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Effects of the type of sintering atmosphere on the chromium leachability of thermal-treated municipal solid waste incinerator fly ash
Waste Management 21 (2001) 85±91
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Eects of the type of sintering atmosphere on the chromium leachability of thermal-treated municipal solid waste incinerator ¯y ash Kuen-Sheng Wang, Chang-Jung Sun *, Chung-Yu Liu Graduate Institute of Environmental Engineering National Central University Chung-Li, Taiwan, ROC Accepted 26 April 2000
Abstract The sintering process oers an opportunity to combine detoxi®cation and resource recovery for the treatment of municipal solid waste (MSW) incinerator ¯y ash. However, the chromium (Cr) in the sintered ¯y ash becomes more readily leachable with increasing sintering time and temperature, thus posing severe threats to the environment and human health when the sintered ash is recycled or reused. This study investigated the enhanced leachability of ¯y ash containing Cr, by heating the chromium (III) oxide (Cr2O3)-spiked ¯y ash to 800 C in atmospheres containing air, nitrogen gas (N2), and 5% H2+95% N2, respectively. The results indicated that trivalent chromium was converted to its soluble hexavalent form during sintering in the air atmosphere; whereas sintering in a nitrogen atmosphere signi®cantly reduced the leachability of Cr due to lack of oxygen (O2) to oxidize. The eects of the sintering temperature on the total chromium content and the leaching concentration in the toxicity characteristic leaching procedure (TCLP) extract are also discussed. # 2000 Elsevier Science Ltd. All rights reserved. Keywords: MSW; Incinerator; Fly ash; Sintering; Leachability; Atmosphere; Chromium
1. Introduction Limited land®ll sites and increasing disposal costs have hastened eorts to adopt incineration technologies and energy recovery strategies for managing municipal solid waste (MSW) in Taiwan. There will be a total of 36 municipal solid waste incinerators (MSWI) on this island by 2003, including 21 waste-to-energy incinerators, proposed in 1991 by Environmental Protection Administration (EPA), and an additional 15 incinerators adopting build-own-operate (BOO) or build-own-transfer (BOT) strategies [1]. These incinerators will be capable of processing 90% of the island's MSW with production of more than 2000 tons of incinerator residues each day, including 400 tons of hazardous ¯y ash to be further properly managed. The MSWI ¯y ash contains high levels of leachable heavy metals and salts, and is usually classi®ed as hazardous [2±4]. Various approaches have been used to solidify/stabilize the hazardous ¯y ash, which include acid*Corresponding author. Tel.: +886-3-436-1070, ext. 571; fax: +8863-456-3674. E-mail address: [email protected] (C.-J. Sun).
extraction, neutralization with exhaust gases, chemical ®xation, sintering and melting. Of these alternatives, sintering process has become promising because it combines an opportunity of detoxi®cation and recycling of the ash as construction materials [5]. This process can bond ¯y ash together by ®ring at high temperatures, resulting in sintered matrices with sucient strength and extremely low heavy metal leachability [6,7]. In general, the heavy metal leachability of the sintered MSWI ¯y ash is dependent on the heavy metal speciation formed during the sintering process. Moreover, the variation of speciation is then aected by the sintering atmosphere and sintering temperature [7]. Experimental laboratory studies have shown that the leachability of most heavy metals decreases with increased sintering time and sintering temperature [8]. However, the leachability of chromium (Cr) species shows an increase with increasing temperature. To ensure the safe disposal or utilization of the sintered MSWI residues, it is essential to have a better understanding of variations in chromium leaching behavior during sintering. This paper presents experimental results that help explain why the chromium species of the MSWI ¯y ash, such as Cr2O3, selected as a spike in this study, and the
0956-053X/00/$ - see front matter # 2000 Elsevier Science Ltd. All rights reserved. PII: S0956-053X(00)00041-6
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K.-S. Wang et al. / Waste Management 21 (2001) 85±91
mechanism by which sintering parameters, mainly sintering time, temperature, and atmosphere, function to convert it into a readily soluble form. Experiments included spiking the MSWI ¯y ash with Cr2O3 to a concentration detectable by X-ray diraction (XRD) techniques [9,10], and sintering the spiked ash at 600±1000 C in various atmospheres such as air, nitrogen gas (N2) and 5% H2+95% N2. The conversion of Cr2O3 to a readily soluble form was con®rmed, and the eects of sintering atmosphere containing oxygen (O2) illustrated [11], providing useful information on reducing the leachability of ¯y ash containing Cr during sintering. 2. Materials Sintering tests were performed with cyclone ¯y ash collected from a mass burning MSW incinerator located in the northern part of the island. The incinerator is capable of processing 1350 tons of local MSW per day and is equipped with a set of air pollution control devices consisting of a cyclone, an adsorption reactor, and a fabric ®lter. The MSWI ¯y ash after collecting was homogenized, then oven dried at 105 C for 24 h. After being dried and cooled, it was again ground, homogenized, and ®nally, desiccated within a week. The samples used in this study were cylindrical specimens 1.5 cm high and 1 cm in diameter, obtained by compacting the above MSWI ¯y ash at 5000 psi (35 MPa). 3. Methods The sintering tests were performed in an electric-heated tubular furnace under various operational parameters such as ash composition, type of atmosphere, sintering temperature and sintering time, as shown in Table 1. Three kinds of ash compositions were used including MSWI ¯y ash, and ash spiked with 5% (w/w) of chromic oxide (Cr2O3) and chromium chloride (CrCl3), respectively. The types of atmosphere used in this study were air, N2, and 5% H2+95% N2. The sintering temperature ranged from 600 to 1000 C, and the sintering time from Table 1 Experimental processing conditions Sintering material
Atmosphere
Sintering temperature ( C)
Sintering time (min)
FAa FA FA FA FA+5% Cr2O3 FA+5% CrCl3
Air Air N2 5% H2+95% N2 Air Air
600, 800, 900, 1000 600, 800 600, 800 600, 800 800 800
5±240 60 60 60 5±240 5±240
a
FA=MSWI ¯y ash.
5 to 240 min. Depending on the purpose of the experiments, some experiments were conducted for a speci®ed sintering time and temperature, as listed in Table 1. All the samples were analyzed after sintering to determine their total chromium content and the chromium leaching concentration of the TCLP extract. The sintered samples were also analyzed using XRD techniques to determine the variation of the chromium species during the sintering process. The composition of MSWI ¯y ash, the heavy metal contents (Cd, Pb, Zn, Cu, and Cr), and the leaching concentrations of the TCLP extracts were analyzed using US EPA Method SW-846: the digestion for the total heavy metal content was conducted according to SW846 3050b; the digestion for hexavalent chromium (Cr+6), SW-846 3060A; the toxicity characteristic leaching procedure, SW-846 1311; the leaching concentrations for Cr+6, Pb, Cd, Cr, Cu, and Zn, 7196A, 7421, 7131A, 7191, 7211, 7951, respectively. 4. Apparatus The apparatus used in this study was composed of an electric-heated tube furnace for research purposes shown in Fig. 1. The heart of the furnace was a quartz tube burner, 100 cm long and 5.5 cm in inner diameter, housed in a glass®ber-liner insulated steel shell. A quartz boat moved by quartz rod was designed to feed the samples into the combustion chamber. The sintering temperature at the center inside the burner tube was monitored by a thermocouple and controlled by a programmed temperature controller. A quartz-®ber ®lter followed the heating chamber. To prevent the ¯ue gases from condensing while ¯owing through, the ®lter together with its holder was heated by a heating band to a temperature higher than 120 C. The ®lter eciency was 99.99% for the removal of 0.3-micron particles during the test conditions. A sampling train consisting of six impingers in a series followed by the ®lter was used. Multiple heavy metals and their compounds in the ¯ue gases were trapped using a modi®ed USEPA method 5 (MM 5 method) [12]. Accordingly, the ®rst impinger was left empty, while the second to the fourth impingers were ®lled with a combined solution of 5% HNO3 (nitric acid) and 10% H2O2 (hydrogen peroxide) for trapping most of the heavy metals and their compounds in the combustion gases. The ®nal impinger was ®lled with silica gel to remove the moisture content from the gases. 5. Results and discussion 5.1. Characterization of the ¯y ash The MSWI ¯y ash used in this study had a pH of 10.15 in 0.01 M calcium chloride (CaCl2) solution,
K.-S. Wang et al. / Waste Management 21 (2001) 85±91
87
Fig. 1. Tube furnace and the sampling devices.
showing the basic nature of the ¯y ash. The loss on ignition was found to be less than 1.6%. The major chemical composition of the ¯y ash, as listed in Table 2, indicates that the major elements are silicon (Si), aluminum (Al), calcium (Ca) and chlorine (Cl). Table 3 summarizes the heavy metal content in the ¯y ash, showing higher concentrations of lead (Pb) and zinc (Zn) (1537 and 7815 mg/kg, respectively) in the MSWI ¯y ash. The concentration of Cd in the TCLP leachate exceeded the current US EPA regulatory threshold. In addition, Zn also showed a high leachability. Table 4 lists the major species found in the MSWI ¯y ash used Table 2 Major chemical compositions in MSWI ¯y ash Element
Concentration (%)
Element
Concentration (%)
Al Si Ca Na K
3.06 10.10 7.78 3.23 2.13
Fe Mg P C Cl
1.27 1.66 1.24 0.53 7.60
in this study. The species were compared with that identi®ed in incinerator ash generated by burning simulated wastes and various spiked chloride amounts [13]. In addition, the species were also compared with those predicted by the Thermal Dynamic Equilibrium Model (Chemical Equilibrium with Transport Properties, 1989; CET 89), developed by the National Aeronautics and Space Administration in 1976 [14]. 1976). The comparison of compounds formed in the ¯y ash during waste combustion suggests that one of the possible chromium compounds in the incinerator ash was Cr2O3. Fig. 2 identi®es the major compounds in the MSWI ¯y ash without spiking. No chromium compound was found by the XRD techniques due to its concentration being too low to be detected, though chromium was detected via the analysis of the total chromium content. 5.2. Chromium leachability of sintered ¯y ash in an air atmosphere Chromium-related compounds in the MSWI ¯y ash were evaluated by their residual content in the sintered
Table 3 Heavy metal contents and TCLP leachate concentrations of MSWI ¯y ash
Metal content (mg/kg) TCLP leachate concentration (mg/l) Regulatory thresholds a
Regulatory thresholds for sludge.
Pb
Cd
Cr
Cu
Zn
1537.25.7 0.530.07 5.0
122.70.7 2.270.08 1.0
309.93.0 1.350.07 5.0
1127.933.8 0.340.03 15.0a
7814.9227.9 14.291.0 25.0a
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Table 4 Major species in the MSWI ¯y ash as a function of chlorine content of the waste Model prediction
XRD identi®cation Fly ash
C2Cl4 (%) 0.8
Cd
Zn
Cr
Cu
Cd
Zn
Cr
Cu
Cd
Zn
Cr
Cu
KCl NaCl
KCl NaCl Zn ZnO
KCl NaCl Cr2O3
NaCl KCl Cu CuCl Cu2S
n.d.a KCl
KCl KCl ZnO
n.d. KCl
n.d. KCl
KO2 KO2
CrO3 CrO3
KCl
KCl NaCl
KCl NaCl
KCl NaCl CuCl
n.d.
ZnO KCl KO2 ZnO KCl KO2 ZnO
n.d. CuO Na2O K2O K2O CuO Cu2O
NaCl KCl FeO FeCl2 FeCl3
KCl NaCl Zn ZnO FeO Fe2O3 FeCl3
KCl NaCl Cr2O3 FeO Fe2O3 FeCl3
NaCl KCl Cu CuCl Cu2S FeO FeCl2 FeCl3
n.d. KCl
KCl KCl NaCl KCl NaCl
n.d. KCl NaCl KCl NaCl
n.d. KCl NaCl KCl NaCl CuCl
KO2 KO2 Fe2O3 Fe2O3
ZnO Fe2O3 Fe3O4 Fe2O3 Fe3O4
CrO3 CrO Fe2O3 CrO Fe2O3
n.d. CuO Fe2O3 CuO Cu Fe2O3
NaCl KCl
KCl NaCl Zn ZnO
KCl NaCl Cr2O3
NaCl KCl Cu CuCl Cu2S
n.d. KCl NaCl CdCl2 KCl NaCl
KCl KCl NaCl
n.d. KCl NaCl
n.d. KCl NaCl
KO2 NaCl
KCl NaCl ZnO
KCl NaCl
KCl NaCl CuCl
NaCl
ZnO KO2 KCl ZnO KO2 ZnO
CrO3 Cr2O3 CrO NaCl Cr2O3 CrO3
n.d. NaO2 CuO Cu2O NaCl CuO
KCl NaCl Zn ZnO
KCl NaCl Cr2O3
NaCl KCl Cu CuCl Cu2S
n.d. KCl NaCl
KCl KCl NaCl
n.d. KCl
n.d. KCl NaCl
KO2 KO2 KCl
ZnO KCl ZnO
n.d. K2O CuO
KCl
KCl NaCl
KCl NaCl
KCl NaCl CuCl
KCl
KCl ZnO
CrO3 Cr2O3 CrO CrO3 KCl
1.6 FeCl3 (%) 0 0.8 1.6
NaCl (%) 0
1.6 KCl (%) 0 0.8
KCl NaCl
1.6
a
Bottom ash
KCl NaCl
CrO3 Cr2O3
KCl KOH CuO
n.d., not detected.
ash and their concentration in the TCLP leachate. The risk posed by heavy metals in the sintered MSWI ash can not be assessed by the residual metal content of the sintered product alone since a greater residual content does not necessarily mean greater leaching concentration. The leachability is dependent on the solubility of the heavy metal-related compounds formed after sintering. In general, heavy metals and/or their compounds in the incinerator ash will evaporate, depending on their volatility during the sintering process. Accordingly, the heavy metal content tends to decrease with increasing sintering temperature for the targeted heavy metals such as Cd, Pb, and copper (Cu), as shown in Fig. 3. However, the chromium content in the sintered ash increases with elevated sintering temperatures. On the other hand, the leachability of Cd and Pb in the sintered ash showed a decreasing trend with increasing temperature. However, as indicated in Fig. 4, Cu and Cr leachability showed tendency to increase
with increasing temperatures, up to 900 C, followed by a decreasing trend with still higher temperatures. The high leachability of the chromium and copper compounds after sintering has raised environmental risk concerns as to thermal treatment for the resource recycling of incinerator ¯y ash. Based on the identi®cation of chromium compounds in incinerator ash by XRD techniques, and on the con®rmation of their presence by the thermal dynamic equilibrium model (CET 89), Cr2O3 was selected as a spike in this study, to investigate the leaching behavior of chromium after sintering, with a CrCl3 spike as a possible contrast. The variation in the total chromium content for Cr2O3 and CrCl3 spikes after sintering is shown in Fig. 5. The total chromium content in ¯y ash samples spiked with 5% (w/w) Cr2O3 (50,000 mg/kg) was found to be only 496 mg/kg before sintering, implying that Cr2O3 is almost wholly undigested by nitric acid, due to its slight
K.-S. Wang et al. / Waste Management 21 (2001) 85±91
89
Fig. 2. The major compounds in MSW incinerator ash identi®ed by XRD techniques.
Fig. 3. The heavy metal content of MSW incinerator ¯y ash after being sintered at various temperatures for 4 h.
Fig. 4. The heavy metal leaching concentrations in TCLP leachate from MSW incinerator ¯y ash sintered at various temperatures for 4 h.
solubility. However, after sintering for 20 and 240 min, the total chromium content increased to 9161.5 mg/kg and 25,865 mg/kg (see Fig. 5), respectively, suggesting that the thermal treatment of the ash signi®cantly enhance the digestibility of Cr. On the other hand, Fig. 6 shows that the chromium concentration of the TCLP leachate varies with time, as the ¯y ash was sintered at 800 C in an air atmosphere. The enhancement of chromium leachability after sintering in an air atmosphere was studied by analyzing which chromium compounds were formed after sintering. In experiments spiked with Cr2O3, the hexavalent chromium concentration and the total chromium concentration in the TCLP leachate were very similar. This suggests that the total chromium extracted by the TCLP test is primarily of the hexavalent form.
Comparatively, in experiments spiked with CrCl3, the extractable total chromium concentration showed an increase with increased sintering time; however, an extremely low or no hexavalent chromium concentration was noted. The results indicate that, during the sintering process, the Cr2O3 in the ¯y ash was converted to its more extractable hexavalent form. In related experiments, the CrCl3 from the ash was also converted to a soluble but not a hexavalent form. The hexavalent form of the chromium compound was identi®ed as the readily soluble K2CrO4 by XRD analysis of the ¯y ash before and after sintering, as shown in Fig. 7. The K2CrO4 peak increased while the Cr2O3 peaks decreased. However, in the case of CrCl3, no signi®cant peak, thus no crystalline form was identi®ed by the XRD analysis.
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Fig. 5. The Cr content of Cr2O3- and CrCl3- spiked MSW incinerator ¯y ash being sintered at 800 C.
all chromium oxides, a sintering atmosphere with or without oxygen is expected to signi®cantly aect the formation of hexavalent chromium. The total chromium contents of the ¯y ash, sintered in atmospheres of air, N2, and 5% H2+95% N2, are shown in Fig. 8. It was noted that at 800 C, the total chromium content tended to decrease as the sintering atmosphere varied from air to N2 or to 5% H2+95% N2. However, no signi®cant variation in the total chromium content was observed between the N2 and 5% H2+95% N2 atmospheres. On the other hand, in the experiments sintered at 600 C, the total chromium content showed a slight increase, as the atmosphere shifted from air to N2 or to 5% H2+95% N2. The increase was also con®rmed by sintering experiments conducted in an air atmosphere or a reduced-pressure air atmosphere, at 600 C, but the possible reasons still remain to be studied. On the other hand, the eects of sintering atmosphere on the leachability of chromium were demonstrated by comparing the chromium leaching concentration of ash sintered in an atmosphere of air and N2, as shown in
Fig. 6. The Cr concentrations in TCLP leachate from Cr2O3- and CrCl3-spiked MSW incinerator ¯y ash being sintered at 800 C.
Fig. 8. The Cr content of MSW incinerator ¯y ash after being sintered at dierent sintering atmospheres for 1 h.
5.3. Eects of sintering atmosphere on the leachability of chromium Previous experiments revealed that sintering in an air atmosphere converted Cr2O3 to its more extractable hexavalent form (K2CrO4). Since the major hexavalent chromium compounds, chromate and dichromate, are
Fig. 7. XRD patterns of MSW incinerator ¯y ash before and after sintering showing the conversion of Cr2O3 to K2CrO4.
K.-S. Wang et al. / Waste Management 21 (2001) 85±91
91
of ¯y ash containing chromium in an atmosphere without oxygen, such as an N2 atmosphere, and in a reducing atmosphere, can eectively prevent the oxidation of trivalent chromium to its more readily soluble hexavalent form. Acknowledgements
Fig. 9. The Cr concentration in TCLP leachate from MSW incinerator ¯y ash after being sintered at 600 and 800 C in dierent sintering atmospheres for 1 h.
Fig. 9. When sintered at 600 C in an air atmosphere, only a small amount of chromium was oxidized to its soluble or digestible form, resulting in a relative low leaching concentration for chromium, as compared to the case sintered at 800 C. Similar trends in leaching concentration were noted for atmospheres of N2 and 5% H2+95% N2, a slightly more reducing atmosphere. However, as the sintering temperature was raised to 800 C, the chromium leachability of the sintered ash was dramatically increased due to the conversion of chromium to it soluble form, such as K2CrO4, as described above. Yet, when the air atmosphere was replaced with N2 or 5% H2+95% N2, the chromium leaching concentration was signi®cantly reduced. The reduction can be adequately explained by no oxygen being supplied by the sintering atmosphere to combine with the chromium compounds to form soluble hexavalent chromium or other soluble forms, thus greatly reducing chromium leachability. However, a reducing atmosphere, created by replacing 5% N2 with H2, shows no signi®cant eects on decreasing the chromium leachability as compared to the N2 atmosphere. 6. Conclusions The sintering process was performed with the cyclone ash collected from a mass burning MSW incinerator for the combined purposes of heavy metal ®xation and resource recovery of sintered ash. However, the leachability of ash containing chromium compounds was enhanced by this thermal treatment, which could pose an environmental risk. In this study, the major chromium compound in the incinerator ash, Cr2O3, was spiked in order to investigate variations in chromium leachability during sintering. The experimental results revealed that the slightly soluble Cr2O3 was converted to K2CrO4, a readily soluble and more toxic hexavalent form. The pronounced conversion took place via the oxidation of trivalent chromium to its hexavalent form, at sintering temperatures higher than 800 C. The thermal treatment
The authors gratefully acknowledge the ®nancial support (under the grant NSC-87-2211-E008-007) of the Science Council of Republic of China in Taiwan and the editorial services of Mrs. Debbie. References [1] ROCEPA (ROC Environmental Protection Administration) Home Page. http://www.epa.gov.tw/burning/home.html (accessed April 1999). [2] Bruggen BV, Vogels G, Herck V, Vandecasteele C. Simulation of acid washing of municipal solid waste incineration ¯y ashes in order to remove heavy metals. Journal of Hazardous Materials 1998;57:127±44. [3] Chan C, Jia CQ, Graydon JW, Kirk D. The behavior of selected heavy metal in MSW incineration electrostatic precipitator ash during roasting with chlorinating agents W. Journal of Hazardous Materials 1996;50:1±13. [4] Eighmy TT, Eusden JD, Krzanowski JE, Domingo DS, Stamp¯i D, Martin JR, Erickson PM. Comprehensive approach toward understanding element speciation and leaching behavior in municipal solid waste incineration Electrostatic Precipitator Ash. Environmental Science & Technology 1995;29:629±46. [5] Jakob A, Stucki S, Kuhn P. Evaporation of heavy metals during the heat treatment of municipal solid waste incinerator ¯y ash. Environmental Science & Technology 1995;29:2429±36. [6] Wang KS, Chiang KY, Perng JK, Sun CJ. The characteristics study on sintering of municipal solid waste incinerator ashes. Journal of Hazardous Materials 1998;59:201±10. [7] Wang KS, Sun CJ, Yeh CC, Chiang KY. Sintering behavior and particle size in¯uence in MSW incinerator ash. In 91st Annual Meeting of Air and Waste Management Association (CD-ROM), San Diego, USA, 1998. [8] Winter RM, Mallepalli RR, Hellem KP. Determination of As, Cd, Cr, and Pb species form in a combustion environment. Combustion Science Technology 1994;101:45±58. [9] Lee CC. A Model Analysis of partitioning in a hazardous waste Incineration System. JAPCA 1998;38:941±5. [10] Perry R. H.and Green, D. W. McGraw-Hill Malaysia: Perr's Chemical Engineers' Handbook, 1984. [11] USEPA (US Environmental Protection Agency). Test methods for evaluating solid waste, SW-846, 1994. [12] Gordon S, McBride B. Measurement methodology for toxic metals from municipal waste combusts. In: Proceeding of the International Conference on Municipal Waste Combustion, vol. 2, 1989. p. 5C1±5C15. [13] Chiang K.Y. Speciation and partitioning metals aected by chloride content during a simulated municipal solid waste incineration process. Ph.D. Thesis, National Central University at Chung-Li, 1997. [14] NASA. Computer program for calculation of complex chemical equilibrium compositions, rocket performance, incident and re¯ected shocks, and Chapman-Jouguet detonations. NASA SP273. National Aeronautics and Space Administration, 1976.
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Eects of the type of sintering atmosphere on the chromium leachability of thermal-treated municipal solid waste incinerator ¯y ash Kuen-Sheng Wang, Chang-Jung Sun *, Chung-Yu Liu Graduate Institute of Environmental Engineering National Central University Chung-Li, Taiwan, ROC Accepted 26 April 2000
Abstract The sintering process oers an opportunity to combine detoxi®cation and resource recovery for the treatment of municipal solid waste (MSW) incinerator ¯y ash. However, the chromium (Cr) in the sintered ¯y ash becomes more readily leachable with increasing sintering time and temperature, thus posing severe threats to the environment and human health when the sintered ash is recycled or reused. This study investigated the enhanced leachability of ¯y ash containing Cr, by heating the chromium (III) oxide (Cr2O3)-spiked ¯y ash to 800 C in atmospheres containing air, nitrogen gas (N2), and 5% H2+95% N2, respectively. The results indicated that trivalent chromium was converted to its soluble hexavalent form during sintering in the air atmosphere; whereas sintering in a nitrogen atmosphere signi®cantly reduced the leachability of Cr due to lack of oxygen (O2) to oxidize. The eects of the sintering temperature on the total chromium content and the leaching concentration in the toxicity characteristic leaching procedure (TCLP) extract are also discussed. # 2000 Elsevier Science Ltd. All rights reserved. Keywords: MSW; Incinerator; Fly ash; Sintering; Leachability; Atmosphere; Chromium
1. Introduction Limited land®ll sites and increasing disposal costs have hastened eorts to adopt incineration technologies and energy recovery strategies for managing municipal solid waste (MSW) in Taiwan. There will be a total of 36 municipal solid waste incinerators (MSWI) on this island by 2003, including 21 waste-to-energy incinerators, proposed in 1991 by Environmental Protection Administration (EPA), and an additional 15 incinerators adopting build-own-operate (BOO) or build-own-transfer (BOT) strategies [1]. These incinerators will be capable of processing 90% of the island's MSW with production of more than 2000 tons of incinerator residues each day, including 400 tons of hazardous ¯y ash to be further properly managed. The MSWI ¯y ash contains high levels of leachable heavy metals and salts, and is usually classi®ed as hazardous [2±4]. Various approaches have been used to solidify/stabilize the hazardous ¯y ash, which include acid*Corresponding author. Tel.: +886-3-436-1070, ext. 571; fax: +8863-456-3674. E-mail address: [email protected] (C.-J. Sun).
extraction, neutralization with exhaust gases, chemical ®xation, sintering and melting. Of these alternatives, sintering process has become promising because it combines an opportunity of detoxi®cation and recycling of the ash as construction materials [5]. This process can bond ¯y ash together by ®ring at high temperatures, resulting in sintered matrices with sucient strength and extremely low heavy metal leachability [6,7]. In general, the heavy metal leachability of the sintered MSWI ¯y ash is dependent on the heavy metal speciation formed during the sintering process. Moreover, the variation of speciation is then aected by the sintering atmosphere and sintering temperature [7]. Experimental laboratory studies have shown that the leachability of most heavy metals decreases with increased sintering time and sintering temperature [8]. However, the leachability of chromium (Cr) species shows an increase with increasing temperature. To ensure the safe disposal or utilization of the sintered MSWI residues, it is essential to have a better understanding of variations in chromium leaching behavior during sintering. This paper presents experimental results that help explain why the chromium species of the MSWI ¯y ash, such as Cr2O3, selected as a spike in this study, and the
0956-053X/00/$ - see front matter # 2000 Elsevier Science Ltd. All rights reserved. PII: S0956-053X(00)00041-6
86
K.-S. Wang et al. / Waste Management 21 (2001) 85±91
mechanism by which sintering parameters, mainly sintering time, temperature, and atmosphere, function to convert it into a readily soluble form. Experiments included spiking the MSWI ¯y ash with Cr2O3 to a concentration detectable by X-ray diraction (XRD) techniques [9,10], and sintering the spiked ash at 600±1000 C in various atmospheres such as air, nitrogen gas (N2) and 5% H2+95% N2. The conversion of Cr2O3 to a readily soluble form was con®rmed, and the eects of sintering atmosphere containing oxygen (O2) illustrated [11], providing useful information on reducing the leachability of ¯y ash containing Cr during sintering. 2. Materials Sintering tests were performed with cyclone ¯y ash collected from a mass burning MSW incinerator located in the northern part of the island. The incinerator is capable of processing 1350 tons of local MSW per day and is equipped with a set of air pollution control devices consisting of a cyclone, an adsorption reactor, and a fabric ®lter. The MSWI ¯y ash after collecting was homogenized, then oven dried at 105 C for 24 h. After being dried and cooled, it was again ground, homogenized, and ®nally, desiccated within a week. The samples used in this study were cylindrical specimens 1.5 cm high and 1 cm in diameter, obtained by compacting the above MSWI ¯y ash at 5000 psi (35 MPa). 3. Methods The sintering tests were performed in an electric-heated tubular furnace under various operational parameters such as ash composition, type of atmosphere, sintering temperature and sintering time, as shown in Table 1. Three kinds of ash compositions were used including MSWI ¯y ash, and ash spiked with 5% (w/w) of chromic oxide (Cr2O3) and chromium chloride (CrCl3), respectively. The types of atmosphere used in this study were air, N2, and 5% H2+95% N2. The sintering temperature ranged from 600 to 1000 C, and the sintering time from Table 1 Experimental processing conditions Sintering material
Atmosphere
Sintering temperature ( C)
Sintering time (min)
FAa FA FA FA FA+5% Cr2O3 FA+5% CrCl3
Air Air N2 5% H2+95% N2 Air Air
600, 800, 900, 1000 600, 800 600, 800 600, 800 800 800
5±240 60 60 60 5±240 5±240
a
FA=MSWI ¯y ash.
5 to 240 min. Depending on the purpose of the experiments, some experiments were conducted for a speci®ed sintering time and temperature, as listed in Table 1. All the samples were analyzed after sintering to determine their total chromium content and the chromium leaching concentration of the TCLP extract. The sintered samples were also analyzed using XRD techniques to determine the variation of the chromium species during the sintering process. The composition of MSWI ¯y ash, the heavy metal contents (Cd, Pb, Zn, Cu, and Cr), and the leaching concentrations of the TCLP extracts were analyzed using US EPA Method SW-846: the digestion for the total heavy metal content was conducted according to SW846 3050b; the digestion for hexavalent chromium (Cr+6), SW-846 3060A; the toxicity characteristic leaching procedure, SW-846 1311; the leaching concentrations for Cr+6, Pb, Cd, Cr, Cu, and Zn, 7196A, 7421, 7131A, 7191, 7211, 7951, respectively. 4. Apparatus The apparatus used in this study was composed of an electric-heated tube furnace for research purposes shown in Fig. 1. The heart of the furnace was a quartz tube burner, 100 cm long and 5.5 cm in inner diameter, housed in a glass®ber-liner insulated steel shell. A quartz boat moved by quartz rod was designed to feed the samples into the combustion chamber. The sintering temperature at the center inside the burner tube was monitored by a thermocouple and controlled by a programmed temperature controller. A quartz-®ber ®lter followed the heating chamber. To prevent the ¯ue gases from condensing while ¯owing through, the ®lter together with its holder was heated by a heating band to a temperature higher than 120 C. The ®lter eciency was 99.99% for the removal of 0.3-micron particles during the test conditions. A sampling train consisting of six impingers in a series followed by the ®lter was used. Multiple heavy metals and their compounds in the ¯ue gases were trapped using a modi®ed USEPA method 5 (MM 5 method) [12]. Accordingly, the ®rst impinger was left empty, while the second to the fourth impingers were ®lled with a combined solution of 5% HNO3 (nitric acid) and 10% H2O2 (hydrogen peroxide) for trapping most of the heavy metals and their compounds in the combustion gases. The ®nal impinger was ®lled with silica gel to remove the moisture content from the gases. 5. Results and discussion 5.1. Characterization of the ¯y ash The MSWI ¯y ash used in this study had a pH of 10.15 in 0.01 M calcium chloride (CaCl2) solution,
K.-S. Wang et al. / Waste Management 21 (2001) 85±91
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Fig. 1. Tube furnace and the sampling devices.
showing the basic nature of the ¯y ash. The loss on ignition was found to be less than 1.6%. The major chemical composition of the ¯y ash, as listed in Table 2, indicates that the major elements are silicon (Si), aluminum (Al), calcium (Ca) and chlorine (Cl). Table 3 summarizes the heavy metal content in the ¯y ash, showing higher concentrations of lead (Pb) and zinc (Zn) (1537 and 7815 mg/kg, respectively) in the MSWI ¯y ash. The concentration of Cd in the TCLP leachate exceeded the current US EPA regulatory threshold. In addition, Zn also showed a high leachability. Table 4 lists the major species found in the MSWI ¯y ash used Table 2 Major chemical compositions in MSWI ¯y ash Element
Concentration (%)
Element
Concentration (%)
Al Si Ca Na K
3.06 10.10 7.78 3.23 2.13
Fe Mg P C Cl
1.27 1.66 1.24 0.53 7.60
in this study. The species were compared with that identi®ed in incinerator ash generated by burning simulated wastes and various spiked chloride amounts [13]. In addition, the species were also compared with those predicted by the Thermal Dynamic Equilibrium Model (Chemical Equilibrium with Transport Properties, 1989; CET 89), developed by the National Aeronautics and Space Administration in 1976 [14]. 1976). The comparison of compounds formed in the ¯y ash during waste combustion suggests that one of the possible chromium compounds in the incinerator ash was Cr2O3. Fig. 2 identi®es the major compounds in the MSWI ¯y ash without spiking. No chromium compound was found by the XRD techniques due to its concentration being too low to be detected, though chromium was detected via the analysis of the total chromium content. 5.2. Chromium leachability of sintered ¯y ash in an air atmosphere Chromium-related compounds in the MSWI ¯y ash were evaluated by their residual content in the sintered
Table 3 Heavy metal contents and TCLP leachate concentrations of MSWI ¯y ash
Metal content (mg/kg) TCLP leachate concentration (mg/l) Regulatory thresholds a
Regulatory thresholds for sludge.
Pb
Cd
Cr
Cu
Zn
1537.25.7 0.530.07 5.0
122.70.7 2.270.08 1.0
309.93.0 1.350.07 5.0
1127.933.8 0.340.03 15.0a
7814.9227.9 14.291.0 25.0a
88
K.-S. Wang et al. / Waste Management 21 (2001) 85±91
Table 4 Major species in the MSWI ¯y ash as a function of chlorine content of the waste Model prediction
XRD identi®cation Fly ash
C2Cl4 (%) 0.8
Cd
Zn
Cr
Cu
Cd
Zn
Cr
Cu
Cd
Zn
Cr
Cu
KCl NaCl
KCl NaCl Zn ZnO
KCl NaCl Cr2O3
NaCl KCl Cu CuCl Cu2S
n.d.a KCl
KCl KCl ZnO
n.d. KCl
n.d. KCl
KO2 KO2
CrO3 CrO3
KCl
KCl NaCl
KCl NaCl
KCl NaCl CuCl
n.d.
ZnO KCl KO2 ZnO KCl KO2 ZnO
n.d. CuO Na2O K2O K2O CuO Cu2O
NaCl KCl FeO FeCl2 FeCl3
KCl NaCl Zn ZnO FeO Fe2O3 FeCl3
KCl NaCl Cr2O3 FeO Fe2O3 FeCl3
NaCl KCl Cu CuCl Cu2S FeO FeCl2 FeCl3
n.d. KCl
KCl KCl NaCl KCl NaCl
n.d. KCl NaCl KCl NaCl
n.d. KCl NaCl KCl NaCl CuCl
KO2 KO2 Fe2O3 Fe2O3
ZnO Fe2O3 Fe3O4 Fe2O3 Fe3O4
CrO3 CrO Fe2O3 CrO Fe2O3
n.d. CuO Fe2O3 CuO Cu Fe2O3
NaCl KCl
KCl NaCl Zn ZnO
KCl NaCl Cr2O3
NaCl KCl Cu CuCl Cu2S
n.d. KCl NaCl CdCl2 KCl NaCl
KCl KCl NaCl
n.d. KCl NaCl
n.d. KCl NaCl
KO2 NaCl
KCl NaCl ZnO
KCl NaCl
KCl NaCl CuCl
NaCl
ZnO KO2 KCl ZnO KO2 ZnO
CrO3 Cr2O3 CrO NaCl Cr2O3 CrO3
n.d. NaO2 CuO Cu2O NaCl CuO
KCl NaCl Zn ZnO
KCl NaCl Cr2O3
NaCl KCl Cu CuCl Cu2S
n.d. KCl NaCl
KCl KCl NaCl
n.d. KCl
n.d. KCl NaCl
KO2 KO2 KCl
ZnO KCl ZnO
n.d. K2O CuO
KCl
KCl NaCl
KCl NaCl
KCl NaCl CuCl
KCl
KCl ZnO
CrO3 Cr2O3 CrO CrO3 KCl
1.6 FeCl3 (%) 0 0.8 1.6
NaCl (%) 0
1.6 KCl (%) 0 0.8
KCl NaCl
1.6
a
Bottom ash
KCl NaCl
CrO3 Cr2O3
KCl KOH CuO
n.d., not detected.
ash and their concentration in the TCLP leachate. The risk posed by heavy metals in the sintered MSWI ash can not be assessed by the residual metal content of the sintered product alone since a greater residual content does not necessarily mean greater leaching concentration. The leachability is dependent on the solubility of the heavy metal-related compounds formed after sintering. In general, heavy metals and/or their compounds in the incinerator ash will evaporate, depending on their volatility during the sintering process. Accordingly, the heavy metal content tends to decrease with increasing sintering temperature for the targeted heavy metals such as Cd, Pb, and copper (Cu), as shown in Fig. 3. However, the chromium content in the sintered ash increases with elevated sintering temperatures. On the other hand, the leachability of Cd and Pb in the sintered ash showed a decreasing trend with increasing temperature. However, as indicated in Fig. 4, Cu and Cr leachability showed tendency to increase
with increasing temperatures, up to 900 C, followed by a decreasing trend with still higher temperatures. The high leachability of the chromium and copper compounds after sintering has raised environmental risk concerns as to thermal treatment for the resource recycling of incinerator ¯y ash. Based on the identi®cation of chromium compounds in incinerator ash by XRD techniques, and on the con®rmation of their presence by the thermal dynamic equilibrium model (CET 89), Cr2O3 was selected as a spike in this study, to investigate the leaching behavior of chromium after sintering, with a CrCl3 spike as a possible contrast. The variation in the total chromium content for Cr2O3 and CrCl3 spikes after sintering is shown in Fig. 5. The total chromium content in ¯y ash samples spiked with 5% (w/w) Cr2O3 (50,000 mg/kg) was found to be only 496 mg/kg before sintering, implying that Cr2O3 is almost wholly undigested by nitric acid, due to its slight
K.-S. Wang et al. / Waste Management 21 (2001) 85±91
89
Fig. 2. The major compounds in MSW incinerator ash identi®ed by XRD techniques.
Fig. 3. The heavy metal content of MSW incinerator ¯y ash after being sintered at various temperatures for 4 h.
Fig. 4. The heavy metal leaching concentrations in TCLP leachate from MSW incinerator ¯y ash sintered at various temperatures for 4 h.
solubility. However, after sintering for 20 and 240 min, the total chromium content increased to 9161.5 mg/kg and 25,865 mg/kg (see Fig. 5), respectively, suggesting that the thermal treatment of the ash signi®cantly enhance the digestibility of Cr. On the other hand, Fig. 6 shows that the chromium concentration of the TCLP leachate varies with time, as the ¯y ash was sintered at 800 C in an air atmosphere. The enhancement of chromium leachability after sintering in an air atmosphere was studied by analyzing which chromium compounds were formed after sintering. In experiments spiked with Cr2O3, the hexavalent chromium concentration and the total chromium concentration in the TCLP leachate were very similar. This suggests that the total chromium extracted by the TCLP test is primarily of the hexavalent form.
Comparatively, in experiments spiked with CrCl3, the extractable total chromium concentration showed an increase with increased sintering time; however, an extremely low or no hexavalent chromium concentration was noted. The results indicate that, during the sintering process, the Cr2O3 in the ¯y ash was converted to its more extractable hexavalent form. In related experiments, the CrCl3 from the ash was also converted to a soluble but not a hexavalent form. The hexavalent form of the chromium compound was identi®ed as the readily soluble K2CrO4 by XRD analysis of the ¯y ash before and after sintering, as shown in Fig. 7. The K2CrO4 peak increased while the Cr2O3 peaks decreased. However, in the case of CrCl3, no signi®cant peak, thus no crystalline form was identi®ed by the XRD analysis.
90
K.-S. Wang et al. / Waste Management 21 (2001) 85±91
Fig. 5. The Cr content of Cr2O3- and CrCl3- spiked MSW incinerator ¯y ash being sintered at 800 C.
all chromium oxides, a sintering atmosphere with or without oxygen is expected to signi®cantly aect the formation of hexavalent chromium. The total chromium contents of the ¯y ash, sintered in atmospheres of air, N2, and 5% H2+95% N2, are shown in Fig. 8. It was noted that at 800 C, the total chromium content tended to decrease as the sintering atmosphere varied from air to N2 or to 5% H2+95% N2. However, no signi®cant variation in the total chromium content was observed between the N2 and 5% H2+95% N2 atmospheres. On the other hand, in the experiments sintered at 600 C, the total chromium content showed a slight increase, as the atmosphere shifted from air to N2 or to 5% H2+95% N2. The increase was also con®rmed by sintering experiments conducted in an air atmosphere or a reduced-pressure air atmosphere, at 600 C, but the possible reasons still remain to be studied. On the other hand, the eects of sintering atmosphere on the leachability of chromium were demonstrated by comparing the chromium leaching concentration of ash sintered in an atmosphere of air and N2, as shown in
Fig. 6. The Cr concentrations in TCLP leachate from Cr2O3- and CrCl3-spiked MSW incinerator ¯y ash being sintered at 800 C.
Fig. 8. The Cr content of MSW incinerator ¯y ash after being sintered at dierent sintering atmospheres for 1 h.
5.3. Eects of sintering atmosphere on the leachability of chromium Previous experiments revealed that sintering in an air atmosphere converted Cr2O3 to its more extractable hexavalent form (K2CrO4). Since the major hexavalent chromium compounds, chromate and dichromate, are
Fig. 7. XRD patterns of MSW incinerator ¯y ash before and after sintering showing the conversion of Cr2O3 to K2CrO4.
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of ¯y ash containing chromium in an atmosphere without oxygen, such as an N2 atmosphere, and in a reducing atmosphere, can eectively prevent the oxidation of trivalent chromium to its more readily soluble hexavalent form. Acknowledgements
Fig. 9. The Cr concentration in TCLP leachate from MSW incinerator ¯y ash after being sintered at 600 and 800 C in dierent sintering atmospheres for 1 h.
Fig. 9. When sintered at 600 C in an air atmosphere, only a small amount of chromium was oxidized to its soluble or digestible form, resulting in a relative low leaching concentration for chromium, as compared to the case sintered at 800 C. Similar trends in leaching concentration were noted for atmospheres of N2 and 5% H2+95% N2, a slightly more reducing atmosphere. However, as the sintering temperature was raised to 800 C, the chromium leachability of the sintered ash was dramatically increased due to the conversion of chromium to it soluble form, such as K2CrO4, as described above. Yet, when the air atmosphere was replaced with N2 or 5% H2+95% N2, the chromium leaching concentration was signi®cantly reduced. The reduction can be adequately explained by no oxygen being supplied by the sintering atmosphere to combine with the chromium compounds to form soluble hexavalent chromium or other soluble forms, thus greatly reducing chromium leachability. However, a reducing atmosphere, created by replacing 5% N2 with H2, shows no signi®cant eects on decreasing the chromium leachability as compared to the N2 atmosphere. 6. Conclusions The sintering process was performed with the cyclone ash collected from a mass burning MSW incinerator for the combined purposes of heavy metal ®xation and resource recovery of sintered ash. However, the leachability of ash containing chromium compounds was enhanced by this thermal treatment, which could pose an environmental risk. In this study, the major chromium compound in the incinerator ash, Cr2O3, was spiked in order to investigate variations in chromium leachability during sintering. The experimental results revealed that the slightly soluble Cr2O3 was converted to K2CrO4, a readily soluble and more toxic hexavalent form. The pronounced conversion took place via the oxidation of trivalent chromium to its hexavalent form, at sintering temperatures higher than 800 C. The thermal treatment
The authors gratefully acknowledge the ®nancial support (under the grant NSC-87-2211-E008-007) of the Science Council of Republic of China in Taiwan and the editorial services of Mrs. Debbie. References [1] ROCEPA (ROC Environmental Protection Administration) Home Page. http://www.epa.gov.tw/burning/home.html (accessed April 1999). [2] Bruggen BV, Vogels G, Herck V, Vandecasteele C. Simulation of acid washing of municipal solid waste incineration ¯y ashes in order to remove heavy metals. Journal of Hazardous Materials 1998;57:127±44. [3] Chan C, Jia CQ, Graydon JW, Kirk D. The behavior of selected heavy metal in MSW incineration electrostatic precipitator ash during roasting with chlorinating agents W. Journal of Hazardous Materials 1996;50:1±13. [4] Eighmy TT, Eusden JD, Krzanowski JE, Domingo DS, Stamp¯i D, Martin JR, Erickson PM. Comprehensive approach toward understanding element speciation and leaching behavior in municipal solid waste incineration Electrostatic Precipitator Ash. Environmental Science & Technology 1995;29:629±46. [5] Jakob A, Stucki S, Kuhn P. Evaporation of heavy metals during the heat treatment of municipal solid waste incinerator ¯y ash. Environmental Science & Technology 1995;29:2429±36. [6] Wang KS, Chiang KY, Perng JK, Sun CJ. The characteristics study on sintering of municipal solid waste incinerator ashes. Journal of Hazardous Materials 1998;59:201±10. [7] Wang KS, Sun CJ, Yeh CC, Chiang KY. Sintering behavior and particle size in¯uence in MSW incinerator ash. In 91st Annual Meeting of Air and Waste Management Association (CD-ROM), San Diego, USA, 1998. [8] Winter RM, Mallepalli RR, Hellem KP. Determination of As, Cd, Cr, and Pb species form in a combustion environment. Combustion Science Technology 1994;101:45±58. [9] Lee CC. A Model Analysis of partitioning in a hazardous waste Incineration System. JAPCA 1998;38:941±5. [10] Perry R. H.and Green, D. W. McGraw-Hill Malaysia: Perr's Chemical Engineers' Handbook, 1984. [11] USEPA (US Environmental Protection Agency). Test methods for evaluating solid waste, SW-846, 1994. [12] Gordon S, McBride B. Measurement methodology for toxic metals from municipal waste combusts. In: Proceeding of the International Conference on Municipal Waste Combustion, vol. 2, 1989. p. 5C1±5C15. [13] Chiang K.Y. Speciation and partitioning metals aected by chloride content during a simulated municipal solid waste incineration process. Ph.D. Thesis, National Central University at Chung-Li, 1997. [14] NASA. Computer program for calculation of complex chemical equilibrium compositions, rocket performance, incident and re¯ected shocks, and Chapman-Jouguet detonations. NASA SP273. National Aeronautics and Space Administration, 1976.