Effect of Mechanical Activation on Electrochemical Properties of Chalcopyrite in Iron-Containing Sulfuric Acid Solutions

Abstract

Mechanical activation significantly enhances the leaching of chalcopyrite, a process that is fundamentally electrochemical in nature. Thus, a comprehensive understanding of its impact on the electrochemical behavior of chalcopyrite in leaching systems is crucial. This study examines the effect of mechanical activation on the electrochemical and semiconductor properties of chalcopyrite in H₂SO₄ solutions containing Fe²⁺ or/and Fe³⁺ at pH = 1.5. Mechanical activation was carried out using a planetary ball mill at 700 rpm for durations ranging from 0 to 2.5 h to reduce particle size and induce lattice distortion, thereby increasing its electrochemical activity. In iron-containing electrolytes, mechanically activated chalcopyrite is more readily reduced, releasing Fe²⁺ and leading to a higher surface concentration of Fe²⁺, which consequently increases the diffusion coefficient at the solid–liquid interface. Mott–Schottky analysis revealed a decrease in flat band potentials (from 261.7 mV to 131.2 mV in 0.1 mol/L Fe³⁺ after 1.0 h of activation) and an elevation in Fermi levels. As a result, mechanical activation markedly accelerates the corrosion rate of chalcopyrite in ferric solutions—the corrosion current increased from 40.27 µA to 70.71 µA in 0.1 mol/L Fe³⁺ after 1.0 h of activation. These findings provide valuable insights for developing strategies to enhance mineral dissolution, and advance the hydrometallurgical processing of chalcopyrite.

1. Introduction

Chalcopyrite hydrometallurgy has been limited by its low leaching rate, which hinders large-scale implementation [1,2,3,4]. The H₂SO₄-Fe³⁺ system is widely employed for chalcopyrite leaching, but even with biological assistance, it fails to address the issue of slow dissolution kinetics. This may be due to the unique crystal structure of chalcopyrite and the formation of a ‘passivation’ layer during leaching. Metal-deficient sulfides and polysulfides are major contributors to reduced dissolution kinetics and a low leaching rate [5,6,7,8]. Additionally, semiconductor properties and electronic structure impact the leaching of chalcopyrite. From a semiconductor band theory perspective, natural chalcopyrite exhibits an electronic structure that impedes anodic current conduction in electrolyte solutions, resulting in slow dissolution rates [9,10,11,12].

Inhibiting the passivation product during chalcopyrite leaching and altering its microscopic electronic structure are crucial factors in improving its leaching kinetics. Researchers have devoted significant efforts to enhancing the leaching rate of chalcopyrite through mechanical activation pretreatment and controlling the redox potential within the leaching solution. Mechanical activation induces changes in microcrystal structure and physicochemical properties by applying high-strength mechanical force to minerals, resulting in reduced particle size, enhanced structural disorder, and modified semiconductor properties [13,14,15,16]. These alterations lead to increased reactivity and facilitate the dissolution of chalcopyrite [17]. Research findings from Bai et al. [14] and Granata et al. [18] demonstrated that mechanical activation significantly increased the crystal lattice distortion of chalcopyrite, thereby greatly accelerating its leaching kinetics and effectively increasing the leaching rate. Cao et al. [19] found that mechanical activation promoted the redox reaction on the surface of chalcopyrite, accelerated its dissolution, and mitigated passivation during bioleaching, resulting in an increase in the leaching rate from 9.39% to 87.41%. Yang et al. [20] investigated the impact of mechanical activation mode on the microstructure and leaching rate of chalcopyrite. They concluded that both dry- and wet-milling cause “sulfur channels” on the surface of chalcopyrite to weaken the “passivation” effect, but dry-milling led to more severe surface oxidation and selective fracture within its lattice, yielding a better leaching effect.

When Fe²⁺, Fe³⁺, and Cu²⁺ coexist in a solution, the potential is primarily determined by the concentration of Fe³⁺ and Fe²⁺ [21,22,23,24,25,26]. The ratio of Fe³⁺/Fe²⁺ significantly influences the leaching of chalcopyrite. Vilcaez et al. [22,23] have reported that when the solution potential exceeded 450 mV (vs. Ag/AgCl)—corresponding to high initial Fe³⁺ concentrations (5000 mg/L)—chalcopyrite underwent direct oxidation by Fe³⁺, resulting in ineffective leaching. Conversely, at potentials below 450 mV (vs. Ag/AgCl) with low initial Fe³⁺ concentrations (200 mg/L), chalcopyrite initially forms intermediate species that are readily oxidized and decomposed, thereby enhancing dissolution. Córdoba et al. [25] have validated the aforementioned model by demonstrating that effective leaching becomes increasingly difficult when the solution potential exceeds 500 mV (vs. Ag/AgCl). Additionally, Santos et al. [26] have discovered that the leaching rate of chalcopyrite reaches its peak at a solution potential of 410–470 mV (vs. Ag/AgCl) with an Fe²⁺ concentration between 0.125 and 0.215 mol/L, once again highlighting the crucial role played by both the presence and redox potential of Fe²⁺ in oxidation and dissolution processes.

As reported in previous work [27,28], mechanical activation significantly refines chalcopyrite particles. However, as activation time increases, particle size tends to stabilize, possibly due to inhibited agglomeration. XRD analysis in [28] showed decreased diffraction peak intensity and peak broadening, indicating altered internal crystal structure (reduced grain size and increased micro-strain).

However, there has been limited research on electrochemical behavior and electronic structure analysis regarding mechanically activated chalcopyrite in a leaching system. Therefore, this study investigated the effect of mechanical activation on the electrochemical behavior of chalcopyrite in the leaching system, aiming to explore the electrochemical properties and semiconductor properties of mechanically activated chalcopyrite in the H₂SO₄-Fe³⁺ or/and Fe²⁺ system, which may help to improve its hydrometallurgical process and use electrochemical technology to explore the effect of mechanical activation, which is of great significance for chalcopyrite leaching.

2. Materials and Methods

2.1. Chalcopyrite Samples

The chalcopyrite sample was obtained from Dongchuan City, Yunnan Province, China. The particle size of the chalcopyrite sample used for mechanical activation testing is less than 0.15 mm. The sample was mainly chalcopyrite peaks without other obvious impurity peaks, as detailed in previous work [27,28]. Chemical analysis demonstrated that the sample comprised 32.75% Cu, 28.60% Fe, 31.78% S, and minor impurities accounting for 3.01%. These analytical findings indicate that the purity of the chalcopyrite samples meets test requirements.

2.2. Mechanical Activation Experiments

The mechanical activation parameters (700 rpm, 0.5–2.5 h) were selected based on previous optimization studies [14,19]. The mechanical activation test was conducted using a YXQM planetary ball mill (Mitr, Changsha, China), wherein a 100 g grinding ball and 5 g chalcopyrite sample with a particle size smaller than 0.15 mm were added to a 100 mL zirconia ball mill (Mitr, Changsha, China). The grinding ball consisted of zirconia and comprised balls with diameters of 3 mm, 5 mm, and 10 mm with proportions of 30%, 50%, and 20%, respectively. The mechanical activation speed was set at 700 rpm, while the activation time varied between 0.5 and 2.5 h at intervals of 0.5 h. Subsequent analysis and testing were performed immediately after the completion of the mechanical activation test.

2.3. Electrochemical Experiments

The working electrode used in the electrochemical test is a 4 mm diameter glassy carbon electrode. In order to achieve a contamination-free, mirror-like surface on the glassy carbon electrode, it was initially polished using alumina powder with particle sizes of 1.0 μm and 0.3 μm for 5 min each. The electrode was then ultrasonically oscillated in deionized water, concentrated nitric acid (volume ratio: 1:1), anhydrous ethanol, and deionized water for 30 s before drying under an infrared lamp. A chalcopyrite suspension was prepared by mixing 10 mg of chalcopyrite, 0.5 mL of anhydrous ethanol, 1.5 mL of deionized water, and 50 μL of Nafion membrane solution in a 10 mL serum bottle, followed by 15 min of ultrasonic oscillation for uniform distribution. Exactly 8 μL of the prepared chalcopyrite suspension droplets were deposited onto the surface of the glassy carbon electrode, ensuring even distribution across its entire area. Finally, the electrode was dried again under an infrared lamp to obtain the working electrode. The electrode preparation method was referred to in the literature [29,30,31].

The electrochemical test utilizing a three-electrode system was conducted on a Gamry electrochemical workstation (Gamry, Philadelphia, PA, USA), where the prepared glassy carbon electrode served as the working electrode, while graphite electrodes and mercurous sulfate electrodes (Sat K₂SO₄) were used as counter electrodes and reference electrodes, respectively [29,30,31]. The electrolyte consisted of a pH = 1.5 sulfuric acid solution along with a concentration of Fe²⁺ = 0.1 mol/L, or Fe²⁺: Fe³⁺ = 0.05 mol/L:0.05 mol/L, or Fe³⁺ = 0.1 mol/L. The pH of the electrolyte is measured with a magnetic-thunder E-201C pH meter (Leici, Shanghai, China). Before each measurement, the electrode is calibrated with a standard buffer solution to ensure accurate and reliable measurement results. The cyclic voltammetry tests scan from the open-circuit potential towards the anode. The cyclic voltammetry curve (scanning rate of 20 mV/s, scanning range from −400 to +1000 mV, consistent with previous studies on sulfide minerals [32]). Mott–Schottky tests were conducted at a frequency of 1 kHz, and the potential was converted to hydrogen standard potential (SHE). Electrochemical tests were carried out in an open atmosphere at room temperature.

3. Results and Discussion

3.1. Cyclic Voltammetry of Chalcopyrite After Mechanical Activation

The electrochemical behavior of chalcopyrite was investigated under various mechanical activation conditions in different systems. The cyclic voltammetry curve is presented in Figure 1. As depicted in Figure 1a, the non-activated chalcopyrite exhibited three anodic oxidation peaks (A₁–A₃) and four cathodic reduction peaks (C₁–C₄) in the H₂SO₄ system.

Upon scanning from the open circuit potential towards the positive potential direction, the initial appearance of oxidation peak A₁ (740 mV) indicated preferential dissolution of iron from the chalcopyrite lattice, resulting in the formation of metal-deficient sulfides (Cu_1−xFe_1−yS_2−z, y > x) [32,33,34,35,36] (Equation (1)). Simultaneously, chalcopyrite also underwent oxidation to form covellite (CuS) [37] (Equation (2)). As the scanning potential increased to 1000 mV, A₂ indicated complete oxidation of chalcopyrite and the release of iron and copper ions (Equations (3) and (4)). At this time, covellite (CuS) formed by oxidation at peak A₁ will also dissolve [38,39] (Equation (5)). The current densities of oxidation peaks A₁ and A₂ exhibited an increase with prolonged mechanical activation time.

Shifting the anode scan from 1000 mV to the cathode resulted in a reduction peak C₁ emerging at approximately 640 mV, indicating the conversion of Fe³⁺ produced by chalcopyrite oxidation to Fe²⁺ [35,39] (Equation (6)). Reduction peaks C₂ and C₃ occurred within a potential range of 360–400 mV, signifying the reduction in Cu²⁺ and S to CuS, respectively, along with further reactions between Cu²⁺ and generated covellite leading to the formation of Cu₂S [33,40,41] (Equations (7) and (8)). A continuing scan towards the cathode revealed a C₄ peak appearing around −110 mV, indicating the reduction in chalcopyrite to bornite Cu₅FeS₄ and chalcocite Cu₂S [32,37] (Equations (9) and (10)). Additionally, there may be a reaction where chalcopyrite is reduced to Cu₉Fe₈S₁₆ [42] (Equation (11)). The reduction peak current for mechanically activated chalcopyrite increased with longer activation times; however, the intensity of the reduction peak C₂ decreased, disappearing after 0.5 h of activation.

When the scanning direction is reversed back to the anode at about 390 mV, oxidation peak A₃ is observed, which involves the conversion of H₂S it formed by reduction in chalcopyrite to bornite, chalcocite, etc., into elemental S [32,34,36,40] (Equation (12)), and the oxidation of chalcocite through a series of reactions [40,42,43] (Equations (13)–(15)). The oxidation peak currents of mechanically activated chalcopyrite increased with prolonged activation time; however, no new oxidation or reduction peaks were generated.

$$\mathrm{CuFe}{\mathrm{S}}_2\to {\mathrm{Cu}}_{1\mathit{-}x}{\mathrm{Fe}}_{1\mathit{-}y}{\mathrm{S}}_{2\mathit{-}z}+x{\mathrm{Cu}}^{2+}+y{\mathrm{Fe}}^{2+}+z{\mathrm{S}}^0+2(x\mathit{+}y){\mathrm{e}}^{-}$$

(1)

$$\mathrm{CuFe}{\mathrm{S}}_2\to 0.75\mathrm{Cu}\mathrm{S}+0.25{\mathrm{Cu}}^{2+}+{\mathrm{Fe}}^{2+}+1.25{\mathrm{S}}^0+2.5{\mathrm{e}}^{-}$$

(2)

$$\mathrm{CuFe}{\mathrm{S}}_2\to {\mathrm{Cu}}^{2+}+{\mathrm{Fe}}^{3+}+2{\mathrm{S}}^0+5{\mathrm{e}}^{-}$$

(3)

$$\mathrm{CuFe}{\mathrm{S}}_2+8{\mathrm{H}}_2\mathrm{O}\to \mathrm{C}{\mathrm{u}}^{2+}+\mathrm{F}{\mathrm{e}}^{3+}+2\mathrm{S}{\mathrm{O}}_4^{2-}+16{\mathrm{H}}^{+}+17{\mathrm{e}}^{-}$$

(4)

$$\mathrm{CuS}+4{\mathrm{H}}_2\mathrm{O}\to \mathrm{C}{\mathrm{u}}^{2+}+\mathrm{S}{\mathrm{O}}_4^{2-}+8{\mathrm{H}}^{+}+8{\mathrm{e}}^{-}$$

(5)

$$\mathrm{F}{\mathrm{e}}^{3+}+{\mathrm{e}}^{-}\to \mathrm{F}{\mathrm{e}}^{2+}$$

(6)

$$\mathrm{C}{\mathrm{u}}^{2+}+{\mathrm{S}}^0+2{\mathrm{e}}^{-}\to \mathrm{CuS}$$

(7)

$$\mathrm{C}{\mathrm{u}}^{2+}+\mathrm{Cu}\mathrm{S}+2{\mathrm{e}}^{-}\to \mathrm{C}{\mathrm{u}}_2\mathrm{S}$$

(8)

$$2\mathrm{CuFe}{\mathrm{S}}_2+3\mathrm{C}{\mathrm{u}}^{2+}+4{\mathrm{e}}^{-}\to \mathrm{C}{\mathrm{u}}_5\mathrm{Fe}{\mathrm{S}}_4+\mathrm{F}{\mathrm{e}}^{2+}$$

(9)

$$\mathrm{CuFe}{\mathrm{S}}_2+3\mathrm{C}{\mathrm{u}}^{2+}+4{\mathrm{e}}^{-}\to 2\mathrm{C}{\mathrm{u}}_2\mathrm{S}+\mathrm{F}{\mathrm{e}}^{2+}$$

(10)

$$9\mathrm{CuFe}{\mathrm{S}}_2+4{\mathrm{H}}^{+}+2{\mathrm{e}}^{-}\to \mathrm{C}{\mathrm{u}}_9\mathrm{F}{\mathrm{e}}_8{\mathrm{S}}_{16}+2{\mathrm{H}}_2\mathrm{S}+\mathrm{F}{\mathrm{e}}^{2+}$$

(11)

$${\mathrm{H}}_2\mathrm{S}\to {\mathrm{S}}_2^{2-}\to {\mathrm{S}}_{\mathrm{n}}^{2-}\to {\mathrm{S}}^0$$

(12)

$$\mathrm{C}{\mathrm{u}}_2\mathrm{S}\to \mathrm{C}{\mathrm{u}}_{1.92}\mathrm{S}+0.08\mathrm{C}{\mathrm{u}}^{2+}+0.16{\mathrm{e}}^{-}$$

(13)

$$\mathrm{C}{\mathrm{u}}_{1.92}\mathrm{S}\to \mathrm{C}{\mathrm{u}}_{1.6}\mathrm{S}+0.32\mathrm{C}{\mathrm{u}}^{2+}+0.64{\mathrm{e}}^{-}$$

(14)

$$\mathrm{C}{\mathrm{u}}_{1.6}\mathrm{S}\to \mathrm{CuS}+0.6\mathrm{C}{\mathrm{u}}^{2+}+1.2{\mathrm{e}}^{-}$$

(15)

In the Fe²⁺ = 0.1 mol/L system, compared to the sulfuric acid system, the current densities of oxidation peaks A₁ and A₂ are significantly enhanced and increase with prolonged mechanical activation time. The A₁ peak indicates preferential dissolution of iron from the chalcopyrite lattice and oxidation of Fe²⁺, while A₂ represents complete oxidation of chalcopyrite. The reduction peak remains similar to that observed in the sulfuric acid system, suggesting comparable reduction reactions of chalcopyrite in both systems.

In the Fe²⁺: Fe³⁺ = 0.05 mol/L:0.05 mol/L and Fe³⁺ = 0.1 mol/L systems, the oxidation peak labeled as A₄ was observed at 510–570 mV, with both peaks (A₃ and A₄) indicating exclusive oxidization of chalcocite only. The intensities for oxidation peak A₂ and reduction peak C₄ increased significantly along with extended activation time; however, other peaks showed no sensitivity towards activation time without significant differences in intensity.

In the three iron-containing systems, the oxidation peak (A1) current decreases as the concentration of Fe³⁺ increases. This can be attributed to the increase in redox potential of the solution with an increasing Fe³⁺ concentration. Prior to reaching the peak potential of oxidation, partial oxidation occurs on the surface of chalcopyrite, resulting in a reduction in the oxidation peak current. Conversely, the reduction peak current increases with higher Fe³⁺. Simultaneously, mechanical activation induces lattice expansion and defects within chalcopyrite, thereby fundamentally driving varying degrees of increase in its electrochemical activity and reactivity towards redox reactions.

According to Randles–Sevcik Equation (16) [39], the cyclic voltammetry curves of chalcopyrite were measured in different systems with Fe²⁺ = 0.1 mol/L, Fe²⁺:Fe³⁺ = 0.05 mol/L:0.05 mol/L, and Fe³⁺ = 0.1 mol/L systems at various scanning rates (10 mV/s, 20 mV/s, 30 mV/s, 40 mV/s, and 50 mV/s) (Figure 2, Figure 3 and Figure 4). The currents of reduction peak C₁ and C₄ were plotted against the square root of the scanning rate (Figure 5 and Figure 6). Data fitting results are presented in

Table 1. Randles–Sevcik calculation data for the reduction peak C₁ peak equation.

Time/h	Fe2+:Fe3+ = 0.05 mol/L:0.05 mol/L		Fe3+ = 0.1 mol/L
	K	R2	K	R2
0	0.077	0.994	0.096	0.979
0.5	0.078	0.970	0.100	0.998
1.0	0.080	0.994	0.109	0.996
1.5	0.082	0.959	0.109	0.994
2.0	0.082	0.983	0.114	0.999
2.5	0.090	0.993	0.112	0.999

and

Table 2. Randles–Sevcik calculation data for the reduction peak C₄ peak equation.

Time/h	Fe2+ = 0.1 mol/L		Fe2+:Fe3+ =0.05 mol/L:0.05 mol/L		Fe3+ = 0.1 mol/L
	K	R2	K	R2	K	R2
0	0.039	0.973	0.059	0.993	0.077	0.959
0.5	0.081	0.961	0.124	0.992	0.148	0.989
1.0	0.123	0.952	0.133	0.992	0.201	0.974
1.5	0.159	0.967	0.202	0.959	0.266	0.949
2.0	0.133	0.914	0.185	0.873	0.206	0.923
2.5	0.149	0.950	0.195	0.932	0.242	0.892

to evaluate the electrochemical activity of mechanically activated chalcopyrite. The slope in Figure 5 and Figure 6 is represented as K = 2.99 × 10⁵α^1/2AD^1/2C, which can be utilized as an indicator for changes in diffusion coefficient to demonstrate the impact of mechanical activation on the electrochemical activity of chalcopyrite in leaching systems.

i_p = 2.99 × 10⁵α^1/2AD^1/2CV^1/2

(16)

In the formula, i_p represents the peak current (A); α denotes the transfer coefficient of electrode reaction, typically ranging from 0.3 to 0.7, and in this case, we assume it as 0.5; A signifies the electrode area (cm²), which is precisely measured as 0.1256 cm²; D stands for the diffusion coefficient of the material (cm²/s); C represents the concentration (mol/cm³); and V indicates the scanning speed (mV/s).

The reduction peak C₁ corresponds to the reduction in Fe³⁺ on the surface of chalcopyrite (Figure 5). Table 1 demonstrates that in an electrolyte solution containing Fe²⁺: Fe³⁺ = 0.05 mol/L:0.05 mol/L and Fe³⁺ = 0.1 mol/L, there is a high degree of fitting between the current and scan rate for reduction peak C₁.

In the same electrolyte solution, the slope of mechanically activated chalcopyrite slightly increases compared with that of non-mechanically activated chalcopyrite. In Fe²⁺: Fe³⁺= 0.05:0.05 mol/L: K increased from 0.077 (0 h) to 0.090 (2.5 h). In Fe³⁺ = 0.1 mol/L: K increased from 0.096 (0 h) to 0.112 (2.5 h). This indicates that the effect of mechanical activation on the reduction in Fe³⁺ on the surface of chalcopyrite can be ignored. Under identical activation conditions, the K value in Fe³⁺ = 0.1 mol/L was greater than that in the Fe²⁺: Fe³⁺ = 0.05 mol/L:0.05 mol/L system, suggesting that as the concentration of Fe³⁺ increases, it reduces more quickly to form Fe²⁺ on mechanically activated chalcopyrite surfaces.

The reduction peak C₄ is associated with the conversion of chalcopyrite into intermediate products such as chalcocite and bornite, indicating that ion diffusion occurs at the solid–liquid interface (Figure 6). A higher K value corresponds to a faster reduction rate of chalcopyrite; meanwhile, the released ferrous ions diffuse into the bulk solution, according to the findings in Table 2, which demonstrate a better fit between current values for reduction peak C₄ and the scan rates employed during testing procedures. Mechanically activated chalcopyrite displays a significantly steeper slope compared to non-activated samples when tested under identical electrolyte solutions, suggesting that mechanical activation greatly enhances electrochemical activity within these materials. As activation time increases up until 1.5 h, both slopes rise accordingly, reaching their peaks at this point. Indicative evidence suggests an increasing diffusion coefficient for Fe²⁺ ions along with faster rates of reducing chalcopyrite, ultimately leading to easier formation processes involving intermediate products.

The slope (K) of mechanically activated chalcopyrite exhibits an increasing trend in different electrolyte solutions under identical activation conditions. Specifically, this trend is observed in systems with Fe²⁺ = 0.1 mol/L, Fe²⁺: Fe³⁺ = 0.05 mol/L:0.05 mol/L, and Fe³⁺ = 0.1 mol/L, respectively. These findings suggest that the surface reduction product Fe²⁺ demonstrates rapid diffusion kinetics within the Fe³⁺ system.

3.2. Effect of Mechanical Activation on the Semiconductor Properties of Chalcopyrite

The natural chalcopyrite can be categorized as n-type and p-type semiconductors [44], with its redox reaction closely linked to electronic structure and semiconductor properties. The mechanical activation enhances the lattice distortion of chalcopyrite. Upon contact between the chalcopyrite surface and the electrolyte solution, the total capacitance of the semiconductor–solution interface is represented by Equation (17). Mott–Schottky analysis is an electrochemical method used to measure the capacitance on the electrode surface when an alternating current potential is applied. The Mott–Schottky curve was utilized to investigate the semiconductor properties of chalcopyrite (Equations (18) and (19)), which facilitates the analysis of the influence of mechanical activation on the carrier concentration, flat-band potential and semiconductor type of chalcopyrite [45].

$$C=C_{\mathrm{SC}}+C_{\mathrm{H}}+C_{\mathrm{D}}$$

(17)

p-type semiconductor:

$${\displaystyle \frac{1}{C_{\mathrm{SC}}^2}}=-{\displaystyle \frac{2}{{\epsilon }_0{\epsilon }_{\mathrm{r}}{A}^2 N_{\mathrm{A}}}}(E-E_{\mathrm{FB}}-{\displaystyle \frac{\mathit{kT}}{e}})$$

(18)

n-type semiconductor:

$${\displaystyle \frac{1}{C_{\mathrm{SC}}^2}}={\displaystyle \frac{2}{{\epsilon }_0{{\epsilon }_{\mathrm{r}}A}^2 N_{\mathrm{D}}}}(E-E_{\mathrm{FB}}-{\displaystyle \frac{\mathit{kT}}{e}})$$

(19)

In the formula, C_SC is the capacitance of the space charge region (F); C_H is the Helmholtz layer capacitance (F); C_D is the diffusion layer capacitance (F); ${\epsilon }_0$ is the vacuum dielectric constant (8.854 × 10⁻¹² F/m); ${\epsilon }_{\mathrm{r}}$ is the relative dielectric constant (F/m); $A$ is the electrode area (m²); $N_{\mathrm{D}}$ is the donor concentration; $N_{\mathrm{A}}$ is the acceptor concentration; E is the applied voltage (V); $E_{\mathrm{FB}}$ is flat band potential; $k$ is the Boltzmann constant (1.38 × 10⁻²³ J/K); T is the thermodynamic temperature in Kelvin (K); e is the elementary charge (1.602 × 10⁻¹⁹ C); the value of $\mathit{kT}$/e at room temperature is negligible and typically disregarded.

The Mott–Schottky curve of mechanically activated chalcopyrite in the H₂SO₄ system is illustrated in Figure 7. Within the potential range of 400–480 mV, the M–S curve for non-activated chalcopyrite exhibits a positive slope, indicating n-type behavior. However, within the range of 480–560 mV, the slope becomes negative and suggests a p-type characteristic for chalcopyrite. Transitions from n-type to p-type behavior are observed at voltages between 560 and 600 mV, resulting in multiple changes in semiconductor type within the measured potential range. When subjected to mechanical activation times ranging from 0.5 h to 2.5 h, chalcopyrite demonstrates n-type behavior at potentials between 400 and 500 mV and switches to p-type at potentials between 500 and 600 mV.

According to

Table 3. Mott–Schottky curve calculation data of chalcopyrite.

Time/h	Flat Band Potential (EFB)/mV	Semiconductor Physics Type
0	289.2	n
0.5	259.6	n
1.0	244.1	n
1.5	247.0	n
2.0	237.1	n
2.5	242.0	n

, it can be observed that during a mechanical activation time period of 0 h–1.0 h, chalcopyrite acts as an n-type semiconductor, with its flat band potential decreasing as activation time increases. This indicates an increase in Fermi level and greater instability in electronic energy levels for this semiconductor material under such conditions. Furthermore, higher electrochemical activity corresponds to easier reactivity with particles present in the solution. After a duration of 1.5 h during chalcopyrite activation, only minimal differences in flat band potential values are observed, suggesting that prolonged mechanical activation time has a limited impact on enhancing electrochemical activity beyond a certain threshold range.

The variations in the energy band structure of chalcopyrite in different electrolyte solutions before and after mechanical activation were investigated. The conditions for mechanical activation included an activation time of 1.0 h and a rotation speed of 700 rpm. A comparison was made between the Mott–Schottky curves of chalcopyrite in different solutions before and after mechanical activation (Figure 7), with corresponding calculation results presented in

Table 4. Calculation results of Mott–Schottky curve of chalcopyrite in different solutions.

Leaching System	Time/h	Flat Band Potential (EFB)/mV	Semiconductor Type
Fe2+ = 0.1 mol/L	0	149.2	n
	1.0	118.4	n
Fe2+: Fe3+ = 0.05 mol/L:0.05 mol/L	0	192.5	n
	1.0	123.8	n
Fe3+ = 0.1 mol/L	0	261.7	n
	1.0	131.2	n

. The results depicted in Figure 8 demonstrate that within the potential range of 550–650 mV, under Fe²⁺ = 0.1 mol/L, Fe²⁺: Fe³⁺ = 0.05 mol/L:0.05 mol/L and Fe³⁺ = 0.1 mol/L, chalcopyrite exhibits n-type semiconductor behavior. Table 4 reveals that mechanical activation leads to a reduction in the flat band potential of chalcopyrite across various systems. This mechanical activation elevates the Fermi level of semiconductors and renders electronic energy levels more unstable [46]. Consequently, mechanically activated chalcopyrite exhibits enhanced electrochemical activity, facilitating its reaction with ions present in the solution. Notably, in the Fe³⁺ = 0.1 mol/L solution, chalcopyrite experiences the most significant decrease in flat band potential, indicating an increase in reactivity towards Fe³⁺.

3.3. Tafel Curve of Chalcopyrite After Mechanical Activation

Previous research indicates that the leaching rate of chalcopyrite in the Fe²⁺ system is higher than that in the Fe³⁺ system, but the mechanically activated chalcopyrite has a higher leaching rate in the Fe³⁺ system [28]. Therefore, the influence of mechanical activation time on the electrochemical corrosion of chalcopyrite in Fe²⁺ and Fe³⁺ systems was studied. The Tafel curves of mechanically activated chalcopyrite in Fe²⁺ = 0.1 mol/L and Fe³⁺ = 0.1 mol/L solution were plotted in Figure 9a,b, and the corrosion kinetic parameters, including corrosion potential, corrosion current, and polarization resistance, were calculated (Figure 9c,d) [47,48]. It can be observed from Figure 9c that obvious changes occur in corrosion potential, corrosion current, and polarization resistance after mechanical activation in Fe²⁺ = 0.1 mol/L systems, but the Tafel curves of chalcopyrite after mechanical activation are almost the same, with prolonged mechanical activation time from 0.5 to 2.5 h, and there is little variation observed in terms of these corrosion kinetic parameters.

In Figure 9b we calculate the relevant parameters in the weakly polarized region less than 10 mV. The anode branch shows that the chalcopyrite has undergone a polysulfide–copper sulfide–elemental sulfur transition [49]. The reduction behavior of ferrous ions is basically consistent with that of cathode branching. The Tafel curve of chalcopyrite in the Fe³⁺ system is significantly different from that in the Fe²⁺ system, especially as the passivation–activation–passivation phenomenon occurs in the anode branch (Figure 9b). However, as the mechanical activation time increases, the passivation near the corrosion potential weakens. In an Fe³⁺ = 0.1 mol/L system, both the corrosion currents increase with prolonged mechanical activation time; however, they return to non-activated levels after exceeding 2 h of activation time, indicating that there is an effective range for mechanical activation duration beyond which it affects the dissolution rate of chalcopyrite (Figure 9d). Compared to the Fe²⁺ = 0.1 mol/L solution, the corrosion current showed a significant increase in the Fe³⁺ = 0.1 mol/L solution. This suggests that mechanical activation can accelerate the dissolution rate of chalcopyrite in ferric solution.

4. Conclusions

Mechanical activation significantly modifies the electrochemical and semiconductor properties of chalcopyrite. Through particle size reduction and the introduction of lattice distortion, this pretreatment markedly alters the electrochemical behavior of chalcopyrite in H₂SO₄-Fe²⁺/Fe³⁺ systems. Cyclic voltammetry results demonstrate intensified oxidation and reduction peaks with extended mechanical activation, indicating a substantial enhancement in redox activity. Specifically, after 1.5 h of activation at 700 rpm, the Randles–Sevcik slope (K) in the Fe³⁺ system increased from 0.039 to 0.266, which is equivalent to a significant increase in the ion diffusion rate at the solid–liquid interface. Additionally, Mott–Schottky analysis indicated a decrease in the flat band potential from 261.7 mV to 131.2 mV, which promotes more efficient electron transfer and facilitates electron capture by Fe³⁺ from the chalcopyrite surface. These quantitative results confirm that mechanical activation considerably enhances the electrochemical reactivity of chalcopyrite, leading to an accelerated corrosion rate in ferric sulfate solutions. In summary, mechanical activation pretreatment effectively enhances the redox activity of chalcopyrite.