hydrogeochemical considerations related to disposal of...
TRANSCRIPT
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Hydrogeochemical Considerations related to Disposal of Thickened
Tailings, including Paste
Rens VerburgGolder Associates, Redmond, USA
15th ANNUAL BC/MEND ML/ARD WORKSHOPDecember 3rd and 4th, 2008
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Presentation Overview
IntroductionThickened Tailings
CharacteristicsPotential Benefits
Environmental BenefitsCase Study ACase Study B
Conclusions
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Introduction
Increased pressure for development of innovative mine waste disposal practicesPrompted by
Regulatory pressureSustainability principlesRisk aversionCost
Tailings: dry disposal
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Comparison of CharacteristicsSlurry Thickened Paste Filter Cake
Segregation High Slight None NoneSolids Content 30-50% 60-65% 70-75% 75-80%Supernatant Considerable Some None NonePost-Placement Shrinkage
High Some Insignificant None
Seepage High Some Insignificant NoneRehab Timing Delayed, often
considerableAlmost
ImmediatelyAlmost
ImmediatelyImmediately
Angle of Repose Up to 1o 1o to 3o 3o to 10o Up to Erosion Angle
Potential Water Recovery
Up to 53% Up to 67% Up to 82% Generally above 82%
Adopted from J. Johnson, Golder - Denver
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Potential Benefits of Dry Disposal
Siting flexibilityMinimal liquid/solid segregationMinimal particle segregationDenser/lower permeabilitySmaller footprintReduced need for damsExtend life of facilityReduced care and maintenance
Less water to manageLess seepage/ contaminant migrationWater conservationEngineered materialLess reactive (sulfide oxidation)Concurrent/progressive closureImproved aestheticsPotential for co-disposalEnhanced sustainability
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Fact or Fiction?
Assumed environmental benefit:Sulfide oxidation/ARD generation retarded due to high degree of saturation
However:Lack of operational verificationLack of systematic studies
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Case Study A
Arid region20-week HCT program
Three thickened tailings samplesFilter cakePasteThickened tailings
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Case Study A – Tailings Properties
Moisture content:Filter cake: 16%Paste: 22%Thickened tailings: 29%
Sulfide Sulfur: 31%NNP: -957 kg CaCO3/tonNPR: < 0.1
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HCT Results - pH
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HCT Results - Sulfate
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Case Study B - Neves Corvo Mine
Underground high-grade Cu-(Sn)-Zn mine in Iberian Pyrite Belt since 1989Lundin MiningVolcanogenic Massive Sulfide (VMS)
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Tailings ManagementUnderground paste backfill and in unlined tailings impoundment (135 ha, 15 Mt)Production of 42 Mt anticipated (14 Mt underground)Sustainable operational and post-closure tailings management: dry disposal vs. subaqueous deposition
No requirement for new dam raises (cost, risk)No increase in footprintNo requirement for maintaining pond in perpetuity (arid climate)Co-mixing with PAG waste rockConcurrent reclamation Regulatory pressures
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Case Study B – Tailings Properties
Moisture content:Filter cake: 18%Paste: 26%Thickened tailings: 31%
Sulfide Sulfur: 29%NNP: -880 kg CaCO3/tonNPR: < 0.1
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Tailings Environmental Stability
Bench-Scale (2000) Field Cell (2002 - 2005)
Paste Pilot (2005 - current)
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Bench-Scale Study - pH
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Bench-Scale Study - SC
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Paste Trial
35,000 m3 in 1-hectare areaObjectives:
Experience with plant operation/placementEnvironmental monitoring• Suction lysimeters, piezometers,
standpipes• Runoff collection
Geotechnical monitoring• Tensiometers• Berm design (PAG waste rock)
Trials of cover designs• Low-flux cover without capillary break• Low-flux cover with capillary break• Barrier cover (sand/bentonite)
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Pilot Cell Construction
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Cover Placement and Instrumentation
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Paste Trial
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Draindown Modeling
Unsaturated flow modeling for estimation of seepage
Ward Wilson (UBC)Fernando Junqueira (Golder-Burnaby)
SVFluxTM - SoilVision30-meter paste layer
No coverLow-flux coverBarrier cover
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100-Year Saturation Profile – No Cover
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100-Year Saturation Profile – All Scenarios
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Paste Trial - Pore Water pH TrendsControl (no cover)
250 mm 500 mm 1,000 mmWaste Rock
Barrier Cover (with bentonite)
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Paste Trial - Pore Water pH Trends
250 mm 500 mm 1,000 mm 250 mm 500 mm
250 mm 500 mm250 mm 500 mm
Waste Rock
Waste Rock Waste Rock
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Fact or Fiction?Fiction:
ARD not a concern when using thickened tailingsSaturation can be maintained in arid climate without engineered controls
Fact:Lag time to ARD is proportional to moisture content – “operational window”With engineered controls, saturation and prevention of ARD is feasibleCover design is critical• in arid climate, low-flux cover better than
barrier
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Conceptual Paste Placement
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Conceptual Paste Placement (cont’d)
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Conceptual Paste Placement (cont’d)
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Acknowledgments
Mafalda Oliveira (Lundin Mining)
Ward Wilson (UBC)
Fernando Junqueira (Golder – Burnaby)
Cheryl Ross (Golder – Redmond)
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Thank you for your attention
Any questions?