On the Mass Transport and the Crystal Growth in a Freeze Lining of an Industrial Nonferrous Slag

Lab scale freeze layers of an industrial nonferrous slag with as main components Al₂O₃-CaO-FeO _x -MgO-SiO₂-ZnO were studied to explore the mass transport in an industrial freeze lining. The freeze layers were formed by submerging a water-cooled probe into a liquid slag bath. In previous research,[1] two distinct types of microstructures were observed in these freeze layers: one with only melilite columnar crystals and one with both melilite and olivine columnar crystals. In this article, the mass transport during the formation of these microstructures is investigated. The element distribution, the phase fraction, and the crystal morphology were examined using electron microscopy and electron probe microanalysis. In addition, thermodynamic software was used to calculate the solidification path of the slag. The impact of the mass transport on the growth of a freeze lining depends on the solidification rate. For high solidification rates, only short-range mass transport is observed and small crystals form. For lower solidification rates, long-range mass transport is observed, resulting in an exchange of components between the freeze layer and liquid slag bath and in the formation of large crystals. Different growth mechanisms are observed for melilite and olivine crystals. The broad large melilite crystals have to exchange components with the liquid slag bath to grow, because the amount of liquid slag between the crystals is limited, while the long thin olivine crystals can exchange components with a large amount of liquid slag in between the crystals. Furthermore, the mass transport of minor elements may be very important, because these elements can pile up at the crystal-liquid slag interface and hamper the crystal growth.