Formation of a Mineral Layer during Coke Dissolution into Liquid Iron and Its Influence on the Kinetics of Coke Dissolut
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INTRODUCTION
COKE is the primary solid material remaining at the level of the hearth and throughout the lower zones of the blast furnace. It is the principal source of fuel for the furnace and provides mechanical support for the burden above it. Additionally, coke supplies the carbon required for the carbonization of the liquid hot metal in the hearth.[1] This investigation is focused on the coke-metal reactions occurring in the hearth below the slag layer. Liquid iron entering this region contains more than 2 mass pct C and picks up its final carbon while percolating through the packed-coke bed in the deadman and hearth of the blast furnace.[1] Metallurgical coke typically contains 8 to 12 pct by mass inorganic mineral matter,[2] derived from the mineral matter of the parent coals.[3,4] As coke is dissolved in the liquid iron, there is potential for the insoluble components of this inorganic mineral matter to form a layer at the surface of the coke, inhibiting carbon dissolution.[5–12] II.
PREVIOUS WORK
There is a significant body of research that has focused on the kinetics of coke dissolution into MICHAEL W. CHAPMAN, Research Student, BRIAN J. MONAGHAN, Senior Lecturer, and SHARON A. NIGHTINGALE, Associate Professor, are with the Pyrometallurgical Research Group, University of Wollongong, Wollongong, NSW 2522, Australia. Contact e-mail: [email protected] JOHN G. MATHIESON, Manager, Iron and Steelmaking Research, and ROBERT J. NIGHTINGALE, Blast Furnace Technology Manager, are with BlueScope Steel Limited, Port Kembla, NSW 2505, Australia. Manuscript submitted on September 18, 2007. Article published online May 21, 2008. 418—VOLUME 39B, JUNE 2008
iron.[6,10–11,13–18] Coke dissolution into iron is generally considered to be controlled by first-order kinetics and is frequently described by the rate of carbon dissolution if liquid side mass-transfer limits the dissolution reaction, as presented in the following equations: J¼
D ½Csat ½Cbulk d
or J ¼ km ½Csat ½Cbulk
½1
Under the assumption of ideal mixing within the melt, the mass balance for carbon can be integrated (assuming A, V, and km are all time independent) as follows: ½Csat ½C0 V ln ½2 ¼ km t A ½Csat ½Cbulk where J is the flux (compositionÆmÆs-1), D is the diffusion coefficient of carbon in liquid iron (m2Æs-1), d is the effective boundary-layer thickness (m), [C]sat is the carbon concentration at carbon saturation (mass pct), [C]bulk is the bulk-carbon concentration in melt (mass pct), [C]0 is the initial carbon concentration of bulk at t = 0 (mass pct), V is the volume of melt (m3), A is the area of reaction interface (m2), t is the time (s), and km is the mass-transfer coefficient (mÆs-1). The generic term, ‘‘ash,’’ as used in the iron-making literature, can sometimes be misconstrued as the mineral-matter residue from coke combustion (reaction with a gas phase), as opposed to a residual product of cokeliquid reactions, as in the case for coke dissolution in liquid iron. In this article when discussing our results, the inorganic and m
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