Separation of neodymium and dysprosium by forming coordination polymers

Abstract

Abstract The increasing demand for neodymium (Nd)-magnets containing dysprosium (Dy) has necessitated the recovery of Nd and Dy from magnet scraps to ensure their supply. Thus, it is required to develop an environmentally friendly method for separating Nd3+ and Dy3+. Herein, we show that the formation of coordination polymers based on di(2-ethylhexyl) phosphoric acid (HDEHP) enables the fractional precipitation of Nd and Dy in ethanol–water solution because the solubility of the Dy coordination polymer is significantly lower than that of the Nd coordination polymer owing to coordination preference. The separation performance was found to be better in HNO3 than in HCl media. The characterization of the solid and liquid phases obtained from the Nd3+ and Dy3+ precipitation systems suggested that Nd3+– NO 3 - association inhibits the formation of Nd coordination polymers. The partial replacement of dehp− in the Nd coordination polymer by NO 3 - may weaken the binding strength of the framework, resulting in higher solubility. In addition, the formation of Nd(NO3)2+ in solution may shift the equilibrium against the precipitation reaction. However, when Nd3+ and Dy3+ coexist, Nd3+ precipitation is not accompanied by the replacement of dehp− by NO 3 - in the solid phase. This may be attributed to the incorporation of Nd3+ into the framework of the Dy coordination polymer. We conclude that the formation of Nd(NO3)2+ shifts the solution equilibrium and plays an important role in enhancing the separation performance in HNO3 than in HCl when Nd3+ and Dy3+ coexist.