Fig. 5 shows the spectra for the thermal decomposition of the coal-derived pyrite at 470, 495 and 515 ◦C. The absorption bands of volatile for the sample also appear to be at the same wave numbers, while the diversities of the absorbance only exist in 1450–1300 cm−1 region. The release of water and CO2 (bands at 3964–3500 and 1800–1300 cm−1 region) is less violent, while the relative intensity of SO2 firstly increases and then decreases. As mentioned earlier, it should be considered the occurrence of reaction (1) in the presence of oxygen atmosphere whereas the reaction Fig. 5. FT-IR spectra of gas released out at 470, 495 and 515 ◦C. (2) must be taken into account in an oxygen-free atmosphere. It is quite conceivable that the reactions (1) and (2) occur simultaneously (410–470 ◦C) while the reaction (3) follows the earlier ones. Therefore, the thermal process at 470–515 ◦C is attributed to the evolution of oxidized sulfur stemmed from the last step the decomposed of the pyrite particle without oxygen.