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TRODUCTION

AMMONIA has been used widely as an effective lixiviant in a number of hydrometallurgical processes for many years.[1] For instance, the Sherritt Gordon process employs ammonia leaching in an autoclave at high pressure and temperature to recover copper, nickel, and cobalt.[2] The Caron process was developed to recover nickel and cobalt from low-grade oxide ores such as laterite by using an ammonia-ammonium carbonate solution.[3] Most of the hydrometallurgical processes in ammoniacal solutions occur at elevated temperatures.[4] Hence, it is useful to determine the thermodynamic data for ammines at elevated temperatures to evaluate their stability regions, often with Pourbaix diagrams, such that leaching processes may be altered or optimized. Principally, this evaluation occurs through the estimation of heat capacity data and the subsequent calculation of free energies. Although the nickel system has been studied previously,[4] there is little information regarding the cobalt and iron systems. Because nickel, cobalt, and iron usually are processed together, this study addresses a gap in our ability to model hightemperature industrial processes. The Criss–Cobble model is based on a general correspondence principle for ionic entropies over wide ranges of temperature.[5] Thus, if an ionic entropy is known at a given temperature, then it can be calculated at another. The Criss–Cobble model is highly ion-type dependent such that different empirically based parameters are employed depending on whether, for G. NAZARI, Student, and E. ASSELIN, Assistant Professor, are with the Department of Materials Engineering, University of British Columbia, Vancouver, British Columbia, Canada V6T 1Z4. Contact e-mail: [email protected]. Manuscript submitted May 12, 2009. Article published online February 24, 2010. 520—VOLUME 41B, JUNE 2010

example, cations or anions are of interest. Historically, the Criss–Cobble model for simple cations (SCCC) has been used to model the thermochemical data for metal ammines even though the degree of uncertainty associated with this approximation has not been elucidated fully.[6] Osseo-Asare and Asihene have presented a comparison of experimental stability data for the nickelous ammines with the predictions from the SCCC.[4] As a result of this comparison, they developed the so-called ‘‘nickel model’’ to represent more accurately the experimental data measured by Letowski.[7] This model used an alternate formulation of the Criss– Cobble model, known as the Lewis formulation,[6] to calculate the heat capacity of the nickel complexes up to 200 C more accurately than the SCCC.[4] Thus, the general correspondence principle as conceived by Criss and Cobble was followed, but the formulation was different, and the parameters used were also different (i.e., based on the empirical data specific to ammine complexes). Criss and Cobble determined the correspondence principle coefficients of all ions for each temperature by the value of the entropy of the hydrogen ion at that temperature. They found that