Alumina is a common substance deporting in copper smelting slags when various secondary copper fractions, e.g. e-scrap or WEEE, are used as feedstock as such or along with primary sulphide concentrates. Properties of iron-silicate slags at high alumina concentrations, in the iron-alumina spinel saturation, have been studied at 1300 °C by a high temperature equilibration-quenching method combined with EPMA (electron probe microanalysis) phase composition data from the polished sections. The equilibrations were performed in fixed oxygen activity with platinum or palladium powder, which dissolved iron from the slag and generated a heterogeneous equilibrium system, characterised by the general equilibrium criterium in isothermal and isobaric conditions, as. μalloy(Fe) = μslag(Fe) = μspinel(Fe). This criterium was used for measuring experimentally iron activities of molten silicate slags. The locations of the spinel-liquid slag tie-lines were also determined in the oxygen partial pressure range of 10−6–10−10 atm. A comparison with the recent critical thermodynamic assessments of the Fe–O–Al2O3 system indicates that the iron-alumina spinel-corundum phase boundary in silica-containing systems as a function of oxygen partial pressure is too steep and thus the assessed databases do not match with the experimental data of this study. The liquid slag domain from silica to iron oxide saturation is also smaller than expected earlier, as the spinel primary phase boundary locates at higher silica concentrations than e.g. obtained in the assessments of the Mtox database.
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