Periodic thermal instability during the isothermal oxidation of pyrite
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A typical thermal-analysis record is shown in Fig. 1. The DTA trace shows that the temperature of the sample was unstable during the first 40 rain after the operating temperature was attained; it can also be seen that the peaks in this trace match the steps present in the TGA trace. XRD analysis of partly oxidized particles indicated the presence of hematite and pyrite. These phases were also found, in the temperature range 370 to 480 ~ by Schorr and Everhart, 9 who used high-temperature XRD and a thermobalance to study the oxidation of pyrite. In our work, any pyrrhotite, magnetite, or sulfate, if present, was below the limit of detection (about 5 pct). Polished sections of grains that had been oxidized for 20 h in air at 435 ~ are shown in Figs. 2 and 3. These micrographs show the generally continuous nature of the oxide layers (2 to 5 t~m ~vide), the cracks between them, and occasional breaks in the layers (Fig. 3) that allowed oxygen to enter. Thermalanalysis records and micrographs similar to those shown in Figs. 1 to 3 were obtained over a range of experimental temperatures (420 to 470 ~ At temperatures below 420 ~ the rates of reaction were too slow for convenient measurement, while at temperatures above 470 ~ it proved impossible to obtain any semblance of temperature control. McLaughlin's explanation of periodic thermal instability is not thought to apply to the present results; the
Fig. 1--Thermal-analysis record, Mount Morgan pyrite in air (sample weight 17.1 mg, temperature 470 ~ particle size 53 to 74/~m, air flow rate 120 ml/min).
Fig. 2--Partially oxidized pyrite grains (optical micrograph).
ISSN 0360-2141/81 / 1211-0769500.75/0 9 1981 AMERICAN SOCIETY FOR METALS AND VOLUME 12B, DECEMBER 1981--769 THE METALLURGICAL SOCIETY OF AIME
and thus the present findings cast doubt on the validity of earlier treatments ~4,~5in which simple isothermal diffusion models were applied to the analysis of kinetic data from the oxidation of pyrite in the vicinity of 450 ~
Fig. 3--Enlarged view of the oxide layer/pyrite interface in Fig. 2.
period of oscillation was too long (namely 4 to 8 mix at 470 ~ and even longer at lower temperatures) and the maximum reaction rates were an order of magnitude less than those calculated for limiting oxygen diffusion into the crucible. Rather, it is thought that in this instance the periodic thermal instability was caused by cracking of the oxide. Such cracking has been ascribed to the pressure of sulfur 1~a n d / o r sulfur dioxide ~1at the interface between the pyrite and the oxide. The latter explanations were applied at temperatures higher than those in the present study, and if they were operative in this case, one would expect more evidence of eruptions and radial cracking than is shown in Fig. 2. Thus, in this instance, the most likely explanation is that the cracking of the oxide layer resulted from stresses produced by crystallographic and volume mismatch between the pyrite and the oxide. Taking into account the stoichiometry of the reaction and the density ~2of the p
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