XXXI International Mineral Processing Congress 2024 Proceedings/Washington, DC/Sep 29–Oct 3 1261
ions, while the partially-hydrated ions adsorbed on the edge
surfaces cannot. The extraction process is essentially based
on an ion exchange (IEX) mechanism driven by large dif-
ferences between the hydration enthalpies (DH°) of the
tri- and monovalent cations. As compiled by Smith (1977),
the DH° values of La3+ and NH4+ ions are 3,296 and 307
kJmole–1, respectively, indicating that the former has a sub-
stantially higher affinity for water. The problems associated
with the IEX leaching process are that the concentrations
of (NH4)2SO4 required to achieve high REE+Y recoveries
exceed 0.5 M and that 3 moles of NH4+ ions are required
to remove one more Ln3+ ions to keep the stoichiometric
balance, which entails high regent consumptions and cre-
ates sustainability issues.
Seredin and Dai (2012) reported that many coal depos-
its around the world contain 0.1% REE+Y with HREE
contents and generated interest in extracting rare earths
from coal and coal byproducts. Bryan et al. (2015) analyzed
the USGS coal database and found an excellent correlation
between the REE+Y and aluminum (Al) contents, which
led to the conclusion that most of the REEs in coal are
partitioned to kaolin clay. It appeared that this conclusion
was supported by the seemingly successful IEX leaching
tests conducted by Rozelle et al. (2016). The authors were
able to extract 60–70% of the REEs from two U.S. coal
byproducts using the standard IEX leaching process using
(NH4)2SO4 as lixiviant. These results, coupled with the
finding that clay minerals constitute 60–70% of the min-
eral matter in coal (Renton, 1982), led to extensive studies
to extract REEs from thickener underflows, in which clayey
refuse materials congregate, with high hopes of extracting
REE+Y using the simple IEX leaching process.
The authors of this communication found that REEs
are indeed partitioned to ash-forming clays rather than car-
bonaceous materials, as suggested by Bryan et al. (2015),
and that the clayey materials show higher HREE-to-LREE
ratios than conventional rare earth resources, e.g., monazite
and bastnäsite. It appears that these findings agree with the
recent work of Foley et al. (2015), who showed that the cli-
mate and geological conditions of the Central Appalachia
region are close to those of South China, where 80% of
the HREEs are produced commercially. These investigators
discovered 4 to 5 small IAC deposits in the Appalachian
Mountains in Virginia and South Carolina and showed
that much of the REEs can be readily extracted from the
IAC samples by ion-exchange leaching using (NH4)2SO4
and complexing agents as lixiviants.
It has been found, however, that the clayey coal
byproducts taken from operating coal preparation plants
did not respond as well as the IAC samples taken from the
Appalachian Mountains. The authors of the present inves-
tigation carried out fundamental studies to identify the
Figure 1. A model for ion-adsorption clay (Borst et al., 2020): HREE (red), LREE
(pink), oxygen (grey), blue (hydrogen)
ions, while the partially-hydrated ions adsorbed on the edge
surfaces cannot. The extraction process is essentially based
on an ion exchange (IEX) mechanism driven by large dif-
ferences between the hydration enthalpies (DH°) of the
tri- and monovalent cations. As compiled by Smith (1977),
the DH° values of La3+ and NH4+ ions are 3,296 and 307
kJmole–1, respectively, indicating that the former has a sub-
stantially higher affinity for water. The problems associated
with the IEX leaching process are that the concentrations
of (NH4)2SO4 required to achieve high REE+Y recoveries
exceed 0.5 M and that 3 moles of NH4+ ions are required
to remove one more Ln3+ ions to keep the stoichiometric
balance, which entails high regent consumptions and cre-
ates sustainability issues.
Seredin and Dai (2012) reported that many coal depos-
its around the world contain 0.1% REE+Y with HREE
contents and generated interest in extracting rare earths
from coal and coal byproducts. Bryan et al. (2015) analyzed
the USGS coal database and found an excellent correlation
between the REE+Y and aluminum (Al) contents, which
led to the conclusion that most of the REEs in coal are
partitioned to kaolin clay. It appeared that this conclusion
was supported by the seemingly successful IEX leaching
tests conducted by Rozelle et al. (2016). The authors were
able to extract 60–70% of the REEs from two U.S. coal
byproducts using the standard IEX leaching process using
(NH4)2SO4 as lixiviant. These results, coupled with the
finding that clay minerals constitute 60–70% of the min-
eral matter in coal (Renton, 1982), led to extensive studies
to extract REEs from thickener underflows, in which clayey
refuse materials congregate, with high hopes of extracting
REE+Y using the simple IEX leaching process.
The authors of this communication found that REEs
are indeed partitioned to ash-forming clays rather than car-
bonaceous materials, as suggested by Bryan et al. (2015),
and that the clayey materials show higher HREE-to-LREE
ratios than conventional rare earth resources, e.g., monazite
and bastnäsite. It appears that these findings agree with the
recent work of Foley et al. (2015), who showed that the cli-
mate and geological conditions of the Central Appalachia
region are close to those of South China, where 80% of
the HREEs are produced commercially. These investigators
discovered 4 to 5 small IAC deposits in the Appalachian
Mountains in Virginia and South Carolina and showed
that much of the REEs can be readily extracted from the
IAC samples by ion-exchange leaching using (NH4)2SO4
and complexing agents as lixiviants.
It has been found, however, that the clayey coal
byproducts taken from operating coal preparation plants
did not respond as well as the IAC samples taken from the
Appalachian Mountains. The authors of the present inves-
tigation carried out fundamental studies to identify the
Figure 1. A model for ion-adsorption clay (Borst et al., 2020): HREE (red), LREE
(pink), oxygen (grey), blue (hydrogen)