the effects of pyrite inclusions, dissolved oxygen and ferric ions on chalcopyrite electrochemistry
TRANSCRIPT
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The effects of pyrite inclusions, dissolved oxygen and ferric ions on chalcopyrite electrochemistry
Daniel Majuste, André Leite, Maria Dantas and Virginia Ciminelli Universidade Federal de Minas Gerais and National Institute of Science and Technology on Mineral Resources, Water and Biodiversity – INCT – Acqua, Brazil
Kwadwo Osseo‐Asare Department of Materials Science and Engineering, Department of Energy and Mineral Engineering, The Pennsylvania State University, USA
ABSTRACT
The present paper discusses the separate and combined effects of naturally associated pyrite (FeS2) micro‐crystallites, dissolved oxygen, and ferric ions on the electrochemical behaviour of chalcopyrite (CuFeS2) electrodes. The use of chalcopyrite electrodes with pyrite micro‐inclusions better represents the sulphide association in the ore. This also maximises the electrical contact between these two phases, thus minimising the voltage losses across the mineral‐mineral contact. The electrochemical properties of particular interest are represented by the mixed potential (EM) and the current density at EM, denoted as dissolution current density (idiss) in this paper. All the measurements were carried out in 0.1 mol/L H2SO4 solutions at room temperature (26 ± 1°C). Potentiometry and linear sweep voltammetry showed the effects of pyrite, dissolved oxygen (0.001 mol/L) and ferric ions (0.01‐0.05 mol/L) on the EM and idiss values of chalcopyrite to be significant. The mixed potential of chalcopyrite increased up to 26% by combining ferric ions and oxygen. The pyrite‐chalcopyrite association led to an increase of up to 75% by combining these oxidants. The dissolution current density of chalcopyrite increased up to 56 times in the presence of ferric ions plus oxygen, and about 91 times in the presence of ferric ions plus oxygen with about 42% pyrite on the electrode surface. Electrochemical models and schematic diagrams were utilised to elucidate the observed trends.
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INTRODUCTION
Chalcopyrite (CuFeS2), the main copper mineral [1], does not dissolve significantly under the heap bioleaching conditions. At low temperatures, chalcopyrite dissolution rate in acidic media is known to be very slow and often tends to decline with time [2‐7]. In an attempt to overcome this constraint, a number of investigations have been conducted aimed at understanding the kinetics and mechanisms of dissolution of this sulfide. Most of the studies ascribe the slow leaching rate to the formation of an insoluble, non‐porous layer, which prevents further mineral dissolution.
The use of catalysts has been investigated in order to improve the chalcopyrite dissolution kinetics. It is well established that the galvanic interactions increase the dissolution rate of one of the minerals that constitute a galvanic couple. The enhancement of copper extraction from chalcopyrite when in contact with pyrite (FeS2) [8‐13] has been attributed to galvanic effects. The mineral with the higher mixed potential acts as the cathode in the galvanic couple and is protected, while the mineral with the lower mixed potential serves as the anode and thus dissolves preferentially.
Berry et al. (1978) demonstrated that FeS2 in intimate contact with CuFeS2 in acidic sulfate media, under atmospheric conditions, enhances the dissolution rate of CuFeS2. These results indicated that chalcopyrite reacts anodically (Eq. 1), while pyrite acts as the cathodic site for the oxygen reduction reaction (Eq. 2).
CuFeS2 → Cu2+(aq) + Fe2+(aq) + 2S + 4e‐ (1)
O2(g) + 4H+(aq) + 4e‐ → 2H2O (2)
Mehta & Murr (1983) investigated the galvanic effect involving CuFeS2 and FeS2 by evaluating the mixed potential of this couple in acidic sulfate media, under atmospheric conditions. These authors connected a pyrite and a chalcopyrite electrode by a copper wire. The mixed potential was found to be 0.56V vs. the Standard Hydrogen Electrode (SHE), which represents an intermediate value between those obtained for FeS2 and CuFeS2 (0.63 and 0.52V vs. SHE, respectively). By using a similar approach (i.e., electrodes connected by Cu wire), You et al. (2007) investigated the effect of pH and ferric ion concentration on the mixed potential and the corresponding current density of the chalcopyrite‐pyrite couple, under atmospheric conditions. It was noticed that the lower the pH and higher the concentration of ferric ion, the higher the current density and the more positive the mixed potential.
In the GalvanoxTM process, the copper extraction from chalcopyrite is conducted in the presence of pyrite particles in ferric sulfate media. High copper extractions have been reported, depending on the leaching time, acidity, ferric ion concentration, and pyrite addition [12,13]. The enhanced rate of chalcopyrite leaching has been ascribed to the faster reduction rate of Fe3+ ion (Eq. 3) on the surface of galvanically‐coupled pyrite particles.
Fe3+(aq) + e‐ → Fe2+(aq) (3)
– 3 –
Littlejohn & Dixon (2008) demonstrated that the presence of pyrite in ferrous sulfate solutions has an enhancing effect on the initial kinetics of Fe2+ ion oxidation (Eq. 4). The oxidation rate depends on the ferrous ion concentration, acidity, leaching time and pyrite addition.
4Fe2+(aq) + O2(g) + 4H+(aq) → 4Fe3+(aq) + 2H2O (4)
It has been proposed that chalcopyrite dissolution involves an electron‐independent formation of hydrogen sulfide (H2S), and that elemental sulfur (S) is formed from the oxidation of H2S [14‐15]. Nicol et al. (2010) [16] showed that pyrite acts as a catalyst for sulfide oxidation. It was observed that in the absence of FeS2, the oxidation rate of sulfide (S2‐) ions (i.e., consumption rate of oxygen in the solution) is very low, while in the presence of fine pyrite particles this process is favored. The authors also demonstrated that for chalcopyrite leaching in the presence of pyrite particles, in acidic chloride media under oxidizing, atmospheric conditions, most of the elemental sulfur was found associated with FeS2.
It is evident from the above review the existing controversies involving the mechanisms by which FeS2 affect the CuFeS2 dissolution. The effect of FeS2 on the dissolution rate of CuFeS2 may go beyond the recognized galvanic interaction, and such effects are expected to be magnified by the occurrence of a permanent contact between the two mineral phases. Since pyrite is a mineral phase often found in copper sulfide deposits [17], a better understanding of the galvanic interaction involving these sulfides may help to improve chalcopyrite leaching rate from low‐grade ores.
In this context, the purpose of this investigation is to quantify the magnitude of the galvanic effect of FeS2 on the electrochemistry of CuFeS2 in acidic sulfate media under typical, low‐temperature leaching conditions, with a method that better represents the association in the ore. By investigating FeS2 micro‐crystallites naturally associated with chalcopyrite ore, the electrical contact between the sulfides is optimized, which minimizes the voltage losses across the mineral‐mineral contact and, consequently, the effect on the magnitude of the galvanic current. This investigation focuses on the separate and combined effects of pyrite inclusions, dissolved oxygen, and ferric ion on fundamental properties of chalcopyrite, such as the mixed potential (EM) and dissolution current density (i0). The results were compared with the properties of pyrite‐free chalcopyrite and chalcopyrite‐free pyrite electrodes, which were selected after detailed surface analyses of mineral samples.
METHODOLOGY
Mineral electrodes
All the mineral samples used in this study were obtained from Ward’s Natural Science, N.Y. The preparation of the pyrite‐free chalcopyrite (CE), chalcopyrite‐free pyrite (PE), and mixed (pyrite‐chalcopyrite) (ME) electrodes involved the following procedures. Firstly, massive samples were cut using a diamond wafering blade (Buehler®, n.11‐4246), thus obtaining regular mineral specimens (1.0 cm2 of exposed area and 0.5 cm thick), which were rinsed with double‐distilled water and dried with analytical grade acetone 100% (Synth®). Next, a Cu wire within a glass tube was connected to the specimens using a conducting silver paint (Dotite®, D‐550). The specimens were mounted in Epoxy resin (Epofix®, Struers), and fresh electrode surfaces were prepared by wet mechanical polishing using fine silicon carbide papers (grit sizes 1200 and 2400) and alumina paste (1.0 μm). Next, the
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electrode surfaces were rinsed with double‐distilled water in an ultrasonic bath for 15 min, dried with analytical grade ethyl alcohol 95% (Synth®) and kept under vacuum at room temperature.
Characterization of the mineral electrodes
Prior to the electrochemical measurements, the mineral electrodes (CE, PE and ME) were analyzed by scanning electron microscopy (SEM) coupled with energy dispersive spectrometry (EDS), using a JEOL (JSM 6360LV) Microscope and a Thermo Noran (Quest) Spectrometer; optical microscopy, using a Leica (Metallux DFC 290) Microscope, equipped with lenses of 50 and 100x; and Raman spectroscopy (RS), using a Horiba Jobin Yvon (Labram HR800) Spectrograph, equipped with a 633 nm He‐Ne laser (20mW power). Raman signal was collected by an Olympus (BHX) Microscope, equipped with lenses of 50 and 100x. The spectral resolution was about 2 cm‐1 and a minimum of 10 scans with 60s integration time were recorded. The grating angle was calibrated by the 520cm‐1 Si band. Mineral samples containing other impurities than pyrite were discarded. Particularly for the ME electrodes, the Standard Test Method ASTM E562 (2008) [18] was used for estimating the FeS2 content on the electrode surfaces. The point counting was carried out using optical microscopy and a square test grid of 9 intersections. 50 photomicrographs were taken from each electrode.
Electrochemical measurements
The electrochemical measurements were conducted at 26±1°C by the conventional three electrode cell (i.e., mineral working, platinum auxiliary and Ag/AgCl/KCl (3.0mol/L) reference electrodes) and an Autolab® (Eco Chemie, PGSTAT20) Potentiostat. The potential difference between the CE, PE, and ME working electrodes and reference electrode was obtained by Luggin capillary, in fixed cell geometry. In all the experiments, ultra‐pure nitrogen (99.999%) was bubbled in the electrolyte solution (0.1 mol/L H2SO4) for at least 20 min before the start of the measurements. The open circuit potentials (OCPs) of freshly polished surfaces were obtained by potentiometry (zero current). The mixed potentials (EM) were measured after 60min of immersion of each electrode in the solution. The dissolution current densities (idiss) were obtained by linear sweep voltammetry at low anodic and low cathodic overpotentials (η ± 150mV), and at 5 mV/s scan rate. The measurements were conducted after an equilibration time of 20 min, being EM defined as the vertex of the polarization. The value of idiss was calculated after extrapolation of the Tafel regions (i.e., the anodic and cathodic linear correlations of log |i| with overpotential) to the EM value. The linear correlations were obtained by determining the best data fitting, using the Microcal OriginTM (v.8) software. In order to achieve reproducibility, the freshly polished electrodes were immersed in the solution for at least 10min before sweeping. In order to evaluate the effect of dissolved oxygen, ultra‐pure oxygen (99.999%) was bubbled in the solution for at least 20min prior to the start of the measurements and when required. The dissolved oxygen concentration, measured by using a dissolved oxygen sensor (Corning®, 312), was found to be about 0.001 mol/L. The effect of ferric ion was investigated by adding 0.01 or 0.05 mol/L Fe3+ ion as iron(III) sulfate hydrate 97%, Fe2(SO4)3.xH2O (Sigma‐Aldrich®) in the acidic solution. All the solutions were prepared with double‐distilled water and using analytical grade sulfuric acid 96% (FMaia®). All the potentials were converted to the SHE scale. The measurements were performed in duplicate and, therefore, average bars are provided.
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RESULTS AND DISCUSSION
Characterization of the mineral electrodes
RS and EDS analyses were employed to assess the features of the mineral electrodes. Figure 1a shows typical micrograph of a mixed pyrite‐chalcopyrite (ME) electrode. It can be seen a smooth surface, with micrometric dark points, which represent cavities or impurities. The matrix of the electrode, large impurity and massive dark point were shown by EDS analyses to be constituted by, respectively, Cu (32.1% wt.), Fe (27.9% wt.) and S (34.4% wt.) (area 1); Fe (42.9% wt.) and S (49.3% wt.) (area 2); and Si (31.5% wt.), Ca (24.8% wt.), O (22.8% wt.) and Mg (9.2% wt.) (area 3). The Raman spectra obtained for areas 1, 2, and 3 are exhibited in Figure 1b. For area 1, it may be noticed one intense band at 294 cm‐1 and additional bands of lower intensity at 268, 322, 360, and 378cm‐1, which represent the characteristics bands of chalcopyrite [19,20]. The spectrum obtained for area 2 indicated intense bands at 353 and 387cm‐1 and additional band of lower intensity at 445 cm‐1, which can be assigned to pyrite [19]. The Raman bands depicted in the spectrum obtained for area 3 are characteristic of diopside, a calcium and magnesium silicate (CaMgSi2O6) [21]. The Raman bands obtained in this investigation were assigned to an iron‐bearing diopside, due to the Fe content obtained via EDS (4.6% wt.). Therefore, the EDS analyses are consistent with the Raman findings. In the present investigation, three ME electrodes were selected and then submitted to the electrochemical measurements in order to evaluate the magnitude of the galvanic effect of pyrite inclusions on the mixed potential and dissolution current density of chalcopyrite, in the absence and in the presence of oxidants. Samples containing about 14% (ME1), 31% (ME2), and 42% (ME3) FeS2 dispersed on the chalcopyrite surface area were selected.
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Figure 1 (a) Back‐scattered electron micrograph of mixed (ME) electrode and selected areas analyzed by EDS; and (b) Raman spectra obtained for areas 1, 2, and 3.
Similar Raman spectra to those obtained for areas 1 and 2 were exhibited by pyrite‐free chalcopyrite (CE) and chalcopyrite‐free pyrite (PE) electrodes, respectively. EDS analyses of Cu (32.9% wt.), Fe (28.9% wt.), and S (33.6% wt.) for the CE electrode, and Fe (42.6% wt.), and S (50.1% wt.) for the PE electrode, are also consistent with the Raman findings. Slight differences in intensity, broadness and position of the bands discussed here in relation to spectra exhibited in the literature may be related to deviations in the elemental stoichiometric ratio and the conditions of measurements [19, 22].
Mixed potential measurements
Effects of pyrite inclusions, dissolved oxygen and ferric ion
The potentials on the CE, PE, and ME electrodes in 0.1mol/L H2SO4 solutions, in the absence and in the presence of oxidants, were followed as a function of time. The results are shown in Figure 2. In the absence of oxidants, the mixed potential (EM) increased as follows: EM CE < EM ME1 < EM ME2 < EM ME3 < EM PE. The average EM of the ME1, ME2 and ME3 electrodes were, respectively, about 80, 100 and 120 mV higher than the potential measured on the CE electrode (i.e., 0.47V). The effect of the pyrite inclusions represented increases of 17, 21, and 26%, respectively. The results achieved with the ME electrodes, intermediate values between the mixed potential found for the CE and PE (i.e., 0.61V)
1
23
Intensity (a.u.)
Raman shift (cm‐1)
200 400 600 800 1000
294
268
360
378
322
386 320
101
666
504
445
387
353
Area 1Chalcopyrite
Area 2Pyrite
Area 3Diopside
(a)
(b)
– 7 –
electrode, agreed well with the findings of Mehta & Murr (1983). The mixed potential of attached pyrite‐chalcopyrite electrodes was found to be 0.56V vs. SHE.
Figure 2 Mixed potentials of the CE, PE and ME electrodes in 0.1mol/L H2SO4 solutions under atmospheric conditions
In the presence of oxidants (Figure 2), it can be noticed that the average mixed potential of the CE and PE electrodes increased, respectively, about 80 and 90mV in the presence of oxygen. The effect of the oxygen bubbling on the mixed potentials of the pyrite‐chalcopyrite (ME) electrodes is relatively inferior to that exhibited for the CE and PE electrodes. The potentials of all the electrodes increased largely in the presence of ferric ion: about 160 and 170 mV, at 0.01 mol Fe3+/L; and about 210 and 200 mV, at 0.05mol Fe3+/L, for the CE and PE electrodes, respectively. The increase of the mixed potential of chalcopyrite by ferric ion addition was also reported by others [2, 6, 11].
Therefore, the magnitude of the effect of Fe3+ ion addition at the investigated conditions was larger than that of dissolved oxygen. This result may be associated to the higher concentration of ferric ion (0.01‐0.05 mol/L) as compared to oxygen (0.001 mol/L). The reaction rate of the ferric and oxygen cathodic reductions on the mineral surfaces can not be discussed at this point. In an attempt to better simulate practical leaching conditions, the magnitude of the combined effects of FeS2 inclusions, O2 and Fe3+ ion on the mixed potential of CuFeS2 was evaluated. The potentials measured in the presence of both the oxidants (0.001 mol O2/L + 0.05 mol Fe3+/L) are shown in Figure 2. It can be noted that the average mixed potential increased notably for all samples. For the CE and PE electrodes, the potential was 250 and 240 mV (53 and 39% increase) respectively, higher than the values measured in the absence of oxidants. For the ME3 electrode, the potential was about 350 mV higher (75% increase) than the potential measured on the CE electrode, in the absence of oxidants. Thus, considering the individual effect of Fe3+ ion at 0.05mol/L, this noteworthy effect could be associated with the oxidation of Fe2+ to Fe3+ ions (Eq. 4), with further discharge of Fe3+ ions on pyrite, cathodic site of the galvanic couple.
0,47
0,550,57
0,590,61
0,55
0,58
0,610,63
0,700,68
0,750,77 0,78
0,81
0,63
0,71 0,720,74
0,78
0,72
0,78
0,81 0,82
0,85
0,4
0,5
0,6
0,7
0,8
0,9C
E
ME
1
ME
2
ME
3
PE
Pote
ntia
l (Vo
lts)
Electrode
Legend:
No oxidant
0.001mol O2/L
0.01mol Fe3+/L
0.05mol Fe3+/L
0.001mol O2/L + 0.05mol Fe3+/L
– 8 –
Dissolution current density measurements
Effects of pyrite inclusions, dissolved oxygen and ferric ion
Figure 3 shows the dissolution current density (idiss) obtained for the CE and ME electrodes in 0.1mol/L H2SO4 solutions in the absence and in the presence of oxidants. It may be noticed that the presence of FeS2 inclusions caused an increase in the dissolution current density of chalcopyrite. Moreover, it can be seen that the idiss values for the ME electrodes are higher than that obtained for chalcopyrite, as follows: idiss CE < idiss ME1 < idiss ME2 < idiss ME3. Important increases by the order of 15, 24, and 40 times were obtained, respectively, for the ME1 (14% FeS2), ME2 (31% FeS2), and ME3 (42% FeS2) electrodes.
Figure 3 Dissolution current densities of CE and ME electrodes in 0.1mol/L H2SO4 solutions under atmospheric conditions
From Figure 3, it can be seen that the presence of oxidants caused an increase in the dissolution current density value calculated for the chalcopyrite (CE) electrode. For this sample, increases by the order of 18, 31, 36, and 56 times were calculated, respectively, in the presence of 0.001 mol O2/L, 0.01 mol Fe3+/L, 0.05 mol Fe3+/L and 0.001 mol O2/L plus 0.05 mol Fe3+/L. Regarding the magnitude of the combined effects of FeS2 and oxidants on the dissolution current density of CuFeS2, a still more notable result was achieved. When approximately 14% FeS2 is dispersed on the chalcopyrite surface (ME1 electrode), the idiss value increased about 23, 43, 57, and 71 times, respectively, in solutions containing 0.001mol O2/L, 0.01 mol Fe3+/L, 0.05mol Fe3+/L, and 0.001 mol O2/L plus 0.05 mol Fe3+/L. Higher increases by the order of 34, 49, 67, and 80 times were calculated for the ME2 electrode (about 31% FeS2), respectively. For the ME3 electrode (approximately 42% FeS2), increases of about 47, 64, 77, and 91 times were calculated, respectively.
Legend:
No oxidant
0.001mol O2/L
0.01mol Fe3+/L
0.05mol Fe3+/L
0.001mol O2/L + 0.05mol
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DISCUSSION
Effects of oxidants on chalcopyrite electrochemistry
It is known that at steady‐state conditions, where the OCP (i.e., potential exhibited by the electrode surfaces in the absence of externally applied potential) stabilizes at the mixed potential (EM), the net current is zero – the anodic and cathodic current densities are equal and of opposite sign [23]. At this condition (i.e., low potential values), it is assumed that the dissolution of chalcopyrite occurs as a simultaneous combination of an anodic non‐stoichiometric oxidation (Eq. 5), with (i) the cathodic reduction of residual or bubbled oxygen on the surfaces (Eq. 6), respectively, when no oxidants are added and when oxygen is bubbled in the solution; with (ii) the cathodic reduction of ferric ion (Eq. 7), when this oxidant is added; and with (iii) parallel cathodic reductions of dissolved oxygen and ferric ion, when both these oxidants are added.
CuFeS2 = Cu1‐xFe1‐yS2‐z + x Cu2+(aq) + y Fe2+(aq) + z S + 2(x + y) e‐ (5)
O2(g) + 4 H+(aq) + 4 e‐ = 2 H2O (6)
Fe3+(aq) + e‐ = Fe2+(aq) (7)
As discussed elsewhere [24], the anodic and cathodic current densities can be described in terms of the Butler‐Volmer equation, with the assumptions that the mineral oxidation is irreversible (i.e., the cathodic reduction of the mineral can be ignored at the potentials encountered during oxidation); and the surface areas available for both half‐cell reactions are equivalent (i.e., Aanodic ≈ Acathodic). It can be demonstrated that at the different steady‐state conditions, the mixed potential and dissolution current density values measured on the electrode surfaces depend on the rate constants (k) of all the contributing half‐cell reactions occurring on the mineral, hydrogen ion and oxygen concentrations. These electrochemical properties may be expressed by Eqs. (8) and (9), in the absence of oxidants other than O2:
⎟⎟⎠
⎞⎜⎜⎝
⎛⎟⎠⎞
⎜⎝⎛=
+
1
'x'x'22
M k][H][Okln
FRTE (8)
( )[ ]21'x'x'2210 ][H][Okki += (9)
Or by Eqs. (10) and (11), when ferric ion is added:
⎟⎟⎠
⎞⎜⎜⎝
⎛
++
⎟⎠⎞
⎜⎝⎛= +
++
'''x'2'31
''x'33
'x'x'22
M ][Fekk][Fek][H][Okln
FRTE (10)
k1
k1’
k2’
k2
k3’
k3
– 10 –
21
'''x'23'1
''x'33
'x'x'22
10 ][Fekk][Fek][H][Okki ⎟⎟
⎠
⎞⎜⎜⎝
⎛
++
= +
++
(11)
where k is the rate constant, F the Faraday constant, R the universal gas constant, T the absolute temperature and x the reaction order. In the absence of measured data, these electrochemical models were obtained by adopting αanodic = αcathodic = 0.5, where α represents the symmetry of the activation barrier for the anodic and cathodic charge transfer reactions [23]. The assumption of nanodic = ncathodic = 1, where n represents the number of electrons involved in the corresponding electrochemical reaction, was based on the fact that one electron transfer reactions are thermodynamically more favorable [23].
The equilibrium described by Eqs. (10) and (11) assumes that ferric ion reduction is reversible, since the oxygen present in the solution favors the reoxidation of Fe2+ to Fe3+ ions. As predicted by all these equations and verified experimentally by others [11], when the acidity of the solution increases, the mixed potential and dissolution current density of chalcopyrite also increase. The observed increase when oxygen and ferric ion were added in the solution may also be predicted. The electrochemical properties were slightly affected by the presence of dissolved oxygen at low concentrations. In the case of ferric ion, if the Fe3+ ion concentration increases, the values of the mixed potential and dissolution density increase, as verified experimentally. In the presence of both oxidants, the increases could be associated with the reduction rates of the two oxidants on the solid surface and, in addition, the Fe2+ oxidation rate.
At the investigated conditions, the effects of Fe3+ ion addition on chalcopyrite electrochemistry are more pronounced than the effects of oxygen bubbling. This result could be related to the higher concentration of ferric ion (0.01‐0.05 mol/L) as compared to dissolved oxygen (0.001 mol/L). At such conditions, it was verified a slower oxygen reduction kinetics on the mineral surface, and a faster kinetics of ferric ion reduction [24], on the basis of Tafel plots.
Effects of pyrite inclusions on chalcopyrite electrochemistry
FeS2, when galvanically‐coupled to CuFeS2, provides a more favorable site for reduction reactions. Thus, the Fe3+ ion and oxygen cathodic reductions occur with a lower activation overpotential on the FeS2 surface than on CuFeS2 surface [24]. A description of the galvanic interaction established between these sulfides, according to the kinetics of the anodic and cathodic reactions occurring on the FeS2‐CuFeS2 couple, is shown in Figure 4. The galvanic coupling causes a positive shift of EM to EG (the galvanic potential), resulting in increase of the anodic current density (from ia to ia’) and the cathodic current density (from ic to ic’), but at the galvanic potential the net current remains equal to zero. Holmes & Crundwell (1995) [25] used a similar approach to discuss the galvanic interaction between galena and pyrite.
Regarding the increase of the mixed potential and dissolution current density when the FeS2 content on the chalcopyrite surface increases, it was assumed that FeS2 acts only as a cathodic site in this coupling. Hence, there are no anodic processes on FeS2 surface. In this case, one may assume that Acathodic > Aanodic since the reduction reactions are expected to occur on the cathodic sites on CuFeS2 as well. As Ianodic/Aanodic = ‐Icathodic/Acathodic = idiss, the cathodic current (i.e., the flow of electrons from the electrode to solution) is higher than the anodic current. Thus, the greater the cathodic discharge area available on the surface of ME electrodes, the higher the magnitude of the cathodic
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current and in turn, the higher the anodic current density (ia’) or chalcopyrite dissolution rate. The magnitude of the increase of the electrochemical properties of CuFeS2 with the increase of FeS2 content may be related to this enhancement of the ferric and oxygen reduction rates on the electrode surface, mainly on pyrite.
Figure 4 Electrochemical theory of the galvanic interaction between CuFeS2 and FeS2. Legend: EM = mixed potential; EG = galvanic potential; ia = anodic current density; and ic = cathodic current density
CONCLUSIONS
The magnitude of the galvanic effect of FeS2 on the electrochemistry of CuFeS2 in acidic sulfate solutions under typical, low‐temperature leaching conditions was investigated. Mineral electrodes were prepared with chalcopyrite samples containing various proportions of pyrite micro inclusions, in a way that the voltage losses across the mineral‐mineral contact were minimized. The positive effect of combining ferric ion and dissolved oxygen on chalcopyrite oxidation was quantified by measuring the increase in the mixed potential and dissolution current density. The effect was largely magnified by the galvanic pyrite‐chalcopyrite interaction. The findings of the present investigation may help to improve chalcopyrite leaching rate from low‐grade ores.
ACKNOWLEDGEMENTS
The authors are grateful to INCT, CNPq, CAPES and FAPEMIG for financial support.
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ic
ic’
ia
ia’
E
i
CuFeS2 = Cu1‐xFe1‐yS2‐z + x Cu2+(aq) + y Fe2+(aq) + z S + 2(x + y)
1: Oxidant + e‐ = Reductant (CuFeS2 surface)
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1 2
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– 12 –
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