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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.

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.

The disposal of zinc-containing residues is an important sustainability issue currently facing the metallurgical industry. These residues can be treated by using zinc slag-fuming processes in which heavy metals are chemically reduced and evaporated from a molten slag bath. Continuous operation of these processes requires high zinc-fuming rates while retaining vessel integrity. To meet these challenges, a new generation of technologies is being developed that depends on the formation of a freeze lining on the internal reactor walls. A general zinc-fuming process model that simultaneously describes the chemical reactions, phase equilibria, and thermal- and heat-transfer outcomes of these processes has been developed using the FactSage thermodynamic databank system and the ChemApp programmer’s library for thermochemical applications. This simultaneous description of these processes allows the model to be used as part of a new approach to the design of freeze linings, namely, the use of slag engineering and composition adjustment to obtain optimum process efficiency and freeze lining behavior.

A new high-temperature submerged plasma zinc-fuming process has been developed for the treatment of zinc leach residues, electric arc furnace (EAF) dusts, and other zinc-containing waste materials. Continuous operation of this process requires high zinc-fuming rates while retaining vessel integrity through the formation of a stable freeze lining. A zinc-fuming process model using the FactSage thermodynamic databases and ChemApp thermodynamic software has been developed, which simultaneously describes chemical, thermal, and heat-transfer outcomes of this process. The model has been used to systematically investigate the potential effects of operating parameters such as feed composition, fuel/oxygen ratio, electrical power and fluxing parameters on bath temperature, heat loss, Zn concentration and content in slag, Cu concentration, and content in matte.

Furnace protection by water-cooled freeze linings becomes increasingly important as the metal producing industry attempts to achieve higher process intensities. Systematic investigations of the growth and the resulting microstructure and compositional profile of freeze linings are necessary to understand the behavior of freeze linings, their relation with the industrial process, and their interaction with the wall cooling system. We have developed a technique based on the submergence of a water-cooled probe into a liquid slag bath. Freeze linings of two industrial nonferrous slags have been produced using this technique and their growth, microstructural, and compositional profiles as a function of submergence time were determined. Thermodynamic equilibrium for the investigated slag systems was calculated and compared with the observed microstructures. The freeze linings form in approximately 15 minutes. Close to the water cooling, the freeze linings are predominantly amorphous in structure. With increasing distance from the water cooling, the proportion of crystalline phases increases and bath material is entrapped in the microstructure. Cellular crystals are observed close to the bath. The freeze linings exhibit an approximate homogeneous composition. The results demonstrate that the technique is a successful tool in obtaining information on the growth, microstructure, and composition of freeze linings in industrial water-cooled furnaces.