In a governmental funded 3-year research project a recycling process for nickel-metal hydride batteries (NiMH) has been developed. Today the discarded batteries are used in the steel industry as a cheap nickel-source. Cobalt is not paid for and rare earths (RE) are slagged and lost for reuse. Spent NiMH batteries contain 36-42% nickel, 3-4% cobalt and 8-10% mischmetal consisting of lanthanum, cerium, praseodymium and neodymium. The dismantled and processed NiMH are melted in a dc electric arc furnace producing a nickel-cobalt alloy and a slag phase highly enriched with RE-oxides. The key for success in this process was to find a suitable slag system which ensures best separation of NiCo alloy from the rare earth oxides. Different oxidic and halogenic slag systems are investigated on a laboratory and technical scale. The slag systems CaO-CaF2 and CaO-SiO2 were determined to be suitable for subsequent pilot plant trials. In these trials, the CaO-CaF2 system has shown a better melting behaviour than the CaO-SiO2 system. Nickel and cobalt were found nearly completely in the metal phase whereas the rare earths were transferred into the slag as oxides. Pyrometallurgical refining of the nickel-cobalt alloy did not make sense as investigated in technical scale trials. Within the scope of a scenario calculation the economical feasibility was demonstrated for a model scenario. (c) 2005 Published by Elsevier B.V.