Evaluation of processing options to avoid the passivation of chalcopyrite

• We compared oxidative leaching kinetics of chalcopyrite in sulfate medium.
• Amorphous iron precipitates are not important in passivation.
• Passivation occurs even with minimum amounts of jarosite or elemental sulfur.
• A. thiooxidans does not avoid passivation of chalcopyrite.
• Shrinking core model fits results, indicating an undetected polysulfide layer.

The feasibility of two options to avoid the passivation of a chalcopyrite concentrate was evaluated in an oxidative process by ferric ion at 30 °C. These options attempt to reduce the formation of a layer of iron precipitates, and sequentially inoculate the bacterium Acidithiobacillus thiooxidans to consume the elemental sulfur layer, without modifying the oxidizing power of the leaching solution.QEMSCAN analysis revealed a reduction in ferric compound precipitation over the mineral surface from 32% to 2% by mass by controlling the formation of precipitates. However, the increase in the rate of copper extraction was insignificant, increasing this only from 3.8% to 4.0% in 30 days. The observed precipitates were mostly of type Fe-sulfates, with minor appearance of jarosite. In neither case, QEMSCAN analysis revealed formation of elemental sulfur on the mineral. Consequently, the inoculation of sulfur oxidizing bacterium A. thiooxidans in a controlled precipitation system produced no improvement in the extraction of copper.The leaching behavior fitted with the model of SCM (shrinking core model), indicating the formation of a layer of undetected compounds which exerts an effect on the diffusion of the leaching agent. Therefore, none of the options taken to avoid the passivation of chalcopyrite were successful, indicating that passivation occurs even in the absence of iron precipitation and the presence of sulfur oxidizing bacteria.