In the field of pyrometallurgy, the presence of multiple molten phases with distinct chemical
compositions, such as slags, mattes, speiss liquids, metals and molten salts, is a well-known
phenomenon. These phases exhibit strongly non-ideal solution behaviour and are mutually
miscible to varying degrees. To describe these complex molten liquids, the Modified
Quasichemical Model (MQM) has proven effective and has been applied to numerous binary,
ternary and several higher-order multicomponent systems related to pyrometallurgy. In recent
decades, the complexity of high-temperature processes has escalated due to increased impurity
concentrations in primary ores, the incorporation of recycled consumer products and the
integration of metallurgical plants through by-product and waste exchanges. To address these
issues, PYROSEARCH laboratory has been developing large 20-component thermodynamic
database using a generalised CALPHAD (CALculation of PHAse Diagrams) methodology
integrated with experimental investigations. Over the past decade, strategic decisions were made
balancing the ability to generate experimental data, prediction power, accuracy, stability of
calculations, as well as computational time, which are discussed in the paper. Examples provided
for issues of >3 liquid immiscibility, non-ionic behaviour and exaggerated ‘sharp’ enthalpies of
mixing in silicate slags, dealing with multiple oxidation states, as well as comparative analysis of
pair and quadruplet approximation in MQM.