XXXI International Mineral Processing Congress 2024 Proceedings/Washington, DC/Sep 29–Oct 3 1373
and Jacobs, at conditions of 14 MPa and 50 °C, the com-
plex is entirely miscible (Wai, Gopalan et al. 2003).
Equations 1 and 2 reflect TBP and HNO3 acting as
chelating agents by interacting with the trivalent lan-
thanides to produce a CO2-soluble complex. The nitrate
anions act to form a nitrate salt with the trivalent lantha-
nide, and then with polar phosphate group on the TBP
molecules substituting for coordinated water around the
metal cation, the complex then becomes soluble (Sinclair,
Baek et al. 2017). The change in solubility of the metal cat-
ion is due to the shielding mechanism provided by the non-
polar butyl groups, allowing charge neutralization of the
ion to become soluble in scCO2 (Burford, Ozel et al. 1999,
Sinclair, Baek et al. 2017). This shielding mechanism is
essential for the extraction as it makes the solubilisation of
the metal cation into scCO2. This solubility is highly influ-
enced by the extraction conditions, however, due to metal
organophosphate complexes involving ion-pair extraction,
it is challenging to obtain solubility data (Smart, Carleson
et al. 1997).
REE O 6HNO 2REE 6NO 3H O
2 3 3
3+
3 2 "+++-(1)
REE^NO 4TBP REE TBP
3 3 3 3 4 "+^NO h h (2)
There are two mechanisms for deploying chelating agents
in SFE, these are in-situ or on-line. In-situ is where scCO2
is introduced and interacts with the chelating agent first,
or where the chelating agent interacts with the metal prior
to the introduction of scCO2 as a batch process. The on-
line method feeds the extraction system with scCO2 and
metal complexes at specified flow rates through a mixing
joint, Sawada et al. utilised this method when conduct-
ing an Nd extraction with SFE (Sawada, Hirabayashi et al.
2008, Ding, Liu et al. 2017). The main distinction between
the techniques is the different mixing and dissolution pro-
cesses, however it has been noted within literature that both
methods have led to efficient extraction.
A chelating agent is a type of chemical compound
such as Di-(2ethylhexyl)phosphoric acid (D2EHPA) and
TBP which are explored for REE extraction. These com-
pounds have the ability to bond with metal ions by form-
ing multiple bonds between the agent and the metal. This
bond involves coordination between the metal ion and the
donor atoms in the chelating agent, forming a stable com-
plex. This complex enables the metal to become soluble in
scCO2. Figure 1 compares the recovery of REEs using two
organophosphate chelating agents, D2EHPA and TBP.
It is evident that TBP has resulted in higher extraction
efficiencies for the lighter REEs, which gradually decrease
for the heavier REEs. Overall, D2EHPA reflects the higher
extraction efficiencies, especially for the heavier REEs. This
may be due to D2EHPA creating a more stable and soluble
REE complex to be extracted. The use of chelating agent
will depend on which REE extraction is most desired as
different chelating agents will have greater selectivity and
interaction with some REE over others.
Pre-Treatment
Pre-treatment is required for REEs because these elements
occur together in minerals and or secondary sources and
50
55
60
65
70
75
80
85
90
95
100
La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu Th U
REE
TBP
D2EHPA
Figure 1. Effect of chelating agents on the recovery of REEs by scCO
2 extraction (Chart plotted using data
from (Samsonov, Trofimov et al. 2015, Samsonov, Trofimov et al. 2016)
RECOVERY
(%)
and Jacobs, at conditions of 14 MPa and 50 °C, the com-
plex is entirely miscible (Wai, Gopalan et al. 2003).
Equations 1 and 2 reflect TBP and HNO3 acting as
chelating agents by interacting with the trivalent lan-
thanides to produce a CO2-soluble complex. The nitrate
anions act to form a nitrate salt with the trivalent lantha-
nide, and then with polar phosphate group on the TBP
molecules substituting for coordinated water around the
metal cation, the complex then becomes soluble (Sinclair,
Baek et al. 2017). The change in solubility of the metal cat-
ion is due to the shielding mechanism provided by the non-
polar butyl groups, allowing charge neutralization of the
ion to become soluble in scCO2 (Burford, Ozel et al. 1999,
Sinclair, Baek et al. 2017). This shielding mechanism is
essential for the extraction as it makes the solubilisation of
the metal cation into scCO2. This solubility is highly influ-
enced by the extraction conditions, however, due to metal
organophosphate complexes involving ion-pair extraction,
it is challenging to obtain solubility data (Smart, Carleson
et al. 1997).
REE O 6HNO 2REE 6NO 3H O
2 3 3
3+
3 2 "+++-(1)
REE^NO 4TBP REE TBP
3 3 3 3 4 "+^NO h h (2)
There are two mechanisms for deploying chelating agents
in SFE, these are in-situ or on-line. In-situ is where scCO2
is introduced and interacts with the chelating agent first,
or where the chelating agent interacts with the metal prior
to the introduction of scCO2 as a batch process. The on-
line method feeds the extraction system with scCO2 and
metal complexes at specified flow rates through a mixing
joint, Sawada et al. utilised this method when conduct-
ing an Nd extraction with SFE (Sawada, Hirabayashi et al.
2008, Ding, Liu et al. 2017). The main distinction between
the techniques is the different mixing and dissolution pro-
cesses, however it has been noted within literature that both
methods have led to efficient extraction.
A chelating agent is a type of chemical compound
such as Di-(2ethylhexyl)phosphoric acid (D2EHPA) and
TBP which are explored for REE extraction. These com-
pounds have the ability to bond with metal ions by form-
ing multiple bonds between the agent and the metal. This
bond involves coordination between the metal ion and the
donor atoms in the chelating agent, forming a stable com-
plex. This complex enables the metal to become soluble in
scCO2. Figure 1 compares the recovery of REEs using two
organophosphate chelating agents, D2EHPA and TBP.
It is evident that TBP has resulted in higher extraction
efficiencies for the lighter REEs, which gradually decrease
for the heavier REEs. Overall, D2EHPA reflects the higher
extraction efficiencies, especially for the heavier REEs. This
may be due to D2EHPA creating a more stable and soluble
REE complex to be extracted. The use of chelating agent
will depend on which REE extraction is most desired as
different chelating agents will have greater selectivity and
interaction with some REE over others.
Pre-Treatment
Pre-treatment is required for REEs because these elements
occur together in minerals and or secondary sources and
50
55
60
65
70
75
80
85
90
95
100
La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu Th U
REE
TBP
D2EHPA
Figure 1. Effect of chelating agents on the recovery of REEs by scCO
2 extraction (Chart plotted using data
from (Samsonov, Trofimov et al. 2015, Samsonov, Trofimov et al. 2016)
RECOVERY
(%)