2
it is essential to assess and adopt greener alternative meth-
ods, like bioleaching or using less harmful reagents [15],
[16].
DESs are systems formed from eutectic mixtures of Lewis
or Brønsted acids and bases, which can comprise various
anionic and cationic species.[17]. The eutectic combina-
tions have melting points lower than their components,
giving a room-temperature ionic liquid [18]. They are liq-
uid at temperatures lower than 150 °C, but most are liquid
between room temperatures and 70 °C [19]. DES is typi-
cally created by combining a quaternary ammonium salt
with metal salts or a hydrogen bond donor (HBD) that
can form a complex with the quaternary ammonium salt’s
halide anion. The lower melting point of the mixture com-
pared to its components is because of the charge delocaliza-
tion occurring through the hydrogen bonding of the halide
ion and the hydrogen donor [20]. DESs are defined using
the general formula Cat+X–zY where Cat+ is an ammo-
nium, phosphonium, or sulfonium cation. X is a Lewis
base, a halide anion generally, Y is a Brønsted acid, while
z denotes the number of Y molecules interacting with the
anion [17]. Although DESs differ slightly from traditional
ionic liquids in that they are mixtures rather than isolated
salt compounds, their anhydrous nature and shared char-
acteristics make them suitable for metal recovery. Choline
Chloride [ChCl, HOC2H4N+(CH3)3Cl) is widely used as
a halide salt. Figure 1 shows the typical structures of some
halide salts and hydrogen bond donors for DES synthe-
ses, while Table 1 shows the classification of DESs formed
based on the nature of the complexing agent used.
A few non-hydrated metal halides have sufficiently
low melting points to produce Type I DESs [17]. Due to
the relatively low cost of the hydrated metal halides, the
Type II DESs can be used in large-scale industrial pro-
cesses. Type III DESs have attracted significant interest
because of their capacity to solvate various transition metal
species, including chlorides [22] and oxides [23]. Reline,
Ethaline, and Oxaline, which are Type III DESs, have been
widely used for their efficiency [17], [19] [22] because it
has shown excellent efficiency for dissolution and leaching
of various commonly available metal oxides, precious met-
als (e.g., Au), and valuable rare earth elements (e.g., Y, La,
Ce, Nd, and Sm) from ores or metal-bearing solids [24].
This research will focus on The Type III DESs due to their
advantages over the Type I and II DES.
The Type III DESs are biodegradable, relatively
cheaper, adaptable for specific applications, have a rela-
tively wide potential window, and can solvate a wide range
of transition metal species, including chlorides [21], [25],
[26], [27].
This work was designed to find a suitable DES to
achieve high-efficiency REE extraction from their complex
ore. Five different DESs were synthesized and tested. After
determining the best DES, more in-depth assessments will
be conducted to evaluate the impact of process conditions
on total REE recovery.
MATERIALS AND METHODS
This chapter discusses experimental procedures for the
research, including the chemicals and characterization
instruments used.
Figure 1. Typical structures of some halide salts and
hydrogen bond donors for DES syntheses [25]
Table 1 General Formula for Classification of DESs [21]
Type General Formula Terms
Type I Cat+X–
Z MCl
x M =Zn, Sn, Fe, Al, Ga
Type II Cat+X–ZMClx.yH2O M =Cr, Co, Cu, Ni, Fe
Type III Cat+X–ZRZ Z= CONH2, COOH, OH
Type IV MCl
x +RZ =MCl
x-1 *RZ +MCl
x-1 M =Al, Zn and Z =CONH
2 ,OH
it is essential to assess and adopt greener alternative meth-
ods, like bioleaching or using less harmful reagents [15],
[16].
DESs are systems formed from eutectic mixtures of Lewis
or Brønsted acids and bases, which can comprise various
anionic and cationic species.[17]. The eutectic combina-
tions have melting points lower than their components,
giving a room-temperature ionic liquid [18]. They are liq-
uid at temperatures lower than 150 °C, but most are liquid
between room temperatures and 70 °C [19]. DES is typi-
cally created by combining a quaternary ammonium salt
with metal salts or a hydrogen bond donor (HBD) that
can form a complex with the quaternary ammonium salt’s
halide anion. The lower melting point of the mixture com-
pared to its components is because of the charge delocaliza-
tion occurring through the hydrogen bonding of the halide
ion and the hydrogen donor [20]. DESs are defined using
the general formula Cat+X–zY where Cat+ is an ammo-
nium, phosphonium, or sulfonium cation. X is a Lewis
base, a halide anion generally, Y is a Brønsted acid, while
z denotes the number of Y molecules interacting with the
anion [17]. Although DESs differ slightly from traditional
ionic liquids in that they are mixtures rather than isolated
salt compounds, their anhydrous nature and shared char-
acteristics make them suitable for metal recovery. Choline
Chloride [ChCl, HOC2H4N+(CH3)3Cl) is widely used as
a halide salt. Figure 1 shows the typical structures of some
halide salts and hydrogen bond donors for DES synthe-
ses, while Table 1 shows the classification of DESs formed
based on the nature of the complexing agent used.
A few non-hydrated metal halides have sufficiently
low melting points to produce Type I DESs [17]. Due to
the relatively low cost of the hydrated metal halides, the
Type II DESs can be used in large-scale industrial pro-
cesses. Type III DESs have attracted significant interest
because of their capacity to solvate various transition metal
species, including chlorides [22] and oxides [23]. Reline,
Ethaline, and Oxaline, which are Type III DESs, have been
widely used for their efficiency [17], [19] [22] because it
has shown excellent efficiency for dissolution and leaching
of various commonly available metal oxides, precious met-
als (e.g., Au), and valuable rare earth elements (e.g., Y, La,
Ce, Nd, and Sm) from ores or metal-bearing solids [24].
This research will focus on The Type III DESs due to their
advantages over the Type I and II DES.
The Type III DESs are biodegradable, relatively
cheaper, adaptable for specific applications, have a rela-
tively wide potential window, and can solvate a wide range
of transition metal species, including chlorides [21], [25],
[26], [27].
This work was designed to find a suitable DES to
achieve high-efficiency REE extraction from their complex
ore. Five different DESs were synthesized and tested. After
determining the best DES, more in-depth assessments will
be conducted to evaluate the impact of process conditions
on total REE recovery.
MATERIALS AND METHODS
This chapter discusses experimental procedures for the
research, including the chemicals and characterization
instruments used.
Figure 1. Typical structures of some halide salts and
hydrogen bond donors for DES syntheses [25]
Table 1 General Formula for Classification of DESs [21]
Type General Formula Terms
Type I Cat+X–
Z MCl
x M =Zn, Sn, Fe, Al, Ga
Type II Cat+X–ZMClx.yH2O M =Cr, Co, Cu, Ni, Fe
Type III Cat+X–ZRZ Z= CONH2, COOH, OH
Type IV MCl
x +RZ =MCl
x-1 *RZ +MCl
x-1 M =Al, Zn and Z =CONH
2 ,OH