A Study on the Preparation of Rare Earth Oxide Powder for Rare Earth Precipitates Recovered from Spent Ni-MH Batteries

Abstract

<p>We report a method for preparing rare earth oxides (Re<sub>x</sub>O<sub>y</sub>) from the recycling process for spent Ni-metal hydride (Ni-MH) batteries. This process first involves a leaching of spent Ni-MH powders with sulfuric acid at 90°C, resulting in rare earth precipitates (i.e., NaRE(SO<sub>4</sub>)<sub>2</sub>·H<sub>2</sub>O, RE = La, Ce, Nd), which are converted into rare earth oxides via two different approaches: i) simple heat treatment in air, and ii) metathesis reaction with NaOH at 70°C. Not only the morphological features but also the crystallographic structures of all products are systematically investigated using field-emission scanning electron microscopy (FESEM) and X-ray diffraction (XRD); their thermal behaviors are also analyzed. In particular, XRD results show that some of the rare earth precipitates are converted into oxide form (such as La<sub>2</sub>O<sub>3</sub>, Ce<sub>2</sub>O<sub>3</sub>, and Nd<sub>2</sub>O<sub>3</sub>) with heat treatment at 1200°C; however, secondary peaks are also observed. On the other hand, rare earth oxides, Re<sub>x</sub>O<sub>y</sub> can be successfully obtained after metathesis of rare earth precipitates, followed by heat treatment at 1000°C in air, along with a change of crystallographic structures, i.e., NaRE(SO<sub>4</sub>)<sub>2</sub>·H<sub>2</sub>O → RE(OH)<sub>3</sub> →Re<sub>x</sub>O<sub>y</sub>.</p>