Chalcopyrite Bioleaching Using Adapted Mesophilic Microorganisms: Effects of Temperature, Pulp Density, and Initial Ferrous Concentrations

Abstract

This study examines the effects of several operating parameters on copper leaching from chalcopyrite ores using an adapted mesophilic bacterial culture. Three temperatures (35, 40, and 45°C), three pulp density (1, 2, and 4% (w/v)), and three initial ferrous ion (Fe(II)) concentrations (5, 10, and 20 g/L) were employed as variable parameters, and their effects on the bioleaching efficiency of chalcopyrite were investigated. After 14 days, the maximum copper bioleaching efficiency was estimated to be ∼64% at a temperature of 45°C, a pH of 1.5, an initial ferrous concentration of 5 g/L, and a pulp density of 4%. More specifically, the chalcopyrite dissolution tests conducted at different temperatures showed a minimal effect of temperature and low leaching efficiency (<20%) regardless of temperature. The trend of chalcopyrite dissolution at different pulp densities showed that Cu extraction tended to increase with increases in pulp density. Moreover, the Cu leaching efficiency associated with mesophilic microorganisms largely decreased when the initial Fe(II) concentration was greater than 10 g/L. The Cu leaching behavior in different test conditions was evalauted with concentrations of total iron (Fe), Fe(II), and ferric ions (Fe(III)), as well as the oxidation-reduction potential (ORP) of the solution used in the test. The Cu leaching rate increased under lower ORP conditions, lower Fe(III):Fe(II) ratios, and balanced Fe(II)–Fe(III) cycles.

1. Introduction

Current mineral processing industries focus on developing advanced, low cost, less energy intensive, and eco-friendly technologies.^1–5) Since pyrometallurgy had an advantage that was unaffected by the type of feed materials, it was a technique that had previously been extensively used for metal extraction. However, due to the high initial investment cost and the high energy consumption, a hydrometallurgy technique has been reported, since it requires relatively low initial investment cost and less energy consumption compared to pyrometallurgy. Moreover, it has been proven for low grade ores,^6,7) which allows the technique to be more widely used for various minerals. A significant amount of effort has been directed toward developing a hydrometallurgical process suitable to leach copper (Cu) from sulfide ores.^8,9) However, as environmental legislation has become stricter, eco-friendly techniques for leaching copper are in higher demand, and consequently, a biohydrometallurgy technique has been considered suitable.^2,10–13) Additionally, the biohydrometallurgy has been reported as cost effective and is applicable for low-grade ores^6,14) because it requires less energy. Hence, studies have also reported that biohydrometallurgy could be a suitable technology for Cu leaching from low-grade chalcopyrite.^4,15)

The bioleaching of chalcopyrite using mesophilic microorganisms such as Acidithiobacillus ferrooxidans (A. ferrooxidans), Acidithiobacillus thiooxidans (A. thiooxidans) and Leptospirillum ferriphilum (L. ferriphilum) has been extensively studied.^16–20) The bioleaching of chalcopyrite includes two steps: the reduction of chalcopyrite to chalcocite (Cu₂S) (eq. (1)); and then dissolution of Cu(II) from the chalcocite (eq. (2)). The role of microorganisms during the bioleaching of chalcopyrite is to generate sulfuric acid that provides protons for mineral hydrolysis and to produce Fe(III) for the oxidation of the mineral.²¹⁾ (eqs. (3) and (4)):   

\begin{equation} \text{CuFeS$_{2}$} + \text{3Cu$^{2+}$} + \text{3Fe$^{2+}$} \rightarrow \text{2Cu$_{2}$S} + \text{4Fe$^{3+}$} \end{equation} (1)

  

\begin{equation} \text{2Cu$_{2}$S} + \text{8Fe$^{3+}$} \rightarrow \text{4Cu$^{2+}$} + \text{2S$^{0}$} + \text{8Fe$^{2+}$} \end{equation} (2)

  

\begin{equation} \text{4Fe$^{2+}$} + \text{4H$^{+}$} + \text{O$_{2}$} \xrightarrow{\text{Bacteria}} \text{4Fe$^{3+}$} + \text{2H$_{2}$O} \end{equation} (3)

  

\begin{equation} \text{S$^{0}$} + \text{3/2O$_{2}$} + \text{H$_{2}$O} \xrightarrow{\text{Bacteria}} \text{H$_{2}$SO$_{4}$} \end{equation} (4)

However, under mesophilic conditions, chalcopyrite is often characterized by slow kinetics and low Cu recovery due to the refractory nature of chalcopyrite in the leaching process.²²⁾ The primary reason for this refractory characteristic is the formation of a passivation layer, such as Cu polysulfide, sulfide, elemental sulphur, and iron hydroxyl that precipitates on the mineral surface, which limits both reagent contact and diffusion, and ultimately slows the dissolution rate of chalcopyrite.^22,23) Previous studies have demonstrated that the ferric:ferrous (Fe(III):Fe(II)) ratio and the controlling ORP solution both play a fundamental role in determining the leaching rate.^24–27) The reactions listed above (eqs. (1)–(4)) reflect the influence of Fe(II) and Fe(III) concentrations on chalcopyrite dissolution. More specifically, it has been reported that chalcopyrite dissolution is inhibited when the ORP is higher than a critical value, and at a Fe(III):Fe(II) ratio of approximately 1:1.^25,28,29) Kametani and Aoki (1985) identified a critical ORP of ∼450 mV,²⁶⁾ although a relatively low ORP in the range of 400 to 425 mV has also been reported as favorable for chalcopyrite dissolution,³⁰⁾ which suggests the values for effective bioleaching could be system-dependent. Moreover, the bioleaching of sulfide minerals is known as a complex process with a number of factors such as pH, temperature, and oxidation-reduction potential (ORP) affecting bacterial activity and chemical dissolution in minerals.^17,31–33)

Thus, the present study was designed as a preliminary study of column bioleaching to obtain insight into the effects of several operating parameters on Cu dissolution from chalcopyrite ores. The tests were conducted using a batch system approach with mesophilic microorganisms pre-adapted to Cu and chalcopyrite.

2. Materials and Methods

2.1 Mineral samples and microorganisms

All the tests were conducted with chalcopyrite ore obtained from China. The Cu and sulphur (S) concentrations in the chalcopyrite ore were 2.29 mass% and 27.71 mass%, respectively. The x-ray diffraction (XRD) was used to analyze the mineralogy of the ore;³⁴⁾ the results indicated that the raw chalcopyrite ore consisted primarily of quartz, jarosite and pyrite (Fig. 1). The mineral was sieved in dry conditions to obtain a size fraction of 37–74 µm and then was washed three times consecutively with 1 M HCl, then once with pure ethanol, and was rinsed with distilled water to sterilize the samples.³⁵⁾ Then, the sterilized samples were dried at room temperature in a vacuum desiccator until the use of the samples. All the chemicals used in the experiments were reagent grade.

Fig. 1

X-ray diffraction patterns of a chalcopyrite ore within the 2θ range of 5°–90° in steps of 0.03°, with a counting time of 85 s/step. All X-ray diffraction peaks correspond to those in the standard spectrum of Quartz (JCPDS #33-1161), Jarosite (JCPDS#10-0443), and Pyrite (JCPDS# 65-3321).

A mix of mesophilic acidophiles was cultured in a mineral salt medium 0K enriched with ferrous sulfate (FeSO₄) at 5 g/L. The mesophilic culture contains strains of Acidithiobacillus caldus (A. caldus) and (L. ferriphilum), and the microbe’s tolerance to Cu was brought up to 40 g/L and 10% (w/v) of solid concentration by continuous sub-culturing in a stirred tank reactor. The optimum pH for the growth of A. caldus and L. ferriphilum are known as 1.0–3.5 and 1.3–1.8, respectively.³⁶⁾ As far as the optimum temperature is concerned, A. caldus is more effective at 45°C while L. ferriphilum optimum growth temperature ranges between 30 and 37°C.³⁶⁾

2.2 Bioleaching experiments

The bioleaching experiments were conducted in 500 mL Erlenmeyer flasks with 250 mL of operating volume. The medium was sterilized in an autoclave at 0.1 MPa and 121°C for 30 min.³⁷⁾ Then, the solution was enriched with FeSO₄ as an energy source and incubated in rotary shakers at 200 rpm. Components of medium included the following basal salts: (NH₄)₂SO₄, KCl, K₂HPO₄, MgSO₄.7H₂O, Ca(NO₃)₂, distilled water and 10 N H₂SO₄.³⁸⁾ The initial pH was adjusted to 1.5 with sulfuric acid. The series of bioleaching experiments were designed at different conditions: temperature, 35, 40, and 45°C; pulp density, 1%, 2%, and 4% (w/v); and initial Fe(II) concentration, 5 g/L, 10 g/L, and 20 g/L. Evaporation was compensated with acidified water. All the experiments were performed at least in triplicate.

2.3 Analytical methods

Soluble Cu was determined by inductively coupled plasma (ICP-OES, Optima 7300DV, PerkinElmer). Total Fe and Fe(II) were determined by 1-10-phenanthroline method.³⁹⁾ The pH was measured at each sampling interval using a pH probe (ORION 4STARS, Thermo). Silver/silver chloride double reference oxidation-reduction probe (Hanna Instruments, model 2211) was used to measure the ORP. Raw samples and leach residues were characterized using X-ray Power Diffraction (XRD) analysis within the 2θ range of 5°–90° in steps of 0.03°, with a counting time of 85 s step⁻¹.

3. Results and Discussion

3.1 Effect of temperature

Figures 2(a)–(d) show the Cu leaching efficiency, ORP, Fe(III):Fe(II) ratio, and total Fe concentration as a function of time. The tests were conducted at three different temperatures (35, 40, and 45°C) while other parameters held constant (i.e., initial pH = 1.5, solid concentration = 1% (w/v), initial Fe(II) concentration = 5 g/L). Overall, Cu leaching tended to increase with increasing operating temperature (Fig. 2(a)), and the leaching efficiency at 19 days was <20% at all test conditions. However, the difference was not statistically significant.

Fig. 2

Cu leaching efficiency (a), variations in ORP (b), Fe(III):Fe(II) ratio (c), and total Fe concentration (d) in leachate over time at different temperatures during chalcopyrite bioleaching. For all the cases, the experiments were conducted at an initial pH of 1.5, solid concentration of 1% (w/v), and an initial Fe(II) concentration of 5 g/L.

The slow leaching rate at all temperatures could be attributed to the formation of a passivation layer on the chalcopyrite surface, which is known to inhibit the contact between the microorganisms and the mineral surface.²⁴⁾ Previous studies have reported that the passivation phenomenon in chalcopyrite bioleaching is related to a high ORP, (i.e., the higher ORP, the higher the probability of formation of passivation layers).²⁴⁾ Consistent with this report, the ORP values observed in this study sharply increased to >600 mV after 2 days of leaching and remained constant until the end of the test. This was likely due to the high oxidation rate of Fe(II) to Fe(III) promoted by high bacterial activity, which was very pronounced from the beginning of the test.^11,12) This high ORP value is consistent with low Cu bioleaching (<20%) over the entire test period. It is important to note that the steady ORP value (>600 mV) was much greater than the critical maximum ORP value of ∼400–450 mV, which has been previously reported as the critical value for effective bioleaching of chalcopyrite.^26,30) Here, it should be noted that the pH values at the end of this study (∼1.2–1.48) were in the reported optimum range of pH (see Section 2.1); hence, the bacterial activities were not inhibited by pH.

Previous studies reported that iron (Fe) species plays a key role in the bioleaching of chalcopyrite.^35,40) Fe(III) dissolves chalcopyrite to generate Cu and Fe(II) ions. Fe(II) as the energy source of bioleaching microorganisms is oxidized to regenerate the Fe(III) needed for the reaction. This implies that the Fe(II)–Fe(III) cycle is important to the effectiveness of chalcopyrite bioleaching, and thus, we examined the change in the concentration of total Fe, Fe(III), and Fe(II). As Figs. 2(c) and (d) show, the total Fe concentration sharply decreased from 4000 mg/L to 1000–2000 mg/L after 2 days of testing, and the Fe(III):Fe(II) ratio increased above the critical Fe(III):Fe(II) ratio of 1:1, which has been reported as a maximum for effective chalcopyrite bioleaching.^25,28,29) After 2 days of testing, the total Fe concentration continuously decreased over time, and the Fe(III):Fe(II) ratio remained much greater than the critical 1:1 value. It should be noted that the critical Fe(III):Fe(II) ratio is closely related to the critical ORP value because they are interconnected. Such a high Fe(III):Fe(II) ratio and the dramatic decrease in total Fe concentration after 2 days of testing provided valuable insight. First, Fe(III) was the dominant species of Fe in solution over the entire test period, which eventually led to a high ORP value in the solution. Second, it is likely the abundance of Fe(III) in solution and high ORP led to the formation and precipitation of stable Fe(III) compound (e.g., jarosite) that caused chalcopyrite passivation. All of these reasons accordingly resulted in the observed low Cu bioleaching rate (<20%) over the entire test period. This is consistent with the findings of Hiroysoshi et al. (2000)²⁸⁾ which reported that the inhibition of chalcopyrite dissolution during bioleaching with mesophilic microorganisms is due to the excessive biooxidation of Fe(II).

The XRD analysis in Fig. 3 supports the formation of a passivation layer further. Comparison of the peak ratio of quartz and jarosite (I_Quartz:I_Jarosite) suggests that jarosite precipitation on the mineral surface is more readily observed with bioleaching residues than raw chalcopyrite samples. The ratio of I_Quartz:I_Jarosite was determined to be 0.48, 0.29, and 0.43 for the residues treated at 35°C, 40°C, and 45°C while the ratio for raw sample 0.09. This observation is not consistent with a previous study that reported greater jarosite formation at low temperatures.⁴¹⁾ In addition, Wang et al. (2006) reported that when the temperature increased from 36°C to 45°C, schwertmannite was substituted with ammonium/hydronium jarosite.⁴²⁾ This substitution requires the presence of monovalent ions and an increase in the temperature to facilitate the process.

Fig. 3

X-ray diffraction patterns of bioleaching residues at different temperatures (35, 40, and 45°C) within the 2θ range of 5°–90° in steps of 0.03°, with a counting time of 85 s/step. The ratio of relative intensity of quartz and jarosite (I_Quartz:I_Jarosite) was determined with the prime peaks in (1 0 1) plane of quartz and (1 1 3) plane of jarosite. The pattern of chalcopyrite ores is also presented for comparison.

The results suggest that despite the positive action of microorganisms in generating the leaching agent for chalcopyrite dissolution, the increase in Fe(III) concentration provokes chemical instability in the system, favoring the formation of a passivation layer of chalcopyrite. To increase the Cu dissolution rate, the chalcopyrite surface needs to be activated by increasing the Fe(II) concentration. Accordingly, in subsequent tests, the leaching behavior at higher pulp densities and higher initial Fe(II) concentrations were evaluated.

3.2 Effect of pulp density

Akcil et al. (2007) suggested that, with increasing pulp density, the bacteria-to-solids ratio would likely become too low to generate sufficient Fe(III) and, consequently, maintain an ORP lower than the critical value under which chalcopyrite dissolution is more favorable.⁴³⁾ Hence, to test this hypothesis, the chalcopyrite bioleaching experiments were conducted under different pulp densities of 1%, 2%, and 4% (w/v). Figures 4(a)–(d) show Cu leaching efficiency, ORP, total Fe concentration, and the Fe(III):Fe(II) ratio as a function of time at different pulp densities, while other parameters were held constant (i.e., initial pH = 1.5, temperature = 45°C, initial Fe(II) concentration = 5 g/L). Final Cu leaching efficiency increased with increasing pulp densities, as follows: 11%, 48%, and 64% at pulp densities of 1, 2, and 4%, respectively. More specifically, Cu leaching was always lower than 20% at a pulp density of 1%. On the other hand, similar Cu leaching rates were observed for 2% and 4% pulp density conditions after 6 days of testing, and thereafter, a higher leaching rate was observed at 4% pulp density compared to the rate at a 2% pulp density.

Fig. 4

Cu leaching efficiency (a), variations of ORP (b), Fe(III):Fe(II) ratio (c), and total Fe concentration (d) in leachate over time at different pulp densities (1, 2, and 4% (w/v)) during chalcopyrite bioleaching. For all the cases, the experiments were conducted at an initial pH of 1.5, a temperature of 45°C, and initial Fe(II) concentration of 5 g/L.

The difference in the observed Cu leaching trend according to pulp density is very consistent with analysis of the ORP presented in Fig. 4(b). At a 1% pulp density, the solution ORP exceeded the critical ORP value (∼400–450 mV) after only 2 days of testing, and it was maintained at ∼700 mV for the entire test period. Accordingly, the Cu leaching rate was very slow, with a relatively low final leaching efficiency that was <20%. Meanwhile, the solution ORP at 2% and 4% pulp densities did not exceed the critical value until the sixth day of testing, which is consistent with the similar Cu leaching rates in both conditions (Fig. 4(a)). However, the ORP at 2% pulp density eventually exceeded the critical value, while remaining lower than the critical value at 4% pulp density. Consequently, Cu leaching consistently increased at a 4% pulp density during the entire test period, whereas the leaching rate suddenly dropped after 6 days of testing for 2% pulp density. It is worth nothing that the solution pH observed at the three pulp densities under analysis showed similar trends (data not shown); the profiles increased at the beginning of the test and dropped after 6 days, and the pH values were remained at the optimum range indicated in Section 2.1. The solution pH at each pulp density value at the end the test were as follows: 1%, 1.19; 2%, 1.21; and 4%, 1.57.

To understand the Cu leaching trend further, total Fe concentration and the Fe(III):Fe(II) ratio over time were examined, and these results are presented in Figs. 4(c) and 4(d), respectively. The critical value of the Fe(III):Fe(II) ratio (∼1) appears to still be effective in our system when comparing Cu leaching efficiency and the Fe(III):Fe(II) ratio. Specifically, the Fe(III):Fe(II) ratio was lower than the critical value at both 2% and 4% pulp densities after 6 days of testing. Thereafter, the ratio became higher than the critical value at a pulp density of 2%, while it remained lower than the critical value over the entire test period at a 4% pulp density. This is consistent with the ORP and Cu leaching trend, such that the higher Fe(III):Fe(II) ratio correlates with the higher ORP, the lower leaching rate, and with the sudden drop in the leaching rate when it exceeds the critical value. Change in the total Fe concentration over time (Fig. 4(d)) also explains the observed Cu leaching trend. Quantitatively, the rapid decrease from 4000 mg/L to <2000 mg/L at a pulp density of 1% indicates the loss of Fe(III) ions via Fe(III)-associated compound, which led to the formation of a passivation layer and consequently the low leaching rate. On the other hand, at both 2% and 4% pulp densities, the total Fe concentration statistically follows the same trend during 6 days of testing. It first increased, which implies no precipitation occurred, or the iron release rate from the chalcopyrite was greater than the precipitation rate, then reached a plateau. Thereafter, over time, total Fe concentration started to decrease in both conditions, with a greater rate of decrease at a pulp density of 2%. This trend correlates well with the leaching rate, the ORP, and the Fe(III):Fe(II) ratio, and the leaching rate at a 2% pulp density began to slow down significantly after 6 days of testing (Fig. 4(a)), where the ORP and Fe(III):Fe(II) ratio exceeded their critical values (Figs. 4(b) and 4(c)), and the chalcopyrite passivation then largely occurred. Moreover, the XRD results of bioleaching residues in Fig. 5 show that the I_Quartz:I_Jarosite was clearly higher at a 1% pulp density in contrast to 2 and 4% conditions, which strongly supports the lowered leaching rate at a 1% pulp density (I_Quartz:I_Jarosite = 0.48, 0.23, and 0.06 for the residues with 1, 2, and 4% pulp density, respectively).

Fig. 5

X-ray diffraction pattern of bioleaching residues at different pulp densities (1, 2, and 4% (w/v)) within the 2θ range of 5°–90° in steps of 0.03°, with a counting time of 85 s/step. The ratio of relative intensity of quartz and jarosite (I_Quartz:I_Jarosite) was determined with the prime peaks in (1 0 1) plane of quartz and (1 1 3) plane of jarosite. The pattern of chalcopyrite ores is also presented for comparison.

3.3 Effect of Fe(II) addition

The results obtained by varying the pulp density clearly demonstrated the importance of the Fe(III):Fe(II) ratio on Cu bioleaching from chalcopyrite. Hence, we investigated the effect of the initial Fe(II) concentration on chalcopyrite bioleaching. Figures 6(a)–(d) show Cu leaching efficiency, ORP, total Fe concentration, and Fe(III):Fe(II) ratio as a function of time with different initial Fe(II) concentrations of 5, 10, and 20 g/L. Other parameters were constantly maintained (i.e., initial pH = 1.5, solid concentration = 4%, temperature = 45°C). Based on the Cu leaching results, two distinct trends were observed. Cu leaching continuously increased over time when the initial Fe(II) dosage was 5 and 10 g/L. On the other hand, at initial Fe(II) concentrations of 20 g/L, Cu leaching efficiency initially increased and then reached a plateau, with an inflection point observed at 5–7 days of testing. The leaching efficiencies at the end of test (14 days) were ∼64%, ∼61%, and 41% at 5, 10, and 20 g/L Fe(II), respectively.

Fig. 6

Cu leaching efficiency (a), variations of ORP (b), Fe(III):Fe(II) ratio (c), and total Fe concentration (d) in leachate through time at different initial Fe(II) concentrations (5, 10, and 20 g/L) during chalcopyrite bioleaching. For all cases, the experiments were conducted at an initial pH of 1.5, a temperature of 45°C, and a solid concentration of 4% (w/v).

To better interpret the cause of the differences in the Cu leaching rate over initial Fe(II) levels, the concentration of total Fe, Fe(III), and Fe(II) as well as the ORP were analyzed. Figure 6(b) shows the ORP change in reactors with different initial Fe(II) concentrations as a function of time. In all the cases, the ORP sharply increased until the third day of testing, and tended to slowly increase over time, and finally reached 430–450 mV after 14 days. Similar results were observed with changes in the initial Fe(II) concentration, and all the data are distributed around the critical ORP value (400–450 mV), However, close inspection of the data reveals that the trend can be divided into two groups (i.e., low Fe(II) (5 and 10 g/L) and high Fe(II) (20 g/L)), and the difference in the ORP began to occur on the fifth day of testing, with the difference becoming larger over time. For instance, the ORP was ∼10 mV and ∼20 mV higher with high initial concentrations of Fe(II) after 5 and 14 days of testing. To complement the ORP measurement, we analyzed the Fe(III):Fe(II) ratio and total Fe concentration further, and the results are presented in Figs. 6(c) and 6(d), respectively. The Fe(III):Fe(II) ratio sharply increased at the beginning of tests for all cases. However, the ratio gradually increased over time and got close to the critical value (∼1) at 20 g/L Fe(II). By contrast, the ratio did not continuously increase and reached a plateau with a ratio much lower than the critical value at 5 and 10 g/L conditions.

The comparison of the ORP and Fe(III):Fe(II) ratio with Cu leaching rate further supports the greater Cu leaching from chalcopyrite ores when the ORP and the Fe(III):Fe(II) ratios were lower as described in sections 3.1 and 3.2. The change in the total Fe concentration is more obvious and noteworthy. Specifically, while negligible change was observed over time at 5 and 10 g/L Fe(II) conditions, total Fe concentration continuously decreased until day 7 of testing and remained constant for the remainder of the test period at 20 g/L Fe(II). The significant decrease in total Fe concentration at 20 g/L Fe(II) suggests there is an imbalance in the Fe(II)–Fe(III) cycle during bioleaching (i.e., the Fe(II) oxidation rate is much higher than the Fe(III) reduction rate, which is likely due to Fe(III) precipitation). The XRD results in Fig. 7 strongly suggest the Fe(III) is largely precipitated as jarosite at 20 g/L Fe(II); the I_Quartz:I_Jarosite at 20 g/L Fe(II) was 0.5, which is much greater than those at 5 and 10 g/L (e.g., I_Quartz:I_Jarosite = 0.09 and 0.07 at 5 and 10 g/L, respectively).

Fig. 7

X-ray diffraction pattern of bioleaching residues at different initial Fe(II) concentrations (5, 10, and 20 g/L) within the 2θ range of 5°–90° in steps of 0.03°, with a counting time of 85 s/step. The ratio of relative intensity of quartz and jarosite (I_Quartz:I_Jarosite) was determined with the prime peaks in (1 0 1) plane of quartz and (1 1 3) plane of jarosite. The pattern of chalcopyrite ores is also presented for comparison.

4. Conclusions

In the present study, the effects of temperature (35, 40, and 45°C), pulp density (1, 2, and 4% (w/v)), and initial Fe(II) concentration (5, 10, and 20 g/L) on Cu dissolution from chalcopyrite ores were investigated as a preliminary study of column bioleaching. The findings are as follows:

1. (1)    The highest Cu bioleaching efficiency obtained in the testing was ∼64% at 45°C, a pH of 1.5, an initial Fe(II) concentration of 5 g/L, and a pulp density of 4%.
2. (2)    The effect of temperature on chalcopyrite dissolution was almost negligible, while both pulp density and initial Fe(II) concentration considerably influenced Cu bioleaching: higher Cu leaching under the condition of high pulp density and a low initial Fe(II) concentration. The leaching efficiency was closely related to the ORP and Fe(II)–Fe(III) distribution in the leachate, both of which can facilitate/inhibit passivation of the chalcopyrite surface by controlling the formation of Fe(III)-associated compounds (e.g., jarosite).

The results obtained from these experiments strongly suggest that the composition and concentration of Fe ions play important roles in chalcopyrite dissolution. The over-oxidation of the leaching solution favored the precipitation of Fe on chalcopyrite surfaces followed by passivation layering. Therefore, balancing Fe(II)–Fe(III) distribution in leaching solutions is critical for delaying Fe(III) precipitation and ultimately preventing the formation of a passivation layer.

Acknowledgments

This work was supported by the Korea Energy and Mineral Resources Engineering Program (KEMREP), the Research Base Construction Fund Support Program funded by Chonbuk National University in 2017, and the National Research Foundation of Korea (NRF) grant funded by the Korea government (MSIP) (No. 2017R1D1A1B03030796).

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