Ore Knob Mine Tailings Pile â Characterization and Restoration ...
Ore Knob Mine Tailings Pile – Characterization and Restoration Alternatives
Prepared by: Robert C. Borden and Mehnroosh Behrooz Department of Civil, Construction and Environmental Engineering North Carolina State University Campus Box 7908 Raleigh, NC 27606
December 12, 2008
ACKNOWLEDGEMENTS We gratefully acknowledge the financial and technical support provided by the U. S. Environmental Protection Agency and the North Carolina Division of Water Quality 319 Program. Special thanks also to Mr. Russell Reeves for providing access to his property for this research.
i
TABLE OF CONTENTS LIST OF TABLES .............................................................................................................................iv LIST OF FIGURES ...........................................................................................................................v 1.0 1.1 1.2 1.3
INTRODUCTION AND SITE HISTORY ................................................................1 Geology, Site History, and Physical Characteristics .................................................1 Production of Acid Mine Drainage ............................................................................2 Prior Site Restoration Activities ................................................................................4
2.0 2.1 2.2 2.3.1 2.3.2 2.3.3 2.3.4 2.3.5 2.3.6 2.3.7 2.4 2.4.1 2.4.2 2.5
HYDROLOGIC CHARACTERIZATION OF TAILINGS PILE.............................6 Overview of Site Hydrology .....................................................................................6 Soil Boring and Monitor Well Installation ................................................................8 Visual Evaluation of Surface Soils ............................................................................10 Blow count analysis ...................................................................................................11 Moisture Content .......................................................................................................12 Particle Characteristics and Engineering Classification ............................................12 In Situ Hydraulic Conductivity Tests ........................................................................14 Water Table Elevations in Monitor Wells .................................................................16 Ground Water Velocity and Flowrate ........................................................................19 Soil Infiltration Capacity............................................................................................20 Infiltration Test Methods ...........................................................................................21 Infiltration Results .....................................................................................................22 Conceptual Model of Tailings Pile Hydrology ..........................................................23
3.0 3.1 3.2 3.2.1 3.2.2 3.3 3.3.1 3.3.2 3.3.3 3.3.4 3.3.5 3.4
TAILINGS PILE GEOCHEMISTRY .......................................................................24 Summary of site characterization activities ...............................................................24 Sediment Analyses .....................................................................................................24 Sediment pH...............................................................................................................24 Sediment Cations, Anions and Metals .......................................................................25 Ground Water Monitoring .........................................................................................27 Data Analysis ............................................................................................................27 Redox Potential and Dissolved Oxygen ....................................................................29 pH, Iron, Sulfate and Acidity .....................................................................................30 Heavy Metals .............................................................................................................31 Major Cations and Anions .........................................................................................31 Conceptual Model of Acidity Generation within Tailings Pile .................................32
4.0 4.1 4.2 4.2.1 4.2.2 4.2.3 4.3
SURFACE WATER GEOCHEMISTRY ..................................................................34 Summary of Surface Water Monitoring Activities ....................................................34 Surface Water Monitoring .........................................................................................35 pH, Iron, Sulfate and Acidity .....................................................................................36 Heavy Metals .............................................................................................................38 Major Cations and Anions .........................................................................................38 Tailings Pile Pollutant Budget ...................................................................................39 ii
4.4
Conceptual Model of Surface Water Geochemistry ..................................................40
5.0 5.1 5.1.1 5.1.2 5.2 5.3 5.4 5.5 5.6
WATERSHED RESTORATION PLAN...................................................................41 Management Approaches...........................................................................................41 Methods for Controlling Acidity Release ..................................................................41 Methods for Controlling Copper and Zinc Release ...................................................42 Required Pollutant Load Reductions .........................................................................43 Technical and Financial Support Required ................................................................43 Public Education ........................................................................................................43 Schedule and Milestones............................................................................................43 Performance Criteria and Monitoring Program .........................................................43
6.0
REFERENCES ..........................................................................................................44 APPENDIX A -- Surface and Ground Water Monitoring Data APPENDIX B – Geotechnical and Slope Stability Evaluation
iii
LIST OF TABLES Table 1.1 Table 2.1 Table 2.2 Table 2.3 Table 2.4 Table 2.5 Table 2.6 Table 2.7 Table 3.1 Table 4.1 Table 4.2
Common sulfide bearing minerals ...........................................................................3 Characteristics of watersheds contributing flow to the tailings pile ........................6 Monitor well characteristics.....................................................................................9 Summary of surficial soil characteristics .............................................................10 Slug test results for monitor wells .........................................................................15 Water table elevations in tailings pile monitor wells June 2007 to 2008 (ft above MSL) .......................................................................................................16 Ground water flow velocity and rate through the tailings pile ..............................19 Infiltration results in different points of the tailing pile.........................................22 Average values of ground water parameters in monitor wells in Ore Knob tailings pile ............................................................................................28 Average parameter values in surface water monitoring stations within the Peak Creek watershed ...........................................................................36 Average dissolved pollutant budget for Ore Knob Mine tailings pile ...................40
iv
LIST OF FIGURES Figure 1.1 Figure 2.1 Figure 2.2 Figure 2.3 Figure 2.4 Figure 2.5 Figure 2.6 Figure 2.7 Figure 2.8 Figure 2.9 Figure 2.10 Figure 2.11 Figure 2.12 Figure 2.13 Figure 2.14 Figure 2.15 Figure 2.16 Figure 3.1 Figure 3.2
Figure 3.3 Figure 3.4 Figure 3.5 Figure 3.6 Figure 4.1
Topographic map of Ore Knob Branch watershed (From Laurel Springs NC Quadrangle, 1984) .........................................................2 Aerial photograph of tailings pile showing watershed boundary and location where four primary tributaries enter tailings pile (A, B, C and D) ............7 Monitor well locations .............................................................................................8 Photograph of sediment sampling and monitor well installation Ore Knob tailing pile- June 2007 .............................................................................9 Stratification observed in test pit during site exploration in 2002 (USACE, 2003) ......................................................................................................10 Uncorrected and corrected SPT values (a typical 60% efficiency is assumed) ...................................................................11 Profiles of water content versus depth below ground surface at each boring location .......................................................................................................12 Magnified image of the tailings .............................................................................12 Particle size distribution (Numbers in the legend correspond to boring depth, ft bgs) .................................13 Pressure response and Hvorslev’s analysis of slug test results for MW11 and MW12 .................................................................................................14 Comparison of well hydraulic conductivity and boring silt content ......................15 Observed water table elevations in monitor wells .................................................16 Water table fluctuations in monitor wells and average rainfall data from nearby weather stations .................................................................................18 Longitudinal water table profile through Ore Knob tailing pile ............................19 Infiltration test locations ........................................................................................20 Double ring infiltrometer used to measure infiltration rate ...................................21 Schematic of Amoozemeter and photograph showing field installation at Ore Knob .........................................................................................22 Variation in sediment pH with depth below ground surface in Ore Knob monitor wells .........................................................................................................25 Extractable iron (Fe), total sulfur (tS), calcium (Ca), copper (Cu), potassium (K), magnesium (Mg), zinc (Zn), aluminum (Al), manganese (Mn), sodium (Na), cadmium (Cd), chromium (Cr), lead (Pb) and silica (Si) in sediment samples from boring MW-9. Values are % by weight of total extractable material........................................................................26 Average values of redox potential and dissolved oxygen in ground water samples ............................................................................................29 Average values of field pH, dissolved iron, hot acidity and dissolved sulfate in ground water samples ............................................................................................30 Average values of aluminum, copper and zinc in ground water samples ..............31 Average concentration of calcium (Ca), potassium (K), magnesium (Mg) and sodium (Na) in ground water samples ............................................................32 Surface water monitoring locations directly adjoining the tailings pile and within the Peak Creek watershed ......................................................................................34 v
Figure 4.3 Figure 4.4 Figure 4.5
Average values of field pH, dissolved iron, hot acidity and dissolved sulfate in surface water samples ............................................................................................37 Average values of aluminum, copper and zinc in surface water samples .............38 Average concentration of calcium (Ca), potassium (K), magnesium (Mg) and sodium (Na) in surface water samples ............................................................39
vi
1.0
INTRODUCTION AND SITE HISTORY
Ore Knob Branch and Peak Creek, located in Ashe County, NC are impaired due to low pH and toxic levels of dissolved copper, iron and zinc released from mine works and a large tailings pile associated with the former Ore Knob Mine. This study was conducted to develop information on pollutant sources within the Ore Knob Branch watershed, with a focus on understanding of the hydrology and geochemistry of the mine tailings pile. 1.1
Geology, Site History, and Physical Characteristics
Ore Knob, near Jefferson, North Carolina, is the location of a massive fissure-type sulfide deposit. The steeply dipping vein varies from 8 to 18 ft thick and extends over 4000 feet long along the contact between the Carolina gneiss and the adjoining muscovite – biotite schist. Pyrrhotite, pyrite, chalcopyrite, quartz, biotite and amphiboles are the principal minerals in the vein (Kinkel, 1967; Rankin and Stuckey, 1943). The bedrock underlying the tailings impoundment is the Carolina gneiss which consists of highly metamorphosed alternating muscovite schist and quartz-biotite granitic gneisses. The Ore Knob deposit was discovered before the Civil War. Between 1871 and 1883, it was worked intensively, yielding twenty-five million lbs of copper ore from 11 openings and one main shaft. Further mining activity was limited until the mine was reopened in the late 1950s. From 1957 to 1962, Appalachian Sulphides, Inc. operated a mine and processing facility at the site. The extracted ore was ground in a processing facility located in the Little Peak Creek watershed. Copper, gold, and silver were extracted using a froth flotation and cyanide leaching process. Most waste tailings were pumped to a large tailings impoundment located on Ore Knob Branch. However, a portion of the tailings were dumped in a small hollow adjacent to the processing facility. Acid mine drainage from the processing area and associated waste piles has impaired water quality in Little Peak Creek. However, water quality conditions in the Little Peak Creek are outside the scope of this study. A topographic map of the Ore Knob Branch watershed is shown in Figure 1.1. At the point where Ore Knob Branch joins Peak Creek, the watershed drainage area is 0.5 square miles. The mine shaft, multiple mine adits and the former tailings impoundment are all located in the upper end of the watershed. The tailings pile covers approximately 22 acres with a maximum tailings depth of approximately 70 ft at the center of the embankment face. Available information suggests that the impoundment was constructed by first installing a small dam across Ore Knob Branch at the approximate location of the existing embankment face. A drop-inlet and 24-inch reinforced concrete pipe (RCP) were installed to provide drainage. Over time, the embankment forming the dam was progressively raised to provide additional storage for the accumulated tailings. The drop-inlet is located about 1,600 feet from the face of the tailings. However, the outlet of the pipe is currently covered by tailings, probably as a result of slope failure on the tailings embankment face. The surface elevation near the embankment face is approximately 20 ft higher than near the drop-inlet causing was to pool in this area, forming a small wetland.
1
Figure 1.1
1.2
Topographic map of Ore Knob Branch watershed (From Laurel Springs NC Quadrangle, 1984)
Production of Acid Mine Drainage
Mining of base and precious metals results in the production of immense quantities of waste rock, mill tailings, and waste related to refining processes. In many cases, waste rock and mill tailings contain an abundance of sulfide minerals (see Table 1.1), the most common being pyrite 2
and pyrrhotite. Mine wastes are typically deposited in impoundments or piles. Exposure of sulfide minerals in the piles to atmospheric oxygen (O2), and rain percolation through the tailings ultimately leads to the oxidation of the tailings and formation of acid mine drainage (AMD). AMD is low-pH water that typically contains high concentrations of SO4, ferrous iron (Fe+2), ferric iron (Fe+3), and other metals. A variety of other metals may also be present in the tailings and can be mobilized as a result of sulfide oxidation or due to the low pH of the AMD. Table 1.1
Common sulfide bearing minerals Chemical Name of formula mineral Pyrite FeS2 Marcasite FeS2 Pyrrhotite FexSx Chalcocite Cu2S Chalcopyrite CuFeS2
Chemical formula MoS2 NiS PbS ZnS FeAsS
Name of mineral Molybdenite Millerite Galena Sphalerite Arsenopyrite
Pyrite oxidation and the resulting AMD production are commonly envisioned as a sequence of three interrelated reactions. Reaction 1 Reaction 2 Reaction 3
2 FeS2 + 7 O2 + 2 H2O → 2 Fe2+ + 4 SO42- + 4 H+ 4 Fe2+ + O2 + 4 H+ → 4 Fe3+ + 2 H2O FeS2 + 14 Fe3+ + 8 H2O → 15 Fe2+ + 2 SO42- + 16 H+
In the first reaction, pyrite (FeS2) reacts with oxygen and water, releasing ferrous iron (Fe+2), sulfate (SO42-) and protons (H+). In the presence of oxygen, ferrous iron will be oxidized to ferric iron (Fe+3) following reaction 2. Ferric iron can then react with pyrite releasing additional ferrous iron, sulfate and acidity (reaction 3). In many cases, reaction 1 is relatively slow. However, reaction 3 takes place very rapidly, and continues until either ferric iron or pyrite is depleted. As a consequence, ferric iron production through reaction 2 is often the rate limiting step. In the absence of bacteria, reaction 2 is often slow at the low pH values associated with AMD. However in natural environments, iron oxidizing bacteria (Thiobacillus ferrooxidans) may accelerate reaction 2 by orders of magnitude ( Kiby and Elder Brady, 1998). Bacterial catalysis therefore enables rapid pyrite oxidation at low pH( pH
Prepared by: Robert C. Borden and Mehnroosh Behrooz Department of Civil, Construction and Environmental Engineering North Carolina State University Campus Box 7908 Raleigh, NC 27606
December 12, 2008
ACKNOWLEDGEMENTS We gratefully acknowledge the financial and technical support provided by the U. S. Environmental Protection Agency and the North Carolina Division of Water Quality 319 Program. Special thanks also to Mr. Russell Reeves for providing access to his property for this research.
i
TABLE OF CONTENTS LIST OF TABLES .............................................................................................................................iv LIST OF FIGURES ...........................................................................................................................v 1.0 1.1 1.2 1.3
INTRODUCTION AND SITE HISTORY ................................................................1 Geology, Site History, and Physical Characteristics .................................................1 Production of Acid Mine Drainage ............................................................................2 Prior Site Restoration Activities ................................................................................4
2.0 2.1 2.2 2.3.1 2.3.2 2.3.3 2.3.4 2.3.5 2.3.6 2.3.7 2.4 2.4.1 2.4.2 2.5
HYDROLOGIC CHARACTERIZATION OF TAILINGS PILE.............................6 Overview of Site Hydrology .....................................................................................6 Soil Boring and Monitor Well Installation ................................................................8 Visual Evaluation of Surface Soils ............................................................................10 Blow count analysis ...................................................................................................11 Moisture Content .......................................................................................................12 Particle Characteristics and Engineering Classification ............................................12 In Situ Hydraulic Conductivity Tests ........................................................................14 Water Table Elevations in Monitor Wells .................................................................16 Ground Water Velocity and Flowrate ........................................................................19 Soil Infiltration Capacity............................................................................................20 Infiltration Test Methods ...........................................................................................21 Infiltration Results .....................................................................................................22 Conceptual Model of Tailings Pile Hydrology ..........................................................23
3.0 3.1 3.2 3.2.1 3.2.2 3.3 3.3.1 3.3.2 3.3.3 3.3.4 3.3.5 3.4
TAILINGS PILE GEOCHEMISTRY .......................................................................24 Summary of site characterization activities ...............................................................24 Sediment Analyses .....................................................................................................24 Sediment pH...............................................................................................................24 Sediment Cations, Anions and Metals .......................................................................25 Ground Water Monitoring .........................................................................................27 Data Analysis ............................................................................................................27 Redox Potential and Dissolved Oxygen ....................................................................29 pH, Iron, Sulfate and Acidity .....................................................................................30 Heavy Metals .............................................................................................................31 Major Cations and Anions .........................................................................................31 Conceptual Model of Acidity Generation within Tailings Pile .................................32
4.0 4.1 4.2 4.2.1 4.2.2 4.2.3 4.3
SURFACE WATER GEOCHEMISTRY ..................................................................34 Summary of Surface Water Monitoring Activities ....................................................34 Surface Water Monitoring .........................................................................................35 pH, Iron, Sulfate and Acidity .....................................................................................36 Heavy Metals .............................................................................................................38 Major Cations and Anions .........................................................................................38 Tailings Pile Pollutant Budget ...................................................................................39 ii
4.4
Conceptual Model of Surface Water Geochemistry ..................................................40
5.0 5.1 5.1.1 5.1.2 5.2 5.3 5.4 5.5 5.6
WATERSHED RESTORATION PLAN...................................................................41 Management Approaches...........................................................................................41 Methods for Controlling Acidity Release ..................................................................41 Methods for Controlling Copper and Zinc Release ...................................................42 Required Pollutant Load Reductions .........................................................................43 Technical and Financial Support Required ................................................................43 Public Education ........................................................................................................43 Schedule and Milestones............................................................................................43 Performance Criteria and Monitoring Program .........................................................43
6.0
REFERENCES ..........................................................................................................44 APPENDIX A -- Surface and Ground Water Monitoring Data APPENDIX B – Geotechnical and Slope Stability Evaluation
iii
LIST OF TABLES Table 1.1 Table 2.1 Table 2.2 Table 2.3 Table 2.4 Table 2.5 Table 2.6 Table 2.7 Table 3.1 Table 4.1 Table 4.2
Common sulfide bearing minerals ...........................................................................3 Characteristics of watersheds contributing flow to the tailings pile ........................6 Monitor well characteristics.....................................................................................9 Summary of surficial soil characteristics .............................................................10 Slug test results for monitor wells .........................................................................15 Water table elevations in tailings pile monitor wells June 2007 to 2008 (ft above MSL) .......................................................................................................16 Ground water flow velocity and rate through the tailings pile ..............................19 Infiltration results in different points of the tailing pile.........................................22 Average values of ground water parameters in monitor wells in Ore Knob tailings pile ............................................................................................28 Average parameter values in surface water monitoring stations within the Peak Creek watershed ...........................................................................36 Average dissolved pollutant budget for Ore Knob Mine tailings pile ...................40
iv
LIST OF FIGURES Figure 1.1 Figure 2.1 Figure 2.2 Figure 2.3 Figure 2.4 Figure 2.5 Figure 2.6 Figure 2.7 Figure 2.8 Figure 2.9 Figure 2.10 Figure 2.11 Figure 2.12 Figure 2.13 Figure 2.14 Figure 2.15 Figure 2.16 Figure 3.1 Figure 3.2
Figure 3.3 Figure 3.4 Figure 3.5 Figure 3.6 Figure 4.1
Topographic map of Ore Knob Branch watershed (From Laurel Springs NC Quadrangle, 1984) .........................................................2 Aerial photograph of tailings pile showing watershed boundary and location where four primary tributaries enter tailings pile (A, B, C and D) ............7 Monitor well locations .............................................................................................8 Photograph of sediment sampling and monitor well installation Ore Knob tailing pile- June 2007 .............................................................................9 Stratification observed in test pit during site exploration in 2002 (USACE, 2003) ......................................................................................................10 Uncorrected and corrected SPT values (a typical 60% efficiency is assumed) ...................................................................11 Profiles of water content versus depth below ground surface at each boring location .......................................................................................................12 Magnified image of the tailings .............................................................................12 Particle size distribution (Numbers in the legend correspond to boring depth, ft bgs) .................................13 Pressure response and Hvorslev’s analysis of slug test results for MW11 and MW12 .................................................................................................14 Comparison of well hydraulic conductivity and boring silt content ......................15 Observed water table elevations in monitor wells .................................................16 Water table fluctuations in monitor wells and average rainfall data from nearby weather stations .................................................................................18 Longitudinal water table profile through Ore Knob tailing pile ............................19 Infiltration test locations ........................................................................................20 Double ring infiltrometer used to measure infiltration rate ...................................21 Schematic of Amoozemeter and photograph showing field installation at Ore Knob .........................................................................................22 Variation in sediment pH with depth below ground surface in Ore Knob monitor wells .........................................................................................................25 Extractable iron (Fe), total sulfur (tS), calcium (Ca), copper (Cu), potassium (K), magnesium (Mg), zinc (Zn), aluminum (Al), manganese (Mn), sodium (Na), cadmium (Cd), chromium (Cr), lead (Pb) and silica (Si) in sediment samples from boring MW-9. Values are % by weight of total extractable material........................................................................26 Average values of redox potential and dissolved oxygen in ground water samples ............................................................................................29 Average values of field pH, dissolved iron, hot acidity and dissolved sulfate in ground water samples ............................................................................................30 Average values of aluminum, copper and zinc in ground water samples ..............31 Average concentration of calcium (Ca), potassium (K), magnesium (Mg) and sodium (Na) in ground water samples ............................................................32 Surface water monitoring locations directly adjoining the tailings pile and within the Peak Creek watershed ......................................................................................34 v
Figure 4.3 Figure 4.4 Figure 4.5
Average values of field pH, dissolved iron, hot acidity and dissolved sulfate in surface water samples ............................................................................................37 Average values of aluminum, copper and zinc in surface water samples .............38 Average concentration of calcium (Ca), potassium (K), magnesium (Mg) and sodium (Na) in surface water samples ............................................................39
vi
1.0
INTRODUCTION AND SITE HISTORY
Ore Knob Branch and Peak Creek, located in Ashe County, NC are impaired due to low pH and toxic levels of dissolved copper, iron and zinc released from mine works and a large tailings pile associated with the former Ore Knob Mine. This study was conducted to develop information on pollutant sources within the Ore Knob Branch watershed, with a focus on understanding of the hydrology and geochemistry of the mine tailings pile. 1.1
Geology, Site History, and Physical Characteristics
Ore Knob, near Jefferson, North Carolina, is the location of a massive fissure-type sulfide deposit. The steeply dipping vein varies from 8 to 18 ft thick and extends over 4000 feet long along the contact between the Carolina gneiss and the adjoining muscovite – biotite schist. Pyrrhotite, pyrite, chalcopyrite, quartz, biotite and amphiboles are the principal minerals in the vein (Kinkel, 1967; Rankin and Stuckey, 1943). The bedrock underlying the tailings impoundment is the Carolina gneiss which consists of highly metamorphosed alternating muscovite schist and quartz-biotite granitic gneisses. The Ore Knob deposit was discovered before the Civil War. Between 1871 and 1883, it was worked intensively, yielding twenty-five million lbs of copper ore from 11 openings and one main shaft. Further mining activity was limited until the mine was reopened in the late 1950s. From 1957 to 1962, Appalachian Sulphides, Inc. operated a mine and processing facility at the site. The extracted ore was ground in a processing facility located in the Little Peak Creek watershed. Copper, gold, and silver were extracted using a froth flotation and cyanide leaching process. Most waste tailings were pumped to a large tailings impoundment located on Ore Knob Branch. However, a portion of the tailings were dumped in a small hollow adjacent to the processing facility. Acid mine drainage from the processing area and associated waste piles has impaired water quality in Little Peak Creek. However, water quality conditions in the Little Peak Creek are outside the scope of this study. A topographic map of the Ore Knob Branch watershed is shown in Figure 1.1. At the point where Ore Knob Branch joins Peak Creek, the watershed drainage area is 0.5 square miles. The mine shaft, multiple mine adits and the former tailings impoundment are all located in the upper end of the watershed. The tailings pile covers approximately 22 acres with a maximum tailings depth of approximately 70 ft at the center of the embankment face. Available information suggests that the impoundment was constructed by first installing a small dam across Ore Knob Branch at the approximate location of the existing embankment face. A drop-inlet and 24-inch reinforced concrete pipe (RCP) were installed to provide drainage. Over time, the embankment forming the dam was progressively raised to provide additional storage for the accumulated tailings. The drop-inlet is located about 1,600 feet from the face of the tailings. However, the outlet of the pipe is currently covered by tailings, probably as a result of slope failure on the tailings embankment face. The surface elevation near the embankment face is approximately 20 ft higher than near the drop-inlet causing was to pool in this area, forming a small wetland.
1
Figure 1.1
1.2
Topographic map of Ore Knob Branch watershed (From Laurel Springs NC Quadrangle, 1984)
Production of Acid Mine Drainage
Mining of base and precious metals results in the production of immense quantities of waste rock, mill tailings, and waste related to refining processes. In many cases, waste rock and mill tailings contain an abundance of sulfide minerals (see Table 1.1), the most common being pyrite 2
and pyrrhotite. Mine wastes are typically deposited in impoundments or piles. Exposure of sulfide minerals in the piles to atmospheric oxygen (O2), and rain percolation through the tailings ultimately leads to the oxidation of the tailings and formation of acid mine drainage (AMD). AMD is low-pH water that typically contains high concentrations of SO4, ferrous iron (Fe+2), ferric iron (Fe+3), and other metals. A variety of other metals may also be present in the tailings and can be mobilized as a result of sulfide oxidation or due to the low pH of the AMD. Table 1.1
Common sulfide bearing minerals Chemical Name of formula mineral Pyrite FeS2 Marcasite FeS2 Pyrrhotite FexSx Chalcocite Cu2S Chalcopyrite CuFeS2
Chemical formula MoS2 NiS PbS ZnS FeAsS
Name of mineral Molybdenite Millerite Galena Sphalerite Arsenopyrite
Pyrite oxidation and the resulting AMD production are commonly envisioned as a sequence of three interrelated reactions. Reaction 1 Reaction 2 Reaction 3
2 FeS2 + 7 O2 + 2 H2O → 2 Fe2+ + 4 SO42- + 4 H+ 4 Fe2+ + O2 + 4 H+ → 4 Fe3+ + 2 H2O FeS2 + 14 Fe3+ + 8 H2O → 15 Fe2+ + 2 SO42- + 16 H+
In the first reaction, pyrite (FeS2) reacts with oxygen and water, releasing ferrous iron (Fe+2), sulfate (SO42-) and protons (H+). In the presence of oxygen, ferrous iron will be oxidized to ferric iron (Fe+3) following reaction 2. Ferric iron can then react with pyrite releasing additional ferrous iron, sulfate and acidity (reaction 3). In many cases, reaction 1 is relatively slow. However, reaction 3 takes place very rapidly, and continues until either ferric iron or pyrite is depleted. As a consequence, ferric iron production through reaction 2 is often the rate limiting step. In the absence of bacteria, reaction 2 is often slow at the low pH values associated with AMD. However in natural environments, iron oxidizing bacteria (Thiobacillus ferrooxidans) may accelerate reaction 2 by orders of magnitude ( Kiby and Elder Brady, 1998). Bacterial catalysis therefore enables rapid pyrite oxidation at low pH( pH
