Freeze-Lining Formation of a Synthetic Lead Slag: Part II. Thermal History
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INTRODUCTION
SEVERAL pyrometallurgical processes operate under high-intensity conditions, such as high process temperatures, strong convection in the bath, and aggressive process materials. To extend the life of the refractory walls, these processes often use cooled reactor walls. In the case of extensive cooling, a layer of process material solidifies on the reactor wall. This layer is referred to as a freeze lining. For example, slag cleaning,[1,2] zinc fuming,[3,4] ilmenite smelting,[5–7] and the Hall–He´roult process[8–12] use freeze linings. The stability of a freeze lining has a direct impact on the corrosion of the refractory. In order to predict the freeze-lining behavior in modeling work, a study of the freeze-lining microstructure (phase distribution and composition) is important. The microstructure determines the properties of the freeze lining and provides information on its thermal history and, thus, on freezelining formation and evolution. MIEKE CAMPFORTS, formerly Research Assistant, Materials Science Department, Katholieke Universiteit Leuven, is Project Leader, Umicore Research, 2250 Olen, Belgium. Contact e-mail: [email protected] EVGUENI JAK, Professor, is with the Pyrometallurgy Research Centre, University of Queensland, 4072 Brisbane St. Lucia, QLD, Australia. BART BLANPAIN and PATRICK WOLLANTS, Professors, are with the Centre for High Temperature Processes, Metallurgy and Refractory Materials, Department of Metallurgy and Materials Engineering, Katholieke Universiteit Leuven, 3001 Leuven, Belgium. Manuscript submitted December 19, 2008. Article published online July 22, 2009. 632—VOLUME 40B, OCTOBER 2009
The models developed to predict freeze-lining behavior can generally be divided into three groups: thermal,[2,9,13–18] process,[3–7] and freeze-lining models.[10,12] Thermal models (numerical as well as physical) model the freeze lining as a solid layer the thickness of which is determined by a heat balance, e.g., between the heat input from the process and the heat removal by the cooling equipment. The freeze lining grows/melts when the temperature at the bath–freeze-lining interface evolves below or above a fixed temperature, which is often taken equal to the liquidus temperature[2,9,13,14] and, in some cases, equal to the solidus temperature.[15–18] Process models, in addition to a thermal balance, include the reactions taking place in the bath.[3–7] Here, the freeze lining is assumed to be in equilibrium with the bath. As a result, the thickness of the freeze lining is defined by a heat balance and by the composition and temperature of the bath. The process models also predict the freeze-lining composition and the bath–freeze-lining interface temperature, assuming they equal the primary phase composition and the liquidus temperature, respectively, of the bath material. Freeze-lining models describe freeze-lining behavior, including heat transfer, mass transfer, and thermodynamic equilibrium calculations.[10,12] Here, the bath– freeze-lining interface temperature equals the liquid
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