Recovery of MnO2 from a spent alkaline battery leach solution via ozone treatment

Abstract

This work investigates the reaction rate of Mn(II) to generate solid manganese dioxide (MnO2) as a function of the gaseous ozone mass flow rate (27.5–77 g h−1). The experimental studies were carried out in a semi-continuous reactor, using a synthetic solution (300 mL of 1 M H2SO4 with 6000 ppm of Mn(II) added as MnSO4) that simulated the composition of an acid leaching solution from spent alkaline battery material (SBM). It was observed that at 1.3–1.45 V/SHE and pH < 1.0 a selective formation of MnO2 powder was obtained; at values greater than 1.45 V/SHE, permanganate ion (MnO41−) was formed. On the other hand, a linear relation was perceived between the volumetric mass transfer coefficient (kLa) and the ozone mass flow rate (19.3–77 g h−1 in 600 mL of the 1 M H2SO4 solution). The rate constant (k) was determined in the presence and absence of nonporous plastic spheres (D = 3 mm). In both cases the rate of Mn(II) conversion increased proportionally with the ozone mass flow rate, although the conversions obtained with non-porous plastic spheres (x = 82%) were always higher than those without non-porous plastic spheres (x = 72%). A pseudo-homogenous mass transfer model adequately approximated the experimental data.