Since iron oxide and silicon oxide are major gangue components in various types of ore, FeO-SiO2
base slag, ie fayalite slag, has been often encountered in pyrometallurgical processes of non-ferrous
metals. During most matte production processes such as the smelting of Cu, Pb, and Ni, the FeOSiO2
based slag is inevitably formed and plays an important role in refining crude metal as well as
engineering aspects. Recently, similar pyrometallurgical approaches have been attempted in the
recovery of valuable metals (Ni, Co, Cr) from wasted secondary batteries. Thus, the viscosity of the
FeO-SiO2 based slag has great significance in properly controlling smelting processes. However,
when molten metal or matte does not coexist with the slag during the measurement, there has been
a serious difficulty in maintaining Fe valance and consequently, the slag composition.
In the present research, the viscosity of molten FeO-SiO2 slag was measured in 1573~1673 K using
a rotational viscometer. In order to keep the state of iron oxides, the oxygen partial pressure was
controlled using CO/CO2 gas mixture. The optimised ratio of CO/CO2 could be calculated using
Fe/Fe2+ and Fe2+/Fe3+ equilibria at each temperature. The composition of the slag, particularly Fe
valance, after the measurement was analysed with the aid of an inductively coupled plasma – optical
emission spectroscopy (ICP-OES) and an automatic potentiometric titrator. The measurement
showed that the FeO-SiO2 slag has considerably low viscosity values in the molten state, and it
sharply increases as the temperature decreases. In spite of the careful oxygen partial pressure
control, a part of Fe2+ was found to be oxidised to Fe3+, with a maximal case reaching about
20 per cent. The increase in viscosity of FeO-SiO2 at low temperatures can be attributed to the
composition change due to an unavoidable Fe2+ oxidation. In addition, the relationship between the
change in the viscosity and the liquidus temperature of FeO-SiO2 based slag was discussed to
understand the composition dependence of viscosity.