Selective recovery of rare earth elements (REEs) from spent NiMH batteries by two-stage acid leaching

Abstract

The current study investigated selective recovery of rare earth elements (REEs) from spent nickel metal hydride (NiMH) batteries using two-stage acid leaching followed by precipitation at acidic pH. The first-stage conventional leaching using 2.5 M of waste phosphoric acid (H3PO4) at 25 °C for 60 min removed 60–100% heavy metals, such as Ni, Co, Zn and Cd, and converted more than 90% REEs into insoluble phosphate precipitates, REE(PO4). Subcritical water extraction (SWE) using mild concentration of sulfuric acid (H2SO4) in the second stage leached out all REEs. The leaching efficiency of REEs increased by increasing acid concentration, and 100% leaching efficiency of REEs, Ni, Co and Mn were achieved using 1 M of H2SO4, solid concentration of 20 g/L, at 125 °C for 30 min. As predicted by species distribution of metals at equilibrium, REEs in the leaching solution were selectively precipitated as NaREE(SO4)2·H2O at low pH (0.5–2) by the addition of NaOH solution, and separated from other metals (Ni, Co, Mn). Precipitation at pH 1 resulted in 82.59% of La, 90.75% of Ce, and 85.97% of Nd recovered as hexagonal rod crystals. This study provides an alternative process to selectively recover REEs from electronic waste (e-waste).