Novel Aqueous Processing of the Reverted Turbine-Blade Superalloy

Jul 2, 2016 - for Rhenium Recovery. Rajiv Ranjan Srivastava,. †,‡. Min-seuk Kim,. †,‡ and Jae-chun Lee*,†,‡. †. Resources Recycling, Kor...
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Novel Aqueous Processing of the Reverted Turbine-Blade Superalloy for Rhenium Recovery Rajiv Ranjan Srivastava,†,‡ Min-seuk Kim,†,‡ and Jae-chun Lee*,†,‡ †

Resources Recycling, Korea University of Science and Technology (UST), Daejeon 305-350, Republic of Korea Mineral Resources Research Division, Korea Institute of Geoscience and Mineral Resources (KIGAM), Daejeon 305-350, Republic of Korea



ABSTRACT: A preliminary study on the hydrometallurgical recovery of rhenium from an engine-reverted superalloy, CMSX-4, was performed. The novelty of this work dealt with (i) the applicability of electro-generated Cl2 in rhenium leaching and (ii) the maneuvering of the organic extractants based on the metal speciation in leach liquor. In a two-step HCl leaching, more than 98% rhenium was selectively leached in the presence of dissolved chlorine (Cl2aq. and Cl−3aq.) in the second step, which was performed after a leaching of thermal-barrier and base-alloy elements in 4 mol L−1 HCl. The leaching kinetics of rhenium indicated a shift from surface diffusion to a chemically controlled reaction with a variance in temperature from 313 to 353 K. The first use of phosphinic acid in molybdenum separation from rhenium in the leach liquor yielded a high separation value (βMo/Re = 839) using 0.067 mol L−1 extractant; whereas the mixed-phosphine oxides showed a tremendous selectivity for separating rhenium from the base-metal impurities in the Mo-depleted solution. The extraction of rhenium increased from 9% to 98% with an increase in extractant concentration from 0.005 to 0.08 mol L−1 in the organic phase. On a metal-to-metal basis, the recovery value of 93% rhenium as NH4ReO4 (9.5 g L−1 Re in the solution) demonstrated the potential of the present process.

1. INTRODUCTION

The reviewed articles on recycling of superalloys revealed that the most scraps and reverted materials are currently being recycled via remelting.3,4 However, an approximate 20% volatilization loss of the scarce elements, such as rhenium, molybdenum, and tungsten, with high energy consumption and purity concerns has caused the remelting process to be nonsustainable.3 The hydrometallurgical recycling process has been identified as a better option,4 albeit the astounding toughness of superalloys creates problems for the smooth processing and recovery of the maximum amount of rare and critical metals alloyed therein. A literature survey revealed that the current research on hydrometallurgical recycling of superalloys is insufficient to provide a sustainable alternate for the typical commercial process of remelting.3,4 This provided the motivation to seek an aqueous processing route that could

Approximately 78% of the worldwide rhenium production is being used for the manufacture of single-crystal superalloys.1 Rhenium’s rare crustal abundance (0.7−1.0 parts per billion) and its specific characteristics of strengthening a superalloy to work under the adverse conditions of a highly oxidative and corrosive environment in the temperature range of 263−1650 K (mainly in jet engines and gas turbines of power plants) have presented it as a critical metal to be alloyed in superalloys.2 The major exploitation of rhenium as a byproduct of molybdenites processing with their limited availability in certain geopolitical areas has remained the factual criticality in rhenium metallurgy.1−5 A high rhenium content in modern superalloys (usually 3−6 wt %, which contributes to >10-fold higher value than the total cost of other 97 wt % alloyed elements),4 make their reclamation from reverted superalloys much attractive. Nevertheless, the toughness and refractory nature of superalloys have endured the recovery of critical metals, which is currently a much demanding form of recycling. © XXXX American Chemical Society

Received: February 26, 2016 Revised: June 30, 2016 Accepted: July 2, 2016

A

DOI: 10.1021/acs.iecr.6b00778 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Figure 1. Closed loop aqueous processing scheme for in situ Cl2 generation−leaching−solvent extraction operations to recover rhenium from reverted turbine-blade superalloys.

Figure 2. (a) As-received reverted turbine-blade superalloy CMSX-4 and (b) the drilled samples of superalloy used in the leaching study.

an emphasis on rhenium recovery, the results presented have the potential to expand an efficient hydrometallurgical recycling process of reverted superalloys. Moreover, the plausible reuse of unconsumed acid, Cl2, and regenerated organic solvents presents an immense advantage of this aqueous processing.

address the above-mentioned problems. However, the separation of metals, those occurring together in natural minerals, has been challenging from an economic perspective and due to their complex chemistry.6 Therefore, we herein report a cutting-edge leaching−solvent extraction process in the context of rhenium recovery (scheme shown in Figure 1) from a reverted turbine-blade superalloy, CMSX-4. In this study, the novel applicability of electro-generated Cl2 in rhenium leaching from the superalloy was investigated. The critical problem of rhenium and molybdenum separation was addressed by maneuvering the organic extractants. Notably, to achieve the liquid−liquid separation of metals, a correlation among previous studies on rhenium solvent extraction and the aqueous chemistry of the metals was referenced.5,7−12 In the present study, an HCl leaching was performed to selectively extract the base-alloy element (nickel) and thermalbarrier elements (e.g., Al, Cr, Co, and W) before they attacked on the rhenium.13 The obtained residues underwent the next step of leaching in the presence of electro-generated chlorine for rhenium extraction in the HCl solution. For this purpose, the effect of influential parameters on rhenium leaching such as temperature, acid concentration, and chlorine supply rate has been presented in detail. Subsequently, the second-step leach liquor was processed for the separation and recovery of rhenium by employing the solvent extraction techniques. First, we revealed the plausible separation of molybdenum and rhenium by employing a phosphinic acid extractant, followed by rhenium recovery with a mixture of 4-trialkyl phosphine oxides.14 The required concentration of extractants was optimized, and the extraction stoichiometry was investigated using supportive spectroscopic analysis. Furthermore, a batch counter-current processing of rhenium-bearing leach liquor at different circuits of solvent extraction was also performed to better understand the distribution of metals at each stage. With

2. EXPERIMENTAL SECTION 2.1. Materials. The reverted turbine-blade superalloy of type CMSX-4 (shown in Figure 2a) was received from Japan. To prepare the sample for leaching studies, the as received superalloy was drilled using a tungsten carbide drill. The chemical composition of the drilled sample (in the form of swarf as shown in Figure 2b) used is given in Table 1. The Table 1. Elemental Composition of the Superalloy CMSX-4 (in wt %) Sample Used in This Study metal in wt % Ni

Co

Cr

Ta

W

Al

Re

Ti

Mo

61.6

9.0

6.5

6.5

6.0

5.6

3.0

1.0

0.6

electrochemical cell used for in situ generation of gaseous chlorine was made of high-purity polymethylmetaacrylate, in which the anion exchanger membrane, Neosepta AMX, was employed to separate the cathodic and anodic compartments of the electrochemical cell. The in situ generated chlorine gas was supplied to the leaching reactor for rhenium leaching from the HCl leached residue of the superalloy. The lixiviant, HCl for the leaching of the superalloy, distilled kerosene as organic diluent, strippent NH4OH, and other reagents (H3PO4, H2SO4, HNO3, and HF) for analysis purpose were supplied by Junsei Chemical Co., Japan. The organic extractants bis(2,4,4trimethylpentyl) phosphinic acid (molecular weight 290 g mol−1; purity 85%, specific gravity 0.92) and mixture of four B

DOI: 10.1021/acs.iecr.6b00778 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Industrial & Engineering Chemistry Research trialkyl phosphine oxides (molecular weight 348 g mol−1; purity 93%, specific gravity 0.88) were supplied by Cytec Canada Inc. All the reagents were used without further purification, and the aqueous dilutions were prepared in deionized water. 2.2. Methods. 2.2.1. Chemical Analysis of the Solid Sample. Due to the complexity of the superalloy, its chemical analysis was carefully performed by knowing the chemistry of alloyed elements in it. For the analysis purpose, a 0.5 g (drilled) sample was placed in a 250 mL glass beaker (covered with a watch glass) in which the concentrated minerals acids were introduced in the following order: (i) 50 mL of HCl, (ii) 10 mL of HNO3, (iii) 50 mL of H2O, and (iv) 50 mL of acid mixture of H3PO4:H2SO4:H2O = 10:3:12. The solution was intensely heated to dissolve the sample and to reduce the aliquot volume to yield the dense fumes of phosphoric acid. Then the aliquot was removed from heating, and 25 mL of dil. H2SO4 (35 vol % in water) was added and slowly heated for 10 min. The aliquot was filtered with Whatman 40 filter paper and collected in a 250 mL volumetric flask. After the filter paper was washed with hot water, it was transferred to a Pt-crucible and burnt in a flame before the ash was treated with 5 mL of HF. When the HF was evaporated, 10 mL of dil. H2SO4 (35 vol %) was added in the Pt-crucible and boiled. Thus, the obtained aliquot was mixed with the earlier-collected aliquot in a 250 mL volumetric flask, and after cooling, the volume was made up with distilled water. Then, the mixed aliquot was subjected to proper dilution with 5 vol % of the above-mentioned acid mixture. The metal contents of the prepared sample were analyzed by the inductively coupled plasma−atomic emission spectroscopy (ICP-AES, Model: iCAP6000 series, Thermo Scientific). 2.2.2. Leaching. The leaching experiments were performed in a 500 mL batch scale using a Pyrex reactor fitted with a water bath (Lab. Companion CW-10G) and a mechanical agitator in a closed system. A fixed amount of ground samples of superalloy was charged under stirring (300 rpm) into the preheated HCl solution at the fixed temperature. Leaching of superalloy for the extraction of base-alloy metals was carried out at 20 g L−1 solid-to-liquid ratio (S/L) in the HCl solutions. The leached slurry was filtered to separate the solid and liquid; further, the solid was treated in caustic solution to dissolve the precipitates of tungstic acid and washed with distilled water before undergoing the next leaching step. Thus, the obtained residues were subjected to chloro-leaching to extract rhenium into the HCl solution. The gaseous chlorine was in situ generated in the anodic compartment of an electrochemical cell filled with 1 mol L−1 HCl; whereas, the cathodic compartment contains 6 mol L−1 HCl acting as a reservoir of chloride ions by dissociating HCl via passing the electrical current.15 Three graphite electrodes of 8 mm diameter in each compartment were dipped into HCl solutions by an exposed surface area of 20.6 cm2, and the electrical circuit was connected to a DC power supply (Agilent E3633A, Germany). When a certain amount of current was applied, the HCl gets dissociated into H+ and Cl−. At that time, the following cathodic reaction occurs: 2H+ + 2e− → H 2(g)

The electro-generation of chlorine and its supply rate was varied by varying the current applied in the range of 0.7−2.8 A and calculated using the following equation: Q = It

where Q = charge in coulombs; I = current applied in amperes; t = time in seconds. Thus, generated in situ chlorine was sparged into the leaching reactor connected to polytetrafluoroethylene (PTFE) tubes and valves. The gaseous chlorine solubilizes in HCl solutions as follows: Cl 2(g) → Cl 2(aq)

(4)

Cl 2(aq) + Cl− → Cl−3

(5)

The unconsumed chlorine of the second-step leaching from the reactor was recycled back into the cathodic compartment, making the process eco-friendly, cost efficient, and overall sustainable, according to the following reaction:15 Cl 2(g) + 2e− → 2Cl−

(6)

The leaching efficiencies of metals in each step were determined by the analysis of their concentration in the leach liquor and calculated for individual metals as follows:16 % Leaching = ⎛ Absolute metal in the sample − Absolute metal in leach liquor ⎞ ⎜ ⎟ × 100 Absolute metal in the sample ⎝ ⎠

(7)

For this, the leaching samples were properly diluted with HCl (5 vol %) and analyzed by the ICP-AES. 2.2.3. Solvent Extraction. For the separation and recovery of rhenium the solvent extraction technique was employed in the rhenium-bearing leach liquor of the second-step leaching, and experiments were performed at room temperature (298 ± 2 K). A 20 mL solution of each phase was equilibrated into a 60 mL separating funnel (Pyrex) for 5 min; then after both the phases were settled for 5 min, they were separated. The aqueous phase was analyzed by the ICP-AES to determine the metal concentration therein; accordingly, the percentage extraction of each metal was calculated as follows:16 ⎛ ⎞ [M]org. ⎟⎟ × 100 % Extraction = ⎜⎜ ⎝ [M]org. + [M]aq. ⎠

(8)

where [M]org. and [M]aq. are the concentration of a particular metal into the organic and aqueous phase, respectively. The separation factor of molybdenum-to-rhenium (βMo/Re) was obtained as βMo/Re =

[Mo]org. /[Mo]aq. [Re]org. /[Re]aq.

(9)

The infrared spectra of the organic phases before and after metal extraction were acquired using Fourier transform infrared spectroscopy (Model: NICOLET 380 FT-IR, Thermo Electron Corp.).

(1)

3. RESULTS AND DISCUSSION 3.1. Leaching Studies of Superalloy. The electronic configuration of rhenium [Xe]4f145d56s2 shows that it is a highly stable metal and prevents the oxidation. The half-filled 5d-orbital and fully occupied 6s-orbital extend the need of highly oxidizing environment to solubilize the rhenium into

The chloride ions pass from the cathodic to anodic compartment via a 28 cm2 anionic membrane, which separates the two compartments, where at the anodic electrode surface, chlorine get evolved as 2Cl− → Cl 2(g) + 2e−

(3)

(2) C

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Industrial & Engineering Chemistry Research aqueous solutions;17 whereas, the chlorine nucleus has a capacity to pull the outer electrons closer with its +7 charge. The conversion of hard metal rhenium into an aqueous species by leaching in a HCl solution in the presence of chlorine was supposed to occur in several reaction steps (eqs 10−14), whose thermodynamic values were also calculated as follows:8,18,19

molybdenum could be leached from the superalloy, CMSX-4. A negligible amount of tungsten was analyzed in the leach liquor, even after 240 min of prolonged leaching. But interestingly, almost all the tungsten was extracted from the superalloy, which precipitates as WO3·H2O under the leaching conditions due to the reverse solubility of tungstic acid in HCl solution.21 After a solid−liquid separation and dilute acid washing of the remaining solids, the precipitated tungsten was separated by alkali dissolution.22 The leaching of the superalloy with HCl solution alone (in the absence of chlorine), however, showed selectivity, with ∼20 mg L−1 rhenium found in the leach liquor. The leached residues containing 26.5% rhenium (full composition is given in Table 2a) were subsequently submitted to the next leaching step. 3.1.2. Electro-Chloro Leaching of Rhenium. The advantages of in situ generation of electro-Cl2 during leaching can be understood by the risks that are eliminated regarding the transportation and handling of chlorine and the plausible recirculation of unconsumed Cl2 sent back to the system as presented in eq 6. A relation between the supply rate of in situ generated chlorine and the acid concentration on rhenium leaching as a function of time (shown in Figure 4) was indicated for a negligible effect of the chlorine supply rate on rhenium leaching at higher acidic concentration (8 mol L−1 HCl). Under the investigated range of electro-chlorine supply rates from 0.215−0.85 mmol min−1, the rhenium leaching kinetics was almost the same; however, it improved as the HCl concentration decreased to 4 mol L−1 in lixiviant solution (Figure 4). Regardless the increased solubility of chlorine with respect to increasing HCl concentration, a decrease in rhenium leaching behavior can be corroborated with the plausible passivation on the rhenium surfaces under higher acidic conditions.23 Furthermore, the influence of temperature on rhenium leaching from the HCl-leached residue of superalloy CMSX-4 was investigated. The results (presented in Figure 5) interestingly showed the slowest kinetics at 333 K. Above and below that temperature, the rates of rhenium leaching were almost similar at 313 and 353 K. At these temperatures, ∼98% rhenium could be leached within 60 min, while the extraction efficacy was 333 K was greatly dominated by the temperature effect, which enhanced the leaching rate, regardless of the lower solubility of chlorine at a higher temperature. This can also be understood as the leaching kinetics shifts from the diffusion-controlled to the chemically controlled region with a variance in temperature.24 After obtaining the basic optimization data for rhenium leaching in HCl medium, with a strong oxidizing environment provided by the electro-generated chlorine, a leaching was performed under the optimized conditions of variants. As depicted from Figure 6, within 30 min of chlorine leaching, the maximum amount of rhenium could be leached along with 85% molybdenum from the residues of the first-step leaching. The remaining nickel, cobalt, aluminum, and chromium components (a minor amount could not be leached out in the firststage of HCl leaching) were also extracted in the presence of

HCl + 4H 2O → H 9O+4 + Cl− ΔG°298K = −35.15 kJ mol−1 Cl− + Cl 2 → Cl−3

Re0 + H 9O+4 + 2Cl−3 +

ΔG°298K = 3.77 kJ mol−1

(10) (11)

3 Cl 2 → ReO−4 + 9HCl 2

ΔG°298K = −696.4 kJ mol−1

(12)

ReO−4 + H+ → HReO4 ΔG°298K = −1465.8 kJ mol−1

(13)

HReO4 + HCl → ReO3Cl + H 2O ΔG°298K = −79.3 kJ mol−1

(14)

Notably, the positive value of the standard Gibbs free energy for eq 11 indicated a quasi state of Cl−3aq. whose distribution was 28% along with 72% of Cl2aq. in a solution of 2 mol L−1 HCl, which increased further with an increasing concentration of HCl.20 The oxidizing environment created by chlorine, − preferably as the soluble chlorine species Cl2aq. and Cl3aq. dissolved in high HCl solution, was therefore found to be suitable to extract the metallic rhenium (Re0) into the readily soluble species of Re(VII). Most of the base-alloy elements in the superalloy (∼62 wt % Ni) were easily leached into the acidic solution (without chlorine). 3.1.1. Prior Leaching of the Thermal-Barrier and BaseAlloy Elements. In the first step of leaching, the base-alloy element nickel along with the thermal-barrier metals (cobalt, aluminum, chromium, tungsten) were extracted into the HCl solutions. For this, the superalloy sample was leached in a 4 mol L−1 HCl solution by maintaining an S/L of 20 g L−1 and a temperature of 363 K. The leaching kinetics under these conditions as a function of time is presented in Figure 3. It was observed that the maximum of nickel (99.7%), aluminum (97.5%), cobalt (96.9%), chromium (99.2%), and titanium (89.2%) were leached into HCl within 120 min, while ∼47%

Figure 3. Leaching behavior of metals from the superalloy CMSX-4 as a function of time (conditions: HCl concentration = 4 mol L−1, S/L = 20 g L−1, temperature = 363 K, agitation speed = 300 rpm). D

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Industrial & Engineering Chemistry Research Table 2. Elemental Composition of the Different Solid Residues after the Leaching of Each Stepa

a

entry no.

sample name

Ni

Co

Cr

Ta

W

Al

Re

Ti

Mo

2a 2b

leach residue-1 leach residue-2

1.7 -----

2.6 -----

0.48 -----

62.1 96.4

0.94 -----

1.3 -----

26.5 0.9

1.1 1.6

3.0 0.6

Dotted line in entry 2b represents 0 to 14) rhenium dominates only as the anionic perrhenate, ReO4−.8,12 However, at higher acidic concentration, molybdenum forms cationic MoO22+;26 whereas rhenium tends to form the neutral species (HReO4 and ReO3Cl) of Re(VII).12 In this study, ∼3 mol L−1 free acid was analyzed in the (second step) leach liquor, which clearly indicated that the favorable conditions to separate the Mo−Re were based on their different forms of aqueous species. At 3 mol L−1 HCl (in the leach liquor), the dominance of HReO4 (approximately 90%) has been reported, and it has been observed that the chloride ions have the least influence on rhenium extraction with a phosphine oxides mixture.12 Therefore, it was decided to use a suitable cation exchange organic extractant for the prior removal of molybdenum, and then, a prominent extractant would be employed to uptake the neutral species of rhenium. 3.2.1. Prior Separation of Molybdenum Using a Phosphinic Acid Extractant. Among the numerous available cationic extractants, bis(2,4,4trimethylpentyl)phosphinic acid is a superior one in terms of greater stability under higher acidic conditions (www.cytec.com); thus, it was employed to achieve the desired separation of molybdenum over rhenium. The phosphinic acid remains in a dimerized form, which is caused by H-bonds between the oxygen atoms of the PO groups and protons of the OH groups of neighboring molecules. Incredibly, the extractant known to extract molybdenum from acidic solutions has not been reported for its separation from rhenium and is reported herein for the first time.14 The extraction and separation behavior of molybdenum and rhenium was therefore investigated by varying the extractant concentration within the range of 0.25−4.0 vol % (equivalent to 0.006−0.107 mol L−1) and equilibrated in a unit phase ratio with the leach liquor. The molybdenum extraction was

Figure 4. Leaching kinetics of rhenium as a function of (a) supply rate of in situ generated Cl2 and (b) HCl concentration (conditions for a: HCl = 8 mol L−1, S/L = 5 g L−1, temperature = 363 K, agitation speed = 300 rpm; conditions for b: supply rate of the electro-generated Cl2 = 0.43 mmol min−1, S/L = 5 g L−1, temperature = 363 K, agitation speed = 300 rpm).

Figure 5. Leaching kinetics of rhenium as a function of temperature (conditions: HCl concentration = 4 mol L−1, S/L = 5 g L−1, supply rate of the electro-generated Cl2 = 0.43 mmol min−1, agitation speed = 300 rpm).

Figure 6. Leaching behavior of rhenium and molybdenum from the HCl-leached residues as a function of time (conditions: HCl concentration = 4 mol L−1, S/L = 5 g L−1, Cl2 supply rate = 0.43 mmol min−1, temperature = 363 K, agitation speed = 300 rpm).

E

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POH, respectively.28 The corroborated medium bands at 816 cm−1, 1366 and 1490 cm−1 were for PCH3 bonds. In contrast, the corresponding bands due to the POH vibration and POH stretching at 1600−1800 and 1238 cm−1, respectively, were absent in the spectra of the molybdenumloaded organic phase. The additional bands at 918 and 951 cm−1 correspond to the symmetric and asymmetric valence vibrations of the MoO bond, respectively.29 The analysis of Figures 7 and 8 revealed that when the dimeric molecules of bis(2,4,4trimethylpentyl)phosphine acid are complexed with the metal ion, the proton of P−O−H was displaced by the MoO22+. Thus, following the cation exchange mechanism as

increased from 5% to 95% over the investigated range. This effect can be corroborated to the shift in distribution curve of molybdenum due to the increasing concentration of extractant molecules into the organic phase.27 The results showed that, with the use of up to 0.067 mol L−1 extractant, a negligible amount of rhenium was coextracted and further reached to 3.7% at 0.107 mol L−1 extractant in the organic phase, regardless of the positive effect on the molybdenum extraction. As also shown in Figure 7, the best separation between these

MoO2 2 + + 2 (HR)2 = MoO2 (R·HR)2 + 2H+

(15)

3.2.2. Rhenium Separation over Base Metal Impurities from the Mo-Depleted Leach Liquor. In commercial solvent extraction of rhenium, high aqueous solubility of tributyl phosphate with slow extraction kinetics and the backsolidification of tri-n-octyl phosphine oxide at lower temperature (99% efficacy) over base metals (Ni, Co, and Al). The ammonia stripping of rhenium from the loaded organic phase yielded an overall 93% recovery on a metal-tometal basis. (vi) The novel reclamation of superalloys by the in situ electro-generation of chlorine in a leaching step, the obtainment of a high β(Mo/Re), and the selective recovery of rhenium over other base metal impurities to yield a pure NH4ReO4 product, incorporated with the use of recyclable organic extractants, eventually dispensed the process that is sustainable and promising compared to the widely adopted remelting of superalloys.



is thankful to KIGAM for awarding a research fellowship during this work.



REFERENCES

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AUTHOR INFORMATION

Corresponding Author

*Jae-chun Lee. Tel.: +82428683613. Fax: +82428683705. Email: [email protected]. Notes

The authors declare no competing financial interest.



ACKNOWLEDGMENTS This work was supported by the Korea Institute of Energy Technology Evaluation and Planning (KETEP) and the Ministry of Trade, Industry & Energy (MOTIE) of the Republic of Korea (No. 20165020101170). The author R.R.S. H

DOI: 10.1021/acs.iecr.6b00778 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.iecr.6b00778 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX