1264 XXXI International Mineral Processing Congress 2024 Proceedings/Washington, DC/Sep 29–Oct 3
validating the IEX mechanism represented by Reaction
[1]. The XPS spectra presented in Figure 4b show, on the
other hand, that in the presence of 0.025 M NaH2PO4,
the characteristic peaks remained on the surface, indicat-
ing that La3+ ions were not removed from the surface. The
results suggest that the PO43– ions acted as a passivating
agent by forming LaPO4. Since the La3+ ion is a hard acid
and the PO43– ion is a hard base (Pearson, 1968), the reac-
tion product is extremely stable therefore, Reaction [1]
cannot proceed to release La3+ ions into the solution. The
IEX leaching tests were conducted with an IAC sample
prepared by adding an appropriate amount of LaCl3 to a
kaolinite suspension to obtain a synthetic IAC sample with
~500 ppm La, including the 115 ppm La present in the
commercial product (Kaofine) produced by Thiele Kaolin
Company, GA.
The best-known IAC ores are mined commercially
in South China to meet 80% of the global demands for
HREEs. The authors of this communication acquired a
sample from this region and subjected it to a standard IEX
leaching test at 0.5 M (NH4)2SO4 and pH 4, with the
results presented in Figure 5 (yellow bars). Note here that
the REE+Y recoveries dropped precipitously in the pres-
ence of PO43– ions (green bars) most probably due to the
formation of a passivating layer of LaPO4(s) on the surface.
Also shown in Figure 5a is that EDTA can readily over-
ride the passivating effects of the PO43– ions on IEX leach-
ing. Interestingly, the passivated IAC sample responded
better to the EDTA leaching at pH 10 (purple bars) than
to the ammonium sulfate leaching at pH 4 (orange bars).
In fact, the REE+Y recoveries obtained with the passivated
IAC sample were significantly higher (violet bars) than the
recoveries obtained with the fresh IAC sample (orange bars)
that had not been exposed to PO43– ions. It is possible that
part of the REE+Y in the IAC ore sample may be in col-
loidal forms that do not respond well to the ammonium
sulfate leaching process. The Eh-pH diagram shown in
Figure 5b explains the beneficial effect of using EDTA to
overcome the passivation effects created by the PO43– ions.
Another approach taken to overcome the passivation
effect was to use a strong base, i.e., OH– ions, to displace
PO43– ions. The former is a stronger base with pKb =0 than
the latter with pKb =2. Figure 6 shows an IEX leaching
test conducted on a coal byproduct sample taken from an
Upper Freeport coal sample, WV. The sample was activated
first in a mixed solution of 10% NaOH and 0.1 M EDTA
at 60°C overnight. The filtrate was analyzed for dissolved
metals, and the residue was subjected to IEX leaching at
0.5 M (NH4)2SO4 for 1 hr at pH 3. The results presented
in Figure 6 show high recoveries for LREEs. Note here that
part of the REEs was removed during the activation step
(the Ist step) however, most of the REEs were recovered
during the IEX leaching step (2nd Step).
Rare Earth Minerals
The results presented in Figure 6 show low HREE recoveries,
as anticipated in view of declining ionic radii with increas-
ing atomic numbers and, hence, higher bond strengths.
This problem is addressed in the industry by increasing
the acid/base strengths and temperatures during leaching.
At Bayan Obo, mixed bastnäsite-monazite concentrates
are roasted at 300°C in the presence of H2SO4 (Cen et
al., 2021). Monazite concentrates are frequently ground
to 10 mm, and are cracked in 70% NaOH solutions at
150°C for 2 hrs. These are substantially more aggressive
conditions than employed in the present work to extract
Figure 5. (a) An IAC sample from South China leaches well at 0.5 M ammonium sulfate (orange) PO43– ions passivates the
leaching (green) and EDTA overrides the passivation effect. (b) Eh-pH diagram for the La(PO4)(s)-EDTA system
validating the IEX mechanism represented by Reaction
[1]. The XPS spectra presented in Figure 4b show, on the
other hand, that in the presence of 0.025 M NaH2PO4,
the characteristic peaks remained on the surface, indicat-
ing that La3+ ions were not removed from the surface. The
results suggest that the PO43– ions acted as a passivating
agent by forming LaPO4. Since the La3+ ion is a hard acid
and the PO43– ion is a hard base (Pearson, 1968), the reac-
tion product is extremely stable therefore, Reaction [1]
cannot proceed to release La3+ ions into the solution. The
IEX leaching tests were conducted with an IAC sample
prepared by adding an appropriate amount of LaCl3 to a
kaolinite suspension to obtain a synthetic IAC sample with
~500 ppm La, including the 115 ppm La present in the
commercial product (Kaofine) produced by Thiele Kaolin
Company, GA.
The best-known IAC ores are mined commercially
in South China to meet 80% of the global demands for
HREEs. The authors of this communication acquired a
sample from this region and subjected it to a standard IEX
leaching test at 0.5 M (NH4)2SO4 and pH 4, with the
results presented in Figure 5 (yellow bars). Note here that
the REE+Y recoveries dropped precipitously in the pres-
ence of PO43– ions (green bars) most probably due to the
formation of a passivating layer of LaPO4(s) on the surface.
Also shown in Figure 5a is that EDTA can readily over-
ride the passivating effects of the PO43– ions on IEX leach-
ing. Interestingly, the passivated IAC sample responded
better to the EDTA leaching at pH 10 (purple bars) than
to the ammonium sulfate leaching at pH 4 (orange bars).
In fact, the REE+Y recoveries obtained with the passivated
IAC sample were significantly higher (violet bars) than the
recoveries obtained with the fresh IAC sample (orange bars)
that had not been exposed to PO43– ions. It is possible that
part of the REE+Y in the IAC ore sample may be in col-
loidal forms that do not respond well to the ammonium
sulfate leaching process. The Eh-pH diagram shown in
Figure 5b explains the beneficial effect of using EDTA to
overcome the passivation effects created by the PO43– ions.
Another approach taken to overcome the passivation
effect was to use a strong base, i.e., OH– ions, to displace
PO43– ions. The former is a stronger base with pKb =0 than
the latter with pKb =2. Figure 6 shows an IEX leaching
test conducted on a coal byproduct sample taken from an
Upper Freeport coal sample, WV. The sample was activated
first in a mixed solution of 10% NaOH and 0.1 M EDTA
at 60°C overnight. The filtrate was analyzed for dissolved
metals, and the residue was subjected to IEX leaching at
0.5 M (NH4)2SO4 for 1 hr at pH 3. The results presented
in Figure 6 show high recoveries for LREEs. Note here that
part of the REEs was removed during the activation step
(the Ist step) however, most of the REEs were recovered
during the IEX leaching step (2nd Step).
Rare Earth Minerals
The results presented in Figure 6 show low HREE recoveries,
as anticipated in view of declining ionic radii with increas-
ing atomic numbers and, hence, higher bond strengths.
This problem is addressed in the industry by increasing
the acid/base strengths and temperatures during leaching.
At Bayan Obo, mixed bastnäsite-monazite concentrates
are roasted at 300°C in the presence of H2SO4 (Cen et
al., 2021). Monazite concentrates are frequently ground
to 10 mm, and are cracked in 70% NaOH solutions at
150°C for 2 hrs. These are substantially more aggressive
conditions than employed in the present work to extract
Figure 5. (a) An IAC sample from South China leaches well at 0.5 M ammonium sulfate (orange) PO43– ions passivates the
leaching (green) and EDTA overrides the passivation effect. (b) Eh-pH diagram for the La(PO4)(s)-EDTA system