A New Approach in Hydrometallurgy for the Solvent Extraction of Cu(II) from Alkaline Solutions Leached with Tartrate Using Phenyl-2-Pyridyl Ketoxime

Abstract

For the first time, an alternative and sustainable approach is reported using phenyl-2-pyridyl ketoxime (PPKO) as a selective extracting agent for the recovery of Cu(II) from alkaline solutions in the presence of tartrate ions. The advantages relative to conventional processes carried out in acidic media are outlined. Through potentiometric and spectrophotometric analyses, the sequential formation of a 1:2 metal cation–ligand Cu(II)-(PPKO)₂ complex was identified as the predominant species in alkaline aqueous solutions. The high removal capacity of the extractant for Cu(II), as assessed from liquid–liquid extraction, and its efficient performance are comparable to widely used commercial extractants. Thermodynamic studies of the complexation between the copper(II) ion and PPKO demonstrated that the process exhibits an endothermic character. A progressive decrease in the performance of the extractant was observed after reuse without a regenerative treatment. This deterioration was partially reversed through a controlled reprotonation process using an acetate buffer solution. Overall, the results support the potential of PPKO as an effective and selective alternative ligand for hydrometallurgical applications in alkaline medium.

1. Introduction

The extraction of copper ions from aqueous media has been an area of interest in hydrometallurgy and remains essential due to the high dynamic demand from the electronics industry, which has significantly driven the advancement of science and technology [1]. Currently, 20% of the world’s primary copper production is processed through leaching, solvent extraction, and electrowinning (SX-EW) [2,3]. Traditionally, acidic leaching with sulfuric acid has dominated the processing of oxidized copper ores. However, acid leaching still presents significant challenges, including the high consumption of sulfuric acid, the generation of leach residues, and acid mine drainage. These by-products, resulting from improper disposal practices or the abandonment of mining operations during closure stages, can become persistent sources of heavy metals and pollutants, posing a considerable environmental risk to surrounding ecosystems [4,5,6]. Alkaline alternatives have been reported in scientific studies, such as the recovery of copper from oxidized or mixed ores using leaching solutions based on glycinate [7], tartrate, oxime derivatives [8], and ammonia [9,10], have emerged as options to improve selectivity and to reduce negative environmental impacts on aquatic ecosystems and soils in case of spillage. However, these emerging leaching solutions present both advantages and disadvantages compared to the conventional use of sulfuric acid in leaching processes; therefore, their application must be evaluated within an operational and environmental context.

It has been demonstrated that tartrate solutions in alkaline medium possess the capacity to form complexes with metal ions at high pH values above 10, through the formation of polydentate complexes, favoring the existence of the [Cu(OH)₂C₄H₄O₆]²⁻ anion in solution and preventing precipitation as copper hydroxide [11]. This property has been investigated in the environmental remediation of contaminated waters [12], reflecting the potential of tartrate use in leaching processes.

Likewise, solvent extraction (SX) continues to play a fundamental role in the recovery of valuable metals. In industrial practice, mixtures of aldoximes and ketoximes, such as LIX 54-100, LIX 984, LIX 84-I, and Acorga M5640, have demonstrated an improvement in kinetics extraction, phase stability, and reasonable operating costs. This context has also spurred great interest in scientific research, proposing extractants such as 2,6-Bis(4-Methoxybenzoyl)-Diaminopyridine [13]; 1,1′-dialkyl-2,2′-bibenzimidazoles [14] for the recovery of Cu (II); and N-benzoyl-N′,N′-diethyl thiourea, N-benzoyl-N′,N′-dibutyl thiourea, and N-benzoyl-N′-butyl thiourea, which have been applied to water treatment [15] in acidic mediums, and complementarily, the solvent extraction of copper(II) from alkaline aqueous solutions has shown high efficiencies when using oxime-based extractants [16], with the aim of improving selectivity against interfering species, increasing loading capacity, or adapting to specific process conditions. Although the commercial viability of these compounds remains limited, recent studies on novel extractants in hydrometallurgical processes provide an opportunity to deepen our understanding of metal–ligand coordination principles. Moreover, these advances allow for the assessment of extractant performance under controlled conditions or model systems, offering a foundation for future developments that could enhance the efficiency and sustainability of industrial processes.

Extractants such as phenyl-2-pyridyl ketoxime (PPKO), as proposed in the present study, have demonstrated considerable potential as selective ligands in coordination chemistry and trace metal analysis. This capability arises from the presence of donor atoms, specifically the nitrogen of the pyridine ring, as well as the nitrogen and oxygen atoms of the oxime group. These features enable the ligand to adopt bidentate (N, N) or even tridentate (N, N, O) coordination modes. Such configurations are influenced by the protonation state of the ligand and the nature of the metal ion involved. Previous studies have reported the formation of stable complexes with metal ions such as Ni²⁺, Cd²⁺, Ln³⁺, Co²⁺, and Zn²⁺, which typically adopt mononuclear structures in which PPKO acts predominantly as a bidentate ligand, coordinating through pyridine nitrogen and oxime nitrogen. Recent advances in the field of hydrometallurgy have focused on optimizing extraction operating conditions and developing extractants with enhanced selectivity [17,18,19], capable of adapting to aqueous solutions derived from leaching processes carried out in acidic, neutral, and alkaline media [20,21,22,23]. Precisely within this context, the aim of this work is to investigate, through liquid–liquid extraction, the recovery of copper (II) from a leach solution produced by the combined action of tartrate in alkaline medium, as well as phenyl-2-pyridyl ketoxime (PPKO) as an extractant in chloroform, as an approach to develop a promising alternative to the conventional use of acidic leaching systems, opening new perspectives on the practice of sustainable hydrometallurgical processes.

Therefore, the experimental approach encompassed the determination of the stoichiometry of the Cu(II):PPKO complex by potentiometric and spectrophotometric analyses, the evaluation of the equilibrium time and maximum loading capacity, the construction of extraction and stripping isotherms, and the implementation of a reconditioning procedure to assess the regeneration and reusability of the extractant over multiple cycles. In addition, the effect of temperature on the complexation–extraction process was examined.

2. Materials and Methods

2.1. Materials

The experimental tests developed in the present research were carried out using analytical-grade chemical reagents without prior treatment. These included sodium potassium tartrate, C₄H₄KNaO₆·4H₂O (SOLUTEST—Lab Chem, Zelienople, PA, USA); sodium hydroxide, NaOH (Riedel-de Haën, Hannover, Germany, 98% purity); potassium chloride, KCl (JT Baker, Phillipsburg, NJ, USA, 100.3% purity); glacial acetic acid, C₂H₄O₂ (JT Baker, Phillipsburg, NJ, USA, ≥99.9% purity); sodium acetate, NaCH₃COO (JT Baker, Phillipsburg, NJ, USA, ≥99% purity); disodium salt of ethylenediaminetetraacetic acid dihydrate, C₁₀H₁₄O₈Na₂N₂·2H₂O (Daryaganj, New Delhi, India (CDH, 99.5% purity)); basic copper carbonate (malachite), CuCO₃·Cu(OH)₂ (Phillipsburg, NJ, USA (JT Baker)); phenyl-2-pyridyl ketoxime, C₁₂H₁₀N₂O (Saint Louis, MO 63103, USA (Sigma-Aldrich)); chloroform, CHCl₃ (Phillipsburg, NJ, USA (JT Baker)), and ethanol, C₂H₆O (Phillipsburg, NJ, USA (JT Baker, 96% purity)).

Additionally, potentiometric measurements were conducted using a digital pH meter, JENCO, model 1671, Shanghai, China, with a combined Ag/AgCl glass electrode (HANNA—HI 1610D, Mexico City, Mexico); a Cu(II) ion-selective electrode (HANNA—HI 4108, Woonsocket, RI, USA); and a scanning spectrophotometer (VSP-UV—VERNIER, Beaverton, OR, USA) (equipped with Vernier Spectral Analysis software, available online: https://www.vernier.com/product/spectral-analysis/ (accessed on 26 August 2024)), fitted with a quartz cell with a 1.0 cm path length. For the homogenization of reaction mixtures, a digital mixer (Cole Parmer EW-50006-03, Vernon Hills, IL, USA), a magnetic stirrer (IKA—COMBIMAG-RCH, Burladingen, Germany), and a vortex mixer (GENIE—WINN, Tolbert, Netherlands) were used. For the precise quantification of copper(II) ions present in the pregnant leach solution, raffinate, and enriched (stripping) solution, a centrifuge (HETTICH–ROTOFIX 32A, Tuttlingen, Germany) and an atomic absorption spectrophotometer (PerkinElmer AA PinAAcle 500, Waltham, MA, USA) were employed.

2.2. Methodology

2.2.1. Experimental Determination of the Coordination Number of the Cu(II):PPKO Complex Through Simultaneous Potentiometric and Spectrophotometric Analysis in Ethanolic Solution

In order to establish the stoichiometry or composition of the complex Cu(II) ion and phenyl-2-pyridyl ketoxime (PPKO), potentiometric measurements were conducted at 298.15 K, over a 1.5–12 pH range. The experiments were carried out in a glass vessel with a water jacket, under constant magnetic stirring, using a calibrated digital pH meter. A standard NaOH solution (C_NaOH = 0.010 mol·dm⁻³) was gradually added via a piston burette connected to a fine capillary, which remained submerged in the reaction medium. pH readings were recorded three minutes after each addition, allowing chemical equilibrium to be established, following procedures described in the literature [24]. The experimental system consisted of a solution of PPKO (C_L = 0.010 mol·dm⁻³) dissolved in ethanol and Cu(II) (C_M = 0.005 mol·dm⁻³), with a total reaction volume of 100 cm³. NaOH was added until a pH of 11 was reached, a condition that simulates the alkaline environment of a loaded leach solution, as employed in liquid–liquid extraction tests. This setup enabled the study of the complexation behavior under representative conditions for the selective extraction of Cu(II). During the titration process, parallel spectrophotometric measurements were performed to analyze the formation of the Cu–PPKO complex throughout the pH adjustment. These readings confirmed the presence of a complex of 1:2 stoichiometry Cu:(PPKO)₂.

2.2.2. Evaluation of Equilibrium Time and Maximum Loading Capacity in the Extraction of Cu(II) Using Phenyl-2-Pyridyl Ketoxime (PPKO)

The equilibrium time study for the extraction of Cu(II) ions was carried out using a synthetic aqueous solution prepared from copper sulfate, with a Cu(II) concentration of 2.00 g/L, in the presence of tartrate ions and adjusted to pH ≈ 11. The organic phase consisted of a 2.0% (w/v) solution of phenyl-2-pyridyl ketoxime (PPKO) dissolved in chloroform. The mixtures were agitated at 600 rpm and maintained at 298.15 ± 1 K, conditions that were consistently applied across all experiments. In all trials, an organic-to-aqueous phase ratio (O:A) of 1:1 was maintained, with a total system volume of 40 mL. To evaluate the effect of contact time, a batch experimental design was implemented with agitation intervals of 1.0, 2.0, 3.0, 4.0, 5.0, and 6.0 min. At the end of each interval, the phases were separated using a separatory funnel. The residual Cu(II) concentration in the aqueous phase was determined by atomic absorption spectroscopy (AAS). Based on these measurements, the extraction efficiency at each time point was calculated in order to determine the minimum contact time required to reach extraction equilibrium.

Subsequently, the maximum loading capacity of phenyl-2-pyridyl ketoxime (PPKO) was assessed using a liquid–liquid extraction system designed to facilitate the transfer of copper ions from an aqueous phase to an organic phase. The same concentration of extractant (2.0% w/v in chloroform) was used for the organic phase, while the aqueous phase consisted of alkaline copper (II) solutions prepared from basic copper carbonate (2.0 g/L), in the presence of tartrate, to simulate conditions representative of an oxidized copper ore leachate. The phases were agitated for 5 min, a duration considered optimal for the extraction process based on previously established procedures in the literature [25], even in systems different from the one evaluated in this study. An organic-to-aqueous phase ratio of 1:1 was maintained in all experiments.

The loading capacity was evaluated through successive extraction cycles. In each cycle, the phases were separated by decantation, and the loaded organic phase was brought into contact with a fresh Cu(II) solution, maintaining identical conditions for each evaluated cycle. This procedure was repeated for five successive cycles without including a stripping stage, allowing the progressive accumulation of copper in the organic phase to be assessed (Figure 1). The aqueous solutions that were not extracted in each cycle were collected in a single container, and their concentration was quantified at the end of each cycle. The obtained data allowed for the precise determination of the amount of copper accumulated in the loaded organic phase, based on the amount of aqueous phase feed accumulated and introduced into the system. A metallurgical mass balance was conducted for each stage using Equation (1), and copper concentration profiles (g/L) were calculated according to Equation (2).

$$\sum \left[{\mathrm{m}}_{{\mathrm{Cu}}_{\left(\mathrm{aq}\right)}^{2+}}\right]=\sum _{\mathrm{i}=1}^{\mathrm{n}}\left[{\mathrm{m}}_{{\mathrm{Cu}}_{\left(\mathrm{raff}\right)-\mathrm{n}}^{2+}}\right]+\sum _{\mathrm{i}=1}^{\mathrm{n}}\left[{\mathrm{m}}_{{\mathrm{Cu}}_{\left(\mathrm{org}\right)-\mathrm{n}}^{2+}}\right]$$

(1)

$$\sum \left[{\displaystyle \frac{{\mathrm{m}}_{{\mathrm{Cu}}_{\left(\mathrm{aq}\right)}^{2+}}}{\mathrm{V}}}\right]=\sum _{\mathrm{i}=1}^{\mathrm{n}}\left[{\displaystyle \frac{{\mathrm{m}}_{{\mathrm{Cu}}_{\left(\mathrm{raff}\right)-\mathrm{n}}^{2+}}}{\mathrm{V}}}\right]+\sum _{\mathrm{i}=1}^{\mathrm{n}}\left[{\displaystyle \frac{{\mathrm{m}}_{{\mathrm{Cu}}_{\left(\mathrm{org}\right)-\mathrm{n}}^{2+}}}{\mathrm{V}}}\right]$$

(2)

where

• ${\mathrm{m}}_{{\mathrm{C}\mathrm{u}}_{\left(\mathrm{a}\mathrm{q}\right)}^{2+}}$ is the mass of Cu(II) in the aqueous phase entering stage n.
• ${\mathrm{m}}_{{\mathrm{C}\mathrm{u}}_{\left(\mathrm{r}\mathrm{a}\mathrm{f}\mathrm{f}\right)-\mathrm{n}}^{2+}}$ is the mass of copper ions in the raffinate at the outlet of the stage.
• ${\mathrm{m}}_{{\mathrm{C}\mathrm{u}}_{\left(\mathrm{o}\mathrm{r}\mathrm{g}\right)-\mathrm{n}}^{2+}}$ is the mass of copper ions in the organic phase at the outlet of stage “n”.
• V is the volume of the phase.

2.2.3. Determination of the Extraction and Stripping Isotherms

For the determination of the extraction isotherms, an aqueous phase obtained from the leaching process was prepared in a 700 cm³ reactor equipped with a vertical stirrer. The reactor was loaded with oxidized ore (50 g; particle size, 45 µm) and 500 cm³ of a leaching solution containing tartrate (0.425 mol dm⁻³, 80 g/L) in NaOH (0.06 mol dm⁻³, 2.5 g/L, pH ≈ 11). The reaction mixture was stirred at 600 rpm for 48 h and then centrifuged at 2500 rpm for 15 min. The supernatant was filtered under reduced pressure. The supernatant was quantified by complexometric titration using an EDTA solution (0.01 mol dm⁻³), a 3% Fast Sulphon Black F indicator, and a pH 10 buffer solution for copper ion quantification. To complement the quantification of copper present in the pregnant leach solution, an atomic absorption spectrophotometry analysis was also conducted.

The extraction distribution isotherm was obtained by mixing the organic phase (2.0% w/v phenyl-2-pyridyl ketoxime dissolved in chloroform) with a diluted aqueous leach solution containing 2.00 g/L of Cu(II), in the presence of tartrate ions, and adjusted to pH ≈ 11. The experiments were conducted at 298.15 ± 1 K using various organic-to-aqueous phase ratios (O:A) ranging from 1:1 to 1:2. Each system was stirred for 5 min, after which copper concentrations in both phases were quantified. The obtained data were used to construct a McCabe–Thiele extraction diagram, reflecting the distribution behavior of Cu(II) between the organic and aqueous phases.

The stripping distribution isotherm was determined using a previously loaded organic phase and a 15% (w/v) aqueous oxalic acid solution as the stripping agent. The procedure was performed under a volumetric aqueous-to-organic ratio (A:O) of 1:1 with a contact time of 5 min. Copper recovery in the aqueous stripping phase was quantified via atomic absorption spectroscopy (AAS).

2.2.4. Reconditioning Procedure for Extractant Regeneration and Reuse

Subsequently, to maintain the efficiency of the extractant phenyl-2-pyridyl ketoxime (PPKO) and enable its reuse over multiple cycles, a chemical reconditioning procedure was implemented at the end of each stripping stage. This consisted of washing the organic phase with a sodium acetate buffer solution adjusted to pH 5 in order to restore the protonated state of the oxime functional group, thereby compensating for the loss of protonation and the positive charge caused during the stripping step with oxalic acid. In order to evaluate the recycling capacity of the extractant, five consecutive extraction–stripping–reconditioning cycles were carried out. In each cycle, the reconditioned organic phase was reused under identical experimental conditions (Figure 2). The copper-loading capacity of the extractant in each cycle was determined indirectly by measuring the concentration of Cu(II) in the aqueous phase before and after contact with the organic phase, using atomic absorption spectroscopy (AAS). This information allowed for the calculation of the amount of copper transferred to the organic phase at each stage.

2.2.5. Evaluation of the Temperature Effect on the Complexation Equilibrium of Cu(II)

To determine the reaction enthalpy (endothermic or exothermic) for the Cu(II) ion complexation process, the effect of temperature on the reaction equilibrium was evaluated. A combined Cu(II) ion-selective electrode coupled to a data acquisition system (model 1671, JENCO Instruments) was employed. The experimental assays consisted of a solution containing 20 mL of 10% w/v potassium nitrate, 20 mL of 96% v/v ethanol, and 1.0 mL of 0.3 mol dm⁻³ copper (II) sulfate, placed in a double-jacketed glass vessel. Measurements were performed at three temperatures: 288.15 K, 298.15 K, and 308.15 K.

For the thermodynamic analysis, the modified Van ’t Hoff equation (Equation (4)) [26] was applied, potential values were monitored over time, and the reaction time was defined as the moment the system reached a potential of 246 mV, assuming that the reaction with the complexing agent followed first-order kinetics. Measurements were initiated immediately after the addition of 6.0 mL of a 2.0% w/v PPKO solution in ethanol.

$$\mathrm{Ln}\left({\displaystyle \frac{{\mathrm{k}}_2}{{\mathrm{k}}_1}}\right)=-{\displaystyle \frac{\Delta \mathrm{H}}{\mathrm{R}}}\left({\displaystyle \frac{1}{{\mathrm{T}}_2}}-{\displaystyle \frac{1}{{\mathrm{T}}_1}}\right)$$

(3)

Since, in a first-order reaction, the rate constant, k, is inversely proportional to the characteristic reaction time (k ∝ 1/t), the equation can be rewritten as

$$\mathrm{Ln}\left({\displaystyle \frac{{\mathrm{t}}_1}{{\mathrm{t}}_2}}\right)=-{\displaystyle \frac{\Delta \mathrm{H}}{\mathrm{R}}}\left({\displaystyle \frac{1}{{\mathrm{T}}_2}}-{\displaystyle \frac{1}{{\mathrm{T}}_1}}\right)$$

(4)

where

• t₁ and t₂ are the times required to reach the steady-state potential at temperatures T₁ and T₂, respectively.
• ΔH is the reaction enthalpy (J·mol⁻¹).
• R is the universal gas constant (8.314 J·mol⁻¹·K⁻¹).

3. Results and Discussion

3.1. Formation and Stability of the Cu²⁺ Complex with Phenyl-2-Pyridyl Ketoxime for Solvent Extraction in the Hydrometallurgical Process

The potentiometric titration of the Cu²⁺–phenyl-2-pyridyl ketoxime (PPKO) system in absolute ethanol showed two inflection points corresponding to the sequential formation of complexes [CuHL]²⁺, [CuL]⁺, and [CuL₂]. These complexes are formed through coordination with the nitrogen atoms of the pyridine ring and the oxime group. In Figure 3, it is evident that upon the addition of NaOH, the first inflection region in the curve occurred at 6.5 mL, with a resulting pK₁ ≈ 4.65, marking the formation of the [CuL]⁺ system (Equation (6)). This involves the formation of a monovalent positively charged complex due to the loss of the first hydrogen from the oxime group and the possible formation of a hydrogen bridge with the proton and available oxygen atoms in the system. The second inflection occurred upon the addition of 9.5 mL of NaOH, reaching pK₂ ≈ 11.5, indicating the formation of the second system [CuL₂] (Equation (7)), where the ligand loses the second hydrogen from the oxime group, resulting in a neutral complex whose charge is balanced by the Cu²⁺ ion. These results are similar to those reported by [26], and the observed outcome is significant because neutral complexes favor solubility in hydrocarbons commonly used in hydrometallurgical recovery processes [27,28].

$${\mathrm{Cu}}^{2+}+\mathrm{HPPKO}\stackrel{{\mathrm{CH}}_3\mathrm{OH}}{\to }{\left[\mathrm{Cu}\left(\mathrm{HPPKO}\right)\right]}^{2+}$$

(5)

$${\left[\mathrm{Cu}\left(\mathrm{HPPKO}\right)\right]}^{2+}+{\mathrm{OH}}^{-}\stackrel{{\mathrm{CH}}_3\mathrm{OH}}{\to }{\left[\mathrm{Cu}\left(\mathrm{PPKO}\right)\right]}^{+}+{\mathrm{H}}_2\mathrm{O}$$

(6)

$${\left[\mathrm{Cu}\left(\mathrm{PPKO}\right)\right]}^{+}+{\mathrm{OH}}^{-}\stackrel{{\mathrm{CH}}_3\mathrm{OH}}{\to }\left[\mathrm{Cu}({\mathrm{PPKO})}_2\right]+{\mathrm{H}}_2\mathrm{O}$$

(7)

$${\mathrm{Cu}}^{2+}+2\left(\mathrm{PPKO}\right)+{2\mathrm{OH}}^{-}\stackrel{{\mathrm{CH}}_3\mathrm{OH}}{\to }\left[\mathrm{Cu}({\mathrm{PPKO})}_2\right]+{2\mathrm{H}}_2\mathrm{O}$$

(8)

3.2. Determination of the Coordination Number of the Cu(II):PPKO Complex to Be Extracted

The combined application of potentiometric and spectrophotometric techniques [29,30,31] enabled the characterization of the sequential formation of complexed species between the Cu(II) ion and the ligand phenyl-2-pyridyl ketoxime (PPKO), revealing that pH greatly influences the type of complexes formed [32,33]. At pH 1.65, the formation of a monoligand complex, [CuHL]²⁺, was observed, evidenced by a well-defined UV–Vis spectrum, consistent with a coordinated species still protonated at the oxime group. The subsequent addition of NaOH up to pH 2.03 showed a decrease in absorbance without a shift in the maximum wavelength, which can be attributed to a dilution effect in the system rather than significant structural changes. With a progressive increase in pH up to 5.05, close to the first experimentally determined pKa value (4.65), a bathochromic shift in the UV–Vis spectrum was identified, indicating the deprotonation of the oxime group and the formation of the [CuL]⁺ species. This spectral change supports the existence of an acid–base equilibrium that modulates the coordination to the metal ion. Finally, at pH 9.75, close to the second pKa (11.5), a significant change in the spectral profile was recorded, attributable to the formation of a bidentate complex [CuL₂], characterized by its high stability in an alkaline medium. These findings confirm the existence of a dynamic pH-dependent equilibrium among Cu–PPKO species, in which [CuHL]²⁺, [CuL]⁺, and [CuL₂] predominate in acidic, neutral, and alkaline ranges, respectively. The 1:2 metal-to-ligand coordination ratio under basic conditions was confirmed by the UV–Vis spectrum recorded at pH 11, which shows a characteristic profile of the [Cu(PPKO)₂] species (Figure 4).

From a hydrometallurgical perspective, the results obtained allow this study to assess the significant implications involved in the design and optimization of liquid–liquid extraction processes. The predominance of the [Cu(PPKO)₂] complex under alkaline conditions, particularly in media enriched with tartrate ions and at pH ≈ 11, indicates that this species constitutes the most stable and effective form for transferring Cu(II) ions from the aqueous phase to the organic phase. This observation is supported by previous studies using commercial extractants such as Mextral 84H and Mextral 54-100 [7], which have demonstrated high extraction efficiencies of Cu(II) in alkaline glycinate solutions. Taking into account the methodology employed, the results obtained contribute to the rational design of more selective and efficient extractive systems for the recovery of copper(II) from leach solutions in alkaline medium, positioning PPKO as a competitive and versatile ligand for selective hydrometallurgical applications for metals.

3.3. Effect of Contact Time

In the present study, it was observed that the extraction of Cu(II) ions using phenyl-2-pyridyl ketoxime (PPKO) reached equilibrium, with no significant changes detected between 3 and 5 min of agitation. Extending the contact time beyond this range did not result in any additional increase in extraction efficiency or adverse effects, indicating that the system stabilizes within a short period. This behavior is consistent with the findings reported by [34], who investigated oxime-derived extractants dissolved in heptane and toluene and concluded that the extraction equilibrium is achieved within the first minute of agitation, with no improvements observed at prolonged contact times. Similarly, ref. [35] documented that commercial extractant systems typically reach equilibrium between 1 and 5 min, depending on the type of organic diluent and operational temperature.

In our system, the rapid stabilization can be attributed to diffusion-controlled mass transfer, where the interfacial transport of Cu(II) to the organic phase occurs rapidly due to the high affinity of the oxime group for the metal ion. This phenomenon highlights the operational advantage of using fast-kinetic extractants, particularly in hydrometallurgical processes where contact time efficiency is critical. In this context, the molecular structure of the extractant and its selectivity toward the metal ion play a decisive role in determining the time required to reach equilibrium [36].

The selection of a standard agitation time of 5 min (Figure 5) for all subsequent experiments ensured equilibrium conditions without the need to apply complex kinetic models. This decision is supported by previous studies indicating that, in systems employing extractants with high affinity for Cu(II), such as oxime-based compounds, detailed kinetic modeling may be unnecessary and may not provide additional value in terms of process optimization [37].

3.4. Evaluation of Cu(II) Transfer Efficiency and Extractant Performance

The experimental findings enabled the determination of the maximum loading capacity of the extractant phenyl-2-pyridyl ketoxime (PPKO) for Cu(II) ions in organic–aqueous biphasic systems. Notable variations in copper accumulation within the organic phase were identified as a function of the organic-to-aqueous phase ratio (O/A), thereby underscoring the significant impact of the phase ratio on the loading performance of the extraction system.

At an organic-to-aqueous phase ratio (O/A) of 1:1, the copper concentration in the organic phase reached a maximum of 4.25 g/L during the third contact cycle and remained stable in subsequent cycles. This behavior suggests that the extractant approached its saturation point under these conditions. In contrast, when using an O/A ratio of 1:2, a more pronounced increase in loading capacity was observed, also reaching 7.00 g/L in the third cycle, with no significant changes in the following cycles. This difference can be attributed to the greater excess of aqueous phase available, which enhances the mass transfer of Cu(II) ions into the organic phase, thereby promoting more efficient saturation of the extractant. Figure 6 shows the evolution of copper loading over successive contact cycles, while

Table 1. Distribution of Cu²⁺ between organic and aqueous phases during successive extraction cycles at different O:A ratios.

Nº Cycles	O/A 1:1			O/A 1:2
	[Cu2+]aq, gpl	[Cu2+]raff, gpl	[Cu2+]org, gpl	[Cu2+]aq, gpl	[Cu2+]raff, gpl	[Cu2+]org, gpl
1	2.00	0.10	1.90	2.00	0.20	3.60
2	4.00	0.70	3.30	4.00	1.15	5.70
3	6.00	1.75	4.25	6.00	2.50	7.00
4	8.00	3.65	4.35	8.00	4.43	7.15
5	10.00	5.60	4.40	10.00	6.33	7.35

summarizes the hydrometallurgical balance, specifying the accumulated concentrations per cycle in the aqueous and loaded organic phases. Furthermore, an effective loading capacity of 0.44 gpl of Cu(II) per volume of organic phase (gpl per v/o) was calculated, normalized with respect to the volumetric phase ratio. This parameter provides a valuable metric for comparing extractant performance under equivalent experimental conditions. Additionally,

Table 2. Comparative evaluation of the maximum effective loading capacity (g/L per v/o) between commercial extractants and the Cu²⁺:PPKO system for copper(II) recovery.

Extractant	% in Diluent	Aqueous Medium	pH	Uptake (g/L)	gpl Per v/o	Reference
Acorga M5640	30% v/v	Alkaline glycinate	~11	17–18	~0.57–0.61	[7]
LIX 984N	10% v/v	Sulfate	~2.5	5–7	~0.40–0.45	[38]
LIX 860N-I	20% v/v	Sulfate–chloride	~2.2	5.5–5.9	~0.35–0.38	[39]
PPKO	2% w/v	Alkaline tartrate	~11	4–7	~0.40–0.50	Our experiment

presents comparative values derived indirectly from experimental data reported in the literature, processed using an analytical approach analogous to that employed in the present study. The results confirm that PPKO exhibits a high affinity for Cu(II) ions, achieving higher loading capacities than those reported for other oxime-based extractants under comparable conditions.

The reported information confirms the high affinity of phenyl-2-pyridyl ketoxime for Cu²⁺ ions due to the presence of the oxime group and the pyridine ring, and that the organic-to-aqueous ratio has a significant impact on the extraction process. Loading capacity data have also been reported for commercial oximes such as LIX 984 N and LIX 64 N at equilibrium pH values between 1.7 and 2.1 [40,41,42]. Additionally, during the evaluation cycles, limitations in extraction were observed starting from the third cycle, where a third component appeared at the organic–aqueous interface. This phenomenon indicates non-ideal behavior in the system due to the formation of dimers or supramolecular aggregates, which are activated by the presence of hydrogen bonding, competing with active sites for coordination with the metal ion and reducing extraction efficiency [43,44]. Although the literature suggests adding octanol or isotridecanol to improve phase separation efficiency—as studied with Acorga M5640 in kerosene and demonstrated experimentally [45]—the objective of our work was not to develop and validate a phase equilibrium model but rather to demonstrate the feasibility of phenyl-2-pyridyl ketoxime for the recovery of Cu²⁺ ions from a loaded leach solution in the presence of tartrate ions under alkaline conditions (Figure 7).

3.5. Leaching and Selective Extraction of Copper(II) Ions

The leachate was generated from an oxidized ore subjected to a leaching process in the presence of tartrate ions in an alkaline medium. The resulting concentration of the loaded leachate was 6.285 g/L. From this loaded leach solution, experimental tests were carried out after diluting it with distilled water to reach a concentration of 2 g/L of copper ions. Preliminary extraction results using phenyl-2-pyridyl ketoxime in chloroform show that copper is strongly extracted at a pH close to 11 in a hydrometallurgical process, which is consistent with expectations for the extractant based on its structure due to the presence of the oxime group (–C=NOH), which is highly effective for the formation of the Cu:PPKO complex. This efficiency is also supported by the presence of tartrate ions in the solution, which contributes to the stability of copper in solution, allowing phenyl-2-pyridyl ketoxime to selectively extract copper(II) ions. The formation of the Cu:PPKO complex from the leach solution is shown in Figure 8. Copper extraction using hydroxyoximes has been widely studied and reported [46], although ketoximes have received attention in coordination and structural chemistry approaches [47,48,49], their application in hydrometallurgical processes under alkaline conditions has not been addressed and constitutes the main focus of the present work.

3.6. Extraction and Stripping Isotherm

The number of stages required for copper extraction was determined, and a McCabe–Thiele diagram was constructed for organic-to-aqueous phase ratios (O:A) ranging from 1:1 to 1:2 while keeping the total phase volume constant. Figure 9 indicates the presence of 0.005 and 0.1 g/L of copper(II) ions in the raffinate after the second and first extraction stages for O:A ratios of 1:2 and 1:1, respectively, suggesting the potential to increase copper concentration in the organic phase. The analysis of copper ion concentrations in the organic phase confirmed extractions of 4 and 2 g/L of copper(II) ions in the tests conducted at O:A ratios of 1:2 and 1:1, respectively. During the stripping process, a stripping efficiency of 99.5% was observed at an aqueous-to-organic phase ratio (A:O) of 1:1. Figure 10 shows that stripping occurs in two stages for organic phases loaded with 4.0 g/L of copper ions, and the organic phase was subsequently recovered and reused.

3.7. Effect of Reconditioning on the Reusability of the PPKO Extractant

The results obtained reveal a 20% decrease in the copper-loading capacity of the extractant phenyl-2-pyridyl ketoxime (PPKO) by the fifth reuse cycle compared to the initial maximum value. This loss of efficiency suggests a progressive reduction in the number of active sites available on the extractant for the formation of the Cu²⁺:PPKO complex. This behavior is consistent with previous reports on oxime-type extractants, in which gradual chemical degradation of the oxime functional group has been documented after multiple extraction and stripping cycles, thereby compromising its coordination capacity with Cu²⁺ ions [50,51,52].

However, treatment of the extractant with an acetate buffer solution at pH 5 enabled the recovery of approximately 75% to 80% of its original extraction capacity. This restoration can be attributed to the effective reprotonation of the oxime group in PPKO, which undergoes deprotonation during the liquid–liquid extraction process (Figure 11). Although oxalic acid, employed as a stripping agent, facilitates the removal of Cu²⁺ ions from the metal–ligand complex, it does not guarantee complete ionic exchange to restore the protonated form of the extractant. In contrast, the acetate buffer—by maintaining a pH close to the first experimentally determined pKa of PPKO (~4.65), as established through potentiometric titrations—provides more favorable conditions for reprotonation of the oxime functional group (–C=NOH). This strategy contributes to a more complete regeneration of the extractant in its active form and enhances its performance across successive extraction cycles.

3.8. Thermodynamic Study of the Cu(II)–PPKO System in Ethanolic Medium

The thermodynamic evaluation of the Cu(II)–PPKO complexation system was conducted using an approach analogous to stoichiometric studies of the interaction between Cu(II) ions and PPKO. The results allowed for an accurate characterization of the thermodynamic behavior of the reaction, providing key insights into its role within a liquid–liquid extraction process, where PPKO is dissolved in chloroform in the organic phase under hydrometallurgical conditions.

The temperature-dependent complexation of Cu(II) ions with PPKO was analyzed through a plot of Ln(1/t) versus 1/T (Figure 12), which exhibited a linear fit, indicating a strong correlation between Cu(II)–PPKO complex formation and the system temperature. The reaction times required to reach a potential of 246 mV were found to be 60, 20, and 8 s at 287 K, 298.5 K, and 306 K, respectively, enabling the determination of whether the process is exothermic or endothermic. The linearization yielded a negative slope, directly associated with the −ΔH/R term of the modified Van ’t Hoff equation, from which a reaction enthalpy of +75.73 kJ·mol⁻¹ was calculated. The positive value of ΔH indicates that the Cu(II)–PPKO complexation process is endothermic, implying that an increase in temperature promotes complex formation.

4. Conclusions

Based on the preceding discussions, the following conclusions are established:

i.

A new methodology was successfully developed based on a solvent, using phenyl-2-pyridyl ketoxime (PPKO) as a selective extractant of Cu²⁺ from leach solutions containing tartrate in an alkaline medium derived from oxidized ores. PPKO demonstrated high efficiency in the transfer of copper(II) without the need for acidic media or conventional commercial extractants.

ii.

It was confirmed that phenyl-2-pyridyl ketoxime (PPKO) forms a stable 1:2 stoichiometric complex with Cu(II) under alkaline conditions, identified as [Cu(PPKO)₂], with maximum stability at pH 11, as evidenced by UV–Vis spectra and potentiometric curves.

iii.

The effective loading capacity of PPKO reached an extraction efficiency of approximately 0.44 gpl per v/o, comparable to commercial systems, validating its competitiveness in terms of performance.

iv.

A progressive decrease in the extraction efficiency of PPKO was observed after several operational cycles, attributed to the sustained deprotonation of the oxime functional group during the extraction process. However, reconditioning of the extractant with an acetate buffer solution at pH 5 enabled the restoration of up to 80% of its original extractive capacity through a reprotonation process associated with an ion exchange mechanism. This strategy contributes to preserving the functionality of the extractant and ensures its efficient reuse in successive extraction cycles.

v.

The Cu(II)–PPKO complexation is an endothermic process, strongly favored by increasing temperature, that reduces the reaction time and enhances the efficiency of solvent extraction under hydrometallurgical conditions.

vi.

Furthermore, the results of this research lay the groundwork for implementing the Cu(II)–PPKO system in a tartrate–alkaline medium, demonstrating it as a promising alternative for the selective recovery of copper in a novel, environmentally friendly hydrometallurgical process.