There were some problems with the experiment of element sulfur. Because sulfur might evaporate during the thermo-analysis, almost no meaningful results were observed from the mixture of sulfur and hematite with ratio of 1:6. While the amount of sulfur in the mixture was increased, the situation changed. Taking the mixture with a mole ratio of 1:1 as a sample, the large weight-loss occurred during 200–300 °C and the corresponding endothermic peak was attributed to the evaporation of element sulfur (as shown in Fig. 5c). The remaining melt sulfur then underwent phase transformation from β-octasulfur into γ-sulfur, so there was no change in weight but a wide exothermic peak before 525 °C. The second weight-loss stage started at 525 °C could attribute to the reaction between S and hematite (Eq. (2)). After evaporation, the remaining S in the mixture was about 2.33% before the reduction reaction. The calculated weight-loss by Eq. (2) is 4.66%, close to the actual measurement value of 4.51%. The XRD of the final product disclosed that magnetite was the majority in the mixture of hematite and magnetite.
equation(2)
S + 6Fe2O3 = 4Fe3O4 + SO2
3.3. Co-roasting of pyrite and hematite
The mixtures of hematite and pyrite with different mole ratio were put into an N2-protected tube furnace and roasted at different temperature for a certain time. XRD patterns of those products are shown in Fig. 6. It was obvious that temperature was the most important factor for the transformation of hematite into magnetite. Hematite in the mixture almost did not change after being roasted at 500 °C for 1 h (Fig. 6a), which could be explained that pyrite hardly decomposed at this temperature (as shown in Fig. 3). Since pyrite obviously decomposed from 525 °C, magnetite could be found in the roasted product at 550 °C although hematite was also found. If the temperature reached 600 °C, the transformation was completed even in half an hour (Fig. 6c).