1366 XXXI International Mineral Processing Congress 2024 Proceedings/Washington, DC/Sep 29–Oct 3
experimental parameters will be compared and discussed in
the later sections.
CONVENTIONAL RECOVERY METHODS
OF REES
The extraction of REEs from both primary and secondary
resources has been studied extensively due to the growing
necessity for the resources. Traditional extraction meth-
ods for REEs from ores such as monazite and bastnaesite
are well-established, and have been in use for many years
(Gupta and Krishnamurthy 2005). Originally, the refining
methods were predicated on the leaching of components
with high-strength mineral acids, namely sulfuric acid,
nitric acid, or hydrochloric acid (HCl). The current meth-
ods used for extraction vary with ore type. For instance,
there are two major extraction methods for bastnaesite
((La,Y) (CO3)F). One was developed by Molycorp for their
Mountain Pass mine which utilised HCl to extract REEs as
chlorides and then extract fluorides by converting them to
hydroxides using sodium hydroxide, from which they are
leached to chlorides (Gupta and Krishnamurthy 2005, De
Lima 2015). The second industrial method is sulfuric acid
roasting, which is when bastnaesite concentrate is heated
at high temperatures in 98% sulfuric acid (Gupta and
Krishnamurthy 2005). However, this process poses serious
environmental and health concerns, as the process releases
hydrogen fluoride gas.
The extraction measures of monazite have additional
concerns due to the presence of radioactive minerals such
as uranium and thorium, with thorium content being
present between 4 and 12% (De Lima 2015). One of the
main leaching technologies currently used is the alkaline
method. The REEs are present as refractory phosphates,
and they are digested in a hot sodium hydroxide solu-
tion at 140˚C (Gupta and Krishnamurthy 2005, Kuzmin,
Pashkov et al. 2012). The residue is then washed with hot
water and leached with a mineral acid of choice. This pro-
cess results in extraction rates of up to 98%, with even
low-grade monazite ore, but it also results in the extrac-
tion of thorium, which leads to safety concerns during the
separation process, in which thorium accumulation can be
dangerous (Kuzmin, Pashkov et al. 2012, De Lima 2015).
A few bioprocessing technologies have also been researched
(Corbett 2017, Corbett, Eksteen et al. 2017, Corbett,
Eksteen et al. 2018, Fathollahzadeh 2018, Fathollahzadeh
2018, Fathollahzadeh, Eksteen et al. 2019, Fathollahzadeh,
Khaleque et al. 2019, Van Alin, Corbett et al. 2023, Van
Alin, Corbett et al. 2023), which utilise phosphate solu-
bilising microorganisms to leach REEs from their phos-
phate minerals. However, leach rates are low, and nutrients
and energy sources have to be provided contrary to sulfide
mineral-solubilising bacteria, where the chemical energy is
provided by the bio-oxidation of the minerals themselves.
Traditional solvent extraction methods use large
quantities of acids and organics, a primary environmental
concern as they produce toxic waste. Increasing environ-
mental awareness has caused significant increases in legal
requirements regarding the disposal of conventional liquid
solvents that are considered hazardous, inherently driving
the increase in the cost of using these types of solvents for
extraction (Wai, Gopalan et al. 2003).
The negative implications of traditional commercial
REE extraction methods have led to an increasing focus on
alternative technologies that enable similar or higher effi-
ciencies using greener methods. Supercritical fluid extrac-
tion is a green alternative extraction technique that has
reflected promising results.
SUPERCRITICAL FLUID EXTRACTION
Supercritical fluid extraction (SFE) is an emerging green
technology that has been studied extensively for selective
metal extraction, particularly metal chelates. Industrial-
scale SFE has been conducted for organics extraction since
the 1930s–1940s (Fox, Ball et al. 2004). Supercritical fluid
is used to replace conventional solvents, where its super-
critical state enables it to have enhanced solvating capabili-
ties. To achieve the supercritical state, the fluid is heated
and compressed above its critical temperature and pressure,
allowing the supercritical fluid to attain characteristics of
both liquids and gases at these conditions in a dynamic
equilibrium. The conditions used in SFE to extract REEs
from their feed sources depend on various factors such as
the type of chelating agents, operating temperature, pres-
sure, and flow rate (Yao, Farac et al. 2018). Supercritical
fluid utilises gas-like diffusivity, liquid-like mass transfer
and the effective solvating capability to be a more efficient
solvent over traditional organic solvents as well as enable
the transfer of solutes from porous solids. Supercritical fluid
technology underpins the pathway to clean energy. For
extraction of REEs by SFE, the quantities of solvent used
in conventional processes are reduced significantly, hence,
reducing hazardous material utility costs while improving
poor environmental practices presently used (Wai, Gopalan
et al. 2003).
In some applications, SFE has shown to consume less
energy and result in higher recovery than other extraction
techniques, such as acid leaching, which could lead to
high energy costs and environmental impacts (Yao, Farac
et al. 2018). Phase separation of the solvent and solute is
achieved easily through depressurisation (Wai, Gopalan et
experimental parameters will be compared and discussed in
the later sections.
CONVENTIONAL RECOVERY METHODS
OF REES
The extraction of REEs from both primary and secondary
resources has been studied extensively due to the growing
necessity for the resources. Traditional extraction meth-
ods for REEs from ores such as monazite and bastnaesite
are well-established, and have been in use for many years
(Gupta and Krishnamurthy 2005). Originally, the refining
methods were predicated on the leaching of components
with high-strength mineral acids, namely sulfuric acid,
nitric acid, or hydrochloric acid (HCl). The current meth-
ods used for extraction vary with ore type. For instance,
there are two major extraction methods for bastnaesite
((La,Y) (CO3)F). One was developed by Molycorp for their
Mountain Pass mine which utilised HCl to extract REEs as
chlorides and then extract fluorides by converting them to
hydroxides using sodium hydroxide, from which they are
leached to chlorides (Gupta and Krishnamurthy 2005, De
Lima 2015). The second industrial method is sulfuric acid
roasting, which is when bastnaesite concentrate is heated
at high temperatures in 98% sulfuric acid (Gupta and
Krishnamurthy 2005). However, this process poses serious
environmental and health concerns, as the process releases
hydrogen fluoride gas.
The extraction measures of monazite have additional
concerns due to the presence of radioactive minerals such
as uranium and thorium, with thorium content being
present between 4 and 12% (De Lima 2015). One of the
main leaching technologies currently used is the alkaline
method. The REEs are present as refractory phosphates,
and they are digested in a hot sodium hydroxide solu-
tion at 140˚C (Gupta and Krishnamurthy 2005, Kuzmin,
Pashkov et al. 2012). The residue is then washed with hot
water and leached with a mineral acid of choice. This pro-
cess results in extraction rates of up to 98%, with even
low-grade monazite ore, but it also results in the extrac-
tion of thorium, which leads to safety concerns during the
separation process, in which thorium accumulation can be
dangerous (Kuzmin, Pashkov et al. 2012, De Lima 2015).
A few bioprocessing technologies have also been researched
(Corbett 2017, Corbett, Eksteen et al. 2017, Corbett,
Eksteen et al. 2018, Fathollahzadeh 2018, Fathollahzadeh
2018, Fathollahzadeh, Eksteen et al. 2019, Fathollahzadeh,
Khaleque et al. 2019, Van Alin, Corbett et al. 2023, Van
Alin, Corbett et al. 2023), which utilise phosphate solu-
bilising microorganisms to leach REEs from their phos-
phate minerals. However, leach rates are low, and nutrients
and energy sources have to be provided contrary to sulfide
mineral-solubilising bacteria, where the chemical energy is
provided by the bio-oxidation of the minerals themselves.
Traditional solvent extraction methods use large
quantities of acids and organics, a primary environmental
concern as they produce toxic waste. Increasing environ-
mental awareness has caused significant increases in legal
requirements regarding the disposal of conventional liquid
solvents that are considered hazardous, inherently driving
the increase in the cost of using these types of solvents for
extraction (Wai, Gopalan et al. 2003).
The negative implications of traditional commercial
REE extraction methods have led to an increasing focus on
alternative technologies that enable similar or higher effi-
ciencies using greener methods. Supercritical fluid extrac-
tion is a green alternative extraction technique that has
reflected promising results.
SUPERCRITICAL FLUID EXTRACTION
Supercritical fluid extraction (SFE) is an emerging green
technology that has been studied extensively for selective
metal extraction, particularly metal chelates. Industrial-
scale SFE has been conducted for organics extraction since
the 1930s–1940s (Fox, Ball et al. 2004). Supercritical fluid
is used to replace conventional solvents, where its super-
critical state enables it to have enhanced solvating capabili-
ties. To achieve the supercritical state, the fluid is heated
and compressed above its critical temperature and pressure,
allowing the supercritical fluid to attain characteristics of
both liquids and gases at these conditions in a dynamic
equilibrium. The conditions used in SFE to extract REEs
from their feed sources depend on various factors such as
the type of chelating agents, operating temperature, pres-
sure, and flow rate (Yao, Farac et al. 2018). Supercritical
fluid utilises gas-like diffusivity, liquid-like mass transfer
and the effective solvating capability to be a more efficient
solvent over traditional organic solvents as well as enable
the transfer of solutes from porous solids. Supercritical fluid
technology underpins the pathway to clean energy. For
extraction of REEs by SFE, the quantities of solvent used
in conventional processes are reduced significantly, hence,
reducing hazardous material utility costs while improving
poor environmental practices presently used (Wai, Gopalan
et al. 2003).
In some applications, SFE has shown to consume less
energy and result in higher recovery than other extraction
techniques, such as acid leaching, which could lead to
high energy costs and environmental impacts (Yao, Farac
et al. 2018). Phase separation of the solvent and solute is
achieved easily through depressurisation (Wai, Gopalan et