Thermodynamic analysis of iron and arsenic species distribution and phase variation of pyrite and arsenopyrite under acidic oxidation relevant to refractory gold ores

This paper presents a novel investigation into the species distribution and stability phase diagrams for the Fe-S-O-H and Fe-As-S-O-H systems under conditions below 300 °C, using available thermodynamic data. This range of temperatures is important for this system, as it informs the behavior of iron and arsenic species in the pressure oxidation of complex refractory ores and in the fixation of arsenic. When calculated based on the established models, the estimated solubility parameters for pure scorodite (FeAsO 4 ∙2H 2 O), a stable form of arsenate, at 25 °C were in good agreement with experimental results below pH 2, but diverged by up to two orders of magnitude above pH 2. In this work, this gap has been narrowed by incorporating the precipitation of ferrihydrite and the equilibrium with scorodite into the model, indicating the critical role of ferrihydrite and incongruent dissolution in scorodite solubility. Furthermore, between pH 0 and 4, the temperature-driven increase in the stability range of Fe(II) in the form of FeH 2 AsO 4 + in the Fe-As-S-O-H system is much larger compared to the Fe-S-O-H system, demonstrating that the complexation of Fe and As can significantly alter its stability and aqueous mobility. Basic ferric sulfate minerals, such as fibroferrite and metahohmannite, exhibit enhanced stability as the pH decreases below 1.5. Conversely, jarosite displays increased stability in acidic environments with rising temperatures, up to around 220 °C, before undergoing a reversal. These novel findings provide valuable new insights into the thermodynamic behavior of iron and arsenic species, advancing the field of geochemical modeling and mineral stability. • Starting point of incongruent dissolution of scorodite is about pH 2. • Improved data fitting by accounting for scorodite-ferrihydrite equilibrium. • Stability field of Fe(II) (aq) increases dramatically due to the existence of As. • Various BFS minerals are more stable than jarosite under strong acidic conditions.