The physico-chemical property, viscosity, is essential for pyrometallurgical processes and influences
the settling rate of valuable metal droplets in slag, affecting the metal recovery. The copper slag from
smelting furnaces typically contains between 1 and 2 wt per cent Cu, which equals to or is higher
than in most sulfidic copper ores mined today, meaning the slag is a valuable resource for copper
extraction. A low viscosity is desirable for slag-cleaning through a settling furnace where entrained
droplets settle under gravity. One approach to control the viscosity is through the slag composition.
The literature has limited data on the correlation between viscosity, slag structure, and optical
basicity of iron silicate slag with FeO/SiO2, CaO, and Al2O3 as factors. Therefore, the present study
evaluated the influence of these factors on viscosity and correlations with slag structure and optical
basicity. The FeO/SiO2 ratio (at%/at%) and the CaO and Al2O3 content were altered in two levels,
resulting in a two-level full factorial design with three factors. The study was done by synthesising
FeO–SiO2–Al2O3–CaO–MgO–Cr2O3 melts, followed by water granulation. The melt structure of the
samples was analysed with the help of Raman Spectroscopy. The viscosity was then estimated with
a high-temperature rheometer using a Mo spindle and crucible. The results showed a correlation
between the optical basicity, FeO/SiO2 ratio, and viscosity, where a high optical basicity and
FeO/SiO2 ratio were beneficial for achieving a low melt viscosity.