On the Microstructure of a Freeze Lining of an Industrial Nonferrous Slag
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SEVERAL pyrometallurgical processes operate with high-intensity conditions, such as a high process temperature, a strong convection in the bath, and aggressive process materials. Examples are slag cleaning,[1,2] zinc fuming,[3,4] ilmenite smelting,[5–7] and the Hall–He´roult process.[8–11] These processes pose challenges related to reactor integrity and the life of the refractory reactor wall. To limit the corrosion of the refractory wall and extend its life, the wall is often cooled. As a result, a solid crust of process material, also referred to as a freeze lining, a freeze layer, or a side ledge, may form on the hot side of the refractory wall. In some processes, such as ilmenite smelting and the Hall–He´roult process, a freeze lining is even the best solution for reaching an acceptable campaign time.[6–9] A freeze lining is the result of a thermal balance between the heat input from the liquid bath and the heat removal by the cooling medium, as is illustrated in Figure 1. In steady state, the heat input equals the heat removal and the freeze lining thickness remains MIEKE CAMPFORTS, Research Assistant, and BART BLANPAIN and PATRICK WOLLANTS, Full Professors, are with the Centre for High Temperature Processes, Department of Metallurgy and Materials Engineering, Katholieke Universiteit Leuven, B-3001 Leuven, Belgium. Contact e-mail: Mieke.Campforts@mtm. kuleuven.be. KAREL VERSCHEURE and TIM VAN ROMPAEY, Project Leaders, and EDDY BOYDENS, Manager Analytical Laboratory, are with Umicore Research, B-2250 Olen, Belgium. Manuscript submitted May 16, 2007. Article published online November 20, 2007. METALLURGICAL AND MATERIALS TRANSACTIONS B
constant. Any unbalance between the heat input and the heat removal results in the growth or melting of the freeze lining to re-establish steady state at another thickness. Furthermore, industrial process materials are often multicomponent systems. Therefore, the freeze lining composition may not equal the composition of the bath material. In this case, components have to be exchanged between the freeze lining and the bath, when the freeze lining grows or melts. Thus, the growth of the freeze lining can be a complex combination of mass transport and heat transport. Because the process materials in the high-intensity processes are highly corrosive for the refractory wall, the formation of a freeze layer has to be guaranteed. The process material has to form a layer of solids on the reactor wall. Furthermore, this layer has to remain attached to the refractory wall and, ideally, does not spall. If the layer does spall, a new layer should form immediately. A better understanding of freeze lining behavior is thus necessary to establish the process conditions for an optimal protection of the refractory wall. The microstructure of the freeze layer contains information on the freeze layer formation and determines its physical, mechanical, and chemical properties, and therefore is very meaningful. The microstructure of a freeze lining has been studied for cryolite salts of the Hall
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