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The study of the sphalerite bioleaching mechanism showed that the oxidation state of the iron in solution is fundamental because it controls the relative rate of the oxidation reactions participating in the process. In addition, it was observed that the leaching of sphalerite is controlled by a layer of elemental sulphur formed on the surface, which controls diffusion. Microbial attachment on the pyrite component of the zinc concentrate relates directly with the leaching rate of the sphalerite due to the liberation of Fe²⁺ from contact bioleaching of the pyrite. Bioleaching of the sphalerite concentrate takes place via a cooperative bioleaching strategy involving contact bioleaching of the pyritic phase of the concentrate and the indirect bioleaching of the sphalerite.