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%).