The Microscopic Role of a Liquid Phase for Enhancement of Melting and Reduction Rates of Carbon Composite Iron Ores

Abstract

Microscopically, Fe-C melt and molten slag can play important roles in accelerating of the rates of iron melting and iron oxide reduction of carbon composite iron ores in a smelter. In order to make an “in-situ observation” of the micro-scale phenomena at 1523K in a simple iron-slag-carbon system representing the carbon composite iron ore, a confocal laser microscope combined with an image furnace was used in all experiments.Iron melting initiation and slag separation can be explained by molten slag wetting behavior in carbon composite iron ores, which is governed by its interfacial energy with iron or carbonaceous materials. CaO-SiO2-Al2O3 slag has good wettability with solid iron but not with liquid one. On the other hand, FeO-CaO-SiO2-Al2O3 slag has good wettability with both of them. The wettability behavior of the later slag gets closer to that of the former as FeO content is decreased by its reduction with Fe-C melt. As a result, the slag separation also takes place after the reduction completion. Coke ash facilitates coke’s wettability to slag but hinders coke’s carbon matrix from coming into contact with solid iron. Carburization-melting process of solid Fe under the co-existence of graphite and w㉨stite has been investigated. In the experiments, strong stirring flow at the surface of the Fe-C melt is observed. This flow is found to be Marangoni flow and is introduced by the surface tension gradient due to the difference of oxygen concentration between the Fe-C melt interface with graphite and solid iron. The melting rate of Fe is analyzed by computational fluid flow simulations of the Fe-C melt. Based on these results, it is confirmed that the carburization and melting rate of solid Fe can be enhanced by allowing Fe-C melt to come into contact with w㉨stite and graphite simultaneously. It is known that the molten iron oxide reduction by Fe-C melt shows quite different behavior depending on whether the surface of Fe-C melts is fully covered with molten iron oxide or not. In the case of the molten iron oxide reduction by Fe-C melt with the existence of free surface, the reduction appears to be controlled by the reaction of C + O ∪ CO(g). The rate of this reaction has not been fully established. In the present study, based on the re-examination of overall reaction rates of previous two studies by Dancy and Lloyd et. al., the forward and backward reaction rate constants of the reaction C + O ∪ CO(g) at 1873K have been evaluated by applying the order of magnitude evaluation method. The forward rate constant, in the unit of mol/m2s, is given by: k = 1.33≠108 exp(-250,000/RT), and the backward rate constant, in the unit of mol/m2s, is deduced to: kCO = 2.4≠1011 exp(-277,000/RT).