be highly toxic and harmful to
the health of these animals. Tailings ponds are used to store the
waste made from separating minerals from rocks. Tailings are
sometimes mixed with other materials such as
bentonite to form a thicker slurry that slows the
release of impacted water to the environment.
There are many different subsets of this method, including valley
impoundments, ring dikes, in-pit impoundments, and specially-dug
pits. The most common is the valley pond, which takes advantage of
the natural topographical depression in the ground. Large earthen
dams may be constructed and then filled with the tailings.
Exhausted open pit mines may be refilled with tailings. In all
instances, due consideration must be made to contamination of the
underlying water table, amongst other issues. Dewatering is an
important part of pond storage, as the tailings are added to the
storage facility the water is removed - usually by draining into
decant tower structures. The water removed can thus be reused in
the processing cycle. Once a storage facility is filled and
completed, the surface can be covered with topsoil and revegetation
commenced. However, unless a non-permeable capping method is used,
water that infiltrates into the storage facility will have to be
continually pumped out into the future.
The
biggest danger of tailings ponds is dam failure, with the most
publicized failure in the US being the failure of a coal slurry dam
in the West
Virginia
Buffalo Creek
disaster
, which killed 125 people; other collapses include
the Ok Tedi environmental
disaster on New
Guinea
, which destroyed the fishery of the Ok Tedi River. On the average,
worldwide, there is one big accident involving a tailings dam each
year. Tailings ponds can also be a source of
acid drainage, leading to the need for
permanent monitoring and treatment of water passing through the
tailings dam; the cost of mine cleanup has typically been 10 times
that of mining industry estimates when acid drainage was
involved.
Dry stacking
Tailings do not have to be stored in ponds or sent as slurries into
oceans, rivers or streams. There is a growing use of the practice
of dewatering tailings using vacuum or pressure filters so the
tailings can then be stacked. This saves water, reduces the impacts
on the environment in terms of space used, leaves the tailings in a
dense and stable arrangement and eliminates the long-term liability
that ponds leave after mining is finished.
Storage in underground workings
While disposal into exhausted open pits is generally a
straightforward operation, disposal into underground voids is more
complex. A common modern approach is to mix a certain quantity of
tailings with waste aggregate and cement, creating a product that
can be used to backfill underground voids and
stopes. A common term for this is
HDPF - High Density Paste Fill. HDPF is a more expensive method of
tailings disposal than pond storage, however it has many other
benefits – not just environmental but it can significantly increase
the stability of underground excavations by providing a means for
ground stress to be transmitted across voids - rather than having
to pass around them – which can cause mining induced seismic events
like that suffered previously at the
Beaconsfield Mine Disaster
Riverine tailings
Usually called RTD – Riverine Tailings Disposal.
In most environments,
not a particularly environmentally sound practice, it has seen
significant utilisation in the past, leading to such spectacular
environmental damage as done by the Mount Lyell Mining and
Railway Company in Tasmania
to the
King River, or the poisoning
from the Panguna
mine on
Bougainville
Island
, which led to lage-scale civil unrest on the
island, and the eventual permanent closing of the
mine.
As of
2005, only three mines operated by international companies
continued to use river disposal: The Ok
Tedi mine, the Grasberg
mine
and the Porgera
mine, all on New Guinea.
Submarine tailings
Commonly referred to as STD (Submarine Tailings Disposal) or DSTD
(Deep Sea Tailings Disposal). Tailings can be conveyed using a
pipeline then discharged so as to eventually descend into the
depths. Practically, it is not an ideal method, as the close
proximity to off-shelf depths is rare. When STD is used, the depth
of discharge is often what would be considered shallow, and
extensive damage to the seafloor can result due to covering by the
tailings product. It is also critical to control the density and
temperature of the tailings product, to prevent it from travelling
long distances, or even floating to the surface.
The Solwara project being commenced in the Bismarck Sea
by Nautilus
Minerals proposes to use a modified STD method back down to
depths below 1500 metres.
This
method is used by the gold mine on Lihir Island
; its waste disposal has been viewed by
environmentalists as highly damaging, while the owners claim that
it is not harmful.
Phytostabilization
Phytostabilization is a form of
phytoremediation that uses plants for
long-term stabilization and containment of tailings, by
sequestering pollutants in soil near the roots. The plant's
presence can reduce wind erosion, or the plant's roots can prevent
water erosion, immobilize metals by adsorption or accumulation, and
provide a zone around the roots where the metals can precipitate
and stabilize. Pollutants become less bioavailable and livestock,
wildlife, and human exposure is reduced. This approach can be
especially useful in dry environments, which are subject to wind
and water dispersion. New work is also being done by Pan Pacific in
the development of algal sequestration for plutonium and uranium
tailings.
Different methods
Considerable effort and research continues to be made into
discovering and refining better methods of tailings disposal.
Research at the
Porgera Gold Mine
is focusing on developing a method of combining tailings products
with coarse waste rock and waste muds to create a product that can
be stored on the surface in generic-looking waste dumps or
stockpiles. This would allow the current use of rivering disposal
to cease. Considerable work remains to be done. However,
co-disposal has been successfully implemented by several designers
including
AMEC at, for example, the Elkview
Mine in British Columbia.
Tailings reprocessing
As mining techniques and the price of minerals improve, it is not
unusual for tailings to be reprocessed using new methods, or more
thoroughly with old methods, to recover additional minerals.
Extensive tailings dumps of Kalgoorlie / Boulder in Western
Australia were re-processed profitably in the 1990s by KalTails
Mining.
See also
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