Exploring As-Cast PbCaSn-Mg Anodes for Improved Performance in Copper Electrowinning
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INTRODUCTION
ELECTROWINNING (EW) is the process used to produce a significant proportion, about 17 pct, of worldwide copper (Cu) production. Research interest in Cu EW continues to increase since the development of enhanced Cu leaching technologies.[1–7] However, despite the growing interest, Cu EW is still considered an energy intensive process with the associated electricity cost requiring up to ~40 pct of the total operational cost.[8–11] This issue, along with the fluctuating energy prices, warrants more research to improve the energy efficiency of Cu EW. In addition, any developments in the Cu EW efficiency also reduce its impact to the environment.[8,11] Under the conventional EW mode of a constant applied current density, there are three contributions of potential, which dictate the power requirements in Cu EW: (1) the reversible decomposition potential; (2) the activation overpotential for the oxygen evolution reaction (OER) and Cu deposition; and (3) the ohmic potential drop.[9] In a mechanistic sense, the minimization of energy consumption could be
JODIE A. YUWONO, MARIE CLANCY, XIAOBO CHEN, and NICK BIRBILIS are with the Department of Materials Science and Engineering, Monash University, Clayton, VIC, 3800, Australia. Contact e-mail: [email protected] Manuscript submitted June 17, 2017.
METALLURGICAL AND MATERIALS TRANSACTIONS B
achieved when the anodes have a small OER overpotential. This can be accomplished by facilitating the OER process through the development of new alloys for more energy-efficient anodes.[12–18] The typical anodes used in EW are nominally lead (Pb)-based anodes as they are conductive, economical, and have good stability in acidic electrolytes and at high potentials. It was understood that a high anode is not desired because it decreases the cell efficiency and increases the anode dissolution rate. Meanwhile, a low overpotential is desired because it decreases the overall cell voltage and the required power consumption for EW. There are some characteristics required for designing new anodes, i.e., alloy workability, mechanical and electrochemical stability, environmental hazards, and manufacturing cost.[19–22] Previous studies have identified two important criteria for designing a new Pb-based anode material: (1) the ability to minimize anode dissolution rate and (2) the ability to promote the formation of dense PbO2 layer.[23,24] Ivanov et al. emphasized the importance of alloying elements electronegativity relative to Pb since it affects the anode corrosion susceptibility.[25] The present industrial standard anode, PbCaSn, used for Cu EW has a nominal composition of Pb-alloyed with 0.08 wt pct Ca and 1.5 wt pct Sn.[9,26] The justification for the Ca and Sn alloying in modern Cu EW anodes is typically based on the investigations of their performance when studied as a binary alloy system. Calcium (Ca) alloying, which is more electrochemically active than Pb, improves the
alloy strength by refining the grain size, while decreasing the anode potential and forming a thick oxide layer in
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