Two-stage leaching of spent NiMH batteries for selective recovery of REEs. First-stage using waste H 3 PO 4 for heavy metals, while converting REEs to REE (PO 4 ). Subcritical water extraction using H 2 SO 4 in second-stage to leach out all metals. Selective precipitation of REEs in at low pH as NaREE (SO 4) 2 ·H 2 O precipitates.
Total metal content of spent NiMH batteries is shown in Table 1 that indicates Ni constituted 345.08 mg/kg in the sample and three REEs, including 89.60 mg/kg of La, 26.89 mg/kg of Ce, and 26.36 mg/kg of Nd. The REEs content in the spent NiMH batteries is in agreement with literature , .
Nickel metal hydride (NiMH) batteries are mainly used to power the hybrid electric vehicle and they contain various critical and valuable elements, including REEs of La, Ce, ND, and other metals .
Spent NiMH batteries powder was obtained from the waste recycling company, Yen-Long Renewable Technology Co., Ltd in Kaohsiung, Taiwan. Spent NiMH batteries were collected by the company, and were discharged in saturated salt solution firstly.
Yun et al. studied REE extraction from spent NiMH batteries using acid leaching and extraction with a primary amine extractant of N1923. Results showed that 99.98% of REEs were extracted after a five-stage counter-current extraction followed by HCl stripping and oxalic precipitation.
A NiMH rechargeable battery consists of a nickel oxyhydroxide (NiOOH) cathode and a hydrogen-absorbing alloy anode. In a single battery, the total metal content is about 60%, with REEs accounting for 17.3% . The anode contains an alloy of the “AB 5 ” category wherein approximately 33 wt% of REEs is found.