Advances in the Development of Hydrometallurgical Processes in Acidic and Alkaline Environments for the Extraction of Copper from Tailings Deposit

Abstract

The geopolitical and economic situation impacts raw materials demand. As principal ore deposits reach exhaustion, the study of new sources of raw materials becomes essential. Therefore, mining wastes emerge as alternative sources of raw materials. Their physicochemical properties, such as small particle size or concentration of some metals of interest, enhance reprocessing. A number of critical raw materials (As, Co, Cu, Sb) and base metals (Pb, Zn), as well as precious metals (Ag), were found present in an abandoned tailing deposit composed by finely grounded washed roasted pyrites within the Iberian Pyrite Belt. Copper leaching from a sample of this deposit was investigated. Two hydrometallurgical approaches were studied: acidic leaching with and without activated carbon; and alkaline leaching with glycine solutions. Leaching tests were carried out during 24 h at ambient and moderate temperatures (60 °C). In acidic medium, the maximum copper extraction varied from 88 to 92.5%, while in alkaline medium, the maximum copper extraction was in the range of 71%–76%. Using activated carbon and H₂O₂ seemed to slightly promote the copper extraction with the maximum extraction (92.5%) after 2 h of leaching at 60 °C. Complementarily, above 50% of the zinc and cobalt contained were extracted. In contrast, temperature in alkaline conditions played a key role in reaction speed, but also in precipitation of copper insoluble compounds. In addition, the glycine solution at pH 10–10.5 showed high selectivity for copper over zinc, iron, lead, arsenic, and antimony. Two extra tests at pH above 12 showed arsenic dissolution (up to 51% at pH 12.5).

1. Introduction

Raw materials are the foundation of humankind progress. Ensuring and securing access to a stable raw materials supply, is nowadays a major concern for the European Union (EU) [1]. In fact, in 2011 the EU launched a list of fourteen critical raw materials (CRM) [2], which has been updated four times since then. The latest list was issued at the beginning of 2023 and contains twenty additional materials compared to the 2011 list [3]. CRM are considered materials of high economic importance for the EU and those which present a potential supply risk. One of the novelties is the inclusion of copper into the latest list [3]. Although copper does not meet the CRM threshold, this time, it has been included as a strategic material for the EU. Copper has been selected due to the fact that it is widely used for electrification across all strategic technologies (i.e., digitalization or green transition), and it is considered a challenge to substitute it due to its unique properties in electrical applications. In 2020, twenty million tonnes of copper were used for electrification. Chile was identified as the main copper supplier worldwide accounting for a 28% share. Yet, the principal copper supplier to Europe was Poland with a 19% share, followed by Chile (14%), Perú (10%), and Spain (8%) [4].

Although the copper market is expected to grow by a compound annual growth rate of 3.26% during the 2021–2026 period [5], primary sources with high grades are becoming depleted. New deposits present challenges due to mineralogical complexity, liberation grades or just low metal concentrations. In this context, mine wastes are attracting the interest of the industry as they host materials which in the past were not economically or technically extractable, materials that were not valuable at the time, which could not be identified with historical techniques or the cost of analyzing them was simply too high [6,7,8,9,10,11].

Residues and tailings from mining and metallurgical processes were mainly stored in waste rock dumps and/or tailings dams. Waste rock dumps often host material extracted from the mine which corresponds to gangue, low-grade ores, or simply complex ores difficult to treat with the available technologies. Tailings deposits instead store material rejected during the separation/concentration of the minerals of interest. Consequently, tailings characteristics vary depending on the ore mined and the treatment applied to it [12].

It is known in the industry that hydrometallurgical processes are able to extract metals of interest from low-grade ores and materials such as wastes, slags, or tailings [13,14,15]. Sulphuric acid has been the most widely employed leaching agent due to its availability, dissolution capacity, and cost [16,17]. However, new leaching approaches have been studied to reduce or replace sulphuric acid in processes, while the challenges of maintaining metal extraction, improving selectivity among elements, or increasing metals extraction from refractory ores remain the same [18,19]. These new approaches include the use of advanced leaching agents to promote kinetics, alkaline leaching, or bioleaching [20,21,22].

The use of activated carbon in acidic leaching was proposed due to its ability to donate, accept or transfer electrons to or from its surrounding environment [22]. In a recent study, it was shown that the Eh of sulphide minerals leaching system was reduced by the addition of specific carbon materials, which resulted in a change in the reaction mechanism increasing copper extraction [23,24]. Activated carbons are hence used as leaching promoters in the case of leaching copper bearing ores (i.e., chalcopyrite or enargite) [23,24,25,26,27]. The effect of carbon materials in the redox potential reduction could be due to galvanic interactions between sulphide and carbon structures, where carbon is nobler than the sulphides and forms a galvanic couple, hence providing a larger surface for ferric ions reduction, and thus improving sulphides dissolution [28,29]. Furthermore, some carbon materials are able to immobilize impurities such as iron or arsenic on their surface [22]. The use of activated carbon in the leaching of more oxidized mineral matrices, such as roasted pyrite residues, with a high content of impurities, has been not studied. We hypothesize that activated carbon addition to the leaching solution could improve the leaching of the more refractory copper and also reduce the presence of elements such as As in the leaching medium. Activated carbon, despite its high price, can be recovered and regenerated.

As an alternative to acidic leaching, glycine has been used as a precious and base metals leaching agent in alkaline media for more than 10 years [30]. Glycine (NH₂CH₂COOH; GlyH), the simplest amino acid, is composed by a carboxylic group and an amine group joined by a central carbon. Glycine appears in three different forms if dissolved in water: glycinate anion (H₂NCH₂COO⁻; Gly⁻), zwitterion (⁺H₃NCH₂COO⁻; H(Gly)) and glycinium cation (⁺H₃NCH₂COOH; H₂(Gly)⁺). When dissolved in water, glycine ligands demonstrate strong affinity by complexation to different metals [31], especially precious (for example Au) and base (Cu, Zn) metals [30]. Often, glycine leaching systems require the addition of additives to promote the mobilization of metals from their matrixes to the leaching solution, such as oxidants/reductants, catalysts, pH modifiers, or combinations of them. Finally, depending on the metal bearing ore, in the case of copper extraction from minerals (native, oxides or primary/secondary sulphides), different leaching behaviours and extractions have been reported [32].

The principal objectives of this study were: (1) to assess the performance of two different hydrometallurgical approaches for copper extraction from mining tails stored in an abandoned tailings deposit within the IPB; (2) to undertake preliminary assessment of the leaching behaviour, in both acidic and alkaline conditions, of other elements and impurities with significant concentration in the residue (Zn, Pb, Fe, As, Sb, Co, and Ag).

2. Materials and Methods

In this study, two hydrometallurgical approaches have been applied to a sample from an abandoned tailings deposit. The sample belongs to a deposit located in the southwest of Spain, within the well-known Iberian Pyritic Belt (IPB), specifically in the municipality of Minas de Riotinto, Huelva (Spain), close to the original Riotinto Mine. The abandoned deposit hosts mainly finely grounded washed roasted pyrites [6].

2.1. Materials Selection and Characterization

2.1.1. Selected Sample

The studied sample corresponded to a blend of samples collected at an abandoned tailings deposit in the Iberian Pyritic Belt. The tailings deposit stores fine and ultrafine material (D₈₀ < 25 µm) from a roasted pyrites washing plant. The 2 kg sample was sieved at 2 mm, then manually homogenized. The homogenized sample was subsequently divided into representative sub-samples. Three of the sub-samples were employed for: (1) chemical (by ICP-OES and XRF), (2) mineralogical (by XRD) and (3) physical and electrochemical characterization [6]. The rest of these were used for the present study. The elements: Fe, As, Pb, Cu, Zn, Sb, Ag, and Co, shown in

Table 1. Main properties of selected mining tailing sample [6].

Concentration
Fe (wt%)	As (wt%)	Pb (wt%)	Cu (wt%)	Zn (wt%)	Sb (wt%)	Ag (ppm)	Co (ppm)	O (wt%)	S (wt%)
25.3	2.97	2.59	0.29	0.20	0.15	199	35	28.48	5.00
pH *	Eh (mV) *	Mineralogy-XRD					D80 (µm)	S.G. **
2.0	665	Py	Hem	Qtz	Ms	Gp	23.53	2.97

* Determined in a water sample solution (4 gL⁻¹); ** S.G.: specific gravity; Py: pyrite; Hem: hematite; Qtz: quartz; Ms: muscovite; Gp: gypsum.

were determined inductively by coupled plasma optical emission spectrometry (ICP Avio 500, Perkin Elmer Inc., Waltham, MA, USA) (ICP-OES) with a multi-acid digestion procedure with HCl, HNO₃, HF and HClO₄.

A SEM-EDS analysis was performed at SCAI (Málaga University) using a Helios Nanolab 650 dual-beam microscope from FEI company with a Schottky field emission source for SEM (FESEM) and a Tomahawk focused ion beam (FIB) (FEI company, Hillsboro, OR, USA). The microscope was equipped with an energy-dispersive X-ray detector (EDS) and an electron-backscatter diffraction detector (EBSD) from Oxford Company (Oxford Lab products, Waples, San Diego, CA, USA).

2.1.2. Leaching Agents

All tests were performed using solutions prepared from analytical grade reagents and distilled water. Sulphuric acid (H₂SO₄) from Sigma-Aldrich^® (St. Louis, MO, USA) and ferric sulphate hydrate (Fe₂(SO₄)₃·3H₂O) from Labkem (Labbox Labware S.L., Barcelona, Spain) were analytical-grade, with purities of 97% and 99.5%, respectively. Glycine reagent, NaOH and hydrogen peroxide 30% (v/v) of analytical grade were supplied by Labkem (Labbox Labware S.L., Barcelona, Spain).

2.1.3. Leaching Promoter—Activated Carbon

The selected commercial activated carbon (W35) was supplied by Norit company (Marshall, TX, USA). A broad characterization (

Table 2. Main properties of selected commercial activated carbon (W35).

Property	Value
pH	10.3
Eh (mV)	253
SBET (m2/g)	985.3
Ash (wt%)	10.5
C (%)	82.4
H (%)	0.38
N (%)	0.26
O (%)	6.30
H/C	0.06
O/C	0.06

) was performed according to the following parameters: pH, redox potential (Eh), electrical conductivity (EC), ash content, ultimate analysis (C, H, O, N, S) and BET-specific surface area. The ultimate analysis was carried out using a LECO CHNS 932 analyser (St. Joseph, MI, USA). Ash content was determined by combustion of sample (0.5 g) in a furnace (HOBERSAL-Model HD-150) at 850 °C for 24 h. O (%) was calculated by difference as follows: O (%) = 100% − (%C + %H + %N + %S + %Ash). Finally, the N₂ adsorption isotherm was determined using a Porosimetry System ASAP 2420 Micromeritics (Malvern Panalytical, Malvern, UK).

W35 has a high surface area (985.3 m²/g) and basic pH (10.3). The carbon content was 82.4%, whereas the inferred oxygen content was 6.30%, as shown in Table 2.

2.1.4. Leaching Tests

Ten subsamples of 5 g each were prepared and weighted in ten 250 mL Erlenmeyer flasks. Ten leaching tests were performed using magnetic mixers (stirring speed 250 rpm) at room and moderate temperature (60 °C), in acidic and alkaline media. Different leaching agents (Fe³⁺/H₂SO₄ with and without H₂O₂ and glycine/H₂O₂) as well as activated carbon, were tested (

Table 3. Leaching tests initial conditions.

ID	Leaching Agent	Oxidant Agent	Activated Carbon	Temperature (°C)	pH
E-I	Fe3+/H2SO4	-	-	60	0.76
E-II	Fe3+/H2SO4	H2O2	-	60	0.88
E-III	Fe3+/H2SO4	-	W35	60	1.15
E-IV	Fe3+/H2SO4	H2O2	W35	60	0.95
E-V	Glycine	H2O2	-	60	10.0
E-VI	Glycine	H2O2	-	60	10.5
E-VII	Glycine	H2O2	-	Room temperature	10.0
E-VIII	Glycine	H2O2	-	Room temperature	10.5
E-IX	Glycine	H2O2	-	Room temperature	12.14
E-X	Glycine	H2O2	-	Room temperature	12.5

). Temperature, pH, and redox potential were monitored at different times (2, 4, 6, and 24 h). Temperature, pH, and redox potential were measured three times per sampling time. Final results correspond to an average of the three measurements.

The acidic leaching agent employed (100 mL) was prepared by a 0.5 M H₂SO₄ solution with concentration of 2.5 g·L⁻¹ of Fe³⁺. W35 (0.10 g) and 1% vol of H₂O₂ were also used in three tests (Table 3). The alkaline medium employed (100 mL) was a solution containing 31.6 × 10⁻³ M glycine and 1% vol of H₂O₂ (Table 3). Glycine concentration was based on the molar ratio Gly:(Cu + Zn + Co + Ag) = 8:1. NaOH was used to adjust pH.

To assess the reaction kinetics, 1 mL of pregnant leach solution (PLS) was sampled at different reaction times (2, 4, 6, and 24 h). The sampling was performed as follows: (1) stirring was stopped, (2) after the solid had settled, 1 mL of PLS was removed and transferred to a 25 mL graduate flask filling up the volume with distilled water, and (3) to balance the volume removed from the experiment, 1 mL of the respective leaching solution was added.

For Cu, Zn, Fe, As, Sb, Pb, and Ag extraction calculation, the following Formula (1) was applied:

$$Elementextraction\left(\%\right)={\displaystyle \frac{{EM}_{PLS}}{{EM}_{PLS}+{EM}_R}}\times 100$$

(1)

EM_PLS: element mass in the PLS (g); EM_R: element mass in the residue (g).

In the case of Co, the Formula (2) was adapted as concentration in the residue was below detection limit, and therefore the Co extraction calculation was as follows:

$$Coextraction\left(\%\right)={\displaystyle \frac{{Co}_{PLS}}{{Co}_F}}\times 100$$

(2)

Co_PLS: cobalt mass in the PLS (g); Co_F: initial cobalt mass in the feed (g).

The pH and Eh of the PLS were monitored during the experiment with Crison micro pH 2000 and pH 60 DHS instruments, respectively. Finally, the concentrations of Cu, Zn, Pb, Fe, As, Sb, Co, and Ag in the PLS were determined by ICP-MS, while those present in the residue were determined by ICP-OES.

3. Results and Discussion

3.1. Characteristics of the Sample

The sample gathered for the study showed significant concentrations of CRM (2.97% As, 0.29% Cu, 0.15% Sb, 35 ppm Co), precious metals (199 ppm Ag), and base metals (2.59% Pb and 0.20% Zn). Mineralogical analysis reported the presence of quartz, pyrite, iron oxides, iron sulphates, and calcium sulphates. Furthermore, Pb was found to be associated with O+S, while Cu and Zn were identified in very low concentrations. Although very low quantities were observed in the mineralogical analysis, copper-bearing materials were defined as, but not limited to, secondary copper sulphides (chalcocite and covelline) and primary copper sulphide (chalcopyrite) with particle sizes between 0.5 and 10 µm. Furthermore, the sample had acidic pH (2.0), high Eh (665 mV), and specific gravity of 2.97. A wide characterization of this tailing deposit was performed in [6].

Figure 1 shows different micrographs obtained by SEM-EDS analysis from the sample selected for leaching tests. The sample demonstrated an inequigranular distribution and a great presence of fine particles (Figure 1d). It can be seen that most of the particles are smaller than 30 µm (Figure 1b,c) showing the fine and ultrafine material rejected by a washing plant nearby.

Fine particle sizes are considered ideal for metal extraction by hydrometallurgy techniques as there is greater surface area exposed to leaching agents.

3.2. Copper Extraction in Acidic Conditions

In Figure 2, Figure 3 and Figure 4, the Cu extraction kinetics in an acidic medium are presented, as well as the behaviour of pH and Eh at selected times (2, 4, 6, and 24 h). Tests E-I and E-II provide baselines regarding copper extraction in the absence of the activated carbon. Extractions of 87.9% and 89.3% were achieved in the first 2 h, although the final extractions reported at 24 h were 84.7% and 85.2%, respectively. Otherwise, test E-IV with activated carbon and hydrogen peroxide showed the highest Cu extraction, 92.5%, after 2 h of leaching at 60 °C. Test E-III showed improved copper extraction after 24 h compared to test E-IV, 88.5% and 84.9%, respectively.

When active carbon is used, there is no decrease in the concentration of Cu in the solution after 2 and 4 h. In this case, a progressive increase in the Cu content in the leach liquor was found.

Figure 3 shows pH of the leaching solution at different reaction times. The initial pH varies between 0.8 and 1.2. The addition of activated carbon slightly increases the initial pH of leaching solution due to the alkaline pH (10.3) of selected activated carbon (Table 2). In the first 2 h of leaching, the pH increases, possibly due to acid consumption. However, the pH of the medium subsequently decreases and after 24 h, the final pH varies from 1.1 to 1.2.

Figure 4 shows Eh of the leaching solutions at different reaction times. Redox potential is lower in cases where activated carbon was employed due to carbon-reducing characteristics of W35 (Table 2). Reduction in Eh in the leaching system is also aligned with the potential galvanic effect of this type of materials [25,33]. Nakazawa et al. [25] studied leaching chalcopyrite by adding carbon black to a sulfuric acid media at 50 °C. These authors proposed that the enhanced leaching kinetics of chalcopyrite could be due to dissolution reactions at low redox potential in conjunction with the galvanic interaction between carbon black structures and chalcopyrite.

The other elements studied, Co, Zn, As, Sb, Pb, and Ag, showed extractions lower than Cu after 24 h (Figure 5). All tests in acidic conditions used in this research gave fairly low-to-low extractions of As (2%–4%) and Sb (4%), or <2% in the case of Pb and Ag. Zn and Co like Cu had higher recoveries (47%–54% and 47%–53%, respectively) after 24h. In the case of Zn, particularly in E-III when activated carbon without H₂O₂ was used, the extraction is 53.7%. Otherwise, Co reached the highest extraction (53.3%) in E-IV, with the combination of activated carbon and hydrogen peroxide after 6 h leaching.

Figure 5 shows the low extraction of elements As and Sb, considered as impurities and which must be eliminated before the electrowinning stage. Low As extractions were achieved with activated carbon and H₂O₂. Therefore, leaching under these conditions would minimize the purification costs necessary before electrowinning.

3.3. Copper Extraction in Alkaline Conditions

3.3.1. Copper Extraction as a Function of Time

Figure 6, Figure 7 and Figure 8 show the copper extraction in an alkaline glycine solution and the behaviour of pH and Eh during the tests (2, 4, 6, and 24 h). At pH 10–10.5, the highest copper extractions of each test (71.3%–76.2%) were achieved within the first 6 h. In contrast, at pH > 12, the highest copper extractions achieved during the leaching tests were 22.2% and 3.65%, respectively.

In an early stage (2 h), test E-VIII at pH 10.5 and room temperature, reported the highest copper extractions of 76.2%. After 4 h leaching, all tests at pH 10–10.5 showed a drop-in copper extraction except for test E-VII, which reported a peak of 75.5% of copper extracted. This drop continued in the case of those tests at moderate temperature, which resulted in copper extraction below 2% after 24 h of leaching. Tests at room temperature and pH 10–10.5 experienced a drop in copper extraction after 6 h; however, the negative trend changed, and a new increase in copper extraction after 24 h leaching was reported (Figure 6).

Therefore, the highest copper extractions at pH 10–10.5 were observed in tests E-V (74.2%) and E-VIII (76.2%) in an early stage (2 h). The tests at pH 10–10.5 showed two different behaviours after 6 h leaching: in those at moderate temperatures, copper extraction dropped significantly below 2% after 24 h. On the other hand, although tests at room temperature experienced a drop after 6 h leaching, there was a change in the extraction trend, which resulted in higher extractions after 24 h. Finally, tests at pH above 12 had copper extractions lower than 25%. Test E-IX experienced a positive trend in terms of Cu extraction over time, while test E-X did the opposite.

In conclusion, time as a factor should not be assessed alone but in conjunction with other factors, such as temperature and/or electrochemistry.

3.3.2. Effect of Temperature in Copper Extraction

To assess the impact of temperature in Cu extraction, four tests at pH 10–10.5 were performed: two at 60 °C and two at room temperature (Table 3). Figure 9 shows the copper extraction at different temperatures and pHs. It is seen that at pH 10, after 2 h, higher temperature enhanced initial Cu extraction. However, this effect was not observed at pH 10.5. O’Connor et al. (2018) [34] explained how the ratio of glycinate anion over zwitterion increases as the pH and temperature do. Glycinate anion dominates when pH is greater than the pKa of 9.8, but also when temperature increases, as the pKa decreases, for instance, from pKa 9.8 to 9.0 when temperature increases from 25 °C to 60 °C. In the case of pH 10, the observed effect is aligned with O’Connor et al. (2018) explanation [34].

However, moderate temperature (60 °C) was also found to be detrimental to Cu extraction after 2 h of reaction. At both pHs (10–10.5), Cu extraction started to decrease after 2 h, reaching minimums below 2% at 24 h. Khezri et al. (2020) [35] noticed that extended leaching times at pH above 10 led to a drop-in copper extraction, proposing that it was due to copper precipitation. The same behaviour was reported by Barragán et al. (2024) [36] at pH 10, 10.5, and 11.

3.3.3. Effect of Electrochemistry in Copper Extraction

In order to evaluate the effect of pH in Cu extraction, four tests were carried out at room temperature and different pHs (Table 3). In Figure 6 and Figure 7, it is shown how pH plays a critical role in copper extraction. Tests E-VII and E-VIII at initial pH 10–10.5 achieved Cu extractions above 70% at different times over the experiments. On the other hand, at more alkaline pH (>12), the highest copper extractions were 22% (test E-XI) and 3.7% (test E-XII) over 24 h of leaching.

Figure 10 shows that at pH 10–11 and positive Eh, glycinate anions are the predominant species. Additionally, Figure 10 depicts the pH and Eh of tests E-IX and E-X; these two tests are within the CuO(s) domain. Previous studies reported that copper could precipitate as copper oxides at pH above 11 or at the alkaline pH but higher temperatures [30,32,34]. Authors indicated that at a pH beyond 11, copper glycinate stability starts to change, so species such as CuO and Cu₂O could precipitate. These insoluble species could occupy and prevent active surfaces to be leached. These precipitates may grow at a point of passivating active copper surfaces, resulting in poor to zero Cu recoveries, which is consistent with tests E-IX and E-X results.

However, is worth noting that this correlation between pH and Cu extraction could be reversible, which would explain the leaching behaviour observed in test E-IX, where Cu extraction increased up to 22.2% when the pH fell from 12.40 to 11.64 (Figure 6, Figure 7 and Figure 10).

Copper leaching in acidic conditions demonstrated that certain impurity elements are also dissolved. Such elements may require additional stages for removal and disposal, thus increasing the cost of treatment. Hydrometallurgical processes include an initial leaching stage of the parent metal(s), followed by one or more purification/concentration stages before the final precipitation stage. If the leaching is not selective, given the complexity of the wastes and the low concentration of the parent metal, the costs of the purification steps can make hydrometallurgical processes unviable. Therefore, efficient and selective leaching is essential. The results shown in Figure 11, as well as the Eh–pH diagrams of a water–glycine–cobalt/arsenic systems (Figure 12 and Figure 13), indicate that glycine leaching systems allow the selective leaching of specific elements (Cu and Co), while impurities like Fe, As, and Sb are reported in the solid residue. Only at pH > 12, As showed a different behaviour than the rest of elements, with a maximum of 50.9% extraction after 24 h at room temperature and pH 12.5.

Cobalt was the only element studied that reported higher levels of extraction at pH 10–10.5 and room temperature (Figure 11) which is within the domain of Co(Gly)₃⁻. The extractions reached accounted for 10.40% and 7.94% at 24 h. During tests E-IX and E-X, cobalt extractions were negligible as it precipitated in the form of hydroxide at pH above 12 (Figure 11 and Figure 12).

Eksteen et al. (2020) [20] studied the Ni and Co leaching in glycine systems. They found that molar ratios [Gly: (Ni + Co + Cu) ≥ 5:1] are favourable for Ni and Co leaching. Although passivation was not reported, they found that Ni and Co needed extended leaching times (346 h) to extract over 60% of each element. In another study, Oraby et al. (2023) [37] reported that different pH modifiers also enhanced global Co recoveries up to 10%; the glycine–ammonia system was identified as the best combination, 86% Co extraction, followed by glycine–NaOH and glycine–KOH systems, with 76% and 72% of cobalt extractions, respectively. Additionally, particle size was identified as key with regard to reaction. Four particle sizes were investigated (10, 13, 15, and 23 µm), from which it was concluded that particle sizes below 15 µm yield maximum recoveries, above 80%, after 48 h, considerably reducing the leaching time.

In summary, although we used suitable conditions found by other researchers, molar ratio [Gly/Cu + Zn + Co + Ag ] above 5:1, low particle size (D₈₀ = 23 µm), glycine–NaOH system as leaching solution, the Co extractions were only 10.4% and 7.9% at pH 10 and 10.5 after 24 h. According to Eksteen et al. (2020) [20], longer leaching times (346 h) would be needed in order to extract over 60% of the Co. Further investigations are suggested on where the Co is hosted, as some of the cobalt could be within the sulphides; however, most could be present in phases more challenging to leach.

Another CRM was leached when tests were carried out at much higher pH (>12), only arsenic (recently included in the last list of CRM by the European Commission [3]) was dissolved under such conditions.

It is worth recalling that the sample studied in this research belongs to an abandoned tailings deposit that hosts washed roasted pyrites from the IPB. Yesares et al. (2024) [38] in a wide characterization study of this type of residues within the IPB, indicated that As is the main component of the secondary arsenate beudantite, and the sulphates: plumbojarosite and anglesite. Authors also stated that this type of residue from the IPB yields a rather homogenous residue, highlighting that the pyrite roasting process produced consistent residues compared to other types of wastes [38].

In Figure 13 it is observed the fact that most likely species of As in the leaching tests were arsenates As (V), as As (III) does not occur above pH 7 in positive Eh ranges. In natural environments, the dominant As forms are the inorganic species, arsenite—As (III) and arsenate—As (V). Arsenite is considered more pollutant and mobile than arsenate. With regard to As speciation, pH and redox potential (Eh) are the most important factors (Figure 13). Arsenic solubility is very low at neutral or slightly acidic/basic pH but increases considerably under both strongly acidic and alkaline conditions [39,40]. For instance, species like AsO₄³⁻ are present in extremely acidic and alkaline conditions due to its amphoteric characteristics. Gersztyn et al. (2013) [39] studied the influence of pH on the solubility of As in heavily contaminated soils. They found that under alkaline conditions, arsenic release from soil could be assigned to the mechanism of pH-related anion desorption, meaning that a replacement of arsenite and arsenate ions are bound in the sorption complexes by OH⁻.

Leaching tests E-IX and E-X at room temperature and pH 12.4 and 12.5 reported As extractions of 21.2% and 50.9%, respectively. An increment of pH resulted in an increase in As extraction of almost 30% (Figure 11). Although arsenic leaching in alkaline conditions is not new in the field of tailings treatment as it has been previously studied. Wang et al. (2021) [41] studied the arsenic leaching with NaOH. In such study, NaOH concentration, liquid/solid ratio and leaching time were evaluated. The conclusion was that high NaOH concentration, a liquid/solid ratio of 5/1, and 3 h of leaching reaction were the optimum to effectively extract over 90% of As from a gold-bearing sludge.

Therefore, As leaching in high alkaline conditions with NaOH could be expected due to the amphoteric and leachability characteristics of certain species. Major factors to be considered would be pH and redox potential (Eh), followed by leaching agent concentration (NaOH). Additionally, minor factors such as liquid/solid ratio, as well as leaching time, should be considered as key to successfully extract the metalloid.

The findings from leaching with glycine in alkaline conditions provide a new approach to reprocess existing tailings deposits of this type.

Three CRM (Cu, Co, and As) were identified as extractable from the studied sample. Cu extraction was found to be highly selective over other elements such as Zn, Fe, Pb, and Sb. Results reported over 70% of copper extraction, while for the rest of elements, extractions were below 5%. Cobalt was also found to be fairly extractable; however, longer leaching times should be studied in order to determine if higher extraction could be achieved. Finally, arsenic was found to be leachable under extremely alkaline conditions (pH 12.5) (Figure 11). Arsenic leaching was very selective too, as it was recovered up to 50% while the rest of elements could not achieve recoveries greater than 1%. The base (Zn and Pb) and precious (Ag) metals in the sample did not report recoveries above 1% in any case.

In summary, CRM, such as Cu, Co, or As, can be selectively extracted from this residue, while impurities are reported for the solid residue and additional efforts would be required to extract them in further stages. This approach would not only aim to benefit the supply of CRM but also to enhance mining soils remediation.

To conclude, it is worth noting that the use of glycine also presents certain conveniences compared to classic leaching agents from an environmental point of view. Studies demonstrated that leaching of base metals may occur at atmospheric pressure and room temperature [42,43], which reduces the energy consumption of the leaching process and the associated CO₂ emissions. Second, glycine can be recycled and reused [32]. Additionally, it is worth mentioning that new processes for the sustainable industrial production of glycine are being investigated [44,45]. And third, the use of glycine is potentially significant in the remediation of these types of residues as this amino acid works as bio-stimulant in plants’ growth [46,47]. Barragán-Mantilla et al. (2024) [36] carried out a post-leaching study on similar residues leached in alkaline conditions, using glycine as a lixiviant. This study showed that alkaline leaching at pH 9.5 produced residues that had the potential to be used as phytostimulants that could favour plant growth. On the downside, the pH of the original sample needs to be considered, since glycine leaching is carried out at alkaline pHs that needs to be adjusted in liquid wastes before disposal.

4. Conclusions

This study aimed: (1) to assess the performance of two different hydrometallurgical approaches for copper extraction from mining tails stored in an abandoned tailing deposit within the IPB; (2) to undertake preliminary assessment of the leaching behaviour, in both acidic and alkaline conditions, of other elements and impurities with significant concentration in the residue (Zn, Pb, Fe, As, Sb, Co, and Ag).

The key findings of the study are described below:

• Three critical raw materials (Cu, Co, and As) and one base metal (Zn) were identified as extractable under the conditions studied. In an acidic medium, Cu, Co, and Zn gave extractions of 92.5%, 52.6%, and 53.7%, respectively. In contrast, under alkaline conditions, extractions of 76.2%, 10.4%, and 50.9% were achieved for Cu, Co, and As.
• The highest extraction of Cu, 92.5%, was achieved in acidic leaching conditions after 2 h when activated carbon and hydrogen peroxide were employed.
• Glycine was demonstrated to be a promising alternative for tailings reprocessing as its use allowed the selective extraction of Cu, Co, and As over impurities such as Fe, Zn, Pb, or Sb. The highest Cu extraction 76.2% was achieved at pH 10.5, room temperature, after 2 h leaching. Cobalt was able to be extracted up to 10.4% at pH 10 and room temperature after 24 h leaching. Finally, 50.9% of As extraction was achieved at pH 12.5, room temperature, after 24 h leaching time.
• Temperature and pH were identified as key factors for copper extraction in alkaline conditions. Temperature could enhance solubility at early stages; however, it could be the main reason of Cu species precipitation observed in some conditions.
• Further optimization of conditions, such as shorter reaction time, solid/liquid ratios, pH, and temperature should be explored in future investigations in order to optimize alkaline leaching of these wastes with glycine.