Pressure oxidation of pyrite in sulfuric acid media: a kinetic study

Abstract

The oxidation kinetics of a massive pyrite (FeS 2) sample from Zacatecas, Mexico were investigated in sulfuric acid solution under oxygen pressure. The effects of temperature (170–230°C), particle size (49–125 μm diameter), agitation speed (650–950 rpm), oxygen partial pressure (345–1035 kPa) and pulp density (1–20 g/L) were evaluated. The catalytic effect of Cu(II) was also observed. Fe(III) was found to be the initial product of pyrite oxidation, although the proportion of total dissolved iron as Fe(III) did reach a minimum during each test, indicating the generation and subsequent oxidation of reduced sulfur species. Pyrite oxidation kinetics are limited by the rate of reaction at the pyrite surface, with an activation energy of 33.2 kJ/mol (7.9 kcal/mol) with respect to dissolved oxygen concentration, or 41.7 kJ/mol (10.0 kcal/mol) with respect to oxygen partial pressure, over the temperature range 170–230°C. The reaction order with respect to oxygen partial pressure was found to be 0.5 at 210°C, indicating the first charge transfers of both the anodic and cathodic half-cell reactions to be the most likely rate-controlling steps. Conversion data conform to the shrinking sphere model initially, but deviate at higher conversions, indicating passivation of the mineral surface, most likely by elemental sulfur, which may precipitate via the disproportionation of other reduced sulfur species (e.g., thiosulfate) that form as intermediate products of pyrite oxidation. In order to account quantitatively for this passivation phenomenon, a new “passivating shrinking sphere” model is proposed which fits the conversion data precisely over the entire range.