Chemical and bacterial leaching of metals from black schist sulfide minerals in shake flasks

A finely ground black schist ore sample was subjected to chemical and bacterial leaching ± additional ferrous or ferric sulfate in shake flask experiments. Pyrrhotite (Fe1 − xS) and pyrite (FeS2) were the main Fe-sulfides in the ore matrix, with lesser amounts of sphalerite (ZnS), pentlandite ((Ni, Fe, Co)9O8), violarite (FeNi2S4), and chalcopyrite (CuFeS2). The main silicate phases were micas, quartz, and feldspars. The leaching of the sulfide minerals was monitored by analysis of dissolved Fe, Ni, Zn, Cu, and Co. Their dissolution was clearly pH-dependent and strongly enhanced when inoculated by acidophilic Fe- and S-oxidizing bacteria. Analysis of pooled Cu dissolution data suggested chalcopyrite passivation. X-ray diffraction analysis was performed on solid leach residues. Sulfide minerals and graphite could not be detected by XRD after an extended contact time of 100 days. The mica phase (biotite/phlogopite) was altered to a mixed interlayer structure with vermiculite as the expanded end member. Dissolved Ca concentration reached the saturation index for gypsum precipitation.

► This study evaluated acid leaching (±bacteria) of metals from a black schist sample. ► Metal dissolution was highly pH dependent but varied with the specific metal. ► The results demonstrated associations between dissolved Fe, redox potential and pH. ► Cu dissolution data and S accumulation suggested chalcopyrite passivation. ► Mica phases were altered to mixed interlayer structures containing vermiculite.