Solvent extraction for high separation strategy of light and heavy rare earth elements from a sulfate-leached solution of low-grade monazite

Abstract

A novel separation strategy for light REEs (Ce4+ and La3+) and heavy REEs (Y3+ and Eu3+) from a low-grade monazite ore leached solution in a sulfate medium was studied to achieve a higher selectivity of metal ions. A prior separation of higher valence Ce4+ was performed using 8.0 vol% Cyanex 923, 0.2 mol/L H2SO4 in the aqueous feed, an organic-to-aqueous (O/A) phase ratio of 1, and a temperature of 25 °C, yielding the separation factor β(Ce4+/REEs3+) = 9424. A reductive back-extraction of Ce2(SO4)3 was efficiently performed using a solution mixture of H2SO4 + H2O2 with concentrations ≥1.0 mol/L each. Further, Y3+ from Ce4+-depleted solution was selectively extracted using 2.0 vol% Cyanex 572 organic extractant and 0.2 mol/L H2SO4 in aqueous feed at an O/A ratio of 1, yielding a separation factor of 6438 at 25 °C. The subsequent extraction of Eu3+ over La3+ was carried out at 2.5 vol% D2EHPA, 0.05 mol/L H2SO4 in the aqueous feed, an O/A ratio of 1, and a temperature of 55 °C, yielding β(Eu3+/La3+) = 5787. Both Y3+ and Eu3+ from the individually loaded organics were stripped with an efficiency of ∼ 99.9% by contact with 2.0 mol/L (COOH)2 prepared in a 1.0 mol/L HCl solution. The remaining La3+ in the raffinate was directly precipitated by adding ≥0.01 mol/L (COOH)2 crystals at 90 °C. This article provides an efficient and highly selective recovery of REEs from a low-grade monazite leached solution with higher separation values β(Ce4+/REEs3+) = 9424 and β(Eu3+/La3+) = 5787.