Freeze-Lining Formation of a Synthetic Lead Slag: Part I. Microstructure Formation
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INTRODUCTION
SEVERAL pyrometallurgical processes operate with high-intensity conditions, such as a high process temperature, strong convection in the bath, and aggressive process materials. To extend the life of the refractory wall, these processes often use a cooled reactor wall. In the case of extensive cooling, a protecting 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[8–11] use freeze linings. The stability of the freeze lining has a direct impact on the corrosion of the refractory. In order to control the freeze-lining stability, a study of the freeze-lining microstructure (phase distribution and composition) is important. A study of this type determines the properties of the freeze lining and provides information on the thermal history of the freeze lining and, thus, on MIEKE CAMPFORTS, formerly Research Assistant, Materials Science Department, Katholieke Universiteit Leuven, is Project Leader, Umicore Research, Olen 2250, Belgium. Contact e-mail: mieke. [email protected] 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, Leuven 3001, Belgium. EVGUENI JAK, Professor, is with the Pyrometallurgy Research Centre, University of Queensland, Brisbane St. Lucia, QLD 4072, Australia. Manuscript submitted December 19, 2008. Article published online July 16, 2009. METALLURGICAL AND MATERIALS TRANSACTIONS B
freeze-lining formation and evolution during the process. In the literature, freeze-lining microstructures have been studied for cryolite salts of the Hall–He´roult process[10,11] and for some industrial nonferrous slags.[12,13] Thonstad and Rolseth[11] studied lab-scale freeze linings of cryolite salt formed under fast and slow cooling rates. At fast cooling, the layer consisted of amorphous material with a composition close to the bath composition. At slow cooling, the layer consisted of cryolite crystals oriented parallel to the heat flux, with a composition close to that of pure cryolite being the primary phase of the studied salt. The authors also studied industrial freeze-lining samples. Here, layers of both slow- and fast-cooled microstructures were observed, indicating that both cooling conditions can occur during freeze-lining formation. Solheim and Støen[10] studied the freeze-lining formation of synthetic cryolite salts. They observed that cryolite dendrites were formed during freeze-lining growth. Here, the freezelining composition deviates from the cryolite composition due to the entrapment of liquid between the dendrites. Campforts et al.[12,13] studied freeze-lining formation for an industrial nonferrous slag. Lab-scale freeze linings were obtained using the watercooled probe technique.[14] In the microstructure, from the cooled probe to the liquid slag bath, a glass la
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