Untangling galvanic and passivation phenomena induced by sulfide minerals on precious metal leaching using a new packed-bed electrochemical cyanidation reactor

Abstract

Because permanent galvanic contacts between gold rotating disc electrodes and slurried sulfide-rich ores are uneasy to achieve, the standard gold rotating disc electrode/slurry cyanidation arrangement has a tendency to inflate overly the importance of passivation phenomena over the corrective trend of galvanic interactions inherently present within the ore grains. A new packed-bed electrochemical reactor (PBER) was thus developed and tested to decouple and quantify the individual contributions of passivation phenomena (PP) and galvanic interactions (GI) on precious metal (gold and silver, PM) leaching rates during the cyanidation of sulfide-rich ores. The PBER was filled with homogenized mixtures of minerals (pyrite, chalcopyrite, sphalerite, chalcocite, galena, stibnite, and industrial ore), gold and silver powders, which were arranged as electrically isolated sulfide-quartz segregated bilayer(s) with permanent (inter)particle–particle electrical contacts among all constituents in each layer. Implantation of PM powders successively in the quartz layer and then in the sulfide layer, enabled PP and GI to be singled out and quantified separately for each sulfide. Galvanic interactions were found to ameliorate, to various degrees, the leaching of PM. Locating the PM powders in the quartz layer emphasized the negative impact of PP of the sulfide layer on Au and Ag dissolution, except for galena which enhanced gold leaching kinetics. Galvanic interactions due to pyrite, chalcopyrite and an industrial ore were so positive that they largely outweighed the negative impact of PM passivation. The galvanic current measured between Au/Ag electrode and each of the above minerals confirmed the positive effect of GI on gold leaching rate. In close agreement with PM recoveries, stronger galvanic currents were measured for Au-pyrite, Au-chalcopyrite and Au-industrial ore galvanic couples in comparison to those for Au-sphalerite, Au-chalcocite and Au-stibnite.