Improved Break Temperature Determination of Slags Through Comprehensive Rheological Characterization
Rheological properties of slags and furnace linings are important for technical design in the field of metal and steel processing. Slag viscosities need to match the process design, as they affect refining efficiency, corrosiveness and flowability. Further, melting/crystallization temperatures define ideal operating temperature windows. Still, partial melting, shear-induced crystallization, and instantaneous solidification complicate technical design and operation parameters. Break temperatures are often used to indicate the solidification temperature, thus defining operation temperature lower limits. This report shows the capabilities of the FRS (Furnace Rheometer Systems) from Anton Paar to characterize melting temperatures, flow behavior of pure melts and crystal-melt suspensions, and break temperatures.
High temperature processing in blast furnaces allows for the extraction and the refining of metals from raw materials. Both metals and the residue (metal-depleted slags) are recovered for further processing (e.g. steel and slag cement). During slag processing, low slag viscosities enhance refining efficiency but induce higher corrosiveness regarding crucible refractories. This is because both convective and diffusive exchange processes are faster, which is important for steel melt purification.
During metal extraction at constant temperature, the rheological properties of the system may change due to both the changing chemical composition of the slag and the transition from a one-phase liquid to a multi-phase suspension via crystallization. Several models for predicting viscosities are available for both pure melts [e.g. 1] and crystal-bearing melts [e.g. 2]. Reasonable predictions for process design are possible for crystal-free melts. However, with increasing crystal content, the crystal shapes as well as the crystal size distribution strongly depend on thermal history (e.g. cooling rates, thermal gradients and cycling, flow conditions, or rest) and are difficult to predict. Existing models are unable to fully cover all influencing factors; and therefore direct measurements and experimental investigations are indispensable.
In contrast to one-phase silicate liquids, the rheological behavior of semi-solid products is shear-rate dependent. Shear-thinning is typically observed for solid fractions from 1 % to 40 %, and shear-thickening is possible for higher solid fractions. Knowledge about rheological properties has the potential to optimize slag processing, especially in the sub-liquidus, multi-phase region.
A common characterization method for slags is the determination of the temperature dependence of viscosity using a rotating concentric cylinder (CC) measuring geometry . During a defined cooling process with a set cooling rate, the viscosities are measured based on torque, rotational speed, and measuring geometry dimensions (ISO 3219, DIN 53018).
In this study, rotational and oscillatory concentric cylinder measurements were used and compared  to rheologically characterize two slags and two furnace lining materials from super-liquidus to sub-solidus conditions. A robust and automated procedure to determine the break temperature, including the possibility to simulate slag behavior at static and non-static conditions, is suggested.
In the past, the break temperature used to be assumed as the temperature at which crystallization starts. This assumption has been reevaluated and the break temperature is now interpreted to represent the critical crystal fraction leading to a dramatic viscosity increase where flowability ceases [6, 7]. It is often defined by the intersection of the two tangents of the curve branches before and after the dramatic increase .
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