Joan E. Thomas

The role of surface sulfur species in the inhibition of pyrrhotite dissolution in acid conditions

Abstract Pyrrhotite, in anoxic acidic conditions, exhibits an induction period before rapid dissolution occurs. The length of the induction period is controlled by the amount of surface oxidation products on the mineral surface, acid strength, and temperature. During the induction period there is slow release of iron but little or no production of H 2 S. The induction period is best described as a period of inhibited dissolution, before the onset of H 2 S production and increased rate of iron release of at least 2 orders of magnitude. X-ray photoelectron spectroscopic (XPS) analysis of the acid-reacted surface shows the progress of the dissolution. Four stages of dissolution have been identified. (1) The immediate dissolution of an outermost layer of oxidised iron hydroxide/oxyhydroxide species and oxy-sulfur species. (2) Inhibited, diffusion limited dissolution during an induction period due to iron diffusion through the metal-deficient layer and oxidative dissolution of the polysulfide species. (3) Rapid, acid-consuming reaction of mono-sulfide species under nonoxidative or reductive conditions with production of H 2 S. (4) Inhibited dissolution due to reoxidation of the sulfide surface by oxidising solution species (i.e., Fe 3+ , residual oxygen) to produce polysulfide, elemental sulfur, and oxy-sulfur species. Dissolving synthetic pyrrhotite in similar, but aerated, acidic conditions, results in inhibited dissolution characterised by a lower rate of Fe release, minimal release of SO 4 2− and no release of H 2 S . The XPS sulfur (S2p) spectrum shows sulfate and a form of elemental sulfur on the reacted surface. Only the first two stages of dissolution occur. The second stage differs in this case in that there is a plentiful supply of oxidising species (O 2 ). Two reaction mechanisms are proposed for the dissolution of the iron sulfide lattice of pyrrhotite in acidic conditions. The mechanisms are oxidative and nonoxidative dissolution. Two distinct activation energies are associated with the two regimes. A lower activation energy corresponds to inhibited dissolution with no production of H 2 S. A t 1/2 rate law describes dissolution in air saturated solutions and supports diffusion controlled dissolution under these conditions. A higher activation energy corresponds to rapid dissolution with H 2 S production. The mechanism of dissolution is determined by the state of the surface, particularly the sulfur species.

A comparison of the dissolution behavior of troilite with other iron(II) sulfides; implications of structure

Abstract Further knowledge as to the nature of the structure of a terrestrial sample of troilite, FeS [stoichiometric iron(II) sulfide] is revealed by a combination of XPS studies and dissolution studies in acid. The XPS analysis of a pristine troilite surface (the sample being cleaved under high vacuum) is compared to that of a surface polished in an inert atmosphere and a surface after reaction in deoxygenated acid. Further comparison is made with polished and acid-reacted surfaces of pyrrhotite (Fe1-xS) and pyrite (FeS2). The pristine troilite S2p spectrum comprises mainly monosulfide 161.1 eV, within the reported range of monosulfide, together with evidence of an unsatisfied monosulfide surface state arising from S–Fe bond rupture. Small, higher oxidation state sulfur contributions, including a disulfide-like state are also present, which suggest the presence of defects due to some nonstoichiometry. The dissolution studies showed that the troilite, in addition to dissolving in acid as an ionic solid to produce H2S, also exhibits some oxidation of sulfur in the surface layers. In addition, a study of the dissolution behavior of troilite under the influence of cathodic applied potential supported the existence of a proportion of the sulfur within troilite needing reduction before dissolution forming HS− or H2S can occur. A significant increase in the dissolution rate was observed with application of −105 mV (SHE), but further stepped decreases in potential to −405 mV and −705 mV resulted in a decreased rate of dissolution, a response typical of an ionic solid. The results of the studies emphasise the viewing of iron(II) sulfides as a continuum. Pyrrhotite has been reported previously to dissolve in acid both oxidatively (like pyrite) and nonoxidatively (like troilite) on the same surface. Dissolution studies using troilite, in Ar-purged acid, indicate that dissolution of this material may not be uniformly nonoxidative. XPS evidence of restructuring of the surface of troilite to pyrrhotite and the surface of pyrrhotite towards a FeS2 type structure, after exposure to Ar-purged acid, is presented.

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

A reductive mechanism is proposed to explain the sudden changes from oxidative (acid-produc- ing) to nonoxidative (acid-consuming) dissolution that can occur with pyrrhotite. Typically, in acidic conditions, an initial period of slow dissolution involving no release of H 2S can suddenly change to nonoxidative dissolution, with release of H2S and greatly increased rates of release of both iron and sulfur species. Observations of the change from oxidative to nonoxidative dissolution of pyrrhotite in deoxygenated acid show that the process is temperature sensitive, with solution temperatures of at least 40°C required. The mechanism is correlated with the observation from XPS analysis that pyrrhotite surfaces exhibit metastable chemical states that have trapped electrons. The same negative charge shift is measured for all C, Fe, and S chemical states implying a crystal-wide space-charge surface region. The accumulation of this surface charge during dissolution appears to result in the reduction of oxidised disulfide and polysulfide species back to sulfide, thus inducing nonoxidative dissolution. Reduction is favoured on natural pyrrhotite surfaces polished in an oxygen-free atmosphere. Reduction also occurs with synthetic pyrrhotite that, before dissolution in acid, has undergone only limited oxidation. The mechanism is minimal or nonexistent if, before dissolution 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 conditions. Reduction of pyrite only occurs with the application of a sufficiently cathodic potential. Copyright

Kinetic factors for oxidative and non-oxidative dissolution of iron sulfides☆

Abstract The dissolution behaviour of iron sulfides is significant in the minerals industry for both the recovery of more precious metals and the treatment of waste materials (pyrite being the major contributor to acid rock drainage). The dissolution behaviour of pyrrhotite, Fe (1−x) S, and troilite, FeS, in deoxygenated acid was studied using approaches established for the study of binary metal oxides in acid conditions. A feature of pyrrhotite (Fe (1−x) S) is that a relatively slow dissolution rate (10 −8 mol m −2 s −1 ) can increase suddenly by three orders of magnitude (to 10 −5 mol m −2 s −1 ) in the same solution. The sudden increases have been shown to be due to the onset of non-oxidative dissolution. Correlation was found between the non-oxidative dissolution of pyrrhotite and that of ionic and semi-conducting binary metal oxides dissolving in acid with reaction controlled kinetics. In both situations -log (Rate) was proportional to pH, implying that the rate-determining step is ionic transfer across the surface - electrolyte interface. The strongly ionic nature of the S 2− bonding within semi-conducting troilite is made evident by dissolution rates of the order of 10 −4 –10 −3 mol m −2 s −1 . As with ionic oxides with dissolution rates of this magnitude, the dissolution kinetics of troilite are no longer reaction-controlled, but are controlled by bulk solution diffusion factors.