Ripening Kinetics of Iron Sulfides in the Presence/Absence of a Carbonyl Group

Abstract

The ripening of mackinawite (Fe₁+ₓS) to pyrite (FeS₂) or greigite (Fe²⁺Fe³⁺ ₂S₄) was examined at temperatures between 23 and 90°C to better understand the kinetics of low temperature iron sulfide crystallization in the presence or absence of aldehydes. Mackinawite starting material was synthesized in a glass, thermostatted reaction vessel from aqueous hydrogen sulfide and ferrous ammonium sulfate. Formaldehyde, which acts as an organic switch leading to greigite formation instead of pyrite, was added to the reaction vessel to explore changes in kinetics which produce greigite and suppress pyrite production in the presence of carbonyls. Samples of iron sulfides were collected at predetermined intervals and analyzed by powder XRD. Two distinct ripening pathways were found: if a carbonyl (formaldehyde) was present, mackinawite ripened to greigite. If a carbonyl was absent, the mackinawite ripened to pyrite with small amounts of smythite and pyrrhotite produced along the way. Application of the Johnson-Mehl-Avrami-Kolmogorov equation to better understand the solid-state reaction kinetics suggests that nucleation of the greigite or pyrite begins at the surface of the mackinawite grains. That is consistent with oxidation of the Fe²⁺ by the carbonyl group to form greigite, or oxidation of the S²⁻ by H₂S to form pyrite as suggested by previous workers.