Extraction of rare earth and cobalt from leach residue of Nd-Fe-B waste by reductive leaching with iron powder

After recovering rare earth from neodymium–iron–boron (Nd–Fe–B) magnet waste via oxidation roasting–hydrochloric acid leaching, extracting rare earths and cobalt from remaining leach residue, containing about 0.67% ( w /w) RE and 0.36% Co, is environmentally and economically important. However, extracting valuable elements from low–grade minerals remains challenging. The purpose of this study is to accelerate the dissolution of rare earths and cobalt from leach residue of Nd–Fe–B waste (MWLR) under low acidity by adding iron as a reductant, which is conducive to destroying the main phase iron oxide. To optimize the leaching process of MWLR in a sulfuric acid system, the effects of reaction temperature, reaction time, sulfuric acid dosage, solid/liquid ( S / L) ratio, reductant addition time and reductant dosage were investigated. Circa 87.9% Fe, 99.7% Co, 99.4% Nd, 73.2% Pr and 87.9% Ce were leached. Furthermore, kinetic results indicated that the leaching process, controlled by a combination of internal diffusion and surface chemical reaction, changed to a surface chemical reaction control in the presence of Fe added as the reductant. This research proposed and verified a way to extract low–grade rare earths and cobalt from MWLR, and investigated the mechanism of reductive leaching process. • A reductive leaching method to extract rare earth and cobalt from leaching residues of Nd-Fe-B waste. • The leaching efficiencies of Co and RE increased from 86.4%, 64.3% to 99.7%, 92.1% under the addition of reductant. • Delayed addition of reductant is more conducive to the reaction between Fe and Fe 3+ . • The addition of reductant can alter the rate-controlling step during the leaching process of Co, Pr and Ce.