Iron carbide 및 manganese carbide의 반응 메커니즘 연구

Abstract

The iron carbide and manganese carbide can be available materials as alternative iron-making resource, however, their mechanism of formation condition should be investigated since both metallic carbides are very difficult to be formed. Iron carbide, cementite, can decompose easily to metallic iron and graphite due to thermodynamic metastability. On the other hand, the relatively stable manganese carbide has difficulty that it could not be formed at a reasonable formation rate kinetically. The experiments for iron carbide by Fe3O4 carburization have been carried out in condition of high pressure of CO-CO2 gas atmosphere under 0.5 and 1.0 MPa. The carbon activity as the most critical factor for Fe3C formation thermodynamically can be increased from high pressure of carburizing gas. It is absolutely for the direct Fe3C formation from Fe3O4 without metallic Fe phase acting as a catalyst for carbon deposition. Once carbon deposition occurs at the reacting surface, Fe3C formation is terminated. In the experiment of the carburization of the single crystal Fe3O4 at 773K under the pressure of the 0.5 MPa, the formed interface between the Fe3C and the Fe3O4 was examined by using TEM. It was found that the formed Fe3C was directly contacted with Fe3O4 without observation of metallic Fe at the interface. This was explained by considering reaction rates (k2) of Fe3C formation from Fe faster than that (k1) of Fe formation from Fe3O4 relatively, Fe3O4 →(k1) Fe→(k2)→Fe3C. The amount of the initially produced Fe is limited so that all of the produced Fe can be consumed after a particular time. This can be the possible reason that Fe layer was not found in TEM observation. The carbothermic reduction of manganese oxide with graphite was also carried out at solid state for manganese carbide formation since reduction gases such as H2 and CO cannot go beyond stable MnO phase. The overall rate of carbothermic reduction of MnO2 to MnO was much faster than the carbon gasification in temperature range of 1273-1573K, resulting in which was controlled by the carbon gasification. It was confirmed by the generation of negligible amount of CO compared with CO2 in the analysis of product gases by using QMS. The oxygen partial pressure (PO2) was increased by high ratio CO2/CO produced during the reduction of Mn3O4 to MnO, which can make the carburization of MnO to Mn7C3 difficult. In the phase diagram of Mn-O-C, the oxygen partial pressure is very important factor for the formation of manganese carbide. In other words, the Mn7C3 formation was decided absolutely by CO/CO2 ratio because MnO was mostly carburized by CO gas produced by carbon gasification with CO2. In order to overcome this problem, hydrogen was used as atmosphere gas in carbothermic reduction of MnO2 because it helps to increase CO2 mass transfer rate or to generate CH4 (0.01~0.03 atm) acting as a means for carbon transportation accelerating the overall reduction rate. In addition, Fe-C melt by Fe addition can play important role in rate increase of MnO carburization. It was explained by that the direct carburization of MnO through dissolved carbon is more efficient than the carburization by CO in regard of faster formation rate of manganese carbide. From the above investigation about mechanism of carbide formation, the application of manganese iron carbide to dephosphorization process can be possible at solid state due to the repulsive force exerted between carbon and phosphorus. In the experiment of carbothermic reduction of MnO2, Fe2O3 and Ca3(PO4)2 at 1200oC in H2 atmosphere employing TGA, the phosphorus did not dissolve into (Mn,Fe)7C3 according to EPMA line mapping even though it has the high reactivity with Mn and Fe. Instead, the phosphorus generated by the decomposition of Ca3(PO4)2 reacted with Fe and Mn to form (Fe,Mn)3P without carbon. That is, it is confirmed that the solubility of phosphorus was low in carbide.