In 2021, BHP and Metso examined processing of high-MgO nickel sulfide concentrates using
Metso’s Ausmelt Top-Submerged-Lance (TSL) technology. Test work was conducted in Metso’s pilot
test work facility in Dandenong, Australia to explore operability of the SiO2-FeO-MgO-CaO-NiO slag
system across a wide range of compositions, temperatures and bath oxygen potentials. Pilot-scale
testing aimed to define slag ‘operability limits’, representing the lowest bath temperature at which
stable process and equipment operation could be maintained. This work was supported by
FactSage™, version 8.2 (by Thermfact/CRCT and GTT-Technologies) thermodynamic modelling,
slag viscosity measurements, physical characterisation of quenched slag samples performed by the
University of Queensland and benchmarking of commercial-scale TSL nickel smelting operations.
A wide range of slag compositions were examined, with Fe/SiO2 ratios varying from 0.4–1.1, CaO
content from 0.8–7.0 wt per cent and MgO content from 6–19 wt per cent. Slag SiO2/MgO ratio,
wt per cent CaO and matte grade were found to have the greatest impacts on identified operability
limits.
Operability limits were found to be influenced by both the solids content in slag and viscosity of the
remaining liquid slag phase, with the relative contribution of these parameters heavily influenced by
the slag composition. In the majority of trials, limits were defined by a theoretical solids content of
40–50 per cent, however in trials with a low Fe/SiO2 ratio and/or low wt per cent CaO, limits were
characterised by a much lower solids content due to the increased effect of the liquid slag viscosity
in determining the behaviour of these slags.
The test work highlighted inherent flexibility of the Ausmelt TSL process to operate across a wide
slag range of slag compositions and recover from process disturbances without an interruption to
feeding. The trials also demonstrated the possibility for Ausmelt TSL technology to process
concentrates with an Fe/MgO ratio as low as 1.4, which has important implications to the
commercial-scale processing of high MgO feeds. Arsenic rejection across the trials was very good,
with only 30 per cent of arsenic in the feed inputs reporting to the matte phase. Such high levels of
arsenic removal provide the Ausmelt TSL process with a notable advantage over alternative smelting
technologies.