Characteristics of ashes from different locations at the MSWincinerator equipped with various air pollution control devices

Geum-Ju Songa, Ki-Heon Kima, Yong-Chil Seoa,*, Sam-Cwan Kimb

aDepartment of Environmental Engineering, Yonsei University, Yonsei Institute of Environmental Science & Technology, Wonju 220-710,

South KoreabNational Institute of Environmental Research, Inchon 404-170, South Korea

Accepted 7 April 2003

Abstract

The characteristics of ashes from different locations at a municipal solid waste incinerator (MSWI) equipped with a water spraytower (WST) as a cooling system, and a spray dryer adsorber (SDA), a bag filter (BF) and a selective catalytic reactor (SCR) as airpollution control devices (APCD) was investigated to provide the basic data for further treatment of ashes. A commercial MSWI

with a capacity of 100 tons per day was selected. Ash was sampled from different locations during the normal operation of theMSWI and was analyzed to obtain chemical composition, basicity, metal contents and leaching behavior of heavy metals. Basicityand pH of ash showed a broad range between 0.08–9.07 and 3.5–12.3, respectively. Some major inorganics in ash were identified

and could affect the basicity. This could be one of the factors to determine further treatment means. Partitioning of hazardousheavy metals such as Pb, Cu, Cr, Hg and Cd was investigated. Large portions of Hg and Cd were emitted from the furnace whileover 90% of Pb, Cu and Cr remained in bottom ash. However 54% of Hg was captured by WST and 41% by SDA/BF and 3.6%was emitted through the stack, while 81.5% of Cd was captured by SDA/BF. From the analysis data of various metal contents in

ash and leach analysis, such capturing of metal was confirmed and some heavy metals found to be easily released from ash. Basedon the overall characteristics of ash in different locations at the MSWI during the investigation, some considerations and sugges-tions for determining the appropriate treatment methods of ash were made as conclusions.

# 2003 Elsevier Ltd. All rights reserved.

1. Introduction

Because of growing urbanization and industrializa-tion, the amount of municipal solid wastes (MSWs) hasincreased very rapidly and their compositions are verycomplex. Therefore, various treatment methods forMSWs have been investigated and utilized. At the endof 1999, the main portion of waste in Korea had beentreated with landfill (52%) but there have been manyproblems such as acceptance at landfill sites, shortageand limits for land usage. In particular, the governmentwas faced with treatment problems due to the odor andleachate at sites. The most preferable method of treat-ment for the MSWs is recycling and the next soundmethod could be the volume-reduction of MSWs byincineration. The MSWs were generated at a rate of

4600 tons/day in 1999. In Korea the waste generated byMSWs was treated as follows, 52% of the waste wastreated as landfill, 38% was recycled and 10% wasincinerated (Ministry of Environment, 2000a). Theincinerated MSWs amounted to 1,083,854 tons/yearand the ash generated amounted to 177,870 tons/year.Ninety percent of the total was bottom ash and 10%was fly ash (Ministry of Environment, 2000b). Usually,the fly ash is transported and dumped into speciallandfill sites with consideration as hazardous wastewithout any tests and the bottom ash is transported anddumped into landfill sites based on the results of leach-ing tests, when the leaching value of Pb did not excee-ded the regulatory limit value. However they currentlyare not being sent to landfill sites because the leachingvalue of Pb in bottom ash has sometimes exceeded theregulatory limit value (Ministry of Environment,2000c).Finally the bottom ash is piling up at incinerator

facilities in Korea and has caused other sources of

0956-053X/03/$ - see front matter # 2003 Elsevier Ltd. All rights reserved.

doi:10.1016/S0956-053X(03)00073-4

Waste Management 24 (2004) 99–106

www.elsevier.com/locate/wasman

* Corresponding author. Tel.: +82-33-760-2438; fax: +82-33-763-

5224.

E-mail address: seoyc@dragon.yonsei.ac.kr (Y.-C. Seo).

environmental problems. The final management of ashdepends on site-specific factors, regulatory requirementsand further utilization or disposal options. In the USA,the predominant MSWI residue stream can be accoun-ted for as a combined ash, that is, a mixture of bottomash, grate siftings and APC residue, and this residue isusually dumped into landfills. Some European countriesare separating major streams of ash into bottom ashand APC residue for further management and treat-ment. About 50% or more of treated MSWI bottom ashis used as secondary building material or for similarpurposes, in road sub-bases and the construction ofembankments, wind and noise barriers and other civilengineering applications (Hjelmar, 1996; Wiles, 1996;Chimenos et al., 2000).The method for the applications is derived from the

research of chemical, physical and mineralogical char-acteristics of each ash. Chimenos et al. (1999) havereported that the major components in bottom ash weremainly SiO2, CaO, Al2O3 and SiO2 increased as theparticle size of bottom ash increased but CaO increasedas the particle size of bottom ash decreased. The heavymetals are mainly found in the fine fraction. Also, theinternational ash working group has also reported thatthe major components in heat recovery system ash aremainly SiO2, SO3, K2O, CaO and that the major com-ponent in SDA/BF ash is mainly CaCl2. Generally,APCD residues contain a much higher proportion ofsoluble components than bottom ash. Water solubilityis a very important property of APCD residues becauseit strongly influences the options available for treat-ment, disposal and possible utilization of the residues(International Ash Working Group, 1997).The chemical elements in the incineration are classi-

fied as followed according to the theory of Miguel et al.(1992). (1) The elements with a high boiling point thatare not volatilized in the combustion area and comprisethe matrix of bottom ash. (2) The elements volatilizedduring combustion, of which small amounts remain inthe bottom ash. These metals condense on the surface ofthe fly ash particles when the combustion gas streamcools down. (3) The elements remaining in the gas phasethroughout the entire process. These metals undergovolatilization but not condensation. (4) The elementshaving a combination of two or more of the threeclasses.It is reported by Johnson et al. (1996) that the leach-

ing of contaminants depends on a combination of phy-sical and chemical parameters that cannot be discernedfrom a single leaching test. Calcium compounds play amajor role in controlling solution chemistry both inMSWI bottom ash and fly ash. Also, Akiko et al. (1996)have reported that the leaching properties of heavymetals from incinerator residues depend internally onthe composition and chemical states of the metalsthemselves and externally in relation to the leaching

environment on pH, ionic strength, oxidation/reductionpotential of the contact solvent or liquid/solid ratio.In this study, the chemical compositions, character-

istics of metal content and leaching behavior of heavymetal of ashes from different locations in the incinera-tion process (which was in normal operation on a com-mercial basis) were investigated to help to choose anyappropriate methods for further treatment and disposal.Therefore, the basic data for the recycling and treatmentof ash through the analysis of characteristics are intro-duced. Based on the results of investigation, some sig-nificant factors for further treatment of ash weresuggested as conclusions.

2. Materials and methods

2.1. Facility used

The facility used in this study is a commercial scaleMSWI having a treatment capacity of 100 tons/day,equipped with a traveling grate (grade 0 degree), whichconsists of continuous metal belt conveyors and movesalong the longitudinal axis of a furnace. The air pollu-tion control devices (APCD) are composed of a SDA, aBF and a SCR. As a cooling system it also has a waterspray-cooling tower (WST) and an air pre-heater. TheMSWI is designed and featured as follows: combustionrate of combustion zone is maximum 185kg/m3.h, heatload factor in combustion zone is 508,573 kJ/m3/h,retention time of raw gas and waste in combustion zoneis 2.0 s and 40 min, respectively. Fig. 1 shows a flowdiagram of the MSWI process. Table 1 shows the con-centration of pollutants in the different locations andTable 2 shows emission limit values of pollutants fromMSWI in Korea. The properties of incinerated MSW inthis study are shown in Table 3.

2.2. Sampling and analysis

Ashes from different streams in the MSWI were sam-pled to analyze the following characteristics accordingto the available standard methods. Ashes were collectedfrom the bottom ash pit and each collection hopper ofthe air pollution control devices, as marked ‘‘S’’ inFig. 1.

2.2.1. Chemical compositionsThe bottom ash was classified into various sizes of

diameter and dried at a temperature of 100–110� for aduration of 24 h. The fly ash collected from differentlocations was also dried under the same conditions asbottom ash. After drying, the bottom ash was pulver-ized to a size of <300 mm and fly ash were screenedthrough a size of 300 mm. The chemical compositions ofash were analyzed with X-ray fluorescence (XRF). The

100 G.-J. Song et al. /Waste Management 24 (2004) 99–106

analyzer used was a Philips PW-2400 and the quantita-tive method was Pellet and Uni Quant.

2.2.2. Metal contentThe classified and dried ash was digested and ana-

lyzed according to EPA Method 3050B. The analyzerused was an inductively coupled plasma/mass spectro-metry (ICP/MS, model: Elan-6000). Mercury was ana-lyzed with an atomic absorption auto mercury analyzer(NIC co. Ltd., model SP-3D) in 253.7 nm.

2.2.3. Leaching test of heavy metalThe leaching test of ash was carried out according to

the Korean Standard Testing Method for solid waste.The Korean Standard Leaching Test (KSLT) uses aleachate value of pH 5.8–6.3, solid weight and leachatevolume ratio is 1:10 (W:V), leaching time is 6h and itagitates horizontally with 200 rpm. The leachate is

filtered with 1-mm glass fiber filter. The analysis of heavymetals utilized ICP/MS.

3. Result and discussion

The amount of bottom ash generated from the incin-erator was about 16% of incinerated waste and that offly ash collected from different air pollution controldevices was 1.2% of incinerated waste. Various analyseswere performed as introduced methods.

3.1. Chemical composition and basicity of different ashes

Table 4 shows the result of XRF analysis for the ashesfrom different locations and different sizes of bottomash. Chemical compositions of ash and the basicity ofash are shown in Table 4. The quantity of oxide mate-rial is estimated with the total amount of analyzed ele-ment and basicity of ash calculated from the ratio ofCaO and SiO2. SiO2 was a major compound in bottomash and showed the least amount in the ash from BF.Only a small amount of Cl was indicated in bottom ashand a large amount in the ash from BF and SDA. Fromthese results one could infer that the air pollution con-trol devices removed a significant portion of metal and

Table 1

Concentration of pollutants in the different locations of MSWI

Location

Temp.

(�C)

O2(%)

CO2(%)

COa

(ppm)

NOx

(ppm)

SOxa

(ppm)

Water

(%)

Hg

(mg/Nm3)

Dusta

(mg/Nm3)

Outlet of combustor

878.4 11.8 6.6 6.0 131.1 3.9 17.7 81.43 4267.6

Outlet of WST

246 11.9 7.6 1.0 130 9.5 37.6 35.07 1435.6

Stack

196 11.8 9.1 10.6 28.8 1.7 34.9 5.46 2.8

a Calculated by the basis of 12% O2.

Table 2

Emission limit value of gas phase pollutants from MSWI in Korea

(2002) (Unit: ppm)

NH3

CO HCl Cl SO2 NO2 As Hg Dust

100

600a 50a 10 300a 200a 3a 5 mg/Nm3a 100 mg/Nm3a

a Calculated by the basis of 12% O2.

Fig. 1. A schematic diagram of the MSWI process experimented.

G.-J. Song et al. /Waste Management 24 (2004) 99–106 101

organic chlorides with deposition into solid particles.CaO was found in almost the same amount in both thebottom ash and the ash from WST, but larger amountof CaO in the ash from SDA was shown due to injectedlime solution to remove acid gas. Most of the metaloxides having low volatility such as Fe2O3 and CuO,were found with large amounts in the bottom ash. Thebasicity of bottom ashes ranged from 0.08 to 1.46. Thecommon basicity of bottom ashes was indicated as <1(Masayuki, 1994). However bottom ash had differentchemical compound concentrations and contents ofheavy metals with particle sizes. Metal oxide and basi-city were increased as the particle size of bottom ashdecreased, with the exception of SiO2, Na2O and CuO.The ash from WST was indicating higher basicity thanbottom ash. The ash from SDA showed the highestbasicity due to injected lime solution and the basicity ofthe ash from BF was also high.

3.2. Metal distribution in ashes of different locations

The metal content was investigated to observe thedistribution characteristic of the remaining metals in

bottom ash and fly ash. The concentration of heavymetals increased as the particle size of bottom ashesdecreased (Park, 1996) as shown in Table 5 and Fig. 2.It is thought that the coarser particles have a longerretention time so that the metal has more thermal vola-tilization; on the other hand, the fine particles do nothave sufficient retention time to become volatile andsoon dropped down to the bottom ash pit. In case of flyash, most of the heavy metal concentrations were higherthan those in bottom ash as shown in Table 5. This couldbe explained by the fact that the vaporized metals werecondensed and aggregated on the surface of fly ash due tothe temperature in flue gas cooling system. The highestmetal concentration was observed at the bag filter, whichcollected the particles having large specific surface area.The material balances of Pb, Cd, Cu, Cr and Hg,

which have been regulated in the emission from MSWcombustion, are shown in Fig. 3 (Kim et al., 2001). Thematerial balances were obtained from the measurementsof concentration of metals in gas streams before andafter each APCD and the collected analysis data ofmetal contents and the amount of ash in different sam-pled locations. From such heavy metal distributions onecan classify them into two groups of metals. As a groupof non-volatile metals, Pb, Cu and Cr were retained atabout 88–97% in bottom ash due to their low volatilities.The rate of vaporization/dispersion was only 3–12%, sothat the most of them were collected in bottom ash. Thenon-volatile metals were collected between values of0.5–7% at WST and were emitted at rates of 0.009–2%at stack. Cr was less affected by temperature because itis known to convert into CrO3, which is a relatively morestable compound in high temperature oxidation condi-tions (Chang and Biswas, 1993). The collection rate of

Table 4

Results of XRF analysis for ashes from different locations (Unit: dry wt.%)

Bottom ash

Fly ash

>5 mm

2–5 mm 0.85–2 mm <0.85 mm WST SDA BF

pH

3.46 11.7 12.2 12.3 5.5 12.3 11.6

SiO2

53.16 34.82 33.10 25.30 16.61 5.83 4.73

TiO2

0.18 0.90 1.55 2.08 4.06 1.94 1.17

Al2O3

3.98 8.58 10.32 10.77 8.82 2.79 1.77

Fe2O3

1.40 5.99 5.75 4.49 3.63 2.36 0.95

Cr2O3

0.13 0.14 – – 0.16 0.07 0.20

K2O

0.94 1.93 2.29 2.19 1.98 1.53 10.57

Na2O

6.67 2.24 1.94 2.07 1.44 – 10.92

P2O5

5.00 10.07 7.02 6.45 4.75 1.42 1.20

CaO

19.13 27.88 30.43 37.04 33.96 52.90 25.16

MgO

1.49 2.17 2.27 3.30 6.02 2.55 1.25

CuO

1.48 0.76 0.45 0.33 0.11 0.07 0.20

PbO

0.08 0.11 0.16 0.15 0.15 0.12 0.49

ZnO

0.67 1.70 1.32 0.95 1.04 0.54 0.85

SO3

5.27 1.51 1.95 2.84 7.02 6.69 7.98

Cl

0.41 1.20 1.45 2.03 8.58 20.32 31.72

Basicitya

0.36 0.08 0.92 1.46 2.04 9.07 5.36

a Basicity: CaO/SiO2.

Table 3

Properties of incinerated MSW

Item

Unit Value Element Unit Composition

Bulk density

ton/m3 0.37 C Wt.% 46.00

Moisture

Wt.% 56.14 H Wt.% 6.75

Combustible

Wt.% 36.05 N Wt.% 1.85

Fixed carbon

Wt.% 7.81 S Wt.% 0.26

High heating value

kJ/kg 8904 O Wt.% 35.35

Low heating value

kJ/kg 6947 Cl Wt.% 1.97

102 G.-J. Song et al. /Waste Management 24 (2004) 99–106

Table 5

Metal concentrations in bottom ash and fly ash from MSW

Bottom ash (mg/kg)

Fly ash (mg/kg)

>5 mm

5–2 mm 2–0.85 mm <0.85 mm Mixture WST SDA BF

Ba

11.246 36.660 61.568 66.133 43.405 63.780 41.980 33.983

Na

436.126 5118.000 7414.517 10 348.965 5739.613 9028.000 6178.000 37 122.575

Mg

552.179 3029.000 5290.942 7662.234 4115.366 11 030.000 8851.000 6681.664

Al

1800.175 31 696.830 37 182.563 45110.000 28 055.941 25 690.000 10 240.000 6430.714

Ca

28 628.549 77 110.000 91 581.684 119 188.081 78 013.105 57 720.000 117 100.000 65 206.959

V

9.973 10.430 10.499 19.662 12.985 10.160 6.993 4.166

Cr

0.343 80.373 408.078 536.186 355.721 115.100 169.070 183.303

Mn

166.633 875.900 889.000 978.902 703.075 803.100 580.300 315.437

Fe

8906.000 12 048.000 25 050.000 23 860.450 17 521.312 3427.000 2006.000 761.848

Co

1.311 2.294 2.354 5.282 2.824 6.975 3.275 1.874

Cu

682.391 1196.320 2367.197 4597.282 2246.780 70.570 406.700 601.880

Zn

578.900 1935.000 4918.016 9879.000 4405.403 4516.000 9036.000 12 813.437

Sr

41.303 134.500 153.269 220.278 135.299 121.800 138.300 108.978

Cd

3.875 5.341 19.069 25.346 13.667 14.190 15.690 190.162

Sn

72.901 448.900 922.416 972.003 597.957 447.000 366.700 767.147

Sb

30.508 22.800 54.159 59.194 42.498 46.570 75.420 157.768

Pb

53.808 325.800 1542.691 1596.840 888.567 227.900 254.000 2053.589

Hg

0.001 0.034 0.005 0.047 0.021 0.021 11.443 48.445

Fig. 2. Metal concentrations in different size particles of bottom ash from MSWI.

G.-J. Song et al. /Waste Management 24 (2004) 99–106 103

Cu showed slightly higher values than other studies(Brunner and Monch, 1985). It is thought that Cu con-verted to Cu2O, which has a relatively high boilingpoint, so that the volatile compound was relatively less.As another classified group of semi-volatile or volatile

metals, Cd and Hg remained at values of about 0.8–

18.36% in bottom ash, and the rate of vaporization/dispersion from the furnace was in the range of 81.64–99.2%. However, Cd showed a comparatively lowemission portion, 0.03%, at stack since it was controlledat air pollution control devices with the condensation/precipitation on the fly ash surface by the temperature

Fig. 3. Distribution of major heavy metals at different locations in MSWI.

Table 6

Comparison between total concentration and leachate concentration by KSLT

Ashes

Concentrationsa Pb Cd Cr Cu Zn Hg Fe Mn

BF ash

Total Conc. 2053.6 190.2 183.3 601.9 12813.4 48.4 3762 315.4

Leachate Conc.

94.2 0.3 1.6 0.4 9.7 0.08 0.8 0.4

Ratiob

45.89 1.38 8.75 0.65 0.75 1.61 0.22 1.20

SDA ash

Total Conc. 254.0 15.7 169.1 406.7 9036.0 11.4 2006 580.3

Leachate Conc.

3.862 0.286 2.094 0.253 7.229 0.003 0.429 0.596

Ratio

15.20 18.23 12.39 0.62 0.80 0.26 0.21 1.03

WST ash

Total Conc. 227.9 14.2 115.1 70.6 4516.0 0.02 3427 803.1

Leachate Conc.

1.2 0.2 0.6 0.02 3.2 – 0.5 0.8

Ratio

5.42 16.42 5.22 0.30 0.71 – 0.14 1.00

Bottom ash >5 mm

Total Conc. 53.8 3.9 184.8 682.4 578.9 0.001 8906 166.6

Leachate Conc.

0.1 0.04 0.2 0.3 0.2 – 0.3 0.01

Ratio

1.82 9.29 0.90 0.41 0.37 – 0.04 0.06

Bottom ash 5–2 mm

Total Conc. 325.8 5.3 294.8 1196.3 1935.0 0.03 12048 875.9

Leachate Conc.

0.1 0.04 0.2 0.4 0.5 0.001 0.06 0.02

Ratio

0.39 8.24 0.72 0.37 0.28 29.41 0.01 0.02

Bottom ash 2–0.85 mm

Total Conc. 1543.0 19.1 408.1 2367.2 4918.0 0.005 25050 889.0

Leachate Conc.

0.2 0.05 0.3 0.5 1.4 – 0.8 0.02

Ratio

0.14 2.46 0.67 0.22 0.29 – 0.03 0.02

Bottom ash <0.85 mm

Total Conc. 1596.8 25.3 536.2 4597.3 9879.0 0.05 23860 978.9

Leachate Conc.

0.3 0.06 0.3 0.9 3.0 0.001 0.6 0.02

Ratio

0.194 2.25 0.55 0.20 0.30 21.28 0.03 0.02

Regulatory limit of leachate

3 0.3 1.5c 3 – 0.005 – –

a Total concentration: mg/kg, Leachate concentration: mg/l.b Ratio:%=100�leachate conc.�leachate vol.�sample weight/total conc.c as Cr+6.

104 G.-J. Song et al. /Waste Management 24 (2004) 99–106

cooling down. Most of the Hg in MSW was volatilizedand about 54.2% was removed by WST. In total, 41.4%was collected by SDA/BF and about 3.6% was emittedinto the atmosphere. Mercury in MSW was easily con-verted into mercury vapor in the combustor and thevapor reacted with HCl in off-gas to produce mostlyHgCl2 (80–90%). HgCl2 could then be removed in WSTbecause it has high solubility in water (Atherton, 1940).It was emitted with higher concentration (3.6%) thanother metals at stack, because it is difficult to captureelemental mercury form by adsorption or absorption.The behavior of heavy metals found in this study wassimilar to the results of other studies (Kazuo, 1994), butsome difference would be shown due to different wastecomposition, conditions and incineration processes.

3.3. Leaching behavior of heavy metal from ashes

Leaching characteristics of eight metals are shown inTable 6 with the regulatory limit values of the KSTL fornon-hazardous solid waste in Korea. Fly ash, in mostcases, had higher leaching ratios of metal than bottomash. The ash from BF had a high leaching ratio of Pbbecause of high content and more leachable pH and theash from WST showed increased leaching ratio of mer-cury because the water-soluble mercury leached out. Inthe results of analysis for metal content and leachingbehavior of heavy metals, bottom ash showed highconcentrations of Cu, Zn and Fe, but low leaching ratioof these metals. The ash from WST and SDA showedhigh contents of Zn, Fe and Mn but high leaching ratiosof Pb, Cd and Cr. The ash from BF had high con-centrations of Pb and Zn and high leaching ratios of Pband Cr. It could be difficult to explain all the leachingcharacteristics of heterogeneous ash from MSWIbecause the leaching of ashes depends on many physico-chemical influences (Johnson et al., 1996; Akilo et al.,1996). However, these results are worthwhile to showjust irregularity of metals leaching characteristics fromthe ashes collected at different locations in a commercialMSWI. One could predict better trends and character-ization of such ashes to understand metal behavior inMSWI with the addition and analyzing of more data.From chemical compositions, metal contents andleaching behavior of different ashes from MSWI, theinvestigation suggests some special care and furthertreatment for different ash. Such brief considerationson different ash from various locations are summar-ized for intermediate treatment or final disposal in theconclusion.

4. Conclusion

The characteristics of ashes from different locations inthe commercial MSWI equipped with WST, SDA, BF

and SCR as air pollution control devices were investi-gated. From the obtained results, the following conclu-sions could be made with some suggestions forsubsequent treatment:

1. Bottom ash had a large amount of SiO2 and

Al2O3 and low basicity and different character-istics with particle sizes. Metal oxide and basicityboth increased as the particle size of bottom ashdecreased, with the exception of SiO2, Na2O andCuO. Therefore the treatment of bottom ashmight be considered for different particle sizes.

2. Major chemical compositions of the ash from

WST were SiO2, Al2O3 and CaO. The compo-nents of SO3, Ca, Mg and Na, which were solublein water, were high and the leaching of toxicheavy metals affecting the environment was alsohigh. Therefore, the treatment of ashes fromWST might be considered with care of extrac-table compounds by water.

3. The ash from SDA and BF contained a large

amount of CaO, Na2O, K2O, Cl, Zn, Pb and Hg.and showed very high basicity. It was predictedthat the ash from SDA and BF contained thesoluble salts such as NaCl and KCl, and metals(Jakob et al., 1995) and organic chlorides (Sakaiet al., 1996) because of increasing Cl in the ashes.Therefore, the treatment of ash from SDA andBF could be necessary to stabilize such solublecompounds and highly leachable heavy metals.

4. The coarser particles in bottom ash had higher

heavy metal contents than the finer ones. It wasconsidered that the coarser particles had longerretention time in the furnace so that metalswould get more chances of thermal volatilization,on the other hand, the fine particles had insuffi-cient retention time to become volatile anddropped down to the bottom ash pit.

5. Due to low volatilization, about 88–97% of Pb,

Cu and Cr remained in bottom ash, and theirvaporization/dispersion ratios ranged from 3 to12%. It could be suggested that the aging (Shin etal., 2000) treatment before their reuse or recy-cling is needed to decrease the leachability ofthese metals.

6. Due to high volatilization of Cd and Hg, about

18% of Cd and less than 1% of Hg were left inbottom ash, and their vaporization/dispersionratios were 82 and 99%, respectively. Also theirleaching ratios were very high, therefore, the flyash requires stabilization with proper methods.

7. The leaching ratios of metals in bottom ash were

lower than those in fly ashes because of the highamount of SiO2 having net structure existed inbottom ash. Usually most of the heavy metalswere fixed in the net structure (Kim, 1996) so that

G.-J. Song et al. /Waste Management 24 (2004) 99–106 105

the fixed heavy metals were not easily extracted.The investigation also suggests a simple treat-ment such as aging or recycling.

Acknowledgements

This work was supported by Yonsei UniversityResearch Fund of 2000 and BK21 Project.

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