Water balance and cost evaluation
for different scenarios of
impermeable covers (raincoats) in
heap leach pad operations
Daniel
a e Pulcha
uc a
Carlos César
Denys Parra
Anddes Asociados SAC, Lima, Peru
Content
• Introduction
• Hydrology
• Water balance scenarios
• Cost evaluation
• Conclusions and recommendations
Introduction
•
Some years ago, the use of impermeable covers or raincoats in
heap leach pad was restricted to minimum areas for cost reasons.
•
Experience indicates that the use of raincoats reduce the longterm operating costs.
•
Water balance refers to the interconnections among the heap
leach pad, the pregnant leach solution (PLS) pond, the
intermediate leach solution (ILS) pond if any, the stormwater
pond, and the raincoat pond.
•
Two different scenarios with raincoat placement in heap leach
pads were analyzed; copper process in Brazil and gold process in
Peru.
Hydrology:
and Evaporation
y
gy Precipitation
p
p
First Case
Second Case
Water balance
Based on the following basic equation:
Inflow − Outflow = Storage Change
Parameters and simulation criteria
The water balance model depends on: ore production plan,
stacking plan in the heap, raincoat installation area, ore
properties, irrigation type, precipitation, evaporation, size of the
ponds and their initial storage capacity
ponds,
capacity.
Water balance scenarios
Four water balance scenarios were analyzed:
• Scenario 1, the base case, heap leach pad without raincoats
• Scenarios 2, 30% of raincoats.
• Scenarios 3, 50% of raincoats.
• Scenarios 4
4, 80% of raincoats
raincoats.
Pond sizing
• Pregnant leach solution (PLS). The capacity depends on
l
leaching
hi operating
ti conditions,
diti
it iis th
the draindown.
d i d
• Stormwater pond. Sizing based on the largest volume
for
precipitation
determined
f maximum
i
i it ti contingency,
ti
d
t
i d ffor
the most unfavorable monthly sequence in wet seasons.
• Raincoat pond
pond. Sizing based on scenarios 2
2, 3
3, and 4
(30, 50 and 80%), with a raincoat efficiency of 90%, a
g storm event,, and 2-hour p
g
periodic monitoring.
design
Water balance results
The evaluations were performed for the following maximum,
average, and
d minimum
i i
variable
i bl values:
l
• Operation and contingency total maximum volume.
• Fresh water demand.
• Water discharge needs of pad-ponds system.
Total storage volume in water balance (m3)
The table shows the water balance storage volumes based on the
most critical hydrological situation for each case being analyzed.
Fresh water demands (m3/h)
The table shows fresh water demands for the system in dry
season, considered as the most critical hydrological situation
Water balance summary
Water balance shows relationships between stored volumes in stormwater
and raincoat ponds and water treatment (detoxification) plant capacity for
the simulated scenarios.
Cost evaluation
Capex and Opex were estimated for each scenario:
•
Capex: construction cost of stormwater pond
pond, raincoat pond and Year 1
treatment plant..
•
Opex: raincoat system cost per year, assuming 30% of geomembrane can
be reused or recovered.
•
Sustaining capital cost: treatment plant cost per stages after Year 1.
•
First case (copper process): treatment cost US$ 2.5/m3 and 100 m3/h
treatment plant cost US$ 10 million.
•
Second case (gold process): treatment cost US$
$ 3.0/m3 and 100 m3/h
treatment plant cost US$ 2 million.
Estimated cost – first case
Description
No raincoats (US$)
Stormwater pond
Earthworks
Geosynthetics
Raincoat pond
Earthworks
Geosynthetics
Raincoat system
Treatment plant and
discharge volumes
Total cost
871,693.9
476,594
395 100
395,100
0
0
0
0
30% of raincoats 50% of raincoats 80% of raincoats
(US$)
(US$)
(US$)
871,693.9
444,262.5
384,521.4
476,594
254,812
224,321
395 100
395,100
189 450
189,450
160 200
160,200
196,277.8
275,844.4
473,246.2
154,428
210,144
368,981
41 850
41,850
65 700
65,700
104 265
104,265
760,099
1,261,082
2,017,731
65,552,735
44,394,095
32,288,795
10,000,000
66,424,429
46,222,166
34,269,984
12,875,499
Estimated cost – second case
Description
No raincoats (US$)
Stormwater pond
Earthworks
Geosynthetics
Raincoat pond
Earthworks
Geosynthetics
y
Raincoat system
Treatment plant and
discharge volumes
Total cost
2,047,752
1,889,965
157,787
0
0
0
0
30% of raincoats 50% of raincoats 80% of raincoats
(US$)
(US$)
(US$)
1,857,689
1,848,618
1,840,965
1,700,660
1,692,394
1,686,427
157,029
156,224
154,538
422,700
463,688
499,126
404,623
423,966
451,345
18,077
39,721
47,781
528,606
881,010
1,409,616
7,800,689
4,297,942
2,500,283
0
9,848,441
7,106,937
5,693,599
3,749,707
Total cost summary (US$)
S
Scenario
i
Fi case
First
S d case
Second
No raincoats
66,424,429
9,848,441
30% off raincoats
i
46 222 166
46,222,166
7 106 937
7,106,937
50% of raincoats
34,269,984
5,693,599
80% off raincoats
i t
12 875 499
12,875,499
3 749 707
3,749,707
Conclusions
• Fresh water entrance is required every month, even in wet
year conditions.
• Earthworks and geosynthetics costs for pond construction
(stormwater and raincoat) are very low compared with
operating costs.
• The higher the raincoat coverage in the heap, the lower the
total project cost (Capex + Opex).
• If water treatment or plant costs are higher than those
considered in this analysis, the differences between
scenarios would be even higher.
Recommendations
In heap leaching projects located in rainy regions, the use of
g y recommended in order to:
raincoats is strongly
• Minimize the process solution dilution.
• Reduce the need for stormwater pond storage and thereby
the size of storage ponds.
• Reduce the treatment plant size.
size
• Reduce the water treatment cost.