A study on the zinc sulfide dissolution kinetics with biological and chemical ferric reagents

The leaching kinetics of a sphalerite concentrate containing 38.25% zinc was studied in the presence of biological and chemical ferric reagents. To produce the biological ferric reagent (BFR), a pyrite concentrate sample was oxidized to ferric ions by iron and sulfur oxidizing bacteria, and this pregnant leach solution was then applied as oxidizing reagent in ZnS leaching. This process is commonly referred to as two-step bioleaching. This biological reagent contained 12.75 g/l ferric and its pH was 0.86. The chemical ferric reagents (CFR) were made by dissolution of Fe-2(SO4)(3) and FeCl3 salts in deionized water. Leaching experiments were carried out at different temperatures to study the mechanism of ZnS dissolution and its kinetics. The kinetic modeling of ZnS dissolution with BFR followed the interfacial transfer and diffusion across the product layer mechanism within the first minutes (about 60 min) while it changed to the diffusion-control mechanism after passing this initial period. On the other hand, the ZnS dissolution in presence of ferric sulfate was described by a diffusion mechanism. The surface analysis by SEM and FTIR confirmed that sulfur layer formation on the mineral surfaces could prevent the solvent diffusion to the minerals surface, and consequently it controls the dissolution reaction. The highest zinc recoveries were 70%, 99% and 83% in presence of biological ferric reagent, ferric sulfate and ferric chloride at 90 degrees C after 200 min, respectively. The zinc recovery for one-step bioleaching was 90% and was achieved after 20 days at 35 degrees C, by iron and sulfur oxidizing bacteria.