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MATERIAL CHARACTERISATION OF THE MUNICIPAL SOLID WASTE INCINERATOR ASH

A.P. Bayuseno

Department of Mechanical Engineering, Faculty of Engineering, Diponegoro University

Jl. Prof. Soedarto SH, Kampus Tembalang Semarang Email: apbayuseno@gmail.com

Abstract

This report presents preliminary results of materials characterisation on the municipal solid waste incinerator (MSWI) ash, namely bottom ash (BA) and fly ash (FA). Our work focuses on the application of state-of-the-art x-ray diffraction full profile analysis techniques for mineralogical phase analysis. Additionally, we characterised the samples by Scanning Electron Microscopy (SEM) with energy-dispersive spectrometry (EDS) to study the particle morphology and surfaces of the raw material. The bulk elemental compositions of the ash were determined by X-ray fluorescence (XRF). The mineralogical phase assemblage of MSWI bottom ash as well as of the fly ash is apparently unstable and reactive. Within three months significant changes in phase composition have been observed.

Key words: “Bottom ash”, “Fly ash” 1. Introduction

Municipal solid waste (MSW) is generally combusted in an incinerator at temperature between 850 and 1000OC, producing large amount of bottom ash (BA) and some amount of fly ash (FA) residues containingsignificant heavy metals (e.g. As, Pb, Sb, Sn, Sr) as well as trace amounts of organic pollutants (e.g. polychlodibenzo-dioxins and-furanes) (Rincon et.al, 1999). These residues are classified as hazardous special waste in most countries (Hjelmar, 1996). These materials must be subsequently subjected to an intermediate treatment to detoxify or decontaminate those incorporated heavy metals prior to disposal. In addition, the considerable amounts of fly ash must be deposited in special landfills equipped with careful control of the effluents (Andreola, et.al, 2001; Rincon et.al, 1999).However, the disposal strategy of MSWI residues by landfilling may have long term-consequences for the environment, due to the potential leaching of contaminant. Therefore, the resource recover of MSWI by converting these residues into new raw materials is currently a major concern. The recent development studies on MSWI materials have been long focussed on recycling

technologies, including vitrification, devitrification and the incorporation into ceramic matrices in order to destroy the organic component, and immobilise toxic elements and minerals by structural incorporation in a solid state lattice (Barbieri, et.al, 1997; Scarinci, et.al, 2000). This technique looks promizing in the way of stabilisation of complex materials such as fly ash and bottom ash. An alternative approach for treating the waste materials may also include a hydrothermal process which converts it to zeolite-like materials through synthesis (Yang and Yang, 1998). In order to develop effectively for a subsequent treatment of the MSWI ash, it requires a comprehensive knowledge of the mineralogy and chemical properties. Here, state of knowledge about these properties may provide useful information how to access chemical and physical durability when used as a resource due to the enrichment of scarce elements over their abundances in average soils and rocks. This report present result of characterisation on the basic physicochemical properties of the inorganic portion of MSWI ash from a MHKW facility in the Ruhr industrial area, Germany. The major element chemistry and quantification of the mineralogy and

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distribution of elements among various chemical phases are described. Aim of this study is to explore qualitatively and quantitatively the phase abundance within the materials in order to be able to develop a long-term ash stabilisation method.

2. Materials and methods

MSWI ash sampling The MSWI ashes selected in the study were bottom ash (BA) and fly ash (FA), which were collected from an incinerator plant in the Ruhr industrial area, Germany. Approximately 15 kg of both freshly wetted ashes were collected as initial material for the experiment. Fractionation Due to the heterogeneity of MSWI materials, fractionation procedures have been applied to increase the opportunity for detection and identification of phases occurring in lower concentration in the materials. Magnetic particles (<300 µm) were separated with a hand magnet. Components of the FA have also been chemically separated in a Soxleth extractor. The FA was initially placed in a paper filter capsule in the extractor. Water steam rising from boiling water in a flask is condensed in a Liebig cooler. The condensing H2O continuously drips down into the filter capsule and fills the cylindrical glass container around the filter until a certain fluid level is reached, and the fluid drains into the boiling flask. This process repeats itself periodically. Soluble components are thus quickly transported from the sample to the solution in the boiling flask. The precipitated particles were then filtered and dried at room temperature for subsequent XRD analyses. Chemical analysis Total elemental chemistry (mass of elemental/dry weight of ash) was conducted by wavelength dispersive X-ray fluorescence (XRF). For all size fractions two type’s materials were prepared. Eight grams of the

dried ash materials were mixed with 2 gram/100 ml acetone as a binder. The mixture was finely homogenised in an agate bowl. The powder pellets were pressed at 20 Kg/cm2 for 2 minutes and then analysed by XRF for the elements. SEM analysis A LEO DSM microscope with EDS Link ISIS system was used for SEM/EDS analysis. Samples were placed on the sample holders supported by carbon paint followed by 1-µm sputter coating of gold. Mineralogical analysis All MSWI materials including the residues obtained by the fractionation processes were subjected to X-ray diffraction analysis. For each fresh ash, it was combined with 0,100 gram of rutile internal standard. Each sample was then ground 10 min with a mortar and pestle. The powder of each specimen was loaded into an aluminium well mount and flattened and compacted with a glass slide. Data collection was performed using Cu-Kα monochromated radiation in a conventional Bragg-Brentano (BB) parafocusing geometry (a Philips MPD and Siemens D500 Diffractometer). The scan parameters (5 –85o 2θ, 0,020-0,022 steps, 12-15 s/step) were selected as required for both types of samples. Data were recorded digitally, and peak positions and intensities were identified either using the peakfinder feature or on screen in the software. A PC-based search match program, the Philips X’Pert Software (Philips Electronics N.V) was used to help identify possible crystalline phases in the ash samples. The possible crystalline phase candidates suggested by search match procedures were subsequently judged by the Rietveld full profile fitting analysis. In this technique, the full x-ray diffraction contribution of each phase to the profile is calculated reliably from available crystal structure data.

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Each XRD data set was evaluated by the Rietveld analysis using the FullProf code. (Rodriguez-Carvajal, 1998). The starting structure models for each phase identified in qualitative analysis were taken from literature, or from the ICSD database (1999). These structure data sets were also tested by performing Rietveld refinements on various pure phases and synthetic phase mixtures. 3. Results and Discussion

Bulk elemental composition Table 1 provides information on the total elemental composition of the municipal solid waste incineration ash as a result of XRF analysis. The composition is given in the form of weight percent, independent of the actual form of chemical binding in the ash. Oxygen was not analysed. However, the cations are mainly bound in oxidic compounds (oxides, silicates, sulphates) while some chlorides (NaCl, KCl) and chloro-complexes are also present. The major constituents (>1 %) in the BA samples collected generally comprised Si, Al, Fe, Mg, and Na. Minor constituents (0,1-1%) include Ba, Cu, Mn and P. The volatile elements such as Cl, S, K, Na, Cd, Pb and Zn show a fractionation into the FA while refractories containing Si, Al and Fe concentrate in the BA. The fractionation depends on combustion temperature and burnout at the MSW incineration facility (Eighmy et.al, 1995). These elements are volatile due to low melting and boiling points of the metals Pb and Zn and compounds like PbO and NaCl. Additional elements which appear to be volatilised during combustion include F, As, Cr, Cs. Sn, as indicated by the higher value of this concentration in FA samples, compared to those in the BA samples.

Table 1. Total Composition of Municipal Solid Waste Incineration Ash

BA FA BA FA element % % Element % % Si 22,38 5,634 Gd <0,001 0,001 Ti 0,811 1,018 Hf 0,004 0,003 Al 6,363 2,427 Ho <0,001 <0,001 Fe 8,916 3,707 Mo 0,010 0,049 Mg 1,475 1,078 Nb <0,001 Mn 0,138 0,122 Nd 0,002 0,002 K 0,853 6,110 Ni 0,036 0,061 Na 3,347 10,378 Pb 0,102 1,363 P 0,391 0,400 Pr 0,001 0,001 Ca 9.726 12.411 Rb 0,003 0,026 As 0,002 0,031 S 0,495 4,114 Ba 0,393 0,347 Sb 0,007 0,163 Bi 0,020 Sc 0,002 0,006 Cd 0,001 0,046 Sm 0,001 Ce 0,005 Sn 0,004 0,033 Cl 0,320 8,320 Sr 0,040 0,054 Co 0,007 0,026 Tb <0,001 <0,001 Cr 0,116 0,203 U <0,001 Cs 0,001 0,011 V 0,008 0,009 Cu 0,774 0,351 W <0,001 Dy <0,001 <0,001 Y 0,008 0,089 Er <0,001 Yb <0,001 0,001 Eu <0,001 Zn 0,773 4,907 F 0,044 0,304 Zr 0,031 0,023 Ga 0,003 The results show that any element abundant in the earth’s crust occurs in the incineration ashes and fractionation of volatiles into the fly ash leads to an unpleasant concentration of heavy metals in the fly ash. Morphology and Particle Chemistry The grains of the BA samples (Figure 1) consist of agglomerates of smaller aggregates (5-100µm), which themselves consist of small flaky particles. This aggregates identified by EDS comprise largely O, Mg, Ca, Cl, Si, Al, Na, Fe and C. Additionally, Mg and Ti were commonly associated with Fe-rich grains as were occasional small concentrations of Mn and /or Cr.

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Figure 1. SEM micrograph of the as-

received bottom ash sample

In contrast to the BA, the FA consists largely of 5-100 µm spheres (Figure 2) which are commonly coated or agglomerated with very fine-grained particles < 0,1µm. The submicron particles always form larger aggregates. SEM/EDS revealed that the aggregates largely contain O, K, Na, Zn, Cl, Ca and S in the FA samples. The spherical particles were more clearly observed in FA samples reacted with water. The spherical particles come from melt droplets which form during combustion. Figure 2. SEM micrograph of the as-

received fly ash sample XRD Mineralogy by Rietveld Full Profile After we identified potential phases contained in the ashes by XRD classical search match methods, we verified the presence of possible phases by full profile Rietveld refinement. In cases of doubt, we verified the XRD profiles

by analysing synthetic pure phases or minerals from the mineral collection, as well as synthetic mixtures. The reliability of the phase identification can be judged from the difference between the observed and calculated XRD profiles. Bottom ash Figure 3 shows a representative Rietveld refinement plot for the bottom ash. Note that the intensity of all major peaks is well represented in the calculated diffractogram. Additional work is necessary to substantiate a few minor peaks, though. Figure 3. Quality of the Rietveld pattern-

fitting results for the full diffractogram. Top curve: observed (����������������) and calculated (-----) patterns respectively. Hash marks indicate XRD peak positions for mineralogical phase compositions (from top to bottom): rutile (TiO2) internal standard, quartz (SiO2),augite (Ca,Mg,Fe2+,Al)2(Si,Al)2O6, anhydrite (CaSO4), calcite (CaCO3), magnetite (Fe3O4), corundum, (Al2O3), gehlenite (Ca2Al2SiO7), haüyne (Na6 Ca2 Al6Si6O24 (SO4)2), magnesioferrite (MgFe3+

2O4), tricalcium aluminate (Ca3Al2O6), silicon carbide (SiC), along with the associated difference plot.

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Figure 4 a and b present the diffractograms of the as-received BA samples and the same sample three-months later, respectively. Figure 4. X-ray diffractograms for (a) the

as-received bottom ash and (b) the as-aged bottom ash. The peaks are labelled; A (corundum), An (anhydrite), Au (augite), C (calcite), Cr (cristobalite), E (ettringit), H (hematite), Hy (hydrocalumite), L (lepidocrocite), M (magnetite), P(phengite), Q (quartz), R (rosenhahnite), Ri (riebeckite), Ru (rutile) as internal standard, T (tricalcium aluminate), Sc (silicon carbide).

Comparison of the diffractograms shows that the principal minerals identified by XRD analysis are different indicating an alteration process in the sample. The ashes are

metastable products, the phase composition changes while in contact with moisture and potentially due to instability of the high-temperature mineral paragenesis formed rapidly during burning, which transforms into an association of minerals typical for ambient/humid conditions. The principal minerals in as-received BA materials that were identified by XRD Rietveld analysis, although not all are labelled in the diffractogram, were anhydrite (CaSO4), augite (Ca,Mg,Fe2+,Al)2(Si,Al)2O6, calcite (CaCO3), corundum, (Al2O3), gehlenite (Ca2Al2SiO7), hematite (Fe2O3), haüyne (Na6 Ca2 Al6Si6O24 (SO4)2), iron (Fe), magnetite (Fe3O4), magnesioferrite (MgFe3+

2O4), quartz (SiO2), silicon carbide (SiC), tricalcium aluminate (Ca3Al2O6). Rutile (TiO2) was added as a internal standard. However, after 3-months, the phase composition has transformed, and the new phases are in particular ettringite (6CaO.Al2O3.32H2O),phengite(KAl2Si3AlO10(OH)2), rosenhahnite (Ca3Si3O8(OH)2), lepidocrocite (Fe+3O(OH)), riebeckite (Na1.38K0.13CaMg0.25Mg2.8Fe1.66Fe0.48Al0.04Si7.94

O22(OH)2=) and Hydrocalumite (Ca8 Al 4(OH)24(CO3)Cl2(H2O)1.6(H2O)8), the amount of calcite has significantly increased. The alteration process (ageing process) in bottom ash is thus related to, but not restricted to, the formation of hydrated minerals (Speiser et.al, 2000). Since we do not observe any significant reduction in the content of a crystalline phase, we propose that the alteration consumes the glass phases, which are not readily visible in XRD. Magnetic separation was also done to examine the heavy magnetic fraction. The magnetic separate contains magnetite (Fe3O4), wustite (FeO), iron (Fe), magnesioferrite spinel (MgFe3+

2O4), and the weakly magnetic hematite (Fe2O3). The spinel mineral may contain other elements such as Ca, Al, Ti, based on EDS analysis. Non-magnetic crystalline phases which were attached to the magnetic phases are corundum, (Al2O3) and quartz (SiO2).

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Fly ash A typical Rietveld plot for the as-received FA sample refinements is shown in Figure 5. The results of phase identified for the both samples are then given in the diffractograms of Figure 6, for the as-received and aged FA samples, respectively. Many phases identified during search-match and Rietveld profile matching is not labelled in this figure due to extensive peak overleap. Comparison of the diffractograms provides information on an alteration process also occurring in this material. Figure 5. Quality of the Rietveld pattern-

fitting results for for the full diffractogram. Top curve: observed (����������������) and calculated (-----) patterns, respectively. Hash marks indicate XRD peak positions for mineralogical phase compositions (from top to bottom): rutile (TiO2) as a internal standard, quartz (SiO2), calcite (CaCO3), anhydrite (CaSO4), halite (NaCl), lime (CaO), sylvite (KCl), muscovite (Kal3Si3O10(OH)2), cristobalite (�-SiO2), gehlenite (Ca2Al2SiO7), diopside (CaMg(SiO3)2), hematite (Fe2O3), potassium tetrachlorozincate (K2ZnCl4), ulvospinel (FeTiO4), kalsilite (KAlSiO4) along with the associated difference plot.

Figure 6. X-ray diffractograms for (a) the

as-received fly ash and (b) the aged fly ash. The peaks are labelled: Al (alunite), An (anhydrite), C (calcite), CAO (calcium aluminum oxide hydroxide hydrate), Cr (cristobalite), D (diopside), G (gordaite), Ht (halite), Hy (hydrocalumite), Lm (lime), O (olivine), PzC (potassium tetrachlorozincate), Q (quartz), Ru (rutile as internal standard), Us (ulvospinel), S (sylvite), Sd (sodalite), Sy (syngenite), T (tricalcium aluminate) and Tm (thomsonite).

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The principal minerals in the as-received FA samples are anhydrite (CaSO4), calcite (CaCO3), cristobalite (SiO2), gehlenite (Ca2Al2SiO7), hematite (Fe2O3, lime (CaO), magnetite (Fe3O4), olivine (Mg,Fe)2SiO4, quartz (α-SiO2), rutile (Ti2O) as a internal standard, tricalcium aluminate (Ca3Al2O6), ulvospinel (Fe2

2+TiO4), in addition to highly soluble sodium and potassium salts such as halite (NaCl), sylvite (KCl) and potassium tetrachlorozincate (K2ZnCl4). K2ZnCl4 is not known as mineral and its identification in fly ash appears unusual. However, from the Rietveld refinement of the synthetic compound, which is in excellent agreement with the corresponding pattern observed in the ashes, the identification is certain. The minerals found here were also reported by Henry et.al (1983) and by Kirby and Rimstidt (1993). This chloride “mineral” may be formed in the fly ash as a result of condensation from the hot flue gas by heterogeneous nucleation on particles (Eighmy et.al, 1995). Further minerals that were obtained include diopside (CaMgSi2O6) and kalsilite (KAlSiO4) as well as sodalite (Na4Al3Si3O12Cl) and muscovite (KAl 3Si3O10(OH)2). In comparison to the as-received FA (Figure 6 a) many of the soluble minerals like K2ZnCl4 have disappeared after 3 months (Figure 6 b). Consequently, some new phases are formed such as the sulphates syngenite (K2Ca(SO4)2.H2O), gordaite (NaZn4(SO4)(OH)6Cl(H2O)6) and alunite (KAl 3(SO4)2(OH)6, a calcium aluminum oxide hydroxide hydrate (CaO)-Ca6Al 2O6(OH)6.32H2O), and the zeolite thomsonite (NaCa2Al 5Si4O20.6H2O). Thus the alteration is primarily related to the dissolution/reprecipitation reaction of salts and their chemical components (Speiser et.al, 2000). The leaching experiments in the Soxleth apparatus provide mainly gypsum (CaSO4.2H2O) as a precipitated mineral in the aqueous phase, while the leached ash residue

contains anhydrite (CaSO4), calcite (CaCO3), cristoballite (α-SiO2), ettringite (6Ca.Al2O3.32H2O), gehlenite (Ca2Al2SiO7), magnetite (Fe3O4), quartz (SiO2), gypsum (CaSO4.2H2O) and wustite (FeO). In comparison to the as-received fly ash, many of the soluble minerals like NaCl were dissolved. 4. Conclusion

Fly ash and bottom ash from the same MSW incinerator have been investigated with respect to chemical composition and mineralogical phase composition, and with respect to ageing of the materials in time. The chemical fractionation is obvious: refractory materials and heavy particles are concentrated in the bottom ash and volatile compounds in the extremely fine-grained fly-ash. However, understanding the structural association of the chemical components into mineralogical phases is far from straightforward. It needs to be known to comprehend and prevent long-term reactions occurring in the ashes when deposited or reused as building materials. While we highly improved the reliability of the XRD phase identification using Rietveld profile matching techniques and synthetic reference samples, further work is still needed. The major elemental constituents of both materials are Si, Al, Fe, Ca, Na, which are present as oxides, silicates, sulphates, and chlorides, usually in the form of common rock-forming minerals. In the fresh fly ash, the compound K2ZnCl4 has been positively identified, which does not occur in nature. During three months, there is considerable mineralogical alteration of both fly ash and bottom ash. New minerals forming in the bottom ash are calcite, ettringite, gedrite, and rosenhahnite. In the fly ash, minerals forming or during ageing are anhydrite, the hydrous sulphates syngenite and gordaite (the latter containing the zinc of the decomposed K2ZnCl4) and hydrocalumite, a hydrous chloridic carbonate of calcium and aluminium.

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For both BA and FA materials, the new-formed minerals are all characteristic phases growing at ambient conditions from unstable high-temperature products such as cement clinker or from volcanites in nature. To develop appropriate stabilisation methods for waste recycling or deposition management, the phase instability and alteration reactions need to be evaluated in further detail. Using XRD so far as the main tool of investigation, we have not been able to characterize the amorphous phase contents which are present in the samples. The amorphous phases are expected to be unstable, notably in the presence of the salts, and thus they may be involved in the long-term alteration reactions. We therefore plan studies of reaction mechanisms on the grain scale by polarisation microscopy and electron microprobe analysis in parallel to further improvement of our XRD phase identification. 5. Acknowledgements

The authors also thank Dr. Neuser and Dr. Bernhardt for helping with the collection of electron microscope and microprobe analysis data; Dr. Müllejans from MVA Iserlohn, providing with the sample. 6. References

Andreola, F., Barbieri, L., Corradi, A, Lancellotti, I, and Manfredini, T., 2001. The Possibility to Recycle Solid Residues of the Municipal Waste Incineration into a Ceramic Tile Body. Journal of Materials Science, 36, 4869-4873.

Barbieri, L., Manfredini, T., Queralt, I., Rincon, J.Ma. and Romero, M., 1997. Vitrification of Fly Ash from Thermal Power Stations. Glass Technology, , 38, 165-170.

Carvajal, J.R., 1998. Program FullProf. Laboratoire Leon Brillouin (CEA-CNRS).

Eighmy, T., Eusden, J.D., Krzanowski, J.E., Dominggo, D.S., Stampfli, D., Martin, J. and Erickson, P.M., 1995. Comprehensive Approach toward Understanding Element Speciation and Leaching Behaviour in Municipal Solid Waste Incinerator Electrostatic Precipitator Ash. Environmental Science and Technology, 29,629-646.

Hjelmar, O., 1996. Disposal Strategies for Municipal Solid Waste Incineration Residues. Journal of Hazardous Materials, 47, 345-368.

Henry, W,M., Barbour, R,L., Jakobsen, R.J., Schumacher, P.M. Inorganic Compound Identification of Fly Ash Emissions from Municipal Incinerators. U.S. EPA Project 600/S3-83-095;U.S.EPA: Washington, DC.

Kirby, C.S. and Rimstidt, J.D., 1993. Mineralogy and Surface Properties of Municipal Solid Waste Ash. Environmental Science and Technology, 29,629-646.

Rincon, J.MA., Romero, M.and Boccaccini, A.R., 1999. Microstructural Characterisation of a Glass and a Glass-Ceramic obtained from Municipal Incinerator Fly Ash. Journal of Materials Science, 34, 4413-4423.

Scarinci, G., Brusatin, G., Barbieri, L., Corradi, A., Lancellotti, I., Colombo, P., Hreglich, S. and Dall’Igna, R., 2000. Vitrification of Industrial and Natural Wastes with Production of Glass Fibers. Journal of the European Ceramic Society, 20, 2485-2490.

Speiser, C., Baumann, T and Niessner, R., 2000. Morphological and Chemical Characterisation of Calcium-Hydrate Phases Formed in Alteration Processes of Deposited Municipal Solid Waste Incinerator Bottom Ash. Environmental Science and Technology, 34, 5030-5037.

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Speiser, C., Baumann, T and Niessner, R., 2001. Characterisation of Municipal Solid Waste Incineration (MSWI) bottom ash by scanning electron microscopy and quantitative energy dispersive X-ray microanalysis (SEM/EDX). Fresenius Journal Analytical Chemistry, 370, 752-759.

Yang, G.C.C. and Yang, T.Y., (1998). Synthesis of Zeolites from Municipal Incinerator Fly Ash. Journal of Hazardous Materials, 62, 75-89.