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I.

INTRODUCTION

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possible alternative method for treating galena could be the production of sufficiently soluble lead salts, from which very pure metal can be obtained by electrowinning. Nonoxidative leaching of galena with HC1 in the presence of chlorides has been the object of considerable interest, since it permits the direct conversion of lead sulfide into chloride: PbS + 2HC1 ~ PbCI2 + H2S The accelerating effect of chlorides in the HC1 solutions used for leaching galena is well known tr-6] and has been employed in processes on a pilot-plant scale. The first of these was the so-called Christensen process, tT] which used HC1-NaC1 solutions. The process was developed around 1925, but, owing to the lack of corrosion-resistant materials required for working with highly concentrated acid-chloride solutions, the pilot plant was forced to close. The availability of new materials resistant to the corrosion produced by the types of solutions used and the possibility of selectively dissolving PbS and ZnS present in chalcopyrite concentrates has recently led various authors to turn their attention to this reaction. t3] The principal line of research has been the replacement of NaC1 with other chlorides, such as CaC12 and MgC12. In favor of MgC12 is the possibility of regenerating HC1 from the hydrated chloride by decomposition at moderate temperatures via the Amon process. [81 It is well known that chlorides with divalent cations increase the reaction rate more than monovalention chlorides, although the reasons for this have not yet been fully explained. Another reason for this recent interest in the study of the possibilities offered by this reaction is the appearance of processes for obtaining metallic lead from its chloride by direct electrolysis. The considerable removal of im-

C. NOIqEZ, Full Professor and Chairman, F. ESPIELL, Professor, and J. GARdA-ZAYAS, Associate Professor, are with the Department of Chemical Engineering and Metallurgy, Barcelona University, Avenida Diagonal 647, Barcelona 08028, Spain. Manuscript submitted June 22, 1988. METALLURGICAL TRANSACTIONS B

purities achieved on crystallizing concentrated lead chloride solutions by cooling is utilized simply and directly in these processes without the need to resort to the lengthy series of operations involved in the pyrometallurgical process. [9,10,11] Kinetic studies have been carried out [121 on the leaching of galena in acid solutions of HCIO4 and acidcomplexing solutions of HC1 and HC1-NaC1, in which a clearly determined reaction order has not been established for the acid activity over a wide range of concentrations. Thus, Awakura et al. tlel have concluded that there is a first-order dependence between the rate of galena leaching and the individual activity of the hydronium ion in HC1 solutions, either alone or with the addition of NaC1, although they admit that this order must be greater than 1 at higher concentrations. The individual activities of the H § and C1- ions had to be determined experimentally by Awakura et al. by electrochemic