Further Experimental Investigation of Freeze-Lining/Bath Interface at Steady-State Conditions
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INTRODUCTION
MODERN solidification theory has successfully explained the microstructures formed, and the rates of crystal growth form melts on cooling.[1,2] The formation of stagnant deposits or freeze linings on the inner surfaces of reaction vessels containing liquid solution represents a particular class of transformations that are important but are less well defined. The reaction vessels typically contain complex liquid solutions maintained at temperature, while the exteriors of the vessels are at a lower temperature. A positive temperature gradient is thus established across the system boundary, that is, across the reactor wall, the deposit, and the liquid solution. The removal of heat creates the thermodynamic driving force for crystallization of species from the liquid solutions. At steady-state conditions, the temperature profile remains unchanged with time, and the stagnant deposit thickness remains constant. Examples of these phenomena include scale formation in heat exchangers and crystallisers,[3–6] entrained flow gasification process, and externally cooled high-temperature reactors used in metal smelting.[7–14] The specific example considered in the present study is the formation of ATA FALLAH-MEHRJARDI, Ph.D in Metallurgical Engineering, Postdoctoral Research Fellow, PETER HAYES, Xstrata Professor, and EVGUENI JAK, Professor in Pyrometallurgy, are with the PYROSEARCH, Level 3, School of Chemical Engineering, The University of Queensland, St Lucia, QLD, Australia. Contact e-mail: [email protected] Manuscript submitted May 6, 2014. Article published online August 6, 2014. 2040—VOLUME 45B, DECEMBER 2014
slag freeze linings on the reactor walls during copper smelting. For improved control and optimization of the freezelining formed in a given chemical system, it is desirable to identify the key factors that influence important operational parameters, such as, the steady-state freeze lining deposit thickness, the interface temperature, and the rate of heat removal from the bath through the deposit. This improved understanding offers the potential to further lower operating temperatures, energy costs, and to reduce the environmental impact of these technologies.[9–14] The current designs of freeze linings[15–22] are based on heat transfer considerations with the assumption that, at steady-state conditions, the interface between the melt and the deposit is at the liquidus temperature of the melt, Tliquidus. Implicitly, it has also been assumed that the material at the deposit interface consists of the primary phase that is the first phase to form on crystallization of the melt at the liquidus temperature. However, there are relatively few published articles available to discuss the effects of melt chemistry on the deposit microstructures.[9–14,23–28] The recent laboratory-based studies by the authors[9–12] have shown, however, that the phase assemblage formed at the stagnant interface of deposit/ bath at steady-state conditions is not necessarily solely that of the primary phase. The microstructural examinations a
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