Viscosity is one of the most important physicochemical properties during pyrometallurgical
operations. In the sub-liquidus regime, an important factor is the presence of suspended solid
particles, yet its influence is not well-described in earlier literature, despite the significance of this
factor. This project aims to determine the influence of spinel particles (Zn(Al,Fe)2O4) on the rheology
of a multiphase synthetic PbO-SiO2-ZnO-Fe2O3-CaO-Al2O3 slag system. Three data sets with a wellcontrolled
methodology and three different spinel sizes (small (13 μm), medium (34 μm) and large
particles (76 μm)) were published previously, using a custom-built high-temperature rheometer. This
study presents an additional data set of slag viscosity measurements, performed using a different
apparatus, the Anton Paar FRS 1800, to verify the consistency of the earlier determined viscosity
behaviour. Within this data set, no consistent trend could be found between the predicted wt per cent
spinel and the relative viscosity. Therefore, additional quenching experiments were conducted to
identify the morphology and experimentally determined volume fraction of the spinel particles in the
slag. These measurements revealed a difference in spinel size across the samples and
corresponding viscosity behaviour, emphasizing the importance of spinel particle size on slag
viscosity. Once the spinel size was taken into account, consistent viscosity results (relative apparent
viscosity and flow index) were observed across the two devices. This study confirms that, once all
parameters affecting slag viscosity are considered, the viscosity behaviour of a heterogeneous slag
can be uniquely determined. The resulting optimised slag viscosity models can be used to predict
viscosity before processing, addressing relevant phenomena such as slag tapping, slag foaming and
copper droplet settlement.