A Preliminary Study on the Solvent Extraction of Molybdenum and Rhenium from an Industrial Pregnant Leach Solution Using Alamine336 as the Extractant and the Ionic Liquid 1-Octyl-3-Methylimidazolium Bis(trifluoromethylsufonyl)imide as the Diluent

Abstract

A study on the selective solvent extraction (SX) of molybdenum (Mo) and rhenium (Re) from two industrial pregnant leach solutions (PLSs) was carried out using Alamine 336 as the extractant and the ionic liquid (IL) 1-octyl-3-methyl Imidazolium bis (trifluoromethylsulfonyl) imide [Omim][Tf₂N] as the diluent. One industrial PLS was rich in Mo (VI) (PLS-Mo) and the second one rich in Re (VII) (PLS-Re). Experiments were carried out in open vials in which the concentration of Alamine336 in the diluent, the aqueous-to-organic ratio (A/O), and the stripping with ammonium carbonate (${\left(\mathrm{N}{\mathrm{H}}_4\right)}_2\mathrm{C}{\mathrm{O}}_3$) were carried out systematically. Results indicate that decreasing the aqueous-to-organic (A/O) ratio led to an enhancement in the extraction performances of both Mo (VI) and Re (VII), reaching recoveries of 95%–98% at an A/O ratio of 1:1. However, differences between PLSs became evident at higher ratios, as Re extraction declined more sharply than Mo. Third-phase formation was observed only in the Mo-containing PLS. The PLS–Re system did not exhibit the formation of a third phase due to a lower concentration of metal (1 g/L Mo). The use of ammonium carbonate for stripping led to enhanced recoveries, achieving 84.4% for Re and 46.8% for Mo. A total of 50 extraction-stripping cycles were carried out in this work. These demonstrated nearly total initial extraction, but performance decreased over the cycles because of insufficient stripping and solvent loading. Overall, [Omim][Tf₂N] proved to be an effective and environmentally friendly alternative to conventional diluents for Mo and Re separation and recovery from industrial leach solutions.

1. Introduction

Molybdenum (Mo) occurs mainly in porphyry copper deposits, where it is extracted through flotation to create a copper–molybdenum concentrate, which is then separated from copper during the processing phase [1]. As an important refractory metal, Mo is extensively utilized in metallurgy (including steel, cast iron, and superalloys) and in chemical industries for applications such as pigments, lubricants, and catalysts [2]. Since molybdenite (MoS₂) is frequently found in conjunction with rhenium sulfide (ReS₂), molybdenum ores are also a key source of rhenium [3]. In this sense, the extraction of molybdenum and rhenium is performed using solvent extraction (SX).

Rhenium (Re), recognized as a crucial and rare dispersed metal, is widely used in high-tech industries such as aerospace, petrochemicals, and electronics due to its exceptional physicochemical properties [4,5]. It is a key component in high-temperature superalloys for aircraft engines, petroleum refining catalysts, and various electronic devices [6,7]. Due to its extremely low natural abundance, absence of primary deposits, and occurrence as a trace constituent in molybdenum and copper ores, rhenium is predominantly recovered as a byproduct of Mo and Cu smelting [8,9]. Given that its global annual production is only around 50 tons [10], improving recovery efficiency and minimizing production costs are critical for its sustainable use.

Mo and Re are extracted from molybdenite leaching [11,12], resulting in a pregnant leach solution (PLS) that contains Mo, Re, Cu, and Fe [12]. Re-bearing solutions are generally obtained from roasting fume/dust, leaching residues, and pregnant leach solutions [10]. In molybdenum concentrates, Mo and Re can be leached simultaneously by pressure oxidative leaching methods like pressure aqueous oxidation of molybdenite with oxygen, acid pressure leaching [13], or alkaline pressure leaching [14]. Given the increasing consumption, high cost, and limited availability of Re, its supply, extraction, and recycling have received significant attention from industry [15].

Mo (VI) and Re (VII) ions in aqueous solutions are recovered from PLS through SX using various extractants like trioctylmethylammonium chloride [Aliquat 336], Cyanex 923, trioctylmethylammonium benzoate [TOMA][BA], and bis(2,4,4-trimethylpentyl) phosphinic acid D2EHPA [16,17,18]. In aqueous environments, Re (VII) primarily occurs as perrhenate (ReO₄⁻) [19], while Mo (VI) is found mainly in cationic form (MoO⁴⁺ > MoO₂²⁺ > MoO₃) [20]. However, at low pH values in sulfuric acid solutions, Mo (VI) is found as heteropoly acid anions $\left[\mathrm{M}{\mathrm{o}}_2{\mathrm{O}}_5(\mathrm{S}{\mathrm{O}}_4{)}_2{]}^{2-}\right.$ and $\left[\mathrm{M}\mathrm{o}{\mathrm{O}}_2(\mathrm{H}\mathrm{S}{\mathrm{O}}_4{)}_4{]}^{2-}\right.$ [21]. The extraction process predominantly involves anion exchange with amine-based extractants and neutral complexation with phosphine oxides. Nevertheless, due to the similar chemistry of their oxyanions, achieving a selective separation poses a significant challenge, underscoring the necessity for new extractant systems. Conventional diluents such as kerosene exhibit high toxicity, volatility, and flammability [22], while an aliphatic alcohol is often added as a phase modifier to prevent third-phase formation, further complicating the system. As a result, current research trends are aimed at substituting kerosene with ionic liquids (ILs) as diluents, which are non-volatile, thermally stable, and environmentally friendly, providing a more sustainable option for solvent extraction [23,24,25,26,27].

Several studies have explored the selective separation of Mo and Re. Nabardi, S et al. [16] examined the solvent extraction of Mo and Re using the bifunctional Bif-IL [Aliquate336] [Cyanex272] diluted in xylene. Joo et al. [26] employed selective precipitation to separate Mo and Re from PLS. The study by Quijada-Maldonado et al. [28] was conducted on the selective SX of Mo and Re from synthetic pregnant leach solution using two diluents—kerosene and the IL 1-octyl-3-methyl-imidazolium bis(trifluoromethylsulfonyl)imide [Omim][Tf₂N]—and two extractants—di(ethylhexyl) phosphoric acid D2EHPA and the IL tri-octylmethylammonium di-(2-ethylhexyl) phosphate [TOMA][D2EHP]. Khoshnevisan et al. [11] achieved Mo-Re separation using commercial extractants dissolved in kerosene. Akram Cheema et al. [29] selectively extracted Re from Mo and other metals present in flue dust leach liquors. To the best of our knowledge, no prior studies have reported the use of a combination of ionic liquid (IL) as diluent in place of kerosene and Alamine 336 as the extractant for the extraction of Re (VII) and Mo (VI) from industrial pregnant leach solutions (PLSs) derived from molybdenite leach liquors. Alamine 336 was selected because it is a well-established commercial extractant widely used in industry for recovering Mo (VI) and Re (VII) from leach liquors, offering high extraction performances.

Thus, the aim of this work is to evaluate the extracting phase composed by Alamine 336 as the extractant and IL 1-octyl-3-methylimidazolium bis(trofluoromethylsulfonyl)imide [Omim][Tf₂N] as the diluent in the SX of Mo (VI) and Re (VII) from two PLSs obtained from molybdenite leach liquors. This IL ([Omim][Tf₂N]) was chosen because it is sufficiently hydrophobic to form a distinct organic phase, with a low solubility in water of 0.91 w/w % [30]. This high hydrophobicity minimizes the loss of the IL to the aqueous phase during extraction, due to its poor ability to extract metal ions by itself [31]. [Omim][Tf₂N] has a relatively low viscosity of approximately 88.6 ± 0.37 mPa·s at 298.15 K [32], compared with other ILs. It is commercially available and has been widely used as a diluent in SX studies of various metals [33]. The weak coordinating ability of the [Tf₂N]⁻ anion does not influence the selectivity toward any specific metal in the PLS. To achieve this, the extraction percentages (% E), selectivity (S), and the formation of a third liquid phase or precipitates are experimentally evaluated as a function of the extractant concentration and the aqueous-to-organic ratio (A/O) in both PLSs. Finally, the stripping efficiencies achieved when employing a stripping solution containing ammonium carbonate was studied. More importantly, to evaluate this extracting phase for future industrial applications, the extracting phase was evaluated up to 50 extraction-stripping cycles. With this, it is expected to deliver a more sustainable phase, containing a common industrial extractant, for industrial Mo and Re purification.

2. Methodology

2.1. Materials and Chemicals

Two PLSs obtained from molybdenite leach liquors were kindly provided by Molymet^® (Santaigo, Chile). The PLS samples were prepared by acid leaching of a molybdenite concentrate containing trace Re. The mineral was finely ground and leached with dilute sulfuric acid under oxidizing conditions to dissolve Mo and Re ions. After solid–liquid separation, the resulting solutions were analyzed to obtain the metal concentrations shown in

Table 1. Concentration of the components in the industrial PLS obtained from molybdenite leach liquors.

	Concentrations (mg/L)
Metal Ion	PLS-Mo	PLS-Re
Mo (VI)	4.216	1.775
Re (VII)	0.02	0.308
Cu (II)	2.679	0.008
Fe (III)	3.479	0.002
pH	0.21	0.94

.

For the stripping steps, ammonium carbonate (NH₄)₂CO₃, was purchased from Merck Co. (Heilbronn, Germany). The extracting phase consisted of Alamine336^® (Santiago, Chile), which was provided by BASF^® (Santiago, Chile), as the extractant. The IL 1-octyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide [Omim][Tf₂N] (purity > 98%) was purchased from Iolitec GmbH (Heilbronn, Germany).

2.2. Experimental Procedure

The extracting phase was prepared by combining Alamine 336 with the chosen diluent [Omim][Tf₂N], and the extractant concentration in the diluent was determined with an analytical balance with a precision of ±0.0001 g (Radwag^® Model AS 220.R2) (Radwag, Radom, Poland). The initial pH of both PLSs was measured by using a pH meter (HI 2221, Hanna Instruments, Shanghai, China) equipped with a microelectrode (HI 1330B, Hanna Instruments, Shanghai, China). Ultimately, the stripping experiments were conducted utilizing a single stripping agent: For the stripping process, ammonium carbonate (1.0 M, Merck, Heilbronn, Germany) was utilized to disrupt the metal–extractant complex at elevated pH and extract metal ions from the Alamine 336/[Omim][Tf₂N] organic phase.

The liquid–liquid extraction experiments (or SX experiments) were conducted at 25 °C in an open flask by combining 2 mL of the aqueous solution with 2 mL of the extracting solution, followed by stirring at 1400 revolutions per minute (rpm) for 40 min to promote interaction between the phases. The vials were double-jacketed to ensure a constant temperature during experiments. After stirring, the mixture was centrifuged for an additional 40 min to ensure the complete separation of the aqueous and organic phases. Throughout the process, the pH of the aqueous PLS was continuously monitored using a HANNA HI4212 (Shanghai, China) pH meter equipped with a microelectrode. After phase separation, the aqueous layer was carefully withdrawn, and the concentrations of all remaining metal species in the PLS were measured using Inductively Coupled Plasma Atomic Emission Spectroscopy (ICP-AES; Varian Inc., Palo Alto, California, USA, Liberty Series II model). Each extraction experiment was performed in duplicate to ensure the reliability of the results.

The performance of the liquid–liquid extractions and percentage of re-extraction (% R) was evaluated using Equations (1)–(5), which define the extraction percentage (% E_i), selectivity (S_i,j), and distribution ratio (D_i) for each metal ion i. In this study, i corresponds to Mo, Re, Cu, and Fe.

$$\%{ E}_i={\displaystyle \frac{D_i}{D_i+\raisebox{1ex}{$V_{aq}$}\!\left/ \!\raisebox{-1ex}{$V_{org}$}\right.}}\ast 100$$

(1)

$$S_{ij}={\displaystyle \frac{D_i}{D_j}}$$

(2)

$$D_i={\displaystyle \frac{\left[M_I\right]org}{\left[M_I\right]aq}}$$

(3)

$$\%R_i={\displaystyle \frac{{D_i}^{org}}{{D_i}^{org}+\raisebox{1ex}{$V_{org}$}\!\left/ \!\raisebox{-1ex}{$V_{aq}$}\right.}}\times 100$$

(4)

$${D_i}^{org}={\displaystyle \frac{\left[M_I\right]aq}{\left[M_I\right]org}}$$

(5)

where V_org and V_aq represent the volume of the organic and aqueous phases, respectively. [M_I]org and [M_I]aq are the concentration of the metal in the organic phase after extraction, and the concentration of the free metal ion in the aqueous phase after extraction, respectively.

Finally, the loss of the IL to the aqueous phase was determined by sampling the aqueous phase at equilibrium and measuring the amount of $\left[\mathrm{T}{\mathrm{f}}_2\mathrm{N}{]}^{-}\right.$ that had transferred from the IL diluent into the aqueous phase. The measurement was conducted with a Metrohm^® Compact 930 IC Flex Ion Chromatograph (Herisau, Switzerland), employing a MetroSep A Supp 5—250/4.0 (Herisau, Switzerland), anionic column.

3. Result and Discussion

3.1. Extraction and Losses of Ionic Solvent

The results obtained in this study, as seen in

Table 2. Extraction and IL losses were performed at 25 °C, with an A/O ratio = 1:1 and without using phase modifiers in both PLSs.

Extractant	PLS	% v/v	Form 3rd Phase	Ionic s. Losses (ppm)	pH eq.
Alamine 336	Mo	5	No	103.3	0.28
	Mo	10	No	666.6	0.35
	Re	5	No	404.9	1.12
	Re	10	No	1118.8	1.28
	Re	5	No	319.7	1.09
	Re	10	No	1272.5	1.26

and

Table 3. Extraction and selectivity were performed at 25 °C, with an A/O ratio = 1:1 and without using phase modifiers.

PLS	% v/v	Extraction Percentage (%)				Selectivity
		Re	Cu	Fe	Mo	Mo-Re	Mo-Cu	Mo-Fe
Mo	5	95.5	2.9	0	96.6	1.4	976.2	Very selective
Mo	10	95.5	6.4	0	97.9	2.2	691.1	Very selective
Re	5	93.5	0	0	95.5	1.5	Very selective	Very selective
Re	10	86.7	0	0	95.0	2.9	Very selective	Very selective
Re	5	93.7	0	0	95.2	1.3	Very selective	Very selective
Re	10	86.0	0	0	94.7	2.9	Very selective	Very selective

, demonstrate that the use of the Alamine 336 diluent in [Omim][Tf₂N] system enables highly efficient extraction of Mo (VI) and Re (VII), achieving recoveries above 90%–95% even at low equilibrium pH values (see Table 1), while maintaining negligible co-extraction of Cu and Fe. Olazabal et al. [34] reported that Mo (VI) was effectively extracted by Aliquat 336 at low pH values, indicating that the extraction mechanism involves anion exchange between the protonated extractant and molybdate anions present in the acidic solution. Also, similar behavior is consistent with the findings of Quijada-Maldonado et al. [35], who reported that the use of [Omim][Tf₂N] as a diluent enhanced Mo (VI) extraction compared to kerosene, due to the weaker extractant–diluent interaction that facilitates metal–extractant complex formation. Previous studies have shown that D2EHPA can achieve high extraction efficiencies for Mo (VI), exceeding 80% when the extractant concentration is above 10% (v/v) [36,37]. However, this performance is often accompanied by the co-extraction of other metal species, such as Cu (II) and Fe (III) in sulfate media, owing to its nature as a pH-selective cation-exchange extractant [38,39].

Another important aspect to analyze is that in these extractions, the use of a phase modifier is not required. This is certainly an advantage when compared to the extracting phase used in industry (Alamine336-isodecanol-kerosene) as less chemicals are used [40]. But, more importantly, the phase modifier is used for avoiding a third-liquid-phase formation, and the amount added depends on the concentration of Mo (VI) in the PLS. This concentration often changes, and the concentration of the phase modifier also has to change to avoid this formation. This operational difficulty leads to a large amount of solvent loss, with associated economic and environmental issues.

The increase in equilibrium pH observed in both the Mo and Re systems, together with the greater ionic losses at higher levels of extractant concentrations, can be explained by the stoichiometry of the extraction reactions and the proton exchange mechanism involved. During extraction, Alamine 336 acts as a basic extractant that becomes protonated in the presence of hydrogen ions according to the equilibrium [41].

$${{2 H}^{+}}_{aq} + {2 \mathrm{A}\mathrm{l}\mathrm{a}\mathrm{m}\mathrm{i}\mathrm{n}\mathrm{e} 336}_{org}\rightleftharpoons +{2Alamine336H^{+}}_{org}$$

(6)

The two protons involved in Equation (7a) align with the negative charge of Mo anions in sulfuric acid solutions at low pH values (pH ≈ 0.3). Thus, Mo predominantly exists as the anionic complex, which is stable at low pH values and readily participates in ion-pair formation with the protonated amine extractant with the IL. According to Wang et al. [21], two possible Mo (VI) species may exist in sulfuric acid solution, namely ${\left[{\mathrm{M}\mathrm{o}}_2{\mathrm{O}}_5{\left({\mathrm{S}\mathrm{O}}_4\right)}_2\right]}^{2-}$ and ${\left[\mathrm{M}\mathrm{o}{\mathrm{O}}_2{\left(\mathrm{H}\mathrm{S}{\mathrm{O}}_4\right)}_4\right]}^{2-}$. In this study, the latter species is considered because, as indicated in [21], at a pH value of 0.49, the ratio of SO₄⁻² to HSO₄⁻¹ is 3 to 97 (%), and at a pH value of 0.94, this ratio is 9 to 91 (%), respectively, and thus the predominant species at the pH values of industrial PLSs is $\left[\mathrm{M}\mathrm{o}{\mathrm{O}}_2(\mathrm{H}\mathrm{S}{\mathrm{O}}_4{)}_4{]}^{2-}\right.$. SX experiments with pure [Omim][Tf₂N] were not performed, since this IL alone is not expected to extract Mo (VI) or Re (VII) [28]. Alamine 336 is the commercially available extractant used in industry, which is known to be selective towards Mo and Re.

$${{\left[Mo{O}_2{\left(HS{O}_4\right)}_4\right]}^{2-}}_{\mathrm{aq}}{+2\left(Alamine336H^{+}\right)}_{\mathrm{org}} \rightleftharpoons \left[Mo{O}_{2 }{\left(HS{O}_4\right)}_4\right]{\left(Alamine336H\right)}_2\mathrm{o}\mathrm{r}\mathrm{g}$$

(7a)

Additional IL co-transfer equilibrium:

$$\left[{Omim}^{+}\right]{\left[T{f}_2{N}^{-}\right]}_{org}\rightleftharpoons \left[{Omim}^{+}\right]\left(\mathrm{aq}\right) + \left[T{f}_2{N}^{-}\right]\left(\mathrm{aq}\right)$$

(7b)

$$\mathrm{Re}{O_4^{-}}_{aq}+{\left(Alamine336H^{+}\right)}_{org}\rightleftharpoons {\left[\mathrm{R}\mathrm{e}{O}_4^{-}. Alamine336H\right]}_{org}$$

(8)

Equations (6) and (7a,b) describe the Mo extraction process, the rise in equilibrium pH relative to both PLS systems, and the partial release of the IL into the aqueous phase. This release is not associated with the extraction mechanism but rather with the intrinsic solubility of the IL in water, which is extremely low—typically ranging from 3.2 × 10⁻⁵ to 1.1 × 10⁻³ (mole fraction) [42]. As the concentration of the organic phase rises from 5% to 10% (v/v), the increased interfacial area and enhanced interaction between the phases lead to more significant proton exchange and a greater solubility of the extractant or IL components in the aqueous phase. Moreover, the ionic losses were consistently higher in the PLS-Re compared to the PLS-Mo. This variation is primarily linked to the lower equilibrium pH of the Re solutions, which facilitates the protonation and solubility of the amine extractant within the aqueous phase. Furthermore, in Equation (8), the presence of perrhenate ions (ReO₄⁻) promotes the creation of soluble ion pairs with the extractant cations, which further contributes to the transfer of the extractant into the aqueous phase. Hidaya N et al. [43] focused on the extraction of rare earth elements (REE) with Alamine 336-[Emim] [Tf₂N] (1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide). Results indicated ILs enhance extraction performance, e.g., in rare earth element recovery with A336, but they also increase losses into the aqueous phase, showing a trade-off between performance and solvent stability. Overall, the extractants demonstrated strong selectivity and performance for the recovery of Mo and Re from PLS. A 10% (v/v) extractant in [Omim][Tf₂N] significantly improves selectivity, with higher concentrations performing even better, making it a strong kerosene alternative for Mo (VI) recovery [28].

3.2. Effect of % Extractant and Aqueous/Organic Ratio

Figure 1 illustrates the extraction performance of a 5% v/v concentration of the extractant Alamine 336 in the diluent [Omim][Tf₂N] in PLS-Re for Mo, Re, Cu, and Fe, assessed at varying (A/O) phase ratios (1:1, 2:1, and 4:1). with the aim of reducing the amount of solvent needed for the SX. At an (A/O) ratio of 4:1, the extraction performances for Mo (VI) and Re (VII) were relatively low, suggesting that increased A/O ratios negatively impact the extraction performances of both metals. The high interaction between the extractant and diluent is the reason for the decreased extraction percentages of all metal species [35]. When the A/O ratio was reduced to 2:1, a noticeable improvement was observed for Re extraction, which increased from 87.34% to higher values, while Mo extraction remained fairly stable at around 93.6%. Further decreasing the A/O ratio to 1:1 resulted in the highest extraction performances for both Re (93.7%) and Mo (95.2%). This trend demonstrates that decreasing the A/O ratio enhances extraction performance by providing a higher concentration of extractant relative to the metal ions, facilitating better phase interaction and mass transfer. Nevertheless, this phenomenon has led to enhanced selectivity for Mo-Re, particularly when employing low extractant concentrations in [Omim][Tf₂N]] [28]. This pattern aligns with the results reported by Zhang-Fang et al. [3], who reported that Re extraction performance decreased markedly from 97% at A/O ratios of 1:2–1:1 to 76.7% at 2:1, due to the limited loading capacity of the extractant and changes in activity coefficients at elevated aqueous-to-organic ratios. Similarly, in this study, maintaining an A/O ratio of 1:1 allowed high extraction of both Re and Mo, while higher A/O ratios disturbed the phase balance and reduced extraction performance, especially for Mo. Similarly, Adavodi R et al. [44] found that increasing the A/O ratio reduced Mo, Co, and Ni extraction due to limited extractant capacity and poorer phase mixing.

The selectivity analysis (

Table 4. Selectivity from the different metals at PLS-Re.

A/O	Mo—Re	Mo—Cu	Mo—Fe
1:1	1.34	Very selective	Very selective
2:1	2.15	Very selective	Very selective
4:1	4.91	Very selective	Very selective

) revealed that for PLS-Re, the Mo/Re selectivity increased from 1.34 to 4.91 at A/O = 1:1 to 4:1, while Mo/Cu and Mo/Fe selectivity remained very high at all ratios. This is a promising result as Cu and Fe are considered impurities in Mo and Re SX plants. This suggests that controlling the phase ratio can significantly enhance Mo selectivity relative to Re, which is also required for downstream processing when Mo and Re are separated and purified. Using 5% (v/v) of the extractant Alamine 336 in [Omim][Tf₂N] enhances selectivity, making it a promising alternative to kerosene for the hydrometallurgical recovery of Mo (VI) from Mo [28]. The [Tf₂N]⁻ anion is large, weakly coordinating [45], and significantly delocalized, which decreases the polarity of the IL phase and minimizes direct interactions with metal oxyanions. A somewhat similar trend has been observed in studies of SX of Mo (VI) by novel ILs: for example, Emama et al. [46] reported that a new IL ([Aliq-336]⁺[HCit]⁻) shows effective Mo (VI) extraction from mixtures with many other metal ions (Co, Ni, Fe, etc.), with very good separation performance. On the other hand, the increase in the A/O ratio for the PLS-Mo is discussed in the coming section.

3.3. Third Phase Formation

As illustrated in Figure 2 and

Table 5. SX of Re (VII) and Mo (VI) from industrial PLS-Mo.

PLS	A/O	% v/v Ext	3rd Phase Form?
PLS-Mo	2:1	5	Yes
	4:1	5	Yes
	2:1	10	Yes
	4:1	10	Yes
	2:1	5	Yes
	4:1	5	Yes
	2:1	5	Yes
	4:1	5	Yes
	2:1	5	Yes
	2:1	5	Yes
	2:1	5	Yes
	2:1	5	Yes
	1.5:1	5	Yes
	1.5:1	5	Yes
	2:1	10	Yes
	1.5:1	10	Yes
	2:1	10	Yes
	2:1	15	Yes
	1.5:1	15	Yes
PLS-Re	2:1	5	No
	4:1	5	No

, the occurrence of a third phase was consistently observed in the industrial Mo-containing PLS at extractant concentrations ranging from 5 to 15% (v/v). This behavior can be ascribed to the strong affinity of Mo ions for Alamine 336, which produces the formation of bulky ion-pair complexes that tend to aggregate. At higher extractant concentrations, these aggregates become unstable and separate from the bulk organic phase, resulting in a distinct third phase. The IL [Omim][Tf₂N] serves as a non-polar, hydrophobic diluent and does not itself significantly interact with Mo (VI) to generate a third phase; therefore, the observed phase splitting is attributed mainly to the poor solubility of the metal complex in the IL phase, especially when the A/O ratio is increased and there is less IL to dissolve complexes. The absence of a phase modifier, which typically improves phase stability by preventing extractant–complex aggregation, further contributes to this instability. In contrast, the PLS–Re system did not exhibit third-phase formation, likely because the concentration of Mo (VI) in the aqueous phase (−1 g/L) was much lower than that of Mo (VI) in PLS–Mo (−4 g/L). As a result, the solubility limit of the Re–Alamine 336 complex in the IL phase was not exceeded, while the Mo–Alamine 336 complex, being less soluble at higher metal concentrations, promoted third-phase formation. Similar third-phase formation behavior has been reported by Olea et al. [22], where Lidocaine Lid-based hydrophobic deep eutectic solvents systems produced insoluble Cu (II) [47] and Fe (III) [48] complexes, leading to gelled or colored third phases. In the present study, Mo appears to play a comparable role in destabilizing the organic phase through the formation of insoluble Mo–extractant complexes, suggesting that third-phase phenomena are a general outcome of strong metal–extractant interactions exceeding solubility limits. Similar observations have been reported in the literature, where Mo (VI) forms bulky ion-pair species such as dialkylammonium molybdates [R₃NH]₂[Mo₇O₂₄] and polymolybdates [R₃NH]₂[HMoO₄]₂, and in the organic phase during extraction with tertiary amines [49,50]. These polymeric or dimeric complexes tend to aggregate at high metal loadings, leading to the separation of a viscous or gelled third phase [51,52]. Therefore, the experimental evidence of third-phase formation observed here is consistent with previously reported Mo–amine complexation behavior.

The extractant percentage of each metal extracted using 5% (v/v) Alamine 336 in [Omim][Tf₂N] in the PLS-Mo system is shown in Figure 3. Mo (VI) was almost completely extracted (98%), while Re (VII) reached 95% extraction. Similarly to previous findings [28], an increase in the extraction percentages of Mo (VI) and Re (VII) was observed due to strong extractant–diluent interactions between [TOMA][D2EHP] and [Omim][Tf₂N] compared to kerosene; our system, using Alamine 336 with [Omim][Tf₂N], exhibited significantly better performance. Although the Mo–Re selectivity (1.37) in

Table 6. Selectivity of the PLS-Mo sample.

Selectivity
A/O	Mo—Re	Mo—Cu	Mo—Fe
1:1	1.37	976.24	>1000

was moderate due to their similar oxyanion chemistry, the system still achieved a very high selectivity of Mo over Cu and Fe. Comparable behavior was also observed by Zhang et al. [53], who employed quaternary ammonium salt-based hydrophobic DES systems to selectively extract Mo (VI) from a spent-catalyst solution containing Al (III) and Co (II). They reported very high distribution ratios for Mo (D ≈ 1000) and strong selectivity over base metals. Overall, these results validate the ionic solvent as a suitable system for the selective extraction of Mo and Re for industry, particularly in solutions where copper and iron are present in large excess and must be effectively excluded.

3.4. Stripping

Figure 4 illustrates the re-extraction performance of Mo and Re from the loaded Alamine 336/[Omim][Tf₂N] solvent when using approximately 1M ammonium carbonate as a stripping agent, which demonstrates its effectiveness in IL-based systems. At an A/O ratio of 1:1, Re was re-extracted more effectively (84.4%) than Mo (46.8%). Similarly, the prior study [16], with [Aliquate336][Cynax272] in ammonium carbonate, achieved 100% Re recovery with negligible Mo, indicating that although both systems confirm the performance of ammonium carbonate for selective Re recovery. The [Omim][Tf₂N]-based solvent enhances overall stripping but promotes some Mo co-stripping and recoveries above 100%, likely due to mass balance or re-dissolution effects. These findings align well with earlier studies indicating that ammonium carbonate serves as the most effective stripping agent for IL systems, achieving approximately 90% re-extraction of Mo in a single stage when using [Omim][Tf₂N] as the diluent together with D2EHPA [35]. The performance of ammonium carbonate is due to its basic nature, as higher pH levels cause the breakdown of organometallic complexes and lead to the liberation of [MoO₂ (HSO₄)₄]²⁻ ions into the aqueous phase. Previous studies have effectively extracted the metal ion from the organic phase by utilizing ammonium carbonate solutions [54,55]. In this study, the behavior was further examined in the Alamine 336/[Omim][Tf₂N] system, revealing that ReO₄⁻ was preferentially extracted compared to MoO⁴⁺. This underscores the capability of ammonium carbonate to facilitate the selective separation of Re from Mo within IL-based solvent extraction systems. Finally, these results provide insights for the downstream purification of Re and Mo, in which adding more equilibrium stages in a countercurrent stripping operation could achieve very high Mo and Re purities in ammonium carbonate.

3.5. Stability of Ionic Solvent Across Cycles

The performance of the IL in cyclic extraction for Mo and Re in PLS-Re is illustrated in Figure 5. In the initial cycle, extraction rates were nearly complete, with Mo at 99% and Re at 96%. However, the extraction performance significantly declined in the following cycles, falling to 70% for Mo and below 50% for Re after five cycles. After more than 10 cycles, the system reached a constant performance, with Mo extraction remaining at 50% and Re at 40%. This decline is primarily attributed to incomplete metal removal during the stripping process, which leaves the IL partially loaded with Mo and Re. As a result, in subsequent cycles, the extraction performance decreases due to the increased activity coefficient of the metal complexes in the partially loaded solvent. The sharper decline in Re compared to Mo indicates a stronger binding affinity of the solvent for Mo species relative to perrhenate, which aligns with the stripping results where Re was more readily displaced from the organic phase. This trend aligns with the study [56], where Mo leaching and extraction remained high (>98%) but gradually decreased due to competition from other metal cations, while Re concentration in the raffinate increased to 42.13 mg/L over seven cycles, highlighting effective Re enrichment. Both studies demonstrate that while Mo maintains relatively strong retention, Re is more easily displaced or enriched, confirming the selectivity of the extraction and cyclic enrichment strategies.

4. Conclusions

This research demonstrated the selective SX of Mo (VI) and Re (VII) from two PLS–Mo and PLS–Re using Alamine 336 diluted in the IL [Omim][Tf₂N]. High % E of Mo and Re were achieved, while Cu and Fe impurities were not extracted. Additionally, the results show that reducing the A/O ratio significantly improved the extraction performances of both Mo (VI) and Re (VII). Consequently, at lower A/O ratios, higher extraction performances were achieved, reaching 95%–98% at an A/O ratio of 1:1. This indicates that extractant availability and phase contact balance strongly influence overall recovery. Third-phase formation was detected exclusively in the Mo-containing PLS, which is linked to the strong interaction between molybdate ions and Alamine 336, resulting in the creation of large complexes that aggregate and separate. The PLS–Re system did not show the development of a third phase because the lower concentration of metal (1 g/L Mo) kept the Re–Alamine 336 complex dissolved in the IL. A key finding of this work is that no phase modifier was used, yet stable operation and high extraction performances were achieved. Increasing the extractant concentration (5%–15% v/v) led to higher solvent losses, particularly in the PLS–Re system, where the lower equilibrium pH favored extractant protonation and solubilization.

Stripping with ammonium carbonate resulted in the preferential recovery of Re (84.4%) compared to Mo (46.8%). Across 50 cyclic extractions, initial recoveries of 99% Mo and 96% Re gradually decreased due to incomplete stripping, stabilizing at 50% and 40%, respectively, after several runs. From a technical and economic perspective, the use of [Omim][Tf₂N] as a diluent offers significant advantages, including a negligible volatility, reusability, and a reduced solvent loss, which lower environmental and operational costs compared to kerosene systems. However, large-scale applications will require additional studies on pilot plant tests for the Mo and Re extraction. Future work should focus on continuous extraction trials, IL-recycling stability, and process integration into molybdenite hydrometallurgical circuits to validate scalability and long-term economic performance.