An investigation of the thermodynamics and kinetics of the ferric chloride brine leaching of galena concentrate
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[-72100"~-[ [2.3 x 10~2[Fe+3]o.2~ r0 exp~----ff~) J t
The rate deviates from the 0.21 order for Fe ยง concentrations greater than 0.6 M. The deviation from the surface model at higher values of PbS conversion is due to the presence of solid PbC12 in the pores of the reacting particles.
I.
INTRODUCTION
PYROMETALLURGICAL processes for smelting lead sulfide to primary lead produce significant quantities of sulfur dioxide and particulate lead which must be captured and treated. The smelting method is also limited in treating complex sulfides as well as low grade ores, both of which will become increasingly important as higher grade materials are depleted. These and other restrictions have provided the impetus for the development of hydrometallurgical alternafives such as the dissolution of lead sulfide in chloridebearing lixiviants.l-10 For example, the United States Bureau of Mines m has developed a process involving the leaching of galena concentrate with ferric chloride-brine solution. The CLEAR 3 and CYMET 4 processes using ferric chloride solution to leach copper concentrates have been operated commercially. The ELKEM process was developed in Norway in order to treat complex sulfide ores (Cu/Pb/Zn) using ferric chloride and appears to be promising but as yet has operated only on a pilot plant scale. 5 Several advantages of ferric chloride over other reagents as a leaching media are that it exhibits substantially faster dissolution rates for most sulfides, it is regenerated easily by chlorination of ferrous chloride leaching byproducts, and it has greater potential for the treatment of complex sulfides. Several kinetic investigations on the ferric chloride leaching of galena have been reported. 6'7'8Murray, 6 for example, has measured dissolution rates for galena concentrate in SEON-HYO KIM, Graduate Student, H. HENEIN, Assistant Professor, and G.W. WARREN, Associate Professor, are with the Department of Metallurgical Engineering and Materials Science, Carnegie-Mellon University, Pittsburgh, PA 15213. Manuscript submitted April 23, 1985.
METALLURGICALTRANSACTIONS B
acidified ferric chloride solutions without the addition of NaC1. Based on the apparent activation energy of 56.5 kJ/mole and the nonlinear kinetics observed, the rate limiting step was suggested to be diffusion of reactant or product ions through the elemental sulfur layer. 6 Chen7 has come to a similar conclusion using a larger particle diameter (512 pm). The apparent activation energy in this case was determined to be 28 kJ/mole. On the other hand, for similar leaching conditions, Demopoulos 8 has found that the rate is chemically controlled in 3.0 M FeC13. In addition, for lower ferric concentrations the rate also appears to be under chemical control at least initially, and this part of the dissolution process is extended as the ferric chloride concentration is increased. This would be in reasonable agreement with the relatively large activation energy of 56.6 kJ/mole observed in 3.0 M FeC13,8 For smaller FeC13 concentrations mostly parabolic
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