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 SO₄²⁻ concentrations and not solely on pH. A two-stage leaching for industrially crushed NiMH waste is performed: The first stage consists of H₂SO₄ leaching (2 M H₂SO₄, L/S = 10.4, V = 104 ml, T = 30 °C) and the second stage of H₂O leaching (V = 100 ml, T = 25 °C). Moreover, precipitation experiments are separately performed as a function of added Na₂SO₄ and H₂SO₄. 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 H₂SO₄ and Na₂SO₄ by 1.59 M and 0.35 M, respectively - results in a 21.8 times Na (as Na₂SO₄) and 58.3 times SO₄ 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.