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