Figure 2 BR leaching process in bench scale with Ionic liquid HbetTf2N
[Davris et al, 2018]
The proposed IL process (Figure 2) provides selectivity, recyclability
of the leaching agent and more efficient acid input in the process. Yet
the high price of the ionic liquid in conjunction with the unavoidable
IL losses in each contact with aqueous phases (in total 3-10% IL losses
can be expected per cycle) makes this process financially unattractive.
Future studies are focused on minimizing IL losses while maintaining the
advantages of this newly developed process.
Leaching with
NaHCO3
Leaching BR studies at Russia indicated that selective Sc leaching can
be achieved avoiding the acidification stage through alkaline leaching
using NaHCO3 as the leaching agent and utilizing
CO2 to regulate pH. It has been found that Sc has
higher solubility in NaHCO3 (16.7 g/L) than in
Na2CO3 (0.43g/L) solutions, (Petrakova
et al., 2016; Suss et al., 2018; Yatsenko and Pyagai, 2010). During
alkaline leaching, part of NaHCO3 is converted into
Na2CO3, reducing Sc solubility. With the
addition of CO2 gas to the pulp, the
NaHCO3 content is increased allowing for higher Sc
recovery. The flowsheet and the pilot plant developed by RUSAL are shown
in Figure 3. During leaching, 26% of Sc is selectively recovered at 60
°C with pCO2=6 atm and liquid to solid ratio 1:4
(Petrakova et al., 2016). About 30% Na, 68% Zr and 6% Ti are
dissolved and extracted during the purification of the pregnant
solution. Efforts to avoid Sc losses during the leaching are made by
introducing resins directly into the pulp (Suss et al., 2018). At
present, this process is the only Sc extraction application from BR at a
semi- industrial scale, allowing Rusal to produce 3 tn of
Sc2O3 per year.
The low recovery of Sc ~25% and the zero recovery of
the other REE are the main drawbacks in this case. The advantage of this
process is the production of Sc with no acid addition, while the final
pH=8 of BR after the process results in a less environmentally
aggressive by-product to be handled. In addition, this method is one of
the few that allows for actual CO2 sequestration. It
should be noted that this technology is effective when Sc is associated
with Aluminium mineral phases. Tests in the Greek BR (where Sc is in the
mineral phases of Fe) did not result in significant Sc dissolution
(<3%).