The kinetics and mechanism of the pyrite-to-pyrrhotite transformation

The kinetics of the transformation of pyrite to pyrrhotite have been investigated. The study was performed using thermogravimetric analysis over the temperature range of 620 to 973 K in atmospheres of H₂, He, Ar, and in vacuo over a wide range of pressures: 0.20 Pa to 4.24 MPa. Based on the kinetic results, a mechanistic picture of the various steps exerting control over the transformation is proposed. The thermal decomposition proceeds via a two-step, consecutive process. The rate-controlling step is the desorption of sulfur vapor from the surface. The presence of H₂ introduces different rate-controlling steps into the sequence, providing the H₂ exists at a pressure sufficiently high to suppress the rate of thermal decomposition. Rates at which the H₂ reduction occurs with pyrite samples from different sources depends upon the samples’ impurity level and the extent to which various crystallographic faces are exposed.