The Importance of Slag Engineering in Freeze-Lining Applications

Freeze linings are frequently used to protect the reactor wall in pyrometallurgical processes. In order to minimize reactor wall corrosion, the stability of a freeze lining has to be guaranteed. To illustrate the importance of slag engineering in the optimization of freeze-lining behavior, the freeze-lining formation of six synthetic lead slags is studied. Lab-scale freeze linings are obtained using a cooled-probe technique and their microstructures are characterized using light optical microscopy (LOM) and microprobe analysis. The results show that slag engineering can have a major impact on the operative freeze-lining formation mechanisms. Some slag properties found to affect the freeze-lining formation are the viscosity, the temperature stability range of the relevant phases, the type of phases that form (interlocking or not), and the crystallization behavior of the slag. The operational demands of a protective freeze lining are defined by the authors as follows: (1) a rapid formation to limit the contact between the reactor wall and the corrosive bath material and (2) a sufficient stability during changes in heat input from the bath and in bath composition. From the comparison of the microstructural features of the freeze linings formed with the studied slags, it is concluded that these demands can be fulfilled with the growth of an initial layer dominated by the presence of interlocking crystals in combination with the subsequent formation of a high-melting crystalline layer at the bath–freeze-lining interface, which is in equilibrium with the slag bath and the composition of which differs sufficiently from the bath composition.