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