Mitigating Acid Rock Drainage From Mine Facilities in a Tropical Climate. Larry Breckenridge, P.E.

Transcription

1 Mitigating Acid Rock Drainage From Mine Facilities in a Tropical Climate Larry Breckenridge, P.E.

2 Project Background Proposed 70,000 TPD open pit mine Copper and Gold 20 year mine life ~540 million cubic meter pit lake ~1 billion tonnes of waste rock Tropical climate 3.5 meters of precipitation per year 2

3 Project Goals Define geochemical properties of site materials Determine environmental impacts of post closure pit lake Determine quality and quantity of waste rock dump leachate Mitigate ARD in the pit lake and waste dumps Design with clean closure in mind 3

4 Geochemical Testing: Project Overview Waste Rock Static Testing Kinetic Testing Pit Lake Static Testing Kinetic Testing Water Quality Testing Phased sampling approach for additional data 4

5 Static Testing Categories Whole Rock Analysis Assay Metals by ICP-MS Metals by Aqua Regia Leachate Analysis ph and conductivity Leach extraction for inorganics and metals 5

6 Kinetic Testing Categories Multiple phases Laboratory humidity cell tests On-site kinetic leach tests Tests run long term 2+ years 6

7 Oxide Saprolite Material is heavily leached Average 0.03% sulfur Metals concentrations very low No neutralization capacity Leachate is benign ph of 5.6 (equivalent to rain water) Low TDS 7

8 Sulfide Saprolite Acid generating Paste ph<3.0 High TDS leachate Consistent over site 8

9 Weathered Rock Generally acid producing Averages 2.3% Sulfide T/KT Net Neutralization Potential Some CaCO 3 Hosts aquifer beneath the project 9

10 Unweathered Rock Increasing NP with depth Some unweathered sulfide Forms the majority of the waste and pit wall 10

11 Geochemical Characterization over Time and Space Mine plan Geologic block model Waste rock hauling plan Geochemical characterization 11

12 Multidisciplinary Modeling Methods Hydrological modeling Water balance Groundwater modeling (MODFLOW-Surfact) Pit filling Lake/groundwater interactions after filling Limnilogical modeling (THERMS & Lake Number) Thermal stratification Stratification stability Geochemical modeling (PHREEQCI) Pit lake water chemistry 12

13 Pit Wall Geology Oxide saprolite (not acid generating) Sulfide saprolite (acid generating) Acid generating rock Moderate acid generating rock Neutralizing rock 13

14 Surface flows, storm water spillover Cold, low TDS precipitation Wind-induced water mixing m Limited acid production Oxide Saprolite Sulfide Saprolite Fractured Rock Unweathered Rock Surface Water Flow Precipitation Wind Limited acid production Acid Generation Acid 14 Neutralization Mixing Zone Reactive wall rock isolated by engineering controls (8,460 m 2 ) Wind and rainwater mixing Warm/Cold Water Thermocline Stagnant Zone Water is anoxic, acid generation reactions stop 1-year after stratification Calcite from wall rock pores ~20m Pit Lake Conceptual Model Acid from wall rock pores Acid from wall rock pores

15 Pit Lake Hydrology With berm, pit lake will be nearly a terminal sump Excess water discharged from spillway Insignificant groundwater/lake water exchange 15

16 The Path to Clean-Closure Rapid filling stratifies lake Oxic water in contact with leached-out soil Reactive soil and rock in anoxic, stagnant zone One-time treatment with lime Water in contact with environment stays clean No perpetual treatment required 16

17 Pyramid Building: Waste Rock Dump ~ 1 billion tonnes of waste Oxide saprolite stockpiled Lower benches sulphide saprolite (comes out first) Buttress of Non-PAG Covered by mixed PAG, Non-PAG hard rock 17

18 Waste Characterization Results: Mixed Waste Non-PAG placed on buttresses changes chemistry of mixed waste Nearly all waste material averages to acidgenerating range of NNP values 18

19 Flow Model Inputs Climate data: average of Geotechnical testing on oxide saprolite Prior hydrogeologic testing Current WRD plans and sections New waste characterization 19

20 Flow Modeling Inputs (cont.) Hydraulic properties of material tested under saturated, unsaturated conditions Moisture content and infiltration compared to on-site lysimeter Compaction key: all materials are reworked 20

21 WRD Conceptual Flow Model 21

22 Model Calibration to Encapsulation Cell 2200 Millimeters of Water Modeled Precipitation Measured Precipitation Modeled Evaporation Measured Pan Evaporation Modeled Storage Measured Storage Modeled Runoff Measured Runoff Modeled Percolation Measured Percolation /24/2006 5/13/2006 7/2/2006 8/21/ /10/ /29/2006 1/18/2007 Days 22

23 Results During Operations Uncapped PAG waste creates: High oxygen diffusion High infiltration rate (43% of precipitation) Poor water quality ph mg/l Acidity 260 L/s 23

24 Designing A Closure Cap Must mitigate ARD Oxygen prevention barrier Must be constructible with site materials (oxide saprolite) Must support vegetation Evaporation from Soil Surface Soil Moisture Storage High Soil Moisture Storage Precipitation Relative Humidity Runoff Vertical Infiltration Coarse Material Creates Capillary Break Coarse Waste Rock Compacted Oxide Saprolite Solar Radiation Plant Transpiration Topsoil 24

25 Excellent Oxygen Limitation Oxygen limited without clay soil amendments 10-6 cm/s cap not required Elevation (m) Gas Concentration 10-6 Cap Gas Concentration Ox. Sap. Only Gas Concentration ET Cover Gas Concentration (g/m 3 ) 25

26 WRD Leachate Comparison of Oxide Saprolite Cover and ET Cover: Mass Balance Water Balance or Chemical Parameters Total Leachate Volume Runoff Units Oxide Saprolite Cover and Operations Phase ET Cover Liters/s % 29% Infiltration Percent of 42% 13% Evaporation and Precipitation 58% 58% Transpiration Leachate ph Units

27 Monitored Natural Attenuation 27

28 Controlling Tropical ARD: The Stakes are High Severe ARD in tropical climates requires action early Geochemical, hydrogeologic, and geotechnical characterization are critical Recommend integrated modeling approach 28

29 Questions? 29