Copper Dissolution from Sulfide Ore with Deep Eutectic Solvents Based on Choline Chloride

Abstract

Copper is a critical resource for the energy transition and the development of novel sustainable processes for its recovery must be a focus of research. The use of deep eutectic solvents (DES) is an alternative for the solvometallurgical extraction of copper from sulfide ores with low or zero water consumption. The objective of this research is to study the dissolution of low-grade copper sulfide ore (0.83% Cu) using deep eutectic solvents. Laboratory scale agitation leaching tests were performed using different DES based on choline chloride (ChCl), namely ChCl-ethylene glycol, ChCl-citric acid, and ChCl-urea, at different temperatures (25, 50, and 60 °C). The effect of water and hydrogen peroxide was also studied in some systems. The best copper extractions were achieved with ChCl-citric acid > ChCl-urea > ChCl-ethylene glycol, reaching ≈99% copper extraction in some cases. This mineral leaching process offers an alternative to the processing of sulfide minerals and could be a technique that allows the use of solvent extraction and electrodeposition facilities available at a metallurgical plant, with less water consumption than the traditional leaching process.

1. Introduction

Climate deregulation is attributed to the effect of humans and industrial development, exploiting the Earth beyond its finite resources. Various initiatives worldwide were established to mitigate this situation. The United Nations Organization established, in 2015, the Sustainable Development Goals (SDGs), with 17 objectives and 169 goals set to be achieved by 2030 aiming to promote social, environmental, and economic benefits [1,2]. Furthermore, the circular economy model is being applied in many industries, which seek to optimize the use of resources, recognizing their finite nature and reformulating traditional processes to eliminate/minimize waste from the production processes [3]. In addition to this, environmental regulations in most countries are becoming more demanding, requiring that industries update their processes to the new reality, including the mining industry [4].

Chile is one of the main mining powers worldwide and has large reserves of copper, lithium, iodine, gold, silver, molybdenum, and nitrates, among other metallic and non-metallic minerals, which allows it to be the leader in copper and iodine production and second in lithium [5]. Mining must be a protagonist in the path of sustained and sustainable growth of the Chilean economy. Thus, efficient mining processes that increase the recovery of metals, consume less water, and use less polluting chemicals, are important to develop safe and more sustainable processes. Chile established its mining roadmap [6] defining priority topics to be developed. Among these are hydrometallurgy and green mining. To address these two issues, it is necessary to create new mineral process strategies that are more environmentally friendly, with lower emissions, and that can implement emerging technologies at an industrial scale [4,7].

Currently, the oxidized copper ores that are processed hydrometallurgically are running out, which will cause industrial plants to stop being used, leaving idle capacity installed. Sulfide copper minerals, which are increasing in Chilean deposits, are treated by flotation and part of the copper concentrate is processed by pyrometallurgy, a highly energy-demanding process that produces gases that must be captured and treated.

Green or sustainable chemistry [8], which emerged as a concept in the 1990s, provides an opportunity to develop new green solvents that can be obtained using renewable raw materials that are safe for living beings, non-toxic, biodegradable, and easy to prepare at low cost [9,10]. Deep eutectic solvents (DES) [11,12], first introduced by Abbot et al. [13], were proposed as green solvents. They are a mixture of two or more solids that melt at lower temperatures than those predicted by the ideal solution model due to negative deviations from thermodynamic ideality resulting from hydrogen bonding between a hydrogen bond donor (HBD) and hydrogen bond acceptor (HBA). The properties of DES are determined by the choice of its precursors. If carefully selected, the resulting DES can present moderate toxicity and volatility, whilst being thermally stable, biocompatible, and biodegradable. DES are classified into five types: type I composed of a quaternary ammonium salt and a metal chloride; type II composed of a quaternary ammonium salt and a metal chloride hydrate; type III composed of a quaternary ammonium salt and an HBD; type IV composed of a metal chloride hydrate and an HBD; and type V composed only of nonionic molecular HBA and HBD. This wide range of possible solvent composition has seen DES being used in numerous applications including the synthesis of nanomaterials, metallurgy and electrodeposition, gas separation and capture, medical and pharmaceutical research, and biocatalysis and energy systems [14].

Several studies have been conducted using DES in the dissolution of minerals and residues. Some researchers studied the use of these solvents in dissolution/recovery of metals (iron (Fe), zinc (Zn), lead (Pb), copper (Cu), cobalt (Co), nickel (Ni), molybdenum (Mo), lithium (Li), silver (Ag)) from minerals (oxides, pyrite (FeS₂), covellite (CuS), chalcopyrite (CuFeS₂), chalcocite (Cu₂S)) [15,16,17], mining waste (anode sludge, furnace dust, conversion slag) [18,19,20,21], and waste from electrical and electronic equipment (printed circuit board (PCB), lithium batteries) [22,23,24,25] with promising results. The use of these solvents provides an interesting alternative solution because it is possible to reduce water consumption compared to traditional hydrometallurgical processes. The concept of solvometallurgy, defined as processing minerals or wastes for the recovery of metals [26] with little or no water use, is now well established and DES are emerging for this purpose [27,28]. DES based on choline chloride (ChCl) as an HBA combined with HBDs such as alcohols (e.g., ethanol), carboxylic acids (e.g., malonic acid), and others (e.g., urea, ethylene glycol) were shown to solubilize various metal oxides. This observation was further confirmed for several metal oxides with DES based on various HBDs such as urea, malonic acid (MA), and ethylene glycol (EG) [29]. Interestingly, the solubility of each metal oxide is quite different depending on the DES, which could be used to selectively leach the metals and avoid the expensive separation process to separate each metal from the aqueous solution.

Table 1. Use of different DES as solvent in metal leaching.

DES Type	Solid Matrix	Experimental Conditions	Results	Authors
ChCl-ethylene glycol ChCl-urea ChCl-oxalic acid dihydrate.	Chalcocite, covellite, and chalcopyrite	Electrochemical measurements by cyclic voltammetry	Dissolution was achieved for all three solid samples	Anggara et al. [30]
ChCl-urea	Copper conversion slag	Leaching time (2–72 h), temperature (25–95 °C), and pulp density (1/10–1/40 g/mL).	65.3% Zn and 89.9% Cu at 48 h, 95 °C, 600 rpm and 1/20 g/mL	Topçu et al. [31]
ChCl-based deep eutectic solvents (DES) with urea and ethylene glycol	Copper anode slime	Leaching time (4–48 h), temperature (25; 50; 75; 95 °C), solid/liquid ratio (1/10–1/25 g/mL), DES composition (ChCl-urea, ChCl-urea-water, ChCl-ethylene glycol, ChCl-ethylene glycol-urea)	97% Cu in ChCl-urea, 95 °C, 4 h and 1 g in 25 mL 91% Ag in ChCl-urea, 95 °C, 48 h and 1 g in 10 mL.	Topçu et al. [18]
ChCl-ethylene glycol	Chalcopyrite concentrate (26.5% Cu)	Ambient pressure, temperature (19.5; 50; 80 and 90 °C)	16% Cu at 90 °C, 9.88 mol DES in mol initial Cu in chalcopyrite	Carlesi et al. [11]
ChCl-ethylene glycol	Sulfide concentrate mainly composed by sphalerite, pyrite, chalcopyrite, and galena	Oxidants (CuCl2, FeCl3, H2O2, I2, and O2) in 300 rpm, 2/100 solid liquid ratio	Dissolution at 80 °C of the following: galena ≈ 100% with I2 pyrite ≈ 5% with CuCl2 chalcopyrite > 95% with CuCl2 and I2 sphalerite > 75% with I2 gold > 60% with I2 tellurium ≈ 70% with FeCl3 and I2 silver > 65% with I2	Bidari et al. [17]
ChCl-urea ChCl-ethylene glycol ChCl-glycerol	Polymetallic concentrates (sulfates, oxides, sulfides) composed of Cu, Fe, Pb, and Zn	0.5 g/20 g DES, 30 °C, 100 rpm and 24 h	Sulfates dissolution: 6.7 g Cu/kg ChCl-urea 17 mg Fe/kg ChCl-urea 3.2 g Zn/kg ChCl-urea 3.4 g Pb/kg ChCl-ethylene glycol Metal oxides dissolution: 232 mg Fe/kg ChCl-urea 527 mg Pb/kg ChCl-urea 777 mg Zn/kg ChCl-urea 53 mg Cu/kg ChCl-ethylene glycol Sulfide dissolution: 22 mg/kg ChCl-urea from chalcopyrite 286 mg Pb/kg ChCl-urea from galena 50 mg Fe/kg ChCl-glycerol from pyrite 25 mg Fe/kg ChCl-glycerol from sphalerite 31 mg Fe/kg ChCl-ethylene glycol from chalcopyrite 13 mg Zn/kg ChCl-ethylene glycol from sphalerite	Aragón-Tobar et al. [32]
ChCl-ethylene glycol ChCl-oxalic acid ChCl-ethylene glycol-oxalic acid	Chalcopyrite concentrate	Temperature (50–80 °C), leaching time (24–72 h), 500 rpm, pulp density ratio 1:6	83% Cu at 80 °C, 72 h, ChCl-oxalic acid	Ghadamgahi et al. [33]
ChCl-ethylene glycol ChCl-oxalic acid ChCl-malonic acid ChCl-ethylene glycol-oxalic acid ChCl-ethylene glycol-malonic acid ChCl-oxalic acid-malonic acid	Chalcopyrite concentrate	1st stage: 75 °C, 48 h, 1 g/20 mL, 400 rpm. 2nd stage: DES ChCl-ethylene glycol-oxalic acid, temperature (45, 55, 65 and 75 °C), leaching time (2, 6, 12, 24, 48 and 72 h), solid liquid ratio (0.025; 0.05; 0.075; 0.1 g/mL), water addition (5; 20; 35; 50%v/v)	86% Cu at 75 °C, 48 h, 0.025 g/mL, ChCl-ethylene glycol-oxalic acid−20% vol. water	Shiri et al. [34]
ChCl-oxalic acid-30% water ChCl-ethylene glycol-30% water ChCl-urea-30% water ChCl-thiourea-30% water	CuO and CoO (analytical grade, ≥99%), Cu-Co ore	CuO and CoO: 60 °C, 400 rpm, 6 h, 1:10 solid/liquid ratio. One test was carried out using only 1 M H2SO4 as comparison. Cu-Co ore: temperature (30; 45; 60; and 75 °C), solid/liquid ratio (1:5; 1:10 and 1:20), leaching time (1–8 h), 400 rpm.	89.2% Cu from CuO and 92.4% Co from CoO at 60 °C, 400 rpm, 6 h, 1:10 solid/liquid ratio, −75 + 53 µm, ChCl-oxalic acid-30% water	Oke et al. [35]
ChCl-maleic acid	Chalcopyrite concentrate	Leaching time (2–24 h), temperature (100–200 °C), mol ratio ChCl to maleic acid (1:2; 1:1 and 2:1)	52.6% Cu obtained in ChCl-maleic acid ratio of 1:1, 150 °C, 24 h	Karimi et al. [36]
ChCl-ρ-toluenesulfonic acid	Chalcopyrite concentrate	Temperature (40; 60; 80; 100 and 120 °C), leaching time (1; 2; 3; 4 and 5 h), stirrer speed (100; 300; 500; 700 and 900 rpm), mass ratio DES/chalcopyrite (20; 40; 60; 80 and 100 g/g)	73.6% Cu in 1 h, mass ratio DES/chalcopyrite of 100, 120 °C, 100 rpm	Behnajady et al. [37]
ChCl-maloni acid-ρ-toluenesulfonic acid	Chalcopyrite concentrate	Leaching time (5; 30; 55; 80 and 105 min), milling time (0; 2; 4; 6 and 8 h), temperature (40; 60; 80; 100 and 120 °C), mass ratio concentrate/DES (0.005; 0.03; 0.055; 0.08 and 0.105 g/g), 500 rpm	83.9% Cu and 87.2% Fe in 80 min, 6 h, 100 °C, 0.03 g concentrate/g DES	Moradi et al. [38]

shows studies carried out using DES for metal dissolution.

About the mechanism of dissolution, some authors have reported certain mechanisms using DES but with other solid matrices [34,35,39]. Shiri et al. [34] studied the dissolution of copper concentrate using different DES and determined a chemically reaction-controlled mechanism using a DES composed of choline chloride, ethylene glycol, and oxalic acid. Oke et al. [35] studied the dissolution mechanism of a Cu and Co (oxide) ore sample using different DES (ChCl-urea, ChCl-ethylene glycol, ChCl-oxalic acid, and ChCl-thiourea). The authors suggest that dissolution is promoted by low pH, ligand exchange processes, and surface complexation. Svärd et al. [39] explained reaction mechanisms for the dissolution in DES of metals from Li-ion battery (cathode material) using organic acid and polyalcohol as HBD.

Despite these studies, studies using Chilean ores are scarce. The objective of this work is to study the dissolution of low-grade copper sulfide ore using deep eutectic solvents (choline chloride-ethylene glycol, choline chloride-citric acid, choline chloride-urea) in agitation leaching, determining efficiencies of the process at different experimental conditions: temperature (25, 50 and 60 °C), addition of water in systems with citric acid (20 and 35%) and urea (1 and 5%), and addition of hydrogen peroxide (0 and 5%) to attempt to identify a new leaching solvent that can be the basis for the design of a novel and sustainable process at laboratory scale.

The DES studied were selected from a preliminary literature search [11,30,40]. In systems with citric acid and urea, water is added to the DES to lower its viscosity and allow for improved solid–liquid mass transfer. Hydrogen peroxide was studied to determine its efficiency as an oxidant in the system, as reported in other studies [17]. Mixtures of two DES were tested to eliminate the addition of water and lower the viscosity of the leaching agent.

2. Materials and Methods

2.1. Ore

The solid used corresponds to a copper sulfide ore from a mining site in the Antofagasta region, Chile. This ore was crushed and milled to obtain a particle size of 95% < 150 μm. The ore sample was characterized by X-ray diffractometer (XRD, Shimadzu 6100, Kyoto, Japan, with a detection limit from 3% by volume), scanning electron microscope with an energy dispersive analyzer (SEM-EDS, Carl Zeiss EVO MA10, Oberkochen, Germany, with a detection limit from 0.1% by mass). The total copper, soluble copper in acid, and total iron content in the ore was quantified using atomic absorption spectrometry (AAS, AA-6880 Shimadzu, Kyoto, Japan).

2.2. Reagents

For the preparation of the DES, choline chloride (ChCl, C₅H₁₄ClNO, Sigma Aldrich, St. Louis, MO, USA, 98%), ethylene glycol (EG, C₂H₆O₂, Merck, Darmstadt, Germany, 99%), citric acid monohydrate (CA, C₆H₈O₇·H₂O, Merck, Darmstadt, Germany, 99%), and urea (U, CO(NH₂)₂, Merck, 99%) were used. Hydrogen peroxide (H₂O₂, Winkler, Freilassing, Germany, 30% 100 vol.) was used in some tests. Hydrochloric acid (HCl, Merck, Darmstadt, Germany, 37%) and distilled water were used to dilute the liquid samples.

2.3. Preparation of DES

All deep eutectic solvents, namely ChCl-EG in a molar ratio of 1:2, ChCl-CA in a molar ratio of 2:1, and ChCl-U in a molar ratio of 1:2, were prepared gravimetrically using a Mettler Toledo Co., Columbus, OH, USA, model AX204 analytical balance with a precision of 0.07 mg. The molar ratio of DES was chosen from the literature [11,21,40]. For the preparation, both reagents are mixed in a glass beaker, which is placed on a heating plate with magnetic stirring. The glass is sealed with Parafilm and stirring begins at 300 rpm, at a temperature of 80 °C, until the sample is homogeneous. First, DES composed of ChCl–EG, ChCl–CA, and ChCl–U were prepared separately. Due to the high viscosity of ChCl–CA and ChCl–U, a controlled amount of water was added during mixing [41]. Subsequently, new DES was prepared from a mixture of ChCl-EG and ChCl–CA. This mixture was prepared at a volume ratio of 70% to 30%, respectively. All DES types used in the leaching test can be seen in

Table 2. Copper extraction obtained from leaching tests carried out at 72 h, 3 g ore/ 30 mL and 300 rpm.

N°	DES	ρDES (g/mL)	ηDES (mPa s)	H2O (%)	H2O2 (%)	Temperature (°C)	Cu Ext. (%)
1	ChCl-EG	1.11639	44.37	0	0	25	4.6
2	ChCl-EG	1.10221	18.12	0	0	50	24.6
3	ChCl-EG	1.09661	13.60	0	0	60	42.7
4	ChCl-CA	1.21891	254.24	20	0	25	75.4
5	ChCl-CA	1.20442	70.66	20	0	50	99.5
6	ChCl-CA	1.19864	46.76	20	0	60	99.6
7	ChCl-U	1.19375	597.28	1	0	25	37.9
8	ChCl-U	1.18019	96.09	1	0	50	43.1
9	ChCl-U	1.17486	18.80	1	0	60	69.3
10	ChCl-CA	1.17354	18.41	35	0	25	48.2
11	ChCl-CA	1.15939	8.15	35	0	50	99.5
12	ChCl-CA	1.15357	5.66	35	0	60	99.8
13	ChCl-U	1.18624	164.02	5	0	25	24.0
14	ChCl-U	1.17269	40.99	5	0	50	81.0
15	ChCl-U	1.16739	14.97	5	0	60	83.1
16	ChCl-EG and ChCl-CA	1.16013	198.44	4	0	25	63.9
17	ChCl-EG and ChCl-CA	1.14579	61.12	4	0	50	99.6
18	ChCl-EG and ChCl-CA	1.14016	41.97	4	0	60	99.3
19	ChCl-EG and ChCl-CA	-	-	4	5	25	72.6
20	ChCl-EG and ChCl-CA	-	-	4	5	50	74.1
21	ChCl-EG and ChCl-CA	-	-	4	5	60	71.0

. The kinematic viscosity of each DES were measured with a calibrated micro-Ostwalt viscometer with a Schott-Gerate automatic measuring unit (model AVS 310, Schott-Gerate, Westerwald, Germany). This equipment has a thermostat (Schott-Gerate, model CT 52, Westerwald, Germany) for temperature control within ±0.05 °C. Absolute viscosities were obtained by multiplying kinematic viscosity and corresponding density with ±5·10⁻³ mPa·s precision for viscosity measurements. Densities were measured with a Mettler Toledo (model DE-50, Greifensee, Switzerland) vibrating tube density meter with ±5·10⁻⁵ g/mL precision. Density meter had self-contained Peltier systems for temperature control with ±0.01 °C precision. The properties were measured by triplicate. The uncertainties were 0.023 mPa·s for viscosity and 0.0011 g/mL for density.

2.4. Leaching Procedure

Leaching tests were performed in 50 mL jacketed glass beakers under constant stirring at 300 rpm, connected to a thermostatic bath with water recirculation to control the temperature with an accuracy of ±0.1 °C. The leaching agent was added to the beaker. When the temperature was reached (controlled by thermometer inside the beaker), according to the test to be carried out, the ore was added, and the stirring and measuring the reaction time was started. The solid–liquid ratio used for all tests was 1:10, using 3 g of ore for 30 mL of leaching agent, with a leaching time of 72 h. In selected tests where hydrogen peroxide was used, it was added with a dropper to the DES up to the desired percentage, achieving 30 mL of leaching solution; then, the ore was added. In tests where water was added, the DES plus the added distilled water correspond to a volume of 30 mL. After 72 h of leaching, the suspension was filtered using a Kitasato flask connected to a vacuum pump, fitted with a Buchner funnel with filter paper. Once the sample was filtered, 5 mL of the leached solution was taken, which was diluted in a 0.5 M HCl solution, making up to 100 mL. This solution was analyzed to determine the copper concentration by AAS. Considering the sample dilution factor and the copper concentration, the copper extraction is calculated using the following equation.

$$Cuextraction\left(\%\right)={\displaystyle \frac{\left[{Cu}^{2+}\right]·V}{m_{Cuinitial}}}a·100$$

(1)

All leaching experiments were conducted in duplicate.

3. Results and Discussions

3.1. Characterization of the Ore

The copper sulfide ore’s chemical analysis determined a total copper grade of 0.83%, sulfuric acid-soluble copper of 0.08%, and total iron grade of 0.85%. Dissolving copper in sulfuric acid is an indirect method of determining primarily oxidized mineralogical species. With a result of 0.08% soluble Cu, it can be inferred that most of the copper species are sulfide minerals since the total copper represents 0.83%. This sample corresponds to an ore with a low grade of copper and a low percentage of iron. Copper is mainly contained as a sulfide mineral, due to the low grade of acid-soluble copper. The mineralogical characterization by X-ray diffraction of the sample is indicated in Figure 1.

According to XRD results, the main mineralogical phases present in the sample ore were quartz (SiO₂), albite (NaAlSiO₈), and muscovite (H₂KAl₃(SiO₄)₃). The predominance of silicates is evident, with copper absent due to its low concentration and/or potential amorphic.

The results obtained by SEM-EDS analysis of the ore are shown in Figure 2.

Figure 2 shows that the ore sample contains a large presence of Si, Al, K, and Na, which is consistent with the presence of silicate gangues. A low presence of copper, iron and sulfur is observed.

3.2. Leaching Results

Table 2 shows the results obtained for copper extraction from leaching tests.

In Table 2, the system composed of DES ChCl-EG at 25 °C was the one that had the lowest process performance. This could be explained by how it is a poor lixiviant for most minerals due to its approximately neutral pH and/or the poorer complexing capabilities of EG, compared to other HBDs such as carboxylic acids. Pateli et al. [42] studied the dissolution of metal oxides (cobalt, manganese, nickel, iron, copper, zinc, and lead) in different DES (choline chloride-ethylene glycol, oxalic acid, acetic acid, lactic acid, levulinic acid, glycerol, and urea) at 50 °C. The increase in the solubility of metal oxide was dependent on proton activity. Solutions with lower pH obtained higher solubility. Using ChCl-EG, there is a low proton (H⁺) activity, which limits the ability to act as an O²⁻ acceptor and driving the dissolution reaction. According to Bidari et al. [17], the system composed of ChCl-EG presents a low redox potential of 147 mV (Ag/AgCl), with a low dissolution of sulfide concentrates composed of chalcopyrite, sphalerite, and pyrite at 40 and 80 °C.

Several systems achieved maximum copper extraction under the conditions studied (tests 5, 6, 11, 12, 17, 18) which can be attributed to the acidity present in the leaching solution which is selective for metals such as copper.

It can be observed that the density of the DES decreases as the temperature increases, and the same trend occurs with the viscosities. The system with the highest viscosity at 25 °C is the one composed of ChCl-U–1% H₂O, followed by ChCl-CA–20% H₂O. It can be observed that in these systems, as the temperature increases, the viscosity decreases considerably, which can favor the dissolution of copper in the system. This behavior was reported by Xie et al. [43] who show that the ChCl–U system, by increasing the amount of water and temperature, drastically decreases the viscosity from 1571 mPa s without water at 25 ° C, to 323.9 mPa s with 0.15 mole fraction of water; similar behavior is observed in Table 2, where at 25 ° C with 1% water, the viscosity goes from 597.28 mPa s to 164.02 mPa s for the DES with 5% water. For system ChCl–CA–H₂O, the same trend is observed in the work reported by Ninayan et al. [44].

The variables studied will be analyzed hereafter.

3.3. Effect of the Type of DES and Temperature

Figure 3 shows the copper extraction result obtained after 72 h of leaching using different DES as leaching solution at a temperature of 25, 50, and 60 °C, respectively.

Figure 3 shows that the highest extraction of copper at 25 °C was obtained with the system ChCl-CA-20% H₂O with a yield of 75.4%, followed by the system composed by the mixture of two DES ChCl-EG and ChCl-CA-4% H₂O-5% H₂O₂, with a yield of 72.6%, and the system with DES ChCl-EG and ChCl-CA-4% H₂O, which extracted 63.9% of the copper. The system of ChCl-EG achieved a maximum extraction of just 4.6% Cu. The better performance of the DES with citric acid can be explained by its proton donor structure, which, besides its chelating properties, allows it to generate complexes with metal ions, improving the solubility of copper from the sulfide matrix. In the case of urea, Topcu et al. [18] report that its leaching capacity is limited at temperatures below 75 °C. The same situation can be attributed to the very low level of Cu extraction obtained with the ethylene glycol system. For the cases of ChCl-U with 1 or 5% H₂O, these low yields may also be associated with the complexities arising from the high viscosity of DES at 25 °C, as reported in previous studies [31] and because the system has low acidity and limited chelating capacity, which results in less leaching efficiency [35]. At 50 °C, three systems allowed for a complete copper extraction, namely the systems composed by DES of ChCl-CA-20% H₂O, ChCl-CA-35% H₂O, and the mix of two DES (ChCl-EG and ChCl-CA-4% H₂O). On the other hand, DES composed of ChCl-U-1% H₂O obtained a recovery of 43.1% while DES composed by ChCl-U-5% H₂O extracted 81.0% Cu. This could indicate that the increase in water in the system modifies the interactions between the DES components and the mineral, affecting the solubility of the system, as explained by Di Pietro et al. [45]. The mix of DES (ChCl-EG and ChCl-CA-4% H₂O-5% H₂O₂) extracted 74.1% Cu. The system ChCl-EG extracted just 24.6% of the initial copper. This result shows that the combination of the chelating capacity of citric acid and the increased interaction between particles that produces the increase in temperature and decrease in viscosity allows the total extraction of copper from the ore. It should also be mentioned that the pH provided by citric acid has a great impact on the chemical dissolution reactions from ores, being useful for the chemical environment conducive to better extraction. DES composed by ChCl-EG perform better than at room temperature but still produce a low extraction compared to the other DES studied. At 60 °C, Figure 3 shows that magnitudes of copper extraction increase for DES composed by ChCl-U and ChCl-EG. A copper extraction of 69.3% is reported to ChCl-U-1% H₂O, 83.1% Cu to ChCl-U-5% H₂O, and 42.7% Cu to ChCl-EG. ChCl-CA with 20 and 35% H₂O maintain their total copper extraction at 99.6 and 99.8%, respectively, and similar results were obtained for ChCl-EG and ChCl-CA-4% H₂O with 99.3% Cu. This confirms the trend of the effect of DES on sulfide leaching, showing that the highest extraction capacity is achieved for DES ChCl-CA-20 or 35% H₂O. These results are in accordance with the literature, which highlights aspects such as the ability to release protons [46] and to contribute with a lower pH as relevant characteristics of acids in their role of generating a favorable chemical environment for leaching [31].

The results obtained by the system of ChCl-EG were 4.6% Cu at 25 °C, 24.6% Cu at 50 °C and 42.7% Cu at 60 °C. There is a clear trend of increasing copper extraction with increasing temperature. This indicates that as with other reagents, ethylene glycol improves its leaching capacity as the temperature of the system increases. However, it generally maintains a low extraction performance, so in future studies, this DES should not even be considered for Cu leaching, because it is not stable a high temperatures [47].

A significant influence of the temperature on the DES ChCl-CA-20% H₂O and ChCl-CA-35% H₂O systems were identified, allowing for a total copper extraction at the temperatures of 50 °C, and 60 °C, while the extraction at 25 °C reaches 75.4% and 48.2%, respectively. According to the results, it is evident that a temperature of 50 °C would be sufficient to obtain the best extraction result. It should be noted that this system, without the addition of water and at a temperature of 25 °C, presents a high viscosity, which complicates its handling in the leaching process (stirring and solid–liquid separation). It is for this reason that in this study, water was added to the system in different proportions. Although this prevents the use of a water-free system, it allows us to decrease the total cost of the solvent and improves its potential.

In the system ChCl-U-1% H₂O, copper extraction obtained were 37.9% at 25 °C, 43.1% at 50 °C, and 69.3% at 60 °C. It can be observed that at temperatures of 60 °C, the system begins to be more efficient with the dissolution of copper from ore, which agrees with the results obtained by Topçu et al. [21,31] using copper slag. The number of collisions of moving particles will increase with increasing temperature in chemical processes [31]. Furthermore, the viscosity of the system decreases with increasing temperature and also decreases with the addition of water [43]. This system composed of ChCl-U can be used in high temperature processes due to its high thermal decomposition temperature [48]. Copper extractions of 24.0% at 25 °C, 81.0% at 50 °C, and 83.1% at 60 °C were obtained in the system ChCl-U-5% H₂O. The results show that the effect of copper leaching is highly dependent on temperature, since increasing the temperature can reduce the viscosity of DES ChCl-U and increase the reaction rate.

The same trend discussed above is observed in the system composed of ChCl-EG (70%) and ChCl-CA-4% H₂O (30%), the increase in temperature increases copper extraction under the conditions studied. Extractions of 63.9% Cu are achieved at 25 °C, 99.6% at 50 °C, and 99.3% at 60 °C.

To the system composed of ChCl-EG and ChCl-CA-4% H₂O-5% H₂O₂, the results indicate a copper extraction of 72.6% at 25 °C, 74.1% at 50 °C, and 71.0% at 60 °C. In this system, the increase in the temperature did not favor the copper dissolution, as is the case when copper is extracted with sulfuric acid and hydrogen peroxide as the oxidizing agent [49]. Hydrogen peroxide in contact with the other compounds of the leaching system will probably decompose rapidly, in the first hours of the process, which occurs more quickly at higher temperatures [50,51].

3.4. Effect of Water Addition

Figure 4 shows the copper extraction results obtained after 72 h of leaching with addition of water (20% or 35%) at different temperatures, 25, 50 and 60 °C, using the system composed of ChCl-CA. Figure 5 shows the copper extraction results obtained after 72 h of leaching with addition of water (1% or 5% H₂O) at different temperatures, 25, 50 and 60 °C, using the system composed of ChCl-U.

The water content present in each mixture has a direct influence on the viscosity and ionic conductivity of the mixture [43,44]. Increasing the percentage of water leads to a decrease in extraction; that is quite clear at 25 °C, from 75.4% to 48.2% (Figure 4). This is because the acid concentration decreases, decreasing the proton activity in the mineral solution. With this, it can be inferred that the increase in the water percentage at 25 °C is deleterious to the performance of this DES. At 50 °C and 60 °C, ≈99% extraction was achieved regardless of the percentage of water. Although there is no variation in the extraction yield, it must be taken into consideration that the ChCl-CA system is a very viscous system [41], for which working without adding water is a very difficult task. Considering the reduction in water consumption in the dissolution process by working with an efficient system, it is recommended to work with the minimum amount of water possible, which, under the conditions studied, corresponds to 20%.

For the ChCl-U system (Figure 5), there is a decrease in copper extraction with increasing water concentration at 25 °C, like the DES ChCl-CA, inferring that this condition has a concentration-dependent behavior. At 25 °C, the increase in the water content decreases the urea concentration, which acts as a complexing of metals from ore. While at temperatures of 50 and 60 °C, there is an increase in copper extraction with increasing water from 1% to 5%, increasing the recovery from 43.1 to 81.0% at 50 °C and from 69.3 to 83.1% at 60 °C, showing a temperature-dependent behavior [21].

3.5. Effect of Combining Two DES

Figure 6 shows the copper extraction result obtained after 72 h of leaching at 25, 50 and 60 °C, respectively, comparing three DES composed by ChCl-EG, ChCl-CA-20% H₂O, and ChCl-EG and ChCl-CA-4% H₂O.

The systems studied at 25 °C show an extraction of 4.7% Cu using DES ChCl-EG, 75.2% Cu using DES ChCl-CA-20% H₂O and 64.6% Cu for the system studied as a mixture of DES ChCl-EG and ChCl-CA-4% H₂O. It is observed that the DES mixture achieves a better performance than DES ChCl-EG, but not better than the system ChCl-CA-20% H₂O. This confirms the leaching capacity of the DES composed of citric acid over the other DES studied. This can be explained because the DES mixture used dilutes the activity of the citric acid at 25 °C. It is worth mentioning that the objective of this mixture is to lower the water content in the leaching agent and decrease the viscosity of the leaching system, which can be achieved by adding ethylene glycol. The DES mixture under study shares the ChCl base as a hydrogen bond acceptor and incorporates two hydrogen bond donors with different characteristics such as ethylene glycol and citric acid. The fact that the mixture at 25 °C does not produce better results than citric acid alone may be due to the less favorable pH conditions created by the organic nature of the ethylene glycol when compared to the aqueous media.

The results obtained at 50 °C show a copper extraction of 24.8% for ChCl-EG, while for the other systems studied, the copper extraction was 100%. This suggests that despite the difficulties derived from incorporating the DES with ethylene glycol and its negative contribution to pH and leaching capacity, the increase in temperature improves the extraction, allowing them to achieve a complete leaching through the DES mixture.

Tests at a temperature of 60 °C resulted in a copper extraction of 42.4% for the DES ChCl-EG system, 100% for the ChCl-CA + 20% H₂O system, and 99.0% for the DES mixture system.

3.6. Effect Hydrogen Peroxide

Figure 7 shows the copper extraction result obtained after 72 h of leaching with and without hydrogen peroxide at different temperatures, 25, 50 and 60 °C, using the DES mixture of ChCl-EG (70%) and ChCl-CA-4% H₂O (30%) system.

According to the results shown in Figure 7, at the temperature of 25 °C, there is an improvement with the addition of hydrogen peroxide from 64.6% to 73% Cu, while at higher temperatures (50 and 60 °C), the addition of hydrogen peroxide decreases the extraction. This could be explained because hydrogen peroxide decomposes as the temperature of the system increases [49], probably producing decomposition reactions of the DES mixture, worsening its mineral dissolution power compared to when hydrogen peroxide is not present in the system. Furthermore, it has been reported that hydrogen peroxide decomposes easily in the presence of metal ions [52], especially iron and copper, whose concentrations increase as dissolution progresses. In the systems with hydrogen peroxide and at 50 and 60 °C, ore must have been dissolved at the beginning, with metals passing from the solid to the solution. This could have affected the leaching power of the system. Since a kinetic study was not performed, this hypothesis cannot be verified, which should be addressed in a subsequent study. According to the experimental conditions used in this study, it is recommended to work without the addition of hydrogen peroxide at temperatures of 50 and 60 °C.

This copper sulfide ore leaching process offers a solvometallurgical alternative to traditional mineral processing and could be a technique that allows the use of solvent extraction and electrowinning facilities available at a metallurgical plant.

The leaching solutions could be processed by solvent extraction [53] and electrowinning or crystallization. It has been reported in the literature that leaching and electrowinning can be performed in the same step [54,55]. Furthermore, there are studies using biphasic aqueous systems or others [40,56,57], which could be achieved using a novel DES that can serve as a selective extractant.

Leaching with DES could offer a less water-intensive alternative to the traditional leaching process, if the study variables can be optimized to obtain maximum copper extraction, using a dissolution-efficient DES with low water consumption in its preparation and adequate viscosity. However, questions remain to be answered, and further study of these processes with new solvents is necessary. Aspects such as the recyclability of the lixiviant, its thermal and chemical stability when in contact with different mineralogies, and its regeneration in an integrated metal dissolution–recovery system are issues to be determined [14,39,48]. Furthermore, it is necessary to consider other more complex mineral matrices such as chalcopyrite and enargite, studying the complete dissolution, purification and recovery process in a continuous system, including an economic study and sustainability indicators (water consumption, energy consumption, solid waste stability, emissions, among others), as well as including new variables such as mineral particle size or the solid–liquid ratio.

4. Conclusions

In this work, copper sulfide ore dissolution using deep eutectic solvents at different temperatures was studied. According to the results obtained and conditions studied, the highest copper extractions at any of the temperatures studied were for DES: ChCl-Citric Acid > ChCl-Urea > ChCl-ethylene glycol. The best copper extraction achieved (99.8%) was with the ChCl–CA–35% H₂O at 60 °C.

The systems with citric acid improved copper extractions because of its proton donor structure which, along with its chelating properties, allows it to generate complexes with metal ions, improving the solubility of copper from the sulfide matrix, achieving a total copper extraction (≈99%) at 50 °C.

The effect of increasing temperature is significant in increasing copper extraction in systems without the addition of hydrogen peroxide. Furthermore, the addition of hydrogen peroxide in the DES mixture positively affects the copper extraction at 25 °C. However, at higher temperatures (50 °C and 60 °C), it presented lower copper extractions, which could be explained by a decomposition of hydrogen peroxide at high temperatures, which slows down the oxidation rate of the copper ore.

The effect of adding water in the ChCl-CA and ChCl-U systems is positive at 50 °C and 60 °C but not at 25 °C, due to the decrease in the acid concentration, decreasing the proton activity in the mineral solution. At 50 °C and 60 °C, the results showed temperature-dependent behavior, decreasing the viscosity of the leaching system that helps the copper dissolution.

It is necessary to optimize the process by studying other variables that can reduce dissolution time, such as increasing the acid concentration in the medium, decreasing viscosity by increasing the ethylene glycol or water concentration at 25 °C, particle size, and others. Based on these results, this study requires further investigation.