Lanthanide-alkali double sulfate precipitation from strong sulfuric acid NiMH battery waste leachate.

In NiMH battery leaching, rare earth element (REE) precipitation from sulfate media is often reported as being a result of increasing pH of the pregnant leach solution (PLS). Here we demonstrate that this precipitation is a phenomenon that depends on both Na and SO concentrations and not solely on pH. A two-stage leaching for industrially crushed NiMH waste is performed: The first stage consists of HSO leaching (2 M HSO, L/S = 10.4, V = 104 ml, T = 30 °C) and the second stage of HO leaching (V = 100 ml, T = 25 °C). Moreover, precipitation experiments are separately performed as a function of added NaSO and HSO. During the precipitation, higher than stoichiometric quantities of Na to REE are utilized and this increase in both precipitation reagent concentrations results in an improved double sulfate precipitation efficiency. The best REE precipitation efficiencies (98-99%) - achieved by increasing concentrations of HSO and NaSO by 1.59 M and 0.35 M, respectively - results in a 21.8 times Na (as NaSO) and 58.3 times SO change in stoichiometric ratio to REE. Results strongly indicate a straightforward approach for REE recovery from NiMH battery waste without the need to increase the pH of PLS.