Stripping conditions to prevent the accumulation of rare earth elements and iron on the organic phase in the solvent extraction circuit at Skorpion Zinc

The concentrations of the rare earth elements (particularly Y, Yb, Er and Sc) in the zinc-stripped organic phase streams in the solvent extraction process at Skorpion Zinc mine have increased gradually over the past four years. Iron is the only other impurity present in notable quantities in the organic phase after washing and scrubbing prior to zinc stripping. This project aimed to evaluate the effects that rare earth elements and iron in the organic phase have on the zinc solvent extraction process and to subsequently find appropriate stripping conditions for the removal of these elements from the zinc-stripped organic phase.Results obtained by performing laboratory scale batch tests indicated that the viscosity of the organic phase doubled and the phase disengagement time increased from 100 s to 700 s when the total rare earth elements and iron concentration in the organic phase was increased from 3100 mg/L to 6350 mg/L. The zinc loading capacity of the organic phase after two extractions furthermore decreased by a value between 1 g/L and 3 g/L, depending on the composition of the pregnant leach solution. The stripping of low concentrations of rare earth elements and iron from 40% di-2-ethylhexyl phosphoric acid (D2EHPA) diluted in kerosene was evaluated using two different stripping agents (H2SO4 and HCl) with concentrations between 1 M and 7 M, organic-to-aqueous ratios between 0.5 and 4, and temperatures between 30 °C and 50 °C. The highest stripping percentages were achieved at acid concentrations greater than 5 M, organic-to-aqueous ratios of less than 0.5 and high temperatures.

► Rare earths and Fe accumulation on D2EHPA reduces Zn loading capacity by 1–3 g/L.
► Viscosity and phase disengagement time increased with increasing REE and Fe loading.
► Y and Er stripping exceeding 95% achieved with H2SO4 concentration of 5 M or higher.
► Compared to H2SO4, HCl achieves lower REE stripping but improved Fe stripping.