As shown by other curves in Fig. 3, if the mole ratio of pyrite to hematite in the mixture was changed from 1:1 to 1:5, 1:10 and 1:20, which meant decreasing the content of pyrite in the mixture, the shape of TG-DTA curves changed obviously. It demonstrated that the weight-loss reduced as the content of pyrite decreased. For the mixtures with the ratio other than 1:1, DTA curves display two distinguished peaks in the range of 500–650 °C. The intensity of the peak at lower temperature decreased and the other one increased with reducing the content of pyrite. The coupling FTIR detected that SO2 was evolved after the temperature was higher than 600 °C. The XRD pattern disclosed that there was magnetite (Fe3O4) formed in the final products, but hematite remained in the mixture with the ratio of 1:20.
It is easy to understand that the first endothermic peak corresponded to the thermal decomposition of pyrite in the mixture. Because hematite alone had no thermo-reaction before 800 °C, it was deduced that the second endothermic peak was related to the reaction between hematite and the decomposed products of pyrite.
3.2. The decomposed products of pyrite react with hematite
As shown in Fig. 4a, the decomposed residue of pyrite under anaerobic roasting at 700 °C is confirmed to be the mixture of pyrite (FeS2), pyrrhotite (Fe7S8 or Fe0.95S1.05), and iron monosulfide (FeS), while the element sulfur (S) was evaporated at 300–400 °C.