1372 XXXI International Mineral Processing Congress 2024 Proceedings/Washington, DC/Sep 29–Oct 3
Temperature
Temperature is a critical factor in SFE, as it directly affects
the solubility of the REEs, the viscosity and density of the
scCO2, and the diffusion rates. In general, high operating
temperature results in high extraction efficiency of REEs
in SFE. As temperature increases, the solubility of REEs
in scCO2 increases due to the weakening of intermolecu-
lar forces as well as the viscosity and density of the scCO2
decrease, leading to better penetration of scCO2 and
hence, improved extraction efficiency and faster extraction.
However, there is an upper limit to temperature as high
temperatures can cause thermal degradation of the system
(Shimizu, Sawada et al. 2005).
The effect of temperature on the extraction efficiency
of REEs was investigated by Zhu et al., where the pressure
was fixed at 21 MPa and the temperature varied between
35 and 60 °C for the scCO2 extraction of Nd2O3 with a
TBP-HNO3 adduct (Zhu, Duan et al. 2009). Zhu and co-
authors concluded that an increase in temperature leads to
greater extraction efficiency, which is consistent with the
findings by others (Zhu, Duan et al. 2009, Duan, Cao et al.
2010). This effect on the system may be due to the increased
reaction between chelating complex and the metal oxide to
form the metal-chelating agent complex (Zhu, Duan et al.
2009). The increase in temperature would lead to a signifi-
cant increase in operational cost and safety concerns for the
system. To optimise the system both operational efficiency
and cost efficiencies must be considered for the overall via-
bility of the project.
Pressure
Pressure is an important parameter as pressure affects the
solubility of the REEs in scCO2, which, in turn, impacts
extraction efficiencies of REEs. An increase in pressure gen-
erally increases the density of scCO2, making it easier to
dissolve the REEs. Shimizu and co-authors confirmed that
higher pressure levels result in higher extraction efficiencies
by comparing the extraction of REEs at atmospheric and
critical pressures. It was found that greater extraction effi-
ciency could be achieved with pressures within the super-
critical range. The results reflected that Y and Eu extraction
efficiencies were 37.4 and 36.8% for atmospheric pressure,
respectively, compared to 99.7 and 99.8% for the same feed
material under supercritical pressure of 15 MPa (Shimizu,
Sawada et al. 2005). These results confirm the theoretical
understanding of supercritical fluid behaviour with fluids
experiencing enhanced solvating and diffusion capabilities.
The varying pressures from 15 to 30 MPa were inves-
tigated at a constant temperature of 50 °C by Zhu et
al. This report found that the increase in pressure had a
negative impact on extraction efficiency. It was noted that
the change in pressure may have impacted the reactivity of
the TBP-HNO3 complex to be reduced (Zhu, Duan et al.
2009). Typically, pressure would have a positive impact on
SFE system, however this study reflected that the additional
components such as the chelating agent used in the system
are important in determining the optimal conditions as
they directly impact the extraction efficacy.
Based on the literature in Table 1, the optimal scCO2
extraction pressure will be within 15 to 25 MPa to achieve
the highest extraction efficiencies of REEs. Higher pres-
sures for this system would also be avoided if lower pres-
sures were as efficient due to the increased operational cost
and safety concerns with higher pressures.
Chelating Agents
The use of chelating agents has been investigated through-
out literature for SFE with the chosen optimal agent being
highly dependent on the system. The selection is essential
for the efficiency and efficacy of the SFE, and extraction
would not be possible without these chelating agents,
where the scCO2 serves as the solvent and diluent for these
ligands. The ligands widely explored for use in SFE include
dithiocarbonates, β-diketones, organophosphorus agents,
and macrocyclic compounds (Burford, Ozel et al. 1999).
Organophosphorus agents are the most used chelating
agents for both conventional solvent extraction and SFE.
Generally, REEs in their oxide form are extracted by scCO2
with tributyl phosphate (TBP) as a chelating agent which
is classified as an organophosphorus agent (Ding, Liu et
al. 2017). The high extraction efficiencies of REEs using
chelating agents are evident in Table 1.
Baek et al. investigated TBP and nitric acid (TBP-
HNO3) as chelating agents, modifying it to create a new
adduct TBP-[(HNO3)1.7(H2O)0.6], which was prepared
by using fuming (90%) HNO3 and TBP. The extraction
occurred at 65.85 °C under 34.5 MPa. Baek et al. reported
under these conditions collectively one of the most optimal
recovery efficiencies seen within the literature for light REE
extraction. The efficiencies were Y (99%), Ce (0.12%),
Eu (99%), Tb (92.1%) and Dy (98.5%) (Baek, Fox et al.
2016).
The efficiencies achieved through scCO2 extraction are
highly dependent on the choice of agent as the solubility
of agent in scCO2. The greater the solubility of the agent
in scCO2 the higher efficiencies that can be achieved (Zhu,
Duan et al. 2009). This explains the optimal recoveries
achieved by Baek et al. as TBP and HNO3 have high solu-
bility in scCO2 and notably, according to Wai, Gopalan
Temperature
Temperature is a critical factor in SFE, as it directly affects
the solubility of the REEs, the viscosity and density of the
scCO2, and the diffusion rates. In general, high operating
temperature results in high extraction efficiency of REEs
in SFE. As temperature increases, the solubility of REEs
in scCO2 increases due to the weakening of intermolecu-
lar forces as well as the viscosity and density of the scCO2
decrease, leading to better penetration of scCO2 and
hence, improved extraction efficiency and faster extraction.
However, there is an upper limit to temperature as high
temperatures can cause thermal degradation of the system
(Shimizu, Sawada et al. 2005).
The effect of temperature on the extraction efficiency
of REEs was investigated by Zhu et al., where the pressure
was fixed at 21 MPa and the temperature varied between
35 and 60 °C for the scCO2 extraction of Nd2O3 with a
TBP-HNO3 adduct (Zhu, Duan et al. 2009). Zhu and co-
authors concluded that an increase in temperature leads to
greater extraction efficiency, which is consistent with the
findings by others (Zhu, Duan et al. 2009, Duan, Cao et al.
2010). This effect on the system may be due to the increased
reaction between chelating complex and the metal oxide to
form the metal-chelating agent complex (Zhu, Duan et al.
2009). The increase in temperature would lead to a signifi-
cant increase in operational cost and safety concerns for the
system. To optimise the system both operational efficiency
and cost efficiencies must be considered for the overall via-
bility of the project.
Pressure
Pressure is an important parameter as pressure affects the
solubility of the REEs in scCO2, which, in turn, impacts
extraction efficiencies of REEs. An increase in pressure gen-
erally increases the density of scCO2, making it easier to
dissolve the REEs. Shimizu and co-authors confirmed that
higher pressure levels result in higher extraction efficiencies
by comparing the extraction of REEs at atmospheric and
critical pressures. It was found that greater extraction effi-
ciency could be achieved with pressures within the super-
critical range. The results reflected that Y and Eu extraction
efficiencies were 37.4 and 36.8% for atmospheric pressure,
respectively, compared to 99.7 and 99.8% for the same feed
material under supercritical pressure of 15 MPa (Shimizu,
Sawada et al. 2005). These results confirm the theoretical
understanding of supercritical fluid behaviour with fluids
experiencing enhanced solvating and diffusion capabilities.
The varying pressures from 15 to 30 MPa were inves-
tigated at a constant temperature of 50 °C by Zhu et
al. This report found that the increase in pressure had a
negative impact on extraction efficiency. It was noted that
the change in pressure may have impacted the reactivity of
the TBP-HNO3 complex to be reduced (Zhu, Duan et al.
2009). Typically, pressure would have a positive impact on
SFE system, however this study reflected that the additional
components such as the chelating agent used in the system
are important in determining the optimal conditions as
they directly impact the extraction efficacy.
Based on the literature in Table 1, the optimal scCO2
extraction pressure will be within 15 to 25 MPa to achieve
the highest extraction efficiencies of REEs. Higher pres-
sures for this system would also be avoided if lower pres-
sures were as efficient due to the increased operational cost
and safety concerns with higher pressures.
Chelating Agents
The use of chelating agents has been investigated through-
out literature for SFE with the chosen optimal agent being
highly dependent on the system. The selection is essential
for the efficiency and efficacy of the SFE, and extraction
would not be possible without these chelating agents,
where the scCO2 serves as the solvent and diluent for these
ligands. The ligands widely explored for use in SFE include
dithiocarbonates, β-diketones, organophosphorus agents,
and macrocyclic compounds (Burford, Ozel et al. 1999).
Organophosphorus agents are the most used chelating
agents for both conventional solvent extraction and SFE.
Generally, REEs in their oxide form are extracted by scCO2
with tributyl phosphate (TBP) as a chelating agent which
is classified as an organophosphorus agent (Ding, Liu et
al. 2017). The high extraction efficiencies of REEs using
chelating agents are evident in Table 1.
Baek et al. investigated TBP and nitric acid (TBP-
HNO3) as chelating agents, modifying it to create a new
adduct TBP-[(HNO3)1.7(H2O)0.6], which was prepared
by using fuming (90%) HNO3 and TBP. The extraction
occurred at 65.85 °C under 34.5 MPa. Baek et al. reported
under these conditions collectively one of the most optimal
recovery efficiencies seen within the literature for light REE
extraction. The efficiencies were Y (99%), Ce (0.12%),
Eu (99%), Tb (92.1%) and Dy (98.5%) (Baek, Fox et al.
2016).
The efficiencies achieved through scCO2 extraction are
highly dependent on the choice of agent as the solubility
of agent in scCO2. The greater the solubility of the agent
in scCO2 the higher efficiencies that can be achieved (Zhu,
Duan et al. 2009). This explains the optimal recoveries
achieved by Baek et al. as TBP and HNO3 have high solu-
bility in scCO2 and notably, according to Wai, Gopalan