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Waste Management 26 (2006) 1462–1467

Diopside-based glass-ceramics from MSW fly ash and bottom ash

Guangren Qian a,*, Yu Song a, Cangang Zhang b, Yuqin Xia b,Houhu Zhang a, Pengcheong Chui c

a School of Environment Engineering, Shanghai University, No. 149 Yangchang Road, Shanghai 200072, Chinab Administration of Solid Waste Management, Pudong New Area, Shanghai 200135, Chinac School of Civil & Environment Engineering, Nanyang Technological University, Singapore

Accepted 16 December 2005Available online 20 February 2006

Abstract

By utilising MSW fly ash from the Shanghai Yuqiao municipal solid waste (MSW) incineration plant as the main raw material, diop-side-based glass-ceramics were successfully synthesized in the laboratory by combining SiO2, MgO and Al2O3 or bottom ash as condi-tioner of the chemical compositions and TiO2 as the nucleation agent. The optimum procedure for the glass-ceramics is as follows:melting at 1500 �C for 30 min, nucleating at 730 �C for 90 min, and crystallization at 880 �C for 10 h. It has been shown that the diop-side-based glass-ceramics made from MSW fly ash have a strong fixing capacity for heavy metals such as lead (Pb), chromium (Cr), cad-mium (Cd) etc.� 2006 Elsevier Ltd. All rights reserved.

1. Introduction

Fly ash from municipal solid wastes (MSW) incinerationplants contains different kinds of toxic heavy metals, suchas Cd, Cr, Pb, etc., which exist in fly ash as oxides and chlo-rides. These heavy metals have a high leachability. Besidesheavy metals, MSW fly ash may also contain a myriad oftoxic chlorinated organic compounds, such as dioxinsand furans (Kirby and Rimstidt, 1993). For these reasons,MSW fly ash is classified as hazardous waste and must bedisposed of safely.

Stabilization/solidification is the usual method for treat-ing and disposing MSW fly ash. This includes cement solid-ification, lime solidification, thermoplastic encapsulationand vitrification. Compared with cement solidification,the vitrification technology has the advantages of reducingthe ash volume, detoxifying the ash, and reducing the spacerequired for waste landfill. At the present time, there aremore than 100 vitrification plants for MSW fly ash in

0956-053X/$ - see front matter � 2006 Elsevier Ltd. All rights reserved.

doi:10.1016/j.wasman.2005.12.009

* Corresponding author. Fax: +86 21 56333052.E-mail address: grqian@mail.shu.edu.cn (G. Qian).

Japan that are operating commercially. However, thepotential returns of products obtained by the vitrificationalone are still not attractive, as the glass-like slag producedusually has been either used as a blend material for manu-facturing cement and fine aggregates for civil engineeringpurposes or landfilled with no significant economicbenefits.

To improve the potential market value of vitrificationproducts from MSW fly ash, glass-ceramics may be a betterchoice because glass-ceramics can be easily formed throughcontrolled de-vitrification of glass. The structure of glass-ceramics is composed by finely grained particles withrandomly oriented crystals, with some residual glass butwithout voids, micro-cracks. Glass-ceramics made withMSWI fly ash tend to be highly value-added and have beenextensively used in commercial uses, e.g., electronic prod-ucts, household decoration and other uses (Endo et al.,1997; Leroy and Ferro, 2001).

A number of studies have been focused on glass-ceram-ics using waste materials such as coal ash, sewage sludgeash and other materials (Ribeiroa et al., 2002; Leroy andFerro, 2001). The feasibility of using MSW fly ash to pro-duce glass-ceramics has been explored in laboratory-scale

Table 2Leaching of MSWI fly ash (mg/l)

Element Pb Cr Cd

Leaching of MSW fly ash 14.35 2.97 2.57

Table 3Designed mix proportions for glass-ceramics using MSW fly ash (dryweight basis, wt%)

Sample no. Fly ash SiO2 MgO TiO2 Al2O3

1 88 5 5 2 02 83 10 5 2 03 78 15 5 2 04 73 20 5 2 05 68 25 5 2 06 83 5 5 2 57 78 10 5 2 58 73 15 5 2 59 68 20 5 2 5

10 63 25 5 2 5

G. Qian et al. / Waste Management 26 (2006) 1462–1467 1463

tests (Park and Heo, 2002a,b; Park et al., 2003; Cheng andChen, 2003; Cheng et al., 2002; Gutman, 1995). The glass-ceramics produced in these studies not only possessed goodmechanical properties, the material exhibited very lowleachability of heavy metals.

However, the technology in developing glass-ceramicsusing MSW fly ash cannot be applied to different typesof MSW fly ash even if using the same operating parame-ters, due to differences in the fly ash characteristics. Thecomposition of MSW fly ash is dependent on the constitu-ents of the incinerated refuse, thermodynamic conditionsduring the combustion process, flue gas treatment technol-ogy and other factors. Hence, the heat treatment condi-tions for glass-ceramics, such as the parameters ofnucleation and crystal growth, are different depending onthe source of the fly ash. In China, the source separationof municipal solid wastes is not well practiced, so thatthe contents of heavy metals in the ash may vary signifi-cantly from one city to another. For instance, high concen-trations of heavy metals have been detected in the MSW flyash from an incinerator receiving electronic wastes(E-wastes) from a city with a well developed electronicsindustry. It is thus necessary to also investigate the poten-tial of glass-ceramics in immobilizing high concentrationsof heavy metals.

In this paper, the technical parameters for producingglass-ceramics using the fly ash from Shanghai MSW areexplored, and the immobilizing capacity of diopside-basedglass-ceramics on heavy metals is also reported.

2. Materials and methods

2.1. Properties of MSW fly ash and bottom ash

The MSW fly ash and bottom ash used in this studywere obtained from the Yuqiao municipal waste incinera-tion plant in Shanghai. The appearance of the fly ash wasa gray fine powder. The bottom ash also appeared as grayin color but was grainy, containing some sundries such ascullet, tiles, rusty nails, broken porcelain pieces, etc. Themain chemical composition of the MSW fly ash and bot-tom ash is given in Table 1.

Leaching tests were conducted in accordance with theChina leachability toxicity standard method (GB/T15555.1–15555.11). Leaching liquor in the leaching test isdiluent HCl solution, with a pH of 5.8–6.3 and liquid-to-solid ratio of 20:1. In the test, the vessel of liquor wasshaken flatly for 8 h and then held still for 16 h. The con-centrations of selected heavy metals (Pb, Cr, and Cd) lea-ched from MSW fly ash are shown in Table 2.

Table 1Main chemical compositions of MSW fly ash and bottom ash (dry weight bas

Compound SiO2 CaO MgO Al2O3

Fly ash 18.22 25.34 2.39 6.74Bottom ash 45.21 26.81 1.64 11.20

2.2. Determination of mix proportions for glass-ceramics

made from MSW fly ash and bottom ash

The proportions of raw materials, i.e. fly ash and bot-tom ash, were determined based on their inherent chemicalcompositions, as well as the desired major crystalline phaseof the glass-ceramics product. The diopside phase wasdesigned as the major crystalline phase of the glass-ceram-ics. Two experimental approaches were adopted.

(1) Fly ash as main raw material. When MSW fly ash wasused as the main raw material, other supplement con-stituents, namely SiO2, MgO, Al2O3, and TiO2, werealso added to result in suitable mix proportions of thediopside-based glass-ceramics. Table 3 shows thedesigned mix proportions of the glass-ceramics madefrom MSW fly ash.

(2) Fly ash and bottom ash as the main raw material. AsSiO2 content in the bottom ash exceeds 45%, it maybe possible to use both MSW fly ash and bottomash together as the main raw materials to preparediopside-based glass-ceramics without the need forSiO2 supplement. Table 4 shows the designed mixproportions of glass-ceramics made from MSW flyash and bottom ash.

(3) Mixes with high concentrations of heavy metals.

Although the concentration of heavy metals in theMSW fly ash, as listed in Table 2, is quite significant,their concentration in the MSW fly ash could varysubstantially, depending on the origin of the munici-pal solid wastes and the incineration process. In order

is, wt%)

Fe2O3 Na2O K2O Cl� SO3

3.65 5.51 4.34 12.29 13.016.48 1.33 1.85 10.89 12.25

Table 4Mix proportions of glass-ceramics from MSW fly ash and bottom ash (dryweight basis, wt%)

Sample No. Fly ash Bottom ash MgO TiO2 Al2O3

11 63 30 5 2 012 53 40 5 2 013 43 50 5 2 014 33 60 5 2 015 23 70 5 2 016 58 30 5 2 517 48 40 5 2 518 38 50 5 2 519 28 60 5 2 520 18 70 5 2 5

1464 G. Qian et al. / Waste Management 26 (2006) 1462–1467

to evaluate if glass-ceramics can immobilize a highconcentration of heavy metals, separate mixtureswere prepared by spiking samples 1 and 2 with 1%Pb(NO3)2, 1% K2Cr2O7, and 1% Cd(NO3)2 (byweight). The content of Pb, Cr, and Cd in the modi-fied mixtures was 6256, 3537, and 4746 mg/kg of mix-ture, respectively.

2.3. Preparation of glass-ceramics

To ensure that the raw material was melted completelyand eliminate the stress of the glass formed, the procedurefor the preparation of glass was as follows: the raw mix-tures of the glass-ceramics were first melted in an aluminacrucible at 1500 �C for 30 min, and then were poured ontoa copper mould in air to form a glass matrix with a regularshape. The melting was undertaken in an oxideatmosphere.

In order to enhance the conversion effectiveness of theglass matrixes into glass-ceramics products, the processesfor nucleation and crystal growth of the glass must be care-fully controlled. The key parameters of heat treatment,including the temperature and the duration time, must bedetermined experimentally. To meet this objective, a NEI-

Fig. 1. TG and DTA curves of gla

ZSCH STA 449C thermal analyzer was used for determin-ing the kinetic parameters, such as temperature andduration time, of nucleation and crystal growth.

2.4. Evaluation of glass-ceramics properties

The density of glass-ceramics was measured by theArchimedes method with deionised water as the medium.The crystallized phases of the glass-ceramics were analyzedby an XRD diffraction instrument and identified by theJoint Committee on powder diffraction standards (JCPDS)data files. The leached properties of Cd, Cr, and Pb fromthe glass-ceramics were evaluated according to the ChinaLeachability Toxicity Standard method (GB/T 15555.1–15555.11). The concentrations of leached heavy metalswere measured using a Perkin–Elmer 5100PC atomicabsorption spectrometer.

3. Results and discussion

3.1. Heat treatment parameters

For the determination of temperatures of nucleation andcrystallization, a differential thermal analysis (DTA) wasused to obtain the weight-loss and heat-effect curves, whichwere produced in the process of heat treatment. As shownin Fig. 1, there is an obvious endothermic peak in the tem-perature range of 700–750 �C, which is in accordance withthe nucleation temperature of glass-ceramics. There is alsoanother exothermic peak in the temperature range of 870–920 �C, indicating that at that temperature, the glass beganto be crystallized and the rate of crystallization graduallyreached its maximum.

Combined with the results of observation of the appear-ance of the glass-ceramics by the naked eye and the recrys-tallization features by polarized microscopy, the heattreatment process of glass-ceramics can be taken as fol-lows: nucleation taking place at 730 �C for 90 min andcrystallization followed at 880 �C for 10 h. In order to

ss-ceramics for Sample No. 16.

5 10 15 20 2510

15

20

25

30

35

40

45

50

55

6018 20 22 24 26 28 30 32 34 36 38

Com

pres

sive

Str

engt

h (M

Pa)

Amount of SiO2 added (wt %)

no Al2O

3 added

5% Al2O

3 added

Total amount of SiO2 (wt %)

Fig. 3. Compressive strength of glass-ceramics as a function of theamount of SiO2 added.

G. Qian et al. / Waste Management 26 (2006) 1462–1467 1465

favor glass-ceramic crystallization, the rate of temperature-rise was 10 �C/min before the endothermic peak and wasdecreased to 6 �C/min after the endothermic peak. To elim-inate the stress in the formation process of glass-ceramics,the glass-ceramics was annealed slowly to roomtemperature.

3.2. Major crystalline phase of glass-ceramics

Four representative samples (No. 1, No. 6, No. 11, andNo. 16) were characterized by XRD phase analysis. Asshown in Fig. 2, they had very similar features on boththe peak positions and the peak intensity in the XRD spec-trum. Compared to the JCPDS card, the strong peak linesin the locations of d = 3.25, 3.02, 2.97, 2.92, 2.57, 2.53, and2.04 A represent that the major crystalline phase of theglass-ceramics was diopside (CaMgSi2O6).

Based on designed chemical composition, the contentof Al2O3 in sample Nos. 6 and 16 were higher than thatin sample Nos. 1 and 11. Sample No. 6 was made fromMSW, but No. 16 was from bottom ash. Meanwhile,the addition of bottom ash as the raw material in mixtureswill make the chemical compositions of glass-ceramicsmore complex. Despite these differences, the major crystal-line phases of all of the samples were the same diopside(CaMgSi2O6). This shows that certain variations in chem-ical composition have a lesser influence on the formationof major crystalline phase diopside. It was obviouslybeneficial for the application of bottom ash as theraw material.

3.3. Compressive strength of glass-ceramics

Compressive strength is an important parameter forevaluating the mechanical properties of glass-ceramics.As shown in Fig. 3, there is a relation between compressivestrength and the amount of SiO2 added to the mixtures.

20 30 40 50 60 70 80

NO1

3.25

CM

S2

NO 6

NO11

2.92

CM

S2

2.97

CM

S2

2.52

CM

S2

2.04

CM

S 2

2.58

CM

S23.

02 C

MS

2

NO16

Fig. 2. XRD pattern of glass-ceramic for samples.

The compressive strength of glass-ceramics was found tobe directly proportional to the amount of SiO2 added tothe mixtures, i.e., proportional to the total content ofSiO2 in the mixtures. For sample No. 10 with 25% SiO2

and 5% Al2O3, its compressive strength could reach53.9 MPa. According to the theory of glass chemistry,SiO2 is the principal component of the interior frameworkof glass-ceramics. The bonds between micro-crystals wouldget stronger with the increase of SiO2 content (McMillan,1964), resulting in an increase in compressive strength ofglass-ceramics.

When MSW fly ash only was used as the main raw mate-rial, the compressive strength of glass-ceramics had a sig-nificant increase with a 5% addition of Al2O3 over thatwithout an Al2O3 addition. Furthermore, the compressivestrength of sample No. 10 was 252% higher than sampleNo. 5. This could be attributed to the fact that the additionof an appropriate amount of Al2O3 favors the formation ofthe crystalline phase and promotion of adhesion betweenglass-ceramics particles. Based on the observations of crys-tallization by polarized microscopy, it was found that theaddition of a suitable Al2O3 content had a positive rolein the crystallization of the diopside phase.

The compressive strengths of glass-ceramics producedfrom both bottom ash and fly ash were evidently higherthan from fly ash only. The relationships between compres-sive strength and the addition of bottom ash are shown inFig. 4. The compressive strength of glass-ceramics with70% bottom ash reached 69.04 MPa, while the maximumcompressive strength with fly ash only was 53.9 MPa. Thiscould be due to the fact that bottom ash contained otherminor metal elements such as Fe and Mn. During the crys-tallization process, these minor metals would decrease theviscosity of the hydraulic phase, which promoted crystalli-zation and improved the density of glass-ceramics leading

30 40 50 60 7010

20

30

40

50

60

70

Com

pres

sive

str

engt

h (M

Pa)

Bottom ash content (wt %)

no Al2O3 added

5% Al2O3 added

Fig. 4. Compressive strength of glass-ceramics as a function of bottomash content.

Table 6Selected heavy metal leaching from glass-ceramics (lg/l)

Element Pb Cr Cd

Sample 1 spiked with heavy metals 23.0 13.6 2.4Sample 2 spiked with heavy metals 18.3 18.2 1.4

1466 G. Qian et al. / Waste Management 26 (2006) 1462–1467

to a stronger adhesion within the glass-ceramics (McMil-lan, 1964).

However, the compressive strength of glass-ceramicsproduced from the mixture of bottom ash and fly ashhad no significant increase when Al2O3 was added. Accord-ing to chemical compositions, the content of Al2O3 in thebottom ash is higher than that in the fly ash. Therefore,it may mean that Al2O3 had no additional benefit to theformation of diopside-based glass-ceramics.

3.4. Heavy metals leaching from glass-ceramics

The leaching test is an important way to evaluate theimmobilization capacity of heavy metals by the glass-ceramics. Fly ash generated in the process of MSW inciner-ation has a relatively high concentration of heavy metals.In this study, Pb, Cr, and Cd were chosen as the targetedheavy metals. The leachates of heavy metals from thetwo groups are given in Table 5.

By comparing the results with the original MSW fly ash,several to dozens of ppb of heavy metals were leached fromthe glass-ceramics, far lower than the limits specified in theChinese standards.

To verify the fixing capacity of diopside-based glass-ceramics for high concentrations of heavy metals, an addi-tional 1% (by wt) K2Cr2O7, 1% Pb(NO3)2 and 1%Cd(NO3)2 were spiked into samples No. 1 and No. 2.As seen in Table 6, in these samples the leaching of heavy

Table 5Results of heavy metal leaching from glass-ceramics (lg/l)

Sample No.

1 2 3 6 7 8

Pb 28.7 31.4 30.7 16.9 20.3 28.2Cr 12.6 5.9 9.1 <0.1 <0.1 4.6Cd 2.8 1.9 1.2 1.1 0.4 1.1

metals from the glass-ceramics was still rather low: Pb 18–23 lg/l, Cr 13–18 lg/l, and Cd 1–2 lg/l, although high con-centrations of heavy metals were added into the mixtures.These were far lower than the limits specified in the Chinesestandards. It was also noted that volatilization of heavymetals simultaneously occurred in the melting process.When 1% Pb(NO3)2, 1% K2Cr2O7, and 1% Cd(NO3)2 werespiked into raw mixtures, 60% of Pb and 58% of Cd in theraw mixtures volatilized while the volatilization from Crwas less at 1500 �C for 90 min, based on the results ofheavy metals volatilization (Song, 2004). Remaining heavymetals had been fixed in the matrixes of glass-ceramicsexcept for volatilization and final leaching. The resultsfurther established that diopside-based glass-ceramics havea high immobilization capacity for heavy metals suchas Pb, Cr, and Cd.

4. Conclusions

Diopside-based glass-ceramics can be produced byusing MSW fly ash and bottom ash as raw materials.The optimum controlling parameters for producing diop-side-based glass-ceramics are melting at 1500 �C for30 min, nucleating at 730 �C for 90 min, and crystallizingat 880 �C for 10 h. Under these conditions, high compres-sive strengths of glass-ceramics can be achieved fromMSW fly ash with 5% additions of Al2O3, and from themixture of fly ash and bottom ash. Meanwhile, the leach-ing of heavy metals from glass-ceramics was far lowerthan the specified limits in the Chinese standards, indicat-ing that the diopside-based glass-ceramics from MSW flyash have a strong fixing capacity for heavy metals such asPb, Cr, and Cd.

Acknowledgement

The authors acknowledge that this project No.032312043-2 has been financially supported by Science &Technology Committee, Shanghai, China.

11 12 13 16 17 18

24.0 20.7 41.1 36.6 29.9 43.415.6 14.6 22.4 1.2 <0.1 <0.15.1 4.1 2.6 1.9 0.5 0.7

G. Qian et al. / Waste Management 26 (2006) 1462–1467 1467

References

Cheng, T.W., Chen, Y.S., 2003. On formation of CaO–Al2O3–SiO2 glass–ceramics by vitrification of incinerator fly ash. Chemosphere 51 (9),817–824.

Cheng, T.W., Ueng, T.H., Chen, Y.S., Chiu, J.P., 2002. Production of glass-ceramic from incinerator fly ash. Ceramics International 28 (7), 779–783.

China Leachability Toxicity Standard Method (GB/T 15555.1–15555.11),1995.

Endo, H., Nagayoshi, Y., Suzuki, K., 1997. Production of glass ceramicsfrom sewage sludge. Water Science and Technology 36 (11), 235–241.

Gutman, R., 1995. Recycling of municipal solid waste incinerator ash bythermal separation and vitrification. In: Proceedings of R’95 Confer-ence, On Recovery, Recycling and Reintegrating, EMPA, SwitzerlandIV, pp. 9–11.

Kirby, C.S., Rimstidt, J.D., 1993. Mineralogy and surface properties ofmunicipal solid waste ash. Environmental Science and Technology 27(4), 652–660.

Leroy, C., Ferro, M.C., 2001. Production of glass-ceramics fromcoal ashes. Journal of European Ceramic Society 21 (2), 195–202.

McMillan, P.W., 1964. Glass-ceramics. Academic Press, London.Park, Y.J., Heo, J., 2002a. Conversion to glass-ceramics from glasses

made by MSW incinerator fly ash for recycling. Ceramics International28 (6), 689–694.

Park, Y.J., Heo, J., 2002b. Vitrification of fly ash from municipal solidwaste incinerator. Journal of Hazardous Materials B91 (1–3), 83–93.

Park, Y.J., Moon, S.O., Heo, J., 2003. Crystalline phase control of glassceramics obtained from sewage sludge fly ash. Ceramics International29 (2), 223–227.

Ribeiroa, M.J., Tulyaganovb, D.U., Ferreirab, J.M., Labrinchab, J.A.,2002. Recycling of Al-rich industrial sludge in refractory ceramicpressed bodies. Ceramics International 28 (3), 319–326.

Song, Y., 2004. Utilization of MSW fly ash as diopside based glassceramic. Master Thesis, Shanghai University.