Transforming electrolytic manganese residue into Mn₃O₄ via acid activation: Structural evolution and leaching kinetics

Electrolytic manganese residue (EMR), a hazardous solid waste from electrolytic manganese metal (EMM) production, poses serious environmental risks due to its complex mineralogy and heavy metal mobility. However, it also holds potential as a secondary resource. We establish an end-to-end waste-to-materials flowsheet—sulfuric-acid curing → water leaching → impurity removal → one-step conversion—that selectively recovers Mn from EMR and upgrades it to phase-pure, high-value Mn₃O₄ nanomaterials. The effects of curing temperature and acid dosage on the leaching behaviors of Mn, Fe, Al, and Si were systematically investigated. At 240 °C and an acid dosage of 2.5 times the stoichiometric requirement, Mn leaching efficiency reached 95.35 %, while Si leaching remained below 50 % due to silicate encapsulation and gelation. Kinetic modeling using the shrinking core model revealed that Mn dissolution was primarily controlled by product-layer diffusion, with an apparent activation energy of 14–18 kJ·mol⁻¹. FTIR, XRD, SEM–EDS, and BET analyses showed that acid curing disrupted the dense silicate matrix and increased surface area from 9.4 to 55.6 m²·g⁻¹. Mn²⁺ in the purified leachate was directly precipitated and oxidized using an NH₃·H₂O–H₂O₂–EDTA system, producing uniformly sized Mn₃O₄ nanoparticles. Rather than a stand-alone synthesis, the impurity-tolerant process with defined operating windows is the core contribution, with the Mn₃O₄ product validating this waste-to-value pathway. This integrated route offers a scalable framework for hazardous-waste valorization while clarifying sulfuric-acid-curing transformation and leaching kinetics, advancing sustainable metal recovery.