Water, Air, & Soil Pollution

, Volume 223, Issue 8, pp 5283–5288

Tracking of Chromium in Plasma co-Melting of Fly Ashes and Sludges

Authors

  • Yeu-Juin Tuan
    • Department of Environmental EngineeringNational Cheng Kung University
    • Sustainable Environment Research CenterNational Cheng Kung University
    • Department of Environmental EngineeringNational Cheng Kung University
  • Juu-En Chang
    • Department of Environmental EngineeringNational Cheng Kung University
    • Sustainable Environment Research CenterNational Cheng Kung University
Article

DOI: 10.1007/s11270-012-1278-2

Cite this article as:
Tuan, Y., Wang, H.P. & Chang, J. Water Air Soil Pollut (2012) 223: 5283. doi:10.1007/s11270-012-1278-2

Abstract

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.

Keywords

ChromiumPlasma meltingThermal stabilizationXANESEXAFS

1 Introduction

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.

2 Experimental

In the plasma melting process, the incineration fly ash and wastewater sludge wastes packed in cartons (125 kg/h) were fed into the melting chamber with a screw conveyer. The melting chamber is integrated with an air pollution control device which is consisted of a quench tower, wet scrubber, activated carbon adsorber unit, and oxidizer. The incineration fly ash discharged from the bag-house filters was thermally stabilized with inorganic wastewater sludge in the plasma melting chamber. Glass wastes were used for adjusting silicon fractions in the feed. The plasma melting chamber lined with refractory materials can be operated at a high-temperature range of 1,700–1,800 K. A brief diagram of the plasma-assisted melting chamber and sampling points is shown in Fig. 1. Two torches were used alternatively to ensure a continuous operation. The fly ash, sludge, and glass wastes having the weight ratio of 1:1:2 were fed to the plasma melting chamber at the temperature of 1,773 K. Slags at the temperature zone between 1,100–1,700 K in the melting chamber were also sampled for chemical structure studies of chromium. Concentrations of leachable toxic metals from the slag samples were measured on an inductively coupled plasma emission spectrometer (Varian, Model VISTA-MPX) following the Taiwan EPA toxicity characteristic leaching procedure (Taiwan EPA 2009).
https://static-content.springer.com/image/art%3A10.1007%2Fs11270-012-1278-2/MediaObjects/11270_2012_1278_Fig1_HTML.gif
Fig. 1

Diagram of the plasma melting chamber and slag sampling points

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 slag are shown in Table 1. It is clear that the concentration of leachable chromium from the fly ash is excess the Taiwan EPA limit (5.0 mg/L). To optimize the high-temperature plasma melting operation, fractions of SiO2 in the feed were adjusted by addition of wastewater sludge and waste glass. The concentrations of leachable chromium in the slags discharged from plasma co-melting of the fly ash and sludge are effectively reduced.
Table 1

Concentrations of leachable chromium in the fly ash, sludge, and slags

 

Leachable chromium (mg/L)

Incineration fly ash

17

Wastewater sludge

0.07

Fly ash/sludge/glass (1:1:2)

4.4

Plasma melting slags

0.13

Taiwan EPA limit

5.0

In Fig. 2, Cr2O3 is the main chromium crystalline in the slags. Sodium silicates and SiO2 are also observed. NaCl which is found in the slag I (near the surface of the melting zone at 1,773 K) is original in the fly ash.
https://static-content.springer.com/image/art%3A10.1007%2Fs11270-012-1278-2/MediaObjects/11270_2012_1278_Fig2_HTML.gif
Fig. 2

XRD patterns of the slags a I, b II, c III, and d IV. (1 SiO2, 2 Cr2O3, 3 NaCl, 4 NaAlSiO4, 5 Na4AlSi3O12Cl)

To better understand chemical structure of chromium in the plasma melting process, the representative slags sampled at the temperature zones of 1,100–1,700 K were studied by component-fitted XANES spectroscopy. In Fig. 3, Cr2O3 and Cr are the main chromium compounds in the slags. During the high-temperature melting treatments, under the reducing environment, chromium compounds such as CrO3 in the fly ash (see Fig. 3b) may be reduced to Cr2O3, and are, to some extent, further reduced to metallic chromium (Cr). Chromium compounds (Cr(OH)3, Cr(NO3)3, and CrCl3) in the sludge are also decomposed and reduced to form Cr2O3 and Cr during the plasma melting treatments. Note that Cr is relatively more soluble which may be associated with the fact of increasing concentrations of leachable chromium from the high-density Cr slag at the bottom of the melting chamber.
https://static-content.springer.com/image/art%3A10.1007%2Fs11270-012-1278-2/MediaObjects/11270_2012_1278_Fig3_HTML.gif
Fig. 3

Component-fitted XANES spectra of chromium in a sludge, b fly ash, and c slags

Table 2 shows that the bond distances and coordination number (CN) of chromium species in the slags. Over 99 % reliability of the EXAFS data fitting for chromium has been obtained. In all EXAFS data reported in this work, the Debye–Waller factors (σ2) are less than 0.007 Å2. Notably, in the second shell, the Cr-(O)-Si species in the slags having the bond distances of 3.06–3.11 Å are observed, suggesting that chromium may be inserted into the silicate matrix. It is noted that the bond distance of the Cr-(O)-Si increases as the slag temperature in the melting zone is decreased between 1,300 and 1,700 K, which may also lead to an increase of the chromium leachability. 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 different melting temperatures between 1,300 and 1,700 K. It is worth noting that the slag at the bottom of the melting chamber (slag IV) has less CN of Cr-(O)-Si than other level slags, suggesting that the leachable chromium in this slag may not be encapsulated in the silicate matrix. Furthermore, the unencapsulated Cr in the slag also contributes its leachability.
Table 2

Speciation parameters of chromium in the slags (determined by refined EXAFS)

 

Shells

Bond distance (Å)

Coordination number

σ22)

Slags I (low-density slag)

Cr-O

2.00

2.4

0.003

Cr-(O)-Cr

2.74

5.1

0.001

Cr-(O)-Si

3.06

7.8

0.001

Cr-Cr

2.25

1.6

0.004

Slag II

Cr-O

2.02

3.6

0.001

Cr-(O)-Cr

2.74

5.0

0.006

Cr-(O)-Si

3.09

8.4

0.003

Cr-Cr

2.25

1.8

0.004

Slag III

Cr-O

2.00

3.4

0.001

Cr-(O)-Cr

2.75

5.1

0.007

Cr-(O)-Si

3.10

8.8

0.004

Cr-Cr

2.22

1.5

0.003

Slag IV (high-density slag)

Cr-O

2.02

3.3

0.001

Cr-(O)-Cr

2.77

3.3

0.004

Cr-(O)-Si

3.11

5.6

0.001

Cr-Cr

2.24

1.4

0.003

Figure 4 shows the correlation between the molar ratios of Cr/Cr2O3 and concentrations of leachable chromium in the slags at 1,100–1,700 K. It is also worth noting that at 1,700 K, the chromium leachability of the slag can be significantly reduced. At 1,100 K, the relatively high leachable chromium concentration may be associated the high Cr fraction in the slag. 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 temperature may be increased to 1,300 K.
https://static-content.springer.com/image/art%3A10.1007%2Fs11270-012-1278-2/MediaObjects/11270_2012_1278_Fig4_HTML.gif
Fig. 4

Correlation between molar ratios of Cr/Cr2O3 and concentrations of leachable chromium in the slags at 1,100–1,700 K

4 Conclusions

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.

Acknowledgments

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.

Copyright information

© Springer Science+Business Media B.V. 2012