GEOCHEMISTRY AND ISOTOPIC CHEMISTRY OF GEOCHEMISTRY AND ISOTOPIC CHEMISTRY OF ACID ROCK DRAINAGE AND THE EVALUATION OF ACID ROCK DRAINAGE AND THE EVALUATION OF PYRITE OXIDATION RATE AT PYRITE OXIDATION RATE AT MINE DOYON, MINE DOYON, QUÉBEC, CANADA QUÉBEC, CANADA Ondra Srace Ondra Srace k k 1 , Ren , Ren é é Lefebvre Lefebvre 2 1 Palack Palack ý University, Olomouc, ý University, Olomouc, Czech Republic Czech Republic 2 Université Laval, Québec, Canad Université Laval, Québec, Canad a a
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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 Sracek 1, René.
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GEOCHEMISTRY AND ISOTOPIC CHEMISTRY OF GEOCHEMISTRY AND ISOTOPIC CHEMISTRY OF ACID ROCK DRAINAGE AND THE EVALUATION OF ACID ROCK DRAINAGE AND THE EVALUATION OF
• 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 2 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.
• 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;
(1) POR from T and O(1) POR from T and O22 profiles (TOP) 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 heat transport,
• Calculated POR at Site 7 is 4.4x10-10 mol(O2)·kg-1·s-1,
A comparison between calculated and measured A comparison between calculated and measured profiles of T and Oprofiles of T and O22 – Site 7 – Site 7
0
5
10
15
20
25
30
0.00 0.05 0.10 0.15 0.20 0.25
Oxygen (% vol.)
Dep
th (
m)
0
5
10
15
20
25
30
0 10 20 30 40 50
Temperature (oC)
Dep
th (
m)
0
5
10
15
20
25
30
0.0 0.2 0.4 0.6 0.8 1.0
Oxidation (kg Py/m3*y)
Dep
th (
m)
• Oxidation of pyrite for pH < 3.0 is described as:
FeS2 + 3.5O2 + H2O 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 pyrite content and pyrite content at time zero; calculated POR is an average value for whole oxidation period.
(2) POR from mass balance for sulfate/pyrite (SMB/PMB)(2) POR from mass balance for sulfate/pyrite (SMB/PMB)
•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.
Role of secondary minerals in mass balance calculationsRole of secondary minerals in mass balance calculations
• 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);
(3) POR from oxygen consumption method(3) POR from oxygen consumption method (OCM)(OCM)
computer for data logging
time
V
Reactor containing sample
voltmeter
O2 sensor
Rock Sample
Mesurement of oxygen consumptionMesurement of oxygen consumption
39.4
39.6
39.8
40
40.2
40.4
40.6
40.8
41
41.2
0 20000 40000 60000 80000 100000
time [s]
volt
age
[mV
]
• Fresh material from Mine Doyon, size of particles 4.5 cm:
Voltage vs. time plotVoltage vs. time plot
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.
Site TBT (core)
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*
& - based on 5 maximum values of pyrite content in deep unoxidized zone, n.a.-not available, **- fresh material, 5 runs, *- weathered slope material, 4 runs
Comparison of various methods of POR Comparison of various methods of POR [[mol(Omol(O22))··kgkg-1-1··ss-1-1] ]
determinationdetermination
• 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,
CONCLUSIONSCONCLUSIONS
• 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 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 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.