Pyrite oxidation in alkaline solutions: Nature of the product layer

The nature of the oxides formed during pyrite oxidation by molecular oxygen in alkaline solutions has been investigated with the aid of Eh-pH diagrams and direct analysis of the solid products. X-ray diffraction (XRD) and infrared analyses indicate that the products formed are determined by solution composition. In hydroxide medium, hematite, as the main phase, and small amounts of ferrihydrite are identified. In contrast, in carbonate medium, the main constituent is ferrihydrite, with some iron hydroxide carbonate phase also present. In calcium hydroxide medium, only calcium carbonate was detected on the surface of oxidized pyrite in an amount that increased when the system was opened to the atmosphere. Only by diffuse reflectance infrared spectroscopy (IR) was it possible to identify carbonate compounds among the products formed during pyrite oxidation in aqueous solutions. The morphology of the product layers was also affected by solution composition. In NaOH solutions, the particles are initially covered by a thin oxide layer that fractures after longer reaction times. Most of the oxide reports to the solution, where it remains as a stable suspension. Pyrite oxidation in Na₂CO₃/NaHCO₃ solutions results in particles that are initially covered by a discontinuous oxide coating that grows with reaction time, thus increasing the overall pyrite surface coverage. In this case, a precipitation confined to the solid/liquid interface is favored by the higher ionic strength of the sodium carbonate solutions.