Effects of contaminant metal ions on precipitation recovery of rare earth elements using oxalic acid

Solution equilibrium calculations were performed in this study to understand the impact of contaminant metal ions on the precipitation efficiency of selected rare earth elements (Ce3+, Nd3+, and Y3+) using oxalic acid as a precipitant. Trivalent metal ions, Al3+ and Fe3+, are found to considerably affect the precipitation efficiency of REEs. When Al3+ and Fe3+ concentrations are increased by 1 x 10(-4) mol/L, in order to achieve an acceptable cerium recovery of 93% from solutions containing 1 x 10(-4) mol/L Ce3+, oxalate dosage needs to increase by 1.2 x 10(-4) and 1.68 x 10(-4) mol/L, respectively. Such great impacts on the required oxalate dosage are also observed for Nd3+ and Y3+, which indicates that oxalic acid consumption and cost will be largely increased when the trivalent metal ions exist in REE-concentrated solutions. Effects of the divalent metal ions on the oxalate dosage are minimal. Furthermore, solution equilibrium calculation results show that the precipitation of Fe3+ and Ca2+ (e.g., hematite and Ca(C2O4)center dot H2O(s)) likely occurs during the oxalate precipitation of REEs at relatively high pH (e.g., pH 2.5), which will reduce rare earth oxalate product purity. In addition to the metal ions, anionic species, especially SO42- , are also found to negatively affect the precipitation recovery of REEs. For example, when 0.1 mol/L SO42- occurs in a solution containing 1 x 10(-4) mol/L Ce3+ and 4 x 10(-4) mol/L oxalate, the pH needs to be elevated from 2.0 to 3.3 to achieve the acceptable recovery. Overall, findings from this study provide guidance for the obtainment of high-purity rare earth products from solutions containing a considerable amount of contaminant metal ions by means of oxalic acid precipitation. (C) 2020 Chinese Society of Rare Earths. Published by Elsevier B.V. All rights reserved.

Automated summary

This study investigated the impact of contaminant metal ions on the precipitation efficiency of selected rare earth elements using oxalic acid as a precipitant. The findings showed that trivalent metal ions had a significant influence on the precipitation efficiency, while divalent metal ions had minimal effects. Additionally, anionic species were found to negatively affect the precipitation recovery of rare earth elements.