Polarization Behavior of Lead-Silver Anodes in Zinc Electrowinning Electrolytes

Abstract

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...

The influence of alloying elements on the electrochemistry of lead anodes for electrowinning of metals: A review

“…An Ag addition of 0.5-1.0 wt. % helps to reduce corrosion (Ivanov et al, 2000b;Zhang and Houlachi, 2010) and the anodic overpotential by as much as 120 mV (Ivanov et al, 2000a;Tunnicliffe et al, 2012). However, if more than this could be added it could potentially reduce the amount of Pb contamination in the deposit on the cathode.…”

“…The dense, thin layer of MnO 2 that can form on the surface of the Pb anode can have good adherence and as it is non-conductive, it protects the underlying anode (Tunnicliffe et al, 2012;Zhang and Houlachi, 2010). An anode that is coated in MnO 2 has a significantly lower operating voltage than typical anodes (Eggett and Naden, 1975).…”

“…Lead (Pb) based anodes are currently the preferred choice for the industrial electro-winning (EW) of many metals as they are conductive, economical and relatively stable at high potentials and low pH (Tunnicliffe et al, 2012). Approximately 20% of the world's Cu is produced by the EW process (Moats and Free, 2007).…”

“…It has been suggested that reducing the anode potential promotes the formation of a denser oxide layer, which consequently will have better adherence and lower dissolution. Insufficient adherence with the anode may cause the oxide to detach, thus exposing the underlying Pb anode and potentially contaminating the metal deposited on the cathode (Tunnicliffe et al, 2012).…”

Manganese consumption during zinc electrowinning using a dynamic process simulation

“…The MnO 2 scale on the anode provides valuable corrosion protection and lowers the oxidation overpotential of the water electrolysis reaction (Cachet et al, 1999;Tunnicliffe et al, 2012;Yu and O'Keefe, 2002). The corrosion protection reduces lead from dissolving and entering solution which will reduce contamination of the cathode product as well as reduce the volume of Pb precipitated contributing to the cell mud.…”