The Oxidation Behavior of a Selection of South African Chromites

Fifteen commercial concentrates that are consumed in ferrochrome pre-oxidative sintering pelletizer operations were characterized, and exposed to oxidative roasting environments. Significant variance was found in the thermogravimetric behavior of the ores, which was observed to be strongly correlated with the starting Cr:Al ratio of the spinel (r = 0.84, P < 0.001). The formation of the sesquioxide phase during roasting was studied through comprehensive XRD and SEM–EDS analysis. Counter-diffusion of Mg2+ and Fe2+ cations with only limited diffusion of Cr3+ and Al3+ was observed. The final mass fraction of sesquioxide present after conversion was determined by calculations using SEM–EDS analyses and was found to be between 35 and 54 pct after oxidation at 1200 °C for 5 minutes. The mass fraction of sesquioxide was found to be most strongly correlated with the total Fe content of the starting chromite spinel (r = 0.93, P < 0.001). The sesquioxide phase that forms was confirmed to be a solid solution between Al2O3–Cr2O3–Fe2O3 with no evidence of pure hematite (Fe2O3) or eskolaite (Cr2O3) precipitate found.