Influence of Mn2+ ions on the corrosion mechanism of lead-based anodes and the generation of heavy metal anode slime in zinc sulfate electrolyte

The influence of Mn²⁺ ions on the generation of heavy metal anode slime during zinc electrolysis industry was extensively investigated using several electrochemical methods, electron microscope technologies, and particle size analysis. Results showed that the Mn²⁺ could obviously promote oxygen evolution reaction (OER) and thereby weaken oxidation efficiency of Mn²⁺ (ηMₙO₂) and dissolution of Pb²⁺. The significant improvement in kinetic parameters for OER was found in electrolytes of 1 and 3 g/L Mn²⁺, but became unstable as the Mn²⁺ concentration increased to 10 g/L. This result was correlated with much different properties of oxide layers that its changes of microstructure are involved in, since it confirmed that the positive role of compact oxide layers in contributing to high corrosion resistance and activity for OER, but excessive Mn²⁺, resulted in its micromorphology of overthickness and instability. Such differences resulted from the effect of the Mn²⁺ concentration fluctuation on kinetic rates of the nucleation growth process. The formation and adsorption of intermediate MnO₂–OHₐdₛ identified as the controlled step for Mn²⁺ catalyzing OER was also recommended. The generation mechanism of anode slime was found to be changed in essence due to varying Mn²⁺ concentrations. In electrolyte of 1 g/L Mn²⁺, results revealed that the root cause of excessive small suspended anode slime (around 20 μm) was the change of the initial pathway of Mn²⁺ electro-oxidation, whereas, it showed great improvement in the settling performance as the Mn²⁺ concentration was increased to 10 g/L. Considering the potential of optimizing Mn²⁺ concentrations as a cleaner approach to control anode slime, deepening the understanding of the impact mechanism of Mn²⁺ can provide new insights into intervention in the generation of anode slime.