Effects of Hydrothermal Treatment on the Major Heavy Metals in Fly

Nov 29, 2012 - In this work, the hydrothermal stabilization of heavy metals in fly ash collected from two types of MSWI plants is investigated. The co...
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Effects of Hydrothermal Treatment on the Major Heavy Metals in Fly Ash from Municipal Solid Waste Incineration Yu-qi Jin,* Xiao-jun Ma, Xu-guang Jiang, Hong-mei Liu, Xiao-dong Li, Jian-hua Yan, and Ke-fa Cen State Key Laboratory of Clean Energy Utilization, Institute for Thermal Power Engineering, Zhejiang University, Hangzhou 310027, People’s Republic of China ABSTRACT: Circulating fluidized bed (CFB) and stoker grate are the two major municipal solid waste incineration (MSWI) technologies currently being used in China. In this work, the hydrothermal stabilization of heavy metals in fly ash collected from two types of MSWI plants is investigated. The components and mineralogical phases of the fly ash are studied by means of X-ray fluorescence and X-ray diffraction spectrometry. The distribution patterns of Cr, Cu, Zn, and Pb in MSWI fly ash before and after the hydrothermal process are evaluated using a modified Bureau Communautaire de Référence sequential extraction procedure. In addition, the environmental risk is assessed using individual contamination factors and risk assessment codes. The results indicate the following: (1) In comparison to raw fly ash, the speciation distributions of Cr, Cu, Zn, and Pb from a hydrothermally treated CFB fly ash sample are more stable than those from a hydrothermally treated stoker grate fly ash sample. (2) Co-firing municipal solid waste and coal changes the characteristics of the fly ash and is beneficial to heavy metal stabilization using the hydrothermal process. (3) The two types of hydrothermally treated fly ash samples in this study are considered to pose little or no contamination risk to the environment.

1. INTRODUCTION The lack of availability of land space for new landfill sites in China means that municipal solid waste incineration (MSWI) is playing an increasingly important role in municipal solid waste (MSW) management.1 It has been estimated that, in 2010, there were 104 MSWI plants, with a combined daily incineration capacity of 84 940 tons, accounting for 18.8% of the total MSW treatment in 2010.2 The two major MSWI technologies currently being used in China use the circulating fluidized-bed (CFB) and stoker grate methods. The former is almost entirely based on domestic technologies. In contrast, the latter typically uses imported technology.1 In addition, the emissions of toxic materials from the MSWI, including persistent organic pollutants (POPs), heavy metals, etc., have attracted wide attention.3−6 The majority of those hazardous compounds in flue gas are usually enriched in fly ash; therefore, it has been classified as a hazardous waste because of its high heavy metal content and significant traces of toxic POPs. Therefore, MSWI fly ash must be treated before disposal in landfills or secondary use in accordance with stringent regulations.7 In recent years, hydrothermal processes have become a promising application for the treatment of coal fly ash. This is because, using hydrothermal crystallization under alkaline conditions, the treatment can convert the fly ash into more stable mineral forms.8−10 The action of the hydrothermal process on heavy metals in MSWI fly ash is a disputed issue, because the heavy metals could either be stabilized11−13 or extracted by the hydrothermal process.14,15 Generally, the toxicity and risk assessment of heavy metals in hydrothermally treated fly ash are specified using the toxicity characteristic leaching procedure (TCLP) or total contents. However, measurements of total metal content are a poor indicator of metal mobility, toxicity, bioavailability, etc., and the TCLP does © 2012 American Chemical Society

not provide any insight into speciation distribution of heavy metals. Sequential extraction procedures are widely used for the evaluation of availability and mobility of heavy metals in soils, sediments, and fly ash.16−19 Considering the diversity of procedures and lack of uniformity in different sequential extraction methods, a standardized sequential extraction method was adopted by Ure et al.,20 which was proposed by the Bureau Communautaire de Référence (BCR, the European Commission’s Bureau of Reference). To the best of our knowledge, the speciation of heavy metals before and after hydrothermal stabilization in MSWI fly ash has been little reported in the literature.14 Previous results indicate that the hydrothermal products from MSWI fly ash collected from CFB incineration are stabilized with respect to trace heavy metals.13 In the current work, two types of MSWI fly ash samples, collected from CFB (co-firing with MSW and auxiliary coal) and stoker grate incineration facilities, were treated by a hydrothermal process. Subsequently, the fractions of the heavy metals (Cr, Cu, Zn, and Pb) present and their redistribution after the hydrothermal treatment were investigated with the aid of the BCR sequential extraction process. Finally, the degree of potential fly ash environmental contamination was computed in terms of individual contamination factors (ICFs) and risk assessment codes (RACs).

2. METHODS AND PROCEDURES 2.1. Sampling and Hydrothermal Treatment. One fly ash sample was collected from a CFB incineration plant in Changxing, Zhejiang province, China (henceforth referred to as “CFB fly ash”). Received: July 20, 2012 Revised: November 28, 2012 Published: November 29, 2012 394

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The plant co-fires a total daily capacity of 600 tons of MSW and coal (5:1 weight ratio of MSW/coal). It is equipped with an air pollution control (APC) system consisting of a semi-dry scrubber, an activated carbon injection, and a fabric filter. Another fly ash sample (“stoker grate fly ash”) was collected from a stoker grate incineration plant in Hangzhou, Zhejiang province, China, with a total daily capacity of 400 tons of MSW combusted directly. Its APC system consists of a dry scrubber, an activated carbon injection, and a fabric filter. Both samples used were obtained from the baghouse filter. A 316L stainless-steel autoclave of 1 L capacity was employed for hydrothermal treatment. Batches of 50 g of fly ash were prepared and then mixed with doubly deionized water in a liquid/solid ratio of 10 mL/g. The pressure inside the autoclave changed along with the temperature in the range of 0−10 MPa. The heating rate used was 3.0 °C min−1 until 150 °C, at which temperature it was then held for 12 h. After the hydrothermal treatment, the autoclave was cooled to atmospheric conditions. Then, the suspension was centrifuged to separate the liquid and solid (referred to as “hydrothermally treated fly ash”), which was dried at 105 °C for 24 h. 2.2. Reagents and Instrumentation. Doubly deionized water was used to prepare all solutions and for all dilutions. The acetic acid (CH3COOH), hydroxylammonium chloride (H2NOH·HCl), hydrogen peroxide (H2O2), ammonium acetate (CH3COONH4), HF, HClO4, and HNO3 used were all of reagent grade. All of the glassware and plastic vessels were treated with dilute (1:1) HNO3 for 24 h and then rinsed with distilled water before use. The major elements in the fly ash before and after the hydrothermal process were determined using an X-ray fluorescence (XRF) spectrometer (ThermoFisher, IntelliPower 4200). The mineralogy was determined using X-ray diffraction (XRD, Rigaku Rotaflex) using Cu Kα radiation at 40 kV and 250 mA. The concentration of heavy metals in the extracted solutions was measured using inductively coupled plasma mass spectrometry (ICP−MS, Thermo Scientific XII). For the sequential extraction, a horizontal mechanical water bath shaker (THZ-82, Zhenjiang) was employed. A centrifuge (TD5M-WS, Shanghai, China) was used for separation of the solid phase from the extraction liquid. 2.3. BCR Sequential Extraction Procedure. A modified BCR sequential extraction procedure was followed to obtain information on the speciation distributions of heavy metals in the fly ash.20,21 The extraction reagents and the residue fractions targeted are shown in Table 1. The residues from the fourth step were digested with HF/

heavy metals removed in each step of the procedures to the results of the pseudo-total digestion. The recovery of the sequential extraction method was calculated as follows:

recovery (%) = 100 × (Cstep 1 + Cstep 2 + Cstep 3 + Cresidue) /C pseudo‐total digestion

3. RESULTS AND DISCUSSION 3.1. Characteristics of Fly Ash. The chemical compositions of fly ash samples, with and without the hydrothermal process, as determined by XRF are listed in Table 2. In the original fly ash, CaO sprayed in the scrubber systems induces a high content of CaO and a high pH value in both types of the ash. The high concentrations of SiO2 and Al2O3 in the original CFB fly ash sample are due to the MSW and coal co-firing in the CFB incineration system. Among the contents of the heavy metals determined by XRF, only ZnO and PbO are present in significant amounts, comprising 0.4 and 0.2%, respectively. Of the non-metallic elements, Cl is present in the largest quantity, i.e., 3.9%. In contrast, the composition of SiO2 and Al2O3 in the original stoker grate fly ash sample was smaller at 8.3 and 2.3%, respectively. Cl is present in a high concentration (23.5%) and results from the formation of soluble salts, such as NaCl, KCl, etc., because of high Na and K contents. Similarly, ZnO and PbO are present in significant amounts, accounting for 0.9 and 0.3%, respectively. After treatment, the percentage of Na, K, and Cl decreased in both types of hydrothermal-treated fly ash samples, which may be attributed to dissolution of the salts during the treatment. However, the percentage decrease caused by dissolution of these salts is greater in the hydrothermal-treated stoker grate fly ash than that in the CFB fly ash. The contents of other components in the hydrothermal-treated CFB fly ash sample do not change significantly, while those in the stoker grate sample generally increase. XRD analyses of the two types of fly ash samples, with and without the hydrothermal process, were carried out to obtain detailed information regarding the mineral composition of the fly ash (Figure 1). When the XRD results of original fly ash samples are compared, all fractions have high Ca and soluble salt contents. The feed coal (CFB fly ash) is quite rich in quartz. However, there is only a little quartz in the stoker grate fly ash. After the hydrothermal process, soluble salts decrease for both of the two types of hydrothermal-treated fly ash. Aluminosilicate mineral [Na8(Al6Si6O24)] formed only in the hydrothermal-treated CFB fly ash. However, the intensities of the other peaks were comparable in the hydrothermal-treated stoker grate fly ash to those in the raw fly ash. 3.2. Effect of the Hydrothermal Treatment. 3.2.1. Distribution of Heavy Metals in the CFB Fly Ash Sample. After the hydrothermal treatment, the mass loss is small. The average concentration of heavy metals in four CFB fly ash samples determined using the BCR method before and after the hydrothermal treatment is listed in Table 3. The amount of heavy metals in raw fly ash and hydrothermal-treated fly ash is compared in Figure 2. The sums of the four fractions are in good agreement with the total digestion results, with satisfactory recoveries (93.6−100.6%). For the original fly ash, the dominant fraction of Cr, Cu, Zn, and Pb is residual, accounting for 60.2, 84.6, 83.5, and 96.6%,

Table 1. Reagents and BCR Three-Stage Sequential Extraction Scheme extraction step

extraction reagent and method

first step

HOAc (0.11 mol/L), pH 2.85, 16 h

second step third step

NH2OH·HCl (0.1 mol/L), pH 2, 16 h

residual

30% H2O2 (8.8 mol/L), 2 h at 85 °C, and then NH4OAc (1.0 mol/L) at pH 2, 16 h mix acid HF/HNO3/HClO4

(1)

nominal target phase(s) exchangeable/ carbonates oxides Fe/Mn organic matter and sulfides metals bound in lithogenic minerals

HNO3/HClO4, and the pseudo-total metal digestion of the samples was performed in a similar way. For each extraction, separation was achieved by centrifuging at 3000 rpm for 15 min and filtering the supernatant liquid through 0.45 μm of glass fiber filter. The extracts were stored in polypropylene tubes and kept at 4 °C before measurement. Finally, the heavy metal concentrations in each fraction in the supernatant liquid were analyzed by ICP−MS. The reproducibility of the BCR sequential extraction procedure was checked by subjecting triplicate samples to each procedure. In addition, an internal check was performed on the results of the sequential extraction by comparing the sum of the amounts of the 395

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Table 2. Chemical Compositions of the Raw Materials in Two Kinds of Fly Ash (%) stoker grate raw fly ash

CFB

hydrothermally treated fly ash

raw fly ash

hydrothermally treated fly ash

constituent

weight (%)

standard error

weight (%)

standard error

weight (%)

standard error

weight (%)

standard error

CaO Cl Na2O SiO2 K2O SO3 Al2O3 MgO Fe2O3 ZnO PbO CuO Cr2O3

34.99 23.47 11.94 8.28 7.24 5.34 2.31 2.27 1.08 0.86 0.30 0.08 0.02

0.24 0.21 0.16 0.14 0.13 0.11 0.08 0.07 0.05 0.04 0.02 0.004 0.001

53.01 6.37 7.33 14.79 2.14 2.71 4.05 3.96 1.58 1.25 0.44 0.13 0.03

0.25 0.12 0.13 0.18 0.07 0.08 0.10 0.10 0.06 0.06 0.02 0.006 0.002

26.80 3.85 2.21 34.0 1.79 2.54 16.29 3.11 4.45 0.41 0.21 0.09 0.05

0.22 0.10 0.07 0.24 0.07 0.08 0.18 0.09 0.10 0.02 0.01 0.004 0.002

20.29 0.73 1.52 36.6 1.17 0.90 15.89 2.58 3.42 0.32 0.17 0.07 0.03

0.20 0.03 0.18 0.24 0.05 0.05 0.18 0.08 0.09 0.02 0.01 0.004 0.002

Figure 1. XRD patterns from (a) raw and (b) hydrothermal-treated CFB fly ash. Similar XRD patterns for stoker grate fly ash are shown for (c) raw ash and (d) treated ash.

exchangeable/carbonate fraction first dissolved into the water and were then precipitated, because the hydrothermal treatment involves solutions with high pH values. When aluminosilicate minerals are formed during the treatment, they can become attached to those precipitates, so that finally the precipitates are physically encapsulated in those minerals.12,13 In addition, metals that bind to organic and/or inorganic compounds in the fly ash are released and precipitated. As a result, a large proportion of those heavy metals are transformed into the residue fraction after the hydrothermal treatment. This indicates that the hydrothermal

respectively. This is due to the formation of minerals in the fly ash (such as glass phase, magnetite, melilite, and silicates), which occur because coal is rich in Si and Al. This is especially so for low-volatile metals, which are preferentially incorporated into special mineralogical forms at the incineration temperatures used (850−1000 °C).12 As seen in Table 3 and Figure 2, heavy metals in the ash transform from the first three steps into the residue fraction after treatment. This process may be attributed to the formation of aluminosilicate minerals in the hydrothermal process (in Figure 1). Most of the heavy metals in the 396

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Table 3. Average Concentration of Heavy Metals in Four CFB Fly Ash Samples Determined Using the BCR Method before and after Treatment raw fly ash (mg kg−1) heavy metals first step second step third step residual three steps + residual pseudo-total recovery (%)

Cr 7.75 2.02 15.97 39.00 64.74

± ± ± ± ±

Cu 0.54 0.09 1.28 3.66 1.59

64.33 ± 3.43 100.63

1.76 28.70 1.76 177.56 209.79

± ± ± ± ±

hydrothermally treated fly ash (mg kg−1)

Zn 0.31 2.28 0.16 6.89 3.14

218.38 ± 2.39 96.07

60.23 81.23 9.81 765.34 916.61

± ± ± ± ±

Pb 7.90 2.88 0.51 26.46 11.76

978.74 ± 25.11 93.65

3.14 18.03 4.35 726.55 752.35

± ± ± ± ±

Cr 0.54 0.07 0.53 35.46 17.54

792.10 ± 10.74 94.98

0.74 0.68 8.73 46.07 56.21

± ± ± ± ±

Cu 0.50 0.07 1.18 4.41 1.96

51.67 ± 6.64 108.78

1.21 12.56 0.60 176.36 190.73

± ± ± ± ±

Zn 0.12 1.63 0.08 21.33 10.38

205.89 ± 2.13 92.63

0.41 20.98 2.64 811.67 835.70

± ± ± ± ±

Pb 0.15 2.75 0.31 40.74 19.87

909.92 ± 16.02 91.84

0.28 7.65 4.62 677.66 690.22

± ± ± ± ±

0.16 0.55 0.99 50.34 24.89

749.87 ± 12.94 92.05

Figure 2. Comparison of the amount of heavy metal in the CFB fly ash sample before and after treatment for (a) raw fly ash and (b) hydrothermal-treated fly ash.

Figure 3. Comparison of the amount of heavy metal in the stoker grate fly ash sample before and after treatment for (a) raw fly ash and (b) hydrothermal-treated fly ash.

process has stabilized the heavy metals in the fly ash collected from the CFB co-firing of coal and MSW. 3.2.2. Distribution of Heavy Metals in the Stoker Grate Fly Ash Sample. Average concentrations of heavy metals in four stoker grate samples were determined using the BCR method before and after treatment, and they are listed in Table 4; a comparison of the amount of heavy metals in the raw fly ash and the hydrothermal-treated fly ash is shown in Figure 3. Again, the sums of the four fractions are in good agreement with the total digestion results (with recoveries of 71.3− 115.1%). Approximately 50% of the mass is lost for the stoker grate fly ash after hydrothermal treatment, which is larger than that for CFB fly ash. This is probably due to the dissolution of soluble inorganic compounds and decomposition of organic compounds in the ash.14 The decrease in the quantities of soluble

salts analyzed by XRF and XRD in the stoker grate raw fly ash (in Table 2 and Figure 1) agrees with this phenomenon. Moreover, some of the heavy metals leach into the liquid during the hydrothermal treatment. In addition, large mass loss during the hydrothermal process results in some parts of the total metal content in the hydrothermal-treated fly ash being higher than those in the raw fly ash, especially for Pb. Cr is extracted predominantly in the third step of the BCR method for both the raw fly ash and hydrothermal-treated fly ash. The proportion of the first three fractions somewhat decreases, and the residual fraction increases after the hydrothermal process. Copper shows a marked presence in forms that are bound to the Fe−Mn oxides in fly ash before and after treatment. The solubility and mobility of Cu is therefore controlled by the Fe− Mn oxides, which are easy to reduce via chemical reaction. This

Table 4. Average Concentration of Heavy Metals in Four Stoker Grate Fly Ash Samples Determined Using the BCR Method before and after Treatment raw fly ash (mg kg−1) heavy metals first step second step third step residual three steps + residual pseudo-total recovery (%)

Cr 1.55 3.29 12.79 15.70 33.33

± ± ± ± ±

Cu 0.13 0.17 1.24 3.79 1.72

38.85 ± 2.26 85.80

2.65 195.15 4.59 103.73 306.12

± ± ± ± ±

hydrothermally treated fly ash (mg kg−1)

Zn 0.09 9.18 4.77 6.67 3.85

332.49 ± 63.91 92.07

11.47 823.47 23.62 1474.91 2333.47

± ± ± ± ±

Pb 0.17 16.80 20.77 78.89 34.33

2539.08 ± 100.97 91.90

158.29 66.77 60.75 232.62 518.43

± ± ± ± ±

Cr 9.44 5.81 31.44 30.58 13.59

551.58 ± 34.42 93.99 397

1.46 3.89 15.26 25.72 46.33

± ± ± ± ±

Cu 0.16 0.21 0.51 2.55 1.14

40.25 ± 6.20 115.09

1.74 202.11 7.29 139.48 350.61

± ± ± ± ±

Zn 0.06 2.06 0.67 7.88 3.57

293.15 ± 6.87 119.60

1.57 912.80 101.82 1737.32 2753.50

± ± ± ± ±

Pb 1.07 14.56 8.03 52.75 23.10

2764.96 ± 59.73 99.59

14.38 31.19 50.65 568.03 664.26

± ± ± ± ±

10.45 2.97 1.49 10.93 4.92

931.22 ± 32.11 71.33

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agrees with the copper distribution in the MSWI fly ash studied by Wang et al.22 However, Cu is mainly bound to sulfides or organic matter in nature.23 This work shows that both incineration and hydrothermal treatment can change the form of distribution of the copper present. In addition, the speciation distribution of Cu has no obvious change after treatment compared to that in the raw fly ash. Zinc is extracted predominantly in the reducible and residual fractions. The speciation distribution of Zn is similar to that of Cu. The reducible fraction is about 35.3 and 33.2% in ash before and after treatment, respectively. The fraction of the first and the third steps is less than 4%. The oxidizable fraction increases from 1.01 to 3.70 mg kg−1 after hydrothermal treatment. Zn found in these phases is expected to be mainly adsorbed on solid phases, such as hydrated oxides of Fe and Mn, clay, and humic acids.24 The concentration of Pb in the original fly ash in the exchangeable/carbonate fraction is 30.5%. However, after the hydrothermal process, the concentration of Pb in the first three steps decreases (especially in the first), while the concentration in the residual fraction increases. The high leaching concentration of Pb (5.29 mg/L) in the remaining liquid indicates that parts of the Pb in the first three steps leaches into the water during the hydrothermal process. This finding is in good agreement with the fact that Pb easily leaches out of the municipal incinerator “reaction products”, which have a high pH value.25 3.3. Environmental Implication. The mobility and immobility of heavy metals in fly ash as well as the corresponding toxicity greatly depend upon the types of binding involved. Fraction 1 consists of exchangeable metals and those soluble in water or slightly acidic conditions. This fraction has the most labile bond to the waste and, therefore, is the most bioavailable and dangerous to the environment. Fractions 2 and 3 can also pose a threat depending upon environmental conditions. Fraction 2 is associated with Fe and Mn oxides that can be released if conditions change from an oxic to an anoxic state. Fraction 3 is associated with sulfides and organic matter that may be released under oxidizing conditions. Finally, fraction 4 corresponds to those metals strongly associated with the crystalline structures of minerals, which are therefore unlikely to be released from the waste.26,27 Assuming that bioavailability is related to solubility, then metal bioavailability decreases in the order of exchangeable forms > acid reduction forms > organic forms > residual forms. The residual phase represents metals largely embedded in the crystal lattice of the fly ash and should not be available for remobilization, except under very harsh conditions.28 3.3.1. ICFs. In this work, to characterize the mobility of heavy metals in the MSWI fly ash before and after the hydrothermal process, the ICFs of the heavy metals are used to express the level of environmental contamination. The ICF is calculated by dividing the sum of the first three extractions (including exchangeable/carbonates and reducible and oxidizable organic forms) by the residual fraction.28 Table 5 compares the ICF values for fly ash samples from different incinerator technologies before and after hydrothermal treatment. The percentage of the metals in terms of relative abundance in the mobile phase of the raw fly ash and hydrothermal-treated fly ash are in the following order based on ICF values. For the CFB raw fly ash

Table 5. Comparison of the ICF Values for Fly Ash Samples from Different Incinerator Types before and after Hydrothermal Treatment incinerator types CFB stoker grate

raw fly treated raw fly treated

ash fly ash ash fly ash

Cr

Cu

Zn

Pb

0.66 0.22 1.12 0.80

0.18 0.08 1.95 1.51

0.20 0.03 0.58 0.58

0.04 0.02 1.23 0.17

The Cr contents based on the average values in the first and third fractions (7.75 and 15.97 mg/kg, respectively) are found to be higher than in the second fraction. However, Cu, Zn, and Pb, which are mainly in the second fraction (13.68, 8.86, and 2.43 mg/kg, respectively), are dominated in the mobile phase. For the hydrothermally treated CFB fly ash Cr (0.22) > Cu (0.08) > Zn (0.03) > Pb (0.02)

Cr in the exchangeable/carbonate and reducible fractions decreases significantly. However, the main remaining part is the oxidizable fraction (8.73 mg/kg). A comparison of the oxidizable and reducible fractions from the BCR extraction process indicates that Cr is in the organic fraction and Cu, Zn, and Pb are in the Fe−Mn oxide fraction in the CFB ashes before and after treatment. In addition, the residual concentrations of Cr, Zn, Cu, and Pb show obvious improvement after treatment, indicating that the non-mobile fraction increased, leading to a decreased possible risk to the environment from these metals after treatment. Table 5 indicates that the highest ICFs were obtained for Cu and Pb in the original fly ash collected from stoker grate incineration. The percentage of the metals in terms of the relative abundance in the mobile phase of the stoker grate fly ash before and after treatment based on ICF values are in the following order. For the raw fly ash collected from stoker grate incineration Cu (1.95) > Pb (1.23) > Cr (1.12) > Zn (0.58)

The mobility of each metal in the stoker grate fly ash is higher compared to those in CFB fly ash. The mobility order is also different from that in CFB fly ash. Because MSW is added to the stoker grate incinerator for direction combustion, the components of stoker grate fly ash are significantly different from CFB fly ash. Therefore, the yield of aluminosilicate minerals is small during the stoker grate incineration process. This has the result that the residual fraction decreases during the BCR process. The higher ICF values also explain this phenomenon. For the hydrothermally treated stoker grate fly ash Cu (1.51) > Cr (0.80) > Zn (0.58) > Pb (0.17)

After treatment, the percentage of the mobility phases is reduced. However, the effect of the treatment on the stabilization of heavy metals is not obvious compared to that for CFB fly ash. This result also indicates that the physical and chemical properties of MSW fly ash, especially with respect to Si and Al contents, have significantly influenced the stabilization of heavy metals during the hydrothermal process. Also, the stabilization efficiency is different for the various types of metal. Considering the change in the ICF value after treatment, the order of the stabilization effect on the metals (from the most to the least) is Pb > Cr > Cu > Zn.

Cr (0.66) > Zn (0.20) > Cu (0.18) > Pb (0.04) 398

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Table 6. Comparison of the RAC Values for Fly Ash Samples from Different Incinerator Types before and after Hydrothermal Treatmenta incinerator types CFB stoker grate

raw fly treated raw fly treated

Cr ash fly ash ash fly ash

Cu

M (11.97) L (1.31) L (4.65) L (3.16)

N N N N

Zn

(0.84) (0.63) (0.86) (0.50)

L N N N

(6.57) (0.05) (0.49) (0.06)

Pb N (0.42) N (0.04) H (30.53) L (2.17)

a

N,