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A kinetic–thermodynamic study of silver leaching in thiosulfate–copper–ammonia–EDTA solutions
Hydrometallurgy 134–135 (2013) 124–131
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A kinetic–thermodynamic study of silver leaching in thiosulfate–copper–ammonia–EDTA solutions D.M. Puente-Siller, J.C. Fuentes-Aceituno ⁎, F. Nava-Alonso Centro de Investigación y de Estudios Avanzados del IPN Unidad Saltillo, Av. Industria Metalúrgica Nº 1062, Parque Industrial Ramos Arizpe, Ramos Arizpe, Coahuila, 25900, Mexico
a r t i c l e
i n f o
Article history: Received 25 April 2012 Received in revised form 31 January 2013 Accepted 14 February 2013 Available online 27 February 2013 Keywords: Leaching Thiosulfate EDTA Silver
a b s t r a c t In this research, an analysis of the effect of ethylenediaminetetraacetic acid (EDTA), thiosulfate and cupric ions on the silver leaching kinetics was performed. For that purpose leaching experiments with pure metallic silver were carried out at different concentrations of these reagents at room temperature. The results of this study showed that small amounts of EDTA accelerate the leaching of silver more efficiently than higher concentrations due to an increase in the oxidation potential of the leaching solution. These results were in accord with Pourbaix diagrams and redox potential measurements. A synergistic effect was found in the silver leaching kinetics when the thiosulfate and EDTA concentrations were both decreased. On the other hand, the silver leaching kinetics was reduced at low Cu(II) concentrations due to the decrease in the oxidizing ability of the leaching solution. Characterization by SEM and EDXS of the silver in the first minutes of leaching revealed that the silver particles were coated by a layer of copper sulfides and copper oxides. Furthermore, characterization of the solid residue during the precipitation of silver in the leaching experiment was performed by the same analytical techniques, showing the presence of silver sulfide on the unreacted silver surface. The mechanism by which the silver sulfide is precipitated was found to be related to the copper sulfide formation. It was also observed that an increase in the concentration of EDTA promotes the silver dissolution avoiding the formation of copper sulfides or oxides; in these cases the process is controlled by the chemical reaction. These observations were also supported with Pourbaix and species distribution diagrams. © 2013 Elsevier B.V. All rights reserved.
1. Introduction Cyanide leaching is effective for gold and silver extraction from minerals in aqueous solutions. The precious metals are generally recovered with cyanide as complexing agent and oxygen as oxidant (Zhang, 2004). The need of processing difficult-to-treat ores as well as the environmental risks associated to cyanide use has encouraged the research and development of new leaching systems for precious metals. The thiosulfate–copper–ammonia system is considered a promising alternative to cyanide (Alonso-Gómez and Lapidus, 2009). The method has been studied formally by different research groups since 1980 (Abbruzzese et al., 1995; Feng and Van Deventer, 2006). In this process cupric ions oxidize the precious metals while thiosulfate forms complexes with them. Ammonia is also added to stabilize the cupric ions in the alkaline solution, avoiding their precipitation as copper hydroxide (Feng and Van Deventer, 2007). Only few oxidizing agents, such as cupric ions, are adequate for working in ammoniacal solutions (Abbruzzese et al., 1995); the presence of cupric ions enhances
⁎ Corresponding author. Tel.: +52 844 438 9600x8512. E-mail address: [email protected] (J.C. Fuentes-Aceituno). 0304-386X/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.hydromet.2013.02.010
the gold dissolution kinetics by 18 to 20 times (Aylmore and Muir, 2001). The chemistry of this alternative leaching system is very complicated due to the occurrence of several chemical and electrochemical reactions during the precious metal leaching. In addition to the silver or gold oxidation reaction, some thiosulfate oxidative degradation occurs (Heath et al., 2008) involving the formation of tetrathionate and other sulfur compounds (Aylmore and Muir, 2001), as well as several silver and copper complexes with ammonia and thiosulfate. It has been reported that the leaching kinetics of silver can be favored by an increase in the thiosulfate concentration (Zipperian et al., 1988); furthermore, the silver pregnant solution can be continuously electrodeposited avoiding the silver re-adsorption in the ore (Preg-Robbing phenomenon) (Fuentes-Aceituno et al., 2004). These findings suggest that the leaching of silver in thiosulfate solutions would be a very promising alternative process; however, the high consumption of cupric ions and thiosulfate has made the thiosulfate leaching system uneconomical compared with the cyanide leaching, so the process has not been widely used on a commercial scale (Aylmore and Muir, 2001). Consequently, scientists and engineers have focused the research on increasing the oxidizing capability of this leaching system and, at the same time, trying to avoid the oxidative loss of thiosulfate. Several
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investigations have been carried out to find an additional reagent for complexing cupric ions with the aim of increasing the silver and gold leaching kinetics but avoiding the thiosulfate oxidation (Alonso-Gómez and Lapidus, 2009; Feng and Van Deventer, 2010). Several studies have shown that EDTA (ethylenediaminetetraacetic acid) is an effective complexing agent for increasing the extraction kinetics of heavy metals such as Pb, Cu, Zn and Cd (Manouchehri et al., 2006). EDTA is more efficient than other complexing agents like citrate (Labanowski et al., 2008). In the precious metals leaching with the thiosulfate system it has been demonstrated that the presence of EDTA decreases the thiosulfate oxidation rate by the formation of complexes of EDTA with the Cu(II) ions; the oxidizing potential of the leaching solution decreases (Alonso-Gómez and Lapidus, 2009; Feng and Van Deventer, 2010). It has been also established that the addition of small amounts of EDTA improves substantially the thiosulfate leaching of gold (Feng and Van Deventer, 2011); however, there is very little information as well as a lack of understanding about the phenomena that controls the silver leaching kinetics with EDTA as cupric ion complexing agent. The objective of this study is to elucidate the phenomena involved in the kinetics of silver dissolution with the thiosulfate–copper–ammonia– EDTA system, in an attempt to increase the silver recovery and to minimize the reagent consumption. 2. Materials and methods For the leaching tests the composition of the solution was changed in order to study the effect of thiosulfate, EDTA and copper on the silver leaching. Pourbaix and species predominance diagrams were constructed for the system, and the kinetic model of shrinking core was applied to the results obtained. 2.1. Leaching tests All the silver leaching experiments were performed in an agitated batch reactor (250 mL glass beaker), magnetically stirred at 400 rpm. The reactor was provided with a pH electrode (SENSOREX) and a redox potential electrode with calomel reference (Cole-Parmer). The general procedure for the leaching experiments was as follows: 200 mL of the solution containing cupric sulfate, thiosulfate, EDTA and ammonium were placed in the reactor at 25 °C. The concentrations of cupric ions, thiosulfate and EDTA were varied as follows: 7.87 × 10 −4 M and 0.05 M for the cupric sulfate; 0.1 M, 0.2 M and 0.3 M for thiosulfate; and 1.25 × 10 −4 M, 0.025 M and 0.05 M for EDTA. All solutions were prepared with reagent grade chemicals and deionized water, and the pH value was adjusted to 10.2 with ammonium hydroxide. After adjusting pH, 0.125 g of metallic silver (Alfa Aesar, 99.999%) (1.3–3.2 μm) was placed in the reactor while stirring. The pH and redox potential were measured before the silver addition and during the leaching experiments. Samples of the leach solution were withdrawn at different times during the leach period for analysis of silver by atomic absorption spectrophotometry (VARIAN Spectra AA 240). All samples for silver analysis were analyzed at the end of the experiment, and before the analysis they were kept in dark vessels to prevent the precipitation of silver by light. Once the experiment was concluded, the residue was filtered, rinsed with deionized water and left to air dry. The solid residues were characterized chemically and morphologically by SEM (Phillips XL30ESEM). The local chemical composition of the residues was determined by EDXS (EDAX Genesis). 2.2. Elaboration of the thermodynamic diagrams In order to elucidate the predominant species in each leaching system, Pourbaix and species distribution diagrams were constructed using the Medusa© Software. These diagrams are also useful for determining the oxidizing ability of the leaching systems, as well as
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the possible redox reactions that could take place. The diagrams were constructed considering the equilibrium data contained in the Hydra database of the Medusa© suite software (Puigdomenech, 2004), complemented with thermodynamic data reported by Aylmore and Muir (2001). Table 1 presents the Gibbs free energy data complemented and used in this work. The software is based in an algorithm developed by Eriksson (1979), which minimizes the Gibbs free energy of reactions in equilibrium that can occur in the aqueous leaching systems, and determines the predominating species under specific solution conditions. 3. Results and discussion The results of this study are discussed in four parts; in the first part appears the effect of the thiosulfate and EDTA concentrations on the silver leaching; the second section analyzes the kinetic data with the shrinking core model; the third section presents the thermodynamic analysis of the studied systems, and the fourth, the effect of the Cu(II) concentration on the silver leaching. 3.1. Effect of EDTA and thiosulfate concentrations on the silver leaching Fig. 1 illustrates the effect of three different EDTA concentrations (1.25 × 10−4 M, 0.025 M and 0.05 M) on the silver dissolution kinetics at room temperature. The thiosulfate and copper concentrations were kept constant at 0.2 M and 0.05 M respectively. From these experiments it was observed that the leaching system with the lowest EDTA concentration (1.25 × 10−4 M) enhanced the silver leaching kinetics. This behavior was also observed in the leaching experiments for 0.1 M and 0.3 M thiosulfate and the same EDTA concentrations (Figs. 2 and 3 respectively). The leaching solution with 0.1 M thiosulfate and 1.25 × 10−4 M EDTA reached a silver extraction of 100% in the first 20 min (see Fig. 2); after this time the silver extraction decreased to 90%, probably due to the precipitation of silver, which suggests an unstable leaching system. All the leaching experiments carried out with solutions containing 1.25 × 10−4 M and 0.025 M EDTA showed some oscillations in the silver extraction curves (Figs. 1, 2 and 3) indicating reprecipitation and, in some cases, redissolution of the silver. A possible explanation to this phenomenon is proposed by Abbruzzese et al. (1995): in the initial stage of the leaching process the dissolution of metallic silver (Eq. (1)) occurs simultaneously with the reduction of cupric ions to cuprous species (Eq. (2)). Ag þ 3S2 O32− →AgðS2 O3 Þ35− þ e
−
ð1Þ
−
CuðNH3 Þ42þ þ 3S2 O32− þ e →CuðS2 O3 Þ35− þ 4NH3
ð2Þ
Besides these reactions, other simultaneous reactions may occur such as the oxidative decomposition of thiosulfate involving the formation of additional sulfur compounds such as tetrathionate, as well as the precipitation of copper sulfides species such as CuS or Cu2S (Eqs. (3) and (4)). 2þ
Cu
2−
2−
þ
þ S2 O3 þ H2 O→CuS þ SO4 þ 2H
ð3Þ
Table 1 Gibbs free energy for thiosulfate and silver species used in the construction of thermodynamic diagrams (25 °C, Aylmore and Muir, 2001). Chemical species
ΔG°298 (kJ/mol)
S2O32−(a) Ag2S Ag2+(a) Ag+(a) Ag(S2O3)35−(a) Ag(S2O3)23−(a) Ag(S2O3)−(a) Ag2O3(c)
−532.2 −40.5 268.2 77.2 −1598.3 −1058.6 −506.3 87.0
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Fig. 1. Effect of EDTA concentration on silver leaching for a solution of 0.2 M thiosulfate (0.125 g silver, 200 mL solution, 0.05 M Cu(II), 0.6 M NH4OH, pH = 10.2, room temperature and 400 rpm).
þ
þ
2Cu þ S2 O32− þ H2 O→Cu2 S þ SO42− þ 2H
ð4Þ
Once the copper sulfide species are formed, the silver contained in the thiosulfate complex (Ag(S2O3)35−) could precipitate as silver sulfide according to Eq. (5) (Zipperian et al., 1988): Cu2 S þ 2AgðS2 O3 Þ35− →Ag2 S þ 2CuðS2 O3 Þ35− :
ð5Þ
This reaction would explain the decrease on silver concentration in the leaching curves of Figs. 1, 2 and 3. Once the silver sulfide is formed, it is possible to redissolve it by a substitution of copper by silver in the sulfide matrix, with the consequent formation of a silver thiosulfate complex in solution (Aylmore and Muir, 2001): þ
þ
2Cu þ Ag2 S→2Ag þ Cu2 S þ 6S2 O32− →2AgðS2 O3 Þ35− :
ð6Þ
In spite of the instability of the leaching systems presented in Figs. 1–3, it is possible to say that the leaching system with 0.1 M thiosulfate (Fig. 2) presents fewer oscillations (dissolution–precipitation phenomena) than the system with 0.2 M thiosulfate (Fig. 1). From a practical point of view, this leaching system with 0.1 M thiosulfate would be more interesting than the systems with 0.2 and
Fig. 2. Effect of EDTA concentration on silver leaching for a solution of 0.1 M thiosulfate (0.125 g silver, 200 mL solution, 0.05 M Cu(II), 0.6 M NH4OH, pH = 10.2, room temperature and 400 rpm).
Fig. 3. Effect of EDTA concentration on silver leaching for a solution of 0.3 M thiosulfate (0.125 g silver, 200 mL solution, 0.05 M Cu(II), 0.6 M NH4OH, pH = 10.2, room temperature and 400 rpm).
0.3 M thiosulfate because of its lower reagent consumption and higher silver leaching kinetics; nevertheless, from the perspective of chemical reaction kinetics, it could be expected that an increase in the thiosulfate concentration would enhance the silver leaching kinetics. These results suggest that in systems with 0.2 M and 0.3 M thiosulfate, thiosulfate could be decomposed more than in the system with 0.1 M. This behavior does not imply that there is no thiosulfate decomposition at 0.1 M, but that this decomposition is smaller and can only be noticed further (after 4 h in Fig. 2, for 0.025 M EDTA), when the silver leaching decreases. It can also be observed, in the systems with 0.1 M thiosulfate and 1.25 × 10 −4 M or 0.025 M EDTA, that the silver extraction reaches an almost constant maximum value of silver extraction of 92 and 80% respectively. This behavior is probably related to the precipitation of silver and copper as sulfides or oxides on the unreacted silver particle surface, generating passive layers which inhibit a complete silver extraction (see Fig. 2). The EDTA concentration affects the silver dissolution. It was observed in Figs. 1, 2 and 3 that an increase in the EDTA concentration decreases the silver extraction. This phenomenon can be explained in terms of thermodynamics and electrochemistry: the Cu (II) complex with EDTA is more stable in 0.05 M EDTA than in 0.025 M EDTA solutions. Besides, the oxidizing ability of the leaching solution decreases in the system with 0.05 M EDTA, causing the diminution in silver leaching kinetics under this condition. Further evidence of this phenomenon is presented in Fig. 4a and b, and will be discussed later. From these results three important characteristics for the thiosulfate–EDTA leaching system can be highlighted: a) a decrease in the thiosulfate and EDTA concentrations favors the silver leaching; b) the leaching solution with 0.1 M thiosulfate is more stable than the other two thiosulfate concentrations; and c) a catalytic effect was found in the silver leaching kinetics when EDTA concentration was lowered (see Figs. 1, 2 and 3). If this last statement is true, there should be a difference in the redox potentials of the leaching solutions, i.e. the redox potential should become more positive when the EDTA is decreased, revealing that the oxidizing ability of the leaching solution is enhanced, and therefore the silver dissolution kinetics could be favored. Redox potential measurements were performed for the leaching solutions in the tests of Figs. 1–3. For example, the leaching solution with 0.025 M EDTA and 0.2 M thiosulfate had a redox potential of 0.4018 V vs SHE; when the EDTA concentration was increased to 0.05 M for the same thiosulfate concentration (0.2 M), the redox potential decreased to 0.323 V vs SHE. This difference in the redox potential indicates that the EDTA concentration has an important effect on the oxidizing ability of the leaching solution, and consequently, on the silver dissolution
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with the shrinking core model. It was found that in some cases the process was controlled by the chemical reaction. Eq. (7) is proposed for a process whose kinetics is controlled by a chemical reaction (Levenspiel, 1972): 1=3 t bks C Al =Rp ρB ¼ 1–ð1–XÞ
ð7Þ
where t = reaction time, b = reaction stoichiometric coefficient, ks = rate constant for the surface reaction, CAl = concentration of the reactive fluid species, Rp = particle radius, ρB = molar density of silver in the solid sample and X = fractional conversion (extraction) of the silver. On the other hand, Eq. (8) can be used for processes whose kinetics is controlled by the diffusion of the fluid species through a porous layer (Levenspiel, 1972): 2 2=3 t 6bDe C Al =Rp ρB ¼ 1–3ð1–XÞ þ 2ð1–XÞ
Fig. 4. Effect of the EDTA concentration on the copper species predominance and oxidizing ability of the leaching solution at 25 °C a) Pourbaix diagram for the Cu–NH3– EDTA–H2O system with 0.025 M EDTA, 0.6 M NH4OH and 0.05 M Cu(II). b) Pourbaix diagram for the same system with 0.05 M EDTA.
kinetics; i.e. if the redox potential is reduced from 0.4018 to 0.323 V vs SHE, the silver dissolution kinetics will be slower. A thermodynamic analysis with the aid of Pourbaix diagrams was carried out in order to understand better the low leaching efficiency obtained at high EDTA concentrations (0.05 M). Fig. 4 presents two predominance or Pourbaix diagrams for the Cu–EDTA–NH3–H2O system at 0.025 and 0.05 M EDTA (Fig. 4a and b respectively). In these diagrams the copper species Cu(EDTA) 2− and Cu(NH3)2+ predominate in the pH range from 1 to 10.4, while the copper oxides predominate at pH values above 10.4. It is important to mention that both species represent the half cell reduction reaction for the silver oxidation. It can be observed that the equilibrium potential for the Cu(EDTA)2−/Cu(NH3)2+ couple is modified considerably with the modification of EDTA concentration. An increase in the EDTA concentration from 0.025 to 0.05 M (Fig. 4a–b) causes a more negative equilibrium potential for the Cu(EDTA)2−/Cu(NH3)2+ couple (from −0.125 to −0.35 V vs SHE approximately). According to the kinetic theory of electrochemical reactions, it is well known that a decrease in the redox potential of a system will affect exponentially the oxidation kinetics of the chemical species, causing, in this case, the slower silver leaching kinetics observed. This is in accordance with the leaching results shown before as well as with the redox potential measurements carried out.
3.2. Kinetics of the silver leaching with 0.2 M thiosulfate In order to understand the phenomena that control the silver leaching kinetics, some silver leaching experiments were analyzed
ð8Þ
where De is the effective diffusion coefficient of the reactive fluid species through the porous layer. The silver leaching results shown in Fig. 1 for 0.2 M thiosulfate and two EDTA concentrations (0.025 M and 0.05 M) were fitted to the shrinking core model using Eqs. (7) and (8) for the initial leaching stage (30 min). It was decided to use these EDTA concentrations, and only the initial leaching data, to avoid the interference of the silver precipitation as proposed in Eq. (5). It is important to remember that at 0.025 M EDTA the copper can form oxides as shown in Fig. 4a. Furthermore the formation of copper sulfides is also possible as shown in Eqs. (3) and (4). These copper species could represent a physical barrier between the fluid reactive species and the unreacted silver sites. Therefore, Fig. 5 shows the fitted data for the leaching solution with 0.025 M EDTA (with Eq. (8)) where it can be observed that the regression line does not represent adequately the porous layer diffusion mechanism. This result suggests that the oxidizing agent (cupric ions) concentration is changing during the silver leaching due to the formation of copper oxides and sulfides. According to Senanayake (2007), one of the requirements for using the shrinking core model is that the concentration of reagents responsible for the surface reaction remains constant; therefore a different kinetic model is necessary to represent better the silver leaching in this dynamic system (0.025 M EDTA). On the other hand, the data of the silver leaching with 0.2 M thiosulfate and 0.05 M EDTA were fitted to Eq. (7) and can be seen in Fig. 6. In this case, the leaching kinetics is controlled by the silver dissolution chemical reaction (Eq. (1)). This result is interesting: if the
Fig. 5. Silver leaching data fitted to Diffusion Control Model, shrinking core in spherical particles. Silver dissolution during the first 30 min of the leaching experiment (0.125 g silver, 200 mL solution, 0.2 M thiosulfate, 0.025 M EDTA, 0.05 M Cu(II), 0.6 M NH4OH, pH = 10.2, room temperature and 400 rpm).
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process is controlled by the chemical reaction, the silver dissolution is expected to occur faster for 0.05 M EDTA solutions than for 0.025 M EDTA due to the absence of copper oxides and sulfides on the silver particle surface; the presence of copper oxides or sulfides would indeed decrease the Cu (II) concentration in the leaching solution. However, the silver extraction curves of Fig. 1 showed that silver dissolution for 0.05 M EDTA reached only a 30% in 6 h. This behavior can be explained by the decrease in the oxidizing ability of the leaching solution when the EDTA is increased from 0.025 to 0.05 M, as was shown in the Pourbaix diagram of Fig. 4b. It is important to note that silver dissolution is an electrochemical nature reaction, and hence it is expected that a decrease in the redox potential of the leaching solution (when EDTA is increased from 0.025 to 0.05 M) will slow down the silver leaching kinetics. 3.3. Thermodynamic study of the Cu–NH3–EDTA–H2O, Cu–S–NH3-EDTA– H2O and Ag–S2O3–NH3–EDTA–H2O systems It was observed that the silver extraction curves showed some oscillations (precipitation–redissolution of silver). In addition to this, the precipitation of copper oxides and sulfides affects the silver leaching process with a 0.2 M thiosulfate solution containing 0.025 M EDTA. The formation of this solid species can be explained thermodynamically by the Pourbaix diagrams of Fig. 4, where the presence of copper oxides occurs at pH values of 10.2 and higher. This pH is very similar to that of the leaching conditions, suggesting that the copper oxides form on the silver particle surface causing a decrease on the silver leaching kinetics. Fig. 7 presents a species distribution diagram for the Cu–NH3– EDTA–H2O system at 25 °C, considering the next concentrations: 0.025 M EDTA, 0.05 M Cu(II), 0.6 M NH3 and a fixed potential of 0.4 V vs SHE (similar to the redox potential of the leaching solution). The copper complexation with EDTA and ammonia as well as the presence of CuO are observed at a pH of 10.2. Furthermore the copper species Cu(EDTA) 2− and CuO predominate at this pH condition. This is indicative that the copper in solution can be stabilized with EDTA, but also can precipitate as oxides. From an economic point of view this system would imply a loss of the oxidizing agent (Cu(II) complexes in solution), increasing the costs of the process. On the other hand, Fig. 8 shows the species distribution diagram under similar conditions as those of Fig. 7, but in this case the EDTA concentration was increased to 0.05 M and the potential was fixed at 0.32 V vs SHE (the redox potential measured for this leaching solution). In this diagram, the copper oxide CuO does not appear in the
Fig. 6. Silver leaching data fitted to Chemical Reaction Control Model, shrinking core in spherical particles. Silver dissolution during the first 30 min of the leaching experiment (0.125 g silver, 200 mL solution, 0.2 M thiosulfate, 0.05 M EDTA, 0.05 M Cu(II), 0.6 M NH4OH, pH = 10.2, room temperature and 400 rpm).
Fig. 7. Distribution of copper species at different pH values for the Cu–NH3–EDTA–H2O system. The conditions for the diagram construction were 0.025 M EDTA, 0.05 M Cu(II), 0.6 M NH4OH, and EH = 0.40 V at 25 °C.
solution; all the copper being complexed by the EDTA and ammonia. This diagram indicates that the low efficiency of the leaching systems shown in Figs. 1, 2 and 3 is not related to the copper oxide precipitation but to the decrease in the redox potential of the couple Cu(EDTA) 2 −/Cu(NH3)2+, as was shown in Fig. 4b. Consequently, the silver dissolution reaction is the rate determining step for the leaching system with 0.05 M EDTA, as shown in Fig. 6. The absence of CuO in the speciation diagram under this leaching condition suggests that the silver particle is not coated by any ash layer. All the silver leaching experiments with 1.25 × 10 −4 and 0.025 M EDTA presented oscillations. The oscillations were related to the dissolution–precipitation phenomenon: the dissolution of silver with thiosulfate (Eq. (1)), and the precipitation of silver sulfide (Eq. (5)). In this section further evidence is presented for a better understanding of the phenomena that cause the so-called oscillations in the leaching curves. Figs. 9, 10 and 11 show the Pourbaix diagrams for the Ag–S– EDTA–H2O systems at 0.2, 0.1 and 0.3 M thiosulfate respectively (0.025 M EDTA, 1 × 10 −5 M Ag). The equilibrium potential for the redox couple Ag(S2O3)35 −/Ag at pH 10.2 corresponds to − 0.089, − 0.044 and − 0.119 V vs SHE respectively. Furthermore, the leaching solutions for 0.1 M, 0.2 M and 0.3 M thiosulfate at 0.025 M EDTA presented a redox potential of 0.4259, 0.4018 and 0.323 V vs SHE respectively; if these redox potential operation conditions are visualized on the Pourbaix diagrams for each system, it can be seen that the silver predominant species is the complex Ag(S2O3)35− (see Figs. 9, 10 and 11). This result suggests that metallic silver is oxidized to form the complex Ag(S2O3)35− in accordance with Eq. (1). However the precipitation of the silver–thiosulfate complex to form silver sulfide is supposed to occur in these systems by the presence of copper sulfide (see Eqs. (3)–(5)). According to Abbruzzese et al. (1995) the precipitation
Fig. 8. Distribution of copper species at different pH values for the Cu–NH3–EDTA–H2O system. The conditions for the diagram construction were 0.05 M EDTA, 0.05 M Cu(II), 0.6 M NH4OH and EH = 0.32 V at 25 °C.
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Fig. 9. Pourbaix diagram for the Ag–S–EDTA–H2O system. The conditions for the diagram construction were 0.2 M thiosulfate, 0.025 M EDTA and 1 × 10−5 M Ag at 25 °C.
Fig. 11. Pourbaix diagram for the Ag–S–EDTA–H2O system. The conditions for the diagram construction were 0.3 M thiosulfate, 0.025 M EDTA, and 1 × 10−5 M Ag at 25 °C.
of copper sulfides can occur by the oxidative degradation of the thiosulfate in the leaching solution. On the other hand, the Pourbaix diagram for the Cu–S–NH3–EDTA–H2O system at 0.2 M thiosulfate and 0.025 M EDTA presented in Fig. 12a shows that the equilibrium potential for the redox couple Cu(EDTA)2−/Cu2S is −0.0632 V vs SHE at pH 10.2 approximately. These results indicate that it is possible that the reduction of Cu(EDTA)2− to Cu2S and the oxidation of Ag to Ag(S2O3)35− occur simultaneously; therefore, the copper sulfide species is also probably formed by this reduction reaction in addition to that mentioned in Eqs. (3) and (4). Once this copper sulfide species is formed during the leaching process, the silver complex Ag(S2O3)35− can precipitate to form Ag2S according to Eq. (5). Two solid samples corresponding to the leaching experiment at 0.2 M thiosulfate and 0.025 M EDTA at 10 and 50 min (Fig. 1) were characterized by SEM and EDXS (Fig. 13a, b, c and d). Fig. 13a shows a spherical morphology of the silver particles after 10 min leaching. The EDXS analysis (see Fig. 13b) revealed the presence of silver, copper, oxygen and sulfur; these last three elements could be related to the formation of copper sulfides and copper oxide species on the silver surface. These results are in accordance with the thermodynamic predictions and support the proposed idea that the silver particle is being coated by copper sulfides and oxides. Fig. 13c shows the silver particles coated by a precipitated solid after 50 min of the leaching process. The EDXS analysis (Fig. 13d) revealed the presence of silver and sulfur, which could be the result of the precipitation of the silver sulfide (Eq. (5)). Fig. 13d did not show any copper sulfide on the residue, which was expected as the copper sulfide had reacted in order to form the silver sulfide.
Contrary to the 0.025 M EDTA system, the leaching system with 0.05 M EDTA did not show oscillations in any case (Figs. 1, 2 and 3). This can be explained thermodynamically: for the leaching solution containing 0.2 M thiosulfate and 0.025 M EDTA the Pourbaix diagram of Fig. 9 predicts an equilibrium potential of −0.089 V vs SHE at pH 10.2 for the redox couple Ag(S2O3)35−/Ag; on the other hand, the diagram for the same thiosulfate concentration and 0.05 M EDTA shows that the equilibrium potential for the redox couple Cu(EDTA)2−/Cu2S is −0.112 V vs SHE at pH 10.2 (Fig. 12b). These results indicate that
Fig. 10. Pourbaix diagram for the Ag–S–EDTA–H2O system. The conditions for the diagram construction were 0.1 M thiosulfate, 0.025 M EDTA, and 1 × 10−5 M Ag at 25 °C.
Fig. 12. Effect of the EDTA concentration on the copper sulfide formation at 25 °C. a) Pourbaix diagram for the Cu–S–NH3–EDTA–H2O system with 0.2 M thiosulfate, 0.025 M EDTA, 0.6 M NH4OH and 0.05 M Cu(II). b) Pourbaix diagram for the same system with 0.05 M EDTA.
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Fig. 13. Effect of leaching time on the chemical composition of the solid residue. Experimental conditions: 0.2 M thiosulfate, 0.025 M EDTA, 0.6 M NH4OH and 0.05 M Cu(II). a) Morphology at 10 min leaching, b) chemical composition of the residue at 10 min, c) morphology at 50 min leaching, d) chemical composition for the residue at 50 min.
the reduction of Cu(EDTA) 2− to Cu2S cannot occur simultaneously with the oxidation of Ag to Ag(S2O3)35−. In addition, the redox potential measurements showed that an increase in the EDTA concentration decreases the redox potential of the solution and hence the oxidation ability of the system. Eqs. (3) and (4) show the possibility to form copper sulfide species as a consequence of the thiosulfate oxidative destruction; however the decrease in the oxidation ability of the system makes the formation of these copper sulfides less probable. These results are consistent with the findings mentioned when discussing
Fig. 14. Effect of the thiosulfate concentration on the silver extraction. Experimental conditions: 0.125 g silver in 200 mL thiosulfate solution 1.25 × 10−4 M EDTA, 0.6 M NH4OH, 7.87 × 10−4 M Cu(II) at room temperature and 400 rpm.
Fig. 6: the silver leaching under these conditions is controlled by the chemical reaction in the initial stage. 3.4. Effect of copper concentration on the silver leaching In the previous sections it was observed that a decrease of the thiosulfate and EDTA concentrations enhances the silver leaching kinetics. This section deals with the effect of decreasing the cupric ion concentration from 0.05 M to 7.87 × 10−4 M. Fig. 14 presents the silver
Fig. 15. Silver leaching data fitted to Diffusion Control Model, shrinking core in spherical particles. Silver dissolution during the first 30 min of the leaching experiment (0.125 g silver, 200 mL solution, 0.1 M thiosulfate, 1.25 × 10−4 M EDTA, 7.87 × 10−4 M Cu(II), 0.6 M NH4OH, pH = 10.2, room temperature and 400 rpm).
D.M. Puente-Siller et al. / Hydrometallurgy 134–135 (2013) 124–131
leaching for the systems containing 0.1 M, 0.2 M and 0.3 M thiosulfate, 1.25 × 10−4 M EDTA and 7.87 × 10 −4 M Cu(II). All experiments showed low silver leaching values: less than 12% in 6 h. If these results are compared with those of Fig. 2 for 1.25 × 10−4 M EDTA, it is clear that the decrease in cupric ion concentration affects significantly the leaching of silver. This behavior is associated with the decrease in the oxidizing ability of the leaching solution, and is in accord with the findings of Abbruzzese et al. (1995), who observed that the presence of a high concentration of cupric ions promotes fast leaching kinetics. Some oscillations also appear in these silver extraction curves, related to the silver precipitation as silver sulfide, as mentioned in the last section. The experimental data for the silver leaching with solutions 0.1 M, 0.2 M and 0.3 M thiosulfate, 1.25 × 10 −4 EDTA and 7.87 × 10 −4 Cu(II) were fitted to the shrinking core model in the same way as in the cases of Figs. 5 and 6. The results for the first 30 min are presented in Fig. 15. This range of time was chosen to avoid the interference of any silver precipitation (passivation phenomena). From Fig. 15 it can be said that during the initial stage of the leaching the kinetics of the process is controlled by the diffusion of the fluid species through the porous layer. As exposed previously, this porous layer is composed of copper oxides. The leaching process is passivated after 1 h for the tests of curves 0.2 and 0.3 M thiosulfate (Fig. 14), probably due to the loss of the oxidizing agent (cupric ions). According to Feng and Van Deventer (2010), the precipitation of cupric ions occurs in the thiosulfate systems. These results suggest that the leaching of silver would proceed faster with 0.05 M Cu(II) than with 7.87 × 10 −7 M Cu(II) (see Figs. 2 and 14). 4. Conclusions On the basis of the results obtained for the metallic silver leaching with the system thiosulfate–EDTA–Cu(II)–ammonia, the following could be concluded: 1.- In the range of thiosulfate and EDTA concentrations studied (0.1 M, 0.2 M and 0.3 M for thiosulfate and 1 × 10−4 M, 0.025 M and 0.05 M for EDTA), the silver leaching kinetics is enhanced by the lowest EDTA concentration, reaching near 100% of extraction in the first 20 min of leaching. In the same way the leaching solution with 0.1 M thiosulfate presented faster leaching kinetics than with 0.2 or 0.3 M thiosulfate. A synergistic effect in the silver leaching kinetics was found when the EDTA and thiosulfate concentrations were decreased to 0.1 and 1.25 × 10−4 M respectively. In these leaching conditions the silver precipitation is minimized, suggesting that this system is more stable than when the thiosulfate concentration is higher. 2.- The precipitation of copper oxides occurs for the leaching systems with 1.25 × 10 −4 and 0.025 M EDTA. According to the SEM results, this precipitation takes place on the surface of the metallic silver particle. On the other hand, when the EDTA concentration is increased up to 0.05 M, the process is controlled by the silver dissolu-
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tion reaction: there is no formation of a copper oxide layer on the metallic silver particle surface. 3.- In the leaching systems with EDTA concentrations lower than 0.025 M, the precipitation of copper sulfides species also occurs. These copper sulfide species proceed from the silver–thiosulfate complex. 4.- Finally, the slowest silver leaching kinetics was found when the cupric ion concentration was decreased from 0.05 M to 7.87 × 10−4 M. This behavior is related to the precipitation of copper oxides on the silver particle surface, as well as to the decrease in the oxidizing ability of the leaching system. Acknowledgments The authors are grateful to CONACYT (Mexico) for the postgraduate scholarship awarded to Damaris M. Puente-Siller. The authors also wish to thank Dr. Alejandro Uribe-Salas, Dr. Roberto Pérez-Garibay and M.Sc. Juan Antonio González-Anaya for their continued support regarding the laboratory materials. References Abbruzzese, C., Fornari, P., Massidda, R., Veglió, F., Ubaldini, S., 1995. Thiosulphate leaching for gold hydrometallurgy. Hydrometallurgy 39, 265–276. Alonso-Gómez, A.R., Lapidus, G.T., 2009. Inhibition of lead solubilization during the leaching of gold and silver in ammoniacal thiosulfate solutions (effect of phosphate addition). Hydrometallurgy 99, 89–96. Aylmore, M.G., Muir, D.M., 2001. Thiosulfate leaching of gold. A review. Miner. Eng. 14, 135–174. Eriksson, G., 1979. An algorithm for the computation of aqueous multicomponent, multiphase equilibria. Anal. Chim. Acta 112, 375–383. Feng, D., Van Deventer, J.S.J., 2006. Ammoniacal thiosulphate leaching of gold in the presence of pyrite. Hydrometallurgy 82 (3–4), 126–132. Feng, D., Van Deventer, J.S.J., 2007. Effect of hematite on thiosulphate leaching of gold. Int. J. Miner. Process. 82, 138–147. Feng, D., Van Deventer, J.S.J., 2010. Thiosulphate leaching of gold in the presence of ethylendiaminetetraacetic acid (EDTA). Miner. Eng. 23, 143–150. Feng, D., Van Deventer, J.S.J., 2011. The role of amino acids in the thiosulphate leaching of gold. Miner. Eng. 24, 1022–1024. Fuentes-Aceituno, J.C., Frade-Chávez, J.G., Mendoza-Hernández, R.L., Lapidus-Lavine, G.T., 2004. Pregrobbing en los sistemas de tiosulfato y tiourea. XIV Congreso Internacional de Metalurgia Extractiva (Pachuca, Hidalgo, México). Heath, F.A., Jeffrey, M.I., Zhang, H.G., Rumball, J.A., 2008. Anaerobic thiosulfate leaching: development of in situ gold leaching systems. Miner. Eng. 21 (6), 424–433. Labanowski, J., Monna, F., Bermond, A., Cambier, P., Fernandez, C., Lamy, I., Van Oort, F., 2008. Kinetic extractions to assess mobilization of Zn, Pb, Cu, and Cd in a metalcontaminated soil: EDTA vs. citrate. Environ. Pollut. 152, 693–701. Levenspiel, O., 1972. Chemical Reaction Engineering, 2nd edition. John Wiley and Sons, Inc., New York, pp. 361–377. Manouchehri, N., Besancon, S., Bermond, A., 2006. Major and trance metal extraction from soil by EDTA: equilibrium and kinetic studies. Anal. Chim. Acta 559, 105–112. Puigdomenech, I., 2004. Make Equilibrium Diagrams Using Sophisticated algorithms (MEDUSA), Inorganic Chemistry. Royal Institute of Technology, Stockholm, Sweden. Senanayake, G., 2007. Review of rate constants for thiosulphate leaching of gold from ores, concentrates and flat surfaces: effect of host minerals and pH. Miner. Eng. 20, 1–15. Zhang, S., 2004. Oxidation of Refractory Gold Concentrates and Simultaneous Dissolution of Gold in Aerated Alkaline Solutions. Ph. D. Thesis. Murdoch University Western Australia, Australia. Zipperian, D., Raghavan, S., Wilson, J.P., 1988. Gold and silver extraction by ammoniacal thiosulfate leaching from a rhyolite ore. Hydrometallurgy 19 (3), 361–375.
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Hydrometallurgy journal homepage: www.elsevier.com/locate/hydromet
A kinetic–thermodynamic study of silver leaching in thiosulfate–copper–ammonia–EDTA solutions D.M. Puente-Siller, J.C. Fuentes-Aceituno ⁎, F. Nava-Alonso Centro de Investigación y de Estudios Avanzados del IPN Unidad Saltillo, Av. Industria Metalúrgica Nº 1062, Parque Industrial Ramos Arizpe, Ramos Arizpe, Coahuila, 25900, Mexico
a r t i c l e
i n f o
Article history: Received 25 April 2012 Received in revised form 31 January 2013 Accepted 14 February 2013 Available online 27 February 2013 Keywords: Leaching Thiosulfate EDTA Silver
a b s t r a c t In this research, an analysis of the effect of ethylenediaminetetraacetic acid (EDTA), thiosulfate and cupric ions on the silver leaching kinetics was performed. For that purpose leaching experiments with pure metallic silver were carried out at different concentrations of these reagents at room temperature. The results of this study showed that small amounts of EDTA accelerate the leaching of silver more efficiently than higher concentrations due to an increase in the oxidation potential of the leaching solution. These results were in accord with Pourbaix diagrams and redox potential measurements. A synergistic effect was found in the silver leaching kinetics when the thiosulfate and EDTA concentrations were both decreased. On the other hand, the silver leaching kinetics was reduced at low Cu(II) concentrations due to the decrease in the oxidizing ability of the leaching solution. Characterization by SEM and EDXS of the silver in the first minutes of leaching revealed that the silver particles were coated by a layer of copper sulfides and copper oxides. Furthermore, characterization of the solid residue during the precipitation of silver in the leaching experiment was performed by the same analytical techniques, showing the presence of silver sulfide on the unreacted silver surface. The mechanism by which the silver sulfide is precipitated was found to be related to the copper sulfide formation. It was also observed that an increase in the concentration of EDTA promotes the silver dissolution avoiding the formation of copper sulfides or oxides; in these cases the process is controlled by the chemical reaction. These observations were also supported with Pourbaix and species distribution diagrams. © 2013 Elsevier B.V. All rights reserved.
1. Introduction Cyanide leaching is effective for gold and silver extraction from minerals in aqueous solutions. The precious metals are generally recovered with cyanide as complexing agent and oxygen as oxidant (Zhang, 2004). The need of processing difficult-to-treat ores as well as the environmental risks associated to cyanide use has encouraged the research and development of new leaching systems for precious metals. The thiosulfate–copper–ammonia system is considered a promising alternative to cyanide (Alonso-Gómez and Lapidus, 2009). The method has been studied formally by different research groups since 1980 (Abbruzzese et al., 1995; Feng and Van Deventer, 2006). In this process cupric ions oxidize the precious metals while thiosulfate forms complexes with them. Ammonia is also added to stabilize the cupric ions in the alkaline solution, avoiding their precipitation as copper hydroxide (Feng and Van Deventer, 2007). Only few oxidizing agents, such as cupric ions, are adequate for working in ammoniacal solutions (Abbruzzese et al., 1995); the presence of cupric ions enhances
⁎ Corresponding author. Tel.: +52 844 438 9600x8512. E-mail address: [email protected] (J.C. Fuentes-Aceituno). 0304-386X/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.hydromet.2013.02.010
the gold dissolution kinetics by 18 to 20 times (Aylmore and Muir, 2001). The chemistry of this alternative leaching system is very complicated due to the occurrence of several chemical and electrochemical reactions during the precious metal leaching. In addition to the silver or gold oxidation reaction, some thiosulfate oxidative degradation occurs (Heath et al., 2008) involving the formation of tetrathionate and other sulfur compounds (Aylmore and Muir, 2001), as well as several silver and copper complexes with ammonia and thiosulfate. It has been reported that the leaching kinetics of silver can be favored by an increase in the thiosulfate concentration (Zipperian et al., 1988); furthermore, the silver pregnant solution can be continuously electrodeposited avoiding the silver re-adsorption in the ore (Preg-Robbing phenomenon) (Fuentes-Aceituno et al., 2004). These findings suggest that the leaching of silver in thiosulfate solutions would be a very promising alternative process; however, the high consumption of cupric ions and thiosulfate has made the thiosulfate leaching system uneconomical compared with the cyanide leaching, so the process has not been widely used on a commercial scale (Aylmore and Muir, 2001). Consequently, scientists and engineers have focused the research on increasing the oxidizing capability of this leaching system and, at the same time, trying to avoid the oxidative loss of thiosulfate. Several
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investigations have been carried out to find an additional reagent for complexing cupric ions with the aim of increasing the silver and gold leaching kinetics but avoiding the thiosulfate oxidation (Alonso-Gómez and Lapidus, 2009; Feng and Van Deventer, 2010). Several studies have shown that EDTA (ethylenediaminetetraacetic acid) is an effective complexing agent for increasing the extraction kinetics of heavy metals such as Pb, Cu, Zn and Cd (Manouchehri et al., 2006). EDTA is more efficient than other complexing agents like citrate (Labanowski et al., 2008). In the precious metals leaching with the thiosulfate system it has been demonstrated that the presence of EDTA decreases the thiosulfate oxidation rate by the formation of complexes of EDTA with the Cu(II) ions; the oxidizing potential of the leaching solution decreases (Alonso-Gómez and Lapidus, 2009; Feng and Van Deventer, 2010). It has been also established that the addition of small amounts of EDTA improves substantially the thiosulfate leaching of gold (Feng and Van Deventer, 2011); however, there is very little information as well as a lack of understanding about the phenomena that controls the silver leaching kinetics with EDTA as cupric ion complexing agent. The objective of this study is to elucidate the phenomena involved in the kinetics of silver dissolution with the thiosulfate–copper–ammonia– EDTA system, in an attempt to increase the silver recovery and to minimize the reagent consumption. 2. Materials and methods For the leaching tests the composition of the solution was changed in order to study the effect of thiosulfate, EDTA and copper on the silver leaching. Pourbaix and species predominance diagrams were constructed for the system, and the kinetic model of shrinking core was applied to the results obtained. 2.1. Leaching tests All the silver leaching experiments were performed in an agitated batch reactor (250 mL glass beaker), magnetically stirred at 400 rpm. The reactor was provided with a pH electrode (SENSOREX) and a redox potential electrode with calomel reference (Cole-Parmer). The general procedure for the leaching experiments was as follows: 200 mL of the solution containing cupric sulfate, thiosulfate, EDTA and ammonium were placed in the reactor at 25 °C. The concentrations of cupric ions, thiosulfate and EDTA were varied as follows: 7.87 × 10 −4 M and 0.05 M for the cupric sulfate; 0.1 M, 0.2 M and 0.3 M for thiosulfate; and 1.25 × 10 −4 M, 0.025 M and 0.05 M for EDTA. All solutions were prepared with reagent grade chemicals and deionized water, and the pH value was adjusted to 10.2 with ammonium hydroxide. After adjusting pH, 0.125 g of metallic silver (Alfa Aesar, 99.999%) (1.3–3.2 μm) was placed in the reactor while stirring. The pH and redox potential were measured before the silver addition and during the leaching experiments. Samples of the leach solution were withdrawn at different times during the leach period for analysis of silver by atomic absorption spectrophotometry (VARIAN Spectra AA 240). All samples for silver analysis were analyzed at the end of the experiment, and before the analysis they were kept in dark vessels to prevent the precipitation of silver by light. Once the experiment was concluded, the residue was filtered, rinsed with deionized water and left to air dry. The solid residues were characterized chemically and morphologically by SEM (Phillips XL30ESEM). The local chemical composition of the residues was determined by EDXS (EDAX Genesis). 2.2. Elaboration of the thermodynamic diagrams In order to elucidate the predominant species in each leaching system, Pourbaix and species distribution diagrams were constructed using the Medusa© Software. These diagrams are also useful for determining the oxidizing ability of the leaching systems, as well as
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the possible redox reactions that could take place. The diagrams were constructed considering the equilibrium data contained in the Hydra database of the Medusa© suite software (Puigdomenech, 2004), complemented with thermodynamic data reported by Aylmore and Muir (2001). Table 1 presents the Gibbs free energy data complemented and used in this work. The software is based in an algorithm developed by Eriksson (1979), which minimizes the Gibbs free energy of reactions in equilibrium that can occur in the aqueous leaching systems, and determines the predominating species under specific solution conditions. 3. Results and discussion The results of this study are discussed in four parts; in the first part appears the effect of the thiosulfate and EDTA concentrations on the silver leaching; the second section analyzes the kinetic data with the shrinking core model; the third section presents the thermodynamic analysis of the studied systems, and the fourth, the effect of the Cu(II) concentration on the silver leaching. 3.1. Effect of EDTA and thiosulfate concentrations on the silver leaching Fig. 1 illustrates the effect of three different EDTA concentrations (1.25 × 10−4 M, 0.025 M and 0.05 M) on the silver dissolution kinetics at room temperature. The thiosulfate and copper concentrations were kept constant at 0.2 M and 0.05 M respectively. From these experiments it was observed that the leaching system with the lowest EDTA concentration (1.25 × 10−4 M) enhanced the silver leaching kinetics. This behavior was also observed in the leaching experiments for 0.1 M and 0.3 M thiosulfate and the same EDTA concentrations (Figs. 2 and 3 respectively). The leaching solution with 0.1 M thiosulfate and 1.25 × 10−4 M EDTA reached a silver extraction of 100% in the first 20 min (see Fig. 2); after this time the silver extraction decreased to 90%, probably due to the precipitation of silver, which suggests an unstable leaching system. All the leaching experiments carried out with solutions containing 1.25 × 10−4 M and 0.025 M EDTA showed some oscillations in the silver extraction curves (Figs. 1, 2 and 3) indicating reprecipitation and, in some cases, redissolution of the silver. A possible explanation to this phenomenon is proposed by Abbruzzese et al. (1995): in the initial stage of the leaching process the dissolution of metallic silver (Eq. (1)) occurs simultaneously with the reduction of cupric ions to cuprous species (Eq. (2)). Ag þ 3S2 O32− →AgðS2 O3 Þ35− þ e
−
ð1Þ
−
CuðNH3 Þ42þ þ 3S2 O32− þ e →CuðS2 O3 Þ35− þ 4NH3
ð2Þ
Besides these reactions, other simultaneous reactions may occur such as the oxidative decomposition of thiosulfate involving the formation of additional sulfur compounds such as tetrathionate, as well as the precipitation of copper sulfides species such as CuS or Cu2S (Eqs. (3) and (4)). 2þ
Cu
2−
2−
þ
þ S2 O3 þ H2 O→CuS þ SO4 þ 2H
ð3Þ
Table 1 Gibbs free energy for thiosulfate and silver species used in the construction of thermodynamic diagrams (25 °C, Aylmore and Muir, 2001). Chemical species
ΔG°298 (kJ/mol)
S2O32−(a) Ag2S Ag2+(a) Ag+(a) Ag(S2O3)35−(a) Ag(S2O3)23−(a) Ag(S2O3)−(a) Ag2O3(c)
−532.2 −40.5 268.2 77.2 −1598.3 −1058.6 −506.3 87.0
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Fig. 1. Effect of EDTA concentration on silver leaching for a solution of 0.2 M thiosulfate (0.125 g silver, 200 mL solution, 0.05 M Cu(II), 0.6 M NH4OH, pH = 10.2, room temperature and 400 rpm).
þ
þ
2Cu þ S2 O32− þ H2 O→Cu2 S þ SO42− þ 2H
ð4Þ
Once the copper sulfide species are formed, the silver contained in the thiosulfate complex (Ag(S2O3)35−) could precipitate as silver sulfide according to Eq. (5) (Zipperian et al., 1988): Cu2 S þ 2AgðS2 O3 Þ35− →Ag2 S þ 2CuðS2 O3 Þ35− :
ð5Þ
This reaction would explain the decrease on silver concentration in the leaching curves of Figs. 1, 2 and 3. Once the silver sulfide is formed, it is possible to redissolve it by a substitution of copper by silver in the sulfide matrix, with the consequent formation of a silver thiosulfate complex in solution (Aylmore and Muir, 2001): þ
þ
2Cu þ Ag2 S→2Ag þ Cu2 S þ 6S2 O32− →2AgðS2 O3 Þ35− :
ð6Þ
In spite of the instability of the leaching systems presented in Figs. 1–3, it is possible to say that the leaching system with 0.1 M thiosulfate (Fig. 2) presents fewer oscillations (dissolution–precipitation phenomena) than the system with 0.2 M thiosulfate (Fig. 1). From a practical point of view, this leaching system with 0.1 M thiosulfate would be more interesting than the systems with 0.2 and
Fig. 2. Effect of EDTA concentration on silver leaching for a solution of 0.1 M thiosulfate (0.125 g silver, 200 mL solution, 0.05 M Cu(II), 0.6 M NH4OH, pH = 10.2, room temperature and 400 rpm).
Fig. 3. Effect of EDTA concentration on silver leaching for a solution of 0.3 M thiosulfate (0.125 g silver, 200 mL solution, 0.05 M Cu(II), 0.6 M NH4OH, pH = 10.2, room temperature and 400 rpm).
0.3 M thiosulfate because of its lower reagent consumption and higher silver leaching kinetics; nevertheless, from the perspective of chemical reaction kinetics, it could be expected that an increase in the thiosulfate concentration would enhance the silver leaching kinetics. These results suggest that in systems with 0.2 M and 0.3 M thiosulfate, thiosulfate could be decomposed more than in the system with 0.1 M. This behavior does not imply that there is no thiosulfate decomposition at 0.1 M, but that this decomposition is smaller and can only be noticed further (after 4 h in Fig. 2, for 0.025 M EDTA), when the silver leaching decreases. It can also be observed, in the systems with 0.1 M thiosulfate and 1.25 × 10 −4 M or 0.025 M EDTA, that the silver extraction reaches an almost constant maximum value of silver extraction of 92 and 80% respectively. This behavior is probably related to the precipitation of silver and copper as sulfides or oxides on the unreacted silver particle surface, generating passive layers which inhibit a complete silver extraction (see Fig. 2). The EDTA concentration affects the silver dissolution. It was observed in Figs. 1, 2 and 3 that an increase in the EDTA concentration decreases the silver extraction. This phenomenon can be explained in terms of thermodynamics and electrochemistry: the Cu (II) complex with EDTA is more stable in 0.05 M EDTA than in 0.025 M EDTA solutions. Besides, the oxidizing ability of the leaching solution decreases in the system with 0.05 M EDTA, causing the diminution in silver leaching kinetics under this condition. Further evidence of this phenomenon is presented in Fig. 4a and b, and will be discussed later. From these results three important characteristics for the thiosulfate–EDTA leaching system can be highlighted: a) a decrease in the thiosulfate and EDTA concentrations favors the silver leaching; b) the leaching solution with 0.1 M thiosulfate is more stable than the other two thiosulfate concentrations; and c) a catalytic effect was found in the silver leaching kinetics when EDTA concentration was lowered (see Figs. 1, 2 and 3). If this last statement is true, there should be a difference in the redox potentials of the leaching solutions, i.e. the redox potential should become more positive when the EDTA is decreased, revealing that the oxidizing ability of the leaching solution is enhanced, and therefore the silver dissolution kinetics could be favored. Redox potential measurements were performed for the leaching solutions in the tests of Figs. 1–3. For example, the leaching solution with 0.025 M EDTA and 0.2 M thiosulfate had a redox potential of 0.4018 V vs SHE; when the EDTA concentration was increased to 0.05 M for the same thiosulfate concentration (0.2 M), the redox potential decreased to 0.323 V vs SHE. This difference in the redox potential indicates that the EDTA concentration has an important effect on the oxidizing ability of the leaching solution, and consequently, on the silver dissolution
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with the shrinking core model. It was found that in some cases the process was controlled by the chemical reaction. Eq. (7) is proposed for a process whose kinetics is controlled by a chemical reaction (Levenspiel, 1972): 1=3 t bks C Al =Rp ρB ¼ 1–ð1–XÞ
ð7Þ
where t = reaction time, b = reaction stoichiometric coefficient, ks = rate constant for the surface reaction, CAl = concentration of the reactive fluid species, Rp = particle radius, ρB = molar density of silver in the solid sample and X = fractional conversion (extraction) of the silver. On the other hand, Eq. (8) can be used for processes whose kinetics is controlled by the diffusion of the fluid species through a porous layer (Levenspiel, 1972): 2 2=3 t 6bDe C Al =Rp ρB ¼ 1–3ð1–XÞ þ 2ð1–XÞ
Fig. 4. Effect of the EDTA concentration on the copper species predominance and oxidizing ability of the leaching solution at 25 °C a) Pourbaix diagram for the Cu–NH3– EDTA–H2O system with 0.025 M EDTA, 0.6 M NH4OH and 0.05 M Cu(II). b) Pourbaix diagram for the same system with 0.05 M EDTA.
kinetics; i.e. if the redox potential is reduced from 0.4018 to 0.323 V vs SHE, the silver dissolution kinetics will be slower. A thermodynamic analysis with the aid of Pourbaix diagrams was carried out in order to understand better the low leaching efficiency obtained at high EDTA concentrations (0.05 M). Fig. 4 presents two predominance or Pourbaix diagrams for the Cu–EDTA–NH3–H2O system at 0.025 and 0.05 M EDTA (Fig. 4a and b respectively). In these diagrams the copper species Cu(EDTA) 2− and Cu(NH3)2+ predominate in the pH range from 1 to 10.4, while the copper oxides predominate at pH values above 10.4. It is important to mention that both species represent the half cell reduction reaction for the silver oxidation. It can be observed that the equilibrium potential for the Cu(EDTA)2−/Cu(NH3)2+ couple is modified considerably with the modification of EDTA concentration. An increase in the EDTA concentration from 0.025 to 0.05 M (Fig. 4a–b) causes a more negative equilibrium potential for the Cu(EDTA)2−/Cu(NH3)2+ couple (from −0.125 to −0.35 V vs SHE approximately). According to the kinetic theory of electrochemical reactions, it is well known that a decrease in the redox potential of a system will affect exponentially the oxidation kinetics of the chemical species, causing, in this case, the slower silver leaching kinetics observed. This is in accordance with the leaching results shown before as well as with the redox potential measurements carried out.
3.2. Kinetics of the silver leaching with 0.2 M thiosulfate In order to understand the phenomena that control the silver leaching kinetics, some silver leaching experiments were analyzed
ð8Þ
where De is the effective diffusion coefficient of the reactive fluid species through the porous layer. The silver leaching results shown in Fig. 1 for 0.2 M thiosulfate and two EDTA concentrations (0.025 M and 0.05 M) were fitted to the shrinking core model using Eqs. (7) and (8) for the initial leaching stage (30 min). It was decided to use these EDTA concentrations, and only the initial leaching data, to avoid the interference of the silver precipitation as proposed in Eq. (5). It is important to remember that at 0.025 M EDTA the copper can form oxides as shown in Fig. 4a. Furthermore the formation of copper sulfides is also possible as shown in Eqs. (3) and (4). These copper species could represent a physical barrier between the fluid reactive species and the unreacted silver sites. Therefore, Fig. 5 shows the fitted data for the leaching solution with 0.025 M EDTA (with Eq. (8)) where it can be observed that the regression line does not represent adequately the porous layer diffusion mechanism. This result suggests that the oxidizing agent (cupric ions) concentration is changing during the silver leaching due to the formation of copper oxides and sulfides. According to Senanayake (2007), one of the requirements for using the shrinking core model is that the concentration of reagents responsible for the surface reaction remains constant; therefore a different kinetic model is necessary to represent better the silver leaching in this dynamic system (0.025 M EDTA). On the other hand, the data of the silver leaching with 0.2 M thiosulfate and 0.05 M EDTA were fitted to Eq. (7) and can be seen in Fig. 6. In this case, the leaching kinetics is controlled by the silver dissolution chemical reaction (Eq. (1)). This result is interesting: if the
Fig. 5. Silver leaching data fitted to Diffusion Control Model, shrinking core in spherical particles. Silver dissolution during the first 30 min of the leaching experiment (0.125 g silver, 200 mL solution, 0.2 M thiosulfate, 0.025 M EDTA, 0.05 M Cu(II), 0.6 M NH4OH, pH = 10.2, room temperature and 400 rpm).
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process is controlled by the chemical reaction, the silver dissolution is expected to occur faster for 0.05 M EDTA solutions than for 0.025 M EDTA due to the absence of copper oxides and sulfides on the silver particle surface; the presence of copper oxides or sulfides would indeed decrease the Cu (II) concentration in the leaching solution. However, the silver extraction curves of Fig. 1 showed that silver dissolution for 0.05 M EDTA reached only a 30% in 6 h. This behavior can be explained by the decrease in the oxidizing ability of the leaching solution when the EDTA is increased from 0.025 to 0.05 M, as was shown in the Pourbaix diagram of Fig. 4b. It is important to note that silver dissolution is an electrochemical nature reaction, and hence it is expected that a decrease in the redox potential of the leaching solution (when EDTA is increased from 0.025 to 0.05 M) will slow down the silver leaching kinetics. 3.3. Thermodynamic study of the Cu–NH3–EDTA–H2O, Cu–S–NH3-EDTA– H2O and Ag–S2O3–NH3–EDTA–H2O systems It was observed that the silver extraction curves showed some oscillations (precipitation–redissolution of silver). In addition to this, the precipitation of copper oxides and sulfides affects the silver leaching process with a 0.2 M thiosulfate solution containing 0.025 M EDTA. The formation of this solid species can be explained thermodynamically by the Pourbaix diagrams of Fig. 4, where the presence of copper oxides occurs at pH values of 10.2 and higher. This pH is very similar to that of the leaching conditions, suggesting that the copper oxides form on the silver particle surface causing a decrease on the silver leaching kinetics. Fig. 7 presents a species distribution diagram for the Cu–NH3– EDTA–H2O system at 25 °C, considering the next concentrations: 0.025 M EDTA, 0.05 M Cu(II), 0.6 M NH3 and a fixed potential of 0.4 V vs SHE (similar to the redox potential of the leaching solution). The copper complexation with EDTA and ammonia as well as the presence of CuO are observed at a pH of 10.2. Furthermore the copper species Cu(EDTA) 2− and CuO predominate at this pH condition. This is indicative that the copper in solution can be stabilized with EDTA, but also can precipitate as oxides. From an economic point of view this system would imply a loss of the oxidizing agent (Cu(II) complexes in solution), increasing the costs of the process. On the other hand, Fig. 8 shows the species distribution diagram under similar conditions as those of Fig. 7, but in this case the EDTA concentration was increased to 0.05 M and the potential was fixed at 0.32 V vs SHE (the redox potential measured for this leaching solution). In this diagram, the copper oxide CuO does not appear in the
Fig. 6. Silver leaching data fitted to Chemical Reaction Control Model, shrinking core in spherical particles. Silver dissolution during the first 30 min of the leaching experiment (0.125 g silver, 200 mL solution, 0.2 M thiosulfate, 0.05 M EDTA, 0.05 M Cu(II), 0.6 M NH4OH, pH = 10.2, room temperature and 400 rpm).
Fig. 7. Distribution of copper species at different pH values for the Cu–NH3–EDTA–H2O system. The conditions for the diagram construction were 0.025 M EDTA, 0.05 M Cu(II), 0.6 M NH4OH, and EH = 0.40 V at 25 °C.
solution; all the copper being complexed by the EDTA and ammonia. This diagram indicates that the low efficiency of the leaching systems shown in Figs. 1, 2 and 3 is not related to the copper oxide precipitation but to the decrease in the redox potential of the couple Cu(EDTA) 2 −/Cu(NH3)2+, as was shown in Fig. 4b. Consequently, the silver dissolution reaction is the rate determining step for the leaching system with 0.05 M EDTA, as shown in Fig. 6. The absence of CuO in the speciation diagram under this leaching condition suggests that the silver particle is not coated by any ash layer. All the silver leaching experiments with 1.25 × 10 −4 and 0.025 M EDTA presented oscillations. The oscillations were related to the dissolution–precipitation phenomenon: the dissolution of silver with thiosulfate (Eq. (1)), and the precipitation of silver sulfide (Eq. (5)). In this section further evidence is presented for a better understanding of the phenomena that cause the so-called oscillations in the leaching curves. Figs. 9, 10 and 11 show the Pourbaix diagrams for the Ag–S– EDTA–H2O systems at 0.2, 0.1 and 0.3 M thiosulfate respectively (0.025 M EDTA, 1 × 10 −5 M Ag). The equilibrium potential for the redox couple Ag(S2O3)35 −/Ag at pH 10.2 corresponds to − 0.089, − 0.044 and − 0.119 V vs SHE respectively. Furthermore, the leaching solutions for 0.1 M, 0.2 M and 0.3 M thiosulfate at 0.025 M EDTA presented a redox potential of 0.4259, 0.4018 and 0.323 V vs SHE respectively; if these redox potential operation conditions are visualized on the Pourbaix diagrams for each system, it can be seen that the silver predominant species is the complex Ag(S2O3)35− (see Figs. 9, 10 and 11). This result suggests that metallic silver is oxidized to form the complex Ag(S2O3)35− in accordance with Eq. (1). However the precipitation of the silver–thiosulfate complex to form silver sulfide is supposed to occur in these systems by the presence of copper sulfide (see Eqs. (3)–(5)). According to Abbruzzese et al. (1995) the precipitation
Fig. 8. Distribution of copper species at different pH values for the Cu–NH3–EDTA–H2O system. The conditions for the diagram construction were 0.05 M EDTA, 0.05 M Cu(II), 0.6 M NH4OH and EH = 0.32 V at 25 °C.
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Fig. 9. Pourbaix diagram for the Ag–S–EDTA–H2O system. The conditions for the diagram construction were 0.2 M thiosulfate, 0.025 M EDTA and 1 × 10−5 M Ag at 25 °C.
Fig. 11. Pourbaix diagram for the Ag–S–EDTA–H2O system. The conditions for the diagram construction were 0.3 M thiosulfate, 0.025 M EDTA, and 1 × 10−5 M Ag at 25 °C.
of copper sulfides can occur by the oxidative degradation of the thiosulfate in the leaching solution. On the other hand, the Pourbaix diagram for the Cu–S–NH3–EDTA–H2O system at 0.2 M thiosulfate and 0.025 M EDTA presented in Fig. 12a shows that the equilibrium potential for the redox couple Cu(EDTA)2−/Cu2S is −0.0632 V vs SHE at pH 10.2 approximately. These results indicate that it is possible that the reduction of Cu(EDTA)2− to Cu2S and the oxidation of Ag to Ag(S2O3)35− occur simultaneously; therefore, the copper sulfide species is also probably formed by this reduction reaction in addition to that mentioned in Eqs. (3) and (4). Once this copper sulfide species is formed during the leaching process, the silver complex Ag(S2O3)35− can precipitate to form Ag2S according to Eq. (5). Two solid samples corresponding to the leaching experiment at 0.2 M thiosulfate and 0.025 M EDTA at 10 and 50 min (Fig. 1) were characterized by SEM and EDXS (Fig. 13a, b, c and d). Fig. 13a shows a spherical morphology of the silver particles after 10 min leaching. The EDXS analysis (see Fig. 13b) revealed the presence of silver, copper, oxygen and sulfur; these last three elements could be related to the formation of copper sulfides and copper oxide species on the silver surface. These results are in accordance with the thermodynamic predictions and support the proposed idea that the silver particle is being coated by copper sulfides and oxides. Fig. 13c shows the silver particles coated by a precipitated solid after 50 min of the leaching process. The EDXS analysis (Fig. 13d) revealed the presence of silver and sulfur, which could be the result of the precipitation of the silver sulfide (Eq. (5)). Fig. 13d did not show any copper sulfide on the residue, which was expected as the copper sulfide had reacted in order to form the silver sulfide.
Contrary to the 0.025 M EDTA system, the leaching system with 0.05 M EDTA did not show oscillations in any case (Figs. 1, 2 and 3). This can be explained thermodynamically: for the leaching solution containing 0.2 M thiosulfate and 0.025 M EDTA the Pourbaix diagram of Fig. 9 predicts an equilibrium potential of −0.089 V vs SHE at pH 10.2 for the redox couple Ag(S2O3)35−/Ag; on the other hand, the diagram for the same thiosulfate concentration and 0.05 M EDTA shows that the equilibrium potential for the redox couple Cu(EDTA)2−/Cu2S is −0.112 V vs SHE at pH 10.2 (Fig. 12b). These results indicate that
Fig. 10. Pourbaix diagram for the Ag–S–EDTA–H2O system. The conditions for the diagram construction were 0.1 M thiosulfate, 0.025 M EDTA, and 1 × 10−5 M Ag at 25 °C.
Fig. 12. Effect of the EDTA concentration on the copper sulfide formation at 25 °C. a) Pourbaix diagram for the Cu–S–NH3–EDTA–H2O system with 0.2 M thiosulfate, 0.025 M EDTA, 0.6 M NH4OH and 0.05 M Cu(II). b) Pourbaix diagram for the same system with 0.05 M EDTA.
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Fig. 13. Effect of leaching time on the chemical composition of the solid residue. Experimental conditions: 0.2 M thiosulfate, 0.025 M EDTA, 0.6 M NH4OH and 0.05 M Cu(II). a) Morphology at 10 min leaching, b) chemical composition of the residue at 10 min, c) morphology at 50 min leaching, d) chemical composition for the residue at 50 min.
the reduction of Cu(EDTA) 2− to Cu2S cannot occur simultaneously with the oxidation of Ag to Ag(S2O3)35−. In addition, the redox potential measurements showed that an increase in the EDTA concentration decreases the redox potential of the solution and hence the oxidation ability of the system. Eqs. (3) and (4) show the possibility to form copper sulfide species as a consequence of the thiosulfate oxidative destruction; however the decrease in the oxidation ability of the system makes the formation of these copper sulfides less probable. These results are consistent with the findings mentioned when discussing
Fig. 14. Effect of the thiosulfate concentration on the silver extraction. Experimental conditions: 0.125 g silver in 200 mL thiosulfate solution 1.25 × 10−4 M EDTA, 0.6 M NH4OH, 7.87 × 10−4 M Cu(II) at room temperature and 400 rpm.
Fig. 6: the silver leaching under these conditions is controlled by the chemical reaction in the initial stage. 3.4. Effect of copper concentration on the silver leaching In the previous sections it was observed that a decrease of the thiosulfate and EDTA concentrations enhances the silver leaching kinetics. This section deals with the effect of decreasing the cupric ion concentration from 0.05 M to 7.87 × 10−4 M. Fig. 14 presents the silver
Fig. 15. Silver leaching data fitted to Diffusion Control Model, shrinking core in spherical particles. Silver dissolution during the first 30 min of the leaching experiment (0.125 g silver, 200 mL solution, 0.1 M thiosulfate, 1.25 × 10−4 M EDTA, 7.87 × 10−4 M Cu(II), 0.6 M NH4OH, pH = 10.2, room temperature and 400 rpm).
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leaching for the systems containing 0.1 M, 0.2 M and 0.3 M thiosulfate, 1.25 × 10−4 M EDTA and 7.87 × 10 −4 M Cu(II). All experiments showed low silver leaching values: less than 12% in 6 h. If these results are compared with those of Fig. 2 for 1.25 × 10−4 M EDTA, it is clear that the decrease in cupric ion concentration affects significantly the leaching of silver. This behavior is associated with the decrease in the oxidizing ability of the leaching solution, and is in accord with the findings of Abbruzzese et al. (1995), who observed that the presence of a high concentration of cupric ions promotes fast leaching kinetics. Some oscillations also appear in these silver extraction curves, related to the silver precipitation as silver sulfide, as mentioned in the last section. The experimental data for the silver leaching with solutions 0.1 M, 0.2 M and 0.3 M thiosulfate, 1.25 × 10 −4 EDTA and 7.87 × 10 −4 Cu(II) were fitted to the shrinking core model in the same way as in the cases of Figs. 5 and 6. The results for the first 30 min are presented in Fig. 15. This range of time was chosen to avoid the interference of any silver precipitation (passivation phenomena). From Fig. 15 it can be said that during the initial stage of the leaching the kinetics of the process is controlled by the diffusion of the fluid species through the porous layer. As exposed previously, this porous layer is composed of copper oxides. The leaching process is passivated after 1 h for the tests of curves 0.2 and 0.3 M thiosulfate (Fig. 14), probably due to the loss of the oxidizing agent (cupric ions). According to Feng and Van Deventer (2010), the precipitation of cupric ions occurs in the thiosulfate systems. These results suggest that the leaching of silver would proceed faster with 0.05 M Cu(II) than with 7.87 × 10 −7 M Cu(II) (see Figs. 2 and 14). 4. Conclusions On the basis of the results obtained for the metallic silver leaching with the system thiosulfate–EDTA–Cu(II)–ammonia, the following could be concluded: 1.- In the range of thiosulfate and EDTA concentrations studied (0.1 M, 0.2 M and 0.3 M for thiosulfate and 1 × 10−4 M, 0.025 M and 0.05 M for EDTA), the silver leaching kinetics is enhanced by the lowest EDTA concentration, reaching near 100% of extraction in the first 20 min of leaching. In the same way the leaching solution with 0.1 M thiosulfate presented faster leaching kinetics than with 0.2 or 0.3 M thiosulfate. A synergistic effect in the silver leaching kinetics was found when the EDTA and thiosulfate concentrations were decreased to 0.1 and 1.25 × 10−4 M respectively. In these leaching conditions the silver precipitation is minimized, suggesting that this system is more stable than when the thiosulfate concentration is higher. 2.- The precipitation of copper oxides occurs for the leaching systems with 1.25 × 10 −4 and 0.025 M EDTA. According to the SEM results, this precipitation takes place on the surface of the metallic silver particle. On the other hand, when the EDTA concentration is increased up to 0.05 M, the process is controlled by the silver dissolu-
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tion reaction: there is no formation of a copper oxide layer on the metallic silver particle surface. 3.- In the leaching systems with EDTA concentrations lower than 0.025 M, the precipitation of copper sulfides species also occurs. These copper sulfide species proceed from the silver–thiosulfate complex. 4.- Finally, the slowest silver leaching kinetics was found when the cupric ion concentration was decreased from 0.05 M to 7.87 × 10−4 M. This behavior is related to the precipitation of copper oxides on the silver particle surface, as well as to the decrease in the oxidizing ability of the leaching system. Acknowledgments The authors are grateful to CONACYT (Mexico) for the postgraduate scholarship awarded to Damaris M. Puente-Siller. The authors also wish to thank Dr. Alejandro Uribe-Salas, Dr. Roberto Pérez-Garibay and M.Sc. Juan Antonio González-Anaya for their continued support regarding the laboratory materials. References Abbruzzese, C., Fornari, P., Massidda, R., Veglió, F., Ubaldini, S., 1995. Thiosulphate leaching for gold hydrometallurgy. Hydrometallurgy 39, 265–276. Alonso-Gómez, A.R., Lapidus, G.T., 2009. Inhibition of lead solubilization during the leaching of gold and silver in ammoniacal thiosulfate solutions (effect of phosphate addition). Hydrometallurgy 99, 89–96. Aylmore, M.G., Muir, D.M., 2001. Thiosulfate leaching of gold. A review. Miner. Eng. 14, 135–174. Eriksson, G., 1979. An algorithm for the computation of aqueous multicomponent, multiphase equilibria. Anal. Chim. Acta 112, 375–383. Feng, D., Van Deventer, J.S.J., 2006. Ammoniacal thiosulphate leaching of gold in the presence of pyrite. Hydrometallurgy 82 (3–4), 126–132. Feng, D., Van Deventer, J.S.J., 2007. Effect of hematite on thiosulphate leaching of gold. Int. J. Miner. Process. 82, 138–147. Feng, D., Van Deventer, J.S.J., 2010. Thiosulphate leaching of gold in the presence of ethylendiaminetetraacetic acid (EDTA). Miner. Eng. 23, 143–150. Feng, D., Van Deventer, J.S.J., 2011. The role of amino acids in the thiosulphate leaching of gold. Miner. Eng. 24, 1022–1024. Fuentes-Aceituno, J.C., Frade-Chávez, J.G., Mendoza-Hernández, R.L., Lapidus-Lavine, G.T., 2004. Pregrobbing en los sistemas de tiosulfato y tiourea. XIV Congreso Internacional de Metalurgia Extractiva (Pachuca, Hidalgo, México). Heath, F.A., Jeffrey, M.I., Zhang, H.G., Rumball, J.A., 2008. Anaerobic thiosulfate leaching: development of in situ gold leaching systems. Miner. Eng. 21 (6), 424–433. Labanowski, J., Monna, F., Bermond, A., Cambier, P., Fernandez, C., Lamy, I., Van Oort, F., 2008. Kinetic extractions to assess mobilization of Zn, Pb, Cu, and Cd in a metalcontaminated soil: EDTA vs. citrate. Environ. Pollut. 152, 693–701. Levenspiel, O., 1972. Chemical Reaction Engineering, 2nd edition. John Wiley and Sons, Inc., New York, pp. 361–377. Manouchehri, N., Besancon, S., Bermond, A., 2006. Major and trance metal extraction from soil by EDTA: equilibrium and kinetic studies. Anal. Chim. Acta 559, 105–112. Puigdomenech, I., 2004. Make Equilibrium Diagrams Using Sophisticated algorithms (MEDUSA), Inorganic Chemistry. Royal Institute of Technology, Stockholm, Sweden. Senanayake, G., 2007. Review of rate constants for thiosulphate leaching of gold from ores, concentrates and flat surfaces: effect of host minerals and pH. Miner. Eng. 20, 1–15. Zhang, S., 2004. Oxidation of Refractory Gold Concentrates and Simultaneous Dissolution of Gold in Aerated Alkaline Solutions. Ph. D. Thesis. Murdoch University Western Australia, Australia. Zipperian, D., Raghavan, S., Wilson, J.P., 1988. Gold and silver extraction by ammoniacal thiosulfate leaching from a rhyolite ore. Hydrometallurgy 19 (3), 361–375.