Sustainable closed‐loop process for recovering base oil, rare‐earth salts, and flocculants from oily <scp>NdFeB</scp> waste

Abstract BACKGROUND ‐Oily Nd–Fe–B machining sludge contains oil–metal capsules that hinder conventional washing and acid leaching. Calcination is often used to remove the oil component but causes secondary pollution, lowers rare‐earth recovery, and requires costly equipment. To address these challenges, we developed a noncalcination HCl–CCl 4 coleaching route that simultaneously separates oil and valorizes metals from real oily Nd–Fe–B sludge. RESULTS The sludge contained 11.7% water, 10.44% oil, 55.82% Fe, 8.28% Nd, and 4.48% Pr. Pure HCl or conventional cleaners achieved &lt;80% rare‐earth leaching and left viscous residues. In contrast, HCl–CCl 4 fully dissolved the sludge and produced clear phase separation: an organic phase yielding base oil after CCl 4 evaporation (91.6% solvent recovery) and an acidic aqueous phase enriched in metals (74 g L −1 Fe, 11 g L −1 Nd, 5 g L −1 Pr, 1.9 g L −1 Al). Adding Na 2 SO 4 (60 g L −1 ) rapidly precipitated rare‐earths as double sulfates with 98.3% Nd and 98.9% Pr recovery, outperforming K 2 SO 4 and (NH 4 ) 2 SO 4 . After that, the resulting supernatant contained ~96.5% Fe 2+ of total Fe and, after nitrite‐oxygen oxidation, yielded a ferric‐rich flocculant suitable for wastewater treatment. CONCLUSION The HCl–CCl 4 ‐ coleaching route induces synergistic delayering of oil and oxyhydroxide shells, enabling clean base oil recovery, near‐quantitative rare‐earth precipitation, and effective utilization of the residual solution as a flocculant without generating secondary wastes. The demonstrated high recoveries and solvent recirculation underscore strong industrial potential for the valorization of oily metallic sludge. © 2025 Society of Chemical Industry (SCI).