Chromium reaction mechanisms for speciation using synchrotron in-situ high-temperature x-ray diffraction

We Use in situ high-temperature X-ray diffraction (HT-XRD), ex-situ XRD and synchrotron X-ray absorption near edge structure spectroscopy (XANES) to derive fundamental insights into mechanisms of chromium oxidation during combustion of solid fuels. To mimic the real combustion environment, Mixtures of pure eskolaite (Cr23+O3), lime (CaO) and/or kaolinite [Al2Si2O5(OH)(4)] have been annealed at 600-1200 degrees C in air versus 1 O-2 diluted by N-2. Our results confirm for the first time that (1) the optimum temperature for Cr6+ formation is 800 degrees C for the coexistence Of lime and eskolaite; (2) upon addition of kaolinite into oxide mixture, the temperature required to produce chromatite shifts to 1000 degrees C with a remarkable reduction in the fraction of Cr6+. Beyond 1000 degrees C, transient phases are formed that bear Cr in intermediate valence states, which convert to different species other than Cr6+ in the cooling stage; (3) of significance to Cr mobility from the waste products generated by combustion, chromatite formed at >1000 degrees C has a glassy disposition that prevents its water-based leaching; and (4) Increasing temperature facilitates the migration of eskolaite particles into bulk lime and enhances the extent to which Cr3+ is oxidized, thereby completing the oxidation of Cr3+ to Cr6+ within 10 min.