Effects of Microwave Energy and MnO₂ from Deep-Sea Polymetallic Nodules as an Oxidizing Agent on the Leaching of Chalcopyrite Concentrate

Abstract

The mineral chalcopyrite (CuFeS₂) is inherently resistant to conventional leaching techniques, necessitating the intensification of the leaching process to achieve efficient metal recovery. Microwave-assisted leaching, combined with the application of a suitable oxidizing agent, presents a viable approach to enhancing the dissolution rate of metals in solutions. The objective of this study is to investigate the effect of microwave irradiation on the leaching behavior of chalcopyrite concentrate in a hydrochloric acid (HCl) medium, employing deep-sea polymetallic nodules (DSP) as the oxidizing agent. The influence of acid concentration and microwave power on copper extraction efficiency was examined. Optimal copper extraction was observed at an HCl concentration of 5 M and a microwave power of 750 W. The results indicate that DSP nodules serve as a more effective oxidizing agent than pyrolusite in acidic oxidative microwave-assisted leaching of chalcopyrite, particularly in terms of copper recovery. Analytical techniques employed for the characterization of leach residues and solutions included Atomic Absorption Spectroscopy (AAS) and Scanning Electron Microscopy (SEM) coupled with Energy-Dispersive X-ray Spectroscopy (EDS).

1. Introduction

Leaching of chalcopyrite (CuFeS₂) is challenging due to its inherently slow dissolution rate and the formation of passivation layers that hinder the efficient release of copper. In an effort to overcome these limitations, several researchers have focused on optimizing leaching conditions using various approaches such as different leaching media, oxidizing agents, or leaching conditions [1,2].

Sokić et al. and Petrović et al. [3,4] investigated chalcopyrite leaching in sulfuric acid (H₂SO₄) in the presence of hydrogen peroxide (H₂O₂) as an oxidizing agent. Both studies consistently demonstrated that H₂O₂ significantly accelerates the dissolution process and improves copper recovery. Additionally, temperatures of 75–80 °C and a 50% H₂SO₄ concentration were identified as key factors for achieving maximum process efficiency. The use of H₂O₂ also enables selective copper extraction, reducing the co-dissolution of undesired metals such as iron.

Copper leaching from chalcopyrite in nitrous-sulfuric acid is effective due to the autocatalytic effect of nitrite in an acidic environment. The process follows a diffusion-controlled shrinking core model with an activation energy of approximately 34 kJ/mol and involves the formation of a passivating elemental sulfur layer [5].

Mechanochemical treatment of chalcopyrite before leaching was examined by Tešinský et al. [6]. This treatment induces microfractures and increases the reactivity of chalcopyrite with sulfuric acid. Even mild mechanical grinding helps to remove passivation layers from the mineral surface, thereby enhancing the kinetics of its dissolution in H₂SO₄ [7]. The effect of chalcopyrite’s structure on the leaching rate was studied in work [8], where the authors found that differences in crystal structure and plane orientation significantly affect dissolution rates and reactivity.

Karimi et al. and Behnajady et al. [9,10] confirmed the positive effect of green deep eutectic solvents (DES) based on choline chloride (ChCl) and ρ-toluenesulfonic acid (ρ-TSA) on chalcopyrite leaching. Choline chloride interacts with Cu²⁺ and Fe³⁺ ions, facilitating their release into solution. This process is further enhanced by the oxidative mechanism of ρ-TSA, which stabilizes metal complexes and reduces the formation of passivating layers, thereby improving leachability.

Innovative approaches aimed at improving chalcopyrite leaching include the application of ozone [11], microwave (MW) irradiation [12,13], electroreduction pretreatment [14], and leaching in NaCl media [15]. Ozone acts as a strong oxidizing agent, significantly increasing the oxidation and dissolution rates of copper. It also helps suppress passivation, improving acid accessibility to active surface sites on chalcopyrite [11]. Microwave irradiation was found to have only a modest effect on activation energy (39.1 kJ/mol vs. 43.9 kJ/mol with conventional heating), but it substantially increased the boiling point of the leaching system. This, combined with selective microwave heating, led to a rise in interfacial temperature and surface energy of chalcopyrite, enhancing dissolution intensity and copper recovery efficiency [12]. Electroreduction of chalcopyrite before leaching in ammoniacal chloride media reduces surface passivation and significantly increases mineral reactivity [14]. The presence of sodium chloride (NaCl) during chalcopyrite leaching improves ion transport between the mineral surface and the leaching medium, enhances copper oxidation, and minimizes surface passivation, resulting in faster dissolution rates [15].

In recent years, the presence of manganese dioxide (MnO₂) in deep-sea polymetallic (DSP) nodules has been shown to significantly enhance the dissolution kinetics of chalcopyrite [16]. DSP nodules, also referred to as manganese nodules, are spherical to ellipsoidal concretions found on the ocean floor, typically measuring 10 to 15 cm. They cover vast regions of the Pacific and Indian Ocean seabed at depths below 3500 m [17]. These nodules are primarily composed of Mn, Fe, Cu, Ni, and Co [14]. The main mineral constituents of DSP nodules include MnO₂, FeOOH (iron oxyhydroxide), NiO (nickel(II) oxide), CuS (copper(II) sulfide), and other sulfides and oxides that serve as key sources for the extraction of valuable metals [18,19]. Given their economic and strategic importance, multiple international initiatives have been established to systematically explore these resources. One such initiative is the Interoceanmetal Joint Organization (IOM), an intergovernmental consortium, including Slovakia among its members, tasked with the exploration, assessment, and eventual exploitation of polymetallic nodules in the Clarion–Clipperton Zone of the Northeastern Pacific Ocean [20,21]. Various methods have been developed to utilize DSP nodules for the extraction of heavy metals, most commonly involving leaching with sulfuric acid or hydrochloric acid (HCl) in the presence of iron-based reducing agents such as Fe²⁺ or through biological processes aimed at recovering manganese, nickel, cobalt, and copper [22,23,24].

The presence of MnO₂ derived from deep-sea polymetallic (DSP) nodules in the leaching medium can enhance the dissolution of chalcopyrite through several mechanisms [25]. During leaching in HCl solution, MnO₂ reacts with HCl, leading to the generation of chlorine gas (Cl₂), which facilitates the breakdown of chalcopyrite. As reported by the authors [26], the dissolution of chalcopyrite proceeds via three primary pathways: galvanic interaction between chalcopyrite and MnO₂, redox reactions involving Fe³⁺/Fe²⁺ ions, and the action of Cl₂ generated from the reaction between MnO₂ and HCl.

According to Torres et al. [27], a temperature of 80 °C positively influences the dissolution of CuFeS₂, whereas the concentration of MnO₂ did not have a significant effect. The highest copper extraction (71%) was achieved at 80 °C, with a particle size of −47 + 38 µm, an MnO₂/CuFeS₂ ratio of 5:1, and 1 mol/L H₂SO₄. The leaching of chalcopyrite in the presence of pure MnO₂ and the microorganism Acidithiobacillus thiooxidans was investigated by the authors [28]. The presence of microorganisms enhances bio-oxidative reactions, leading to higher copper and manganese recoveries even at lower temperatures.

These intensified leaching approaches for copper sulfides represent promising methods for improving the efficiency of copper extraction, each contributing to reduced environmental and economic costs compared to conventional techniques [29]. Advances in biotechnology, electrochemical methods, and hydrometallurgy demonstrate substantial potential for more environmentally friendly and efficient metal recovery from deep-sea concretions [30]. Future research will be crucial to further enhance these technologies, particularly in optimizing leaching conditions and mitigating environmental risks associated with mining DSP nodules from the seafloor. A balanced approach is required that prioritizes responsible resource extraction while protecting marine ecosystems. The implementation of stricter regulatory and governance structures, advances in environmentally conscious mining technologies, and a commitment to preventive scientific research can provide solutions [31,32].

Results also indicate that the application of microwave (MW) irradiation during leaching enables a more rapid onset of chemical reactions between reactants and achieves a higher percentage of copper dissolution in a shorter time compared to conventional leaching. The temperature increase caused by MW irradiation accelerates reaction kinetics and enhances the penetration of the leaching agent into the mineral structure. The authors [33] emphasize that microwave-assisted leaching selectively heats the mineral, increasing its surface energy and promoting more efficient metal extraction. Microwave heating efficiency depends on several factors. The most important are the ability to absorb microwave radiation, the microwave frequency, the electric power of microwave radiation, the mass of the sample [34], and the depth of penetration of the microwave energy into the minerals [35]. The rate and selectivity of microwave heating are the most significant advantages of utilizing microwave energy in mineral processing [36].

A review of the current literature highlights a significant gap in alternative approaches to the acidic leaching of chalcopyrite concentrate using microwave-assisted techniques in the presence of DSP nodules (MnO₂). Existing hydrometallurgical strategies predominantly rely on conventional leaching methods, and studies exploring the synergistic effects of MnO₂ under microwave irradiation remain scarce.

The study aims to investigate the effect of leaching parameters on the extraction of CuCl₂ from chalcopyrite concentrate. A thermodynamic analysis of the CuFeS₂–HCl–MnO₂ system was conducted to identify leaching conditions under which the proposed transformation is thermodynamically favorable. DSP nodules were incorporated into the leaching system as a natural source of MnO₂. Various analytical techniques, including SEM, EDX, particle size distribution analysis, AAS, and Eh–pH diagrams, were employed to study the thermodynamics of chalcopyrite leaching and to predict the formation of reaction products under specific conditions.

This work presents a new approach where DSP nodules serve as both an oxidizing agent and a source of valuable metals, improving the efficiency of leaching. Additionally, it demonstrates the synergistic effect of MnO₂ in DSP nodules on increased manganese recovery and copper extraction efficiency, as well as the use of microwave radiation to mitigate the formation of passivating sulfur layers. This study offers an innovative approach to copper and manganese recovery from low-grade ores using sustainable resources.

2. Materials and Methods

2.1. Experimental Samples and Reagents

This study was carried out using a chalcopyrite concentrate from a Slovinky site (Slovakia, Central Europe), obtained after mineral processing procedures. The deep-sea polymetallic nodules sample was collected from the Pacific Ocean by the IOM [25,37]. In this study, analytical grade hydrochloric acid (37% HCl) (Mikrochem, Pezinok, Slovakia) was used as the leaching medium.

2.2. Analytical and Experimental Methods

The particle size distribution (PSD) of the sample was analyzed with the use of a Malvern Mastersizer 3000 laser diffraction analyzer (Malvern Instruments Ltd., Malvern, Worcestershire, UK). Each sample underwent five separate measurements, and the final particle size distribution was obtained by averaging the results from all 25 measurements.

The concentrations of copper, manganese, nickel, and zinc in the samples were determined using atomic absorption spectroscopy (AAS), carried out on a Varian AA20+ spectrophotometer (Varian, Belrose, Australia), with a detection limit ranging from 0.3 to 6 ppb. Before the measurements, the optimal operating parameters for fuel flow, acetylene supply, and burner height were experimentally established using calibration solutions containing the highest concentration of each respective element to maximize AAS efficiency. Each leachate sample was analyzed in triplicate to ensure the reliability of the results.

Both the input DSP nodules and the solid residue products were analyzed using Scanning Electron Microscopy (SEM) and Energy-Dispersive X-ray (EDX) Spectroscopy. The morphology and particle size were examined using a MIRA3 FE-SEM scanning electron microscope (TESCAN, Prague, Czech Republic), which also enabled semi-quantitative elemental analysis via EDX. The EDX data was processed with AZtec software v6.0 (Oxford Instruments, Oxford, UK).

The morphology of the particles was analyzed using a Dino-Lite ProAM4113T optical stereo microscope (AnMo Electronics Corporation, Hsinchu, Taiwan, magnifications below 100×).

The redox potential (Eh) and pH of leachates were determined with an Orion Lab Star PH111 pH meter (Thermo Fisher Scientific, Waltham, MA, USA). Pourbaix Eh–pH diagrams were designed to study the thermodynamics of chalcopyrite leaching and the prediction of the formation of products under specific conditions using HSC Chemistry, version 10.0.2.3, with a University Basic License (Metso Finland Oy, Espoo, Finland).

2.3. Procedure of Leaching Experiments

The experimental setup followed the apparatus described by the authors in [34]. A glass reactor was placed in a microwave heating device with an adjustable power range from 90 to 900 W and a frequency of 2.45 GHz. A volume of 200 mL of the leaching solution was poured into the reactor and heated at 750 W until vigorous boiling occurred. This heating period lasted approximately two minutes, after which the required amount of sample was added. Once the experiment duration (120 min) was set, the process was initiated. The temperature was measured with an infrared non-contact thermometer, Solight TE47 (Solight Holding, Bratislava, Slovakia), reaching a consistent value of 104 °C at 750 W. At a power setting of 350 W, the temperature stabilized at 100 °C. Microwave radiation acts on the reactor continuously during leaching, depending on the set power intensity [34].

Hydrochloric acid with concentrations of 2, 3, and 5 mol/L was used as the leaching solution. Sets of experiments were performed using 12.5 g of chalcopyrite concentration, thus the liquid-to-solid (L/S) ratio was 16:1. The feed material consisted of a mixture of chalcopyrite and DSP nodules in a fixed mass ratio of 1:4, meaning four parts of nodules were used per one part of chalcopyrite concentrate. This ratio was based on theoretical calculations and literature data [25].

Mixing was achieved through the vortex created during boiling. Initially, the slurry agitation was vigorous, but it decreased in intensity as the experiment progressed. Liquid samples of 5 mL were taken at selected time intervals. The copper concentration in the leachate was then determined using atomic absorption spectroscopy (AAS). In addition to copper, the extraction of Ni, Zn, and Mn was also evaluated.

The pH and redox potential (Eh) of the liquid samples were measured throughout the experiment. The pH value remained at 0.1, while the redox potential was 0.415 V. After the experiment, the slurry was filtered, and the solid residue was washed with distilled water and dried at 60 °C.

3. Results and Discussion

To comprehensively evaluate the leaching behavior of chalcopyrite concentrate under selected conditions (HCl concentration, microwave irradiation intensity, and liquid-to-solid ratio), this study integrates a combination of analytical methods. Particle size distribution analysis was used to characterize the grain size input of the chalcopyrite concentrate and DSP nodules. The characterization of particle size, surface morphology, and chemical composition is crucial for understanding the reactivity of materials during leaching. Studies show that smaller particles provide a larger surface area for interaction with acid and oxidizing agents [38,39]. Scanning Electron Microscopy (SEM) provided insights into the surface morphology of the solid phases. Energy-Dispersive X-ray Spectroscopy (EDS) provided an overview of the elemental composition of the solid residues, and atomic absorption spectroscopy (AAS) was used for quantitative analysis of metal concentrations in solutions. In addition, Eh–pH diagrams were generated to thermodynamically interpret the behavior of the system and to predict the stability of the reaction products under different leaching conditions.

Each of these techniques is discussed in a separate section of this work to provide a comprehensive understanding of the leaching process and the role of DSP nodules as an oxidizing agent.

3.1. Deep-Sea Polymetallic Nodules and Chalcopyrite Concentrate Characterization

For both the efficient extraction of metals from chalcopyrite concentrate and the reactivity of MnO₂ derived from deep-sea polymetallic (DSP) nodules, the particle size distribution is a critical factor.

3.1.1. Deep-Sea Polymetallic Nodules Analysis

Figure 1a presents the morphology of the DSP nodules before leaching, as observed using an optical stereo microscope. The particles in the sample vary in size and can be grouped into distinct fractions based on their dimensions.

Figure 1b displays the graphical output of the particle size distribution obtained through sieve analysis and laser diffraction, illustrating the granulometric characteristics of the milled DSP nodules. The distribution exhibits a trimodal pattern (blue curve), which is typical for heterogeneous materials such as DSP nodules. The sample contains three fractions. The fine fraction around 5–10 µm—the peak for this fraction is faint, it is more of a shoulder than a peak. Coarse fraction around 40–80 µm—this middle fraction forms the dominant fraction, and it is the ideal size for leaching. The presence of the coarsest fraction of approximately 200–400 µm indicates that the grinding was not completely homogeneous or efficient, which could be due to the more resistant mineral phases in the DSP nodules (e.g., some silicates or quartz) being more difficult to grind and remaining coarser.

The cumulative curve (green line) increases steadily and confirms the trimodal distribution, leveling off at 100% of the particle volume at approximately 700–800 µm. From the cumulative curve, it is evident that 80% of the particle volume consists of particles with diameters less than or equal to 266 µm. It is generally known that the optimal grain size for leaching chalcopyrite concentrate is in the range of 70–80 µm [40]. In the sample studied, the fraction of particles in the size range of 40–80 μm accounts for nearly 70% of the material, which provides favorable conditions for effective interaction between MnO₂ particles and CuFeS₂ in an HCl medium.

It is also well known that the DSP nodules have a complex composition, containing a mixture of organic, colloidal, biogenic, and mineral fragments. The identified crystalline mineral phases are todorokite (a manganese mineral) and quartz, with amorphous phases dominating at 89–91% of the composition. Muscovite and vernadite are present in small amounts [41].

The AAS analysis of DSP nodules’ major elements,

Table 1. Chemical analysis of major elements in DSP nodules by the AAS method.

Content (wt.%)	Mn	Fe	Cu	Zn	Ni	Co	Mo	Mg	Residue
Average	21.74	3.282	1.004	0.14	1.15	0.156	0.242	1.336	70.95
Standard Deviation	1.041	0.153	0.015	0	0.026	0.008	0.004	0.035	-
Variance	1.085	0.234	0.22·10−3	0	0.68·10−3	0.64·10−4	0.16·10−5	0.12·10−2	-

, shows that the DSP nodule contains an average of 21.74 wt.% of manganese (Mn), 1 wt.% of copper (Cu), 3.282 wt.% of ferrous (Fe) and 0.14 wt.% of zinc (Zn), and 1.15% nickel (Ni) which proves that DSP nodules are an important source of metals, especially Mn. These values, characterized by a relative standard deviation below 2% for five replicates, validate the reliability of the analysis. The elemental profile obtained is in agreement with literature data [41], which reports the presence of manganese and iron-rich phases such as todorokite, vernadite, and minor quartz and muscovite. The presence of these minerals supports the identified elements and confirms the polymetallic nature of the DSP nodules.

These findings were further supported by SEM and EDX analyses of the DSP nodule sample before leaching. SEM imaging, Figure 2a, revealed a heterogeneous mixture of fine and coarse particles, predominantly smaller than 200 µm, with an almost spherical morphology typical of DSP nodules. The EDX analysis, presented in Figure 2b, confirmed the presence of manganese, iron, silicon, nickel, and copper, in line with the AAS results. The good agreement between EDX and AAS confirms the reliability of the elemental composition data and further supports the presence of mineral phases reported in the literature.

3.1.2. Chalcopyrite Concentrate Characterization

The grain size of the chalcopyrite concentrate obtained via flotation was in the range of −80 + 63 μm [13], and this size fraction was used consistently in all leaching experiments to ensure uniform reactivity.

The chemical composition of the sample, determined by elemental analysis (

Table 2. Chemical analysis of major elements in the chalcopyrite concentrate by the AAS method.

Content (wt.%)	Fe	Cu	Zn	Pb	As	Ba	Ag	Residue
Average	29.7	29.1	2.4	0.43	0.03	0.002	0.02	38.18
Standard Deviation	0.1	0.15	0.88	0.01	0.01	0	0.02	-
Variance	6.6·10−3	1.55·10−2	0.1302	2.22·10−5	6.6·10−5	0	2.2·10−5	-

), revealed average contents of 29.1 wt.% copper (Cu), 29.7 wt.% iron (Fe), and 2.4 wt.% zinc (Zn), indicating that the Slovinky chalcopyrite concentrate is a rich source of copper.

According to Laubertova et al., the elemental composition is consistent with the typical mineralogical profile of flotation-derived chalcopyrite concentrates as reported in the literature [13,25]. According to these references, the main phase is chalcopyrite (CuFeS₂), accompanied by minor amounts of sphalerite (ZnS), and trace phases of pyrite (FeS₂), galena (PbS), and hematite (Fe₂O₃). These previously reported XRD-based findings agree with the observed chemical composition of the Slovinky sample and substantiate its mineralogical character.

3.2. Thermodynamics Study of the Chalcopyrite Concentrates Leaching

The thermodynamic behavior of chalcopyrite concentrate leaching was assessed using HSC Chemistry software (version 10.0.2.3), which relies on Gibbs free energy minimization to perform thermochemical equilibrium calculations and includes a comprehensive set of built-in databases [42]. The evaluation followed a defined procedure: based on previous studies, hydrochloric acid was selected as the leaching agent, and manganese dioxide (MnO₂) as the oxidant. To determine the optimal leaching temperature, Eh–pH diagrams were constructed for both 25 °C and 100 °C, as shown in Figure 3. The main component concentrations were set at 1 mol/kg of water, with the pressure maintained at 101.325 kPa.

The Eh–pH diagrams, Figure 3, show that copper is present in the solution only as Cu²⁺_(aq) at a temperature of 25 °C in the pH range 0–3.2 and at a temperature of 100 °C at a pH of 0–2.15. Potential Eh was 0.38–1.36 V for 25 °C, and Eh 0.38–1.3 V for 100 °C. Since the work is focused on the leaching of chalcopyrite using microwave irradiation, the temperatures during the process will reach 70 to 100 °C. The Eh–pH diagram at 100 °C better reflects the actual experimental conditions and is key for assessing the stability of copper compounds in this environment. For processes using microwave irradiation, the Eh–pH diagram shown in Figure 3b is therefore more relevant.

The stability forms of copper under aqueous conditions were analyzed using Eh–pH diagrams, which led to the formulation of a relevant chemical equation. The thermodynamic evaluation based on the J. H. van’s Hoff equation indicates that reactions (1) through (4), as listed in

Table 3. Predicted chemical reactions and values of Gibbs energy.

Predicted Chemical Reaction	ΔGT [kJ/mol]		Equation
	25 °C	100 °C
${\mathrm{M}\mathrm{n}{\mathrm{O}}_2}_{\left(\mathrm{s}\right)}+4{\mathrm{H}\mathrm{C}\mathrm{l}}_{\left(\mathrm{a}\mathrm{q}\right)}={\mathrm{M}\mathrm{n}{\mathrm{C}\mathrm{l}}_2}_{\left(\mathrm{a}\mathrm{q}\right)}+{{\mathrm{C}\mathrm{l}}_2}_{\left(\mathrm{g}\right)}+2{\mathrm{H}}_2\mathrm{O}$	−201.778	−220.863	(1)
${\mathrm{C}\mathrm{u}\mathrm{F}\mathrm{e}{\mathrm{S}}_2}_{\left(\mathrm{s}\right)}+{{2\mathrm{C}\mathrm{l}}_2}_{\left(\mathrm{g}\right)}={\mathrm{C}\mathrm{u}{\mathrm{C}\mathrm{l}}_2}_{\left(\mathrm{a}\mathrm{q}\right)}+{\mathrm{F}\mathrm{e}{\mathrm{C}\mathrm{l}}_2}_{\left(\mathrm{a}\mathrm{q}\right)}+2{\mathrm{S}}_{\left(\mathrm{s}\right)}$	−305.401	−282.813	(2)
${\mathrm{C}\mathrm{u}\mathrm{F}\mathrm{e}{\mathrm{S}}_2}_{\left(\mathrm{s}\right)}+{{2.5\mathrm{C}\mathrm{l}}_2}_{\left(\mathrm{g}\right)}={\mathrm{C}\mathrm{u}{\mathrm{C}\mathrm{l}}_2}_{\left(\mathrm{a}\mathrm{q}\right)}+{\mathrm{F}\mathrm{e}{\mathrm{C}\mathrm{l}}_3}_{\left(\mathrm{a}\mathrm{q}\right)}+2{\mathrm{S}}_{\left(\mathrm{s}\right)}$	−396.733	−340.173	(3)
${\mathrm{C}\mathrm{u}\mathrm{F}\mathrm{e}{\mathrm{S}}_2}_{\left(\mathrm{s}\right)}+{2.5\mathrm{M}\mathrm{n}{\mathrm{O}}_2}_{\left(\mathrm{s}\right)}+10{\mathrm{H}\mathrm{C}\mathrm{l}}_{\left(\mathrm{a}\mathrm{q}\right)}={\mathrm{C}\mathrm{u}{\mathrm{C}\mathrm{l}}_2}_{\left(\mathrm{a}\mathrm{q}\right)}+{\mathrm{F}\mathrm{e}{\mathrm{C}\mathrm{l}}_3}_{\left(\mathrm{a}\mathrm{q}\right)}+2{\mathrm{S}}_{\left(\mathrm{s}\right)}$+ $5{\mathrm{H}}_2\mathrm{O}+{2.5\mathrm{M}\mathrm{n}{\mathrm{C}\mathrm{l}}_2}_{\left(\mathrm{a}\mathrm{q}\right)}$	−901.179	−892.330	(4)

, are highly favorable for product formation. The generation of Cl₂ through the interaction between MnO₂ and HCl plays a crucial role in facilitating chalcopyrite dissolution.

3.3. Effect of HCl Concentration on Cu Extraction Efficiency

The leaching behavior of chalcopyrite concentrates under microwave irradiation at 750 W, in the presence of DSP nodules and at a liquid-to-solid (L/S) ratio of 16:1, is shown in Figure 4a. The leaching profiles exhibit hyperbolic trends for HCl concentrations of 2, 3, and 5 M, with copper extraction proceeding most rapidly within the first 5 min. The initial phase, up to 40 min, represents the most efficient leaching interval, with the maximum copper recovery (~40%) observed for 5 M HCl. Increasing the acid concentration beyond this point does not significantly enhance extraction efficiency.

The rapid initial dissolution is attributed to the oxidative action of MnO₂ in acidic media, which promotes in situ generation of Cl₂ (Equation (1)). Chlorine then reacts with chalcopyrite, enhancing its decomposition and accelerating copper release (Equations (2) and (3)).

As leaching progresses, leaching kinetics slow down, likely due to the depletion of reactive species, formation of a passivating elemental sulfur layer (S₍ₛ₎) on the mineral surface, or due to the reduced availability of the oxidizing agent. Inflection points observed around the 40-min mark indicate a pronounced deceleration of the leaching process. Beyond this point, copper recovery either reaches a plateau or slightly declines, depending on specific experimental conditions. This behavior suggests that the system approaches kinetic or chemical limitations, where further dissolution of copper becomes inefficient despite prolonged leaching time.

To evaluate the reaction mechanism, the apparent order of chemical reaction concerning HCl concentration was determined in the range of 2–5 mol/L at 373 K, Equation (4), Table 3. The apparent reaction order “n” was calculated based on the initial acid concentration according to the following:

$$v=k·c^n,$$

(5)

where:

• v—rate of reaction (4)
• k—rate constant
• c—the concentration of hydrochloric acid
• n—the order of reaction [5,43]

Taking the logarithm of relation (5) gives the following form:

$$\mathit{ln}v=\ln k+n·\ln c,$$

(6)

where:

n corresponds to the slope of the linear plot of the relationship between $\ln v_{{CuFeS}_2}=f{c}_{HCl}$.

Based on the measured Cu concentration values, Figure 4a, at the 5th minute of leaching, the dependence was constructed: $\ln v_{{CuFeS}_2}=f{c}_{HCl}$, shown in Figure 4b. The values obtained from the equation of the line in Figure 4b were as follows: n = 0.4448 and $\ln k=-7.3777$ (the correlation index is 0.887). After substituting it into Equation (6), we obtain the following:

$$\mathit{ln}v=-7.3777+0.4448·\mathit{ln}{c}_{HCl},$$

(7)

The dissolution rate of CuFeS₂ at the temperature of 373 K can be represented by the following equation:

$$v=6.25·{10}^{-4}·{c_{HCl}}^{0.4448},$$

(8)

From the above, it can be concluded that at the ratio HCl: chalcopyrite concentrate = 16:1, n = 0.4448, which indicates that the HCl concentration no longer affects the reaction rate significantly. According to Dutrizac [1], during the leaching of chalcopyrite with ferric chloride, elemental sulfur (a surface layer on fine particles of the mineral) is formed. This passivation effect is not affected at all by changing the concentration of ferric chloride. This means that the presence of other components/factors, such as MnO₂, may have a greater impact on the reaction rate than HCl under these conditions.

3.4. Effect of the HCl Concentration on Mn Extraction Efficiency

Figure 5a presents manganese recovery during leaching at a L/S ratio of 16:1 under microwave irradiation at 750 W. The curve (2M HCl) exhibits a typical hyperbolic trend, with a rapid initial increase in manganese recovery. The curves (3M HCl and 5M HCl) have an increasing trend; however, at 40 min, there is a gradual stabilization or decrease over time. At a HCl concentration of 2 M, manganese recovery remains relatively low; however, significantly higher recoveries are achieved at 3 M and 5 M HCl. Within the first 5 min of leaching, the difference in manganese extraction between the 2 M solution and higher acid concentrations reaches up to 30%.

The data also indicates that further increases in HCl concentration beyond 3 M do not significantly enhance copper recovery from chalcopyrite. This is attributed to early saturation of the system, with steady state conditions being approached within the first 15 min of leaching at both 3 M and 5 M HCl concentrations.

Although numerous studies have explored chalcopyrite leaching and the role of oxidizing agents, only a limited number have investigated the effectiveness of MnO₂ derived from DSP nodules in chloride-rich environments [26,27]. The findings from the present study are consistent with those previously reported and further support the potential of DSP-sourced MnO₂ as an effective oxidant under acidic chloride conditions.

Based on the mathematical Formulas (5) and (6), the apparent order of reaction (4) was calculated using the same procedure for the L/S ratio of 16:1 for the manganese recovery into the leachate. The calculations were performed under the same conditions. At the ratio HCl: chalcopyrite concentrate = 16:1, the apparent order for Mn²⁺ is 0.491, Figure 5b, which indicates that the presence of MnO₂ affects the reaction kinetics.

When comparing the Mn and Cu recoveries, Figure 6 shows that increasing concentration positively affects the solubility of MnO₂ in the solution, and with its increase, the Cu recoveries also increase in direct proportion. This confirms the positive effect of MnO₂ in DSP nodules on increasing the reaction rate and enabling better dissolution of chalcopyrite. Another trace metal in the nodules is cobalt. However, cobalt does not play the same role in the oxidation process as manganese and therefore should not affect the leaching process.

3.5. Effect of Microwave Energy Intensity on the Leaching Efficiency of Cu

The effect of microwave (MW) power on copper recovery was investigated across a range of power levels: 350 W, 750 W, and 900 W. Copper recoveries at different MW power settings are shown in Figure 7, at a L/S ratio of 16:1 under microwave irradiation at 750 W and concentration 5 M HCl.

Despite the increase in microwave power, the maximum leaching temperature stabilized around 104 °C in all cases, indicating limited thermal differentiation between settings. The variation in MW power did not result in a significant increase in copper recovery, suggesting that beyond a certain threshold, higher radiation intensity does not enhance the dissolution efficiency of chalcopyrite.

From a technical and operational perspective, 750 W was identified as the optimal power setting. While 900 W represents the upper limit of the system’s capability, it leads to excessive equipment heating, posing risks of thermal stress and increased energy consumption, which makes the process less economically viable. Therefore, 750 W offers a favorable balance between efficiency, equipment safety, and operational cost. Znamenačková et al. [44,45] confirm that, in consequence of microwave heating, the stress and thermal dilatation at the interface of mineral grains arise. Microwaves cause minerals to heat up more quickly, which can disrupt the surface of metallic minerals, increase their reaction surface, and improve the release of elements.

Figure 8a,b present the SEM and EDX analysis of the solid residue after leaching the chalcopyrite concentrate (leaching condition at 750 W, 5 M HCl and L/S ratio of 8:1). Surface analysis of the mineral reveals the absence of an elemental sulfur layer, which is commonly observed during conventional leaching in media such as HCl, H₂SO₄, or iodized salts, as reported in previous studies [46,47]. This suggests that microwave (MW) irradiation applied during chalcopyrite leaching disrupts the formation and accumulation of this passivating sulfur layer on the mineral surface.

We hypothesize that the interaction of microwave energy with the CuFeS₂ matrix facilitates continuous surface renewal, thereby preventing the stable formation of elemental sulfur coatings. This effect is consistent with the findings of Wen et al. [48], who reported that microwave irradiation can enhance leaching efficiency by eliminating passivation layers, increasing the number of active sites, and enlarging the effective reaction surface.

4. Conclusions

This study investigated the influence of hydrochloric acid (HCl) concentration on the microwave-assisted leaching of chalcopyrite concentrate in the presence of DSP nodules, which contain MnO₂ as an oxidizing agent. The results are as follows:

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Under optimized conditions (HCl concentration of 5 mol/L, L/S ratio of 16:1, and microwave power of 750 W), copper recovery reached a maximum of 40% within 40 min, while manganese recovery from DSP nodules increased by up to 30% compared to 2 mol/L HCl, confirming the synergistic oxidative role of MnO₂.

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Kinetic analysis revealed that the apparent reaction order with respect to HCl concentration was n = 0.4448 for Cu and n = 0.491 for Mn, indicating a decreasing sensitivity of the reaction rate to acid concentration at higher molarities and underscoring the growing importance of the oxidant (MnO₂) in the leaching process.

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The key scientific contribution of this work is the innovative use of deep-sea polymetallic nodules (DSP) as a leaching agent with a dual function, serving simultaneously as an oxidizing agent and as an additional source of valuable metals (Mn: 21.74 wt.%, Cu: 1.004 wt.%, Ni: 1.15 wt.%).

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The use of microwave radiation at a power of 750 W proved effective in disrupting the formation of a passivating sulfur layer, which commonly occurs during conventional leaching processes. This facilitated a more effective interaction between the oxidizing agent and chalcopyrite, promoting copper extraction.

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Further optimization of the leaching system could be achieved by reducing the particle size of the DSP nodules below 266 µm. Finer particles are expected to exhibit improved surface contact with chalcopyrite, enhancing Cl2 generation and thus intensifying the oxidative leaching process. Additionally, smaller DSP particles may absorb microwave energy more efficiently, leading to localized temperature increases that could accelerate reaction kinetics.

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The practical significance lies in the fact that DSP nodules represent a sustainable source of oxidizing agents and metals, support the circular economy, and, under optimal leaching conditions, provide an efficient and easily scalable process for processing low-grade copper ores, which can be further improved by reducing particle size.

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Future experiments should explore not only reducing the particle size of the feedstock and determining its limits, but also increasing the amount of DSP nodules in the batch. In addition to serving as a source of MnO₂, these DSP nodules contain appreciable quantities of copper, offering a dual benefit in terms of oxidizing capability and metal recovery potential.