Some researchers suggest that the oxidation of pyrites proceeds primarily through the formation of sulphates [FeSO4, Fe2(SO4)3] which later decompose to ferric oxide (Fe2O3) and sulphur dioxide (SO2). Others hold that the primary products of oxidation are the oxides (FeO, Fe2O3) which subsequently react with sulphur dioxide to produce sulphates [46,47]. According to the reported by Yan et al. [3], it would be very difficult to figure out the decomposition mechanism of pyrite under real coal thermal condition. In addition, the reaction is more complicated and difficult to study in the presence of coal and only sporadic information can be found in the literature. It is also reported that individual pyrite does not provide the interactions between pyrite and the coal carbon matrix. It is also believed that oxidation of pyrites is accompanied by the thermal decomposition to pyrrhotite.