Abstract
The decomposition of chromite ore in platinum group metal (PGM) concentrates and the further partitioning of chromium between PGM matte and gangue were investigated under conditions typically found in the black top of primary PGM smelters. UG–2 and Platreef PGM concentrates were fired under isothermal conditions at eight different temperatures from 800 up to 1480 °C, in sealed steel capsules. The microstructural and chemical evolution of chromite, as well as the partitioning of chromium, was studied by characterizing the PGM concentrates before and after firing, using SEM–EDS analysis. Results showed that between 800 and 1100 °C (i.e., below the solidus temperature of the silicate portion of the concentrates), the chromite particles exhibited limited reduction of Fe3+ to Fe2+ at the chromite surface, similar to the early stages of chromite reduction observed in the ferrochrome industry. After heating at 1200 °C and above, the formation of a liquid silicate was confirmed. This liquid phase acts as a solvent for the spinel components, which are subsequently transported through this liquid silicate phase to the sulfide phases and undissolved enstatite particles. At temperatures of 1300, 1400 and 1480 °C, the dissolved Al2O3 and FeO diffuse away from the chromite surface. This leads to the saturation of the adjacent liquid silicate in MgO and Cr2O3, and subsequently, MgCr2O4–rich spinel crystals precipitate. A maximum of 1.1 mass% Cr was found in the matte at 1480 °C. Chromium was transferred to the matte through reaction of the liquid matte with solid chromium-containing spinel phases and through partitioning between the liquid silicate gangue and the liquid matte.
Published Version
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