Sodium Carbonate-Assisted Roasting and Sulfuric Acid-Leaching of Waelz Kiln Clinker for Enhanced Zinc Recovery

Waelz kiln clinker is a complex Fe-Ca-Si-rich metallurgical residue, with zincite- and ferrite-type phases, identified by XRD, hampering its direct hydrometallurgical treatment. The effect of Na2CO3-assisted roasting (600–900 °C), followed by water and sulfuric acid leaching, on Zn extraction was studied in the present work. XRD indicated the formation of sodium ferrite and sodium silicate, alongside zincite, while SEM-EDX showed heterogeneous aggregates after roasting. Roasting at 800 °C increased Zn extraction by water leaching from 1.8% (raw clinker) to 16.4%, but Fe extraction was about 5.1%. Acid leaching of the 800 °C product with 1.5 M H2SO4 in a liquid-to-solid ratio of 10 mL/g provided a balance between zinc recovery and iron dissolution, with approximately 82% Zn and 28% Fe leached at 25 °C for 60 min, which increased to 95% Zn and 44% Fe at 75 °C for 120 min. Original-scale weighted-fitting supported product-layer diffusion as the principal interpretation of the initial stage, with an apparent activation energy of 20.1 kJ mol−1; model deviations remained at 25 °C. Under identical acid-leaching conditions, Zn extraction increased from 33% for raw clinker and 42% after roasting without soda to 82% after soda roasting, demonstrating the benefit of Na2CO3 pretreatment at 800 °C.