Published in J. Environ. Qual. 32:2410-2413 (2003).
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TECHNICAL REPORTS
Waste Management
Use of Alkaline Extraction to Quantify Sulfate Concentration in Oxidized Mine Tailings
Guohong Yin and
Lionel J. J. Catalan*
Dep. of Chemical Engineering, Lakehead Univ., 955 Oliver Road, Thunder Bay, ON, Canada P7B 5E1
* Corresponding author (Lionel.Catalan{at}lakeheadu.ca).
Received for publication November 21, 2002.
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ABSTRACT
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An alkaline extraction method has been developed for the determination of total sulfate in mine tailings containing secondary sulfate minerals formed by the oxidation of primary sulfides. Oxidized tailings were extracted with a 0.3 M NaOH solution at a liquid/solid ratio of 30 at room temperature for 16 h. The sulfate concentration in the extracts was determined by ion chromatography (IC). The coefficient of variation for sulfate determinations ranged from 1.9 to 3.2% for five tailings samples collected at two tailings impoundments. Mineralogical analysis of the tailings by scanning electron microscopy/X-ray energy dispersive spectrometry (SEM/EDS) demonstrated that the extraction of sulfate was complete, with the exception of extremely insoluble barite. The proposed method is simple, yields an accurate yet rapid measurement of sulfate, and involves a safer laboratory operation than conventional methods that make use of strong HCl acid solutions. Moreover, this method allows the specific measurement of sulfate in the extract, whereas conventional methods are generally limited to the measurement of total S by inductively coupled plasma atomic emission spectrometry (ICP–AES) due to the interference of chloride with sulfate in IC.
Abbreviations: EDS, X-ray energy dispersive spectrometry • IC, ion chromatography • ICP–AES, inductively coupled plasma–atomic emission spectrometry • SEM, scanning electron microscope
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INTRODUCTION
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A LARGE VARIETY of secondary sulfate minerals are commonly found near the surface of sulfidic mine tailings impoundments that have been exposed to the atmosphere for several years (Alpers et al., 1994; Bigham, 1994; Bigham and Nordstrom, 2000; Boulet and Larocque, 1998; Jambor et al., 2000; McGregor et al., 1998; VanHuyssteen, 1998). Secondary sulfate minerals form by the precipitation of constituents in acid sulfate drainage derived primarily from the oxidation of pyrite (FeS2) and pyrrhotite (Fe1-xS) (Jambor, 1994). These minerals vary widely in solubility and include highly soluble iron sulfate salts such as melanterite (FeSO4·7H2O), less soluble iron oxyhydroxysulfates such as jarosite [MFe3(SO4)2(OH)6 where M represents K, Na, or H3O], and relatively insoluble sulfates such as anglesite (PbSO4) (Alpers et al., 1994). Metal ions such as Cu, Mn, Ni, Pb, and Zn often form solid solutions in iron sulfate minerals or may occasionally form non-Fe-bearing sulfates. Sulfate adsorption on iron oxyhydroxide surfaces accounts for a significant fraction of the sulfate present in iron oxyhydroxysulfate phases resulting from acid mine drainage (Rose and Ghazi, 1997; Webster et al., 1998).
Secondary sulfate minerals can store metals and acidity during dry periods and later release them by dissolution during wetting events with undesirable impacts on biota (Alpers et al., 1994). Their presence also affects the selection and implementation of site rehabilitation methods. For example, directly flooding oxidized tailings to minimize further sulfide oxidation has the undesired effect of dissolving soluble sulfate minerals, thus releasing metals and acidity to the water (Catalan et al., 2000). Hence, alkaline amendments such as hydrated lime or calcite are often added to the tailings as part of reclamation activities (Amyot and Vézina, 1997; Catalan et al., 2001; Davis et al., 1999) to neutralize the stored acidity, increase pH, and immobilize soluble metals. Although sulfate itself does not contribute to the stored acidity, the required amounts of lime or calcite amendment are related to the concentrations in both soluble metal sulfate minerals and iron oxyhydroxysulfates, since the latter can account for the majority of the alkalinity consumption (Catalan et al., 2002; Catalan and Yin, 2003). Hence, measurements of total sulfate in oxidized tailings are useful for predicting environmental impacts and for planning reclamation.
Sulfate in mine tailings is traditionally measured by acid extraction with HCl solutions (4–12 M) (e.g., CANMET, 1991; Rose and Ghazi, 1997; Sobek et al., 1978). Extreme caution is required because of the strong acid solutions. Furthermore, if the acid extract is analyzed by IC, the occurrence of very high chloride concentrations may interfere with the detection of sulfate. It is therefore not recommended to use IC for anion analysis, unless the extract solution is greatly diluted to ensure complete separation of the sulfate and chloride peaks. However, this dilution inevitably introduces significant experimental error, and for this reason, HCl extracts are mostly analyzed by ICP–AES. Because HCl may also leach highly reactive sulfides (CANMET, 1991), sulfate concentrations based on measurements of total S in the extracts by ICP–AES may be overestimated.
Previous studies of sulfate extraction by alkaline solutions are scarce. Blowes and Jambor (1990) measured total sulfate in mine tailings by liquid chromatography using samples digested in a sodium carbonate solution. Rose and Ghazi (1997) found that solutions of NaOH, HCl–HNO3–H2O2, and sodium oxalate provided comparable sulfate extractions from amorphous iron oxyhydroxide precipitates formed in acid drainage from coal mining. By comparison, a solution of hydroxylamine in acetic acid resulted in lower sulfate extractions.
The proposed sulfate extraction method is based on the empirical observation (Catalan et al., 2002; Catalan and Yin, 2003) that iron oxyhydroxysulfate minerals react with hydroxide ions at alkaline pH by incongruent dissolution, as shown below for jarosite:
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This reaction suggests that the sulfate in jarosite can be completely released to solution if sufficient hydroxide is added. Geochemical equilibrium calculations with the software MINTEQ (Allison et al., 1991) show that the above reaction is thermodynamically favored and complete at alkaline conditions. Similar reactions can be written for other oxyhydroxysulfate minerals. Furthermore, desorption of sulfate adsorbed on oxyhydroxide surfaces has been reported to occur when aging oxyhydroxysulfates at alkaline pH (Webster et al., 1998). The present study was performed to verify that an excess alkali addition to the extracting solution would result in the complete conversion of oxyhydroxysulfates to oxyhydroxides and in the release of sulfate and dissociated metal ions (M+). Furthermore, soluble iron sulfate salts are completely dissociated in the extracting solution and therefore contribute to the dissolved sulfate. The alkaline extract is well suited for IC analysis, thus allowing a direct measurement of the sulfate concentration. The complete removal of sulfate in the extracted tailings was assessed by quantitative electron scanning microscopy. To our knowledge, no detailed description and quantitative evaluation of an alkaline extraction method for total sulfate in mine tailings have been previously reported.
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MATERIALS AND METHODS
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Materials
Five different samples of oxidized tailings, denoted as KK-1, KK-2, KK-3, KK-4, and WL were used for this study. The KK samples were collected in April 2001 at the Kam Kotia Mine site (Timmins, ON) at three different locations in an unimpounded tailings area where tailings had been deposited between 1961 and 1972. Sampling was performed in the top 0.3 m of oxidized material, which was most often unsaturated except for a few weeks during snowmelt in the spring. The WL sample was a composite of 12 individual tailings samples collected at the Winston Lake Mine site (Schreiber, ON). This sample was representative of the oxidized layer, which ranged in thickness from 0.03 to 0.5 m depending on location. The WL tailings were deposited between 1988 and 1999 and had therefore oxidized for a shorter time than KK tailings. Both sites were mined for zinc and copper sulfide ores. The samples were transported to the laboratory in doubled plastic bags, homogenized, and oven dried at 50°C. Dried samples were stored in sealed plastic jars at room temperature (18–29°C). The elemental compositions of the tailings were measured by ICP–AES following microwave-assisted acid digestion using EPA Method 3051 (USEPA-Office of Solid Waste, 2002) (Table 1).
Alkaline Extractions
Known masses of dried tailings between 1.000 and 1.500 g (±0.001 g) were extracted with 0.3 M NaOH solution at a liquid/solid (L/S) ratio of 30 at room temperature in 50-mL centrifuge tubes. To prevent the oxidative dissolution of primary sulfide minerals by dissolved O2, whose rate is enhanced at alkaline pH (Bonnissel-Gissinger et al., 1998), the tubes containing the tailings slurries were purged with N2 for 2 min, sealed, and then placed on an end-over-end rotator at 45 rotations per minute (rpm) for 16 h. Extractions were performed at least in quadruplicate for each tailings sample. The pH of the alkaline supernatants (extracts) was measured with a Metrohm Solitrode 6.0220.100 combined glass electrode connected to a Metrohm 713 pH meter (Metrohm, Herisau, Switzerland). The extracts were filtered through a 0.2-µm nylon membrane, diluted 100 fold with distilled dionized water, and analyzed for sulfate by IC. The IC analyses were done on a Dionex DX 120 ion chromatograph equipped with an AS 40 autosampler (Dionex, Sunnyvale, CA). The IC conditions were: 50-µL sample loop, 3.5 M Na2CO3/1.0 M NaHCO3 eluent, 1.0 mL min-1 eluent flow rate, AS14 analytical column, AG14 guard column, ASRS-II suppressor, and conductivity detector (Dionex, Sunnyvale, CA). The background conductivity was 15 to 18 µS.
Scanning Electron Microscopy/Energy Dispersive Spectrometry
Samples of tailings before and after alkaline extraction were impregnated with epoxy resin, polished using kerosene to obtain smooth grain sections without dissolving water-soluble minerals, and coated with carbon. The polished sections were then examined with a JEOL JSM 5900 scanning electron microscope (SEM) (JEOL, Akishima, Japan) to assess compositional and mineralogical changes caused by alkaline extractions. The elemental compositions of mineral phases were determined by X-ray energy dispersive spectrometry (EDS) with an Oxford Link ISIS system (Oxford Instruments, Witney, UK). A series of standards were selected for calibration of elemental compositions. Pyrite was used for Fe and S, wollastonite for Ca, chalcopyrite for Cu, galena for Pb, sphalerite for Zn, barite for O and Ba, periclase for Mg, and orthoclase for Al, K, Na, and Si.
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RESULTS AND DISCUSSION
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Measurements of sulfate in the tailings samples were quite reproducible with coefficients of variation ranging from 1.9 to 3.2% (Table 2). The pH of the alkaline extracts was between 13.1 and 13.5. To check that the sulfate concentration in the extracts was not limited by the solubility of sulfate minerals that may have been present in the extracted solids, the following procedure was performed. The solids were separated from the alkaline extracts by centrifugation at 3000 x g (4500 rpm) for 15 min, and the wet solids retained in the tubes were weighed to determine the liquid carryover from the alkaline extraction. Fresh deionized distilled water was then added in the tubes at a liquid/solid ratio of 30, and the tubes were rotated for 16 h on an end-over-end rotator at 45 rpm. The resulting water extracts were separated and analyzed for sulfate by IC. Three consecutive water extractions were performed on each solid sample. We found that the amount of sulfate released during the consecutive water extractions was simply equal to the amount of sulfate present in the liquid carried over from the alkaline extraction. This indicates that the sulfate concentration in the alkaline extract was not limited by solubility. Thus, consecutive water extractions are not necessary, since they do not result in additional sulfate recovery from the solids.
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Table 2. Sulfate concentrations measured by ion chromatography after alkaline extraction of oxidized tailings samples.
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Backscattered electron photomicrographs of tailings grains in Sample KK-1 before alkaline extraction showed that primary sulfides were nearly absent due to extensive weathering (Fig. 1)
. Quantitative EDS analysis of the sulfate-bearing phases identified jarosite with a molar composition K0.027Fe0.149S0.1O0.715. This composition is intermediate between K-jarosite (K0.05Fe0.15S0.10O0.70) and H-jarosite (Fe0.15S0.10O0.75) (Note that H cannot be quantified by EDS). Another common iron oxyhydroxysulfate phase, noted as Sc in Fig. 1, was characterized by Fe/S mole ratios ranging from 6.5 to 12. This is slightly greater than the range of Fe/S mole ratios of 5 to 8 reported for schwertmannite (Bigham et al., 1990; Bigham et al., 1996). Jarosite and Sc phases accounted for most of the sulfate present in KK tailings.
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Fig. 1. Backscattered electron image of the edge of a typical grain in unextracted KK-1 tailings. J is jarosite; Sc is an iron oxyhydroxysulfate phase with Fe/S mole ratios between 6.5 and 12.
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Jarosite was not detected in SEM examinations of KK-1 tailings after alkaline extraction, which indicates that the reaction of this phase with the added hydroxide was complete. Moreover, ICP analyses of the extracts (results not shown) indicated that 98.5% of the K initially contained in the unextracted tailings was released to the extract solutions. This further corroborates that jarosite completely reacted. Phases with the same visual appearance as the Sc phase in the unextracted tailings remained after alkaline extraction (Fig. 2)
, but contained no detectable sulfate and had Fe/O mole ratios compatible with goethite (FeOOH) and ferrihydrite (Fe(OH)3). These phases are noted as F in Fig. 2. Therefore, our observations indicate that iron oxyhydroxysulfates were completely converted to oxyhydroxides by incongruent dissolution reactions and that sulfate initially adsorbed on oxyhydroxides was exchanged for hydroxide. Both mechanisms resulted in the liberation of sulfate to the alkaline extract. The only sulfate detected in extracted KK-1 tailings was associated with a very small number of extremely insoluble barite (BaSO4) particles.
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Fig. 2. Backscattered electron image of the interior of a typical grain in extracted KK-1 tailings. F is an iron oxyhydroxide phase; Q is quartz.
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KK-2 and WL tailings differed from KK-1 tailings in that they contained significant amounts of pyrite and some chalcopyrite (FeCuS2). Sulfide crystals exhibited alteration rims, some of which likely consisted of melanterite (FeSO4) based on molar composition (Fe0.19S0.16O0.64). The Fe/S mole ratios measured for the Sc phases in WL unextracted tailings covered a wider range (3.8 to 17.2) compared with KK tailings. Examinations of extracted KK-2 and WL tailings samples showed that jarosite and Sc phases were replaced by iron oxyhydroxides (Fig. 3)
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Fig. 3. Backscattered electron image of the interior of a typical grain in extracted KK-2 tailings. Ch is chalcopyrite; F is an iron oxyhydroxide phase; Py is Pyrite; Q is quartz.
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Comparison of sulfate extracted by the alkaline method (Table 2) with total S (Table 1) shows that sulfate accounted for 99.6, 15.3, 95.8, 16.9, and 26.9% of total S in Samples KK-1, KK-2, KK-3, KK-4 and WL, respectively. These results are consistent with SEM observations. The total S remaining in extracted tailings was mainly contained in sulfide minerals.
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CONCLUSIONS
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The alkaline extraction method provided reproducible measurements of sulfate concentrations in a set of five oxidized tailings samples collected at two tailings impoundments. Mineralogical analysis of the tailings by SEM/EDS showed that the sulfate present in the unextracted tailings was mostly contained in jarosite and an iron oxyhydroxysulfate phase with Fe/S mole ratios ranging from 3.8 to 17.2. Alkaline extractions resulted in the complete conversion of all oxyhydroxysulfates to oxyhydroxides, with a corresponding release of sulfate to solution. Sulfate in the extracted tailings was only detected in extremely insoluble barite particles. Compared with the conventional acid extraction methods, the alkaline method is considerably safer and leads to more reliable determinations since IC can be used to measure specifically the sulfate concentration in the extracts.
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ACKNOWLEDGMENTS
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The authors thank A. Hammond, J. Joncas, A. Raitsakas, and A. Mackenzie for assisting with the preparation of polished sections, ICP, IC, and SEM/EDS analyses. This research was funded through an operating grant from the Natural Sciences and Engineering Research Council of Canada (NSERC).
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ABSTRACT
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