XXXI International Mineral Processing Congress 2024 Proceedings/Washington, DC/Sep 29–Oct 3 1375
an experimental study undertaken by Zhang, Anawati and
Azimi confirmed the importance of an acidic system as it
found that REE extraction systems are notably sensitive
to increases in pH. An increase in pH caused the REE-
phosphates to precipitate, resulting in high calcium extrac-
tion and a significant reduction in REE extraction efficiency
(Zhang, Anawati et al. 2022).
The influence of pH on the formation of REE cation
was investigated by Vincent et al. A lower pH has been
demonstrated to promote higher ionisation which enabled
a higher conversion of REE cations to form complexes that
could be extracted by scCO2 (Vincent, Mukhopadhyay et
al. 2009).
Solvent Modifier
The use of solvent modifiers in extracting REEs by SFE
has been found to significantly enhance the efficiency and
selectivity of the process. Most commonly, methanol, eth-
anol, acetone, and various surfactants are used as solvent
modifiers. Solvent modifiers change polarity of supercriti-
cal fluid and react with metal complexes (Ding, Liu et al.
2017). Solvent modifiers have been demonstrated to greatly
enhance extraction efficiency, as evidenced by Yao and co-
authors. Specifically, the addition of methanol increased
extraction efficiency by 20% (Yao, Farac et al. 2018). The
positive impact was also noted in a study extracting Hg2+
where 5% methanol addition improved the SFE, however,
addition of too much may have a negative impact (Ding,
Liu et al. 2017).
CONCLUSION
Supercritical CO2 extraction has been demonstrated to
recover REEs with relatively high extraction efficiency from
various primary and secondary resources. Supercritical
CO2 is a more environmentally and economically sustain-
able alternative to traditional solvents. The extraction effi-
ciency of REEs using scCO2 extraction is highly dependent
on experimental conditions and the feed sources of REEs.
The literature review has highlighted the many influenc-
ing factors that require optimisation to achieve the high
extraction efficiencies of REEs using scCO2 extraction.
This work has provided valuable insights for future work
into the extraction of REEs and aided in understanding the
impact extent of these influencing factors on REEs extrac-
tion by scCO2 technique. This mini review reflected that
the choice of chelating agent is the most highly influen-
tial parameter on the system and that most studies have
been conducted using TBP as the chosen chelating agent
due to its high solubility in scCO2. Furthermore, tempera-
ture and pressure of the solvent system play the next most
critical roles in determining the extraction efficiency as they
maintain the system within a supercritical state. The other
parameters less significantly aid in achieving an optimally
efficient system and are highly influenced by the type of
feed source.
REFERENCES
Baek, D. L., R. V. Fox, M. E. Case, L. K. Sinclair, A. B.
Schmidt, P. R. McIlwain, B. J. Mincher and C. M.
Wai (2016). “Extraction of Rare Earth Oxides Using
Supercritical Carbon Dioxide Modified with Tri--
Butyl Phosphate-Nitric Acid Adducts.” Industrial &
Engineering Chemistry Research 55(26): 7154–7163.
Beh, C. C., R. Mammucari and N. R. Foster (2017).
“Neoteric Media as Tools for Process Intensification.”
29th Symposium of Malaysian Chemical Engineers
(Somche) 2016 206.
Beh, C. C., M. G. Wong, V. Olet and N. Foster (2019).
“Development of a novel continuous dense gas pro-
cess for the production of residual solvent-free self-
assembled nano-carriers.” Chemical Engineering and
Processing-Process Intensification 143.
Binnemans, K., P. T. Jones, B. Blanpain, T. Van Gerven, Y.
X. Yang, A. Walton and M. Buchert (2013). “Recycling
of rare earths: a critical review.” Journal of Cleaner
Production 51: 1–22.
Brown, D., B. M. Ma and Z. M. Chen (2002).
“Developments in the processing and properties of
NdFeb-type permanent magnets.” Journal of Magnetism
and Magnetic Materials 248(3): 432–440.
Burford, M. D., M. Z. Ozel, A. A. Clifford, K. D. Bartle, Y.
H. Lin, C. M. Wai and N. G. Smart (1999). “Extraction
and recovery of metals using a supercritical fluid with
chelating agents.” Analyst 124(4): 609–614.
Corbett, M. K., J. J. Eksteen, X. Z. Niu, J. P. Croue and E.
L. J. Watkin (2017). “Interactions of phosphate solu-
bilising microorganisms with natural rare-earth phos-
phate minerals: a study utilizing Western Australian
monazite.” Bioprocess and Biosystems Engineering 40(6):
929–942.
Corbett, M. K., J. J. Eksteen, X. Z. Niu and E. L. J. Watkin
(2018). “Syntrophic effect of indigenous and inocu-
lated microorganisms in the leaching of rare earth ele-
ments from Western Australian monazite.” Research in
Microbiology 169(10): 558–568.
Corbett, M. K., Eksteen, J.J., Niu, X.Z., Watkin, E.L.J.
(2017). “Incorporation of indigenous microorganisms
increases leaching rates of rare earth elements from
Western Australian monazite.” Solid State Phenomena
262: 294–298.
an experimental study undertaken by Zhang, Anawati and
Azimi confirmed the importance of an acidic system as it
found that REE extraction systems are notably sensitive
to increases in pH. An increase in pH caused the REE-
phosphates to precipitate, resulting in high calcium extrac-
tion and a significant reduction in REE extraction efficiency
(Zhang, Anawati et al. 2022).
The influence of pH on the formation of REE cation
was investigated by Vincent et al. A lower pH has been
demonstrated to promote higher ionisation which enabled
a higher conversion of REE cations to form complexes that
could be extracted by scCO2 (Vincent, Mukhopadhyay et
al. 2009).
Solvent Modifier
The use of solvent modifiers in extracting REEs by SFE
has been found to significantly enhance the efficiency and
selectivity of the process. Most commonly, methanol, eth-
anol, acetone, and various surfactants are used as solvent
modifiers. Solvent modifiers change polarity of supercriti-
cal fluid and react with metal complexes (Ding, Liu et al.
2017). Solvent modifiers have been demonstrated to greatly
enhance extraction efficiency, as evidenced by Yao and co-
authors. Specifically, the addition of methanol increased
extraction efficiency by 20% (Yao, Farac et al. 2018). The
positive impact was also noted in a study extracting Hg2+
where 5% methanol addition improved the SFE, however,
addition of too much may have a negative impact (Ding,
Liu et al. 2017).
CONCLUSION
Supercritical CO2 extraction has been demonstrated to
recover REEs with relatively high extraction efficiency from
various primary and secondary resources. Supercritical
CO2 is a more environmentally and economically sustain-
able alternative to traditional solvents. The extraction effi-
ciency of REEs using scCO2 extraction is highly dependent
on experimental conditions and the feed sources of REEs.
The literature review has highlighted the many influenc-
ing factors that require optimisation to achieve the high
extraction efficiencies of REEs using scCO2 extraction.
This work has provided valuable insights for future work
into the extraction of REEs and aided in understanding the
impact extent of these influencing factors on REEs extrac-
tion by scCO2 technique. This mini review reflected that
the choice of chelating agent is the most highly influen-
tial parameter on the system and that most studies have
been conducted using TBP as the chosen chelating agent
due to its high solubility in scCO2. Furthermore, tempera-
ture and pressure of the solvent system play the next most
critical roles in determining the extraction efficiency as they
maintain the system within a supercritical state. The other
parameters less significantly aid in achieving an optimally
efficient system and are highly influenced by the type of
feed source.
REFERENCES
Baek, D. L., R. V. Fox, M. E. Case, L. K. Sinclair, A. B.
Schmidt, P. R. McIlwain, B. J. Mincher and C. M.
Wai (2016). “Extraction of Rare Earth Oxides Using
Supercritical Carbon Dioxide Modified with Tri--
Butyl Phosphate-Nitric Acid Adducts.” Industrial &
Engineering Chemistry Research 55(26): 7154–7163.
Beh, C. C., R. Mammucari and N. R. Foster (2017).
“Neoteric Media as Tools for Process Intensification.”
29th Symposium of Malaysian Chemical Engineers
(Somche) 2016 206.
Beh, C. C., M. G. Wong, V. Olet and N. Foster (2019).
“Development of a novel continuous dense gas pro-
cess for the production of residual solvent-free self-
assembled nano-carriers.” Chemical Engineering and
Processing-Process Intensification 143.
Binnemans, K., P. T. Jones, B. Blanpain, T. Van Gerven, Y.
X. Yang, A. Walton and M. Buchert (2013). “Recycling
of rare earths: a critical review.” Journal of Cleaner
Production 51: 1–22.
Brown, D., B. M. Ma and Z. M. Chen (2002).
“Developments in the processing and properties of
NdFeb-type permanent magnets.” Journal of Magnetism
and Magnetic Materials 248(3): 432–440.
Burford, M. D., M. Z. Ozel, A. A. Clifford, K. D. Bartle, Y.
H. Lin, C. M. Wai and N. G. Smart (1999). “Extraction
and recovery of metals using a supercritical fluid with
chelating agents.” Analyst 124(4): 609–614.
Corbett, M. K., J. J. Eksteen, X. Z. Niu, J. P. Croue and E.
L. J. Watkin (2017). “Interactions of phosphate solu-
bilising microorganisms with natural rare-earth phos-
phate minerals: a study utilizing Western Australian
monazite.” Bioprocess and Biosystems Engineering 40(6):
929–942.
Corbett, M. K., J. J. Eksteen, X. Z. Niu and E. L. J. Watkin
(2018). “Syntrophic effect of indigenous and inocu-
lated microorganisms in the leaching of rare earth ele-
ments from Western Australian monazite.” Research in
Microbiology 169(10): 558–568.
Corbett, M. K., Eksteen, J.J., Niu, X.Z., Watkin, E.L.J.
(2017). “Incorporation of indigenous microorganisms
increases leaching rates of rare earth elements from
Western Australian monazite.” Solid State Phenomena
262: 294–298.