Sustainable recovery of rare earth elements from Ni-MH batteries: Flux-free thermal isolation and Subsequent hydrometallurgical refinement

Abstract

This study details a sustainable approach for efficient recovery of highly pure rare earth elements oxides (REOs) from Ni-MH batteries. REOs (i.e., La, Ce, Sm, Nd, and Pr oxides) were thermally isolated from spent Ni-MH batteries into an oxide phase with the REO concentration of 67.4 wt%. Subsequently, separate leaching processes were conducted using sulfuric acid and hydrochloric acid solutions, and the results were compared in terms of efficiency and feasibility. Sodium REEs sulfate hydrate (NaRE(SO4)2.xH2O) was precipitated from the sulfuric acid leachate by adjusting the pH to 1.5 using sodium hydroxide, while the addition of oxalic acid to the hydrochloric acid leachate enabled the recovery of REEs as oxalate hydrate (RE2(C2O4)3.xH2O). The resulting sulfate salt was mixed with a sodium hydroxide solution, leading to the formation of REEs hydroxide. The REEs hydroxide and oxalate were calcined at 1000 °C, resulting in the production of REOs with a purity exceeding 99.7%. This research demonstrates the feasibility of developing an efficient, environmentally friendly, and sustainable approach for the recovery of highly pure REOs from Ni-MH batteries which potentially can minimize the environmental impact associated with the extraction of REOs from virgin sources, while addressing the challenge of effective recycling of end-of-life Ni-MH batteries.