Model for the ferric chloride leaching of galena
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I.
INTRODUCTION
The ferric chloride leaching of galena has received considerable attention over the last 20 years or so. A large part of this is due to the interest in developing a hydrometallurgical alternative to the current pyrometallurgical methods for the extraction and recovery of lead from sulfide ores.[1–8] This has also been partly spurred by conflicting findings on the reaction rates, reaction mechanisms, and parametric dependencies of oxidative leaching.[9–30] Nevertheless, some consensus seems to have been reached on some aspects. Most studies show that the leaching rate shows a strong dependence on the ferric ion concentration below about 0.1 mol/L FeCl3, but is virtually independent of the amount of the oxidant above this level.[11,20,22,30] On the other hand, the leaching rate increases continually as the NaCl (or LiCl) level is raised.[12,20–23,28,30] The biggest questions for researchers have been to explain the reason for the change in behavior at 0.1 mol/L FeCl3 and to pin down the controlling mechanism for FeCl3 concentrations above 0.1 mol/L. From most of the reported studies, the answer to the second question comes down to a choice between rate control by either diffusion of the aqueous lead chloride reaction products through the S7 layer or rate control by surface reaction kinetics. Kobayashi et al.[27] reviewed the previous work done on this system and re-examined the reported leaching data, but could not conclude which mechanism is more appropriate. However, a few other studies have seemed to clarify the situation somewhat. Sohn and Baek[25] were able to fit a mixed kinetic-diffusion model to experimental data and found from their analysis that the leaching rate was controlled primarily by diffusion at higher temperatures (50 7C and above) and large particle sizes (126 mm and above). More recently, Cano and Lapidus[30] developed a transient reaction-transport model for the FeCl3 leaching of galena and validated it against experimental data obtained on suspensions of monosized galena concentrate. The strength MARK PRITZKER, Associate Professor, is with the Department of Chemical Engineering, University of Waterloo, Waterloo, ON, Canada N2L 3G1. Manuscript submitted October 20, 1997. METALLURGICAL AND MATERIALS TRANSACTIONS B
of their approach was the rigorous and comprehensive nature of the model. The model explicitly incorporates speciation of the various aqueous chloride complexes in the product layer and bulk solution, accumulation and depletion of the various species in the bulk solution, transport of all species through the product layer, and reaction kinetics at the PbS/S7 interface. Furthermore, it tracks the possibility of the precipitation of PbCl2 in the product layer during leaching and accounts for its effect on limiting the lead solubility from the point of precipitation onward. With this framework, the model has predictive capability that is not possible with the traditional shrinking core approach. In their experiments, Cano and Lapidus observed the same change in leaching be
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