The dissolution of galena in ferric chloride media

The dissolution of galena (PbS) in ferric chloride-hydrochloric acid media has been investigated over the temperature range 28 to 95 °C and for alkali chloride concentrations from 0 to 4.0 M. Rapid parabolic kinetics were observed under all conditions, together with predominantly (>95 pet) elemental sulfur formation. The leaching rate decreased slightly with increasing FeCl₃ concentrations in the range 0.1 to 2.0 M, and was essentially independent of the concentration of the FeCl₂ reaction product. The rate was relatively insensitive to HCl concentrations <3.0 M, but increased systematically with increasing concentrations of alkali or alkaline earth chlorides. Most significantly, the leaching rate decreased sharply and linearly with increasing initial concentrations of PbCl₂ in the ferric chloride leaching media containing either 0.0 or 3.0 M NaCl. Although the apparent activation energy was in the range 40 to 45 kJ/mol (∼10 kcal/mol), this value was reduced to 16 kJ/mol (3.5 kcal/mol) when the influence of the solubility of lead chloride on the reaction rate was taken into consideration. The experimental results are consistent with rate control by the outward diffusion of the PbCl₂ reaction product through the solution trapped in pores in the constantly thickening elemental sulfur layer formed on the surface of the galena.