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Review
Nonaqueous Solvent Extraction for Enhanced Metal Separations:
Concept, Systems, and Mechanisms
Zheng Li,*,∇ Brecht Dewulf,∇ and Koen Binnemans
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sı Supporting Information
*
ABSTRACT: Efficient and sustainable separation of metals is gaining increasing attention,
because of the essential roles of many metals in sustainable technologies for a climateneutral society, such as rare earths in permanent magnets and cobalt, nickel, and manganese
in the cathode materials of lithium-ion batteries. The separation and purification of metals
by conventional solvent extraction (SX) systems, which consist of an organic phase and an
aqueous phase, has limitations. By replacing the aqueous phase with other polar solvents,
either polar molecular organic solvents or ionic solvents, nonaqueous solvent extraction
(NASX) largely expands the scope of SX, since differences in solvation of metal ions lead to
different distribution behaviors. This Review emphasizes enhanced metal extraction and
remarkable metal separations observed in NASX systems and discusses the effects of polar
solvents on the extraction mechanisms according to the type of polar solvents and the type
of extractants. Furthermore, the considerable effects of the addition of water and
complexing agents on metal separations in terms of metal ion solvation and speciation are
highlighted. Efforts to integrate NASX into metallurgical flowsheets and to develop closed-loop solvometallurgical processes are also
discussed. This Review aims to construct a framework of NASX on which many more studies on this topic, both fundamental and
applied, can be built.
where clp and cmp, Vlp and Vmp are, respectively, analytical
concentrations and volumes in the organic (less polar) phase
and the aqueous (more polar) phase, respectively; DA and DB
are distribution ratios of metals A and B, respectively.
Both high extraction efficiency and high separation factors
are desired in SX. There are multiple methods to optimize an
SX system, and the focus has been mainly on the composition
of the organic phase. Most common approaches include (1)
modifying the structures of the extractants to improve
selectivity;22−25 (2) using a second extractant to enable
synergism (synergistic solvent extraction);4,13,26 (3) adding
modifiers to prevent the third phase formation or enhance
stripping;27−29 and (4) changing diluents.30,31
Compared to the modifications of the organic phase, the
number of studies investigating alterations to the aqueous
phase is limited. Adjustment of pH is most effective for
improving separations in SX systems involving acidic
extractants. The change of salts concentration (ionic strength)
is useful for neutral and basic extractants due to the salting-out
effect. A less-common practice is the use of complexing agents,
1. INTRODUCTION
Solvent extraction (SX) or liquid−liquid extraction is one of
the most widely performed hydrometallurgical techniques for
the separation and purification of precious metals,1−3 base
metals,4,5 rare-earth elements (REEs),6−9 actinides,10−12 alkali
and alkaline-earth metals,13−20 and so on. This unit operation
is based on differences in distributions of metal ions between
two immiscible phases, typically an aqueous feed phase
containing the metals to be separated and an organic extract
phase containing an extractant diluted in a diluent.21 Metal
ions are often highly hydrated in the aqueous phase and, hence,
are highly hydrophilic and insoluble in the organic phase. The
transfer of metal ions from the aqueous phase to the organic
phase is facilitated by the coordination of the metal ions to the
extractant to form hydrophobic complexes.
The percentage extraction (%E), distribution ratio (D), and
separation factor (α) are the most important parameters in
solvent extraction. They are defined as follows:
c lp
D=
cmp
(1)
%E =
α=
c lp·Vlp
c lp·Vlp + cmp·Vmp
DA
DB
Received: June 13, 2021
Revised: October 6, 2021
Accepted: October 27, 2021
× 100
(2)
(3)
© XXXX The Authors. Published by
American Chemical Society
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NASX is largely different from SX from aqueous solutions, and
enhanced separations compared to conventional SX systems
are often observed. However, strictly anhydrous conditions are
not required to perform NASX, the presence of a small
quantity of water in a NASX system is not only tolerable but
can also be beneficial in some cases. It can reduce the viscosity
of the more polar phase, reduce the miscibility of the two
phases, increase the solubility of salts in the system, and even
enhance metal separations by tuning metal ion solvation.
In this Review, we comprehensively summarize NASX of a
wide range of metals from both PMOSs and ionic solvents
[molten inorganic salts, molten hydrates, and ionic liquids
(ILs)], encompassing publications from the earliest explorations in the 1950s to the latest research in 2021. Emphasis is
given to enhanced extractions and improved separations, and
the chemical mechanisms behind the unusual extraction
performance are explained. Finally, the integration of NASX
to metallurgical flowsheets is discussed.
e.g., lactic acid, EDTA, in the aqueous phase to selectively
complex metal ions to enhance the separation. The most
notable process using this approach is the TALSPEAK process
used in the processing of spent nuclear fuel, where the
separation of lanthanides and actinides is enhanced using
aqueous complexing agents.11,32 This approach was also used
for REEs separations.33,34
Researchers have also observed that the addition of polar
organic solvents to the aqueous solution modifies the
extraction behavior.35−41 Because of the addition of polar
organic solvents, the aqueous−organic mixture could no longer
be called aqueous phase, hence Alian et al. named it the “polar
phase”, and the organic phase is correspondingly called the
“nonpolar phase”, although the phase is also slightly polar.42
Later, the two phases are renamed the “more polar phase” and
the “less polar phase” by Batchu et al.43 The addition of polar
organic solvents influences the extraction performance from
several dimensions:44 (1) lowering water activity; (2) changing
(lowering) the dielectric constants that affect the stability of
complexes; (3) changing the mutual solubility of the two
phases; and (4) altering the interfacial tensions. Also of
importance is the solvation of the metal ion by the polar
solvent. The Gutmann donor number, which is a measure for
the basicity of a solvent, can be a reference for solvation
strength. It is defined as the negative enthalpy value for the 1:1
adduct formation in dichloroethane between a Lewis base (the
solvent) and SbCl5, which is a standard Lewis acid.45 These
factors are not independent but are closely connected.
Consequently, no simple correlations with a single factor can
be made.
Water has been the default solvent for the more polar phase
in solvent extraction of metals. However, it is not the only
suitable solvent. Those solvents that can form two immiscible
phases with the less polar phase can, in principle, substitute
water. The use of polar molecular organic solvents (PMOSs)
instead of water can be understood as the use of an aqueous−
organic mixture that proceeds to the extreme of zero water. On
the other hand, when the addition of salts to the aqueous phase
goes to the extreme, the more polar phase becomes purely
(molten) inorganic salts. Moreover, organic salts (ionic
liquids) can form two immiscible phases with the less polar
phase as well. SX of metals from pure nonaqueous solvents, or
with a small quantity (<50 vol %) of water, is called nonaqueous
solvent extraction (NASX).46 Contrary to a conventional SX
system, which contains an organic phase and an aqueous
phase, a NASX system has two nonaqueous phases (Figure 1).
2. EXTRACTANTS AND EXTRACTION MECHANISMS
There are four main types of extractants in SX systems, namely,
acidic extractants (cation exchangers), neutral extractants
(solvating extractants), basic extractants (anion exchangers),
and binary extractants (acid−base extractants). Each type of
extractant extracts metal ions via different mechanisms, and
they have been overviewed by, for instance, Wilson et al.47 and
Eyal et al.48 All four types of extractants have been applied in
NASX systems.
The mechanism of metal extraction by an acidic extractant
(denoted by HL) can be expressed in a simplified way as
Mn + + nHL ⇆ MLn + nH+
(4)
where Mn+ is a metal cation, and the overbar indicates that the
species reside in the organic phase. During the extraction, the
proton of the extractant is replaced by the metal cations. This
is why acidic extractants are also called cation exchangers. The
extraction and stripping equilibria are controlled by pH, as
shown by eq 4. Bis(2-ethylhexyl)phosphoric acid (D2EHPA),
bis(2,4,4-trimethylpentyl)phosphinic acid (Cyanex 272), and
Versatic Acid 10 (a mixture of C10 carboxylic acids; see Figure
2) are the most common acidic extractants.
The extraction of metal ions by a neutral extractant
(denoted by S), in this example from nitrate media, proceeds
as
Mn + + n(NO3)− + xS ̅ ⇆ M(NO3)n ·Sx
(5)
Neutral extractants solvate the metal ions; hence, they are
also called solvating extractants. The extraction equilibrium is
heavily dependent on the activity of the counteranion, e.g.,
nitrate in eq 5. On the other hand, the extracted metals can be
stripped by acids, because neutral extractants have a tendency
to interact with acids. Tri-n-butyl phosphate (TBP), tri-n-octyl
phosphine oxide (TOPO), and Cyanex 923 (a mixture of
trialkyl phosphine oxides) are the most common neutral
extractants (Figure 2).
Amines (primary, secondary, tertiary, and quaternary) are
classified as basic extractants, although quaternary amines are
not Brønsted bases. Take Aliquat 336 (a mixture of
methyltrialkylammonium chloride where the alkyls are nhexyl and n-octyl, denoted by NR4Cl, Figure 2) as an example.
The extraction reaction from chloride media is
Figure 1. Comparison of (left) conventional solvent extraction and
(right) nonaqueous solvent extraction (NASX) systems.
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Figure 2. Structures of typical extractants.
Mn + + nCl− + x NR 4Cl ⇆ [NR 4]x ·[MCl n + x]
3. POLAR MOLECULAR ORGANIC SOLVENTS
3.1. Formation of Two Immiscible Liquid Phases. To
perform NASX, first of all, a pair of two immiscible liquid
phases should be selected. The miscibility of two solvents is
governed by the rule of “like dissolves like”. The Hansen
solubility parameters are one of the most popular criteria to
determine the likeness of solvents and solutes. Three
parameters, δd, δp, and δh, which represent the energy from
dispersion force, dipolar intermolecular force, and hydrogen
bonds between molecules, respectively, are utilized. The total
parameter of a compound, δT, can be calculated from the three
parameters. The closer the parameters of a solute to that of a
solvent, the higher solubility the solute has in the solvent.52
The Hansen solubility parameters of the involved solvents in
NASX systems are given in Table S1 in the Supporting
Information. Based on these empirically regressed parameters,
this methodology allows one to quantitatively estimate the
mutual solubility of a wide range of solvents.
The miscibility of two solvents can be approximated more
simply based on the polarity and hydrophobicity of the
solvents. Polarity is reflected by the dipole moment of the
solvent (Table S1), or δp of the Hansen solubility parameters.
Generally, compounds with symmetric molecular structures
have low polarity. For example, linear saturated hydrocarbons
are nonpolar (δp = 0). Hydrophobicity (lipophilicity) is most
commonly expressed by the distribution (partition) of the
solvent between octanol and water.53 The higher the
distribution ratio of the solvent, the more hydrophobic the
solvent. For compounds with the same functional groups,
longer carbon chains facilitate higher hydrophobicity. Hydrophobicity is also related to the dielectric constant (ε).
Generally, a higher hydrophobicity corresponds to a lower
(6)
Although it is disputable whether the anionic complex is the
most extractable species in the aqueous phase,49 the observed
complex in the organic phase is an ion-pair complex, as if the
chloride anion of Aliquat 336 is exchanged with the
chlorometallate anion (e.g., [FeCl4]−). Hence, quaternary
amines are also called anion exchangers. Primary, secondary,
and tertiary amines (e.g., tri-n-octyl amine, TOA, Figure 2)
extract metal ions following a similar mechanism as quaternary
amines except that the former amines have to first take up
protons to transform into the ammonium form. Phosphonium
extractants (e.g., Cyphos IL 101) are also regarded as basic
extractants considering the similarity of phosphonium and
ammonium ions.
Binary extractants are composed of the cation of a basic
extractant and a deprotonated acidic extractant; hence, they are
also called acid−base extractants.48 A binary extractant works in
a way similar to an acidic extractant, but it extracts salts instead
of metal cations. The extraction of salts by an ammoniumbased binary extractant (denoted by NR4L) from chloride
media can be written as
Mn + + nCl− + n NR 4L ⇆ MLn + n NR 4Cl
(7)
Cyphos IL 104 is an example of a binary extractant (Figure
2). However, the boundary between the different categories of
extractants is not very sharp. For instance, acidic extractants
can act as solvating extractants if the acidity of the feed
solution is high.50,51
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Supporting Information. Larsen performed the first exploratory
study on NASX for the separation of Zr(IV) and Hf(IV) using
a system consisting of isoamyl ether and acetonitrile as two
immiscible phases.66 The utilization of NASX instead of a
conventional organic/water SX system was driven by the fact
that ZrCl4 and HfCl4 hydrolyze strongly in aqueous solutions,
which can be avoided when less-reactive solvents than water
are used. Both ZrCl4 and HfCl4 preferentially resided in the
acetonitrile phase and a separation factor αHf/Zr of 1.8 was
obtained. Following the same route, distributions of several
metals (Co(II), Fe(III), Mo(V), Mo(VI), Sn(II), Sn(IV)) as
chlorides, bromides, nitrates, and thiocyanates were examined
between a diethyl ether phase and a more polar phase
containing either 2-aminoethanol, formamide, or hexanedinitrile.67 All the salts distributed mainly in the more polar phase,
except for SnCl4, which showed some preference for the
diethyl ether phase. The low distribution ratios of the metals
are caused by the low polarity and high hydrophobicity of
diethyl ether, as indicated by the low dipole moment and low
dielectric constant (Table S1). However, SnCl4, as a Lewis
acid, can react readily with organic Lewis bases, such as diethyl
ether,68 leading to a higher distribution ratio of SnCl4.
A two-phase system formed by methyl isobutyl ketone
(MIBK) and formamide was used to separate Tl(III) from
In(III), Ga(III), Fe(III), Sn(II), and Sn(IV).69 These metals
were first loaded to MIBK from an aqueous phase containing
2.0 mol L−1 HBr, presumably as ion pairs (i.e., [H(MIBK)2][TlBr4]), and subsequently stripped to a formamide phase with
a lower HBr concentration. Tl(III) was largely maintained in
the MIBK phase, while other metals were quantitatively
stripped to the formamide phase, leading to efficient separation
of Tl(III) from other metals. The retention of Tl(III) in the
MIBK phase is most likely due to the stronger tendency of
Tl(III) to form anionic bromo-complexes than other metal
ions. However, no comparison of separation capabilities of the
NASX system with aqueous SX systems was performed.
3.3. Systems with Neutral Extractants. Liquid ammonia
was used as a polar solvent at −40 °C for the extraction of
metals by undiluted TBP. The distribution ratios of all alkalimetal thiocyanates were very low (<0.1), and the extraction
order was Li < Na < K < Rb.70,71 The extraction was not
directly compared with the extraction of alkali-metal
thiocyanates from aqueous solutions, because of the lack of
relevant literature data. However, the extraction of alkali-metal
halides from aqueous solutions follows the opposite order, i.e.,
Li > Na > K > Cs.72 The different anions (chloride and
thiocyanate) might play a role in the opposite extraction order
of the two systems. More importantly, although solvations of
the alkali metals in ammonia and water follow the same
order,73,74 the solvation strength in the polar solvents relative
to the solvation with TBP is an important factor affecting the
extraction sequence. The extraction of Zn(II), Co(II), and
Ca(II) thiocyanates to the TBP phase from ammonia was
negligible, because of the strong solvation of these metals in
ammonia.71
Matsui et al. studied the extraction of Zn(II) and Cd(II) by
TOPO dissolved in toluene from ethylene glycol (EG)
solutions containing either HCl, HBr, or alkali-metal chlorides
or bromides. With the addition of acid or salt, the extraction
efficiency of both Zn(II) and Cd(II) shows a maximum, but
the maximum was reached at a lower acid (or salt)
concentration than that in extraction from aqueous solutions.75,76 The same trend was observed for the extraction of
dielectric constant (Table S1). Therefore, the miscibility of
two solvents can be easily estimated based on their chemical
structures and their polarity and hydrophobicity parameters.
Hydrocarbon diluents in the less polar phase generally are
hydrophobic solvents with low polarity. Therefore, the less
polar phase can readily form immiscible phases with ionic
solvents, such as molten inorganic salts and molten hydrates,
because of the big difference in polarity and hydrophobicity
between these two types of solvents. However, note that, when
the cations of the salts are extractable (e.g., Ca(NO3)2), the
change of volumes of the two immiscible phases can be nonnegligible.54 For this reason, alkali-metal salts, such as LiNO3
or KNO3, are preferred for molten inorganic salts, because of
their low extractability.55 Hydrophilic ILs are also polar and,
hence, insoluble in the less polar phase, although they are
organic salts. For example, tetraethylammonium chloride is
insoluble in p-cymene. However, when the alkyl chains are
longer, the hydrophobicity increases, resulting in a high
solubility in the less polar phase, e.g., Aliquat 336 readily
dissolves in p-cymene.
PMOSs are more likely to be miscible with the less polar
phase, because of the relatively small difference in polarity.
Many solvents, such as ethanol, are miscible with some
hydrocarbon diluents, despite being miscible with water as
well. Only those solvents with both low hydrophobicity and
high polarity are immiscible with the less polar phase. The
effect of the addition of extractants is also worth mentioning.
For example, toluene and ethylene glycol have very low mutual
solubilities. However, since Aliquat 336 is largely miscible with
both solvents, the addition of Aliquat 336 to the SX system
increases the mutual solubility of the two phases and even
leads to the merging of the two phases. In this case, the
amount of the extractant should be limited to minimize the
mutual solubility of the two phases. Moreover, nonpolar
diluents can sometimes be problematic, in combination with
polar extractants. For example, n-dodecane is nonpolar and is
immiscible with ethylene glycol. However, the addition of the
extractant Aliquat 336 to n-dodecane induces the formation of
a third phase. In this case, the replacement of the nonpolar ndodecane with the slightly polar toluene would diminish the
third phase. Finally, the addition of salts, especially lithium
salts (e.g., LiCl, LiNO3) could largely enhance the immiscibility due to the salting-out effect of salts. Lithium salts are
often selected because of their high solubility in many PMOSs.
The salt-induced phase separation in the methanol/ndodecane system was described in detail by Macchieraldo et
al.56
In short, the polarity and hydrophobicity of the nonaqueous
solvents are the most important properties for forming
immiscible phases with the less polar phase, and the effect of
extractants and salts should also be taken into account. There
are many empirical thermodynamic models to correlate the
phase equilibria of ternary, quaternary, and even morecomplicated systems with nonelectrolytes57−61 and electrolytes,62,63 and some predictive models for phase equilibria of
nonelectrolytes.64,65 However, quantitative studies on thermodynamic phase equilibria of NASX systems are lacking. It
would be worthwhile to perform an in-depth study of the
phase equilibria of NASX systems involving many extractants,
nonaqueous solvents, and salts, to establish a thermodynamic
equilibria library of NASX systems.
3.2. Systems without Extractants. A summary of NASX
systems comprising PMOSs is given in Table S2 in the
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Mn(II) by TOPO from EG solutions.77 This observation
shows that the metal halide complexes are more stable in EG
solution than in water. The stability of ZnCl+ and ZnCl2 in
propylene glycol (PG), EG, and an EG−water mixture
(volume ratio = 9:1) was further estimated based on solvent
extraction of Zn(II) by TOPO from these polar solvents.78,79
The stability order was found to be PG > EG > EG−water
mixture, which is consistent with the increasing dielectric
constant. Zn(II) was extracted as ZnX2·(TOPO)2 (X = Cl, Br),
which is the same as for extraction from aqueous solutions.
The decrease of metal extraction at higher acid (or salt)
concentrations was attributed to the formation of anionic
complexes (i.e., ZnCl3−, ZnCl42−) in the more polar phase.75
Batchu et al. developed a NASX system that showed
superior separation of rare-earth elements (REEs).43 Rareearth nitrates, dissolved in EG with LiNO3 as the salting-out
agent, were extracted by Cyanex 923 dissolved in n-dodecane.
Interestingly, heavy rare-earth elements (HREEs) were more
efficiently extracted while light rare-earth elements (LREEs)
were less efficiently extracted when compared with the
extraction from aqueous solutions, leading to a better
separation of the two groups of elements (Figure 3). Moreover,
Review
from aqueous solutions.81 EG, PG, and poly(ethylene glycol)
200 (PEG 200) were tested as the solvent of the more polar
phase for the separation of Eu(III) and Y(III) in chloride
media by Cyanex 923. The extraction efficiency followed this
order: PEG 200 > PG > EG > water. Despite higher extraction
efficiency from PEG 200 and PG solution, the separation factor
for Y(III)/Eu(III) was low. As the EG system combined a
satisfactory extraction efficiency with a high separation factor, a
flowsheet was developed and tested on laboratory-scale mixersettlers for the separation of the two elements.82 While pure
nonaqueous systems enhance extraction of most elements
through weakening the solvation and increasing ion-pair
formation with the anions, the addition of water to the polar
organic solvent can enhance the separation between two metal
ions. In the extraction of Nd(III) and Dy(III) by Cyanex 923,
both elements were quantitatively extracted from PEG 200
solution (with 1.0 mol L−1 LiCl), while neither element could
be extracted from aqueous solution (with 1.0 mol L−1 LiCl).
However, with the addition of 30−40 vol % water in PEG 200
(with 1.0 mol L−1 LiCl), efficient extraction of Dy(III) and
good separation of Dy(III)/Nd(III) could be concurrently
obtained, with a separation factor of up to 69 (Figure 4).83 The
Figure 4. Extraction of Nd(III) and Dy(III) by Cyanex 923 from a
mixture of PEG and water solutions. [Adapted from Dewulf et al.83]
Figure 3. Separation of REEs by Cyanex 923 from aqueous and
ethylene glycol solutions. [Adapted from Batchu et al.43]
extracted complex from the PEG 200 system was found to be
[LnCl3·L4] (L represents Cyanex 923),83 which is the same as
the complexes formed in the extraction from the EG system.81
Based on the fact that the complexes formed in the LP phase
are not influenced by MP phase composition, the differences in
extraction efficiency can be ascribed, on the one hand, to the
different solvation in the MP phase, and on the other hand, to
the lower dielectric constants of the polar organic solvents, that
increases the stability of inner-sphere chloride complexes.
3.4. Systems with Basic Extractants. Next to neutral
extractants, basic extractants have also been applied to NASX.
The extraction of Ni(II) from methanol halide solutions by
trioctylamine (TOA) in cyclohexane was found to be
enhanced significantly, compared to extraction from aqueous
solutions (Figure 5). The extracted species is [NiX4]2−, the
same as that extracted from an aqueous solution.84 The higher
extraction efficiency can be attributed to the stronger
formation of [NiX4]2−, facilitated on the one hand by the
weaker solvation with methanol compared to water, and on the
other hand, by the lower dielectric constant of methanol, that
enhances the inner-sphere complex formation. Although the
Gutmann donor number of water is slightly smaller than that
of methanol (Table S1), metal cations generally coordinate
stronger to water than to alcohols.44,85 Similar enhancement of
the separation of neighboring REEs was also enhanced. The
extracted complexes in the Cyanex 923 phase were found to be
[Ln(NO3)3L3] (L represents Cyanex 923), which is the same
as that in the extraction from aqueous solutions. Therefore, the
enhanced separation is very likely caused by weaker solvation
in the EG solution than in aqueous solutions, although further
investigations are needed. The EG solution with LiNO3 was
further applied for the separation of Nd(III) and Dy(III) by a
group of phosphine oxide, phosphinate, phosphonate, and
phosphate with octyl chains. All four extractants showed better
separation of the two elements, compared to extraction from
the aqueous solutions.80 Note that, combined with the EG
solutions, the phosphinate and phosphonate extractants are
more suitable for the separation of the two elements than the
phosphine oxide and phosphate extractants, because the
extraction by the phosphine oxide is too strong for both
elements and the extraction by phosphates is too weak for both
elements.
The EG solution could also enable the extraction of rareearth chlorides by Cyanex 923 dissolved in n-dodecane (with
10 vol % decanol as a modifier to prevent third phase
formation) with the assistance of LiCl as a chloride source,
whereas the extraction of rare-earth chlorides is not feasible
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amine < secondary amine < tertiary amine < quaternary
ammonium, which is similar to the extraction from aqueous
solutions. The extraction of Zn(II) and Cd(II), as a function of
the HCl concentration, showed a maximum, similar to the
extraction from aqueous solutions, yet at a lower concentration
of HCl, compared to aqueous systems. There are two
explanations for the decreased extraction of metals at higher
HCl concentrations. First, there is the competition for
extractants by the coextraction of HCl in the form of
[HCl2]−. Second, [ZnCl4]2− is assumed to be stabilized in
the MP phase at higher HCl concentrations, hence its
extraction is lowered. This assumption is explained in detail
by a new extraction model.49 The extraction of Zn(II) and
Cd(II) as a function of LiCl concentration also showed a
maximum,88 which is different from the extraction from
aqueous solutions, where the extraction increases monotonously. It is plausible to attribute the decrease of extraction to
the stabilization of [ZnCl4]2− in the EG solution at higher LiCl
concentrations, considering the enhanced formation of the
chlorometallate anions in solutions having a lower dielectric
constant. The extraction of Co(II) did not show a decrease
because the LiCl concentration was not sufficient for
stabilization of [CoCl4]2−. In another study, the extraction of
Co(II) by Aliquat 336 from methanol and formamide solutions
also showed a maximum at a higher LiCl concentration (6.0
mol L−1).89 In conclusion, the use of polar organic solvents
instead of water enhances the formation of chlorometallate
anions, which, on the one hand, can enhance the extraction of
metals; and on the other hand, can stabilize the anions in the
MP phase at higher chloride concentrations (or suppress the
extraction).
Superior separation was observed for the extraction of
Co(II) and Sm(III), making use of NASX combined with a
basic extractant.90 The LP phase of the system was Aliquat 336
Figure 5. Extraction of Ni(II) from aqueous and methanolic solutions
by Alamine 336. Data were taken from Florence and Farrar.84
the extraction by TOA was observed for Mn(II), Cr(III), and
Th(IV) halides.86 The extraction efficiency of Ni(II) and
Cr(III) by addition of halide salts followed the order: LiCl <
LiBr < LiI. However, the reverse order was observed for
Mn(II) and Th(IV) extraction. Mn(II) and Cr(III) were
extracted as [MnX4]2− and [CrX5]2−, respectively, whereas
Th(IV) was likely polymerized in the TOA phase. Extraction
of CuCl2 from methanol solutions by the secondary amine
bis(3,5,5-trimethylhexyl)ammonium chloride also proceeded
via the formation of the tetrachlorocuprate(II) anion
[CuCl4]2−.87 However, the extraction was very low (DCu <
0.3), because of the high solubility of the secondary amine in
methanol.
Primary, secondary, tertiary amines, and quaternary
ammonium in toluene were tested for the extraction of Zn(II),
Cd(II), and Co(II) chlorides from EG and PG solutions.88
The Zn(II) extraction efficiency followed the order of primary
Figure 6. Extraction of La(III) and Ni(II) by Aliquat 336 from various polar molecular organic solvents. [Adapted from Li et al.89]
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in toluene and the MP phase was EG with LiCl as the chloride
source, which also reduced the mutual solubility of the two
phases. Compared to the aqueous SX system, extraction of
Co(II) was enhanced, while the extraction of Sm(III) was
reduced to naught, leading to complete separation of Co(II)
and Sm(III) in a single extraction step. The system was found
to be generally suitable for the separation of transition metals
(except for Ni(II)) from REEs, such as Zn(II)/Eu(III) and
Fe(III)/Nd(III). The extracted species in the less polar phase
were the same as those extracted from aqueous solutions;
therefore, the enhanced extraction originates from the
increased formation of chlorometallate complexes in the EG
phase.
A similar NASX system, using Cyphos IL 101 instead of
Aliquat 336, was applied to the separation of In(III) and
Zn(II). The extraction of In(III) from EG solution was more
efficient than from aqueous solution, whereas the extraction of
Zn(II) was less efficient, leading to enhanced separation of
these two elements.91 The species of In(III) in EG was found
to change from the bridging (InCl3)2(EG)3 or mononuclear
(InCl3)(EG)2 complex at low LiCl concentrations to [In(EG)Cl4]− at high LiCl concentrations.91 In contrast, In(III)
exists in aqueous HCl solution as mixed octahedral complexes,
[In(H2O)6−nCln]3−n, in which n increases from 0 to 6 as the
concentration of HCl increases in the range of 0−12 mol
L−1.92 Given the presence of the same species of In(III) (i.e.,
[InCl4]−) in the LP phase in both SX systems, the different
speciation of In(III) in aqueous and EG solutions is
responsible for the different extraction behavior of In(III) in
the two systems.
The NASX system with EG and Aliquat 336 was expanded
to include other PMOSs, namely methanol, formamide, and Nmethylformamide (NMF), to separate transition metals and
REEs.89 The change of polar solvents in the MP phase
significantly affects the separation of metals. For example, in
Figure 6, La(III) is overall more efficiently extracted than
Ni(II) from aqueous solutions, but Ni(II) is more efficiently
extracted than La(III) from methanol solution, neither of the
two is extracted from EG solution, and only Ni(II) is extracted
from NMF, leading to excellent separation of the two elements.
The extraction of transition metals and REEs are affected by
polar solvents in two different ways. Transition metals are
extracted by Aliquat 336 as chlorometallate anions (e.g.,
[CoCl4]2−). The strength for the formation of [CoCl4]2− in
polar solvents follows the order NMF > methanol > formamide
> EG > water, which is a result of different solvations and the
physicochemical properties of the polar solvents (dielectric
constants, molarity, etc.). For instance, consider the effect of
the dielectric constant. Density functional theory (DFT)
calculations show that chlorometallate anions (e.g., [CoCl4]2−)
are more stable in solvents of lower dielectric constants.89 This
observation is consistent with the above discussions on the
effect of the dielectric constant. In contrast, Ln(III)
coordinates very weakly to Cl− ions; hence, it is extracted to
the LP phase as solvated cationic or neutral complex (i.e.,
[LnClx]3−x, in which 0 ≤ x ≤ 3, solvating solvent molecules are
omitted). As the extraction mechanism of transition metals and
REEs differ, the two groups of metals are influenced differently
by changing the polar solvents, creating the possibility of
tuning metal separations.
Review
4. IONIC SOLVENTS
Ionic solvents are essentially different from molecular solvents
in that the former is composed of ionic bonds, while the latter
is made entirely of covalent bonds. Molten inorganic salts,
molten salt hydrates, ionic liquids, and deep-eutectic solvents
are ionic solvents and they have been used as the solvent of the
MP phase in NASX systems. These NASX systems have been
summarized in Table S3 in the Supporting Information.
4.1. Low Melting Inorganic Salts. 4.1.1. KNO3−LiNO3
Eutectic. 4.1.1.1. Neutral Extractants. Isaac et al. were the first
to explore the use of a KNO3−LiNO3 eutectic mixture at a
molar ratio of ∼3:4, as the solvent of the MP phase in the
extraction of lanthanides, actinides, and cobalt using TBP at
150 °C.55 The use of molten salts was driven by the
assumption that molten salts should have the maximum
“salting-out” effect, based on the observation that the presence
of salts considerably increases the distribution ratio of metal
ions. Besides having a relatively low melting point (120 °C),
the KNO3−LiNO3 eutectic is poorly extractable to the TBP
phase, compared to alkaline-earth metal nitrates, making it a
suitable candidate as the solvent of the MP phase. Polyphenyl
hydrocarbons were used as diluents in the LP phase because
they have high boiling points. Similarly, TBP was chosen as the
extractant because of its thermal stability, and it had been
proven efficient in the PUREX process as well. Distribution
coefficients of Co(II), Eu(III), Nd(III), Am(III), Cm(III),
Np(VI), and U(VI) between the nitrate eutectic and the TBP
phase were found to be higher by a factor of 102−103,
compared to concentrated aqueous nitrate solutions. However,
the number of TBP molecules associated with the metal ions in
the LP phase was found to be the same as those reported for
the aqueous nitrate SX system. Consequently, the enhanced
extraction of metals from the eutectic was attributed to the
absence of water in the SX system.
From the same nitrate eutectic, Am(III), Cm(III), and
Cf(III) was extracted by TOPO and several diphenyl
diphosphine dioxides ((C6H5)2PO(CH2)nPO(C6H5)2, n = 2,
3, 4, 6, and denoted as n-DPO, respectively) at 160 °C.93 The
extraction of these elements by 2-DPO, 3-DPO, and 4-DPO
was more efficient than the extraction by TOPO, but 6-DPO
did not extract these elements. The same SX system was also
applied to the extraction of Co(II), Pr(III), Eu(III), and
Tm(III) from the nitrate eutectic.94 However, the performance
of these NASX systems was not compared with the
corresponding aqueous SX systems.
Extraction of HgX2 (X = Cl, Br, I, or a mixture of any two
anions) from the KNO3−LiNO3 eutectic was investigated
using a polyphenyl eutectic consisting of 48 mol % o-terphenyl,
15 mol % m-terphenyl, and 37 mol % biphenyl at 150−200
°C. 95−97 HgX 2 exists in the molten eutectic salt as
[HgX2(NO3)2]2−, but was extracted as the covalent HgX2
into the polyphenyl eutectic. The extraction of HgX2 from the
eutectic salt was considerably higher than that from dilute
aqueous solutions. The higher extraction was also attributed to
the “salting-out” effect, based on two observations: (1)
addition of KNO3 and LiNO3 mixtures (in the same ratio as
the KNO3−LiNO3 eutectic) to aqueous solutions linearly
increased the DHgCl2; and (2) extrapolation of DHgCl2 to 90 °C
for the extraction from the KNO3−LiNO3 eutectic intersects
with the extrapolation of DHgCl2 at the same temperature from
aqueous solution to full salt (same as the KNO3−LiNO3
eutectic). The linearity of “salting-out” of HgX2 is due to the
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of actinides. In contrast, the loading of Cu(I) into TBP and
HDBP was lower, facilitating the extraction of actinide ions.
4.1.4. CsOAc−NaOAc−KOAc Eutectic. Extraction of Cr(III), Fe(II), Co(II), and Ni(II) from molten cesium acetate−
sodium acetate−potassium acetate eutectic (50−25−25
mol %) at 140−180 °C into n-dodecane solutions containing
Cyanex 272, D2EHPA, or TOPO was studied.107 The metal
cations in the eutectic melt were in the form of negatively
charged complexes, whereas they are hydrated in aqueous
solutions. TOPO did not extract any metals, D2EHPA formed
a third phase, and Cyanex 272 extracted metals in this order:
Co(II) > Fe(II) > Cr(III) > Ni(II). The extracted complexes
were the same as the extraction from aqueous solutions, and
acetate was not involved in the complexes. However, the
distribution ratios were <0.1 for all metals, which might be due
to the difficulty of controlling the acidity of the salt melt and
the negatively charged complexes being too stable in the
molten eutectic phase.
4.2. Molten Salt Hydrates. 4.2.1. Molten Nitrate
Hydrates. Molten inorganic salts or eutectics, especially the
KNO3−LiNO3 eutectic, have been shown to enhance the
extraction of a variety of metals, compared to extraction from
aqueous solutions. However, the high melting points of the
inorganic salts are associated with high energy consumption
and extractant decomposition. Molten nitrate salt hydrates, also
called hydrate melts, are considered to lie between anhydrous
molten salts and concentrated aqueous electrolyte solutions.
The low melting points of the molten hydrates (Table S3)
make them interesting substitutes for molten inorganic salts,
particularly in the extraction of lanthanides and actinides by
TBP for the processing of nuclear waste.
Extraction of lanthanides and actinides by TBP from molten
nitrate hydrates was more efficient than from typical aqueous
solutions.108−112 As the complexes in the LP phase extracted
from molten nitrate hydrates or aqueous solutions are
identical, the improved extraction was attributed to the
increased chemical activity of the nitrate ions due to the
water deficiency of the system.108,110,112 TBP is poorly soluble
in the molten hydrates. For example, the solubility of TBP in
Ca(NO3)2·4H2O is only 0.05 g L−1 at 45 °C.118 However, the
nitrate salts are well extractable to the TBP phase, e.g., up to
1.45 mol L−1 Mn(NO3)2 is extracted into pure TBP, leading to
a 10% volume increase of the TBP phase.54 The high
extraction of molten nitrate hydrates hinders the extraction
of target metal ions and makes the SX system complicated for
fundamental studies. Yamana et al. suggested that the
coextraction of Ca(II) from Ca(NO3)2·4H2O can be neglected
only when the concentration of TBP in the less polar phase is
lower than 0.10 mol L−1.112
As water is the dominating component in molten hydrates,
in terms of mole fraction (e.g., xH2O is 0.8 in Ca(NO3)2·4H2O),
the water activity in molten melts is not as low as one might
expect (e.g., aH2O is ∼0.30 in Ca(NO3)2·4H2O113). The water
activity of Ca(NO3)2·nH2O (n is the number of water
molecules), as a function of the water mole fraction at 70
°C, has been given in Figure 7.114 The water activity decreases
with decreasing water mole fraction, from 1.0 for pure water to
∼0.30 for Ca(NO3)2·4H2O and further decreases in a slower
manner to <0.20. The relatively low water activity of
Ca(NO3)2·4H2O, or water deficiency, explains the higher
extraction of metals from molten nitrate hydrates than from
aqueous solutions. However, metal ions in Ca(NO3)2·4H2O
covalent binding of HgX2, whereas many other salts are
hydrated in aqueous solutions. The extraction of HgX2 by the
polyphenyl eutectic as covalent molecules is similar to the
extraction of solid GaCl3 as covalent dimers by aliphatic
hydrocarbons, although the latter extraction must be
performed under anhydrous conditions, because of the water
sensitivity of GaCl3.98
4.1.1.2. Basic Extractants. Tetraheptylammonium nitrate
(THAN) and tetraoctylphosphonium nitrate (TOPN) dissolved in a polyphenyl eutectic or 1-nitronaphthalene were
used to extract [ReO4]− and [AgCl2]− from the KNO3−LiNO3
eutectic in the presence of KCl−LiCl salts at 150 °C. The
extraction was found to proceed as an anion exchange, i.e.,
[ReO4]− and [AgCl2]− exchange with [NO3]− of THAN or
TOPN, similar to extraction from aqueous solutions.99,100
4.1.1.3. Synergistic Solvent Extraction. Extraction of some
REEs from KNO3−LiNO3 eutectic by a synergistic SX system
consisting of 2-thenoyltrifluoroacetone (HTTA) and 2-DPO
in polyphenyl eutectic was examined at 160 °C. The synergistic
factors in the HTTA-2-DPO system were 2−10, which is
smaller than extraction from aqueous solutions.101 The low
synergism is caused by the protons released by HTTA during
the extraction, which are not neutralized in the molten salt
phase. The difficulty of neutralizing protons and controlling
pH is a general problem in NASX. A possible solution to this
problem is to use saponified extractants, which have not been
investigated in detail yet.
4.1.2. KSCN−NaSCN Eutectic. The extraction of Fe(III),
Co(II), and Ni(II) from a molten KSCN−NaSCN eutectic
mixture at 150−170 °C into TBP dissolved in biphenyl
showed a marked enhancement when compared with
extraction from aqueous systems, which could be explained
by the salting-out effect.102 The temperature was controlled
below 170 °C to avoid decomposition of the eutectic. The
species in the eutectic were elucidated by spectrophotometric
measurements to be [Fe(SCN)6]3−, [Co(SCN)4]2−, and
[Ni(SCN)4]2−, respectively. The extracted species were the
same as in the eutectic phase but charge neutrality was
achieved by solvated Na+ or K+ ions, e.g., [Na(TBP)x]2[Co(SCN)4]. This is not the conventional solvation mechanism,
but it is similar to the synergistic extraction of Li+ by TBP and
Fe(III) from aqueous chloride media in the form of
[Li(TBP)x][FeCl4].103
Zn(II) was extracted from KSCN at 195 °C by TOPO and
several other uncommon neutral extractants dissolved in
phenanthrene.104 The extracted species was proposed to be
Zn(TOPO)2(SCN)2. The different complexes of transition
metals formed with TBP and TOPO might be caused by the
different coordination capabilities of these extractants.
Basic extractants were also investigated for the extraction of
metal ions from the KSCN−NaSCN eutectic at 150 °C.
Co(II) in KSCN−NaSCN eutectic was extracted by di-ndodecylammonium thiocyanate ([NR2H2][SCN]) dissolved in
chloronaphthalene in the form of [NR2H2]2[Co(SCN)4] via
an anion exchange mechanism.105
4.1.3. KCl−CuCl Eutectic. Distribution ratios of U(IV),
U(III), Pu(IV), Pu(III), and Am(III) between the KCl−CuCl
eutectic at 180 °C and an LP phase containing TBP,
trioctylamine (TOA), or di-n-butyl phosphoric acid (HDBP)
dissolved in biphenyl were determined.106 The amount of
Cu(I) extracted into the TOA phase was comparable to the
total amount of TOA, because of the formation of ion-pair
complexes, e.g., [(TOA)2Cu][CuCl2], hindering the extraction
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Extraction of REEs from Ca(NO3)2·nH2O by [A336][NO3]
at 53 °C was investigated by Rout and Binnemans.120 The
extraction was highly efficient, although slightly lower than the
extraction from molten inorganic nitrates. This extraction
efficiency agrees well with the fact that molten nitrate hydrates
lie between concentrated aqueous nitrate solutions and molten
inorganic nitrates. The extraction efficiency of the REEs
decreases as the atomic number increases, which is the same as
the sequence of extraction from aqueous solutions.121,122 The
decrease of extraction efficiency might be explained by the
hydration of REEs in the molten nitrate hydrates, as studies by
Fujii et al.115 indicate; these studies show that there are still 5−
7 water molecules hydrated to REEs in molten Ca(NO3)2·
4H2O. Furthermore, the addition of Mg(NO3)2·6H2O or water
to Ca(NO3)2·4H2O both reduce the extraction efficiency,
because of the increase in water activity. The loading of Ca(II)
to the [A336][NO3] phase was not studied. However, it seems
that coextraction of Ca(II) did not hinder the extraction of
REEs, because more than 120 g L−1 Nd(III) could be loaded
to the [A336][NO3] phase.
4.2.2. Molten Chloride Hydrates. Molten chloride hydrates,
such as CaCl2·4H2O and CaCl2·6H2O, were also used as the
solvent of the MP phase in the extraction of lanthanides,
actinides, and some fission products by TBP and [A336][Cl].54,111,120 The extraction of actinides and lanthanides was
much less efficient than from molten nitrate hydrates, because
these elements are less able to coordinate to Cl− ions than to
nitrate ions.85 The effect of water was similar to the case of
molten nitrate hydrates, as discussed above. The loading of
Ca(II) to the TBP phase was also significant; 1.03 mol L−1 was
loaded to 75 vol % TBP.54
4.3. Ionic Liquids. 4.3.1. Immiscible Biphasic ILs or
Inorganic Salt Systems. Ionic liquids (ILs) are solvents
composed entirely of ions. They are organic salts with lower
melting points than inorganic salts, and many of them are
liquids at room temperature or slightly elevated temperatures.
Hydrophobic ILs are often used as extractants, such as Aliquat
336 and Cyphos IL 101. However, the use of hydrophilic ILs
as the solvent of the MP (feed) phase is rare. Arce et al.123
discovered three types of biphasic ILs mixtures: (1) a
hydrophobic and a hydrophilic IL that share the same anion,
e.g., [P666,14]Cl and [Cnmim]Cl, (where n < 6); (2) two
hydrophobic ILs with a common anion, e.g., [C2mim][NTf2]
and ([P666,14][NTf2]); and (3) two ILs with four different ions,
e.g., [C2mim][OSO2CH3] and [P666,14][PO2(C8H17)2]. Many
more biphasic ILs were created afterward.124−128 While both
the anions and the cations affect the formation of biphasic IL
systems,128 only mixtures of two ILs with significantly
structurally different IL cations or of highly different
hydrogen-bond acidity can undergo liquid−liquid phase
separation, highlighting the determining role of the IL
cation.127 Despite having two phases, all biphasic IL systems
display significant ion exchange, and all display an upper critical
solution temperature (UCST)-type behavior. Since both IL
phases are polar and it is not straightforward to tell which
phase is more polar, here, we can use the feed phase and the
extract phase to distinguish the two phases.
Wellens et al.129 performed a proof-of-concept study for the
separation of Co(II) and Ni(II) using the biphasic IL system
consisting of Cyphos IL 104 and [C2mim]Cl. Co(II) and
Ni(II) formed [CoCl4]2− and [NiCl4]2−, respectively, in the
[C2mim]Cl phase. Co(II) was efficiently extracted as [Co(R2POO)2], while the extraction of Ni(II) was low, leading to
Figure 7. Water activity in Ca(NO3)2·nH2O, as a function of water
mole fraction at 70 °C. [Data were taken from Yamana et al.114]
Figure 8. Hydration number of Eu(III) and Dy(III) in Ca(NO3)2
nH2O at 50 °C. [Adapted from Fujii et al.115]
are not free of hydration, since the water activity is still
considerable. The hydration of Eu(III) and Dy(III), as a
function of water activity, has been investigated by Fujii et al.
(see Figure 8).115 The number of hydrated water molecules in
the first coordination sphere of Eu(III) and Dy(III) decreases
from 8−9 in dilute aqueous solutions to 5−7 in Ca(NO3)2·
4H2O, which is still significant. The extraction of lanthanides
from Ca(NO3)2·4H2O by TBP first increases with increasing
atomic number, and then decreases, which is the same pattern
as the extraction from dilute aqueous solutions. However, the
extraction from molten KNO3−LiNO3 eutectic and highly
concentrated nitric acid solutions (e.g., 18.5 mol L−1 HNO3)
increased monotonically.54,112,116 The decrease of the extraction efficiency for heavier elements from the molten nitrate
hydrates might be due to stronger hydration of these elements,
as the hydration energy increases with increasing atomic
numbers.117,118 The water activity in highly concentrated nitric
acid solution is lower than that of Ca(NO3)2·4H2O; for
example, aH2O is 0.19 in 15 mol L−1 HNO3 solution.119 Hence,
extraction of REEs from highly concentrated nitric acid is less
affected by hydration, showing monotonically increasing
extraction efficiency in the entire lanthanide series. Similarly,
actinides are more sensitive to water activity in molten nitrate
hydrates than lanthanides. A varying water activity helps to
enhance the separation of the two groups of elements.111 The
water content in Ca(NO3)2·nH2O also affects the reaction rate.
Equilibrium of REEs extraction by TBP from Ca(NO3)2·nH2O
with n = 4 can be reached within 2 h, however, it takes more
than 8 h to reach equilibrium when n < 3, mainly because of
the higher viscosity of the drier molten nitrate hydrate.114
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species of Co(II) and Fe(III) in EAN were proposed as
[Co(NO3)4]2− and [Fe(NO3)4]−, respectively. The enhanced
extraction is caused by the exclusion of other ligands (i.e.,
solvent molecules) to solvate the metal cations, besides nitrate
ions. The addition of H2O to EAN reduces the extraction
because the metal cations coordinate more strongly to water
than to nitrate ions. Co(II) speciation converts from
[Co(NO3)4]2− in EAN to [Co(H2O)6]2+ with the addition
of water to the EAN phase. Interestingly, different metals
respond differently to the water content; therefore, the
addition of water leads to enhanced separations for some
metals pairs. For example, the extraction of Ni(II) decreased
from 84% to 21% with the addition of 5 vol % water, but the
extraction of Fe(III) remained almost the same at ∼99%,
leading to enhanced separation of Fe(III)/Ni(II).
EAN was found to be a perfect solvent for studying the
sequence of lanthanides extraction by quaternary ammonium
nitrate ILs (e.g., [A336][NO3]).138 The extraction of
lanthanides by [A336][NO3] from aqueous nitrate solutions
shows a negative sequence (i.e., light lanthanides are more
efficiently extracted than heavy lanthanides),121,122 which has
confused researchers in the field for decades, because it
conflicts with the “lanthanide contraction”. However, the
extraction of lanthanides from EAN by [A336][NO3] shows a
positive sequence, which is converted to a negative sequence
with the addition of water (Figure 9). The transformation from
a separation factor of >200. The high separation factor is not
surprising, because phosphinic acids (e.g., Cyanex 272) are
known to have a high Co(II)/Ni(II) selectivity.130 Increasing
the temperature from 95 °C to 140 °C reduced the
distribution of Co(II), because of increased solubility of
phosphinate anions in the [C2mim]Cl phase. The solubility
increase with increasing temperature is consistent with the
common UCST behavior of biphasic IL systems. The same
biphasic IL system was further used for the extraction of
REEs.131 REEs in the [C2mim]Cl phase were expected to form
anionic [LnCl6]3− complexes, because of the absence of water,
although the speciation was not comprehensively characterized. All tested REEs were efficiently extracted from the
[C2mim]Cl phase to the Cyphos IL 104 phase, and could also
be separated from Ni(II).
Biphasic IL systems can further form three- or four-phase
immiscible liquid systems with water and a nonpolar alkane,
such as the pentane−[P666,14][NTf2]−water−[C2mim][NTf2]
system.123 Vander Hoogerstraete et al.132 made use of the
triphasic [Hbet][NTf2]−H2O−[P666,14][NTf2] system to
separate Sn(II), Y(III), and Sc(III), which were enriched in
the [Hbet][NTf2] phase, the aqueous phase, and the
[P666,14][NTf2] phase, respectively. Strictly speaking, the IL−
H2O−IL system is not a NASX system, because of the
involvement of an aqueous phase.
Similar to ILs, inorganic salts could also form two immiscible
phases, although a higher temperature is needed. KBr and
AlBr3 form two immiscible phases, with one phase being
mainly AlBr3 (>99% mol %) and the other phase having ∼20
mol % AlBr3 and 80 mol % KBr. Ammon studied the
distribution of 14 metal halides in this system at 200 °C and
found that ionic compounds prefer the polar KBr-rich phase,
while covalent-type compounds are enriched in the AlBr3-rich
phase, facilitating the separation of different types of metal
halides.133 The highest separation factor was found for Cs(I)/
Zr(IV), reaching 126. Smith investigated the distribution of
PdBr2, RhBr3, and RuBr3 between two phases of the same
system at 110 °C. However, these halides exhibited similar
distributions and, hence, the separation was poor. This poor
separation might be explained by the similar property of these
halides in that they are all ionic compounds, e.g., [PdBr4]2−
and [RuBr6]2− were the main species in both phases, as
determined by the spectroscopic study.134 The ternary system
LiCl−KCl−AlCl3 could form two immiscible polar phases with
the KAlCl4 phase on the top and the LiCl phase at the bottom.
Distributions of some metal chlorides and oxychlorides
(UO2Cl2) in this system were determined by Moore.135 The
distribution ratios range from 0.014 for Sr(II) to 18.1 for
Cs(I), allowing possible separations of different metal
chlorides. Because of the resistance of molten salts to radiation
damage, immiscible salt systems might find applications in
processing radioactive materials.
4.3.2. Ethylammonium Nitrate. Ethylammonium nitrate
(EAN) is a hydrophilic IL with a low melting point (12 °C)
and low viscosity (32 cP at 27 °C).136 EAN is a suitable
solvent for the feed phase of an SX system, because it does not
need heating, in contrast to molten inorganic salts, and it does
not contain water in contrast to molten nitrate hydrates. EAN
was utilized as a polar solvent for the extraction of transitionmetal nitrates by TBP.137 The extraction from EAN is
considerably stronger than that from a range of polar molecular
solvents, which is consistent with the high extraction of Co(II)
from molten KNO3−LiNO3 eutectic by TBP. The main
Figure 9. Transformation of lanthanides extraction sequence from
positive to negative by addition of water. [Adapted from Li and
Binnemans.138]
positive to negative sequences reveals that the negative
sequence is caused by the hydration of lanthanide ions.138
This observation is consistent with the effect of water on
lanthanides extraction from molten nitrate hydrates.120 The
EAN−[A336][NO3] system contains only nitrate ions as
ligands for metal cations and, hence, is a good simplified model
system for many fundamental studies.
4.4. Deep-Eutectic Solvents. Deep-eutectic solvents
(DESs) formed by a eutectic mixture of Lewis or Brønsted
acids and bases can contain a variety of anionic and/or cationic
species. DESs are a different type of solvent compared to ILs,
which are formed from systems composed primarily of one
type of discrete anion and cation.139 Foreman investigated the
extraction of metals from the lactic acid-choline chloride-based
DES.140 With the increase of the DES in the water−DES
mixture, the extraction of Zn(II) and Cd(II) by Aliquat 336
dissolved in toluene decreased, while extraction of Fe(III) and
Mn(II) increased, and the extraction of Co(II) and Cu(II)
peaked at 80 vol % DES. It is known that these metals are
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extracted by Aliquat 336 via the formation of chlorometallate
anions that bind the ammonium cation of Aliquat 336. The
different extraction efficiency of these metals reflects the
different capabilities for the formation of their corresponding
chlorometallate anions. One important factor affecting the
formation of chlorometallate anions is the changing Cl− ion
concentration (and, hence, its activity) as the DES-to-water
ratio changes in the DES-water mixture.141,142 Besides, water
molecules and lactate can coordinate to the metal cations as
well. The detailed coordination of each metal ion in such a
mixture remains unknown. The extraction of Co(II) by Aliquat
336 (dissolved in ethylbenzene) from loaded DES after
leaching metallic cobalt would result in accumulation of
lactate, which can inhibit the extraction of Co(II) due to the
formation of CoL2 (where L represents a lactate anion).143
The accumulation of lactate can be avoided by using D2EHPA
as an extractant. On the other hand, the extraction of these
metals from the water−DES mixture by D2EHPA was less
sensitive to the DES content,140 because the extraction by
D2EHPA is not dependent on the formation of chlorometallate anions, but on the acidity of the MP phase.
The lactic acid-choline chloride-based DES was also
employed for the dissolution of NdFeB magnets, followed by
two steps of SX for the recovery and separation of the
metals.144 The first step used [A336][SCN] to extract Fe(III),
Co(II), and B(III), and the second step used Cyanex 923 to
recover Nd(III) and Dy(III). Despite also efficiently extracting
Nd(III) and Dy(III) from the DES, D2EHPA was not selected
because of the difficulty of stripping. Compared with the
extraction of these metals from aqueous solutions, higher
extraction efficiency for all elements and enhanced separation
for Nd(III) and Dy(III) were obtained for the extraction from
the DES. More interestingly, while Fe(III) was extracted in the
form of [Fe(SCN)6]3− from aqueous solutions, it was extracted
as [FeCl4]− from the DES solutions. These observations
indicate that the DES alters the speciation of metals in the MP
phase, affecting the extraction mechanism and extraction
efficiency.
Review
The use of salts or complexing agents in the MP phase could
also enhance the metal separations. In the extraction of
transition metals from EAN by TBP, the addition of LiCl
significantly increased the separation factors of Fe(III)/Zn(II),
Fe(III)/Cu(II), Fe(III)/Co(II), Fe(III)/Ni(II), and Mn(II)/
Zn(II).137 The separation factors of metal pairs with 0.50 mol
L−1 LiCl in the MP phase are given in Table 1. Again, take
Table 1. Separation Factors (αM1/M2) of Six Transition
Metals with 0.5 mol L−1 LiCl in EAN
M1
M2
αM1/M2
Mn(II)
Fe(III)
Co(II)
Ni(II)
Cu(II)
Zn(II)
Mn(II)
Fe(III)
Co(II)
Ni(II)
Cu(II)
Zn(II)
1.0
−
17.1
7.4
43.6
104.2
38.5
1.0
659.0
283.2
1680.7
4012.2
−
−
1.0
−
2.6
6.1
−
−
2.3
1.0
5.9
14.2
−
−
−
−
1.0
2.4
−
−
−
−
−
1.0
Co(II) as an example, the addition of LiCl converts
[Co(NO3)4]2− in the feed phase to [CoCl4]2−. The latter is
more stable, hence more difficult to be extracted by TBP as
[Co(NO3)2(TBP)x] (x = 2, 3). These transition metals have
different capabilities to coordinate to Cl− ions, leading to the
enlargement of separation factors by the addition of LiCl.
The same methodology applies to the separation of
transition metals from REEs, since these two groups of
elements have remarkably different coordination capabilities.
Hydrophilic ILs were found to be suitable complexing agents
in the MP phase to enhance the separation. For instance, both
Co(II) and Sm(III) can be efficiently extracted by Cyanex 923
from EG solutions with LiCl as a chloride source. However,
the addition of tetraethylammonium chloride (TEAC) to the
MP phase holds back the extraction of Co(II) through the
formation of [N2222]2[CoCl4] without affecting the extraction
of Sm(III), leading to highly efficient separation of the two
elements (Figure 10).146 However, the role of the cations, i.e.,
why [N2222]+ is more efficient than Li+ to stabilize [CoCl4]2− in
the more polar phase, has not been understood yet.
Computational methods, such as molecular dynamics (MD)
simulations, seem to be a powerful tool to gain deeper insights
into the stability of complexes in solutions.147 Therefore, it is
recommended that more computational investigations should
be conducted in future studies on NASX systems.
5. COMPLEXING AGENTS IN THE FEED PHASE
Not only using alternative solvents in the MP phase affects the
extraction efficiency, the addition of salts or complexing agents
also does. The addition of Cl− to the KNO3−LiNO3 eutectic55
or EAN137 significantly reduces the extraction of transition
metals [Mn(II), Fe(III), Co(II), Ni(II), Cu(II), Zn(II)] by
TBP. This is because of the formation of more stable
chlorometallate complexes (e.g., [CoCl4]2−) in the MP
phase. Extraction of Hg(II) from the KNO3−LiNO3 eutectic
by polyphenyl is also subject to the formation of
chlorometallate complexes (e.g., [HgCl3]−, [HgCl4]2−) upon
the addition of LiCl.95,97 However, the addition of Cl− to the
KSCN−NaSCN eutectic does not reduce the extraction of
transition metals,102 because transition metals preferentially
coordinate to thiocyanate ions.85 Nevertheless, the addition of
a small amount of cyanide ions significantly lowers the
extraction of transition metals by both neutral phosphorus
extractants104 and quaternary ammonium salts,145 because
cyanide ions coordinate stronger to transition metals than
thiocyanate ions do. The effect of the anions in the MP phase
on the extraction of transition metals follows the sequence of
their coordination ability to transition metals: CN− > SCN− >
Cl− > NO3−.85
6. NASX IN METALLURGICAL FLOWSHEETS
A hydrometallurgical process from minerals to marketable salts
or metals typically includes leaching, SX, and metal recovery
by, for instance, precipitation or electrodeposition. NASX and
other solvometallurgical unit processes can be used to enhance
such existing metallurgical processes. As such, solvometallurgy
is complementary to hydro- and pyro-metallurgy, and it can be
applied whenever it provides economic and technical
advantages. In terms of scaling up, it has been demonstrated
that NASX systems can be operated smoothly in mixer-settlers,
such as the Cyanex 923 and DES system for the separation of
Dy(III) and Nd(III),144 the Cyanex 923 and DES system for
the separation of Fe(III) from Zn(II) and Pb(II),148 and the
Cyanex 923 and EG system for the separation of Y(III) and
Eu(III).82
Complete solvometallurgical processes (Figure 11) have not
been developed, yet efforts have been made toward this end.
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Review
Figure 10. Addition of TEAC to EG solution enhances the separation of Co(II) and Sm(III). [Adapted from Li et al.146]
In a similar study, Nd−Fe−B permanent magnets were leached
by a DES composed of choline chloride and lactic acid (molar
ratio of 1:2).144 Fe(III), Co(II), and B(III) in the leachate
were removed using the IL [A336][SCN], while Nd(III) and
Dy(III) were further recovered by NASX using Cyanex 923.
The loaded metals and B(III) in [A336][SCN] and Cyanex
923 were selectively stripped by aqueous solutions.
Besides the recovery of valuable metals from magnets,
Peeters et al. studied solvometallurgical recovery of cobalt from
lithium cobalt oxide (LiCoO2).151 A DES composed of choline
chloride−citric acid (molar ratio of 2:1) diluted with 35 wt %
water (to reduce viscosity) was used to leach LiCoO2 with the
assistance of metallic aluminum and copper as reducing agents.
Cu(I/II) and Co(II) in the DES were recovered by LIX 984
and Aliquat 336, respectively, after which both were
precipitated by aqueous oxalic acid solutions.
Recovery of platinum group metals (PGMs) from spent
automotive catalysts by solvometallurgy was also investigated.152 After two steps of solvoleaching of the automotive
catalysts using acetonitrile and FeCl3, and removal of
acetonitrile by distillation, the solid residue was dissolved in
EG. The EG solution containing Fe(III), Pt(IV), and Rh(III)
was treated by NASX using [A336][Cl] to recover Fe(III) and
Pt(IV), leaving behind Rh(III) in the raffinate. The loaded
Fe(III) and Pt(IV) were scrubbed by water and stripped by
thiourea solution, respectively.
All of the above processes consist of nonaqueous leaching
and NASX, but the stripping used aqueous solutions, which
may introduce water to the LP phase of NASX systems. The
loaded metal ions in the LP phase could also be stripped to the
nonaqueous MP phase, in which the ions could be recovered
by nonaqueous electrodeposition. For example, Cu(I) and
In(III) were directly electrodeposited from EG solution153 and
PEG 400 solution,154 respectively.
Figure 11. Flowsheet of a solvometallurgical process.
Orefice et al. used a mixture of aqueous HCl and EG to leach
Co−Sm permanent magnets.149 The resulting pregnant
leachate was treated by SX using Aliquat 336 to recover
Co(II), Cu(II), and Fe(III), which were then separated by
selective stripping. The extraction from a mixture of water and
EG was more efficient than from purely aqueous solutions, as
discussed in Section 3.4. In another example, pyridine
hydrochloride (PyHCl) was used to leach the production
scrap of Nd−Fe−B permanent magnets at 165 °C for the
recovery of REEs.150 Dy(III) and Nd(III) in the pregnant
leachate were recovered by NASX using PC 88A (2ethylhexylphosphonic mono-2-ethylhexyl ester), and Fe(III)/
Fe(II) was recovered using Cyphos IL 101. The loaded
Dy(III) and Nd(III) were precipitated by aqueous oxalic acid
solutions, and Fe(III)/Fe(II) was precipitated by aqueous
NH3 solutions. It is worth mentioning that REEs could not be
precipitated as oxalates in PMOSs, e.g., ethylene glycol,
because of the high solubility of rare-earth oxalates in
PMOSs, although this is typically done in aqueous solutions.
7. CONCLUSIONS AND OUTLOOK
NASX systems can be developed starting with choosing the
appropriate more polar and less polar phases to form
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immiscible two-phase systems, based mainly on the polarity
and hydrophobicity of the two phases. A variety of NASX
systems involving polar molecular organic solvents and ionic
solvents (molten inorganic salts, molten salt hydrates, ionic
liquids, and deep eutectic solvents) as the solvent of the more
polar phase have been investigated using four different types of
extractants for the extraction of a wide range of metals.
The use of polar molecular organic solvents generally
enhances the extraction of metals, mainly due to the higher
stability of inner-sphere metal−ligand complexes in polar
molecular organic solvents caused by the lower dielectric
constants. This effect enables the extraction of REEs chlorides
by Cyanex 923 and enhances extraction of transition metals,
but also slightly suppresses extraction of transition metals (e.g.,
Zn(II)) at high chloride solutions, because of the anionic
complexes (e.g., [ZnCl4]2−) being stable in the polar molecular
organic solvents. Because different metals behave differently in
terms of speciation at different anion and water concentrations
in various solvents, NASX offers an extra dimension to improve
metal separations. This effect leads to more efficient
separations of REEs by NASX using polar molecular organic
solvents and neutral extractants, and highly efficient separations of transition metals from REEs using polar molecular
organic solvents and basic extractants.
Extraction of metals from inorganic salts, molten salt
hydrates, and ionic liquids is much more efficient, compared
to extraction from aqueous systems. The addition of water to
these systems, on the one hand, reduces the extraction, but
may also enhance the separation of metals. The NASX system
consisting of [A336][NO3] and ethylammonium nitrate
reveals that hydration of REEs counteracts the separation of
REEs, which indicates that weaker solvation in the more polar
phase is beneficial for separations. The [A336][NO3] and
ethylammonium nitrate system can be a model for the study of
ion transport, because it includes only one ligand. Furthermore, the introduction of complexing agents, either inorganic
salts or ionic liquids, may enhance separations, because of the
complexing of metal ions to the anions, but the role of the
cations has not been understood yet.
Studies on NASX so far have covered a wide scope
encompassing three aspects: (1) the use of molecular solvents,
(2) the use of ionic solvents, and (3) the introduction of
complexing agents. More work must be done to fully
understand NASX and make better use of it for the separation
of metals.
Review
More low-melting hydrophilic ILs should be explored
for use as the more polar phase, such as chloride ILs.
(5) More efforts should be made to develop either fully
solvometallurgical processes or nonaqueous systems that
can be integrated practically into existing metallurgical
flowsheets. Those investigations should focus on
attaining a high technology readiness level (TRL).
■
ASSOCIATED CONTENT
* Supporting Information
sı
This material is available free of charge via the Internet at
http://pubs.acs.org/. The Supporting Information is available
free of charge at https://pubs.acs.org/doi/10.1021/acs.iecr.1c02287.
Physical and chemical properties of common polar
molecular organic solvents (PMOSs) and some diluents;
nonaqueous solvent extraction (NASX) systems with
polar molecular organic solvents (PMOSs); NASX
systems using ionic solvents in the more polar phase
(PDF)
■
AUTHOR INFORMATION
Corresponding Author
Zheng Li − Department of Chemistry, KU Leuven, B-3001
Heverlee, Belgium; orcid.org/0000-0002-7882-5999;
Email: zheng.li@kuleuven.be
Authors
Brecht Dewulf − Department of Chemistry, KU Leuven, B3001 Heverlee, Belgium; orcid.org/0000-0002-4325273X
Koen Binnemans − Department of Chemistry, KU Leuven, B3001 Heverlee, Belgium; orcid.org/0000-0003-47683606
Complete contact information is available at:
https://pubs.acs.org/10.1021/acs.iecr.1c02287
Author Contributions
∇
These authors contributed equally.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
Z.L. was supported by the Senior FWO Postdoctoral
Fellowship (No. 181203/12ZI920N). K.B. acknowledges
funding from the European Research Council (ERC) under
the European Union’s Horizon 2020 Research and Innovation
Programme: Grant Agreement 694078Solvometallurgy for
critical metals (SOLCRIMET).
■
(1) The construction of correlation-based and even
predictive thermodynamic models to describe the
phase equilibria in NASX systems involving a range of
polar solvents (molecular and ionic), including the effect
of extractants and salts.
(2) Speciation of metals in the more polar phase and the
solvation energy should be studied in more detail, by
experimental and computational methods, to quantitatively understand the effect of the polar solvents on
metal ion coordination.
ABBREVIATIONS
a = activity
Alamine 336 = N,N-dioctyl-1-octanamine, also known as trin-octyl amine (TOA)
Aliquat 336 = tricaprylmethylammonium chloride, where
capryl is a mixture of n-octyl and n-decyl
Aliquat 4 = dodecyltrimethylammonium chloride
bipy = 2,2′-bipyridyl
Cyanex 272 = bis(2,4,4-trimethylpentyl)phosphinic acid
Cyanex 301 = bis(2,4,4-trimethylpentyl)dithiophosphinic
acid
■
(3) Neutral and basic extractants are mainly used, so far.
The use of acidic extractants, either as such or in the
saponified forms (or binary extractants), in NASX
should be explored.
(4) So far, ethylammonium nitrate is the only ionic liquid
(IL) that can be used at room temperature for NASX.
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ShellSol A150 = C9−C10 aromatic hydrocarbon solvent
UCST = upper critical solution temperature
Versatic Acid 10 = a mixture of carboxylic acids with the
common structural formula C10H20O2
vol % = volume percent
wt % = weight percent
x = mole fraction
α = separation factor
δd = Hansen solubility parameter representing dispersion
forces
δp = Hansen solubility parameter representing intermolecular forces
δh = Hansen solubility parameter representing hydrogen
bonding forces
δT = total Hansen solubility parameter
%E = percentage extraction
[A336][NO3] = Aliquat 336 nitrate
[Cnmim]Cl = 1-alkyl-3-methylimidazolium chloride
[Hbet][NTf2] = betaine bis(trifluoromethylsulfonyl)imide
[NR2H2][SCN] = di-n-dodecylammonium thiocyanate
Cyanex 923 = mixture of trialkylphosphine oxides with the
alkyl chains being n-hexyl and n-octyl
Cyphos IL 101 = trihexyl(tetradecyl)phosphonium chloride,
[P666,14]Cl
Cyphos IL 104 = trihexyl(tetradecyl)phosphonium bis(2,4,4-trimethylpentyl)phosphinate
D = distribution ratio
D2EHPA = bis(2-ethylhexyl) phosphoric acid
DESs = deep eutectic solvents
DMF = N,N-dimethylformamide
DMSO = dimethyl sulfoxide
EAN = ethylammonium nitrate
EDTA = ethylenediaminetetraacetic acid
EG = ethylene glycol
GS190 = aliphatic diluent derived from gas-to-liquid
technology having low levels of sulfur, olefins, and aromatics
HDBP = di-n-butyl phosphoric acid
HL = an acidic extractant molecule
HMPT = hexamethylphosphorotriamide
HREE = heavy rare-earth elements
HTTA = 2-thenoyltrifluoroacetone
ILs = ionic liquids
LIX 984 = 1:1 volume blend of LIX 860 (5-dodoecylsalicylaldoxime) and LIX 84 (2-hydroxy-5-nonylacetophenone
oxime)
Ln(III) = trivalent lanthanide ion
LP = less polar
LREE = light rare-earth elements
MIBK = methyl isobutyl ketone
mol % = mole percent
MP = more polar
NASX = nonaqueous solvent extraction
n-DPO = diphenyldiphosphine dioxides, (C6H5)2PO(CH2)nPO(C6H5)2
NMF = N-methylformamide
NR4Cl = quaternary ammonium chloride
NR4L = a binary extractant with the cation being a
quaternary ammonium
PC 88A = 2-ethylhexylphosphonic mono-2-ethylhexyl ester
PEG 200 = poly(ethylene) glycol with an average molecular
weight of 200
PEG 400 = poly(ethylene) glycol with an average molecular
weight of 400
PG = propylene glycol
PGMs = platinum-group metals
phen = 1,10-phenanthroline
PMOSs = polar molecular organic solvents
Progil pure = 2,4,5,7-tetramethyloctane
PyHCl = pyridine hydrochloride
REEs = rare-earth elements
S = neutral extractant molecule
SX = solvent extraction
TALSPEAK = trivalent actinide-lanthanide separation with
phosphorus-reagent extraction from aqueous complexes
TBAI = tetra-n-butylammonium iodide
TBP = tri-n-butyl phosphate
TEAC = tetraethylammonium chloride, [N2222]Cl
THAN = tetraheptylammonium nitrate
TOA = trioctyl amine, also known as Alamine 336
TOPN = tetraoctylphosphonium nitrate
TOPO = tri-n-octyl phosphine oxide
TPPO = triphenyl phosphine oxide
TRL = technology readiness level
■
REFERENCES
(1) Demopoulos, G. P. Solvent Extraction in Precious Metals
Refining. JOM 1986, 38 (6), 13−17.
(2) Saguru, C.; Ndlovu, S.; Moropeng, D. A Review of Recent
Studies into Hydrometallurgical Methods for Recovering PGMs from
Used Catalytic Converters. Hydrometallurgy 2018, 182, 44−56.
(3) Nguyen, V. T.; Riaño, S.; Binnemans, K. Separation of Precious
Metals by Split-Anion Extraction Using Water-Saturated Ionic
Liquids. Green Chem. 2020, 22 (23), 8375−8388.
(4) Cheng, C. Y.; Barnard, K. R.; Zhang, W.; Robinson, D. J.
Synergistic Solvent Extraction of Nickel and Cobalt: A Review of
Recent Developments. Solvent Extr. Ion Exch. 2011, 29 (5−6), 719−
754.
(5) Banza, A. N.; Gock, E.; Kongolo, K. Base Metals Recovery from
Copper Smelter Slag by Oxidising Leaching and Solvent Extraction.
Hydrometallurgy 2002, 67 (1), 63−69.
(6) Cheisson, T.; Schelter, E. J. Rare Earth Elements: Mendeleev’s
Bane, Modern Marvels. Science 2019, 363 (6426), 489−493.
(7) Qi, D. Extractants Used in Solvent Extraction−Separation of
Rare Earths: Extraction Mechanism, Properties, and Features. In
Hydrometallurgy of Rare Earths: Extraction and Separation; Elsevier:
Amsterdam, 2018; Chapter 2, pp 187−389.
(8) Doidge, E. D.; Carson, I.; Love, J. B.; Morrison, C. A.; Tasker, P.
A. The Influence of the Hofmeister Bias and the Stability and
Speciation of Chloridolanthanates on Their Extraction from Chloride
Media. Solvent Extr. Ion Exch. 2016, 34 (7), 579−593.
(9) Rout, A.; Binnemans, K. Influence of the Ionic Liquid Cation on
the Solvent Extraction of Trivalent Rare-Earth Ions by Mixtures of
Cyanex 923 and Ionic Liquids. Dalt. Trans. 2015, 44 (3), 1379−1387.
(10) Ansari, S. A.; Pathak, P.; Mohapatra, P. K.; Manchanda, V. K.
Chemistry of Diglycolamides: Promising Extractants for Actinide
Partitioning. Chem. Rev. 2012, 112 (3), 1751−1772.
(11) Nash, K. L. The Chemistry of TALSPEAK: A Review of the
Science. Solvent Extr. Ion Exch. 2015, 33 (1), 1−55.
(12) Mathur, J. N.; Murali, M. S.; Nash, K. L. Actinide
PartitioningA Review. Solvent Extr. Ion Exch. 2001, 19 (3), 357−
390.
(13) Su, H.; Li, Z.; Zhang, J.; Liu, W.; Zhu, Z.; Wang, L.; Qi, T.
Combining Selective Extraction and Easy Stripping of Lithium Using
a Ternary Synergistic Solvent Extraction System through Regulation
of Fe3+ Coordination. ACS Sustainable Chem. Eng. 2020, 8 (4), 1971−
1979.
(14) Li, Z.; Binnemans, K. Selective Removal of Magnesium from
Lithium-Rich Brine for Lithium Purification by Synergic Solvent
Extraction Using β-Diketones and Cyanex 923. AIChE J. 2020, 66
(7), e16246.
N
https://doi.org/10.1021/acs.iecr.1c02287
Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX
Industrial & Engineering Chemistry Research
pubs.acs.org/IECR
(15) Li, Z.; Mercken, J.; Li, X.; Riaño, S.; Binnemans, K. Efficient
and Sustainable Removal of Magnesium from Brines for Lithium/
Magnesium Separation Using Binary Extractants. ACS Sustainable
Chem. Eng. 2019, 7 (23), 19225−19234.
(16) Li, Z.; Binnemans, K. Opposite Selectivities of Tri-n-Butyl
Phosphate (TBP) and Cyanex 923 in Solvent Extraction of Lithium
and Magnesium. AIChE J. 2021, 67, e17219.
(17) Wesselborg, T.; Virolainen, S.; Sainio, T. Recovery of Lithium
from Leach Solutions of Battery Waste Using Direct Solvent
Extraction with TBP and FeCl3. Hydrometallurgy 2021, 202, 105593.
(18) Masmoudi, A.; Zante, G.; Trébouet, D.; Barillon, R.; Boltoeva,
M. Solvent Extraction of Lithium Ions Using Benzoyltrifluoroacetone
in New Solvents. Sep. Purif. Technol. 2021, 255, 117653.
(19) Torrejos, R. E. C.; Nisola, G. M.; Song, H. S.; Limjuco, L. A.;
Lawagon, C. P.; Parohinog, K. J.; Koo, S.; Han, J. W.; Chung, W.-J.
Design of Lithium Selective Crown Ethers: Synthesis, Extraction and
Theoretical Binding Studies. Chem. Eng. J. 2017, 326, 921−933.
(20) Li, Z.; Pranolo, Y.; Zhu, Z.; Cheng, C. Y. Solvent Extraction of
Cesium and Rubidium from Brine Solutions Using 4-Tert-Butyl-2-(αMethylbenzyl)-Phenol. Hydrometallurgy 2017, 171, 1−7.
(21) Rydberg, J.; Cox, M.; Musikas, C.; Choppin, G. R. Solvent
Extraction Principles and Practice, 2nd Edition; Marcel Dekker: New
York, 2004.
(22) Chandrasekar, A.; Rao, C. V. S. B; Sundararajan, M.; Ghanty, T.
K.; Sivaraman, N. Remarkable Structural Effects on the Complexation
of Actinides with H-Phosphonates: A Combined Experimental and
Quantum Chemical Study. Dalton Trans. 2018, 47 (11), 3841−3850.
(23) Omelchuk, K.; Szczepański, P.; Shrotre, A.; Haddad, M.;
Chagnes, A. Effects of Structural Changes of New Organophosphorus
Cationic Exchangers on a Solvent Extraction of Cobalt, Nickel and
Manganese from Acidic Chloride Media. RSC Adv. 2017, 7 (10),
5660−5668.
(24) Nifant’ev, I. E.; Minyaev, M. E.; Tavtorkin, A. N.; Vinogradov,
A. A.; Ivchenko, P. V. Branched Alkylphosphinic and Disubstituted
Phosphinic and Phosphonic Acids: Effective Synthesis Based on αOlefin Dimers and Applications in Lanthanide Extraction and
Separation. RSC Adv. 2017, 7 (39), 24122−24128.
(25) Du, R.; Yu, D.; An, H.; Zhang, S.; Lu, R.; Zhao, G.; Xiao, J.-C.
α,β-Substituent Effect of Dialkylphosphinic Acids on Lanthanide
Extraction. RSC Adv. 2016, 6 (61), 56004−56008.
(26) Petrova, M. A.; Kurteva, V. B.; Lubenov, L. A. Synergistic Effect
in the Solvent Extraction and Separation of Lanthanoids by 4-(4Fluorobenzoyl)-3-Methyl-1-Phenyl-Pyrazol-5-One in the Presence of
Monofunctional Neutral Organophosphorus Extractants. Ind. Eng.
Chem. Res. 2011, 50 (21), 12170−12176.
(27) Perera, J. M.; Stevens, G. W. The Role of Additives in Metal
Extraction in Oil/Water Systems. Solvent Extr. Ion Exch. 2011, 29 (3),
363−383.
(28) Cheng, C. Y.; Boddy, G.; Zhang, W.; Godfrey, M.; Robinson,
D. J.; Pranolo, Y.; Zhu, Z.; Wang, W. Recovery of Nickel and Cobalt
from Laterite Leach Solutions Using Direct Solvent Extraction: Part 1
Selection of a Synergistic SX System. Hydrometallurgy 2010, 104
(1), 45−52.
(29) Swami, K. R.; Venkatesan, K. A.; Antony, M. P. Role of Phase
Modifiers in Controlling the Third-Phase Formation During the
Solvent Extraction of Trivalent Actinides. Solvent Extr. Ion Exch. 2019,
37 (7), 500−517.
(30) Petrova, M. A.; Lachkova, V. I.; Vassilev, N. G.; Varbanov, S. G.
Effect of Diluents on the Synergistic Solvent Extraction and
Separation of Trivalent Lanthanoids with 4-Benzoyl-3-Phenyl-5Isoxazolone and Tert-Butylcalix[4]Arene Tetrakis(N,N-Dimethyl
Acetamide) and Structural Study of Gd(III) Solid Complex by IR
and NMR. Ind. Eng. Chem. Res. 2010, 49 (13), 6189−6195.
(31) Kuipa, P. K.; Hughes, M. A. Diluent Effect on the Solvent
Extraction Rate of Copper. Sep. Sci. Technol. 2002, 37 (5), 1135−
1152.
(32) Weaver, B. Solvent Extraction in the Separation of Rare Earths
and Trivalent Actinides. In Ion Exchange and Solvent Extraction;
Review
Marinsky, J. A., Marcus, Y., Eds.; Marcel Dekker: New York, 1974; pp
189−277.
(33) Nishihama, S.; Hirai, T.; Komasawa, I. Selective Extraction of Y
from a Ho/Y/Er Mixture by Liquid−Liquid Extraction in the
Presence of a Water-Soluble Complexing Agent. Ind. Eng. Chem.
Res. 2000, 39 (10), 3907−3911.
(34) Kashi, E.; Habibpour, R.; Gorzin, H.; Maleki, A. Solvent
Extraction and Separation of Light Rare Earth Elements (La, Pr and
Nd) in the Presence of Lactic Acid as a Complexing Agent by Cyanex
272 in Kerosene and the Effect of Citric Acid, Acetic Acid and
Titriplex III as Auxiliary Agents. J. Rare Earths 2018, 36 (3), 317−
323.
(35) Hala, J. Solvent Extraction of Europium from Aqueous-Organic
Solutions by Solvating Extractants. J. Radioanal. Nucl. Chem. 1998,
230 (2), 135−141.
(36) Mousa, S. B.; Altakrory, A.; Abdel Raouf, M. W.; Alian, A.
Extraction and Separation of Zinc and Cadmium Chlorides by TOPO
from Mixed Media. J. Radioanal. Nucl. Chem. 1998, 227, 143−146.
(37) Shukla, J. P.; Kedari, C. S. Solvent Extraction of Plutonium(IV)
into Dodecane by Bis(2-Ethylhexyl)Sulfoxide From Mixed AqueousOrganic Solutions. Radiochim. Acta 1992, 56 (1), 21−24.
(38) Kamitani, M.; Shibata, J.; Sano, M.; Nishimura, S. Extraction of
Metal Ions with D2EHPA from Mixed Aqueous-Organic Media.
Solvent Extr. Ion Exch. 1988, 6 (4), 605−619.
(39) Alian, A.; Sanad, W.; Khaufa, H. Extraction of Certain Elements
from Aqueous Methanol, Ethanol and Acetone by Tridodecylamine
and Tributyl Phosphate. Talanta 1968, 15 (2), 249−255.
(40) Hála, J.; Stodola, P. Solvent Extraction of Hafnium(IV) by TBP
and TOPO from Acidic Organic-Aqueous Solutions. J. Radioanal.
Chem. 1983, 80, 31−41.
(41) Habana, R. S.; Ruf, H. Distribution from Mixed Solvents
Analogy between Liquid-Liquid Extraction of Np-IV and Its Sorption
on Solid Resins. Radiochim. Acta 1976, 23, 123−126.
(42) Alian, A.; Sanad, W.; Shabaya, R. Extraction of Protactinium
from Mineral Acid-Alcohol Media. Talanta 1968, 15 (7), 639−651.
(43) Batchu, N. K.; Vander Hoogerstraete, T.; Banerjee, D.;
Binnemans, K. Non-Aqueous Solvent Extraction of Rare-Earth
Nitrates from Ethylene Glycol to n-Dodecane by Cyanex 923. Sep.
Purif. Technol. 2017, 174, 544−553.
(44) Hala, J. Solvent Extraction from Aqueous-Organic Media. In
Ion Exchange and Solvent Extraction: A Series of Advances; Marinsky, J.,
Marcus, Y., Eds.; Marcel Dekker.: New York, 1981; pp 369−410.
(45) Gutmann, V. The Donor-Acceptor Approach to Molecular
Interactions; Plenum Press: New York, 1978.
(46) Binnemans, K.; Jones, P. T. Solvometallurgy: An Emerging
Branch of Extractive Metallurgy. J. Sustain. Metall. 2017, 3 (3), 570−
600.
(47) Wilson, A. M.; Bailey, P. J.; Tasker, P. A.; Turkington, J. R.;
Grant, R. A.; Love, J. B. Solvent Extraction: The Coordination
Chemistry behind Extractive Metallurgy. Chem. Soc. Rev. 2014, 43
(1), 123−134.
(48) Eyal, A. M.; Bressler, E.; Bloch, R.; Hazan, B. Extraction of
Metal Salts by Mixtures of Water-Immiscible Amines and Organic
Acids (Acid-Base Couple Extractants). 1. A Review of Distribution
and Spectroscopic Data and of Proposed Extraction Mechanisms. Ind.
Eng. Chem. Res. 1994, 33 (5), 1067−1075.
(49) Lommelen, R.; Vander Hoogerstraete, T.; Onghena, B.; Billard,
I.; Binnemans, K. Model for Metal Extraction from Chloride Media
with Basic Extractants: A Coordination Chemistry Approach. Inorg.
Chem. 2019, 58 (18), 12289−12301.
(50) Sato, T.; Nakamura, T. The Extraction of Titanium(IV) and
Aluminium(III) from Sulphuric Acid Solutions by Di-(2-Ethylhexyl)Phosphoric Acid. Anal. Chim. Acta 1975, 76 (2), 401−408.
(51) Sato, T.; Nakamura, T.; Ikeno, M. The Extraction of Iron(III)
from Aqueous Acid Solutions by Di(2-Ethylhexyl)Phosphoric Acid.
Hydrometallurgy 1985, 15 (2), 209−217.
(52) Hansen, C. M. Hansen Solubility Parameters: A User’s
Handbook, 2nd Edition; CRC Press: Boca Raton, FL, 2007.
O
https://doi.org/10.1021/acs.iecr.1c02287
Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX
Industrial & Engineering Chemistry Research
pubs.acs.org/IECR
(53) Amézqueta, S.; Subirats, X.; Fuguet, E.; Rosés, M.; Ràfols, C.
Octanol−Water Partition Constant. In Liquid-Phase Extraction; Poole,
C. F., Ed.; Elsevier: Amsterdam, 2020; Chapter 6, pp 183−208.
(54) Akatsu, E.; Asano, M. Radiochemical Studies on the Solvent
Extraction of Several Elements in Mn(NO 3 ) 2 .6H 2 O- and
CaCl2.6H2O-Tri-n-Butyl Phosphate Systems. Anal. Chim. Acta 1971,
55 (2), 333−340.
(55) Isaac, N. M.; Fields, P. R.; Gruen, D. M. Solvent Extraction of
Actinides and Lanthanides from Molten Salts. J. Inorg. Nucl. Chem.
1961, 21, 152−168.
(56) Macchieraldo, R.; Gehrke, S.; Batchu, N. K.; Kirchner, B.;
Binnemans, K. Tuning Solvent Miscibility: A Fundamental Assessment on the Example of Induced Methanol/n-Dodecane Phase
Separation. J. Phys. Chem. B 2019, 123 (20), 4400−4407.
(57) Sørensen, J. M.; Arlt, W. Liquid-Liquid Equilibrium Data
Collection-Ternary and Quaternary Systems; Behrens, D., Eckermann,
R., Eds.; DECHEMA: Frankfurt, Germany, 1980.
(58) Sørensen, J. M.; Arlt, W. Liquid-Liquid Equilibrium Data
Collection-Ternary Systems; Behrens, D., Eckermann, R., Eds.;
DECHEMA: Frankfurt, Germany, 1980.
(59) Renon, H.; Prausnitz, J. M. Local Compositions in
Thermodynamic Excess Functions for Liquid Mixtures. AIChE J.
1968, 14 (1), 135−144.
(60) Chen, J.; Mi, J.; Fei, W.; Li, Z. Liquid-Liquid Equilibria of
Quaternary and Quinary Systems Including Sulfolane at 298.15 K. J.
Chem. Eng. Data 2001, 46 (1), 169−171.
(61) Li, Z.; Mumford, K. A.; Smith, K. H.; Chen, J.; Wang, Y.;
Stevens, G. W. Solution Structure of Isoactivity Equations for LiquidLiquid Equilibrium Calculations Using the Nonrandom Two-Liquid
Model. Ind. Eng. Chem. Res. 2016, 55 (10), 2852−2859.
(62) Song, Y.; Chen, C.-C. Symmetric Electrolyte Nonrandom TwoLiquid Activity Coefficient Model. Ind. Eng. Chem. Res. 2009, 48 (16),
7788−7797.
(63) Pitzer, K. S. Activity Coefficients in Electrolyte Solutions, 2nd
Edition; CRC Press: Boca Raton, FL, 1991.
(64) Klamt, A. The COSMO and COSMO-RS Solvation Models.
Wiley Interdiscip. Rev.: Comput. Mol. Sci. 2011, 1 (5), 699−709.
(65) Eckert, F.; Klamt, A. Fast Solvent Screening via Quantum
Chemistry: COSMO-RS Approach. AIChE J. 2002, 48 (2), 369−385.
(66) Larsen, E. M.; Trevorrow, L. V. E. The Systems Formed by
Zirconium and Hafnium Tetrachloride with Acetonitrile and Isoamyl
Ether. J. Inorg. Nucl. Chem. 1956, 2 (4), 254−259.
(67) Latimer, G. W. Distribution of Ion Pairs between Immiscible
Nonaqueous Solvents. Anal. Chem. 1963, 35 (12), 1983−1983.
(68) Fǎrcaşiu, D.; Leu, R.; Ream, P. J. The 1:1 and 2:1 Complexes of
Diethyl Ether with Tin Tetrachloride and Their Stability, Studied by
119
Sn NMR Spectroscopy. J. Chem. Soc., Perkin Trans. 2 2001, 2, 427−
432.
(69) Dean, J. A.; Eskew, J. B. Solvent Extraction with Two
Immiscible Organic Solvents Separation of Thallium(III). Anal. Lett.
1971, 4 (11), 737−743.
(70) Hála, J.; Tuck, D. G. Solvent Extraction Studies. Part VIII. The
Partition of Alkali-Metal and Ammonium Thiocyanates between Trin-Butyl Phosphate and Liquid Ammonia. J. Chem. Soc. A 1971, 0,
3437−3441.
(71) Hála, J.; Tuck, D. G. Extraction Equilibria and Solvation Effects
in the System Liquid Ammonia−Tri-n-Butyl Phosphate. In Solvent
Extraction: Proceedings of the International Solvent Extraction Conference, ISEC 71: The Hague, April 19−23, 1971; Society of Chemical
Industry, 1971; Vol. 1, pp 197−203.
(72) Baldwin, W. H.; Higgins, C. E.; Soldano, B. A. The Distribution
of Monovalent Electrolytes between Water and Tributyl Phosphate. J.
Phys. Chem. 1959, 63 (1), 118−123.
(73) Fawcett, W. R. Thermodynamic Parameters for the Solvation of
Monatomic Ions in Water. J. Phys. Chem. B 1999, 103 (50), 11181−
11185.
(74) Orabi, E. A.; Lamoureux, G. Molecular Dynamics Investigation
of Alkali Metal Ions in Liquid and Aqueous Ammonia. J. Chem.
Theory Comput. 2013, 9 (5), 2324−2338.
Review
(75) Matsui, M.; Aoki, T.; Enomoto, H.; Shigematsu, T. Nonaqueous Liquid-Liquid Extraction: Extraction of Zinc from Ethylene
Glycol Solution of Chloride by Trioctylphosphine Oxide. Anal. Lett.
1975, 8 (4), 247−255.
(76) Matsui, M.; Aoki, T.; Inoue, O.; Shigematsu, T. Nonaqueous
Liquid-Liquid Extraction. Extraction of Zinc and Cadmium from
Ethylene Glycol Solution of Bromide by Trioctylphosphine Oxide.
Bull. Inst. Chem. Res., Kyoto Univ 1974, 52, 5−6.
(77) Aoki, T. Nonaqueous Liquid-Liquid Extraction of Manganese
from Ethylene Glycol Solution with Trioctylphosphine Oxide. Bull.
Inst. Chem. Res., Kyoto Univ 1981, 59 (3), 191−195.
(78) Doe, H.; Matsui, M.; Shigematsu, T. Nonaqueous LiquidLiquid Extraction: Solvent Extraction Studies of Zinc Chloride and
Perchlorate Complexes in Glycols. Bull. Inst. Chem. Res., Kyoto Univ
1979, 57 (5−6), 343−348.
(79) Doe, H.; Matsui, M.; Shigematsu, T. Nonaqueous Extraction of
Zinc from Ethylene Glycol Solution of Lithium Perchlorate and/or
Chloride. Bull. Inst. Chem. Res., Kyoto Univ 1980, 58 (2), 133−139.
(80) Batchu, N. K.; Li, Z.; Verbelen, B.; Binnemans, K. Structural
Effects of Neutral Organophosphorus Extractants on Solvent
Extraction of Rare-Earth Elements from Aqueous and Non-Aqueous
Nitrate Solutions. Sep. Purif. Technol. 2021, 255, 117711.
(81) Batchu, N. K.; Vander Hoogerstraete, T.; Banerjee, D.;
Binnemans, K. Separation of Rare-Earth Ions from Ethylene Glycol
(+LiCl) Solutions by Non-Aqueous Solvent Extraction with Cyanex
923. RSC Adv. 2017, 7 (72), 45351−45362.
(82) Batchu, N. K.; Dewulf, B.; Riañ o, S.; Binnemans, K.
Development of a Solvometallurgical Process for the Separation of
Yttrium and Europium by Cyanex 923 from Ethylene Glycol
Solutions. Sep. Purif. Technol. 2020, 235, 116193.
(83) Dewulf, B.; Batchu, N. K.; Binnemans, K. Enhanced Separation
of Neodymium and Dysprosium by Nonaqueous Solvent Extraction
from a Polyethylene Glycol 200 Phase Using the Neutral Extractant
Cyanex 923. ACS Sustainable Chem. Eng. 2020, 8 (51), 19032−19039.
(84) Florence, T. M.; Farrar, Y. J. Liquid-Liquid Extraction of Nickel
with Long-Chain Amines from Aqueous and Nonaqueous Halide
Media. Anal. Chem. 1968, 40 (8), 1200−1206.
(85) Diaz-Torres, R.; Alvarez, S. Coordinating Ability of Anions and
Solvents towards Transition Metals and Lanthanides. Dalt. Trans.
2011, 40 (40), 10742−10750.
(86) Florence, M.; Farrar, Y. J. Liquid-Liquid Extraction of
Manganese, Chromium, and Thorium by Long-Chain Amines from
Concentrated Halide Media. Aust. J. Chem. 1969, 22, 473−476.
(87) Burns, A. R.; Cattrall, R. W. The Distribution of Copper(II)
Between Methanolic Lithium Chloride Solutions and Benzene
Solutions of Bis(3,5,5-Trimethylhexyl)Ammonium Chloride. J. Inorg.
Nucl. Chem. 1973, 35 (7), 2489−2496.
(88) Wada, Y.; Aoki, T.; Kumagai, T.; Matsui, M. Nonaqueous
Liquid-Liquid Extraction of Zinc, Cadmium and Cobaltous Ions from
Ethylene Glycol and Propylene Glycol Solutions of Chloride by Long
Chain Alkyl Amine and Alkyl Ammonium Compound. Bull. Inst.
Chem. Res., Kyoto Univ 1985, 62 (5−6), 348−355.
(89) Li, Z.; Zhang, Z.; Smolders, S.; Li, X.; Raiguel, S.; Nies, E.; De
Vos, D.; Binnemans, K. Enhancing Metal Separations by Liquid-liquid
Extraction Using Polar Solvents. Chem. - Eur. J. 2019, 25 (39), 9197−
9201.
(90) Li, Z.; Li, X.; Raiguel, S.; Binnemans, K. Separation of
Transition Metals from Rare Earths by Non-Aqueous Solvent
Extraction from Ethylene Glycol Solutions Using Aliquat 336. Sep.
Purif. Technol. 2018, 201, 318−326.
(91) Deferm, C.; Onghena, B.; Nguyen, V. T.; Banerjee, D.;
Fransaer, J.; Binnemans, K. Non-Aqueous Solvent Extraction of
Indium from an Ethylene Glycol Feed Solution by the Ionic Liquid
Cyphos IL 101: Speciation Study and Continuous Counter-Current
Process in Mixer-Settlers. RSC Adv. 2020, 10 (41), 24595−24612.
(92) Deferm, C.; Onghena, B.; Vander Hoogerstraete, T.; Banerjee,
D.; Luyten, J.; Oosterhof, H.; Fransaer, J.; Binnemans, K. Speciation
of Indium(III) Chloro Complexes in the Solvent Extraction Process
P
https://doi.org/10.1021/acs.iecr.1c02287
Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX
Industrial & Engineering Chemistry Research
pubs.acs.org/IECR
Review
Ca(NO3)2 and CaCl2. J. Radioanal. Nucl. Chem. 2011, 288 (1), 181−
187.
(112) Yamana, H.; Kaibuki, T.; Moriyama, H. Distribution
Equilibrium of Lanthanides in the Liquid-Liquid Extraction System
of TBP and Molten Ca(NO3)2·4H2O. Radiochim. Acta 1999, 84 (4),
191−200.
(113) Yamana, H.; Kaibuki, T.; Miyashita, Y.; Shibata, S.; Moriyama,
H. Equilibrium Distributions of Trivalent Actinides and Lanthanides
in the Liquid-Liquid Extraction System of TBP and Molten
Ca(NO3)2·4H2O. J. Alloys Compd. 1998, 271−273, 707−711.
(114) Yamana, H.; Asano, H.; Fujii, T.; Goto, R.; Moriyama, H. TBP
Extraction of Lanthanides from Molten Calcium Nitrate Hydrate.
Radiochim. Acta 2002, 90 (2), 87−94.
(115) Fujii, T.; Asano, H.; Kimura, T.; Yamamoto, T.; Uehara, A.;
Yamana, H. Spectroscopic Study of Trivalent Rare Earth Ions in
Calcium Nitrate Hydrate Melt. J. Alloys Compd. 2006, 408−412,
989−994.
(116) Peppard, D. F.; Driscoll, W. J.; Sironen, R. J.; McCarty, S.
Nonmonotonic Ordering of Lanthanides in Tributyl Phosphate-Nitric
Acid Extraction Systems. J. Inorg. Nucl. Chem. 1957, 4 (5−6), 326−
333.
(117) Regueiro-Figueroa, M.; Esteban-Gómez, D.; de Blas, A.;
Rodríguez-Blas, T.; Platas-Iglesias, C. Understanding Stability Trends
along the Lanthanide Series. Chem. - Eur. J. 2014, 20 (14), 3974−
3981.
(118) Cosentino, U.; Villa, A.; Pitea, D.; Moro, G.; Barone, V.
Extension of Computational Chemistry to the Study of Lanthanide(III) Ions in Aqueous Solution: Implementation and Validation of a
Continuum Solvent Approach. J. Phys. Chem. B 2000, 104 (33),
8001−8007.
(119) Davis, W., Jr.; De Bruin, H. J. New Activity Coefficients of 0−
100% Aqueous Nitric Acid. J. Inorg. Nucl. Chem. 1964, 26 (6), 1069−
1083.
(120) Rout, A.; Binnemans, K. Separation of Rare Earths from
Transition Metals by Liquid-Liquid Extraction from a Molten Salt
Hydrate to an Ionic Liquid Phase. Dalton Trans. 2014, 43 (8), 3186−
3195.
(121) Preston, J. S.; Du-Preez, A. C. Solvent Extraction Processes for
the Separation of Rare Earth Metals. In Solvent Extraction 1990;
Sekine, T., Ed.; Elsevier Science Publishers B.V.: Amsterdam, 1992;
pp 883−894.
(122) Larsson, K.; Binnemans, K. Separation of Rare Earths by SplitAnion Extraction. Hydrometallurgy 2015, 156, 206−214.
(123) Arce, A.; Earle, M. J.; Katdare, S. P.; Rodríguez, H.; Seddon,
K. R. Mutually Immiscible Ionic Liquids. Chem. Commun. 2006,
No. 24, 2548−2550.
(124) Arce, A.; Earle, M. J.; Katdare, S. P.; Rodríguez, H.; Seddon,
K. R. Phase Equilibria of Mixtures of Mutually Immiscible Ionic
Liquids. Fluid Phase Equilib. 2007, 261 (1−2), 427−433.
(125) Annat, G.; Forsyth, M.; MacFarlane, D. R. Ionic Liquid
Mixtures-Variations in Physical Properties and Their Origins in
Molecular Structure. J. Phys. Chem. B 2012, 116 (28), 8251−8258.
(126) Omar, S.; Lemus, J.; Ruiz, E.; Ferro, V. R.; Ortega, J.; Palomar,
J. Ionic Liquid Mixtures - An Analysis of Their Mutual Miscibility. J.
Phys. Chem. B 2014, 118 (9), 2442−2450.
(127) Neves, C. M. S. S.; Silva, A. M. S.; Fernandes, A. M.;
Coutinho, J. A. P.; Freire, M. G. Toward an Understanding of the
Mechanisms behind the Formation of Liquid−Liquid Systems
Formed by Two Ionic Liquids. J. Phys. Chem. Lett. 2017, 8 (13),
3015−3019.
(128) Shimomura, T.; Sugiyama, M. Phase Behavior of ImidazoliumBased Ionic Liquid + Phosphonium-Based Ionic Liquid Mixtures with
a Common Anion: Effects of the Alkyl-Chain Length of Cation. J.
Chem. Eng. Data 2018, 63 (2), 402−408.
(129) Wellens, S.; Thijs, B.; Moller, C.; Binnemans, K. Separation of
Cobalt and Nickel by Solvent Extraction with Two Mutually
Immiscible Ionic Liquids. Phys. Chem. Chem. Phys. 2013, 15 (24),
9663−9669.
from Chloride Aqueous Solutions to Ionic Liquids. Dalt. Trans. 2017,
46 (13), 4412−4421.
(93) Foos, J.; Mesplede, J. Solvent Extraction from Molten Eutectic
LiNO3-KNO3 of Some “5f” Elements by Diphenyldiphosphine
Dioxides and Tri-n-Octylphosphine Oxide. J. Inorg. Nucl. Chem.
1972, 34 (6), 2051−2056.
(94) Mesplede, J.; Porthault, M. Extraction Liquide-Liquide a Partir
de Nitrates Alcalins Fondus: Utilisation de Diphosphine Dioxyde
Comme Agent d’extraction. J. Inorg. Nucl. Chem. 1971, 33 (12),
4275−4282.
(95) Zangen, M.; Marcus, Y. Solvent Extraction from Molten Salts I.
Mercury (II) Chloride, Bromide and Iodide. Isr. J. Chem. 1964, 2 (2),
49−55.
(96) Zangen, M. Solvent Extraction from Molten Salts. II. Mixed
Mercury (II) Halides. Isr. J. Chem. 1964, 2 (3), 91−97.
(97) Zangen, M.; Marcus, Y. Solvent Extraction from Molten Salts.
III [1, 2]. Formation of Anionic Mercury (II) Mixed-Halide
Complexes. Isr. J. Chem. 1964, 2 (4), 155−165.
(98) Li, Z.; Bruynseels, B.; Zhang, Z.; Binnemans, K. Separation of
GaCl3 from AlCl3 by Solid−Liquid Extraction and Stripping Using
Anhydrous n-Dodecane and NaCl. Ind. Eng. Chem. Res. 2019, 58 (27),
12459−12464.
(99) Gal, I. J.; Mendez, J.; Irvine, J. W. Distribution of Some Simple
and Complex Anions between Molten Lithium Nitrate-Potassium
Nitrate and Tetraheptylammonium Nitrate in Polyphenyl. Inorg.
Chem. 1968, 7 (5), 985−991.
(100) Tan, Z. C. H.; Irvine, J. W. Extraction of Some Anions from
Molten Lithium Nitrate-Potassium Nitrate by Tetraoctylphosphonium Nitrate in Polyphenyl or 1-Nitronaphthalene Solvent. Inorg.
Chem. 1972, 11 (7), 1701−1707.
(101) Targhetta, J.; Mesplede, J.; Porthault, M. Synergistic Solvent
Extraction of Some “4f” Elements from Molten Alkali Nitrates at
160°C. J. Inorg. Nucl. Chem. 1974, 36 (2), 445−450.
(102) de Haas, K. S.; Brink, P. A.; Crowther, P. Solvent Extraction of
Iron, Cobalt and Nickel from Thiocyanate Melts. J. Inorg. Nucl. Chem.
1971, 33 (12), 4301−4309.
(103) Zhou, Z.; Qin, W.; Chu, Y.; Fei, W. Elucidation of the
Structures of Tributyl Phosphate/Li Complexes in the Presence of
FeCl3 via UV-Visible, Raman and IR Spectroscopy and the Method of
Continuous Variation. Chem. Eng. Sci. 2013, 101, 577−585.
(104) Durand, G.; Billon, M.; Trémillon, B. Extraction Par Solvants
Fondus: I - É quilibres de Distribution Du Zinc(II) Entre Le
Thiocyanate de Potassium et Le Phénanthrène Fondus (à 195°C).
Anal. Chim. Acta 1976, 82 (2), 349−367.
(105) Zangen, M.; David-Auslaender, J.; Kertes, A. S. Solvent
Extraction from Molten Salts-XI. Extraction of Cobalt(II) by Di-nDodecylammonium Thiocyanate. J. Inorg. Nucl. Chem. 1974, 36 (1),
218−220.
(106) Borkowska, Z.; Mielcarski, M.; Taube, M. High Temperature
Organic Extraction of Uranium, Plutonium and Americium Fused
Chlorides. J. Inorg. Nucl. Chem. 1964, 26 (2), 359−371.
(107) Maroni, V. A.; Philbin, C. E.; Yonco, R. M. Extraction of 3d
Transition Metals from Molten Cesium-Sodium-Potassium/Acetate
Eutectic Into Dodecane Using Organophosphorous Ligands. Sep. Sci.
Technol. 1983, 18 (14−15), 1699−1713.
(108) Mitsugashira, T.; Kamoshida, M.; Suzuki, Y.; Satoh, I.
Extraction of Americium and Lanthanides with Tributylphosphate
from Water-Deficient Nitrate Media. J. Alloys Compd. 1994, 213-214
(C), 347−350.
(109) Akatsu, E.; Aratono, Y. Radiochemical Studies on the LiquidLiquid Extraction of Several Elements in Fused Manganese Nitrate
Hexahydrate-Tri-n-Butyl Phosphate in Various Diluents. Anal. Chim.
Acta 1972, 62 (2), 325−335.
(110) Aratono, Y.; Akatsu, E. Liquid-Liquid Extraction of Several
Elements in the System of Fused Ca(NO3)2. 4H2O and Tri-n-Butyl
Phosphate. J. Inorg. Nucl. Chem. 1974, 36 (5), 1141−1146.
(111) Fujii, T.; Okude, G.; Uehara, A.; Sekimoto, S.; Hayashi, H.;
Akabori, M.; Minato, K.; Yamana, H. Coordination Characteristics of
Trivalent Lanthanides and Actinides in Molten Hydrate Salts of
Q
https://doi.org/10.1021/acs.iecr.1c02287
Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX
Industrial & Engineering Chemistry Research
pubs.acs.org/IECR
(130) Preston, J. S. Solvent Extraction of Cobalt and Nickel by
Organophosphorus Acids I. Comparison of Phosphoric, Phosphonic
and Phosphinic Acid Systems. Hydrometallurgy 1982, 9 (2), 115−133.
(131) Rout, A.; Wellens, S.; Binnemans, K. Separation of Rare Earths
and Nickel by Solvent Extraction with Two Mutually Immiscible Ionic
Liquids. RSC Adv. 2014, 4 (11), 5753−5758.
(132) Vander Hoogerstraete, T.; Blockx, J.; De Coster, H.;
Binnemans, K. Selective Single-Step Separation of a Mixture of
Three Metal Ions by a Triphasic Ionic-Liquid−Water−Ionic-Liquid
Solvent Extraction System. Chem. - Eur. J. 2015, 21 (33), 11757−
11766.
(133) Ammon, R. V. Die Verteilung Einiger Kationen Zwischen Den
Unmischbaren Flüssigen Phasen Im System AlBr3−KBr. J. Inorg. Nucl.
Chem. 1966, 28 (11), 2569−2578.
(134) Smith, F. J. Distribution of PdBr2, RhBr3 and RuBr3 between
the Two Immiscible Liquid Phases in the AlBr3-KBr System. J. LessCommon Met. 1984, 97, 21−26.
(135) Moore, R. H. Distribution Coefficients for Certain Actinide
and Fission Product Chlorides in the Immiscible Salt System: LiCl−
KAlCl4. J. Chem. Eng. Data 1964, 9 (4), 502−505.
(136) Greaves, T. L.; Drummond, C. J. Protic Ionic Liquids:
Properties and Applications. Chem. Rev. 2008, 108 (1), 206−237.
(137) Li, Z.; Binnemans, K. Ethylammonium Nitrate Enhances the
Extraction of Transition Metal Nitrates by Tri-n-Butyl Phosphate.
AIChE J. 2021, 67, e17213.
(138) Li, Z.; Binnemans, K. Hydration Counteracts the Separation
of Lanthanides by Solvent Extraction. AIChE J. 2020, 66 (9), e16545.
(139) Smith, E. L.; Abbott, A. P.; Ryder, K. S. Deep Eutectic
Solvents (DESs) and Their Applications. Chem. Rev. 2014, 114 (21),
11060−11082.
(140) Foreman, M. R. S. Progress towards a Process for the
Recycling of Nickel Metal Hydride Electric Cells Using a Deep
Eutectic Solvent. Cogent Chem. 2016, 2 (1), 1139289−1139299.
(141) Foreman, M. R. S. J.; Holgersson, S.; McPhee, C.;
Tyumentsev, M. S. Activity Coefficients in Deep Eutectic Solvents:
Implications for the Solvent Extraction of Metals. New J. Chem. 2018,
42 (3), 2006−2012.
(142) Cen, P.; Spahiu, K.; Tyumentsev, M. S.; Foreman, M. R. S. J.
Metal Extraction from a Deep Eutectic Solvent, an Insight into
Activities. Phys. Chem. Chem. Phys. 2020, 22 (19), 11012−11024.
(143) Albler, F.-J.; Bica, K.; Foreman, M. R. S.; Holgersson, S.;
Tyumentsev, M. S. A Comparison of Two Methods of Recovering
Cobalt from a Deep Eutectic Solvent: Implications for Battery
Recycling. J. Cleaner Prod. 2017, 167, 806−814.
(144) Riano, S.; Petranikova, M.; Onghena, B.; Vander
Hoogerstraete, T.; Banerjee, D.; Foreman, M. R. S.; Ekberg, C.;
Binnemans, K. Separation of Rare Earths and Other Valuable Metals
from Deep-Eutectic Solvents: A New Alternative for the Recycling of
Used NdFeB Magnets. RSC Adv. 2017, 7 (51), 32100−32113.
(145) Lessing, J. G. V.; Brink, P. A.; Fouché, K. F. Influence of
Cyanide on the Extraction of Transition Elements from Molten and
Aqueous Thiocyanate with Quaternary Amines. J. Inorg. Nucl. Chem.
1973, 35 (6), 2009−2015.
(146) Li, Z.; Onghena, B.; Li, X.; Zhang, Z.; Binnemans, K.
Enhancing Metal Separations Using Hydrophilic Ionic Liquids and
Analogues as Complexing Agents in the More Polar Phase of LiquidLiquid Extraction Systems. Ind. Eng. Chem. Res. 2019, 58 (34),
15628−15636.
(147) Estes, S. L.; Qiao, B.; Jin, G. B. Ion Association with Tetra-nAlkylammonium Cations Stabilizes Higher-Oxidation-State Neptunium Dioxocations. Nat. Commun. 2019, 10 (1), 59.
(148) Spathariotis, S.; Peeters, N.; Ryder, K. S.; Abbott, A. P.;
Binnemans, K.; Riaño, S. Separation of Iron(III), Zinc(II) and
Lead(II) from a Choline Chloride−Ethylene Glycol Deep Eutectic
Solvent by Solvent Extraction. RSC Adv. 2020, 10 (55), 33161−
33170.
(149) Orefice, M.; Audoor, H.; Li, Z.; Binnemans, K. Solvometallurgical Route for the Recovery of Sm, Co, Cu and Fe from SmCo
Permanent Magnets. Sep. Purif. Technol. 2019, 219, 281−289.
Review
(150) Orefice, M.; Binnemans, K. Solvometallurgical Process for the
Recovery of Rare-Earth Elements from Nd−Fe−B Magnets. Sep. Purif.
Technol. 2021, 258, 117800.
(151) Peeters, N.; Binnemans, K.; Riaño, S. Solvometallurgical
Recovery of Cobalt from Lithium-Ion Battery Cathode Materials
Using Deep-Eutectic Solvents. Green Chem. 2020, 22 (13), 4210−
4221.
(152) Nguyen, V. T.; Riaño, S.; Aktan, E.; Deferm, C.; Fransaer, J.;
Binnemans, K. Solvometallurgical Recovery of Platinum Group
Metals from Spent Automotive Catalysts. ACS Sustainable Chem.
Eng. 2021, 9 (1), 337−350.
(153) Li, X.; Monnens, W.; Li, Z.; Fransaer, J.; Binnemans, K.
Solvometallurgical Process for Extraction of Copper from Chalcopyrite and Other Sulfidic Ore Minerals. Green Chem. 2020, 22, 417−426.
(154) Monnens, W.; Deferm, C.; Sniekers, J.; Fransaer, J.;
Binnemans, K. Electrodeposition of Indium from Non-Aqueous
Electrolytes. Chem. Commun. 2019, 55 (33), 4789−4792.
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