Impact of Phase Evolution in Platreef and UG-2 Concentrates on Matte Drainage in the Black Top of a Platinum Group Metal Smelter

Abstract

The melting behavior of Platreef and UG–2 platinum group metal (PGM) concentrates was studied under conditions typically found in the black top of PGM smelters. In order to understand how the phase changes influence the separation between matte and slag, the concentrates were characterized before and after firing at eight temperatures from 800°C to 1480°C. The PGM concentrates were isothermally heated in either silica (Platreef) or magnesia crucibles (UG–2), which in turn were sealed in steel capsules under argon atmosphere. This configuration prevented the escape of sulfur–containing gases during the experiments. Observed phase relations were compared against phase relations predicted by FactSage® under oxygen partial pressures in the range of 10–18 to 10–9 bar and sulfur partial pressures in the range of 10–9 to 10–3.5 bar. The results showed the effective separation of matte from the gangue minerals at sub–liquidus temperatures of the gangue (1480°C for UG–2 concentrate, 1400°C and 1480°C for Platreef concentrate). The PGM concentrates did not sinter at low temperatures (800–1100°C); however, from 1200°C the formation of a liquid phase promoted liquid phase sintering. The collection of matte in a button at the bottom of the crucible was facilitated by the formation of a continuous liquid phase in the sintered concentrate, through which the matte droplets could descend. This occurred after the main gangue phase, enstatite, melted incongruently.