XXXI International Mineral Processing Congress 2024 Proceedings/Washington, DC/Sep 29–Oct 3 2283
maximum adsorption density for a monolayer of horizon-
tally-oriented SHA molecules is shown in blue representing
a Theoretical Adsorption Density (TAD) of approximately
0.8×10–10 mol/cm2. TAD estimations take into account
the SHA molecules adsorbing into a face-centered cubic
arrangement but, because it is chemisorption, it involves
steric impedance caused by SHA orientation as well as the
lattice spacing between REE atoms at the REM surface.
Because adsorption densities are above the LB monolayer,
it is likely that surface precipitation also occurred (Trant et
al. 2018 Sime 2018).
Under all circumstances, it seems that SHA adsorption
is faster and greater on Ln2Si2O7 types of RESs compared
to Ln4.67(SiO4)3O types for each of the REEs (La, Nd and
Dy) examined in this study. For example, adsorption densi-
ties for each type of Ln4.67(SiO4)3O were below TAD for
as long as an hour but, for each type of Ln2Si2O7, the time
was no more than 0.5 hours to exceed the LB monolayer.
When compared to the REOs, REPs and RECs,
Ln4.67(SiO4)3O exhibits SHA adsorption behavior that is
similar to RECs and Ln2Si2O7 exhibits behavior that is
similar to REOs (Galt 2017). In this regard, it appears that
Ln4.67(SiO4)3O and Ln2Si2O7 have coordination numbers
of 10 and 6–7, respectively, depending on the REE that
is present (compare Figure 2). This concurs with Felsche
(1970) who studied polymorphism and crystal data of the
rare-earth disilicates (Ln2Si2O7).
These results suggest that the increasing amount silicate
polymerization (as determined by Si:O ratio) corresponds
to decreasing CNs and therefore to decreasing REE ionic
sizes explaining why REE ores containing silicates do not
always respond well to flotation (LeVier, 2023 Pickarts
2023). Consequently, it can be concluded that LRESs of the
SiO4-type (CN =10) as well as the HRESs of Si2O7-type
(CN =6) will not float as well with SHA similar to HREOs
and LRECs. Hence, to float all REMs, flotation will require
collector blends (Young et al. 2023), particularly if at least
one of the collectors targets LRECs and LRESs of the SiO4-
type and another collector targets HREOs and HRESs of
the Si2O7-type (see Figure 2).
CONCLUSIONS
SHA adsorption has been investigated on a range of REEs
utilizing pure chemical powders as REMs. Previous studies
discovered that results obtained for REOs, RECs and REPs
varied depending on the coordination number (CN) and
ionic diameter of the REEs (i.e., Lanthanide Contraction,
LC). This study continued those efforts by examining syn-
thetic RESs made of La, Nd and Dy. The same phenomena
were observed but are dependent on the RES type. In this
regard, it was found that Ln4.67(SiO4)3O exhibited simi-
lar behavior to RECs due to REEs having a coordination
number of 10. Likewise, Ln2Si2O7 was noted to have coor-
dination numbers of 7 or 6 because they behaved similar
to LREOs of La and Nd as well as MREOs of Dy, respec-
tively. Ultimately, the results depend on the silicate polym-
erization as measured by the silicon to oxide ratio (Si:O).
Because adsorption densities appeared to exceed monolayer
coverages, adsorption is attributed to chemisorption and
surface precipitation. In order to improve metallurgical
performance, it is suggested the collector blends be used so
that LRECs and LRESs of the SiO4-type as well as HREOs
and HRESs of the Si2O7-type can be targeted to float better.
ACKNOWLEDGMENTS
The research was sponsored by the Army Research
Laboratory and was accomplished under Cooperative
Agreement Number W911NF-15-2-0020. The views and
conclusions contained in this document are those of the
authors and should not be interpreted as representing the
official policies, either expressed or implied, of the Army
Research Laboratory or the U.S. Government. The U.S.
Government is authorized to reproduce and distribute
reprints for Government purposes notwithstanding any
copyright notation herein.
REFERENCES
Anderson, C. D., 2015. Improved understanding of rare
earth surface chemistry and its application to froth flo-
tation. Golden, CO: Colorado School of Mines.
Anon, 2024, History and Future of Rare Earth Elements,
History and Future of Rare Earth Elements |Science
History Institute, (Accessed: January, 10 2024)
Anon, 2024, Rare Earth Elements, Rare Earth Elements
(mit.edu), (Accessed: January 10 2024)
Felsche, J., 1970. Polymorphism and crystal data of the
rare-earth disilicates of type RE2Si2O7. Journal of the
Less Common Metals, 21(1), pp.1–14.
Fleet, M.E. and Liu, X., 2001. High-pressure rare earth dis-
ilicates REE2Si2O7 (REE= Nd, Sm, Eu, Gd): type K.
Journal of Solid State Chemistry, 161(1), pp.166–172.
Table 2. BET Surface Areas (cm2/g) of the Synthetic RESs
Silicate
Surface Area (cm2/g)
1st 2nd 3rd Average
La4.67(SiO4)3O 39030 38150 37240 38140
La
2 Si
2 O
7 21350 21230 20260 20950
Nd
4.67 (SiO
4 )
3 O 58120 38590 48200 48300
Nd2Si2O7 20630 21430 20350 20800
Dy4.67(SiO4)3O 24950 26360 31970 27760
Dy
2 Si
2 O
7 21850 20310 20670 20940
maximum adsorption density for a monolayer of horizon-
tally-oriented SHA molecules is shown in blue representing
a Theoretical Adsorption Density (TAD) of approximately
0.8×10–10 mol/cm2. TAD estimations take into account
the SHA molecules adsorbing into a face-centered cubic
arrangement but, because it is chemisorption, it involves
steric impedance caused by SHA orientation as well as the
lattice spacing between REE atoms at the REM surface.
Because adsorption densities are above the LB monolayer,
it is likely that surface precipitation also occurred (Trant et
al. 2018 Sime 2018).
Under all circumstances, it seems that SHA adsorption
is faster and greater on Ln2Si2O7 types of RESs compared
to Ln4.67(SiO4)3O types for each of the REEs (La, Nd and
Dy) examined in this study. For example, adsorption densi-
ties for each type of Ln4.67(SiO4)3O were below TAD for
as long as an hour but, for each type of Ln2Si2O7, the time
was no more than 0.5 hours to exceed the LB monolayer.
When compared to the REOs, REPs and RECs,
Ln4.67(SiO4)3O exhibits SHA adsorption behavior that is
similar to RECs and Ln2Si2O7 exhibits behavior that is
similar to REOs (Galt 2017). In this regard, it appears that
Ln4.67(SiO4)3O and Ln2Si2O7 have coordination numbers
of 10 and 6–7, respectively, depending on the REE that
is present (compare Figure 2). This concurs with Felsche
(1970) who studied polymorphism and crystal data of the
rare-earth disilicates (Ln2Si2O7).
These results suggest that the increasing amount silicate
polymerization (as determined by Si:O ratio) corresponds
to decreasing CNs and therefore to decreasing REE ionic
sizes explaining why REE ores containing silicates do not
always respond well to flotation (LeVier, 2023 Pickarts
2023). Consequently, it can be concluded that LRESs of the
SiO4-type (CN =10) as well as the HRESs of Si2O7-type
(CN =6) will not float as well with SHA similar to HREOs
and LRECs. Hence, to float all REMs, flotation will require
collector blends (Young et al. 2023), particularly if at least
one of the collectors targets LRECs and LRESs of the SiO4-
type and another collector targets HREOs and HRESs of
the Si2O7-type (see Figure 2).
CONCLUSIONS
SHA adsorption has been investigated on a range of REEs
utilizing pure chemical powders as REMs. Previous studies
discovered that results obtained for REOs, RECs and REPs
varied depending on the coordination number (CN) and
ionic diameter of the REEs (i.e., Lanthanide Contraction,
LC). This study continued those efforts by examining syn-
thetic RESs made of La, Nd and Dy. The same phenomena
were observed but are dependent on the RES type. In this
regard, it was found that Ln4.67(SiO4)3O exhibited simi-
lar behavior to RECs due to REEs having a coordination
number of 10. Likewise, Ln2Si2O7 was noted to have coor-
dination numbers of 7 or 6 because they behaved similar
to LREOs of La and Nd as well as MREOs of Dy, respec-
tively. Ultimately, the results depend on the silicate polym-
erization as measured by the silicon to oxide ratio (Si:O).
Because adsorption densities appeared to exceed monolayer
coverages, adsorption is attributed to chemisorption and
surface precipitation. In order to improve metallurgical
performance, it is suggested the collector blends be used so
that LRECs and LRESs of the SiO4-type as well as HREOs
and HRESs of the Si2O7-type can be targeted to float better.
ACKNOWLEDGMENTS
The research was sponsored by the Army Research
Laboratory and was accomplished under Cooperative
Agreement Number W911NF-15-2-0020. The views and
conclusions contained in this document are those of the
authors and should not be interpreted as representing the
official policies, either expressed or implied, of the Army
Research Laboratory or the U.S. Government. The U.S.
Government is authorized to reproduce and distribute
reprints for Government purposes notwithstanding any
copyright notation herein.
REFERENCES
Anderson, C. D., 2015. Improved understanding of rare
earth surface chemistry and its application to froth flo-
tation. Golden, CO: Colorado School of Mines.
Anon, 2024, History and Future of Rare Earth Elements,
History and Future of Rare Earth Elements |Science
History Institute, (Accessed: January, 10 2024)
Anon, 2024, Rare Earth Elements, Rare Earth Elements
(mit.edu), (Accessed: January 10 2024)
Felsche, J., 1970. Polymorphism and crystal data of the
rare-earth disilicates of type RE2Si2O7. Journal of the
Less Common Metals, 21(1), pp.1–14.
Fleet, M.E. and Liu, X., 2001. High-pressure rare earth dis-
ilicates REE2Si2O7 (REE= Nd, Sm, Eu, Gd): type K.
Journal of Solid State Chemistry, 161(1), pp.166–172.
Table 2. BET Surface Areas (cm2/g) of the Synthetic RESs
Silicate
Surface Area (cm2/g)
1st 2nd 3rd Average
La4.67(SiO4)3O 39030 38150 37240 38140
La
2 Si
2 O
7 21350 21230 20260 20950
Nd
4.67 (SiO
4 )
3 O 58120 38590 48200 48300
Nd2Si2O7 20630 21430 20350 20800
Dy4.67(SiO4)3O 24950 26360 31970 27760
Dy
2 Si
2 O
7 21850 20310 20670 20940