The effect of experimental variables on the mechanism of the oxidation of pyrite: Part 2. Oxidation of particles of size 90–125 μm

Abstract

The oxidation of pyrite of particle size 90–125 μ m has been studied by simultaneous thermogravimetry-differential thermal analysis (TG-DTA). Partially oxidised samples were characterized by qualitative and quantitative X-ray diffraction (XRD) and scanning electron microscopy (SEM). The first weight loss, of 4%, occurred at 425–435 °C, and was associated with a weak exotherm. XRD analysis indicated 11% hematite present at 453 °C, with 86% pyrite unreacted. The SEM micrographs of samples heated to 453 °C were visually identical to those of unreacted pyrite, although analysis by energy dispersive X-ray spectrometry showed some rim oxidation. Between 450 and 515 ° C there was a gradual weight loss of 55%, with an associated exothermic drift. The SEM micrographs revealed a pyrite core surrounded by a thick product coating. By 520 °C, 75% hematite was present as determined by XRD, with 16% pyrite remaining. The next weight loss, of 30%, occurred between 515 and 535 °C, and was coincident with the major exothermic peak observed. The hematite content increased to 89% by 550 °C, and no unreacted pyrite was detected. There was significant porosity apparent in the core of the particles. The final weight loss of 11% occurred above 565 ° C. No DTA response was evident. Increasing the heating rate between 2.5 and 40 °C min − increased the third weight loss at the expense of the second. A similar effect was noted on changing the atmosphere from air to oxygen. A comparison of the oxidation of pyrite of particle size < 45 and 90–125 μm has been made. The effects of changing the particle size, heating rate and atmosphere are discussed. Depending on the conditions chosen, one of the three following mechanisms of reaction operates: (i) slow combustion, controlled by oxygen diffusion; (ii) pyrolytic decomposition of pyrite and oxidation of the resulting pyrrhotite; and (iii) ignition. The TG-DTA records show distinct differences according to the prevailing mechanism.