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Cyclic voltammetry applied to evaluate reactivity in sulfide mining residues
Applied Geochemistry 16 (2001) 1631–1640 www.elsevier.com/locate/apgeochem
Cyclic voltammetry applied to evaluate reactivity in sulfide mining residues Roel Cruz a, Blanca A. Me´ndez b, Marcos Monroy b, Ignacio Gonza´lez a,* a
Universidad Auto´noma Metropolitana-Iztapalapa, Depto. De Quı´mica, Apdo. Postal 55-534, 09340 Me´xico D.F., Mexico b Universidad Auto´noma de San Luis Potosı´, Instituto de Metalurgia. Av. Sierra Leona No. 550, Col. Lomas 2a Seccio´n, 78210 San Luis Potosı´, S.L.P., Mexico Received 15 May 2000; accepted 8 January 2001 Editorial handling by R. Fuge
Abstract Oxidation of sulfide present in mining residues can generate contaminating acid effluent known as acid rock drainage. Prediction and control of acid rock drainage are critically important to the mining industry because of the environmental impact resulting from the sulfide oxidation. Due to its particular reaction kinetics, once acid drainage has begun, it is very difficult to control without a substantial economic investment. For this reason, efficient prediction and prevention programs, which monitor mining waste reactivity, are required to limit the oxidation of sulfide-bearing residues before damage to the environment occurs. In this work, the authors evaluated mining waste reactivity under oxidizing conditions as a function of its voltammetric behavior before and during alteration under simulated natural conditions produced in the laboratory. This method is supported by conventional mineralogical characterization of the mineral samples and chemical quality of the effluents produced during the simulated alteration process kinetics. # 2001 Elsevier Science Ltd. All rights reserved.
1. Introduction One of the major environmental problems confronting the mining industry is the generation of acid rock drainage (ARD). ARD, which results from the oxidation of sulfide minerals contained in mining wastes, is often characterized by high concentrations of metals and sulfates. Such solutions can potentially contaminate ground and surface waters, as well as soils. Pyrite (FeS2) and pyrrhotite (Fe1xS) are by far the most abundant sulfide minerals present in sulfidic mining wastes. Oxidation of these minerals has been extensively described (Buckley et al., 1988; Ahlberg et al., 1990; Li and Wadsworth, 1993; Pratt et al., 1994; Evangelou, 1995; Nicholson et al., 1998). The general mechanisms involved in the chemical and biological dissolution of pyrite and the subsequent formation of acidic drainage are well established (Lowson, 1982; Skousen, 1995). However, application of scientific knowledge in predicting ARD generation * Corresponding author. Fax:+52-580-44666. E-mail address: [email protected] (I. Gonza´lez).
is very difficult, due to the heterogeneous nature of the mining wastes and the great number of possible reactions and interactions of the minerals found in mining wastes. The procedure most commonly used to predict ARD consists of both static and kinetic tests (Morin and Hutt, 1997). The static tests determine potential ARD generation by evaluating the balance between acid producing and acid neutralizing minerals. Samples identified as potential ARD generators according to the static tests are submitted to kinetic tests. These tests monitor acidity production and metal dissolution from the mining wastes which have undergone alteration in humidity cells over long periods of time (at least 40 weeks). Mathematical models are then applied to the results obtained in both static and kinetic tests to predict ARD generation and the chemical quality of the contaminating acid effluent. However, this procedure is limited in that it provides information only on the final oxidation products, while evolution of the surface state of sulfides, changes in mineralogical sample composition and the effect of these factors on mineral reactivity and ARD generation can not established.
0883-2927/01/$ - see front matter # 2001 Elsevier Science Ltd. All rights reserved. PII: S0883-2927(01)00035-X
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In a previous study, application of a cyclic voltammetry technique with carbon paste electrodes and typical mineralogical techniques were used to study the reactivity evolution of pyritic samples with different mineralogical composition (Cruz et al., 2000). The carbon paste electrodes contained sulfide mineral sample recovered from leaching tests after different times of leaching under simulated environmental conditions. For each cell, the evolution of the effluent chemical quality monitored by acidity production and metal liberation from altered samples was evaluated. The results obtained from this study showed that reactivity of pyrite-rich samples was mainly affected by the presence of sulfide mineral impurities (like sphalerite ZnS and galena PbS) and by the formation of Fe oxihydroxy coatings, which passivate the pyrite surface (Cruz et al., 2000). Despite these findings, the application of this methodology in the prediction of ARD generation from mining wastes has still to be demonstrated. The aim of the present work was to use this methodology in the study of the reactivity evolution of sulfide mine wastes (mill tailings) subjected to oxidising conditions, in order to assess the procedure for prediction of ARD generation. The electrochemical and mineralogical data could further represent valuable data with application in the validity of ARD predictive models.
2. Methodology 2.1. Mineral samples Samples were provided as mill tailings by 3 mill plants in Mexico: sample MT1 came from the processing of a vulcanogenic massive sulfide type ore; sample MT2 came from a Pb-Zn skarn type ore; and sample MT3 came from a Au–Ag epithermal type ore. The mineralogical composition of each sample is shown in Table 1. The MT1 sample had a high percentage of pyrite (66.0%) and a significant concentration of sphalerite (2.1%), while its carbonate content was relatively low (0.5%). Pyrite and pyrrhotite represented 39.0% of the mineral content in the MT2 sample with 1.6% sphalerite, and a carbonate and silicate content of 56.6%. The MT3 sample contained the greatest quantity of carbonates (9.6%) and silicates (86.9%) and the lowest sulfide concentration (<3.5%). Mill tailings samples were received as pulp, filtered, dried at 35 C, and homogenized before analysis.
Particle size distribution of each mill sample was analyzed using a Shimadzu SALD-1100 size analyzer. The size distribution for MT1 sample indicated that 80% (P80) of the particles was less than 20.14 mm. The P80 for the for MT2 and MT3 samples were 21.5 and 18.75 mm, respectively. Prepared samples were kept in N2 in order to avoid subsequent alteration. 2.2. Acid-base accounting The acid-base accounting (ABA) is the method most routinely used to perform static tests for ARD prediction. The method consists of determining the balance between acid producing mineral components (acid potential, AP) and the acid-consuming (neutralizing) minerals (neutralization potential, NP) in a given sample. To determine NP, the modified method of Sobek (Morin and Hutt, 1997) was used. Calculation of the neutralization potential was performed based on the acid utilized in digestion of the neutralizing species according to the following formula: NP (Kg CaCO3/t)=[((Nvol. HCl)– (Nvol. NaOH))50] / [sample weight]. In this formula, N is the normality of the HCl and NaOH solutions used in the digestion and titration and vol. is the volume of each solution used. The acid potential (AP) of the sample was calculated from the measured concentrations of total sulfur-sulfide in the sample. Finally, the ratio NP/AP, known as the neutralization potential ratio (NPR), was used as the criterion to evaluate the capacity of the material to generate acidic drainage (Price et al., 1997). From this criterion, the samples classified as likely ARD generators were submitted to kinetic studies as described below. 2.3. Kinetic test (alteration of sulfide samples) Evolution of mining wastes reactivity were evaluated using electrochemical and mineralogical studies for unaltered samples, and using samples submitted to alteration in kinetic tests. The apparatus and leaching procedure used were designed to simulate natural oxidation of primary mineral samples, similar to the conditions produced for kinetic assays in humidity cells [American Society for Testing and Materials (ASTM), 1996; Morin and Hutt, 1997). The leaching apparatus consisted of a 5-cm diameter Buchner funnel containing 20 g of mineral sample
Table 1 Mineralogical composition of mill tailings (% weight) Sample
Pyrite
Pyrrhotite
Sphalerite
Galena
Arsenopyrite
Carbonate
Silicate
MT1 MT2 MT3
66.0 18.0 3.8
0.0 21.0 0.0
2.1 1.6 0.01
0.4 0.3 0.01
0.6 2.3 0.02
0.5 18.5 9.6
30.2 38.1 86.9
R. Cruz et al. / Applied Geochemistry 16 (2001) 1631–1640
supported on 0.45 mm pore size filter paper. Samples were submitted to cyclic leaching periods. The procedure consisted of 1 day of leaching, followed by 3 days of exposure to atmospheric conditions, then 1 day of leaching again, and 2 days under atmospheric conditions. Subsequently, leaching cycles were carried out twice a week. Data obtained were analyzed as a function of the leaching time, which was defined as the time between the initial cycle of leaching and removal of the sample for analysis. The initial leach solution was prepared before use and formulated to simulate rainwater: distilled water with a pH adjusted to 5.5 by the addition of purified CO2 (g). The leaching of samples was carried out by adding 15 ml of leach solution to each sample container for each cycle. The samples were left inundated with the leach solution for 3 h during each leach cycle. Leach solution was extracted from each device by vacuum suction, refiltered (0.45 mm) and analyzed. Experiments were run in triplicate for each sample. Mineral samples were taken after different leaching times. All leached samples were dried and preserved under an inert atmosphere (N2) for electrochemical analysis and SEM observations. 2.4. Chemical and mineralogical characterization Chemical and mineralogical characterization of each sample was performed before and after leaching. In this characterization, the composition, occurrence and form of minerals making up each sample were determined. The surface state of the mineral particles after different leaching times was then evaluated. Mineralogical observations were carried out using a Versamet Union optical microscope and a Philips XL30 scanning electron microscope (SEM). Microanalyses were performed using an energy dispersion X-ray fluorescence spectrometer (EDAX 4 Dix) coupled to an SEM. The content of heavy metals was determined for the leachate solutions using a Perkin Elmer 5000 Atomic Absorption spectrometer. The pH of the leachate solution samples was determined using a Beckman f320 pH meter. For solid samples, total S was analyzed using a Leco 244 SC. Sulfur in solution was determined with plasma spectroscopy using a Jarell Ash Iris 25 spectrometer. Solid sulfates were analyzed using the standard gravimetric method. The content of sulfide-sulfur in solid samples was derived by subtracting S in the form of SO4 from total S. 2.5. Electrochemical experimentation system The reactivity of mill tailings was measured for both unleached samples and after selected times of leaching. Electrochemical analyses were carried out by cyclic voltammetry. This electrochemical technique consisted of imposing a sweep potential to the sample contained in the working electrode in order to reach the interfacial energetic conditions that promote the electrochemical
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reactions. The transferred charges (electrons) associated with the electrochemical reactions are registered as a function of an imposed potential (i.e. current vs. potential, voltammograms). These voltammetric studies do not simulate the ARD process, they are only used to characterize the surface state evolution and oxidation capacity of mineral samples through alteration processes. An EG&G PAR 273 potentiostat coupled to a PC with the M270 software was used to impose a sweep rate of 20 mVs1 and record the voltammetric response. A typical 3-electrode cell was used at room temperature and N2 atmosphere. A C paste electrode (CPE — sample, 50% wt) was used as the working electrode. The feasibility and preparation details of the CPE have been previously reported by La´zaro et al. (1995). A graphite rod (Æesar, 99.999%) was used as counter electrode. The reference electrode consisted of a Hg/Hg2SO4/K2SO4(sat) electrode, where (sat) indicates saturated K2SO4 solution. This electrode is identified as SSE and has a potential of 0.615 V vs. a standard H electrode. All potentials in this investigation are referred to this electrode. The electrode system was placed in a PyrexTM glass cell containing an electrolyte solution of 0.1 M NaNO3 at a pH of 6.5 normally used as a characterization electrolyte. Even though this solution was not pH buffered, changes in the pH were not significant due to the microelectrolysis conditions of the voltammetric assays. Purified N2 gas was bubbled through the electrolyte solution for 45 min before the start of the experiments and a N2 atmosphere was maintained within the cell during the tests.
3. Results 3.1. Acid base accounting (ABA) The results of the ABA are presented in Table 2. This table also shows the screening criteria for the identification of potentially acid generating materials (Price et al., 1997). On the basis of these results, with a NPR of 0.005, MT1 sample could be classified as having the greatest ARD generating potential. The MT2 sample had a NPR of 0.36, which also corresponds to a high ARD generating potential, although to a lesser extent than the MT1 sample. The MT3 sample proved to represent no risk of ARD generation with a NPR of 7.26. The experience indicates that for materials with a NPR higher than 4.0, it is not necessary to carry out further kinetic tests (Price et al., 1997). On the basis of these results, kinetic tests in mini-cells were only performed for the MT1 and MT2 samples. 3.2. Chemical evolution of the leachate Fig. 1 shows the pH evolution of leachates for the MT1 and MT2 samples submitted to alteration conditions. Fig. 1 shows that the pH evolution of both leachates is
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Table 2 Potential for acid rock drainage generation based on the acidbase accounting analysis Sample
NPa
APb
NPRc
ARD potentiald
MT1 MT2 MT3
4.1 197.3 78.5
756.2 540.3 10.8
0.005 0.36 7.26
Likely Likely None
a
NP, neutralization potential in kg of CaCO3/ton of mineral. b AP, acid potential in kg of CaCO3/ton of mineral. c NPR, neutralization potential ratio, NP/AP. d Screening criteria for identification of potentially ARD generating (ASTM, 1996; Morin and Hutt, 1997): NPR < 1:1, likely; 1:1 to 2:1, possibly; 2:1 to 4:1: low; > 4:1, none.
Fig. 1. Evolution of pH of the leachate solution as a function of leaching time for mill tailing samples: MT1 and MT2.
quite different. While the pH of the leachate solution for the MT2 sample rises from 6.5 to almost 7.0, for the MT1 sample the pH decreases immediately to values close to 3.0 after the first week of leaching. Figs. 2 and 3 show the chemical evolution for dissolved Fe, Zn, and Pb ions in leachates from the MT1 and MT2 samples under alteration. In the MT1 sample, high Zn dissolution occurred during the first week of leaching (Fig. 2). The Zn concentration in solution gradually diminished until reaching a stable level in the sixth week of leaching. A high level of Fe dissolution was also observed for the first week of leaching followed by a decrease in the concentration of dissolved Fe. A barely perceptible increase in Fe dissolution occurred during the last 8 weeks of leaching. Lead in the leachate from the MT1 sample was highest in concentration (2.5 mg/l) for the first two weeks, with lesser dissolution (<0.3 mg/l) occurring steadily for the rest of the alteration process. For the MT2 sample, Zn dissolution followed the same pattern as that observed for MT1, however the Zn concentration in the leachate obtained from the MT2
Fig. 2. Chemical evolution of Fe, Zn and Pb ions in the leachate solution of mill tailing MT1 as a function of leaching time. Inset shows a low concentration zone for the Pb in solution.
Fig. 3. Chemical evolution of Fe, Zn and Pb ions in the leachate solution of mill tailings sample MT2 as a function of leaching time.
sample was as much as 500 times lower than that from MT1 (Fig. 3). Iron dissolution was low for the MT2 sample during the first week of leaching, increasing during the second and fourth weeks, and again diminishing to low concentrations from the fifth week of alteration. Lead dissolution was low throughout the 10 weeks of leaching. 3.3. Electrochemistry of mining wastes Fig. 4 shows the voltammetric behavior of all the fresh samples without leaching. The MT1 and MT2 samples (curves a and b, respectively) exhibit a voltammetric behavior similar to that reported for pyrite in previous studies (Cruz et al., 2000). A high oxidation process (O1) could be associated with pyrite oxidation. This process is referred to as a transpassive oxidation and it is characterized by an aggressive electrochemical oxidation of pyrite (Zhu and Wadsworth, 1993). The
R. Cruz et al. / Applied Geochemistry 16 (2001) 1631–1640
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Fig. 4. Typical voltammograms obtained for carbon paste electrodes containing mill tailings samples from each mining site: (a) MT1; (b) MT2; (c) MT3. The voltammetric response of carbon paste electrode (CPE) without mineral in 0.1 N NaNO3 is also shown (d). The scan potential was initiated in the positive direction at 20 mV s1. Inset shows low current area of curves for MT3 and CPE. Arrows indicate the scan potential direction.
reaction products in this electrochemical oxidation are Fe(III) oxides, SO2 4 ion and partially oxidized S intermediates. The proposed reactions are as follow (Zhu and Wadsworth, 1993): FeS2 þ 2H2 O ! FeOOH þ 2S0 þ 3Hþ þ 3e FeS2 þ 10H2 O ! FeOOH þ
2SO¼ 4
þ
ð1Þ
þ 19H þ 15e
ð2Þ 2
xFeS2 þ xH2 O () xFeOH
þ
2S2 x
þ
þ xH
þ ð3x 4Þe
ð3Þ
xFeS2 þ ðx þ 12ÞH2 O () xFeOH2 þ 2Sx O2 6 þ ðx þ 24ÞHþ þ ð3x þ 20Þe
ð4Þ
The transpassive voltammetry oxidation of pyrite is characterized by the following behavior: current in the oxidation branch obtained in the reverse scan potential is higher than that obtained in forward scan. However, oxidation process O1 shows an inverse behavior in the oxidation branch (Fig. 4, curve a). This could indicate the presence of another oxidation process (see below). When the scan is reversed, two barely perceptible reduction peaks (R1 and R2) appear in these voltammograms. These reduction peaks correspond to the reduction of oxidation products. The presence of an inflection (O2) is significant in the oxidation process of
the MT2 sample at potentials 0.4 V. In previous work a similar shoulder has been observed at 0.33 V, this oxidation process was attributed to the presence of galena in a pyrite sample (Cruz et al., 2000). However, electrochemical studies of pyrrhotite and pyrite–pyrrhotite mixtures have shown that the oxidation process of pyrrhotite is carried out at a very similar potential to the O2 process (Cruz et al., 1999). Moreover, it is important to notice that pyrrhotite concentration is higher than galena in the MT2 sample (Table 1). For this reason, in the present work, the O2 oxidation process is related to electrochemical oxidation of pyrrhotite present in this sample. Figure 4, curve c represents the voltammetric behavior of the MT3 sample. It is important to observe that for current scales in mA, this mineral could be considered as having no voltammetric response. However, for low currents (Fig. 4, inset), a voltammetric response is observed for this sample compared to the response for the CPE without mineral (Fig. 4, curve d). In order to compare the voltammetric responses, parameters based on the oxidation process characteristics were established. These parameters were: the required potential to reach 5 mA (EI=5mA) for the forward scan in the oxidation process, the oxidation rate (I/E rate) of the mineral sample, and the charge (Q) associated to the oxidation process. The I/E rate was calculated from the slope of the current-potential response during the forward scan at 0.55–0.65 V potential range. The Q was evaluated from the surface area below the oxidation process. These parameters are reported in Table 3.
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Table 3 Electrochemical parameters of the oxidation process for mining wastes before leaching and after 2, 6 and 10 weeks of leaching EI=5mA (V/SSE)a
I/E (mA/mV)b
Q (mC)c
Sample
Fresh
2 weeks
6 weeks
10 weeks
Fresh
2 weeks
6 weeks
10 weeks
Fresh
2 weeks
6 weeks
10 weeks
MT1 MT2 MT3
0.20 0.30 0.66
0.29 0.39 NAd
0.31 0.39 NA
0.23 0.36 NA
1.71 2.3 0.04
3.71 1.4 NA
2.8 1.2 NA
4.0 1.4 NA
23.8 11.8 0.08
10.2 2.5 NA
7.4 2.2 NA
12.7 2.9 NA
a b c d
EI=5mA, potential required to reach 5 mA during the direct sweep in the oxidation process. I/E, current released as the potential increased. Q, charge associated with the oxidation process. NA, not analyzed.
To reach an oxidation current of 5 mA, the MT1 sample required a potential of 0.2 V, while for the MT2 sample a higher potential ( 0.31 V) was required. Finally, the MT3 sample requires the highest potential ( 0.6 V). This trend is associated with the oxidation capacity of each sample, thus, the higher the required potential for the oxidation process, the lower the oxidation rate (I/E) and Q (Table 3).
4. Discussion
3.4. Evolution of electrochemical reactivity of the mining wastes
The low pH of the leachate solution from MT1 is due to the high pyrite content and the low amount of neutralizing species in this sample. This pH condition provokes partial dissolution of oxihydroxy species formed during the leaching process in agreement with:
Evolution of electrochemical reactivity as a function of leaching time for the MT1 and MT2 samples is presented in Figs. 5 and 6, respectively. Since we are concerned only with the oxidative character of reactivity, these figures show the region corresponding to the oxidation process for the different voltammetric responses. Fig. 5 illustrates evolution of reactivity for the MT1 sample after 2, 6 and 10 weeks of leaching. The response after 2 weeks of leaching shows a significant decrease in charge associated with the oxidation process (Q) and an increase in potential where the oxidation process begins (Fig. 5, curve b, and Table 3). After 6 weeks of leaching, a decrease in reactivity of the residues is observed (Fig. 5, curve c). This is demonstrated by a significant decrease in Q as well as in the oxidation rate (I/E). In addition, a greater overpotential (EI=5mA) is required at this time. After 10 weeks of leaching, a gain in reactivity is observed compared to the 2 and 6 week periods as indicated by the increase in oxidation rate and Q (Fig. 5, curve d, Table 3). On the other hand, the transpassive behavior (higher current for the reverse scan) is recovered as the leaching time increases (Fig. 5). The MT2 sample also showed a significant decrease in the oxidation process after 2 weeks of leaching (Fig. 6, curve b). This reactivity decrease is continued after 6 weeks of leaching (curve c). After 10 weeks of leaching, the MT2 sample showed an increase in oxidative capacity (curve d) although changes in the electrochemical parameters (EI, I/E and Q) were practically negligible (Table 3).
The mechanism associated with acid generation from Fe sulfide is represented by: þ FeS2 þ 5=2H2 O þ 15=4O2 ! FeOOH þ 2SO¼ 4 þ 4H
ð5Þ
FeOOH þ 3Hþ () Fe3þ þ 2H2 O
ð6Þ
The high Fe concentration in the leachate solution from MT1 supports this hypothesis. On the other hand, the high level of neutralizing species in the MT2 sample provokes a high pH in this sample leachate solution. Thus, the Fe oxihydroxy species are stable on the surface and the dissolved Fe is considerably lower than in the MT1 leachate solution. 4.1. The MT1 sample Static tests for the MT1 sample indicate that this sample possesses a high potential for generating ARD (Table 2). This is principally due to the low concentration of neutralizing species and high sulfide concentration (Table 1). The fact that the MT1 sample has a greater potential for ARD generation than the other samples should implicate greater reactivity for this sample, as corroborated by the electrochemical study (Fig. 4). However, the electrochemical behavior of the unleachated sample could be strongly influenced by the presence of surface oxidized species, which could have been formed as an intermediate product during the mill process (Fig. 4, curve a). The presence of such surface species was demonstrated by scanning electron microscopy (SEM)
R. Cruz et al. / Applied Geochemistry 16 (2001) 1631–1640
Fig. 5. Evolution of electrochemical reactivity of the MT1 sample: (a) before leaching; (b) after 2 weeks of leaching; (c) 6 weeks of leaching; (d) 10 weeks of leaching. The voltammograms were obtained from CPE-mill tailing sample in 0.1 M NaNO3. The scan potential was initiated in the positive direction at 20 mV s1.
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Fig. 6. Evolution of electrochemical reactivity for the MT2 sample: (a) before leaching; (b) after 2 weeks of leaching; (c) 6 weeks of leaching; (d) 10 weeks of leaching. The scan potential was initiated in the positive direction at 20 mV s1.
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Fig. 7. Scanning electron microscopy images of mineral particles in the mill tailings sample MT1: (a) oxidized Fe species on the unleached pyrite surface; (b) favored corrosion of sphalerite on sample after leaching.
observations (Fig. 7). Thus, the O1 process could be associated with a simultaneous oxidation of surface species and pyrite. For this reason, the typical transpassive voltammetric behavior was not observed (Fig. 5, curve a). Dissolution of these Fe species during the first week of leaching is shown by the rapid increase in the leachate Fe concentration (Fig. 2). Thus, the changes observed in the voltammetric behavior after 2 weeks appear to be associated with a decrease in the concentration of electroactive species present on the mineral surface. This provokes the recovering of a transpassive voltammetric response (Fig. 5, curve b). The decrease in reactivity of the sample after 6 weeks leaching (Fig. 5, curve c, Table 3) could be related to the formation of an Fe oxihydroxy coating at the mineral surface during the leaching process. These Fe oxihydroxy coatings could correspond to goethite, in agreement with reaction (5), . or ferrihydrite, (5Fe3+ 2 O3 9H2O) and lepidocrocite (Fe3+O(OH)), which have been reported to be present in weathered sulfide tailings (Hammarstrom and Smith, 2000). Moreover, amorphous Fe oxihydroxy compounds could also be formed in agreement with the follow reaction (Hammarstrom and Smith, 2000): FeS2 þ 15=4 O2 þ 7=2 H2 O () FeðOHÞ3 þ2H2 SO4 ð7Þ Some authors have observed that when two sulfide minerals with different rest potentials (corrosion potential) come into contact, the mineral with the higher rest potential is galvanically protected, favoring oxidation of the mineral with lower rest potential (Metha and Murr, 1983, Nowak et al., 1984). As the sphalerite has a lower rest potential than pyrite (Chander, 1988; Metha and Murr, 1983,), this sulfide offers galvanic protection to pyrite in the MT1 sample, such as is demonstrated by favorable Zn dissolution from sphalerite and low Fe dissolution (Fig. 2). It is important to recall that Fe in solution in the first stages
of leaching, comes from the dissolution of surface species developed during the mill process. Favorable corrosion of sphalerite was also observed using SEM [Fig. 7(b)]. When most of the sphalerite had dissolved after 7 weeks of leaching, sample reactivity increased, as observed in the voltammogram corresponding to 10 weeks of leaching (Table 3). It is important to notice even when the chemistry of the leachate does not suggest changes in the oxidation of pyrite present in the sample after 4 weeks leaching, the electrochemical study provides evidence of important changes in sample reactivity. 4.2. The MT2 sample The ABA analysis for the MT2 sample revealed that this sample possessed a high potential for ARD generation despite its elevated carbonate concentration (Table 1). On the other hand, the chemical quality of the leachate indicates that the MT2 sample did not generate acidity during the period of alteration (Fig. 1). In addition, the sulfide dissolution was very slow for this sample, as indicated by the low metal concentration in the leachate (Fig. 3). These results could indicate a low potential for ARD generation, in contrast to the results of the ABA analysis. The voltammetric study for the MT2 sample showed a strong decrease in mineral reactivity after 2 weeks of leaching (Fig. 6 and Table 3). This is most certainly due to the formation of Fe oxihydroxy precipitates formed on the surface of the mineral as a result of dissolved Fe present and the system conditions of neutral pH. Besides the Fe precipitates in this system, CaSO4 was also formed, contributing to the decreased reactivity of the MT2 sample. Formation of Fe and Ca precipitates was also demonstrated by SEM observations (Fig. 8). The passivation state of the MT2 sample, induced by the formation of precipitates on the sulfide surface, was maintained during the 10 weeks of alteration as indicated by the leachate chemistry, the voltammetric study and the SEM observations (Figs. 3, 6 and 8 and Table 3).
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Fig. 8. Scanning electron microscopy images of mineral particles in mill tailings sample MT2 after different leaching time: (a) 2 weeks; (b) 10 weeks. The oxidized species formed during the leaching process are indicated.
Although the results of the kinetic study of the MT2 sample did not demonstrate ARD generation potential, according to the static tests and electrochemical reactivity, this sample had a high potential for ARD generation. This discrepancy between the results obtained from each technique is mainly due to the neutralizing species (carbonate) in the MT2 sample had not been consumed at the end of the leaching period of this study. Certainly, oxidation of sulfides occurred during the first 2 weeks of leaching, however, this fact is not completely reflected in the chemistry of the leachate. The products of dissolution of sulfide minerals must be immediately neutralized by carbonate, resulting in the precipitation of gypsum and ferric oxihydroxy, such as is suggested by the SEM images. For this reason, it is suggested that the alteration kinetics should be observed for a greater period of time to establish the actual evolution of reactivity of the sulfides present in the MT2 sample, in anticipation of the neutralization or passivation of carbonates and therefore the reactivation of sulfides, mainly pyrite and pyrrhotite. 4.3. The MT3 sample The high concentration of neutralizing, non-electroactive species and the low concentration of sulfides in the MT3 sample, indicate that the ARD generation potential is essentially null for this sample, as predicted by the results of the static tests (Table 2). Experience with this kind of sample indicates that it does not represent an acid generation risk. The voltammetric response of this sample evidences the low sulfide concentration contributing to the lowest electrochemical reactivity for this mill waste. For some voltammetric assays of this sample, a slight response of the sulfides occurred, however, only the reproducible behavior of this mill waste is shown (Fig. 4, curve c). Although an electrochemical voltammetric study could be performed using samples recovered from kinetic tests, the effect of sulfide passivation, similar to
that observed for the MT2 sample, must be more significant in this sample due to the higher concentration of carbonate and the very low concentration of sulfide minerals. The passivation of sulfides combined with their low concentration in this sample must decrease the electrochemical reactivity of sulfides and consequently the reproducibility of the electrochemical results.
5. Conclusions Cyclic voltammetry offers a good alternative method for monitoring the evolution of reactivity of sulfide mining wastes. In this work, by using electrochemical methods the authors showed evidence of the reactivity potential of mill wastes subjected to alteration conditions, even if no reactivity was shown by conventional kinetic tests. The main advantage of incorporating cyclic voltammetry into the conventional methods of ARD prediction is its capacity to describe the factors that influence sulfide reactivity which are not evaluated by the traditionally utilized prediction techniques. Hence, reactivity evolution of mill tailings and the effect of the galvanic protection offered by associated impurities, as well as the passivation by Fe oxihydroxy coatings could be established. The use of the methodology presented here could provide interesting information in acid rock drainage prediction from sulfide mining wastes. Acknowledgements Financial support for this work comes from CONACyT (grant 485100-5-25715B). R.C. and B.A.M. also thank CONACyT for scholarships. References Ahlberg, E., Forssberg, R.S.E., Wang, X., 1990. The surface oxidation of pyrite in alkaline solution. J. Appl. Electrochem. 20, 1033–1039.
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American Society for Testing and Materials (ASTM), 1996. ASTM Designation: D5744-96 — Standard Test Method for Accelerated Weathering of Solid Materials Using Modified Humidity Cell, ASTM, West Conshocken, PA. Buckley, A.N., Hamilton, I.C., Woods, R., 1988. Studies of the surface oxidation of pyrite and pyrrhotite using X-Ray photoelectron and linear potential sweep voltammetry. In: Woods, R., Richardson, P. (Eds.), Internat. Symp. Electrochemistry in Minerals and Metals Processing II, 1988. 21, The Electrochemical Society, pp. 234–246. Chander, S., 1988. Electrochemistry of sulfide flotation. Minerals and Metallurgical Processing 8, 104–113. Cruz, R., Bertrand, V, Monroy M., Gonzalez, I., 2001. Effect of sulfide impurities on the reactivity of pyrite and pyritic concentrates: a multi-tool approach. Appl. Geochem., in press. Cruz, R., Monroy, M., Gonza´lez, I., 1999. Modificacio´n de la reactividad de Pirita y Pirrotita por efectos galva´nicos con otros sulfuros minerales. Proceedings of XIII National Meeting of Mexican Electrochemical Society (SME), Me´rida. Yuc., Me´xico. CD-ROM. Evangelou, V.P., 1995. Pyrite Oxidation and its Control. CRC Press, USA p. 293. Hamarstrom, J.M., Smith, K.S., 2000. Basics of solids geochemistry and characterization. In: Geoenvironmental Analysis of Ore Deposits, Short Course Notes. 5th Internat. Conf. Acid Rock Drainage, Denver, CO, USA. Li, J., Zhu, X., Wadsworth, M.E., 1993. Raman spectroscopy of natural and oxidized metal sulfides. In: Hager, J.P. (Ed.), EPD Congress. The Minerals, Metals and Materials Society, pp. 229–244. La´zaro, I., Martı´nez-Medina, N., Arce, E., Rodrı´gez, I., Gonza´lez, I., 1995. The use of carbon paste electrodes with non-
conducting binder for the study of minerals: Chalcopyrite. Hydrometal 38, 3, 275–285. Lowson, R.T., 1982. Aqueous oxidation of Pyrite by molecular oxygen. Chem. Rev. 82, 5. Mehta, A.P., Murr, L.E., 1983. Fundamentals studies of the contribution of galvanic interactions to acid-bacterial leaching of mixed metal sulfides. Hydrometal 9, 235–256. Morin, K.A., Hutt, N.M., 1997. Environmental Geochemistry of Minesite Drainage: Practical and Case Studies. MDAG Publising, Vancouver, BC, Canada, p. 333. Nicholson, R.V., Scharer, J.M. Kwong, E.C.M., Janzen, M.P., 1998. Laboratory studies of pyrrhotite oxidation. MEND Project 1.21.2, 1998. Nowak, P., Krauss, E., Pomianowsky, A., 1984. The electrochemical characteristics of the galvanic corrosion of sulfide minerals in short-circuited model galvanic cells. Hydrometal 12, 95–110. Pratt, A.R., Nesbitt, H.W., Muir, I.J., 1994. Generation of acids from mine waste: oxidative leaching of pyrrhotite in dilute H2SO4 solution at pH 3.0. Geochim. Cosmochim. Acta 58, 23, 5147. Price, W.A., Morin, K., Hutt, N., 1997 Guide lines for the prediction of acid rock drainage Part II. Recommended procedures for static and kinetic testing. In: 4th Internat. Conf. on Acid Rock drainage, Vancouver, BC, Vol. I, 15-30. Skousen, J.G., 1995. Acid mine drainage. In: Skousen, J.G. and Ziemkiewicz, P.F. (Eds.), Acid Mine Drainage, Control and Treatment. West Virginia Univ. and the National Mine Land Reclamation Center (Chapter 2, pp. 91–12). Zhu, X., Li, J., Wasdworth, M.E., 1993. Kinetics of transpassive oxidation of pyrite. In: Hager. J.P. (Ed.), EPD Congress. The Minerals, Metals and Materials Society (pp. 355–368).
Cyclic voltammetry applied to evaluate reactivity in sulfide mining residues Roel Cruz a, Blanca A. Me´ndez b, Marcos Monroy b, Ignacio Gonza´lez a,* a
Universidad Auto´noma Metropolitana-Iztapalapa, Depto. De Quı´mica, Apdo. Postal 55-534, 09340 Me´xico D.F., Mexico b Universidad Auto´noma de San Luis Potosı´, Instituto de Metalurgia. Av. Sierra Leona No. 550, Col. Lomas 2a Seccio´n, 78210 San Luis Potosı´, S.L.P., Mexico Received 15 May 2000; accepted 8 January 2001 Editorial handling by R. Fuge
Abstract Oxidation of sulfide present in mining residues can generate contaminating acid effluent known as acid rock drainage. Prediction and control of acid rock drainage are critically important to the mining industry because of the environmental impact resulting from the sulfide oxidation. Due to its particular reaction kinetics, once acid drainage has begun, it is very difficult to control without a substantial economic investment. For this reason, efficient prediction and prevention programs, which monitor mining waste reactivity, are required to limit the oxidation of sulfide-bearing residues before damage to the environment occurs. In this work, the authors evaluated mining waste reactivity under oxidizing conditions as a function of its voltammetric behavior before and during alteration under simulated natural conditions produced in the laboratory. This method is supported by conventional mineralogical characterization of the mineral samples and chemical quality of the effluents produced during the simulated alteration process kinetics. # 2001 Elsevier Science Ltd. All rights reserved.
1. Introduction One of the major environmental problems confronting the mining industry is the generation of acid rock drainage (ARD). ARD, which results from the oxidation of sulfide minerals contained in mining wastes, is often characterized by high concentrations of metals and sulfates. Such solutions can potentially contaminate ground and surface waters, as well as soils. Pyrite (FeS2) and pyrrhotite (Fe1xS) are by far the most abundant sulfide minerals present in sulfidic mining wastes. Oxidation of these minerals has been extensively described (Buckley et al., 1988; Ahlberg et al., 1990; Li and Wadsworth, 1993; Pratt et al., 1994; Evangelou, 1995; Nicholson et al., 1998). The general mechanisms involved in the chemical and biological dissolution of pyrite and the subsequent formation of acidic drainage are well established (Lowson, 1982; Skousen, 1995). However, application of scientific knowledge in predicting ARD generation * Corresponding author. Fax:+52-580-44666. E-mail address: [email protected] (I. Gonza´lez).
is very difficult, due to the heterogeneous nature of the mining wastes and the great number of possible reactions and interactions of the minerals found in mining wastes. The procedure most commonly used to predict ARD consists of both static and kinetic tests (Morin and Hutt, 1997). The static tests determine potential ARD generation by evaluating the balance between acid producing and acid neutralizing minerals. Samples identified as potential ARD generators according to the static tests are submitted to kinetic tests. These tests monitor acidity production and metal dissolution from the mining wastes which have undergone alteration in humidity cells over long periods of time (at least 40 weeks). Mathematical models are then applied to the results obtained in both static and kinetic tests to predict ARD generation and the chemical quality of the contaminating acid effluent. However, this procedure is limited in that it provides information only on the final oxidation products, while evolution of the surface state of sulfides, changes in mineralogical sample composition and the effect of these factors on mineral reactivity and ARD generation can not established.
0883-2927/01/$ - see front matter # 2001 Elsevier Science Ltd. All rights reserved. PII: S0883-2927(01)00035-X
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In a previous study, application of a cyclic voltammetry technique with carbon paste electrodes and typical mineralogical techniques were used to study the reactivity evolution of pyritic samples with different mineralogical composition (Cruz et al., 2000). The carbon paste electrodes contained sulfide mineral sample recovered from leaching tests after different times of leaching under simulated environmental conditions. For each cell, the evolution of the effluent chemical quality monitored by acidity production and metal liberation from altered samples was evaluated. The results obtained from this study showed that reactivity of pyrite-rich samples was mainly affected by the presence of sulfide mineral impurities (like sphalerite ZnS and galena PbS) and by the formation of Fe oxihydroxy coatings, which passivate the pyrite surface (Cruz et al., 2000). Despite these findings, the application of this methodology in the prediction of ARD generation from mining wastes has still to be demonstrated. The aim of the present work was to use this methodology in the study of the reactivity evolution of sulfide mine wastes (mill tailings) subjected to oxidising conditions, in order to assess the procedure for prediction of ARD generation. The electrochemical and mineralogical data could further represent valuable data with application in the validity of ARD predictive models.
2. Methodology 2.1. Mineral samples Samples were provided as mill tailings by 3 mill plants in Mexico: sample MT1 came from the processing of a vulcanogenic massive sulfide type ore; sample MT2 came from a Pb-Zn skarn type ore; and sample MT3 came from a Au–Ag epithermal type ore. The mineralogical composition of each sample is shown in Table 1. The MT1 sample had a high percentage of pyrite (66.0%) and a significant concentration of sphalerite (2.1%), while its carbonate content was relatively low (0.5%). Pyrite and pyrrhotite represented 39.0% of the mineral content in the MT2 sample with 1.6% sphalerite, and a carbonate and silicate content of 56.6%. The MT3 sample contained the greatest quantity of carbonates (9.6%) and silicates (86.9%) and the lowest sulfide concentration (<3.5%). Mill tailings samples were received as pulp, filtered, dried at 35 C, and homogenized before analysis.
Particle size distribution of each mill sample was analyzed using a Shimadzu SALD-1100 size analyzer. The size distribution for MT1 sample indicated that 80% (P80) of the particles was less than 20.14 mm. The P80 for the for MT2 and MT3 samples were 21.5 and 18.75 mm, respectively. Prepared samples were kept in N2 in order to avoid subsequent alteration. 2.2. Acid-base accounting The acid-base accounting (ABA) is the method most routinely used to perform static tests for ARD prediction. The method consists of determining the balance between acid producing mineral components (acid potential, AP) and the acid-consuming (neutralizing) minerals (neutralization potential, NP) in a given sample. To determine NP, the modified method of Sobek (Morin and Hutt, 1997) was used. Calculation of the neutralization potential was performed based on the acid utilized in digestion of the neutralizing species according to the following formula: NP (Kg CaCO3/t)=[((Nvol. HCl)– (Nvol. NaOH))50] / [sample weight]. In this formula, N is the normality of the HCl and NaOH solutions used in the digestion and titration and vol. is the volume of each solution used. The acid potential (AP) of the sample was calculated from the measured concentrations of total sulfur-sulfide in the sample. Finally, the ratio NP/AP, known as the neutralization potential ratio (NPR), was used as the criterion to evaluate the capacity of the material to generate acidic drainage (Price et al., 1997). From this criterion, the samples classified as likely ARD generators were submitted to kinetic studies as described below. 2.3. Kinetic test (alteration of sulfide samples) Evolution of mining wastes reactivity were evaluated using electrochemical and mineralogical studies for unaltered samples, and using samples submitted to alteration in kinetic tests. The apparatus and leaching procedure used were designed to simulate natural oxidation of primary mineral samples, similar to the conditions produced for kinetic assays in humidity cells [American Society for Testing and Materials (ASTM), 1996; Morin and Hutt, 1997). The leaching apparatus consisted of a 5-cm diameter Buchner funnel containing 20 g of mineral sample
Table 1 Mineralogical composition of mill tailings (% weight) Sample
Pyrite
Pyrrhotite
Sphalerite
Galena
Arsenopyrite
Carbonate
Silicate
MT1 MT2 MT3
66.0 18.0 3.8
0.0 21.0 0.0
2.1 1.6 0.01
0.4 0.3 0.01
0.6 2.3 0.02
0.5 18.5 9.6
30.2 38.1 86.9
R. Cruz et al. / Applied Geochemistry 16 (2001) 1631–1640
supported on 0.45 mm pore size filter paper. Samples were submitted to cyclic leaching periods. The procedure consisted of 1 day of leaching, followed by 3 days of exposure to atmospheric conditions, then 1 day of leaching again, and 2 days under atmospheric conditions. Subsequently, leaching cycles were carried out twice a week. Data obtained were analyzed as a function of the leaching time, which was defined as the time between the initial cycle of leaching and removal of the sample for analysis. The initial leach solution was prepared before use and formulated to simulate rainwater: distilled water with a pH adjusted to 5.5 by the addition of purified CO2 (g). The leaching of samples was carried out by adding 15 ml of leach solution to each sample container for each cycle. The samples were left inundated with the leach solution for 3 h during each leach cycle. Leach solution was extracted from each device by vacuum suction, refiltered (0.45 mm) and analyzed. Experiments were run in triplicate for each sample. Mineral samples were taken after different leaching times. All leached samples were dried and preserved under an inert atmosphere (N2) for electrochemical analysis and SEM observations. 2.4. Chemical and mineralogical characterization Chemical and mineralogical characterization of each sample was performed before and after leaching. In this characterization, the composition, occurrence and form of minerals making up each sample were determined. The surface state of the mineral particles after different leaching times was then evaluated. Mineralogical observations were carried out using a Versamet Union optical microscope and a Philips XL30 scanning electron microscope (SEM). Microanalyses were performed using an energy dispersion X-ray fluorescence spectrometer (EDAX 4 Dix) coupled to an SEM. The content of heavy metals was determined for the leachate solutions using a Perkin Elmer 5000 Atomic Absorption spectrometer. The pH of the leachate solution samples was determined using a Beckman f320 pH meter. For solid samples, total S was analyzed using a Leco 244 SC. Sulfur in solution was determined with plasma spectroscopy using a Jarell Ash Iris 25 spectrometer. Solid sulfates were analyzed using the standard gravimetric method. The content of sulfide-sulfur in solid samples was derived by subtracting S in the form of SO4 from total S. 2.5. Electrochemical experimentation system The reactivity of mill tailings was measured for both unleached samples and after selected times of leaching. Electrochemical analyses were carried out by cyclic voltammetry. This electrochemical technique consisted of imposing a sweep potential to the sample contained in the working electrode in order to reach the interfacial energetic conditions that promote the electrochemical
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reactions. The transferred charges (electrons) associated with the electrochemical reactions are registered as a function of an imposed potential (i.e. current vs. potential, voltammograms). These voltammetric studies do not simulate the ARD process, they are only used to characterize the surface state evolution and oxidation capacity of mineral samples through alteration processes. An EG&G PAR 273 potentiostat coupled to a PC with the M270 software was used to impose a sweep rate of 20 mVs1 and record the voltammetric response. A typical 3-electrode cell was used at room temperature and N2 atmosphere. A C paste electrode (CPE — sample, 50% wt) was used as the working electrode. The feasibility and preparation details of the CPE have been previously reported by La´zaro et al. (1995). A graphite rod (Æesar, 99.999%) was used as counter electrode. The reference electrode consisted of a Hg/Hg2SO4/K2SO4(sat) electrode, where (sat) indicates saturated K2SO4 solution. This electrode is identified as SSE and has a potential of 0.615 V vs. a standard H electrode. All potentials in this investigation are referred to this electrode. The electrode system was placed in a PyrexTM glass cell containing an electrolyte solution of 0.1 M NaNO3 at a pH of 6.5 normally used as a characterization electrolyte. Even though this solution was not pH buffered, changes in the pH were not significant due to the microelectrolysis conditions of the voltammetric assays. Purified N2 gas was bubbled through the electrolyte solution for 45 min before the start of the experiments and a N2 atmosphere was maintained within the cell during the tests.
3. Results 3.1. Acid base accounting (ABA) The results of the ABA are presented in Table 2. This table also shows the screening criteria for the identification of potentially acid generating materials (Price et al., 1997). On the basis of these results, with a NPR of 0.005, MT1 sample could be classified as having the greatest ARD generating potential. The MT2 sample had a NPR of 0.36, which also corresponds to a high ARD generating potential, although to a lesser extent than the MT1 sample. The MT3 sample proved to represent no risk of ARD generation with a NPR of 7.26. The experience indicates that for materials with a NPR higher than 4.0, it is not necessary to carry out further kinetic tests (Price et al., 1997). On the basis of these results, kinetic tests in mini-cells were only performed for the MT1 and MT2 samples. 3.2. Chemical evolution of the leachate Fig. 1 shows the pH evolution of leachates for the MT1 and MT2 samples submitted to alteration conditions. Fig. 1 shows that the pH evolution of both leachates is
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Table 2 Potential for acid rock drainage generation based on the acidbase accounting analysis Sample
NPa
APb
NPRc
ARD potentiald
MT1 MT2 MT3
4.1 197.3 78.5
756.2 540.3 10.8
0.005 0.36 7.26
Likely Likely None
a
NP, neutralization potential in kg of CaCO3/ton of mineral. b AP, acid potential in kg of CaCO3/ton of mineral. c NPR, neutralization potential ratio, NP/AP. d Screening criteria for identification of potentially ARD generating (ASTM, 1996; Morin and Hutt, 1997): NPR < 1:1, likely; 1:1 to 2:1, possibly; 2:1 to 4:1: low; > 4:1, none.
Fig. 1. Evolution of pH of the leachate solution as a function of leaching time for mill tailing samples: MT1 and MT2.
quite different. While the pH of the leachate solution for the MT2 sample rises from 6.5 to almost 7.0, for the MT1 sample the pH decreases immediately to values close to 3.0 after the first week of leaching. Figs. 2 and 3 show the chemical evolution for dissolved Fe, Zn, and Pb ions in leachates from the MT1 and MT2 samples under alteration. In the MT1 sample, high Zn dissolution occurred during the first week of leaching (Fig. 2). The Zn concentration in solution gradually diminished until reaching a stable level in the sixth week of leaching. A high level of Fe dissolution was also observed for the first week of leaching followed by a decrease in the concentration of dissolved Fe. A barely perceptible increase in Fe dissolution occurred during the last 8 weeks of leaching. Lead in the leachate from the MT1 sample was highest in concentration (2.5 mg/l) for the first two weeks, with lesser dissolution (<0.3 mg/l) occurring steadily for the rest of the alteration process. For the MT2 sample, Zn dissolution followed the same pattern as that observed for MT1, however the Zn concentration in the leachate obtained from the MT2
Fig. 2. Chemical evolution of Fe, Zn and Pb ions in the leachate solution of mill tailing MT1 as a function of leaching time. Inset shows a low concentration zone for the Pb in solution.
Fig. 3. Chemical evolution of Fe, Zn and Pb ions in the leachate solution of mill tailings sample MT2 as a function of leaching time.
sample was as much as 500 times lower than that from MT1 (Fig. 3). Iron dissolution was low for the MT2 sample during the first week of leaching, increasing during the second and fourth weeks, and again diminishing to low concentrations from the fifth week of alteration. Lead dissolution was low throughout the 10 weeks of leaching. 3.3. Electrochemistry of mining wastes Fig. 4 shows the voltammetric behavior of all the fresh samples without leaching. The MT1 and MT2 samples (curves a and b, respectively) exhibit a voltammetric behavior similar to that reported for pyrite in previous studies (Cruz et al., 2000). A high oxidation process (O1) could be associated with pyrite oxidation. This process is referred to as a transpassive oxidation and it is characterized by an aggressive electrochemical oxidation of pyrite (Zhu and Wadsworth, 1993). The
R. Cruz et al. / Applied Geochemistry 16 (2001) 1631–1640
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Fig. 4. Typical voltammograms obtained for carbon paste electrodes containing mill tailings samples from each mining site: (a) MT1; (b) MT2; (c) MT3. The voltammetric response of carbon paste electrode (CPE) without mineral in 0.1 N NaNO3 is also shown (d). The scan potential was initiated in the positive direction at 20 mV s1. Inset shows low current area of curves for MT3 and CPE. Arrows indicate the scan potential direction.
reaction products in this electrochemical oxidation are Fe(III) oxides, SO2 4 ion and partially oxidized S intermediates. The proposed reactions are as follow (Zhu and Wadsworth, 1993): FeS2 þ 2H2 O ! FeOOH þ 2S0 þ 3Hþ þ 3e FeS2 þ 10H2 O ! FeOOH þ
2SO¼ 4
þ
ð1Þ
þ 19H þ 15e
ð2Þ 2
xFeS2 þ xH2 O () xFeOH
þ
2S2 x
þ
þ xH
þ ð3x 4Þe
ð3Þ
xFeS2 þ ðx þ 12ÞH2 O () xFeOH2 þ 2Sx O2 6 þ ðx þ 24ÞHþ þ ð3x þ 20Þe
ð4Þ
The transpassive voltammetry oxidation of pyrite is characterized by the following behavior: current in the oxidation branch obtained in the reverse scan potential is higher than that obtained in forward scan. However, oxidation process O1 shows an inverse behavior in the oxidation branch (Fig. 4, curve a). This could indicate the presence of another oxidation process (see below). When the scan is reversed, two barely perceptible reduction peaks (R1 and R2) appear in these voltammograms. These reduction peaks correspond to the reduction of oxidation products. The presence of an inflection (O2) is significant in the oxidation process of
the MT2 sample at potentials 0.4 V. In previous work a similar shoulder has been observed at 0.33 V, this oxidation process was attributed to the presence of galena in a pyrite sample (Cruz et al., 2000). However, electrochemical studies of pyrrhotite and pyrite–pyrrhotite mixtures have shown that the oxidation process of pyrrhotite is carried out at a very similar potential to the O2 process (Cruz et al., 1999). Moreover, it is important to notice that pyrrhotite concentration is higher than galena in the MT2 sample (Table 1). For this reason, in the present work, the O2 oxidation process is related to electrochemical oxidation of pyrrhotite present in this sample. Figure 4, curve c represents the voltammetric behavior of the MT3 sample. It is important to observe that for current scales in mA, this mineral could be considered as having no voltammetric response. However, for low currents (Fig. 4, inset), a voltammetric response is observed for this sample compared to the response for the CPE without mineral (Fig. 4, curve d). In order to compare the voltammetric responses, parameters based on the oxidation process characteristics were established. These parameters were: the required potential to reach 5 mA (EI=5mA) for the forward scan in the oxidation process, the oxidation rate (I/E rate) of the mineral sample, and the charge (Q) associated to the oxidation process. The I/E rate was calculated from the slope of the current-potential response during the forward scan at 0.55–0.65 V potential range. The Q was evaluated from the surface area below the oxidation process. These parameters are reported in Table 3.
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Table 3 Electrochemical parameters of the oxidation process for mining wastes before leaching and after 2, 6 and 10 weeks of leaching EI=5mA (V/SSE)a
I/E (mA/mV)b
Q (mC)c
Sample
Fresh
2 weeks
6 weeks
10 weeks
Fresh
2 weeks
6 weeks
10 weeks
Fresh
2 weeks
6 weeks
10 weeks
MT1 MT2 MT3
0.20 0.30 0.66
0.29 0.39 NAd
0.31 0.39 NA
0.23 0.36 NA
1.71 2.3 0.04
3.71 1.4 NA
2.8 1.2 NA
4.0 1.4 NA
23.8 11.8 0.08
10.2 2.5 NA
7.4 2.2 NA
12.7 2.9 NA
a b c d
EI=5mA, potential required to reach 5 mA during the direct sweep in the oxidation process. I/E, current released as the potential increased. Q, charge associated with the oxidation process. NA, not analyzed.
To reach an oxidation current of 5 mA, the MT1 sample required a potential of 0.2 V, while for the MT2 sample a higher potential ( 0.31 V) was required. Finally, the MT3 sample requires the highest potential ( 0.6 V). This trend is associated with the oxidation capacity of each sample, thus, the higher the required potential for the oxidation process, the lower the oxidation rate (I/E) and Q (Table 3).
4. Discussion
3.4. Evolution of electrochemical reactivity of the mining wastes
The low pH of the leachate solution from MT1 is due to the high pyrite content and the low amount of neutralizing species in this sample. This pH condition provokes partial dissolution of oxihydroxy species formed during the leaching process in agreement with:
Evolution of electrochemical reactivity as a function of leaching time for the MT1 and MT2 samples is presented in Figs. 5 and 6, respectively. Since we are concerned only with the oxidative character of reactivity, these figures show the region corresponding to the oxidation process for the different voltammetric responses. Fig. 5 illustrates evolution of reactivity for the MT1 sample after 2, 6 and 10 weeks of leaching. The response after 2 weeks of leaching shows a significant decrease in charge associated with the oxidation process (Q) and an increase in potential where the oxidation process begins (Fig. 5, curve b, and Table 3). After 6 weeks of leaching, a decrease in reactivity of the residues is observed (Fig. 5, curve c). This is demonstrated by a significant decrease in Q as well as in the oxidation rate (I/E). In addition, a greater overpotential (EI=5mA) is required at this time. After 10 weeks of leaching, a gain in reactivity is observed compared to the 2 and 6 week periods as indicated by the increase in oxidation rate and Q (Fig. 5, curve d, Table 3). On the other hand, the transpassive behavior (higher current for the reverse scan) is recovered as the leaching time increases (Fig. 5). The MT2 sample also showed a significant decrease in the oxidation process after 2 weeks of leaching (Fig. 6, curve b). This reactivity decrease is continued after 6 weeks of leaching (curve c). After 10 weeks of leaching, the MT2 sample showed an increase in oxidative capacity (curve d) although changes in the electrochemical parameters (EI, I/E and Q) were practically negligible (Table 3).
The mechanism associated with acid generation from Fe sulfide is represented by: þ FeS2 þ 5=2H2 O þ 15=4O2 ! FeOOH þ 2SO¼ 4 þ 4H
ð5Þ
FeOOH þ 3Hþ () Fe3þ þ 2H2 O
ð6Þ
The high Fe concentration in the leachate solution from MT1 supports this hypothesis. On the other hand, the high level of neutralizing species in the MT2 sample provokes a high pH in this sample leachate solution. Thus, the Fe oxihydroxy species are stable on the surface and the dissolved Fe is considerably lower than in the MT1 leachate solution. 4.1. The MT1 sample Static tests for the MT1 sample indicate that this sample possesses a high potential for generating ARD (Table 2). This is principally due to the low concentration of neutralizing species and high sulfide concentration (Table 1). The fact that the MT1 sample has a greater potential for ARD generation than the other samples should implicate greater reactivity for this sample, as corroborated by the electrochemical study (Fig. 4). However, the electrochemical behavior of the unleachated sample could be strongly influenced by the presence of surface oxidized species, which could have been formed as an intermediate product during the mill process (Fig. 4, curve a). The presence of such surface species was demonstrated by scanning electron microscopy (SEM)
R. Cruz et al. / Applied Geochemistry 16 (2001) 1631–1640
Fig. 5. Evolution of electrochemical reactivity of the MT1 sample: (a) before leaching; (b) after 2 weeks of leaching; (c) 6 weeks of leaching; (d) 10 weeks of leaching. The voltammograms were obtained from CPE-mill tailing sample in 0.1 M NaNO3. The scan potential was initiated in the positive direction at 20 mV s1.
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Fig. 6. Evolution of electrochemical reactivity for the MT2 sample: (a) before leaching; (b) after 2 weeks of leaching; (c) 6 weeks of leaching; (d) 10 weeks of leaching. The scan potential was initiated in the positive direction at 20 mV s1.
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Fig. 7. Scanning electron microscopy images of mineral particles in the mill tailings sample MT1: (a) oxidized Fe species on the unleached pyrite surface; (b) favored corrosion of sphalerite on sample after leaching.
observations (Fig. 7). Thus, the O1 process could be associated with a simultaneous oxidation of surface species and pyrite. For this reason, the typical transpassive voltammetric behavior was not observed (Fig. 5, curve a). Dissolution of these Fe species during the first week of leaching is shown by the rapid increase in the leachate Fe concentration (Fig. 2). Thus, the changes observed in the voltammetric behavior after 2 weeks appear to be associated with a decrease in the concentration of electroactive species present on the mineral surface. This provokes the recovering of a transpassive voltammetric response (Fig. 5, curve b). The decrease in reactivity of the sample after 6 weeks leaching (Fig. 5, curve c, Table 3) could be related to the formation of an Fe oxihydroxy coating at the mineral surface during the leaching process. These Fe oxihydroxy coatings could correspond to goethite, in agreement with reaction (5), . or ferrihydrite, (5Fe3+ 2 O3 9H2O) and lepidocrocite (Fe3+O(OH)), which have been reported to be present in weathered sulfide tailings (Hammarstrom and Smith, 2000). Moreover, amorphous Fe oxihydroxy compounds could also be formed in agreement with the follow reaction (Hammarstrom and Smith, 2000): FeS2 þ 15=4 O2 þ 7=2 H2 O () FeðOHÞ3 þ2H2 SO4 ð7Þ Some authors have observed that when two sulfide minerals with different rest potentials (corrosion potential) come into contact, the mineral with the higher rest potential is galvanically protected, favoring oxidation of the mineral with lower rest potential (Metha and Murr, 1983, Nowak et al., 1984). As the sphalerite has a lower rest potential than pyrite (Chander, 1988; Metha and Murr, 1983,), this sulfide offers galvanic protection to pyrite in the MT1 sample, such as is demonstrated by favorable Zn dissolution from sphalerite and low Fe dissolution (Fig. 2). It is important to recall that Fe in solution in the first stages
of leaching, comes from the dissolution of surface species developed during the mill process. Favorable corrosion of sphalerite was also observed using SEM [Fig. 7(b)]. When most of the sphalerite had dissolved after 7 weeks of leaching, sample reactivity increased, as observed in the voltammogram corresponding to 10 weeks of leaching (Table 3). It is important to notice even when the chemistry of the leachate does not suggest changes in the oxidation of pyrite present in the sample after 4 weeks leaching, the electrochemical study provides evidence of important changes in sample reactivity. 4.2. The MT2 sample The ABA analysis for the MT2 sample revealed that this sample possessed a high potential for ARD generation despite its elevated carbonate concentration (Table 1). On the other hand, the chemical quality of the leachate indicates that the MT2 sample did not generate acidity during the period of alteration (Fig. 1). In addition, the sulfide dissolution was very slow for this sample, as indicated by the low metal concentration in the leachate (Fig. 3). These results could indicate a low potential for ARD generation, in contrast to the results of the ABA analysis. The voltammetric study for the MT2 sample showed a strong decrease in mineral reactivity after 2 weeks of leaching (Fig. 6 and Table 3). This is most certainly due to the formation of Fe oxihydroxy precipitates formed on the surface of the mineral as a result of dissolved Fe present and the system conditions of neutral pH. Besides the Fe precipitates in this system, CaSO4 was also formed, contributing to the decreased reactivity of the MT2 sample. Formation of Fe and Ca precipitates was also demonstrated by SEM observations (Fig. 8). The passivation state of the MT2 sample, induced by the formation of precipitates on the sulfide surface, was maintained during the 10 weeks of alteration as indicated by the leachate chemistry, the voltammetric study and the SEM observations (Figs. 3, 6 and 8 and Table 3).
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Fig. 8. Scanning electron microscopy images of mineral particles in mill tailings sample MT2 after different leaching time: (a) 2 weeks; (b) 10 weeks. The oxidized species formed during the leaching process are indicated.
Although the results of the kinetic study of the MT2 sample did not demonstrate ARD generation potential, according to the static tests and electrochemical reactivity, this sample had a high potential for ARD generation. This discrepancy between the results obtained from each technique is mainly due to the neutralizing species (carbonate) in the MT2 sample had not been consumed at the end of the leaching period of this study. Certainly, oxidation of sulfides occurred during the first 2 weeks of leaching, however, this fact is not completely reflected in the chemistry of the leachate. The products of dissolution of sulfide minerals must be immediately neutralized by carbonate, resulting in the precipitation of gypsum and ferric oxihydroxy, such as is suggested by the SEM images. For this reason, it is suggested that the alteration kinetics should be observed for a greater period of time to establish the actual evolution of reactivity of the sulfides present in the MT2 sample, in anticipation of the neutralization or passivation of carbonates and therefore the reactivation of sulfides, mainly pyrite and pyrrhotite. 4.3. The MT3 sample The high concentration of neutralizing, non-electroactive species and the low concentration of sulfides in the MT3 sample, indicate that the ARD generation potential is essentially null for this sample, as predicted by the results of the static tests (Table 2). Experience with this kind of sample indicates that it does not represent an acid generation risk. The voltammetric response of this sample evidences the low sulfide concentration contributing to the lowest electrochemical reactivity for this mill waste. For some voltammetric assays of this sample, a slight response of the sulfides occurred, however, only the reproducible behavior of this mill waste is shown (Fig. 4, curve c). Although an electrochemical voltammetric study could be performed using samples recovered from kinetic tests, the effect of sulfide passivation, similar to
that observed for the MT2 sample, must be more significant in this sample due to the higher concentration of carbonate and the very low concentration of sulfide minerals. The passivation of sulfides combined with their low concentration in this sample must decrease the electrochemical reactivity of sulfides and consequently the reproducibility of the electrochemical results.
5. Conclusions Cyclic voltammetry offers a good alternative method for monitoring the evolution of reactivity of sulfide mining wastes. In this work, by using electrochemical methods the authors showed evidence of the reactivity potential of mill wastes subjected to alteration conditions, even if no reactivity was shown by conventional kinetic tests. The main advantage of incorporating cyclic voltammetry into the conventional methods of ARD prediction is its capacity to describe the factors that influence sulfide reactivity which are not evaluated by the traditionally utilized prediction techniques. Hence, reactivity evolution of mill tailings and the effect of the galvanic protection offered by associated impurities, as well as the passivation by Fe oxihydroxy coatings could be established. The use of the methodology presented here could provide interesting information in acid rock drainage prediction from sulfide mining wastes. Acknowledgements Financial support for this work comes from CONACyT (grant 485100-5-25715B). R.C. and B.A.M. also thank CONACyT for scholarships. References Ahlberg, E., Forssberg, R.S.E., Wang, X., 1990. The surface oxidation of pyrite in alkaline solution. J. Appl. Electrochem. 20, 1033–1039.
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