An Improved Understanding of Chalcopyrite Leaching Mechanisms: The Inﬂuence of Anisotropic Crystal Planes

: Chalcopyrite (CuFeS 2 ) particles, exposing anisotropic crystal planes during the grinding process, possess comprehensive surface properties that affect their leaching behaviors. In order to investigate the inﬂuence of anisotropic crystal planes on the leaching mechanisms, CuFeS 2 particles with anisotropic crystal planes were produced by employing three-head laboratory grinder mill (TM), rod mill (RM), and ball mill (BM) and were then leached in a sulfuric acid solution at pH = 1. Based on the XRD, SEM, XPS, and simulation results, (112), (102), (312), (110), (116), (100), and (001) planes were mainly exposed on the CuFeS 2 surface during the crushing and grinding process. In addition, fewer (112), (102), (312), and (110) planes but more (116), (100), and (001) planes were exposed on the CuFeS 2 samples in the TM method than in the RM and BM methods. Since the leaching rates were in the order of (001) > (100) > (116) > (110) > (312) > (102) > (112) planes, the Cu extractions followed an order of TM > RM > BM. This study, therefore, provides an excellent theoretical basis for the effect of anisotropic crystal planes on CuFeS 2 leaching, further improving the understanding of CuFeS 2 leaching mechanisms.


Introduction
Copper is an important nonferrous metal that has been widely used in various areas such as the light, electrical, communications, construction, and machinery manufacturing industries and other industries [1].Copper metal is mainly purified from chalcopyrite (CuFeS 2 ), since approximately 70% of the Earth's copper resources are found as CuFeS 2 [2,3].The average copper grades are about 0.4% and 1~2% for open-cast and underground CuFeS 2 deposits [1], respectively.Generally, a flotation process is employed to obtain a CuFeS 2 concentrate [4], followed by the metallurgy process for copper production [5].In addition, pyrometallurgical methods are preferentially considered for high-grade CuFeS 2 concentrates (greater than 20%) [6].However, these methods lead to extensive environmental problems due to the emission of SO 2 [7,8].As an alternative to pyrometallurgy, hydrometallurgy has been paid much attention due to its advantages, such as being a more economical method of treating low-grade ores and being more environmentally friendly [3,5].
However, hydrometallurgical processing of CuFeS 2 has not been widely adopted in the industry due to the extremely slow leaching kinetics [9][10][11][12][13][14], mainly due to the formation of passivation layers on the CuFeS 2 surface [15][16][17][18][19][20][21].Thus, the investigation of the formation and elimination mechanisms of the passivation layer is very important for the application of CuFeS 2 hydrometallurgy.In recent years, various analytical methods have been applied to investigate the properties of the passivating layer on the CuFeS 2 surface [22][23][24][25], such as scanning electron microscopy (SEM), synchrotron scanning photoelectron microscopy (SPEM), transmission electron microscopy (TEM), and X-ray photoelectron spectroscopy Minerals 2023, 13, 1461 2 of 12 (XPS).However, there is no generally accepted theory as to the composition of the passivation layer.Córdoba et al. [3] reported that the passivation layers were predominantly composed of bimetallic sulfide, iron-deficient copper polysulfide, and elemental sulfide.Li et al. [5] reported that the formation of elemental sulfide, disulfide, polysulfide, and iron hydroxy-oxide inhibited the leaching of CuFeS 2 .In this case, it is extremely urgent to clarify the surface properties of CuFeS 2 to better understand the leaching mechanisms.
In addition, the surface properties of minerals are closely associated with the mineral's exposed planes.For instance, Wei et al. [26] indicated that the MoS 2 face (i.e., (001) plane) exhibited hydrophobic properties while the MoS 2 edges (i.e., (103), (105), (100), and (110) planes) exhibited hydrophilic properties.Therefore, investigations on the anisotropic crystal planes are very important for determining the surface properties of CuFeS 2 .Normally, the CuFeS 2 (112), ( 102), (312), ( 110), ( 116), (100), and (001) planes are exposed during the crushing and grinding process [27][28][29][30].The properties of these crystal planes have been widely investigated using density functional theory (DFT) calculations from the viewpoint of theory.For example, de Oliveira et al. [31] reported three different reconstruction mechanisms for CuFeS 2 -typical planes, wherein the reconstructed planes presented different properties.Wei et al. [27] investigated the relaxation and oxidation of the CuFeS 2 (001) and (112) planes on exposure to O 2 and H 2 O, indicating that the formation of disulfide (S 2 2− ) on the CuFeS 2 (001) plane and polysulfide (S n 2− ) on the CuFeS 2 (112) plane inhibited the leaching process.Meanwhile, the CuFeS 2 (001) plane per unit area was more easily oxidized than the (112) plane.However, the influence of the anisotropic crystal planes' properties on the actual leaching performance of CuFeS 2 and the underlying mechanisms remain unclear up to now.
In this study, three different grinding methods (i.e., a three-head laboratory mill with a grinder mill (TM), a rod mill (RM), and a ball mill (BM) were considered to obtain CuFeS 2 particles with anisotropic crystal planes, followed by the leaching experiment to investigate the influence of the anisotropic crystal plane on the leaching behavior of CuFeS 2 .The anisotropy of the CuFeS 2 raw samples was analyzed using X-ray diffraction (XRD) and SEM; XPS and DFT simulation were employed to investigate the anisotropic oxidation properties of the CuFeS 2 planes.This study, therefore, provides an excellent theoretical basis in revealing the influence of anisotropic crystal planes on CuFeS 2 leaching mechanisms, providing new insights for the application of CuFeS 2 hydrometallurgy.

CuFeS 2 Samples
The high-purity chunks of CuFeS 2 samples (Figure S1 and Table S1, Supplementary Materials, the content of CuFeS 2 was nearly 98 wt%) obtained from the Araci Mine in Peru were crushed to small particles (around 5 mm) and milled in three ways, i.e., TM (RK/XPM 120 × 3 three-head grinding machine, Wuhan Rock Grinding Equipment Manufacturing Co., Ltd., Wuhan, China), RM (XMB-70, three-roller, four-barrel rod mill, Wuhan Prospecting Machinery Factory, Wuhan, China), and BM (XMB-70, three-roller, four-barrel ball mill, grinding mediums using ceramic balls), followed by wet sieving to obtain particles with two size fractions of 38~75 µm and 75~150 µm.These samples were treated using ultrasonication (JP-020, Shenzhen Jie Meng Cleaning Equipment Co., Ltd, Shenzhen China) under ethanol solution conditions at 40 KHz for 2 mins to remove the attached fine particles from the sample surfaces, which were freeze-dried (LGJ-12, vacuum freeze dryer, Beijing Songyuanhuaxing Technology Develop Co., Ltd., Beijing, China) to avoid further surface oxidation.

Leaching Experiments
Briefly, 500 mL glass reactor vessels with a 5-port lid were used for the leaching experiments.A 1 g CuFeS 2 sample and 400 mL of a sulfuric acid solution at pH = 1 were poured into the reactor vessel to avoid interference from other ions [5] and were then heated using a thermostatically controlled silicone oil bath at 75 • C. The 5-port lid provided access Minerals 2023, 13, 1461 3 of 12 to the leach solution; three ports were used to house a thermometer, a Teflon impeller, and a reflux condenser, and the remaining two ports were covered with a plug to avoid moisture evaporation.The four-blade Teflon impeller was driven by a digitally controlled stirrer at a constant agitation speed of 170 rpm.
Subsequently, 5 mL solution samples were taken twice a day at intervals of 8 h and 16 h, respectively.Then, the solution was filtered through a 0.22 µm membrane for pH and Cu-concentration tests.The pH was maintained at 1 by adding 5 M H 2 SO 4 [5].

Bulk and Surface Analyses
In order to determine the exposed crystal planes for the CuFeS 2 samples treated through the TM, RM and BM methods, CuFeS 2 samples with size fractions of 38~75 µm and 75~150 µm were directly tested using XRD (D8 Advance, Bruker, Germany, with 2θ from 10 • to 70 • (Cu Kα) at a scanning speed of 0.2 • /min).The obtained data were analyzed using Jade 6.0 software (Materials Data, Inc., Livermore, CA, USA) to determine the exposed crystal planes corresponding to each diffraction peak based on the standard XRD pattern.Since the preferred orientation may exist in the XRD test for bigger CuFeS 2 particles, side-loading methods and the rotation of the sample table were considered to avoid the preferred orientation [32].SEM analyses (Zeiss Ultra Plus, Oberkochen, Germany) were conducted on the surface of leached CuFeS 2 , with an acceleration voltage of 30 keV.
A Thermo Scientific K-Alpha (Thermo Fisher-VG Scientific, Waltham, MA, USA) was used for XPS analyses of the leached CuFeS 2 .The XPS instrument was operated using an Al K-Alpha X-ray source (hv = 1486.6eV) at 12 kV and 6 mA with a pressure of 2.0 × 10 −7 m Bar.Next, 2.0 g of samples with a particle size of 38~75 µm were conditioned.The obtained spectra were analyzed using Avantage software (Thermo Fisher-VG Scientific, Thermo Fisher Scientific, Waltham, MA, USA).The binding energies were referenced to the C 1s peak at 284.8 eV.

Anisotropic Leaching Results
In order to investigate the influence of the anisotropic crystal plane on CuFeS 2 leaching, three different grinding methods (TM, RM, and BM) were employed to obtain CuFeS 2 particles with anisotropic crystal planes, which were then leached in a H 2 SO 4 solution.For the three CuFeS 2 samples with a particle size of 38~75 µm, a similar increasing trend of Cu extractions was obtained (Figure 1a), giving Cu extractions at around 76.2%, 78.9%, and 87.5% at 648 h for the TM, RM, and BM methods, respectively.In contrast, significantly different evolutions of Cu extractions were observed for the three CuFeS 2 samples with a particle size of 75~150 µm (Figure 1b), giving a Cu extraction order of TM > RM > BM.It should be noted that Cu was nearly completely extracted within 500 h for the TM method, while the Cu extractions were around 94% and 53% at 648 h for the RM and BM methods, suggesting that the TM method was beneficial for the leaching of bigger CuFeS 2 particles.while the Cu extractions were around 94% and 53% at 648 h for the RM and BM methods, suggesting that the TM method was beneficial for the leaching of bigger CuFeS2 particles.In addition, the leaching rate was higher for smaller CuFeS2 particles in the BM method due to their bigger surface area [5,35].However, the bigger CuFeS2 particles obtained through the TM and RM methods exhibited a higher leaching rate than the smaller particles, especially for the TM method, suggesting that the surface properties of the CuFeS2 particles in the three grinding methods led to significantly different leaching results.

Anisotropy of CuFeS2 Raw Samples
In order to investigate the differences in the surface properties, XRD and SEM tests were employed for the CuFeS2 raw samples.Figure 2 shows the XRD patterns; the typical exposed planes for all the CuFeS2 raw samples were the (112), ( 102), (312), ( 110), ( 116), (100), and (001) planes [27][28][29][30].For the size fraction of 38~75 µm, all diffraction peaks at various planes of the three CuFeS2 samples exhibited a similar intensity, suggesting that the exposed percentages of each plane for the three smaller CuFeS2 particles were similar (Figure 2a), since the intensity of the diffraction peak is highly related to the exposed percentage of the planes [27,33,36].In contrast, the intensities of diffraction peaks at various planes for the three CuFeS2 samples with a particle size of 75~150 µm were significantly different (Figure 2b), suggesting that bigger CuFeS2 particles from all three grinding methods had different exposed percentages for each plane, thus presenting different leaching rules as shown in Figure 1b.In other words, the anisotropic crystal planes of the CuFeS2 particles seemed to dominate their leaching behavior.In addition, the leaching rate was higher for smaller CuFeS 2 particles in the BM method due to their bigger surface area [5,35].However, the bigger CuFeS 2 particles obtained through the TM and RM methods exhibited a higher leaching rate than the smaller particles, especially for the TM method, suggesting that the surface properties of the CuFeS 2 particles in the three grinding methods led to significantly different leaching results.

Anisotropy of CuFeS 2 Raw Samples
In order to investigate the differences in the surface properties, XRD and SEM tests were employed for the CuFeS 2 raw samples.Figure 2 shows the XRD patterns; the typical exposed planes for all the CuFeS 2 raw samples were the (112), (102), (312), (110), ( 116), (100), and (001) planes [27][28][29][30].For the size fraction of 38~75 µm, all diffraction peaks at various planes of the three CuFeS 2 samples exhibited a similar intensity, suggesting that the exposed percentages of each plane for the three smaller CuFeS 2 particles were similar (Figure 2a), since the intensity of the diffraction peak is highly related to the exposed percentage of the planes [27,33,36].In contrast, the intensities of diffraction peaks at various planes for the three CuFeS 2 samples with a particle size of 75~150 µm were significantly different (Figure 2b), suggesting that bigger CuFeS 2 particles from all three grinding methods had different exposed percentages for each plane, thus presenting different leaching rules as shown in Figure 1b.In other words, the anisotropic crystal planes of the CuFeS 2 particles seemed to dominate their leaching behavior.In order to further investigate the anisotropy of the raw CuFeS2 samples, the normalized percentages of the typically exposed planes for three CuFeS2 samples based on the diffraction peak intensity of each plane were calculated (Figure 3 and Tables S2 and S3).Obviously, the normalized percentage rules of the ( 112), ( 102), ( 312), ( 110), ( 116), (100), In order to further investigate the anisotropy of the raw CuFeS 2 samples, the normalized percentages of the typically exposed planes for three CuFeS 2 samples based on the diffraction peak intensity of each plane were calculated (Figure 3 and Tables S2 and  S3).Obviously, the normalized percentage rules of the (112), ( 102), (312), ( 110), ( 116), (100), and (001) planes for the three CuFeS 2 samples with particle sizes of 38~75 µm and 75~150 µm were different, e.g., the differences in the normalized percentages between the standard value and the experimental value of the (112) plane for the TM, RM, and BM methods were −9.279%, −8.210%, and −7.456% for the three smaller CuFeS 2 particles, while they were −15.357%, −13.541%, and −6.307% for the three bigger CuFeS 2 particles, respectively, indicating that the three bigger CuFeS 2 particles exhibited greater differences in their exposure percentages.For the (001) plane, the three smaller CuFeS 2 samples still exhibited similarly normalized percentages for the TM, RM, and BM methods (i.e., 3.856%, 3.908%, and 3.264%, respectively).However, the three bigger CuFeS 2 particles exhibited different normalized percentages of the exposed planes for the TM, RM, and BM methods (i.e., 3.613%, 2.959%, and 2.053%, respectively), suggesting that the three bigger CuFeS 2 particles exhibited greater anisotropy than the smaller CuFeS 2 particles.In addition, the total percentages of the (112), (102), and (312) planes for the three bigger CuFeS 2 particles were in the order of TM > RM > BM, while the (110), ( 116), (100), and (001) planes exhibited a total percentage order of TM < RM < BM (Figure 3), further demonstrating the crystal plane anisotropy for the three bigger CuFeS 2 particles.In order to further investigate the anisotropy of the raw CuFeS2 samples, the normalized percentages of the typically exposed planes for three CuFeS2 samples based on the diffraction peak intensity of each plane were calculated (Figure 3 and Tables S2 and S3).Obviously, the normalized percentage rules of the (112), ( 102), (312), ( 110), ( 116), (100), and (001) planes for the three CuFeS2 samples with particle sizes of 38~75 µm and 75~150 µm were different, e.g., the differences in the normalized percentages between the standard value and the experimental value of the (112) plane for the TM, RM, and BM methods were −9.279%, −8.210%, and −7.456% for the three smaller CuFeS2 particles, while they were −15.357%, −13.541%, and −6.307% for the three bigger CuFeS2 particles, respectively, indicating that the three bigger CuFeS2 particles exhibited greater differences in their exposure percentages.For the (001) plane, the three smaller CuFeS2 samples still exhibited similarly normalized percentages for the TM, RM, and BM methods (i.e., 3.856%, 3.908%, and 3.264%, respectively).However, the three bigger CuFeS2 particles exhibited different normalized percentages of the exposed planes for the TM, RM, and BM methods (i.e., 3.613%, 2.959%, and 2.053%, respectively), suggesting that the three bigger CuFeS2 particles exhibited greater anisotropy than the smaller CuFeS2 particles.In addition, the total percentages of the (112), (102), and (312) planes for the three bigger CuFeS2 particles were in the order of TM > RM > BM, while the (110), ( 116), (100), and (001) planes exhibited a total percentage order of TM < RM < BM (Figure 3), further demonstrating the crystal plane anisotropy for the three bigger CuFeS2 particles.Figure 4 presents the SEM results of raw CuFeS 2 samples at 75~150 µm for the TM, RM, and BM methods.It can be clearly observed that the samples in the TM and RM methods were mainly long, irregular particles, while the samples in the BM method were mainly rounded particles.As shown in Table 1, the length-width ratios based on the average length and width for raw CuFeS 2 samples in the TM, RM, and BM methods were 1.776, 1.759, and 1.299, respectively, suggesting that the exposure percentage for each plane was related to the length-width ratio of the particles.Combining the normalized percentage of the typical exposed plane and the length-width ratio of the raw CuFeS 2 samples, the long, irregular particles in the TM method exposed fewer (112), (102), (312), and (110) planes but more (116), (100), and (001) planes than the rounded particles in the BM method.1.776, 1.759, and 1.299, respectively, suggesting that the exposure percentage for each plane was related to the length-width ratio of the particles.Combining the normalized percentage of the typical exposed plane and the length-width ratio of the raw CuFeS2 samples, the long, irregular particles in the TM method exposed fewer (112), ( 102), (312), and (110) planes but more (116), (100), and (001) planes than the rounded particles in the BM method.

XPS Analysis
The three bigger CuFeS2 samples were tested using XPS to investigate the species differences on the CuFeS2 surface.Figure 5 and Table 2 show the XPS survey spectra of the CuFeS2 surface and the elemental quantification (at %) of the CuFeS2 surfaces, respectively.The O 1s peak at 532.1 eV was found on the CuFeS2 surface, and the O contents were 43.87%, 53.37%, and 70.40% on the surface of CuFeS2 treated using the BM, RM, and TM methods, respectively, indicating that the order of the oxidation degree of the CuFeS2 surface treated using different methods was TM > RM > BM.

XPS Analysis
The three bigger CuFeS 2 samples were tested using XPS to investigate the species differences on the CuFeS 2 surface.Figure 5 and Table 2 show the XPS survey spectra of the CuFeS 2 surface and the elemental quantification (at %) of the CuFeS 2 surfaces, respectively.The O 1s peak at 532.1 eV was found on the CuFeS 2 surface, and the O contents were 43.87%, 53.37%, and 70.40% on the surface of CuFeS 2 treated using the BM, RM, and TM methods, respectively, indicating that the order of the oxidation degree of the CuFeS 2 surface treated using different methods was TM > RM > BM.The Fe contents on the surface of CuFeS2 treated using the BM, RM, and TM methods were determined to be 8.56%, 9.48%, and 4.88%, respectively, indicating that Fe was more readily leached from the surface of CuFeS2 when treated with the TM method.The Cu contents on the surface of the CuFeS2 treated using the BM, RM, and TM methods were found to be 38.31%,32.22%, and 16.64%, respectively, suggesting that Cu was more easily leached from the surface of CuFeS2 when subjected to the TM method, compared with both the BM and RM methods.This observation is consistent with the leaching results presented in Figure 5.The Fe contents on the surface of CuFeS 2 treated using the BM, RM, and TM methods were determined to be 8.56%, 9.48%, and 4.88%, respectively, indicating that Fe was more readily leached from the surface of CuFeS 2 when treated with the TM method.The Cu contents on the surface of the CuFeS 2 treated using the BM, RM, and TM methods were found to be 38.31%,32.22%, and 16.64%, respectively, suggesting that Cu was more easily leached from the surface of CuFeS 2 when subjected to the TM method, compared with both the BM and RM methods.This observation is consistent with the leaching results presented in Figure 5.
Figure 6 and Table 3 show the fitted S 2p spectra and S species of the bigger CuFeS 2 particles treated using the BM, RM, and TM methods.The S 2p 3/2 component located at 161.4 eV for CuFeS 2 was attributed to the monosulfide S 2− species [37,38].The 2p 3/2 spectra due to S 2 2− , S n 2− , S 0 , SO 3 2− , SO 4 2− and the energy loss of CuFeS 2 were located at 162.2, 163.2, 163.7, 166.3, 168.8, and 165.1 eV, respectively [38][39][40].The concentrations of S 2 2− , S n 2− , S 0 , SO 3 2− , and SO 4 2− on the BM-treated CuFeS 2 surface were 8.60%, 0%, 23.57%, 3.81%, and 34.59%, respectively (Table 3).The presence of oxidized S species indicated that the surface S 2− of CuFeS 2 was oxidized to S 2 2− , S n 2− , and S 0 , which were the main substances forming the passivation layer [41].The passivation layer composed of S 2 2− , S n 2− , and especially S 0 , hindered the further leaching of CuFeS 2 , which was the main reason for the low leaching rate of CuFeS 2 treated using the BM method.The concentrations of S2 2− , Sn 2− , S 0 , SO3 2− , and SO4 2− on the RM-treated CuFeS2 surface were 10.32%, 5.50%, 8.12%, 0.54%, and 39.02%, respectively (Table 3).The S species content on the surface of CuFeS2 treated using the TM method was comparable to that observed on the surface of CuFeS2 treated using the RM method, except for a higher presence of SO4 2− compounds.The low concentration of S 0 and the high concentration of SO4 2− indicated that the surface of CuFeS2 treated using the RM and TM methods had reduced passivation layer formation, enhancing the generation of highly oxidized SO4 2− during leaching, further facilitating the leaching process.The highest SO4 2− content was found on the CuFeS2 surface treated using the TM method, indicating a significant oxidation reaction  The concentrations of S 2 2− , S n 2− , S 0 , SO 3 2− , and SO 4 2− on the RM-treated CuFeS 2 surface were 10.32%, 5.50%, 8.12%, 0.54%, and 39.02%, respectively (Table 3).The S species content on the surface of CuFeS 2 treated using the TM method was comparable to that observed on the surface of CuFeS 2 treated using the RM method, except for a higher presence of SO 4 2− compounds.The low concentration of S 0 and the high concentration of SO 4  2− indicated that the surface of CuFeS 2 treated using the RM and TM methods had reduced passivation layer formation, enhancing the generation of highly oxidized SO 4 2− during leaching, further facilitating the leaching process.The highest SO 4 2− content was found on the CuFeS 2 surface treated using the TM method, indicating a significant oxidation reaction and the maximum leaching rate.Overall, the CuFeS 2 samples treated using the BM, RM, and TM methods in the same leaching conditions underwent varying degrees of oxidation, especially showing different evolutions for the S species on the chalcopyrite surface.These results indicate that the anisotropic crystal planes of CuFeS 2 significantly affected its leaching behavior.

The Properties of Anisotropic Crystal Planes
In order to further investigate the reactivities of CuFeS 2 planes, the properties of the CuFeS 2 planes were investigated through the same calculation methods as in our previous research (e.g., only Fe and S sites were considered for the adsorption of O 2 ) [27].Due to the symmetry of the CuFeS 2 structure (i.e., each Cu/Fe atom is bonded with four surrounding S atoms, each S atom is bonded with two surrounding Cu atoms and two surrounding Fe atoms) [31], the CuFeS 2 (112), (312), ( 116), (100), and (001) planes exposed both sulfurterminated (only exposing S atoms) and metal-terminated (only exposing Fe/Cu atoms) planes, while the CuFeS 2 (102) and (110) planes exposed mixed terminated (exposing both S and Fe/Cu atoms) planes during the cleavage process (Figure S2), which fully exhibited the anisotropic structure of CuFeS 2 planes.
Table 4 exhibits the surface energies of the CuFeS 2 planes, giving the order of (112)-S < (112)-M < (102) < (312)-M < (312)-S < (110) < (116)-S < (116)-M < (100)-S < (100)-M < (001)-S < (001)-M planes, suggesting that the stabilities of the CuFeS 2 planes were in the order of (112) > (102) > (312) > (110) > (116) > (100) > (001) planes [27,[42][43][44], consistent with the exposed percentage of CuFeS 2 planes for all the grinding methods (Figure 3), e.g., the CuFeS 2 (112) plane obtained the lowest surface energy (0.923 J•m −2 for the (112)-S plane and 1.018 J•m −2 for the (112)-M plane) and was, thus, preferentially exposed during the grinding process [27], occupying the highest exposed percentage (more than 40%, Table S1).It should be noted that the CuFeS 2 (001) planes with the highest surface energy (1.495 J•m −2 for the (001)-S plane and 1.556 J•m −2 for the (001)-M plane) were less easily exposed during the grinding process, occupying the lowest exposed percentage (less than 5%, Table S1).27,34,35].For example, de Lima et al. [34,45] reported that the adsorption energies were −142.12,−105.34, and 95.30 kJ•mol −1 for SO 4 2− , HSO 4 − , and H 2 O at Fe sites on the CuFeS 2 (001) plane.Wei et al. [27] reported that the adsorption energies of H 2 O and O 2 at Fe sites on the CuFeS 2 (001) plane were −153.94 and −556.12kJ•mol −1 , respectively.In addition, the reactivity of the Fe sites was stronger than that of the Cu sites on the CuFeS 2 metal-terminated planes [27].In this case, only the adsorption of O 2 at Fe and S sites on the CuFeS 2 planes was considered to compare the reactivity of each plane, and the results are shown in Table 4 and Figures 7 and S3.It was clearly seen that the adsorption energy per unit area at Fe sites on each plane was more negative than that at S sites, suggesting that these Fe sites were more easily oxidized than the S sites due to the higher affinity of Fe sites for O2 than that of S sites for O2 [38].In addition, it was clearly seen that the adsorption energies per unit area at Fe sites It was clearly seen that the adsorption energy per unit area at Fe sites on each plane was more negative than that at S sites, suggesting that these Fe sites were more easily oxidized than the S sites due to the higher affinity of Fe sites for O 2 than that of S sites for O 2 [38].

Figure 4 .
Figure 4. SEM images of raw CuFeS2 samples (75~150 µm) obtained through three grinding methods: (a) TM; (b) RM; (c) BM.The average length and width of each raw CuFeS2 sample was calculated based on 5 lengths and 5 widths as shown in (a) (the red lines), respectively.

Figure 4 .
Figure 4. SEM images of raw CuFeS 2 samples (75~150 µm) obtained through three grinding methods: (a) TM; (b) RM; (c) BM.The average length and width of each raw CuFeS 2 sample was calculated based on 5 lengths and 5 widths as shown in (a) (the red lines), respectively.

Minerals 2023 ,
13,  x FOR PEER REVIEW 10 of 13 adsorption of O2 at Fe and S sites on the CuFeS2 planes was considered to compare the reactivity of each plane, and the results are shown in Table4and Figures7 and S3.

Table 1 .
Average length, width, and length-width ratio for three raw CuFeS 2 samples.

Table 1 .
Average length, width, and length-width ratio for three raw CuFeS2 samples.

Table 3 .
S species (at %) on CuFeS2 surfaces treated under different conditions.

Table 3 .
S species (at %) on CuFeS 2 surfaces treated under different conditions.

Table 4 .
Area, surface energy, and adsorption energy per unit area of O 2 for CuFeS 2 planes.Since the leaching experiments were performed in a sulfuric acid solution at pH = 1, components such as H 2 O, O 2 , SO 4 2− , and HSO 4 − that could react with the CuFeS 2 planes were present in the solution.Research indicated that O 2 was adsorbed on the CuFeS 2 planes prior to the adsorption of H 2 O, SO 4 2− , and HSO 4 − [ [27]ese data have been published in our previous research[27].