Effect of the Flux-Assisted Thermal Treatment of Industrial Ammonium Jarosite: Thermodynamic Mechanisms and Mineralogical Evolution

Jarosite-type residues generated during zinc hydrometallurgical processing represent a significant environmental liability and a latent source of valuable metals (Zn, Pb, Ag). In this study, the thermal decomposition and mineralogical evolution of an industrial ammonium jarosite residue were investigated to 600–1400 °C. The behavior of the as-received residue was compared against a designed flux-assisted formulation comprising 45 wt% jarosite, 40 wt% Na2CO3, and 15 wt% SiC. The conventional roasting of pure jarosite forms refractory zinc ferrite (ZnFe2O4) and releases SO2 above 800 °C, while the flux-assisted route stabilized the sulfur as Na2SO4 and CaSO4, decreasing toxic gas emissions. Concurrently, the reducing effect of the SiC significantly inhibited bulk zinc ferrite formation up to 1200 °C and favored the partial reduction of iron to magnetite (Fe3O4). Thermodynamic assessment using FactSage reasonably matches experimental results by X-ray diffraction and SEM-EDS measurements. The thermodynamic evaluation predicts the formation of elemental silver available for subsequent pickup by a collector metal and a liquid slag phase at approximately 1000 °C for the flux-assisted jarosite samples. The ammonium jarosite flux-assisted roasting strategy enhances the potential for metal recovery while increasing environmental sulfur fixation in the slag, aligning with sustainable circular economy principles in non-ferrous metallurgy.