Dichloromethane-assisted phase separation for simultaneous recovery of oil and rare earth elements from oil-rich grinding sludge

Grinding sludge is a gel-like hazardous waste generated in large quantities during metal cutting processes. Conventional calcination and hydrometallurgical treatment of oil-rich grinding sludges emit flue gases and generate insoluble residues. This study proposes CH2Cl2-assisted phase separation under mild conditions to enhance oil removal and metal recovery by overcoming encapsulation effects. Neodymium iron boron grinding sludge, containing 10.4% oil, 8.3% neodymium, 4.5% praseodymium, and 55.8% Fe, was selected as a representative material. Complete dissolution was achieved using a dichloromethane/hydrochloric acid mixed solvent, producing an organic phase enriched with oil, carbon and grease, and an aqueous phase rich in Nd, Pr, and Fe. The aqueous phase was processed to sequentially precipitate a double salt containing 98.58% Nd/Pr and an iron yellow product with 99.97% Fe. This approach provides an effective and economical strategy for the simultaneous recovery of oil and metallic products from grinding sludge, avoiding harsh conditions or costly equipment, offering significant potential for practical on-site industrial applications.