Lead anodes performance is important in zinc electrowinning (EW) as it greatly influences the final product quality and operating costs. Pb-based alloys are used in EW because they are cheap, conductive, and relatively stable at high potentials and low pH. In this study, polarization behavior and the oxygen evolution reaction (OER) on a Pb-Ag alloy were examined as a function of H 2 SO 4 , Mn 2+ , Cl − , Zn 2+ concentrations and solution temperature. The slope of the oxygen evolution region and the OER potential were determined using potentiodynamic experiments. Corrosion products were studied using potentiostatic experiments along with scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDS) and X-ray diffractometry (XRD). The relationship between surface morphology of the electrode and operating potential was studied.The London Metal Exchange (LME) reported that 12 million tons of metallic zinc was produced in 2007 and, of that, 7 million tons were recovered through aqueous EW. 1 The largest producers were: Australia, Canada, China, Peru, and the United States. 2 Zinc usually exists in nature as sphalerite (ZnS) and is commonly recovered using a roast-leach-EW process. Otherwise, the concentrate is leached in an autoclave, solids and liquids are separated, and zinc is recovered using EW. 3 Fundamentally, EW is an electrolytic process composed of a metallic reduction at the cathode (reaction 1) and oxygen evolution reaction (OER) (reaction 2) at the anode. The standard state thermodynamic cell potential based on these oxidation and reduction reactions is −1.99 V. In order to produce metallic zinc, a constant current between 400 and 600 A/m 2 is applied to the cell which results in a cell potential between 3.2 V and 3.7 V. This high potential is required to achieve desired kinetics during EW.An EW cell contains acidic electrolyte and utilizes Pb-based anodes and aluminum cathodes. Few anode materials are insoluble in sulfate based electrolytes making Pb-based alloys a primary candidate for this application. Pure Pb lacks the physical properties required for anodes but Pb-based alloys are used because they are inexpensive, insoluble, conductive, electrocatalytic, and stable at high potentials. 4 Pb-based alloys have been used as anodes since 1909 when zinc was first produced industrially. 5 A protective corrosion layer creates a physical barrier between the Pb alloy and electrolyte in service. Pb-based anodes' corrosion resistance and physical properties are enhanced by the addition of Ag. Ag improves the Pb alloys electrical conductivity as well as enhances the OER. 4 Ag content varies in electrodes, but industrial anodes commonly contain between 0.5 to 1wt% silver. 4-10 The cost of alloying Pb with Ag becomes substantial because of the high price of metal and the large amount required for the over 10,000 anodes typically used in an EW plant.Assuming that the processing plant is capable of recovering the majority of the used Pb anodes, each new anode costs approximately $300. The cost of ano...
Due to their insolubility, lead and its alloys have always been candidates for anodes in electrowinning plants. However, corrosion problems associated with lead anodes cost millions of dollars for industries annually. Cast and rolled lead-silver and lead-calcium anodes were tested under typical nickel electrowinning conditions. Effects of sulfuric acid and chloride concentrations on overall corrosion performance of the anode materials were investigated. Moreover, oxygen evolution reaction rate and overpotential were determined for specific electrode material/solution conditions. Galvanostatic experiments were performed so the actual conditions that the electrodes are exposed to were simulated. Corrosion rates of the electrodes were calculated using the discharge plateau recorded during OCP measurements following the galvanostatic experiments. SEM and XRD were used to study the correlation between the surface morphology of the electrodes, their phase composition and their corrosion rates.
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