Article pubs.acs.org/jced
Solvent Extraction of Rhenium in Ionic Liquid with N235 Da-wei Fang, Zong-ren Song, Zhong-kai Zhou, Wei-jun Shan, and Jun Li* Institute of Rare and Scattered Elements, College of Chemistry, Liaoning University, Shenyang, China, 110036 ABSTRACT: Extraction of rhenium into the ionic liquids phase from aqueous solutions was investigated using N235 as extractant. At temperatures from 293.15 to 318.15 K, the equilibrium molalities of ReO4− were measured at an ionic strength from 0.2 to 2.0 mol·kg−1 in the aqueous phase containing NH4Cl as supporting electrolyte. The standard extraction constants K0 at various temperatures were obtained by methods of the Pitzer polynomial approximation. Thermodynamic properties for the extraction process were calculated.
1. INTRODUCTION Rhenium is an important rare material that has been widely used in modern industry. It exists in low levels in the earth’s crust and is mainly found in chalcopyrite, molybdenite, and copper−molybdenum deposits. Probing a valid method for the recycle and enrichment of rhenium is significant owing to the increasingly demands for high purity rhenium. Many methods, such as capillary electrophoresis, chemical deposition, liquid chromatography, ion exchange, and solvent extraction, are used to separate and enrich rhenium. Solvent extraction provides an effective and simple separation method; it is often used for the recovery of rhenium from molybdenum.1−4 From an environmental viewpoint, ionic liquids (ILs) are green solvents with many unmatched properties, such as negligible vapor pressure, recyclability, and high thermal stability. ILs can be regarded as a sustainable alternative to the generally hazardous molecular diluents. This study adopts 1-pentyl-3-methyl-imidazolium hexafluorophosphate [C5mim][PF6] for its immiscible property with water. In particular, the ionic liquid can be reused in this experiment. In our study, we focused on a fast, simple, and sustainable strategy for rhenium extraction by means of solvent extraction in [C5mim][PF6] with N235. As a continuation of our previous work,5−7 we measured a concentration of perrhenate in the aqueous phase at different ionic strengths in a hydrochloric acid system. The standard extraction constants K0 at various temperatures are obtained by methods of polynomial approximation.8−10 Thermodynamic characters for the extraction process are calculated. In the presence of excessive extractant N235 (structure of N235 in Figure 1), abbreviated as NR3, the extraction reaction is
Figure 1. Structure of N235 (n = 6−8).
where (aq) and (org) refer to the aqueous and organic phase, respectively, NR3 is the extractant N235, and H+·NR3·ReO4− is the extraction complex. The standard equilibrium constant K0 is given by log K 0 = log[m{H+· NR3·ReO4 −}] − log[m{H+} ·m{ReO4 −} ·m{NR3}] + log[γ {H+·NR3·ReO4 −}] − log[γ {H+} ·γ {ReO4 −} ·γ {NR3}]
where γ is the activity coefficient in the molality scale, and m is the molality.
2. EXPERIMENTAL SECTION Water used in the extraction system was doubly deionized, and its conductivity was 1.5 × 10−4 Ω−1·m−1 (Ω is ohm). The HCl was of GR (guarantee reagent) grade, and the ammonium chloride was of AR grade. The [C5mim][PF6], 99% mass pure, Received: April 17, 2017 Accepted: July 18, 2017 Published: August 1, 2017
NR3(org) + H+(aq) + ReO4 −(aq) = H+·NR3·ReO4 −(org) (1) © 2017 American Chemical Society
(2)
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DOI: 10.1021/acs.jced.7b00359 J. Chem. Eng. Data 2017, 62, 2423−2427
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used as the diluent was purchased from Lanzhou Institute of Chemical Physics. The experimental process referenced published articles,4 The difference is that the organic phase was prepared by dissolving N235 in [C5mim][PF6]. The results are shown in Figure 2 and Table 1.
Table 1. Values of pH and Effective Ionic Strength (I) at Temperatures in the Range 293.15−318.15 Ka T/K
temperatures.(■,
Figure 2. Percentage of extraction at different K; red ●, 1298.15 K; blue ▲, 2303.15 K; green ◀, 4313.15 K; chartreuse ▶, 5318.15 K).
▼,
293.15 3308.15 K; pink
The equilibrium molalities (m{i} for the species i) in the organic phase were calculated from the initial molalities a, b, and m{ReO4−} in the aqueous phase: m{H+·NR3·ReO4 −} = [a − m{ReO4 −}]/ρ
(3)
m{NR3} = b − [a − m{ReO4 −}]/ρ
(4)
where ρ is the density of the organic phase.
3. RESULTS AND DISCUSSION The values of pH measured at the different temperatures (in the range 293.15−318.15 K) for several total ionic strengths in the range 0.2−2.0 mol kg−1 are listed in Table 1, where each value of pH is the mean of three replicate measurements with the uncertainty ±0.01. From Table 1 and Figure 2, we can easily discover that the pH values decreased with increasing temperature and ionic strength, and the remaining perrhenate increases with increasing temperature, which meant that low temperature and low ionic strength benefits the solvent extraction system. The extraction efficiencies are mostly over 97% at low temperature, which indicated that the extraction performed completely. Furthermore, perrhenate molalities in the aqueous phase are small at lower temperatures and lower ionic strengths. The best extraction condition with a extraction ratio of 97.07% is at 293.15 K. So in industry, the extraction system should be kept at a high acidity with little impurities at low temperatures. 3.1. Polynomial Approximation to Determine K0. There were four ionic species (H+, NH4+, ReO4− and Cl−) in the equilibrium aqueous phase. The molalities and activity coefficients are m{H+}, m{NH4+}, m{ReO4−}, and m{Cl−}, and γ{H+}, γ{NH4+}, γ{ReO4−}, and γ{Cl−}, respectively. The
293.15
I′ pH ReO4− (10−5)
0.197 1.28 2.933
I′ pH ReO4− (10−5)
0.402 1.26 3.119
I′ pH ReO4− (10−5)
0.500 1.28 3.274
I′ pH ReO4− (10−5)
0.604 1.25 3.305
I′ pH ReO4− (10−5)
0.810 1.21 3.583
I′ pH ReO4− (10−5)
1.011 1.19 3.738
I′ pH ReO4− (10−5)
1.212 1.20 3.893
I′ pH ReO4− (10−5)
1.419 1.17 4.048
I′ pH ReO4− (10−5)
1.517 1.18 4.017
I′ pH ReO4− (10−5)
1.622 1.15 4.358
I′ pH ReO4− (10−5)
1.827 1.14 4.543
I′ pH ReO4− (10−5)
2.033 1.09 4.760
298.15
303.15
I 0.199 1.25 3.119 I 0.407 1.21 3.243 I 0.506 1.23 3.428 I 0.611 1.19 3.398 I 0.816 1.17 3.800 I 1.012 1.18 3.955 I 1.219 1.15 4.110 I 1.427 1.12 4.296 I 1.523 1.14 4.327 I 1.628 1.11 4.667 I 1.834 1.10 4.853 I 2.035 1.08 5.039
= 0.2 0.205 1.19 3.614 = 0.4 0.411 1.18 3.738 = 0.5 0.515 1.15 3.924 = 0.6 0.620 1.13 3.955 = 0.8 0.817 1.16 4.296 = 1.0 1.021 1.12 4.482 = 1.2 1.226 1.11 4.667 = 1.4 1.434 1.08 4.884 = 1.5 1.539 1.05 4.915 = 1.6 1.642 1.04 5.256 = 1.8 1.844 1.05 5.442 = 2.0 2.041 1.05 5.658
308.15
313.15
318.15
0.207 1.18 4.110
0.211 1.18 4.420
0.212 1.14 5.411
0.414 1.16 4.234
0.416 1.16 4.636
0.419 1.12 5.535
0.516 1.15 4.358
0.517 1.15 4.853
0.516 1.15 5.751
0.620 1.13 4.543
0.620 1.15 5.070
0.620 1.13 5.906
0.820 1.14 4.760
0.818 1.17 5.287
0.824 1.12 6.154
1.023 1.11 4.946
1.022 1.13 5.256
1.024 1.11 6.371
1.231 1.08 5.256
1.223 1.14 5.689
1.222 1.14 6.618
1.440 1.05 5.442
1.432 1.10 5.906
1.437 1.07 6.588
1.532 1.09 5.658
1.533 1.09 6.123
1.542 1.04 7.021
1.644 1.03 5.689
1.638 1.07 6.371
1.647 1.02 7.269
1.850 1.02 6.030
1.848 1.04 6.464
1.854 1.00 7.517
2.046 1.03 6.216
2.001 1.01 6.897
2.056 1.01 7.733
a
T is Kelvin temperature, and I is ionic strength (molar concentration). Standard uncertainties u are u(T) = 0.05 K, u(p) = 10 kPa, and the expanded uncertainty is U(m) = 5 × 10−7 mol·kg−1, U(pH) = 0.01 (0.95 level of confidence).
effective ionic strength I′ in the equilibrium aqueous phase can then be calculated as I′ =
1 1 ΣmiZi 2 = (m{ReO4−} + m{NH4 +} + m{Cl−} + m{H+}) 2 2 (5)
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DOI: 10.1021/acs.jced.7b00359 J. Chem. Eng. Data 2017, 62, 2423−2427
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m{NH4 +} = m{NH4Cl} + a
(6)
m{Cl−} = m{NH4Cl} + c
(7)
to I/m0, and is the third virial coefficient similarly defined but for three ions with charges not all of the same sign. According to Pitzer and Kim,15
+
and the [H ] was obtained by pH values. The calculated values of I′ are listed in Table 1. Equation 2 could be expressed as log K 0 = log[m{H+· NR3·ReO4 −}] +
−
− log[m{H } ·m{ReO4 } ·m{NR3}] +
−
− log[γ {H } ·γ {ReO4 }]
(8)
γ{ReO4−}
′ = βca(1)y2 Bca
(16)
y1 = 2[1 − (1 + α{I /m0}1/2 ) exp( −α{I /m0}1/2 ) /(α 2{I /m0})
× exp( −α{I /m0}1/2 )]/(α 2{I /m0}2 )
a
+
∑ (mc /m )(2ΦMc + ∑ (ma /m )ψMca) 0
0
c
+
In using Pitzer’s equations to determine K it is assumed that (1) the effective ionic strength is regarded as the total ionic strength in the aqueous phase; (2) interactions between ions can be regarded as those between ReO4−, H+, and the ions of the supporting electrolyte; (3) following the advice of Pitzer and Mayorga,16,17 ΨMca, is considered to be independent of ionic strength. In estimating γ{ReO4−} and γ{H+}, all the mixed parameters (Φij, Φ′ij, and ψijk) are neglected, so that the pertinent combination of activity coefficients may be written as
∑ ∑ (ma /m0)(ma′/m0)ψMaa′ a′
+ |Z M| ∑ ∑ (mc /m0)(ma /m0)Cca c
ln γX = z X 2F +
a
(9)
∑ (mc /m0)(2BCX + ZCcX ) c
+
∑ (ma /m0)(2ΦMa + ∑ (mc /m0)ψcX a) a
+
ln γMX = |z Mz X|F
c
+ (vM /v) ∑ ma [2BMa + ZCMa + 2(vX /v)ΦXa ]
∑ ∑ (mc /m0)(mc ′/m0)ψcc ′ X c
a
c′
+ |ZX | ∑ ∑ (mc /m0)(ma /m0)Cca c
a
+ (vX /v) ∑ mc[2BCx + ZCCx + 2(vM /v)ΦMc] c
(10)
+
where F=f +
∑ ∑ (ma /m )(mc /m )B′ca 0
a
+ +
0
+
c
+
c′
a′
+ (2/1.2)ln[1 + 1.2(I /m0)1/2 ]
∑ (mc /m0)|Zc| = ∑ (ma /m0)|Za|
CijP = CijP/2(|zizj|)1/2
∑ ∑ mama′(vM/v)ψMaa′ + 2ψMaa′
+ 2 ∑ mn(vMλnm + vXλn X )/v
(11)
n
a
(19)
Then, substitution of eq 19 into eq 8, yields a working equation
f r = −AP(I /m0)1/2 /[1 + 1.2(I /m0)1/2 ]
c
∑ ∑ mcmc ′(vX /v)ψcc ′ X a
