An Electrochemical Quartz Crystal Microbalance Investigation of Manganese Oxide Deposition and Dissolution in Sulfuric Acid Relevant for Zinc Electrowinning

Abstract

Manganese(II)-ions are present in varying amounts in the electrolyte used for zinc electrowinning, based on the manganese content in the ore. The manganese ions are oxidized at the anode during electrowinning to form manganese dioxide alongside the oxygen evolution reaction. This has both positive and negative consequences for the lead-based anodes used in the industry today. The manganese dioxide prevents corrosion of the lead and hence lowers the amount of lead as an impurity in the produced zinc. Manganese dioxide also increases the selectivity of the oxygen evolution reaction to the chlorine evolution reaction. On the other hand, the manganese dioxide layer grows in thickness over time leading to an increase in cell resistance in addition to a possibility of flaking off and short circuiting the cell. The anodes therefore have to be removed from the cells at regular intervals to physically remove the MnO2 layer, which is a labor intensive task. Lead-based electrodes are not ideal as anode materials for several reasons. They are not very active for oxygen evolution and they introduce lead impurities to the produced metal. Lead is also unwanted for environmental reasons. It is desirable to replace them with electrochemically active dimensionally stable anodes (DSAs). The aim of this study is to investigate manganese oxide deposition and dissolution on noble metals by using conventional electrochemical methods and electrochemical quartz crystal microbalance (EQCM). In this work EQCM was used to provide additional information about the manganese oxide deposition and dissolution from a sulfuric acid solution. The electrode materials studied were platinum and gold. The electrolyte consisted of 0.1 M H2SO4 with 5 mM Mn2+-ions and the experiments were conducted at room temperature. Cyclic voltammograms were recorded for a number of different upper reversal potentials and sweep rates. In addition, sweep-hold and step-hold experiments were conducted. In these experiments the potential was swept or stepped to various potentials at which manganese oxide deposition occurred and held for a given time. These sequences were followed by a negative-going sweep from the hold potential. To complement the EQCM experiments, manganese oxide was deposited on a platinum disc at two different potentials (1.45 V and 1.55 V) and studied by scanning electron microscopy (SEM) as a function of deposition time. The cyclic voltammograms showed an oxidation peak commencing at about 1.42 V and peaking around 1.46 V in the positive-going scan (Fig 1), as observed elsewhere [1]. The oxidation peak coincided with an increase in mass. Clear indications of a nucleation, growth and collision mechanism were observed when the upper reversal potential was chosen from the rising part of the oxidation peak, i.e. below 1.46 V. This peak occurs below the onset of oxygen evolution and is thus related to the onset and continuous formation of manganese oxide. The shape and size of the oxidation peak were similar on gold and platinum. Subsequently, a cathodic peak (reduction of manganese oxide) was observed in the negative-going scan at potentials just positive of 1.3 V, which also appeared similarly on both electrode materials. Interestingly, a second reduction peak at around 1.2 V was observed at both electrodes, but only when the upper limit was sufficiently positive (above 1.5 V). Voltammograms recorded at different sweep rates showed no clear trend in the peak current for the oxidation peak. However, the mass deposited did show a very clear dependence and was inversely proportional to the sweep rate. The SEM images showed that the deposit formed at the higher potential had more dendrites than the deposit formed at the lower potential although almost the same amount of charge was passed in the deposition experiments. The change from one to two reduction peaks when reversing the sweep at potentials above 1.55 V indicates a change in the reduction and deposition mechanism. The lack of sweep-rate dependence of the oxidation peak shows that it is at least not entirely governed by bulk diffusion of Mn2+. To be able to implement DSAs in zinc electrowinning it is important to understand the mechanism of manganese oxide deposition and dissolution. W.H. Kao and V.J. Weibel, Electrochemical oxidation of manganese(II) at a platinum electrode, J. Appl. Electrochem., 22 (1992) 21-27. Figure 1