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GEOCHEMISTRY AND ISOTOPIC CHEMISTRY OF ACID ROCK DRAINAGE AND THE EVALUATION OF PYRITE OXIDATION RATE AT MINE DOYON, QUÉBEC, CANADA Ondra Sracek1, René Lefebvre2 Palacký University, Olomouc, Czech Republic Université Laval, Québec, Canada Mine Doyon pile - description(1) • Mine Doyon is a gold mine, located north of Montreal, in Abitibi region, • Average height of south pile is 30-35 m, rocks are sericite schists with pyrite content up to 7.0 wt % (locally 9.0 wt %), initial calcite content about 2.0 wt % has been generally depleted, • AMD appeared years after construction of pile and reached steady-state in 1988; typical pH values are about 2,0 and concentrations of sulfate in pore water are > 200 g/L; under such low pH conditions, behavior of Al and Mg can be considered conservative in long term Mine Doyon pile – description (2) • Behavior of sulfate in the pile is not conservative due to the precipitation of gypsum (CaSO4*2H2O) and K-jarosite (KFe3(SO4)2(OH)6), • Precipitation of these minerals occurs within metamorphic foliation, with resulting decomposition of rocks and increased access of oxidants to unoxidized pyrite; this process increases greatly pyrite oxidation rate; Mine Doyon pile: sampling and mesurement sites D-302 D-512 D-510 BH-91-101 W3 BH-107 BH-1 BH-2 Site TBT BH-102 BH-5 BH-3 BH-4 BH-106 Site BH-7 BH-6 D-301 Site BH-103 D-309 D-511 BH-104 BH-91-105 100 m Conceptual model of processes in Mine Doyon pile (1) Oxygen diffusion Waste rock Baserock (2) Production and conduction of heat (3) Infiltration of water (4) Oxygen convection Two types of oxygen transport are evident in temperature and O2 profiles • Convection oxygen transport close to slope (Site 6) • Diffusion oxygen transport in the core (Site 7) Temperature (oC) 10 20 30 40 50 60 70 0 5 10 10 15 D e p th (m ) D e p th (m ) Oxygen (% vol.) 10 15 20 15 20 20 25 25 Site Site 30 30 25 Profiles of dissolved species concentrations a) Site Ca (mg/L) pH 300 600 900 Mg (mg/L) 10000 20000 Al (mg/L) 30000 0 5 10 10 10 10 15 15 15 15 20 20 20 20 25 25 25 25 30 30 30 35 35 35 0 10000 10000 20000 30000 October 1995 April 1996 D e p th (m ) 30 Saturated Zone 35 Eh (mV) 300 D e p th (m ) 10 600 0 10 100000 200000 300000 Fe3+ (mg/L) Fe (mg/L) SO4 (mg/L) 900 10000 20000 30000 0 5 10 10 15 15 15 15 20 20 20 20 25 25 25 25 30 30 30 30 35 35 35 35 20000 30000 SI values for K-jarosite and Si concentrations a) b) K-Jarosite Saturation Index -4 -3 -2 -1 0 5 10 10 Depth (m) Depth (m) -5 Silica (mg/L) 15 20 25 30 35 25 50 75 15 20 25 Saturated Zone 30 35 Saturated Zone 100 125 a) Dry period (Summer 1995) a) Deuterium (per mil.) Isotopes D and 18O Local Meteoric Water Line Internal Evaporation Line Shallow Lysimeters (10 m) Snow -25 -50 D=7.02 18 O + 7.67 -75 D=2.4 -100 -125 18 O - 66.0 Summer 1995 -150 -20 -16 -12 18 b) Recharge period (Spring 1996) Deuterium (per mil.) b) -8 -50 Oxygen (per mil.) Local Meteoric Water Line Internal Evaporation Line Shallow Lysimeters (10 m) Snow -25 -4 D=7.02 18 O + 7.67 -75 D=2.4 -100 -125 18 O - 66.0 Spring 1996 -150 -20 -16 -12 18 -8 Oxygen (per mil.) -4 Determination pyrite oxidation rate (POR) There are principal possibilities in waste rock piles: (1) Interpretation of temperature and oxygen profiles (TOP) (2) Mass balance for sulfate/pyrite (SMB/PMB) (3) Oxygen consumption method in laboratory (OCM) Experiments in different scales generally produce different results (Malmström et al., 2000) (1) POR from T and O2 profiles (TOP) • When O2 is dominated by diffusion, O2 concentration profile is more or less exponential, • Analytical solution of Crank can be applied to determine transport parameters, • In Mine Doyon pile, O2 profile at Site indicates O2 transport by diffusion, • At this site analytical solution works for O2 profile, but not for T profile - there probably is lateral heat transport, • Calculated POR at Site is 4.4x10-10 mol(O2)·kg-1·s-1, A comparison between calculated and measured profiles of T and O2 – Site Oxidation (kg Py/m *y) Temperature ( C) 5 10 10 10 15 10 20 30 40 50 Depth (m) 0 15 0.0 0.00 0.05 0.10 0.15 0.20 0.25 Depth (m) Depth (m) o Oxygen (% vol.) 15 20 20 20 25 25 25 30 30 30 0.2 0.4 0.6 0.8 1.0 (2) POR from mass balance for sulfate/pyrite (SMB/PMB) • Oxidation of pyrite for pH < 3.0 is described as: FeS2 + 3.5O2 + H2O → Fe2+ + 2SO42- + 2H+ (oxidation of mol of pyrite produces moles of sulfate), • When behavior of sulfate is conservative, the amount of sulfate flushed out of the pile can be converted to oxidized pyrite, e.g sulfate mass balance (SMB), • As an alternative, pyrite mass balance (PMB) can be used, assuming known initial pyrite content and pyrite content at time zero; calculated POR is an average value for whole oxidation period Role of secondary minerals in mass balance calculations •At Mine Doyon, precipitation of gypsum, CaSO4.2H2O, and jarosite, KFe3(SO4)2(OH)6, in the pile affects mass balance for sulfate (SMB) and values of POR are underestimated, • For this reason, pyrite mass balance (PMB) was a better option (3) POR from oxygen consumption method (OCM) • Sample of a broken waste rock is placed into a closed chamber, sprinkled with water and changes in headspace oxygen concentration are recorded, • A graph of voltage as a function of time is plotted and straight line in the graph is used to calculate POR in mol(O2)·kg-1·s-1, • Scale of the test is small compared to field and POR values are generally high (oxygen transport limitations are not present); Mesurement of oxygen consumption O2 sensor V time voltmeter computer for data logging Reactor containing sample Rock Sample Voltage vs time plot • Fresh material from Mine Doyon, size of particles 4.5 cm: voltage [mV] 41.2 41 40.8 40.6 40.4 40.2 40 39.8 39.6 39.4 20000 40000 60000 time [s] 80000 100000 Comparison of various methods of POR [mol(O2)·kg-1·s-1] determination Site/method Temperature and O2 profiles (TOP) Pyrite mass balance (PMB)& Oxygen consumption method (OCM) Site (slope) 3.9x10-9 2.2x10-9 n.a n.a 1.8x10-9 n.a Site (core) 4.4x10-10 2.0x10-9 n.a Pile average 1.5x10-9 2.0x10-9 6.6x10-8** 3.4x10-9* Site TBT (core) - based on maximum values of pyrite content in deep unoxidized zone, n.a.-not available, **- fresh material, runs, *- weathered slope material, runs & CONCLUSIONS • In Mine Doyon waste rock pile oxygen convection plays an important role,temperature and oxygen concentration profiles indicate convection close to slope and diffusion in the pile core, • Concentrations of contaminants in pore water (sulfate, iron, aluminum etc.) are extremely high close to slope an in dry period and decrease in recharge, high concentrations are linked to internal evaporation of pore water in the pile due to high temperature (up to 67º C), • Internal evaporation is also confirmed by enriched values of isotopes D and 18O, • Principal secondary minerals are gypsum and jarosite, their precipitation within metamorphic foliation breaks rocks and facilitates the access of oxidants to the surface of unoxidized pyrite, CONCLUSIONS (continuation) • POR values obtained by different methods are comparable, • PMB method with a minimum precision can still distinguish between POR values close to slope (Site 6) from central zone (Sites and TBT), • When oxidized material from the slope is used, results obtained by OCM are consistent with TOP results from Site close to slope, • This means that when transport of O2 is not a limiting factor, values of POR obtained from small scale experiments may give representative results; this is different from sites like Aitik in Sweden with oxygen transport dominated by diffusion, where POR values depend very much on the scale of experiments [...]...(1) POR from T and O2 profiles (TOP) • When O2 is dominated by diffusion, O2 concentration profile is more or less exponential, • Analytical solution of Crank can be applied to determine transport parameters, • In Mine Doyon pile, O2 profile at Site 7 indicates O2 transport by diffusion, • At this site analytical solution works for O2 profile, but not for T profile - there probably is lateral... weathered slope material, 4 runs & CONCLUSIONS • In Mine Doyon waste rock pile oxygen convection plays an important role,temperature and oxygen concentration profiles indicate convection close to slope and diffusion in the pile core, • Concentrations of contaminants in pore water (sulfate, iron, aluminum etc.) are extremely high close to slope an in dry period and decrease in recharge, high concentrations... (oxygen transport limitations are not present); Mesurement of oxygen consumption O2 sensor V time voltmeter computer for data logging Reactor containing sample Rock Sample Voltage vs time plot • Fresh material from Mine Doyon, size of particles 4.5 cm: voltage [mV] 41.2 41 40.8 40.6 40.4 40.2 40 39.8 39.6 39.4 0 20000 40000 60000 time [s] 80000 100000 Comparison of various methods of POR [mol (O2 )·kg-1·s-1]... linked to internal evaporation of pore water in the pile due to high temperature (up to 67º C), • Internal evaporation is also confirmed by enriched values of isotopes D and 1 8O, • Principal secondary minerals are gypsum and jarosite, their precipitation within metamorphic foliation breaks rocks and facilitates the access of oxidants to the surface of unoxidized pyrite, CONCLUSIONS (continuation) • POR... POR from oxygen consumption method (OCM) • Sample of a broken waste rock is placed into a closed chamber, sprinkled with water and changes in headspace oxygen concentration are recorded, • A graph of voltage as a function of time is plotted and straight line in the graph is used to calculate POR in mol (O2 )·kg-1·s-1, • Scale of the test is small compared to field and POR values are generally high (oxygen... values obtained by different methods are comparable, • PMB method with a minimum precision can still distinguish between POR values close to slope (Site 6) from central zone (Sites 7 and TBT), • When oxidized material from the slope is used, results obtained by OCM are consistent with TOP results from Site 6 close to slope, • This means that when transport of O2 is not a limiting factor, values of POR obtained... pyrite content and pyrite content at time zero; calculated POR is an average value for whole oxidation period Role of secondary minerals in mass balance calculations •At Mine Doyon, precipitation of gypsum, CaSO4.2H 2O, and jarosite, KFe3(SO4)2(OH)6, in the pile affects mass balance for sulfate (SMB) and values of POR are underestimated, • For this reason, pyrite mass balance (PMB) was a better option (3)... (SMB/PMB) • Oxidation of pyrite for pH < 3.0 is described as: FeS2 + 3. 5O2 + H 2O → Fe2+ + 2SO42- + 2H+ (oxidation of 1 mol of pyrite produces 2 moles of sulfate), • When behavior of sulfate is conservative, the amount of sulfate flushed out of the pile can be converted to oxidized pyrite, e.g sulfate mass balance (SMB), • As an alternative, pyrite mass balance (PMB) can be used, assuming known initial... [mol (O2 )·kg-1·s-1] determination Site/method Temperature and O2 profiles (TOP) Pyrite mass balance (PMB)& Oxygen consumption method (OCM) Site 6 (slope) 3.9x10-9 2.2x10-9 n.a n.a 1.8x10-9 n.a Site 7 (core) 4.4x10-10 2.0x10-9 n.a Pile average 1.5x10-9 2.0x10-9 6.6x10-8** 3.4x10-9* Site TBT (core) - based on 5 maximum values of pyrite content in deep unoxidized zone, n.a.-not available, **- fresh material,... heat transport, • Calculated POR at Site 7 is 4.4x10-10 mol (O2 )·kg-1·s-1, A comparison between calculated and measured profiles of T and O2 – Site 7 Oxidation (kg Py/m *y) Temperature ( C) 5 5 5 10 10 10 15 10 20 30 40 50 Depth (m) 0 0 15 0 0.0 0.00 0.05 0.10 0.15 0.20 0.25 0 Depth (m) Depth (m) 3 o Oxygen (% vol.) 15 20 20 20 25 25 25 30 30 30 0.2 0.4 0.6 0.8 1.0 (2) POR from mass balance for sulfate/pyrite
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