The kinetics of hematite reduction and cementite formation with CH4-H2-Ar gas mixture

•Iron carbide at a temperature of 750 °C in 15 min is complete formed.•Time and temperature are two important parameters on the formation of iron carbide.•Chemical reaction and diffusion at the interface kinetic model was well fitted with experimental data.

The kinetics of hematite reduction and formation cementite were studied by a gas mixture of 30 vol% CH4, 60 vol% H2 and 10 vol% Ar, inside a fixed-bed reactor at 700–800 °C and different times in this research work. The percentage of reduction was determined by thermogravimetry method with a precision of 0.001 g. The results obtained showed that the hematite pellets were first reduced to metallic iron and then cementite is formed. The kinetic modeling has shown that the reduction rate of hematite pellets is controlled by the mixed mechanism of chemical reaction-diffusion at the interface of and the rate of the formation of cementite is controlled by the chemical reaction of methane adsorption on the surface of reduced iron. On the other hand, the results showed that the temperature and time of the process are the two very important factors in the formation of cementite and its stability. In fact, by increasing the temperature (from 700 °C to 800 °C) led to increase in the rate of cementite formation. But, selection of an appropriate temperature can be very effective in the rate of the decomposition of cementite into metallic iron and graphite. The results of XRD and Feritscope demonstrated that at 750 °C over time even for 80 min, a negligible percentage of cementite is decomposed. Furthermore, the images obtained from SEM and RLM showed that an increase in time can also change the formed cementite morphology from layer state to block state.