VERSION CORRIGÉE CORRECTED VERSION The influence of kinetic test type on the geochemical response of low acid generating potential tailings Mathieu Villeneuve, Bruno Bussière and Mostafa Benzaazoua Industrial NSERC Polytechnique-UQAT Chair on Environment and Mine Waste Management, UQAT, 445 University blvd, Rouyn-Noranda, Québec, Canada, J9X 5E4. Michel Aubertin Industrial NSERC Polytechnique-UQAT Chair on Environment and Mine Waste Management, École Polytechnique, C.P. 6079, Succ. Centre-Ville, Montréal, Québec, Canada, H3C 3A7 Marcos Monroy ABSTRACT: This paper presents a comparison of the geochemical responses of five different tailings submitted to five different kinetic tests. The five kinetic tests were done using column tests, humidity-cells, shaking flasks, modified Soxhlet extractors and mini-alteration cells. The different tailings have all AP values ranging from 23.9 to 178.8 kg CaCO3/t and NP values from 13.9 to 93.9 kg CaCO3/t. Evolution of the calcium, magnesium, manganese and sulfate loads were compared between each kinetic test and each of the tailings. It was found that, although the aggressiveness of the various kinetic tests was very different, the ratio of the acid production rate to the neutralization rate was similar from test to test. 1 INTRODUCTION The production of acid mine drainage (AMD) remains one of the most important environmental problems facing the mining industry. Acid mine drainage occurs when sulfide minerals contained in mine wastes react with oxygen and water to produce dissolved sulfuric acid. The production of acid lowers pH, which in turn increases the dissolution of metals contained in the mine waste. Improper management of acid producing waste can be a source of serious damage to the adjacent ecosystems. Many different remedial techniques have been proposed to control the production of AMD, such as sub-aquatic storage, barriers that inhibit the infiltration of water or gases, and collection of the effluents followed by chemical or passive treatment (MEND, 2001). Remediation costs alone for an acid generating tailings impoundment can range from 30 and 250 k CAN $ per hectare (Aubertin et al., 2002). The rehabilitation strategy for closing a mine and the associated costs depend on the acid generating potential (AGP) of the wastes. The determination of a material’s AGP can be done either by static or kinetic testing. A static test consists of measuring the balance between acid producing and acid neutralizing potentials. By comparing the acidity potential (AP) to the neutralization potential (NP), the net acid generating potential of a specific waste can be estimated. However, these tests have a relatively large uncertainty zone, where it is difficult to determine if a specific waste has acid generating potential or not (Miller et al. 1991, Adam et al. 1997). In such cases, it is useful to run kinetic tests to quantify evolution of water geochemistry with time. Such measurements can help in predicting the reaction rates of the acid producing and neutralizing minerals. Also, kinetic tests may be used to predict mineral depletion, to determine if and when AMD will occur. The present work focuses on the comparison of 5 different kinetic tests that were performed on 5 tailings which either had low AP, or gave results in the uncertainty zone for static testing. The first part of this study describes the main properties of the five tailings used in the experiments as well as the kinetic testing procedures used, i.e.: column tests, humidity-cells, shaking flasks, modified Soxhlet extractors and mini-alteration cells. Each of the kinetic test procedures varies in both aggressiveness and its ability to represent natural processes. Following these descriptions, the cumulative results from the different kinetic tests are presented and then analyzed. 2 MATERIALS AND METHODS 2.1 Methods This section describes the different chemical and physical characterization conducted on the tailings and leachates. 2.1.1 Tailings characterization 2.1.2 Water chemistry Physical properties Humid tailings samples were oven dried at 45 °C for 24 hours to determine their water content and to prepare dry samples for other analyses. The sample’s particle size distribution was determined using a Malvern Mastersizer laser particle size analyzer. The solid grain relative density was measured with a Micromeritics Accupyc 1330 helium gas pycnometer. pH and Eh Leachate samples were filtered with a 0.45 µm nylon mesh filter immediately after collection. Sample pH was read by a combination pH electrode with temperature compensation. Redox potential was determined with a Pt/Ag/AgCl electrode. The results were then corrected for the standard hydrogen electrode (SHE) to obtain Eh (expressed in mV). Chemical composition and mineralogy The chemical composition of the different tailings was analyzed with a Perkin Elmer Optima 3100 RL ICP-AES following a total HNO3/Br2/HF/HCl digestion. Dilute HCl was used to extract sulfates and the solution obtained was analyzed by ICP-AES. Silica content was determined by ICP-AES following a Na2O2/NaOH fusion. The initial tailings mineralogy was determined by a combination of visual observations, with a polarizing microscope and X-Ray diffraction spectroscopy. Acid-Base accounting Neutralization potential (NP) was determined using the modified static test proposed by Lawrence (1990) for each different tailing sample. The NP analyses were run in duplicate, and results were expressed in kg CaCO3/t. Acidity potential (AP), also expressed in kg CaCO3/t, was calculated by using the sulfide portion of total sulfur, obtained by subtracting the sulfate sulfur from the total sulfur (see Equations 1 and 2). The net neutralization potential (NNP) was calculated by subtracting the AP value from the NP value, as shown by Equation (3). It is postulated here that values of NNP < -20 kg CaCO3/t indicate an acid producing material, whereas materials with NNP > 20 kg CaCO3/t are acid consuming. Hence, an uncertainty zone for this technique would lie between 20 > NNP > -20 kg CaCO3/t (Miller et al. 1991). %Ssulfide = %S total - %Ssulfate (1) AP (kg CaCO 3 /t) = 31.25 × %Ssulfide (2) NNP (kg CaCO 3 /t) = NP (kg CaCO 3 /t) - AP (kg CaCO 3 /t) (3) Another useful way to evaluate the AMD production potential using static tests is the NP to AP ratio. Typically, the material is considered non acidgenerating if NP/AP > 2.5, uncertain if 2.5 > NP/AP > 1 and acid generating if NP/AP < 1 (Adam et al. 1997). Metals and sulfates Filtered leachates were acidified to 2 % HNO3 to insure metal solubilization. The resulting solutions were analysed with a Perkin Elmer Optima 3100 RL ICP-AES to determine metal and sulfate concentrations, results are expressed in mg/L. Geochemical modeling The aqueous geochemical equilibrium model Visual MINTEQ version 2.14, which is a Visual Basic version of the previous version of MINTEQ (Felmy et al., 1984), was used to evaluate the saturation indices (SI) of possible secondary minerals in the kinetic tests leachates. A SI value greater than 0 for a given mineral indicates that it would precipitate under these conditions. A negative SI value means that the mineral would dissolve. A SI equal to 0 indicates equilibrium. The main model assumption is that mineral dissolution or precipitation reactions occur rapidly enough to be in equilibrium with the leachate. 2.1.3 Kinetic tests procedures Several different kinetic tests were used in this study. The following section contains a brief description of each test used. A more detailed description of the protocols used can be found in Villeneuve (in prep). Columns Four of the five column tests were conducted in 1 m tall Plexiglas tubes, 10 cm in diameter. Approximately 16 kg (dry weight) of GRE-M1, LAR-M3, MAT-M1 and MAT-M2 tailings were placed in each of these columns. Column UQ-8 was run in a separate investigation and used a Plexiglas column 0.7 m tall by 10 cm in diameter filled with 8 kg (dry weight) of tailings. The bottom of all columns included a porous ceramic plate to simulate a water table 2 m below the column base. Two liters of deionized water were added to the top of each column and allowed to drain, while the resulting leachate was collected. This process was repeated twelve times over a period of about one and a half year. The top of each column was opened between flushes. A detailed description of the column tests can be found in Benzaazoua et al. (2001). Humidity-cells Testing in the humidity-cells was done following the procedure described in Morin & Hutt (1997). Plexiglas cells, 14 cm in diameter and 30 cm in height, were filled with approximately 1 kg (dry weight) of fresh tailings. A full cycle consisted of 3 days of dry air at 1 L/min, 3 days of humid air at 1 L/min and leaching of the cells on the seventh day. The leaching was done by inundation and soaking of the tailings for 4 hours with 500 mL of deionized water. The resulting leachate was then collected by overpressurization to about 35 kPa. of the Buchner funnel. The solution was recovered by applying suction on a filtering flask after 3 h of contact with the tailings. Figure 2 shows a photograph of a typical mini-alteration cell used in this study. Shaking flasks The shaking flasks procedure was inspired by the work of Gleisner (2001). The ratio of solids to water used was 1:10, sampling was periodic and compensated. Five 1 L Erlenmeyer flasks containing 50 g of dry tailings and 500 mL of deionized water were placed on a reciprocating shaker. 50 mL water samples were taken once a week and are compensated by the addition of 50 mL of fresh deionized water. The test flasks were adjusted to their original weight weekly by addition of deionized water to compensate for evaporation. The dilution is taken into account in later load calculations. Modified Soxhlet extractors A specially designed modified Soxhlet extractor, similar to the one used by Sullivan & Sobek (1982), was used. This modified Soxhlet varies from the classic design in that the extraction chamber is located outside the path of the rising water vapor (see Figure 1). Also, there is no siphon tube, so the water passes straight through the sample and back into the boiling flask. The main reason for these modifications was to maintain a realistic temperature in the sample. About 6 g of dry tailings were placed in a glass extraction thimble and 200 mL of deionized water were placed in the boiling flask. Sampling of the extracted solution was done with a syringe through a self-sealing septum placed on the boiling flask. Sampling was done on days 1, 3, 7 and 14; 50 mL of the extracted solution was sampled and later compensated by the addition of 50 mL of fresh deionized water. Again, the dilution is taken into account in later load calculations. Mini-alteration cells Mini-alteration cells similar to those of Cruz et al. (2001) were used. The aggressiveness of the method lies in the use of a thin layer of sample and frequent leaching-drying cycles. About 67 g (dry weight) of tailings were placed in a 100 mm diameter Buchner funnel equipped with a glass-fiber filter. A 7 day cycle consisted of 2 days of exposure to ambient air, leaching on the 3rd day, 3 days of exposure to air and finally, leaching on the last day. The flushes were done by adding 50 mL of deionized water to the top Figure 1. Photographs of the modified Soxhlet extractor used: a) global view of the apparatus, b) modified extraction chamber. Figure 2. Photograph of a typical mini-alteration cell. 2.2 Materials Following is a description of the tailings characteristics, for more detailed results, see Villeneuve (2003, Masters Thesis, to be submitted). 2.2.1 Tailings UQ-8 tailings were sampled in an actual tailings impoundment from the Abitibi-Témiscamingue region in the province of Québec, Canada. Tailings GREM1, LAR-M3, MAT-M1 and MAT-M2 were manufactured by mixing desulfurized and sulfurized fractions to obtain low AP tailings. Desulphurization of the original fresh tailings was conducted with a laboratory flotation bench using a xanthate collector and methylisobutyl carbinol (or MIBC) as frother (more details on the desulphurization by flotation can be found in Benzaazoua et al., 2000). 2.2.2 Physical properties Table 1 presents, for each of the tailings, the solid grain relative density (Dr) and the main characteristics of the particle size analysis. Table 1. Physical properties of the five tailings studied. Dr % under (-) (%) Material UQ-8 GRE -M1 3.04 2.90 94.6 94.0 LAR -M3 2.78 89.2 MAT -M1 3.02 90.3 MAT -M2 3.07 83.6 (µm) (µm) (µm) 1.1 11.9 54.8 3.5 22.4 83.3 2.0 15.7 78.4 2.4 22.8 104 80 µm D10 D50 D90 3.9 20.4 63.8 These tailings are typical of fine tailings from a hard rock mines (Vick, 1983; Aubertin et al. 2002), with D10 ranging from 1.1 to 3.9 µm and percent passing 80 µm between 80 and 95 %. Relative densities are very similar from one material to the other, ranging from 2.78 to 3.07. Generally, a larger sulfides content induces an increased density of the tailings. 2.2.3 Chemical and mineralogical properties Results from chemical analysis of the five tailings are given in Table 2. Table 2. Chemical composition of the five tailings studied UQ-8 Al As B Ba Ca Cd Co Cr Cu Fe K Mg Mn Na Ni Pb SiO2 Stotal Ssulfate Ssulfide (wt%) (wt%) (wt%) (wt%) (wt%) (wt%) (wt%) (wt%) (wt%) (wt%) (wt%) (wt%) (wt%) (wt%) (wt%) (wt%) (wt%) (wt%) (wt%) (wt%) 3.81 0.027 0.068 0.006 3.43 0.015 0.004 0.008 0.012 17.1 n/d 1.65 0.456 1.8 0.005 0.008 n/d 7.09 1.37 5.72 GREM1 7.51 0 0.077 0.03 2.89 0.002 0.001 0.006 0.03 11.5 1.24 2.87 0.196 1.36 0.017 0.034 53.20 1.09 0.240 0.850 LARM3 5.87 0 0.129 0.031 1.68 0.002 0 0.003 0.023 2.83 1.25 0.730 0.043 1.37 0.008 0.046 68.81 0.816 0.050 0.766 MATM1 3.16 0 0.086 0.005 3.79 0.002 0.003 0.008 0.026 16.6 0.180 3.15 0.434 1.01 0.007 0.028 53.29 1.90 0.190 1.71 MATM2 2.99 0.001 0.081 0.006 3.60 0.002 0.004 0.008 0.028 17.9 0.170 2.94 0.409 1.01 0.007 0.028 46.82 2.85 0.324 2.53 Zn (wt%) 0.007 0.159 0.108 0.234 0.276 As can be seen in Table 2, the present study focuses on relatively low-sulfide wastes, with sulfide content varying from 0.766 to 5.72 %. All materials, except UQ-8, had low sulfate contents (0.05 to 0.324 wt%), typical of fresh tailings. UQ-8 (1.37 wt% Ssulfate) was oxidized prior to being used in the present work. Calcium, magnesium and manganese contents of the tailings are tracers to their neutralization potential as these elements are generally found in carbonate minerals. Table 3 shows the results of the mineralogical characterization that was conducted by the Géoberex Recherche firm in Montréal (Bernier, 2002) on the five tailings. For each one, the principal minerals found by the mineralogical analysis were quartz, chlorite, feldspars, carbonates (mainly ankerite and calcite) and micas. The main sulfide mineral was pyrite, with traces of pyrrhotite for tailings MAT-M1 and MAT-M2. The main Ca, Mg and Mn bearing minerals found were carbonates. Table 3. Mineralogical analysis of the tailings (Bernier, 2002). Mineral Albite\Orthose Amphibole Anhydrite Ankerite/dolo mite Barite Biotite Calcite Chlorite Clinopyroxène Epidote Gypsum Magnetite Muscovite\Illite Pyrite Pyrrhotite Quartz Sepiolite Siderite Talc UQ-8 M --- GREM1 L L -- LARM3 M --- MATM1 L L -- MATM2 L L Tr M -L -Tr --Tr -- L --Tr M Tr Tr --- ? -Tr Tr M L ---- M -Tr Tr M tr Tr -Tr M -Tr Tr M Tr Tr -Tr -M -A -L -- L L -A ---- L Tr Tr A ---- -L ? A L --- -L ? A L --- A: abundant (30-50 wt%); M: moderately abundant (10-30 wt%); L: low abundance (2-10 wt%); Tr: trace (<2 wt%); ?: suspected 2.2.4 Acid-Base accounting The results from acid-base accounting are shown in Table 4. Table 4. Acid-base accounting from the five tailings. AP UQ-8 NP NNP (kg CaCO3/t) 178.8 64.2 -115 NP/AP (-) 0.359 2.7 0.568 1.76 1.15 Using the criteria of Miller et al. (1991), only the UQ-8 tailings are considered to be acid generating, with an NNP of -115 kg CaCO3/t. Materials LARM3 (-10.3 kg CaCO3/t) and MAT-M2 (12.1 kg CaCO3/t) are in the uncertainty zone. Tailings GREM1 (45.1 kg CaCO3/t) and MAT-M1 (40.5 kg CaCO3/t) are non acid generating. Using the NP to AP ratio and the criteria of Adam et al. (1997), UQ-8 (NP/AP = 0.359) and LAR-M3 (NP/AP = 0.568) should be acid generating. Tailings MAT-M1 (NP/AP = 1.76) and MATM2 (NP/AP = 1.15) are in the uncertainty zone, whereas GRE-M1 tailings (NP/AP = 2.7) are non acid generating. These results indicate that kinetic tests should be run to better asses the nature of these tailings in terms of their acid generating nature. Nevertheless, the choice of which kinetic testing procedure to use is not always evident. Therefore, the influence of the kinetic test procedure on the geochemical response of these low AP tailings was investigated. 3.1 pH and Eh measurements For all of the kinetic tests conducted on all of the tailings sample in this study, pH typically started near neutrality (pH 7) and rose to alkaline values (up to pH 9.8 for the GRE-M1 modified Soxhlet extractor test). In general, Eh readings oscillated between around 300 and about 520 mV in all tests and for all the tailings. The modified Soxhlet extractors run on the five tailings had both decreasing Eh values in time and consistently lower Eh readings than the other kinetic tests results. 3.2 Cumulative sulfate, Ca, Mg and Mn loads Figures 3 through 7 show the evolution of the cumulative sulfate load normalized by the dry weight of sample for each kinetic test run on each of the tailings. 3 RESULTS 40000 35000 30000 25000 20000 15000 10000 5000 0 0 100 200 300 400 Time (d) Figure 3. Evolution of the cumulative sulfate loads for the UQ8 tailing. Column Humidity-Cell Shake Flask Soxhlet Mini-Cell 4 2- (mg/kg) GRE-M1 Cumulative SO The following results have been obtained from ongoing kinetic tests, except for the columns and modified Soxhlet extractors which were run to completion (1 year and 14 days, respectively). For each kinetic test run on the tailings, pH, Eh, conductivity, punctual metal concentration and acidity and alkalinity were analyzed for each leachate sample. This data was processed to compute instantaneous and cumulative loads as well as elemental depletion curves based on the geochemistry of the leachates. The apparatus used for each completed kinetic test was dismantled at the end and a complete characterization of the post-tests tailings was done. Detailed results are found in Villeneuve (2003, Masters Thesis, to be submitted). Results presented in this paper are expressed in cumulative sulfate load and in cumulative calcium, magnesium and manganese added loads. The former represents the sulfide minerals oxidation products and therefore acid production. The later represents the by-products of acid neutralization by carbonates. Results are normalized by kilogram (dry weight) of tailings tested. This technique has been used in the past to aid interpretation and comparison between different kinetic testing protocols (Morin et Hutt, 1997; Benzaazoua et al. 2001). The following sections detail the geochemical results of each of the five types of kinetic tests for each of the tailings. Column Humidity-Cell Shake Flask Soxhlet Mini-Cell UQ -8 (mg/kg) 45.1 -10.3 40.5 12.1 2- 71.7 13.6 93.9 91.0 4 26.6 23.9 53.4 78.9 Cumulative SO GRE-M1 LAR-M3 MAT-M1 MAT-M2 8000 7000 6000 5000 4000 3000 2000 1000 0 0 100 200 300 400 500 Time (d) Figure 4. Evolution of the cumulative sulfate loads for the GRE-M1 tailing. Column Humidity-Cell Shake Flask Soxhlet Mini-Cell (mg/kg) LAR-M3 6000 4 4000 Cumulative SO 2- 5000 3000 fate load evolution is not linear and slope breaks are often observed (Figures 4 to 7). In instances where a break in slope is observed, it becomes difficult to obtain relevant information from the slopes. Figures 8 to 12 show the evolution of cumulative Ca, Mg and Mn added loads for each test run on each of the tailings. 2000 0 100 200 300 400 500 Time (d) Figure 5. Evolution of the cumulative sulfate loads for the LAR-M3 tailing. Column Humidity-Cell Shake Flask Soxhlet Mini-Cell (mg/kg) MAT-M1 10000 2- 8000 16000 14000 12000 10000 8000 6000 4000 2000 0 0 100 200 300 400 Time (d) Figure 8. Evolution of the cumulative added Ca, Mg and Mn loads for the UQ-8 tailing. 6000 4 Cumulative Ca+Mg+Mn (mg/kg) 0 Cumulative SO Column Humidity-Cell Shake Flask Soxhlet Mini-Cell UQ -8 1000 4000 0 100 200 300 400 500 600 Time (d) Figure 6. Evolution of the cumulative sulfate loads for the MAT-M1 tailing. Column Humidity-Cell Shake Flask Soxhlet Mini-Cell 14000 12000 10000 8000 6000 4000 2000 0 4000 3000 2000 1000 0 0 100 200 300 400 500 Time (d) Figure 9. Evolution of the cumulative added Ca, Mg and Mn loads for the GRE-M1 tailing. Column Humidity-Cell Shake Flask Soxhlet Mini-Cell LAR-M3 0 200 400 600 Time (d) Figure 7. Evolution of the cumulative sulfate loads for the MAT-M2 tailing It can be observed on Figures 3 to 7 that many similarities can be found from tailings to tailings. Based on the total quantity of sulfates leached, the following general gradation can be obtained: columns < mini-alteration cells < humidity-cells < shake flasks < modified Soxhlet extractors. The order of increasing slope angle of the sulfates vs. time plot is: columns < humidity-cells < mini-alteration cells < shake flasks < Soxhlet extractors. It is also apparent that for many kinetic tests (mainly humidity-cells, mini-alteration cells and shake flasks), sul- Cumulative Ca+Mg+Mn (mg/kg) 4 2- (mg/kg) MAT-M2 Cumulative Ca+Mg+Mn (mg/kg) 0 Cumulative SO Column Humidity-Cell Shake Flask Soxhlet Mini-Cell GRE-M1 2000 2500 2000 1500 1000 500 0 0 100 200 300 400 500 Time (d) Figure 10. Evolution of the cumulative added Ca, Mg and Mn loads for the LAR-M3 tailing. Column Humidity-Cell Shake Flask Soxhlet Mini-Cell Cumulative Ca+Mg+Mn (mg/kg) MAT-M1 4000 3000 2000 1000 0 0 200 400 600 Time (d) Figure 11. Evolution of the cumulative added Ca, Mg and Mn loads for the MAT-M1 tailing. Column Humidity-Cell Shake Flask Soxhlet Mini-Cell Cumulative Ca+Mg+Mn (mg/kg) MAT-M2 5000 4000 3000 2000 1000 0 0 200 400 600 Time (d) Figure 12. Evolution of the cumulative added Ca, Mg and Mn loads for the MAT-M2 tailing. The evolution of cumulative Ca, Mg and Mn added loads for each test run on all tailings presented in Figures 8 to 12 shows the same overall tendencies as for sulfates. Progressive increase of the plotted slope angle is generally: columns < humidity-cells < mini-alteration cells < shake flasks < modified Soxhlet extractors. Based on total quantity of Ca, Mg and Mn, the same general order as for sulfates is obtained: columns < mini-alteration cells < humidity-cells < shake flasks < modified Soxhlet extractors. Breaks in the slope of the curves representing the humidity-cells, mini-alteration cells and shake flasks were also frequently observed (Figures 9 to 12). Some exceptions to the general gradations presented earlier did occur. It was the case of the minialteration-cell test results run on the LAR-M3 tailings. This test extracted sulfates and neutralization products at an uncharacteristically low rate relative to the same test conducted on the other materials. Also, the modified Soxhlet extractor conducted on the MAT-M2 sample extracted a relatively small quantity of reaction products per kilo versus other Soxhlets. The results presented here are assessed in more detail in the following sections. 4 DISCUSSION In this section, further analysis of the results is presented. A classical interpretation regarding each sample’s AMD generation potential is first given. Then, the cumulative loads graphs are evaluated. Finally, the global geochemical response of the five tailings during the kinetic tests is discussed. It should be recalled that results and interpretation (for depletion and precipitation) are based on the geochemical responses and modeling from the leachate collected. Most of these tests are still ongoing and therefore, it was not possible to validate precipitation and depletion by analyzing the final altered tailings sample. 4.1 AMD potential The limit often used to characterize onset of AMD is a pH lower than 5 (SRK, 1989). None of the kinetic tests in the present study yielded such an acidic pH. The metals loads remained low in every test due to the high pH. Heavy metals were often on or under detection limits for the ICP analysis. Therefore, we can conclude that evidence of the AMD generating potential of the 5 tailings studied was not observed during the course of the 5 different kinetic testing protocols. However, these observations do not allow stipulation of the long term acid generating potential of the tailings. 4.2 Geochemical data analysis During a kinetic test, the main variables that influence the geochemical composition of the leachate are: depletion of the reactants, depletion of ultra-fine particles, precipitation of secondary minerals (Aubertin et al. 2002) or passivation of the reacting minerals surfaces (Cruz et al. 2001). Depletion of ultrafines can only be detected by the particle size distribution analysis of the post-dismantlement sample; therefore this hypothesis will not be investigated in this study, but will be the focus of future work. Note that for all of the tests, the quantity of cumulative sulfates collected exceeded the quantity of soluble sulfates originally found in the tailings. This means that sulfates found in leachates have been mainly produced by oxidation of pyrite. 4.2.1 Elemental depletion Table 5 lists the depletion of total sulfur and neutralizing elements (Ca+Mg+Mn) for all of the tailings samples and kinetic tests. These results indicate that modified Soxhlet extractors may be used to maximize depletion of minerals in the course of a laboratory kinetic test. Columns tests induced the lowest depletion rates (% depleted vs. duration) of all five tests. Generally, it has been found that the increasing order of depletion rates is: columns < humidity-cells < mini-alteration cells < shake flasks < modified Soxhlet extractor. However, variations do occur. They could be caused by precipitation of reaction products which compromises the water quality analysis or physical problems encountered during the tests. Table 5. Computed % remaining in total sulfur and Ca+Mg+Mn for the kinetic tests run on the five tailings. Kinetic test Duration (days) C 550 Tailings UQ-8 Parameter % remaining Stotal Ca+Mg+Mn Stotal Ca+Mg+Mn Stotal Ca+Mg+Mn Stotal Ca+Mg+Mn Stotal Ca+Mg+Mn 95.5 96.1 93.2 98.5 93.3 97.3 93.7 98.5 96.9 98.5 GREM1 LARM3 MATM1 MATM2 H.C. 225 83.3 86.0 77.8 94.9 83.9 92.2 85.8 96.0 85.8 94.0 M.C. 77 S.F. 105 Sox 14 88.0 83.4 80.1 95.5 94.5 97.1 88.9 97.0 89.6 96.1 82.7 74.4 79.2 94.9 87.5 93.3 87.9 95.9 82.9 94.5 81.0 71.2 76.4 93.8 73.6 92.7 83.6 94.9 82.3 94.9 C.: Column; H.C.: Humidity-Cell; M.C.: Mini-alteration cell; S.F.: Shake flask; Sox.: Modified Soxhlet extractor. Table 5 also shows that the total sulfur depletion rates are greater than the neutralizing elements depletion rates for the tailings and tests evaluated here (except for UQ-8). 4.2.2 Geochemical responses UQ-8 tailings Analysis of the pH, Eh and the curves on Figures 3 and 8 tend to indicate that the acidity was produced by oxidation of the sulfide minerals. Acid was immediately neutralized by the carbonates in the sample. Both acid production and consumption reactions followed an almost constant rate through all of the kinetic tests, suggesting that neither the AP nor the NP were fully consumed (see section 4.2.1). Similar results have been observed by Benzaazoua et al. (2001). GRE-M1 tailings Breaks in the slope, like the ones on curves shown in Figures 4 and 9, are most important in the case of the humidity-cell, the mini-alteration cells and the shake flask. Depletion alone can not explain the observed slope changes, because neither acid producing nor neutralizing minerals were found to be depleted (see section 4.2.1). Precipitation of metals inside the test chamber or in the sample voids would reduce the amounts found in the leachate. This would affect the shape of a resulting curve on a load vs. time graph. Geochemical modeling was conducted based on the results. Modeling results show that, in all cases, calcite and dolomite are close to equilibrium (SI ~ 0) throughout the tests. Also, gypsum was found to be close to saturation in the first part of these tests, but not near the end. Hence, it can be inferred that the breaks in slope were not caused by precipitation. From the combined facts that there is a greater probability of precipitation happening before the break in slope and that the reactants are not depleted, it can be assumed that the reduction in reaction rate may be related to some form of passivation. During the first weeks of the tests, gypsum, iron hydroxides and other secondary minerals slowly precipitated on the surface of the pyrite and this coating may have impeded further reaction with water and oxygen. Acid production was slowed down and consequently, neutralization was reduced as well. This would cause a reduction in the slopes of the SO42and Ca, Mg, Mn vs. time graphs. Such passivation was measured using cyclic voltametry on altered pyrite by Cruz et al. (2001). LAR-M3 tailings In Figures 5 and 10, breaks in slope are most evident in the case of the humidity-cell test and to a lesser degree in the shake flask test. Again, depletion calculations indicate that none of the elements of interest were depleted during the course of the different kinetic tests (see section 4.2.1). As for the GRE-M1 tailing, the geochemical modeling results show that precipitation is more probable at the beginning of the humidity-cell test. Various aluminum and iron hydroxides were found to be either oversaturated or near equilibrium in the first stages of the humidity-cell test. No sulfate bearing secondary minerals were found to precipitate by the Visual MINTEQ model. This may indicate that passivation of the pyrite surfaces could be responsible for the diminution in sulfate production rates. Less acid production entails less neutralization and as a result, less Ca, Mg and Mn in the flush waters. The low reactivity observed in the mini-alteration cells is believed to be due to a physical wetting problem encountered during the course of the test that lead to inadequate dissolution of reaction products. MAT-M1 and MAT-M2 tailings Both the MAT-M1 and the MAT-M2 tailings have similar behavior on Figures 6, 7, 11 and 12. For both samples, a change in the kinetic rate is observed with the humidity-cell and the mini-alteration cell tests results. In both cases the reactants were far from complete depletion. Geochemical modeling of the rinse waters of humidity-cells run on both samples showed the same pattern. In the beginning, various 4.3 Comparison of the kinetic test protocols 4.3.1 Relative aggressiveness Based on geochemical responses (reactive rates or total cumulative loads extracted vs. test time) and depletion data obtained in this study, a general trend regarding the different kinetic tests relative aggressiveness becomes apparent. The following gradation was observed: columns < humidity-cells < minialteration cells < shake flasks < modified Soxhlet extractors. 4.3.2 Acid production to neutralization ratio To evaluate the ability of a kinetic test to simulate the acid production and the consequent neutralization, the cumulative loads of Ca, Mg and Mn (neutralization by-products) are plotted against the cumulative SO42- (acid production product). Therefore, if neutralization is in response to the acid production, Ca+Mg+Mn vs. SO42- plots should be linear as long as the reactants are not depleted and precipitation is not significant (Benzaazoua et al. 2001). The main hypothesis behind this interpretation is that the tailings contain a considerable amount of NP, the Ca, Mg and Mn loads in the leachates are only attributable to dissolution of carbonate minerals. In the case of near neutral drainage pH, it has been noted that neutralization comes mainly from the carbonates and silicate minerals reaction rates are low (Sverdrup, 1990; Kwong, 1993; Sherlock et al. 1995; Paktunc, 1999, Benzaazoua et al. 2001). UQ-8 Tailings Figure 13 shows the Ca+Mg+Mn cumulative and normalized loads as a function of the cumulative normalized Sulfate load for each of the kinetic tests run on the UQ-8 tailings. Cumulative Ca+Mg+Mn (mg/kg) UQ -8 16000 14000 12000 10000 8000 6000 4000 2000 0 Column Humidity-Cell Shake Flask Soxhlet Mini-Cell 0 10000 20000 30000 40000 50000 2- Cumulative SO 4 (mg/kg) Figure 13. Cumulative added Ca, Mg and Mn loads vs. cumulative sulfate load for the UQ-8 tailing. The fact that the curves shown on Figure 13 are all linear and in the same axis implies that, no mater how aggressive the kinetic testing procedure used to alter the UQ-8 tailing, the ratio of the acid production rate to the neutralizing rate is constant. The column experiment is often considered to be the closest to natural oxidation-neutralization conditions. As the other curves are in line with the end portion of the column’s curve, it can be tentatively stated that, for the UQ-8 tailing, natural conditions are preserved even in the more aggressive tests. GRE-M1 tailings The graph of normalized cumulative Ca, Mg and Mn loads as a function of the cumulative normalized SO42- for the different kinetic tests run on the GREM1 tailings is shown on Figure 14. GRE-M1 Cumulative Ca+Mg+Mn (mg/kg) Al and Fe hydroxides, as well as various Ca, Mg, Mn and Zn carbonates were found to be oversaturated or near equilibrium. Near the end of the humidity-cell experiment, all the oversaturated minerals were soluble and near equilibrium. Modeling of the mini-cell results showed little or no change in secondary minerals saturation indices during the entire test. Various hydroxides remained oversaturated and gypsum remained near equilibrium throughout the tests. On Figures 7 and 12, it can be noted that the modified Soxhlet extractor yielded less total cumulative sulfates and Ca+Mg+Mn than the humidity-cell (a phenomenon not observed for the other tailings). This is probably due to water flow problem observed with the MAT-M2 sample. In fact, the MAT-M2 sample was submerged with water through most of the test, preventing oxygen intake. In the case of the MAT-M1 and MAT-M2 tailings, the geochemical response was deemed to have been influenced by both precipitation of metals as secondary minerals and passivation of the primary minerals’ surfaces. 4000 Column Humidity-Cell Shake Flask Soxhlet Mini-Cells 3000 2000 1000 0 0 1000 2000 3000 4000 5000 6000 7000 8000 Cumulative SO 4 2- (mg/kg) Figure 14. Cumulative added Ca, Mg and Mn loads vs. cumulative sulfate load for the GRE-M1 tailing. Figure 14 shows that, in the case of the column, the humidity-cell and the mini-alteration cell tests, the ratio of acid production to neutralization rates is maintained. However, near the end of the humidity- LAR-M3 tailings Figure 15 presents the cumulative normalized Ca+Mg+Mn vs. SO42- curves for all of the kinetic tests run on the LAR-M3 samples. Cumulative Ca+Mg+Mn (mg/kg) MAT-M1 4000 Column Humidity-Cell Shake Flask Soxhlet Mini-Cell 3000 2000 1000 0 0 2000 Cumulative Ca+Mg+Mn (mg/kg) 6000 8000 10000 Figure 16. Cumulative added Ca, Mg and Mn loads vs. cumulative sulfate load for the MAT-M1 tailing. Column Humidity-Cell Shake Flask Soxhlet Mini-Cell MAT-M2 5000 4000 3000 2000 1000 0 0 LAR-M3 4000 Cumulative SO 4 2- (mg/kg) Cumulative Ca+Mg+Mn (mg/kg) cell test we see an upward shift towards the neutralization products. As mentioned before, modeling showed that none of the sulfate bearing secondary minerals was oversaturated. This response could be explained by the passivation of the sulfide surfaces, but the natural dissolution of the carbonates could have still taken place. In the case of the shake flask and the modified Soxhlet extractor tests, the curves on Figure 14 are shifted towards higher Ca+Mg+Mn values than the end portion of the column experiment curve. In these two tests, which are more aggressive, an exaggeration of the carbonates dissolution is observed. Note that the GRE-M1 material has a very low sulfide content (0.850 %Ssulfide), which gives it an AP of only 26.6 kg CaCO3/t and a relatively large NP of 71.7 kg CaCO3/t. It seems that, in the case of a low AP-large NP sample, Soxhlets and shake flasks do not adequately simulate the natural oxidationneutralization processes. 5000 10000 15000 Cumulative SO 4 2- (mg/kg) 2500 Figure 17. Cumulative added Ca, Mg and Mn loads vs. cumulative sulfate load for the MAT-M2 tailing. 2000 1500 Column Humidity-Cell Shake Flask Soxhlet Mini-Cell 1000 500 0 0 1000 2000 3000 4000 5000 6000 2- Cumulative SO 4 (mg/kg) Figure 15. Cumulative added Ca, Mg and Mn loads vs. cumulative sulfate load for the LAR-M3 tailing. Again, in Figure 15, one can see that results from the column and humidity-cell tests are well correlated. Solution in the boiling flask of the Soxhlet extraction has reached saturation and precipitation of various Ca, Mg and Mn carbonates, whereas sulfate bearing minerals were still soluble. This explains the behavior of the Soxhlet test curve. Notice also the shake flask’s tendency to exaggerate carbonate dissolution when the material has a low AP. MAT-M1 and MAT-M2 tailings Figures 16 and 17 show the different kinetic tests’ Ca+Mg+Mn cumulative normalized loads vs. the SO42- cumulative normalized loads curves for the MAT-M1 and MAT-M2 tailings. Analysis of Figures 16 and 17 shows that most kinetic tests were affected by precipitation and redissolution. However, the main trend remains apparent for all the kinetic tests. Shake flasks tests show signs of what is believed to be sulfate precipitation in the first weeks, then re-dissolution. Both modified Soxhlets and shake flasks tests show signs of carbonate over-dissolution as well. Again, one can see that in the Soxhlet tests, sulfate becomes saturated in the boiling flask near the end of the 14 day period. Mini-alteration cells show a good correlation with the column test results until near the end of the test where neutralization products were found more saturated. 5 CONCLUSIONS The main findings of this study can be summarized as follows: − Based on the geochemical behaviors (cumulative curves and depletion data) observed in this study, the following trend has been obtained for the relative aggressiveness of the kinetic testing procedures: columns < humidity-cells < mini-alteration − − − − cells < shake flasks < modified Soxhlet extractors. The plot of the cumulative and normalized Ca, Mg and Mn loads vs. cumulative normalized load in sulfate is a valuable tool to assess a kinetic test’s ability to simulate the natural acid production to neutralization ratio. For tailings with a medium AP and a medium NP, the ratio of acid production to acid neutralization rates appears to be constant for all the kinetic test procedures used in this study. The more aggressive kinetic test procedures (shake flasks and modified Soxhlets) tend to exaggerate dissolution of carbonate minerals in the case of low AP materials. Mini-alteration cell tests are a valid alternative to humidity cell tests, especially when: a small quantity of the sample is available, one needs to alter a sample faster than in the humidity-cell, a large number of samples must be tested. Results from the mini-cells are very similar to those of the humidity-cells. 6 ACKNOWLEDGEMENTS The authors would like to acknowledge the financial contribution of the Industrial NSERC Polytechnique-UQAT Chair on Environment and Mine Wastes Management (www.polymtl.ca/envirogeremi) and the “Unité de Recherche et de Service en Technologie Minérale” for its technical support. REFERENCES Adam, K., Kourtis, A., Gazea, B. and Kontopoulos, A. 1997. Evaluation of static tests used to predict the potential for acid drainage generation at sulfide mine sites. Trans. Inst. Min. Metall. sect. 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