Figure 3 Sc pilot production at Rusal facilities (Petrakova et al., 2016)

Comparison of leaching methods

Table 2 presents a comparison study of direct BR leaching. In most REE studies it is reported that the direct BR leaching by inorganic acids (HCl, HNO3 and H2SO4) is more advanced than the use of organic acids such as citric acid, acetic acid, etc. (Borra et al., 2015; Ujaczki et al., 2017; Yatsenko and Pyagai, 2010). However, the recovery of REE with some of the organic acids is comparable or even better to those of inorganic acids, especially under a high temperature leaching process. Nevertheless, the use of H2SO4 is considered to be most appropriate for BR leaching in terms of low cost and easy to handle process (Petrakova, 2014).
According to the chemical cost, it seems that citric acid leaching presents a double price compared to bio-leaching, while H2SO4 leaching is the most economically viable solution (Qu et al., 2015). The main disadvantages of acid extraction are the consumption of large acid quantities for the BR’s neutralization, the waste manipulation and the acidic residue’s reuse after leaching (Table 2) (Petrakova, 2014). In addition, acid leaching dissolves large amounts of Fe, Ti and Al by reducing selectivity (Wang and Cheng, 2011; Wang et al., 2011). The high concentration of these metals in the pregnant solution reduces the recovery efficiency during refining with ion-exchange resins, while increasing the acid consumption during the washing-regeneration phase of the resins. Unlike the acidic route, alkali extraction such as NaHCO3 is advantageous due to the lack of neutralization and the production of less caustic BR, and while the scandium recovery in this process is quite low, it seems to be profitable as Sc2O3 can be produced at cost of well below US$ 900 / kg. However, this process is only effective in certain BRs from Russia. The patent of Orbite Aluminae Inc. is also very promising, as it promotes metal recovery and acid reuse, resulting in the minimization of the waste production. However, the use of concentrated HCl requires corrosion resistant equipment, increasing the total investment and operational cost. The IL leaching process is also very promising in delivering a concentrated Sc solution with low Fe, Ti, Al impurities and overall low acid consumption, but it is uneconomic given the reported IL losses in each processing cycle and the current cost of the IL reagents.
Table 2 Comparison of direct BR leaching methods