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Case Study: Site-Wide Water Balance of the Pierina Gold Mine, Peru
Liane George Geological Engineer, WMC/SWS
Denver, Colorado
William Ludwick Manager of Hydrology, South American Region, Barrick Gold
Lima, Peru
John Chahbandour Regional Operations Manager-US/Mexico, WMC/SWS
Denver, Colorado
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Introduction
Produced ~ 45,000 tonnes per day of gold-silver ore in 2007 Began operations in 1998 Active mining will continue until 2011Mine split between two watersheds:
The open pit in Puca Uran watershedProcess facilities are in Pacchac watershed
Focus of projectProcess facilities cover an area of about 5.4 km2
Waste dump split between both watersheds
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Puca UranWatershed
Pacchac Watershed
Open Pit
Processing Facilities
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Scope
Tool for daily solution management Compliance with the International Cyanide CodeHelp prevent unintentional releases:
failure of pumping equipment extreme precipitation events
Account for mine expansionEstimate make-up water or excess solution in the leach pad
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Climate
PrecipitationWet season from October to AprilDry season from May to September Average annual is approximately 1.16 m
EvaporationAverage annual pan evaporation is approximately 1.11 mPond (open water) evaporation
0.8*(pan evap) = 0.88 mNet positive
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Climate
Pacchac Meterological Station Average Monthly Precipitation and Evaporation (mm)
0
50
100
150
200
250
300
Aver
age
Dept
h (m
m)
Precipitat ion 134.3 176.1 239.7 119.3 37.5 8.8 2.2 10.5 51.5 100.3 103.3 173.1
Evaporat ion 105.4 59.4 48.3 68.1 96.8 87.0 123.7 126.4 92.5 119.9 111.9 72.8
Janurary February M arch April M ay June July August September October November December
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Site Facilities
Six holding pondsLeach pad and caisson platformProcessing plantAcid Rock Drainage (ARD) treatment plant Two diversion ditches (SDD and NDD)
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Conceptual Model
Site-wide water balance model Individual facility water balances integrated into one modelIncludes flows that are not currently in use, but are required for predictive purposes Run on a daily time-step
For use as a daily solution management tool
Complex water and solution flow
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Arrows indicate direction of flow for solution/ water on site.
Main facilities shown in bold.
All flows indicated are represented in the model.
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Climate Management
Historical climate data were used when available Statistics from historic daily data (2000-2006) used for predictive model runs:
mean standard deviation precipitation - normal distributionevaporation - log-normal distribution, inversely related to precipitationcreates storm events
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Surface Water Management
Surface water runoff volumesNRCS curve number method
Antecedent Moisture ConditionsModel looks at the five previous days Higher runoff if previous days are wet
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Process Water Flow Management
Flow through the leach padDelayed by:
pore-space wet-up flow-throughand drain-down.
Complex leaching schedule based on actual process at mine
Piping/diversionsFlows are instantaneousOn a daily time-step, this travel time is not a major concern
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Model Calibration
Match the simulated water elevations to historically recorded elevations in:
4 of the 6 holding ponds No historical data available to calibrate the sedimentation and raincoat pondsCaissons in the PLS solution dike
January to August, 2006 most comprehensive datasetnormal operating conditions
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Calibration Approach
Some process flows were not recorded, or had significant measurement error
reasonable adjustments to the historical data based on quality of the data evaluated determined possible error (+/-)removed outliers
maximum flows estimated typical pump rates limited to 150 m3/hrflow limit for most of Pierina’s pumps and conveyance systems
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Results
Caissons, collection and polishing pondsDifference between was typically less than one meterTrends well matched
Sludge ponds Difference of up to two metersTotal volume of the sludge ponds is relatively small when compared to the other storage facilities Trend well matched
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Collection Pond Water Level (m)
385038523854385638583860
12/31/2005 3/1/2006 4/30/2006 6/29/2006 8/28/2006
Date
Stag
e (m
) AM
SL
Historical Stage (m) Modeled Stage (m)
Caisson Water Level (m)
39723974397639783980398239843986
12/31/2005 3/31/2006 6/29/2006
Date
Stag
e (m
) AM
SL
Historical Stage (m) Modeled Stage (m)Sludge Ponds Water Level (m)
3920
3922
3924
3926
3928
12/31/2005 3/31/2006 6/29/2006 9/27/2006
Date
Stag
e (m
) AM
SL
Historical Stage (m) Modeled Stage (m)
Polishing Pond Water Level (m)
38233824382538263827382838293830
12/31/2005 3/31/2006 6/29/2006
Date
Stag
e (m
) AM
SL
Historical Stage (m) Modeled Stage (m)
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Predictive Runs
Loss of PLS Caisson Pumping CapacityGoal – dike can not overtopThree scenarios:
barren solution application continues as normalbarren solution application reduced by 50 percentbarren solution application ceases
Extreme precipitation eventGoal – no offsite discharge
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Loss of PLS Pumping Capacity
Caisson Platform Pumping ceasedStart at high elevation for normal operating conditions available storage volume of ~350,000 m3
Results barren solution application continues as normal
Overtops in 7 daysNot recommended
barren solution application reduced by 50 percentOvertops in 12 days Will give operators more time (almost two weeks) for repairs
barren solution application ceasesDoes not overtop within 20 days (max pump shutdown period in model)Should be enough time to make needed repairs to system
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Extreme precipitation event
Synthetic 100-year, 24-hr storm 98 mm (3.9 inches)during the wet season The starting elevations of water in the ponds based on normal operating levels during the wet season
ResultsSedimentation and raincoat ponds overflow Water not transferred to the SDD Enough storage available in collection pondNo off-site discharge necessary
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Conclusions
The Site-Wide Water BalanceProvides information required to safely manage daily solution transfer and volumes. Can be used as a decision-making tool by creating predictive runs. Can be used by mine personnel for day-to-day solution management
Graphical User Interface (GUI) easy to learneasy to use
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Model GUI
Questions?Comments?