Efficient separation of heavy metals from gypsum residue and secondary zinc oxide fume based on synergistic sulfidation

• • Zinc sulfidation rate of 90.39% achieved at 700 °C with 30% carbon powder, 10% Na2CO3, gypsum residue to secondary zinc oxide mass ratio of 1.4:1, and 2 h roasting time, enabling high-efficiency conversion of ZnO to ZnS for subsequent flotation recovery. • • Simultaneous removal of 76.32% F, 72.11% Cl, and 93.41% As under the same optimized conditions, mitigating impurity accumulation that typically degrades zinc electrowinning performance and reduces product quality. • • ZnS grain size increased from 5 to 10 μm via controlled cooling at 1 °C/min and sodium salt addition, which is critical for effective flotation separation, as fine grains are notoriously difficult to float and result in high losses. • • Thermodynamic enhancement of ZnO sulfidation by increasing CaSO4 and carbon dosage within 500−800 °C, with TG/DTG−DSC and 3D FTIR confirming conversion of CaSO4 to CaCO3 and suppression of CO2 and SO2 emissions, addressing environmental compliance and carbon footprint concerns.