The theoretically calculated weight-loss would be 26.7% if pyrite totally decomposed into iron monosulfide (FeS). Since the actual weight-loss was less than that value, it was inferred that non-stoichiometric iron(II) sulfides (pyrrhotite, FeSx, x = 1–2) were produced before 700 °C. As shown in Eq. (1), several metastable pyrrhotite polytypes might form in the range of 450–750 °C [15].
equation(1)
FeS2 = FeSx + (2 − x)S x = 1–2
The above results demonstrated that pyrite could be thermo-decomposed into iron monosulfide, pyrrhotite and element sulfur during heat-treatment, all of which have the reduction ability to react with hematite. Fig. 3a shows the TG-DTA curves of the mixture of pyrite and hematite in mole ratio of 1:1. A weak endothermic peak and a corresponding weight-loss occurred at about 280 °C, which attributes to the desulfurization of pyrite. A larger weight-loss of 13% occurred in the range of 430–600 °C, which happened to be half of the weight-loss for pyrite alone. However, the endothermal peak occurred at lower temperature than that for pyrite alone, and there was another endothermal peak as a shoulder appearing near 610 °C.