Selective neodymium recovery from model permanent magnets using cost-effective organic acid systems

Abstract

Recovery of critical metals from waste is becoming very important to bridge the gap between the limited natural resources available and their ever-increasing demand. One such vital metal is neodymium (Nd), which plays an essential role in advancing sustainable clean energy technologies. Therefore, in this work, the key parameters to selectively recover Nd over iron (Fe) from their oxides, as model systems were investigated. By investigating the effect of key parameters, we aim to understand the underpinning science principles necessary for the safe and efficient recovery of critical metals from secondary sources. A series of deep eutectic solvents, consisting of a hydrogen bond donor (HBD), lactic acid or acetic acid, and hydrogen bond acceptor (HBA), guanidine hydrochloride (GUC), have been investigated in HBA–HBD combinations, and individually in the presence of water to determine the role of the HBD and HBA towards Nd and Fe leaching efficiency and selectivity. The combination of GUC with HBDs was less beneficial for the leaching of Nd₂O₃, with a maximum value of 78% in comparison with the individual systems, in the absence of GUC, which demonstrated a maximum dissolution of 95%. Among the different combinations, the acetic acid aqueous solution led to the highest dissolution efficiency and selectivity, probably due to the high basicity and strong stability constants for Nd-acetate complexes. Other parameters, such as the impact of the molar ratio Nd₂O₃ : Fe₂O₃ were also explored, and a synergetic effect that promotes Nd₂O₃ solubility at 1 : 1 Nd : Fe weight ratio is observed across the samples. However, when increasing the amount of Fe₂O₃ in the mixture to simulate realistic ratios present in spent magnets, selectivity is strongly affected, and only the acetic acid solution is capable of selectively dissolving Nd₂O₃ with a separation factor of up to 5038, which is higher than the current state of the art (1608). Finally, the acetic acid concentration was also studied as a factor to assess its effect on selectivity while also reducing cost.