Deposition and removal of manganese oxide in zinc electrowinning

Zinc electrowinning in sulfate -based electrolytes produces zinc at the cathode and oxygen gas at the anode. Lead anodes are commonly used despite its low activity towards oxygen evolution and potential environmental hazards. Recent advances in materials for oxygen evolution in acidic electrolytes offers a wide variety of alternative anode catalysts with much higher performance than lead. However, significant formation of manganese oxide alongside oxygen evolution at the anode prevents introduction of new and more active catalytic coatings. In this work, we investigate manganese oxide deposition at IrO2-Ta2O5 coated Ti electrodes using chronoamperometry, linear sweep voltammetry and rotating disc electrode. The morphologies of the electrode and deposited manganese oxide are also examined. Oxygen evolution is observed to primarily occur at the electrode and not on the growing manganese oxide deposit. We provide experimental evidence that deposition of manganese oxide gradually becomes limited by diffusion of Mn2+ within the growing manganese oxide deposit itself. We interpret this to be due to formation of a poorly conducting outer part of the deposit. An electrochemical sequence for removal of deposited manganese oxide is developed and validated in synthetic electrolyte (2 mol L-1 H2SO4 + 0.15 mol L-1 Mn2+) and industrial electrolyte. Most importantly the sequence includes formation of a prelayer of manganese oxide at a low electrode potential (1000 s at 1.45 V or 7 A m-2) as we show this to reduce more easily. Furthermore, a short reduction step (100 s at 0.4 V or -35 A m-2) is then sufficient to diminish the adhesion of remaining manganese oxide and allow for easy mechanical removal. Implementing this method can make it possible to replace the currently used anodes in zinc electrowinning with more energy efficient and environmentally friendly anodes.