Figure 1 Leaching process with concentrated HCl developed by Orbite Technology Inc. (Borra et al., 2016a)

Direct leaching with organic acids

BR leaching using organic acids has been studied in the same way as inorganic acids. Greek BR treatment with citric, methanesulfonic and acetic acids can correspond to inefficient rare earths leaching, resulting in lower recoveries compared to inorganic acids. Exceptionally, high REE recoveries, which are comparable to inorganic acids, can be achieved after citric acid treatment under rising temperature (Borra et al., 2015). In contrast to these results BR studies from Russia revealed that a mixture of formic -acetic acid at 80°C, pH 1-3,6 obtained 63-74% Sc recovery and similar Al recovery (yield), while the recoveries of other REE are not reported; further increase of pΗ during the leaching led to silica gel formation with filtration disorder (Bogomazov and Senyuta, 2017). Use organic acids for BR leaching results in more expensive process as the cost of the solvents is higher and additional recycling steps with distillation are needed to recover and reuse the organic solvents after the leaching stage.
Bioleaching process is based on a more indirect use of BR leaching with organic acids. During bioleaching, metals are extracted with microorganisms (bacteria or fungi) which produce organic acids. Fungi are mainly used for the BR leaching because they can survive at high pH and produce organic acids, amino acids and proteins in the presence of an organic agent forming complexes with metal ions. The produced organic acids are usually a mixture of citric, oxalic and gluconic acid. REE were leached from BR with the nematode fungi RM-10 in the presence of a sucrose agent (Qu and Lian, 2013). The authors found that leaching in two steps (pre fungi culture followed by leaching) was better than the one step leaching (culture in the presence of BR). Similarly, Aspergillus niger fungi was used (Qu et al., 2013) both in batch and continuous leaching. During the batch leaching at 2% w/v pulp density after 10 days recovery of Sc is 44%, while the increase of pulp density to 10% w/v reduce the Sc recovery at 30% under continuous operation.

Leaching with ionic liquids

Ionic liquids (IL’s) are versatile solvents consisting solely of ions, having superior properties against classic organic solvents in terms of low vapor pressure and inflammability. Ionic liquids have been applied as lixiviants to selectively dissolve rare earth elements (REE) from bauxite residue.
A new leaching process that developed is based on a hydrophobic ionic liquid betainium bis(trifluoromethylsulfonyl)imide [Hbet][Tf2N] (Davris et al, 2016 and 2018). Leaching bauxite residue with HbetTf2N-water mixture (40%v/v water) at autoclave conditions of 150°C for 4h at 10% pulp density (w/v) can selectively dissolve REE (>70%) against Fe, Al, Ti and Si in the Greek BR. Scandium followed a different leaching behavior from the rest REE with maximum 45% recovery upon leaching, whereas Ca and Na content are totally dissolved. The metals ions present in HbetTf2N leachate obtained after filtration, are stripped with an aqueous acidic solution while hydrophobic HbetTf2N is regenerated for reuse. During stripping and upon pH adjustment a preconcentrated Sc solution with fewer impurities is generated. Where the concentration of the Sc in the organic phase before the stripping was 5-6 mg/l (directly comparable with the mineral acid selective leaching presented in section 1.1.2), in the aqueous strip solution concentrations of 100 mg/l Sc have been achieved. This significant increase in concentration along with the complete avoidance of silica gel formation are the most promising factors of this novel approach