Environ. Sci. Technol. 2003, 37, 1408-1413
Comparison of Calcite to Quicklime for Amending Partially Oxidized Sulfidic Mine Tailings before Flooding LIONEL J. J. CATALAN* AND GUOHONG YIN Department of Chemical Engineering, Lakehead University, 955 Oliver Road, Thunder Bay, Ontario, Canada P7B 5E1
Flooding partially oxidized mine tailings for the purpose of mitigating further oxidation of sulfide minerals and generation of acid drainage is generally preceded by treatment with alkaline amendments to prevent releasing previously accumulated acidity to the water cover. This work compares the ability of calcite (CaCO3) and quicklime (CaO), two common amendments, to establish and maintain pH conditions and dissolved metal concentrations within environmentally acceptable ranges over long time periods. Although higher initial pH values were obtained with quicklime, the pH of quicklime treated tailings decreased over time. This was attributed to the low buffering capacity of quicklime treated tailings and to the consumption of hydroxide ions by incongruent dissolution of water-insoluble iron oxyhydroxysulfate minerals. In contrast, the pH of tailings treated with calcite increased initially and then remained stable at pH ≈ 6.7. This pH behavior was due to the lower reactivity of iron oxyhydroxysulfates with calcite, the increased buffering capacity provided by bicarbonate ions, and the incomplete dissolution of calcite. Overall, calcite was found preferable to quicklime for maintaining long-term neutral pH conditions in the treated tailings. With the exception of zinc, acceptable dissolved metal concentrations were achieved with calcite treated tailings.
Introduction Water-soluble secondary sulfate salts (e.g., melanterite FeSO4‚ 7H2O) are commonly found in the oxidized tailings layer at the surface of sulfidic mine tailings impoundments exposed to the atmosphere (1, 2). These minerals are produced by the oxidation of primary sulfides, mainly pyrite FeS2 and pyrrhotite Fe1-xS, in the presence of oxygen and water (3, 4). Metal ions (e.g., Cu, Mn, Ni, Pb, and Zn) are often incorporated as solid solutions in iron sulfate salts or may occasionally form non-iron-bearing sulfates (5). Because soluble sulfate salts can store metals and acidity during dry periods and then release them by dissolution during wetting events (5), they constitute long-term sources of acid drainage and thus a significant environmental challenge at many metal mining sites. During remedial activities aimed at mitigating acid and metal releases from oxidized tailings, large quantities of alkaline amendments are often added to the tailings to increase pH and immobilize the metals. The most common * Corresponding author phone: (807)343-8573; fax: (807)343-8928; e-mail:
[email protected]. 1408
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amendments are calcite CaCO3 (limestone), quicklime CaO, and hydrated lime Ca(OH)2. Amendments are generally tilled into the tailings by plowing for depths up to 1 m, although surface application has been used for tailings depths less than 10 cm (6, 7). A field investigation carried out 5 years after treating partially oxidized tailings with mixtures of CaCO3, CaO, and Ca(OH)2 revealed that the pore water in shallow amended tailings was neutral or slightly alkaline (pH ) 6.4-7.2) and contained lower metal concentrations than in the deeper unamended tailings where acidic conditions prevailed (6). The decrease in pore water metal concentrations was attributed to sorption on ferric hydroxides and/or coprecipitation. However, increased arsenic mobility in tailings amended with CaCO3 and Ca(OH)2 has been linked to desorption of arsenic from oxide minerals and layer silicates at pH values ranging from 7.7 to 9.8 during column experiments (8). At mine sites with positive hydrologic balance and suitable topography, further oxidation of the sulfide minerals remaining in partially oxidized tailings can be drastically diminished by establishing a permanent water cover over the amended tailings. Oxygen consumption in flooded tailings is reduced by factors ranging from 70 to 2000 when compared to well drained tailings exposed to the atmosphere (9-11) due to the much lower concentration and diffusion coefficient of oxygen in water compared to the atmosphere. However, directly flooding oxidized tailings without prior amending could release metals and sulfate initially contained in the pore water and in soluble secondary sulfate minerals to the water cover (12). Amending and flooding partially oxizided tailings has been implemented as part of closure activities at a number of mining sites, but post-closure studies are scarce. At the Solbec site near Stratford (Que´bec), calcite dust and granules were incorporated into the exposed tailings and hydrated lime was dumped in the pond water before the entire site was flooded in 1994 (7). On average, 230 tonnes of alkaline material per hectare were applied. Between the summer of 1995 and the fall of 1996, all water quality parameters in the water cover met the applicable effluent regulations, but the oxidized tailings pore water remained slightly acidic (pH ) 6.1) and contained elevated iron and zinc concentrations. Monitoring data for later years have not been published. At the Quirke tailings site in Elliot Lake (Ontario), the exposed tailings surface in Cell 14 was dressed with limestone before flooding in 1992 (13, 14). Between 1993 and 1999, the pH in the water cover ranged from 6.0 to 8.1 (except for one measurement at pH ) 5 in April 1996), and solute concentrations decreased to below effluent discharge limits. The pore water pH in the top 0.15 m of tailings increased from 3.2 before flooding to 7.5 in 1999, with a concurrent decrease in trace metal concentrations. The improvements in water cover and pore water quality over time have been attributed to several factors, including dilution of the water cover by precipitation and snowmelt, flushing of solutes by water infiltration from the cover to the tailings, and removal of metals by precipitation and sorption to mineral phases. Although alkaline amending of mine tailings is commonly practiced, the geochemical reactions between various types of alkaline amendments and secondary sulfate minerals in the tailings have received little attention. In a recent study (15), we found that the neutralization of soluble sulfate salts, including ferrous and ferric sulfates, accounted for less than half of the total alkalinity consumption when quicklime was used as amendment. The majority of the hydroxide alkalinity 10.1021/es020781z CCC: $25.00
2003 American Chemical Society Published on Web 02/20/2003
TABLE 1. Elemental Composition of Composite Tailings Samplea Al
Ca
Cd
Co
Cr
Cu
Fe
K
Mg
Mn
Na
Ni
Pb
S
Ti
Zn
16.1
6.6
0.02
0.22
0.04
1.0
183
4.4
19.2
0.30
1.1
0.07
0.05
96.3
1.2
6.4
a
All concentrations are in mg/g tailings.
provided by quicklime was consumed by incongruent dissolution reactions with less soluble iron oxyhydroxysulfate minerals such as jarosite KFe3(SO4)2(OH)6 to form iron oxyhydroxides and gypsum, as shown in eq 1 below.
KFe3(SO4)2(OH)6(s) + 3OH- f
3FeOOH(s) + K+ + 2SO42- + 3H2O (1)
Hence, the efficiency of quicklime as a neutralizing amendment is low since the majority of it is “wasted” in reactions with iron oxyhydroxysulfates, which, because of their low solubility (5), contribute much less metal and acidity loadings to the drainage and pore water than soluble sulfate salts. The same conclusion holds true for hydrated lime since both types of lime are strong bases and completely dissociate into calcium and hydroxide ions in water. By contrast, calcite may be more efficient than quicklime and hydrated lime for reacting selectively with soluble sulfate salts. This is because the low solubility of CaCO3
CaCO3(s) ) Ca2+ + CO32-
(2)
and the carbonate system equilibria
CO32- + H2O ) HCO3- + OH-
(3)
HCO3- + H2O ) H2CO3 + OH-
(4)
combine to limit the concentration of hydroxide ions in solution at neutral and alkaline conditions. It is hypothesized that the lower hydroxide concentration may reduce the reactivity of iron oxyhydroxysulfates, while still allowing complete neutralization of soluble sulfate salts and the establishment of alkaline or neutral pH conditions required to reduce dissolved metal concentrations to acceptable values. This work examines the validity of this hypothesis by comparing the results of neutralization tests carried out with quicklime, calcite, and combinations of both amendments. Moreover, the long-term effectiveness of the different amendments is also assessed by comparing the buffering capacity of treated tailings.
Materials and Methods Tailings. All the tests were carried out with a composite tailings sample consisting of a mixture of individual samples collected in the Spring of 2000 at 12 different locations within the oxidized tailings area of the Winston Lake Mine site located in Schreiber, Northwestern Ontario. This site was mined for zinc and copper sulfide ores between 1988 and 1999. The oxidized tailings occupied an area of approximately 14 400 m2 and ranged in thickness from 0.03 to 0.5 m depending on location. The closure activities that took place at the Winston Lake site in the summer of 2000 (after our samples were collected) involved raising the level of the water in the tailings pond and relocating tailings from exposed areas into the pond to establish a permanent water cover over all the tailings. Oxidized tailings were amended with hydrated lime applied as a slurry or spread on the oxidized tailings surface at a ratio of 13 kg lime per tonne of oxidized tailings prior to submergence (12).
The elemental composition of the composite tailings sample (Table 1) was determined by inductively coupled plasma atomic emission spectrometry (ICP-AES) following microwave-assisted acid digestion using EPA Method 3051 (16). The sulfate content was obtained by alkaline extraction of the tailings and analysis of the extracts by ion chromatography (IC) (17). The total sulfur (S) and sulfate (SO42-) contents were 96.3 and 18.9 mg S/g of tailings, respectively. The sulfide content was estimated by difference to be 77.4 mg S/g tailings. Water-soluble acidity, metals, and sulfate were quantified by the multiple water extraction procedure described in ref 15. Results are shown in Table S-1, Supporting Information. The cumulative soluble acidity was 14.7 mg of CaCO3/g tailings after four consecutive extractions. Water-soluble metals were dominated by iron (4.26 mg/g), followed by zinc (3.17 mg/g) and calcium (3.17 mg/g). The ferrous iron concentration measured in the first water extract by the phenanthroline method (18) indicated that most of the watersoluble iron occurred as Fe2+. Water-soluble sulfate was 10.6 mg S/g, which corresponds to 56% of the total sulfate content. The micromineralogy of the tailings was characterized by Scanning Electron Microscopy (SEM) and Energy Dispersive Spectrometry (EDS) using sample preparation and analysis methods described in ref 15. Typical backscattered electron photomicrographs of partially oxidized tailing grains are shown in Figure 1. Primary sulfides included pyrite FeS2, pyrrhotite Fe1-xS, and chalcopyrite CuFeS2. Most sulfide crystals exhibited alteration rims and substantial amounts of secondary minerals forming interstitial cements. Some of these alteration rims (Figure 1a) likely consisted of melanterite FeSO4 based on their molar composition obtained by quantitative EDS (Fe0.19S0.16O0.64). Significant quantities of jarosite, a common sulfide oxidation product (3), were also present as coatings on quartz and other gangue particles (Figure 1b), as interstitial cements, and as individual crystals (Figure 1c). The average molar composition of the jarosite phases was determined by EDS to be K0.028Na0.005Fe0.148S0.100O0.708. By comparison, the molar compositions of pure K-jarosite KFe3(SO4)2(OH)6, Na-jarosite NaFe3(SO4)2(OH)6, and H-jarosite (H3O)Fe3(SO4)2(OH)6 are given by K0.05Fe0.15S0.10O0.70, Na0.05Fe0.15S0.10O0.70, and Fe0.15S0.10O0.75, respectively (excluding hydrogen, which is not quantified by EDS). Hence, the observed jarosite phases are likely mixtures of these three jarosite minerals. Other iron oxyhydroxysulfate phases containing more iron (40-53 wt %) and much less sulfur (1.4-7.4 wt %) than jarosite were also abundant in the tailings and are denoted “Sc” in Figure 1b-d. The Fe/S mole ratios of these phases ranged from 3.8 to 17.2, with an average of 9.6 (Table S-2, Supporting Information). By comparison, the range of Fe/S mole ratios for schwertmannite (Fe16O16(OH)12 (SO4)2 to Fe16O16(OH)10(SO4)3) is 5-8 (19). From these data, it is uncertain whether schwertmannite was present in the tailings. In addition to forming specific oxyhydroxysulfate minerals, sulfate may also be adsorbed on goethite FeOOH and ferrihydrite Fe(OH)3 since iron oxyhydroxides have positive net surface charges at low pH (20, 21). Neutralization Tests. Tailings were dried at 50 °C and mixed with reagent grade water (Nanopure Diamond water system) at liquid-to-solid ratios (L/S) of 5 and 15 in 50-mL centrifuge tubes. Reagent grade CaO and CaCO3 were mixed VOL. 37, NO. 7, 2003 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 1. Backscattered electron images of untreated partially oxidized tailings from the Winston Lake site showing (a) altered pyrrhotite crystal surrounded by a rim of melanterite, (b) jarosite coating the edge of a tailings grain and “Sc” phases present as interstitial cements between silicate particles, (c) jarosite crystals, “Sc” phases, and silicate particles, and (d) weathered pyrrhotite crystals and secondary iron oxyhydroxysulfates (“Sc” phases) forming interstitial cements, tin particle, and silicate minerals. in the slurries in amounts corresponding to alkalinity addition levels of 0, 8, 15, 20, 25, 30, 35, and 60 mg of CaCO3 (equiv)/g tailings. Note that alkalinities are reported in units of mg CaCO3 (equiv)/g tailings throughout this paper, independently of the type of amendment used in particular tests. With these alkalinity units, 1 mg of CaO corresponds to an alkalinity addition of 100/56 ) 1.79 mg of CaCO3 (equiv) since the molecular weights of CaCO3 and CaO are 100 and 56 g/mol, respectively. After adding the amendments, the centrifuge tubes were purged with nitrogen for two minutes, sealed, and placed on an end-over-end rotator operated at 16 rpm at room temperature. The continuous mixing of the slurries ensured uniformity of neutralization conditions throughout the tests. Although L/S ratios for tailings deposited in the field are typically less than unity, slurries with L/S ratios below 5 were found to be too thick to be mixed uniformly and were therefore not used. The nitrogen atmosphere prevented unrealistically high oxidation rates that would have resulted from the much higher exposure of suspended tailings grains to dissolved oxygen in the slurries by comparison to the field situation where oxygen transport is limited by diffusion in the pore water and in the laminar layer above the flooded tailings/water interface (9-11). The pH of the slurries was measured every 24 h. Purging with nitrogen was repeated after each pH measurement. It was found that the pH values varied by less than 0.05 units between consecutive measurements after 11 days. After this time, the slurries were filtered through 0.2-µm nylon mem1410
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branes, and the filtrates were analyzed for metals and sulfate by ICP-AES and IC, respectively. Filtrates were also titrated to an end point pH of 4.3 with a 0.01 N HCl solution to determine dissolved alkalinity. Dissolved alkalinity determinations were done in duplicate. A second series of neutralization tests investigated the pH evolution of the slurries over a much longer period of time (>120 days). These tests were carried out in 125-mL Erlenmeyer flasks with CaO and CaCO3, an L/S ratio of 15, and alkalinity addition levels of 0, 15, 35, and 60 mg of CaCO3 (equiv)/g tailings. Nitrogen purge and mixing of the slurries were similar to the first test series. Duplicate runs were carried out for the alkalinity addition level of 35 mg of CaCO3 (equiv)/g tailings. In a third series of neutralization tests, the tailings were treated with a combination of CaCO3 and CaO at an L/S ratio of 15. The CaCO3 addition level was kept constant at 35 mg of CaCO3 (equiv)/g tailings, and CaO was added at alkalinity levels of 5, 15, and 25 mg of CaCO3 (equiv)/g tailings. The pH evolution of the slurries was measured over a time period of 107 days. Slurries were purged with nitrogen and continuously mixed on an end-over-end rotator as in the previous test series. Liquid aliquots collected after 11 days were analyzed by ICP-AES. To compare the changes in mineral composition of the tailings caused by carbonate and hydroxide alkalinity addition, samples of tailings treated with CaCO3 and CaO at an alkalinity addition level of 60 mg of CaCO3 (equiv)/g tailings
FIGURE 2. Slurry pH versus alkalinity addition after 11 days for CaO and CaCO3 treated tailings and L/S ratios equal to 5 and 15.
FIGURE 3. Evolution of slurry pH with time for CaO and CaCO3 treated tailings, L/S ) 15.
for 11 days were oven dried at 40 °C and characterized by SEM/EDS.
Results and Discussion Slurry pH. The pH results after 11 days are shown as a function of alkalinity addition in Figure 2. In tailings treated with CaCO3, the pH reached an asymptotic value of approximately 6.7 for an alkalinity addition of 35 mg of CaCO3 (equiv)/g tailings. This behavior is consistent with pH buffering by the H2CO3/HCO3- pair (pKA ) 6.38 at 20 °C). The concentration of bicarbonate ion, which is the main source of dissolved alkalinity in the pH range of our tests, also reached an asymptotic value of 408 mg/L for an alkalinity addition level of 35 mg of CaCO3 (equiv)/g tailings (Figure S-1, Supporting Information). Further CaCO3 addition to the system beyond this level remained undissolved and, therefore, did not increase the pH of the slurry. By contrast, the slurry pH of tailings treated with CaO kept increasing with increased levels alkalinity addition, was higher than the pH of CaCO3 treated tailings for alkalinity addition levels larger than 15 mg of CaCO3 (equiv)/g tailings, and reached pH ) 10.9 for 60 mg of CaCO3 (equiv)/g tailings. The pH data were similar at L/S ) 5 and L/S ) 15 for CaO treated tailings. For CaCO3 treated tailings, pH values at L/S ) 5 were only slightly higher than at L/S ) 15. Therefore, the L/S ratio is not a critical parameter for assessing the pH behavior of either system. The results obtained after 11 days show that in the shortterm CaO provides higher pH values than CaCO3 for equivalent amounts of alkalinity added. The pH data measured over a period of 120 days indicate, however, that the pH of slurries treated with CaO decreased slowly but steadily with time for alkalinity addition levels of 35 and 60 mg of CaCO3 (equiv)/g tailings (Figure 3). The complete pH data for the 120-day neutralization tests are provided in Table S-3, Supporting Information. For an alkalinity addition level of 35 mg of CaCO3 (equiv)/g tailings, the pH of CaO treated tailings decreased from 9.4 after 2 days to 7.1 after 120 days. During the same time period, tailings treated with an equivalent amount of CaCO3 increased their slurry pH from 6.3 to 6.7. For tailings treated with mixtures of CaO and CaCO3, the initial slurry pH increased with the amount of CaO (Figure 4). The initial slurry pH was 8.2 when the CaO addition was 25 mg of CaCO3 (equiv)/g tailings. However, the pH stabilized at approximately 6.7 after 90 days or less. Hence, treating the tailings with mixtures of CaCO3 and CaO did not result in higher long-term pH values than treating with CaCO3 alone.
FIGURE 4. Evolution of slurry pH with time for tailings treated with CaO-CaCO3 mixtures, L/S ) 15. Tailings treated with calcite have significant buffering capacity provided by high HCO3- concentrations in the aqueous phase and the presence of undissolved CaCO3 particles identified by SEM/EDS analyses (see below). By comparison, the slurries treated with CaO have very little buffering capacity because all their soluble alkalinity consists of hydroxide ions, which are present at much smaller concentrations than bicarbonate ions in CaCO3 treated slurries. This is illustrated by the titration curves of filtrates from slurries treated with each type of amendment (Figure 5): the volume of 0.001 N HCl solution required to decrease the pH to 6.0 was seven times lower for filtrates of CaO treated slurries. Moreover, within the range of pH values and alkalinity addition levels practically relevant for tailings amendment, CaO is completely dissolved and, unlike CaCO3, cannot be held “in reserve” in the tailings. Hence, CaCO3 is much better suited than CaO for maintaining the pH at close to neutral pH in the long term. Our results are consistent with field measurements by Davis et al. (6) who found that pore waters in tailings amended with CaCO3 remained oversaturated with respect to calcite 5 years after amending, thus demonstrating that the amendment was still continuing to produce alkalinity after this time. Metals Immobilization. Table 2 shows the dissolved concentrations of selected metals after 11 days for alkalinity addition levels of 0, 35, and 60 mg of CaCO3 (equiv)/g tailings. Authorized concentration limits prescribed by the Canadian Metal Mining Liquid Effluent Regulations (MMLER) (22) for As, Cu, Ni, Pb, and Zn are also provided for reference. The VOL. 37, NO. 7, 2003 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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TABLE 2. Dissolved Metal Concentrations after 11 Days for CaO and CaCO3 Treated Tailings in L/S )15 Slurriesa alkalinity addition mg of CaCO3 (equiv)/g tailings 0.0
3.9
35.0 60.6 35.4 60.2 MMLER limit a
pH
Al 27.5
As
Cd
0.25
0.70
6.6 6.6
0.13 0.14
0.16 0.13
0.026 0.022
8.4 10.9 >6.0
0.17 0.19
