Extraction of Gold(III) from Acidic Chloride Media Using Phosphonium

Jan 13, 2015 - The present work addresses Au(III) extraction from chloride media using phosphonium-based ionic liquid (Cyphos IL 109) as a novel anion...
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Extraction of Gold(III) from Acidic Chloride Media Using Phosphonium-based Ionic Liquid as an Anion Exchanger Viet Tu Nguyen,†,‡ Jae-chun Lee,*,†,‡ Jinki Jeong,‡ Byung-Su Kim,‡ Gérard Cote,§ and Alexandre Chagnes§ †

Resources Recycling, Korea University of Science and Technology (UST), Daejeon 305-350, Korea Mineral Resources Research Division, Korea Institute of Geoscience and Mineral Resources (KIGAM), Daejeon 305-350, Korea § Institut de Recherche de Chimie Paris, PSL Research University, Chimie ParisTech−CNRS, 11 rue Pierre et Marie Curie, 75005 Paris, France ‡

ABSTRACT: The present work addresses Au(III) extraction from chloride media using phosphonium-based ionic liquid (Cyphos IL 109) as a novel anion exchanger diluted in xylene. Gold(III) is extracted into the organic phase as [P66614+][AuCl4− ] and can easily be stripped by reduction to Au(I) with CS(NH2)2/HCl or Na2S2O3. A series of extraction-stripping cycles show a possible recirculation of used solvent without loss of performance, as far as the extraction of gold(III) from HCl/Cl− media is concerned. Au(III) was quantitatively (99.4%) extracted from 0.1 mol L−1 HCl initially containing 100 mg L−1 Au(III) with 0.8 g L−1 Cyphos IL 109 with two counter-current stages at O/A = 1, whereas total stripping as Au(I) with 0.02 mol L−1 CS(NH2)2 in 5% HCl or 0.02 mol L−1 Na2S2O3 also needed two counter-current stages, at O/A = 3. Simulations of counter-current modes reveal Cyphos IL 109 can be considered a promising new extractant for complete recovery of Au(III) from Cl− media in pilot scale.

1. INTRODUCTION With the growth of the electronics industry, the demand for gold has increased markedly from year to year. Recovery of gold from low-content ores and secondary resources such as electronic scrap and waste electroplating solutions has become a strategic topic that has received significant attention in recent decades. Although low-grade ores are usually treated by bioleaching or cyanide processes, electronic wastes are leached with acidic solutions. Subsequently, gold can be recovered from the leachates via a number of processes, including adsorption,1−3 liquid−liquid extraction,4−7 ion exchange,8−10 and supported liquid membrane.11,12 Among these processes, solvent extraction is considered one of the most powerful techniques, offering several advantages such as operation in a continuous mode and use of relatively simple equipment at both laboratory and industrial scales.13 Selective extraction of gold(III) from acidic solutions has been extensively studied using liquid anion exchangers (protonated Alamine 336, Aliquat 336)14 and solvating extractants (Cyanex 921, Cyanex 923).15,16 For sustainability of the liquid−liquid extraction, it is necessary to find “greener and safer” extractants to replace the traditional organic solvents. Room temperature ionic liquids (RTILs) are attracting increasing attention in various fields because they constitute a new class of solvents with negligible vapor pressures, nonflammability, high polarity, high thermal stability and tunable properties.17−21 Various authors have proposed to use RTILs directly as organic phases (i.e., without dilution in an organic diluent) in liquid−liquid extraction.22−26 In particular, Rout et al. have proposed the separation of rare earths and nickel using a liquid−liquid system consisting of two immiscible ionic liquids.26 © 2015 American Chemical Society

However, RTILs are highly viscous and most authors take advantage of RTILs by considering them as new extractants to be used in the classical way, i.e., diluted in organic diluents. In particular, phosphonium-based salts recently developed by Cytec have been studied as extractants in various diluents. These phosphonium-based salts are a new subclass of ionic liquids with three-dimensional cation structures, offering good miscibility with nonpolar diluents and low solubility in aqueous phases. More precisely, Cytec proposed three phosphoniumbased ionic liquids: Cyphos IL 101 (tetradecyl(trihexyl)phosphonium chloride, [P 66614 + ]Cl − ), Cyphos IL 109 (trihexyl)tetradecylphosphonium bis (trifluoromethylsulfonyl) imide, [P66614+][NTf2−]) and Cyphos IL 104 (trihexyl(tetradecyl)phosphonium bis-2,4,4-trimethylpentyl)phosphinate, [P 6 6 6 1 4 + ][R 2 POO − ]). Cyphos IL 101 ([P66614+]Cl−) has been used after dilution in an appropriate organic diluent for the extraction of various metals, including palladium(II), zinc(II) and iron from chloride solutions.27−30 Cyphos IL 104 (trihexyl(tetradecyl)phosphonium bis(2,4,4trimethylpentyl)phosphinate, [P66614+][R2POO−]), was also tested for extraction of the metals palladium(II), cobalt(II), nickel(II) and iron.28,30−32 Some RTILs were also used after immobilization on supporting polymers (e.g., Amberlite XAD7, biopolymer capsules)33−35 or inclusion in polymer inclusion membranes.30 In most of the cases studied, the ionic liquids can extract metal ions (under the form of anionic complexes) via an ion exchange mechanism. Campos et al.34 investigated solvent Received: Revised: Accepted: Published: 1350

November 20, 2014 January 13, 2015 January 13, 2015 January 13, 2015 DOI: 10.1021/ie5045742 Ind. Eng. Chem. Res. 2015, 54, 1350−1358

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Industrial & Engineering Chemistry Research Table 1. Characteristics of the Phosphonium-based Ionic Liquid, Cyphos IL 10937

2. MATERIALS AND METHODS 2.1. Materials. The ionic liquid Cyphos IL 109 was kindly supplied by Cytec Industries Inc. (Canada) and was used without further purification. The structure and physicochemical properties of [P66614+][NTf2−] are summarized in Table 1.37 Because the solubility of [P66614+][NTf2−] in aliphatic kerosene is limited, extra pure xylene (>99%, b.p.p 412 K, Junsei Chemical Co., Japan) was used as the diluent to overcome some drawbacks of the high viscosity typical of RTILs. Stock solutions of 100 mg L−1 (i.e., 0.51 mmol L−1) Au(III) were prepared from HAuCl4 (Merck A. R.) in HCl solutions. NaCl (>99.9%, Junsei Chemical Co., Japan) was used for adjusting chloride concentration in feed solution. All other chemicals used were of analytical grade (Junsei Chemical Co., Ltd.). 2.2. Methods. Before the extraction experiments, the organic phases were pre-equilibrated with water and then 1.0 mol L−1 HCl and/or 1.0 mol L−1 NaCl solution. The concentrations of chloride ions in the aqueous solutions were determined by means of the Mohr method (direct precipitation titration with AgNO3 using potassium dichromate as an indicator). The preliminary tests confirmed that there is no risk of conversion of [P66614+][NTf2−] into [P66614+]Cl− during the pre-equilibration treatment. Batch extraction experiments were conducted using 10 mL vials. Unless otherwise stated, equal volumes of Au(III) solutions and [P66614+][NTf2−] dissolved in xylene were shaken on a Recipro Shaker (RS-1, Jeio Tech, Korea) at room temperature (298 ± 1 K) for 10 min to ensure equilibrium. After phase disengagement, the aqueous phase was properly diluted and the metal content was measured using an atomic absorption spectrometer (PerkinElmer model AAnalyst-400). The concentration of metals in the organic phase was calculated

extraction for the recovery of gold(III) from a chloride solution using a solution of [P66614+]Cl− in toluene and hexane immobilized in biopolymers. AuCl4− was bound to [P66614+] through an electrostatic attraction mechanism with a 1:1 stoichiometry. In another study, Navarro et al.35 used [P66614+]Cl− impregnated on Amberlite XAD-7 for Au(III) recovery from HCl solutions. The Au(III) recovery proceeded through a combination of extraction processes including sorption onto the polymer matrix and extraction by the ionic liquid. Gold(III) was extracted via an ion exchange mechanism with 1:1 interaction between AuCl4− and the [P66614+] cation. To date, no studies have been performed on solvent extraction of gold(III) using Cyphos IL 109. More common than most other anions in ILs, [NTf2−] has attracted tremendous interest, mostly due to its favorable properties. The presence of [NTf2−] as counteranion increases the solubility of the ionic liquids in nonpolar solvents as well as their thermal stability. In addition, [NTf2−] is considered a noncoordinating anion, which interacts weakly with phosphonium cation and favors Au(III) extraction from aqueous solution via anion-exchange mechanisms.36 Here for the first time, we report systematic studies on the solvent extraction process for the recovery of gold(III) from a chloride medium using a phosphonium-based ionic liquid, [P66614+][NTf2−]. Numerical treatments and graphical methods have been used to determine the stoichiometry of the complexes extracted, then the stoichiometry has been confirmed by Job’s method and Fourier transform infrared (FTIR) spectra. Thermodynamic parameters were estimated, providing useful insights into the solvent extraction system. In addition, stripping of gold(III) from loaded organics was considered, and the recycling of the organic phase was investigated in a series of extraction-stripping cycles. 1351

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Industrial & Engineering Chemistry Research from the difference between the metal concentration in the aqueous phase before and after extraction. FTIR spectra of the organic phases were recorded with a Nicolet 6700 FT-IR spectrometer (Thermo Scientific). FTIR samples were prepared by the procedure of spreading one drop on germanium disk (optically transparent in the range 6000−450 cm−1), which can remove the diluent prior to FTIR spectral analysis. The extraction efficiency (E) was calculated using eq 1: E (%) =

D × 100 D + (Vaq /Vorg)

(1)

where Vaq and Vorg refer to the volumes of aqueous and organic phases, respectively. The distribution ratio, D, was defined as the ratio of the concentration of metal present in the organic phase to the concentration of metal present in the aqueous phase at equilibrium. The stripping experiments were carried out by mixing individual strip solutions such as Na2SO3, Na2S2O3 and thiourea/HCl at O/A = 1 with the previously loaded organic phase using the procedure similar to the procedure described for the extraction experiments. All experiments were performed in triplicate and the results averaged. The estimated error bars were less than 5%. 2.3. Computing. The speciation of gold(III) in aqueous solution was computed by using the MEDUSA software and its database.38

Figure 1. Distribution of Au(III) species as a function of HCl concentration. Total AuCl4− concentration = 100 mg L−1 (0.51 mmol L−1); ionic strength = 1.0 mol L−1; T = 298 K.

situation is favorable for the extraction of gold(III) by a mechanism of anion-exchange. 3.2. Influence of Various Parameters on the Extraction of Au(III). 3.2.1. Effect of Equilibration Time. The kinetics of Au(III) extraction from HCl solution with Cyphos IL 109, fresh or recycled, were investigated. A feed solution of 100 mg L−1 Au(III) (i.e., 0.51 mmol L−1) in 1.0 mol L−1 HCl was contacted with organic phase of 0.4 g L−1 Cyphos IL 109 (i.e., 0.52 mmol L−1), fresh or recycled, in xylene, at a O/A phase volume ratio of 1. Samples were taken at 0.5, 1, 2, 3, 5 and 10 min. The percentage of Au(III) extraction reached 63.7% after mixing for 0.5 min and remained almost constant thereafter. These results indicate that the Au(III) extraction kinetics by Cyphos IL 109 are fast, as expected from an anion exchange mechanism. To ensure attainment of the equilibrium under all conditions, a 10 min contact time was invariably maintained in further experiments. 3.2.2. Effect of Cyphos IL 109 Concentration. To investigate the effect of initial extractant concentration on the Au(III) extraction efficiency and distribution, the Cyphos IL 109 concentration in xylene was varied between 0.2 and 4.0 g L−1. Other parameters, meanwhile, were kept constant at an O/ A phase ratio of 1 and 0.1 mol L−1 HCl. Figure 2 shows that extraction efficiency increases sharply with increasing Cyphos IL 109 concentration from 0.2 up to 2.0 g L−1, where the yield of the extraction has already reached 97.8%. Above 2.0 g L−1 Cyphos IL 109, the yield of the extraction is close to 100% within the experimental error, but the distribution ratio (D) of Au(III) seems to reach a plateau (D = 45.7 at 4.0 g L−1 Cyphos IL 109), whereas one would expect that its values would continue to increase more significantly between 2.0 and 4.0 g L−1 Cyphos IL 109 by mass effect. Finally, neither stable emulsion nor third phase formation was observed in this series of experiments. For further studies of Au(III) extraction, an organic phase containing 0.8 g L−1 Cyphos IL 109 (i.e., 1.03 mmol L−1) was chosen. 3.2.3. Effect of HCl Concentration. The effect of HCl concentration on the extraction behavior of Au(III) using the Cyphos IL 109/xylene system was investigated. The Cyphos IL 109 concentration in the organic phase was 0.8 g L−1 (i.e., 1.03 mmol L−1), while the HCl concentration was varied over the range 0.1−4.0 mol L−1. Examination of Figure 3 shows the

3. RESULTS AND DISCUSSION 3.1. Speciation of Au(III) in HCl Media. The speciation of Au(III) in HCl plays a crucial role as far as extraction with Cyphos IL 109 is concerned, as only anions are expected to be extracted. Gold is present mainly in aqueous chloride solutions as square-planar Au(III) complex ions [AuCli(OH)4−i]−, where 1 ≤ i ≤ 4, depending on the conditions of acidity and chloride concentration. Because [Au(OH)4]− appears above pH = 11, only Au(III)-chloro and chloro-hydroxy complexes are taken into account in the present study (eqs 2−5).39 Au 3 + + Cl− + 3H 2O ⇌ [AuCl(OH)−3 ] + 3H+ log K1 = −0.78

(2)

Au 3 + + 2Cl− + 2H 2O ⇌ [AuCl 2(OH)−2 ] + 2H+ log K 2 = 7.28

(3)

Au 3 + + 3Cl− + H 2O ⇌ [AuCl3(OH)− ] + H+ log K3 = 14.31

Au 3 + + 4Cl− ⇌ [AuCl−4 ] + 4H+

(4)

log K4 = 20.31

(5)

where Ki (1 ≤ i ≤ 4) refers to the equilibrium constants for an ionic strength of 1.0 mol L−1 and a temperature of 298 K. The speciation of gold(III) in aqueous HCl solutions reported in Figure 1 was calculated without correction of activity coefficients for a total concentration of gold(III) of 100 mg L−1 (0.51 mmol L−1) [initially introduced as HAuCl4] and a concentration of HCl ranging between 10−4 and 10 mol L−1.38 Even if deviation due to a variation of the activity coefficients can be expected, examination of this figure shows that gold(III) exists only as AuCl4− above approximately 10−2 mol L−1 HCl, i.e., in the solutions of interest for hydrometallurgy. Such a 1352

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Figure 4. Effect of NaCl concentration on Au(III) extraction. Organic phase = 0.8 g L−1 Cyphos IL 109 (i.e., 1.03 mmol L−1 P66614+); aqueous = 100 mg L−1 Au(III); [HCl] = 0.1 mol L−1; [NaCl] = 0.1− 4.0 mol L−1; O/A = 1; t = 10 min; T = 298 K.

Figure 2. Effect of Cyphos IL 109 concentration on Au(III) extraction. Organic phase = 0.2−4.0 g L−1 Cyphos IL 109; aqueous = 100 mg L−1 Au(III); [HCl] = 0.1 mol L−1; O/A = 1; t = 10 min; T = 298 K.

concentration is responsible for a decrease of Au(III) extraction efficiency. The percentage of extraction decreased from 78.5 to 54.9%, or in terms of distribution ratio (D), from 3.71 to 1.23 when the concentration of NaCl was increased from 0.1 to 4.0 mol L−1. The extraction curve reported in Figure 4 is qualitatively similar to the extraction curve shown in Figure 3. Such observations indicate that the concentration of chloride ion has a strong influence on the yield of extracted Au(III) by Cyphos IL 109. In other words, the effect of HCl concentration reported in Figure 3 is mainly due to the concomitant variation in the chloride ion concentration. The negative effect of chloride ion on the extraction efficiency certainly arises from the limited distribution ratio of [NTf2−] into the aqueous phase at high Cl− concentration. 3.2.5. Effect of Gold(III) Concentration and Mechanism of Extraction. To determine the amount of gold that can be extracted by the Cyphos IL 109-based system, a series of experiments was carried out with various Au(III) concentrations in the feed solution (0.1 mol L−1 HCl), the other conditions being kept constant. The concentration of Cyphos IL 109 was equal to 0.8 g L−1 (i.e., 1.03 mmol L−1). Figure 5 depicts an almost linear decrease in extraction percentage from 97.3 to 38.9% as the initial Au(III) concentration was increased over the range 50−500 mg L−1. Further examination of Figure 5 shows that the amount of Au(III) extracted reaches a plateau corresponding to 195 mg L−1 Au (i.e., 0.99 mmol L−1) and chiefly to a molar ratio Au/Cyphos IL 109 equal to 1, as expected from an anion exchange mechanism leading to the formation of [P66614+][AuCl4−] as already reported by Navarro et al.35 in the case of gold(III) extraction from HCl media with Cyphos IL 101. The stoichiometry of the extracted species was also investigated by the Job’s method.40 Examination of Figure 6 plotted for a phase volume ratio of 1 shows that the (extrapolated) maximum of extraction occurs for [AuCl4−]/ ([AuCl4−] + [Cyphos IL109]) close to 0.52, which confirms the 1:1 Au/Cyphos IL 109 stoichiometry reported above. The extracted species was also characterized by FTIR spectroscopy. Figure 7 shows the infrared spectra of the organic phases before and after extraction at maximum loading capacity. The vibration bands of interest are listed in Table 2.

Figure 3. Effect of HCl concentration on Au(III) extraction. Organic phase = 0.8 g L−1 Cyphos IL 109 (i.e., 1.03 mmol L−1 P66614+); aqueous = 100 mg L−1 Au(III); [HCl] = 0.1−4.0 mol L−1; O/A =1; t = 10 min; T = 298 K.

decreasing trend in Au(III) extraction with the increase in the acidity of the aqueous solution and/or the chloride ion concentration, which varied in parallel with the HCl. At 0.1 mol L−1 HCl, the extraction efficiency of Au(III) was 83.5%, but the extraction efficiency gradually decreases to 62.8% at 4.0 mol L−1 HCl. As a result, the distribution ratio (D) decreased from 5.38 to 2.06. In contrast, Campos et al.34 reported that varying the HCl concentration over a similar range (0.1−5.0 mol L−1) did not significantly affect the extraction efficiency of Au(III) using [P66614+]Cl−. To separate the effects of acidity and chloride ion concentration, a series of experiments was performed by keeping the concentration of HCl constant and increasing the concentration of the chloride ions. 3.2.4. Effect of Chloride Ion Concentration at Constant HCl Concentration. The effect of chloride ion concentration on Au(III) extraction with 0.8 g L−1 Cyphos IL 109 in xylene was determined at a constant HCl concentration (i.e., 0.1 mol L−1 HCl) by adding different amounts of NaCl (0.1−4.0 mol L−1). As shown in Figure 4, an increase of chloride 1353

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The transmission peaks obtained at different wave numbers were found to be in good agreement with the available literature.41,42 Table 2. Vibration Bands Associated with Cyphos IL 109

Figure 5. Effect of Au(III) concentration in the feed solution on the Au(III) extraction. Organic phase = 0.8 g L−1 Cyphos IL 109 (i.e., 1.03 mmol L−1 P66614+); aqueous = 50−500 mg L−1 Au(III); [HCl] = 0.1 mol L−1; O/A =1; t = 10 min; T = 298 K.

bands (cm−1)

vibration

observation

2955 2925 2858 1460 1349 1224 1189 1136 720

C−H stretching of CH3 C−H stretching of CH2 C−H stretching of CH2 P−CH2− deformation band −SO2− stretching, 1349 cm−1 CF3 stretching −SO2− stretching, 1189 cm−1 CF3 stretching long-chain −(CH2)n−, (n > 3)

s, asymmetric s, asymmetric s, symmetric s s, asymmetric s, asymmetric s, symmetric s, symmetric m, in-phase rock

The presence of Au(III) in the organic phase does not modify the FTIR vibration bands of the [P66614+] cation. Conversely, the intensities of the −SO2− (1349 and 1189 cm−1) and CF3 (1224 and 1136 cm−1) vibration bands of the [NTf2−] anion decrease sharply after Au(III) extraction, indicating a decrease in the [NTf2−] concentration in the organic phase during Au(III) extraction. Therefore, FTIR spectroscopy confirms the presence of an anion exchange equilibrium between AuCl4− and [NTf2−] during Au(III) extraction as in eq 6: AuCl4 − + [P66614 +][NTf 2−] ⇌ [P66614 +][AuCl4 −] + [NTf 2−]

(6)

3.2.6. Effect of Temperature. To study the effect of temperature on the Au(III) extraction by Cyphos IL 109, experiments were carried out in a water-jacketed reactor. Temperature was varied over the range 288−348 K using a circulating water bath (±0.2 °C). Other parameters, meanwhile, were kept constant at 100 mg L−1 Au(III) in 0.1 mol L−1 HCl, 0.8 g L−1 Cyphos IL 109 in xylene, and O/A = 1. Figure 8a shows that an increase in temperature from 288 to 348 K leads to a decrease in the percentage of Au(III) extraction from 83.6 to 61.9%. Furthermore, the extraction equilibrium constant, Kex, can be derived from eq 6:

Figure 6. Job’s plot for the Au(III)−Cyphos IL 109 system. Molar fraction of Au(III) was varied from 0 to 1, while keeping the total molar concentration of Au(III) and Cyphos IL 109 at 1.5 mmol L−1; [HCl] = 0.1 mol L−1; O/A = 1; t = 10 min; T = 298 K.

Kex = =

[[P66614 +][AuCl4 −]· [[NTf 2−]] [AuCl4 −]· [[P66614 +][NTf 2−]] D·[[NTf 2−]] [[P66614 +][NTf 2−]]

(7)

Taking logarithm and rearranging eq 7, we have log Kex = log D + log[[NTf 2−]] − log[[P66614 +][NTf 2−]] (8)

The study of Kex variation as a function of temperature allows calculating the change in enthalpy ΔH° for the Au(III) extraction with [P66614+][NTf2−] using the van’t Hoff equation: log Kex =

Figure 7. FTIR spectra of the organic phase before and after extraction at maximum loading capacity.

−ΔH ° +C 2.303RT

(9)

where R is the universal gas constant and C is the integration constant, which was assumed to be constant at a particular temperature under the experimental conditions 1354

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Figure 9. Effect of strip solution concentrations on the stripping of Au(III). Loaded organic phase = 100 mg L−1 Au(III); aqueous = strippant concentration from 10−3−10−1 mol L−1; O/A = 1; t = 10 min; T = 298 K.

Figure 8. (a) Effect of temperature on the extraction of Au(III); (b) Plot of log Kex vs 1/T·103 for Au(III) extraction. Organic phase = 0.8 g L−1 Cyphos IL 109 (i.e., 1.03 mmol L−1 P66614+); aqueous = 100 mg L−1 Au(III); [HCl] = 0.1 mol L−1; O/A = 1; t = 10 min; T = 288−348 K.

with 0.1 mol L−1 thiourea). Compared to cyanide, thiourea has certain advantages such as higher gold recovery, lower toxicity and more friendly toward the environment. The electron configuration of the complexing (or donor) ligand is closely related to the formation of stable complexes with either Au(I) or Au(III). The so-called soft electron donor ligands of the thiourea type prefer metal ions of low valence like Au(I), whereas hard electron donor ligands such as chloride and the other halides prefer high-valence metal ions like Au(III).43 As reported by Argiropoulos et al., during the stripping of Au(III) from the loaded organic, thiourea itself acts as the complexing agent, forming strong cationic complexes with gold(I) and the reductive agent, which reduces Au(III) to Au(I) with the formation of formamidine as the oxidation product.44 The reduction of Au(III) to Au(I) followed by Au(I) complexation with thiourea during the desorption/stripping of gold bound to an anionic exchange resin with acidic thiourea solution was also reported by Alguacil et al.45 Not only thiourea but also thiosulfate has been considered seriously as a potential substitute for cyanide because thiosulfate generally causes fewer environmental impacts. Stripping by means of thiosulfate exhibits less interference from foreign cations and poses fewer pollution concerns. Because thiosulfate is classified as a soft electron donor ligand like thiourea, the stable complex Au(S2O3)23− with low valence Au(I) was supposed to be formed during the stripping process.43 In further studies, the solutions of 0.02 M thiourea in HCl and Na2S2O3 have been chosen. During the stripping, [P66614+][NTf2−] initially present in Cyphos IL 109 is converted into [P66614+][Cl−] as found in Cyphos IL 101. As shown below, it is not necessary to regenerate [P66614+][NTf2−] to recycle the organic phase. 3.4. Extraction-Stripping Cycles. To verify the recyclability of the organic phase, a series of extraction-stripping experiments was performed as follows. A fresh solution of Cyphos IL 109 was equilibrated with an equal volume of feed solution and after separation, Cyphos IL 109 solution was stripped with either 0.02 mol L−1 CS(NH2)2 in 5% HCl or 0.02 mol L−1 Na2S2O3 solution at an O/A ratio of 3/1. The stripped organic phase was then reused for a new extraction under similar experimental conditions (Figure 10). Examination of

Figure 8b shows that log Kex vs 1/T varies linearly with a slope of 0.821 equal to −ΔH°/2.303R (eq 9). Therefore, the extraction process is exothermic with ΔH° = −15.7 kJ·mol−1. The change in standard Gibbs free energy (ΔG°) and entropy (ΔS°) can be obtained from eq 10. ΔG° = −2.303RT log Kex = ΔH ° − T ΔS°

(10)

The standard Gibbs free energy change (ΔG° = −2.43 kJ mol−1) indicates the Au(III) extraction reaction by Cyphos IL 109 occurs spontaneously. The value of ΔS° (−44.5 J K−1 mol−1) suggests that the degree of order has increased during the extraction process. 3.3. Stripping of Gold from the Loaded Organic Phase. To recycle the organic phase and operate extractionstripping cycles, the stripping step is a critical property. Thus, a loaded organic solution (100 mg L−1 Au(III)) obtained from the extraction stage was used for testing deionized water, HCl, H2SO4 and HNO3 (6.0 mol L−1) as stripping solutions. However, the stripping efficiency of gold was lower than 8% with all of the above stripping solutions, probably because of the strong ionic bonds between [P66614+] and [AuCl4−] in the organic phase. Therefore, reductive stripping was considered using an acidic thiourea, sodium thiosulfate or sodium sulfite solution at various concentrations in the range 10−3−10−1 mol L−1. In the three cases, the stripping kinetics was very fast (equilibrium of gold stripping was achieved within 1 min). Neither emulsion nor precipitation was observed. As shown in Figure 9, the stripping efficiency of gold increased very sharply with an increasing concentration of the stripping agent. In particular, the gold stripping percentage rose significantly to 98.0 and 93.6% using 0.02 mol L−1 acidic thiourea or sodium thiosulfate solution, respectively. In addition, thiosulfate leads to the same stripping behavior as acidic thiourea solution. Meanwhile, stripping with Na2SO3 solution exhibits the lowest stripping efficiency (less than 60.4% Au(III) is stripped even when 0.1 mol L−1 Na2SO3 is used). Under acidic conditions (5% v/v of HCl), thiourea leads to fast and efficient stripping of gold (stripping efficiency = 99.9% 1355

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0.8 g L−1 Cyphos IL 109), X− = NTf2− with fresh Cyphos IL 109 solutions and X− = Cl− with recycled solutions) at different phase ratios from 1/5 to 5/1 while keeping the total volume of phases constant. Two theoretical stages are needed for quantitative extraction of Au(III) using 1.03 mmol L−1 [P66614+][X−] at an O/A ratio of 1/1 while three countercurrent stages are required at O/A = 2/3. Obviously, decreasing the O/A ratio leads to an increase in the number of theoretical stages required for quantitative extraction. Considering the phase ratio as well as the number of theoretical stages, the O/A ratio of 1 is more preferable than a ratio of 2/3 for Au(III). Subsequently, a two-stage counter-current study was performed to confirm the McCabe−Thiele diagram prediction. The steady state was attained within five complete cycles of operation. Figure 12 shows the profile of Au(III) concentration Figure 10. Extraction of Au(III) with regenerated [P66614+][NTf2−]. Organic phase = 0.8 g L−1 Cyphos IL 109 (i.e., 1.03 mmol L−1 P66614+); aqueous = 100 mg L−1 Au(III); [HCl] = 0.1 mol L−1; O/A =1; t = 10 min; T = 298 K.

this figure shows that the recycled organic phase exhibits the same extractive behaviors as the fresh solution of Cyphos IL 109, which indicates that the conversion of [P66614+][NTf2−] into [P66614+][Cl−] has no negative effect and that the organic phase can easily be recycled. However, the Au(III) extraction percentage with the organic phase treated with Na2S2O3 is a little higher than the extraction percentage obtained with organic phases stripped with acidic thiourea, possibly because a small amount of thiourea still remained in the organic phase after stripping, which causes a lower extraction efficiency of gold(III). 3.5. McCabe−Thiele Diagrams. 3.5.1. Extraction Isotherm and McCabe−Thiele Diagram. The Au(III) extraction distribution isotherm and the McCabe−Thiele diagram obtained with fresh or recycled organic Cyphos IL 109 solution are shown in Figure 11. The feed solution containing 100 mg L−1 Au(III) in 0.1 mol L−1 HCl was equilibrated with the organic phase (1.03 mmol L−1 [P66614+][X−] in xylene (initially

Figure 12. Counter-current experiment with two stages for Au(III) extraction. Organic phase = 0.8 g L−1 Cyphos IL 109 (i.e., 1.03 mmol L−1 P66614+); aqueous = 100 mg L−1 Au(III); [HCl] = 0.1 mol L−1; O/ A =1; t = 10 min; T = 298 K.

in the organic and aqueous phases at a steady state in mixerssettlers. The final raffinate (R2−5) containing less than 1 mg L−1 Au(III) corresponds to over 99.0% extraction of Au(III) into the ionic liquid phase. An analysis of the loaded organic phase (LO1−5) containing 99.4 mg L−1 Au(III) confirms the mass balance during the extraction stage. The gold extraction in the first and second stage was found to be 75.2 and 99.4%, respectively, in line with predicted data in the McCabe−Thiele plot. 3.5.2. McCabe−Thiele Diagram and Stripping Simulation. To investigate the number of stages required for stripping of gold in the continuous mode, the McCabe−Thiele diagram was plotted as in Figure 13. The loaded organic phases (100 mg L−1 Au(III)) were mixed with either 0.02 mol L−1 CS(NH2)2 in 5% HCl or 0.02 mol L−1 Na2S2O3 at various O/A ratios from 1/5 to 5/1 while keeping the total volume of phases constant. As observed in Figure 13, two-theoretical stages are needed to quantitatively strip Au(III) from the loaded organic phase at an O/A ratio of 3/1 in both cases. In fact, the two stripping isotherms are very similar. After the first stripping stage, the gold concentrations remaining in the organic were less than 5.0 and 10 mg L−1 using acidic thiourea and thiosulfate solution, respectively. Subsequently, two-stage counter-current stripping experiments were carried out using either 0.02 mol L−1 CS(NH2)2 in 5% HCl or 0.02 mol L−1 Na2S2O3 solution at the O/A ratio of 3/1. Equilibrium gold concentrations in the strip liquor and stripped organic phase are shown in Figure 14. The results match closely with the prediction obtained from the McCabe− Thiele diagram. Furthermore, the mass balance during the stripping was also taken into account. The ultimate strip liquor contained over 298 mg L−1 gold for both cases, which indicated more than 99.9% stripping efficiency. The gold concentration

Figure 11. McCabe−Thiele diagram for Au(III) extraction. Organic phase = 0.8 g L−1 Cyphos IL 109 (i.e., 1.03 mmol L−1 P66614+); aqueous = 100 mg L−1 Au(III); [HCl] = 0.1 mol L−1; O/A = 1/5 to 5/ 1; t = 10 min; T = 298 K. 1356

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Industrial & Engineering Chemistry Research

extraction of gold(III) from HCl/Cl− media is concerned. For instance, Au(III) was quantitatively (99.4%) extracted from 0.1 mol L−1 HCl initially containing 100 mg L−1 Au(III) (i.e., 0.51 mmol L−1) with 1.03 mmol L−1 Cyphos IL 109 in xylene with two counter-current stages at O/A = 1, whereas its total stripping as gold(I) with 0.02 mol L−1 CS(NH2)2 in 5% HCl or 0.02 mol L−1 Na2S2O3 also required two counter-current stages at O/A = 3. Finally, the calculated thermodynamic parameters (ΔH°, ΔS° and ΔG°) suggest that the extraction of gold is spontaneous (−ΔG°) in nature and the process is exothermic (−ΔH°) with the formation of highly ordered complexes (−ΔS°) in the organic phase.

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AUTHOR INFORMATION

Corresponding Author

*J.-c. Lee. Tel. +82-42-868-3613. Fax: +82-42-868-3705. Email: [email protected]. Figure 13. McCabe−Thiele diagram for the stripping of Au(III). Loaded organic phase = 100 mg L−1 Au(III); aqueous = 0.02 mol L−1 thiourea in 5% HCl and 0.02 mol L−1 Na2S2O3; O/A = 1/5 to 5/1; t = 10 min; T = 298 K.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the basic research program of the Korea Institute of Geoscience and Mineral Resources (KIGAM). One of the authors, Viet Tu Nguyen, expresses his warmest thanks to KIGAM for awarding a research fellowship during the course of this study.

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REFERENCES

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Figure 14. Counter-current experiment with two stages for Au(III) stripping from loaded organic using (a) acidic thiourea solution; (b) thiosulfate solution. Loaded organic phase = 100 mg L−1 Au(III); aqueous = 0.02 mol L−1 thiourea in 5% HCl and 0.02 mol L−1 Na2S2O3; O/A = 3/1; t = 10 min; T = 298 K.

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4. CONCLUSIONS As a source of P66614+ cations, phosphonium-based ionic liquid Cyphos IL 109 can be used as a novel ionic exchanger for the liquid−liquid extraction of Au(III) from hydrochloric acid solutions. Gold(III) is extracted into the organic phase as [P66614+][AuCl4−] and can easily be stripped by reduction into gold(I) with 0.02 mol L−1 CS(NH2)2 in 5% HCl or 0.02 mol L−1 Na2S2O3. A series of extraction-stripping cycles showed that the organic phase can be recycled without loss of performance compared to fresh solutions of Cyphos IL 109, as far as the 1357

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