Liquid Slag Viscosity Calculation

This post discusses the calculator that can be used to estimate the liquid viscosity of a metallurgical slag.

These viscosity calculations are also available as Excel functions through an add-in discussed and available for download here.

Slag viscosity is an important process variable in pyrometallurgical processes, with the typical importance of relating the slag composition with the ease with which it could be tapped from the furnace. Material’s viscosity is determined by its chemical composition and the temperature. In the case of metallurgical slags, a silicate network forms increasing the viscosity. Monoxides have a destructive effect on the silicate bonds, with their presence decreasing the viscosity. These same monoxides however, increase the liquidus temperature, stabilising the formation of solid solution phases which has an increasing effect on effective viscosity.

In practice, the ability to estimate a slag’s viscosity based on its composition and temperature enables better and more consistent decision making when based on researched relationships, rather than rules of thumb. This also makes it possible to evaluate slags outside the normal operating conditions where experience has not yet been gained.

Numerous research results could be found in literature attempting to derive viscosity models for specific slag systems. A general model based on the Urbain formalism is presented in the Slag Atlas, 2nd Edition (1995) by dr. K.C. Mills, (Ch.9, pp. 349 – 354). This model groups the amounts of input species into glass formers (SiO2, P2O5), modifiers (CaO, MgO, Na2O, K2O, CaF2, FeO, MnO, TiO2, and ZrO2), and amphoterics (Al2O3, Fe2O3, and B2O3). Parameters were derived and tuned for these groups from experimental data to model the viscosity from the mole fraction of each component and the temperature. The software presented here provides a tool to estimate the slag viscosity using this Urbain model as published by dr. K.C. Mills.

This calculator allows for the following two calculations:

  • Liquid viscosity: Calculates the liquid viscosity for the input chemical composition (mass %’s of 0 to 100 allowed) and temperature (°C) using the Urbain model publish by dr. K.C. Mills in the Slag Atlas, 2nd Ed. It is called the liquid viscosity, based on the assumption that the slag specified is fully liquid at the temperature at which viscosity is calculated (temperature is higher than liquidus).
  • Temperature for viscosity: The viscosity model above is used to solve the temperature (°C) at which a slag will have the target viscosity specified. This is useful in operation to calculate the temperature that a slag need to be knowing the typical viscosity value predicted by this specific model for slag classified as easy to tap.
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