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Electrochemical hysteresis and bistability in chalcopyrite passivation
Hydrometallurgy 105 (2010) 140–147
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Hydrometallurgy j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / h yd r o m e t
Electrochemical hysteresis and bistability in chalcopyrite passivation Gonzalo Viramontes-Gamboa a,⁎, Marycarmen M. Peña-Gomar a, David G. Dixon b a b
Facultad de Ciencias Físico-Matemáticas, UMSNH, Campus Universitario, Edificio L, Francisco J. Mujica s/n, C.P. 58000, Morelia, Michoacán México Department of Materials Engineering, University of British Columbia, 309-6350 Stores Road, Vancouver, BC, Canada V6T 1Z4
a r t i c l e
i n f o
Article history: Received 8 February 2010 Received in revised form 25 August 2010 Accepted 25 August 2010 Available online 31 August 2010 Keywords: Chalcopyrite Passivation Voltammetry Electrochemistry
a b s t r a c t A literature review of studies where the kinetic response of chalcopyrite leaching has been investigated as a function of controlled slurry potential, and the results of potentiostatic anodic polarizations, show that this mineral passivates at potentials higher than either ~ 440 mV or ~ 510 mV(SCE). These two values have been consistently observed in leaching practice regardless of the geological source of the mineral, which indicates that its passivation potential is little affected by impurity content. The factors or variables determining which one of these two passivation potentials will actually be observed remain unexplained. This paper uses steady-state sampled-current voltammetry to demonstrate that chalcopyrite active–passive behavior at high temperatures displays considerable hysteresis in response to an externally applied potential. When it is polarized in the positive (anodic) direction from the open circuit potential, the active– passive transition is observed at ~ 510 mV. When polarized in the negative (cathodic) direction, from high passive potentials to the open circuit potential, the passive–active transition (depassivation) is observed at ~ 440 mV. Hence, a potential range exists, approximately between 440 and 510 mV, where chalcopyrite presents thermodynamic bistability, and can be either passive or active depending on how it was brought to that particular potential. © 2010 Elsevier B.V. All rights reserved.
Due to the economic importance of chalcopyrite, which constitutes most of the world's copper reserves, hydrometallurgical alternatives to smelting have been vigorously pursued by the world's major copper producers. The perennial problem with chalcopyrite leaching is passivation at high solution potentials. The leaching response of chalcopyrite to various oxidative solutions of industrial relevance strongly depends on the redox potential. Particularly, in Fe(III)/Fe(II) sulfate leaching media, chalcopyrite displays classical active–passive and passive–transpassive transitions as observed in passivating metals. According to both leaching and electrochemical experiments, a passivation potential, Epp, exists which demarcates the transition between the active and passive states, delineating the boundary between the regions of potential where chalcopyrite leaches effectively and where it does not. Due to the crucial importance of Epp, the objective of many recent studies in the field of copper hydrometallurgy has been the determination of exactly where chalcopyrite passivation begins and its behavior with different variables; both controllable, such as temperature and acidity, and uncontrollable, such as the concentration of dissolved impurities and ore composition. Both leaching and electrochemical experiments have been used to obtain this information.
In this paper we first present a literature review of studies where the kinetics of chalcopyrite dissolution have been determined as a function of redox potential (in leaching experiments) and electrode potential (in electrochemical experiments). The main conclusion obtained from the review is that some naturally occurring chalcopyrite samples passivate at ~440 mV (SCE)1 while others passivate at ~ 510 mV1. The fact that two different values for Epp have been consistently reported is an industrially relevant and intriguing question. It is of major fundamental and practical importance to know whether a particular ore of chalcopyrite can be polarized in a leaching operation up to 510 mV or not. These two values are consistently observed in leaching practice for many different geological sources of the mineral, which indicates that Epp is largely unaffected by the type or content of solid state impurities. This is a remarkable fact, since the response of semiconductor materials is commonly very sensitive to impurities. Thereafter we demonstrate, using electrochemical techniques, that the active–passive transition of chalcopyrite displays bistability and hysteresis. Epp is sensitive to the direction of polarization; transitions from active to passive (passivation) and from passive to active (depassivation) do not take place at the same potential. The objective of the electrochemical experiments was to study the oxidative behavior of chalcopyrite in order to compare the passivation potentials obtained by electrochemical methods with those reported in leaching
⁎ Corresponding author. Tel./fax: + 52 443 316 7257. E-mail addresses: [email protected] (G. Viramontes-Gamboa), [email protected] (D.G. Dixon).
1 Every potential value along the paper is referred to the Standard Calomel Electrode (SCE), unless other scale is being explicitly indicated.
1. Introduction
0304-386X/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.hydromet.2010.08.012
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experiments. When the potential of chalcopyrite is increased from the active state, passivation at temperatures equal to or higher than 50 °C is observed around 510 to 515 mV. However if chalcopyrite is already passivated and the potential is decreased to bring it back to the active state, it reactivates around 440 mV. This hysteresis offers an explanation to the existence of two different values of Epp. Phenomenologically this is similar to the solid–liquid transition of agar (Selby and Selby, 1999) or the externally induced magnetization of ferromagnetic materials (Stöhr and Siegmann, 2006). When agar is heated from room temperature it melts at 85 °C, but when the temperature is decreased the molten agar remains liquid down to 40 °C. Between 40 and 85 °C agar can be either solid or liquid, depending on how it was brought to that temperature. Similarly, between 440 and 515 mV chalcopyrite can be either passive or active, depending on how it was brought to that potential. The last part of the paper discusses the relevance of chalcopyrite bistability and hysteresis to leaching practice. The hysteresis and passivation bistability of chalcopyrite have not been reported previously. Besides being industrially relevant these two phenomena are fundamental to understanding chalcopyrite leaching and passivation behavior. The reasons causing such bistability and hysteresis can only be a matter of conjecture and are beyond the scope of this study. 2. A brief review of the effect of potential on chalcopyrite leaching The objective of this review is to collect the passivation potentials observed in different studies of chalcopyrite oxidation in ferric sulfate media, to establish which are the major factors and variables affecting its behavior. The review includes results from both electrochemical and leaching experiments. Some of the leaching studies implemented very accurate potential controlling techniques, which allow them to report very precise Epp's (Kametani and Aoki, 1985; Rivera-Vasquez et al., 2006; Stott et al., 2003; Viramontes-Gamboa et al., 2007). Others report Epp's reflecting more the authors' own experience than the result of carefully conducted experiments (Ahonen and Tuovinen, 1993; Ballester and Córdoba, 2005; Córdoba et al., 2009; Hunter, 2006; Pinches et al., 2001a and 2001b). Credit is given to both approaches. Each value for Epp will be shown as a point in Fig. 1, which is a kind of comprehensive map having the concentration of H2SO4 in the abscissa and Epp in the ordinate, with five isotherms at 25, 40, 50, 60 and 80 °C. 2.1. Leaching and electrochemistry The white open symbols joined by solid lines in Fig. 1 represent the passivation potentials recently reported by Viramontes-Gamboa et al. (2007) who undertook a comprehensive effort to determine by electrochemical methods how Epp was affected by temperature and acidity. In their study potentiostatic anodic curves displaying a clear active–passive transition were systematically generated (using the same potentiostatic techniques described in this paper) at the five different temperatures and acid concentrations represented in Fig. 1. They concluded that chalcopyrite passivation onsets at one of two values, either ~440 at temperatures lower than 50 °C or ~515 mV at higher temperatures. 50 °C was identified as a transition temperature between these two Epp values. Acid concentration was reported not to have a significant effect on Epp, except at 50 °C where it drives the transition. All their anodic curves were generated scanning the potential in the positive direction, starting 30 mV above EOCP and passing through the passive and transpassive states. They did not study chalcopyrite depassivation. Kametani and Aoki (1985) authored one of the first papers reporting the effect of controlled solution potential on chalcopyrite leaching. The authors controlled the slurry potential from 300 to 650 mV using KMnO4 for ferric regeneration, at irregular steps ranging from 20 to 50 mV. Their experiments were conducted at 90 °C and
Fig. 1. Behavior of the passivation potential of chalcopyrite with acid and temperature. Open symbols indicate the electrochemical predictions of (Viramontes-Gamboa et al., 2007). Filled symbols represent passivation potentials taken from literature and measured under many different conditions (refer to the text for details).
1.0 M of H2SO4. In that study it was reported that chalcopyrite leaching reached a maximum rate at 400 to 430 mV, and that a marked decrease takes place at around 450 mV. In the current context of chalcopyrite oxidation, this value indicates the onset of passivation. In Fig. 1, Kametani and Aoki's suggestion is taken as 440 mV, joined to a 20 mV error bar, which indicates the accuracy of their potential control. Their study can be considered to be one of the most important contributions to the understanding of chalcopyrite leaching. When the literature is reviewed, it becomes obvious that their work has been very influential, and consequently many people take 450 mV as the potential for chalcopyrite passivation. However it is very important to notice that the authors never reported the true extraction of copper, but only the consumption of KMnO4. They assumed that copper extraction can be deducted from, and was directly related to, the consumption of permanganate, which is not necessarily true. In a similar study, RiveraVasquez et al. (2006) and Viramontes-Gamboa et al. (2006) leached chalcopyrite concentrates at 80 °C and 30 g/L of H2SO4, controlling the solution potential from 430 to 700 mV at steps from 20 to 30 mV. KMnO4 was also used as the oxidant for ferric regeneration. The passivation potential reported in their study was 510 mV, with the size of the passive window around 65 mV. Rivera-Vasquez et al. (2006) also showed that under conditions similar to those used by Kametami and Aoki a very considerable amount of potassium jarosite is precipitated. Interestingly, the consumption of KMnO4 did not reflect the extraction of copper, as Kametani and Aoki assumed. Even more, the trends followed by KMnO4 consumption and copper extraction with solution potential were very different. It is quite possible that Kametami and Aoki's curves, which only indicate KMnO4 consumption and were not corrected by jarosite precipitation, are not an accurate indicator of copper extraction. Ballester and Córdoba (2005) reported a series of chemical leaching experiments at different initial solution potentials ranging from 255 to 555 mV at steps of 100 mV. Although the authors did not control the
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potential, but allowed it to evolve freely from its initial value, they were able to show that chalcopyrite leaching is disfavored above a certain value of solution potential. Their best copper recoveries were obtained at an initial potential of 455 mV at 35 °C and 355 mV at 68 °C. The authors tested only four initial potentials, and their technique does not allow precise identification of the potential above which chalcopyrite passivates. However from the behavior of the potential with time (which asymptotically drifted towards a final common value, independent of the initial value) and its correlation with the leaching rate, the authors report that at 68 °C the leaching of chalcopyrite stops at potentials around 455 mV. This value is indicated in Fig. 1 as representing Ballester and Córdoba's observations, with no error bars since potential was actually not controlled. In a more recent study, also conducted at 68 °C, Córdoba et al. (2009) associated chalcopyrite passivation with the precipitation of jarosites, and recommended 405 mV as the best potential for bioleaching. Ahonen and Tuovinen (1993) studied the effect of redox potential on a pyrite-rich chalcopyrite-sphalerite sample. In their mini-column leaching experiments the potential was controlled by regulating the rate of aeration to the leaching solution. Their methodology did not allow accurate control around a specific set point, with only small deviation from it; instead, quite a large range of potentials was obtained. In their experiment at low potential the authors tried to control the potential at around 500 mV, by increasing the aeration when the potential decreased to lower values as a result of leaching, and ceasing aeration once that value was reached; however, the potential drifted towards ~540 mV. In their high potential experiment the potential was held around 620 mV by continuous aeration. They concluded that better extraction rates of copper are achieved at low redox potentials, 500 to 550 mV (SCE), rather than at high potentials, 600 to 650 mV. Although the authors do not identify the potential where the transition from the active dissolution to passivation takes place, their results show that leaching kinetics at low potentials is faster. In Fig. 1 their observation for better leaching is indicated at 520 mV, with a 40 mV wide error bar, indicating the low accuracy of potential control in their experiments. In 2003, Okamoto et al. (2003) presented the results of a study they conducted on the effect of solution potential on chalcopyrite leaching, both in semi-batch flask and column leaching tests. They did not control the potential, but recorded its behavior and correlated it to the leaching rate. The flask leaching tests were conducted at 30 °C and pH ~ 1.5, adjusted with sulfuric acid. Column leaching tests were conducted at the same pH; but temperature was not indicated. Their work was part of a broader project, headed by a Japanese research team, whose main claim is that chalcopyrite leaches faster at potentials lower than a critical value because it transforms to chalcocite, Cu2S, an intermediate product which is easier to leach. According to them, chalcopyrite leaches fastest when the potential lies between Eox b E b Ec, where Eox is the oxidation potential of Cu2S and Ec is the critical potential of chalcocite formation. Those potentials (in the standard hydrogen electrode scale) are given by h i 0:75 0:25 Ec = 0:681 + 0:059 log aCu2+ aFe2+ h i 0:5 Eox = 0:561 + 0:059 log aCu2+ ; Since they suggest this expression rather than a specific value, a point representing these authors' results has not been included in Fig. 1. According to this theoretical expression the passivation potential of chalcopyrite can have practically any value, depending on the activities of cupric and ferrous ions used to evaluate it. These authors are the only ones who propose a passivation potential so flexible and versatile to experimental control. In a recent study led by the same Japanese team, Hiroyoshi et al. (2007) studied the effect of many dissolved ions typically found in
actual leaching systems on the passivation potential of chalcopyrite. In their leaching experiments, conducted at room temperature and 0.1 mol/L H2SO4, the potential was not controlled, but the initial and final redox potentials were reported. Under those conditions their curves show that chalcopyrite passivation began at initial potentials as low as 330 mV and higher (with no Bi3+ or Ag+ ions). They concluded that the addition of Ag+ and Bi3+ increases Epp, while Pd2+, Hg2+, Cd2+, Zn2+, Ni2+, Co2+, and Mn2+ have no noticeable effect. In a previous publication Hiroyoshi et al. (2004) reported an electrochemical study, conducted at room temperature and 0.1 mol/L H2SO4, whose main objective was to discern the role of coexisting cupric and ferrous ions on the active–passive behavior of chalcopyrite. They concluded that chalcopyrite only displays active–passive behavior when these two ions coexist in the leaching solution, and not otherwise, and that passivation begins at around 400 mV when 0.1 mol/L each of cupric and ferrous ions are present. Viramontes-Gamboa et al. (2007) reported that Hiroyoshi's claims were irreproducible, and demonstrated that the active–passive behavior of chalcopyrite can be perfectly observed in the virtual absence of both cupric and ferrous ions. Compared with the results of others, Hiroyoshi's passivation potentials lie very low in Fig. 1. Recently, yet another paper authored by Hiroyoshi et al. (2008) studied the effect of the concentration of sulfuric acid, Fe2+ and Cu2+ ions on the passivation potential of chalcopyrite at room temperature. They again found that chalcopyrite only displays active–passive behavior when Cu2+ and Fe2+ coexist in the leaching solution, and gave the following expression (in SHE) for the best potential to leach chalcopyrite: h i h i 2+ 2+ Eoptimum = 0:691 + 0:030 log Cu + 0:013 log Fe : They also reported that the concentration of sulfuric acid has no practical effect on the passivation potential. This expression gives optimum potentials from 374 to 440 mV (SCE) at fixed [Fe2+] = 0.1 M, [H2SO4] = 0.1 M, and [Cu2+] ranging from 0.01 to 1.0 M. 2.2. Bioleaching A series of patents also include among their main claims the control of potential as a key factor for chalcopyrite leaching. Recently Hunter (2006) patented a process for the heap leaching of chalcopyrite assisted by a sulfide oxidizing bacterial culture that either does not oxidize ferrous to ferric, or is inefficient at doing so. The inventor claims that in his process a pond is required where the redox potential (and pH) of the leaching solution should be controlled at a maximum potential of 455 mV. Since the particular bacterial culture claimed in the patent does not regenerate ferric and leaching of chalcopyrite produces ferrous, the leaching reactions are conducted at this maximum potential. The purpose of this, according to the author, is to avoid chalcopyrite passivation. In Fig. 1 Hunter's potential value for optimal leaching of chalcopyrite is indicated as 455 mV. Pinches et al. (2001a,b) patented a process to bio-leach copper from chalcopyrite, whose main claim is the control of the solution potential. They claim that their technology has potential use for both stirred-tank and heap leaching. Rather than a systematic study to identify where the transition to passivation occurs, and how it behaves with temperature and acidity, the inventors loosely define the broad range of potential ranging from 350 to 450 mV as the best in their US patent, and from 335 to 435 mV in their world patent. Although they report some experiments at different pH values (1.5 to 2) and temperatures (35 to 80 °C), they do not identify the effect of these variables on the transition to passivation, and recommend 435 and 450 mV as the maximum potential, above which passivation occurs. Pinches' recommendations are indicated in Fig. 1 at 435 and 450 mV, respectively. The authors also commented that the best potential window varies for different chalcopyrite samples, but unfortunately
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they offer no discussion of the typical size of those variations and show no results. Stott et al. (2003) conducted a bioleaching study whose objective was to compare the leaching performance of 11 species of acidophilic bacteria and archaea and to understand chalcopyrite passivation mechanisms. The temperature was selected according to the bacterial species used and ranged from 30 to 70 °C. The authors did not control the solution potential at any specific set points, but allowed it to evolve freely while relating its behavior with the onset of passivation. The study concluded that chalcopyrite passivation is due to the precipitation of any one or a combination of H+, K+, or NH+ 4 jarosite. Hence the authors paid special attention to any correlation between changes in the solution potential and the observed precipitation of jarosite. In the abstract the authors mention that the highest leaching rate of chalcopyrite is achieved at high temperatures (70 °C) at potentials below 505 mV, which is mentioned as a top limit for best recoveries. This value is indicated in Fig. 1 as a representation of Stott's suggestion, however it should be kept in mind that that suggestion is based on those authors' experience rather than being based on precise control of the redox potential. Third et al. (2002) implemented a computer-controlled system to control the solution potential at constant values by regulating oxygen flow into the solution. The authors conducted both biological and chemical leaching tests, and compare the effect of ferrous and ferric concentrations on the leaching rate. Temperature was 37 °C and pH was around 1.5. They concluded that high potentials are unfavorable for chalcopyrite leaching and recommended not to exceed 375 mV. 2.3. Key conclusions from the review The points scattered all over Fig. 1 reflect the experience of many authors regarding the maximum potential for best chalcopyrite leaching. Even though Hiroyoshi's and Third's points lie at very low values, all others lie within experimental error either around 440 or 510 mV. Hence from Fig. 1 it may be concluded that, at least according to most leaching experiments carried out under very different conditions and using samples from all over the world, some chalcopyrite samples passivate at approximately 440 mV while others passivate at 510 mV (SCE). These two values have been predicted by Viramontes-Gamboa et al., (2007) using the electrochemical methodology described above in the experimental section, which demonstrates quite good agreement between leaching and electrochemical approaches. However there is an important point of disagreement which deserves deeper analysis for its understanding. According to the electrochemical results (white open symbols in Fig. 1), below 50 °C Epp is 440 mV and above 50 °C Epp is 510 to 520 mV. However it is notable that some high-temperature leaching experiments reported in the literature found passivation potentials closer to 440 mV rather than 510 mV; the study by Kametani and Aoki (1985) made at 90 °C is a good example of this. On the other hand, some studies leaching chalcopyrite close to room temperature report passivation potentials closer to 510 mV, rather than to 440 mV; the study by Ahonen and Tuovinen (1993) is a good example of this. The origin of this disagreement motivated the present study. 3. Experimental Chalcopyrite oxidative behavior was studied in sulfuric acid solutions at controlled temperature using a 1.0 L electrochemical cell with a common three electrode arrangement. The reference was a Standard Calomel Electrode placed inside a Luggin capillary, both filled with saturated KCl solution. This electrode will be used along the paper as the main reference for potential, otherwise will be explicitly indicated. Oxygen was purged from the solution before and during each experiment by continuous sparging of high-purity nitrogen.
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The working electrodes were massive samples of naturally occurring chalcopyrite from Chihuahua, Mexico, cut into nearly cubical shape and mounted on epoxy resin, with only one side of the sample exposed to the solution after being polished with carbide paper # 600. A quantitative phase analysis using the Rietveld–XRD method on ground samples detected 98.6% CuFeS2, with 1.4% SiO2 as the only impurity. The electrical contact between the chalcopyrite and the wire was made using copper welding, but conductive silver-loaded epoxy resin works equally well. Many electrodes were prepared, and only those with the lowest electrical resistivity were chosen. This selection process was made to ensure that the IR drop across the body of the electrode and the electrical contact between the mineral and the wire (which was continuously monitored) was never larger than 2 mV for each reported experiment. The potentiostat used was the PARSTAT 2273 from Princeton Applied Research. Steady-state sampled-current voltammetries only covered oxidative ranges of potentials, starting 30 mV above the open circuit potential (EOCP) and ending at 650 mV at 23 °C, at 650 and 690 mV at 50 °C (two different experiments), and at 710 mV at 80 °C. These upper limits for the potential were selected because they lie within the passive window and belong to very well passivated states at each respective temperature. Both the open circuit potential and the potential window where passivation is observed in chalcopyrite generally displace to more positive potentials with increasing temperature (Viramontes-Gamboa et. al., 2007), so the potential window analyzed was different for 23 °C, 50 °C and 80 °C. The passivation and depassivation transitions always lie within the selected range of potentials for each temperature. Electrochemical reduction and transpassive reactions of chalcopyrite were not analyzed. The program used to generate the steady-state sampled-current voltammetries consisted of the following steps: 1) EOCP is measured. 2) The first potentiostatic measurement of current vs. time begins 30 mV above EOCP, lasting exactly 1 h. 3) Immediately after this first hour, the potential is suddenly increased 10 mV (or, in some cases, 20 mV) and held there for 1 h. 4) Step 3 is repeated until the upper potential limit is reached. 5) The entire procedure is repeated in reverse, beginning at the upper potential limit and decreasing by decrements of 10 mV for one hour each until the starting potential (30 mV above EOCP) is attained. This procedure completes one whole cycle. The current recorded for each potential at the end of each hour is referred to as the “steadystate current” and is used to generate anodic dissolution curves of current vs. potential. Results are shown for 23 and 80 °C where two continuous cycles were generated. The stationary current–potential anodic curves generated by this method, according to our experience, predict the leaching kinetics of chalcopyrite very well within experimental error (results not yet published), unlike rapid positive potential scans, which are usually very far from equilibrium and bear little relevance to leaching kinetics. In this study special emphasis is placed on the effect of the direction of polarization. The steady-state voltammetries were generated by scanning the active and passive ranges of potentials both in the positive (anodic) and negative (cathodic) directions. The potential scans in the positive direction began 30 mV above EOCP and proceeded to the upper potential limit indicated above; they show the transition from the active to the passive state at 440 mV at 23 °C, and at about 510 mV at 50 and 80 °C. The potential scans in the negative direction began at the upper potential limit, with a well passivated surface, and proceeded to 30 mV above EOCP; they show the depassivation process (transition from the passive to the active state) at around 440 mV at all temperatures. Interestingly the passivation and depassivation transitions are asymmetrical and do not take place at the same potential.
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4. Results and discussion Now the electrochemical results that help us to understand the causes producing two passivation potentials in the broad variety of leaching experiments represented in Fig. 1 will be discussed. 4.1. Passive behavior at room temperature Fig. 2a illustrates the active–passive behavior of chalcopyrite at 23 °C and 30 g/L of sulfuric acid as obtained by the method of sampled-current voltammetry described above. Current was sampled after 1 h of potentiostatic polarization at steps of 10 mV. The study consisted of the generation of two uninterrupted cycles covering only the oxidative range of potentials. The scan was begun in the positive direction 30 mV above EOCP and taken up to 630 mV, where the scanning direction was reversed and brought down to 360 mV, where it was reversed again and taken up to 560 mV, where it was finally reversed one last time and taken down to 340 mV. The two scans in the positive direction are very similar, showing an active range where the current increases exponentially with potential following the Tafel relationship up to about 440 mV. Current has its maximum value at this potential. At potentials higher than 440 mV the currents decrease with increasing potential, gradually approaching the passive state. The two potential scans in the negative direction began at potentials within the passive range, just before the sudden onset of transpassivity. Essentially both of them have the same behavior, which resembles the curves scanned in the positive direction. The only
difference is that the curves scanned in the negative direction have currents slightly lower than those scanned in the positive direction. Everything else behaves the same, regardless of the scanning direction. Fig. 2b shows the behavior with time of the currents scanned in the positive (at the top) and negative (at the bottom) directions at five selected potentials (400, 440, 480, 520, and 560 mV). Upon a 10-mV step-change in potential, currents scanned in the positive direction change rapidly from their previous state, then consistently show a gradual decrease with time which gives straight lines when plotted in a log–log scale. Similarly, currents scanned in the negative direction change rapidly, then show a gradual increase with time which also gives straight lines on a log–log scale. Both curves gradually approach a common steady state, which is typically not completely reached after 1 h at potentials above the transition to passivation. Currents scanned in the positive direction approach the steady state from above, while currents scanned in the negative direction approach it from below. Hence in Fig. 2a positive and negative scans have slightly different currents because the steady state is approached asymptotically, but has not been yet reached after 1 h. However the difference is so small (around 0.3 μA/cm2) that positive and negative scans can be considered coincident with each other. This steadystate voltammetry in the oxidative range of potentials show that at room temperature the transition from the active to the passive state (passivation) takes place at the same potential than the transition from the passive to the active state (depassivation), which is around 440 mV. Hence chalcopyrite passivation and depassivation appear symmetrical. 4.2. Passive behavior at 50 °C
Fig. 2. Chalcopyrite active–passive behavior at 23 °C and 30 g/L of H2SO4; a) Two-cycle steady-state sampled-current voltammetry showing symmetrical passivation and depassivation. b) Current density vs. time behavior at selected potentials.
The symmetrical scenario between the active–passive and passive– active transitions valid at room temperatures drastically changes when temperature is increased. Fig. 3 shows the oxidative behavior of chalcopyrite at 50 °C and 30 g/L of sulfuric acid. To generate the onecycle sampled-current voltammetry results shown in Fig. 3a, EOCP was first measured, then the polarization scan was initiated at 340 mV (just 30 mV above EOCP) at steps of 10 mV per hour in the positive direction (with some initial steps of 20 mV). Polarization continued up to 690 mV within the passive range (after 28 h), where the scanning direction was reversed. The current–potential anodic curve scanned in the positive direction shows three potential ranges where chalcopyrite oxidation behaves differently. First the quasi-stationary current increases exponentially with the applied potential up to 440 mV, following the Tafel relationship as in Fig. 2. Then a tilted plateau showing two weak maxima at 440 and 510 mV is observed, with the maximum current observed at 510 mV. Finally a gradual transition to passivation begins immediately after this potential. When the direction of the potentiostatic scan is reversed at 690 mV the electrode retraces its previous behavior only in the passive range of potentials. Below 510 mV important differences arise. The potential scan in the negative direction follows its depassivating trend up to 440 mV, where the maximum current is now observed. The dual-maxima tilted plateau observable in the positive scan was not detected in the negative scan. It is also worth noting that below 440 mV the negative scan shows much more active Tafel behavior than the positive scan; almost a threefold increase. For the moment we have no explanation for this attractive result, deeper physicochemical studies of the behavior of the chalcopyrite/solution interface must be conducted to reach such an understanding, which is beyond the scope of this publication. Results in Fig. 3a show that the potential where the oxidation current is maximum, (which is by definition Epp), is strongly dependent on the scanning direction. This is 440 and 510 mV for the negative and positive potential scans, respectively. In order to examine the reproducibility of these results, the experiment was repeated at the same temperature and acidity following a slightly
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exponential behavior was observed. The scanning direction was reversed to positive at 330 mV, ultimately increasing up to 630 mV, well within the passive region. Within the active range of potentials, below 440 mV, the positive scan retraces back the same trajectory. Notably, the transition from the active to the passive state does not take place at the same potential where depassivation was observed. In the positive direction, passivation starts at around 500 mV. Although the 1-h sampled-current voltammetries in Fig. 3a and b look quite different, both of them consistently show that chalcopyrite passivation and depassivation are asymmetrical. It is also notable that hysteresis is present in both voltammetries, with the magnitude of the currents being very different between 440 and 510 mV. Fig. 3c shows the current vs. time behavior at some selected potentials (410, 440, 470, 500, 530, and 560 mV) for the experiments in Fig. 3b. In the Tafel active range (below 440 mV) the steady state is reached very quickly and currents generated in the positive and negative scans coincide with each other within a few minutes. The same happens well within the passive state. The most interesting range of potentials is observed between 440 and 510 mV, where the difference in current is the largest. At these potentials, currents generated in the positive and negative directions for the same potential differ by up to 130 μA/cm2, and they approach each other very slowly following power laws, i = i0tβ, with absolute values for β very small, ranging from 0.05 to 10−9. If current vs. time curves are plotted in a log–log scale and the trend of the last 30 min is extrapolated as a means to estimate how different their thermodynamic states are, these curves will cross each other only after very long times, on the order of 108 s (3 years). This indicates that the thermodynamic states of chalcopyrite in the positive and negative directions for a particular potential between 440 and 510 mV are very different. The origin of the difference lies within the previous history of polarization and how the electrode was brought to that potential. 4.3. Passive behavior at 80 °C Fig. 4 shows a two-cycle sampled-current voltammetry showing the effect of the polarization direction at 80 °C and 30 g/L of sulfuric acid. At this temperature chalcopyrite also displays hysteresis and asymmetry in its passivation–depassivation behavior. The maximum current is observed at two different potentials, indicating the passivation and depassivation transitions, which depend on how the electrode was polarized. The transition to passivation is observed at around 510 mV in positive potential scans, while the transition to depassivation is observed at around 460 mV in the negative potential
Fig. 3. Hysteresis in chalcopyrite active–passive behavior at 50 °C and 30 g/L H2SO4; a) Positive scan → negative scan. b) Negative scan → positive scan. c) Current density vs. time behavior.
different strategy; Fig. 3b shows the results. First a freshly polished electrode was polarized for 20 h at 650 mV (directly from EOCP) in order to bring its surface to the passive state. The inset in Fig. 3b shows in a log–log scale how the electrode gradually approaches its steadystate passive state. (Evidently, 104 s are enough for the electrode to reach steady-state; it is not strictly necessary to wait for 28 h to go from 340 to 690 mV.) Once the electrode was passivated a negative potential scan was initiated, beginning at 650 mV and decreasing to 330 mV, with the objective to evaluate first where the depassivation transition takes place. The maximum current was observed at around 450 mV – passive currents were always smaller at potentials higher than 450 mV – while at potentials lower than 450 mV active Tafel
Fig. 4. Hysteresis in chalcopyrite active–passive behavior at 50 °C and 30 g/L H2SO4; Positive scan → negative scan.
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scans. At this temperature the difference between the magnitude of the currents generated by positive and negative potential scans, respectively, between 460 and 510 mV (the hysteresis zone) can be as large as 103 μA/cm2, with the larger current observed during the positive potential scans. At this temperature the current vs. time behavior also gives curves running virtually parallel to each other, separated by hundreds of μA/cm2, which indicates how different the two thermodynamic states can be at exactly the same potential. The difference lies again in how the external driving force (electrochemical potential) was applied. Figs. 2 to 4 consistently show that at temperatures of 50 °C and higher the transitions to passivation and depassivation always occur at different potentials. This behavior was always observed and is very reproducible. However, the behavior of the current with applied potential in higher currents in the active range of potentials (below 440 mV) — after the electrode has been polarized to passivation in the positive direction the first time. Fig. 3b shows the same currents in the active (below 440 mV) and passive (above 550 mV) ranges of potentials, and very different currents only in the transition to passivation. Fig. 4 also shows important differences between 500 mV and 570 mV. We always observed a range of potential where the magnitude of the current was different, but the magnitude itself was not so reproducible, especially at 50 °C. It is possible that this current behavior with potential is related to the content of impurities at the surface of the electrode. It is well known that some properties of semiconductors, particularly electrical conductivity, are extraordinarily sensitive to the content of impurities. Misra and Fuerstenau (2005) demonstrated that nanosize particles of SiO2, the main impurity of our samples, do catalyze chalcopyrite leaching. Other impurities at concentrations below the detectable limit of Rietveld analysis, and generally associated to chalcopyrite like atomic inclusions of silver, might also contribute to change the magnitude of the electric current. 4.4. Key observations One of the most astonishing features of these experiments is that in the positive potential scans, begun just above the open circuit potential, passivation onsets at about 510 mV at high temperatures. However, in the negative potential scans imposed on a previously passivated surface, depassivation only occurs at about 440 mV. Hence between 440 and 510 mV chalcopyrite can be either active or passive, depending on the previous history of the electrode. The fact that these two passivation potentials have often been reported in independent leaching studies indicates that the observed hysteresis is not a mere electrochemical curiosity, but an important property of chalcopyrite behavior, with relevant consequences to chalcopyrite leaching. Another equally significant result is that in the active range of potentials at temperatures higher than 50 °C, between EOCP and 440 mV, chalcopyrite electrodes generally become much more active after the first potential scan. In chalcopyrite leaching experiments in sulfate media, a kind of incubation period is often observed after which the leaching rate increases considerably. Electrochemical hysteresis, similar to that reported here for the first time for chalcopyrite, is also displayed by other compounds, such as Ni(OH)2 (Srinivasan et al., 2001), which also shows different potentials for passivation and depassivation transitions. A comparison between the curves in Fig. 4 and those presented by Stöhr and Siegmann, (2006) and by Srinivasan et al., (2001) shows that the hysteresis loops for almost pure iron and Ni(OH)2 are smoother and more well-behaved than hysteresis loops for chalcopyrite. For chalcopyrite, different passivation potentials have been reported by many different authors, but the pattern we can observe is that most studies agree well with two different values, which are close to 440 and 510 mV. According to our results these two potentials
arise naturally as a consequence of chalcopyrite bistability and hysteresis. Although at this point the relevance of this phenomenon to potential control is clear, its relevance to leaching kinetics is less clear, since oxidation currents measured electrochemically do not necessarily produce reproducible patterns, as shown in Fig. 3a and b. Further comparison between very well controlled leaching and electrochemical experiments are required in order to define the effects of passivation loops to leaching kinetics. 5. Conclusions A literature review has shown that the oxidative leaching of some chalcopyrite concentrates in ferric sulfate media drastically slows down at above about 440 mV, while others are well active at that potential and only slow down above about 510 mV. This study demonstrates that this is a consequence of the fact that chalcopyrite passivation displays bistability and hysteresis. Below 440 mV chalcopyrite is always in its active state, regardless of the geological source of the mineral, acidity or temperature. In the range of potentials between 440 and 510 mV chalcopyrite is a bistable system, and can be passive or active depending on how it was brought to that potential. Above about 510 mV and below the transpassive potential, chalcopyrite is always passive and leaching is very slow. In a bistable system many variables can be the determining factors driving the system to one of the two available states. One of these factors, which can be easily controlled, is the route by which the slurry is brought to the desired potential for leaching. Acknowledgments The authors acknowledge the supporters of the Natural Sciences and Engineering Research Council of Canada (NSERC) (Discovery Grant 155122-02), as well as PROMEP PTC-222 (Mexico), and CONACyT (Mexico) Proyecto de Ciencia Básica 2007 # 79608 for providing the financial support to carry out this research. References Ahonen, L., Tuovinen, O., 1993. Redox potential-controlled bacterial leaching of chalcopyrite ores. In: Torma, A., Wey, J., Lakshmanan, V. (Eds.), Biohydrometallurgical Technologies, vol. 1. TMS, Warrendale, pp. 571–578. Ballester, A., Córdoba, E., 2005. Hidrometalurgia de la calcopirita. In: Menacho, J., Casas de Prada, J. (Eds.), Hydrocopper 2005. Universidad de Chile, Santiago, pp. 19–41. Córdoba, E.M., Muñoz, J.A., Blázquez, M.L., González, F., Ballester, A., 2009. Passivation of chalcopyrite during its chemical leaching with ferric ion at 68 °C. Miner. Eng. 22, 229–235. Hiroyoshi, N., Kuroiwa, S., Miki, H., Tsunekawa, M., Hirajima, T., 2004. Synergistic effect of cupric and ferrous ions on active–passive behavior in anodic dissolution of chalcopyrite in sulfuric acid solutions. Hydrometallurgy 74, 103–116. Hiroyoshi, N., Kuroiwa, S., Miki, H., Tsunekawa, M., Hirajima, T., 2007. Effects of coexisting metal ions on the redox potential dependence of chalcopyrite leaching in sulfuric acid solutions. Hydrometallurgy 87, 1–10. Hiroyoshi, N., Kitagawa, H., Tsunekawa, M., 2008. Effect of solution composition on the optimum redox potential for chalcopyrite leaching in sulfuric acid solutions. Hydrometallurgy 91, 144–149. Hunter, C., 2006. Method for the bacterially assisted heap leaching of chalcopyrite. US Patent No. 7,022,504. Kametani, H., Aoki, A., 1985. Effect of suspension potential on the oxidation rate of copper concentrate in a sulfuric acid solution. Metall. Trans. B 16B, 695–705. Misra, M., Fuerstenau, M.C., 2005. Chalcopyrite leaching at moderate temperature and ambient pressure in the presence of nanosize silica. Miner. Eng. 18 (3), 293–297. Okamoto, H., Nakayama, R., Tsunekawa, M., Hiroyoshi, N., 2003. Improvement of chalcopyrite leaching in acidic sulfate solutions by redox potential control. In: Riveros, P., Dixon, D., Dreisinger, D., Menacho, J. (Eds.), Copper 2003, vol. VI. CIM, Montreal, Quebec, Canada, pp. 67–81. Pinches, A., Gericke, M., van Rooyen, J., 2001a. A method of operating a bioleach process with control of redox potential, World Patent, WO01/31072 A1. Pinches, A., Myburgh P., van der Merwe, C., 2001b. Process for the rapid leaching of chalcopyrite in the absence of catalyst. U.S. Patent No 6,277,341 B1. Rivera-Vasquez, B.F., Viramontes-Gamboa, G., Dixon, D.G., 2006. Leaching of chalcopyrite concentrates in acidic ferric sulfate media at controlled redox potentials. In: Domic, E., Casas de Prada, J. (Eds.), Gecamin, Santiago, Chile, pp. 287–299.
G. Viramontes-Gamboa et al. / Hydrometallurgy 105 (2010) 140–147 Selby, Selby, 1999. Agar. In: Whister (Ed.), Industrial Gums. Academic Press Inc., New York, N.Y. Srinivasan, V., Weidner, J.W., Newman, J., 2001. Hysteresis during cycling of nickel hydroxide active material. J. Electrochem. Soc. 148, A969–A980. Stöhr, J., Siegmann, H.C., 2006. Magnetism, from Fundamentals to Nanoscale Dynamics. Springer-Verlag. chapter 11. Stott, M., Sutton, D., Watling, H., Franzmann, P., 2003. Comparative leaching of chalcopyrite by selected acidophilic bacteria and archaea. Geo. J. 20, 215–230.
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Third, K., Cord-Ruwisch, R., Watling, H., 2002. Control of the redox potential by oxygen limitation improves bacterial leaching of chalcopyrite. Biotechnol. Bioeng. 78, 433–441. Viramontes-Gamboa, G., Rivera-Vasquez, B., Dixon, D.G., 2006. The active-to-passive transition of chalcopyrite. ECS Trans. 2, 165–175. Viramontes-Gamboa, G., Rivera-Vasquez, B.F., Dixon, D.G., 2007. The active–passive behavior of chalcopyrite, comparative study between electrochemical and leaching responses. J. Electrochem. Soc. 154 (6), C299–C311.
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Hydrometallurgy j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / h yd r o m e t
Electrochemical hysteresis and bistability in chalcopyrite passivation Gonzalo Viramontes-Gamboa a,⁎, Marycarmen M. Peña-Gomar a, David G. Dixon b a b
Facultad de Ciencias Físico-Matemáticas, UMSNH, Campus Universitario, Edificio L, Francisco J. Mujica s/n, C.P. 58000, Morelia, Michoacán México Department of Materials Engineering, University of British Columbia, 309-6350 Stores Road, Vancouver, BC, Canada V6T 1Z4
a r t i c l e
i n f o
Article history: Received 8 February 2010 Received in revised form 25 August 2010 Accepted 25 August 2010 Available online 31 August 2010 Keywords: Chalcopyrite Passivation Voltammetry Electrochemistry
a b s t r a c t A literature review of studies where the kinetic response of chalcopyrite leaching has been investigated as a function of controlled slurry potential, and the results of potentiostatic anodic polarizations, show that this mineral passivates at potentials higher than either ~ 440 mV or ~ 510 mV(SCE). These two values have been consistently observed in leaching practice regardless of the geological source of the mineral, which indicates that its passivation potential is little affected by impurity content. The factors or variables determining which one of these two passivation potentials will actually be observed remain unexplained. This paper uses steady-state sampled-current voltammetry to demonstrate that chalcopyrite active–passive behavior at high temperatures displays considerable hysteresis in response to an externally applied potential. When it is polarized in the positive (anodic) direction from the open circuit potential, the active– passive transition is observed at ~ 510 mV. When polarized in the negative (cathodic) direction, from high passive potentials to the open circuit potential, the passive–active transition (depassivation) is observed at ~ 440 mV. Hence, a potential range exists, approximately between 440 and 510 mV, where chalcopyrite presents thermodynamic bistability, and can be either passive or active depending on how it was brought to that particular potential. © 2010 Elsevier B.V. All rights reserved.
Due to the economic importance of chalcopyrite, which constitutes most of the world's copper reserves, hydrometallurgical alternatives to smelting have been vigorously pursued by the world's major copper producers. The perennial problem with chalcopyrite leaching is passivation at high solution potentials. The leaching response of chalcopyrite to various oxidative solutions of industrial relevance strongly depends on the redox potential. Particularly, in Fe(III)/Fe(II) sulfate leaching media, chalcopyrite displays classical active–passive and passive–transpassive transitions as observed in passivating metals. According to both leaching and electrochemical experiments, a passivation potential, Epp, exists which demarcates the transition between the active and passive states, delineating the boundary between the regions of potential where chalcopyrite leaches effectively and where it does not. Due to the crucial importance of Epp, the objective of many recent studies in the field of copper hydrometallurgy has been the determination of exactly where chalcopyrite passivation begins and its behavior with different variables; both controllable, such as temperature and acidity, and uncontrollable, such as the concentration of dissolved impurities and ore composition. Both leaching and electrochemical experiments have been used to obtain this information.
In this paper we first present a literature review of studies where the kinetics of chalcopyrite dissolution have been determined as a function of redox potential (in leaching experiments) and electrode potential (in electrochemical experiments). The main conclusion obtained from the review is that some naturally occurring chalcopyrite samples passivate at ~440 mV (SCE)1 while others passivate at ~ 510 mV1. The fact that two different values for Epp have been consistently reported is an industrially relevant and intriguing question. It is of major fundamental and practical importance to know whether a particular ore of chalcopyrite can be polarized in a leaching operation up to 510 mV or not. These two values are consistently observed in leaching practice for many different geological sources of the mineral, which indicates that Epp is largely unaffected by the type or content of solid state impurities. This is a remarkable fact, since the response of semiconductor materials is commonly very sensitive to impurities. Thereafter we demonstrate, using electrochemical techniques, that the active–passive transition of chalcopyrite displays bistability and hysteresis. Epp is sensitive to the direction of polarization; transitions from active to passive (passivation) and from passive to active (depassivation) do not take place at the same potential. The objective of the electrochemical experiments was to study the oxidative behavior of chalcopyrite in order to compare the passivation potentials obtained by electrochemical methods with those reported in leaching
⁎ Corresponding author. Tel./fax: + 52 443 316 7257. E-mail addresses: [email protected] (G. Viramontes-Gamboa), [email protected] (D.G. Dixon).
1 Every potential value along the paper is referred to the Standard Calomel Electrode (SCE), unless other scale is being explicitly indicated.
1. Introduction
0304-386X/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.hydromet.2010.08.012
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experiments. When the potential of chalcopyrite is increased from the active state, passivation at temperatures equal to or higher than 50 °C is observed around 510 to 515 mV. However if chalcopyrite is already passivated and the potential is decreased to bring it back to the active state, it reactivates around 440 mV. This hysteresis offers an explanation to the existence of two different values of Epp. Phenomenologically this is similar to the solid–liquid transition of agar (Selby and Selby, 1999) or the externally induced magnetization of ferromagnetic materials (Stöhr and Siegmann, 2006). When agar is heated from room temperature it melts at 85 °C, but when the temperature is decreased the molten agar remains liquid down to 40 °C. Between 40 and 85 °C agar can be either solid or liquid, depending on how it was brought to that temperature. Similarly, between 440 and 515 mV chalcopyrite can be either passive or active, depending on how it was brought to that potential. The last part of the paper discusses the relevance of chalcopyrite bistability and hysteresis to leaching practice. The hysteresis and passivation bistability of chalcopyrite have not been reported previously. Besides being industrially relevant these two phenomena are fundamental to understanding chalcopyrite leaching and passivation behavior. The reasons causing such bistability and hysteresis can only be a matter of conjecture and are beyond the scope of this study. 2. A brief review of the effect of potential on chalcopyrite leaching The objective of this review is to collect the passivation potentials observed in different studies of chalcopyrite oxidation in ferric sulfate media, to establish which are the major factors and variables affecting its behavior. The review includes results from both electrochemical and leaching experiments. Some of the leaching studies implemented very accurate potential controlling techniques, which allow them to report very precise Epp's (Kametani and Aoki, 1985; Rivera-Vasquez et al., 2006; Stott et al., 2003; Viramontes-Gamboa et al., 2007). Others report Epp's reflecting more the authors' own experience than the result of carefully conducted experiments (Ahonen and Tuovinen, 1993; Ballester and Córdoba, 2005; Córdoba et al., 2009; Hunter, 2006; Pinches et al., 2001a and 2001b). Credit is given to both approaches. Each value for Epp will be shown as a point in Fig. 1, which is a kind of comprehensive map having the concentration of H2SO4 in the abscissa and Epp in the ordinate, with five isotherms at 25, 40, 50, 60 and 80 °C. 2.1. Leaching and electrochemistry The white open symbols joined by solid lines in Fig. 1 represent the passivation potentials recently reported by Viramontes-Gamboa et al. (2007) who undertook a comprehensive effort to determine by electrochemical methods how Epp was affected by temperature and acidity. In their study potentiostatic anodic curves displaying a clear active–passive transition were systematically generated (using the same potentiostatic techniques described in this paper) at the five different temperatures and acid concentrations represented in Fig. 1. They concluded that chalcopyrite passivation onsets at one of two values, either ~440 at temperatures lower than 50 °C or ~515 mV at higher temperatures. 50 °C was identified as a transition temperature between these two Epp values. Acid concentration was reported not to have a significant effect on Epp, except at 50 °C where it drives the transition. All their anodic curves were generated scanning the potential in the positive direction, starting 30 mV above EOCP and passing through the passive and transpassive states. They did not study chalcopyrite depassivation. Kametani and Aoki (1985) authored one of the first papers reporting the effect of controlled solution potential on chalcopyrite leaching. The authors controlled the slurry potential from 300 to 650 mV using KMnO4 for ferric regeneration, at irregular steps ranging from 20 to 50 mV. Their experiments were conducted at 90 °C and
Fig. 1. Behavior of the passivation potential of chalcopyrite with acid and temperature. Open symbols indicate the electrochemical predictions of (Viramontes-Gamboa et al., 2007). Filled symbols represent passivation potentials taken from literature and measured under many different conditions (refer to the text for details).
1.0 M of H2SO4. In that study it was reported that chalcopyrite leaching reached a maximum rate at 400 to 430 mV, and that a marked decrease takes place at around 450 mV. In the current context of chalcopyrite oxidation, this value indicates the onset of passivation. In Fig. 1, Kametani and Aoki's suggestion is taken as 440 mV, joined to a 20 mV error bar, which indicates the accuracy of their potential control. Their study can be considered to be one of the most important contributions to the understanding of chalcopyrite leaching. When the literature is reviewed, it becomes obvious that their work has been very influential, and consequently many people take 450 mV as the potential for chalcopyrite passivation. However it is very important to notice that the authors never reported the true extraction of copper, but only the consumption of KMnO4. They assumed that copper extraction can be deducted from, and was directly related to, the consumption of permanganate, which is not necessarily true. In a similar study, RiveraVasquez et al. (2006) and Viramontes-Gamboa et al. (2006) leached chalcopyrite concentrates at 80 °C and 30 g/L of H2SO4, controlling the solution potential from 430 to 700 mV at steps from 20 to 30 mV. KMnO4 was also used as the oxidant for ferric regeneration. The passivation potential reported in their study was 510 mV, with the size of the passive window around 65 mV. Rivera-Vasquez et al. (2006) also showed that under conditions similar to those used by Kametami and Aoki a very considerable amount of potassium jarosite is precipitated. Interestingly, the consumption of KMnO4 did not reflect the extraction of copper, as Kametani and Aoki assumed. Even more, the trends followed by KMnO4 consumption and copper extraction with solution potential were very different. It is quite possible that Kametami and Aoki's curves, which only indicate KMnO4 consumption and were not corrected by jarosite precipitation, are not an accurate indicator of copper extraction. Ballester and Córdoba (2005) reported a series of chemical leaching experiments at different initial solution potentials ranging from 255 to 555 mV at steps of 100 mV. Although the authors did not control the
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potential, but allowed it to evolve freely from its initial value, they were able to show that chalcopyrite leaching is disfavored above a certain value of solution potential. Their best copper recoveries were obtained at an initial potential of 455 mV at 35 °C and 355 mV at 68 °C. The authors tested only four initial potentials, and their technique does not allow precise identification of the potential above which chalcopyrite passivates. However from the behavior of the potential with time (which asymptotically drifted towards a final common value, independent of the initial value) and its correlation with the leaching rate, the authors report that at 68 °C the leaching of chalcopyrite stops at potentials around 455 mV. This value is indicated in Fig. 1 as representing Ballester and Córdoba's observations, with no error bars since potential was actually not controlled. In a more recent study, also conducted at 68 °C, Córdoba et al. (2009) associated chalcopyrite passivation with the precipitation of jarosites, and recommended 405 mV as the best potential for bioleaching. Ahonen and Tuovinen (1993) studied the effect of redox potential on a pyrite-rich chalcopyrite-sphalerite sample. In their mini-column leaching experiments the potential was controlled by regulating the rate of aeration to the leaching solution. Their methodology did not allow accurate control around a specific set point, with only small deviation from it; instead, quite a large range of potentials was obtained. In their experiment at low potential the authors tried to control the potential at around 500 mV, by increasing the aeration when the potential decreased to lower values as a result of leaching, and ceasing aeration once that value was reached; however, the potential drifted towards ~540 mV. In their high potential experiment the potential was held around 620 mV by continuous aeration. They concluded that better extraction rates of copper are achieved at low redox potentials, 500 to 550 mV (SCE), rather than at high potentials, 600 to 650 mV. Although the authors do not identify the potential where the transition from the active dissolution to passivation takes place, their results show that leaching kinetics at low potentials is faster. In Fig. 1 their observation for better leaching is indicated at 520 mV, with a 40 mV wide error bar, indicating the low accuracy of potential control in their experiments. In 2003, Okamoto et al. (2003) presented the results of a study they conducted on the effect of solution potential on chalcopyrite leaching, both in semi-batch flask and column leaching tests. They did not control the potential, but recorded its behavior and correlated it to the leaching rate. The flask leaching tests were conducted at 30 °C and pH ~ 1.5, adjusted with sulfuric acid. Column leaching tests were conducted at the same pH; but temperature was not indicated. Their work was part of a broader project, headed by a Japanese research team, whose main claim is that chalcopyrite leaches faster at potentials lower than a critical value because it transforms to chalcocite, Cu2S, an intermediate product which is easier to leach. According to them, chalcopyrite leaches fastest when the potential lies between Eox b E b Ec, where Eox is the oxidation potential of Cu2S and Ec is the critical potential of chalcocite formation. Those potentials (in the standard hydrogen electrode scale) are given by h i 0:75 0:25 Ec = 0:681 + 0:059 log aCu2+ aFe2+ h i 0:5 Eox = 0:561 + 0:059 log aCu2+ ; Since they suggest this expression rather than a specific value, a point representing these authors' results has not been included in Fig. 1. According to this theoretical expression the passivation potential of chalcopyrite can have practically any value, depending on the activities of cupric and ferrous ions used to evaluate it. These authors are the only ones who propose a passivation potential so flexible and versatile to experimental control. In a recent study led by the same Japanese team, Hiroyoshi et al. (2007) studied the effect of many dissolved ions typically found in
actual leaching systems on the passivation potential of chalcopyrite. In their leaching experiments, conducted at room temperature and 0.1 mol/L H2SO4, the potential was not controlled, but the initial and final redox potentials were reported. Under those conditions their curves show that chalcopyrite passivation began at initial potentials as low as 330 mV and higher (with no Bi3+ or Ag+ ions). They concluded that the addition of Ag+ and Bi3+ increases Epp, while Pd2+, Hg2+, Cd2+, Zn2+, Ni2+, Co2+, and Mn2+ have no noticeable effect. In a previous publication Hiroyoshi et al. (2004) reported an electrochemical study, conducted at room temperature and 0.1 mol/L H2SO4, whose main objective was to discern the role of coexisting cupric and ferrous ions on the active–passive behavior of chalcopyrite. They concluded that chalcopyrite only displays active–passive behavior when these two ions coexist in the leaching solution, and not otherwise, and that passivation begins at around 400 mV when 0.1 mol/L each of cupric and ferrous ions are present. Viramontes-Gamboa et al. (2007) reported that Hiroyoshi's claims were irreproducible, and demonstrated that the active–passive behavior of chalcopyrite can be perfectly observed in the virtual absence of both cupric and ferrous ions. Compared with the results of others, Hiroyoshi's passivation potentials lie very low in Fig. 1. Recently, yet another paper authored by Hiroyoshi et al. (2008) studied the effect of the concentration of sulfuric acid, Fe2+ and Cu2+ ions on the passivation potential of chalcopyrite at room temperature. They again found that chalcopyrite only displays active–passive behavior when Cu2+ and Fe2+ coexist in the leaching solution, and gave the following expression (in SHE) for the best potential to leach chalcopyrite: h i h i 2+ 2+ Eoptimum = 0:691 + 0:030 log Cu + 0:013 log Fe : They also reported that the concentration of sulfuric acid has no practical effect on the passivation potential. This expression gives optimum potentials from 374 to 440 mV (SCE) at fixed [Fe2+] = 0.1 M, [H2SO4] = 0.1 M, and [Cu2+] ranging from 0.01 to 1.0 M. 2.2. Bioleaching A series of patents also include among their main claims the control of potential as a key factor for chalcopyrite leaching. Recently Hunter (2006) patented a process for the heap leaching of chalcopyrite assisted by a sulfide oxidizing bacterial culture that either does not oxidize ferrous to ferric, or is inefficient at doing so. The inventor claims that in his process a pond is required where the redox potential (and pH) of the leaching solution should be controlled at a maximum potential of 455 mV. Since the particular bacterial culture claimed in the patent does not regenerate ferric and leaching of chalcopyrite produces ferrous, the leaching reactions are conducted at this maximum potential. The purpose of this, according to the author, is to avoid chalcopyrite passivation. In Fig. 1 Hunter's potential value for optimal leaching of chalcopyrite is indicated as 455 mV. Pinches et al. (2001a,b) patented a process to bio-leach copper from chalcopyrite, whose main claim is the control of the solution potential. They claim that their technology has potential use for both stirred-tank and heap leaching. Rather than a systematic study to identify where the transition to passivation occurs, and how it behaves with temperature and acidity, the inventors loosely define the broad range of potential ranging from 350 to 450 mV as the best in their US patent, and from 335 to 435 mV in their world patent. Although they report some experiments at different pH values (1.5 to 2) and temperatures (35 to 80 °C), they do not identify the effect of these variables on the transition to passivation, and recommend 435 and 450 mV as the maximum potential, above which passivation occurs. Pinches' recommendations are indicated in Fig. 1 at 435 and 450 mV, respectively. The authors also commented that the best potential window varies for different chalcopyrite samples, but unfortunately
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they offer no discussion of the typical size of those variations and show no results. Stott et al. (2003) conducted a bioleaching study whose objective was to compare the leaching performance of 11 species of acidophilic bacteria and archaea and to understand chalcopyrite passivation mechanisms. The temperature was selected according to the bacterial species used and ranged from 30 to 70 °C. The authors did not control the solution potential at any specific set points, but allowed it to evolve freely while relating its behavior with the onset of passivation. The study concluded that chalcopyrite passivation is due to the precipitation of any one or a combination of H+, K+, or NH+ 4 jarosite. Hence the authors paid special attention to any correlation between changes in the solution potential and the observed precipitation of jarosite. In the abstract the authors mention that the highest leaching rate of chalcopyrite is achieved at high temperatures (70 °C) at potentials below 505 mV, which is mentioned as a top limit for best recoveries. This value is indicated in Fig. 1 as a representation of Stott's suggestion, however it should be kept in mind that that suggestion is based on those authors' experience rather than being based on precise control of the redox potential. Third et al. (2002) implemented a computer-controlled system to control the solution potential at constant values by regulating oxygen flow into the solution. The authors conducted both biological and chemical leaching tests, and compare the effect of ferrous and ferric concentrations on the leaching rate. Temperature was 37 °C and pH was around 1.5. They concluded that high potentials are unfavorable for chalcopyrite leaching and recommended not to exceed 375 mV. 2.3. Key conclusions from the review The points scattered all over Fig. 1 reflect the experience of many authors regarding the maximum potential for best chalcopyrite leaching. Even though Hiroyoshi's and Third's points lie at very low values, all others lie within experimental error either around 440 or 510 mV. Hence from Fig. 1 it may be concluded that, at least according to most leaching experiments carried out under very different conditions and using samples from all over the world, some chalcopyrite samples passivate at approximately 440 mV while others passivate at 510 mV (SCE). These two values have been predicted by Viramontes-Gamboa et al., (2007) using the electrochemical methodology described above in the experimental section, which demonstrates quite good agreement between leaching and electrochemical approaches. However there is an important point of disagreement which deserves deeper analysis for its understanding. According to the electrochemical results (white open symbols in Fig. 1), below 50 °C Epp is 440 mV and above 50 °C Epp is 510 to 520 mV. However it is notable that some high-temperature leaching experiments reported in the literature found passivation potentials closer to 440 mV rather than 510 mV; the study by Kametani and Aoki (1985) made at 90 °C is a good example of this. On the other hand, some studies leaching chalcopyrite close to room temperature report passivation potentials closer to 510 mV, rather than to 440 mV; the study by Ahonen and Tuovinen (1993) is a good example of this. The origin of this disagreement motivated the present study. 3. Experimental Chalcopyrite oxidative behavior was studied in sulfuric acid solutions at controlled temperature using a 1.0 L electrochemical cell with a common three electrode arrangement. The reference was a Standard Calomel Electrode placed inside a Luggin capillary, both filled with saturated KCl solution. This electrode will be used along the paper as the main reference for potential, otherwise will be explicitly indicated. Oxygen was purged from the solution before and during each experiment by continuous sparging of high-purity nitrogen.
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The working electrodes were massive samples of naturally occurring chalcopyrite from Chihuahua, Mexico, cut into nearly cubical shape and mounted on epoxy resin, with only one side of the sample exposed to the solution after being polished with carbide paper # 600. A quantitative phase analysis using the Rietveld–XRD method on ground samples detected 98.6% CuFeS2, with 1.4% SiO2 as the only impurity. The electrical contact between the chalcopyrite and the wire was made using copper welding, but conductive silver-loaded epoxy resin works equally well. Many electrodes were prepared, and only those with the lowest electrical resistivity were chosen. This selection process was made to ensure that the IR drop across the body of the electrode and the electrical contact between the mineral and the wire (which was continuously monitored) was never larger than 2 mV for each reported experiment. The potentiostat used was the PARSTAT 2273 from Princeton Applied Research. Steady-state sampled-current voltammetries only covered oxidative ranges of potentials, starting 30 mV above the open circuit potential (EOCP) and ending at 650 mV at 23 °C, at 650 and 690 mV at 50 °C (two different experiments), and at 710 mV at 80 °C. These upper limits for the potential were selected because they lie within the passive window and belong to very well passivated states at each respective temperature. Both the open circuit potential and the potential window where passivation is observed in chalcopyrite generally displace to more positive potentials with increasing temperature (Viramontes-Gamboa et. al., 2007), so the potential window analyzed was different for 23 °C, 50 °C and 80 °C. The passivation and depassivation transitions always lie within the selected range of potentials for each temperature. Electrochemical reduction and transpassive reactions of chalcopyrite were not analyzed. The program used to generate the steady-state sampled-current voltammetries consisted of the following steps: 1) EOCP is measured. 2) The first potentiostatic measurement of current vs. time begins 30 mV above EOCP, lasting exactly 1 h. 3) Immediately after this first hour, the potential is suddenly increased 10 mV (or, in some cases, 20 mV) and held there for 1 h. 4) Step 3 is repeated until the upper potential limit is reached. 5) The entire procedure is repeated in reverse, beginning at the upper potential limit and decreasing by decrements of 10 mV for one hour each until the starting potential (30 mV above EOCP) is attained. This procedure completes one whole cycle. The current recorded for each potential at the end of each hour is referred to as the “steadystate current” and is used to generate anodic dissolution curves of current vs. potential. Results are shown for 23 and 80 °C where two continuous cycles were generated. The stationary current–potential anodic curves generated by this method, according to our experience, predict the leaching kinetics of chalcopyrite very well within experimental error (results not yet published), unlike rapid positive potential scans, which are usually very far from equilibrium and bear little relevance to leaching kinetics. In this study special emphasis is placed on the effect of the direction of polarization. The steady-state voltammetries were generated by scanning the active and passive ranges of potentials both in the positive (anodic) and negative (cathodic) directions. The potential scans in the positive direction began 30 mV above EOCP and proceeded to the upper potential limit indicated above; they show the transition from the active to the passive state at 440 mV at 23 °C, and at about 510 mV at 50 and 80 °C. The potential scans in the negative direction began at the upper potential limit, with a well passivated surface, and proceeded to 30 mV above EOCP; they show the depassivation process (transition from the passive to the active state) at around 440 mV at all temperatures. Interestingly the passivation and depassivation transitions are asymmetrical and do not take place at the same potential.
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4. Results and discussion Now the electrochemical results that help us to understand the causes producing two passivation potentials in the broad variety of leaching experiments represented in Fig. 1 will be discussed. 4.1. Passive behavior at room temperature Fig. 2a illustrates the active–passive behavior of chalcopyrite at 23 °C and 30 g/L of sulfuric acid as obtained by the method of sampled-current voltammetry described above. Current was sampled after 1 h of potentiostatic polarization at steps of 10 mV. The study consisted of the generation of two uninterrupted cycles covering only the oxidative range of potentials. The scan was begun in the positive direction 30 mV above EOCP and taken up to 630 mV, where the scanning direction was reversed and brought down to 360 mV, where it was reversed again and taken up to 560 mV, where it was finally reversed one last time and taken down to 340 mV. The two scans in the positive direction are very similar, showing an active range where the current increases exponentially with potential following the Tafel relationship up to about 440 mV. Current has its maximum value at this potential. At potentials higher than 440 mV the currents decrease with increasing potential, gradually approaching the passive state. The two potential scans in the negative direction began at potentials within the passive range, just before the sudden onset of transpassivity. Essentially both of them have the same behavior, which resembles the curves scanned in the positive direction. The only
difference is that the curves scanned in the negative direction have currents slightly lower than those scanned in the positive direction. Everything else behaves the same, regardless of the scanning direction. Fig. 2b shows the behavior with time of the currents scanned in the positive (at the top) and negative (at the bottom) directions at five selected potentials (400, 440, 480, 520, and 560 mV). Upon a 10-mV step-change in potential, currents scanned in the positive direction change rapidly from their previous state, then consistently show a gradual decrease with time which gives straight lines when plotted in a log–log scale. Similarly, currents scanned in the negative direction change rapidly, then show a gradual increase with time which also gives straight lines on a log–log scale. Both curves gradually approach a common steady state, which is typically not completely reached after 1 h at potentials above the transition to passivation. Currents scanned in the positive direction approach the steady state from above, while currents scanned in the negative direction approach it from below. Hence in Fig. 2a positive and negative scans have slightly different currents because the steady state is approached asymptotically, but has not been yet reached after 1 h. However the difference is so small (around 0.3 μA/cm2) that positive and negative scans can be considered coincident with each other. This steadystate voltammetry in the oxidative range of potentials show that at room temperature the transition from the active to the passive state (passivation) takes place at the same potential than the transition from the passive to the active state (depassivation), which is around 440 mV. Hence chalcopyrite passivation and depassivation appear symmetrical. 4.2. Passive behavior at 50 °C
Fig. 2. Chalcopyrite active–passive behavior at 23 °C and 30 g/L of H2SO4; a) Two-cycle steady-state sampled-current voltammetry showing symmetrical passivation and depassivation. b) Current density vs. time behavior at selected potentials.
The symmetrical scenario between the active–passive and passive– active transitions valid at room temperatures drastically changes when temperature is increased. Fig. 3 shows the oxidative behavior of chalcopyrite at 50 °C and 30 g/L of sulfuric acid. To generate the onecycle sampled-current voltammetry results shown in Fig. 3a, EOCP was first measured, then the polarization scan was initiated at 340 mV (just 30 mV above EOCP) at steps of 10 mV per hour in the positive direction (with some initial steps of 20 mV). Polarization continued up to 690 mV within the passive range (after 28 h), where the scanning direction was reversed. The current–potential anodic curve scanned in the positive direction shows three potential ranges where chalcopyrite oxidation behaves differently. First the quasi-stationary current increases exponentially with the applied potential up to 440 mV, following the Tafel relationship as in Fig. 2. Then a tilted plateau showing two weak maxima at 440 and 510 mV is observed, with the maximum current observed at 510 mV. Finally a gradual transition to passivation begins immediately after this potential. When the direction of the potentiostatic scan is reversed at 690 mV the electrode retraces its previous behavior only in the passive range of potentials. Below 510 mV important differences arise. The potential scan in the negative direction follows its depassivating trend up to 440 mV, where the maximum current is now observed. The dual-maxima tilted plateau observable in the positive scan was not detected in the negative scan. It is also worth noting that below 440 mV the negative scan shows much more active Tafel behavior than the positive scan; almost a threefold increase. For the moment we have no explanation for this attractive result, deeper physicochemical studies of the behavior of the chalcopyrite/solution interface must be conducted to reach such an understanding, which is beyond the scope of this publication. Results in Fig. 3a show that the potential where the oxidation current is maximum, (which is by definition Epp), is strongly dependent on the scanning direction. This is 440 and 510 mV for the negative and positive potential scans, respectively. In order to examine the reproducibility of these results, the experiment was repeated at the same temperature and acidity following a slightly
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exponential behavior was observed. The scanning direction was reversed to positive at 330 mV, ultimately increasing up to 630 mV, well within the passive region. Within the active range of potentials, below 440 mV, the positive scan retraces back the same trajectory. Notably, the transition from the active to the passive state does not take place at the same potential where depassivation was observed. In the positive direction, passivation starts at around 500 mV. Although the 1-h sampled-current voltammetries in Fig. 3a and b look quite different, both of them consistently show that chalcopyrite passivation and depassivation are asymmetrical. It is also notable that hysteresis is present in both voltammetries, with the magnitude of the currents being very different between 440 and 510 mV. Fig. 3c shows the current vs. time behavior at some selected potentials (410, 440, 470, 500, 530, and 560 mV) for the experiments in Fig. 3b. In the Tafel active range (below 440 mV) the steady state is reached very quickly and currents generated in the positive and negative scans coincide with each other within a few minutes. The same happens well within the passive state. The most interesting range of potentials is observed between 440 and 510 mV, where the difference in current is the largest. At these potentials, currents generated in the positive and negative directions for the same potential differ by up to 130 μA/cm2, and they approach each other very slowly following power laws, i = i0tβ, with absolute values for β very small, ranging from 0.05 to 10−9. If current vs. time curves are plotted in a log–log scale and the trend of the last 30 min is extrapolated as a means to estimate how different their thermodynamic states are, these curves will cross each other only after very long times, on the order of 108 s (3 years). This indicates that the thermodynamic states of chalcopyrite in the positive and negative directions for a particular potential between 440 and 510 mV are very different. The origin of the difference lies within the previous history of polarization and how the electrode was brought to that potential. 4.3. Passive behavior at 80 °C Fig. 4 shows a two-cycle sampled-current voltammetry showing the effect of the polarization direction at 80 °C and 30 g/L of sulfuric acid. At this temperature chalcopyrite also displays hysteresis and asymmetry in its passivation–depassivation behavior. The maximum current is observed at two different potentials, indicating the passivation and depassivation transitions, which depend on how the electrode was polarized. The transition to passivation is observed at around 510 mV in positive potential scans, while the transition to depassivation is observed at around 460 mV in the negative potential
Fig. 3. Hysteresis in chalcopyrite active–passive behavior at 50 °C and 30 g/L H2SO4; a) Positive scan → negative scan. b) Negative scan → positive scan. c) Current density vs. time behavior.
different strategy; Fig. 3b shows the results. First a freshly polished electrode was polarized for 20 h at 650 mV (directly from EOCP) in order to bring its surface to the passive state. The inset in Fig. 3b shows in a log–log scale how the electrode gradually approaches its steadystate passive state. (Evidently, 104 s are enough for the electrode to reach steady-state; it is not strictly necessary to wait for 28 h to go from 340 to 690 mV.) Once the electrode was passivated a negative potential scan was initiated, beginning at 650 mV and decreasing to 330 mV, with the objective to evaluate first where the depassivation transition takes place. The maximum current was observed at around 450 mV – passive currents were always smaller at potentials higher than 450 mV – while at potentials lower than 450 mV active Tafel
Fig. 4. Hysteresis in chalcopyrite active–passive behavior at 50 °C and 30 g/L H2SO4; Positive scan → negative scan.
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scans. At this temperature the difference between the magnitude of the currents generated by positive and negative potential scans, respectively, between 460 and 510 mV (the hysteresis zone) can be as large as 103 μA/cm2, with the larger current observed during the positive potential scans. At this temperature the current vs. time behavior also gives curves running virtually parallel to each other, separated by hundreds of μA/cm2, which indicates how different the two thermodynamic states can be at exactly the same potential. The difference lies again in how the external driving force (electrochemical potential) was applied. Figs. 2 to 4 consistently show that at temperatures of 50 °C and higher the transitions to passivation and depassivation always occur at different potentials. This behavior was always observed and is very reproducible. However, the behavior of the current with applied potential in higher currents in the active range of potentials (below 440 mV) — after the electrode has been polarized to passivation in the positive direction the first time. Fig. 3b shows the same currents in the active (below 440 mV) and passive (above 550 mV) ranges of potentials, and very different currents only in the transition to passivation. Fig. 4 also shows important differences between 500 mV and 570 mV. We always observed a range of potential where the magnitude of the current was different, but the magnitude itself was not so reproducible, especially at 50 °C. It is possible that this current behavior with potential is related to the content of impurities at the surface of the electrode. It is well known that some properties of semiconductors, particularly electrical conductivity, are extraordinarily sensitive to the content of impurities. Misra and Fuerstenau (2005) demonstrated that nanosize particles of SiO2, the main impurity of our samples, do catalyze chalcopyrite leaching. Other impurities at concentrations below the detectable limit of Rietveld analysis, and generally associated to chalcopyrite like atomic inclusions of silver, might also contribute to change the magnitude of the electric current. 4.4. Key observations One of the most astonishing features of these experiments is that in the positive potential scans, begun just above the open circuit potential, passivation onsets at about 510 mV at high temperatures. However, in the negative potential scans imposed on a previously passivated surface, depassivation only occurs at about 440 mV. Hence between 440 and 510 mV chalcopyrite can be either active or passive, depending on the previous history of the electrode. The fact that these two passivation potentials have often been reported in independent leaching studies indicates that the observed hysteresis is not a mere electrochemical curiosity, but an important property of chalcopyrite behavior, with relevant consequences to chalcopyrite leaching. Another equally significant result is that in the active range of potentials at temperatures higher than 50 °C, between EOCP and 440 mV, chalcopyrite electrodes generally become much more active after the first potential scan. In chalcopyrite leaching experiments in sulfate media, a kind of incubation period is often observed after which the leaching rate increases considerably. Electrochemical hysteresis, similar to that reported here for the first time for chalcopyrite, is also displayed by other compounds, such as Ni(OH)2 (Srinivasan et al., 2001), which also shows different potentials for passivation and depassivation transitions. A comparison between the curves in Fig. 4 and those presented by Stöhr and Siegmann, (2006) and by Srinivasan et al., (2001) shows that the hysteresis loops for almost pure iron and Ni(OH)2 are smoother and more well-behaved than hysteresis loops for chalcopyrite. For chalcopyrite, different passivation potentials have been reported by many different authors, but the pattern we can observe is that most studies agree well with two different values, which are close to 440 and 510 mV. According to our results these two potentials
arise naturally as a consequence of chalcopyrite bistability and hysteresis. Although at this point the relevance of this phenomenon to potential control is clear, its relevance to leaching kinetics is less clear, since oxidation currents measured electrochemically do not necessarily produce reproducible patterns, as shown in Fig. 3a and b. Further comparison between very well controlled leaching and electrochemical experiments are required in order to define the effects of passivation loops to leaching kinetics. 5. Conclusions A literature review has shown that the oxidative leaching of some chalcopyrite concentrates in ferric sulfate media drastically slows down at above about 440 mV, while others are well active at that potential and only slow down above about 510 mV. This study demonstrates that this is a consequence of the fact that chalcopyrite passivation displays bistability and hysteresis. Below 440 mV chalcopyrite is always in its active state, regardless of the geological source of the mineral, acidity or temperature. In the range of potentials between 440 and 510 mV chalcopyrite is a bistable system, and can be passive or active depending on how it was brought to that potential. Above about 510 mV and below the transpassive potential, chalcopyrite is always passive and leaching is very slow. In a bistable system many variables can be the determining factors driving the system to one of the two available states. One of these factors, which can be easily controlled, is the route by which the slurry is brought to the desired potential for leaching. Acknowledgments The authors acknowledge the supporters of the Natural Sciences and Engineering Research Council of Canada (NSERC) (Discovery Grant 155122-02), as well as PROMEP PTC-222 (Mexico), and CONACyT (Mexico) Proyecto de Ciencia Básica 2007 # 79608 for providing the financial support to carry out this research. References Ahonen, L., Tuovinen, O., 1993. Redox potential-controlled bacterial leaching of chalcopyrite ores. In: Torma, A., Wey, J., Lakshmanan, V. (Eds.), Biohydrometallurgical Technologies, vol. 1. TMS, Warrendale, pp. 571–578. Ballester, A., Córdoba, E., 2005. Hidrometalurgia de la calcopirita. In: Menacho, J., Casas de Prada, J. (Eds.), Hydrocopper 2005. Universidad de Chile, Santiago, pp. 19–41. Córdoba, E.M., Muñoz, J.A., Blázquez, M.L., González, F., Ballester, A., 2009. Passivation of chalcopyrite during its chemical leaching with ferric ion at 68 °C. Miner. Eng. 22, 229–235. Hiroyoshi, N., Kuroiwa, S., Miki, H., Tsunekawa, M., Hirajima, T., 2004. Synergistic effect of cupric and ferrous ions on active–passive behavior in anodic dissolution of chalcopyrite in sulfuric acid solutions. Hydrometallurgy 74, 103–116. Hiroyoshi, N., Kuroiwa, S., Miki, H., Tsunekawa, M., Hirajima, T., 2007. Effects of coexisting metal ions on the redox potential dependence of chalcopyrite leaching in sulfuric acid solutions. Hydrometallurgy 87, 1–10. Hiroyoshi, N., Kitagawa, H., Tsunekawa, M., 2008. Effect of solution composition on the optimum redox potential for chalcopyrite leaching in sulfuric acid solutions. Hydrometallurgy 91, 144–149. Hunter, C., 2006. Method for the bacterially assisted heap leaching of chalcopyrite. US Patent No. 7,022,504. Kametani, H., Aoki, A., 1985. Effect of suspension potential on the oxidation rate of copper concentrate in a sulfuric acid solution. Metall. Trans. B 16B, 695–705. Misra, M., Fuerstenau, M.C., 2005. Chalcopyrite leaching at moderate temperature and ambient pressure in the presence of nanosize silica. Miner. Eng. 18 (3), 293–297. Okamoto, H., Nakayama, R., Tsunekawa, M., Hiroyoshi, N., 2003. Improvement of chalcopyrite leaching in acidic sulfate solutions by redox potential control. In: Riveros, P., Dixon, D., Dreisinger, D., Menacho, J. (Eds.), Copper 2003, vol. VI. CIM, Montreal, Quebec, Canada, pp. 67–81. Pinches, A., Gericke, M., van Rooyen, J., 2001a. A method of operating a bioleach process with control of redox potential, World Patent, WO01/31072 A1. Pinches, A., Myburgh P., van der Merwe, C., 2001b. Process for the rapid leaching of chalcopyrite in the absence of catalyst. U.S. Patent No 6,277,341 B1. Rivera-Vasquez, B.F., Viramontes-Gamboa, G., Dixon, D.G., 2006. Leaching of chalcopyrite concentrates in acidic ferric sulfate media at controlled redox potentials. In: Domic, E., Casas de Prada, J. (Eds.), Gecamin, Santiago, Chile, pp. 287–299.
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Third, K., Cord-Ruwisch, R., Watling, H., 2002. Control of the redox potential by oxygen limitation improves bacterial leaching of chalcopyrite. Biotechnol. Bioeng. 78, 433–441. Viramontes-Gamboa, G., Rivera-Vasquez, B., Dixon, D.G., 2006. The active-to-passive transition of chalcopyrite. ECS Trans. 2, 165–175. Viramontes-Gamboa, G., Rivera-Vasquez, B.F., Dixon, D.G., 2007. The active–passive behavior of chalcopyrite, comparative study between electrochemical and leaching responses. J. Electrochem. Soc. 154 (6), C299–C311.