XXXI International Mineral Processing Congress 2024 Proceedings/Washington, DC/Sep 29–Oct 3 1367
al. 2003). This thus motivated the research into the poten-
tial to use scCO2 as a possible mobilising agent for REEs.
As the maturity of the application of scCO2 in this field is
limited, the current review may be valuable as a foundation
for continued research.
Supercritical Carbon Dioxide Extraction for the
Recovery of REEs
Carbon dioxide is commonly used in SFE due to its wide
availability, inexpensive, non-toxic, non-flammable, inert,
and ease of recyclability (Beh, Mammucari et al. 2017, Beh,
Wong et al. 2019). Supercritical CO2 has a moderate critical
temperature of 31.1 °C and critical pressure of 7.37 MPa,
implying that it is easy to obtain supercritical conditions for
this fluid. Supercritical CO2 is used in various applications
including the pharmaceutical, food, and agriculture indus-
tries for extracting organics such as organic solvents from
pharmaceutical products as well as essential oils from herbs
and flowers such as rosemary, turmeric and chamomile
(Das, Gaustad et al. 2018). Supercritical CO2 extraction
is also useful in the energy industry for the direct lique-
faction of coal (Zhang, Anawati et al. 2022). In addition,
scCO2 has been applied as a solvent for extracting inorgan-
ics, including REEs, under various experimental conditions
(Das, Gaustad et al. 2018). The high solvation strength of
scCO2 on REEs has been demonstrated with recovery effi-
ciencies as high as 99% from the literature. Supercritical
CO2 extraction requires minimal reagent input, especially
compared to conventional extraction.
The use of scCO2 as a solvent for inorganic material
has been developed as a selective extraction method due
to its tuneable properties by varying the density of scCO2.
Generally, scCO2 is a poor solvent for polar or ionic com-
pounds due to CO2 being a linear molecule with no dipole
moment. Therefore, chelating agents are required to dis-
solve the REEs as CO2-soluble metal chelates in scCO2.
Subsequently, the metal-chelates in scCO2 can be chemi-
cally reduced to the elemental state for metal deposition in
the fluid phase (Wai, Gopalan et al. 2003). Lastly, REEs can
be recovered upon depressurisation of the scCO2 extraction
system.
Laintz and co-authors reported the first use of a che-
lating method to extract transition metals with scCO2 in
1992. The study investigated the impact of introducing
fluorinated dithiocarbamate chelating agent into scCO2
extraction system to extract metal Cu2+ ions from both
an aqueous solution and solid surface. The study found
that the absence of the chelating agent led to no metal
ion extraction (Laintz, Wai et al. 1992). In addition, other
chelating agents such as β-diketones, organophosphosrus
reagents, and macrocyclic ligands were also studied for the
extraction of metal species using scCO2 extraction (Wai
2002, Vincent, Mukhopadhyay et al. 2009). In the recent
decade a novel chelating agent tributyl phosphate-nitric
adduct (TBP-HNO3), has been investigated for the extrac-
tion of REEs from their oxide form using scCO2. Table 1
summarises the recent literature recovering REEs by scCO2
extraction under various operating conditions.
PARAMETERS AFFECTING RECOVERY
OF REES BY SUPERCRITICAL CO2
EXTRACTION
Supercritical fluid extraction has been shown to have a
high potential as an effective and environmentally friendly
technique for extracting REEs from multiple sources. The
extraction process is influenced by various parameters such
as temperature, pressure, REE sources, chelating agents,
mechanical activation, impurities, water content, pH, resi-
dence time, agitation, presence of solvent modifier and flow
rate of solvent. The major parameters are described in the
sections below.
REE Sources
Sources of REEs are found all over the world, with the
largest reserves located in China. Other significant sources
of REEs include Australia, United States, Brazil, India,
and Malaysia. These elements can be found in primary
sources such as minerals and clays within the earth’s crust.
Secondary sources such as the recycling of electronic prod-
ucts is also becoming an important source of REEs. As
the demand for REEs continues to grow, it is important
to diversify the sources of these crucial minerals to prevent
over-reliance on any one country, region, or source (Gupta
and Krishnamurthy 2005).
Primary Sources
Mineralised ore bodies are considered the primary sources
of REEs as well as in phosphates and ion adsorption clays
deposits. These top three sources with high rare earth oxide
(REO) contents are bastnaesite [(Ce, La,Y)CO3(OH,F)],
monazite [(REEs,Th)PO4], and xenotime [Y(PO4)] (Gupta
and Krishnamurthy 2005).
Bastnaesite deposits are flurocarbonates with approxi-
mately 70% REO by weight in the mineralisation, with
mostly lighter element and lower or no thorium concentra-
tions. Monazite deposits contain REEs that are phosphates
with the presence of thorium. The presence of significant
levels of fluoride and thorium in these ore bodies make
them toxic and radioactive, which pose safety challenges
for processing. Xenotime deposits exhibit similar properties
al. 2003). This thus motivated the research into the poten-
tial to use scCO2 as a possible mobilising agent for REEs.
As the maturity of the application of scCO2 in this field is
limited, the current review may be valuable as a foundation
for continued research.
Supercritical Carbon Dioxide Extraction for the
Recovery of REEs
Carbon dioxide is commonly used in SFE due to its wide
availability, inexpensive, non-toxic, non-flammable, inert,
and ease of recyclability (Beh, Mammucari et al. 2017, Beh,
Wong et al. 2019). Supercritical CO2 has a moderate critical
temperature of 31.1 °C and critical pressure of 7.37 MPa,
implying that it is easy to obtain supercritical conditions for
this fluid. Supercritical CO2 is used in various applications
including the pharmaceutical, food, and agriculture indus-
tries for extracting organics such as organic solvents from
pharmaceutical products as well as essential oils from herbs
and flowers such as rosemary, turmeric and chamomile
(Das, Gaustad et al. 2018). Supercritical CO2 extraction
is also useful in the energy industry for the direct lique-
faction of coal (Zhang, Anawati et al. 2022). In addition,
scCO2 has been applied as a solvent for extracting inorgan-
ics, including REEs, under various experimental conditions
(Das, Gaustad et al. 2018). The high solvation strength of
scCO2 on REEs has been demonstrated with recovery effi-
ciencies as high as 99% from the literature. Supercritical
CO2 extraction requires minimal reagent input, especially
compared to conventional extraction.
The use of scCO2 as a solvent for inorganic material
has been developed as a selective extraction method due
to its tuneable properties by varying the density of scCO2.
Generally, scCO2 is a poor solvent for polar or ionic com-
pounds due to CO2 being a linear molecule with no dipole
moment. Therefore, chelating agents are required to dis-
solve the REEs as CO2-soluble metal chelates in scCO2.
Subsequently, the metal-chelates in scCO2 can be chemi-
cally reduced to the elemental state for metal deposition in
the fluid phase (Wai, Gopalan et al. 2003). Lastly, REEs can
be recovered upon depressurisation of the scCO2 extraction
system.
Laintz and co-authors reported the first use of a che-
lating method to extract transition metals with scCO2 in
1992. The study investigated the impact of introducing
fluorinated dithiocarbamate chelating agent into scCO2
extraction system to extract metal Cu2+ ions from both
an aqueous solution and solid surface. The study found
that the absence of the chelating agent led to no metal
ion extraction (Laintz, Wai et al. 1992). In addition, other
chelating agents such as β-diketones, organophosphosrus
reagents, and macrocyclic ligands were also studied for the
extraction of metal species using scCO2 extraction (Wai
2002, Vincent, Mukhopadhyay et al. 2009). In the recent
decade a novel chelating agent tributyl phosphate-nitric
adduct (TBP-HNO3), has been investigated for the extrac-
tion of REEs from their oxide form using scCO2. Table 1
summarises the recent literature recovering REEs by scCO2
extraction under various operating conditions.
PARAMETERS AFFECTING RECOVERY
OF REES BY SUPERCRITICAL CO2
EXTRACTION
Supercritical fluid extraction has been shown to have a
high potential as an effective and environmentally friendly
technique for extracting REEs from multiple sources. The
extraction process is influenced by various parameters such
as temperature, pressure, REE sources, chelating agents,
mechanical activation, impurities, water content, pH, resi-
dence time, agitation, presence of solvent modifier and flow
rate of solvent. The major parameters are described in the
sections below.
REE Sources
Sources of REEs are found all over the world, with the
largest reserves located in China. Other significant sources
of REEs include Australia, United States, Brazil, India,
and Malaysia. These elements can be found in primary
sources such as minerals and clays within the earth’s crust.
Secondary sources such as the recycling of electronic prod-
ucts is also becoming an important source of REEs. As
the demand for REEs continues to grow, it is important
to diversify the sources of these crucial minerals to prevent
over-reliance on any one country, region, or source (Gupta
and Krishnamurthy 2005).
Primary Sources
Mineralised ore bodies are considered the primary sources
of REEs as well as in phosphates and ion adsorption clays
deposits. These top three sources with high rare earth oxide
(REO) contents are bastnaesite [(Ce, La,Y)CO3(OH,F)],
monazite [(REEs,Th)PO4], and xenotime [Y(PO4)] (Gupta
and Krishnamurthy 2005).
Bastnaesite deposits are flurocarbonates with approxi-
mately 70% REO by weight in the mineralisation, with
mostly lighter element and lower or no thorium concentra-
tions. Monazite deposits contain REEs that are phosphates
with the presence of thorium. The presence of significant
levels of fluoride and thorium in these ore bodies make
them toxic and radioactive, which pose safety challenges
for processing. Xenotime deposits exhibit similar properties