Water balance and cost evaluation for different scenarios of impermeable covers (raincoats) in heap leach pad operationsDaniel Pulcha

heap leach pad operationsa e u c a

Carlos CsarDenys ParraDenys ParraAnddes Asociados SAC, Lima, Peru

Content

Introduction

Content

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.

Introduction

Experience indicates that the use of raincoats reduce the long-term 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 inpads were analyzed; copper process in Brazil and gold process in Peru.

Hydrology: Precipitation and EvaporationFirst Case

y gy p pSecond Case

Water balanceWater balanceBased on the following basic equation:

Inflow Outflow Storage ChangeInflow Outflow = Storage Change

Parameters and simulation criteriaParameters and simulation criteriaThe 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 capacityponds, and their initial storage capacity.

Water balance scenariosFour water balance scenarios were analyzed:

Water balance scenarios

Scenario 1, the base case, heap leach pad without raincoats

Scenarios 2, 30% of raincoats.

Scenarios 3, 50% of raincoats.

Scenarios 4 80% of raincoatsScenarios 4, 80% of raincoats.

Pond sizing Pregnant leach solution (PLS). The capacity depends on

l hi ti diti it i th d i d

Pond sizing

leaching operating conditions, it is the draindown.

Stormwater pond. Sizing based on the largest volume f i i it ti ti d t i d ffor maximum precipitation contingency, determined for the most unfavorable monthly sequence in wet seasons.

Raincoat pond Sizing based on scenarios 2 3 and 4 Raincoat pond. Sizing based on scenarios 2, 3, and 4 (30, 50 and 80%), with a raincoat efficiency of 90%, a design storm event, and 2-hour periodic monitoring.g , p g

Water balance resultsThe evaluations were performed for the following maximum,

d i i i bl l

Water balance results

average, and minimum variable values:

Operation and contingency total maximum volume.

Fresh water demand.

Water discharge needs of pad-ponds system.

Total storage volume in water balance (m3)Total storage volume in water balance (m )

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)Fresh water demands (m /h)

The table shows fresh water demands for the system in dry season, considered as the most critical hydrological situation

Water balance summaryWater balance summary

Water balance shows relationships between stored volumes in stormwaterWater balance shows relationships between stored volumes in stormwaterand raincoat ponds and water treatment (detoxification) plant capacity for the simulated scenarios.

Cost evaluationCost evaluationCapex and Opex were estimated for each scenario:

Capex: construction cost of stormwater pond raincoat pond and Year 1 Capex: construction cost of stormwater 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.

$ 3 3 Second case (gold process): treatment cost US$ 3.0/m3 and 100 m3/h treatment plant cost US$ 2 million.

Estimated cost first caseEstimated cost first case

Description No raincoats (US$) 30% of raincoats (US$) 50% of raincoats

(US$) 80% of raincoats

(US$) Description No raincoats (US$) (US$) (US$) (US$) Stormwater pond 871,693.9 871,693.9 444,262.5 384,521.4

Earthworks 476,594 476,594 254,812 224,321 Geosynthetics 395 100 395 100 189 450 160 200 Geosynthetics 395,100 395,100 189,450 160,200 Raincoat pond 0 196,277.8 275,844.4 473,246.2

Earthworks 0 154,428 210,144 368,981 Geosynthetics 0 41 850 65 700 104 265 Geosynthetics 0 41,850 65,700 104,265

Raincoat system 0 760,099 1,261,082 2,017,731 Treatment plant and discharge volumes 65,552,735 44,394,095 32,288,795 10,000,000

Total cost 66,424,429 46,222,166 34,269,984 12,875,499

Estimated cost second caseEstimated cost second case

Description No raincoats (US$) 30% of raincoats (US$) 50% of raincoats

(US$) 80% of raincoats

(US$) (US$) (US$) (US$) Stormwater pond 2,047,752 1,857,689 1,848,618 1,840,965

Earthworks 1,889,965 1,700,660 1,692,394 1,686,427 Geosynthetics 157,787 157,029 156,224 154,538 Geosynthetics 157,787 157,029 156,224 154,538 Raincoat pond 0 422,700 463,688 499,126

Earthworks 0 404,623 423,966 451,345 Geosynthetics 0 18,077 39,721 47,781 y

Raincoat system 0 528,606 881,010 1,409,616 Treatment plant and discharge volumes 7,800,689 4,297,942 2,500,283 0

Total cost 9,848,441 7,106,937 5,693,599 3,749,707

Total cost summary (US$)Total cost summary (US$)

S i Fi S d Scenario First case Second case

No raincoats 66,424,429 9,848,441

30% f i 46 222 166 7 106 937 30% of raincoats 46,222,166 7,106,937

50% of raincoats 34,269,984 5,693,599

80% f i t 12 875 499 3 749 707 80% of raincoats 12,875,499 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.

RecommendationsIn heap leaching projects located in rainy regions, the use of raincoats is strongly recommended in order to:g y

Minimize the process solution dilution.

Reduce the need for stormwater pond storage and thereby Reduce the need for stormwater pond storage and thereby the size of storage ponds.

Reduce the treatment plant size Reduce the treatment plant size.

Reduce the water treatment cost.