The effect of chloride ion on the ferric chloride leaching of galena concentrate

Abstract

Previous investigations of the ferric chloride brine leaching of galena concentrate have shown that additions of chloride ion result in accelerated dissolution rates. The current study has provided the necessary information to extend and modify these previous results by incorporating the important effect of chloride ion on the dissolution kinetics. As part of this study the solubility of lead chloride in ferric chloride-brine solutions has been determined and results indicate that additions of either FeCl3 or NaCl increase the PbCl2 solubility. This is attributed to the effect of complexing on the level of free chloride ion. In addition, the dissolution kinetics of elemental lead and lead chloride were also determined and compared with the kinetics of PbS dissolution. It is significant that the rate of dissolution of PbCl2 decreases as the concentration of Cl− is decreased and as the concentration of dissolved lead increases. These results along with SEM examination of partially reacted Pb shot show that solid PbCl2 forms on the surface long before the bulk solution is saturated with lead. The PbCl2 is proposed to form by a direct electrochemical reaction between Cl− and PbS prior to the formation of dissolved lead. The reaction was determined to be first order with respect to Cl− and closely obeys the following kinetic model based on a rate limiting charge transfer reaction at the surface: $$1 - (1 - a)^{1/3} \left[ {\frac{{5.01x10^{11} }}{{r_0 }}\left[ {Fe^{3 + } } \right]_T^{0.21} \left[ {Cl^ - } \right]_T^{1.0} \exp \left( {\frac{{ - 72100}}{{RT}}} \right)} \right]t$$ The model is in excellent agreement with experimental results up to about 95 pct reaction as long as the solubility of PbCl2 is greater than about 0.051 M. Where these conditions are not met, deviation from the surface reaction model occurs due to the extremely slow dissolution rate of PbCl2. Therefore the effect of Cl− on the brine leaching of PbS is attributed to two factors, the direct reaction of Cl− with the pbS surface and the effect of Cl− on the dissolution rate of PbCl2. The overall dissolution process is viewed as occurring in three stages; in the first stage the reaction is controlled by the surface reaction and described by the model above, then as solid PbCl2 is produced the diffusion of Cl− would be equal in importance with the surface reaction,i.e, the second stage. As the reaction proceeds further, a shift in the rate-limiting step from surface reaction to product layer or pore diffusion occurs, the third stage. Thus the rate-determining step would no longer be just the surface reaction as observed experimentally at longer reaction times. The practical implications of these results for the processing of a complex sulfide concentrate using sequential, selective, or total leach approaches are also discussed.