Extraction of Cu(II), Fe(III), Zn(II), and Mn(II) from Aqueous Solutions with Ionic Liquid R 4 NCy

: The leaching of copper ores produces a rich solution with metal interferences. In this context, Fe(III), Zn(II), and Mn(II) are three metals contained in industrial copper-rich solutions in high quantities and eventually can be co-extracted with the copper. The purpose of the current study was to determine the feasibly of solvent extraction with the use of ionic liquid methyltri-octyl/decylammonium bis (2,4,4-trimethylpentyl)phosphinate (R 4 NCy) as an extractant of Cu(II) in the presence of Fe(III), Zn(II), and Mn(II). In general terms, the results showed a high single extraction efﬁciency of all the metals under study. In the case of Fe(III) and Zn(II), the extraction was close to 100%. On the contrary, the stripping efﬁciency was poor to Fe(III) and discrete to Zn(II), but very high to Cu(II) and Mn(II). Finally, the ﬁndings of this study suggest that the ionic liquid R 4 NCy is feasible for the pre-treatment of the copper solvent extraction process to remove metal impurities such as Fe(III) and Zn(II).


Introduction
Hydrometallurgy is an important technique used in mineral processing to obtain pure metal from ores. The hydrometallurgical route commonly involves ore leaching, solvent extraction (SX), and pure metals' electrowinning [1][2][3]. SX is a technique with several advantages, it is a continuous operation, efficient, high production, low cost, and employs of simple equipment to recover valuable elements from impurities [4,5]. Opposite, SX has industrial-scale severe problems, such as emulsification, solvent volatilization, or degradation of extractant. The negative effect of SX results in decreased production, an increase of production cost, low quality of final products, and environmental implications, mainly by loss of volatile compounds. Hydroxyoxime type extractants are one of the large-scale copper extractants. Currently, a variety of hydroxy oximes are used for the copper industry, either ketoxime or aldoxime or a combination of both [6,7].
The recent advance in metal extraction with SX finds to improve the separation, extraction efficiencies, and selectivity, mainly involving the use of synergistic extractants, ionic liquids, and both, that is, a mixture of extractant and ionic liquids [8][9][10].
Ionic liquids (ILs) are a green kind of reagents, comprising organic cations and organic/inorganic anions, with a high potential to reduce or replace the hazardous organic solvents [11][12][13]. The properties of ILs have globally captured the attention, negligible

Reagents and Synthesis of Ionic Liquids
The SX experiments were carried out by a synthetic pregnant leach solution (PLS). The PLS was prepared according to the information of Chilean mining reported by Quijada-Maldonado et al. [41]. The concentrations of Cu(II), Zn(II), Mn(II), and Fe(III) of the PLS are summarized in Table 1. The reagents used in the aqueous phases were copper(II) sulfate pentahydrate (CuSO 4 ·5H 2 O), Fe(III) sulfate hydrate (Fe 2 (SO 4 ) 3 ·nH 2 O), Mn(II) sulfate monohydrate (MnSO 4 ·H 2 O), Zn sulfate heptahydrate (ZnSO 4 ·7H 2 O), sulfuric acid, and distilled water. All reagents used in aqueous phases were analytical grade purchased from Merck. The reagents used in the organic phase were the commercial extractants trialkyl methylammonium chloride (Aliquat 336), bis 2,4,4-trimethylpentyphosphinic acid (Cyanex 272), kerosene, and sodium hydrogen carbonate. All the reagents were provided by Sigma-Aldrich (St. Louis, MO, USA), except Cyanex 272, provided by Solvay (Santiago, Chile). Aliquat 336 and Cyanex 272 (commercial grade) were used without further purification; all other used compounds were analytical grade.
Ionic liquid methyltrioctyl/decylammonium bis 2,4,4-(trimethylpentyl)phosphinate denoted as R 4 NCy was synthesized according to procedures published in the literature [35,36], mixing equimolar ratio of Cyanex 272 (HCy) and Aliquat 336 (R 4 NCl), and dissolved the mixture in kerosene to obtain the desired IL concentration. The mixture of Cyanex 272 and Aliquat 336 was washed twice with sodium bicarbonate 0.5 M to remove the chloride anion and the proton from the organic phase. The general reaction of ionic liquid formation is shown in the following equation [39]: Schematic structures of the ionic liquids and reagent used in this work are depicted in Figure 1. To identify the formation of R4NCy, the FT-IR spectra was performed on the IL synthesized. The concentration of IL in kerosene was 0.1 M. The frequencies of the char-acteristic stretching vibrational bands of this extractant in kerosene are presented in Figure 2. The reagents used in the organic phase were the commercial extractants trialkyl methylammonium chloride (Aliquat 336), bis 2,4,4-trimethylpentyphosphinic acid (Cyanex 272), kerosene, and sodium hydrogen carbonate. All the reagents were provided by Sigma-Aldrich (St. Louis, MO, USA), except Cyanex 272, provided by Solvay (Santiago, Chile). Aliquat 336 and Cyanex 272 (commercial grade) were used without further purification; all other used compounds were analytical grade.
Ionic liquid methyltrioctyl/decylammonium bis 2,4,4-(trimethylpentyl)phosphinate denoted as R4NCy was synthesized according to procedures published in the literature [35,36], mixing equimolar ratio of Cyanex 272 (HCy) and Aliquat 336 (R4NCl), and dissolved the mixture in kerosene to obtain the desired IL concentration. The mixture of Cyanex 272 and Aliquat 336 was washed twice with sodium bicarbonate 0.5 M to remove the chloride anion and the proton from the organic phase. The general reaction of ionic liquid formation is shown in the following equation [39]: R4NCl + HCy + NaHCO3 = R4NCy + NaCl + CO2 + H2O (1) Schematic structures of the ionic liquids and reagent used in this work are depicted in Figure 1    The reagents used in the organic phase were the commercial extractants trialkyl methylammonium chloride (Aliquat 336), bis 2,4,4-trimethylpentyphosphinic acid (Cyanex 272), kerosene, and sodium hydrogen carbonate. All the reagents were provided by Sigma-Aldrich (St. Louis, MO, USA), except Cyanex 272, provided by Solvay (Santiago, Chile). Aliquat 336 and Cyanex 272 (commercial grade) were used without further purification; all other used compounds were analytical grade.

Liquid/Liquid Extraction
Ionic liquid methyltrioctyl/decylammonium bis 2,4,4-(trimethylpentyl)phosphinate denoted as R4NCy was synthesized according to procedures published in the literature [35,36], mixing equimolar ratio of Cyanex 272 (HCy) and Aliquat 336 (R4NCl), and dissolved the mixture in kerosene to obtain the desired IL concentration. The mixture of Cyanex 272 and Aliquat 336 was washed twice with sodium bicarbonate 0.5 M to remove the chloride anion and the proton from the organic phase. The general reaction of ionic liquid formation is shown in the following equation [39]: R4NCl + HCy + NaHCO3 = R4NCy + NaCl + CO2 + H2O (1) Schematic structures of the ionic liquids and reagent used in this work are depicted in Figure

Liquid/Liquid Extraction
The solvent extraction tests were performed in shake flasks (50 mL), at ambient temperature (22 ± 1 • C), with a magnetic stirring set to 200 rev/min for 20 min. The elemental analysis was measured in an aqueous solution by atomic absorption spectrometry (PinAAcle 900F, Perkin Elmer, Waltham, MA, USA). For extraction tests, 10 mL of aqueous phase containing metal was contacted with a volume of the organic phase to obtain the organic/aqueous phase ratio (O/A). After extraction, two phases were centrifuged at 3000 rpm for 5 min for total phase separation. The aqueous sample was analyzed for metal concentration. The liquid-liquid extraction tests were carried out in duplicate, and a standard deviation of ±2% was obtained for all the tests. The performance of the liquidliquid extractions was determined by Equations (2)-(4) as the extraction percentage, the stripping percentage, and distribution ratio of the specie (D). The stripping procedure was similar to that of extraction tests. The mass balance was confirmed by measuring the concentration of the metals in the stripped aqueous phase: where Otherwise, the mass balances incorporate the phase volumes. Table 2 shows the single metals extraction as a function of the different molar concentrations of R 4 NCy in the organic phase. It is observed that R 4 NCy strongly extracts Fe (III) and Zn(II) in all ranges of concentration of IL. In the case of Cu(II) and Mn(II), the extraction increases when the concentration of R 4 NCy increases, from 44.14% and 30.63% with 0.1 M of extractant to achieving 97.64% and 93.22% respectively with 0.54 M of extractant. These results are totally surprising, considering that the purpose of the current study was to determine the feasibly of solvent extraction with the use of ionic liquid R 4 NCy as an extractant of Cu(II) in the presence of Fe(III), Zn(II), and Mn(II). Therefore, the R 4 NCy would not replace the commercial extractants, for example, LIX and ACORGA type, widely used in the hydrometallurgical copper industry. Anyway, the more efficient extraction of Fe and Zn could be positive, considering that are potential impurities that could affect the process SX.

Effect of the R 4 NCy Concentration in Metal Extraction
The trend to the high efficiency of extraction of Fe(III) and Zn(II) over the Cu(II) and Mn(II), is reported in literature using similar ILs. Devi [42], despite working with lower copper and ionic liquid concentrations, reported that some ILs showed a similar tendency for Cu extraction. In the case of Fe(III) and Zn(II), Regel-Rosocka et al. [43] report high extraction percentage in chloride media using phosphonium ILs (Cyphos IL 101 and Cyphos IL 104). Another study similar to the previous one, Baczyńska et al. [44], uses three phosphonium ILs (Cyphos IL 101, Cyphos IL 104, and Cyphos IL 167) to extract Zn(II) and Fe(III) in membrane process, the result showed a good extraction efficiency. In the case of extraction of Mn(II) and Fe(III), Ola et al. [45] report a similar tendency, the Fe(III) is extracted more efficiently than Mn(II). Ola et al. used Cyphos IL 101 in chloride media. Finally, Nguyen and Lee report good extraction of Mn(II) with R 4 NCy from leaching solution containing Co(II), Ni(II), Mn(II), and Li(I) [46]. The mechanism of metal extraction with R 4 NCy involves both the metal and the medium in which the tests are carried out. Previous work [35] reported of Cu(II) extraction mechanism with R 4 NCy as shown below: This mechanism can be generalized to the rest of the metals in this study, as shown below: nR 4 NCy + M n+ + SO 2− 4 = MCy n · (R 4 N) n SO 4 (6)

Effect of Initial pH in Metal Extraction
The results shown in Figure 3 indicate two hugely different single metal extraction performance tendencies with R 4 NCy. In the case of Fe(III) and Zn(II), the efficiency of extraction is not influenced by initial pH in the range 1 to 3; this result confirms the affinity of R 4 NCy for Fe(III) and Zn(II). The mechanism of metal extraction with R4NCy involves both the metal and the medium in which the tests are carried out. Previous work [35] reported of Cu(II) extraction mechanism with R4NCy as shown below: This mechanism can be generalized to the rest of the metals in this study, as shown below:

Effect of Initial pH in Metal Extraction
The results shown in Figure 3 indicate two hugely different single metal extraction performance tendencies with R4NCy. In the case of Fe(III) and Zn(II), the efficiency of extraction is not influenced by initial pH in the range 1 to 3; this result confirms the affinity of R4NCy for Fe(III) and Zn(II).  On the contrary, the efficiency of extraction of Cu(II) and Mn(II) is clearly influenced by the initial pH of the aqueous solution, decrease the percentage from close 97% at pH 2 and 3 to 65% and 30% at pH 1, respectively. This makes it possible to carry out a multi-element Metals 2021, 11, 1585 6 of 10 extraction at pH values from 2 or, failing that, selective extraction of Fe(III) and Zn(II) at pH 1, like a pretreatment of Cu PLs or recovery of Fe and Zn.
Previous work [35] demonstrates the influence of pH on the Cu(II) extraction using R 4 NCy. A similar tendency of pH effect on Cu extraction is reported by Devi [42]. This research uses the same IL for Cu(II) extraction in a diluted system with low IL concentration. The results are very similar in this study; the efficiency of Cu(II) extraction decrease at low pH. This fact is because the solutions with acid pH and high sulfate concentrations produce the undissociated neutral molecule CuSO 4aq . In the case of Zn(II) and Mn(II), Zhu et al. [47] report a higher Zn(II) extraction over Mn(II) as a function of pH. This research used Cyphos IL 101 as extractant.

Effect of O/A Ratio in Single Metal Extraction
The results show in Table 3 show a total extraction of Fe(III) and Zn(II). This result is impressive because the 1/6 is a high O/A ratio, and the extraction is not affected, again the R 4 NCy demonstrates the high affinity for these cations. On the other hand, Cu(II) and Mn(II) extraction efficiency decreases significantly with the increment of the O/A ratio. The results of these sets of tests show a clear tendency to the high efficiency of extraction of Fe(III) and Zn(II) over the Cu(II) and Mn(II). In this context, to study the O/A ratio, Foltova et al. [48] used quaternary ammonium ionic liquids to extract Co and Sm. The results of Foltova show a similar tendency of this study, the Cu extraction efficiency decay when the O/A relationship is under 0.25 (or 1/4).

Effect of Sulfuric Acid in Metal Stripping
Each loaded organic phase contained 3.25 g/L Cu(II), 1.55 g/L Fe(III), 0.96 g/L Mn(II), and 0.106 g/L Zn(II). Figure 4 shows the stripping performance as a function of the sulfuric acid. It is observed that the sulfuric acid concentration of 2 M all metals are removed over 90% of organic phases. If the acid concentration is reduced to 1 and 0.5 M, the performance decrease significantly in the case of Fe(III); the other metals maintain the best stripping efficiency.
The most interesting aspect of this graph is sensitivity to acid concentration for the stripping Fe(III) loaded IL and the contrast with the stripping performance of the other metals in the study. Cu(II), Zn(II), and Mn(II) were easily stripped from loaded ILs for all acid concentrations studied. In the case of Fe(III), the stripping efficiency is near 95% for 2 M of H 2 SO 4 but decreases significantly at 1 and 0.5 M of acid concentration (65% and 20% respectively). This result is relevant for subsequent studies since it is possible to perform a selective stripping of iron concerning the remaining metals extracted in the IL but at higher sulfuric acid concentrations.  The most interesting aspect of this graph is sensitivity to acid concentration for the stripping Fe(III) loaded IL and the contrast with the stripping performance of the other metals in the study. Cu(II), Zn(II), and Mn(II) were easily stripped from loaded ILs for all acid concentrations studied. In the case of Fe(III), the stripping efficiency is near 95% for 2 M of H2SO4 but decreases significantly at 1 and 0.5 M of acid concentration (65% and 20% respectively). This result is relevant for subsequent studies since it is possible to perform a selective stripping of iron concerning the remaining metals extracted in the IL but at higher sulfuric acid concentrations.
In the literature it has been reported that HNO3, HCl, and H2SO4 solutions can strip the metals in this study from the loaded ionic liquids. In this context, Nguyen and Lee report the stripping of Mn(II) from ALiCY with HCl, and the best performance of 1 M of acid [46]. Tran  , ALiCy, ALiD2, and ALiPC. The results of the stripping tests are very heterogeneous, but in general terms, the best performance of stripping from Fe(III) loaded ILs was using H2SO4 [49].

Effect of O/A in Metal Stripping
The loaded organic phases were similar to the previous point. To concentrate the metals in stripping solution, the O/A ratio was investigated from 1/1 to 6/1 ( Figure 5). The tests showed a different high performance for metals in the study. It is possible to obtain relatively high stripping efficiency for Mn(II) and Cu(II), even at high O/A ratios (6/1), which reach 60% and 30%, respectively. For Zn(II) and Fe(III), the stripping at 3/1 of O/A ratio were 20% and 2%, respectively, and for 6/1 ratio, both metals were not stripped.
To evaluate the effect of the O/A ratio in the stripping efficiency, the relation of organic and aqueous phases was variate from 1/1 to 6/1. Figure 5 shows a clear trend of decreasing stripping efficiency when increasing the O/A ratio from 1/1 to 6/1, but at the same time, the aqueous phase is concentrated with the discharged metals. The Cu(II) and especially the Mn(II) maintain relative ease for being stripped of the organic phase. On the other hand, Zn(II) and Fe(III) have a poor stripping efficiency; both metals practically are not stripped from load IL over 3/1 O/A ratio. In the literature it has been reported that HNO 3 , HCl, and H 2 SO 4 solutions can strip the metals in this study from the loaded ionic liquids. In this context, Nguyen and Lee report the stripping of Mn(II) from ALiCY with HCl, and the best performance of 1 M of acid [46]. Tran [49].

Effect of O/A in Metal Stripping
The loaded organic phases were similar to the previous point. To concentrate the metals in stripping solution, the O/A ratio was investigated from 1/1 to 6/1 ( Figure 5). The tests showed a different high performance for metals in the study. It is possible to obtain relatively high stripping efficiency for Mn(II) and Cu(II), even at high O/A ratios (6/1), which reach 60% and 30%, respectively. For Zn(II) and Fe(III), the stripping at 3/1 of O/A ratio were 20% and 2%, respectively, and for 6/1 ratio, both metals were not stripped. This result is not entirely evident in solvent extraction systems. For example, Quijada et al. [41] reported a stripping efficiency of Fe(III) close to 50% for similar operational conditions, but in this case, the researchers used TFA as extractant and a Bmim-based ionic liquid as diluent. Finally, these results may be useful to design a selective process of separation of metals contained in a PLS of copper, or a pretreatment of this solution to remove To evaluate the effect of the O/A ratio in the stripping efficiency, the relation of organic and aqueous phases was variate from 1/1 to 6/1. Figure 5 shows a clear trend of decreasing stripping efficiency when increasing the O/A ratio from 1/1 to 6/1, but at the same time, the aqueous phase is concentrated with the discharged metals. The Cu(II) and especially the Mn(II) maintain relative ease for being stripped of the organic phase. On the other hand, Zn(II) and Fe(III) have a poor stripping efficiency; both metals practically are not stripped from load IL over 3/1 O/A ratio.
This result is not entirely evident in solvent extraction systems. For example, Quijada et al. [41] reported a stripping efficiency of Fe(III) close to 50% for similar operational conditions, but in this case, the researchers used TFA as extractant and a Bmim-based ionic liquid as diluent. Finally, these results may be useful to design a selective process of separation of metals contained in a PLS of copper, or a pretreatment of this solution to remove Fe(III) and Zn(II).

Conclusions
The present research aimed to evaluate the extraction and stripping of ionic liquid R 4 NCy in the presence of a simulated commercial PLS of copper industry containing Cu(II), Mn(II), Zn(II), and Fe(III). The main conclusions are the following: (1) The extraction tests showed a strongly selective extraction of Fe(III) and Zn(II) over Cu(II) and Mn(II). A practically total extraction of Fe(III) and Zn(II) was obtained for all the conditions under study. In the case of Cu(II) and Mn(II), the extraction efficiency is strongly influenced by the R 4 NCy concentration, the PLS pH, and the O/A ratio.