1868 XXXI International Mineral Processing Congress 2024 Proceedings/Washington, DC/Sep 29–Oct 3
impact of hydrometallurgical processes, deep eutectic sol-
vents (DESs), with good thermal and chemical stability,
low vapor pressure, and low or practically negligible toxic-
ity, have potential as a green solvent for cleaner processes
(Martín et al., 2023). The DESs comprise eutectic mix-
tures of Lewis or Brønsted acids and bases, encompassing
diverse anionic and/or cationic species. The solvents have
a lower melting point than the pure substances due to the
charge delocalization caused by the hydrogen bond formed
between hydrogen bond acceptor (HBA) and hydrogen
bond donor (HBD), and therefore DESs can exist as a liq-
uid at room temperature (Smith et al., 2014). Among 5
types of DESs, type III DES, which uses carboxylic acid
as HBD, is excel in dissolving transition metals species,
encompassing chlorides and oxides (Smith et al., 2014
Yuan et al., 2022).
There have been prior studies on complete or selective
leaching of REEs from NdFeB magnets using DESs (Riaño
et al., 2017 Liu et al., 2020 Yang et al., 2024), but the pre-
treatment process was limited to oxidative roasting and was
only performed on a small leaching scale. Therefore, in this
study, DES and pretreatment method that can selectively
leach REEs from NdFeB magnets were selected. In addi-
tion, in the expanded leaching scale, the leaching behavior
over time was confirmed and assessed the effect of solid/
liquid (S/L) ratio.
EXPERIMENTAL
Preparation of DESs
In this study, ethylene glycol (EG)-maleic acid (MA) and
guanidine hydrochloride (GUC)-lactic acid (LA) DESs
were used, which are effective in leaching light rare earth
oxides (REOs) (Chen et al., 2019 Liu et al., 2020). For
each DESs, EG and GUC were used as HBA, and MA and
LA were used as HBD. EG-MA and GUC-LA were mixed
at a molar ratio of 4:1 and 1:2, at 60 and 70°C, respectively,
until homogeneous, and then cooled at room temperature
before use.
Materials
The NdFeB permanent magnets used in this study were
provided by a Korean permanent magnet manufacturer.
The magnets were crushed using a jaw crusher and ground
via a roll mill and vibrating cup mill. After that, the magnets
were ground using a shatter box and the particle below 325
mesh (44 μm) was used. The composition of the NdFeB
magnet shown in Table 1 was analyzed using inductively
coupled plasma optical emission spectrometer (ICP-OES,
Perkin Elmer, Optima 5300 DV) and inductively coupled
plasma mass spectrometer (ICP-MS, Perkin Elmer, Elan
DRC II) after dissolution.
Pretreatment
The pretreatment was applied to convert the metal elements
in NdFeB magnet into oxide form. First, the most common
methods, oxidative roasting, was performed at 900°C for 5
h using a muffle furnace. The second is a method that com-
bines NaOH digestion and oxidative roasting. The magnet
powder was added to a 50% NaOH solution at a 0.1 g/g
S/L ratio and stirred at 145°C for 5 h. After washing and
drying, the powder was roasted at 450°C for 3 h. The crys-
tal structure of magnet powder after pretreatment was ana-
lyzed using XRD (D8 ADVANCE, DRUKER).
Leaching
Leaching test was performed using glass vials and a double-
jacketed reactor. The preliminary dissolution tests using
REOs and iron oxides (Fe2O3 and Fe3O4) and leaching
tests to select a pretreatment method were conducted in
a 40 mL glass vial applying ultrasonication at a 0.01 g/g
S/L ratio and 70°C for 5 h. The leaching behavior of opti-
mal pretreated magnet powder was investigated in a 250
mL double-jacketed reactor at 70°C and 250 rpm for 3h
to assess the effect of S/L ratio. Samples were collected at
regular time intervals and filtered through a 0.2 μm syringe
filter. Post leaching, the mixture was centrifuged at 17000
rpm for 20 min and filtered through a glass fiber filter. The
leaching solution was then diluted in 2% nitric acid, and
the metallic element concentrations were analyzed using
ICP-OES. The leaching residue was washed, dried, and
then dissolved for analyzing the content of metal elements
using ICP-OES. The leaching efficiency (L) was calculated
according to Eq. 1:
%h L M M
M
L R
L =+^(1)
where ML and MR represents the metal mass in the leach-
ing solution (g) and the leaching residue (g), respectively.
RESULTS AND DISCUSSION
Preliminary Dissolution Tests
In DESs, metal oxides have different solubility depend-
ing on their type, and this difference in solubility can be
utilized for selective leaching of a target metal. Therefore,
the leaching efficiency of Nd, the target element, and Fe,
Table 1. Chemical composition of the NdFeB magnet
Element Fe Nd Pr Dy Tb
Content, wt.% 67.7 26.9 1.39 2.68 0.038
impact of hydrometallurgical processes, deep eutectic sol-
vents (DESs), with good thermal and chemical stability,
low vapor pressure, and low or practically negligible toxic-
ity, have potential as a green solvent for cleaner processes
(Martín et al., 2023). The DESs comprise eutectic mix-
tures of Lewis or Brønsted acids and bases, encompassing
diverse anionic and/or cationic species. The solvents have
a lower melting point than the pure substances due to the
charge delocalization caused by the hydrogen bond formed
between hydrogen bond acceptor (HBA) and hydrogen
bond donor (HBD), and therefore DESs can exist as a liq-
uid at room temperature (Smith et al., 2014). Among 5
types of DESs, type III DES, which uses carboxylic acid
as HBD, is excel in dissolving transition metals species,
encompassing chlorides and oxides (Smith et al., 2014
Yuan et al., 2022).
There have been prior studies on complete or selective
leaching of REEs from NdFeB magnets using DESs (Riaño
et al., 2017 Liu et al., 2020 Yang et al., 2024), but the pre-
treatment process was limited to oxidative roasting and was
only performed on a small leaching scale. Therefore, in this
study, DES and pretreatment method that can selectively
leach REEs from NdFeB magnets were selected. In addi-
tion, in the expanded leaching scale, the leaching behavior
over time was confirmed and assessed the effect of solid/
liquid (S/L) ratio.
EXPERIMENTAL
Preparation of DESs
In this study, ethylene glycol (EG)-maleic acid (MA) and
guanidine hydrochloride (GUC)-lactic acid (LA) DESs
were used, which are effective in leaching light rare earth
oxides (REOs) (Chen et al., 2019 Liu et al., 2020). For
each DESs, EG and GUC were used as HBA, and MA and
LA were used as HBD. EG-MA and GUC-LA were mixed
at a molar ratio of 4:1 and 1:2, at 60 and 70°C, respectively,
until homogeneous, and then cooled at room temperature
before use.
Materials
The NdFeB permanent magnets used in this study were
provided by a Korean permanent magnet manufacturer.
The magnets were crushed using a jaw crusher and ground
via a roll mill and vibrating cup mill. After that, the magnets
were ground using a shatter box and the particle below 325
mesh (44 μm) was used. The composition of the NdFeB
magnet shown in Table 1 was analyzed using inductively
coupled plasma optical emission spectrometer (ICP-OES,
Perkin Elmer, Optima 5300 DV) and inductively coupled
plasma mass spectrometer (ICP-MS, Perkin Elmer, Elan
DRC II) after dissolution.
Pretreatment
The pretreatment was applied to convert the metal elements
in NdFeB magnet into oxide form. First, the most common
methods, oxidative roasting, was performed at 900°C for 5
h using a muffle furnace. The second is a method that com-
bines NaOH digestion and oxidative roasting. The magnet
powder was added to a 50% NaOH solution at a 0.1 g/g
S/L ratio and stirred at 145°C for 5 h. After washing and
drying, the powder was roasted at 450°C for 3 h. The crys-
tal structure of magnet powder after pretreatment was ana-
lyzed using XRD (D8 ADVANCE, DRUKER).
Leaching
Leaching test was performed using glass vials and a double-
jacketed reactor. The preliminary dissolution tests using
REOs and iron oxides (Fe2O3 and Fe3O4) and leaching
tests to select a pretreatment method were conducted in
a 40 mL glass vial applying ultrasonication at a 0.01 g/g
S/L ratio and 70°C for 5 h. The leaching behavior of opti-
mal pretreated magnet powder was investigated in a 250
mL double-jacketed reactor at 70°C and 250 rpm for 3h
to assess the effect of S/L ratio. Samples were collected at
regular time intervals and filtered through a 0.2 μm syringe
filter. Post leaching, the mixture was centrifuged at 17000
rpm for 20 min and filtered through a glass fiber filter. The
leaching solution was then diluted in 2% nitric acid, and
the metallic element concentrations were analyzed using
ICP-OES. The leaching residue was washed, dried, and
then dissolved for analyzing the content of metal elements
using ICP-OES. The leaching efficiency (L) was calculated
according to Eq. 1:
%h L M M
M
L R
L =+^(1)
where ML and MR represents the metal mass in the leach-
ing solution (g) and the leaching residue (g), respectively.
RESULTS AND DISCUSSION
Preliminary Dissolution Tests
In DESs, metal oxides have different solubility depend-
ing on their type, and this difference in solubility can be
utilized for selective leaching of a target metal. Therefore,
the leaching efficiency of Nd, the target element, and Fe,
Table 1. Chemical composition of the NdFeB magnet
Element Fe Nd Pr Dy Tb
Content, wt.% 67.7 26.9 1.39 2.68 0.038