Rheological Characterization of Slags
High temperature rheometry is a precise technique used to measure visco-elasticity of materials with high melting temperatures like slags, silicates and metal melts. Multicomponent silicate melts are, in contrast to e.g. single oxide melts or pure metals, usually characterized by a crystallization temperature interval, where the sample is a mixture of liquid melt and solid crystals, with an increasing crystal percentage at lower temperature.
Slag processing in blast furnaces allows for the extraction of metal from ore materials. Both metals and the residue (metal-depleted slags) are recovered for further processing (e.g. steel and slag cement). During slag processing, lower slag viscosity means higher corrosiveness regarding crucible refractories; but melt refining is more efficient when melt viscosity is low. This is because both convective and diffusive exchange processes are therefore faster, which is important for steel melt purification. Moreover, the viscosity affects the types of mineral inclusions in the final slag.
During metal extraction, the rheological properties of the system change due both to the transition from a one-phase liquid to a multi-phase suspension and to the changing chemical composition of the slag. Several models for predicting viscosities are available for both pure melts and crystal-bearing melts. Reasonable predictions for process tailoring 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).
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