Environ. Sci. Technol. 2005, 39, 4913-4920
Characterizing and Quantifying Controls on Arsenic Solubility over a pH Range of 1-11 in a Uranium Mill-Scale Experiment B R E T T J . M O L D O V A N * ,†,‡ A N D M. JIM HENDRY‡ Cameco Corporation, 2121 11th Street West, Saskatoon, SK, Canada, S7M 1J3, and Department of Geological Sciences, University of Saskatchewan, Saskatoon, SK, Canada, S7N 5E2
A mill-scale hydrometallurgical experiment (2700 m3 of effluent treated/day) was conducted for three months at the Rabbit Lake uranium mine site located in northern Saskatchewan, Canada, to determine the controls on the solubility of dissolved arsenic over a pH range of 1-11 and to develop a thermodynamic database for the dominant mineralogical controls on arsenic in the mill and the resulting mill tailings. The arsenic concentrations in the mill ranged from 526 mg/L at pH 1.0 (initial) to 1.34 mg/L at pH 10.8 (final discharge). Geochemical modeling of the chemistry data shows that arsenic solubility is controlled by the formation of scorodite (FeAsO4‚2H2O) from pH 2.4 to pH 3.1, with 99.8% of dissolved arsenic precipitated as scorodite. Model results show that scorodite is unstable (releasing arsenic back in to solution) above pH 3.1 and arsenic adsorption to the surface of 2-line ferrihydrite is the dominant controlling factor in the solubility of arsenic from pH 3.2 to pH 11.0, with 99.8% of dissolved arsenic removed from solution via this mechanism. Finally, model results show ∼0.2% of the total dissolved arsenic adsorbs to the surface of amorphous aluminum hydroxide from pH 5.0 to pH 8.0. Minor alterations to the thermodynamic properties of arsenite and arsenate adsorption to 2-line ferrihydrite allowed the fit between measured mill-scale and modeled concentrations for the pH range of 3.2-11.0 to be optimized.
Introduction Arsenic is ubiquitous in the Earth’s crust, with an average terrestrial concentration of 2 µg/g, and is a toxic impurity in many sulfide ore deposits. The concentration of arsenic in base metal and uranium ores is often 2-3 wt % (1), and in gold ores, the arsenic concentration can be as high as 11 wt % (2). Sulfuric acid-based leaching processes of ores that contain arsenic-bearing minerals results in solubilization of arsenic. Removal of this arsenic is achieved through the addition of ferric iron during the lime neutralization process to form ferric arsenates. Hydrous ferric oxide (Hfo) (or, more specifically, ferrihydrite) forms after the rapid hydrolysis of ferric iron at ambient temperatures under oxidizing conditions (3-5) and has a strong affinity for arsenate at a pH * Corresponding author. Phone: (306) (306) 966-8593; E-mail:
[email protected]. † Cameco Corporation. ‡ University of Saskatchewan. 10.1021/es0482785 CCC: $30.25 Published on Web 06/04/2005
966-5720;
2005 American Chemical Society
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range of 4-8 (6). Ferrihydrite formed at temperatures of 7 and exhibits maximum adsorption capacity over the pH range of 8-9 (10, 11). The arsenate anion forms an inner-sphere complex with a bidentate binuclear configuration on the surface of amorphous aluminum hydroxide and ferrihydrite, independent of pH and ionic strength (10, 11). The formation of inner-sphere complexes occurs via a ligand exchange reaction with a surface functional group; no water molecules are present between surface functional groups and adsorbate ions (12). Conversely, the arsenite anion forms outer-sphere complexes via electrostatic interactions, with decreasing adsorption as ionic strength increases. Because of the presence of water molecules between the adsorbate and the adsorbent functional group, arsenite is more weakly bound to amorphous aluminum hydroxide and ferrihydrite than arsenate (10). Despite the wide spread occurrence of arsenic adsorption on surfaces of ferrihydrite and amorphous aluminum hydroxides in mine wastes, surface water, and groundwater, most knowledge on the sorption equilibrium is derived from laboratory-scale experiments (5, 8, 10, 11, 13-15). The validity of the physiochemical and thermodynamic parameters obtained from these studies to evaluate the long-term stability of arsenic at larger scales, such as that observed in mine wastes, is unproven (5, 10). For example, one study showed that scorodite (FeAsO4‚2H2O) is the dominant compound that controls the solubility of arsenic when acidic hydrometallurgical solutions are neutralized to pH 8 (15). In contrast, another study concluded that scorodite is unstable at pH >2, with Hfo being the stable solid phase that controls the solubility of arsenic (16). Other studies have concluded that ferric arsenate solids formed at pH 5 and at 8 show the formation of precipitates that contain cabrerite [(Ni,Mg,Fe)3(AsO4)2‚8H2O] (15). Our objectives are (i) to determine concentration profiles of arsenic, aluminum, and iron in acidic hydrometallurgical solutions over the pH range of 1-11; (ii) to identify the VOL. 39, NO. 13, 2005 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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TABLE 1. Precipitation Profile for the Neutralization of Raffinate in the Rabbit Lake mill process used during the mill-scale experiment Test Phase pH Profile process solution
operational pH month 1 month 2 month 3
raffinate solution Pachcua 1 solution Pachcua 2 solution Pachcua 3
1.0 6.5 8.5 11.0
1.0 2.5 5.0 8.5
1.0 3.0 6.0 10.0
1.0 4.0 7.0 11.0
relevant mineralogical controls, with respect to pH; and (iii) to assess the solubility product constants of secondary minerals formed during the neutralization process. We conducted a unique, large-scale continuous flow experiment in the mill at the Rabbit Lake uranium mine in northern Saskatchewan, Canada (58°,15′′ N and 103°,40′′ E) and used our results as input variables in geochemical modeling of secondary arsenic-, aluminum-, and iron-bearing minerals to estimate the associated chemical equilibrium constants. The thermodynamic data will be used to develop a hydrogeochemical model to predict the long-term fate and transport of arsenic that is contained within the Rabbit Lake tailings facility.
Materials and Methods Mill-Scale Experiment. At the Rabbit Lake mill, ground ore is mixed with water to produce an admixture of 55% solids, 45% of which pass through a 75-µm sieve (18). Leaching the slurry with 93% sulfuric acid and sodium chlorate as the oxidant (Eh maintained at +660 mV) results in the dissolution of uranium, as well as the primary arsenic-, aluminum-, and iron-bearing minerals. The insoluble barren fraction is separated from the uranium-bearing soluble fraction in the counter-current decantation (CCD) process. The soluble fraction (pregnant aqueous) reports to the solvent extraction process where uranium is extracted. The resulting uraniumrich solution is further purified in an impurity precipitation circuit and precipitated from solution as uranyl peroxide, using hydrogen peroxide that is maintained at a pH value of 4.5 with milk of magnesia (Mg(OH)2). The barren solution (raffinate) from the solvent extraction circuit, containing the leached aluminum, arsenic, and iron is neutralized with lime (Ca(OH)2) at ∼25 °C in three solution pachucas with incrementally increasing pH (retention time in each is ∼3 h; see Table 1). The raffinate solution is continuously pumped to the first pachuca, and the neutralized raffinate solution and the associated chemical precipitates are advanced through the solution pachucas via gravity flow. Precipitated solids are thickened in a solution recovery thickener, combined with lime-neutralized leach residue solids (pH 11) from the CCD circuit and discharged to the Rabbit Lake inpit tailings management facility. Our mill-scale experiment was conducted from July 1999 to September 1999, during which time 235 000 m3 of raffinate was processed (sustained continuous flow rate of 110 m3/h; 395 m3 per pachuca). During the mill-scale test, the geochemical characteristics of the ore fed to the mill was held constant to miminize variability in the test. The pH profile in the solution pachucas was modified on a monthly basis (see Table 1) to study the effects of pH on the precipitation profile of aluminum, arsenic, and iron among other metals. Samples of raffinate solution and neutralized raffinate solutions from the three pachucas were collected every 4 h. pH and Eh measurements were made immediately following sample collection, using an Orion model 250 pH/ion meter, an Orion combination pH/ATC electrode for pH measurements, and a Cole Palmer ORP platinum electrode for Eh measurements. The accuracy of the measurements was 4914
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determined using standard buffer solutions for pH and a reference solution for Eh (Eh ) +420 mV @ 25 °C). The accuracy of these measurements was (0.2 pH units for pH measurements and (20 mV for Eh measurements. Samples were filtered through 0.45-µm Supor filters. An aliquot was collected for analysis of Fe2+, Fe3+, NO3-, SO42-, and Cl-, and the remainder was acidified to pH 400 mg/L; see Table 4) and Na, K, Mg, Cu, Pb, Zn, NO3, Cl, Mo, and U were identified as minor species (98% of the arsenic in the raffinate solution is present in the pentavalent form H3AsO4 (20). Calculated saturation index (SI) values from PHREEQC with no equilibrium phases specified in the input file suggest that scorodite (FeAsO4‚2H2O), ferrihydrite [Fe(OH)3(a)], goethite (FeOOH), hematite (Fe2O3), an iron hydroxide [Fe3(OH)8] (mixture of Fe2+ and Fe3+), gibbsite (Al(OH)3), and amorphous aluminum hydroxide (Al(OH)3(a)) are possible iron and aluminum phases that could precipitate from the neutralized raffinate solution. Pichler et al. (7) showed, through XRD analysis of the fresh mill tailings (pH 10.8), that 2-line ferrihydrite was the only iron phase present. As a result, only scorodite and ferrihydrite were considered as the iron phases that control the solubility of arsenic in our model simulation. Model results indicate that scorodite could precipitate in the pH range of 2.4-5.5; as the alkalinity of the solution increases from pH 5.6 to pH 10.8, scorodite could dissolve, which is consistent with the stability field diagram. Furthermore, ferrihydrite could precipitate from pH 3.1 to pH 10.8 and both goethite and hematite could precipitate from pH 2.0 to pH 10.8. The iron hydroxide phase was determined to be stable from pH 4.4 to pH 10.8. Finally, gibbsite and amorphous aluminum hydroxide could precipitate from pH 4 to pH 11 and from pH 5 to pH 8, respectively. Results of the modeling suggest that the only arsenic-bearing minerals with the potential to precipitate (SI > 1) under conditions present in the Rabbit Lake mill are scorodite (from pH 2.5 to pH 5.5) and Ca3(AsO4)2‚4H2O (at pH >10.5). Fe-As System. The initial model input (including equilibrium, with respect to gypsum in the input file) was modified to assess the controls on the solubility of arsenic. The modified models included (i) scorodite, (ii) ferrihydrite (with arsenic adsorption), and (iii) scorodite + ferrihydrite (with arsenic adsorption) as equilibrium phases by setting the saturation index of the respective phase to SI ) 0. Scorodite was included as an equilibrium phase from pH 2.4 to pH 5.5, using a solubility product of Ksp ) 10-21.689 (PHREEQC database) (see Table 2). Our Fe-As modeling exercise shows that iron precipitates from solution from pH 2.4 to pH 5.5 as scorodite and results in a minimum concentration of 471 mg/L (0.007 mol Fe precipitated; see Figure 3). From pH 6.0 to pH 11.0, this iron is released back into solution as scorodite dissolves; all precipitated iron is released back into solution by pH 8.5. Overall, simulated iron concentrations were well above those measured in the mill experiment for the entire pH range of 1.0-11.0. 4916
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Furthermore, the results of this modeling exercise show that arsenic precipitates from solution as scorodite between pH 2.4 and pH 5.5 (Figure 4); the simulated concentration reaches a minimum of 1.0 mg/L at pH 4.4. The mass of arsenic precipitated from solution was 0.007 mol, correlating well with a 1:1 Fe/As molar ratio in scorodite. Scorodite begins to dissolve at pH 6.0, and by pH 8.5, all of the arsenic is released back into solution. Results of this modeling exercise show that scorodite could not exclusively control the solubility of arsenic during the neutralization of the raffinate; previous mineralogical studies have not observed scorodite in fresh or aged (up to 10 y) Rabbit Lake mine tailings (7, 20). In the next modelling exercise, only ferrihydrite was considered to be the phase that controls the solubility of arsenic and iron in solution. Ferrihydrite was included as an equilibrium phase over the pH range of 3-11. The surface complexation constants for arsenite and arsenate adsorption to ferrihydrite from Dzombak and Morel (5) were used (see Table 3). Results of this modeling exercise suggest that iron precipitates from solution as ferrihydrite over a pH range of 3.1-11.0 (see Figure 3), and the simulated iron concentration is reduced to 8.1, ferrihydrite fully deprotonates and attains a net negative surface charge, resulting in the release of arsenate anions. Results of the charge-density calculation support the arsenic concentration profile observed in the mill solutions from pH 3.2-11.0. Although the fits between the PHREEQC model simulations and the measured data are good and are consistent with the solids chemistry of the mill tailings (7, 20), these simulations cannot be presumed to represent unique solutions. Recent research by Chen et al. (26), using synchrotron-
based X-ray absorption studies of chemical precipitates collected from neutralized raffinates under mill conditions representative of the Rabbit Lake mill, support the precipitation of scorodite at low pH and its dissolution above pH 4 with the associated precipitation of ferrihydrite with arsenic adsorption. Implications for the Solubility and Long-Term Stability of Arsenical Ferrihydrite in Mine Wastes. Our research suggests that ferrihydrite is the dominant phase that controls the solubility of arsenic in Rabbit Lake mill tailings from pH 3.2 to pH 11.0, accounting for 99.8% of the nonaqueous phase arsenic. At low pH (pH 2.4-3.1), scorodite is the dominant phase that controls the solubility of arsenic in the tailings. The inclusion of gibbsite and amorphous aluminum hydroxide in the model suggests that they have a limited effect on arsenic concentrations, with only 0.2% of the arsenic adsorbed to aluminum hydroxide over a pH range of 5.08.0. Based on PHREEQC modeling of the neutralization of acidic arsenic- and iron-bearing hydrometallurgical solutions, we modified the complexation constants for arsenic adsorption to hydrous ferric oxides to optimize the simulated fit to the measured data. The optimized constants were slightly greater over the pH range of 3.2-11.0 for both the arsenite and arsenate anions of Dzombak and Morel (5). The modified thermodynamic properties for the adsorption of arsenic to ferrihydrite may be applicable for estimating arsenic solubility over over a pH range of 3.2-11.0 in large-scale studies, although further testing is required.
Acknowledgments The authors acknowledge Cameco Corporation for the use of the data from the mill-scale experiment and Cameco Corporation and the Cameco-NSERC Research Chair for financial support.
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Received for review November 4, 2004. Revised manuscript received May 1, 2005. Accepted May 4, 2005. ES0482785
