In situ anodic leaching of Nd-Fe-B sludge with fast leaching kinetics for selective recovery of rare-earth elements

Abstract

Neodymium-iron-boron (Nd-Fe-B) sludge is an important secondary resource of rare-earth elements (REEs). However, the state-of-the-art recycling method, i.e., HCl-preferential dissolution faces challenges such as slow leaching kinetics, excessive chemical consumption and wastewater generation. In this work, the in situ anodic leaching of Nd-Fe-B sludge was developed to selectively recover REEs with high efficiency. The leaching rates of the REEs are 2.4–9.0 times higher using the in situ anodic leaching at the current density from 10 to 40 mA/cm² than using conventional chemical leaching under the maintained pH of 3.7. Mechanism studies reveal that the anode-generated H⁺ plays the key role during the in situ anodic leaching process that locally increases the H⁺ concentration at the interface of sludge particles, accelerating the leaching kinetics. By achieving a total leaching efficiency of Nd-Fe-B sludge close to 100% and the Fe deposition efficiency in the range of 70.9%–74.3%, selective leaching of REEs is successfully realized and thus largely reduces chemical consumption. Additionally, a two-step recycling route involving electrolysis-selective precipitation was proposed that enables a stable REEs recovery of 92.2% with recyclable electrolyte. This study provides a novel and environmentally-friendly strategy for the efficient recovery of REEs from secondary resources.