Water-cooled probe technique for the study of freeze lining formation
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TRODUCTION
IN modern high-temperature metallurgical processes, water cooling of furnaces is increasingly used to protect the furnace wall from thermal, chemical, and mechanical wear. It not only reduces refractory wear but leads in some cases to the formation of a freeze lining: a relatively stationary crust of solidified slag formed against the refractory or directly against the inside of the cooled furnace shell. In stationary conditions, the thickness of the freeze lining remains constant and is determined by the equilibrium between heat input from the superheated bath and heat extraction through the cooled wall (Figure 1). In transient conditions, due to process changes or changes in water cooling, the thickness of the freeze lining changes through partial solidification of bath material or through melting or dissolution of the freeze lining. In some metallurgical processes such as the Hall–He´roult process, there are no other realistic alternatives than using a freeze lining at all times. Various other pyrometallurgical processes, however, use a combination of a refractory lining (for example, during the first months of the furnace campaign) and a freeze lining (when sufficient wear of the refractory has occurred). Although it may in principle be possible to reline the furnace after this period of refractory wear, cooling the furnace with water and forming a freeze lining offers numerous advantages, in that increased process KAREL VERSCHEURE and MIEKE CAMPFORTS, Research Assistants, FREDERIK VERHAEGHE, Research Assistant of the Research Foundation–Flanders (FWO), EDDY BOYDENS, Research Associate, BART BLANPAIN, Professor, and PATRICK WOLLANTS, Full Professor, are with the Department of Metallurgy and Materials Science, Division of Thermodynamics in Materials Engineering, Katholieke Universiteit Leuven, B-3001 Heverlee (Leuven), Belgium. Contact e-mail: karel. [email protected] MAURITS VAN CAMP, Senior Metallurgist, is with Umicore RDI, B-2250 Olen, Belgium. Manuscript submitted April 17, 2006. METALLURGICAL AND MATERIALS TRANSACTIONS B
intensities, longer campaign times, increased freedom in slag chemistry, and smaller, high-intensity vessels with low capital cost are possible. Although there are a number of drawbacks, including reduced safety due to explosion risk in the case of water leaks inside the furnace, high pumping costs, and high heat losses when the slag is severely superheated, it is believed that the majority of these issues can be tackled by improving the current insight in the solidification processes taking place at the inside of the cooled furnace shell. This article is a next step in this direction and presents an experimental technique to systematically investigate the microstructure, composition, and heat transfer of freeze linings of slags. Extensive work in the area of furnace protection with freeze linings has been performed for the Hall–He´roult process.[1–15] In this process, dissolved aluminum oxide (Al2O3) is electrochemically reduced from a complex fluoride melt through electroly
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