Formation of a Mineral Layer during Coke Dissolution into Liquid Iron and Its Influence on the Kinetics of Coke Dissolution Rate

Abstract

The formation and development of the mineral layer that forms between coke and liquid iron during carbon dissolution has been characterized. Coke particles (-2 mm, +0.5 mm) were added to the top surface of an iron 2 mass pct C melt at representative iron-making temperatures, for periods of time between 2 and 120 minutes, before being quenched. The quenched samples were then sectioned, and the solidified coke-melt interfacial region analyzed in the scanning electron microscope (SEM). Analysis showed that a mineral layer was present at the interface at all experimental temperatures (1450°C to 1550°C) from 2 minutes and persisted beyond 120 minutes. The mineral layer was found to be composed of calcium aluminate phases, with the proportions of these phases dictating its morphology. Further, changes observed in the rate of carbon dissolution from the coke were related to the composition and morphology of the mineral layer. The effect of this mineral layer on the rate of carbon dissolution has been interpreted as a change in the reaction control mechanism.

Liquid Iron Wetting of Calcium Aluminates

“…No measureable change in weight of these graphite materials was observed after an experiment when using a balance accurate to 0.0001 g This indicates that the furnace environment was in essence free of oxygen. Given these conditions the gas pO 2 Assuming an activity for graphite of 1, a pCO of 1 atm and using the thermodynamic data for reaction 5 given in Turkdogan,15) the maximum pO 2 in the gas phase would range from 1.3ϫ10 Ϫ16 to 3.0ϫ10 Ϫ16 atm over the temperature range of 1 450 to 1 550°C.…”

“…The channels represent a necessary transfer path if significant coke (carbon) dissolution is to be maintained after layer formation. It is assumed here that the mineral layer is solid, as was the case in the previous studies by Chapman and Monaghan, 2,4) but this would be at least in part dependant on the July 20, 2010 ) An investigation has been carried out to assess the wetting behaviour of liquid iron carbon alloys on alumina, CA6 (CaO · 6Al 2 O 3 ), CA2 (CaO · 2Al 2 O 3 ) and CA (CaO · Al 2 O 3 ). The melt compositions studied were 2 and 5 mass% [C] over a temperature range of 1 450 to 1 550°C.…”

“…1) Recent studies focussed on mineral layer development as coke dissolves in liquid iron have resulted in new information on how the layer grows, evolves and effects carbon dissolution rates. [2][3][4] In these studies it was found that the composition and morphology of the mineral layer formed on dissolution had a profound effect on the kinetics of reaction. Moreover it was found that the layer was primarily calcium aluminate based and that over time (0-120 min) became progressively enriched with calcium.…”

“…Specific details of the minerals structures are given in Table 1. 5,6) It was argued 2,4) that this densification due to the change in mineral structure caused the decrease in the coke dissolution kinetics. On the basis of rate constant plots corrected for thermodynamic driving force effects, the coke dissolution kinetics was characterised as a two stage process.…”

“…It should be noted the growth of the mineral layer in itself could slow the rate of dissolution but no correlation between the thickness of the layer and rate of carbon transfer was found. 2,4) It was argued that the change of phase to CA after a period of time densified the mineral layer making analysis of the growth (thickness) with time problematic.…”