A critical updated review of the hydrometallurgical routes for recycling zinc and manganese from spent zinc-based batteries

•Revision of Zn and Mn hydrometallurgical recovery processes from Zn-based batteries.•Revision of literature between 2007 and 2019.•Studies dealing with leaching and/or metals separation are critically reviewed.•Alkaline-, complexation-, acid- and reductive-acid-assisted leaching strategies.•Metals purification by solvent extraction and chemical precipitation. This review paper aims to present and analyse data from the most recent literature (between 2007 and 2019) published on the topic of manganese (Mn) and zinc (Zn) recovery from zinc-based spent batteries through hydrometallurgical methods. In a first attempt, a detailed comparative assessment of the metals leaching performance (as well as the experimental variables that influence its performance) reported in the various studies with strong acid or bases, potentially supplemented by complexing or reducing agents, as well as the reactions involved, are reviewed and discussed. All data point out that the use of a reductant is needed to fully solubilize Mn from spent batteries during the leaching process. Comparison of the data seem to indicate that most reductants have similar performance and, therefore, the choice of a reductant should be focused on low cost or even waste materials. In a second attempt, the separative processes mostly described in the literature to recover Mn and Zn from leachates are reviewed emphasizing the strengths and weaknesses of each technique. Solvent extraction is the most widely tested process for this aim. A thorough comparison of existing data indicates that, in general, neutral extractants have higher potential for selective separation of Zn and Mn. Furthermore, although chemical precipitation is a simple process, low pure final metal hydroxide products are expected to be achieved when alkaline precipitation is implemented comparatively to the Mn oxidative precipitation where Mn can be recovered selectively as a solid of manganese (IV) oxide.