1346 XXXI International Mineral Processing Congress 2024 Proceedings/Washington, DC/Sep 29–Oct 3
and Dyer 2023 Lim and Alorro 2021). Treating these low-
grade feedstocks through a pyrometallurgical route would
not be economically feasible because of their concentration
(Binnemans and Jones 2017). In the case of hydrometallur-
gical processing, the consumption of aqueous acidic solu-
tion would be significantly high for low-grade feedstocks as
these solutions have very poor selectivity (Binnemans and
Jones 2017). This would cause many impurities and gangue
minerals to go into the solution, producing impure leach-
ates (Binnemans and Jones 2017). Alkaline solutions have
increased selectivity only for certain metals. Therefore, it is
advisable to look for other alternative options for process-
ing from low-grade materials (Binnemans and Jones 2017).
In an era where sustainability and economic viability
are prioritised, the quest for green processes for a clean
and more efficient route has become crucial. One such
green process is solvometallurgy, which involves the usage
of non-aqueous solvents for processing. This includes
organic solvents, ionic liquids, deep eutectic solvents, and
inorganic solvents such as concentrated sulphuric acid,
supercritical carbon dioxide, etc. (Binnemans and Jones
2017). These solvents do not indicate that they work only
under anhydrous conditions but with a low water content.
Solvometallurgical processes are similar to hydrometallurgi-
cal ones except for using non-aqueous solvents (Binnemans
and Jones 2017). Several studies have been done with ionic
liquids and deep eutectic solvents to extract rare earth ele-
ments. Ionic liquids are selective and an excellent choice due
to their low vapour pressure (Vanda et al. 2018). However,
these solvents are quite expensive and can be toxic to the
environment (Vanda et al. 2018). Deep eutectic solvents
have overcome these limitations and have gained attention
recently because of their low toxicity and low cost of con-
stituent components (Vanda et al. 2018).
Deep Eutectic Solvents (DES) are a class of green sol-
vents characterised by significant depressions in melting
points compared to their individual components. Abbott
et al. (2003) described DES as a mixture of hydrogen bond
acceptors (HBA) and hydrogen bond donors (HBD) at a
certain composition. There are four types of DES, among
which Type I DES have been extensively studied, combining
a quaternary ammonium salt and a metal chloride (Abbott
et al. 2003 Binnemans and Jones 2017). Type II DES con-
sists of a quaternary ammonium salt and a metal chloride
hydrate, Type III DES consists of a quaternary ammonium
salt and an HBD (an organic molecule like carboxylic
acid, amide, or polyol), and Type IV DES is a mixture of
metal chloride hydrate and an HBD (Binnemans and Jones
2017). The commonly followed preparation method for
DES involves heating and stirring the constituents of the
DES together under an inert atmosphere until a homoge-
neous liquid is formed (Binnemans and Jones 2017 Abbott
et al. 2003 2004). A few other methods of preparation of
DESs include vacuum evaporation, grinding, and freeze-
drying (Zhang et al. 2012).
A few studies have been explored for extracting rare
earth elements using DES. Chen et al. (2019) studied
the solubilities of different rare earth oxides with DES.
They observed that ethylene glycol and maleic acid (4:1)
were good solvents for separating rare earth oxides. They
achieved a good separation between the light (La2O3 and
CeO2) and heavy rare earths (Y2O3 and Sm2O3) because
of the difference in solubilities (Chen et al. 2019). Liu et
al. (2020) studied the extraction of neodymium from used
NdFeB magnets by dissolving it in guanidine hydrochlo-
ride (GUC) based DES. They observed that combining
GUC and lactic acid achieved a very high separation factor
between Fe and Nd (Liu et al., 2020). Another study used
single carbonate salts of La, Y, Ce, Nd, and Sm (Entezari-
Zarandi and Larachi, 2019). They obtained a better dis-
solution rate with DES based on choline chloride, malonic
acid, and urea. Also, they observed that the DES selectively
dissolved heavy rare earth elements compared to light rare
earth elements (Entezari-Zarandi and Larachi 2019). These
studies proposed that oxalic acid precipitation would be
suitable for recovering these elements from the leach solu-
tion (Entezari-Zarandi and Larachi 2019 Chen et al. 2019
Liu et al. 2020). Also, a levulinic acid-choline chloride sys-
tem was used to recover yttrium and europium from spent
fluorescent lamps, and they noted that the solubility of
YOX phosphor was high. In contrast, the HALO phosphor
was low (Pateli et al., 2020). On optimising the parameters,
they found that the results were comparable to the ionic
liquids ([Hbet][NTf2]) however, the DES is much cheaper
(Pateli et al. 2020). For this DES system, oxalic acid was
unsuitable for precipitating the elements as it dissolved
some of the ChCl (Pateli et al., 2020). The authors sug-
gested that the metals could be extracted using D2EPHA
for solvent extraction and stripped using an aqueous hydro-
chloric acid (Pateli et al., 2020).
From the studies conducted, it was seen that the deep
eutectic solvents have increased selectivity over certain ele-
ments. This increased selectivity can aid in the separation
of rare earth elements. Also, the literature survey shows
that extracting rare earth elements using deep eutectic sol-
vents has been explored primarily for secondary sources
like NdFeB magnets, discarded e-waste, etc. Not many
studies have been reported on the extraction from primary
sources and primary wastes, such as ores and mine wastes.
Therefore, this study would investigate the applicability
and Dyer 2023 Lim and Alorro 2021). Treating these low-
grade feedstocks through a pyrometallurgical route would
not be economically feasible because of their concentration
(Binnemans and Jones 2017). In the case of hydrometallur-
gical processing, the consumption of aqueous acidic solu-
tion would be significantly high for low-grade feedstocks as
these solutions have very poor selectivity (Binnemans and
Jones 2017). This would cause many impurities and gangue
minerals to go into the solution, producing impure leach-
ates (Binnemans and Jones 2017). Alkaline solutions have
increased selectivity only for certain metals. Therefore, it is
advisable to look for other alternative options for process-
ing from low-grade materials (Binnemans and Jones 2017).
In an era where sustainability and economic viability
are prioritised, the quest for green processes for a clean
and more efficient route has become crucial. One such
green process is solvometallurgy, which involves the usage
of non-aqueous solvents for processing. This includes
organic solvents, ionic liquids, deep eutectic solvents, and
inorganic solvents such as concentrated sulphuric acid,
supercritical carbon dioxide, etc. (Binnemans and Jones
2017). These solvents do not indicate that they work only
under anhydrous conditions but with a low water content.
Solvometallurgical processes are similar to hydrometallurgi-
cal ones except for using non-aqueous solvents (Binnemans
and Jones 2017). Several studies have been done with ionic
liquids and deep eutectic solvents to extract rare earth ele-
ments. Ionic liquids are selective and an excellent choice due
to their low vapour pressure (Vanda et al. 2018). However,
these solvents are quite expensive and can be toxic to the
environment (Vanda et al. 2018). Deep eutectic solvents
have overcome these limitations and have gained attention
recently because of their low toxicity and low cost of con-
stituent components (Vanda et al. 2018).
Deep Eutectic Solvents (DES) are a class of green sol-
vents characterised by significant depressions in melting
points compared to their individual components. Abbott
et al. (2003) described DES as a mixture of hydrogen bond
acceptors (HBA) and hydrogen bond donors (HBD) at a
certain composition. There are four types of DES, among
which Type I DES have been extensively studied, combining
a quaternary ammonium salt and a metal chloride (Abbott
et al. 2003 Binnemans and Jones 2017). Type II DES con-
sists of a quaternary ammonium salt and a metal chloride
hydrate, Type III DES consists of a quaternary ammonium
salt and an HBD (an organic molecule like carboxylic
acid, amide, or polyol), and Type IV DES is a mixture of
metal chloride hydrate and an HBD (Binnemans and Jones
2017). The commonly followed preparation method for
DES involves heating and stirring the constituents of the
DES together under an inert atmosphere until a homoge-
neous liquid is formed (Binnemans and Jones 2017 Abbott
et al. 2003 2004). A few other methods of preparation of
DESs include vacuum evaporation, grinding, and freeze-
drying (Zhang et al. 2012).
A few studies have been explored for extracting rare
earth elements using DES. Chen et al. (2019) studied
the solubilities of different rare earth oxides with DES.
They observed that ethylene glycol and maleic acid (4:1)
were good solvents for separating rare earth oxides. They
achieved a good separation between the light (La2O3 and
CeO2) and heavy rare earths (Y2O3 and Sm2O3) because
of the difference in solubilities (Chen et al. 2019). Liu et
al. (2020) studied the extraction of neodymium from used
NdFeB magnets by dissolving it in guanidine hydrochlo-
ride (GUC) based DES. They observed that combining
GUC and lactic acid achieved a very high separation factor
between Fe and Nd (Liu et al., 2020). Another study used
single carbonate salts of La, Y, Ce, Nd, and Sm (Entezari-
Zarandi and Larachi, 2019). They obtained a better dis-
solution rate with DES based on choline chloride, malonic
acid, and urea. Also, they observed that the DES selectively
dissolved heavy rare earth elements compared to light rare
earth elements (Entezari-Zarandi and Larachi 2019). These
studies proposed that oxalic acid precipitation would be
suitable for recovering these elements from the leach solu-
tion (Entezari-Zarandi and Larachi 2019 Chen et al. 2019
Liu et al. 2020). Also, a levulinic acid-choline chloride sys-
tem was used to recover yttrium and europium from spent
fluorescent lamps, and they noted that the solubility of
YOX phosphor was high. In contrast, the HALO phosphor
was low (Pateli et al., 2020). On optimising the parameters,
they found that the results were comparable to the ionic
liquids ([Hbet][NTf2]) however, the DES is much cheaper
(Pateli et al. 2020). For this DES system, oxalic acid was
unsuitable for precipitating the elements as it dissolved
some of the ChCl (Pateli et al., 2020). The authors sug-
gested that the metals could be extracted using D2EPHA
for solvent extraction and stripped using an aqueous hydro-
chloric acid (Pateli et al., 2020).
From the studies conducted, it was seen that the deep
eutectic solvents have increased selectivity over certain ele-
ments. This increased selectivity can aid in the separation
of rare earth elements. Also, the literature survey shows
that extracting rare earth elements using deep eutectic sol-
vents has been explored primarily for secondary sources
like NdFeB magnets, discarded e-waste, etc. Not many
studies have been reported on the extraction from primary
sources and primary wastes, such as ores and mine wastes.
Therefore, this study would investigate the applicability