A mechanism to explain sudden changes in rates and products for pyrrhotitedissolution in acid solution

A reductive mechanism is proposed to explain the sudden changes from oxidat ive (acid-producing) to nonoxidative (acid-consuming) dissolution that can occur with pyrrhotite. Typically, in acidic conditions, an initial period o f slow dissolution involving no release of H2S can suddenly change to nonox idative dissolution, with release of H2S and greatly increased rates of rel ease of both iron and sulfur species. Observations of the change from oxida tive to nonoxidative dissolution of pyrrhotite in deoxygenated acid show th at the process is temperature sensitive, with solution temperatures of at l east 40 degreesC required. The mechanism is correlated with the observation from XPS analysis that pyr rhotite surfaces exhibit metastable chemical states that have trapped elect rons. The same negative charge shift is measured for all C, Fe, and S chemi cal states implying a crystal-wide space-charge surface region. The accumul ation of this surface charge during dissolution appears to result in the re duction of oxidised disulfide and polysulfide species back to sulfide, thus inducing nonoxidative dissolution. Reduction is favoured on natural pyrrho tite surfaces polished in an oxygen-free atmosphere. Reduction also occurs with synthetic pyrrhotite that, before dissolution in acid, has undergone o nly limited oxidation. The mechanism is minimal or nonexistent if, before d issolution in acid, the pyrrhotite (natural or synthetic) is ground either in air or in a N-2 atmosphere. No evidence for this mechanism is found with either polished or ground pyrite dissolving in acid under the same conditi ons. Reduction of pyrite only occurs with the application of a sufficiently cathodic potential. Copyright (C) 2001 Elsevier Science Ltd.