Factors influencing slag viscosity measurements using the Concentric Cylinder Measuring System at high temperatures

Abstract

This paper examines the factors that influence viscosity measurements of oxide melts at high temperatures and may lead to inaccurate results. These include the chemical instability of iron oxide in FeO-bearing slags under low oxygen partial pressures and its reaction with crucible materials, particularly molybdenum, leading to contamination and the formation of molybdenum oxides and iron–molybdenum alloys. These reactions increased viscosity values in the liquid-dominated region by up to 50 %, with molybdenum contents reaching up to 45 at % in alloys and 11.8 at % in oxides after 4 h at high temperature. The use of platinum crucibles also proved unsuitable under these conditions due to crucible breakage. In fluorine-containing mold powders, the evaporation of volatile components such as fluorine and sodium significantly affected viscosity, with the loss of fluorine and sodium causing a 19 % increase in viscosity within one hour. For CaO–SiO₂ melts, measurements at a constant cooling rate of 10 K/min showed viscosity reductions of up to 25 mPa·s compared to equilibrium conditions, alongside transient viscosity fluctuations. A reduced homogenization time caused a shift in the break point temperature by up to 50 °C, though liquid-phase viscosity remained stable. Experimental findings were supported by thermochemical simulations using FactSage 8.1 and validated via scanning electron microscopy of quenched slag samples to confirm the presence of metallic phases. Based on the results, recommendations are proposed to minimize the influence of these factors and improve the reliability of high-temperature viscosity measurements.