Leaching mechanisms of industrial powders of spent nickel metal hydride batteries in a pilot‐scale reactor

In view of a sustainable recycling process, the leaching mechanisms of nickel and rare‐earth elements (REEs) contained within industrial samples of spent nickel metal hydride battery powders were investigated in HCl and H2SO4, under mild temperature (25–60 °C) and pH (3–5.5). First, in‐depth characterization of the heterogeneous battery powder was carried out with powder XRD, SEM, electron probe microanalyzer wavelength‐dispersive spectroscopy (EPMA‐WDS) quantitative analyses of individual particles, and inductively coupled plasma optical emission spectrometry (ICP‐OES) elemental analysis. An unusual result is the identification of particles that exhibit a core–shell structure, which is related to anode active mass aging mechanisms. Then, a leaching study in a 10 L pilot‐scale reactor demonstrated the selective dissolution of REEs, with respect to nickel, at pH 3, which is attributed to 1) the kinetic inhibition of nickel metal dissolution, and 2) the specific core–shell structure of aged mischmetal particles. Furthermore, the use of H2SO4 led to coprecipitation of lanthanide–alkali double sulfates and nickel salts.