Chromium Reaction Mechanisms for Speciation using Synchrotron in-Situ High-Temperature X-ray Diffraction

Abstract

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 (Cr(3+)2O3), lime (CaO) and/or kaolinite [Al2Si2O5(OH)4] have been annealed at 600-1200 °C in air versus 1% O2 diluted by N2. Our results confirm for the first time that (1) the optimum temperature for Cr(6+) formation is 800 °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 °C with a remarkable reduction in the fraction of Cr(6+). Beyond 1000 °C, transient phases are formed that bear Cr in intermediate valence states, which convert to different species other than Cr(6+) in the cooling stage; (3) of significance to Cr mobility from the waste products generated by combustion, chromatite formed at >1000 °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 Cr(3+) is oxidized, thereby completing the oxidation of Cr(3+) to Cr(6+) within 10 min.

Mechanism and Prediction of Cr and Zn Immobilization from Industrial Residues by Glass-Ceramics

“…The formation of Cr 6+ could be explained by the reaction of Cr 2 O 3 as an acid oxide and CaO as an alkaline oxide with the aid of O 2 via eq . It should be noted that the formation of Cr 6+ was undesirable due to the higher toxicity and mobility of Cr 6+ than Cr 3+ . , This reaction mainly occurred on the surface of glass-ceramics, while it was unfavorable in the bulk phase due to the deficiency of oxygen. The absence of CaCrO 4 peaks in XRD patterns also confirmed its relatively low concentration.…”

Further Insight into the Formation and Oxidation of CaCr2O4 during Solid Fuel Combustion

“…In Ca-rich fuel, chromium was probably oxidized to form calcium chromate during combustion. , An effective method to suppress the oxidation of chromium is to decrease the free CaO content, by separating Ca-rich components from the fuel or enhancing the interactions of Si/Al- or P-compounds with free CaO. , In addition, increasing the operating temperature could partly decrease the fraction of chromate in ash residues. This effect was attributed to the rapid decomposition of oxidized CaCrO 4 .…”

“…On the basis of these theories, Mao et al limited the potential of CaO to promote Cr 2 O 3 oxidation by converting free CaO into phosphate minerals . Similarly, Low et al suppressed the oxidation of Cr 2 O 3 through scavenging of free lime by reaction with kaolinite . Therefore, the fate of calcium likely determined the oxidation of CaCr 2 O 4 , and measures could be taken to inhibit CaCr 2 O 4 oxidation based on our understanding of calcium behavior in the process.…”

Chemical speciation and risk assessment of heavy metals in biochars derived from sewage sludge and anaerobically digested sludge

“…However, the concentration of Cr increased by 33.83% after the pyrolysis. The stabilization of Cr might be attributed to the formation of stable CaCrO 4 or CaCr 2 O 4 through the reaction of Cr 3þ and CaO produced by the decomposition of CaCO 3 in the sludge (Low et al 2015).…”