The role of migration in shrinking-core leaching reactions controlled by pore transport
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INTRODUCTION
SHRINKING-CORE models have proved to be extremely useful in describing the leaching behavior of many ores and flotation concentrates5 ~,2,3] Despite this success, there are still some shortcomings in the way that they have been conventionally applied. Perhaps the biggest shortcoming is the fact that they have been used as empirical tools rather than in any predictive way. Generally, one obtains batch leaching data, attempts to fit them according to the different shrinking-core equations, and then chooses the one that does the best job of doing so. There are no reliable criteria to predict a priori the rate-controlling regime in which a given system will operate. Clearly, obtaining these criteria will require a greater understanding of the controlling processes on a microscopic level. In fairness, this situation is not unique to the analysis of leaching reactions, but is common to the way in which virtually all chemical reactions are treated. Part of this difficulty with regard to leaching reactions is no doubt due to their heterogeneous nature and the variability from one ore or concentrate to another. However, another part of this difficulty stems from the fact that the conventional shrinking-core equations have been narrowly focused on the dissolved reactant and the particles being leached. Yet, experience has shown that the nature of the electrolyte can have dramatic effects on leaching behavior. For example, the ferric leaching of chalcopyrite is controlled by pore diffusion when it is carried out in a sulfate solution, whereas it is kinetically controlled when a chloride medium is used. t4,51 Moreover, there are substantial differences in the controlling surface reactions and morphology of leaching. Another important aspect related to the influence of the electrolyte that has been recognized and, in fact, used in some cases, is complex formation.[6,7,s] This can complicate
MARK D. PRITZKER, Associate Professor, is with the Department of Chemical Engineering, University of Waterloo, Waterloo, ON, Canada N2L 3G1. Manuscript submitted May 3, 1994. METALLURGICAL AND MATERIALS TRANSACTIONS B
the application of a model such as the shrinking core. In a recent study, for example, on the cupric chloride leaching of sphalerite in ammoniacal solutions, Lapidus and de Lourdes Mosqueirafgi reported some deviation from the expected behavior, which was attributed to the formation of ammine complexes and the resulting limitations on zinc solubility. There have been efforts in the last few years to incorporate some of these effects directly into the shrinking-core framework. [9-13]Some of the refinements that have been considered are multiple surface reactions, homogeneous reactions to account for complex formation, the inclusion of electric migration and convection as transport processes, and the accounting for a distribution of particle sizes in the solid feed. Migration of charged species in an electric field has long been considered an important transport mode in electrochemical cells, ion exchange resi
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