Comparative leaching of spent zinc-manganese-carbon batteries using sulfur dioxide in ammoniacal and sulfuric acid solutions

The non-magnetic fraction of spent zinc-manganese-carbon batteries containing 20.8% Zn, 22.7% Mn, 2.65% Fe, and < 0.1% Hg, Ni, Co, Cu and Pb was leached in H2SO4, H2SO4/SO2, NH3 and NH3/SO2 at 30-60 °C. In acid media the complete dissolution of zinc is unaffected by SO2. However, the reductive role of SO2 increases the leaching of manganese in H2SO4 from 25% to 100% and iron from 1% to 25%. Literature results of leaching with other reductants are compared. The XRD analysis of leach residues from ammonia solutions shows the conversion of Zn5(OH)8Cl2·H2O in the feed to ZnO with low dissolution of zinc, manganese and iron. Low leaching of iron in NH3/SO2 is due to the formation of Fe(NH4)2(SO3)2 and Fe(NH4)2(SO4)2 identified by XRD analysis of leach residues. However, the formation of Zn(NH3)42+ facilitates the selective leaching of zinc in buffered SO2/NH3 solutions that contains NH4+. Complete dissolution of copper also occurs in both H2SO4/SO2 and NH3/SO2. The dissolution of mercury by H2SO4 is retarded in the presence of SO2, and enhanced by NH3/SO2. Lead remains insoluble in all media, whilst the partial dissolution of nickel and cobalt is retarded by NH3/SO2.