Purification of MnSO4 solution obtained from manganese sludge using sulfide precipitation and sulfuric acid-assisted crystallization

Manganese-rich anode sludge is a common by-product of the zinc roasting-leaching-electrowinning (RLE) process. It primarily consists of oxidized manganese, along with smaller amounts of lead, zinc, calcium, and other minor metals. The presence of these impurities necessitates the development of new recycling strategies to enable efficient utilization of the sludge. This study presents a novel purification method for MnSO 4 solution aimed at producing battery-grade MnSO 4 ·H 2 O (Mn > 32 wt-%, transition metal < 10 mg/kg each). The process involves sulfide precipitation followed by selective crystallization, facilitated by the addition of H 2 SO 4 . Sulfide precipitation was studied in two stages as commercial iron-containing MnS was ineffective for iron removal. Both commercial and self-synthesized MnS were efficient in removing over 99% of zinc with a single addition. However, particle passivation occurred during the process, requiring multiple additions of MnS to initiate partial iron removal. Ultimately, the iron content of the pregnant leaching solution (PLS) was minimized by adjusting the pH to almost 4 prior to filtration. Additionally, Mn separation from Ca, K, and Mg was examined through sulfuric acid-assisted reactive crystallization at varying sulfuric acid concentrations and temperatures. The method achieved good separation from Ca and K, whereas Mg was incorporated into MnSO 4 ·H 2 O crystals via lattice substitution. To avoid multiple crystallization cycles, water-soluble Mg was removed from the anode sludge by acid washing. Even though transition metals, Ca, Mg and K were kept within target impurity limits, manganese content remained slightly below 32 wt-%, most likely due to residual sulfuric acid and/or acid-rich mother liquor associated with the final product after sulfuric acid-assisted crystallization.