Investigation of the effects of thermal treatment on the leachability of Zn and Mn from discarded alkaline and Zn[sbnd]C batteries

Abstract

Spent alkaline and ZnC batteries are a potential source of Zn and Mn. This study reports on the effects of thermal treatment on the leachability of metals from such products. Black mass originating from alkaline and ZnC batteries was thermally treated under four different atmospheres: O2, N2, H2 and air at different temperatures (300–950 °C). Samples without prior thermal treatment and solid residues after thermal treatment were subsequently leached with H2SO4. Leaching of the samples without the thermal treatment was done to determine the reference leaching conditions, which were established to be 0.5 M H2SO4 at 25 °C for 60 min. Subsequently, samples which had been thermally treated were leached using the aforementioned reference conditions. The effect of thermal treatment on the leaching of Mn and Zn was studied to determine which conditions may improve or hinder subsequent hydrometallurgical processing of alkaline batteries. The use of O2 during the thermal treatment led to formation of ZnMn2O4, which inhibited effective leaching. ZnMn2O4 was decomposed in N2 atmosphere and in the presence of C, improving further dissolution of Mn. The use of H2 led to formation of Mn3O4. This compound showed low dissolution, being possible to be separated from impurities such as Fe, Ni and K by leaching with 0.5 M H2SO4. To increase the purity of recovered Mn3O4, oxalic acid was used as reduction agent. A final solid product, Mn3O4, with a purity of 99.95%, was obtained. Data provided by this research can lead to improvements of existing recycling processes, allowing for higher leaching efficiency of alkaline and ZnC battery materials. Also, overall material recovery rates can be increased by producing manganese oxide with high purity.