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Geotechnical News • September 2018
WASTE GEOTECHNICS
tailings initially. This and the difficulty
of recovering water from the pit, mean
that the tailings may remain flooded
and under-consolidated, rapidly filling
the pit, mainly with water. This is
partially overcome by thickening the
coal tailings prior to in-pit disposal,
although coal mineral-rich coal tail-
ings have proven difficult to thicken.
A number of newer coal projects in
Eastern Australia has adopted belt
press filtering of the tailings, and the
mixing of the output with coarse reject
for disposal within the spoil piles.
However, Smectite-rich coal tailings
have proved difficult to filter, resulting
in a wet mixture of tailings and coarse
reject, and hence a wet co-disposed
mixture within the spoil. A small num-
ber of coal mines in Eastern Australia
has employed either on-off temporary
surface tailings storage cells in which
the tailings are desiccated and har-
vested periodically and dumped with
coarse-grained wastes, or pressure
plate filtration. The tailings manage-
ment options considered are:
1. A series of surface TSFs.
2. An in-pit TSF.
3. On-off temporary surface tailings
storage cells.
4. Pressure filtration and “dry” dis-
posal of the tailings filter cake with
coarse-grained wastes.
5. A surface TSF until a completed
pit becomes available for tailings
disposal.
NPV analysis of capital and
operating costs
Figure 1 illustrates NPV capital
and operating cost comparisons for
Discount Factors of 2.5% (close to the
Consumer Price Index), 5% and 10%,
applied to the tailings management
options considered. These compari-
sons do not include closure and reha-
bilitation costs.
The high costs of the series of surface
TSFs would surprise many, but arise
from the flat terrain of the Eastern
Australian coalfields providing limited
“free” valley storage for tailings and
requiring an ever-increasing length of
dam around the perimeter of a surface
TSF. This limits the height of each
TSF due to the excessive cost of dam
raising and the ever-increasing length
of perimeter dam required, forcing a
new surface TSF to be constructed.
An in-pit TSF is the least expensive
option, but this assumes that a pit
is available from the start, which
is unlikely. The other three options
attract similar capital and operating
costs. The costs of on-off temporary
tailings cells are dominated by high
re-handling costs, although drying by
the sun and wind is relatively robust,
with high rainfall affecting less than
30 days per year on average in the
Eastern Australian climate. The costs
of pressure filtration
are dominated
by the high
up-front cost
of the equip-
ment, which is
the main reason
why this option
is often removed
from consider-
ation early in
the selection
process. Pressure
filtration is also
sensitive to the
inevitably vari-
able input stream, resulting in opera-
tional difficulties, particularly for clay
mineral-rich coal tailings. The initial
use of a surface TSF until a completed
pit becomes available for tailings
disposal appears to be marginally the
best option.
Including closure and rehabilitation
costs
Figures 2 and 3 show, respectively,
NPV comparisons at Discount Fac-
tors of 10% and 2.5%, for a series of
surface TSFs, including operational
and rehabilitation costs. Three surface
TSFs are operated during the 20-year
life of the coal mine, the first to year
7, the second from year 7 to year 15,
and the last from year 15 to year 20.
Rehabilitation of the TSFs is carried
out progressively, the first in year
7, the second in year 15 and the last
in year 20. A 10% Discount Factor
obscures the cost of TSF rehabilita-
tion, which is by far the major cost
impost in undiscounted terms, while
a more realistic 2.5% Discount Factor
better reflects the real and substantial
cost of rehabilitation. Leaving the
rehabilitation of all three TSFs to the
end of the mine life would reduce the
heavily discounted cost of rehabilita-
tion further, while likely to raise the
actual cost.
Rehabilitation security deposit
New South Wales and Queensland
Regulators require coal mine opera-
tors to assess and submit a Security
Deposit against future mine site reha-
bilitation. Both the New South Wales
Rehabilitation Cost Estimation Tool
,
and the Queensland Mining Financial
Assurance Calculator
provide the same
indicative costs for reshaping, cap-
ping/sealing tailings:
• AUD170 000/ha for tailings likely
to present considerable difficulties
due to reactive and/or soft tailings.
Figure 1. NPV comparisons of alternative tailings storage
scenarios.
