Conversion of hematite to iron carbides by gas phase carbidization

The intrinsic conversion rates of hematite (Fe2O3) to carbide (primarily cementite) with a CO-H-2 feed-gas have been measured in the temperature range 550 to 650 degrees C, by employing a micro-thermogravimetric system. As a preliminary analysis phase stability diagrams were developed in a triangular representation to overcome limitations of binary-type diagrams already available in the open literature. In general the reaction sequence was identified as consisting of a high-rate conversion of Fe2O3 to Fe (33-75%.min(-1)) followed by conversion of Fe to Fe3C. Two stages of carbidization were identified. For the first stage, the conversion rates were higher, from 55 to 75%.min(-1), depending on the reactor temperature. The rate of carbidization in the second stage region was lower, in the-range of 25 to 35%.min(-1). The rate of carbidization was found to increase as temperature decreases, within the range from 600 to 640 degrees C. A model based on adsorption kinetics was developed which qualitatively describes the behavior observed. After conversion of Fe to Fe3C carbon deposition (sooting) was evident. The catalytic role of cementite (in contrast to Fe) in the heterogeneous sooting-reaction has also been addressed.