On the microstructure of a freeze lining of an industrial nonferrous slag

Labscale freeze layers of an industrial nonferrous slag with Al2O3-CaO-FeOx-MgO-SiO2-ZnO as main components are studied to explore the microstructure and the composition of an industrial freeze lining. The freeze layers were formed by submerging a watercooled probe into a liquid slag bath. The influence of submergence time, of heat input from the furnace, and of the rotational speed of the crucible is studied. The microstructures of the freeze layers are examined using scanning electron microscopy (SEM), electron probe microanalysis (EPMA), and X-ray diffraction (XRD). Thermodynamic software is used to interpret the solidification microstructure. The results show that the freeze layer microstructure consists of different zones, depending on the local thermal history. These zones show different growth morphologies and different microstructure scales, from an amorphous matrix with small crystals to large columnar crystals. Furthermore, two microstructure types are observed, one with melilite columnar crystals and the other with olivine columnar crystals. These microstructure types appear for similar experimental conditions and are even observed within the same freeze layer. An increase in submergence time or in heat input from the slag bath does not seem to favor a particular microstructure type. A high rotational speed of the crucible resulting in a higher convection in the slag bath seems to favor the microstructure type with olivine columnar crystals.