Tracking of Chromium in Plasma co-Melting of Fly Ashes and Sludges
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- Tuan, Y., Wang, H.P. & Chang, J. Water Air Soil Pollut (2012) 223: 5283. doi:10.1007/s11270-012-1278-2
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Leachable chromium in the incineration fly ash and wastewater sludge has been thermally stabilized by plasma melting at the temperature of 1,773 K. To better understand how chromium is stabilized with the high-temperature treatment, chemical structure of the slags sampled at temperature zones of 1,100–1,700 K has been studied by synchrotron X-ray absorption spectroscopy. The component-fitted X-ray absorption near edge structure spectra of chromium indicate that the main chromium compounds in the sludge and fly ash are Cr(OH)3, Cr2O3, and CrCl3. A small amount of toxic CrO3 is also observed in the fly ash. In the plasma melting chamber under the reducing environment, the high-oxidation state chromium is not found. The slags in the plasma melting chamber have much less leachable chromium, which is due to chemical interactions between chromium and SiO2 in the slags. The existence of the interconnected Cr-O-Si species is observed by refined extended X-ray absorption fine structure spectroscopy. In the Cr2O3 phase of the slags, their bond distances, and coordination numbers for the first (Cr-O) and second (Cr-(O)-Cr) shells have insignificant perturbation when experienced with different melting temperatures between 1,300 and 1,700 K. It seems that Cr2O3 and chromium encapsulated in the silicate matrix of the slags have relatively much lower leachability. With this concept, to obtain a low chromium leachability slag from the plasma melting process, the residence time of the melting chamber may be decreased, and the slag discharge temperatures may be increased to 1,300 K. This work also exemplifies utilization of molecule-scale data obtained from synchrotron X-ray absorption spectroscopy to reveal how chromium is thermally stabilized in a commercial scale plasma melting process.
KeywordsChromiumPlasma meltingThermal stabilizationXANESEXAFS
To treat university laboratory wastes in Taiwan, an integrated incineration (375 kg/h), plasma melting (125 kg/h), and inorganic wastewater treatment system (625 kg/h) was designed and constructed at An-Nan campus of the National Cheng Kung University in 2004. Organic wastes can be destructed in the incineration process. However, toxic metals in the incineration air pollution control devices may be enriched in the fine fly ash discharged from the bag-house filters unit (Lampris et al. 2011; Fedje et al. 2010; Yang et al. 2009; Liu et al. 2009; Chou et al. 2009). Moreover, toxic metals in the sludges discharged from the inorganic wastewater treatment process are also needed to be chemically stabilized for final disposal. Under high temperatures (1,700–1,800 K) in the plasma melting process, toxic inorganic compounds may be thermally stabilized (Kourti et al. 2011; Yang et al. 2010; Gomez et al. 2009; Kuo et al. 2008; Park and Heo 2002). It is of great importance and interest to understand how toxic metals can be stabilized with the high-temperature thermal treatments (Abielaala et al. 2011; Kuo et al. 2009; Moustakas et al. 2008; Lin and Chang 2006; Kuo et al. 2004; Ku et al. 2003).
Chromium and chromate compounds are used widely in pigment manufacturing, plating, and stainless steel production and found frequently in dusts of spraying, cutting, and welding (Cohen et al. 1993). Excess exposure of chromium species may cause lung cancer (Costa and Klein 2006; Wise et al. 2002). By synchrotron X-ray absorption near edge structure (XANES) or extended X-ray absorption fine structure (EXAFS) spectroscopy, detailed local chemical structure information such as bond distance, coordination, and oxidation states of select elements in the complicated environmental samples can be observed. In separate experiments, by XANES and EXAFS, speciation of copper in catalytic oxidation of chlorophenols and reduction of NO was studied to reveal the nature of active sites and reaction paths involved (Lin and Wang 2001, 2000a, b; Chiu et al. 2011). During incineration of plating sludges, it was found that >50 % of Cr(III) was oxidized to Cr(VI) (Wei et al. 2007). Generally, high-oxidation state chromium (i.e., Cr(VI)) has a greater toxicity than other chromium compounds (Ku et al. 2003; Cohen et al. 1993; Costa et al. 2003). To better understand how chromium in the incineration fly ash and wastewater sludge can be thermally stabilized by plasma melting, their refined EXAFS and component-fitted XANES spectra were studied.
Chemical structure of the incineration fly ash, inorganic wastewater sludge, and plasma melting slags was studied by X-ray diffraction (XRD) (Bruker, Model D8 Advanced) spectroscopy. The EXAFS spectra of chromium in the samples were recorded on the Wiggler beamline at the Taiwan National Synchrotron Radiation Research Center. A Cr foil was used for absorption energy calibration at absorption of 5,989 eV. Because of low concentrations of chromium in the slag samples, the fluorescence mode was used in the EXAFS data accumulation. The Fourier transformed EXAFS spectra of chromium were measured in the range of 1.5–10.0 Å−1. Detailed EXAFS data treatments have been described in the literature (Rehr et al. 1995; Koningsberger and Prins 1998; Teo 1986). The raw experimental data were calculated and fitted using the WinXAS 2.0 simulation programs. The XANES spectra of chromium model compounds such as Cr, CrCl2, CrCl3, Cr(NO3)3, Cr(OH)3, Cr2O3, CrO3, K2CrO7, and Na2CrO4 were also determined for component fittings.
3 Results and discussion
Concentrations of leachable chromium in the fly ash, sludge, and slags
Leachable chromium (mg/L)
Incineration fly ash
Fly ash/sludge/glass (1:1:2)
Plasma melting slags
Taiwan EPA limit
Speciation parameters of chromium in the slags (determined by refined EXAFS)
Bond distance (Å)
Slags I (low-density slag)
Slag IV (high-density slag)
During the plasma melting treatments, under the reducing environment, chromium compounds such as the toxic CrO3 in the fly ash can be reduced to Cr2O3 and Cr. Chromium in the sludge can also be decomposed and reduced to form Cr2O3 and Cr. The slags in the plasma melting chamber have much less leachable chromium concentrations, which is due to insertion of chromium in the silicate matrix. The existence of the interconnected Cr-O-Si species is observed by refined EXAFS. In the Cr2O3 phase of the slags, their bond distances and CN for the first (Cr-O) and second (Cr-(O)-Cr) shells have insignificant perturbation when experienced at different melting temperatures between 1,100 and 1,700 K. It seems that the relatively more soluble Cr in the slags may be reduced by decreasing the residence time of the melting chamber to have the slag discharged temperature between 1,100 and 1,300 K in the plasma melting process.
The financial supports of Taiwan National Science Council, Bureau of Energy, and National Taiwan Synchrotron Radiation Research Center (NSRRC) are gratefully acknowledged. We also thank Prof. Jyh-Fu Lee of the NSRRC for his EXAFS experimental assistance.