Integrated recovery of rare earth elements and transition metals from spent NiMH batteries

Abstract Spent nickel-metal hydride batteries are concentrated secondary resources rich in transition metals (TM) and critical rare earth elements (REE). This study compares two integrated hydrometallurgical routes exploiting different solution chemistries: (i) sulfuric-acid leaching followed by bulk REE precipitation and (ii) methanesulfonic-acid (MSA) leaching followed by targeted oxidative removal of Ce. In the sulfate route, TM dissolution was favored (approximately 98% Ni at 60 °C), whereas REE extraction remained within approximately 40–60% over the investigated temperature range. La extraction further decreased to approximately 42% at 3 mol.L −1 H 2 SO 4 , consistent with the limited solubility of REE sulfate species. Following leaching and pH adjustment, the precipitation percentages of La, Ce, and Nd exceeded 98%. In the MSA route, the high solubility of metal methanesulfonates prevented this sulfate related limitation and enabled approximately 95–99% extraction of La, Ni, and Co at 60 °C, together with near-complete La dissolution at a 1/30 g.mL −1 solid-to-liquid ratio. Subsequent addition of 6 vol.% H 2 O 2 at pH 4 resulted in 94% Ce removal, consistent with selective oxidative precipitation, while most other metals remained predominantly in solution. The sulfate route therefore introduces TM–REE selectivity during leaching and enables collective REE recovery, whereas the MSA route prioritizes comprehensive dissolution and postpones selectivity to a targeted downstream step. These results show that the preferred route depends on the desired product streams: early group separation and bulk REE recovery, or near-complete metal dissolution followed by Ce-first separation.