Estimating oxygen decay coefficient for acid generating waste rock 1

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1 Estimating oxygen decay coefficient for acid generating waste rock 1 Song, Qing 2 and Ernest K. Yanful 3 2 Geotechnical Research Center, Dept. of Civil & Environmental Engineering, University of Western Ontario, London, ON N6A 5B9, Canada, qsong3@uwo.ca 3 Geotechnical Research Center, Dept. of Civil & Environmental Engineering, University of Western Ontario, London, ON N6A 5B9, Canada, eyanful@eng.uwo.ca ABSTRACT Measuring or predicting the distribution of oxygen concentration in waste rock underneath a soil cover is an important aspect of cover performance evaluation. The oxygen decay coefficient is a key parameter in the prediction. In the present study, a method calculating oxygen decay coefficient from oxygen consumption rate is developed. The decrease in oxygen concentration over time measured above a thin layer of waste rock (<12 mm) in a box was used to calculate oxygen consumption rate and oxygen decay coefficient for waste rock from Mattabi mine near Ignace, Ontario, Canada. The results indicate that the calculated oxygen decay coefficient from the proposed method is similar to that obtained from fitting of measured oxygen concentrations with the computer program POLLUTEv.6. The oxygen decay coefficient is influenced not only by oxygen consumption rate but also by physical characteristics of waste rock piles. Additional Key Words: Mattabi, oxidation, oxygen consumption rate, sulphide mineral. INTRODUCTION The performance of a soil cover is usually evaluated based on the oxygen flux penetrating into the waste rock (Nicholson et al., 1989; Yanful, 1993; Bussiere et al., 2003). In order to calculate this oxygen flux, the oxygen concentrations in the waste should be known. The oxygen diffusion coefficient and oxygen decay coefficient are two key parameters used to predict the oxygen concentration distribution spatially and temporarily in waste rock dumps. Oxygen diffusion coefficients of porous media are usually measured using a chamber with source and base reservoirs (Yanful, 1993; Cabral et al., 2000; Jones et al., 2003). These devices have also been employed to measure oxygen decay coefficients for reactive materials (Mbonimpa et al., 2003). Fick s second law and its modified form with a reaction term provide the theory for analyzing these measurements and calculating the appropriate parameters. Although most of the research mentioned in the above papers concentrated on fine materials, such as clay and mine tailings, the basic equations can still be applied to waste rock despite the obvious differences 1 Paper presented at Securing the Future and 8th ICARD, June 23-26, 2009, Skellefteå, Sweden. 1

2 between the waste rock and tailings. The oxygen decay coefficient, also called the reaction rate constant or coefficient, is directly related to the oxidation rate of the waste rock. However, little work has been done to directly obtain the oxygen decay coefficient from the measured oxidation rate of the waste rock during routine characterization of the waste rock. Collin (1987, 1998) developed a model to directly calculate the oxygen decay coefficient, but the oxidation rate of the pyrite was considered constant in the model; as a result, it does not account for differences in oxidation rates for various waste rock materials or piles. The objective of the present paper is to develop a method of estimating the oxygen decay coefficient of waste rock from oxygen consumption rates. Sealed chamber tests (box tests) were employed to measure the oxygen concentration change with time and oxygen consumption rate. The oxygen decay coefficient calculated from the proposed method was compared to those obtained from other methods. The factors influencing the calculation of oxygen decay coefficient are also discussed. METHODS AND MATERIALS Oxygen Decay Coefficient Estimation Methods In this work, four methods of calculating the oxygen decay coefficient are introduced: (i) simplified analytical method; (ii) modeling using the computer program POLLUTEv.6; (iii) Collin (1987,1998) method; (iv) oxygen uptake method that takes into account waste rock physical characteristics (proposed method). (i) Simplified Analytical method The oxygen concentration can be expressed as a modified Fick s second law when the kinetic reaction in a reactive material is assumed as a first-order reaction (Elberling and Nicholson, 1996; Mbonimpa et al., 2003): where C (z,t) is the interstitial oxygen concentration in the air phase at depth z and time t; where D * and D e are the bulk and effective diffusion coefficients; λ * and λ are the bulk and effective oxygen decay coefficients, respectively, and θ eq is the equivalent porosity for oxygen transport, which is defined as (Aubertin et al., 2000): (1) (2) (3) (4) 2

3 where K H is the dimensionless Henry s equilibrium constant (K H is approximately 0.03 at 20 ); θ a and θ w are volumetric air and water content in waste rock respectively (with n=θ a + θ w and n is the porosity of the waste rock; θ a is also called the air-filled porosity). Similar to that of a general radioactive material, the oxygen decay coefficient is defined using the half-life time: where t * 1/2 is the bulk half-life during which the oxygen concentration in the closed chamber decreases by half due to oxygen consumption from sulphide oxidation. If the oxygen diffusion part is negligible, Eq. 1 reduces to Eq. 6 and its solution is Eq.7. (5) where C 0 is the oxygen concentration in the chamber at the start of the test (that is, t=0). Eq. 7 indicates that the measured oxygen concentration in the chamber can be fitted with an exponential equation to get the bulk oxygen decay coefficient. However, Eq. 7 does not consider the physical characteristics of the waste rock. It should be noted that if the chamber has the headspace as a source reservoir (as used in this experiment), the bulk oxygen decay coefficient obtained using Eq.7 is not entirely representative of the oxygen decay coefficient of the waste rock layer because the measured oxygen concentration was related to the total volume of the chamber (including the source reservoir and the waste rock layer). In this case, the fitted oxygen decay coefficient in the source reservoir can be transformed into the oxygen decay coefficient of the waste rock layer using Eq. 8 when the source reservoir and the waste rock layer have the same area. (6) (7) (8) in which H 2 is the height of the source reservoir; H 1 is the height of the waste rock layer; and λ * w is the bulk oxygen decay coefficient for the waste rock layer. Eq. 8 indicates that when H 2 is zero, λ * w equals to λ *. (ii) Modeling using the computer program POLLUTEv.6 The commercial computer code POLLUTEv.6 (Rowe et al., 1994) has been extensively used to model the oxygen transport through saturated-unsaturated media in laboratory tests (MacKay et al., 1998; Yanful et al., 1999; Aubertin et al., 2000). For a reactive material, POLLUTEv.6 employs a semi-analytic method to solve Eq. 1 to obtain D * and λ * simultaneously (Mbonimpa et al., 2003). When only λ * is estimated in POLLUTEv.6, D * has to be known. In the present study, the effective oxygen diffusion coefficient was estimated following the method proposed by Aachib et al. (2004), and the top and the bottom boundary conditions were set to the no flux boundaries and the initial concentration of oxygen in the whole box (including the waste rock 3

4 layer) was set to equal to the oxygen concentration in the atmosphere. Sensitivity analysis conducted in the present study indicated that, under these boundary conditions, the simulated oxygen concentration in the test boxes was found to be relatively insensitive to the oxygen diffusion coefficient used in the modeling. (iii) Collin (1987, 1998) Method The model proposed by Collin (1987, 1998) is related to the total porosity, pyrite content, and an equivalent diameter of the grain size (Eq. 9). (9) where k is the reactivity of pyrite with oxygen (k is approximately 15.8x10-3 m 3 (O 2 ) m -2 (pyrite) year -1 ); and C py is the pyrite content per mass of dry waste rock; D H is defined as Eq. 10. where C u is the coefficient of uniformity, and d 10 is the grain size at which 10% of the waste rock is finer. (iv) Oxygen uptake method that takes into account waste rock physical characteristics The measured oxygen concentration versus time data can also be related to the oxygen consumption rate in the closed chamber when oxygen diffusion is negligible (Eq. 11). (10) (11) where ρ d is the dry density of the waste rock in the chamber; R O2 is the oxygen consumption rate of the waste rock (mass of oxygen over time per mass of dry waste rock). The negative symbol in the right hand of Eq. 11 indicates that the oxygen concentration would decrease due to the oxygen consumption. The solution for Eq. 11 is Eq. 12. (12) from the definition of half-life, when t=t * 1/2, C=1/2C 0, which yields, (13) Substituting Eq. 13 into Eq. 5 yields, (14) Eq. 14 builds the relationship between the bulk oxygen decay coefficient, λ *, and the oxygen consumption rate, R O2. The value of R O2 can be obtained easily in measuring the oxygen 4

5 concentration in the box at two different times and expressing the difference as mass per time. The advantage of Eq. 14 is that one does not need to know the effective diffusion coefficient of oxygen through the waste rock. Waste Rock Material The waste rock used in the experiment was obtained from Mattabi mine site, located approximately 70 km northeast of Ignace, Ontario, Canada. The Mattabi deposits were identified as Archean volcanogenic massive sulphide (Franklin et al., 1981). The waste rock was first sent from Mattabi mine site to a stone quarry near the University of Western Ontario, Ontario, Canada to be crushed; then the crushed waste rock was taken to the laboratory and sieved through 12.5 mm opening sieve (No. ½ inch). Only waste rock finer less than 12.5 mm was used in the experiments. After sieving, the waste rock was washed with tap water to remove debris and any material coating the particle surfaces before the rock was used in the experiment. Experimental Setup and Measurements The boxes or chambers used in the experiment measured 23 cm x 13 cm x 6 cm (length x width x height). An oxygen sampling port was installed in the middle of the top cap for each box. Four boxes (Box1 to Box 4) were used in the experiments including two control test boxes (Box 1 and Box 4) and two test boxes (Box 2 and Box 3). Box 1 was completely empty (no waste rock and no water); while Box 4 contained only 20 ml distilled water (no waste rock). The purpose of the control tests was to monitor the oxygen concentration in the empty and water-filled boxes to provide estimates for oxygen uptake of the box and water for comparison with oxygen consumed by the waste rock. Four hundred grams of air dried waste rock was placed in Box 2 and in Box 3 to provide a thin layer (<12 mm) of waste rock at the base of each box. Then 20 ml of distilled water was sprayed on the waste rock in Box 3 while no water was added in Box 2. All caps of the boxes were sealed with silicon glue to prevent air from entering the boxes or leaving the boxes. The configurations of waste rock and test purposes for the box tests are listed in Table 1. The oxygen sampling ports in the box tests, equipped with rubber septa to seal automatically after sampling, were similar to those described by Yanful (1993) and Yanful et al. (1999). Before extracting the gas for each measurement, the syringe was pulled and emptied several times to minimize the impact of the gas in the dead space. All box tests, except the test in Box 4, lasted 3 months, from June 3, 2008 to September 3, The test for Box 4 began on July 29, 2008 and ended on September 3, Only oxygen concentrations were measured in the box tests. Approximately 2 ml of gas was withdrawn from the box with a syringe and injected into an oxygen analyzer (Model 905V, Quantek Instruments) to measure gaseous oxygen concentration. The analyzer was calibrated in the atmosphere prior to sampling each port. The oxygen analyzer had a 0.1% resolution and needed less than 1 ml of gas for measurement of the oxygen concentration. 5

6 Table 1. Configurations of waste rock and test purposes for box tests Box No. Box 1 Box 2 Box 3 Box 4 Content Empty (no waste Air dried waste rock, Air dried waste rock, Distilled rock and no water) 400g. 400g; distilled water, water only, 20 ml. 20 ml. Test purpose Control test O 2 consumption due to O 2 consumption due to Control test oxidation of waste rock oxidation of waste rock Height of waste rock (H 1, cm) / / Height of headspace (H 2, cm) / / Dry density (ρ d, g/cm 3 ) / / Porosity (n) / / Equivalent air porosity (θ eq ) / / RESULTS The particle size analysis indicates that the waste rock used in the experiment had d 10 of 0.16 mm, and d 60 of 6 mm. X-ray diffraction analysis showed that the dominant sulphide-bearing minerals in the waste rock were pyrite, pyrrhotite, and small amount of sphalerite, chalcopyrite, and galena. Gaugue minerals included quartz, chlorite, and sericite. The paste ph of the waste rock was 5.8. The modified acid base accounting test (MEND, 1991) indicated that the waste rock had a neutralization potential (NP) of 29 tonnes CaCO 3 per 1000 tonnes of waste rock, and an acid potential (AP) of 600 tonnes CaCO 3 per 1000 tonnes of waste rock. The NP/AP ratio was less than 0.65, which demonstrates the waste rock is a potential source of acidic drainage. The measured oxygen concentrations with time in the experimental boxes (Boxes 1-4) shown in Fig. 1 indicates that the oxygen concentrations in the two control boxes (Box 1 and Box 4) were close to the atmospheric oxygen concentration (20.9%) at all times, which implied that the experimental boxes themselves and the distilled water did not consume much oxygen. Thus, the observed oxygen concentration decreases in the boxes containing the air dried waste rock (Box 2, with 0.14% gravimetric water content) and moist waste rock (Box 3, with 5.13% gravimetric water content) were mainly due to oxygen consumption through the oxidation of sulphide minerals present in the waste rock. In spite of above mentioned facts, the measured oxygen concentrations in Boxes 2 and 3 were corrected for possible external effects such as oxygen consumption by the experimental boxes themselves, according to the following equation (Eq. 15): where C o m is the measured oxygen concentration (% by volume) in Box 2 or 3; C o R1 is the direct (15) 6

7 reading of the oxygen concentration (%) from the oxygen analyzer in Box 1 (control, empty box); and C o R is the direct reading of the oxygen concentration (%) from the oxygen analyzer in Box 2 or 3. To facilitate the analysis, the corrected C o m (% by volume) was converted to mg/l assuming 20.9% of oxygen by volume in air equals to 280 mg/l of oxygen. Figure 1. Measured oxygen concentrations in experimental boxes According to the simplified analytical method (Eq. 7), the fitted exponential equations to the measured oxygen concentrations in the test boxes with the air dried waste rock and moist waste rock (Boxes 2 and 3) are presented in Fig. 2. After correction using the configuration parameters of the waste rock listed in Table 1 and Eq. 8, the effective oxygen decay coefficients obtained for the air dried and moist waste rock are 2.66 x10-8 s -1 and 3.63 x 10-7 s -1, respectively, as shown in Table 2. Figure 2. Measured oxygen concentrations in the experiment boxes with the air dried waste rock and moist waste rock and fitted exponential equations (Eq. 7) The measured oxygen concentration versus time data from the box tests were fitted with POLLUTEv.6 to back calculate the oxygen decay coefficient. In the simulation with POLLUTEv.6, the half-life of the waste rock was first obtained; then the oxygen decay 7

8 coefficient was calculated from half-life based on Eq. 5. The estimated half-lives for the air dried and moist waste rock obtained using POLLUTEv.6 are 185 day and 11.2 day, respectively, and the calculated effective oxygen decay coefficients are 2.61 x10-8 s -1 for air dried waste rock and 3.92 x 10-7 s -1 for moist waste rock correspondingly (Table 2). The air dried and moist waste rock have the same effective decay coefficient, 9.85 x 10-7 s -1, according to Collin s Model (Eq. 9). Table 2. Oxygen decay coefficients for the waste rock obtained from different methods Methods Effective oxygen decay coefficients (s -1 ) Air dried waste rock (Box 2) Moist waste rock (Box 3) Simplified analytical method 2.66 x x 10-7 Modeling using the computer program POLLUTEv x x 10-7 Collin method 9.85 x x 10-7 Oxygen uptake method that takes into account waste rock physical characteristics (proposed method) 3.74 x x 10-7 The calculated oxygen consumption rate of the moist waste rock (Box 3) was 2.50 mg(o 2 ) /d (or mg (O 2 )/kg/week), while that of the air dried waste rock was only 0.23 mg(o 2 ) /d (or 4.03 mg (O 2 )/kg/week) (Fig. 3). Apparently, the measured oxygen concentration in the box containing the moist waste rock (Box 3) decreased much more quickly than in the box with the air dried waste rock (Box 2), implying that the oxidation reaction in the moist waste rock was much faster than in the air dried waste rock. Since the mass of oxygen consumed in the test boxes was independent of the volume of the box, the oxygen consumption can better represent the oxidation of the waste rock than the measured oxygen concentration in the test box. The calculated oxygen decay coefficients of air dried and moist waste rock are 3.74 x10-8 s -1 and 3.99 x 10-7 s -1, respectively, as shown in Table 2. Figure 3. Calculated oxygen consumption over time in the experiment boxes with the air dried waste rock and moist waste rock (Boxes 2 and 3) 8

9 DISCUSSION AND CONCLUSIONS The oxygen decay coefficients obtained from the proposed method (oxygen uptake method) are very close to those obtained from the simplified analytical method and from the modeling using the computer program POLLUTEv.6 in the present study for both the air dried and moist waste rock, which implies that the proposed method is reasonable for calculating the oxygen decay coefficient. Moreover, the proposed method may be further used to calculate the oxygen decay coefficient from other expression of the oxidation rate, such as sulphate release rate, after conversion to the oxygen consumption rate. Eq. 14 indicates that, besides oxygen consumption rate, the oxygen decay coefficient is also influenced by other characteristics of the waste rock, such as dry density and porosity, so the oxygen decay coefficient is a site-specific parameter. Thus it may be difficult to compare the oxygen decay coefficients in different waste rocks piles. Except for the value obtained from the model of Collin (1987,1998), the effective oxygen decay coefficients obtained for the moist waste rock (Box 3) using the various methods are approximately times larger than those in the air dried waste rock (Box 2), indicating that water content has a significant effect on the oxygen decay coefficient. Although Collin s Model predicted an oxygen decay coefficient for the moist waste rock (Box 3) similar to those obtained from the other methods, it produced a much larger oxygen decay coefficient for the air dried waste rock (Box 2) compared to values obtained using the other approaches. Collin s Model does not take into account the change in oxidation rate with the water content of the waste rock. Instead it employs a constant oxidation rate and, as a result, predicts the same oxygen decay coefficients for both the air dried waste rock (Box 2) and the moist waste rock (Box 3). However, the test results in this study do not support a constant oxidation rate for the dried and moist waste rock samples, as shown in Fig. 3. The proposed method (Eq. 14) was developed in the present study based on measuring changes in oxygen concentration in a closed headspace over a thin layer of waste rock, in which oxygen diffusion was assumed to be negligible. Consequently the oxygen concentration over time in the test boxes is mainly controlled by oxygen decay due to the oxidation of the sulphide minerals in the waste rock. Under these conditions, the oxygen concentration over time is approximately linear. Sensitivity analysis using POLLUTEv.6 indicated the validity of the above assumption was dependent on the combination of the thickness, the diffusion coefficient and the oxygen decay coefficient of the waste rock pile. Further research is however necessary to test whether the calculated oxygen decay coefficient from the proposed method combined with calculated oxygen diffusion coefficient (such as proposed by Aachib et al. (2004)) can be used to predict the oxygen concentration in waste rock piles for general situations. Additional research could also look at the role of temperature and bacterial activity. The experiments in the present study were carried out at laboratory temperature (23 2 ), which was fairly constant. Although not quantified, bacteria present in the natural rock likely contributed to the reported oxygen decay coefficient. A comparison of the proposed method (Eq. 14) with the POLLUTE results indicates that the oxygen decay coefficient of waste rock can be calculated from the oxygen consumption rate when oxygen diffusion is negligible. The oxygen decay coefficient is influenced not only by the 9

10 oxygen consumption rate but also by waste rock physical characteristics such as the dry density. Within a certain range, water content also has the influence on the oxygen decay coefficient. REFERENCES Aachib, M., Mbonimpa, M., and Aubertin, M Measurement and prediction of the oxygen diffusion coefficient in unsaturated media, with applications to soil covers. Water, Air, and Soil Pollution, 156: Aubertin, M., Aachib, M., and Authier, K Evaluation of diffusive gas flux through covers with a GCL. Geotextiles and Geomebranes, 18: Bussière, B., Aubertin, M., and Chapuis, R.P The behavior of inclined covers used as oxygen barriers. Canadian Geotechnical Journal, 40: Cabral, A., Racine, I., Burnotte, F, and Lefebvre, G Diffusion of oxygen through a pulp and paper residue barrier. Canadian Geotechnical Journal, 37: Collin, M Mathematical modeling of water and oxygen transport in layered soil covers for deposits of pyretic mine tailings. Licenciate Treatise, Department of Chemical Engineering, Royal Institute of Technology, Stockholm, Sweden. Collin, M The Bersbo Pilot Project. Numerical simulation of water and oxygen transport in the soil covers at mine waste deposits. Report 4763, Swedish Environmental Protection Agency, Stockholm, Sweden. Elberling, B. and Nicholson, R.V Field determination of sulphide oxidation rates in mine tailings. Water Resources Research, 32: Franklin, J.M., Lydon, J.W., and Sangster, D.F Volcanic-Associated massive Sulfide Deposits. p In B.J. Skinner (ed.), Economic Geology Seventy-fifth Anniversary Volume Jones, S.B., Or, D., and Bingham, G.E Gas diffusion measurement and modeling in coarse-textured porous media. Vadose Zone Journal, 2: Mackay, P.L., Yanful, E.K., Rowe, R.K., and Badv, K A new apparatus for measuring oxygen diffusion and water retention in soils. Geotechnical Testing Journal, 21: Mbonimpa, M., Aubertin,M., Aachib, M., and Bussiere, B Diffusion and consumption of oxygen in unsaturated cover materials. Canadian Geotechnical Journal, 40: MEND Acid rock drainage prediction manual. MEND Project b. Coastech Research Inc. Nicholson, R.V., Gillham, R.W., Cherry, J.A., and Reardon, E.J Reduction of acid generation in mine tailings through the use of moisture-retaining cover layers as oxygen barriers. Canadian Geotechnical Journal, 26: 1-8. Rowe, R.K., Booker, J.R., and Fraser, M.J POLLUTEv6 and POLLUTE-GUI user s guide. GAEA Environmental Rngineering Ltd., London, Ontario, Canada. Yanful, E.K Oxygen diffusion through soil covers of sulphidic mine tailings. Journal of Geotechnical Engineering, 119: Yanful, E.K., Simms, P.H., and Payant, S.C Soil covers for controlling acid generation in mine tailings: a laboratory evaluation of the physics and geochemistry. Water, Air, and Soil Pollution, 114: