Comparative Study of Sulfur-Based Reductive Agents for Pyrolusite Leaching

To minimize impurity introduction during the leaching of pyrolusite and mitigate the adverse effects of impurities on downstream processing, sodium thiosulfate (Na2S2O3, ST), sodium sulfite (Na2SO3, SS), and sodium disulfite (Na2S2O4, SD) were systematically evaluated as reductive leaching agents. A comparative analysis revealed distinct leaching performances under optimized conditions: for ST, a molar dosage of 1.4 × relative to MnO2, a reaction time of 80 minutes, an H2SO4 concentration of 3 mol L⁻1, and a temperature of 363 K yielded a manganese extraction efficiency of 90.57 pct; for SS, under identical dosage and time but at 2 mol L⁻1 H2SO4 and 323 K, extraction reached 95.42 pct; and for SD, a lower dosage (0.8 × MnO2 molar ratio), a shorter duration (20 minutes), milder acidity (2 mol L⁻1 H2SO4), and ambient temperature (293 K) achieved 88.03 pct extraction. Kinetic analysis indicated apparent activation energies of 30.84 and 24.83 kJ·mol⁻1 for ST- and SS-mediated leaching, respectively, confirming their higher thermal dependence compared to SD. XRD and SEM–EDS mapping analyses of raw pyrolusite and post-leaching residues confirmed structural transformation and compositional evolution; the residue exhibited a porous, irregular morphology with SiO2 identified as the predominant phase. Notably, SD enabled rapid, low-temperature leaching—attributed to its participation in multiple concurrent redox pathways—whereas ST and SS required elevated temperatures to overcome kinetic barriers and attain high extraction yields. This study offers mechanistic insights and practical guidance for selecting sulfur-based reductants in pyrolusite hydrometallurgy.