On the Development of Thermo-Kinetic Eh-pH Diagrams
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INTRODUCTION
SINCE the pioneering work of Garrels and Christ in 1965 and Peters’ classic 1976 AIME publication on the thermodynamics of sulphide leaching, the hydrometallurgical community has used Pourbaix (Eh-pH) diagrams for the interpretation and design of leaching systems.[1,2] Around the same time, electrochemical methods such as cyclic and potentiodynamic polarization for the characterization of leaching kinetics were beginning to gain acceptance and are now well established. For the hydrometallurgist it is clear that kinetics and thermodynamics are inextricably linked: the interpretation of electrochemical kinetic data is contingent on an understanding of aqueous speciation while thermodynamic predictions are not useful without the surety that they will be kinetically favorable. Thermodynamic predictions are only as good as the data used to make them and much of the data required for the generation of hydrometallurgically-relevant Pourbaix diagrams are either not available or of questionable origin. In the event that thermodynamic data are not available it becomes difficult to make confident process decisions. In this paper we present the possibility of using electrochemical kinetic data for the generation of new diagrams, which we will call thermokinetic diagrams. These diagrams are presented in the Eh-pH domain but do not rely on thermodynamic data. The first use of this type of measurement was by Pourbaix when comparing his diagrams to actual corrosion behavior.[3] We will use the Fe-NH3-CO2-H2O system as an example of the application of this method. The advantage to the use of this system here is that it has been SUBRATA ROY, M.A.Sc. Student, HAMIDREZA ZEBARDAST, Ph.D. Candidate, and EDOUARD ASSELIN, Associate Professor, are with the Department of Materials Engineering, The University of British Columbia, Vancouver, BC V6T 1Z4, Canada. Contact e-mail: [email protected] Manuscript submitted October 5, 2011. Article published online August 11, 2012. METALLURGICAL AND MATERIALS TRANSACTIONS B
extensively studied in the past. Aside from the original studies by Caron, Osseo-Asare and co-workers have published systematic thermodynamic and electrochemical studies of this system.[4–10] More recently Nicol et al. have thoroughly investigated the passivation of iron in Caron solutions.[11–14] In this work, measured thermokinetic data are compared to purely thermodynamic predictions from the literature. We proceed by measuring the electrochemical data and comparing to other known sources. We then establish that the potentials where active dissolution is measured are consistent with known thermodynamic processes and that the measured potentials reflect a steady state that is relevant to leaching.
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EXPERIMENTAL
Specimens of iron (~99.83 pct) measuring 1 9 1 9 1 cm3 were cut from Armco iron bar stock. The electrodes were mounted in epoxy resin so as to present a planar surface to the solution. The electrodes were polished to 600 and then 1200 grit SiC. The polished electrodes were then rinsed with methanol and w
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