Solvent extraction
The solvent extraction experiments were performed with 1 mL
of a mixture containing 25.0 g L1 dysprosium and 3.0 g L1 cobalt. A higher concentration of cobalt, in comparison with the
concentrations found aer the leaching process, was chosen to
obtain a peak in the TXRF spectrum with an intensity that is
high enough to come out of the higher background spectrum
due to the very high rare-earth concentrations in the solution.
Dysprosium was chosen instead of neodymium to avoid peak
overlap between the L lines of neodymium and the K line of
cobalt in the TXRF spectrum. Trihexyl(tetradecyl)phosphonium
chloride has been chosen as ionic liquid because it is hydrophobic,
non-uorinated and it can be used in undiluted form.
The advantages of using this ionic liquid are describe elsewhere.61–63
The aqueous phase contained 3.5 M NH4Cl and the
HCl concentration was ranged between 0 and 1 M HCl. The
organic phase consists of 1 mL of trihexyl(tetradecyl)phosphonium
chloride which was presaturated with water containing
the same NH4Cl and HCl concentration as used in the extraction
experiments. The volume ratio was one and the mixtures
were shaken for 1 h at 1800 rpm at 60 C. Aerwards, they were
centrifuged for 5 min at 5300 rpm and the metal ion concentration
was measured with TXRF. The separation factor aCo,Dy is
dened as:
aCo;Dy ¼ DCo
DDy
(4)
with the distribution ratios DM dened as:
DM ¼ Corg
Caq
(5)
where Corg and Caq the concentrations of the metal in the
organic and aqueous phase, respectively.