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 × 10⁻⁴ mol/L, in order to achieve an acceptable cerium recovery of 93% from solutions containing 1 × 10⁻⁴ mol/L Ce³⁺, oxalate dosage needs to increase by 1.2 × 10⁻⁴ and 1.68 × 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)∙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 × 10⁻⁴ mol/L Ce³⁺ and 4 × 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.