Recovery of metals from zinc-carbon and alkaline spent batteries by using automotive shredder residues

The main aim of this work is the thermal recovery of manganese and zinc from a mixture of zinc-carbon and alkaline spent batteries containing 40.9% of Mn and 30.1% of Zn after a preliminary physical treatment followed by the removal of mercury. Separation of the metals is carried out on the basis of their different phase change temperatures, in fact the boiling point of mercury and zinc are 357 °C and 906 °C, respectively, and the melting point of Mn3O4, the main Mn-bearing phase in the mixture, is 1564 °C. After wet comminution and sieving to remove the anodic collectors and most of the chlorides contained in the mixture, chemical and X-ray powder diffraction (XRPD) analyses were performed. The mixture was initially heated in an air flow at temperatures ranging from 300 °C to 400 °C to eliminate mercury, then, the flow of air was turned into an inert carbon dioxide atmosphere. The volatilization of metallic zinc starts at a temperature 850 °C. At higher temperature the reduction of zinc oxide and subsequent volatilization of metallic zinc is carried out by the carbon present in the original mixture or in the automotive shredder residue (car fluff) added from the outside. By optimization of temperature, stoichiometric ratio and residence time, a product suitable for production of new batteries after refining was obtained. The treatment residue consisted of manganese and iron oxides that could be used to produce manganese-iron alloys.