Kinetics of the direct synthesis of molycarbide by reduction-carburization of molybdenite in the presence of lime

Kinetic studies were conducted on the carbon monoxide reduction of molybdenite in the presence of lime. Contrary to the expectation that the MoS₂ (s)+CaO (s)+CO (g) reaction will result in metal formation, molycarbide was found to form and no Mo was detected in the product. This is explained on the basis of thermochemical considerations, which indicate that the Mo₂C formation is more feasible by eight orders of magnitude. The effects of quantity of lime in the charge, CO flow rate, temperature (1123 to 1298 K), and time of reduction have been studied. Kinetic analysis reveals that the results on the MoS₂ (s) conversion to Mo₂C (s) fit into a modified parabolic rate law. Based on the thermochemical calculations and experimental findings, the probable reaction scheme has been identified. Molycarbide appears to form by a three-successive solid-gas reaction path consisting of (1) metal formation by the MoS₂ (s)+CO (g) reaction followed by (2) in-situ carburization of Mo metal by CO (g), and finally (3) the scavenging of the COS (g) by lime, resulting in CaS (s). The latter two reactions drive the overall reaction forward. Further, out of these three consecutive reactions, progress of the overall MoS₂+CaO+CO reaction seems to be governed by the intrinsic kinetics of the first one. Calcium molybdate, which forms as a transitory phase, plays a significant role by modifying the linear kinetics of the MoS₂ (s)+CO (g) to one of parabolic nature.