Insights into the mechanism of separation of Co, Nd, and Dy by different crystal structures of zirconium phosphate in bead form

Abstract

The increasing demand for Co, Nd, and Dy, particularly in neodymium iron boron (NdFeB) magnets, has heightened concerns over their supply risks. Developing efficient, environmentally friendly methods to recover Co, Nd, and Dy from spent NdFeB magnets is therefore critical. This study presents the synthesis of two types of crystalline zirconium phosphate (ZrP) encapsulated in polyacrylonitrile (PAN) composite beads. The structure, morphology, and chemical properties were analyzed using X-ray diffraction (XRD), thermogravimetry (TG), Fourier-transform infrared spectroscopy (FT-IR), scanning electron microscopy (SEM), X-ray computed tomography (XCT), transmission electron microscopy (TEM), Brunauer-Emmett-Teller (BET), and 31P nuclear magnetic resonance (NMR). The intercalation of dimethylformamide (DMF) enlarged the interlayer spacing of α-ZrP and γ-ZrP, thereby enhancing their ion exchange properties. Batch sorption experiments revealed that α-ZrP/PAN outperformed γ-ZrP/PAN in both sorption and separation efficiency. α-ZrP/PAN beads exhibited high selectivity for Nd3+ and Dy3+, with minimal Co2+ sorption (8.57 %). Stepwise elution studies on α-ZrP/PAN demonstrated the obtained metal purities to be ∼100 wt% for Co, 64.1 wt% for Nd, and 96.5 wt% for Dy. Sorbent regeneration was investigated over ten sorption–desorption cycles, which revealed that α-ZrP/PAN retained 72.72 % of its sorption capacity, demonstrating excellent reusability and stability, while γ-ZrP/PAN's capacity declined to 68.95 % after five cycles. The ion-exchange reaction accompanied by shrinking of the interlayer space was illustrated by combining the batch sorption experiments, column operations, and analyses from XRD, FT-IR, and X-ray photoelectron spectroscopy (XPS). Overall, α-ZrP/PAN shows promise for Co, Nd, and Dy separation from NdFeB magnets, although further optimization of sorbents and elution agents is necessary.