Obtention of Suitable Pregnant Leach Solution (PLS) for Copper Solvent Extraction Plants from Copper Concentrate Using Hydrogen Peroxide and Iodine in a Sulfuric Acid–Chloride Medium

Abstract

Copper leaching presents an environmentally friendly alternative to traditional sulfide ore processing methods. This study investigates an efficient leaching process for copper concentrate, utilizing a solution of sulfuric acid (H₂SO₄) and potassium iodide (KI) in a chloride medium (NaCl), enhanced by hydrogen peroxide (H₂O₂) at room temperature. A significant aspect of this research was optimizing the KI concentration to minimize iodide sublimation into iodine gas (I₂). Through the experimental design, the optimal dosages of reagents were determined, leading to maximized copper extraction of approximately 27% in 45 min of testing at room temperature. The results showed that it is possible to obtain a suitable pregnant leach solution (PLS) (i.e., in the range of 3 to 8 g/L of Cu) for treatment in available copper solvent extraction (SX) plants with a cost of less than 4.5 USD/t Cu, according to the economic analysis carried out. The results of this study determine the most effective operational conditions for leaching and ensure a suitable PLS for SX plants in a cost-effective and environmentally friendly manner. This approach could significantly contribute to more sustainable practices in the mining and processing of copper ores.

1. Introduction

An explosive growth in raw material demand, especially driven by developing countries, has been seen in recent decades [1]. Copper plays a crucial role in various sectors due to its unique properties, such as high electrical and thermal conductivity, corrosion resistance, and durability. It is widely used in electrical wiring, telecommunications, construction, and transportation. The significant role of copper in renewable energy, particularly in solar and wind power systems, is noteworthy [2]. Additionally, this metal is essential for modern technologies like smartphones and computers.

Copper can be found in nature as native copper or minerals, such as copper sulfides and copper oxides, which are processed in concentrators and hydrometallurgical plants, respectively. With the increase in demand, a decrease in ore grade, and mineralogy changes, a rise in energy and water consumption in their processes has been observed [3,4]. Specifically in the case of Chile, the world’s leading copper producer [5], the projections show that copper production via hydrometallurgical processes will decrease from 27.3% in 2019 to 8.1% in 2031 of the total production, leading to an increase in copper concentrates via concentrators [6] corresponding to an unused hydrometallurgical capacity which corresponds to 2.5 million tons of copper cathodes. Hydrometallurgical processes, like leaching, offer additional benefits over traditional smelting for extracting copper from chalcopyrite. They are more flexible with ore variations, reduce emissions, and have a lower environmental impact. Consequently, many mining companies are researching these methods for their environmental and economic advantages, especially utilizing unused plants [7,8,9,10].

Challenges remain and several studies are being conducted to explore alternative hydrometallurgical methods for treating low-grade copper ores or from complex ore bodies, with the aim of maintaining production targets and ensuring the quality of the metal. For example, Harichandan and Mandre [11] studied the recovery of copper from a low-grade mixed sulfide-oxide ore in order to identify the most influential process parameters through a statistical design model and surface analysis. The results showed that time, temperature, and particle size can affect the leaching performance. Additionally, in the literature review of Ji et al. [10], a review of the hydrometallurgical leaching of low-grade complex chalcopyrite is studied in which the advantages and disadvantages of mainstream leaching processes such as oxidation leaching, coordination leaching, and biological leaching are analyzed.

One of the primary challenges in chalcopyrite leaching is the formation of a passivation layer, which impedes copper dissolution and complicates the industrialization of the leaching process. Consequently, ores containing chalcopyrite receive significant attention, with efforts being focused on developing a technologically and economically feasible process. Studies of leaching in acidic media have explored chalcopyrite leaching using sulphate or chloride solutions, often in combination with oxidants [12,13,14,15]. Generally, it is concluded that refractory ores, such as chalcopyrite and bornite, leach slowly in sulfate solutions; therefore, oxidizing agents like hydrogen peroxide and iron ions can increase the leaching rate. In chloride media, the passivation of chalcopyrite has also been observed, although to a lesser degree than in sulfate systems. Chloride media can stabilize cuprous ions and increase the solubility of copper, resulting in better copper extractions [14,16]. In addition to low-grade ore, the leaching of copper concentrates has been explored. For instance, Lu and Dreisinger [17] investigated the leaching of chalcopyrite concentrate in a ferric and cupric chloride media. They employed a two-stage countercurrent leach circuit, operating at high temperatures in an oxygen-free atmosphere, achieving copper extractions above 95%.

Furthermore, other novel leaching reagents, such as ionic liquids, glycine, methanesulfonic acid, and ammoniacal solutions, are being explored for use with both concentrates and chalcopyrite-containing ores. However, operational, environmental, and economic aspects still require further studies [18].

Other oxidants investigated include nitrate and nitrite salts, as well as iodine salts. These chemical species have a higher oxidative potential than other oxidants, such as ferric or cupric ions [19,20,21,22]. These salts can be found in large ore deposits in the north of Chile in the forms of NaNO₃, NaCl, KI, and NaIO₃. The Chilean roadmap, as defined in 2019 [23], prioritizes the development of new green technologies in the field of hydrometallurgy. Leveraging these salts in hydrometallurgical applications not only aligns with this strategic direction but also places Chile in a strategically advantageous and beneficial position on the global stage. Regarding the latter, Moraga et al. [24] studied the effect of low concentrations of H₂O₂ in the extraction of copper from a sulfide concentrate. In combination with iodine-based oxidants, obtained from northern Chile, in an acid chloride media, suitable pregnant leach solutions for hydrometallurgical plants could be obtained. However, the iodide losses by sublimation and the high costs associated with reagent consumption used in maximizing the copper extraction must be addressed to develop a sustainable process.

Hence, this research aimed to develop a sustainable leaching process for a copper concentrate to obtain suitable pregnant leach solutions [PLS], for later processing in SX plants. Using a leaching solution based on sulfuric acid and oxidizing salts, such as potassium iodide, in a chloride medium and an oxygen-supplying agent, hydrogen peroxide was optimized in terms of concentration. A statistical analysis of the experimental design plus a critical economic evaluation were carried out to identify the best conditions for the proposed process and the main mechanisms involved in the leaching process, where several interactions occur between reagents and copper sulfide species. Leaching sulfide copper ores, predominantly chalcopyrite, needs oxidizing reagents in an acidic environment. For instance, the utilization of hydrogen peroxide in a sulfuric acid medium as a leaching agent partially facilitates chalcopyrite leaching, as represented by the following [25]:

$$H_2{O}_2+2H^{+}+2\mathrm{e}\to 2H_{2}O$$

(1)

$$2CuFe{S}_2+10H^{+}+5H_2{O}_2\to 2C{u}^{2+}+{2Fe}^{3+}+4S+10H_{2}O$$

(2)

$$2CuFe{S}_2+2H^{+}+17H_2{O}_2\to 2C{u}^{2+}+{2Fe}^{3+}+2S+18H_{2}O$$

(3)

Furthermore, a chloride medium enhances the solubility of copper, iron, and other metals. Ferric ions act as an oxidizing agent, leaching the chalcopyrite. The general equation for chalcopyrite dissolution in ferric media is:

$$CuFe{S}_2+\left(6x+4\right){Fe}^{3+}+4x{H}_{2}O={Cu}^{2+}+\left(6x+5\right){Fe}^{2+}+8x{H}^{+}+xS{O}_4^{2-}+\left(2-x\right)S^0$$

(4)

In a chloride medium, elemental sulfur (S⁰) is the dominant sulfur species in the residue (i.e., x = 0). Chalcopyrite leaching in sulfate media exhibits refractory behavior. According to related studies, the dissolution of copper in a chloride medium can be improved up to five times under the same leaching conditions compared to leaching in sulfide media [18].

Its redox properties also improve because both cuprous and cupric ions stabilize as chloro-complexes. In this context, the Cu(I)/Cu(II) redox couple establishes sulfide oxidation. At moderated potentials, the CuCl⁺ complex is stable and interacts with chalcopyrite [26]:

$$CuFe{S}_2+{3CuCl}^{+}+{Cl}^{-}\to 4CuCl+{Fe}^{2+}+2S^0$$

(5)

This finding aligns with the results presented by Hirato et al. [27] in which an elevation in the redox potential in the leaching solution was observed with the augmentation of the chloride concentration (as CuCl₂), consequently enhancing copper extraction.

The leaching process proposed involves the use of iodide salts as oxidants. According to the available literature, iodine, present as either diiodine or triiodide, is identified as the active oxidant essential for the dissolution of chalcopyrite. Its effectiveness is dependent on the potential solution, as defined by Winarko et al. [28]. These reactions, represented from Equation (6) to Equation (8), play a significant role in the dissolution of chalcopyrite, as detailed in Winarko et al. [22].

$$I_2+{ I }^{-}=I_3^{-}$$

(6)

$$CuFe{S}_2+2I_3^{-}=C{u}^{2+}+F{e}^{2+}+2S^0+6I^{-}$$

(7)

$$CuFe{S}_2+2I_2={Cu}^{2+}+F{e}^{2+}+2S^0+4I^{-}$$

(8)

Thus, the dissolution of copper involves multiple mechanisms, each playing a significant role. A main objective of this study was to clarify the predominant role of each mechanism in the extraction of copper. This was achieved through a detailed experimental approach coupled with an electrochemical analysis, which will be discussed in the following sections. By providing a detailed analysis of these aspects, the study aims to contribute significantly to the field of copper extraction, enhancing both the theoretical understanding and practical applications of these mechanisms.

2. Materials and Methods

2.1. Copper Concentrate Characterization

Copper concentrate from a mine in northern Chile was used for the leaching tests. The sample was characterized using quantitative evaluation of minerals by scanning electron microscopy (QEMSCAN, Model Zeiss EVO 50, Zeiss, Oberkochen, Germany) and X-ray diffraction (XRD-7000, Shimadzu Corporation, Kyoto, Japan). The results are presented in

Table 1. Mineral characterization of the copper concentrate.

Mineral Specie	Formula	%Base Material	%Base Sulfide	%Base Sulfide Cu	%Cu
Chalcopyrite	CuFeS2	63.7	68.8	84.4	22.0
Bornite	Cu5FeS4	6.4	17.8	8.5	4.1
Digenite	Cu9S5	3.3	3.6	4.4	2.6
Enargite	Cu3AsS4	1.2	1.3	1.6	0.6
Covellite	CuS	0.8	0.9	1.0	0.5
Pyrite	FeS2	16.4	6.9
Molybdenite	MoS2	0.3	0.3
Sphalerite	ZnS	0.2	0.2
Pyrargyrite	Ag3SbS3	0.1	0.1
Magnetite	Fe3O4	0.1
Hematite	Fe2O3	0.1
Gangue		7.30
Total		100	100	100	29.8

.

The total copper content in the concentrate was quantified using atomic absorption spectrometry (AAS, AA-6880 Shimadzu, Tokyo, Japan). The analysis showed a total copper concentration of 29.79% (w/w). The particle size corresponds to an 80% passing size (P80) of 61 microns.

2.2. Experimental Methods

The previous work of Moraga et al. [24] demonstrated the effect of ions such as iodate, iodide, chloride, and hydrogen peroxide on the kinetics and the resulting copper concentration in PLS. Also, it was demonstrated that the reagent concentration could be adjusted to decrease costs and losses by sublimation. For these reasons, the following experimental designs, described in detail in the following sections, consider refine intervals in which the reagent concentration was evaluated to improve the leaching performance operationally and economically. The reagents used in the test were potassium iodide (Merck-Rahway, NJ, USA, 99%), sulfuric acid (Winkler-USA, 95–98%), hydrogen peroxide (Winkler-USA, 30%), and sodium chloride (Winkler-USA, 99%). The experimental error of this study is 2.9% on average.

2.2.1. Determination of Optimal Concentration of Iodide Salt

To determine the optimal KI concentration and avoid iodine loss, five leaching tests were conducted using variable iodide concentrations in the form of KI, as detailed in

Table 2. KI concentrations employed in the leaching tests to minimize iodide sublimation during the process.

Test	KI (ppm)
T1	0.00
T2	100
T3	400
T4	917
T5	1758

. Each test was conducted in an isolated 2 L flask containing 100 g of concentrate dissolved in 1 L of leaching solution at room temperature. The leaching solution had a composition of 26.32 g/L of sulfuric acid (H₂SO₄), 15 g/L of hydrogen peroxide (H₂O₂), and 90 g/L of sodium chloride (NaCl). The agitation speed was maintained at 600 rpm. After 45 min, samples from the solutions were taken to measure the iodide concentration using the standardized method from INEN [29].

2.2.2. Determination of Residence Time

Leaching tests were performed to assess the impact of variable reagent concentrations, both high and low, on the dissolution kinetics of copper, as detailed in

Table 3. Composition of the leaching solution for the residence time study.

Compound	Test
	Low P1 (g/L)	High P2 (g/L)
NaCl	0.00	90.00
KI	0.00	0.52
H2O2	3.00	15.00
H2SO4	5.26	26.32

. The results of these tests were helpful in determining the optimal residence time for future experimental setups. For these tests, 100 g of concentrate was used in 1 L of leaching solution, maintained at room temperature with an agitation speed of 600 rpm. The iodide concentration was determined based on findings from the prior section. Solution samples were taken at intervals of 15, 45, and 150 min, as well as at 48 and 72 h, to analyze the copper concentration by AAS.

2.2.3. Evaluation of Reagent Concentration Effect on Copper PLS

After determining the optimal iodide concentration and residence time for the leaching tests, a full 3k factorial design was established (tests labeled “L”). The levels for each factor are detailed in

Table 4. Concentrations of reagents tested in leaching solutions with different levels of each according to the experiment factorial design.

Variable	Levels
	Lower (g/L)	Medium (g/L)	Upper (g/L)
NaCl	0.00	45.00	90.00
KI	0.00	0.13	0.52
H2O2	0.00	3.00	15.00
H2SO4	5.26	12.21	26.32

. The experimental conditions were the same as described in the previous section. For each test, the concentrations of copper and iron in PLS were quantified by AAS. Additionally, the conductivity (measured with an Orion 170 conductivity meter) and redox potential (measured with a Hanna portable pH/ORP meter, model HI991003, Hanna, St. Louis, MO, USA) were recorded. Subsequently, the response data were analyzed statistically using the Minitab version 18 software.

3. Results

3.1. Optimal Iodide Salt Dosage

These experiments were designed to identify the optimal range that minimizes losses through sublimation. As highlighted in the earlier research by Moraga et al. [24], losses of iodine due to sublimation can impact both the environment and the operational cost of the process. The rise in temperature within the system, caused by exothermic reactions, can promote the sublimation of any excess iodine. Therefore, when determining the operating range, it is essential to account for this negative effect, especially when considering its impact on large-scale operations. Figure 1 shows the results of the concentrate leaching test using the reagent concentrations provided in Table 2 from Section 2.2.1.

Referring to Figure 1, the segmented line illustrates how copper extraction slightly increased with the rise in iodide concentration in the leaching solution. Additionally, with this growth in copper extraction, the black line represents a corresponding rise in profitability. However, this enhanced extraction led to an associated iodine loss into the environment, as indicated by the light gray line, which demands attention. When the iodide concentration was increased to 1758 ppm, copper extraction rose by seven percentage points. However, this enhanced extraction led to a considerable loss of iodide. Specifically, in test T3, raising the iodide concentration resulted in a four percentage-point increase in copper extraction, led to a 2.78% improvement in profitability compared to the baseline T1 test. Nevertheless, this translates to an iodide loss of approximately 330 ppm, which is close to 80% of the initial iodide concentration in the leaching solution. Such a loss represents a significant environmental concern. Therefore, it was determined that the optimal concentration of iodide in the leaching solution should range between 100 and 400 ppm. This concentration effectively limits iodine losses to a maximum of 300 ppm, while simultaneously allowing for an increase in copper yield by up to 4 percentage points, attributable to the effects of the iodide.

3.2. Study of Leaching Kinetics and Determination of Optimal Residence Time

Figure 2 illustrates the rapid dissolution in the P2 test, achieving up to 25% copper extraction (equivalent to 6.8 g/L of Cu in the PLS). Following this, the dissolution rate slowed, eventually reaching a copper concentration of 7.7 g/L. By 45 min, over 80% of the leaching extraction was attained, enabling us to determine a reasonable residence time for subsequent tests. This is indicated with a circle and zoomed in portion in Figure 2. The significance of the reagent concentration in leaching is also evident; using the maximum concentration in the leaching solution can increase the copper concentration by seven times, compared to tests with a lower reagent content, such as the P1 test.

3.3. Species Concentration Effect on the Concentrate Leaching

The results obtained for the 3k factorial design are presented in

Table 5. Copper concentration (g/L) obtained in PLS at different leaching solutions according to the experimental design.

Test L	(g/L)
	NaCl	KI	H2O2	H2SO4	Cu in PLS
L1	0.00	0.00	0.00	5.26	0.39
L2	0.00	0.00	0.00	12.21	0.90
L3	0.00	0.00	0.00	26.32	1.94
L4	0.00	0.00	3.00	5.26	1.25
L5	0.00	0.00	3.00	12.21	1.75
L6	0.00	0.00	3.00	26.32	2.77
L7	0.00	0.00	15.00	5.26	4.63
L8	0.00	0.00	15.00	12.21	5.22
L9	0.00	0.00	15.00	26.32	6.20
L10	0.00	0.13	0.00	5.26	0.41
L11	0.00	0.13	0.00	12.21	0.92
L12	0.00	0.13	0.00	26.32	1.96
L13	0.00	0.13	3.00	5.26	1.27
L14	0.00	0.13	3.00	12.21	1.78
L15	0.00	0.13	3.00	26.32	2.82
L16	0.00	0.13	15.00	5.26	4.71
L17	0.00	0.13	15.00	12.21	5.20
L18	0.00	0.13	15.00	26.32	6.26
L19	0.00	0.52	0.00	5.26	0.46
L20	0.00	0.52	0.00	12.21	0.96
L21	0.00	0.52	0.00	26.32	1.99
L22	0.00	0.52	3.00	5.26	1.30
L23	0.00	0.52	3.00	12.21	1.81
L24	0.00	0.52	3.00	26.32	2.87
L25	0.00	0.52	15.00	5.26	4.72
L26	0.00	0.52	15.00	12.21	5.31
L27	0.00	0.52	15.00	26.32	6.36
L28	45.00	0.00	0.00	5.26	0.62
L29	45.00	0.00	0.00	12.21	1.12
L30	45.00	0.00	0.00	26.32	2.18
L31	45.00	0.00	3.00	5.26	1.49
L32	45.00	0.00	3.00	12.21	1.96
L33	45.00	0.00	3.00	26.32	2.99
L34	45.00	0.00	15.00	5.26	4.92
L35	45.00	0.00	15.00	12.21	5.46
L36	45.00	0.00	15.00	26.32	6.52
L37	45.00	0.13	0.00	5.26	0.64
L38	45.00	0.13	0.00	12.21	1.14
L39	45.00	0.13	0.00	26.32	2.19
L40	45.00	0.13	3.00	5.26	1.49
L41	45.00	0.13	3.00	12.21	2.01
L42	45.00	0.13	3.00	26.32	3.01
L43	45.00	0.13	15.00	5.26	4.88
L44	45.00	0.13	15.00	12.21	5.40
L45	45.00	0.13	15.00	26.32	6.47
L46	45.00	0.52	0.00	5.26	0.68
L47	45.00	0.52	0.00	12.21	1.19
L48	45.00	0.52	0.00	26.32	2.24
L49	45.00	0.52	3.00	5.26	1.55
L50	45.00	0.52	3.00	12.21	2.04
L51	45.00	0.52	3.00	26.32	3.10
L52	45.00	0.52	15.00	5.26	5.00
L53	45.00	0.52	15.00	12.21	5.53
L54	45.00	0.52	15.00	26.32	6.55
L55	90.00	0.00	0.00	5.26	0.84
L56	90.00	0.00	0.00	12.21	1.35
L57	90.00	0.00	0.00	26.32	2.42
L58	90.00	0.00	3.00	5.26	1.71
L59	90.00	0.00	3.00	12.21	2.19
L60	90.00	0.00	3.00	26.32	3.28
L61	90.00	0.00	15.00	5.26	5.13
L62	90.00	0.00	15.00	12.21	5.62
L63	90.00	0.00	15.00	26.32	6.71
L64	90.00	0.13	0.00	5.26	0.87
L65	90.00	0.13	0.00	12.21	1.38
L66	90.00	0.13	0.00	26.32	2.42
L67	90.00	0.13	3.00	5.26	1.72
L68	90.00	0.13	3.00	12.21	2.26
L69	90.00	0.13	3.00	26.32	3.24
L70	90.00	0.13	15.00	5.26	5.13
L71	90.00	0.13	15.00	12.21	5.63
L72	90.00	0.13	15.00	26.32	6.68
L73	90.00	0.52	0.00	5.26	0.93
L74	90.00	0.52	0.00	12.21	1.43
L75	90.00	0.52	0.00	26.32	2.49
L76	90.00	0.52	3.00	5.26	1.79
L77	90.00	0.52	3.00	12.21	2.29
L78	90.00	0.52	3.00	26.32	3.32
L79	90.00	0.52	15.00	5.26	5.21
L80	90.00	0.52	15.00	12.21	5.67
L81	90.00	0.52	15.00	26.32	6.72

. The maximum extraction was 26.9%, with a concentration of copper in the PLS of 6.72 g/L, which was achieved using the highest levels of reagents in the leaching solution at 20 °C and 45 min. This PLS is suitable for industrial solvent extraction processes. According to industrial data reported in the literature [30], hydrometallurgical plants with a suitable PLS ranging from 3 to 7 g/L exist. On the other hand, Figure 3 shows the main effect of species concentration on the PLS copper concentration. In order of importance, the species most favorable to leaching are H₂O₂, H₂SO₄, NaCl, and KI. It can be seen that a low concentration of H₂O₂ had a negligible effect on the Cu concentration. These results are consistent with those obtained in the previous work of Moraga et al. [24]. Tests with a high peroxide content in the leaching solution allow for a suitable PLS. Conversely, on average, the KI had less effect on the process due to the low concentration, which was selected to avoid losses directly into the environment. Additionally, Figure 4 shows the interaction plot representing the dependence between two factors on a response. Parallel lines indicate no interaction between the factors, while the degree of interaction increases as the difference between the slope lines increases. Therefore, there was no dependence between the factors affecting copper extraction.

3.4. Leaching Reagents Effect on Iron Concentration and PLS Redox Potential

The leaching of copper concentrates results in the co-dissolution of iron, among other metals, into the PLS. According to the literature, iron as ferric ions can enhance chalcopyrite leaching at moderated concentrations. Various authors reported different concentrations of iron from 0.001 to 0.1 M used for the dissolution of copper sulfides, which depends on the type of solid (ore or concentrate) and operational conditions (temperatures, solid–liquid ratio, sulfate or chloride media) [15,18,31,32,33,34,35].

Conversely, high concentrations of iron ions and uncontrolled pH and redox potential conditions can cause precipitates that directly affect the copper-leaching kinetics [35]. Additionally, the effect of the redox potential in chalcopyrite has been studied extensively in the literature, concluding that moderated levels of potential maximize chalcopyrite dissolution and avoid the formation of precipitates, such as jarosite, and subsequent chalcopyrite passivation. Figure 5 shows the main effect of the leaching reagent concentration on the total iron concentration and the redox potential of the PLS; see Figure 5a,b, respectively. The results show that the acid concentration impacted the iron concentration more significantly than the copper concentration, increasing the iron concentration five times. Other species had a negligible effect. On the other hand, hydrogen peroxide, a powerful oxidant, caused the largest effect on the PLS redox potential, increasing the potential from 530 mV to 600 mV SHE. At low hydrogen peroxide concentrations, the copper concentration in the PLS was low. This is in agreement with the results of Velasquez-Yévenes et al. [36] in which the rate and extraction were enhanced in the redox potential range of 550 to 620 mV. Therefore, it is crucial to analyze the factors that affect copper extraction and the side effects that can cause passivation.

3.5. Electrochemistry Analysis of the Leaching Tests

Figure 6 shows the Pourbaix diagram for the Cu-Cl-Fe-I system at 20 °C. These diagrams were developed using the HSC Chemistry v9 software [37], module Eh pH Diagrams. After the leaching test was completed, the pH and redox potential (in mV SHE) were measured and are represented by a square in each diagram in Figure 6. The diagram was created using the ion concentrations from the pregnant leaching solution of the L81 test, which yielded the best copper concentration results in the PLS. The ion concentrations in the PLS used for constructing the Pourbaix diagrams were as follows: 0.404 g/L for iodine (I), 54.6 g/L for chloride (Cl), 0.08 g/L for iron (Fe), and 6.71 g/L for copper (Cu).

At the defined ion concentration and the resulting pH and redox potential, chloro-complexes such as CuCl+ and FeCl₂+ were stable. Similar results were obtained in Hashemzadeh and Liu [38], where copper and iron speciation for chloride solutions at high ionic strength was analyzed, with cathodic and anodic half reactions being shown to be responsible for copper leaching from chalcocite in the chloride media. Thus, chalcopyrite leaching was enhanced by the formation of the CuCl+ complex, as proposed in Equation (5). On the other hand, Figure 6c shows the stable iodine species in which triiodide (I3⁻) is the predominant ionic specie. According to Equations (5)–(7), the iodine reacts with the iodide to form diiodine or triiodide, both oxidants enhancing chalcopyrite dissolution. Consistent results were obtained in the work of Winarko et al. [22] in which the rate of chalcopyrite extraction was studied using iodine in a ferric sulfate media as a leaching solution.

4. Economic Analysis

The economic analysis mainly comprised the cost of reagent consumption. To obtain the economic estimation for the conditions under study, the unit cost in USD/t of copper was calculated using the following equation:

$$Unit cost=\frac{\sum _k{C}_k\ast P_k}{C_{Cu}}$$

(9)

where C_k is the concentration of reagent k in g/L, P_k is the reagent price in USD/t, and C_Cu is the copper concentration in the PLS in g/L. The reagent prices used in this study are shown in

Table 6. Prices of the reagents according to the actual market.

Reagent	Price (USD/t)
H2SO4 98% w/w	250
H2O2 50% w/w	500
KI	800
NaCl	50

.

For all tests, from L1 to L81, the unit cost was calculated using Equation (9). The results are shown in Figure 7.

According to Schlesinger et al. [30], current leaching plants produce a PLS with a copper concentration in the range of 3 to 8 g/L. In Figure 7, the tests demarcated by segmented lines indicate leaching tests that yielded a PLS appropriate for solvent extraction plants. The specific tests (i.e., tests labeled “L”) that produced a PLS within this concentration range are detailed in

Table 7. Leaching tests with a suitable PLS for solvent extraction plants.

Tests L	Concentration (g/L)					(USD/t)
	NaCl	KI	H2O2	H2SO4	Cu in PLS	Cost
L8	0.00	0.00	15.00	12.21	5.22	3.46
L27	0.00	0.52	15.00	26.32	6.36	3.46
L18	0.00	0.13	15.00	26.32	6.26	3.46
L26	0.00	0.52	15.00	12.21	5.31	3.48
L9	0.00	0.00	15.00	26.32	6.20	3.48
L16	0.00	0.13	15.00	5.26	4.71	3.49
L17	0.00	0.13	15.00	12.21	5.20	3.49
L7	0.00	0.00	15.00	5.26	4.63	3.52
L25	0.00	0.52	15.00	5.26	4.72	3.55
L36	45.00	0.00	15.00	26.32	6.52	3.66
L45	45.00	0.13	15.00	26.32	6.47	3.70
L54	45.00	0.52	15.00	26.32	6.55	3.70
L35	45.00	0.00	15.00	12.21	5.46	3.72
L53	45.00	0.52	15.00	12.21	5.53	3.75
L34	45.00	0.00	15.00	5.26	4.92	3.77
L44	45.00	0.13	15.00	12.21	5.40	3.78
L52	45.00	0.52	15.00	5.26	5.00	3.80
L43	45.00	0.13	15.00	5.26	4.88	3.83
L63	90.00	0.00	15.00	26.32	6.71	3.89
L72	90.00	0.13	15.00	26.32	6.68	3.92
L81	90.00	0.52	15.00	26.32	6.72	3.95
L51	45.00	0.52	3.00	26.32	3.10	3.95
L42	45.00	0.13	3.00	26.32	3.01	3.97
L62	90.00	0.00	15.00	12.21	5.62	4.02
L71	90.00	0.13	15.00	12.21	5.63	4.02
L80	90.00	0.52	15.00	12.21	5.67	4.05
L61	90.00	0.00	15.00	5.26	5.13	4.06
L79	90.00	0.52	15.00	5.26	5.21	4.07
L70	90.00	0.13	15.00	5.26	5.13	4.08
L60	90.00	0.00	3.00	26.32	3.28	4.29
L78	90.00	0.52	3.00	26.32	3.32	4.37
L69	90.00	0.13	3.00	26.32	3.24	4.38

, with the most cost-effective tests highlighted in bold and circled in Figure 7. When hydrogen peroxide is incorporated into the leaching solution, it is feasible to produce a PLS suitable for solvent extraction plants at a reduced unit cost. Additionally, the cost impact of iodide remains negligible due to its minimal concentration in the leaching process. It is evident that as sodium chloride is introduced to the leaching solution, the unit cost rises in direct correlation with the copper concentration in the PLS. Through a refined experimental approach, a leaching process was devised that produces copper concentrations compatible with subsequent solvent extraction processes (shown in a circle in Figure 7). These experiments also evidenced a decline in unit costs, dropping below 4.5 USD/t Cu. Furthermore, it is viable to produce an appropriate PLS at diminished chloride levels, thereby mitigating the potential impacts of the cupric chloride complex during the solvent extraction phase, as observed by Shakibania [39].

Table 8. Tests used in cost analysis charts.

Figures		Concentration (g/L)
	Test	KI	NaCl	H2SO4	H2O2
Figure 8a	C4.1	0.00	0.00	5.26	0.00
	C4.2	0.00	0.00	5.26	3.00
	C4.3	0.00	0.00	5.26	15.00
	C5.1	0.00	0.00	12.21	0.00
	C5.2	0.00	0.00	12.21	3.00
	C5.3	0.00	0.00	12.21	15.00
	C6.1	0.00	0.00	26.32	0.00
	C6.2	0.00	0.00	26.32	3.00
	C6.3	0.00	0.00	26.32	15.00
Figure 8b	C7.1	0.13	45.00	5.26	0.00
	C7.2	0.13	45.00	5.26	3.00
	C7.3	0.13	45.00	5.26	15.00
	C8.1	0.13	45.00	12.21	0.00
	C8.2	0.13	45.00	12.21	3.00
	C8.3	0.13	45.00	12.21	15.00
	C9.1	0.13	45.00	26.32	0.00
	C9.2	0.13	45.00	26.32	3.00
	C9.3	0.13	45.00	26.32	15.00
Figure 8c	C10.1	0.52	90.00	5.26	0.00
	C10.2	0.52	90.00	5.26	3.00
	C10.3	0.52	90.00	5.26	15.00
	C11.1	0.52	90.00	12.21	0.00
	C11.2	0.52	90.00	12.21	3.00
	C11.3	0.52	90.00	12.21	15.00
	C12.1	0.52	90.00	26.32	0.00
	C12.2	0.52	90.00	26.32	3.00
	C12.3	0.52	90.00	26.32	15.00

displays the tests labeled “C” that are organized to assess the effect of each reagent on the total unit cost, as visualized in Figure 8. The main difference between Figure 8a–c lies in the varying concentrations of NaCl and KI in the leaching solution. By maintaining consistent levels of hydrogen peroxide and acid, an increase in the concentration of the remaining reagents resulted in a slight increase in the copper concentration in the PLS, relative to the base case C4.1, where leaching included only acid. Due to the negligible change in copper extraction and the associated costs and dosages of these reagents, especially NaCl, the unit cost rose significantly. This increase was less prominent in tests with elevated hydrogen peroxide concentrations (15 g/L). As highlighted earlier, the main effect plot (i.e., Figure 3) underscores the crucial role of hydrogen peroxide in copper extraction. Sections i, ii, and iii of Figure 8 illustrate the reagent’s dual impact on both unit cost and extraction. Elevated concentrations of hydrogen peroxide correlate with increased extractions, and consequently, a reduction in the total unit cost.

5. Discussion and Future Prospects

This work focuses on studying copper extraction from copper sulfide concentrate from Northern Chile using hydrometallurgy. Building on previous research by the authors, which demonstrated the feasibility of copper extraction using oxidants such as hydrogen peroxide and iodine salts in chloride media leaching, this study aimed to refine the operational conditions to reach pilot scale. It addresses sublimation losses from high iodine concentrations and operational conditions arising during experimentation. The goal of this study was to achieve similar results while minimizing the environmental impact and develop a cost-effective method. Both this and the earlier study successfully produced suitable PLS for SX plants, yet challenges in maximizing total copper extraction remain. In this study, the maximum copper extraction achieved was approximately 27% at 45 min of testing, outperforming conventional sulfide leaching at room temperature over a short period. However, retardation of the leaching kinetics was observed after 150 min, indicating the need for future research to investigate the surface of the spent concentrate for potential passivating species. Species such as covellite, chalcocite, jarosite, and elemental sulfur have been reported in the literature and explanations of their formation mechanisms are varied, but are still not fully elucidated [8,40]. To mitigate passivation in the leaching of chalcopyrite, several advanced methods have been developed. Researchers have implemented pretreatment stages that use chemical agents in combination with temperature control to improve leaching kinetics [41]. Additionally, pre-leaching grinding processes have been employed to increase the surface area for exchange, thus enhancing kinetics. Also, the mechanical activation of chalcopyrite is used to modify the chalcopyrite crystal and lattice structure [42]. Another approach involves using mechanical friction and collisions with silica particles to reduce passivation effects, as discussed by Misra and Fuerstenau [43]. Despite these advancements, challenges persist in fully understanding and preventing passivation, especially in large-scale operations, underscoring the need for ongoing research in this area.

6. Conclusions

This study aimed to upgrade the previous work of Moraga et al. [19] in order to develop an agitated leaching process for copper concentrates using KI and H₂O₂ in an acidic chloride media at room conditions, which can obtain a suitable PLS at a lower cost and in an environmentally friendly manner. The agitated leaching tests were designed in different steps to study the kinetics, electrochemistry, and the main effect of reagents in the leaching process. The results showed that it is possible to obtain a suitable PLS (i.e., in the range of 3 to 8 g/L of Cu) after 45 min of agitated leaching at room conditions for a copper concentrate composed mainly of chalcopyrite, bornite, and pyrite. The higher PLS concentration (i.e., 6.72 g/L) was obtained using 90 g/L of NaCl, 0.52 g/L of KI, 15 g/L of H₂O₂, and 26.32 g/L of H₂SO₄. The H₂O₂ had the largest effect on copper extraction due to it being the main component responsible for promoting all the potential dependent mechanisms compared to the acid, sodium chloride, and potassium iodide effects. The economic analysis showed that it is possible to obtain a PLS appropriate for SX plants at a cost below 4 USD/t Cu. Even though the extraction was superior with the maximum concentration of reagents defined in the experimental design, the costs increased considerably. For this reason, the best economic condition was low levels of NaCl and KI at high concentrations of H₂O₂. Moreover, these operation conditions also benefit the environment, decreasing possible iodine sublimation and chloride side effects in SX plants.