1652 XXXI International Mineral Processing Congress 2024 Proceedings/Washington, DC/Sep 29–Oct 3
between 6,600 to 20,400 tons/year of Sc based on the cur-
rent RM production rates [15].
The main constituents of RM typically include
Hematite, Goethite, Rutile, Anatase, Anhydrite, Cancrinite,
Gibbsite, Diaspore, Katoite, and Kaolinite [17–19]. Fe is
primarily present in Hematite and Goethite, as indicated
by Scanning Electron Microscopy – Energy Dispersive
Spectroscopy (SEM-EDS) and time-of-flight secondary ion
mass spectroscopy (TOF-SIMS) [17–22]. Studies by Vind
et al., Zhang et al., and Liu et al. suggest that Sc3+ sub-
stitutes for Fe3+ in Hematite by isomorphous substitution
while also being adsorbed on the surface of Goethite [17–
19]. Additionally, minor associations of Sc with zircon and
Ti-phases have been observed, although their distribution
varies significantly among RM. The significant challenges
for the recovery of Sc from RM are 1) its high alkalinity,
resulting in substantial chemical consumption in the leach-
ing process, and 2) its high Fe content, which is co-leached
with Sc due to their similar chemical behavior.
Direct leaching (DL) and acid baking and water leach-
ing (ABWL) are the most common approaches to recover-
ing Sc from RM discussed in the literature. Wang et al.
conducted RM leaching experiments over 2 hours using a
1:20 solid-to-liquid (S/L) ratio, demonstrating that H2SO4
is the most effective lixiviant for leaching Sc, followed by
HNO3 and HCl [39]. The main species forming during the
DL or ABWL of RM with H2SO4 include REE2(SO4)3,
Fe2(SO4)3, Al2(SO4)3, CaSO4, Na2SO4, HPO3 (+SO3
(g)), MnSO4, TiOSO4 [15, 22–24]. It can be observed that
the H2SO4 ABWL process is not highly selective toward
the formation of water-soluble species. Borra et al. dem-
onstrated that selective Sc leaching of up to 50% can be
achieved with 4% Fe recovery while using H2SO4 concen-
tration of 1N or less and similarly for up to 1.5N HCl [24].
This selectivity was attributed to the potential presence of
Sc on the surface of Fe, facilitating its quick initial recovery.
However, the selectivity decreases as the acid concentration
increases while increasing the Sc recovery [24, 40–42].
Kappusamy et al. showed that ABWL consumes as little
as 4% of the acid while recovering 2.8 times greater REEs
as compared to DL [25]. ABWL process was further inves-
tigated by Borra et al. and Liu et al. [23, 24]. They reported
that acid baking between 600 °C and 750 °C leads to dis-
sociation of Fe2(SO4)3 to Fe2O3, reducing Fe recovery to
less than 1%. However, some Sc2(SO4)3 also decomposed
in this temperature range to Sc2O3, leading to Sc recov-
ery dropping from 65% at 650°C baking to 58% at 700°C
according to Borra et al., while Liu et al. showed Sc recov-
ery close to 70% at 750°C and 40% at 600°C [23, 24]. At
temperatures below 600°C, this selectivity effect was not
observed. Anawati et al. noted that during H2SO4 ABWL,
Sc recovery reached approximately 60% at 200°C, increas-
ing to 80% at 400°C while keeping all other conditions the
same [37]. This was attributed to forming (H3O)Fe(SO4)2
at 200°C, which consumed more SO4 per mole of Fe. This
phase was decomposed to Fe2(SO4)3 at 400°C, consum-
ing less SO4 per mole of Fe, thereby favoring the sulfation
of more Fe. Narayanan et al. demonstrated that enhanc-
ing the surface area during leaching through ultrasonics or
ball mill leaching can improve Sc recovery from RM [35].
Kappusamy et al. indicated that REE recovery from coal
tailings via ABWL is limited by the mass diffusion of acid
through the sulfated product layer by comparing a TABWL
process to a single-stage acid bake-leach process [25].
Based on the above-reported studies, it is hypoth-
esized that Sc recovery from RM at less than 400°C can be
improved by exposing the Sc in bulk and creating a new
surface. Fe would also need to be leached to gain access to
Sc in the bulk phase. Therefore, this study aims to develop
a more efficient H2SO4 ABWL process to achieve higher
Sc recovery without high temperatures or excess acid. The
practical implications of this research are significant, as it
could lead to a more sustainable and cost-effective method
of Sc recovery.
MATERIALS AND METHODS
Sample Preparation and Characterization
A US-sourced RM supplied by Lawrence Livermore
Laboratory as a wet pulp in vacuum-sealed buckets was used
in this study. One kg of the RM was sampled and dried at
60°C in an air-drying oven, and the dry agglomerates were
broken down by mild grinding with a mortar and pestle.
The rest of the wet RM was stored in vacuum-sealed bags.
The ground RM was sealed in air-tight bags. HNO3 (67 -
70% trace metal grade) was diluted to 5% concentration to
preserve the leachate samples. DI water at a resistivity of 15
MΩ sourced from Avidity Science ® GinoTM II was used for
water leaching (WL) experiments and for diluting HNO3.
RM was first digested using basaltic digestion for head
sample analysis and analyzed for REEs in Agilent 7800
Inductivity coupled plasma-mass spectroscopy (ICP-MS).
The primary composition of the head sample, especially
Si, was determined using LiBO2 digestion and a Thermo
iCAP 7400 inductivity coupled plasma-atomic emission
spectroscopy (ICP-AES). All leachates from the various
processes were analyzed for their elemental composition
using ICP-MS. Mineral characterization of the RM was
performed using a cobalt radiation source in Malvern
Panalytical Empyrean 2 x-ray diffraction (XRD), and the
data was analyzed in MDI-Jade ® software. 20% ZnO by
between 6,600 to 20,400 tons/year of Sc based on the cur-
rent RM production rates [15].
The main constituents of RM typically include
Hematite, Goethite, Rutile, Anatase, Anhydrite, Cancrinite,
Gibbsite, Diaspore, Katoite, and Kaolinite [17–19]. Fe is
primarily present in Hematite and Goethite, as indicated
by Scanning Electron Microscopy – Energy Dispersive
Spectroscopy (SEM-EDS) and time-of-flight secondary ion
mass spectroscopy (TOF-SIMS) [17–22]. Studies by Vind
et al., Zhang et al., and Liu et al. suggest that Sc3+ sub-
stitutes for Fe3+ in Hematite by isomorphous substitution
while also being adsorbed on the surface of Goethite [17–
19]. Additionally, minor associations of Sc with zircon and
Ti-phases have been observed, although their distribution
varies significantly among RM. The significant challenges
for the recovery of Sc from RM are 1) its high alkalinity,
resulting in substantial chemical consumption in the leach-
ing process, and 2) its high Fe content, which is co-leached
with Sc due to their similar chemical behavior.
Direct leaching (DL) and acid baking and water leach-
ing (ABWL) are the most common approaches to recover-
ing Sc from RM discussed in the literature. Wang et al.
conducted RM leaching experiments over 2 hours using a
1:20 solid-to-liquid (S/L) ratio, demonstrating that H2SO4
is the most effective lixiviant for leaching Sc, followed by
HNO3 and HCl [39]. The main species forming during the
DL or ABWL of RM with H2SO4 include REE2(SO4)3,
Fe2(SO4)3, Al2(SO4)3, CaSO4, Na2SO4, HPO3 (+SO3
(g)), MnSO4, TiOSO4 [15, 22–24]. It can be observed that
the H2SO4 ABWL process is not highly selective toward
the formation of water-soluble species. Borra et al. dem-
onstrated that selective Sc leaching of up to 50% can be
achieved with 4% Fe recovery while using H2SO4 concen-
tration of 1N or less and similarly for up to 1.5N HCl [24].
This selectivity was attributed to the potential presence of
Sc on the surface of Fe, facilitating its quick initial recovery.
However, the selectivity decreases as the acid concentration
increases while increasing the Sc recovery [24, 40–42].
Kappusamy et al. showed that ABWL consumes as little
as 4% of the acid while recovering 2.8 times greater REEs
as compared to DL [25]. ABWL process was further inves-
tigated by Borra et al. and Liu et al. [23, 24]. They reported
that acid baking between 600 °C and 750 °C leads to dis-
sociation of Fe2(SO4)3 to Fe2O3, reducing Fe recovery to
less than 1%. However, some Sc2(SO4)3 also decomposed
in this temperature range to Sc2O3, leading to Sc recov-
ery dropping from 65% at 650°C baking to 58% at 700°C
according to Borra et al., while Liu et al. showed Sc recov-
ery close to 70% at 750°C and 40% at 600°C [23, 24]. At
temperatures below 600°C, this selectivity effect was not
observed. Anawati et al. noted that during H2SO4 ABWL,
Sc recovery reached approximately 60% at 200°C, increas-
ing to 80% at 400°C while keeping all other conditions the
same [37]. This was attributed to forming (H3O)Fe(SO4)2
at 200°C, which consumed more SO4 per mole of Fe. This
phase was decomposed to Fe2(SO4)3 at 400°C, consum-
ing less SO4 per mole of Fe, thereby favoring the sulfation
of more Fe. Narayanan et al. demonstrated that enhanc-
ing the surface area during leaching through ultrasonics or
ball mill leaching can improve Sc recovery from RM [35].
Kappusamy et al. indicated that REE recovery from coal
tailings via ABWL is limited by the mass diffusion of acid
through the sulfated product layer by comparing a TABWL
process to a single-stage acid bake-leach process [25].
Based on the above-reported studies, it is hypoth-
esized that Sc recovery from RM at less than 400°C can be
improved by exposing the Sc in bulk and creating a new
surface. Fe would also need to be leached to gain access to
Sc in the bulk phase. Therefore, this study aims to develop
a more efficient H2SO4 ABWL process to achieve higher
Sc recovery without high temperatures or excess acid. The
practical implications of this research are significant, as it
could lead to a more sustainable and cost-effective method
of Sc recovery.
MATERIALS AND METHODS
Sample Preparation and Characterization
A US-sourced RM supplied by Lawrence Livermore
Laboratory as a wet pulp in vacuum-sealed buckets was used
in this study. One kg of the RM was sampled and dried at
60°C in an air-drying oven, and the dry agglomerates were
broken down by mild grinding with a mortar and pestle.
The rest of the wet RM was stored in vacuum-sealed bags.
The ground RM was sealed in air-tight bags. HNO3 (67 -
70% trace metal grade) was diluted to 5% concentration to
preserve the leachate samples. DI water at a resistivity of 15
MΩ sourced from Avidity Science ® GinoTM II was used for
water leaching (WL) experiments and for diluting HNO3.
RM was first digested using basaltic digestion for head
sample analysis and analyzed for REEs in Agilent 7800
Inductivity coupled plasma-mass spectroscopy (ICP-MS).
The primary composition of the head sample, especially
Si, was determined using LiBO2 digestion and a Thermo
iCAP 7400 inductivity coupled plasma-atomic emission
spectroscopy (ICP-AES). All leachates from the various
processes were analyzed for their elemental composition
using ICP-MS. Mineral characterization of the RM was
performed using a cobalt radiation source in Malvern
Panalytical Empyrean 2 x-ray diffraction (XRD), and the
data was analyzed in MDI-Jade ® software. 20% ZnO by