New understanding of the reduction mechanism of pyrolusite in the Acidithiobacillus ferrooxidans bio-leaching system

Abstract

The reductive dissolution of pyrolusite in simulated Acidithiobacillus ferrooxidans bio-leaching medium was investigated. This study was performed in three stages. First, the advantageous electrochemical test conditions, parallel to the optimal bio-leaching conditions and adopting the Mn reduction rate, were determined by imitated electrolysis. The facilitation of A. ferrooxidans on MnO2 reduction is sensitive to pH and Fe(III) concentration. Second, electrochemical tests revealed that the reductive dissolution of manganese dioxide incorporated two single electron and proton steps-the first exchange of MnO2 to MnO·OH, and then conversion to Mn(OH)2 for diffusion. The results of transient and steady electrochemical measurements indicated that the first electron-transfer significantly affects the rate controlling step of Mn-leaching in control(9K) medium, while using A. ferrooxidans and Fe(III) in the solution tends to enable the leaching rate to be controlled by the latter electron transfer step. Third, the analysis of semiconductor and carrier properties of passive films of pyrolusite formed in the different solutions, illustrated that the reductive dissolution of manganese dioxide tends to depend on the movement of the holes. The first electron preferentially reacts with the shallow energy level of the O-vacancy to form MnO2·-, which then absorbs H+ to become MnO·OH. The second electron participates in the transformation of MnO·OH to (MnOH)(OH) and then to Mn(OH)2. A. ferrooxidans increases the carrier densities of the passivating film accelerating electron and proton transfer and Fe(III) primarily influences the shallow donor density of oxygen during the first electron-exchange. Additionally, the synergistic effect of A. ferrooxidans and Fe(III) on manganese dioxide ore reductive leaching is confirmed.