A computational approach to predicting the formation of iron sulfide species using stability diagrams

A program has been developed for generating stability diagrams that combine the principles of the Pourbaix E-pH diagrams with a rigorous and predictive thermodynamic model for multicomponent, nonideal aqueous solutions. Since the diagrams are based on a realistic model for the aqueous phase, they are referred to as real-solution stability diagrams. They are valid for solutions ranging from dilute to concentrated (up to 30 mol kg(-1)) at temperatures up to 300 degrees C and pressures up to 1 kbar. The stability diagrams are used to predict the conditions that favor the stability of various iron sulfide species. For this purpose, the applicability of the diagrams is extended to include the prediction of both stable and metastable products. The diagrams indicate that the formation of iron monosulfide follows the FeHS+ --> amorphous FeS --> mackinawite --> pyrrhotite replacement sequence. It is predicted that a transformation of iron monosulfides to pyrite may occur through greigite and/or marcasite. Greigite is predicted to be absent in strictly reducing environments. The predictions are in agreement with experimental data on iron sulfide formation in solution and/or at the iron/solution interface. (C) 1997 Elsevier Science Ltd.