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

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.

Selective leaching of Zn from spent alkaline batteries using environmentally friendly approaches.

The main aim of this work was to evaluate the possibility of using microwave or ultrasound to assist the efficient and selective leaching of Zn from spent alkaline batteries and compare the results with those obtained using the conventional method. Two different strategies were applied: acid leaching of a washed residue and alkaline leaching of the original residue. In both (acid and alkaline) approaches, the use of microwave- or ultrasound-assisted leaching increased the extraction of Zn compared with the best results obtained using conventional leaching [acid leaching (1.5mol/L H2SO4, 3h, 80°C), 90% of Zn extracted; alkaline leaching (6mol/L NaOH, 3h, 80°C), 42% of Zn extracted]. With acid leaching, 94% of the Zn was extracted using microwave-assisted leaching (1 cycle, 30s, 1mol/L H2SO4), and 92% of the Zn was extracted using ultrasound-assisted leaching (2min, 0.1p, 20% amplitude, 1mol/L H2SO4). Ultrasound-assisted leaching resulted in a more selective (Zn/Mn ratio of 5.1) Zn extraction than microwave-assisted leaching (Zn/Mn ratio of 3.5); both processes generated a concentrated Zn solution (⩾18.7g/L) with a purity (83.3% and 77.7%, respectively) that was suitable for electrowinning. With alkaline leaching, microwave- (1 cycle, 3 min, 4mol/L NaOH) and ultrasound-assisted (14min, 0.1p, 20% amplitude, 4mol/L NaOH) leaching extracted about 80% of the Zn and less than 0.01% of the Mn, which resulted in lesser concentrated Zn solutions (approximately 16.5g/L) but with high purity (>99.5%) that was suitable for the recovery of Zn by precipitation. The microwave- and ultrasound-assisted leaching strategies used in this work proved to be efficient and environmentally-friendly approaches for the extraction of Zn from spent alkaline residues since a concentrated Zn solution with adequate purity for subsequent Zn recovery was obtained using significantly decreased leaching times and concentrations of chemicals.