Separation of neodymium and dysprosium by solvent extraction using ionic liquids combined with neutral extractants: batch and mixer-settler experiments

Abstract

A solvent extraction method based on the combination of the ionic liquid trihexyl(tetradecyl)phosphonium thiocyanate or nitrate ([C101][SCN], [C101][NO₃]) and the neutral extractants Cyanex 923 or tri-n-butyl phosphate (TBP) has been investigated for the separation of Nd(iii) and Dy(iii) from chloride media. High distribution ratios and separation factors were obtained when using Cyanex 923 diluted in [C101][SCN] 40 : 60 (wt%) and extracting from chloride media. The addition of Cyanex 923 to the ionic liquid has four advantages: (1) increase in the distribution ratios of the rare earths, (2) decrease of the viscosity of the organic phase, hence an improved mass transfer, (3) increase in the loading capacity of the ionic liquid and (4) improvement of the coalescence and phase disengagement, which is of importance when carrying out separations in continuous mode. Different extraction parameters were optimized: concentration of Cyanex 923, chloride concentration in the aqueous phase, equilibration time, pH of the aqueous phase, type of scrubbing and stripping agents. The ionic liquid combined with Cyanex 923 was recycled up to three times without losing its extraction efficiency. McCabe-Thiele diagrams were constructed to determine the number of stages needed for the separation of Nd(iii) and Dy(iii). Stripping of Dy(iii) from the organic phase was easily achieved with water. The feasibility to run this process in continuous mode was tested in a battery of small mixer-settlers (0.12 L and 0.48 L effective volume in the mixer and the settler, respectively). As a result, this process constitutes a novel and scalable alternative for the separation of Nd(iii) and Dy(iii).

Fig. 1. Effect of the concentration of Cyanex® 923 diluted in [C101][SCN] on the separation of Nd(iii) and Dy(iii) in 2.5 M CaCl₂. Equilibration time: 60 min. Shaking speed: 2000 rpm at 25 °C.

Fig. 2. Effect of the CaCl₂ concentration on the separation of Nd(iii) and Dy(iii). Equilibration time: 60 min. Shaking speed: 2000 rpm at 25 °C, 40 wt% Cyanex 923 in [C101][SCN].

Fig. 3. Influence of time on the separation of Nd(iii) and Dy(iii) from a 2.5 M CaCl₂ matrix to an organic phase composed of 40 wt% Cyanex 923, 60 wt% [C101][SCN] shaking speed: 2000 rpm at 25 °C.

Fig. 4. McCabe–Thiele diagram for the extraction of Dy(iii) from a mixture of Dy(iii) and Nd(iii) to the organic phase (40 wt% Cyanex 923 and 60 wt% [C101][SCN]). CaCl₂ in the aqueous phase = 2.5 M. Contact time: 60 min, O : A ratios varied between 1 : 10 and 5 : 1.

Fig. 5. McCabe–Thiele diagram for the stripping of dysprosium from a loaded organic phase (40 wt% Cyanex 923, 60 wt% [C101][SCN], 11 g L⁻¹ Dy(iii)). CaCl₂ in the aqueous phase = 1.0 M. Phase contact time: 60 min, O : A ratios varied between 0.5 : 5 and 5 : 1.5.

Fig. 6. Conceptual flow sheet of the developed and optimized process for the extraction and purification of Nd(iii) and Dy(iii) mixtures in concentrated chloride media.

Fig. 7. Metal concentration in the aqueous phase of each mixer-settler stage after 10 h of equilibration. Organic phase: 40 wt% Cyanex 923 and 60 wt% [C101][SCN], CaCl₂ in the aqueous phase = 2.5 M. Retention time in each mixer: 15 min. O : A ratio = 1 : 1.

Fig. 8. Metal concentration in the organic phase of each mixer-settler stage after 16 h of equilibration. Loaded organic phase: 40 wt% Cyanex 923 and 60 wt% [C101][SCN], 11 g L⁻¹. Stripping agent: CaCl₂ 1.0 M. Retention time in each mixer: 15 min. O : A ratio = 1 : 1.